For the stability and commercial development of the perovskite solar cells (PVK-SCs), synthesizing high-efficiency dopant-free hole-transport materials (DF-HTMs) and exploring how the DF-HTM structure affects the photovoltaic performance is inevitable. Two small-molecule DF-HTMs based on 2,2′-bithiophene as a central part (denoted by BT-MTP and DFBT-MTP) were designed and synthesized. DFBT-MTP, with two more fluorine atoms substituted on the 2,2′-bithiophene group, exhibited enhanced photovoltaic property as DF-HTMs, including larger backbone planarity, declining highest occupied molecular orbit (HOMO) energy level, increasing hole transportation, more effective passivation, and efficient charge extraction. With fluorinated DFBT-MTP being applied as DF-HTMs in p–i–n PVK-SCs, an efficiency of 20.2% was achieved, showing ∼35% efficiency increase compared with the nonfluorinated BT-MTP-based devices. The leading power conversion efficiency (PCE) indicates that the fluorinated compounds should be a promising direction for exploring high-performance DF-HTMs in the p–i–n PVK-SCs.

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Hole-transport materials (HTMs) play an important role in perovskite solar cells (PSCs) to enhance the power conversion efficiency (PCE). The innovation of HTMs can increase the hole extraction ability and reduce interfacial recombination. Three organic small molecule HTMs with 4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) as the central unit was designed and synthesized, namely, CPDTE-MTP (with the 2-ethylhexyl substituent and diphenylamine derivative end-group), CPDT-MTP (with the hexyl substituent and diphenylamine derivative end-group), and CPDT-PMTP (with the hexyl substituent and triphenylamine derivative end-group), which can form bifunctional and robust hole transport layer (HTL) on ITO and is tolerable to subsequent solvent and thermal processing. The X-ray photoelectron spectroscopy (XPS) results proved that CPDT-based HTMs can both interact with ITO through the nitrogen element in them and the tin element in ITO and passivate the upper perovskite layer. It is worth noting that the champion efficiency of MAPbI3 PSCs based on CPDT-PMTP achieved 20.77%, with an open circuit voltage (V OC) of 1.10 V, a short-circuit current (J SC) of 23.39 mA cm–2, and a fill factor (FF) of 80.83%, as three new materials were introduced into p–i–n PSCs as dopant-free HTMs.

To develop low-cost and highly efficient hole-transport materials (HTMs), a series of linear HTMs based on a fluorenyl core were synthesized with different alkyl side chains substituted for the middle carbon atom, from methyl (FMT-M), and 2-ethylhexyl (FMT-EH) to dodecyl (FMT-D). Together with the hexyl substituent one (FMT), the photophysical and electronic properties, and hole-mobility of the compounds were investigated to determine the influence of the alkyl side chain on the performance of the material as dopant-free HTMs in p-i-n perovskite solar cells (pero-SCs) with the structure of ITO/HTM/MAPbI 3-x Cl x /C 60 /BCP/Ag. All the linear molecules exhibit promising HTM properties including high transmittance, hole-mobility, and optimal energy level alignment with perovskites (MAPbI 3-x Cl x ). As the alkyl chain gets longer, the highest efficiency of corresponding p-i-n pero-SCs based on FMT-M, FMT-EH, FMT, and FMT-D increases from 16.68 and 17.60 to 18.57 and 19.99%, respectively. Through the investigation, it is determined that the alkyl side chain will affect the thermal stability, the melting point, and the intermolecular stacking of the linear molecules; thus, the corresponding HTMs obtain different transparency, hole-transport mobility, energy level, and film morphology, which together will affect the p-i-n pero-SCs performance.

The utilization of hole-transport materials (HTMs) in perovskite solar cells (pero-SCs) is essential for the betterment of hole extraction and hole transport, both of which are critical for enhancing efficiency. To commercialize pero-SCs, the improvement in highly efficient and cost-effective HTMs without dopants is essential. As the HTMs used in inverted pero-SCs, two dibenzothiophene (DBT)-based small linear compounds, denoted DBT-MOP (with the methoxyphenyl in the peripheral) and DBT-2F-MOP (with fluorinated methoxyphenyl in one branch), are designed and synthesized in this study. Compared to fluorinated DBT-2F-MOP, DBT-MOP shows an amorphous state, increasing hole-transport mobility, and greater charge extraction; thus, the inverted device based on DBT-MOP exhibits a remarkable efficiency of 19.5%, while the devices based on DBT-2F-MOP show an efficiency only reaching up to 15.19%. The efficiency of the DBT-MOP-based devices is among the top efficiencies of the DBT-based HTMs. However, the introduction of the fluoride atom near the methoxy group harms the enhanced efficiency, which may originate from the crystallization of DBT-2F-MOP, leading to a rugged film morphology of DBT-2F-MOP, thus further affecting the crystallization of the on-top perovskite layer.