Thin film solar photovoltaic technologies

June 11th, 2014, Published in Articles: Energize

 

The number of projects worldwide using thin film PV in preference  to silicon wafer technology is increasing, and record  efficiencies for different technologies are being claimed. Thin film PV continues to show advantages over other forms, both in manufacturing and performance. This article looks at some of the latest developments in both products and technologies, which promise to offer both reduced cost and greater flexibility in PV products.

Thin film PV (TFPV) is fast becoming a major factor in the solar PV market. With projects of 550 MW already under construction, and future projects of 750 MW planned, [1], TFPV seems to be the technology of choice for large scale PV installations, and there are claims by some analysts that thin film is likely to overtake silicon wafer based PV systems in utility scale applications as well as in smaller commercial and industrial applications in the near future. TFPV seems to have overcome some of the disadvantages of the past, and offers efficiency of production as well as installation, all contributing to lower total installed costs.

Advantages of thin film

Production methods and cost

Thin film PV is generally produced by applying a thin layer of active material to a rigid or flexible substrate. This allows a continuous production process, with no interruption  between different stages of the process.  Most commercially produced technologies are produced as single modules. The goal of continuous production of rolls of thin film PV material seems to have eluded manufacturers, with the exception of one producer of flexible amorphous silicon (a-Si) TFPV, although this was widely quoted as a future scenario during the early days of the technology. Several manufacturers have gone bankrupt trying to achieve this goal. The structure of the modules means that many different materials could be used as substrate and also that relatively cheap materials, such as ordinary glass could be used as a substrate. Flexible panels are produced for small scale applications, but for larger utility scale applications rigid substrates are used. Most modules are produced using vapour deposition as the prime process, which means production of single modules. Several other methods such as electroplating, printing and  painting on of active material have been tried but with limited success. One of the most obvious advantages is the reduced amount of material used, and the simplified production process, which lends itself to automation. A manufacturing cost advantage of up to 35% compared to wafer c-Si., has been claimed [2].

Performance and temperature dependence

The operating temperature of a PV module or system is a crucial parameter for its energy output. PV modules are in fact usually rated at standard test conditions (STC = 1000 W/m2, air mass (AM) of 1,5, temperature 25°C), but their operating temperatures are usually significantly higher. The nominal operating cell temperature (NOCT) gives an indication of the operating temperature of a PV device and is therefore a useful parameter for the PV system designers. NOCT of open-rack mounted modules is measured, according to IEC 61215 and 61646 design qualification and type approval standards, at 45° tilt, 800 W/m2 with a wind speed of 1 m/s, an ambient temperature of 20°C, and in open-circuit. Typical NOCT values are around 50°C and may be a few degrees lower for devices with a polymer/glass structure (conventional c-Si modules) and slightly higher for glass/glass devices, as thin film devices usually are. However, real operating temperatures of PV devices may by far exceed NOCT values, particularly in summer days, depending on several factors, as wind speed, irradiance, ambient temperature, etc., during summer days thin film glass/glass modules may reach temperatures of up to 70°C [3].

The performance of PV is temperature dependant and decreases with increased cell or module temperature. This can be important for roof mounted solar or solar installed in desert areas. Even under ideal conditions the cell temperature can easily exceed the standard temperature. Operation in high temperatures can have a marked impact on the overall performance of an installation. The temperature dependence of PV is measured by means of the temperature coefficient (Tco) , expressed in terms of the change in output power per degree temp variation from the standard.

With the exception of  thin film crystaline silicon, all thin film solar technologies exhibit a lower temperature coefficient than c-Si. This is seen as an advantage, as under high temperature conditions in an installation,  a thin film module may have an efficiency comparable to a crystalline product. Even under mild ambient temp conditions the module temp can rise well above ambient and this can effect the performance of an installation. This is a major factor to take into account when considering utility scale installations, many of which are planned for installations in high insolation areas, which are associated with high temperatures.

Fig. 1: Modules’ maximum power (Pmax) as a function of the temperature for the different technologies [3].

Fig. 1: Modules’ maximum power (Pmax) as a function of the temperature for the different technologies [3].

Fig. 1 shows the temperature performances for several technologies compared with crystalline silicon, studied under laboratory conditions [3]. It can be seen that a-Si shows the lowest temperature sensitivity, followed by CdTe and CIGS, with crystalline and thin film silicon performing the worst. Table 1 compares the performance at 60°C and the effect on relative cell efficiencies to give the same output at 60°C. Crystalline silicon is taken as the reference point. The implication of this is that although a TFPV module may have a lower efficiency than c-Si at STC, under real operating conditions there may be little difference in efficiency.

Low light performance and tuning the absorption spectrum

Thin film PV offers better performance at low light levels than c-Si. This achieved by tuning the absorption spectrum to respond to a wider range of frequencies present in sunlight. Overcast conditions  absorb visible light but not infrared (IR). TFPV can be tuned  to absorb a wider range of IR  than c-Si and thus provides greater output under low light conditions  [4].

Disadvantages

Degradation

Degradation is the decrease in efficiency with time, a factor which all PV technologies exhibit. This was a problem in the early days of a-Si, and is known as the Staebler–Wronski effect [5]. Modules exhibited a relatively high initial degradation followed by a stabilised slower rate. This problem seemed to have been solved with of thin film although there is still a high degree of uncertainty about actual rates experienced under different field conditions. Several studies suggest that CIGS modules show  no degradation with time, and that actual degradation is highly dependant on conditions prevailing at the site where PV is installed [5].

Efficiencies

One of the prime disadvantages of thin film PV is the lower efficiency than crystalline silicon. Efficiency of thin film technologies has shown a marked increase since the first products appeared on the market years ago. Fig. 1 shows best research cell efficiencies for a range of solar PV technologies [6].

Fig. 2: Research (laboratory) cell efficiencies for PV technologies [2].

Fig. 2: Research (laboratory) cell efficiencies for PV technologies [2].

Fig. 2 shows that the efficiency of TFPV is approaching that of single junction polycrystalline silicon (c-Si). These results are for small scale laboratory produced samples only. Efficiencies for production modules are lower, for instance the laboratory record for CdTe is about 20%, whereas typically production modules achieve approximately 13%. Other technologies exhibit similar differences between laboratory and production samples.

Thin film technologies

Amorphous silicon (a-Si)

In amorphous silicon, the atoms form a continuous random network, rather than the well ordered crystalline structure characteristic of silicon. One of the earliest technologies to be commercially produced , a-Si continues to lead the TFPV market, although the efficiency is lower than that of CdTe and GIS technologies. A-Si has been produced in many forms, including flexible continuous rolls. Fig. 3 shows an early flexible a-Si panel.

Copper indium gallium selenide (CIGS)

PV devices based on CIGS have the highest conversion efficiency among all thin film technologies. Modules  can be manufactured on low cost glass substrates, which enables access to the largest PV markets. The CIGS module is compatible with existing PV system infrastructure and has the ability to dominate the building integrated photovoltaic (BIPV) market in the near future. CIGS uses homogeneous multiphase semiconductor alloys containing five chemical elements, namely copper (Cu), indium (In), gallium (Ga), selenium (Se) and sulphur (S) rather than a single material. Manufacturing appears to be limited to rigid substrate single modules and several companies are delivering product.

Cadmium telluride (CdTe)

Consists of a junction formed by CdTe and CdS with various other materials , this  is the leading non-silicon based TFPV technology and its market continues to grow. This technology is essentially based on rigid substrate modules, and vapour deposition appears to be the manufacturing process of choice. There are several companies delivering product which has enjoyed success in the utility scale market recently. The main disadvantage is perceived to be the use of cadmium, but the small quantities involved do not seem to pose a problem.

Crystaline silicon thin film (CSiTF)

One of the less well known technologies, this nonetheless holds the promise of delivering the same performance as silicon wafer technology, but with a greatly reduced usage of silicon. Crystalline silicon thin-film solar cells have the potential to combine the positive features of wafer and thin-film solar cells combining high efficiency with very low silicon consumption. Manufacturing involves depositing an amorphous film on a cheap substrate such as glass, and then treating  the film to form crystals. The most promising seems to a process called liquid phase crystalisation [7] which can be achieved by one of two means. The first involves heating the film to melting point of 900°C, usually in an oven type structure, with controlled cooling to allow crystallisation.

The problem here is that the glass substrate cannot handle temperatures of this nature for long periods. The second method makes use of a bar line focussed laser beam applied for a very short period which ensures that only the silicon layer is raised to melting point. The recent development of line-focus diode-lasers makes it possible to overcome this restriction. Diode-lasers can deliver power sufficient to heat silicon films up to melting point. A line focus beam, which can be from a few millimetres to tens of centimetres long, is ideal for a large-area, thin-film device treatment, while laser scanning speeds allow control of the treatment time (or exposure) in the milliseconds range (1 to 100 ms).

In this range, the heat transfer is limited to a few tens of microns, and thus mostly confined to a thin film itself. As a result, higher temperatures can be applied to the silicon film while keeping the glass at a far lower temperature. The laser scans over the module with exposure time of any zone limited. Crystal growth of acceptable size has been achieved by these methods. The process will allow a full thin film module of the size currently produced to be scanned in a very short time. It is expected that production efficiencies approaching that of polycrystalline wafer technology will be achieved (>10%). No commercially available product as yet, although several companies are pursuing the technology.

Emerging technologies

Several technologies are emerging at laboratory level. The most promising is the perovskite cell which has the potential to reach production status in a very short time [8]. Perovskite is a mineral first found in the Ural mountains in 1839. The new cells are made from a relative of the mineral, having the same crystal structure but consisting of different elements. Small but vital changes to the material allow it to absorb sunlight very efficiently. The material is also easy to fabricate using liquids which can be printed on substrates like ink in a printing press, or made from simple evaporation. These properties suggest an easy, affordable route to solar cells.

By playing with the elemental composition, it is also possible to tune the perovskite material to access different parts of the sun’s spectrum. That flexibility can be crucial, because it means that the material can be changed by deliberately introducing impurities, and in such a way that it can be used in multi-junction solar cells that have ultra-high efficiencies.

In four years, perovskite’s conversion efficiency has grown from 3,8% in 2009 to just over 16%, with unconfirmed reports of even higher efficiencies arriving regularly. The theoretical maximum efficiency of a perovskite-based solar cell is about 31%. Multi-junction cells based on perovskites could attain higher efficiencies still.

Several companies are already interested in forming cooperative research and development agreements so they can work with NREL on perovskite.

Which technology?

The factors affecting the performance of TFPV modules makes the choice of technology a lot more complicated than simple rand/W. Temperature dependence, degradation and low light performance can all affect the performance of an installation in a particular site, and a more representative measure would be the kWh produced per annum per unit cost or kWh/pa/R. Calculation of this value may prove complex and would probably require a pilot installation to gather the necessary info.

SA to produce TFPV?

PTiP

A semi-commercial plant for the production of a South African-developed thin-film solar module technology has been opened in Stellenbosch. The pilot production line for the manufacturing of thin-film solar modules was launched by South African technology development and intellectual holding company photovoltaic technology intellectual property (PTiP) in partnership with German engineering company Singulus technologies. The development of the thin-film technology was initiated in 1993 by the University of Johannesburg’s Prof. Vivian Alberts, whose work paved the way for the establishment of a pilot plant at the University.

The novel process technology was patented during the R&D phase to produce homogeneous quaternary [Cu(In,Ga)Se2 and CuIn(Se,S)2] and pentenary [Cu(In,Ga)(Se,S)2] chalcopyrite alloys. The homogeneous alloys have unique semiconductor features, which have superior advantages in PV cells. These specific alloys are currently utilised in specialised thin film PV devices. Device efficiencies above 16% have been produced with these materials, which ranks amongst the best in the world [9].

The success demonstrated by the pilot plant secured further financial backing from the government, and led to the partnership with Singulus Technologies and the establishment of the demonstration plant in Stellenbosch. The university, the Industrial Development Corporation (IDC) and the DST, together invested R180-million in the plant. Singulus technologies supplied the engineering technology and support for the key production processes.

The immediate goal is to set up a commercially viable production plant for thin-film solar modules in order to supply products with high local content to existing and future solar photovoltaic (PV) projects in South Africa. The key advantage of the PTIP process technology, besides the homogeneity of the semiconductor absorber alloys, is the use of commercially available production equipment during the mass production of the modules.

A second player?

At a recent local solar power conference an established German company with a working production line and proven products announced its intention to set up a CIGS manufacturing facility in East London. The company has product in service world wide [2].

References

  1. KS Dicks: “Thin Film Brief 5 – 18 March 2014”, PV Insider, http://news.pv-insider.com/thin-film-pv/thin-film-brief-5-%E2%80%93-18-march-2014
  2. C Kuhn: “CIGS-State of the art thin film PV”, Africa PVSEC, Durban 28 March 2014.
  3. A Virtuani, D Pavanello, and G Friesen: “Overview of temperature coefficients of different thin film photovoltaic technologies”, 25th European Photovoltaic Solar Energy Conference and Exhibition /5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain www.isaac.supsi.ch/ISAAC/Pubblicazioni/Fotovoltaico/Conferences/Valencia%20(Spain)%20-%2025%20EU%20PVSEC%20-20September%202010/4av3.83%20overview%20of%20temperature%20coefficients%20of%20different%20thin%20film%20photovoltaic%20technologies%20(a.%20virtuani).pdf
  4. J Runyon: “Staying Alive: Could Thin-film Manufacturers Come Out Ahead in the PV Wars?”, Renewable energy world, 14 May 2014.
  5. D Jordan and S Kurtz: “Photovoltaic Degradation Rates — An Analytical Review”, NREL www.nrel.gov/docs/fy12osti/51664.pdf
  6. NREL: “Best research cell efficiencies” www.nrel.gov/ncpv/images/efficiency_chart.jpg
  7. S Varlamov, et al: “Diode laser processed crystalline silicon thin-film solar cells” Proceedings of the SPIE, 2013.
  8. B Scanlon: “NREL Unlocking Secrets of New Solar Material”, Renewable energy world, 14 April 2014 www.renewableenergyworld.com /rea/news/article/2014/04/nrel-unlocking-secrets-of-new-solar-material
  9. SouthAfrica.info: “Pilot plant for SA thin-film solar technology” www.southafrica.info/about/science/solar-040214.htm

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