Solar Cells Based on CuInSe₂ and Related Compounds: Material and Device Properties and Processing

Abstract

This paper summarizes recent material and device results obtained at the Institute of Phy sical Electronics at Stuttgart University (lP£). Properties of the mat erial system Cu(ln, Ga)(S, Se h were analy sed and wherever possible a correlation bet ween the material properties and the device characteristics is made. Different high-vacuum techniques of absorber preparation are presented and compared. The f ormation of different alloy s of the family Cu(ln, Ga)(S, S eh is possible for the co-evaporation and at least one of the sequential evaporation techniques. The model f or Cu-rich grow th of Cul nSe2 kno wn from the co-evaporation process can also be used for the Cu-rich growth in the sequential evaporation processes. The surf ace composition of slightly (In, Gas-rich bulk compositions is always determined to be the defect chalcopyrite Cu(/n, GahSe5. Solar cells prepared with different processes and therefore different morphologie s yielded similar device performance. An exponential decay of the density of states from the valence and conduction bands was obtained. Improved cell performance is achieved using absorber layers with higher carrier concentrations. The carrier concentration can be increased by using Na-containing substrates or by utilizing a new Cd-free buffer layer. Device efficiencies in the range of 15% were achieved using the Cd-free buffer layer.1 -5 and 12% for CulnS2.6 But, not only for single cells on a laboratory scale, the material systems passed the status of only being a promising candidate for solar cell applications. Modules with more than 40 W have been realized." Submodules of 50 em? revealed efficiencies of more than 10%7 and the upscaling process to full-size modules is ongoing. To overcome the actual limitations of these cells it is very important to understand the correlation between crystallographical, electrical and optical properties and the achieved solar cell data. This paper tries to illustrate some of these connections.

Effects of Sodium on Polycrystalline Cu(In,Ga)Se2 and Its Solar Cell Performance

“…As noted above, a higher oxygen content induces more diffusion of Na during growth, which in turn induces further oxidation. This interpretation is further supported by recent results showing that Cu(In x Ga 1±x ) 3 Se 5 films with 0 < x < 0.3, which are normally n-type, are converted to p-type if a Na 2 S precursor is used. [38] Moreover, also in this case Na presence was accompanied by O.…”

“…[1] In particular, solar cells based on Cu(In,Ga)Se 2 (CIGS) have attracted considerable attention due to their high conversion efficiency. Such cells with a conversion efficiency of~17 % or more (under standard test conditions) are now being produced in Europe, [2,3] the United States, [4] and Japan. [5] This efficiency approaches the best achieved by polycrystalline silicon cells.…”

“…ii) They act as traps for recombination of photogenerated electrons with holes, which has been identified as a major loss mechanism in polycrystalline photovoltaic devices in general and in CIGS-based photovoltaic devices in particular. [18] Fortunately, these sites are neutralized by oxide, according to Equation 2, where the adsorbed oxide is obtained from molecular oxygen by the sodiumcatalyzed reaction given in Equation 3.…”

Optimization of Processing and Modeling Issues for Thin-Film Solar Cell Devices; Annual Report, 3 February 1997-2 February 1998

“…The growth of films with Cu/In > 1.1 followed by a KCN etch to remove segregated CuS from the surface is considered necessary for high efficiency devices. The CuS enhances the growth of the CuInS 2 resulting in larger grains [107]. The highest reported efficiency device for Cu/In precursors reacted in H 2 S is 10.5% [108].…”

Junction formation in CuInSe[sub 2]-based thin-film devices

“…The CdS/CIS junction has been treated as an abrupt heterojunction, and this assumption has never been put to rigorous experimental test. The existence of ordered vacancy phases (OVC) such as CuIn 3 Se 5 and CuIn 5 Se 8 at the surface region of the chalcopyrite is thought to yield a buried junction (3,4). However, when these compounds are synthesized separately, they do not show sufficiently high conductivity or definite conductivity type to persuade us to believe they can create an efficient junction with the (112) chalcopyrite phase (5).…”