實現15％效率之混合式有機矽異質接面太陽能電池

Strategies to achieve 15% efficiency hybrid organic-silicon heterojunction solar cells

賴逸群(Yi-Chun Lai)

指導教授 ： 余沛慈 ； 紀國鐘

國立交通大學/電機學院/光電工程系所/博士(2018年)

摘要

根據美國綠能科技(Greentech Media, GTM)研究報導指出，2015年全球安裝的太陽能光伏總量為59千兆瓦比2014年相比增加了34 %，安裝總能達到了256千兆瓦的太陽能光伏，而GTM研究學者更預測了未來10年，所安置的太陽能光伏能源每一年會有10 到 15 %的成長，直到2020年將會來到754千兆瓦。此外美國能源資訊管理局指出，2016年美國發電總能統計上，太陽能產生了9.5千兆瓦的能源正式超過當年度天然氣的能源產能，由此可知未來太陽能將取代掉天然氣主宰了未來替代的能源市場，其中矽晶太陽能電池主宰了太陽能電池的市場，市佔比例高達80%。然而，目前矽太陽能電池的售價仍然偏高而無法在民生普及，因此本研究使用低溫溶液製程搭配矽晶片製作高效率有機矽混合型太陽能電池來大幅降低矽太陽能電池的發電成本達成太陽能發電普及的目標。
在本論文裡，首先我們在矽基板蝕刻出奈米結構，再旋塗高導電有機分子PEDOT:PSS形成異質接面混合型太陽能電池。由於混合式矽太陽能電池的載子在介面傳輸的時候，有很嚴重的載子複合的情形，我們為了解決這個問題，在矽晶片背面利用水溶液刮刀製程技術做了介面的處理，我們利用甲醇來溶解有機小分子Alq3和OXD-7製作成水溶液，並利用刮刀自動塗佈製程來塗佈有機分子在矽晶片的背面，隨著刮刀速度由快到慢，我們可以塗佈不同的厚度的有機分子層在矽晶片的背面，我們發現塗佈Alq3的元件效率最好可以達到11.89 %，而塗佈OXD-7最好可以達到12.90 %，接著我們利用電容電壓的量測得知，塗佈了此兩種有機分子層後，會有效的增加內建電場(Built-in potential)，此額外增加的內建電場有助於載子的分離，並且OXD-7還可以扮演很好的電洞阻擋層(Hole Blocking Layer)來提升少數載子的生命期並提高元件的開路電壓。而為了更有效的提升太陽能電池的效率我們使用了有摻雜奈米碳管的高導電有機分子，在有較佳的透光度與導電度下，我們找出優化後適合的矽奈米線深度並且利用退火來調變適合的功函數，使最好的效率可以來到14.42 %，在完成元件後，再鍍上30 nm的SiOx薄膜來做抗反射層，弭補優化後矽奈米線深度較淺的缺點，使元件的最高效率可以來到15.16 %。為了進一步的得到正電極更佳的導電特性，我們在平面矽太陽能電池的正面貼上石墨烯/PET。利用化學氣相沉積成長高品質石墨烯，再利用簡易的轉印手法，將石墨烯轉印至PET上，並將其貼合在元件的正面，來提升元件效率。在兩層石墨烯的幫助下，元件的效率有53%的提升，平面矽太陽能電池效率來到了8.95%。
利用這些簡易、快速的水溶液製程來提升元件的效率，這些水溶液製程不需要高真空的製程及可提升元件的效率，對於未來我們希望達成的高效率、低成本的混和式矽太陽能電池有很大的幫助，未來也會致力於了解研究更多的製程技術來提升元件的效率。

關鍵字

混和式太陽能電池 ； 異質接面太陽能電池 ； 奈米結構 ； 石墨烯 ； 奈米碳管

並列摘要

According to the report of Greentech Media (GTM) research in America, 59 gigawatts of solar Photovoltaics (PV) were installed globally in 2015, a 34 percent increase over 2014’s total, the total installed solar PV reached 256 gigawatts. GTM Research expected global PV demand between 2015 and 2020 will be increased by an average of 10 percent to 15 percent annually and the cumulative worldwide PV installation to reach 754 gigawatts in 2020. Moreover, U.S. Utility-scale solar photovoltaic power produced more than 9.5 gigawatts in 2016, which exceeded the capacity of natural gas, making PV the most dominant new fuel source for the first time, according to the U.S. Energy Information Administration (EIA). Wafer-based silicon photovoltaics currently dominate the solar cells industry around 80% of the market. However, the price of silicon photovoltaics needs to be further decreased to accelerate the wide-spread use. Therefore, in our research, we aim to lower the cost of silicon photovoltaics by using low-temperature, solution processes to realize high efficiency hybrid organic silicon heterojunction solar cells.
In this thesis, we firstly etch the surface of silicon substrates to make nano-textures, and then spin-cast high conductive organic polymer (PEDOT:PSS) on silicon to form hybrid heterojunction solar cells. Due to the carrier recombination loss at the interface, we applied rear interfacial engineering on silicon via a soluble blade coating process. There, we dissolve organic small molecule powder, Alq3 and OXD-7 with methanol, and use an automatic blade-coating process to coat the organic thin film on the rear surface of silicon. By tuning the coating thickness with various coating speeds, we successfully achieve the best power conversion efficiency (PCE) of 11.89% for the Alq3 devices, and 12.90% for the OXD-7 devices. The characteristics of the hybrid solar cell devices with the two rear interlayers can be further explored through a capacitance-voltage (C-V) measurement, we found the molecular layers can effectively increase built-in potential which is helpful to separate carriers. Moreover, OXD-7 also permits the interlayer to function as a hole-blocking layer (HBL) which could increase the minority carrier lifetime and boost the open-circuit voltage for solar cells. Furthermore, highly transparent and conductive Carbon-nanotube (CNT) doped PEDOT:PSS is employed to realize the solution-processed hybrid silicon heterojunction solar cells. After optimization of depth of silicon nanowires and suitable work-function turning, the best PCE can reach 14.42%. To offset the sacrifice of shorter length nanowires, we deposit a 30 nm thick SiOx layer for antireflection layer and the efficiency can reach to 15.16%. In order to improve the carrier collection and conductivity of front contact, we apply transparent conductive graphene/PET on the planar silicon hybrid solar cells. The high quality graphene is grown by chemical vapor deposition (CVD), transferred to PET thin films, and then attached onto the front surface of device. With the help of two layers graphene, the efficiency of planar silicon hybrid solar cell achieves 8.95%, showing a 53% enhancement factor.
Throughout these efforts, we have successfully demonstrated various low-temperature, solution process approaches in order to address the goals of high efficiency, low-cost hybrid organic silicon heterojunction solar cells.

並列關鍵字

Hybrid silicon solar cell ； Heterojunction solar cell ； Nanostructure ； Graphene ； Carbon nanotube

參考文獻

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