Anomalous Transient Photocurrent

The operating principle in conventional optoelectronic devices is based on steady-state photocurrent. In these devices, photogenerated carriers have to travel long distances across the devices. Various dissipation mechanisms such as traps, scattering and recombination dissipate these carriers during transport, and decrease device response speed as well as optoelectronic conversion efficiency, especially in organic devices (Forrest & Thompson, 2007; Pandey et al., 2008; Saragi et al., 2007; Spanggaard & Krebs, 2004; Xue, 2010). Such organic devices have received considerable attention due to their potential for of large-area fabrication, combined with flexibility, low cost (Blanchet et al., 2003), and so on. Efforts to substitute inorganic materials by organic ones in optoelectronics have encountered a serious obstacle, i.e., poor carrier mobility that prevents photogenerated carriers from travelling a long distance across the devices. Typically, exciton diffusion length in organic materials is approximately 10-20 nm (Gunes et al., 2007). Internal quantum efficiency decreases with the increase in film thickness (Slooff et al., 2007) since recombination will occur prior to exciton dissociation if photogenerated excitons are unable to reach the region near the electrodes. Therefore, though a thicker film can result in an enhanced light harvesting, collecting carriers using electrodes becomes difficult. In addition, the poor mobility of organic materials always triggers the formation of space charges in thin film devices, and the space charges additionally limit the photocurrent (Mihailetchi et al., 2005). In this chapter, we introduce an anomalous transient photocurrent into optoelectronics based on Maxwell’s theory on total current, which consists of conduction and displacement current. In contrast to organic optoelectronic devices based on conduction photocurrent, which sufferrs from poor carrier mobility, the anomalous photocurrent can contribute to optoelectronic conversion and “pass” through an insulator. Though such anomalous photocurrent, or photoinduced displacement current, has received previous attention (Andriesh et al., 1983; Chakraborty & Mallik, 2009; Iwamoto, 1996; Kumar et al., 1987; Sugimura et al., 1989; Tahira & Kao, 1985), its mechanism and characteristics are still largely unresolved. We systematically explained this phenomenon based on our theoretic analyses and experiments on an organic radical 4’4-bis(1,2,3,5-dithiadiazolyl) (BDTDA) (Bryan et al., 1996) thin film device. A double-layer model was introduced, and a new type of device with structure of metal/blocking layer/semiconductor layer/metal was developed to reproduce the anomalous photocurrent (Hu et al., 2010b). The photocurrent transient is observed to