Highly‐efficient Cd‐free CuInS2 thin‐film solar cells and mini‐modules with Zn(S,O) buffer layers prepared by an alternative chemical bath process

Recent progress in fabricating Cd‐ and Se‐free wide‐gap chalcopyrite thin‐film solar devices with Zn(S,O) buffer layers prepared by an alternative chemical bath process (CBD) using thiourea as complexing agent is discussed. Zn(S,O) has a larger band gap (E_g = 3·6–3·8 eV) than the conventional buffer material CdS (E_g = 2·4 eV) currently used in chalcopyrite‐based thin films solar cells. Thus, Zn(S,O) is a potential alternative buffer material, which already results in Cd‐free solar cell devices with increased spectral response in the blue wavelength region if low‐gap chalcopyrites are used. Suitable conditions for reproducible deposition of good‐quality Zn(S,O) thin films on wide‐gap CuInS₂ (‘CIS’) absorbers have been identified for an alternative, low‐temperature chemical route. The thickness of the different Zn(S,O) buffers and the coverage of the CIS absorber by those layers as well as their surface composition were controlled by scanning electron microscopy, X‐ray photoelectron spectroscopy, and X‐ray excited Auger electron spectroscopy. The minimum thickness required for a complete coverage of the rough CIS absorber by a Zn(S,O) layer deposited by this CBD process was estimated to ∼15 nm. The high transparency of this Zn(S,O) buffer layer in the short‐wavelength region leads to an increase of ∼1 mA/cm² in the short‐circuit current density of corresponding CIS‐based solar cells. Active area efficiencies exceeding 11·0% (total area: 10·4%) have been achieved for the first time, with an open circuit voltage of 700·4 mV, a fill factor of 65·8% and a short‐circuit current density of 24·5 mA/cm² (total area: 22·5 mA/cm²). These results are comparable to the performance of CdS buffered reference cells. First integrated series interconnected mini‐modules on 5 × 5 cm² substrates have been prepared and already reach an efficiency (active area: 17·2 cm²) of above 8%. Copyright © 2006 John Wiley & Sons, Ltd.     

Criteria for improving the properties of ZnGeAs2 solar cells

We explore the potential utility of the II‐IV‐V semiconductor ZnGeAs₂ as the absorber material in solar cells. As‐deposited ZnGeAs₂ films prepared by pulsed laser deposition are amorphous because of the limited substrate temperature that can be used without the rapid loss of volatile Zn and As. Thermal processing above 450 °C results in crystallization and improved electrical properties with hole mobilities as high as 58 cm²/V s. The annealed films were used to fabricate p‐type ZnGeAs₂: n‐type CdS cells on SnO₂‐buffered borosilicate glass substrates in the so‐called superstrate geometry. Light‐induced currents of up to ~2 mA/cm² and open‐circuit voltages of up to 470 mV were observed using backside illumination, indicating that these nascent devices hold potential for realizing high performance solar cells from earth‐abundant elements. The performance of the devices fabricated to‐date is degraded by conduction through shorts resulting from the presence of micron‐sized pinholes in the absorber layer. Copyright © 2012 John Wiley & Sons, Ltd.     

Above 16% efficient sequentially grown Cu(In,Ga)(Se,S)2‐based solar cells with atomic layer deposited Zn(O,S) buffers

We report the development of Cd‐free buffers by atomic layer deposition for chalcopyrite‐based solar cells. Zn(O,S) buffer layers were prepared by atomic layer deposition on sequentially grown Cu(In,Ga)(Se,S)₂ absorbers from Bosch Solar CISTech GmbH. An externally certified efficiency of 16.1% together with an open circuit voltage of 612 mV were achieved on laboratory scale devices. Stability tests show that the behavior of the ALD‐Zn(O,S)‐buffered devices can be characterized as stable only showing a minor drift of the open circuit voltage and the fill factor. Copyright © 2015 John Wiley & Sons, Ltd.     

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.     

Growth kinetics,properties, performance,and stability of atomic layer deposition Zn–Sn–O buffer layers for Cu(In,Ga)Se2 solar cells

A new atomic layer deposition process was developed for deposition of Zn–Sn–O buffer layers for Cu(In,Ga)Se₂ solar cells with tetrakis(dimethylamino) tin, Sn(N(CH₃)₂)₄, diethyl zinc, Zn(C₂H₅)₂, and water, H₂O. The new process gives good control of thickness and [Sn]/([Sn] + [Zn]) content of the films. The Zn–Sn–O films are amorphous as found by grazing incidence X‐ray diffraction, have a high resistivity, show a lower density compared with ZnO and SnO_x, and have a transmittance loss that is smeared out over a wide wavelength interval. Good solar cell performance was achieved for a [Sn]/([Sn] + [Zn]) content determined to be 0.15–0.21 by Rutherford backscattering. The champion solar cell with a Zn–Sn–O buffer layer had an efficiency of 15.3% (V_oc = 653 mV, J_sc(QE) = 31.8 mA/cm², and FF = 73.8%) compared with 15.1% (V_oc = 663 mV, J_sc(QE) = 30.1 mA/cm², and FF = 75.8%) of the best reference solar cell with a CdS buffer layer. There is a strong light‐soaking effect that saturates after a few minutes for solar cells with Zn–Sn–O buffer layers after storage in the dark. Stability was tested by 1000 h of dry heat storage in darkness at 85 °C, where Zn–Sn–O buffer layers with a thickness of 76 nm retained their initial value after a few minutes of light soaking. Copyright © 2011 John Wiley & Sons, Ltd.     

Thin‐film Cu(In,Ga)(Se,S)2‐based solar cell with (Cd,Zn)S buffer layer and Zn1−xMgxO window layer

(Cd,Zn)S buffer layer and Zn_1−x Mg_x O window layer were investigated to replace the traditional CdS buffer layer and ZnO window layer in Cu(In,Ga)(Se,S)₂ (CIGSSe)‐based solar cell. (Cd,Zn)S with band‐gap energy (E _g) of approximately 2.6 eV was prepared by chemical bath deposition, and Zn_1−x Mg_x O films with different [Mg]/([Mg] + [Zn]) ratios, x , were deposited by radio frequency magnetron co‐sputtering of ZnO and MgO. The estimated optical E _g of Zn_1−x Mg_x O films is linearly enhanced from 3.3 eV for pure ZnO (x  = 0) to 4.1 eV for Zn_0.6Mg_0.4O (x  = 0.4). The quality of the Zn_1−x Mg_x O films, implied by Urbach energy, is severely deteriorated when x is above 0.211. Moreover, the temperature‐dependent current density‐voltage characteristics of the CIGSSe solar cells were conducted for the investigation of the heterointerface recombination mechanism. The external quantum efficiency of the CIGSSe solar cell with the (Cd,Zn)S buffer layer/Zn_1−x Mg_x O window layer is improved in the wavelength range of 320–520 nm. Therefore, a gain in short‐circuit current density up to about 5.7% was obtained, which is higher conversion efficiency of up to around 5.4% relative as compared with the solar cell with the traditional CdS buffer layer/ZnO window layer. The peak efficiency of 19.6% was demonstrated in CIGSSe solar cell with (Cd,Zn)S buffer layer and Zn_1−x Mg_x O window layer, where x is optimized at 0.211. Copyright © 2017 John Wiley & Sons, Ltd.     

The Zn(S,O,OH)/ZnMgO buffer in thin film Cu(In,Ga)(S,Se)2‐based solar cells part I: Fast chemical bath deposition of Zn(S,O,OH) buffer layers for industrial application on Co‐evaporated Cu(In,Ga)Se2 and electrodeposited CuIn(S,Se)2 solar cells

This paper is focused on the basic study and optimization of short time (<10 min) Chemical Bath Deposition (CBD) of Zn(S,O,OH) buffer layers in co‐evaporated Cu(In,Ga)Se₂ (CIGSe) and electrodeposited CuIn(S,Se)₂ ((ED)‐CIS) solar cells for industrial applications. First, the influence of the deposition temperature is studied from theoretical solution chemistry considerations by constructing solubility diagrams of ZnS, ZnO, and Zn(OH)₂ as a function of temperature. In order to reduce the deposition time under 10 min, experimental growth deposition studies are then carried out by the in situ quartz crystal microgravimetry (QCM) technique. An optimized process is performed and compared to the classical Zn(S,O,OH) deposition. The morphology and composition of Zn(S,O,OH) films are determined using SEM and XPS techniques. The optimized process is tested on electrodeposited‐CIS and co‐evaporated‐CIGSe absorbers and cells are completed with (Zn,Mg)O/ZnO:Al windows layers. Efficiencies similar or even better than CBD CdS/i‐ZnO reference buffer layers are obtained (15·7% for CIGSe and 8·1% for (ED)‐CIS). Copyright © 2009 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.     

Structure of chemically deposited Zn(S,O,OH) buffer layer and the effects on the performance of Cu(in,Ga)Se2 solar cell

The effects of the immersion into a NH₃ aqueous solution on the structural characteristics of the chemically deposited Zn(S,O,OH) layer and photovoltaic performance of the CIGS/Zn(S,O,OH) solar cells were investigated with structural and electrical characterizations. The as‐deposited‐Zn(S,O,OH) layer possessed a layered structure of upper Zn(OH)₂ and Zn(S,O) layers, and the upper Zn(OH)₂ layer was removed by the immersion. The conversion efficiency for the CIGS solar cell was improved from 6.8% to 13.7% by removing the upper Zn(OH)₂ layer during the immersion. Copyright © 2015 John Wiley & Sons, Ltd.     

Secondary phase formation in Zn‐rich Cu2ZnSnSe4‐based solar cells annealed in low pressure and temperature conditions

Zn‐rich Cu₂ZnSnSe₄ (CZTSe) films were prepared by a two‐step process consisting in the DC‐magnetron sputtering deposition of a metallic stack precursor followed by a reactive anneal under a Se + Sn containing atmosphere. Precursor composition and annealing temperature were varied in order to analyze their effects on the morphological, structural, and optoelectronic properties of the films and solar cell devices. Raman scattering measurements show the presence of ZnSe as the main secondary phase in the films, as well as the presence of SnSe at the back absorber region of the films processed with lower Zn‐excess values and annealing temperatures. The ZnSe phase is found to accumulate more towards the surface of the absorber in samples with lower Zn‐excess and lower temperature annealing, while increasing Zn‐excess and annealing temperature promote its aggregation towards the back absorber region of the devices. These measurements indicate a strong dependence of these process variables in secondary phase formation and accumulation. In a preliminary optimization of both the composition and reactive annealing process, a solar cell with 4.8% efficiency has been fabricated, and potential mechanisms limiting device efficiency in these devices are discussed. Copyright © 2014 John Wiley & Sons, Ltd.     

Improved fill factor and open circuit voltage by crystalline selenium at the Cu(In,Ga)Se2/buffer layer interface in thin film solar cells

A surface treatment by evaporated selenium on Cu(In,Ga)Se₂ (CIGS) is shown to improve open circuit voltage, V_oc, and in some cases fill factor, FF, in solar cells with CdS, (Zn,Mg)O or Zn(O,S) buffer layers. V_oc increases with increasing amount of crystalline Se, while FF improves only for small amounts. The improvements are counteracted by a decreasing short circuit current assigned to absorption in hexagonal Se. Improved efficiency is shown for device structures with (Zn,Mg)O and Zn(O,S) buffer layers by atomic layer deposition. Analysis by grazing incidence X‐ray diffraction and photoelectron spectroscopy show partial coverage of the CIGS surface by hexagonal selenium. The effects on device performance from replacing part of the CIGS/buffer interface area by a Se/buffer junction are discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Record 1.0 V open‐circuit voltage in wide band gap chalcopyrite solar cells

Tandem solar cell structures require a high‐performance wide band gap absorber as top cell. A possible candidate is CuGaSe₂, with a fundamental band gap of 1.7 eV. However, a significant open‐circuit voltage deficit is often reported for wide band gap chalcopyrite solar cells like CuGaSe₂. In this paper, we show that the open‐circuit voltage can be drastically improved in wide band gap p‐Cu(In,Ga)Se₂ and p‐CuGaSe₂ devices by improving the conduction band alignment to the n‐type buffer layer. This is accomplished by using Zn_1−x Sn_x O_y , grown by atomic layer deposition, as a buffer layer. In this case, the conduction band level can be adapted to an almost perfect fit to the wide band gap Cu(In,Ga)Se₂ and CuGaSe₂ materials. With an improved buffer band alignment for CuGaSe₂ absorbers, evaporated in a 3‐stage type process, we show devices exhibiting open‐circuit voltages up to 1017 mV, and efficiencies up to 11.9%. This is to the best of our knowledge the highest reported open‐circuit voltage and efficiency for a CuGaSe₂ device. Temperature‐dependent current‐voltage measurements show that the high open‐circuit voltage is explained by reduced interface recombination, which makes it possible to separate the influence of absorber quality from interface recombination in future studies.     

The effect of Zn1‐xMgxO buffer layer deposition temperature on Cu(In,Ga)Se2 solar cells: A study of the buffer/absorber interface

The effect of atomic layer deposition temperature of Zn_1‐xMg_xO buffer layers for Cu(In,Ga)Se₂ (CIGS) based solar cell devices is evaluated. The Zn_1‐xMg_xO films are grown using diethyl zinc, bis‐cyclopentadienyl magnesium and water as precursors in a temperature range of 105 to 180°C. High efficiency devices are produced in the region from 105 up to 135°C. At a Zn_1‐xMg_xO deposition temperature of 120°C, a maximum cell efficiency of 15·5% is reached by using a Zn_1‐xMg_xO layer with an x‐value of 0·2 and a thickness of 140 nm. A significant drop in cell efficiency due to large losses in open circuit voltage and fill factor is observed for devices grown at temperatures above 150°C. No differences in chemical composition, structure and morphology of the samples are observed, except for the samples prepared at 105 and 120°C that show elemental selenium present at the buffer/absorber interface. The selenium at the interface does not lead to major degradation of the solar cell device efficiency. Instead, a decrease in Zn_1‐xMg_xO resistivity by more than one order of magnitude at growth temperatures above 150°C may explain the degradation in solar cell performance. From energy filtered transmission electron microscopy, the width of the CIGS/Zn_1‐xMg_xO chemical interface is found to be thinner than 10 nm without any areas of depletion for Cu, Se, Zn and O. Copyright © 2008 John Wiley & Sons, Ltd.     

High‐rate deposition of microcrystalline silicon in a large‐area PECVD reactor and integration in tandem solar cells

We study the high‐rate deposition of microcrystalline silicon in a large‐area plasma‐enhanced chemical‐vapor‐deposition (PECVD) reactor operated at 40.68 MHz, in the little‐explored process conditions of high‐pressure and high‐silane concentration and depletion. Due to the long gas residence time in this process, the silane gas is efficiently depleted using moderate feed‐in power density, thus facilitating up‐scaling of the process to large surfaces. As observed in more traditional deposition processes, the deposition rate and performance of device‐quality material are limited by the inter‐electrode gap of the reactor. We significantly increase the cell performances by reducing this gap. X‐ray diffractometry (XRD) and secondary ion mass spectroscopy (SIMS) are used to characterize the microcrystalline material deposited in the modified reactor at a rate of 1 nm/s. Comparison with a microcrystalline process at a low deposition rate demonstrates that the crystallographic orientation of the absorbing layer of the cell and the concentrations of contaminants are strongly correlated and dependent on the process. We use microcrystalline cells with absorber layer grown at a rate of 1 nm/s integrated as bottom cells in amorphous‐microcrystalline (micromorph) tandem solar cells using the superstrate configuration. We report an initial efficiency of 10.8% (9.6% stabilized) for a tandem cell with 1.2 cm² surface. Copyright © 2010 John Wiley & Sons, Ltd.     

Compositional engineering of chemical bath deposited (Zn,Cd)S buffer layers for electrodeposited CuIn(S,Se)2 and coevaporated Cu(In,Ga)Se2 solar cells

A comparative study of chemical bath deposition (CBD) of ZnS, CdS, and a mixture of (Cd,Zn)S buffer layers has been carried out on electrodeposited CuIn(S,Se)₂ (CISSe) and coevaporated Cu(In,Ga)Se₂ (CIGS) absorbers. For an optimal bath composition with the ratio of [Zn]/[Cd] = 25, efficiencies higher than those obtained with CdS and ZnS recipes, both on co‐evaporated CIGS and electrodeposited CISSe, have been obtained independent of the absorber used. In order to better understand the (Cd,Zn)S system and its impact on the increased efficiency of cells, predictions from the solubility diagrams of CdS and ZnS in aqueous medium were made. This analysis was completed by in situ growth studies with varying bath composition by quartz crystal microbalance (QCM). The morphology and composition of the films were studied using scanning electron microscopy (SEM) and X‐ray photoelectron spectra (XPS) techniques. Preliminary XPS studies showed that films are composed of a mixture of CdS and Zn(O,OH) phases and not a pure ternary Cd_{1 − x}Zn_xS compound. The effect of the [Zn]/[Cd] molar ratio on properties of the corresponding CISSe and CIGS solar cells was investigated by current voltage [J(V)] and capacitance voltage [C(V)] characterizations. The origin of optimal results is discussed. Copyright © 2008 John Wiley & Sons, Ltd.     

Impact of vacuum and nitrogen annealing on HVE SnS photoabsorber films

Researchers worldwide focus on new earth abundant and cheap absorber materials for use in thin film solar cells that allow wider use of photovoltaics in energy production. SnS is one of such promising absorber materials that comprises earth abundant elements (Sn, S). We describe here the effect of annealing of high vacuum evaporated (HVE) SnS thin films in vacuum and nitrogen atmosphere with relatively high pressures of nitrogen. SnS thin films with a thickness of 500 nm were deposited onto the surface of glass by HVE at a substrate temperature of 300 °C. The as-deposited SnS thin films were annealed at 500 °C and 550 °C for 1 h in vacuum as well as in nitrogen with respect to ambient (N₂) pressure that varied in the range of 500–2000 mbar. We analyze crystalline quality, crystal structure, elemental and phase compositions, and electrical properties of SnS films before and after the annealing process and their changes. Our results show that the use of pressurized inert ambient, such as nitrogen, improves the crystalline quality as well as the electrical properties of SnS thin films. The enhanced growth of crystals and modification of microstructural properties of SnS thin films as a function of annealing conditions (type of ambient, annealing temperature and ambient pressure) are discussed in detail.     

Minimizing metastabilities in Cu(In,Ga)Se2/(CBD)Zn(S,O,OH)/i‐ZnO‐based solar cells

Chemical bath deposited (CBD)Zn(S,O,OH) is among the alternatives to (CBD)CdS buffer layers in Cu(In,Ga)Se₂(CIGSe)‐based devices. Nevertheless, the performances reached by devices buffered with (CBD)Zn(S,O,OH) vary strongly from one sample to another and from one laboratory to another, indicating that parameters of minority impact with (CBD)CdS‐buffered devices have major influence when buffered with (CBD)Zn(S,O,OH). Moreover, the literature reports, but not systematically, the requirement of substituting the standard resistive intrinsic ZnO by (Zn,Mg)O and/or soaking the devices in ultraviolet‐containing light in order to reach optimal device operation. The present study investigates the impact of the three following parameters on the optoelectronic behavior of the Cu(In,Ga)Se₂/(CBD)Zn(S,O,OH)/i‐ZnO‐based solar cells: (i) CIGSe surface composition; (ii) (CBD)Zn(S,O,OH) layer thickness; and (iii) i‐ZnO layer resistivity. The first conclusion of this study is that all of these parameters are observed to influence the electrical metastabilities of the devices. The second conclusion is that the light soaking time needed to achieve optimal photovoltaic parameters is decreased by (i) using absorbers with Cu content close to stoichiometry, (ii) increasing the buffer layer thickness, and (iii) increasing the resistivity of i‐ZnO. By optimizing these trends, stable and highly efficient Zn(S,O,OH)‐buffered CIGSe solar cells have been fabricated. Copyright © 2014 John Wiley & Sons, Ltd.     

The Zn(S,O,OH)/ZnMgO buffer in thin‐film Cu(In,Ga)(Se,S)2‐based solar cells part II: Magnetron sputtering of the ZnMgO buffer layer for in‐line co‐evaporated Cu(In,Ga)Se2 solar cells

A ZnS/Zn_1‐xMg_xO buffer combination was developed to replace the CdS/i‐ZnO layers in in‐line co‐evaporated Cu(In,Ga)Se₂(CIGS)‐based solar cells. The ZnS was deposited by the chemical bath deposition (CBD) technique and the Zn_1‐xMg_xO layer by RF magnetron sputtering from ceramic targets. The [Mg]/([Mg] + [Zn]) ratio in the target was varied between x = 0·0 and 0·4. The composition, the crystal structure, and the optical properties of the resulting layers were analyzed. Small laboratory cells and 10 × 10 cm² modules were realized with high reproducibility and enhanced stability. The transmission is improved in the wavelength region between 330 and 550 nm for the ZnS/Zn_1‐xMg_xO layers. Therefore, a large gain in the short‐circuit current density up to 12% was obtained, which resulted in higher conversion efficiencies up to 9% relative as compared to cells with the CdS/i‐ZnO buffer system. Peak efficiencies of 18% with small laboratory cells and 15·2% with 10 × 10 cm² mini‐modules were demonstrated. Copyright © 2009 John Wiley & Sons, Ltd.     

Characterisation of CuInS2/ Zn(Se,O)/ZnO solar cells as a function of Zn(Se,O) buffer deposition kinetics in a chemical bath

Thin‐film solar cells of CuInS₂/Zn(Se,O)/ZnO configuration have been studied from the point of view of their dependence on the Zn(Se,O) chemical bath deposition (CBD) conditions. The kinetics of deposition of the Zn(Se,O) buffer is followed during cell processing with a quartz crystal microbalance (QCM). Two different CBD growth mechanisms yield buffer layers with different properties. Under a predominant electroless deposition reaction, the resulting buffer layer has mixed ZnSe–ZnO composition. The solar cells with this buffer type show higher fill factor (FF) and lower open‐circuit voltage (V_oc). Under a chemical growth regime, the buffer layer has higher ZnSe proportion, giving rise to cells with higher V_OC, but lower FF and stability. The parameters of this second type of cell also show major dependence on illumination effects (light‐soaking effects). Electron‐beam‐induced current (EBIC) and cathodoluminescence (CL) measurements are carried out to characterise the CuInS₂/Zn(Se,O) junctions formed under the two buffer growth regimes. Cross‐sectional EBIC shows a wider space charge region (SCR) than expected for p‐CuInS₂ in contact with Zn(Se,O), and the p–n junction is driven within the CuInS₂ phase. These results reflect a chemical modification of CuInS₂, most probably caused by the ammonia of the bath solution. CL shows more defective interfaces when Zn(Se,O) is deposited under the chemical mechanism (slower deposition rate, hence longer contact time of the CuInS₂ with the bath solution) than under the electroless kinetics (faster deposition rate). Copyright © 2002 John Wiley & Sons, Ltd.     

High potential of thin (<1 µm) a‐Si: H/µc‐Si:H tandem solar cells

Silicon based thin tandem solar cells were fabricated by plasma enhanced chemical vapor deposition (PECVD) in a 30 × 30 cm² reactor. The layer thicknesses of the amorphous top cells and the microcrystalline bottom cells were significantly reduced compared to standard tandem cells that are optimized for high efficiency (typically with a total absorber layer thickness from 1.5 to 3 µm). The individual absorber layer thicknesses of the top and bottom cells were chosen so that the generated current densities are similar to each other. With such thin cells, having a total absorber layer thickness varying from 0.5 to 1.5 µm, initial efficiencies of 8.6–10.7% were achieved. The effects of thickness variations of both absorber layers on the device properties have been separately investigated. With the help of quantum efficiency (QE) measurements, we could demonstrate that by reducing the bottom cell thickness the top cell current density increased which is addressed to back‐reflected light. Due to a very thin a‐Si:H top cell, the thin tandem cells show a much lower degradation rate under continuous illumination at open circuit conditions compared to standard tandem and a‐Si:H single junction cells. We demonstrate that thin tandem cells of around 550 nm show better stabilized efficiencies than a‐Si:H and µc‐Si:H single junction cells of comparable thickness. The results show the high potential of thin a‐Si/µc‐Si tandem cells for cost‐effective photovoltaics. Copyright © 2010 John Wiley & Sons, Ltd.