Side-Chain Polymers as Dopant-Free Hole-Transporting Materials for Perovskite Solar Cells—The Impact of Substituents’ Positions in Carbazole on Device Performance

Abstract

Side-chain polymers have the potential to be excellent dopant-free hole-transporting materials (HTMs) for perovskite solar cells (PSCs) because of their unique characteristics, such as tunable energy levels, high charge mobility, good solubility, and excellent film-forming ability. However, there has been less research focusing on side-chain polymers for PSCs. Here, two side-chain polystyrenes with triphenylamine substituents on carbazole moieties were designed and characterized. The properties of the side-chain polymers were tuned finely, including the photophysical and electrochemical properties and charge mobilities, by changing the positions of triphenylamine substituents on carbazole. Owing to the higher mobility and charge extraction ability, the polymer P2 with the triphenylamine substituent on the 3,6-positions of the carbazole unit showed higher performance with power conversion efficiency (PCE) of 18.45%, which was much higher than the PCE (16.78%) of P1 with 2,7-positions substituted. These results clearly demonstrated that side-chain polymers can act as promising HTMs for PSC applications and the performance of side-chain polymers could be optimized by carefully tuning the structure of the monomer, which provides a new strategy to design new kinds of side-chain polymers and obtain high-performance dopant-free HTMs.

Progress in Perovskite Solar Cells towards Commercialization—A Review

“…The majority of PSCs with high PCE still use conventional organic HTLs; however, the instability of the devices resulting from additives cannot be ignored. Wu et al designed and synthesized two new dopant-free polymers as HTMs by attaching a carbazole-based hole-transporting unit to a polystyrene chain ( Figure 6 e,f) [ 134 ]. The polymer P2 with the triphenylamine substituent at the 3,6-positions of the carbazole unit achieved a PCE of 18.45%.…”

“…The polymer P2 with the triphenylamine substituent at the 3,6-positions of the carbazole unit achieved a PCE of 18.45%. However, two novel side chain polymers both showed excellent long-term stability; the PSC devices without encapsulation retained more than 80% of the initial PCE after 30 days in an ambient environment with approximately 30% humidity ( Figure 6 g) [ 134 ]. The application of Li + -free hydrophobic EH44 has been used as a more reliable HTM.…”

A Review on Solution‐Processable Dopant‐Free Small Molecules as Hole‐Transporting Materials for Efficient Perovskite Solar Cells

“…Although a large amount of polymeric HTMs have achieved very high PCEs, [70][71][72][73][74] the small molecules possess much increased reproductivity on their photovoltaic performances due to the well-defined molecular weights and structures compared with polymers. Moreover, since spiro-OMeTAD is also a kind of small molecule, it is very reasonable to carry out an exploration of dopant-free candidates based on small molecules.…”

Highly Stable and Efficient Perovskite Solar Cells with 22.0% Efficiency Based on Inorganic–Organic Dopant‐Free Double Hole Transporting Layers

“…[1][2][3][4][5][6] polymers have been vigorously investigated. [20,21] Compared with the small molecules, conjugated polymers hold the advantages of better film-uniformity, high solution-process ability, and high mechanical flexibility. [22][23][24] However, to deposit a full coverage and compact film on the surface of the underlying perovskite layers, it needs to increase the thickness of the conjugated polymer films, leading to the reduction of PCEs in PSCs due to the relatively low hole mobility of conjugated polymers films without dopant.…”