The objective of this project is to develop highly efficient, low-toxic, stable, cost effective thin film solar cells based on CuGaSe2 and CuGaSe2 / CulnSe2 tandem structures. The wide-gap CuGaSe2 (Eg =1.68 eV) based solar cell will be processed at low temperatures and should exceed the best PV parameters, achieved so far. We aim in particular for an open circuit voltage of more than 1 V and a short-circuit current density of more than 15mA/cm2 (under AM1.5 illumination).
In order to study the influence of source materials and deposition techniques on device performances, MOCVD and PVD will be investigated. Polycrystalline CuGaSe2 and defect chalcopyrite layers will be deposited on glass/Mo and on glass/n+ -ZnO:AI (to form a tunnel junction), and also on glass/p+ -SiC (in order to develop transparent back contacts). For reference reasons the same layers wil be grown on GaAs-substrates by MBE and MOCVD. In some PVD processes various additive materials acting as fluxing agents will be used during the growth of CuGaSe2 thin films (flux-assisted growth). In the MOCVD-system p-CuGaSe2 /n-ZnSe structuires will be deposited in one multilayer-process.
With different characterisation methods a study of material, interface and device properties will be performed. The material quality prior to and after deposition will be analyzed by mass spectrometry and nuclear mass resonance. With the help of ion-beam analysis the hydrogen content depth profile in the complete solar cell structures will be studied quantitatively . The stoichiometric composition at the interface between window layers and CuGaSe2 will be analyze by secondary ion mass spectroscopy (SIMS) together with sputtering and Rutherford backscattering (RBS). Optoelectronic scanning tunneling microscopy (STM), atomic force microscopy (AFM) and in-situ photoelectron spectroscopy will be used to investigate the junction formation. The devices will be characterized by spectral response and temperature dependent IV-measurements.
EXPECTED ACHIEVEMENTS AND EXPLOITATION
It is expected that the wide-gap CuGaSe2 based solar cells developed in this project will exhibit better PV parameters than any state-of-the-art wide gap chalcopyrite device processed so far.
If MOCVD-grown wide gap chalcopyrites can be processed into highly efficient solar cells, AIXTRON sees the opportunity to offer MOCVD equipment (with large planetary reactor) and transfer the developed technology with the equipment. The precursor materials developed at EPICHEM could be exploited using batch scale production with a relatively short induction period of 6 to 12 months.
Funding SchemeCSC - Cost-sharing contracts