Toward Fully Solution-Processed Organic Photovoltaics: Spectroscopic Studies on Electronic Structures for Organic Solar Cells with Graphene Electrodes

本文探討了有機體異質接面太陽電池中能階匹配對其開路電壓之影響與機制，並透過修飾其內部各分界面之性質，減少光生載子於內部傳輸時的能量損耗來提升光電轉換效率，從能量角度歸納出元件構型與材料選用之參考準則，藉此規劃元件結構達到適當之能階匹配，進而不需經由真空製程即可實現高效率之半透明有機太陽電池。 本文首先探討作為太陽電池核心之光敏層其能階對光生激子分離的影響，以摻雜電子施體或受體來改變體異質接面的能階結構，擴大內建電位差達到效率提升之目的；同時分析元件內部於萃取光生載子時所經過之其他接合界面，其能階匹配與能帶組合對該元件效能之影響，並結合光電子能譜與阻抗分析，歸納出利於載子和能量傳輸之能帶組合來減少內部損耗，達到透過能帶工程來優化元件之目的；此外，一套有效調變石墨烯功函數之轉印方法被開發並整合至有機體異質接面太陽電池之製程，驗證電極功函數之改變所產生的界面能障亦會對元件效率產生可觀影響。基於上述實驗與發現，工程上成功地結合了能階調配後的元件結構與石墨烯，有效地將石墨烯薄膜作為透明導電陰極與陽極並成功地轉印至有機物質之上，開發出與傳統元件效率相當之石墨烯有機光伏元件，更進一步地發展出半透明有機太陽電池，達到全溶液製程而不需使用真空系統製備。如此降低時間成本與製程開銷，增添了大面積製備之可能性，更提供了可與建築、外觀整合的嶄新應用，對製作高效率之有機體異質接面太陽電池和其實際應用提供了更多元化的選擇。Heterostructures based on organic semiconductors are fundamentally different and more flexible than those made from conventional covalently bonded inorganics. In particular, organic bulk heterojunction (BHJ) solar cells have been known for their potential as a time-saving, low-cost and lightweight technology. In this work, the electronic properties of internal junctions including donor-acceptor heterojunction, organic/organic and organic/inorganic junctions at each interface within a BHJ solar cell have been studied in detail via photoemission spectroscopy. Promising ways for performance improvement have been developed based on the incorporation of energy ladder, interfacial modification, as well as forming an energetically favorable band alignment by tuning the work function of electrodes. In addition to investigations from the aspect of electronic structure, loss mechanisms induced by interfacial barrier due to band mismatch have been clarified with better understanding of capacitive response provided by impedance spectroscopy. A facile approach that can transfer high-quality graphene along with effective doping was also established based on aqueous intercalation process, enabling n-doped and p-doped graphene for transparent electrode applications. Accordingly, the incorporation of both graphene anode and graphene cathode was successfully carried out to achieve high-performance organic BHJ solar cells with semi-transparency, showing high compatibility with solution-processed organic optoelectronics. Such proposed scheme benefited the integration of graphene with organic BHJ solar cells, which was further developed into a vacuum-free process for device fabrication. Therefore, the development of fully solution-processed semitransparent device distinguishes organic BHJ solar cells as a feasible green energy source for flexible optoelectronics, wearable battery as well as facade integration