Carrier drift length

In the presence of an electric field the drift length is the mobility-lifetime product times the electric field A.mfp = prE [576]. With typical values of pz and E the mean free path usually exceeds by far the thickness of the solar cell, and virtually all photogenerated carriers can be collected. However, under certain operating conditions, field-free regions in the / -layer may exist, and the collection efficiency is decreased because the diffusion lengths of the carriers are much smaller than the thickness of the solar cell [11, 577]. 

First, the drift current is calculated in the case of a constant electrical field, as one would expect for very thin bulk heterojunction solar cells. If the width W of the active layer is similar to the drift length of the carrier, the device will behave as a MIM junction, where the intrinsic semiconductor is fully depleted. The current is then determined by integrating the generation rate G = —dP/dx over the active layer, where P is the photon flux ... 

In most a-Si H solar cells, a built-in electric field (F) assists in the collection of photogenerated carriers, and efficient collection occurs as long as the drift length (/itF) is significantly larger than the film thickness. Crandall (1982) has shown that the transport in p-i-n cells can be charac-... 

In a drift dominated case the recombination probability depends on the mean drift length Wh/e = rih/eth/eF. If the mean drift length of one or both of the charge carriers is smaller than the device length d the recombination is significant. The saturated photocurrent density is given by [60],... 

Global AMI.5 sun illumination of intensity 100 mW/cm ). The DOS (or defect) is found to be low with a dangling bond (DB) density, as measured by electron spin resonance (esr) of - 10 cm . The inherent disorder possessed by these materials manifests itself as band tails which emanate from the conduction and valence bands and are characterized by exponential tails with an energy of 25 and 45 meV, respectively the broader tail from the valence band provides for dispersive transport (shallow defect controlled) for holes with alow drift mobiUty of 10 cm /(s-V), whereas electrons exhibit nondispersive transport behavior with a higher mobiUty of - 1 cm /(s-V). Hence the material exhibits poor minority (hole) carrier transport with a diffusion length <0.5 //m, which puts a design limitation on electronic devices such as solar cells. 

The numerous defects inherent in organic polymers creates the donor or acceptor impurity levels. The low drift mobilities of the order 10-7-10-12 m2 V-1 s"1 lead to the paradoxical situation where the length of the free travel distance for the charge carrier becomes less than the size of the separate molecule links. So the hopping or activated models are the most acceptable ones for polymers in such circumstances. 

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