In the past 30 years, new and renewable energy have been considerably the emerging realm of studies along with declination of fossil fuel reserves. Triggering by oil crisis in the early 1970’s, intensive efforts on renewable energy research and development as the alternative energy for substituting oil has been initiated in the developed countries. Almost at the same time, world faced the increasing of carbon emission from industries, houses and passenger cars which has been flooded the air with pollution and lately be the source of green-house effect. On the other hand, rapid increasing of world’s population has warned government, politicians, scientists and industries to provide more energy supply and also security for the future. Those situations then boosted the public awareness on employing energy without any detrimental effects for environment, save and sustainable which recently called by green energy or sustainable energy concept.
Recently, nuclear energy, wind power, coals, geothermal, hydrogen fuel cell are some of the field of research interests which are aimed to provide appropriate green energy for fulfilling world energy demand and distribution as well as established water power plant. However, those are depending on area characteristics which can not possibly be applied on the all geographical regions.
Since 1956 when Bell Laboratories researchers incidentally discovered that pn junction diode generated a voltage when a room lights were on, the way of thinking about energy has been changed. Solar electricity that generated from the pn junction brought a new vision about how to generate electricity from sunlight which later on called with Photovoltaic (PV).
Nowadays, 50 years from its first discovery, PV helps us avoid most of threats associated with our present techniques of electricity production and also has many other benefits. PV has shown that it can generate electricity for human race for a wide range of applications, scales, climates and geographic locations. PV is able to bring electricity to a rural and remote area far away form the nearest electric grid connection, thus allowing the population there to have clean electric lights instead of kerosene lamps or just to listen to the radio as well as major cities populations do. PV allows homes and business a new level of guaranteed energy availability and security as well. These benefits are not able to attain by other kind of energies.
However, to achieve situation where renewable energy, such as PV, substitutes fossil fuels place, it needs long term scenario and effort involving research and development, government policy and funding for at least 20 years from now.
Present Status of Photovoltaic Technologies
Recently, fossil fuels provided 80% of the total, with oil (35%) in frst place, followed by coal (23%) and natural gas (21%). The renewable energies collectively provided 14% of the primary energy, mostly in the form of traditional biomass (10%). Only 2% of the electricity was provided by the renewable energy (small hydro, biomass, geothermal, wind, solar and tidal energy). It demonstrates that the portion of renewable energy still below one digit and fossil fuel still predominance the world’s energy and electricity supply. PV, represented by solar energy item, is the second-largest energy source among the renewable energies after geothermal.
PV as the apparatus for directly converting sunlight into electricity consists of many types regarding to its materials and processing route. Those are silicon solar cell, amorphous silicon, thin film silicon, thin film CuInSe2/CuInS2 (CIS) solar cell, CdTe and others. It doesn’t quite surprising that in fact silicon has played such a dominant role in the market. The main reason is that silicon technology has already been highly developed before the advent of PV and high-quality material is being produced in large quantities for the microelectronics market. On the other hand, silicon feedstock is considerably large in quantity to supply the demands either for microelectronics industry or solar cell manufacture.
However, due to indirect band gap materials, silicon is generally perceived that the thickness of silicon required to absorb usable sunlight should be large enough. Although silicon still plays a role as major solar absorber materials for commercial solar cell, its optical absorption is considerably small for converting most of photon energy into electricity. In semiconductor manufacturing, commercial silicon wafers has a thickness of ~ 250 microns, while based on calculation, sufficient absorption will be high if the Si thickness is achieved 700 microns. This is quite a huge of thickness for Si wafers and is not desirable for commercial production of solar cells due to the wafer cost can be very high.
A lot of efforts have been going on and is still going into the search for new materials. It has been calculated that an ideal PV material should satisfy the following: (1) band gap between 1.1 and 1.7 eV (2); direct band structure (3); consisting of readily available, non-toxic materials; (4) easy, reproducible deposition technique, suitable for large area production; (5) good photovoltaic conversion efficiency; and (6) long-term stability.
Future PV cell research and development will be focused on PV with less expensive due to less material used, but highly absorb sunlight. Thin film PV type due to its light-weight, so that needs less materials, and high absorption coefficient, for absorbing necessity portion of sunlight, shows promising techniques to satisfy the above-mentioned prerequisites. But this situation still encounters some challenges regarding to thin film PV materials properties.
So far, the efficiency of thin film PV is considerably not comparable with silicon PV. Also, thin film PV has much less-developed knowledge and technology base compared to silicon technology, respectively.