Minority Carrier Blocking Research Articles

Enhanced seebeck coefficient and low thermal conductivity of Cu2SexTe1-x solid solutions via minority carrier blocking and interfacial effects

Abstract The investigation of state-of-the-art copper chalcogenides (Cu2-yX; X = S, Se or Te) based thermoelectric (TE) materials can direct to improve the performances of TE devices. Herein, we report the synthesis of Cu2SexTe1-x solid solutions prepared by two-step ball-milling method. The effect of Se concentration on their TE performance as a function of temperature, was investigated. XRD and HRTEM analyses confirmed the size reduction and the presence of amorphous regions with polycrystalline behavior of the samples. To evaluate their TE performance, as-prepared powder samples were cold-pressed and annealed at 773 K. The high figure of merit was obtained for Cu2Se0·7Te0.3 solid solution as 0.3 at 500 K due to the synergy of high Seebeck coefficient (174 μVK−1 at 500 K) and low thermal conductivity (1.16 Wm−1K−1 at 500 K). The high Seebeck coefficient with optimum carrier concentration and low thermal conductivity was achieved due to wide range of phonon scattering. The results suggested that, Cu2SexTe1-x solid solutions were efficient for low grade (300–500 K) waste heat recovery.

Gigantic Phonon-Scattering Cross Section To Enhance Thermoelectric Performance in Bulk Crystals.

In thermoelectric energy conversions, thermal conductivity reduction is essential for enhancing thermoelectric performance while maintaining a high power factor. Herein, we propose an approach based on coated-grain structures to effectively reduce the thermal conductivity to a much greater degree when compared to that done by conventional nanodot nanocomposite. By incorporating CdTe coated layers on the surface of SnTe grains, the thermal conductivity is as low as 1.16 W/m-K at 929 K, resulting in a thermoelectric figure of merit, i.e., zT, of 1.90. According to our developed theory, phonons scatter coherently due to the phase lag between phonons passing through and around the coated grain. Such scattering is induced by the acoustic impedance mismatch between the coated layer and the grain, resulting in a gigantic phonon-scattering cross section. The phonon-scattering cross section of the coated grains is several orders of magnitude larger than that of the nanodots with the same impurity concentration. The power factor was also slightly increased by the energy filtering effect at the coated surface and additional minority carrier blocking by the heterointerfaces. This scheme can be utilized for various bulk crystals, meaning a broad range of materials can be considered for thermoelectric applications.

Antimony-induced heterogeneous microstructure of Mg2Si0.6Sn0.4 thermoelectric materials and their thermoelectric properties

In order to achieve enhancements in thermoelectric efficiency, microstructures that can form numerous interfaces have been investigated intensively for controlling the transport of charge carriers and heat-carrying phonons. In this paper, we report the heterogeneous microstructure of Mg2Si0.6Sn0.4 thermoelectric materials synthesized by a simple B2O3 encapsulation method and investigation of its influence on thermoelectric properties. The addition of Sb causes the evolution of a Sn-rich secondary phase and a heterogeneous microstructure consisting of Sn-deficient grains and a Sn-rich boundary phase, with coherent interfaces between them. The secondary phase induced by Sb doping suppressed the bipolar effect and reduced the thermal conductivity because of minority carrier blocking and phonon scattering at phase boundaries. However, high concentration of Sb in Sn-rich phase led to insufficient doping in Si-rich main phase and electron-hole compensation by Mg vacancies, resulting in decrease of the doping efficiency of Sb.

Minority Carrier Blocking to Enhance the Thermoelectric Performance of Solution-Processed BixSb2-xTe3 Nanocomposites via a Liquid-Phase Sintering Process.

Minority carrier blocking through heterointerface barriers has been theoretically proposed to enhance the thermoelectric figure of merit (ZT) of bismuth telluride based nanocomposites at elevated temperatures recently (Phys. Rev. B 2016, 93, 165209). Here, to experimentally realize the minority carrier blocking, a liquid-phase sintering process enabled by excess Te is applied to the solution-processed BixSb2-xTe3 nanocomposites to introduce interfacial energy barriers. The controlling parameters in the liquid-phase sintering process such as the amount of excess Te, sintering temperature and holding time, and the Bi composition (x) are systemically tuned and investigated to fully understand the minority carrier blocking mechanism. These interface-engineering parameters are optimized for introducing maximum lattice imperfections and band-bending interfaces that are responsible for blocking the minority carrier and wide-range scattering of the phonons toward enhanced thermoelectric performance. High ZT > 1.4 at 375 K is realized in the Bi0.5Sb1.5Te3 sample, which is much higher than those of the state-of-the-art commercial ingots (ZT ∼ 1) and other solution-processed nanocomposites. The enhanced ZT at elevated temperatures is mostly due to the suppression of bipolar thermal conductivity by minority carrier blocking as well as the reduction of lattice thermal conductivity. Adapting this solution synthesis process to design favorable heterointerfaces for minority carrier blocking in the liquid-phase sintering process holds promise to further enhance the ZT values.

Minority carrier blocking to enhance the thermoelectric figure of merit in narrow-band-gap semiconductors

We present detailed theoretical predictions on the enhancement of the thermoelectric figure of merit by minority carrier blocking with heterostructure barriers in bulk narrow-band-gap semiconductors. Bipolar carrier transport, which is often significant in a narrow-band-gap material, is detrimental to the thermoelectric energy conversion efficiency as it suppresses the Seebeck coefficient and increases the thermal conductivity. When the minority carriers are selectively prevented from participating in conduction while the transport of majority carriers is relatively unaffected by one-sided heterobarriers, the thermoelectric figure of merit can be drastically enhanced. Thermoelectric transport properties such as Seebeck coefficient, electrical conductivity, and electronic thermal conductivity including the bipolar term are calculated with and without the barriers based on the near-equilibrium Boltzmann transport equations under the relaxation time approximation to investigate the effects of minority carrier barriers on the thermoelectric figure of merit. For this, we provide details of carrier transport modeling and fitting results of experimental data for three important material systems, $\mathrm{B}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{3}$-based alloys, $\mathrm{M}{\mathrm{g}}_{2}\mathrm{S}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{n}}_{x}$, and $\mathrm{S}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{G}{\mathrm{e}}_{x}$, that represent, respectively, near-room-temperature (300 K--500 K), midtemperature (600 K--900 K), and high-temperature ($g1000\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) applications. Theoretical maximum enhancement of thermoelectric figure of merit that can be achieved by minority carrier blocking is quantified and discussed for each of these semiconductors.

(Invited) Advanced Nanostructured Thermoelectric Materials for Energy Conversion

The rapid development of nanomaterials in the past decade has provided a possibility of directly converting waste heat back to electricity based on the thermoelectric Seebeck effect. In the past four years, we have developed a transformative approach to pioneered low cost and scalable solution-phase growth methods to mass produce thermoelectric nanowires and nanowire heterostructures to match the physical and economic magnitudes of energy use and economical entertainment in the manufacture/recycling. These nanostructured thermoelectric materials show a significantly enhanced performance compared to the bulk crystals based on the quantum confinement and minority carrier blocking.

Theoretical Aspects of Minority Carrier Extraction in Unipolar Barrier Infrared Detectors

We have examined, theoretically, minority carrier collection in unipolar barrier infrared photodetectors. In barrier infrared detectors, for example the nBn, the unipolar barrier should block only majority carriers and allow unimpeded flow of minority carriers. However, an imperfect barrier would also block minority carriers, resulting in higher than expected turn-on bias. Minority carrier blocking can be caused by barrier doping or unintended band offset between the barrier and the absorber. The distinct manner in which these two mechanisms affect device performance were investigated. We found that introduction of an appropriate amount of barrier doping can reduce depletion dark current without increasing turn-on bias. We examined the effects of band structure on conductivity effective masses when the n-type absorber was a type-II superlattice (T2SL). We showed that for a long-wavelength infrared InAs/GaSb T2SL the vertical conductivity hole effective mass can be much smaller than that predicted by the simple band-edge effect mass picture, implying that the vertical hole mobility estimated from the band-edge effective mass can be unduly pessimistic.

Minority carrier barrier heterojunctions for improved thermoelectric efficiency

We propose and demonstrate the beneficial use of minority carrier blocking layers to lessen bipolar conduction at elevated temperatures and increase the Seebeck coefficient in a thermoelectric material. Bipolar conduction, caused by thermally activated intrinsic carriers in semiconductors, causes a detrimental “roll-over” in the Seebeck coefficient and increase in thermal conductivity, measured as a function of temperature. We propose that these negative effects can be lessened or delayed in a small band gap host matrix by incorporating wider band gap semiconductor layers to impede the thermal diffusion of minority carriers over majority carriers. To prove this concept, a composite material grown by molecular beam epitaxy of p-type InAs layers and electron blocking p-type AlSb layers were grown and measured. We show improved thermoelectric properties over just p-type InAs in measurements from 110K to 600K.

Enhanced thermoelectric properties in bulk nanowire heterostructure-based nanocomposites through minority carrier blocking.

To design superior thermoelectric materials the minority carrier blocking effect in which the unwanted bipolar transport is prevented by the interfacial energy barriers in the heterogeneous nanostructures has been theoretically proposed recently. The theory predicts an enhanced power factor and a reduced bipolar thermal conductivity for materials with a relatively low doping level, which could lead to an improvement in the thermoelectric figure of merit (ZT). Here we show the first experimental demonstration of the minority carrier blocking in lead telluride-silver telluride (PbTe-Ag2Te) nanowire heterostructure-based nanocomposites. The nanocomposites are made by sintering PbTe-Ag2Te nanowire heterostructures produced in a highly scalable solution-phase synthesis. Compared with Ag2Te nanowire-based nanocomposite produced in similar method, the PbTe-Ag2Te nanocomposite containing ∼5 atomic % PbTe exhibits enhanced Seebeck coefficient, reduced thermal conductivity, and ∼40% improved ZT, which can be well explained by the theoretical modeling based on the Boltzmann transport equations when energy barriers for both electrons and holes at the heterostructure interfaces are considered in the calculations. For this p-type PbTe-Ag2Te nanocomposite, the barriers for electrons, that is, minority carriers, are primarily responsible for the ZT enhancement. By extending this approach to other nanostructured systems, it represents a key step toward low-cost solution-processable nanomaterials without heavy doping level for high-performance thermoelectric energy harvesting.

Enhancing the thermoelectric figure of merit through the reduction of bipolar thermal conductivity with heterostructure barriers

In this paper, we present theoretically that the thermoelectric figure of merit for a semiconductor material with a small band gap can be significantly enhanced near the intrinsic doping regime at high temperatures via the suppression of bipolar thermal conductivity when the minority carriers are selectively blocked by heterostructure barriers. This scheme is particularly effective in nanostructured materials where the lattice thermal conductivity is lowered by increased phonon scatterings at the boundaries, so that the electronic thermal conductivity including the bipolar term is limiting the figure of merit zT. We show that zT can be enhanced to above 3 for p-type PbTe, and above 2 for n-type PbTe at 900 K with minority carrier blocking, when the lattice thermal conductivity is as low as 0.3 W/m K.

Electronic structure of strained [formula omitted] superlattices : S. Ciraci, O. Gulseren and S. Ellialtioglu. Solid St. Commun.65, 1285 (1988)

N-type SIPOS and poly-silicon emitters on silicon show potential for improved minority carrier blocking properties over conventional diffused emitters. This paper discusses experiments designed to elucidate the physical mechanisms responsible for this improvement and to optimize the process conditions. Emitters both with and without an intentionally grown chemical oxide under the SIPOS or poly-silicon film are investigated. Both poly-silicon and SIPOS emitters, in their optimized form, can achieve Joe of less than 2 × 10−14 A/cm2, an improvement of several decades over shallow diffused emitters.

N-type SIPOS and poly-silicon emitters

N-type SIPOS and poly-silicon emitters on silicon show potential for improved minority carrier blocking properties over conventional diffused emitters. This paper discusses experiments designed to elucidate the physical mechanisms responsible for this improvement and to optimize the process conditions. Emitters both with and without an intentionally grown chemical oxide under the SIPOS or poly-silicon film are investigated. Both poly-silicon and SIPOS emitters, in their optimized form, can achieve J oe of less than 2 × 10 −14 A/cm 2, an improvement of several decades over shallow diffused emitters.

Effect of the back-surface field on the open-circuit voltages of p +-n-n + and n +-p-p + silicon solar cells

The effect of the back-surface field (BSF) on the open-circuit voltages V oc of front-illuminated p +-n-n + (p/n BSF) and n +-p-p + (n/p BSF) silicon solar cells was investigated. A theory based on the assumption that the front emitter has a unit injection efficiency was formulated and this theory can be applied to both low and high level conditions in the bulk regions of p/n and n/p BSF cells. The effect of perfect and imperfect minority carrier blocking at the low-high (L-H) junction end of the bulk region was analysed. The theory gives some insight into the physics of the BSF cells. It shows that the minority carrier blocking at the L-H back junction both by itself and by causing a back reflection of minority carriers towards the front-junction end of the bulk region leads to an improvement in the open-circuit voltage of the cells. The contribution of back reflection is independent of the intensity of illumination and the resistivity ϱ B of the bulk region, whereas the contribution of minority carrier blocking increases with both the intensity of illumination and ϱ B . For low level conditions the improvement in V oc is mainly due to the contribution of back reflection. However, for high level conditions the contribution of minority carrier blocking can be quite large. For high level conditions the increase in recombination at the front surface and in the front region is more harmful to the V oc value of a p/n BSF cell. However, a decrease in the minority carrier blocking at the L-H junction is more harmful to the V oc value of an n/p BSF cell. The fact that V oc for BSF cells is independent of ϱ B is attributed to the high level conditions in their bulk regions.