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.
The degradation of poly(3-hexylthiophene) (P3HT)− titania-based solid-state dye-sensitized solar cells (ssDSSCs) is studied to better understand device aging mechanisms. The correlation of temporal evolution between P3HT crystallite structures and device performance is discussed for the first time using in situ measurements. For comparison, two types of mesoporous titania photoanodes with different pore sizes are prepared. Grazing incidence wide-angle X-ray scattering is used in situ under continuous solar illumination to obtain information about the impact of pore size on P3HT crystalline order and on temporal evolution of the P3HT crystallites. The development of the photovoltaic characteristics is explored in parallel. The lattice constants, crystal sizes, and volume fraction of crystalline P3HT in the large-pore active layer remain stable over 30 min, while the volume fraction of crystalline P3HT decreases in the small-pore active layer. Thus, the pore size of titania photoanodes is important for the stability of P3HT−titania-based ssDSSCs.
From a morphological
perspective, the understanding of the influence of the [6,6]-phenyl
C71-butyric acid methyl ester (PC71BM) content
on the morphology of the active layer is not complete in organic solar
cells (OSCs) with bulk heterojunction (BHJ) configuration based on
the low-bandgap polymer poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl] (PTB7-Th). In this work, we
obtain the highest power conversion efficiency (PCE) of 10.5% for
BHJ organic solar cells (OSCs) with a PTB7-Th/PC71BM weight
ratio of 1:1.5. To understand the differences in PCEs caused by the
PC71BM content, we investigate the morphology of PTB7-Th/PC71BM blend films in detail by determining the domain sizes,
the polymer crystal structure, optical properties, and vertical composition
as a function of the PC71BM concentration. The surface
morphology is examined with atomic force microscopy, and the inner
film morphology is probed with grazing incidence small-angle X-ray
scattering. The PTB7-Th crystal structure is characterized with grazing
incidence wide-angle X-ray scattering and UV/vis spectroscopy. X-ray
reflectivity is employed to yield information about the film vertical
composition. The results show that in PTB7-Th/PC71BM blend
films, the increase of PC71BM content leads to an enhanced
microphase separation and a decreased polymer crystallinity. Moreover,
a high PC71BM concentration is found to decrease the polymer
domain sizes and crystal sizes and to promote polymer conjugation
length and formation of fullerene-rich and/or polymer-rich layers.
The differences in photovoltaic performance are well explained by
these findings.
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