Influence of the backbone structure of the donor material and device processing conditions on the photovoltaic properties of small molecular BHJSCs

Small molecule donor materials, ICT18, ICT19 and ICT20 are synthesized to explore the effect of donor material architecture/backbone length and nature of central donor moiety on their thermal, optical, electronic and photovoltaic properties. Compared to ICT18 (epsilon, 0.83 x 10(5) mol(-1) cm(-1)), ICT19 (2.26 x 10(5) me(-1) cm(-1)) has higher light harvesting ability indicating that higher conjugation length is essential for better exciton generation and is further enhanced by replacing the BDT unit of ICT19 with DTP unit of ICT20 (2.50 x 10(5) mol(-1) cm(-1)). Thermal stability of these materials is enhanced and their HOMO level is destabilized with an increase in their conjugation length. BHJSCs are fabricated using these materials as donor and PC71BM as acceptors. Under the optimized conditions, BHJSCs with ICT18, ICT19 and ICT20 donors show PCE of 5.69, 6.92 and 8.13%, respectively. ICT19 and ICT20 with large conjugation length and symmetrical A-D-1-D-2-D-1-A architecture show higher hole conductivity over ICT18 with low conjugation length and unsymmetrical D-2-D-1-A structure. Photo transient experiments are performed to understand the excited state dynamics of the synthesized materials. The enhancement in the PCE is due to higher light harvesting ability of the active layer with efficient and balanced charge transport and effective exciton dissociation at the donor-acceptor interface.