Jianli Hua

Influence of the Donor Size in D−π–A Organic Dyes for Dye-Sensitized Solar Cells

We report two new molecularly engineered push-pull dyes, i.e., YA421 and YA422, based on substituted quinoxaline as a π-conjugating linker and bulky-indoline moiety as donor and compared with reported IQ4 dye. Benefitting from increased steric hindrance with the introduction of bis(2,4-dihexyloxy)benzene substitution on the quinoxaline, the electron recombination between redox electrolyte and the TiO2 surface is reduced, especially in redox electrolyte employing Co(II/III) complexes as redox shuttles. It was found that the open circuit photovoltages of IQ4, YA421, and YA422 devices with cobalt-based electrolyte are higher than those with iodide/triiodide electrolyte by 34, 62, and 135 mV, respectively. Moreover, the cells employing graphene nanoplatelets on top of gold spattered film as a counter electrode (CE) show lower charge-transfer resistance compared to platinum as a CE. Consequently, YA422 devices deliver the best power conversion efficiency due to higher fill factor, reaching 10.65% at AM 1.5 simulated sunlight. Electrochemical impedance spectroscopy and transient absorption spectroscopy analysis were performed to understand the electrolyte influence on the device performances with different counter electrode materials and donor structures of donor-π-acceptor dyes. Laser flash photolysis experiments indicate that even though the dye regeneration of YA422 is slower than that of the other two dyes, the slower back electron transfer of YA422 contributes to the higher device performance.

Colorimetric and ratiometric red fluorescent chemosensor for fluoride ion based on diketopyrrolopyrrole.

Three new diketopyrrolopyrrole (DPP) compounds are shown to be colorimetric and ratiometric red fluorescent sensors for fluoride anions with high sensitivity and selectivity. The recognition mechanism is attributed to the intermolecular proton transfer between a hydrogen atom on the lactam N positions of the DPP moiety and the fluoride anion.

New starburst sensitizer with carbazole antennas for efficient and stable dye-sensitized solar cells

A new starburst organic sensitizer (I) with carbazole units as antennas and the corresponding dyes (II and III) with starburst triphenylamine or one triphenylamine moiety (for the purpose of comparison) were designed and synthesized, in which carbazole or triphenylamine moieties were used as the electron donor, thiophene units as the π-conjugated bridge, and a cyanoacrylic acid group as the electron acceptor. The photophysical and electrochemical properties of the dyes were investigated by UV-vis spectrometry and cyclic voltammetry. Electrochemical measurement data indicate that the tuning of the HOMO and LUMO energy levels can be conveniently accomplished by alternating the donor moiety. Photovoltaic devices with carbazole dye I showed a maximum monochromatic incident photon to current efficiency (IPCE) of 89% and an overall conversion efficiency of 4.4% under full sunlight (AM 1.5G, 100 mW cm−2) irradiation. The dyes were also tested in quasi-solid-state devices and showed good efficiencies. The stability of devices was performed over 1200 h in full sunlight and at 50 °C. The sensitizer I with carbazole antennas exhibited good stability, retaining 95% of its initial efficiency, while dyes II and III retained 82% and 46% after 1200 h of irradiation, respectively. This result means that carbazole is a photostable hole-transporting moiety for use in dye-sensitized solar cells.

Efficient CdSe quantum dot-sensitized solar cells prepared by a postsynthesis assembly approach

A postsynthesis assembly approach, an ex situ ligand exchange route, was developed for fast (within 2 h) and high loading (34% coverage) deposition of CdSe QDs on TiO(2) films. With the combination of high-quality QD sensitizers and the effective deposition technique, a record photovoltaic performance with an efficiency of 5.4% was observed for the resulting cell device.

Stable Dyes Containing Double Acceptors without COOH as Anchors for Highly Efficient Dye-Sensitized Solar Cells†

The electron acceptor 2-(1,1-dicyanomethylene) rhodanine is a promising alternative to cyanoacrylic acid as an anchoring group for organic dyes. For example, the RD-II-based dye-sensitized solar cell has an overall conversion efficiency of 7.11 % and long-term stability.

Bithiazole-bridged dyes for dye-sensitized solar cells with high open circuit voltage performance

Five new metal-free organic dyes (T1–T5) containing bithiazole moieties were synthesized and used for dye-sensitized solar cells (DSSCs). Their absorption spectra, electrochemical and photovoltaic properties were fully characterized. Electrochemical measurement data indicate that the tuning of the HOMO and LUMO energy levels can be conveniently accomplished by alternating the donor moiety. All of these dyes performed as sensitizers for the DSSC test, and the photovoltaic performance data of these bithiazole-bridged dyes showed higher open circuit voltages (745–810 mV). Among the five dyes, T1 showed the best photovoltaic performance: a maximum monochromatic incident photon-to-current conversion efficiency (IPCE) of 83.8%, a short-circuit photocurrent density (Jsc) of 11.78 mA cm−2, an open-circuit photovoltage (Voc) of 810 mV, and a fill factor (ff) of 0.60, corresponding to an overall conversion efficiency of 5.73% under standard global AM 1.5 solar light condition, which reached 93% with respect to that of an N719-based device fabricated under similar conditions. The result shows that the metal-free dyes based on bithiazole π-conjugation are promising candidates for improvement of the performance of DSSCs.

Starburst triphenylamine-based cyanine dye for efficient quasi-solid-state dye-sensitized solar cells

A new starburst triphenylamine-based unsymmetrical organic cyanine sensitizer I and the corresponding cyanine dye II containing one triphenylamine unit for the purpose of comparison were designed and synthesized. They were successfully applied in quasi-solid-state dye-sensitized solar cells (DSSCs). The absorption spectra, electrochemical and photovoltaic properties of I and II were extensively investigated. It was found that the HOMO and LUMO energy levels tuning can be conveniently accomplished by alternating the donor moiety, which was confirmed by electrochemical measurements. The quasi-solid-state DSSCs based on the dye I showed the better photovoltaic performance: a maximum monochromatic incident photon-to-current conversion efficiency (IPCE) of 83%, a short-circuit photocurrent density (Jsc) of 9.12 mA cm−2, an open-circuit photovoltage (Voc) of 0.54 V, and a fill factor (ff) of 0.64, corresponding to an overall conversion efficiency of 3.19% under standard global AM 1.5 solar conditions. This work suggests that the cyanine dyes based on the starburst triarylamine donor are promising candidates for improvement of the performance of the quasi-solid-state DSSCs.

Series of new D-A-π-A organic broadly absorbing sensitizers containing isoindigo unit for highly efficient dye-sensitized solar cells.

In this work, six new D-A-π-A sensitizers (ID1-ID6), with triarylamine as the electron donor; isoindigo as a auxiliary electron withdrawing unit; thiophene, furan, and benzene as the linker; and cyanoacrylic acid as the anchoring group, were synthesized through simple synthetic procedures and with low cost. Their absorption spectra were broad with long wavelength absorption maximum approximately at 589 nm and the absorption onset at 720 nm on the TiO(2) film. Electrochemical experiments indicate that the HOMO and LUMO energy levels can be conveniently tuned by alternating the donor moiety and the linker. All of these dyes performed as sensitizers for the DSSCs test under AM 1.5 similar experimental conditions, and a maximum overall conversion efficiency of 5.98% (J(sc) = 14.77 mA cm(-2), V(oc) = 644 mV, ff = 0.63) is obtained for ID6-based DSSCs when TiO(2) films were first immersed for 6 h in 20 mM CDCA ethanol solution followed by 12 h of dipping in the dye CH(2)Cl(2) solution. Electrochemical impedance measurement data implies that the electron lifetime can be increased by coadsorption of CDCA, which leads to a lower rate of charge recombination and thus improved V(oc).

Benzotriazole-bridged sensitizers containing a furan moiety for dye-sensitized solar cells with high open-circuit voltage performance.

Two new benzotriazole-bridged sensitizers are designed and synthesized (BTA-I and BTA-II) containing a furan moiety for dye-sensitized solar cells (DSSCs). Two corresponding dyes (BTA-III and BTA-IV) with a thiophene spacer were also synthesized for comparison. All of these dyes performed as sensitizers for DSSCs, and the photovoltaic performance data of these benzotriazole-bridged dyes showed a high open-circuit voltage (V(oc): 804-834 mV). Among the four dyes, DSSCs based on BTA-II, with a furan moiety and branched alkyl chain, showed the highest V(oc) (834 mV), a photocurrent density (J(sc)) of 12.64 mA cm(-2), and a fill factor (FF) of 0.64, corresponding to an overall conversion efficiency (η) of 6.72%. Most importantly, long-term stability of the BTA-I-IV-based DSSCs with ionic-liquid electrolytes under 1000 h light-soaking was demonstrated, and BTA-II exhibited better photovoltaic performance of up to 5.06% power conversion efficiency.

New pyrido[3,4-b]pyrazine-based sensitizers for efficient and stable dye-sensitized solar cells

A series of new pyrido[3,4-b]pyrazine-based organic sensitizers (PP-I and APP-I–IV) containing different donors and π-spacers have been synthesized and employed in dye-sensitized solar cells (DSSCs). The absorption spectra properties of dyes are analysed by density functional theory (DFT). The calculated results in combination with the experiments suggest that the absorption characteristics and excited state features will mainly be dominated by charge transfer transitions from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) and to higher LUMO orbitals. Furthermore, attaching the octyloxy groups significantly extends the π-conjugation of the donor in APP-IV, which raises the HOMO energy and facilitates its oxidation. As a consequence, APP-IV exhibits the lowest HOMO–LUMO energy gap among all dyes, which, in turn, corresponds well with the red shift of the absorption spectra. Transient photovoltage and photocurrent decay experiments as well as electrochemical impedance spectroscopy indicate that the electron lifetime and charge recombination resistance are increased due to the introduction of octyloxy chains on the donor unit, resulting in the high photovoltage based on APP-IV. It was found that APP-IV based DSSCs with liquid electrolyte display the highest power conversion efficiency (PCE) of 7.12%. Importantly, a PCE of 6.20% has been achieved for APP-IV based DSSCs with ionic-liquid electrolytes and retained 97% of the initial value after continuous light soaking for 1000 h at 60 °C. This renders these pyrido[3,4-b]pyrazine-based sensitizers quite promising candidates for highly efficient and stable DSSCs.