In the current study, a nanocomposite of molybdenum disulfide and graphene (MoS₂/RGO) was proposed for the first time as the counter electrode (CE) catalyst in dye-sensitized solar cells (DSSCs) to speed up the reduction of triiodide (I₃ ⁻) to iodide (I⁻). This novel catalyst was synthesized by simply mixing graphene oxide nanosheets with a solution of ammonium tetrathiomolybdate and then converting the solid intermediate into MoS₂/RGO nanocomposite in a H₂ flow at 650 °C. Atomic force microscopy, X-ray powder diffraction and X-ray photoemission spectroscopy confirmed that MoS₂ nanoparticles were deposited onto the graphene surface. The extensive cyclic voltammograms (CV) showed that the cathodic current density of the MoS₂/RGO CE was higher than those of MoS₂, RGO and sputtered Pt CEs, due to the increased active surface area of the former. Moreover, the peak current densities of the MoS₂/RGO CE showed no sign of degradation after 100 consecutive CV tests, suggesting the great electrochemical stability of the MoS₂/RGO CE. Furthermore, the MoS₂/RGO CE demonstrated an impressively low charge-transfer resistance (0.57 Ω cm²) for I₃ ⁻ reduction. Finally, the DSSC assembled with the MoS₂/RGO CE showed a high power conversion efficiency of 6.04%, which is comparable to the DSSC with a Pt CE (6.38%).

The emerging dye-sensitized solar cells, perovskite solar cells, and organic solar cells have been regarded as promising photovoltaic technologies. The device structures and components of these solar cells are imperative to the device’s efficiency and stability. Polymers can be used to adjust the device components and structures of these solar cells purposefully, due to their diversified properties. In dye-sensitized solar cells, polymers can be used as flexible substrates, pore- and film-forming agents of photoanode films, platinum-free counter electrodes, and the frameworks of quasi-solid-state electrolytes. In perovskite solar cells, polymers can be used as the additives to adjust the nucleation and crystallization processes in perovskite films. The polymers can also be used as hole transfer materials, electron transfer materials, and interface layer to enhance the carrier separation efficiency and reduce the recombination. In organic solar cells, polymers are often used as donor layers, buffer layers, and other polymer-based micro/nanostructures in binary or ternary devices to influence device performances. The current achievements about the applications of polymers in solar cells are reviewed and analyzed. In addition, the benefits of polymers for solar cells, the challenges for practical application, and possible solutions are also assessed.

In the current study, the nanocomposite of molybdenum disulfide and multi-walled carbon nanotubes (MWCNT@MoS₂) was proposed for the first time as a counter electrode (CE) catalyst in dye-sensitized solar cells (DSSCs) to speed up the reduction of triiodide (I₃ ⁻) to iodide (I⁻). This novel catalyst was synthesized by simply mixing MWCNTs and MoS₂ in an acidic solution and then converting the solid intermediate into the MWCNT@MoS₂ nanocomposite in a H₂ flow at 650 °C. X-ray powder diffraction, Raman and X-ray photoemission spectroscopy confirmed the composition and the structure of the MWCNT@MoS₂ nanocomposite. The microstructure details of the nanocomposite were studied by transmission electron microscopy, showing that only a few-layers of the MoS₂ nanosheets were formed on the MWCNT surface. This unique structure is beneficial to the improvement of the catalytic activity of MWCNT@MoS₂ towards the reduction of I₃ ⁻. The extensive cyclic voltammograms (CV) showed that the cathodic current density of the MWCNT@MoS₂ CE was higher than those of MoS₂, MWCNT and sputtered Pt CEs due to the increased active surface area of the former. Moreover, the peak current densities of the MWCNT@MoS₂ CE showed no sign of degradation after consecutive 100 CV tests, suggesting the great electrochemical stability of the MWCNT@MoS₂ CE. Furthermore, the MWCNT@MoS₂ CE demonstrated an impressive low charge-transfer resistance (1.69 Ω cm²) for I₃ ⁻ reduction. Finally, the DSSC assembled with the MWCNT@MoS₂ CE showed a high power conversion efficiency of 6.45%, which is comparable to the DSSC with Pt CE (6.41%).

A high porous molybdenum sulfide–carbon (MoS₂–C) hybrid film was prepared by using an in situ hydrothermal route. The MoS₂–C hybrid film served as a low-cost and high efficient platinum-free counter electrode for a dye-sensitized solar cell (DSSC). The cyclic voltammetry, electrochemical impedance spectroscopy and Tafel curve analysis indicate that the MoS₂–C electrode possesses low charge transfer resistance on the electrolyte–electrode interface, high electrocatalytic activity and fast reaction kinetics for the reduction of triiodide to iodide at the counter electrode, which is due to large specific surface area and special structure and compositions of MoS₂–C film. A DSSC with the novel MoS₂–C counter electrode achieve a high power conversion efficiency of 7.69% under standard light illumination, which exceeds that of the DSSC with a Pt counter electrode (6.74%).

A transparent MoS₂–graphene nanosheet (GNS) nanocomposite counter electrode (CE) was incorporated into a Pt-free dye-sensitized solar cell (DSC). The DSC assembled with the transparent MoS₂–GNS CE therefore exhibited an impressive photovoltaic conversion efficiency of 5.81%, up to 93% of that obtained using the conventional Pt CE (6.24%).