Owing to its unlimited abundance, the successful conversion of solar energy to electric current with high efficiency have been a topic of interest in recent days. A conventional solar cell made of crystalline Si is limited by the Shockley-Queisser limit of only 33%, which states the highest attainable conversion efficiency (from solar energy to electricity) of the solar cell material. The value obtained is based under the assumption that upon absorption of a single electron-hole pair by a single photon in a conventional solar cell, the major portion of photon that was absorbed is wasted by means of phonon scattering and thermal radiation (thermal losses). The unfavorable efficiency coupled with cost factors associated with respect to starting material, high temperature processing steps and strict parameter controls questions the sustainability of conventional cells in commercial domain. Reduction of high energy loss and non-dissipative recombination mechanisms coupled with prospects to enhance energy harvesting are some of the futuristic approach in the design of efficient photovoltaic devices. Though efforts are made with triple junction devices such as integrated devices based on GaInP and GaInAs and Ge which could attain an efficiency of 40.7% under concentrated sunlight, costs associated with production does not favor sustainable commercialization. Henceforth, it is realized that ideally a single junction solar cell circumventing the aforementioned losses by utilization of the excess energy of hot carriers by either Carrier Multiplication or Multi Excition Generation which basically yields higher photo-voltage via extraction of hot carriers prior to thermalization and/ or generate multiple electron-hole pairs per photon enhancing the photocurrent, is desired for sustainable solar cell production and energy conservation. Quantum cutting materials which basically adopts downconversion phenomena with the absorption of a single photon whilst emitting multiple photons coupled with quantum efficiency higher than 100% is often found attractive for this application. PbS and CdSe and lanthanides are the usual materials utilized for this purposes however, owing to the high costs of starting materials and concerns over toxicity, the sustainability and cost-effectiveness of these materials are highly questionable. Though not widely studied, reports on earth abundant materials containing Si, Ge and C and its prospects as highly efficient and nontoxic spectral converter have been studied by Timmermann et al and others. In our present study, single source precursor containing the elements desired Si, Ge and C have been synthesized and fabricated into Si1-x-yGexCy thin films by Low Pressure Chemical Vapour Deposition onto two different substrates. Thin films attained in both substrates exhibit quantum cutting behavior with QE exceeding that of 400% and 500% in some cases. The compositional analysis, thin film thickness, morphology and particle size examined by various characterization techniques provide insights into the po...
Unique properties of lead chalcogenides have enabled multiple exciton generation (MEG) in their nanocrystals that can be beneficial in enhancing the efficiency of third-generation solar cells. Although the intrinsic electric field plays an imperative role in a solar cell, its effect on the multiple exciton generation (MEG) has been overlooked, so far. Using EOM-CCSD as a many-body approach, we show that any electric field can affect the absorptivity spectra of the lead chalcogenide nanocrystals (Pb Te , Pb Se , and Pb S ). The same electric field, however, has insignificant effects on the MEG quantum probabilities and the thresholds in these nanocrystals. Furthermore, simulations show that Pb Te , among the aforementioned nanocrystals, has the lowest MEG threshold and the strongest absorptivity peak that is located in the multi-excitation window, irrespective of the field strength, making it the most suitable candidate for MEG applications. Simulations also demonstrate that an electric field affects the MEG characteristics in the Pb Te nanocrystal, in general, less than it perturbs MEG characteristics in Pb Se and Pb S nanocrystals. Our results can have a great impact in designing optoelectronic devices whose performance can be significantly influenced by MEG.
The multiple exciton generation (MEG) in Si (0 x 7) Ge (7Àx) nanoclusters using the equation-of-motion coupled-cluster (EOM-CCSD) method has been studied here. Simulation results indicate that the energy normalized to the optical bandgap, at which the absorptivity profile peaks, depends on the elemental structure of the nanocluster (NC). Moreover, the results show that the larger the number of Si atoms (x) in the NC, the larger the normalized MEG threshold to the optical energy gap (E Th ), and the larger the optical absorption cross section. As an example, the maximum absorptivity in Si 7 Ge 0 nanocluster is about 2.37 times that in Si 0 Ge 7 , and E Th for the former NC is larger than that of the latter. Hence, the superior optical absorptivity of Si 7 shows, despite its larger normalized MEG threshold, it is the most desirable option for the MEG process in light-harvesting devices, including solar cells. This is contrary to the concluding remark in our previous study that was made solely on the basis of comparing the normalized MEG thresholds.
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