Graded Bandgap Perovskite with Intrinsic n–p Homojunction Expands Photon Harvesting Range and Enables All Transport Layer‐Free Perovskite Solar Cells

Abstract

Organic–inorganic halide perovskites are promising materials for next‐generation photovoltaic device due to their attractive photoelectrical properties such as strong light absorption, high carrier mobility, and tunable bandgap. Generally, perovskite solar cells require carrier transport layers (CTL) to provide a built‐in electric field and reduce the recombination rate. However, the construction of suitable electron‐ and hole‐transport layers is not cost effective, impairing the commercial application of the devices. An n–p perovskite homojunction absorber with a graded bandgap is developed by introducing a three‐step dynamic spin‐coating strategy and variable valence Sn elements. The bandgap of the perovskite absorber is gradually manipulated from 1.53 eV (the bottom) to 1.27 eV (the top). The electronic behavior is also transformed from n‐type (excess PbI2, the bottom) to p‐type (Sn vacancy, the top) in a very short distance (50 nm). This designed perovskite homojunction electronic structure not only expands the light harvesting range from 800 to 970 nm which provides potential to break the PCE limits, but also promotes oriented carrier transportation and weakens the dependence on CTL. The demonstrated asymmetrical active layer shows a brand‐new approach to simplify the device structure and boost the performance of CTL‐free perovskite solar cells.

Secondary Ion Mass Spectrometry (SIMS) for Chemical Characterization of Metal Halide Perovskites

“…[79] An n-p homojunction MHP with a graded bandgap was also developed by a dynamic spin-coating method. [80] The homojunction is formed due to a preferential Sn distribution at the MHP film surface, which is confirmed by ToF-SIMS. [80] In Figure 10g,h, it is seen that the Sn is distributed in the whole film in the mixed Pb/Sn MHP and no Sn in the Pb-based MHP.…”

“…[80] The homojunction is formed due to a preferential Sn distribution at the MHP film surface, which is confirmed by ToF-SIMS. [80] In Figure 10g,h, it is seen that the Sn is distributed in the whole film in the mixed Pb/Sn MHP and no Sn in the Pb-based MHP. [80] However, a graded Sn distribution at the surface is observed in the graded Pb/Sn MHP, indicating the formation of homojunction in the graded Pb/Sn MHP.…”

“…[80] In Figure 10g,h, it is seen that the Sn is distributed in the whole film in the mixed Pb/Sn MHP and no Sn in the Pb-based MHP. [80] However, a graded Sn distribution at the surface is observed in the graded Pb/Sn MHP, indicating the formation of homojunction in the graded Pb/Sn MHP. When applying this graded Pb/Sn MHP to solar cell, a stabilized PCE of 20.98% was achieved.…”

Perovskite Nano‐Heterojunctions: Synthesis, Structures, Properties, Challenges, and Prospects

“…This gradient perovskite heterojunction achieved a PCE of 21.4%. [ 197 ] In another strategy to prepare perovskite gradient heterojunctions, CsPbBr 3 perovskite NWs’ colloidal solution was dripped on to the MAPbI 3 or MA 0.3 FA 0.7 PbI 3 film followed by annealing, resulting in a lateral homogeneity film. A high PCE up to 21.4% and high operational stability were achieved (Figure 28b).…”

“…Reproduced with permission. [ 197 ] Copyright 2020, WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim. b) Schematic illustration of the preparation of perovskite thin films with graded incorporation of CsPbBr 3 nanocrystals at the top film surface.…”