Table 1 shows the composition content of Ag2Se thin films 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 films were named by using nominal atomic ratio as Ag1.65Se, Ag1.75Se, Ag1.85Se, Ag1.95Se, Ag2.05Se, Ag2.15Se and Ag2.25Se, respectively.
Table 1
Nominal atomic ratio of Ag and Se powder before evaporate and the actual content of the thin films measured by EDS
Nominal atomic ratio (Ag : Se)
|
1.65
|
1.75
|
1.85
|
1.95
|
2.05
|
2.15
|
2.25
|
Ag(at%)
|
61.5
|
62.2
|
65.6
|
66.3
|
67.3
|
68.3
|
70.0
|
Se(at%)
|
38.5
|
37.8
|
34.4
|
33.7
|
32.7
|
31.8
|
30.0
|
Actual atomic ratio (Ag : Se)
|
1.6
|
1.65
|
1.91
|
1.97
|
2.06
|
2.15
|
2.33
|
Figure 1(a) shows the X-ray diffraction patterns of thin films, and indicates three main peaks located at ~ 33.5°, ~ 34.7° and ~ 36.9° for all the patterns, corresponding to the (112), (121) and (013) planes of Ag2Se polycrystalline (PDF#24-1041) [37]. The Ag1.65Se and Ag1.75Se samples contain a weak impurity peak at ~ 29.5°, belonging to Se phase (PDF#06-0362), and it disappears after further increased Ag content. Meanwhile, all the samples have Ag impurity peak at ~ 38.1° and the peak (PDF#04-0783) 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) confirm 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 film and confirms as Ag clusters [30], indicating that there are some component defects in the thin film deposited at room temperature, which are mainly due to insufficient atomic energy as shown from the surface morphology in Fig. S1 (Supporting information).
Figure 2(a) shows the room-temperature electrical conductivity σ, Seebeck coefficient S, and power factor PF of the Ag2Se based thin films. The σ increases with the rising of Ag content, while the S has a negative change trend. As a comprehensive result, PF firstly increases, reach a maximum value of 6.27 µWcm− 1K− 2, and then decreases. According to the Mott equation [1], both the σ and S is determined by carrier concentration n and mobility µ:
(2)
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 Ag1.55Se is 3.3×1018 cm− 3, and greatly increases to over 14.0 ×1018 cm− 3 after the atomic ratio raised over 2.05, while µ decreases from to 650 cm− 2V− 1s− 1 to 400 cm− 2V− 1s− 1. 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 films 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 efficient way that can reduce the component defects and increase the grain size of the thin films, leading to high mobility which benefits to achieve high TE performance [22]. Thus, Ag2.05Se 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. Figure 3(a) shows the σ, S, and PF as function of annealing temperature. It indicates that both σ and S are increased after annealing, and all the annealed thin films have higher σ and S values than that of the as-deposited sample. A maximum value of 17.62 µWcm− 1K− 2 is obtained from the sample annealed at 423 K, which is over 200 % enhancement compare with the as-deposited sample. Figure 3(b) shows the n and µ as a function of annealing temperature. The n firstly increases and then decreases with the increasing annealing temperature, which is well matched with the change of σ. EDS measurement of Ag2.05Se films annealed at different temperature as shown in the 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− 2V− 1s− 1 as-deposited sample to over 600 cm− 2V− 1s− 1 with the slightly affect in the carrier concentration when the annealing temperature was over 393 K, benefiting 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(c) and indicates an increasing trend with the increasing test temperature. The σ of 1526.5 Scm− 1, S of 115.9 µVK− 1 are obtained at 393 K, contributing to a maximum PF of 20.51 µWcm− 1K− 2. The achieved PF values at room temperature and 393 K are the record high values of the Ag2Se thin films prepared by thermal evaporation method as shown in Fig. 3(d).
Figure 4(a) shows the X-ray diffraction patterns of the annealed thin films. All the thin films show the primary Ag2Se 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 Ag2Se. The binding states of Ag and Se elements in the Ag2.05Se thin film 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 3d5/2 and ~ 374.2 eV of Ag 3d3/2, 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 identified into two symmetric peaks to be assigned to Se 3d5/2 and Se 3d3/2 located at ~ 54.2 and ~ 54.9 eV, which is the characteristic shape of Se(−Ⅱ) 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 films are Ag+ and Se2−, respectively. SEM images of the samples are shown in Fig. S2 and indicates that the surface of all the thin films 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 film 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 Ag2Se (In-situ XRD is shown in Fig. S4). Thus, in order to further investigate the factor, the unannealed Ag2.05Se 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 film. Moreover, Ag vacancies in the lattice are observed in Fig. 5(a), which are furthered confirmed by the intensity line profile 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 film is slight Ag-rich. As mentioned above in SEM analysis (Fig. 1) that some independent Ag clusters distributes in the thin film’s surface due to the limit of diffusion energy of the atoms when the thin film deposited at room temperature. For annealed Ag2.05Se film, 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 beneficial 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 Ag2Se with Ag vacancy has smaller bandgap than that of the complete Ag2Se. 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 confirms that the films 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 intensified atomic diffusion at the annealing temperature over phase change temperature will reduce dislocation defects and Ag vacancy defects in the Ag2Se thin film, thus contributes in the enhancement of carrier mobility and results in relative high S.