Photogenerated Carrier Concentration Research Articles

Study of the Contributions of Donor and Acceptor Photoexcitations to Open Circuit Voltage in Bulk Heterojunction Organic Solar Cells

One of the key parameters in determining the power conversion efficiency (PCE) of bulk heterojunction (BHJ) organic solar cells (OSCs) is the open circuit voltage . The processes of exciting the donor and acceptor materials individually in a BHJ OSC are investigated and are found to produce two different expressions for . Using the contributions of electron and hole quasi-Fermi levels and charge carrier concentrations, the two different expressions are derived as functions of the energetics of the donor and acceptor materials and the photo-generated charge carrier concentrations, and calculated for a set of donor-acceptor blends. The simultaneous excitation of both the donor and acceptor materials is also considered and the corresponding , which is different from the above two, is derived. The calculated from the photoexcitation of the donor is found to be somewhat comparable with that obtained from the photoexcitation of the acceptor in most combinations of the donor and acceptor materials considered here. It is also found that the calculated from the simultaneous excitations of donor and acceptor in BHJ OSCs is also comparable with the other two . All three thus derived produce similar results and agree reasonably well with the measured values. All three depend linearly on the concentration of the photoexcited charge carriers and hence incident light intensity, which agrees with experimental results. The outcomes of this study are expected to help in finding materials that may produce higher and hence enhanced PCE in BHJ OSCs.

Structure and thickness-dependent gas sensing responses to NO2 under UV irradiation for the multilayered ZnO micro/nanostructured porous thin films

The structure and thickness of the chemiresistive thin films can significantly affect their gas sensing performances for the heating-typed sensors. Under light irradiation, however, their influences are still to be addressed. In present paper, the multilayered ZnO porous thin films with different (three types) micro/nanostructures and controllable thickness are fabricated via layer by layer construction of the self-assembled colloidal-layers. The structural and thickness effects of such films on the gas sensing performances to NO2 under ultraviolet (UV) illumination are experimentally studied. It has been found that under UV irradiation, the responses of the ZnO porous thin films to NO2 increase upto the maxima with the rising film thickness. Further increasing the thickness would lead to the insignificantly or gradually decreasing responses. The film thicknesses corresponding to the maximal responses are associated with the porous structures and the porosity of the thin films. The films with the higher porosity would lead to the higher maximal responses and the larger corresponding film-thicknesses, or vice versa. Such thickness and porous-structure dependences of the responses are attributed to the ever-decaying light intensity (and hence ever-decreasing photo-generated carrier concentration) in the films along the depth from the films’ surface. This study is of importance in design and development of the light illuminating-typed gas sensing devices with high performances.

(Invited) Contacts to and Contact-Less Characterization of Transition Metal Dichalcogenides: Challenges Abound

Transition metal dichalcogenides (TMD) such as MoS2 or WSe2 have often been billed as the semiconducting counterpart – or even successor - of graphene. While the semiconducting nature of the corresponding bulk materials is beyond doubt, the understanding of what semiconducting means for a 2 dimensional material is emerging only slowly. In this presentation I will talk about the formation of contacts to TMD materials and the associate shift of the Fermi level. Experimental techniques used to obtain the data includes X-ray photoelectron spectroscopy and transport measurements. The results shed light on fundamental challenges of transport and devices utilizing these materials. In addition, I will also talk about surface acoustic wave measurements that reveal photogenerated carrier concentrations in TMD materials. These measurements can be performed without the need for metal contacts and, thus, do conceptually not suffer from the limitations described above.

Influence of active layer thickness on contribution of hole and electron trapping to threshold voltage instability in organic field effect transistors

Effect of active layer thickness on threshold voltage stability was studied. Threshold voltage instability was found to be minimum for an optimum thickness of 15nm when operated under normal room light conditions. Influence of active layer thickness on individual contribution of hole and electron trapping effect to hysteresis was estimated. Hole and electron trap concentrations were found to exhibit different pattern of variation with thickness compared to each other. Film with 15nm thickness exhibited minimum hole trapping and maximum electron trapping effect. This could be attributed to reduced grain boundary density and availability of large photogenerated carrier concentration at the dielectric/semiconductor interface. Along with electron trap, hole trap concentrations were also shown to be significantly influenced by photogenerated carrier concentration at the interface. For further increase in thickness, increase in hole trapping and reduction in electron trapping were observed again. This could be attributed to (a) increase in grain boundary density due to volume effect and (b) lesser number of photogenerated carriers at the interface due to limitations in exciton quenching length. Hole and electron trapping effect were suggested to be mutually affecting each other, depending on active layer thickness. Effect of active layer thickness was further confirmed with frequency and voltage dependent capacitance measurements.

Advanced Photoconductive Terahertz Optoelectronics Based on Nano-Antennas and Nano-Plasmonic Light Concentrators

High power sources and high sensitivity detectors are highly in demand for terahertz imaging and sensing systems. Use of nano-antennas and nano-plasmonic light concentrators in photoconductive terahertz sources and detectors has proven to offer significantly higher terahertz radiation powers and detection sensitivities by enhancing photoconductor quantum efficiency while maintaining its ultrafast operation. This is because of the unique capability of nano-antennas and nano-plasmonic structures in manipulating the concentration of photo-generated carriers within the device active area, allowing a larger number of photocarriers to efficiently contribute to terahertz radiation and detection. An overview of some of the recent advancements in terahertz optoelectronic devices through use of various types of nano-antennas and nano-plasmonic light concentrators is presented in this article.

Influence of equilibrium charge reservoir formation on photo generated charge transport in TiO2/organic devices

Charge transport measurements have been performed using the photo induced charge extraction by linearly increasing voltage (photo-CELIV) technique on indium tin oxide/titanium dioxide/poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester/copper (ITO/TiO2/P3HT:PCBM/Cu) devices. By adjusting the offset voltage such that holes are accumulated at the ITO/TiO2 contact we obtain space charge limited current (SCLC) extraction in the dark. Using photo-generation the current response is limited by SCLC extraction at low carrier concentrations but becomes purely recombination limited at high photo-generated carrier concentration. A 1-D drift diffusion model has been developed to simulate our results and we show that the hole blocking ITO/TiO2 contact is responsible for the SCLC behavior. The highly reduced recombination of charges seen in P3HT:PCBM devices is necessary to obtain the large extraction current transients that are seen in the experimental measurements. By comparing the simulated dark CELIV and photo-CELIV we show that photo-generated extraction is more sensitive towards changes in the surface recombination velocity.

Minimized open-circuit voltage reduction in GaAs/InGaAs quantum well solar cells with bandgap-engineered graded quantum well depths

The use of InGaAs quantum wells with composition graded across the intrinsic region to increase open-circuit voltage in p-i-n GaAs/InGaAs quantum well solar cells is demonstrated and analyzed. By engineering the band-edge energy profile to reduce photo-generated carrier concentration in the quantum wells at high forward bias, simultaneous increases in both open-circuit voltage and short-circuit current density are achieved, compared to those for a structure with the same average In concentration, but constant rather than graded quantum well composition across the intrinsic region. This approach is combined with light trapping to further increase short-circuit current density.

Spectral and Injection Level Dependence of Recombination Lifetimes in Silicon Measured by Impedance Spectroscopy

The effect of photo-generated carrier concentration on recombination lifetime is studied on silicon wafers using the impedance spectroscopy technique. The induced p <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^+$</tex></formula> -p-n structures are created by depositing semitransparent layers of high (Pd) and low (Al) work function (w.r.t silicon) metal layers on crystalline silicon (c-Si) wafers. The measurements are carried out by illuminating the device from the p <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^+$</tex></formula> -p side. The impedance data are analyzed by assuming a network consisting of a number of RC circuits, which, in turn, give recombination times (τ) controlled by defects in bulk and at the interfaces of the device. The maximum value of the effective recombination lifetime (τ) is found to be 12.3 ± 0.4 μs at an intensity of ∼100 mW/cm <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^2$</tex></formula> , which matches closely with the maximum value 11.4 ± 1.0 μs obtained under forward bias (+ 0.4 V) conditions in the same sample in the dark. These values match well with the effective minority carrier lifetime obtained using the microwave photoconductive decay technique on the same silicon wafer prior to device fabrication.

Hierarchical (0 0 1) facet anatase/rutile TiO2 heterojunction photoanode with enhanced photoelectrocatalytic performance

A hierarchical heterojunction TiO2 photoanode with large surface/body ratio is reported to exhibit high oxidation activity due to the constructing of anatase TiO2 with exposed (001) facets. The mixed-phase photoanode is fabricated through surfactant-assisted anchoring ultrathin anatase nanosheets on vertically ordered rutile nanorod arrays. This cactaceae-like TiO2 possesses high-exposed (001) facets outer layer, large specific surface area (375m2g−1), efficient photo-to-current conversion (8.2%) and excellent photocatalytic ability to degrade bisphenol A. The greatly promoted photoelectric and photocatalytic performance results from the synergetic effects of the architecture design of high-active (001) facets and hierarchical heterojunctions. The mechanism analysis reveals that the remarkable increase of photogenerated carrier concentration (2.40×1022cm−3) improves photocatalytic activity, by virtue of constructing staggered energy levels, suppressing the recombination of electrons and holes, and extending the electron lifetime (133ms).

Enhanced photovoltaic effect of TiO2-based composite ZnFe2O4/TiO2

Abstract A TiO 2 -based composite, ZnFe 2 O 4 /TiO 2 , was prepared with a commercial TiO 2 (Degussa P25) and ZnFe 2 O 4 nanopowders using a simple method. The composite was characterized by X-ray diffraction (XRD), UV–vis absorption spectrum, surface photovoltage spectroscopy (SPS) and SEM. The composite presents an enhanced photovoltaic effect compared with the pure TiO 2 . The trap distributions of TiO 2 and ZnFe 2 O 4 /TiO 2 composite were investigated and the energy levels of TiO 2 and ZnFe 2 O 4 were analyzed. The results indicate that the enhancement of photovoltaic effect is mainly due to the increase of photo-generated carrier concentration and improvement of the separation efficiency of photo-generated carriers by the addition of ZnFe 2 O 4 .

Extraction of photogenerated charge carriers by linearly increasing voltage in the case of Langevin recombination

Charge extraction by linearly increasing voltage (CELIV) is a powerful and widely used technique for studying charge transport physics, particularly in disordered systems such as organic semiconductors. In this article, we show that CELIV photocurrent transients are strongly dependent on experimental conditions, such as the light intensity and absorption profile. With this in mind, we introduce a universal correction factor that qualitatively extends previously derived CELIV equations, allowing carrier mobility to be estimated at various photogenerated carrier concentrations and, most importantly, photogeneration profiles. In addition, we demonstrate how the CELIV technique can be conveniently used to determine precisely the presence of Langevin bimolecular carrier recombination.

Optical generation of free charge carriers in nanocrystalline tin oxide for gas sensor application

Nanocrystalline films of tin oxides SnO 2 with nanocrystal sizes of about 100 nm and thickness of 600 nm prepared by thermal oxidation of metallic tin layers were investigated by using the infrared (IR) spectroscopy, Raman scattering and current–voltage measurements in different molecular environments. An increase of the absorption in the middle IR spectral range was detected at room temperature after illumination of the samples by UV light with photon energy of 3.3 eV, which was below the band gap of SnO 2 . The effect was explained as an evidence of the absorption by non-equilibrium free charge carriers (electrons) in SnO 2 nanocrystals. The concentration of photo-generated free charge carriers was calculated from the IR-spectroscopy data according to the Drude model and it was up to 10 18 cm −3 . An effect of the molecular adsorption of iodine on the electrical conductivity of the films was investigated by current–voltage measurements, which revealed a strong increase of the sensor response for nanocrystalline SnO 2 under the UV excitation at room temperature. Possible mechanisms of the observed photo-generation of free charge carriers in SnO 2 nanocrystals and feasible applications of this effect in gas sensors are discussed.

Passivation of defects at the SrTiO3/Si interface with H and H2

A silicon surface covered with ∼150 Å of strontium titanium oxide was exposed to both atomic and molecular hydrogen at temperatures between 20 and 300 °C. A rf probe was used to continuously monitor changes in charge-carrier recombination centers at the SrTiO3/Si interface by following the steady-state photogenerated carrier concentration in the silicon. Independent passivation of interfacial defects was observed by both atomic and molecular hydrogen.

Evaluation of Effective Recombination Velocity Related to the Potential Barrier in n/n+ Silicon Epitaxial Wafers

For silicon n/n+ epitaxial wafers (36/0.017 Ωcm), the effective recombination velocity Snn+ related to the potential barrier height at the n/n+ interface has been studied by detecting the leakage currents which are collected at Schottky barrier diodes formed on the substrate. The experimental results obtained are Snn+ =10.6-21.8 cm/s, which increases linearly with increasing photo-generated carrier concentration in the epitaxial layer. These values may be slightly smaller than the real values of effective recombination velocity at the interface, because the recombination effect in the n/n+ transition layer is not taken into account the experimental results. The experimental values are about five times the values expected from a theoretical analysis. These results are discussed in relation to the potential barrier height at the n/n+ junction.

Rate constants for the reaction of H2 with defects at the SiO2/Si(111) interface

A radio frequency probe has been used to monitor changes in charge-carrier recombination centers at a SiO2/Si interface by following the steady-state photogenerated carrier concentration in the silicon. A silicon surface covered with ∼200 Å of thermal oxide was exposed to gaseous molecular hydrogen at temperatures between 135 and 300 °C. The reaction was found to occur with a nonexponential rate law that could be described by a cluster of rate constants governed by a Gaussian distribution of activation energies. At high temperatures, the dominant reaction was found to be that of “Pb” centers with a rate law that is consistent with that previously obtained by electron paramagnetic resonance. At low temperatures the reaction is dominated by the passivation of a carrier recombination center with a much lower activation energy. This new species makes up about 25% of the total recombination center concentration at the Si/SiO2 interface. Almost 20% of the recombination centers could not be passivated by H2.

Behavior of Si Photoelectrodes under High Level Injection Conditions. 1. Steady-State Current−Voltage Properties and Quasi-Fermi Level Positions under Illumination

The behavior of the quasi-Fermi levels of electrons and holes at various semiconductor/liquid interfaces has been probed through the use of thin, high purity, low dopant density single crystal Si photoelectrodes. Since standard Air Mass 1.5 illumination is sufficient to produce high level injection conditions in such samples, minimal electric fields can be present near the solid/liquid interface. Under these conditions, efficient charge separation relies on establishment of kinetic asymmetries at the back contacts while effectively sustaining photogenerated carrier concentration gradients in the photoelectrode. These conditions were achieved for Si/CH3OH interfaces in contact with the 1,1‘-dimethylferrocene+/0, cobaltocene+/0, methyl viologen2+/+, and decamethylferrocene+/0 redox couples. For redox couples having energies near the top of the Si valence band, such as 1,1‘-dimethylferrocene+/0, the sample acted like an n-type photoelectrode, yielding large photovoltages for collection of electrons at the ba...

Monitoring of the formation and removal of bulk, surface, and interfacial carrier traps on silicon(100)

An RF probe was used to monitor the steady-state photo-generated carrier concentration in silicon with and without an oxide layer. The changes in this steady-state concentration during exposure to gaseous molecular and atomic species such as He, H2, O2, H, and O at temperatures between 25 and 450 °C are interpreted in terms of changes in the bulk silicon, interface, and exposed surface. The initial experiments established the relationship between the carrier concentration and the RF-probe signal, and distinguished between changes in the bulk, and on silicon surfaces and interfaces. It was found that in the case of hydrogen, bulk passivation and depassivation by H2 can be observed only at elevated temperatures, with or without an oxide layer. H-atom depassivation can be observed at all temperatures and is irreversible at room temperature. The formation of an oxide layer by exposure to O atoms at 25 and 450 °C was followed, and the oxides formed were then treated in an attempt to create an SiO2/Si interface with an acceptable density of interfacial states.

Relaxation of magnetization in Cd1−xMnxTe diluted magnetic semiconductors under illumination

Relaxation of thermoremanent magnetization (TRM) of Cd1−xMnxTe diluted magnetic semiconductors (DMS) in the spin-glass state have been studied under light illumination. The relaxation of TRM can be described well by a power law decay, M(t)=M(t0)t−α (t≳t0, t0∼2 s). The variations of the decay parameter α with the illumination light intensity has been measured and a relation which indicates that α is proportional to the photogenerated carrier concentration n has been observed.

Relaxation of spin-glass magnetization in Cd1-xMnxTe diluted magnetic semiconductors.

Relaxation of thermal-remanent magnetization and isothermal remanent magnetization of spin glass in ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te diluted magnetic semiconductors have been studied at different conditions. Magnetization relaxation can be described by a power-law decay, M(t)=${\mathit{M}}_{0}$${\mathit{t}}^{\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\alpha}}}$ (tg${\mathit{t}}_{0}$ and ${\mathit{t}}_{0}$=2 s) with small values of \ensuremath{\alpha} or a logarithmic decay M(t)=${\mathit{M}}_{0}$(1-\ensuremath{\alpha} lnt) (tg${\mathit{t}}_{0}$ and ${\mathit{t}}_{0}$=2 s). Temperature and applied magnetic-field dependencies of the decay parameter \ensuremath{\alpha} have been measured. The above-band-gap photoexcitation has been used to generate free carriers (electrons and holes) in the sample and their effects on the spin-spin interaction of Mn ions have been studied. The dependence of the power-law decay parameter \ensuremath{\alpha} on excitation light intensity has also been measured. It is found that \ensuremath{\alpha} is proportional to the photogenerated carrier concentration. Furthermore, aging effects in the spin-glass state have also been studied by varying the time duration of the applied magnetic field. The transient responses of magnetization in the spin-glass state upon applying a magnetic field in the dark and under illumination have been measured and are found to follow a power-law time dependence, M(t)=${\mathit{M}}_{0}$+${\mathit{M}}_{1}$${\mathit{t}}^{\mathrm{\ensuremath{\beta}}}$. Temperature and the applied magnetic-field dependencies of the transient parameter \ensuremath{\beta} have been measured. A mechanism, involving an increased spin domain size under light illumination via free-carrier-Mn spin interaction, can explain our results very well.

Observation of charge screening in semiconductor nanocrystals.

Charge screening in semiconductor nanocrystals is observed using a technique based on the optical erasure of optically encoded second-harmonic generation in semiconductor microcrystallite-doped glasses. The optical erasure rate for illumination above the band gap (\ensuremath{\Elzxh}\ensuremath{\omega}&gt;${\mathit{E}}_{\mathit{g}}$) is dominated by the recombination of charges trapped outside the nanocrystals with photogenerated free carriers. The measured erasure rate is found to decrease with increasing intensity at photogenerated carrier concentrations which correspond to screening lengths of the order of the crystallite diameter. Rate calculations based on the recombination of electrons, trapped outside the crystallite, with quantized hole states, perturbed by a screened electric field, correctly predict the observed behavior.