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Light trapping for thin film crystalline silicon solar cells
Thin film (~10-20um) c-Si can potentially decrease the cost of modules and enables mechanical flexibility and light-weight for low-cost installation. Silicon with its indirect bandgap is a poor light absorber. In bulk silicon solar cells, light absorption at longer wavelengths can be achieved using random pyramid surface texturing with a pyramid size distribution of 4-10um. It is clear that these structures are not suitable for thin film c-Si solar cells. Therefore, various nanophotonic structures have been suggested for advanced light trapping in thin films c-Si, including nanowires, nanocones, nanodomes, inverted pyramids, and nanoholes. In APP, we particularly interested in light trapping structures that can be applied on thin film crystalline silicon surfaces with novel low-cost fabrication techniques.
Electromagnetics and electrical simulation of solar cells
Recent studies on light-trapping in solar cells have been primarily based on either optical simulations, or fabrication of light-trapping various structures (nanowires, nanoholes, nanocones, etc) and absorption measurements. However, design of an efficient solar cell must consider both the electromagnetic wave propagation and carrier transport properties simultaneously. In APP, we perform combined electromagnetic and carrier transport simulations of solar cells to gain insights on the optimum light trapping structures, and doping and contact schemes. Various loss mechanisms (surface, bulk and Auger recombinations), mobility decrease and band gap shrinking with doping are being implemented in our models. We are aiming at extending our model to heterojunction solar cells.
Organic-inorganic perovskite materials have been introduced as the new class of low cost photovoltaic devices to have the promise to compete with current commercialized inorganic solar cells. The unique optoelectronic properties of perovskite absorber layer stem from its desirable band gap, high absorption coefficient over a broad visible range of the sunlight spectrum and from the high minority carrier lifetime and mobilities within the material. Among all, CH3NH3PbI3 has emerged as highly attractive perovskite material used in PV researches with already various fabrication protocols and a favorite direct band gap of 1.55 eV. In APP, along with solution processing method, we use thermal co-evaporation technique to prepare CH3NH3PbI3. This technique offers good crystallinity, morphology, uniformity and high reproducibility, which is particularly suitable for large-area applications. Moreover, due to its band gap tunability, we have been using perovskite in tandem with crystalline Si in order to harvest more of the sunlight spectrum leading to higher Jsc and Voc and consequently higher efficiencies.
Liquid Phase Epitaxy of Germanium on Silicon
Germanium is a group IV element compatible with CMOS fabrication technology and provides infrared photodetectors and high-speed transistors with the advantage of its smaller band gap and higher carrier mobility compared to silicon. It can also be used as a substrate for the growth of III-V compounds on silicon due to lattice-matching. These benefits can be achieved by growing crystalline germanium on silicon, which is a low cost CMOS material. In APP, we are studying the formation of single crystal germanium on insulator layers using liquid phase epitaxy enabled by crystalline silicon seed layer and rapid melting. Effects of the seed window, insulator and capping materials are also aimed to be investigated.
Thin Films Silicon Lift-off by Stress Induced Layer
An important factor for cost reduction of solar energy is the reduction of silicon material quantity that used for making a solar cell. Thin film silicon(20-30 micron) prevents material loss on the other hand these layers can be used for flexible solar cell so that solar cell usage areas can be spread. However, an effective production method for thin film silicon layer has not been developed yet. In APP, we are working on thin film silicon lift-off by stress induced layer which is occurred due to difference between thermal coefficient of expansion of two different materials. The main idea of this project, create a misfit stress on a structure which is combined of epoxy based polymer(as stress induced layer) and silicon by applying thermal process. We studied simulations of the model based on linear thermoelasticiy by using XFEM (Extended Finite Element Method) to understand fracture mechanics. Experiments are also performed parallel to simulations. With the help of this project efficient, flexible and light-weight solar cells can be produced with using thin silicon layers.
UPS and XPS
Various materials which exhibit photovoltaic properties including silicon and perovskite and their hetero-structures show several unique physical properties at their interfaces. In order to fabricate modern devices with superior photovoltaic features, understanding the band alignment of the hetero-junction is of vital importance. Ultraviolet Photoelectron Spectroscopy (UPS) allows the detection of valance band electrons at higher intensities than X-Ray Photoelectron Spectroscopy (XPS). On the other hand, XPS provides for the detection of core level electrons to investigate the elemental and chemical state of the materials. In APP, using UPS/XPS we analyze the work function and valance band structure of various solar materials.