Electron-beam-induced current at absorber back surfaces of Cu (In,Ga) Se2 thin-film solar cells

The following article appeared in Journal of Applied Physics 115.1 (2014): 014504 and may be found at http://scitation.aip.org/content/aip/journal/jap/115/1/10.1063/1.4858393The present work reports on investigations of the influence of the microstructure on electronic properties of Cu(In,Ga)Se2 (CIGSe) thin-film solar cells. For this purpose, ZnO/CdS/CIGSe stacks of these solar cells were lifted off the Mo-coated glass substrates. The exposed CIGSe backsides of these stacks were investigated by means of electron-beam-induced current (EBIC) and cathodoluminescence (CL) measurements as well as by electron backscattered diffraction (EBSD). EBIC and CL profiles across grain boundaries (GBs), which were identified by EBSD, do not show any significant changes at Σ3 GBs. Across non-Σ3 GBs, on the other hand, the CL signals exhibit local minima with varying peak values, while by means of EBIC, decreased and also increased short-circuit current values are measured. Overall, EBIC and CL signals change across non-Σ3 GBs always differently. This complex situation was found in various CIGSe thin films with different [Ga]/([In]+[Ga]) and [Cu]/([In]+[Ga]) ratios. A part of the EBIC profiles exhibiting reduced signals across non-Σ3 GBs can be approximated by a simple model based on diffusion of generated charge carriers to the GBs.This work was supported in part by the BMU projects comCIGS and comCIGSII. R.C. acknowledges financial support from Spanish MINECO within the program Ramon y Cajal (RYC-2011-08521)

Luminescence properties of Ga graded Cu In,Ga Se2 thin films

Cathodoluminescence CL has been measured at 10 K in cross section and plan view configuration on Cu In,Ga Se2 thin films with Ga grading as they are used in high efficiency solar cells. Measurements on cross section samples show the vertical profile of the emission energy to correspond to the band gap profile of the film as calculated from measurements of the Ga grading. Hence, the CL method is capable to directly measure the band gap grading in semiconductor thin films, but often the influence of the grading on the emission energy is completely ignored in recent literature. At the same time, we observe a strong drift of excited charge carriers towards the minimum of the band gap. The transport process can be explained by the quasi electric field induced by the Ga grading and applied to determine transport properties of the Cu In,Ga Se2 material. Implications for luminescence investigations on band gap graded thin films are discusse

Grain boundary character distribution and correlations with electrical and optoelectronic properties of CuInSe2 thin films

Thin amp; 64257;lm solar cells based on polycrystalline Cu In,Ga Se2 absorbers exhibit record conversionef amp; 64257;ciencies of up to 22.6 . There is still a lack of a quantitative connection between the grain boundary character distribution GBCD and the corresponding electrical and optoelectronic properties. The present work uses microstructural data from a CuInSe2 thin amp; 64257;lm acquired by electron backscatter diffraction EBSD to evaluate the GBCD. The most prominent features of the GBCD of CuInSe2 are S3 twin boundaries and the S9 and S27a symmetric tilt grain boundaries. Moreover, combining EBSD with electron beam induced current and cathodoluminescence measurements on the same identical area on a CuInSe2 Mo glass stack provide the means to relate the grain boundary character with the corre sponding electrical and optoelectronic signals across the grain boundary. In part, determining this relationship is accomplished by means of correlation analysis using measurement data from more than 100 grain boundaries. However, the crystallographic, electrical and optoelectronic data showed no strong correlations, which is attributed to atomic reconstruction found in atomic planes adjacent to planar defects in polycrystalline CuInSe2 thin amp; 64257;lms and corresponding reductions of excess charge densities at these defects

Near interface doping by ion implantation in Cu In,Ga Se2 solar cells

Cu In,Ga Se2 absorber layers were implanted with 20 keV Cd ions in order to investigate the influence of changes in the near interface doping profile. Modifications in this region are shown by AMPS 1D simulations to have substantial impact on solar cell properties. Ion implantation and subsequent thermal annealing steps were monitored by SIMS measurements to control the thermal diffusion of the dopant. Solar cells both with and without CdS buffer layer were made from the implanted absorbers and characterized by j V and EQE measurements. These experimental results in conjunction with simulations of the quantum efficiency show that a well defined type inversion of the implanted layer can be achieved by low energy ion implantatio

Increased homogeneity and open circuit voltage of Cu In,Ga Se2 solar cells due to higher deposition temperature

Cu In,Ga Se2 absorber deposition on commonly used soda lime glass is constrained in temperature by the softening of the substrate. To overcome this limitation, a high temperature resistant glass was employed as a substrate for the growth of Cu In,Ga Se2 absorbers by multi stage coevaporation at standard 530 C and elevated 610 C temperatures. Absorbers were investigated using cathodoluminescence and X ray diffraction and compared to the performance of solar cells fabricated from these absorbers. The higher deposition temperature is shown to lead to an increased homogeneity of the absorber layer both laterally and vertically and strongly enhanced open circuit voltage. A best certified efficiency of 19.4 is reache

Buffer free Cu In,Ga Se2 solar cells by near surface ion implantation

High efficiency Cu In,Ga Se2 thin film solar cells typically include CdS buffer layers deposited in a chemical bath. In this work, Cu In,Ga Se2 devices are presented in which the CdS buffer layer was omitted completely. Instead, low energy ion implantation of group II elements Cd, Zn, and Mg is applied in order to establish an n type surface layer in p type Cu In,Ga Se2 absorber layers. Therefore, thermal annealing procedures were developed which lead to a full recovery of the implantation induced defects and simultaneously minimize the diffusion of the dopants. Such a treatment is shown to provide high quality p n junction functionality and buffer free Cu In,Ga Se2 thin film solar cells with opencircuit voltages close to 600 mV and efficiencies exceeding 1

comCIGS INTEGRATIVE FRAMEWORK OF EXPERIMENTAL AND VIRTUAL LAB

An overview is given on the present status of the comCIGS project funded by the German BMU Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit . Five partners are heading this effort which represents a framework of virtual and experimental Labs IBM Germany, Schott AG, Helmholtz Zentrum Berlin, Universities of Mainz and Jena. All the partners have capabilities for computational Materials Science, whereas the HZ Berlin and University of Jena have pilot lines for CIGS processes. Schott AG mainly concentrates on optimization and production of specialty glass substrates for CIGS manufacturing. Aim of the project is an improvement of the overall efficiency of CIGS solar cells by replacing the presently used CdS buffer layer, optimizing CIGS layer process and using a specially designed glass substrate. As first results a number of gt;200 ternary alloys Heusler phases were identified by numerical simulation that could replace CdS based on the lattice structure and band gap. The improvement of the CIGS process itself seems to be mainly driven by the combination of a specialty glass substrate with defined TG, CTE and Na content, together with a CIGS layer process that produces a homogeneous layer at elevated process temperatures gt; 600 C. Solar cell efficiencies of amp; 951; 19.2 could be obtained using process parameters that are applicable to a manufacturing environment

Compositional and electrical properties of line and planar defects in Cu In,Ga Se2 thin films for solar cells a review

The present review gives an overview of the various reports on properties of line and planar defects in Cu In,Ga S,Se 2 thin films for high efficiency solar cells. We report results from various analysis techniques applied to characterize these defects at different length scales, which allow for drawing a consistent picture on structural and electronic defect proper ties. A key finding is atomic reconstruction detected at line and planar defects, which may be one mechanism to reduce excess charge densities and to relax deep defect states from midgap to shallow energy levels. On the other hand, nonra diative Shockley Read Hall recombination is still enhanced with respect to defect free grain interiors, which is correlated with substantial reduction of luminescence intensities. Com parison of the microscopic electrical properties of planar defects in Cu In,Ga S,Se 2 thin films with two dimensional device simulations suggest that these defects are one origin of the reduced open circuit voltage of the photovoltaic devices