Abstract
In this work, the ultrafast transient absorption spectroscopy (TAs) was utilized to first investigate the charge transfer from the emerging FAPbI3 (FA = CH(NH2)2) perovskite quantum dots (PQDs) to charge transport layers. Specifically, we compared the TAs in pure FAPbI3 PQDs, PQDs grown with both electron and hole transfer layers (ETL and HTL), and PQDs with only ETL or HTL. The TA signals induced by photoexcited electrons decay much faster in PQDs samples with the ETL (~20 ps) compared to the pure FAPbI3 PQDs (>1 ns). These results reveal that electrons can effectively transport between coupled PQDs and transfer to the ETL (TiO2) at a time scale of 20 ps, much faster than the bimolecular charge recombination inside the PQDs (>1 ns), and the electron transfer efficiency is estimated to be close to 100%. In contrast, the temporal evolution of the TA signals in the PQDs with and without HTL exhibit negligible change, and no substantive hole transfer to the HTL (poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine], PTAA) occurs within 1 ns. The much slower hole transfer implies the further potential of increasing the overall photo-carrier conversion efficiency through enhancing the hole diffusion length and fine-tuning the coupling between the HTL and PQDs.
Highlights
Perovskite solar cells (PSCs) have made significant achievements in the certified power conversion efficiency (PCE), which increases from 3.8% to 25.2% [1] in recent several years
Unlike the exciton or charge diffusion in the MAPbI3 thin films as the active layer of PSCs, the diffusion process in the active layer of FAPbI3 perovskite quantum dots (PQDs) critically relies on the electronic couplings between the PQDs before the free carriers are injected into the charge transport layer
Energy level values of FAPbI3 PQDs and TiO2 are −3.94 eV and −4.15 eV [25,33,34], respectively, which are in favor of the charge transfer
Summary
Perovskite solar cells (PSCs) have made significant achievements in the certified power conversion efficiency (PCE), which increases from 3.8% to 25.2% [1] in recent several years. Xing et al reported the charge-carrier transfer time of 0.40 ns and efficiency of 92% for MAPbI3 with selective electron layers ([6,6]-phenyl-C61-butyric acid methyl ester, PCBM) [3]. FAPbI3 can be fabricated as quantum dots (QDs) which possess proper phase structure and crystalline orientation without the need of high temperature annealing [23,24,25]. The investigation of CT processes to reveal the intrinsic transfer time and efficiency is still lacking, which hinders the clarification of the important factors limiting the PCEs. Unlike the exciton or charge diffusion in the MAPbI3 thin films as the active layer of PSCs, the diffusion process in the active layer of FAPbI3 PQDs critically relies on the electronic couplings between the PQDs before the free carriers are injected into the charge transport layer. We did not observe significant hole transfer to PTAA within our experimental time window of ~1 ns
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