Recombination Current Research Articles

The effect of solar cell shunt resistance change on the bus voltage ripple in spacecraft power system

The reliability of the solar array, being the primary energy source, is a key factor that may affect the operation of the spacecraft. The equivalent internal series and shunt resistance play a vital role in health monitoring of solar cells in the spacecraft power system. With time, the performance degradation of solar cells with increased carrier recombination and leakage current, result in a decrease of the internal shunt resistance. However, the real-time detection of this change in the internal shunt resistance is very difficult. In this paper, a relationship between the shunt resistance change and bus voltage ripple has been investigated, and based on the principle that the shunt resistance monotonous change during the solar cell degradation, the proposed relationship can be used for the evaluation of the health state of the solar cell.

Investigation of Forward Tunneling Characteristics of InGaN/GaN Blue Light-Emitting Diodes on Freestanding GaN Detached from a Si Substrate.

We report forward tunneling characteristics of InGaN/GaN blue light emitting diodes (LEDs) on freestanding GaN detached from a Si substrate using temperature-dependent current–voltage (T-I-V) measurements. T-I-V analysis revealed that the conduction mechanism of InGaN/GaN LEDs using the homoepitaxial substrate can be distinguished by tunneling, diffusion and recombination current, and series resistance regimes. Their improved crystal quality, inherited from the nature of homoepitaxy, resulted in suppression of forward leakage current. It was also found that the tunneling via heavy holes in InGaN/GaN LEDs using the homoepitaxial substrate can be the main transport mechanism under low forward bias, consequentially leading to the improved forward leakage current characteristics.

Temperature-dependent contact resistance of carrier selective Poly-Si on oxide junctions

Carrier selective junctions using a poly-silicon/ silicon oxide stack on crystalline silicon feature low recombination currents J0 whilst allowing for low contact resistivity ρC. We describe the limiting current transport mechanism as a combination of homogeneous tunneling through the interfacial silicon oxide layer and transport through pinholes where the interfacial silicon oxide layer is locally disrupted. We present an experimental method and its theoretical basis to discriminate between homogenous tunneling and local pinhole transport mechanisms on n + /n or p + /p junctions by measuring the temperature-dependent contact resistance. Theory predicts opposing trends for the temperature dependencies of tunneling and pinhole transport. This allows identifying the dominant transport path. For the contact resistance of two differently prepared poly-Si/ silicon oxide/ c-Si junctions we either find clear pinhole-type or clear tunneling-type temperature dependence. Pinhole transport contributes more than 94% to the total current for the sample with a 2.1 nm-thick interfacial silicon oxide that we anneal at a temperature of 1050 °C to achieve highest selectivity. In contrast pinhole transport contributes less than 35 % to the total current for the sample with a 1.7 nm-thick silicon oxide that we annealed at only 700 °C in order to avoid pinholes.

Effect of Electron Irradiation on Electrical and Electroluminescent Properties of n<sup>+</sup>p 4H-SiC Structures

A study of how electron irradiation affects the current-voltage (I-V) and electroluminescence (EL) characteristics of two types of 4H-SiC n+p structures with p-base and base doping to ~5∙1015 cm-3 is presented. The characteristics were measured prior to irradiation and after each of five stages of irradiation with 0.9 MeV electrons at doses in the range from 1∙1015 to 1.1∙1016 cm-2. The irradiation leads to an increase in the recombination current, decrease in the intensity of the edge EL (hνmax≈3.18 eV), and increase in the intensity of the infra-red (IR) EL (hνmax≈1.35 eV), which starts to predominate. Presumably, this indicates that the nonequilibrium carrier lifetime decreases and the concentration of acceptor type defects grows as a result of the irradiation. The IR EL, attributed to a complex defect containing a silicon vacancy, is of interest for development of single-photon sources of light.

In-situ characterization of electron-assisted regeneration of Cz-Si solar cells

We examine the regeneration kinetics of passivated emitter and rear solar cells (PERCs) fabricated on boron-doped p-type Czochralski-grown silicon wafers in darkness by electron injection via application of a forward bias voltage at elevated temperature (140 °C) in order to discriminate between electronic and photonic effects. Based on these dark regeneration experiments, we address the existing inconsistency regarding the measured linear dependence of the regeneration rate constant on the excess carrier density. Using the method of dark regeneration by current injection into the solar cell, we are able to measure the total recombination current of the solar cell at the actual regeneration temperature under applied voltage, i.e., at the physically relevant regeneration conditions. The direct comparison of the regeneration rate constant as a function of electronically injected carrier concentration in the dark and the regeneration rate constant during illumination clearly shows that the regeneration is a purely electronically stimulated effect and that photons are not directly involved.

The Effect of Nonuniform Emitter Sheet Resistance on PERC Solar Cell Performance

The aim of this research work is to study the effects of non-uniform emitter sheet resistance on the performance of PERC solar cells. For this purpose, we used different simulation techniques including EDNA 2, MATLAB and Griddler 2.5 Pro. We calibrated the phosphorous doping profiles with Nmax of 4E20, 3.5E20, 3E20, 2.5E20, 2E20, 1.5E20 and 1E20 by using EDNA 2. The results show that the emitter saturation current densities are decreased for corresponding increase in sheet resistance during phosphorous diffusion. In this way we find a relationship between emitter sheet resistances and recombination current densities and developed a MATLAB function from it. By assuming maximum emitter sheet resistance and the corresponding minimum recombination current density at the centre of the cell, we determined their average values from the same MATLAB function. These average values were then used as input parameters for computer simulations Griddler 2.5 Pro, to assess the effect of non-uniformities in emitter sheet resistance and recombination currents on the performance of p- PERC solar cells. The open-circuit voltage, short-circuit current density, fill factor and efficiency are calculated as a function of the difference between the maximum and minimum value of emitter sheet resistance. The results show that the open-circuit voltage and the efficiency of p-PERC solar cell are decreased if the difference between maximum and minimum value of emitter sheet resistance is increased from 0.45 ohm/sq to 19.3 ohm/sq. Whereas the fill factor is increased for the corresponding increase in the difference between maximum and minimum value of emitter sheet resistance.

Performance Simulation of Unipolar InAs/InAs1−xSbx Type-II Superlattice Photodetector

This paper reports performance simulation of a unipolar tunable band gap InAs-InAsSb type-II superlattice (T2SL) infrared photodetector. The generation-recombination and surface leakage currents limit the performance of T2SL photodiodes. Unipolar nBn device design incorporating a suitable barrier layer in the diode structure is taken to suppress the Auger recombination and tunneling currents. At low reverse bias, the generation-recombination current is negligible in the absence of a depletion region, but the dark current is dominated by the diffusion current at higher operation temperatures. The composition, band alignment, barrier width, doping level and thickness of the absorber region are optimized here to achieve low dark current and high quantum efficiency at elevated operating temperatures. Thin unipolar T2SL absorbers are placed in a resonant cavity to enhance photon-material interaction, thus allowing complete absorption in a thinner detector element. It leads to the reduction in the detector volume for lower dark current without affecting the quantum efficiency. It shows an improvement in the quantum efficiency and reduction in the dark current. Dark current density ∼ 10−5 A/cm2 is achievable with low absorber thickness of 2 μm and effective lifetime of 250 ns in the InAs/InAs0.6Sb0.4/B-AlAs1−xSbx long wave length T2SL detector at 110 K.

Origin of Nonideal Graphene-Silicon Schottky Junction

In this paper, we investigate the origin of the nonideal turn- ON characteristics of the graphene-silicon Schottky junction. Native oxide (SiO2) is proved to play a critical role in determining the behavior of a graphene-Si junction. Within the metal–oxide–semiconductor structure, the effective voltage drop across the junction degrades due to the capacitor network, which contributes to an increased ideality factor. Residual metal catalysts are detected, which act as recombination centers in the silicon and further degrade the ideality factor by enhancing the recombination current. The recombination current is found to be the dominant factor in making the junction nonideal. Forming gas annealing and the insertion of an interfacial dielectric restore the ideality of a graphene-Si junction by reducing the interface states and bulk recombination centers.

Structure of high asymmetric p-n-junction taking into account the charge of movable carriers

A refined analytical model is proposed for calculating the structure of the spatial charge area of a semiconductor p-n junction with allowance for the spatial charge of movable carriers. The work was carried out in the context of refinement of the analytical estimation of p-n junction recombination current, and also in order to estimate the Schottky model applicability limits when calculating the recombination currents in the spatial charge area of a sharply asymmetric semiconductor p-n junction.

Catalyst Deposition on Photoanodes: The Roles of Intrinsic Catalytic Activity, Catalyst Electrical Conductivity, and Semiconductor Morphology

Semiconducting oxide photoanodes are used to drive the oxygen evolution reaction (OER) in water-splitting systems. The highest-performing systems use nanostructured semiconductors coated with water-oxidation catalysts. Despite much work, the design principles governing the integration of catalysts with semiconductors are poorly understood. Using hematite as a model system, we show how semiconductor morphology and electrical conductivity of the catalyst affect the system photoresponse. Electrically conductive catalysts can introduce substantial “shunt” recombination currents if they contact both the semiconductor surface and the underlying conducting-glass substrate, leading to poor performance. This recombination can be largely eliminated by using pinhole-free semiconductors, using selective photoassisted electrodeposition of thin catalyst layers on the semiconductor surface, using electrically insulating catalyst layers, or adding an intermediate insulating oxide layer. The results of this study are used...

Different Roles of Fe1–xNixOOH Cocatalyst on Hematite (α-Fe2O3) Photoanodes with Different Dopants

Transparent Fe1-xNixOOH overlayers (~2 nm thick) were deposited photoelectrochemically on (001) oriented heteroepitaxial Sn- and Zn-doped hematite (Fe2O3) thin film photoanodes. In both cases, the water photo-oxidation performance was improved by the co-catalyst overlayers. Intensity modulated photocurrent spectroscopy (IMPS) was applied to study the changes in the hole current and recombination current induced by the overlayers. For the Sn-doped hematite photoanode, the improvement in performance after deposition of the Fe1-xNixOOH overlayer was entirely due to reduction in the recombination current, leading to a cathodic shift in the onset potential. For the Zn-doped hematite photoanode, in addition to a reduction in recombination current, an increase in the hole current to the surface was also observed after the overlayer deposition, leading to a cathodic shift in the onset potential as well as an enhancement in the plateau photocurrent. These results demonstrate that Fe1-xNixOOH co-catalysts can play different roles depending on the underlying hematite photoanode. The effect of the co-catalyst is not always limited to changes in the surface properties, but also to an increase in hole current from the bulk to the surface that indicates a possible crosslink between surface and bulk processes.

60Co gamma radiation total ionizing dose combined with conducted electromagnetic interference studies in BJTs

The influence of combined 60Co gamma radiation total ionization dose (TID) and conducted electromagnetic interference (EMI) in bipolar transistors was studied. The BJTs were set at forward active bias during the entire irradiation processes to investigate the current gain degradation. The experimental results demonstrated that the base current increases under the influence of EMI, TID and combined TID with EMI due to the recombination currents in the emitter-base spacer of the transistor. The ideality factor n investigated showed that TID was approximately equal to 2 and the combination of TID and EMI was greater than 2. Meanwhile, the degradation quantity of the device current gain βTID+EMI tested with combined TID and EMI irradiation was observed to be more severe than that of βTID or βEMI tested only with TID or EMI at low bias VBE region (<0.65 V). However, the degradation trends of the dc characteristics of the tested devices at high VBE region (>0.6 V) became smaller under different experimental conditions (EMI, TID, TID + EMI). Finally, the current gain degradation levels of the tested devices were compared (βTID+EMI > βTID > βEMI).

Comparison of Free Energy Loss Analysis and Synergistic Efficiency Gain Analysis for PERC Solar Cells

The optimization of a solar cell requires a detailed knowledge of the efficiency limiting power losses. The free energy loss analysis (FELA) and the synergistic efficiency gain analysis (SEGA) both allow for studying these power losses. We compare both approaches for industrial passivated emitter and rear solar cells. The resistive losses calculated with both approaches are in good agreement with each other. In contrast, we find deviations of more than 20% for losses because of charge carrier recombination. From an analytical description of both methods, we conclude that these deviations are caused by a shift of the maximum power point voltage in an improved cell which is not considered in the FELA. The deviation of the losses critically depends on the specific loss channel investigated. In contrast with implied front-side power gains which are usually underestimated, rear-side or absorber gains can be over or underestimated by the FELA depending on the magnitude of the power loss. These differences follow from the Fermi level splitting which is affected differently by recombination currents at different locations. For an accurate determination of the efficiency improvement potential of individual recombination loss channels, we therefore recommend to perform a SEGA.

All-organic bipolar vertical transistor with sulfonated polyaniline base energy barriers favoring recombination emitter-collector current

Abstract We report the development and characterization of an all-organic vertical transistor, with tris(8-hydroxyquinoline) aluminum (Alq3) as emitter, poly(bithiophene) (PBT), as collector material and a sulfonated polyaniline (SPAN) layer as base material in the structure Au/PBT/SPAN/Alq3/CsO/Al. A thin cesium oxide (CsO) layer provides improved electron injection from the Al into the Alq3. The transistor is operated in the usual common-emitter mode were the output characteristic evidences the modulation of the collector current through the applied voltage, showing common-emitter gain of 160 at 4.5 V. The incremented gain is due to an increased recombination current, consequence of the arrangement of the frontier orbitals of PBT, SPAN and Alq3, which favors the confinement of electrons and holes in the SPAN layer.

Contact Selectivity Engineering in a 2 μm Thick Ultrathin c-Si Solar Cell Using Transition-Metal Oxides Achieving an Efficiency of 10.8.

In this paper, the integration of metal oxides as carrier-selective contacts for ultrathin crystalline silicon (c-Si) solar cells is demonstrated which results in an ∼13% relative improvement in efficiency. The improvement in efficiency originates from the suppression of the contact recombination current due to the band offset asymmetry of these oxides with Si. First, an ultrathin c-Si solar cell having a total thickness of 2 μm is shown to have >10% efficiency without any light-trapping scheme. This is achieved by the integration of nickel oxide (NiOx) as a hole-selective contact interlayer material, which has a low valence band offset and high conduction band offset with Si. Second, we show a champion cell efficiency of 10.8% with the additional integration of titanium oxide (TiOx), a well-known material for an electron-selective contact interlayer. Key parameters including Voc and Jsc also show different degrees of enhancement if single (NiOx only) or double (both NiOx and TiOx) carrier-selective contacts are integrated. The fabrication process for TiOx and NiOx layer integration is scalable and shows good compatibility with the device.

Charged grain boundaries and carrier recombination in polycrystalline thin film solar cells.

We present analytical relations for the dark recombination current of a junction with positively charged columnar grain boundaries in the high defect density regime. We consider two defect state configurations relevant for positively charged grain boundaries: a single donor state and a continuum of both acceptors and donors. Compared to a continuum of acceptor+donor states, or to the previously studied single acceptor+donor state, the grain boundary recombination of a single donor state is suppressed by orders of magnitude. We show numerically that superposition holds near the open-circuit voltage , so that our dark relations determine for a given short circuit current . We finally explicitly show how depends on the these two grain boundary defect state configurations.

A New Method for Fitting Current\u2013Voltage Curves of Planar Heterojunction Perovskite Solar Cells

Herein we propose a new equivalent circuit including double heterojunctions in series to simulate the current–voltage characteristic of P–I–N planar structure perovskite solar cells. This new method can theoretically solve the dilemma of the parameter diode ideal factor being larger than 2 from an ideal single heterojunction equivalent circuit, which usually is in the range from 1 to 2. The diode ideal factor reflects PN junction quality, which influences the recombination at electron transport layer/perovskite and perovskite/hole transport layer interface. Based on the double PN junction equivalent circuit, we can also simulate the dark current–voltage curve for analyzing recombination current (Shockley–Read–Hall recombination) and diffusion current (including direct recombination), and thus carrier recombination and transportation characteristics. This new model offers an efficacious and simple method to investigate interfaces condition, film quality of perovskite absorbing layer and performance of transport layer, helping us further improve the device efficiency and analyze the working mechanism.

Advanced Local Characterization of Silicon Solar Cells

Solar cells made from multicrystalline silicon material, which still represent the majority of solar cells produced today, are by nature inhomogeneous devices. Their bulk excess carrier lifetime, which decisively influences the short circuit current density, the saturation current density, and the effective bulk diffusion length, may vary from position to position by an order of magnitude or more. Moreover, such cells may contain ohmic shunts and, in particular in the edge regions, positions of significant depletion region recombination current. The classical local characterization method for solar cells is light beam‐induced current (LBIC) mapping. This method does not give direct information on factors influencing the dark current density. A reliable quantitative local characterization of inhomogeneous cells can be performed by combining dark lock‐in thermography (DLIT) with electro‐ and photoluminescence (EL and PL) imaging. In this contribution, the well‐established and the most recent findings on the application of these methods are reviewed and typical application examples are presented. It is shown that luminescence imaging is most appropriate for local series resistance imaging and for qualitatively imaging of all kinds of recombination‐active defects. DLIT, on the other hand, shows a lower spatial resolution, but is most appropriate for imaging local two‐diode parameters like depletion region‐ and bulk saturation current densities, the latter allowing to draw conclusions also to the local short circuit current density. A comprehensive local characterization of silicon solar cells is possible by applying both DLIT and luminescence imaging, which complement each other.

Quantification of pn-Junction Recombination in Interdigitated Back-Contact Crystalline Silicon Solar Cells

Interdigitated back-contact (IBC) solar cells based on diffused crystalline silicon comprise a series of pn -junctions which border at the rear surface of the wafer. In this work, it is established that the presence of these pn -junctions can induce significant additional charge-carrier recombination, which affect the conversion efficiency of IBC cells through a reduction in fill factor and open-circuit voltage. Using specialized test structures with varying length of pn -junctions per area of solar cell (i.e., with varying junction density), the magnitude of the recombination at the pn -junction was determined. For nonpassivated rear surfaces, a second-diode recombination current density per unit of junction density J 02 of ∼61 nA·junction–1·cm–1 was measured, whereas for surfaces which were passivated by either SiN $_{x}$ or Al 2O3/SiN $_{x}$ , J 02 was reduced to ∼0.4 nA·junction–1·cm–1. Therefore, passivation of defects at the rear surface was proven to be vital in reducing this characteristic recombination current. Finally, by optimizing the p - and n -type dopant diffusion process recipes, J 02 recombination could be suppressed. The improved doping recipes led to an increase in conversion efficiency of industrial “mercury” IBC solar cells by ∼1% absolute.

Comparison between conventional and inverted solar cells using open circuit voltage decay transients

In the development of new structures for solar cells, it has become important to extract the true ideality factor of a diode, disentangling it from other loss mechanisms in the device. We use the open circuit voltage decay (OCVD) transient to obtain ideality factors and isolate the internal loss mechanisms in P3HT:PCBM bulk heterojunction solar cells. We compare two different structures using OCVD transients over more than six orders in timescale. The equivalent circuit parameters of diodes can be reliably extracted from such decays. Specifically, the numerical solution of the transient allows the determination of the diode ideality factor, and the saturation leakage current. In addition, this technique makes it possible to determine the diode current in the presence of excess carriers under photo irradiance, and hence, the light induced recombination current and shunt resistance can be separately extracted. We compare the decay transients of an efficient device with a leaky device and demonstrate that the diode current changes in the same manner in both the cases. We study in detail the intensity power-law dependence of the recombination current that controls the decay transient and observe that it increases significantly faster in leaky devices with light intensity.