Silicon-in-silica spheres via axial thermal gradient in-fibre capillary instabilities

The ability to produce small scale, crystalline silicon spheres is of significant technological and scientific importance, yet scalable methods for doing so have remained elusive. Here we demonstrate a silicon nanosphere fabrication process based on an optical fibre drawing technique. A silica-cladded silicon-core fibre with diameters down to 340 nm is continuously fed into a flame defining an axial thermal gradient and the continuous formation of spheres whose size is controlled by the feed speed is demonstrated. In particular, spheres of diameter \u3c 500 nm smaller than those produced under isothermal heating conditions are shown and analysed. A fibre with dual cores, p-type and n-type silicon, is drawn and processed into spheres. Spatially coherent break-up leads to the joining of the spheres into a bispherical silicon \u27p-n molecule\u27. The resulting device is measured to reveal a rectifying I-V curve consistent with the formation of a p-n junction

A Two-Step Absorber Deposition Approach To Overcome Shunt Losses in Thin-Film Solar Cells: Using Tin Sulfide as a Proof-of-Concept Material System

As novel absorber materials are developed and screened for their photovoltaic (PV) properties, the challenge remains to reproducibly test promising candidates for high-performing PV devices. Many early-stage devices are prone to device shunting due to pinholes in the absorber layer, producing “false negative” results. Here, we demonstrate a device engineering solution towards a robust device architecture, using a two-step absorber deposition approach. We use tin sulfide (SnS) as a test absorber material. The SnS bulk is processed at high temperature (400˚C) to stimulate grain growth, followed by a much thinner, low-temperature (200˚C) absorber deposition. At lower process temperature, the thin absorber overlayer contains significantly smaller, densely packed grains, which are likely to provide a continuous coating and fill pinholes in the underlying absorber bulk. We compare this two-step approach to the more standard approach of using a semi-insulating buffer layer directly on top of the annealed absorber bulk, and demonstrate a more than 3.5x superior shunt resistance Rsh with smaller standard error σRsh. Electron-beam induced current (EBIC) measurements indicate a lower density of pinholes in the SnS absorber bulk when using the two-step absorber deposition approach. We correlate those findings to improvements in the device performance and device performance reproducibility.Chemistry and Chemical Biolog

Nanostructured photovoltaics : improving device efficiency and measuring carrier transport

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2017.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 153-163).Photovoltaics (PV) offer a promising route to combat climate change. However, the growth rate of the dominant commercial photovoltaic (PV) technology is limited by large capital expenditure requirements. This motivates fundamental research into thin-film materials, such as lead sulfide (PbS) quantum dots (QDs), that are composed of earth-abundant elements, can be produced through low-cost deposition techniques, and are stable under operating conditions. In this thesis, a device architecture that combines a zinc oxide (ZnO) nanowire ordered bulk heterojunction (OBHJ) architecture with band alignment engineering of the PbS QD film to enhance charge extraction is demonstrated. This approach results in PV devices with photocurrent density greater than 30 mA/cm2, which represents a 15% improvement compared to planar devices and enables solar cells with power conversion efficiency up to 9.6%. This photocurrent density is the highest achieved for QDs with a 1.3 eV band gap, which is the optimal band gap in the detailed balance limit. The enhanced photocurrent in the nanowire devices is shown to be a result of both improved light harvesting due to improved in-coupling of light after the addition of the ZnO nanowire array and improved carrier collection due to the bulk heterojunction effect. Furthermore, electron beam-induced current (EBIC) was used to study charge transport in PbS QD films. It is shown that holes are the minority carrier in PbS QD films treated with tetrabutylammonium iodide (TBAI). This finding indicates that the thickness of OBHJ devices composed of a PbS-TBAI film paired with an n-type nanowire array are constrained by minority carrier transport. Moreover, quantitative EBIC was applied for the first time on PbS QD diodes to measure the bulk minority carrier diffusion length (Lbulk). Lbulk was extrapolated by comparing the effective diffusion length measured at different beam energies. EBIC injection leads to high-level injection conditions, therefore a lower bound for the hole diffusion length in PbS-TBAI QD films is established, with Lbulk e 110 nm. This provides a critical design parameter for OBHJ solar cells. This thesis motivates further work on optimization of ZnO nanowire arrays for PbS QD OBHJ solar cells through array patterning, acceptor-doping, and passivation of the nanowire surface. Furthermore, the EBIC technique developed in this work can be applied to quantitatively measure nanoscale carrier diffusion lengths in other thin-film PV materials.by Paul Harlan Rekemeyer.Ph. D

Improved efficiency in organic/inorganic hybrid solar cells by interfacial modification of ZnO nanowires with small molecules

We demonstrate improved photovoltaic performance of ZnO nanowire/poly(3-hexylthiophene) (P3HT) nanofiber hybrid devices using an interfacial modification of ZnO nanowires. Formation of cascade energy levels between the ZnO nanowire and P3HT nanofiber was achieved by interfacial modification of ZnO nanowires using small molecules tetraphenyldibenzoperiflanthene (DBP) and 3,4,9,10-perylenetetracarboxylic bisbenzimidazole (PTCBI). The successful demonstration of improved device performance owing to the cascade energy levels by small molecule modification is a promising approach toward highly efficient organic/inorganic hybrid solar cellsclose1

Simultaneous high crystallinity and sub-bandgap optical absorptance in hyperdoped black silicon using nanosecond laser annealing

Hyperdoped black silicon fabricated with femtosecond laser irradiation has attracted interest for applications in infrared photodetectors and intermediate band photovoltaics due to its sub-bandgap optical absorptance and light-trapping surface. However, hyperdoped black silicon typically has an amorphous and polyphasic polycrystalline surface that can interfere with carrier transport, electrical rectification, and intermediate band formation. Past studies have used thermal annealing to obtain high crystallinity in hyperdoped black silicon, but thermal annealing causes a deactivation of the sub-bandgap optical absorptance. In this study, nanosecond laser annealing is used to obtain high crystallinity and remove pressure-induced phases in hyperdoped black silicon while maintaining high sub-bandgap optical absorptance and a light-trapping surface morphology. Furthermore, it is shown that nanosecond laser annealing reactivates the sub-bandgap optical absorptance of hyperdoped black silicon after deactivation by thermal annealing. Thermal annealing and nanosecond laser annealing can be combined in sequence to fabricate hyperdoped black silicon that simultaneously shows high crystallinity, high abovebandgap and sub-bandgap absorptance, and a rectifying electrical homojunction. Such nanosecond laser annealing could potentially be applied to non-equilibrium material systems beyond hyperdoped black silicon.Engineering and Applied Science

Minority Carrier Transport in Lead Sulfide Quantum Dot Photovoltaics

Lead sulfide quantum dots (PbS QDs) are an attractive material system for the development of low-cost photovoltaics (PV) due to their ease of processing and stability in air, with certified power conversion efficiencies exceeding 11%. However, even the best PbS QD PV devices are limited by diffusive transport, as the optical absorption length exceeds the minority carrier diffusion length. Understanding minority carrier transport in these devices will therefore be critical for future efficiency improvement. We utilize cross-sectional electron beam-induced current (EBIC) microscopy and develop methodology to quantify minority carrier diffusion length in PbS QD PV devices. We show that holes are the minority carriers in tetrabutylammonium iodide (TBAI)-treated PbS QD films due to the formation of a p–n junction with an ethanedithiol (EDT)-treated QD layer, whereas a heterojunction with n-type ZnO forms a weaker n⁺–n junction. This indicates that modifying the standard device architecture to include a p-type window layer would further boost the performance of PbS QD PV devices. Furthermore, quantitative EBIC measurements yield a lower bound of 110 nm for the hole diffusion length in TBAI-treated PbS QD films, which informs design rules for planar and ordered bulk heterojunction PV devices. Finally, the low-energy EBIC approach developed in our work is generally applicable to other emerging thin-film PV absorber materials with nanoscale diffusion lengths