Si nanostructures based solar cells
Date of Issue2014
School of Electrical and Electronic Engineering
A*STAR Singapore Institute of Manufacturing Technology
To address the current problems of increasing energy shortages and serious environmental pollutions, clean energy option such as photovoltaic has been actively explored. Currently, the single crystalline silicon (Si) solar cell dominates the photovoltaic market. Unfortunately, its high cost has seriously impeded its competition with fossil fuels and prevented its wide-spread application. To address the cost issue, nanostructures based Si solar cells have been actively researched as the strong light trapping capability of the nanostructures will potentially allow a thinner layer of Si to be employed. In order to further reduce the cost of Si nanostructures based solar cells, hybrid Si/organic cells incorporating conductive polymers have been pursued, leveraging on the benefits of polymers which include low temperature, simple and solution based process capability, and low cost. In this work we study high efficiency hybrid solar cells based on Si nanohole (SiNH) structures and poly (3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS). The SiNH structures are fabricated on Si substrates using the metal-catalyzed electroless etching (MCEE) method, and the cells are obtained by spin coating of PEDOT:PSS on top. The hybrid cells are optimized by varying the hole depth, and a maximum power conversion efficiency (PCE) of 8.3% is achieved at a hole depth of 1 μm. To better understand the experimental results, we have also simulated the optical characteristics of SiNH structures using the finite element method, and the results are discussed in the context of light trapping and scattering. The SiNH structures fabricated using the MCEE technique are generally non-periodic. In this work we have also studied the fabrication of periodic SiNH structures with hexagonal unit cell, using a monolayer of patterned polystyrene (PS) spheres as a mask. Periodic SiNH structures with different structural dimensions have been fabricated and investigated in terms of their microstructures and optical reflectance. Compared with planar Si wafer, such structures exhibit much lower reflectance due to the enhanced scattering introduced by the SiNHs. Concurrently we have carried out optical simulation using the finite element method to optimize the structural periodicity and hole diameter for maximum light absorption. The optimum structure is found to have a periodicity of 693 nm and hole diameter of 560 nm, and it yields the highest ultimate efficiency of 28.8%. Besides the vertical SiNH structures, we have investigated the light absorption characteristics of several other nanoholes structures to examine their potential applications in solar cells. These include (i) slanting SiNH structures where the longitudinal axis of the SiNHs is not aligned perpendicular to the surface of the Si film, (ii) random SiNH structures where randomness has been introduced to the radius, depth and position of the SiNHs, to simulate more realistic non-periodic structures that are achieved using low cost approaches such as the MCEE technique, and (iii) a hybrid solar cell structure that comprises nanopyramids and nanoholes. For the slanting SiNH structures, the optimal condition for maximum absorption of solar energy is achieved when the periodicity is 700 nm and diameter/periodicity ratio is 0.85. The highest ultimate efficiency achieved is 32.9%, which is higher than that of the vertical counterpart of 29.7%. Therefore, the slanting SiNH structure is a potential approach to further improve the efficiency of Si solar cell. As for the random SiNH structures, it is found that when randomness is introduced to the hole radius, depth and position, the light absorption is significantly improved compared to the periodic nanohole array due to reduced reflectance, additional resonances induced and broadening of the existing resonance. Therefore, structural randomness in SiNH structures is beneficial for light absorption, and hence the use of high cost techniques to fabricate periodic SiNH structures for solar cell application may not be necessary. Lastly, we proposed a hybrid solar cell structure that comprises nanopyramids and nanoholes. The hybrid nanostructure is designed with the smaller NH structure to suppress light reflectance for short wavelength light and the larger NP structure to enhance light trapping for long wavelength light. The optimized structure has a periodicity of 800 nm and nanohole diameter of 560 nm. The hybrid nanostructure has light absorption that is substantially increased above the Lambertian limit, achieving an ultimate efficiency of 38.3%. The ultimate efficiency is maintained above the Lambertian limit even for incident angle up to 50 degree for TM polarized sunlight. The results suggest that the proposed hybrid nanostructure is very promising to achieve high performance solar cell. To further reduce the cost of Si/organic hybrid solar cells, it is important that the cells are fabricated on Si thin films, rather than bulk Si wafers. In this work we also study the formation of poly-crystalline Si (poly-Si) based on laser annealing of amorphous silicon (a-Si) thin films. Besides achieving crystallization, our approach of using the laser annealing process has also resulted in the formation of nanostructures on the surface of the poly-Si films, which is important for light trapping and enhancement of optical absorption. In the first study, a 400 nm thick a-Si film is crystallized using a diode-pumped solid-state neodymium doped yttrium orthvanadate (DPSS Nd:YVO4) UV laser. It is found that with pulse energy of 380 mJ/cm2, the a-Si is converted to poly-Si with concurrent nanodome texturing with periodicity of 300 to 500 nm. The absorption efficiency (ultimate efficiency) of the illuminated Si thin film is enhanced by ~ 200% as compared with the untreated a-Si. In our second set of experiments, a femtosecond laser is used to crystallize a-Si film with a thickness of 1.6 μm, which simultaneously leads to the formation of micro/nanocone structures on the surface with diameters varying from 160 nm to 1.4μm. The micro/nanocone structures have resulted in significantly enhanced light absorption as compared with the untreated film. We have also used the same femtosecond laser to fabricate periodic ripple structure on crystalline Si wafer surface with a structural periodicity of 600 nm and ripple modulation height of 300 nm. Compared with planar Si substrate, the ripple structures have substantially suppressed light reflectance, leading to a 41% improvement in light absorption.
DRNTU::Engineering::Electrical and electronic engineering::Microelectronics