Space Charge Limited Current Method Research Articles

Tuning the charge transport properties of non-planar zinc(ii) complexes of azadipyrromethenes using solubilizing groups

Side chain engineering of non-planar zinc(ii) complexes of azadipyrromethene enables either very high hole mobility, high electron mobility or both as estimated by the space-charge limited current (SCLC) method in diodes.

Cellulose‐Assisted Formation of 2D Hybrid Halide Perovskite Nanocrystals with Enhanced Stability for Light‐Emitting Devices

AbstractA common approach to enhance the stability of metal halide perovskites (MHPs) implemented in optical and optoelectronic devices is to incorporate polymer additives into the perovskite layer. A β‐(1,4) cellulose oligosaccharide (COS) synthesized by mechanocatalytic depolymerization of cellulose has been incorporated to films of the BA2MA4Pb5I16 (BA = n‐butylammonium and MA = methylammonium) two‐dimensional (2D) Ruddlesden‐Popper (RP) hybrid perovskite. Scanning Electron Microscopy (SEM) images displayed a three‐fold reduction of the 2D RP perovskite grains (≈30–40 nm), close to the quantum confinement scale. The analysis of the dark J–V curves of single carrier devices by using the space charge limited current (SCLC) method resulted in a rise of the defect concentration. A notable 14‐fold increase in the photoluminescence (PL) signal at high COS content is detected. Moreover, the analysis of the temperature dependence PL measurements (80–300 K) resulted in a larger exciton binding energy, Eb = 180 to 370 meV, at high COS content. Light emitting diodes of the 2D RP perovskites (PeLEDs) are fabricated w/o the COS compound. The stability test performed under operation (5 V) displays 20 times higher operational lifetimes at high COS content while the luminance is also increased in the devices with the COS compound.

Organic Field Effect Transistors Comprising Copolymers Synthesized without Structure-Directing Moieties with Enhanced Carrier Mobility

Conjugated materials are synthesized by C–C bond formation reactions. Trimethyltin and boronic acid are two widely used structure-directing moieties. While using the ethynyl moiety as a part of the monomer, additional structure-directing moieties are not needed. However, ethynylene will be part of the conjugated material. Often, the polymers with an ethynylene moiety negatively impact the polymers’ properties. However, the ethynylene moiety could minimize the dihedral angle if the monomer has steric functionalities. For example, BODIPY is an attractive monomer due to its high molar extinction coefficient, partial quinoidal character, and high quantum yield. However, materials based on BODIPY exhibited poor charge carrier mobility due to steric hindrance generated by four methyl groups. Herein, we copolymerize BODIPY with ethynylene comprising i-indigo and thieno-i-indigo. The copolymer with thieno-i-indigo showed increased molecular weight and significantly reduced band gap compared to the copolymer with i-indigo. The copolymer with i-indigo showed immeasurably low hole transport mobility. On the other hand, the copolymer with thieno-i-indigo exhibited 0.003 cm2V–1 s–1. These measurements were made using field effect transistors. We also measured the charge carrier mobility using the space charge-limited current method. Both copolymers exhibited a mobility of 10–3 cm2 V–1 s–1.

Ambipolar Columnar Self-assembled Organic Semiconductors based on Heteroatom bay-Annulated Perylene Bisimides.

Highly electron-deficient heteroatom (N, S, Se) bay-annulated PBIs exhibiting ordered columnar phase over a wide mesomorphic range including ambient temperature are reported in this manuscript. These compounds with six peripheral n-decyloxy chains exhibited absorption spectra with high molar extinction coefficients, electron-deficient nature and self-assembling behaviour. A detailed comparison with the PBIs bearing six peripheral n-decyl chains was also carried out to get the valuable insights on the structure-property relations in this important class of organic semiconductors. Both of the PBI series were tested for their charge carrier mobility by space charge limited current method and found that they exhibit ambipolar conductivity. This is in contrary to the vast body of literature, where most of the PBI based semiconductors exhibit electron transport behaviour. In general, PBIs derived from tri-n-alkyl anilines exhibit higher mobility values than the PBIs derived from tri-n-alkoxy anilines. Especially, the ambipolar S-annulated PBI derived from tri-n-alkyl aniline exhibited highest hole (8.39×10-3 cm2 /V.s) and electron (1.5×10-2 cm2 /V.s) mobility values and promising for the application in organic electronics.

Effects of electron- and hole-current hysteresis on trap characterization in organo-inorganic halide perovskite

Charge trap density and carrier mobility of perovskite materials are the critical properties of perovskite solar cells. The space charge limited current (SCLC) method, which measures a dark current–voltage (I-V) curve of a single-carrier device has found extensive use for studying the trap density and charge carrier mobility in perovskite materials. Herein, it was found that the electron- and hole-current in organo-lead perovskite-based single-carrier device undergoes significant hysteresis under forward and reverse scanning due to the mobile ions. In addition, it was also observed that measuring history has a detrimental effect on hysteresis resulting in possible overestimation or underestimation of the extracted electrical values from the SCLC measurement. In the forward/reverse scanning process, the mobile ionic defects enhance/shield the charge in the traps due to ionic charging/discharging, thereby increasing/reducing the interface barrier and net charge in the I-V scanning, which in turn affects the determination of transport properties of the carrier. These results raise quite a few doubts over the direct application of classical SCLC measurements for the accurate characterization of intrinsic transport properties of the mixed ionic-electronic perovskite.

Anisotropic mobilities with different packing motifs of the thin films of fused-selenophenes measuring from the SCLC method

In this article, we attempt to establish a correlation between the structural property of three polycyclic arene-fused selenophenes with different crystal packing such as naphthoselenophene ( Nap-Se ), phenanthroselenophene ( Phe-Se ), and naphtho-bis(selenophene) ( Nap-Se2 ) and their mobility measured via the space charge limited current (SCLC) method. The energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the fused selenophenes was a governing factor of their hole mobilities. We also correlate their theoretically calculated anisotropic mobility to obtain some initial idea about their performance as a donor in solar cell devices. The naphtho-bis(selenophene) ( Nap-Se2 ) exhibit the highest mobility by maintaining superiority with all the factors. • The mobility of a semiconductor depends on its HOMO-LUMO energy gap. • Naphtho-bis(selenophene) with 1-D lamellar crystal packing showed the highest mobility. • Vertically measured SCLC mobility provides a lower limit of mobility. • Anisotropic mobility is dictated by the effective charge transfer integral between the dimers.

Ternary Planar Heterojunction Organic Solar Cells Based on the Ternary Active Layers: α-6T/AlPcCl/C60

Ternary planar heterojunction organic solar cells (PHJ-OPVs) were fabricated using three organic small molecules, alpha-sexithiophene (α-6T), aluminum phthalocyanine chloride (AlPcCl) and fullerene (C60). These molecules can be easily sublimated under a vacuum; they have complementary optical absorption spectra and their energy band structure alignment is favorable for electronic charge transfers. Moreover, α-6T and AlPcCl have almost the same HOMO, which is desirable to avoid any decrease in open circuit voltage. The AlPcCl intercalated layer bridges the energy levels of the electron donor, α-6T, and the electron acceptor, C60, which facilitates charge transport through the energy cascade effect. Moreover, the charge carrier mobility measurements of AlPcCl, using the space charge limited current method, demonstrated that it iss ambipolar. All these properties combine to improve the power conversion efficiency (PCE) of PHJ-OPVs by moving from binary structures (α-6T/C60, α-6T/AlPcCl and AlPcCl/C60) to ternary ones (α-6T/AlPcCl/C60). We show, in this study, that both interfaces of the ternary PHJ-OPVs are efficient for carrier separation. After optimization of the different layer thickness, we show that, by comparing the optimum efficiencies of the binary PHJ-OPVs, the realization of ternary PHJ-OPVs, based on the active layers α-6T/AlPcCl/C60, using the following optimized method, allows us to achieve a PCE of 4.33%.

Benzothiophene and benzosulfone fused pyrazino[2,3-g]quinoxaline: Synthesis and semiconducting properties

A novel air-stable n-type benzothiaphene endcapped azaarene ( BTPQ ) and its sulfonated derivative ( BSPQ ) were prepared via two pathways and characterized by NMR, UV-Vis, fluorescence and cyclic voltammetry spectroscopy. Symmetrically introducing four nitrogen atoms into acenes, the semiconductor properties could be changed from p-type to n-type detected through the space charge limited current (SCLC) method. After sulfonation of BTPQ, BSPQ is with deeper frontier orbital energy levels and enhanced the electron mobility. Two novel air-stable n-type azaarenes with deep frontier orbital energy levels were prepared. This part will be used for Graphical Contents.

Insight into the Role of Guanidinium and Cesium in Triple Cation Lead Halide Perovskites

The overall impact of the partial replacement (5–15%) of methylammonium (MA) in the MAPbI3 perovskite by cesium or guanidinium (Gua) cations to fabricate thin films of triple cation Cs x Gua y MA1–x–y PbI3 perovskites is studied. The structural changes are investigated by using X‐ray diffraction measurements revealing shrinkage or expansion of the unit cell upon Cs or Gua incorporation, respectively. The optoelectronic properties are characterized with photoluminescence (PL) time‐resolved spectroscopy and the space charge limited current (SCLC) method. Shorter PL time constants are obtained for the samples with only Cs, while longer PL decays are measured for the perovskites containing additional Gua cation. The SCLC measurements reveal a larger density of trap states in the Cs x MA1–x PbI3 perovskites compared to the MAPbI3 material. The PSCs fabricated with the different mixed cation Cs x Gua y MA1–x–y PbI3 perovskites reveal a good correlation with the measured optoelectronic properties. The power conversion efficiency (PCE) improves from an average value of 18.6% for the MAPbI3 to a value of 20.0% for the Cs0.05Gua0.05MA0.90PbI3 perovskite with a champion cell delivering 21.2%. On the opposite, the PCE decreases to a value of 17.3% for the double cation perovskite with Cs.

The effect of calcination temperature on the photophysical and mechanical properties of copper iodide (5 mol%)–doped hydroxyapatite

Nanocomposite consisting of 5 mol% crystalline CuI-doped hydroxyapatite (HA) was prepared for the first time by facile chemical method and calcined at different temperatures such as 300 °C, 500 °C, 700 °C and 900 °C. In this study, HA played a role as the matrix and CuI was the reinforcement. The effect of calcination temperature on XRD pattern, TEM, SEM, BET, FTIR, luminescence intensity, photoluminescence quantum yield (PLQY), fluorescence lifetime, charge mobility measurements, electronic speckle pattern interferometry and mechanical properties were investigated. The modified Scherrer equation of the composites showed that the size of the particles increased with increasing calcination temperature from 66.64 to 87.20 nm. The photoluminescence intensity of CuI (5 mol%)/HA was quenched at room temperature, while the represented constant decay time for the calcined compound was increased at 700 °C. Furthermore, the CIE coordinates were shown 0.39333, 0.18493 for CuI (5 mol%)/HA calcined at 700 °C. The charge mobility values of the nanocomposites were extracted by space-charge limited current (SCLC) method and the range of effective mobility was from 3.645 × 10−4 to 6.697 × 10−4 cm2V−1s−1. Mechanical properties were fully discussed via ASTM-E9 standard and CuI (5 mol%)/HA calcined at 900 °C was in better range than other compounds, the σyc and hardness values were reported 7.32 Mpa and 40.81 HV respectively. Speckle interferometry was used to demonstrate that there was no large imperfection in the surface of each sample. The components were very sensitive to the calcination temperatures. In other words, the combination of CuI (5 mol%)/HA showed that the replacement of Cu ions with Ca was helpful to improve the photophysical and mechanical properties.

Improvement in power conversion efficiency of all-polymer solar cells enabled by ultrafast channels for charge dynamics

Despite the rapid breakthrough of polymerized fused-ring small molecular acceptors, developing accessible and inexpensive polymer acceptors for high performance all-polymer solar cells (all-PSCs) and revealing the fundamental work mechanism of all-PSCs is highly desirable. In addition, the isomeric issue of polymerized fused-ring small molecular acceptors still needs to be concerned. In this work, we report an accessible and inexpensive polymer acceptor P2TF by polymerization of 2,2'-((2Z,2Z)-((4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno [1,2-b:5,6-b']dith-iophene-2,7-diyl)bis (me-thanylylidene))bis (3-oxo-2,3-dihydro-1H-indene-2,1-diylide-ne)) dimalononitrile-based small molecular. Molecular electrostatic potentials and permanent dipole moments are evaluated by theoretical calculations, revealing that structural isomerization has a negligible impact on their optoelectronic properties. Femtosecond-resolved transient absorption, device charge dynamic measurements and space-charge limited current method are performed to systematically investigate charge dynamics in all-PSCs. The PM6:P2TF-based as-cast device exhibits poor power conversion efficiency of 4.51%, while the optimized device achieves a superior power conversion efficiency of 10.65%. The significantly improved performance of the optimized device originates from ultrafast channels of charge dynamics for ultrafast charge transfer, efficient charge transport and suppressed charge recombination, which is strongly related to the well-established morphology induced by optimal treatment. Furthermore, the PM6:P2TF also has excellent bending property, indicating its great potential for application in flexible all-PSCs.

Growth and photodetection properties of Cs2AgBiBr6 crystals with large flat (1 1 1) plane grown from the solution by adding toluene

Cs2AgBiBr6 crystals with the maximum dimension of 10 mm and flat plane have been grown by the solution cooling method with additive toluene. The crystal structure and crystallinity were characterized by the X-ray diffraction of the powder and flat plane, respectively. The optical transmission and temperature dependent photo-emission spectra were measured, and the binding energy was estimated by fitting the Arrhenius equation. Comparative research on the trap densities, carrier mobilities and resistivities of our crystals between Ag or Au electrodes were undertaken using the space charge limited current method. The photodetection properties induced by UV light and X-ray were also investigated. Furthermore, the transient response of the X-ray detectors is very stable at room temperature under air condition with the on-off ratio of 165. This study shows that Cs2AgBiBr6 crystal grown by additive toluene exhibits good promising application for X-ray detection.

Towards radiation detection using Cs2AgBiBr6 double perovskite single crystals

In this work, we studied the optical- and electrical- properties of emerging Cs2AgBiBr6 double perovskite single crystals and demonstrated their potential for detecting ionizing radiation. We prepared Cs2AgBiBr6 double perovskite single crystals from a saturated aqueous solution. Low-temperature photoluminescence (PL) was employed to determine the bandgap energies of Cs2AgBiBr6, which are 2.00 eV (indirect) and 2.26 eV (direct) respectively. Using the space charge limited current method, we estimated the density of trap states and mobility of charge carriers as 1.44 × 1010 cm−3 and 7.02 cm2/V-s respectively. A lower bound value of the mobility-lifetime (μ-τ) product of 2.48 × 10−3 cm2/V was determined using 450 nm laser excitation, which was sufficient for ensuring a long drift distance of charge carriers for several radiation detector applications. Furthermore, we tested the direct response of Cs2AgBiBr6 single crystals to X-ray radiation. Our Cs2AgBiBr6 single crystal device with gold electrodes deposited on the two parallel surfaces exhibited excellent linear response to low energy X-rays.

Highly Efficient CsPbBr3 Planar Perovskite Solar Cells via Additive Engineering with NH4SCN.

Improving stability is a major aspect for commercial application of perovskite solar cells (PSCs). The all-inorganic CsPbBr3 perovskite material has been proven to have excellent stability. However, the CsPbBr3 film has a small range of light absorption and serious charge recombination at the interface or inside the device, so the power conversion efficiency is still lower than that of the organic-inorganic hybrid one. Here, we successfully fabricate high-quality CsPbBr3 films via additive engineering with NH4SCN. By incorporating NH4+ and pseudo-halide ion SCN- into the precursor solution, a smooth and dense CsPbBr3 film with good crystallinity and low trap state density can be obtained. At the same time, the results of a series of photoluminescence and electrochemical analyses including electrical impedance spectroscopy, space-charge limited current method, Mott-Schottky data, and so on reveal that the NH4SCN additive can greatly reduce the trap state density of the CsPbBr3 film and also effectively inhibit interface recombination and promote charge transport in the CsPbBr3 planar PSC. Finally, the CsPbBr3 planar PSC prepared with a molar ratio of 1.5% NH4SCN achieves a champion efficiency of 8.47%, higher than that of the pure one (7.12%).

Efficient inverted perovskite solar cells with preferential orientation and suppressed defects of methylammonium lead iodide by introduction of phenothiazine as additive

High crystalline quality of perovskite thin film is essential for efficient metal halide perovskite solar cells (PSCs). In this investigation, we report a simple anti-solvent washing treatment using a versatile electron-rich heterocycle (phenothiazine, PTZ) as additive in sec-butyl alcohol to improve methylammonium lead iodide (MAPbI3) perovskite crystal growth. As a result, the PTZ-modified PSCs exhibit average power conversion efficiency (PCE) of 15.50%, obviously surpassing 14.38% of the reference device, which is due to the synergistic effects of the PTZ on the crystallization orientation and defects passivation of the perovskite films. The strong interaction between PTZ and Pb2+ ions not only promotes the size of the MAPbI3 crystals to increase from 0.58 μm to 0.73 μm and thus reduces the grain boundaries, is but also beneficial for the formation of a preferential orientation perovskite film, thereby delivering improved electronic properties. Moreover, as Lewis base, PTZ passivates the defects among perovskite crystals consistent with the decreased defect density calculated using the space charge limited current (SCLC) method. The result indicates using PTZ as additive into the perovskite precursor solution a highly effective and facile method to obtain high performance PSCs.

High charge carrier mobility in solution processed one-dimensional lead halide perovskite single crystals and their application as photodetectors.

Organic inorganic hybrid metal halide perovskites have emerged as promising candidates for photovoltaics and light-emitting diodes. Recently, interest has been growing in the properties of low-dimensional metal halide perovskites, and one-dimensional versions with strong quantum confinement have demonstrated highly efficient broadband luminescence. Nevertheless, the charge transport mechanism in these low dimensional perovskites remains unclear. In this work, we characterised the charge mobility in one-dimensional perovskite single crystals using a space charge limited current method. Temperature dependent charge mobility measurements indicated that localized polarons at high temperature are replaced by delocalized polarons at low temperature with extended states in the polaronic band. A minimum mobility of 4.51 cm2 V-1 s-1 was measured at room temperature. UV photodetectors based on these crystals show an ultrahigh photoresponsivity of 132.3 A W-1. These findings show the promise of high mobility low dimensional perovskite materials for optoelectronic applications.

Ultra-fast measurement circuit for transient space charge limited current in organic semiconductor thin films

The charge carrier mobility is a crucial parameter determining the device performance for numerous different semiconductor applications. Consequently, an accurate measurement of this quantity is crucial. For this purpose, the transient space charge limited current (SCLC) method is commonly applied and is preferable over, for example, Hall or field effect measurements, as the analyzed current direction is in line with typical device architectures. For the transient SCLC method, a voltage step is applied and the transit time of injected charge carriers is determined using displacement currents. Consequently, the difficulty of this method is the use of an adequate RC time constant for the sample charging, as it needs to be much shorter than the transit time. This parameter generally limits the application of transient SCLC strongly, in terms of obtainable charge carrier mobility or minimum required film thickness. Here, we demonstrate a measurement circuit with a low RC time constant, which works in a wide current range (1 A–0.5 A) and thus allows for significant flexibility in terms of minimum film thickness or detectable charge carrier mobility. The circuit is fast enough to measure, for example, charge carrier mobilities of up to for a 76 nm thick 4,4’,4”-Tris[phenyl(m-tolyl)amino]triphenylamine (MTDATA) layer, without using limited bridge circuitry. For this purpose, a capacitor coupled fast transistor switch generates a voltage step to avoid voltage oscillations and a fast operational amplifier is used for amplification of the voltage over a variable measurement resistor. We demonstrate the circuit working principle by measuring benchmarked MTDATA diodes and discuss its range of application.

Energy level tuning of aromatic polyamines by [2 + 2] cycloaddition-retroelectrocyclization for the optimization of device performances

The highest occupied molecular orbital (HOMO) level of hole-transporting polymeric materials was tuned by postfunctionalization based on a highly-efficient [2 + 2] cycloaddition-retroelectrocyclization (CA-RE) reaction. After the CA-RE reaction, the HOMO level of an aromatic polyamine, determined by the electrochemical first oxidation potential, was lowered from −5.30 eV to −5.37 eV. This enabled the smooth hole injection from the perovskite layer of organic-inorganic perovskite solar cells. Although the hole mobility of the aromatic polyamine, determined by the space-charge limited current (SCLC) method, decreased by this postfunctionalization, the energy level adjustment was more significant for the improvement of the solar cell performances. The device based on the postfunctionalized polyamine with the HOMO level of −5.37 eV showed more than twice the photoconversion efficiency than that based on the precursor polyamine with the HOMO level of −5.30 eV.

Enhancing photovoltaic performance of perovskite solar cells utilizing germanium nanoparticles

Morphological and crystalline control over hybrid organic-inorganic perovskite films is pivotal for efficient photovoltaic (PV) performance devices. Yet, this remains very challenging for solution processed perovskite solar cells (PVSCs), especially mesoscopic PVSCs, due to the complicated crystallization kinetics of hybrid semiconductor materials within dynamic spin-coating and post annealing. In this work, colloidal Ge nanoparticles (NPs) were added onto a mesoporous TiO2 (m-TiO2) electron transporting layer (ETL) to regulate perovskite crystal growth. Systematic investigation and optimization disclose that incorporation of an appropriate ratio of Ge NPs onto the m-TiO2 ETL can simultaneously increase the size of the CH3NH3PbI3 crystals, decrease the number of the grain boundaries and promote the interfacial properties of perovskite/m-TiO2. The related mechanisms are clarified through detailed morphology and crystal structure analyses. The electron mobility of the perovskite absorber, determined using the space charge limited current (SCLC) method, was increased by over 5 times when an optimized amount of Ge NPs were employed. Average power conversion efficiency (PCE) of 18.59% was achieved from 16 cells and the best PCE of 19.6% was attained via the addition of the optimized amount of Ge NPs. We study the fundamentals of optics and physics behind the PVSC device based on the high refractive index Ge NPs. This work offers an innovative scenario to enhance the performance of perovskite based optoelectronics by employing optically stable, chemically inert, low-cost and green semiconductor NPs.

Columnar Iridium(III) Metallomesogens Based on Polycatenar Pyridyltetrazolate with Ambipolar Carrier Mobility Behavior.

In this paper, we have designed and synthesized a series of neutral liquid-crystalline iridium(III) complexes based on polycatenar 2,5-diphenylpyridine and pyridyltetrazolate derivatives. Iridium(III) complexes all display highly emissive behavior with photoluminescence quantum yields in the range of 0.45-0.66 and a maximum emission wavelength at ∼563 nm. Hexagonal columnar mesophases of iridium(III) complexes can be obtained by changing the number and length of peripheral alkoxyl chains attached to a 2,5-diphenylpyridine ligand (main ligand) and a pyridyltetrazolate ligand (auxiliary ligand). Moreover, experimental results of the charge transport properties for these iridium(III) complexes, which were measured by the space charge limited-current method, exhibit ambipolar carrier mobility behavior. In particular, the liquid-crystalline iridium(III) complexes can self-organize into one-dimensional (1D) nanostructure after thermal annealing treatment in their liquid-crystalline phase. The devices based on liquid crystal film display improved charge transport behavior compared with that of the devices based on polycrystalline film, indicating 1D nanostructure is beneficial to charge carrier injection and transportation.