Solar

Enhanced cell efficiency holds promise for thin film silicon

Thin-film silicon solar technology is highly robust and well-characterized, and the demonstrated improvements in efficiency portend further market expansion for this technology.

Thin-film silicon solar technology is highly robust and well-characterized, and the demonstrated improvements in efficiency portend further market expansion for this technology.

CHRIS CONSTANTINE, CHRIS O’BRIEN, Oerlikon Solar, Trübbach, Switzerland

The achievement of a stabilized efficiency >10%—confirmed by the National Renewable Energy Laboratory for single-junction amorphous silicon photovoltaic (PV) cells—will be a key factor in driving down costs and assuring the long-term competitiveness of thin film silicon technologies in coming years.

The achievement resulted from an extensive research and development program funded by a leading manufacturing equipment supplier, and demonstrates the potential leverage of equipment suppliers to accelerate technology improvements and scale up. This achievement was a result of two key factors: an optimized amorphous silicon junction, and optimized transparent conducting oxide (TCO) layers (zinc oxide) used for the front and back contacts for the cell.

An advanced low pressure chemical vapor deposition (LPCVD) process for TCO deposition results in high light capture and light trapping within the silicon junctions, leading to high overall conversion efficiency. By optimizing the synergy between TCO and single junction amorphous silicon device, efficiency gains will improve the performance of commercial thin-film silicon PV modules, both single junction type and tandem Micromorph type.

Outlook for thin film PV

As the world emerges from the deep financial crisis of 2008 to 2009, it is widely anticipated that solar PV markets will resume rapid growth in 2010. As the cost of solar-generated energy continues to drop and approach the cost of conventional energy sources in many regions, we expect to see a prolonged period of accelerated market growth. In the past, the level of solar technology deployed has been a small percentage of the global energy production (Fig. 1), but large increases are forecast for the coming years.

Figure 2 shows that renewable energy is expected to grow at >30% year-over-year. Thin film PV technologies are well positioned in today’s market (~ 25% of total shipments in 2009). This demand is fueling the need for further research and development to provide technology improvements to make solar power economically viable. Achieving improved thin-film silicon cell efficiency will further drive down the cost of PV-generated energy.

 

Thin film silicon (single-junction or Tandem/Micromorph) is just one of three leading thin film PV technologies, along with cadmium telluride (CdTe) and copper, indium, gallium selenides (CIGS). All have advantages that allow them to be viable. CdTe and CIGS are direct bandgap semiconductors, which offer the promise of high efficiency due to the ease at which a photon of light can cause formation of electron hole pairs and subsequent electricity generation. These technologies, however, are also compound polycrystalline materials, where careful control over composition, crystallinity, and thickness make manufacturability difficult to control.

Thin film silicon PV technology has a long history, and silicon may be the most widely studied material known to modern science—thanks in part to the vast and open semiconductor industry, which has pushed silicon technology steadily for 50 years. Even though silicon is an indirect bandgap material, recorded cell efficiencies for crystal silicon are currently quite high (~ 25%) and, ultimately, there is no known constraint barring thin film silicon material from achieving the same efficiency level as conventional crystalline PV.

The Micromorph structure

Figure 3 represents the current structure of a high-efficiency, thin film silicon solar cell. The initial amorphous silicon layer is augmented with a thicker microcrystalline thin film silicon layer, which responds to longer wavelength sunlight (near infrared). Together, this is known as a Micromorph Tandem Junction structure, and historically allows for an ~ 50% increase in light conversion efficiency versus a single-junction, amorphous silicon layer alone. By combining progressive improvements in amorphous silicon solar cells with improvements in front and back contact TCO, high-efficiency, large-area devices are possible.

Silicon film deposition. By introducing a modified single-chamber plasma-enhanced chemical vapor deposition (PECVD) process at VHF excitation frequency (40MHz), it was possible to deposit intrinsic amorphous silicon absorber layers leading to high performance P-i-n solar cell devices (Fig. 4, pg. 17). The single-chamber process enables control of the important interface properties. The design allows a cleaning after each cell deposition run, avoiding effects due to the history of the chamber; thus, good process control can be obtained.

TCO for front and back contacts. Deposition of zinc oxide (ZnO) on the front and back of the thin film silicon layers using a low-pressure chemical vapor deposition (LPCVD) process has been shown to result in consistent high quality and high performance for thin film silicon PV cells and modules. A TCO layer using ZnO is highly transparent and highly conductive. When grown by LPCVD, the ZnO layer is polycrystalline in nature, and the “facets” of the crystal structure are controllable in height and period on the as-grown surface. This allows the TCO to be actively tuned to scatter and reflect specific parts of the spectrum. In contrast, conventional as-grown sputter-based ZnO films are not able to optimize these effects.

Figure 5 demonstrates that similar Tandem Junction Micromorph devices are improved with proper use of optimized TCO layer properties. ZnO films are typically characterized by the “haze” level they generate during a full-spectrum, light-scatter measurement; the higher the “haze,” the more scattering is involved. Type A TCO is a 12% “haze” film, and is optically quite optimized to the lower wavelength (“blue”) a-Si layer. Conversely, Type B TCO is a 40% “haze” film, and optically matches the longer wavelength (“red”) μc-Si layer within the Micromorph Tandem Junction device.

By carefully optimizing the TCO layers within the a-Si or Tandem Junction (a-Si/μc-Si) device, a significant increase in overall efficiency can be achieved, even with a single junction structure [2].

Conclusion

The achievement of a >10% stabilized efficiency thin film silicon single-junction cell is evidence of the progress that is being made through concentrated research and development efforts by leading equipment suppliers to improve the performance and drive down the cost of thin-film silicon PV. Thin-film silicon solar technology is highly robust and well-characterized, and the demonstrated improvements in efficiency portend further market expansion for this technology. The progressive improvements made by equipment suppliers and their customers is driving down the cost of thin film silicon and improving factory throughput at the same time. This allows production fabrication of low-cost solar modules suitable for a variety of applications, and especially suited for large-scale, utility projects.

Acknowledgments

Micromorph is a registered trademark of Oerlikon Solar. KAI is a trademark of Oerlikon Solar.

References

  1. J. Meier, J. Bailat, L. Castens, S. Benagli, U. Kroll, J. Hötzel, et al., “High-efficiency Micromorph Tandem Developments in KAI-M PECVD Reactors,” Proc., 24th EU PVSEC, Sept. 23, 2009.
  2.  S. Benagli, D. Borrello, E. Vallat-Sauvain, J. Meier, U. Kroll, J. Hötzel, et al., “High-efficiency Amorphous Silicon Devices on LPCVD-ZNO TCO Prepared in Industrial KAI-M R&D Reactor,” Proc., 24th EU PVSEC.

Chris Constantine received his BS in chemistry from Fordham U. and a PhD in physical chemistry from the City University of New York (CCNY), and an MBA from the Wharton School of Business. He is director of new technologies within the solar segment of Oerlikon Corp.

Christopher O’Brien received an engineering degree from Dartmouth College and an MBA from Stanford U. He is head of market development, North America, at Oerlikon Solar USA, Inc., 700 12th Street NW, Suite 700, Washington, DC.  20005 USA; [email protected]

 

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