Kobsak Sriprapha

High efficiency protocrystalline silicon/microcrystalline silicon tandem cell with zinc oxide intermediate layer

The authors develop a hydrogenated protocrystalline silicon (pc-Si:H)/hydrogenated microcrystalline silicon (μc-Si:H) double-junction solar cell structure employing a boron-doped zinc oxide (ZnO:B) intermediate layer. Highly stable intrinsic pc-Si:H and μc-Si:H absorbers are prepared by a 60MHz very-high-frequency plasma-enhanced chemical vapor deposition technique. Degenerate ZnO:B intermediate and back reflectors are deposited via a metal organic chemical vapor deposition technique. Because the ZnO:B intermediate layer reduces the potential thickness for the pc-Si:H absorber in the top cell, this double-juncion structure is a promising candidate to fabricate highly stable Si-based thin-film solar cells. Consequently, the high conversion efficiency of 12.0% is achieved.

Effects of Temperature and Spectral Irradiance on Performance of Silicon-Based Thin Film Multijunction Solar Cells

A numerical study using Analysis of Microelectronic and Photonic Structures (AMPS) device simulator has been carried out to examine how the operating temperature and spectral irradiance affect the behavior of a hydrogenated amorphous silicon/ hydrogenated microcrystalline silicon (a-Si:H/mc-Si:H) tandem cell. We have proposed a new tunnel-recombination junction (TRJ) model to calculate the characteristics of the tandem cell as a complete single device. We have shown that our new TRJ model is reliable and can be used for the simulation of the tandem cell. From the calculated results, we have observed that the performance of the tandem cell is sensitive to the spectral irradiance distribution. Furthermore, we have also observed that the current matching condition slightly affects the temperature coefficient of the tandem cell. We have considered that a toplimited cell has a lower temperature coefficient than the bottom-limited and current-matched cells. [DOI: 10.1143/JJAP.46.1398]

Temperature Dependence of Protocrystalline Silicon/Microcrystalline Silicon Double-Junction Solar Cells

We have investigated the temperature dependence of hydrogenated protocrystalline silicon (pc-Si:H)/hydrogenated microcrystalline silicon (µc-Si:H) double-junction solar cells. A boron-doped zinc oxide (ZnO:B) film is employed as an intermediate layer between the pc-Si:H top cell and the µc-Si:H bottom cell. The fabricated solar cells exhibit good stability against light soaking. Photocurrent density–voltage (photo J–V) characteristics are measured under AM 1.5 illumination at ambient temperatures in the range of 25–75 °C. The double-junction solar cells show a temperature coefficient (TC) for conversion efficiency (η) of around -0.30 to -0.45%/°C, which between the TC for η of the pc-Si:H top and µc-Si:H bottom cells. It is found that the values of TC for η are inversely proportional to the initial open-circuit voltage (Voc). In contrast, these values become higher with the increase in the crystalline volume fraction (Xc) of the i-µc-Si:H in the bottom cells. Since the pc-Si:H/µc-Si:H double-junction solar cells exhibit high η, low light-induced degradation, and low temperature dependence, they are good candidates for solar cells that operate in high-temperature or tropical regions.

Temperature Dependence of Si-Based Thin-Film Solar Cells Fabricated on Amorphous to Microcrystalline Silicon Transition Phase

The temperature dependence of silicon (Si)-based thin-film single-junction solar cells, whose intrinsic absorbers were fabricated on the transition phase between hydrogenated amorphous silicon (a-Si:H) to hydrogenated microcrystalline silicon (µc-Si:H), was investigated. By varying the hydrogen dilution ratio, wide-band-gap protocrystalline silicon (pc-Si:H) and µc-Si:H absorber layers were obtained. Photo-current density–voltage (Photo-J–V) characteristics were measured under AM1.5 illumination at ambient temperatures in the range of 25–75 °C. We found that the solar cells with pc-Si:H, which exists just below the a-Si:H to µc-Si:H transition boundary, showed the lowest temperature coefficient (TC) for conversion efficiency (η) and open-circuit voltage (Voc), while the solar cells fabricated at the onset of the a-Si:H to µc-Si:H phase transition exhibited a relatively high TC for η and Voc. Experimental results indicated that pc-Si:H is a promising material for the absorber layer of the single junction or the top cell of tandem solar cells that operate in high temperature regions.

An Improved PV Output Forecasting Model by Using Weight Function: A Case Study in Cambodia

This paper proposes a new concept to improve accuracy of PV forecasting model. The model was implemented by MATLAB/Simulink software using solar irradiance and module temperature as measurement parameters for calculation. The model was developed by single-diode equivalent circuits (5-p model) for simulated PV module power output and compared with other software programs for validation which showed correct PV characteristics. To achieve high accuracy, the model was improved by weight function using one-year measured data. The accuracy of our developed model was verified by comparison with four commercial simulator software programs and the results from real system which were measured and recorded for 1 year. It was found that the model output was in a good agreement with the measured data. This research can be utilized in another area by adjusting the PV equation with weight function of that area.

Effect of Ambient Temperature on Performance of Grid-Connected Inverter Installed in Thailand

The effects of temperature on performance of a grid-connected inverter, and also on a photovoltaic (PV) system installed in Thailand have been investigated. It was found that the maximum efficiency of the inverter showed 2.5% drop when ambient temperature was above 37°C. The inverter performed efficiently in November and December, the months of high irradiance, and monthly average ambient temperature of lower than 35°C, allowing relatively high system performance ratio in this period. Our results show that high temperature provides negative impacts not only on the PV modules, but also on the performance of the inverter. Thus, the effect of temperature on the inverter efficiency should be taken into account when predicting energy yield or analyzing losses of the PV systems—especially in high temperature regions.

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.

The Study of Decrement in Insulation Resistance of PV String and its Effects on PV System Degradation

This paper presents the investigated variations in insulation resistances of amorphous silicon (a-Si), multi crystalline silicon (MC-Si) and hybrid solar cell (HIT). The insulation resistance of PV string of each system was measured and used to represent leakage current in photovoltaic system and the analysis was done in accordance with IEC 61215 Standard. The 10.152 kW multi-PV cell technologies grid connected system supports the energy demand of the 10 kW Building at School of Renewable Energy Technology (SERT), Naresuan University since July 2005 till date. In general the system was found effectively working under hot and humid climatic conditions of Thailand for a long duration. However the decrement in insulation resistance (MΩ) of a-Si string, MC-Si string and HIT string have affected electrical parameter of each system. The decrements in fill factor (F.F) over the 10 years period (July 13, 2005 – January 25, 2015) of a-Si, MC-Si and HIT were-27.4%, -21.9% and-6.2% respectively.