Direct Electrical Evidence of Plasmonic Near-Field Enhancement in

Jul 8, 2014 - E-mail: [email protected]., *E-mail: [email protected]. .... Till Jägeler-Hoheisel , Johannes Benduhn , Christian Körner , Karl Leo...
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Direct Electrical Evidence of Plasmonic Near-Field Enhancement in Small Molecule Organic Solar Cells Till Jag̈ eler-Hoheisel,* Franz Selzer, Moritz Riede,† and Karl Leo* Institut für Angewandte Photophysik, Technische Universität Dresden, George-Bähr-Strasse 1, 01069 Dresden, Germany S Supporting Information *

ABSTRACT: We present a simple and versatile technique to introduce plasmonic silver nanoparticles into organic thin film devices by in situ vacuum deposition. Silver particles with 80 nm diameter at the back of small molecule organic solar cells increase the power conversion efficiency (PCE). Doped organic transport layers allow one to separate electrical and optical effects. By a systematic variation of the position of the silver particles within the solar cell stack, we can thus clearly distinguish a near-field photocurrent gain in the IR that decays to one-half on length scales of around 4 nm, and a less distancedependent selective mirror effect for short wavelength, which allows one to optimize devices for different wavelengths simultaneously. Device optimization reveals that plasmonic increased absorption can be used to significantly reduce the thickness of the absorber layers and gain efficiency through improved transport properties. A plasmonic zinc phthalocyanine fullerene-C60 solar cell that yields improved photocurrent, fill factor, and PCE of 2.6% includes one-half of the absorber material of an optimized reference device with PCE of 2.4%. The design priciples for plasmonic solar cells are general and were confirmed in thin devices containing zinc 1,8,15,22-tetrafluorophthalocyanine, improving the PCE from 2.7% to 3.4%.

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INTRODUCTION Organic solar cells (OSCs) have great potential for resourcefriendly conversion of solar energy due to their very low material consumption and process temperatures.1 Despite absorption coefficients >105 cm−1 of many organic materials, it is often difficult to absorb sufficient light in OSCs, because comparatively small charge carrier mobilities2,3 and exciton diffusion lengths in the order of 10 nm4,5 limit the possible thickness of the active absorber layer. Therefore, light trapping techniques on the subwavelength scale are desirable. The use of plasmonic structures that exhibit collective oscillations of metal electrons and electromagnetic waves6 has been studied since the early days of OSCs.7,8 The main goal is to concentrate light on a subwavelength scale in the region of the absorber layers to overcome the trade-off between absorber layer thickness and transport properties, which usually limits the power conversion efficiency (PCE).9−11 Plasmonic approaches have also been discussed to broaden the absorption spectra. Especially the enhancement of photogeneration in the IR is needed for most thin film photovoltaic technologies.12,13 The tuning of particle plasmon resonances to those wavelengths is possible by suitable particle shape14−18 or aggregation of particles.19,20 Simulations reveal possible high absorption gains within organic layers especially for thin absorber layers21 for a variety of plasmonic geometries, for example, plasmonic gratings,22 nanodisks, and nanoholes.23 However, the introduction of plasmonic structures into organic solar cells is challenging:24 Exciton or charge-carrier recombination at the metal surface requires passivation of the metal structures,25,26 unless they can be used as selective contacts. Additional demanding requirements © 2014 American Chemical Society

on material purity and morphological properties like layer smoothness and phase separation in bulk heterojunctions (BHJ) complicate the use of prestructured substrates27,28 or limit the size of the plasmonic structures. For OSCs including plasmonic metal structures, a variety of effects have been reported, ranging from enhanced photogeneration,29−33 shorter exciton lifetimes,33,34 changes in morphology,25,35 improved charge carrier extraction,31,33,35 to charge generation at the surface of the metal structures.36,37 Usually the plasmonic nature of the enhancement is deduced from comparing spectroscopic features in external quantum efficiency of the devices and absorption of the metal structures34,38 or by optical simulations of the plasmonic structures.26,39 Here, we present a simple and versatile technique to introduce plasmonic silver particles in evaporated small molecule OSCs. Because of the excellent control of the electrical and optical properties of the OSC stacks using doped transport layers, we are able to differentiate between the various effects of plasmonic nanostructures discussed above. We introduce the silver particles at the back of the solar cell stack after depositing the absorber layer to enshure that absorber layer thickness and morphology remain unaffected. In this study, we first investigate the effect of particle size and thus wavelength of plasmon resonance on absorption and photocurrent production in zinc phthalocyanine (ZnPc) fullerene−C60 (C60) devices. In a second step, we vary the position of the plasmonic particles with respect Received: March 12, 2014 Revised: June 4, 2014 Published: July 8, 2014 15128

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at rates of 0.02 nm s−1. During evaporation of Ag at low rates of 0.003 nm s−1, the substrate is heated to 80 °C to adjust the size and distribution of the particles. Ag and Al were purchased from Kurt J. Lesker, UK. To adjust the particle to absorber distance, we use HTL spacer layers of 0, 2, 4, 10, and 21 nm before depositing dewetting layers of either 2 or 4 nm V2O5 and Ag thickness of 3, 5, and 10 nm for particle formation. As depicted in Figure 1, we attempt to compensate for the introduction of additional layers and different spacer layers by adjusting the final HTL to add to an overall 50 nm nominal thickness corresponding to the optimized reference device, neglecting the actual coarse morphology of the silver structures, before evaporating the Al back contact (100 nm). Solar cells are deposited through shadow masks onto a 15 cm × 15 cm glass wafer with prestructured indium tin oxide (ITO) (90 nm) contacts (Thin Film Devices, U.S.) defining four independent devices in each sample at pressures below 2 × 10−7 mbar in a custom-made vacuum system (K.J. Lesker). OSCs for current−voltage (I−V), absorption, and EQE measurements are produced on one wafer together with samples for SEM, AFM, and integrated reflection spectra, which were masked during deposition of the final HTL and the Al back contact. Samples for device optimization and absorber thickness variation are produced as follows: OSCs with 40 nm “thick” BHJ absorber layer: ITO (90 nm)/W2(hpp)4 (1 nm)/C60 (30 nm)/ ZnPc: C60 (1:1, 40 nm)/ZnPc (3 nm)/BF-DPB: F6-TCNNQ (10 wt %, 2 nm)/V2O5 (2 nm)/Ag (3 nm)/BF-DPB: F6-TCNNQ (10 wt %, 55 nm)/Ag (100 nm). OSCs with 20 nm “thin” BHJ absorber layer: ITO (90 nm)/W2(hpp)4 (1 nm)/C60 (5 nm)/ ZnPc: C60 (1:1, 20 nm)/ZnPc (3 nm)/BF-DPB: F6−TCNNQ (10 wt %, 2 nm)/V2O5 (2 nm)/Ag (3 nm)/BF-DPB: F6-TCNNQ (10 wt %, 55 nm)/Ag (100 nm). The Ag (3 nm) layer is omitted in the devices without nanoparticles. The 28 nm sample without nanoparticles is from another deposition process that generally yielded lower FF. To check the reproducibility and general applicability of the results, we build devices with and without silver nanoparticles using either 30, 35, or 45 nm BHJ layers of the more efficient fluorinated F4−ZnPc mixed in a 1:1 mixing ratio with C60. These devices have the layer sequence: ITO (90 nm)/ C60: W2(hpp)4 (2 wt %, 5 nm)/C60 (30 nm)/BHJ/F4−ZnPc (3 nm)/BF-DPB: F6-TCNNQ (10 wt %, 2 nm)/V2O5 (2 nm)/ Ag (3 nm)/BF-DPB: F6−TCNNQ (10 wt %, 40 nm)/Al (100 nm). Here, the introduction of V2O5 and Ag in the plasmonic device is compensated by supplentary 5 nm of final HTL in the reference devices. Layer thickness is monitored during evaporation with quartz crystal microbalances. The substrate is rotated during evaporation to ensure homogeneity of the layers. The uncertainty in layer thickness can be expected to be below 5%. Characterization. The active area of 6.44 mm2 is defined by the overlap of the ITO-contacts with a common back electrode. Possible effects of silver conductivity on the effective active area are excluded by remeasuring using aperture masks. I−V curves are recorded using a Keithley 2400 SMU and a sunlight simulator 16S-003-300-AM1.5 (Solar Light Co., U.S.) and can be found in the Supporting Information. Efficiencies are determined with respect to a calibrated S1337-33BQ photodiode (Hamamatsu) using a common mismatch correction.41 Setting the intensity of the sunlight simulator is the main source of error in determining absolute current densities and thus

to the absorber layer by adjusting the thickness of spacer layers to rigorously prove the near-field enhancement and distinguish it from reflection and scattering at the nanoparticles. As a last part of the study, we focus on device optimization, varying the thickness of the absorber layers. We aim to understand whether plasmonic photocurrent enhancement can also be used for new optimized devices with very low absorber layer thickness and improved transport properties. Although we have studied the effects here in a simple reference materials system, it is worth noting that photocurrents and efficiencies are reported with respect to reference devices that have already been optimized. This approach helps to classify effects of plasmonic enhancement with respect to other means of device optimization and specify conditions under which it has the best effect. The application of the derived design principles in devices containing the more efficient absorber material zinc 1,8,15,22-tetrafluoro-phthalocyanine(F4−ZnPc)40 confirms that the results are readily usable for optimized materials with higher efficiency.

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EXPERIMENTAL SECTION Solar Cell and Nanoparticle Deposition. As depicted in Figure 1, a ZnPc:C60 solar cell stack is used to investigate the

Figure 1. Organic solar cell stack of the reference device without silver particles and the plasmonic device containing different amounts of silver for particle formation and different spacer layer thickness to set the particle to absorber distance.

effect of different amounts of Ag for particle formation and to introduce spacer layers of different thickness to adjust the distance between the silver nanoparticles and the absorber layer. The stack consists of n-doped electron transport layers: tetrakis[1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinato] ditungsten(II) (W2(hpp)4) (Novaled, Ger.) (1 nm)/C60 (CreaPhys, Ger.) doped with 2 wt % of W2(hpp)4 (10 nm), intrinsic absorber layers: C60 (30 nm)/a BHJ layer of ZnPc (Fraunhofer-COMEDD, Ger.) mixed in 1:1 volume ratio with C60 (30 nm)/ZnPc (5 nm). Nanoparticles are introduced into a hole transport layer (HTL) of N,N′-((diphenyl-N,N′-bis)9,9,dimethyl-fluoren-2-yl)-benzidine) (BF-DPB) doped with 10 wt % of 2,2′-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6-TCNNQ) (Novaled). The HTL is deposited in two steps: a HTL spacer layer between the absorber layer and the particles and a final HTL between particles and the Al back contact. To ensure the growth of well-separated nanoparticles, a dewetting layer of vanadium(V) oxide (V2O5) (Sigma-Aldrich 99.99%) is evaporated 15129

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layer,43 overcompensates the short wavelength absorption of V2O5 in this best reference device.44,45 With respect to this reference, which has a PCE of (2.25 ± 0.05)%, a short circuit current density (jsc) of (8.1 ± 0.1) mA cm−2, a FF of 54%, and an open circuit voltage (Voc) of 0.52 V, the deposition of Ag slightly reduces FF by 4% and Voc by 10 mV, possibly by Ag atoms diffusing into the HTL material. Thus, a net gain in PCE for this stack design results only for devices including 3 nm of Ag on 2 nm V2O5 with 2 and 4 nm HTL spacer layers, which have jsc of around (9.0 ± 0.1) mA cm−2 and PCEs of (2.33 ± 0.05)% and (2.28 ± 0.05)%. More details on solar cell performance can be found in the Supporting Information. Although the gain in PCEs remains marginal in this stack design, the effects of particle size and particle to absorber distance on enhanced photocurrent generation are discussed in the framework of the above-mentioned devices. Further increased PCEs are discussed in the Absorber Layer Thickness section where particle size and position are fixed and the thickness of the absorber layer is varied. Silver Nanoparticle Size. The growth of evaporated silver layers depends on the surface energies of the metal and the substrate enabling island growth on V2O5.46,47 The density of the particles can be designed by controlling substrate temperature, substrate roughness, and evaporation rate; the size of the particles is then determined by the total amount of Ag evaporated.48,49 SEM images in Figure 3a−c show the Ag particles formed from 10, 5, and 3 nm of Ag, respectively, deposited onto a solar

PCEs in samples containing ZnPc:C60. For devices containing F4−ZnPc, deviations in the fill factor of the four devices on each samples add to the total error. Absorption spectra of the solar cells are determined from direct reflection using a combined halogen and deuterium lamp Avalight-DH-S-BAL (Avantes) coupled via a Y-fiber to a custom built spectrometer (OMT, Ger.) using an Al mirror as a reflection reference. Angle integrated reflection spectra are recorded using a UV-3100 UV−vis−NIR spectrophotometer with MPC-3100 integrating sphere unit (both Shimadzu) and a glass substrate as reference. EQE spectra are recorded with respect to a S1337-33BQ photodiode using a Xe arc-lamp (Oriel), a Cornerstone 260 1/4m monochromator (Newport), an optical chopper (Scitec instruments), an aperture mask, a 5182 preamplifier, and a 7265 DSP lock-in amplifier (both Signal Recovery). Results are averaged over four identical solar cells produced on each sample. Scanning electron microscopy (SEM) pictures are recorded using a GSM 982 Gemini (Zeiss). Atomic force microscopy (AFM) images are recorded in tapping mode on a Combiscope1000 (AIST-NT).

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RESULTS AND DISCUSSION Electrical Effects of Nanoparticle Device Integration. The introduction of nanoparticles leads to enhanced photocurrents in the OSCs exept for the devices containing 10 nm of Ag and devices with spacer layers of 21 nm where the photocurrent is comparable to the reference or slightly reduced. Figure 2 shows current density voltage (J−V) characteristics of representative devices under AM1.5 illumination conditions.

Figure 2. lluminated (AM1.5) current density voltage characteristics of the reference device and devices comprising different amounts of silver for nanoparticle formation, either with or without 2 nm of HTL spacer layer, which is necessary to achieve good fill factors.

Figure 3. Scanning electron microscopy images of silver clusters for different nominal Ag thickness deposited onto an organic solar cell stack: (a) 10 nm Ag on 4 nm V2O5, (b) 5 nm Ag on 4 nm V2O5, (c) 3 nm Ag on 4 nm V2O5, and (d) 3 nm Ag on 2 nm V2O5.

The peak short current densities of (9.1 ± 0.1) mA cm−2 in the device with 3 nm of Ag on 2 nm of V2O5 and (8.5 ± 0.1) mA cm−2 in the device with 5 nm of Ag on 4 nm of V2O5 are found in devices without HTL spacer layer. However, to provide good charge carrier extraction for the operation at the maximum power point, a spacer layer of at least 2 nm HTL between the absorber layer and the V2O5 dewetting layer is necessary; we assume that this layer reduces recombination at the V2O5 layer, which has very low lying energy levels with respect to the organic materials.42 As a reference device, we use 2 nm of V2O5 on 2 nm HTL spacer layer as depicted on the left in Figure 1 because a slightly increased fill factor (FF), presumably due to an additional doping effect of the transparent metal oxide on the thin HTL

cell stack using a dewetting layer of 4 nm of V2O5, and the particles formed from 3 nm of Ag on 2 nm of V2O5 are presented in Figure 3d. The particles are well separated and do not show a periodic order. It is unlikely that they are closely spaced: this can be deduced from a minimum in the autocorrelation function of the images (e.g., AFM image in the Supporting Information), which is a direct result of the growth mode where occurrence of seed clusters lowers the probability of new seed clusters of critical size in the near surrounding.49−51 The mean ellipticity of the particles in the plane of the picture is 0.8 ± 0.15. 15130

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Table 1. Mean Physical Parameters with Standard Deviations of Nanoparticles Formed on V2O5 Thickness (tV2O5) from Nominal Ag Thickness (tAg) Deposited onto an Organic Solar Cell Stack tV2O5 (nm)

tAg (nm)

2 4 4 4

3 3 5 10

diameter (nm) 87 100 114 138

± ± ± ±

20 27 27 41

height (nm)

density (μm−2)

± ± ± ±

19.1 12.2 12.3 19.0

33.3 40.2 50.3 57.7

6.4 6.9 9.7 9.5

The mean physical dimensions derived by SEM and AFM are presented in Table 1. With the use of more Ag, larger particles form, but the ratio of height to diameter remains around 0.4. The thinner V2O5 layer yields a higher cluster density, which could be attributed to higher roughness or defect density. For the 10 nm of Ag sample, the increased particle density might be due to a slightly lower temperature at the edge of the 15 cm × 15 cm substrate. The number of particles per surface area has been drastically reduced with respect to previous studies by choosing the dewetting layer, an elevated substrate temperature, and a very low deposition rate allowing the Ag atoms to diffuse over long distances; at fixed surface density, the diameter of the elliptical particles is defined by their aspect ratio and the total volume of Ag deposited. Absorption spectra of OSCs containing different amounts of Ag without HTL spacer layer are shown in Figure 4. As compared

Figure 5. External quantum efficiency (EQE) of the reference device and solar cells including different amounts of silver for particle formation on either 2 nm or 4 nm V2O5.

to 20% at 775 nm and a maximum relative increase of more than 50% at 800 nm. When the EQEs are integrated over an AM1.5 sun spectrum, they reproduce the measured short current densities. The short wavelength photocurrent enhancement in the device containing 10 nm of Ag does not completely compensate the reduced EQE in the main ZnPc absorption band where most of the photocurrent is produced in the reference device. This is consistent with an altered thin film optic, and the slightly increased absorption between 600 and 700 nm in devices containing 10 nm of Ag presented in Figure 4 is likely to be outbalanced by parasitic absorption of the particles. Integrated reflection spectra of samples produced without final HTL and withut reflecting aluminum (Al) back contact are shown in Figure 6. The reflection around wavelength of 500 nm

Figure 4. Absorption spectra of the reference device and solar cells including different amounts of silver for nanoparticle formation on either 2 nm or 4 nm V2O5. Particles of different size are sketched in the inset.

to the reference device, the introduction of nanoparticles leads to a broadly enhanced absorption exhibiting a new near-infrared (IR) peak that shifts from 775 nm for the smallest particles to longer wavelengths up to around 1000 nm with increasing amounts of Ag. The external quantum efficiency (EQE) response of devices without HTL spacer layer presented in Figure 5 increases in the short wavelength region (300−550 nm) for all devices especially for increasing amounts of Ag. Another significant increase of the EQE can be found at the IR edge of the ZnPc signature when less Ag is used and the near IR absorption peak and the EQE response of the solar cell overlap. For the device including 3 nm of Ag, the photocurrent is enhanced for all wavelengths with a maximum absolute EQE increase from 14%

Figure 6. Angle-integrated reflection spectra of samples without aluminum back contact containing different amounts of silver for particle formation on 4 nm of V2O5.

increases with increasing amount of Ag, resembling the short wavelength enhancement of the EQE in Figure 5.

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DISTANCE DEPENDENCE To understand in detail the effects that lead to enhanced photocurrent generation, we analyze OSC samples with different particle to absorber layer distances. The particle to absorber distance was adjusted by introducing HTL spacer layers between 15131

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the reflection at the particles is blue-shifted with respect to the additional absorption in Figure 4 and the maximum of particle reflection in Figure 6 peaking at 500 nm due to the overlap with the 450 nm absorption peak52 of the C60 layer facing the incoming light and reduced parasitic short wavelength absorption in the final HTL.53 The EQE enhancement factors of the devices including 3 nm of Ag and 4 nm of V2O5 in Figure 9 show the same behavior,

the absorber layers and the V2O5 layer according to Figure 1. The thickness of the V2O5 dewetting layer adds to the total particle to absorber distance. EQE spectra and EQE enhancement factors, normalized to the EQE of the reference device, for different spacer layer thickness are shown in Figures 7 and 8 for the devices including 3 nm of Ag on 2 nm of V2O5.

Figure 7. EQE spectra for different particle to absorber distances ranging from 2 to 23 nm of solar cells including nanoparticles formed from nominally 3 nm Ag on 2 nm V2O5 and the reference device. The distance variation is sketched in the inset.

Figure 9. EQE enhancement factor for different particle to absorber distances ranging from 4 to 25 nm of solar cells including nanoparticles formed from nominally 3 nm Ag on 4 nm V2O5 calculated by normalizing to the reference device.

but the short wavelength enhancement is smaller due to more absorption of V2O5. This selective mirror effect presents a general way of broadening the absorption of organic solar cells by optimizing their thin film optics for different wavelengths simultaneously. It could be especially useful in tandem devices. The long wavelength regime (>600 nm) of the EQE enhancement in Figures 8 and 9 is strongly dependent on the distance between Ag nanoparticles and the absorber layer and can thus be attributed to absorption enhancement in the optical near field of the plasmonic particles. A gain in this wavelength region is present for particle to absorber distances of up to 8 nm. When the IR enhancement factor of the high particle to absorber distances of about 0.86 at wavelength between 800 and 850 nm is taken as a baseline, accounting for some absorption of the nanoparticles, the enhancement can be described with an exponential decay dropping to one-half every 4 nm of distance increase. Although the change of field intensity at the position of the nanoparticles due to the interference pattern inside the OSC stack should have a minor effect for short particle to absorber distances, it would have to be taken into account in a more detailed interpretation when the particles are located close to the reflecting back contact. Although we compensated the thickness of the spacer layer by reducing the thickness of the final HTL, we present the EQE of a study without nanoparticles54 varying the overall thickness of the final transport layer in the Supporting Information. In contrast to the plasmonic near-field effect, changing the overall thickness of the transport layer uniformly affects the EQE for wavelength longer than 600 nm by thin film interference without revealing enhancement features that correspond to infrared absorption peaks present in the plasmonic devices in Figure 4. The length scale on which the near-field photocurrent enhancement drops is comparable to values derived by thicknessdependent absorption measurements of copper-phthalocyanine on

Figure 8. EQE enhancement factor for different particle to absorber distances ranging from 2 to 23 nm of solar cells including nanoparticles formed from nominally 3 nm Ag on 2 nm V2O5 calculated by normalizing to the reference device.

Regarding the distance dependence of the EQE enhancement factor in Figure 8, we can distinguish a short and a long wavelength regime. The EQE enhancement in the short wavelength region (