Does the Temperature Dependence of the Charge Carrier Mobility in Disordered Organic Semiconductors at Large Carrier Concentrations Obey the Meyer-Neldel Compensation Law?

The temperature-activated charge transport in disordered organic semiconductors at large carrier concentrations has been thoroughly considered, by using a recent analytical model [Phys.Rev.B 76, 045210 (2007)] assuming a Gaussian density-of-states (DOS) distribution and Miller-Abrahams jump rates. We demonstrate that the apparent Meyer-Neldel compensation rule is recovered with regard for the temperature dependences of the charge carrier mobility upon varying the carrier concentration, but not for varying the DOS distribution width. We show that this phenomenon is entirely due to the evolution of the occupational DOS distribution as a function of the state filling. Predictions of the model are in a quantitative agreement with available experimental results.status: publishe

Electric field dependence of charge carrier hopping transport within the random energy landscape in an organic field effect transistor

We extended our analytical effective medium theory [ Phys. Rev. B 81 045202 (2010)] to describe the temperature-dependent hopping charge carrier mobility at arbitrary electric fields in the large carrier density regime. Special emphasis was made to analyze the influence of the lateral electric field on the Meyer–Neldel (MN) phenomenon observed when studying the charge mobilities in thin-film organic field-effect transistors (OFET). Our calculations are based on the average hopping transition time approach, generalized for large carrier concentration limit finite fields, and taking into account also spatial energy correlations. The calculated electric field dependences of the hopping mobility at large carrier concentrations are in good agreement with previous computer simulations data. The shift of the MN temperature in an OFET upon applied electric field is shown to be a consequence of the spatial energy correlation in the organic semiconductor film. Our calculations show that the phenomenological Gill equation is clearly inappropriate for describing conventional charge carrier transport at low carrier concentrations. On the other hand a Gill-type behavior has been observed in a temperature range relevant for measurements of the charge carrier mobility in OFET structures. Since the present model is not limited to zero-field mobility, it allows a more accurate evaluation of important material parameters from experimental data measured at a given electric field. In particular, we showed that both the MN and Gill temperature can be used for estimating the width of the density of states distribution.status: publishe

Unified description for hopping transport in organic semiconductors including both energetic disorder and polaronic contributions

We developed an analytical model to describe hopping transport in organic semiconductors including both energetic disorder and polaronic contributions due to geometric relaxation. The model is based on a Marcus jump rate in terms of the small-polaron concept with a Gaussian energetic disorder, and it is premised upon a generalized effective medium approach yet avoids shortcomings involved in the effective transport energy or percolation concepts. It is superior to our previous treatment [ Phys. Rev. B 76 045210 (2007)] since it is applicable at arbitrary polaron activation energy Ea with respect to the energy disorder parameter σ. It can be adapted to describe both charge-carrier mobility and triplet exciton diffusion. The model is compared with results from Monte Carlo simulations. We show (i) that the activation energy of the thermally activated hopping transport can be decoupled into disorder and polaron contributions whose relative weight depend nonlinearly on the σ/Ea ratio, and (ii) that the choice of the density of occupied and empty states considered in configurational averaging has a profound effect on the results of calculations of the Marcus hopping transport. The σ/Ea ratio governs also the carrier-concentration dependence of the charge-carrier mobility in the large-carrier-concentration transport regime as realized in organic field-effect transistors. The carrier-concentration dependence becomes considerably weaker when the polaron energy increases relative to the disorder energy, indicating the absence of universality. This model bridges a gap between disorder and polaron hopping concepts.status: publishe

Interplay between hopping and band transport in high-mobility disordered semiconductors at large carrier concentrations: The case of the amorphous oxide InGaZnO

We suggest an analytic theory based on the effective medium approximation (EMA) which is able to describe charge-carrier transport in a disordered semiconductor with a significant degree of degeneration realized at high carrier concentrations, especially relevant in some thin-film transistors (TFTs), when the Fermi level is very close to the conduction-band edge. The EMA model is based on special averaging of the Fermi-Dirac carrier distributions using a suitably normalized cumulative density-of-state distribution that includes both delocalized states and the localized states. The principal advantage of the present model is its ability to describe universally effective drift and Hall mobility in heterogeneous materials as a function of disorder, temperature, and carrier concentration within the same theoretical formalism. It also bridges a gap between hopping and bandlike transport in an energetically heterogeneous system. The key assumption of the model is that the charge carriers move through delocalized states and that, in addition to the tail of the localized states, the disorder can give rise to spatial energy variation of the transport-band edge being described by a Gaussian distribution. It can explain a puzzling observation of activated and carrier-concentration-dependent Hall mobility in a disordered system featuring an ideal Hall effect. The present model has been successfully applied to describe experimental results on the charge transport measured in an amorphous oxide semiconductor, In-Ga-Zn-O (a-IGZO). In particular, the model reproduces well both the conventional Meyer-Neldel (MN) compensation behavior for the charge-carrier mobility and inverse-MN effect for the conductivity observed in the same a-IGZO TFT. The model was further supported by ab initio calculations revealing that the amorphization of IGZO gives rise to variation of the conduction-band edge rather than to the creation of localized states. The obtained changes agree with the one we used to describe the charge transport. We found that the band-edge variation dominates the charge transport in high-quality a-IGZO TFTs in the above-threshold voltage region, whereas the localized states need not to be invoked to account for the experimental results in this material.status: publishe

The impact of hot charge carrier mobility on photocurrent losses in polymer-based solar cells

A typical signature of charge extraction in disordered organic systems is dispersive transport, which implies a distribution of charge carrier mobilities that negatively impact on device performance. Dispersive transport has been commonly understood to originate from a time-dependent mobility of hot charge carriers that reduces as excess energy is lost during relaxation in the density of states. In contrast, we show via photon energy, electric field and film thickness independence of carrier mobilities that the dispersive photocurrent in organic solar cells originates not from the loss of excess energy during hot carrier thermalization, but rather from the loss of carrier density to trap states during transport. Our results emphasize that further efforts should be directed to minimizing the density of trap states, rather than controlling energetic relaxation of hot carriers within the density of states