Development of a Hetero-Junction Oxide-Based Solar Cell Device

Engelsk versjon: Throughout the course of the HeteroSolar project, we built significant knowledge working with various material problems and challenges on Cu-oxide containing solar cell structures which currently puzzle the international PV research community. More specifically, successful growth of high quality Cu2O and ZnO films was achieved by use of reactive magneton sputtering. The electronic and atomic structure of the thin films and their interfaces was studied in depth and gave new insight published in internationally highly renowned journals. Further, one PhD thesis was accomplished (defence is planned for early-2017). A major finding is the growth of a few nanometer thick layer of CuO under optimised growth condition for Cu2O on Cu2O-ZnO interfaces independently of deposition sequence. Previously, the observation of co-oriented growth of Cu2O and ZnO by X-ray diffraction (XRD) was taken as an indication of high quality interfaces between the two phases due to a relative good lattice match between (111) Cu2O and (0001) ZnO surfaces. However, our in depth studies have shown that this conclusion is not valid. Thin interfacial CuO layers were not detectable by XRD, however, the co-oriented growth previously reported by others is retained with the presence of CuO. Our experimental and theoretical studies have shown that the presence of CuO results in epitaxial interfaces towards both ZnO and Cu2O with less lattice strain and hence lower the energy of the interface region. The existence of CuO at the interfaces is critical for the electronic properties of the heterojuction and is seen as a contributing factor for the large discrepancy between the theoretical and experimentally obtained solar conversion efficiencies. The interfacial electronic structure of the CuO film is influenced by the proximity of the Zn atoms in the ZnO film. The CuO presence causes a band offset that is detrimental to open circuit voltage and rectification of the Cu2O/ZnO junction and therefore, the PV efficiency of such a structure. We devoted a significant effort trying to resolve the problem as we were ultimately aiming to demonstrate success by developing "proof of concept" devices. With the new understanding and increased knowledge of the structure and insight into the relationship with optoelectronic properties, alternative structures were grown where direct contact between Cu2O and ZnO was avoided. A range of devices and structures were made and tested. However, they did not show high PV effiecience (below 2 %) and as such, they were in line with results from the whole international scientific community working with thin-film Cu-O cells. Towards the end of the project, more intense efforts were made to achieve rectification by doping of the Cu2O film. Indeed, rectification of several orders was achieved in structures of Cu2O with moderately doped ZnO and Si and detailed studies have shown that the achieved rectification was owing to the moderate doping of ZnO and Si. The energy band alignment in these structures is such that the depletion region near the heterojunction is expanded into ZnO and Si, rather than into Cu2O. This explains the low PV effect in the structures. These findings are also of significant importance. This last part of the study has shown promising results, but has not been completed as it took place towards the end of the project.

This project aims to contribute towards developing a new generation solar cell with reduced cost and improved energy conversion efficiency. The main focus is on implementation of micro-engineering and nanotechnology for formation of heterostructures based on oxide thin films and metal-oxide nanostructures with properties optimized for PV. Besides the bulk of the thin films, interfaces within such solar cells play a crucial role both during manufacturing and operation. Their complex structure demands syner gy between processing, characterisation and theory and should be probed at the atomic level in order to understand the mechanisms governing their formation and function. We intend to address the local atomic arrangement of the materials by high resolution electron microscopy and nanoscale electron diffraction. Their electronic structure will be probed by high spatial resolution electron energy loss spectroscopy and electron holography together with x-ray and ultraviolet photoelectron spectroscopy. These t echniques have high spatial and/or energy resolution, and are able to probe both the bulk film and the interfaces. The materials will be also fully characterised optoelectronically. The main theoretical tool to be used is density functional theory (DFT), which can predict atomistic structures and electronic structures without any adjustable parameters. This will be complemented with more accurate methods like hybrid functional and GW if e.g. precise predictions of band gaps are required. We intend to star t our studies with the ZnO/Al and ZnO/Ag systems by exploiting consortiums experience in both processing and characterisation of similar systems. Studies of the novel ZnO/Ag2O and ZnO/Cu2O structures will follow. The results will be demonstrated by develo ping first a Si-based solar cell with ZnO(Al) antireflective coating followed by the development of a multijunction solar cell with oxide-based front cell and Si-based back cell.