Brian C. O'Regan

Catalysis of Recombination and Its Limitation on Open Circuit Voltage for Dye Sensitized Photovoltaic Cells Using Phthalocyanine Dyes

In order to increase the energy efficiency of dye-sensitized solar cells beyond 10%, an improved dye needs to be developed with greater light absorption in the red and near-infrared. Many dyes have been tested for this purpose; however, no dye with significant absorption beyond 750 nm has functioned properly. We have examined a series of ruthenium phthalocyanines, a dye class with large and tunable absorption in the red. For these dyes we observe a large reduction in the output voltage of the cells relative to the benchmark dye (N719). By examination of photovoltage transients and charge density measurements, we demonstrate that this reduction in voltage is caused by a 100-fold increase in the rate constant for recombination (iodine reduction) at the TiO2/electrolyte interface. N719, however, does not seem to catalyze this reaction. By examination of the literature, we propose that catalysis of the recombination reaction may be occurring for many other classes of potentially useful dyes including porphyri...

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.

Kinetic competition in liquid electrolyte and solid-state cyanine dye sensitized solar cells

The photovoltaic performance of liquid electrolyte and solid-state dye sensitized solar cells, employing a squarilium methoxy cyanide dye, are evaluated in terms of interfacial electron transfer kinetics. Dye adsorption to the metal oxide film resulted in a mixed population of aggregated and monomeric sensitizer dyes. Emission quenching data, coupled with transient absorption studies, indicate that efficient electron injection was only achieved by the monomeric dyes, with the aggregated dye population having an injection yield an order of magnitude lower. In liquid electrolyte devices, transient absorption studies indicate that photocurrent generation is further limited by slow kinetics of the regeneration of monomeric dye cations by the iodide/iodine redox couple. The regeneration dynamics are observed to be too slow (≫ 100 µs) to compete effectively with the recombination of injected electrons with dye cations. In contrast, for solid-state devices employing the organic hole conductor spiro-OMeTAD, the regeneration dynamics are fast enough (≪ 1 µs) to compete effectively with this recombination reaction, resulting in enhanced photocurrent generation.

A new ruthenium polypyridyl dye, TG6, whose performance in dye-sensitized solar cells is surprisingly close to that of N719, the ‘dye to beat’ for 17 years

A new ruthenium polypyridine sensitizer for dye-sensitized solar cells (DSSCs) is proposed containing a hexasulfanyl–styryl-modified bipyridyl group as an ancillary ligand. The advantages of this dye are the much larger absorption coefficient and the small shift of the absorption envelope to the red. We compare this new dye, TG6 (cis-bis(thiocyanato)(2,2′-bipyridyl-4,4′-dicarboxylato){4,4′-bis[2-(4-hexylsulfanylphenyl)vinyl]-2,2′-bipyridine}ruthenium(II) mono(tetrabutylammonium) salt), to the current best-performing dye, N719 (cis-bis(thiocyanato)bis(2,2′-bipyridine-4,4′-dicarboxylato)ruthenium(II) bis(tetrabutylammonium) salt). We have applied a suite of evaluation tools including: varying the electrolyte, varying the TiO2 film thickness, charge density and recombination rate constant measurements, fluorescence lifetime and magnitude, and transient absorption techniques. The combined results indicate that TG6, as presently constructed, can surpass the performance of N719 under some conditions, but is likely to need some modification before surpassing cells designed to give record energy efficiency using N719. The higher absorption coefficient may be relevant to mass-produced DSSCs on metal where thinner TiO2 films are advantageous. The main disadvantage is the slight catalysis of the electron/electrolyte recombination, which is possibly due to the extended π-system. A factor that requires further optimization involves the complex interaction of the slightly lower LUMO position, the composition of the electrolyte, the band edge position of the TiO2, and the electron injection rate. We show why the maximum output from TG6 cells will not occur using the exact electrolytes used to maximize N719 cells.

Dye adsorption, desorption, and distribution in mesoporous TiO2 films, and its effects on recombination losses in dye sensitized solar cells

Recombination rates through bare, partially dyed, and fully dyed mesoporous TiO2 films have been measured, as well as the penetration kinetics of dye molecules into the films. We give evidence that homogeneous partial dye coverage cannot be achieved by short dyeing times or dilute dye solutions, due to the high sticking coefficient of the dyes on TiO2. A new method has been developed which gives homogenous partial dye coverage, based on full dyeing followed by partial desorption. Using this method we show that dyeing with N719, C101, or TG6 dyes provides only minor blocking of recombination, relative to bare TiO2. We also show that recombination rates across the bare TiO2 surface can vary strongly after treatment with mild base, acid, or a Lewis acid/base buffer. We find these treatments can change the density of trap states as well as the conduction band edge potential. We present a method, based on charge extraction, that corrects for changes in trap density when estimating shifts in the conduction band potential. The combined results suggest that the industrially favored fast dyeing procedures can lead to higher recombination rates, especially in the absence of the typical TiCl4 treatment, and under low (e.g. indoor) light levels. The results also explain some anomalous features seen in studies of exciton and hole conduction along partial coverage dye layers which were previously assumed to be homogenous.

Hole‐Transporting Transistors and Circuits Based on the Transparent Inorganic Semiconductor Copper(I) Thiocyanate (CuSCN) Processed from Solution at Room Temperature

The wide bandgap and highly transparent inorganic compound copper(I) thiocyanate (CuSCN) is used for the first time to fabricate p-type thin-film transistors processed from solution at room temperature. By combining CuSCN with the high-k relaxor ferroelectric polymeric dielectric P(VDF-TrFE-CFE), we demonstrate low-voltage transistors with hole mobilities on the order of 0.1 cm(2) V(-1) s(-1) . By integrating two CuSCN transistors, unipolar logic NOT gates are also demonstrated.

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.

Improved environmental stability of organic lead trihalide perovskite-based photoactive-layers in the presence of mesoporous TiO2

Impressive hybrid photovoltaic device performances have been realised with the methylammonium lead triiodide (MAPbI3) perovskite absorber in a wide range of device architectures. However, the question as to which of these systems represents the most commercially viable long-term prospect is yet to be answered conclusively. Here, we report on the photoinduced charge transfer processes in MAPbI3 based films measured under inert and ambient conditions. When exposed to ambient conditions, the coated mesoporous Al2O3 and bilayer systems show a rapid and significant degradation in the yield of long-lived charge separation. This process, which does not affect sensitized-mesoporous TiO2 films, is only found to occur when both light and oxygen are present. These observations indicate that the presence of a mesostructured TiO2 electron acceptor to rapidly extract the photoexcited electron from the perovskite sensitizer may be crucial for fundamental photovoltaic stability and significantly increases innate tolerance to environmental conditions. This work highlights a significant advantage of retaining mesoscale morphological control in the design of perovskite photovoltaics.