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Engineering Schottky contacts in open-air fabricated heterojunction solar cells to enable high performance and ohmic charge transport.

Hoye RL, Heffernan S, Ievskaya Y, Sadhanala A, Flewitt A, Friend RH, MacManus-Driscoll JL, Musselman KP - ACS Appl Mater Interfaces (2014)

Bottom Line: The efficiencies of open-air processed Cu2O/Zn(1-x)Mg(x)O heterojunction solar cells are doubled by reducing the effect of the Schottky barrier between Zn(1-x)Mg(x)O and the indium tin oxide (ITO) top contact.This work therefore shows that the Zn(1-x)Mg(x)O window layer sub-bandgap state density and thickness are critical parameters that can be engineered to minimize the effect of Schottky barriers on device performance.More generally, these findings show how to improve the performance of other photovoltaic system reliant on transparent top contacts, e.g., CZTS and CIGS.

View Article: PubMed Central - PubMed

Affiliation: Department of Materials Science and Metallurgy, University of Cambridge , 27 Charles Babbage Road, Cambridge CB3 0FS, United Kingdom.

ABSTRACT
The efficiencies of open-air processed Cu2O/Zn(1-x)Mg(x)O heterojunction solar cells are doubled by reducing the effect of the Schottky barrier between Zn(1-x)Mg(x)O and the indium tin oxide (ITO) top contact. By depositing Zn(1-x)Mg(x)O with a long band-tail, charge flows through the Zn(1-x)Mg(x)O/ITO Schottky barrier without rectification by hopping between the sub-bandgap states. High current densities are obtained by controlling the Zn(1-x)Mg(x)O thickness to ensure that the Schottky barrier is spatially removed from the p-n junction, allowing the full built-in potential to form, in addition to taking advantage of the increased electrical conductivity of the Zn(1-x)Mg(x)O films with increasing thickness. This work therefore shows that the Zn(1-x)Mg(x)O window layer sub-bandgap state density and thickness are critical parameters that can be engineered to minimize the effect of Schottky barriers on device performance. More generally, these findings show how to improve the performance of other photovoltaic system reliant on transparent top contacts, e.g., CZTS and CIGS.

No MeSH data available.


Band-diagram of ITO/Cu2O/SAALDZn0.8Mg0.2O/ITO with thin (<30 nm) and thick(>30 nm) Zn0.8Mg0.2O. (a) The Zn0.8Mg0.2O is thinnerthan the full depletion width of the Zn0.8Mg0.2O/ITO Schottky barrier, and so the full built-in potential at the p–n junction cannot form. By (b)making the Zn0.8Mg0.2O at least as thick asthe Schottky barrier full depletion width (i.e., >30 nm), the fullbuilt-in potential of the p–n junction can form, leading to increased current densities. CB isthe conduction band, VB the valence band and FL the Fermi level.
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fig5: Band-diagram of ITO/Cu2O/SAALDZn0.8Mg0.2O/ITO with thin (<30 nm) and thick(>30 nm) Zn0.8Mg0.2O. (a) The Zn0.8Mg0.2O is thinnerthan the full depletion width of the Zn0.8Mg0.2O/ITO Schottky barrier, and so the full built-in potential at the p–n junction cannot form. By (b)making the Zn0.8Mg0.2O at least as thick asthe Schottky barrier full depletion width (i.e., >30 nm), the fullbuilt-in potential of the p–n junction can form, leading to increased current densities. CB isthe conduction band, VB the valence band and FL the Fermi level.

Mentions: For such a claim that the Schottky barrier is influencingthe currentdensities to hold true, the Schottky barrier width must be at or nearthe 30 nm cutoff point. The barrier width can be estimated from thePoisson equation, where full carrier depletion is assumed,33 and is estimated to be between 20 and 50 nm(see the Supporting Information, SectionS3). Thus, when the films are 30 nm or thinner, the band-bending dueto the Schottky barrier should overlap with the band-bending at the p–n junction and therefore reducethe built-in potential (Figure 5a). To verifythis concept, the built-in potentials (VBI) were estimated from the voltage at which the light and dark currentdensities are equal (Figure 4c). It can beseen that there was an increase in the VBI from the 10 to 30 nm thick films. The series resistance of the deviceswas 20–40 Ω·cm2, from which a 0.04 Vincrease in the VBI corresponds to a maximumincrease of 1.3 mA·cm–2 in the JSC (calculation given in Section S3 of the Supporting Information), which more than accountsfor the increase in JSC that occurred(Figure 3a). Thus, spacing the Schottky barrieraway from the p–n junctionallows a larger built-in potential to form at the heterojunction (Figure 5b), producing a larger driving force for electronsto overcome the series resistance of the device. The VBI change is not large enough compared to the factorsgoverning the VOC loss mechanisms to leadto a significant change in the VOC inFigure 3b.


Engineering Schottky contacts in open-air fabricated heterojunction solar cells to enable high performance and ohmic charge transport.

Hoye RL, Heffernan S, Ievskaya Y, Sadhanala A, Flewitt A, Friend RH, MacManus-Driscoll JL, Musselman KP - ACS Appl Mater Interfaces (2014)

Band-diagram of ITO/Cu2O/SAALDZn0.8Mg0.2O/ITO with thin (<30 nm) and thick(>30 nm) Zn0.8Mg0.2O. (a) The Zn0.8Mg0.2O is thinnerthan the full depletion width of the Zn0.8Mg0.2O/ITO Schottky barrier, and so the full built-in potential at the p–n junction cannot form. By (b)making the Zn0.8Mg0.2O at least as thick asthe Schottky barrier full depletion width (i.e., >30 nm), the fullbuilt-in potential of the p–n junction can form, leading to increased current densities. CB isthe conduction band, VB the valence band and FL the Fermi level.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4333600&req=5

fig5: Band-diagram of ITO/Cu2O/SAALDZn0.8Mg0.2O/ITO with thin (<30 nm) and thick(>30 nm) Zn0.8Mg0.2O. (a) The Zn0.8Mg0.2O is thinnerthan the full depletion width of the Zn0.8Mg0.2O/ITO Schottky barrier, and so the full built-in potential at the p–n junction cannot form. By (b)making the Zn0.8Mg0.2O at least as thick asthe Schottky barrier full depletion width (i.e., >30 nm), the fullbuilt-in potential of the p–n junction can form, leading to increased current densities. CB isthe conduction band, VB the valence band and FL the Fermi level.
Mentions: For such a claim that the Schottky barrier is influencingthe currentdensities to hold true, the Schottky barrier width must be at or nearthe 30 nm cutoff point. The barrier width can be estimated from thePoisson equation, where full carrier depletion is assumed,33 and is estimated to be between 20 and 50 nm(see the Supporting Information, SectionS3). Thus, when the films are 30 nm or thinner, the band-bending dueto the Schottky barrier should overlap with the band-bending at the p–n junction and therefore reducethe built-in potential (Figure 5a). To verifythis concept, the built-in potentials (VBI) were estimated from the voltage at which the light and dark currentdensities are equal (Figure 4c). It can beseen that there was an increase in the VBI from the 10 to 30 nm thick films. The series resistance of the deviceswas 20–40 Ω·cm2, from which a 0.04 Vincrease in the VBI corresponds to a maximumincrease of 1.3 mA·cm–2 in the JSC (calculation given in Section S3 of the Supporting Information), which more than accountsfor the increase in JSC that occurred(Figure 3a). Thus, spacing the Schottky barrieraway from the p–n junctionallows a larger built-in potential to form at the heterojunction (Figure 5b), producing a larger driving force for electronsto overcome the series resistance of the device. The VBI change is not large enough compared to the factorsgoverning the VOC loss mechanisms to leadto a significant change in the VOC inFigure 3b.

Bottom Line: The efficiencies of open-air processed Cu2O/Zn(1-x)Mg(x)O heterojunction solar cells are doubled by reducing the effect of the Schottky barrier between Zn(1-x)Mg(x)O and the indium tin oxide (ITO) top contact.This work therefore shows that the Zn(1-x)Mg(x)O window layer sub-bandgap state density and thickness are critical parameters that can be engineered to minimize the effect of Schottky barriers on device performance.More generally, these findings show how to improve the performance of other photovoltaic system reliant on transparent top contacts, e.g., CZTS and CIGS.

View Article: PubMed Central - PubMed

Affiliation: Department of Materials Science and Metallurgy, University of Cambridge , 27 Charles Babbage Road, Cambridge CB3 0FS, United Kingdom.

ABSTRACT
The efficiencies of open-air processed Cu2O/Zn(1-x)Mg(x)O heterojunction solar cells are doubled by reducing the effect of the Schottky barrier between Zn(1-x)Mg(x)O and the indium tin oxide (ITO) top contact. By depositing Zn(1-x)Mg(x)O with a long band-tail, charge flows through the Zn(1-x)Mg(x)O/ITO Schottky barrier without rectification by hopping between the sub-bandgap states. High current densities are obtained by controlling the Zn(1-x)Mg(x)O thickness to ensure that the Schottky barrier is spatially removed from the p-n junction, allowing the full built-in potential to form, in addition to taking advantage of the increased electrical conductivity of the Zn(1-x)Mg(x)O films with increasing thickness. This work therefore shows that the Zn(1-x)Mg(x)O window layer sub-bandgap state density and thickness are critical parameters that can be engineered to minimize the effect of Schottky barriers on device performance. More generally, these findings show how to improve the performance of other photovoltaic system reliant on transparent top contacts, e.g., CZTS and CIGS.

No MeSH data available.