Influence of residual gas composition and background pressure in a multi-stage co-evaporation chamber on the quality of Cu(In, Ga)Se<inf>2</inf> thin films and their device performance

Abstract

-Thin film solar cells with Cu(In,Ga)Se 2 (CIGSe) absorbers prepared by co-evaporation reach efficiencies above 21% [1]. Typical multi-stage co-evaporation chambers are MBElike (ultra-)high vacuum systems with individual effusion sources for each element. Cleanliness of the process chamber and the background pressure during the co-evaporation process could be of importance for the chamber design and a fair comparison of production costs when comparing different PV/Chalcopyrite technologies.Here we study the influence of the background pressure quality on the electronic and structural properties of the deposited absorber layer. To achieve this, we analyzed the residual gas composition before and the background pressure during consecutive co-evaporation processes and investigate the effect of a combined cleaning (mechanical and electro-chemical) of the chamber walls together with a simple conditioning of the chamber after opening the chamber and re-filling the crucibles.Cleaning of the chamber yielded a significant reduction in carbon species and an overall lower base pressure. The background pressure during the process was reduced from ~6*10 -6 mbar (before cleaning with water cooling shroud) to 1*10 -7 mbar (after cleaning with LN 2 filled cooling shroud). The type and amounts of contaminants in the absorber layer are characterized by laser ablation inductively coupled plasma mass spectroscopy (LA ICP-MS). The impact of the process pressure on the growth of the CIGSe layer is analyzed with respect to preferential orientation (using XRD), grain-size (using SEM), in-depth elemental gradients (using GDOES) and the electronic quality (using TRPL, C-V). Analysis of completed solar cell devices shows that the absorber band-gap is hardly affected by the chamber conditions, whereas we see an improved collection of charge carriers generated by photons in the infra-red spectral range from the conditioned chamber, also resulting in slightly higher j sc . The major effect is an increase in median V oc values from 585mV (before cleaning and conditioning) to 635mV (after cleaning and condition). The overall solar cell efficiency is increased by 18% (relative).

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“…23 Water typically is omnipresent inside an SEM chamber and is considered the main contributor to the SEMs base pressure, based on MS investigations. [224][225][226] Even though the vapor pressure of water is 32 mbar at room temperature, water films of approximately 0.7 nm, corresponding to two double-layers of water molecules, are observed down to a critical pressure of 2 × 10 -7 mbar on SiO2/Si(100) substrates. [227][228] In most FEBID studies the background pressure is higher than the aforementioned critical pressure, thus potential reactions with water have to be always considered.…”