Optical order of the polymer phase within polymer/fullerene blend films

Abstract

This study reports on how the degree of polymer order within a polymer/fullerene blend can be investigated by spectroscopic methods. Non-annealed blend compositions with 0-80 wt % fullerene content were analyzed using temperature dependent photoluminescence (PL) and room temperature spectroscopic ellipsometry (SE) measurements. To evaluate the SE data with respect to the optical order, an optical model was developed, including a lower and higher ordered polymer phase within a fullerene matrix. This was done using an effective medium approach describing the polymer by combining lower and higher ordered polymer properties (polymer-EMA). The polymer/fullerene blend was then evaluated using another EMA consisting of the polymer-EMA and the dielectric function of the disordered fullerene. The degree of optical order obtained by SE, was confirmed using another independent measurement, photoluminescence spectroscopy, according to the method of Francis C. Spano (2005). The volume fraction of the ordered polymer within the polymer-EMA was found to be between 70 and 60 vol % for fullerene contents lower than 20 wt % in the polymer/fullerene blend. Above 20 wt % fullerene, the optical order of the polymer strongly decreases all the way down to 0 vol %. In contrast to the complementary performed X-ray diffraction measurements, which address only the longrange structural order of the blends, we give quantitative information on the optical order, including information on the composition, that is, volume fractions of the higher and lower ordered polymer. The gained information on the tilt of the polymer molecules with respect to the substrate is discussed comparing XRD results from the literature with those obtained by our SE model. Finally, the developed model is used to describe the influence of the P3HT molecular weight on the optical order. Results obtained with our model were compared to the structural data and mobility data in the literature.

Real‐Time Photoluminescence Studies of Structure Evolution in Organic Solar Cells

“…This assumption is based on earlier reports which have shown that the fullerene PL is significantly smaller than most polymer PL. [ 40,46 ] Especially in solution the fullerene absorbance at the excitation wavelength of 488 nm is negligible compared with the polymer absorption. Additionally, collisional quenching of fullerene molecules in solution might reduce radiative recombination further, see the Supporting Information.…”

“…[ 39 ] The spectral shapes of the p-polarized transmission and the PL (Figure 1 e,f) can both be used to reveal the degree of local P3HT order which we will hereafter refer to as "aggregation" in the strict optical sense. [25][26][27]40 ] Consistent with earlier reports, P3HT aggregation in the case of the BHJ w/ ODT is not signifi cant until the fi lm is almost dry, and it occurs under conditions of extreme supersaturation during a small time False color representation of in-situ a) s-polarized refl ection, b) p-polarized transmission, and c) PL acquired simultaneously during fi lm drying of a P3HT/PCBM BHJ with 2% ODT-"BHJ w/ ODT." d) Selected refl ection spectra, e) transmission spectra and f) PL spectra.…”

Performance evaluation of PTB7 : PC71BM based organic solar cells fabricated by spray coating method using chlorine free solvent

“…This often results in no uniform phase segregation due to large fullerene / polymer rich grains in the cast films, reveals limiting the device performance 32 This critical issue was solved by adding the more suitable additive with solvent and it enhances the fullerene solubility and exhibit enhanced device performance. 13,[34][35][36][37][38][39][40][41][42][43] Therefore, development of polymer solar cell using chlorine-free solvents by spray coating is capable of delivering large-area, uniform films with small amounts of material, which is having the compatibility for roll-to-roll fabrication is crucial. 24,27 However, spray-coating techniques are being explored for large-area, uniform thin film and wastage of material is low compare with other method.…”

Structural, optical, and conductive properties of a poly(styrene)‐b‐poly(thiophene) copolymer doped with fullerenes under different conditions

“…Excitons created in all cases must diffuse on the interface to enable charge separation. 8 Organic solar cells are considered as low cost alternative materials with mechanical flexibility [9][10][11][12] and potential variation of their bandgap through the incorporation of different polymers, in order to harvest more efficiently the solar spectrum. 7 These materials should be preferably mixed into a bicontinuous interpenetrating network in order to achieve higher efficiency.…”

“…Polymer-fullerene solar cells were among the first materials applied in the aforementioned type III organic solar cells. 10,11 Poly(thiophene) is one of the most applied polymers for organic solar cells, 13 since it presents desirable optical properties, thermal, and environmental stability with potential applications on electronics, photovoltaics, sensors, and transistors. It should be mentioned that polymeric heterojunctions can exhibit different architectures, such as: (a) bilayer (with limitations, due to the active layer length), (b) bulk heterojunction, and (c) ordered heterojunction.…”