Synthesis and Fabrication of low-cost, Metal Oxides-based Charge Transport Materials for Efficient and Stable Perovskite Solar Cells

Abstract

Organic-inorganic metal halide perovskite solar cells (PSCs) have attained widespread recognition in the photovoltaic (PV) market owing to their higher power conversion efficiencies (PCEs). The high absorption coefficient, long charge carrier diffusion length, ambipolar nature, and superior optoelectronic properties of perovskite materials are the key motivations behind the enhancement in photovoltaic performances. However, several fundamental issues related to charge transport materials (CTMs) such as high sintering temperature, poor stability, degradation, and excessive cost of materials restrict the commercialization of perovskite solar cell technology. Further, the complex synthesis procedures and toxic materials involved are not beneficial in fabricating lowcost and environment-friendly solar devices. The mismatched energy band levels and aggravated J-V hysteresis in perovskite devices also limit the performances of PSCs. The introduction of scalable, inexpensive, stable, and low-temperature solution-processed, charge transport materials for the electron transport layer (ETL) and hole transport layer (HTL) using facile deposition techniques would increase the possibility of commercialization of the PSCs. This experimental research investigates the potential of low-cost and stable, inorganic metal oxide (MOX) semiconductors (TiO2, α-Fe2O3) as ETL and (Fe3O4) HTL in perovskite solar by spin coating deposition technique. The in-depth analysis of ETL and HTL using MOX charge transport layers (CTLs) was conducted by simply tuning the optoelectronic properties and optimizing the experimental parameters to develop a low-cost, environment-friendly, efficient and stable perovskite solar cell. First, the high-purity iron oxide nanoparticles, hematite (α-Fe2O3), and magnetite (Fe3O4) were synthesized by a facile top-down approach using a high-energy ball mill. The high-purity hematite and magnetite nanoparticles were subsequently investigated for structural, morphological, thermal, optoelectrical, and magnetic properties and thus recommended for optoelectronic devices as plausible candidates. Further, the regular planar perovskite solar cells were fabricated by employing widely reported conventional TiO2 ETL with suitable dopant and spiro-OMeTAD as HTL using a lowtemperature solution process to investigate the charge collection and recombination reactions at interfaces. Moreover, the solvent-assisted crystallization of synthetic α-Fe2O3 ETLs was conducted to probe the impact of solvent and its concentration on the morphology, crystallinity, and film thicknesses. The perovskite solar cells were fabricated in an n-i-p architecture and analyzed for iv PCEs, stability, and J-V hysteresis. The optimal device exhibited a PCE of 11.65 % with a very low hysteresis index of 0.04. Furthermore, the high-purity natural hematite was employed in perovskite solar cells and analyzed for charge carrier kinetic and interfacial recombination. The entire device fabrication process was conducted at a temperature below 150 °C. The natural hematite ETL with 10 mg/ml concentration demonstrated a power conversion efficiency (PCE) of 13.3 %, fill factor of 68 %, and VOC of 1.03 V due to high crystallinity, fast carrier extraction from the perovskite to the ETL, and less charge recombination. Finally, Fe3O4/spiro-OMeTAD architecture was proposed to develop a solution-processed inorganic-organic double HTL in regular planar PSCs. A thin layer of magnetite nanoparticles was spin-coated between the perovskite and spiro layers. The Fe3O4/spiro-modified bilayer HTL-based perovskite devices exhibited higher PCE, and improved stability as compared to the unmodified devices owing to improved surface roughness and reduce trap density favorable in fast hole extraction and transport. Briefly, metal oxide charge transport materials were developed as ETL and HTL in regular planar PSCs using the spin-coating deposition technique. These low-cost, ecofriendly, and stable materials can be beneficial in the fabrication of solar devices with higher PCEs and good stability.