利用表面處理優化混合型有機/矽奈米線異質接面太陽能電池

Abstract

混合型有機矽異質接面太陽能電池具有低溫低製程成本的優勢，有潛力成為下世代的矽薄膜光伏元件。在本論文中，我們首先優化元件面積1cm×1cm的混合型太陽能電池。為了增加元件的光吸收，我們在矽晶片上蝕刻矽奈米線結構，並使用平面迴轉式振盪器來穩定蝕刻均勻性。藉由控制蝕刻時間與溫度，可以準確控制矽奈米線的深度與深寬比，我們成功在蝕刻溫度20˚C蝕刻時間3分鐘條件下，轉換效率可達12.53 (±0.12)%。 另外，矽晶片存在嚴重的表面載子復合問題，使得照光產生的載子易被侷限與復合，我們尋求以表面鈍化層與表面電場兩種方法解決此問題。在表面鈍化層研究上，我們利用原子層沉積系統在矽晶片上沉積Al2O3當作鈍化層。隨後利用爐管進行(450度C for 20 minutes)退火，形成高品質且緻密的Al2O3。由Sinton WCT-120 QSSPC量測結果發現，在半導體級矽晶片上的等效載子生命週期可達3.69ms；而在太陽能級單晶方片上可達2.44ms。目前我們正著手研究將這層高品質的鈍化層應用於混合型太陽能電池。在表面電場研究上，我們先利用高溫製程方式 (840度C) 於矽晶片背表面上摻雜磷形成n++背表面電場，以及沉積n+ a-Si來鈍化表面，並觀察到開路電壓、短路電流、與填充因子皆有提升，元件的轉化效率最好可由12.26%提升13.37%。然而為了延續低溫低能的元件製程，我們嘗試使用刮刀製程在矽基板與背電極之間，刮上一層有機小分子寬能帶材料: OXD-7或是Alq3以減少少數載子在背電極界面的復合。這兩種電子傳導層材料皆能以溶液製程製作，最好的元件轉化效率能提升至12.83%。

Hybrid organic-silicon solar cells offer low temperature and low cost fabrication process for next generation silicon thin film photovoltaics. In this study, we first optimize the device performance with an active area of 1×1cm2. In order to increase light absorption, silicon nanowire templates are prepared by a metal-assisted chemical etch method. By using a digital rotator to stabilize the etching uniformity, we can successfully control the etch depths and aspect ratio of the nanowires by adjusting the etch time and solution temperature. Consequently, we demonstrate hybrid solar cells with high power conversion efficiency of 12.53 (±0.12)% obtained by etching nanowires in 20˚C solution for 3 minutes. Next, as silicon wafers often suffer from serious surface recombination issues, we seek solutions via surface passivation and surface field approaches. In the former, we employ a thin Al2O3 layer for surface passivation using an atomic layer deposition method, followed by annealing in 450 C for 20 minutes to form high quality and high density Al2O3. Based on the measurement results of quasi-steady-state photoconductance (QSSPC), the effective carrier lifetime of crystalline silicon wafer and solar grade wafer can reach 3.69ms, and 2.44ms, respectively. The application of this high quality passivation layer on hybrid solar cells is on progress. For the introduction of surface field, we first use the standard high-temperature process (840 C) to diffuse Phosphor into the rear surface of silicon wafers to form an n++ back surface field, and also deposit an n+ a-Si to passivate surface. We observe enhanced open-circuit voltage (Voc), short-circuit current (Jsc), and fill-factor (FF). The power conversion efficiency can be improved from 12.26% to 13.37% for the champion device. However, to continue the route for low manufacturing energy footprint, we employ a soluble, blade coating process to deposit a small-molecule, wide-bandgap organic material, either OXD-7 or Alq3 on the rear surface, between silicon and the aluminum cathode. These two materials can serve as both electron-transport and hole-blocking layers, which can improve the power conversion efficiency to 12.83% under best processing conditions.