Hybrid Organic-Inorganic Solar Cells

Abstract

This thesis explores the feasibility of a hybrid metal-insulator-semiconductor (MIS) diode with poly(3,4-ethylendioxythiophene) (PEDOT) conductor for photovoltaic (PV) applications and advances the understanding of conductive polymers and their effects on hybrid systems. In a proof-of-concept experiment it was demonstrated that a photosensitive MIS junction can be created on n-type silicon with thermally-grown oxide and a transparent conductive polymer, PEDOT doped with polystyrene sulfonate (PSS). The measured devices had an estimated efficiency of 1.65%. The predicted efficiency for an optimized device was estimated at nearly 12%. A variety of PEDOT formulations were explored. An in situ polymerized PEDOT dispersed in a silica matrix prepared in a sol-gel process was deemed to be the best candidate due to its superior adhesion to a SiO2 surface, compatibility with nanostructured surfaces and highly adjustable conductivity and transparency. For the first time the operation of a hybrid MIS diode with PEDOT:PSS conductor was compared to an inorganic equivalent with gold. For this purpose two sets of devices Au/SiO2/Si and PEDOT:PSS/SiO2/Si were fabricated in parallel and analyzed using a purpose-built, physics-based simulator. Measurement of these devices show lower current densities in the hybrid devices as compared to inorganic ones. The simulations reveal that the lower current density in the hybrid devices is not due to effective lowering of the work function, but rather due to the limited extent of the energy bands in the conductive polymer, PEDOT:PSS, which cannot be ignored for predicting device performance in photovoltaics as well as organic and hybrid electronics. The feasibility of nanostructuring the silicon substrate in the MIS device architecture for the purpose of increasing the photovoltaic efficiency was also investigated. The possibility of the effective increase in surface recombination due to surface nanostructuring hindering device performance was assessed. It was determined that the PV efficiency increase due to the enhanced light capture outweighs the surface effects. In the context of a hybrid MIS device, conductive polymer transparency was identified as the factor limiting potential efficiency improvement. A transparency/conductivity optimization scheme is proposed for optimizing the nanostructured hybrid MIS design.