二維材料在光學元件的應用

Abstract

本研究系統性的探討了二維材料於光學元件可能的應用，其中包括透明電極、垂直與橫向光電偵測器，以及柔性元件等。在透明導電電極的部分，我們使用單層二硫化鉬作為薄金與鈣鈦礦太陽能電池主動層間的介面層，透過二維材料表面的凡德瓦磊晶的幫助，< 10 nm 的薄金展現出良好的導電特性，以薄金/單層二硫化鉬作為透明導電電極，我們成功的製作出雙面鈣鈦礦太陽能電池。利用轉移單層二硫化鉬製備的雙面太陽能電池，在沉積 8 nm 薄金電極後達成 89.6% 的高雙面發電效率，其優異表現可歸因於薄金屬膜的高光學穿透率與良好匹配的二硫化鉬/金界面，有效抑制載子復合。在光偵測器的應用，我們透過高溫硫化非晶二硫化鉬薄膜，成功的成長出晶圓級多層二硫化鉬，其層數可達到 30 層，我們將所成長出的多層二硫化鉬薄膜應用於垂直型光伏與橫向型光導元件中，並探討其在不同結構下的光電特性。垂直元件採用簡單的金屬/半導體/金屬（金/二硫化鉬/鋁）架構，藉由上下電極的功函數差異觸發有效的光伏反應；橫向元件則結合多層二硫化鉬為光吸收層與石墨烯為載子傳輸層，實現高效的載子分離與高響應度，並進一步分析二硫化鉬吸光層的層數對於元件反應時間的影響，我們也透過不同的元件製作流程來最佳化橫向二維材料光偵測器的元件表現。最後，我們也將具不同吸光材料的石墨烯/過渡金屬硫族化物異質結構光電偵測器製作於可撓性基板上，得益於過渡金屬硫族化物層可調變的能隙與石墨烯高的載子遷移率，這些元件展現出偵測波長可調性、高響應度以及高偵測度的優異表現，相較於製作於剛性基板上的元件，製作於可撓性聚對苯二甲酸乙二酯基板上的元件依然維持 102-103 A/W 的高響應度，並展現出在機械彎曲條件下的穩定光電性能。特別是以二硫化鎢與二硒化鎢為吸光層的元件於不同彎曲狀態下僅顯示出極小的性能衰退，突顯其在柔性與穿戴式光子元件中之機械穩定性與整合潛力。綜合而言，本研究驗證了單層及多層二維材料於透明電極、光伏結構以及高性能光電偵測器等不同的潛在應用。其可擴展的材料堆疊順序及層數、優異的機械柔韌性、波長選擇性以及高光響應度等特性，皆奠定了其於新世代剛性與柔性光電元件應用中的發展潛力。

In this thesis, we have investigated the applications of 2D materials for optical devices including transparent electrodes, vertical and lateral photodetectors and flexible devices. On transparent electrodes, with the assist of van der Waals epitaxy on 2D material surfaces, thin and conductive gold (Au) film can be grown on mono-layer MoS2 surfaces. By using 8 nm Au/mono-layer MoS2 as transparent electrodes, bifacial perovskite solar cells with high bifaciality 89.6 % are fabricated. The good light transmittance of the Au/MoS2 electrode and the minimum carrier recombination in the thin mono-layer MoS2 layer are the key parameters resulting in the high performances of the device. Through the sulfurization of amorphous MoS2 films, wafer-scale and multi-layer MoS2 with layer numbers up to 30 can be grown on sapphire substrates. Based on the multi-layer MoS2, vertical photodetectors with photovoltaic responses and lateral photodetectors with high responsivities are fabricated. The vertical devices utilized a simple Au/MoS2/Al configuration, where the work function difference between the electrodes enabled efficient photovoltaic response. The lateral devices adopted a hybrid structure with multi-layer MoS2 as the light absorption layer and graphene as the carrier transport layer, enabling efficient carrier separation and high responsivities. We have also investigated the influence of MoS2 layer numbers to response times and optimized the device performances through different device fabrication procedures. By using different transition metal dichalcogenides (TMDs) such as WS2, MoS2 and WSe2, graphene/TMD heterostructure photodetectors are fabricated on PET substrates. The devices exhibit tunable detection wavelengths based on the bandgaps of different TMDs. Compared with the devices fabricated on rigid substrates, the flexible 2D material photodetectors exhibit high responsivities up to 102-103 A/W, which indicate their high endurance to mechanical deformations. Devices with WS2 and WSe2 light absorption layers exhibited minimal performance degradation under various bending conditions, indicating the mechanical robustness and integration potential of these heterostructures for flexible and wearable photonic applications. In conclusion, this study demonstrates the potential of 2D materials for optical device applications. Their scalable synthesis, easy stacking, and wavelength-tunable photo-response make 2D materials promising candidates for next-generation optical device applications on either rigid or flexible substrates.