High Ideality Factor Research Articles

Role of A-Site Cation and X-Site Halide Interactions in Mixed-Cation Mixed-Halide Perovskites for Determining Anomalously High Ideality Factor and the Super-linear Power Law in AC Ionic Conductivity at Operating Temperature

Mixed-cation mixed-halide perovskites have gained a lot of attention due to their superior device properties, optoelectronic properties, and improved structural stabilities over methyl ammonium lea...

Atomic Layer Deposition of AlN Thin Films on GaN and Electrical Properties in AlN/GaN Heterojunction Diodes

AlN thin films were deposited with different thicknesses (5 and 20 nm) by atomic layer deposition and we explored the electron transport mechanisms in AlN/GaN Schottky junctions. Higher ideality factor and higher m values were observed in the current–voltage plots when the thickness of the AlN was 5 nm. The reverse bias leakage current for 5-nm-thick AlN was explained based on the Poole–Frenkel emission. Capacitance–voltage analysis revealed a higher interface state density for 5 nm thick AlN. These results might be associated with the defective interfacial layer present near the AlN/GaN interface. Because of the strain relaxation, the upper region of the AlN layer was degraded in the case of 20-nm-thick AlN, which may have strongly affected the interfacial properties of the Pt/AlN junction strongly.

SCHOTTKY DIODE FABRICATION VIA COLD SUBSTRATE EVAPORATED Ag ON SOL-GEL DERIVED ZnO ULTRA-THIN FILMS FOR SEMICONDUCTOR DEVICES

ZnO thin films were grown on glass and SnO2 substrate by using Sol-gel method with a simple, low cost-temperature and high controlled away. ZnO thin films were contacted with Ag in two different temperatures at 200K (cold substrate) and 300K (room temperature) by PVD Cold Substrate Method. Characterization of the obtained films and Schottky diodes studied by XRD, SEM, Optical and Electrical measurements. The average particle size and film thickness of the films were calculated as ~25 and ~300nm. Bandgap of the ZnO films coated on different substrates was calculated to be around 3.29-3.31 eV. Peaks of various types of defects were observed in different wavelengths from photoluminescence measurements. The barrier height and ideal factor values of Ag/ZnO Schottky diodes at 200K and 300K were (0.48-0.43eV; 15.60) and (0.66-0.15eV; 8.6- 7.41), respectively. The series resistance values (4.46Ω; 4.88Ω) at 200K and (3.65Ω; 4.78Ω) at 300K were obtained from two different methods, respectively. Diodes made at 200K temperature were found to have a higher ideality factor. However, while 75% of all produced Schottky diodes at 200K substrate temperature show rectifying feature, this rate is only 25% in contacts made at 300K substrate temperature.

Carrier selective solution processed molybdenum oxide silicon heterojunctions solar cells with over 12% efficiency

We have fabricated and characterized the performance of carrier selective molybdenum oxide (MoOx)/n-Si heterojunction solar cells (HSCs), with the MoOx prepared by both E-beam evaporation (e-MoOx) and solution process (s-MoOx) techniques. The s-MoOx/Si HSC demonstrates the best power conversion efficiency of 12.5 %, with an open circuit voltage of 555 mV, short circuit current density of 33.3 mA cm−2, and fill factor of 67.4% and it is higher than the efficiencies reported to date for s-MoOx/Si HSC. The efficiency achieved is approaching the 13.3% obtained for the e-MoOx/Si HSC. The slightly lower efficiency of s-MoOx/Si HSC is attributed to its carrier extraction loss arising from a higher contact resistivity at the MoOx/n-Si interface. This is also reflected in the selectivity of the contacts, which measures 10.1 for the s-MoOx contact, relative to 11.5 for the e-MoOx contact. Trap-assisted tunneling at the interface is believed to be the main mechanism that influences the cell performance such as open circuit voltage and fill factor, and results in a high ideality factor of ∼3.

Asymmetric Double‐Gate β‐Ga2O3 Nanomembrane Field‐Effect Transistor for Energy‐Efficient Power Devices

AbstractThe ultra‐wide bandgap and cost‐effective melt‐growth of β‐Ga2O3 ensure its advantages over other wide bandgap materials, and competitive electrical performance has been demonstrated in various device structures. In this paper, an asymmetric double‐gate (ADG) β‐Ga2O3 nanomembrane field‐effect transistor (FET) comprised of a bottom‐gate (BG) metal‐oxide field‐effect transistor and a top‐gate (TG) metal‐semiconductor field‐effect transistor (MESFET) is demonstrated. Schottky contact properties are validated by characterizing the lateral Schottky barrier diode (SBD), which exhibits high rectification ratio and low ideality factor. The top‐gate β‐Ga2O3 MESFET shows reasonable electrical performance with a high breakdown voltage, as anticipated by three terminal off‐state breakdown measurement. These properties are further enhanced by double‐gate operation, and superior device performance is demonstrated; positive‐shifted threshold voltage and reduced subthreshold slope enable the asymmetric double‐gate β‐Ga2O3 FET to operate at low power, and almost twice as much transconductance is demonstrated for high‐frequency operation. These results show the great potential of asymmetric double‐gate β‐Ga2O3 FETs for energy‐efficient high‐voltage and ‐frequency devices with optimal material and structure co‐designs.

Photovoltaics literature survey (no. 149)

Photovoltaics literature survey (no. 149)

Simulation of solar cells by delocalized recombination model and its applications

For lattice-mismatch, high-defect-density solar cells, the current-voltage curve is not perfects, and it always has high ideality factor and low shunt resistance. In traditional one-dimension circuit model of solar cells, there is a paralleled resistor Rp and recombination diodes with high ideality factor to simulate this imperfection. In this research, a delocalized recombination model is built based on the assumption that recombination could occur between trapped electrons from n type sides and trapped holes from p type side. The model with different parameters (i.e. defect volume w and recombination parameter Cp) is then added into current continuity equations and I-V curves of solar cells were calculated. With the increase of Cp, fill factor (FF) decreases obviously, followed by the decrease of short-circuit current and open-circuit voltage. Larger defect volume w would cause higher ideality factor. The effects of mobility on I-V curves were also calculated, low mobility leads to decrease of shunt resistance and fill factor. Two type of solar cells with different configurations were irradiated by 70 keV and the I-V curves show different degradation behaviors. Type A cell shows large decrease of shunt resistance indicating loss of mobility while type B cell shows increase of n(V) due to delocalized recombination. The difference is related to different thicknesses of GaInP.

Real reason for high ideality factor in organic solar cells: Energy disorder

Organic semiconductor has recently emerged as a promising and versatile material for solar cells. This paper shows that the structural order of the electron transport layers has a significant influence on the ideality factor and the power conversion efficiency of organic solar cells. By using Fermi-Dirac distribution, we demonstrate that both ideality factor and degeneracy will up to 3, even 6, when the energy disorder of the transport layer is large enough. By comparing the numerical values of 15 materials with experimental data, our results show that the ideality factor is almost the same value as the degeneracy when the light intensity gets near to 1 mW/cm2. Additionally, both degeneracy and ideality factor have a positive correlation with the energy disorder. However, the power conversion efficiency will decrease as the energy disorder increasing. These results shed light on the real reason for high ideality factor and the origin of the power conversion efficiency loss, and provide a direction to increase the power conversion efficiency of the organic solar cells.

Non-approximated series resistance evaluation by considering high ideality factor in organic solar cell

The quality performance analysis of any solar cell can only be predicted through the parameters extracted from its current-voltage characteristics. Herein, we demonstrate an efficient analytical method to calculate the series resistance of organic solar cells without any pre approximation. It has been shown that the inaccuracy in series resistance takes place due to the high ideality factor of the organic based solar cells, usually ranging from 2-5. Using a systemic approach, we solved the single diode solar cell model to determine the series resistance expression. The dependence of series resistance on ideality factor of the device and the reverse saturation current calculation through the illuminated current-voltage characteristics has been presented here for the first time. The entire method has been programmatically realized in MATLAB. The extracted parameters values have been compared to the other methods for the proof of validation and it shows good agreement where as some outputs are better resolved. Additionally, the effectiveness of the present method is also matched with NREL certified silicon solar cell (reference cell model 60623) parameter extracted from K24XX cell software provided with PET cell tester model, CT200AAA solar simulators. The present proposed method is not restricted to the organic solar cell only but it can also be applied for the other type of solar cell as well. Therefore, the proposed method shows the relevance in the present scenario of solar cell research and development.

Electrical Properties of Au/n-GaN Schottky Junctions with an Atomic-Layer-Deposited Al2O3 Interlayer

The temperature-dependent electrical properties of Au/GaN Schottky diodes with an Al2O3 layer prepared by using atomic layer deposition (ALD) were investigated. Compared to the Au/GaN junction, the Au/Al2O3/GaN junction was found to have lower barrier heights and higher ideality factors. For the Au/GaN junction, the native oxide and the thin surface barrier formed by surface states caused a large leakage current and a small capacitance. Due to surface passivation through the ALD process, the leakage current for the Au/Al2O3/GaN junction was reduced. The barrier height reduction for the Au/Al2O3/GaN junction was associated with interface dipole formation at the Al2O3/GaN interface.

Characterization of Al/In:ZnO/p-Si photodiodes for various In doped level to ZnO interfacial layers

The detection of the light by a device is so important for industrial applications such as energy harvesting, sensing and switching. For that aim, we have introduced various In doped (0%, 0.1%, 0.5% and 1.0%) nanostructure ZnO thin films which was prepared by the sol-gel spin coating technique as interfacial materials between the Al metal and p-type Si for investigation photodetection properties of the material. According to morphological results of the In doped ZnO thin films at AFM, undoped and In doped ZnO thin films formed as fiber like structures. The obtained optical band gap energy for undoped, 0.1%, 0.5% and 1.0% In doped ZnO thin films were determined as 3.293 eV, 3.283 eV, 3.272 eV and 3.263 eV, respectively. The Al/In:ZnO/p-Si devices were characterized with I–V and C–V measurements. The I–V data was acquired under various illumination conditions to see the response of the devices to the light. The I–V characteristics have revealed that the devices have high ideality factors and, their values usually increased with increasing In doping level, but the rectifying properties decreased. In addition, barrier heights and series resistance values decreased with increasing In doping level. Also, the device parameters were calculated via Cheung and Norde methods for accuracy of the results. The current transient measurements highlighted that In doping provided to increase of light response. The C–V measurements have imparted that the capacitance values are strong function of the frequency and voltage for various In doping level. The devices can be thought and improved as photodiode and photodetector applications in the industry.

High temperature current transport in gate oxides based (GaN)/AlGaN/GaN Schottky diodes

We have prepared high temperature stable Schottky MOS diodes on AlGaN/GaN heterostructure with Ir-Al oxide based gate oxide layer and analyzed current transport mechanism in these diodes up to very high temperatures. The thermionic emission analysis of I-V curves gave high ideality factor in between 2 and 3 decreasing with increasing temperature. Simulation of the thermionic transport with the barrier height extracted from the measured curves and ideality factor unity gave the simulated current densities many orders of magnitude higher than experimentally measured. We explained experimental data by a bias dependent barrier height as a result of thin dielectric layer and a bias dependent charge localized in interface traps between the semiconductor and this dielectric layer. The measured data are then explained by the thin dielectric layer at the interface between (GaN)AlGaN and the metal electrode. Increasing forward voltage and moving Fermi level towards conduction band the charge in the traps becomes negative and increases the effective barrier height for electrons moving from the semiconductor into the metal. By this mechanism also a linear increase of reverse current with bias can be explained. The opposite is true for the reverse voltage.

Observation of anomalous transport characteristics in graphene-oxide thinfilm

This paper describes experimental results of an anomalous electrical transport properties of graphene-oxide (GO) thinfilm. A nonlinear current-voltage (I-V) characteristic has been observed and analyzed with various current-transport mechanisms such as thermionic emission, space-charge limited conduction (SCLC), and Poole-Frenkel (P-F) conduction. Observation of high ideality factor reveals that the current transport is not influenced by thermionic emission. Interestingly, a characteristic transition of current from Ohmic to SCLC has been noticed. P-F conduction has been evidenced through the straight line fit observed between ln(I/V) and V1/2. The recombination tunneling with SCLC is found to be the main conduction process compared to thermionic emission and P-F conduction. The charge traps present in the GO bulk causes SCLC and P-F conduction. A plausible mechanism for each current transport phenomenon is discussed in detail. The presence of charge traps in GO is further evidenced through Raman mapping analysis. Our study further advances the understanding of the fundamental charge transport mechanisms appeared in GO thinfilms which will be an essential parameter in the development resistive memory switching applications.

Low‐Temperature Fabrication and Performance of Polycrystalline CuI Films as Transparent p‐Type Semiconductors

Transparent p‐type polycrystalline CuI films with high hole mobility are produced by solid iodination of Cu3N precursor layers and subsequent heat treatment at 120 °C. The hole mobility reached 20 cm2 V−1 s−1 in the CuI films fabricated on both glass and polyethylene terephthalate substrates. Furthermore, transparent p–n diodes with sufficiently high rectification ratio (6 × 106) and ideality factor (1.6) are successfully fabricated by employing a heterojunction of p‐type CuI and n‐type amorphous In–Ga–Zn–O (a‐IGZO) layers. The CuI/a‐IGZO heterojunction exhibits the photovoltaic effect, with a short‐circuit current of 0.33 µA and an open‐circuit voltage of 10 mV under UV illumination at a wavelength of 365 nm and intensity of 1.9 mW cm−2. The photoresponse at zero bias is sufficiently quick, and thus, the photovoltaic properties of the CuI/a‐IGZO diodes rendere them potential candidates for visible‐light‐blind and self‐powered UV photodetectors. These findings suggest that CuI is an excellent transparent p‐type semiconductor that can be fabricated at low temperature.

Device physics of van der Waals heterojunction solar cells

Heterostructures based on atomically thin semiconductors are considered a promising emerging technology for the realization of ultrathin and ultralight photovoltaic solar cells on flexible substrates. Much progress has been made in recent years on a technological level, but a clear picture of the physical processes that govern the photovoltaic response remains elusive. Here, we present a device model that is able to fully reproduce the current–voltage characteristics of type-II van der Waals heterojunctions under optical illumination, including some peculiar behaviors such as exceedingly high ideality factors or bias-dependent photocurrents. While we find the spatial charge transfer across the junction to be very efficient, we also find a considerable accumulation of photogenerated carriers in the active device region due to poor electrical transport properties, giving rise to significant carrier recombination losses. Our results are important to optimize future device architectures and increase power conversion efficiencies of atomically thin solar cells.

Origin of Voltage‐Dependent High Ideality Factors in Graphene–Silicon Diodes

AbstractUndoubtedly graphene–silicon (GS) heterostructure devices will play significant roles as future rectifiers, potential barrier modulators, photodetectors, photovoltaic devices, biochemical sensors, and so on. However, typical GS devices suffer from unusually wide‐range voltage‐dependent high ideality factors (η = 1.1–33.5). To overcome this hurdle, the origin of this wide‐range voltage‐dependent ideality factor should first be identified but this has not yet been fully studied. This study focuses on identifying the origin using impedance spectroscopy in conjunction with current–voltage, Raman, and X‐ray photoemission spectroscopy. The impedance spectra are analyzed with an equivalent distributed circuit model that accounts for the voltage‐dependent resistance and capacitance of graphene, the graphene–metal contact, the silicon interface states, and the nonequilibrium behavior in GS junction. This study clearly shows that the voltage‐dependent resistance and capacitance of interface states, graphene, and graphene–metal contact are responsible for the wide‐range voltage‐dependent high ideality factors. This study provides a potential method to overcome the drawbacks of GS devices.

Influence of the transition region between p- and n-type polycrystalline silicon passivating contacts on the performance of interdigitated back contact silicon solar cells

Passivating contacts based on thin tunneling oxides (SiOx) and n- and p-type semi-crystalline or polycrystalline silicon (poly-Si) enable high passivation quality and low contact resistivity, but the integration of these p+/n emitter and n+/n back surface field junctions into interdigitated back contact silicon solar cells poses a challenge due to high recombination at the transition region from p-type to n-type poly-Si. Here, the transition region was created in different configurations—(a) p+ and n+ poly-Si regions are in direct contact with each other (“pn-junction”), using a local overcompensation (counterdoping) as a self-aligning process, (b) undoped (intrinsic) poly-Si remains between the p+ and n+ poly-Si regions (“pin-junction”), and (c) etched trenches separate the p+ and n+ poly-Si regions (“trench”)—in order to investigate the recombination characteristics and the reverse breakdown behavior of these solar cells. Illumination- and injection-dependent quasi-steady state photoluminescence (suns-PL) and open-circuit voltage (suns-Voc) measurements revealed that non-ideal recombination in the space charge regions with high local ideality factors as well as recombination in shunted regions strongly limited the performance of solar cells without a trench. In contrast, solar cells with a trench allowed for open-circuit voltage (Voc) of 720 mV, fill factor of 79.6%, short-circuit current (Jsc) of 41.3 mA/cm2, and a conversion efficiencies (η) of 23.7%, showing that a lowly conducting and highly passivating intermediate layer between the p+ and n+ poly-Si regions is mandatory. Independent of the configuration, no hysteresis was observed upon multiple stresses in reverse direction, indicating a controlled and homogeneously distributed breakdown, but with different breakdown characteristics.

Performance of CZTSSe thin film solar cells fabricated using a sulfo-selenization process: Influence of the Cu composition

In this work, earth-abundant CZTSSe thin film solar cells were fabricated by sulfo-selenization of the Mo/Zn/Cu/Sn/Cu metallic precursors. The influences of morphological and compositional properties of the absorbers on performance of solar cells were investigated by tuning Cu content in the films. The Raman analysis showed that absorbers consist of a kesterite CZTSSe phase with ZnSe as a minor secondary phase. X-ray photoelectron spectroscopy (XPS) analyses revealed that the surfaces are Cu depleted and Zn enriched compared with the bulk composition of the absorbers. The results indicate that during sulfo-selenization the Cu diffused into the film and the Zn towards the film surface. The performance of the solar cells initially improved with the increasing of the Cu content and then decreased. By tuning the Cu content in the absorbers, the minority-carrier life time improved from 0.8 to 1.6 ns. The power conversion efficiency increased from 5.1 to 8.03% with fine controlling of Cu composition of the CZTSSe absorbers. The diode-ideality factors are higher than 2, suggesting an increased interfacial recombination in the devices. The high ideality-factors A and low minority carrier life times may originate from surface and bulk related defects, which in turn limits the Voc and the achievable high conversion efficiency for the CZTSSe thin film solar cells.

Fabrication and Characterization of Ultra-wide Bandgap AlN-Based Schottky Diodes on Sapphire by MOCVD

AlN Schottky diodes with various device geometries were fabricated on sapphire substrate and their temperature-dependent current–voltage characteristics were analyzed. At forward bias, high ideality factors were obtained, indicating a large deviation from the ideal thermionic emission model. At reverse bias, the breakdown voltage showed a negative temperature dependence, and the leakage current was well described using a 2-D variable-range hopping conduction model. Furthermore, the breakdown voltages and leakage currents of the devices showed a strong dependence on the surface distance between the ohmic and Schottky contacts, but a relatively small dependence on the area of the Schottky contacts. These results suggest surface states between ohmic and Schottky contacts play a more important role than the metal/AlN interface in determining the reverse breakdown and leakage current of AlN Schottky diodes. A quantitative study of AlN Schottky diodes at high temperature reveals a geometry-dependent surface breakdown electric field and surface leakage current. Surface passivation and treatments may enhance the device performances and impact the reverse breakdown and current leakage mechanisms. These results will serve as the guidance for the design and fabrication of future AlN electronic devices.

Theoretical Models for Anomalously High Ideality Factor in a Au/SnO2-Si(n)/Al Solar Cell

Interestingly, anomalously high ideality factors (n > 2) in the prepared Au/SnO2-Si(n)/Al solar cell junction in the interim bias voltage range were obtained in our previous paper. Theoretical models were proposed to clarify the much higher ideality factors. The temperature dependence of the solar cell ideality factor can give valuable information about the main recombination mechanism in the cell. We find a significant agreement of the temperature dependence with a theory derived by Padovani et al. We conclude that tunneling currents play an important role inside the cell.