The photovoltaics of organic–inorganic lead halide perovskite materials have shown rapid improvements in solar cell performance, surpassing the top efficiency of semiconductor compounds such as CdTe and CIGS (copper indium gallium selenide) used in solar cells in just about a decade. Perovskite preparation via simple and inexpensive solution processes demonstrates the immense potential of this thin-film solar cell technology to become a low-cost alternative to the presently commercially available photovoltaic technologies. Significant developments in almost all aspects of perovskite solar cells and discoveries of some fascinating properties of such hybrid perovskites have been made recently. This Review describes the fundamentals, recent research progress, present status, and our views on future prospects of perovskite-based photovoltaics, with discussions focused on strategies to improve both intrinsic and extrinsic (environmental) stabilities of high-efficiency devices. Strategies and challenges regardi...

Halide Perovskite Photovoltaics: Background, Status, and Future Prospects

With rapid progress in a power conversion efficiency (PCE) to reach 25%, metal halide perovskite-based solar cells became a game-changer in a photovoltaic performance race. Triggered by the development of the solid-state perovskite solar cell in 2012, intense follow-up research works on structure design, materials chemistry, process engineering, and device physics have contributed to the revolutionary evolution of the solid-state perovskite solar cell to be a strong candidate for a next-generation solar energy harvester. The high efficiency in combination with the low cost of materials and processes are the selling points of this cell over commercial silicon or other organic and inorganic solar cells. The characteristic features of perovskite materials may enable further advancement of the PCE beyond those afforded by the silicon solar cells, toward the Shockley-Queisser limit. This review summarizes the fundamentals behind the optoelectronic properties of perovskite materials, as well as the important approaches to fabricating high-efficiency perovskite solar cells. Furthermore, possible next-generation strategies for enhancing the PCE over the Shockley-Queisser limit are discussed.

High-Efficiency Perovskite Solar Cells.

Methylammonium Chloride Induces Intermediate Phase Stabilization for Efficient Perovskite Solar Cells

All highly-efficient organic–inorganic halide perovskite (OIHP) solar cells to date are made of polycrystalline perovskite films which contain a high density of defects, including point and extended imperfections. The imperfections in OIHP materials play an important role in the process of charge recombination and ion migration in perovskite solar cells (PSC), which heavily influences the resulting device energy conversion efficiency and stability. Here we review the recent advances in passivation of imperfections and suppressing ion migration to achieve improved efficiency and highly stable perovskite solar cells. Due to the ionic nature of OIHP materials, the defects in the photoactive films are inevitably electrically charged. The deep level traps induced by particular charged defects in OIHP films are major non-radiative recombination centers; passivation by coordinate bonding, ionic bonding, or chemical conversion have proven effective in mitigating the negative impacts of these deep traps. Shallow level charge traps themselves may contribute little to non-radiative recombination, but the migration of charged shallow level traps in OIHP films results in unfavorable band bending, interfacial reactions, and phase segregation, influencing the carrier extraction efficiency. Finally, the impact of defects and ion migration on the stability of perovskite solar cells is described.

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Imperfections and their passivation in halide perovskite solar cells

Defects play an important role in the degradation processes of hybrid halide perovskite absorbers, impeding their application for solar cells. Among all defects, halide anion and organic cation vacancies are ubiquitous, promoting ion diffusion and leading to thin-film decomposition at surfaces and grain boundaries. Here, we employ fluoride to simultaneously passivate both anion and cation vacancies, by taking advantage of the extremely high electronegativity of fluoride. We obtain a power conversion efficiency of 21.46% (and a certified 21.3%-efficient cell) in a device based on the caesium, methylammonium (MA) and formamidinium (FA) triple-cation perovskite (Cs0.05FA0.54MA0.41)Pb(I0.98Br0.02)3 treated with sodium fluoride. The device retains 90% of its original power conversion efficiency after 1,000 h of operation at the maximum power point. With the help of first-principles density functional theory calculations, we argue that the fluoride ions suppress the formation of halide anion and organic cation vacancies, through a unique strengthening of the chemical bonds with the surrounding lead and organic cations. Defects and defect migration are detrimental for perovskite solar cell efficiency and long-term stability. Li et al. show that fluoride is able to suppress the formation of halide anion and organic cation vacancy defects by restraining the relative ions via ionic and hydrogen bonds.

Cation and anion immobilization through chemical bonding enhancement with fluorides for stable halide perovskite solar cells

Organic–inorganic halide perovskite is believed to be a potential candidate for high efficiency solar cells because power conversion efficiency (PCE) was certified to be more than 22%. Nevertheless, mismatch of PCE due to current density (J)–voltage (V) hysteresis in perovskite solar cells is an obstacle to overcome. There has been much lively debate on the origin of J–V hysteresis; however, effective methodology to solve the hysteric problem has not been developed. Here we report a universal approach for hysteresis-free perovskite solar cells via defect engineering. A severe hysteresis observed from the normal mesoscopic structure employing TiO2 and spiro-MeOTAD is almost removed or does not exist upon doping the pure perovskites, CH3NH3PbI3 and HC(NH2)2PbI3, and the mixed cation/anion perovskites, FA0.85MA0.15PbI2.55Br0.45 and FA0.85MA0.1Cs0.05PbI2.7Br0.3, with potassium iodide. Substantial reductions in low-frequency capacitance and bulk trap density are measured from the KI-doped perovskite, which is ...

Universal Approach toward Hysteresis-Free Perovskite Solar Cell via Defect Engineering

Chemical interaction at a heterojunction interface induced by an appropriate chemical linker is of crucial importance for high efficiency, hysteresis-less, and stable perovskite solar cells (PSCs). Effective interface engineering in PSCs is reported via a multifunctional chemical linker of 4-imidazoleacetic acid hydrochloride (ImAcHCl) that can provide a chemical bridge between SnO2 and perovskite through an ester bond with SnO2 via esterification reaction and an electrostatic interaction with perovskite via imidazolium cation in ImAcHCl and iodide anion in perovskite. In addition, the chloride anion in ImAcHCl plays a role in the improvement of crystallinity of perovskite film crystallinity. The introduction of ImAcHCl onto SnO2 realigns the positions of the conduction and valence bands upwards, reduces nonradiative recombination, and improves carrier life time. As a consequence, average power conversion efficiency (PCE) is increased from 18.60% ± 0.50% to 20.22% ± 0.34% before and after surface modification, respectively, which mainly results from an enhanced voltage from 1.084 ± 0.012 V to 1.143 ± 0.009 V. The best PCE of 21% is achieved by 0.1 mg mL-1 ImAcHCl treatment, along with negligible hysteresis. Moreover, an unencapsulated device with ImAcHCl-modified SnO2 shows much better thermal and moisture stability than unmodified SnO2 .

Multifunctional Chemical Linker Imidazoleacetic Acid Hydrochloride for 21% Efficient and Stable Planar Perovskite Solar Cells.

Anomalous current–voltage (J–V) hysteresis in perovskite (PSK) solar cell is open to dispute, where hysteresis is argued to be due to electrode polarization, dipolar polarization, and/or native defects. However, a correlation between those factors and J–V hysteresis is hard to be directly evaluated because they usually coexist and are significantly varied depending on morphology and crystallinity of the PSK layer, selective contacts, and device architecture. In this study, without changing morphology and crystallinity of PSK layer in a planar heterojunction structure employing FA0.9Cs0.1PbI3, a correlation between J–V hysteresis and trap density is directly evaluated by means of thermally induced PbI2 regulating trap density. Increase in thermal annealing time at a given temperature of 150 °C induces growth of PbI2 on the PSK grain surface, which results in significant reduction of nonradiative recombination. Hysteresis index is reduced from 0.384 to 0.146 as the annealing time is increased from 5 to 100 ...

The Interplay between Trap Density and Hysteresis in Planar Heterojunction Perovskite Solar Cells.

Interface engineering to form an interlayer via ion exchange reaction is reported. A FA0.88 Cs0.12 PbI3 formamidinium (FA) perovskite layer is first prepared, then FAPF6 solution with different concentrations is spin-coated on top of the perovskite film, which leads to a partial substitution of iodide by PF6- ion. The second phase with nominal composition of FA0.88 Cs0.12 PbI3-x (PF6 )x is grown at the grain boundary, which has island morphology and its size depends on the FAPF6 solution concentration. The lattice is expanded and bandgap is reduced due to inclusion of larger PF6- ions. The power conversion efficiency (PCE) is significantly enhanced from 17.8% to 19.3% as a consequence of improved fill factor and open-circuit voltage (Voc ). In addition, current-voltage hysteresis is reduced. Post-treatment with FAPF6 reduces defect density and enhances carrier lifetime, which is responsible for the improved photovoltaic performance and reduced hysteresis. The unencapsulated device with post-treated perovskite film demonstrates better stability than the pristine perovskite, where the initial PCE retains over 80% after 528 h exposure under relative humidity of around 50-70% in the dark and 92% after 360 h under one sun illumination.

FA0.88Cs0.12PbI3−x(PF6)x Interlayer Formed by Ion Exchange Reaction between Perovskite and Hole Transporting Layer for Improving Photovoltaic Performance and Stability

Metal halide perovskite materials have shown versatile functionality for a variety of optoelectronic devices. Remarkable progress in device performance has been achieved for last few years. Their high performance in combination with low production cost puts the perovskite optoelectronics under serious consideration for possible commercialization. A fundamental question that remains unanswered is whether these materials can sustain their optoelectronic properties during harsh and prolonged operational conditions of the devices. A major concern stems from an unprecedented and unique feature of perovskite materials, which is migration of ionic species (or charged defects). Recent studies have indicated that the ion migration might be a limit factor for long-term operational stability of the devices. In this regard, herein we have reviewed important studies on discovery, quantification, and mitigation of the ion migration process in metal halide perovskite materials. A possible emerging application using the ion migration is also briefly introduced.

Seul-Gi Kim

Papers

Universal Approach toward Hysteresis-Free Perovskite Solar Cell via Defect Engineering

Multifunctional Chemical Linker Imidazoleacetic Acid Hydrochloride for 21% Efficient and Stable Planar Perovskite Solar Cells.

The Interplay between Trap Density and Hysteresis in Planar Heterojunction Perovskite Solar Cells.

FA0.88Cs0.12PbI3−x(PF6)x Interlayer Formed by Ion Exchange Reaction between Perovskite and Hole Transporting Layer for Improving Photovoltaic Performance and Stability

Verification and mitigation of ion migration in perovskite solar cells