Nickel oxide for inverted structure perovskite solar cells

Abstract

The emergence of inverted perovskite solar cells (PSCs) has attached great attention derived from the potential in improving stability. Charge transporting layer, especially hole transporting layer is crucial for efficient inverted PSCs. Organic materials were used as hole transporting layer previously. Recently, more and more inorganic hole transporting materials have been deployed for further improving the device stability. Nickel oxide (NiOx) as p-type metal oxide, owning high charge mobility and intrinsic stability, has been widely adopted in inverted PSCs. High performance over 20% efficiency has been achieved on NiOx base inverted PSCs. Herein, we have summarized recent progresses and strategies on the NiOx based PSCs, including the synthesis or deposition methods of NiOx, doping and surface modification of NiOx for efficient and stable PSCs. Finally, we will discuss current challenges of utilizing NiOx HTLs in PSCs and attempt to give probable solutions to make further development in efficient as well as stable NiOx based PSCs.

Introduction

Perovskite solar cells (PSCs) have made tremendous progresses in photoelectric efficiency from 3.8% to 25.2% in a decade [1,2]. The high efficiencies of perovskite in fast development is mainly due to its outstanding optoelectronic properties: 1) excellent light absorption coefficient (~10⁵ cm⁻¹); 2) long carrier diffusion length (>1 µm) in layer structured films; 3) fairly high defects tolerance [3,4,11]. Generally, PSCs employ an organic-inorganic hybrid halide perovskite with formula of AMX₃ (A = methylammonium ‘MA’, formamidinium ‘FA’, Cesium ‘Cs’ and Rubidium ‘Rb’ or these cations with different ratio in A side; B = Lead ‘Pb’ and/or Tin ‘Sn’; X = Iodine ‘I’ and/or Bromine ‘Br’) as light absorber layer.

The perovskite is sandwiched by n-type electron transporting (ETL) layer and p-type hole transporting layer (HTL). Depending the sequence charge transporting layer, the configuration of PSCs has developed two types of structure: n-i-p structure (conventional/regular) and p-i-n structure (inverted) [5,6,7]. Nowadays, the champion efficiency of PSCs is delivered from the conventional structure. Meanwhile, inverted planar structure has been a hotpot on PSCs due to its advantages such as simple device fabrication, high stability and negligible hysteresis effect [8,9,110]. In addition, high quality of n-type metal oxide such as SnO₂ and ZnO could be easily deposited on the surface of perovskite layer at low temperature, which make it feasible for making top illumination of PSCs for tandem solar cells. While the p-type metal oxide such as NiOx usually need high temperature for the processing and also the solvent used could destroy the perovskite layer, which make the n-i-p structure not explored deeply in tandem structure [10,11,118,119].

In inverted structure PSCs, poly (3,4-ethylene dioxythiophene): poly (4-styrenesulfonate) (PEDOT: PSS) was initially used as hole transporting layer, while the open circuit voltage was always low, due to the inferior band gap alignment and also the serious interface recombination [108,117]. Thereby, poly-bis (4-phenyl) (2,4,6-trimethylphenyl) amine (PTAA) is used frequently to substitute PEDOT: PSS. And the most recent progress of PTAA inverted device showed certificated 22.3% power conversion efficiency [12,13,14,20]. However, the organic HTMs possess several drawbacks: high cost in raw materials and preparation, difficult deposition on large area substrate and potential degradation in moisture and high temperature environment [15]. In addition, some molecular organic based hole transport layer such as PTAA shows hydrophobic property, which makes the wet chemical deposition of perovskite a great challenge [12,20]. These factors confine the applications and commercialization of PSCs. Alternatively, inorganic p-type semiconductors could replace these organic HTMs for inverted PSCs because of higher stability and intrinsically higher carrier mobility and conductivity. So far, several inorganic materials with high transparency for visible solar light region and tunable energy level could be candidates, including copper compounds (CuO, CuO₂, CuSCN, CuI) [16,17], nickel oxide (NiOx) and some other metal oxides like V₂O₅ and MoOx [18,19]. The energy level alignment of different commonly used HTMs is illustrated in Fig. 1.

Among the mentioned inorganic transport layer, the NiOx hole transporting layer for PSCs have been extensively investigated. And the highest power conversion efficiency based on NiOx is also beyond 20% with negligible hysteresis [12,20,21]. These works demonstrate that NiOx is a promising p-type candidate HTM for inverted PSCs, since NiOx has following merits: 1) Large band gap (>3.5 eV) exhibit good transmittance in visible region; 2) A deep-lying valence-band maximum (V_BM~5.4 eV), making it energetically favorable energy level alignment with most perovskite absorbers [22,23,24]; 3) Outstanding chemical stability compared with other inorganic HTM like V₂O₅, MoOx. NiOx could not react with or exchange atom with the perovskite layer; 4) Facile synthesis and depositions via a variety method, such as chemical solution processes (sol-gel and sintered nanoparticles) and series of physical vapor deposition methods. Moreover, the valued ratio cost of average NiOx based modules also provides promising commercial potential (Fig. 2) [22,25]. These appealing features make NiOx HTLs suitable for the fabrication of low-cost, flexible, large area PSCs with negligible hysteresis, long-term stability.

Nevertheless, besides the advantages aforementioned, there are several problems need to be overcome for achieving high performance of PSCs based on NiOx HTL. The critical issues are: 1) The relatively low intrinsic conductivity (~10⁻⁴ S/cm); 2) The valance band maximum (V_BM) obtained NiOx is often not real deep (-5.4 eV); 3) Too much surface defects on NiOx [26,27]. Influenced by these factors, the non-radiation recombination at interface could increase heavily, which are detrimental to open circuit voltage (V_oc) and fill factor (FF) even short circuit current density (J_sc) of PSCs. It is very essential to address these issues for its further application in PSCs. In this work, we review the recent progresses in using NiOx as hole transport layer in inverted structure PSCs as well as the related device performance, long-term stability and hysteresis. The strategies enhancing the NiOx will be discuss in detail, including synthesis, deposition routes, extrinsic doping, surficial modification/passivation of NiOx and other proposals about NiOx based PSCs. Finally, the challenges and future outlooks on further improvements of NiOx based PSCs will also be discussed.