Abstract
Harnessing electrical energy from solar radiation using photovoltaic (PV) technology promises to be an essential component of future global energy production. For PV devices to become a sustainable energy source that can supply our daily energy needs, it must be made economically competitive with fossil fuels and other emerging renewable energy technologies. Semiconducting polymers and small molecule organic materials offer an alternative means of fabricating low cost organic solar cells. But these photoactive organic materials are characterised by large optical band gaps (1.5 eV-3.0 eV), low photo absorption and low charge carrier mobility (mu). These imply low radiation to electrical power conversion efficiencies (eta) in such devices. It is well known that using poly (3,4-ethylenedioxythiophene) poly (styrene sulfonate) (PEDOTrPSS) with higher work function (5.1 eV) as a buffer layer facilitates hole extraction and converts blocking contacts into ohmic by modifying the surface of ITO (4.7 eV). In this thesis, we present and discuss the enhancement of the power conversion efficiency of organic solar cells through nano-imprinting of geometrical patterns into the PEDOT:PSS layer. Poly (3-hexylthiopene-2,5-diyl) (P3HT)/[6,6]-phenyl-C61-butyric acid ester (PCBM) based solar cells were fabricated and the PEDOT:PSS buffer layers between the photoactive material and the ITO anode were modified through patterning by soft lithography. Poly (dimethylsiloxane) (PDMS) was used to make the stamps for the imprinting and two approaches of soft lithography; the "Lift-up" and "put-down" processes were used. Several bulk heterojunction organic solar cells were fabricated at different stamping temperatures to investigate the influence of temperature on the stamped features as it affects the device performance. One of the device parameters that significantly limit the efficiency of polymer/fullerene-based solar cells is the open-circuit voltage (Voc), which does not obey the metal-insulator-metal principle due to its complicated characteristics. The results show that modifying the PEDOT:PSS anode buffer layer enhances the short circuit current density, the open circuit voltage and the fill factor of the device. The efficiency of polymer PVs have been improved by reducing the device series resistance and increasing its shunt resistance through patterning. This modification can be helpful when optimizing devices. Further studies on the optical and chemical modifications of the device interfaces provided insights to additional effects due to patterning and helped to further explain the hypothesis made from these experiments. Application of these findings may eventually lead to fabrication of large area, flexible and cheap organic solar cells.