History of solar cells

The development of photovoltaic technology up to today's 42.8% efficient cells starts with the Becquerel's discovery of photogalvanic effect in 1839 when he realized that the conductance of an electrolyte increased under illumination. Then in 1877 Adams and Day (UK) were to discover the photovoltaic effect on a junction between selenium and platinum which was the first observation of photovoltaic effect in solids. After several years of research and development in 1954, first 6% efficient Si solar cells were reported by Bell Labs, USA at the same time with 6% efficient Cu2S/CdS solar cells by Air Force, USA.
In early years, most of the photovoltaic research was aimed for space applications with high cost and relatively high efficiency. As the technology began to mature in 1980's, low cost solar cells for everyday usage emerged with first MW plants installed in USA and Saudi Arabia. The emerge of concentrated multijunction solar cells at 1980's enabled higher efficiencies from smaller areas of illumination and together with thin film solar cells they both decreased the raw material usage hence decreasing the initial costs. Then in 1990's low cost-low efficiency organic solar cells hit the market which offered ease of application and flexibility in large area integration.

Photovoltaic Market Today

Despite its fifty years of research and development, photovoltaic technology, like all other green technologies, was first considered to be alternative to the conventional fossil energy in the beginning of 1990's when EU recognized the CO2 emissions are at dangerous rate for the earth's near future which will cause an estimated global warming of 1-4 Co by 2030. Until 1990's, photovoltaic applications on Earth were dominated with off-grid applications such as rural communication systems, meteorological systems, water pumping power supplies and small electronic devices. However by 1999 the installed PV generator capacity in PVPS (Photovoltaic Power Systems Program) countries was estimated to be a total of 500 MWp including off-grid and grid applications. Today, the total amount of installation reaches 23 GWp around the world

Total Installed PV capacity in the world

The crystalline silicon technology still dominates the world PV market today. According to PVPS, the wafer based module production was 2400 MW in 2007 while it is 300 MW for thin film based module production.

Market shares of different photovoltaic technologies by the year 2008

The first reason why both multi and mono c-Si solar cells dominate the market is the abundance of Si which constitutes a big portion of the Earth's crust. Secondly, silicon is an element that has been used and studied for more than 30 years in semiconductor electronic industry so every step of silicon processing has matured more than any other elements.

Photovoltaic Technologies

1) Single & Multicrystalline Silicon Solar Cells

Single & Multicrystalline Silicon Solar Cells

Single and multicrystalline silicon solar cells are both wafer based technologies which are considered as the first generation solar cells. While single crystalline silicon solar cells posses' higher efficiencies compared to multicrystalline silicon solar cells, the cost of producing single crystalline wafers is much higher than producing multicrystalline wafers. Consequently multicrystalline solar cell production holds the biggest share in the world market today compared to other technologies.

Wafer based silicon solar cells have reached more than 24% cell efficiency and a 19.3% panel efficiency using different metallization and doping designs such as "Metallization Wrap Through", "Emitter Wrap Through" and back contact designs. All of those designs commonly solve the shading problems on the front surface, increasing the p-n junction area and easing the module production hence increasing the efficiency.

p-n junction

2) Thin Film Solar Cells

The second generation photovoltaic cells are in the form of thin films which are deposited as a thin layer of photovoltaic material on thicker substrates. Some examples of solar cells in this group are a-Si solar cells, Cd-Te solar cells, CIGS solar cells. The main advantage of thin film solar cells is their cost effectiveness due to low raw material usage. However, difficulty of deposition onto large areas is one of the problems that thin film industry faces today. In addition, the efficiency of thin film solar cells is relatively low as compared to their wafer based counterparts which is a major drawback of thin film technology. Despite these difficulties, low cost and ability to be deposited on flexible substrates make thin film solar cells desirable when compared to wafer based silicon solar cells which must be joined together and laminated to form rigid solar panels.

p-n junction

3) III-V semiconductors

III-V semiconductors such as GaAs, InAs, InP can be engineered to obtain different bandgaps from the same material family. III-V semiconductors with different bandgaps can be grown onto each other enabling different wavelengths of solar spectrum to be collected efficiently at each layer. However, due to high cost of the production of these materials, the solar devices of this kind are used under high concentration of sunlight to obtain higher power from smaller areas, reducing the cost of energy production. This type of solar cell system, which is commonly called Concentrated Photovoltaics (CPV), reaches record high efficiency values exceeding 40 %.


4) Organic Solar Cells

Organic solar cells can be produced using low cost polymers and can be deposited as very thin layers due to their high absorption coefficients. Therefore they are considered as the cheapest solar cells that can be produced. However, the efficiency and lifetime of these solar cells are very low compared to other solar cells. The highest efficiency reached with organic solar cells is about 6.5% obtained from a tandem structure. The low efficiency and low lifetime of these cells can be compensated by application to large areas on flexible substrates, which is a very easy and low cost process based on roll to roll production technique.

5) Dye sensitized solar cells

Dye sensitized solar cells (DSSC) have emerged as an attractive low cost solar cell system recently. In this type of solar cells, dye molecules attached to TiO2 nanoparticles are used to convert the solar radiation to electrical charge and collect them as an electrical current. Efficiency values up to 10% have been reported for DSSC. Easy production process and relatively high efficiency values have made DSSC an attractive solar cell alternative to current technologies. DSSC can be fabricated with roll to roll production lines or by inject-printing. They can be used as transparent solar cells over windows enabling a more aesthetic and large surface integration in Building Integrated Photovoltaic (BIPV) applications.