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Cost-Effective CIGS Film Boosts Solar Cell Performance
The Pitch Compared with traditional silicon photovoltaic (PV) manufacturing, thin-film PV uses roughly 1% of the active (and costly) PV material to convert the sun’s energy into electricity. Copper-indiumgallium selenide (CIGS) with high conversion efficiencies has long been the most promising thin-film material. The company HelioVolt has patented a process that prints high-quality CIGS thin films. Called FASST (field-assisted simultaneous synthesis and transfer), it is a rapid manufacturing process that can be scaled up effectively to meet the needs of the $36 billion photovoltaic market. Observations drawn from the HelioVolt work on device-quality CIGS show large, columnar grains and an overall copper deficiency compared to the structure of the conventional α-phase copper indium di-selenide (CuInSe2). Confirmed by tests conducted with the National Renewable Energy Laboratory (NREL), the intraabsorber junction (IAJ) model shows that the different charges of CIGS elements cause the molecules in the material to spontaneously arrange themselves into a percolation network, α so-called nanoscale expressway for electricity. The IAJ model shows that compositions lie in the equilibrium of a two-phase domain, α + β, and form a nanoscale p-n junction network or nanodiode network (percolation network). The n-type networks act as preferential electron pathways while the p-type networks act as preferential hole pathways. Positive and negative charges travel to the contacts in physically separate paths, reducing recombination and improving efficiency. By designing the FASST process around these findings, HelioVolt is able to significantly reduce the cost of thin-film CIGS devices and to control the quality of highthroughput manufacturing. High-quality, low-cost CIGS thin films deliver electricity that is cost competitive with other conventional fossil-fuel–based sources and open the photovoltaic market to regions with less solar irradiation or more diffuse light conditions. The Technology FASST is a two-stage reactive transfer printing method involving (1) the deposition of two separate precursors, and (2) the chemical reaction between the precursor films to form CIGS, as shown in
Precursor 1 on Print Plate
Heat
Precursor 2 on Substrate
Print Plate
Electrostatic & Mechanical Pressure
CIGS on Substrate
Figure 1. Schematic of the FASST® process. CIGS stands for copper indium gallium selenide.
Figure 1. In the first stage, two Cu-In-GaSe–based precursor layers forming the chemical basis of CIGS are deposited onto a substrate and a print plate. The use of two separate precursors provides the benefits of an independently optimized composition, structure, deposition method, and processing conditions for each; eliminates pre-reaction prior to the second-stage FASST; and facilitates optimized CIGS formation in the second stage. Furthermore, precursors can be deposited at a low substrate temperature, saving energy and costs. In the second stage, these precursors are
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