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Quantum-efficiency measurements on carbon–hydrogen-alloy-based solar cells
Abstract
Solar cell devices have been fabricated with amorphous, hydrogenated carbon (a-C:H) as the primary semiconducting material. These devices clearly demonstrate photovoltaic behavior as determined by their I–V curves. To identify photon energies that contribute to the photogenerated current, quantum efficiency measurements have been performed. The fabricated solar cells exhibit a maximum quantum efficiency response in the ultraviolet region. Using the measured quantum efficiency curves, the short-circuit currents for a global AM1.5 spectrum at an irradiance of 1000W/m2 have been calculated. These values are comparable to the actually measured short-circuit currents.
... In this respect, some efforts have been made, which can be retrieved from recent publications that relate to the fabrication of solar cell devices using ta-C or a-C : H, such as, Simens AG, Germany, which described the fabrication of heterojunction solar cells a-C : H/GaAs and a-C : H/InP in their US patent 5206534 [1]. Maldei and Ingram reported their a-C : H-based solar cells in 1998 [2]. Cheah et al. reported ta-C/Si heterojunction solar cells in 1998 [3]. ...
This paper reports on the successful deposition of amorphous carbon nitride thin films (a-CNx) and fabrication of ITO/a-CNx/Al Schottky thin-film solar cells by using the technique of ion beam sputtering. XPS and Raman spectra are used to characterize the deposited thin films. Nitrogen atoms are incorporated into the films in the form of carbon–nitrogen multiple bands. Their optical properties are also investigated using a spectroscopic ellipsometer and UV/VIS/NIR spectrophotometer. The refraction of the carbon nitride thin films deposited lies in the range of 1.7–2.1. The Tauc optical band gap is about 0.6eV. The photovoltaic values of the device, short-circuit current and open-circuit voltage are 1.56μA/cm2 and 250mV, respectively, when exposed to AM1.5 illumination (100mW/cm2, 25°C).
Amorphous carbon (a-C) is a potential material for the development of low cost solar cells. The heterojunction n-C/p-Si solar cell has been recently developed by Krishna et al. It has been shown that the maximum quantum efficiency (25%) appears at wavelength λ (600 nm). In the present work, theoretical quantum efficiency has been calculated taking into account the contribution of hole photocurrent density, electron photocurrent density and the photocurrent within the depletion region. The variation of quantum efficiency with wavelength is found to be qualitatively similar to the experimentally observed variation. The solar cell parameters namely Voc, Isc, FF and efficiency have also been calculated and compared with the experimental values.
Photoresponse characteristics of heterostructure solar cells, fabricated by depositing a phosphorous (P) doped carbon (n-C) layer on a p-type silicon (Si) substrate (n-C/p-Si cell), have been studied. The camphoric carbon (CC) targets containing varying amounts of P ranging from 1% to 7% by mass were used in a pulsed laser deposition chamber for the deposition of the carbon layers of the cells under analysis. The quantum efficiency of these cells was measured in the ultraviolet–visible–infrared region (300–1200nm), which is found to vary with the P content in the carbon layer. The individual contributions of the carbon and silicon regions to the overall photoresponse are extracted by deconvoluting the overall photoresponse spectra. The relative contribution of the carbon region is found to increase with the P content up to 5wt.% P in the carbon layer of the cell and decreases thereafter. This trend is similar to that of the optoelectronic properties of the P-doped CC films.
Amorphous carbon nitride films (a-CNx) were synthesized by using single ion beam sputtering of a graphite target in argon and nitrogen sputtering gases. This thin film could be used as a novel photovoltaic material. The films were characterized with the technique of laser Raman, spectroscopic ellipsometry and electron spin resonance spectrometer (ESR). In this paper we report the effects of ion impacting and nitrogenation on the microstructure, density of defect states, bonding character, optical and photovoltaic properties. Effective decreasing of intensity of the ESR signal and formation of C-N bonding were observed, which could be attributed to the increment of the impinging ions on the growing films. The nitrogenation of a-CNx films could decrease the Tauc optical gap (0.62∼0.86eV) and the intensity of ESR signal, increase photon absorption coefficient of the films (106∼104cm−1).
The primary photovoltaic values of the devices having Schottky structure of ITO/CNx/Al are Isc 1.56 μ A/cm2 and Voc 250 mV, respectively, when exposed to AM1.5 illumination ( 100mW/cm2, 25°C).
Recent studies have shown the application of amorphous carbon as a semiconductor in C/Si heterojunction photovoltaic solar cells. In this letter, we report the rectifying current–voltage characteristics of the phosphorus-doped carbon/undoped-carbon (n-C/p-C) junction. The p- and n-carbon films were deposited by pyrolysis and ion-beam sputtering, respectively, on a p-Si substrate, using camphor as a natural carbon precursor. The preliminary photovoltaic characteristics of the cell reveals a short-circuit current density of 17.1 mA/cm2, open-circuit voltage of 0.339 V, and photoelectrical conversion efficiency of 1.82%, a reproducible result, under air mass zero and 1 sun illumination conditions. The spectral photoresponse characteristics of the cell of the above configuration was explained in terms of transmission/absorption characteristics of the two individual carbon layers. © 2000 American Institute of Physics.
Amorphous hydrogenated carbon-nitrogen alloy (a-CNx:H) thin films
have been deposited on silicon substrates by improved dc magnetron
sputtering from a graphite target in nitrogen and hydrogen gas
discharging. The films are investigated by using Raman
spectroscopy, x-ray photoelectron spectroscopy, spectral
ellipsometer and electron spin resonance techniques. The optimized
process condition for solar cell application is discussed. The
photovoltaic property of a-CNx:H/silicon heterojunctions can be
improved by the adjustment of the pressure ratio of hydrogen to nitrogen and
unbalanced magnetic field intensity. Open-circuit voltage and short-circuit
current reach 300 mV and 5.52 mA/cm2, respectively.
Heterojunction photodiodes were prepared by r.f. plasma-enhanced chemical vapour deposition of a-C:H films on single crystalline p-type silicon substrates (NA = 1014 cm−3, orientation 〈111〉) and a-Si:H substrates. We studied the quantum efficiency of devices working in photovoltaic and reverse-biased mode in the wavelength region between 350 and 1200 nm. Post-deposition treatment of a-C:H films with hydrogen at high pressures (up to 300 bar) and elevated temperatures (up to 650 K) changed the bulk properties of the a-C:H films significantly. Photoconductivity was observed within the films after the hydrogen processing. The responsivity of the photoconducting a-C:H films, however, is too small to account for the observed quantum efficiencies (~ 20–30%) of the devices. Multiplication of charge carriers is indicated by quantum efficiencies exceeding 100%, which are observed for reverse-biased devices after the hydrogen treatment. According to C-V measurements the formation of the heterojunction leads to an inversion layer on the silicon surface which is essential for properly working devices. In our paper we propose a model for the operation of photodiodes which is supported by optical beam-induced current measurements.
The photovoltaic behavior and spectral response of n‐type (nitrogen‐doped) tetrahedral amorphous carbon (ta‐C)/p‐type crystalline silicon heterojunction photodiodes are reported. Abrupt step junction type characteristics are observed with ta‐C films ranging in thickness from 40 to 160 nm. The photovoltage increases and the prominent peak in responsivity shifts from 800 nm to longer wavelengths of 1000 nm as the doping in the ta‐C films is increased, indicative of a widening of the depletion region in the Si. Use of the responsivity vs wavelength data with the Donnelly and Milnes model for an abrupt heterojunction is successful in predicting the depletion width in the Si and the doping level in the higher doped ta‐C. The latter value coupled with the x‐ray photoemission spectroscopy derived N concentration in the ta‐C films enables an estimation of the doping efficiency. Secondary ion mass spectroscopy studies also confirm the abrupt nature of the junctions.
The structure and properties of thin amorphous carbon films are critically dependent upon the preparation conditions. Hydrogenated amorphous carbon films, prepared by both ion beam sputtering and glow discharge techniques, have been investigated by solid‐state 13C magic angle spinning nuclear magnetic resonance measurements of the sp2 and sp3 bonding sites. Film hardness and density correlate with the incorporated hydrogen, whereas the optical band gap is controlled by the fraction of tetrahedral (sp3) versus graphitic (sp2) bonding. It is shown that structural trade‐offs prevent the formation of so‐called amorphous diamond, i.e., a material with simultaneous extreme hardness and wide optical band gap.
The electronic structure of amorphous carbon and hydrogenated amorphous carbon (a-C:H) has been investigated through calculations on a number of model structures containing different configurations of sp2 and sp3 sites. We find that the most stable arrangement of sp2 sites is in compact clusters of fused sixfold rings, i.e., graphitic layers. The width of the optical gap is found to vary inversely with the sp2 cluster size, and the ∼0.5-eV optical gap of evaporated amorphous carbon is found to be consistent with a model of disordered graphitic layers of about 15 Å in diameter, bounded by sp3 sites. It is argued that a-C forms such finite clusters in order to relieve strain. It is then shown that the 1.5–2.5-eV optical gap of a-C:H is unusually small and requires that both its valence and conduction band consist of π states on sp2 sites and that these sites must also be significantly clustered, such as in graphitic clusters containing four or more rings. In other words, the optical gap of both a-C and a-C:H depends on their degree of medium-range order, rather than just on their short-range order as is the case in most amorphous semiconductors. We have also studied the nature of states away from the gap in order to interpret the photoemission data and the carbon 1s core-level absorption spectra. The nature of defects and midgap states is discussed, and it is predicted that the defect density decreases with increasing band gap. Finally it is argued that the doping of a-C:H by group-III and -V elements proceeds via a substitution mechanism, as in a-Si:H, in spite of the coordination disorder present in a-C:H. Doping is also expected to be accompanied by an increase in gap states, as in a-Si:H.