A moth-eye nanostructured mp-TiO2 film using conventional lithography, nano-imprinting and polydimethyl-siloxane (PDMS) stamping methods is demonstrated for the first time. Power conversion efficiency of the moth-eye patterned perovskite solar cell is improved by ≈11%, which mainly results from increasing light harvesting efficiency by structural optical property.
The preparation of highly efficient perovskite nanocrystal light-emitting diodes is shown. A new trimethylaluminum vapor-based crosslinking method to render the nanocrystal films insoluble is applied. The resulting near-complete nanocrystal film coverage, coupled with the natural confinement of injected charges within the perovskite crystals, facilitates electron–hole capture and give rise to a remarkable electroluminescence yield of 5.7%.
Graphitic carbon nitride (g-C3N4) is applied in bulk heterojunction polymer solar cells for the first time by doping g-C3N4 quantum dots (QDs) in the P3HT:PC61BM, PBDTTT-C:PC71BM, or PTB7-Th:PC71BM active layer. This results in obvious efficiency enhancement, as S. F. Yang and co-workers show on page 1719. Thus, the novel application of g-C3N4 in energy conversion beyond the commonly used photocatalysts is demonstrated.
A plasma-induced p-type MoS2 flake and n-type ZnO film diode, which exhibits an excellent rectification ratio, is demonstrated. Under 365 nm optical irradiation, this p–n diode shows a strong photoresponse with an external quantum efficiency of 52.7% and a response time of 66 ms. By increasing the pressure on the junction to 23 MPa, the photocurrent can be enhanced by a factor of four through the piezophototronic effect.
Efficient homo-tandem and triple-junction polymer solar cells are constructed by stacking identical subcells composed of the wide-bandgap polymer PBDTTPD, achieving power conversion efficiencies >8% paralleled by open-circuit voltages >1.8 V. The high-voltage homo-tandem is used to demonstrate PV-driven electrochemical water splitting with an estimated solar-to-hydrogen conversion efficiency of ≈6%.
High-quality perovskite monocrystalline films are successfully grown through cavitation-triggered asymmetric crystallization. These films enable a simple cell structure, ITO/CH3NH3PbBr3/Au, with near 100% internal quantum efficiency, promising power conversion efficiencies (PCEs) >5%, and superior stability for prototype cells. Furthermore, the monocrystalline devices using a hole-transporter-free structure yield PCEs ≈6.5%, the highest among other similar-structured CH3NH3PbBr3 solar cells to date.
The density of trap states within the bandgap of methylammonium lead iodide single crystals is investigated. Defect states close to both the conduction and valence bands are probed. Additionally, a comprehensive electronic characterization of crystals is carried out, including measurements of the electron and hole mobility, and the energy landscape (band diagram) at the surface.
NiO is a promising hole transporting material for perovskite solar cells due to its high hole mobility, good stability, easy processibility, and suitable Fermi level for hole extraction. However, the efficiency of NiO-based cells is still limited by the slow hole extraction due to the poor perovskite/NiO interface and the inadequate quality of the two solution-processed material phases. Here, large influences of a monolayer surface modification of NiO nanocrystal layers with ethanolamine molecules are demonstrated on the enhancement of hole extraction/transport and thus the photovoltaic performance. The underlying causes have been revealed by a series of studies, pointing to a favorable dipole layer formed by the molecular adsorption along with the enhanced perovskite crystallization and the improved interface contact. Comparatively, the solar cells based on a diethanolamine-modified NiO layer have achieved a rather high fill factor, indeed one of the highest among NiO-based perovskite solar cells, and high short-circuit photocurrent density (Jsc), resulting in a power conversion efficiency of ≈16%, most importantly, without hysteresis.
Diethanolamine (DEA) modification of the thin NiO film surface in perovskite solar cells enhances the interfacial hole extraction rate and thus the photovoltaic performance. Perovskite film quality and the contact at the perovskite/NiO interface are greatly improved through the chemical coordination of Ni and Pb with the –NH– and –OH groups of DEA, respectively.
The Al2O3 passivation layer is beneficial for mesoporous TiO2-based perovskite solar cells when it is deposited selectively on the compact TiO2 surface. Such a passivation layer suppressing surface recombination can be formed by thermal decomposition of the perovskite layer during post-annealing.
The first two asymmetric-indenothiophene-based donor–acceptor copolymers (PITBT and PITFBT) are prepared through Stille coupling reactions between distannyl indenothiophene and brominated benzothiadiazole derivatives. The best performing solar cell fabricated from PITFBT exhibits a power conversion efficiency of 9.14% which demonstrates a great potential of the asymmetric indenothiophene for high-performance copolymers.
The successful application of a Ni/Au transparent electrode for fabricating efficient perovskite-based solar cells is demonstrated. Through interdiffusion of the Ni/Au bilayer, Au forms an interconnected metallic network structure as the transparent electrode. Ni diffuses to the bilayer surface and oxidizes into NiOx becoming an appropriate electrode interlayer. These ITO- and PEDOT:PSS-free devices have potential applications in the design of future cost-effective, low-weight, and stable solar cells.
Recently, Kovalenko and co-workers and Li and co-workers developed CsPbX3 (X = Cl, Br, I) inorganic perovskite quantum dots (IPQDs), which exhibited ultrahigh photoluminescence (PL) quantum yields (QYs), low-threshold lasing, and multicolor electroluminescence. However, the usual synthesis needs high temperature, inert gas protection, and localized injection operation, which are severely against applications. Moreover, the so unexpectedly high QYs are very confusing. Here, for the first time, the IPQDs' room-temperature (RT) synthesis, superior PL, underlying origins and potentials in lighting and displays are reported. The synthesis is designed according to supersaturated recrystallization (SR), which is operated at RT, within few seconds, free from inert gas and injection operation. Although formed at RT, IPQDs' PLs have QYs of 80%, 95%, 70%, and FWHMs of 35, 20, and 18 nm for red, green, and blue emissions. As to the origins, the observed 40 meV exciton binding energy, halogen self-passivation effect, and CsPbX3@X quantum-well band alignment are proposed to guarantee the excitons generation and high-rate radiative recombination at RT. Moreover, such superior optical merits endow them with promising potentials in lighting and displays, which are primarily demonstrated by the white light-emitting diodes with tunable color temperature and wide color gamut.
A room-temperature supersaturated recrystallization method is developed to rapidly synthesize all-inorganic halide perovskite QDs with blue, green, and red luminescent quantum yields of 70%–95% and line-widths less than 35 nm. The origins of the optical superiority are proposed to be the observed 40 meV exciton binding energy, surface self-passivation effect, and quantum-well band alignment. Such superior optical merits endow them with promising potentials in healthy lighting and wide-color-gamut displays, which are primarily demonstrated by the color-temperature-tunable white light-emitting diodes.
High performance solution-processed fluorescent and phosphorescent organic light emitting diodes (OLEDs) are achieved by water solution processing of lacunary polyoxometalates used as novel electron injection/transport materials with excellent electron mobilities and hole blocking capabilities. Green fluorescent OLEDs using poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1′,3}-thiadiazole)] (F8BT) as the emissive layer and our polyoxometalates as electron transport/hole blocking layers give a luminous efficiency up to 6.7 lm W−1 and a current efficiency up to 14.0 cd A−1 which remained nearly stable for about 500 h of operation. In addition, blue phosphorescent OLEDs (PHOLEDs) using poly(9-vinylcarbazole) (PVK):1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (OXD-7) as a host and 10.0 wt% FIrpic as the blue dopant in the emissive layer and a polyoxometalate as electron transport material give 12.5 lm W−1 and 30.0 cd A−1 power and luminous efficiency, respectively, which are among the best performance values observed to date for all-solution processed blue PHOLEDs. The lacunary polyoxometalates exhibit unique properties such as low electron affinity and high ionization energy (of about 3.0 and 7.5 eV, respectively) which render them as efficient electron injection/hole blocking layers and, most importantly, exceptionally high electron mobility of up to 10−2 cm2 V−1 s−1.
High performance, solution-processed, fluorescent, and phosphorescent organic light emitting diodes are achieved by water solution processing of lacunary polyoxometalates used as novel electron injection/transport materials with excellent electron mobilities and hole blocking capabilities.
Highly efficient P-I-N type perovskite/bulk-heterojunction (BHJ) integrated solar cells (ISCs) with enhanced fill factor (FF) (≈80%) and high near-infrared harvesting (>30%) are demonstrated by optimizing the BHJ morphology with a novel n-type polymer, N2200, and a new solvent-processing additive. This work proves the feasibility of highly efficient ISCs with panchromatic absorption as a new photovoltaic architecture and provides important design rules for optimizing ISCs.
Nature Materials 15, 247 (2016). doi:10.1038/nmat4572
Authors: Aslihan Babayigit, Anitha Ethirajan, Marc Muller & Bert Conings
In the last few years, the advent of metal halide perovskite solar cells has revolutionized the prospects of next-generation photovoltaics. As this technology is maturing at an exceptional rate, research on its environmental impact is becoming increasingly relevant.
Chemical doping is often used to enhance electric conductivity of the conjugated molecule as hole-transporting material (HTM) for the application in optoelectronics. However, chemical dopants can promote ion migration at the electrical field, which deteriorates the device efficiency as well as increases the fabrication cost. Here, two star HTMs, namely 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine) 9,9′-spirobifluorene (Spiro-OMeTAD) and poly(triarylamine) are subjeted to chemical combination to yield dopant-free N2,N2,N2′,N2′,N7,N7,N7′,N7′-octakis(4-methoxyphenyl)-10-phenyl-10H-spiro[acridine-9,9′-fluorene]-2,2′,7,7′-tetraamine (SAF-OMe). The power conversion efficiencies (PCEs) of 12.39% achieved by solar cells based on pristine, dopant-free SAF-OMe are among the highest reported for perovskite solar cells and are even comparable to devices based on chemically doped Spiro-OMeTAD (14.84%). Moreover, using a HTM comprised of SAF-OMe with an additional dopant results in a record PCE of 16.73%. Compared to Spiro-OMeTAD-based devices, SAF-OMe significantly improves stability.
A dopant-free hole-transporting material (HTM), N2,N2,N2′,N2′,N7,N7,N7′,N7′-octakis(4-methoxyphenyl)-10-phenyl-10H-spiro[acridine-9,9′-fluorene]-2,2′,7,7′-tetraamine (SAF-OMe), is developed and implemented for use in perovskite solar cells. The power conversion efficiency (PCE) achieved by using SAF-OMe as HTM is 12.39%, more than two times the reference 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9′-spirobifluorene (Spiro-OMeTAD).
This work proposes a new perovskite solar cell structure by including lithium-neutralized graphene oxide (GO-Li) as the electron transporting layer (ETL) on top of the mesoporous TiO2 (m-TiO2) substrate. The modified work-function of GO after the intercalation of Li atoms (4.3 eV) exhibits a good energy matching with the TiO2 conduction band, leading to a significant enhancement of the electron injection from the perovskite to the m-TiO2. The resulting devices exhibit an improved short circuit current and fill factor and a reduced hysteresis. Furthermore, the GO-Li ETL partially passivates the oxygen vacancies/defects of m-TiO2 by resulting in an enhanced stability under prolonged 1 SUN irradiation.
Lithium-neutralized graphene oxide (GO-Li) as electron transporting layer in perovskite solar cells is reported. The proposed device conjugates the extraordinary conduction properties of graphene based materials with the exceptional harvesting behavior of organoleadtrihalide compounds and shows enhanced power conversion efficiency and improved long term stability under operative conditions.
This study demonstrates the formation of extremely smooth and uniform formamidinium lead bromide (CH(NH2)2PbBr3 = FAPbBr3) films using an optimum mixture of dimethyl sulfoxide and N,N-dimethylformamide solvents. Surface morphology and phase purity of the FAPbBr3 films are thoroughly examined by field emission scanning electron microscopy and powder X-ray diffraction, respectively. To unravel the photophysical properties of these films, systematic investigation based on time-integrated and time-dependent photoluminescence studies are carried out which, respectively, bring out relatively lower nonradiative recombination rates and long lasting photogenerated charge carriers in FAPbBr3 perovskite films. The devices based on FTO/TiO2/FAPbBr3/spiro-OMeTAD/Au show highly reproducible open-circuit voltage (Voc) of 1.42 V, a record for FAPbBr3-based perovskite solar cells. Voc as a function of illumination intensity indicates that the contacts are very selective and higher Voc values are expected to be achieved when the quality of the FAPbBr3 film is further improved. Overall, the devices based on these films reveal appreciable power conversion efficiency of 7% under standard illumination conditions with negligible hysteresis. Finally, the amplified spontaneous emission (ASE) behavior explored in a cavity-free configuration for FAPbBr3 perovskite films shows a sharp ASE threshold at a fluence of 190 μJ cm−2 with high quantum efficiency further confirming the high quality of the films.
The fabrication of smooth and uniform formamidinium lead bromide perovskite films exhibiting strong emission and long charge carrier lifetime paves the way for high open-circuit voltage and amplified spontaneous emission with a low threshold carrier density.
The detailed characterization of a dialkoxyphenylene-difluorobenzothiadiazole based conjugated polymer poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2FBT) is reported. PPDT2FBT closely tracks theoretical photocurrent production while maintaining a high fill factor in remarkably thick films. In order to understand the properties that enable PPDT2FBT to function with thick active layers, the effect of film thickness on the material properties and device parameters was carefully studied and compared to three benchmark polymers. Optical modeling, grazing incidence wide angle X-ray scattering, cross-sectional transmission electron microscopy, transient photoconductivity, and extensive device work were carried out and have clarified the key structural features and properties that allow such thick active layers to function efficiently. The unique behavior of thick PPDT2FBT films arises from high vertical carrier mobility, an isotropic morphology with strong, vertical π–π stacking, and a suitable energy band structure. These physical characteristics allow efficient photocurrent extraction, internal quantum efficiencies near 100% and power conversion efficiencies over 9% from exceptionally thick active layers in both conventional and inverted architectures. The ability of PPDT2FBT to function efficiently in thick cells allows devices to fully attenuate incident sunlight while providing a pathway to defect-free film processing over large areas, constituting a major advancement toward commercially viable organic solar cells.
A unique conjugated polymer (PPDT2FBT) is found to closely track theoretical photocurrent production and maintain a high fill factor in remarkably thick films. The unique behavior of thick PPDT2FBT films arises from high vertical carrier mobility and a morphology with strong, vertical π–π stacking, allowing efficient photocurrent extraction and internal quantum efficiencies near 100% from exceptionally thick active layers.
Two-dimensional inorganic materials are emerging as a premiere class of materials for fabricating modern electronic devices. The interest in 2D layered transition metal dichalcogenides is especially high. Particularly, 2D MoS2 is being heavily researched due to its novel functionalities and its suitability for a wide range of electronic and optoelectronic applications. In this article, the progress in mono/few layer(s) MoS2 research is reviewed by focusing primarily on the layer dependent evolution of crystal, phonon, and electronic structure. The review includes extensive detail into the methodologies adapted for single or few layer(s) MoS2 growth. Further, the review covers the versatility of 2D MoS2 for a broad range of device applications. Recent advancements in the field of van der Waals heterostructures are also highlighted at the end of the review.
The recent emergence of atomically thin 2D materials beyond graphene opened up the platform for new device designs. This review covers recent progress in atomically thin MoS2 research, including a description of layer-dependent evolution of the crystal structure, phonon properties and electronic structure, and various methodologies adapted for single- or few-layer MoS2 growth. It also covers the versatility of 2D MoS2 for a broad range of electronic and optoelectronic device applications.
Nature Photonics 10, 115 (2016). doi:10.1038/nphoton.2015.243
Authors: Son-Tung Ha, Chao Shen, Jun Zhang & Qihua Xiong
