(Invited) Porous Semiconducting Nanotube Interfaces for Device Applications
Authors: Gonzalez-Rodriguez Roberto , Hathaway Evan , Lin Yuankun , Cui Jingbiao , Coffer Jeffery 
Published: December 2024

Non-lithographic routes to creating curved, non-planar nanostructures and associated interfaces are appealing from both fundamental scientific and applied technological perspectives. For some time, we have been interested in the creation of hollow Si nanotubes with sensitive control of inner tube diameter, wall thickness, and tube length using the sacrificial etching of zinc oxide (ZnO) from Si-coated ZnO nanowires (NWs). With suitable nanotube wall thinness (< 10 nm), porosity in the sidewall morphology is retained as well (pSiNTs). Such structures have been exploited for selected applications relevant to drug delivery1, lithium storage/recycling,2 and optoelectronics/solar cells.3

For the latter category, charge carrier energetics also require careful control of the interfacial composition of the surrounding layers as well as their structure. This is especially true for semiconductors known as metal halide perovskites, where power conversion efficiencies (PCEs) for solar cells above 30%4 and external quantum efficiencies (EQEs) for LEDs above 10% have been reported recently.5

In this work we demonstrate proof-of-concept for nanotube geometries of such devices produced non-lithographically by creating porous nanotubes of either CeO2 or NiO following the sacrificial templating process outlined above, followed by formation of selected perovskites (MAPbI3 or MAPbBr3) within their hollow interior. Structural characterization and evaluation of their respective performance as a PV cell (MAPbI3/CeO2) or LED (MAPbBr3/NiO) is described, along with possible refinements and/or opportunities for the future.
As an n-type semiconductor, the thermal stability and transparency of cerium oxide makes it an excellent candidate as an electron transport layer (ETL), and in planar perovskite solar cells it has been shown to offer protection of perovskites to UV light, heat, and moisture degradation. Cerium oxide was deposited onto the ZnO nanowires (100 nm diameter) by a spin coating process using a dilute solution of Ce3+, followed by OH- addition, ample rinsing to remove excess precursors, thereby forming cerium hydroxide on the surface of the ZnO NW (1 cycle). This process can be repeated amply until the desired wall thickness is obtained. To obtain CeO2, it is necessary to anneal the cerium hydroxide at 500°C in air. CeO2 NTs are subsequently formed by etching away the ZnO core with 1% HCl.
Sidewall porosity facilitates formation of a desired metal halide perovskite (such as MAPbI3) by infiltration of a precursor solution, spin coating to remove excess, and thermal annealing at 500oC. Such MAPbI3/CeO2 NT structures have been thoroughly characterized by a combination of SEM, TEM, XRD, XPS, Raman, and PL spectroscopy. We use an inverted p-i-n configuration for an initial photovoltaic cell evaluation; specifically, a non-optimized device architecture of FTO-CuO-MAPbI3/CeO2 NTs-C60-Ag produces a Voc 0.81 ± 0.04 V and a PCE % 11.32 ± 1.35.
From the complementary perspective of a hole transport layer for perovskite devices, one appealing candidate is nickel oxide (NiO) in nanotube form: it has high carrier mobility, good stability and processability; prior successful use in planar devices with suitable band level energetics has been demonstrated; and its use avoids the hygroscopic and acidic substrate-degrading nature of organic counterparts such as PEDOT:PSS. Similar to the CeO2 system, formation of NiO NTs is achieved by controlled exposure of preformed ZnO NW arrays to dilute aqueous solutions of a suitable metal salt (in this case nickel chloride) which is then followed by base exposure, wash steps, and a thermal anneal (at 500°C) to form a Ni(OH)2 shell. Etching in 1% HCl then occurs (to remove the ZnO), followed by another 500oC anneal, thereby creating the desired NiO NT. Nanotube wall thickness is controlled by duration of Ni precursor solution exposure. These pNiO NTs have been characterized by a combination of SEM, TEM, XRD, and XPS. Infiltration of these NTs to suitable perovskite precursors, followed by wash steps and a thermal anneal yields nanoscale MAPbBr3 formed within porous NiO nanotubes. Utilizing a device configuration of FTO/TiO2/MAPbBr3-pNiOx NTs/Spiro-OMeTAD/Ag, measurement of the electroluminescence (EL) properties of MAPbBr3 in these porous NiO nanotubes reveal a current efficiency (CE) of 5.99 Cd/A and external quantum efficiency (EQE) of 3.9%.
Refinement of each type of structure, including interfacial chemical modification to reduce defects at these curved surfaces, is underway.

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