BiCuOSe and SnS are layered, moderate band gap (ǫ G ≈ 1 eV) semiconductors that exhibit intrinsic... more BiCuOSe and SnS are layered, moderate band gap (ǫ G ≈ 1 eV) semiconductors that exhibit intrinsic p type conductivity. Doping of BiCuOSe with Ca results in a slight expansion of the lattice and an increase of the hole concentration from 10 18 cm −3 to greater than 10 20 cm −3. The large carrier density in undoped films is the result of copper vacancies. Mobility is unaffected by doping, remaining constant at 1.5 cm 2 V −1 s −1 in both undoped and doped films, because the Bi-O layers serve as the source of carriers, while transport occurs within the Cu-Se layers. Bi possesses a 6s 2 lone pair that was expected to hybridize with the oxygen p states at the top of the valence band, resulting in high hole mobility as compared to similar materials such as LaCuOSe, which lack this lone pair. However, both LaCuOSe and BiCuOSe have similar hole mobility. X-ray absorption and emission spectroscopy, combined with density functional theory calculations, reveal that the Bi 6s states contribute deep within the valence band, forming bonding and anti-bonding states with O 2p at 11 eV and 3 eV below the valence band maximum, respectively. Hence, the Bi lone pair
The electronic structure of the oxychalcogenides LaCuOSe and BiCuOSe has been studied using O K-e... more The electronic structure of the oxychalcogenides LaCuOSe and BiCuOSe has been studied using O K-edge x-ray emission spectroscopy, x-ray absorption spectroscopy, and density functional theory, in order to examine the effects of the M 3+ ion configurations. The known distortion of the BiO layers in BiCuOSe compared to the LaO layers in LaCuOCh; the significantly smaller band gap of BiCuOSe (0.9 eV) compared to LaCuOSe (2.8 eV); and similar hole transport properties of the two compounds are explained in terms of the electron lone pairs associated with the Bi d 10 s 2 p 0 electronic configuration. The Bi 6s orbitals are chemically active and form bonding and antibonding states with the oxygen 2p orbital. The structural distortion facilitates the interaction between the 6p orbital with 6s via the antibonding state. For BiCuOSe, the majority of the Bi 6s orbital character (i.e., the bonding state) lies below the valence band, with the antibonding state lying below the valence band maximum (VBM). The similar hole transport properties between the two compounds is a consequence of the Bi 6s contributing little to the Cu 3d-Se 4p derived VBM. Finally, the band gap narrowing of BiCuOSe compared to LaCuOSe is mostly due to the low energy of the unoccupied Bi 6p orbitals along with the upshift of the VBM due to the presence of the O 2p-Bi 6s antibonding states.
Polycrystalline thin films of tin sulfide were grown on fused quartz substrates from an Sn2S3 tar... more Polycrystalline thin films of tin sulfide were grown on fused quartz substrates from an Sn2S3 target by pulsed laser deposition at temperatures ranging from 200C to 500C and pulse rates between 3Hz and 10Hz. 100nm thick films absorb roughly 50% of incident light in the 400 to 700nm range, and have an optical band gap of approximately 1.5eV. Hall measurements
Emergence of a terawatt scalable photovoltaic (PV) thin film technology is currently impeded by t... more Emergence of a terawatt scalable photovoltaic (PV) thin film technology is currently impeded by the limited supply of relatively rare elements like In or Te, which has spurred active research in recent years on earth-abundant PV materials. Instead of searching for alternative PV materials, we approach the problem here by structural modification through alloying of a known PV material, namely, tin sulfide. Although SnS is a strong visible light absorber that is naturally p-doped, its indirect band gap reduces the open circuit voltage of SnS-based solar cells. The anisotropic crystal structure results in undesirable anisotropic transport properties. Based on the observation that the isoelectronic sulfides MgS, CaS, and SrS assume the rock-salt structure, we use ab initio calculations to explore the structure and electronic properties of metastable Sn 1Àx (II) x S (II ¼ Mg, Ca, Sr) alloys, finding that the isotropic rock-salt phase is stabilized above x ¼ 0.2-0.3, and predicting direct band gaps in the range of interest for PV applications, i.e., 0.6-1.5 eV for Ca and Sr alloying. We subsequently synthesized such Sn 1Àx (Ca) x S films by pulsed laser deposition, confirmed the cubic rock-salt structure, and observed optical band gaps between 1.1 and 1.3 eV. These results highlight the potential of structural modification by alloying as a route to widen the otherwise limited materials base for promising earth-abundant materials. V
ABSTRACT Thin films of Ca-doped BiCuOSe were grown on (001) MgO and SrTiO3 substrates via pulsed ... more ABSTRACT Thin films of Ca-doped BiCuOSe were grown on (001) MgO and SrTiO3 substrates via pulsed laser deposition. X-ray absorption and emission spectroscopy were used to analyze the O K-edge and Cu L3,2-edge in order to determine the bulk electronic structure of BiCuOSe. Analysis of the O K-edge XAS/XES spectra yield a band gap of ˜1 eV, consistent with optical measurements on thin films. Optical measurements on single crystals show a band gap of ˜0.83 eV. XAS/XES results show the presence of strongly hybridized Bi 6s - O 2p orbitals in undoped samples. This hybridization is diminished in doped samples, providing strong evidence of Ca^2+ on the Bi^3+ site. X-ray diffraction measurements show that the films are highly oriented, with rocking curves around the (003) peak having a FWHM of 1 . Expansion of the c-axis is observed as Ca concentration is increased. All films show p-type conductivity and develop more metallic character as calcium doping increased. Ca concentration was determined by EPMA, which shows non-stoichiometric transfer of Ca from the target into the films.
SnS is a potential earth-abundant photovoltaic (PV) material. Employing both theory and experimen... more SnS is a potential earth-abundant photovoltaic (PV) material. Employing both theory and experiment to assess the PV relevant properties of SnS, we clarify on whether SnS has an indirect or direct band gap and what is the minority carrier effective mass as a function of the film orientation. SnS has a 1.07 eV indirect band gap with an effective absorption onset located 0.4 eV higher. The effective mass of minority carrier ranges from 0.5 m 0 perpendicular to the van der Waals layers to 0.2 m 0 into the van der Waals layers. The positive characteristics of SnS feature a desirable p-type carrier concentration due to the easy formation of acceptor-like intrinsic Sn vacancy defects. Potentially detrimental deep levels due to Sn S antisite or S vacancy defects can be suppressed by suitable adjustment of the growth condition towards S-rich. V
BiCuOSe and SnS are layered, moderate band gap (ǫ G ≈ 1 eV) semiconductors that exhibit intrinsic... more BiCuOSe and SnS are layered, moderate band gap (ǫ G ≈ 1 eV) semiconductors that exhibit intrinsic p type conductivity. Doping of BiCuOSe with Ca results in a slight expansion of the lattice and an increase of the hole concentration from 10 18 cm −3 to greater than 10 20 cm −3. The large carrier density in undoped films is the result of copper vacancies. Mobility is unaffected by doping, remaining constant at 1.5 cm 2 V −1 s −1 in both undoped and doped films, because the Bi-O layers serve as the source of carriers, while transport occurs within the Cu-Se layers. Bi possesses a 6s 2 lone pair that was expected to hybridize with the oxygen p states at the top of the valence band, resulting in high hole mobility as compared to similar materials such as LaCuOSe, which lack this lone pair. However, both LaCuOSe and BiCuOSe have similar hole mobility. X-ray absorption and emission spectroscopy, combined with density functional theory calculations, reveal that the Bi 6s states contribute deep within the valence band, forming bonding and anti-bonding states with O 2p at 11 eV and 3 eV below the valence band maximum, respectively. Hence, the Bi lone pair
The electronic structure of the oxychalcogenides LaCuOSe and BiCuOSe has been studied using O K-e... more The electronic structure of the oxychalcogenides LaCuOSe and BiCuOSe has been studied using O K-edge x-ray emission spectroscopy, x-ray absorption spectroscopy, and density functional theory, in order to examine the effects of the M 3+ ion configurations. The known distortion of the BiO layers in BiCuOSe compared to the LaO layers in LaCuOCh; the significantly smaller band gap of BiCuOSe (0.9 eV) compared to LaCuOSe (2.8 eV); and similar hole transport properties of the two compounds are explained in terms of the electron lone pairs associated with the Bi d 10 s 2 p 0 electronic configuration. The Bi 6s orbitals are chemically active and form bonding and antibonding states with the oxygen 2p orbital. The structural distortion facilitates the interaction between the 6p orbital with 6s via the antibonding state. For BiCuOSe, the majority of the Bi 6s orbital character (i.e., the bonding state) lies below the valence band, with the antibonding state lying below the valence band maximum (VBM). The similar hole transport properties between the two compounds is a consequence of the Bi 6s contributing little to the Cu 3d-Se 4p derived VBM. Finally, the band gap narrowing of BiCuOSe compared to LaCuOSe is mostly due to the low energy of the unoccupied Bi 6p orbitals along with the upshift of the VBM due to the presence of the O 2p-Bi 6s antibonding states.
Polycrystalline thin films of tin sulfide were grown on fused quartz substrates from an Sn2S3 tar... more Polycrystalline thin films of tin sulfide were grown on fused quartz substrates from an Sn2S3 target by pulsed laser deposition at temperatures ranging from 200C to 500C and pulse rates between 3Hz and 10Hz. 100nm thick films absorb roughly 50% of incident light in the 400 to 700nm range, and have an optical band gap of approximately 1.5eV. Hall measurements
Emergence of a terawatt scalable photovoltaic (PV) thin film technology is currently impeded by t... more Emergence of a terawatt scalable photovoltaic (PV) thin film technology is currently impeded by the limited supply of relatively rare elements like In or Te, which has spurred active research in recent years on earth-abundant PV materials. Instead of searching for alternative PV materials, we approach the problem here by structural modification through alloying of a known PV material, namely, tin sulfide. Although SnS is a strong visible light absorber that is naturally p-doped, its indirect band gap reduces the open circuit voltage of SnS-based solar cells. The anisotropic crystal structure results in undesirable anisotropic transport properties. Based on the observation that the isoelectronic sulfides MgS, CaS, and SrS assume the rock-salt structure, we use ab initio calculations to explore the structure and electronic properties of metastable Sn 1Àx (II) x S (II ¼ Mg, Ca, Sr) alloys, finding that the isotropic rock-salt phase is stabilized above x ¼ 0.2-0.3, and predicting direct band gaps in the range of interest for PV applications, i.e., 0.6-1.5 eV for Ca and Sr alloying. We subsequently synthesized such Sn 1Àx (Ca) x S films by pulsed laser deposition, confirmed the cubic rock-salt structure, and observed optical band gaps between 1.1 and 1.3 eV. These results highlight the potential of structural modification by alloying as a route to widen the otherwise limited materials base for promising earth-abundant materials. V
ABSTRACT Thin films of Ca-doped BiCuOSe were grown on (001) MgO and SrTiO3 substrates via pulsed ... more ABSTRACT Thin films of Ca-doped BiCuOSe were grown on (001) MgO and SrTiO3 substrates via pulsed laser deposition. X-ray absorption and emission spectroscopy were used to analyze the O K-edge and Cu L3,2-edge in order to determine the bulk electronic structure of BiCuOSe. Analysis of the O K-edge XAS/XES spectra yield a band gap of ˜1 eV, consistent with optical measurements on thin films. Optical measurements on single crystals show a band gap of ˜0.83 eV. XAS/XES results show the presence of strongly hybridized Bi 6s - O 2p orbitals in undoped samples. This hybridization is diminished in doped samples, providing strong evidence of Ca^2+ on the Bi^3+ site. X-ray diffraction measurements show that the films are highly oriented, with rocking curves around the (003) peak having a FWHM of 1 . Expansion of the c-axis is observed as Ca concentration is increased. All films show p-type conductivity and develop more metallic character as calcium doping increased. Ca concentration was determined by EPMA, which shows non-stoichiometric transfer of Ca from the target into the films.
SnS is a potential earth-abundant photovoltaic (PV) material. Employing both theory and experimen... more SnS is a potential earth-abundant photovoltaic (PV) material. Employing both theory and experiment to assess the PV relevant properties of SnS, we clarify on whether SnS has an indirect or direct band gap and what is the minority carrier effective mass as a function of the film orientation. SnS has a 1.07 eV indirect band gap with an effective absorption onset located 0.4 eV higher. The effective mass of minority carrier ranges from 0.5 m 0 perpendicular to the van der Waals layers to 0.2 m 0 into the van der Waals layers. The positive characteristics of SnS feature a desirable p-type carrier concentration due to the easy formation of acceptor-like intrinsic Sn vacancy defects. Potentially detrimental deep levels due to Sn S antisite or S vacancy defects can be suppressed by suitable adjustment of the growth condition towards S-rich. V
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