Jason Alexander Röhr

Jason Alexander Röhr
University of Pennsylvania | UP · School of Engineering & Applied Sciences

PhD

About

43
Publications
21,642
Reads
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2,273
Citations
Additional affiliations
January 2019 - present
New York University
Position
  • PostDoc Position
Description
  • Perovskite and organic solar cells. Fundamental properties of MXenes.
January 2018 - December 2018
Yale University
Position
  • PostDoc Position
Description
  • Metal oxide charge transport and photo catalysis
October 2012 - December 2017
Imperial College London
Position
  • PhD Student
Description
  • Space-charge-limited current measurements; theory and experiments. Non-fullerene organic solar cells.
Education
October 2014 - July 2018
Imperial College London
Field of study
  • Physics
October 2013 - October 2014
Imperial College London
Field of study
  • Physics
August 2011 - August 2013
University of Copenhagen
Field of study
  • Nanoscience

Publications

Publications (43)
Article
Full-text available
Utilizing single-carrier devices for mobility determination, via space-charge-limited current measurements, has become commonplace in the organic, inorganic and hybrid semiconductor communities. However, since there is often a difference in contact work functions of the singe-carrier devices used for the measurements, a built-in voltage is commonly...
Article
Full-text available
While space-charge-limited current measurements are often used to characterize charge-transport in relatively intrinsic, low-mobility semiconductors, it is currently difficult to characterize lightly or heavily doped semiconductors with this method. By combining the theories describing ohmic and space-charge-limited conduction, we derive a general...
Article
Full-text available
In perovskite solar cells, doped organic semiconductors are often used as charge-extraction interlayers situated between the photoactive layer and the electrodes. The π-conjugated small molecule 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9-spirobifluorene (spiro-OMeTAD) is the most frequently used semiconductor in the hole-conducting layer1...
Preprint
Full-text available
Improving solar energy collection in aquatic environments would allow for superior environmental monitoring and remote sensing, but the identification of optimal photovoltaic technologies for such applications is challenging as evaluation requires either field deployment or access to large water tanks. Here, we present a simple bench-top characteri...
Preprint
Full-text available
Space-charge-limited current (SCLC) measurements are used to estimate charge carrier mobilities and electronic trap densities of semiconductors by analysing the current density-voltage (JV) relationship for unipolar devices predicted by the Mott-Gurney (MG) law. However, the interpretation of SCLC measurements for metal-halide perovskites is proble...
Article
Full-text available
The growth in electrified transportation has benefited from the massive worldwide research efforts used to discover and improve electrode materials and electrolytes. Nevertheless, lithium-ion batteries still suffer from a slow-charging limitation. Recently, it has been demonstrated that white light illumination of LiMn2O4 provokes faster charging,...
Article
Full-text available
Solution‐processed quasi‐2D perovskites contain multiple quantum wells with a broad width distribution. Inhomogeneity results in the charge funneling into the smallest bandgap components, which hinders deep‐blue emission and accelerates Auger recombination. Here, a synthetic strategy applied to a range of quasi‐2D perovskite systems is reported, th...
Article
CdTe is a high‐efficiency thin‐film photovoltaic technology that has seen tremendous commercial success over the past decade. Yet despite the improvement of other device characteristics, the fabrication of an ohmic hole back contact layer has remained a challenge due to the high ionization potential of CdTe which limits the external potential that...
Article
Full-text available
Improving solar energy collection in aquatic environments would allow for superior environmental monitoring and remote sensing, but the identification of optimal photovoltaic technologies for such applications is challenging as evaluation requires either field deployment or access to large water tanks. Here, we present a simple bench-top characteri...
Preprint
Full-text available
By considering the changes in the interface charge-carrier densities of a single-carrier device as a function of injection-barrier heights and comparing these to the equilibrium, background charge-carrier density of a device with Ohmic contacts, we calculate simple conditions for when these barriers are expected to limit injection and therefore sig...
Article
Tin dioxide (SnO2) is a frequently reported electron transporting material for perovskite solar cells that yields high performance devices and can be solution processed from aqueous colloidal solutions. While being very simple to process, electron transport layers deposited in this manner often lead to non-uniform film morphology, significantly aff...
Article
Full-text available
Developing materials that possess high electrical conductivities (σ) and Seebeck coefficients (S), low thermal conductivities (κ), and excellent mechanical properties is important to realize practical thermoelectric (TE) devices. Here, 3D hierarchical architectures consisting of hybrid molybdenum disulfide (MoS2)/carbon nanotubes (CNTs) films are f...
Preprint
Full-text available
While space-charge-limited current measurements are often used to characterize charge-transport in relatively intrinsic, low-mobility semiconductors, it is currently difficult to characterize lightly or heavily doped semiconductors with this method. By combining the theories describing ohmic and space-charge-limited conduction, we derive a general...
Article
Despite huge improvements in power conversion efficiencies of perovskite solar cells, the technology is still limited by fill factors at around 80%. Here, we report perovskite solar cells having exceptionally high fill factors of 85% and enhanced open-circuit voltage without sacrificing short-circuit current through a polymer-capped solvent anneali...
Article
Two-dimensional transition metal carbides and nitrides (MXenes) have accumulated tremendous interest recently due to their high conductivity and excellent figures of merit in electromagnetic interference shielding and other applications. Large-area freestanding films of MXenes are important for versatility in application; however, alternative proce...
Article
Two-dimensional (2D) materials such as graphene, MXenes, and transition-metal dichalcogenides have recently gained substantial attention for their superlative physicochemical properties. Assembling 2D mate-rials into functional composites is of great importance, which could be the key to breakthroughs in emerging technologies and applications.The l...
Article
Operation of underwater vehicles and autonomous systems is currently limited by the lack of long-lasting power sources. These systems could potentially be powered using underwater solar cells, but the material requirements to achieve their full potential are not well-understood. Using detailed-balance calculations, we show that underwater solar cel...
Preprint
Operation of underwater vehicles and autonomous systems is currently limited by the lack of long-lasting power sources. These systems could potentially be powered using underwater solar cells, but the material requirements to achieve their full potential are not well-understood. Using detailed-balance calculations, we show that underwater solar cel...
Article
Full-text available
Water-splitting by photocatalyst particles has attracted much attention recently for its potential to produce renewable H 2 at scale. However, the correlation between the energetics at photocatalyst/co-catalyst/water interfaces and their interfacial...
Article
Full-text available
The performance of kesterite (Cu2ZnSn(S,Se)4 ,CZTSSe)) solar cells is hindered by relatively low open circuit voltage (Voc).A reliable quantification of the voltage deficit would be useful to improve kesterite absorbers. The conventional subtraction of qVoc from the absorber band gap is too inaccurate for compositionally complex absorbers like kest...
Article
Full-text available
Zinc oxide is a versatile semiconductor with an expansive range of applications including lighting, sensing and solar energy conversion. Two central phenomena coupled to its performance that remain heavily investigated are the origin of its sub-band-gap green emission and the nature of its conductivity. We report photoluminescence and dark conducti...
Article
Full-text available
Photocatalytic water splitting using particulate suspensions is a promising approach for achieving large-scale production of renewable hydrogen fuels. Previous studies have hypothesized that band energy levels of such photocatalysts in...
Article
Development of low overpotential, non-precious metal electrocatalysts is important for the sustainable water oxidation using renewable energy. Herein, we report the fabrication of nano-interfaces between MnOx nanoscale islands and NiFe layered double hydroxide (LDH) nanosheets, chosen as baseline electrocatalysts for OER activity tuning. The MnOx n...
Article
Full-text available
Core-shell architectures are used to modulate injection and recombination in dye-sensitized photoelectrochemical cells. Here, we demonstrate that exposing SnO2-core/ZrO2-shell films to acid permits photoinduced electron transfer through ZrO2-shells at least...
Thesis
Full-text available
This thesis discusses the accuracy of a number of analytical models commonly used to analyse current density-voltage curves from space-charge-limited current measurements, such as Ohm's law and the Mott-Gurney law. We studied the accuracy of the models using a numerical drift-diffusion solver which can calculate single-carrier devices containing ei...
Article
Accomplishing acid-stable water oxidation is a critical matter for achieving both long-lasting water splitting devices and other fuel-forming electro- and photo-catalytic processes. Since water oxidation releases protons into the local electrolytic environment, it becomes increasingly acidic during device operation, which leads to corrosion of the...
Article
Full-text available
Extracting charge-carrier mobilities for organic semiconductors from space-charge-limited conduction measurements is complicated in practice by nonideal factors such as trapping in defects and injection barriers. Here, we show that by allowing the bandlike charge-carrier mobility, trap characteristics, injection barrier heights, and the shunt resis...
Article
Full-text available
A new synthetic route, to prepare an alkylated indacenodithieno[3,2‐b]thiophene‐based nonfullerene acceptor (C8‐ITIC), is reported. Compared to the reported ITIC with phenylalkyl side chains, the new acceptor C8‐ITIC exhibits a reduction in the optical band gap, higher absorptivity, and an increased propensity to crystallize. Accordingly, blends wi...
Article
Using drift-diffusion simulations, we investigate the voltage dependence of the dark current in single carrier devices, typically used to determine charge-carrier mobilities. For both low and high voltages, the current increases linearly with the applied voltage. Whereas the linear current at low voltages is mainly due to space charge in the middle...
Article
Full-text available
Environmental stability is a common challenge for the commercialisation of low cost, encapsulation-free organic opto-electronic devices. Understanding the role of materials degradation is the key to address this challenge, but most such studies have been limited to conjugated polymers. Here we quantitatively study the role of the common fullerene d...
Article
We discuss the approach of determining the charge-carrier density of a single-carrier device by combining Ohm's law and the Mott-Gurney law. We show that this approach is seldom valid, due to the fact that whenever Ohm's law is applicable the Mott-Gurney law is usually not, and vice versa. We do this using a numerical drift-diffusion solver to calc...
Article
Technological deployment of organic photovoltaic modules requires improvements in device light-conversion efficiency and stability while keeping material costs low. Here we demonstrate highly efficient and stable solar cells using a ternary approach, wherein two non-fullerene acceptors are combined with both a scalable and affordable donor polymer,...
Article
Full-text available
Organic photovoltaics is an emerging solar power technology which embodies properties such as transparency, flexibility, and rapid, roll to roll manufacture, opening the potential for unique niche applications. We report a detailed techno-economic analysis of one such application, namely the photovoltaic greenhouse, and discuss whether the unique p...
Article
A novel small molecule, FBR, bearing 3-ethylrhodanine flanking groups was synthesized as a non-fullerene electron acceptor for solution-processed bulk heterojunction organic photovoltaics (OPV). A straightforward synthesis route was employed, offering the potential for large scale preparation of this material. Inverted OPV devices employing P3HT as...
Article
Full-text available
Accurate characterization and reporting of organic photovoltaic (OPV) device performance remains one of the important challenges in the field. The large spread among the efficiencies of devices with the same structure reported by different groups is significantly caused by different procedures and equipment used during testing.The presented article...
Thesis
Full-text available
Space-charge-limited current (SCLC) measurements were realised on thin films of solution processed antimony sulphide which was deposited from a xanthate precursor decomposed at low temperatures. The electron mobility was determined to be 4.05 x 10^4 cm^2/Vs, indicating that this easy processing route yielded poly-crystalline thin films with high mo...
Thesis
Full-text available
Novel symmetric polymer electron-only devices were realised using caesium carbonate as electron injecting interlayers spun from solution. Asymmetric electron-only devices were realised using calcium and aluminium as contact materials. Steady-state space-charge-limited currents in the dark were observed for both the symmetric and asymmetric single-c...
Article
Full-text available
The incorporation paths of Be in GaAs nanowires grown by the Ga-assisted method in molecular beam epitaxy has been investigated by electrical measurements of nanowires with different doping profiles. We find that Be atoms incorporate preferentially via the nanowire side facets, while the incorporation path through the Ga droplet is negligible. We a...

Questions

Questions (6)
Question
Hi all,
I'm interested in simulating CdTe solar cells with drift-diffusion modelling (coupled with transfer-matrix modelling). I have been able to find old papers where they explain the roll-over effect observed in J-V curves (near the Voc) due to a bad back contacts (such as the papers by Burgelman), but I haven't been able to find anything recent. I'm assuming the way CdTe layers are processed have changed a lot since back in the late 90's early 2000s.
Any pointers towards a recent 2016+ paper where they manage to model the roll-over effect due to the back contact would be greatly appreciated.
Thank you!
Jason
Question
Hi there,
I am currently using Alfa Aesar fused quartz slides for my UV-vis and PL measurements of TiO2, but it seems that their slides emit in the UV around the same wavelength region where I would expect emission from TiO2.
Could someone recommend another type of slide? Maybe fused silica? A link to a vendor would be much appreciated.
Thank you,
Jason
Question
I have never conducted a meta analysis before, so I was wondering how to approach it.
Let's say I wanted to look a how a specific metric has evolved over the past 20 years, for example the reported polymer molecular weight for the highest tensile strength (the question here is arbitrary). Any pointers to how I one would go about that? Seems pretty straight forward to brute force it, albeit very time consuming.
Any help would be appreciated.
Question
Hi there,
I'm looking to buy various photodiodes/solar cells for some experiments.
The cells I'm looking for are InGaP, CZAS (and CZT) and amorphous silicon.
Any pointers to where I could buy those type of cells? The higher the efficiency of the cells, the better but not the most important factor.
Thank you in advance.
J

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Projects

Projects (7)
Project
Applications of MXenes and other 2D materials for catalysis, interference shielding and thermoelectrics.
Project
Projects aimed at understanding underwater solar cells.
Project
Various projects aimed towards commercialisation of emerging solar cell technologies.