Hybrid ZnO-organic semiconductor interfaces in photodetectors: A comparison of two near-infrared donor-acceptor copolymers

Abstract

Hybrid organic-inorganic photodiode interfaces have gained significant interest due to their unique physical properties such as mechanical flexibility and high photosensitivity. Two diketopyrrolopyrrole (DPP)-based donor-acceptor copolymers with different backbone conformations are characterized in an inverted non-fullerene photodiode architecture using ZnO nano-patterned films as the electron transport layer. The DPP copolymer with a thienothiophene unit (PBDT-TTDPP) is more planar and rigid compared to the DPP system with a thiophene unit connecting the donor and acceptor moieties within the monomer (PBDT-TDPP). The hybrid interfaces were optimized by using poly(3-hexylthiophene) (P3HT) as the p-type layer for monitoring the critical thickness and morphology of the ZnO layer. The maximum photoresponsivity from a P3HT:ZnO photodiode was found to be 56 mA/W. The photoresponsivity of PBDT-TTDPP:ZnO photodiodes were found to be more than two orders of magnitude higher than PBDT-TDPP:ZnO photodiodes, which is attributed to an enhanced transport of carriers due to the planar backbone conformation of the PBDT-TTDPP copolymer. Capacitance-voltage measurements from hybrid Schottky barrier interfaces further shed light into the nature of photocarriers and device parameters. First-principles time-dependent density-functional theoretical calculations yield a higher absorptivity for the PBDT-TTDPP dimer compared to PBDT-TDPP.