Fei Zheng

Dual Förster resonance energy transfer effects in non-fullerene ternary organic solar cells with the third component embedded in the donor and acceptor

Non-fullerene ternary organic solar cells (OSCs) are new promising candidates for future applications in the area of organic photovoltaics. However, their low short-circuit current (JSC) values impede efforts at increasing their power conversion efficiency (PCE) levels. Maximizing the JSC is one of the critical elements for enabling high performances of OSCs. To improve the JSC of the non-fullerene ternary OSCs based on poly[[2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b:3,3-b]dithiophene][3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]:3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene (PTB7-Th:ITIC), a polymer of poly[N-9′′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) was added to the binary system. The PCDTBT was shown to embed in both PTB7-Th and ITIC, and introduced dual Forster resonance energy transfer (FRET) effects in the resulting ternary system. In addition, the PCDTBT may have decreased the molecular coherence lengths of PTB7-Th and ITIC and influenced the charge transport. Furthermore, the PCDTBT apparently also decreased the d-spacing of the π–π stacking of PTB7-Th and ITIC, which likely increased the intermolecular charge hopping efficiency. Doping an appropriate amount of PCDTBT in the system yielded an increase in the JSC from 13.89 to 16.71 mA cm−2. The increased JSC resulted in a 15% enhancement of the PCE, which indicated that introducing dual FRET effects is an effective way to enhance the JSC and thus the performance of the OSCs.

Improving the Compatibility of Donor Polymers in Efficient Ternary Organic Solar Cells via Post-Additive Soaking Treatment

In dual-donor ternary organic solar cells, the compatibility between the donor polymers plays important roles to control the conformational change and govern the photophysical behavior in the blend films. Here, we apply a post-additive soaking (PAS) approach to reconstruct the morphology in a ternary organic photovoltaic BHJ of PTB7-Th: PCDTBT: PC71BM. The PAS-treated device has a maximum power conversion efficiency (PCE) of about 8.7% in this ternary system. From the analyses of GIWAXS and GISAXS, the superior device performance is attributed to the favorable nanomorphology with optimum crystallinity of PTB7-Th and good intermixing of PCDTBT with PTB7-Th:PC71BM, leading to improved charge transport in the vertical direction. AFM and TRPL measurements clearly demonstrate PAS-treated film envisages a homogeneous distribution of smaller PC71BM aggregates to facilitate the exciton dissociation and carrier extraction at the interface. The increased PCE ascribed to not only the enhancement of absorption and nonradiative Förster resonance energy transfer (FRET) between two donors (PCDTBT and PTB7-Th) but also the formation of a bicontinuous interpenetrating network of PC71BM.

Charge transfer from poly(3-hexylthiophene) to graphene oxide and reduced graphene oxide

The energy levels of graphene oxide (GO) and reduced graphene oxide (rGO) were estimated in this work. We demonstrate that the energies of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of GO become closer to each other upon reduction. GO and rGO were blended with poly(3-hexylthiophene) (P3HT) to form composite films and their optoelectronic properties were evaluated. It was found that both electrons and holes from photoexcited P3HT can transfer to rGO leading mostly to recombination while only electrons transfer to GO.

The structure and optical properties of regio-regular poly(3-hexylthiophene) and carboxylic multi-walled carbon nanotubes composite films

The introduction of small amounts of carboxylic multi-walled carbon nanotubes into regio-regular poly(3-hexylthiophene) (P3HT) can vary the structure and optical properties of the polymer. The microstructure of the composite film was analysed by various techniques including x-ray diffraction, ultraviolet–visible optical absorption, Raman spectroscopy, and Fourier-transform infrared spectroscopy. It is shown that the presence of carbon nanotubes can improve the crystallinity and increase the conjugation length of P3HT in the films. Furthermore, an obvious photoluminescence quenching is observed in the P3HT/CNTs composite films, suggesting the possibility of photoinduced electron transfer between P3HT and carbon nanotubes due to the π–π interaction of these two components. These results are important for applications in bulk heterojunction solar cells and organic photo-detectors.

Effects of Processing Solvent on the Photophysics and Nanomorphology of Poly(3-butyl-thiophene) Nanowires:PCBM Blends.

We report the effect of the processing solvent on the nanoscale morphology and photophysical dynamics of poly(3-butyl-thiophene) nanowires (P3BT-nw). P3BT-nw assembled in ortho-dichlorobenzene (ODCB) show higher crystallization and a longer conjugation length with increased exciton delocalization compared with those assembled in chlorobenzene (CB). It is proposed that this solvent effect is associated with the higher ordered structures formed from ODCB solution state. Charge-transfer dynamics and phase separation for P3BT-nw:PCBM blends were investigated by ultrafast fluorescence techniques. The more efficient fluorescence quenching observed in P3BT-nw:PCBM blend films processed from ODCB suggests that there is intimate contact between P3BT-nw and PCBM that facilitates charge transfer. The superior performance of organic photovoltaic devices based on P3BT-nw:PCBM bulk heterojunctions processed using ODCB is attributed to the higher crystallization of P3BT-nw, optimized phase separation, and more efficient charge transfer from P3BT-nw to PCBM.

Structural and optical properties of conjugated polymer and carbon-based non-fullerene material blend films for photovoltaic applications

In this work, the structural and optical properties of bulk heterojunctions (BHJ) composed of MEH-PPV and carbon-based non-fullerene nanodiamond (ND) have been investigated. The steady-state absorption, temperature-dependent fluorescence and Raman spectroscopy demonstrate the conjugated length is increased and more ordered crystalline domains are formed in MEH-PPV films with the increment of ND content. The time-resolved fluorescence spectroscopy and 2D microscopy indicate that the BHJs with higher concentrations of ND may enhance the donor/acceptor interfacial contact which favors the dissociation of excitons into free charge carriers. Solar cells based on MEH-PPV:ND BHJs were fabricated to explore the interplay between structural, photophysical properties, and ultimately, device performances. The results show that optimization of blend composition and structures with ND may improve the efficiency of the thin film photovoltaic devices.

Efficient photoinduced charge transfer in chemically-linked organic-metal Ag-P3HT nanocomposites

A novel nanocomposite of P3HT and Ag nanoparticles (Ag-P3HT) has been synthesized by laser ablation. The fluorescence wavelength of the nanocomposite can be tuned by varying the ablation duration. The steady-state emission maximum at 580 nm for pristine P3HT shows significant blueshift to wavelengths ranging from 520 to 550 nm for Ag-P3HT. NMR, FTIR and XPS spectroscopy confirm that chemical links are formed between P3HT and silver nanoparticles (NPs) in the nanocomposite. For the sample with an emission maximum at 530 nm, 1H NMR spectra indicate that 66% of the P3HT thiophene ring protons are replaced by Ag NPs. Time-resolved spectroscopy demonstrates that charge transfer efficiency increases for Ag-P3HT and this is attributed to the enhanced intimate interfacial contact between the chemically-bonded metal NPs and polymer chains. The synthetic route outlined provides a one-pot and green strategy for producing metal-organic polymeric materials, with controlled luminescence wavelengths and efficient photoinduced charge transfer that meet the requirement for developing high-performance organic light-emitting and photovoltaic devices.

Impact of solvent additive on exciton dissociation in P3HT : EP-PDI blend film via controlling morphology

Suitable solvent additives provide an effective means to control the morphology of polymer blend films. In this work, we systematically investigate the impact of the solvent additive chloronaphthalene (CN) on the morphology of P3HT : EP-PDI blends. The optimum volume fraction of solvent additive CN was found to be 0.5 vol% by atom force microscopy and transmission electron microscopy. UV–visible absorption spectroscopy and grazing incidence x-ray diffraction indicate that the crystallinity of both the P3HT and EP-PDI domains significantly decreased in the blends with 0.5 vol% CN. Grazing incidence small-angle x-ray scattering results show that the size of the small EP-PDI aggregation decreases from 44–22 nm with the addition of CN. Time-resolved photoluminescence measurement reveals that the decreased EP-PDI domains give rise to increased donor–acceptor interfacial areas, which not only facilitate the exciton dissociation, but suppresses the formation of the EP-PDI intermolecular state.

Improved compatibility of DDAB-functionalized graphene oxide with a conjugated polymer by isocyanate treatment

2-Chlorophenyl isocyanate (CI) reacts with didodecyl dimethyl ammonium bromide (DDAB) functionalized graphene oxide (DDAB-GO) dispersed in ortho-dichlorobenzene under mild conditions. The CI treatment complements DDAB functionalization to further decrease the hydrophilicity of pristine GO sheets. The resulting CI–DDAB-GO exhibits improved compatibility with the conjugated polymer poly(3-hexylthiophene) (P3HT), compared to DDAB-GO. CI–DDAB-GO sheets can be homogeneously dispersed when blended with P3HT, resulting in an improved morphology compared to P3HT:DDAB-GO composites. The incorporation of CI–DDAB-GO can effectively reduce the dark current of photo-diodes based on P3HT composites, showing potential to enhance the performance of electronic devices based on conjugated polymer composites.

Femtosecond laser processing induced low loss waveguides in multicomponent glasses

We have investigated that femtosecond laser induced optical waveguides in AF32 and in Borofloat-33 glasses. Type-I positive and type-II negative refractive index change mechanisms were established in AF32 and in Borofloat-33 glass waveguides, respectively. Lowest propagation loss of 1.1 ± 0.31 dB/cm could be attained in AF32 glass, which has a typically higher index change than in Borofloat-33 glass. Resultant losses are directly correlated with densification and non-bridging oxygen hole centers, which can be authenticated by Raman and photoluminescence studies.