Kinetics of the recombination at TiO2/dye/electrolyte interfaces is vital to establish reliable strategy for reducing energy loss in dye sensitized solar cell (DSCs). The semilogarithmic plots of open circuit voltage–light intensity indicate that the DSCs exhibit different recombination behaviors in high and low voltage regions. Even so, the average orders of recombination reaction in the two regions are both found to be expressed as the summation of the apparent charge transfer coefficient and the averaged density distribution parameter. This statement is based upon the fact that the “steady-state” electron lifetimes in cobalt complex based DSCs are well interpreted by using the Butler–Volmer model. Combining the results of electron lifetime, the order of recombination reaction, and density distribution, we addressed that the dominant pathway of recombination in cobalt complex based DSCs is via the trapping states rather than the conduction band. In addition, the variation of the Fermi level in TiO2 film could affect the adsorbing configurations of 4-tert-butylpyridine molecules and Li+/H+ cations at TiO2 surfaces and hence engenders the nonexponential distribution of electron density at higher Fermi level.

Organolead iodide perovskite showing tubular morphology was fabricated on mesoporous TiO2 film via a one-step spin-coating procedure. These perovskite tubes with the external diameter of 1 μm were found to facilitate the charge separation at perovskite/hole-transport material (HTM) interfaces. This engenders a tubular perovskite device showing a higher fill factor over the device with planar perovskite, and improves power conversion efficiency accordingly.

•Two new triarylamine sensitizers for dye-sensitized solar cells were synthesized.•Carbazole as electron donor is superior to phenothiazine.•An efficiency of 9.58% was achieved for the M36 sensitized cobalt cell.•Marcus inverted behavior of cobalt(III) species are observed in studied DSCs.•Marcus inverted behavior of cobalt(III) species depends strongly on the dye layer.A new generation of dye-sensitized solar cells (DSCs) is based on a combination of D–π–A organic dyes in conjunction with cobalt-based redox mediators. Here, two new triarylamine organic dyes (M36 and M37) toward cobalt electrolytes are constructed and employed as photosensitizers for dye-sensitized solar cells. The photoelectrochemical properties and photovoltaic performance of dyes are sensitive to the slightly structural modification of the terminal donor in triarylamine. Recombination kinetics of cobalt(III) complexes at titania/dye interface are also studied using electrochemical impedance spectroscopy and controlled intensity modulated photovoltage spectroscopy measurements. Our results show that, for M36 sensitized DSCs, a Marcus inverted region can be reached for the charge recombination kinetics behavior of cobalt(III) species. While that for DSCs based on M37 just lies in the Marcus normal region. The results can be attributed to differences in the retarding charge recombination ability of the dye layer. Benefiting from a Marcus inverted region behavior, the M36 dye exhibits a good compatibility with the [Co(phen)3]2+/3+ redox couples, achieving a high overall power conversion efficiency (PCE) of 9.58% under full sun illumination.

We prepared nitrogen doped graphene (NG) by reacting pristine graphene oxide (GO) with urea hydrothermally and elucidated its usage as a Pt-free cathode material in the DSCs based on cobalt complex redox shuttles. In the process of hydrothermal reaction, the graphene oxide sheets are kept in a flocculating state. This experimental protocol prevents the graphene sheets from gelation and yields a maximum nitrogen content of 7.6 at%. The resultant NG sediments can be readily deposited onto conductive glass sheet for fabricating a porous cathode. The content ratio of the doped nitrogen atoms to the residual oxygen atoms is demonstrated as a determinant factor affecting the electrocatalytic activities of the as-prepared NG sheets. Annealing treatment to the NG cathodes gives rise to a remarkable increase of exchange current density. Moreover, the XPS results indicate that the electrochemically active pyridinic-N groups formed in the early stage of hydrothermal reaction are unstable to annealing. However, their thermal stability can be improved by extending the time of the hydrothermal reaction. By optimizing the composition of the embedded nitrogen species and oxygen containing groups, the DSCs with the NG cathode yield a maximal device efficiency of 8.2%.

A simplified reaction–diffusion model is proposed to show the origins of inefficient charge transport in dye-sensitized solar cells (DSCs) based on bulky complex redox shuttles. In this model, diffusion length of the oxidized species in electrolyte solution is defined and assumed to depend on the average density of the conduction band electrons. The resulting quasi-linear equations enable the density profiles of electrons and redox shuttles to be calculated directly. When the charge transport of a DSC is under diffusion limitation, diffusion overpotential is shown to serve as an additional potential, which maintains a high level of electron density in TiO2 film, even at short circuit. The photocurrent output is therefore considerably limited because of the accelerated recombination reaction. Moreover, short circuit current density is confined by the diffusion limited current density under inefficient charge transport, and close to the photocurrent density at high diffusivity of redox shuttles. For the DSC with slow redox shuttles, a critical thickness of TiO2 film is observed. This is the maximum thickness allowing the charge transport to be free from diffusion limitation. The optimized film thickness yielding the maximum power conversion efficiency is shown to be slightly less than the critical thickness, which implies the potential usage of the critical thickness in designing the state-of-art DSC.

Poly(N-propyl-vinylimidazolium iodide-co-poly(ethylene glycol) methyl ether methacrylate) with a comb-like configuration was synthesized. The copolymer and its blend with poly(ethylene oxide) (PEO) were used to prepare the polymer electrolytes for solid dye-sensitized solar cells. The amorphous characteristics and the ion-crosslinking effects of the copolymer and its blend electrolyte were analyzed by WAXS, DSC and FT-IR. The copolymer and PEO in the blend electrolyte were shown to play the roles of decreasing the crystallinity and breaking the ion-crosslinking, respectively. The blend electrolytes thus presented the highest ionic conductivity among the prepared electrolytes. The as-prepared DSSCs yielded the energy conversion efficiency of 3.46% at 100 mW cm−2. The impedance results indicated the improved photovoltaic performance of the blend electrolyte was originated from the increased ionic diffusion coefficient and the prevented downward movement of the conduction band edge of TiO2.Highlights► A comb-like copolymer with PEGMA and imidazolium iodide moieties was synthesized. ► Copolymer/poly(ethylene oxide) blend electrolytes yields the highest conductivity. ► Blending makes polymer electrolyte to be amorphous and reduces ion-crosslinking. ► An efficiency of 3.46% for the blend electrolyte based DSSC was obtained.

Oligo(ethylene glycol) terminated by pyridine derivatives was designed and synthesized for improving the performance of the dye-sensitized solar cells (DSCs) with poly(ethylene oxide) based electrolyte. Effects of the plasticizer on retarding the recombination reaction in DSCs were characterized by current density–voltage characteristics. Combined with the results on electron density measurements, photovoltage–intensity characteristics correlate the retarded electron recombination with the upward movement of the conduction band edge and the reduced order of recombination reaction. The increased electron lifetimes of the DSCs with plasticizer modified electrolyte were confirmed by a small perturbation voltage decay technique. Additionally, WAXS measurements show that the presence of the plasticizer decreases the crystallinity of PEO electrolyte, which facilitates the mass transport of the redox species as impedance spectra indicated. By introducing guanidinium thiocyanate into the plasticizer modified PEO electrolyte, the performance of the DSCs is further improved, which yields the highest efficiency of 3.5%.Highlights► Oligo(ethylene glycol) terminated by pyridine derivatives was synthesized as plasticizer. ► PEO electrolyte for dye sensitized solar cell (DSC) was modified with the plasticizer. ► The plasticizer in DSC retards recombination reaction and leveling up the band edge. ► Addition of GuSCN into the modified electrolyte yields the highest efficiency of 3.5%.

Organolead iodide perovskite showing tubular morphology was fabricated on mesoporous TiO2 film via a one-step spin-coating procedure. These perovskite tubes with the external diameter of 1 μm were found to facilitate the charge separation at perovskite/hole-transport material (HTM) interfaces. This engenders a tubular perovskite device showing a higher fill factor over the device with planar perovskite, and improves power conversion efficiency accordingly.