Transition metal nanohybrid as efficient and stable counter electrode for heterostructure quantum dot sensitized solar cells: A trial

Highlights

• Heterostructure in-situ sensitization of quantum dots.

• Metal sulphide nano-hybrids as counter electrode.

• Hall effect measurements to determine electron mobility.

• Enhanced PCE over reported literatures.

Abstract

Counter electrodes are critical components of third generation solar cells. A simple strategy of utilizing isostructural analogue of graphene, namely 2D molybdenum di-sulphide as nanohybrid with CuS as counter electrode is explored in this work. Formation of heterostructure photoanode with CdSe/CdS quantum dots along with nano-hybrid counter electrode has significantly boosted current density and open circuit voltage. The designed heterostructure and the counter electrode performs exceptionally well with good stability due to synergistic effect, thus spiking new hopes in achieving high efficiency in quantum dot solar cells.

Introduction

Rise in energy demand and concerns such as global warming have attracted broad interest in research on renewable energy resources. Solar energy is among the most promising renewable option for tackling these problems. Quantum dots (QDs) have become important as light harvesting material because of their appealing and unique characteristics such as multiple exciton generation, band gap tuning, low cost and easy manufacturing (Ka et al., 2016, Yu et al., 2003). Though the overall cell efficiency of quantum dot sensitized solar cells (QDSSC) remain, as low as 13%, QDs are still considered suitable for fostering. QDSSC’s main shortcomings (photoanode, quantum dots, hole transport layer, and counter electrode) prevent them from being marketed. This necessitates the search for appropriate materials that are thermodynamically and kinetically favourable for overall performance of the device.

Counter electrodes assumes a crucial role of drawing electrons at the external circuit and regeneration of polysulphide electrolyte. High surface area (active catalytic sites), good conductivity and stability are the basic criteria in choosing material for counter electrodes (Milan et al., 2016). Catalytic activity and conductivity of CE material towards electrolyte regeneration determines the over potential and loss of energy due to the transport of electrons between CE and electrolyte interface and has critical effect on all photovoltaic parameters considerably. Till date metal sulphides, conducting polymers, carbon materials, metal oxide heterostructures, and noble metals have been explored as CE materials (Hwang and Yong, 2015). Carbon CEs have proved to be the efficient ones with reported efficiency of 11% with nitrogenated mesoporous carbon (Du et al., 2016). Among all these, Cu_XS (x = 1–2) is preferred and the most used CE material because of enhanced catalytic activity towards polysulphide electrolyte. But these materials undergo chemical corrosion under continuous exposure to polysulphide and degrade performance of the device (Hessein et al., 2017). Replacing these polysulphides with solid state hole transport layer like conducting ceramics has proved to be an excellent material for stability as reported by Kusuma and Geetha Balakrishna (2020). Copper deficient (CuS) source is more stable than brass/Cu₂S (copper-rich source) and hence has been chosen in our studies (Hessein et al., 2017, Ye et al., 2015, Zhang et al., 2015).

To enhance optical absorption, which is one of the other major challenges in QDSSC, utilization of QDs having different diameter as sensitizer has always been attempted and are either used in the form of core–shell or in the form of the heterostructures. Heterostructures have been extensively explored and reported to have enhanced performance of photovoltaic device. Ex-situ methods of deposition results in lower loading of QDs and this can block some of the pores of TiO₂, leaving a large portion of the area being exposed to hole transport layer (that forms direct contact). These factors increase recombination and limit Voc across the device (Zhang et al., 2018). Direct adsorption allows better and uniform loading of QDs resulting in better photon capture and electron transfer (Wang et al., 2017). This can possibly cover a large portion uniformly, avoid many shunt paths, recombination centres and any unfavourable charge dissociation (Li et al., 2017, Mohamed Mustakim et al., 2018). Huang et al. (2016) studied CdS/CdSe core–shell QDs with the insertion of ZnSe layers reporting an efficiency of 7.2%. Savariraj et al. (2014) also established a device for CdS/CdSe which has just limited efficiency of around 4.5%. Yun et al., utilized the CdSe/CdS core–shell which forms type II heterojunction reporting higher efficiency of 2.83% compared to CdSe (0.48%) QDs. Dao et al., reports efficiency upto 5% for CdS/CdSe with noble metals as counter electrodes (Dao et al., 2015a, Dao et al., 2015b, Dao et al., 2016, Dao et al., 2018). This clearly shows that heterostructures enhances photon absorption resulting in higher efficiency (Yu et al., 2012). We adopted the strategy of combining CdSe with CdS forming type I heterojunction.

2D materials have gained exceptional importance and significance in last few decades due to its fascinating properties and potential applications. Isostructural analogues of Graphene namely MoS₂ (molybdenum disulphide) have gained wide importance as catalysts, lubricants, and energy based materials (Edney Geraldo da Silveira Firmiano, 2014) because of its chemical versatility and measurable bandgaps unlike chemically inert and gapless semimetal Graphene (Kumar et al., 2015). They have rich chemistry because of the number of oxidation states they exhibit (Saji and Lee, 2012) and –S-Mo-S sheets are held by Van der Waals’ force of attraction (Ghosh et al., 2013). Their anisotropic bonding shows excellent physicochemical properties (Gong Zhanga et al., 2016). The drawback of exfoliated sheets of MoS₂ is its inherent restacking property, which shuts down its activity and this has been addressed in various reports by utilizing them as templates or hybrids or as composites (Kumar et al., 2015). Integration of Cu metal into the active edge sites of layered MoS₂ has been reported to significantly improve the efficiency in hydrogen evolution reactions (Hong et al., 2017) and hence it might be a promising application for photo catalysis/ photovoltaics as well. Wu et al. (2017) designed an in situ wet method to grow vertically few-layer of MoS₂ on 3D graphene, for its use as CE material. The material showed improvement in efficiency by 40–60% (4.3%) compared to platinum and pure MoS₂. Also Zhen et al. (2017) used MoS₂-graphene hybrids as CE material for CdS QDSSC to achieve an efficiency enhancement by 70% (2.21%) over bare MoS₂. D'Souza et al. (2015) developed GO-Cu₂S CE which was able to increase the performance of device from 0.49 to 0.77. Kamaja et al. (2017) explored MoS₂/CuS/Cu through in-situ technique and reports an efficiency of 5%. In our work, we have prepared 2D MoS₂ –CuS nanohybrid CE material by simple method, and it functioned as quasi co-catalyst in shuttling electrons between FTO and CuS with an efficiency of 6.7%.