Laju Bu

Monodisperse Co-oligomer Approach toward Nanostructured Films with Alternating Donor-Acceptor Lamellae

A series of donor-acceptor (D-A) co-oligomers with oligo(fluorene-alt-bithiophene) and perylene diimide as donor and acceptor segments, respectively, have been designed and synthesized. They can self-assembly into alternating D-A lamellar nanostructured films with the periods depending on the molecular length. These films have been successfully used in fabrication of high-performance single-molecular solar cells with power conversion efficiency up to 1.50%.

The Role of Additive in Diketopyrrolopyrrole‐Based Small Molecular Bulk Heterojunction Solar Cells

A new diketopyrrolopyrrole-based small molecule for solution-processed OPVs is synthesized. The chemical additive has a profound effect in refining the morphology and, thus, significantly increases the efficiencies of the devices. The role of the additive is characterized by various techniques and the additive-driven structure-property relationship is established, which reveals that the additive affects the crystallization, PCBM aggregation, and phase separation.

Green light-emitting polyfluorenes with improved color purity incorporated with 4,7-diphenyl-2,1,3-benzothiadiazole moieties

By incorporating 4,7-diphenyl-2,1,3-benzothiadiazole instead of 2,1,3-benzothiadiazole into the backbone of polyfluorene, we developed a novel series of green light-emitting polymers with much improved color purity. Compared with the state-of-the-art green light-emitting polymer, poly(fluorene-co-benzothiadiazole) (λmax = 537 nm), the resulting polymers (λmax = 521 nm) showed 10–20 nm blueshifted electroluminescence (EL) spectra and greatly improved color purity because the insertion of two phenylene units between the 2,1,3-benzothiadiazole unit and the fluorene unit reduced the effective conjugation length in the vicinity of the 2,1,3-benzothiadiazole unit. As a result, the resulting polymers emitted pure green light with CIE coordinates of (0.29, 0.63), which are very close to (0.26, 0.65) of standard green emission demanded by the National Television System Committee (NTSC). Moreover, the insertion of the phenylene unit did not affect the photoluminescence (PL) and EL efficiencies of the resulting polymers. PL quantum efficiency in solid films up to 0.82 was demonstrated. Single-layer devices (ITO/PEDOT/polymer/Ca/Al) of these polymers exhibited a turn-on voltage of 4.2 V, luminous efficiency of 5.96 cd A−1 and power efficiency of 2.21 lm W−1. High EL efficiencies and good color purities made these polymers very promising for display applications.

Bulk Interpenetration Network of Thermoelectric Polymer in Insulating Supporting Matrix

Thermoelectric properties of conjugated polymers are found to improve upon homogeneously distributing conjugated polymer into an insulating supporting matrix. The local one-dimensional charge transport along the interpenetration conductive network simultaneously leads to lower thermal conductivity, higher electrical conductivity without sacrifice of Seebeck coefficient, and thus a higher figure of merit ZT, as compared with neat conjugated polymer.

Novel spiro-fluorenes from tandem radical addition for liquid crystalline monodisperse conjugated oligomers

3′-Nonafluorobutylmethyl-4′-methyl-spiro[cyclopentyl-9,1′]fluorenes were successfully synthesized via tandem radical-addition reactions between 9,9-diallylfluorenes and perfluorobutyl iodide in the presence of a radical initiator followed by reduction under mild conditions. Single crystal analysis indicates that two substituents at 3,4-positions of cyclopentane are in a maleinoid form. Accordingly, four oligo(fluorene-co-bithiophene)s with the same molecular length of ∼10 nm (7 fluorene units and 12 thiophene units) containing one to three novel spiro-fluorene units were synthesized. The introduction of the spiro-fluorene units results in noticeable enhancement of both glass transition temperature (Tg) and clearing point temperature (Tc) of the oligomers, but has little effect on their photophysical properties. Exchanging three 9,9-dioctylfluorene units with 3′-nonafluorobutylmethyl-4′-methyl-spiro[cyclopentyl-9,1′] fluorene units results in a 37 °C enhancement of Tg and a 61 °C enhancement of Tc. All these results indicate that this new spiro-fluorene unit is an attractive building block for liquid crystalline conjugated polymers/oligomers with both high Tg and Tc.

Organic-semiconductor: Polymer-electret blends for high-performance transistors

As compared with polymer semiconductors, solution-processed small-molecule semiconductors usually have poorer film-formation properties, which induces wide variations in device performance in terms of mobility and threshold voltage, thus severely limiting their commercial applications. In this work, we propose an easily accessible method to improve the performance and reduce the variability of small-molecule organic field-effect transistors (OFETs) by blending organic semiconductors with an insulator polymer, which is subsequently post-treated by gate stress to generate an electret. By blending the organic semiconductor 2,7-didodecyl[1]benzothieno[3,2-b][1]benzothiophene (C12-BTBT) with the insulator polystyrene, uniform transport layers with vertically phase segregated morphology are obtained, from which the mobility and threshold voltage of OFETs are largely manipulated. The OFETs exhibit field-effect mobilities as high as 7.5 cm2·V−1·s−1 with an on/off ratio exceeding 106 in ambient conditions. This double-layer structure provides an appropriate architecture for applying gate-stress to inject charges into the insulating layer, forming an electret. The generation of the electret is thermally accelerated and thus can be easily realized under moderate gate-stress at elevated temperature (e.g., 60 °C). After cooling, the electret layer serves as a floating-gate, which not only continuously tunes the threshold voltage and field-effect mobility, but also helps minimize the contact resistances and optimize the subthreshold swing. As an application of this method, a digital inverter is built and its performance is optimized via in situ tuning of its individual transistors.

Reconstructing Space- and Energy-Dependent Exciton Generation in Solution-Processed Inverted Organic Solar Cells

Photon absorption-induced exciton generation plays an important role in determining the photovoltaic properties of donor/acceptor organic solar cells with an inverted architecture. However, the reconstruction of light harvesting and thus exciton generation at different locations within organic inverted device are still not well resolved. Here, we investigate the film depth-dependent light absorption spectra in a small molecule donor/acceptor film. Including depth-dependent spectra into an optical transfer matrix method allows us to reconstruct both film depth- and energy-dependent exciton generation profiles, using which short-circuit current and external quantum efficiency of the inverted device are simulated and compared with the experimental measurements. The film depth-dependent spectroscopy, from which we are able to simultaneously reconstruct light harvesting profile, depth-dependent composition distribution, and vertical energy level variations, provides insights into photovoltaic process. In combination with appropriate material processing methods and device architecture, the method proposed in this work will help optimizing film depth-dependent optical/electronic properties for high-performance solar cells.

Field-effect Charge Transport in Doped Polymer Semiconductor-Insulator Alternating Bulk Junctions with Ultrathin Transport Layers

Conjugated-polymer field-effect transistors are attractive for flexible electronics. However, relatively high chemical doping (oxidation) concentration of p-type polymer semiconductors is usually not compatible with good transistor performance, due to poor switching-off capability and short-channel performance. Here, we propose a combined simulation and experimental investigation on charge transport in a semiconductor-insulator alternating bulk junction composed of repeating semiconductor and insulator regions, which shows better transistor performance at higher doping levels, as compared with traditional planar transistors. Moreover, the doped semiconductor transport layers in the junction should be less than 2 nm thick to ensure sufficient pinch-off capability. Using some semiconductors including poly(3-hexylthiophene), we utilize a fast solvent evaporation approach to obtain semiconductor-insulator alternating bulk junctions with ultrathin (thickness < 2 nm) semiconductor crystallites and with vertical gradients of both morphology and electronic properties. Doping with a concentration of up to 1019 cm-3 simultaneously induces the improvement of field-effect mobility, on/off ratio, and subthreshold swing, which leads to long-term (>1 year) stability, without lowering the short-channel performance. Moreover, these heterojunctions are optically transparent, nearly colorless, and flexible, thus could be exploited for wide electronic and photonic applications.

Gate-voltage-dependent charge transport in multi-dispersed polymer thin films

In semiconductor polymers, charge transport usually occurs via hopping between localized states, which are generally multi-dispersed due to multi-dispersed chemical structures, crystallinities, and phase segregations. We report a combined modeling and experimental study to investigate gate-voltage-dependent charge transport in field-effect transistors based on multi-dispersed polymers including semiconductor:semiconductor and semiconductor:insulator blends. Film-depth-dependent charge accumulation and transport are correlated with vertical composition profiles and film-depth-dependent energetic distribution of localized states. Even low gate-voltage could accumulate charges in any depth of the films, greatly increasing charge density in some (sub-) components for effective charge transport. Therefore, neither overall high crystallinity nor molecular ordering near the semiconductor-dielectric interface is necessarily required for high field-effect mobility (μFET). This study not only proposes a model for h...

Probing film-depth-related light harvesting in polymer solar cells via plasma etching

Light harvesting is the first step of photovoltaic process in polymer solar cells. However, such donor: acceptor bulk junction layers are usually featured with vertical phase segregation as well as film-depth-dependent molecular aggregation, chain orientation and crystallinity, leading to a significant variation of photon absorption and exciton generation at different film-depths. We propose an experimentally and numerically accessible method to investigate the depth-dependent light harvesting behaviors in the active layer in polymer solar cells. A low-pressure oxygen plasma is utilized to etch the active layer gradually which is monitored by a light absorption spectrometer. Including the obtained sublayer absorption spectra into transfer matrix optical model yields depth-dependent optical properties and exciton generation profiles, which contribute to quantum efficiency and short-circuit current. This approach is helpful to optimize vertical material variation and provide insights into photovoltaic process.