Complete Report - University of New South Wales

ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTSynthetic silicon. Portion of the PeriodicTable showing pathways to engineeringsemiconductors with 4 valence electrons/atom.Figure 4.4.3.14.4.3 Earth-Abundant PhotovoltaicDevices (CZTS)University Staff• Dr Xiaojing Hao• Prof. Martin GreenPostgraduate Research StudentXiaolei Liu4.4.3.1 Strand OutlineAll successfully commercialised non-concentratingphotovoltaic technologies to date are based onsilicon or the chalcogenides (semiconductorscontaining Group VI elements, specifically Te, Seand S). As indicated by Figure 4.4.3.1, the successfulchalcogenide materials, CdTe and CuInSe 2, can beregarded as “synthetic silicon” where the balancebetween atoms in these materials provides thesame average number of valence band electrons asin silicon.Unfortunately Cd and Se are toxic “heavy metals”while Te and In are amongst the 12 most rareelements in the Earth’s crust, factors that wouldseem to limit the long-term potential of theestablished chalcogenide technologies. However,as indicated in Figure 4.4.3.1, by investigating moredeeply into the Periodic Table, the compoundCu 2ZnSnS 4is uncovered with the same number ofvalence band electrons on average but involvingearth-abundant, non-toxic elements.Although the potential of this material is relativelyunexplored for photovoltaics, initial results havebeen promising with a group at IBM reporting 9.7%efficiency for small cells based on related materials(alloy of the above sulphide and the correspondingselenide). The appearance of the cells involved(Fig. 4.4.3.2) is quite close to the appearance ofcells made using CIGS technology (CuInSe 2plusCuGaSe 2alloy).The Centre’s work in this new strand of activitytakes a different direction from most of the presentinternational work in this area. The starting point isa viable high throughput manufacturing process.Efforts are to develop technology suitable for thedesired process rather than persevering with solutiongrowth processes that have given the best laboratoryresults to date. Materials selection would beguided by stable compositions formed in naturallyoccurring minerals such as kesterite and stannites.Another key area that is to be investigated in detailis epitaxial relationships to silicon to investigate thesuitability for tandem cell stacks, including stacksinvolving silicon as the lowermost layer.4.4.3.2 Simplified fabrication of CZTSthin film cellsThough various approaches have been appliedto fabricate CIGS solar cell, CIGS with the highestefficiency (up to 20.3% so far) have been achievedby the physical vapour deposition (PVD) approach,with the first 10% efficient cell demonstrated in1980, the first 15% cell in 1993 and the first 20% cellin 2008. As a substitute for CIGS thin film solar cells,CZTS has many similarities to CIGS as mentionedabove. As such, it is logical that the full potentialof high efficiency CZTS thin film solar cells will beextracted using the PVD approach.Though a major milestone was reached at theend of 2009 with the demonstration of 9.7% CZTSefficiency (some Se also in device) by a researchteam at IBM by a “hydrazine-based solution”approach [4.4.3.1], solution growth is believedinherently incapable of giving the material controlrequired for best-possible performance. Evidencefor this arises in the similar case of CIGS where thesame IBM team holds the record of 12.2% efficiencyfor solution-deposited CIGS (again hydrazine–based) [4.4.3.2], while in contrast the PVD approachgives 20.3% efficiency.Evaporation and sputtering are the two mostimportant PVD techniques. The disadvantage ofevaporation is that it is very difficult to control theCu evaporation source; and the lack of commerciallyavailable equipment for large area thermalevaporation. In contrast, magnetron sputtering is59