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abstract: Abstract This paper presents results of measurements of optical absorption in undoped epitaxial GaAs for photon energies below the band gap. Absorption spectra were determined from transmission spectra of a thin GaAs layer at several temperatures between 25 degC and 205 degC. We optimized our experiment to investigate the long wavelength part of the spectrum, where the absorption is relatively low, but significant from the point of view of applications of GaAs in semiconductor lasers. Absorption of 100 cm 1 was observed over 30 nm below the band gap at high temperatures.
author_names: Michal Wasiak

Jaroslaw Walczak

Marcin Motyka

Filip K Janiak

Artur Trajnerowicz

Agata Jasik
title: Below band gap absorption in undoped GaAs at elevated temperatures
year: 2017

abstract: Multi photon absorption and multiple exciton generation represent two separate strategies for enhancing the conversion efficiency of light into usable electric power. Targeting below band gap and above band gap energies, respectively, to date these processes have only been demonstrated independently. Here we report the combined interaction of both nonlinear processes in CsPbBr3 perovskite nanocrystals. We demonstrate nonlinear absorption over a wide range of below band gap excitation energies (0.5 0.8 Eg) Interestingly, we discover high order absorption processes, deviating from the typical two photon absorption, at specific energetic positions. These energies are associated with a strong enhancement of the photoluminescence intensity by up to 105. The analysis of the corresponding energy levels reveals that the observed phenomena can be ascribed to the resonant creation of multiple excitons via the absorption of multiple below band gap photons. This effect may open new pathways for the efficient conversion of optical energy, potentially also in other semiconducting materials.The effects of the combined interplay of below band gap multi photon absorption and multiple exciton generation have not been investigated. Here, the authors show the combined interaction of these processes in perovskite nanocrystals, leading to an enhanced interband photoluminescence emission under resonant conditions.
author_names: Aurora Manzi

Yu Tong

Julius Feucht

En Ping Yao

Lakshminarayana Polavarapu

Alexander S Urban

Jochen Feldmann
title: Resonantly enhanced multiple exciton generation through below band gap multi photon absorption in perovskite nanocrystals
venue: Nature Communications
year: 2018

abstract: Summary Recent advances in the development of polymer acceptors and the investigation of molecular mass have boosted the power conversion efficiency (PCE) of all polymer solar cells (all PSCs) to approximately 11% Here, a fused aromatic ring constructed polymer acceptor PYT (Poly[(2,2' (2Z,2'Z) (12,13 bis(2 octyldodecyl) 3,9 diundecyl 12,13 dihydro[1,2,5]thiadiazolo[3,4e]thieno[2',3':4',5']thieno[2',3':4,5]pyrrolo[3,2 g]thieno[2',3':4,5]thieno[3,2 b] indole 2,10 diyl)bis(methanylylidene))bis(3 oxo 2,3 dihydro 1H indene 2,1 diylidene) dimalononitrile alt 2,5 thiophene) is reported, while a series of PYT polymers with different molecular masses (designated as PYTL, PYTM, and PYTH) are prepared to fine tune the molecular crystallinity and miscibility. Benefiting from the advantages of PYT series, which possess broad absorption with a narrow band of 1.40 1.44 eV and high absorption coefficients of over 1.00 x 105 cm 1, we investigated the blend miscibility and device performance of all PSCs based on a wide band gap polymer donor, PM6. The PYTM based all PSCs exhibit an excellent PCE of 13.44% outperforming those with PYTL (12.55% and PYTH (8.61% Our results provide insight into polymer acceptor backbone and molecular mass and suggest guidelines to rationally select polymers for all PSCs.
author_names: Wei Wang

Qian Wu

Rui Sun

Jin-tang Guo

Yao Wu

Mumin Shi

Wenyan Yang

Hongneng Li

Jie Min
title: Controlling Molecular Mass of Low Band Gap Polymer Acceptors for High Performance All Polymer Solar Cells
year: 2020

abstract: Multilayer van der Waals (vdW) heterostructures assembled by diverse atomically thin layers have demonstrated a wide range of fascinating phenomena and novel applications. Understanding the interlayer coupling and its correlation effect is paramount for designing novel vdW heterostructures with desirable physical properties. Using a detailed theoretical study of two dimensional (2D) MoS2 graphene (GR) based heterostructures based on state of the art hybrid density functional theory, we reveal that for 2D few layer heterostructures, vdW forces between neighboring layers depend on the number of layers. Compared to that in the bilayer, the interlayer coupling in trilayer vdW heterostructures can significantly be enhanced by stacking the third layer, directly supported by short interlayer separations and more interfacial charge transfer. The trilayer shows strong light absorption over a wide range <700 nm) making it great potential for solar energy harvesting and conversion. Moreover, the Dirac point of GR
author_names: Liang Xu

Wei-Qing Huang

Wangyu Hu

Bing-Xin Zhou

Anlian Pan

Gui-Fang Huang
title: Two Dimensional MoS2 Graphene Based Multilayer van der Waals Heterostructures: Enhanced Charge Transfer and Optical Absorption, and Electric Field Tunable Dirac Point and Band Gap
year: 2017

abstract: (BiO)2CO3 hierarchical microspheres and (BiO)2CO3 nanoparticles with wide band gaps were fabricated by a simple hydrothermal method. The former showed higher visible light photocatalytic activity towards the removal of NO than the latter due to the enhanced extrinsic absorption benefiting from the strong light reflecting and scattering effects. The extrinsic absorption over (BiO)2CO3 hierarchical microspheres can induce the production of holes and electrons with visible light, subsequently generating active species to participate in photocatalytic reactions.
author_names: Ting Xiong

Fan Dong

Zhongbiao Wu
title: Enhanced extrinsic absorption promotes the visible light photocatalytic activity of wide band gap (BiO)2CO3 hierarchical structure
year: 2014

abstract: Confirmation of 13CO2 photoconversion into a 13C product is crucial to produce solar fuel. However, the total reactant and charge flow during the reaction is complex; therefore, the role of light during this reaction needs clarification. Here, we chose Ag ZrO2 photocatalysts because beginning from adventitious C, negligible products are formed using them. The reactants, products, and intermediates at the surface were monitored via gas chromatography mass spectrometry and FTIR, whereas the temperature of Ag was monitored via Debye Waller factor obtained by in situ extended X ray absorption fine structure. With exposure to 13CO2, H2, and UV visible light, 13CO selectively formed, while 8.6% of the 12CO mixed in the product due to the formation of 12C bicarbonate species from air that exchanged with the 13CO2 gas phase during a 2 h reaction. By choosing the light activation wavelength, the CO2 photoconversion contribution ratio was charge separated at the ZrO2 band gap (l 248 nm) 70% localized at the Ag surface plasmon resonance (LSPR) (330 l 580 nm) 28% and characterized by a thermal energy of 295 K: 2% LSPR at the Ag surface was converted to heat at temperatures of up to 392 K, which provided an efficient supply of activated H species to the bicarbonate species, combined with separated electrons and holes above the ZrO2, which generated CO at a rate of 0.66 mmol h 1 gcat 1 with approximately zero order kinetics. Photoconversion of 13CO2 using moisture was also possible. Water photo oxidation step above ZrO2 was rate limited, and the side reactions that formed H2 above the Ag were successfully suppressed instead to produce CO via the Mg2+ addition to trap CO2 at the surface.
author_names: Hongwei Zhang

Takao Itoi

Takehisa Konishi

Yasuo Izumi
title: Dual Photocatalytic Roles of Light: Charge Separation at the Band Gap and Heat via Localized Surface Plasmon Resonance To Convert CO2 into CO over Silver Zirconium Oxide.
venue: Journal of the American Chemical Society
year: 2019

abstract: Two dimensional binary MX2 (M Ni, Pd and Pt; X P and As) exhibiting a beautiful pentagonal ring network is discussed through first principles calculations. We predict that MX2 sheets possess narrow direct band gaps of about 0.3 0.8 eV and ultrahigh mobilities both for holes and electrons. The hole mobility of PdAs2 is even up to 50 x 104 cm2 V 1 s 1 and the electron mobility of PtAs2 is up to 25 x 104 cm2 V 1 s 1. Besides fascinating electronic properties, 2D MX2 also exhibits optical absorption over the entire visible light range. Additionally, like graphene and carbon nanotubes, a MX2 sheet can be rolled into various forms of MX2 nanotubes as well. Interestingly, NiP2 nanotubes exhibit rich band gap behaviour which can be adjusted by the roll up vector. With a desired band gap, high carrier mobility and optical properties, 2D MX2 sheets promise to be potential candidates for electronic devices.
author_names: Shifeng Qian

Xiaowei Sheng

Xian Xu

Yuxiang Wu

Ning Lu

Zhengbo Qin

Jian Wang

Caixia Zhang

Er-yin Feng

Wuying Huang

Yong Zhou
title: Penta MX2 (M Ni, Pd and Pt; X P and As) monolayers: direct band gap semiconductors with high carrier mobility
year: 2019

abstract: Two dimensional materials with an appropriate band gap and high carrier mobility are urgently desired in the field of nanoelectronics. We propose a novel two dimensional crystal monolayer TlP5, which is dynamically and thermodynamically stable and possesses a direct band gap of 2.02 eV with high carrier mobilities (13 960 cm2 V 1 s 1 for electrons and 7560 cm2 V 1 s 1 for holes) comparable to that of phosphorene. The band gap value and band characteristics of monolayer TlP5 can be adjusted by biaxial and uniaxial strains, and excellent optical absorption over the visible light range is predicted. These properties, especially the balanced high mobilities for not only the electrons but also the holes, make monolayer TlP5 an exciting functional material for future applications in nanoelectronics and optoelectronics.
author_names: Jun-Hui Yuan

Alessandro Cresti

Kan-Hao Xue

Hai-Lei Su

Li-Heng Li

Naihua Miao

Zhimei Sun

Jia-Fu Wang

Xiang-shui Miao
title: TlP5: an unexplored direct band gap 2D semiconductor with ultra high carrier mobility
year: 2019

abstract: Perovskite oxide materials have grabbed enormous attention from various research groups all over the world due to their large application areas. The band gap engineering of those materials are important for optoelectronic researches especially for ferroelectric (FE) solar cells that have unique features such as having higher open circuit voltages than the band gap and their spontaneous polarization which leads to photovoltaic effect. Nevertheless, the most of the perovskite FE materials have wide band gaps that hamper the absorption of large solar spectrum. In the present study, it has been demonstrated the band gap of YMnO3 (YMO) which is one of the mostly studied FE materials, can be tuned via doping osmium (Os) into manganese (Mn) site. The band gap of YMO, 2.10 eV successfully is lowered to 1.61 eV. Polycrystalline YMnO3 and YMn1 xOsxO3 (YMOO) (x 0.01, 0.05, 0.10) thin films were synthesized on indium tin oxide (ITO) substrates at 500 degC by magnetron sputtering method. Their structural, chemical and optical band gap properties were studied and the results showed the Os doped YMO compounds could be a potential candidate for future ferroelectric solar cell studies.
author_names: Ozgur Polat

F M Coskun

Mustafa Coskun

Zehra Durmus

Yasemin Caglar

Mujdat Caglar

A Turut
title: Tailoring the band gap of ferroelectric YMnO3 through tuning the Os doping level
venue: Journal of Materials Science: Materials in Electronics
year: 2019

abstract: Abstract In the present work and for the first time, tuning of the band gap width of the SrTiO3 (STO) perovskite to a value suitable for photocatalytic (PC) and photovoltaic (PV) applications is accomplished by the incorporation of Fe cation. Nanocrystalline SrTi0.9Fe0.1O2.968 (STFO) was prepared by a modified solid state reaction process including successive sequences of milling and calcinations at high temperature. The X ray diffraction (XRD) pattern revealed the formation of a single cubic perovskite phase of STFO with average crystallite size equaling ~30 nm. The local lattice strain on (h00) and (hh0) planes was found to decrease by Fe doping. The absorption spectrum deduced from diffused reflectance showed high intense broad structure extending over the range ~0.5 ~6 eV, whereas pure STO gave strong absorption only at the UV region (l
author_names: K Sedeek

Sh A Said

Taghreed Zaghloul Amer

N Makram

Hanan Hassan Hantour
title: Band gap tuning in nanocrystalline SrTi0.9Fe0.1O2.968 perovskite type for photocatalytic and photovoltaic applications
year: 2019