This paper presents a detailed study on the role of various annealing treatments on organic poly(3-hexylthiophene) and [6]-phenyl-C 61 -butyric acid methyl ester blends under different experimental conditions. A combination of analytical tools is used to study the alteration of the phase separation, structure and photovoltaic properties of the P3HT:PCBM blend during the annealing process. Results showed that the thermal annealing yields PCBM ''needle-like'' crystals and that prolonged heat treatment leads to extensive phase separation, as demonstrated by the growth in the size and quantity of PCBM crystals. The substrate annealing method demonstrated an optimal morphology by eradicating and suppressing the formation of fullerene clusters across the film, resulting in longer P3HT fibrils with smaller diameter. Improved optical constants, PL quenching and a decrease in the P3HT optical bad-gap were demonstrated for the substrate annealed films due to the limited diffusion of the PCBM molecules. An effective strategy for determining an optimized morphology through substrate annealing treatment is therefore revealed for improved device efficiency.
IntroductionOrganic photovoltaic (OPV) solar cells have attracted significant attention due to their great potential for large-area, light-weight, flexible, and low-cost devices [1][2][3][4]. Recently, OPV research has been dominated by the poly(3-hexylthiophene):[6]-phenyl C 61 -butyric acid methyl ester (P3HT: PCBM) blend reaching power conversion efficiencies (PCEs) of 5-7 % [5,6]. Most recently a record PCE of about 8.13 and 8.5 % has been reported [7, 8]. Despite recent achievements, improvement in PCEs, stability and lifetime of the devices are necessary before organic solar cells become commercially viable [9][10][11]. The PCE of the solar cells based on the P3HT/fullerene system depends strongly on the processing conditions [12] and can be improved by increasing the crystalline content of the P3HT. Different approaches have been suggested for the optimization of the morphology, such as thermal annealing [5,6,[13][14][15]