Charge Transport and Defects in Sulfur-Deficient Chalcogenide Perovskite <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>Ba</mml:mi><mml:mi>Zr</mml:mi><mml:mi mathvariant="normal">S</mml:mi></mml:mrow></mml

Exploring the conduction mechanism in the chalcogenide perovskite <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mi>Ba</a:mi><a:mi>Zr</a:mi><a:mi mathvariant="normal">S</a:mi></a:mrow></a:math><d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:msub><d:mi/><d:mn>3</d:mn></d:msub></d:math> is of significant interest due to its potential suitability as a top absorber layer in silicon-based tandem solar cells and other optoelectronic applications. Theoretical and experimental studies anticipate native ambipolar doping in <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:mrow><f:mi>Ba</f:mi><f:mi>Zr</f:mi><f:mi mathvariant="normal">S</f:mi></f:mrow></f:math><i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:msub><i:mi/><i:mn>3</i:mn></i:msub></i:math>, although experimental validation remains limited. This study reveals a transition from highly insulating behavior to <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mi>n</k:mi></k:math>-type conductivity (approximately <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:mn>100</m:mn></m:math> S/cm) in <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:mrow><o:mi>Ba</o:mi><o:mi>Zr</o:mi><o:mi mathvariant="normal">S</o:mi></o:mrow></o:math><r:math xmlns:r="http://www.w3.org/1998/Math/MathML" display="inline"><r:msub><r:mi/><r:mn>3</r:mn></r:msub></r:math> thin films through annealing in an <t:math xmlns:t="http://www.w3.org/1998/Math/MathML" display="inline"><t:mrow><t:mrow><t:mi mathvariant="normal">S</t:mi></t:mrow></t:mrow></t:math>-poor environment. <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"><w:mrow><w:mi>Ba</w:mi><w:mi>Zr</w:mi><w:mi mathvariant="normal">S</w:mi></w:mrow></w:math><z:math xmlns:z="http://www.w3.org/1998/Math/MathML" display="inline"><z:msub><z:mi/><z:mn>3</z:mn></z:msub></z:math> thin films are synthesized via a two-step process: co-sputtering of <bb:math xmlns:bb="http://www.w3.org/1998/Math/MathML" display="inline"><bb:mi>Ba</bb:mi></bb:math>-<db:math xmlns:db="http://www.w3.org/1998/Math/MathML" display="inline"><db:mi>Zr</db:mi></db:math> followed by sulfurization at <fb:math xmlns:fb="http://www.w3.org/1998/Math/MathML" display="inline"><fb:mn>600</fb:mn><fb:msup><fb:mspace width="0.1em"/><fb:mo>∘</fb:mo></fb:msup><fb:mrow><fb:mi mathvariant="normal">C</fb:mi></fb:mrow></fb:math>, and subsequent annealing in high vacuum. Ultraviolet-visible spectroscopy measurements reveal a red shift of approximately <jb:math xmlns:jb="http://www.w3.org/1998/Math/MathML" display="inline"><jb:mn>100</jb:mn></jb:math> meV in the band gap concurrent with sample-color darkening after vacuum annealing. The increase in defect density from the order of <lb:math xmlns:lb="http://www.w3.org/1998/Math/MathML" display="inline"><lb:msup><lb:mn>10</lb:mn><lb:mn>17</lb:mn></lb:msup></lb:math> to <nb:math xmlns:nb="http://www.w3.org/1998/Math/MathML" display="inline"><nb:msup><nb:mn>10</nb:mn><nb:mn>21</nb:mn></nb:msup><nb:mspace width="0.2em"/><nb:msup><nb:mi>cm</nb:mi><nb:mrow><nb:mo>−</nb:mo><nb:mn>3</nb:mn></nb:mrow></nb:msup></nb:math> with vacuum annealing, coupled with the low activation energy (approximately <qb:math xmlns:qb="http://www.w3.org/1998/Math/MathML" display="inline"><qb:mn>8</qb:mn></qb:math> meV), and the <sb:math xmlns:sb="http://www.w3.org/1998/Math/MathML" display="inline"><sb:mi>n</sb:mi></sb:math>-type character of the defects, strongly suggests that sulfur vacancies (<ub:math xmlns:ub="http://www.w3.org/1998/Math/MathML" display="inline"><ub:msub><ub:mi>V</ub:mi><ub:mrow><ub:mi mathvariant="normal">S</ub:mi></ub:mrow></ub:msub></ub:math>) are responsible for the <xb:math xmlns:xb="http://www.w3.org/1998/Math/MathML" display="inline"><xb:mi>n</xb:mi></xb:math>-type doping, in agreement with theoretical predictions. Temperature-dependent Hall measurement shows that phonon scattering governs charge transport at room temperature in <zb:math xmlns:zb="http://www.w3.org/1998/Math/MathML" display="inline"><zb:mrow><zb:mi>Ba</zb:mi><zb:mi>Zr</zb:mi><zb:mi mathvariant="normal">S</zb:mi></zb:mrow></zb:math><cc:math xmlns:cc="http://www.w3.org/1998/Math/MathML" display="inline"><cc:msub><cc:mi/><cc:mn>3</cc:mn></cc:msub></cc:math> films and that <ec:math xmlns:ec="http://www.w3.org/1998/Math/MathML" display="inline"><ec:mrow><ec:mrow><ec:mi mathvariant="normal">S</ec:mi></ec:mrow></ec:mrow></ec:math> vacancies are shallow donor defects acting as a weak impurity metal. The shift of the valence-band maximum (VBM) with respect to the Fermi level, quantified by hard x-ray photoelectron spectroscopy (<hc:math xmlns:hc="http://www.w3.org/1998/Math/MathML" display="inline"><hc:mi>Ga</hc:mi></hc:math> <jc:math xmlns:jc="http://www.w3.org/1998/Math/MathML" display="inline"><jc:mrow><jc:mi mathvariant="normal">K</jc:mi></jc:mrow><jc:mi>α</jc:mi></jc:math>, 9.25 keV), further corroborates the induced <mc:math xmlns:mc="http://www.w3.org/1998/Math/MathML" display="inline"><mc:mi>n</mc:mi></mc:math> type of conductivity in annealed samples. Our findings indicate that vacuum annealing induces <oc:math xmlns:oc="http://www.w3.org/1998/Math/MathML" display="inline"><oc:msub><oc:mi>V</oc:mi><oc:mrow><oc:mi mathvariant="normal">S</oc:mi></oc:mrow></oc:msub></oc:math> defects that dominate charge transport, resulting in <rc:math xmlns:rc="http://www.w3.org/1998/Math/MathML" display="inline"><rc:mi>n</rc:mi></rc:math>-type conductivity in <tc:math xmlns:tc="http://www.w3.org/1998/Math/MathML" display="inline"><tc:mrow><tc:mi>Ba</tc:mi><tc:mi>Zr</tc:mi><tc:mi mathvariant="normal">S</tc:mi></tc:mrow></tc:math><wc:math xmlns:wc="http://www.w3.org/1998/Math/MathML" display="inline"><wc:msub><wc:mi/><wc:mn>3</wc:mn></wc:msub></wc:math> under <yc:math xmlns:yc="http://www.w3.org/1998/Math/MathML" display="inline"><yc:mrow><yc:mrow><yc:mi mathvariant="normal">S</yc:mi></yc:mrow></yc:mrow></yc:math>-poor conditions. These discoveries will inform important design choices, such as selecting appropriate contact materials, for integrating <bd:math xmlns:bd="http://www.w3.org/1998/Math/MathML" display="inline"><bd:mrow><bd:mi>Ba</bd:mi><bd:mi>Zr</bd:mi><bd:mi mathvariant="normal">S</bd:mi></bd:mrow></bd:math><ed:math xmlns:ed="http://www.w3.org/1998/Math/MathML" display="inline"><ed:msub><ed:mi/><ed:mn>3</ed:mn></ed:msub></ed:math> into solar cell structures or other devices.