Thin-film solar cells are preferable for their cost-effective nature, least use of material, and an optimistic trend in the rise of efficiency. This paper presents a holistic review regarding 3 major types of thin-film solar cells including cadmium telluride (CdTe), copper indium gallium selenide (CIGS), and amorphous silicon (α-Si) from their inception to the best laboratory-developed module. The remarkable evolution, cell configuration, limitations, cell performance, and global market share of each technology are discussed. The reliability, availability of cell materials, and comparison of different properties are equally explored for the corresponding technologies. The emerging solar cell technologies holding some key factors and solutions for future development are also mentioned. The summarized part of this comparative study is targeted to help the readers to decipher possible research scopes considering proper applications and productions of solar cells.
Auger processes involving the filling of holes in the valence band are thought to make important contributions to the low-energy photoelectron and secondary electron spectrum from many solids. However, measurements of the energy spectrum and the efficiency with which electrons are emitted in this process remain elusive due to a large unrelated background resulting from primary beam-induced secondary electrons. Here, we report the direct measurement of the energy spectra of electrons emitted from single layer graphene as a result of the decay of deep holes in the valence band. These measurements were made possible by eliminating competing backgrounds by employing low-energy positrons (<1.25 eV) to create valence-band holes by annihilation. Our experimental results, supported by theoretical calculations, indicate that between 80 and 100% of the deep valence-band holes in graphene are filled via an Auger transition.
The density of defect states near the valence band of the hole blocking layer (commonly called the n-layer) is determined by analyzing the transient dark current behavior of multilayer amorphous selenium (a-Se) X-ray image detectors.The previous transient dark current model (Mahmood et al. Appl. Phys. Lett. 92, 223506 (2008)) is modified and compared with recently published experimental transient dark currents on commercial n-i-p and cold deposited n-i a-Se detector structures to determine the energy distributed deep defect densities in these two types of n-layer. The peak defect state exists at 0.75 and 0.78 eV from the valence band mobility edge in alkaline doped and cold deposited n-layers, respectively. The peak trap density in these n-layers varies in the range of 5 × 10 16 -5 × 10 17 cm −3 eV −1 . The energy depths of the trap centers should be ϳ(0.75-0.8) eV from the valence band mobility edge for a requirement of less transient time to reach a plateau. The shallower trap levels are unable to retain sufficient trapped charge to reduce the dark current, and the deeper trap centers create longer transient times to reach a steady level of dark current.
A theoretical model for describing bias-dependent transient and steady-state behaviors of dark current in amorphous selenium (a-Se) avalanche detector structures has been developed. The analytical model considers bulk thermal generation current from mid-gap sates, transient carrier depletion, and carrier injection from the electrodes incorporating avalanche multiplication. The proposed physics-based dark current model is compared with the published experimental results on three potential a-Se avalanche detector structures. The steady-state dark current is the minimum for the structures that have effective blocking layers for both holes and electrons. The transient decay time to reach a plateau decreases considerably with increasing electric field.
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