P3HT-b-PS Copolymers as P3HT/PCBM Interfacial Compatibilizers for High Efficiency Photovoltaics

Abstract

To control the donor-acceptor phase separation for more efficient organic bulk heterojunction photovoltaic cells, poly(3-hexylthiophene)-block-polystyrene (P3HT-b-PS) diblock copolymer was added to serve as a compatibilizer in a P3HT/ [6,6]-phenyl-C61-butyric acid methyl ester fullerene derivative (PCBM) blend. An addition of 5 wt% of P3HT-b-PS copolymer in the P3HT/PCBM blend improved the power-conversion efficiency from 3.3% to 4.1% due to an enhancement of both the short-circuit current density and fill factor compared to that of a pristine P3HT/PCBM solar cell. Grazing incidence x-ray scattering (GIXS), absorption spectroscopy and carrier mobility studies reveal that the crystallinity and orientation of P3HT were improved, thereby enhancing hole transport in the P3HT polymer, and leading to a better balance between the electron and hole mobilities in the P3HT/PCBM active layer. Neutron reflectometry (NR) experiments demonstrate that a distinct scattering length density profile shows the highest PCBM concentration in the middle layer region and a more compact and homogeneous layer, presumably due to an increase in miscibility of P3HT and PCBM driven by the copolymer compatibilizer, while adding 5 wt% of P3HT-b-PS copolymer in the P3HT/PCBM blend. Quantum density functional theory calculations show that the P3HT-b-PS additive tends to promote microphase segregation, with the PCBM attracted to the PS block, and the P3HT stacking onto the P3HT block, which presumably leads to improvements in long-range “crystallinity”, consistent with the GIXS findings. Overall, the results for P3HT-b-PS copolymer in a P3HT/PCBM blend demonstrate that tailored block copolymers can act as an effective compatibilizer in blended systems to further improve solar cell performance