Distribution Of Tail States Research Articles

Opto-electronic properties of solution-processed zinc oxide thin films: role of solvents and doping

Undoped zinc oxide (ZnO) and nitrogen-doped zinc oxide (NZO) thin films were prepared on transparent conducting oxide-coated glass substrates by employing sol–gel technique. The effect of different solvents and nitrogen doping on the optical, structural, and electrical properties was investigated by UV–visible absorption spectroscopy, atomic force microscopy (AFM), X-ray diffraction (XRD), profilometry, and Hall effect studies. ZnO films yielded transmittance above 85% and the bandgap of ZnO thin films decreased with doping. XRD pattern confirmed hexagonal wurtzite structure of ZnO. NZO thin films were found to be in the nano-thin film phase with thickness of 40 nm. Hall effect studies yielded carrier concentration of 1.2 × 1015 cm−3 and 2.03 × 1014 cm−3, respectively, for undoped and doped ZnO thin films. The changes in vibrational modes of ZnO due to nitrogen doping were detected using Fourier transform infrared (FTIR) analysis. It was found that p-type doping, leading to an improved surface morphology, led to a reduction in optical bandgap and an increased charge carrier mobility. The choice of the solvent was found to have a profound influence on the surface morphology, optical bandgap, tail states distribution, and charge carrier mobility.

Trap-state suppression and band-like transport in bilayer-type organic semiconductor ultrathin single crystals

Fundamental study of the carrier transport in both the channel layer and the electrode-channel contact of organic semiconductor crystals is indispensable for achieving high-performance organic field-effect transistors. In this paper, we report the temperature-dependent carrier transport of high-mobility organic semiconductors called $2\text{--}\text{decyl}\text{--}7\text{-phenyl-}[1]\text{benzothieno}[3,2\text{--}b][1]\mathrm{benzothiophene}$ (Ph-BTBT-C10) and $3\text{-decyl-}9\text{-phenyl-}[1]\text{benzothieno}[3,2\text{--}b]\text{naphtho}[2,3\text{--}b]\text{thiophene}$ (Ph-BTNT-C10). The use of single-crystal films with controlled bilayer-number thickness enabled the simultaneous study of intralayer and interlayer transport at cryogenic temperatures. Four-probe measurement of two-bilayer- and three-bilayer-thick films of Ph-BTBT-C10 suggests that the access resistance is dominated by tunneling transport across the insulating alkyl-chain layers. Single-crystal thin-film transistors of these materials showed band-like carrier transport down to 80 K and the carrier mobility of Ph-BTBT-C10 reached $34\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{V}\phantom{\rule{0.16em}{0ex}}\mathrm{s}$. Detailed analysis of the low-temperature characteristics revealed small activation energy of approximately 5 meV and a sharp distribution of band tail states. These findings suggest that high crystallinity owing to the bilayer-type crystal structure effectively suppresses the localization of gate-induced carriers.

Charge transport and dynamic response of organic and polymer transistors

It is shown that the dynamic response of a thin-film transistor (TFT) comprises a polymer or organic semiconductor and can be directly correlated with charge transport parameters. Charge transport is assumed to take place in accordance with the often employed multiple trap and release model. We can simulate the non-quasistatic response of TFTs for devices in which the mobility is not constant, but depends on carrier density and other parameters. The previous model of Burns has been significantly improved and made applicable to organic and polymer transistors. Our approach works for an arbitrary density of states including both a relatively shallow exponential distribution of tail states and deeper Gaussian trap states. We set the gate voltage, VG, in our TFTs to be zero. The source voltage, VS, is varied and, thus, the value of gate-to-source voltage is varied. The predictive power of the model that is presented in this paper will be very useful for accurate device modeling and for charge transport studies.

Tail state limited photocurrent collection of thick photoactive layers in organic solar cells

We analyse organic solar cells with four different photoactive blends exhibiting differing dependencies of short-circuit current upon photoactive layer thickness. These blends and devices are analysed by transient optoelectronic techniques of carrier kinetics and densities, air photoemission spectroscopy of material energetics, Kelvin probe measurements of work function, Mott-Schottky analyses of apparent doping density and by device modelling. We conclude that, for the device series studied, the photocurrent loss with thick active layers is primarily associated with the accumulation of photo-generated charge carriers in intra-bandgap tail states. This charge accumulation screens the device internal electrical field, preventing efficient charge collection. Purification of one studied donor polymer is observed to reduce tail state distribution and density and increase the maximal photoactive thickness for efficient operation. Our work suggests that selecting organic photoactive layers with a narrow distribution of tail states is a key requirement for the fabrication of efficient, high photocurrent, thick organic solar cells.

High-mobility, trap-free charge transport in conjugated polymer diodes

Charge transport in conjugated polymer semiconductors has traditionally been thought to be limited to a low-mobility regime by pronounced energetic disorder. Much progress has recently been made in advancing carrier mobilities in field-effect transistors through developing low-disorder conjugated polymers. However, in diodes these polymers have to date not shown much improved mobilities, presumably reflecting the fact that in diodes lower carrier concentrations are available to fill up residual tail states in the density of states. Here, we show that the bulk charge transport in low-disorder polymers is limited by water-induced trap states and that their concentration can be dramatically reduced through incorporating small molecular additives into the polymer film. Upon incorporation of the additives we achieve space-charge limited current characteristics that resemble molecular single crystals such as rubrene with high, trap-free SCLC mobilities up to 0.2 cm2/Vs and a width of the residual tail state distribution comparable to kBT.

Full characterization of electronic transport properties in working polymer light-emitting diodes via impedance spectroscopy

The electron and hole drift mobilities of organic semiconductor layers, localized tail state distributions, and bimolecular recombination constants in working polymer light-emitting diodes (PLEDs) are determined simultaneously using impedance spectroscopy (IS). The organic light-emitting layers of these PLEDs are composed of poly(9,9-dioctylfluorene-alt-benzothiadiazole). Electron and hole transit time effects are observed in the capacitance-frequency characteristics of the PLEDs, and their drift mobilities are determined over wide temperature and electric field ranges. The drift mobilities exhibit thermally activated behavior, and the localized tail state distributions from the conduction band and valence band mobility edges are then determined from analysis of the electric field dependences of the activation energies. The bimolecular recombination constants are determined from the inductive response of the impedance-frequency characteristics. The IS technique is also applicable to degradation analysis of the PLEDs; changes in the mobility balance, the localized tail state distributions, and the bimolecular recombination constant caused by aging are all shown.

Experimental validation of the surface state distribution model in the Suzuki theory to qualify the thin film surface materials

The relevance of the theoretical Suzuki’s model of surface state density to determine the surface state distribution at the active layer/silicon dioxide interfaces is carried out in polycrystalline silicon thin film transistors (TFTs) issued from two different crystallization technologies of the active layer: furnace solid phase crystallized (FSPC) and laser solid phase crystallized (LSPC) TFTs. The experimental validation of this model is demonstrated using the field effect conductance method. Results show that distribution is higher for the FSPC TFTs in relationship with the process crystallization of the active layer. In addition, it is shown that the theoretical surface state model allows discriminating dangling bond state and tail state distributions and acts as relevant model to qualify the surface of thin film material.

Modeling and characterization of the low frequency noise behavior for amorphous InGaZnO thin film transistors in the subthreshold region

An analytical model of the low-frequency noise (LFN) for amorphous InGaZnO (a-IGZO) thin film transistors (TFTs) in the subthreshold region is developed. For a-IGZO TFTs, relations between the device noise and the subgap defects are characterized based on the dominant multiple trapping and release (MTR) mechanism. The LFN is considered to be contributed from trapping/detrapping of carriers both into the border traps and the subgap density of states (DOS). It is revealed that the LFN behavior of a-IGZO TFTs in the subthreshold region is significantly influenced by the distribution of tail states, where MTR process prevails. The 1/f α (with α < 1) spectrum of the drain current noise is also related to the characteristic temperature of the tail states. The new method is introduced to calculate the LFN of devices by extracting the LFN-related DOS parameters from the current-voltage characteristics.

Analysis of low frequency noise characteristics inp-type polycrystalline silicon thin film transistors

Low frequency noises in the p-type polycrystalline silicon thin film transistors are investigated. It shows a pure 1/f[Formula: see text] (with [Formula: see text] near one) noise behavior which can be explained by emission and trapping processes of carriers between trapping states. Subsequently, the gate voltage-dependent drain current noise power spectral densities closely follow the mobility fluctuation model, and the average Hooge’s parameter is then extracted. By considering traditional tunneling processes, the flat-band voltage spectral density is extracted and the concentration of traps in the grain boundary is calculated to be [Formula: see text]. By converting the frequency to tunneling depth of carriers in the gate oxide, the spatial distribution of gate oxide trapped charges are obtained. Finally, the distribution of localized states in the energy band is extracted. The experimental results show an exponential deep states and tail states distribution in the band gap while [Formula: see text] is about [Formula: see text], [Formula: see text] is [Formula: see text][Formula: see text]617 K, [Formula: see text] is [Formula: see text] and [Formula: see text] is [Formula: see text][Formula: see text]265 K.

Localized tail state distribution and hopping transport in ultrathin zinc-tin-oxide thin film transistor

Carrier transport properties of solution processed ultra thin (4 nm) zinc-tin oxide (ZTO) thin film transistor are investigated based on its transfer characteristics measured at the temperature ranging from 310 K to 77 K. As temperature decreases, the transfer curves show a parellel shift toward more postive voltages. The conduction mechanism of ultra-thin ZTO film and its connection to the density of band tail states have been substantiated by two approaches, including fitting logarithm drain current (log ID) to T−1/3 at 310 K to 77 K according to the two-dimensional Mott variable range hopping theory and the extraction of density of localized tail states through the energy distribution of trapped carrier density. The linear dependency of log ID vs. T−1/3 indicates that the dominant carrier transport mechanism in ZTO is the variable range hopping. The extracted value of density of tail states at the conduction band minimum is 4.75 × 1020 cm−3 eV−1 through the energy distribution of trapped carrier density. The high density of localized tail states in the ultra thin ZTO film is the key factor leading to the room-temperature hopping transport of carriers among localized tail states.

Characterization of the dominating bulk recombination in bulk-heterojunction blends using photoinduced absorption

In this work we show how to clarify the dominating bulk recombination in organic solar cells by using photoinduced absorption. We show how to use the intensity and frequency dependence of the in-phase and quadrature signals to obtain the effective reaction order. For trap-assisted recombination, we can show using a multiple trapping and retrapping model with an exponential tail-state distribution that a temperature dependent reaction order is obtained which allows for determination of the characteristic energy of the exponential distribution of trap-states. In the model system pBTTT:PC60BM, we show that trap-assisted recombination is the dominating bulk recombination in 1:1 blends with a characteristic energy of the exponential trap distribution Ech=44±5 meV. The 1:4 blend, on the other hand, shows temperature independent behavior in good agreement with a dominating 2D Langevin bulk recombination.

Impact of a Low Concentration of Dopants on the Distribution of Gap States in a Molecular Semiconductor

We investigate the distribution of valence and tail states in copper phthalocyanine (CuPc) upon the introduction of minute amounts of the p-dopant molybdenum tris[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd)3), using a combination of electron spectroscopy and carrier transport measurements. Density of gap states, conductivity, and hole-hopping activation energy are measured. We observe the progressive filling (and deactivation) of the deepest tail states by charges introduced by the dopants, as well as significant broadening of the CuPc density of states. Simulations relate this broadening to the electrostatic and structural disorder induced by the dopant in the CuPc matrix.

Extraction of density of localized states in indium zinc oxide thin film transistor

Density of localized states (DOS) over the band-gap determines the electrical and instability characteristics in the indium zinc oxide thin film transistor (IZO TFT). In order to propose an accurate extraction method for DOS in the bulk region, low frequency noise and multi-frequency capacitance voltage characteristics are measured and analyzed in this paper. Firstly, the relationship between surface potential and gate voltage is extracted based on subthreshold I-V characteristics. The extraction results show that the surface potential increases with the increase of gate voltage in the sub-threshold region. When the Fermi level is close to the bottom of conduction band, the increase of surface potential should be saturated. Secondly, drain current noise power spectral densities in the IZO TFTs under different operation modes are measured. Based on carrier number fluctuation mechanism, the flat-band voltage noise power spectral density is extracted and localized state near IZO/SiO2 interface is then calculated. By considering the emission and trapping processes of carriers between localized states, the distribution of bulk trap density in the band-gap is extracted based on low frequency noise measurement results. The experimental results show an exponential tail state distribution in the band-gap while NTA is about 3.421020 cm-3eV-1 and TTA is about 135 K. Subsequently, contact resistances are then extracted by combining capacitance-voltage characteristics with I-V characteristics in the linear region. The extrinsic parasitic resistances at gate, source, drain are separated. By considering charges trapped in the localized states and free carriers, the distributions of deep states and tail states in the active layer of IZO TFT are extracted through multi-frequency capacitance-voltage characteristics. The experimental results also show an exponential deep state and tail state distribution in the band-gap while NDA is about 5.41015 cm-3eV-1, TDA is about 711 K, NTA is about 1.991020 cm-3eV-1, and TTA is about 183 K. The above two proposed extraction methods are compared and analyzed. The deviation between two extraction results may relate to the existence of neutral trap in the gate dielectric which is also an important source of low frequency noise in the IZO TFT.

Full Analytical Evaluation of the Einstein Relation for Disordered Semiconductors

We present a simple analytical method for the evaluation of the Einstein relation for disordered semiconductors with exponential distribution of tail states. The proposed analytical method is based on the binomial expansion theorems, and the calculation result permitted an accurate evaluation of the validity of the analytical approximation, as well as further improvement of the theoretical formulas. The accuracy and the efficiency of the obtained formulas of the Einstein relation for disordered semiconductors are demonstrated with the comparison of the numerical and analytical results. The formulas can be used to analyze the experimental results.

Band tail recombination in polymer:fullerene organic solar cells

Recombination through band tail localized states is studied analytically and by measurement of the forward-bias dark current as a function of temperature in three different organic bulk-heterojunction solar cells. The Shockley-Read-Hall mechanism is analyzed for the specific case of recombination between mobile carriers and an exponential distribution of localized band tail states. The analysis gives a simple relation between the dark current ideality factor and the band tail slope. Assumptions of the model are verified by numerical drift-diffusion modeling. Diode current-voltage measurements give good agreement with the analytical model, confirming that the band tail recombination mechanism applies to at least some organic solar cells. Deep traps provide a secondary recombination channel in some devices.

Recombination dynamics of a localized exciton bound at basal stacking faults within the m-plane ZnO film

We investigated the carrier dynamics near basal stacking faults (BSFs) in m-plane ZnO epitaxial film. The behaviors of the type-II quantum wells related to the BSFs are verified through time-resolved and time-integrated photoluminescence. The decay time of the emission of BSFs is observed to have a higher power law value and longer decay time than the emission of the donor-bound excitons. The spectral-dependent decay times reveal a phenomenon of carriers migrating among band tail states, which are related to the spatial distribution of the type-II quantum wells formed by the BSFs. A high density of excited carriers leads to a band bending effect, which in turn causes a blue-shift of the emission peak of BSFs with a broadened distribution of band tail states.

Dependence of the dynamics of exciton transport, energy relaxation, and localization on dopant concentration in disordered C545T-doped Alq_3 organic semiconductors

The dynamics of exciton transport, energy relaxation, and localization in disordered Tris(8-quinolinolato)-aluminum (Alq3) organic semiconductors with different 10-(2-benzothiazolyl)-1, 1, 7, 7-tetramethyl-2, 3, 6, 7-tetrahydro-lH, 5H, 11H-benzo[l] pyrano[6, 7, 8-і ј] quinolizin-11-one (C545T) dopant concentrations were reported. The increasing trend of the Stokes Shift (737~764 meV) with increasing dopant concentrations is consistent with the degree of disorder and a more effective Forster energy transfer from Alq3 to C545T. In addition, a dynamic scenario representing possible paths of the exciton transport (hopping) among host molecules and the competition of the exciton transport from host molecules into the deep site traps (localized states) and aggregations was proposed to elucidate the recombination dynamics in disordered C545T-doped Alq3 organic semiconductors. The early-stage decay times, decreasing with increasing emission photon energy, show the characteristic of the exciton hopping and energy relaxation processes within the inhomogeneously broadened density-of-states in organic semiconductors. Because the current-voltage (J-V) characteristics of the C545T-doped organic light emitting diode (OLED) fitted well with the power law J~Vm (m>2), the carrier transport behaviours can be described by the trapped-control mode and the tail state distribution can be approximated by the exponential trap distribution. With the approximation of an exponential distribution for the tail states, the characteristic energy (Em), radiative recombination lifetime (τrad), and localization depth (E0) associated with the dynamics of exciton energy relaxation and localization can be quantitatively determined. The much larger E0 (40~120 meV), increasing with the dopant concentration, than other disordered semiconductors (2~34 meV) indicates a strong localization effect in such doped organic semiconductors. Also, the strong dependence of Em on the dopant concentration shows that a relatively small dopant concentration can enhance the degree of disorder and greatly affect the recombination dynamics. Furthermore, the observed optical properties and dynamic scenario of C545T-doped Alq3 films are found to be consistent with the carrier transport and recombination dynamics of C545T-doped Alq3 OLEDs.

Analytical Drain Current Model for Poly-Si Thin-Film Transistors Biased in Strong Inversion Considering Degradation of Tail States at Grain Boundary

This paper presents an analytical hot-carrier effect model for n-channel poly-Si thin-film transistors biased in strong inversion based on degradation of tail states at grain boundary near drain after stress. Via a stress-time-dependent tail state model, the degradation of tail state distribution and drain current after stress could be predicted as verified by the experiment data. Based on this model, both the tail state density and the characteristic decay energy of the tail state distribution in the damaged region increase with the stress time, which determine the degradation of the drain current.

Localized tail state distribution in amorphous oxide transistors deduced from low temperature measurements

In this paper, we extract density of localized tail states from measurements of low temperature conductance in amorphous oxide transistors. At low temperatures, trap-limited conduction prevails, allowing extraction of the trapped carrier distribution with energy. Using a test device with a-InGaZnO channel layer, the extracted tail state energy and density at the conduction band minima are 20 meV and 2 × 1019 cm−3 eV−1, respectively, which are consistent with values reported in the literature. Also, the field-effect mobility as a function of temperature from 77 K to 300 K is retrieved for different gate voltages, yielding the activation energy and the percolation threshold.

Transient photoconductivity responses in amorphous In-Ga-Zn-O films

We studied the photoconductivity responses in amorphous In-Ga-Zn-O (a-IGZO) films using a time-resolved microwave photoconductivity decay (μ-PCD) technique. The a-IGZO film characteristics are correlated with three components in the photoconductivity response: the peak value and two decay constants. The peak value originated from the density of the photo-generated free carriers through carrier generation and recombination processes during laser pulse irradiation. Power law characteristics indicated that the peak values are attributed to recombination process related to the exponential distribution of the conduction band tail states. After the laser pulse was turned off, the reflectivity signal decreased rapidly, indicating fast recombination of the photo-generated carriers. This fast decay component is suggested to be related to the recombination processes through the deep level states. Following the fast decay, a slow decay with a decay constant on the order of microseconds appeared. This slow decay was attributed to the reemission of trapped carriers with an activation energy of ∼0.2 eV. In addition, both the fast and slow decays for the wet annealed a-IGZO film were longer than those of the as-deposited a-IGZO film. The decay constants are considered to reflect the density of the subgap states that act as trapping or recombination centers. The μ-PCD method provides a useful estimation of the film quality, such as the density of the defect states, and the physical properties of electronic devices using a-IGZO films.