Introduction of Graphene Oxide as Buffer Layer in Perovskite Solar Cells and the Promotion of Soluble n-Butyl-substituted Copper Phthalocyanine as Efficient Hole Transporting Material

A review of graphene derivative enhancers for perovskite solar cells

Due to the finite nature, health and environmental hazards currently associated with the use of fossil energy resources, there is a global drive to hasten the development and deployment of renewable energy technologies. One such area encompasses perovskite solar cells (PSCs) that have shown photoconversion efficiencies (PCE) comparable to silicon-based photovoltaics, but their commercialisation has been set back by short-term stability and toxicity issues, among others. A tremendous potential to overcome these drawbacks is presented by the emerging applications of graphene derivative-based materials in PSCs as substitutes or components, composites with other functional materials, and enhancers of charge transport, blocking action, exciton dissociation, substrate coverage, sensitisation and stabilisation. This review aims to illustrate how these highly capable carbon-based materials can advance PSCs by critically outlining and discussing their current applications and strategically identifying prospective research avenues. The reviewed works show that graphene derivatives have great potential in boosting the performance and stability of PSCs through morphological modifications and compositional engineering. This can drive the sustainability and commercial viability aspects of PSCs.

Solution-processed two-dimensional materials for next-generation photovoltaics

In the ever-increasing energy demand scenario, the development of novel photovoltaic (PV) technologies is considered to be one of the key solutions to fulfil the energy request. In this context, graphene and related two-dimensional (2D) materials (GRMs), including nonlayered 2D materials and 2D perovskites, as well as their hybrid systems, are emerging as promising candidates to drive innovation in PV technologies. The mechanical, thermal, and optoelectronic properties of GRMs can be exploited in different active components of solar cells to design next-generation devices. These components include front (transparent) and back conductive electrodes, charge transporting layers, and interconnecting/recombination layers, as well as photoactive layers. The production and processing of GRMs in the liquid phase, coupled with the ability to "on-demand" tune their optoelectronic properties exploiting wet-chemical functionalization, enable their effective integration in advanced PV devices through scalable, reliable, and inexpensive printing/coating processes. Herein, we review the progresses in the use of solution-processed 2D materials in organic solar cells, dye-sensitized solar cells, perovskite solar cells, quantum dot solar cells, and organic-inorganic hybrid solar cells, as well as in tandem systems. We first provide a brief introduction on the properties of 2D materials and their production methods by solution-processing routes. Then, we discuss the functionality of 2D materials for electrodes, photoactive layer components/additives, charge transporting layers, and interconnecting layers through figures of merit, which allow the performance of solar cells to be determined and compared with the state-of-the-art values. We finally outline the roadmap for the further exploitation of solution-processed 2D materials to boost the performance of PV devices.

Low-cost air-stable perovskite solar cells by incorporating inorganic materials

Herein, we demonstrate a new fabrication strategy for low-cost and stable-operation perovskite solar cells (PSCs) suitable for commercialization.

Performance of Graphene-CdS Hybrid Nanocomposite Thin Film for Applications in Cu(In,Ga)Se2 Solar Cell and H2 Production

A graphene-cadmium sulfide (Gr-CdS) nanocomposite was prepared by a chemical solution method, and its material properties were characterized by several analysis techniques. The synthesized pure CdS nanoparticles (NPs) and Gr-CdS nanocomposites were confirmed to have a stoichiometric atomic ratio (Cd/S = 1:1). The Cd 3d and S 2p peaks of the Gr-CdS nanocomposite appeared at lower binding energies compared to those of the pure CdS NPs according to X-ray photoelectron spectroscopy analyses. The formation of the Gr-CdS nanocomposite was also evidenced by the structural analysis using Raman spectroscopy and X-ray diffraction. Transmission electron microscopy confirmed that CdS NPs were uniformly distributed on the graphene sheets. The absorption spectra of both the Gr-CdS nanocomposite and pure CdS NPs thin films showed an absorption edge at 550 nm related to the energy band gap of CdS (~2.42 eV). The Cu(In,Ga)Se₂ thin film photovoltaic device with Gr-CdS nanocomposite buffer layer showed a higher electrical conversion efficiency than that with pure CdS NPs thin film buffer layer. In addition, the water splitting efficiency of the Gr-CdS nanocomposite was almost three times higher than that of pure CdS NPs.

Molecular materials as interfacial layers and additives in perovskite solar cells

Here we review the recent strategies for developing organic and inorganic molecular materials for application as electron and hole transport layers and as additives to achieve high efficiency and stability perovskite solar cells.

On the assessment of incorporation of CNT-TiO2 core-shell structures into nanoparticle TiO2 photoanodes in dye-sensitized solar cells

Herein, we report dye-sensitized solar cells (DSCs) based on conventional nanocrystalline TiO2 photoanodes decorated with one-dimensional (1D) CNT-TiO2 core-shell structures (CTH). The core-shell nanotubes are synthesized by a simple sol-gel template-assisted method via in situ deposition of TiO2 on the surface of non-covalently functionalized CNTs. The core-shell nanotubes are well characterized by various techniques. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images show that formation of the TiO2 shell on the surface of the CNT core follows a layer or Frank-van der Merwe growth mode, resulting in a highly uniform interface with excellent charge transfer from the TiO2 conduction band into the CNTs. The thickness and crystal structure of the TiO2 shell can be tailored by controlling the processing parameters. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy verify that CNTs have no surface defects and are well preserved using the employed method and the subsequent heat treatment in air, respectively. UV-vis spectroscopy and photoluminescence spectroscopy reveal an extension to visible regions with an increase in overall intensity and a significant reduction in charge recombination due to a shift of the Fermi level toward positive potentials. We find an increase by up to 37% in the DSC device's power conversion efficiency by incorporating the CNT-TiO2 core-shell nanotubes into the nanoparticle TiO2 photoanode due to the charge recombination reduction and electron injection enhancement.

Phthalocyanines and porphyrinoid analogues as hole- and electron-transporting materials for perovskite solar cells

Organic-inorganic lead halide perovskite absorbers in combination with electron and hole transporting selective contacts result in power conversion efficiencies of over 23% under AM 1.5 sun conditions. The advantage of perovskite solar cells is their simple fabrication through solution-processing methods either in n-i-p or p-i-n configurations. Using TiO₂ or SnO₂ as an electron transporting layer, a compositionally engineered perovskite as an absorber layer, and Spiro-OMeTAD as a HTM, several groups have reported over 20% efficiency. Though perovskite solar cells reached comparable efficiency to that of crystalline silicon ones, their stability remains a bottleneck for commercialization partly due to the use of doped Spiro-OMeTAD. Several organic and inorganic hole transporting materials have been explored to increase the stability and power conversion efficiency of perovskite solar cells. IIn this review, we analyse the stability and efficiency of perovskite solar cells incorporating phthalocyanine and porphyrin macrocycles as hole- and electron transporting materials. The π-π stacking orientation of these macrocycles on the perovskite surface is important in facilitating a vertical charge transport, resulting in high power conversion efficiency.

Study of Hole-Transporter-Free Perovskite Solar Cells based on Fully Printable Components

Hole-transporter-free perovskite solar cells carrying a carbon back contact electrode provide the possibility of making full printable low cost and stable devices, even though their efficiency is substantially lower than those made in the standard configuration. The present work searched for simple and easy routes for constructing such devices, demonstrating that organic components do enhance device efficiency but only to a level that is not worth the trouble nor the cost. Devices based on a triple mesoporous layer of titania/zirconia/carbon with perovskite infiltration gave an efficiency of 10.7%. After 180 days of storing under ambient conditions, a small loss of efficiency has been observed for a cell made in June, in spite of the fact that in going from June to December, a large increase of the ambient humidity took place, thus verifying the protective effect that the carbon electrode is providing. The addition of spiro-OMeTAD to the hole-transporter-free device resulted in increasing the efficiency by about 10%, a change which is appreciated to be of low importance given the cost of this material. This increase mainly derived from an increase in the current. Devices of different sizes have been constructed by screen printing, using home-made pastes for all the components making the cell scaffold, i.e., for titania, zirconia, and carbon layers.

The charge carrier dynamics, efficiency and stability of two-dimensional material-based perovskite solar cells

Recent advances in the use of two-dimensional (2D) materials for perovskites solar cells (PSCs) are summarized. The effects of their unique optical and electrical properties on the charge carrier dynamics of PSCs are detailed.

MoS2 Quantum Dot/Graphene Hybrids for Advanced Interface Engineering of a CH3NH3PbI3 Perovskite Solar Cell with an Efficiency of over 20

Interface engineering of organic-inorganic halide perovskite solar cells (PSCs) plays a pivotal role in achieving high power conversion efficiency (PCE). In fact, the perovskite photoactive layer needs to work synergistically with the other functional components of the cell, such as charge transporting/active buffer layers and electrodes. In this context, graphene and related two-dimensional materials (GRMs) are promising candidates to tune "on demand" the interface properties of PSCs. In this work, we fully exploit the potential of GRMs by controlling the optoelectronic properties of molybdenum disulfide (MoS₂) and reduced graphene oxide (RGO) hybrids both as hole transport layer (HTL) and active buffer layer (ABL) in mesoscopic methylammonium lead iodide (CH₃NH₃PbI₃) perovskite (MAPbI₃)-based PSCs. We show that zero-dimensional MoS₂ quantum dots (MoS₂ QDs), derived by liquid phase exfoliated MoS₂ flakes, provide both hole-extraction and electron-blocking properties. In fact, on one hand, intrinsic n-type doping-induced intraband gap states effectively extract the holes through an electron injection mechanism. On the other hand, quantum confinement effects increase the optical band gap of MoS₂ (from 1.4 eV for the flakes to >3.2 eV for QDs), raising the minimum energy of its conduction band (from -4.3 eV for the flakes to -2.2 eV for QDs) above the one of the conduction band of MAPbI₃ (between -3.7 and -4 eV) and hindering electron collection. The van der Waals hybridization of MoS₂ QDs with functionalized reduced graphene oxide (f-RGO), obtained by chemical silanization-induced linkage between RGO and (3-mercaptopropyl)trimethoxysilane, is effective to homogenize the deposition of HTLs or ABLs onto the perovskite film, since the two-dimensional nature of RGO effectively plugs the pinholes of the MoS₂ QD films. Our "graphene interface engineering" (GIE) strategy based on van der Waals MoS₂ QD/graphene hybrids enables MAPbI₃-based PSCs to achieve a PCE up to 20.12% (average PCE of 18.8%). The possibility to combine quantum and chemical effects into GIE, coupled with the recent success of graphene and GRMs as interfacial layer, represents a promising approach for the development of next-generation PSCs.

Construction of Perovskite Solar Cells Using Inorganic Hole-Extracting Components

A NiO _x -graphene oxide (NiO _x -GO) hybrid has been prepared by a simple solution-processed method and was used as hole-extraction material in perovskite solar cells with either gold or carbon as back contact electrode. The impact of GO content on the optoelectronic behavior of NiO _x and the photovoltaic performance of the fabricated devices has been studied. Thus, GO incorporation showed a significant improvement in the performance of NiO _x -based devices. The best attained efficiency was 13.3%, and it was 45% higher than that with pure NiO _x . This is attributed to a significant improvement in the hole extraction, recombination resistance, and energy-level matching in comparison to pure NiO _x . In addition, NiO _x -GO/Au-based perovskite solar cell devices showed a negligible hysteresis effect and high reliability and repeatability. When carbon was used as back contact electrode, the obtained efficiencies were lower, but it leaves space for improvement. Devices based on inorganic hole transporters NiO _x or NiO _x -GO demonstrated higher stability in ambient air compared to a standard cell based on spiro-OMeTAD.

A turn-on chemiluminescence biosensor for selective and sensitive detection of adenosine based on HKUST-1 and QDs-luminol-aptamer conjugates

In this work, HKUST-1 and QDs-luminol-aptamer conjugates were prepared. The QDs-luminol-aptamer conjugates can be adsorbed by graphene oxide through π-π conjugation. When the adenosine was added, the QDs-luminol-aptamer conjugates were released from magnetic graphene oxide (MGO), the chemiluminescent switch was turned on. It was reported that HKUST-1 can catalyze the chemiluminescence reaction of luminol-H₂O₂ system in an alkaline medium, and improve the chemiluminescence resonance energy transfer (CRET) between chemiluminescence and QDs indirectly. Thus, the adenosine can be detected sensitively. Based on this phenomenon, the excellent platform for detection of adenosine was established. Under the optimized conditions, the linear detection range for adenosine was 1.0 × 10^-12-2.2 × 10^-10 mol/L with a detection limit of 2.1 × 10^-13 mol/L. The proposed method was successfully used for adenosine detection in biological samples.

Efficient Carbon-Based CsPbBr3 Inorganic Perovskite Solar Cells by Using Cu-Phthalocyanine as Hole Transport Material

Metal halide perovskite solar cells (PSCs) have attracted extensive research interest for next-generation solution-processed photovoltaic devices because of their high solar-to-electric power conversion efficiency (PCE) and low fabrication cost. Although the world's best PSC successfully achieves a considerable PCE of over 20% within a very limited timeframe after intensive efforts, the stability, high cost, and up-scaling of PSCs still remain issues. Recently, inorganic perovskite material, CsPbBr₃, is emerging as a promising photo-sensitizer with excellent durability and thermal stability, but the efficiency is still embarrassing. In this work, we intend to address these issues by exploiting CsPbBr₃ as light absorber, accompanied by using Cu-phthalocyanine (CuPc) as hole transport material (HTM) and carbon as counter electrode. The optimal device acquires a decent PCE of 6.21%, over 60% higher than those of the HTM-free devices. The systematic characterization and analysis reveal a more effective charge transfer process and a suppressed charge recombination in PSCs after introducing CuPc as hole transfer layer. More importantly, our devices exhibit an outstanding durability and a promising thermal stability, making it rather meaningful in future fabrication and application of PSCs.

The role of a vapor-assisted solution process on tailoring the chemical composition and morphology of mixed-halide perovskite solar cells

Herein, we report a modified two-step method to construct a uniform and pinhole-free polycrystalline perovskite film with large grains up to the microscale using lead mixed-halide (PbI₂–PbCl₂) precursor solutions to guarantee the device functioning.

Improvement of the photovoltaic parameters of perovskite solar cells using a reduced-graphene-oxide-modified titania layer and soluble copper phthalocyanine as a hole transporter

Graphene modified mesoporous titania for perovskite solar cells.

Oxasmaragdyrins as New and Efficient Hole-Transporting Materials for High-Performance Perovskite Solar Cells

The high performance of the perovskite solar cells (PSCs) cannot be achieved without a layer of efficient hole-transporting materials (HTMs) to retard the charge recombination and transport the photogenerated hole to the counterelectrode. Herein, we report the use of boryl oxasmaragdyrins (SM01, SM09, and SM13), a family of aromatic core-modified expanded porphyrins, as efficient hole-transporting materials (HTMs) for perovskite solar cells (PSCs). These oxasmaragdyrins demonstrated complementary absorption spectra in the low-energy region, good redox reversibility, good thermal stability, suitable energy levels with CH₃NH₃PbI₃ perovskite, and high hole mobility. A remarkable power conversion efficiency of 16.5% (V_oc = 1.09 V, J_sc = 20.9 mA cm^-2, fill factor (FF) = 72%) is achieved using SM09 on the optimized PSCs device employing a planar structure, which is close to that of the state-of-the-art hole-transporting materials (HTMs), spiro-OMeTAD of 18.2% (V_oc = 1.07 V, J_sc = 22.9 mA cm^-2, FF = 74%). In contrast, a poor photovoltaic performance of PSCs using SM01 is observed due to the interactions of terminal carboxylic acid functional group with CH₃NH₃PbI₃.