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Self-Functionalization Behind a Solution-Processed NiOx Film Used As Hole Transporting Layer for Efficient Perovskite Solar Cells John Ciro, Daniel Ramírez, Mario Alejandro Mejía Escobar, Juan Felipe Montoya, Santiago Mesa, Rafael Betancur, and Franklin Jaramillo* Centro de Investigación, Innovación y Desarrollo de MaterialesCIDEMAT, Facultad de Ingeniería, Universidad de Antioquia (UdeA), Calle 70 52-21, Medellín, Colombia S Supporting Information *
ABSTRACT: Fabrication of solution-processed perovskite solar cells (PSCs) requires the deposition of high quality films from precursor inks. Frequently, buffer layers of PSCs are formed from dispersions of metal oxide nanoparticles (NPs). Therefore, the development of trustable methods for the preparation of stable colloidal NPs dispersions is crucial. In this work, a novel approach to form very compact semiconducting buffer layers with suitable optoelectronic properties is presented through a self-functionalization process of the nanocrystalline particles by their own amorphous phase and without adding any other inorganic or organic functionalization component or surfactant. Such interconnecting amorphous phase composed by residual nitrate, hydroxide, and sodium ions, proved to be fundamental to reach stable colloidal dispersions and contribute to assemble the separate crystalline nickel oxide NPs in the final film, resulting in a very homogeneous and compact layer. A proposed mechanism behind the great stabilization of the nanoparticles is exposed. At the end, the selffunctionalized nickel oxide layer exhibited high optoelectronic properties enabling perovskite p-i-n solar cells as efficient as 16.6% demonstrating the pertinence of the presented strategy to obtain high quality buffer layers processed in solution at room temperature. KEYWORDS: hole transport layer, nickel oxide, PIN perovskite solar cell, colloidal nanoparticles stability, self-functionalization
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are their low processing temperature (sub-150 °C) and processability in solution. However, these kind of materials are expensive and poorly stable leading to a premature degradation of the solar cell device.17,18 Alternatively, stable transition metal oxides, such as Cu2O, CuO, or NiOx, can be applied as functional and low cost HTLs.19,20 However, the processing to these materials normally require high sintering temperature (≥300 °C) limiting the possibility of application on flexible substrates.21−27 Thus, the deposition of metal oxide layers at low temperature is crucial for the development of flexible, stable, efficient, and low cost PSCs. Nickel oxide (NiOx) has been demonstrated as a promising HTL candidate for PSCs.28−30 Some desirable properties of NiOx are its high transparency, wide bandgap, high hole mobility and appropriate alignment of its valence band with that of the CH3NH3PbI3 perovskite.31−33 The deposition of NiOx films by different techniques, such as sputtering,34,35 electrodeposition,20 atomic layer deposition,36 or pulse laser deposition,28 allowed obtaining highly efficient PSC devices. However, these physical methods are highly expensive and
INTRODUCTION Organometal trihalide perovskites were synthesized for the first time in 1978.1 Although some aspects of solution processing and properties of these materials were reported in the 1990s,2 they have been extensively applied just in the past few years.3−5 The swift surge in the research on perovskites has been promoted by its application in optoelectronic devices, especially in solar cells.6−8 Nowadays, perovskite solar cells (PSCs) have reached a certified power conversion efficiency of 22.1%.9 Commonly, the most efficient PSC devices require buffer layers in order to effectively transport the charges generated in the perovskite absorber to the electrodes.10−12 Hence, the design of functional charge transport layers with suitable optoelectronic properties is a critical aspect. The main features of an efficient charge transport layer are high transparency, wide bandgap, effective alignment with the energy levels of the active layer, high electron/hole mobility, and optimal morphological properties in order to avoid shunt pathways within the device.12,13 Some organic polymers or molecules, such as poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) or 2,2′,7,7′-tetrakis(N,N-di-p-methoxiphenylamine)-9,9′-spirobifluorene (spiro-MeOTAD) have been successfully applied as hole transport materials (HTL) in PSCs.14−16 The main advantages of these organic materials © XXXX American Chemical Society
Received: December 13, 2016 Accepted: March 17, 2017
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DOI: 10.1021/acsami.6b15975 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. Compositional analysis of the synthesized sf-NiOx powder. (a) Top: EDS analysis of the synthesized sample (sf-NiOx) exhibiting sodium and nitrogen traces. Bottom: reference NiO directly obtained from calcination of commercial Ni(OH)2 Inset: Corresponding atomic concentration. (b) Diffractogram of the obtained powder. Inset: Ratio of the main peaks (200)/(111).
(XRD) and energy-dispersive X-ray spectroscopy (EDS), respectively. Transmision electron microscopy (TEM) analysis revealed a self-functionalized NiOx NPs (sf-NiOx) consisting of nanocrystalline NiOx nanoparticles embedded in an amorphous interconnected phase. Such amorphous phase was relevant for the formation of stable colloidal dispersions. Thermogravimetric analysis (TGA), Fourier transform infrared (FTIR), and Raman in situ spectroscopy characterization of the sf-NiOx showed the presence of some reaction byproducts in the amorphous phase, especially nitrates, hydroxide and sodium. We found that these byproducts in the amorphous phase are fundamental for the formation of stable colloidal dispersion to form interconnected particles and therefore uniform films. Atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM) characterization of the deposited sf-NiOx films revealed optimal morphological and electronic properties, especially a very low roughness of around 3.5 nm and a high hole mobility of 0.048 cm2 V−1 s−1 were calculated. Our sf-NiOx films were then applied as HTL for PSCs. Efficiencies as high as 16.6% with null hysteresis and high reproducibility were achieved. Notoriously, a better average PCEs was reached implementing the sf-NiOx NPs compared with previous publications using NiOx films made by more expensive physical methods.36 We propose the self-functionalization of NiOx nanoparticles as a proof concept method to achieve stable colloidal dispersions of metal oxides suitable for application as charge transport layers in solution processed optoelectronic devices.
require stringent conditions such as high vacuum and high temperature. As an alternative, some research groups have developed solution processing methods for the deposition of NiO x films obtaining PSCs with average photovoltaic conversion efficiency (PCEs) ranging from 7.6 to 16.47%.22,24,26,27,30,37−40 The precursors used for solution processing of NiOx films are nickel organic sols and colloidal dispersions of nanocrystalline NiOx. While films deposited with the former precursor require annealing at high temperatures (>300 °C) to obtain the desired crystalline phase of NiOx,37 films made from the later precursor require very soft annealing conditions (
