Enhanced performance of pseudo-bilayer organic photovoltaic devices via small molecule doping

Controlling both the film crystallinity of the active layer for better charge transport and the interdiffusion between donor and acceptor materials for optimal bicontinuous networks is essential in producing pseudo-bilayer polymer solar cells. In this work, we investigated the influence of a doping solution-processable small molecule with high carrier mobility, 5,11-bis(triethylsilylethynyl) anthradithiophene (TES-ADT), on the performance of pseudo-bilayer polymer solar cells made of an underlayer of poly(3-hexylthiophene) (P3HT) and an upper layer of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). By analysis of the X-ray diffraction and UV-vis absorbance spectra of P3HT:TES-ADT blend films it was demonstrated that the film crystallinity was enhanced by TES-ADT doping in the P3HT underlayer. The hole mobility extracted from the current density-voltage curves of hole-only devices based on P3HT:TES-ADT demonstrated an optimized value with proper TES-ADT doping and thermal annealing. An intermixed photoactive layer was observed for the annealed device, indicating the occurrence of interdiffusion with a large interfacial area. With improved film crystallinity and interdiffusion, the optimal device performance was obtained when 5% TES-ADT was blended with P3HT and a thermal annealing treatment at 150 °C for 1 min was conducted. At that optimal condition, the mean crystallite size was increased by 35%, and hence the enhancement of 8% and 14% in power conversion efficiency and short-circuit current density was observed, respectively.