Dual Förster resonance energy transfer effects in non-fullerene ternary organic solar cells with the third component embedded in the donor and acceptor

Abstract

Non-fullerene ternary organic solar cells (OSCs) are new promising candidates for future applications in the area of organic photovoltaics. However, their low short-circuit current (J_SC) values impede efforts at increasing their power conversion efficiency (PCE) levels. Maximizing the J_SC is one of the critical elements for enabling high performances of OSCs. To improve the J_SC of the non-fullerene ternary OSCs based on poly[[2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b:3,3-b]dithiophene][3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]:3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene (PTB7-Th:ITIC), a polymer of poly[N-9′′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) was added to the binary system. The PCDTBT was shown to embed in both PTB7-Th and ITIC, and introduced dual Förster resonance energy transfer (FRET) effects in the resulting ternary system. In addition, the PCDTBT may have decreased the molecular coherence lengths of PTB7-Th and ITIC and influenced the charge transport. Furthermore, the PCDTBT apparently also decreased the d-spacing of the π–π stacking of PTB7-Th and ITIC, which likely increased the intermolecular charge hopping efficiency. Doping an appropriate amount of PCDTBT in the system yielded an increase in the J_SC from 13.89 to 16.71 mA cm⁻². The increased J_SC resulted in a 15% enhancement of the PCE, which indicated that introducing dual FRET effects is an effective way to enhance the J_SC and thus the performance of the OSCs.