Using 1D and 2D Nanomaterials in Halide Perovskite Solar Cells to Enhance Light Harvesting and Charge Collection

• Author / Creator

  Ujwal Kumar Thakur

• The increasing global energy demand and scarcity of available energy resources have spurred research efforts to find cheap and abundant alternative energy sources. Solar energy is the inexhaustible renewable energy resource that has the potential to supply more than 15 TW/annum of carbon-neutral energy. Among solar energy harvesting devices, solar cells are the most efficient, directly converting solar energy into electricity.
  Inorganic solar cells based on materials such as crystalline silicon, cadmium telluride and copper indium germanium selenide constitute mature technologies that exhibit a relatively high-power conversion efficiency of around 12−20% in deployed modules and thus dominate commercially available photovoltaic technologies. However, the relatively long energy payback times of inorganic solar cells have partially impeded their pace to widespread deployment, and thus alternative approaches are being explored. Organic photovoltaics (OPV), dye-sensitized solar cells (DSSCs), halide perovskite solar cells (HPSCs) and quantum-dot solar cells are examples of next-generation solution-processable solar cell technologies that have emerged as low cost, short energy payback time alternatives to replace conventional solar cells. Among these low-cost solar cells, HPSCs have revolutionized the field of photovoltaics by exhibiting the potential for ultralow cost fabrication and short energy payback times while simultaneously achieving solar-to-electricity power conversion efficiencies comparable to inorganic solar cells. Two major issues preventing deployment and commercialization of perovskite solar cell technology are stability and efficiency. Simple and economical encapsulation technologies have been developed to provide excellent encapsulation of OLED displays in commercial smartphones and OPV modules. It is therefore reasonable to expect that such techniques will be adapted to encapsulate HPSCs and thus potentially solve the stability issue. On the other hand, boosting the efficiency of HPSCs to their theoretical limit is more challenging. The efficiency of such solar cells mainly depends on (i) their ability to capture and trap the photons of the light (ii) the ability to manipulate the directionality, entropy and quantum efficiency of re-radiated photons (i.e. fluorescence) and (iii) the efficient separation and collection of photogenerated charge carriers.
  HPSCs contain a thin film of the halide perovskite active layer sandwiched between an electron transport layer and a hole transport layer. Halide perovskites have different electron and hole diffusion lengths because of which a nanostructured charge transporting layer is generally used to improve the charge collection efficiency. In the state-of-the-art n-i-p type solar cells, mesoporous TiO2 is used as the electron transport layer which suffers from poor charge transport (due to a random-walk type dispersive hopping of charge carriers) and poor light management inside the solar cells. Vertically aligned and near horizontally aligned TiO2 nanorod arrays were fabricated and used as the electron transporting material in the perovskite solar cells to realize high-efficiency n-i-p type perovskite solar cells. 17.6 % champion efficiency was realized with a solar cell architecture comprising monocrystalline vertically standing TiO2 nanorods (TNRs) infiltrated with perovskite and synthesized using facile solution processing without non-routine surface conditioning. TNR ensembles are desirable as electron transporting layers (ETLs) HPSCs because of potential advantages such as vectorial electron percolation pathways to balance the longer hole diffusion lengths in certain halide perovskite semiconductors, ease of incorporating nanophotonic enhancements, and optimization between a high contact surface area for charge transfer (good) vs. high interfacial recombination (bad). These advantages arise from the tunable morphology of hydrothermally grown rutile TNRs, which is a strong function of the conditions of growth. Horizontal nanowires have received much less attention despite their higher photonic strength due to overlapping electric and magnetic dipolar Mie resonance modes. Horizontal TiO2 nanorods were fabricated on FTO substrates via a facile hydrothermal route. The HATNRs are employed as the ETL to achieve 15.03% power conversion efficiency (PCE) in HPSCs which is higher than the PCE of compact TiO2 based devices (10.12 %) by a factor of nearly 1.5. Also, the hole transporting property of a NiO nanostructure with Ni3+ defect deposited by the solvothermal method has been studied in p-i-n type perovskite solar cells. With an optimized nanostructure, a high-efficiency solar cell with a power conversion efficiency ~34% higher than that of the conventional compact NiO based perovskite solar cells have been fabricated. Also, the effects of g-C3N4 and C3N5 doping on the charge transport properties of the perovskite active layer and hence on photovoltaic properties of perovskite solar cells have been studied.

• Subjects / Keywords

  • Solar Cells
  • Characterization
  • Nanorods
  • Fabrication
  • Perovskite
  • Metal Oxides

• Graduation date

  Spring 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-xh3s-ne34

• License

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