Energy
Solar Photovoltaics
In this thesis, various nano- and micro-architectures are investigated to optically improve the efficiency of solar PV and PECs. A fabrication method for facilitating multiple sizes and spacing of these triangular Ag contacts was developed to extend their applicability to other solar PV and PEC technologies.
Context
Numerical Approach
Role of Recombination Pathways in Optimal Bulk Doping
In low bulk doping, the primary recombination pathway at open circuit voltage is surface recombination accounting for nearly 50%. In Fig. 2.3(c), the open circuit voltage was observed to peak at a bulk doping of 1017 cm.
Conclusion
Introduction
Researchers have explored various nanostructured antireflection and light trapping strategies for thin film Si crystal [44], [19]. Such bare films exhibited close to 4n2 light trapping compared to volume-equivalent planar Si thin film.
Fabrication
Some recent studies have shown limitations in light-trapping structures arising from parasitic absorption, as opposed to limitations in the Si-absorbing material of the cell. An increase in plasma O2 content improves sidewall protection during low temperature etching and therefore results in a change in microwire taper. The etching rate remained constant at 1 μm/min, resulting in microwires with a total height of 30 μm. the business.
Optical Characterization
The angular reflection without and with SiNx anti-reflection coating is shown in Figure 3.4 (a) and (b) and Figure 3.4 (c) and (d) in linear and log plots, respectively. In the 1000nm – 1100nm wavelength range, where the absorption depth is on the order of hundreds of microns, reflectance ranged from 3.6% at normal incidence to 2.7% at 50o for uncoated microcones, which is limited by reflection from the back surface of the ~300 µm thick sample.
Wave Optic Simulations
Total generation was slightly lower in the top and bottom ~5 µm of the wire compared to the generation in the middle. For λavg = 940 nm, there was no significant generation concentration in the wire tip relative to the rest of the microwire volume. For ϼ = 1.6, significant deposition was observed from the top of the wire to the axial center and also in the axial lower quarter.
Potential Applications
In summary, photoelectrochemical deposition of Au on cylindrical p-Si microwire arrays resulted in a spatially anisotropic metal coating on the surface of the microwire, where the localization of the coating was a function of the illumination wavelength. The spatial distribution of photoinduced charge carrier generation rates in the wires, as derived from computer simulations using the FDTD method, correlated well with the localization of metal deposition observed experimentally. Analog simulations of the generation rate of spatially separated photocarriers in an array of related p-Si microwire arrays with pointed (non-cylindrical) wires were also performed.
Status of Silicon Photocathodes
Light-limited current densities up to |Jph | = 43 mA cm-2 was reported for Si µpyramids coated with a highly transparent and potentially antireflective MoSxCly catalyst grown by chemical vapor deposition [167]. For Si microwires (μ-wires), JRHE = -35.5 mA cm-2 was obtained by optimizing the coverage of the electrodeposited Ni-Mo catalyst on the μ-wires and the pitch of the μ-wires [162]. Replacing the catalyst in these structures with thin Pt (~5 nm or less), which is widely used for its high activity and stability in corrosive environments, leads to significant losses in optical absorption and reflection, resulting in a reduction of 5–7 mA cm 2 and JRHE.
Homojunction Si photocathodes with sputtered Pt catalyst
Of the three photocathode geometries, the μ-cone array exhibited the highest light current density, limited. For planar and μ-pyramidal geometries, -JRHE decreased to < 10 mA cm-2 when the Pt layer thickness reached 8 nm or 16 nm, respectively. In contrast, little or no loss of Jph or fill factor accompanied the increased Pt thickness (16 nm) required for stable performance of n+p-Si/Pt μ-conical array photocathodes.
Heterojunction Si photocathodes with electrodeposited Co-P catalyst
Arrays of bare p-Si μ-cones predominantly absorbed light at 625 nm at the tips of the μ-cones [31]. Therefore, even after catalyst deposition, Si μ-cone arrays had better light-trapping properties compared to bare Si pyramidal structures. The photovoltage of Si µ-cone arrays can be improved by optimizing the homojunction doping distribution.
Optical Simulation Results
Compared to the structure in Figure 6.1(c) without nanocones and the same amount of Ni loading as Figure 6.1(b), the transmitted photon flux was 23.3% and 24.8% in air and water, respectively. Strong coupling of the incident light occurs at different radii for different wavelengths as expected from a nanocone Figure 6.4: (c), (d), (e) and (f) show the electric field profiles along the cross section shown in (a) at the maximum in the transmission spectrum of Si with TiO2 and Ni nanocones in air. Compared to the field profiles in Figure 6.4, the profiles in Figure 6.5 show a higher electric field intensity in the middle of the nanocones.
Fabrication of TiO 2 nanocone photoanodes
Therefore, the incorporation of Ni between the TiO2 nanocones results in a photocurrent loss of ~5 mA cm-2, but compared to the array of Ni holes without TiO2 nanocones, the photocurrent increase was >300%. This behavior of the TiO2 nanocones cannot be explained by effective medium theory, and wave-optical simulations show that the nanocones act as an antenna for the incoming radiation, coupling the light to the waveguide modes and providing a path for the light to reach the underlying Si substrate despite the 54% surface coverage of Ni. Since the TiO2 nanocones were electrically insulated using thin SiO2, Ni was preferentially deposited between the TiO2 nanocones on the Si substrate with an increased concentration closer to the cone, as shown in Figures 6.6 (b) and (d).
Optical and electrochemical measurements
Reflectance measurements were made on Si with TiO2 nanocone samples before and after Ni deposition for those shown in Figure 6.6, along with a Ni hole array prepared via electron beam patterning of PMMA on a Ni sputtered Si sample. 50 nm. The simulations were done under a coherent illumination while the experimental measurement was not, and the TiO2 and Ni nanocones as can be seen in the SEM images in Figure 6.6 are not perfect cones and perfectly flat layers, respectively. This indicates that the TiO2 nanocones act as antireflective structures that can couple the incoming light and enable light transmission to the underlying Si light attenuator in this case even when ~54% of the surface was loaded with a thick Ni layer.
ETCs on planar Si heterojunction solar cells
Then the stamp was placed on the ITO patch of the solar cell and silver ink was filled from the side by capillary action. The short-circuit current density is 33 mA cm-2 and is therefore 2 mA cm-2 lower than the cell with only the ITO layer and no metal contacts (blue curve). The active surface of the measured cells was 5 Figure 7.2: Current-voltage characteristics of silicon heterojunction solar cells with three different front contacts.
ETC implementation over other solar cells
The superstrate way of implementing ETCs could be useful for very rough Si solar cells. Therefore, the ability to implement ETC via textured Si and perovskite solar cells can improve light management and enable record efficiency in a 4-terminal tandem device. Recently, it has been demonstrated that a dense array of ETCs over thin Si solar cells and the backside of a bifacial solar cell can improve the light trapping properties and result in higher efficiency [236, 237].
Fabrication of Si masters for ETCs
The aspect ratio and taper of the triangular lines etched into the silicon sample can be adjusted by varying the SF6/O2 ratio in the plasma. The two images in Figure 7.6 are designed for a 50% coverage of ETCs designed via electron beam lithography. These samples used a grid of line pattern and therefore horizontal triangular groves can be seen intersecting the triangular grove facing out of plane on this side.
Printing ETCs from etched Si masters
ETC superstrates
As previously mentioned, the production of Si masts with a width of 30 – 50 µm has not yet been realized, and is therefore not yet ready for use on Si homojunction solar cells. Currently, efforts are being made to fabricate large size triangles in Si or to use 3D printing to realize hard masters with 1:3 aspect ratio triangles with a 30 – 50 µm base.
Future Work on ETCs
Another key question that has not been explored is whether ETCs can survive the lamination process after application onto a solar cell. All solar cell technologies are encapsulated in a polymer to prevent physical and chemical degradation due to external factors, and demonstrating that ETCs can withstand the lamination process will be an important step to convince us researchers and the photovoltaic industry that they can be realized outside. laboratory for real-world applications. In summary, this thesis mainly deals with optical strategies using tipped structures for light trapping, which are generally applicable to solar photovoltaics and/or photoelectrochemical systems.
Semiconducting micro- and nano- cones
Pt and Co-P hydrogen evolution catalysts were encapsulated with Si microcones to demonstrate that high loadings of light-blocking highly active catalysts can be employed with minimal photocurrent loss. Therefore, a series of photocathodes with Au deposited at various locations across a cone can be explored to find the best architecture for Si cones using gold for CO2 reduction. Therefore, photoelectrochemical Au depositions over these nanocones can be performed to experimentally show the region of high light confinement, potentially depositing Au at different radius on the nanocone depending on the wavelength of illumination.
Dielectric nanocones
Unlike Si, direct band gap semiconductors such as GaAs and InP demonstrate higher light confinement. In terms of cost reduction and scalability, these structures can in principle be fabricated using nanoimprint lithography [241]. Nanoimprint can also be a milder process than etching in terms of causing damage to the underlying light absorber due to the criticality of stopping etching immediately when the dielectric is etched.
Metal Triangles
The nanocones used in the work presented in this thesis were produced by dry etching. A previous study of Au and Pd nanocones showed a field-induced decrease in overvoltage due to reagent concentration [242] . Finally, the work presented in this thesis demonstrates several light-trapping strategies applicable to both photovoltaics and photoelectrochemical cells.
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Trupke et al., "Temperature dependence of the radiative recombination coefficient of intrinsic crystalline silicon," Journal of Applied Physics, vol. Boettcher et al., "Photo-electrochemical Hydrogen Evolution Using Si Microwire Arrays," Journal of the American Chemical Society, vol. Yang et al., "Synthesis of PbTe Nanowire Array Using Lithographically Patterned Nanowire Electrodeposition," Nano Letters, vol.
