Department of Physics and Astronomy, Stony Brook University , Stony Brook, New York 11974, United States.
ACS Nano. 2014 Jan 28;8(1):323-31. doi: 10.1021/nn404172m. Epub 2013 Dec 30.
The effects of thin-film confinement on the material properties of ultrathin polymer (electron donor):fullerene (electron acceptor) bulk heterojunction films can be important for both fundamental understanding and device applications such as thin-film photovoltaics. We use variable angle spectroscopic ellipsometry and near edge X-ray absorption fine structure spectroscopy to measure the optical constants, donor-acceptor volume fraction profile, and the degree of interchain order as a function of the thickness of a poly(3-hexythiophene-2,5-diyl) and phenyl-C61-butyric acid methyl ester bulk heterojunction film. We find that as the thickness of the bulk heterojunction film is decreased from 200 nm to the thickness confinement regime (less than 20 nm), the vertical phase segregation gradient of the donor and acceptor phases becomes less pronounced. In addition, observing the change in exciton bandwidth and the shift of absorption resonances (0-0 and 0-1) relative to neat donor and acceptor films, we find that the conjugation length and disorder in ultrathin films (20 nm) are less affected than thicker (200 nm) films by the addition of fullerene into the polymer. We believe that these findings could be important for discovering methods of precisely controlling the properties of bulk heterojunction films with crucial implications for designing more efficient organic-based photovoltaics.
薄膜限制对超薄聚合物（给体）：富勒烯（受体）体异质结薄膜的材料性质的影响对于基础理解和设备应用都很重要，例如薄膜光伏。我们使用变角光谱椭圆偏振术和近边 X 射线吸收精细结构光谱来测量光学常数、给体-受体体积分数分布以及作为聚（3-己基噻吩-2,5-二基）和苯基-C61-丁酸甲酯体异质结薄膜厚度的函数的链间有序程度。我们发现，随着体异质结薄膜的厚度从 200nm 减小到厚度限制区（小于 20nm），给体和受体相的垂直相分离梯度变得不那么明显。此外，通过观察激子带宽的变化和吸收共振（0-0 和 0-1）相对于纯给体和受体薄膜的位移，我们发现，与较厚的（200nm）薄膜相比，富勒烯添加到聚合物中对超薄薄膜（20nm）的共轭长度和无序的影响较小。我们相信这些发现对于发现精确控制体异质结薄膜性质的方法很重要，这对设计更高效的有机基光伏具有重要意义。
