Si wire arrays based solar energy harvesting systems

Abstract

Si nano- and microwire arrays, which have superior light absorption ability and carrier collection efficiency, have been emerged as promising building blocks for solar energy harvesting systems. Recently, Si wire arrays used as two critical roles in solar cell such as antireflection coating and light absorber. Antireflective Si wire textured on the surface of Si substrate could effectively reduce the light reflection by two dominant mechanisms including gradual reduction of refractive index mismatch between air and Si substrate and multiple light scattering which can enhance the optical path length in the absorber media. Light absorber is another usage of Si wire arrays in solar cells. Cylinder shaped geometry of Si wire provides superior light absorption ability over broad wavelengths by increase of light path length and light resonance. In addition, employing radial p-n junction in Si wire causes efficient carrier collection due to short distance of minority carrier path length. These two critical benefits allow using the low quality and/or thin Si substrate which could reduce fabrication cost and material consumption. In this thesis, we study the optical and electrical properties of Si wire as roles of antireflection coating and light absorber in solar cells. Silicon nanowires (SiNWs) and microwires (MWs) are cost-effectively integrated on a four inch wafer using metal-assisted electroless etching for solar cell applications. In chapter 2, we study antireflective SiNW arrays based solar cell. A tapering feature of SiNW made refractive indexes graded between Si and air such that the mismatching of optical impedance was alleviated, resulting in a strong antireflection ability. We suggest a design guideline that optimally combines light absorptance and electrical performance in SiNW solar cells. The design was experimentally developed using a tradeoff relation between wire length and space-filling ratio. A short length (~500 nm) design using nanopillars rather than nanowires effectively diminished the surface and bulk recombinations to achieve a high short circuit current while maintaining good light absorptance. In addition, introducing a ZnSe quantum dot (QD) layer over SiNW solar cells, which provide the combination of two major benefits of superior light trapping and photon down-conversion, could enhance considerably the conversion efficiency. In chapter 3, we investigate the optical and electrical properties of free-standing Si wires which are suitable for light absorber due to their superior light absorptance. We demonstrate that novel design of flexible solar cell based upon the free-standing Si wire arrays embedding into polymer film provides opportunities to reduce the material consumption while maintaining superior light absorptance. Superior light scattering of the randomly arrayed SiNWs significantly improved the light path length, which allow to obtain the great light absorptance from only using short length of SiNW (<12 m), especially in short wavelengths (<700 nm). Adoption of the rugged metallic back surface for exciting the surface plasmon polaritons along the interface between the metal and Si showed a plasmonic potential to enhance light absorption in long wavelengths (>700 nm). This flexible film yielded a light absorption of ~92.6% over broad wavelength at a wire length (12 m, the use of only ~5% of the silicon required for conventional solar cells). Furthermore, we have developed a cost-efficient way to fabricate a waferscale co-integrated wire array of radial p-n junction MWs and bulk p-n junction tapered NWs for wired photovoltaic applications. A co-integrated, tapered NWs and MWs (CNMW) achieved a considerable performance of photovoltaic devices with a CE of 7.19%. Two major benefits, i.e., optical absorption enhanced by bulk-junction NWs and efficient carrier collection by radial junction MWs, were confirmed using the co-integrated wire sample (CNMW) made up of MWs and tapered NWs. To boost the photovoltaic performance of Si wire solar cells, we propose the wrap-around top metal contacts for radial junction silicon wire solar cells of which a thin Ag sheet surrounds the silicon wires close to the planar bottom contacts. A fill factor of 65.4% was obtained, which is higher than that of conventional top contacts made using transparent conductive oxides and metal grids. Beyond photovoltaic cells application, in chapter 4, photoelectrochemical cells, one of a solar energy harvesting system, which can directly convert the sun light into fuel, were developed based upon the Si nanostructures. The effective photocathode adopting a tapered Si nanohole (SiNH) array has been demonstrated for photoelectrochemical water splitting. Compared to a planar counterpart, the formation of NHs caused an increase in optical bandgap, which could generate surface passivation effect (for lowering dark current) while increasing a photovoltage (for anodic shift of onset voltage). As a result, photogenerated current was improved in tapered SiNHs while reducing overpotential required for H2 evolution. SiNHs also adopted in thin c-Si (20 m in thickness) based photocathode as a strategy for reducing the material consumption. We found that SiNHs significantly improve light absorption and reduce the surface recombination loss, which enable to obtain a high photocurrent (23 mA/cm2 at 0V vs. RHE) even in thin c-Si, approaching a comparable performance level of generally used thick c-Si (100200 m).