Highly efficient solution processed nanorice structured NiS counter electrode for quantum dot sensitized solar cells

Abstract

Facile and nano-rice structured nickel sulfide (NiS) thin films were synthesized and utilized as counter electrode (CE) in quantum dot sensitized solar cells (QDSSCs) for polysulfide redox reactions. NiS thin films deposited on fluorine-doped tin oxide substrate results in formation of highly efficient CE for liquid-junction QDSSCs. Under illumination of 1 sun (100 mW cm⁻²), the QDSSC based NiS CE exhibited power conversion efficiency (η) of 2.97%, which is higher than platinum (1.45%) CE. Electrochemical impedance spectroscopy affirms that high value of η in NiS CE is due to the elevated electro-catalytic activity towards polysulfide electrolyte reduction and also due to the scale-down in the charge transfer resistance (3.16 Ω) at the CE-electrolyte interface.

Introduction

Dye-sensitized solar cells (DSSCs) have a promising future in third- generation solar cell applications owing to their low-cost production, usage of medium purity materials and comparable incident photons to electrical current conversion efficiency [1], [2]. Many efforts have been made to increase the efficiency and stability of the solar cells based on the TiO₂ photo electrode structures [3], employing different types of redox couples [4], synthesizing new sensitizers [5], electrolyte development [6], [7] and different types of counter electrodes (CEs) [8]. As an alternative to dye molecules, quantum dots (QDs) such as CdS [9], CdSe [10], PbS [11] and InP [12] have been employed in quantum dot sensitized solar cells (QDSSCs). The QDs have size dependent optical band gap, hot electron injection and design of the hierarchical multilayer absorber structure [13], [14] makes as the hot choice in solar cell fabrication. In a mesoscopic oxide film, proper aggregation and ordering of semiconductor QDs play a pivotal role in the design of QDSSCs [15]. For QDs solar cells, CdS and CdSe have exhibited exceptional performance and ZnS layer acts as a passivation layer, which enhances the photovoltaic properties of solar cells [16]. Photo-degradation and corrosion are the problems occurred in QDs with I₃^-/I⁻ redox couple [17]. Consequently, to minimize the photocorrosion, the polysulfide electrolyte has been tested for the CdS/CdSe/ZnS QDSSCs [16]. However, the photovoltaic performance of most QDSSCs remains lower than those of DSSCs [18]; this low photovoltaic performance may be ascribed to electron loss occurring through charge recombination at TiO₂-electrolyte and QD-electrolyte interfaces, and inner energy loss at electrolyte-CE interfaces [19], [20]. Theoretically, the photo-conversion efficiency of the QDSSCs can reach 44% considerably higher than that of DSSCs [21]. Platinum (Pt) is widespread CE in the DSSCs for iodine/triiodide (I₃^-/I⁻) redox electrolyte shows the low charge transfer resistance (R_ct) at the CE/electrolyte interface. Pt electrodes are expensive and have short life times in QDSSCs when I₃^-/I⁻ is used as the electrolyte. Nevertheless, Pt electrodes with the polysulfide electrolyte exhibit low electrocatalytic activity results in reduction of total power conversion efficiency of the solar cell; because, sulfur containing compounds adsorb preferably and strongly on Pt surfaces reduce the surface activity and conductivity of the electrodes [22]. Inexpensive metal sulfides, such as NiS, CuS and CoS have been used as counter electrodes in QDSSC [23] which have low charge transfer resistance and high electrocatalytic activity for the reduction of polysulfide electrolyte boost up the solar cell efficiency with simple fabrication processes. Among the metal sulfides, NiS is also found to be one the most efficient electrocatalytic active material.

Chemical bath deposition (CBD) has emerged as most common method for the deposition of metal chalcogenide, metal oxide and metal sulfide thin films and is currently attracting considerable attention as it is relatively inexpensive, simple and convenient for large area deposition [24]. In this study, we have fabricated nanorice structured NiS CE by CBD method using urea or urea/triethanolamine (TEA) at different deposition times, Use of urea as a reagent is the uniqueness of our report. Urea increases the rate of thioacetamide (TAA) decomposition and thereby increasing the concentration of S⁻² ions and hence employed in our work [25]. The TEA was used to adjust the pH of the solution. A schematic structure of the TiO₂/CdS/CdSe/ZnS QDSSC with nanorice structure NiS CE is depicted in Fig. 1. Upon illumination of 1 sun (100 mW cm⁻²), photons are absorbed by the QDs generating excitons and the electrons are driven toward the TiO₂ conduction band and hole transfer into the electrolyte. The injected electron flows through the semiconductor network to arrive at the back contact and then through the load to the CE. At the CE, reduction of the oxidized redox system regenerates the reduced redox species. The NiS CE was used in conjunction with aqueous polysulfide electrolyte in QDSSC. The low charge transfer resistance (R_ct) at the NiS CE/electrolyte interface was observed based on the electrochemical impedance spectroscopy, indicating a good electrocatalytic activity of NiS CE for the reduction of oxidized redox couple.