J. Phys. Chem. C 2007, 111, 14001-14010
14001
Influence of the TiCl4 Treatment on Nanocrystalline TiO2 Films in Dye-Sensitized Solar Cells. 2. Charge Density, Band Edge Shifts, and Quantification of Recombination Losses at Short Circuit Brian C. O’Regan,*,† James R. Durrant,† Paul M. Sommeling,‡ and Nicolaas J. Bakker‡ Department of Chemistry, Imperial College London, Exhibition Road, SW7 2AZ, London, United Kingdom, and Energy Research Centre Netherlands, P.O. Box 1, 1755 ZG Petten, The Netherlands ReceiVed: April 20, 2007; In Final Form: July 16, 2007
Chemical bath deposition of TiO2 from TiCl4 is an often used treatment that improves the photocurrent from dye-sensitized TiO2 solar cells. In this paper, charge density and kinetic data are used to show that the main effects of this treatment are an 80 mV downward shift in the TiO2 conduction band edge potential and a 20-fold decrease in the electron/electrolyte recombination rate constant. Together, these changes increase the quantum efficiency of charge separation at the interface, thus providing the observed increase in the photocurrent. The reduction in the recombination rate constant allows a greater concentration of electrons to accumulate at Voc, thus offsetting the Voc loss otherwise expected from the conduction band edge shift. Photocurrent transients and charge extraction data are used to show that the TiCl4 treatment has little effect on the transport of electrons at short circuit. The electron/electrolyte recombination rate constant at short circuit has been measured with the CCTPV (Constant Current Transient PhotoVoltage) technique. The results further confirm that any improvements in transport could not cause the beneficial effect of the TiCl4 treatment. Verification of the CCTPV technique is undertaken by comparison to transient absorption and by a model of the technique. Charge separation in dye-sensitized cells concerns two steps, charge injection and dye regeneration. Transient optical experiments to determine which process is improved by the TiCl4 treatment are discussed.
Introduction Research on dye-sensitized solar cells (DSSC) as a less expensive replacement for silicon-based PV has resulted in more than 1500 journal articles in the past decade. Despite this avalanche, there are still several fundamental questions which are not yet answered. Several of these questions concern the role of additives and treatments that have been empirically developed to increase the efficiency of the cells. The most efficient dye-sensitized cells are based on a nanoporous TiO2 film that serves both as a substrate for the dye and as the electron conductor. One fabrication step that has been very common for over 15 years is a posttreatment of the nanoporous TiO2 films with a solution of TiCl4. This treatment was developed to rectify a problem that arose during the fabrication of “thick” (>1 µm) films of colloidal particles on glass substrates. When dye sensitization was first applied to films of colloidal particles on tin oxide glass (in 1988) the particles were synthesized via the hydrolysis of titanium isopropoxide in the presence of nitric acid, followed by peptization at 80 °C for 12 h.1,2 This recipe results in approximately 7 nm partially crystalline anatase particles. Films of this solution can be spin coated onto TCO glass. During heating to 400 °C, necessary to further crystallize the particles and strengthen the film, the shrinkage of these films is g20%. It was found that films over 0.4 µm cracked badly due to this shrinkage. None the less, by application of 10 spin coating layers and 10 firing steps, films of 4 µm could be made. These were very useful for fundamental studies of dye* Address correspondence to this author. E-mail:
[email protected]. † Imperial College London. ‡ Energy Research Centre Netherlands.
sensitization processes.2,3 Further, by using organic electrolytes the efficiency of electron injection from the dye to the TiO2 was increased to near 100%. To make thicker films more easily, autoclaving was used to further crystalize the particles, and to increase the particle size to 12-15 nm. This was successful, and with the addition of organic binders, films of over 12 µm could be applied and heated in one step. Initial results, however, showed that the photocurrent from these films decreased by ∼20% relative to similar films made from non-autoclaved particles.4 Various unsuccessful attempts were made to rectify this by recreating the processing history of the non-autoclaved films, for example, repeated exposure to nitric acid followed by heating to 400 °C. Eventually, it was concluded that the autoclaving had “spoiled” the TiO2 surface, which needed to be covered with new TiO2 material. Cathodic electrodeposition of a new layer of TiO2 was attempted, under the rationale that hot spots for cathodic electrodeposition might also be hot spots for electron recombination with the dye or triiodide. This direction did not succeed, partially due to inhomogeneity in the resulting layers.4 A recipe for anodic electrodeposition of TiO2 with use of TiCl3 was developed.5 A few nanometer thick film deposited from TiCl3, followed by rinsing and additional heating, finally returned the autoclaved particle films to the same photocurrent efficiency shown by the nonautoclaved particles. The TiCl3 route required use of an oxygen free deposition chamber and thus was replaced quickly when it was found that similar results could be obtained by chemical bath deposition from TiCl4.6 The TiCl4 treatment results in an improvement in photocurrent, normally between 10% and 30%. Depending on the quality
10.1021/jp073056p CCC: $37.00 © 2007 American Chemical Society Published on Web 08/25/2007
14002 J. Phys. Chem. C, Vol. 111, No. 37, 2007 SCHEME 1
O’Regan et al. luminescence, thermal decay to the ground state, and possibly electron transfer to electron acceptors in solution or on the interface. The second is between the reduction of the oxidized dye by iodide and geminate or nongeminate recombination with an electron from the TiO2. After the charge separation event, some electrons will diffuse to the SnO2 and create photocurrent in the external circuit, and some will recombine with the oxidized species in the electrolyte. The fraction of the separated charges measured as photocurrent under short circuit conditions is termed the collection efficiency (ΦCOL). The collection efficiency is controlled by the recombination losses, which are equal to the product of the charge density under short circuit conditions and the (pseudo-first-order) rate constant that holds for that charge density. The charge density at short circuit is in turn controlled by the transport rate. If the transport rate is slow, then a large charge density is required to carry the photocurrent, and the recombination losses will be higher. The total photocurrent measured is proportional to the product of the light harvesting efficiency, the charge separation efficiency, and the collection efficiency:
Jsc ) IoΦLHΦCSΦCOL
of the TiO2 used to make the initial film, the extrema of the improvement can be from 200%. The largest improvements come when using the poorest quality TiO2 films. In view of the importance and wide application of the TiCl4 treatment, it is surprising that only recently has it been studied in detail. We have recently published an article detailing the material effects of the TiCl4 treatment on the TiO2 film (thickness, mass, porosity, dye absorption, etc.).7 In this article we continue with results concerning the kinetic (charge recombination and charge transport) and surface effects of the TiCl4 treatment. The operating mechanisms of dye-sensitized solar cells (DSSC) have been previously described; only relevant elements will be summarized below.8,9 Standard dye-sensitized cells consist of a transparent conductive glass substrate, a nanoporous TiO2 layer with attached dye, an iodide/iodide electrolyte, and a counter electrode. Light absorption occurs in a monolayer of dye at the interface between the TiO2 and electrolyte. Sufficient light absorption is achieved by using a thick layer (∼10 µm) of nanoparticles (∼20 nm), wherein all the internal surface is coated with the dye. The fraction of incident light absorbed by the dye is termed the light harvesting efficiency (ΦLH) and is determined by the amount of dye present and the degree to which light absorption is increased by light scattering or other factors. After light absorption, the excited dye molecule can inject an electron into the TiO2 and the thus oxidized dye can be regenerated by electron transfer from the electrolyte. The quantum efficiency of this charge separation event (ΦCS) depends on two kinetic competitions (Scheme 1). One is between the electron injection into the TiO2 and all other routes for deactivation of the excited dye. These latter routes include
(1)
where Io is the incident photon flux expressed as Coulombs/s. We have shown previously that the TiCl4 treatment does not increase the amount of dye present by more than 5%, or significantly increase the light scattering. The increased photocurrent must then stem from improvements in either charge separation or collection efficiency. Heretofore, collection efficiency has not been quantified for DSSCs. Collection efficiency can be estimated by comparing the charge transport and recombination time constants when both values are measured under short circuit conditions. A Constant-Current TransientPhotoVoltage (CCTPV) experiment has recently been suggested as a method to measure recombination time constants at short circuit.10 We employ this method herein. As this method has not previously been critically examined, we show new measurements and modeling results which verify that the CCTPV technique does give an accurate measurement of recombination rates. Results from CCTPV experiments show that it is highly unlikely that the TiCl4 treatment increases the collection efficiency by more than 5%. This leads us to the conclusion that the major effect of the TiCl4 treatment must be on the charge separation efficiency. We show charge density, photocurrent transient, and recombination rate data that support this conclusion. These data also suggest a mechanism by which TiCl4 creates the increase in ΦCS. Methods Cells were fabricated as in the previous study.7 Transparent conductive SnO2 glass, LOF Tec 8, was purchased from Pilkington. TiO2 particles were synthesized from Ti-isopropoxide following the nitric acid/acetic acid route, followed by autoclaving.1,5,11 Layers of the TiO2 particles (∼3 µm) were deposited by screen printing. The cell geometry was 5 × 0.5 cm, with printed silver current collectors, resulting in a series resistance of less than 2 Ω. The TiO2 layers were heated by ramping quickly to 570 °C, holding for 10 min, and then cooling slowly. The TiCl4 treatment was applied by soaking in a 50 mM TiCl4 solution for 30 min at 70 °C, followed by a water rinse and an identical heat treatment. The dye was applied in a special apparatus as previously described.12 The dye was N719 purchased from Solaronix under the name Ruthenium 535. The electrolyte “Li” was acetonitrile, 0.6 M propylmethylimidazo-
Effects of TiCl4 Treatment on Nanocrystalline TiO2 Films lium iodide, 0.5 M tert-butylpyridine, 0.05 M iodine, and 0.1 M LiI. The electrolyte “Gu” substituted 0.1 M guanadinethiocyanate for the LiI. Photocurrent and photovoltage transients were generally taken with use of a pump pulse generated by an array of 1 W red LEDs controlled by a fast solid-state switch, as previously described.10 White bias light was supplied by 10 W “Solarc” lamps (WelchAllyn), which are of the metal halogen type. The bias light was attenuated when needed by neutral density filters. The pulse and bias light were incident on the SnO2 side of the cell. Pulse lengths of 10 to 100 µs were used, with rise and fall times of e1 µs. The pulse intensity was controlled in order to keep the height of the photovoltage transient below 10 mV, except where noted. The magnitudes of the photocurrent transients were less than 10% of the bias photocurrent. Transients were recorded on a potentiostat (Autolab, Ecochemie) with a resolution of 20 µs. This system was used due to its superior noise rejection, and no signal averaging was required for transients larger than 1 mV. In all cases, photovoltage decays were single exponential and the time constant was extracted by fitting. Photocurrent transients at Voc were measured as previously described.13 Transient absorption (TAS) was measured with use of the same bias light and pulse source described above. To measure the transient absorption an additional beam of monochromatic light was directed through the sample and focused onto a photodetector. The output of the photodetector was connected to an oscilloscope and recorded for averaging. To use the same low amplitude pulses required for the photovoltage and photocurrent studies, the TAS signal was averaged for 400 to 1000 pulses to get an acceptable signal-to-noise ratio. To calculate the capacitance at each Voc we have used C ) ∆Qp/∆Vmax, where ∆Qp is the number of electrons injected by the pulse and ∆Vmax is the peak height of the transient photovoltage. We find ∆Qp by integrating the short-circuit photocurrent transient caused by an identical pulse. To calculate the DOS (in electron states/cm3‚V), we have used DOS ) (6.24 × 1018)C/[d(1 - p)], where C is the capacitance/cm2, d is the thickness of the TiO2 film in cm, p is the porosity, and the conversion factor is the number of electrons per Coulomb. The total capacitance of the cell also includes a parallel capacitance from the interface between the SnO2 substrate and the electrolyte. This capacitance is known to be ∼15 µF/cm2.14,15 Total charge present in the cell at the 1 sun Voc and Jsc was also measured by the “charge extraction” technique.16 To measure the recombination rate constant at short circuit we have employed a Constant-Current Transient-PhotoVoltage (CCTPV) experiment as previously described.10 The cell was placed under 1 sun bias illumination and attached to a constant current source. The current source was set to apply exactly the short circuit photocurrent generated by the bias illumination. The applied voltage is thus zero as long as the illumination is kept constant. When a small pulse of additional light is applied, the current source must apply a small voltage increase to keep the photocurrent constant. The constant current source (Autolab, Ecochemie) has a response time of 2 µs, during which time
