Improved exciton dissociation efficiency by a carbon-quantum-dot doped workfunction modifying layer in polymer solar cells

Abstract In addition to the phase-separated morphology of donor and acceptor, the internal field created by work-function difference between cathode and anode can also influence the exciton dissociation probability. In this study, we have demonstrated enhanced photovoltaic performance by increasing exciton dissociation efficiency. To improve both work-function modification effect and charge transport properties, we have incorporated novel carbon quantum dots (CQD) having NH2 ligands into the polyethyleneimine (PEI) work-function modifying layer as a dopant. A study of net photocurrent density as a function of effective voltage showed that devices with a CQD-doped PEI layer had a much higher charge separation probability compared to devices with a pristine PEI layer. A Kelvin-probe force microscopy study demonstrated that a CQD-doped PEI layer induced lower work-function of ITO than that of ITO with a pristine PEI, which induced a stronger internal field. This strengthened internal field induced better exciton dissociation efficiency, thereby improving solar cell performance.