Which solar cell technology is the best?

While crystalline silicon still remains the premium technology for critical applications in remote areas, thin film will be a strong contender in the consumer market due to its lower price

Like with every new technology, solar energy is also facing hurdles in its initial stages of adoption in India. Yet, despite several pitfalls, players are betting big on it. Solar technologies have garnered interest from various segments in India including the government, and efforts are being made to propagate and make the use of solar energy a household practice. While time will decide the fate of solar energy in India, as the industry needs some time to build consumer confidence around the various solar technologies, let’s study the different cell technologies—their merits, demerits and suitability considering local climatic conditions.

Pros and cons of solar cell technologies

Solar cells, also called photovoltaic (PV) cells, convert the sun’s energy into electrical energy by the PV effect. Basically, there are two main categories of technology—crystalline silicon (c-Si) solar cells and thin film or amorphous solar cells. Crystalline silicon cells come in two variants—monocrystalline and polycrystalline or multi-crystalline solar cells. The major difference between the two is that monocrystalline cells are sliced from a single large silicon crystal, while multi-crystalline cells are cut from ingots made from smaller crystals.

Crystalline silicon solar cells

This is the most common and popular cell technology, having been adopted by almost 80 per cent of the market, globally. Most manufacturers in India make c-Si solar cells. The light absorbing material here is originally created in bulk form, sliced into wafers and then positioned as a layer in the solar cell structure. Crystalline PV has been steadily growing since the early days of solar energy generation, both in terms of technological developments and market acceptance.

Advantages: Crystalline silicon solar cells represent a time tested technology and have proved convenient because they yield stable solar cells with good efficiencies of 15-17 per cent. Between the two types, monocrystalline cells are more efficient as they are made of the most pure silicon. Besides, their high efficiency ensures that the use of this technology requires less space to produce the desired capacity. These cells are also normally backed by a warranty of 25 years. On the other hand, panels from polycrystalline solar cells have higher heat tolerance and tend to perform better at high temperatures.

Disadvantages: From a financial standpoint, the high price has always hindered the adoption of this technology. Until recently, monocrystalline solar panels were the most expensive.

“Even though the production of polysilicon is energy intensive, there are hardly any costs apart from the abundantly available raw material of quart silica in the earth’s crust. This means that the polysilicon production costs per kg could be well below US$ 20. Crystalline technology offers greater scope for price reduction. The aggressive R&D on the efficiencies of crystalline PV was also underestimated by the thin film industry.”

Other disadvantages of this technology are that if the monocrystalline panels are partially covered with shade, dirt or snow, the entire circuit breaks. The efficiency of silicon solar cells reduces with increasing temperatures. This affects the performance at various locations. This means that solar cells located at different climatic regions will have differing outputs. Crystalline silicon cells are efficient up to 36°C but beyond that, with every increase in temperature, the conversion efficiency of c-Si decreases. Hence, prior to opting for c-Si solar panels for installation, it is necessary to know the temperature and relative humidity at the installation spot. In places like Gujarat and Rajasthan, where surface temperatures can go up to 65°C, C-Si cell yields drop dramatically.

Thin film solar cells

Thin film solar cells are made of thin films of PV material attached to a substrate. In contrast to crystalline silicon solar cells, the light absorbing materials incorporated into thin film solar cells are applied to substrate surfaces using deposition techniques. The different types of thin film solar cells can be categorised by what type of PV material is attached to the substrate. The most common light absorbing materials used in thin film solar cells are cadmium telluride (CdTe), copper indium gallium diselenide (CIGS) and amorphous silicon (a-Si), a non-crystalline form of silicon.

The total installed capacity of thin films is India is 55 per cent. Moser Baer was the pioneer in this field, establishing itself as a major thin film cell manufacturer in India. 
“Accepting new technology always takes time. There is misinformation about thin film technology in India. Once there is awareness, people will realise the benefits of this technology.”

Advantages: The biggest advantage of this technology is that it is lower priced than crystalline silicon. This technology is best suited for Indian weather conditions. The temperature coefficient of thin film is 0.22, whereas for crystalline silicon panels, the figure is 0.45. This means that with the increase in temperature, the drop in crystalline silicon’s output is double that of thin film’s.

Also, thin film responds to diffused sunlight much better than c-Si. In cloudy weather, or the moment the sun sets, c-Si shuts down, as for C-Si the incidence of sun rays is of much importance. While conversion efficiency of thin film works well green at down or dusk.

Disadvantages: The disadvantages of thin film technology are its lower efficiency (7 per cent) and the uncertainty about its durability. Lower efficiency implies that more space and mounting hardware are required to produce the same amount of power. Thin film materials tend to be less stable than crystalline, degrading much faster over time. Ongoing research on new thin film products will surely lead to improved technology. Hence, we can expect thin film cells with higher efficiency and warranties within a few years. According to Vivek Chaturvedi, “The challenge facing thin film technology is that there aren’t any large installations that have been around for a long period of time. Hence, customers and financial institutions have no data to evaluate the viability of this technology. While there are some experiments done in the US on solar farms that are as old as 30 years, in India we only have the Mahagenco PV plant (in Maharashtra) and a plant in Tamil Nadu that have used thin film technology.”

Concentrated photovoltaics

Concentrated photovoltaics (CPV) is another developing technology that uses optics (such as lenses) to concentrate large amounts of sunlight onto a small area of solar PV materials to generate electricity. CPV systems operate most efficiently in concentrated sunlight, as long as the solar cell is kept cool. With an efficiency of 20 per cent, this technology can be widely adopted. But being new, there is a lot of speculation regarding its durability and lifespan. CPV systems are categorised into low, medium and high concentration CPV, based on the concentration of solar energy.

CPV systems are often much cheaper to produce because the concentration allows for the production of a much smaller area of solar cells.

Disadvantages: Deploying this technology on a wide scale is difficult. One of the limiting factors is its need for direct beam radiation, which make this technology suitable only for limited geographies. Moreover, diffused light, which occurs in cloudy and overcast conditions, cannot be concentrated. To reach their maximum efficiency, CPV systems must be located in areas that receive plenty of direct sunlight. The design of PV concentrators poses yet another challenge as the optical design has to be efficient, suitable for mass production, capable of high concentration, robust enough to withstand mounting inaccuracies and capable of providing uniform illumination of the cell.

While these cell technologies continue to compete, PV experts generally agree that crystalline silicon will remain the premium technology for critical applications, particularly in remote areas. Crystalline PV has successfully passed the phase of establishing and proving itself for large scale deployment. With the focus on grid parity, cost reduction has begun by way of optimising the use of raw materials. While crystalline PV has proved to be a more commercially viable manufacturing approach than any of the other current technologies, thin film will be a strong contender in the consumer market where price is a critical factor.

What to consider while selecting a particular solar technology

It is necessary to evaluate the pros and cons of the different cell technologies before installation. While a particular technology may suit one user, it may not suit your needs. Hence, you should evaluate the following criteria:

Efficiency: While the yield of crystalline solar cells is 15-17 per cent, that of thin film is 7 per cent.

Climatic conditions: Everything in this technology is related to the sun and the intensity of heat, hence temperature plays a vital role. The efficiency of cell technology is directly related to the climate and temperature of the location. If you are planning to install solar panels at locations with a higher temperature, opt for thin film solar cells rather than c-Si cells.

Affordability: Another important factor is the cost of cell technology and the RoI. In a clear comparison between the two technologies, thin film has an edge. However, your decision should not be based just on the cost but you should also evaluate the returns on your investment.

Space and installation: While installing solar panels, space is the major hurdle. Hence, one should opt for either of the cell technologies based on the availability of space. Thin film panels require more number of panels compared with crystalline silicon cells to yield the same amount of energy. In case of space constraints, opt for crystalline solar panels.

Maintenance: Maintaining solar panels by keeping them free from dust, bird droppings, leaves and other debris is a tough task. Dusty solar panels will decrease the output of electricity and the larger the number of panels installed, the more is the maintenance required. Hence, before considering the technology to be adopted, it is necessary to evaluate this aspect.

R&D projects on solar technologies

Various ongoing research projects in India, which focus on solar technology, are listed below:

• *  R&D on thin film solar cells at the National Physical Laboratory (NPL), New Delhi, involving the development of cost effective and efficient thin film silicon solar cells based on two distinct micro structures of silicon, and CIGS-based solar cells and modules.

• *  Advanced research on thin film silicon solar cells and PV systems by the Bengal Engineering & Science University, Kolonia, which aims at the upgradation of existing technologies and the development of new technologies in 3G solar cells, and the design and development of PV systems including solar storage technologies for different applications.

• *  A project on dye sensitised solar cells (DSSC) covering quantum dots at the Indian Institute of Chemical Technology, Hyderabad and the National Chemical Laboratory, Pune. The objective of the project is to develop cost effective, durable and more efficient DSSC by implementing the holistic approach of integrating novel materials with knowledge intensive characterisation and processes. The final target would be to assess the feasibility of large scale production of PV modules based on this technology.

Design and development of organic solar cell sub-modules by the Indian Institute of Technology Kanpur. The project objectives are: (a) lab scale development of low cost, light weight and flexible solar cells with at least ~ 5 per cent efficiency and a 5-year lifespan; (b) sub-modules of 10 cm x 10 cm with 4 per cent efficiency, compatibility with commercial specifications and with a 5-year life span.

The development of stable and low cost thin film solar cells using the automated spray technique by the Cochin University of Science & Technology. The proposal is to develop industrially viable and stable all sprayed thin film solar cells with moderate efficiencies in a lab scale, using an automated process by working on non-silicon compound semiconductors deposited using the CSP technique.

Source: MNRE