Developing novel hybrid heterojunctions for high efficiency photovoltaics
Date
2014
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Photovoltaic (PV) devices hold great promise for the future of renewable energy--especially with an ever increasing societal demand for electricity. In order for PV to be able to compete with incumbent energy sources it must be financially competitive. With today's state-of-the-art technology, high efficiency devices reduce balance of system costs, but this also comes with a higher initial cost. In order to make PV competitive, then, the trend between high efficiency and high cost must be broken to introduce high efficiency devices that are also low cost. In order to address this issue, work on hybrid organic/inorganic PV devices will be presented here. This work begins with the concept of the induced junction--or heterojunction--device. A layer of amorphous material is deposited on a crystalline substrate. This amorphous material performs two functions: passivating the substrate and causing band bending at the crystal surface. Traditionally, this device has used amorphous silicon deposited on crystalline silicon (c-Si), with impressive results. In this work, the amorphous Si is replaced with an organic layer to perform the same device functionality. The organic compound quinhydrone (QHY) is a high-quality c-Si passivant, and, for this work, was broken into its constituent components--p-benzoquinone (BQ) and hydroquinone (HQ)--for the study of its bonding mechanisms. After examining the effects of time and light on the passivation by BQ and HQ of c-Si, a bonding mechanism is proposed and BQ is shown to be the active passivant. Density functional theory confirms the role of light in the surface reaction and its effective passivity, and surface analysis through XPS experimentally shows the types of bonds being formed. Hybrid devices were also fabricated, using BQ as both passivant and bandbending layer in an organic/inorganic hybrid device. A Poly(3,4ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) layer is the contact layer. Experimental analysis of methods for the improvement of open-circuit voltage ( Voc ), short-circuit current density (Jsc ), and fill factor (FF ) is performed, and pathways for device improvement presented. Theoretical modeling of these devices is also included, using the finite element method software Sentaurus TCAD. The expected characteristics and performance of the proposed device structure based upon the devices fabricated are calculated, and shown to corroborate the experimental results. This analysis will assist in the direction of future work on these hybrid organic/inorganic structures.