Co‐Evaporated MAPbI₃ with Graded Fermi Levels Enables Highly Performing, Scalable, and Flexible p‐i‐n Perovskite Solar Cells

Abstract

Recent progress of vapor-deposited perovskite solar cells (PSCs) has proved the feasibility of this deposition method in achieving promising photovoltaic devices. For the first time, it is probed the versatility of the co-evaporation process in creating perovskite layers customizable for different device architectures. A gradient of composition is created within the perovskite films by tuning the background chamber pressure during the growth process. This method leads to co-evaporated MAPbI 3 film with graded Fermi levels across the thickness. Here it is proved that this growth process is beneficial for p-i-n PSCs as it can guarantee a favorable energy alignment at the charge selective interfaces. Co-evaporated p-i-n PSCs, with different hole transporting layers, consistently achieve power conversion efficiency (PCE) over 20% with a champion value of 20.6%, one of the highest reported to date. The scaled-up p-i-n PSCs, with active areas of 1 and 1.96 cm 2 , achieved the record PCEs of 19.1% and 17.2%, respectively, while the flexible PSCs reached a PCE of 19.3%. Unencapsulated PSCs demonstrate remarkable long-term stability, retaining ≈90% of their initial PCE when stored in ambient for 1000 h. These PSCs also preserve over 80% of their initial PCE after 500 h of thermal aging at 85 °C.

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“…[86] One issue with thermal evaporation is that, while inorganic cesium halide and lead halide salts evaporate in a ballistic way and their evaporation rate can be controlled by quartz crystal microbalances (QCMs), the evaporation of organic halide salts is more difficult to control due to their high vapor pressure under vacuum, low sticking coefficient on QCMs or chamber walls and varying sticking coefficient on different substrates. [158,159] To circumvent this issue, Mitzi and co-workers [160] developed a single-source flash evaporation technique to deposit a wide range of layered halide perovskites. A pre-synthesized perovskite material, for example, by a solution-based route, is placed on a metal heater in a vacuum and rapidly evaporated.…”

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“…83 The Fermi levels were close to the conduction band minimum energies which shows that all of the films had n-type doping. 84 Interestingly, the Fermi level energies decreased as the MG content increased, which may be due to electron donation from the MGs. The difference between the energy of the conduction band minimum and the Spiro HOMO increased as the MG content increased, which could contribute to an increase in V oc .…”

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“…[ 55 ] Since then, the field has grown rapidly with flexible PSCs reaching PCEs above 20% [ 56 ] from solution process technique and 19% from coevaporation process. [ 57 ] while flexible minimodule has been demonstrated, with PCEs of 15.22% over 30 cm 2 active area. [ 58 ] The flexible PSCs with high power‐per‐weight enables specific application scenarios where light weight, high flexibility, and versatile conformability are indispensable.…”