使用散射式近場光學顯微鏡探討薄膜有機太陽能pBCN:PCBM異質接面之奈米微域結構

Revealing Nanodomains of Bulk Heterojunction pBCN:PCBM films with Scattering-type Scanning Near-field Optical Microscopy

黃政嘉(Cheng-Chia Huang)

指導教授 ： 王玉麟

國立臺灣大學/理學院/應用物理研究所/碩士(2020年)

https://doi.org/10.6342/NTU202002658

摘要

在新世代太陽能電池的發展中，因為有機太陽能電池的吸收波長易於調整和可撓曲的特性，應用在室內和健康管理設備等電源中十分具有潛力。有機太陽能電池在性能表現有一個重大突破關鍵是活化層改由電子予體和電子受體材料混合組成的異質接面，其可促進激子分離而使能量轉換效率提高。對於發展新的有機導電高分子材料和加工方法以增進能量轉換效率中，檢測異質接面之奈米微域結構是一重要步驟。因予體和受體的材料元素很類似，使用一般的電子顯微鏡和X光繞射分析無法清楚區分。而散射式近場光學顯微鏡是利用介電常數來區分奈米微域的不同材料，加上它的水平解析度可達到5奈米以下，對於揭開有機材料奈米微域的面貌是一個合適的檢測工具。
此研究乃以散射式近場光學顯微鏡觀測新的有機高分子聚合物pBCN作為電子予體與PCBM作為電子受體混合的異質接面奈米微域結構，以了解不同混合比例及熱退火或溶劑退火後處理對其微域結構的影響。根據靜電偶極子模型以及pBCN和PCBM的介電常數，得知pBCN近場訊號的強度比PCBM強，而且近場訊號的相位領先於PCBM。從pBCN:PCBM薄膜的近場強度和相位影像同時觀察到兩種材料微域的區別。值得注意的是，高強度的區域和領先相位的區域是符合的，而且高強度（領先相位）的區域比率隨著製作pBCN:PCBM薄膜中pBCN的重量混合比率上升而增加。這兩個顯著之處可以推導出近場影像中高強度（領先相位）的區域為pBCN的區域。另外，pBCN的區域面積比率都比pBCN的重量混和比率高，意味著薄膜頂面佔有比較多pBCN的區域。尤其是經由溶劑退火的pBCN:PCBM薄膜中，即使pBCN的重量混和比率只有50%，pBCN的區域面積比率可高達約80%。這些結果指出退火過程在製作異質接面有機高分子太陽能電池中，不僅能促進電子受體和予體微域的形成也促使這些微域分布不均。這發現和異質接面反式結構太陽能電池的能量轉換效率比標準結構高的結果一致。這研究更進一步證明散射式近場光學顯微鏡是一個強而有力的奈米尺度特性分析工具，可以廣泛的應用於其它異質接面太陽能電池中。

關鍵字

異質接面 ； pBCN:PCBM ； 近場光學 ； 散射式近場光學顯微鏡 ； 有機導電高分子太陽能電池

並列摘要

During the development of new generation solar cells, owing to wavelength tunability and flexibility propensities of polymer solar cells, they are considered as one of potential candidates for harvesting artificial lighting energy as the energy source for indoor and health-care devices. One key breakthrough in their performance is the formation of separate nanodomains of donor and acceptor materials in their active layer—bulk heterojunction (BHJ)—to boost exciton dissociation and improve the power conversion efficiency (PCE). Revealing nanostructural morphology of BHJ active layer is an important step to the development of new polymers and processing methods to further PCE. However, because of the element similarity in organic materials, examining these nanodomains with conventional electron microscopy and X-ray scattering is futile. Scattering-type scanning near-field optical microscopy (s-SNOM)—differentiating the separate nanodomains of donor and acceptor due to their different dielectric constants and bearing a lateral resolution of <5 nm—is an appropriate characterization tool to unveil the organic nanomorphology.
This study shows the revelation of the nanodomains of a BHJ system, which was formed by blending a new donor polymer (pBCN) with PBCM as an acceptor, with use of s-SNOM and the effect of nanodomains with different mixing percentages and different annealing treatments (solvent and thermal annealing). Theoretically predicted near-field amplitude and phase signals based on point-dipole model and the dielectric constants of pBCN and PCBM show that the amplitude signal of pBCN is larger than that of PCBM while the phase of pBCN leads that of PCBM. The observed near-field amplitude and phase images of pBCN:PCBM films exhibit two separate domains. Noticeably, the high-amplitude regions coincide with the led-phase regions; the area percentage of the high-amplitude (led-phase) regions increases with the weight percentage of pBCN in the preparation of the pBCN:PCBM films. These two conspicuous features lead to the assignment of the high-amplitude (led-phase) regions of the near-field images to pBCN domains. The area percentage of the pBCN domains is consistently higher than the weight percentage of pBCN, suggesting that pBCN domains occupy more on the top of the pBCN:PCBM films. Particularly, the area percentage of pBCN in the pBCN:PCBM film prepared with solvent annealing can reach ~80% even for its weight percentage of 50%. The result indicates that post annealing in the fabrication of BHJ polymer solar cells not only facilitates the formation of appropriate donor and acceptor domains but also induces uneven distribution of these nanodomains. This finding agrees with the fact that the PCE of BHJ pBCN:PCBM solar cells with an inverse device structure—the holes are collected from the top anode while the electrons are collected from the bottom cathode—is higher than that with a standard device structure. This study further proves that s-SNOM is a powerful nanometer-scaled characterization tool that can be generally applied to other BHJ polymer solar cells.

並列關鍵字

Bulk heterojunction ； pBCN:PCBM ； near-field ； s-SNOM ； polymer solar cells

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