Pawel Zabierowski

Modeling metastabilities in chalcopyrite-based thin film solar cells

Cu(In,Ga)Se2-based thin film solar cell devices exhibit metastable electrical behavior. This behavior is often ascribed to intrinsic defects that can change configuration accompanied by large lattice relaxations. We extended the thin film solar cell simulation software scaps to enable the simulation of the metastable behavior of this kind of defects. The statistics that are needed to describe metastable defects are discussed. The procedure that has been implemented is introduced, and special attention is paid to the convergence of the method for high defect densities. The model is demonstrated by simulating the effect of voltage induced metastabilities on the capacitance-voltage characteristics. Some of the features present in the measured apparent doping density profiles can be directly related to presence of metastable defects.

The Influence of the n-Side Doping on Metastable Defect Concentrations in Cu(In,Ga)Se 2 Evaluated From Space Charge Profiles

In this study, we investigate, through numerical simulations, the influence of the n-side of the Cu(In,Ga)Se2 (CIGS)-based photovoltaic devices on capacitance space charge profiles, with a special emphasis on evaluating metastable defect concentrations. We calculated that for plausible doping level of the CdS buffer ranging from 1016 to 1018 cm-3, the values of hole concentrations in the absorber delivered by capacitance can be underestimated, and the ZnO/CdS/CIGS devices should not be treated a priori as n+-p junctions. We showed how this effect can influence the values of metastable defect concentrations extracted from capacitance-voltage (CV) profiles. We indicated that the apparent nonuniformity of experimental CV profiles might be due to an incomplete depletion of the buffer layer. Based on our results, we proposed experiments that can help unveil the correct shallow acceptor and metastable defect concentration in CIGS.

Impact of the Cu-poor to Cu-rich transition speed and Mo back contact porosity on the electrical and structural properties of CIGS-based solar cells

The influence of the transition speed from low to high copper content and two different contents of sodium in the 3-stage co-evaporation process on the Cu(In, Ga)Se2 solar cells has been carefully analyzed. In this work we present results of the electrical and structural measurements of samples deposited under such conditions. From the current-voltage curves we infer the impact of the transition speed and sodium content on the transport mechanism and relative content of the metastable defects. Results of the IQE and XRD measurements indicate the possible difference between slow and fast transition in the band gap grading.

Fill factor metastabilities in CIGSe-based solar cells investigated by means of photoluminescence techniques

In this paper we aim at a deeper understanding of fill factor (FF) deterioration under the absence of high energy photons in CIGSe-based solar cells (so-called “red kink” effect). In order to elucidate the mechanisms responsible for this phenomenon we take advantage of voltage dependent photoluminescence (PL-V) in combination with current voltage characteristics (I-V). Basing on a close correlation between PL-V and light I-V characteristics we show that the “red kink” effect is due to the redistribution of the electrical field in the absorber space charge region which in turn influences carrier collection. An important observation is that both, PL-V and light I-V, exhibit the same metastable behavior under light soaking (LS) and reverse bias treatment (REV). Since these metastabilities are caused by a defect charge state redistribution in the close-to-interface CIGSe layer (p+ layer), it allows us to locate the source of FF metastable behavior on the absorber side of the junction. Basing on numerical modeling of PL-V and light I-V characteristics we discuss presented results within a framework of existing models in which the redistribution of the negative charge in the window/buffer/absorber interface region has a direct influence on carrier collection. We show that the best agreement with experimental results can be achieved by a combination of a barrier at the window/buffer interface with the p+ layer model.

Finding Relevant Parameters for the Thin-film Photovoltaic Cells Production Process with the Application of Data Mining Methods

A data mining approach is proposed as a useful tool for the control parameters analysis of the 3‐stage CIGSe photovoltaic cell production process, in order to find variables that are the most relevant for cell electric parameters and efficiency. The analysed data set consists of stage duration times, heater power values as well as temperatures for the element sources and the substrate – there are 14 variables per sample in total. The most relevant variables of the process have been found based on the so‐called random forest analysis with the application of the Boruta algorithm. 118 CIGSe samples, prepared at Institut des Matériaux Jean Rouxel, were analysed. The results are close to experimental knowledge on the CIGSe cells production process. They bring new evidence to production parameters of new cells and further research.

Quantitative analysis of the persistent photoconductivity effect in Cu(In,Ga)Se2

The magnitude of the persistent photoconductivity effect (PPC) in two sets of Cu(In,Ga)Se2 samples, differing in the amount of cadmium and sodium, was measured. Using equations describing the magnitude of PPC, metastable defect and shallow acceptor densities were calculated. The method of the analysis of PPC in the presence of a deep acceptor level was presented. Based on obtained results, we drew conclusions about reasons of decreased PPC in Cu(In,Ga)Se2 without sodium as well as the role of (VSe-VCu) complexes in establishing the carrier concentration in Cu(In,Ga)Se2 with and without sodium.

Advancing the Understanding of Reverse Breakdown in Cu(In,Ga)Se 2 Solar Cells

Reverse breakdown is investigated in multiple Cu(In,Ga)Se₂ solar cells with varying buffer layer thicknesses. A method to extract transition voltage, which marks the change of conduction mechanism that leads to electrical breakdown, is described as an alternative to the often less-meaningful breakdown voltage. Transition voltages for samples with CdS and Zn<italic>_x</italic>Sn_1-<italic>_x</italic>O<italic> _y</italic> buffers are extracted from breakdown measurements performed in darkness and under illumination. The electric field is calculated for ZTO-based samples measured in darkness, and its implications for the energy band structure are examined. Fowler–Nordheim tunneling and Poole–Frenkel conduction are considered as candidates for the main breakdown mechanism in darkness. A model combining the two conduction mechanisms is proposed, and fits for experimental data are presented and discussed. Involvement of defects is debated, and defect-and-breakdown-related phenomena are showcased.

Depth resolved preferential orientation of Cu(In,Ga)Se2 thin films based on the 112 peak model

X-ray diffraction is a commonly used technique to characterize the Cu(In,Ga)Se2 thin films. It allows to derive information on the pureness of deposited layers, their preferential orientation and a degree of crystallization. In this work we present a detailed analysis of the shape of a very characteristic 112 peak and its dependence on the compositional profile of indium and gallium. Contrary to standard X-ray analysis we go one step further and from the comparison of calculated and measured 112 peaks we are able to determine the evolution of preferential orientation along the thickness of Cu(In,Ga)Se2 thin film.

Dependence of defect signature on conductivity of polycrystalline Cu(In,Ga)Se2 layers by photocurrent spectroscopy

Photocurrent spectroscopy - a method sensitive only to bulk levels - was used to investigate defect spectrum of CIGS polycrystalline films. Low temperature signal, previously labeled E4, with parameters similar to the N1 was observed in all samples, irrespective of their preparation details. We show that sensitivity to the hole concentration is its characteristic feature. Similar dependence of emission rates on doping changes induced by light soaking has been commonly observed for N1 level in solar cells. Our findings point toward common origin for the N1 level and E4 signal observed by photocurrent methods in the epitaxial and polycrystalline CIGS.

On the magnitude of the persistent photoconductivity (PPC) effect in CIGS layers with and without sodium

In this contribution we analyze the influence of the presence of sodium on the magnitude of the persistent photoconductivity effect (PPC) in Cu(In,Ga)Se2 layers. Results of DLCP measurements on CIGS samples with and without sodium are presented. By solving the occupation equations we show that the VSe - related model alone is not sufficient to explain large differences between the samples with and without sodium. We propose that the decrease of the of the magnitude of the PPC effect in Na-free samples by a factor of 10 is caused by very deep donor level located in the range of 0.2 eV - 0.3 eV above the valence band. We show a numerical example of the influence of a deep donor on the magnitude of the PPC effect.