ChunHung Law

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 ∼14u2009ps. 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 ∼0.1–1u2009ms, faster than most observed hysteresis.

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.

Water-based electrolytes for dye-sensitized solar cells.

ANDERSON MA, 1988, J MEMBRANE SCI, V39, P243, DOI 10.1016-S0376-7388(00)80932-1; Regan B. O., 1991, NATURE, V353, P737, DOI [DOI 10.1038-353737A0, 10.1038-353737a0]; Ardo S, 2009, CHEM SOC REV, V38, P115, DOI 10.1039-b804321n; Barnes PRF, 2009, NANO LETT, V9, P3532, DOI 10.1021-nl901753k; Barnes PRF, 2009, J PHYS CHEM C, V113, P1126, DOI 10.1021-jp809046j; Green MA, 2009, PROG PHOTOVOLTAICS, V17, P320, DOI 10.1002-pip.911; Hagfeldt A, 2000, ACCOUNTS CHEM RES, V33, P269, DOI 10.1021-ar980112j; Jung YS, 2009, ELECTROCHIM ACTA, V54, P6286, DOI 10.1016-j.electacta.2009.06.006; JURIS A, 1988, COORDIN CHEM REV, V84, P85, DOI 10.1016-0010-8545(88)80032-8; Kaneko M, 2003, MACROMOL RAPID COMM, V24, P444, DOI 10.1002-marc.200390059; Koops SE, 2009, J AM CHEM SOC, V131, P4808, DOI 10.1021-ja8091278; LISKA P, 1988, J AM CHEM SOC, V110, P3686, DOI 10.1021-ja00219a068; Liu Y, 1998, SOL ENERG MAT SOL C, V55, P267, DOI 10.1016-S0927-0248(98)00111-1; Macht B, 2002, SOL ENERG MAT SOL C, V73, P163, DOI 10.1016-S0927-0248(01)00121-0; Matar F, 2008, J MATER CHEM, V18, P4246, DOI 10.1039-b808255c; Mikoshiba S, 2005, CURR APPL PHYS, V5, P152, DOI 10.1016-j.cap.2004.06.023; NAZEERUDDIN MK, 1990, HELV CHIM ACTA, V73, P1788, DOI 10.1002-hlca.19900730624; OREGAN BC, 1990, THESIS U WISCONSIN; ORegan BC, 2004, J PHYS CHEM B, V108, P4342, DOI 10.1021-jp035613n; ORegan BC, 2009, J AM CHEM SOC, V131, P3541, DOI 10.1021-ja806869x; ORegan BC, 2007, J PHYS CHEM C, V111, P14001, DOI 10.1021-jp073056p; Saito H, 2004, ELECTROCHEMISTRY, V72, P310; Tropsha YG, 1997, J PHYS CHEM B, V101, P2259, DOI 10.1021-jp9629856; VLACHOPOULOS N, 1988, J AM CHEM SOC, V110, P1216, DOI 10.1021-ja00212a033; Wang P, 2005, APPL PHYS LETT, V86, DOI 10.1063-1.1887825

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.

The mechanism behind the beneficial effect of light soaking on injection efficiency and photocurrent in dye sensitized solar cells

Electrical and luminescence characterization was performed on 16 dye sensitized solar cells with different formulations, from different industrial and academic sources. Most of the cells were fabricated in pre-industrial pilot lines. The cells were put through a light soaking period up to 150 hours and then re-characterized. The results show the commonly observed increase in Jsc with light soaking is due to a decrease in the conduction band energy (with respect to the electrolyte) and an increase in the injection rate and efficiency. The strong correlation between the luminescence decay lifetime (<200 ps to 5 ns) and the photocurrent (7 to 13 mA cm−2) shows that the luminescence decay is a useful monitor of injection rates in these cells. The very slow injection shown by some cells implies substantial losses at the injection step. The data point to a need to understand and improve the TiO2 processing and dyeing conditions in the industrial setting as well as the need to focus injection studies on the full range of dynamics present in the cells.

Managing wetting behavior and collection efficiency in photoelectrochemical devices based on water electrolytes; improvement in efficiency of water/iodide dye sensitised cells to 4%

We have fabricated Dye Sensitised Solar Cells (DSSCs) using only water as the solvent and guanidinium iodide–iodine as the redox couple that operate at 4% energy efficiency under 1-sun illumination. This result is ∼5 times higher than the best previously reported values. We show that it is critical to facilitate the wetting of water electrolytes into the mesoporous TiO2 dye films, especially when using hydrophobic dyes. We show chenodeoxycholic acid to be a good surfactant for this purpose. By separate variation of iodide and iodine concentrations in series of cells we show that the optimum concentrations for water based DSSCs are quite different from those used in organic electrolytes. We argue that this is due to the much lower stability constant of tri-iodide in water, relative to organic solvent. Finally we also vary the TiO2 thickness and pore structure to achieve the above stated efficiency.

Rediscovering a Key Interface in Dye-Sensitized Solar Cells: Guanidinium and Iodine Competition for Binding Sites at the Dye/Electrolyte Surface

We propose a new mechanism by which the common electrolyte additive guanidinium thiocyanate (GdmSCN) improves efficiency in dye-sensitized solar cells (DSSCs). We demonstrate that binding of Gdm(+) to TiO2 is weak and does not passivate recombination sites on the TiO2 surface as has been previously claimed. Instead, we show that Gdm(+) binds strongly to the N719 and D131 dyes and probably to many similar compounds. The binding of Gdm(+) competes with iodine binding to the same molecule, reducing the surface concentration of dye-I2 complexes. This in turn reduces the electron/iodine recombination rate constant, which increases the collection efficiency and thus the photocurrent. We further observe that GdmNO3 can increase efficiency more than the current Gdm(+) source, GdmSCN, at least in some DSSCs. Overall, the results point to an improved paradigm for DSSC operation and development. The TiO2/electrolyte surface has long been held to be the key interface in DSSCs. We now assert that the dye layer/electrolyte interaction is at least, and probably more, important.

2000 hours photostability testing of dye sensitised solar cells using a cobalt bipyridine electrolyte

DSSCs incorporating Co(II/III) tris(bipyridine) redox couple in acetonitrile and the Z907 dye were subjected to ≥2000 hours of light soaking at 20 °C and ∼1 sun light intensity from white LEDs (∼no UV component). Initial energy efficiencies were near 6.3%. After 2000 hours, the best acetonitrile based cells maintained ∼66% of the initial efficiency. Both JSC and fill factor (FF) declined, while the VOC remained highly stable. In comparison, the best Z907/cobalt cells with 3-methoxypropionitrile (MPN) as a solvent, maintained 91% of the initial efficiency after 2000 hour light soaking. Only FF declined in MPN based cells. In follow up testing of similar ACN cells at the maximum power point, at 30 °C, the best cells maintained ∼100% of the initial efficiency after 1000 hours. Impedance, JSCvs. intensity and charge extraction data are consistent with a decrease in the Co(III) concentration, or a restriction in Co(III) diffusion, during light soaking.

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

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

Brief air heating of TiO2/dye films, to 120–250 °C; the effect on resulting liquid junction dye sensitised solar cells (DSSCs) and melt-processed solid-state DSSCs

TiO2/Z907 and TiO2/D149 films have been subjected to 1 minute heating in room air to 120 °C, 190 °C, and 255 °C. Liquid electrolyte dye-sensitised solar cells (DSSCs), fabricated after the heating step, are used to assess the damage to the TiO2/dye film by the heat. N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) and spiro-OMeTAD (SOT) hole conductors are also melt processed onto the TiO2/dye films, requiring 190 °C and 255 °C respectively. The hole conductors have been removed by dissolution and liquid electrolyte DSSCs have been fabricated from the TiO2/dye films. The effect of the 120 °C heat step is minimal. After 190 °C heating the efficiency loss is 10–20% depending on dye and film thickness. After 255 °C heating the efficiency loss is from 27 to 65%. The loss of efficiency occurs almost entirely due to a decrease in recombination lifetime, up to 100 fold in the worst case. This causes a decrease in Voc, and also in collection efficiency for thicker films. Solid state DSSCs made with melt processed spiro-OMeTAD on 2 μm TiO2/Z907 films are shown, with efficiency of 0.45%. Efficiency is limited by a thick capping layer and base (4-tert-butylpyridine, TBP) boil-off during processing.