Deposition of zinc oxide as an electron transport layer in planar perovskite solar cells by spray and SILAR methods comparable with spin coating

Controlling surface morphology of Ag-doped ZnO as a buffer layer by dispersion engineering in planar perovskite solar cells

In recent years, the power conversion efficiency (PCE (%)) of perovskite solar cells (PSCs) has improved to over 26%. To enhance the photovoltaic properties of PSCs, several materials for the electron transport layer (ETL) have been investigated. Zinc oxide (ZnO) is a significant ETL due to its high electron mobility and optical transparency in PSCs. As a result of various deposition methods, ZnO ETL can be processed at low temperatures. On the other hand, based on several studies, metal-doped ZnO can facilitate electron transfer, thereby improving the performance of un-doped ZnO ETL-based PSCs. Here, to improve the PCE (%) and long-term stability of un-doped ZnO ETL-PSCs, silver (Ag)-doped ZnO 1wt% as a buffer layer is examined. In this paper, with the addition of an organic solvent (ethanol) to the dispersion of Ag-doped ZnO 1 wt% nanoparticles (NPs) in deionized (DI) water, the morphology of the buffer layer (Ag-doped ZnO 1 wt%) can be controlled. This approach focuses on reducing the wettability of the ZnO/Ag-doped ZnO 1 wt% bilayer ETLs and enhancing the stability of un-doped ZnO ETL-PSCs. According to the results, the ZnO/H2O-ethanol mixtures-Ag-doped ZnO 1 wt% bilayer ETL leads to the formation of high-quality perovskite with low defects, reducing the recombination rate, and long-term stability of un-doped ZnO ETL-PSCs in ambient conditions.

A Review of the Impact of Zinc Oxide Nanostructure Morphology on Perovskite Solar Cell Performance

Zinc oxide (ZnO) has been widely studied over the last decade for its remarkable properties in optoelectronic and photovoltaic devices because of its high electron mobility and excitonic properties. It has probably the broadest range of nanostructured forms that are also easy and cheap to synthesize using a wide variety of methods. The volume of recent work on ZnO nanostructures and their devices can potentially overshadow significant developments in the field. Therefore, there is a need for a concise description of the most recent advances in the field. In this review, we focus on the effect of ZnO nanostructure morphologies on the performance of ZnO-based solar cells sensitized using methylammonium lead iodide perovskite. We present an exhaustive discussion of the synthesis routes for different morphologies of the ZnO nanostructure, ways of controlling the morphology, and the impact of morphology on the photoconversion efficiency of a given perovskite solar cell (PSC). We find that although the ZnO nanostructures are empirically similar, one-dimensional structures appear to offer the most promise to increasing photoconversion efficiency (PCE) by their proclivity to align and form vertically stacked layers. This is thought to favor electron hopping, charge mobility, and conductivity by allowing multiple charge conduction pathways and increasing the effective junction cross-sectional area. The combined effect is a net increase in PCE due to the reduced surface reflection, and improved light absorption.

Hydrodynamic Analysis of the Thickness Variation in a Solid Film Formed by a Spin Coating Process

The surface profile of the film formed by spin coating is experimentally investigated in this paper. The unavoidable wavy form at the surface was observed when the ultraviolet curable resin was used. In addition, the surface thickness variation was directly related to the viscosity, disk rotation speed, and disk size. Fluid dynamic theory with non-dimensional analysis was conducted to describe the surface profile after the spin coating process. It was found that the film had been thickened until the viscosity force and Coriolis force were balanced. The Coriolis force, however, also affected the flow instability during the spinning of the disk. The film thickness variation is successfully described by using the non-dimensional factors. In addition, the edge bump which is induced by hydraulic jump is expressed by the relation of power law of Ekman, Weber, and Reynolds numbers. In this paper, the thickness variation and edge bump position are expressed by using hydrodynamic theory. It is also reveals that the Coriolis force acts based on the magnitude of thickness variation, and the surface tension affects the edge bump position. The presented relationships will contribute further understanding of the spin coating process. The outcome of this paper supports the cost-effective productions of electronic microcircuits and solar cells.

Role of a Solution-Processed V2O5 Hole Extracting Layer on the Performance of CuO-ZnO-Based Solar Cells

In this research, a heterostructure of the CuO-ZnO-based solar cells has been fabricated using low-cost, earth-abundant, non-toxic metal oxides by a low-cost, low-temperature spin coating technique. The device based on CuO-ZnO without a hole transport layer (HTL) suffers from poor power conversion efficiency due to carrier recombination on the surface of CuO and bad ohmic contact between the metal electrode and the CuO absorber layer. The main focus of this research is to minimize the mentioned shortcomings by a novel idea of introducing a solution-processed vanadium pentoxide (V2O5) HTL in the heterostructure of the CuO-ZnO-based solar cells. A simple and low-cost spin coating technique has been investigated to deposit V2O5 onto the absorber layer of the solar cell. The influence of the V2O5 HTL on the performance of CuO-ZnO-based solar cells has been investigated. The photovoltaic parameters of the CuO-ZnO-based solar cells were dramatically enhanced after insertion of the V2O5 HTL. V2O5 was found to enhance the open-circuit voltage of the CuO-ZnO-based solar cells up to 231 mV. A detailed study on the effect of defect properties of the CuO absorber layer on the device performance was theoretically accomplished to provide future guidelines for the performance enhancement of the CuO-ZnO-based solar cells. The experimental results indicate that solution-processed V2O5 could be a promising HTL for the low-cost, environment-friendly CuO-ZnO-based solar cells.

ZnO nanostructured materials for emerging solar cell applications

Zinc oxide (ZnO) has been considered as one of the potential materials in solar cell applications, owing to its relatively high conductivity, electron mobility, stability against photo-corrosion and availability at low-cost. Different structures of ZnO materials have been engineered at the nanoscale, and then applied on the conducting substrate as a photoanode. On the other hand, the ZnO nanomaterials directly grown on the substrate have been attractive due to their unique electron pathways, which suppress the influence of surface states typically found in the former case. Herein, we review the recent progress of ZnO nanostructured materials in emerging solar cell applications, such as sensitized and heterojunction architectures, including those embedded with promising perovskite materials. The remarkable advancement in each solar cell architecture is highlighted towards achieving high power conversion efficiency and operational stability. We also discuss the foremost bottleneck for further improvements and the future outlook for large-scale practical applications.

High-performance hole conductor-free perovskite solar cell using a carbon nanotube counter electrode

Carbon-based perovskite solar cells (C-PSCs) are the most promising photovoltaic (PV) due to their low material and manufacturing cost and superior long-term stability. This work compares the performance between gold (Au) and multi-wall carbon nanotube (MWCNT) electrodes for hole transport material (HTM)-free PSCs. Based on the obtained results, C-PSCs showed remarkable power conversion efficiency (PCE) and negligible hysteresis. Indeed, under optimized conditions, MWCNTs demonstrated superior performance as a counter electrode (CE) for HTM-free PSCs, leading to a PCE of 15.56%, which is comparable to the current state-of-the-art materials. Also, the presence of MWCNTs in the cell architecture enhances the collection and injection of holes at the perovskite/MWCNT interface and as a result, improves the external quantum efficiency (EQE) and current density because the recombination process is quenched. This improvement is confirmed by impedance spectroscopy (EIS), photoluminescence (PL), current/voltage (J–V), and EQE measurements. Moreover, MWCNTs could act as a protective layer and enhance the PSC stability. C-PSC was more stable than that of traditional PSC based on Au, which could maintain 80% of its primary PCE for long-periods of storage in moist conditions.

Carbon-based perovskite solar cells (C-PSCs) are the most promising photovoltaic (PV) due to their low material and manufacturing cost and superior long-term stability.

Influence of back donation effects on the structure of ZnO nanoclusters

The structure and properties of ZnO quantum dots is a very popular and rapidly growing field of research for which accurate quantum calculations are challenging to perform. Since the dependence between system size and wall time scales nonlinearly, certain compromises have to be made. A particularly important limiting factor is the size of the basis used, this is especially the case if accurate large calculations are to be carried out. In our work, we discovered that an important O(2p)->Zn(4p) back donation, which greatly influences the strength of the ZnO bond, can be reproduced only if diffuse functions are added to the basis set. We further tested the basis dependence for the magic-sized wurtzite nanophase ZnO clusters which were previously shown to be able to accurately reproduce the magnetically doped II-IV Q-dots. In this work, we outline the minimal basis sets required to properly describe ZnO bonds in a nanocluster. It was demonstrated that the rock salt nanophase is incorrectly stabilized if a basis set does not contain sufficiently diffuse functions while the correct wurtzite phase is stabilized when diffuse functions are added. This tendency, similar to that in the ZnO dimer case, was shown to stem from the incorrect lack of Zn(4p) electron density in calculations when using the diffuse-free basis set.

Optimizing the Performance of CsPbI3-Based Perovskite Solar Cells via Doping a ZnO Electron Transport Layer Coupled with Interface Engineering

Interface engineering has been regarded as an effective and noninvasive means to optimize the performance of perovskite solar cells (PSCs). Here, doping engineering of a ZnO electron transport layer (ETL) and CsPbI3/ZnO interface engineering via introduction of an interfacial layer are employed to improve the performances of CsPbI3-based PSCs. The results show that when introducing a TiO2 buffer layer while increasing the ZnO layer doping concentration, the open-circuit voltage, power conversion efficiency, and fill factor of the CsPbI3-based PSCs can be improved to 1.31 V, 21.06%, and 74.07%, respectively, which are superior to those of PSCs only modified by the TiO2 buffer layer or high-concentration doping of ZnO layer. On the one hand, the buffer layer relieves the band bending and structural disorder of CsPbI3. On the other hand, the increased doping concentration of the ZnO layer improves the conductivity of the TiO2/ZnO bilayer ETL because of the strong interaction between the TiO2 and ZnO layers. However, such phenomena are not observed for those of a PCBM/ZnO bilayer ETL because of the weak interlayer interaction of the PCBM/ZnO interface. These results provide a comprehensive understanding of the CsPbI3/ZnO interface and suggest a guideline to design high-performance PSCs.