Optimization and characterization of Organic Solar cells based on Regio-P3HT and C66-PCBM blends

Bulk Hetero-Junction - (BHJ) - Organic photovoltaic (OPV) cells based on the conjugated polymer poly (3-hexylthiophene-2, 5- diyl) (region-P3HT) also referred to as P3HT and a fullerene derivative [6, 6] Phenyl-C61-butyric acid methyl ester (C60-PCBM) also referred to as PCBM. The optimization, realization and characterization of these cells were done on both glass and aluminium substrates. The aluminium provided a flexible substrate for the cells whilst the glass provided a rigid substrate and acted as a control. The thin films for the solar cells were deposited by means of the spin coating method.
The integrity of the starting materials were evaluated using Fourier Transform infra-red (FTIR) and x-ray diffraction (XRD) spectroscopy whilst the particle size and distribution, were examined using XRD spectroscopy and transmission electron microscopy (TEM). The electrical characterization was done using a 4-point probe and gas sensing methods. The morphology and roughness of the local composition of TiO2 thin films before and after annealing were evaluation by means of atomic force microscopy (AFM).
The study eventually evaluated the effects of blend ratio and annealing temperature on photo-absorption of the blends. Blend ratios examined in the study fell within the range of 1:0.8 to 1:1 by weight of P3HT to PCBM. The range was divided into 11 intervals with a step increment of 0.02 for PCBM while P3HT was kept constant. The optimum blend ratio was determined and found to be 1:0.96 at 130 oC annealing temperature and duration of 30 minutes. The annealing temperature dependence of each blend ratio with respect to photon-absorbance was found not to be symmetric but random. Each blend ratio was found to have own optimum annealing temperature.
The efficiency of the cells was enhanced by the use of TiO2 thin film as a back reflector and optical spacer on aluminium substrate. The study compared the performance of cells on glass substrate to those on aluminium substrate. The cells built on aluminium substrate showed a power conversion efficiency of 2.8 % whilst those on glass substrate showed 1.9 % efficiency.
The main objective of the study was to determine the optimum blend ratio, the optimum annealing temperature, and to eventually build solar cells from the optimized parameters.