Synthetic Approach to Achieve a Thin-Film Red-Selective Polymer Photodiode: Difluorobenzothiadiazole-Based Donor-Acceptor Polymer with Enhanced Space Charge Carriers

Abstract

We suggest a synthetic strategy to accelerate charge collection narrowing (CCN) of a polymer photodiode so that a well-defined color selectivity can be realized even with a thin film thickness, which is the most important requirement for commercial application of polymer photodiodes. A new polymer semiconductor POFPhDT2FBT is synthesized by copolymerizing a difluorobenzothiadiazole acceptor with a difluorinated donor moiety. A polymer photodiode with a structure of indium tin oxide/ZnO/POFPhDT2FBT:PC70BM bulk-heterojunction (BHJ) (550 nm)/MoO3/Ag exhibits a high peak detectivity of ∼6 × 1012 jones at 650 nm with a narrow full width at half-maximum <80 nm as well as a low noise equivalent power (4.83 × 10-14 W Hz-0.5), implying that CCN is achieved. This result is valuable as usually a BHJ film with a thickness of 550 nm exhibits a detectivity spectrum similar in shape to the absorption spectrum. We attribute the origin of the thin-film CCN in the POFPhDT2FBT:PC70BM BHJ to a weakened intramolecular charge transfer between the donor and acceptor of POFPhDT2FBT, leading to near-isotropic molecular orientations with a short-range π-π ordering, as confirmed by grazing-incidence X-ray diffraction. In other words, in the case of POFPhDT2FBT, a substantial number of photogenerated charges can remain as space charge carriers, thus enabling the CCN without using a thickness larger than 1 μm. This study shows the possibility of efficient realization of color selectivity of a polymer photodiode even at a film thickness of 550 nm of the active layer, which can contribute to the improvement of the integration of organic image sensors in the near future. Copyright © 2018 American Chemical Society.