materials Review An Update Review on N-Type Layered Oxyselenide Thermoelectric Materials 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/ s S k 10.3390/ma14143905 tities: electrical conductivity ( ), Seebeck coefficient ( ) and thermal conductivity ( ), ZT = sS2T/k, 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 semiconductor 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. Materials 2021, 14, x FOR PEER REVIEW 3 of 19 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 (s) and Seebeck with temperature, coefficient ( Sshowing) for Fe- semiconductordoped samples. conduction The s of all behavior. samples Mean increaseswhile, monotonically at the same temperature, with temperature, the σ increases showing withsemiconductor the increasing conduction Fe content,