Carrier Selectivity Research Articles

Elimination of burn-in open-circuit voltage degradation by ZnO surface modification in organic solar cells.

Photodegradation of inverted organic solar cells based on ZnO as an electron transport layer (ETL) was studied over short time scales of 5 min and 8 h. Devices with ZnO as ETL reproducibly exhibited a steep loss of open-circuit voltage, VOC, and shunt resistance, RSH, in a matter of minutes upon illumination. Removing the UV-content of illumination minimized VOC loss and impact on the device's shunting behavior, indicating its role in the loss. Application of an ultrathin layer of Al on ZnO led to almost negligible photoinduced VOC loss up to 8 h of exposure. By applying the fundamental Shockley diode equation, we approximated the VOC loss to be caused by dramatic increases in reverse saturation current I0. We attribute the increased rate of recombination to diminished carrier selectivity at the ZnO/organic interface. Devices with Al modified ZnO ETL demonstrated remarkable RSH (1.4 kΩ cm(2) at 1 sun), rectification ratio (10(6)) and reverse saturation current density (2.1 × 10(-7) mA/cm(2)).

Contact‐Induced Mechanisms in Organic Photovoltaics: A Steady‐State and Transient Study

The role of the contacts in thin‐film, blended heterojunctions (<100 nm thick) organic photovoltaics is explored, specifically considering concepts of carrier selectivity, injection, and extraction efficiency, relative to recombination. Contact effects are investigated by comparing two hole‐collecting interlayers: a phosphonic acid monolayer on indium tin oxide (ITO) and a nickel oxide thin film. The interlayers have equivalent work functions (≈5.4 eV) but widely variant energy band offsets relative to the lowest unoccupied molecular orbital of the acceptor (electron blocking versus not), which are coupled to large differences in carrier density. Trends in open‐circuit voltages (VOC) as a function of light intensity and temperature are compared and it is concluded that the dominant mechanism limiting VOC for high density of states contacts is free carrier injection, not surface recombination or extraction barriers. Transient photocurrent decay measurements confirm excess reinjected carriers decrease the extraction efficiency via increased recombination and decrease free carrier lifetime, even at high internal electric fields, due to space charge accumulation. These results demonstrate that the energetics and injection dynamics of the interface between interlayers and high carrier density electrodes (typically ITO and metals) must be considered with fabrication and processing of interlayers, in addition to possible carrier selectivity and the interface with the active layer.

Evaluation of iron- and manganese-based mono- and mixed-metallic oxygen carriers for chemical looping combustion

Chemical looping combustion (CLC) is an emerging technology for clean combustion of fossil fuels with inherent CO2 capture. In the present work, we investigate the use of iron and manganese based mixed oxides (MnxFe1−x–CeO2) supported on CeO2 as oxygen carriers in CLC. The low cost and low toxicity of iron and manganese make them interesting candidates for CLC, but both mono-metallic carriers suffer from issues of low reactivity, and manganese is additionally prone to form undesired spinel structures with typical oxide supports. Mono- and bimetallic oxygen carriers were synthesized across the entire spectrum of compositions from pure Mn to pure Fe (with x=0, 0.1, 0.33, 0.5, 0.8, 0.9, 1), characterized, and tested in thermogravimetric and fixed-bed reactor studies using H2 and CH4 as fuels. We find that the use of ceria as support results in stable operation for all compositions of the metal phase, including pure Mn. Bimetallic carriers with high Fe content, which contain a FeMnO3 phase, exhibit an unusual, reversible de-alloying/re-alloying behavior during cyclic redox operation, which precludes any synergistic effects between the two metals and results in slowed reduction kinetics. However, Mn-rich carriers show a pronounced increase in carrier reactivity and selectivity for total oxidation of methane due to the addition of small amounts of Fe, indicating the promise of appropriately designed FeMn carriers as low-cost, environmentally benign oxygen carrier materials for chemical looping combustion.

Efficient carrier-selective p- and n-contacts for Si solar cells

The successful realization of carrier-selective contacts is a crucial prerequisite to approaching the theoretical efficiency limit of silicon solar cells. The herein proposed carrier-selective contacts resemble the polysilicon contacts, including a tunnel oxide, but make use of a wide bandgap semi-crystalline Si layer thereby forming a heterostructure with the crystalline silicon substrate. It will be shown that these tunnel oxide passivated contacts enable a low interface recombination, e.g. the electron contact achieved implied Voc (iVoc) of about 720mV. On the other hand, the interface passivation quality provided by the hole contact was significantly lower (iVoc=680mV) and it seems that its design is more challenging. The critical aspects which might explain the lower performance are addressed in this paper. On solar cell level good carrier selectivity was demonstrated which means that the passivated contacts effectively reduced minority carrier recombination and were transparent for majority carriers as well. The latter manifests itself in an external Voc of 694mV and a FF of 81%. Raman spectroscopy revealed that the Si layer can partially contain crystalline Si phases and it will be demonstrated that this significantly improved the cell׳s blue response. In combination with the higher thermal stability these carrier-selective contacts could be a very appealing option for next generation high-efficiency silicon solar cells once the limitations for the p-contact are overcome.

Extraction of Lignosulfonate using TOA-Kerosene-PVDF in Supported Liquid Membrane Process

Lignosulfonate is a major byproduct from the sulphite pulping process which is the most abundant biopolymer and largely unused. Although lignosulfonate is nontoxic, it impacts brownish black colour to water and makes the water unsuitable for reuse. However, lignosulfonate have a wide range application, such as production of vanillin, animal feed pellets binder and pesticides. Therefore, an efficient separation technique of lignosulfonate from the wastewater is necessary in order to meet the wastewater treatment requirement and as a source of valuable material for industrial applications. In this study, lignosulfonate was extracted from aqueous solution through flat sheet supported liquid membrane (SLM) using trioctylamine (TOA) as carrier. SLM has great potential for the separation lignosulfonate as it offers advantages such as simultaneous extraction and stripping steps, low consumption of carrier and high selectivity. The important parameters governing the extraction process are the types of support material, types of solvents, types of stripping agent, feed phase flow rate and pH were investigated. The favorable condition for extraction was obtained by using TOA-Kerosene-PVDF membrane system, 0.5M NaOH as stripping agent, 100 mL/min of flow rate and feed at pH 2 with 37.5% of lignosulfonate removal. The result demonstrated that the membrane support remains stable for more than 9 hours, hence demonstrating promising separation technique for lignosulfonate.

Tuning Contact Recombination and Open-Circuit Voltage in Polymer Solar Cells via Self-Assembled Monolayer Adsorption at the Organic–Metal Oxide Interface

We adsorbed fluorinated-alkyl and hydrogenated-alkyl phosphonic acid derivatives onto indium tin oxide (ITO) to form self-assembled monolayers (SAMs). Polymer solar cells having these treated ITOs as anodes display open-circuit voltages (Vocs) that are higher than those with bare ITO as anodes. Although the work function of ITO can be significantly tuned by SAM adsorption, the position of the Fermi level of the anode with respect to the hole transport level in the polymer active layer is essentially the same in all of the devices, suggesting that changes in the work function of the anode are not responsible for the Voc variation. Rather, the barrier for minority carrier transport to ITO is altered through SAM adsorption. The adsorption of fluorinated-alkyl phosphonic acid on ITO, in particular, induces a barrier of 2.4 eV for minority carrier transport, which effectively increases carrier selectivity at the anode and increases the Voc in polymer solar cells comprising such treated ITO as anodes compared to those with untreated anodes.

Electrodeposited NiO anode interlayers: Enhancement of the charge carrier selectivity in organic solar cells

Nickel oxide (NiO) thin films prepared by cathodic electrodeposition exhibit superior electrical performace than PEDOT:PSS when used as anode interlayers of bulk-heterojunction solar cells. Devices incorporating 30nm-thick NiO films firstly annealed at 320°C in air and posteriorly treated with UV-O3 reach power conversion efficiencies comparable to that obtained for PEDOT:PSS-based cells. NiO interlayers enhance contact selectivity by simulataneously increasing shunt resistance (lower leakage current related to electron-blocking ability), and reducing hole-extraction resistance. Carrier selectivity is quantified from the resistance components associated with the impedance response of the anode contacts. The versatile electrodeposition technique of NiO interlayers permits avoiding PEDOT:PSS use as it presents disadvantages related to its acid character and hygroscopic nature.

Synergistic effects of buffer layer processing additives for enhanced hole carrier selectivity in inverted Organic Photovoltaics

Solution based inverted Organic Photovoltaic (OPVs) usually use Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) derivatives combined with pristine processing additives as hole selective contact on top of the hydrophobic conjugated polymer:fullerene active layer. In this study, PEDOT:PSS based hole selective contact is treated with two different boiling point additives, 2,5,8,11-tetramethyl-6-dodecyn-5,8-diol ethoxylate (Dynol) and Zonyl FS-300 fluorosurfactant (Zonyl). Although corresponding inverted OPVs using the above PEDOT:PSS:Additives show similar power conversion efficiency (PCE) values, the mechanisms of their implementation on inverted OPV operation are not identical. By understanding the synergistic effects of PEDOT:PSS processing additives on the hole selectivity of inverted OPVs we demonstrate a novel combination of PEDOT:PSS additives mixture as an effective route to further increase the hole selectivity, reliability andpower conversion efficiency of inverted OPVs.

Interplay between Fullerene Surface Coverage and Contact Selectivity of Cathode Interfaces in Organic Solar Cells

Interfaces play a determining role in establishing the degree of carrier selectivity at outer contacts in organic solar cells. Considering that the bulk heterojunction consists of a blend of electron donor and acceptor materials, the specific relative surface coverage at the electrode interfaces has an impact on the carrier selectivity. This work unravels how fullerene surface coverage at cathode contacts lies behind the carrier selectivity of the electrodes. A variety of techniques such as variable-angle spectroscopic ellipsometry and capacitance-voltage measurements have been used to determine the degree of fullerene surface coverage in a set of PCPDTBT-based solar cells processed with different additives. A full screening from highly fullerene-rich to polymer-rich phases attaching the cathode interface has enabled the overall correlation between surface morphology (relative coverage) and device performance (operating parameters). The general validity of the measurements is further discussed in three additional donor/acceptor systems: PCPDTBT, P3HT, PCDTBT, and PTB7 blended with fullerene derivatives. It is demonstrated that a fullerene-rich interface at the cathode is a prerequisite to enhance contact selectivity and consequently power conversion efficiency.

Lanthanides: new metallic cathode materials for organic photovoltaic cells

Organic photovoltaics (OPVs) are compliant with inexpensive, scalable, and environmentally benign manufacturing technologies. While substantial attention has been focused on optimization of active layer chemistry, morphology, and processing, far less research has been directed to understanding charge transport at the interfaces between the electrodes and the active layer. Electrical properties of these interfaces not only impact efficiency, but also play a central role in stability of organic solar cells. Low work function metals are the most widely used materials for the electron transport layer with Ca being the most common material. In bulk heterojunction OPV devices, low work function metals are believed to mirror the role they play in OLEDs, where such metals are used to control carrier selectivity, transport, extraction, and blocking, as well as interface band bending. Despite their advantages, low work function materials are generally prone to reactions with water, oxygen, nitrogen, and carbon dioxide from air leading to rapid device degradation. Here we discuss the search for a new metallic cathode interlayer material that increases device stability and still provides device efficiency similar to that achieved with a Ca interlayer.

Technology-compatible hot carrier solar cell with energy selective hot carrier absorber and carrier-selective contacts

We propose a hot carrier solar cell based on epitaxial growth of a quantum well superlattice and adjacent contact barriers. The concept fulfills required electronic, optical, and several phononic criteria. The first superlattice miniband determines the absorption threshold. The second miniband with appropriate energy width and position provides energy selectivity in situ; contacts are optimized for carrier selectivity exclusively. Electronic transport properties were investigated including elastic random electron–electron scattering, random layer thickness deviation, and illumination as differential absorption per quantum well using a Monte-Carlo code. Carrier extraction probability and energy selectivity strongly suggest a practical implementation of the proposed concept.

Selective Interlayers and Contacts in Organic Photovoltaic Cells.

Organic photovoltaic cells (OPVs) are promising solar electric energy conversion systems with impressive recent optimization of active layers. OPV optimization must now be accompanied by the development of new charge-selective contacts and interlayers. This Perspective considers the role of interface science in energy harvesting using OPVs, looking back at early photoelectrochemical (photogalvanic) energy conversion platforms, which suffered from a lack of charge carrier selectivity. We then examine recent platforms and the fundamental aspects of selective harvesting of holes and electrons at opposite contacts. For blended heterojunction OPVs, contact/interlayer design is especially critical because charge harvesting competes with recombination at these same contacts. New interlayer materials can modify contacts to both control work function and introduce selectivity and chemical compatibility with nonpolar active layers and add thermodynamic and kinetic selectivity to charge harvesting. We briefly discuss the surface and interface science required for the development of new interlayer materials and take a look ahead at the challenges yet to be faced in their optimization.

Engineering of hybrid interfaces in organic photovoltaic devices

In this study, we engineer and investigate the interface structure and chemistry at the indium tin oxide (ITO) anode (front-side electrode) as well as at the Mg−Ag cathode (back-side electrode) in metal phthalocyanine (MePc)/C 60 organic solar cells (OSCs). For the front-side electrode, Zn-phthalocyaninetetraphosphonic acid (Zn-PTPA) and Sn-phthalocyanine axially substituted with tartaric acid (Sn-PTA) have been used for the surface termination of ITO coated glass substrates. Both terminations yielded OSCs with higher fill factors and open circuit voltages, thus increasing the power conversion efficiency by 33% and 67%, respectively. A possible influence of a chemisorbed Zn-PTPA on the film growth of the adjacent ZnPc absorber in the vicinity of the hybrid interface is discussed using X-ray reflectivity and near edge X-ray absorption fine structure data. Distinct effects of the Zn-PTPA and Sn-PTA terminations on the electronic properties of the ITO surface were found by X-ray photoelectron spectroscopy (XPS) measurements at the valence band edge. We demonstrate the possibility to engineer the hybrid interface without additional buffer. For the back-side electrode we report the formation of buffer-free charge carrier selective Mg−Ag cathodes, which are applied for bulk heterojunction organic absorbers consisting of copper phthalocyanine (CuPc) donor and fullerene C 60 acceptor materials. The chemical and structural properties of the CuPc:C 60/Mg−Ag interface are investigated by element depth profiling using secondary ion mass spectrometry (SIMS), grazing incidence X-ray diffraction analysis (GI-XRD) and XPS. We demonstrate that an optimum charge carrier selectivity is achieved with Mg:Ag/Ag cathode structures, where the Mg:Ag alloy layer has a composition close to that of Ag 3Mg. In addition, Mg diffusion into CuPc:C 60 layer is observed. As a result, an interaction between Mg and Cu 2+ with a concurrent change in oxidation state of both metals takes place. However, no formation of MgPc is observed. The findings of this work are discussed against the background of the performance and electrical properties of the corresponding MePc/C 60-based organic solar cells.

Formation of charge-selective Mg–Ag electrodes to CuPc:C60 blend layers

The formation of buffer-free charge carrier selective Mg–Ag cathodes for donor-copper phthalocyanine (CuPc):acceptor-C60 bulk heterojunction organic solar cells is investigated. An optimum charge carrier selectivity is achieved with Mg:Ag/Ag cathode structures, where the Mg:Ag alloy layer has a composition close to that of Ag3Mg. In addition, Mg diffusion into CuPc:C60 layer is observed. As a result, an interaction between Mg and Cu2+ with a concurrent change in oxidation state of both metals takes place. However, no formation of magnesium phthalocyanine is observed.

Study of the potentiometric response of the doped spinel Li 1.05Al 0.02Mn 1.98O 4 for the optimization of a selective lithium ion sensor

In this paper, we studied the development of a selective lithium ion sensor constituted of a carbon paste electrode modified (CPEM) with an aluminum-doped spinel-type manganese oxide (Li 1.05Al 0.02Mn 1.98O 4) for investigating the influence of a doping ion in the sensor response. Experimental parameters, such as influence of the lithium concentration in the activation of the sensor by cyclic voltammetry, pH of the carrier solution and selectivity for Li + against other alkali and alkaline-earth ions were investigated. The sensor response to lithium ions was linear in the concentration range 5.62 × 10 −5 to 1.62 × 10 −3 mol L −1 with a slope 100.1 mV/decade over a wide pH 10 (Tris buffer) and detection limit of 2.75 × 10 −5 mol L −1, without interference of other alkali and alkaline-earth metals, demonstrating that the Al 3+ doping increases the structure stability and improves the potentiometric response and sensitivity of the sensor. The super-Nernstian response of the sensor in pH 10 can be explained by mixed potential arising from two equilibria (redox and ion-exchange) in the spinel-type manganese oxide.

Synthesis and ligand properties of thianthrenophane.

Thianthrenophane 1 has a cavity which offers enough room to potentially enable endohedral coordination to small ions or molecules. For the complexation of silver(I) perchlorate the complex stability constants of 1 logK1=5.45 +/- 0.13 and of thianthrene logK2=9.16 +/- 0.10 were determined by UV/Vis titration. Single competition transport experiments with ten metal salts demonstrate a very high selectivity of 1 as a carrier for silver(I) and a distinctly higher transport rate compared to carriers such as thianthrene and 1,4,8,11-tetrathiacyclotetradecane (14-ane-S4). Although the X-ray crystal structure analysis of the polymeric [Ag(1)]ClO4.(dioxane)7 complex shows an exohedral coordination to silver(I) we suggest that the formation of an endohedral [Ag(1)]+ complex is the explanation for the unusual carrier selectivity of silver(I) by 1 in bulk liquid membrane.

Cytosine substituted calix[4]pyrroles: neutral receptors for 5'-guanosine monophosphate.

The synthesis and characterization of two cytosine-substituted calix[4]pyrrole conjugates, bearing the appended cytosine attached at either a beta- or meso-pyrrolic position, is described. These systems were tested as nucleotide-selective carriers and as active components of nucleotide-sensing ion-selective electrodes at pH 6.6. Studies of carrier selectivity were made using a Pressman-type model membrane system consisting of an initial pH 6.0 aqueous phase, an intervening dichloromethane barrier containing the calix[4]pyrrole conjugate, and a receiving basic aqueous phase. Good selectivity for the Watson-Crick complementary nucleotide, 5'-guanosine monophosphate (5'-GMP), was seen in the case of the meso-linked conjugate with the relative rates of through-membrane transport being 7.7:4.1:1 for 5'-GMP, 5'-AMP, and 5'-CMP, respectively. By contrast, the beta-substituted conjugate, while showing a selectivity for 5'-GMP that was enhanced relative to unsubstituted calix[4]pyrrole, was found to transport 5'-CMP roughly 4.5 times more quickly than 5'-GMP. Higher selectivities were also found for 5'-CMP when both the beta- and meso-substituted conjugates were incorporated into polyvinyl chloride membranes and tested as ion selective electrodes at pH 6.6, whereas near-equal selectivities were observed for 5'-CMP and 5'-GMP in the case of unsubstituted calix[4]pyrroles. These seemingly disparate results are consistent with a picture wherein the meso-substituted cytosine calix[4]pyrrole conjugate, but not its beta-linked congener, is capable of acting as a ditopic receptor, binding concurrently both the phosphate anion and nucleobase portions of 5'-GMP to the calixpyrrole core and cytosine "tails" of the molecule, respectively, with the effect of this binding being most apparent under the conditions of the transport experiments.

Na/K competitive transport selectivity of (221) C lo-cryptand: effects of pH and carrier concentration

The kinetics of the competitive transport of Na + and K + ions across the membrane of large unilamellar vesicles (LUV) were determined when transport was induced by (221)C 1O-cryptand, an ionizable mobile carrier. The experiments were performed at various pH values (7.7 and 8.7) and carrier concentrations (0.1, 0.5 and 1.0 μM) in order to quantify the effects of these parameters on the Na/K competitive transport selectivity of this mobile carrier. At any given pH and carrier concentration, the apparent affinity of (221)C 10 for Na + was higher and less dependent on the concentration of the other competing ion than that for K +. The Na/K competitive transport selectivity ( S c (Na/K)) of (221)C 0 increased linearly with the Na + concentrations, decreased hyperbolically with increasing those of K + and was independent of the pH and of the carrier concentration. In equimolecular ionic mixtures, this competitive selectivity amounted to about 1.5 and when the pH rose, the carrier selectivity for Na + over K + ions was enhanced by cation competition compared to transport of cations as unique substrates. Equations were established to describe the variations of the competitive transport selectivity ( S c ) of cryptands, and for comparison of their noncompetitive selectivity ( S, NC), with the ionic concentrations, the Michaelis parameters of the cations and the pH. The reaction order in Na + ( n(Na)) increased significantly with decreasing the pH and the K + concentration. The results are discussed in terms of the structural, physico-chemical and electrical characteristics of carriers and complexes.

Electrophysiological investigation of the amino acid carrier selectivity in epithelial cells from Xenopus embryo.

The electrical responses induced by external applications of neutral amino acids were used to determine whether different carriers are expressed in the membrane of embryonic epithelial cells of Xenopus laevis. Competition experiments were performed under voltage-clamp conditions at constant membrane potential. Gly, L-Ala, L-Pro, L-Ser, L-Asn and L-Gln generate electrical responses with similar apparent kinetic constants and compete for the same carrier.They are [Na]o and voltage-dependent, insensitive to variations in [Cl]o and [HCO3]o, inhibited by pHo changes, by amiloride and, for a large fraction of the current, by MeAIB. The increase in [K]o at constant and negative membrane potential reduces the response, whereas lowering [K]o augments it. L-Leu, L-Phe and L-Pro appear to compete for another carrier. They generate electrogenic responses insensitive to amiloride and MeAIB, as well as to alterations of membrane potential, [Na]o and [K]o. Lowering [Cl]o decreases their size, whereas increasing [HCO3]o at neutral pHo increases it. It is concluded that at least two and possibly three transport systems (A, ASC and L) are expressed in the membrane of the embryonic cells studied. An unexpected electrogenic character of the L system is revealed by the present study and seems to be indirectly linked to the transport function. L-Pro seems to be transported by system A or ASC in the presence of Na and by system L in the absence of Na. MeAIB induces an inward current.

Effects of diffusion current on characteristics of metal-oxide (insulator)-semiconductor transistors : H. C. Pao and C. T. Sah, Solid St. Electron., 9, (1966), p. 927

We use temperature-dependent contact resistivity (ρc) measurements to systematically assess the dominant electron transport mechanism in a large set of poly-Si passivating contacts, fabricated by varying (i) the annealing temperature (Tann), (ii) the oxide thickness (tox), (iii) the oxidation method, and (iv) the surface morphology of the Si substrate. The results show that for silicon oxide thicknesses of 1.3–1.5 nm, the dominant transport mechanism changes from tunneling to drift-diffusion via pinholes in the SiOx layer for increasing Tann. This transition occurs for Tann in the range of 850°C-950 °C for a 1.5 nm thick thermal oxide, and 700°C-750 °C for a 1.3 nm thick wet-chemical oxide, which suggests that pinholes appear in wet-chemical oxides after exposure to lower thermal budgets compared to thermal oxides. For SiOx with tox = 2 nm, grown either thermally or by plasma-enhanced atomic layer deposition, carrier transport is pinhole-dominant for Tann = 1050 °C, whereas no electric current through the SiOx layer could be detected for lower Tann. Remarkably, the dominant transport mechanism is not affected by the substrate surface morphology, although lower values of ρc were measured on textured wafers compared to planar surfaces. Lifetime measurements suggest that the best carrier selectivity can be achieved by choosing Tann right above the transition range, but not too high, in order to induce pinhole dominant transport while preserving a good passivation quality.