Carrier generation in photoconductive poly (N -vinylcarbazole) as revealed by multifrequency time-resolved ESR

Using the multifrequency time-resolved electron-spin-resonance (TRESR) method, we have investigated the carrier-generation dynamics in a photoconductive film sample of poly (N -vinylcarbazole). The TRESR spectra, due to spin-polarized geminate electron-hole (e-h) pairs captured in deep-trap sites, were observed at several fields that were achieved using microwaves with different frequencies. The spectra detected at room temperature were interpreted in terms of the S T0 polarization in trapped e-h pairs and the S T+ polarization induced in canonical e-h pairs. The spectral simulation clarified the distribution of e-h separation distance (r) and the r -dependent recombination of the trapped pairs. The initial spatial distribution of the trapped e-h pairs exponentially decreased with an increase in r. There is no large population of long-distant e-h pairs with r>1.5 nm. Based on the appearance of the S T+ polarization in the long-distant pairs that is generated in middle-distant pairs and the initial exponential distribution of the trapped pairs, it was concluded that stepwise hole hops rather than a long-range hole jump represent the proper dynamics of the geminate pair in the charge-separation process. The r -dependent recombination rate suggests a single-step tunneling recombination of the trapped pairs.