Influence of the Acceptor on Electrical Performance and Charge

Feb 5, 2014 - Konkuk University-Fraunhofer ISE Next Generation Solar Cell Research Center (KFnSC), Konkuk University, 120 Neungdong-ro, Gwangjin-gu, ...
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Influence of the Acceptor on Electrical Performance and Charge Carrier Transport in Bulk Heterojunction Solar Cells with HXS‑1 Helene Ahme,†,‡ Myounghee Lee,‡ Chan Im,‡,§ and Uli Würfel*,†,∥ †

Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstr. 2, 79110 Freiburg, Germany Konkuk University-Fraunhofer ISE Next Generation Solar Cell Research Center (KFnSC), Konkuk University, 120 Neungdong-ro, Gwangjin-gu, Seoul 143-701, Korea § Department of Chemistry, Konkuk University, 120 Neungdong-ro, Gwangjin-gu, Seoul 143-701, Korea ∥ Freiburg Materials Research Centre (FMF), University of Freiburg, Stefan-Meier-Str. 21, 79104 Freiburg, Germany ‡

S Supporting Information *

ABSTRACT: Enhancing the open-circuit voltage (VOC) is one way to increase the efficiency of organic solar cells. In cells with the polymer poly(2-(5-(5,6-bis(octyloxy)-4-(thiophen-2yl)benzo[c][1,2,5]thiadiazol-7-yl)thiophen-2-yl)-9-octyl-9H-carbazole) (HXS-1) this can be achieved by replacing the acceptor [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM) with indene-C60-bis-adduct (ICBA). The lowest unoccupied molecular orbital (LUMO) of ICBA is located at a higher energy, which leads to an increase of VOC from 0.86 to 1.05 V. However, the short-circuit current density (JSC) and fill factor (FF) are significantly lower in HXS-1:ICBA cells when compared with HXS-1:PC71BM cells, and thus the overall efficiency drops from almost 5% to 2.5%. Despite the smaller LUMO−LUMO offset between HXS-1 and ICBA, strong photoluminescence quenching as well as transient absorption studies indicate efficient and fast exciton dissociation in cells with either fullerene. The slope of the current−voltage characteristics of HXS1:ICBA cells at short-circuit conditions and the lower dark current in forward direction suggest poor charge carrier transport. These findings were reproduced by a reduction of the electron mobility in electrical simulations. Furthermore, results from Suns-VOC measurements reveal a dramatically increased transport resistance in cells with ICBA when compared with devices using PC71BM. The observed effects could at least be partially due to a finer morphology in the HXS-1:ICBA layer, which is supported by AFM images.

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INTRODUCTION The efficiency of organic solar cells (OSC) has been increasing continuously in the past years, currently reaching values of up to 12%.1 One path to improve their performance is to increase the open-circuit voltage (VOC) by tailoring the HOMO and LUMO levels of the active layer materials while simultaneously maintaining a high short-circuit current (JSC) and fill factor (FF). For achieving a high VOC, the difference between the donor HOMO and the acceptor LUMO (i.e., the effective band gap) has to be as large as possible. At the same time, the LUMO−LUMO offset has to be sufficient to allow for an effective exciton dissociation. For a high JSC and FF, the morphology of the bulk heterojunction (BHJ) plays a crucial role. If the donor and acceptor materials are too finely intermixed, continuous percolation pathways from the molecular interface to the respective electrodes cannot be formed and the interface area is larger. As a consequence, bimolecular recombination of charge carriers is increased which deteriorates the charge transport and hence the FF and JSC. In this work, we study the polymer poly(2-(5-(5,6bis(octyloxy)-4-(thiophen-2-yl)benzo[c][1,2,5]thiadiazol-7-yl)thiophen-2-yl)-9-octyl-9H-carbazole) (HXS-1, Figure 1a) because of its promising characteristics for the use in solar cells: a balanced charge transport due to planar polymer stacking, good © 2014 American Chemical Society

solubility in common solvents, and a lower HOMO (5.2 eV) than, for example, P3HT which allows for a higher VOC when blended with [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM) (Figures 1b and 2).2 Because of the high VOC in HXS-1:PC71BM blends, efficiencies of 5.4% were reached.2 Since the LUMO of indene-C60-bis-adduct (ICBA, Figure 1c) is higher than that of PC71BM, VOC can be increased even further. The efficiency of P3HT solar cells rose to 6.5% by exchanging PC71BM with ICBA.3 The high efficiency of P3HT:ICBA cells shows that the intrinsic electron mobility in ICBA is not a limiting factor. However, blending polymers other than P3HT with ICBA led to reduced performance compared to PC71BM in many cases,4 mainly because of a very low JSC and also a decreased FF.5 This behavior has been discussed in the literature recently. One possible reason that has been suggested is an insufficient driving force for exciton dissociation if the LUMO−LUMO offset ΔLL is smaller than a value that was reported to be in the range from 0.1 to 0.9 eV.6−9 Miller et al. attribute the lower efficiency with ICBA to poor charge carrier transport,5 while Xin et al. attribute it to Received: August 30, 2013 Revised: January 21, 2014 Published: February 5, 2014 3386

dx.doi.org/10.1021/jp408705r | J. Phys. Chem. C 2014, 118, 3386−3392

The Journal of Physical Chemistry C

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Figure 1. Molecular structures of (a) HXS-1, (b) ICBA, and (c) PC71BM.

1:ICBA cells were dried after encapsulation at 70 °C for 10 min. The active cell area was 9 mm2. Current−voltage (J−V) characteristics were measured under simulated “1 sun” AM1.5G illumination. The intensity of the solar simulator (Newport 94062 A) was adjusted to 1 sun using an Oriel reference cell (Newport). No correction for spectral mismatch was applied. Absorption was measured with a NEOSYS-2000 double-beam UV−vis spectrophotometer (Scinco). The spectra were recorded in transmission mode next to the Al and ITO pads on the cell, so that the part of the cell that was actually measured was the structure glass/ PEDOT:PSS/active layer/glass encapsulation cap. The absorption spectra were baseline corrected using a blank sample (glass substrate/glass encapsulation cap in the case of pristine HXS-1 films, glass/PEDOT:PSS/glass encapsulation cap in the case of the HXS-1:fullerene solar cells). Photoluminescence (PL) measurements were performed with a FluoroMate FS-2 fluorescence spectrometer (Scinco). For the Suns-V OC measurement, a WCT-120 stage (Sinton Instruments) was used, and the measurements were carried out at room temperature. Atomic force microscope (AFM) images were recorded with a XE-70 scanning probe microscope (Park Systems) in tapping mode. The fundamental beam in the pump−probe transient absorption (TA) setup was generated by a Libra ultrafast amplifier laser system (Coherent), providing pulses with an energy of 1 mJ at a repetition rate of 1 kHz at 800 nm, with pulse widths below 100 fs. The beam was split with a beam splitter into a pump and a probe beam. To set the pump wavelength of 510 nm, a TOPAS optical parametric amplifier (Coherent) was used. The pump fluence was around 5 μJ/cm2. The white light continuum for the probe beam was generated in two different sapphire crystals for VIS and NIR white light, respectively. The TA spectra and kinetics were measured with a HELIOS transient absorption spectrometer (Ultrafast Systems). Each data point was integrated for 2 s, and each full scan was repeated nine times to increase the signal-to-noise ratio. The measurements were carried out at room temperature with encapsulated samples. Like for steady-state absorption, the spectra were recorded in transmission mode next to the Al and ITO pads on the cell, so that the part of the cell that was actually measured was the structure glass/PEDOT:PSS/active layer/glass encapsulation cap. For the bias dependence only, the TA was measured in reflection mode directly on the active area of the cell with the structure glass/ITO/PEDOT:PSS/ active layer/LiF/Al. Different biases were applied to the cell using a Keithley 2400 SourceMeter.

Figure 2. HOMO and LUMO energy levels of HXS-1, ICBA, and PC71BM.2,11

geminate recombination and therefore decreased charge carrier generation.10 The data presented here from spectroscopy experiments, electrical characteristics, and morphology studies of HXS1:PC71BM and HXS-1:ICBA solar cells strongly suggest that HXS-1:ICBA cells are mainly limited by poor charge carrier transport.

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EXPERIMENTAL METHODS Poly(2-(5-(5,6-bis(octyloxy)-4-(thiophen-2-yl)benzo[c][1,2,5]thiadiazol-7-yl)thiophen-2-yl)-9-octyl-9H-carbazole) (HXS-1) was synthesized as described previously.2 [6,6]-Phenyl-C71 butyric acid methyl ester (PC71BM) was purchased from nanoC, indene-C60-bis-adduct (ICBA) from Luminescence Technology. All chemicals were used as received. The substrates were cleaned in ultrasonic baths using deionized water and detergent, deionized water, acetone, and IPA in sequence. Pristine HXS-1 films were spin-coated on glass substrates from an o-dichlorobenzene (oDCB, SigmaAldrich) solution at 90 °C. For the solar cells, clean patterned indium tin oxide (ITO)-covered glass substrates (Samsung Corning, sheet resistance 7 Ω/sq) were treated with O2 plasma for 10 min. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Starck, Clevios AI 4083) was spin-coated immediately after the plasma treatment to give a layer thickness of around 35 nm. The PEDOT:PSS layer was subsequently baked at 140 °C for 10 min. The HXS-1:fullerene solutions were made by mixing 5 mg of HXS-1 and 12.5 mg of PC71BM or ICBA, respectively, into 1 mL of oDCB and 1,8diiodooctane (DIO, Sigma-Aldrich). The DIO concentration was 2.5 vol %. After stirring overnight in a nitrogen atmosphere at 90 °C, these solutions were spin-coated at 90 °C on the PEDOT:PSS to give an active layer thickness of around 105 nm. The HXS-1:PC71BM layer was dried at 90 for 10 min in a nitrogen atmosphere. After being stored under vacuum overnight, the electron contact consisting of ≈0.3 nm lithium fluoride (LiF) and 120 nm aluminum (Al) was evaporated thermally. Then the cells were encapsulated under nitrogen using glass encapsulation caps and UV curing epoxy. The HXS-

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RESULTS AND DISCUSSION The current−voltage (J−V) characteristics of HXS-1:PC71BM and HXS-1:ICBA solar cells show the influence of the fullerene on the electrical performance of the cells (Figure 3). The electrical parameters of both cells are given in Table 1. The 3387

dx.doi.org/10.1021/jp408705r | J. Phys. Chem. C 2014, 118, 3386−3392

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Another reason for the low JSC could be a driving force that is too small for efficient exciton dissociation because of the smaller LUMO−LUMO offset ΔLL between HXS-1 and ICBA (Figure 2). This has been discussed for other polymer:ICBA cells before.6,8 If the exciton dissociation at the interface is not efficient, there should be photoluminescence (PL) from the fraction of the remaining excitons that recombine radiatively. As can be seen in Figure 5, the PL of both HXS-1:PC71BM and

Figure 3. J−V curves of HXS-1:PC71BM and HXS-1:ICBA solar cells. Shown is also the J−V curve of HXS-1:ICBA normalized to JSC of HXS-1:PC71BM to demonstrate the slope around 0 V and the lower FF.

Table 1. Electrical Performance of HXS-1:PC71BM and HXS-1:ICBA Cells under Simulated AM1.5G Conditions cell

VOC [V]

JSC [mA/cm2]

FF

η [%]

HXS-1:PC71BM HXS-1:ICBA

0.86 1.05

10.0 5.0

0.56 0.47

4.8 2.5

Figure 5. Photoluminescence (PL) spectra of pristine HXS-1 and cells with HXS-1:PC71BM and HXS-1:ICBA. The excitation wavelength was 540 nm. To account for different excitation densities in the different samples, the PL signal of each sample was normalized to its respective absorption at 540 nm.

open-circuit voltage VOC increases from 0.86 V for HXS1:PC71BM to 1.05 V for HXS-1:ICBA. This is caused by the higher LUMO of ICBA compared to PC71BM and therefore a larger effective band gap of the photoactive layer (difference between the donor HOMO and the acceptor LUMO) (Figure 2). On the other hand, the short-circuit current density JSC decreases from 10 mA/cm2 for HXS-1:PC71BM to 5 mA/cm2 for HXS-1:ICBA, and the fill factor FF decreases from 0.56 to 0.47, reducing the overall efficiency from 4.8% for HXS1:PC71BM to 2.5% for HXS-1:ICBA. It may appear that the lower JSC could be a consequence of a lower absorption of HXS-1:ICBA. However, Figure 4 shows

HXS-1:ICBA is almost completely quenched, which is a sign of very efficient exciton dissociation. The very slightly higher PL of HXS-1:ICBA could be due to the smaller ΔLL, but again, this effect is much too small to explain the low JSC of HXS1:ICBA. Thus, neither lower absorption nor inefficient exciton dissociation is the reason for the lower JSC in HXS-1:ICBA when compared with HXS-1:PC71BM cells. By studying the J− V curves in more detail, information about possible loss mechanisms in the cells can be extracted. The current density of the HXS-1:ICBA device has a larger slope around 0 V than that of HXS-1:PC71BM and is therefore more dependent on the voltage. This can be seen more clearly when the curve is normalized to JSC of HXS-1:PC71BM (dashed line in Figure 3). Figure 6 shows that the current density of HXS-1:ICBA cells increases to at least 7.5 mA/cm2 at larger negative biases, where the dark current characteristics still show a rectifying diode behavior. Hence, more photogenerated charge carriers can be

Figure 4. Absorption of HXS-1:PC71BM and HXS-1:ICBA, which are clearly superpositions of the pristine HXS-1 and respective fullerene absorption. The absorption of ICBA and PC71BM is normalized to the respective fullerene peaks in the absorption curves of HXS-1:ICBA and HXS-1:PC71BM.

that the absorption of HXS-1:ICBA is similar to that of HXS1:PC71BM. The higher absorption of PC71BM in the visible range around 500 nm is too small an effect to account for the fact that the HXS-1:PC71BM JSC value is double that of HXS1:ICBA (cf. Table 1). The polymer absorption that peaks at 540 and 580 nm is even slightly lower for HXS-1:PC71BM.

Figure 6. Dark and illuminated J−V curves of an HXS-1:ICBA solar cell. With larger negative bias more photogenerated charge carriers contribute to the current. 3388

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extracted at reverse bias. This may be due to either fieldenhanced charge carrier separation12 or reduced charge carrier (bimolecular) recombination. The reason for the voltage dependence of the current has not been clearly identified.13 Albrecht et al. attribute the lower current density around the short-circuit condition to losses by nongeminate recombination,14 and also Mauer et al. and Etzold et al. attribute the bias dependence to bimolecular recombination.15,16 When comparing the dark current of the HXS-1:PC71BM and HXS-1:ICBA cells in forward direction at VOC + 0.4 V (to account for the difference in VOC), the current density of HXS1:ICBA with 33 mA/cm2 is approximately 33% lower than that of HXS-1:PC71BM (49 mA/cm2). This is a sign of a poorer charge carrier transport in these cells compared to HXS1:PC71BM. To elucidate the effect of a poor charge carrier transport on the J−V curve of a BHJ OSC, numerical simulations were carried out with the commercial semiconductor simulation tool Sentaurus Device from Synopsys. 17 The basis of the calculations was a single effective semiconductor (ES model). The generation rate was assumed to be homogeneous throughout the whole active layer. More information about the simulation technique and the parameters are given elsewhere.18 The only parameter varied here was the electron mobility. The results, depicted in Figure 7, show that indeed the same trend of lower JSC, lower FF, and a slope of the J−V curve at short circuit conditions can be reproduced with a lower electron mobility.

to reach the same current density. However, in a solar cell this comes at a price: A higher gradient is built up by the accumulation of electrons as they do not leave the cell as easily as in the case of the higher mobility. First, and somehow as a side effect, this leads to a buildup of space charge, which makes it also harder for the holes (which we assume to have a higher mobility in this example) to be transported to the hole contact. Second, any increase of the charge carrier concentrations immediately causes a higher recombination rate. Whether this additional recombination, due to poor transport properties, affects only the fill factor or also leads to a reduction of JSC depends on the magnitude of the described effect. As can be clearly seen from Figure 7, the lower the mobility, the more it affects the J−V curve already at low voltages. In order to gain more insight into the transport of the charge carriers in the photoactive layer, Suns-VOC measurements were carried out. The most important output of these measurements is a pseudo J−V curve representing the curve that the device would have without series resistance. In the Suns-VOC method, the open-circuit voltage of the solar cell is monitored while the illumination intensity is varied over several orders of magnitude. As no current flows, the results are independent of the series resistance of the device and thus solely reflect generation and recombination processes. The series resistance is the sum of the series resistance of the circuitry (usually independent of voltage) and the transport resistance of the photoactive layer (dependent on voltage). Recently, this measurement technique, which is frequently used for the characterization of inorganic solar cells, has been successfully applied to organic solar cells.19 It could be shown that the transport resistance of the photoactive layer of a BHJ OSC can make a significant contribution to the overall series resistance of the device. Suns-VOC curves are shown in the Supporting Information (Figure S5). Despite the difference of FF between HXS1:PC71BM and HXS-1:ICBA (0.56 vs 0.47), the pseudo FF measured with Suns-VOC is identical for both devices and is much higher (0.82). This shows that the FF of both cells is clearly transport limited, and this effect is even more pronounced in the case of HXS-1:ICBA. The series resistance can be extracted from the voltage difference of the J−V and the Suns-VOC curves as a function of the current (refer to Schiefer et al.19 for details). It has a value of 25 Ω cm2 at the maximum power point (MPP) for HXS-1:PC71BM and 127 cm2 for HXS1:ICBA. As the series resistance of the circuitry is the same for both devices (it can be estimated to be