Chalcopyrite-based Thin Film Solar Cells Research Articles

Band offsets and point-defect charges of the aluminum and hafnium oxides in contact with the Cu(In,Ga)Se2 chalcopyrite

Surface passivation of CuInSe2 (CIS) and related Cu(In,Ga)Se2 (CIGS) chalcopyrite materials by depositing selected dielectric layers has been a major research activity aiming to reduce interface recombination and increase the electrical efficiency of chalcopyrite-based thin-film solar cells. The present study reports calculations based on density-functional theory and ab-initio thermodynamics that examine the origin of field-effect passivation from alumina and hafnia two wide-gap, predominantly ionic insulators that have exhibited promising passivation qualities in silicon-based microelectronics. The source of fixed charges within the bulk lattices of both oxides was studied by determining the thermodynamically most favorable charge states of their native defects within the admissible ranges of the metal and oxygen chemical potentials. An alignment of the electron bands based on the branch-point energies was performed in order to correctly place the defect charge-transition levels with respect to the band edges of the CIS and the CIGS materials. The trends and predictions of the sign of the fixed charges in either insulator were obtained as a function of temperature, oxygen partial pressure and Fermi-level position inside the band gaps of CIS and CIGS. The findings are discussed in connection with existing experimental studies that extracted the magnitude and polarity of the fixed charges of both alumina and hafnia by analyzing the electrical properties of the CIGS/insulator interfaces.

Interface Formation between CdS and Alkali Postdeposition-Treated Cu(In,Ga)Se2 Thin-Film Solar Cell Absorbers-Key To Understanding the Efficiency Gain.

A combination of X-ray photoelectron/Auger electron spectroscopy and soft X-ray emission spectroscopy has been employed to investigate the impact of different alkali postdeposition treatments (PDTs) on the chemical structure of the (buried) CdS/Cu(In,Ga)Se2 heterojunction: the key interface in chalcopyrite-based thin-film solar cells. Chemical bath deposited (CBD) CdS layers of different thicknesses on NaF PDT (CIGSeNaF) and NaF + KF PDT (CIGSeNaF+KF) Cu(In,Ga)Se2 absorbers prepared at low temperature (to facilitate the use of flexible, e.g., polyimide, substrates) were studied. While we find the CdS/CIGSeNaF interface to be mainly free of significant chemical interaction, in the proximity of the CdS/CIGSeNaF+KF interface, an elemental redistribution involving Cd, In, K, S, and Se is revealed. For the early stages of the CBD-CdS process, our findings are in agreement with the conversion of the K-In-Se-type layer present on the CIGSeNaF+KF surface into a mixed Cd-In-(O,OH,S,Se)-type layer, probably having some Cd-In and (S,O)-Se composition gradients. For long CBD times-independent of employed PDT-we find the buffer material to be best described by a Cd(O,OH,S)-like species rather than by a pure CdS buffer. These findings shed light on the observed performance leap of corresponding CdS/CIGSeNaF+KF-based solar cells.

Solution-processed In2S3buffer layer for chalcopyrite thin film solar cells

We report a route to deposit In2 S3 thin films from air-stable, low-cost molecular precursor inks for Cd-free buffer layers in chalcopyrite-based thin film solar cells. Different precursor compositions and processing conditions were studied to define a reproducible and robust process. By adjusting the ink properties, this method can be applied in different printing and coating techniques. Here we report on two techniques, namely spin-coating and inkjet printing. Active area efficiencies of 12.8% and 12.2% have been achieved for In2 S3 -buffered solar cells respectively, matching the performance of CdS-buffered cells prepared with the same batch of absorbers.

Impact of sulfur and gallium gradients on the performance of thin film Cu(In,Ga)(Se,S)2 solar cells

A graded bandgap structure proved to be an important factor for increasing an overall efficiency of the chalcopyrite-based thin film solar cells. This contribution is focused on the effects of sulfur incorporation into the surface region of industrial sequentially grown Cu(In,Ga)(Se,S) 2 absorbers. A front grading due to such a sulfurization step enhances the bandgap in the space charge region, whereas the bulk of the absorber exhibits a lower bandgap which determines absorption and photocurrent. It will be demonstrated that such graded bandgap structures allow separating the absorption and recombination processes, therefore resulting in highly efficient solar cells with improved open circuit voltages without compromising short circuit currents. Moreover, a segregation of a gallium rich layer at the back contact as a result of sequential deposition reactions is discussed in terms of a back contact passivation that prevents injection of electrons to the back contact and suppresses phototransistor effects often observed at low temperatures. Furthermore, an influence of longer diffusion times on gallium distribution throughout the absorber layer has been investigated. High temperature deposition processes for prolonged time enhance gallium diffusion towards the absorber/buffer interface therefore leading to an overall increase of the absorber bandgap energy when both recombination and absorption processes are being affected. • Separation of the recombination and absorption processes due to S incorporation • A gallium-rich layer at the back contact suppresses phototransistor effects. • Ga-diffusion at high temperature affects both electrical and optical bandgaps.

Modelling and measurement of the metastable defect distribution in chalcopyrite-based thin film solar cells

Metastable behaviour in chalcopyrite-based solar cells is often related to the presence of intrinsic defects in the absorber layer which can change configuration through thermally activated transitions. The facilities to model these defects have recently been introduced in the software SCAPS (a one-dimensional solar cell simulator developed at the University of Gent and freely available to the research community). The presence of these defects leads to strongly non-uniform defect densities which can explain admittance spectroscopy measurement results under different bias conditions. In particular the appearance of a peak with a characteristic energy of about 0.13eV which only appears when the spectrum is measured under forward bias conditions can be related to the presence of the IIICu-defect.

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.

Preparation and properties of radio-frequency-sputtered half-Heusler films for use in solar cells

The class of half-Heusler compounds opens possibilities to find alternatives for II–VI or III–V compound semiconductors. We aim to find suitable substitutes for the cadmium sulphide buffer layer in chalcopyrite-based thin film solar cells, where the buffer layer is located between the p-type chalcopyrite absorber and an n-type transparent window layer. We report here the preparation of radio-frequency-sputtered lithium copper sulphide “LiCuS” and lithium zinc phosphide “LiZnP” films. The optical analysis of these films revealed band gaps between 1.8 and 2.5eV, respectively. Chemical properties of the film surface and both interfaces between the film and a Cu(In,Ga)Se2 layer and between the film and an (Zn,Mg)O layer were investigated by in-situ photoelectron spectroscopy. The valence band offsets to the Cu(In,Ga)Se2 layer were estimated to be (0.4±0.1) eV for “LiCuS”/Cu(In,Ga)Se2 and (0.5±0.8) eV for “LiZnP”/Cu(In,Ga)Se2. This leads to positive conduction band offsets of >1eV. These rather large offsets are not compatible with efficient solar cell devices.Under atmospheric conditions “LiCuS” and “LiZnP” films show rapid decomposition.

Electronic level alignment at the deeply buried absorber/Mo interface in chalcopyrite-based thin film solar cells

We have investigated the electronic structure of the absorber/back contact interface for S-free [Cu(In,Ga)Se2 (“CIGSe”)] and S-containing [Cu(In,Ga)(S,Se)2 (“CIGSSe”)] chalcopyrites with direct and inverse photoemission. Comparison of the electronic levels of the cleavage planes reveals a pronounced cliff in the conduction band at the CIG(S)Se∕Mo interface. For the valence band, we find a flat alignment and a small spike for the CIGSe- and CIGSSe-based structures, respectively.

Cd2+/NH3 treatment of Cu(In,Ga)(S,Se)2 thin-film solar cell absorbers: a model for the performance-enhancing processes in the partial electrolyte

To obtain highly efficient chalcopyrite-based thin-film solar cells where the conventionally used CdS buffer is replaced by a ZnO layer prepared by the ILGAR (ion layer gas reaction) process, the Cu(In,Ga)(S,Se)2 absorber has to be pretreated in a Cd2+/NH3 solution. Based on the measured characteristics of the pH-value in the Cd2+/NH3 solution during the treatment, a model of the processes in the bath can be established. The conclusions are correlated with results from X-ray-photoelectron and X-ray-excited Auger electron spectroscopy of the Cd2+/NH3-treated Cu(In,Ga)(S,Se)2 surface, giving an explanation for the observed formation of Cd-compounds on the surface of the absorber. Copyright © 2005 John Wiley & Sons, Ltd.

Chemical bath process for highly efficient Cd-free chalcopyrite thin-film-based solar cells

The highest efficiency for Cu(Ga,In)Se2 (CIGS) thin-film-based solar cells has been achieved with CdS buffer layers prepared by a solution growth method known as the chemical bath deposition (CBD). With the aim of developing Cd-free chalcopyrite-based thin-film solar cells, we describe the basic concepts involved in the CBD technique. The recipes developed in our laboratory for the heterogeneous deposition of good-quality thin films of ZnO, ZnSe, and MnS are presented. In view of device optimization, the initial formation of chemical-bath-deposited ZnSe thin films on Cu(Ga,In)(S,Se)2 (CIGSS) and the subsequent development of the ZnSe/CIGSS heterojunctions were investigated by X-ray photoelectron spectroscopy (XPS). The good surface coverage was controlled by measuring changes in the valence-band electronic structure as well as changes in the In4d, Zn3d core lines. From these measurements, the growth rate was found to be around 3.6 nm/min. The valence band and the conduction band-offsets ΔEV and ΔEC between the layers were determined to be 0.60 and 1.27 eV, respectively for the CIGSS/ZnSe interface. The energy-band diagram is discussed in connection with the band-offsets detemined from XPS data. A ZnSe thickness below 10 nm has been found to be optimum for achieving a homogeneous and compact buffer layer on CIGSS with a total area efficiency of 13.7%.PACS No.: 42.70

13·7%-Efficient Zn(Se,OH)x/Cu(In,Ga)(SSe)2 thin-film solar cell

Cu(In,Ga)Se2 (CIGS) and related semiconducting compounds have demonstrated their high potential for high-efficiency thin-film solar cells. The highest efficiency for CIGS-based thin-film solar cells has been achieved with CdS buffer layers prepared by a solution growth method known as chemical bath deposition (CBD). With the aim of developing Cd-free chalcopyrite-based thin-film solar cells, Zn(Se,OH)x buffer layers were deposited by CBD on polycrystalline Cu(In,Ga)(S,Se)2 (CIGSS). A total-area conversion efficiency of 13·7% was certified by the Frauenhofer Institute for Solar Energy Systems. The CIGSS absorber was fabricated by Siemens Solar Industries (California). For device optimization, the thickness and good surface coverage were controlled by XPS–UPS photoemission spectroscopy. A Zn(Se,OH)x thickness below 7 nm has been found to be optimum for achieving a homogeneous and compact buffer film on CIGSS, with open-circuit photovoltage Voc=535 mV, fill factor FF=70·76% and a high short-circuit photocurrent density Jsc=36·1 mA cm−2. Copyright © 1998 John Wiley & Sons, Ltd.

Phase and Interface Stability Issues in Chalcopyrite-Based Thin Film Solar Cells

AbstractHow stable are Cu(In,Ga)Se2 solar cells? CuInSe2 is known also as an ionic Cu conductor and the Cu-In-Se system has a rich phase diagram. Two stability-related problems are treated from a thermodynamic point of view: -a- Cu migration, -b- phase segregation. We show how Cu migration imparts self-stabilization on the system. Based on recently reported results on the room temperature phase diagram of CuInSe2, we explain why an overlayer of an ordered vacancy compound (OVC) is not to be expected to appear as a separate phase, even though the surface composition corresponds to that of the I-III3V5 OVC compound.