By Xu Yunlong

Solar energy is becoming cost-effective thanks to recent industry advancements, in technology and commercial scaling. Both will enable the attainment of its promise as a key sustainable resource.

Essential photovoltaic components

The development of photo- voltaic solar cells can be roughly divided into three generations. The first generation encompasses crystalline silicon (c-Si) cells, while the second has arrived in the form of thin-film solar cells (TFSCs). Diverse new technologies, such as high-concentration cells, organic solar cells, flexible solar cells, and dye-sensitized solar cells are collectively referred to as the third generation. First-gen cells are still the mainstream, with TFSCs seeing a growing market share. Third-generation cells remain largely confined to the lab, with the exception of high-concentration units.

Crystalline silicon cells

Monocrystalline silicon technologies are the most mature c-Si solar cells. Their efficiency and cost are primarily affected by the manufacturing process, which consists of ingot casting, slicing, diffusion, texture etching, screen printing, and sintering. Solar cells produced from such standard processes typically feature photoelectric conversion efficiencies of 16 to 18%.

Monocrystalline cells are the most efficient among their silicon brethren, but also the most expensive. Polysilicon solar cells feature a much lower cost and much greater scalability thanks to the large square silicon ingots involved; this reduces equipment and manufacturing complexity as well as energy and material costs, while also lowering the bar for material quality.

Solar cell costs can be reduced primarily by reducing material consumption (thinner silicon wafers) and increasing conversion efficiency, the latter of which can be done in various ways, one of which is enhancement of the cell's light absorption, either through texture etching of an antireflection layer or narrowing the width of front-surface electrodes. Another method is to reduce the recombination of photo-generated carriers and increase photo utilization through techniques such as emitter passivation. Conversion efficiency can also be enhanced through reduced resistance values and increased electrode absorption of photocurrent through techniques such as doping by region and back-surface field.

The highest documented photoelectric conversion rate for monocrystalline silicon is 24.7%, attained by PERL (passivated emitter, rear locally diffused) silicon solar cells at the University of New South Wales. These cells feature less phosphorous doping on the silicon surface to reduce recombination and prevent the development of a "dead layer," both of which can occur on the surface. They also feature high-concentration diffusion under front- and rear-surface electrodes to reduce local recombination and create a good ohmic contact, along with narrower front-surface electrodes, created through lithography, to increase the light absorption area.

Other features include a combination of more suitable metals such as titanium, palladium, and silver to reduce the contract resistance between the electrode and the silicon, and the use of silicon dioxide and point contact on the front and rear cell surfaces to reduce surface recombination. However, PERL technology has yet to be commercialized.

Non-PERL high-conversion technologies include surface-grooved & textured solar cells, as well as emitter wrap through (EWT) technologies from BP Solar. The former reduces the front surface electrode width through laser grooving, which increases the sunlight absorption area; volume production has been able to achieve an 18.3% conversion efficiency. EWT involves cells with laser-etched holes that lead from front-surface electrodes to the rear surface, thereby increasing the light absorption area on the front surface and achieving a 21.3% efficiency.

TFSCs

Crystalline silicon solar cells are efficient and remain dominant in large-scale applications and industrial production. However, it is very difficult to reduce their expense thanks to high silicon material costs. As an alternative to such cells, cheaper TFSCs have come into being. Mainstream TFSCs fall under cadmium tellurium (CdTe), copper indium gallium selenide (CIGS), or silicon-based varieties.

The thickness of a silicon-based TFSC is roughly two microns, making the material consumption 1.5% of what it would be for a 200 micron c-Si cell. Silicon-based TFSCs are categorized by their number of PN junctions into single-, tandem-, and multi-junction. Different PN junctions absorb sunlight at different wavelengths, as they are composed of different materials. Single-junction TFSCs have efficiencies of up to 7%, while those for dual-junctions may reach 10%.

Thanks to their substrate's light absorption rate, CdTe TFSCs are more conversion-efficient than their silicon-based counterparts (up to 12% presently). However, cadmium is a carcinogen while naturally occurring tellurium is rare, which limits the efficacy of both as a solar cell substrate.

CIGS is believed to be the future of the TFSC, as its sunlight absorption rate can be increased through mere adjustments to the manufacturing process. Today, its conversion efficiency can reach 20.1% in the lab and 13-14% commercially; both are the respective peaks among all TFSCs.

Third-generation cells

Third-generation technologies can theoretically reach higher conversion efficiencies than their predecessors, but only high-concentration cells are commercially available at present. These cells are typically based on III-V semiconductors, primarily because they feature much greater heat resistance than silicon, as well as high photoelectric conversion efficiencies in low-light conditions, a multi-junction structure that gives them an absorption spectrum nearly identical to that for sunlight, and a theoretical conversion efficiency of up to 68%. The most commonly used high-concentration cells have three PN junctions consisting of three different semiconductors [germanium, gallium arsenide (GaAs) and gallium indium phosphide (GaInP)], which can reach an efficiency of up to 40% in scale production.

Through packaging, solar cells become solar modules. Specific solar cell applications depend on solar cell characteristics and changing market demands. Early solar energy applications revolved around base stations and satellites, after which consumer applications such as solar roofing came into being. Such scenarios involve small mounting areas and high energy density requirements, which has pushed crystalline silicon modules into the forefront. As large solar power stations and solar-powered buildings have developed, total cost has replaced energy density as the primary consideration, which is pushing TFSC applications back into relevance.

Photovoltaic technology applications

A full set of photovoltaic systems is required to convert solar energy into electricity for the home or workplace. Photovoltaic cells serve as the foundation of any such system, but inverters, batteries, monitors, and distribution systems are also involved.

Photovoltaic systems

Photovoltaic systems can be on-grid or off-grid; off-grid systems include independent photovoltaic and hybrid power supply (HPS) systems. Independent photovoltaic systems are typically used for base stations, streetlights, and remote power supplies. All use solar energy as their power source. Such systems primarily consist of solar modules, inverters, controllers, batteries, distribution systems, and lightning protection systems. Among them, energy storage equipment (batteries) and controllers are the key factors that determine system costs and lifecycle. HPS systems comprise diesel and/or wind power generators that supplement solar cells.

On-grid system technologies are typically used for solar roofs and large-scale power stations. These systems need no energy storage equipment, which makes them less expensive. Primary components include solar modules, inverters, distribution systems, lightning protection systems, and monitoring systems. Today, on-grid systems represent 80% of all solar energy applications.

Supplementary technologies

In addition to photovoltaic technologies, inverters, grid access, energy storage, and intelligent monitoring technologies are all tied to the application and growth of solar photovoltaic systems. Solar cells output power intermittently and variably as sunlight intensity varies. The grid is impacted by large-scale grid access, making island protection and grid access control critical. Solar modules also output direct current, which must be converted into alternating current through inverters, leading to greater demands on the quality of inverted current. Module power output depends on factors such as temperature and light level; solar array voltage mismatch can occur, making system monitoring and alarm important. Finally, most photovoltaic systems are remote, making their control ever the more vital.

Status and outlook for photovoltaic generation

Solar technologies have taken off in the 21st century, both in terms of efficiency and cost. The world's total installed capacity increased to 22.8GW in 2009, up from 1.4GW in 2000. European countries, led by Germany, Italy, and Spain, are the largest markets. The EU plans to increase solar's contribution to its total power supply to 12% by 2020. Developing countries such as China and India have also launched solar energy development plans. In addition to the usual remote applications, solar power is now widely used to power various mobile devices such as cell phone chargers.

As an alternative and supplementary energy source, solar power has entered a period of rapid growth and decreasing generation costs. Thanks to its clean and renewable nature, the photovoltaic industry is sure to become a key energy player for the next century.