Origin of Carrier Types in Intrinsic Organic Semiconductors

Abstract

What determines carrier types in organic semiconductors? This question is growing in the recent studies of organic field-effect transistors (FETs), because ambipolar operation has been effected in FETs of several organic semiconductors. [1][2][3][4][5][6][7] This means that the carrier type -electron or holecould be controlled by the external voltage although each organic semiconductor is considered to have either p-or n-character in normal conduction. The origin of such a native carrier type is, in most cases, empirically explained by the energy positions of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) relative to the work function of the electrode metal. This answer is, however, too naïve and not quantitative, judging from the recent surface-science results [8][9][10][11][12][13][14] that point out the absence of universal rule for the energy alignment at the metal-organic semiconductor interfaces. In this study, we succeeded in changing the carrier injection barriers and observing the resultant change of carrier type in an organic FET. The key factor is the usage of an identical specimen for both electrical and electronic measurements. The field-effect mobilities, evaluated from conventional three-terminal measurements, decreased exponentially with precisely evaluated charge injection barrier, independent of carrier type; this suggests that the carrier type of intrinsic organic semiconductors is not determined a priori but is governed by the formation of an interface with a metal electrode.

The Influence of Deposition Rates on Properties of AlPcCl Thin Films and on the Performance of Planar Organic Solar Cells

“…Therefore, it can be concluded from this study that the hole mobility of AlPcCl is significantly improved when films are deposited slowly. It must be noted that the values estimated are in the range of values measured for other groups . Therefore, the calculated values shown in Table are credible.…”

“…It must be noted that the values estimated are in the range of values measured for other groups. [22] Therefore, the calculated values shown in Table 3 are credible. Even if they are only indicative, they can be used to draw comparisons with similar materials.…”

“…The carrier mobility value is central to the J sc value. As is the case for most organic materials, the conductivity of AlPcCl layers is low, which limits the J sc and FF values . Thus, any improvement made to the hole mobility of AlPcCl should improve the performance of the OPVC.…”

Impact of structural imperfections on the energy-level alignment in organic films

“…Such an increase in the HIB by annealing was also observed for ClAlPc films/Au polycrystalline electrode in the bottom-contact OFETs, and resulted in a change of the major charge carrier from hole to electron. 23 To elucidate the origin of the difference between δ-dependent ELAs of the AG and AN films, we discuss the details of the electronic structure of these films below. .…”

“…For example, Kaji et al demonstrated recently for chloroaluminum phthalocyanine (ClAlPc) thin-films-based organic field-effect transistors (OFETs) that the Fermi level could be controlled widely in between the highest occupied molecular orbital (HOMO) and the lowest unoccupied MO of the films by applying an annealing procedure, and succeeded in tuning the type of charge carrier and the device performance. 23 Among several successful uses of polar molecules for organic devices, [23][24][25][26][27][28][29] the correlation between their film structures and the ELA is not yet well explored and thus still remains vague. For ultrathin films of various polar metal-Pc (p-MPc) molecules deposited on a molybdenum disulfide (MoS 2 ) (ClAlPc [30][31][32][33] ) and a highly oriented pyrolytic graphite (HOPG) (OTiPc, [34][35][36][37] OVPc, 38 ClAlPc, 39,40 and PbPc 41 ), we have thoroughly studied their film structure and growth, and evolution of electronic structure in these films using mainly ultraviolet photoelectron spectroscopy (UPS), metastable atom electron spectroscopy (MAES), and other electron spectroscopic methods.…”