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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

Chronophotoamperometry measurements of a perovskite-based cell.(a) The measurement sequence in a d-TiO2/CH3NH3PbI3/spiro-OMeTAD/Au cell is indicated; measured temperatures (to the nearest 0.5 °C) of the devices were −9.5 (dark blue), −5.5, 0.5, 5, 10.5, 15, 19.5, 24.5, 30, 40 and 50 °C (dark red). The dark current under forward bias was very sensitive to fluctuations in the controlled temperature; no time constants were taken from the dark current. (b) Arrhenius plot of the rates of photocurrent relaxation. Fits (purple lines) to the fast (k1) and slow (k2) components of bi-exponential to the photocurrent rise at 1 sun equivalent light intensity following reverse bias at −0.5 V in the dark (open circles and squares, respectively). The activation energy of the fast component evaluated between −9.5 and 50 °C was EA=0.62 eV. The activation energy for the slow component evaluated between 15 and 50 °C was EA=0.60 eV (the measurement duration was insufficient to reliably estimate the slow component at lower temperatures). The photocurrent rise at 50 °C did not reach a stable plateau and started to decline after its peak; we did not fit to this portion of the curve. The red crosses show the rates inferred from a single exponential fit to the tail of the photocurrent decay (k3) following forward bias at 1 V in the dark; the corresponding activation energy is 0.68 eV (red line) evaluated between 15 and 50 °C. Given the spread of points, we consider that the range of activation energies determined is similar to the uncertainty of the estimation.
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f4: Chronophotoamperometry measurements of a perovskite-based cell.(a) The measurement sequence in a d-TiO2/CH3NH3PbI3/spiro-OMeTAD/Au cell is indicated; measured temperatures (to the nearest 0.5 °C) of the devices were −9.5 (dark blue), −5.5, 0.5, 5, 10.5, 15, 19.5, 24.5, 30, 40 and 50 °C (dark red). The dark current under forward bias was very sensitive to fluctuations in the controlled temperature; no time constants were taken from the dark current. (b) Arrhenius plot of the rates of photocurrent relaxation. Fits (purple lines) to the fast (k1) and slow (k2) components of bi-exponential to the photocurrent rise at 1 sun equivalent light intensity following reverse bias at −0.5 V in the dark (open circles and squares, respectively). The activation energy of the fast component evaluated between −9.5 and 50 °C was EA=0.62 eV. The activation energy for the slow component evaluated between 15 and 50 °C was EA=0.60 eV (the measurement duration was insufficient to reliably estimate the slow component at lower temperatures). The photocurrent rise at 50 °C did not reach a stable plateau and started to decline after its peak; we did not fit to this portion of the curve. The red crosses show the rates inferred from a single exponential fit to the tail of the photocurrent decay (k3) following forward bias at 1 V in the dark; the corresponding activation energy is 0.68 eV (red line) evaluated between 15 and 50 °C. Given the spread of points, we consider that the range of activation energies determined is similar to the uncertainty of the estimation.

Mentions: By measuring the temperature- and time-dependent photocurrent following forward and reverse biasing in the dark, the rate at which the cell relaxes to equilibrium can be determined and thus activation energies for the relaxation of the device can be estimated (see Fig. 4a,b). The temperature range covered in this study is representative of typical device-operating temperatures but below the second-order tetragonal-to-cubic phase transition. Following dark reverse bias conditions, the photocurrent rise at short circuit was fitted well by a bi-exponential. The time constant for the initial part of the photocurrent rise after reverse biasing varied between 53 s at −9.5 °C and 0.36 s at 50 °C. The time constant for the slower phase of the rise was six times longer, with values very similar to those obtained by fitting to single exponential functions to the tail of the photocurrent decay following forward biasing in the dark (Fig. 4a). The shape of the photocurrent relaxations remained relatively constant but scaled with time according to the temperature, implying that only the kinetics of the relaxation process had changed. The activation energies for short-circuit photocurrent relaxation from both forward (1 V) and reverse bias (−0.5 V) preconditioning were similar: 0.68 eV and 0.60–0.62 eV, respectively. This similarity, combined with the observation that the slow phases of both the rise and fall of the photocurrent have approximately the same time constants for a given temperature, suggests that the underlying process controlling the relaxation is reversible.


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)

Chronophotoamperometry measurements of a perovskite-based cell.(a) The measurement sequence in a d-TiO2/CH3NH3PbI3/spiro-OMeTAD/Au cell is indicated; measured temperatures (to the nearest 0.5 °C) of the devices were −9.5 (dark blue), −5.5, 0.5, 5, 10.5, 15, 19.5, 24.5, 30, 40 and 50 °C (dark red). The dark current under forward bias was very sensitive to fluctuations in the controlled temperature; no time constants were taken from the dark current. (b) Arrhenius plot of the rates of photocurrent relaxation. Fits (purple lines) to the fast (k1) and slow (k2) components of bi-exponential to the photocurrent rise at 1 sun equivalent light intensity following reverse bias at −0.5 V in the dark (open circles and squares, respectively). The activation energy of the fast component evaluated between −9.5 and 50 °C was EA=0.62 eV. The activation energy for the slow component evaluated between 15 and 50 °C was EA=0.60 eV (the measurement duration was insufficient to reliably estimate the slow component at lower temperatures). The photocurrent rise at 50 °C did not reach a stable plateau and started to decline after its peak; we did not fit to this portion of the curve. The red crosses show the rates inferred from a single exponential fit to the tail of the photocurrent decay (k3) following forward bias at 1 V in the dark; the corresponding activation energy is 0.68 eV (red line) evaluated between 15 and 50 °C. Given the spread of points, we consider that the range of activation energies determined is similar to the uncertainty of the estimation.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f4: Chronophotoamperometry measurements of a perovskite-based cell.(a) The measurement sequence in a d-TiO2/CH3NH3PbI3/spiro-OMeTAD/Au cell is indicated; measured temperatures (to the nearest 0.5 °C) of the devices were −9.5 (dark blue), −5.5, 0.5, 5, 10.5, 15, 19.5, 24.5, 30, 40 and 50 °C (dark red). The dark current under forward bias was very sensitive to fluctuations in the controlled temperature; no time constants were taken from the dark current. (b) Arrhenius plot of the rates of photocurrent relaxation. Fits (purple lines) to the fast (k1) and slow (k2) components of bi-exponential to the photocurrent rise at 1 sun equivalent light intensity following reverse bias at −0.5 V in the dark (open circles and squares, respectively). The activation energy of the fast component evaluated between −9.5 and 50 °C was EA=0.62 eV. The activation energy for the slow component evaluated between 15 and 50 °C was EA=0.60 eV (the measurement duration was insufficient to reliably estimate the slow component at lower temperatures). The photocurrent rise at 50 °C did not reach a stable plateau and started to decline after its peak; we did not fit to this portion of the curve. The red crosses show the rates inferred from a single exponential fit to the tail of the photocurrent decay (k3) following forward bias at 1 V in the dark; the corresponding activation energy is 0.68 eV (red line) evaluated between 15 and 50 °C. Given the spread of points, we consider that the range of activation energies determined is similar to the uncertainty of the estimation.
Mentions: By measuring the temperature- and time-dependent photocurrent following forward and reverse biasing in the dark, the rate at which the cell relaxes to equilibrium can be determined and thus activation energies for the relaxation of the device can be estimated (see Fig. 4a,b). The temperature range covered in this study is representative of typical device-operating temperatures but below the second-order tetragonal-to-cubic phase transition. Following dark reverse bias conditions, the photocurrent rise at short circuit was fitted well by a bi-exponential. The time constant for the initial part of the photocurrent rise after reverse biasing varied between 53 s at −9.5 °C and 0.36 s at 50 °C. The time constant for the slower phase of the rise was six times longer, with values very similar to those obtained by fitting to single exponential functions to the tail of the photocurrent decay following forward biasing in the dark (Fig. 4a). The shape of the photocurrent relaxations remained relatively constant but scaled with time according to the temperature, implying that only the kinetics of the relaxation process had changed. The activation energies for short-circuit photocurrent relaxation from both forward (1 V) and reverse bias (−0.5 V) preconditioning were similar: 0.68 eV and 0.60–0.62 eV, respectively. This similarity, combined with the observation that the slow phases of both the rise and fall of the photocurrent have approximately the same time constants for a given temperature, suggests that the underlying process controlling the relaxation is reversible.

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