Rational Design and Materials Simulation of Semiconductors for Energy Conversion Applications

Author:He Xin

Supervisor:zhang li jun


Degree Year:2019





The increasing challenges in energy demands and environmental concerns due to the consumption of fossil fuels have invigorated growing awareness in the past few decades.At present,the world’s energy demand is largely dependent on fossil fuels,such as petroleum,coal,and natural gas,which are rapidly being depleted.As a result,novel discoveries and frontlines in materials science and ngineering have been pursued to overcome the obstacle for effective energy conversion and environmental protection.Among various renewable energy projects,semiconductor based photovoltaic and photocatalysis,in which the inexhaustible and clean solar energy can be harvested as a feasible technology,has gained considerable interdisciplinary attention for its diverse potential in energy and environmental applications.Until now,the direct conversion of solar energy to energy fuels and chemical energy has been regarded as one of the green sustainable avenues to address the energy and environmental crisis in the future.Thermoelectric materials,which can generate electricity from waste heat or be used as solid-state Peltier coolers,could play an important role in a global sustainable energy solution.The global demand for energy is rapidly increasing.Screening and designing new high-efficiency energy materials has become a research hotspot.This paper starts with the optimization design of novel photovoltaic,photocatalytic and thermoelectric conversion materials.The main work is as follows:(1)Dielectric behavior as a screen in rational searches for novel photovoltaic and thermoelectric materials:metal-pnictide-sulfosalts.Dielectric screening plays an important role in reducing the strength of carrier scattering and trapping by point defects for many semiconductors such as the halide perovskite.However,dielectric screening was never considered as a criterion to find/design new electronic material.In this work,we first time take dielectric screening as characteristic property and based on high-throughput materials calculation method to design high performance semiconductors in metal-V-sulfide temery system.The elements of group V have ns2 lone pair electrons and the significant cross-band gap hybridization between itsp state and S-p state is the main reason for the high dielectric constant of this kind of materials.Meanwhile,based on our designed results,Sn-Sb-S system has been proved to be a solar photovoltaic material with high efficiency(solar cell efficiency>5%).This study provides a new perspective for the optimal design of novel electronic semiconductor.(2)Solid salt confinement effect:an effective strategy to fabricate high crystalline polymer carbon nitride for enhanced photocatalytic hydrogen evolution.As one type of metal-free photocatalysts for solar energy utilization,heptazine-based polymer carbon nitrides(PCN)have attracted intense interest due to their extraordinary chemical stability,facile preparation,visible light response and tunable electronic structure.These characteristics have endowed PCN with great potentials in artificial photocatalysis.Nevertheless,the photocatalytic activity over pristine PCN is limited by several inherent issues,such as low optical absorption threshold,high electron-hole recombination rate and sluggish surface reaction kinetics.One of the major reasons is that PCN prepared by thermal-induced polymerization typically shows low crystallinity.The high-density defects serve as recombination centers of photogenerated electron-hole pair,thus leading to low transfer efficiency of charge carriers in water splitting reaction.Herein,we present a solid-salt-assisted growth strategy to improve the crystallinity of PCN under an atmospheric environment.The solid salt with high melting point acts as an easily removable and recyclable template to guide the growth of crystalline polymer carbon nitride(CPCN).The fabricated CPCN samples have large grain sizes up to 400 nm.We resolve the CPCN crystalline structure with the aid of first principles calculations to be a conjugation structure based on chain-like melon motifs with K cations intercalated.It reduced the distance between the layer and influence the electron distribution,provides a favorable transmission channel for photogenic carriers,greatly reduces the probability of recombination,and much enhanced the photocatalytic performance for hydrogen ecolution reaction(by the factor of 22)under visible light illuminated.Our work provides a facile approach to enhance crystalline quality of polymer carbon nitrides and other polymeric photocatalysts for photocatalytic activity improvement.(3)We have built reasonable structure model for Zn-Sb system,prediced optimal carrier concentration for high thermoelectric performance,provided reference for the experiment.Zn-Sb binary materials have been widely studied for their excellent thermoelectric properties.The figure of merit ZT of β-Zn4Sb3 can reach 1.3.The research is devoted to finding new Zn-Sb materials and further improving the thermoelectric properties of known materials by doping.In recent years,novel material(?)-Zn8Sb7 havs been synthesized.It also has excellent thermoelectric properties.The existence of interstitial Zn atoms in B-phase materials has hindered the theoretical research.We build reasonable structure models for(?)-phase,combined with Boltzmann transport theory,to predict their thermoelectric properties and the optimal doping concentration,providing guidance for the experiment.The structure of the metastable phase Zn3Sb2 had been reported in the experiment without structure information.Was based on the particle swarm intelligence optimization algorithm to predict its structure.All the phases are found to be semiconducting with bandgaps in the range of 0.06-0.77 eV.This semiconducting behavior is understood in Zintl terms as a balance between the Zn:Sb and Sb3-:1/2(Sb2)4-ratios in the stable crystal structures.With the exception of Zn3Sb2,which has a small gap,all the compounds have electronic properties favorable for thermoelectric performance.(4)Identification of critical condition for drastically enhancing thermoelectric powerfactor of two-dimensional layered materials.As early as 1993,professor Dresselhaus and his student Hicks proposed that the enhanced electronic density of states per unit volume in two-dimensional(2D)quantum well and superlattice materials would give rise to highly increased power factors over the bulk values.[Phys.Rev.B 47,12727(1993)],which provided a very important theoretical guidance for the acquisition of high-performance thermoelectric materials.But no conclusive evidence has been obtained in experimental and theoretical studies.Recent theory argued that the discrepancy between theory and experiments arises from a competition between the quantum confinement length L and the thermal de Broglie wavelength ξ.Here we present a joint theory-experiment study of the thickness dependence of thermoelectric transport properties in 2D InSe at various temperatures and gate-tunable carrier densities.Therefore,it is crucially important to experimentally identify the critical condition associated with thickness of 2D indium selenide(InSe)below which the power factor could be greatly enhanced over its bulk form.More importantly,we verify that the power factor in 2D InSe can be drastically enhanced when the confinement length of sample is shorter than the thermal de Broglie wavelength in agreement with recently proposed theory.Our results demonstrate a promising path for enhancing the power factor and thus thermoelectric performance of 2D materials and may pave the way toward utilizing 2D materials in practical thermoelectric devices.