Balancing Energy-Level Difference for Efficient n-i-p Perovskite Solar Cells with Cu Electrode

Developing low cost and stable metal electrode is crucial for mass production of perovskite solar cells (PSCs). As an earth-abundant element, Cu becomes an alternative candidate to replace noble metal electrodes such as Au and Ag, due to its comparable physiochemical properties with simultaneously good stability and low cost. However, the undesirable band alignment associated with the device architecture impedes the exploration of efficient Cu-based n-i-p PSCs. Here, we demonstrated the ability of tuning the Fermi level (EF) of hole transport layer (HTL) to reduce the energy level difference (Schottky barrier) between HTLs and Cu. Further, we identified that the balance of energy level difference between HTL and adjacent layers (including perovskite and Cu) is crucial to efficient carrier transportation and photovoltaic performance improvement in the PSCs. Under the optimized condition, we achieve a device power conversion efficiency (PCE) of 20.10%, which is the highest on the planar n-i-p PSCs with Cu electrode. Meanwhile, the Cu-based PSCs can maintain 92% of their initial efficiency after 1000 h storage, which is comparable with Au-based devices. The present work not only extends the understanding on the band alignment of neighboring semiconductor functional layer in the device architecture to improve the resulting performance but also suggests great potential of Cu electrode for application in PSCs community

Reversible Phase Transition for Durable Formamidinium-Dominated Perovskite Photovoltaics

Phase instability is one of the major obstacles to the wide application of formamidinium (FA)-dominated perovskite solar cells (PSCs). An in-depth investigation on relevant phase transitions is urgently needed to explore more effective phase-stabilization strategies. Herein, the reversible phase-transition process of FA1−xCsxPbI3 perovskite between photoactive phase (α phase) and non-photoactive phase (δ phase) under humidity, as well as the reversible healing of degraded devices, is monitored. Moreover, through in situ atomic force microscopy, the kinetic transition between α and δ phase is revealed to be the “nucleation–growth transition” process. Density functional theory calculation implies an enthalpy-driven α-to-δ degradation process during humidity aging and an entropy-driven δ-to-α healing process at high temperatures. The α phase of FA1−xCsxPbI3 can be stabilized at elevated temperature under high humidity due to the increased nucleation barrier, and the resulting non-encapsulated PSCs retain >90% of their initial efficiency after >1000 h at 60 °C and 60% relative humidity. This finding provides a deepened understanding on the phase-transition process of FA1−xCsxPbI3 from both thermodynamics and kinetics points of view, which also presents an effective means to stabilize the α phase of FA-dominated perovskites and devices for practical applications

Impacts of alkaline on the defects property and crystallization kinetics in perovskite solar cells

Defect density reduction is pertinent for halide perovskite solar cells but a universal strategy has not been exploited. Here Chen et al. show that by fine tuning the alkaline environment in precursor solution, they can greatly suppress defects density and obtain high certified efficiency of 20.87%

The Impacts and Origins of A-site Instability in Formamidinium-Cesium Lead Iodide Perovskite Solar Cells Under Extended Operation

Improved understanding of the origins of instability during photovoltaic operation of perovskite solar cell materials must be established to overcome barriers to commercialization. In this study, we analyze the microscopic mechanisms of degradation in high-performing methylammonium free (FA0.9Cs0.1PbI3) perovskite solar cells (PSC) over 600 hours of operation under stressors inherent to PV operation, including heat, illumination, and a load while excluding atmospheric effects by testing in a water-and oxygen-free atmosphere. While the PSCs exhibit reasonable thermal stability, they show considerable performance loss under constant illumination or stable power output. Synchrotron-based nanoprobe X-ray fluorescence and X-ray beam induced current (XRF/XBIC) measurements reveal segregation of current-blocking Cs-rich phases during stress testing. The decrease in performance correlates with the resulting number density of the Cs-rich clusters, which varies by stress condition. These findings unveil cation-dependent instability in FA0.9Cs0.1PbI3 perovskites and provide a framework for understanding the energy landscape in alloy perovskites to guide the engineering of long-lived halide perovskite devices

Phase transformation barrier modulation of CsPbI3 films via PbI3− complex for efficient all-inorganic perovskite photovoltaics

Cesium lead iodide (CsPbI3) has gained great attention due to its thermal stability and appropriate bandgap (≈1.73 eV) at black (γ) phase potentially suitable for tandem solar cells. However, it is challenging to obtain CsPbI3 film with desired black phase. Herein, we fabricate kinetically favorable γ-CsPbI3 thin films by stoichiometry modulation, where in-situ 2D GIWAXS measurement was innovatively performed to illustrate the phase transition process of the precursor films, to aid a full picture study on the entire film evolution process. Conceptually different from introducing other extrinsic species, the cogenetic doping by excessive cesium iodide is found to tailor energy barriers for phase transformations during both the film formation and ageing process simultaneously. During film growth, excessive CsI affects the formation of Pb−I complex in the precursor solution, which facilitates the δ to γ phase transformation. Also, the Cs-rich resultant film could suppress γ to δ phase transformation. The corresponding CsPbI3 solar cells deliver a PCE of 16.68% without performance loss at continuous maximum power point output (MPP) for ~175 h under continuous illumination in a N2 glovebox. This work highlights the importance of precursors chemistry and provides guidelines to adjust the phase transformation barrier in CsPbI3 films without any foreign additives

Microscopic Degradation in Formamidinium-Cesium Lead Iodide Perovskite Solar Cells under Operational Stressors

The most important obstacle to widespread use of perovskite solar cells is their poor stability under operational stressors. Here, we systematically monitor the evolution of the photovoltaic performance of perovskite solar cells based on formamidinium-cesium lead iodide (FA0.9Cs0.1PbI3) for 600 h, under a series of controlled operational stressors. Although these devices exhibit reasonable thermal stability, their stability under illumination or stabilized power output (SPO) is far from commercial demands. Synchrotron-based nanoprobe X-ray fluorescence and X-ray-beam-induced current measurements reveal that current-blocking Cs-rich phases segregate during stress tests. The decrease in performance is in line with the increasing density of the Cs-rich clusters in area upon illumination. Theoretical calculations indicate that light-generated carriers provide the thermodynamic driving force for that phase segregation. Our findings correlate device performance to microscopic behavior and atomistic mechanisms and shed light on inhibiting the cation-dependent phase segregation during device operation

Stress compensation based on interfacial nanostructures for stable perovskite solar cells

Abstract The long‐term stability issue of halide perovskite solar cells hinders their commercialization. The residual stress–strain affects device stability, which is derived from the mismatched thermophysical and mechanical properties between adjacent layers. In this work, we introduced the Rb2CO3 layer at the interface of SnO2/perovskite with the hierarchy morphology of snowflake‐like microislands and dendritic nanostructures. With a suitable thermal expansion coefficient, the Rb2CO3 layer benefits the interfacial stress relaxation and results in a compressive stress–strain in the perovskite layer. Moreover, reduced nonradiative recombination losses and optimized band alignment were achieved. An enhancement of open‐circuit voltage from 1.087 to 1.153 V in the resultant device was witnessed, which led to power conversion efficiency (PCE) of 22.7% (active area of 0.08313 cm2) and 20.6% (1 cm2). Moreover, these devices retained 95% of its initial PCE under the maximum power point tracking (MPPT) after 2700 h. It suggests inorganic materials with high thermal expansion coefficients and specific nanostructures are promising candidates to optimize interfacial mechanics, which improves the operational stability of perovskite cells

Promoting Energy Transfer via Manipulation of Crystallization Kinetics of Quasi-2D Perovskites for Efficient Green Light-Emitting Diodes

Quasi-2D (Q-2D) perovskites are promising materials applied in light-emitting diodes (LEDs) due to their high exciton binding energy and quantum confinement effects. However, Q-2D perovskites feature a multiphase structure with abundant grain boundaries and interfaces, leading to nonradiative loss during the energy-transfer process. Here, a more efficient energy transfer in Q-2D perovskites is achieved by manipulating the crystallization kinetics of different-n phases. A series of alkali-metal bromides is utilized to manipulate the nucleation and growth of Q-2D perovskites, which is likely associated with the Coulomb interaction between alkali-metal ions and the negatively charged PbBr64– frames. The incorporation of K+ is found to restrict the nucleation of high-n phases and allows the subsequent growth of low-n phases, contributing to a spatially more homogeneous distribution of different-n phases and promoted energy transfer. As a result, highly efficient green Q-2D perovskites LEDs with a champion EQE of 18.15% and a maximum brightness of 25 800 cd m–2 are achieved. The findings affirm a novel method to optimize the performance of Q-2D perovskite LEDs

Cation and anion immobilization through chemical bonding enhancement with fluorides for stable 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