Spin‐Coated Small Molecules for High Performance Solar Cells

Organic solar cells (OSCs) have attracted signifi cant attention as a clean and competitive renewable energy source due to their attractive features such as low-cost, light weight, solution processability and high mechanical fl exibility. [ 1–4 ] Benchmark power conversion effi ciencies (PCEs) of 10% or higher have been predicted if a suitable low bandgap donor material can be designed and implemented. [ 5 ] More recently, bulk heterojunction (BHJ) OSCs using solution-processed small molecules as the donor have attracted great attention. [ 6–8 ] This long-time but recently increased interest lies in the fact that solutionprocessed small molecule based OSCs have numerous advantages, such as relatively simple synthesis and purifi cation methods, monodispersity and well defi ned structures, high open circuit voltage and charge carrier mobilities, and better batch-to-batch reproducibility. [ 6–8 ] However, solution-processed small molecule OSCs have not met such high expectations as those of their polymeric counterparts due to their limited PCEs. In most cases, small molecule devices using solution processing always seem to have poorer fi lm quality than that of their polymeric counterparts in BHJ OSCs. [ 6 , 7 ] It is thus expected that better PCE could be achieved if the intrinsic poor fi lm quality and morphology in BHJ architecture could be improved. However, in order to achieve this, careful molecule design has to be carried out to address many factors simultaneously, including the material’s solar light absorption associated with its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) positions, mobility, fi lm forming quality, electronic band structure and morphology compatibility with the acceptors, and so on. Indeed, several families of solution processed small molecules, such as oligothiophenes, [ 9 ]