Two perylene dyes characteristic of electron-donors phenanthrocarbazole (PC) and carbazyl functionalized PC are selected to study the complicated dynamics of excited states and charge carriers, which underlie the photovoltaic parameters of dye-sensitized solar cells (DSCs). We have combined femtosecond fluorescence up-conversion and time-resolved single-photon counting techniques to probe the wavelength-dependent photoluminescence dynamics of dye molecules not only dissolved in THF but also grafted on the surface of oxide nanoparticles. Excited state relaxation and electron injection both occur on a similar timescale, resulting in a very distributive kinetics of electron injection. It is also found that the carbazyl ancillary electron-donor causes a faster electron injection, which over-compensates the adverse impact of a slightly shorter lifetime of the equilibrium excited state. Nanosecond transient absorption and transient photovoltage decay measurements have shown that conjugating carbazyl to PC can effectively slow down the kinetics of charge recombination of electrons in titania with both photo-oxidized dye molecules and triiodide anions, improving the cell photovoltage.

Molecular engineering of a photosensitizing dye plays a vital role in the control of excited states and charge carriers in dye-sensitized solar cells (DSCs) for performance improvement. Herein we report three metal-free donor–acceptor organic dyes by conjugating electron donors triphenylamine–cyclopentadithiophene (TPA–CPDT), triphenylamine–phenanthrocarbazole (TPA–PC), and di(biphenyl)amine–phenanthrocarbazole (DBPA–PC) with electron acceptor 4-(7-ethynylbenzo[c][1,2,5]thiadiazol-4-yl)benzoic acid (EBTBA). The influences of electron donors on the energy levels, light absorption, dynamics of excited states and photogenerated charges, and photovoltaic parameters of DSCs were systematically analyzed via theoretical calculations and physical measurements. A DSC with dye SC-3 employing the bulky DBPA–PC electron donor achieved a high power conversion efficiency (PCE) of 11.5% measured under the air mass 1.5 global (AM1.5G) conditions, owing to the significantly reduced interfacial charge recombination. Femtosecond fluorescence upconversion measurements have unraveled that energy relaxation and electron injection both occur for dye molecules in the nonrelaxed hot excited states. Also, the rate constants of injecting electrons from dyes in the excited states to titania are very dispersive by over 1 order of magnitude, which could be mainly ascribed to the broad energy distribution of excited states.

Apart from the harvesting of more infrared solar photons for a higher photocurrent, improving the photovoltage and thermal stability of a dye-sensitized solar cell are the other two key challenging issues for its performance enhancement. Herein we report a metal-free donor–acceptor dye (SC-4) characteristic of a triphenylamine-dihexylbithiophene electron donor and a benzothiadiazole-benzoic acid electron acceptor. This organic dye can be utilized for the fabrication of sensitized titania solar cells exhibiting excellent photovoltages of 1005 and 825 mV, respectively, when a volatile tris(1,10-phenanthroline)cobalt-based electrolyte and a nonvolatile iodine-based ionic liquid composite electrolyte are applied, respectively. With respect to the control dye C239 using the traditional electron acceptor cyanoacrylic acid, dye SC-4 displays not only an enhanced photovoltage owing to slower interfacial charge recombination but also an improved stability of photocurrent and efficiency even under a long-term thermal aging at 85 °C, because of negligible desorption of dye molecules from the surface of titania. The photovoltage drop of dye-sensitized solar cells under the thermal stress is identified for the first time as the intrinsic instability of the interface between titania and electrolyte, which needs to be judiciously passivated in a future study. Ultrafast PL measurements and theoretical calculations have unveiled that torsional energy relaxation and electron injection occur from the multiple nonequilibrium excited states of organic dyes, resulting in a highly distributive kinetics of electron injection.Keywords: charge transfer; excited state; organic dye; solar cell; torsional relaxation; ultrafast spectroscopy;

Three new N-annulated perylene (NP) substituted porphyrin dyes WW-7–WW-9 with different linking modes and accepting groups were synthesized and applied in Co(II)/(III) based dye sensitized solar cells (DSCs). The bay-linked porphyrins WW-7 and WW-8 exhibited moderate power conversion efficiency (PCE = 4.4% and 4.8%, respectively), while the peri-linked porphyrin dye WW-9 showed a PCE up to 9.2% which is slightly lower than that of our reference dye WW-6. Detailed physical measurements (optical and electrochemical), DFT calculations, and photovoltaic characterizations were performed to understand how the structural changes affect their light-harvesting ability, molecular orbital profile, energy level alignment, and eventually the photovoltaic performance. It turned out that the lower efficiencies of the cells based on WW-7 and WW-8 could be ascribed to the weak π-conjugation between the bay-substituted NP and phenylethynyl substituted porphyrin unit. The introduction of a benzothiadiazole acceptor at the anchoring group has induced a significant red shift of the IPCE action spectra of WW-8 and WW-9, by about 90 nm and 50 nm as compared to that of WW-7 and WW-6, respectively. However, less efficient electron injection was observed. Our studies gave some insight into the important role of electronic interactions between different components when one designs a dye for high-efficiency DSCs.

In this work, by conjugating 2-cyanoacrylic acid (CA), 4-(benzo[c][1,2,5]thiadiazol-7-yl)benzoic acid (BTBA), 4-(7-ethynylbenzo[c][1,2,5]thiadiazol-4-yl)benzoic acid (EBTBA), and 4-((7-ethynylbenzo[c][1,2,5]thiadiazol-4-yl)ethynyl)benzoic acid (EBTEBA) to a binary electron-donor diphenylamine-phenanthrocarbazole (DPA-PC), we systematically investigate the impacts of electron-acceptors upon energy level, energy gap, light-harvesting ability, photovoltaic parameter, and cell stability of donor–acceptor dyes in photoelectrochemical cells. In conjunction with an ionic liquid composite electrolyte, the DPA-PC dye with EBTEBA as electron-acceptor yields a high power conversion efficiency of 8% and an outstanding stability after a 1000 h aging test under the soaking of full sunlight at 60 °C in a dye-sensitized solar cell. Femtosecond fluorescence up-conversion measurements have suggested that energy relaxation and electron injection both occur to dye molecules in the nonequilibrium excited states. Moreover, the time constants of injecting electrons from dye molecules in the excited states to titania are very dispersive for over 1 order of magnitude, mainly owing to the broad energy distribution of excited states.Keywords: charge transfer; excited state; perylene; solar cell; ultrafast spectroscopy;

4-(Benzo[c][1,2,5]thiadiazol-4-ylethynyl)benzoic acid (BTEBA) as a promising electron acceptor has been used in the highly efficient organic dye-sensitized solar cells (DSCs) recently. Because of its strong electron-deficient character, BTEBA could bring forth a remarkable decline in the energy level of the lowest unoccupied molecular orbital (LUMO) and further reduce the energy gap of dye molecules significantly. In this contribution, two metal-free organic dyes WEF1 and WEF2 were synthesized by simply combining BTEBA with two slightly tailored electron-releasing moieties: 4-hexylphenyl substituted indaceno[1,2-b:5,6-b′]dithiophene (IDT) and cyclopenta[1,2-b:5,4-b′]dithiophene[2′,1′:4,5]thieno[2,3-d]thiophene (CPDTDT), which were screened rationally from an electron-donor pool via computational simulation. With respect to those of WEF1, WEF2-sensitized solar cells demonstrate a far better short-circuit photocurrent density (JSC) and open-circuit photovoltage (VOC), resulting in a ∼50% improved power conversion efficiency of 10.0% under irradiance of 100 mW cm–2 AM1.5G sunlight. We resorted to theoretical calculations, electrical measurements, steady-state, and time-resolved spectroscopic methods to shed light on the fatal influences of elaborately modulating electron donors on light absorption, interfacial energetics, and multichannel charge-transfer dynamics.

Three new N-annulated perylene (NP) substituted porphyrin dyes WW-7–WW-9 with different linking modes and accepting groups were synthesized and applied in Co(II)/(III) based dye sensitized solar cells (DSCs). The bay-linked porphyrins WW-7 and WW-8 exhibited moderate power conversion efficiency (PCE = 4.4% and 4.8%, respectively), while the peri-linked porphyrin dye WW-9 showed a PCE up to 9.2% which is slightly lower than that of our reference dye WW-6. Detailed physical measurements (optical and electrochemical), DFT calculations, and photovoltaic characterizations were performed to understand how the structural changes affect their light-harvesting ability, molecular orbital profile, energy level alignment, and eventually the photovoltaic performance. It turned out that the lower efficiencies of the cells based on WW-7 and WW-8 could be ascribed to the weak π-conjugation between the bay-substituted NP and phenylethynyl substituted porphyrin unit. The introduction of a benzothiadiazole acceptor at the anchoring group has induced a significant red shift of the IPCE action spectra of WW-8 and WW-9, by about 90 nm and 50 nm as compared to that of WW-7 and WW-6, respectively. However, less efficient electron injection was observed. Our studies gave some insight into the important role of electronic interactions between different components when one designs a dye for high-efficiency DSCs.

Two perylene dyes characteristic of electron-donors phenanthrocarbazole (PC) and carbazyl functionalized PC are selected to study the complicated dynamics of excited states and charge carriers, which underlie the photovoltaic parameters of dye-sensitized solar cells (DSCs). We have combined femtosecond fluorescence up-conversion and time-resolved single-photon counting techniques to probe the wavelength-dependent photoluminescence dynamics of dye molecules not only dissolved in THF but also grafted on the surface of oxide nanoparticles. Excited state relaxation and electron injection both occur on a similar timescale, resulting in a very distributive kinetics of electron injection. It is also found that the carbazyl ancillary electron-donor causes a faster electron injection, which over-compensates the adverse impact of a slightly shorter lifetime of the equilibrium excited state. Nanosecond transient absorption and transient photovoltage decay measurements have shown that conjugating carbazyl to PC can effectively slow down the kinetics of charge recombination of electrons in titania with both photo-oxidized dye molecules and triiodide anions, improving the cell photovoltage.

Molecular engineering of a photosensitizing dye plays a vital role in the control of excited states and charge carriers in dye-sensitized solar cells (DSCs) for performance improvement. Herein we report three metal-free donor–acceptor organic dyes by conjugating electron donors triphenylamine–cyclopentadithiophene (TPA–CPDT), triphenylamine–phenanthrocarbazole (TPA–PC), and di(biphenyl)amine–phenanthrocarbazole (DBPA–PC) with electron acceptor 4-(7-ethynylbenzo[c][1,2,5]thiadiazol-4-yl)benzoic acid (EBTBA). The influences of electron donors on the energy levels, light absorption, dynamics of excited states and photogenerated charges, and photovoltaic parameters of DSCs were systematically analyzed via theoretical calculations and physical measurements. A DSC with dye SC-3 employing the bulky DBPA–PC electron donor achieved a high power conversion efficiency (PCE) of 11.5% measured under the air mass 1.5 global (AM1.5G) conditions, owing to the significantly reduced interfacial charge recombination. Femtosecond fluorescence upconversion measurements have unraveled that energy relaxation and electron injection both occur for dye molecules in the nonrelaxed hot excited states. Also, the rate constants of injecting electrons from dyes in the excited states to titania are very dispersive by over 1 order of magnitude, which could be mainly ascribed to the broad energy distribution of excited states.