Achieving Ultrahigh Power Factor In n-type Ag2Se Thin Films By Carrier Engineering

Ag 2 Se is a promising n-type material which has been proposed for thermoelectric (TE) application. Achieving high TE power factor for Ag 2 Se thin lm to use in micro and wearable electronic systems has recently attracted great attention. In present work, Ag 2 Se thin lms were prepared via a simple co-evaporation method, which provides an effective way for adjusting its composition. By selective modication of Ag content, the carrier concentration is optimized, leading to a PF of 6.27 μWcm -1 K -2 . Furthermore, the carrier mobility increased while carrier concentration is maintained after performing an annealing process, thus contributes to relatively high Seebeck coecient and decent electrical conductivity for Ag 2.05 Se lm annealed at 423 K. As a result, a record-high power factor of 20.51 μWcm 1 K -2 at 393 K is achieved, which is the best result of the Ag 2 Se thin lm prepared by evaporation method. This work has opened the way for environmentally friendly room-temperature thermoelectricity.


Introduction
Thermoelectric (TE) materials can utilize Seebeck effect to generate energy from the conversion between heat and electricity [1][2][3][4][5]. TE devices fabricated based on TE materials have wide applications in daily life, aerospace, military and other elds due to the advantages of long service life, and non-polluting [6][7][8][9][10]. The performance of TE material is determined by the dimensionless gure of merit ZT (ZT = S 2 σT/κ), which de ned by conductivity σ, Seebeck coe cient S, thermal conductivity κ and absolute temperature T, respectively [11][12][13][14]. For low dimensional TE materials, especially thin lms, power factor (PF = S 2 σ) is more usually used to characterize their TE performance.
Due to potential applications in miniature and wearable devices, achieving high performance TE thin lm is highly desired. Despite high ZT value thin lms fabricated based on traditional Te-based materials have been reported, non-environmentally friendly and poor mechanical property still limits their application [15]. Silver selenide (Ag 2 Se) is a promising n-type TE material and a high ZT of 0.96 at 390 K with excellent mechanical property was investigated by X. Shi [16]. Therefore, various methods in recent years have been employed to fabricated Ag 2 Se thin lms in order to achieve high TE performance and mechanical properties thin lm to meet the demand of device manufacturing [17][18][19][20][21][22]. For instance, Ding et al. have produced this exible substance on the Nylon layer by hot-pressed. The power factor of their exploration is 9.87 µWcm − 1 K − 2 at room temperature [21]. Zhou et al. synthesized the Ag 2 Se thermoelectric thin lms by using pulsed laser deposition method and achieved high PF about 17.5 µWcm − 1 K − 2 at room temperature [20]. Gao et al. achieved high PF of 24.51 µWcm − 1 K − 2 by hydrothermal method of paper-supported Ag 2 Se lm [22].
Since both Ag and Se are easy oxidized, it can be expected that the vacuum physical vapor deposition technique should be more suitable for the preparation of Ag 2 Se thin lm materials [23][24][25][26]. De nitely, Gonzalez et al. obtained a record power factor of 24.4 µWcm − 1 K − 2 in stoichiometric Ag 2 Se lm grown by pulsed hybrid reactive magnetron sputtering, which is comparable with that of the state-of-the-art bulk Ag 2 Se [17]. Thermal evaporation method is one of the most commonly used vacuum physical vapor deposition techniques that have been reported for preparing excellent TE performance thin lms, such as Bi 2 Te 3 , Cu 2 Se and Sb 2 Se 3 [27][28][29]. However, Ag 2 Se thin lms prepared by this method has low power factor [30][31][32]. Optimization of the carrier concentration and mobility can effectively improve the thermoelectric performance of TE thin lms to some extent [33]. To further enhance the TE performance of evaporated Ag 2 Se thin lms, it is imperative to integrate multiple approaches to tune the highly interconnected thermoelectric properties [34][35][36]. In this work, Ag 2 Se thin lms were prepared by a facile thermal co-evaporation method, instead of using single evaporated, which provides an effective way to adjust composition. The selective modi cation of Ag content can markedly increase the carrier concentration and enhance the electrical conductivity. Moreover, further annealing can effectively decrease the micro-structure defects of the lms, leading to the enhancement of Seebeck coe cient.
Accordingly, the power factor of 20.51 µWcm − 1 K − 2 is achieved at 393 K, which is a record value of the Ag 2 Se thin lms prepared by evaporation methods. Experiment Ag 2 Se based thin lms were deposited at room temperature by thermal co-evaporation method. High purity Ag powder (99.99 %) and Se powder (99.99 %) were xed in a vacuum deposition chamber by using tantalum evaporator boats. The BK7 glass was used as substrates with a dimension of 20 mm × 20 mm × 2 mm and ultrasonic cleaned for 10 minutes sequentially in acetone, ethanol, and deionized water. The background pressure was maintained at 6.5 × 10 − 4 Pa. The working current of silver source is 160 A, and selenium source stabilized at 40 A with the deposition time was both 15 min. All the Ag powder and Se powder were evaporated after deposition process. The weight ratio of Ag powder and Se powder was reasonably regulated in order to control the composition of Ag and Se in the thin lms.
Annealing process was further employed in the glove compartment after the optimal composition ratio was con rmed.
X-ray diffraction (XRD, D/max 2500 Rigaku Corporation, CuK α radiation with the angle of 20° − 60° under 0.02° per step) was applied to analyze the crystal orientation. The surface morphology and element distribution were characterized by a scanning electron microscope (SEM, Zeiss supra55), transmission electron microscopy (TEM, Titan Cubed Themis G201, FEI) with an energy dispersive spectrometer (EDS, Bruker EDS QUANTAX). The electrical conductivity and Seebeck coe cient were measured by utilizing a Seebeck coe cient and electrical conductivity apparatus (SBA458, Nezsch). Van der Pauw Hall measuring instrument (HL5500 PC, Nanometrics) was applied to investigate the carrier concentration and mobility. X-ray photoelectron spectroscopy (XPS, Thermo escalab 250Xi Thermo Fisher) provided semiquantitative information of the elemental valence states. Variable temperature XRD (SmartLab 3KW Rigaku Corporation) reacts the fact that the sample undergoes a phase transition at 406 K. The bandgap was determined from the re ection spectra obtained on UV-VIS-NIR spectrometer (UV-3600Plus Shimadzu Corporation). Table 1 shows the composition content of Ag 2 Se thin lms measured by EDS and it can be seen that the actual atomic ratio is close to the nominal atomic ratio of the powder. In order to better recognize, the lms were named by using nominal atomic ratio as Ag intensity increase with increase of Ag content as shown from the illustrate inset in the right side of Fig.  1(a). The elemental mappings as shown in Fig. 2(b) con rm some Se clusters in the Se-rich sample [38], and no Se-rich when Ag increases, which matched the XRD result. However, some big particles are observed in the surface of Ag-rich thin lm and con rms as Ag clusters [30], indicating that there are some component defects in the thin lm deposited at room temperature, which are mainly due to insu cient atomic energy as shown from the surface morphology in Fig. S1 (Supporting information).

Results And Discussion
Figure 2(a) shows the room-temperature electrical conductivity σ, Seebeck coe cient S, and power factor PF of the Ag 2 Se based thin lms. The σ increases with the rising of Ag content, while the S has a negative change trend. As a comprehensive result, PF rstly increases, reach a maximum value of 6.27 µWcm − 1 K − 2 , and then decreases. According to the Mott equation [1], both the σ and S is determined by carrier concentration n and mobility µ: m * is the effective mass of electrons. Thus, the Hall measurement is analyzed and Fig. 2(b) displays the n and µ as function of Ag to Se atomic ratio. The n of the Ag 1.55 Se is 3.3×10 18 cm − 3 , and greatly increases to over 14.0 ×10 18 cm − 3 after the atomic ratio raised over 2.05, while µ decreases from to 650 Comparatively, the change of carrier concentration is more distinct than the mobility due to disappeared Se defect and increase of Ag content, thus attributing to the greatly enhancement of σ [33,34,39].
Although thin lms deposited at room-temperature have high carrier concentration, some Ag clusters component defects observed from the SEM results, resulting low µ mobility, and thus cause the low power factor [33]. Annealing has been reported as an e cient way that can reduce the component defects and increase the grain size of the thin lms, leading to high mobility which bene ts to achieve high TE performance [22]. Thus, Ag 2.05 Se sample with maximum PF value was annealed and the temperature was set as 375 K, 393 K, 403 K, 413 K, 423 K, 453 K, 483 K, 513 K and 543 K, respectively.  Table S1 indicates the Se content slightly decreased after annealing, resulting in the decreased of n. Especially, carrier mobility has greatly increased from of 400 cm − 2 V − 1 s − 1 as-deposited sample to over 600 cm − 2 V − 1 s − 1 with the slightly affect in the carrier concentration when the annealing temperature was over 393 K, bene ting to achieve high S and results in relatively high PF. Additionally, temperature dependence TE performance of sample annealed at 423 K is shown in Fig. 3 Figure 4(a) shows the X-ray diffraction patterns of the annealed thin lms. All the thin lms show the primary Ag 2 Se phase with a weak impurity Ag phase related peak. With the increase of annealing temperature, the intensity of (112) peak increases and (121) peak decrease, which is more closed to the typical α-phase Ag 2 Se. The binding states of Ag and Se elements in the Ag 2.05 Se thin lm are investigated by XPS and the results are illustrated in Fig. 4(b) and 4(c). As shown in Fig. 4(b), the core level spectrums reveal that the sample have two strong peaks located at ~ 368.4 eV of Ag 3d 5/2 and ~ 374.2 eV of Ag 3d 3/2 , which agree with the spin-orbit phenomena of Ag and Ag + , respectively [40]. A broad peak ranging from 52 to 56 eV is observed and can be identi ed into two symmetric peaks to be assigned to Se 3d 5/2 and Se 3d 3/2 located at ~ 54.2 and ~ 54.9 eV, which is the characteristic shape of Se(− ) in a consistent bonding environment as shown in Fig. 4(c) [40]. Thus, these analyses indicate that the chemical states of the elements of the thin lms are Ag + and Se 2− , respectively. SEM images of the samples are shown in Fig. S2 and indicates that the surface of all the thin lms have Ag clusters.
However, more Ag spherical-liked clusters is observed when the annealing temperature was over 483 K. These independence Ag clusters in the thin lm surface (Fig. S3) will act as a combining center, thus causes the decrease of electrical conductivity [30]. The content of Se is slightly decreased after annealing, as shown in EDS results (Table S1), suggesting that the aggravation of element diffusion during the annealing process led to the strength of Ag clusters and the loss of Se. Similar phenomenon is also reported by Jindal et al. [30,32].
It is worth noting that the annealing temperature corresponding to the sharp increase in the carrier mobility is near the phase-transition temperature from α-phase to β-phase of Ag 2 Se (In-situ XRD is shown in Fig. S4). Thus, in order to further investigate the factor, the unannealed Ag 2.05 Se sample and annealed sample at 423 K have been analyzed by TEM. As shown in Fig. 5 (a), screw dislocations with length of ~ 100nm are observed for unannealed thin lm. Moreover, Ag vacancies in the lattice are observed in Fig.  5(a), which are furthered con rmed by the intensity line pro le of the square root of STEM intensity. It can be speculated that there is still a lack of Ag in some regions due to the Ag-clusters, despite the thin lm is slight Ag-rich. As mentioned above in SEM analysis (Fig. 1) that some independent Ag clusters distributes in the thin lm's surface due to the limit of diffusion energy of the atoms when the thin lm deposited at room temperature. For annealed Ag 2.05 Se lm, no dislocation defects are observed in the measurement region and Ag vacancy also disappeared from the grains, indicating the redistribution of atoms during the annealing process. The reduction of defects is bene cial to transport of carriers, and the vanishment of Ag vacanicies leads to the decrease of electron concentration. The results consist with the transport properties as displayed in Fig. 3. Meanwhile, the calculation in Fig. 5(c) and Fig. 5(d) shows that the Ag 2 Se with Ag vacancy has smaller bandgap than that of the complete Ag 2 Se. We have established a cell with a volume of 2×2×1. And the K point we selected is 3×3×3. We followed the geometrical optimization method BFGS which the convergence standard is the energy of a single atom of 1.0×10 − 5 eV, the interaction force between atoms of 0.03 eVnm − 1 , the stress in the crystal of 0.05 GPa, and the maximum displacement of atoms of 0.0001 nm. We used Generalized Gradient Approximation (GGA) in the form of Perdew-Burke-Ernzerhof (PBE) to describe the exchange correlation energy: and used the ultra-soft pseudopotential to express the interaction between electrons and ions [34]. The electron wave function generated by plane waves with truncate energy of 300 eV. In Fig. S5, the measured optical bandgap con rms that the lms have larger optical bandgap after annealing, which match the calculated result. Additionally, more Ag atoms can enter into the β-phase lattice than the α-phase as reported in the literatures [35,36]. Therefore, it can be inferred that the intensi ed atomic diffusion at the annealing temperature over phase change temperature will reduce dislocation defects and Ag vacancy defects in the Ag 2 Se thin lm, thus contributes in the enhancement of carrier mobility and results in relative high S.

Conclusion
In summary, we fabricated Ag 2 Se based thin lm at room temperature by using a facile thermal coevaporation method. High electrical conductivity of Ag 2 Se thin lms is achieved after carrier concentration optimization by controlling Ag content, resulting in a maximum power factor of 6.27 µWcm − 1 K − 2 . Subsequently, the S of the prepared thin lms are enhanced via annealing according to the relative high carrier mobility due to the reduced dislocation defects and Ag vacancy defects after annealing. As a result, an ultrahigh PF of 17.62 µWcm − 1 K 2 at room temperature and 20.51 µWcm − 1 K 2 at 393 K has been obtained. The overall performance of TE properties studied in this work is comparable or even higher than that of previously reported Ag 2 Se thin lms prepared by thermal evaporation method.
Declarations Figure 1 (a) X-ray diffraction patterns of Ag2Se thin lms with different nominal ratio of Ag to Se. (b) Surface morphology and elemental mappings of Ag1.75Se, Ag1.95Se and Ag2.15Se thin lms

Supplementary Files
This is a list of supplementary les associated with this preprint. Click to download. supportinginformation0817.docx