Direct Observation of Intragrain Defect Elimination in FAPbI₃ Perovskite Solar Cells by Two-Dimensional PEA₂PbI₄

Abstract

Two-dimensional perovskites have widely been used to improve the efficiency and stability of perovskite solar cells and are generally believed to passivate defects at the grain boundaries of threedimensional perovskites. Herein, we studied introducing various combinations of two-dimensional phenyl ethylammonium lead iodide (PEA 2 PbI 4 ) and methylammonium chloride (MACl) to the formamidinium lead iodide (FAPbI 3 ) precursor solution. Ultralow dose selected area electron diffraction studies prove that the high density of intragrain planar defects in FAPbI 3 can be strongly reduced by adding PEA 2 PbI 4 and fully eliminated by adding further MACl. Although PEA + is too large to incorporate into FAPbI 3 , PEA 2 PbI 4 not only improves crystallization but also suppresses intragrain defect formation. As a result, a longer charge carrier lifetime, higher photoluminescence quantum yield, lower Urbach energy, and current−voltage hysteresis are achieved, resulting in a champion PCE of 23.69%, with an improvement of humidity stability.

Heterogeneity of Light-Induced Open-Circuit Voltage Loss in Perovskite/Si Tandem Solar Cells

“…However, there are fewer defects at GIs, so there is no significant V OC drop, and even a certain increase in V OC with the continuous time, which may be attributed to the formation of the bromine-rich phase here . Previous reports have shown that 2D perovskites are mainly distributed at the GBs. ,,, Similarly, our results also show that the FPEAI treatment modifies the electronic properties of the GBs, resulting in higher CPD and significantly higher V OC and better V OC stability under continuous light at GBs compared to the control samples (Supplementary Note 3).…”

“…13 Previous reports have shown that 2D perovskites are mainly distributed at the GBs. 28,29,32,61 Similarly, our results also show that the FPEAI treatment modifies the electronic properties of the GBs, resulting in higher CPD and significantly higher V OC and better V OC stability under continuous light at GBs compared to the control samples (Supplementary Note 3).…”

“…Due to the upward band bending at the grain boundaries, photoexcited holes tend to migrate to the boundaries, resulting in a larger surface work function and a lower CPD than at the interiors. In contrast, the target film has a lower work function and higher CPD at the GB, which is consistent with what has previously been reported. ,, It is clear that the average CPDs of the grain interiors are much less sensitive to the illumination duration than those of the grain boundaries, suggesting that the grain boundaries are active sites for iodide ion migration and undergo more severe changes in composition, consistent with previous studies.…”

“…31,34 However, for the target sample, the opposite CPD is observed, suggesting that GBs modified by FPEAI increase the migration barrier for the iodine ions, which effectively eliminates the occurrence of photoinduced phase segregation. 28,29 In comparison to the surface potential map in the absence of light, the overall CPD values decreased under initial illumination, primarily due to the presence of surface photovoltage. 31 This suggests that upon exposure to illumination, the energy band on the pristine surface exhibits a greater downward bending compared to the bulk material.…”

“…In this work, we unveil the impact of light-induced phase segregation in a 1.67 eV Cs 0.17 FA 0.83 Pb­(I 0.80 Br 0.20 ) 3 mixed-halide perovskite on the local V OC loss across the film, as well as in entire macroscopic solar cell devices. This is achieved through a combination of (1) inhibiting phase segregation and passivating grain boundaries by introducing FPEAI during the antisolvent step of the perovskite deposition, in turn improving device performance − while otherwise keeping the device materials and configuration nominally the same, (2) using a customized c-AFM system to locally measure current–voltage ( I – V ) curves and surface potential distribution at both grain boundaries and grain interiors (GIs) in the dark and under different illumination times, and (3) correlating these to the defects, phase segregation, and V OC loss of macroscopic solar cell devices. We find that iodide-rich phases predominately segregate at the GBs of our control samples, causing significant V OC loss with extended light exposure at the GBs, but we do not observe such segregation and the corresponding instability and V OC loss at GIs.…”