M. Ruckh

ZnO/CdS/CuInSe2 thin‐film solar cells with improved performance

An important milestone in the development of photovoltaic thin‐film solar cells is the achievement of 15% conversion efficiency. This letter describes the highest efficiency single junction thin‐film cell reported to date. An active area efficiency of 14.8% is obtained with the cell structure n‐ZnO/n‐CdS/p‐CuInSe2 deposited on a soda‐lime glass substrate. The current achievements are due to improved properties of the CuInSe2 layer and the heterojunctions compared to previously reported results. The rate and substrate temperature profiles used during the coevaporation process yield a relatively large‐grained material with very strong 〈112〉 orientation and low porosity. This results in reduced recombination rates, hence higher open circuit voltage and fill factor.

A novel cadmium free buffer layer for Cu(In,Ga)Se2 based solar cells

Abstract Solar cells based on Cu(In,Ga)Se 2 were prepared replacing the “standard buffer layer” CdS with a In x (OH,S) y thin film. The film is deposited in a chemical bath (CBD) process using an aqueous solution containing InCl 3 and thioacetamide. X-ray photoemission spectroscopy measurements were performed in order to characterize the growth kinetics and the chemical composition. The influence of different concentrations of InCl 3 and thioacetamide in the solution on the electrical properties of the solar cells was studied by measuring the j-V characteristics and the spectral quantum efficiencies. Capacitance-voltage ( C-V ) measurements indicate that the high V ∞ values of devices with the novel buffer layer are correlated with narrower space charge widths and higher effective carrier concentrations in the absorber materials. The achieved conversion efficiency of 15.7% (active area) using the cadmium free In x (OH,S) y buffer demonstrates the potential of this process as an alternative to the standard chemical bath deposition of CdS.

ZnO/CdS/Cu(In,Ga)Se/sub 2/ thin film solar cells with improved performance

This paper reports results from experiments concerning the growth of CuInSe/sub 2/ films on different substrate materials, uncoated, and coated with molybdenum. Specifically the effect on the structure, i.e. preferred orientation, of the polycrystalline films is investigated. It is found that soda-lime float glass results in the most oriented films and also that the highest solar cell conversion efficiency is obtained with devices made from such films. In another set of experiments the effect of various deposition conditions for the ZnO window layer is studied. It is found that optimum performance is not strongly dependent on the deposition process. The highly doped part of the window, ZnO:Al, has been replaced with ITO on some devices and a comparison is made. Finally, ZnO/CdS/CuInSe/sub 2/ and ZnO/CdS/Cu(In,Ga)Se/sub 2/ thin film devices exhibiting active area conversion efficiencies of 15.4% and 16.9%, respectively, are demonstrated.<<ETX>>

Photoemission studies on Cu(In, Ga)Se2 thin films and related binary selenides

Abstract The aim of this work is to provide a data basis for both X-ray and UV photoelectron spectroscopy on chalcopyrite thin films. A model for the segregation behaviour at the surfaces of polycrystalline thin films of I–III–VI 2 -chalcopyrites (I = Cu; III = In, Ga; VI = Se) based on surface analysis data is presented. In situ photoemission measurements on a variety of Cu(In, Ga)Se 2 samples, as well as on Cu 2− x Se ( x ≈ 0.15), In 2 Se 3 and Ga 2 Se 3 thin film samples, clearly prove the existence of Cu 2− x Se on the surfaces of all Cu-rich thin films (i.e. Cu (In + Ga) ). The surface composition of Cu-poor thin films generally deviates strongly from the bulk stoichiometry. It has been found that the as grown surfaces of Cu-poor thin films are in most cases covered by In Ga-rich defect compounds. The surface of bulk Cu-poor CuInSe 2 has been identified as CuIn 3 Se 5 . For bulk Cu-poor CuGaSe 2 , the surface composition is determined by the conditions of film growth; it ranges between stoichiometric CuGaSe 2 and CuGa 5 Se 8 . This study is based on photo e lectron d istribution c urves (EDCs) which have been measured for photon energies of hv = 21.2, 40,8 and 1253.6 eV. The photoionization cross-sections of the atomic levels comprising the valence bands are strongly dependent on hv . This dependence is exploited to correctly interpret the observed valence band features of the different surface species investigated. The energy positions of the valence band maxima, which are of great practical importance considering heterojunction devices have been determined. For Cu-rich thin films these values could be extracted from the measurements by numerical subtraction of the EDC of the overlaying Cu 2− x Se from the EDCs of Cu-rich surfaces. The binding energies of the core levels and the kinetic energies of the main Auger structures are given for all the materials examined.

Influence of substrates on the electrical properties of Cu(In,Ga)Se2 thin films

Abstract Thin films based on Cu(In,Ga)Se 2 prepared on alkali free substrates are compared to films prepared on soda lime glass. On the latter, the presence of sodium species as detected with X-ray photoelectron spectroscopy is correlated with an enhanced formation of Se O, In O and Ga O bonds at the surface after several days air exposure. The electrical conductivity is also one order of magnitude higher for films on soda lime glass. Solar cells prepared on these substrates exhibit increased open circuit voltages. Capacitance-voltage characteristics on junctions prepared on alkali free substrates show an increased space charge width. The observations can be explained in terms of an increased net acceptor density in polycrystalline Cu(In,Ga)Se 2 when prepared on soda lime glass substrates. An alkali-metal-promoted oxidation of the surface is discussed.

XPS and IR analysis of thin barrier films polymerized from C2H4/CHF3 ECR‐plasmas

Thin fluorocarbon polymer films are prepared on PE-foils in low-pressure electron cyclotron resonance plasmas using ethylene (C2H4) and trifluoromethane (CHF3) as monomers. The thin fluorinated hydrocarbon layers strongly reduces the permeability of polyethylene to alkanes. For example, the permeation of toluene was decreased by a factor of about 100 by a single, thin fluorocarbon layer. A further reduction of the permeation down to a factor of 1600 can be obtained by a multilayer coating. X-ray photoelectron spectroscopy and Fourier transform IR spectroscopy are used to characterize the plasma polymerized films. It is shown that the addition of CHF3 to a C2H4 plasma leads to an increase of CF3—, CF2—, and CF— groups and to a decrease of CH3— and CH2— groups in the film. The chemical composition of the polymer layers and their toluene permeabilities are discussed.

Small- and large-area CIGS modules by co-evaporation

A system for large-area deposition of Cu(In,Ga)Se/sub 2/ by co-evaporation has been built and taken into operation. The system is designed after a patented approach, to use the kind of in-line process we are using in our small-area research work. Small-area laboratory cells have been made from CuInSe/sub 2/ and Cu(In,Ga)Se/sub 2/ films fabricated by the in-line process reaching active area efficiencies as high as 15.4% (CuInSe/sub 2/) and 17.6% (Cu(In,Ga)Se/sub 2/). Our Cu(In,Ga)Se/sub 2/ module technology is described and the best results are reported. Best obtained aperture area efficiency for small modules (15 cm/sup 2/) is 12.4% made on Cu(In,Ga)Se, films from our small-area deposition system. The large-area deposition system has produced CuInSe/sub 2/ from which small-area cells with conversion efficiencies of 8.3% have been obtained so far.

Applications of ZnO in Cu(In,Ga)Se/sub 2/ solar cells

The influence of preparation conditions of the transparent conductive oxide (TCO) ZnO in a magnetron sputtering process on the properties of solar cells based on Cu(In,Ga)Se/sub 2/ absorbers is examined. Criteria for optimized ZnO layers for high efficiency applications are given. Preparing at substrate temperatures below 200/spl deg/C, the sputtering conditions have only a minor effect on the device properties. A novel high rate reactive DC-sputtering process has been developed leading to highly transmissive conductive ZnO layers with no additional substrate heating. TCO layers suitable for module applications have been deposited in less than 5 minutes. Solar cell device properties using ZnO prepared with this process are comparable to cells with standard RF-sputtered ZnO. This demonstrates the possibility to apply a scalable high deposition rate ZnO process to high efficient Cu(In,Ga)Se/sub 2/ solar cells. A significant cost reduction for large area applications is expected with this novel process.

Thin film solar modules based on CIS prepared by the co-evaporation method

CuInSe/sub 2/ (CIS) and related compounds have demonstrated their high potential for high efficiency thin film solar cells. A key issue for the development of modules is upscaling of the CIS absorber deposition. Different preparation methods have been applied. For transferring all process steps on large area with high throughput and process yield, the strategy is to optimize the methods available in view of highest device performance. For the deposition of the different films, standard techniques for optimized small area devices have been applied. For the upscaling of CIS deposition, a quasi in-line system for 10 cm/spl times/10 cm substrates was constructed. In a second step, a real in-line system for 30 cm/spl times/30 cm substrates with linear evaporation sources was developed. This system is already in operation and uniform Cu(In,Ga)Se/sub 2/ (CIGS) films have already been produced. Based on laboratory processes, static and dynamic, about 50 modules with an aperture area up to 90 cm/sup 2/ and 10 to 15 interconnected cells have been prepared. For the 30 cm/spl times/30 cm technique, all film depositions from the gas phase are based on in-line processes. Mo and ZnO films are sputtered from linear magnetron targets and CIS is deposited by co-evaporation onto moving substrates. All process steps including patterning, electrical contacts and encapsulation have been set up and films with sufficient quality and uniformity for 30 cm/spl times/30 cm solar cell modules have been produced.

A comprehensive characterization of the interfaces in Mo/CIS/CdS/ZnO solar cell structures

The alignment of energy bands in a Mo/CIS/CdS/ZnO solar cell structure is presented. Special attention is paid to the surface chemistry of the molybdenum coated substrate. In this study we have performed in situ analyses on polycrystalline thin films starting from the molybdenum coated soda lime glass. The different films have been deposited sequentially under ultra high vacuum conditions and analyzed with photoelectron spectroscopy techniques. We have found that Mo surface is significantly oxidized. Air annealing at 200/spl deg/C leads to a diffusion of sodium through the Mo layer. Mo-O and Mo-Se compounds are formed and present during the evolution of the Mo-CIS interface. Band lineups have been determined for the CIS/CdS/ZnO interfaces of the solar cell structure. We have found no indication for band discontinuities deteriorating the solar cell device performance.