A comparative study of Cd‐ and Zn‐compound buffer layers on Cu(In1−x,Gax)(Sy,Se1−y)2 thin film solar cells

This paper reports a comparative study of Cu(In,Ga)(S,Se)₂ (CIGSSe) thin‐film solar cells with CBD‐CdS, CBD‐ZnS(O,OH) and ALD‐Zn(O,S) buffer layers. Each buffer layer was deposited on CIGSSe absorber layers which were prepared by sulfurization after selenization (SAS) process by Solar Frontier K. K. Cell efficiencies of CBD‐CdS/CIGSSe, CBD‐ZnS(O,OH)/CIGSSe and ALD‐Zn(O,S)/CIGSSe solar cells exceeded 18%, for a cell area of 0.5 cm². The solar cells underwent a heat‐light soaking (HLS) post‐treatment at 170 °C under one‐sun illumination in the air; among the three condtions, the ALD‐Zn(O,S)/CIGSSe solar cells showed the highest cell efficiency of 19.78% with the highest open‐circuit voltage of 0.718 V. Admittance spectroscopy measurements showed a shift of the N₁ defect's energy position toward shallower energy positions for ALD‐Zn(O,S)/CIGSSe solar cells after HLS post‐treatment, which is in good agreement with their higher open‐circuit voltage and smaller interface recombination than that of CBD‐ZnS(O,OH)/CIGSSe solar cells. Copyright © 2015 John Wiley & Sons, Ltd.     

CuIn1−xGaxSe2‐based thin‐film solar cells by the selenization of sequentially evaporated metallic layers

Polycrystalline CuIn_1−xGa_xSe₂ (CIGS) thin films were deposited by the non‐vacuum, near‐atmospheric‐pressure selenization of stacked metallic precursor layers. A study was carried out to investigate the influence of significant factors of the absorber on the solar cells performance. An efficiency enhancement was obtained for Cu/(In+Ga) atomic ratios between 0·93 and 0·95. The slope of the observed energy bandgap grading showed a strong influence on the V_OC and the short circuit current density J_SC. An increase of the Ga content in the active region of the absorber was achieved by the introduction of a thin Ga layer on the Mo back contact. This led to an improvement of efficiency and V_OC. Furthermore, an enhanced carrier collection was detected by quantum efficiency measurements when the absorber layer thickness was slightly decreased. Conversion efficiencies close to 10% have been obtained for these devices. Copyright © 2005 John Wiley & Sons, Ltd.     

SHORT COMMUNICATION: ACCELERATED PUBLICATION: Diode characteristics in state‐of‐the‐art ZnO/CdS/Cu(In1−xGax)Se2 solar cells

We report a new state of the art in thin‐film polycrystalline Cu(In,Ga)Se₂‐based solar cells with the attainment of energy conversion efficiencies of 19·5%. An analysis of the performance of Cu(In,Ga)Se₂ solar cells in terms of some absorber properties and other derived diode parameters is presented. The analysis reveals that the highest‐performance cells can be associated with absorber bandgap values of ∼1·14 eV, resulting in devices with the lowest values of diode saturation current density (∼3×10⁻⁸ mA/cm²) and diode quality factors in the range 1·30 < A < 1·35. The data presented also support arguments of a reduced space charge region recombination as the reason for the improvement in the performance of such devices. In addition, a discussion is presented regarding the dependence of performance on energy bandgap, with an emphasis on wide‐bandgap Cu(In,Ga)Se₂ materials and views toward improving efficiency to > 1;20% in thin‐film polycrystalline Cu(In,Ga)Se₂ solar cells. Published in 2005 John Wiley & Sons, Ltd.     

Growth of 1‐eV GaNAsSb‐based photovoltaic cell on silicon substrate at different As/Ga beam equivalent pressure ratios

We report the performance of 1‐eV GaNAsSb‐based photovoltaic samples grown on a Si substrate using molecular beam epitaxy at different As/Ga beam equivalent pressure (BEP) ratios. The light current–voltage curve and spectral response of the samples were measured. The sample grown at an As/Ga BEP ratio of 10 showed the highest energy conversion efficiency with an open circuit voltage (V_OC) of 0.529 V and a short circuit current density of 17.0 mA/cm². This measured V_OC is the highest ever reported value in GaNAsSb 1‐eV photovoltaic cell, resulting in the lowest ever reported E_g/q‐V_OC of 0.50 eV. The increase in the As/Ga BEP ratio also resulted in an increase in the bandgap‐voltage offset value (E_g/q‐V_OC) and a decrease in quantum efficiency up to As/Ga BEP ratio of 18. Copyright © 2015 John Wiley & Sons, Ltd.     

Composition and bandgap control in Cu(In,Ga)Se2‐based absorbers formed by reaction of metal precursors

The control of composition and bandgap in chalcopyrite thin‐film absorber layers formed by a metal precursor reaction is addressed. Two processes using reaction with either H₂Se or H₂S as the final step of a three‐step reaction process were compared as follows: a three‐step H₂Se/Ar/H₂S reaction and a three‐step H₂Se/Ar/H₂Se reaction. In both processes, significant Ga homogenization was obtained during the second‐step Ar anneal, but the third‐step selenization resulted in Ga depletion near the Cu(InGa)Se₂ surface, whereas the third‐step sulfization did not. Solar cells were fabricated using absorbers formed using each method, and the surface Ga depletion significantly affected device performances. The solar cell incorporating the sulfization yielded a better device performance, with an efficiency of 14.4% (without an anti‐reflection layer) and an open‐circuit voltage of 609 mV. The bandgap control in the metal precursor reaction is discussed in conjunction with the device behavior. Copyright © 2014 John Wiley & Sons, Ltd.     

Interface Analysis of Cu(In,Ga)Se2 and ZnS Formed Using Sulfur Thermal Cracker

We analyzed the interface characteristics of Zn‐based thin‐film buffer layers formed by a sulfur thermal cracker on a Cu(In,Ga)Se₂ (CIGS) light‐absorber layer. The analyzed Zn‐based thin‐film buffer layers are processed by a proposed method comprising two processes — Zn‐sputtering and cracker‐sulfurization. The processed buffer layers are then suitable to be used in the fabrication of highly efficient CIGS solar cells. Among the various Zn‐based film thicknesses, an 8 nm–thick Zn‐based film shows the highest power conversion efficiency for a solar cell. The band alignment of the buffer/CIGS was investigated by measuring the band‐gap energies and valence band levels across the depth direction. The conduction band difference between the near surface and interface in the buffer layer enables an efficient electron transport across the junction. We found the origin of the energy band structure by observing the chemical states. The fabricated buffer/CIGS layers have a structurally and chemically distinct interface with little elemental inter‐diffusion.     

High‐efficiency Cu2ZnSn(S,Se)4 solar cells fabricated through a low‐cost solution process and a two‐step heat treatment

A method for fabricating high‐efficiency Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells is presented, and it is based on a non‐explosive, low‐cost, and simple solution process followed by a two‐step heat treatment. 2‐Methoxyethanol was used as a solvent, and Cu, Zn, Sn, chloride salts, and thiourea were used as solutes. A CZTSSe absorber was prepared by sulfurising and then selenising an as‐coated Cu₂ZnSnS₄ (CZTS) film. Sulfurisation in a sulfur vapour filled furnace for a long time (2 h) enhanced the crystallisation of the as‐coated CZTS film and improved the stability of the CZTS precursor, and selenisation promoted further grain growth to yield a void‐free CZTSSe film. Segregation of Cu and S at the grain boundaries, the absence of a fine‐grain bottom layer, and the large grain size of the CZTSSe absorber were the main factors that enhanced the grain‐to‐grain transport of carriers and consequently the short‐circuit current (J_sc ) and efficiency. The efficiency of the CZTS solar cell was 5.0%, which increased to 10.1% after selenisation. For the 10.1% CZTSSe solar cell, the external quantum efficiency was approximately 80%, the open‐circuit voltage was 450 mV, the short‐circuit current was 36.5 mA/cm², and the fill factor was 61.9%. Copyright © 2016 John Wiley & Sons, Ltd.     

Impacts of surface sulfurization on Cu(In1−x,Gax)Se2 thin‐film solar cells

In this work, the impacts of surface sulfurization of high‐quality Cu(In_1−x,Ga_x)Se₂ (CIGS) thin films deposited by three‐stage process on the film properties and the cell performance were investigated. The CIGS thin films were sulfurized at 550 °C for 30 min using H₂S gas. The X‐ray photoelectron spectroscopy analysis revealed that sulfur atoms diffused into the CIGS surface layer and that the valence band minimum was lowered by the film sulfurization. The open circuit voltage (V_oc) drastically increased from 0.590 to 0.674 V as a result of the sulfurization process. Temperature‐dependent current–voltage and capacitance–frequency measurements also revealed that interface recombination was drastically decreased by the lowering of the defect's activation energy level at the vicinity of the buffer/CIGS interface after the sulfurization. Copyright © 2014 John Wiley & Sons, Ltd.     

Solution processed high band‐gap CuInGaS2 thin film for solar cell applications

A high band‐gap (~1.55 eV) chalcopyrite compound film (CuInGaS₂) was synthesized by a precursor solution‐based coating method with an oxidation and a sulfurization heat treatment process. The film revealed two distinct morphologies: a densely packed bulk layer and a rough surface layer. We found that the rough surface is attributed to the formation of Ga deficient CuInGaS₂ crystallites. Because of the high band‐gap optical property of the CuInGaS₂ absorber film, a solar cell device with this film showed a relatively high open circuit voltage (~787 mV) with a power conversion efficiency of 8.28% under standard irradiation conditions. Copyright © 2013 John Wiley & Sons, Ltd.     

Improved performance in ZnO/CdS/CuGaSe2 thin‐film solar cells

We report the growth and characterization of improved efficiency wide‐bandgap ZnO/CdS/CuGaSe₂ thin‐film solar cells. The CuGaSe₂ absorber thickness was intentionally decreased to better match depletion widths indicated by drive‐level capacitance profiling data. A total‐area efficiency of 9·5% was achieved with a fill factor of 70·8% and a V_oc of 910 mV. Published in 2003 by John Wiley & Sons, Ltd.     

Interface engineering and characterization at the atomic‐scale of pure and mixed ion layer gas reaction buffer layers in chalcopyrite thin‐film solar cells

In this work, we investigate the p–n junction region for two different buffer/Cu(In,Ga)(Se,S)₂ (CIGSSe) samples having different conversion efficiencies (the cell with pure In₂S₃ buffer shows a lower efficiency than the nano‐ZnS/In₂S₃ buffered one). To explain the better efficiency of the sample with nano‐ZnS/In₂S₃ buffer layer, combined transmission electron microscopy, atom probe tomography, and X‐ray photoelectron spectroscopy studies were performed. In the pure In₂S₃ buffered sample, a CuIn₃Se₅ ordered‐defect compound is observed at the CIGSSe surface, whereas in the nano‐ZnS/In₂S₃ buffered sample no such compound is detected. The absence of an ordered‐defect compound in the latter sample is explained either by the presence of the ZnS nanodots, which may act as a barrier layer against Cu diffusion in CIGSSe hindering the formation of CuIn₃Se₅, or by the presence of Zn at the CIGSSe surface, which may disturb the formation of this ordered‐defect compound. In the nano‐ZnS/In₂S₃ sample, Zn was found in the first monolayers of the absorber layer, which may lead to a downward band bending at the surface. This configuration is very stable (Fermi level pinning at the conduction band, as observed for Cd in Cu(In,Ga)Se₂) and reduces the recombination rate at the interface. This effect may explain why the sample with ZnS nanodots possesses a higher efficiency. This work demonstrates the capability of correlative transmission electron microscopy, atom probe tomography, and X‐ray photoelectron spectroscopy studies in investigating buried interfaces. The study provides essential information for understanding and modeling the p–n junction at the nanoscale in CIGSSe solar cells. Copyright © 2014 John Wiley & Sons, Ltd.     

Progress toward 20% efficiency in Cu(In,Ga)Se2 polycrystalline thin‐film solar cells

This short communication reports on achieving 18·8% total‐area conversion efficiency for a ZnO/CdS/Cu(In,Ga)Se₂/Mo polycrystalline thin‐film solar cell. We also report a 15%‐efficient, Cd‐free device fabricated via physical vapor deposition methods. The Cd‐free cell includes no buffer layer, and it is fabricated by direct deposition of ZnO on the Cu(In,Ga)Se₂ thin‐film absorber. Both results have been measured at the National Renewable Energy Laboratory under standard reporting conditions (1000 W/m², 25°C, ASTM E 892 Global). The 18·8% conversion efficiency represents a new record for such devices (Notable Exceptions) and makes the 20% performance level by thin‐film polycrystalline materials much closer to reality. We allude to the enhancement in performance of such cells as compared to previous record cells, and we discuss possible and realistic routes to enhance the performance toward the 20% efficiency level. Published in 1999 by John Wiley & Sons, Ltd. This article is a US government work and is in the public domain in the United States.     

Bandgap optimization of submicron‐thick Cu(In,Ga)Se2 solar cells

Reducing Cu(In,Ga)Se₂ (CIGS) absorber thickness into submicron regime provides an opportunity for reducing CIGS solar cell manufacturing time and cost. However, CIGS with submicron‐thick absorber would suffer strong absorption loss in the long‐wavelength region. In this paper, we report a new fabrication route for CIGS solar cells on soda‐lime glass substrates with different Ga content (0.3 < [Ga]/([Ga] + [In]) < 0.6), all with absorber thicknesses around 0.9 µm. Efficiency of 17.52% has been achieved for cells with high Ga content of [Ga]/([Ga] + [In]) = 41%, which is currently the best reported efficiency for submicron‐thick CIGS solar cells. Unlike the normal‐thickness absorber (2–3 µm) that has an optimal [Ga]/([Ga] + [In]) of ~32%, the increased value of optimal [Ga]/([Ga] + [In]) in submicron‐thick absorber greatly enhances the open‐circuit voltage, by nearly 15% compared with that of samples with Ga content optimized at normal absorber thickness. This large gain in V_OC well compensates the absorption loss in the long‐wavelength region and contributes to the enhancement of final solar cell efficiency. Copyright © 2014 John Wiley & Sons, Ltd.     

High efficiency low temperature grown Cu(In,Ga)Se2 thin film solar cells on flexible substrates using NaF precursor layers

We report a total‐area efficiency of 15.9% for flexible Cu(In,Ga)Se₂ thin film solar cells on polyimide foil (cell area 0.95 cm²). The absorber layer was grown by a multi‐stage deposition process at a maximum nominal process temperature of 420°C. The Na was added via evaporation of a NaF layer prior to the absorber deposition leading to an enhanced V_oc and FF. Growth conditions and device characterization are described. Copyright © 2011 John Wiley & Sons, Ltd.     

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.     

The Role of Sulfur in Solution‐Processed Cu2ZnSn(S,Se)4 and its Effect on Defect Properties

Understanding the electrically active defects in kesterite Cu₂ZnSn(S,Se)₄(CZTSSe) is critical for the continued development of solar cells based on this material, but challenging due to the complex nature of this polycrystalline multinary material. A comparative study of CZTSSe alloys with three different bandgaps, made by introducing different fractions of sulfur during the annealing process, is presented. Using admittance spectroscopy, drive level capacitance profiling, and capacitance‐voltage profiling, the dominant defect energy level present in the low sulfur content device is determined to be 0.134 eV above the valence band maximum, with a bulk defect density of 8 × 10¹⁴ cm^?3, while the high sulfur content device shows a deeper defect energy level of 0.183 eV and a higher bulk defect density, 8.2 × 10¹⁵ cm^?3. These findings are consistent with the current density–voltage characteristics of the resulting solar cells and their external quantum efficiency. It suggests that as the sulfur content increases, the bandgap of the absorber is enlarged, leading to an increasing open‐circuit voltage (V_oc), that is accompanied by stronger recombination due to the higher defect density of the sulfur‐rich absorber. This is reflected in large V_oc deficit and poor carrier collection of the high sulfur content device.     

Aluminum‐doped Zn1 − xMgxO as transparent conductive oxide of Cu(In,Ga)(S,Se)2‐based solar cell for minimizing surface carrier recombination

A 19.5%‐efficient Cu(In,Ga)(S,Se)₂ (CIGSSe)‐based solar cell is obtained by replacing traditional CdS/ZnO buffer layers with Cd_0.75Zn_0.25S/Zn_0.79Mg_0.21O buffer layers for increasing short‐circuit current density because band‐gap energies of Cd_0.75Zn_0.25S and Zn_0.79Mg_0.21O are wider than those of CdS and ZnO, respectively. This yields the increase in external quantum efficiency in a short wavelength range of approximately 320 to 550 nm. Moreover, difference of conduction band minimum (E _C) between Zn_1 − x Mg_x O:Al (transparent conductive oxide, TCO) layer and CIGSSe absorber is optimized by varying [Mg]/([Mg] + [Zn]), x . It is revealed that Zn_1 − x Mg_x O:Al films with [Mg]/([Mg] + [Zn]) in a range of 0.10 to 0.12, enhancing E _g from 3.72 to 3.76 eV, are appropriate as TCO because of their enhanced mobility and decreased carrier density. Addition of 12% Mg into ZnO:Al to form Zn_0.88Mg_0.12O:Al as TCO layer effectively decreases surface carrier recombination and improves photovoltaic parameters, especially open‐circuit voltage and fill factor. This is the first experimental proof of the concept for optimizing E _C difference between TCO and absorber to minimize surface carrier recombination. Ultimately, conversion efficiency (η ) of CIGSSe solar cell with alternative Cd_0.75Zn_0.25S/Zn_0.79Mg_0.21O/Zn_0.88Mg_0.12O:Al (TCO) layers is enhanced to 20.6%, owing to control of total E _C alignment, which is higher η up to 12.6% relative as compared with the solar cell with traditional CdS/ZnO/ZnO:Al layers.     

Growth of Heteroepitaxial ZnO Thin Films on GaN‐Buffered Al2O3 (0001) Substrates by Low‐Temperature Hydrothermal Synthesis at 90 °C

Heteroepitaxial ZnO films are successfully grown on nondoped GaN‐buffered Al₂O₃ (0001) substrates in water at 90 °C using a two‐step process. In the first step, a discontinuous ZnO thin film (ca. 200 nm in thickness) consisting of hexagonal ZnO crystallites is grown in a solution containing Zn(NO₃)·6 H₂O and NH₄NO₃ at ca. pH 7.5 for 24 h. In the second step, a dense and continuous ZnO film (ca. 2.5 μm) is grown on the first ZnO thin film in a solution containing Zn(NO₃)·6 H₂O and sodium citrate at ca. pH 10.9 for 8 h. Scanning electron microscopy, X‐ray diffraction, UV‐vis absorption spectroscopy, photoluminescence spectroscopy, and Hall‐effect measurement are used to investigate the structural, optical, and electrical properties of the ZnO films. X‐ray diffraction analysis shows that ZnO is a monocrystalline wurtzite structure with an epitaxial orientation relationship of (0001)[11 0]_ZnO∥(0001)[11 0]_GaN. Optical transmission spectroscopy of the two‐step grown ZnO film shows a bandgap energy of 3.26 eV at room temperature. A room‐temperature photoluminescence spectrum of the ZnO film reveals only a main peak at ca. 380 nm without any significant defect‐related deep‐level emissions. The electrical property of ZnO film showed n‐type behavior with a carrier concentration of 3.5 × 10¹⁸ cm^–3 and a mobility of 10.3 cm² V^–1 s^–1.     

ILGAR‐ZnO Window Extension Layer: an adequate substitution of the conventional CBD‐CdS buffer in Cu(In,Ga) (S,Se)2‐based solar cells with superior device performance

A novel Window Extension Layer (WEL) concept for chalcopyrite‐based thin‐film solar cell devices has been developed. The optimization of its deposition is presented. WEL means the replacement of the conventional buffer layer by a layer consisting of the same material as the window, i.e., a part of the window is directly deposited onto the absorber by a soft process, such as ILGAR (Ion Layer Gas Reaction). This sequential cyclic technique has been applied to Cu(In,Ga)(S,Se)₂ absorber substrates. The ILGAR procedure was optimized with respect to the efficiency of the resulting Mo/Cu(In,Ga)(S,Se)₂/WEL/ZnO solar cells. The devices were characterized by J–V (under AM 1.5 and without illumination) as well as by quantum efficiency measurements. Devices with ZnO WEL yield total area (0.5 cm²) efficiencies of 14.6% (best cell) without any anti‐reflecting coating. The efficiencies are superior to those of the corresponding devices with CBD (Chemical Bath Deposition)‐CdS buffer (14.1%, best cell). Thus, in contrast to other ZnO buffers, ILGAR‐ZnO achieves record efficiencies exceeding those of CBD‐CdS buffered reference cells. Copyright © 2001 John Wiley & Sons, Ltd.     

Effects of Na and MoS2 on Cu2ZnSnS4 thin‐film solar cell

Cu₂ZnSnS₄ (CZTS)‐based materials have a useful band gap and a high absorption coefficient; however, their power conversion efficiency is low compared with that of CdTe and Cu(In,Ga)Se₂‐based solar cells. Two of the factors that strongly affect CZTS solar cell characteristics are the MoS₂ layer and the presence of defects. In this study, Mo back‐contact layers were annealed to control MoS₂ layer formation and the Na content in the Mo layer before the absorber precursor layer was deposited. The increase in oxygen content in the Mo layer suppressed MoS₂ layer formation. In addition, the increase in Na diffusion during the initial stage of the absorber precursor deposition decreased the defect density in the absorber layer and in the absorber–buffer interface. These results were verified through measurements of the external quantum efficiency, the temperature dependence of the open‐circuit voltage (V_OC), and admittance spectra. The current densities (J_SC) and V_OC, as well as the power conversion efficiencies, improved as the annealing temperature of the Mo layer increased, which suggests that CZTS solar cell characteristics can be improved by suppressing MoS₂ layer formation and increasing Na content in the Mo layer before deposition of the absorber precursor layer. Copyright © 2014 John Wiley & Sons, Ltd.