Doped Organic Semiconductors: Trap‐Filling, Impurity Saturation, and Reserve Regimes

Abstract

A typical human being carries billions of silicon‐based field‐effect transistors in his/her pockets. What makes these transistors work is Fermi level control, both by doping and field effect. Organic semiconductors are the core of a novel flexible electronics age, but the key effect of doping is still little understood. Here, precise handling is demonstrated for molar doping ratios as low as 10−5 in p‐ and n‐doped organic thin‐films by vacuum co‐sublimation, allowing comprehensive studying of the Fermi level control over the whole electronic gap of an organic semiconductor. In particular, dopant saturation and reserve regimes are observed for the first time in organic semiconductors. These results will allow for completely new design rules of organic transistors with improved long term stability and precise parameter control.

Impact of doping on the density of states and the mobility in organic semiconductors

“…The agreement and deviations in DOS shape at lower and higher concentrations, respectively, rationalize the consistent agreement and deviations between model II and the MC simulations at these limits. The roughly exponential shape of the dopant-induced tail states is likely the reason for the apparent success of mobility models that phenomenologically include an exponential or broad Gaussian tail of dopant-induced states [10,18,19]. Finally, we note that the strongest increase in effective width of the DOS occurs at the highest doping concentrations, in line with the results reported by Mityashin et al on basis of a more detailed atomistic model [14].…”

“…These models were extended by Schmechel [10] and later Tietze et al [18,19] by phenomenological inclusion of dopant-induced trap levels inside the bandgap. None of these models has been benchmarked against numerically exact kinetic Monte Carlo (MC) or similar calculations.…”

Passivation of Molecular n‐Doping: Exploring the Limits of Air Stability

“…However, an equal dopant aggregation causing this effect in all the investigated systems seems unlikely, but cannot be excluded, particularly at the high doping ratios used. Furthermore, since dopant saturation has been concluded from E F versus molecular doping ratio correlations for these n-doped systems, [ 24 ] further experimental and theoretical studies, in particular on the doping effi ciency, are required in order to establish a complete picture.…”

“…The shift of the Fermi level upon air exposure is further illustrated in Figure 3 b, i.e., compared to values determined from fresh samples with varying doping concentrations. [ 24 ] Already after the fi rst 15 s of air exposure, the Fermi level is shifted below values that have been measured for fresh samples with doping concentrations of one hundredths of 18 wt%, i.e., only at most 1% of the initial W 2 (hpp) 4 molecules still provide free electrons. With increasing air exposure time, this trend proceeds and after 1095 s, the n-doping effect completely vanishes.…”

“…[ 21,22 ] In contrast, the di-metal complex W 2 (hpp) 4 with its remarkable low ionization energy of just 2.4 eV is not only able to effi ciently n-dope C 60 , [ 23 ] but also host materials with much lower EA s like zinc-phthalocyanine (ZnPc, EA = 3.4 eV) or even P5 ( EA = 2.7 eV). [ 1,24 ] While this dopant has the drawback of immediate degradation in atmosphere, [ 25,26 ] we showed that the strong n-doping effect in C 60 :W 2 (hpp) 4 thin fi lms, which disappears upon air exposure, can partially be recovered by returning the doped fi lms back to vacuum and subsequent thermal annealing. [ 26 ] This unexpected effect was referred to as a selfpassivation mechanism of molecular n-doping and explained either by charge transfer from W 2 (hpp) 4 to C 60 accompanied by a down shift of the dopant energy levels or by the formation of hybridized W 2 (hpp) 4 -C 60 electronic states, [ 27 ] protecting the dopant species from oxidation in air.…”