Perovskite Photovoltaics for Artificial Light Harvesting

Perovskites have encountered a growing interest as light-absorbing materials for harvesting and recycling ambient light. This interest comes from the distinct and impressive intrinsic properties of these materials and is necessitated by the need for reliable and sustainable power supplies for wearable and portable internet of things (IoT) applications. Perovskite artificial light cells (PALCs; i. e., indoor perovskite photovoltaics) have been intensively explored, and thus their device performance has been rapidly augmented. In this review, we summarize and consolidate the research outputs of PALCs by focusing on the tuning of photo-absorber and interlayer materials for efficient harvest and collection of artificial light sources. We also emphasize various challenges and possible future research directions that should be addressed for realizing a large market uptake of PALCs.     

A Layered Hybrid Perovskite Solar‐Cell Absorber with Enhanced Moisture Stability

Two‐dimensional hybrid perovskites are used as absorbers in solar cells. Our first‐generation devices containing (PEA)₂(MA)₂[Pb₃I₁₀] ( 1 ; PEA=C₆H₅(CH₂)₂NH₃⁺, MA=CH₃NH₃⁺) show an open‐circuit voltage of 1.18 V and a power conversion efficiency of 4.73 %. The layered structure allows for high‐quality films to be deposited through spin coating and high‐temperature annealing is not required for device fabrication. The 3D perovskite (MA)[PbI₃] ( 2 ) has recently been identified as a promising absorber for solar cells. However, its instability to moisture requires anhydrous processing and operating conditions. Films of 1 are more moisture resistant than films of 2 and devices containing 1 can be fabricated under ambient humidity levels. The larger bandgap of the 2D structure is also suitable as the higher bandgap absorber in a dual‐absorber tandem device. Compared to 2 , the layered perovskite structure may offer greater tunability at the molecular level for material optimization.     

A Fast Deposition‐Crystallization Procedure for Highly Efficient Lead Iodide Perovskite Thin‐Film Solar Cells

Thin‐film photovoltaics based on alkylammonium lead iodide perovskite light absorbers have recently emerged as a promising low‐cost solar energy harvesting technology. To date, the perovskite layer in these efficient solar cells has generally been fabricated by either vapor deposition or a two‐step sequential deposition process. We report that flat, uniform thin films of this material can be deposited by a one‐step, solvent‐induced, fast crystallization method involving spin‐coating of a DMF solution of CH₃NH₃PbI₃ followed immediately by exposure to chlorobenzene to induce crystallization. Analysis of the devices and films revealed that the perovskite films consist of large crystalline grains with sizes up to microns. Planar heterojunction solar cells constructed with these solution‐processed thin films yielded an average power conversion efficiency of 13.9±0.7 % and a steady state efficiency of 13 % under standard AM 1.5 conditions.     

Carbon‐Electrode‐Tailored All‐Inorganic Perovskite Solar Cells To Harvest Solar and Water‐Vapor Energy

Moisture is the worst enemy for state‐of‐the‐art perovskite solar cells (PSCs). However, the flowing water vapor within nanoporous carbonaceous materials can create potentials. Therefore, it is a challenge to integrate water vapor and solar energies into a single PSC device. We demonstrate herein all‐inorganic cesium lead bromide (CsPbBr₃) solar cells tailored with carbon electrodes to simultaneously harvest solar and water‐vapor energy. Upon interfacial modification and plasma treatment, the bifunctional PSCs yield a maximum power conversion efficiency up to 9.43 % under one sun irradiation according to photoelectric conversion principle and a power output of 0.158 μW with voltage of 0.35 V and current of 0.45 μA in 80 % relative humidity through the flowing potentials at the carbon/water interface. The initial efficiency is only reduced by 2 % on exposing the inorganic PSC with 80 % humidity over 40 days. The successful realization of physical proof‐of‐concept multi‐energy integrated solar cells provides new opportunities of maximizing overall power output.     

Enhanced Performance of Camphorsulfonic Acid-Doped Perovskite Solar Cells

High-quality perovskite film with large grains and therefore reduced grain boundaries plays a significant role in improving the power conversion efficiency (PCE) and ensuring good long-term stability of the perovskite solar cells. In this work, we found that adding camphorsulfonic acid (CSA), a Lewis base, to the perovskite solution results in the crystallization of larger perovskite grains. By varying the concentration of CSA, we found that the optimal concentration of the additive is 1 mg/mL, which leads to an 20% increase in PCE of the cells compared to the reference CSA-free cell. Interestingly, we observed that the PCE of cells with an excess of CSA was initially poor, but may increase significantly over time, possibly due to CSA migration to the hole-transporting layer, leading to an improvement in its conductivity.     

Hexaazatrinaphthylene Derivatives: Efficient Electron‐Transporting Materials with Tunable Energy Levels for Inverted Perovskite Solar Cells

Hexaazatrinaphthylene (HATNA) derivatives have been successfully shown to function as efficient electron‐transporting materials (ETMs) for perovskite solar cells (PVSCs). The cells demonstrate a superior power conversion efficiency (PCE) of 17.6 % with negligible hysteresis. This study provides one of the first nonfullerene small‐molecule‐based ETMs for high‐performance p–i–n PVSCs.     

A Lead Iodide Perovskite Based on a Large Organic Cation for Solar Cell Applications

Methylammonium (CH₃NH₃⁺) and formamidinium ((NH₂)₂CH⁺) based lead iodide perovskites are currently the two commonly used organic–inorganic lead iodide perovskites. There are still no alternative organic cations that can produce perovskites with band gaps spanning the visible spectrum (that is, <1.7 eV) for solar cell applications. Now, a new perovskite using large propane‐1,3‐diammonium cation (1,3‐Pr(NH₃)₂²⁺) with a chemical structure of (1,3‐Pr(NH₃)₂)_0.5PbI₃ is demonstrated. X‐ray diffraction (XRD) shows that the new perovskite exhibits a three‐dimensional tetragonal phase. The band gap of the new perovskite is about 1.6 eV, which is desirable for photovoltaic applications. A (1,3‐Pr(NH₃)₂)_0.5PbI₃ perovskite solar cell (PSC) yields a power conversion efficiency (PCE) of 5.1 %. More importantly, this perovskite is composed of a large hydrophobic cation that provides better moisture resistance compared to CH₃NH₃PbI₃ perovskite.     

3,4‐Phenylenedioxythiophene (PheDOT) Based Hole‐Transporting Materials for Perovskite Solar Cells

Two new electron‐rich molecules based on 3,4‐phenylenedioxythiophene (PheDOT) were synthesized and successfully adopted as hole‐transporting materials (HTMs) in perovskite solar cells (PSCs). X‐ray diffraction, absorption spectra, photoluminescence spectra, electrochemical properties, thermal stabilities, hole mobilities, conductivities, and photovoltaic parameters of PSCs based on these two HTMs were compared with each other. By introducing methoxy substituents into the main skeleton, the energy levels of PheDOT‐core HTM were tuned to match with the perovskite, and its hole mobility was also improved (1.33×10^?4 cm² V^?1 s^?1, being higher than that of spiro‐OMeTAD, 2.34×10^?5 cm² V^?1 s^?1). The PSC based on MeO‐PheDOT as HTM exhibits a short‐circuit current density (J_sc) of 18.31 mA cm^?2, an open‐circuit potential (V_oc) of 0.914 V, and a fill factor (FF) of 0.636, yielding an encouraging power conversion efficiency (PCE) of 10.64 % under AM 1.5G illumination. These results give some insight into how the molecular structures of HTMs affect their performances and pave the way for developing high‐efficiency and low‐cost HTMs for PSCs.     

Steric Mixed‐Cation 2D Perovskite as a Methylammonium Locker to Stabilize MAPbI3

The reduced dimension perovskite including 2D perovskites are one of the most promising strategies to stabilize lead halide perovskite. A mixed‐cation 2D perovskite based on a steric phenyltrimethylammonium (PTA) cation is presented. The PTA‐MA mixed‐cation 2D perovskite of PTAMAPbI₄ can be formed on the surface of MAPbI₃ (PTAI‐MAPbI₃) by controllable PTAI intercalation by either spin coating or soaking. The PTAMAPbI₄ capping layer can not only passivate PTAI‐MAPbI₃ perovskite but also act as MA⁺ locker to inhibit MAI extraction and significantly enhance the stability. The highly stable PTAI‐MAPbI₃ based perovskite solar cells exhibit a reproducible photovoltaic performance with a champion PCE of 21.16 %. Such unencapsulated devices retain 93 % of initial efficiency after 500 h continuous illumination. This steric mixed‐cation 2D perovskite as MA⁺ locker to stabilize the MAPbI₃ is a promising strategy to design stable and high‐performance hybrid lead halide perovskites.     

Perovskite as Light Harvester: A Game Changer in Photovoltaics

It is not often that the scientific community is blessed with a material, which brings enormous hopes and receives special attention. When it does, it expands at a rapid pace and its every dimension creates curiosity. One such material is perovskite, which has triggered the development of new device architectures in energy conversion. Perovskites are of great interest in photovoltaic devices due to their panchromatic light absorption and ambipolar behavior. Power conversion efficiencies have been doubled in less than a year and over 15 % is being now measured in labs. Every digit increment in efficiency is being celebrated widely in the scientific community and is being discussed in industry. Here we provide a summary on the use of perovskite for inexpensive solar cells fabrication. It will not be unrealistic to speculate that one day perovskite‐based solar cells can match the capability and capacity of existing technologies.     

Hole‐Transport Materials for Perovskite Solar Cells

The pressure to move towards renewable energy has inspired researchers to look for ideas in photovoltaics that may lead to a major breakthrough. Recently the use of perovskites as a light harvester has lead to stunning progress. The power conversion efficiency of perovskite solar cells is now approaching parity (>22 %) with that of the established technology which took decades to reach this level of performance. The use of a hole transport material (HTM) remains indispensable in perovskite solar cells. Perovskites can conduct holes, but they are present at low levels, and for efficient charge extraction a HTM layer is a prerequisite. Herein we provide an overview of the diverse types of HTM available, from organic to inorganic, in the hope of encouraging further research and the optimization of these materials.     

A Purified,Solvent‐Intercalated Precursor Complex for Wide‐Process‐Window Fabrication of Efficient Perovskite Solar Cells and Modules

A high‐purity methylammonium lead iodide complex with intercalated dimethylformamide (DMF) molecules, CH₃NH₃PbI₃?DMF, is introduced as an effective precursor material for fabricating high‐quality solution‐processed perovskite layers. Spin‐coated films of the solvent‐intercalated complex dissolved in pure dimethyl sulfoxide (DMSO) yielded thick, dense perovskite layers after thermal annealing. The low volatility of the pure DMSO solvent extended the allowable time for low‐speed spin programs and considerably relaxed the precision needed for the antisolvent addition step. An optimized, reliable fabrication method was devised to take advantage of this extended process window and resulted in highly consistent performance of perovskite solar cell devices, with up to 19.8 % power‐conversion efficiency (PCE). The optimized method was also used to fabricate a 22.0 cm², eight‐cell module with 14.2 % PCE (active area) and 8.64 V output (1.08 V/cell).     

The Effect of Humidity upon the Crystallization Process of Two‐Step Spin‐Coated Organic–Inorganic Perovskites

Moisture is shown to activate the reaction between PbI₂ and methylammonium halides. In addition, two activating mechanisms are proposed for the formation of CH₃NH₃PbI₃ and CH₃NH₃PbI_3?xCl_x films from a series of carefully controlled experiments. When these rapidly formed perovskite films are directly fabricated into the devices, poor photovoltaic properties are found, due to heavy surface charge recombination. However, the cell performance can be significantly enhanced to 13.63 % and to over 12 % in the steady state for CH₃NH₃PbI₃ and to 15.50 % and over 14 % in the steady state for CH₃NH₃PbI_3?xCl_x, if the rapidly formed perovskite film is annealed. Thus, it is believed that moisture (below 60 % RH) is not a problem for the fabrication of highly efficient perovskite solar cells.     

Inorganic Pb‐Free Perovskite Light Absorbers for Efficient Perovskite Solar Cells with Enhanced Performance

Recently, enormous efforts have been made to develop the efficient, lead (Pb) free and stable perovskite solar cells (PSCs). In this regards, various strategies were applied and the optoelectronic properties of various Pb free perovskites such as (CH₃NH₃)₃Sb₂I₉, (CH₃NH₃)₃Bi₂I₉, Cs₃Sb₂I₉, Cs₃Bi₂I₉, CH₃NH₃SnI₃ and CH₃NH₃GeI₃ etc have been investigated. However, the photovoltaic performance of the developed PSCs was still low and presence of organic moieties in common hole‐transport materials (HTMs) shows poor stability against moisture and heat. Herein, we have investigated the optoelectronic properties of all inorganic Pb free perovskites (Cs₃Sb₂I₉=1 and Cs₃Bi₂I₉=2) and employed novel strategies (dissolution‐recrystallization) to prepare the efficient Pb free PSCs. The band gaps of the 1 and 2 were found to be 2.2 eV and 2.0 eV, respectively. The developed PSCs with 1 and 2 exhibited the power conversion efficiency of 0.68% and 1.087%, respectively.     

Unraveling the Role of Monovalent Halides in Mixed‐Halide Organic–Inorganic Perovskites

The performance of perovskite solar cells is strongly influenced by the composition and microstructure of the perovskite. A recent approach to improve the power conversion efficiencies utilized mixed‐halide perovskites, but the halide ions and their roles were not directly studied. Unraveling their precise location in the perovskite layer is of paramount importance. Here, we investigated four different perovskites by using X‐ray photoelectron spectroscopy, and found that among the three studied mixed‐halide perovskites, CH₃NH₃Pb(I_0.74Br_0.26)₃ and CH₃NH₃PbBr_3?xCl_x show peaks that unambiguously demonstrate the presence of iodide and bromide in the former, and bromide and chloride in the latter. The CH₃NH₃PbI_3?xCl_x perovskite shows anomalous behavior, the iodide content far outweighs that of the chloride; a small proportion of chloride, in all likelihood, resides deep within the TiO₂/absorber layer. Our study reveals that there are many distinguishable structural differences between these perovskites, and that these directly impact the photovoltaic performances.     

低温原子层沉积封装技术在OLED上的应用及对有机、钙钛矿太阳能电池封装的启示

有机半导体光电器件在基础研究和工业应用方面进展迅速, 相关方向已经成为一个较为成熟的新兴领域. 然而, 这类器件中的关键有机材料存在对空气中的水、氧十分敏感的问题, 严重影响了器件的长期工作性能. 除了选择合适的传输层材料、界面层结构, 利用界面工程提高器件水氧耐受能力之外, 对器件进行可靠的封装是隔绝空气中水、氧的另一个有效手段. 原子层沉积(atomic layer deposition, ALD)是一种近乎完美的薄膜沉积封装技术, 这种技术所生长的薄膜具有独特的层-层(layer by layer)生长特性, 而且可以在低温下沉积出厚度可控、重复率高、均匀致密的薄膜, 使得该技术在半导体行业已经得到广泛应用. 在此, 我们将回顾ALD封装技术在有机发光二极管(organic light emitting diode, OLED)、有机太阳能电池(organic photovoltaics, OPV)和钙钛矿太阳能电池(perovskite solar cell, PSC)中的应用, 并进一步讨论现阶段应用于OLED的相对比较成熟的ALD封装技术, 及其对OPV和PSC封装的启示性意义.     

Optimizing the Photovoltaic Performance of Organic Solar Cells for Indoor Light Harvesting

Organic solar cells (OSCs) harvesting indoor light are highly promising for emerging technologies, such as internet of things. Herein, the photovoltaic performance of PTB7-Th:PC₇₁BM solar cells constructed using “optimized (with 1,8-diiodooctane (DIO))” and “non-optimized (without DIO)” processing conditions are compared for indoor and outdoor applications. We find that in comparison to the “optimized” solar cell, the “non-optimized” solar cell is less efficient under simulated solar light illumination (100 mW cm⁻², spectral range 350–1100 nm), owing to significant bimolecular charge carrier recombination losses. However, under simulated indoor illumination (3.28 mW cm⁻², spectral range 400–700 nm), bimolecular recombination losses are effective suppressed, thus the power conversion efficiency of the solar cell without DIO was increased to 14.7 %, higher than that of the solar cell with DIO (14.2 %). These results suggest that the common strategy used to optimize the OSCs could be undesired for indoor OSCs. We demonstrate that the efforts for realizing the desired “morphology” of the active layer for the outdoor OSCs may be unnecessary for indoor OSCs, allowing us to realize high-efficiency indoor OSCs using a non-halogenated solvent.     

Beyond Perovskite Solar Cells: Tellurium Iodide as a Promising Light‐Absorbing Material for Solution‐Processed Photovoltaic Application

Searching new light‐absorbing materials to replace toxic lead halide in solar cells is very important and highly desirable. In this research, we firstly demonstrated that tellurium iodide (TeI₄) could be used as a light‐absorbing material in solar cells due to its suitable optical band gap and the active lone‐pair electron orbital in Te⁴⁺. The best power conversion efficiency (PCE=3.56%) was achieved with a concentration of 0.9 M TeI₄ in DMF:DMSO (4 : 1, v,v) without any heat treatment or antisolvent dripping. Our study indicates the promising potential of TeI₄ for photovoltaic and optoelectronic applications.     

Alkyl‐Chain‐Regulated Charge Transfer in Fluorescent Inorganic CsPbBr3 Perovskite Solar Cells

Improved charge extraction and wide spectral absorption promote power conversion efficiency of perovskite solar cells (PSCs). The state‐of‐the‐art carbon‐based CsPbBr₃ PSCs have an inferior power output capacity because of the large optical band gap of the perovskite film and the high energy barrier at perovskite/carbon interface. Herein, we use alkyl‐chain regulated quantum dots as hole‐conductors to reduce charge recombination. By precisely controlling alkyl‐chain length of ligands, a balance between the surface dipole induced charge coulomb repulsive force and quantum tunneling distance is achieved to maximize charge extraction. A fluorescent carbon electrode is used as a cathode to harvest the unabsorbed incident light and to emit fluorescent light at 516 nm for re‐absorption by the perovskite film. The optimized PSC free of encapsulation achieves a maximum power conversion efficiency up to 10.85 % with nearly unchanged photovoltaic performances under 80 %RH, 80 °C, or light irradiation in air.