arXiv subjects

Ole Hansen

Publications and source records attributed to Ole Hansen.

17 recordsLinked to original sources

arXiv2024

Variable Temperature and Carrier-Resolved Photo-Hall Measurements of High-Performance Selenium Thin-Film Solar Cells

Selenium is an elemental semiconductor with a wide bandgap suitable for a range of optoelectronic and solar energy conversion technologies. However, developing such applications requires an in-depth understanding of the fundamental material properties. Here, we study the properties of the majority and minority charge carriers in selenium using a recently developed carrier-resolved photo-Hall technique, which enables simultaneous mapping of the mobilities and concentrations of both carriers under varying light intensities. Additionally, we perform temperature-dependent Hall measurements to extract information about the acceptor level and ionization efficiency. Our findings are compared to results from other advanced characterization techniques, and the inconsistencies are outlined. Finally, we characterize a high-performance selenium thin-film solar cell and perform device simulations to systematically address each discrepancy and accurately reproduce experimental current-voltage and external quantum efficiency measurements. These results contribute to a deeper understanding of the optoelectronic properties and carrier dynamics in selenium, which may guide future improvements and facilitate the development of higher-efficiency selenium solar cells.

cond-mat.mtrl-sci↗

arXiv2024

Increasing the Collection Efficiency in Selenium Thin-Film Solar Cells Using a Closed-Space Annealing Strategy

Elemental selenium is a promising wide-bandgap ($E_\mathrm{G}\approx$ 1.95 eV) photovoltaic material for the next generation of thin-film solar cells. To realize high-efficiency selenium solar cells, it is crucial to optimize the crystallization process of the selenium thin-film photoabsorber. However, the high vapor pressure of selenium restricts the processing conditions to a compromise between the growth of large crystal grains and the formation of pinholes. In this study, we introduce a closed-space annealing (CSA) strategy designed to suppress the sublimation of selenium, enabling thermal annealing processes at higher temperatures and for longer periods of time. As a result, we consistently improve carrier collection and the overall photovoltaic device performance in our selenium solar cells. By characterizing the carrier dynamics in our devices, we conclude that the observed improvements result from a reduction in charge transfer resistance rather than an increase in carrier diffusion length. The CSA strategy is a promising method for controlling surface morphology and roughness without reducing crystal grain sizes, which paves the way for further advancements in the efficiency and reproducibility of selenium thin-film solar cells.

cond-mat.mtrl-sci↗

arXiv2023

Monolithic Selenium/Silicon Tandem Solar Cells

Selenium is experiencing renewed interest as a promising candidate for the wide bandgap photoabsorber in tandem solar cells. However, despite the potential of selenium-based tandems to surpass the theoretical efficiency limit of single junction devices, such a device has never been demonstrated. In this study, we present the first monolithically integrated selenium/silicon tandem solar cell. Guided by device simulations, we investigate various carrier-selective contact materials and achieve encouraging results, including an open-circuit voltage of V$_\text{oc}$=1.68 V from suns-V$_\text{oc}$ measurements. The high open-circuit voltage positions selenium/silicon tandem solar cells as serious contenders to the industrially dominant single junction technologies. Furthermore, we quantify a pseudo fill factor of more than 80% using injection-level-dependent open-circuit voltage measurements, indicating that a significant fraction of the photovoltaic losses can be attributed to parasitic series resistance. This work provides valuable insights into the key challenges that need to be addressed for realizing higher efficiency selenium/silicon tandem solar cells.

cond-mat.mtrl-sci↗

arXiv2023

Laser-Annealing and Solid-Phase Epitaxy of Selenium Thin-Film Solar Cells

Selenium has resurged as a promising photovoltaic material in solar cell research due to its wide direct bandgap of 1.95 eV, making it a suitable candidate for a top cell in tandem photovoltaic devices. However, the optoelectronic quality of selenium thin-films has been identified as a key bottleneck for realizing high-efficiency selenium solar cells. In this study, we present a novel approach for crystallizing selenium thin-films using laser-annealing as an alternative to the conventionally used thermal annealing strategy. By laser-annealing through a semitransparent substrate, a buried layer of high-quality selenium crystallites is formed and used as a growth template for solid-phase epitaxy. The resulting selenium thin-films feature larger and more preferentially oriented grains with a negligible surface roughness in comparison to thermally annealed selenium thin-films. We fabricate photovoltaic devices using this strategy, and demonstrate a record ideality factor of n=1.37, a record fill factor of FF=63.7%, and a power conversion efficiency of PCE=5.0%. The presented laser-annealing strategy is universally applicable and is a promising approach for crystallizing a wide range of photovoltaic materials where high temperatures are needed while maintaining a low substrate temperature.

cond-mat.mtrl-sci↗

arXiv2021

Gettering in polySi/SiOx passivating contacts enables Si-based tandem solar cells with high thermal and contamination resilience

Multijunction solar cells in a tandem configuration could further lower the costs of electricity if crystalline Si (c-Si) is used as bottom cell. However, for direct monolithic integration on c-Si, only a restricted number of top and bottom cell architectures are compatible, due to either epitaxy or high temperature constraints, where the interface between subcells is subject to a trade-off between transmittance, electrical interconnection, and bottom cell degradation. Using polySi/SiOx passivating contacts for Si, this degradation can be largely circumvented by tuning the polySi/SiOx stacks to promote gettering of contaminants admitted into the Si bottom cell during the top cell synthesis. Applying this concept to the low-cost top cell chalcogenides Cu2ZnSnS4 (CZTS), CuGaSe2 (CGSe) and AgInGaSe2 (AIGSe), fabricated under harsh S or Se atmospheres above 550 {\deg}C, we show that increasing the heavily-doped polySi layer thickness from 40 to up to 400 nm prevents a reduction in Si carrier lifetime by one order of magnitude, with final lifetimes above 500 us uniformly across areas up to 20 cm2. In all cases, the increased resilience was correlated with a 99.9% reduction in contaminant concentration in the c-Si bulk, provided by the thick polySi layer, which acts as a buried gettering layer in the tandem structure without compromising the Si passivation quality. The Si resilience decreased as AIGSe > CGSe > CZTS, in accordance with the measured Cu contamination profiles and higher annealing temperatures. An efficiency of up to 7% was achieved for a CZTS/Si tandem, where the Si bottom cell is no longer the limiting factor.

physics.app-ph↗

arXiv2021

Assessing the role of quantum effects in 2D heterophase MoTe$_2$ field effect transistors

The two-dimensional transition metal dichalcogenides (TMDs) have been proposed as candidates for the channel material in future field effect transistor designs. The heterophase design which utilizes the metallic T- or T' phase of the TMD as contacts to the semiconducting H phase channel has shown promising results in terms of bringing down the contact resistance of the device. In this work, we use ab-initio calculations to demonstrate how atomic-scale and quantum effects influence the ballistic transport properties in such heterophase transistors with channel lengths up to 20 nm. We investigate how the charge transfer depends on the carrier density both in T'-H MoTe$_2$ Schottky contacts and planar T'-H-T' MoTe$_2$ transistors. We find that the size of the Schottky barrier and the charge transfer is dominated by the local atomic arrangements at the interface and the doping level. Furthermore, two types of quantum states have a large influence on the charge transport; interface states and standing waves in the semiconductor due to quantum confinement. We find that the latter can be associated with rises in the current by more than an order of magnitude due to resonant tunneling. Our results demonstrate the quantum mechanical nature of these 2D transistors and highlight several challenges and possible solutions for achieving a competitive performance of such devices.

cond-mat.mes-hall↗

arXiv2020

Monolithic thin-film chalcogenide-silicon tandem solar cells enabled by a diffusion barrier

Following the recent success of monolithically integrated Perovskite/Si tandem solar cells, great interest has been raised in searching for alternative wide bandgap top-cell materials with prospects of a fully earth-abundant, stable and efficient tandem solar cell. Thin film chalcogenides (TFCs) such as the Cu2ZnSnS4 (CZTS) could be suitable top-cell materials. However, TFCs have the disadvantage that generally at least one high temperature step (>500 C) is needed during the synthesis, which could contaminate the Si bottom cell. Here, we systematically investigate the monolithic integration of CZTS on a Si bottom solar cell. A thermally resilient double-sided Tunnel Oxide Passivated Contact (TOPCon) structure is used as bottom cell. A thin (<25 nm) TiN layer between the top and bottom cells, doubles as diffusion barrier and recombination layer. We show that TiN successfully mitigates in-diffusion of CZTS elements into the c-Si bulk during the high temperature sulfurization process, and find no evidence of electrically active deep Si bulk defects in samples protected by just 10 nm TiN. Post-process minority carrier lifetime in Si exceeded 1.5 ,s. i.e., a promising implied open-circuit voltage (i-Voc) of 715 mV after the high temperature sulfurization. Based on these results, we demonstrate a first proof-of-concept two-terminal CZTS/Si tandem device with an efficiency of 1.1% and a Voc of 900 mV. A general implication of this study is that the growth of complex semiconductors on Si using high temperature steps is technically feasible, and can potentially lead to efficient monolithically integrated two-terminal tandem solar cells.

cond-mat.mtrl-sci↗

arXiv2020

Nitride-based interfacial layers for monolithic tandem integration of new solar energy materials on Si: The case of CZTS

The monolithic tandem integration of third-generation solar energy materials on silicon holds great promise for photoelectrochemistry and photovoltaics. However, this can be challenging when it involves high-temperature reactive processes, which would risk damaging the Si bottom cell. One such case is the high-temperature sulfurization/selenization in thin film chalcogenide solar cells, of which the kesterite Cu2ZnSnS4 (CZTS) is an example. Here, by using very thin (<10 nm) TiN-based diffusion barriers at the interface, with different composition and properties, we demonstrate on a device level that the protection of the Si bottom cell is largely dependent on the barrier layer engineering. Several monolithic CZTS/Si tandem solar cells with open-circuit voltages (Voc) up to 1.06 V and efficiencies up to 3.9% are achieved, indicating a performance comparable to conventional interfacial layers based on transparent conductive oxides, and pointing to a promising alternative design in solar energy conversion devices.

cond-mat.mtrl-sci↗

arXiv2020

Persistent double layer formation in kesterite solar cells: a critical review

In kesterite CZTSSe solar cell research, an asymmetric crystallization profile is often obtained after annealing, resulting in a bilayered or double-layered absorber. So far, only segregated pieces of research exist to characterize this double layer, its formation dynamics and its effect on the performance of devices. Here, we review the existing research on double-layered kesterites and evaluate the different mechanisms proposed. Using a cosputtering-based approach, we show that the two layers can differ significantly in morphology, composition and optoelectronic properties, and complement the results with a statistical dataset of over 850 individual CZTS solar cells. By reducing the absorber thickness from above 1000 nm to 300 nm, we show that the double layer segregation is alleviated. In turn, we see a progressive improvement in the device performance for lower thickness, which alone would be inconsistent with the known case of ultrathin CIGS solar cells. By comparing the results with CZTS grown on monocrystalline Si substrates, without a native Na supply, we show that the alkali metal supply does not determine the double layer formation, but merely reduces the threshold for its occurrence. Instead, we propose that the main formation mechanism is the early migration of Cu to the surface during annealing and formation of Cu2-xS phases, in a self-regulating process akin to the Kirkendall effect. We comment on the generality of the mechanism proposed, comparing our results to other synthesis routes, including our own in-house results from solution processing and pulsed laser deposition of sulfide and oxide-based targets. Although the double layer occurrence largely depends on the kesterite synthesis route, the common factors determining the double layer occurrence appear to be the presence of metallic Cu and/or a chalcogen deficiency in the precursor matrix.

cond-mat.mtrl-sci↗

arXiv2020

Thermal radiation dominated heat transfer in nanomechanical silicon nitride drum resonators

Nanomechanical silicon nitride (SiN) drum resonators are currently employed in various fields of applications that arise from their unprecedented frequency response to physical quantities. In the present study, we investigate the thermal transport in nanomechanical SiN drum resonators by analytical modelling, computational simulations, and experiments for a better understanding of the underlying heat transfer mechanism causing the thermal frequency response. Our analysis indicates that radiative heat loss is a non-negligible heat transfer mechanism in nanomechanical SiN resonators limiting their thermal responsivity and response time. This finding is important for optimal resonator designs for thermal sensing applications as well as cavity optomechanics.

physics.app-ph↗

arXiv2019

Tunable MEMS VCSEL on Silicon substrate

We present design, fabrication and characterization of a MEMS VCSEL which utilizes a silicon-on-insulator wafer for the microelectromechanical system and encapsulates the MEMS by direct InP wafer bonding, which improves the protection and control of the tuning element. This procedure enables a more robust fabrication, a larger free spectral range and facilitates bidirectional tuning of the MEMS element. The MEMS VCSEL device uses a high contrast grating mirror on a MEMS stage as the bottom mirror, a wafer bonded InP with quantum wells for amplification and a deposited dielectric DBR as the top mirror. A 40 nm tuning range and a mechanical resonance frequency in excess of 2 MHz are demonstrated.

physics.app-ph↗

arXiv2017

Interface band gap narrowing behind open circuit voltage losses in Cu$_2$ZnSnS$_4$ solar cells

We present evidence that band gap narrowing at the heterointerface may be a major cause of the large open circuit voltage deficit of Cu$_2$ZnSnS$_4$/CdS solar cells. Band gap narrowing is caused by surface states that extend the Cu$_2$ZnSnS$_4$ valence band into the forbidden gap. Those surface states are consistently found in Cu$_2$ZnSnS$_4$, but not in Cu$_2$ZnSnSe$_4$, by first-principles calculations. They do not simply arise from defects at surfaces but are an intrinsic feature of Cu$_2$ZnSnS$_4$ surfaces. By including those states in a device model, the outcome of previously published temperature-dependent open circuit voltage measurements on Cu$_2$ZnSnS$_4$ solar cells can be reproduced quantitatively without necessarily assuming a cliff-like conduction band offset with the CdS buffer layer. Our first-principles calculations indicate that Zn-based alternative buffer layers are advantageous due to the ability of Zn to passivate those surface states. Focusing future research on Zn-based buffers is expected to significantly improve the open circuit voltage and efficiency of pure-sulfide Cu$_2$ZnSnS$_4$ solar cells.

cond-mat.mes-hall↗

arXiv2015

Full-field hard x-ray microscopy with interdigitated silicon lenses

Full-field x-ray microscopy using x-ray objectives has become a mainstay of the biological and materials sciences. However, the inefficiency of existing objectives at x-ray energies above 15 keV has limited the technique to weakly absorbing or two-dimensional (2D) samples. Here, we show that significant gains in numerical aperture and spatial resolution may be possible at hard x-ray energies by using silicon-based optics comprising 'interdigitated' refractive silicon lenslets that alternate their focus between the horizontal and vertical directions. By capitalizing on the nano-manufacturing processes available to silicon, we show that it is possible to overcome the inherent inefficiencies of silicon-based optics and interdigitated geometries. As a proof-of-concept of Si-based interdigitated objectives, we demonstrate a prototype interdigitated lens with a resolution of ~255 nm at 17 keV.

physics.optics↗

arXiv2013

Enhanced light-matter interaction in graphene-covered gold nanovoid arrays

The combination of graphene with noble-metal nanostructures is currently being explored for strong light-graphene interaction enhanced by plasmons. We introduce a novel hybrid graphene-metal system for studying light-matter interactions with gold-void nanostructures exhibiting resonances in the visible range. Strong coupling of graphene layers to the plasmon modes of the nanovoid arrays results in significant frequency shifts of the underlying plasmon resonances, enabling more than 30% absolute light absorption in a single layer of graphene and up to 700-fold enhancement of the Raman response of the graphene. These new perspectives enable us to verify the presence of graphene on gold-void arrays and the enhancement even allows us to accurately quantify the number of layers. Experimental observations are further supported by numerical simulations and perturbation-theory analysis. The graphene gold-void platform is beneficial for sensing of molecules and placing R6G dye molecules on top of the graphene, we observe a strong enhancement of the R6G Raman fingerprints. These results pave the way toward advanced substrates for surface-enhanced Raman scattering (SERS) with potential for unambiguous single-molecule detection on the atomically well-defined layer of graphene.

cond-mat.mes-hall↗

arXiv2013

Experimental observation of plasmons in a graphene monolayer resting on a two-dimensional subwavelength silicon grating

We experimentally demonstrate graphene-plasmon polariton excitation in a continuous graphene monolayer resting on a two-dimensional subwavelength silicon grating. The subwavelength silicon grating is fabricated by a nanosphere lithography technique with a self-assembled nanosphere array as a template. Measured transmission spectra illustrate the excitation of graphene-plasmon polaritons, which is further supported by numerical simulations and theoretical prediction of plasmonband diagrams. Our grating-assisted coupling to graphene-plasmon polaritons forms an important platform for graphene-based opto-electronics applications.

physics.optics↗

arXiv2006

Two models with rescattering for high energy heavy ion collisions

The effects of hadronic rescattering in RHIC-energy Au+Au collisions are studied using two very different models to describe the early stages of the collision. One model is based on a hadronic thermal picture and the other on a superposition of parton-parton collisions. Operationally, the output hadrons from each of these models are used as input to a hadronic rescattering calculation. The results of the rescattering calculations from each model are then compared with rapidity and transverse momentum distributions from the RHIC BRAHMS experiment. It is found that in spite of the different points of view of the two models of the initial stage, after rescattering the observed differences between the models are mostly "washed out" and both models give observables that agree reasonably well with each other and experiment.

nucl-th↗

arXiv1994

Comparison of Experimental Data to the Relativistic Quantum Molecular Dynamics Model For Si+Au Collisions at 14.6 A GeV/c

Predictions from the RQMD model are systematically compared to recently published charged hadron distributions of AGS Experiment 802 for central Si+Au collisions at 14.6$A$ GeV/$c$, taking into account both the experimental trigger condition and acceptance. The main features of the data, including K$^+$ production, can be understood quantitatively to better than 20\% within the framework of the model, although several discrepancies are found, most importantly for the proton spectra.

nucl-th↗