Wei-Long Xu

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.

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.

Morphology, photophysics and optoelectronics of P3HT nanoparticles and TiO2 nanorods composite

Morphology and photophysics of polymers are critical to the performance of organic optoelectronics. In this work, poly(3-thiophene) (P3HT) nanoparticles were successfully fabricated by the miniemulsion and reprecipitation methods. The P3HT nanoparticles demonstrated uniform distribution with the domain size of ∼40 nm. The photophysics of P3HT nanoparticles was investigated by absorption and steady-state and time-resolved PL spectroscopy. P3HT nanoparticles showed more ordered and longer conjugation length than pristine P3HT. At the same time, P3HT nanoparticles showed aggregate species which are favorable for interchain charge transfer. The organic/inorganic hybrid photodetectors based on P3HT and TiO2 nanorods were fabricated. The superior performance of the photodetector based on P3HT nanoparticle and TiO2 nanorods comes from the efficient charge transfer and large donor/acceptor interface.

New design graded refractive index antireflection coatings for silicon solar cells

Reflectance spectrum of nanoporous silicon dioxide (SiO2) double layer was calculated by using the matrix method. The results were compared with the corresponding spectrum of silicon oxynitride (SiOxNy)–porous silicon (PS) double layer which deposited on nanostructured black silicon coatings. The nanoporous silicon dioxide (SiO2) double layer deposited on nanostructure black silicon antireflection coating presents a lower reflectance in a broad range of solar spectrum. This research outcome may find a wide application in solar cell industry.

Performance Enhancement in Polymer-Based Organic Optoelectronic Devices Enabled By Discontinuous Metal Interlayer

To realize substantial performance enhancement in the poly (3-hexylthiophene)/[6, 6]-pheny-C71-butyric acid methyl ester (P3HT/PC71BM)-based organic optoelectronic devices, a discontinuous layer consisting of nickel nanoparticles (Ni NPs) was introduced to embellish the indium tin oxide electrode. When the Ni layer with a thickness of 3 nm was introduced, photocurrent gain was enhanced nearly 300% at low bias without sacrificing any response speed in the obtained organic photodetectors, and power conversion efficiency of organic solar cells increased nearly by 30%. The improved performance of the polymer photodetectors was mainly attributed to the injection of secondary electrons due to hole trap states introduced by the poor interfacial contact between the active layer and the Ni layer. On the other hand, the incident light was scattered over a longer propagation path by Ni NPs in the active layer, and therefore, the performance of polymer solar cells was enhanced.