In the field of organic solar cells, it has been generally accepted until recently that a difference in band energies of at least 0.3 eV between the highest occupied molecular orbital (HOMO) level of the donor and the HOMO of the acceptor is required to provide adequate driving force for efficient photoinduced hole transfer due to the large binding energy of excitons in organic materials. In this work, we investigate polymeric donor:non-fullerene acceptor junctions in binary and ternary blend polymer solar cells, which exhibit efficient photoinduced hole transfer despite negligible HOMO offset and demonstrate that hole transfer in this system is dependent on morphology. The morphology of the organic blend was gradually tuned by controlling the amount of ITIC and PC 70 BM. High external quantum efficiency was achieved at long wavelengths, despite ITIC-to-PC 70 BM ratio of 1:9, which indicates efficient photoinduced hole transfer from ITIC to the donor despite an undesirable HOMO energy offset. Transient absorption spectra further confirm that hole transfer from ITIC to the donor becomes more efficient upon optimizing the morphology of the ternary blend compared to that of donor:ITIC binary blend.

Two random copolymers based on the (2,5-difluorophenylene)dithiophene and dialkoxybenzothiadiazole with benzodithiophene (P1) or thiophene (P2) as third conjugated bridges having sulfur and fluorine (S···F) and/or oxygen (S···O) non-covalent intramolecular interaction are synthesized and characterized. In despite of molecular weight difference over three times between both polymers, P1 and P2 possess similar solubility in organic solvents and thermal stability (Td~320 o C), which means probably due to that P1 with bulky alkylthiophene substituted benzodithiophene as a third conjugated bridge has less non-covalent intramolecular interaction than that of P2 with thiophene as a bridge. Both polymers were used as electron donors in bulk heterojunction organic photovoltaics (BHJ OPV) with PC71BM as an acceptor. From the photovoltaic measurements, it reveals that P2 shows higher power conversion efficiency (PCE) of up to 6.82% than that of P1 (2.44%). After 1,8-diiodooctane (DIO) treatments as a processing additive, the P1 and P2 devices show a significantly improved PCE of 5.95% for P1 and 7.71% for P2. The surface morphology analysis of the blend films using the atomic force microscope (AFM) reveals that P1:PC71BM film shows a macrophase separation, while the P2 film has a smooth morphology. After DIO treatment, morphology of both polymer blend films is improved with better bi-continuous nanosclae networks. Charge carrier mobilities through the space charge limited current (SCLC) method demonstrate that P2 with thiophene bridge has higher charge carrier mobilities than that of P1. In particular, inverted structured BHJ OPV with P2 exhibits a PCE of 8.50%, which is the highest PCE reported in the literature regarding random copolymers.

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