A Review of Transition Metal Sulfides as Counter Electrodes for Dye-Sensitized and Quantum Dot-Sensitized Solar Cells

Abstract

Third-generation solar cells, including dye-sensitized solar cells (DSSCs) and quantum dot-sensitized solar cells (QDSSCs), have been associated with low-cost material requirements, simple fabrication processes, and mechanical robustness. Hence, counter electrodes (CEs) are a critical component for the functionality of these solar cells. Although platinum (Pt)-based CEs have been dominant in CE fabrication, they are costly and have limited market availability. Therefore, it is important to find alternative materials to overcome these issues. Transition metal chalcogenides (TMCs) and transition metal dichalcogenides (TMDs) have demonstrated capabilities as a more cost-effective alternative to Pt materials. This advantage has been attributed to their strong electrocatalytic activity, excellent thermal stability, tunability of bandgap energies, and variable crystalline morphologies. In this study, a comprehensive review of the major components and working principles of the DSSC and QDSSC are presented. In developing CEs for DSSCs and QDSSCs, various TMS materials synthesized through several techniques are thoroughly reviewed. The performance efficiencies of DSSCs and QDSSCs resulting from TMS-based CEs are subjected to in-depth comparative analysis with Pt-based CEs. Thus, the power conversion efficiency (PCE), fill factor (FF), short circuit current density (J_sc) and open circuit voltage (V_oc) are investigated. Based on this review, the PCEs for DSSCs and QDSSCs are found to range from 5.37 to 9.80% (I^-/I₃^- redox couple electrolyte) and 1.62 to 6.70% (S^-2/S_x^- electrolyte). This review seeks to navigate the future direction of TMS-based CEs towards the performance efficiency improvement of DSSCs and QDSSCs in the most cost-effective and environmentally friendly manner.

Keywords: counter electrode; dye-sensitized solar cell; materials performance; polysulfide electrolyte; quantum dot-sensitized solar cell; transition metal sulfide.

Figure 1

Typical representation of DSSC showing the main components, comprising a photoanode, a semiconducting oxide layer (TiO₂), molecular dye, a redox-couple electrolyte, and counter electrode. Electrons exit and make re-entry through the photoanode and counter-electrode systems, respectively. Modified after reference [36].

Figure 2

The energy band positions of commonly used semiconductors indicate ZnO as a promising alternative to TiO_2, given their matching energy band structures. Modified after reference [34].

Figure 3

Schematic representation of the DSSC working principle with indications of electron movements from the HOMO to LUMO regions of the cell and passing through the external circuitry system for accumulation at the CE.

Figure 4

Current–voltage curve for evaluating DSSC performance using the cell parameters. Modified after reference [37].

Figure 5

Schematic representation of a typical QDSSC and its associated working principle. Modified after reference [13].

Figure 6

(a) XRD of the synthesized product and standard Ag₂S (JCPDS No. 14–72) in black and blue line, respectively. and (b) TEM of Ag₂S nano- crystals. Adapted with permission from reference [108]. Copyright 2022, Wiley Publishing.

Figure 7

(a) TEM and (b) HRTEM images of as-prepared SnS2 nanosheets. Adapted with permission from [112]. Copyright 2022, Wiley Publishing.

Figure 8

The PCE Vs FF performance for TMS-based CE and Pt-based CE in DSSCs.

Figure 9

The V_oc Vs J_sc for TMS-based CE and Pt-based CE in DSSCs.

Figure 10

FESEM snaps of (a–c) FTO/MCS (3, 6, and 9 cycles) and (d) FTO/Pt CEs. Adapted with permission from reference [129]. Copyright 2022, IOP Publishing.

Figure 11

SEM of (a,c) CNTs/MnCo₂S₄ and (b,d) MnCo₂S₄. Adapted with permission from [130] Copyright 2022, American Chemical Society.

Figure 12

The PCE Vs FF performance for TMS-based CE and Pt-based CE in QDSSCs.

Figure 13

The V_oc Vs J_sc for TMS-based CE and Pt-based CE in QDSSCs.