Perovskite solar cells: A new paradigm in Energy sector
G. Grancini, C. Roldán-Carmona, I. Zimmermann and M. K. Nazeeruddin
Group for Molecular Engineering of Functional Materials, Institute of Chemical Sciences and Engineering, Ecole polytechnique fédérale de Lausanne, CH-1951 Sion, Switzerland.
ABSTRACT: Organo-metal trihalide perovskites have revolutionized the field of thin film solar cells due to their meteoric rise of power conversion efficiency (PCE) of a record value over 22%.1 The advantage of perovskite material is low-cost precursors, and capable of being processed a variety of scalable methods.2-3 Despite impressive photovoltaic performances of perovskite solar cells, most reported devices are not stable under operation. Here we report stable perovskite solar cells by engineering an ultra-stable 2D/3D HOOC(CH2)2NH3)2PbI4/CH3NH3PbI3 perovskite junction. The 2D/3D perovskite films are produced in a single step from the solution containing a mixture of 3% HOOC(CH2)2NH3I, methylammonium iodide and PbI2. The composite mixture self-assembles into gradually organized multidimensional structure that yields up to 14.6% employing mesoporous TiO2 and spiro-MeOTAD as electron and hole specific contacts, respectively. The XRD and photoluminescence analysis demonstrate the unique role of the 2D perovskite, anchored at the interface with the oxide nanoparticle network, in templating and stabilizing the 3D CH3NH3PbI3 phase. The resulting 2D/3D metal halide perovskite solar cells in a hole-conductor free architecture exhibit a record stability under AM 1.5 sunlight. Also, we address hysteresis by using molecularly engineered novel hole transporting materials.4
(2). Lee, M. M., Teuscher, J., Miyasaka, T., Murakami, T. N. & Snaith, H. J. Science 338, 643−647 (2012).(3). Burschka, J., Pellet, N., Moon, S-J., Humphry-Baker, R., Gao, P., Nazeeruddin &Grätzel, M, Nature 499, 316−319 (2013).(4). A molecularly engineered hole-transporting material for efficient perovskite solar cells, Michael Saliba & Mohammad Khaja Nazeeruddin, Nature Energy 1, Article number: 15017 (2016), doi:10.1038/nenergy.2015.17
(3). Burschka, J., Pellet, N., Moon, S-J., Humphry-Baker, R., Gao, P., Nazeeruddin &Grätzel, M, Nature 499, 316−319 (2013).
(4). A molecularly engineered hole-transporting material for efficient perovskite solar cells, Michael Saliba & Mohammad Khaja Nazeeruddin, Nature Energy 1, Article number: 15017 (2016), doi:10.1038/nenergy.2015.17
Hybrid metal halide perovskites (stoichiometry AMX3) have recently emerged as low-cost active materials in PV cells with power conversion efficiencies in excess of 20%. We discuss how parameters essential for photovoltaic operation, such as crystallinity, photostability, charge carrier mobility and diffusion lengths are altered with substitutions of the organic A cation (e.g. methylammonium versus formamidinium), the metal M cation (e.g. Pb2+ or Sn2+) and the halide X anion (I- versus Br-). We focus on two 3D perovskite systems that have attracted interest lately, lead-free ASnI3 (optical bandgap ~1.3 eV) and the mixed organic lead iodide/bromide system APb(BryI1-y)3 whose band gap can be tailored between ~1.5 eV (FAPbI3) and ~2.3 eV (FAPbBr3). We show that unintentional hole doping in tin iodide perovskites introduces fast recombination pathways that limit the charge-carrier diffusion length. However, changes in crystal structure appear to subtly influence the relative alignment of dopant levels with respect to the valence band, offering a route to reduced background hole densities . In addition, we demonstrate that such hole doping introduces a radiative quasi-monomolecular charge recombination channel that supports efficient light emission even in the low charge-carrier density regime . In addition, we demonstrate that charge-carrier diffusion and recombination in FAPb(BryI1-y)3 depends on a complex interplay between changes in morphology and electronic bandstructure with bromide fraction y . In particular, a “stability gap” that leads to photo-induced halide segregation in the central region (y=0.3–0.5) is associated with low crystallinity and charge-carrier mobility [4,5]. We show that the replacement of a small fraction of FA with caesium (e.g. FA0.83Cs0.17Pb(I0.6Br0.4)3) lifts this instability allowing for high charge-carrier mobilities (21 cm2/(Vs)) and diffusion lengths . We find that the substitution range of 10-30% caesium fraction is associated with higher crystallinity which correlates with improved optoelectronic properties and photo-stability .
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 Rehman, McMeekin, Patel, Milot, Johnston, Snaith, Herz, Energy Environ. Sci. ASAP (2017).
An unexpected and impressive evolution in organic-inorganic perovskite solar cells (PSCs) with a vertical rise in power conversion efficiency (PCE) from 3.8% to 22.1% has energized the photovoltaic community to fabricate low-cost devices from this material with excellent optoelectronics properties. The highly efficient PSCs have been fabricated by using mesoporous TiO2 electron transport layers (ETLs), requiring annealing at high temperatures while mesoporous-free “planar” TiO2 ETL has suffered from a pronounced hysteresis. On the other hand, recent studies have shown the potential of low-temperature planar SnO2 ETLs in highly efficient PSCs with less hysteresis than planar TiO2. However, there is a lack of understanding of this improvement and detailed investigations of SnO2 layer and the interface between SnO2 and perovskite layers are needed.
Here, we fabricated PSCs using SnO2 prepared by atomic layer deposition (ALD) at low temperature as an ETL and triple cation and mixed halide perovskite as an absorber layer. The SnO2 layers were annealed at different temperatures in order to investigate if the hysteresis is resulting from the mismatch of energy levels of perovskite and SnO2 which can be changed by different annealing temperatures. Later, we have characterized the SnO2 layers with XRD and XPS, ensuring the formation of pure SnO2. Moreover, our UPS measurements showed that the band alignment between perovskite absorber and SnO2 layer match quite well, resulting in less hysteresis. Therefore, low-temperature SnO2 represents a significant contribution, offering a suitable energetics and improvements in series resistance at the ETL/perovskite interface on the way towards industrialization of PSCs.
Two-dimensional layered perovskites can be obtained by replacing common small singly charge cations such as methylammonium, cesium or formamidinium by a larger cation, for instance butyl ammonium. Mixtures of small and large cations can results in so-called Ruddlesden-Popper structures where multiple layers of inorganic perovskite are separated by organic layers. This approach can be used to tune the absorption and emission properties but in almost all examples the organic part has no specific functionality. In this work we explore possibilities to introduce specific functionality in these sidechains. Using pulse-radiolysis time-resolved microwave conductivity measurements we can study the mobility of charges and their recombination kinetics. As a first example, we discuss the effect of the length of the alkyl chains that separate the inorganic layers on the recombination of charge. This is related to the inter-layer transport in these materials, which is a key parameter in the efficiency of light emitting diodes. In the second example we explore the introduction of more functionality in the organic part using electronic structure calculations. For instance the introduction of a strongly electron accepting chromophore can lead to enhanced exciton dissociation in the 2D perovskites. This is important since it has been shown that exciton binding energies in these materials are so high that most charges exist as bound excitons at room temperature. Overall, we demonstrate that the opto-electronic properties of 2-D perovskites can be tuned, not only by varying the thickness and composition of the inorganic part but also by introducing specific functionality in the organic component.
The high efficiencies and expected low production costs together with fast energy payback time of (Pb-based) halide perovskite (HaP) photovoltaic cells bode well for commercialization of these cells. The main question mark remaining as to if these cell can be commercialized is whether they will exhibit the 20+ years lifetime required of all large-scale photovoltaic cells to date or not. Stability studies on HaP materials and cells show a wide range of behaviors; however, some studies do show promising stability over at least the medium time frame (months). I discuss some of these studies and then focus on our own results relevant to HaP (both Pb- and Sn-based) materials and cells. Since the effect of a range of incident radiation is important (e.g. e-beam and XRD) also for the ability to reliably measure these materials (such as in electron microscopy and XPS), this talk will include the effects of such radiation sources on the HaPs as well as that of solar radiation.
Work done in collaboration with David Cahen
To improve the performance of existing perovskite solar cells (PSCs), a detailed understanding of the underlying device physics during their operation is essential.
As a first step, we have developed and validated a device model that describes the operation of PSCs and quantitatively explains the role of contacts and of (doped) transport layers, carrier generation, drift and diffusion of carriers and recombination. We fit the simulation to experimental data of vacuum deposited CH3NH3PbI3 solar cells over multiple thicknesses. By doing so, we identify a unique set of parameters and physical processes that describe these solar cells. Trap-assisted recombination at material interfaces (HTL/perovskite and perovskite/ETL) is the dominant recombination channel limiting the device performance and passivation of these traps increases the power conversion efficiency (PCE) of these devices by 40%. Finally, we issue guidelines to increase performance and show that a PCE beyond 25% is within reach.
Grain boundaries (GBs) are ubiquitous in polycrystalline films and are studied extensively in CIGS, poly-Si and CdTe solar cells. The Seto model is able to successfully describe the GB physics in these solar cells. However, PSCs are different. Perovskites are lightly doped materials and due to the presence of ionic defects it is likely that the traps at GBs are charged when empty and neutral when filled, in contrast to the basis of the classic Seto model. Therefore, a different perspective on GB physics is essential for PSCs.
We include grain boundaries in our model and fit the simulation to the experimental data of vacuum deposited CH3NH3PbI3 solar cells in p-i-n and n-i-p configuration. Our model quantitatively explains (for both p-i-n & n-i-p cells) the light intensity dependence of the VOC and FF, delineating the recombination dynamics at GBs and interfaces under different operating conditions. We find that despite the presence of traps at GBs, their neutral (when filled) disposition along with the long-lived nature of holes leads to the high-performance of PSCs. We also give an estimate of the defect ion density in these solar cells.
Furthermore, we shed light on the role of charged grain boundaries which may exist under some conditions (under/over stoichiometric preparation).
 T. S. Sherkar, C. Momblona, L. Gil-Escrig, H. J. Bolink, L. J. A. Koster (submitted)
 J. Y. Seto, J. Appl. Phys. 1975, 46, 5247-5254.
 T. S. Sherkar, C. Momblona, L. Gil-Escrig, M. Sessolo, H. J. Bolink, L. J. A. Koster (in preparation)
Organometal halide perovskites are promising materials for photovoltaic devices and have demonstrated a rapid increase in performance in the last decade. Recently, perovskite solar cells have passed the threshold of 20 % power conversion efficiency (PCE) by optimized processing and device structures. Despite the high PCE, perovskite solar cells are still not competitive to their inorganic counterparts in terms of production scalability and lifetime.
Most of the devices reported in literature are fabricated by small-scale solution-based processing techniques (e.g. spin-coating). Perovskite solar cells produced by vacuum thermal evaporation (VTE) have also been attracting considerable attention, due to uniformly deposited layers, high PCE and reproducibility. Regarding the co-evaporation of the perovskite constituents, this technology is challenged by large differences in thermodynamic characteristics of the two species. While the organic components evaporate instantaneously, higher temperatures are needed for reasonable deposition rates of the metal halides. An option to overcome the mentioned issues are vapor phase based processes, which have been proven to be a desirable choice for industrial large-area production.
In this study, we present a setup for the deposition of methylammonium lead iodide (MAPbI3) via chemical vapor deposition employing nitrogen as carrier gas. Therefore, we developed evaporation sources for temperatures up to 500 °C in case of lead iodide and 150 °C for methylammonium iodide. The deposition rates can be easily controlled by adjusting carrier gas flows. In the pressure regime of 10 – 20 mbar, the deposition of the perovskite layer is carried out on 2.5 cm x 2.5 cm large either fluorine-doped tin oxide on glass or silicon substrates involving a reaction of both co-deposited components. The substrate can be heated or cooled to control layer formation and reaction kinetics. We discuss the impact of the simultaneous and alternating deposition of the precursors on the resulting films and point out the effect of the substrate temperature on the structural properties. X-Ray diffraction measurements are used to verify perovskite formation. In order to investigate the crystal quality and structural properties scanning electron microscopy is carried out.
By optimizing the deposition parameters, we produced uniform perovskite films at a deposition rate of 30 nm/h. Furthermore, the developed CVD process can be easily scaled up to larger substrates, thus rendering this technique a promising candidate for manufacturing large-area devices. Moreover, CVD of perovskite solar cells can overcome most of the limitations of liquid processing, e.g. the need for appropriate and orthogonal solvents.
Issues on stability and durability of hybrid perovskites are dramatically delaying the large dissemination of low cost/high yield related technologies for photovoltaic, light sensing and emitting purposes. Thereby, the advance of the technologies is currently forced to pass through the rationalization of the phenomena occurring into the hybrid lattice under conditions which mimic the material operation. In this framework, we study the structural modifications of MAPbI3 layers by in-situ structural and optical analyses upon recursive thermal cycles from 30°C to 80°C in different annealing environments. We reveal a hitherto unknown phenomenon consisting in an acceleration of the material degradation, above what expected, as the threshold of the tetragonal to cubic transition (50°C) is surpassed. This produces discontinuities in the degradation rate, bandgap and dielectric behavior of the material. The phenomenon is put in relationship with the order-disorder lattice modifications described by Car-Parrinello molecular dynamics calculations, and reveals that the action of species from humid air becomes largely more effective above 50°C for reasons related to the increased accessibility/reactivity of the lattice which, in turn, rises defects generation.
The materials, called perovskites, are particularly good at light absorbing and has stunned the photovoltaic community. Solar cells made of this materials, namely the currently termed perovskite solar cells (PSCs) as evolved from dye-sensitized solar cells (DSSCs), was recognized as one of Science’s 10 breakthroughs in 2013. However, only by an innovative strategy we developed to make the highest quality single-crystal perovskites was us able to study the photovoltaic merits of this materials in their purest form – perfect single crystals . By using a combination of laser-based techniques to track down the rapid motion of charge carriers in the material, we determined the mobility—how fast the carriers can move through the material as well as the diffusion length—how far carriers can travel without getting trapped by imperfections in the material. Our work identifies the bar for the ultimate solar energy-harvesting potential of perovskites.
On the other hand, despite intense research efforts, the performance of spiro-OMeTAD as the most commonly used hole-transporting layer (HTL) in PSCs and DSSCs has remained stagnant, creating a major bottleneck for improving solar cell efficiency. Thinking that the material has given all it has to offer, many researchers have begun investigating alternative materials to replace spiro-OMeTAD in future solar cells. But in a new study published in Science Advances , we have taken a closer look at spiro-OMeTAD and found that it still has a great deal of untapped potential. For the first time, we have grown single crystals of the pure material, and in doing so, we have made the surprising discovery that spiro-OMeTAD's single-crystal structure has a hole mobility that is three orders of magnitude greater than that of its thin-film form (which is currently used in solar cells). This work reports a major breakthrough for the fields of perovskite and solid-state dye-sensitized solar cells by finally clarifying the potential performance of the material and showing that improving the crystallinity of the hole transport layer is the key strategy for further breakthroughs in device engineering of these solar cells. The findings suggest that, at least in the short term, the time-consuming process of designing and synthesizing radically new organic hole conductors as replacements to spiro-OMeTAD may not be necessary.
 D. Shi, et al., Science 347, 519 – 522 (2015).
 D. Shi, et al., Science Advances 2, e1501491 (2016).
In the past few years, organic-inorganic halide perovskite solar cells have been attracted much attention due to their rapidly increasing power conversion efficiencies and compatibility with low cost fabrication. High performance achieved in perovskite solar cells has been attributed to high absorption coefficient and high mobilities for both electrons and holes. Surfaces and interfaces in these solar cell devices can have a strong impact on device performance. Also, many of the pressing challenges (e.g., stability, hysteresis, etc.) in organometal halide perovskite solar cell research are closely correlated with surfaces and interfaces. My group at OIST investigates relevant surfaces and interfaces for better understanding about perovskite materials and solar cells. Based on the findings, we develop strategies to further improve device performance. By creating a connection between surface and interface science studies and device fabrication and characterization, we have obtained deepened understanding about perovskite solar cells. In this presentation, I will introduce our recent research findings.
The impressively fast technological progress of highly efficient perovskite solar cells based on organic-anorganic lead halides such as CH3NH3PbI3 (MAPI) has made a huge impact on the photovoltaic research community. There are two main concerns linked to this technology: The device stability and the involvement of the heavy metal Pb. In consequence, major research efforts are put into the development of new material solutions and enhanced device architectures. One promising class of candidates are double perovskites, where two Pb ions (group IV) are replaced by one group III ion and one group V ion. In our contribution, we report on the successful synthesis of Cs2AgBiBr6 single crystals and their fundamental opto-electronic characterization. This lead-free double perovskite has an indirect band gap of 1.95 eV and is reported to have better stability than MAPI. Here, we report on our structural characterization by X-ray diffraction and a detailed study of the vibrational modes investigated by multi-wavelength Raman spectroscopy. In addition, we present an effective methodology to detect secondary phases in this material. We find the single crystals to be stable at ambient air conditions up to a temperature of approximately 200ºC. Finally, the potential of this material for photovoltaic applications will be discussed.
High crystal orientation in perovskite absorber layers is beneficial for photovoltaic device performance. Recently, we have shown perfectly oriented MAPbBr3 perovskite crystallites incorporated in solar cells which resulted in internal quantum efficiencies approaching unity. Here, we have developed a simple and fast route for MAPbI3 in order to tune the crystal orientation and crystal size independently. With this route, we show that the orientation of the long c-axis is switchable for the tetragonal MAPbI3 crystallites, where we can align it parallel to the substrate as well as the usual orientation with the c-axis perpendicular to the substrate. Our approach is based on different interactions of solvents and the Pb(Ac)2 precursor during perovskite crystal growth. We specified the influence of these parameters, grain size and crystal orientation in photovoltaic devices and investigated their optoelectronic characteristics with Time of Flight (ToF). Films with large and perfectly oriented grains show very high charge carrier mobility exceeding 8 cm2/Vs, approximately an order of magnitude higher than standard disordered samples. When applied in planar heterojunction solar cells, the short-circuit currents exceed 22 mA/cm2 with open-circuit voltages about 1.1 V. This results promote a further understanding of perovskite thin film crystallization behaviour and its resulting different film properties.
 N. Giesbrecht, J. Schlipf, L. Oesinghaus, A. Binek, T. Bein, et al. ACS Energy Lett., 2016, 1, 150-154
Methylammonium lead halide perovskites have become one of the most promising photovoltaic materials, with power conversion efficiencies now beyond 20%. Most of these certified solar cells are processed from solution, and the perovskite layer is deposited from toxic solvents such as dimethylformamide (DMF) or dimethyl sulfoxide(DMSO) on (mesoscopic) metal oxides. Vacuum techniques are solvent-free with unique intrinsic characteristics such as a precise control over the film thickness and composition, low-temperature processing, and the possibility of preparing multilayer structures. These advantages make vacuum techniques a viable alternative to solution-based processing for perovskite solar cell preparation. Here, we present the preparation and optimization of fully vacuum deposited perovskite solar cells employing doped organic charge transport layers. In this work we perform a direct comparison between two common configurations, one inverted with respect to the other (p-i-n and n-i-p), where the intrinsic layer (i) is a methylammonium lead iodide film. This configuration leads to planar solar cells without hysteresis and high efficiencies, 15% for p-i-n and 18% for n-i-p on average.
Methylammonium lead halide perovskites have been considered to be promising photovoltaic materials due to their large absorption coefficient, high carrier mobility, high carrier diffusion length, and direct band gap. We investigated the electrical properties of lead-free CH3NH3Sn(I,Br)3 perovskite materials under ambient conditions. The highest photo-conversion efficiency of the solar cells was 2.1% with TiO2 mesoporous structure. The work function and the local current were measured by Kelvin probe force microscopy and conductive atomic force microscopy. Surface potential distributions on grains and near grain boundaries of the Sn-based perovskite show a similar behaviors in the Pb-based perovskite [1,2]. The analysis of the physical quantities depending on time yields to us the extent of decomposition. An additional value of the surface potential shows up as the materials goes to degradation simultaneously with a sudden increase and subsequent relaxations of local current. The temporal behaviors of the surface potential and local current represent the instability of the perovskite material. Based on the understanding of the degradation mechanism, we will pave a way for improved photo-conversion efficiency in environment-friendly perovskite solar cells.
 G. Y. Kim, S. H. Oh, B. P. Nguyen, W. Jo, B. J. Kim, D. G. Lee, H. S. Jung, “Efficient Carrier Separation and Intriguing Switching of Bound Charges in Inorganic−Organic Lead Halide Solar Cells”, Journal of Physical Chemistry Letters, 6, 2355 (2015). Daehan Kim, Gee Yeong Kim, Changhyun Ko, Seong Ryul Pae, Yun Seog Lee, Oki Gunawan, D. Frank Ogletree, W. Jo, and Byungha Shin, “The effect of post-synthesis annealing duration on grain properties and photovoltaic performance of organic-inorganic hybrid perovskites”, Journal of Physical Chemistry-C, 120, 21330 (2016).
Here, we report the processing of efficient and relatively stable perovskite solar cells with both conventional and inverted structures using simple and low temperature solution-process buffer layers. Pure or copper-doped nickel oxide (NiO or NiO:Cu) and zinc oxide (ZnO) or niobium-doped titanium dioxide (TiO2:Nb) films were used as a hole transport layer (HTL) and electron transport layers (ETL), respectively. By using alcohols with different boiling points as solvents for the buffer layers, they can be used for all type of processing tools such as spin coating, slot-dye or Roll to Roll printers. The electrical conductivity of the buffer layers are sufficient enough giving the possibility to use them within different thicknesses leading to optimizing the optical cavity of full device. As photo-active layer, smooth and pinhole-free perovskite absorbing layers ware fabricated by anti-solvent method on rigid Glass/ITO substrates. This results can lead to fully solution-processed perovskite solar cells reducing manufacturing cost and high processing speed.
Despite the promising progress in improving the efficiency of CH3NH3PbI3 solar cells, long-term stability is one of the key issues that must be addressed before any market viability of perovskite solar cells could be suggested.
Incorporation of organic cations larger than methylammonium, leads to the formation of multi-dimensional perovskites (A2A’n-1BnX3n+1) which may be seen to be derived from their 3D counterparts (ABX3) by introducing appropriate organic cations (A, A’) .Mixed-dimensional perovskites could be rendered chemically stable at higher temperature due to the lower volatility of the larger cations.
One of the first work in mixed-dimensionality perovskite, a mixture of layered (2D) and 3D perovskites was reported in the form of (PEA)2(MA)2[Pb3I10], (with PEA = C6H5(CH2)2NH3+ and MA = CH3NH3+) . This material displayed a 2D and 3D mixed perovskite structure with promising efficiency, and stability against moisture. We have recently pursued a similar approach where mixed dimensionality perovskites were formed as (MA)n-1(EAI)2PbnX3n+1, with EAI = CH3CH2NH3I , where tunability of bandgaps and improved stability was brought about. Yet another approach of formation of bilayers (in nanoparticles) was also demonstrated by our group recently where a 2D layered perovskite (OA)2PbBr4 was formed on top of MAPbBr3 perovskite, OA =CH3(CH2)7NH3.
This presentation will outline a broad palette of elemental substitutions, solid solutions, and multidimensional families  that will provide the next step towards the advances of the perovskite solar cells and light-emitting devices. Challenges and opportunities in perovskite materials beyond methyl ammonium lead iodide,4-7 with particular emphasis on their recombination dynamics, optoelectronic properties, and integration into solar cells and light-emitting devices , will also be addressed.
 I.C. Smith, E.T. Hoke, D. Solis-Ibarra, et al. “A Layered Hybrid Perovskite Solar-Cell Absorber with Enhanced Moisture Stability”, Angew. Chem. Int. Ed., 2014, 53(42), 11232-11235.
 T.M. Koh, V. Shanmugam, J. Schlipf et al. “Nanostructuring Mixed-Dimensional Perovskites: A Route Toward Tunable, Efficient Photovoltaics”, Adv. Mater., 2016, 28(19), 3653-3661.
 S Bhaumik, SA Veldhuis, YF Ng, MJ Li, SK Muduli, TC Sum, B Damodaran, SG Mhaisalkar, N Mathews, Highly stable, luminescent core-shell type methylammonium-octylammonium lead bromide layered perovskite nanoparticles, Chem Comm, 52(44): 7118-7121, 2016
 P.P. Boix, S. Agarwala, T.M. Koh, N. Mathews, S.G. Mhaisalkar. “Perovskite Solar Cells: Beyond Methylammonium Lead Iodide”, Feature Article - J. Phys. Chem. Lett., 2015, 6(5): 898-907.
 S Veldhius, PP Boix, N Yantara, M Li, TC Sum, N Mathews, and SG Mhaisalkar, “Perovskite Materials for Light-Emitting Diodes and Lasers,” Advanced Materials, 2016, DOI: 10.1002/adma.201600669.
There is increasing evidence that the presence of mobile ions in perovskite solar cells can cause a current-voltage curve hysteresis. It is however still under debate how exactly mobile ions influence the device operation [1, 2]. We use drift-diffusion simulations incorporating mobile ions to describe IV-curves of preconditioned methylammonium lead iodide perovskite solar cells and compare them with experimental results. The occurrence of hysteresis is also related to the contact layer materials . Hereby the following question arises: If mobile ions in the bulk are responsible for the IV-curve hysteresis, why does the hysteresis depend on the contact materials? And why do highly efficient devices generally show low hysteresis?
Our simulation results show that the hysteresis depends on the extent of surface recombination and on the diffusion-length of charge carriers. We provide a detailed explanation for the reduced hysteresis of perovskite solar cells with high power conversion efficiencies. We find that in high-efficiency solar cells ion migration is still present, but does not cause a hysteresis effect. This finding is consistent with findings from Calado et al. that showed the presence of mobile ions in devices without IV-curve hysteresis . In such devices charge extraction is mainly driven by diffusion of free electrons and holes.
 W. Tress, N. Marinova, T. Moehl, S. M. Zakeeruddin, M. K. Nazeeruddin, M. Grätzel, Energy Environ. Sci., 2015, 8, 995.
 D. W. deQuilettes, W. Zhang, V. M. Burlakov, D. J. Graham, T. Leijtens, A. Osherov, V. Bulovic, H. J. Snaith, D. S. Ginger, S. D. Stranks, Nature Comm., 2016, 7, 11683.
 W. Nie, H. Tsai, R. Asadpour, J.-C. Blancon, A. J. Neukirch, G. Gupta, J. J. Crochet, M. Chhowalla, S. Tretiak, M. A. Alam, H.-L. Wang, A. D. Mohite, Science, 2015, 347, 522-525.
 P. Calado, A. M. Telford, D. Bryant, X. Li, J. Nelson, B. C. O’Regan, P. R. F. Barnes, arXiv:1606.00818.
Metal-halide perovskites are promising materials for opto-electronic applications. Their mechanical and electronic properties are directly connected to the nature of their lattice vibrations. Whereas the mid infrared (IR) range contains mainly information on the internal vibrations of the methylammonium cation,1–3 the lead-halide lattice vibrations are located in the far IR.
We will report far-IR spectroscopy measurements of CH3NH3Pb(I/Br/Cl)3 thin films and single crystals at room temperature and a detailed quantitative analysis of the spectra.4 We find strong broadening and anharmonicity of the lattice vibrations for all three halide perovskites. We determine for the first time the frequencies of both the transversal and longitudinal optical phonons, and use them to calculate the static dielectric constants, polaron masses, and upper limits for the phonon-scattering limited charge carrier mobilities.
We will show how mid-IR spectra can be used to determine the exact stoichiometry of MAPbX3 films. Furthermore, we will demonstrate how the impact of external stimuli can be tracked with mid-IR spectroscopy and correlated to electronic properties. We recently found that water can infiltrate methylammonium lead iodide with surprising ease.3 This infiltration has a strong impact on the opto-electronic properties of the material, possibly via photochemical processes, as demonstrated by measuring the change of photocurrent in lateral devices.
(1) Glaser, T.; Müller, C.; Sendner, M.; Krekeler, C.; Semonin, O. E.; Hull, T. D.; Yaffe, O.; Owen, J. S.; Kowalsky, W.; Pucci, A.; LovrinÄiÄ‡, R. Infrared Spectroscopic Study of Vibrational Modes in Methylammonium Lead Halide Perovskites. J. Phys. Chem. Lett. 2015, 6 (15), 2913–2918.
(2) Bakulin, A. A.; Selig, O.; Bakker, H. J.; Rezus, Y. L. A.; Müller, C.; Glaser, T.; Lovrincic, R.; Sun, Z.; Chen, Z.; Walsh, A.; Frost, J. M.; Jansen, T. L. C. Real-Time Observation of Organic Cation Reorientation in Methylammonium Lead Iodide Perovskites. J. Phys. Chem. Lett. 2015, 6 (18), 3663–3669.
(3) Müller, C.; Glaser, T.; Plogmeyer, M.; Sendner, M.; Döring, S.; Bakulin, A. A.; Brzuska, C.; Scheer, R.; Pshenichnikov, M. S.; Kowalsky, W.; Pucci, A.; LovrinÄiÄ‡, R. Water Infiltration in Methylammonium Lead Iodide Perovskite: Fast and Inconspicuous. Chem. Mater. 2015, 27 (22), 7835–7841.
(4) Sendner, M.; Nayak, P. K.; Egger, D. A.; Beck, S.; Müller, C.; Epding, B.; Kowalsky, W.; Kronik, L.; Snaith, H. J.; Pucci, A.; LovrinÄiÄ‡, R. Optical Phonons in Methylammonium Lead Halide Perovskites and Implications for Charge Transport. Mater Horiz 2016, 3 (6), 613–620.
In spite of the substantial progress that has been made in improving power conversion efficiencies of perovskite-based solar cells and understanding the photo-physics of perovskites, there are only a handful of reports investigating the kinetics of charge transfer from perovskite to charge-specific transport materials (TMs). In order to rationally design efficient and stable perovskite-based solar cells, it is crucial to understand processes occurring at the perovskite/TM interfaces, such as charge transfer and interfacial recombination. Some groups have extracted transfer rates or yields from global analysis of spectroscopic data. However, a quantitative description accounting both for the dynamics of charges in the perovskite itself and the transfer to charge-specific electrodes is still lacking. In this work, Time-Resolved Microwave Conductivity measurements are performed to investigate interfacial processes for methylammonium lead iodide and various organic TMs. Both the frequently used hole transporting material Spiro-OMeTAD and the recently reported low-cost alternatives H101 and EDOT-OMeTPA are investigated. Similarly, state-of-the-art PCBM and C60 are compared to less commonly used electron transport materials such as ICBA and bis-PCBM. We introduce a global kinetic model to describe both the dynamics of excess charges in the perovskite layer and transfer to charge-specific TMs. Hence, we find the rates of charge transfer and interfacial recombination for the above mentioned organic TMs. Additionally, this model enables us to separate the electron and hole mobilities and to deduce the charge collection efficiency as a function of charge carrier density (i.e., illumination intensity). We conclude that for state-of-the-art materials, such as Spiro-OMeTAD and PCBM, charge extraction is not significantly affected by intra-band gap traps. That is, for trap densities under 1015 cm-3, the trapping rates (<107 s-1) are substantially lower than the transfer rates (typically ~ 108 s-1). Finally, our results show that transfer rates to C60, PCBM, EDOT-OMeTPA and Spiro-OMeTAD are sufficient to outcompete second order recombination under excitation densities representative for illumination by AM1.5. These results pave the way for rational design of perovskite-based solar cells with balanced extraction of charges, which is essential for avoiding accumulation of charges at one of the electrodes.
In this talk we will discuss about the optical properties of ABX3 perovskites and their potential integration as light harvesters in tandem solar cells. We will present how our optical model can be employed in order to extract which are the best configurations for both perovskite-silicon and perovskite-perovskite tandem devices. We make use of semi-analytical models based on the transfer matrix to describe light distribution within the device. This allows us to analyse the effect of different electron and hole selective materials in the performance of the cell due to the parasitic absorption of light that they introduce in the system. In addition, these tandem solar cells are complex multilayered stacks in which unwanted reflections appear and can reduce their final efficiency. In this regard, we will present a roadmap to optimise the absorption of light in the active layers depending on the incident angle of the light beam. All our calculations take into account the AM 1.5 solar spectrum and the Shockley-Queisser theory to provide the audience with realistic values.
The existence of slow dynamic processes in perovskite solar cells is well-known. This is most commonly observed as hysteresis in the current-voltage curve during device efficiency measurements. Slow processes have also been observed in a range of frequency and time domain measurements. Whilst there is considerable evidence linking the origins of these observed processes to the migration of ions within the perovskite, the exact nature of their interaction with the electronic structure of the device is still unclear.
In this work a range of complimentary frequency and time domain characterization techniques have been utilized to help understand the wide range of effects this electronic-ionic coupling has upon recombination in perovskite solar cells. By combining the use of transient photovoltage (TPV) and intensity modulated voltage spectroscopies (IMVS), over a wide range of temperatures from 77â€‘323 K, relaxations with time constants on microsecond, millisecond and second timescales have been observed. Measurements are complimented further with the use of impedance spectroscopy, large amplitude open-circuit photovoltage decay and current-voltage measurements.
Excellent agreement is shown between the time and frequency domain results, with the combination of techniques being used to aid understanding. A range of architectures have been studied including planar and mesoporous based TiO2 and organic contact cells. Similar processes are observed in all types highlighting that the underlying physical process is intrinsic to the perovskite. The recombination rate is found to be frequency/time dependent, relating to the particular ionic environment and its impact on electronic band structure. The high frequency response is representative of recombination in a fixed ionic system. On longer timescales the relaxation of the electronic and ionic species to re-establish electrochemical equilibrium results in a reduction in recombination rate.
Perovskite solar cells with all-organic transport layers have shown efficiencies rivalling their counterparts that employ inorganic transport layers while avoiding high temperature processing. In this contribution we show that reduced non-radiative recombination as a result of better blocking behaviour at the interface between the perovskite and fullerenes is the determining factor for high open-circuit voltages in our (p-i-n) perovskite solar cells. Exemplary, we employ UPS/IPES to determine the energetics of the contact layers adjacent to the perovskite and correlate this with the fluorescence lifetime, which allows us to enhance the open-circuit voltage while preserving good charge colletion efficiency in our devices (i.e. high fill-factors above 70%). Consequently we produce solar cells with efficiencies up to 19.4% accompanied by open-circuit voltages as high as 1.16V reflected in external radiative efficiencies of up to 0.3% at carrier densities equivalent to solar AM1.5G conditions. This work highlights the importance of well-chosen selective contacts in achieving highly efficient devices, adresses the losses in perovsktie solar cells and seeks to redress some of the misleadings regarding figures-of-merit often presented (e.g. photoluminescence quenching).
In the context of the increase of the photovoltaic (PV) technologies, the question of the cost remains central. Indeed, the price of the electricity reached by this technology remains high with regard to the conventional energies.
The current strategy to remain competitive articulates around two main strategies: the increase of the efficiency of the PV cells and the decrease of the costs linked to the raw material. In this context, our teams work to combine on one hand heterojunction amorphous / crystalline silicon cells (HJT) and on the other hand perovskite (CH3NH3PbI3) ones to obtain a 2-terminal tandem device in standard architecture with the target of highly efficient conversion up to 30 %.
The first stage of the project concerned the perovskite sub-cell development for which we concentrated on two axes of studies: i) the elaboration of a transparent hole extraction layer (P-type) at the front side of the cell as well as ii) the elaboration of a Transparent Conductive Oxyde (TCO) front electrode.
Concerning the P type interfacial layer, we suggested studying 3 main classes of materials: a conductive oxide (WO3), an organic molecular semiconductor (Spiro-MeOTAD) and a conductive polymer (PEDOT:PSS). These 3 materials show a high transparency in the wavelength range of the absorption of the active perovskite layer and can be processed in solution. After being characterized in PV device, the PEDOT: PSS containing cell gave the best performances in terms of conversion efficiency. The performances were comparable to those obtained in the standard conditions for single junction opaque cell.
About the TCO, Indium Tin Oxyde (ITO) layers were processed by PVD (Physical Vapor Deposition). We had to adapt the process for a deposition at room temperature taking into account the risk of degradation of the high-temperature sensitive perovskite material if processed above 150°C. Thus we optimized the parameters to improve the transparency/resistivity of the layer by varying the power of the plasma as well as the flows of argon and oxygen. Finally it was possible to obtain highly transparent (>80%) and low resistive (1mOhm.cm) ITO in optimized conditions without degradation of the properties of the perovskite layer nor the performances of PV cells as prepared.
The next step is now to integrate the perovskite sub-cell onto HJT to elaborate tandem device to reach high performances.
The outstanding efficiencies of organic-inorganic perovskite thin film based solar cells led to interest in increasing the range of perovskite materials. One such family of materials is the all inorganic CsPbX3 colloidal nanocrystals (NCs). Despite the recent surge of synthetic protocols producing different shapes and crystal structures, there are still significant gaps in the understanding of nucleation and growth processes involved in their formation. Here we try to address the mechanisms and the common factors behind these multiple synthetic pathways.We have identified that the formation of CsPbX3 NCs follows through two separate stages. First, seed mediated nucleation through the formation of metal Pb NCs. Second, further growth is attained through oriented attachment. We were able to investigate the two stages independently by significantly slowing down the kinetics and by the separate successive additions of ligands. It was found that the critical factor determining the size, shape and crystallographic structure of the CsPbX3 NCs is the polarity of the ligands. Using this understanding we could synthesize other novel materials such as CsPbBr3 nanowires, CsPbCl3 bulk-like crystals and CsPbI3 orthorhombic nanowires of length ranging from 200 nm to several microns.We believe this work will lead to the development of improved strategies for rational design of synthesis and more importantly, the stabilization of perovskite NCs which may offer an advantage for perovskite NCs over thin films for photovoltaics and other potential applications.
The enormous recent interest in the Halide Perovskites is mostly motivated by the prospect of high efficiency solar cells. However, while solar cells operate under steady state (or CW) illumination conditions, much of the research in the optoelectronic properties on these materials and cells relies on using illumination pulses of very short duration. Among these studies time-resolved photoluminescence has become one of the standard characterization tools of these systems. However, unlike the qualitative understanding and quantitative models that connect the cell parameters and quantities that are measured by steady state phototransport methods, the relation between these parameters and quantities derived by the transient parameters is not as clear. Hitherto, no model of the steady state recombination scenario in the Halide Perovskites has been proposed. In this work we present such a model that is based on a single type of recombination center, which is deduced from our measurements of the illumination intensity dependence of the photoconductivity and the ambipolar diffusion length in those systems. We also discuss the relation between the present results and those from time-resolved measurements that are commonly reported in the literature.
Recent advances in organolead halide perovskite solar cells have led to a remarkable improvement in the cell efficiency . Yet, it is still challenging to achieve good moisture resistance of the perovskite films while obtaining high cell efficiency [2, 3, 4]. Here we address the issue through a simple post-deposition passivation treatment of perovskite films with small-sized amine molecules containing benzene rings . We compared three structurally similar aromatic molecules, including aniline, benzylamine and phenethylamine, and observed a drastic difference in their passivation effect. Through density functional theory calculations we found that the efficacy of the moisture resistance and defect passivation is extremely sensitive to the steric arrangement of the amine molecules and that only benzylamine provides the optimal configuration. Solar cells based on benzylamine modified formamidinium lead iodide (FAPbI3) perovskite films exhibit a champion efficiency of 19.2% and an open-circuit voltage (Voc) of 1.12 V, revealing extremely low loss-in-potential (0.36 eV) in the cells. The modified FAPbI3 films exhibit no degradation in their structural and electronic properties after >2800 hours air exposure. The study elucidates the molecular passivation principles to achieve simultaneous efficiency and stability improvement.
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In recent years, halide perovskite solar cells (PSCs) have been studied steadily due to their potential properties: high power conversion efficiency (PCE) to over 21% and low processing cost . However, the stability of PSCs under operating conditions is the main challenge to be addressed before commercialization . It has been suggested that ionic migration could impact optoelectronic performance and affect device operation and long-term stability . In this study, glow-discharge optical emission spectroscopy(GDOES) is used to analyze the depth profiles of constituent elements in halide perovskite films with applied voltage. Solar cells based on halide perovskite (CH3NH3PbI3-XClX) are fabricated by one-step solution-process with PCE of 12.7% (active area of 0.28 cm2) and short-circuit current density (Jsc) of 21.9 mA/cm2. A shift of iodide and chloride ions distribution in perovskite films is observed depending on applied voltage. In this communication, it is shown that GDOES is a powerful method to investigate ionic migration in PSCs under operating conditions.
 E. Edri, S. Kirmayer, S. Mukhopadhyay, K. Gertsman, G. Hodes and D. Cahen, Nature communications, 2013, 5, 3461.
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 S. Meloni, T. Moehl, W. Tress and M. Gratzel, Nature communications, DIO:10.1038/ncomms10334.
The first report on quantum mechanical calculations of methylammonium lead iodide was in 2004. Since then, many millions of processing hours have been spent calculating the properties of these intriguing compounds.
I will critically review the contribution of materials modelling to the field of hybrid perovskite solar cells and assess the current status and challenges. This will include research from my group, where we have been developing and applying from multi-scale simulation techniques [2-5]. The temporal behaviour of hybrid perovskites predicted by theory has recently been validated through a combination of neutron scattering, time-resolved vibrational spectroscopy, and kinetic measurements of the current-voltage response. The implications for achieving high-efficiency stable photovoltaic devices will be discussed.
This research has been supported by the ERC, Royal Society and ESPRC, with a wide collaboration network including simulations by Jarvist Frost, Jonathan Skelton, Federico Brivio and Keith Butler.
1. "First-principles study of the structural and the electronic properties of the lead-halide-based inorganic-organic perovskites (CH3NH3)PbX3 and CsPbX3 (X = Cl, Br, I)" J. Korean Phys. Soc. 44, 889 (2004)
2. “Atomistic origins of high-performance in hybrid halide perovskite solar cells” Nano Letters, 14, 2584 (2014)
3. “Molecular ferroelectric contributions to anomalous hysteresis in hybrid perovskite solar cells” APL Materials 2, 081506 (2014)
4. "Dynamic disorder, phonon lifetimes, and the assignment of modes to the vibrational spectra of methylammonium lead halide perovskites" Phys. Chem. Chem. Phys. 18, 27051 (2016)
5. "Direct observation of dynamic symmetry breaking above room temperature in methylammonium lead iodide perovskite" ACS Energy Lett. 1, 880 (2016)
Since the advent of organic–inorganic hybrid methylammonium (MA) lead halide MAPbX3 perovskites as active materials for photovoltaic applications in 2009,  the PCE of perovskite-based solar cells (PSCs) has dramatically increased from the initial 3.8% to a recently certified 22.1%.  Emerging from this groundbreaking discovery, an unprecedented scientific research has sprouted in the field of photovoltaics due to their exceptional physical properties. Therefore, the development of cost-effective HTMs with high efficiency along with a good stability is an important task to address.Planar and sulfur-rich polycyclic aromatic hydrocarbons bearing arylamine moieties have demonstrated to be a successful approach for designing new highly efficient HTMs for PSCs.  Conventionally, the π-extended conjugation associated with the planar and electron-rich structure of the fused heterocycles enable them to show strong stacking through intermolecular interactions (πâ€’π, S···S), thereby bestowing enhanced hole-carrier mobilities. This behaviour is beatifully exemplified by anthra[1,2-b:4,3-b′:5,6-b′′:8,7-b′′′]tetrathiophene-based (ATT) HTMs.  Here we report a readily available new class of multi-armed, sulfur-rich hole transporting materials based on pi-conjugated central cores. Devices were fabricated using state-of-the-art mixed anion mixed halide perovskite composition with the nominal formula [FAPbI3]0.85[MAPbBr3]0.15 (FA = formamidinium, MA = methylammonium). The performance of the solar cells employing the novel HTMs were measured under simulated 1 sun irradiation and conversion efficiencies of up to 19 % were obtained.
 Kojima A., Teshima K., Shirai Y. and Miyasaka T., “Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells”,J. Am. Chem. Soc., 131, 17, (2009), pp 6050-6051. National Renewable Energy Laboratory, N.R.E.L. Molina-Ontoria A., Zimmermann I., Garcia-Benito I., Gratia P, Roldán-Carmona C., Aghazada S., Graetzel M., Nazeeruddin M.K., and Martín, “Benzotrithiophene-Based Hole-Transporting Materials for 18.2 % Perovskite Solar Cells”, Angew. Chem. Int. Ed., 55, 21, (2016), pp 6270-6274. Zimmermann I, Urieta-Mora J., Gratia P., Aragó J., Grancini G., Molina-Ontoria A., Ortí E., Martín N. and Nazeeruddin M. K., “High-Efficiency Perovskite Solar Cells Using Molecularly Engineered, Thiophene-Rich, Hole-Transporting Materials: Influence of Alkyl Chain Length on Power Conversion Efficiency”, Adv. Energy Mater., (2016), DOI: 10.1002/aenm.201601674.
The application of methylammonium lead (II) iodide as a light-absorbing layer in solid-state solar cells leads to impressive efficiency of over 22% in laboratory devices. However, for industrial applications, fundamental issues regarding their thermal and moisture stability need to be addressed. MAPbI3 belongs to the perovskite family of materials with the general formula ABX3 ,where is the organic cation (methylammonium) which is reported to be a major source of instability. In this work, a variety of alkyammonium lead (II) iodide materials have been synthesized by changing the organic cation, to study the relationship between the structural and physical properties of these materials. Methylammonium, ethylammonium and propylammonium were used for the [(A)PbI3] series. In another set of experiments, butyldiammonium, hexyldiammonium, and octyldiammonium cations were studied as (A)PbI4 perovskite materials. Various dimensionalities for the structures of these materials were found; three dimensional (3D) networks (MAPbI3, MAPbBr3), two dimensional (2D) layered systems (BdAPbI4, HdAPbI4, OdAPbI4), and one dimensional (1D) columns (EAPbI3, PAPbI3, EAPb2I6). Several new lower dimensional materials (2D and 1D) were investigated and reported for the first time. X-ray single crystallography was used to obtain the detailed structures. Bulk structures were confirmed by comparison of the X-ray diffraction patterns with single crystal data. [PbI6] octahedral structural units were repeated through the material network depending on the dimensionality and connectivity of the materials. Where a bulkier cation was introduced, the crystallographic unit cell increased in size which resulted lower symmetry crystals. The connectivity of the unit cells along the material networks was found to be based on corner-sharing and face-sharing. Lower dimensionality resulted in larger bandgaps and lower photoconductivity, and hence a lower light conversion efficiency for the related solar cells. The thermal and moisture stability was greater in the 1D and 2D materials with bulkier organic cations than with methylammonium. The electronic structure of the new 2D layered perovskites was investigated by X-ray photoelectron spectroscopy, X-ray absorption spectroscopy and X-ray emission spectroscopy. These findings were in agreement with the experimental part, indicating that the valence band is composed mainly of iodine orbitals, while lead orbitals predominate in the conduction band. The iodide/lead ratio obtained from surface analysis of the material deposited on the TiO2 films matched the proposed general formula from single crystal data. In total, an overview is provided of the relationship between the chemical dimensionality and physical properties of the organic-inorganic lead halide materials with focus on the solar cell application.
The performance of perovskite solar cells are continuously increasing, recently reaching efficiencies of over 20%. However, the stability of these devices is not as impressive as their efficiency: more comprehensive studies on degradation mechanisms, and a deeper understanding of the weak spots of the different available stacks are necessary to bring perovskite solar cells to the next level.
We studied the stability of perovskite solar cells for two different structures: N-I-P (glass/ITO/TiO2/Perovskite/Spiro-OMeTAD, Au/barrier) and - P-I-N (glass/ITO/NiO:Cu/Perovskite/PCBM/ZnO/Al/barrier). In several parallel experiments, we stressed both structures with light or with temperature, and we compared the results with devices under shelf-life conditions. We found that for both architectures, the main stress factor was light. Looking at the whole set of experiments, we observed different degradation pathways for P-I-N and N-I-P structures. For P-I-N devices we found the Aluminum contact as the main degradation factor, while for the N-I-P devices there are strong indications that the photocatalytic property of the TiO2 layer is the major cause of fast degradation. A second set of devices was stressed in dark, at temperatures increasing from 45 °C to 100°C: for P-I-N and N-I-P devices we observed that the deviation between shelf life and temperature induced degradation happened at different temperatures. The P-I-N devices, compared with the N-I-P devices, shown higher stability.
These experiments helped us to identify the weak spots in our devices. Moreover, we gained a better comprehension of the degradation factors for our devices: this helps us to choose the best ageing conditions for accelerated lifetime tests, allowing a faster screening of new stacks and helping us to focus our work on the most promising stacks. The understanding of degradation pathways will also help up define encapsulation requirements and specifications.
Metal halide perovskites have emerged as promising photovoltaic materials, with record efficiencies beyond 20%. Among other, one of the unique properties of perovskites is their tunable band gap. While most studies have been directed towards the developments of small bandgap absorber, wide bandgap systems have not been explored in details. Realizing efficient solar cells with bandgap of about 2 eV would enable the fabrication of perovskite-perovskite tandem cells with efficiencies exceeding 23%, hence surpassing the efficiency of single junction devices. In this work we combine the beneficial stabilizing effects of Cs and FA cations to study the band gap variation and the properties of the perovskite compound Cs0.15FA0.85Pb(BrxI1-x)3 as a function of the halides composition, with the aim of developing an efficient complementary absorber for MAPbI3 in a perovskite tandem device. We have found the perovskite stoichiometry Cs0.15FA0.85Pb(Br0.7I0.3)3 to be a promising candidate, thanks to its band gap of approximately 2 eV. We also present an efficient monolithic tandem solar cells based on Cs0.15FA0.85Pb(BrxI1-x)3 and MAPbI3. By employing doped organic semiconductors, an efficient extraction of the photogenerated charge carriers is ensured, while carrier recombination at the perovskite interfaces is prohibited by using intrinsic and selective charge transport layers. Despite the non-ideal combination of bandgaps in the two subcells, we demonstrate perovskite-perovskite tandem cells delivering an average PCE of 15%, with a maximum efficiency obtained of 18%.
The success of metal halide perovskite solar cells stems from a very high absorption coefficient combined with a relatively low recombination rate. The slow recombination leads to experimental open-circuit voltages close to the thermodynamic limit. Despite the fact that this property is inherent to the perovskite material, the choice of selective contacts (electron and hole transporting layers) is critical to achieve high voltages according to experimental evidence. In this work we investigate the recombination kinetics using temperature-dependent voltage measurements and small-perturbation optoelectronic techniques. The recombination resistance and the open-circuit voltage are measured for two excitation wavelengths (blue and red light), two illumination directions (back and front) and at different temperatures. The influence of two perovskite compositions (MAPbI3 and FAPbI3)0.85(MABr3)0.15) and two hole selective layers (Spiro-OmeTAD and P3HT) have been studied. Our results point to a recombination process controled by the bulk of the perovskite layer via a trap-limited mechanism. The effect of the hole transporting layer and the relative contribution of surface recombination is also discussed
Perovskite solar cells are surprising the photovoltaic community as unconventional behaviors have been reported. In most of the cases the origin of these behaviors is not completely understood and also, very important for the final application, how them influence the final performance of the device. Hysteresis, preconditioning effect or large low frequency capacitances have received high attention but other phenomena as inductive loops and/or negative capacitance have been studied significantly less, in part, as their implications in the final performance of the device were not clearly established. Here we unambiguously demonstrate a direct correlation between the observation of the negative capacitance and a corresponding decrease on performance of the device, manifested by a reduction of open circuit potential and fill factor. On the other hand we discuss the influence of injection processes, more concretely between TiO2 electron selective contact and perovskite, in the observation of this phenomena. This presentation stress the necessity of establishing the physical origin of negative capacitance in perovskite solar cells due to the important implications in the final performance.
The concept of polycrystalline all-thin-film multi-junction photovoltaic devices has been proposed for long time to revolutionize the solar energy harvesting by offering very high efficiencies at low production costs. Until recently successful development was hindered by rather low efficiency and/or low sub-bandgap transmittance of the wide band gap top cell(s). With the discovery and incredibly fast progress of halide perovskite-based solar cells the situation has changed considerably and the development of polycrystalline all-thin-film multi-junction devices is worth revisiting. In this contribution the combination of wide band gap perovskite top cells with low band gap chalcogenide bottom cells is discussed and different device configurations are presented. In particular the architecture of semi-transparent halide perovskite top cells grown in “inverted” p-i-n substrate configuration where light enters the solar cell from the film side will be shown. This device configuration not only allows direct (monolithic) growth onto the Cu(In,Ga)Se2 bottom cell but additionally also permits the development of flexible perovskite devices on non-transparent substrate materials. Substrate configuration perovskite solar cells with efficiency above 16% (MPP measurement) and average sub-bandgap transmittance >80% are achieved. Light soaking mechanism with implications for further device optimization as well as thermal and photo-stability properties will be presented for devices in substrate configuration. In combination with Cu(In,Ga)Se2 bottom cells tandem efficiencies above 22% are realized.
We present a new strategy of perovskite films preparation for optoelectronic applications in particular solar cells using melts of novel reactant compounds and lead precursors The new reactant exhibits very low melting points and high chemical activity and selectivity. Being liquid at room temperature it renders superfluous the use of a solvent in the conversion reaction.
Using this radically new approach we demonstrate that a thin film of metallic lead can be converted at once in a single step into polycrystalline perovskite films of high electronic quality at room temperature. The reaction between the metallic lead film and the newfound compounds proceeds rapidly without using any solvents or additional agents. Depending on the preparation conditions, two types of morphology are obtained, including sphere-like and cubic crystals of perovskite. The latter case gives high quality perovskite films with large crystals exhibiting intensive photoluminescence and a lifetime of charge carriers above 200 ns.
The advantage of the proposed method is the use of lead or other precursors as initial compounds while lead can be deposited by highly controlled way using a large number of standard techniques such as sputtering, galvanostatic deposition, etc. on different substrates, including flexible ones.
We also show that mixed perovskites MAxFA1-xPbI3-xBrx can also be easily obtained using this strategy. In particular, single-phase MA-stabilized FAPbI3 with long charge carriers lifetimes was obtained. Moreover, the proposed new strategy is not limited to the treatment of metallic lead and opens, for the first time, an access to a new field of efficient methods of perovskite preparation and other classical melt techniques which are well known for semiconductors.
We illustrate the results of ab initio molecular dynamics simulations coupled to first principles electronic structure calculations on the effect of light absorption on the electronic and dynamical properties of organohalide lead perovskites. The role of the organic cation dynamics and of ion/defect migration is analyzed in relation to photoinduced structural transformations and solar cell operation. It is found that Frenkel defects, relatively abundant in MAPbI3 and related perovskites, undergo a light-induced dynamical transformation which may account for the observed enhanced photoluminescence quantum yield following sample irradiation. Finally, we show how most of perovskites unusual properties in terms of defects and trapping dynamics can be explained by the close similarity between the perovskite properties and the photochemistry of iodine.
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Physical modeling of hysteretic behavior in I-V curves of perovskite solar cells (PSCs) is necessary for further understanding of power generation mechanism and improvements of power conversion efficiencies (PCEs). In some cases, reduction of hysteresis in planar structure PSC (p-PSCs) has been achieved by using PCBM layer mostly for inverted structure. In such cases, the opposite trend of the I-V hysteresis, where the forward scan shows higher efficiency than that of reverse scan has been observed. In this paper, an equivalent circuit model with inductance was newly proposed. This model consists of Schottky diode (SBD) involving parasite inductance focusing PCBM/Al(Ca) interface and well represents the opposite I-V hysteresis of the p-PSC with inverted structure. Furthermore, based on the equivalent circuit we have constructed physical real device composed of 2 Si photo diodes and a diode, an inductor and resistor. Photovoltaic response for modeled perovskite solar cells based on equivalent circuit with 2 Si photo diodes, a diode, an inductor and resistor. The I-V curves for this device well supported our equivalent circuit having 2 Si photodiode and SBD diode, and conclude that the inverted trend of hysteresis is coming from the inductance originated from the interface PCBM/Al(Ca).
Solution based processes show great potential for synthesis of mixed-halide perovskite thin-films, as they do not need cost intensive vacuum technology and no high-temperature annealing steps. However little is known about the exact mechanisms during film formation. We present a detailed investigation of the chlorine derived synthesis of MAPbI3 perovskite. By simultaneously applying synchrotron based in-situ quick scanning X-ray absorption spectroscopy (QEXAFS), X-ray diffraction (XRD) as well as X-ray fluorescence (XRF) in combination with in-situ optical reflection and photoluminescence spectroscopy a conclusive picture of the film formation can be drawn. Product and educt phases as well as their reaction kinetics were identified by combining these measurements with ex-situ high energy resolution fluorescence detected (HERFD)-XAS. The reaction is conducted in a custom designed in-situ reactor with control of temperature and atmosphere. Using this set of complementary real-time time -resolved characterization techniques simultaneously, we can correlate the evolution of the chemical reaction from XAS, the formation of crystalline phases from XRD and the chemical composition from XRF with the evolution of the optoelectronic properties of the film on one single timeline.Combining complementary results we are able to draw a comprehensive picture describing the film formation and reaction route. Furthermore, the combination of compositional and structural properties with the evolution of optoelectronic properties allows a meaningful interpretation of the optoelectronic data. Hence, we are able to correlate the delayed formation of crystalline MAPbI3 with the decreasing chlorine content of the sample and will present a detailed view on the kinetics of all involved formation and decomposition reactions. Correlating changes of the optoelectronic properties with simultaneously measured structural and compositional properties allows an interpretation of lab-based in-situ optical spectroscopy. Our results give insight into the detailed formation process and can help to provide a mechanistic understanding of the reactions and intermediates involved in the formation of MAPbI3 from Cl-containing precursors. This knowledge could be utilized to optimize synthesis strategies and tunability of the optoelectronic properties of organic-metal-halide perovskite semiconductors.
Recently, inorganic-organic based perovskite solar cells (PSCs) have attracted incredible attention due to its unprecedented rise in the PCE value, from 3.8% to 22% in a short span of time. 
Phthalocyanines (Pcs) are outstanding dye candidates in for dye sensitized solar cells (DSSCs) due to their high extinction coefficient in the visible and near-infrared spectral region and to their high thermal and chemical stabilities. Pcs incorporated in DSSCs have achieved PCEs as high as 6.4 %,  still far away from the 12.75% PCE obtained by porphyrins, their closest relatives. An elegant strategy to improve the light-harvesting ability of the Pcs is the extension of the aromatic structure by generation aromatic-fused analogues of Pc having a red-shifted absorption. The same strategy will be also tested in PSCs as electron donor systems.
On the other hand, perylenediimides (PDIs) are quite promising n-semiconducting materials as non-fullerene acceptors. Efficiencies higher than 8.4 % have been achieved in organic solar cells. On the other hand, PDIs have been used in perovskite solar cells as ionic interlayers, which can successfully replace the use of reactive electrodes, since they facilitate the electron extraction while reducing the non-radiative recombination at the electron transport interface.
Herein, we will report our more recent results related with the synthesis of different substituted phthalocyanines and perylenediimides and the improvement in efficiency in PSCs.
 National Renewable Energy Labs (NREL) Efficiency Chart, NREL 2016.
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Photovoltaics (PV) using tin halide perovskites as the light harvesting material have not yet benefitted from the same intensive research effort that has propelled lead perovskite PVs to >20% power conversion efficiency for three reasons: (i) the susceptibility of tin perovskites to oxidation in air.; (ii) the low energy of defect formation; and (iii) the difficultly in forming pin-hole free films, the latter two of which seriously undermine device fill-factor. This talk will report two distinct findings relating to the advancement of CsSnI3 perovskite PV: (1) A new strategy for fabricating CsSnI3 based PV devices with high fill factor that removes both the requirement for an electron blocking layer at the hole-extracting electrode and the need for an additional processing step to minimise the density of pinholes in the perovskite film.; (2) Demonstration that the stability of unencapsulated CsSnI3 PV devices based on a simplified electron-blocking layer free device architecture is improved by at least an order of magnitude as compared to lead based perovskite PV with the same architecture when tested under continuous simulated solar illumination in ambient air at 50oC.
(1) Nature Energy 1, Article number: 16178 (2016) DOI:10.1038/nenergy.2016.178
Halide perovskite (HaP)-based photovoltaic (PV) devices have reached >21% small area power conversion efficiency. However, HaP layers still suffer from limited basic knowledge of the effects of their defect chemistry and physics on their PV behavior and in particular the charge transport mechanism(s). Understanding the electronic, ionic and in particular the coupled electronic-ionic processes involved in the opto-electronic characteristics for the films, used for devices, can provide guidance for improvements in cell preparation and device performance.
Work function measurements on MAPbI3 layers show that changing the atmosphere in which the sample is stored / measured, affects its Fermi level position. The Fermi level position in the forbidden gap will influence charge recombination and Voc values of PV cells, made with those layers. Surface potentials of MAPbI3 layers that were exposed to high vacuum (HV; ~10-6 mbar), O2, humid N2 , or to I2 in dry inert carrier gas environment were characterized by Kelvin probe, measurements of the contact potential difference (CPD), UPS/ XPS and I-V measurements. During O2 exposure the WF increased by about 100 mV toward p-type. This effect is fully reversible. Humid N2 causes a dramatic increase of MAPbI3 film resistance up to a point that the film behaves as a capacitor. MAPbI3 layers which were exposed to I2 were doped permanently to 4.8eV, ~150 mV shift towards the VB. We postulate that the I2 decreases the density of I vacancies [VI-] in the lattice. The electrical characteristics of TiO2 / MAPbI3 / Spiro-OMeTAD/ Au PV cell, decrease after I2 doping which may hint at a positive role of ionic conduction.
Methylammonium lead iodide perovskites (MAPI) are generally considered direct bandgap semiconductors. However, theoretical calculations have predicted a slight indirect bandgap for MAPI as a consequence of spin-orbit coupling resulting in Rashba-splitting of the conduction band. Currently there is limited experimental evidence to support this theoretical prediction. Using pressure-dependent absorption and emission measurements, we show that a weakly indirect bandgap around 60 meV below the direct bandgap transition is present. Under hydrostatic pressure from ambient to 325 MPa, Rashba splitting is reduced due to a pressure-induced reduction in electric field around the Pb atom. The indirect nature of the bandgap is suppressed, leading to five times faster charge carrier recombination, and a two-fold increase of the radiative efficiency. At hydrostatic pressures above 325 MPa, a reversible phase transition of MAPI occurs, resulting in a purely direct bandgap semiconductor.The finding of an indirect bandgap in MAPI sheds light on the apparent contradiction of strong absorption and long charge carrier lifetime. Novel epitaxial and synthetic routes to higher efficiency optoelectronic devices might be developed based on the pressure-induced changes we observe.
In inorganic semiconductor junction devices, capacitance spectroscopy is one of the most fundamental means to uncover the defect states and the doping concentrations. Of course, it has been already applied to perovskite photovoltaic (PKPV) devices as well, providing the energy levels and densities of the trap states. However, the technique has not been fully utilized to enhance the understanding of the PKPV operation. For example, a comparative study of the information on the defect states and the power conversion efficiency is still lacking.
The formulation of the capacitance spectroscopy is well developed and should be readily applicable to planar type PKPVs. We have been successful in preparing simple planar type PKPVs, which are free of hysteresis and have diode characteristics quantitatively comparable with those of GaAs p-i-n junctions. We applied the capacitance spectroscopy to our samples prepared under systematically varied conditions, e.g., samples prepared in different procedures or samples at different stages of degradation. It was found that the efficiency is related to the bulk defect conditions while the degradation leaves stronger signature at the perovskite/charge-extraction layer interface. Numerical simulations using SCAPS (Solar Cell Capacitance Simulator) were performed assuming defect states in the bulk or at the interface. The resulting macroscopic electronic properties are qualitatively in agreement with the observation.
Empirical and semi-empirical models are particularly suited to address issues related to complex structures. Based on a general symmetry analysis, herein we build an empirical sp3 tight-binding (TB) model for the reference Pm-3m cubic phase of halide perovskite structures of general formula ABX3. This TB model includes 16 and 32 basis functions without and with spin orbit coupling (SOC), respectively. The 16 basis functions are made of one "s" and three "p" orbitals for the metal atom B, and the same for three halide atoms X atoms. No basis function is taken into account for the A organic molecule, which position cannot be fixed in the cubic phase. The TB electronic band diagram, with and without SOC of MAPbI3, is determined based on state of the art density functional theory results that include many body correction (DFT+GW). The band gap is obtained at the R-point with Eg=1.603eV. Close to the band gap, effective masses are: mh*=0.215m0 for the valence band and me*=0.218m0 for the conduction band, which leads to a reduced effective mass of 0.108m0, all data being consistent with experimental values.
This TB model affords an atomic-scale description to computing various properties, including distorted structures, at a significantly reduced computational cost. The powerfulness of the present TB Hamiltonian is exemplified with the calculation of band-to-band absorption spectra, the variation of the band gap under volumetric strain, as well as the Rashba effect for a uniaxial symmetry breaking. Compared to first-principles approaches, such a semi-empirical model permits us to tackle more difficult issues in terms of size with complex heterostructures, nanostructures or composite materials as well as diversity of physical phenomenon under investigation. It should be as relevant to the future of perovskite device modeling, as it has proved efficient for conventional semiconductors.
This work has been performed within the GOTSOLAR project, which has received funding from the European Union’s Horizon 2020 research and innovation Programme under the grant agreement No 687008. The information and views set out in this abstract are those of the authors and do not necessarily reflect the official opinion of the European Union. Neither the European Union institutions and bodies nor any person acting on their behalf may be held responsible for the use which may be made of the information contained herein.
In recent years, the interest in hybrid organic - inorganic perovskites rose at a rapid pace due to their tremendous success in the field of photovoltaics. In addition to the thin film properties of the active layer, the performance of optoelectronic devices strongly depends on the appropriate energetic alignment between the active- and adjacent layers. In order to choose adequate transport materials for the increasingly complex hybrid perovskite compositions in a non-trial-and-error fashion, it is important to understand how the induced changes in band gap relate to shifts in the valence and/or conduction band.
In this talk, I will discuss recent findings regarding measurements of the electronic structure of various hybrid perovskites, covering lead as well as tin based systems and a variety of halogens. Hereby, I will combine reports from literature with our own results based on UV-, inverse, and x-ray photoelectron spectroscopy measurements (UPS/IPES/XPS). Furthermore, using these surface sensitive techniques, not only the energetic values can be extracted but the alignment at interfaces between different layers can be probed in-situ as well by a stepwise film preparation.
Looking at the bottom contact, it became increasingly obvious in the last year that the substrate can have a strong influence on the stability of the adjacent perovskite film and that chemical interactions, band bending, and interface dipole formation play an important role. I will show that the nature of the substrate not only determines the energetic alignment but can create gap states and influence film formation and crystallinity, all of which makes the investigation of the interface to the substrate a highly relevant topic for current studies.
The development of organic-inorganic lead halide perovskites with very large efficiency requires us to understand the operation of the solar cell. We have used the measurement of impedance spectroscopy in combination with current-voltage scans and physical modeling to obtain insights about the physical processes dominating the photovoltaic operation. This class of semiconductors presents remarkable bulk electronic and optical properties, but the contacts to the device are a key aspect of the operation and show important dynamic interactions. We provide an interpretation of capacitances as a function of frequency both in dark and under light, and we discuss the meaning of resistances and how they are primarily related to the operation of contacts in many cases. The capacitance reveals a very large charge accumulation at the electron contact, which has a great impact in the cell measurements, both in photovoltage decays, recombination, and hysteresis. We aim to present a global view of the formation of photovoltage and the kinetic processes governing the operation of the solar cell, providing a suitable unified explanation to the many strange observations reported in the last few years.
Ronen Gottesman, Pilar Lopez-Varo, Laxman Gouda, Juan A. Jimenez-Tejada, Jiangang Hu, Shay Tirosh, Arie Zaban, Juan Bisquert, Dynamic Phenomena at Perovskite/Electron-Selective Contact Interface as Interpreted from Photovoltage Decays, Chem , 1, 776-789 (2016)
Hui-Seon Kim, In-Hyuk Jang, Namyoung Ahn, Mansoo Choi, Antonio Guerrero, Juan Bisquert, Nam-Gyu Park, Control of I/V Hysteresis in CH3NH3PbI3 Perovskite Solar Cell, The Journal of Physical Chemistry Letters, 6, 4633-4639 (2015)
Isaac Zarazúa, Guifang Han, Pablo P. Boix, Subodh Mhaisalkar, Francisco Fabregat-Santiago, Ivan Mora-Sero, Juan Bisquert, and Germà Garcia-Belmonte, Surface Recombination and Collection Efficiency in Perovskite Solar Cells from Impedance Analysis, J. Phys. Chem. Lett., 10.1021/acs.jpclett.6b02193 (2016)