Thin Film Photovoltaic Cells Engineering Essay

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The increase in demand and rising cost of silicon has made thin-film photovoltaics a contending substitute to traditional silicon solar modules. Thin film PV modules are now seen as a realistic alternative for a cost effective transformation of radiation (solar energy) into electrical energy. PV cost is already competitive for peaking power in locations with high electricity prices such as California and Hawaii.

For the past 25 years thin-film photovoltaic devices have demonstrated steady developmental progress. Photovoltaic modules based on copper indium gallium diselenide (CIGS), copper indium gallium selenide (CIS) and cadmium telluride are already being produced with high-quality and solar conversion efficiency values in the range of 20% for in-laboratory devices. In 2008, scientists at the National Renewable Energy Laboratory (NREL) developed a highly efficient, ultra-thin and light solar cell known as the Inverted Metamorphic Multi-junction Solar Cell which set a record solar conversion efficiency rate that exceeded 40%. Increased knowledge of optical, chemical, electronic and structural properties of the materials, as well as evolutions in device design and solar cell fabrication technologies have contributed remarkably to the ability to achieve higher efficiencies at lower cost. CIGS solar cells are currently the most efficient and reliable thin-film solar technology available with the ability to convert incident sunlight into an electrical current, even in low-light conditions. For this reason, this report will focus on the function, design, applications and limitations of Thin Film CIGS solar cells.

TECHNICAL EXPLANATION

Semiconductor materials possess a property known as the photoelectric effect which enables them to absorb photons of light and release electrons. The atomic level conversion of light into electricity is called photovoltaics. The term "band gap" refers to the energy difference between the valence band and the conduction band of an element. Materials with a small band gap, which more easily allows the excitation of electrons into their conduction bands, are called semiconductors.

When sunlight photons contact the semiconductor materials of a PV cell, they increase the energy of the electrons in the band gap causing the movement of the electrons to the conduction band of the material in turn creating an electric field within the cell. Photons with lower energy than the band gap will simply pass through the solar cell. Photons with higher energy than the band gap are absorbed by the semiconductor material but most of their energy is lost in the form of heat. When electrodes are connected to the top and bottom layers of the solar cell, the free electrons in the n-type layer will flow out of the cell creating a small current which may be used to power a load, then either run to ground, a battery, to the commercial power grid or back into the cell via the electrode connected to the p-type side.

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Figure 1. Function of a typical solar cell. [8]

Today's most common PV devices use a single junction interface to create an electric field within a semiconductor such as a PV cell. Only photons with energy levels equal to or greater than the band gap  of the PV cell materials have the ability to free electrons for an electric circuit.

A typical PV cell produces 0.5 â€" 0.6 volts DC under no-load conditions. The current output of a PV cell depends on its efficiency and surface area, and is proportional to the intensity of sunlight in contact with the absorber surface of the cell. Under peak sunlight conditions, an average commercial PV cell with a surface area of (25x25) inches will produce nearly 2 watts of peak power. [9]

MATERIALS

All modern thin-film solar cells are constructed using a stacking technique consisting of several layers of material. Some favorable materials for thin film solar include amorphous silicon, cadmium telluride and CuInSe2 and its alloys. Each of these materials has a low-band gap, but polycrystalline structures, tend toward a loss in efficiency due to grain boundary recombination. Amongst polycrystalline thin film solar, CIGS solar cells are unrivalled in efficiency.

CIGS thin film belongs to the multinary Cu-chalcopyrite system, where the band gap can be modified by varying the periodic Table Group III element cations such as In, Ga, and Al and the anions between Se and S. A wide range of band gaps can be accomplished using different compositional combinations. The ideal theoretical band gap range for this technology is from 1 to 1.7 eV. [7]

Typical construction of a CIGS solar cell consists of a dispersed deposition of molybdenum back contact material on a substrate. The substrate material can be a soda-lime glass or a flexible polyimide. Molybdenum dispersion is followed by an absorber layer of p-type concentrated CuInGaSe2, a concentrated n-type buffer layer of CdS, and the final contact layers of Zinc and Tin Oxides. Molybdenum is used as a back contact for its ability to form an ohmic contact and its resistance to corrosive gasses. Cadmium sulfide is used to form a concentrated n-type semiconductor material along with the p-type CuInGaSe2 alloy absorber material. Some of the methods for depositing Cadmium Sulfide include closed-space sublimation (CSS), chemical bath deposition (CBD), and sputtering. Zinc and Tin Oxides possess increased electrical and optical properties and are therefore used as the frontal contact materials.

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Figure 2: Stack construction of a typical CIGS thin-film solar cell. [6]

MANUFACTURING

Thin film solar cells may never achieve the conversion efficiency reached by crystalline silicon cells, but the advantage of thin film cells is that they can be manufactured quickly and in large volumes using materials that cost a fraction of the price of silicon.

Many current manufacturers of CIGS solar cell modules employ an efficient roll-to-roll photolithography technique when constructing their solar cells. In this process CIGS cells are manufactured in layers using ink-jet and ultrasonic technology to apply metallic inks in separate layers directly onto a substrate material as it passes from one roll to another through the processing system. The film layers are quickly bonded under heat and pressure forming large-grain CIGS crystals. A laser-scribing process is used to weld the electrical connections between the individual cells of a module. The process can be completed in minutes at significantly lower temperatures than traditional silicon solar cell manufacturing which requires hours at temperatures 500-700 degrees Celsius higher, vacuum processing, evaporation and other substantially more costly steps. [10]

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Figure 3. Roll-to-roll manufacturing process of CIGS solar modules at HelioVolt, an NREL partner company. [11]

CURRENT STATE OF TECHNOLOGY

Currently, thin-film technology has a much smaller market share as compared to traditional silicon-based cells due to lower industrial production backing and its lesser degree of efficiency. The current possibility of a shortage in silicon supply provides a window of opportunity for thin-film solar cells to gain a significant percentage of market shares within the Si technology field. Due to predictions of a Si shortage in the near future, some experts anticipate that current Si module manufacturers will increase their production significantly within the next decade therefore further decreasing the amount of solar grade Si available for production.

Several advantages of thin-film technologies that will stimulate continued development over Si technologies are:

Very low material consumption per area.

Numerous methods for deposition with room for breakthrough improvement.

Fabrication methods inherently lend themselves to larger unit production and integrated cell interconnection.

Shorter pay-back times on investment.

Flexible substrate leads to diverse applications.

As production capacities increase, module manufacturing cost is lower.

The definition of efficiency as it relates to a solar cell is the ratio of electrical power output to the solar power input. The average efficiency of a CIGS PV module at market is currently 13-16%. At this rate of efficiency, when 750 W of sunlight is comes into contact with one square meter of solar film (typical sunlight intensity in most non-desert regions), the solar module would produce 97.5-120 Watts of power per square meter. [12]

Figure 4. Current in-laboratory efficiencies for various PV devices. [14]

APPLICATIONS

The development of thin, flexible and lightweight photovoltaic films has created an exponential increase in the ability of PV technology to be integrated into an infinite number of applications previously unreachable by traditional Si solar systems. Some applications of current CIGS thin-film PV modules are as follows:

Building integrated photovoltaic systems such as awnings, shingles and facades.

Commercial and residential rooftops.

Personal and commercial vehicles, public transportation and aircraft.

Remote applications where a power source is not readily available.

Foldable flex portable chargers for mobile applications.

Military applications and space exploration.

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Figure 5. Examples of CIGS thin-film solar applications. [4]

LIMITATIONS

Even though the module efficiency of CIGS has potential to exceed those of Si based technologies, there are constraints that limit the technology from becoming the primary choice of second generation photovoltaic systems. The largest constraint is material availability, namely indium. For future work, there must be further characterization done on the CIGS cells to more accurately estimate the cost per watt and area of CIGS cells. Although Indium is the limiting quantity by mass, Gallium and Selenium are nearing their limits as well, although Gallium and Selenium are currently limited by production capabilities, not by element scarcity. If commercial production of CIGS is to see a formidable increase in the future, manufacturers must secure large supplies of Indium, Gallium and Selenium. [14]

The high efficiency, flexible PV module has also been hindered by apparent vulnerability to moisture of CIGS technology. Potential causes range from the packaging process of the modules that allow for moisture to enter into the PV cells to transparent conductive Tin and Zinc Oxide materials. Zinc Oxide conductor layers have also shown to possess a high series resistance due to hydrolysis.

FUTURE DIRECTIONS

In the future, both Si and thin film technologies will be major components of solar cell technology, but the market will favor thin film technologies due to lower price. Thin film technologies are expected to have 10% market share in 2010 and will have increasing growth through 2030 with market share exceeding Si technologies. Although thin-films have lower efficiency than Si, significant investment interest in thin-film technologies by small investors and large corporations validates the availability of CIGS and thin-film photovoltaics in the marketplace. This is supported by the fact that thin film technologies based on copper-indium-gallium-diselenide (CIGS) and cadmium telluride (CdTe) have already reached sufficient research maturity to be commercialized and have gained a secure foothold in the solar technology market over the past several years.

[14]

The PV seller’s market of 2008 has transitioned to a buyer’s market beginning with the drastic Spanish Feed-in Tariff cuts and followed by the global financial crisis. Reductions in PV module and system prices have confirmed the elasticity of PV demand though well designed and implemented government subsidy programs which remain crucial for rapid PV deployment and market growth.[5]

Conclusion:

Over the past decade, rapid progress has been achieved in both the Si and in Cu(In,Ga)Se2 (CIGS) thin-film PV technologies. Advances have been made in the following areas:

Material delivery, manufacturing processes and film growth.

Manipulation of material properties at the microscopic and nanoscopic levels.

Understanding of device physics and how to improve the properties of individual layers.

Fundamental device stability and reliability of prototype modules.

The results from these advances have helped both technologies evolve successfully from the laboratory to the marketplace. The existing industry, joined by new start-up entities supported by venture capital, continues to work toward expanded capacity from initial production to first-time manufacturing and beyond.

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