Optical management in high‐efficiency thin‐film silicon micromorph solar cells with a silicon oxide based intermediate reflector

In the effort to increase the stable efficiency of thin film silicon micromorph solar cells, a silicon oxide based intermediate reflector (SOIR) layer is deposited in situ between the component cells of the tandem device. The effectiveness of the SOIR layer in increasing the photo‐carrier generation in the a‐Si:H top absorber is compared for p–i–n devices deposited on different rough, highly transparent, front ZnO layers. High haze and low doping level for the front ZnO strongly enhance the current density (J_sc) in the μc‐Si:H bottom cell whereas J_sc in the top cell is influenced by the angular distribution of the transmitted light and by the reflectivity of the SOIR related to different surface roughness. A total J_sc of 26.8 mA/cm² and an initial conversion efficiency of 12.6% are achieved for 1.2 cm² cells with top and bottom cell thicknesses of 300 nm and 3 μm, and without any anti‐reflective coating on the glass. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

NaCl/Ca/Al as an efficient cathode in organic light-emitting devices

An efficient cathode NaCl/Ca/Al used to improve the performance of organic light-emitting devices (OLEDs) was reported. Standard N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′ biphenyl 4,4′-dimaine (NPB)/tris-(8-hydroxyquinoline) aluminum (Alq₃) devices with NaCl/Ca/Al cathode showed dramatically enhanced electroluminescent (EL) efficiency. A power efficiency of 4.6 lm/W was obtained for OLEDs with 2 nm of NaCl and 10 nm of Ca, which is much higher than 2.0 lm/W, 3.1 lm/W, 2.1 lm/W and 3.6 lm/W in devices using, respectively, the LiF (1 nm)/Al, LiF (1 nm)/Ca (10 nm)/Al, Ca (10 nm)/Al and NaCl (2 nm)/Al cathodes. The investigation of the electron injection in electron-only devices indicates that the utilization of the NaCl/Ca/Al cathode substantially enhances the electron injection current, which in case of OLEDs leads to the improvement of the brightness and efficiency.     

n‐type silicon solar cells with surface‐passivated screen‐printed aluminium‐alloyed rear emitter

Aluminium‐doped p‐type (Al‐p⁺) silicon emitters fabricated by means of a simple screen‐printing process are effectively passivated by plasma‐enhanced chemical‐vapour deposited amorphous silicon (a‐Si). We measure an emitter saturation current density of only 246 fA/cm², which is the lowest value achieved so far for a simple screen‐printed Al‐p⁺ emitter on silicon. In order to demonstrate the applicability of this easy‐to‐fabricate p⁺ emitter to high‐efficiency silicon solar cells, we implement our passivated p⁺ emitter into an n⁺np⁺ solar cell structure. An independently confirmed conversion efficiency of 19.7% is achieved using n‐type phosphorus‐doped Czochralski‐grown silicon as bulk material, clearly demonstrating the high‐efficiency potential of the newly developed a‐Si passivated Al‐p⁺ emitter. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Bathocuproine/Ag复合电极对于聚合物光伏器件效率和稳定性的影响

采用Bathocuproine/Ag (BCP/Ag)复合电极代替Ca/Al复合电极, 制备PTB7:PC71BM 作为光敏层的聚合物光伏器件, 并通过改变BCP薄膜厚度来研究BCP/Ag复合电极对于器件光电转换器和稳定性的影响. 研究发现: 在光敏层和金属电极之间插入BCP修饰层后, 器件性能得到了显著的改善, 在BCP厚度为5 nm时, 器件的效率达到了6.82%, 且略高于Ca/Al复合电极的器件效率; 相比于采用Ca/Al复合电极的器件, BCP/Ag复合电极增大了器件的短路电流和外量子效率, 使器件效率得到提高; 同时器件的稳定性得到了显著的改善, BCP/Ag 复合电极器件的衰减速率几乎和未插入BCP的器件衰减速率相同, 相对于Ca/Al复合电极器件大幅提高.     

Post-fabrication annealing effects on the performance of P3HT:PCBM solar cells with/without ZnO nanoparticles

In this study, P3HT:PCBM organic photovoltaic (OPV) devices, with or without ZnO nanoparticles buffer layer between the photoactive layer (P3HT:PCBM) and the cathode (Al top electrode), were fabricated. The devices were annealed at 145 °C either before or after depositing the top electrode. The objective of this study was to investigate the effects of the ZnO buffer layer and pre-/post-fabrication annealing on the general performance of these devices. The short-circuit current density (J_SC), open-circuit voltage (V_OC) and the external quantum efficiency (EQE) of the OPV devices were improved by the insertion of the ZnO layer and post-fabrication annealing. The post-fabrication annealed devices, with or without the ZnO layer, exhibited higher values of J_SC, V_OC and EQE than those of similar devices annealed before depositing the Al metal. This can be attributed to, among other things, improved charge transport across the interface between the photoactive layer and the Al top electrode as a result of post-annealing induced modification of the interface morphology.     

Temperature‐dependent electroluminescence intensity in green and blue (In,Ga)N multiple‐quantum‐well diodes

The electroluminescence (EL) intensity has been investigated of green and blue (In,Ga)N multiple‐quantum‐well diodes grown on c ‐plane sapphire over a wide temperature range and as a function of current between 0.01 mA and 10 mA. The EL intensity of the green diode with p‐(Al,Ga)N electron blocking layer does not show low‐temperature quenching, especially at low injection levels, previously observed for the blue (In,Ga)N quantum‐well diodes. This finding rules out possi‐ bilities that the freeze‐out of holes at deep Mg acceptor levels and the failure of hole injections through the p‐(Al,Ga)N layer are directly responsible for the EL quenching at temperatures below 100 K. Variations of the EL efficiency with current level suggest that capture/escape efficiencies of injected carriers by the wells play an important role for the determination of EL external quantum efficiency. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Integrated self‐aligned conjugated polymer fiber laser devices

In this letter we present a fully integrated self‐aligned distributed feedback (DFB) fiber laser device, which was directly imprinted in the conjugated polymer poly[2‐methoxy‐5 (2‐ethyl‐hexyloxy)‐1,4‐phenylenevinylene] (MEH‐PPV) on the top of optical fibers by using the soft lithographic technique of “liquid imprinting”. For this process master gratings (360 and 380 nm) for the feedback structure were fabricated via e‐beam lithography, transferred to an elastomeric stamp and used to imprint the grating into the highly luminescent conjugated polymer. Such second order gratings were photo‐pumped with a frequency doubled Nd:YAG laser, the laser emission (around 640 nm, depending on the used grating) was directly coupled into the waveguide and detected via a CCD spectrometer at the end of the fiber. The threshold of the laser devices was found to be in the range of 4.3 mJ/cm² and exhibited a line width of approximately 2 nm. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Decoupling crystalline volume fraction and VOC in microcrystalline silicon pin solar cells by using a µc‐Si:F:H intrinsic layer

Microcrystalline silicon thin film pin solar cells with a highly crystallized intrinsic μc‐Si:F:H absorber were prepared by RF‐plasma enhanced chemical vapour deposition using SiF₄ as the gas precursor. The cells were produced with a vacuum break between the doped layer and intrinsic layer depositions, and the effect of different subsequent interface treatment processes was studied. The use of an intrinsic μc‐Si:H p/i buffer layer before the first air break increased the short circuit current density from 22.3 mA/cm² to 24.7 mA/cm². However, the use of a hydrogen‐plasma treatment after both air breaks without an interface buffer layer improved both the open circuit voltage and the fill factor. Although the material used for the absorber layer showed a very high crystalline fraction and thus an increased spectral response at long wavelengths, an open‐circuit voltage (V_OC) of 0.523 V was nevertheless observed. Such a value of V_OC is higher than is typically obtained in devices that employ a highly crystallized absorber as reported in the literature (see abstract figure). Using a hydrogen‐plasma treatment, a single junction μc‐Si:F:H pin solar cell with an efficiency of 8.3% was achieved.

     

Novel post‐process for the passivation of a CMOS biosensor

Sensors, which are designed and fabricated in complementary metal oxide semiconductor (CMOS) technology, have become increasingly important in the field of bioelectronics. The standardized industry processes enable a fast, cheap, and reliable fabrication of biosensor devices with integrated addressing and processing units. However, the interfacing of such chips with a liquid environment has been a challenge in recent years. Especially for interfacing living cells with CMOS biosensors different elaborate post‐processes have been proposed. In this article we describe a novel and single step passivation of a CMOS biosensor using a bio‐compatible high‐permittivity thin film, which can be directly applied to the top aluminium layer of a CMOS process. The aluminium oxide and hafnium oxide multi‐layer thin films were prepared using atomic layer deposition at low process temperatures. Electrical I –V and capacitance measurements as well as electrochemical leakage current measurements were performed on films grown on aluminium bottom electrodes. The films showed a very low leakage current and were stable up to 6 V at a thickness of just 50 nm. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thin Film Solar Cell Based on ZnSnN2/SnO Heterojunction

In this article, we report the growth of zinc‐tin nitride (ZnSnN₂) thin films as a potential absorber for photovoltaic applications by fabricating a heterojunction of n‐ZnSnN₂/p‐SnO. The performance of the heterojunction has been monitored through selective deposition of top electrode with different materials (Ni/Au or Al). The electron‐transfer process from the ZnSnN₂ layer to the cathode is facilitated by selecting metal electrode with relatively low work function, which also boosts up the electron injection or/and extraction. The diode exhibits a good J–V response in the dark with a rectification ratio of 3 × 10³ at 1.0 V and an ideality factor of 4.2 in particular with Al as the top electrode. Under illumination, the heterostructure solar cell demonstrates a power conversion efficiency of ≈0.37% with an open circuit voltage of 0.25 V and a short circuit current density of 4.16 mA cm^?2. The prime strategies, on how to improve solar cell efficiency concerning band offsets and band alignment engineering are also discussed.     

Organic‐inorganic hybrids based on phenanthrene‐functionalized gold nanoparticles for OLEDs

The electroluminescence intensity of the phenanthrene‐functionalized gold nanoparticles, PMPT‐Au nanoparticles/CPB: Ir(PIA)₂ (acac) film, was increased by 4.9 times compared with control device, CPB: Ir(PIA)₂ (acac) due to coupling between the excitons of emissive layer and localized surface plasmonic resonance of PMPT‐Au NPs. The maximum luminous efficiencies of devices II to IV with PMPT‐Au NPs were 39.2 cd A^?1 (11.8 V), 40.1 cd A^?1 (10.5 V), and 43.1 cd A^?1 (9.0 V), respectively. The increment of current efficiency with PMPT‐Au NP coated devices was strongly related to the energy transfer between the radiated light generated from CBP: Ir(PIA)₂ (acac) emissive layer and localized surface plasmonic resonance excited by PMPT‐Au NP layer.     

Ultra‐thin (002)‐oriented Al‐doped zinc oxide transparent electrode grown on oxygen‐controlled homo‐seed layer

Highly (002)‐oriented Al‐doped zinc oxide (AZO) thin films with the thickness of less than 200 nm have been deposited on an oxygen‐controlled homo‐seed layer at 200 °C by DC magnetron sputtering. With the homo‐seed layer being employed, the full‐width at half maximum (FWHM) of the (002) diffraction peak for the AZO ultra‐thin films decreased from 0.33° to 0.22°, and, the corresponding average grain size increased from 26.8 nm to 43.0 nm. The XRD rocking curves revealed that the AZO ultra‐thin film grown on the seed layer deposited in atmosphere of O₂/Ar of 0.09 exhibited the most excellent structural order. The AZO ultra‐thin film with homo‐seed layer reached a resistivity of 4.2 × 10^–4 Ω cm, carrier concentration of 5.2 × 10²⁰ cm^–3 and mobility of 28.8 cm² V^–1 s^–1. The average transmittance of the AZO ultra‐thin film with homo‐seed layer reached 85.4% in the range of 380–780 nm including the substrate. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Modeling the formation of local highly aluminum‐doped silicon regions by rapid thermal annealing of screen‐printed aluminum

The formation of local highly aluminum‐doped (Al‐p⁺) regions by rapid thermal annealing (firing) of screen‐printed aluminum strongly depends on the temperature profile and the contact geometry. We measure the local Al‐p⁺ layer thickness W_Al‐p+ as a function of the point and line contact size. Using quantitative yet simple analytical modeling, the time‐dependent silicon concentration in the Al melt is described by elementary differential equations. From this we calculate W_Al‐p+ and find agreement with the measurements. In contrast to the formation of full area Al‐p⁺ layers we find a smaller silicon concentration at the end of the firing process compared to the equilibrium concentration. This is a result of the process dynamics such as the dissolution rate of solid silicon and the transport of silicon in the Al melt. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Contact passivation in silicon solar cells using atomic‐layer‐deposited aluminum oxide layers

Atomic‐layer‐deposited aluminum oxide (AlO_x) layers are implemented between the phosphorous‐diffused n⁺‐emitter and the Al contact of passivated emitter and rear silicon solar cells. The increase in open‐circuit voltage V_oc of 12 mV for solar cells with the Al/AlO_x/n⁺‐Si tunnel contact compared to contacts without AlO_x layer indicates contact passivation by the implemented AlO_x. For the optimal AlO_x layer thickness of 0.24 nm we achieve an independently confirmed energy conversion efficiency of 21.7% and a V_oc of 673 mV. For AlO_x thicknesses larger than 0.24 nm the tunnel probability decreases, resulting in a larger series resistance. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Improved yield in molecular electronic devices using amino-style molecules

Molecular electronic devices were fabricated with amino-style derivatives as redox-active components. These molecules are amphiphilic to allow monolayer formation by the Langmuir–Blodgett (LB) method, and this LB monolayer is inserted between two metal electrodes. On measuring the current–voltage (I–V) characteristics, it was found that the Al/amino style LB monolayer/Al devices show remarkable hysteresis and switching behavior, so that they can be used as memory devices at ambient conditions, when an aluminum oxide layer exists on the bottom electrode. From the results of I–V measurements, we acquired values of the switching voltage and some large on/off ratios in the case of the ASBC-18 molecule. Also, we improved the yield of the molecular electronic device by reducing the area of the device and by inserting a Ti protecting layer between the top metal electrode and the amino style LB monolayer.     

Impact of amorphous titanium oxide film on the device stability of Al/TiO2/Al resistive memory

We have investigated the role of amorphous titanium oxide film in the reliable bipolar resistive switching of Al/TiO₂/Al resistive random access memory devices. As TiO₂ deposition temperature decreased, a more stable endurance characteristic was obtained. We proposed that the degradation of the bipolar resistive switching property of Al/TiO₂/Al devices is closely related to the imperfect migration of oxygen ions between the top insulating interface layer and the oxygen-deficient titanium oxide during the set and reset operations. In addition, the dependence of the TiO₂ film thickness on the switching property was also studied. As the thickness of the film increased, a reduction in the resistance of the high resistance state rapidly appeared. We attribute the improved endurance performance of thin and low-temperature grown TiO₂ devices to the amorphous state with a low film density.     

Efficient and long‐life green light‐emitting diodes comprising tridentate thiol capped quantum dots

In this paper, the green quantum dots capped with the ligand, tris(mercaptomethyl)nonane (TMMN), are fabricated as the light‐emitting layer for efficient and bright light‐emitting diodes. These TMMN‐capped quantum dots exhibit well‐preserved photoluminescence properties with quantum yields of ∼90% after ligand exchange. The light‐emitting diodes based on TMMN‐capped quantum dots are reported with a maximum external quantum efficiency of 16.5% corresponding to a power efficiency and current efficiency of 57.6 lm W^–1 and 70.1 cd A^–1, respectively. The devices exhibit high color stability that is not markedly affected by the increase of applied voltage, thus leading to a high color reproducibility. Most importantly, the devices exhibit high environmental stability. For the highest luminance devices (with emitting layer thickness of 25 nm) and the highest power efficiency devices (with emitting layer thickness of 38 nm), the lifetimes are > 480 000 h and > 110 000 h, respectively.

     

Bi‐directional organic light‐emitting diodes with nanoparticle‐enhanced light outcoupling

An effective method is presented for enhancing the outcoupling efficiency of translucent/bi‐directional organic light‐emitting diodes (TL/BD‐OLEDs) with a bottom indium tin oxide (ITO) anode and a top cathode comprised of a thin Ag layer covered with an organic capping layer. Upon insertion of a nanoparticle (NP)‐based scattering layer (NPSL) between the substrate and the ITO anode, the TL/BD‐OLEDs exhibit significantly enhanced external quantum efficiency (EQE) in both emission directions. Furthermore, the NPSL improves the color stability of the TL/BD‐OLEDs over a wide range of viewing angles. Simulations based on geometrical and statistical optics are performed to elucidate the mechanism by which the efficiency is enhanced and to establish strategies for further optimization. Simulations performed on the scattering layers with varying NP volume percentage reveal that the bottom‐side emission is governed by competition between waveguide‐mode extraction and backward scattering by NPs in the film, while the top‐side emission is largely dominated by the latter. Optimized bi‐directional OLEDs achieve a 1.64‐fold enhanced EQE compared to reference devices without NPSL.     

Microcrystalline silicon‐carbon alloys as anti‐reflection window layers in high efficiency thin film silicon solar cells

Microcrystalline silicon‐carbide (μc‐SiC:H) films were prepared using hot wire chemical vapor deposition at low substrate temperature. The μc‐SiC:H films were employed as window layers in microcrystalline silicon (μc‐Si:H) solar cells. The short‐circuit current density (J_SC) in these n‐side illuminated n–i–p cells increases with increasing the deposition time t_W of the μc‐SiC:H window layer from 5 min to 60 min. The enhanced J_SC is attributed to both the high transparency and an anti‐reflection effect of the μc‐SiC:H window layer. Using these favourable optical properties of the μc‐SiC:H window layer in μc‐Si:H solar cells, a J_SC value of 23.8 mA/cm² and cell efficiencies above 8.0% were achieved with an absorber layer thickness of 1 μm and a Ag back reflector. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Bright flexible organic light-emitting devices on copy paper substrates

We have developed flexible top emission organic light-emitting devices (OLEDs) using copy paper substrates. Poly-2-chloro-p-xylylene and SiO₂ buffer layers were coated on the copy paper for protecting the water absorption and improving the barrier and morphological characteristics. Sputtered Ni and thermally evaporated Ca/Ag films were used as anode and semitransparent cathode, respectively. The electrical conduction, luminance, and efficiency characteristics of the copy paper-based flexible OLEDs were investigated. The SiO₂ buffer layer was critically important in obtaining the high performance devices. The device exhibited a high luminance of 2200 cd/m² at a driving voltage of 13 V.