Responsivity reverse bias

The simplest and most widely used model to explain the response of organic photovoltaic devices under illumination is a metal-insulaior-metal (MIM) tunnel diode [55] with asymmetrical work-function metal electrodes (see Fig. 15-10). In forward bias, holes from the high work-function metal and electrons from the low work-function metal are injected into the organic semiconductor thin film. Because of the asymmetry of the work-functions for the two different metals, forward bias currents are orders of magnitude larger than reverse bias currents at low voltages. The expansion of the current transport model described above to a carrier generation term was not taken into account until now. 

The Schottky barriers were excellent diodes for films annealed at 600 °C, with turn on voltages of 0.6-0.8V and minimal reverse bias leakage.48 However, many of the contacts on the as-deposited films gave large reverse bias currents and nearly ohmic responses. This behavior is indicative of degeneracy of the semiconductor because of a high carrier density resulting from native defects. The improvement in the diode behavior of the annealed films is attributed to enhanced crystallinity and reduction of defects. 

CdO, a degenerate n-type semiconductor, was chemically deposited on single-crystal p-type Si [40]. The junction showed clear diode behavior, and, although no photovoltaic effect was observed, photocurrent was generated under reverse bias. From the spectral response of the photocurrent, almost all of the current generation occurred in the Si. 

When a reverse bias of 2-5 V is applied to the devices with polymer-C o blends, the external photosensitivity is 0.2-0.3 AAV and the external quantum yield is 50-80% el/ph. Because polymer photodiodes exhibit a relatively flat response over a broad spectral range, they are especially suitable as detectors for spectroscopic applications. They are particularly advantageous compared to silicon for detecting light in the blue and ultraviolet regions of the spectrum. At 370 nm the photosensitivity is approximately 0.3 AAV for a polymer photodiode and 0.05 AAV for a UV-enhanced silicon photodiode [69]. 

The response speed of OPD was evaluated in the frequency range up to 100 MHz using a sinusoidal modulated laser (A,=650nm). The results obtained at the reverse bias of 0.0,1.0,3.0, and 7.0 V are shown in Figure 15.10a. As the reverse bias is increased, the response increases toward the highspeed regions. The intensity of output power P of OPD, which depends on the frequency/is described as... 

Fig. 6.7 Band diagram of a n-type photoanode at (a) flat-band potential, (b) a potential sufficient to separate charge carriers and drive photocurrent, and (c) large reverse bias potential sufficient to saturate the photocurrent response. The corresponding hypothetical j-V curve is...

The working mechanism of PLECs discussed earlier is responsible for their unique structures and characteristics, which have both advantages and drawbacks in performance compared to PLEDs. The PLECs exhibit a bipolar behavior due to the insensitivity to the work function of electrode. Under both forward and reverse bias, PLECs show strong electroluminescence, which has been observed in masses of PLECs constructed with various... 

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