Current‐voltage characteristics of high‐concentration,photovoltaic arrays

The current–voltage ( I–V ) characteristics of photovoltaic (PV) systems have always been a good indicator of the overall performance of a system. The aim of this paper is to give an overview and elucidate the use of the I–V characteristics of concentrator PV (CPV) modules and arrays as an important diagnostic tool to identify factors that lower a system's performance and the types of mismatch that exist between series‐connected single‐junction cells within a module. Possible causes for mismatch between cells include factors such as; misalignment of optical elements and cells, nonuniform cell material parameters, uneven cell illumination due to dew, dust or degradation of the secondary and main optical elements. The different types of mismatch typically found in CPV are categorized and their effects on the resultant module I–V curves are discussed and shown. The effect of bypass diodes on the module's I–V curves is also illustrated. This paper also reports on, and interprets I–V measurements that were recorded for a commercially available point‐focus concentrator module under various real outdoor conditions. Copyright © 2004 John Wiley & Sons, Ltd.     

Native Defect‐Induced Hysteresis Behavior in Organolead Iodide Perovskite Solar Cells

Despite the high power conversion efficiency and ease of fabrication, planar‐junction organolead halide perovskite solar cells often exhibit anomalous hysteretic current–voltage (I–V) characteristics. In this work, the origin of the I–V hysteresis is studied by fine‐tuning the precursor ratio of methylammonium lead iodide and thus varying the native defects in the material. It is shown that the perovskites synthesized from “PbI₂ excess,” “methylammonium iodide excess,” and “stoichiometric” precursors exhibit identical film morphology but different I–V hysteresis in a planar solar cell configuration. Through a comparative analysis on the temperature‐dependent continuous and stepwise‐stabilized I–V responses of the three devices, a model involving transport and trapping of the ionic native defects is proposed. The active energy of the transport process is estimated to be between 0.10 and 0.18 eV, most likely associated with the vacancy‐mediated iodide ion migration. The lower activation energy of the “PbI₂ excess” and “Stoichiometric” samples indicates that the presence of methylammonium vacancies may provide a favorable pathway for the migration of iodide ions due to reduced steric hindrance. Furthermore, the slow trapping and release processes of iodide ions at the TiO₂/perovskite interface are accounted for the long time scale current decay (or raise) following a voltage change.     

Quantitative Analysis of Current–Voltage Characteristics of Semiconducting Nanowires: Decoupling of Contact Effects

A metal‐semiconductor‐metal (M‐S‐M) model for quantitative analysis of current–voltage (I–V) characteristics of semiconducting nanowires is described and applied to fit experimental I–V curves of Bi₂S₃ nanowire transistors. The I–V characteristics of semiconducting nanowires are found to depend sensitively on the contacts, in particular on the Schottky barrier height and contact area, and the M‐S‐M model is shown to be able to reproduce all experimentally observed I–V characteristics using only few fitting variables. A procedure for decoupling contact effects from that of the intrinsic parameters of the semiconducting nanowires, such as conductivity, carrier mobility and doping concentration is proposed, demonstrated using experimental I–V curves obtained from Bi₂S₃ nanowires and compared with the field‐effect based method.     

Effect of voltage sweep direction on the performance evaluation of P3HT : PCBM solar cells

Investigations on the effect of direction of voltage sweeps, on the current density–voltage (J–V) characteristics in polymer bulk‐heterojunction solar cells, based on the blend of poly(3‐hexylthiophene) (P3HT) and phenyl [6,6] C₆₁ butyric acid methyl ester (PCBM), are reported with time. On the freshly prepared device, the direction of the voltage sweep did not have any effect; however, as the device started degrading, the change in direction of the voltage sweep resulted into different characteristics. Analysis beyond complete degradation, when all the photovoltaic parameters reduced to zero, revealed some interesting results. The J–V characteristics, measured with voltage sweep from −ve to +ve voltage, both in the dark and under illumination, were observed to pass through the second quadrant. On the other hand, with the change in the direction of voltage sweep, viz. from +ve to −ve voltage, the characteristics both in the dark and under illumination passed through the fourth quadrant. These results have been explained on the basis of polarization of the degraded active layer due to applied external voltage. This is an important effect and is observed to depend on the applied voltages during performance evaluation and becomes more prominent with time. This effect puts a question mark on the correctness of the method for calculation of the parameters of a degraded device. Studies on degradation of P3HT : PCBM solar cells showed that both the short circuit current density (J_sc) and the power conversion efficiency (η) decay exponentially, whereas the open circuit voltage (V_oc) decays almost linearly with time. Copyright © 2012 John Wiley & Sons, Ltd.     

Bacteriorhodopsin‐Monolayer‐Based Planar Metal–Insulator–Metal Junctions via Biomimetic Vesicle Fusion: Preparation,Characterization, and Bio‐optoelectronic Characteristics

A reliable and reproducible method for preparing bacteriorhodopsin (bR)‐containing metal–biomolecule–monolayer‐metal planar junctions via vesicle fusion tactics and soft deposition of Au top electrodes is reported. Optimum monolayer and junction preparations, including contact effects, are discussed. The electron‐transport characteristics of bR‐containing membranes are studied systematically by incorporating native bR or artificial bR pigments derived from synthetic retinal analogues, into single solid‐supported lipid bilayers. Current–voltage (I–V) measurements at ambient conditions show that a single layer of such bR‐containing artificial lipid bilayers pass current in solid electrode/bilayer/solid electrode structures. The current is passed only if retinal or its analogue is present in the protein. Furthermore, the preparations show photoconductivity as long as the retinal can isomerize following light absorption. Optical characterization suggests that the junction photocurrents might be associated with a photochemically induced M‐like intermediate of bR. I–V measurements along with theoretical estimates reveal that electron transfer through the protein is over four orders of magnitude more efficient than what would be estimated for direct tunneling through 5 nm of water‐free peptides. Our results furthermore suggest that the light‐driven proton‐pumping activity of the sandwiched solid‐state bR monolayer contributes negligibly to the steady‐state light currents that are observed, and that the orientation of bR does not significantly affect the observed I–V characteristics.     

On the voltage‐dependent series resistance of a planar Schottky barrier diode

The series resistance of a planar Schottky barrier diode fabricated on p‐type silicon is investigated by analysing the current–voltage characteristics of the device. Different characterisation techniques have been applied to obtain the value of the series resistance of the device. It is found that the existing techniques are either not applicable for the present device or yield unreliable value for the series resistance. A numerical analysis of the I–V data reveals unusual voltage dependence of the series resistance of the device. The anomaly has been resolved by postulating a potential barrier at the ohmic contact and drawing analogy to serially connected high‐ and low‐barrier diodes in a back‐to‐back configuration. It is found that the voltage dependence of the series resistance of the device can be described by certain empirical law, which also applies to device on GaN. The measured voltage behaviour of the ac resistance and capacitance of the device at different frequencies have been found to be consistent with those of a serial combination of diodes considered to verify the postulate made in interpreting the I–V data.     

Low‐Operating‐Voltage Organic Transistors Made of Bifunctional Self‐Assembled Monolayers

Self‐assembled monolayers (SAMs) are molecular assemblies that spontaneously form on an appropriate substrate dipped into a solution of an active surfactant in an organic solvent. Organic field‐effect transistors are described, built on an SAM made of bifunctional molecules comprising a short alkyl chain linked to an oligothiophene moiety that acts as the active semiconductor. The SAM is deposited on a thin oxide layer (alumina or silica) that serves as a gate insulator. Platinum–titanium source and drain electrodes (either top‐ or bottom‐contact configuration) are patterned by using electron‐beam (e‐beam) lithography, with a channel length ranging between 20 and 1000 nm. In most cases, ill‐defined current–voltage (I–V) curves are recorded, attributed to a poor electrical contact between platinum and the oligothiophene moiety. However, a few devices offer well‐defined curves with a clear saturation, thus allowing an estimation of the mobility: 0.0035 cm² V^–1 s^–1 for quaterthiophene and 8 × 10^–4 cm² V^–1 s^–1 for terthiophene. In the first case, the on–off ratio reaches 1800 at a gate voltage of –2 V. Interestingly, the device operates at room temperature and very low bias, which may open the way to applications where low consumption is required.     

Negative differential resistance,rectifying performance and switching behaviour in carbon-chain based molecular devices

Using non equilibrium Green's function formalism coupled with density functional theory, we carry out electronic transport calculation in two types of molecular devices, one constructed by linear monoatomic carbon chain (···C–C–C–C ···) and the other by two carbon chains capped with a phenyl ring (···C–C–Ph–C–C···), sandwiched between two z-shape electrodes, constructed by zigzag-armchair-zigzag (zz-ac-zz) graphene nanoribbons (GNRs). The potential difference between the z-shape contacts can be varied by employing an external d.c. voltage source. Thus, one may observe the variation of conductivity through the channels. The current–voltage (I–V) characteristics of the proposed resistors show N-type negative differential resistance (NDR), within a particular voltage region. The figure of merit or PVR (peak to valley) ratio (I_peak/I_valley) gets significantly increased, on capping the chains with phenyl ring. A higher value of PVR in I–V characteristics enhances the possibility of applications utilizing NDR. The calculated I–V characteristic is asymmetric and the rectification ratio is found to be 7, in case of the linear carbon chain.The rectification ratio R(V) = I(V)/I(−V) is an important parameter which determines, its suitability as rectifying device. It has been demonstrated that on varying the conformation of the phenyl ring with respect to the plane of electrodes, the transport properties of the system can be modulated. Interestingly, I–V characteristics are asymmetric and show dual NDR peaks in perpendicular conformation of the phenyl ring, with respect to the electrodes in the (···C–C–Ph–C–C···) system. The figure of merit is found to be respectively 8 and 51 for the first and second NDR regions. The later value is extremely high, making it an excellent candidate for potential applications. Moreover, the multi peak NDR device may be widely used in multiple-valued logics. Only a limited number of multiple NDR peak molecular-based nano systems have so far been reported, which are quite complex; by contrast the present system seems to be quite simple. The physical phenomenon of NDR was explained in the light of molecular projected self-consistent Hamiltonian (MPSH) and also the evolution of the frontier molecular orbitals (HOMO–LUMO) as well as transmission under various external bias voltages.     

Current–voltage curve translation by bilinear interpolation

By means of bilinear interpolation and four reference current–voltage (I–V) curves, an I–V curve of a photovoltaic (PV) module is translated to desired conditions of irradiance and PV module temperature. The four reference I–V curves are measured at two irradiance and two PV module temperature levels and contain all the essential PV module characteristic information for performing the bilinear interpolation. The interpolation is performed first with respect to open‐circuit voltage to account for PV module temperature, and second with respect to short‐circuit current to account for irradiance. The translation results over a wide range of irradiances and PV module temperatures agree closely with measured values for a group of PV modules representing seven different technologies. Root‐mean‐square errors were 1·5% or less for the I–V curve parameters of maximum power, voltage at maximum power, current at maximum power, short‐circuit current, and open‐circuit voltage. The translation is applicable for determining the performance of a PV module for a specified test condition, or for PV system performance modeling. Copyright © 2004 John Wiley & Sons, Ltd.     

Electrical properties of Au–Sb/p-GaSe:Gd Schottky barrier diode

Experimental results of the fabricated Schottky barrier diode on a GaSe:Gd substrate are presented. The electrical analysis of Au–Sb/p-GaSe:Gd structure has been investigated by means of current–voltage (I–V) and capacitance–voltage (C–V) measurements at 296 K temperature. The diode ideality factor and the barrier height have been obtained to be 1.07 and 0.85 eV, respectively, by applying a thermionic emission theory. At high currents in the forward direction, the series resistance effect has been observed. The series resistance has been determined from I–V measurements using Cheung's method.     

Estimation of multi‐junction solar cell parameters

The paper deals with the parameter estimation of InGaP/GaAs/Ge multi‐junction solar cell and is based on minimizing the difference between the measured I–V and the theoretical I–V characteristics—the objective function. The parameter estimation was first performed on a multi‐junction solar cell represented by a single‐diode model containing eight parameters: five conventional parameters and three additional parameters for the negative diode breakdown voltage. An extended model is also presented for detailed analysis of the multi‐junction cell containing three subcells connected in series. In this model, each subcell is represented by eight parameters, and therefore a total of 24 parameters describe the cell. The parameter estimation procedure requires derivatives of the first and the second order of an objective function, filtering of noisy measurements, iteration algorithm, guessing of initial parameters, zero finding, and stopping criteria. The paper presents a mathematical method and a procedure for extracting solar cell parameters based on I–V measured data. The parameters' values may be used for analysis of the current mismatch of the subcells, the power loss, the output power of the multi‐junction cell for different environmental conditions, and to some extent, for cell fabrication. Copyright © 2012 John Wiley & Sons, Ltd.     

Influence of Hole‐Transport Layers and Donor Materials on Open‐Circuit Voltage and Shape of I–V Curves of Organic Solar Cells

The effect of injection and extraction barriers on flat heterojunction (FHJ) and bulk heterojunction (BHJ) organic solar cells is analyzed. The barriers are realized by a combination of p‐type materials with HOMOs varying between –5.0 and –5.6 eV as hole‐transport layer (HTL) and as donor in vacuum‐evaporated multilayer p‐i‐metal small‐molecule solar cells. The HTL/donor interface can be seen as a model for the influence of contacts in organic solar cells in general. Using drift‐diffusion simulations we are well able to reproduce and explain the experimental I–V curves qualitatively. In FHJ solar cells the open‐circuit voltage (V_oc) is determined by the donor and is independent of the HTL. In BHJ solar cells, however, V_oc decreases if injection barriers are present. This different behavior is caused by a blocking of the charge carriers at a spatially localized donor/acceptor heterojunction, which is only present in the FHJ solar cells. The forward current is dominated by the choice of HTL. An energy mismatch in the HOMOs leads to kinks in the I–V curves in the cases for which V_oc is independent of the HTL.     

Origin of Hysteresis in CH3NH3PbI3 Perovskite Thin Films

Organic–inorganic hybrid perovskite solar cells are attracting the attention of researchers owing to the high level of performance they exhibit in photovoltaic device applications. However, the attainment of an even higher level of performance is hindered by their anomalous current–voltage (I–V) hysteresis behavior. Even though experimental and theoretical studies have suggested that the perovskite materials may have a ferroelectric nature, it is still far from being fully understood. In this study, the origin of the hysteresis behavior in CH₃NH₃PbI₃ perovskite thin films is investigated. The behavior of ferroelectricity using piezoresponse force microscopy is first examined. Then, by comparing the scan‐rate‐dependent nano/macroscopic I–V curves, it is found that ion migration assisted by the grain boundaries is a dominant origin of I–V hysteresis from a macroscopic viewpoint. Consequently, the observations suggest that, even though ferroelectricity exists in the CH₃NH₃PbI₃ perovskite materials, ion migration primarily contributes to the macroscopic I–V hysteresis. The presented results can provide fundamental guidelines to the resolution of hysteresis issues in organic–inorganic hybrid perovskite materials.     

A Large Magnetoresistance Effect in p–n Junction Devices by the Space‐Charge Effect

The finding of an extremely large magnetoresistance effect on silicon based p–n junction with vertical geometry over a wide range of temperatures and magnetic fields is reported. A 2500% magnetoresistance ratio of the Si p–n junction is observed at room temperature with a magnetic field of 5 T and the applied bias voltage of only 6 V, while a magnetoresistance ratio of 25 000% is achieved at 100 K. The current‐voltage (I–V) behaviors under various external magnetic fields obey an exponential relationship, and the magnetoresistance effect is significantly enhanced by both contributions of the electric field inhomogeneity and carrier concentrations variation. Theoretical analysis using classical p–n junction transport equation is adapted to describe the I–V curves of the p–n junction at different magnetic fields and reveals that the large magnetoresistance effect origins from a change of space‐charge region in the p–n junction induced by external magnetic field. The results indicate that the conventional p–n junction is proposed to be used as a multifunctional material based on the interplay between electronic and magnetic response, which is significant for future magneto‐electronics in the semiconductor industry.     

Spectral mismatch correction and spectrometric characterization of monolithic III–V multi‐junction solar cells

III–V monolithic multi‐junction (MJ) solar cells reach efficiencies exceeding 30% (AM 1.5 global) and have applications in space and in terrestrial concentrator systems. The subcells of monolithic MJ cells are not accessible separately, which presents a challenge to measurement systems and procedures. A mathematical approach is presented which enables a fast way of spectral mismatch correction for MJ cells, thereby significantly reducing the time required for calibration. Moreover, a systematic investigation of the I–V parameters of a MJ solar cell with variation of the incident spectrum is possible, herein called ‘spectrometric characterization’. This analysis method visualizes the effects of current limitation and shifting of the operating voltage, and yields precise information about the current‐matching of the subcells. MJ cells can hereby be compared without the need to match the current of the structures to a reference spectrum in advance. Further applications of the spectrometric characterization are suggested, such as for the determination of the radiation response of the subcells of MJ space solar cells or for the prediction of the annual power output of terrestrial MJ concentrator cells. Copyright © 2002 John Wiley & Sons, Ltd.     

Method for photovoltaic parameter extraction according to a modified double‐diode model

The variables presented in the current–voltage equation of a photovoltaic (PV) device are usually called PV parameters. There are several different methods for PV parameter extraction from measured data according to different models. However, many of these methods provide results that do not represent I–V curves of thin films devices correctly. This can occur because either the applied model or the PV parameter extraction methods are not suitable. It is also possible that the extracted parameters provide a good mathematical representation of the curves but without physical meaning (e.g. negative series resistance). This work presents a method for PV parameter extraction based on a modified double‐diode model. In this model, the ideality factor related to the recombination of the charge carriers in the space‐charge region is assumed as a variable. This method has been tested for different I–V curves of different PV module technologies providing very good results and parameters with physical meaning in all the cases. Copyright © 2012 John Wiley & Sons, Ltd.     

Molecular n‐Type Doping of 1,4,5,8‐Naphthalene Tetracarboxylic Dianhydride by Pyronin B Studied Using Direct and Inverse Photoelectron Spectroscopies

Molecular n‐type doping of 1,4,5,8‐naphthalene tetracarboxylic dianhydride (NTCDA) by pyronin B (PyB) is investigated using ultraviolet photoelectron spectroscopy (UPS), inverse photoelectron spectroscopy (IPES), and current–voltage (I–V) measurements. Deposition of small amounts (< 2 Å) of PyB on pristine NTCDA films leads to a shift of all the molecular levels away from the Fermi level by nearly 0.20 eV, indicative of n‐type doping of NTCDA by PyB. Interface and bulk energy levels of films formed by co‐evaporation of host and dopant show similarly efficient n‐doping. The spectroscopic measurements are confirmed by I–V measurements, which show a four‐orders‐of‐magnitude increase in current in doped films. The comparison of data obtained from UPS of the neat PyB film with the results of density functional theory calculations confirm that two species of PyB are evaporated and condensed into the solid state, with one species primarily responsible for doping.     

CMOS dual output current mode half-wave rectifier

A CMOS dual output current mode half-wave rectifier is presented. The proposed rectifier is composed of three main components: a dual output V–I converter, two half-wave current rectifiers and two I–V converters. A voltage input signal is changed into two current signals by the V–I converter. The current rectifiers rectify these current signals, resulting in positive and negative half-wave current signals that are converted to positive and negative half-wave voltage signals by the I-Vconverters. The theory of operation is described, and the simulated results obtained from the PSPICE program are used to verify the theoretical prediction. Simulated rectifier performance with a 0.5μm MOSFET model using ±1.2V supply voltage demonstrates good rectifier integrity at operating frequencies up to 100MHz.     

Organic/Inorganic Hybrid Nanochannels Based on Polypyrrole‐Embedded Alumina Nanopore Arrays: pH‐ and Light‐Modulated Ion Transport

Inspired by the asymmetric structure and responsive ion transport in biological ion channels, organic/inorganic hybrid artificial nanochannels exhibiting pH‐modulated ion rectification and light‐regulated ion flux have been constructed by introducing conductive polymer into porous nanochannels. The hybrid nanochannels are achieved by partially modifying alumina (Al₂O₃) nanopore arrays with polypyrrole (PPy) layer using electrochemical polymerization, which results in an asymmetric component distribution. The protonation and deprotonation of Al₂O₃ and PPy upon pH variation break the surface charge continuity, which contributes to the pH‐tunable ion rectification. The ionic current rectification ratio is affected substantially by the pH value of electrolyte and the pore size of nanochannels. Furthermore, the holes (positive charges) in PPy layer induced by the cooperative effect of light and protons are used to regulate the ionic flux through the nanochannels, which results in a light‐responsive ion current. The magnitude of responsive ionic current could be amplified by optimizing this cooperation. This new type of stimuli‐responsive PPy/Al₂O₃ hybrid nanochannels features advantages of unique optical and electric properties from conducting PPy and high mechanical performance from porous Al₂O₃ membrane, which provide a platform for creating smart nanochannels system.     

Downscaling and Charge Transport in Nanostructured Ferroelectric Memory Diodes Fabricated by Solution Micromolding

Ferroelectric polymer memory diodes are interface devices where charge injection into the organic semiconductor is controlled by the stray electric field of the ferroelectric polymer. Key to high current density and current modulation is the areal density of well‐defined interfaces. Here, bistable diodes are fabricated by using the soft lithography method solution micromolding. First, the semiconducting polymer poly(9,9‐dioctylfluorene) is patterned into linear gratings. Subsequently, bilinear arrays are obtained by backfilling with the ferroelectric polymer poly(vinylidenefluoride‐co‐trifluoroethylene). The lateral feature size is scaled down from 2 μm to 500 nm. Comprising memory diodes show rectifying J–V characteristics with an On‐current density larger than 10³ A m^?2 and an On/Off current ratio exceeding 10³. The charge transport is explained by 2D numerical simulations. Since the dependence of polarization on electric field is explicitly taken into account, entire J–V characteristics can be quantitatively described. The simulations reveal that rectifying J–V characteristics are inherently related to the concave shape of the patterned ferroelectric polymer. It is argued that the exponential increase in current density with decreasing feature size can be due to confinement of the semiconductor. High On‐current density combined with downscaling, rectification, and simple fabrication yield new opportunities for low‐cost integration of high‐density solution‐processed memories.