Texture and Se vacancy optimization induces high thermoelectric performance in Bi_(2)Se_(3) flexible thin films

Bi_(2)Se_(3)-based flexible thin film with high thermoelectric performance is promising for the waste heat recovery technology.In this work,a novel post-selenization method is employed to prepare n-type Bi_(2)Se_(3)flexible thin films with highly textured structure.The strengthened texture and Se vacancy optimization can be simultaneously achieved by optimizing the selenization temperature.The highly oriented texture leads to the increased carrier mobility and results in a high electric conductivity of~290.47 S·cm^(-1)at 623 K.Correspondingly,a high Seebeck coefficient(>110μW·K-1)is obtained due to the reduced carrier concentration,induced by optimizing vacancy engineering.Consequently,a high power factor of 3.49μW·cm^(-1)·K^(-2)at 623 K has been achieved in asprepared highly-bendable Bi_(2)Se_(3)flexible thin films selenized at 783 K.This study introduces an effective post-selenization method to tune the texture structure and vacancies of Bi_(2)Se_(3)flexible thin films,and correspondingly achieves high thermoelectric performance.

Isovalent co-alloying contributes to considerable improvement in thermoelectric performance of BiSe bulks with weak anisotropy

BiSe with intrinsic low thermal conductivity has considered as a promising thermoelectric(TE)material at nearly room temperature.To improve its low thermoelectric figure of merit(zT),in this work,Sb and Te isovalent co-alloying was performed and significantly optimized its TE property with weakly anisotropic characteristic.After substituting Sb on Bi sites,the carrier concentration is suppressed by introduction of Sbsingle bond Se site defects,which contributes to the increased absolute value of Seebeck coefficient(|S|).Further co-alloying Te on Se of the optimized composition Bi_(0.7)Sb_(0.3)Se,the carrier concentration increased without affecting the|S|due to the enhanced effective mass,which leads to a highest power factor of 12.8μW/(cm·K^(2))at 423 K.As a result,a maximum zT of∼0.54 is achieved for Bi_(0.7)Sb_(0.3)Se_(0.7)Te_(0.3) along the pressing direction and the average zT(zTave)(from 300 K to 623 K)are drastically improved from 0.24 for pristine BiSe sample to 0.45.Moreover,an energy conversion efficiency∼4.0%is achieved for a single leg TE device of Bi_(0.7)Sb_(0.3)Se_(0.7)Te_(0.3)when applied the temperature difference of 339 K,indicating the potential TE application.

Realizing high thermoelectric performance via selective resonant doping in oxyselenide BiCuSeO

Tuning the charge carrier concentration is imperative to optimize the thermoelectric(TE)performance of a material.For BiCuSeO based oxyselenides,doping efforts have been limited to optimizing the carrier concentration.In the present work,dual-doping of In and Pb at Bi site is introduced for p-type BiCuSeO to realize the electric transport channels with intricate band characteristics to improve the power factor(PF).Herein,the impurity resonant state is realized via doping of resonant dopant In over Pb,where Pb comes forward to optimize the Fermi energy in the dual-doped BiCuSeO system to divulge the significance of complex electronic structure.The manifold roles of dual-doping are used to adjust the elevation of the PF due to the significant enhancement in electrical properties.Thus,the combined experimental and theoretical study shows that the In/Pb dual doping at Bi sites gently reduces bandgap,introduces resonant doping states with shifting down the Fermi level into valence band(VB)with a larger density of state,and thus causes to increase the carrier concentration and effective mass(m*),which are favorable to enhance the electronic transport significantly.As a result,both improved ZTmax=0.87(at 873 K)and high ZTave=0.5(at 300–873 K)are realized for InyBi(1−x)−yPbxCuSeO(where x=0.06 and y=0.04)system.The obtained results successfully demonstrate the effectiveness of the selective dual doping with resonant dopant inducing band manipulation and carrier engineering that can unlock new prospects to develop high TE materials.

Realizing high thermoelectric performance in n-type Bi_(2)Te_(3)based thin films via post-selenization diffusion

Thermoelectric thin film has attracted a lot of attention due to its potential in fabricating micropower generator in chip sensors for internet of things(IoT).However,the undeveloped performance of n-type thermoelectric thin film limits its widely application.In this work,a facile post-selenization diffusion reaction method is employed to introduce Se into Bi_(2)Te_(3)thin films,in order to optimize the carrier transport properties.Experimental and theoretical calculation results indicate that the carrier concentration decreases and density of states increases after Se doping,leading to the enhancement of Seebeck coefficient.Further,adjusting the diffusion reaction temperature can maintain the carrier concentration while increasing the mobility simultaneously,resulting in a high power factor of 1.5 mW/(m·K^(2)),which is eight times higher than that of the pristine Bi_(2)Te_(3)thin films.Subsequently,a thin film device fabricated by the present Se-doped Bi_(2)Te_(3)thin films shows the highest output power of 60.20 nW under the temperature difference of 37 K,indicating its potential for practical use.

Enhanced thermoelectric properties of n-type Bi_(2)O_(2)Se by KCl doping

Bi_(2)O_(2)Se is considered one of the most promising thermoelectric(TE)materials for combining with p-type BiCuSeO in a TE module given its unique chemical and thermal stability.However,the enhancement of its dimensionless figure of merit,zT value,remains a challenge because of its low electrical conductivity.Herein,we introduce KCl into Bi_(2)O_(2)Se,synthesized by solid-state reaction and spark plasma sintering method,to improve its TE properties.The synthesized samples show an outstanding enhancement in electrical conductivity,carrier concentration,and power factor after KCl doping.The Bi_(2)O_(2)Se-based sample with a 0.05%KCl doping content possesses a high zT value of~0.58 at 773 K,which is over 50%enhancement compared with the pristine Bi_(2)O_(2)Se sample.We also prove that the K element substitutes the Bi site,and Cl replaces the Se site by X-ray diffraction results and density functional theory calculation,supporting that K can improve the electrical conductivity by the position of Fermi level which is above the conduction band minimum.Experimental and theoretical results indicate the success of co-doping with a small amount of KCl and show a huge potential of this novel method for Bi_(2)O_(2)Se TE performance improvement.

SnSe+Ag_2Se composite engineering with ball milling for enhanced thermoelectric performance

Earth-abundant IV-VI semiconductor SnSe is regarded as a promising thermoelectric material due to its intrinsic low thermal conductivity. In this report, the highly textured SnSe/Ag2Se composites were first designed by solid solution method followed by spark plasma sintering （SPS） and their thermoelectric properties in two directions were investigated, and then, the performance of composites was further optimized with an additional ball milling. The coexistence of SnSe and Ag2Se phases is clearly confirmed by energy-dispersive X-ray spectroscopy （EDX） in transmission electron microscopy （TEM）. After ball milling, the size of SnSe grains as well as the incorporated Ag2Se particles reduces effectively, which synergistically optimizes the electrical and thermal transport properties at high temperature range. As a result, a maximum ZT of -0.74 at 773 K for SnSe ＋ 1.0%AgzSe in the direction vertical to the pressing direction is achieved. Composite engineering with additional ball milling is thus proved to be an efficient way to improve the thermoelectric properties of SnSe, and this strategy could be applicable to other thermoelectric systems.

High thermoelectric and mechanical performance in the n-type polycrystalline SnSe incorporated with multi-walled carbon nanotubes

In this study,we introduce multi-walled carbon nanotubes(MWCNTs)in Pb/I co-doped n-type polycrys-tal SnSe to simultaneously improve its thermoelectric and mechanical properties for the first time.The introduced MWCNTs act as the“bridges”to accelerate the electron carrier transport between SnSe grains,leading to significantly increased electrical conductivity from 32.6 to 45.7 S cm^(−1) at 773 K,which con-tributes to an enhanced power factor of∼5.0μW cm^(−1) K^(−2) at this temperature.Although MWCNTs possess high intrinsic thermal conductivities,these MWCNTs,acting as nanoinclusions in the SnSe matrix to form the dense interfaces between SnSe and MWCNTs,provide extra heat-carrying phonon scattering centers,leading to a slightly reduced lattice thermal conductivity of only 0.34 W m^(−1) K^(−2) at 773 K and in turn,a high ZT of∼1.0 at this temperature.Furthermore,the introduced MWCNTs can simultane-ously act as the“binders”to bond adjacent grains,significantly improving the mechanical properties of SnSe by boosting its Vickers hardness from 39.5 to 50.5.This work indicates that our facile approach can achieve high thermoelectric and mechanical properties in n-type SnSe polycrystals with a considerable potential for applying to thermoelectric devices as n-type elements.

In-situ growth of high-performance(Ag,Sn)co-doped CoSb_(3)thermoelectric thin films

Owing to the unique features,such as mechanically robust,low-toxic,high stability,and high thermoelectric performance,CoSb_(3)-based skutterudite materials are among art-of-the state thermoelectric candidates.In this work,we develop a facile in-situ method for the growth of well-crystallinity(Ag,Sn)co-doped CoSb_(3)thin films.This preparation method can efficiently control the dopant concentration and distribution in the thin films.Both the density functional theory calculation and the experimental results suggest that Sn and Ag dopants trend to enter the lattice and preferentially fill interstitial sites.Additionally,band structure calculation results suggest that the Fermi level moves into the conduction bands due to co-doping and eventually induces the increased electrical conductivity,which agrees with the optimization of carrier concentration.Moreover,an increase in the density of state after co-doping is responsible for the increased Seebeck coefficient.As a result,the power factors of(Ag,Sn)co-doped CoSb_(3)thin films are greatly enhanced,and the maximum power factor achieves over 0.3 m W m^(-1)K^(-2)at 623 K,which is almost two times than that of the un-doped CoSb_(3)film.Multiple microstructures,including Sb vacancies and Ag/Sn interstitial atoms as point defects,and a high density of lattice distortions coupled with nano-sized Ag-rich grains,lead to all scale phonon scatterings.As a result,a reduced thermal conductivity of~0.28 W m^(-1)K^(-1)and a maximum ZT of~0.52 at 623 K are obtained from(Ag,Sn)co-doped CoSb_(3)thin films.This study indicates our facile in-situ growth can be used to develop high-performance dual doped CoSb_(3)thins.