Thin-film solar cells

As opposed to first-generation crystalline silicon solar cells (c-Si), which use wafers up to 0.2 millimetres thick, thin-film solar cells (2nd generation) are composed of a thin layer (tens of nanometres to tens of micrometres) of photovoltaic material deposited on a substrate (glass, plastic or metal).

Types of thin-film solar cells include cadmium telluride (CdTe), copper indium gallium diselenide (CIGS), and amorphous thin-film silicon (a-Si, TF-Si). Thin-film solar cells can be laminated onto glazing material, or sandwiched between panes of glass, such as in large solar PV power stations.

The gains in price and flexibility has been offset by shorter lifespan and lower efficiency than conventional c-Si technology. Efficiencies have been rising, with CdTe and CIGS now higher than 21%, which is better than most-commonly used multicrystalline silicon. Market share never exceeded 20% and is currently falling.

Third generation PV cells include organic, and dye-sensitized, as well as quantum dot, copper zinc tin sulfide, nanocrystal, micromorph, and perovskite solar cells.

In November, 2018, the Centre for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW) and the Nanoelectronics Research Centre (imec) from Belgium announced they had developed a tandem thin-film solar cell with a record efficiency of 24.6 percent. The tandem cell consists of a perovskitz cell from imec and a ZSW solar cell with a semiconductor made of copper, indium, gallium and selenium (CIGS). The Perovskitz cell was developed in partnership with the Belgian organisations EnergyVille and Solliance.

In the record tandem cell, the perovskite solar cell is located above the CIGS cell. The tandem cell has four connections and is based on a fully scalable coating concept. This allows the process to be used industrially. The scientists achieved the new peak efficiency thanks to several innovations. On the one hand, they improved the transmission of the perovskite cell for light in the near infrared spectrum through improved transparent electrodes. On the other hand, they increased the band gap of the perovskite material to 1.72 eV. The result is a higher efficiency of the tandem solar cell.

The 0.5 cm2 CIGS cell was manufactured in the ZSW’s high-efficiency facility using all the optimized processes required to produce CIGS record cells. Further improvements in this technology will ultimately pave the way for thin film solar cells with efficiencies in excess of 30%.

In 2016, China/Taiwan produced 68% of the world’s new PV modules, while Europe produced only 4% (fallen from 5% in 2015), and USA/Canada only 6%. Europe installed 33% (down from 40% in 2015) of PV installations, and China 26% (up from 21% in 2015). Multi-crystalline Si-wafer based technology accounts for around 75% of all PV cells.

Solar Cell Efficiency

Best in lab: silicon wafer-based mono-crystalline = 26.7%, multi-crystalline = 21.9%. Thin-film: CIGS = 21.7%, CdTe = 21.0%. High-concentration multi-junction solar cells have efficiencies as high as 46%, and concentrator modules 38.9%.

CIGS: copper, indium, gallium, selenide. CIGS is made by depositing a thin layer of the four metals on a substrate, such as rigid glass or flexible plastic. The higher absorption coefficient of the material permits the cell to be thinner than conventional solar cells. EMPA (Swiss Federal Laboratories for Materials Science and Technology) and ZSW (Zentrum für Sonnenenergie und Wasserstoff Forschung, Deutschland) have both achieved efficiencies in excess of 20% for CIGS. EMPA achieved 20.4% efficiency with a polymer substrate, and ZSW 21.7% with a glass substrate.

CdTe: cadmium telluride. In multi-kilowatt systems, CdTE has lower costs than conventional crystalline silicon PV. Its shorter pay-back time is a major advantage, but the presence of the toxic metal cadmium obliges the owner to recycle the material at the end of life. Tellurium is also a rare metal, so places a limit on this technology’s potential for large-scale applications. CdTe thin film technology accounts for 50% of all the thin film market, but still only 5% of world PV production, despite it being used in some of the world’s largest photovoltaic power stations, such as the Topaz Solar Farm.

Crystalline silicon: more than 90% of worldwide PV production. Since being introduced around 1990, multicrystalline has gradually increased its share of the market compared to monocrystalline silicon, overtaking it around 2005. In 2014, the global market share of crystalline silicon cells was about 90%, with multi-Si making up 55% and mono-Si 35%. According to forecasts, silicon cells will continue to be the dominant photovoltaic technology in the long term and will be the “workhorse” of power generation together with wind power plants.

Commercial wafer-based silicon modules are returning around 17%, super-mono 21%, CdTe 16%. Energy payback (generated energy = manufacturing energy cost) is 1 year for multi-Si modules in high-solar regions such as Sicily. Inverter efficiency is now 98%.

At the end of 2016 photovoltaic plants with an output of 303 GW were installed worldwide. It is expected that the annual increase will reach 100 GW by 2020 and that the installed capacity will reach between 3,000 and 10,000 GW by 2030. In 2014, the global market share of crystalline silicon cells was about 90%. According to forecasts, silicon cells will continue to be the dominant photovoltaic technology in the long term and will be the “workhorse” of power generation together with wind power plants.