Tuning Photoresponse Through Size Distribution Control Of Silicon Quantum Dots In Nanocrystalline Silicon Thin Films

The emergence of semiconductor nanocrystals as the building blocks of nanotechnology has opened up new ways in next generation solar cells. Nanocrystalline Si (nc-Si) thin film, a mixed-phase material consisting of Si quantum dots embedded in amorphous tissue, presents very promising features in solving the unavoidable disadvantages of low energy conversion efficiency and light-induced degradation of commercial amorphous Si thin film solar cells. Size quantization effect allows one to tune the photoresponse easily, making nc-Si thin film a good light harvester. Strong optical absorption and high photocurrent have been found in nc-Si films and attributed to the enhancement of the optical absorption cross section and good carrier conductivity in the nanometer grains. There are numbers of attempts to realize high efficiency and good stability single-junction and tandem third generation nc-Si thin film solar cells, ready to make a substantial contribution to the world's photovoltaic market.In this dissertation, we report a detailed experimental and theoretical investigation on the photocurrent characteristics of nanocrystalline Si thin films, with the emphasis on the effect of Si dot size distribution. In previous work, the deviation of Si grain sizes has been well characterized by Raman mapping technique and the film uniformity versus doping concentration was correlated in terms of the growth mechanism. Here, with the aid of the existence of multiple energy states identified from the current-voltage measurements, we are able to decompose the photocurrent spectra, where the photocurrent characteristics (peak position, linewidth and intensity) have been well explained by the average dots size and grain size deviation through the quantum confinement effect. We will show that broader photocurrent response occurs in Si quantum dots with larger size dispersion due to the improvement of light harvest. Meanwhile, as a result of tunneling loss in the expanded energy distribution, we have demonstrated that there is a tradeoff between the absorption enhancement and reduced transport for the photocurrent intensity.While modulation of band energies through size quantization effect offers a good way to control the photoresponse and photoconversion efficiency of quantum dot solar cells, we present in this Letter a novel strategy to maximize the photoresponse through size distribution control of the Si quantum dots. In most quantum dot based optoelectronic devices, it is imperative to achieve good dot size uniformity because the size variation gives rise to an inhomogeneous broadening of optical transitions resulting from carrier recombination or generation. However, the inherent size distribution in quantum dot structures can provide a marked improvement of light harvest for the potential application in quantum dot rainbow solar cells to boost the energy conversion limit. Although expanded energy distribution sacrifices the transport characteristics of the photogenerated carriers, broad size dispersion will provide instinct spread in photoresponse spectra. Therefore, this work indicating that a suitable size dispersion is desirable for the real application of semiconductor nanomaterials in solar cells, where a new approach is developed for maximizing light energy conversion with relative simple and low cost manufacturing process.This work was supported by the Natural Science Foundation of China (contract No. 10734020), National Major Basic Research Project of 2010CB933702, and Shanghai Municipal Commission of Science and Technology Project of 08XD14022.