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While typical perovskite solar cells (PSCs) with doped Spiro-OMeTAD as a hole transport material (HTM) have shown rapid increase in their power-conversion efficiencies (PCEs), their poor stability remains a big concern as the dopants and additives used with Spiro-OMeTAD have a strong tendency to diffuse into and degrade the perovskite active layer under normal operating conditions. Aiming to push forward the development of PSCs, many dopant-free small-molecular HTMs have been reported based on energetic considerations for charge transfer and criteria for charge transport. However, the PCEs of the state-of-the-art PSCs with dopant-free small-molecular HTMs are still inferior to those using doped Spiro-OMeTAD, and little attention has been paid to the interactions between the HTM and perovskite absorber in PSCs. Here, we report a facile design concept to functionalize HTMs so that they can passivate perovskite surface defects and enable perovskite active layers with lower density of surface trap states and more efficient charge transfer to the hole transport layer. As a consequence, perovskite solar cells with a functionalized HTM exhibit a champion PCE of 22.4%, the highest value for PSCs using dopant-free small molecular HTMs to date, and substantively improved operational stability under continuous illumination. With a T 80 of (1617 ± 7) h for encapsulated cells tested at 30 °C in air, the PSCs containing the functionalized HTM are among the most stable PSCs using dopant-free small-molecular HTMs. The effectiveness of our strategy is demonstrated in PSCs comprising both a state-of-the-art MA-free perovskite and MAPbI, a system having more surface defects, and implies the potential generality of our strategy for a broad class of perovskite systems, to further advance highly efficient and stable solar cells. 

Organic semiconductors (OSCs) have shown great promise in a variety of applications. Although solution processing of OSCs has resulted in high‐quality films, exquisite control of structural development to minimize defect formation during large‐scale fabrication remains formidable. Compounding this challenge is the use of halogenated organic solvents, which poses significant health and environmental hazards. However, the solvent‐free techniques introduced thus far impose additional limitations on solidification kinetics; the resulting OSC thin films are often more defective than those processed from solution. Here, a solvent‐free technique is reported to prepare OSC membranes with centimetric crystalline domains. Leveraging the tendency for liquid crystalline materials to preferentially orient, OSCs are “prealigned” by depositing them from the melt over a metal frame to form a freely suspended membrane. Crystallization from this prealigned phase affords membranes with unprecedented structural order across macroscopic distances. Field‐effect transistors comprising membranes of dioctyl[1]‐benzothieno[3,2‐b][1]benzothiophene (C8BTBT) and didodecyl[1]‐benzothieno[3,2‐b][1]benzothiophene (C12BTBT) having centimeter‐sized domains as active layers exhibit a hole mobility of ≈8.6 cm²V⁻¹s⁻¹, superseding the mobility of any transistors whose active layers are deposited from melt. This technique is scalable to yield membranes with large crystalline domains over wafer dimensions, making it amenable for broad applications in large‐area organic electronics.

 

The mechano‐electrical properties of poly(3‐hexylthiophene) thin films are investigated as a function of their tie‐chain content. Tie chains play an indispensable role in enabling strain‐induced structural alignment and charge‐transport enhancement in the strain direction. In the absence of sufficient tie chains, the external mechanical force cannot induce any significant polymer backbone alignment locally or crystallite reorientation at the mesoscale. These samples instead undergo brittle fracture on deformation, with cracks forming normal to the direction of strain; charge transport in this direction is hindered as a consequence. This mechanistic insight on strain alignment points to the promise of leveraging tie‐chain fraction as a practical tuning knob for effecting the mechano‐electrical properties in conjugated polymer systems.