Synthesis and Characterization of Spinel Cobaltite (Co₃O₄) Thin Films for Function as Hole Transport Materials in Organometallic Halide Perovskite Solar Cells

Abstract

Conventional inorganic p-type conductive oxides, for example, NiO, CuO X , and CuCrO X , can serve as low-cost and efficient hole transport materials for wide-bandgap organolead halide perovskites [for example, MAPbI 3 ] but fail for low-bandgap Snrich organometallic perovskites, for example, (FASnI 3 ) 0.6 (MAPbI 3 ) 0.4 , where MA = (CH 3 NH 3 ) and FA = (HC(NH 2 ) 2 ). In this work, we explore spinel Co 3 O 4 -based p-type conductive oxides as hole transport materials in organometallic halide MAPbI 3 and (FASnI 3 ) 0.6 (MAPbI 3 ) 0.4 perovskite solar cells. We examine the structural, crystalline, optical, electrical, photo-electrochemical, and surface chemistry properties of spin-coated Co 3 O 4 films without and with lithium doping. We find that lithium doping improves hole mobilities and film optical transparency and causes a lithium-enriched overlayer (e.g., LiCoO 2 ) forming at the Co 3 O 4 film surface. As a result, lithium doping can maximize the hole transport properties of Co 3 O 4 in our inverted planar perovskite solar cells, achieving about 14 and 7% light-to-electricity power conversion efficiencies (PCEs) for perovskite halides MAPbI 3 and (FASnI 3 ) 0.6 (MAPbI 3 ) 0.4 , respectively. This work underscores that cobaltite spinels hold promise for application as working HTLs for all kinds of organometallic halide perovskites.

Self‐Organized Co3O4‐SrCO3 Percolative Composites Enabling Nanosized Hole Transport Pathways for Perovskite Solar Cells

“…This phenomenon manifests that a large percentage of Co 3 O 4 region do not well contribute to vertical charge transport in solar cells, which may relate to lateral diffusion, surface charge traps, etc. [ 43,44 ] In comparison, Co 3 O 4 ‐SrCO 3 percolative film features more homogeneous currents with the mean value over ≈10 nA (Figure 3e,h). It should be pointed out that the maximum currents of both samples are ≈15 nA, and the current near the grain boundaries of Co 3 O 4 ‐SrCO 3 film are close to that of the bulk grains, which excluded the possibility of forming other highly conductive phases in the Co 3 O 4 ‐SrCO 3 percolative film.…”

Organic‐Inorganic Hybrid Perovskite Nanomaterials: Synthesis and Application

“…(3) Device efficiency issues: The photoelectric conversion efficiency of solar cells based on hybrid perovskite nanomaterials need to be improved. At present, the highest efficiency of hybrid perovskite solar cells based on polycrystalline thin films has reached 24.2 %, [93] but PCE of the perovskite solar cell based on hybrid perovskite nanomaterials only reached 18.7 % [88] . The optical and electrical properties of perovskite nanomaterials are closely related to particle size, morphology and purity after forming film.…”

“…At present, the highest photovoltaic efficiency of single-crystalline solar cells is 21.09 %, [92] lower than that of solar cells based on polycrystalline perovskite thin films, which is 24.2 %. [93] In addition, the high cost of single crystal preparation and the limitations of preparation technology and equipment limit the application of monocrystalline perovskite cells. Compared with the aforementioned bulk polycrystals and single crystals, nanomaterials not only possess some of their characteristics, but also have many unique advantages.…”

“…However, because of the hydration of single crystal surface and the inevitable contamination during the crystal growth process, the performance is seriously affected. At present, the highest photovoltaic efficiency of single‐crystalline solar cells is 21.09 %, [92] lower than that of solar cells based on polycrystalline perovskite thin films, which is 24.2 % [93] . In addition, the high cost of single crystal preparation and the limitations of preparation technology and equipment limit the application of monocrystalline perovskite cells.…”

Core Fusion Engineering of Hole-Transporting Materials for Efficient Perovskite Solar Cells

“…As a solution-processable photovoltaic technology, perovskite solar cells (PSCs) have realized ultrahigh efficiency up to 25.2% during the past years due to the extraordinary optoelectronic properties of perovskite materials and great attention worldwide. − Generally, the structure of PSCs consists of a thick layer of perovskite light-harvesting material sandwiched between electron- and hole-transporting materials and two contact electrodes . Among them, the hole-transporting materials (HTMs) play a considerable role in hole extraction and transport in PSCs, especially for high-performance devices. , Compared to inorganic HTMs, − which usually show more surface defects, organic HTMs possess good solution-processability and tuneable film morphological properties, enabling superior interfacial contact, effective passivation effect, and reduced surface defects. − Among organic HTMs, small molecules exhibit well-defined structures, high purity, and good repeatability, which have been regarded as promising candidates for HTMs. , However, the widely used small molecular HTM spiro-OMeTAD suffers from relatively low thermal stability with the T g of 121 °C and unsatisfactory film morphology, which usually acquire a high concentration of HTM solution (60–80 mg mL –1 ), and the pinhole defects cannot be avoided in some cases . In addition, intrinsic low hole mobility is observed for spiro-OMeTAD due to its symmetric globular structure and the resultant weak π–π interactions. − Hence, it is imperative to explore new HTMs with good hole mobility and excellent film morphology.…”