Nivedita Yumnam

• Organic solar cells have the potential to meet the rising energy demand. They offer great prospects due to inexpensive processing techniques. They can be fabricated at low temperatures, which makes them compatible for large scale roll-to-roll printing. Despite high absorption coefficients of organic semiconductors, there is significant absorption loss. ZnO nanorods can scatter incoming light and elongate optical path length, thereby enhancing absorption in semiconductor layer. Secondly, ZnO nanorods can serve as charge collection pathways to electrodes. For light scattering, it is critical to control dimensions and distribution of ZnO nanorods. In this thesis, a method is introduced to achieve controlled growth of ZnO nanorods via electrochemical deposition. Self-assembled monolayer (SAM) of alkanethiol molecule is applied on substrate to define ZnO nucleation sites. A growth model of ZnO on SAM-modified substrate is developed and it is supported by impedance measurements. The dimensions and nucleation density of nanorods are varied by changing quality of SAM and parameters of electrochemical deposition. Angle-resolved transmission measurements are performed to study light scattering properties of ZnO nanorods. Bulk heterojunction organic solar cells are successfully fabricated with ZnO nanorods and semiconductors poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Charge carrier mobility plays a big role in organic semiconductors. MIS-CELIV (metal-insulator-semiconductor charge carrier extraction by linearly increasing voltage) technique is used to get selective mobility of holes and electrons in semiconductor blend of P3HT:PCBM. A simulation method based on 1-D drift-diffusion equation is used to analyze MIS-CELIV measurements. This method includes doping and trapping, which have been ignored in traditional MIS-CELIV analysis. An analytical model is also introduced which allows determination of semiconductor conductivity.