Photovoltaic (PV) energy can become a very important means to produce 'green' electricity, provided that the manufacturing costs are reduced substantially with respect to the present level. In 2002 the costs of PV modules was about 5 euro/Watt-peak, and crystalline silicon (Si) wafer technology was the most dominant technology with a market share of about 90%. The present cost of these PV-cells and modules is largely determined by the cost of the starting material: crystalline Si wafers. In order to attain a breakthrough to cost levels of about 1 euro/Watt-peak, necessary in order to compete with conventional electricity production, PV production has to abandon Si wafers and change to thin film technology.
One of the first attempts in this direction has been the development of film Si PV, based on amorphous silicon layers. This technology has achieved a substantial reduction of Si feedstock per Watt-peak, but its commercial success is limited because of the rather low conversion efficiencies of these modules. In order to become commercially successful, film Si PV has to attain similar conversion efficiencies as wafer based Si PV. The project MYCRYSTAL aims to make significant progress towards this goal.
In this project we will develop a proof of principle for a new process technology for solar cells based on thin films of silicon (film-Si PV). Per Watt-peak, PV modules based on this technology will require about 1/100 of the silicon feedstock that is required for Si wafer based PV.
The process technology will comprise a number of innovations with respect to common process technology of film Si PV:
1. The technology will be based on microcrystalline Si layers as photon-absorbing material instead of commonly used amorphous silicon layers. Microcrystalline Si has the advantage of high stability under illumination and therefore allows for higher solar cell efficiencies than for solar cells based solely on amorphous silicon. The layers will be grown at low temperatures (room temperature to 300 degrees C) allowing for a large variety of cheap substrates.
2. The technology will be based on continuous processing. In contrast to conventional technology, based on batch processing on glass substrates, continuous processing allows for roll-to-roll processing on light and flexible substrates. PV products based on flexible substrates are easier to integrate into the built environment, and allow for a larger variety of applications.
3. The deposition technology for silicon layers will utilize a microwave plasma source. In comparison to conventional deposition technologies, which are based on Radio-Frequency (RF), the microwave plasma source allows for higher deposition rates (the throughput of this process step is economically a very important issue for film-Si PV) and a better homogeneity on large areas (typically 0.5 m in width).
4. Interconnection technology using screen-printing instead of common photolithographic processing. Photolithography is a complicated and expensive process, which in addition is not compatible with roll-to-roll processing. We will develop an interconnection process using a printing process for both insulating and metal patterns suitable for high-throughput continuous processing.
The project is divided into the following work packages:
I. Design, engineering and construction of an inline, continuous microwave plasma enhanced chemical vapour deposition (MWPECVD) system for growth of intrinsic and doped silicon layers on flexible substrates.
II. Development of deposition process for intrinsic and doped Si layers, with appropriate quality for solar cells, by MWPECVD.
In-situ monitoring of Oxygen will be developed.
III. Development of deposition technology of the passive layers of the solar cells. The passive layers are the Transparent Conducting Oxide (TCO) layers, barrier layers for electrical insulation, and reflectance layers (typically Ag). The deposition technology of these layers will also be based on roll-to-roll processing. The technologies that will be investigated are sputtering (TCO, barrier and metal layer) and spraying (barrier layer).
IV. Development of cell interconnection technology. This cell interconnection process will comprise laser scribing and screen-printing.
V. Encapsulation. Development of an encapsulation process for film Si modules on flexible substrates.
Keywords: solar energy, thin film, photovoltaic (PV) energy, film-Si PV.