Recently, ternary and quaternary compound semiconductors of the type I?III?VI2 have received much attention because of their potential application in optoelectronic devices. The printing of nanocrystal inks is an attractive approach due to the ability to achieve high throughput for relatively low costs. Device efficiency above 13% was achieved with nanoparticle oxide precursors. However, the solution or paste type precursors consisted of nano-particles may leave a residual carbon layer after absorber formation process, which are attributed to the carbon-based dispersant or binder. Although the residual carbon layer might be harmful for the opto-electronic properties of the absorber and therefore its removal is desirable, its role in a solar cell has not been fully understood yet.
In this study, by investigating the selenization kinetics of Cu(In,Ga)S2 (CIGS) nanocrystal precursors printed on Mo coated sodalime glass substrate and selenized by using Se coated cover glass, it was found that the residual carbon layer thickness decreased with Se partial pressure and thinner carbon layer did not always translate into a higher efficiency solar cell.
The precursor films were prepared by printing a solution containing CuIn0.7Ga0.3S2 nanocrystals of about 20 nm in diameter four times to make 2㎛ thickness on 0.6 mm Mo coated sodalime glass substrate. The printing process was interrupted to dry the sample at 400 °C for 2 min in air. The nanocrystal precursor film had composition of Cu/(In+Ga)=1, Ga/(In+Ga)=0.3. The selenization was carried out at 570 °C for 15 min by pairing of the precursor glass covered with a Se-coated glass in a graphite box heated in a two-zone quartz furnace. The Se-coated glass was prepared by evaporating Se of 0.8~2.0 mm thickness onto a bare glass. Subsequently, CdS buffer layer was grown by chemical bath deposition (CBD) and window layer of un-doped ZnO (80 nm) and Al-doped ZnO (500 nm) was deposited to construct a solar cell.
Another study is The production process for large area and low cost CIGS thin film photovoltaic module is based on selenization of Cu(In,Ga) precursor film deposited by sputtering method.
Selenization method that is often used today, is using H2Se gas which is highly toxic and corrosive, furthermore, the material cost is very high. H2Se gas itself is responsible for more than 30% of total material cost to make a CIGS photovoltaic module.
we propose a new selenization system for using elemental Se vapour in a rapid thermal process (RTP) system to produce a CIGS absorber layer. The system was developed to enable uniform distribution of cracked Se over large area and includes a gas-permeable component which was not artificially machined but allows only Se-vapour to pass through and holds any condensed Se phases.
Unlike the stacked elemental layer RTP (SEL-RTP) developed by Avancis, there is no need to deposit Se layer on Cu(In,Ga) precursor by using an additional Se evaporator. In the new system, Se vapour is directly supplied from the gas-permeable component, which provides Se vapour with a long travelling path and generates active Se radicals.
CIGS films selenized by using the new system, were found to have sufficient Se content of more than 50% upto 60%. The solar cell of Ag/Ni/(Ga, Al)-ZnO/i-ZnO/CIGS/Mo/glass realized with the absorber showed best performance of 11.4% efficiency.