Effect of an Adsorbent on Recombination and Band-Edge Movement

J. Phys. Chem. B 2006, 110, 12485-12489

12485

Effect of an Adsorbent on Recombination and Band-Edge Movement in Dye-Sensitized TiO2 Solar Cells: Evidence for Surface Passivation Nikos Kopidakis,* Nathan R. Neale, and Arthur J. Frank* National Renewable Energy Laboratory, Golden, Colorado 80401-3393 ReceiVed: February 3, 2006; In Final Form: April 3, 2006

The mechanism by which the adsorbent guanidinium affects the open-circuit photovoltage of dye-sensitized TiO2 nanocrystalline solar cells was investigated. The influence of the guanidinium cation on the rate of recombination and band-edge movement was measured by transient photovoltage. When guanidinium is present in the electrolyte recombination becomes slower by a factor of about 20. At the same time, the adsorbent causes the band edges to move downward, toward positive electrochemical potentials, by 100 mV. The collective effect of both a downward shift of the band edges and slower recombination, owing to the presence of guanidinium, results in an overall improvement in the open-circuit photovoltage.

Introduction For over the past decade, dye-sensitized nanocrystalline solar cells (DSSCs) have attracted considerable attention from both the academic and the corporate communities. The typical cell consists of a dye-covered mesoporous TiO2 nanoparticle film sandwiched between two transparent electrodes. A liquid electrolyte, traditionally containing the I3-/I- redox couple, fills the pores of the film and contacts the nanoparticles. Photoexcited dye molecules inject electrons into the conduction band of TiO2, and redox species (e.g., I-) in the electrolyte reduce the oxidized dyes back to their original state. The photoinjected electrons diffuse through the TiO2 network, where they are collected at the back-contact (collecting electrode) unless they recombine with oxidized species at the TiO2/electrolyte interface. Redox species (e.g., I3-) in the electrolyte transport the holes from the oxidized dye molecules to the counter electrode. An important photovoltaic parameter determining the overall solar conversion efficiency of a cell is the open-circuit photovoltage (VOC). The open-circuit photovoltage is determined by the difference between the electron quasi-Fermi level in the TiO2 film under illumination and the Fermi level (redox electrolyte level) in the dark.1 A major photovoltage loss mechanism is the disappearance of photoelectron density in the film due to recombination at the TiO2/electrolyte interface. Several strategies involving the treatment of the TiO2 surface have been employed to increase VOC. Depositing a thin, inorganic insulating layer on the surface of the TiO2 particles, such as Al2O32 and Nb2O5,3 is reported to improve photovoltage by slowing recombination. However, if the insulating layer is too thick, it can also reduce the charge-injection efficiency,2 which, in turn, can lower the photovoltage. Adsorption of certain organic molecules has also been thought to insulate the TiO2 surface from recombination. For instance, amphiphilic adsorbents, such as hexadecylmalonic acid4 and decylphosphonic acid,5 when grafted with the sensitizer on the TiO2 surface, are reported to improve VOC. The carboxylic or phosphonic group of the molecules is believed to bind to coordinatively unsaturated Ti species at the TiO2 surface, leaving the hydrophobic end as * Author to whom correspondence should be addressed. E-mail: [email protected]; [email protected].

a buffer between the TiO2 and the electrolyte. The higher photovoltages have been attributed to the shielding of electrons in TiO2 from oxidized species in the electrolyte by the hydrophobic moiety of the adsorbents. Several studies have also reported that adding certain molecular adsorbents to the electrolyte improves the photovoltage by retarding recombination.6-8 In addition to recombination, another important phenomenon affecting VOC is band-edge movement with respect to the redox electrolyte energy.9 Band-edge movement occurs, for example, when a sufficient net number of negative or positive charges (or dipoles) build up on the surface of the particles to induce a change in the potential drop across the Helmholtz layer. A negative surface charge buildup can cause the band edges to shift upward, toward negative electrochemical potentials, leading to a higher photovoltage. It was first shown, for example, that exposing the dye-sensitized TiO2 film to 4-tert-butylpyridine or ammonia, prior to DSSC assembly, causes a significant upward shift of the band edges, resulting in an increased VOC at a fixed photoinduced charge density.9 The band-edge shift was attributed to the amines deprotonating the TiO2, thereby charging it negatively. Coadsorbing chenodeoxycholate with a carboxylated bipyridylruthenium dye onto nanocrystalline TiO2 films was also found to induce an upward band-edge shift, resulting in an improved VOC.10 A recent study has also reported that a zwitterionic molecule, 4-guanidinobutyric acid, coadsorbed with a dye increased VOC by shifting the TiO2 conduction band upward.11 Conversely, a positive surface charge (or dipole) buildup can cause the conduction band edge to move downward, toward positive potentials, favoring a lower photovoltage. For example, surface adsorption of cations, such as Li+ from the electrolyte, is reported to lower VOC;6,12 however, Li+ can also intercalate into the TiO2 lattice, a phenomenon that strongly affects the electron-transport and recombination properties.13 Because guanidinium is not expected to intercalate into TiO2 (as does Li+), guanidinium thiocyanate has recently been used to replace LiI, a traditional electrolytic constituent of DSSCs.11,14-16 In many studies on the effect of molecular adsorbents, the proposed mechanism to account for a change in VOC is based on measurements that do not readily distinguish between

10.1021/jp0607364 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/07/2006

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Figure 1. Dominant resonance structure of the guanidinium cation.

Figure 2. Predicted shifts of log(n) vs VOC curves (dashed lines) caused by band-edge movement induced by the charge or dipole of an adsorbent. The direction of band-edge movement is relative to the case with no adsorbent (solid line). An upward shift corresponds to the band edges moving toward negative potentials.

passivation (or shielding) and band-edge movement. Among the few cases for which such measurements have been performed, the improved photovoltage was found to be due to the adsorbent inducing an upward shift of the band edges.9-11 More importantly, and perhaps surprisingly, even though VOC improved, in two of these cases the adsorbents enhanced recombination.9,10 To date, experimental evidence showing that molecular adsorbents improve VOC in DSSCs by passivating surface recombination sites is inconclusive. In this paper, we present evidence, based on transient photovoltage measurements, which shows that the adsorbent guanidinium (Figure 1), when added to an electrolyte, slows recombination by 1 order of magnitude. In addition, the guanidinium cation was also found to induce a downward shift of the TiO2 conduction band edge. The net effect of these opposing phenomena was an improved VOC. Theory Band-Edge Movement and Recombination. Descriptions of the techniques used to demonstrate band-edge movement are given elsewhere.1,9,10 In brief, evidence for band-edge movement in TiO2 mesoporous films is based on comparing the dependence of VOC on the photoinduced electron density (n) at open circuit in the absence and in the presence of an adsorbent. Inasmuch as n solely determines the difference between the conduction band edge and the electron quasi-Fermi level under illumination, a higher VOC at a constant photoelectron density indicates upward movement of the conduction band edge (Figure 2). Likewise, a lower VOC at a given n indicates a downward shift of the conduction band edge (Figure 2). Therefore, a change in VOC at constant n, upon adding an adsorbent to an electrolyte, can only be attributed to band-edge movement. While the sign of the charge (or dipole orientation) of an adsorbent determines the direction of band-edge movement, the influence of an adsorbent on the recombination kinetics is much less predictable. At open circuit and a constant incident light intensity, the recombination current density (Jr) equals the charge-injection current density (Jinj). Neither Jr nor Jinj can be measured directly but is inferred from measurements of the short-circuit photocurrent density (JSC)17 because recombination at short circuit is negligible.18,19 At light intensities below 1 sun, the charge-injection current density at short circuit and at open circuit are equal,9,17 and the following relationships hold:

JSC ) Jinj ) |Jr| ) kcoxnγ

(1)

where cox is the concentration of the oxidized redox component,

Figure 3. Predicted shifts of log(n) vs log(JSC) curves when an adsorbent retards or enhances recombination (dashed lines) relative to the case without an adsorbent (solid line). JSC determines Jr at open circuit (eq 1).

k is the rate constant, which is related to the microscopic mechanism of recombination, and the exponent γ has typical values of 2-318,20 but can be larger.13 Thus, the recombination current density |Jr| at open circuit has the same value as the short-circuit photocurrent density JSC at the same light intensity. Consequently, measurement of the recombination current density or recombination rate at a given light intensity can be performed without having to assume a particular recombination mechanism; the recombination rate equals |Jr|/qed, where d is the film thickness and qe is the elementary charge. One can assess the influence of an adsorbent on the recombination current density by comparing the dependence of the recombination current density on the electron density n at open circuit in cells with and without an adsorbent, as illustrated schematically in Figure 3. The effect of an adsorbent on recombination kinetics indicates a change in the rate constant k, which results in either an upward or a downward shift in the log(n) versus log(JSC) plot, reflecting either slower or faster recombination, respectively. The change in k can be due to a change in surface shielding or, under certain conditions, a variation in the electron diffusion coefficient.13 If, however, the adsorbent does not affect the electron diffusion coefficient, then k will only vary if the recombination kinetics change at the TiO2/electrolyte interface. For instance, if an adsorbent passivates the surface against recombination at a fixed JSC, then k will decrease and n will increase. Conversely, if an adsorbent enhances recombination, then k will increase and n will decrease. Therefore, a plot of the recombination current density Jr (determined from JSC and eq 1) versus n (determined from transient photovoltage), in cells with and without the adsorbent, is indicative of whether the adsorbent slows or enhances recombination (Figure 3). It should also be noted that the rate constant k is not directly involved in the analysiss only the recombination current density versus n is of interest. Figure 4 depicts the collective effect of band-edge movement and a change in the recombination rate. At constant n, a bandedge shift of ∆V causes the log(JSC) versus VOC curve to shift laterally by ∆V, as indicated by the horizontal arrows. Changing the recombination rate by a factor R alters JSC (eq 1) at constant n and, therefore, shifts the log(JSC) versus VOC curve vertically by an amount log(R), as denoted by the vertical arrows. The combined effect of both the ∆V and the log(R) shifts alters VOC at a constant JSC (i.e., at a constant light intensity) by an amount ∆Vtot, which can be either positive or negative. For instance, Figure 4A shows that the collective effect of both an upward shift of the bands by ∆V and a faster recombination rate by a factor R can improve VOC by an amount of ∆Vtot, which, in this case, is less than ∆V. Generally, if recombination changes, then ∆Vtot does not equal ∆V. Figure 4A describes the situation observed when a TiO2 nanoparticle film is exposed to 4-tertbutylpyridine, ammonia, or chenodeoxycholate.9,10 Under the conditions used, each of these compounds caused the bands to

Effects of Guanidinium Cations on DSSCs

Figure 4. Predicted shifts of log(JSC) vs VOC curves in the absence (solid lines) and in the presence (dashed lines) of an adsorbent. The adsorbent can induce various effects including (A) an upward bandedge shift and faster recombination, (B) a downward band-edge shift and slower recombination, (C) a downward band-edge shift and faster recombination, and (D) an upward band-edge shift and slower recombination. See text for the significance of arrows. ∆V denotes the band-edge shift, R signifies the factor by which the recombination rate changes, and ∆Vtot represents the overall change in VOC at a constant JSC. JSC determines Jr at open circuit (eq 1).

move up and recombination to increase, the overall effect of which was an improved VOC. Figure 4B shows that downward band-edge movement and slower recombination can also lead to an improved VOC. Whether the overall photovoltage improves or becomes worse at a given current density (at the same incident light intensity) will depend, naturally, on the relative extent of band-edge movement and change in the recombination rate. Figure 4C illustrates the worst case in which both downward band-edge movement and faster recombination results in a lower VOC. Figure 4D displays the best scenario in which the adsorbent causes an upward band-edge shift and slower recombination. This scenario is expected to produce the highest photovoltage of the cases considered in Figure 4. None of the cases displayed in Figures 4B, 4C, and 4D has been experimentally substantiated previously. Experimental Section Sample Preparation. Colloidal TiO2 nanoparticles were prepared and analyzed as described elsewhere.5 The TiO2 paste was spread on top of a conducting glass plate (LOF TEC8 F-doped SnO2 glass; Hartford Glass Company) with a doctor blade and sintered at 450 °C for 45 min. Typical film thickness and porosity were 10 µm and 60%, respectively. The films were immersed in ethanol containing 5 × 10-4 M N719 dye (N719 ) TBA2[RuL2(NCS)2], where L is 4-carboxylic acid-4′-carboxylate-2,2′-bipyridine and TBA is tetrabutylammonium) for ca. 18 h. A semitransparent counter electrode was prepared by spreading a droplet of 5 mM H2PtCl6 in 2-propanol onto a conducting glass plate and subsequently firing it at 380 °C for 20 min. The counter electrode and dye-covered electrode were sealed together with a thermal plastic spacer (Dupont Surlyn). The electrolyte was introduced through predrilled holes in the counter electrode, and the holes were subsequently sealed with the same thermoplastic and a microscope slide cover. The electrolyte was composed of 0.6 M 1-butyl-3-methylimidazolium iodide and 0.03 M iodine in 85:15 (v/v) acetonitrile/ valeronitrile. In cells containing guanidinium, 0.1 M guanidinium thiocyanate (Aldrich) was added to the electrolyte.

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Figure 5. Effect of guanidinium on the photoinduced charge density at open circuit as a function of the short-circuit current density. The data are for DSSCs with and without 0.1 M guanidinium thiocyanate in the electrolyte (0.6 M 1-butyl-3-methylimidazolium iodide and 0.03 M iodine in 85:15 (v/v) acetonitrile/valeronitrile). JSC determines Jr at open circuit (eq 1).

Photovoltage Transient Measurements. Recombination times were measured by the transient photovoltage technique detailed elsewhere.13 The samples were probed with a weak laser pulse (pulse duration ∼3 ns) at 540 nm superimposed on a relatively large, background (bias) illumination at 680 nm. The bias light was incident on the cell from the collecting electrode (substrate) side. The short-circuit photocurrent density and opencircuit voltage were measured at a constant bias light intensity. The characteristic time, τR, of the exponential decay of the photovoltage at open circuit induced by the laser pulse (probe light) equals the recombination lifetime of photocarriers at the bias light intensity.1,13 The photoinduced charge density at open circuit is given by the equation13

n ) JSCτR/(qed(1 - P))

(2)

where P is the porosity. The light intensity of the probe light was adjusted so that the collected charge at open circuit induced by the probe light was less than 5% of the steady-state charge as estimated from eq 2. Results and Discussion Figure 5 displays the effect of guanidinium on the photoinduced charge density at open circuit as a function of JSC; JSC is proportional to the recombination current density Jr at open circuit (eq 1). Adding guanidinium had no measurable effect on JSC and, therefore, did not affect the recombination current density (eq 1) at a constant light intensity. However, in light of the discussion of Figure 3, an analysis of the data in Figure 5 reveals that adding guanidinium to the electrolyte of a DSSC slows recombination by a factor of about 20 at the same photoinduced charge density (eq 2). Adding guanidinium did not alter the electron diffusion coefficient. This observation rules out the possibility that variations in the electron diffusion coefficient caused the recombination current density to decrease.13 Therefore, the data indicate that guanidinium suppresses recombination at the TiO2/electrolyte interface (cf.

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Figure 6. Effect of guanidinium on the photoinduced charge density at open circuit over a range of open-circuit voltages. The data are for DSSCs with and without 0.1 M guanidinium thiocyanate in the electrolyte (0.6 M 1-butyl-3-methylimidazolium iodide and 0.03 M iodine in 85:15 (v/v) acetonitrile/valeronitrile).

Figure 3). Using the well-known dependence of VOC on photoinduced charge density1 and the data of Figure 5, we estimate that if the change in recombination were solely responsible for determining VOC, then guanidinium would increase VOC by about 120 mV. To our knowledge, this is the first experimental evidence of a molecular adsorbent that passivates surface recombination sites in DSSCs. Figure 6 shows the effect of guanidinium on the photoinduced charge density at open circuit over a range of open-circuit photovoltages. In view of the discussion of Figure 2, it is seen that adding guanidinium to the electrolyte causes the TiO2 bands to move down by ca. 100 mV at a fixed photoinduced charge density. Using similar arguments and an expression for the surface charge density ∆Q ) C∆V (where ∆V is the change in band-edge potential and C is the Helmholtz capacitance of the TiO2 surface (C ) 10 µF cm-2)) discussed previously,10 we estimate that adding the guanidinium cation resulted in a net gain of positiVe surface charges of about 6 × 1012 cm-2. Because there are roughly 1015 surface atoms/cm2, guanidinium introduces on the order of 6 positive surface charges for every 1000 surface atoms. Thus, a relatively small number of additional surface charges would be sufficient to induce an 100 mV bandedge shift. Furthermore, because the guanidinium cation is very weakly acidic (pKa of 13),21 it is unlikely to increase the net surface charge of TiO2 (isoelectric point of 5.9)22 by protonation. Instead, the guanidinium cation (Figure 1) likely interacts with electron-rich TiO2 surface oxygens via the central carbon atom on which its delocalized positive charge is most strongly associated. Figure 7 shows the overall effect of guanidinium on VOC at several short-circuit photocurrent densities. The collective effect of a downward shift of the conduction band edge of TiO2 by about 100 mV (Figure 6) and slower recombination by a factor of 20 leads to a net improVement in VOC of about 20 mV at a constant light intensity. The combined effect of both the downward band-edge shift and the slower recombination corresponds to the case described in Figure 4B. The net improvement of 20 mV is in good agreement with the estimated 120 mV gain in VOC, owing just to a lower recombination rate, and the 100 mV loss of VOC, resulting solely from a downward band-edge movement.

Kopidakis et al.

Figure 7. Effect of guanidinium on the open-circuit photovoltage over a range of short-circuit photocurrent densities. The data are for DSSCs with and without 0.1 M guanidinium thiocyanate in the electrolyte (0.6 M 1-butyl-3-methylimidazolium iodide and 0.03 M iodine in 85:15 (v/v) acetonitrile/valeronitrile). ∆V denotes the net improvement in VOC at constant light intensity. JSC determines Jr at open circuit (eq 1).

Conclusions The mechanism by which the adsorbent guanidinium affects the open-circuit photovoltage of dye-sensitized TiO2 nanocrystalline solar cells was investigated. Photovoltage transient measurements showed that adding guanidinium to the electrolyte slows recombination by a factor of about 20. Concomitantly, guanidinium was found to cause the TiO2 bands to shift downward, toward positive potentials, by 100 mV. Collectively, both the suppression of surface recombination and the downward shift of the band edges produced a net photovoltage gain of about 20 mV, indicating that surface passivation offsets the unfavorable band-edge movement. This study draws attention to the possibility of enhancing the photovoltage by developing adsorbents that not only shield the surface against recombination but also shift the band edges to negative potentials as depicted in Figure 4D. Acknowledgment. We are grateful to Professor M. Gra¨tzel for valuable discussions. This work was supported by the Office of Science, Division of Chemical Sciences, and the Office of Utility Technologies, Division of Photovoltaics, U. S. Department of Energy, under Contract No. DE-AC36-99GO10337. References and Notes (1) Frank, A. J.; Kopidakis, N.; van de Lagemaat, J. Coord. Chem. ReV. 2004, 248, 1165. (2) Palomares, E.; Clifford, J. N.; Haque, S. A.; Lutz, T.; Durrant, J. R. J. Am. Chem. Soc. 2003, 125, 475. (3) Chen, S. G.; Chappel, S.; Diamant, Y.; Zaban, A. Chem. Mater. 2001, 13, 4629. (4) Wang, P.; Zakeeruddin, S. M.; Comte, P.; Charvet, R.; HumphryBaker, R.; Gra¨tzel, M. J. Phys. Chem. B 2003, 107, 14336. (5) Wang, P.; Zakeeruddin, S. M.; Humphry-Baker, R.; Moser, J. E.; Gra¨tzel, M. AdV. Mater. 2003, 15, 2101. (6) Pelet, S.; Moser, J. E.; Gra¨tzel, M. J. Phys. Chem. B 2000, 104, 1791. (7) Hara, K.; Dan-oh, Y.; Kasada, C.; Ohga, Y.; Shinpo, A.; Suga, S.; Sayama, K.; Arakawa, H. Langmuir 2004, 20, 4205. (8) Nazeeruddin, M. K.; Kay, A.; Rodicio, R.; Humphry-Baker, R.; Mu¨ller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (9) Schlichtho¨rl, G.; Huang, S. Y.; Sprague, J.; Frank, A. J. J. Phys. Chem. B 1997, 101, 8141.

Effects of Guanidinium Cations on DSSCs (10) Neale, N. R.; Kopidakis, N.; van de Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2005, 109, 23183. (11) Zhang, Z.; Zakeeruddin, S. M.; O’Regan, B. C.; Humphry-Baker, R.; Gra¨tzel, M. J. Phys. Chem. B 2005, 109, 21818. (12) van de Lagemaat, J.; Park, N.-G.; Frank, A. J. J. Phys. Chem. B 2000, 104, 2044. (13) Kopidakis, N.; Benkstein, K. D.; van de Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2003, 107, 11307. (14) Wang, P.; Zakeeruddin, S. M.; Humphry-Baker, R.; Gra¨tzel, M. Chem. Mater. 2004, 16, 2694. (15) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Humphry-Baker, R.; Gra¨tzel, M. J. Am. Chem. Soc. 2004, 126, 7164.

J. Phys. Chem. B, Vol. 110, No. 25, 2006 12489 (16) Wang, P.; Humphry-Baker, R.; Moser, J.-E.; Zakeeruddin, S. M.; Gra¨tzel, M. Chem. Mater. 2004, 16, 3246. (17) Huang, S. Y.; Schlichtho¨rl, G.; Nozik, A. J.; Gra¨tzel, M.; Frank, A. J. J. Phys. Chem. B 1997, 101, 2576. (18) Kopidakis, N.; Schiff, E. A.; Park, N.-G.; van de Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2000, 104, 3930. (19) van de Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2001, 105, 11194. (20) Kambili, A.; Walker, A. B.; Qiu, F. L.; Fisher, A. C.; Savin, A. D.; Peter, L. M. Physica E 2002, 14, 203. (21) Vigneron, J.-P. Molecules 1999, 4, 180. (22) Kosmulski, M. AdV. Colloid Interface Sci. 2002, 99, 255.