Enhanced PSCs Performance with Sn and AgCl co-doped Titania Microspheres

Scientists from Various Universities (1. Mössbauer Effect Data Center, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China, 2. Centre for Solar Energy Materials, International Advanced Research Centre for Powder Metallurgy and New Materials, Hyderabad 500005, India, 3. Department of Materials Science and Engineering, City University of Hong Kong, Kowloon Tong, Hong Kong SAR, 4. Tokyo University of Science, Kagulazaka 1-3, Shinjuku, Tokyo 162-0825, Japan) and their research collaboration team have discovered hierarchical Sn and AgCl co-doped TiO₂ microspheres as an electron transport layer for the enhanced perovskite solar cell performance.

Zero carbon emission by low-cost and environmentally-friendly alternative energy resources is required for the growing world’s energy demand. Concerning this, perovskite solar cells (PSCs) have gained more attention in recent years, owing to their low cost, higher absorption coefficient, and high efficiency [1]. In general, PSCs are composed of three layers, the electron transport layer (ETL), the absorber layer (perovskite), and the hole transfer layer. An emerging trend of developing novel and effective electron/hole-transporting materials for the enhancement of material stability and device efficiency of PSCs is carried out. Therefore, to tune and control the growth of these perovskite films, a new hole or electron interfacial layers with suitable energy levels are very essential for high-performance and stability in PSCs. Organic-inorganic halide PSCs provide a greater potential for the photovoltaics applications owing to their unique advantages, like simple process low-cost production with very high efficiency. Titanium dioxide (TiO₂) is widely utilized in PSCs as ETL owing to its low-cost production, photo-stability, nontoxicity, and chemical inertness. TiO₂ has a wide bandgap (Eg = 3.2 eV) which can trigger the UV radiation, electron-hole pairs that can be created at the surface of the outer region which accounts for the solar spectrum of about 3–4% [2].

Ganeshraja et al., and his team have reported about the importance of metal doping, co-doping, and metal complex grafting in TiO₂. For the first time, they reported on the fabrication of PSCs with hierarchical AgCl@Sn-TiO₂ microspheres as photoelectrode via a two-step spin coating process, undermining the need for dry atmosphere and post-annealing. Further, they effectively improved the visible light photocatalytic activity by doping the metal or nonmetal element into TiO₂microspheres which showed a significant enhancement in the photovoltaic properties in comparison with undoped TiO₂nanoparticles [3–5].

Figure 1. Schematic diagram of (a) PSC device and (b) Energy band diagram for the fabricated device [5].

The energy band diagram (Figure 1b) represents the fabricated device, showing the level of highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the perovskite which suitably matched for the electron transporting AgCl@Sn-TiO₂ layer for the hole transport layer which facilitates for efficient dissociation of the exciton and extraction through these electrodes. They have used the standard 2,2′,7,7′-tetrakis(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene(spiro-MeTAD) and with their respective additives as the hole transporting layer with the thermal evaporated gold as a cathode, respectively. Further, they found that the thickness of TiO₂ with perovskite and hole transporting layers in the range of 400 and 180 nm, respectively.

They have utilized the noble metal halide loaded with silver doped metal oxide semiconductor microspheres as photoelectrodes for PSCs. Dopants used in this work are non-toxic, low-cost, and highly photo-stable. A very small amount of dopant inclusion is expected to increase the optical density of the active layer, thus for the improvement of light-harvesting. Therefore, the performance of PSCs has arisen from various multiple factors which include excellent crystallinity, light absorption, carrier dynamics, and surface morphology. Also, the changes in band structure and energy level alignment might contribute to improved solar cell device performance. As it can be seen that, the ETL film morphology and poor perovskite infiltration reduces the device performance, irrespective of its good photocatalytic property in high level loaded AgCl materials due to variation in particle size and porosity. However, the exact mechanism further needs to be investigated. In general, under dark conditions, all their devices are stored at ambient conditions with 30–40% humidity.

To check for the long term stability, they have developed new ETL devices measured without encapsulation. Hence, a low AgCl content in TiO₂ microspheres showed good photovoltaic and magnetic properties. On the other side, high AgCl content in TiO₂is predominantly observed with visible light and play as an efficient photocatalyst [4,5].

Our SNB team recommended this research article to enrich our viewer’s knowledge to know about the hierarchical Sn and AgCl co-doped TiO₂ microspheres and their enhancement of PSCs performance. Further, they summarize that the noble metal halide and tin codoped hierarchical TiO₂microspheres were synthesized via the simple hydrothermal method by a single step mechanism in an efficient manner. Also, the as-synthesized hierarchical TiO₂ microspheres showed efficient photovoltaic activity in the PSCs applications. The use of AgCl nanoparticle loaded and Sn doped TiO₂ ETL increases the optical density of the active layer which in turn slings the efficiency of PSCs. In order to develop for a new generation of materials with highly stable and efficient PSCs, the exploration of AgCl nanoparticles loaded with metal-doped/codoped of metal oxide semiconductors based systems obtained significant potential with tailored optical and electronic characteristics. Finally, they reported that AgCl@Sn-TiO₂ is one of the efficient materials with good potential for application in photo energy conversion devices. This provides new insight into the design of advanced perovskite solar cells with TiO₂based electron transport materials.

References

Z. Wang et al., Angew. Chem. Int. Ed. 56, 1190 (2017).
S. Maniarasu, et al., Renew. Sustain. Energy Rev. 82, 845 (2018).
A.S. Ganeshraja, et al., RSC Adv. 6, 409 (2016).
A.S. Ganeshraja, et al., J. Phys. Chem. C 121, 6662 (2017).
A.S. Ganeshraja, et al., Catalysis Today, 355, 333(2020).

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School of Materials Science and Engineering,

Tianjin University of Technology, China

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