Photocurrent yield, LEDs

To assess the suitability of the nanocrystals as optically active centers for their incorporation into optoelectronic devices, a monolayer of particles was deposited onto mercaptopropionic acid derivatized ITO substrates. Their photoelectrochemical response was assessed under conditions of illumination using LED whose peak intensity (A.pk = 470 nm) is greater than the calculated bandgap. As can be seen from the inset of Fig. 2, upon illumination of the SnS-derivatized electrode the current is observed to quickly increase and remain relatively constant during the illumination time, here 20 s, and upon switching off of the LED the current returns to its preillumination value. This photocurrent response profile is reproducible over many cycles (in a number of trials for periods in excess of an hour). An average photocurrent (current under illumination minus background current) for a number of similarly prepared electrodes has yielded values of between 6 and 8 nA cm . ... 

Photovoltaic cells were prepared with a polymer layer sandwiched between electrodes with different work functions, as in polymer-based LEDs (Fig. 29.2). In the dark, the device exhibits a rectification ratio of 10 at 3.5 V. Under illumination, devices of this type show a strong photoresponse, with open-circuit voltages of 0.6 V and short-circuit currents that correspond to quantum efficiencies of up to 6%. Under forward or reverse biases, the quantum efficiencies rise rapidly, reaching 15% at a reverse bias of 3.5 V, 40% at 10 V. and considerably higher values under forward bias. These performance figures are very much better than those for devices made with aluminum electrodes and either MEH-PPV or CN-PPV alone. Thus, Halls et al. report values for the quantum yield of the short-circuit photocurrent in the MEH-PPV of 0.04% at the peak response wavelength (2.2 eV) and on the order of 10 %... 

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