Influence of poly3-octylthiophene (P3OT) film thickness and preparation method on photovoltaic performance of hybrid ITO/CdS/P3OT/Au solar cells

Chemically synthesized poly3-octylthiophene (P3OT) solution was spin-coated (SC) or drop-cast (DC) on cadmium sulfide thin film to form a hybrid junction of transparent conductive glass (indium-tin oxide (ITO))/CdS/P3OT/Au. XRD patterns of DC P3OT films show a couple of peaks at 2θ between 8° and 13°. It suggests that a long-range order may exist in the slowly dried DC P3OT films. Optical absorption coefficient spectra of P3OT films exhibit the presence of polaron bands in P3OT films with thickness lower than 700 nm, which indicates that thin P3OT films are susceptible to oxygen doping. The microscopically more ordered DC P3OT films show a coplanar photoconductivity, which was absent in the SC P3OT ones. The effect of P3OT film preparation method and ambient conditions was also notable on the photovoltaic performance of CdS/P3OT-based solar cells. The potential barrier of the CdS/P3OT heterojunctions decreased as the P3OT film thickness reduced from 671 to 106 nm, which was related to the diffusion of oxygen into the polymer film. As the P3OT film thickness increased to more than 1000 nm, the open-circuit voltage (VOC) reached to around 1 V, close to the value suggested by fitting the current–voltage curves with the Schottky barrier diode model. But at the same time, the short-circuit current density (JSC) of the same cells is lowered due to the increasing charge recombination effect inside the polymer material. The simple and economic drop-cast preparation method is promising for polymer film deposition in hybrid solar cell fabrication.