Measurements of Efficiency Losses in Blend and Bilayer-Type Zinc

Jul 3, 2012 - The results obtained by using time-resolved techniques such as charge extraction (CE) and photoinduced transient photovoltage (TPV) have...
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Measurements of Efficiency Losses in Blend and Bilayer-Type Zinc Phthalocyanine/C60 High-Vacuum-Processed Organic Solar Cells Antonio Sánchez-Díaz,† Lorenzo Burtone,‡ Moritz Riede,*,‡ and Emilio Palomares*,†,§ †

Institute of Chemical Research of Catalonia (ICIQ), Avenida Països Catalans, 16, Tarragona E-43007, Spain Institut für Angewandte Technische, Universität Dresden, 01062 Dresden, Germany § ICREA (Catalonian Institution for Advanced Research and Studies), Avenida Lluís Companys, Barcelona, E-08010, Spain ‡

S Supporting Information *

ABSTRACT: Losses of charge carriers, due to the interfacial charge recombination processes, in small molecule organic solar cells (SMOSCs) have been investigated under operating conditions. The devices consist of zinc phthalocyanine (ZnPc) as electron donor material and C60 as electron acceptor. The results obtained by using time-resolved techniques such as charge extraction (CE) and photoinduced transient photovoltage (TPV) have been compared to the measurements carried out with impedance spectroscopy (IS) and show good agreement. Significantly, much difference is observed in either the charge density distribution versus the device voltage or the charge carriers lifetime when comparing bulk heterojunction versus bilayer-type ZnPc:C60 devices. The implications of the faster charge carrier recombination with the device fill factor (FF) and the open circuit voltage (VOC) are discussed.

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INTRODUCTION Understanding the electron transfer reactions taking place at the interfaces of the photoactive materials in organic solar cells is a key element to minimize the power conversion efficiency losses.1 During the recent years, much effort has been focused on optimizing materials and molecules to increase the device efficiency. However, only few groups worldwide are capable of achieving certified light-to-energy conversion efficiencies higher than 8% in a reproducible manner.2−4 A step forward to increase the solar cell efficiency is to analyze the processes that limit the device operation under working conditions and, moreover, to understand the close relationship between the interfaces between both photoactive materials and the charge transfer reactions at the nanoscale.5−11 Here, we present two different organic solar cells prepared by the high-vacuum deposition technique and using zinc phthalocyanine (ZnPc) and the fullerene C60 as electron donor and electron acceptor molecules, respectively. Both stacks have identical thickness and metal electrodes; they only differ in the organic layer morphology. The first device has a bialayer structure, with the acceptor evaporated on top of the donor material. The resulting solar cell is composed of a planar heterojunction (PHJ). The second structure is based on a bulk heterojunction (BHJ). This means that both the donor and the acceptor molecules are evaporated together, and the organic active layer presents a blended structure. To avoid a direct contact between the blend layer and the metal contacts, 5 nm of pristine materials is used on both sides, as depicted in Figure 1b. The pristine layers have the function of preventing © 2012 American Chemical Society

Figure 1. Photocurrent−voltage curves for the ZnPc:C60 BHJ (red dots) and the bilayer ZnPc/C60 (PHJ) (blue squares) solar cells. Received: June 4, 2012 Revised: July 3, 2012 Published: July 3, 2012 16384

dx.doi.org/10.1021/jp3054422 | J. Phys. Chem. C 2012, 116, 16384−16390

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Scheme 1. The Device Structures for the PHJ Organic Solar Cellsa

a

(a) ITO/ZnPc(30 nm)/C60(30 nm)/Al and for the BHJ solar cells; (b) ITO/ZnPc(5 nm)/ZnPc:C60(1:2)(50 nm)/C60(5 nm)/Al.

changes in the HOMO−LUMO energy levels when photoactive layers are prepared in blend or bilayer configuration, the observed difference in VOC cannot be explained only in such way. Moreover, Manca and co-workers16 among others18 reported that for a series of semiconductor polymers, the observed VOC can be correlated with the value of the band resulting from the formation of a charge transfer state between the semiconductor polymer and the electron acceptor molecule. In all cases, they used a derivate of the C60 molecule (PCBM: [6,6]-phenyl-C61-butyric acid methyl ester). Taking into account that in our experiment both devices have identical electron donor/electron acceptor materials, a variation of the energy levels due to the different morphology of the active layer is not able to justify alone the large difference in VOC reported in Figure 1. Regarding the fill factor (FF) of the current−voltage characteristics, a significant difference between the two devices is also observed. On one hand, PHJ devices show a FF of 56%, while, in clear contrast, the BHJ solar cells only achieve FF of 30%. This difference is directly correlated with the efficiency in the extraction of photogenerated charges, and an explanation for the differences in VOC and FF between BHJ and bilayer devices observed in this work is the argument of the following discussion. Thus, to make a proper comparison of the density of photogenerated charges in both types of devices, the CE technique can be employed, as previously described by our group among others.19−21 In brief, this technique consists of applying white light to the device for a sufficient period to reach steady-state conditions, usually 5−10 s, keeping the solar cell in open circuit conditions. Thereafter, the device is switched to short circuit, within less than 100 ns, so that the total accumulated charges can flow through the external circuit connected to the solar cell. The flowing current is measured over time, and the measured decay is integrated to obtain the total extracted charge that was accumulated in the device. In our setup, the extracted current is measured using a 40 Ω precision resistance in the discharging circuit. By integrating the voltage drop over the resistor, the charge can be obtained applying eq 1:

the acceptor (or donor) molecules from being in contact with the holes-collecting (electrons-collecting) contact, with consequent loss of performance12 Scheme 1 illustrates the device structure of the two organic solar cells. Our aim in this letter is to investigate the charge recombination process of the electrons and holes after the exciton dissociation at the donor/acceptor interface in complete working devices under illumination conditions. As the devices differ only for the active layer morphology (BHJ versus bilayer structure), we focus on the differences in the carrier recombination lifetime and/or in the distribution of charge carriers at different open circuit conditions. For our first objective, the use of transient photovoltage measurements (TPV) makes it possible to obtain information about the charge carrier lifetime, at different light-induced photovoltages. Second, employing the photoinduced charge extraction technique (CE), the charge carrier density in the device under different light illumination intensities can be estimated. The combination of both measurements permits one to establish a relation between the charge density accumulated in the solar cell and the recombination mechanisms involved during device operation.13,14 The results are validated by obtaining similar conclusions with the impedance spectroscopy (IS) methodology, but the data acquisition time is reduced to minutes in the case of TPV+CE.

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RESULTS AND DISCUSSION Figure 1 shows the photocurrent versus voltage curves under 100 mW/cm2 sun-simulated light irradiation. All devices were prepared by a high-vacuum evaporation process and subsequently encapsulated as previously reported.15 From Figure 1 it is also observed that the BHJ photoactive layer provides a higher short circuit current (4.2 mA) in comparison to the PHJ devices (2.9 mA), as expected from the larger interface present in the BHJ structure.12 On the contrary, the open circuit voltage is higher for the PHJ solar cell that shows at 1 sun a value of VOC = 469 mV, whereas the blend has only a VOC of 197 mV. It is generally assumed that for organic solar cells, the theoretical maximum VOC is given by the highest occupied molecular orbital to lowest unoccupied molecular orbital (HOMO−LUMO) energy difference between the electron donor (ZnPc) and the electron acceptor (C60) material.16 In real devices, the obtained VOC is lower than the theoretical value, and the origin of the open circuit voltage is still a question of active debate.9,12,17 Although previous studies in small molecule organic solar cells (SMOSCs) evidenced

Q=

1 R

∫t

t

v (t ) d t 0

(1)

where R is the resistance connected to the circuit, Q is the charge, and v(t) is the measured voltage. It is necessary to point out that this technique can be used in devices where the recombination kinetics is slow enough to permit a complete extraction of charges, because the discharging process in the external circuit is in competition with the internal recombina16385

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open-circuit conditions, and a white light source is used to induce the desired VOC value. While keeping the device in this steady-state condition, a short light pulse (Δt < 5 ns) is superimposed to the bias light, perturbing the equilibrium. The charges generated by this additional stimulus recombine until steady state is reached again. If the perturbation is small enough, the charge density inside the device is not significantly changed. Thus the decay time of the additional charges can be considered as a characteristic time of the recombination process at this particular VOC. Repeating the voltage decay measurement with different light intensities, it is possible to obtain a characteristic time for various light induced biases. From the CE measurements, VOC is correlated also with the accumulated charges in the device. Combining the two measurements leads to the recombination time as a function of accumulated charges, as presented in Figure 3. As illustrated in Figure 3, at the same charge density, the recombination lifetime for the ZnPc:C60 BHJ device is much faster than that for the ZnPc/C60 bilayer solar cell. More important is the correspondence between charge density and carrier lifetime observed for ZnPc:C60 BHJ devices in comparison with the correlation observed for the ZnPc/C60 bilayer ones. The data suggest that the recombination dynamic for ZnPc:C60 BHJ devices is much faster than in the case of the ZnPc/C60 bilayer organic solar cells. One consequence is that the charge's lifetime dependence on the carrier density is a crucial aspect to be optimized in order to control the charge recombination and improve the ZnPc:C60 BHJ efficiency. Our data suggests that the use of selective materials deposited at the metal contacts before and after the photoactive BHJ layer will help reducing recombination dynamics. The observed slow recombination dynamics for the bilayer devices may be understood considering the lower interfacial area present in these devices, which leads to rapid transport of charges across the pure phase of the organic material in the PHJ structure, avoiding nongeminative recombination losses. This increasing carrier lifetime was also observed for organic small molecule based solar cells with good FF, a key parameter for the overall light-to-energy conversion efficiency. More recently, Ziehlke et al.7 analyzed the charge carrier in SMOSCs and confirmed, by using photoinduced absorption spectroscopy (PIA), the coexistence of slower carrier recombination kinetics with the increase of the FF in the solar cells. Hence, the data shown in Figure 3 is in good agreement with these independent measurements, as the ZnPc/C60 bilayer-type devices showed much higher FF than the BHJ device. To confirm the results obtained by CE and TPV techniques, the same devices are also investigated using IS. The measurement consists of applying a constant bias voltage to set the working point of the device and then superimpose a small sinusoidal signal (10 mV rms) at one single frequency. The response current is then measured, and the impedance function calculated. In this case, the cell is illuminated, and then the bias voltage is set at the open-circuit value. Repeating the measurement for different illumination intensities, a voltage scan of the device response is obtained. For every open circuit voltage value, an impedance arch is measured, and by the appropriated fitting procedure, the values of the equivalent circuit are estimated. In the simplest case, a solar cell can be modeled by one resistance due to the contacts, connected in series with a parallel RC circuit, as shown in Scheme 2. The capacitance takes into account the overall accumulated charges either at the metal contacts or in the photoactive layer.

tion charges. In our case, the recombination dynamics in the BHJ solar cell at 1 sun (device open circuit voltage of 200 mV) are at least 5 times slower than the CE decay (please see Supporting Information). However, we observed that in case of BHJ solar cells, for VOC values below 175 mV, the total extracted charge is below the value expected for a device with a geometrical capacitance of 5nF. This observation implies that, for the BHJ devices, the amount of accumulated charges at low light intensities is below the minimal resolution of our setup, and it is not possible to estimate them with sufficient accuracy when extracted. The measurement accuracy is improved in the case of higher light-intensity because of the higher number of photogenerated charges in the device, as shown in Figure 2.

Figure 2. Distribution of charge density for different light-induced photovoltage in the BHJ device (red circles) and the PHJ solar cell (blue squares). The solid line shows the device charge corresponding to a geometrical capacitance Cg = 5 nF. The gray circles are values for CE measurements at low illumination densities.

For the blend active layer, a severe increase of charge density is observed with the light intensity. On the contrary, for the flat heterojunction device, the charge carrier density grows linearly with voltage in the range from 0 V to 0.35 V. Nonetheless, for voltage values near VOC (from 0.35 V to 0.5 V), the charge density does not follow the previous linear trend. The structure of the BHJ solar cell, allows a higher light absorption in comparison to a planar structure. However, the charge carriers’ density observed at low light intensities is in contrast with the expectations. This result suggests that the charge carrier lifetime plays a significant role in the overall accumulated charge carriers density. We also expect that the recombination dynamic limits both the VOC and the FF of the BHJ solar cell. In fact, as several research groups already demonstrated for organic solar cells, the efficiency losses due to the bimolecular charge recombination reactions are directly correlated to the accumulated charge and also have an important effect over the device VOC.9,22 To demonstrate these hypotheses, TPV measurements are carried out. The devices are connected to an oscilloscope in 16386

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Figure 3. Charge recombination lifetime for BHJ devices (red circles) and bilayer solar cells (blue squares).

Scheme 2. Equivalent Circuit of a Solar Cell Used in the Fitting Procedure of the Measured IS

Table 1

The resistance in parallel to the capacitance can be considered a recombination resistance, because it models the current flow inside the device. The characteristic time of this RC circuit (τ = R·C) is an effective carrier lifetime of the charge density. Moreover, from the capacitance value, it is possible to estimate the total accumulated charge, by multiplying it for the applied bias voltage (Q = C·V). The advantage of using IS to estimate the accumulated charge in the device is represented by the absence of errors due to fast recombination, being IS is a steady-state measurement. Thus, the combination of IS and CE +TPV confirms (a) that both measurements can be used independently to measure efficiency losses due to interfacial charge recombination processes in the device under working conditions and (b) that CE is a valuable technique as long as the recombination kinetics are slower than the extraction time (in the order of some hundred nanoseconds). Table 1 shows the equivalent circuit values resulting from the fitting procedure. As expected, the RC time constant increases for decreasing illumination intensities, because fewer charges are accumulated at the device. A comparison between IS and CE-TPV results is presented in Figure 4. As it can be seen in Figure 4, the results are in good agreement for both techniques with some discrepancy in the results obtained for the BHJ solar cells at low illumination intensities (VOC < 175 mV). As already discussed introducing

bias (mV)

RS (Ω)

441 437 429 414 362 309 283 183

72.53 72.79 72.98 73.01 72.08 71.68 71.57 70.34

175 168 163 154 131 99 50

94.87 96.10 93.33 92.35 88.37 84.45 81.98

CP (nF)

RP (kΩ)

τ (μs)

Flat Heterojunction Solar Cell 8.01 0.553 4.43 7.90 0.660 5.21 7.64 1.050 8.02 7.32 2.186 16.0 6.80 9.100 61.9 6.54 26.19 171 6.45 44.67 288 6.19 280.7 1740 BHJ Solar Cell 5.80 0.229 1.33 5.72 0.256 1.47 5.42 0.301 1.63 5.16 0.411 2.12 4.74 0.879 4.17 4.48 2.996 13.4 4.34 67.44 292

n (1016cm−3) 5.65 5.53 5.25 4.85 3.94 3.24 2.92 1.82 1.62 1.54 °1.42 1.27 0.99 0.71 0.35.

Figure 2, the accumulated charges can not be less than the value given by the geometrical capacitance of the device. For that reason, the CE results can be complemented with the values obtained by IS in the low light intensity region. Moreover, although the carrier recombination lifetime seems faster from the IS measurements, it is important to highlight that this discrepancy is within the measurement error of the CE experimental setup, and the use of the logarithmic scale emphasizes the differences between the results. On the basis of the agreement on the results obtained by using both techniques, the observed differences in device opencircuit voltage between BHJ and bilayer ZnPc-C60 solar cells are now discussed. Although both solar cells are based on the same acceptor and donor materials, the different stack design directly controls the recombination dynamics of photogenerated charges. In PHJ devices at the donor/acceptor interface, the excitons are 16387

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Figure 4. A comparison of (a) charge density versus light bias and (b) the charge recombination lifetime for BHJ devices (red circles) and bilayer solar cells (blue squares) using IS (empty symbols) and the TPV (filled symbols). The solid line indicates the estimated contribution from the cell geometric capacitance (C = 5 nF).

dissociated, and free charges are generated and spatially separated at the same time. This charge carriers’ generation mechanism leads to a shift of the Fermi levels from the intrinsic level in the acceptor and donor layers. The difference between the acceptor and donor Fermi levels defines the VOC. In a BHJ, the exciton generation and dissociation take place in the whole active layer volume and is not spatially separated from the charge carrier tansport. As a consequence, in the blend active layer, both photogenerated electrons and holes are present and need to be transported to the respective contact. From the blend, the photogenerated charges are injected in the

5 nm pristine layers, where only one species of charge carriers is present and the nongeminate recombination prevented. The charge carrier extraction in BHJ solar cells can be considered an efficient process, as confirmed by results in the literature.13,14,17,19,23 Nevertheless, we have shown here that the charge carrier lifetime is strongly reduced in a blend structure when compared with the planar case. This observation can be explained considering also recombination between the charges present in the 5 nm pristine layers and in the blend. Since the spacers between the blend and the contacts are only 5 nm thick, charge carriers accumulated in these layers can easily recombine 16388

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The IS measurements were performed at room temperature with an Autolab PGSTAT302N. The probe signal had an amplitude of 10 mV(rms), and the frequency was varied from 10 Hz to 1 MHz. The complex impedance fitting was performed using the commercial software Zview(R).

with the ones photogenerated in the blend close to the contacts. Charge accumulation in the spacer layers is due to an extraction barrier at the organic/metal interface. This barrier does not play a significant role in the PHJ because the charges stay spatially separated. This explanation is also supported by the results presented in literature, showing that for similar BHJ solar cells, the presence of thicker spacers or doped transport layers prevents losses in the VOC.24,25

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ASSOCIATED CONTENT

S Supporting Information *

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Comparison of TPV and CE decays at 1 sun and the linear representation of the electron lifetime vs the electron density plot.This material is available free of charge via the Internet at http://pubs.acs.org.

CONCLUSIONS Two organic solar cells with different active layer design are compared. The presence of a BHJ structure leads to a higher photogenerated current, but a lower FF and VOC are observed. The use of TPV in combination with CE technique allows for a quantitative comparison of the recombination dynamic. The data are compared with the results obtained by an independent measurement as IS, resulting in a good agreement between the two techniques. It is shown that in a BHJ solar cell, the photogenerated charge carrier lifetime is significantly lower than in a planar structure (PHJ). The strong recombination contrasts the photogenerated charges extraction to the contacts and is translated into a low FF and a small VOC. This is therefore an important aspect for improving the solar cells performance. Yet, the nature of such charge recombination reactions in the BHJ devices measured in this work (for example, if they are in the bulk of the BHJ photoactive layer or between the charges accumulated at the electrodes and the BHJ layer) cannot be inferred from the experimental data on this paper and is a open question for further discussion in the future. Indeed, complementary studies as the ones carried out by Lane et al.26 will further complete the picture of the charge recombination processes in organic small molecule solar cells. Finally, we would like to remark that the possibility to access the same information with different measurements represents an important step to validate and correctly interpret the measured results.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS E.P. would like to acknowledge the financial support from ICIQ, ICREA, and the Spanish MICINN Projects CTQ201018859 and CONSOLIDER CDS-0007 HOPE-2007. E.P. also thanks the EU for the ERCstg Polydot and the Catalan government for the 2009-SGR-207 project. M.R. and L.B. would like to thank the BMBF for funding in the framework of the OPEG project (13N9720).

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EXPERIMENTAL SECTION Devices were built on glass/indium tin oxide (ITO) substrates (Thin Film Devices, USA). Two identical structures are investigated differing only for the active layer morphology, composed by a zinc-phthalocyanine (ZnPc, TCI Europe)/C60 (Bucky USA) heterojunction. The ZnPc and C60 were purified at least twice using train sublimation, and the evaporation was performed under high vacuum conditions (