Defect-Tolerant Sodium-Based Dopant in Charge Transport Layers for Highly Efficient and Stable Perovskite Solar Cells

Abstract

To extract charges more efficiently through charge-transporting layers (CTLs), various dopants are necessary. Lithium bis­(trifluoromethanesulfonyl)­imide (Li-TFSI) is the most widely used dopant in electron- and hole-transporting layers. However, Li+ ions easily migrate into the perovskite and deteriorate the device performance. To address this issue, several efforts such as introducing a buffer layer have been tried, but the issue is still not fully resolved. Thus it is required to find a simple way without additional treatments. In this work, we propose a simple strategy to use defect-tolerant dopant in CTLs, sodium bis­(trifluoromethanesulfonyl)­imide (Na-TFSI), to improve both the efficiency and the stability of perovskite solar cells (PSCs). The PSCs with Na-TFSI for both the electron-transport layer and the hole-transport layer show the highest power conversion efficiency up to 22.4%. In addition, the device with Na-TFSI exhibited better long-term operating stability at 45 °C, maintaining >80% of the initial performance even after 500 h of continuous 1 sun illumination.

Mechanisms and Suppression of Photoinduced Degradation in Perovskite Solar Cells

“…22.4 >500 h FTO/TiO 2 (Na-TFSI)/(FAPbI 3 ) 0.95 (MAPbBr 3 ) 0.05 / spiro-OMeTAD(Na-TFSI)/Au AM 1.5 G, 45 °C TiO 2 (Na-TFSI) [234] (suppressed ions migration by Na-TFSI) 2020 18.2 >500 h FTO/TiO 2 (0.1CL-GP)/Perovskite/spiro-OMeTAD/Au AM 1.5 G, N 2 TiO 2 (0.1CL-GP) [235] Besides spiro-OMeTAD, many small organic HTMs also suffer from similar problems, as the dopants therein usually induce pinholes in the layer or react with perovskite. [251] There are three ways to address these problems: 1) inserting a compact blocking layer between the HTL and perovskite layer, 2) finding a new HTM that is friendly to perovskite layer and compact enough to block the migrating ions, 3) developing new HTMs without dopants.…”

Nonhalide Materials for Efficient and Stable Perovskite Solar Cells

“…For example, sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) was used to dope not only Spiro‐MeOTAD but also TiO 2 ( Figure a). [ 19 ] A high PCE of 22.4% was achieved by simultaneous doping of NaTFSI in ETL and HTL (Figure 8b). ToF‐SIMS showed that Li + ion in Spiro‐MeOTAD was easy to migrate to mesoporous TiO 2 layer through interstitial sites of the perovskite lattice.…”

“…Reproduced with permission. [ 19 ] Copyright 2020, American Chemical Society. d) J‐V curves of the devices using LiTFSI and Zn(TFSI) 2 as dopant for Spiro‐MeOTAD.…”

“…Nevertheless, the role of molecular anions is still blurry up to now. The applications of various molecular anions in PSCs have been explored and attempted, such as BF 4 − , [ 4,8 ] PF 6 − , [ 14 ] SCN − , [ 15 ] HCOO − , [ 16 ] CH 3 COO − (Ac − ), [ 17 ] + NH 3 C 4 H 9 COO − , [ 18 ] bis(trifluoromethane)sulfonimide (TFSI − ), [ 19 ] PO 4 3− , [ 13 ] CO 3 2− , [ 20 ] NO 3 − , [ 21 ] SO 4 2− , [ 13 ] etc.…”

Recent advances in semitransparent perovskite solar cells

“…The inorganic HTMs are used in the PSCs due to their excellent properties such as high hole mobility and low cost. Famous examples of HTMs are CuI, 181 CuO x, 182 CuSCN, 183 triarylamine polymer derivatives, 184 PEDOT:PSS, 185 NiO x, 186 polytriarylamine (PTAA), 187 thiophenes, 188 tetrathiafulvalene, 189 poly‐3‐hexylthiophene (P3HT), 190 spiro‐OMeTAD, 191 organometallic compounds, 192 and so on. During the operation hole transporting layer (HTL) has to facilitate the energy conversion process in different form such as high carrier mobility, long carrier lifetime, compatible valence band level between HTL and perovskite, low material and processing cost, and stability during processing.…”