The deposition and optoelectronic properties of reduced graphene oxide thin films are described. Thin films with thicknesses ranging from 1–10nm have been deposited by the vacuum filtration method. The conductivity of the thin films can be varied over six orders of magnitude by varying the filtration volume of the graphene oxide aqueous suspension while maintaining the transmittance between 60%–95%. In addition, enhancement in the conductance through Cl doping is demonstrated. The combination of the reduction and Cl treatments make the reduced graphene oxide thin films sufficiently conducting to incorporate them as the hole collecting electrode in proof of concept organic photovoltaic devices.

Improvement of the photovoltaic efficiency via exposure of organic poly(3-hexylthiophene) (P3HT) and phenyl-C₆₁-butyric acid methyl ester (PCBM) devices to solvent vapor at room temperature is reported. In situ photoluminescence (PL) and Raman spectroscopy, in conjunction with ex situ optical absorption and atomic force microscopy, have been used to provide insight into the nanoscale morphological changes occurring during solvent vapor annealing. We found that in 1 : 1 composites of P3HT : PCBM, suppression of PL, narrowing in line-width of the 1442 cm⁻¹ P3HT Raman peak, and strong modifications in the optical absorption spectra were observed during solvent vapor annealing, while minimal changes occurred in pure P3HT films. We attribute these spectral modifications to de-mixing of PCBM and subsequent stacking of P3HT in coplanar conjugated segments, similar to what is observed during thermal annealing.

Organic photovoltaics (OPVs) have developed rapidly in the last decades due to their potential for providing cost-efficient, low-energy consumption, and environmentally friendly renewable energy sources. Some research reports have focused on the device physics of organic photovoltaics that governs open circuit voltage (V_oc) and short circuit current (J_sc) to improve their performance. In this review, we focus on the third parameter, fill factor (FF), that is equally important in determining the power conversion efficiency. We discuss the mathematical calculation of the FF and the relationship between the FF and equivalent circuit model elements, namely, shunt resistance, series resistance, and diode ideal factor. In order to provide a strategy toward a high FF for OPVs from the viewpoints of device design and material synthesis, we review important device features and BHJ features that have a large impact on the device FF, including preventing shorting, buffer layer design, domain size or purity, gradated BHJ structures, π–π stacking distance or direction, etc. We hope this article can provide a comprehensive insight into elements controlling the FF of OPVs and give a valuable direction for better device and material design.