Recent advances in two-dimensional photovoltaic devices

Two-dimensional(2D)materials have attracted tremendous interest in view of the outstanding optoelectronic properties,showing new possibilities for future photovoltaic devices toward high performance,high specific power and flexibility.In recent years,substantial works have focused on 2D photovoltaic devices,and great progress has been achieved.Here,we present the review of recent advances in 2D photovoltaic devices,focusing on 2D-material-based Schottky junctions,homojunctions,2D−2D heterojunctions,2D−3D heterojunctions,and bulk photovoltaic effect devices.Furthermore,advanced strategies for improving the photovoltaic performances are demonstrated in detail.Finally,conclusions and outlooks are delivered,providing a guideline for the further development of 2D photovoltaic devices.

UV-ozone-treated MoO_3 as the hole-collecting buffer layer for high-efficiency solution-processed SQ:PC_(71) BM photovoltaic devices

The enhanced performance of a squaraine compound, with 2,4-bis[4-（N,N-diisobutylamino）-2,6-dihydroxyphenyl] squaraine as the donor and [6,6]-phenyl-C71-butyric acid methyl ester （PC71BM） as the acceptor, in solution-processed or- ganic photovoltaic devices is obtained by using UV-ozone-treated MoO3 as the hole-collecting buffer layer. The optimized thickness of the MoO3 layer is 8 nm, at which the device shows the best power conversion efficiency （PCE） among all devices, resulting from a balance of optical absorption and charge transport. After being treated by UV-ozone for 10 min, the transmittance of the MoO3 film is almost unchanged. Atomic force microscopy results show that the treated surface morphology is improved. A high PCE of 3.99% under AM 1.5 G illumination （100 mW/cm2） is obtained.

Optical management in organic photovoltaic devices

Due to their potentials in light‐weight,flexible,and semitransparent devices,organic photovoltaics are of great significance in the field of renewable energy.However,the narrow intrinsic absorption spectrum of organic materials hinders the full utilization of solar energy.To fabricate a highly efficient opaque solar cell,it is greatly necessary to modify the optical properties of the device to improve light absorption.In addition,the growing interest in building‐integrated photovoltaics drives the development of semitransparent devices.The preparation of semitransparent solar cells with excellent performance imposes high requirements on the high efficiency and appropriate visible light transmittance of effective optical management.In this review,the recent research progress of optical management in organic photovoltaics is reviewed,including the design of light‐absorbing materials,the modification of different layers,adding a lighttrapping structure,and changing the light absorption capabilities of specific materials,so as to provide strategies of how to improve the performance of organic photovoltaic devices and present the prospect of the area.

Unveiling the origin of performance enhancement of photovoltaic devices by upconversion nanoparticles

To better utilize the infrared(IR)region in sunlight for photovoltaic devices(PVs),upconversion nanoparticles(UCNPs)have been proposed to improve power conversion efficiency(PCE).However,researchers recently have found that the upconversion(UC)effect is negligible in PVs performance improvement for their ultra-low UC photoluminescence quantum yields of UCNPs solid film,while the real mechanism of UCNPs in PVs has not been clearly studied.Herein,based on the material inorganic perovskitesγ-CsPbI_(3),NaYF_(4):20%Yb^(3+),2%Er^(3+)UCNPs were integrated into different transport layer to optimize device performance.Compared with reference device,the short-circuit current density and PCE of optimized device reached 20.87 mA/cm^(2)(20.39 mA/cm^(2))and 18.34%(17.72%),respectively,without sacrificing open-circuit voltage and filling factor.Further experimental characterizations verified that the improved performance was attributable to enhanced visible light absorption instead of IR.To theoretically explain the statement,the light field distribution in device was simulated and the absorption in different layers was calculated.The results revealed that the introduction of UCNPs with different refractive index from other layers caused light field disturbance,and improved visible light captured by γ-CsPbI_(3).Importantly,through experiments and theoretical calculation,the research deeply explored the potential mechanism of UCNPs in optimizing PVs performance.

Enhanced performance in organic photovoltaic devices with a KMnO_4 solution treated indium tin oxide anode modification

The properties of poly（3-hexylthiophene）：（6,6）-phenyl C61 butyric acid methyl ester （P3HT：PCBM） organic pho- tovoltaic devices （OPVs） with an indium tin oxide （ITO） anode treated by a KMnO4 solution are investigated. The optimized KMnO4 solution has a concentration of 50 rag/L, and ITO is treated for 15 min. The modification of ITO anode results in an enhancement of the power conversion efficiency （PCE） of the device, which is responsible for the increase of the photocurrent. The performance enhancement is attributed to the work function modification of the ITO substrate through the strong oxygenation of KMnO4, and then the charge collection efficiency is improved.

The Improvement of Bulk-Heterojunction Order in Polymer Photovoltaic Device

The blend morphology and vertical arrangement are critical to the performance of organic bulk-heterojunction photovoltaic devices.In the present paper,the authors proposed a new annealing method that controls the blend morphology and vertical arrangement of two materials by means of simultaneously applying external electrical field and violet irradiation on the active layer of poly(3-hexylthiophene) (P3HT) and -phenyl C61-butyric acid methyl ester(PCBM) during annealing process.By using this annealing method,the power conversion efficiency increased by 36%,which was caused by vertical phased-separated blend of crystalline P3HT and PCBM and better charge extraction of electrodes.X-ray photoelectron spectroscopy(XPS) was measured to prove more fullerene derivatives at the organic/cathode interfaces by using this annealing method.The X-ray diffraction(XRD) analysis and UV-Vis absorption spectrum analysis also revealed more ordered polymer crystallization.

Flexible organic photovoltaic devices based on oligothiophene derivatives

For the purpose of developing flexible organic photovoltaic devices, we have fabricated two flexible devices using 5-formyl- 2,2′：5′,2″：5″,2′″-quaterthiophene （4T-CHO）, 5-formyl-2,2′：5′, 2″：5″,2′″：5′″,2″″-quinquethiophene （5T-CHO） and 3,4,9,10-perylenetertracarboxylic dianhydride （PTCDA）. The PET-ITO/4T-CHO/PTCDA/A1 device has an open circuit voltage （Voc） of 1.56 V, photoelectric conversion efficiency of 0.77%. The PET-ITO/5T-CHO/PTCDA/A1 device has a Voc of 1.70 V, photoelectric conversion efficiency of 0.84%. The two flexible devices have high Voc （1.56 and 1.70 V）. It is possible that intermolecular hydrogen bonding between -CHO group of nT-CHO and carboxylic dianhydride of PTCDA contributes to enhancing the efficiency by promoting interfacial electron transfer and eliminating the subconducting band trap sites.

Recent Advances and Challenges Toward Application of Fibers and Textiles in Integrated Photovoltaic Energy Storage Devices

Flexible microelectronic devices have seen an increasing trend toward development of miniaturized,portable,and integrated devices as wearable electronics which have the requirement for being light weight,small in dimension,and suppleness.Traditional three-dimensional(3D)and two-dimensional(2D)electronics gadgets fail to effectively comply with these necessities owing to their stiffness and large weights.Investigations have come up with a new family of one-dimensional(1D)flexible and fiber-based electronic devices(FBEDs)comprising power storage,energy-scavenging,implantable sensing,and flexible displays gadgets.However,development and manufacturing are still a challenge owing to their small radius,flexibility,low weight,weave ability and integration in textile electronics.This paper will provide a detailed review on the importance of substrates in electronic devices,intrinsic property requirements,fabrication classification and applications in energy harvesting,energy storage and other flexible electronic devices.Fiber-and textile-based electronic devices for bulk/scalable fabrications,encapsulation,and testing are reviewed and presented future research ideas to enhance the commercialization of these fiber-based electronics devices.

Surface plasmon excitation in semitransparent inverted polymer photovoltaic devices and their applications as label-free optical sensors

Herein,we report on surface plasmon(SP)-sensitive semitransparent inverted polymer photovoltaic(PV)devices that are based on multilayered material systems consisting of poly(3-hexylthiophene):fullerene-derivative bulk-heterojunction PV layers and thin gold or silver anodes.We demonstrate that these PV devices allow the simultaneous generation of both electrical power and SPs on their anodes for photoexcitation just above the optical absorption edge of the PV layers,resulting not only in attenuated total reflection,but also in attenuated photocurrent generation(APG)under the SP resonance(SPR)condition.Moreover,we also confirm that the biomolecular interaction of biotin–streptavidin on the PV devices can be precisely detected via apparent SPR angle shifts in the APG spectra,even without the need for complex attenuated total reflection configurations.We highlight our view that APG measurements made using these PV devices show great potential for the development of future generations of compact and highly sensitive SPR-based optical sensors.

Heavy metal complex containing organic/polymer materials for bulk-heterojunction photovoltaic devices

The application of heavy-metal complexes in bulk-heterojunction（BHJ） solar cells is a promising new research field which has attracted increasing attention,due to their strong spin-orbit coupling for efficient singlet to triplet intersystem crossing.This review article focuses on recent advances of heavy metal complex containing organic and polymer materials as photovoltaic donors in BHJ solar cells.Platinum-acetylide containing oligomersor and polymers have been firstly illustrated due to the good solubility,square planar structure,as well as the fairly strong Pt-Pt interaction.Then the cyclometalated Pt or Ir complex containing conjugated oligomers and polymers are presented in which the triplet organometallic compounds are embedded into the organic/polymer backbone either through cyclometalated main ligand or the auxiliary ligand.Pure triplet small molecular cyclometalated Ir complex are also briefly introduced.Besides the chemical modification,physical doping of cyclometalated heavy metal complexes as additives into the photovoltaic active layers is finally demonstrated.

Efficient Hybrid Photovoltaic Devices Based on in-situ Electrochemical Copolymerization of 3-Methylthiophene and Bithiophene into Pores of Nanocrystalline TiO2

Novel organic and inorganic hybrid photovoltaic devices were thbricated by in-situ electrochemical copolymerization of 3-methylthiophene（3MT） and bithiophene（BT） into the pores of nanostructured TiO2 sintered on fluorine-doped tin oxide（FTO） substrate. The photoactive layer was investigated by Fourier transform infrared（FTIR） spectroscopy, ultraviolet-visable（UV-Vis） spectrometer, scanning electron microscope（SEM） and cyclic voltammo- gram characterization. Device efficiency based on different molar feed ratios of 3MT and BT during electrochemical polymerization, and the effect of in-situ copolymer state（doped by electrolyte and de-doped） were measured and compared. Under the solar illumination of 100 mW/cm2（AM 1.5）, an optimized device efficiency of 0.938% was obtained when the molar ratio of 3MT to BT was 500：1, polymerization time was 500 s and the system was in doped copolymer state, respectively. The mechanism of overall photovoltaic parameter improvement was discussed.

A thermal stable cathode buffer based on an inexpensive tetranuclear zinc(Ⅱ) complex for organic photovoltaic devices

An inexpensive material, i.e., tetranuclear zinc（Ⅱ） complex, （Zn40（A/D）6） [AID = 7-azaindolate], was utilized as a cathode buffer in organic photovoltaic （OPV） devices, leading to the improvement of device performance. Compared to OPV devices based on a conventional cathode buffer of TPBi （1,3,5-tris（2-N-phenylbenzimidazolyl）benzene）, although the freshly prepared devices showed similar performance, when heated to a series of high temperatures under air, the short circuit current and the open circuit voltage of the Zn40（AID）6 devices dropped more slowly, indicating the superiority of using Zn40（AID）6 as a cathode buffer over TPBi in OPV devices.

Organic photovoltaic device enhances the neural differentiation of rat bone marrow-derived mesenchymal stem cells

Electrical stimulation is known to be involved in stem cell differentiation,particularly neural differentiation.Various electrical stimulation systems and devices have been developed for neural tissue engineering.The organic photovoltaic materials PM6 and Y6 have showed high-power conversion efficiency.In this study,we used PM6 and Y6 to develop an organic photovoltaic device(OPD)to supply electrical stimulation.The photoelectric stimulation by the OPD showed no impact on cell viability.We tested the neural differentiation potential of rat bone marrow-derived mesenchymal stem cells(rBMSCs)under light induced electrical stimulation.The changes in cell morphology suggested that photoelectric stimulation significantly increased the neurite length and the number of extremities of differentiated neural cells.In addition,genes of neuron markers and neurotrophic factors were upregulated when rBMSCs were under photoelectric stimulation.Furthermore,the calcium influx of differentiated cells responding to acetylcholine and the phosphorylation of extracellular-signal-regulated kinase(ERK)1 and 2,protein kinase B(AKT)and mammalian target of rapamycin(mTOR)were significantly elevated after photoelectric stimulation.These findings demonstrated that PM6:Y6 based OPD could provide photoelectric stimulation to enhance rBMSCs neural differentiation,which might be an alternative approach to electrically manipulate stem cells differentiation into neural cells in vitro.

Synthesis and photovoltaic properties of new europium complex Eu(DBM)3(CPyBM)

A new europium（Ⅲ） complex, tris（dibertzoylmethanate）{ 1-[9-hexyl-9-carbazole]-2-（2-pyridyl）-bertzimidazole}europium（Ⅲ） [Eu（DBM）a（CPyBM）] was synthesized and used as an electron-acceptor and electron-transport layer in organic photovoltaic （PV） device. Power conversion efficiency achieved from the device was 1.04% under illumination with 365 nm UV light at 1.6 mW/cm^2. Compared with the previous reported devices based on Eu（Ⅲ） complexes, the PV performances were improved. The working mechanism of the organic PV device was discussed.

Synthesis and Photovoltaic Properties of Novel Porphyrin Derivatives

The porphyrin derivatives, 5,10,15,20-tetra（4-（N-pentane-carboxamide） phenyl） porphyrin（4 NC5-TPP）, 5,10,15,20-tetra（4-（N-dodecane-carboxamide） phenyl） porphyrin（4 NC12-TPP） and their zinc-complexes（4 NC5-TPPZn and 4 NC12-TPPZn）, have been synthesized. Their thermal properties and morphologies were investigated via thermal gravity analysis（TGA）, differential scanning calorimetry（DSC） and polarized optical microscopy（POM）. It was found that the 4 NC5-TPP was amorphous and the 4 NC5-TPPZn was crystalline at room temperature, while the 4 NC12-TPP formed the columnar liquid crystal and the 4 NC12-TPPZn showed the spherulite texture. The electron state density distributions and the optimum configuration of the porphyrin derivatives were calculated by chemical simulation. The electrochemical oxidation and reduction abilities of the porphyrin derivatives were studied by cyclic voltammetry（CV）. It was indicated that the porphyrin derivatives had the potential to develop organic photovoltaic（OPV） devices. Using the porphyrin derivatives as donor materials and the 3,4,9,10-perylenetetracarboxylic dianhydride（PTCDA） as the acceptor material, the OPV devices were fabricated. The device structure is ITO/porphyrin derivatives：PTCDA/Al. The relationship between the morphology and performance of OPV was studied. It was found that the crystalline morphology of the film was beneficial to improve the efficiency of the devices.

Photoconductive Properties of MEH-PPV/CuS-Nanoparticle Composites

Photoconductive properties of photodiodes based on composites of CuS nanoparticles and Poly[2-methoxy,5- （2＇-ethylhexyloxy）-p-phenylenevlnylene] （MEH-PPV） are investigated. By comparing composite devices with different MEH-PPV：CuS weight ratios of l：l （D2-1）, 1：1.25 （D2-2）, 1：2.5 （132-3） and 1：5 （D2-4）, it is found that the device D2 3 exhibited the best performance： the short-circuit current density of 17μA/cm^2 with the light intensity of 16.7mW/cm^2, the highest open-circuit voltage of 0,83 V, and the photosensitivity of 132 at reverse bias of - 1 V. The photosensitivity is improved by a factor of 5 compared with the undoped MEH-PPV device.

Preventing inhomogeneous elemental distribution and phase segregation in mixed Pb-Sn inorganic perovskites via incorporating PbS quantum dots

Inhomogeneous Pb/Sn elemental distribution and the resulted phase segregation in mixed Pb-Sn halide perovskites would result in energy disorder(band structure and phase distribution disorder),which greatly limits their photovoltaic performance.Here,Pb S quantum dot has been synthesized and demonstrated as seeds for modulation crystallization dynamics of the mixed Pb-Sn inorganic perovskites,allowing an enhanced film quality and significantly suppressing phase segregation.With this additive power conversion efficiency of 8%and 6%is obtained under irradiation of full sunlight in planar and mesoporous structured solar cells in combination with CsPb_(0.5) Sn_(0.5)I_(2)Br inorganic perovskite,respectively.Our finding reveals exploring the actual Pb/Sn atoms location in perovskite structure and its influence on developing efficient and stable low-bandgap perovskite solar cells.

Effect of Ultraviolet Light on Hybrid Zinc Oxide Polymer Bulk Heterojunction Solar Cells

Compared to conjugated polymer poly[2-methoxy-5-（3＇ ,7＇-dimethyloctyloxy）-l,4-phenylenevinylene] （MDMO-PPV） solar cells, bulk heterojunction solar cells composed of zinc oxide （ZnO） nanocrystals and MDMO-PPV have a better energy conversion efficiency. However, ultraviolet （UV） light deteriorates the performance of solar cells composed of ZnO and MDMO-PPV. We propose a model to explain the effect of UV illumination on these ZnO：MDMO-PPV solar cells. According to this model, the degradation from UV illumination is due to a decrease of exciton dissociation efficiency. Our model is based on the experimentM results such as the measurements of current density versus voltage, photoluminescence, and photocurrent.

Annealed Treatment Effect in Poly（3-hexylthiophene）：Methanofullerene Solar Cells

Polymer photovoltaic devices based on poly（3-hexylthiophene） （P3HT） ： [6,6]-phenyl-C61-butyricacid methyl ester （PCBM） 1：1 weight-ratio blend are reported. The effects of various annealing treatments on the device performance are investigated. Thermal annealing shows significant improvement of the device performances. For devices at 130℃ annealing, maximum power conversion efficiency （PCE） of 3.3% and All factor up to 60.3% is achieved under air mass 1.5, 100 m W/cm^2 illumination. We discuss the effect of thermal annealing by the results of ultraviolet-visible absorption spectroscopy （UV-vis）, dark current-voltage curve, atomic force microscopy （AFM）.

Defect Detection in Raw Si Substrates Using Transmission Polarimetry

Inline characterization for fabrication of silicon wafer PV （photovoltaic） devices may be used to optimize device efficiencies, reduce their performance variance, and their cost of production. In this article, the frozen in strain from a variety of extended defects in silicon is shown to effect the polarization of light transmitted through a silicon substrate due to the photo-elastic effect. Transmission polarimetry on pre-fabricated silicon substrates may be used for identification of extended defects in the materials using a polarization analysis instrument. Instrumentation is proposed for detection of defects in raw silicon wafers for applications like raw silicon wafer sorting, scanning silicon bricks, and inline inspection prior to solar cell metallization. Such analysis may assist with gettering of silicon solar cells, may be implemented in the sorting and rejection procedures in PV device fabrication, and in general shows advantages for detection of defects in silicon wafer solar cell materials and devices.