Progress in Theoretical Study of Metal Halide Perovskite Solar Cell Materials

Lead halide perovskites have recently emerged as promising absorbers for fabricating low‐cost and high‐efficiency thin‐film solar cells. The record power conversion efficiency of lead halide perovskite‐based solar cells has rapidly increased from 3.8% in 2009 to 22.1% in early 2016. Such rapid improvement is attributed to the superior and unique photovoltaic properties of lead halide perovskites, such as the extremely high optical absorption coefficients and super‐long photogenerated carrier lifetimes and diffusion lengths that are not seen in any other polycrystalline thin‐film solar cell materials. In the past a few years, theoretical approaches have been extensively applied to understand the fundamental mechanisms responsible for the superior photovoltaic properties of lead halide perovskites and have gained significant insights. This review article highlights the important theoretical results reported in literature for the understanding of the unique structural, electronic, optical, and defect properties of lead halide perovskite materials. For comparison, we also review the theoretical results reported in literature for some lead‐free perovskites, double perovskites, and nonperovskites.     

Perovskite Tandem Solar Cells

The meteoric rise of perovskite single‐junction solar cells has been accompanied by similar stunning developments in perovskite tandem solar cells. Debuting with efficiencies less than 14% in 2014, silicon–perovskite solar cells are now above 25% and will soon surpass record silicon single‐junction efficiencies. Unconstrained by the Shockley–Quiesser single‐junction limit, perovskite tandems suggest a real possibility of true third‐generation thin‐film photovoltaics; monolithic all‐perovskite tandems have reached 18% efficiency and will likely pass perovskite single‐junction efficiencies within the next 5 years. Inorganic–organic metal–halide perovskites are ideal candidates for inclusion in tandem solar cells due to their high radiative recombination efficiencies, excellent absorption, long‐range charge‐transport, and broad ability to tune the bandgap. In this progress report, the development of perovskite tandem cells is reviewed, with presentation of their key motivations and challenges. In detail, it presents an overview of recombination layer materials, bandgap‐tuneability, transparent contact architectures, and perovskite compounds for use in tandems. Theoretical estimates of efficiency for future tandem and triple‐junction perovskite cells are presented, outlining roadmaps for future focused research.     

Thermodynamically Self‐Healing 1D–3D Hybrid Perovskite Solar Cells

Thermal degradation in perovskite solar cells is still an unsettled issue that limits its further development. In this study, 2‐(1H‐pyrazol‐1‐yl)pyridine is introduced into lead halide 3D perovskites, which allows 1D–3D hybrid perovskite materials to be obtained. The heterostructural 1D–3D perovskites are proved to be capable of remarkably prolonging the photoluminescence decay lifetime and suppressing charge carrier recombination in comparison to conventional 3D perovskites. The intrinsic properties of thermodynamically stable yet kinetically labile 1D materials allow the system to alleviate the lattice mismatch and passivate the interface traps of heterojunction region of 1D–3D hybrid perovskites that may occur during the crystal growth process. Importantly, the as‐fabricated 1D–3D perovskite solar cells display a thermodynamic self‐healing ability, which is induced through blocking the ion‐migration channels of A‐site ions by the flexible 1D perovskite with less densely close‐packed structure. Particularly, the power conversion efficiency of as‐fabricated unencapsulated 1D–3D perovskite solar cells is demonstrated to be reversible under temperature cycling (25–85 °C) at 55% relative humidity, which largely outperforms the pure 3D perovskite solar cell. The present study provides a facile approach to fabricate 1D–3D perovskite solar cells with high efficiency and long‐term stability.     

Influence of Grain Size on Phase Transitions in Halide Perovskite Films

Grain size in polycrystalline halide perovskite films is known to have an impact on the optoelectronic properties of the films, but its influence on their soft structural properties and phase transitions is unclear. Here, temperature‐dependent X‐ray diffraction, absorption, and macro‐ and micro‐photoluminescence measurements are used to investigate the tetragonal to orthorhombic phase transition in thin methylammonium lead iodide films with grain sizes ranging from the micrometer scale down to the tens of nanometer scale. It is shown that the phase transition nominally at ≈150 K is increasingly suppressed with decreasing grain size and, in the smallest grains, the first evidence of a phase transition is only seen at temperatures as low as ≈80 K. With decreasing grain size, an increasing magnitude of the hysteresis is also seen in the structural and optoelectronic properties when cooling to, and then upon heating from, 100 K. This work reveals the remarkable sensitivity of the optoelectronic, physical, and phase properties to the local environment of the perovskite structure, which will have large ramifications for phase and defect engineering in operating devices.     

Inorganic Halide Perovskite Solar Cells: Progress and Challenges

All‐inorganic perovskite semiconductors have recently drawn increasing attention owing to their outstanding thermal stability. Although all‐inorganic perovskite solar cells (PSCs) have achieved significant progress in recent years, they still fall behind their prototype organic–inorganic counterparts owing to severe energy losses. Therefore, there is considerable interest in further improving the performance of all‐inorganic PSCs by synergic optimization of perovskite films and device interfaces. This review article provides an overview of recent progress in inorganic PSCs in terms of lead‐based and lead‐free composition. The physical properties of all‐inorganic perovskite semiconductors as well as the hole/electron transporting materials are discussed to unveil the important role of composition engineering and interface modification. Finally, a discussion of the prospects and challenges for all‐inorganic PSCs in the near future is presented.     

Record Open‐Circuit Voltage Wide‐Bandgap Perovskite Solar Cells Utilizing 2D/3D Perovskite Heterostructure

In this work, the authors realize stable and highly efficient wide‐bandgap perovskite solar cells that promise high power conversion efficiencies (PCE) and are likely to play a key role in next generation multi‐junction photovoltaics (PV). This work reports on wide‐bandgap (≈1.72 eV) perovskite solar cells exhibiting stable PCEs of up to 19.4% and a remarkably high open‐circuit voltage (V_OC) of 1.31 V. The V_OC‐to‐bandgap ratio is the highest reported for wide‐bandgap organic?inorganic hybrid perovskite solar cells and the V_OC also exceeds 90% of the theoretical maximum, defined by the Shockley–Queisser limit. This advance is based on creating a hybrid 2D/3D perovskite heterostructure. By spin coating n‐butylammonium bromide on the double‐cation perovskite absorber layer, a thin 2D Ruddlesden–Popper perovskite layer of intermediate phases is formed, which mitigates nonradiative recombination in the perovskite absorber layer. As a result, V_OC is enhanced by 80 mV.     

Metal Halide Perovskite Materials for Solar Cells with Long‐Term Stability

Metal halide perovskite solar cells (PSCs) have emerged as promising candidates for photovoltaic technology with their power conversion efficiencies over 23%. For prototypical organic–inorganic metal halide perovskites, their intrinsic instability poses significant challenges to the commercialization of PSCs. Recently, the scientific community has done tremendous work in composition engineering to develop more robust light‐absorbing layers, including mixed‐ion hybrid perovskites, low‐dimensional hybrid perovskites, and all‐inorganic perovskites. This review provides an overview of the impact of these perovskites on the efficiency and long‐term stability of PSCs.     

Organic Gelators as Growth Control Agents for Stable and Reproducible Hybrid Perovskite‐Based Solar Cells

Low‐molecular‐weight organic gelators are widely used to influence the solidification of polymers, with applications ranging from packaging items, food containers to organic electronic devices, including organic photovoltaics. Here, this concept is extended to hybrid halide perovskite‐based materials. In situ time‐resolved grazing incidence wide‐angle X‐ray scattering measurements performed during spin coating reveal that organic gelators beneficially influence the nucleation and growth of the perovskite precursor phase. This can be exploited for the fabrication of planar n‐i‐p heterojunction devices with MAPbI₃ (MA = CH₃NH₃⁺) that display a performance that not only is enhanced by ≈ 25% compared to solar cells where the active layer is produced without the use of a gelator but that also features a higher stability to moisture and a reduced hysteresis. Most importantly, the presented approach is straightforward and simple, and it provides a general method to render the film formation of hybrid perovskites more reliable and robust, analogous to the control that is afforded by these additives in the processing of commodity “plastics.”     

Engineering Multiphase Metal Halide Perovskites Thin Films for Stable and Efficient Solar Cells

The intrinsic instability of lead halide perovskite semiconductors in an ambient atmosphere is one of the most critical issues that impedes perovskite solar cell commercialization. To overcome it, the use of bulky organic spacers has emerged as a promising solution. The resulting perovskite thin films present complex morphologies, difficult to predict, which can directly affect the device efficiency. Here, by combining in‐depth morphological and spectroscopic characterization, it is shown that both the ionic size and the relative concentration of the organic cation, drive the integration of bulky organic cations into the crystal unit cell and the thin film, inducing different perovskite phases and different vertical distribution, then causing a significant change in the final thin film morphology. Based on these studies, a fine‐engineered perovskite is constructed by employing two different large cations, namely, ethyl ammonium and butyl ammonium. The first one takes part in the 3D perovskite phase formation, the second one works as a surface modifier by forming a passivating layer on top of the thin film. Together they lead to improved photovoltaic performance and device stability when tested in air under continuous illumination. These findings propose a general approach to achieve reliability in perovskite‐based optoelectronic devices.     

Dark-Bright Exciton Splitting Dominates Low-Temperature Diffusion in Halide Perovskite Nanocrystal Assemblies

Semiconductor nanocrystals can replace conventional bulk materials completely in displays and light-emitting diodes. Exciton transport dominates over charge carrier transport for materials with high exciton binding energies and long ligands, such as halide perovskite nanocrystal films. Here, how beneficial superlattices – nearly perfect 3D assemblies of nanocrystals - are to exciton transport is investigated. Surprisingly, the high degree of order is not as crucial as the individual nanocrystal size, which strongly influences the splitting of the excitonic manifold into bright and dark states. At very low temperatures, the energetic splitting is larger for the smallest nanocrystals, and dark levels with low oscillator strength effectively trap excitons inside individual nanocrystals, suppressing diffusion. The effect is reversed at elevated temperatures, and the larger nanocrystal size becomes detrimental to exciton transport due to enhanced exciton trapping and dissociation. The results reveal that the nanocrystal size must be strongly accounted for in design strategies of future optoelectronic applications.     

How to Report Record Open‐Circuit Voltages in Lead‐Halide Perovskite Solar Cells

Open‐circuit voltages of lead‐halide perovskite solar cells are improving rapidly and are approaching the thermodynamic limit. Since many different perovskite compositions with different bandgap energies are actively being investigated, it is not straightforward to compare the open‐circuit voltages between these devices as long as a consistent method of referencing is missing. For the purpose of comparing open‐circuit voltages and identifying outstanding values, it is imperative to use a unique, generally accepted way of calculating the thermodynamic limit, which is currently not the case. Here a meta‐analysis of methods to determine the bandgap and a radiative limit for open‐circuit voltage is presented. The differences between the methods are analyzed and an easily applicable approach based on the solar cell quantum efficiency as a general reference is proposed.     

Lewis‐Adduct Mediated Grain‐Boundary Functionalization for Efficient Ideal‐Bandgap Perovskite Solar Cells with Superior Stability

State‐of‐the‐art perovskite solar cells (PSCs) have bandgaps that are invariably larger than 1.45 eV, which limits their theoretically attainable power conversion efficiency. The emergent mixed‐(Pb, Sn) perovskites with bandgaps of 1.2–1.3 eV are ideal for single‐junction solar cells according to the Shockley–Queisser limit, and they have the potential to deliver higher efficiency. Nevertheless, the high chemical activity of Sn(II) in these perovskites makes it extremely challenging to control their physical properties and chemical stability, thereby leading to PSCs with relatively low PCE and stability. In this work, the authors employ the Lewis‐adduct SnF₂·3FACl additive in the solution‐processing of ideal‐bandgap halide perovskites (IBHPs), and prepare uniform large‐grain perovskite thin films containing continuously functionalized grain boundaries with the stable SnF₂ phase. Such Sn(II)‐rich grain‐boundary networks significantly enhance the physical properties and chemical stability of the IBHP thin films. Based on this approach, PSCs with an ideal bandgap of 1.3 eV are fabricated with a promising efficiency of 15.8%, as well as enhanced stability. The concept of Lewis‐adduct‐mediated grain‐boundary functionalization in IBHPs presented here points to a new chemical route for approaching the Shockley–Queisser limit in future stable PSCs.     

Chemical Approaches for Stabilizing Perovskite Solar Cells

Chemical bonding dictates not only the optoelectronic properties of materials, but also the intrinsic and extrinsic stability of materials. Here, the causes of intrinsic and extrinsic instability of perovskite materials are reviewed considering their correlation with the unique chemical‐bonding nature of perovskite materials. There are a number of key standardized stability tests established by the International Electrotechnical Commission for commercialized photovoltaic modules. Based on these procedures, the possible causes and related mechanisms of the material degradation that can arise during the test procedures are identified, which are discussed in terms of their chemical bonds. Based on the understanding of the critical causes, promising strategies for mitigating the causes to enhance the stability of perovskite solar cells are summarized. The stability of the state‐of‐the‐art perovskite solar cells implies a need for the development of improved stability‐testing protocols to move onto the next stage toward commercialization.     

Oxygen Degradation in Mesoporous Al2O3/CH3NH3PbI3‐xClx Perovskite Solar Cells: Kinetics and Mechanisms

The rapid pace of development for hybrid perovskite photovoltaics has recently resulted in promising figures of merit being obtained with regard to device stability. Rather than relying upon expensive barrier materials, realizing market‐competitive lifetimes is likely to require the development of intrinsically stable devices, and to this end accelerated aging tests can help identify degradation mechanisms that arise over the long term. Here, oxygen‐induced degradation of archetypal perovskite solar cells under operation is observed, even in dry conditions. With prolonged aging, this process ultimately drives decomposition of the perovskite. It is deduced that this is related to charge build‐up in the perovskite layer, and it is shown that by efficiently extracting charge this degradation can be mitigated. The results confirm the importance of high charge‐extraction efficiency in maximizing the tolerance of perovskite solar cells to oxygen.