While crystalline silicon contribute about 85% of market share. It is expected that market share of thin film will increase in future. The major reason of attention of thin film technology is due to the increase in efficiencies and declining costs. Recently solar frontier has announced that their CIGS based cells have achieved efficiency of the order of 18%. Thin film technology is not a recent technology NREL has tested thin film cells in mid-80’s and earlier due to poor efficiency this technology never bought attention, however the recent improvements in efficiency and advent of new manufacturing methods have laid thin film attractive to many applications. In this article we would like to highlight some of the advantages and key characteristics of thin film modules.
Higher Module Voltage
A typical crystalline module operate at relatively low voltage and high current, however a thin film module operate at relatively high voltage and low current. It means in a string there are about 16-20 crystalline silicon modules connected in series operating at string voltage of 600 V dc, in case of thin film there are about 4-5 modules connected to provide the same voltage levels. This makes string sizing critical in case of thin film as matching of voltage range with inverter voltage is in very limited band. If we need to accommodate thin film it is advisable to go for higher string voltages and many projects are now considering the string voltage of about 1000V dc. This will not only reduce the BOS cost but also provides better flexibility for string sizing.
fig: I-V curve (Source: Solar Frontier Modules)
Lower Temperature coefficient
The temperature coefficient of thin film is relatively lower as compared to crystalline silicon modules. The lower temperature coefficients helps to improve overall system performance in hot weather conditions. Especially in Indian conditions where the PV modules are expected to operate at cell temperature as high as 75 celsius, the performance of thin film in terms of specific yield(Kwh/Kwp) will be better in thin film as compared to crystalline silicon modules.
Better shade tolerance
Thin film modules are better performing in case of shading and the problems of hot spot formation are not significant. In some of the BIPV applications where shading is unavoidable it is preferred to use thin film modules rather than crystalline silicon modules for their better performance during shading conditions.
Better spectral performance
PV materials absorb and convert light more efficiently within a given wavelength of spectrum. In thin film cells it is possible to deposit different layers of PV materials so that a broader light spectrum can be absorbed. For example the recently launched triple junction technology by some of the manufacturer allow absorber layers to capture blue, green and red portions of visible light spectrum. Some of the companies such as SANYO has come up with hetero-junction intrinsic thin film (HIT) cells which are combination of mono crystalline cells with amorphous silicon deposition to cover wider spectrum of sunlight.
Due to above advantages many developers have used thin film technology for large MW scale power plants. Thin film is also being used in many roof top applications it is possible to have thin film technology modules as partial transparent glass to glass laminates which used for building façade applications. Thin film also available in various colours which makes attractive to architects as part of their BIPV design applications. The PV technology in crystalline silicon and thin film are changing at fast phase and we believe some of the advantages of thin film and some of the advantages of crystalline will come in the form of hybrid which makes solar cells cost effective and more attractive in various applications.
Dr. Sanjay Vashishtha & Rishikesh Muthyal