Improved Charge Generation via Ultrafast Effective Hole‐Transfer in All‐Polymer Photovoltaic Blends with Large Highest Occupied Molecular Orbital (HOMO) Energy Offset and Proper Crystal Orientation

Improved charge generation via fast and effective hole transfer in all‐polymer solar cells (all‐PSCs) with large highest occupied molecular orbital (HOMO) energy offset (ΔE_H) is revealed utilizing ultrafast transient absorption (TA) spectroscopy. Blending the same nonfullerene acceptor poly{[N,N′‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene) (N2200) with three different donor polymers produces all‐polymer blends with different ΔE_H. The selective excitation of N2200 component in blends enables to uncover the hole transfer process from hole polaron‐induced bleaching and absorption signals probed at different wavelength. As the ΔE_H is enhanced from 0.14 to 0.37 eV, the hole transfer rate rises more than one order and the hole transfer efficiency increases from 12.9% to 86.8%, in agreement with the trend of internal quantum efficiency in the infrared region where only N2200 has absorption. Additionally, Grazing‐incidence wide‐angle X‐ray scattering measurements indicate that face‐on crystal orientation in both polymer donor and acceptor also plays an important role in facilitating the charge generation via hole transfer in all‐PSCs. Hence, large ΔE_H and proper crystal orientation should be considered in material design for efficient hole transfer in N2200‐based heterostructures. These results can provide valuable guidance for fabrication of all‐PSCs to further improve power conversion efficiency.