The counter electrode (CE), despite being as relevant as the photoanode in a quantum dot solar cell (QDSC), has hardly received the scientific attention it deserves. In this study, nine CEs (single-walled carbon nanotubes (SWCNTs), tungsten oxide (WO), poly(3,4-ethylenedioxythiophene) (PEDOT), copper sulfide (CuS), candle soot, functionalized multiwalled carbon nanotubes (F-MWCNTs), reduced tungsten oxide (WO), carbon fabric (C-Fabric), and C-Fabric/WO) were prepared by using low-cost components and facile procedures. QDSCs were fabricated with a TiO/CdS film which served as a common photoanode for all CEs. The power conversion efficiencies (PCEs) were 2.02, 2.1, 2.79, 2.88, 2.95, 3.78, 3.66, 3.96, and 4.6%, respectively, and the incident photon to current conversion efficiency response was also found to complement the PCE response. Among all CEs employed here, C-Fabric/WO outperforms all the other CEs, for the synergy between C-Fabric and WO comes to the fore during cell operation. The low sheet resistance of C-Fabric and its high surface area due to the meshlike morphology enables high WO loading during electrodeposition, and the good electrocatalytic activity of WO, the very low overpotential, and its high electrical conductivity that facilitate electron transfer to the electrolyte are responsible for the superior PCE. WO-based electrodes have not been used until date in QDSCs; the ease of fabrication of WO films and their good chemical stability and scalability also favor their application to QDSCs. Futuristic possibilities for other novel composite CEs are also discussed. We anticipate this study to be useful for a well-rounded development of high-performance QDSCs.

A notable strategy to achieve a dramatically high power conversion efficiency (PCE) of 9.76% for a tandem photovoltaic device has been implemented by the use of a nickel phthalocyanine-tetrasulfonic acid tetrasodium salt (NiPcTs) dye-sensitized p-type nickel oxide (NiO) semiconductor-based photocathode supported over carbon (C)-fabric paired with a photoanode scaffold comprising luminescent and conducting core/shell copper@carbon dots (Cu@C-dots) anchored to cadmium sulfide (CdS) quantum dots tethered to n-type titanium oxide (TiO2). The PCE yielded for the n-type quantum dot-sensitized solar cell (n-QDSC) or photoanode-based half-cell (TiO2/CdS/Cu@C-dots-nS2–/S n 2–-C-fabric) is 6.82% with a significant contribution from the light-harvesting capability of the plasmonic Cu core encased within the C-dot shell. In spite of the long-wavelength light-harvesting NiPcTs enabling easier reduction of the polysulfide electrolyte because of the additional photoexcited charge transfer, the p-type solar cell (p-SC) or photocathode-based half-cell (NiO/NiPcTs-nS2–/S n 2–-C-fabric) delivers a relatively lower PCE of 0.039%, but on coassembling the p- and n-half solar cells in a tandem design (TiO2/CdS/Cu@C-dots-nS2–/S n 2–-NiPcTs/NiO/C-fabric) the cell efficiency gets an immense boost under 1 sun illumination in consequence of the maximized range of light absorption afforded by CdS–Cu@C-dots on one side and NiPcTs on the other. This work rationalizes the synergism between the photoanode and photocathode elaborately to obtain a hitherto unmatched solar energy conversion in tandem solar cells.

Selenium nanoparticle-decorated silicon nanowire (Se NPs@Si NWs) electrodes are applied as a photoanode in a liquid-junction photoelectrochemical (PEC) solar cell for the first time. Upon illumination, the Se NPs anchored along the axial length of Si NWs allow fast hole extraction at the radial Se/Si junctions because of the p-type conduction nature of Se NPs, thus enhancing electron–hole separation and simultaneously increasing the population of photoexcited electrons in Si NWs through light scattering that amplifies the effective light absorption of Si NWs. These attributes of Se NPs result in a power conversion efficiency (PCE) of 7.03% for the Se NPs@Si NW-based liquid-junction solar cell encompassing a Br–/Br2 electrolyte and a carbon fabric counter electrode. This PCE is greater by 43% than that of the analogous Si NW-based cell. Se NPs are photoconducting because of facile hole propagation that occurs particularly along the c-axis of trigonal Se NPs with a hexagonal crystal structure and size effects improve the optical path length, factors that lead to a significantly improved performance. Compared to Pt or Au NPs that have been explored previously in combination with Si NWs, where their roles are distinctively different, Se NPs here are not only more cost effective and easy to be synthesized on a large scale but also enable an improvement in PCE of Si NWs by relying on unique mechanisms. Optical, structural, PEC, and impedance studies furnish a deep understanding of the phenomena involved in yielding a superior performing liquid-junction PEC solar cell based on the Se NPs@Si NW photoanode.

A novel assembly of a photocathode and a photoanode is investigated to explore their complementary effects in enhancing the photovoltaic performance of a quantum-dot solar cell (QDSC). While p-type nickel oxide (NiO) has been used previously, antimony selenide (SbSe) has not been used in a QDSC, especially as a component of a counter electrode (CE) architecture that doubles as the photocathode. Here, near-infrared (NIR) light-absorbing SbSe nanoparticles (NPs) coated over electrodeposited NiO nanofibers on a carbon (C) fabric substrate was employed as the highly efficient photocathode. Quasi-spherical SbSe NPs, with a band gap of 1.13 eV, upon illumination, release photoexcited electrons in addition to other charge carriers at the CE to further enhance the reduction of the oxidized polysulfide. The p-type conducting behavior of SbSe, coupled with a work function at 4.63 eV, also facilitates electron injection to polysulfide. The effect of graphene quantum dots (GQDs) as co-sensitizers as well as electron conduits is also investigated in which a TiO/CdS/GQDs photoanode structure in combination with a C-fabric CE delivered a power-conversion efficiency (PCE) of 5.28%, which is a vast improvement over the 4.23% that is obtained by using a TiO/CdS photoanode (without GQDs) with the same CE. GQDs, due to a superior conductance, impact efficiency more than SbSe NPs do. The best PCE of a TiO/CdS/GQDs-nS/S-SbSe/NiO/C-fabric cell is 5.96% (0.11 cm area), which, when replicated on a smaller area of 0.06 cm, is seen to increase dramatically to 7.19%. The cell is also tested for 6 h of continuous irradiance. The rationalization for the channelized photogenerated electron movement, which augments the cell performance, is furnished in detail in these studies.

Silicon nanowire (SiNW) arrays offer a range of exciting opportunities, from maximizing solar spectrum utilization for high-performance liquid-junction solar cells (LJSCs) to functioning as potential micro-supercapacitors in the near future. This work, contrasting strongly with the previously reported studies on SiNW-based LJSCs where electron-conducting nanoparticles of Pt or Au were employed to achieve high efficiencies, aims at tethering relatively inexpensive, hole-conducting, and photoresponsive carbon-coated tellurium nanorods (C@TeNRs) to SiNWs in the quest to achieve an outstanding solar cell performance. A SiNW LJSC (control cell) with a SiNWs/Br–, Br2/carbon-fabric architecture delivers a power conversion efficiency (PCE) of 4.8%. Further, by anchoring C@TeNRs, along the lengths of SiNWs via electrophoresis, a PCE of ∼11.6% is attained for a C@TeNRs@SiNWs/Br–, Br2/carbon-fabric-based LJSC. The multifunctionality of C@Te comes to the fore in this cell where (1) the p-type (hole) conducting nature of C@Te ensures efficient charge separation by rapidly collecting holes from SiNWs (and suppresses recombination), (2) the C@TeNRs are also photoresponsive and increase light-harvesting, and (3) the C coating restricts the chemical corrosion and photo-oxidation of SiNWs and the Te core by the acidic electrolyte, thereby improving the cell’s operational lifetime. This LJSC also serves as an effective stand-alone energy-storage device giving an improved areal specific capacitance of 1605 μF cm–2 (at 1 mA cm–2). This study unravels the pivotal role played by C@TeNRs in controlling the performance of SiNW-based LJSCs.