Self-doping of cations on the tetrahedral and octahedral sites in spinel oxides creates "anti-sit... more Self-doping of cations on the tetrahedral and octahedral sites in spinel oxides creates "anti-site" defects, which results in functional optical, electronic, magnetic, and other materials properties. Previously, we divded the III−II spinel family into four doping types (DTs) based on firstprinciple calculations in order to understand their electrical behavior. Here, we present experimental evidence on two prototype spinels for each major doping type (DT1 and DT4) that test the first principles calculations. For the DT-1 Ga 2 ZnO 4 spinel, we show that the anti-site defects in a stoichiometric film are equal in concentration and compenstate each other, whereas, for nonstoichiometric Cr 2 MnO 4 , a representative DT-4 spinel, excess Mn on the tetrahedral sites becomes electrically inactive as the Mn species switch from (III) to (II). The agreement between experiment and theory validates the Doping Rules distilled from the theoretical framework and significantly enhances our understanding of the defect chemistry of spinel oxides.
Advanced materials with new functionalities often require a combination of several materials prop... more Advanced materials with new functionalities often require a combination of several materials properties that are counterindicated and diffi cult to achieve simultaneously. Transparent conducting oxides (TCOs), particularly the p-type variety, are an example of such materials, requiring simultaneously optical transparency and p-type (holecarrier) electrical conductivity. Although p-type transparent conducting oxides (TCOs) are desirable in many optoelectronic applications, [ 1 ] such materials are much more diffi cult to design and make compared to their n-type counterparts. The diffi culties arise from the generally heavier effective masses of holes compared to electrons, the tendency for self-compensation of p-type doping in oxides, and the frequent occurrence of small-polarons that impede effective hole conduction. Indeed, the basic principles controlling p-type conductivity in wide-gap materials have yet to be established beyond heuristics, so the rational design has thus far been diffi cult. This is refl ected, in part, by numerous attempts at p-type doping of ZnO, which has created intense interest and research activity, but still remains a matter of ongoing debate. [ 2 ] Here we apply the inverse design approach to searching for p-type TCOs, which was proposed to address the inverse problem in materials science that can be formulated as: given the target properties, fi nd the material (chemical space search) or confi guration (structure search) that has this property. Inverse design is an approach powered by theory guiding experiment, and three modalities have been developed for different types of search problems. Modality 1 of inverse design is the inverse band-structure (IBS) method, [ 3 ] which applies to cases [ 3 , 4 ] when one needs to fi gure out the specifi c atomic confi guration of a fi xed chemical system having the target properties. This can be done by combining a confi gurational search, such as genetic algorithms and simulated annealing, with quantum mechanical calculations of the target properties. In Modality 2 of inverse design, considered in this paper, the search is directed to the chemical space consisting of known compounds with currently unknown properties (such as the ≈ 10 5 compounds documented in the inorganic crystal structure database (ICSD)). [ 5 ] Often, the desired functionality requires a non-trivial combination of several target properties, for example, good optical transparency, high charge carrier mobility and concentration for a TCO, or high fi gure of merit
Cation off-stoichiometry leads to high p-type conductivity and enhanced transparency in Co 2 ZnO ... more Cation off-stoichiometry leads to high p-type conductivity and enhanced transparency in Co 2 ZnO 4 and Co 2 NiO 4 thin films
The microscopic cause of conductivity in transparent conducting oxides like ZnO, In 2 O 3 , and S... more The microscopic cause of conductivity in transparent conducting oxides like ZnO, In 2 O 3 , and SnO 2 is generally considered to be a point defect mechanism in the bulk, involving intrinsic lattice defects, extrinsic dopants, or unintentional impurities like hydrogen. We confirm here that the defect theory for Ovacancies can quantitatively account for the rather moderate conductivity and off-stoichiometry observed in bulk In 2 O 3 samples under high-temperature equilibrium conditions. However, nominally undoped thinfilms of In 2 O 3 can exhibit surprisingly high conductivities exceeding by 4-5 orders of magnitude that of bulk samples under identical conditions (temperature and O 2 partial pressure). Employing surface calculations and thickness-dependent Hall measurements, we demonstrate that surface donors rather than bulk defects dominate the conductivity of In 2 O 3 thin films.
Conference Record of the Twenty Sixth IEEE Photovoltaic Specialists Conference - 1997
This report summarizes our recent efforts to produce large-grained CIGS materials from porous nan... more This report summarizes our recent efforts to produce large-grained CIGS materials from porous nanoparticle precursor films. In our approach, a nanoparticle colloid is first prepared via low-temperature solution synthesis. Next, a thin precursor film is prepared by spray deposition of this colloid onto a heated substrate. Post-processing of these precursors is performed to induce phase formation and grain growth. Various colloids have been sprayed onto substrates such as molybdenum/glass and CdS/SnO 2 /glass. Post-processing treatments under controlled gas ambients have been performed according to both standard and rapid-thermal annealing (RTA) approaches. Morphology, phase formation, and chemical composition of these nanoparticle-derived films have been characterized using SEM, XRD, Raman, and AES. Although the majority of the approaches employed did not produce large-grained materials, CIGS colloid sprayed onto CdS/SnO 2 /glass substrates heated at 225°C produced films with a dense morphology as-deposited.
Transparent conducting oxide (TCO) coatings with decreased cost and greater process or performanc... more Transparent conducting oxide (TCO) coatings with decreased cost and greater process or performance versatility are needed for a variety of optoelectronic applications. Among potential new TCO candidates, doped titanium dioxide is receiving particular interest. In this study, niobium-doped titania bilayer structures consisting of a nanoscale seed layer (deposited by atomic layer deposition or RF magnetron sputtering) followed by a thick bulk-like layer were grown directly on glass in order to examine the effects of the seed layer processing on the subsequent crystallization and electrical properties of these heterostructures. Observations from Raman spectroscopy suggest that higher oxygen content in the seed layer suppresses the formation of detrimental titania polymorph phases, found in films produced by annealing directly after synthesis without any exposure to oxygen. Furthermore, our results indicate that the generation of excellent Nb:TiO2 conductors on glass (without breaking v...
Conference Record of the Thirty-first IEEE Photovoltaic Specialists Conference, 2005.
UL 18. NUMBER OF PAGES 19b. TELEPHONE NUMBER (Include area code) Standard Form 298 (Rev. 8/98) Pr... more UL 18. NUMBER OF PAGES 19b. TELEPHONE NUMBER (Include area code) Standard Form 298 (Rev. 8/98) Prescribed by ANSI Std. Z39.18
Conference Record of the Thirty-first IEEE Photovoltaic Specialists Conference, 2005.
Transparent conducting oxides (TCOs) can serve a variety of important functions in thin-film phot... more Transparent conducting oxides (TCOs) can serve a variety of important functions in thin-film photovoltaics such as transparent electrical contacts, antireflection coatings, and chemical barriers. Two areas of particular interest are TCOs that can be deposited at low temperatures and TCOs with high carrier mobilities. We have employed combinatorial highthroughput approaches to investigate both these areas. Conductivities of σ = 2500 Ω-1-cm-1 have been obtained for In-Zn-O (IZO) films deposited at 100°C and σ > 5000 Ω-1-cm-1 for In-Ti-O (ITiO) and In-MoO (IMO) films deposited at 550°C. The highest mobility obtained was 83 cm 2 /V-s for ITiO deposited at 550°C.
materials with ABX stoichiometry are an important class of compounds that is under research for n... more materials with ABX stoichiometry are an important class of compounds that is under research for numerous applications. Systematic search across databases and literature indicates that many ABX materials are unreported [1], for example 29 out of 45 materials in the V-IX-IV 18-electron family. Theoretical calculations reveal that 8 of these V-IX-IV materials are thermodynamically stable, including 4 new materials with half-Heusler structure. Thin film combinatorial synthesis experiments using sputtering, x-ray fluorescence and x-ray diffraction confirm that one of these materials TaCoSn is stable in the predicted half-Hesuler structure. [2] Despite being made of three metallic elements, TaCoSn is a semiconductor, but the band gap of this material is difficult to measure due to a high concentration of interstitial cobalt defects. [1] Adv.
Abstract The successful development of alternative deposition methods for thin-film transparent c... more Abstract The successful development of alternative deposition methods for thin-film transparent conducting oxides (TCOs) has important implications for photovoltaic technologies, such as CIGS. This work examines the use of atmospheric-pressure solution ...
Using direct-write approaches in photovoltaics for metallization and contact formation can signif... more Using direct-write approaches in photovoltaics for metallization and contact formation can significantly reduce the cost per watt of producing photovoltaic devices. Inks have been developed for various materials, such as Ag, Cu, Ni and Al, which can be used to inkjet print metallizations for various kinds of photovoltaic devices. Use of these inks results in metallization with resistivities close to those of bulk materials. By means of inkjet printing a metallization grid can be printed with better resolution, i.e. smaller lines, than screen-printing. Also inks have been developed to deposit transparent conductive oxide films by means of ultrasonic spraying.
We will discuss recent results for the NREL polymer photovoltaics group. The work reported here e... more We will discuss recent results for the NREL polymer photovoltaics group. The work reported here explores the impact of polymer morphology on the physics and performance of polymer-based photovoltaic devices. By varying both the annealing temperatures and the solvent used for polym er deposition, we can alter the polymer chain morphology, thus shifting the absorption onset and shape and improving the charge transport properties of the resulting devices. The higher degree of order in the films results in increased photovoltaic performance.
Bulk heterojunction photovoltaic devices based on blends of conjugated polymers and fullerenes ha... more Bulk heterojunction photovoltaic devices based on blends of conjugated polymers and fullerenes have achieved efficiencies of 3.5% under AM1.5 illumination. This is a result of efficient exciton dissociation at donoracceptor interfaces. However, the intimate blending of the electron and hole transporting species leads to detrimental charge carrier recombination in the bulk of these devices. We are investigating a solution to this problem by fabricating nanostructured oxideconjugated polymer composite structures. Porous SnO2 films with pore diameters of~100 nm have been fabricated. Intercalation of polymers into the pores by adsorption from solution yielded structures with approximately 75% of the free volume filled with polymer. The resulting composite structures are promising candidates for developing polymer-based solar cells with short carrier-to-electrode path lengths while retaining high optical absorption, thus leading to increased efficiencies. Additionally, a carboxylic acid ...
Two cations A and B in A 2 BO 4 spinels appear in precise 2:1 Daltonian ratio ("line compounds") ... more Two cations A and B in A 2 BO 4 spinels appear in precise 2:1 Daltonian ratio ("line compounds") only at very low temperature. More typically, at finite temperature, they tend to become either A rich or B rich. Here we survey the experimentally observed stoichiometry asymmetries and describe the first-principles framework for calculating these. Defect calculations based on first principles are used to calculate the enthalpies of substitution of A atom H (A T d) and B atom H (B O h) and determine their site occupancies leading to (non)-stoichiometry. In Co 2 ZnO 4 , the result of the calculation for site occupancy compares well with that measured via anomalous x-ray diffraction. Further, the calculated phase boundary also compares well with that measured via Rietveld refinement of x-ray diffraction data on bulk ceramic sintered samples of Co 2 ZnO 4 and Rh 2 ZnO 4. These results show that Co 2 ZnO 4 is heavily Co nonstoichiometric above 500 • C, whereas Rh 2 ZnO 4 is slightly Zn nonstoichiometric. We found that, in general, the calculated H (A T d) is smaller than H (B O h), if the A-rich competing phase is isostructural with the A 2 BO 4 host, for example, A 2 AO 4 , whereas B-rich competing phase is not, for example, BO. This observation is used to qualitatively explain nonstoichiometry and solid solutions observed in other spinels.
Cation off-stoichiometry leads to high p-type conductivity and enhanced transparency in Co 2 ZnO ... more Cation off-stoichiometry leads to high p-type conductivity and enhanced transparency in Co 2 ZnO 4 and Co 2 NiO 4 thin films
Self-doping of cations on the tetrahedral and octahedral sites in spinel oxides creates "anti-sit... more Self-doping of cations on the tetrahedral and octahedral sites in spinel oxides creates "anti-site" defects, which results in functional optical, electronic, magnetic, and other materials properties. Previously, we divded the III−II spinel family into four doping types (DTs) based on firstprinciple calculations in order to understand their electrical behavior. Here, we present experimental evidence on two prototype spinels for each major doping type (DT1 and DT4) that test the first principles calculations. For the DT-1 Ga 2 ZnO 4 spinel, we show that the anti-site defects in a stoichiometric film are equal in concentration and compenstate each other, whereas, for nonstoichiometric Cr 2 MnO 4 , a representative DT-4 spinel, excess Mn on the tetrahedral sites becomes electrically inactive as the Mn species switch from (III) to (II). The agreement between experiment and theory validates the Doping Rules distilled from the theoretical framework and significantly enhances our understanding of the defect chemistry of spinel oxides.
Advanced materials with new functionalities often require a combination of several materials prop... more Advanced materials with new functionalities often require a combination of several materials properties that are counterindicated and diffi cult to achieve simultaneously. Transparent conducting oxides (TCOs), particularly the p-type variety, are an example of such materials, requiring simultaneously optical transparency and p-type (holecarrier) electrical conductivity. Although p-type transparent conducting oxides (TCOs) are desirable in many optoelectronic applications, [ 1 ] such materials are much more diffi cult to design and make compared to their n-type counterparts. The diffi culties arise from the generally heavier effective masses of holes compared to electrons, the tendency for self-compensation of p-type doping in oxides, and the frequent occurrence of small-polarons that impede effective hole conduction. Indeed, the basic principles controlling p-type conductivity in wide-gap materials have yet to be established beyond heuristics, so the rational design has thus far been diffi cult. This is refl ected, in part, by numerous attempts at p-type doping of ZnO, which has created intense interest and research activity, but still remains a matter of ongoing debate. [ 2 ] Here we apply the inverse design approach to searching for p-type TCOs, which was proposed to address the inverse problem in materials science that can be formulated as: given the target properties, fi nd the material (chemical space search) or confi guration (structure search) that has this property. Inverse design is an approach powered by theory guiding experiment, and three modalities have been developed for different types of search problems. Modality 1 of inverse design is the inverse band-structure (IBS) method, [ 3 ] which applies to cases [ 3 , 4 ] when one needs to fi gure out the specifi c atomic confi guration of a fi xed chemical system having the target properties. This can be done by combining a confi gurational search, such as genetic algorithms and simulated annealing, with quantum mechanical calculations of the target properties. In Modality 2 of inverse design, considered in this paper, the search is directed to the chemical space consisting of known compounds with currently unknown properties (such as the ≈ 10 5 compounds documented in the inorganic crystal structure database (ICSD)). [ 5 ] Often, the desired functionality requires a non-trivial combination of several target properties, for example, good optical transparency, high charge carrier mobility and concentration for a TCO, or high fi gure of merit
Cation off-stoichiometry leads to high p-type conductivity and enhanced transparency in Co 2 ZnO ... more Cation off-stoichiometry leads to high p-type conductivity and enhanced transparency in Co 2 ZnO 4 and Co 2 NiO 4 thin films
The microscopic cause of conductivity in transparent conducting oxides like ZnO, In 2 O 3 , and S... more The microscopic cause of conductivity in transparent conducting oxides like ZnO, In 2 O 3 , and SnO 2 is generally considered to be a point defect mechanism in the bulk, involving intrinsic lattice defects, extrinsic dopants, or unintentional impurities like hydrogen. We confirm here that the defect theory for Ovacancies can quantitatively account for the rather moderate conductivity and off-stoichiometry observed in bulk In 2 O 3 samples under high-temperature equilibrium conditions. However, nominally undoped thinfilms of In 2 O 3 can exhibit surprisingly high conductivities exceeding by 4-5 orders of magnitude that of bulk samples under identical conditions (temperature and O 2 partial pressure). Employing surface calculations and thickness-dependent Hall measurements, we demonstrate that surface donors rather than bulk defects dominate the conductivity of In 2 O 3 thin films.
Conference Record of the Twenty Sixth IEEE Photovoltaic Specialists Conference - 1997
This report summarizes our recent efforts to produce large-grained CIGS materials from porous nan... more This report summarizes our recent efforts to produce large-grained CIGS materials from porous nanoparticle precursor films. In our approach, a nanoparticle colloid is first prepared via low-temperature solution synthesis. Next, a thin precursor film is prepared by spray deposition of this colloid onto a heated substrate. Post-processing of these precursors is performed to induce phase formation and grain growth. Various colloids have been sprayed onto substrates such as molybdenum/glass and CdS/SnO 2 /glass. Post-processing treatments under controlled gas ambients have been performed according to both standard and rapid-thermal annealing (RTA) approaches. Morphology, phase formation, and chemical composition of these nanoparticle-derived films have been characterized using SEM, XRD, Raman, and AES. Although the majority of the approaches employed did not produce large-grained materials, CIGS colloid sprayed onto CdS/SnO 2 /glass substrates heated at 225°C produced films with a dense morphology as-deposited.
Transparent conducting oxide (TCO) coatings with decreased cost and greater process or performanc... more Transparent conducting oxide (TCO) coatings with decreased cost and greater process or performance versatility are needed for a variety of optoelectronic applications. Among potential new TCO candidates, doped titanium dioxide is receiving particular interest. In this study, niobium-doped titania bilayer structures consisting of a nanoscale seed layer (deposited by atomic layer deposition or RF magnetron sputtering) followed by a thick bulk-like layer were grown directly on glass in order to examine the effects of the seed layer processing on the subsequent crystallization and electrical properties of these heterostructures. Observations from Raman spectroscopy suggest that higher oxygen content in the seed layer suppresses the formation of detrimental titania polymorph phases, found in films produced by annealing directly after synthesis without any exposure to oxygen. Furthermore, our results indicate that the generation of excellent Nb:TiO2 conductors on glass (without breaking v...
Conference Record of the Thirty-first IEEE Photovoltaic Specialists Conference, 2005.
UL 18. NUMBER OF PAGES 19b. TELEPHONE NUMBER (Include area code) Standard Form 298 (Rev. 8/98) Pr... more UL 18. NUMBER OF PAGES 19b. TELEPHONE NUMBER (Include area code) Standard Form 298 (Rev. 8/98) Prescribed by ANSI Std. Z39.18
Conference Record of the Thirty-first IEEE Photovoltaic Specialists Conference, 2005.
Transparent conducting oxides (TCOs) can serve a variety of important functions in thin-film phot... more Transparent conducting oxides (TCOs) can serve a variety of important functions in thin-film photovoltaics such as transparent electrical contacts, antireflection coatings, and chemical barriers. Two areas of particular interest are TCOs that can be deposited at low temperatures and TCOs with high carrier mobilities. We have employed combinatorial highthroughput approaches to investigate both these areas. Conductivities of σ = 2500 Ω-1-cm-1 have been obtained for In-Zn-O (IZO) films deposited at 100°C and σ > 5000 Ω-1-cm-1 for In-Ti-O (ITiO) and In-MoO (IMO) films deposited at 550°C. The highest mobility obtained was 83 cm 2 /V-s for ITiO deposited at 550°C.
materials with ABX stoichiometry are an important class of compounds that is under research for n... more materials with ABX stoichiometry are an important class of compounds that is under research for numerous applications. Systematic search across databases and literature indicates that many ABX materials are unreported [1], for example 29 out of 45 materials in the V-IX-IV 18-electron family. Theoretical calculations reveal that 8 of these V-IX-IV materials are thermodynamically stable, including 4 new materials with half-Heusler structure. Thin film combinatorial synthesis experiments using sputtering, x-ray fluorescence and x-ray diffraction confirm that one of these materials TaCoSn is stable in the predicted half-Hesuler structure. [2] Despite being made of three metallic elements, TaCoSn is a semiconductor, but the band gap of this material is difficult to measure due to a high concentration of interstitial cobalt defects. [1] Adv.
Abstract The successful development of alternative deposition methods for thin-film transparent c... more Abstract The successful development of alternative deposition methods for thin-film transparent conducting oxides (TCOs) has important implications for photovoltaic technologies, such as CIGS. This work examines the use of atmospheric-pressure solution ...
Using direct-write approaches in photovoltaics for metallization and contact formation can signif... more Using direct-write approaches in photovoltaics for metallization and contact formation can significantly reduce the cost per watt of producing photovoltaic devices. Inks have been developed for various materials, such as Ag, Cu, Ni and Al, which can be used to inkjet print metallizations for various kinds of photovoltaic devices. Use of these inks results in metallization with resistivities close to those of bulk materials. By means of inkjet printing a metallization grid can be printed with better resolution, i.e. smaller lines, than screen-printing. Also inks have been developed to deposit transparent conductive oxide films by means of ultrasonic spraying.
We will discuss recent results for the NREL polymer photovoltaics group. The work reported here e... more We will discuss recent results for the NREL polymer photovoltaics group. The work reported here explores the impact of polymer morphology on the physics and performance of polymer-based photovoltaic devices. By varying both the annealing temperatures and the solvent used for polym er deposition, we can alter the polymer chain morphology, thus shifting the absorption onset and shape and improving the charge transport properties of the resulting devices. The higher degree of order in the films results in increased photovoltaic performance.
Bulk heterojunction photovoltaic devices based on blends of conjugated polymers and fullerenes ha... more Bulk heterojunction photovoltaic devices based on blends of conjugated polymers and fullerenes have achieved efficiencies of 3.5% under AM1.5 illumination. This is a result of efficient exciton dissociation at donoracceptor interfaces. However, the intimate blending of the electron and hole transporting species leads to detrimental charge carrier recombination in the bulk of these devices. We are investigating a solution to this problem by fabricating nanostructured oxideconjugated polymer composite structures. Porous SnO2 films with pore diameters of~100 nm have been fabricated. Intercalation of polymers into the pores by adsorption from solution yielded structures with approximately 75% of the free volume filled with polymer. The resulting composite structures are promising candidates for developing polymer-based solar cells with short carrier-to-electrode path lengths while retaining high optical absorption, thus leading to increased efficiencies. Additionally, a carboxylic acid ...
Two cations A and B in A 2 BO 4 spinels appear in precise 2:1 Daltonian ratio ("line compounds") ... more Two cations A and B in A 2 BO 4 spinels appear in precise 2:1 Daltonian ratio ("line compounds") only at very low temperature. More typically, at finite temperature, they tend to become either A rich or B rich. Here we survey the experimentally observed stoichiometry asymmetries and describe the first-principles framework for calculating these. Defect calculations based on first principles are used to calculate the enthalpies of substitution of A atom H (A T d) and B atom H (B O h) and determine their site occupancies leading to (non)-stoichiometry. In Co 2 ZnO 4 , the result of the calculation for site occupancy compares well with that measured via anomalous x-ray diffraction. Further, the calculated phase boundary also compares well with that measured via Rietveld refinement of x-ray diffraction data on bulk ceramic sintered samples of Co 2 ZnO 4 and Rh 2 ZnO 4. These results show that Co 2 ZnO 4 is heavily Co nonstoichiometric above 500 • C, whereas Rh 2 ZnO 4 is slightly Zn nonstoichiometric. We found that, in general, the calculated H (A T d) is smaller than H (B O h), if the A-rich competing phase is isostructural with the A 2 BO 4 host, for example, A 2 AO 4 , whereas B-rich competing phase is not, for example, BO. This observation is used to qualitatively explain nonstoichiometry and solid solutions observed in other spinels.
Cation off-stoichiometry leads to high p-type conductivity and enhanced transparency in Co 2 ZnO ... more Cation off-stoichiometry leads to high p-type conductivity and enhanced transparency in Co 2 ZnO 4 and Co 2 NiO 4 thin films
Uploads
Papers by David Ginley