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 (Egopt), 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-C 70-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 (Egopt = 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 Egopt below 1.4 eV. The optoelectronic properties and photovoltaic device performance for a series of low band gap donor-acceptor polymers based upon dithienogermole are reported. One very low band gap polymer, PDTG-PT, (Egopt = 1.32 eV) is shown to exhibit a promising device efficiency of 6.6% when utilized in inverted photovoltaic devices, making it a promising candidate for incorporation in tandem solar cell devices. © 2013 The Authors. Advanced Functional Materials published by Wiley-VCH Verlag GmbH 8 Co. KGaA Weinheim.