Low-cost, high-efficiency, silicon based photovoltaic devices
The favorable bandgap and natural abundance of silicon, combined with the large expertise on semiconductor wafer processing, have led to the use of wafer-based crystalline silicon (c-Si) in the vast majority of photovoltaic cells and modules produced worldwide. However, the high cost of purifying, crystallizing, and segregating Si wafers has inhibited these photovoltaic energy sources from approaching cost parity with conventional sources of energy. In this dissertation work, we exploit the description and exploration of low-cost, relatively high-efficiency non-conventional silicon based photovoltaic devices. Three different non-conventional approaches were studied, namely, (a.) radial p-n junction wires arrays for effective carrier collection, (b.) plasmonic metal nanoparticles for light trapping, (c.) organic/inorganic hybrid heterojunction, as a means to realize a low cost photovoltaic devices. Radial p-n junctions are potentially of interest in photovoltaics devices as a way to decouple light absorption from minority carrier collection. In a traditional planar design these occur in the same direction, orthogonal to the plane of the solar cell, and this sets a lower limit on absorber material quality, as cells must both be thick enough to effectively absorb the solar spectrum while also having minority-carrier diffusion lengths long enough to allow for efficient collection of the photo-generated carriers. In a radial p-n junction design, decoupling occurs because the direction of light absorption becomes perpendicular to the diffusion direction of minority-carrier transport, allowing the cell to be thick enough for effective light absorption, while also providing a short pathway for carrier collection. Furthermore, plasmonic effects have gained tremendous momentum in solar cells research because they are deemed to be able to dramatically boost the efficiency of photovoltaic devices. A key opportunity of plasmon resonance lies in their ability to condense the conduction electron oscillation strength into the desired spectral range. In the first part of this dissertation work, a novel solar cell device geometry having sub-wavelength nanotextured surfaces in combination with plasmonic gold nanoparticles is studied via simulation. The computation modeling indicates that, the short circuit current density (JSC) and the power conversion efficiency (PCE) values of 31.57mA/cm 2 and 25.42%, respectively, have been achieved in just 2.8 μm thin single crystal silicon film that compare favorably well to the predicted JSC and PCE values of 25.45mA/cm 2 and 20.87%, respectively, for an optimized nanotextured surface without plasmonic effects of Au nanoparticles. A promising power conversion efficiency as high as 13.30% has been demonstrated in the laboratory by employing the experimental techniques described herein which has a noticeable potential for lowering the manufacturing cost. An alternative approach using low-temperature, chemical-solution based organic-semiconductor hybrid devices is potentially more affordable compared to their inorganic counterparts, however organic solar cells are generally considered not very efficient. In the last part of this dissertation work, we explore whether organic semiconductors can be judiciously integrated with silicon to form hybrid organic/silicon solar cells that are both efficient and low-cost. PCE values above 10.50% have been achieved in the described hybrid solar cells based on highly ordered silicon nanopillars (SiNPs) arrays and poly(3,4-ethylene-dioxythiophene):polystyrenesulfonate (PEDOT:PSS) thin films. In addition, an ultrathin, flexible hybrid Si/organic polymer solar cell with a promising PCE has also been demonstrated in sub-ten micrometers free-standing Si membranes.