Carbon materials, with their diverse allotropes, have played significant roles in our daily life and the development of material science. Following 0D C60 and 1D carbon nanotube, 2D graphene materi...

3D Graphene Materials: From Understanding to Design and Synthesis Control.

There has been considerable progress over the last decade in development of the perovskite solar cells (PSCs), with reported performances now surpassing 25.2% power conversion efficiency. Both long-term stability and component costs of PSCs remain to be addressed by the research community, using hole transporting materials (HTMs) such as 2,2′,7,7′-tetrakis(N,N′-di-pmethoxyphenylamino)-9,9′-spirbiuorene(Spiro-OMeTAD) and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA). HTMs are essential for high-performance PSC devices. Although effective, these materials require a relatively high degree of doping with additives to improve charge mobility and interlayer/substrate compatibility, introducing doping-induced stability issues with these HTMs, and further, additional costs and experimental complexity associated with using these doped materials. This article reviews dopant-free organic HTMs for PSCs, outlining reports of structures with promising properties toward achieving low-cost, effective, and scalable materials for devices with long-term stability. It summarizes recent literature reports on non-doped, alternative, and more stable HTMs used in PSCs as essential components for high-efficiency cells, categorizing HTMs as reported for different PSC architectures in addition to use of dopant-free small molecular and polymeric HTMs. Finally, an outlook and critical assessment of dopant-free organic HTMs toward commercial application and insight into the development of stable PSC devices is provided.

Development of Dopant-Free Organic Hole Transporting Materials for Perovskite Solar Cells

Organic–inorganic hybrid lead halide perovskites have been widely investigated in optoelectronics both experimentally and theoretically. The present work incorporates chemically modified graphene into nanocrystal SnO2 as the electron transporting layer (ETL) for highly efficient planar perovskite solar cells. The modification of SnO2 with highly conductive two-dimensional naphthalene diimide-graphene can increase surface hydrophobicity and form van der Waals interaction between the surfactant and the organic–inorganic hybrid lead halide perovskite compounds. As a result, highly efficient perovskite solar cells with power conversion efficiency of 20.2% can be achieved with an improved fill factor of 82%, which could be mainly attributed to the augmented charge extraction and transport.

Efficient Planar Perovskite Solar Cells with Improved Fill Factor via Interface Engineering with Graphene

Organic–inorganic halide perovskite solar cells (PSCs) have achieved great success in recent years with a demonstrated power conversion efficiency (PCE) increasing rapidly from 3.8% to 22.3% for single junction devices. Most high-performance PSCs consist of a perovskite absorber sandwiched between an electron transport layer (ETL) and a hole transport layer (HTL), which extracts electrons (holes) and blocks holes (electrons) from the absorber efficiently. Inorganic hole transport materials have extracted extensive attention due to their higher mobility and better stability. Particularly, the excellent hole selective transport property of nickel oxide (NiOx) has been highlighted by its recent application in organometallic halide PSCs, due to the favorable band alignment formed between the halide perovskite absorber and NiOx HTL. This comprehensive review summarizes the recent progress in the fabrication of NiOx films and their application in PSCs. Special attention is paid to the optoelectronic properties of NiOx films, which strongly depend on the synthesis methods and post-treatment conditions, as well as the resulting photovoltaic device performance. Surface modification and doping strategies that are used to improve the optoelectronic properties of NiOx films and the resulting device performance are discussed with emphasis. Finally, a short perspective of NiOx-based PSCs is also provided.

Nickel Oxide as Efficient Hole Transport Materials for Perovskite Solar Cells

Inverted perovskite solar cells employing doped NiO hole transport layers: A review

Sol-gel-processed yttrium-doped NiO as hole transport layer in inverted perovskite solar cells for enhanced performance

Cesium‐Containing Perovskite Solar Cell Based on Graphene/TiO2 Electron Transport Layer

Highly transparent and crystallized nickel oxide thin film through magnetron sputtering is incorporated in CH3NH3PbI3 planar heterojunction perovskite solar cells (PSCs). In this work, the influence of substrate temperature on NiO film during magnetron sputtering and corresponding performance of PSCs were investigated. NiO thin films were prepared with different substrate temperatures, i.e., 20 °C, 150 °C, and 300 °C, named as 20-NiO, 150-NiO, and 300-NiO, separately. It is found that higher substrate temperature leads to better crystallinity of NiO film, and corresponding a higher PSCs efficiency. PSCs based on 300-NiO thin films’ inorganic hole transport layer show the best efficiency (up to 14.11%), which is about 32% higher than that of device based on 20-NiO. This is mainly due to the fact of 300-NiO thin films with the better light transmittance and higher hole mobility, promoting light absorption and photo-generated charge separation efficiency of perovskite layer, which leads to an improvement in device performance.

Magnetron-sputtered nickel oxide films as hole transport layer for planar heterojunction perovskite solar cells

The chemical doping is an effective strategy to improve the charge transport property of hole transport material (HTM). Herein, tris(2-(1H-pyrazol-1-yl)pyridine]cobalt(III) (FK102) doped 2,2,7,7-tetrakis(N, N-di-p-methoxyphenylamine)-9,9-spirobifluorene (spiro-MeOTAD) as HTM for semi-transparent cesium-containing planar perovskite solar cell (Cs0.1MA0.9PbI3) is demonstrated. Incorporating FK102 realizes efficient doping, which improves the mobility of HTM from 3.948 × 10-3 m2V-1s-1 to 2.22 × 10-2 m2V-1s-1, which is 5.6 times enhancement. As a result, the power conversion efficiency (PCE) is largely improved from 7.58% to 10.09% due to the improved hole transport and extraction.

Cobalt Salt as Efficient Dopant for Spiro-MeOTAD in Cesium-Containing Planar Perovskite Solar Cells.

Low-cost carbon materials (carbon black and graphite power) were applied as substitution of platinum (Pt) in counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). Three fabrication methods, such as ball-milled, pulp-refined, and ultrasonic-crushed, were applied to remove the particle aggregation in the carbon pastes. Then the carbon based pastes were printed on fluorine-doped transparent conducting oxide (FTO) glasses, used as the CEs for DSSCs. Under illumination of 100 mW/cm2, DSSCs with ultrasonic-crushed CEs (U-CEs) show an energy conversion efficiency of 3.57%, which reach to 65.38% of that with conventional sputtered platinum CEs (5.46%). In addition, U-CEs exhibit a higher catalytic activity and a faster charge transfer rate toward the reduction of I-3 to I-.

Zijun Hu

Papers

Sol-gel-processed yttrium-doped NiO as hole transport layer in inverted perovskite solar cells for enhanced performance

Cesium‐Containing Perovskite Solar Cell Based on Graphene/TiO2 Electron Transport Layer

Magnetron-sputtered nickel oxide films as hole transport layer for planar heterojunction perovskite solar cells

Cobalt Salt as Efficient Dopant for Spiro-MeOTAD in Cesium-Containing Planar Perovskite Solar Cells.

Ultrasonic Remove of Particle Aggregation in Carbon Based Counter Electrodes for Dye-Sensitized Solar Cells.