Advanced Stacked Elemental Layer Process for Cu(InGa)Se2 Thin Film Photovoltaic Devices

Copper indium gallium selenide (CIGS) photovoltaic devices made using multistep selenization of nanocrystal films

The power conversion efficiency of photovoltaic devices made with ink-deposited Cu(InxGa1-x)Se2 (CIGS) nanocrystal layers can be enhanced by sintering the nanocrystals with a high temperature selenization process. This process, however, can be challenging to control. Here, we report that ink deposition followed by annealing under inert gas and then selenization can provide better control over CIGS nanocrystal sintering and yield generally improved device efficiency. Annealing under argon at 525 °C removes organic ligands and diffuses sodium from the underlying soda lime glass into the Mo back contact to improve the rate and quality of nanocrystal sintering during selenization at 500 °C. Shorter selenization time alleviates excessive MoSe2 formation at the Mo back contact that leads to film delamination, which in turn enables multiple cycles of nanocrystal deposition and selenization to create thicker, more uniform absorber films. Devices with power conversion efficiency greater than 7% are fabricated using the multiple step nanocrystal deposition and sintering process.

Nanoparticulate film precursors to cis solar cells: Spray deposition of Cu-In-Se colloids

The use of nanoparticle colloids for spray deposition of Cu-In-Se precursor films and subsequent thermal treatment to form CuInSe2 (CIS) films has been investigated. In the present study, the metathesis reaction between Na2Se in methanol and metal salts (i.e., Cu(BF4)2 and/or InI3) in pyridine produced CuSe, In2Se3, and CuInSe2.5 nanoparticle colloids. Purified colloid was sprayed onto heated molybdenum-coated sodalime glass substrates to form CuInSe2.5/Mo and In2Se3/CuSe/Mo precursor films. These precursor films were subjected to various thermal treatments in an effort to produce large-grained CIS films from the nano-sized precursors. The annealed CIS films were characterized by scanning electron microscopy (SEM) and x-ray diffraction (XRD). Results of this continuing effort will be discussed.

Cu(In, Ga)(Se, S)2 Absorbers Formed by Rapid Thermal Processing of Elemental Precursors: Analysis of Thin Film Formation and Implementation of A Large Area Industrial Process

Large area Cu(In, Ga)(Se, S)2 thin films (CIGSSe) for solar modules are processed by rapid thermal annealing of stacked elemental layers. A pilot line for 60×90 cm2 absorbers and 30×30cm2 modules is now up and running. Modules with efficiencies at 12% are fabricated with excellent structural and electrical uniformity. For the optimization of electrical performance the selenization and sulfurization process is analyzed by x-ray fluorescence (XRFA), x-ray photo-electron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS) and current voltage (I-V) measurements. Sequentially processed CIGSSe films show characteristic depth distribution profiles of gallium and sulfur. Based upon our previously published model obtained by In-Situ XRD analysis the Ga profile can be explained by the strongly inhibited formation of binary gallium-selenides. The sulfur incorporation is investigated by comparing XRFA data and SIMS profiles of samples from different sulfurization recipes. A characteristic dependence of S on the Cu/(In+Ga) ratio is observed. Two different mechanisms for sulfur incorporation are proposed. It will be shown that the accumulation of sulfur towards the back is predominantly due to the incorporation into an intermediate molybdenum sulfo-selenide layer. Device characterization shows that the Ga and S profiles lead to a favorable double band gap grading structure.

POLYCRYSTALLINE THIN FILM SOLAR CELLS:Present Status and Future Potential

▪ Abstract  Polycrystalline thin film solar cells on copper indium diselenide (CulnSe2) and its alloys and cadmium telluride (CdTe) appear to be the most promising candidates for large-scale application of photovoltaic energy conversion because they have shown laboratory-efficiences in excess of 15%. Heterojunction devices with n-type cadmium sulfide (CdS) films show very low minority carrier recombination at the absorber grain boundaries and at the metallurgical interface, which results in high quantum efficiences. Open circuit voltages of these devices are relatively low owing to the recombination in the space charge region in the absorber. Further improvements in efficiency can be achieved by reducing the recombination current, especially in devices based on CulnSe2 and its alloys. Low-cost manufacturing of modules requires better resolution of a number of other technical issues. For modules based on CulnSe2 and its alloys, the role of Na and higher deposition rates on device performance need to be better understood. In addition, replacing the chemical bath deposition method for CdS film deposition with an equally effective, but more environmentally acceptable process is needed. For modules based on CdTe, more fundamental understanding of the effect of chloride/oxygen treatment and the development of more reproducible and manufacturable CdTe contacting schemes are necessary.