利用表面電位顯微鏡量測功函數調變對混合式有機/矽太陽能電池特性的影響

Abstract

在近幾年，混合式有機/矽太陽能電池的發展具有顯著的進步，依據有機表面形貌、載子導電度及能帶對準在混合太陽能電池的光電特性扮演重要腳色，此外，各種界面的製程技術和材料也進一步提升功率轉換效率。在本論文中，我們透過表面電位顯微鏡調查混合太陽能電池中有機材料功函數調變的相關性及其特性。首先，我們研究PEDOT:PSS(PH1000)與矽基板的影響，量測原始的PEDOT:PSS功函數為4.98eV，在矽基板上調變PEDOT:PSS的厚度，當PEDOT:PSS厚度為140nm時，功函數為4.85eV，隨著厚度越薄，功函數下降，在PEDOT:PSS厚度為37nm時，功函數下降為4.67eV，藉由功函數改變可以知道PEDOT:PSS與矽基板電荷交換情形；為了做更深入的了解，我們量測PEDOT:PSS與矽基板的截面，利用一次微分得到電場分布，隨著PEDOT:PSS厚度越薄，內建電場越大，在厚度為37nm時，內建電場為0.459V/m，可以了解到厚度與內建電場的關係。 再來，我們研究P型摻雜p-PFO加入小分子F4-TCNQ的功函數調變，透過調整不同的摻雜濃度，該材料可藉由界面復合機制來提高電荷傳輸特性。然而，材料界面能帶對準和F4-TCNQ使用不同溶劑中的溶解度仍影響太陽能電池的性能，且尚未被完全得知。我們先比較PFO溶解在不同溶劑中的功函數變化：氯苯（CB）和四氫呋喃（THF），且混入溶解在THF中的F4 -TCNQ溶液。根據觀察，此共溶劑的方法，在PFO和F4-TCNQ溶解在THF中，可以得到相對均勻的薄膜，且比CB溶劑共混p-PFO析出量少。再者，由於良好的溶解度，共溶劑p-PFO的功函數變化正比於摻雜濃度；相反地，溶於CB的功函數變化不大且被限制在10%濃度結果，該顯著的差異也反映在元件特性上，其中，元件趨勢的短路電流，開路電壓及填充因子對應於功函數調變的趨勢。最後，在使用共混p-PFO溶液中，我們說明刮刀塗佈和旋轉塗佈技術導致不同的表面形貌，該結果些微影響薄膜的功函數。總結，有機材料的功函數調變，藉由界面活性劑、添加劑、摻雜劑、塗佈技術可影響混合太陽能電池的特性。該SKPM技術提供一個簡單的解決方案去了解材料相關性及節省製程週期。

The development of hybrid PEDOT:PSS silicon solar cells has advanced considerably in the past few years. It has found that the organic surface morphology, carrier conductivity, and energy band alignment all play critical roles in depicting the photovoltaic characteristics of the hybrid solar cells. In addition, various interface engineering techniques and materials are also proposed to further boost the power conversion efficiency. In this work, we investigate the correlation between the work function (WF) engineering of organic materials to the characteristics to hybrid solar cells through a scanning kelvin probe microscopy technique. First, we study the influence between PEDOT:PSS (PH1000) and the silicon substrate, the pristine PEDOT:PSS has a WF of 4.98eV. Modulating PEDOT: PSS thickness on silicon substrate, changing the PEDOT:PSS thickness from 140nm to 37nm decreases the WF of PEDOT:PSS to 4.67eV. By changing the work function can know charge exchange situation between PEDOT: PSS and the silicon substrate. Then, we measure cross section with PEDOT:PSS and silicon substrate, using the first differential can get the electric field distribution. When changing the PEDOT:PSS thickness from 140nm to 37nm increases the built-in electric field to 0.459V/m. Next, we study the WF engineering of p-typed-doped fluorescent conjugated polymer poly(9,9-di-n-octylfluorenyl-2,7-diyl) (p-PFO) with small-molecule tetrafluorotetracyano- quinodimethane (F4-TCNQ). By adjusting different doping concentrations, the materials can boost charge transport via an interfacial recombination mechanism. However, the interfacial band alignment and the solubility of F4-TCNQ with different solvent still affect the cell performance and had yet been thoroughly understood. We first compare the WFs of PFO dissolved in different solvents: chlorobenzene (CB) and tetrahydrofuran (THF), which are blended with the F4-TCNQ solution, dissolved in THF. It is observed that the co-solvent approach, both PFO and F4-TCNQ in THF, results in relatively uniform thin films with less precipitation than the co-blending p-PFO. Moreover, the WF variation of co-solvent p-PFO is positively correlated to the doping concentration due to better solubility. In contrast, the WF of CB does not vary much and is clamped at the 10% concentration. The significant differences are also reflected in the PV characteristics, where the trend of short-circuit current, open-circuit voltage, and fill-factor correspond to the trend of WF modifications. Finally, we show that the blend-coating and spin-coating techniques also lead to different surface morphologies for the co-blending p-PFO solutions, which slightly affect the resulting WF of the films. In summary, the WF modification of organic materials via surfactant, additives, dopants, coating techniques can affect the hybrid device characteristics. The SKPM technique offers a straightforward solution to understand the correlation, and thus save the development cycle.