CuInSe₂ (CIS) Thin Film Solar Cells by Direct Coating and Selenization of Solution Precursors

Abstract

CuInSe2 (CIS) absorber layer was formed by a direct nonvacuum coating and a subsequent selenization of precursor solutions of Cu(NO3)2 and InCl3 dissolved in methanol. The viscosity of precursor solutions was adjusted by adding ethyl-cellulose (EC) to be suitable for the doctor-blade coating. During the coating and drying process Cu2+ ions in the starting solution were reduced to Cu+, resulting in precursor films consisting of CuCl crystals and amorphous In compound embedded in EC matrix. Selenization of the precursor films with Se vapor at elevated temperature generated double-layered films with an upper layer of chalcopyrite CIS and a carbon residue bottom layer. Significant In loss was observed during the selenization, which was attributed to the evaporation of the In2Se binary phase, confirmed by investigating the change in the Cu/In ratio of the selenized film as a function of Se flux and substrate temperature. As a proof-of-concept, thin film solar cells were fabricated with the double-layered absorber film and the devices exhibited reproducible conversion efficiency as high as about 2%.

Three dimensional web-like fibrous CuInS2 film

“…It is clear from Fig. 4 that the carbon, which is expected to remain in the fiber structure, cannot form thick layers as reported in previous flat p-n junction structures [8,20]. Carbon in our fiber film is believed to act as a simple conductor rather as a resistive element.…”

“…There are pros and cons for carbon inclusions in copper-indium based solar cells [16,19]. As an impurity, thick carbon layers inside the CuInS 2 layer can hinder electron-hole transport [8,20]. On the other hand, carbon is typically a conductor and can augment electron-hole transport without interfering with the overall conductivity of the CuInS 2 layer.…”

Characteristics of Cu(In,Ga)Se2 (CIGS) thin films deposited by a direct solution coating process

“…In this regard, many efforts have been devoted to develop alternative deposition techniques for thin film CIGS solar cells using non-vacuum coating processes [2][3][4][5][6][7][8][9][10][11][12][13][14], based on their inherent advantages such as no requirement of expensive vacuum equipments, less energy intensive deposition and much better material utilization. These non-vacuum coating processes can be roughly divided into two categories depending on precursor types, in which the first approach uses the true solution precursors [3][4][5][6][7] and the other uses particulate precursors [2,[8][9][10][11][12][13][14].…”

Electrochemical Surface Processes and Opportunities for Material Synthesis

“…Among the I-III-VI 2 compounds, CuInSe 2 has a high absorption coefficient (10 5 cm −1 ) and a layer of only 0.1-0.3 μm is sufficient for significant absorption [139]. Its band gap can be tuned between 1.04 and 1.68 eV by substituting Ga for In.…”