Infrared photodetectors based on single‐layer CVD‐grown graphene and PbS quantum dots, which are fabricated by solution processing, show ultrahigh responsivities of up to 107 A/W under infrared light ...illumination. The devices fabricated on flexible plastic substrates have excellent bending stability. The photoresponse is attributed to the field‐effect doping in graphene films induced by negative charges generated in the quantum dots.
Graphene is the thinnest two-dimensional (2D) carbon material and has many advantages including high carrier mobilities and conductivity, high optical transparency, excellent mechanical flexibility ...and chemical stability, which make graphene an ideal material for various optoelectronic devices. The major applications of graphene in photovoltaic devices are for transparent electrodes and charge transport layers. Several other 2D materials have also shown advantages in charge transport and light absorption over traditional semiconductor materials used in photovoltaic devices. Great achievements in the applications of 2D materials in photovoltaic devices have been reported, yet numerous challenges still remain. For practical applications, the device performance should be further improved by optimizing the 2D material synthesis, film transfer, surface functionalization and chemical/physical doping processes. In this review, we will focus on the recent advances in the applications of graphene and other 2D materials in various photovoltaic devices, including organic solar cells, Schottky junction solar cells, dye-sensitized solar cells, quantum dot-sensitized solar cells, other inorganic solar cells, and perovskite solar cells, in terms of the functionalization techniques of the materials, the device design and the device performance. Finally, conclusions and an outlook for the future development of this field will be addressed.
2D materials have been successfully used in various types of solar cells as transparent electrodes, interfacial and active materials.
Mid‐infrared (MIR) photodetection, covering diverse molecular vibrational regions and atmospheric transmission windows, is vital to civil and military purposes. Versatile use of MIR photodetectors is ...commonly dominated by HgCdTe alloys, InSb, and quantum superlattices, which are limited by strict operation demands, high‐cost, and environmental toxicity. Despite the rapid advances of black phosphorus (BP)‐based MIR photodetectors, these are subject to poor stability and large‐area integration difficulty. Here, the van der Waals (vdW) epitaxial growth of a wafer‐scale 2D platinum ditelluride (PtTe2) layer is reported via a simple tellurium‐vapor transformation approach. The 2D PtTe2 layer possesses a unique mosaic‐like crystal structure consisting of single‐crystal domains with highly preferential 001 orientation along the normal direction, reducing the influence of interface defects and ensuring efficient out‐of‐plane carrier transportation. This characteristic, combined with the wide absorption of PtTe2 and well‐designed vertical device architecture, makes the PtTe2/Si Schottky junction photodetector capable of sensing ultra‐broadband light of up to 10.6 µm with a high specific detectivity. Also, the photodetector exhibits an excellent room‐temperature infrared‐imaging capability. This approach provides a new design concept for high‐performance, room‐temperature MIR photodetection based on 2D layered materials.
Van der Waals epitaxial growth of wafer‐scale mosaic‐like 2D PtTe2 layers is achieved for highly sensitive MIR photodetection. A photodetector based on a PtTe2/Si Schottky junction is capable of sensing ultrabroadband light of up to 10.6 µm with a high specific detectivity. The photodetector arrays also display an excellent room‐temperature MIR imaging capability.
A single-layer graphene film with high conductance and transparency was realized by effective chemical doping. The conductance of single-layer graphene was increased for more than 400% when it was ...doped with Au nanoparticles and poly(3,4-ethylenedioxythiophene): poly(styrene sulfonic acid). Then semitransparent organic solar cells based on poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) were fabricated with single-layer graphene and indium tin oxide (ITO) as the top and bottom electrodes, respectively. The performance of the devices was optimized by tuning the active layer thickness and doping the single-layer graphene electrodes. The maximum efficiency of 2.7% was observed in the devices with the area of 20 mm2 illuminated from graphene electrode under the AM1.5 solar simulator. It is notable that all of the devices showed higher efficiency from the graphene than ITO side, which was attributed to the better transmittance of the graphene electrodes. In addition, the influence of the active area of the organic solar cell on its photovoltaic performance was studied. We found that, when the active areas increased from 6 to 50 mm2, the power conversion efficiencies decreased from 3% to 2.3% because of the increased series resistances and the decreased edge effect of the devices.
Although organic photovoltaic devices (OPVs) have been investigated for more than two decades, the power conversion efficiencies of OPVs are much lower than those of inorganic or perovskite solar ...cells. One effective approach to improve the efficiency of OPVs is to introduce additives to enhance light harvesting as well as charge transportation in the devices. Here, black phosphorus quantum dots (BPQDs) are introduced in OPVs as an additive. By adding 0.055 wt % BPQDs relative to the polymer donors in the OPVs, the device efficiencies can be dramatically improved for more than 10 %. The weight percentage is much lower than that of any other additive used in OPVs before, which is mainly due to the two‐dimentional structure as well as the strong broadband light absorption and scattering of the BPQDs. This work paves a way for using two‐dimentional quantum dots in OPVs as a cost‐effective approach to enhance device efficiencies.
Strong light absorption: The power conversion efficiencies of organic photovoltaics have been improved by introducing black phosphorus quantum dots (BPQDs; 0.055 wt % relative to the donor polymers) due to the boosted light harvesting of the devices. The effect is attributed to the strong light absorption as well as the two‐dimensional structure of the BPQDs. A pronounced size effect of BPQDs on the performance enhancement is observed.
We report a new mechanistic strategy for controlling and modifying the photon emission of lanthanides in a core–shell nanostructure by using interfacial energy transfer. By taking advantage of this ...mechanism with Gd3+ as the energy donor, we have realized efficient up‐ and down‐converted emissions from a series of lanthanide emitters (Eu3+, Tb3+, Dy3+, and Sm3+) in these core–shell nanoparticles, which do not need a migratory host sublattice. Moreover, we have demonstrated that the Gd3+‐mediated interfacial energy transfer, in contrast to energy migration, is the leading process contributing to the photon emission of lanthanide dopants for the NaGdF4@NaGdF4 core–shell system. Our finding suggests a new direction for research into better control of energy transfer at the nanometer length scale, which would help to stimulate new concepts for designing and improving photon emission of the lanthanide‐based luminescent materials.
Up, down, flying around: Photon up‐ and down‐conversion (UC and DC, respectively) have been realized through Gd3+‐mediated interfacial energy transfer (IET) in a core–shell nanoarchitecture. This finding offers a simple, efficient approach for photon management, and enables a fundamental understanding of the interactions between lanthanide ions at nanometer length scale.
To alleviate photoinduced charge recombination in semiconducting nanomaterials represents an important endeavor toward high‐efficiency photocatalysis. Here a judicious integration of piezoelectric ...and photocatalytic properties of organolead halide perovskite CH3NH3PbI3 (MAPbI3) to enable a piezophotocatalytic activity under simultaneous ultrasonication and visible light illumination for markedly enhanced photocatalytic hydrogen generation of MAPbI3 is reported. The conduction band minimum of MAPbI3 is higher than hydrogen generation potential (0.046 V vs normal hydrogen electrode), thereby rendering efficient hydrogen evolution. In addition, the noncentrosymmetric crystal structure of MAPbI3 enables its piezoelectric properties. Thus, MAPbI3 readily responds to external mechanical force, creating a built‐in electric field for collective piezophotocatalysis as a result of effective separation of photogenerated charge carriers. The experimental results show that MAPbI3 powders exhibit superior piezophotocatalytic hydrogen generation rate (23.30 µmol h−1) in hydroiodic acid (HI) solution upon concurrent light and mechanical stimulations, much higher than that of piezocatalytic (i.e., 2.21 µmol h−1) and photocatalytic (i.e., 3.42 µmol h−1) hydrogen evolution rate as well as their sum (i.e., 5.63 µmol h−1). The piezophotocatalytic strategy provides a new way to control the recombination of photoinduced charge carriers by cooperatively capitalizing on piezocatalysis and photocatalysis of organolead halide perovskites to yield highly efficient piezophotocatalysis.
CH3NH3PbI3 exhibits a superior piezophotocatalytic hydrogen generation rate upon concurrent light and mechanical stimulations, much higher than that of piezocatalytic and photocatalytic hydrogen evolution rate as well as their sum. Combining piezocatalysis and photocatalysis of semiconductor photocatalysts to attain a collective piezophotocatalysis may represent an appealing strategy for efficient solar energy conversion, including water splitting, organic fuel production, etc.
Platinum disulfide (PtS2), a new member of the group‐10 transition‐metal dichalcogenides, is studied experimentally and theoretically. The indirect bandgap of PtS2 can be drastically tuned from 1.6 ...eV (monolayer) to 0.25 eV (bulk counterpart), and the interlayer mechanical coupling is almost isotropic. It can be explained by strongly interlayer interaction from the pz orbital hybridization of S atoms.
Gallium selenide, an important second‐order nonlinear semiconductor, has received much scientific interest. However, the nonlinear properties in its two‐dimensional (2D) form are still unknown. A ...strong second harmonic generation (SHG) in bilayer and multilayer GaSe sheets is reported. This is also the first observation of SHG on 2D GaSe thin layers. The SHG of multilayer GaSe above five layers shows a quadratic dependence on the thickness; while that of a sheet thinner than five layers shows a cubic dependence. The discrepancy between the two SHG responses is attributed to the weakened stability of non‐centrosymmetric GaSe in the atomically thin flakes where a layer–layer stacking order tends to favor centrosymmetric modification. Importantly, two‐photon excited fluorescence has also been observed in the GaSe sheets. Our free‐energy calculations based on first‐principles methods support the observed nonlinear optical phenomena of the atomically thin layers.
2D materials: Layer‐dependent second‐order optical nonlinearity has been observed in few‐layer (L) gallium selenide sheets, which is the first observation of second harmonic generation (SHG; see picture) on two‐dimensional GaSe nanosheets because of the absence of the inversion symmetric center for ε‐GaSe. Two‐photon excited fluorescence has also been found in the few‐layer GaSe sheets.
The electrical and optical measurements, in combination with density functional theory calculations, show distinct layer‐dependent semiconductor‐to‐semimetal evolution of 2D layered PtSe2. The high ...room‐temperature electron mobility and near‐infrared photoresponse, together with much better air‐stability, make PtSe2 a versatile electronic 2D layered material.
