Transparent conductors—A status review

Energy harvesting generators are attractive as inexhaustible replacements for batteries in low-power wireless electronic devices and have received increasing research interest in recent years. Ambient motion is one of the main sources of energy for harvesting, and a wide range of motion-powered energy harvesters have been proposed or demonstrated, particularly at the microscale. This paper reviews the principles and state-of-art in motion-driven miniature energy harvesters and discusses trends, suitable applications, and possible future developments.

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Energy Harvesting From Human and Machine Motion for Wireless Electronic Devices

This Review focuses on recent developments in the use of ZnO nanostructures for dye-sensitized solar cell (DSC) applications. It is shown that carefully designed and fabricated nanostructured ZnO films are advantageous for use as a DSC photoelectrode as they offer larger surface areas than bulk film material, direct electron pathways, or effective light-scattering centers, and, when combined with TiO2, produce a core–shell structure that reduces the combination rate. The limitations of ZnO-based DSCs are also discussed and several possible methods are proposed so as to expand the knowledge of ZnO to TiO2, motivating further improvement in the power-conversion efficiency of DSCs.

https://depts.washington.edu/solgel/documents/pub_docs/journal_docs/2009/advanced_material2009.pdf

ZnO Nanostructures for Dye-Sensitized Solar Cells

http://nathan.instras.com/documentDB/paper-44.pdf

Surface photovoltage phenomena: theory, experiment, and applications

To convert the energy of sunlight into chemical energy, the leaf splits water via the photosynthetic process to produce molecular oxygen and hydrogen, which is in a form of separated protons and electrons. The primary steps of natural photosynthesis involve the absorption of sunlight and its conversion into spatially separated electron–hole pairs. The holes of this wireless current are captured by the oxygen evolving complex (OEC) of photosystem II (PSII) to oxidize water to oxygen. The electrons and protons produced as a byproduct of the OEC reaction are captured by ferrodoxin of photosystem I. With the aid of ferrodoxin–NADP+ reductase, they are used to produce hydrogen in the form of NADPH. For a synthetic material to realize the solar energy conversion function of the leaf, the light-absorbing material must capture a solar photon to generate a wireless current that is harnessed by catalysts, which drive the four electron/hole fuel-forming water-splitting reaction under benign conditions and under 1 su...

The artificial leaf

By coupling the results of electrical measurements, such as spectral response, lighted and dark I-V determinations, and deep-level-transient spectroscopy with optical and laser scan photomicroscopy, we have evaluated the effects of grain boundaries and impurities on silicon solar cells. Titanium which produces two deep levels, E_{v} + 0.30 and E_{c} - 0.26 eV, in silicon, degrades cell performance by reducing bulk lifetime and thus cell short-circuit current. Electrically active grain boundaries induce carrier recombination in the bulk as well as in the depletion region of the solar cell. Experimental data imply a small but measurable segregation of titanium into some grain boundaries of the polycrystalline silicon containing high Ti concentration (2 × 1014cm-3). However for the titanium-contaminated polycrystalline material used in this study, solar cell performance is dominated by the electrically active titanium concentration in the grains. Microstructural impacts on the devices are of secondary importance.

The properties of polycrystalline silicon solar cells with controlled titanium additions

This paper shows for the first time a comparison of commercial-ready n-type passivated emitter , rear totally diffused solar cells with boron (B) emitters formed by spin-on coating, screen printing, ion implantation, and atmospheric pressure chemical vapor deposition. All the B emitter technologies show nearly same efficiency of ~20%. The optimum front grid design (5 busbars and 100 gridlines), calculated by an analytical modeling, raised the baseline cell efficiency up to 20.5% because of reduced series resistance. Along with the five busbars, rear point contacts formed by laser ablation of dielectric and physical vapor deposition Al metallization resulted in another 0.4% improvement in efficiency. As a result, 20.9% efficient n-type passivated emitter, rear totally diffused cell was achieved in this paper. Copyright © 2016 John Wiley & Sons, Ltd.

Process development and comparison of various boron emitter technologies for high‐efficiency (~21%) n‐type silicon solar cells

This paper presents the development and analysis of the first 19%-efficient silicon solar cells fabricated by rapid thermal processing (RTP). These cells required no high-temperature conventional furnace processing (CFP). Improved front surface passivation by rapid thermal oxidation (RTO) was found to reduce S/sub front/ from 7.5/spl times/10/sup 5/ to 2/spl times/10/sup 4/ cm/s, decrease J/sub 0e/ by almost an order of magnitude, and increase cell efficiency by about 1% (absolute). An additional 1% gain in efficiency was achieved by applying a thicker screen-printed, RTP-alloyed Al-BSF instead of the standard 1 /spl mu/m evaporated Al-BSF. Because of the rapid heating rate in RTP, the RTP Al-BSF also gives a more uniform and effective BSF compared with the CFP Al-BSF. In comparison with furnace processed cells, this RTP/RTO process reduced the time for diffusion and oxidation from 330 to 10 min without sacrificing efficiency. To reduce back surface recombination further, RTO can be grown on the back. PCD characterization of RTO on 1.3 /spl Omega/cm Si reveals an S/sub back/ well below 100 cm/s, which can result in >20% efficient RTP/RTO cells.

Characterization and application of rapid thermal oxide surface passivation for the highest efficiency RTP silicon solar cells

In this work, we report on ion-implanted, high-efficiency n-type silicon solar cells fabricated on large area pseudosquare Czochralski wafers. The sputtering of aluminum (Al) via physical vapor deposition (PVD) in combination with a laser-patterned dielectric stack was used on the rear side to produce front junction cells with an implanted boron emitter and a phosphorus back surface field. Front and back surface passivation was achieved by thin thermally grown oxide during the implant anneal. Both front and back oxides were capped with SiNx, followed by screen-printed metal grid formation on the front side. An ultraviolet laser was used to selectively ablate the SiO2/SiNx passivation stack on the back to form the pattern for metal–Si contact. The laser pulse energy had to be optimized to fully open the SiO2/SiNx passivation layers, without inducing appreciable damage or defects on the surface of the n+ back surface field layer. It was also found that a low temperature annealing for less than 3 min after PVD Al provided an excellent charge collecting contact on the back. In order to obtain high fill factor of ~80%, an in situ plasma etching in an inert ambient prior to PVD was found to be essential for etching the native oxide formed in the rear vias during the front contact firing. Finally, through optimization of the size and pitch of the rear point contacts, an efficiency of 20.7% was achieved for the large area n-type passivated emitter, rear totally diffused cell. Copyright © 2014 John Wiley & Sons, Ltd.

20.7% efficient ion‐implanted large area n‐type front junction silicon solar cells with rear point contacts formed by laser opening and physical vapor deposition

Low-cost high efficiency solar cells are the key to achieving grid parity with photovoltaic devices. Laser processing in silicon photovoltaics is being incorporated at various stages to achieve this target. This paper details the fabrication, characterization and analysis of 4 cm2 screen printed cells with efficiency over 20% achieved using a UV laser ablation for selective opening of rear dielectric. These cells are compared to cells fabricated using a screen printed etching paste for opening vias through the rear dielectric. Microscopy was used to examine the impact of laser pulses on the silicon surface and quality of the BSF and compare it with vias opened using etching paste. Characterization and analysis of these cells is performed using IQE measurements and supported by PC1D modeling. It was found that while laser ablation had an effect on the morphology of the silicon surface, the overall quality of the local back surface field and dielectric rear passivation were maintained, resulting in high cell efficiencies and V OC .

Ajeet Rohatgi

Papers

The properties of polycrystalline silicon solar cells with controlled titanium additions

Process development and comparison of various boron emitter technologies for high‐efficiency (~21%) n‐type silicon solar cells

Characterization and application of rapid thermal oxide surface passivation for the highest efficiency RTP silicon solar cells

20.7% efficient ion‐implanted large area n‐type front junction silicon solar cells with rear point contacts formed by laser opening and physical vapor deposition

20% efficient screen printed LBSF cell fabricated using UV laser for rear dielectric removal