Efficiency improvement of polymer solar cells by iodine doping

A series of poly(3-hexylthiophene) (P3HT)/(6,6)-phenyl C60 butyric acid methyl ester (PCBM) bulk hetero-junction polymer solar cells were fabricated with different iodine (I2) doping concentrations. The short circuit current density (Jsc) was increased to 8.7 mA/cm2 from 4 mA/cm2, meanwhile the open circuit voltage (Voc) was decreased to 0.52 V from 0.63 V when the iodine doping concentration is 5%. The optimized power conversion efficiency of polymer solar cells (PSCs) with iodine doping is about 1.51%, which should be attributed to the better charge carrier transport and collection, and the more photon harvesting due to the red shift of absorption peaks and the widened absorption range to the longer wavelength. The morphology and phase separation of polymer thin films were measured by atomic force microscopy (AFM). The phase separation of P3HT and PCBM has been distinctly increased, which is beneficial to the exciton dissociation. The photocurrent density of PSCs with iodine doping was increased compared with the PSCs without iodine doping under the same effective voltage.

► Power conversion efficiency of P3HT:PCBM organic solar cells was increased through optimal iodine doping concentration.
► Short circuit current density was increased to 8.7 mA/cm2 from 4 mA/cm2 when iodine doping concentration is 5%.
► These improvements can be attributed to the increase of absorption intensity and range, and better charge carrier transport and collection induced by iodine doping.
► A series of characterizations were carried out to clarify the function of iodine doping in the polymer solar cells.