Elucidating the Effect of the Different Buffer Layers on the Thermal Stability of CIGSe Solar Cells

Abstract

In this contribution, the impact of thermal stress on Cu(In,Ga)Se 2 (CIGSe) thin film photovoltaic devices is investigated. The tolerance of such devices to high temperatures is of particular interest for processing transparent conductive oxides (TCOs) in order to further close the gap to the theoretical efficiency limit and for their potential use as bottom devices in tandem applications in order to overcome the theoretical efficiency limit of single junction solar cells. When CdS-buffered CIGSe high efficiency solar cells are subjected to thermal stress, elemental interdiffusion of Na and Cd between the absorber and the window layers as well as chemical reactions at the CIGSe/CdS interface result in a degraded power conversion efficiency (PCE). Here, we compare the degradation mechanisms of CdS and GaO x buffered CIGSe solar cells under thermal stress. A model explaining the observed degradation behaviors is proposed. Index Terms-Amorphous buffer layer, Cu(In,Ga)Se 2 (CIGSe) solar cells, CdS, thermal stability, GaO x. I. INTRODUCTION C u(In,Ga)Se 2 (CIGSe) thin film solar cells have attracted the attention of science and industry by attaining a maximum Manuscript

The Energy Level Alignment at the Buffer/Cu(In,Ga)Se2 Thin‐Film Solar Cell Interface for CdS and GaOx

“…For the CdS buffer, we find a slightly Cd-rich [Cd]/[S] ratio of 1.1 (±0.1); in agreement (within the experimental uncertainty) with the expected bulk composition ratio. [18] For the GaO x , we derive x to be 1.1 (±0.1), which matches well with the oxygen-deficient (compared to stoichiometric Ga 2 O 3 ) composition previously reported for amorphous GaO x layers deposited in Ar 2 atmosphere (via pulsed laser deposition) [29] or via oxidation of metallic Ga in 1 × 10 −6 mbar partial pressure of O 2 . [30] The UPS and IPES spectra of the CIGSe absorber with and without nominal 50-nm-thick CdS and GaO x buffers, respectively, are plotted in Figure 1 on a common energy scale relative to the E F level.…”

“…This agrees well with the optically derived bulk band gap of 4.71 eV reported for similarly prepared GaO x thin films. [ 18 ] The VBM (±0.10 eV), CBM (±0.20 eV), and E g Surf (±0.25 eV) values for the bare CIGSe absorber (the 50‐nm‐thick CdS/CIGSe sample) are −0.40 (−1.55), 1.15 (0.80), and 1.55 (2.35) eV, respectively, as depicted in Figure 1. The values are in line with previous UPS/IPES measurements for CIGSe absorbers having similar surface CGI and GGI ratios (resulting in E g Surf values between 1.40 and 1.61 eV [ 25–27 ] ) and for CdS buffers (for which E g Surf values between 2.40 and 2.65 eV have been reported [ 25,26 ] ), in particular when considering the experimental uncertainty.…”

“…Solar cells prepared using identical processes resulted in efficiencies of 13.0% for devices with GaO x buffer and 16.2% for CdS buffered cells. [ 18 ]…”

“…Solar cells prepared using identical processes resulted in efficiencies of 13.0% for devices with GaO x buffer and 16.2% for CdS buffered cells. [18] 2.2. Photoelectron Spectroscopy HAXPES measurements were performed using an excitation energy of linearly polarized 5.95 keV X-rays (referred to as 6 keV in this manuscript) at BL15XU of the SPring-8 electron storage ring, [20] equipped with a hemispherical electron analyzer (VG Scienta R4000) oriented perpendicular to the X-ray propagation access and with the analyzer axis and X-ray polarization vector aligned.…”

“…This is a crucial requirement to benefit from high‐temperature deposition of the transparent conductive oxide (used as front contact) or for the application of CIGSe cells in tandem or multijunction devices to avoid negative effects of subsequent high‐temperature process stages (e.g., during the deposition of the top cell). [ 18 ]…”