Origin of Electric Field Dependence of Charge Generation in Organic Photovoltaics with Planar and Bulk Heterojunctions

Efficient, electric-field independent charge generation in the photo-electron conversion process in organic photovoltaics (OPVs) leads to high short-circuit current (<i>J</i>_SC) and fill factor (FF). However, the origin of the electric field-dependent charge generation in OPVs is still unclear.<br/> In this study, we fabricated bulk- and planar-heterojunction (BHJ and PHJ) type OPVs with the same donor (PM6) and acceptor (Y6) materials and investigated their charge generation process. The state energies of the singlet excited (S₁) and charge transfer (CT) states were experimentally quantified. In addition, the molecular orientation was determined by variable angle spectroscopic ellipsometry (VASE).<br/> The charge generation process in the BHJ is electric-field independent, while that in the PHJ is strongly electric-field dependent. The state energy offset between S₁ and CT states in the PHJ is 70 meV smaller than that in the BHJ, indicating that insufficient energy offset is one of the origins of the electric-field dependent charge generation in the PHJ¹. We also fabricated the PHJ with acceptors exhibiting different molecular orientations to figure out the effect of the relative molecular orientation between the acceptor and the donor domain on the charge generation process. We observed that the acceptor with face-on orientation to the donor domain showed field-dependent generation, whereas that with end-on (the direction of the end edge of the molecular long axis) orientated one had field-independent generation. The results suggest that the energy difference between S₁ and CT states and the molecular orientation in the BHJ film must be considered to realize high-performance OPV devices.