Organic semiconductor mobility

Fig. 2.1. I mprovement in organic semiconductor mobility over the last two decades. Rates of mobility increase have been very similar for single crystals, polymers, and small molecules over the same time period [49].

Keywords Hme-of-fllght Organic semiconductor Mobility Charge transport... 

It should be pointed out that for organic semiconductors mobilities of up to 15 cmVVs at room temperature have been reported. However, these high values relate to single crystals with translation symmetry in contrast to PEDOTrPSS as an amorphous solid. Additionally, free-charge carrier interaction can be neglected in such crystalline systems, whereas in PEDOTPSS such an interaction is anticipated due to the high density of oxidized states. This undetermined contribution makes it difficult to translate conductivity models applied for organic semiconductors to PEDOT PSS. 

The development of microelectronics cannot be envisaged without a comprehensive modeling of the devices. The modeling of OFETs is currently hampered by several features. First, charge transport in organic semiconductors is still not completely understood. The situation is clear at both ends of the scale. In high mobility materials (//>IOcnr V-1 s l), transport occurs within delocalized levels when temperature... 

One has to consider that in Eqs. (9.15)—(9.17) the mobility /t occurs as a parameter. As it will be pointed out below, // shows a characteristic dependence on the applied electric field typical for the type of organic material and for its intrinsic charge transport mechanisms. For the hole mobility, //, Blom et al. obtained a similar log///,( ) const. [E dependency [88, 891 from their device modeling for dialkoxy PPV as it is often observed for organic semiconductors (see below). 

Such a log // [E dependency is observed for many organic semiconductors [92-98]. For other polymers with an extraordinarily high mobility, such as m-LPPP (jix I03 cm2 V s ) [99], which is at least one order of magnitude higher than the highest mobility observed in PPV and its derivatives [38, 100, 101], only a weak dependence /<( ) is observed [991. 

We note a temperature dependence of the zero field mobility as exp[—( F()/F)2], a behavior which is indeed encountered in real organic semiconductors, and differs from both Millers-Abrahams fixed range and Moll s variable range hopping models. 

Figure 12.3. Benchmark of peer-reviewed academic reports of organic semiconductor device field-effect mobility versus time of report. All data points are for spin-coated organic semiconducting transistors. Solid points are derived from the benchmark study completed in 2002 by Brazis and Dyrc at Motorola (unpublished). The curve is a calculated estimation, based on these data, of what the expected mobility values will be in the future. The open points are data derived in 2005 from the public journals for verification of the 2002 prediction.6 38... 

Despite the success of the disorder model concerning the interpretation of data on the temperature and field dependence of the mobility, one has to recognize that the temperature regime available for data analysis is quite restricted. Therefore it is often difficult to decide if a In p vs or rather a In p vs representation is more appropriate. This ambiguity is an inherent conceptual problem because in organic semiconductors there is, inevitably, a superposition of disorder and polaron effects whose mutual contributions depend on the kind of material. A few representative studies may suffice to illustrate the intricacies involved when analyzing experimental results. They deal with polyfluorene copolymers, arylamine-containing polyfluorene copolymers, and c-bonded polysilanes. 

Klenkler RA, Xu G, Aziz H, Popovic ZD (2006) Charge-carrier mobility in an organic semiconductor thin film measured by photoinduced electroluminescence. Appl Phys Lett 88 242101... 

Eishchuk II, Arkhipov VI, Kadashchuk A, Heremans P, Bassler H (2007) Analytic model of hopping mobility at large charge carrier concentrations in disordered organic semiconductors polarons versus bare charge carriers. Phys Rev B 76 045210... 

Arkhipov VI, Emelianova EV, Heremans P, Bassler H (2005) Analytic model of carrier mobility in doped disordered organic semiconductors. Phys Rev B 72 235202... 

Sakanoue T, Sirringhaus H (2010) Band-like temperature dependence of mobility in a solution-processed organic semiconductor. Nat Mater 9 736... 

Inokuchi H, Imaeda K, Enoki T, Mori T, Maruyama Y, Saito G, Okada N, Yamochi H, Seki K, Higuchi Y, Yasuoka N (1987) Tetrakis(methyltelluro)tetrathiafulvalene (TTeCjTTF), a high-mobility organic semiconductor. Nature 329 39 0... 

Eley DD (1989) Studies of organic semiconductors for 40 years-I the mobile 7C-electron-40 years on. Mol Cryst Liq Cryst 171 1-21 and references therein... 

Katz FIE, Lovinger AJ, Johnson J, Kloc C, Siegrist T, Li W, Lin Y-Y, Dodabalapur A (2000) A soluble and air-stable organic semiconductor with high electron mobility. Nature 404 478 81... 

Organic semiconductor photovoltaic cells share many characteristics with both DSSCs and conventional cells. Charge generation occurs almost exclusively by interfacial exciton dissociation, as in DSSCs, but, in contrast, OPV cells usually contain no mobile electrolyte and thus rely on Vcharge separation. OPV cells may have planar interfaces, like conventional PV cells, or highly structured interfaces, like DSSCs. They provide a conceptual and experimental bridge between DSSCs and conventional solar cells. 

Identical methods for investigation of photoconductivity can be used for inorganic and organic semiconductors. Polymer semiconductors as a rule have very high resistance. For such materials the main information about the photoconductive mechanisms and properties may be obtained by two methods electrophotographic (or discharge method) and time of flight (or transit method). Both methods are successfully applied for materials with low mobilities, less than lO-4m2 V-1 s-1, which are the usual values for polymer semiconductors. 

Tetradecafluorosexithiophene 428 was considered as a potential n-type semiconductor for FETs for the following reasons sexithiophene is an excellent p-type semiconductor with high hole mobility perfluorination is an effective way to convert a p-type organic semiconductor to a n-type one (01JA4643). The absorption and emission maxima of 428 (421 and 471 nm, respectively) shifted to higher energies relative to those of sexithiophene (435 and 508 nm, respectively). 

A p-type organic semiconductor 461 based on benzodithiophene building blocks which has exceptional thermal stability was reported (97AM36). The highest mobility ( aFFt = 0.04 cm2 V-1 s-1) was obtained for transistors prepared at Tsub = 100°C because of the accompanying changes in morphology at elevated temperatures. 

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