Yanlian Lei

Graphene quantum-dot-doped polypyrrole counter electrode for high-performance dye-sensitized solar cells.

Herein graphene quantum dot (GQD), a graphene material with lateral dimension less than 100 nm, is explored to dope PPy on F-doped tin oxide glass as an efficient counter electrode for high-performance dye-sensitized solar cells (DSSCs). The GQDs-doped PPy film has a porous structure in comparison to the densely structured plain PPy, and displays higher catalytic current density and lower charge transfer resistance than the latter toward I3(-)/I(-) redox reaction. The highest power conversion efficiency (5.27%) for DSSCs is achieved with PPy doped with10% GQDs, which is comparable to that of Pt counter electrode-based DSSCs. This work provides an inexpensive alternative to replace platinum for DSSCs.

Enhancing Crystalline Structural Orders of Polymer Semiconductors for Efficient Charge Transport via Polymer‐Matrix‐Mediated Molecular Self‐Assembly

A facile polymer-matrix-mediated molecular self-assembly of polymer semiconductors into highly crystalline orders for efficient charge transport in organic thin-film transistors is demonstrated. Phenomenal enhancements in field-effect mobility of about one order of magnitude and current on/off ratio of two to three orders of magnitude are realized with polyacrylonitrile-incorporated polymer semiconductor compositions via solution deposition.

Solution-processed donor-acceptor polymer nanowire network semiconductors for high-performance field-effect transistors

Organic field-effect transistors (OFETs) represent a low-cost transistor technology for creating next-generation large-area, flexible and ultra-low-cost electronics. Conjugated electron donor-acceptor (D-A) polymers have surfaced as ideal channel semiconductor candidates for OFETs. However, high-molecular weight (MW) D-A polymer semiconductors, which offer high field-effect mobility, generally suffer from processing complications due to limited solubility. Conversely, the readily soluble, low-MW D-A polymers give low mobility. We report herein a facile solution process which transformed a lower-MW, low-mobility diketopyrrolopyrrole-dithienylthieno[3,2-b]thiophene (I) into a high crystalline order and high-mobility semiconductor for OFETs applications. The process involved solution fabrication of a channel semiconductor film from a lower-MW (I) and polystyrene blends. With the help of cooperative shifting motion of polystyrene chain segments, (I) readily self-assembled and crystallized out in the polystyrene matrix as an interpenetrating, nanowire semiconductor network, providing significantly enhanced mobility (over 8 cm2V−1s−1), on/off ratio (107), and other desirable field-effect properties that meet impactful OFET application requirements.

Identifying the roles of the excited states on the magnetoconductance in tris-(8-hydroxyquinolinato) aluminum

Magnetoconductance response (MC) in illuminated tris-(8-hydroxyquinolinato) aluminum (Alq3) single layer has been studied to clarify the role of excitons and electron-hole (e-h) pairs on the MC generation. By fitting the MC curves at different bias conditions, the contribution of e-h pairs and excitons to the MC can be differentiated in the same material. The fitting results indicated that hyperfine mixing between the singlet and triplet e-h pairs results in a Lorentzian type MC within hyperfine field and charge reaction of triplet excitons induces a high-field MC following a non-Lorentzian shape, respectively. Moreover, the characteristic field width for the high-field MC is very different at forward and reverse bias regime, implying that the rate strength for the hole/electron-exciton reactions in Alq3 should be treated separately.

Large magneto-conductance and magneto-electroluminescence in exciplex-based organic light-emitting diodes at room temperature

In this work, we report on large magneto-conductance (MC) over 60% and magneto-electroluminescence (MEL) as high as 112% at room temperature in an exciplex-based organic light-emitting diode (OLED) with efficient reverse intersystem crossing (ISC). The large MC and MEL are individually confirmed by the current density-voltage characteristics and the electroluminescence spectra under various magnetic fields. We proposed that this type of magnetic field effect (MFE) is governed by the field-modulated reverse ISC between the singlet and triplet exciplex. The temperature-dependent MFEs reveal that the small activation energy of reverse ISC accounts for the large MFEs in the present exciplex-based OLEDs.

Engineering gate dielectric surface properties for enhanced polymer field-effect transistor performance

The modification of silicon dioxide (SiO2) gate dielectrics with silane self-assembled monolayers (SAMs) via silylation was conducted to study their impacts on polymer field-effect transistor (FET) performance. SAMs formed from silylating agents with long alkyl chains such as octadecyl gave high field-effect mobility but a lower on/off ratio because of lower SAM coverage of the gate dielectric surface. In contrast, SAMs from silylating agents with phenyl or medium alkyl chains (octyl) provided a high on/off ratio from high SAM surface coverage but lower mobility due to their inefficiency in promoting molecular ordering of the channel semiconductor. By treating the SiO2 dielectric surface with two silylating agents, one with an octadecyl chain and one with an octyl or phenyl chain, in a proper sequence, a high-performance “hybrid” dual-silane SAM could be created, enabling attainment of both a high mobility and on/off ratio, together with other desirable FET properties.

Photocurrent generation through electron–exciton interaction at the organic semiconductor donor/acceptor interface

In this work, we report our effort to understand the photocurrent generation that is contributed via electron-exciton interaction at the donor/acceptor interface in organic solar cells (OSCs). Donor/acceptor bi-layer heterojunction OSCs, of the indium tin oxide/copper phthalocyanine (CuPc)/fullerene (C60)/molybdenum oxide/Al type, were employed to study the mechanism of photocurrent generation due to the electron-exciton interaction, where CuPc and C60 are the donor and the acceptor, respectively. It is shown that the electron-exciton interaction and the exciton dissociation processes co-exist at the CuPc/C60 interface in OSCs. Compared to conventional donor/acceptor bi-layer OSCs, the cells with the above configuration enable holes to be extracted at the C60 side while electrons can be collected at the CuPc side, resulting in a photocurrent in the reverse direction. The photocurrent thus observed is contributed to primarily by the charge carriers that are generated by the electron-exciton interaction at the CuPc/C60 interface, while charges derived from the exciton dissociation process also exist at the same interface. The mechanism of photocurrent generation due to electron-exciton interaction in the OSCs is further investigated, and it is manifested by the transient photovoltage characteristics and the external quantum efficiency measurements.

Synthesis and properties of benzo[c]-, pyrrolo[3,4-c]-, and thieno[3,4-c]-pyrrole-4,6-dione copolymers

New π-conjugated alternating donor–acceptor polymers, BPD–DTT and PPD–DTT, based respectively on benzo[c]pyrrole-4,6-dione (BPD) and pyrrolo[3,4-c]pyrrole-4,6-dione (PPD) acceptor structures with the 2,5-dithienylthieno[3,2-b]thiophene (DTT) donor structure were synthesized via Stille cross-coupling polymerization to study their physical and optoelectronic properties. For comparison, an analogous polymer, TPD–DTT, derived from thieno[3,4-c]pyrrole-4,6-dione (TPD) was also prepared by the same approach. These polymers displayed reasonably good solubility in common organic solvents (e.g., THF, toluene, etc.), and were thermally stable to about 350 °C in nitrogen or air. Both BPD–DTT and PPD–DTT showed blue shifts in their UV-vis absorptions and photoluminescence compared with those of TPD–DTT, suggesting that both possess lower degrees of π-delocalization than TPD–DTT, and thus shorter effective π-conjugation lengths and higher air stability as evidenced from their lower HOMO energy levels. Optical band gaps of 1.61 eV, 1.82 eV, and 1.73 eV and HOMO levels of −5.06 eV, −5.32 eV, and −5.20 eV were estimated for TPD–DTT, BPD–DTT and PPD–DTT respectively from spectral and electrochemical measurements. Electrical characterization showed that all three polymers behaved predominantly as p-type semiconductors in organic field-effect transistor and bulk-heterojunction organic solar cell devices.

Magnetic field dependence of photocurrent in thermally evaporated rubrene-based devices

The magnetic field response of photocurrent in thermally evaporated rubrene-based single layer devices was investigated, with the results revealing a change of sign in magneto-photocurrent (MPC) with a change of bias. Specifically, under forward bias, there was a positive MPC for |B| > 50 mT, while otherwise there was a negative MPC at smaller fields, forming a “W” shape at low magnetic fields. In contrast, the sign of the MPC was completely changed under reverse bias, forming an “M” shape at low magnetic fields (|B| 50 mT, while otherwise there was a negative MPC at smaller fields, forming a “W” shape at low magnetic fields. In contrast, the sign of the MPC was completely changed under reverse bias, forming an “M” shape at low magnetic fields (|B| < 50 mT). Our study suggests that the observed MPC was not only related to the singlet fission (SF) process itself but also strongly dependent on the subsequent decay routes of triplets in rubrene. The triplet-charge reaction and triplet dissociation superimposed on SF under magnetic fields were proposed to account for the different MPC under forward and reverse bias, respectively. Moreover, the magnitude of both the positive and negative MPC could be promoted by avoiding the triplet loss cha...

Influence of exction recombination zone movement with the changing temperature on the magnetic field effect in OLED

Organic electroluminescence devices, with the structure of ITO/CuPc/NPB/Alq 3 (emitting green light)/Alq 3 :DCM(emitting red light)/LiF/Al(double emitting layers device) and single light-emitting layer reference device, have been fabricated in this paper. The magnetic field effects on electroluminescence and current (MEL and MC) are studied under different temperatures. The results show that the high field effect (B 0 mT) of MEL in double emitting layers device decreases first, then turns to increase with decreasing temperature. In contrast, the single light-emitting layer device increases monotonically, at the same injection currents. Moreover, the two spectral peaks at 533 nm and 600 nm in double emitting layers device coming from DCM and Alq 3 respectively, show one falling and the other rising with decreasing temperature, which indicates the exction recombination region shifting with changing temperature. The nonmonotonic changes in the high field effect of the MEL are discussed, and in terms of the triplet-triplet interaction and carrier mobility change with the working temperature.