Abstract :
[en] Cu(In,Ga)S2 is a chalcopyrite material suitable as the higher bandgap top cell in tandem applications in next generation multijunction solar cells. This owes primarily to the tunability of its bandgap from 1.5 eV in CuInS2 to 2.45 eV in CuGaS2, and its relative stability over time. Currently, a major hinderance to the potential use of Cu(In,Ga)S2 in tandem capacity remains a deficient single-junction device performance in the form of low open-circuit voltage (VOC) and low efficiency. Aside interfacial recombination which leads to losses in the completed Cu(In,Ga)S2 solar cell, deficiencies stems from a low optoelectronic quality of the Cu(In,Ga)S2 absorber quantified by the quasi-Fermi level splitting (QFLS) and which serves as the upper limit of VOC achievable by a solar cell device. In this thesis, the QFLS is compared with the theoretical VOC (SQ-VOC) in the radiative limit, and “SQ-VOC deficit” is defined to compare the difference between SQ-VOC and QFLS as a comparable measure of the optoelectronic deficiency in the absorber material. In contrast to the counterpart Cu(In,Ga)Se2 absorber which has produced highly efficient solar cell devices, the Cu(In,Ga)S2 absorber still suffers from a high SQ-VOC deficit. However, SQ-VOC deficit in Cu(In,Ga)S2 can be reduced by growing the absorbers under Cu-deficient conditions.
For the effective use of Cu(In,Ga)S2 as the top cell in tandem with Si or Cu(In,Ga)Se2 as the bottom cell, an optimum bandgap of 1.6-1.7 eV is required, and this is realized in absorbers with Ga content up to [Ga]/([Ga]+[In]) ratio of 0.30-0.35. However, the increase of Ga in Cu-poor Cu(In,Ga)S2 poses a challenge to the structural and optoelectronic quality of the absorber, resulting from the formation of segregated Ga phases with steep Ga/bandgap gradient which constitutes a limitation to the quality of the Cu(In,Ga)S2 absorber layer with a highSQ-VOC deficit and low open circuit voltage and overall poor performance of the finalized solar cell.
In this work, the phase segregation in Cu(In,Ga)S2 has been circumvented by employing higher substrate temperatures and adapting the Ga flux during the first-stage of deposition when growing the Cu(In,Ga)S2 absorbers. A more homogenous Cu(In,Ga)S2 phase and improved Ga/bandgap gradient is achieved by optimizing the Ga flux at higher substrate temperature to obtain a Cu(In,Ga)S2 absorber with high optoelectronic quality and low SQ-VOC deficit. Additionally, the variation of the Cu-rich phase when growing the Cu(In,Ga)S2 absorber layers was found to not only alter the notch profile and bandgap minimum of the absorbers, but also influence the optoelectronic quality of the absorber. Shorter Cu-rich phase in the absorbers led to narrower notch profile and higher bandgap. Ultimately, several steps in the three-stage deposition method used for processing the Cu(In,Ga)S2 absorbers were revised to enhanced the overall quality of the absorbers. Consequently, the SQ-VOC deficit in high bandgap Cu(In,Ga)S2 absorbers is significantly reduced, leading to excellent device performance.
This thesis also examines the temperature- and compositional-related optoelectronic improvement in pure Cu-rich CuInS2 absorbers without Ga, where improvement in QFLS was initially linked to a reduction of nonradiative recombination channels with higher deposition temperatures and increase in Cu content. Findings through photoluminescence decay measurements show that the origin of the improved QFLS in CuInS2 is rather linked to changes in doping levels with variations of deposition temperature and Cu content.
Finally, in order to understand and gain insight into the influence of Ga in Cu(In,Ga)S2, the electronic structure of CuGaS2 absorbers was investigated in dependence of excitation intensity and temperature by low temperature photoluminescence measurements. A shallow donor level and three acceptor levels were detected. It was found that similar acceptor levels in CuInSe2 and CuGaSe2 which are otherwise shallow become deeper in CuGaS2. These deep defects serve as nonradiative recombination channels and their appearance in the Ga-containing compound is be detrimental to the optoelectronic quality of Cu(In,Ga)S2 absorbers as Ga content is increased therefore limiting the optimum performance of Cu(In,Ga)S2 devices.
