Three-dimensional structure of the buffer/absorber interface in CdS/CuGaSe2 based thin film solar cells

Abstract

The chemical structure of the CdS/CuGaSe2 chalcopyrite solar cell buffer/absorber interface is investigated by combining element depth profiling using elastic recoil detection analysis and surface-near bulk sensitive x-ray emission spectroscopy. Significant Cd and S concentrations (≥0.1 at. %) are found deep in the absorber bulk. The determined high Cd and S diffusion coefficient values at 333 K of 3.6 and 3.4×10−12 cm2/s, respectively, are attributed to diffusion along CuGaSe2 grain boundaries. As a result, a three-dimensional buffer/absorber interface geometry is proposed.

Materials interface engineering for solution-processed photovoltaics

“…In addition to sodium diffusing from the glass substrate 35 , cadmium diffuses into the absorber to a depth of about 10 nm during cadmium-sulphide chemical bath deposition (studied mostly in CIGSSe devices), leading to n-type doping (Cd Cu donor defects) that can invert the top layer of the absorber and increase the band bending, and hence electric field, near the semiconductor junction 59 . Deeper diffusion of cadmium has been reported in films with many grain boundaries -the internal interfaces within the absorber film 60 . …”

Diffusion activation energy of cadmium in thin film CuInGaSe<inf>2</inf>

“…Although diffusion coefficients and activation energies for major elemental constituents like copper and impurities like iron have been extensively quantified for copper indium selenide (CuInSe 2 ) and its CIGS derivative, little attention has been paid to the diffusion activation energy and temperature-dependent diffusivity of cadmium in CIGS, especially in polycrystalline CIGS where grain boundaries are abundant. In addition, there is conflicting information in regard to diffusion activation energies of cadmium in CIS as well as the associated diffusion mechanisms [3]- [6]. The conflicting information must be resolved in order to understand on cadmium's long-term effects on the reliability of CIGS-based devices.…”

EPR g factors and tetragonal distortion for the isoelectronic Ni+ and Cu2+ centers in the CuGaSe2 crystal

“…Since the band gap is 1.68 eV, it exhibits optoelectronic properties that qualify the material to be used in solar cell absorbers [1][2][3][4][5][6][7]. The energy conversion efficiency of CuGaSe 2 single crystal solar cell is about 9.7% [6] and that of CuGaSe 2 thin-film solar cell is about 9.3% [7].…”