Piers R. F. Barnes

Ionic transport in hybrid lead iodide perovskite solar cells

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.

Parameters influencing the efficiency of electron injection in dye-sensitized solar cells.

In this paper we focus upon the electron injection dynamics in complete nanocrystalline titanium dioxide dye-sensitized solar cells (DSSCs) employing the ruthenium bipyridyl sensitizer dye N719. Electron injection dynamics and quantum yields are studied by time-resolved single photon counting, and the results are correlated with device performance. In typical DSSC devices, electron injection kinetics were found to proceed from the N719 triplet state with a half-time of 200 +/- 60 ps and quantum yield of 84 +/- 5%. We find that these injection dynamics are independent of presence of iodide/triiodide redox couple and of the pH of the peptization step used in the synthesis of the TiO(2) nanoparticles. They are furthermore found to be only weakly dependent upon the application of electrical bias to the device. In contrast, we find these dynamics to be strongly dependent upon the concentration of tert-butylpyridine (tBP) and lithium cations in the electrolyte. This dependence is correlated with shifts of the TiO(2) conduction band energetics as a function of tBP and Li(+) concentration, from which we conclude that a 100 meV shift in band edge results in an approximately 2-fold retardation of injection dynamics. We find that the electron injection quantum yield determined from these transient emission data as a function of tBP and Li(+) concentration shows a linear correlation with device short circuit density J(sc). We thus conclude that the relative energetics of the dye excited state versus the titanium dioxide acceptor state is a key determinant of the dynamics of electron injection in DSSC, and that variations in these energetics, and therefore in the kinetics and efficiency of electron injection, impact directly upon device photovoltaic efficiency. Finally, we discuss these results in terms of singlet versus triplet electron injection pathways and the concept of minimization of kinetic redundancy.

The dynamics of methylammonium ions in hybrid organic–inorganic perovskite solar cells

Methylammonium lead iodide perovskite can make high-efficiency solar cells, which also show an unexplained photocurrent hysteresis dependent on the device-poling history. Here we report quasielastic neutron scattering measurements showing that dipolar CH3NH3+ ions reorientate between the faces, corners or edges of the pseudo-cubic lattice cages in CH3NH3PbI3 crystals with a room temperature residence time of ∼14 ps. Free rotation, π-flips and ionic diffusion are ruled out within a 1–200-ps time window. Monte Carlo simulations of interacting CH3NH3+ dipoles realigning within a 3D lattice suggest that the scattering measurements may be explained by the stabilization of CH3NH3+ in either antiferroelectric or ferroelectric domains. Collective realignment of CH3NH3+ to screen a devices built-in potential could reduce photovoltaic performance. However, we estimate the timescale for a domain wall to traverse a typical device to be ∼0.1–1 ms, faster than most observed hysteresis.

Interpretation of Optoelectronic Transient and Charge Extraction Measurements in Dye‐Sensitized Solar Cells

Tools that assess the limitations of dye sensitized solar cells (DSSCs) made with new materials are critical for progress. Measuring the transient electrical signals (voltage or current) after optically perturbing a DSSC is an approach which can give information about electron concentration, transport and recombination. Here we describe the theory and practice of this class of optoelectronic measurements, illustrated with numerous examples. The measurements are interpreted with the multiple trapping continuum model which describes electrons in a semiconductor with an exponential distribution of trapping states. We review standard small perturbation photocurrent and photovoltage transients, and introduce the photovoltage time of flight measurement which allows the simultaneous derivation of both effective diffusion and recombination coefficients. We then consider the utility of large perturbation measurements such as charge extraction and the current interrupt technique for finding the internal charge and voltage within a device. Combining these measurements allows differences between DSSCs to be understood in terms such as electron collection efficiency, semiconductor conduction band edge shifts and recombination kinetics.

Optoelectronic Studies of Methylammonium Lead Iodide Perovskite Solar Cells with Mesoporous TiO2: Separation of Electronic and Chemical Charge Storage, Understanding Two Recombination Lifetimes, and the Evolution of Band Offsets during J–V Hysteresis

Methylammonium lead iodide (MAPI) cells of the design FTO/sTiO2/mpTiO2/MAPI/Spiro-OMeTAD/Au, where FTO is fluorine-doped tin oxide, sTiO2 indicates solid-TiO2, and mpTiO2 is mesoporous TiO2, are studied using transient photovoltage (TPV), differential capacitance, charge extraction, current interrupt, and chronophotoamperometry. We show that in mpTiO2/MAPI cells there are two kinds of extractable charge stored under operation: a capacitive electronic charge (∼0.2 μC/cm(2)) and another, larger charge (40 μC/cm(2)), possibly related to mobile ions. Transient photovoltage decays are strongly double exponential with two time constants that differ by a factor of ∼5, independent of bias light intensity. The fast decay (∼1 μs at 1 sun) is assigned to the predominant charge recombination pathway in the cell. We examine and reject the possibility that the fast decay is due to ferroelectric relaxation or to the bulk photovoltaic effect. Like many MAPI solar cells, the studied cells show significant J-V hysteresis. Capacitance vs open circuit voltage (V(oc)) data indicate that the hysteresis involves a change in internal potential gradients, likely a shift in band offset at the TiO2/MAPI interface. The TPV results show that the V(oc) hysteresis is not due to a change in recombination rate constant. Calculation of recombination flux at V(oc) suggests that the hysteresis is also not due to an increase in charge separation efficiency and that charge generation is not a function of applied bias. We also show that the J-V hysteresis is not a light driven effect but is caused by exposure to electrical bias, light or dark.

Lattice dynamics and vibrational spectra of the orthorhombic, tetragonal, and cubic phases of methylammonium lead iodide

The hybrid halide perovskite CH3NH3PbI3 exhibits a complex structural behaviour, with successive transitions between orthorhombic, tetragonal and cubic polymorphs at ca. 165 K and 327 K. Herein we report first-principles lattice dynamics (phonon spectrum) for each phase of CH3NH3PbI3. The equilibrium structures compare well to solutions of temperature-dependent powder neutron diffraction. By following the normal modes we calculate infrared and Raman intensities of the vibrations, and compare them to the measurement of a single crystal where the Raman laser is controlled to avoid degradation of the sample. Despite a clear separation in energy between low frequency modes associated with the inorganic PbI3 network and high-frequency modes of the organic CH3NH3+ cation, significant coupling between them is found, which emphasises the interplay between molecular orientation and the corner-sharing octahedral networks in the structural transformations. Soft modes are found at the boundary of the Brillouin zone of the cubic phase, consistent with displacive instabilities and anharmonicity involving tilting of the PbI6 octahedra around room temperature.

Evidence for ion migration in hybrid perovskite solar cells with minimal hysteresis

Ion migration has been proposed as a possible cause of photovoltaic current–voltage hysteresis in hybrid perovskite solar cells. A major objection to this hypothesis is that hysteresis can be reduced by changing the interfacial contact materials; however, this is unlikely to significantly influence the behaviour of mobile ionic charge within the perovskite phase. Here, we show that the primary effects of ion migration can be observed regardless of whether the contacts were changed to give devices with or without significant hysteresis. Transient optoelectronic measurements combined with device simulations indicate that electric-field screening, consistent with ion migration, is similar in both high and low hysteresis CH3NH3PbI3 cells. Simulation of the photovoltage and photocurrent transients shows that hysteresis requires the combination of both mobile ionic charge and recombination near the perovskite-contact interfaces. Passivating contact recombination results in higher photogenerated charge concentrations at forward bias which screen the ionic charge, reducing hysteresis.

Simultaneous Transient Absorption and Transient Electrical Measurements on Operating Dye-Sensitized Solar Cells: Elucidating the Intermediates in Iodide Oxidation

Simultaneously acquired transient absorption, photocurrent, and photovoltage signals are used to characterize operational dye-sensitized solar cells (DSSCs) under 1 sun illumination. At open circuit, the biphasic decay of both photovoltage and 980 nm absorption have identical decay profiles. The data show that, in devices incorporating N719 [(Bu4N)2[Ru(dcbpyH)2(NCS)2] (dcbpy = 4,4′-dicarboxy-2,2′-bipyridyl) as the sensitizer, the 950−1020 nm absorption can be assigned to electrons alone. This differs from some previous assignments. At short circuit, the comparison of the integrated photocurrent transient, electron absorption, and the absorption of the oxidized dye (830 nm) can be used to show relative loss of photocurrent to oxidized dyes. In addition, we show that, under operational conditions, there is no evidence for long-lived di-iodine radicals (I2•−). Further, we show that the decay of the transient bleach at 560 nm, previously assigned to the bleach of the ground-state dye, contains additional long...

Re-evaluation of Recombination Losses in Dye-Sensitized Cells: The Failure of Dynamic Relaxation Methods to Correctly Predict Diffusion Length in Nanoporous Photoelectrodes

Photocurrents generated by thick, strongly absorbing, dye-sensitized cells were reduced when the electrolyte iodine concentration was increased. Electron diffusion lengths measured using common transient techniques (L(n)) were at least two times higher than diffusion lengths measured at steady state (L(IPCE)). Charge collection efficiency calculated using L(n) seriously overpredicted photocurrent, while L(IPCE) correctly predicted photocurrent. This has implications for optimizing cell design.

Performance and Stability of Lead Perovskite/TiO2, Polymer/PCBM, and Dye Sensitized Solar Cells at Light Intensities up to 70 Suns

Three organic or hybrid photovoltaic technologies are compared with respect to performance and stability under the harsh regime of concentrated light. Although all three technologies show surprisingly high (and linear) photocurrents, and better than expected stability, no golden apples are awarded.