Organic photovoltaics: An investigation of interface layers and space-charge effects

The research described in this thesis includes interface study of organic photovoltaics, and the study of the space-charge effect on organic electronics from a photonics perspective.

First, dynamic spin coating was used as a deposition technique in the fabrication of p-n bilayer organic photovoltaics, solvents used in the fabrication were carefully selected to generate different p-n interface morphologies from low level inter-digitated to flat, and it was found that the interface morphology can significantly affect the device performance.

Stability optimization was carried out by using electro-polymerized PEDOT:PSS (EPEDOT:PSS) as a replacement for the spin coated PEDOT:PSS (SPEDOT:PSS) as hole transport layer (HTL). The X-ray photoelectron spectroscopy study shows a decrease of PSS to PEDOT ratio as well as a lower Indium atomic concentration in the electro-polymerized film. The device fabricated with EPEDOT:PSS showed an appreciable improvement in the stability property.

The highly conductive PEDOT:PSS was designed to replace the Indium Tin Oxide (ITO)/PEDOT:PSS junction as the new anode to make the device fully flexible. The ITO-free device showed higher power conversion efficiency than the ITO-based device. However, the light-intensity study suggests that due to the less anode conductivity in the ITO-free device, more space-charge effects can be observed and in turn causes a higher bimolecular recombination rate in the flexible device.

Using the flexible substrates with alternative anodes, examining their current voltage performance under different light intensity, the correlation of open circuit voltage (VOC) with anode transport property was studied. A simple model on VOC as a function of space charge density and photo-generated charge density was applied and successfully explained how space-charge affects the changing trend of VOC.