Sorapong Inthisang

Effect of light intensity on performance of silicon-based thin film solar cells

Theoretical and experimental studies were performed to explore the effect of light intensity on the performance of silicon-based thin film solar cells. The theoretical study was conducted using AMPS-1D (Analysis of Microelectronic and Photonic Structures) to analyze how the device structure affects the performance of silicon-based solar cells under various concentration ratios of sunlight. We calculated a-Si:H and μc-Si:H p-i-n type single-junction solar cells, and also experimentally evaluated a-Si:H, a-SiO:H and μc-Si:H solar cells. From both simulation and measurement results, we confirmed that the Voc logarithmically increases with increasing the light intensity. Moreover, from simulation results, we also observed that the defect density and thickness of i-layer strongly influence the light-intensity dependence of a-Si:H solar cells.

High efficiency a-Si:H/a-SiGe:H tandem solar cells fabricated with the combination of V- and U-shaped band gap profiling techniques

Hydrogenated amorphous silicon germanium (a-SiGe:H) films prepared by very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) using a mixture of SiH4, H2, and GeH4 were investigated for their use as the bottom cell of amorphous silicon/amorphous silicon germanium (a-Si:H/a-SiGe:H) tandem solar cell structures. Narrow optical band gaps (Eopt) in the range of 1.5 to 1.6 eV were obtained by varying the GeH4/(SiH4 + GeH4) gas flow rate ratio in low-temperature deposition. The a-SiGe:H films deposited with various GeH4/(SiH4 + GeH4) gas flow rate ratios were used as intrinsic layers for the a-Si:H/a-SiGe:H tandem solar cells with different graded band gaps: V-, VU-, and U-shapes. It was found that using the VU-shape improves the solar cell efficiency owing to a higher Jsc when compared with using V-shape. The VU-shape’s Voc and FF are also improved when compared with the U-shape’s Voc and FF. As a result, a high efficiency of 11.0% (Voc = 1.74 V, Jsc = 9.07 mA/cm2, and FF = 0.70) was successfully achieved with the solar cells fabricated using the VU-shape graded band gap technique.

Wide-Gap p-c-:H Films and Their Application to Amorphous Silicon Solar Cells

Optimization of p-type hydrogenated microcrystalline silicon oxide thin films (p-c-:H) by very high frequency plasma enhanced chemical vapor deposition 40 MHz method for use as a p-layer of a-Si:H solar cells was performed. The properties of p-c-:H films were characterized by conductivity, Raman scattering spectroscopy, and spectroscopic ellipsometry. The wide optical band gap p-c-:H films were optimized by CO2/SiH4 ratio and H2/SiH4 dilution. Besides, the effects of wide-gap p-c-:H layer on the performance of a-Si:H solar cells with various optical band gaps of p-layer were also investigated. Furthermore, improvements of open circuit voltage, short circuit current, and performance of the solar cells by using the effective wide-gap p-c-:H were observed in this study. These results indicate that wide-gap p-c-:H is promising to use as window layer in a-Si:H solar cells.

Development of amorphous silicon-based thin-film solar cells with low-temperature coefficient

In this work, we investigated the temperature dependence of wide bandgap hydrogenated amorphous silicon (a-Si:H)-based, hydrogenated amorphous silicon oxide (a-SiO:H)-based single-junction and hydrogenated protocrystalline silicon/hydrogenated microcrystalline silicon (pc-Si:H/μc-Si:H) double-junction solar cells in order to develop solar cells which are suitable for use in high temperature region. Photo J-V characteristics were measured under AM 1.5 illumination at ambient temperature in the range of 25-75 oC. We found that, the values of temperature coefficient for conversion efficiency (TC for η) of both single- and double-junction solar cells were inversely proportional to the initial open-circuit voltage (Voc). In case of p-i-n single-junction solar cells, the typical pc-Si:H and pc-SiO:H solar cells showed the lowest TC for η of -0.21 and -0.14%/oC, respectively. The smallest TC for η of pc-SiO:H solar cell was attributed to the positive increase in TC for fill factor (FF). The TC for η of typical pc-Si:H/μc-Si:H double-junction solar cells was around -0.35%/oC with initial η around 10-12%. Since high Voc pc-Si:H/μc-Si:H double-junction solar cells exhibit low temperature dependence and highly stable η against light soaking, they are promising for use in high temperature regions. In addition, we conclude that solar cells which are suitable for use in high temperature region must be considered both high η with low temperature dependence.

Effect of the CO2/SiH4 Ratio in the p-μc-SiO:H Emitter Layer on the Performance of Crystalline Silicon Heterojunction Solar Cells

This paper reports the preparation of wide gap p-type hydrogenated microcrystalline silicon oxide (p-c-SiO:H) films using a 40 MHz very high frequency plasma enhanced chemical vapor deposition technique. The reported work focused on the effects of the CO2/SiH4 ratio on the properties of p-c-SiO:H films and the effectiveness of the films as an emitter layer of crystalline silicon heterojunction (c-Si-HJ) solar cells. A p-c-SiO:H film with a wide optical band gap (E04), 2.1 eV, can be obtained by increasing the CO2/SiH4 ratio; however, the tradeoff between E04 and dark conductivity must be considered. The CO2/SiH4 ratio of the p-c-SiO:H emitter layer also significantly affects the performance of the solar cells. Compared to the cell using p-c-Si:H (CO2/SiH4 = 0), the cell with the p-c-SiO:H emitter layer performs more efficiently. We have achieved the highest efficiency of 18.3% with an open-circuit voltage () of 692 mV from the cell using the p-c-SiO:H layer. The enhancement in the and the efficiency of the solar cells verified the potential of the p-c-SiO:H films for use as the emitter layer in c-Si-HJ solar cells.

Advantages of N-Type Hydrogenated Microcrystalline Silicon Oxide Films for Micromorph Silicon Solar Cells

We report on the development and application of n-type hydrogenated microcrystalline silicon oxide films (n μc-SiO:H) in hydrogenated amorphous silicon oxide/hydrogenated microcrystalline silicon (a-SiO:H/μc-Si:H) micromorph solar cells. The n μc-SiO:H films with high optical bandgap and low refractive index could be obtained when a ratio of carbon dioxide (CO2) to silane (SiH4) flow rate was raised; however, a trade-off against electrical property was observed. We applied the n μc-SiO:H films in the top a-SiO:H cell and investigated the changes in cell performance with respect to the electrical and optical properties of the films. It was found that all photovoltaic parameters of the micromorph silicon solar cells using the n top μc-SiO:H layer enhanced with increasing the CO2/SiH4 ratio up to 0.23, where the highest initial cell efficiency of 10.7% was achieved. The enhancement of the open circuit voltage () was likely to be due to a reduction of reverse bias at subcell connection—n top/p bottom interface—and a better tunnel recombination junction contributed to the improvement in the fill factor (FF). Furthermore, the quantum efficiency (QE) results also have demonstrated intermediate-reflector function of the n μc-SiO:H films.

Performance of multi-junction silicon-based thin film solar cells under concentrated sunlight

Multi-junction silicon-based thin-film concentrator solar cells are promising candidate to achieve both low-cost and high-efficiency. For the application of silicon-based thin film solar cells to concentrator photovoltaics, it is required to be revealed the light intensity dependence of the performance of silicon-based thin film solar cells. From these reasons, in this study both calculation and experimental studies were conducted with several types of single-junction and multi-junction tandem solar cells. From both simulation and measurement results, we observed that double- and triple-junction solar cells achieve high open-circuit voltage and large logarithmic increment in open-circuit voltage with increasing light intensities. On the other hand, it became clear that the drop of fill factor is required to be improved for the realization of the multi-junction silicon-based thin-film solar cells with very high efficiency under low concentration ratios of sunlight.

Effect of Hydrogen Dilution on the Metastability of Hydrogenated Amorphous Silicon Oxide Solar Cells

We investigated the effect of H2 dilution on the stability of hydrogenated amorphous silicon oxide (a-Si1-xOx:H) films and solar cells. a-Si1-xOx:H films were prepared by very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) using a gas mixture of silane (SiH4), hydrogen (H2), and carbon dioxide (CO2). The degradation results after light soaking for 310 h under AM1.5, 125 mW/cm2 at 48 °C clearly showed that the film and device stabilities were improved substantially with increasing H2 dilution. Our best stable result with a high open-circuit voltage (Voc) of 1.03 V for a-Si1-xOx:H single-junction solar cells showed a degradation of 16.6%. This result indicates that the stability against the light soaking of the a-Si1-xOx:H material is not inferior to that of a conventional hydrogenated amorphous silicon (a-Si:H) material. The temperature dependences of current–voltage (I–V) characteristics observed before and after light soaking showed that a-Si1-xOx:H solar cells had a similar temperature dependence to conventional a-Si:H solar cells.

Characterization of wide bandgap a-SiO:H films and their application to thin film solar cells

Hydrogenated amorphous silicon oxide (a-SiO:H) films were prepared by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) using mixture of SiH<inf>4</inf>, H<inf>2</inf> and CO<inf>2</inf> source gas. The optical bandgap (Eopt) in the range between 1.90 and 2.02 eV was obtained by the variation of CO<inf>2</inf> flow rate and H<inf>2</inf> dilution. With increasing of H<inf>2</inf> dilution, it was found that Eopt of the films increased and defect density reduced down to 1.69×10¹⁶ cm⁻³. The high open-circuit voltage (V<inf>oc</inf>) of 1.04 V was achieved for a single-junction amorphous silicon solar cells using optimized a-SiO:H as intrinsic layer (i-layer thickness = 120 nm, J<inf>sc</inf> = 7.92 mA/cm², FF = 0.64, η = 5.24%). Experimental results indicated that wide bandgap a-SiO:H films are promising material for use as intrinsic layer in top cell structure of multi-junction solar cells.

Study of an Amorphous Silicon Oxide Buffer Layer for p-Type Microcrystalline Silicon Oxide/n-Type Crystalline Silicon Heterojunction Solar Cells and Their Temperature Dependence

Intrinsic hydrogenated amorphous silicon oxide (i-a-SiO:H) films were used as front and rear buffer layers in crystalline silicon heterojunction (c-Si-HJ) solar cells. The surface passivity and effective lifetime of these i-a-SiO:H films on an n-type silicon wafer were improved by increasing the CO2/SiH4 ratios in the films. Using i-a-SiO:H as the front and rear buffer layers in c-Si-HJ solar cells was investigated. The front i-a-SiO:H buffer layer thickness and the CO2/SiH4 ratio influenced the open-circuit voltage , fill factor (FF), and temperature coefficient (TC) of the c-Si-HJ solar cells. The highest total area efficiency obtained was 18.5% mV, mA/cm2, and . The TC normalized for this c-Si-HJ solar cell efficiency was −0.301%/°C.