Transparent Contact Layer Research Articles

Enhancement of the light output efficiency and thermal stability of AlGaN-based deep-ultraviolet light-emitting diodes with Ag-nanodot-based p-contacts and an 8-nm p-GaN cap layer.

The efficiency of AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) is limited by the high absorption issue of the p-GaN contact layer or poor contact properties of the transparent p-AlGaN contact layer. Enhancement of the light output efficiency and thermal stability of DUV LEDs with an emission wavelength of 272 nm was investigated in this work. Ag nanodots on an 8-nm p-GaN cap layer were used to form ohmic contact, and Al and Mg reflective mirrors were employed to enhance the light output power (LOP) of DUV LEDs. However, serious deterioration of LOP occurred after the high-temperature process for the LEDs with Al and Mg reflective mirrors, which can be attributed to the damage to the ohmic contact properties. A Ti barrier layer was inserted between the Ag/p-GaN and Al layers to prevent the degeneration of ohmic contact. The wall-plug efficiency (WPE) of DUV LEDs fabricated by the Ag-nanodot/Ti/Al electrode is 1.38 times that of LEDs fabricated by adopting a thick Ag layer/Ti/Al at 10 mA after a high-temperature process. The Ag-nanodot/Ti/Al electrode on thin p-GaN is a reliable technology to improve the WPE of DUV LEDs. The experimental and simulated results show that the ohmic contact is important for the hole-injection efficiency of the DUV LEDs when p-GaN is thin, and a slight increase in the contact barrier height will decrease the WPE drastically. The results highlighted the importance of thermally stable ohmic contacts to achieve high-efficiency DUV LEDs and demonstrated a feasible route for improving the LOP of DUV LEDs with a thin p-GaN cap layer and stable reflective electrodes.

Comprehensive analysis of photon dynamics in thin-film GaAs solar cells with planar and textured rear mirrors

This study describes a comprehensive analysis of light trapping and photon recycling (PR) in thin-film GaAs solar cells, explicitly considering the effects of (non-perfect) light scattering at a textured rear mirror. We calculate the probabilities of escape, reabsorption in the absorber and parasitic absorption for internally generated photons, as well as the reflectance, absorptance and parasitic absorptance of externally incident photons. Combined with the internal luminescent efficiency to account for non-radiative recombination non-explicitly, but in a phenomenological manner, device performance metrics such as the short-circuit current density ( J s c ), open-circuit voltage ( V o c ) and its radiative limit, the external luminescent efficiency, the contribution of PR to the V o c and the power conversion efficiency ( P C E ) are derived. We analyze a typical thin-film GaAs solar cell architecture, consisting of an n-GaAs/p-InGaP heterojunction structure cladded by commonly employed front- and rear-side layer stacks. The probabilities of escape, reabsorption and parasitic absorption and the resulting performance metrics are presented as a function of absorber thickness, rear-mirror reflectivity, haze factor, internal luminescent efficiency, front grid coverage and band tailing. The simulations show that the implementation of a textured rear mirror reduces PR and its importance for device performance. In these textured cells, a high rear-mirror reflectivity and, thereby, efficient PR, is less crucial to achieve V o c values close to the radiative limit. The increased J s c in textured cells renders light trapping favorable for the P C E , regardless of absorber thickness and material quality and despite reduced PR. Furthermore, the impact of an absorbing front grid and lateral transport of internal luminescence on PR is estimated, emphasizing the importance of transparent (non-absorbing) contact layers, at the rear, but importantly, also at the front of the device. To conclude, P C E limits of 300-nm-thick GaAs solar cells are deduced in the context of an experimentally realized light-trapping scheme based on a randomly textured rear mirror. P C E s exceeding 25.5% are deemed realistic. • Photon recycling and light trapping are studied in textured thin-film GaAs cells • Ray-optical modeling yields probabilities of escape and (parasitic) absorption • Textured rear mirrors reduce photon recycling and its impact on cell performance • Light trapping improves efficiency for any absorber thickness and material quality • Efficiency limits of 300-nm GaAs cells are currently 25.5% and ultimately ¿28%

Photonic Lift-off Process to Fabricate Ultrathin Flexible Solar Cells.

A microsecond time-scale photonic lift-off (PLO) process was used to fabricate mechanically flexible photovoltaic devices (PVs) with a total thickness of less than 20 μm. PLO is a rapid, scalable photothermal technique for processing extremely thin, mechanically flexible electronic and optoelectronic devices. PLO is also compatible with large-area devices, roll-to-roll processing, and substrates with low temperature compatibility. As a proof of concept, PVs were fabricated using CuInSe2 nanocrystal ink deposited at room temperature under ambient conditions on thin, plastic substrates heated to 100 °C. It was necessary to prevent cracking of the brittle top contact layer of indium tin oxide (ITO) during lift-off, either by using a layer of silver nanowires (AgNW) as the top contact or by infusing the ITO layer with AgNW. This approach could generally be used to improve the mechanical versatility of current collectors in a variety of ultrathin electronic and optoelectronic devices requiring a transparent conductive contact layer.

Real-Time Optimization of Anti-Reflective Coatings for CIGS Solar Cells.

A new method combining in-situ real-time spectroscopic ellipsometry and optical modeling to optimize the thickness of an anti-reflective (AR) coating for Cu(In,Ga)Se2 (CIGS) solar cells is described and applied directly to fabricate devices. The model is based on transfer matrix theory with input from the accurate measurement of complex dielectric function spectra and thickness of each layer in the solar cell by spectroscopic ellipsometry. The AR coating thickness is optimized in real time to optically enhance device performance with varying thickness and properties of the constituent layers. Among the parameters studied, we notably demonstrate how changes in thickness of the CIGS absorber layer, buffer layers, and transparent contact layer of higher performance solar cells affect the optimized AR coating thickness. An increase in the device performance of up to 6% with the optimized AR layer is demonstrated, emphasizing the importance of designing the AR coating based on the properties of the device structure.

Получение проводящих пленок SnO2 с использованием гидрохимического осаждения

Thanks to such unique properties as transparency and conductivity tin dioxide often utilize as transparent contact layer to produce displays, solar cells, and sensor devices. Hydrochemical method of deposition SnO2 films is a perspective due to its simplicity, and economical efficiency. The ionic equilibria analysis was carried out and the boundary conditions of Sn(OH)2 solid phase formation in the «Sn2+ – H2O – OH‾» system calculated. It was established, that tin(II) hydroxide may be obtain in the range 2 < pH < 12. Preliminary results allow to determinate an optimal mixture sourness interval 1 < pH < 5. Revealed, that the thickness of the Sn(OH)2 films strongly depends on the solution pH. Maximum value of 488 nm reached at pH = 8. Conductive SnO2 layers were obtained on a glass and sitall substrates with simultaneously presence of antimony chloride and ammonium fluoride followed by annealing in air. The thickness vs temperature and thickness vs tin initial salt concentration dependences were installed. The uniform tin hydroxide layers with a thickness of ~74 nm may be synthesized under pH = 2 conditions. By the electron microscopy method the average particle size was established changing from 200 to 400 nm for as-synthesized films, to ~20 nm for annealed which indicates the nanostructure nature of the films. The morphology, elemental composition and conductive properties of deposited films were investigated before and after heating stage. Studying the annealing temperature influence at the film resistance were identified a three temperature ranges within which the films sharply differ in their conductive properties, which is associated with phase and structural transformations in them. Shown, that the most conductive SnO2 films with the omic resistance 3-5 kOm/sm were obtained at the temperature range 620-870 K.

Optimization of Transparent Passivating Contact for Crystalline Silicon Solar Cells

A highly transparent front contact layer system for crystalline silicon (c-Si) solar cells is investigated and optimized. This contact system consists of a wet-chemically grown silicon tunnel oxide, a hydrogenated microcrystalline silicon carbide [SiO2/µc-SiC:H( n )] prepared by hot-wire chemical vapor deposition (HWCVD), and a sputter-deposited indium doped tin oxide. Because of the exclusive use of very high bandgap materials, this system is more transparent for the solar light than state of the art amorphous (a-Si:H) or polycrystalline silicon contacts. By investigating the electrical conductivity of the µc-SiC:H( n ) and the influence of the hot-wire filament temperature on the contact properties, we find that the electrical conductivity of µc-SiC:H( n ) can be increased by 12 orders of magnitude to a maximum of 0.9 S/cm due to an increased doping density and crystallite size. This optimization of the electrical conductivity leads to a strong decrease in contact resistivity. Applying this SiO2/µc-SiC:H( n ) transparent passivating front side contact to crystalline solar cells with an a-Si:H/c-Si heterojunction back contact we achieve a maximum power conversion efficiency of 21.6% and a short-circuit current density of 39.6 mA/cm2. All devices show superior quantum efficiency in the short wavelength region compared to the reference cells with a-Si:H/c-Si heterojunction front contacts. Furthermore, these transparent passivating contacts operate without any post processing treatments, e.g., forming gas annealing or high-temperature recrystallization.

13 mW operation of a 295–310 nm AlGaN UV-B LED with a p-AlGaN transparent contact layer for real world applications

Smart, high-power ultraviolet (UV)-B light-emitting diodes (LEDs) are demanded for real world applications, including vitamin D3 production in human skin (295–304 nm), immunotherapy (310 nm) and enriching phytochemicals in plants (310 nm).

Fabrication and Characterization of High-Efficiency Green InGaN Light-Emitting Diodes With Metal-Doped ITO Layer

We investigate a method to improve the efficiency of green InGaN light-emitting diodes (LEDs) using various metal-doped indium tin oxide (ITO) transparent contact layers. The resistance and transmittance of metal-doped ITO with different metal thicknesses and different rapid thermal annealing conditions were studied and analyzed systematically. It is found that extinction coefficient and surface roughness of ITO layer are distinctly reduced by doping suitable metal and annealing in a mixture of N2/O2 (200:35) ambient at 600 °C for 3 min. In addition, it is useful to decrease the sheet resistance and provide more uniform current distribution by using the metal-doped ITO. Compared with conventional green LEDs with 80-nm ITO layer, there are an 18.9% increase in light-output power and a 1.5 V decrease in forward voltage at 120 mA for the green LEDs with 3-nm Al-doped ITO layer.

Improving the Efficiency of AlGaN Deep‐UV LEDs by Using Highly Reflective Ni/Al p‐Type Electrodes

AlGaN‐based deep‐ultraviolet light‐emitting diodes (DUV LEDs) have a wide range of applications and a large market is expected. However, the efficiency of DUV LEDs is still much lower than that of blue LEDs due to the quite low light‐extraction efficiency (LEE). We are developing high LEE DUV LEDs by introducing a transparent contact layer and a highly reflective Ni/Al p‐type electrode. In this work, we investigate optimization of the Ni layer thickness of the highly reflective Ni/Al p‐type electrode for AlGaN DUV LEDs. We find that the reflectivity in the UV region becomes higher as the thickness of the Ni layer becomes smaller; however, the external quantum efficiency (EQE) is reduced if the Ni layer is too thin (<0.8 nm). As a result, we find that the most appropriate Ni thickness for the Ni/Al electrode is 0.9 nm. We fabricate 279 nm AlGaN quantum well (QW) DUV LEDs with the optimized electrodes and demonstrate an enhancement in EQE by a factor of 1.8 and a maximum EQE of 9% under bare wafer measurement conditions. We also demonstrate a high output power of 33 mW under an injection current of 100 mA.

Enhancing the light-extraction efficiency of AlGaN deep-ultraviolet light-emitting diodes using highly reflective Ni/Mg and Rh as p-type electrodes

Improving the light-extraction efficiency (LEE) is a major issue for the development of deep-ultraviolet (DUV) light-emitting diodes (LEDs). For this improvement, we introduced a transparent p-AlGaN contact layer and a reflective p-type electrode. In this work, we investigated the improvements obtained by replacing conventional Ni/Au p-type electrodes with highly reflective Ni/Mg and Rh electrodes. The external quantum efficiencies (EQEs) of 279 nm DUV LEDs were increased from 4.2 to 6.6% and from 3.4 to 4.5% by introducing Ni/Mg and Rh p-type electrodes, respectively. The LEE enhancement factors for the Ni/Mg and Rh electrodes were 1.6 and 1.4, respectively. These results are explained by the fact that the measured reflectances of the Ni/Mg and Rh electrodes were approximately 80 and 55%, respectively. Moreover, it was concluded that a passivation layer is required for Ni/Mg electrodes to prevent the degradation of the LED properties by the oxidation of Mg.

Si‐Doped Cu(In,Ga)Se2 Photovoltaic Devices with Energy Conversion Efficiencies Exceeding 16.5% without a Buffer Layer

AbstractIn this communication, novel and simplified structure Cu(In,Ga)Se2 (CIGS) solar cells, which nominally consist of only a CIGS photoabsorber layer sandwiched between back and front contact layers but yet demonstrate high photovoltaic efficiencies, are reported. To realize this accomplishment, Si‐doped CIGS films grown by the three‐stage coevaporation method, B‐doped ZnO transparent conductive oxide front contact layers deposited by chemical vapor deposition, and heat–light soaking treatments are used. Si‐doping of CIGS films is found to modify the film surfaces and grain boundary properties and also affect the alkali metal distribution profiles in CIGS films. These effects are expected to contribute to improvements in buffer‐free CIGS device performance. Heat–light soaking treatments, which are occasionally performed to improve conventional buffer‐based CIGS device performance, are found to be also effective in enhancing buffer‐free CIGS photovoltaic efficiencies. This result suggests that the mechanism behind the beneficial effects of heat–light soaking treatments originates from CIGS bulk issues and is independent of the buffer materials. Consequently, over 16.5% efficiencies, including an independently certified value, are demonstrated from completely buffer‐free CIGS photovoltaic devices.

Transparent electrode design for AlGaN deep-ultraviolet light-emitting diodes.

Zinc gallate (ZnGa2O4; ZGO) thin films were employed as the p-type transparent contact layer in deep-ultraviolet AlGaN-based light-emitting diodes (LEDs) to increase light output power. The transmittance of 200-nm-thick ZGO in deep-ultraviolet wavelength (280 nm) was as high as 92.3%. Two different ohmic contact structures, a dot-LED (D-LED; ZGO/dot-ITO/LED) and whole-LED (W-LED; ZGO/ITO/LED), exhibited improved light output power and current spreading compared to a conventional ITO-LED (C-LED). At an injection current of 20 mA, the D-LED and W-LED exhibited 33.7% and 12.3% enhancements in light output power, respectively, compared to the C-LED. The enhanced light output power of the D-LED can be attributed to an improvement in current spreading and enhanced light-extracting efficiency achieved by introducing ZGO/dot-ITO.

Growth and Fabrication of High External Quantum Efficiency AlGaN-Based Deep Ultraviolet Light-Emitting Diode Grown on Pattern Si Substrate

Growing III-V semiconductor materials on Si substrates for opto-electronic applications is challenging because their high lattice mismatch and different thermal expansion coefficients cause the epitaxial layers to have low quality. Here we report the growth of a high-quality AlN template on a micro-circle-patterned Si substrate by using NH3 pulsed-flow multilayer AlN growth and epitaxial lateral overgrowth techniques. Then, we fabricated and characterized a deep-ultraviolet light-emitting diode (UV-LED) device using this AlN/patterned Si. By using standard lithography and inductively coupled plasma etching, the Si substrate was prepared with very high pattern density and was made deep enough to grow a thick AlN template with high crystal quality and very few threading dislocations, allowing for further re-growth of the deep UV-LED device. And by combining a transparent p-AlGaN contact layer, an electron blocking layer and using this high quality AlN template: a deep UV-LED device fabricated and showed a strong single sharp electroluminescence (EL) peak at 325 nm and achieved an external quantum efficiency (EQE) of about 0.03%, for a deep UV-LED grown on Si substrate.

High-Performance GaN-Based LEDs With AZO/ITO Thin Films as Transparent Contact Layers

The Al-doped ZnO (AZO)/indium tin oxide (ITO) bilayer films were proposed as transparent contact layers (TCLs) for the fabrication of GaN-based light-emitting diodes (LEDs). In TCLs on the p-type GaN layer, the ITO film serves as the ohmic contact layer with the thickness of only 20 nm, whereas the 300–750nm AZO films act as the current spreading layer. At an optimal AZO thickness of 500 nm, GaN-based LEDs with AZO/ITO TCLs have a forward voltage of 3.36 V, an electroluminescence emission at 526 nm with highest brightness and a large light intensity of 386 mcd at 20 mA, which are almost the same as the commercial LEDs with a 300-nm ITO TCL. The 500-nm AZO/20-nm ITO bilayer films exhibit a high transparency above 90% in the visible region, but they display a low conductivity with resistivity $\sim 7 \times 10^{-3}~\Omega \cdot $ cm. The success of GaN-based LEDs with such a relatively high-resistivity electrode strongly demonstrates that the AZO/ITO film is a very effective and practical TCL on the p-type GaN layer. The indium-saving AZO/ITO TCLs may be actually a universal approach as p-type electrodes in high-performance GaN-based LEDs for commercially mass production with low cost.

Efficient amorphous silicon solar cells: characterization, optimization, and optical loss analysis

Hydrogenated amorphous silicon (a-Si:H) has been effectively utilized as photoactive and doped layers for quite a while in thin-film solar applications but its energy conversion efficiency is limited due to thinner absorbing layer and light degradation issue. To overcome such confinements, it is expected to adjust better comprehension of device structure, material properties, and qualities since a little enhancement in the photocurrent significantly impacts on the conversion efficiency. Herein, some numerical simulations were performed to characterize and optimize different configuration of amorphous silicon-based thin-film solar cells. For the optical simulation, two-dimensional finite-difference time-domain (FDTD) technique was used to analyze the superstrate (p-i-n) planar amorphous silicon solar cells. Besides, the front transparent contact layer was also inquired by using SnO2:F and ZnO:Al materials to improve the photon absorption in the photoactive layer. The cell was studied for open-circuit voltage, external quantum efficiency, and short-circuit current density, which are building blocks for solar cell conversion efficiency. The optical simulations permit investigating optical losses at the individual layers. The enhancement in both short-circuit current density and open-circuit voltage prompts accomplishing more prominent power conversion efficiency. A maximum short-circuit current density of 15.32 mA/cm2 and an energy conversion efficiency of 11.3% were obtained for the optically optimized cell which is the best in class amorphous solar cell.

High-Efficiency Nanowire Solar Cells with Omnidirectionally Enhanced Absorption Due to Self-Aligned Indium-Tin-Oxide Mie Scatterers.

Photovoltaic cells based on arrays of semiconductor nanowires promise efficiencies comparable or even better than their planar counterparts with much less material. One reason for the high efficiencies is their large absorption cross section, but until recently the photocurrent has been limited to less than 70% of the theoretical maximum. Here we enhance the absorption in indium phosphide (InP) nanowire solar cells by employing broadband forward scattering of self-aligned nanoparticles on top of the transparent top contact layer. This results in a nanowire solar cell with a photovoltaic conversion efficiency of 17.8% and a short-circuit current of 29.3 mA/cm2 under 1 sun illumination, which is the highest reported so far for nanowire solar cells and among the highest reported for III-V solar cells. We also measure the angle-dependent photocurrent, using time-reversed Fourier microscopy, and demonstrate a broadband and omnidirectional absorption enhancement for unpolarized light up to 60° with a wavelength average of 12% due to Mie scattering. These results unambiguously demonstrate the potential of semiconductor nanowires as nanostructures for the next generation of photovoltaic devices.

Progress in thin film CIGS photovoltaics – Research and development, manufacturing, and applications

AbstractThis review summarizes the current status of Cu(In,Ga)(S,Se)2 (CIGS) thin film solar cell technology with a focus on recent advancements and emerging concepts intended for higher efficiency and novel applications. The recent developments and trends of research in laboratories and industrial achievements communicated within the last years are reviewed, and the major developments linked to alkali post deposition treatment and composition grading in CIGS, surface passivation, buffer, and transparent contact layers are emphasized. Encouraging results have been achieved for CIGS‐based tandem solar cells and for improvement in low light device performance. Challenges of technology transfer of lab's record high efficiency cells to average industrial production are obvious from the reported efficiency values. One section is dedicated to development and opportunities offered by flexible and lightweight CIGS modules. Copyright © 2016 John Wiley & Sons, Ltd.

Sputter deposition of Sn-doped ZnO/Ag/Sn-doped ZnO transparent contact layer for GaN LED applications

Sputter deposition of Sn-doped ZnO/Ag/Sn-doped ZnO transparent contact layer for GaN LED applications

Sputter deposition of Sn-doped ZnO/Ag/Sn-doped ZnO transparent contact layer for GaN LED applications

Sn-doped ZnO/Ag/Sn-doped ZnO (ZAZ) multilayers prepared using sputtering process was employed in GaN-based blue light-emitting diodes(LEDs) as transparent contact layers (TCLs). The ZAZ layer had better optical and electrical properties without any thermal annealing. The ZAZ TCLs improved the current spreading on p-GaN layer and increased the light output power in the large area devices. The efficiency droop was also quite small in the case of adopting a ZAZ TCL in GaN-based LEDs. These results are promising for the development of a ZAZ TCL using sputtering process for GaN LED applications.

Electrically actuated phase-change pixels for transmissive and reflective spatial light modulators in the near and mid infrared.

Transmissive and reflective spatial light modulators have been designed and simulated for the 1.55 to 2.10 μm spectral region. An electrically actuated layer of phase-change material (PCM) was employed as the electro-optical medium for two-state self-holding "light-to-dark" intensity modulation of free-space light beams. The PCM was sandwiched between transparent conductive N-doped Si or indium tin oxide contact layers in a simple planar structure. A 100 to 500 nm PCM layer of Ge2Sb2Te5 (GST) was employed for optimum performance at 1.55 μm where the transmissive-modulator insertion loss was around 4.5 dB. The GST light-dark contrast was found to be 32 dB. For the GST reflection device, an included metal film (Ag) improved the 1.55 μm performance metrics to 0.7 dB of insertion loss with a contrast around 26 dB. The calculated performance for both types of spatial light modulators was robust to changes in the input incidence angle near normal incidence. Applications include infrared scene generation and signal processing.