The standard technique to separately and simultaneously determine the carrier concentration per unit volume (N, cm ) and the mobility (μ) of doped inorganic single crystals is to measure the Hall effect. However, this technique has not been reported for bulk-doped organic single crystals. Here, the Hall effect in bulk-doped single-crystal organic semiconductors is measured. A key feature of this work is the ultraslow co-deposition technique, which reaches as low as 10 nm s and enables us to dope homoepitaxial organic single crystals with acceptors at extremely low concentrations of 1 ppm. Both the hole concentration per unit volume (N, cm ) and the Hall mobility (μ ) of bulk-doped rubrene single crystals, which have a band-like nature, are systematically observed. It is found that these rubrene single crystals have (i) a high ionization rate and (ii) scattering effects because of lattice disturbances, which are peculiar to this organic single crystal.

16-dione is a soluble precursor of pentacene, which can be converted into pentacene by irradiation in the solid-state. Its photoconversion process in spin-coated films was monitored by UV-visible absorption and IR spectroscopy. A small amount of high-boiling-point additives in the chloroform spin-coating solution promoted photoconversion to obtain high quality films suitable for FETs. The FET mobilities showed a correlation with the additives' boiling points and dissolution abilities, indicating that the retainment of a semidry state during photoconversion is essential to the complete photoconversion. Photoirradiation conditions (irradiation intensity, duration and substrate temperature) were optimized to achieve a field-effect mobility of 0.86 cm 2 V À1 s À1 , comparable to the performance of vacuum-deposited pentacene films. The prepared films have a partially crystalline morphology different from that of vacuum-deposited films. The high FET mobility of the photoconverted film is attributed to continuously connected grain boundaries arising from partial crystallinity.

Hole mobility was evaluated by top-contact bottom gate field effect transistor and time resolved microwave conductivity measurements in 2,6-dithienylanthracene and hexyl-substituted 2,6-dithienylanthracene films prepared by spin-coating of their α-diketone precursors followed by photoirradiation, revealing enough high potentials for semiconducting films with charge carrier mobilities of 0.8-0.9 cm(2) V(-1) s(-1) in the photo-irradiated films.

a b s t r a c tThe effects of doping at concentrations at the ppm level in organic photovoltaic cells were clarified using simple n þ p-homojunctions. With doping from 0 to 10 ppm, the fill factor increased due to the appearance of majority carriers. From 10 to 100 ppm, the photocurrent density increased due to an increase in the built-in potential, i.e., the formation of an n þ p-homojunction. The photocurrent was increased by a factor of 1.3 by directly doping the photoactive co-deposited layer with acceptor molecules at a concentration of 100 ppm.