ZnO Buffer Research Articles

Polymer Solar Cells with Efficiency >10% Enabled via a Facile Solution‐Processed Al‐Doped ZnO Electron Transporting Layer

A facile and low‐temperature (125 °C) solution‐processed Al‐doped ZnO (AZO) buffer layer functioning very effectively as electron accepting/hole blocking layer for a wide range of polymer:fullerene bulk heterojunction systems, yielding power conversion efficiency in excess of 10% (8%) on glass (plastic) substrates is described. The ammonia‐treatment of the aqueous AZO nanoparticle solution produces compact, crystalline, and smooth thin films, which retain the aluminum doping, and eliminates/reduces the native defects by nitrogen incorporation, making them good electron transporters and energetically matched with the fullerene acceptor. It is demonstrated that highly efficient solar cells can be achieved without the need for additional surface chemical modifications of the buffer layer, which is a common requirement for many metal oxide buffer layers to yield efficient solar cells. Also highly efficient solar cells are achieved with thick AZO films (>50 nm), highlighting the suitability of this material for roll‐to‐roll coating. Preliminary results on the applicability of AZO as electron injection layer in F8BT‐based polymer light emitting diode are also presented.

Influence of homo buffer layer thickness on the quality of ZnO epilayers

ZnO buffer layers with different thicknesses were deposited on a-plane sapphire substrates at 300°C. ZnO epilayers were grown on ZnO buffers at 600°C by radio-frequency magnetron sputtering and vacuum annealed at 900°C for an hour. Influence of nucleation layer thickness on the structural and quality of ZnO thin films was investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), and Raman spectroscopy. The best ZnO film quality was obtained with the ZnO buffer layer of 45nm thick which provided the smoothest surface with RMS value of 0.3nm. X-ray diffraction measurements reveal that the films have a single phase wurtzite structure with (0001) preferred crystal orientation. As evident from narrow FWHM of ZnO (0002) rocking curve, ZnO buffer can serve as a good template for the growth of high-quality ZnO films with little tilt. In addition, the micro-Raman scattering measurements at room temperature revealed the existence of Raman active phonon modes of ZnO; A1(TO), A1(LO) and E2(high). The latter two modes were not observed in thin buffer layer beside the dis-appearance of E2(low) mode in all films.

Structural and surface characterization of magnetron sputtered In1−xAlxN films grown on GaAs substrates

This paper reports the structural and surface features of In1−xAlxN films grown on GaAs substrates with ZnO buffer layer by using reactive magnetron co-sputtering of Al and In targets in Ar–N2 at 100°C. The Al sputtering power was varied between 100W and 300W with respect to the fixed In power (50W) to obtain different compositions of the films. X-ray diffraction (XRD) studies revealed the formation of nanostructured In1−xAlxN films along (0002) and (101¯1) planes with composition ‘x’ ranging from 0.4 to 0.9. Intensity of the diffraction peaks and crystallite size were decreased with increase of the Al contents in the film. Field emission scanning electron microscopy (FESEM) results indicated that a compact film structure changes into a relatively loose structure by increasing the Al power.

Effect of buffer layer on growth and properties of ZnO nanorod arrays

ZnO nanorods arrays on ZnO buffered Si substrates have been obtained by conventional chemical vapor deposition. The influence of ZnO buffer layer on the growth and properties of the ZnO nanorods have been studied. The morphology and the alignment ordering of ZnO nanorods arrays were greatly affected by the thickness of ZnO buffers. Photoluminescence and field emission properties can be enhanced by modulating the thicknesses of the ZnO buffer layers. ZnO nanorods with the best crystallinity and the highest ordering were obtained with 50-nm-thick ZnO buffer layer, which exhibits the strongest UV emission.

Energy dissipation channels affecting photoluminescence from resonantly excited Er3+ ions doped in epitaxial ZnO host films

We identified prerequisite conditions to obtain intense photoluminescence at 1.54 μm from Er3+ ions doped in ZnO host crystals. The epitaxial ZnO:Er films were grown on sapphire C-plane substrates by sputtering, and Er3+ ions were resonantly excited at a wavelength of 532 nm between energy levels of 4I15/2 and 2H11/2. There is a threshold deposition temperature between 500 and 550 °C, above which epitaxial ZnO films become free of miss-oriented domains. In this case, Er3+ ions are outside ZnO crystallites, having the same c-axis lattice parameters as those of undoped ZnO crystals. The improved crystallinity was correlated with enhanced emissions peaking at 1538 nm. Further elevating the deposition temperature up to 650 °C generated cracks in ZnO crystals to relax the lattice mismatch strains, and the emission intensities from cracked regions were three times as large as those from smooth regions. These results can be consistently explained if we assume that emission-active Er3+ ions are those existing at grain boundaries and bonded to single-crystalline ZnO crystallites. In contrast, ZnO:Er films deposited on a ZnO buffer layer exhibited very weak emissions because of their degraded crystallinity when most Er3+ ions were accommodated into ZnO crystals. Optimizing the degree of oxidization of ZnO crystals is another important factor because reduced films suffer from non-radiative decay of excited states. The optimum Er content to obtain intense emissions was between 2 and 4 at. %. When 4 at. % was exceeded, the emission intensity was severely attenuated because of concentration quenching as well as the degradation in crystallinity. Precipitation of Er2O3 crystals was clearly observed at 22 at. % for films deposited above 650 °C. Minimizing the number of defects and impurities in ZnO crystals prevents energy dissipation, thus exclusively utilizing the excitation energy to emissions from Er3+ ions.

Significant mobility enhancement in extremely thin highly doped ZnO films

Highly Ga-doped ZnO (GZO) films of thicknesses d = 5, 25, 50, and 300 nm, grown on 160-nm ZnO buffer layers by molecular beam epitaxy, had 294-K Hall-effect mobilities μH of 64.1, 43.4, 37.0, and 34.2 cm2/V-s, respectively. This extremely unusual ordering of μH vs d is explained by the existence of a very high-mobility Debye tail in the ZnO, arising from the large Fermi-level mismatch between the GZO and the ZnO. Scattering theory in conjunction with Poisson analysis predicts a Debye-tail mobility of 206 cm2/V-s at the interface (z = d), falling to 58 cm2/V-s at z = d + 2 nm. Excellent fits to μH vs d and sheet concentration ns vs d are obtained with no adjustable parameters.

Post-deposition annealing of magnetron sputtered InAlN film at different temperatures

Effects of post-deposition annealing on the structural, surface and optical properties of InAlN films have been investigated in this paper. InAlN films were grown on ZnO buffer layers on GaAs (100) substrates by magnetron co-sputtering of In and Al targets in Ar–N2 plasma at 100°C. The as-grown films were annealed at 250°C, 500°C and 700°C for six hours in a quartz tube furnace. X-ray diffraction (XRD) analysis confirmed the formation of InAlN films with preferred orientation along (101¯1) plane. Crystallinity of the films was improved with increase of the annealing temperature. A substantial increase in the crystallinity of InAlN film along (101¯1) plane was noticed at 700°C. Compositional analyses from the XRD and energy dispersive X-ray spectroscopy (EDS) specified higher In contents in the films as compared to the Al. Surface morphology of the films indicated grains growth whereas changes in the surface roughness, calculated using WSxM 5.0 develop 7.0 Nanotech Software, were found to be inconsistent with increase of the annealing temperature. Raman spectroscopy revealed A1 (LO) and E2 (high) phonon modes in the film annealed at 700°C. Photoluminescence spectra of InAlN films were also recorded to determine the band gap.

Ultraviolet lasing action in aligned ZnO nanowall

A vertically aligned ZnO nanowall arrays have been synthesized on a patterned ZnO buffer layer using an interference laser beam. The average height and thickness of the ZnO nanowall were 4 μm and 200 nm, respectively. Room-temperature ultraviolet (UV) lasing was obtained from single piece of ZnO nanowall detached from the substrate. The threshold power density for lasing was estimated to be about 80 kW/cm2. In addition, UV lasing from vertically aligned ZnO nanowall was also observed owing to the presence of holes. The lasing spectra had no regularity in the mode spacing and many different spatial modes, suggesting that the action was supported by the random lasing mechanism. The threshold power density for lasing was about 300 kW/cm2, which was much higher than that of the single piece of ZnO nanowall because of the leakage of the light into buffer layer.

Domain matched epitaxial growth of (111) Ba0.5Sr0.5TiO3 thin films on (0001) Al2O3 with ZnO buffer layer

Epitaxial (111) Ba0.5Sr0.5TiO3 (BST) thin films have been grown by pulsed laser deposition on (0001) Al2O3 substrate with ZnO as buffer layer. The x-ray ω-2θ, Φ-scan and reciprocal space mapping indicate epitaxial nature of BST thin films. The domain matched epitaxial growth of BST thin films over ZnO buffer layer was confirmed using Fourier filtered high resolution transmission electron microscope images of the film-buffer interface. The incorporation of ZnO buffer layer effectively suppressed the lattice mismatch and promoted domain matched epitaxial growth of BST thin films. Coplanar inter digital capacitors fabricated on epitaxial (111) BST thin films show significantly improved tunable performance over polycrystalline thin films.

Catalyst free growth of ZnO nanowires on graphene and graphene oxide and its enhanced photoluminescence and photoresponse

We demonstrate the graphene assisted catalyst free growth of ZnO nanowires (NWs) on chemical vapor deposited (CVD) and chemically processed graphene buffer layers at a relatively low growth temperature (580 °C) in the presence and absence of ZnO seed layers. In the case of CVD graphene covered with rapid thermal annealed ZnO buffer layer, the growth of vertically aligned ZnO NWs takes place, while the direct growth on CVD graphene, chemically derived graphene (graphene oxide and graphene quantum dots) without ZnO seed layer resulted in randomly oriented sparse ZnO NWs. Growth mechanism was studied from high resolution transmission electron microscopy and Raman spectroscopy of the hybrid structure. Further, we demonstrate strong UV, visible photoluminescence (PL) and enhanced photoconductivity (PC) from the CVD graphene–ZnO NWs hybrids as compared to the ZnO NWs grown without the graphene buffer layer. The evolution of crystalinity in ZnO NWs grown with ZnO seed layer and graphene buffer layer is correlated with the Gaussian line shape of UV and visible PL. This is further supported by the strong Raman mode at 438 cm−1 significant for the wurtzite phase of the ZnO NWs grown on different graphene substrates. The effect of the thickness of ZnO seed layers and the role of graphene buffer layers on the aligned growth of ZnO NWs and its enhanced PC are investigated systematically. Our results demonstrate the catalyst free growth and superior performance of graphene–ZnO NW hybrid UV photodetectors as compared to the bare ZnO NW based photodetectors.

Fabrication of inverted bulk heterojunction organic solar cells based on conjugated P3HT:PCBM using various thicknesses of ZnO buffer layer

Recently, there has been tremendous progress among the researchers all over the world in the development of polymer-based organic solar cells as a clean and safe energy source. Besides, it has also attracted a great attention due to its inexpensive manufacturing cost and environment-friendly energy conversion capability. However, they still have low efficiency and the unstable because organic materials are easily oxidized by humidity and UV light under the atmosphere environment. The objective of this study was to investigate the effects of different thickness of the ZnO buffer layer on the general performance of the bulk heterojunction organic solar cell devices with configuration indium tin oxide/zinc oxide buffer layer/poly(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester/gold (ITO/ZnO/P3HT:PCBM/Au). ZnO film acts as a protection layer to prevent the interface of photoactive layer by UV light from the oxidation and to reduce the energy barrier for easily transferring electron between collecting electrode and the LUMO level of the organic acceptor. Also, ZnO film block holes in P3HT from being recombined with electrons in collecting electrode. It is observed that the power conversion efficiency is significantly dependent on the thickness of the buffer layer. The solar cell performance of a short-circuit current density of 1.599mA/cm2, an open-circuit voltage of 119mV and a fill factor of 20.85%, with a power conversion efficiency of about 0.0432% under AM 1.5 illumination (100mW/cm2) is obtained when the thickness of the buffer layer is optimized.

Effect of ZnO nanoparticle morphology and post-treatment with zinc acetate on buffer layer in inverted organic photovoltaic cells

Spherical and rod-shaped ZnO nanoparticles were synthesized through modified sol–gel method to investigate the morphological effect in the buffer layer of inverted organic photovoltaic cells (OPVs). Also the buffer layers fabricated with ZnO nanoparticles were post-treated by using zinc acetate dihydrate in methanol (zinc acetate post-treatment) to survey the influence of surface state on buffer layer. After the zinc acetate post-treatment on ZnO buffer layer, the low series resistance (Rs) and high shunt resistance (Rsh) were obtained due to the enhancement of the electric contiguity at the interface (buffer layer/active layer) and interconnectivity between ZnO nanoparticles. By improving the resistance, the current density (Jsc) and fill factor (FF) of devices were increased. The performance of device fabricated with rod-shaped ZnO particles was higher than that of cell fabricated with spherical ZnO particles because the 1-dimensional rod particles acted as an appropriate bridge to promote the electron mobility. The best power conversion efficiency (2.68%) could be achieved through the facile and effective zinc acetate post-treatment.

Effect of the buffer layer on the metal-semiconductor-metal UV photodetector based on Al-doped and undoped ZnO thin films with different device structures

ZnO and Al-doped ZnO (AZO) thin films were grown on the ZnO buffer layer by a chemical spray pyrolysis method. The metal–semiconductor–metal (MSM) UV photodetector based on ZnO thin films with two device configuration Ag/AZO/ZnO/Ag and Ag/ZnO/ZnO/Ag have been studied. AZO and ZnO thin films were grown on the ZnO buffer layer by a chemical spray pyrolysis technique. The effect of the buffer layer on the physicochemical and UV-sensing properties of the AZO and ZnO thin-film-based UV photodetectors were analyzed. The XRD results suggested that the buffer layer improves the crystalline quality of the ZnO thin films. The device with configuration Ag/AZO/ZnO/Ag exhibits a maximum photocurrent of about 876 µA at 5 V bias at 365 nm peak wavelength. The linear I–V characteristic of the photodetector reveals the good ohmic contacts between metal–semiconductor junctions. The AZO-based UV photodetector with ZnO buffer layer shows a maximum photoresponsivity of about 340 A/W, which is much higher than that of the ZnO-based UV photodetector with ZnO buffer layer. The photoresponse and photoswitching characteristics of the device demonstrate that a ZnO UV photodetector with a buffer layer offers a new way to fabricate devices on a buffer-layer-coated substrate.

Contactless electroreflectance spectroscopy of ZnO/ZnMgO quantum wells: Optical transitions and Fabry–Perot features

Contactless electroreflectance (CER) spectroscopy was applied to study optical transitions in ZnO/ZnMgO quantum wells (QWs) of various widths and barrier contents (Mg = 10, 20, and 30%) grown on sapphire substrates with a Zn0.9Mg0.1O or ZnO buffer layer. Spectral features related to absorption in ZnO QW region and ZnMgO barrier were clearly observed in CER spectra. The spectral position of these features correlates very well with photoluminescence peaks related to emission from ZnO QW and ZnMgO barrier. Besides CER resonances, which are associated with optical transitions, some spectral features related to the Fabry–Perot (F–P) interference effect were clearly observed for these samples. They appear for these structures due to a strong contrast of refractive index between the sapphire and Zn(Mg)O layers and they are observed in the region of transparency for Zn(Mg)O epilayers. For some samples the F–P related signal is quite strong and interferes with CER resonances related to QW transitions. However the two contributions (i.e., CER signal related to optical transitions and the Fabry–Perot effect) can be recognized when the proper analysis of CER spectra is performed. This analysis is presented and discussed in this paper.

Fully understanding the positive roles of plasmonic nanoparticles in ameliorating the efficiency of organic solar cells.

Herein, we constructed inverted PBDTTT-CF:PC70BM bulk-heterojunction organic solar cells by introducing Au nanoparticles to a ZnO buffer layer and a great improvement in energy conversion efficiency has been realized. To discover the positive roles of such plasmonic nanoparticles in the process of solar energy conversion, photovoltaic devices with the same architecture but different sized Au nanoparticles were purposely fabricated and it has been observed that the overall efficiency can be remarkably improved from 6.67% to 7.86% by embedding 41 nm Au nanoparticles in the buffer layer. The devices with other sizes of Au nanoparticles show a relatively low performance. Subsequent investigations including finite difference time domain simulation and transient photoluminescence studies reveal that the existence of the plasmonic particles could not only improve the optical absorption and facilitate the exciton separation, but can also benefit the collection of charge carriers. Thus, this paper provides a comprehensive perspective on the roles of plasmonic particles in organic solar cells and insights into the photo energy conversion process in the plasmonic surroundings.

Well-defined star-shaped conjugated macroelectrolytes as efficient electron-collecting interlayer for inverted polymer solar cells.

A star-shaped monodisperse conjugated macroelectrolyte grafted with cationic side chains, TrNBr, was designed, synthesized, and utilized as efficient electron-collecting cathode interlayers for inverted polymer solar cells. A neutral one composed of identical star-shaped conjugated backbone, TrOH, was also investigated for comparison. The surface properties and the function as interfacial layers on modulating the work function of bottom electrode (indium tin oxide) were systematically studied. Both interfacial electron-selective materials show strongly thickness-dependent performance for inverted polymer solar cells, and the best performance could be achieved via optimizing the thickness with 2.4 nm of TrNBr and 8.7 nm of TrOH. Parallel investigations of optimized TrNBr and TrOH interlayer in inverted architecture with active blend layer of poly(3-hexylthiophene):indene-C60 bisadduct (P3HT:ICBA) demonstrated a remarkable power conversion efficiency (PCE) enhancement (PCE of 4.88% for TrNBr and 4.74% for TrOH) in comparison with those of conventional noninverted devices using Ca/Al cathodes (3.94%) and inverted devices with sol-gel ZnO buffer layer (4.21%). In addition, the inverted devices using the TrNBr and TrOH interlayer exhibited improved device stability in contrast to conventional noninverted devices using Ca/Al cathodes.

Mimicry of sputtered i-ZnO thin films using chemical bath deposition for solution-processed solar cells.

Solution processing provides a versatile and inexpensive means to prepare functional materials with specifically designed properties. The current challenge is to mimic the structural, optical, and/or chemical properties of thin films fabricated by vacuum-based techniques using solution-based approaches. In this work we focus on ZnO to show that thin films grown using a simple, aqueous-based, chemical bath deposition (CBD) method can mimic the properties of sputtered coatings, provided that the kinetic and thermodynamic reaction parameters are carefully tuned. The role of these parameters toward growing highly oriented and dense ZnO thin films is fully elucidated through detailed microscopic and spectroscopic investigations. The prepared samples exhibit bulk-like optical properties, are intrinsic in their electronic characteristics, and possess negligible organic contaminants, especially when compared to ZnO layers deposited by sol-gel or from nanocrystal inks. The efficacy of our CBD-grown ZnO thin films is demonstrated through the effective replacement of sputtered ZnO buffer layers within high efficiency solution processed Cu2ZnSnS4xSe4(1-x) solar cells.

Structural Properties of Ultrasonically Sprayed Al-Doped ZnO (AZO) Thin Films: Effect of ZnO Buffer Layer on AZO

Transparent aluminium-doped ZnO (AZO)-conducting oxide films were deposited on a glass substrate, using an ultrasonic spray pyrolysis (USP) system at 475°C. We investigated the effects of the Al/Zn atomic ratios on the structural properties of the AZO films. All the deposited AZO thin films presented hexagonal wurtzite structure. As Al doping increased in the film, the preferential orientation switched from [002] to [101], and crystallite sizes varied from 31.90 nm to 34.5 nm. Field emission scanning electron microscopy showed a change in the surface morphology of the AZO films with respect to the Al/Zn ratio, and secondary ion mass spectroscopy showed that the amount of Al incorporated into the films was proportional to the concentration of the starting solution. A fast Fourier transform of the AZO film measurements confirmed the presence of (100), (102), and (200) reflections, corresponding to a wurtzite structure of the AZO thin films. The plane corresponding to AZO was simulated, and matched the experimental pattern obtained from high-resolution transmission electron microscopy. An un-doped ZnO layer was deposited onto the AZO film using USP at 400°C, and a bilayer of AZO/ZnO was annealed in vacuum for 20 min at 350°C. The resistivity of these bilayer films was lower than that of a single-layered AZO film, and it further decreased by vacuum annealing.

Metal-oxide broken-gap tunnel junction for copper indium gallium diselenide tandem solar cells

A metal-oxide broken-gap heterojunction between p-Cu2O and n-In2O3 is proposed and studied for use as a tunnel junction in polycrystalline CuIn1−xGaxSe2 (CIGS) based tandem solar cells using numerical device simulation. Specifically, a tandem solar cell with a CuGaSe2 (CGS) absorber top cell and a CuInSe2 (CIS) absorber bottom cell was considered. The ballistic transport model explains well the carrier transport in the broken-gap heterojunction. Broken-gap heterojunctions provide linear current–voltage characteristics even if one side is lightly doped and the junction resistance is much lower than typical values of series resistance in solar cells. However, electron affinities of metal oxides for broken-gap band alignment may induce an energy barrier between the tunnel junction and the top and bottom solar cells, degrading performance. These barriers can be reduced using buffer layers. For example, NiO and graded In2O3–ZnO buffer layers are proposed beneath CGS top solar cell and above the CIS bottom solar cell, respectively. With these buffer layers, the efficiencies of the top and bottom cells are 17.5% and 6.5% respectively. The modeled efficiency of the CGS/CIS tandem solar cell is 24.1% and there is virtually no significant efficiency loss due to the presence of the broken-gap junction.

Solution-processed silver nanowires as a transparent conducting electrode for air-stable inverted organic solar cells

Highly efficient and air-stable inverted organic solar cells (IOSCs) were fabricated using solution-processed silver nanowire electrodes. The electrodes showed a low sheet resistance of ~16Ωsq−1 and a high transmittance of ~95% at a wavelength of 550nm. A solution-processed ZnO buffer layer is typically used for electron transport and effective passivation of the surface of Ag NW electrodes. The device performance of the IOSCs that used these Ag NW electrodes, which were fabricated on a glass or plastic substrate, was >94% of that of devices containing indium tin oxide (ITO) electrodes. This indicates that solution-processed Ag NW electrode can replace commercialized ITO and can be utilized in roll-to-roll and large-area fabrication processes.