arXiv subjects

Bruno Ehrler

Publications and source records attributed to Bruno Ehrler.

25 records · Page 2Linked to original sources

arXiv2019

Local Crystal Misorientation Influences Non-Radiative Recombination

We use ultrasensitive electron backscatter diffraction (EBSD) to map the local crystal orientations, grains, and grain boundaries in CH3NH3PbI3 (MAPI) perovskite thin films. Although the true grain structure is broadly consistent with the morphology visible in scanning electron microscopy (SEM), the inverse pole figure maps taken with EBSD reveal grain structure and internal misorientation that is otherwise hidden. Local crystal misorientation is consistent with the presence of local strain which varies from one grain to the next. We acquire co-aligned confocal optical photoluminescence (PL) microscopy images on the same MAPI samples used for EBSD. We correlate optical and EBSD data, showing that PL is anticorrelated with the local grain orientation spread, suggesting that grains with higher degrees of crystalline orientational heterogeneity (local strain) exhibit more non-radiative recombination. We find that larger grains tend to have larger grain orientation spread, consistent with higher degrees of strain and non-radiative recombination.

cond-mat.mtrl-sci↗

arXiv2019

Crystal orientation and grain size: do they determine optoelectronic properties of MAPbI3 perovskite?

It is thought that growing large, oriented grains of perovskite can lead to more efficient devices. We study MAPbI3 films fabricated via Flash Infrared Annealing (FIRA) consisting of highly oriented, large grains. Domains observed in the SEM are often misidentified with crystallographic grains, but SEM images don't provide diffraction information. We measure the grain size, crystal structure and grain orientation using Electron Back-Scattered Diffraction (EBSD) and we study how these affect the optoelectronic properties as characterized by local photoluminescence (PL) and time-resolved microwave conductivity measurements (TRMC). We find a spherulitic growth yielding large (tens of micron), highly oriented grains along the (112) and (400) planes in contrast to randomly oriented, smaller (400 nm) grains observed in films fabricated via conventional antisolvent (AS) dripping. We observe a local enhancement and shift of the photoluminescence emission at different regions of the FIRA clusters, but these can be explained with a combination of light-outcoupling and self-absorption. We observe no effect of crystal orientation on the optoelectronic properties. Additionally, despite a substantial difference in grain size between our FIRA sample and a conventional AS sample, we find similar photoluminescence and charge carrier mobilities and lifetime for the two films. These findings show that the optoelectronic quality is not necessarily related to the orientation and size of crystalline domains in perovskite films indicating that fabrication requirements may be more relaxed for perovskites.

physics.app-ph↗

arXiv2019

Quantification of Ion Migration in CH3NH3PbI3 Perovskite Solar Cells by Transient Capacitance Measurements

Solar cells based on organic-inorganic metal halide perovskites show efficiencies close to highly-optimized silicon solar cells. However, ion migration in the perovskite films leads to device degradation and impedes large scale commercial applications. We use transient ion-drift measurements to quantify activation energy, diffusion coefficient, and concentration of mobile ions in methylammonium lead triiodide (MAPbI3) perovskite solar cells, and find that their properties change close to the tetragonal-to-orthorhombic phase transition temperature. We identify three migrating ion species which we attribute to the migration of iodide (I-) and methylammonium (MA+). We find that the concentration of mobile MA+ ions is one order of magnitude higher than the one of mobile I- ions, and that the diffusion coefficient of mobile MA+ ions is three orders of magnitude lower than the one for mobile I- ions. We furthermore observe that the activation energy of mobile I- ions (0.29 eV) is highly reproducible for different devices, while the activation energy of mobile MA+ depends strongly on device fabrication. This quantification of mobile ions in MAPbI3 will lead to a better understanding of ion migration and its role in operation and degradation of perovskite solar cells.

cond-mat.mtrl-sci↗

arXiv2018

The Potential of Singlet Fission Photon Multipliers as an Alternative to Silicon-based Tandem Solar Cells

Singlet fission, an exciton multiplication process in organic semiconductors which converts one singlet exciton into two triplet excitons is a promising way to reduce thermalization losses in conventional solar cells. One way to harvest triplet excitons is to transfer their energy into quantum dots, which then emit photons into an underlying solar cell. We simulate the performance potential of such a singlet fission photon multiplier combined with a silicon base cell and compare it to a silicon-based tandem solar cell. We calculate the influence of various loss-mechanisms on the performance potential under real-world operation conditions using a variety of silicon base cells with different efficiencies. We find that the photon multiplier is more stable against changes in the solar spectrum than two-terminal tandem solar cells. We furthermore find that, as the efficiency of the silicon solar cell increases, the efficiency of the photon multiplier increases at a higher rate than the tandem solar cell. For current record silicon solar cells, the photon multiplier has the potential to increase the efficiency by up to 4.2% absolute.

physics.app-ph↗

arXiv2017

The Sobering Reality of Perovskite/Si Tandem Solar Cells under Realistic Operating Conditions

Perovskite/Si tandem solar cells have the potential to considerably out-perform conventional solar cells. Under standard test conditions, perovskite/Si tandem solar cells already outperform the Si single junction. Under realistic conditions, however, as we show, those tandem solar cells are hardly more efficient than the Si cell alone. We model the performance of realistic perovskite/Si tandem solar cells under real-world climate conditions, by incorporating parasitic cell resistances, non-radiative recombination, and optical losses into the detailed-balance limit. We show quantitatively that optimizing these parameters in the perovskite top cell, perovskite/Si tandem solar cells reach an efficiency advantage of up to 14% absolute, even while leaving the Si cell untouched. Despite the rapid efficiency increase of perovskite solar cells, our results emphasize the need for further material development, careful device design, and light management strategies, all necessary for highly efficient perovskite/Si tandem solar cells.

cond-mat.mtrl-sci↗

arXiv2016

Indirect to direct bandgap transition in methylammonium lead halide perovskite

Methylammonium lead iodide perovskites are considered direct bandgap semiconductors. Here we show that in fact they present a weakly indirect bandgap 60 meV below the direct bandgap transition. This is a consequence of spin-orbit coupling resulting in Rashba-splitting of the conduction band. The indirect nature of the bandgap explains the apparent contradiction of strong absorption and long charge carrier lifetime. Under hydrostatic pressure from ambient to 325 MPa, Rashba splitting is reduced due to a pressure induced ordering of the crystal structure. The nature of the bandgap becomes increasingly more direct, resulting in five times faster charge carrier recombination, and a doubling of the radiative efficiency. At hydrostatic pressures above 325 MPa, MAPI undergoes a reversible phase transition resulting in a purely direct bandgap semiconductor. The pressure-induced changes suggest epitaxial and synthetic routes to higher efficiency optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2015

A Silicon-Singlet Fission Parallel Tandem Solar Cell Exceeding 100 % External Quantum Efficiency

Silicon solar cells dominate the solar cell market with record lab efficiencies reaching almost 26%. However, after 60 years of research, this efficiency saturated close to the theoretical limit for silicon, and radically new approaches are needed to further improve the efficiency. Here we present parallel-connected tandem solar cells based on down-conversion via singlet fission. This design allows raising the theoretical power conversion efficiency limit to 45% with far superior stability under changing sunlight conditions in comparison to traditional series tandems. We experimentally demonstrate a silicon/pentacene parallel tandem solar cell that exceeds 100% external quantum efficiency at the main absorption peak of pentacene, showing efficient photocurrent addition and proving this design as a realistic prospect for real-world applications.

cond-mat.other↗