磷化銦鎵/砷化鎵/鍺三接面太陽能電池效率提升之研究

Abstract

太陽能電池(Solar cell) 屬光能轉換電能元件。而又以III-V族太陽能電池具備高轉換效率、高抗輻射效應、直接能隙等優異的特徵，並且太陽光源取之不盡用之不竭且無汙染，安全，方便，讓這環保意識抬頭的年代，太陽能電池發展逐漸受到國際的重視。可惜III-V族太陽能產業在半導體領域，由於材料結構價格成本的關係與傳統矽太陽能電池相比，雖能提供比矽太陽能更高的轉換效率但其成本相對提高許多，因此，高能量轉換效率III-V族太陽能電池之研發具有迫切性的需求。為了獲得具備高能量轉換效率III-V族太陽能電池並想方法結合利用降低成本的方式製作高效率III-V族太陽能電池，進一步降低III-V族太陽能電池成本並且也能提升III-V族太陽能電池整體效率是本論文的主軸。 本論文探討利用三種創新結構設計製作方式來提升III-V族太陽能電池效率，第一種方式採用硫化鎘量子點新穎材料運用在三五族磷化銦鎵/砷化鎵/鍺串接式太陽能電池之表面結構上，利用硫化鎘量子點物理機制原理吸收短波長波段並放出可見光波段來提升電流收集，這量子點機制原理可以改善III-V太陽能電池短波長表面複合速率大造成短波長電流不容易收集的問題，此外硫化鎘量子點寬廣頻譜範圍吸收效果也擁有抗反射層優點存在，可在長波長地方增加吸收來提升太陽能電池電流與整體太陽能電池轉換效應。而在電性的量測的結果可以得到短路電流增加0.33 mA/cm2，硫化鎘量子點尺寸大小為3.5nm. 整體太陽能轉換效率從28.25%提升到29.04%. 第二種方式利用低溫水熱法成長方式來成長氧化鋅奈米管狀結構應用於三五族磷化銦鎵/砷化鎵/鍺串接式太陽能電池來提升太陽能電池效率。從實驗結果中發現使用氧化鋅奈米管狀結構太陽能元件在短波長的部分有較低的反射率呈現，與傳統有加上氮化矽抗反射層的三接面太陽能電池相比較下，尤其是在波段350-500 nm，也意味了氧化鋅奈米管狀結構可以降低短波長的反射率增加整體太陽能電池的入射光。同時也擁有良好的抗反射層效應作用。經論中得知，氧化鋅奈米管狀結構三五族磷化銦鎵/砷化鎵/鍺串接式太陽能電池效率可以提升到4.9%，傳統有加上氮化矽抗反射層的三接面太陽能電池效率可以提升到3.2%與沒有抗反射層的三五族串接式太陽能電池相比較的結果。

第三種方式在探討不同圖形正電極對於聚光型砷化鎵單接面太陽能電池特性之影響。主要分析的太陽能電池輸出參數計有:短路電流(Isc)、開路電壓(Voc)、填充因子(FF)、轉化效率(η)。使用不同圖形正電極時會同時改變入射光遮蔽率與太陽能電池串聯電阻，兩者呈現逆向變化關係，須在權衡兩者下，找出適當的正電極圖形以獲得太陽能電池最高轉換效率。尤其是在高聚光倍率下，太陽能電池會受到串聯電阻強烈的影響而造成太陽能電池效率的降低。經由本研究的調查分析顯示，砷化鎵單接面太陽能電池在低聚光倍率下使用中心圓區域遮蔽率為6.224%的圓環狀正電極圖案時，可得最佳效率為28.5% (34，AM1.5)；但在高聚光倍率下，則使用中心圓區域遮蔽率為7.1%的圓環狀正電極圖案時其可得最佳效率為27% (106，AM1.5)。

Solar cell is a photovoltaic device designed to convert sunlight into electrical power, solar energy benefits have been constaulty discussed, such as inexhaustibility, safety, convenience, non-polluting, and high practicality, etc. Due to these characteristics, solar energy plays an important role on the recycled research. III-V solar cell is known to hance super-high efficiency and radiation damage resistant is now used for space application. The production cost of III-V materials is higher than that of Si solar cells. We study the novel materials, news process ways and news structure design for III-V solar cells to increase the solar cell efficiency, thus reducing the total cost of traditional III-V solar cell. The efficiency improvement on InGaP/GaAs/Ge Triple-junction solar cell study here are divided into three parts Part1 : The optical and electrical characteristics of InGaP/ GaAs/ Ge triplejunction (TJ) solar cells with CdS quantum dots (QDs) fabricated by a novel chemical solution are presented. With antireflective feature at long wavelength and down conversion at UV regime, the CdS quantum dot effectively enhance the overall power conversion efficiency of the III-V solar cell. The measurement under one sun air mass 1.5 global illumination, an increase of 0.33 mA/cm2 in the short circuit current was observed for the triple junction solar cells with CdS QDs of about 3.5nm in diameter. The conversion efficiency was improved from 28.25% to 29.04%.

Part2: A ZnO nanotube, fabricated by the hydrothermal growth method on triple-junction (T-J) solar cell devices to enhance efficiency, is investigated. Compared to those of bare T-J solar cells (without AR coating) and solar cells with Si3N4 AR coatings, the experimental results show that the T-J solar cells, which use a ZnO nanotube as an antireflection (AR) coating, have the lowest reflectance in the short wavelength spectrum. The ZnO nanotube has the lowest light reflection among all experimental samples, especially in the range of 350-500 nm from ultraviolet (UV) to visible light. It was found that a ZnO nanotube can enhance the conversion efficiency by 4.9%, compared with a conventional T-J solar cell. The Si3N4 AR coatings also enhance the conversion efficiency by 3.2%.The results show that a cell with ZnO nanotube coating could greatly improve solar cell performances.

Part3: The effects of front contacts with different grid patterns on the characteristics of light-concentrated type GaAs single-junction solar cell. The device parameters analyzed include the open-circuit voltage (Voc), short-circuit current (Isc), fill factor (FF) and conversion efficiency (η). In this study, we investigate the effect of the shadowing factor of the front grid pattern on concentrated solar cell efficiency, taking the trade-off between the series resistance of the electrodes and the amount of incident light into consideration. We examine the thermal effect with regard to five different circle-grid electrode patterns of the front contact. The front contacts with different grid patterns affect the characteristics of light-concentrated-type GaAs single-junction solar cells. The device parameters analyzed include the open-circuit voltage (Voc), short-circuit current (Isc), fill factor (FF) and conversion efficiency (η). The results of our study show that for a concentration ratio greater than 60x with AM1.5G, the device with a shading factor of 7.1% has the best cell efficiency of 27.05 %, due to the smaller current crowding at the center spot. The results indicated that the conversion efficiency of solar cells can be improved by establishing a compromise between the shading effect and the series resistance effect.