Fabrication and Analysis of GeSe Thin Film Solar Cells Using Close Space Sublimation Deposition

Over the past 6 years, GeSe has emerged as a potential absorber material for thin film solar cells, with a power conversion efficiency (PCE) increase from 1.5% to 6.1% from just over 10 publications on photovoltaic devices worldwide. With well suited base materials properties such as a high absorption coefficient, thin-film carrier mobility over 25cm²V^-1s^-1, intrinsic carrier density of 10¹⁵cm³ and a direct band gap of 1.3eV, it seems that the main limitation of this material as an absorber is the volume of research focussing on it. With a 2-dimensional orthorhombic Pnma structure, covalently bonded in 2-directions with van der Waals interactions in the other, an optimized deposition method and device structure could offer reduced orientation-dependence on device performance compared with Sb₂Se₃ absorbers. Alongside potentially more favourable electronic properties and stereochemically active lone pairs, this could enable the production of photovoltaic devices that outperform those with Sb₂Se₃ absorbers.
Large single crystals of intrinsic, Cu-doped and Ag-doped GeSe have been produced via a vapour transport growth method and these have been analysed in low and high energy X-ray photoelectron spectroscopy, along with electrical characterisation techniques such as Hall effect.

The majority of higher efficiency GeSe photovoltaic absorbers have been produced via vapour transport deposition, mostly in a ‘rapid thermal sublimation’ or ‘close spaced sublimation’ (CSS) based system. The increased performance for this type of deposition had been attributed to a sublimation purification mechanism as a result of high vapour pressure of GeSe and the removal of unwanted Ge and GeSe₂ phases. This work focusses on the optimisation of GeSe film growth on both CdS and TiO₂ substrates using a CSS deposition system similar to that used for deposition of high efficiency Sb₂Se₃ and CdTe devices. Alongside it’s benefits in reducing parasitic absorption in the device, it is shown that TiO₂ acts as a more optimal window layer choice than CdS for GeSe absorber devices due to the incidence of Cd diffusion during deposition with a CdS window layer, resulting in the formation of CdSe grains in the bulk absorber and a poor interface quality at the junction.
Previous works have utilised an Sb₂Se₃ interfacial layer between the window layer and the absorber to enable higher efficiency devices above 3% PCE. In this work, the role of this layer is found to provide an anisotropic seed layer for growth of GeSe thin films which improves the quality of the thin-film growth.
A range of hole-transport layers (HTLs) have been applied to the device structure and the effect of these on device performance have been studied. The application of a Spiro-O-MeTAD HTL enables the production of Cd-free GeSe Devices with PCE of up to 2.51%.