Exploring Deep and Shallow Trap States in a Non-Fullerene Acceptor ITIC-Based Organic Bulk Heterojunction Photovoltaic System

Abstract

Nonfullerene organic molecules that usually act as acceptors have exhibited prominent optoelectronic properties in organic bulk heterojunction (BHJ) solar cells. Although the highly efficient exciton dissociation can happen even with a relatively smaller driving force, the underlying energy loss including exciton dissociation and electron–hole recombination in the nonfullerene acceptor (NFA) system remains unclear. In principle, the energetic disorder characteristic is a common feature in organic semiconductors as well as their organic blends due to the molecular random distribution and the donor–acceptor interpenetration network. Owing to their completely different molecular structures in contrast to the bulky-ball shaped fullerene-derivative counterpart such as PCBM, a comprehensive understanding for the trap-associated energetic disorder characteristic and distribution are merits to further eliminate the open-circuit voltage (Voc) loss. In this work, a highly efficient planar organic BHJ solar cell comprising ITO(glass)/ZnO/PBDB-T:ITIC/MoO3/Al was fabricated. Both the polymeric donor PBDB-T and NFA ITIC form an interpenetrating network. With an assist from the nondestructive impedance spectroscopic technique, surface and bulk trap states can be fully revealed. More importantly, both of them respectively contain deep and shallow traps with different energetic locations in an energy gap. The results help to explain the appearance of double and triple capacitance–voltage (C–V) bands. Furthermore, with pump–probe laser spectroscopic measurements, the trap states may locate far below the photoabsorption band-edge. This work serves an insightful view on the trap states in the NFA-based organic BHJ system, and it is valuable for minimizing the energy loss.