UC Santa Cruz

Stability and reliability of solar cells are crucial for utilizing them for solar energy technology. In this dissertation work photothermal deflection spectroscopy (PDS) technique was used to detect small absorption changes and to investigate trap density changes in three different types of solar cells in the process of light, air, and temperature induced degradation. The light-induced metastable changes in the properties of amorphous silicon and crystallinity effect in microcrystalline silicon were quantified by PDS. The effect of ligands and nanoparticle (NP) size on mid-gap trap states in NP thin films (CdTe and PbS) as it impacts on the performance during degradation were examined. Finally, several most common polymers (P3HT, MEH-PPV, and Polyfluorene Red) films absorption were compared and effect of photo-degradation and photo-oxidation on their trap states were analyzed. The PDS measurement technique is independent of scattering and permits the full band gap of the solar cells to be measured as well as the Urbach energy and the density of mid-gap trap states through analysis of the band gap and the band tail absorption. This work demonstrated that the higher amount of trap states in the material do not necessary limit the efficiency of a solar cell, since material structure, crystallinity, a particle deformation, and a polymer's decomposition may have much higher effect on the solar cells' stability and performance.