Single cell all perovskite triple junction solar cells have the potential to exceed the power conversion efficiency of the most advanced double junction series cells and far exceed the balance limit of single junction batteries. However, their current performance is limited by defects in open circuit voltage in wide bandgap perovskite solar cells, as well as potential limitations in short-circuit current density and filling factor. The author found that even after material synthesis, the heterogeneity of halides still plays a crucial role in interface non radiative recombination and collection efficiency loss under long-term irradiation, especially in Br rich perovskites. So the author introduced a diammonium halide salt, namely propane-1,3-diammonium iodide, during the thin film preparation process and found that it can improve the homogenization of halides in Br rich perovskites, enhance operational stability, and achieve an open circuit voltage of 1.44 V in inverted (p-i-n) devices; For a bandgap of 1.97 eV, it reached 86% of the equilibrium limit. This efficient broadband gap battery makes it possible to prepare single cell all perovskite triple junction solar cells, with an open circuit voltage of 3.33 V and a maximum photoelectric conversion efficiency of 25.1% (23.87% certified quasi steady state efficiency).
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Integrating multiple absorption layers into a multi junction solar cell has the ability to overcome the limitations of carrier heating from high-energy photons and the transmission of low-energy photons observed in single junction photovoltaic cells (PVs). The recent progress in metal halide perovskite semiconductors with bandgap energies of 1.2-1.8 eV has enabled them to be used together with materials such as perovskite, crystalline silicon (c-Si), copper indium gallium selenium (CIGS), and organic photovoltaics (OPV) in series solar cells. Among these materials, all double junction perovskite (2J) series solar cells were coupled with an absorption layer of 1.77/1.22 eV, achieving a steady-state certified photovoltaic conversion efficiency (PCE) of 29%; In contrast, the efficiency of single junction perovskite solar cells (PSCs) is 26.1%, highlighting the potential application of perovskite photovoltaics.
Combining three perovskites in a single solar cell yields higher efficiency than the corresponding 2J counterpart. According to optical and device simulations, the all perovskite triple junction (3J) solar cell uses cascaded 2.0, 1.6, and 1.2 eV absorption layers, with a theoretical efficiency limit of over 50% and is expected to achieve a photoelectric conversion efficiency of over 36%. However, only a limited number of studies have experimentally demonstrated 3J PSCs, and their actual performance is still lower than their single junction and 2J counterparts. Compared with narrow bandgap perovskite solar cells, inverted (p-i-n) single junction perovskite solar cells with a~2 eV bandgap typically have a bromine content of over 60%, a significant open circuit voltage (Voc) loss of over 700mV, and performance loss due to halide separation caused by light. Recent studies have found that the main factors affecting performance are increased band offset in the charge transport layer and defect density in the wide bandgap perovskite absorption layer. The latter is particularly evident in perovskites based on high Br/I ratios, as they are susceptible to uncontrolled growth of various halide species, leading to halide heterogeneity within the body and at the interface.
The author aims to improve the in-phase and interfacial halide homogeneity in~2 eV mixed Br/I perovskite to reduce energy loss and better utilize the potential of 3J PSCs. Previously, vacuum deposition technology had been used to prepare uniform mixed halide perovskites due to its excellent composition control. For the solution treatment process, literature has shown that introducing Lewis acid-base adduct additives can regulate the crystallization process of perovskite; However, few studies have explored the effect of additives on the homogeneity of mixed Br/I perovskites. In the pursuit of reducing defect density and achieving homogenization of high brominated perovskite thin films, the author first used ammonium halide salts, as they have been reported to have both template effects on perovskite growth and the ability to modify surface defects.
Ren é A. J. Janssen from Eindhoven University of Technology in the Netherlands and Edward H. Sargent from the University of Toronto jointly reported a dual in vivo and interface perovskite surface modification technique using diammonium halide salts, which greatly suppressed non radiative recombination within the perovskite body and at the perovskite/charge transport layer heterojunction, achieving a record breaking Voc in p-i-n devices, approximately 86% of the detailed equilibrium (DB) limit of 1.97 electron volts. The author observed that although diammonium salts cannot suppress light induced halide separation, broadband gap perovskite solar cells still maintain high operational stability (>28 h) under long-term irradiation, exceeding the stability of~2 eV PSCs reported in the literature. Further research has found that additives promote halide homogeneity near the perovskite/hole transport layer (HTL) interface and reduce trap mediated recombination, providing a pathway for efficient extraction of charge carriers from halide separation regions. The author prepared a fully perovskite 3J solar cell, combining absorption layers of 1.97, 1.61, and 1.25 eV. Using a pre transparent conductive oxide layer with high carrier mobility and high near-infrared transmittance, the author obtained a 3J device with 3.33 V Voc and 25.1% PCE (23.87% certified).
Literature information
Junke Wang, Lewei Zeng, Dong Zhang, Aidan Maxwell, Hao Chen et al.Halide homogenization for low energy loss in 2-eV-bandgap perovskites and increased efficiency in all-perovskite triple-junction solar cells. Nature Energy (2023).
https://doi.org/10.1038/s41560-023-01406-5
https://www.nature.com/articles/s41560-023-01406-5
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