Uniformity And Bandgap Engineering In Hydrogenated Nanocrystalline Silicon Thin Films For Solar Cell Application

Energy is the basis of our daily life, while the problem of environment pollution and energy source shortage are becoming increasingly serious. Therefore, the exploitation of clean power and renewable energy resources becomes the urgent problem. As an inexhaustibility and green energy, solar energy provides a promising prospect.Due to their advantages over the first generation crystalline Si solar cells in ease of large area and low manufacturing cost, the second generation thin film solar cells, especially those based on Si, have attracted much attention and are ready to make a substantial contribution to the world's photovoltaic market. For commercial amorphous Si thin films, however, there still remain some unavoidable disadvantages, i.e., low energy conversion efficiency, low conductance, and light induced degradation of cell performance (the so-called Staebler-Wronski effect). Nanocrystalline Si thin film, a mixed-phase material consisting of nanocrystals embedded in amorphous tissue, presents very promising features in solving these problems. Extensive optical and electrical investigations of nc-Si thin films have been carried out. Strong optical absorption and high photocurrent are 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.Uniform deposition over a large area, which is one of the principle challenges in applying nc-Si thin film on an industrial scale, plays a significant role in promoting the performance of solar cells. Another critical factor that influences the performance of nc-Si solar cells is the controllable optical bandgap of nc-Si to make full use of the sunlight.In this paper, micro-Raman mapping with a spatial resolution of micrometer and room-temperature visible photoluminescence (PL) has been carried out on phosphorous-doped hydrogenated nanocrystalline silicon (nc-Si:H) thin films grown by plasma enhanced chemical vapor deposition. Both the thin film uniformity and structural properties are revealed.We start with Raman analyses to get information about the structures, i.e., the distribution of grain sizes and crystalline volume fraction, of our P-doped nc-Si:H thin films with different doping concentrations and physically interpreted on the basis of growth mechanism. We notice the first gradual decrease of the average grain size d when CP increases up to 5%, and then gradual increase with the further increase of CP, whereas the average crystalline volume fraction XC decreases continuously from 54.9% to 35.4% as CP increases from 0% to 20%. At the same time, we can identify easily the changing of the average grain size and the linewidth of the distribution, which indicates the standard deviation, with the doping concentration. The intrinsic sample D1 exhibits the narrowest linewidth in the distribution profile, i.e., the most homogeneous with the least relative deviation of only 1.88%, while in doped samples the relative deviation (thin film nonuniformity) first decreases and then enhances with increasing doping concentration. Interestingly, the mean values of d and XC appear to have an opposite dependence on CP to those of strain andΓ3, respectively, which demonstrates that increasing doping concentration reduces the degree of structural order within these nc-Si:H thin films. In order to gain more insight into the optical properties such as the bandgap of these nc-Si:H thin films, we performed photoluminescence measurements. The PL profiles can be well reconstructed by using the model proposed by Islam and Kumar (I-K model), which takes into account the combined effects of quantum confinement, localized surface states, and grain size distribution, together with a lognormal rather than normal crystallite size distribution. The PL profiles yield microstructural information in good agreement with the Raman analysis, demonstrating that PL spectra is a convenient method to acquire the size dispersion, while the Raman mapping can further identify the thin film uniformity in nanomaterials, which is indispensable in solar cell evaluation from the viewpoints of both fundamental physics and future applications. Additionally, we have investigated the shift of the PL peak energy stemming from the variation of P doping concentration, which is determined by the values of d,σPL/d, and C.Therefore, selection of appropriate doping concentration allows the production of samples exhibiting high uniformity and suitable optical bandgap. We expect that this study may benefit to the improvement of the nc-Si:H thin film solar cell performance, although controlling the growth conditions such as gas pressure, hydrogen dilution ratio, and substrate temperature, which are not independent of each other for optimizing the growth of nc-Si:H, still needs further investigations.This work was supported by the National Major Basic Research Project of 2010CB933702, Natural Science Foundation of China (contract No. 10734020), and Shanghai Municipal Commission of Science and Technology Project of 08XD14022.