Hole transport behavior of N-phenylcarbazole-doped poly(bisphenol A carbonate) films in a glass transition region

Hole mobility of a N-phenylcarbazole-doped poly(bisphenol A carbonate) system was measured by the time-of-flight method at −60 to 130 °C, which covers the glass transition temperature Tg of the poly(bisphenol A carbonate) matrix. The hole mobility at T=298 K and E=1.6×105 V cm−1 changes from 1.7×10−8 to 6.6×10−6 cm2 V−1 s−1 with the increase of N-phenylcarbazole concentration from 7.5×10−4 to 2.4×10−3 mol cm−3. Above Tg, the activation energy of hole mobility is smaller than that below Tg. This implies that the micro-Brownian motion of polymer chain in the glass transition region changes the trap level for hole transport, i.e., the trap level becomes shallower in the temperature range above Tg. The shape of the transient photocurrent was also studied at various temperatures. The value of (t1/2−tT) is used as an indicator of the trapping effect, where tT is the transit time and t1/2 is the time when the transient photocurrent becomes half of the photocurrent at tT. The slope of the plot of (t1/2−tT) vs T was found to change discontinuously in the glass transition region. This result also supports the change of the trap level in the glass transition region.