Following the discovery of silicon solar cell, there are many varieties of solar cell regarding to the materials employed as the solar absorber. There are single and polycrystalline silicon, amorphous silicon, crystalline silicon thin film solar cell, CdTe thin film, Cu(In,Ga)Se2 thin film, the other types are organic solar cell and dye sensitized solar cell. Each type of solar cell has its own advantages as well as limitations and will be briefly discussed.Silicon Solar Cell
Indeed, the first solar cell was a (bulk) silicon solar cell. Right after its discovery, silicon solar cell has been utilized on empowering the space satellites. Silicon solar cell technology takes great benefits from high standard silicon technology. The Czochralski (Cz) technology for growing single crystal silicon has been used for many years as main process for supplying silicon as solar absorber.
The production route starts from the so-called solar grade Cz-silicon wafer –either single crystal or polycrystal- which has already doped to p-type by boron with resistivity of 1 Ohmcm. In order to make a pn junction, phosphor is diffused on the wafer surface for growing n layer. This step follows with TiO2 deposition in order to growth anti-reflection coating (ARC) which usually deposit by atmospheric pressure CVD (AP CVD). Nowadays, silicon nitride is used as AR coating instead of TiO2. This route is finished with the metallization (using silver) for front contact, metallization of aluminum for back contact, co-firing and finally testing and sorting process. The best cell’s efficiency has been confirmed 24.7% by University of New South Wales team.
Major challenge for further development of silicon solar cell is regarded to the supply of highly purified silicon wafer for solar cells in the future. For anticipating this uncertainty, research on new crystalline silicon materials of medium thickness has been envisioned to reduce the amount of materials in one cell or module. On the other side, theoretically, silicon possesses low absorption coefficient due to its indirect band gap, low band gap and thus regards as silicon limitation for improving the cell’s efficiency.
Amorphous Silicon Solar Cell
In the early of 1970’s, a new material has presented into the solar cell research community. It was amorphous silicon thin film as a solar absorber in solar cell. In contrast to bulk silicon solar cell, this cell consists of a p-i-n junction which is completely different with bulk one, either in manufacturing process or properties. It answers the bulk silicon solar cell problem in terms of reducing thickness and increasing the optical band gap. Due to the defects associated with the “dangling bond”, the amorphous silicon (a-Si) is hydrogenated to reduce the band gap states and to allow the development of open-circuit voltages (symbolized by a-Si:H).
Amorphous silicon is of an alloy of silicon with hydrogen in the form of thin film. Bond length distribution and bond angles disturb the long range of the crystalline silicon lattice order. This brings consequence of changing the optical and electrical properties. Its band gap can then be tuned from 1.12 to 1.7 eV by varying the hydrogen content. Amorphous silicon has competitive advantages to bulk silicon solar cell; its light optical characteristic make it 100 times more effective absorbing sun’s irradiance than bulk silicon.
The preparation of amorphous silicon film is generally perfumed by means of chemical vapor deposition (CVD) from silane precursor gas. Typical deposition temperature is below 5000C in order to let the hydrogen diffuse effectively into film. The champion efficiency of 9.5% with active area of 1.070 cm2 has been reported by National Renewable Energy Laboratory (NREL), USA . In recent times, the development of this technology has been emphasized on stability issue considering the inherent light instability or losing 50% of power after several hour operations.
Crystalline Silicon Thin Film Solar Cell
In recent times, many efforts have been performed on producing thin film silicon for solar absorber not only to be directed to minimize the silicon thickness, but enhancing the total absorption coefficient as well. The basic ideas are to apply very thin silicon layer while maintaining its absorption coefficient keep high enough as bulk silicon solar absorber does. In recent times, many efforts have been performed on producing thin film silicon for solar absorber not only to be directed to minimize the silicon thickness, but enhancing the total absorption coefficient as well.
The basic component of thin film silicon solar cell is an active silicon layer as a base, intermediate layer as a diffusion barrier and back side reflector, a substrate which can be a glass, and base contact. There are large variety of silicon deposition technologies which can be allocated to the main group liquid phase and gaseous phase for depositing Si on a substrate. In the liquid phase deposition, the respective substrate is brought into contact with a metal melt (Cu, Al. Sn. In) saturated with silicon. By lowering temperature of the melt supersaturation occurs and silicon is deposited on the substrate. The substrate temperature lies within the range of 800 – 1000 deg.C and deposition rates vary from a few micrometers per hour up to 10 microns/h. In the chemical vapor deposition (CVD) method, which is a well-established method in microelectronics, a mixture of H2 and the precursors SiH4, SiH2Cl2, or SiHCl3 is decomposed thermally at the hot surface of the substrate. The most common techniques are low pressure and atmospheric pressure CVD (LP CVD, AP CVD), but also plasma enhanced, ion assisted or hot wire CVD (PE CVD, IA CVD, or HW CVD) are used to deposit silicon films at temperatures between 300 – 1200 deg.C. The best cell’s efficiency is 16.6 % (4.017 cm2) and module’s efficiency is 8.2 % (661 cm2).
As a general trend the cell efficiency increases with the grain size, due to the fact that grain boundaries in their unpassivated state can be very effective recombination centers which reduce the diffusion length of the minority carriers immediately.
Cadmium Telluride (CdTe) Solar Cell
Since the 1960s, CdTe has been a candidate solar cell material. It has firstly applied at terrestrial application as power generator in satellite. The two properties of this material are its near ideal band gap for solar cell conversion efficiency of 1.45 eV and its high optical absorption coefficient. It has been reported that a thin film of CdTe with thickness of approximately 2 microns will absorb nearly 100% of the incident solar radiation.
Thin film CdTe solar cell and modules are typically heterojunctions with CdS being n-type partner or window layer. The preferred configuration of CdTe cell is the superstrate structure with borosilicate glass as the substrate. The highest quality materials and efficiencies are obtained with close spaced sublimation (CSS). CuTe source material is heated by source heater in order to vaporize Cd and Te2 vapor into glass substrate at the temperature of 450 – 6000C with high deposition rate of 1 microns/min.
The active layer of a CdTe based solar cell is deposited on transparent conducting oxide (TCO) of SnO2 or Indium-Tin Oxide (ITO) coated glass. High efficiency cell use very thin chemically deposited CdS. It has been confirmed that the best efficiency of CdTe cell is 16.5% with active area of 1.032 cm2 and of 10.7% for CdTe module. Although there are concerns, perceptions, and misconceptions about the environmental, safety and health effects of the Cd in this device, extensive studies indicate that all safety issues can be handled with modest investment in cost and recycling.
Copper Indium Gallium Diselenide, Cu(In,Ga)Se2 / CIGS
Interest in the Cu-ternary semiconductor began in the early 1970s including CuInX2 (X = S, Se, Te) material. The so called chalcopyrite for this ternary semiconductor has been considerably of interest due to their properties that suited for solar cell prerequisites i.e. optical band gap lies within 1.01 – 1.5 eV. They exhibit direct band gap semiconductors, capable of either n or p –type conduction, possessing high absorption coefficient, stable electro-optical properties and matched band gap to the solar spectrum.
From this materials class, the CuInSe2 (CIS) and Cu(In,Ga)Se2 (CIGS) based solar cells have often been attributed as being among the most promising of solar cell technologies. The reasons are due to their advantages of low cost thin film, high rate deposition on large scale of substrate and less than 2 microns of thickness.
A wide variety of film deposition methods has been used to deposit CIGS thin films. The most promising deposition methods for the commercial manufactures of modules can be divided to two categories. The first category is vacuum co-evaporation in which all the precursors Cu, In, Ga and Se can be simultaneously deposited to substrate at 400–600 deg.C. The second category is two-step process that deposition of elemental Cu, In Ga and Se followed by annealing process at 400 – 600 deg.C as well.
50 nm CdS/CdSe film has been using as a buffer layer and it boosts the CIS/CIGS efficiency as well as the cell’s open circuit voltage. An optimal transparent front junction to the absorber is fulfilled by this film. So that, the complete CIS/CIGS solar cell composes of a antri-reflection coanting (usually MgF2), a transparent conducting oxide (Al-doped ZnO ot Indium-Tin Oxide), a buffer layer, a CIS or CIGS absorber layer, back contact (usually Mo) and substrate that can be either glass, polymer or stainless steel. Recently CIGS has demonstrated very high efficiency over 19% for cell produced by NREL, USA and 13.4% for module produced by Showa Shell.
ye-Sensitized Solar Cell
In 1991, O’Regan and Dr. Gratzel discovered the novel device as an alternative to present solid state solar cell, i.e. dye-sensitized solar cell (DSSC) which relied on the photochemistry phenomena combined with solid state semiconductor. Unlike solid state based solar cells, the DSSC employed a metal-complex dye and electrolyte to harvest the light into electricity. The devise is based on a 10 microns thick, optically transparent film of titanium dioxide (TiO2) particles a few nanometers in size, coated with a monolayer of a charge transfer dye to sensitize the film for light harvesting. In their letter to Nature, the light-to-electric energy conversion yield was 7.1 – 7.9%.
At the heart of the DSSC system is a wide band gap oxide semiconductor as front electrode (F-doped SnO2/ FTO), metal complex dye, electrolyte and platinum film as a counter electrode. The dye which is consisted of Ruthenium complex photosensitizer contributes to photon absorption. The TiO2 nanopowder serves as a conduction band for electrons to reach the front electrode whereas the electrolyte supplies the electrons to the Ru complex from the redox reaction of iodine solution.
DSSC do not use vacuum technique to be produced. TiO2 commercial nanopowder can be employed and TiO2 film can be prepared by colloidal method followed by sintered at 4500C in air. FTO coated glass can be deposited by spray pyrolysis, sputtering, CVD or electrodeposition. Pt counter electrode also produced by either sputtering or colloidal method. By this process, currently, reported efficiency is of 10.4% with active area of ~ 1 cm2 by Sharp, Japan.
Organic Solar Cell
Beside dye-sensitized solar cells, which may be considered as organic/inorganic hybrid cells, other types of organic solar cells currently become of broader interest. These cells can be divided roughly into molecular and polymer organic solar cells or into flat-layer systems and bulk heterojunctions. Organic materials as, e.g. conjugated polymers, dyes or molecular organic glasses can show p or n-type semiconducting properties. Extremely high optical absorption coefficients are possible with these materials, which offers the possibility for the production of very thin solar cells (far below 1 mm), and therefore, only very small amounts of needed materials.
The variability of organic compounds is nearly infinite. Beside this, the large interest in these materials results from technological aspects as the expected ease of large-scale manufacturing at low-temperature processes and very low costs. The up scaling of organic solar cells into large-area devices, always a big challenge with inorganic solar cells, has already been demonstrated to be straightforward. The energetic pay-back time of organic solar cells is expected to be very short.
Considering the fact that light emitting films of plastic materials have been realized there is a realistic chance to achieve efficient photovoltaic conversion also in such materials because this is just the reverse process. Organic solar cells offer the hope of being very inexpensive. Quite a variety of materials, compositions and concepts are being investigated, which reflects the possibilities in terms of device concepts, materials use and materials design. In spite of the many fundamental questions that still exist, these perspectives and the fact that exploration has only just begun, cause a largely growing interest in the development of such solar cells.