利用散射式近場光學顯微鏡探究有機太陽能P3HT:PCBM異質接面之奈米微域結構

Abstract

有機太陽能電池被視為潛力的次世代太陽能電池，自從異質接面的概念被引入後，有機太陽能電池的光電轉換效率大幅提升。有機太陽能電池之異質接面是藉由電子予體和電子受體高分子材料混合而產生之纖維狀結構。在高效率的異質接面太陽能電池中，其奈米微域的尺寸理論上與激子的擴散距離相近---約略為20奈米。因此欲了解有機太陽能電池有效層之奈米微域結構和其光電特性之關聯性，需要可提供至少約略10奈米空間解晰度之顯微量測技術。此研究以Heterodyne散射式近場光學顯微鏡，用633 nm的光激發研究有機太陽能電池異質接面P3HT:PCBM薄膜表面之奈米微域結構，並搭配靜電偶極子模型的計算判讀近場顯微影像。從理論模型計算得知P3HT之近場訊號相對強度大於PCBM，且有序結構之P3HT之近場訊號相對強度大於無序結構之P3HT。從近場顯微影像可得知旋轉塗佈P3HT的薄膜表面出現網狀之奈米微域結構，而P3HT:PCBM薄膜上則出現互不相連之奈米微域，除此之外，此研究發現薄膜表面之P3HT和PCBM的面積比例和旋轉塗佈前材料混合之比例有所差異，可得知P3HT奈米微域並非均勻地分佈於三維的有機太陽能電池有效層中。此論文顯示散射式近場光學顯微鏡可應用於有機太陽能電池表面之奈米微域鑑定。

Polymer-based organic solar cell has been seen as a potential candidate of next-generation photovoltaics. The introduction of bulk-heterojunction (BHJ) concept — a blended film of electron donor and acceptor that forms an interconnected network — is the key to greatly improve the power conversion efficiency (PCE). The domain size in the blended layer of donor and acceptor should be comparable to the exciton diffusion length (~20 nm) for best device performance. Revealing the relationship between nanostructure morphology to the electro-optical properties of organic solar cell device therefore requires a characterization tool with spatial resolution down to less 10 nanometers. In this study, nanostructure morphology of the pristine P3HT and the blended P3HT:PCBM films—a prototypical polymer-based organic solar cell was investigated by a heterodyne-based scattering-type scanning near-field optical microscopy (s-SNOM) with an excitation wavelength of 632.8 nm. The obtained s-SNOM images were interpreted with point-dipole and modified point dipole models. Theoretical prediction based on these two models show that the near-field amplitude and phase signals of P3HT are larger than that of PCBM, while these two near-field signals of ordered P3HT are larger than that of disordered P3HT. Based on these two near-field traits, the s-SNOM image of pristine P3HT displays nanometer-scaled connected network of ordered P3HT, while that of blended P3HT:PCBM shows isolated nanodomains of P3HT. Last but not least, the portrayed area ratio between P3HT and PCBM is smaller than the mixing ratio, indicating that the actual distribution of P3HT domains is not uniform over the whole active layer. This study demonstrates the capability of s-SNOM as a tool in identifying nanostructure morphology of blended P3HT:PCBM film in BHJ polymer solar cells.