Charge Carriers in Pyrene Crystals

From the rectifying behaviour of photoconduction, the sign of charge carriers in molecular crystals is estimated; the ratio of forward photocurrent and reverse one corresponds to the ratio of mobilities μe⁄μh, where μe is the mobility of electron and μh is that of hole. The ratio for pyrene crystal reaches over a unity with repeating the purification, that is, the predominant charge carrier changed from hole to electron with increasing the purity of the crystals. For violanthrene or dibenzpentacene, the ratio is larger than 4 and for quaterrylene, that is almost a unity.