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Jun 22, 2009 - All the experimental IMPS responses are satisfactorily fitted to the previously developed IMPS model for the bilayer cells at Va = 0 V,...
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J. Phys. Chem. C 2009, 113, 12608–12614

Electric Field Effects on Charge Transport in Polymer/TiO2 Photovoltaic Cells Investigated by Intensity Modulated Photocurrent Spectroscopy Chong Chen,†,‡ Ruixiang Peng,†,‡ Huan Wu,†,‡ and Mingtai Wang*,†,‡,§ Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, P. R. China, Key Lab of NoVel Thin Film Solar Cells, Chinese Academy of Sciences, Hefei 230031, P. R. China, and Department of Materials Science and Engineering, Anhui Institute of Architecture & Industry, Hefei 230022, P. R. China ReceiVed: April 8, 2009; ReVised Manuscript ReceiVed: May 25, 2009

The electric field effects on the charge transport dynamics in bilayer polymer/TiO2 photovoltaic cells are studied by intensity modulated photocurrent spectroscopy (IMPS), for the first time, where a small bias voltage (Va) in the range from -0.1 to 0.2 V is applied to a cell during each IMPS measurement. The bilayer cells consisting of poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and nanostructured TiO2 are constructed by using a polymer layer thickness l of 350 nm but varied TiO2 layer thicknesses (d ) 40 and 65 nm). All the experimental IMPS responses are satisfactorily fitted to the previously developed IMPS model for the bilayer cells at Va ) 0 V, providing a detailed analysis of charge transport properties under the applied voltages. It is revealed that the external electric field almost exclusively changes the exciton dissociation rate (S) at the donor/acceptor (D/A) interface, but imposes only a very faint effect on the transport dynamics of photogenerated electrons [φn(ω), τD and De]. A linear correlation is found for the dependence of S (IPCE), τD (fmin), and De on the applied voltages, which is not changed remarkably by the TiO2 thickness. Fitted results show that, as the voltage is changed from -0.1 to 0.2 V, which provides the change from a reverse electric field Er (Va < 0 V) that sweeps the carriers toward electrodes to a forward electric field Ef (Va > 0 V) that drives the carriers toward the D/A interface, the S values linearly decreased from 130 to 96 cm/s (d ) 40 nm) or 100 to 64 cm/s (d ) 65 nm), while the De values slightly decreased from 9 × 10-5 to 8 × 10-5 cm2/s (d ) 40 nm) or 7.5 × 10-5 to 6.6 × 10-5 cm2/s (d ) 65 nm). The IMPS data suggest that the transport of photogenerated electrons in the bilayer MEH-PPV/TiO2 cells proceeds mainly by diffusion rather than drift, and the transport dynamics is hardly altered by the electric field. Apparently, the field-dependent exciton dissociation at the D/A interface is almost the exclusive determining process for the energy conversion efficiency of bilayer cells. 1. Introduction Polymer-based photovoltaic (PV) cells based on conjugated polymers as the donor (D) and nanocrystals as the acceptor (A) have attracted great attention because of their advantages over conventional Si-based cells, such as low cost, easy processability, and capability to make flexible devices.1-5 The conversion of light into electricity by a polymer-based PV cell generally involves the formation of excitons by photon absorption, the exciton diffusion to the D/A interface where exciton dissociation occurs, and the charge transport within D and A semiconductors to the respective electrodes. Even though there have been dramatic improvements in the operating principles, the energy transfer at the D/A interface, the device construction, and the processing of photoactive layers, a deeper level of mechanistic understanding of the charge generation and transport dynamics, which is crucially important for improving cell performance, is still a challenging issue. Current-voltage (J-V) measurements under applied bias voltages (Va) are commonly used to characterize device performance and to analyze the series and shunt resistances in devices.6,7 Several authors have also studied transport properties * Corresponding author. Tel./Fax: 0086-551-5593171; E-mail address: [email protected]. † Institute of Plasma Physics, Chinese Academy of Sciences. ‡ Key Lab of Novel Thin Film Solar Cells, Chinese Academy of Sciences. § Anhui Institute of Architecture & Industry.

on the basis of the J-V characteristics, providing important information on the exciton dissociation at the D/A interface.8-15 During the J-V measurements, the diffusion current of photogenerated carriers must be balanced by a drift current produced by an applied bias voltage (Va) to achieve zero net current. The experimental current under illumination (Ji) in the J-V experiments is the sum of the photogenerated current (Jph) and the dark current injected by the applied voltage (Jd), that is, Ji ) Jph + Jd.12-15 By considering the dependence of Jph on the applied voltages, it was found that the photocurrent in the bilayer12 and bulk13,14 cells is almost exclusively determined by the field-dependent carrier generation at the D/A interface. To the best of our knowledge, however, no comprehensive understanding of the influences imposed by the applied external voltage on the charge generation and transport dynamics in the polymer-based PV cells has been available. Intensity modulated photocurrent spectroscopy (IMPS), a dynamic photoelectrochemical method, has been successfully used as a powerful tool for investigation of charge generation and transport dynamics in dye-sensitized solar cells (DSCs), and has provided mechanistic insight into the basic PV processes.16-24 Moreover, the IMPS method was also used to investigate the electric field effects on electron transport dynamics in DSCs, across which a bias voltage was sourced during IMPS measurements.16,20 However, only a few reports on the IMPS studies of polymer-based cells have been

10.1021/jp9032645 CCC: $40.75  2009 American Chemical Society Published on Web 06/22/2009

Charge Transport Dynamics in Polymer/TiO2 PV Cells

Figure 1. Geometry of the bilayer device under illumination from the ITO side (a), and the carrier fluxes under illumination due to diffusion and drift (b). The coordinates x ) -d, 0, and l indicate the ITO/TiO2 interface, the D/A interface, and the polymer/Au interface, respectively. The reference direction of the external electric field is given according to the applied voltage with Au and ITO as positive and negative electrodes, respectively. Er and Ef show the actual field directions applied to the charge carrier fluxes as the applied voltage changes from negative to positive.

published.25-27 For a detailed analysis of charge transport properties, the experimental IMPS response needs a fit to a suitable theoretical model, which strongly correlates to the cell configuration. The typical DSCs are normally arranged in a sandwich structure, that is, the space between the dye-sensitized nanoporous TiO2 and counter electrodes is filled with an organic solution containing I-/I-3 redox couple as electrolyte. Essentially different from the DSCs, the structure and electron transport in bilayer polymer/TiO2 PV cells are characterized by the fact that the D/A interface almost only exists near the surface of TiO2 film as a result of the difficult penetration of polymers into the TiO2 layer; the exciton generation occurs mainly in the polymer layer, but the charge separation happens at the D/A interface after a certain time delay (i.e., phase shift) as a result of the exciton diffusion, and the electron transportation proceeds with a strongly minimized charge recombination as a consequence of the spatial separation of electrons and holes.1-5 Recently, we developed a dynamic IMPS model for the bilayer polymer/ TiO2 cells by fully considering those characteristics, and the experimental data confirmed all the main expectations of the model, providing dynamic information on the exciton dissociation at the D/A interface and electron transport dynamics in the TiO2 layer.27 In the present paper, the IMPS method is used on the basis of the model to study the influence of the external electric fields on the charge transport dynamics in the bilayer polymer/TiO2 devices. Our results show that the IMPS model developed for the cells at Va ) 0 V is applicable to the devices to which a small bias voltage is applied, and the external applied electric fields greatly affect the charge separation at the D/A interface but impose a very faint effect on the transport dynamics of photogenerated electrons. 2. Experimental Section Poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) (avg. Mn ) 40000-70000) was purchased from Aldrich. Titanium tetraisopropoxide [Ti(Oi-Pr)4] (Acros, 98+%) was used as the TiO2 precursor. The bilayer PV devices, with a structure of ITO/TiO2/MEH-PPV/Au as shown in Figure 1a, were

J. Phys. Chem. C, Vol. 113, No. 28, 2009 12609 constructed by spinning down first a nanostructured titanium dioxide (TiO2) layer and then a MEH-PPV layer over indium tin oxide (ITO, e15 Ω/0, Wuhu Token Sci. Co., Ltd., China) sheet glass, as described elsewhere.27 The thicknesses of the TiO2 and MEH-PPV films were measured by a field-emission scanning electron microscopy (FESEM, FEI Sirion 200). The effective illumination area of a cell was 0.16 cm2. The steady-state current-voltage (J-V) characteristics and dynamic IMPS responses of the cells were measured on a controlled intensity modulated photo spectroscopy (CIMPS) (Zahner Co., Germany) in ambient conditions, as described previously.27 Briefly, the cells were illuminated through the ITO glass side, with an illuminating light intensity I0 ) 15.85 mW/ cm2, and a small sinusoidal perturbation Iac) I0δeiωt (1.54 mW/ cm2) with a depth of ca. δ ) 10% of the dc light intensity I0 was used in IMPS measurements; during the IMPS measurements, the gold and ITO contacts were taken as positive and negative electrodes, respectively, while the electrodes were reversed in the steady-state J-V measurements. In order to study the electric field effects on the charge transport dynamics, a small bias voltage (Va) sourced from -0.1 to 0.2 V was applied across the gold (positive) and ITO (negative) electrodes during each IMPS measurement, giving a reference direction of the external applied electric field reverse to the x axis in the cell geometry, as illustrated in Figure 1b. At Va ) 0 V, the normal IMPS response is measured at short circuit. As Va is applied, the IMPS response under an applied voltage is recorded. The external electric field across the devices may provide additional driving force for charge transport. As shown in the left part of Figure 1b, when Va < 0 V, the external electric field will make the electrons and holes drift away from the D/A interface [reverse bias for p-n junction], in the same direction as their diffusions. We refer to the electric field due to Va < 0 V as a reverse one (Er) by following the definition for the conventional p-n junction.10,28 As shown in Figure 1b, therefore, a reverse electric field (Er) favors the movement of both electrons and holes toward the respective electrodes; while a forward bias (Va > 0 V) will result in a forward electric field (Ef) that drives the carriers to drift toward the D/A interface in the direction reverse to their diffusions. Obviously, the Er will facilitate the interfacial exciton dissociation and the charge transport to electrodes, while the Ef will impose an opposite effect on these processes. 3. Theory IMPS Model. According to the cell geometry in Figure 1a, under a modulated illumination of I(x,t) ) I0(1+δeiωt), where I0 is a larger steady background illumination level, ω ) 2πf is the variable modulation frequency, and δ , 1, in order to allow the linearization of the system response, the position- and timedependent density p(x,t) of photogenerated excitons at the location x away from the D/A interface where x ) 0 can be described by the continuity equation ∂p(x, t) ∂2p(x, t) p(x, t) ) θI(x, t)Rpe-Rpx + Dp 2 ∂t τp ∂x

(1)

where θ is the quantum efficiency of exciton generation by light absorption, Rp is the absorption coefficient of the polymer as a function of photon wavelength λ, and Dp is the diffusion coefficient of excitons. Under the modulated illumination, the exciton density can be written as p(x,t) ) p0(x) + ∆p(x)eiωt, where p0(x) is the steady-state exciton density and is independent

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of time, and pac(x,t) ) ∆p(x)eiωt represents the ac component of exciton density. On the other hand, the excess electron densities in the conduction band of TiO2 [i.e., n(x,t)] and in the trap states [i.e., N(x,t)] are described by the following continuity equations: ∂n(x, t) n(x, t) ∂2n(x, t) ) De - k1n(x, t) + k2N(x, t) 2 ∂t τe ∂x

(2)

∂N(x, t) ) -k2N(x, t) + k1n(x, t) ∂t

(3)

where τe is the electron lifetime in conduction band, and De)Dcbk2/k1 the effective diffusion coefficient of electrons with Dcb being the diffusion coefficient of electrons in conduction band. k1 and k2 are the first-order rate constants for trapping and detrapping, respectively. Since the exciton dissociation at the D/A interface happens after a certain time as a result of exciton diffusion in the polymer layer, the time delay will manifest itself as a frequencydependent phase shift in IMPS response. Therefore, the electron densities in the conduction band and the trap states can be expressed as n(x,t) ) n0(x) + ∆n(x) exp{i[ωt - φn(ω)]} and N(x,t) ) N0(x) + ∆N(x) exp{i[ωt - φN(ω)]}, respectively, where the first terms are the steady-state electron densities, and the second ones are the ac components of the electron densities; φn(ω) and φN(ω) are the phase shifts due to the time delay between the generation and dissociation of excitons, and the term [ωt - φn(ω)] represents the total phase shift between the photogenerated exciton density and the electron density at position x (-d e x e 0). For simplification, it is given that φn(ω) ) πω/ω0, with ω0 (rad/s) is a constant correlating with the exciton diffusion property in the conjugated polymer. On the other hand, the interfacial dissociation of photogenerated excitons at the D/A interface is crucially important for the PV effect.1-5,12-15,29,30 By ignoring the detailed kinetics of exciton dissociation, it is assumed that the excitons dissociate at the D/A interface with a constant rate S (cm/s), similar to the quenching velocity of excitons at an interface.31 With those assumptions and the following boundary conditions,

Dp

∂∆p(x) ∂x

|

) S∆p(0)

x)0

(4)

∆p(l) ) 0

(5)

) S∆p(0)eiφn(ω)

(6)

) kext∆n(-d)

(7)

De

∂∆n(x) ∂x

|

De

∂∆n(x) ∂x

|

x)0

x)-d

the ac component of the photocurrent is derived from the solutions of eqs 1-3 for the diffusion-limited case at short circuit, as the electron extraction to the ITO substrate at the ITO/TiO2 interface (x ) -d) proceeds with a large enough rate constant kext, that is,

∆j(ω) ) qDe

∂∆n(x) ∂x

|

x)-d

(8)

Figure 2. The IMPS response calculated from eq 9 with the following parameters: d ) 100 nm, l ) 220 nm, De ) 5 × 10-5 cm2/s, S ) 800 cm/s, k1 ) 8 × 105 s-1, k2 ) 1.0 × 103 s-1, R ) 20 Ω, C ) 2 × 10-5 F, and ω0 ) 4 × 104 rad/s. In this case, Va ) 0 V.

where q is the elementary charge. The ac photocurrent conversion efficiency is given by φ(ω) ) ∆j(ω)/qδθI0. The experimentally measured IMPS response Φm is often affected by RC attenuation of the electrode, especially toward higher frequencies. In general, to include the RC attenuation, the measured IMPS response Φm is obtained by multiplying φ(ω) with the attenuation factor F(ω) ) (1+iωRC)-1, that is, Φm(ω) ) φ(ω)F(ω). Thus

Φm(ω) ) 2kextS∆p(0)eiφn(ω) δθI0[(Deβ + kext)e

βd

-βd

+ (kext - Deβ)e

1 ] 1 + iωRC ·

(9)

R is the series resistance, and C is the capacitance of the electrode, and β ){[-ω2 + i(k1 + k2)ω]/[De(k2 + iω)] + (Deτe)-1}1/2. Under short circuit conditions, R and C are mainly due to the conducting glass substrate.16,19,32 Here, the following parameters were taken for the theoretical calculations:27 θ ) 1, kext ) ∞, τp ) 300 ps, Rp ) 105 cm-1, Dp ) 3 × 10-3 cm2/s, and τe ) 0.01 s. Theoretical Expectations. As shown in Figure 2, the shape of the IMPS response on a complex plane can be characterized by Phigh, incident photon-to-current conversion efficiency (IPCE), and fmin points, in which the IMPS response crosses with the positive real axis at high (Phigh point) and low (IPCE point) frequencies. The Phigh point location gives an evaluation of the exciton diffusion effect on the electron transport; the IPCE point gives a direct estimation of IPCE;19,27 the mean transit time of electrons (τD) through the TiO2 layer to the collection electrode correlates with the frequency fmin value of the lowest imaginary component by the relation τD ) (2πfmin)-1. The ω0, electron diffusion coefficient (De) in TiO2 layer, TiO2 thickness (d), and exciton dissociation rate (S) at the D/A interface are the main factors affecting the shape of IMPS responses. A smaller ω0, a smaller d or a larger De leads to the bias of the Phigh point away from the origin of IMPS response on the complex plane, while the IPCE value is sensitive to the exciton dissociation rate S at the D/A interface, and a higher S value leads to a higher energy conversion efficiency. The S-dependence of IPCE agrees with the observations in the J-V measurements, where the cell efficiency is determined by the competition between the exciton dissociation and charge recombination.9,11 The changes in the shape of the calculated IMPS responses have provided the dynamic information on the charge transport in the cells at Va ) 0 V. First, the bias of the Phigh point away from the origin is due to the phase shift φn(ω) effect on the electron transport in the TiO2 layer. A more remarkable φn(ω) effect results from either a larger De in TiO2 or a smaller ω0.

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Figure 4. Dependence of the measured IMPS responses on the applied bias voltage. The cell was configured with d ) 65 nm and l ) 350 nm. The solid symbols identify the fmin points.

Figure 3. Steady-state J-V characteristics of the cells with l ) 350 nm and d ) 40 nm (a) or 65 nm (b) measured under illumination and in dark.

Second, a smaller De results in a smaller fmin, indicating a longer electron transit time τD [) (2πfmin)-1], but both ω0 and S do not impose obvious influences on the fmin value. From the cell geometrical point of view, moreover, a larger d value leads to a longer transit time τD, a smaller S (IPCE), and a weaker φn(ω) effect on electron transport as well. 4. Results and Discussion J-V and IMPS Measurements. Two bilayer devices of ITO/ TiO2/MEH-PPV/Au (Figure 1a) with d ) 40 and 65 nm (l ) 350 nm in both cells) were constructed for experiments. Steadystate J-V measurements were carried out under illumination (I0 ) 15.85 mW/cm2) and in the dark, as shown in Figure 3. The cells exhibited an open circuit voltage (Voc) around 0.70 V and a short circuit current Jsc of 20.41 (d ) 40 nm) and 8.79 (d ) 65 nm) µA/cm2. The Voc value is comparable with that of previous reports on similarly structured devices.8,33,34 However, both Voc and Jsc are a bit lower than our previous data obtained with the same d but a smaller l () 220 nm) value,27 which is attributed to the increased resistances in the cells due to a thicker polymer film.6,8 It is essential to guarantee that the total current under illumination (Ji) during the IMPS measurement exclusively originates from the photogenerated electrons, hence the contribution of dark current (Jd) injected by the applied bias should be eliminated. Known from the J-V measurements, the dark current Jd is at least 3 orders of magnitude lower than the total current Ji within Va ) -0.1 - 0.2 V. It is reasonable that the contribution of the Jd to the measured current Ji during the IMPS measurements of the cells that are applied a voltage in the range of -0.1 to 0.2 V can be ignored, that is, Ji ≈ Jph. Therefore, the changes in the experimental IMPS responses within the voltage range do reflect the applied voltage effects on the transport dynamics of photogenerated electrons. Figure 4 illustrates a typical set of IMPS responses at the bias voltages (Va) from -0.1 to 0.2 V. The Phigh points in the measured IMPS plots locate around 5 × 10-5, indicating an obvious phase shift φn(ω) effect due to the exciton diffusion on the electron transport.27 Note, in the cases of DSCs, the phase shift φn(ω) due to the exciton diffusion to D/A interface is

eliminated because the dye molecules only assemble on TiO2 nanoparticle surfaces in the form of a monolayer, resulting in the Phigh point spiraling into the origin on the IMPS plot at short circuit or when an external voltage is applied.16-19 At the first glance, no remarkable influence on the location of Phigh point imposed by the external voltages across the cell is found (Figure 4). However, detailed inspections (inset of Figure 4) show the Phigh point of the IMPS response actually tends slightly toward the origin as the applied voltage changes from -0.1 to 0.2 V, which provides the change from a reverse (Er) to a forward (Ef) electric field (Figure 1b), indicating a weakened φn(ω) effect as the result of the strengthening Ef field. On the basis of the theoretical expectations (Figure 2), the slightly weakened φn(ω) effect reasonably originates from a slightly reduced De due to the electric field change from reverse (Er) to forward (Ef) direction, because d and ω0 values will not be changed in the same device. From the Va dependence of the Phigh point locations, therefore, it is reasonably suggested that applied external electric field only imposes a very faint effect on the electron transport dynamics (De). From the measured IMPS responses, we obtained the dependence of IPCE and fmin on the applied voltages. The Va dependences of the IPCE and fmin values in the different bilayer polymer/TiO2 cells are shown in Figure 5. The IPCE values obtained from the dynamic measurements at Va ) 0 V are 0.29% for d ) 40 nm and 0.22% for d ) 65 nm, and they are close to the data obtained from the steady-state J-V measurements, that is 0.33% (d ) 40 nm) and 0.15% (d ) 65 nm). As the external voltage Va was sourced from -0.1 to 0.2 V, both fmin and IPCE were reduced linearly with the strengthened Ef field. Interestingly, even though the cells are different in the thickness of TiO2 layer (d ) 40 and 60 nm), the Va dependence of either fmin or IPCE is almost identical, as indicated by the slopes of the linear regression lines. Within the tested Va range, an increase in TiO2 thickness d leads to a smaller fmin (or longer τD) and a lowered IPCE, which is consistent with the previous data.27,34 On the basis of the relation τD ) (2πfmin)-1, it is easy to derive, by calculation and regression analysis, that τD values for the two cells increase slightly with the strengthened Ef field by the gradients of 2.8 × 10-3 s/V (d ) 65 nm) and 1.9 × 10-3 s/V (d ) 40 nm), indicating that the applied electric field only imposes a faint effect on the electron transport dynamics, which agrees with the information from the Phigh points (Figure 4). The dependence of IPCE on the applied voltage Va has also been observed in DSCs16 and in the three-dimensional (3D) cells of TiO2 and CuInS2.20 Dloczik and co-workers16 observed that, as the system moves from short circuit condition along the J-V curve toward open circuit (i.e., Va ) 0 - 0.7 V), the IPCE of typical DSCs gradually decreases with Va. Grasso and coworkers20 found in the 3D-structured TiO2/CuInS2 cells, which

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Figure 5. Dependences of the measured IPCE (a) and fmin (b) on the applied bias voltage, obtained from the IMPS measurements. The cells were configured with l ) 350 nm and different d values. The symbols are the experimental data, and the lines were calculated by linear regression for those symbols. The numeral marked above each line is the line slope.

are in principle similar to typical DSCs with CuInS2 bringing together the functions of dye-sensitizer and hole transporter, that the IPCE values change with the applied external voltage (Va ) -0.3, 0, and 0.3 V) in the order 0.3 V < 0 V < -0.3 V (Note, the field directions across the 3D cells were not specified). Even though the origins of the Va-dependent IPCE is not clear in the previous reports,16,20 the fact that the external electric fields impose a great impact on the device IPCE is clearly demonstrated. Exciton Dissociation and Electron Diffusion Coefficient. The experimental IMPS responses were fitted to eq 9, as described previously.27 Except for the De and S values, the parameters (i.e., Rp, Dp, τp, τe, k1, k2, ω0, kext, R, and C) for fitting each cell under the applied voltages were same as those in the case of Va ) 0 V. Figure 6 shows two examples for the fitting processes. The Bode plots of the measured IMPS responses were satisfactorily fitted over the whole frequency range. The fits gave values around 70-80 µF for C and 30-40 Ω for R, in a good accordance with the findings of others.16,19,32 For the cells with the varied d values, the RC effects differ to a certain extent; this is due to the TiO2 thickness influence on the resistance and capacitance of the electrode.6,27 Moreover, the fitted ω0 values for d ) 40 nm (ω0 ) 2.36 × 104 rad/s) and 65 nm (ω0 ) 3.14 × 104 rad/s) also differ a little bit from each other, which is plausible because ω0 correlates with the phase shift φn(ω) () πω/ω0) effect on the electron transport; the slightly weakened φn(ω) effect as a result of the increased d leads inevitably to a bit larger ω0 in calculation (Figure 6). On the other hand, it is reasonable to take kext ) ∞ for the cells with the applied voltages, the same as in the case of Va ) 0 V. Since the contact between the ITO electrode (workfunction -4.8 eV)35 and TiO2 (quasi-Fermi energy level -4.2 eV)8 is nearly Ohmic, because of the lack of energy barrier for electrons to surmount in the transfer from TiO2 to ITO,36 resulting in a large enough kext, and the measured Va dependence of the Phigh location and τD (Figures 4 and 5b) has preliminarily indicated that almost no or only a very faint effect is imposed by the applied electric field on the transport dynamics of photogenerated electrons, the

Chen et al.

Figure 6. The measured and fitted IMPS responses in the form of a Bode plot for the cells with l ) 350 nm and different d values. (a) The cell with d ) 40 nm at Va ) -0.1 V, and the fitting parameters are De ) 9 × 10-5 cm2/s, S ) 129 cm/s, R ) 30 Ω, C ) 8 × 10-5 F, and ω0 ) 2.36 × 104 rad/s. (b) The cell with d ) 65 nm at Va ) 0.2 V, and the fitting parameters are De ) 6.6 × 10-5 cm2/s, S ) 64 cm/s, R ) 40 Ω, C ) 7 × 10-5 F and ω0 ) 3.14 × 104 rad/s. Other parameters for the fitting calculations in both cases are k1 ) 3 × 105 s-1 and k2 ) 8.5 × 102 s-1.

transport can still be regarded as a diffusion-limited case with a large enough kext under the small applied voltages.16,27 Shown in Figure 7 are the Va dependences of S and De values obtained by fitting the experimental IMPS responses of the different bilayer MEH-PPV/TiO2 cells under different applied voltages to eq 9. Analogous to the measured fmin and IPCE (Figure 5), S and De also exhibit a good linear dependence on the applied voltages, and the Va dependence of both S and De is almost identical in the differently configured cells. The fits provide S values of 64-130 cm/s within the applied voltage range; those data are comparable to the quenching velocity (144 cm/s) of the excitons produced in perylene bis(phenethylimide) (PPEI) at the interfaces between PPEI and nonquenching substrates, but much smaller than the exciton quenching velocity of ca. 106 at the interfaces between PPEI and quenching substrates.31 Within the applied voltage range of -0.1 to 2 V, we obtain De values of 6.6 × 10-5 - 9 × 10-5 cm2/s, which agrees with our previous results27 and those obtained in typical DSCs.16,19,32 Surprisingly, the Va dependence of the S value is about 2 orders of magnitude higher than that of De, as indicated by the slopes of the linear regression lines. Therefore, the external electric filed influences very slightly the transport dynamics (De) of photogenerated electrons, but almost exclusively changes the interfacial exciton dissociation rate S. The faint Va dependence of De agrees well with the results derived from the Phigh point (Figure 4) and τD or fmin (Figure 5b). Note, Grasso and co-workers20 also found no obvious electric field effect on the electron transport dynamics in the 3D-structured TiO2/CuInS2 cells, for which the field screening from CuInS2 is suggested, similarly to that from the ionic electrolyte in typical DSCs. On the other hand, the high Va dependence of S is consistent with the conclusion that the photocurrent in the bilayer12 and bulk13,14 cells is almost exclusively determined by

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J. Phys. Chem. C, Vol. 113, No. 28, 2009 12613 5. Conclusion

Figure 7. Dependences of the exciton dissociation rate S (a) and the effective electron diffusion coefficient De (b) on the applied bias voltage for the cells with l ) 350 nm and different d values. The data were obtained by fitting the experimental IMPS responses, in which the fitting parameters for each cell are the same as those in Figure 6. The lines were calculated by linear regression for those data, and the numeral marked above each line is the line slope.

the field-dependent carrier generation at the D/A interface. Therefore, the normally low IPCE of bilayer MEH-PPV/TiO2 cells is very likely due to the rather low exciton dissociation efficiency at the D/A interface,8,33,34 even thought other factors may also be responsible for the low efficiency.26 Known from Figure 7, an increase in TiO2 thickness d reduces S and De. As is previously shown, the accumulation of the space charge due to the low mobility significantly impacts the photocurrent and power conversion efficiency of PV cells.14,25,37,38 The d dependence of the S and De values may be understood by the trapping of electrons in TiO2. More trapping processes will exist in a thicker TiO2 layer, and the multiple trapping effect on the electron transport dynamics will magnify itself to reduce De and prolongs the transit time for charge collection.39,40 As a result of the increase in TiO2 thickness, therefore, a more serious accumulation of electrons in the TiO2 layer will occur to impede the charge travel away from the D/A interface and consequently reduce the S value.27 As is well recognized, the transport of photogenerated charge carriers to the electrodes can be by diffusion alone,41 or may be assisted by an internal macroscopic electric field (Ebi) builtin by the difference between the electrode work functions.42-44 Our dynamic experimental data from the IMPS measurements suggest that the transport of photogenerated electrons in bilayer MEH-PPV/TiO2 cells proceeds mainly by diffusion rather than drift, and the Ebi only contributes very slightly to the charge transport property by assisting the charges to transport toward their correct electrodes. Otherwise, if the field Ebi had an remarkable action on the electron transport in our cases, it would make the electrons drift to the lower function electrode (ITO)8,45 and suppress the charge recombination,46 and the electron transport dynamics would accordingly be changed noticeably upon applying a field Ef, which evidently contradicts the IMPS observation that τD or De only changes very slightly with the applied voltages (Figures 5 and 7).

The effects of the electric fields on the charge transport dynamics in bilayer MEH-PPV/TiO2 PV cells are studied by the IMPS method. It is revealed that the external electric field almost exclusively changes the interfacial exciton dissociation rate (S) at the D/A interface, but imposes only a very faint effect on the transport dynamics [φn(ω), τD, and De] of the photogenerated electrons. A linear correlation is found for the dependence of S (IPCE), τD (fmin), and De on the applied voltages, and it is not changed remarkably by the TiO2 thickness d. As the electric field changes from the reverse Er that sweeps the carriers toward electrodes to the forward Ef that drives the carriers toward the D/A interface, the S decreases remarkably with increasing the forward bias voltage. Even though very faintly, the change from Er to Ef does reduce the effective electron diffusion coefficient De to a certain extent, consequently weakening the phase shift φn(ω) effect on the electron transport but increasing the electron transit time τD in the TiO2 layer. Furthermore, the IMPS data suggest that electron transport in the bilayer MEH-PPV/TiO2 cells proceeds mainly by diffusion rather than drift, and the internal macroscopic electric field due to the workfunction difference between the electrodes only contributes very slightly to the charge transport property by helping the charges transport toward their correct electrodes. The results summarized in this paper demonstrate that IMPS can be a useful tool for the investigation of the charge generation and transport in bilayer polymer/TiO2 devices, even under a small applied voltage. Acknowledgment. This work was supported by the “100Talent Program” of the Chinese Academy of Sciences, the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, and the National Natural Science Foundation of China (No. 20474066). The authors acknowledge the referees involved for their generous advice on revision. References and Notes (1) Coakley, K. M.; McGehee, M. D. Chem. Mater. 2004, 16, 4533– 4542. (2) Gu¨nes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. ReV. 2007, 107, 1324–1338. (3) Thompson, B. C.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. 2008, 47, 58–77. (4) Saunders, B. R.; Turner, M. L. AdV. Colloid Interface Sci. 2008, 138, 1–23. (5) Boucle´, J.; Ravirajan, P.; Nelson, J. J. Mater. Chem. 2007, 17, 3141– 3153. (6) Moliton, A.; Nunzi, J.-M. Polym. Int. 2006, 55, 583–600. (7) Waldauf, C.; Scharber, M. C.; Schilinsky, P.; Hauch, J. A.; Brabec, C. J. J. Appl. Phys. 2006, 99, 104503. (8) Breeze, A. J.; Schlesinger, Z.; Carter, S. A.; Brock, P. J. Phys. ReV. B. 2001, 64, 125205. (9) Barker, J. A.; Ramsdale, C. M.; Greenham, N. C. Phys. ReV. B 2003, 67, 075205. (10) Koehler, M.; Roman, L. S.; Ingana¨s, O.; da Luz, M. G. E. J. Appl. Phys. 2004, 96, 40–43. (11) Rand, B. P.; Burk, D. P.; Forrest, S. R. Phys. ReV. B 2007, 75, 115327. (12) Yin, C.; Piepar, B.; Stiller, B.; Kietzke, T.; Neher, D. Appl. Phys. Lett. 2007, 90, 133502. (13) Mihailetchi, V. D.; Koster, L. J. A.; Hummelen, J. C.; Blom, P. W. M. Phys. ReV. Lett. 2004, 93, 216601. (14) Blom, P. W. M.; Mihailetchi, V. D.; Koster, L. J. A.; Markov, D. E. AdV. Mater. 2007, 19, 1551–1566. (15) Marsh, R. A.; McNeill, C. R.; Abrusci, A.; Campbell, A. R.; Friend, R. H. Nano Lett. 2008, 8, 1393–1398. (16) Dloczik, L.; Ileperuma, O.; Lauermann, I.; Peter, L. M.; Ponomarev, E. A.; Redmond, G.; Shaw, N. J.; Uhlendorf, I. J. Phys. Chem. B 1997, 101, 10281–10289.

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