Chapter 1 Introduction
1.2 Classification of Solar Cells
1.2.1 Classification of Solar Cells Depending on Various PV Materials
In general, the solar cells are mainly classified into few categories (Fig. 1.1), according to the quality, overall thickness and the fabricated approach of the devices [1.5]:
Wafer based crystalline silicon solar cells, which include mono (single)-crystalline and poly (multi)-crystalline silicon.
Thin-film solar cells, which include amorphous silicon (a-Si), cadmium telluride (CdTe), copper indium gallium diselenide (CIGS) etc.
Emerging PV technologies, which include concentrating photovoltaics, dye sensitized solar cells, polymer organic solar cells etc.
Fig. 1. 1 Classification of various solar cell technologies.
Wafer-based crystalline silicon solar cell technology
So far, most technologies used for the solar cell fabrication rely heavily on the microelectronics industry, and that can be mainly classified into two categories as (i)
Mono (single)-crystalline, and (ii) Poly (multi-)-crystalline silicon solar cells.
(i) Mono (single)-crystalline silicon solar cell
This is the most established and efficient solar cell technologies till date, which have module efficiency of 15-18%. The cell and module fabrication technology is well developed and reliable. These cells are manufactured from single silicon crystal, by process called Czochralski process. During the manufacturing, c-Si crystals are cut from cylindrical ingots, they do not completely cover a square solar cell module.
(ii) Poly (multi)-crystalline silicon solar cell
The production of poly-crystalline silicon (poly-Si) cells is more cost-efficient which are manufactured by cooling a graphite mould filled with molten silicon. In this process, liquid silicon is poured into blocks that are subsequently sawed into plates. During solidification of the material, crystal structures of varying sizes are formed, at whose borders defects emerge. These cells have module efficiency of around 12-14%.
Thin film solar cell technology
Thin film solar cells are also called second-generation photovoltaic cells. In this approach thin layers of semiconductor material are deposited onto a supporting substrate, such as a large sheet of glass. Typically, less than a micron thickness of semiconductor material is required. Some of the thin film solar cells in use are as follows: (i) Amorphous silicon (a -Si), (ii) Cadmium telluride (CdTe), and (iii) Copper indium gallium (di)selenide (CIS, CIGS).
(i) Amorphous silicon thin film solar cell
Amorphous Silicon (a-Si) modules are the first thin film solar module to be commercially produced and at present has the maximum market share out of all thin
film solar cell technologies. A-Si solar can be fabricated at a lower deposition temperature hence permits the use of various low cost flexible substrates by easier processing technique. The major concern of a-Si solar cells is their low stabilized efficiency. The overall efficiency drops inevitably at module level and at present the efficiencies of commercial modules are in the range of 4-8%.
(ii) Cadmium telluride (CdTe) thin film solar cell
Being a crystalline compound Cadmium Telluride is a direct band gap semiconductor, which is a strong solar cell material. It is usually sandwiched with cadmium sulfide to form a pn junction PV solar cell. CdTe with laboratory efficiency as high as 16% have been developed at NREL. Multitudes of manufacturing techniques are main advantage of these solar cells which are suitable for large scale production. Limited availability of cadmium and pollution problem associated with cadmium is main concerns with this technology.
(iii) Copper indium gallium diselenide (CIGS) solar cells
This is a new semiconductor material comprising copper, indium, gallium and selenium in a specific order, which is used for solar cell manufacturing. It is one of the most promising thin film technologies due to their high-attained efficiency and low material costs. Amongst thin film solar cells, the advantage of CIGS solar cell is its extended operational lifetime without significant degradation. The inherent properties of CIGS also provide an opportunity for maximizing the efficiency.
Emerging PV technologies
Emerging PV technologies are the techniques which represent the most outstanding ongoing developments, advances, and innovations in PV fields. The following lists some of current emerging PV technologies, which contains (i) concentrating
photovoltaics, (ii) dye sensitized solar cells, and (iii) organic solar cells.
(i) Concentrating photovoltaics
In concentrating photovoltaic system, lenses or mirrors are used to focus sunlight onto a much smaller area of photovoltaic panel. In general, the system uses a tracking device to follow the sun as it moves across the sky. The system’s ability to concentrate sunlight dramatically decreases the size of the receiver; hence it is a good candidate for more expensive, higher efficiency receiver materials including stacks with multiple layers of semiconductors that can absorb much more of the solar spectrum.
(ii) Dye sensitized solar cells
A dye-sensitized solar cell (DSSC) is based on a semiconductor formed between a photo-sensitized anode and an electrolyte. The DSSC has a number of attractive features including being easy for conventional roll-printing techniques, semi-flexible and semi-transparent. However, its liquid electrolyte is limited to the weather condition in the practical applications. Nevertheless, its price/performance ratio should be good enough to allow them to compete with fossil fuel electrical generation by achieving grid parity.
(iii) Organic solar cells
An organic solar cell is a type of polymer solar cell that uses organic electronics for light absorption and charge transport to produce electricity from sunlight by the photovoltaic effect. Despite the device used polyacetylene as the organic layer, its charge collection efficiency was only 0.3% with Al and graphite at the early stage;
recently there is a breakthrough in development that a three-layer fullerene-free stack achieved a conversion efficiency of 8.4% with an open-circuit voltage around 1 V.
Fig. 1. 2 Pie charts showing the various type of solar cells market share from 2008 to 2010.
As can be easily seen from the pie charts in
Fig. 1.2, wafer-based crystalline silicon
solar cells dominated the solar market by a large margin from 2008 to 2010. Even to 2013, wafer-based crystalline silicon solar cells were still up to 90% market share [1.6].The thin film solar cells’ (including a-Si, ribbon Si, CdTe, CIGS etc.) share for all thin film technologies was only 14% in 2010. wafer-based crystalline silicon solar cells’
share has been rapidly increasing the last few years as Chinese manufacturers have come on strong; however, the thin film’s market share is forecast to decline further to 7% by 2017 according to Solarbuzz [1.6].