The Ferroelectric-Ferroelastic Debate about Metal Halide Perovskites

Metal halide perovskites (MHPs) are solution-processed materials with exceptional photoconversion efficiencies that have brought a paradigm shift in photovoltaics. The nature of the peculiar optoelectronic properties underlying such astounding performance is still controversial. The existence of ferroelectricity in MHPs and its alleged impact on photovoltaic activity have fueled an intense debate, in which unanimous consensus is still far from being reached. Here we critically review recent experimental and theoretical results with a two-fold objective: we argue that the occurrence of ferroelectric domains is incompatible with the A-site cation dynamics in MHPs and propose an alternative interpretation of the experiments based on the concept of ferroelasticity. We further underline that ferroic behavior in MHPs would not be relevant at room temperature or higher for the physics of photogenerated charge carriers, since it would be overshadowed by competing effects like polaron formation and ion migration

Reduction of the transverse effective charge of optical phonons in ZnO under pressure

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 96, 231906 (2010) and may be found at https://doi.org/10.1063/1.3447798."From Raman scattering on a-plane wurtzite ZnO crystals we obtained a decreasing splitting between longitudinal and transversal optical phonons with A1 and E1 symmetry as a function of hydrostatic pressure up to 5.5 GPa. Consequently, the transverse effective charge (e∗T) exhibits a strong reduction with increasing pressure, yielding 2.17–14.6×10−3 P/GPa and 2.04–13.7×10−3 P/GPa (in units of the elementary charge) for the A1 and E1 phonons, respectively. We find a clear systematic in the linear pressure coefficient of e∗T with bond polarity for the series of wide-band gap semiconductors SiC, AlN, GaN, and ZnO.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Exploring the origin of high optical absorption in conjugated polymers

Vezie, Michelle S. et al.The specific optical absorption of an organic semiconductor is critical to the performance of organic optoelectronic devices. For example, higher light-harvesting efficiency can lead to higher photocurrent in solar cells that are limited by sub-optimal electrical transport. Here, we compared over 40 conjugated polymers, and found that many different chemical structures share an apparent maximum in their extinction coefficients. However, a diketopyrrolopyrrole-thienothiophene copolymer shows remarkably high optical absorption at relatively low photon energies. By investigating its backbone structure and conformation with measurements and quantum chemical calculations, we find that the high optical absorption can be explained by the high persistence length of the polymer. Accordingly, we demonstrate high absorption in other polymers with high theoretical persistence length. Visible light harvesting may be enhanced in other conjugated polymers through judicious design of the structure.M.S.V. and S. F. are grateful to the Engineering and Physical Sciences Research Council (EPSRC) for a doctoral training award and a CDT studentship (EP/G037515/1) respectively. G.P. and S.C.H. acknowledge the University of Cyprus for funding through the internal grant "ORGANIC". B.D., A.G. and M.C.Q. acknowledge financial support from the Ministerio de Economía y Competitividad of Spain through projects CSD2010–00044 (Consolider NANOTHERM), SEV-2015_0496 and MAT2012–37776 and the European Research Council through project ERC CoG648901. I.M., R.S.A. and I.McC. acknowledge support from the European Commission FP7 Project ArtESun (604397). J.N. is grateful to the Royal Society for a Wolfson Merit Award, and acknowledges financial support from EPSRC grants EP/K030671/1, EP/K029843/1 and EP/J017361/1. The authors thank Dr. Isabel Alonso for performing supplementary ellipsometric measurements; we thank Prof. Thomas Kirchartz, Dr. Jarvist Moore Frost, Dr. Christian Müller and Dr. Isabel Alonso for helpful discussions.Peer reviewe

Comparing different geometries for photovoltaic-thermoelectric hybrid devices based on organics

Coupling thermoelectrics (TE) with photovoltaics (PV) has emerged as an approach to solid-state solar harvesting, directly converting light and infrared heat into electricity. In this work, we compare PV-TE hybrid devices based on organic semiconductors in three different geometries: a reflection geometry, a non-contact transmission geometry, and a contact transmission geometry. The temperature rises of films of common organic thermoelectric materials, including poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), single-walled carbon nanotubes (swCNT), and poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT), were measured in configurations representative of the proposed geometries. Because organic semiconductors possess broadband light absorption and low thermal conductivities, a significant rise in temperature was observed under illumination for all geometries. We find, however, that the best configuration is, in fact, the transmission contact mode because it sums two effects. Operating under 1 sun illumination, the temperature of a commercial organic PV module increased by ≈30 K, which leads to an enhancement in OPV performance compared to room temperature. After attaching a thermoelectric to the OPV module, losses from convection are reduced, and the OPV module heats up even more, further increasing its efficiency while additionally enabling thermoelectric generation. Finally, we calculate theoretical thermoelectric efficiencies for the materials and their respective power densities.The authors acknowledge financial support from the Spanish Ministry Science and Innovation through the “Severo Ochoa” Program for Centers of Excellence in R&D SEV-2015-0496 (FUNMAT) and CEX2019-000917-S (FUNFUTURE), and PGC2018-095411-B-I00 (RAINBOW) projects; from the Generalitat de Catalunya through grants 2017SGR488 and AGAUR 2018 PROD 00191; and from the European Research Council (ERC) under grant agreement no. 648901. This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement no. 713673. J. P. J. has received financial support through the “la Caixa” INPhINIT Fellowship Grant for Doctoral studies at Spanish Research Centers of Excellence (Grant code: LCF/BQ/IN17/11620035), “la Caixa” Banking Foundation (ID100010434), Barcelona, Spain. O. Z. A. acknowledges CONACYT-SENER for his PhD scholarship (no. 472571). J. P. J. acknowledges the departments of Physics, Chemistry and Geology of the Autonomous University of Barcelona (UAB) as coordinators of the PhD programme in Materials Science. The authors thank Dr A. Roig, Dr A. Laromaine and Dr D. Abol-Fotouh (ICMAB-CSIC) for the CNT:cellulose sample preparation and fruitful discussions. The authors thank Dr Aleksandr Perevedentsev for his help with sample preparation, and Mr Pau Molet for his help with the FTIR measurements. We acknowledge support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).Peer reviewe

Electronic wave functions and optical transitions in (In,Ga)As/GaP quantum dots

Robert, Cédric et al.We study the complex electronic band structure of low In content InGaAs/GaP quantum dots. A supercell extended-basis tight-binding model is used to simulate the electronic and the optical properties of a pure GaAs/GaP quantum dot modeled at the atomic level. Transitions between hole states confined into the dots and several XZ-like electronic states confined by the strain field in the GaP barrier are found to play the main role on the optical properties. Especially, the calculated radiative lifetime for such indirect transitions is in good agreement with the photoluminescence decay time measured in time-resolved photoluminescence in the µs range. Photoluminescence experiments under hydrostatic pressure are also presented. The redshift of the photoluminescence spectrum with pressure is also in good agreement with the nature of the electronic confined states simulated with the tight-binding model.A.R.G. and M.I.A. acknowledge support from the Spanish Ministry of Economy and Competitiveness (MINECO) through Grant No. MAT2009-09480 (PIEZOHM) and Severo Ochoa Excellence Centre Award (No. SEV-2015-0496). M.O.N. acknowledges financial support from the Russian Foundation for Basic Research. This research is supported by “Region Bretagne” through the PONANT project including ´ FEDER funds. This paper is also supported by the OPTOSI Agence Nationale pour la Recherche Project No. 12-BS03- 002-02. This work has been performed using HPC resources of GENCI CINES, TGCC/CCRT, and IDRIS under the allocation No. 2013-[x2013096724].Peer reviewe

Photoinduced p- to n-type Switching in Thermoelectric Polymer-Carbon Nanotube Composites

Dörling, Bernhard et al.UV-induced switching from p- to n-type character is demonstrated during deposition of carbon-nanotube–conjugated polymer composites. This opens the possibility to photopattern n-type regions within an otherwise p-type film, which has a potential for complementary circuitry or, as shown here, thermoelectric generators made from a single solution.The authors would like to thank Dr. Isabel Alonso (ICMAB) and Prof. Salvador Ferrer (ALBA) for assisting during the synchrotron sessions as well as for useful discussions. The authors would also like to acknowledge financial support from the Ministerio de Economía y Competitividad of Spain through projects CSD2010–00044 (Consolider NANOTHERM) and MAT2012–37776 and the European Research Council (ERC) under grant agreement nos. 637624 and 648901. C.M. gratefully acknowledges fi nancial support from Formas, the Knut and Alice Wallenberg Foundation through a Wallenberg Academy Fellowship. The transmission X-ray microscopy experiments were performed at MISTRAL beamline at ALBA Synchrotron with the collaboration of ALBA staff. A.E.B. thanks the Egyptian Ministry of Higher Education for funding through the Short-term Scientifi c Mission Postdoctoral program. The authors thank Dr. Elena Bailo (Witec) for assistance with the Raman imaging experiments.Peer reviewe

Localized thinning for strain concentration in suspended germanium membranes and optical method for precise thickness measurement

We deposited Ge layers on (001) Si substrates by molecular beam epitaxy and used them to fabricate suspended membranes with high uniaxial tensile strain. We demonstrate a CMOS-compatible fabrication strategy to increase strain concentration and to eliminate the Ge buffer layer near the Ge/Si hetero-interface deposited at low temperature. This is achieved by a two-steps patterning and selective etching process. First, a bridge and neck shape is patterned in the Ge membrane, then the neck is thinned from both top and bottom sides. Uniaxial tensile strain values higher than 3% were measured by Raman scattering in a Ge membrane of 76 nm thickness. For the challenging thickness measurement on micrometer-size membranes suspended far away from the substrate a characterization method based on pump-and-probe reflectivity measurements was applied, using an asynchronous optical sampling technique.EC/FP7/628197/EU/Heat Propagation and Thermal Conductivity in Nanomaterials for Nanoscale Energy Management/HEATPRONAN

Investigation of proton damage in III-V semiconductors by optical spectroscopy

We studied the damage produced by 2MeV proton radiation on epitaxially grown InGaP/GaAs structure by means of spatially resolved Raman and photoluminescence (PL) spectroscopy. The irradiation was performed parallel to the sample surface in order to determine the proton penetration range in both compounds. An increase in the intensity of longitudinal optical phonons and a decrease in the luminescence were observed. We associate these changes with the creation of defects in the damaged region, also responsible for the observed change of the carrier concentration in the GaAs layer, determined by the shift of the phonon-plasmon coupled mode frequency. From the spatially resolved profile of the PL and phonon intensities, we obtained the proton range in both materials and we compared them with stopping and range of ions in matter simulations. The comparison between the experimentally obtained proton range and simulations shows a very good agreement for GaAs but a discrepancy of 20% for InGaP. This discrepancy can be explained in terms of limitations of the model to simulate the electronic orbitals and bonding structure of the simulated compound. In order to overcome this limitation, we propose an increase in 40% in the electronic stopping power for InGaP.This work was supported by BEC.AR FUNDACION YPF-CONICET 2013 (Argentina) and DFG within Grant No. SFB 787. ARG thanks the Spanish Ministry of Economy and Competitiveness (MINECO) for its support through Grant No. SEV-2015-0496 in the framework of the Spanish Severo Ochoa Centre of Excellence program.Peer reviewe

The effect of Bi doping on the thermal conductivity of ZnO and ZnO:Al thin films

The dissipation of heat generation has been one of the largest obstacles in the design of semiconductor devices and reducing the thermal conductivity is vital for improving thermoelectric efficiency. This work focuses on the Bi doping effect on ZnO, and ZnO:Al thin films produced by magnetron sputtering with thickness varying between 500 and 900 nm. The approach introduces Bi ions, a higher mass element, into the ZnO metal-oxide matrix, to hinder phonon-mediated heat conduction and, consequently, reduce thermal conductivity. Atom probe tomography (APT) was employed to survey Bi doping distribution in ZnO:Al:Bi and ZnO:Bi thin films and to study the morphology of the grain boundaries. The thermal properties of the thin films were measured by frequency-domain thermoreflectance. Based on thermal conductivity results, it is concluded that the doping of ZnO films with Al has a significant effect on thermal conductivity, being reduced from 6.0 W m$^{−1}$ K${^−1}$ in its undoped state to 3.3 W m$^{−1}$ K$^{−1}$ for ZnO with ∼3 at.% of Al, mainly due to alloy scattering of phonons in the wurtzite cell. Further doping with Bi contributes to a slight reduction in the thermal conductivity of ZnO:Al.Bi films (2.9 W m$^{−1}$ K$^{−1}$), due to grain boundary scattering by Bi/Bi$_2$O$_3$ phases. This result is understood as the confluence of two counteracting effects. On the one hand, the thermal conductivity of the film decreases because Bi, unlike Al, is segregated to grain boundaries and does not substitute Zn in the wurtzite crystal lattice, which is unequivocally demonstrated by APT results. On the other hand, the simultaneous presence of Al and Bi triggers a morphological change with the film\u27s microstructure becoming more columnar. This change in microstructure from 3D island growth in ZnO:Al and ZnO:Bi to a more regular columnar structure in ZnO:Al,Bi limits further reduction in the thermal conductivity