Due to excellent electronic and optical properties as well as tunable work functions, graphene and graphene-based materials are highly attractive for applications in enhancement of harvesting solar energy. In particular, they can be used as electron and hole transport materials, buffer layers, and window or/and counter electrodes in solar cells. This research news surveys very recent advances in this emerging fi eld, with emphasis on fundamental understanding of their performance enhancement mechanisms for photovoltaic devices, and discusses future challenges.

The microstructures of photo‐ and counter‐electrodes play critical roles in the performance of dye‐sensitized solar cells (DSSCs). In particular, various interfaces, such as fluorinated‐tin oxide (FTO)/TiO2, TiO2/TiO2, and TiO2/electrolyte, in DSSCs significantly affect the final power conversion efficiency (PCE). However, research has generally focused more on the design of various nanostructured semiconducting materials with emphasis on optimizing chemical or/and physical properties, and less on these interface functionalizations for performance improvement. This work explores a new application of graphene to modify the interface of FTO/TiO2 to suppress charge recombination. In combination with interfaces functionalization of TiO2/TiO2 for low charge‐transport resistance and high charge‐transfer rate, the final PCE of DSSC is remarkably improved from 5.80% to 8.13%, achieving the highest efficiency in comparison to reported graphene/TiO2‐based DSSCs. The method of using graphene to functionalize the surface of FTO substrate provides a better alternative method to the conventional pre‐treatment through hydrolyzing TiCl4 and an approach to reduce the adverse effect of microstructural defect of conducting glass substrate for electronic devices.

A p-type semiconductor photoelectrode in dye sensitized solar cells (DSCs) has a large optical band gap and high ionization potential but suffers from its intrinsically low hole transfer rate, thus resulting in much poorer performance than n-DSCs. Nickel oxide (NiO)/graphene composite is synthesized to offer a larger surface area and higher conductivity than the pristine NiO film. The novel composite as a photoelectrode in p-DSCs demonstrates increment of both short-circuit photocurrent and open-circuit photovoltage, leading to 2 times increase of power conversion efficiency. Electrochemical impedance spectroscopy and open-circuit voltage decay measurements indicate that the charge recombination of the composite-based p-DSCs is significantly suppressed due to enhanced hole transport by the presence of graphene, thus achieving an efficient electron–hole pair charge separation and collection in the composite film electrode for performance-improved NiO-based devices. The proposed mechanism provides physical insight into the enhancement process in p-DSCs.

Dye-sensitized solar cells (DSSCs) have attracted intense research attention because of their low cost and high efficiency in converting solar energy into electricity. [1,2] Typically, a DSSC constitutes a dye-loaded mesoporous TiO 2 photoelectrode, an electrolyte containing iodide/tri-iodide (I − /I 3 − ) redox couples, and a catalytic counter electrode which reduces I 3 − ions. [3,4] Since both the photo and counter electrodes are exposed to the electrolyte, a high DSSC performance is greatly dependent on a fast reduction reaction at the counter electrode to suppress the possible reduction reaction by a charge recombination process at the photoelectrode. [5,6] To achieve such selectivity, noble platinum (Pt) is often used as a catalyst on fluorine-doped tin oxide (FTO) conducting glass to enhance the reduction rate at the counter electrode. [7] However, Pt is expensive and non-sustainable for long-term applications. Carbon-based materials such as polymers, carbon black, and carbon nanotubes have also been reported to replace Pt catalysts. [8,9] However, these carbon materials have much weaker catalytic activities than Pt and thus require large carbon loadings (loading ∼300 to >1000 μg cm −2 ) to compensate for the lowered electrocatalytic activity, resulting in poor device energy density. [9][10][11] Therefore, a low cost and robust alternative counter electrode with high electrocatalytic activity towards I 3 − reduction is in high demand. Transition metal based materials have been demonstrated to possess outstanding catalytic performance in redox reactions because of their multiple oxidation states. [12,13] However, very few of such materials have been studied and employed as a DSSC counter electrode. [14,15] In this study, nickel oxide (NiO) is selected as a potential electrode material because of its good charge capacity, [16] high chemical stability, [17,18] and a valence band energy (∼-4.96 eV) comparable to the redox potential of I − /I 3 − for favorable charge transfer between the metal oxide and redox couples, [19,20] although its low conductivity and poor charge carrier mobility limit its actual charge transfer process. To improve the charge transfer impedance of NiO, a sulfur-doped nickel oxide (S-NiO) thin film is fabricated for a DSSC counter electrode and its electrocatalytic activity towards the I − /I 3 − redox process is investigated. By using an electrophoretic deposition method, a porous S-NiO thin film with a nanorod-like network is fabricated on a FTO substrate to greatly enhance its effective reaction surface area. The surface composition and morphology of the S-NiO thin film are characterized using scanning electron microscopy (SEM), X-ray photoemission spectroscopy (XPS), and energy dispersive spectroscopy (EDS). The results obtained from cyclic voltammetry (CV) and alternating current (A.C.) impedance measurements indicate that the S-NiO\FTO (SNO) electrode possesses high electrocatalytic activity towards the I − /I 3 − redox process as compared to both plain NiO\FTO (NO) and FTO electro...