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 (Delta E-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 Delta E-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 Delta E-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 Delta E-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.