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Semiconducting Polymer and Hydrogenated Amorphous Silicon Heterojunction Solar Cells

A. R. Middya and Eric A. Schiff Department of Physics, Syracuse University, Syracuse, NY 13244 - 1130

ABSTRACT In this work, we report on investigation of p-type semiconducting polymer, {poly(3,4 polyethylenedioxythiophene)-poly(styrenesulfonate)} (PEDOT:PSS) as the p-layer in NIP and PIN hydrogenated amorphous silicon (a-Si:H) solar cells. The rectification ratio of solutioncasted diode is ~ 10, it increases to 3x104 when PEDOT:PSS is deposited by Spin Coating technique. We observed additional photovoltaic effect when light is illuminated through polymer side. So far, best solar cells characteristics observed for PEDOT:PSS/a-Si:H hybrid solar cells are Voc ≈ 720 mV and Jsc ≈ 1 - 2 mA/cm2.

INTRODUCTION Amorphous silicon (a-Si:H) thin films is one of the promising candidates for the application of low-cost solar cells. However, the highest efficiency of amorphous silicon (a-Si) based solar cells is at present limited to 12% [1]. An increase in open-circuit voltage (Voc) is a promising route to increase the power output of a-Si:H solar panel. In principle, increase in Voc should be achievable through increase in built-in potential to 1.3 eV - 1.4 eV from it’s present value of 1.2 eV [2]. In this work, we investigated whether semiconducting polymer can perform as a better player in NIP or PIN a-Si:H solar cells. We attempted molecularly doped p-type semiconducting polymer, {poly(3,4-polyethylenedioxythiophene)-poly(styrenesulfonate)} (PEDOT:PSS) [3,4] as the p-layer in NIP and PIN a-Si:H solar cells. The PEDOT:PSS has simultaneously high conductivity (up to 30 S/cm) and high transparency (T), T > 90% for 6 μm Spin Coated film. The combination of these properties are better than the inorganic p-layer (a-Si:H, μc-Si or aSiC:H) used in state-of-art PIN or NIP a-Si:H solar cells. We would like to investigate additional effect whether p-type semiconducting polymer, PEDOT:PSS can increase the conduction band offset at P/I interface besides increase in built-in potential. An increase in either, or both of these device parameters is expected to lead to improvement in Voc. So far, few attempts were made to investigate the feasibility of semiconducting polymer as the p-layer in a-Si:H or c-Si solar cells [5,6]. The p-type semiconducting polymers have also technological advantages over inorganic

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p-layer, for example, it is easily processable over large area by low-cost printing technique instead of vacuum based technology. In this work, we investigated semiconducting polymer, PEDOT:PSS as the p-layer which is not p-type silicon thin-film, or p-type silicon based alloy, we did experiment to see how PEDOT:PSS works in NIP or PIN a-Si:H solar cells. EXPERIMENTAL DETAILS The state-of-the-art amorphous silicon (a-Si:H) absorber layer and n-type a-Si:H layer have been deposited on top of stainless steel substrate as well as transparent conducting oxide (TCO) coated glass substrate by radio frequency plasma-enhanced chemical vapor deposition (R