Physik-Department, Lehrstuhl für Funktionelle Materialien, Technische Universität München , James-Franck-Str. 1, 85748 Garching, Germany.
Munich School of Engineering, Technische Universität München , Lichtenberg Str. 4, 85748 Garching, Germany.
ACS Appl Mater Interfaces. 2017 Feb 1;9(4):3740-3748. doi: 10.1021/acsami.6b14926. Epub 2017 Jan 23.
The influence of three different solvents and a solvent additive on the morphology and photovoltaic performance of bulk heterojunction films made of the copolymer based on thieno[3,4-b]thiophene-alt-benzodithiophene unit PTB7-F40 blended with [6,6]-phenyl-C71-butyric acid methyl ester (PCBM) is investigated. Optical microscopy and atomic force microscopy are combined with X-ray reflectivity and grazing incidence small and wide-angle X-ray scattering (GISAXS and GIWAXS, respectively), enabling the characterization of the morphology of the whole photoactive film. The detailed study reveals that different length scales of PCBM clusters are observed using different solvents, while adding a solvent additive results in the PCBM clusters being selectively dissolved. Vertical and lateral phase separation occurs during spin coating and depends on the solvent used. A hierarchical morphology is detected within the bulk film through GISAXS measurements. Furthermore, GIWAXS shows that a rather amorphous film with low crystallinity was probed, which substantiates that high crystallinity is not necessarily required for high performance organic solar cells. Different models for the morphology are proposed through the combination of all the findings and correlated with the corresponding device properties. Consequently, the solvent-induced different device performance is mainly ascribed to the varied lateral structure sizes, whereas the highest device performance is a result of the smallest average multilength scale lateral structure sizes with the smallest length scale matching the exciton diffusion length.
研究了三种不同的溶剂和一种溶剂添加剂对基于噻吩并[3,4-b]噻吩-alt-苯并二噻吩单元的共聚物 PTB7-F40 与 [6,6]-苯基-C71-丁酸甲酯（PCBM）共混的体异质结薄膜形貌和光伏性能的影响。光学显微镜和原子力显微镜与 X 射线反射率和掠入射小角 X 射线散射（GISAXS 和 GIWAXS）相结合，能够对整个光活性薄膜的形貌进行表征。详细研究表明，使用不同的溶剂观察到不同长度尺度的 PCBM 团簇，而添加溶剂添加剂会导致 PCBM 团簇选择性溶解。旋涂过程中会发生垂直和横向相分离，这取决于所用的溶剂。GISAXS 测量结果表明在体膜内存在分层形貌。此外，GIWAXS 表明探测到的是结晶度较低的相当非晶态薄膜，这证实了高结晶度对于高性能有机太阳能电池并非必需。通过结合所有发现提出了不同的形态模型，并与相应的器件性能相关联。因此，溶剂引起的不同器件性能主要归因于不同的横向结构尺寸，而最高的器件性能是由于最小的平均多层横向结构尺寸与激子扩散长度匹配的最小长度尺度所致。
