Toward Highly Efficient Heterojunction Solar Devices: An Electrical and Optical Concurrent Design
Date Issued
2016
Date
2016
Author(s)
Wang, Hsin-Ping
Abstract
To achieve high efficient solar devices, concurrent engineering design involving electrical and optical perspectives in parallel is necessary. The heterojunction is presently popular design in the photovoltaics due to the low recombination rate, leading to high open-circuit voltages (VOC). Here, we’ll focus on boosting the efficiency of heterojunction solar devices. First of all, hierarchical structures combining micropyramids and nanowires with appropriate control of surface carrier recombination represent a unique class of architectures for radial p-n junction solar cells that synergizes the advantageous features including excellent broadband, omnidirectional light-harvesting and efficient separation/collection of photoexcited carriers. The heterojunction solar cells fabricated with hierarchical structures exhibit the efficiency of 15.14% using cost-effective as-cut Czochralski n-type Si substrates, which is the second highest reported efficiency among all n-type Si nanostructured solar cells. This is also the first described omnidirectional solar cell that exhibits the daily generated power enhancement of 44.2%, as compared to conventional micropyramid control cells. The concurrent improvement in optical and electrical properties for realizing high-efficiency omnidirectional solar cells using as-cut Czochralski n-type Si substrates demonstrated here makes hierarchical architecture concept promising for large-area and cost-effective mass production. Amorphous Si (a-Si)/ crystalline Si (c-Si) heterojunction (SHJ) photoelectrochemical cells can serve as highly efficient and stable photoelectrodes for solar fuel generation. Low carrier recombination in the photoelectrodes leads to a high photocurrent and high photovoltage. Both SHJ photoanodes and photocathodes are designed for high efficiency oxygen and hydrogen evolution. The SHJ photoanode with sol-gel NiOx as the catalyst shows the current density of 21.48 mA/cm2 at the equilibrium water oxidation potential. The SHJ photocathode displays excellent hydrogen evolution performance with an onset potential of 0.640 V and a solar to hydrogen conversion efficiency of 13.26%, which is the highest ever reported for Si-based photocathodes. Then, we moved to the next promising photovoltaic materials: InP. The thin-film vapor-liquid-solid (TF-VLS) growth technique presents a promising route for high quality, scalable and cost-effective InP thin films for optoelectronic devices. Towards this goal, careful optimization of material properties and device performance is of utmost interest. Here, we show that exposure of polycrystalline Zn-doped TF-VLS InP to a hydrogen plasma (in the following referred to as hydrogenation) results in improved optoelectronic quality as well as lateral optoelectronic uniformity. Notably, hydrogenation reduces the relative intra-gap defect density by one order of magnitude. As a metric to monitor lateral optoelectronic uniformity of polycrystalline TF-VLS InP, photoluminescence and electron beam induced current mapping reveal homogenization of the grain versus grain boundary upon hydrogenation. At the device level, we measured more than 260 TF-VLS InP solar cells before and after hydrogenation to verify the improved optoelectronic properties. Hydrogenation increased the average VOC of individual TF-VLS InP solar cells by up to 130 mV, and reduced the variance in VOC for the analyzed devices. Finally, we develop a growth mode that enables to simultaneously obtain InP in-situ doped with different dopants and different concentrations. The process utilizes templated liquid-phase crystal growth with the spin-on dopant (SOD) as the cap and dopant sources. n-type InP with the doping level from 1.0×1017 to 4.8×1018 cm-3 can be successfully obtained in the same growth run by controlling the dilution of Sn-doped SOD. The doping level of p-type InP could be controlled from 9.0×1016 to 3.0×1018 cm-3. Finally, we perform to simultaneously grow both n-type and p-type InP patterns on the same substrate by defining SOD with the pre-patterning metal templates. This result outlines a promising method to achieve partial in-situ doping of materials for future optoelectronic applications.
Subjects
Subwavelength-antireflective hierarchical structure
heterjunction solar device
surface passivation
grain boundary passivation
thin-film vapor-liquid-solid (TF-VLS) growth technique
SDGs
Type
thesis