Hybrid Multi-Layer Graphene/Si Schottky Junction Solar Cells

Abstract

Hybrid Graphene/Si Schottky junction solar cells are inexpensive alternatives for photovoltaics due to easy and rapid fabrication processes. However, one of the main challenges for graphene-based Schottky junction solar cells is the large sheet resistance of single-layer graphene, leading to low fill factor. Here, we demonstrate multi-layer graphene-based Schottky junction solar cell, which are prepared by stacking monolayer graphene via a chemical vapor deposition (CVD) method on copper foils. The objectives are to improve charge transport and decrease the series resistance of the hybrid solar cell. We present the Raman spectrum results of monolayer graphene and stacked multilayer graphene films, and observe no bonding interaction between the stacked layers. For the optical and electrical properties, the sheet resistance of stacked multilayer graphene is reduced from 3000 Omega/sq to 1500 Omega/sq and 750 Omega/sq with two and three layers, respectively. The reduction of the sheet resistance approaches the properties of highly-oriented pyrolytic graphite (HOPG) when the layer number is more than five. Moreover, from the optical transmittance measurement, the absorption of a monolayer graphene is about 3% at the 555 nm wavelength, giving rise to a transmittance of 90% for triple-layer graphene. The hybrid silicon solar cell with a single-layer graphene achieves a power conversion efficiency of 0.9% with a short-circuit current of 8.75 mA/cm(2), open-circuit voltage of 0.25V, and fill factor of 39% The external quantum efficiency measurement reveals an integrated photocurrent of 18.3 mA/cm(2), which further confirms the potential of the hybrid photovoltaic approach. Device fabrication with multi-layer graphene is still in process, and more data will be presented. Other characterization techniques such as capacitance-voltage and capacitance-frequency measurements will be taken to analyze the performance of the single- and multi-layer graphene solar cells.