We investigated the initial photooxidation mechanism leading to reactive radical formation of polythiophene derivatives by focusing on the differences in the photochemical behaviors of photounstable poly(3-hexylthiophene) (P3HT) and photostable poly(3-octyloxythiophene) (P3OOT). Electron spin resonance measurements revealed that the [P3HT]+· (formed by oxygen doping) decayed, whereas no change was observed in the photostable [P3OOT]+· after light irradiation. Furthermore, the absorption decrease of the [P3HT]+· (P3HT) was suppressed by superoxide dismutase. Therefore, the oxygen dopant is superoxide, which might also initiate the oxidation of P3HT. The photochemical difference between P3HT and P3OOT can be explained by the reaction of an α-proton with superoxide in the case of P3HT.

•We synthesized terminally-modified oligothiophene molecule.•The ortho-biphenyl group promises homogeneous blend film.•Molecular interaction leads to improvement of photovoltaic performance.•Crystalline blend structure decreases carrier trap.To elucidate the optimal bulk heterojunction (BHJ) film for high-performance organic photovoltaics, structural control of BHJ films based on small molecules is one of the great challenges. A homogeneous BHJ structure based on oligothiophene and fullerene has been fabricated by means of terminal modification using the sterically bulky t-butyl group. However, the suppression of interaction among the oligothiophene molecules produces a low fill factor. Molecular design of donor material in order to obtain good fill factor was not clear.Herein, we synthesized oligothiophene with sterically bulky o-biphenyl groups possessing molecular flexibility. We achieved balance between the morphological control of BHJ films and improvement of the fill factor. In this study, we suggest that improvement of fill factor is attributed to increasing hole mobility and crystallinity by ortho-biphenyl group.

The photooxidation of poly(3-hexylthiophene) (P3HT) has previously been investigated and it is known that the sulfur atom of the thiophene ring is oxidized into sulfur oxides by a hydroxyl radical that arises through hydrogen abstraction at the hexyl side chain at the α-position. In this study, in order to further investigate the photooxidation mechanism of the polymer, the side chain at α-position was varied and three different polythiophene derivatives with phenyl (P3PhT), tert-pentyl (P3DMPT), and alkoxy (P3OOT) groups were synthesized with no α-hydrogens. We observed the differences in the photochemical behavior of the polythiophene derivatives by UV–vis–NIR and FT-IR (ATR) spectroscopy during simulated solar light irradiation. It was found that the photooxidation of P3PhT and P3DMPT progressed in the same way as P3HT, whereas the analogous decomposition of P3OOT was not observed.

Organic photovoltaic solar cells based on oligothiophene and fullerene have been investigated intensively. Until now, the morphologies of bulk heterojunction (BHJ) structures based on oligothiophene and fullerene have been difficult to control because the oligothiophene molecules have lacked substituent groups and therefore have interacted strongly with one another to the point of aggregating into microfibril structures.We succeeded in fabricating homogeneous oligothiophene:C60 BHJ films by adding sterically-bulky groups to oligothiophene in order to control the extent of aggregation. We found that the aggregation onset and maximum fill factor occurred at higher oligothiophene concentrations in BHJ films with C60 for the substituted molecules – the ratio that provides as much oligothiophene as possible for good photovoltaic performance but not so much as to incur aggregation-fill factor was improved. Here in, we report the morphologies and photovoltaic performances of single-component oligothiophene films and BHJ films based on oligothiophenes.Graphical abstractHighlights► We examined the oligothiophene added tert-butyl groups in BHJ film. ► We fabricated well-blended BHJ film with oligothiophene volume ratio of 50%. ► The π-stacking interaction in oligothiophene films shows microfibril structure. ► The oligothiophene-rich device performance was obtained by tert-butyl groups.

Multilayered oriented polyfluorene (PF) films were obtained by applying thermal treatment procedure to a multilayered PF film constructed with fluorene derivatives layer formed on top of a highly oriented friction-transferred crystalline poly(9,9-dioctylfluorene) (PF8) film. The orientations in the multilayered PF films were investigated by polarized photoluminescence (PL) spectroscopy and grazing incident X-ray diffraction (GIXD) analysis. The results of the multilayered PF film constructed with spin-coated PF8 on friction-transferred PF8 indicate that the rearrangement of the upper PF8 layer is induced from the orientation of lower PF8 layer by thermal treatment at the nematic phase temperature. Polarized green emission from the multilayered oriented PF film was demonstrated using the blend of PF8 and poly(9,9-dioctylfluorene-co-benzothiadiazol) (F8BT) as green light emitter for upper layer. By this method, the polarized emission color can be tuned using polymer blends for upper layer similar to the liquid-crystalline polymer arrangement without using different materials as an underlying layer such as the rubbed polyimide.

Detailed structures of friction-transferred poly(9,9-dioctylfluorene) (PFO) thin films have been studied using synchrotron-sourced grazing-incidence X-ray diffraction (GIXD). The results of in-plane GIXD were in full agreement with those of electron diffraction in our previous study. We also revealed the structures of liquid-crystalline (LC) and crystalline films that were prepared by rapid and slow cooling from the LC melted state of the friction-transferred films, respectively. For the crystalline films, the results of in-plane GIXD were not in full agreement with those of electron diffraction; additional weak peaks of the odd 00l reflections were observed only in the GIXD profiles. This implies that the twofold screw symmetry along the c-axis of PFO is broken. The rocking profiles for each reflection gave the order parameter of almost one for the crystalline films and 0.67 for the LC films. These results suggest the ability of friction transfer for obtaining single-crystal-like thin films and the importance of carefully designed annealing process for the applications of the films to various optoelectronic devices.

Effects of substituted side-chains of poly(2,5-dialkoxy-1,4-phenylenevinylene) derivatives (PPV) on polymer orientation have been studied using polarized optical spectroscopy and synchrotron-sourced grazing incidence X-ray diffraction (GIXD) analysis. The results of polarized optical spectroscopy indicate that the symmetrically substituted PPV is suitable for homogeneous oriented film preparation by friction-transfer technique. The formed film of poly(2,5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (BEHPPV) exhibited large light emission dichroism of up to 42.0. The results of synchrotron-sourced GIXD for BEHPPV film indicated that the BEHPPV molecules crystallized in not a different manner from that of other PPV.

•We perform Conductive-AFM (C-AFM) and Space Charge Limited Current (SCLC) measurements on the photo-oxidized P3HT and PCBM films.•The conductivity of P3HT decreases upon irradiation, mainly due to a decrease of the hole mobility.•P3HT crystal formation may partially be prevented by the oxidized molecular fragments of reduced molecular weight.Previously, we reported that the photo-oxidation of P3HT:PCBM was shown to induce an increase of the electric resistance at the active layer in the solar cells, resulting in a drop in the device performances. Meanwhile, fragments resulting from the partial molecular scission of P3HT chains were also detected by analyzing the produced materials using MALDI-TOF-MS. However, the way the photo-oxidation of these active materials actually impacts on the electric resistance of the resulting device yet remains to be investigated. In this work, we perform Conductive-AFM (C-AFM) and Space Charge Limited Current (SCLC) measurements on the photo-oxidized P3HT based films; the results indicate that the conductivity of P3HT decreases upon irradiation, mainly due to a decrease of the hole mobility. Even though the number average molecular weight of the P3HT only modestly decreases during the photo-oxidation, UV–vis spectra indicate a significant decrease of the long-range ordering of the P3HT chains. These results inferred that P3HT crystal formation may partially be prevented by the oxidized molecular fragments of reduced molecular weight, resulting in the decrease of P3HT hole mobility, hence leading to the observed resistance increase in these photovoltaic active layers.To understand the effect of the photo-oxidized P3HT:PCBM on the resistance increase of the active layer in the solar cells, we focused on changes of the physical properties and structure of P3HT in photo-oxidized P3HT/PCBM blend films. Conductive-AFM (C-AFM) and Space Charge Limited Current (SCLC) measurements suggest that the conductivity decrease along with irradiation derives from a decrease of P3HT hole mobility. Even though the number average molecular weight of the P3HT is slightly decreased by photo-oxidation, the major reason for the decrease of the electrical properties is attributed to a reduction of the crystallinity of P3HT, compared with that of the non-oxidized polymer, as reflected in the UV-vis spectra. These results indicate that P3HT crystal formation may partially be prevented by the photo-oxidized fragments with low molecular weight, resulting in the decrease of P3HT hole mobility and leading to the resistance increase in the organic films.Download high-res image (335KB)Download full-size image