Dimitrios Hariskos

An 11.4% efficient polycrystalline thin film solar cell based on CuInS2 with a Cd-free buffer layer

The fabrication of a 11.4% efficient thin film solar cell based on CuInS2 with an Inx(OH,S)y buffer layer is described. The device parameters and performance are compared to heterojunctions with a standard Us buffer layer. A junction breakdown at negative bias under illumination is related to the buffer layer. A simple model implying photoconductive shunting paths is presented.

Influence of sodium on the growth of polycrystalline Cu(In,Ga)Se2 thin films

We investigate the influence of Na on the growth of Cu(In,Ga)Se2 thin-films by three model experiments. First, we examine the influence of Na on the Se activity during selenisation of Mo films on soda-lime and borosilicate glass after growth and after thermal treatments. Second, we analyse the location and possible binding partners of Na in polycrystalline Cu(In,Ga)Se2 prior to and after air-exposure. The final experiment focuses on the identification of the chemical state of Na on the surface of as grown and air-exposed films. Our experiments demonstrate that Na influences the growth of CIGS Cu(In,Ga)Se2 due to its interaction with Se. In non-air-exposed films Na is mainly localised in the form of sodium-polyselenides (Na2Sex) at the grain boundaries. We conclude that Na2Sex acts as Se-reservoir during film formation and oxidation.

Model for electronic transport in Cu(In,Ga)Se2 solar cells

Temperature-dependent measurements of the current–voltage characteristics and of the junction admittance of ZnO/CdS/Cu(In,Ga)Se2 heterojunction solar cells are presented, together with numerical modelling of these experimental results. We explain the cross-over between dark and illuminated current–voltage characteristics currently observed for this type of device by the impact of the defect chalcopyrite layer at the surface of the Cu(In,Ga)Se2 absorber. Our model assumes an illumination-dependent voltage drop across a defect layer with a thickness of 15 nm to explain the cross-over. The voltage drop results from the electrical dipole made up of donor-like states at the interface between the defect layer and CdS and deep acceptor states in the defect layer itself. The illumination dependence of this voltage drop is explained by photogenerated holes trapped by the deep acceptor states in the defect layer. Numerical simulations have been carried out using the program SCAPS-1D in order to verify our model assumptions. From our model, indirect conclusions are derived concerning the maximum conduction band offsets between CdS and the defect layer and between CdS and ZnO. Copyright

Improved Photocurrent in Cu(In,Ga)Se 2 Solar Cells: From 20.8% to 21.7% Efficiency with CdS Buffer and 21.0% Cd-Free

New processing developments in the Cu(In,Ga)Se2 (CIGS)-based solar cell technology have enabled best cell efficiencies to exceed 21%. The key innovation involves the alkali post-deposition treatment (PDT) of the CIGS film. Furthermore, the range of optimal CIGS growth parameters and the minimal thickness of the CdS buffer layer is affected by the process modifications. In 2013, we reported a 20.8% record device with PDT. Later optimizations, e.g., in the composition profile and CdS buffer layer thickness as discussed in this study, enabled us to increase the photocurrent density with only a slight loss in open-circuit voltage and unchanged fill factor, resulting in the current world record of 21.7% efficiency. Furthermore, a record efficiency of 21.0% could be achieved with a Cd-free Zn(O,S) buffer layer. This contribution presents measurements, simulations, and a discussion of the photocurrent increase.

CD-free Cu(In,Ga)Se2 thin-film solar modules with In2S3 buffer layer by ALCVD

The atomic layer chemical vapour deposition (ALCVD) technique allows the deposition of highly homogeneous thin-films with an excellent step coverage. This method has already shown promising results for the deposition of cadmium-free buffer layers in Cu(In,Ga)Se2 (CIGS) thin-film solar cells (13.5% efficiency with indium sulphide buffer). In this work, the process has been up-scaled to module areas of up to 30×30 cm2. The indium sulphide buffer layer was deposited at substrate temperatures between 160 and 220 °C using indium acetylacetonate and hydrogen sulphide precursors. An efficiency of η=10.8% (open-circuit voltage, VOC=592 mV; fill factor, FF=62%; current density, jSC=29.5 mA/cm2) for a module area of 30×30 cm2 has been achieved. For laboratory cells even an efficiency of 14.9% was realised. Damp heat stability testing of CIGS mini-modules indicates a similar behaviour of both devices with ALCVD indium sulphide and solution grown cadmium-sulphide buffer layer.

Chemical bath deposition of CdS buffer layer : prospects of increasing materials yield and reducing waste

Abstract The CdS buffer layer for CIGS-based solar cells is grown in an aqueous solution containing a cadmium salt, ammonia, and thiourea. Bottlenecks of this technique called chemical bath deposition (CBD) are the low material yield and the production of toxic CdS-containing waste. To improve yield and reduce waste, the CdS precipitate was separated from the waste after deposition by ultra-filtration, and the permeate, which contains ammonia and thiourea, was used for the next CBD process after addition of cadmium salt. The use of permeate leads to a decrease of the CdS growth rate but has no significant influence on the CdS film composition and on the Cu(In,Ga)Se 2 /CdS/ZnO device performance. The prominent formation of guanidine and urea was identified and quantified by chemical analysis of the permeate. A decrease of the deposition rate is observed as a function of the number of runs, which is related to the enrichment of the permeate with reaction products and to hydroxide ion consumption. The growth rate can be maintained by adjusting the concentrations after each CBD run.

Large-area CIGS modules: processes and properties

Abstract For the photovoltaic (PV) power market, modules with an area in the square meter range are necessary. ZSW and Wurth Solar developed for the first time processes for a Cu(In,Ga)Se2 (CIGS) pilot line for modules of an area up to 1.2×0.6 m2, which is a common size for PV generators. The pilot line has been running since the first half of 2001 and CIGS modules for different applications are realised. This paper describes the baseline processes and discusses the loss mechanisms for modules and cells. The production statistics for the 0.7 m2 modules are given with top aperture efficiencies of over 12% and average values approximately 10%. Failures due to inhomogeneous film properties are discussed with respect to upscaling from cells to modules. New processes, like thin foil substrates, atomic layer chemical vapour deposition of In2S3 instead of CBD CdS, or modifications of the absorber layer are tested in the ZSW line. Data from outdoor measurements demonstrate that CIGS modules also perform well under real conditions.

CIGS Cells and Modules With High Efficiency on Glass and Flexible Substrates

Thin-film solar cells based on Cu(In,Ga)(Se,S) 2 (CIGS) have demonstrated both high efficiencies and a high cost-reduction potential in industrial production. This way, future CIGS module production lines can be profitable even for scales below the GW range. Among the different technologies, only the coevaporation method has demonstrated efficiencies above 20%, approaching the record values of polycrystalline Si cells. The main focus of this contribution is on the new results of the ZSW cell line with efficiencies above 20%, as well as on the mini-module line on glass substrates. Mini modules (10 cm × 10 cm) with efficiencies in the range of 17% give a proof of concept for industrial-sized modules. ZSW is also developing flexible cells and modules, transferring the processes from the glass-based technology. We achieved 18.6% cell efficiency on metal substrates and a 15.4% efficient mini module could be demonstrated with adapted methods of module patterning. In order to develop industrially relevant processes for foils, we are running a roll-to-roll deposition plant. Additionally, we have improved CIGS cell efficiencies with alternative buffers to certified 19.0% for solution-grown Zn(O,S), to 16.4% for sputtered Zn(O,S), and 17.1% for evaporated In 2S 3. Our cells deposited by vacuum-free methods exhibit an efficiency of 8.5% with a nanoparticle-based process.

Chemical bath deposition of Zn(O,S) and CdS buffers: Influence of Cu(In,Ga)Se2 grain orientation

The present contribution discusses buffer growth by chemical bath deposition (CBD) on polycrystalline Cu(In,Ga)Se2 (CIGS) films deposited by in-line co-evaporation with an integral [Ga]/([Ga]+[In]) ratio of 0.3. We report a correlation of the coverage of CBD Zn(O,S) and CdS films with the CIGS grain orientation as determined by electron backscatter diffraction. 〈221〉-oriented CIGS grains are sparsely covered with the CBD films, whereas on 〈100〉/〈001〉- and 〈110〉/〈201〉-oriented CIGS grains, we found very dense coverage of the CIGS surfaces. This result may be explained by lower energies of CIGS {112} surfaces compared with those of {100}/{001} and {110}/{102}.

Method for a High-Rate Solution Deposition of Zn(O,S) Buffer Layer for High-Efficiency Cu(In,Ga)Se 2 -Based Solar Cells

We present a method for high-rate solution growth of the Zn(O,S) buffer layer to achieve deposition rates and material consumptions far below the standard Zn(O,S) and CdS deposition method. We replace the organosulfide thiourea by the more quickly decomposable thioacetamide and control the reaction kinetics by the use of chelating ligands and ammonia. We characterize the produced layers by secondary neutral mass spectrometry, X-ray diffraction, and optical transmission. For cell preparation, we use high-efficiency Cu(In,Ga)Se2 with an alkali-modified surface, as well as industrially relevant inline absorber material. We realize a certified 21% cell efficiency with the standard thiourea-based Zn(O,S) and first cells with over 19 % with the high-rate Zn(O,S) buffer.