A perovskite solar cell-photothermal-thermoelectric tandem system for enhanced solar energy utilization.

Photovoltaic-thermoelectric (PV-TE) tandem system has been considered as an effective way to fully utilize the solar spectrum, and has been demonstrated in a perovskite solar cell (PSC)-thermoelectric (TE) configuration. However, the conventional PSC-TE tandem architecture cannot convert infrared light transmitted through the upper PSC into heat effectively, impeding the heat-electricity conversion of TE devices. Herein, a semi-transparent PSC-photothermal-TE tandem system is designed for improved photothermal utilization. Through optimizing the buffer layer of the back transparent electrode, semi-transparent PSC with a power conversion efficiency (PCE) of 13% and an average transmittance of 53% in the range of 800-1500 nm was obtained. On this basis, a photothermal thin film was introduced between the semi-transparent PSC and the TE device, which increased the efficiency contribution ratio of the TE device from 14% to 19%, showing enhanced utilization of AM 1.5 G solar spectrum and improved photo-thermal-electric conversion efficiency.

We have constructed a semi-transparent perovskite solar cell-photothermal-thermoelectric tandem system through the optimization of transparent back electrode and the introduction of photothermal thin-film, realizing enhanced utilization of solar energy.

Thermoelectric Power Generation of TiS2/Organic Hybrid Superlattices Below Room Temperature.

Recently, the n-type TiS2/organic hybrid superlattice (TOS) was found to have efficient thermoelectric (TE) properties above and near room temperature (RT). However, its TE performance and power generation at the temperature gradient below RT have not yet been reported. In this work, the TE performance and power generation of the TOS above and below RT were investigated. The electrical conductivity (σ) and Seebeck coefficient (S) were recorded as a function of temperature within the range 233-323 K. The generated power at temperature gradients above (at ΔT = 20 and 40 K) and below (at ΔT = -20 and -40 K) RT was measured. The recorded σ decreased by heating the TOS, while |S| increased. The resulting power factor recorded ~100 µW/mK2 at T = 233 K with a slight increase following heating. The charge carrier density and Hall mobility of the TOS showed opposite trends. The first factor significantly decreased after heating, while the second one increased. The TE-generated power of a single small module made of the TOS at ΔT = 20 and 40 K recorded 10 and 45 nW, respectively. Surprisingly, the generated power below RT is several times higher than that generated above RT. It reached 140 and 350 nW at ΔT = -20 and -40 K, respectively. These remarkable results indicate that TOS might be appropriate for generating TE power in cold environments below RT. Similar TE performances were recorded from both TOS films deposited on solid glass and flexible polymer, indicating TOS pertinence for flexible TE devices.

High Thermoelectric Power Generation by SWCNT/PPy Core Shell Nanocomposites.

Polypyrrole (PPy) is a conducting polymer with attractive thermoelectric (TE) properties. It is simple to fabricate and modify its morphology for enhanced electrical conductivity. However, such improvement is still limited to considerably enhancing TE performance. In this case, a single-wall carbon nanotube (SWCNT), which has ultrathin diameters and exhibits semi-metallic electrical conductivity, might be a proper candidate to be combined with PPy as a core shell one-dimensional (1D) nanocomposite for higher TE power generation. In this work, core shell nanocomposites based on SWCNT/PPy were fabricated. Various amounts of pyrrole (Py), which are monomer sources for PPy, were coated on SWCNT, along with methyl orange (MO) as a surfactant and ferric chloride as an initiator. The optimum value of Py for maximum TE performance was determined. The results showed that the SWCNT acted as a core template to direct the self-assembly of PPy and also to further enhance TE performance. The TE power factor, PF, and figure of merit, zT, values of the pure PPy were initially recorded as ~1 µW/mK2 and 0.0011, respectively. These values were greatly increased to 360 µW/mK2 and 0.09 for the optimized core shell nanocomposite sample. The TE power generation characteristics of the fabricated single-leg module of the optimized sample were also investigated and confirmed these findings. This enhancement was attributed to the uniform coating and good interaction between PPy polymer chains and walls of the SWCNT through π-π stacking. The significant enhancement in the TE performance of SWCNT/PPy nanocomposite is found to be superior compared to those reported in similar composites, which indicates that this nanocomposite is a suitable and scalable TE material for TE power generation.

Simultaneous Realization of Flexibility and Ultrahigh Normalized Power Density in a Heatsink-Free Thermoelectric Generator via Fine Thermal Regulation.

Wearable thermoelectric generators (w-TEGs) can incessantly convert body heat into electricity to power electronics. However, the low efficiency of thermoelectric materials, tiny terminal temperature difference, rigidity, and negligence of lateral heat transfer preclude broad utilization of w-TEGs. In this work, we employ finite element simulation to find the key factors for simultaneous realization of flexibility and ultrahigh normalized power density. Using melamine foam with an ultralow thermal conductivity (0.03 W/m K) as the encapsulation material, a novel lightweight π-type w-TEG with no heatsink and excellent stretchability, comfortability, processability, and cost efficiency has been fabricated. At an ambient temperature of 24 °C, the maximum power density of the w-TEG reached 7 µW/cm2 (sitting) and 29 µW/cm2 (walking). Under suitable heat exchange conditions (heatsink with 1 m/s air velocity), 32 pairs of w-TEGs can generate 66 mV voltage and 60 µW/cm2 power density. The output performance of our TEG is remarkably superior to that of previously reported w-TEGs. Besides, the practicality of our w-TEG was showcased by successfully driving a quartz watch at room temperature.

Microstructurally Tailored Thin ß-Ag2 Se Films toward Commercial Flexible Thermoelectrics.

Aiming to overcome both the structural and commercial limitations of flexible thermoelectric power generators, an efficient room-temperature aqueous selenization reaction that can be completed in air within less than 1 min, to directly fabricate thin ß-Ag2 Se films consisting of perfectly crystalline and large columnar grains with both in-plane randomness and out-of-plane [201] preferred orientation, is designed. A high power factor (PF) of 2590 ± 414 µW m-1 K-2 and a figure-of-merit (zT) of 1.2 ± 0.42 are obtained from a sample with a thickness of ≈1 µm. The maximum output power density of the best 4-leg thermoelectric generator sample reach 27.6 ± 1.95 and 124 ± 8.78 W m-2 at room temperature with 30 and 60 K temperature differences, respectively, which may be useful in future flexible thermoelectric devices.

Enhanced thermoelectric performance in polymorphic heavily Co-doped Cu2SnS3 through carrier compensation by Sb substitution.

Heavily acceptor-doped Cu2SnS3 (CTS) shows promisingly large power factor (PF) due to its rather high electrical conductivity (σ) which causes a modest ZT with a high electronic thermal conductivity (κe ). In the present work, a strategy of carrier compensation through Sb-doping at the Sn site in Cu2Sn0.8Co0.2S3 was investigated, aiming at tailoring electrical and phonon transport properties simultaneously. Rietveld analysis suggested a complex polymorphic microstructure in which the cation-(semi)ordered tetragonal phase becomes dominant over the coherently bonded cation-disordered cubic phase, as is preliminarily revealed using TEM observation, upon Sb-doping and Sb would substitute Sn preferentially in the tetragonal structure. With increasing content of Sb, the σ was lowered and the Seebeck coefficient (S) was enhanced effectively, which gave rise to high PFs maintained at ~10.4 µWcm-1K-2 at 773 K together with an optimal reduction in κe by 60-70% in the whole temperature range. The lattice thermal conductivity was effectively suppressed from 1.75 Wm-1K-1 to ~1.2 Wm-1K-1 at 323 K while maintained very low at 0.3-0.4 Wm-1K-1 at 773 K. As a result, a peak ZT of ~0.88 at 773 K has been achieved for Cu2Sn0.74Sb0.06Co0.2S3, which stands among the tops so far of the CTS-based diamond-like ternary sulfides. These findings demonstrate that polymorphic microstructures with cation-disordered interfaces as an approach to achieve effective phonon-blocking and low lattice thermal conductivity, of which further crystal chemistry, microstructural and electrical tailoring are possible by appropriate doping.

Realization of an Ultrahigh Power Factor and Enhanced Thermoelectric Performance in TiS2 via Microstructural Texture Engineering.

Layered 1T-type TiS2 powders were pretreated by an ethanol-based shear pulverization process, which showed outstanding effectiveness in reducing the average grain size and narrowing the size distribution while maintaining high crystallinity and plate-shaped morphology. The resulting bulk ceramics densified by spark plasma sintering possessed a highly (00l)-oriented texture and pronounced anisotropy. They showed a noticeably increased σ and an unaffected S in the in-plane direction due to the increased carrier mobility µ and the constant carrier concentration n, which resulted in a significant enhancement of the in-plane power factor, optimally to an unprecedented high level of 1.6-1.8 mW m-1 K-2 in a range of 323-673 K. Meanwhile, the lattice thermal conductivity was reduced by approximately 20% due to the intensified grain boundary phonon scattering that overwhelmed the effect due to texturing. These effects not only demonstrated the powder shear pulverization pretreatment as a facial and reliable route toward a high-textured TiS2 but also enabled a remarkable increase of ZT record for TiS2-based thermoelectrics (TEs) to approximately 0.7 at 673 K, indicating clearly the significant effect of texture engineering on TE performance.

Distinct anisotropy and a high power factor in highly textured TiS2 ceramics via mechanical exfoliation.

Polycrystalline bulk of TiS2 with a remarkable enhancement of the texture degree was obtained by densifying powders refined by a liquid-based mechanical exfoliation process. As compared to the pristine TiS2, the in-(a-b)-plane mobility in the exfoliation sample increased from 5.9 to 9.8 cm2 V-1 s-1 with an almost unaffected carrier concentration, in spite of the increased scattering due to grain boundaries. As a result, a tremendously high power factor of up to 16 µW cm-1 K-2 at 673 K was achieved, which is 60% higher than that of the pristine TiS2 and is the highest for bulk TiS2 at high temperatures.

Realizing a High ZT of 1.6 in N-Type Mg3Sb2-Based Zintl Compounds through Mn and Se Codoping.

Mg3Sb2-based compounds by virtue of nontoxicity and low-cost have become a promising class of candidates for midtemperature thermoelectric power generation. Here, we successfully fabricated n-type Mg3Sb2-based materials using an inexpensive and efficient approach of one-step ball milling and spark plasma sintering, and demonstrate that a complementary and favorable effect of multiple elements coalloying/-doping leads to an excellent thermoelectric performance. The intrinsic p-type conducting behavior for Mg3Sb2 could be changed to n-type through Bi and Se coalloying on Sb sublattices with excess Mg, resulting from the suppression of Mg vacancies and the formation of Mg interstitial. Furthermore, Mn doping on Mg sublattices could soften the chemical bonds, leading to the increase of carrier mobility and concentration simultaneously. Additionally, multielement coalloying/-doping could significantly increase the lattice disorder, which undoubtedly strengthens the phonon scattering and readily results in a suppressed lattice thermal conductivity. As a result, a highest ZT value of 1.6 at 723 K and an average ZT value up to 1.1 were obtained in the temperature range of 323-723 K in the Mg3.18Mn0.02Sb1.5Bi0.49Se0.01 sample, which is one of the highest values among the Te free Mg3Sb2. This work could give guidance for improving the thermoelectric performance of Zintl phase materials or even others using the multielement codoping/-alloying strategy.

Thermoelectric Flexible Silver Selenide Films: Compositional and Length Optimization.

Silver selenide is considered as a promising room temperature thermoelectric material due to its excellent performance and high abundance. However, the silver selenide-based flexible film is still behind in thermoelectric performance compared with its bulk counterpart. In this work, the composition of paper-supported silver selenide film was successfully modulated through changing reactant ratio and annealing treatment. In consequence, the power factor value of 2450.9 ± 364.4 µW/(mK2) at 303 K, which is close to that of state-of-the-art bulk Ag2Se has been achieved. Moreover, a thermoelectric device was fabricated after optimizing the length of annealed silver selenide film via numerical simulation. At temperature difference of 25 K, the maximum power density of this device reaches 5.80 W/m2, which is superior to that of previous film thermoelectric devices. Theoretically and experimentally, this work provides an effective way to achieve silver-selenide-based flexible thermoelectric film and device with high performance.

Flexible Foil of Hybrid TaS2 /Organic Superlattice: Fabrication and Electrical Properties.

TaS2 nanolayers with reduced dimensionality show interesting physics, such as a gate-tunable phase transition and enhanced superconductivity, among others. Here, a solution-based strategy to fabricate a large-area foil of hybrid TaS2 /organic superlattice, where [TaS2 ] monolayers and organic molecules alternatively stack in atomic scale, is proposed. The [TaS2 ] layers are spatially isolated with remarkably weakened interlayer bonding, resulting in lattice vibration close to that of TaS2 monolayers. The foil also shows excellent mechanical flexibility together with a large electrical conductivity of 1.2 × 103 S cm-1 and an electromagnetic interference of 31 dB, among the highest values for solution-processed thin films of graphene and inorganic graphene analogs. The solution-based strategy reported herein can add a new dimension to manipulate the structure and properties of 2D materials and provide new opportunities for flexible nanoelectronic devices.

Preparation of pH-Responsive Hollow Capsules via Layer-by-Layer Assembly of Exfoliated Layered Double Hydroxide Nanosheets and Polyelectrolytes.

pH-Responsive smart capsules were developed by the layer-by-layer assembly with a colloidtemplating technique. Polystyrene (PS) particles were employed as core templates. Acid-soluble inorganic nanosheets were prepared from Mg-Al layered double hydroxide (LDH) by an exfoliation technique. LDH nanosheets and anionic polyelectrolytes were alternatively deposited on PS core particles by the layer-by-layer assembly using electrostatic interaction. Hollow capsules were obtained by the removal of the PS core particles. The hollow capsules obtained thus were collapsed at acidic conditions by dissolution of LDH nanosheets in the hollow shells. The dissolution rate, i.e., the responsiveness of capsule, is tunable according to the strength of acids.

Ultrahigh thermoelectric power factor in flexible hybrid inorganic-organic superlattice.

Hybrid inorganic-organic superlattice with an electron-transmitting but phonon-blocking structure has emerged as a promising flexible thin film thermoelectric material. However, the substantial challenge in optimizing carrier concentration without disrupting the superlattice structure prevents further improvement of the thermoelectric performance. Here we demonstrate a strategy for carrier optimization in a hybrid inorganic-organic superlattice of TiS2[tetrabutylammonium] x [hexylammonium] y , where the organic layers are composed of a random mixture of tetrabutylammonium and hexylammonium molecules. By vacuum heating the hybrid materials at an intermediate temperature, the hexylammonium molecules with a lower boiling point are selectively de-intercalated, which reduces the electron density due to the requirement of electroneutrality. The tetrabutylammonium molecules with a higher boiling point remain to support and stabilize the superlattice structure. The carrier concentration can thus be effectively reduced, resulting in a remarkably high power factor of 904 µW m-1 K-2 at 300 K for flexible thermoelectrics, approaching the values achieved in conventional inorganic semiconductors.

Eco-friendly p-type Cu2SnS3 thermoelectric material: crystal structure and transport properties.

As a new eco-friendly thermoelectric material, copper tin sulfide (Cu2SnS3) ceramics were experimentally studied by Zn-doping. Excellent electrical transport properties were obtained by virtue of 3-dimensionally conductive network for holes, which are less affected by the coexistence of cubic and tetragonal phases that formed upon Zn subsitition for Sn; a highest power factors ~0.84 mW m-1 K-2 at 723 K was achieved in the 20% doped sample. Moreover, an ultralow lattice thermal conductivity close to theoretical minimum was observed in these samples, which could be related to the disordering of atoms in the coexisting cubic and tetragonal phases and the interfaces. Thanks to the phonon-glass-electron-crystal features, a maximum ZT ~ 0.58 was obtained at 723 K, which stands among the tops for sulfide thermoelectrics at the same temperature.

Dielectric Mismatch Mediates Carrier Mobility in Organic-Intercalated Layered TiS2.

The dielectric constant is a key parameter that determines both optical and electronic properties of materials. It is desirable to tune electronic properties though dielectric engineering approach. Here, we present a systematic approach to tune carrier mobilities of hybrid inorganic/organic materials where layered two-dimensional transition-metal dichalcogenide TiS2 is electrochemically intercalated with polar organic molecules. By manipulating the dielectric mismatch using polar organic molecules with different dielectric constants, ranging from 10 to 41, the electron mobility of the TiS2 layers was changed three times due to the dielectric screening of the Coulomb-impurity scattering processes. Both the overall thermal conductivity and the lattice thermal conductivity were also found to decrease with an increasing dielectric mismatch. The enhanced electrical mobility along with the decreased thermal conductivity together gave rise to a significantly improved thermoelectric figure of merit of the hybrid inorganic/organic materials at room temperature, which might find applications in wearable electronics.

Flexible n-type thermoelectric materials by organic intercalation of layered transition metal dichalcogenide TiS2.

Organic semiconductors are attracting increasing interest as flexible thermoelectric materials owing to material abundance, easy processing and low thermal conductivity. Although progress in p-type polymers and composites has been reported, their n-type counterpart has fallen behind owing to difficulties in n-type doping of organic semiconductors. Here, we present an approach to synthesize n-type flexible thermoelectric materials through a facile electrochemical intercalation method, fabricating a hybrid superlattice of alternating inorganic TiS2 monolayers and organic cations. Electrons were externally injected into the inorganic layers and then stabilized by organic cations, providing n-type carriers for current and energy transport. An electrical conductivity of 790 S cm(-1) and a power factor of 0.45 mW m(-1) K(-2) were obtained for a hybrid superlattice of TiS2/[(hexylammonium)x(H2O)y(DMSO)z], with an in-plane lattice thermal conductivity of 0.12 ± 0.03 W m(-1) K(-1), which is two orders of magnitude smaller than the thermal conductivities of the single-layer and bulk TiS2. High power factor and low thermal conductivity contributed to a thermoelectric figure of merit, ZT, of 0.28 at 373 K, which might find application in wearable electronics.

Electronic conduction in La-based perovskite-type oxides.

A systematic study of La-based perovskite-type oxides from the viewpoint of their electronic conduction properties was performed. LaCo0.5Ni0.5O3±Î´ was found to be a promising candidate as a replacement for standard metals used in oxide electrodes and wiring that are operated at temperatures up to 1173 K in air because of its high electrical conductivity and stability at high temperatures. LaCo0.5Ni0.5O3±Î´ exhibits a high conductivity of 1.9 × 103 S cm-1 at room temperature (R.T.) because of a high carrier concentration n of 2.2 × 1022 cm-3 and a small effective mass m∗ of 0.10 me. Notably, LaCo0.5Ni0.5O3±Î´ exhibits this high electrical conductivity from R.T. to 1173 K, and little change in the oxygen content occurs under these conditions. LaCo0.5Ni0.5O3±Î´ is the most suitable for the fabrication of oxide electrodes and wiring, though La1-x Sr x CoO3±Î´ and La1-x Sr x MnO3±Î´ also exhibit high electronic conductivity at R.T., with maximum electrical conductivities of 4.4 × 103 S cm-1 for La0.5Sr0.5CoO3±Î´ and 1.5 × 103 S cm-1 for La0.6Sr0.4MnO3±Î´ because oxygen release occurs in La1-x Sr x CoO3±Î´ as elevating temperature and the electrical conductivity of La0.6Sr0.4MnO3±Î´ slightly decreases at temperatures above 400 K.

Enhanced thermoelectric performance of Nb-doped SrTiO3 by nano-inclusion with low thermal conductivity.

Authors reported an effective path to increase the electrical conductivity while to decrease the thermal conductivity, and thus to enhance the ZT value by nano-inclusions. By this method, the ZT value of Nb-doped SrTiO3 was enhanced 9-fold by yttria stabilized zirconia (YSZ) nano-inclusions. YSZ inclusions, located inside grain and in triple junction, can reduce the thermal conductivity by effective interface phonon scattering, enhance the electrical conductivity by promoting the abnormal grain growth, and thus lead to the obvious enhancement of ZT value, which strongly suggests that, it is possible to not only reduce the thermal conductivity, but also increase the electrical conductivity by nano-inclusions with low thermal conductivity. This study will give some useful enlightenment to the preparation of high-performance oxide thermoelectric materials.

Thermoelectric performance of SrTiO3 enhanced by nanostructuring-self-assembled particulate film of nanocubes.

Self-assembled particulate films with a uniform structure over a large area were prepared from La-SrTiO3 nanocubes for thermoelectric applications. UV irradiation was used to assist the formation of particulate film for decomposition of the organic phase in situ to obtain a mechanically robust structure at high temperature. The thermoelectric properties of the particulate film were measured after calcination at 1000 °C under a reductive atmosphere (Ar/H2 = 60/40). A Seebeck coefficient of S = -239 ± 24 µV/K, electrical conductivity of σ = 160 ± 5 S/cm, and thermal conductivity of κ ≈ 1.5 W/mK were obtained for a self-assembled particulate film (La: 5%) corresponding to a ZT value of 0.2 at room temperature, which exceeded that of a La-SrTiO3 single crystal with similar composition.

Titanium sulphene: two-dimensional confinement of electrons and phonons giving rise to improved thermoelectric performance.

We investigated the thermoelectric properties of titanium sulphene, namely, few-layered TiS(2) nanosheets, by using density functional theory and the Boltzmann transport equations. The Seebeck coefficient of titanium sulphene was found to increase if the thickness fell below 14 triple layers (~8 nm) and for the monolayer becomes 40% larger than that of the bulk TiS(2). This behavior is attributed to an enhancement in the density of states near the conduction band minimum in the monolayer. Moreover, the acoustic phonon band of the monolayer is more flat than that of the bulk, which results in a 37% reduction of the acoustic phonon group velocity and was beneficial for a low lattice thermal conductivity. Therefore, the combined effects from quantum confinement of both electrons and phonons could lead to a significant enhancement in thermoelectric performance in the two-dimensional titanium sulphene.