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Abstract
Cu(In,Ga)Se2 (CIGS)/CdS thin-film solar cells have reached, at laboratory scale, an efficiency higher than 22.3%, which is one of the highest efficiencies ever obtained for thin-film solar cells. The research focus has now shifted onto fabrication processes, which have to be easily scalable at an industrial level. For this reason, a process is highlighted here which uses only the sputtering technique for both the absorber preparation and the deposition of all the other materials that make up the cell. Particular emphasis is placed on the comparison between different precursors obtained with either In2Se3 and Ga2Se3 or InSe and GaSe as starting materials. In both cases, the precursor does not require any heat treatment, and it is immediately ready to be selenized. The selenization is performed in a pure-selenium atmosphere and only lasts a few minutes at a temperature of about 803 K. Energy conversion efficiencies in the range of 15%–16% are reproducibly obtained on soda-lime glass (SLG) substrates. Similar results are achieved if commercial ceramic tiles are used as a substrate instead of glass. This result is especially useful for the so-called building integrated photovoltaic. Cu(In,Ga)Se2-based solar cells grown directly on ceramic tiles are ideal for the fabrication of ventilated façades in low impact buildings.
Highlights
The price of photovoltaic (PV) systems has decreased by about 75% in the last 10 years
We showed how different compounds were tested as starting materials
We described the process based on In2 Se3 –Ga2 Se3 and its evolution towards the InSe–GaSe system, which granted us better control of the Ga-concentration profile inside the absorber layer
Summary
The price of photovoltaic (PV) systems has decreased by about 75% in the last 10 years. Thin-film solar cells are one of the most promising technologies for the production of “clean” and low-cost energy This is due to a greatly reduced consumption of semiconductor material and to the fact that solar cells can be manufactured on inexpensive large-area substrates. The work’s main objective is to identify which particular sequences of precursor layers—crucial agents for the formation of a CIGS absorber film—yield the highest value for both solar cell photovoltage and photocurrent One such sequence of starting layers, which was found to bring about a V-shaped distribution of Ga inside the CuInGaSe2 film, has been discovered and examined. A high-gap material at the back contact could repel the minority carriers back into the grains, increasing their lifetime If all these requirements are complied with, the resulting solar cells will exhibit energy conversion efficiencies well above 16%. The PV tiles are suitable for the production of ventilated walls, widely used in the construction of low energy-consumption buildings
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