Abstract

The popularity of solar cell has been on the rise over the past few years. In order to be the substitute the fossil fuels, the energy production cost from solar cells must be reduced further. A lot of effort has been made by the scientific community to produce cheap and green energy from solar cells. High absorption coefficient and cost effective deposition technique have made polycrystalline CdTe a suitable applicant for thin-film solar cell technology. On the other hand, bulk heterojunction polymer solar cells based on blends of conjugated polymers (P3HT) and fullerene derivatives (PCBM) have shown great potential to produce efficient organic solar cells.

Low availability of Te and need for enhancement of deposition speed influence to reduce the CdTe absorber layer. However, the overall cell efficiency of CdTe/CdS thin-film solar cell deteriorates unpredictably when the absorber width is reduced to sub-micron level. Deterioration of charge transport properties and their effects on voltage-dependent charge collection in submicron CdTe solar cells are believed to be responsible for this detrimental current-voltage. An analytical model for voltage-dependent photocurrent has been proposed in this thesis, considering partial depletion of the absorber and both electron and hole drift and diffusion in the depleted region and field free region, respectively, by solving continuity equations with proper boundary conditions. Exponential photon absorption and actual solar spectrum are considered to compute the photocurrent densities at entire output voltages. The results of the analytical model are verified by the 2D Taurus Medici device simulator. The model is also compared with recently published experimental results on ultra-thin CdTe solar cells in order to determine the carrier transport properties (defects and carrier lifetimes) and effective doping concentrations.

The efficiency of bulk heterojunction organic solar cells is mostly controlled by the dissociation of bound electron and hole pairs (EHPs) and charge carrier transport properties after dissociation. An analytical model has been developed considering both drift and diffusion of charge carrier to examine the relative effects of EHP dissociation and charge collection on the current-voltage characteristics. The effects of annealing on carrier transport properties are also examined. The theoretical model is also verified by the published experimental data.