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).

Perovskite cells pose an intriguing modelling challenge as the electrical cell properties are governed by both electronic and ionic charge transport and the optical cell properties need to be carefully optimized when seeking record efficiencies in tandem cell configurations with silicon wafer cells. In this contribution, we give an update on recent advances in both electrical and optical modeling and discuss experimental results.

Negative capacitance and inductive loops in impedance spectroscopy in perovskite solar cells have been described in several recent reports, though their origin remained unclear so far. The negative capacitance and inductive loop may be related to one another as they appear in the same samples but at different applied biases. Similarly, we have demonstrated that ion migration is present even in high-efficiency low-hysteresis perovskite cells [1]. We shed light on the likely physical mechanisms behind these observations and compare devices in the frequency domain at different applied bias by employing a mixed electronic-ionic device model that naturally produces inductive loops and negative capacitance allowing us to study correlations with relevant material parameters.

Moreover we present an optical model implemented in the software SETFOS 4.6 [2] for simulating perovskite/silicon monolithic tandem solar cells that exploit light scattering structures [3]. We validate the model with experimental data of tandem solar cells that either use front- or rear-side textures. The software is used to investigate the potential of different monolithic tandem structures. The p-i-n solar cell architecture is the most promising with respect to achievable photocurrent for both flat and textured wafers. Finally, cesium-formamidinium-based perovskite materials with several bandgaps were synthetized, optically characterized [4] and their potential in tandem devices was quantified by simulations. The most promising tandem has a potential of reaching a power conversion efficiency of 31% [5].

[1] M. Neukom, S. Züfle, E. Knapp, M. Makha, R. Hany, B. Ruhstaller, Solar En. Mat. & Solar Cells, 169 159ff (2017)

[2] T. Lanz, B. Ruhstaller, C. Battaglia, and C. Ballif, J. Appl. Phys. 110, 33111 (2011) and SETFOS 4.6 by Fluxim AG, https://www.fluxim.com, Switzerland

[3] S. Altazin, L. Stepanova, K. Lapagna, P. Losio, J. Werner, B. Niesen, A. Dabirian, M. Morales-Masis, S. de Wolf, C. Ballif, B. Ruhstaller, Proc. 32nd Eur. Photovolt. Sol. Energy Conf. 1276 (2016)

[4] J. Werner et al., ACS Energy Lett. 3, 742–747 (2018)

[5] S. Altazin, L. Stepanova, J. Werner, B. Niesen, C. Ballif, and B. Ruhstaller, Optics Express 26 (10), A579 (2018)

Metal halide perovskites are mixed ionic-electronic conductors extremely efficient for making solar cells, due to its strong absorption in the visible and their relatively slow recombination. Processes like transport, recombination, charge accumulation, hysteresis, etc. occur at very different time scales and determine the photovoltaic performance of the solar cell. Small-perturbation, frequency-modulated optoelectronic techniques such as impedance spectroscopy (IS) or intensity-modulated photocurrent spectroscopies (IMPS/IMVS) are especially suited to detect, deconvolute and quantify all these processes.

Nowadays, a full understanding of the typical features observed in the IS and IMPS/IMVS spectra in perovskite solar cells is still missing. Hence, it is not clear yet how properties like the magnitude, locus and the nature of the recombination loss can be identified or quantified from the analysis of the data. Besides, low frequency signals, associated to hysteresis phenomena should also be unambiguously assessed in the spectra. In this talk I discuss the interpretation of the spectra and the use of simple models to rationalize the small-perturbation response of the device and its impact on efficiency-determining dynamic processes.

The phenomenom of hysteresis in the current-voltage characteristics of perovskite solar cells has been discussed by numerous authors. It is agreed by many that mobile ions or ion vacancies play a significant role in the underlying processes causing this hysteresis. Calao et al. have shown that hysteresis will only occur if ion migration alters the rate of surface recombination. The latter is a loss mechanism which occurs at the interfaces of the perovskite absorber with the adjacent materials, usually electron (ETM) and hole (HTM) transporting material and it depends sensitively on the concentration of electrons and holes in the vicinity of such an interface. As Calao et al. could show there is minimal hysteresis if ETM or HTM layers passivate the corresponding interface to the absorber such that surface recombination becomes insignificant regardless of the distribution of mobile ions or ion vacancies.
We built inverted planar p-i-n perovskite solar cells and observed a high open-circuit voltage (Voc) when NiOx was employed as HTM. In contrast, Voc was significantly reduced if NiOx was replaced by PEDOT:PSS. This reduction of Voc could clearly be attributed to surface recombination occurring at the absorber/HTM interface as confirmed by photoluminescence measurements. Interestingly, the PEDOT:PSS based perovskite solar cells nevertheless did not show any hysteresis. To answer the question why the migrating ions do not cause hysteresis in these kind of devices we performed numerical simulations. We can reproduce the experimental finding when implementing interface traps at the perovskite/PEDOT:PSS interface. These states can lead to a fermi level pinning effect such that the ionic distribution has only very little influence on the concentrations of electrons and holes in the vicinity of that recombination active interface.

[1] P. Calado, A. M. Telford, D. Bryant, X. Li, J. Nelson, B. C. O’Regan, P. R.F. Barnes, Nature Communications 7 (2016), 13831

In perovskite solar cells, achieving consistency in device fabrication is very difficult. Although it easy to measure whether a solar cell performs well, it is not always easy to determine what the problem is in case of a bad solar cell. Many devices that show less than ideal performance show a feature called an ‘s-shape’, where in the JV-characteristic a region exists where the second derivative of the current with respect to voltage is negative. If this s-shape occurs below Voc, this will dramatically decrease the fill factor. Until now some possible causes are known. It has been shown using drift diffusion simulations that in perovskite solar cells, s-shapes can exist under forward bias. Also from organic solar cells it is well known that poor charge transport can lead to s-shapes, but no comprehensive explanation has been shown on these s-shapes in perovskites as of yet.

In this contribution we identify all the processes and parameters that yield s-shapes in the JV-characteristic and can therefore give a complete story on the phenomenon of s-shapes in perovskite solar cells. Furthermore, the occurrence of s-shapes is linked to device characteristics such as transport layer mobility and effects such as charge buildup at certain bias voltages. This is done using very large amounts of simulated solar cells with different parameters, where parameters are chosen such that the whole parameter space of solar cells yielding s-shapes is swept. Each individual solar cell simulation is a drift diffusion simulation including mobile ions where n-i-p or p-i-n structures are assumed. The conclusions from the modeling can be used to pinpoint weak spots of fabricated solar cells and help improve device recipes and fabrication.

During the last years, three-dimensional hybrid perovskites were in the spotlight because of their very promising properties as semiconductor materials for solar cells, but also for optoelectronic applications in general. Indeed, with a low temperature and precursor solution based synthesis and certified 20% photovoltaic efficiency, 3D hybrid perovskites have been claimed as the new big thing in photovoltaics¹. Further studies must be conducted to understand in detail the properties of this material, but especially to overcome one of its main flaws: degradation issues.

Currently, two-dimensional hybrid perovskites are getting attention of the scientific community mainly due to two reasons: the improvement of the stability and the chemical composition versatility, much higher than for the 3D-peovoskites. In fact, in the 3D case, the size of the “cages” created by the octahedral network limits the candidates as organic/inorganic cation, which is not the case of layered 2D hybrid perovskites. With these materials, we are expending the field of possibilities and it can lead to better tunability of the photophysical properties.

We are reporting here (time-dependent) density functional theory calculations [(TD)-DFT] on alkyl-ammonium lead iodide perovskites, and more specifically, the influence of the alkyl chain length of the spacer cation on the electronic structure and optical properties of the material. We predicted significant changes using long or short chains. Indeed, if the inorganic layers fix the electronic properties, the length of the organic cation showed to have an indirect effect. With long alkyl chains, as dodecyl chains, an opening of the electronic band gap occurs, due to the influence of the supramolecular packing on the structure organization of the octahedra network. For the case of long organic spacer dodecylammonium lead iodide perovskites, organic chains adopt a polyethylene-like packing, causing distortions in the inorganic frame and leading to the observed electronic band gap opening. These theoretical results are in agreement with experimental data and demonstrate that organic saturated chains can modify the optoelectronic properties of layered halide perovskite semiconductors².

¹ J. Bisquert, Journal of Physical Chemistry Letters 4(15):2597, 2013

² C.Quarti, et al., Journal of Physical Chemistry Letters, DOI: 10.1021/acs.jpclett.8b01309

The analysis of perovskite solar cells by impedance spectroscopy has provided a rich variety of behaviors that demand adequate interpretation. Two main features have been reported: First, different impedance spectral arcs vary in combination; second, inductive loops and negative capacitance characteristics appear as an intrinsic property of the current configuration of perovskite solar cells. Here we adopt a previously developed surface polarization model based on the assumption of large electric and ionic charge accumulation at the external contact interface. Just from the equations of the model, the impedance spectroscopy response is calculated and reproduces and explains the mentioned general features. The inductance element in the equivalent circuit is the result of the delay of the surface voltage and depends on the kinetic relaxation time. The model is therefore able to quantitatively describe exotic features of the perovskite solar cell and provides insight into the operation mechanisms of the device.

Recently, hybrid perovskite solar cells (HPSCs) have achieved conversion efficiencies > 22 % and
have revived the search for clean, affordable and efficient energy. However, the practical realization
of this hope is pending due to problems related to the stability of HPSCs in moisture, heat,
ultraviolet light and oxygen rich environments [1]. Thus, the search is underway for protective coating materials to protect HPSCs. A proper coating should have a wide band gap in order to serve as a good window
material, exhibit minimum lattice mismatch at the coating-perovskite interface and most
importantly, be resistance to environmental conditions. In this study, we consider a series of ABX3 perovskites, with  A = MA/Cs, B = Sn/Pb and X = Cl/Br/I. To select possible protective-coating candidates, we
filtered the large Aflow [2] database to collect materials with wide band gap (≥ 3 eV) and further
sorted them based on their solubility in water, toxicity and abundance. To avoid large lattice
mismatch, we only considered rectangular surfaces of coating materials and limited the mismatch to be
within -5 and 5 %. For instance, for MAPI3, we found the following promising coating candidates, NiO, PbTiO3, BaZrO3 , ZnO and GaN, whose lattice mismatch are -0.35, -0.10, 0.37, -0.04 and 0.64%, respectively.
In addition to protecting the stability of HPSCs, our collected coating materials have the potential to serve as efficient transport layers in the HPSC architecture. Our search will not only improve the stability of HPSCs but also serve as a starting point in the search of novel device materials for emergent HPSC technologies.
Keywords
hybrid, perovskites, resistance, lattice mismatch

Reference
[1] M. A. Green, A. Ho-Baille, and H. J Snaith, Nature Photon. 8, 506(2014).
[2] R. H. Taylor, F. Rose, C. Tohler, O. Levy, K. Yang, M. B. Nardelli, and S. Curtarolo,
Computational Materials Science 93, 178 (2014).

Solution-processed halide perovskites have demonstrated remarkable performances in optoelectronic devices and applications. Despite the extraordinary progress associated with perovskite materials, many questions about the fundamental photophysical processes taking place in these devices remain open. Here we report the results from an in-depth computational study of small polaron formation utilizing information from electronic structure, charge density, and reorganization energy calculations on isolated structures. Local lattice symmetry, electronic structure, and electron phonon coupling are interrelated in polaron formation in hybrid halide perovskites. To illustrate these aspects, first principles calculations are performed on CsPbI₃, CsSnI₃, CsPbBr₃, MAPbI₃, FAPbI₃, MAPbBr₃, FAPbBr₃, MASnI₃, and FASnBr₃. This study will focus on how ionic exchange changes the geometry and polaron binding energy in the material. It is found that in all cases that hole polaron formation is associated with smaller binding energies and lattice contraction, while electron polaron formation exhibits larger polaron binding energies, lattice expansion, and Jahn Teller like distortions.

The development of organic-inorganic lead halide perovskites with very large efficiency requires us to understand the operation of the solar cell. The application of frequency techniques is a major tool for the analysis of perovskite solar cells. Here we describe the general theory and methods of small perturbation frequency modulated techniques. These methods connect a broad range of experimental tools that interrogate the system in specific steady-state conditions.¹ the first method we discuss is the simulation of the dynamic response of the perovskite/contact interface. Here we find that it is necessary to establish the main polarization conditions, and how they are coupled to recombination and charge transfer, eventually leading to a typical behaviour of negative capacitance. Secondly we investigate the physical meaning of light modulated techniques as IMPS and IMVS when applied to charge collection in perovskite solar cells. Third we introduce a light-to-light impedance that is able to provide information on phenomena of photon diffusion as in photon recycling regime.²

(1) Bertoluzzi, L.; Bisquert, J. Investigating the Consistency of Models for Water Splitting Systems by Light and Voltage Modulated Techniques, J. Phys. Chem. Lett. 2017, 8, 172-180.

(2) Ansari-Rad, M.; Bisquert, J. Theory of Light-Modulated Emission Spectroscopy, J. Phys. Chem. Lett. 2017, 8, 3673-3677.

The fundamental nature of charge carrier transport (band-like or polaronic), and the influence thereupon of various scattering mechanisms and defect distributions are of central importance to the operation of semi-conductor based devices. While there have been numerous investigations aiming to understand these effects in hybrid halide perovskites, there remains much to be understood [1,2]. The structural and compositional complexity of perovskite based solar cells renders it extremely difficult to disentangle these effects, and theoretical simulations can provide valuable insights and predictions. So far modelling has focused on atomistic [3] and continuum [4] length scales, but a model bridging these scales, while taking into account all of the aspects described above, is lacking.

Here, we will describe a “device Monte Carlo” meso-scale model, based on well established semi-classical transport theory, which takes into account the band structure of the material, phonon and defect scattering, and electrostatic fields arising from inhomogeneities in defect and carrier concentrations, using parameters derived from experiment and ab initio calculations. We will present the results of the application of this model to charge carrier transport in hybrid halide perovskites, with a particular emphasis on current–voltage characteristics and the experimentally observed effects of changing defect distributions under illuminatation [5].

References

[1] T. Brenner et al., Nat. Rev. Mater. 1 (2016) 15007

[2] L. M. Herz, ACS Energy Lett. 2 (2017) p1539

[3] C. Motta and S. Sanvito, J. Phys. Chem. C 122 (2018) p1361

[4] S. E. J. O’ Kane et al., J. Mater. C 5 (2017) p452

[5] G. Y. Kim et al., Nat. Mater. 17 (2018) p445

Knowledge of the vibrational structure of a semiconductor is essential for explaining its optical and electronic properties and enabling optimized materials selection for optoelectronic devices. However, experimental measurement of the vibrational density of states of nanomaterials is particularly challenging. In this contribtion, I will describe recent work my group has carried out, investigating electron-phonon interactions through a variety of computational and experimental techniques. In particular, we have performed ab-initio molecular dynamics simulations on CsPbBr₃ perovskite nanocrystals in order to gain insight in the electronic and vibrational structure of these materials. We haved studied the influence of the large surface to volume ratio in nanocrystals versus bulk crystals, and shown that our computational results match the first experimental measurements of the vibrational denisty of states of CsPbBr₃ perovskite nanocrystals obtained using inelastic x-ray scattering (IXS).  We also show how the power spectrum of the electron and hole wavefunction dephasing can be used to investigate which phonons couple strongly to electrons and holes and mediate non-radiative transitions. 

The effects of transport layers on perovskite solar cell performance, in particular anomalous hysteresis, are investigated. A model for coupled ion vacancy motion and charge transport is formulated and solved in a three-layer planar perovskite solar cell. Its results are used to demonstrate that the replacement of standard transport layer materials (spiro-OMeTAD and TiO2) by materials with lower permittivity and/or doping leads to a shift in the scan rates at which hysteresis is most pronounced to rates higher than those commonly used in experiment. These results provide a cogent explanation for why organic electron transport layers can yield seemingly “hysteresis-free” devices but which nevertheless exhibit hysteresis at low temperature. In these devices the decrease in ion vacancy mobility with temperature compensates for the increase in hysteresis rate with use of low permittivity/doping organic transport layers. Simulations are used to classify features of the current-voltage curves that distinguish between cells in which charge carrier recombination occurs predominantly at the transport layer interfaces and those where it occurs predominantly within the perovskite. These characteristics are supplemented by videos showing how the electric potential, electronic and ionic charge profiles evolve across a planar perovskite solar cell during a current-voltage scan. Design protocols to mitigate the possible effects of high ion vacancy distributions on cell degradation are discussed. Finally, features of the steady-state potential profile for a device held near the maximum power point are used to suggest ways in which interfacial recombination can be reduced, and performance enhanced, via tuning transport layer properties. 

Perovskite solar cells have gathered a large interest in the last years as a very compelling and promising photovoltaic technology thanks to many interesting properties such as a wide spectrum of deposition techniques, a simple integration with both organic and inorganic materials and, most important of all, a high light power conversion efficiency.

Perovskite materials have also challenged the scientific community due to the many different physical processes that concur to set the optical and electrical properties: from ferroelectricity [1], to ion migration [2], defects and different recombination processes [3]. An important aspect of perovskite films is the presence of interfaces, both due to grain boundaries as well as due to interfaces between the perovskite layer and the charge selective contacts. Although many progresses have been obtained in the quality of the film, still grain boundaries within the perovskite film in fabricated devices are present. These interfaces can play a major role in setting device performances and hysteresis effects.

The effect of these grain boundaries and interfaces have been investigated by many groups, we refer here to just one reference [4], but the effect to free charges and ion migration is still under debate.

In the present work we theoretically investigate the effect of ion migration with the presence of grain boundaries. The analysis is performed using two different models: a drift-diffusion model to study the role of the mesoporous electron selective contact [5] and kinetic Monte Carlo [6] for the effect of grains and grain boundaries.

References

[1] A. Pecchia et al., Nano Lett., 16, 988 (2016)

[2] J. M. Azpiroz et al., Energy & Environmental Science, 8, 2118-2127 (2015)

[3] L. M. Herz, Annual Rev. Phys. Chemistry, 67, 65-89 (2016)

[4] B. Roose et al., Nano Energy, 39, 24-29 (2017)

[5] A. Gagliardi and A. Abate, ACS Energy Lett., 3, 163 (2018)

[6] T. Albes, A. Gagliardi, Physical Chemistry Chemical Physics, 19 (31), 20974-20983 (2017)

Lead halide perovskites are promising materials for new generation photovoltaics, exceeding 22% of efficiency in solar cells devices.[1] The presence of native defects, however, can strongly affect their efficiency due to charge trapping processes which can limit the lifetime of the photogenerated charge carriers. In this work a state of the art Density Functional Theory (DFT) study of native defects in MAPbI₃ and MAPbBr₃ is presented, aimed to unveil the nature of deep charge traps in these materials and the associated defects chemistry. The technical aspects of the computational modelling of defects are illustrated, with particular emphasis on the role of theory in the accurate evaluation of defects properties. The role of spin-orbit and self interactions corrections are discussed, as well as the performance of different corrections schemes in the supercell approach.[2] Good practices and open issues in the technical modelling of defects in these materials are discussed. Thus, a global picture of the defects chemistry in these perovskites is provided by the analysis of the associated formation energies in different conditions of growth and of the thermodynamic ionization levels. Our discussion shows that the defects chemistry of these materials is intrinsically dominated by halide chemistry, that is at the heart of their high defects tolerance.[3]     

References

[1] Wehrenfennig et al. Adv. Mater. 2014, 26, 1584-1589.

[2] Komsa et al. Phys. Rev. B 2012, 86, 045112.

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

Perovskite solar cells (PSCs) are the current rockstar of photovoltaic research attracting more and more attention. With efficiency now reaching up to 23% PSCs are on the way of catching up with classical inorganic solar cells. However, PSCs have not reached their full potential yet. In fact, their efficiency is limited, on the one hand, by non-radiative recombination, mainly via trap states located either at the grain boundaries or at the interface between the perovskite and the transport layers. On the other hand, it is limited by losses due to the poor transport properties of the commonly used transport layers.  Indeed, state-of-the-art transport layers (e.g. TiO₂, PCBM and Spiro-OMeTAD…) suffer from rather low mobilities, typically within 10^-4 – 10^-2 cm² V^-1 s^-1, when compared to the high mobilities, 1 – 10 cm² V^-1 s^-1, measured for perovskite using field-effect transistors or space-charge-limited-current measurement.

In this work, the effect of the mobility, thickness and doping density of the transport layers was investigated by means of a combined experimental and modeling analysis. For the experiment, two sets of devices made of a triple-cation perovskite were studied, including n-i-p and  p-i-n structures demonstrating efficiencies of up to 20%. For the two structures, the thickness and doping density of one of the transport layers were varied in order to understand their effect on the performance and especially on the FF. In addition, we performed a transient extraction experiment to look at the influence of the transport layers properties on the rate of extraction. The experimental results were then reproduced using drift-diffusion simulations to explain how and by how much every single parameter influences the extraction and the performance. A new and simple formula was also introduced to easily calculate the amount of doping necessary to counterbalance the low mobility of the transport layer.

In conclusion, this work presents a comprehensive analysis of the effects of the different properties of a transport layer on the efficiency of PSCs. We also present general guidelines on how to optimize a transport layer to avoid losses.

Experimental and theoretical studies show that the presence of mobile ions in perovskite solar cells (PSCs) modifies the electronic operation of the device [1-3]. The effects of the ion migration on the PSC performance can be enhanced or attenuated with the selective contacts (charge-transport-layer /perovskite heterojunctions) [2]. Thus, the knowledge of the mechanisms that take place at the selective contacts is crucial for the optimization of PSCs.

Anomalous high values of the low-frequency capacitance at open-circuit (OC) and short-circuit (SC) indicate a high accumulation of charge at the heterojunctions, which could hinder the extraction of charge and increase hysteresis in current-voltage curves [2]. This accumulation of charge can be affected by the presence of ionic species. Our goal is to quantify this accumulation of charge as a function of the different physical mechanisms that take place along its bulk and heterojunctions [2]. 

To investigate this issue, we developed a simulation model based on the drift-diffusion equations with specific boundary conditions at the heterojunctions [1-2]. The effect of ion migration on the charge and energy profile distributions along the PSC in OC and SC conditions was analyzed. We conclude that the accumulation of charge at the interfaces is strongly affected by the specific contact materials, and critically depends on a compromise among the presence of ions, the values of the carrier mobility, and the interfacial and bulk recombination parameters.

1. López-Varo, P. et al. ACS Energy Letters 1450-1453(2017)

2. García-Rosell, M. et al. J. Phys. Chem. C. (2018)

3. Reenen, S. et al. J. Phys. Chem. Lett. 6, 3808-3814(2015)

In this work, we have synthesized and characterized a totally inorganic, lead-free and non-soluble in water perovskite with catalytic properties, demonstrated by the photodegradation of an organic dye as crystal violet used, for example, in textile industries. From characterization, XRD revealed the presence of cubic (Pm-3m) and tetragonal (I4/mcm) CsSnBr₃ perovskite. The formation of the perovskite is supported by the Goldschmidt’s tolerance factor and octahedral factor calculations. From XPS, we have observed the presence of Sn²⁺, Sn⁴⁺ and the formation of non-stoichiometric tin oxide on the surface. UV-Vis spectroscopy showed high absorption of light in the visible range of the electromagnetic spectrum, with an optical band gap of 1.74 eV. Adsorption and photocatalysis tests have been performed under photovoltaic standard conditions (1 sun, AM1.5G solar spectrum and 25 ºC). The evolution of the system, followed by UV-Vis spectroscopy, showed a photodegradation of the dye in presence of the perovskite. A maximum of 73.1% of photodegradation of the crystal violet has been reached so the synthesized perovskite is, a priori, a suitable and eco-friendly material for removing dyes and other contaminants from the environment. A possible mechanism for the photocatalytic process has been proposed.

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.

Sample preparation of perovskite materials has a crucial impact on their optoelectronic properties and has indeed been a dominant factor for the rapid advances in photoconversion efficiencies. However, very little is currently known about the microcopic details that determine the nucleation and crystal growth proces. Such a knowledge could be a starting point to enable control over the crystallization process and a rational optimization of preparation conditions.

In principle, molecular dynmaics simulations can provide atomistic insight into complex phenomena but direct simulations of the nucleation process are highly challenging due to the large activation barriers that are involved and the high-dimensionality of the available phase space. Here, we present enhanced sampling molecular dynamics simulations based on well-tempered metadynamics simulations about the nucleation process of lead halide perovskites from solution. Choosing appropriate collective varaibles, it has been possible for the first time to monitor the nucleation and growth of such a multicomponent system (containing, lead ions, halide anions, monovalent cations and solvent molecules). These simulations demonstrate  the influence that different solvents play in this process and reveal a pivotal role of the monovalent cations.