Capturing Mobile Lithium Ions in a Molecular Hole Transporter Enhances the Thermal Stability of Perovskite Solar Cells

Abstract

A thermally stable perovskite solar cell (PSC) based on a new molecular hole transporter (MHT) of 1,3‐bis(5‐(4‐(bis(4‐methoxyphenyl) amino)phenyl)thieno[3,2‐b]thiophen‐2‐yl)‐5‐octyl‐4H‐thieno[3,4‐c]pyrrole‐4,6(5H)‐dione (coded HL38) is reported. Hole mobility of 1.36 × 10−3 cm2 V−1 s−1 and glass transition temperature of 92.2 °C are determined for the HL38 doped with lithium bis(trifluoromethanesulfonyl)imide and 4‐tert‐butylpyridine as additives. Interface engineering with 2‐(2‐aminoethyl)thiophene hydroiodide (2‐TEAI) between the perovskite and the HL38 improves the power conversion efficiency (PCE) from 19.60% (untreated) to 21.98%, and this champion PCE is even higher than that of the additive‐containing 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐MeOTAD)‐based device (21.15%). Thermal stability testing at 85 °C for over 1000 h shows that the HL38‐based PSC retains 85.9% of the initial PCE, while the spiro‐MeOTAD‐based PSC degrades unrecoverably from 21.1% to 5.8%. Time‐of‐flight secondary‐ion mass spectrometry studies combined with Fourier transform infrared spectroscopy reveal that HL38 shows lower lithium ion diffusivity than spiro‐MeOTAD due to a strong complexation of the Li+ with HL38, which is responsible for the higher degree of thermal stability. This work delivers an important message that capturing mobile Li+ in a hole‐transporting layer is critical in designing novel MHTs for improving the thermal stability of PSCs. In addition, it also highlights the impact of interface design on non‐conventional MHTs.

Halogen‐Bonded Hole‐Transport Material Suppresses Charge Recombination and Enhances Stability of Perovskite Solar Cells

“…It is worth noting that we chose spiro-OMeTAD as reference because of its wide use, however other HTMs enabled reaching high efficiency, sometimes even without the use of dopants. [45][46][47][48][49][50] We believe our own concept of halo-functional HTM could be further developed to achieve even higher efficiencies, ideally without the need of doping. Therefore, with this work we wish to show the advantages of this kind of HTM and we hope to pave the way to a new set of halo-functional materials leading to high efficiency and stable devices.…”

Crowning lithium ions in hole transport layer toward stable perovskite solar cells

“…In sharp contrast, Li + ions primarily located within the target HTL and a relatively small portion of Li + ions are diffused out and detected at the SnO 2 layer, indicating the ion migration has been greatly suppressed. 17 The Li + ion signal intensity over the perovskite range in the target device is a little higher than that for the control, which is due to the migration of Li(12-crown-4) + complexes into the inner part of perovskite lm where the 12crown-4 has a high a nity to Li + ions.…”

“…11 Thus far, considerable efforts have been devoted to optimizing the HTL of PSCs for enhanced PCE and stability. [12][13][14] The research community is actively searching for equivalent substitutes [15][16][17][18] to Spiro-OMeTAD for simple synthesis and cost reduction. Notwithstanding the recent success, LiTFSI is essential to these newly synthesized HTLs and Spiro-OMeTAD still represents the best choice for PSCs at the current stage owing to its high compatibility with dopants and energy band matching with perovskite.…”

Crowning Lithium Ions in Hole‐Transport Layer toward Stable Perovskite Solar Cells

“…In sharp contrast, Li + ions primarily located within the target HTL and a relatively small portion of Li + ions are diffused out and detected at the SnO 2 layer, indicating the ion migration has been greatly suppressed. [17] The Li + -ion signal intensity over the perovskite range in the target device is a little higher than that for the control, which is due to the migration of Li(12-crown-4) + complexes into the inner part of perovskite film where the 12-crown-4 has a high affinity to Li + ions.…”

“…[11] Thus far, considerable efforts have been devoted to optimizing the HTL of PSCs for enhanced PCE and stability. [12][13][14] The research community is actively searching for equivalent substitutes [15][16][17][18] to Spiro-OMeTAD for simple synthesis and cost reduction. Notwithstanding the recent success, LiTFSI is essential to these newly synthesized HTLs and Spiro-OMeTAD still represents the best choice for PSCs at the current stage owing to its high compatibility with dopants and energy band matching with perovskite.…”