We discuss the physical origin and effects of micrononuniformities on thin-film photovoltaics. The key factors are the large device area and the presence of potential barriers in the grain boundaries (for polycrystalline films) and in device junctions. We model the nonuniformity effects in the terms of random microdiodes connected in parallel through a resistive electrode. The microdiodes of low open circuit voltages are shown to affect macroscopically large regions. They strongly reduce the device performance and induce its nonuniform degradation in several different modes. We support our predictions by experiments, which show that the device degradation is driven by the light-induced forward bias and is spatially nonuniform.
Large-area, thin-film semiconductor devices often exhibit strong fluctuations in electronic properties on a mesoscale level that originate from relatively weak microscopic fluctuations in material structures such as grain size, chemical composition, and film thickness. Amplification comes from the fact that electronic transport through potential barriers is exponentially sensitive to the local parameter fluctuations. These effects create new phenomena and establish the physics of large-area, thin-film devices as a distinctive field of its own, quite different from that of microelectronics. We show that ͑i͒ large-area semiconductor thin-film devices are intrinsically nonuniform in the lateral directions, ͑ii͒ the nonuniformity can span length scales from millimeters to meters depending on external drivers such as light intensity and bias, and ͑iii͒ this nonuniformity significantly impacts the performance and stability of, e.g., photovoltaics, liquid crystal displays, and light emitting arrays. From the theoretical standpoint our consideration introduces a new class of disordered systems, which are random diode arrays. We propose a theory describing one class of such arrays and derive a figure of merit that characterizes the significance of nonuniformity effects. Our understanding suggests some methods for blocking the effects of nonuniformities.
Spectral ellipsometry at 300 K, in the range 0.75–5.4 eV, has been used to determine the optical constants ε(E)[=ε1(E)+iε2(E)] of a series of CdTe1−xSx (0⩽x⩽1) films fabricated by a laser-deposition process. The measured ε(E) data reveal distinct structures associated with critical points (CPs) at E0 (direct gap), spin-orbit split E1, E1+Δ1 doublet and E2. The experimental data over the entire measured spectral range (after oxide removal) has been fit using the Holden model dielectric function [Phys. Rev. B 56, 4037 (1997)] based on the electronic energy-band structure near these CPs (also E0+Δ0 CP) plus excitonic and band-to-band Coulomb enhancement (BBCE) effects. In addition to evaluating the energies of these various band-to-band CPs, our analysis also makes it possible to obtain information about the binding energies of not only the three-dimensional exciton associated with E0 but also the two-dimensional exciton related to the E1, E1+Δ1 CPs. Our results will be compared to previous experiments and modeling (which neglect the BBCE terms) of ε(E) of CdTe and CdS as well as optical absorption measurements of E0 of CdTe1−xSx (0⩽x⩽1). The results of this experiment demonstrate conclusively that the band-to-band line shape at E0 is BBCE even if the exciton is not resolved.
The proposed CdTe-based large area thin-film detector has a potential of becoming an efficient low-cost electronic portal imaging device for radiation therapy applications.
We propose a principle of photovoltaics utilizing the built-in electric field generated by aligned nanosize dipoles in a semiconductive host, either polymer or liquid, or amorphous, or polycrystalline. Such dipoles are readily found with the wurtzite CdS, CdSe, and some other nanoparticles. We show that their generated field can be uniform and strong enough, ∼3×104V∕cm, to separate electron-hole pairs and run significant drift currents. Our suggested structure does not rely on p-n or Schottky junctions and can be tunable in a broad range of parameters.
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