Monday, September 22, 2008

Solar Power: The next energy source

Photovoltaic technology (PV) offers a secure power source. It is free from the leash of the oil industry, low in maintenance, and high in modularity and scalability. It is the only technology capable of meeting the world's long-term energy needs without emitting greenhouse gases. However, this energy source is more expensive than grid power, lacks a suitable load-balancing solution and requires considerable space for electricity generation.
Generally, there are four categories of solar cells, of which the first three are in full-scale production.
• The materials in the III-V group, also known as multijunction concentrators, are direct bandgap compounds and have the highest conversion efficiency and cost. They are used primarily for satellite and military applications and require light-concentrating optics and sophisticated tracking systems to deliver 40+ percent efficiencies.• Bulk-silicon solar cells, using single-crystalline silicon (cSi) or multicrystalline silicon (mcSi), achieve conversion efficiencies above 22 percent in some cases.• Thin-film solar cells have efficiencies in the range of 10 percent for amorphous Si-based cells to almost 18 percent for the II-VI materials, which include cadmium telluride and copper indium gallium diselenide (CIGS). Thin-film methods are believed to pose the greatest opportunity for cost reduction in the long-term because of their potential for introducing modified materials into existing processes.• The final category is a catchall for emerging technologies, such as dye-sensitized thin-film cells, which already show high conversion efficiencies, and organic-based cells, which have relatively low conversion efficiencies.
Turning photons into current
The simplest definition for photovoltaic conversion efficiency is a single value that shows what fraction of the photons striking the PV array is converted to usable current for the attached load. In practice, efficiency is more complicated. Simply absorbing the light and generating free carriers aren't enough. To generate energy, the electron and hole carriers must reach the cell electrodes. If the electron-hole pairs formed by the incident photon recombine too quickly on their way to the electrode, they cannot contribute to the photocurrent.

Various issues can reduce carrier mobility or increase recombination.
Crystalline-silicon bulk solar cells have the least issues and have very high efficiencies for single-junction cells. This is because moving carriers from the junction, where they are generated to the cell electrodes from which they are supplied to a load, is relatively simple: Electron-hole pairs split and move through the n-type or p-type material as appropriate. mcSi PV cells operate in a similar fashion; but because of the presence of grain boundaries, increased electron-hole pair recombination occurs, reducing mobility and conversion efficiency to a level approaching CIGS-based thin-film cells (approximately 15 percent to 18 percent).
Although thin-film cells use less Si and may provide the best path in the future to grid parity through cost reduction, they exhibit lower conversion efficiencies than do bulk-Si cells. Although CIGS cells approach mcSi in conversion efficiency, the CIGS-based PV-cell processes have shown higher degrees of variability than bulk-Si-based processes. Although the reason is unclear, it is thought that nanoscale-level segregation control of the p-type alpha and n-type, Indium-rich beta phases of CuIn3Se5 may be critical to forming a consistent bandgap of the cells.

Shown is a TEM cross-section of Kaneka Corp.'s multi-layer amorphous Si PV cell. The front transparent conducting oxide electrode consists of SnO with rough interface to increase incoming photon scattering.
Market standing
Bulk-silicon PV cells constitute about 94 percent of global production capacity, of which Japan and Germany possess the lion's share. China is rapidly growing into a PV production powerhouse and will likely surpass Japan within the next few years.
Revenues of $10 billion in 2006 increased to more than $13 billion in 2007. Almost $16 billion was used in capacity expansion and R&D over the same period. For many companies, the importance of the supply chain has been a hard-learned lesson. The reliability of cells and modules depend on the consistency and composition of the starting material.
For China's Suntech Power, expanding the supply chain to beat the increasing demand initially led to Si-procurement and Si-quality issues, prompting the company to address impurity variability with proprietary substrate-treatment processes. These processes have improved efficiency and increased solar cell stability.
The United States has shown strong interest in exploring disruptive technology to achieve grid parity, and with a few notable exceptions, it is not heavily focused on bulk-Si technology. SunPower Corp. manufactures cSi cells with the highest conversion efficiency in the world, which is expected to generate around $1.2 billion in revenue in 2008.
Japan produces 50 percent of the world's solar cells, which puts it first in global production ranking. Although the bulk of this region's production is in bulk-Si-based solar cells—whose major suppliers are Sharp and Sanyo—Japan is also producing thin-film products. Capacity increases are planned for the amorphous thin-film cells manufactured by Kaneka Corp. and the CIGS PV cells manufactured by Showa Shell Sekiyu KK.
- John BoydSemiconductor Insights

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