Mobility Lifetime Research Articles

Photophysics of Molecular‐Weight‐Induced Losses in Indacenodithienothiophene‐Based Solar Cells

The photovoltaic performance and optoelectronic properties of a donor–acceptor copolymer are reported based on indacenodithienothiophene (IDTT) and 2,3‐bis(3‐(octyloxy)phenyl)quinoxaline moieties (PIDTTQ) as a function of the number‐average molecular weight (Mn). Current–voltage measurements and photoinduced charge carrier extraction by linear increasing voltage (photo‐CELIV) reveal improved charge generation and charge transport properties in these high band gap systems with increasing Mn, while polymers with low molecular weight suffer from diminished charge carrier extraction because of low mobility–lifetime (μτ) product. By combining Fourier‐transform photocurrent spectroscopy (FTPS) with electroluminscence spectroscopy, it is demonstrate that increasing Mn reduces the nonradiative recombination losses. Solar cells based on PIDTTQ with Mn = 58 kD feature a power conversion efficiency of 6.0% and a charge carrier mobility of 2.1 × 10−4 cm2 V−1 s−1 when doctor bladed in air, without the need for thermal treatment. This study exhibits the strong correlations between polymer fractionation and its optoelectronics characteristics, which informs the polymer design rules toward highly efficient organic solar cells.

Studies of scattering mechanisms in gate tunable InAs/(Al,Ga)Sb two dimensional electron gases

A study of scattering mechanisms in gate tunable two dimensional electron gases confined to InAs/(Al,Ga)Sb heterostructures with varying interface roughness and dislocation density is presented. By integrating an insulated gate structure the evolution of the low temperature electron mobility and single-particle lifetime was determined for a previously unexplored density regime, 1011–1012 cm−2, in this system. Existing theoretical models were used to analyze the density dependence of the electron mobility and single particle lifetime in InAs quantum wells. Scattering was found to be dominated by charged dislocations and interface roughness. It was demonstrated that the growth of InAs quantum wells on nearly lattice matched GaSb substrate results in fewer dislocations, lower interface roughness, and improved low temperature transport properties compared to growth on lattice mismatched GaAs substrates.

Optimization of shielding electrode lengths of virtual Frisch-grid CdZnTe radiation detector for gamma-ray detection

Room-temperature semiconductor radiation detectors (RTSDs) such as cadmium zinc telluride (CZT) have been under development for several decades to provide a high performance for gamma ray detection, which needs a single crystallinity, a good uniformity, a high stopping power, and a wide band gap. However, the performance of CZT is limited in terms of the energy resolution by an incomplete charge collection owing to the slow mobility of the hole and polarization effects. To overcome such limitation, a virtual Frisch-grid CZT detector using a single polarity charge sensing method has been introduced. In this paper, we optimized the shielding electrode length of a virtual Frisch-grid to enhance the energy resolution and detection efficiency. In addition, the applied bias can be decreased by using the relatively thin (40 μm) encapsulate materials to reduce the system power consumption. The detector was fabricated using 5 × 5 × 14 mm3 CZT from Redlen, and the fabrication process used to make the virtual Frisch-grid CZT radiation detector is described. Characteristics such as the current-to-voltage curve, energy resolution, electron mobility life-time products, and intrinsic detection efficiency are compared by varying the shielding electrode lengths of the virtual Frisch-grid CZT detector. The results from these measurements will give us an optimal shielding electrode length and achieve a higher performance for a room-temperature radiation detector.

Characterization of transport properties of organic semiconductors using impedance spectroscopy

Methods for the determination of drift mobility, localized-state distribution and deep trapping lifetime in organic semiconductors using impedance spectroscopy are reviewed. The theoretical basis is single-injection space-charge-limited current under small sinusoidal voltage perturbation. Major advantages of impedance spectroscopy are: full automatic measurements and simultaneous measurements of these physical quantities. Information on these physical quantities are essential for the understanding of transport properties in organic semiconductors and for the design of organic devices such as organic light-emitting diodes and organic solar cells using a device simulator. The determination of drift mobility, localized-state distribution and deep trapping lifetime from impedance spectra is demonstrated in a molecularly doped polymer. A molecularly doped polymer is a prototypical organic semiconductor and is a good example for the demonstration of simultaneous determination of these quantities. The methods presented here are applicable to insulating semiconductors and thus to inorganic disordered semiconductors such as hydrogenated amorphous silicon and amorphous oxide semiconductors, as well.

Effect of doping on transport properties of nanocrystalline CdSe thin film

The effect of In and Zn doping on the transport properties of nanocrystalline CdSe thin films has been investigated. Thin films are prepared by the physical vapor deposition technique. The films crystallize in a hexagonal structure with spherical particles of nanometer range having different sizes and uniformly distributed all over the surface of the substrates. The experimental data of DC electrical conductivity measurements suggest that the conduction in the high temperature range occurs in the extended states while conduction in the low temperature range takes place through a variable range hopping. The bimolecular recombination nature of the thin films has been revealed by the photoconductivity illumination dependence. The mobility lifetime product and minority carrier diffusion length of thin films have been investigated by the time of flight and steady state photocarrier grating technique respectively. The values of minority carrier diffusion length are found to be 184nm, 107nm and 103nm for nc-CdSe, nc-CdSe:In and nc-CdSe:Zn thin films, respectively.

Electron mobility and spin lifetime enhancement in strained ultra-thin silicon films

Spintronics attracts much attention because of the potential to build novel spin-based devices which are superior to nowadays charge-based microelectronic devices. Silicon, the main element of microelectronics, is promising for spin-driven applications. Understanding the details of the spin propagation in silicon structures is a key for building novel spin-based nanoelectronic devices. We investigate the surface roughness- and phonon-limited electron mobility and spin relaxation in ultra-thin silicon films. We show that the spin relaxation rate due to surface roughness and phonon scattering is efficiently suppressed by an order of magnitude by applying tensile stress. We also demonstrate an almost twofold mobility increase in ultra-thin (001) SOI films under tensile [110] stress, which is due to the usually neglected strain dependence of the scattering matrix elements.

Enhanced performance using an SU-8 dielectric interlayer in a bulk heterojunction organic solar cell.

The effect of inserting an SU-8 dielectric interlayer into inverted bulk heterojunction (BHJ) organic solar cells (OSCs) was studied. Insertion of an ultrathin layer of SU-8 between the zinc oxide (ZnO) electron transport layer and the photoactive layer resulted in a smoother interface and a 14% enhancement in power conversion efficiency. The properties of devices with and without an SU-8 interlayer were investigated using transient photovoltage (TPV) and double injection (DoI) techniques, and it was found that devices with SU-8 show longer carrier lifetimes and greater mobility-lifetime (μ-τ) products than those without. Devices with SU-8 were also found to have improved stability. The results indicate that the insertion of an SU-8 interlayer reduces the recombination rate for photogenerated carriers without affecting the charge transport properties, improving overall performance and stability.

Critical role of domain crystallinity, domain purity and domain interface sharpness for reduced bimolecular recombination in polymer solar cells

Inverted bulk heterojunction solar cells were fabricated using poly(3-hexylthiophene) (P3HT) blended with two different fullerene derivatives namely phenyl-C61-butyric acid methyl ester (PC60BM) and indene-C60 bis-adduct (IC60BA). The effects of annealing temperatures on the morphology, optical and structural properties were studied and correlated to differences in photovoltaic device performance. It was observed that annealing temperature significantly improved the performance of P3HT:IC60BA solar cells while P3HT:PC60BM cells showed relatively less improvement. The performance improvement is attributed to the extent of fullerene mixing with polymer domains. Energy filtered transmission electron microscopy (EFTEM) and x-ray diffraction (XRD) results showed that ICBA mixes with disordered P3HT much more readily than PC60BM which leads to lower short circuit current density and fill factor for P3HT:IC60BA cells annealed below 120°C. Annealing above 120°C improves the crystallinity of P3HT in case of P3HT:IC60BA whereas in P3HT:PC60BM films, annealing above 80°C leads to negligible change in crystallinity. Crystallization of P3HT also leads to higher domain purity as seen EFTEM. Further it is seen that cells processed with additive nitrobenzene (NB) showed enhanced short circuit current density and power conversion efficiency regardless of the fullerene derivative used. Addition of NB led to nanoscale phase separation between purer polymer and fullerene domains. Kelvin probe force microscopy (KPFM) images showed that enhanced domain purity in additive casted films led to a sharper interface between polymer and fullerene. Enhanced domain purity and interfacial sharpness led to lower bimolecular recombination and higher mobility and charge carrier lifetime in NB modified devices.

Effect of total gas pressure in sputtered hydrogenated amorphous silicon

AbstractHydrogenated amorphous silicon (a‐Si:H) thin films are prepared using DC magnetron sputtering method at a substrate temperature of 200 °C using a plasma of argon and hydrogen gas mixture. The effects of the total gas pressure (TGP) during the deposition on structural, optical and electrical properties of the films are investigated. A decrease of the polyhydride bonding groups concentration (SiHn) is observed in FTIR spectra when the TGP increases. The optical gap remains constant. With increasing TGP the dark and photoconductivity increase, the defect density of states determined from constant photocurrent method (CPM) decreases, and the quantum efficiency mobility lifetime product (ημτ) is enhanced by a factor of more than fifty. Thus, the structural and electrical properties are enhanced by only increasing the TGP. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Charge carrier transport and collection enhancement of copper indium diselenide photoactive nanoparticle-ink by laser crystallization

There has been increasing needs for cost-effective and high performance thin film deposition techniques for photovoltaics. Among all deposition techniques, roll-to-roll printing of nanomaterials has been a promising method. However, the printed thin film contains many internal imperfections, which reduce the charge-collection performance. Here, direct pulse laser crystallization (DPLC) of photoactive nanoparticles-inks is studied to meet this challenge. In this study, copper indium selenite (CIS) nanoparticle-inks is applied as an example. Enhanced crystallinity, densified structure in the thin film is resulted after DLPC under optimal conditions. It is found that the decreased film internal imperfections after DPLC results in reducing scattering and multi-trapping effects. Both of them contribute to better charge-collection performance of CIS absorber material by increasing extended state mobility and carrier lifetime, when carrier transport and kinetics are coupled. Charge carrier transport was characterized after DPLC, showing mobility increased by 2 orders of magnitude. Photocurrent under AM1.5 illumination was measured and shown 10 times enhancement of integrated power density after DPLC, which may lead to higher efficiency in photo-electric energy conversion.

Triphenylamine‐Substituted Metalloporphyrins for Solution‐Processed Bulk Heterojunction Solar Cells: The Effect of the Central Metal Ion on Device Performance

AbstractA series of metal(II)‐substituted 5,10,15,20‐tetrakis[4‐(diphenylamino)phenyl]porphyrins (P) were designed and synthesized. Incorporation of the flexible and strong electron‐donating triphenylamine substituent endowed the porphyrins with enhanced solubility and intermolecular interactions. The distinct properties arising from different central metal ions were investigated by studying the absorption spectra, electrochemistry behavior, charge transport properties, thermal stability, and morphology characterization of the porphyrins. The PdII 5,10,15,20‐tetra(4‐diphenylaminophenyl)porphyrins (PPd) were found to have more matching energy levels with methyl [6,6]‐phenyl‐C‐61‐butyrate (PC61BM), higher hole mobility and longer triplet exciton lifetime. Bulk heterojunction solar cells based on porphyrins and PC61BM have been fabricated, with PPd‐based solar cells exhibiting greatly enhanced photovoltaic performance originating from a combination of elements. These results will help guide the future molecular design of porphyrins for application in organic small molecule bulk heterojunction solar cells.

A study on the clustering technology of underwater isomorphic sensor networks based on energy balance.

Nowadays, there is a greater need for energy efficient and stable underwater sensor networks (UWSNs). Underwater sensors usually do not have enough power, so the goal of underwater sensor networks is to make the network have a long lifetime. An underwater heterogeneous sensor network (UWHSN) is one way to cluster the sensors, and the application of UWHSNs is simple and fast, but robots, lifetime and energy-partition are all drawbacks of UWHSNs. In this paper we propose the underwater isomorphic sensor network (UWISN) clustering technology. By analyzing the characteristics of UWISNs, we determine that an UWISN has strong expansibility, mobility, energy-efficiency and long lifetime. An UWISN adopts normal sensor nodes to be cluster heads, and these cluster heads communicate with each other. This paper seeks the optimal number of clusters and uses FCM to elect cluster heads and establish the network. In addition, an idea of real cluster heads and the method to elect them have been proposed. Finally, the simulation results show that the solution is effective and UWISNs can improve the energy consumption of an UWSN.

Oxygen incorporation in rubrene single crystals.

Single crystal rubrene is a model organic electronic material showing high carrier mobility and long exciton lifetime. These properties are detrimentally affected when rubrene is exposed to intense light under ambient conditions for prolonged periods of time, possibly due to oxygen up-take. Using photoelectron, scanning probe and ion-based methods, combined with an isotopic oxygen exposure, we present direct evidence of the light-induced reaction of molecular oxygen with single crystal rubrene. Without a significant exposure to light, there is no reaction of oxygen with rubrene for periods of greater than a year; the crystal's surface (and bulk) morphology and chemical composition remain essentially oxygen-free. Grand canonical Monte Carlo computations show no sorbtion of gases into the bulk of rubrene crystal. A mechanism for photo-induced oxygen inclusion is proposed.

Chromium compensated gallium arsenide detectors for X-ray and γ-ray spectroscopic imaging

Semi-insulating GaAs material of 500μm thickness grown using the Liquid Encapsulated Czochralski (LEC) method has been compensated with chromium to produce high resistivity single crystals suitable for spectroscopic imaging applications. Results are presented for the performance of three small pixel detectors each with 80×80 pixels on a 250μm pitch, fabricated with metal contacts and bonded to a spectroscopic imaging ASIC. Current–voltage measurements demonstrated a material resistivity of 2.5×109Ωcm at room temperature. At an optimised bias voltage, the average energy resolution at 60keV (FWHM) was in the range 2.8–3.3keV per pixel. An analysis of the voltage dependent X-ray spectroscopy suggests that the electron mobility lifetime (μτe) for each detector is in the range 2.1–4.5×10−5cm2V−1. The spectroscopic imaging capability of the detectors is also demonstrated in X-ray absorption spectroscopy measurements.

A Reliable and Efficient Geographic Routing Scheme for Delay/Disruption Tolerant Networks

The research in this letter focuses on geographic routing in Delay/Disruption Tolerant Networks (DTNs), by considering sparse network density. We explore the Delegation Forwarding (DF) approach to overcome the limitation of the geometric metric which requires mobile node moving towards destination, with the Delegation Geographic Routing (DGR) proposed. Besides, we handle the local maximum problem of DGR, by considering nodal mobility and message lifetime. Analysis and evaluation results show that DGR overcomes the limitation of the algorithm based on the given geometric metric. By overcoming the limited routing decision and handling the local maximum problem, DGR is reliable for delivering messages before expiration lifetime. Meanwhile, the efficiency of DGR regarding low overhead ratio is contributed by utilizing DF.

Correlations of Bridgman-Grown Cd0.9Zn0.1Te Properties with Different Ampoule Rotation Schemes

A unique ampoule rotation system was developed at the Center for Materials Research at Washington State University for enhancing convection in the cadmium zinc telluride (CZT) melt by applying different ampoule rotation schemes (RS). Experiments were performed with different initial charge material concentrations and rotation parameters (acceleration, speed, rotation time, etc.). The applied speed and acceleration ranged from 30 rpm to 50 rpm and 30 rpm2 to 200 rpm2, respectively. Zinc (Zn) distribution profiles of radial and axial slices from the same regions in the grown ingot were determined by room-temperature photoluminescence mapping. The results demonstrate the effects of ampoule rotation on Zn segregation and growth interface evolution. The most stable interface propagation was obtained when 0.2 atomic percent (at.%) excess tellurium (Te) was used in the initial charge material along with a trapezoidal RS. Uniform radial Zn distribution was achieved using triangular RS, which is because of the interface flatness near the axis. Comparison of secondary phase (SP) generation for different RS and initial excess Te was performed. Closed-container CZT growth was performed using the trapezoidal RS, which resulted in high single-crystal yield with lower-diameter SP near the last-to-freeze region. High-resistivity (on the order of 1010 Ω-cm) crystals were obtained from all the RS. The mobility–lifetime product (μτ)e of electrons for planar detectors was found to be on the order of 3 × 10−3 cm2/V to 5 × 10−3 cm2/V for all the RS with 3.5 at.% excess Te growths.

Mobility lifetime product in doped and undoped nanocrystalline CdSe

This paper reports the effect of doping on the charge transport in nanocrystalline CdSe thin film. The X-ray study confirms that the doping is achieved and the physical properties are improved. The energy resolution of a semiconductor radiation detector depends on the charge transport properties of the semiconductor and the mobility-lifetime (μτ) product is a key figure of merit for the charge transport. μτ product in nanocrystalline CdSe, CdSe:In and CdSe:Zn thin films has been estimated from temperature dependence of the photoconductivity, which increases with increase in temperature and doping. Also, μτ product of electrons in pure and doped nanocrystalline CdSe thin films has been determined by spectral photoconductivity at different applied voltages. Both the μτ and photoconductivity increase linearly with the bias voltage but the wavelength dependence remains qualitatively similar in all samples. The μτ products increase at photon energies>energy gap, which indicates that the recombination process depends on the excitation energy. The doped CdSe thin films have higher drift length in comparison with undoped films which suggest that these thin films can be used in charge collecting devices.

Relation of Nanostructure and Recombination Dynamics in a Low‐Temperature Solution‐Processed CuInS2 Nanocrystalline Solar Cell

The understanding and control of nanostructures with regard to transport and recombination mechanisms is of key importance in the optimization of the power conversion efficiency (PCE) of solar cells based on inorganic nanocrystals. Here, the transport properties of solution‐processed solar cells are investigated using photo‐CELIV (photogenerated charge carrier extraction by linearly increasing voltage) and transient photovoltage techniques; the solar cells are prepared by an in‐situ formation of CuInS2 nanocrystals (CIS NCs) at the low temperature of 270 °C. Structural and morphological analyses reveal the presence of a metastable CuIn5S8 phase and a disordered morphology in the CuInS2 nanocrytalline films consisting of polycrystalline grains at the nanoscale range. Consistent with the disordered morphology of the CIS NC thin films, the CIS NC devices are characterized by a low carrier mobility. The carrier density dynamic indicates that the recombination kinetics in these devices follows the dispersive bimolecular recombination model and does not fully behave in a diffusion‐controlled manner, as expected by Langevin‐type recombination. The mobility–lifetime product of the charge carriers properly explains the performance of the thin (200 nm) CIS NC solar cell with a high fill‐factor of 64% and a PCE of over 3.5%.

Photoconductivity in the Chalcohalide Semiconductor, SbSeI: a New Candidate for Hard Radiation Detection

We investigated an antimony chalcohalide compound, SbSeI, as a potential semiconductor material for X-ray and γ-ray detection. SbSeI has a wide band gap of 1.70 eV with a density of 5.80 g/cm(3), and it crystallizes in the orthorhombic Pnma space group with a one-dimensional chain structure comprised of infinite zigzag chains of dimers [Sb2Se4I8]n running along the crystallographic b axis. In this study, we investigate conditions for vertical Bridgman crystal growth using combinations of the peak temperature and temperature gradients as well as translation rate set in a three-zone furnace. SbSeI samples grown at 495 °C peak temperature and 19 °C/cm temperature gradient with 2.5 mm/h translation rate produced a single phase of columnar needlelike crystals aligned along the translational direction of the growth. The ingot sample exhibited an n-type semiconductor with resistivity of ∼10(8) Ω·cm. Photoconductivity measurements on these specimens allowed us to determine mobility-lifetime (μτ) products for electron and hole carriers that were found to be of similar order of magnitude (∼10(-4) cm(2)/V). Further, the SbSeI ingot with well-aligned, one-dimensional columnar needlelike crystals shows an appreciable response of Ag Kα X-ray.

On Ultrafast Photoconductivity Dynamics and Crystallinity of Black Silicon

We investigate the carrier dynamics of thin films of black silicon, amorphous hydrogenated silicon which under laser annealing forms a microstructured surface with extremely high broadband optical absorption. We use Raman spectroscopy to determine the degree of crystallinity of the annealed surfaces, and investigate the dependence on crystallinity and fabrication method of the photoconductivity. Time-resolved THz spectroscopy is used to determine the evolution of the carrier scattering time and confinement of carriers on the picosecond time scale. We conclude that a fabrication method with high energy leading edge of the annealing laser results in black silicon with the largest photon-to-electron conversion efficiency, largest mobility, and longest carrier lifetime.