arXiv subjects

Jason A. Röhr

Publications and source records attributed to Jason A. Röhr.

6 recordsLinked to original sources

arXiv2026

On the Role of Color Temperature and Color Rendering Index of White-Light LEDs on the Theoretical Efficiency Limit of Indoor Photovoltaics

As the Internet of Things (IoT) continues to grow, the demand for sustainable indoor power sources is increasing. Indoor photovoltaics (IPVs), which are currently in development, present a renewable solution but must be designed to match specific light sources to maximize efficiency. While previous studies have emphasized the role of white-light LED correlated color temperature (CCT) in determining IPV efficiency and optimum bandgap energy, the role of color rendering index (CRI) remains less understood. In this study, we employ detailed-balance calculations to assess the theoretical maximum efficiency and optimal bandgap energies of IPVs under commercial white-light LED illumination varying in both CCT (2200 K to 6500 K) and CRI (70, 80, and 90). Our results confirm that lower CCTs yield higher efficiencies and lower optimal bandgaps; however, contrary to prior assumptions that CRI has negligible impact on IPV material choice and performance, we demonstrate that high-CRI LEDs necessitate the use of materials with significantly lower bandgap energies for optimum efficiency due to the shift towards red in the higher CRI irradiance spectra. We also evaluate the performance of various IPVs at fixed bandgaps, revealing that while optimal IPV performance is achieved with wide-bandgap materials under lower CRI lighting, mature technologies like silicon and CdTe benefit from high-CRI illumination. These findings underscore the need to consider both CCT and CRI in the design, evaluation, and choice of IPVs for indoor IoT applications.

physics.app-ph↗

arXiv2022

On Injection in Intrinsic Single-Carrier Devices

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 significantly affect space-charge-limited currents in the device. We show that these conditions depend on the device temperature, semiconductor relative permittivity, and effective density of states, but most importantly the thickness of the semiconducting film being probed. This is in accordance with previous observations and similar derived expressions for when defects influence single-carrier devices. The conditions described herein can be used to aid the design of single-carrier devices for space-charge-limited current measurements that are not limited by injection.

cond-mat.mes-hall↗

arXiv2022

Identifying optimal photovoltaic technologies for underwater applications

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 characterization technique that does not require direct access to water and therefore circumvents the need for field testing during initial trials of development. Employing LEDs to simulate underwater solar spectra at various depths, we compare Si and CdTe solar cells, two commercially available technologies, with GaInP cells, a technology with a wide band gap close to ideal for underwater solar harvesting. We use this method to show that while Si cells outperform both CdTe and GaInP under terrestrial AM1.5G solar irradiance, both CdTe and GaInP outperform Si at depths > 2 m, with GaInP cells operating with underwater efficiencies exceeding 51%.

physics.app-ph↗

arXiv2022

The Validity Window of Space-Charge-Limited Current Measurements of Metal Halide Perovskite Devices

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 problematic due to mobile ionic defects which redistribute to screen electrostatic fields in devices during measurements. To overcome this, an SCLC measurement protocol was recently suggested that minimises ionic charge redistribution by probing the current during millisecond voltage pulses superimposed on a background bias. Here, we use drift-diffusion simulations with mobile ions to assess the validity of the MG law for analysing both the standard and new protocol JV measurements. We simulated idealised perovskite devices with differing mobile ion densities and compared them with simulations and measurements of devices with typical contact materials. We found the validity region for the MG law is limited to perovskites with mobile ion densities lower than the device's equilibrium charge carrier density (<10^17 cm-3 for 400 nm thick methylammonium lead iodide films) and contacts with injection/extraction barriers <=0.1 eV. The latter limitation can be partially overcome by increasing the device thickness, whereas the former limitation cannot. This restricts the range of perovskite layer compositions and viable contact materials that can be reliably analysed with the MG law. Approaches such as estimating trap densities from the apparent voltage onset to trap-free SCLC regime should also be critically reviewed since they rely on the same potentially invalid assumptions as the MG law. Our results demonstrate that extracting meaningful and accurate values for metal halide perovskite material properties from SCLC maybe challenging, or often not possible.

cond-mat.mtrl-sci↗

arXiv2020

Analytical Description of Mixed Ohmic and Space-Charge-Limited Conduction in Single-Carrier Devices

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 analytical approach to extract the charge-carrier density, the conduction-band edge and the drift components of the current density-voltage curves of a single-carrier device when the semiconductor is either undoped, lightly doped or heavily doped. The presented model covers the entire voltage range, i.e., both the low-voltage regime and the Mott-Gurney regime. We demonstrate that there is an upper limit to how doped a device must be before the current density-voltage curves are significantly affected, and we show that the background charge-carrier density must be considered to accurately model the drift component in the low-voltage regime, regardless of whether the device is doped or not. We expect that the final analytical expressions presented herein to be directly useful to experimentalists studying charge transport in novel materials and devices.

physics.app-ph↗

arXiv2012

Doping incorporation paths in catalyst-free Be-doped GaAs nanowires

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 also demonstrate that Be can diffuse into the volume of the nanowire giving an alternative incorporation path. This work is an important step towards controlled doping of nanowires and will serve as a help for designing future devices based on nanowires.

cond-mat.mtrl-sci↗