Influence of the Electron Deficient Co‐Monomer on the Optoelectronic Properties and Photovoltaic Performance of Dithienogermole‐based Co‐Polymers

A series of donor–acceptor (D–A) conjugated polymers utilizing 4,4‐bis(2‐ethylhexyl)‐4H‐germolo[3,2‐b:4,5‐b′]dithiophene (DTG) as the electron rich unit and three electron withdrawing units of varying strength, namely 2‐octyl‐2H‐benzo[d][1,2,3]triazole (BTz), 5,6‐difluorobenzo[c][1,2,5]thiadiazole (DFBT) and [1,2,5]thiadiazolo[3,4‐c]pyridine (PT) are reported. It is demonstrated how the choice of the acceptor unit (BTz, DFBT, PT) influences the relative positions of the energy levels, the intramolecular transition energy (ICT), the optical band gap (E_g^opt), and the structural conformation of the DTG‐based co‐polymers. Moreover, the photovoltaic performance of poly[(4,4‐bis(2‐ethylhexyl)‐4H‐germolo[3,2‐b:4,5‐b′]dithiophen‐2‐yl)‐([1,2,5]thiadiazolo[3,4‐c]pyridine)] (PDTG‐PT), poly[(4,4‐bis(2‐ethylhexyl)‐4H‐germolo[3,2‐b:4,5‐b′]dithiophen‐2‐yl)‐(2‐octyl‐2H‐benzo[d][1,2,3]triazole)] (PDTG‐BTz), and poly[(4,4‐bis(2‐ethylhexyl)‐4H‐germolo[3,2‐b:4,5‐b′]dithiophen‐2‐yl)‐(5,6‐difluorobenzo[c][1,2,5]thiadiazole)] (PDTG‐DFBT) is studied in blends with [6,6]‐phenyl‐C₇₀‐butyric acid methyl ester (PC₇₀BM). The highest power conversion efficiency (PCE) is obtained by PDTG‐PT (5.2%) in normal architecture. The PCE of PDTG‐PT is further improved to 6.6% when the device architecture is modified from normal to inverted. Therefore, PDTG‐PT is an ideal candidate for application in tandem solar cells configuration due to its high efficiency at very low band gaps (E_g^opt = 1.32 eV). Finally, the 6.6% PCE is the highest reported for all the co‐polymers containing bridged bithiophenes with 5‐member fused rings in the central core and possessing an E_g^opt below 1.4 eV.