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Ionic transport in hybrid lead iodide perovskite solar cells.

Eames C, Frost JM, Barnes PR, O'Regan BC, Walsh A, Islam MS - Nat Commun (2015)

Bottom Line: Ionic transport has been suggested to be an important factor contributing to these effects; however, the chemical origin of this transport and the mobile species are unclear.Here, the activation energies for ionic migration in methylammonium lead iodide (CH3NH3PbI3) are derived from first principles, and are compared with kinetic data extracted from the current-voltage response of a perovskite-based solar cell.We identify the microscopic transport mechanisms, and find facile vacancy-assisted migration of iodide ions with an activation energy of 0.6 eV, in good agreement with the kinetic measurements.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, University of Bath, Bath BA2 7AY, UK.

ABSTRACT
Solar cells based on organic-inorganic halide perovskites have recently shown rapidly rising power conversion efficiencies, but exhibit unusual behaviour such as current-voltage hysteresis and a low-frequency giant dielectric response. Ionic transport has been suggested to be an important factor contributing to these effects; however, the chemical origin of this transport and the mobile species are unclear. Here, the activation energies for ionic migration in methylammonium lead iodide (CH3NH3PbI3) are derived from first principles, and are compared with kinetic data extracted from the current-voltage response of a perovskite-based solar cell. We identify the microscopic transport mechanisms, and find facile vacancy-assisted migration of iodide ions with an activation energy of 0.6 eV, in good agreement with the kinetic measurements. The results of this combined computational and experimental study suggest that hybrid halide perovskites are mixed ionic-electronic conductors, a finding that has major implications for solar cell device architectures.

No MeSH data available.


Related in: MedlinePlus

Influence of iodide ion vacancies on band energies of a perovskite thin film.(a) Schematic diagrams indicating the influence of vacancy drift on the band energies of a p-i-n device at short circuit. EC is the conduction band energy, EV is the valence band energy and Vbi is the built-in potential. Iodide ion vacancies are represented by the squares with ‘plus' signs. Implicit in the diagram is that the vacancies with effective positive charges are balanced by immobile cation vacancies (not shown) with effective negative charges. (b) Hypothesized energy level configurations corresponding to different bias conditions and times during the chronophotoamperometry measurements. The variation in the conduction and valence bands corresponds to the redistribution of iodide ion vacancies to and from interfaces with different applied potentials and times.
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f5: Influence of iodide ion vacancies on band energies of a perovskite thin film.(a) Schematic diagrams indicating the influence of vacancy drift on the band energies of a p-i-n device at short circuit. EC is the conduction band energy, EV is the valence band energy and Vbi is the built-in potential. Iodide ion vacancies are represented by the squares with ‘plus' signs. Implicit in the diagram is that the vacancies with effective positive charges are balanced by immobile cation vacancies (not shown) with effective negative charges. (b) Hypothesized energy level configurations corresponding to different bias conditions and times during the chronophotoamperometry measurements. The variation in the conduction and valence bands corresponds to the redistribution of iodide ion vacancies to and from interfaces with different applied potentials and times.

Mentions: Migration of iodide ion vacancies under the influence of an electric field could change the photogenerated charge collection efficiency of devices with time and so helps to explain hysteresis. The possible influence of iodide ion vacancies on band energies of a perovskite thin film device and its interfaces is described in Figure 5. We suggest the following model to explain the chronophotoamperometry measurements (see Fig. 5b). When the cell is short-circuited, a built-in electric field is present in the perovskite layer due to the difference in the work functions of the contacts. This could result in the migration of iodide ion vacancies towards the contacts (while immobile vacancies remain fixed), partially screening the field. As discussed above, we expect the magnitude of the short-circuit photocurrent to be controlled by the extent to which the electric field is screened (as also recently suggested by Tress et al.17). A reduced internal field within the device (due to ionic screening) would lead to less efficient collection of photogenerated charge carriers. Prolonged poling of the device under forward bias reduces (or reverses) the built-in field, which could allow a dissipation of ionic charge from the contacts by diffusion. This tempering would result in more efficient collection of photogenerated charges when the device is returned to short circuit, since the built-in field is no longer screened by accumulated ionic or vacancy charge. Conversely, holding the device under reverse bias would accentuate the migration of ions and vacancies to the interfaces, resulting in less efficient collection of photogenerated charges when the device is returned to short circuit.


Ionic transport in hybrid lead iodide perovskite solar cells.

Eames C, Frost JM, Barnes PR, O'Regan BC, Walsh A, Islam MS - Nat Commun (2015)

Influence of iodide ion vacancies on band energies of a perovskite thin film.(a) Schematic diagrams indicating the influence of vacancy drift on the band energies of a p-i-n device at short circuit. EC is the conduction band energy, EV is the valence band energy and Vbi is the built-in potential. Iodide ion vacancies are represented by the squares with ‘plus' signs. Implicit in the diagram is that the vacancies with effective positive charges are balanced by immobile cation vacancies (not shown) with effective negative charges. (b) Hypothesized energy level configurations corresponding to different bias conditions and times during the chronophotoamperometry measurements. The variation in the conduction and valence bands corresponds to the redistribution of iodide ion vacancies to and from interfaces with different applied potentials and times.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4491179&req=5

f5: Influence of iodide ion vacancies on band energies of a perovskite thin film.(a) Schematic diagrams indicating the influence of vacancy drift on the band energies of a p-i-n device at short circuit. EC is the conduction band energy, EV is the valence band energy and Vbi is the built-in potential. Iodide ion vacancies are represented by the squares with ‘plus' signs. Implicit in the diagram is that the vacancies with effective positive charges are balanced by immobile cation vacancies (not shown) with effective negative charges. (b) Hypothesized energy level configurations corresponding to different bias conditions and times during the chronophotoamperometry measurements. The variation in the conduction and valence bands corresponds to the redistribution of iodide ion vacancies to and from interfaces with different applied potentials and times.
Mentions: Migration of iodide ion vacancies under the influence of an electric field could change the photogenerated charge collection efficiency of devices with time and so helps to explain hysteresis. The possible influence of iodide ion vacancies on band energies of a perovskite thin film device and its interfaces is described in Figure 5. We suggest the following model to explain the chronophotoamperometry measurements (see Fig. 5b). When the cell is short-circuited, a built-in electric field is present in the perovskite layer due to the difference in the work functions of the contacts. This could result in the migration of iodide ion vacancies towards the contacts (while immobile vacancies remain fixed), partially screening the field. As discussed above, we expect the magnitude of the short-circuit photocurrent to be controlled by the extent to which the electric field is screened (as also recently suggested by Tress et al.17). A reduced internal field within the device (due to ionic screening) would lead to less efficient collection of photogenerated charge carriers. Prolonged poling of the device under forward bias reduces (or reverses) the built-in field, which could allow a dissipation of ionic charge from the contacts by diffusion. This tempering would result in more efficient collection of photogenerated charges when the device is returned to short circuit, since the built-in field is no longer screened by accumulated ionic or vacancy charge. Conversely, holding the device under reverse bias would accentuate the migration of ions and vacancies to the interfaces, resulting in less efficient collection of photogenerated charges when the device is returned to short circuit.

Bottom Line: Ionic transport has been suggested to be an important factor contributing to these effects; however, the chemical origin of this transport and the mobile species are unclear.Here, the activation energies for ionic migration in methylammonium lead iodide (CH3NH3PbI3) are derived from first principles, and are compared with kinetic data extracted from the current-voltage response of a perovskite-based solar cell.We identify the microscopic transport mechanisms, and find facile vacancy-assisted migration of iodide ions with an activation energy of 0.6 eV, in good agreement with the kinetic measurements.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, University of Bath, Bath BA2 7AY, UK.

ABSTRACT
Solar cells based on organic-inorganic halide perovskites have recently shown rapidly rising power conversion efficiencies, but exhibit unusual behaviour such as current-voltage hysteresis and a low-frequency giant dielectric response. Ionic transport has been suggested to be an important factor contributing to these effects; however, the chemical origin of this transport and the mobile species are unclear. Here, the activation energies for ionic migration in methylammonium lead iodide (CH3NH3PbI3) are derived from first principles, and are compared with kinetic data extracted from the current-voltage response of a perovskite-based solar cell. We identify the microscopic transport mechanisms, and find facile vacancy-assisted migration of iodide ions with an activation energy of 0.6 eV, in good agreement with the kinetic measurements. The results of this combined computational and experimental study suggest that hybrid halide perovskites are mixed ionic-electronic conductors, a finding that has major implications for solar cell device architectures.

No MeSH data available.


Related in: MedlinePlus