Electrochemical CO2 conversion can be coupled with a photovoltaic cell and provide a pathway to utilize solar energy for the chemical synthesis. Ideally, such artificial photosynthesis system want to use CO2 and H2O as feed-stock molecules to produce value-added chemicals such as fuels or raw chemicals. My research team reported a monolithic and stand-alone device composed of a photovoltaic cell module, an Au CO2 reduction, a cobalt oxide anode accomplishing over 4 % conversion efficiency for CO2 conversion to CO production. To improve the solar to chemical conversion efficiency and to increase the feasibility further, we have developed efficient electrocatalysts and replaced the photovoltaic cell with Si modules, achieving ~ 8% of solar-to-CO conversion efficiency.

In addition, in this talk, metal-based electrocatalysts interacting with p-block elements or surface mediated molecules will be discussed for selective CO or C2+ (i.e. ethylene) production from CO2 reduction. The experimental results and theoretical simulation with various different types of metal catalysts (Ag, Zn, and Cu) give insights how to suppress the hydrogen evolution reaction (HER) is crucial to achieve efficient CO2 reduction catalysts. Monodispersed Ag nanoparticles are suggested to have the special interaction between the surface Ag and the surface mediated molecules which can modify the local electronic structure favoring for the selective CO production (up to 95 % of Faradaic efficiency). In addition, in the case of selective ethylene production, special Cu nanostructure formed by in-situ electrochemical fragmentation is demonstrated to be effective for increasing C-C bond coupling (up to 73 % of Faradaic efficiency) and selective ethylene production (up to ~ 60 % of Faradaic efficiency). In-situ X-ray absorption spectroscopy (XAS) studies are performed to understand the catalyst activity. Our series of studies suggests the modification of the metal nanoparticle surface by oxygen atom or surface mediated molecules can be effective strategies to increase CO2 reduction reaction activity and stability.

Chemically synthesized quantum dots (QDs) can potentially enable new classes of highly flexible, spectrally tunable lasers processible from solutions [1,2]. Despite a considerable progress over the past years, colloidal-QD lasing, however, is still at the laboratory stage and an important challenge - realization of lasing with electrical injection - is still unresolved. A major complication, which hinders the progress in this field, is fast nonradiative Auger recombination of gain-active multicarrier species such as trions (charged excitons) and biexcitons [3,4]. Recently, we explored several approaches for mitigating the problem of Auger decay by taking advantage of a new generation of core/multi-shell QDs with a radially graded composition that allow for considerable (nearly complete) suppression of Auger recombination by “softening” the electron and hole confinement potentials [5]. Using these specially engineered QDs, we have been able to realize optical gain with direct-current electrical pumping [6], which has been a long-standing goal in the field of colloidal nanostructures. Further, we apply these dots to practically demonstrated the viability of a “zero-threshold-optical-gain” concept using not neutral but negatively charged particles wherein the pre-existing electrons block either partially or completely ground-state absorption [7]. Such charged QDs are optical-gain-ready without excitation and, in principle, can exhibit lasing at vanishingly small pump levels. All of these exciting recent developments demonstrate a considerable promise of colloidal nanomaterials for implementing solution-processible optically and electrically pumped laser devices operating across a wide range of wavelengths and fabricated on virtually any substrate using a variety of optical-cavity designs.

[1] Klimov, V. I.et al., Optical gain and stimulated emission in nanocrystal quantum dots. Science290, 314 (2000).

[2] Klimov, V. I.et al., Single-exciton optical gain in semiconductor nanocrystals. Nature447, 441 (2007).

[3] Klimov, V. I. et al., Quantization of multiparticle Auger rates in semiconductor quantum dots. Science287, 1011 (2000).

[4] Robel, I., et al., Universal Size-Dependent Trend in Auger Recombination in Direct-Gap and Indirect-Gap Semiconductor Nanocrystals. Phys. Rev. Lett.102, 177404 (2009).

[5] Y.-S. Park, et al., Effect of Interfacial Alloying versus “Volume Scaling” on Auger Recombination in Compositionally Graded Semiconductor Quantum Dots. Nano Lett. 17, 5607 (2017).

[6] Lim, J., et al., Optical Gain in Colloidal Quantum Dots Achieved by Direct-Current Charge Injection. Nat. Mater.17, 42 (2018).

[7] Wu, K., et al., Towards zero-threshold optical gain using charged semiconductor quantum dots. Nat. Nanotechnol.12, 1140 (2017).

Halide Perovskites may be mostly normal (inorganic) semiconductors and, yes, we should be careful to describe them with concepts from organic electronics. HOWEVER, it is remarkable that a material with over-all high quality optoelectronic properties can result from fast, low temperature, solution preparation. Understanding the reason(s) behind this, may help answer the question if this is because Pb is so unique or if we can generalize to find other materials like these. I will consider apparent inconsistencies in what we think we know about their defects, to zoom in on what remains special, and maybe even unique.

work done with Gary Hodes (Weizmann) and many others

Halide Perovskites may be mostly normal (inorganic) semiconductors and, yes, we should be careful to describe them with concepts from organic electronics. HOWEVER, it is remarkable that a material with over-all high quality optoelectronic properties can result from fast, low temperature, solution preparation. Understanding the reason(s) behind this, may help answer the question if this is because Pb is so unique or if we can generalize to find other materials like these. I will consider apparent inconsistencies in what we think we know about their defects, to zoom in on what remains special, and maybe even unique.

Impressive photovoltaic conversion efficiencies have been achieved within a very short time span with solar cells based on metal halide perovskites. Such a development is especially remarkable given the low temperature preparation methods used for these materials.¹ It is generally accepted that defects must be “benign” and thus allow for high-quality devices despite low temperature synthesis. Relatedly, doping in halide perovskites has been achieved by intrinsic defects, however with very limited control over doping levels and spatial profiles.

We will discuss how the soft nature of halide perovskites contributes to the unusual optoelectronic properties.² For doping via intrinsic defects, we developed a procedure to control and determine the exact stoichiometry of halide perovskite films from infrared spectroscopy, which allows us to quantify the MA content in the films. We present the first perovskite-perovskite homojunction obtained by vacuum deposition of stoichiometrically-tuned methylammonium lead iodide films and devices based thereon.³ We analyzed the resulting thin film junctions by cross-sectional scanning Kelvin probe microscopy, and found a pronounced contact potential difference at the interface between the two differently doped perovskite layers.

[1] David Egger, Achintya Bera, David Cahen, Gary Hodes, Thomas Kirchartz, Leeor Kronik, Robert Lovrincic, Andrew Rappe, David Reichman, and Omer Yaffe. What Remains Unexplained about the Properties of Halide Perovskites? Advanced Materials, 30 (20):800691, 2018.

[2] Michael Sendner, Pabitra K. Nayak, David A. Egger, Sebastian Beck, Christian Müller, Bernd Epding, Wolfgang Kowalsky, Leeor Kronik, Henry J. Snaith, Annemarie Pucci, and Robert Lovrincic. Optical Phonons in Methylammonium Lead Halide Perovskites and Implications for Charge Transport. Materials Horizons, 3:613–620, 2016.

[3] Benedikt Dänekamp, Christian Müller, Michael Sendner, Pablo P. Boix, Michele Sessolo, Robert Lovrincic, and Henk Bolink. Perovskite-Perovskite homojunctions via Compositional Doping. The Journal of Physical Chemistry Letters, 9:2770, 2018.

An understanding of charge-carrier recombination processes is essential for the development of hybrid metal halide perovskites for photovoltaic applications. We show that typical measurements of the radiative bimolecular recombination constant in CH₃NH₃PbI₃ are strongly affected by photon reabsorption that masks a much larger intrinsic bimolecular recombination rate constant.

We have used optical-pump THz-probe spectroscopy to study the charge-carrier dynamics in a set of dual-source vapour-deposited CH₃NH₃PbI₃ films whose thicknesses vary between 50 and 533 nm [1]. We find that the bimolecular charge recombination rate appears to slow by an order of magnitude as the film thickness increases. However, by using a dynamical model that accounts for photon reabsorption and charge-carrier diffusion we determine that a single intrinsic bimolecular recombination coefficient of value 6.8 × 10^–10 cm³s^–1 is common to all samples irrespective of film thickness [2].

We therefore postulate that the wide range of literature values reported for such coefficients is partly to blame on differences in photon out-coupling between samples with crystal grains or mesoporous scaffolds of different sizes influencing light scattering, whereas thinner films or index-matched surrounding layers can reduce the possibility for photon reabsorption. We discuss the critical role of photon confinement on free charge-carrier retention in thin photovoltaic layers and highlight an approach to assess the success of such schemes from transient spectroscopic measurement.

References:

1)  Crothers, Timothy W., et al. "Photon reabsorption masks intrinsic bimolecular charge-carrier recombination in CH3NH3PbI3 perovskite." Nano letters 17.9 (2017): 5782-5789.

2)  Davies, Christopher L., et al. "Bimolecular recombination in methylammonium lead triiodide perovskite is an inverse absorption process." Nature communications 9.1 (2018): 293.

Hybrid halide perovskites are being intensively studied as active layers in photovoltaic cells. They combine cost efficiency due to solution processability and high power conversion efficiency due to electrical transport properties and proper band gap. The chemical stability, which is often affected by quality of surfaces and interfaces, still remains a concern. Meanwhile, lead sulfide (PbS) is rocksalt-structured semiconductor with low band gap, which can be easily made as large single crystals. PbS is also well known for quantum dot material with high radiative efficiency. Recently, quantum-dot-in-perovskite crystals reported as promising optoelectronic material as well. Therefore, understanding the interface between lead sulfide and halide perovskite is important. In this study, we perform first-principles density-functional theory (DFT) calculations to investigate atomic contact properties (e.g. interface geometry) and electronic contact properties (e.g. charge redistribution and band offset). We focus on interface between PbS and CsPbBr₃ where epitaxial interface between the both of materials with low lattice strain is possible. Our results predict spontaneous forming of CsPbBr3 - PbS interface is feasible, and consequentially this interface will have a type I band alignment.

The structural phase behavior of high quality single crystals of methylammonium lead iodide (CH₃NH₃PbI₃ or MAPbI₃) was revisited by combining Raman scattering and photoluminescence (PL) measurements under high hydrostatic pressure up to ca. 10 GPa. Both PL and Raman spectra show simultaneous changes in their profiles that indicate the occurrence of three phase transitions subsequently at around 0.4 GPa, 2.7 GPa and 3.3 GPa. At the second phase transition, the Raman spectra exhibit a pronounced reduction in linewidth of the phonon modes of the inorganic cage, similar to the changes observed at the tetragonal-to-orthorhombic phase transition occurring at around 160 K but ambient pressure [1]. This behavior is interpreted as evidence for the locking of the organic cations in the cage voids above 2.7 GPa, due to the reduced volume and symmetry of the unit cell. At the third phase transition, reported here for the first time, the PL is greatly affected, whereas the Raman experiences only subtle changes related to a splitting of some of the peaks. This behavior may indicate a change mostly in the electronic structure with little effect on the crystal structure. Strikingly, no amorphization of the sample was observed up to the highest pressure which reached close to 10 GPa, in frank discrepancy with most of the high-pressure (x-ray) data of the literature [2], which established an onset of 3 GPa for the set in of an amorphous phase in MAPI₃.

[1] Leguy, A.M.A. et al., Phys. Chem. Chem. Phys. 2016, 18, 27051–27066.
[2] Postorino, P. & Malavasi, L., J. Phys. Chem. Lett. 2017, 8, 2613–2622 and references therein.

Perovskite solar cells have been extensively developed for few years but still the ultrafast and fast processes occurring in this system are not fully understood. The main tools providing wide view of a charge transport behavior are transient absorption and time-resolved emission spectroscopies. In the femtoseconds to nanoseconds time range two main transient absorption features dominate. The first is the long-wavelength band edge bleach assigned to a state filling and decaying in nanosecond range [1]. The second one is caused by cooling of charges, it takes place in hundreds of femtoseconds and usually has a shape of a band edge bleach first derivative [2].

Comparison of transient absorption signals for differently sensitized methylammonium lead iodide (MAPbI₃) solar cells (non-stoichiometric and stoichiometric ratio of PbI₂:MAI in precursor solution) will be presented. It was found that modification of the precursor ratio from non-stoichiometric to stoichiometric leads to the drastic changes of transient absorption signal from the typical strong long-wavelength band-edge bleach to the weak derivative-like signal due to the bandgap shift [3].

Transient absorption and emission measurements allows determination of charge injection kinetics for different charge transporting materials and recombination rates within perovskite [2]. Our studies of the hole injection from MAPbI₃ to spiro-OMeTAD and its xanthene derivative X60 [4] as well as the recent results of electron injection from MAPbI₃ to PCBM, PenPTC and SPPO13 will be presented. Moreover, proper determination of the first and the second order recombination rate constants in perovskite materials will be proposed. In particular, the influence of different transient absorption data treatment (band integral, global analysis based on singular value decomposition and bleach minimum amplitude) on the obtained rate constants will be shown.

Acknowledgements

The study was supported by Polish Ministry of Science and Higher Education under project 0019/DIA/2017/46.

[1]  J. Peng, Y. Chen, K. Zheng, T. Pullerits, Z.Liang, Chem. Soc. Rev., 2017, 46, 5714-5729.

[2] K. Pydzińska, J. Karolczak, I. Kosta, R. Tena-Zaera, A. Todinova, J. Idigoras, J.A. Anta, M. Ziółek, ChemSusChem, 2016, 9, 1547-1659.

[3] K. Pydzińska, J. Karolczak, M. Szafrański, M. Ziółek, RCS Advances, 2018, 8, 6479-6487.

[4] K. Pydzińska,  P. Florczak, G. Nowaczyk, M. Ziółek, Synthetic Metals, 2017, 232, 181-187.

Thermodynamic calculations revealed that single junction solar cell conversion efficiencies can exceed the Shockley-Queisser limits and reach around 66% under 1-sun illumination if the excess energy of hot photogenerated carriers is utilized before they cool down to the lattice temperature (i.e., hot-carrier solar cells).¹ Organic–inorganic lead halide perovskite semiconductors have recently emerged as the leading contender in low-cost high-performance solar cells.^2,3 The key for the realization of hot-carrier solar cell include the slow hot-carrier cooling and effective extraction of hot-carrier energies which requires fast hot-carrier injection into charge collection layer before hot-carrier cooling down to the lattice temperature. Emulating semiconductor nanoscience, some interesting questions would be if the hot-carrier cooling rate in halide perovskites could be further modulated through confinement effects, and if these hot-carriers can be efficiently extracted. Here, the hot-carrier cooling dynamics and mechanisms in colloidal CH₃NH₃PbBr₃ nanocrystals of different sizes (with mean radius ~2.5–5.6 nm) and their bulk-film counterpart were compared using room-temperature transient absorption spectroscopy. Our results revealed that the weakly quantum confined CH₃NH₃PbBr₃ nanocrystals are very promising hot-carrier absorber materials (~ 2 orders slower hot-carrier cooling times and around 4 times larger hot-carrier temperatures than their bulk-film counterparts). This is attributed to their intrinsic phonon bottleneck and Auger-heating effects at low and high carrier densities, respectively. Importantly, we demonstrate efficient room temperature hot-electrons extraction (up to about 83%) by an energy-selective electron acceptor layer within ~1 ps from surface-treated perovskite nanocrystal very thin films (~30 nm). These new insights would allow the development of extremely thin absorber and concentrator-type hot-carrier perovskite solar cells. ⁴

References:

[1] Ross, R.T. & Nozik, A.J. Efficiency of Hot-Carrier Solar-Energy Converters. J. Appl. Phys. 53, 3813-3818 (1982).

[2] Lee, M.M., Teuscher, J., Miyasaka, T., Murakami, T.N. & Snaith, H.J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 338, 643-647 (2012).

[3] Zhou, H.P. et al. Interface engineering of highly efficient perovskite solar cells. Science 345, 542-546 (2014).

[4] Li, M. et al. Slow cooling and highly efficient extraction of hot carriers in colloidal perovskite nanocrystals. Nat. Commun. 8, 14350 doi: 10.1038/ncomms14350 (2017).

We have used time-resolved transient spectroscopies to better understand both bulk carrier dynamical processes as well as surface carrier dynamics. We employed transient reflection spectroscopy to measure the surface carrier dynamics in methylammonium lead iodide perovskite single crystals and polycrystalline films. We find that the surface recombination velocity (SRV) in polycrystalline films is nearly an order of magnitude smaller than that in single crystals, likely due to unintended surface passivation of the films during synthesis. In spite of the low SRV, surface recombination limits the total carrier lifetime in polycrystalline thin films, meaning that recombination inside grains and at grain boundaries is less important than top and bottom surface recombination. The suppressed SRV in the polycrystalline films appears to be related to an excess of methylammonium compared to the single crystals surfaces, determined by X-ray photoelectron spectroscopy analysis.  We studied the charge carrier dynamics in 2D Ruddlesden-Popper perovskites PEA2PbI4·(MAPbI3)n−1(PEA, phenethylammonium; MA, methylammonium; n = 1, 2, 3, 4) single crystals. We have also studied carrier dynamics in Perovskite QD samples and will discuss recent results. 

Recently, the low-temperature solution-processed metal-halide perovskites have demonstrated great potential in light harvesting applications. The light to electricity power conversion efficiency up to 23.25% has been achieved. The primary advantages of these lead based perovskites for solar cells are the large photon absorption coefficient, long-range balanced charge carrier diffusion lengths, low trap density, high charge carrier mobility, fast and efficient photo-generated exciton dissociation and slow electron-hole bimolecular recombination. Additional to these properties, the tunable optical direct bandgap over a wide range also makes these perovskites good candidates for using in light emission applications (Lasing & Electroluminescence). However, the relatively slow electron-hole bimolecular recombination in the traditional three-dimensional perovskites that drives their outstanding photovoltaic performance is a fundamental limitation for the light emission. Here, we show that the perovskite light emission efficiency could be greatly enhanced by tailoring the bimolecular-type recombination in three-dimensional crystals to excitonic-type recombination in low-dimensional crystals.

References

[1] Guichuan Xing, Bo Wu, Xiangyang Wu, Mingjie Li, Bin Du, Qi Wei, Jia Guo, Edwin K. L. Yeow, Tze Chien Sum, Wei Huang, Nature Communications, 8, 14558 (2017).

[2] Guichuan Xing, Mulmudi Hemant Kumar, Wee Kiang Chong, Xinfeng Liu, Yao Cai, Hong Ding, Mark Asta, Michael Grätzel, Subodh Mhaisalkar, Nripan Mathews, Tze Chien Sum, Advanced Materials, 28, 8181-8196 (2016).

[3] Guichuan Xing, Nripan Mathews, Swee Sien Lim, Natalia Yantara, Xinfeng Liu, Dharani Sabba, Michael Grätzel, Subodh Mhaisalkar, Tze Chien Sum, Nature Materials, 13, 476-480 (2014).

[4] Guichuan Xing, Nripan Mathews, Shuangyong Sun, Swee Sien Lim, Yeng Ming Lam, Michael Grätzel, Subodh Mhaisalkar, Tze Chien Sum, Science, 342, 344-347 (2013).

[5] Fei Yan, Jun Xing, Guichuan Xing, Lina Quan, Swee Tiam Tan, Jiaxin Zhao, Rui Su, Lulu Zhang, Shi Chen, Yawen Zhao, Alfred Huan, Edward H Sargent, Qihua Xiong, Hilmi Volkan Demir, Nano Lett., 18, 3157-3164 (2018)

Acknowledgements: Financial support from the Macau Science and Technology Development Fund (FDCT-116/2016/A3 and FDCT-091/2017/A2), Research Grant (SRG2016-00087-FST and MYRG2018-00148-IAPME) from the University of Macau, the Natural Science Foundation of China (91733302, 61605073 and 2015CB932200) is gratefully acknowledged.

In less than 10 years, hybrid organic-inorganic perovskites have emerged as a new generation of absorber materials for high-efficiency and low-cost solar cells [1], [2]. More recently, fully inorganic perovskite quantum dots (QD) also led to promising efficiencies [3], [4] and then become a serious alternative to hybrid organic-inorganic perovskites. Currently, the record efficiency for QD solar cells is obtained with CsPbI₃. High resolution in-situ synchrotron XRD measurements have been performed on CsPbI₃ as a function of the temperature and revealed a highly anisotropic variation of the lattice parameters. Moreover, CsPbI₃ can be temporarily maintained in a perovskite-like structure down to room temperature, stabilizing a metastable perovskite polytype (black-phase) crucial for photovoltaic applications. Structural, vibrational and electronic properties of the three experimentally observed black phases are further scrutinized using theoretical approaches [5], [6]. A symmetry-based tight-binding model, calibrated with self-consistent GW calculations including spin-orbit coupling, affords further insight into their electronic properties. A Rashba effect is thus predicted for both cubic and tetragonal phases when using the symmetry breaking structures obtained through frozen phonon calculations.

The ab initio simulations have been performed on HPC resources of CINES under the allocation 2017-[x2017096724] made by GENCI (Grand Equipement National de Calcul Intensif).

[1] A. Kojima, et al., Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J.

Am. Chem. Soc. 2009, 131, 6050−6051.

[2] Best research-cell efficiencies; https://www.nrel.gov/pv/assets/images/efficiency-chart.png (accessed Nov 7, 2017).

[3] H. Bian et al., Graded Bandgap CsPbI2+xBr1-x Perovskite Solar Cells with a Stabilized Efficiency of 14.4%, Joule (2018), https://doi.org/10.1016/j.joule.2018.04.012

[4] E. M. Sanehira, et al., Enhanced Mobility CsPbI3 Quantum Dot Arrays for Record-efficiency, High-voltage Photovoltaic Cells. Sci. Adv. 2017, 3, eaao4204.

[5] A. Marronnier et al., Structural Instabilities Related to Highly Anharmonic Phonons in Halide Perovskites. . J. Phys. Chem. Lett. 2017, 8, 2659−2665

[6] A. Marronnier et al., Anharmonicity and Disorder in the Black Phases of Cesium Lead Iodide Used for Stable Inorganic Perovskite Solar Cells. ACS Nano 2018, 12, 3477−3486

Due to their sizable refractive index, reflectivity of visible light off the lead halide perovskite - air interface exceeds 15%. This has prompted a number of investigations into the prominence of photo-reflective contributions to pump-probe data in these materials, with conflicting results. Here we report experiments aimed at assessing this by comparing transient transmission from lead halide perovskite films and weakly quantum confined nanocrystals of cesium lead iodide (CsPbI3) perovskite. The rationale is to compare pump-probe measurements samples where the relative contributions of changes in the real and imaginary components of the refractive index are very different. The absorption cross section of a nanocrystal of volume v in terms of its complex refractive index and that of its (non-absorbing) surroundings n0 is

is a local field factor which must be included since it depends on the same refractive indices. For a polycrystalline film or a crystal, reflectivity at the interface with a non-absorbing dielectric whose refractive index is n0, is given by the Fresnel relation {2}:

The first derivatives of sabs or of R (both of which reduce transmission) with respect to n or k quantify their sensitivity to variations in either constant. By analyzing how complex refractive index changes impact the two experiments we can show that reflectivity changes due to variations in n would not only differ in amplitude but even in sign in both experiments. The results presented in the figure below demonstrate virtually identical TA data for both samples proving that changes in absorption and not reflection dominate transient transmission measurements in thin films of these materials. None of the characteristic spectral signatures reported in such experiments is exclusively due to, or even strongly affected by changes in sample reflectivity. This finding is upheld by another experiment where a methyl ammonium lead iodide perovskite film was formed on high index flint glass, and probed after pump irradiation from either face of the sample. We conclude that interpretations of ultrafast pump-probe experiments on thin perovskite films in terms of photo-induced changes in absorption alone are qualitatively sound, requiring relatively minor adjustments to factor in photo-reflective effects.

Layered halide hybrid organic−inorganic perovskites [1] have been the subject of intense investigation before the rise of three-dimensional (3D) halide perovskites and their impressive performance in solar cells. Recently, layered perovskites have also been proposed as attractive alternatives for photostable solar cells [2] and revisited for light-emitting devices. Interestingly, these performances can be traced back to extremely efficient internal exciton dissociation through edge states identified on thin films and single crystals [3].

Layered perovskites present fascinating features with inherent quantum and dielectric confinements imposed by the organic layers sandwiching the inorganic core, and computational approaches have successfully help rationalized their properties (excitonic, Rashba effects, etc.) [4-6]. Here, we propose a joint spectroscopic and computational investigation to unravel the origin of the recently identified layer-edge states in layered Ruddlesden-Popper phases with inorganic layers containing n = 1 to 4 octahedra. We show that for n > 2, the system presents a localized surface state within the band gap.

Based on our conclusion, we propose an elastic model providing design principles for future layered perovskites with optimized properties for photovoltaics or light emission.

References

[1] L. Pedesseau et al., ACS Nano (2016), 10, 9776.

[2] H. Tsai et al.,Nature (2016), 536, 312.

[3] J.-C. Blancon et al., Science (2017), 355, 1288.

[4] M. Kepenekian et al., ACS Nano (2015), 12, 11557.

[5] D. Sapori, M. Kepenekian, L. Pedesseau, C. Katan, J. Even, Nanoscale (2016), 8, 6369.

[6] M. D. Smith et al., Chem. Sci. (2017), 8, 1960.

In recent years, the interest in hybrid organic - inorganic perovskites rose at a rapid pace due to their tremendous success in the field of photovoltaic; but other fields, like light emitting diodes, show great potential as well. In such devices, the function and performance depend crucially on the proper alignment of the energy level landscape throughout the device, i.e. allowing for efficient charge transport across the various interfaces. Here, an advantage of these novel semiconductors is that the electronic structure and band gap energy can be readily tuned by changing the compositions of the perovskite.

In this talk, I will discuss recent findings regarding the variations in electronic structure of hybrid perovskites, covering all lead and tin based halide systems using a combined DFT and UV-/ inverse/ x-ray photoelectron spectroscopy study. Furthermore, with these surface sensitive techniques, the energetic alignment at interfaces between different layers can be probed in-situ by performing a stepwise film preparation. Looking at various bottom contacts I will show that chemical interactions, band bending, and interface dipole formation play an important role.

Lead halide materials have seen a recent surge of interest from the photovoltaics community following the observation of surprisingly high photovoltaic performance, with optoelectronic properties similar to GaAs. This begs the question: What is the limit for the efficiency of these materials? It has been known that under 1-sun illumination the efficiency limit of crystalline silicon is ∼29%, despite the Shockley–Queisser (SQ) limit for its bandgap being ∼33%: the discrepancy is due to strong Auger recombination. In this article, we show that methyl ammonium lead iodide (MAPbI3) likewise has a larger than expected Auger coefficient. Auger nonradiative recombination decreases the theoretical external luminescence efficiency to ∼95% at open-circuit conditions. The Auger penalty is much reduced at the operating point where the carrier density is less, producing an oddly high fill factor of ∼90.4%. This compensates the Auger penalty and leads to a power conversion efficiency of 30.5%, close to ideal for the MAPbI3 bandgap.

The development of organic-inorganic lead halide perovskites with very large efficiency requires us to understand the operation of the solar cell. 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 describe the results of analysis of kinetic phenomena using frequency modulated techniques. First with impedance spectroscopy 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.¹ 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 also shows the identification of the impedance of ionic diffusion by measuring single crystal samples.² Working in samples with lateral contacts, we can identify the effect of ionic drift on changes of photoluminescence, by the creation of recombination centers in deffects of the structure.³ We also address new methods of characterization of the optical response by means of light modulated spectroscopy. The IMPS is able to provide important influence on the measured photocurrent. We describe important insinghts to the measurement of EQE in frequency modulated conditions, which shows that the quantum efficiency can be variable at very low frequencies.⁴

(1)         Lopez-Varo, P.; Jiménez-Tejada, J. A.; García-Rosell, M.; Ravishankar, S.; Garcia-Belmonte, G.; Bisquert, J.; Almora, O. Device Physics of Hybrid Perovskite Solar cells: Theory and Experiment, Adv. Energy Mater. 2018, 1702772.

(2)         Peng, W.; Aranda, C.; Bakr, O. M.; Garcia-Belmonte, G.; Bisquert, J.; Guerrero, A. Quantification of Ionic Diffusion in Lead Halide Perovskite Single Crystals, ACS Energy Lett. 2018.

(3)         Li, C.; Guerrero, A.; Zhong, Y.; Gräser, A.; Luna, C. A. M.; Köhler, J.; Bisquert, J.; Hildner, R.; Hüttner, S. Real-Time Observation of Iodide Ion Migration in Methylammonium Lead Halide Perovskites, Small 2017, 1701711.

(4)         Ravishankar, S.; Aranda, C.; Boix, P. P.; Anta, J. A.; Bisquert, J.; Garcia-Belmonte, G. Effects of Frequency Dependence of the External Quantum Efficiency of Perovskite Solar Cells, J. Phys. Chem. Lett. 2018, 3099-3104.

Triple mesoporous layer devices containing a TiO₂ electron transport layer, a ZrO₂ insulating layer and carbon as the hole transporting contact show great promise for scale-up and wide spread implementation. To improve these devices and begin to challenge inorganic PV record efficiencies a deeper understanding of their operation, and in particular sources of performance loss, is needed.
The current state-of-the-art devices use a mixed cation perovskite, consisting of methylammonium and 5-aminovaleric acid (5-AVA). The AVA containing perovskite has been shown to give greater stability and performance – linked to 2D/3D structuring of the perovskite as well as interfacial modifications at the TiO₂ surface. The cells undergo a slow light soaking effect during which time the JV performance of the device is vastly improved. They also show improvement when exposed to a high relative humidity.
A striking feature observed using TPV measurements is the presence of a negative photovoltage transient, comparable to that observed in our previous work on planar TiO₂ devices at low temperature. This behaviour suggests the presence of high rates of interfacial recombination at the TiO₂ surface. In carbon based cells the phenomena is observed at room temperature and is very slow to disappear under continued illumination. For the planar devices the negative transient was shown to diminish over time as ions in the perovskite redistributed, leading to a reduction in the recombination rate. We show that in the carbon devices the exceptionally slow dynamic behaviour observed at room temperature has a similar origin linked to the effects of ion migration – activation energy calculated to be 0.4 eV (in the range of many literature values for iodide migration). However, it takes place at a much slower rate due to the 2D AVA based perovskite hindering iodide ion migration – attempt frequency reduced by several orders of magnitude compared to pure MAPI devices. We show that the inhibited ion migration is the dominant affect rather than the AVA having a direct impact at the TiO₂ interface by adsorption via the carboxylic acid group. This inhibition of iodide migration is also linked to the increased stability demonstrated for these devices.

Lead halide perovskites have long been known to be ionic and electronic semiconductors.^{1, 2} Recently, lead halide perovskite have revolutionized the photovoltaics field with impressive efficiencies reaching ~23 %. Unlike most photoactive materials, ionic conductivity plays a key role in perovskites during photovoltaic device operation. In this presentation it is described how to characterize the ionic properties of lead halide perovskites by advanced electrical and optical techniques.³ Approaches to minimize the electronic contribution to the measured current are used so the ionic current can be probed. For example, Impedance Spectroscopy (IS)  reveals the characteristic signature of ionic diffusion in monocrystalline devices which do not contain HTL. This is the Warburg element and transmission line equivalent circuit in MAPbBr₃ and ion accumulation at the MAPbBr₃/Au interface typical for non-reactive contacts. Alternatively, measurements of confocal photoluminescence (PL) in interdigitated electrodes as a function of the applied bias reveals that the ionic movement dramatically modifies the PL emission properties that also allows the calculation of the diffusion coefficient of the material.⁴

References

1.         Kuku, T. A.; Salau, A. M., Electrical conductivity of CuSnI3, CuPbI3 and KPbI3. Solid State Ionics 1987, 25, 1-7.

2.         Kuku, T. A., Ionic transport and galvanic cell discharge characteristics of CuPbI3 thin films. Thin Solid Films 1998, 325, 246-250.

3.         Peng, W.; Aranda, C.; Bakr, O. M.; Garcia-Belmonte, G.; Bisquert, J.; Guerrero, A., Quantification of Ionic Diffusion in Lead Halide Perovskite Single Crystals. ACS Energy Lett. 2018, 1477-1481.

4.         Li, C.; Guerrero, A.; Zhong, Y.; Gräser, A.; Luna, C. A. M.; Köhler, J.; Bisquert, J.; Hildner, R.; Huettner, S., Real-Time Observation of Iodide Ion Migration in Methylammonium Lead Halide Perovskites. Small 2017, 13, 1701711.

The relatively weak bond of metal-halide perovskites  (MHPs) gives rise to an inherently soft crystal lattice which is naturally prone to disorder, [1] associated to formation of defects. Defects introducing levels in the material’s band-gap may act as traps and recombination centers for photogenerated charge carriers, limiting the device performance and possibly impacting the device temporal stability. Defects may also introduce ionic mobility channels in MHPs. Ion migration is boosted by the presence of vacancies and interstitial defects, acting as shuttles for ion hopping.[2] If the migrating defects are also charge traps, as it occurs for iodine defects in MAPbI₃, one has migrating traps which can respond to the action of an electric field [3] and to the presence of photogenerated carriers.[4, 5] Some of the traps may also undergo photochemical reactions, such as the reported release of molecular iodine under light irradiation[6, 7]. Defects may also lay behind the reported material transformation under light exposure, followed by very slow relaxation to initial conditions.[8,9]

Theoretical and computational modeling is a complementary tool for rationalizing experimental results, on the one hand, and to direct experiments and device fabrication towards innovative concepts, on the other hand.  Several computational studies have already been carried out on native defects in MHPs, employing Density Functional Theory. The complex interplay of electronic structure and dynamical features of MHPs, however, poses challenging problems to the accuracy and reproducibility of these calculations.[10] Here we present what we believe are the “best practices” in defect modeling of metal-halide perovskites with selected examples of applications related to the effect of electric fields and charge carriers on the structural and electronic properties of perovskites relevant to stability and solar cell operation.

References:

[1] Conings, B. et al. Adv. Energy Mater. 2015, 5, 1500477.

[2] Mosconi, E.; De Angelis, F. ACS Energy Lett. 2016, 1, 182-188.

[3] Chen, B. et al. Nat. Mater., 2018, in press.

[4] Birkhold, S.T.  et al. ACS Energy Lett. 2018, 3, 1279−1286

[5] Meggiolaro, D. et al. Energy Environ. Sci. 2018, 11, 702-713.

[6] Meggiolaro, D. et al. ACS Energy Lett., 2018, 3, 447–451.

[7] Kim, G.Y. et al. Nat Mater 2018, 17, 445-449.

[8] Gottesman, R. et al.  J. Phys. Chem. Lett. 2014, 5, 2662-2669.

[9] Tsai, H. et al. Science 2018, 360, 67-70.

[10] Meggiolaro, D.; De Angelis, F. ACS Energy Lett. 2018, 2206-2222.

The high performance of recently emerged lead halide perovskite-based photovoltaic

devices has been attributed to remarkable carrier properties in this kind of material:

long carrier diffusion length, long carrier lifetime, and low electron-hole recombination

rate. However, the mechanism of the charge separation is still not fully understood. In my talk, it will be demonstrated that the charge separation is induced by the structural fluctuation of the inorganic lattice using first-principles molecular dynamics simulations [1]. On the other hand, charge carrier trapping at defects on surfaces or grain boundaries is detrimental for the performance of perovskite solar cells. In practice, it is one of the main limiting factors for carrier lifetime. 　Surface defects responsible for carrier trapping are clarified by comprehensive first-principles investigations and it is proposed that PbI2-rich condition is preferred to MAI-rich one, while intermediate condition has possibility to be the best choice [2].

References

[1] H. Uratani and K. Yamashita, J. Phys. Chem. C, 121, 26648−26654 (2017).

[2] H. Uratani and K. Yamashita, J. Phys. Chem. Lett., 8, 742−746 (2017).

While 3D perovskites are the materials currently leading the field for photovoltaics, 2D hybrid organic/inorganic layered materials have a much broader versatility in accommodating a vast variety of organic molecules and are providing a long-term devices stability. In particular, we obtained a one-year stable perovskite device by engineering an ultra-stable 2D/3D perovskite junction with a PCE of 14.6% in standard mesoporous solar cells.[1] Hybrid organic-inorganic multidimensional perovskites, also known as Ruddlesden-Popper perovskites, are composed of 3D domains separated by large organic cations. The mixing of the two mainly studied types of perovskites supposes the combination of the good properties from each one. Due to the 3D domains, Ruddlesden-Popper perovskites can absorb radiation in a wide range of the electromagnetic spectrum. Moreover, the environmental stability issue characteristic of 3D perovskites is solved thanks to the good stability provided by the 2D domains, in which the larger amount of organic phase acts as barrier against water and moisture penetration.[1] The main problem of this material arises from its photoexcitation and the consequent generation of the electron-hole pairs. Holes and electrons keep confined in the inorganic layers due to the electric isolation of the organic cations in 2D domains, i.e.: while their diffusion along the 3D domains is excellent, it is quite poor across the 2D ones. In order to find a solution, we will explore the possibility of improving the conductivity of charge carriers across 2D domains by the insertion of specifically designed conducting organic cations. The chemical structure of these cations should be suitable for the purpose, for instance by embedding aromatic rings or conjugated multiple bonds and allowing the selective transport of a single carrier type. The desired cation properties will be finely computed in order to realize a Ruddlesden-Popper perovskite with high and selective vertical conduction, see Fig. 1.

[1] Grancini, G.; Roldán-Carmona, C.; Zimmermann, I.; Mosconi, E.; Lee, X.; Martineau, D.; Narbey, S.; Oswald, F.; De Angelis, F.; Graetzel, M., et al. Nat Commun, 8 (2017) 15684.

Both all inorganic and hybrid halide perovskites have undeniably remarkable characteristics for next-generation photovoltaics, which deserve to be better understood. There are many different perovskite structures that are currently widely explored as absorber materials among which 3D AMX₃ and 2D A₂ A’_n-1 M_n X_3n+1 frameworks, where A, A’ are cations, M is a metal, X is a halide. Here, through a couple of recent examples including newly discovered halide perovskite phases [1], we will discuss their optoelectronic properties based on first-principles calculations and semi-empirical modelling. Impact of interfaces [2], structural fluctuations [3], quantum and dielectric confinements [4] on charge carriers and excitons will be inspected. Particular attention will be paid on excitonic effects comparing the results of model calculations with low temperature optical spectroscopy and 60-Tesla magneto-absorption [5]. Theoretical inspection of low energy states associated with electronic states localized on the edges of the perovskite layers [6] will also be shown to provide guidance for the design of new synthetic targets [7] taking advantage of experimentally determined elastic constants [8].

[1] C. M. M. Soe et al. JACS, 139, 16297, 2017; L. Mao et al. JACS, 140, 3775, 2018; X. Li et al. submitted.

[2] W. Nie et al. Adv. Mater. 30, 1703879, 2018.

[3] M. A. Carignano et al. J. Phys. Chem. C, 121, 20729, 2017; A. Marronnier et al. ACS Nano, 12, 3477, 2018; L. Zhou et al. ACS Energy Lett., 3, 787–793, 2018; C. Katan et al. Nature Materials, 17, 377, 2018.

[4] B. Traore et al. ACS Nano, 12, 3321, 2018.

[5] J.-C. Blancon et al. Nature Com. in press (arXiv:1710.07653).

[6] J.-C. Blancon et al. Science, 355, 1288, 2017.

[7] M. Kepenekian et al. arXiv:1801.00704.

[8] A. C. Ferreira et al. arXiv:1801.08701.

In this talk, I will discuss our group’s recent experimental and computational work on understanding electronic and phononic structure nanocrystal thin films and charge transport in these thin films. Using electrochemical-based approaches, we show that we can quantify the electronic density of states and also examine charge-transfer processes across interfaces. Using inelastic x-ray scattering, we quantify the phononic denisty of states. We combine density functional theory calculations and kinetic Monte Carlo simulations to develop a first-principles model for charge transport in nanocrystals solids. We show that these simulations explain temperature-dependent time-of-flight measurements of electron and hole mobility performed on lead sulfide nanocrystal thin films. The combination of experimental and computational work highlights the importance of electron-phonon interactions in nanoscale transport and enables us to determine the relative impact of energetic and positional disorder on transport, providing us with design guidelines on parameters to consider when optimizing nanocrystal synthesis, nanocrystal surface treatments, and nanocrystal thin film preparation for different device applications.

For the fabrication of an integrated solar-to-chemical system, different components should be interfaced together in an orchestrated manner. Photoelectrodes need to absorb in the visible range, with a valence and a conduction band suited for the target reaction. Moreover, the presence of catalysts is required to manage the intrinsic energetic hurdle. Herein, we address the study of the major challenges, namely performance, stability, and interfaces to enable fabrication of integrated solar-to chemical systems. Novel scientific directions for the synthesis of functional interfaces and the development of new tools for their characterization will be addressed. Specifically, we will present a methodology for evaluating corrosion mechanisms and apply it to bismuth vanadate, a state-of-the-art photoanode. Analysis of changing morphology and composition under solar water splitting conditions reveals chemical instabilities that are not predicted from thermodynamic considerations of stable solid oxide phases, as represented by the Pourbaix diagram for the system. These findings are confirmed by in situ electrochemical atomic force microscopy (EC-AFM), which reveals that degradation under operating conditions occurs via dissolution of the film, starting at exposed facets of grains in polycrystalline thin films. In addition, we will present the correlation between morphological and functional heterogeneity in this material by photoconductive atomic force microscopy. We demonstrate that contrast in mapping electrical conductance depends on charge transport limitations, and on the contact at the sample/probe interface. We observe no additional recombination sites at grain boundaries, which indicates high defect tolerance in bismuth vanadate.

Insights into corrosion mechanisms and nanoscale heterogeneity aid development of protection strategies and provide information on how local functionality affects the macroscopic performance. 

Metal-halide perovskites have emerged as promising solution-processable semiconductors for optoelectronics applications. These materials show unexpectedly high luminescence yields and long carrier lifetimes under operating conditions. Facile changes in composition during fabrication can be employed to control their optical properties, and the nature of electronic states. Recently, the ad-mixture of monovalent cations to the precursor solutions has demonstrated enhanced luminescence yields and optoelectronic performance, which harvests photon-recycling effects.

The properties and dynamics of the perovskites’ electronic states are controlled by the crystal structure and symmetry. Strong spin-orbit coupling is predicted to introduce Rashba-type state splitting in the electronic band structure. Together with the soft crystal structure of the perovskite lattice, it is likely that dynamic changes occur in the electronic states during their lifetime. So far, it is not understood how such effects change after optical excitation and how they proceed during relaxation of electronic states.

In this talk I will present how we use spectroscopic techniques to study the dynamics of electronic states and crystal structure in metal-halide perovskites on ultrafast timescales. I will report results on layered and bulk lead-halide perovskites, but also on more sustainable lead-free variants. I will discuss how the crystal structure affects the properties of electronic states, and how we can use these modifications to create novel optoelectronic devices.

Three-dimensional lead-halide perovskites combine solution processing with outstanding optoelectronic properties. Despite their soft ionic nature these materials demonstrate a surprisingly low level of electronic disorder. Understanding how structural and dynamic disorder impacts the optoelectronic properties of these perovskites is important for many applications. Here we combine a number of bulk-sensitive and surface-selective spectroscopic techniques to eluscidate the structure and dynamics of organic cations.

We use ultrafast two-dimensional vibrational spectroscopy and molecular dynamics simulations to study the dynamics of the organic cation orientation in a films of pure and mixed tri-halide perovskite materials. For pure MAPbX3 (X=I, Br, Cl) perovskite films, we observe that the cation dynamics accelerate with decreasing size of the halide atom. Much slower dynamics, up to partial immobilisation of the organic cation, are observed in the mixed MAPb(ClxBr1-x)3 and MAPb(BrxI1-x)3 alloys, which we associate with symmetry breaking within the perovskite unit cell.

We also applied surface sensitive vibrtational sum-frequency generation spectroscopy (VSFG) to address the orientation of cations at the perovrkite active layer interfaces with different electron- and hole-extracting materials.We found that at perovskite spiro interface cations are patially immobilised that can be and evidence of high interfacial charge density. 

The observed dynamics and structural information are essential for understanding the effects of structural and dynamical disorder in perovskite-based optoelectronics.

Within five years, methylammonium lead iodide (MAPbI3) solar cells have quickly reached remarkable power conversion efficiencies rivaling those of established technologies. However, arguably, the toxic and water-soluble lead compound may be an obstacle on their way to market. The quest for alternative, non-toxic photo harvesters is partly hampered by a lack of fundamental understanding of the crystal grain’s characteristics and energy conversion mechanisms. As part of this process, the scientific community controversially discusses the importance of ferroic properties for the exceptional performance of MAPbI3 light-harvesting layers, including claims of non-ferroelectricity, anti-ferroelectricity, ferroelectricity and ferroelasticity. Simulations have predicted ferroelectricity in MAPbI3 with alternating polarized domains ruling the charge carrier transport [1]. Understanding the crystallographic cause and the effects of the crystal’s ferroelectricity would therefore provide helpful guidance for the quest to find non-toxic MAPbI3 replacements.

We explore the ferroic properties of methylammonium lead iodide perovskite solar cells by piezoresponse force microscopy (PFM) [2][3]. In vertical and horizontal PFM imaging, we find 90 nm wide ferroelectric domains of alternating polarization. High-resolution photo-conductive atomic force micrographs under illumination also show alternating charge carrier extraction patterns which we attribute to the local vertical polarization components within the ferroelectric domains.

We apply these techniques to investigate formation of polarized domains during thermal treatment and study their influence on the performance of perovskite solar cells. Annealing steps, commonly only viewed as a means of crystal growth and precursor conversion, prove to directly influence the formation, shape and polarization direction of ferroelectric domains in perovskite thin films.

References:

[1] D. Rossi, A. Pecchia, M. Auf der Maur, T. Leonhard, H. Röhm, M. J. Hoffmann, A. Colsmann, A. D. Carlo, Nano En. (2018).

[2] H. Röhm, T. Leonhard, M.J. Hoffmann, A. Colsmann, Energy Environ. Sci. (2017).

[3] T. Leonhard, A. Schulz, H. Röhm, S. Wagner, F. Altermann, W. Rheinheimer, M.J. Hoffmann, A. Colsmann, submitted.

Slow carrier recombination in metal halide perovskites (MHP) is considered the origin of the observed large charge carrier diffusion length. However, the reasons for this slow charge carrier recombination remains unclear. In this report, on the basis of novel experimental data and the results of modeling, we propose that the observed long luminescence lifetimes in MHP are due to the extremely fast capture of carriers by shallow non-quenching traps, followed by their much slower release back to the conduction band. Multiple (tens to thousands of times) repetition of this loss-free process can explain the observed slow luminescence decay after pulsed photoexcitation (the so-called "delayed luminescence") [1, 2]. In the case of polycrystalline layers and in single crystals of MHP, the result of this multiple capture of carriers by shallow non-quenching traps are rather low values of the diffusion coefficient found for this type of materials [2].

REFERENCES

[1] Chirvony, V. S.; González-Carrero, S.; Suárez, I.; Galian, R. E.; Sessolo, M.; Bolink, H. J.; Martínez-Pastor, J. P.; Pérez-Prieto, J. Delayed luminescence in lead halide perovskite nanocrystals. J. Phys. Chem. C 2017, 121, 13381-13390.

[2] Chirvony, V. S.; Martínez-Pastor, J. P. Trap-limited dynamics of excited carriers and delayed luminescence in metal halide perovskites. J. Phys. Chem. Lett. 2018, accepted.