石墨烯/N型單晶矽蕭特基接面太陽能電池之研究

Abstract

在第三代太陽能電池的「薄膜、高效率、低成本」的主張下，蕭特基接面太陽能電池由於其製程簡單以及低成本的優點而走入人們的眼中。相較於傳統的氧化銦錫/半導體蕭特基接面太陽能電池，石墨烯/半導體蕭特基接面太陽能電池無論是在材料或者是製程上的成本又更加的低廉，進而受到眾多研究團隊的重視。在本論文中，我們成功的利用低壓化學氣相沉積法高溫成長於銅箔上的單層石墨烯，製做出石墨烯/N型矽晶圓蕭特基接面太陽能電池。在Class A，AM1.5G的一個太陽光強度下量測得到0.34V的開路電壓，7.73 mA/cm2的短路電流密度，28.3%的填充因子以及0.7%的光電轉換效率。在元件的進一步優化上，我們使用了許多方法來降低石墨烯的電阻以及提昇元件空乏區的長度:堆疊多層石墨烯可以有效的降低石墨烯層的電阻並且填補轉印破碎，在元件光電轉換效率的量測上，堆疊四層石墨烯後會得到最佳的效率，從原先單層的0.8%提升到2.1% ; 使用硝酸、鹽酸以及過氧化氫氣體，透過分子摻雜的方式來改變石墨烯的功函數。在效率提升度的比較上，我們得到使用硝酸氣體分子，對元件摻雜20秒後可以得到236%的最佳光電轉換效率提升，效率從0.56%提升到1.88%。最後在本論文中我們率先提出使用銀奈米線塗佈的方法，大幅的降低石墨烯薄膜的電阻，提升石墨烯/N型單晶矽蕭特基接面太陽能電池的光電轉換效率。在實驗結果上得到優化的銀奈米線溶液重量百分比為1.25wt%，而光電轉換效率則從0.34%提升到0.68%。

Hybrid Graphene/Si Schottky junction solar cells are inexpensive alternatives for photovoltaics due to easy and rapid fabrication processes. Compared with Indium-tin- oxide Schottky junction solar cells, graphene has advantages in terms of material and fabrication cost. In this thesis, we demonstrate mono- and multi-layer graphene-based Schottky junction solar cell. The fabrication involves growth of graphene on copper foils via a chemical vapor deposition (CVD) method, followed by transferring the heated PMMA/graphene sheet on a silicon substrate. The device power conversion efficiency of the mono-layer graphene is 0.7% with a short-circuit current of 7.73 mA/cm2, open-circuit voltage of 0.34V, and fill factor of 28.3 %. Further, we improve the graphene-based Schottky junction solar cells performance using various methods. First, stacking multi-layer devices can effectively decrease the sheet resistance of mono-layer graphene, and full up the cracks due to the transfer process to increase the fill factor and open-circuit voltage. Second, molecular doping with HNO3, HCL and H2O2 vapor on to graphene sheet could modulate the work function of graphene, hence increasing the depletion region length and decrease the sheet resistance. Finally, we present the characteristics of graphene/Si Schottky junction solar cells with drop-cast silver nano-wires. The external quantum efficiency measurement reveals an integrated photocurrent of 25.3mA/cm2 due to improved carrier collection. In summary, the improvement of power conversion efficiency from deferent approaches is from 0.8% to 2.1% with stacking four-layer graphene, from 0.56% to 1.88% with molecular doping and from 0.34% to 0.68% with silver nano-wire network.