Solvent–Morphology–Property Relationship of PTB7:PC₇₁BM Polymer Solar Cells

Abstract

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.

On the Role of PTB7‐Th:[70]PCBM Blend Concentration in ortho‐Xylene on Polymer Solar‐Cell Performance

“…To ensure ideal cell performance, xylene is typically associated to a very limited amount of additive, typically 1,8‐diiodoctane (DIO), which helps to guarantee proper morphology of the film. DIO ensures suitable solubility and homogeneous dispersion of the acceptor material, which prevents agglomeration of [70]PCBM . Concentrations in the range of 3 to 5 % are commonly found as the optimal concentrations to improve charge transport in BHJs …”

Ternary Blend Strategy for Achieving High‐Efficiency Organic Photovoltaic Devices for Indoor Applications

“…Chem.E ur.J. 2019,25,[6154][6155][6156][6157][6158][6159][6160][6161] www.chemeurj.org pared with the binary blend system of PTB7:PC 71 BM. The OPV device prepared with the ternary blend containing 40 wt %E P-PDI showedt he highest efficiency of 15.68 %, attributedt oa synergistic contribution of the PC 71 BM andE P-PDIa cceptors, whereas the efficiency of the cell based on the binary blend PTB7:PC 71 BM, that is, that without EP-PDI, was only 9.11%.T he overall improved device performance resulting from including EP-PDIw as attributedt ot he synergistic effects of enhanced absorption, better charget ransport, and the suppression of non-geminate bimolecular recombination.…”

“…For the binary blend PTB7:PC 71 BM, we selected the optimum blend ratio (1:1.5 wt %) and the solvent chlorobenzene (CB)/1,8-diiodooctane (DIO) (97:3 vol %) in accord with previousr eports. [6,25] The resulting fabricated device showedaPCE of 9.11%,aV OC of 0.6 V, J SC of 46.1 mAcm À2 ,a nd FF of 54.3 %u nder the simulated indoor illuminationo fa pproximately0 .17 mW cm À2 from a5 00 lx LED light source.Wefound that partial replacement of the acceptor PC 71 BM in the PTB7:PC 71 BM systemw itht he acceptorE P-PDI led to increases in the values of the device parameters, and a 60:40 wt %r atio of these acceptors appeared to be optimum, yieldingaP CE of 15.68 %( and V OC = 0.65 V, J SC = 56.7 mAcm À2 , and FF = 68.5 %). As imilar trend in device performance was also observedu nder one-suni rradiation conditions, with the highest efficiency of around8 .53 %a chieveda tt he same 60:40 wt %r atio of PC 71 BM/EP-PDI, as shown in Table 1.…”

π‐Conjugated Donor Polymers: Structure Formation and Morphology in Solution, Bulk and Photovoltaic Blends

“…Chlorobenzene, 1,2-dichlorobenzene and 1,2,4-trichlorobenzene were investigated. [124] The solventinduced difference in device performance was mainly ascribed to the variation of lateral structure sizes in the active layers. The highest device performance was reported to result from the smallest average multilength scale lateral structure sizes with the smallest length scale matching the exciton diffusion length.…”