Research Article: Low‐Temperature Combustion Synthesis of a Spinel NiCo2O4 Hole Transport Layer for Perovskite Photovoltaics

Date Published: March 03, 2018

Publisher: John Wiley and Sons Inc.

Author(s): Ioannis T. Papadas, Apostolos Ioakeimidis, Gerasimos S. Armatas, Stelios A. Choulis.


The synthesis and characterization of low‐temperature solution‐processable monodispersed nickel cobaltite (NiCo2O4) nanoparticles (NPs) via a combustion synthesis is reported using tartaric acid as fuel and the performance as a hole transport layer (HTL) for perovskite solar cells (PVSCs) is demonstrated. NiCo2O4 is a p‐type semiconductor consisting of environmentally friendly, abundant elements and higher conductivity compared to NiO. It is shown that the combustion synthesis of spinel NiCo2O4 using tartaric acid as fuel can be used to control the NPs size and provide smooth, compact, and homogeneous functional HTLs processed by blade coating. Study of PVSCs with different NiCo2O4 thickness as HTL reveals a difference on hole extraction efficiency, and for 15 nm, optimized thickness enhanced hole carrier collection is achieved. As a result, p‐i‐n structure of PVSCs with 15 nm NiCo2O4 HTLs shows reliable performance and power conversion efficiency values in the range of 15.5% with negligible hysteresis.

Partial Text

Over the last few years, a great deal of effort has been made to improve photovoltaic performance based on organic–inorganic lead halide perovskites, which has been reported to exhibit power conversion efficiencies (PCEs) over 20%.1, 2, 3, 4, 5 The use of organic–inorganic lead halide perovskites has attracted intense interest due to extraordinary characteristics such as high light absorption,6, 7, 8, 9 enhanced charge transport properties, and direct band gap transition. For the fabrication of efficient perovskite solar cells (PVSCs), the so‐called n‐i‐p architecture is widely used.10 For the p‐i‐n‐type PVSCs, also called inverted architecture structure, poly(3,4‐ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) is commonly used as a hole transport layer (HTL). PEDOT:PSS is usually used as HTL for printed electronic due to its facile processing and good electrical conductivity and transparency.11, 12, 13, 14, 15, 16 On the other hand, the hydroscopicity and inhomogeneous electrical properties might limit its performance as a HTL for advanced optoelectronic applications.17, 18 Recently, p‐type metal oxides and complexes, such as NiO, V2O5, CuO, CuSCN, CuPc, and ZnPc,19, 20, 21, 22, 23, 24, 25 have been incorporated as HTLs into PVSCs. Inorganic p‐type semiconductor materials have the advantages of providing energy levels for improved hole selectivity and chemical stability, showing promising performance as HTLs in PVSCs.26, 27

Combustion synthesis has been applied recently for the low‐temperature fabrication of metal oxide thin films.67 In general, solution combustion synthesis has the advantage of rapidly producing homogeneous metal oxide materials with fine grain size, and most significantly at much lower temperature compared with the conventional solid‐state reaction processes and co‐precipitation methods. The structural and morphological characteristics of the resulting materials closely depend on the type and amount of chemicals (fuel, oxidizing agent) used in the synthesis.33, 45 Furthermore, the choice of the fuel regent for the combustion process has an essential role to avoid the formation of large clusters or/and large voids between the grains.46 A fuel, i.e., the substance capable of acting as electrons acceptor, can significantly affect the properties of the final product, such as grain size, surface area, morphology, crystal phase, and degree and nature of particle agglomeration.52, 55

In conclusion, a low‐temperature combustion synthesis method, using for the first time a tartaric acid as a fuel, was successfully developed and applied for the fabrication of compact films of p‐type NiCo2O4 NPs. The size of the NPs was fully controlled due to the usage of tartaric acid leading to the formation of monodispersed NiCo2O4 NPs with a diameter of ≈4 nm. The combustion proceeds under low temperature (250 °C) and within a short reaction time (1 h), produce high‐quality, homogeneous NiCo2O4 NPs films with high electrical conductivity (≈4 S cm−1) and very low roughness (0.56 nm) functional layers were fabricated. The detailed physicochemical characterization of the NiCo2O4 NPs using XRD, EDS, and electron microscopy measurements confirm the high purity, crystallinity, and small grain composition of the NiCo2O4. Furthermore, the proposed synthetic approach allowed the production of compact films using blade coating, which is a large‐scale compatible technique appropriate for the development of printed electronic devices. The impact of NiCo2O4 HTL thicknesses on PVSCs characteristics was also investigated. The optimum thickness is found to be 15 nm showing enhanced charge carrier collection and negligible J–V hysterics, compared to thicker films, delivering reliable p‐i‐n PVSCs with a PCE of 15.5%. We believe that the proposed combustion synthesis method using a tartaric acid as a fuel can provide a route to produce highly reproducible metal oxides suitable for use in a range of advanced materials applications.

Materials: Prepatterned glass‐ITO substrates (sheet resistance 4Ω sq−1) were purchased from Psiotec Ltd., Pb(CH3CO2)2.3H2O from Alfa Aesar, methylammonium iodide (MAI) and methylamonium bromide (MABr) from Dyenamo Ltd., PC[70]BM from Solenne BV. All the other chemicals used in this study were purchased from Sigma‐Aldrich.

The authors declare no conflict of interest.




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