Wen Ma

A study of interface properties in a-Si solar cells with μc-Si(C)

Abstract A series of experimental trials have been carried out on the interface properties of a-Si solar cells by utilization of heavily doped microcrystalline silicon (μ-Si) and its carbon alloys (μc-SiC) in the front electrode TCO and wide gap window layer p type a-SiC interface. As the result, a remarkable improvement of the photovoltaic performance of not only Voc but also FF has been obtained due to the reduction of the surface barrier potential at the TCO/p a-SiC interface. An insertion of a thin n-type μc-Si layer in the TCO/p μc-Si(C) interface is also useful to improve the FF due to improving the tunnel effect. Special effort has also been made on the p/i interface. A series of differently structured buffer layers of a-SiC are formed at the p a-SiC/i a-Si interface for the improvement of the voltage characteristic. As a result, Voc has been raised parallel with the built-in potential. The mechanism of the improvements is discussed on the basis of the systematic diagnostics on the interface properties.

Microcrystalline silicon carbide — New useful material for improvement of solar cell performance

Abstract A new kind of semiconductor — doped microcrystalline silicon carbide (μc−Si(1−X)CX) — which is useful for improvement of photovoltaic performance has been developed. A series of experimental verifications on some unique properties such as high dark conductivity with good optical transparency in the solar spectrum is introduced. Then, the usefulness of this new functional material is demonstrated as the cases of wide gap window, front interface modifier, BSF (Back Surface Field) and RSOC (Rear Side Ohmic Contact) effects in the fabrication processes of a-Si and poly-Si sosed solar cells. The mechanisms of the performance improvement are studied and dicuussed. It could be expected that there exists a quite wide variety of availability for not only amorphous solar cells but also single crystalline and polycrystalline solar cells and photo imaging sensors.

Modulated photocarrier grating technique for diffusion length measurement in amorphous semiconductors

This article presents a method for investigating the diffusion of photocarriers in semiconductors by the analysis of phase shift between a temporally modulated illumination grating and its inducing photocurrent. Experiments on hydrogenated amorphous silicon prove that an accurate measurement of the diffusion length as well as an identification of ambipolar or nonambipolar diffusion can be acquired by using this technique.

Optimum design and its experimental approach of a-Si/ /poly-Si tandem solar cell

Abstract A series of technical data on four-terminal a-Si/ /poly-Si stacked solar cells has been reported. The developed device has some unique significances such as high achievable efficiency, and low cost with almost no light induced degradation. It has been shown on a poly-Si bottom cell that an efficiency of 17.2% has been obtained by employing high conductivity with wide optical band-gap p-type μc-SiC as a window material and n-type μc-SiC as a back ohmic contact with BSF effects. On the optically transparent a-Si top cell, an optimum design has been experimentally made with the device structure of p μc-SiC/p a-SiC/i a-Si/n μc-Si/ITO, and an efficiency of 7.25% has been obtained with a 100 nm thick i-layer, while the best efficiency is 12.3% for p-i-n single-junction solar cell with 500 nm i-layer thickness deviced by Ag back-electrode. With the 100 nm thick ultrathin top cell, a total conversion efficiency as high as 21.0% has been achieved on a-Si/ /poly-Si four-terminal tandem solar cells.

An optimum design of a a-Sipoly-Si tandem solar cell

A systematic investigation of a high efficiency a-Sipoly-Si four-terminal structure tandem solar cell has been made using both theoretical and experimental approaches. It has been shown from optimum design theory, used on a realistically attainable best efficiency, that the best combination of a tandem solar cell is a-Si/poly-Si silicon materials with respect to high achievable efficiency. In the optical design rule, priority of the photon utilization is put on the poly-Si bottom cell. Employing high conductivity with wide optical band gap p type /spl mu/c-SiC as a window material and n type /spl mu/c-Si as back ohmic contact with BSF treatment, a conversion efficiency of 17.2% has been obtained for the poly-Si cell. An optimum design of the a-Si top cell has been experimentally made on a p /spl mu/c-SiC/p a-SiC/i a-Si/n /spl mu/c-Si/ITO structure, and an efficiency of 7.25% has been obtained with a 100 nm thick i-layer top cell. The best efficiency of a p-i-n single-junction solar cell with this structure is 12.3% so far with a 500 nm i-layer thickness device using an Ag back electrode. With a 100 nm thick ultra thin top cell, a total conversion efficiency of an a-Sipoly-Si four-terminal tandem solar cell as high as 21.0% has been achieved.<<ETX>>

Amorphous-Silicon-Based Devices

Publisher Summary This chapter reviews recent advances in the amorphous silicon (a-Si) device applications. This chapter begins with enumerating some unique advantages of these tetrahedrally bonded amorphous semiconductors and explains some concrete evidence from current technologies. Remarkable progress has been seen in the field of disordered materials in both theoretical and experimental aspects. One major reason for this progress is the great advances made in material preparation technologies. These advances are supported by ultra-high-vacuum techniques, ultra-purification of inorganic elements and precisely synthesized heat-treatment technologies, including the rapid quenching of thin-film materials. The recent discovery of an existence of valence controllability in hydrogen passivated (a-Si) strongly promotes the evaluation of amorphous semiconductor as a new electronic material. This new amorphous material is able to form both p-n and p-i-n junctions and has excellent photoconductivity with a considerably high absorption coefficient. These characteristics, coupled with mass-production capabilities of large-area non-epitaxial growth on any substrate material, strongly satisfy the current need for the development of a low-cost solar cell as a new energy resource. These integrated ideas have opened some other new application fields such as thin-film transistor (TFT), electrophotography, three-dimensional integrated devices, and quantum-well devices.

The utilization of microcrystalline Si and SiC for the efficiency improvement in a-Si solar cells

A systematic investigations on the modification of interface potential by introducing doped microcrystalline Si (/spl mu/c-Si) and its carbon alloys (/spl mu/c-SiC) at the TCO/p a-SiC interface has been made. It has been shown that a remarkable increase of the built-in potential with insertion of these materials. A series of technical data on the improvement of photovoltaic characteristics achieved by employing the microcrystalline materials are presented. The mechanism of improvements is discussed on the basis of systematic diagnostics on the interface properties.

Toward 20% Efficiency With a-Si // poly-Si Tandem Solar Cell

A series of experimental investigations has been made on the a-Si // poly-Si tandem solar cell which is one of the most promised candidate of high cost-performance photovoltaic cell, e.g., high efficiency, low cost with almost no light induced degradation. Employing high conductivity with wide optical band gap p type microcrystalline SiC (μ-SiC) as a window material together with a-SiC as an interface buffer layer and also n type μc-Si as a back ohmic contact layer in the poly-Si based bottom cell, the conversion efficiency of 17.2 % has been obtained. Combining an optically transparent a-Si p-i-n cell as a top cell with an optical coupler between the top and the poly-Si bottom cell, a total efficiency of 20.3 % has been obtained so far on the four-terminal stacked mode structure. A systematic technical data for the optimization of cell structure variation on the developed tandem solar cells are presented and further possibility to improving the performance are discussed.

Approaching 20% efficiency with a-Si/poly-Si tandem solar cell

A high-efficiency a-Si/poly-Si four-terminal tandem solar cell has been developed. By the optimization of each layer thickness and optical features of the top and bottom cells, the authors obtained a conversion efficiency of 19.1% with a good stability and repeatability. There appears to be a possibility of achieving conversion efficiency exceeding 20% with the four-terminal tandem device through careful optimization of the a-Si top cell and optical matching between the two cells.<<ETX>>

Improvement of Interface Properties in µc-SiC/poly-Si/µc-Si Double Heterojunction Solar Cell

A new type of double heterojunction thin film solar cell has been developed with a p µ c-SiC/n poly-Si/n µ c-Si/Al structure. By efficiently employing the wide band gap window effect and the back surface field (BSF) effect with hydrogen passivation treatment, a fill factor higher than 80% has been obtained with a conversion efficiency of 17.2%. In order to determine the detailed mechanism of the enhancement of cell performance, a systematic measurement of the rear interface recombination velocity has been performed using poly-Si based solar cells with the p µ c-SiC/a-SiC/n poly-Si/n µ c-Si/Al structure. It is found that the interface recombination velocity decreases by more than one order of magnitude with the insertion of the n µ c-Si rear side layer. Possible mechanisms for the improvement of cell performance are discussed on the basis of a series of characterizations of the devices with several different interface layers.