$U_c$ are delocalized, then it is natural to consider charge transport using the multiple trapping model [@Noolandi:4466], and in the opposite case a hopping transport is the more appropriate model. Recently it was suggested that in some amorphous organic semiconductors there is an exponential tail of the DOS [@Vissenberg:12964; @Tachiya:085201; @Street:165207; @Schubert:024203; @Frost:255; @Kirchartz:165201; @Volker:195336; @Foertig:115302; @Street:075208; @Keivanidis:734]. Even in perfect organic crystals, where the bands are very narrow, charge transport at the room temperature is dominated by hopping due to thermally induced dynamic disorder and polaronic effects [@Troisi:086601; @Troisi:2347; @Ciuchi:245201; @Fratini:2292; @Gershenson:973]. In amorphous organic semiconductors significant static disorder leads to the total localization of all states. For such materials the charge transport is exclusively provided by the hopping mechanism.

Hopping charge transport for the exponential DOS has been studied extensively for many years and is considered as rather well studied area [@Monroe:146; @Rudenko:177; @Baranovskii:283]. The most fundamental fact about such transport is that for low temperature $kT/U_0 < 1$ carriers do not attain a quasi-equilibrium state with constant average velocity but the carrier velocity monotonously decreases with time and, hence, with the thickness $L$ of the transport layer as $$\label{disp-L}
v_L\propto 1/L^{\frac{1}{\alpha}-1}, \hskip10pt \alpha=kT/U_0,$$ indicating the transition to 