materials

Review An Update Review on N-Type Layered

Junqing Zheng, Dongyang Wang * and Li-Dong Zhao *

School of Materials Science and Engineering, Beihang University, Beijing 100191, China; [email protected] * Correspondence: [email protected] (D.W.); [email protected] (L.-D.Z.)

Abstract: Compared with traditional thermoelectric materials, layered oxyselenide thermoelectric materials consist of nontoxic and lower-cost elements and have better chemical and thermal stability. Recently, several studies on n-type layered oxyselenide thermoelectric materials, including BiCuSeO,

Bi2O2Se and Bi6Cu2Se4O6, were reported, which stimulates us to comprehensively summarize these researches. In this short review, we begin with various attempts to realize an n-type BiCuSeO

system. Then, we summarize several methods to optimize the thermoelectric performance of Bi2O2Se, including carrier engineering, band engineering, microstructure design, et al. Next, we introduce

a new type of layered oxyselenide Bi6Cu2Se4O6, and n-type transport properties can be obtained through halogen doping. At last, we propose some possible research directions for n-type layered oxyselenide thermoelectric materials.

Keywords: thermoelectric; n-type oxyselenide; BiCuSeO; Bi2O2Se; Bi6Cu2Se4O6

  1. Introduction Citation: Zheng, J.; Wang, D.; Thermoelectric (TE) materials can achieve the direct transition between heat and Zhao, L.-D. An Update Review on electricity without producing other pollutants, providing an effective solution to the energy N-Type Layered Oxyselenide crisis and environmental problems [1,2]. The dimensionless figure of merit ZT defines Thermoelectric Materials. Materials the efficiency of a thermoelectric device, which derives from three related physical quan- 2021, 14, 3905. https://doi.org/ σ S κ 10.3390/ma14143905 tities: electrical conductivity ( ), Seebeck coefficient ( ) and thermal conductivity ( ), ZT = σS2T/κ, with absolute temperature T. However, the tightly-coupled relationship

Academic Editor: Andres Sotelo among these parameters makes it difficult to improve the overall ZT [2–6]. Compared with traditional thermoelectric materials, such as Bi2Te3 [7–10], PbTe [11–15], Received: 18 June 2021 SnTe [13,16], MgAgSb [17–19], half-Heusler alloys [20–22], Zintl phases [23,24], etc., layered Accepted: 8 July 2021 oxyselenide thermoelectric materials, mainly including BiCuSeO [2,25], Bi2O2Se [26,27] and Published: 13 July 2021 Bi6Cu2Se4O6 [28–30], consist of earth-abundant, nontoxic, light and lower-cost elements, and have better chemical and thermal stability in the middle temperature range (600– Publisher’s Note: MDPI stays neutral 900 K) [2,31]. Therefore, this series was regarded as thermoelectric materials with broad with regard to jurisdictional claims in development prospects and studied extensively. published maps and institutional affil- Intrinsic p-type BiCuSeO possesses a layered ZrCuSiAs structure with iations. space group P4/nmm [2,32]. The special layered crystal structure of BiCuSeO is constituted 2+ 2− by insulative [Bi2O2] layers and conductive [Cu2Se2] layers heaping along the c-axis by turns [33,34]. Due to the weak Van der Waals interaction between layers [9,35–37], the large displacement of the Cu atom [38,39] and the heavy Bi atom [3,40,41], BiCuSeO has intrin- Copyright: © 2021 by the authors. sically low thermal conductivity [42], which is an inherent advantage as a thermoelectric Licensee MDPI, Basel, Switzerland. material. However, most of the reported BiCuSeO are p-type semiconductor materials at This article is an open access article present, and researches on n-type BiCuSeO are relatively few and unsuccessful. The main distributed under the terms and problems are that no effective electronic dopant was found, and BiCuSeO-based materials conditions of the Creative Commons with stable n-type transport properties have not been obtained yet. Attribution (CC BY) license (https:// Different from BiCuSeO, the intrinsic Bi2O2Se exhibits n-type transport properties creativecommons.org/licenses/by/ due to a large number of Se vacancies in the crystal structure [43,44]. However, the crystal 4.0/).

Materials 2021, 14, 3905. https://doi.org/10.3390/ma14143905 https://www.mdpi.com/journal/materials Materials 2021, 14, x FOR PEER REVIEW 2 of 19

Materials 2021, 14, 3905 2 of 18 Different from BiCuSeO, the intrinsic Bi2O2Se exhibits n-type transport properties due to a large number of Se vacancies in the crystal structure [43,44]. However, the crystal structure of Bi2O2Se is very similar to BiCuSeO in which insulative [Bi2+2O2]2+ layers and structure of Bi2O2Se is very similar to BiCuSeO in which insulative [Bi2O2] layers and con- conductive 2[Se]− 2- layers stack along the c-axis alternatively [26,45,46]. Similarly, Bi2O2Se ductive [Se] layers stack along the c-axis alternatively [26,45,46]. Similarly, Bi2O2Se also hasalso intrinsically has intrinsically low thermal low thermal conductivity conductivity due to the due weak to interlayerthe weak interactioninterlayer [interaction26,47–49], 15 −3 but[26,47–49], its intrinsic but carrierits intrinsic concentration carrier concentration (~1.5 × 1015 (~1.5cm− 3×) 10 is too cm low,) is resulting too low, in resulting poor elec- in tricalpoor transportelectrical transport performance performance [50,51]. Therefore,[50,51]. Therefore, current researchcurrent research is mainly is focused mainly onfo- increasingcused on increasing the carrier the concentration, carrier concentration, thus improving thus improving the electrical the electrical transport transport performance per- 2 2 offormance Bi2O2Se. of Bi O Se. 6 2 4 6 BiBi6Cu2SeSe4OO6 isis a anew new type type of oflayered layered oxyselenid oxyselenidee thermoelectric thermoelectric material material [28–30]. [28– 30The]. Thecrystal crystal structure structure of ofBi6 BiCu6Cu2Se24SeO64 Ocan6 can be beregarded regarded as as a a1:2 1:2 ratio ratio of of BiCuSeO andand Bi Bi2O2O22SeSe 2+ 2+ heapingheaping along the cc-axis-axis by by turns turns [28], [28], so so there there are are insulative insulative layers layers [Bi [Bi2O22O] 2 ]andand conduc- con- 2- 2− 2- 2− ductivetive layers layers [Cu2 [CuSe2]2Se and2] [Se]and in [Se] the structure.in the structure. The Bi6Cu The2Se Bi4O66Cu system2Se4O not6 system only maintains not only maintainsthe low thermal the low conductivity thermal conductivity of BiCuSeO of but BiCuSeO also can but utilize also canthe utilizeintrinsic the electron intrinsic carrier elec- tronconcentration carrier concentration of Bi2O2Se, considered of Bi2O2Se, to considered be a promising to be n a-type promising oxyseleniden-type thermoelectric oxyselenide thermoelectricmaterial. At present, material. the Bi At6Cu present,2Se4O6 thesystem Bi6Cu with2Se stable4O6 system n-type with transport stable propertiesn-type trans- can portbe obtained properties by halogen can be obtained doping [29], by halogen and strategies doping to [ 29improve], and strategiesits thermoelectric to improve perfor- its thermoelectricmance need to performancebe further explored. need to be further explored. Recently,Recently, severalseveral studies studies on onn-type n-type layered layered oxyselenide oxyselenide thermoelectric thermoelectric materials materials were reported,were reported, which which motivates motivates us to systematicallyus to systematically summarize summarize the recent the recent progress progress of these of researches.these researches. The outlineThe outline of this of this review review is shown is shown in Figurein Figure1. First,1. First, several several attempts attempts to to realizerealize nn-type-type BiCuSeOBiCuSeO areare summarized.summarized. Then,Then, somesome typicaltypical approachesapproaches toto optimizeoptimize thethe thermoelectricthermoelectric performance performance of of Bi Bi2O2O2Se2Se are are presented. presented. Next, Next, a new a new type type layered layered oxyselenide oxysele- Binide6Cu Bi2Se6Cu4O2Se6 is4O introduced,6 is introduced, and andn-type n-type transport transport properties properties can can be be obtained obtained throughthrough halogenhalogen dopingdoping andand optimizedoptimized byby introducingintroducing transitiontransition metalmetal elements.elements. AtAt last,last, somesome prospectiveprospective andand outlooksoutlooks werewere providedprovidedfor forfuture future research research in in the the end. end.

FigureFigure 1.1. TheThe outlineoutline ofof thisthis review.review.

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Materials 2021, 14, 3905 3 of 18

2. Various Attempts to Realize n-Type BiCuSeO 2.1.2. Various Doping AttemptsFe at Cu Sites to Realize n-Type BiCuSeO 2.1. Doping Fe at Cu Sites Pan et al. obtained n-type transport properties in the BiCuSeO system in the low- temperaturePan etal. range obtained (300–500n-type K) through transport doping properties Fe at Cu in sites the BiCuSeO [52]. Figure system 2a,b display in the low- the temperature-dependenttemperature range (300–500 electrical K) through conductivity doping ( Feσ) atand Cu Seebeck sites [52 coefficient]. Figure2 a,b(S) displayfor Fe- dopedthe temperature-dependent samples. The σ of all samples electrical increases conductivity monotonically (σ) and Seebeck with temperature, coefficient ( Sshowing) for Fe- semiconductordoped samples. conduction The σ of all behavior. samples Mean increaseswhile, monotonically at the same temperature, with temperature, the σ increases showing withsemiconductor the increasing conduction Fe content, behavior. indicating Meanwhile, that Fe at is the an same effective temperature, dopant for the pristineσ increases Bi- with the increasing Fe content, indicating that Fe is an effective dopant for pristine BiCuSeO CuSeO matrix. The peak values of S for samples BiCu1-xFexSeO (x = 0 to 0.03) are larger matrix. The peak values of S for samples BiCu Fe SeO (x = 0 to 0.03) are larger than than 150 μV K−1 and shift towards higher temperature1-x x with the increasing Fe content. The 150 µVK−1 and shift towards higher temperature with the increasing Fe content. The band gap can be estimated by the peak value of S through the Goldsmid–Sharp law as band gap can be estimated by the peak value of S through the Goldsmid–Sharp law Formula (1) [53,54]: as Formula (1) [53,54]: Eg ==2ℯ2e|S||max|T (1)

Figure 2. The curves of ( a) electrical conductivity, andand ((bb)) SeebeckSeebeck coefficientcoefficient varyingvarying withwith temperaturetemperature for for BiCu BiCu1-x1-xFexxSeO (x = 0 to 0.04); (c) Fe content dependences of Hall coefficient for BiCu1-xFexSeO (x = 0 to 0.04) at room temperature. Data (x = 0 to 0.04); (c) Fe content dependences of Hall coefficient for BiCu1-xFexSeO (x = 0 to 0.04) at room temperature. Data presented in (a–c) were adopted from Reference [52]. Copyright 2018, The Royal Society of Chemistry. (d) Calculated band presented in (a–c) were adopted from Reference [52]. Copyright 2018, The Royal Society of Chemistry. (d) Calculated structure of pristine BiCuSeO near the Fermi level. Copyright, the Royal Society of Chemistry. (d) Reproduced with per- band structure of pristine BiCuSeO near the Fermi level. Copyright, the Royal Society of Chemistry. (d) Reproduced with mission from Reference [55]. Copyright 2019, Elsevier Masson SAS. permission from Reference [55]. Copyright 2019, Elsevier Masson SAS. The peak Seebeck coefficient and corresponding temperature increase with the in- The peak Seebeck coefficient and corresponding temperature increase with the increase creasein Fe content, in Fe content, indicating indicating the increase the increase of band gapof band in those gap samples.in those samples. Clearly, n Clearly,-type transport n-type transportproperties properties can be found can inbe thefoundx = in 0.03 the sample x = 0.03 at sample a low-temperature at a low-temperature range, indicating range, indi- that catingappropriate that appropriate Fe content canFe content achieve ncan-type achieve BiCuSeO n-type in aBiCuSeO circumscribed in a circumscribed temperature range. tem- perature range. The variation of Hall coefficient (RH) at room temperature is presented in Figure2c. Expectedly, BiCuSeO-based materials exhibit p-type electrical transport properties, and

Materials 2021, 14, 3905 4 of 18

RH should be positive. However, the RH of BiCu0.97Fe0.03SeO exhibits a negative value, confirming the n-type behavior conjectured by the negative S. The transition of electrical transport properties can be explained by the following two formulas [56,57]:

n µ2 − n µ2 = h h e e RH 2 (2) e(nhµh + neµe) S σ + S σ S = h h e e (3) σh + σe where, subscripts h and e express hole and electron carriers, respectively. In the above two formulas, nh and ne are both positive, while Sh and Se are positive and negative, respectively. The decline of RH and S after doping Fe in Cu sites can be explained by the competition between the intrinsic holes and the electrons introduced by Fe substitution, which indicates that the substitution of Fe for Cu is a kind of donor doping. When the Fe content increases to 0.03, σ increases due to the increasing total carrier concentration (nt = nh + ne); while S declines because the negative contribution of electrons partly counterbalances the positive contribution of holes to S, ultimately switching to negative when electrons become majority carriers. Figure2d exhibits the calculated band structure of pristine BiCuSeO. The valence band maximum (VBM) and conduction band minimum (CBM) are located along the Γ-M and Γ-Z direction, respectively. Larger degeneracy of VBM than CBM leads to a less favorable compromise between large mobility and large effective mass, so the large S values are not easy to obtain n-type transport.

2.2. Doping Co at Cu Sites Ren et al. aimed to increase the Seebeck coefficient by introducing magnetic ions into the BiCuSeO matrix because previous studies on other systems showed that extra spin entropy could be introduced by the magnetic ions, thereby contributing to increasing the Seebeck coefficient [55]. They found that the BiCuSeO system exhibits n-type electrical transport properties below room temperature when the Co content reaches 20%. Figure3a shows the temperature-dependent Seebeck coefficient ( S) for Co-doped samples. For x = 0.05 and 0.10 samples, the S is positive within the entire temperature boundaries, indicating the dominant carrier is p-type. However, when the doping content of Co reaches 15%, the S begins to appear negative, indicating that the system exhibits n-type electrical transport properties. The negative S values of x = 0.20 sample within the entire temperature region indicate stable n-type electrical transport properties in the BiCuSeO system below room temperature. The VBM of BiCuSeO mainly comes from the hybridization between Cu 3d and Se 4p orbitals, while the CBM is derived from the Bi 6p orbital [2,58,59], as shown in Figure3b . For BiCu0.875Co0.125SeO, the DOS close to Fermi level mainly derives from the Co 3d and Se 4p orbitals (Figure3c), and the number of bands crossing the Fermi level is two ( Figure3d ), which are both electron cylinder and hole cylinder, indicating the Co substitution introduces n-type carriers [60]. It can be seen that Co substitution can change the band structure of BiCuSeO and make the Fermi level reach to the CBM. When the Co content is large enough, electrons become majority carriers.

2.3. Doping Halogen (Br, I) at Se Sites Zhang et al. designed a series of continuous experimental steps to obtain n-type BiCuSeO [61]. Firstly, considering that the existence of Bi/Cu vacancies is the main reason for the p-type behavior of pristine BiCuSeO [62,63], extra Bi/Cu was introduced into the matrix to fill the vacancies which may produce holes. Finally, the optimal concentrations of extra Bi and Cu are determined as x = 0.04 and 0.05, respectively. To increase electron carrier concentration, halogen elements (Br, I) were selected as donor dopants at Se sites and introduced into the Bi1.04Cu1.05SeO matrix. The S as a function of temperature for I/Br doped Bi1.04Cu1.05SeO samples is presented in Figure4a,b. The introduction of Br/I can Materials 2021, 14, 3905 5 of 18

successfully transform Bi1.04Cu1.05SeO from p-type to n-type within a certain temperature range, and the negative S for I-doped Bi1.04Cu1.05SeO traverses a narrower temperature range than Br-doped one. In the high-temperature range, p-type behavior appears again, Materials 2021, 14, x FOR PEER REVIEW 5 of 19 indicating that vacancies reproduced as the temperature rises, which may be relevant to the instability of Cu–Br and Cu–I bonds [64,65].

Figure 3. (a(a) )The The temperature-dependent temperature-dependent Seebeck Seebeck coefficient coefficient of ofBiCu BiCu1-xCo1-xxCoSeOxSeO (x =( x0.05–0.20).= 0.05–0.20). Data Datawere wereadopted adopted from Referencefrom Reference [55]. Copyright [55]. Copyright 2019, 2019,Elsevier Elsevier Masson Masson SAS. SAS.Calculated Calculated density density of states of states (DOS) (DOS) of (b of) (BiCuSeOb) BiCuSeO and and (c) BiCu0.875Co0.125SeO, and (d) electronic band structure of BiCu0.875Co0.125SeO. (b–d) Reproduced with permission from Ref- (c) BiCu0.875Co0.125SeO, and (d) electronic band structure of BiCu0.875Co0.125SeO. (b–d) Reproduced with permission from erenceReference [55]. [55 Copyright]. Copyright 2019, 2019, Elsevier Elsevier Masson Masson SAS. SAS.

TheIn order VBM to of further BiCuSeO explore mainly the comesp-n-p-type from behaviorthe hybridization in the obtained between system, Cu 3d aand heating– Se 4p orbitals,cooling measurementwhile the CBM was is derived carried outfrom for the halogen Bi 6p orbital doing BiCuSeO[2,58,59], samplesas shown [66 in], Figure as shown 3b. Forin Figure BiCu04.c,d.875Co As0.125 canSeO, be the seen, DOS Bi 1.04closeCu to1.05 FermiSe0.99X level0.01O mainly (X = Br, derives I) changes from completely the Co 3dfrom and Sen-type 4p orbitals to p-type (Figure transport 3c), and behavior the number after eightof bands heating–cooling crossing the Fermi cycle measurements.level is two (Figure The 3d),above which results are indicate both electron that halogens cylinder are and effective hole cylinder, dopants indicating to obtain nthe-type Co BiCuSeO substitution but introducesexhibit poor n-type stability. carriers To improve [60]. It thecan stabilitybe seen ofthatn-type Co substitution transport, metallic can change particles the band were structureintroduced of intoBiCuSeO the halogen-doped and make the BiFermi1.04Cu level1.05SeO. reach The to the temperature CBM. When dependence the Co content of S for is largeBi1.04 Cuenough,1.05Se0.99 electronsBr0.01O be + comex% Ag majority samples carriers. is negative within the entire temperature range, and the maximum |S| is ~125 µV/K (Figure4e). The maximum ZT ~0.05 can be reached 2.3.at 475 Doping K in BiHalogen1.04Cu 1.05(Br,Se I)0.99 at BrSe0.01 SitesO + 15% Ag (Figure4f). ZhangTo further et al. understand designed the a series instability of cont of halogeninuous dopingexperimental in BiCuSeO, steps theto obtain energy n integrated-type Bi- CuSeOCrystal [61]. Orbital Firstly, Hamiltonian considering Population that the (ICOHP) existence values of Bi/Cu were vacancies calculated is(Figure the main5) [66 reason] . The formore the negative p-type behavior value of of ICOHP pristine indicates BiCuSeO the [62,63], stronger extra bond Bi/Cu strength was [introduced67]. As can into be seen the matrixin Figure to 5fill, after the vacancies halogen doping, which may the ICOHPproduce value holes. decreases Finally, the from optimal ~1.09 concentrations eV for Cu–Se ofto extra ~0.36 Bi eV and for Cu Cu–I, are ~0.25 determined eV for Cu–Br as x = and0.04 ~0.13and 0.05, eV for respective Cu–Cl,ly. respectively, To increase indicating electron carrier concentration, halogen elements (Br, I) were selected as donor dopants at Se sites and introduced into the Bi1.04Cu1.05SeO matrix. The S as a function of temperature for I/Br doped Bi1.04Cu1.05SeO samples is presented in Figure 4a,b. The introduction of Br/I can successfully transform Bi1.04Cu1.05SeO from p-type to n-type within a certain temperature range, and the negative S for I-doped Bi1.04Cu1.05SeO traverses a narrower temperature range than Br-doped one. In the high-temperature range, p-type behavior appears again, indicating that vacancies reproduced as the temperature rises, which may be relevant to the instability of Cu–Br and Cu–I bonds [64,65].

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Materials 2021, 14, x FOR PEER REVIEW 6 of 19 the weaker bond strength due to halogen doping. The weakened bond strength led to the instability of halogen doping in the BiCuSeO system under the heating–cooling cycle.

FigureFigure 4. The 4. temperatureThe temperature dependence dependence of Seebeck of Seebeck coefficient coefficient for for(a) Bi (a)1.04 BiCu1.041.Cu05Se1.051+xSeIxO1+x (IxO( = x0.01–0.05)= 0.01–0.05) and (b) Bi1.04Cuand1.05(Seb)1+x Bi1.04BrxCuO (1.05x =Se 0.01–0.05).1+xBrxO(x The= 0.01–0.05). temperature-dependent The temperature-dependent Seebeck co Seebeckefficients coefficients obtained obtained through through heating-cooling heating- cy- cle measurements:cooling cycle measurements: 8 cycles for (c 8) cyclesBi1.04Cu for1. (05c)Se Bi01.04.99I0Cu.01O1.05 andSe0.99 (dI0.01) BiO1.04 andCu (1d.05)Se Bi1.040.99BrCu01.05.01O,Se 0.99respectively.Br0.01O, respectively. The solid The symbols solid and lines expresssymbols the and heating lines express process, the heating while process,the dashed while symbols the dashed and symbols lines express and lines co expressoling coolingprocess. process. The temperature The temperature depend-

ence ofdependence (e) Seebeck of (coefficientse) Seebeck coefficients and (f) ZT and value (f) ZT ofvalue Bi1.04 ofCu Bi1.041.05SeCu1+x1.05BrSexO1+x +Br xx%O Ag + x% (x Ag = 5–20). (x = 5–20). Data Data shown shown in in (a (,ab,b,e,e,f,f)) were adoptedwere from adopted Reference from Reference [61]. Copyright [61]. Copyright 2017, Elsevier 2017, Elsevier Inc. Data Inc. shown Data shown in (c– ind) ( cwere–d) were adopted adopted from from Reference[66]. Reference [66 ].Copy- right 2019,Copyright Elsevier 2019, Ltd Elsevier and Techna Ltd and Group Techna GroupS.r.l. S.r.l.

In order to further explore the p-n-p-type behavior in the obtained system, a heating– cooling measurement was carried out for halogen doing BiCuSeO samples [66], as shown in Figure 4c,d. As can be seen, Bi1.04Cu1.05Se0.99X0.01O (X = Br, I) changes completely from n-type to p-type transport behavior after eight heating–cooling cycle measurements. The above results indicate that halogens are effective dopants to obtain n-type BiCuSeO but exhibit poor stability. To improve the stability of n-type transport, metallic particles were introduced into the halogen-doped Bi1.04Cu1.05SeO. The temperature dependence of S for Bi1.04Cu1.05Se0.99Br0.01O + x% Ag samples is negative within the entire temperature range,

MaterialsMaterials2021 2021, 14, ,14 3905, x FOR PEER REVIEW 78 of of 18 20

Figure 5. The energy integrated Crystal Orbital Hamiltonian Population (ICOHP) values for (a) Cu–Se bonding in 3. Various Attempts to Enhance Thermoelectric Properties of Bi2O2Se BiCuSeO, (b) Cu–Cl bonding in Bi Cu Se Cl O, (c) Cu–Br bonding in Bi Cu Se Br O and (d) Cu–I 3.1. Introduce1.04 1.05 Bi Deficiencies0.99 0.01 1.04 1.05 0.99 0.01 bonding in Bi1.04Cu1.05Se0.99I0.01O. Data in (a–d) were adopted from Reference [66]. Copyright 2019, Elsevier Ltd and Techna Group S.r.l. Due to a large number of Se vacancies in the crystal structure, the pristine Bi2O2Se exhibits n-type semiconductor behavior [43,44]. To improve the conductivity of Bi2O2Se, 3.thereby Various optimizing Attempts to its Enhance ZT, the Thermoelectricgeneral approach Properties is to do donor of Bi2 Odoping2Se at the Bi/Se sites 3.1.[47,51,68]. Introduce However, Bi Deficiencies Zhan et al. made an innovative attempt that introduces Bi deficien- ciesDue into to Bi a2O large2Se by number components of Se vacancies deviating in from the crystalthe stoichiometric structure, the ratio pristine [46]. In Bi 2fact,O2Se the exhibitsintroductionn-type of semiconductor Bi deficiencies behavior is equivalent [43,44 to]. acceptor To improve doping the conductivity to the matrix, of which Bi2O2 Se,runs therebycounter optimizing to the general its ZT method., the general However, approach the increment is to do of donor Seebeck doping coefficient at the Bi/Seand the sitesdecrease [47,51 ,in68 thermal]. However, conductivity Zhan et caused al. made by th ane innovativeintroduction attempt of Bi deficiencies that introduces have opti- Bi deficienciesmized the intothermoelectric Bi2O2Se by properties components of Bi deviating2O2Se. from the stoichiometric ratio [46]. In fact, theThe introduction introduction of Bi of deficiencies Bi deficiencies is equivalent has little toeffect acceptor on the doping electric to theconductivity matrix, which (σ) of runsthe counterBi2O2Se system, to the general but it can method. significantl However,y change the the increment Seebeck of coefficient Seebeck coefficient(S). The absolute and thevalues decrease of S inincreased thermal first conductivity and then causeddecreased by thewith introduction temperature of but Bi always deficiencies kept havea large optimizedpeak value the ( thermoelectric−445.6, −556.6, properties−490.0 and of −568.8 Bi2O2 μSe.V/K at 773 K, respectively), as shown in FigureThe 6a. introduction When the ofcarrier Bi deficiencies concentration has littlein the effect semiconductor on the electric is very conductivity low, the S ( σcan) of be theevinced Bi2O2Se as system, the following but it can Formula significantly (4) [4]: change the Seebeck coefficient (S). The absolute values of S increased first and then decreased with temperature but always kept a large / peak value (−445.6, −556.6, −490.0 and8𝜋−568.8𝑘 µV/K at𝜋 773 K, respectively), as shown in 𝑆= 𝑚∗𝑇 (4) Figure6a. When the carrier concentration3𝑒ℎ in the semiconductor3𝑛 is very low, the S can be

Materials 2021, 14, x FOR PEER REVIEW 8 of 19

decrease in thermal conductivity caused by the introduction of Bi deficiencies have opti- mized the thermoelectric properties of Bi2O2Se. The introduction of Bi deficiencies has little effect on the electric conductivity (σ) of the Bi2O2Se system, but it can significantly change the Seebeck coefficient (S). The absolute values of S increased first and then decreased with temperature but always kept a large Materials 2021, 14, 3905 peak value (−445.6, −556.6, −490.0 and −568.8 μV/K at 773 K, respectively), as shown8 of 18in Figure 6a. When the carrier concentration in the semiconductor is very low, the S can be evinced as the following Formula (4) [4]: evinced as the following Formula (4) [4]: / 8 = ∗ (4) 8π2k2  π 2/3 S = 3ℎB m∗T 3 (4) 3eh2 3n where kB, e, h, m*, T and n mean Boltzmann constant, electron charge, Plank constant, the whereeffectivekB, masse, h, m* of, Tcarrier,and n meanthe absolute Boltzmann temper constant,ature and electron the carrier charge, concentration, Plank constant, respec- the ef- fectivetively. mass of carrier, the absolute temperature and the carrier concentration, respectively.

FigureFigure 6.6. TheThe temperature-dependenttemperature-dependent thermoelectricthermoelectrictransport transport properties properties of of Bi Bi2-x2-xOO22Se (x = 0–0.10): ( a) Seebeck coefficient,coefficient, ((bb)) powerpower factor, factor, ( c()c total) total thermal thermal conductivity conductivity and and (d )(dZT) ZT. Data. Data shown shown in (ina– d(a)– wered) were adopted adopted from from Reference Reference [46]. [46]. Copyright Copy- 2015,right The2015, American The American Ceramic Ceramic Society. Society.

TheThe SS atat lowlowtemperature temperature increases increases proportionally proportionally with with temperature temperature for for a given a given carrier car- concentrationrier concentration and effectiveand effective mass. mass. The intrinsic The intr excitationinsic excitation at high at temperature high temperature is enhanced, is en- andhanced, the and effective the effective carrier concentrationcarrier concentration increases, increases, so the soS decreases.the S decreases. Furthermore, Furthermore, the Stheof S Bi of deficiencies Bi deficiencies samples samples are are basically basically larger larger than than pristine pristine Bi Bi2O2O2Se2Se within within thethe entireentire temperaturetemperature range,range, which which can can be be attributed attributed to to the the influence influence of m*of .m* Thanks. Thanks to theto the significant signifi- −1 −2 improvementcant improvement of S, of the S,PF thepeak PF peak value value reaches reaches ~0.93 ~0.93µW μ cmW cmK−1 K−2at at 773773 KK (Figure(Figure6 6b),b), −1 −2 whichwhich isis twicetwice that that of of the the intrinsic intrinsic sample sample (~0.45 (~0.45µ WμW cm cm−1K K−2 atat 773 773 K). The Bi deficienciesdeficiencies strengthenstrengthen thethe pointpoint defectdefect scatteringscattering andand leadlead toto aa decreasedecrease of ofκ κtottot (Figure(Figure6 6c).c). Integrating Integrating thethe enhancedenhanced electricalelectrical propertiesproperties andand suppressedsuppressed thermalthermal properties,properties, thethe peakpeak ZTZT valuevalue reachesreaches ~0.12~0.12 in Bi1.91.9O22SeSe at at 773 773 K K (Figure (Figure 66d).d). 3.2. Doping Cl at Se Sites 2− In the special crystal structure of Bi2O2Se, the [Se] layer is considered to be an electron-conducting pathway [43,44,46]. Therefore, effective electron donor dopants can be used to modify the conductive [Se]2− layer to increase the carrier concentration of the Bi2O2Se system, thereby enhancing its thermoelectric performance. Tan et al. doped Cl at Se sites and achieved an extraordinary enhancement in the electrical conductivity of the Bi2O2Se system [51]. Materials 2021, 14, x FOR PEER REVIEW 9 of 19

3.2. Doping Cl at Se Sites

In the special crystal structure of Bi2O2Se, the [Se]2− layer is considered to be an elec- tron-conducting pathway [43,44,46]. Therefore, effective electron donor dopants can be used to modify the conductive [Se]2− layer to increase the carrier concentration of the Bi2O2Se system, thereby enhancing its thermoelectric performance. Tan et al. doped Cl at Materials 2021, 14, 3905 9 of 18 Se sites and achieved an extraordinary enhancement in the electrical conductivity of the Bi2O2Se system [51]. At room temperature, the σ hikes from ~0.019 S cm−1−1 for Bi2O2Se to ~101.6 S cm−−11 for At room temperature, the σ hikes from ~0.019 S cm for Bi2O2Se to ~101.6 S cm for Bi2O2Se0.985Cl0.015, and then declines obviously as the Cl content increases (Figure 7a). One Bi2O2Se0.985Cl0.015, and then declines obviously as the Cl content increases (Figure7a ). One - 2- Cl− dopeddoped into into the the Se Se2 −sitessites can can provide provide an an extra extra electron, electron, and and the the measured measured carrier carrier con- con- 15 −3 20 −3 centration increased fromfrom 1.51.5 ×× 101015 cmcm− to3 to 1.38 1.38 × ×1010 cm20 cm (x− 3= (0.015).x = 0.015). However, However, when when the Clthe content Cl content exceeds exceeds the solubility the solubility limit, limit, the formation the formation of the oflow-conductivity the low-conductivity second second phase Bi12O15Cl6 [69] will reduce the effective doping amount of Cl, thereby deteriorating the σ. phase Bi12O15Cl6 [69] will reduce the effective doping amount of Cl, thereby deteriorating Thethe σ small. The polaron small polaron hopping hopping conduction conduction theory theory was chosen was chosen to study to study the impact the impact of Cl of do- Cl pantdopant on onthe the σ. Thisσ. This theory theory could could be beexpressed expressed as asthe the following following Formula Formula (5) (5)[70]: [70 ]:

 C   −E− σ=== neµ = exp exp a (5) T kBT

where n,, ee,, μµ, ,CC, ,kBk,B E, aE anda and T expressT express the the carrier carrier concentration, concentration, carrier carrier charge, charge, carrier carrier mo- bility,mobility, the the pre-exponential pre-exponential terms, terms, Boltzmann Boltzmann constant, constant, activation activation energy energy and and the the absolute temperature, respectively. Figure Figure 77bb showsshows thethe linearlinear relationshiprelationship betweenbetween ln(ln(σT) andand 1000/TT,, and and the the EEaa cancan be be obtained obtained by by calculating calculating the the slope of the straight line. As shown in the the inset inset of of Figure Figure 7b,7b, the the EEa aofof Cl-doped Cl-doped samples samples is isobviously obviously lower lower than than Bi2O Bi22Se,O2 in-Se, dicatingindicating that that the the introduction introduction of Cl of is Cl conduc is conduciveive to carrier to carrier excitation. excitation. In summary, In summary, the high the σhigh achievedσ achieved in Cl-doped in Cl-doped Bi2O2Se Bi2 isO estimated2Se is estimated to be the to beresult the resultof higher of higher n coupledn coupled with lower with Elowera. Ea.

Figure 7. Thermoelectric transport properties of Bi2O2Se1-xClx (x = 0–0.04): (a) electrical conductivity, (b) the fitting plots of the small polaron model and the activation energy (Ea) shown in the inset, (c) lattice thermal conductivity and (d) ZT. Data shown in (a–d) were adopted from Reference [51]. Copyright 2017, The American Ceramic Society.

Lattice thermal conductivity (κlat) of Bi2O2Se0.985Cl0.015 declines evidently after 423 K, reaching the lowest value ~0.56 W m−1 K−1 at 823 K (Figure7c). This effective de- crease in κlat is derived from point defect scattering introduced by Cl substitution coupled Materials 2021, 14, 3905 10 of 18

with the enhanced grain boundaries scattering. However, for the Bi2O2Se0.98Cl0.02 and Bi2O2Se0.96Cl0.04 sample, the considerable augment in κlat is the result of the secondary phase Bi12O15Cl6. Benefitting from both the enhancement of the σ and the depression of the κtot, the peak ZT value ~0.23 at 823 K is achieved in Bi2O2Se0.985Cl0.015 (Figure7d), which demonstrates that Cl is an effective dopant to optimize the thermoelectric performance of Bi2O2Se.

3.3. Doping Te at Se Sites

The ZT value of pristine Bi2O2Se is primarily restricted by the low electric conduc- tivity (~2.0 S cm−1) mainly caused by the low carrier concentration (~1015 cm−3)[50,51]. Fundamentally, this shortcoming can be attributed to the excessively wide band gap (~1.28 eV)[71]. Bi2O2Te, an isostructure of Bi2O2Se, possesses a narrow band gap (~0.23 eV) and a moderately high room-temperature carrier concentration (~1.06 × 1018 cm−3)[45]. Additionally, p-type BiCuTeO has a narrower band gap (~0.4 eV) compared with BiCuSeO (~0.8 eV) [72,73], and relevant studies have proved that Te substitution can effectively enhance the electrical conductivity of p-type BiCuSeO by narrowing the band gap [74]. Hence, isovalent Te doping at Se sites could be utilized to the n-type Bi2O2Se. The significantly narrowed band gap is conducive for electrons to jump across the band gap and enter the valence band (Figure8a) so that more electrons can be excited and participate in the electrical transport. The measured optical absorption spectrum of Bi2O2Se1-xTex (x = 0.02, 0.03, 0.04, 0.06) samples indicates that the band gap is monotonically reduced from ~1.77 eV for Bi2O2Se to ~0.78 eV for Bi2O2Se0.94Te0.06 with the increasing Te content (Figure8b) [ 44]. Considering the apparent difference of the band gap between Bi2O2Se (~1.77 eV) and Bi2O2Te (~0.23 eV), the band gap engineering can be effectively tuned by a small amount of Te substitution. 15 −3 Ultimately, the low carrier concentration of pristine Bi2O2Se (~10 cm ) was boosted to ~1018 cm−3, which was increased by three orders of magnitude [44]. The greatly increased carrier concentration makes the σ of all the Te-doped samples significantly larger than pristine Bi2O2Se throughout the entire test temperature range (Figure8c). To obtain insight into the electrical transport behavior, the small polaron hopping conduction theory mentioned in the previous work [70] was selected to analyze the electric conductivity. The strong linear correlation between ln(σT) and 1000/T is exhibited in Figure8d, and the curve of calculated activation energy Ea varying with the Te content is depicted in Figure8e. The Ea of electronic conduction declines with the increasing Te content, indicating that the Te substitution is beneficial for the intrinsic excitation of electrons, thereby contributing to the optimized σ. Figure8f plots the temperature dependence of ZT. Due to the increment in σ caused by the narrowing band gap, the thermoelectric performance of Bi2O2Se was enhanced. Ultimately, the highest ZT reaches ~0.28 at 823 K for Bi2O2Se0.96Te0.04.

3.4. Doping Ta at Bi Sites Choosing a suitable dopant to enhance its low carrier concentration has always been an important means to optimize the thermoelectric performance of the Bi2O2Se system. A pentavalent Ta5+ cation doping at Bi sites will provide two extra electrons for the matrix. Moreover, Ta is less electronegative than Bi, thereby easily extracting electrons. Therefore, Tan et al. chose Ta as the dopant to increase the carrier concentration, thereby enhancing the electrical transport properties of Bi2O2Se [68]. −1 Ta doping can significantly increase the σ of Bi2O2Se, from ~0.02 S cm of pristine −1 Bi2O2Se to ~149.3 S cm of Bi1.90Ta0.10O2Se at room temperature (Figure9a). Meanwhile, the temperature-dependent σ transform from semiconductor behavior to mixed-conducting behavior, and finally, Bi1.90Ta0.10O2Se and Bi1.88Ta0.12O2Se even exhibit degenerate semi- conductor behavior. The carrier concentration (nH) and mobility (µH) were measured and exhibited in Figure9b. Ta doping increases the nH by four orders of magnitude, from ~1015 cm−3 to ~1019 cm−3. In addition to the reason that Ta replaces Bi to provide extra Materials 2021, 14, 3905 11 of 18

electrons, authors believe that the formation of the Ta2O5 phase will introduce vacancies into the matrix, and each oxygen vacancy is compensated by two electrons as the Materials 2021, 14, x FOR PEER REVIEWfollowing formula: 11 of 19 × ·· 0 VO → VO + 2e

FigureFigure 8. (a) 8. The(a) schematic The schematic diagram diagram of narrowing of narrowing the the band band gap; gap; (b) ( bthe) the optical optical absorption absorption spectra spectra of of Bi Bi22OO2Se2Se1-x1-xTeTex x(x = 0– 0.06) (andx = 0–0.06 the band) and gap the band varying gap varyingwith Te with content Te content shown shown in the in inset; the inset; (c) (electricalc) electrical conductivity, conductivity, ( d) thethe fittingfitting plot plot of of the electricalthe electrical conductivity conductivity by the by small the small polaron polaron model, model, (e ()e )the the activation activation energyenergy (E (aE) anda) and (f) ZT(f) ofZT Bi of2O Bi2Se2O1-x2SeTex1-x(Tex =x 0–0.06).(x = 0–0.06). Data Datashown shown in (b in–f (b) –weref) were adopted adopted from from Reference Reference [[44].44]. CopyrightCopyright 2017, 2017, The The American American Ceramic Ceramic Society. Society.

Ultimately, the low carrier concentration of pristine Bi2O2Se (~1015 cm−3) was boosted to ~1018 cm−3, which was increased by three orders of magnitude [44]. The greatly in- creased carrier concentration makes the σ of all the Te-doped samples significantly larger than pristine Bi2O2Se throughout the entire test temperature range (Figure 8c). To obtain insight into the electrical transport behavior, the small polaron hopping conduction theory mentioned in the previous work [70] was selected to analyze the elec- tric conductivity. The strong linear correlation between ln(σT) and 1000/T is exhibited in Figure 8d, and the curve of calculated activation energy Ea varying with the Te content is

Materials 2021, 14, 3905 12 of 18

Materials 2021, 14, x FOR PEER REVIEW 13 of 19

2-x x 2 Figure 9. 9. ThermoelectricThermoelectric transp transportort properties properties of Bi of BiTa2-xOTaSexO (2xSe = 0–0.12): (x = 0–0.12): (a) electrical (a) electrical conductivity, conductivity, (b) the carrier (b) the concen- carrier concentrationtration and mobility and mobility at room at temperature, room temperature, (c) Seebeck (c) Seebeck coefficient, coefficient, (d) lattice (d) lattice thermal thermal conductivity, conductivity, (e) phonon (e) phonon mean mean free path (MPF, lph) at room temperature, and (f) ZT. Data shown in (a–f) were adopted from Reference [68]. Copyright 2019, free path (MPF, l ) at room temperature, and (f) ZT. Data shown in (a–f) were adopted from Reference [68]. Copyright WILEY-VCH Verlagph GmbH & Co. KGaA, Weinheim. 2019, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Therefore, both Ta doping and oxygen vacancies lead to the increment of nH. Simul- Therefore, both Ta doping and oxygen vacancies lead to the increment of nH. Simulta- taneously, the stable deterioration of μH implies gradually reinforced carrier scattering, neously, the stable deterioration of µH implies gradually reinforced carrier scattering, but but relatively high μH (>40 cm2 2− V1 −1 −s−11) can be maintained, which is because performing Ta relatively high µH (>40 cm V s ) can be maintained, which is because performing Ta at Bi sites would not introduce lattice defects into the conductive [Se]2− layers.layers. Compared with pristine Bi2O2Se, the absolute value of S decreases when the Ta con- Compared with pristine Bi2O2Se, the absolute value of S decreases when the Ta H tentcontent increases increases (Figure (Figure 9c),9c), which which is iscoincident coincident with with the the increase in nH.. The The calculatedcalculated *3/2*3/2 03/23/2 22 −1−1−1 −1 2 2 weighted mobility ((µμmm )) is is greatly greatly increased fromfrom ~5.41~5.41 mm0 cmcm VV s s forfor Bi BiO2SeO2 Seto 3/2 3/22 −12 −1−1 −1 ~15.44to ~15.44 m0 m 0cm cmV sV fors Bi1for.90Ta Bi01.90.10OTa2Se,0.10 revealingO2Se, revealing that Ta that doping Ta doping in Bi2O in2Se Bi 2canO2Se effec- can tivelyeffectively optimize optimize the electrical the electrical transport transport properties. properties.

Materials 2021, 14, 3905 13 of 18

−1 −1 The κlat continuously decreases as the Ta content increases, reaching ~0.69 W m K for Bi1.88Ta0.12O2Se at 823 K (Figure9d). The phonon mean-free-path ( lph) is calculated by the following Formula (6) [75,76] and plotted in Figure9e as a function of Ta content.

1 κ = C v l (6) lat 3 v a ph

where, Cv and va represent the specific heat capacity per unit volume and average sound speed, respectively. A highly intense phonon scattering process and decrease of κlat in Ta-doped Bi2O2Se can be seen from monotonically reduced lph from ~11.9 Å for Bi2O2Se to ~9.9 Å for Bi1.90Ta0.10O2Se, which mainly results from that Ta substitution introduces multi-scale lattice defects, including the enormous defects, grain boundaries, and phase interfaces [68,77]. The carrier engineering and hierarchical microstructure by Ta doping remarkably enhance the ZT values in Bi1.90Ta0.10O2Se, reaching ~0.30 at 773 K, which is an increase of ~350% compared to pristine Bi2O2Se (Figure9f).

4. Attempts to Realize a New Kind of n-Type Oxyselenide: Bi6Cu2Se4O6 Because of the strong phonon scattering caused by layered structure [9], the lone pair electrons of Bi3+ [78,79], and the local vibration of Cu+ [38], BiCuSeO exhibits inherent low thermal conductivity. Another well-known thermoelectric oxyselenide, Bi2O2Se re- veals stable n-type transport properties due to Se vacancies [43,44]. To fully utilize the features of BiCuSeO and Bi2O2Se, a new type layered oxyselenide Bi6Cu2Se4O6 was syn- thesized through solid state reaction (SSR) with the 1:2 ratio of BiCuSeO and Bi2O2Se (Figure 10a) [28–30], and stable n-type conductive transports were observed in this system through halogen element doping [29].

4.1. Halogen Element Doping at Se Sites

The σ of Cl-doped Bi6Cu2Se4O6 is higher than Br-doped one at high temperature for the doping content x = 0.2. When the doping content x is increased to 0.8, the σ was significantly improved to ~70 S cm−1, and the Br-doped sample was better σ than the Cl-doped one at high temperature (Figure 10b). The Bi6Cu2Se3.8Br0.2O6 exhibits p-type semiconductor characteristics below 673 K and transfer to n-type with the temperature increasing; while Bi6Cu2Se3.8Cl0.2O6 has a negative S value within the entire temperature boundaries indicating that a small amount of Cl doping (x = 0.2) can achieve stable n-type semiconductor behavior (Figure 10c). When the doing content raises up to 0.8, the S of Cl/Br-doped samples has little difference, varying from ~−60 to −160 µVK−1. The maximum ZT value ~0.15 at 823 K is achieved in Bi6Cu2Se3.2Br0.8O6 (Figure 10c).

4.2. Transition Metal Element Doping at Bi Sites

Zheng et al. chose Bi6Cu2Se3.6Cl0.4O6 as the matrix and doped transition metal el- ements (Zr, Ti and Ce) at Bi sites to enhance its thermoelectric performance [30]. The introduction of transition metal elements can effectively increase the carrier concentration (nH) and maintain the carrier mobility (µH; Figure 11a), thereby optimizing the electric conductivity (σ) of the matrix. The S of all doped samples remains negative through- out the entire temperature range, indicating the stable n-type semiconductor properties (Figure 11b). Thanks to the optimized σ and maintained S, the power factor (PF) is effec- tively enhanced (Figure 11c). Finally, due to the enhanced electrical transport properties and reduced thermal conductivity [30], the peak ZT value reached ~0.16 at 873 K in Bi5.9Zr0.1Cu2Se3.6Cl0.4O6 (Figure 11d), which is 60% higher than that in Bi6Cu2Se3.6Cl0.4O6 (~0.10 at 873 K). As a new type of layered oxyselenide thermoelectric material, Bi6Cu2Se4O6 maintains the advantages of BiCuSeO and Bi2O2Se, such as lower-cost and nontoxic elements, better thermal and chemical stability. Moreover, Bi6Cu2Se4O6 can exhibit stable n-type semicon- ductor behavior by simple halogen doping and has intrinsic low thermal conductivity Materials 2021, 14, 3905 14 of 18

due to complex crystal structure. Thereby, Bi Cu Se O is a new kind of n-type layered Materials 2021, 14, x FOR PEER REVIEW 6 2 4 6 15 of 19 oxyselenide thermoelectric material with broad development prospects.

Figure 10. Figure(a) The crystal 10. (structurea) The crystalof Bi6Cu2Se structure4O6. The temperature of Bi Cu dependenceSe O . Theof ther temperaturemoelectric transport dependence properties for of thermoelectric MaterialsBi6Cu 20212Se, 144−,x Mx FORxO6 PEER(M = REVIEWCl/Br, x = 0.2/0.8): (b) electrical conductivity,6 2(c) Seebeck4 6 coefficient, (d) ZT. Data shown16 in of ( b19– d)

were adoptedtransport from Reference properties [29]. Copyright for Bi 2019,6Cu WILEY-VCH2Se4−xMx VerlO6 ag(M GmbH = Cl/Br, & Co. KGaA,x = 0.2/0.8):Weinheim. (b) electrical conductivity, (c) Seebeck coefficient, (d) ZT. Data shown in (b–d) were adopted from Reference [29]. Copyright (Figure4.1. Halogen 11c). Finally, Element due Doping to the atenhanced Se Sites electrical transport properties and reduced ther- 2019, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. mal conductivityThe σ of Cl-doped [30], the peak Bi6Cu ZT2Se value4O6 is reached higher ~0.16 than at Br-doped 873 K in Bione5.9Zr at0. 1highCu2Se temperature3.6Cl0.4O6 for (Figurethe doping 11d), whichcontent is x60% = 0.2. higher When than the that doping in Bi6 contentCu2Se3.6Cl x 0is.4 Oincreased6 (~0.10 at to 873 0.8, K). the σ was signif- icantly improved to ~70 S cm−1, and the Br-doped sample was better σ than the Cl-doped one at high temperature (Figure 10b). The Bi6Cu2Se3.8Br0.2O6 exhibits p-type semiconductor characteristics below 673 K and transfer to n-type with the temperature increasing; while Bi6Cu2Se3.8Cl0.2O6 has a negative S value within the entire temperature boundaries indi- cating that a small amount of Cl doping (x = 0.2) can achieve stable n-type semiconductor behavior (Figure 10c). When the doing content raises up to 0.8, the S of Cl/Br-doped sam- ples has little difference, varying from ~−60 to −160 μV K−1. The maximum ZT value ~0.15 at 823 K is achieved in Bi6Cu2Se3.2Br0.8O6 (Figure 10c).

4.2. Transition Metal Element Doping at Bi Sites

Zheng et al. chose Bi6Cu2Se3.6Cl0.4O6 as the matrix and doped transition metal ele- ments (Zr, Ti and Ce) at Bi sites to enhance its thermoelectric performance [30]. The intro- duction of transition metal elements can effectively increase the carrier concentration (nH) and maintain the carrier mobility (μH; Figure 11a), thereby optimizing the electric conduc- tivity (σ) of the matrix. The S of all doped samples remains negative throughout the entire temperature range, indicating the stable n-type semiconductor properties (Figure 11b). Thanks to the optimized σ and maintained S, the power factor (PF) is effectively enhanced

Figure 11. Thermoelectric transport properties of Bi6Cu2Se3.6Cl0.4O6 and Bi5.9M0.1Cu2Se3.6Cl0.4O6 (M = Ti, Zr, Ce): (a) carrier concentrationFigure (nH) and 11. mobilityThermoelectric (μH) at room temperature, transport (b) Seebeck properties coefficient, of ( Bic) power6Cu2 factorSe3.6 andCl 0.4(d) OZT6. Dataand shown Bi5.9 M0.1Cu2Se3.6Cl0.4O6 in (b–d) were adopted from Reference [30]. Copyright 2021, Acta Materialia Inc. (M = Ti, Zr, Ce): (a) carrier concentration (nH) and mobility (µH) at room temperature, (b) Seebeck

coefficient, (c) powerAs a newfactor type of andlayered (d oxyselenide) ZT. Data thermoelectric shown in material, (b–d) wereBi6Cu2Se adopted4O6 maintains from Reference [30]. Copyright 2021,the advantages Acta Materialia of BiCuSeO Inc. and Bi2O2Se, such as lower-cost and nontoxic elements, better thermal and chemical stability. Moreover, Bi6Cu2Se4O6 can exhibit stable n-type semicon- ductor behavior by simple halogen doping and has intrinsic low thermal conductivity due to complex crystal structure. Thereby, Bi6Cu2Se4O6 is a new kind of n-type layered oxysele- nide thermoelectric material with broad development prospects.

5. Summary and Perspective In this short review, we introduced the latest accomplishments in n-type layered ox- yselenide thermoelectric materials, including BiCuSeO, Bi2O2Se and Bi6Cu2Se4O6. For Bi- CuSeO, many strategies have been used to enhance the thermoelectric performance of p- type systems, but there are few studies on n-type BiCuSeO, and it is difficult to obtain stable n-type semiconductor behavior. For Bi2O2Se, carrier engineering, band engineering, microstructure design, etc., achieved performance enhancements of Bi2O2Se, but the ZT value is still limited to 0.4 [68]. Moreover, a new kind of promising n-type transport prop- erties can be obtained in layered oxyselenide Bi6Cu2Se4O6 through halogen element dop- ing. Apart from the advancements mentioned above, there is still room left for further research, such as make full utilization of the anisotropic transport properties of those com- pounds through texturing microstructure and crystals growth.

Materials 2021, 14, 3905 15 of 18

5. Summary and Perspective In this short review, we introduced the latest accomplishments in n-type layered oxyselenide thermoelectric materials, including BiCuSeO, Bi2O2Se and Bi6Cu2Se4O6. For BiCuSeO, many strategies have been used to enhance the thermoelectric performance of p-type systems, but there are few studies on n-type BiCuSeO, and it is difficult to obtain stable n-type semiconductor behavior. For Bi2O2Se, carrier engineering, band engineering, microstructure design, etc., achieved performance enhancements of Bi2O2Se, but the ZT value is still limited to 0.4 [68]. Moreover, a new kind of promising n-type transport properties can be obtained in layered oxyselenide Bi6Cu2Se4O6 through halogen element doping. Apart from the advancements mentioned above, there is still room left for further research, such as make full utilization of the anisotropic transport properties of those compounds through texturing microstructure and crystals growth.

Author Contributions: All the authors co-edited and wrote the paper. All authors have read and agreed to the published version of the manuscript. Funding: This work was financially supported by the National Natural Science Foundation of China (51772012), the National Key Research and Development Program of China (2018YFA0702100 and 2018YFB0703600), Beijing Natural Science Foundation (JQ18004), National Postdoctoral Program for Innovative Talents (BX20200028), the National Science Fund for Distinguished Young Scholars (51925101), and 111 Project (B17002). This work was also supported by the high-performance computing (HPC) resources at Beihang University. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data of this study are available from the corresponding author upon reasonable request. Conflicts of Interest: The authors declare no conflict of interest.

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