The thin-film solar cell technology pioneered by Solibro Research is based on the semiconductor material Cu(In,Ga)Se2 (commonly referred to as CIGS). This material is deposited on glass substrates in a unique in-line production tool. Additional thin-film layers are then added to form the complete solar cell module.
The core process of the Solibro Research technology is the deposition of the CIGS absorber layer, which is evaporated in one single pass through the production equipment, with no additional annealing steps. Optimizing the performance of this layer is key to making very high efficiency thin-film solar cells.
The CIGS material is uniquely suited to the solar cell application with
The CIGS technology, commercialized by the Solibro companies, originates in the research of the Thin-film Solar Cell Group at the Ångström laboratory, a centre of excellence for material research at Uppsala University. This research group has held a World leading position in the research field over several decades.
Commercialization of the CIGS technology means up-scaling processes from the lab to full-size photovoltaic modules, typically around 1 m2. This larger scale processing brings with it several engineering challenges, like deposition control and uniformity over large areas as well as high demands on process yield and machine uptime.
The CIGS is deposited by evaporating metals from elemental sources onto a train of moving glass substrates, while the selenium is added in abundance so that the Cu(In,Ga)Se2 is formed on the hot substrate surface. This inline deposition process ensures good layer uniformity, while the continuous train of substrates gives high material yield. Source configurations and temperature profiles in the deposition zone are two of the keys to achieving a high efficiency CIGS absorber material.
Apart from the core CIGS technology, every part of the solar cell device needs to be optimized to reach world record level efficiency, from the back contact and substrate material to the transparent front contact materials. The substrate of choice for Solibro Research is soda lime glass, which has excellent properties and is available at low cost.
On the glass substrate a molybdenum layer is deposited by sputtering, and forms the electrical back contact. This contact layer needs to withstand the high temperature process in the CIGS deposition step, where the atmosphere of selenium also is corrosive.
In order to get the sunlight into the solar cell structure, there needs to be front contact layers (often referred to as window layers) that are both transparent in the wavelength region of the solar radiation and highly conductive. There is always a trade-off between these two properties in a transparent conductive oxide (TCO), which means that these layers need to be optimized with respect to the design of the solar cell module, the absorption profile in the CIGS layer and the requirements for a low resistance contact layer.
In order to make a complete photovoltaic module with a suitable voltage and current output, a series connection of many individual cells on each substrate is needed. This is accomplished by patterning the individual semiconductor layers by laser and mechanical scribing. The first scribe line (P1) is used to isolate the molybdenum layer of neighbouring cells, while another scribe (P3) is used to isolate the TCO layers between cells. A third scribe (P2) is used to create an electrical contact from the TCO of one cell to the molybdenum of the next, thus creating a series connection of cells.
To finalize the product, the substrate is laminated with a polymer film and a cover glass that provides mechanical and chemical protection for the semiconductor layers in the module. The polymer and glass sheet also contribute to maximizing the share of the sunlight entering into the active layers. Optionally a frame can be added to facilitate mounting, and a junction box with connectors is placed on the backside.
Lab scale to module size
A major factor contributing to the success of the team at Solibro Research is the complete in-house large area research line, where every process step in the production can be performed, evaluated and optimized. This also enables very quick transfer from the company's high-throughput lab tool to the larger format, cutting development time and streamlining the innovation process.