Cu(In,Ga)Se₂ superstrate solar cells: prospects and limitations

Abstract

Superstrate solar cells were prepared by thermal evaporation of Cu(In,Ga)Se2 onto ZnO coated glass substrates. For the first time, photo-conversion efficiencies above 11% were reached without the necessity of additional light soaking or forward biasing of the solar cell. This was achieved by modifying the deposition process as well as the sodium doping. Limitations of the superstrate device configuration and possible ways to overcome these were investigated by analyzing the hetero-interface with electron microscopy and X-ray photoemission spectroscopy measurements, combined with capacitance spectroscopy and device simulations. A device model was derived that explains how on the one hand the GaO x , which forms at the CIGSe/ ZnO interface, reduces the interface recombination. On the other hand how it limits the efficiency by acting as an electron barrier at the hetero-interface presumably because of a high density of negatively charged acceptor states like Cu Ga . The addition of sodium enhances the p-type doping of the absorber but also increases the net doping within the GaO x . Hence, a trade-off between these two effects is required. The conversion efficiency was found to decrease over time, which can be explained in our model by field-induced diffusion of sodium cations out from the GaO x layer. The proposed device model is able to explain various effects frequently observed upon light soaking and forward biasing of superstrate devices.

Bifacial Cu(In,Ga)Se2 solar cells using hydrogen‐doped In2O3 films as a transparent back contact

“…It was shown before, that the presence of Na (and possibly F) during absorber formation significantly catalyzes the formation of Ga 2 O 3 at the TCO/chalcopyrite interface, which potentially leads to long‐ranging redistributions in Ga and In in the absorber . In order to investigate the effect of the Na supply method and the presence of Al 2 O 3 on the chemical and structural properties, the different samples on IOH thin were analyzed by STEM and EDS.…”

“…An electrically unfavorable back contact junction after high temperature CIGS deposition ( T CIGS > 500°C) was determined as the main obstacle. A supposedly detrimental Ga 2 O 3 formation at the absorber/TCO interface (for AZO and ITO) or a reduced conductivity of FTO by a fluorine loss during high temperature absorber deposition was suggested as possible origin. On the other hand, Mollica et al have demonstrated ohmic behavior of FTO and AZO layers on CIGS (deposited at T CIGS > 500°C) and used these materials as appropriate TBCs in ultra‐thin CIGS devices .…”

Interference effects in photoluminescence spectra of Cu2ZnSnS4 and Cu(In,Ga)Se2 thin films

“…As shown in Figure 6, the PL spectrum for a band gap graded CIGS absorber on Mo back contact is influenced by interferences which are not in phase with the measured 1 À R. The CIGS absorber was deposited by three-stage evaporation as described elsewhere, 19 giving a double band gap grading with the minimum, or notch, at around 300 nm from the absorber surface. Radiative recombination will be highest in this notch and the distribution of luminescence centres can be approximated by a Gaussian centred at this position.…”

Accelerated development of CuSbS2 thin film photovoltaic device prototypes

“…In addition to metallic back contacts, we also studied MoO x and Cu 12 Sb 4 S 13 as charge-selective layers on Mo electrodes (Table II), similar to what has been previously studied for CdTe [41] and CIGS [42]. The PV devices (Table II).…”