太陽光敏化石墨烯電極與矽之蕭基接面高效率光伏元件

Abstract

由單層碳原子所組成的石墨烯，因為特殊的能帶結構以及原子排列方式，而擁有許多優異的性質，例如可調變的功函數、高穿透度、高載子遷移率(carrier mobility)等等，這些性質使得石墨烯具有相當的潛力應用於透明電極領域。然而可調變功函數的特性在p型摻雜(p-type doping)石墨烯陰極已經被廣泛地研究與應用；相對地，由於一般n型摻雜物(n-type dopant)的大氣穩定性與熱穩定性低，且易受到外在環境的影響，所以n型石墨烯電極領域仍有很大的發展空間。 在本論文的第一部分，我們製作出新穎的”太陽光敏化”石墨烯透明電極，藉由摻入TiOx對石墨烯造成n型摻雜外，TiOx本身也會吸收少量的近紫外光而產生光激發電子電洞對，並對石墨烯產生額外的摻雜效果，同時能經由控制照光時間，調變其摻雜程度。我們也將這個新穎的透明導電電極實際與矽晶作結合，形成蕭基接面太陽能電池。石墨烯除了能作為收集光激發載子(exciton)的電極外，也能與矽晶形成蕭基接面，產生電場分離光激發載子。再搭配特殊的介面處理製程消除石墨烯與矽晶間原生氧化層，進而得到n型石墨烯/p型矽晶的結構下目前報導中最高的效率(10.4%)。 雖然本實驗結果是該結構蕭基太陽能電池的一大突破，然而對比發展成熟的p型石墨烯/n型矽蕭基太陽能電池仍有進步空間。於是論文的第二部分即是針對前一部分設計的元件作改良，藉由使用適當的抗反射層材料增加光電流、利用快速退火製程得到更理想的背電極減少介面損耗，以及利用緻密的氧化層做出無介面陷阱(interfacial trap)的元件，不僅能降低介面陷阱造成的影響並保護矽晶表面阻礙原生氧化層的形成，還能作為阻擋層(blocking layer)減少漏電流與再結合現象發生，進而提高元件的表現。

Graphene, which consists of a single atom-thick layer of carbon, has a lot of attracting properties such as tunable work function, high transparency and high carrier mobility etc. All these properties make graphene be a promising material to replacing widely-used ITO as transparent conducting electrode. However, compared to well-developed graphene-based anodes, fabricating a stable graphene-based cathode is more difficult because n-type dopants for graphene have limited thermal and chemical stabilities and are also sensitive to the influence of ambient environment. In the first part of this thesis, we developed a novel “sunlight-activated” graphene-heterostructure transparent electrode. Besides, TiOx was found to be an effective n-type dopant for graphene by surface charge transfer process. With only costing a small amount of ultraviolet, TiOx will photo-generates charges under illumination then are transferred toward graphene and further doped it. This photoactive TiOx/graphene heterostructure transparent electrode exhibits excellent tunable electrical properties and is appropriate to fabricate an n-graphene/p-silicon Schottky junction solar cell, even achieving a record-high power efficiency of graphene/p-silicon structure. In the second part, we aim to improve the performance of device in the first part. With more suitable anti-reflective layers, back contact electrodes, and surface passivation, we demonstrate a “trap-free” photoactive n-graphene/p-Si Schottky solar cell with higher short circuit current and open circuit voltage. This device is also an ideal candidate for future derivatives of tandem cells.