Photocharge Separation Research Articles

Surface photovoltage measurements in μc-Si:H: Manifestation of the bottom space charge region

We discuss results of surface photovoltage (U) measurements for d=10 μm thick layers of undoped hydrogenated microcrystalline silicon (μc-Si:H). By applying excitation with low energetic photons (down to 1.1 eV), i.e., with small absorption coefficient α and a large penetration depth α−1, a photovoltage peak appears on a curve U=U(α). This peak is located at α≳d−1 and its occurrence depends critically on the substrate material. The peak is present in a μc-Si:H film grown on p+ crystalline silicon (c-Si), on the other hand it is missing in the μc-Si:H samples grown on n+ c-Si or ZnO film. We present a mathematical model that enables us to link this peak to photocharge separation in the bottom space charge region at the interface μc-Si:H/substrate. Besides the magnitude of the ambipolar carrier diffusion length L, a parameter particularly critical for the occurrence of the peak turns out to be the ratio n of reverse saturation current densities of the two diodes representing surface and bottom space charge regions. The peak can be observed only when |n| is below a certain threshold value |nth|. In the simplified case when d/L≫1 and when the thicknesses of the upper and lower depletion layers can be neglected, we have found |nth|≈0.135 or 1, depending on the orientation of the top and bottom barriers to each other. However, the magnitude of the peak increases exponentially with further lowering of |n|. Therefore, the surface photovoltage method is suitable not only for evaluating the minority carrier diffusion length L, but also for detecting the occurrence and properties of the bottom space charge region in thin film solar cells.

Photo-transfer of electrons in donor-acceptor systems

We consider here a new type of opto-electronic processes: long-distance photo-assisted transfer of electrons in donor-acceptor structures. This long-distance one- and two-photon transfer may be realized in “localized donor - semiconductor - localized acceptor - semiconductor”, delocalized semiconductor layered structures or a similar kind of superlattice structures. We show that the one-photon electron transfer in this kind of structures is possible for distances 15–30 Å, and the efficiency of two-photon phototransfer does not decrease with the distance, that may lead to very large distances of photo charge separation under high light intensities. We suggest to use a special type of the quantum well structure for the detection of this effect. We propose to employ a thin semiconductor layer with a specific nonhomogeneous spatial distribution of donor and acceptor impurities as IR-visible image converter and an element of optical memory. We also consider two new modifications of polarization four-photon parametric spectroscopy which can be used for detection of long-distance photo-transfer states in proposed structures.

Photovoltaic effect based on wet poly(vinyl alcohol)/merocyanine dye junction

AbstractThe sandwich cell, Au/wet poly(vinyl alcohol) (PVA)/merocyanine (MD)/Au, based on the PVA/MD junction exhibited photovoltaic and rectifying effects. Forward dark current from the MD/Au electrode to the counter PVA/Au electrode through the cell was observed. The Au/MD electrode showed a positive photovoltage with respect to the PVA/Au electrode regardless of the direction of the incident light. It was suggested that a heterojunction barrier similar to the Schottky type formed at polymer/dye interface was responsible for the photo charge separation.