Development of Dopant‐Free Organic Hole Transporting Materials for Perovskite Solar Cells

Abstract

There has been considerable progress over the last decade in development of the perovskite solar cells (PSCs), with reported performances now surpassing 25.2% power conversion efficiency. Both long‐term stability and component costs of PSCs remain to be addressed by the research community, using hole transporting materials (HTMs) such as 2,2′,7,7′‐tetrakis(N,N′‐di‐pmethoxyphenylamino)‐9,9′‐spirbiuorene(Spiro‐OMeTAD) and poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine] (PTAA). HTMs are essential for high‐performance PSC devices. Although effective, these materials require a relatively high degree of doping with additives to improve charge mobility and interlayer/substrate compatibility, introducing doping‐induced stability issues with these HTMs, and further, additional costs and experimental complexity associated with using these doped materials. This article reviews dopant‐free organic HTMs for PSCs, outlining reports of structures with promising properties toward achieving low‐cost, effective, and scalable materials for devices with long‐term stability. It summarizes recent literature reports on non‐doped, alternative, and more stable HTMs used in PSCs as essential components for high‐efficiency cells, categorizing HTMs as reported for different PSC architectures in addition to use of dopant‐free small molecular and polymeric HTMs. Finally, an outlook and critical assessment of dopant‐free organic HTMs toward commercial application and insight into the development of stable PSC devices is provided.

The damaging effects of the acidity in PEDOT:PSS on semiconductor device performance and solutions based on non-acidic alternatives

“…These materials have been discussed in recent review articles. 88,89 However, the processability of such materials is limited -depositing organic solutions on top of a layer of spiro-OMeTAD would be challenging due to dissolution and interlayer mixing when solutions of common solvents are used. Therefore, the discussion is focussed on materials with solution-processing properties typically more appropriate for the majority of organic semiconductor devices.…”

Synergistic Effect of Excited State Property and Aggregation Characteristic of Organic Semiconductor on Efficient Hole‐Transportation in Perovskite Device

“…The synthetic procedures of the molecules are described in the Experimental Section, General Synthetic Procedure, Supporting Information, and Figure S1, Supporting Information. The detailed experimental information about the synthesized molecules such as 1 H NMR, 13 C NMR, and LCMS are presented in Figures S2-S9, Supporting Information. With commercially available starting materials, all molecules were synthesized in a simple two-step reaction method with high reaction yield of 72%.…”

“…In this respect, the development of novel organic HTMs, which have a facile synthesizability (e.g., short reaction steps with conventional simple materials, uncomplicated structure-tunability with various functional groups, low-cost processability, and high yield), along with excellent charge transport capability (e.g., through efficient intermolecular interactions) even without dopants, has become a crucial issue in realizing high performance and stable perovskite PV devices. [13] In addition to those inherent properties of organic HTMs, the importance of their characteristics at photo-excited state, which has been disregarded, recently begins to be recognized. [14,15] Because PV devices are always operated under a solar illumination condition, the status of organic HTM molecule at the excited state becomes effective during the device operation, and especially the transition dipole moment of organic molecule at photo-excited state was revealed to work beneficially, increasing the built-in potential of the PV devices.…”

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

“…Although many well discussed reviews on dopant-free organic HTMs have been published, [66][67][68][69] the review here will specially center on these highly efficient small molecular HTMs (SM-HTMs) with PCEs of over 15% based on the classification of their molecular structures, which is quite different from the previous ones mainly cataloging the HTMs according to their device configurations. In this review, we will first investigate the properties of the solution processed dopantfree SM-HTMs and their relation to the molecular structures.…”