Degradation of Mg-doped zinc oxide buffer layers in thin film CdTe solar cells

Abstract

Cadmium Sulphide is the conventional n-type buffer layer used in thin film Cadmium Telluride solar cells. It is well known that Cadmium Sulphide causes optical losses and sulphur diffuses into the absorber during high temperature activation. Sputterdeposited Mg-doped ZnO (MZO) has been shown to be an attractive buffer layer for Cadmium Telluride solar cells due to its transparency and tuneable band gap. It is also stable to high temperature processing and avoids diffusion of elements into the cadmium telluride absorber during the cadmium chloride activation treatment. However, degradation is observed in solar cells incorporating MZO buffer layers. Analysis of the MZO film surface potential has revealed significant fluctuations in the thin film work function once the layer is exposed to the atmosphere following deposition. These fluctuations are attributed to the high reactivity to water vapour of the MgO contained in the MZO films. This has been analysed using X-ray Photoelectron Spectroscopy to determine corresponding changes in the surface chemistry. The Zinc Oxide component is relatively stable, but the analysis shows that MgO forms a Mg(OH)2 layer on the MZO surface which forms a secondary barrier at the MZO/CdTe interface and/or at the interface between MZO and the Fluorine-doped SnO2. This affects the Fill Factor and as a consequence it degrades the conversion efficiency.

20%-efficient polycrystalline Cd(Se,Te) thin-film solar cells with compositional gradient near the front junction

“…When the ZMO layer, which forms a spike-like CBM alignment with Cd(Se,Te), is too resistive, the Cd(Se,Te) solar cells often show the so-called S-kink character in the photocurrent density-voltage (J-V) curves, which leads to low fill factors (FFs) 16,18,22 . Additionally, ZMO is sensitive to moisture, which may lead to longterm instability issues [23][24][25] . On the other hand, the commercial SnO 2 buffer is very stable and can be easily reproduced with desirable electron conductivity 26,27 .…”

Indium Gallium Oxide Emitters for High-Efficiency CdTe-Based Solar Cells

“…Others have noted that the S-kink in the MZO devices can also be due to metastability in the MZO due to the formation of MgO during processing. 27,28 Thus, both mechanisms depend on device processing, especially when CdCl 2 activation is done in air. We note that the devices reported here were CdCl 2 activated in air and illuminated with an LED solar simulator with a 400 nm low wavelength cutoff but do not show any S-kink-like behavior until the bandgap of the emitter is 4.10 eV.…”

“…For some devices, the S-kink appears to be related to the carrier concentration and Fermi level position in the MZO even when only a small positive CBO exists , when the doping level in MZO is controlled by oxygen vacancy concentration. Others have noted that the S-kink in the MZO devices can also be due to metastability in the MZO due to the formation of MgO during processing. , Thus, both mechanisms depend on device processing, especially when CdCl 2 activation is done in air. We note that the devices reported here were CdCl 2 activated in air and illuminated with an LED solar simulator with a 400 nm low wavelength cutoff but do not show any S-kink-like behavior until the bandgap of the emitter is 4.10 eV.…”

Interrelation of the CdTe Grain Size, Postgrowth Processing, and Window Layer Selection on Solar Cell Performance

“…7 Different processing strategies have emerged among various research groups to prevent this "S" shaped curve such as limiting the layer thickness, 4 reducing the oxygen content, 7 varying the Mg/Zn ratio, 8 and postgrowth annealing. 9 This strong sensitivity to processing conditions as well as indications of degradation due to the MgO content in MZO films reacting with water vapor 10 mean that other alternative partner layers remain worthy of investigation. Devices with a graded CdSe x Te 1−x absorber layer forming a junction with SnO 2 have recently shown high efficiency, 11 demonstrating this as a suitable substrate for growing CdTebased devices.…”