Thin Film

Although the entire solar manufacturing industry, from raw materials to finished modules, has enjoyed strong double-digit growth rates over the past several years, few sectors have soared like the amorphous-silicon thin-film photovoltaic equipment space. Much of this prodigious multibillion-dollar booking activity can be attributed to the acceptance of the turnkey production packages offered by the likes of Applied Materials, Oerlikon and Ulvac. These suppliers’ plug-and-play, standard toolset solutions are attractive to companies seeking to get into the TFPV module business on a fast track and then scale up their capacities in multimegawatt chunks to achieve grid-competitive cost-per-manufactured-watt metrics.

With the thin-film silicon industry facing the problems of high-quality material deposition at high rates and narrowing deposition process windows, the “no-drift regime” is an important part of this development. In the case of the plasma-enhanced chemical vapor deposition (PECVD) of thin silicon films, the inconstancy of the concentration of silicon-containing particles (SCP) in the plasma leads to changes in deposition conditions, causing a deterioration of film properties, and, therefore, decreasing the performance of the solar cells. During the last few decades, evidence about the process instabilities has been accumulated in different laboratories. In this study, Fourier transform infrared absorption spectroscopy (FTIR), optical emission spectroscopy (OES), self-bias voltage and plasma impedance controls were applied as in-situ process diagnostics during the deposition of amorphous and microcrystalline silicon thin-films. Results of the study were then discussed.

The rapidly-growing photovoltaic market has placed a strong demand on manufacturers to decrease solar cell production costs. For thin-film solar cells, this can be achieved by increasing substrate sizes to achieve a better productivity and by adding more advanced layer stack systems to enhance the solar cell’s efficiency. Nearly all required layers of the prominent thin-film-based solar cell types (a-Si/µc-Si, CdTe and CI(G)S) can be deposited by using plasma processes. On the one hand, plasma-enhanced chemical vapor deposition (PECVD) is used for the deposition of a-Si and µc-Si layers. On the other hand, magnetron sputtering is used for coating with transparent conductive oxides as ITO (indium tin oxide) and ZAO (aluminium-doped zinc oxide), metallic back contact layers such as Ti, Al and Mo, or components of the compound semiconductor layers such as Cu and In. Magnetron sputter processes use direct current (DC) or pulsed DC, whereas radio frequency (RF) power is used for PECVD processes. Of utmost importance to get a reliable, high-efficiency solar cell is a good uniformity of the deposited layers and the need for the layer to be defect-free. Defects such as particles and splashes are created inside the plasma when an unwanted local discharge - a so-called arc - occurs. This arc can be eliminated by switching off the power supply. The faster this is done, and the less energy that is delivered into the arc, the smaller and more insignificant the defect creation will be. For this reason, as well as for precise control of electrical power, advanced, fast-reacting arc management is very important to attain high-quality solar cell coatings.

The next two years will be crucial in determining the market viability and future of what many see as the most promising thin-film photovoltaics technology: copper indium gallium (di)selenide (CIGS) and its gallium-free cousin, CIS. With potential conversion efficiencies just below that of crystalline silicon PV, low-cost manufacturing strategies offering a chance to reach sub-dollar-per-watt manufacturing costs on both glass and flexible modules, and applications ranging from utility- and industrial-scale farms to building-integrated commercial and residential uses, the quaternary compound has a large grid-parity upside - if the very real challenges of scaling production to commercial volume can be met.

Thin-film silicon solar cells are a potentially low-cost alternative to solar cells based on bulk silicon that are commonly used in the industry at the present time. However, a major drawback of the current epitaxial semi-industrial screen-printed cells is that they only achieve an efficiency of about 11-12%. By upgrading their efficiency, this kind of solar cell would become more attractive to the photovoltaic industry. The optimization of the front surface texture by dry texturing based on a fluorine plasma and the introduction of an intermediate porous silicon reflector at the epi/substrate interface (multiple Bragg reflector) has proven to result in an efficiency boost up to about 14%.

Until recently, Solyndra had been one of the stealthiest thin-film photovoltaics operators, its glistening, prominently logoed headquarters building reminding tech-savvy commuters plowing up and down the I.880 corridor near Fremont, CA, of how little they knew about the company. But Solyndra has finally let the sunshine in and come out of the closet — even if it hasn't quite changed some of its stealthy ways. After a well-planned media and analyst rollout, the public knows that for this copper-indium-gallium-(di)selenide (CIGS) thin-film PV manufacturer, the world — or at least its solar-module form factor — is not flat. Like many TFPV purveyors, Solyndra loves glass as a substrate, but the company's meter-long CIGS-coated cylindrical modules look like a fluorescent light-bulb tube, not just another rectangular slab of the smooth stuff.