Efficient Passivation of Hybrid Perovskite Solar Cells Using Organic Dyes with COOH Functional Group

Organic–inorganic halide perovskites are efficient absorbers for solar cells. Nevertheless, the trap states at the surfaces and grain boundaries are a detrimental factor compromising the device performance. Here, an organic dye (AQ310) is employed as passivator to reduce the trap states of the perovskites and promote better stability. The results demonstrate that the trap states of perovskite are minimized by the presence of AQ310's ?COOH group and the formation of coordination with under‐coordinated Pb²⁺ ions. The resulting carrier recombination time is prolonged and verified by the photoluminescence and open‐circuit voltage decay measurements. Consequently, the best average power conversion efficiency (PCE) of 19.43% is achieved for the perovskite solar cell (PSC) with AQ310 passivation, as compared with a low average PCE of 17.98% for the PSC without AQ310 passivation.     

Di‐Spiro‐Based Hole‐Transporting Materials for Highly Efficient Perovskite Solar Cells

Hole‐transporting materials (HTMs) are essential for enabling highly efficient perovskite solar cells (PVSCs) to extract and transport the hole carriers. Among numerous HTMs that are studied so far, the single‐spiro‐based compounds are the most frequently used HTMs for achieving highly efficient PVSCs. In fact, all the new spiro‐based HTMs reported so far that render PVSCs over 20% are based on spiro[fluorene‐9,9′‐xanthene] or spiro [cyclopenta [2,1‐b:3,4b′]dithiophene‐4,9′‐fluorene] cores; therefore, there is a need to diversify the design of their structures for further improving their function and performance. In addition, the fundamental understanding of structure–performance relationships for the spiro‐based HTMs is still lagging, for example, how molecular configuration, spiro numbers, and heteroatoms in spiro‐rings impact the efficacy of HTMs. To address these needs, two novel H‐shaped HTMs, G1 and G2 based on the di‐spiro‐rings as the cores are designed and synthesized. The combined good film‐forming properties, better interactions with perovskite, slightly deeper highest occupied molecular orbital, higher mobility and conductivity, as well as more efficient charge transfer for G2 help devices reach a very impressive power conversion efficiency of 20.2% and good stability. This is the first report of demonstrating the feasibility of using di‐spiro‐based HTMs for highly efficient PVSCs.     

Quantifying Efficiency Loss of Perovskite Solar Cells by a Modified Detailed Balance Model

A modified detailed balance model is built to understand and quantify efficiency loss of perovskite solar cells. The modified model captures the light‐absorption‐dependent short‐circuit current, contact and transport‐layer‐modified carrier transport, as well as recombination and photon‐recycling‐influenced open‐circuit voltage. The theoretical and experimental results show that for experimentally optimized perovskite solar cells with the power conversion efficiency of 19%, optical loss of 25%, nonradiative recombination loss of 35%, and ohmic loss of 35% are the three dominant loss factors for approaching the 31% efficiency limit of perovskite solar cells. It is also found that the optical loss climbs up to 40% for a thin‐active‐layer design. Moreover, a misconfigured transport layer introduces above 15% of energy loss. Finally, the perovskite‐interface‐induced surface recombination, ohmic loss, and current leakage should be further reduced to upgrade device efficiency and eliminate hysteresis effect. This work contributes to fundamental understanding of device physics of perovskite solar cells. The developed model offers a systematic design and analysis tool to photovoltaic science and technology.     

Analysis of Ion‐Diffusion‐Induced Interface Degradation in Inverted Perovskite Solar Cells via Restoration of the Ag Electrode

Straightforward evidence for ion‐diffusion‐induced interfacial degradation in inverted perovskite solar cells is presented. Over 1000 h, solar cells inevitably undergo degradation, especially with respect to the current density and fill factor. The Ag electrode is peeled off and re‐evaporated to investigate the effect of the Ag/[6,6]‐phenyl C71 butyric acid methyl ester (PCBM) interfacial degradation on the photovoltaic performance at days 10 (240 h), 20 (480 h), 30 (720 h), and 40 (960 h). The power conversion efficiency increases after the Ag electrode restoration process. While the current density shows a slightly decreased value, the fill factor and open‐circuit voltage increase for the new electrode devices. The decrease in the activation energy due to the restored Ag electrode induces recovery of the fill factor. The diffused I^? ions react with the PCBM molecules, resulting in a quasi n‐doping effect of PCBM. Upon electrode exchange, the reversible interaction between the iodine ions and PCBM causes current density variation. The disorder model for the open‐circuit voltage over a wide range of temperatures explains the open‐circuit voltage increase at every electrode exchange. Finally, the degradation mechanism of the inverted perovskite solar cell over 1000 h is described under the proposed recombination system.     

Surface Electronic Modification of Perovskite Thin Film with Water‐Resistant Electron Delocalized Molecules for Stable and Efficient Photovoltaics

Although the efficiency of perovskite solar cells (PSCs) is close to crystalline silicon solar cells, the instability of perovskite, especially in humid condition, still hinders its commercialization. As an effective method to improve their stability, surface functionalization, by using hydrophobic molecules, has been extensively investigated, but usually accompanied with the loss of device efficiencies owing to their intrinsic electrical insulation. In this work, for the first time, it is demonstrated that 3‐alkylthiophene‐based hydrophobic molecules can be used as both water‐resistant and interface‐modified layers, which could simultaneously enhance both stability and performance significantly. Benefitting from their unique structures of thiophene rings, the π‐electrons are highly delocalized and thus enhance the charge transfer and collection at the interface. The device based on 3‐hexylthiophene treatment exhibits a champion energy conversion efficiency of 19.89% with a dramatic 10% enhancement compared with the pristine one (18.08%) of Cs_0.05 FA_0.81 MA_0.14 PbBr_0.45 I_2.55‐based PSCs. More importantly, the degradation of the long‐term efficiency of unsealed device is less than 20% in Cs_0.05 FA_0.81 MA_0.14 PbBr_0.45I_2.55‐based PSCs after more than 700 h storage in air. This finding provides an avenue for further improvement of both the efficiency and stability of PSCs.     

Understanding the Role of Cesium and Rubidium Additives in Perovskite Solar Cells: Trap States,Charge Transport,and Recombination

Adding cesium (Cs) and rubidium (Rb) cations to FA_0.83MA_0.17Pb(I_0.83Br_0.17)₃ hybrid lead halide perovskites results in a remarkable improvement in solar cell performance, but the origin of the enhancement has not been fully understood yet. In this work, time‐of‐flight, time‐resolved microwave conductivity, and thermally stimulated current measurements are performed to elucidate the impact of the inorganic cation additives on the trap landscape and charge transport properties within perovskite solar cells. These complementary techniques allow for the assessment of both local features within the perovskite crystals and macroscopic properties of films and full devices. Strikingly, Cs‐incorporation is shown to reduce the trap density and charge recombination rates in the perovskite layer. This is consistent with the significant improvements in the open‐circuit voltage and fill factor of Cs‐containing devices. By comparison, Rb‐addition results in an increased charge carrier mobility, which is accompanied by a minor increase in device efficiency and reduced current–voltage hysteresis. By mixing Cs and Rb in quadruple cation (Cs‐Rb‐FA‐MA) perovskites, the advantages of both inorganic cations can be combined. This study provides valuable insights into the role of these additives in multiple‐cation perovskite solar cells, which are essential for the design of high‐performance devices.     

New Strategy for Two‐Step Sequential Deposition: Incorporation of Hydrophilic Fullerene in Second Precursor for High‐Performance p‐i‐n Planar Perovskite Solar Cells

In p‐i‐n planar perovskite solar cells (pero‐SCs) based on methylammonium lead iodide (MAPbI₃) perovskite, high‐quality MAPbI₃ film, perfect interfacial band alignment and efficient charge extracting ability are critical for high photovoltaic performance. In this work, a hydrophilic fullerene derivative [6,6]‐phenyl‐C₆₁‐butyric acid‐(3,4,5‐tris(2‐(2‐(2‐methoxyethoxy)ethoxy)ethoxy)phenyl)methanol ester (PCBB‐OEG) is introduced as additive in the methylammonium iodide precursor solution in the preparation of MAPbI₃ perovskite film by two‐step sequential deposition method, and obtained a top‐down gradient distribution with an ultrathin top layer of PCBB‐OEG. Meanwhile, a high‐quality perovskite film with high crystallinity, less trap‐states, and dense‐grained uniform morphology can well grow on both hydrophilic (poly(3,4‐ethylenedioxythiophene)/poly(styrenesulfonic acid)) and hydrophobic (polytriarylamine, PTAA) hole transport layers. When the PCBB‐OEG‐containing perovskite film (pero‐0.1) is prepared in a p‐i‐n planar pero‐SC with the configuration of ITO/PTAA/pero‐0.1/[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester/Al, the device delivers a promising power conversion efficiency (PCE) of 20.2% without hysteresis, which is one of the few PCE over 20% for the p‐i‐n planar pero‐SCs. Importantly, the pero‐0.1‐based device shows an excellent stability that can retain 98.4% of its initial PCE after being exposed for 300 h under ambient atmosphere with a high humidity, and the flexible pero‐SCs based on pero‐0.1 also demonstrate a promising PCE of 18.1%.