The Effect of Ga Content on the Recombination Behavior of Grain Boundaries in Cu(In,Ga)Se 2 Solar Cells
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The Effect of Ga Content on the Recombination Behavior of Grain Boundaries in Cu(In,Ga)Se2 Solar Cells Harvey Guthrey1, Miguel Contreras1, Mowafak Al-Jassim1 1
National Renewable Energy Laboratory, Golden, CO 80401, U.S.A.
ABSTRACT The highest efficiency CuIn1-xGaxSe2 (CIGS) based solar cells have been produced from films with x~0.3 which gives a value of Eg around 1.1-1.2eV. Increasing the Ga content of the CIGS absorber provides an increase in Voc, allows tuning of the band gap that can enhance performance under actual operating conditions, and potentially makes it possible to use CIGS films in multi-junction devices. However, champion cells have not yet been produced for values of x significantly greater than 0.3. This work focuses on how increased Ga content in CIGS films affects the recombination behavior of grain boundaries. Cathodoluminescence spectral imaging (CLSI) measurements on fully processed devices allow us to compare device properties with recombination behavior and optical properties of grain boundaries in films with different Ga content. Our data suggests that grain boundaries in high efficiency films with x~0.3 exhibit a significant red shift in the CL spectra whereas grain boundaries in films with higher Ga content typically show either a small shift or none at all. This shift indicates band bending near the boundaries which could enhance charge separation and subsequent collection of carriers generated near grain boundaries. This is investigated statistically to identify trends in different regions of the films. INTRODUCTION Grain Boundaries
Grain boundary behavior in CIGS PV devices has been widely debated and remains of great interest. It is well known that polycrystalline CIGS performs better than its single crystal counterpart, however the mechanism responsible for this is not well understood. A study by Hanna et al. showed a possible relationship between the dominant crystallographic orientation of films and the electrical activity of grain boundaries by utilizing TEM-based CL analysis[1]. It was proposed that the work function of boundaries in 112 oriented films induced downward band bending and thus enhanced non-radiative recombination. In 220/204 oriented films the larger work function had the opposite effect of upward band bending that causes grain boundaries to repel minority carriers (electrons) and so non-radiative recombination is reduced. Reduced recombination at grain boundaries has also been attributed to hole barriers that result from Cu depletion (VCu) at CIGS grain boundaries [2, 3]. Cu depletion at grain boundaries has been confirmed in other work [4, 5]. In another study by Nichterwitz et al. EBIC imaging of a CIGS film in cross section revealed a decrease in current collection at a grain boundaries [6]. Scanning kelvin probe microscopy and scanning capacitance microscopy studies have identified built-in grain boundary potentials that should act as hole barriers [7, 8]. One possibility for the variation in experimental results pertaining to the optoelectronic activity
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