Organic Photovoltaic Devices Based on Water-Soluble Copper

ARTICLE pubs.acs.org/JPCC

Organic Photovoltaic Devices Based on Water-Soluble Copper Phthalocyanine S. Schumann, R. A. Hatton, and T. S. Jones* Department of Chemistry, University of Warwick, Coventry, CV4 7AL, United Kingdom ABSTRACT: Organic photovoltaic (OPV) planar heterojunction devices based on an aqueous solution processed 3,40 ,400 ,4000 -copper(II) phthalocyanine-tetrasulfonic acid tetrasodium salt (TSCuPc) donor layer and a fullerene (C60) acceptor have been fabricated demonstrating power conversion efficiencies of up to 0.32% and an open circuit voltage (VOC) of almost 0.6 V. The VOC is significantly larger than that produced by similar cells based on the commonly studied copper phthalocyanine (CuPc)/C60 heterojunction (VOC = 0.46 V). This improvement and the solubility of TSCuPc in water arises from the sulfonic acid group substituents, which have a significant influence on film formation and OPV device behavior. In contrast to other solution-processed molecular semiconductors, TSCuPc forms nanocrystals in solution resulting in dense crystal thin film networks on the substrate surface. This unusual thin film morphology was studied in detail using ultraviolet-visible absorption spectroscopy, X-ray diffraction, and atomic force microscopy. Combined with current density-voltage analysis under 1 sun solar illumination, a deeper understanding of the molecular arrangement of TSCuPc thin films is presented as well as its impact on the resulting device behavior.

1. INTRODUCTION Organic semiconducting low molecular weight molecules, socalled molecular semiconductors, have attracted much attention in organic photovoltaics (OPVs) due to their relatively simple synthesis, molecular tuneability, easy processability, and good absorption coefficients in the visible part of the solar spectrum.1-6 The most common thin film deposition process for molecular semiconductors is high vacuum organic molecular beam deposition (OMBD), a thermal vapor-deposition technique, which enables highly reproducible thin film growth and complete device fabrication.7 The use of solution processed molecular semiconductors for OPV devices has received considerably less attention, despite the potential for cheaper device manufacture.8 Pentacene derivatives such as 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) have been studied, although the primary focus with these materials has been on their application in organic thin film transistors.9-12 Metal phthalocyanines (MPcs) are a different class of molecular semiconductor with good thermal and environmental stability, excellent molecular and electronic tunability, and also high absorption coefficients.13-15 In contrast to most solution processed molecular semiconductors, 3,40 ,400 ,4000 -copper(II) phthalocyanine-tetrasulfonic acid tetrasodium salt (TSCuPc) can be deposited from aqueous solution, which simplifies the device fabrication process and makes it potentially more economically as well as ecologically attractive. TSCuPc has also demonstrated potential for more general applications in organic electronics, with new thin r 2011 American Chemical Society

film structures being produced for electrode modification as well as templated porous and composite structures.16-18 In addition, water-soluble MPcs are nontoxic and find applications as photosensitizers in anticancer drugs.19 Thin film fabrication of watersoluble MPcs has been demonstrated using a number of different methods including Langmuir-Blodgett techniques, allowing precise control over film thickness, molecular stacking, and crystallinity.20,21 Previous studies have successfully demonstrated an improvement in the performance of bulk heterojunction OPV devices through electrode modification by incorporating an interlayer of nanostructured TSCuPc-sensitized multiwall carbon nanotubes (MWCNTs) between the indium-tin oxide (ITO) substrate and a photoactive polymer/fullerene (P3HT/PCBM) blend.16,22,23 The TSCuPc/MWCNT composite interlayer acts as a holeextraction layer due to improved interfacial energy level alignment with the highest occupied molecular orbital (HOMO) of P3HT. More recently, the fabrication of TSCuPc from layer-bylayer deposition was used to generate a modified OPV device architecture, demonstrating a power conversion efficiency (PCE) of 0.01%.24 TSCuPc has also found application in nanosphere templating as one of the few compatible water-soluble infiltration materials to generate porous large area organic Received: October 5, 2010 Revised: December 20, 2010 Published: February 28, 2011 4916

dx.doi.org/10.1021/jp109544m | J. Phys. Chem. C 2011, 115, 4916–4921

The Journal of Physical Chemistry C

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semiconductor thin films with 400 nm down to sub-100 nm open-cellular three-dimensional (3D) networks.17,25 Complete donor/acceptor (D/A) 3D composite structures were also fabricated by backfilling the porous thin films with an appropriate electron acceptor material.18,26 In this work, we present OPV devices fabricated from TSCuPc, using both pH neutral and basic aqueous solution (pH 11). Devices are demonstrated with significantly improved current density-voltage (J-V) characteristics and PCE when compared to the only two other reported devices based on this class of material.23,24 The TSCuPc thin films are also characterized by ultraviolet-visible absorption spectroscopy (UV-vis), atomic force microscopy (AFM) and X-ray diffraction (XRD). J-V analysis of the OPV devices under 1 sun illumination was carried out to develop a deeper understanding of the molecular arrangement as well as its impact on subsequent OPV device behavior.

2. EXPERIMENTAL SECTION The OPV devices were fabricated on precleaned ITO-coated glass substrates ( 18 MΩ, pH 7) and basic solutions (0.1 M NH3, pH 11) with the TSCuPc concentrations ranging from 5 to 20 mg mL-1. The solutions were stirred for 24 h at 50 °C and then filtered (0.2 μm filter) before being spin-coated at 2000 rpm for 1 min in air under ambient conditions. The films were dried for 30 min in air at room temperature, followed by 15 min at 100 °C under an inert atmosphere to remove any remaining water or ammonia. All other materials were deposited by OMBD using a Kurt J. Lesker Spectros system at a base pressure of 10-7 mbar. C60 (Nano-C) and bathocuproine (BCP, Aldrich) films were grown with film thicknesses of 40 and 7 nm, respectively. Al electrodes with a film thickness of ∼100 nm were deposited in situ through a shadow mask. The active area of the OPV devices was 0.16 cm2. J-V measurements were recorded with a computer controlled Keithley 2400 source-meter. Solar irradiation of AM1.5G was simulated with a Newport Oriel solar simulator. The light intensity was set to 1 sun (100 mW cm-2) using neutral density filters and measured with a Fraunhofer calibrated silicon photodiode (PV Measurements Inc.). All devices were tested in a sealed sample holder under nitrogen atmosphere. Further device analysis involved external quantum efficiency (EQE) measurements. Thin film characterization was performed using UV-vis absorption spectroscopy, XRD, and morphology studies with an Asylum Research MFP-3D for AFM in tapping mode. 3. RESULTS AND DISCUSSION In contrast to other molecular semiconductors commonly used in OPVs, and phthalocyanines in particular, TSCuPc is processable from aqueous solution, which makes it potentially attractive for a more sustainable process design. Although TSCuPc (Figure 1) is based on the same core molecule as unsubstituted CuPc, it reveals very different thin film characteristics which are reflected in the UV-vis absorption spectra as well as XRD and AFM data. 3.1. UV-vis Absorption. Figure 2 shows normalized UVvis absorption spectra of TSCuPc in neutral and basic solution as well as a thin film. Both solution spectra reveal an absorption band ranging from about 550 to 700 nm (Q-band) with a

Figure 1. Molecular structure of TSCuPc.

Figure 2. UV-vis absorption spectra of TSCuPc in pH neutral and basic solution as well as the spin-coated thin film. Inset: The solution and thin film spectra of CuPc as a comparison to highlight the different solution characteristics.

dominant maximum at 630 nm and a shoulder at 662 nm, which is slightly more pronounced for the neutral solution. The absorption spectrum for the spin-coated thin film is broader, but similar in shape and shows a blue-shifted maximum at 613 nm with an extended red-shifted and less pronounced shoulder. This suggests that the peak at lower wavelengths corresponds to molecular aggregates, cofacially stacked dimers and oligomers, and the shoulder at higher wavelengths to monomers.16,24 Increased pH should enhance deprotonation of the sulfonic acid substituents, resulting in a higher dissolution potential. However, the shoulder at 662 nm is slightly lower in absorption intensity at pH 11 and the effect is negligible. The inset in Figure 2 shows a CuPc thin film and solution spectrum for direct comparison, with a distinct difference in the monomer to aggregate ratio. The CuPc spectra have a pronounced monomer peak in solution and a mixed distribution in the thin film dominated by aggregates similar in shape to TSCuPc.27 For TSCuPc, the resemblance between the thin film and solution spectra in both shape and peak ratio is rather unexpected and the absence at higher wavelengths of a clear monomer peak in solution is very different from that 4917

dx.doi.org/10.1021/jp109544m |J. Phys. Chem. C 2011, 115, 4916–4921


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Figure 3. XRD of thin films: (i) CuPc and (ii) TSCuPc. The inset shows the extended pattern including the ITO peaks as a reference.

seen for CuPc. The low proportion of monomer in solution and the pronounced absorption peak for aggregates indicate that TSCuPc does not entirely dissolve and forms a suspension containing partially dissolved TSCuPc nanocrystals and clusters.20,21 This finding is further supported by the XRD and AFM data presented in the next sections. Further heat treatment, longer dissolution times, and sonication did not change the TSCuPc solution spectrum. 3.2. XRD. XRD data from thin films of TSCuPc from a 20 mg mL-1 (pH 11) solution (and CuPc as a comparison) are shown in Figure 3. There is a clear trend in decreasing diffraction angle (2θ) from unsubstituted CuPc to TSCuPc, with 2θ values of 6.79° and 5.67°, respectively, for the (100) diffraction peak adopting a slipped stack arrangement. The interplanar spacing of the molecular stacks increases from 13.00 Å (6.79°) for R-CuPc to 15.56 Å (5.67°) for the bulkier TSCuPc substituted molecule with four sulfonic acid groups, which require more space in the molecular arrangement. The XRD data of TSCuPc also reveals a higher crystallinity than for CuPc with the appearance of additional peaks at 9.73° for the merged (10-1) and (101) as well as 11.31° for the (200) plane. The diffraction peaks of TSCuPc are also much broader in appearance than the ones found for CuPc which may indicate that these peaks come from an assembly of nanocrystalline domains with different orientations. The peaks were assigned using the simulated R-CuPc diffraction data (triclinic; two molecules/unit cell) using the unit cell parameters proposed by Hoshino et al.28 and fit very well with TSCuPc under the assumption of the same crystalline packing structure with increased interplanar spaces from the bulky substituents. It is likely that the increased crystallinity in the TSCuPc arises from the existence of nanocrystals suspended in the solution before film formation, as seen in the UV-vis spectrum shown in Section 3.1. 3.3. AFM Morphology. AFM images of TSCuPc thin films spun from pH neutral and basic solution with dye concentrations of 10 mg mL-1 and 20 mg mL-1 are shown in Figure 4. The images reveal a mesh of long crystal-like features with no preferred orientation and complete coverage of the ITO substrate. With increasing TSCuPc concentration, the crystal shape of the large crystals in the film becomes more pronounced and the crystal feature length increases due to the coalescence of several smaller crystal segments to one crystal unit indicating

Figure 4. AFM images of spin coated TSCuPc thin films on ITO, processed from water with concentrations of 10 mg mL-1 (a) pH 11 (NH3), (b) pH 7 and 20 mg mL-1, (c) pH 11 (NH3), and (d) pH 7 with the corresponding height profiles shown below each image to highlight changes in crystal size. (e) AFM image of a bare ITO substrate as a comparison.

crystal ripening upon deposition. As the solution was filtered with a 0.2 μm filter, the crystal size of the remaining nanocrystals in the solution should definitely be much smaller than the ones observed in the AFM images due to the primary limitation to the filter pore size. Higher concentrations increase the number of crystals existing in the solution with little influence of the concentration of the solution on the size. The crystals measured on the surface of a thin film were elongated and much larger than the expected size from the filtering process. The crystal subunits, however, are much smaller, 50 nm up to 100 nm in appearance, which is indicated by the AFM images. Smaller crystals could act 4918



Figure 5. (a) J-V curves under 1 sun illumination for devices with (A) 8 nm, (B) 13 nm, (C) 18 nm, and (D) 30 nm TSCuPc film thickness and device (G), which is based on only C60. The inset shows the J-V curves under dark conditions. (b) Schematic OPV device architecture and (c) electronic energy level diagram.

as seed crystals on the substrate surface for the subsequent network formation of the film with the long and pronounced crystals. For both 10 mg mL-1 solutions, at pH 7 and pH 11, the morphology is very similar with crystal features up to 500 nm long and about 50 nm wide forming a dense network (Figure 4a,b). The root-mean-square roughness (rms) of 2.4-2.5 nm indicates that the films are both much smoother and very different in appearance than bare ITO, which has an rms of 4.4 nm (Figure 4e). The height cross sections confirm the round and smooth crystal shapes embedded in the films. For the films made of 20 mg mL-1 TSCuPc, larger crystals of up to 1 μm in length and 100 nm in width are observed with a clear difference in roughness and crystal appearance for the two different solutions used, pH 7 and pH 11 (Figure 4c,d). Processed from basic solution, the rms roughness remains at 2.5 nm, but increases for pH 7 to 3.4 nm with even larger and better defined crystals. The large crystals found on the TSCuPc film surface are assumed to be formed during the spin-coating and/or drying process because of the prior filtration step. 3.4. J-V Device Characteristics. TSCuPc single layers were incorporated in OPV devices with a planar TSCuPc (d nm)/C60 heterojunction structure, as shown schematically in Figure 5b. The devices fabricated varied in TSCuPc film thickness d and solvent pH: device (A) (8 nm, 5 mg mL-1), (B) (13 nm, 10 mg mL-1, (C) (18 nm, 15 mg mL-1), and (D) (30 nm, 20 mg mL-1) from basic solution and device (E) (13 nm, 10 mg mL-1) and (F) (30 nm, 20 mg mL-1) from pH neutral solution. As the spin speed was set to 2000 rpm for all devices, the film thickness

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Figure 6. (a) Summary of performance parameters for devices (A) to (D), including JSC, VOC, FF, and PCE, with varying TSCuPc concentration (pH 11) and film thickness d. (b) Comparison of the EQE of device (B) with the UV-vis absorption spectra of the appropriate TSCuPc/ C60 bilayer and a single layer TSCuPc thin film.

was varied by changes in concentration. As a reference, device (G) was based on just ITO/C60 (40 nm)/ BCP/Al without the TSCuPc layer. Figure 5a shows the J-V curves of devices (A)-(D) and (G) under 1 sun illumination and the inset under dark conditions. The open-circuit voltage (VOC) increases significantly from the thinnest device (A) with 0.29 V to (B) with 0.52 V and levels off toward (D) with 0.59 V, the thickest TSCuPc layer (d = 30 nm). The short-circuit current (JSC) and fill factor (FF) show the opposite trend. The JSC and FF start at 1.56 mA cm-2 and 0.50 for device (A) and decrease steadily with increased TSCuPc film thickness to 1.32 mA cm-2 and 0.28, respectively, for device (D). The PCE starts at 0.23% for device (A), reaches a maximum of 0.32% for device (B), which is close to 0.30% for device (C), and then decreases to 0.23% for device (D). These trends are plotted in Figure 6a and the complete set of values is summarized in Table 1. Devices processed at pH 7 show a similar trend but demonstrate a generally lower overall performance. Device (E) exhibited a VOC of 0.44 V, a JSC of 1.48 mA cm-2, an FF of 0.42 and a PCE of 0.28%. Device (F) produced a VOC of 0.44 V, a JSC of 1.19 mA cm-2, an FF of 0.27 and a PCE of 0.14%. The reference device (G) based on just a C60 layer gave a VOC of only 0.04 V, a JSC of 0.91 mA cm-2, a FF of 0.30 and a PCE of 0.01%. The PCE of 0.32% for the optimized device (B) with 13 nm TSCuPc thickness and processed from basic solution is much higher than any reported values in the literature for OPV devices fabricated with this material.24 4919



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Table 1. Summary of Device Characteristics for OPVs Based on an ITO/TSCuPc (d)/ C60 (40 nm)/ BCP (7 nm)/ Al Structure with Varying TSCuPc Thickness da device

d (nm)

VOC (V)

JSC (mA cm-2)

FF

PCE (%)

(A)

8

0.29

1.56

0.50

0.23

(B)

13

0.52

1.47

0.41

0.32

(C)

18

0.56

1.44

0.37

0.30

(D)

30

0.59

1.32

0.28

0.23

(E) (F)

13 30

0.44 0.44

1.48 1.19

0.42 0.27

0.28 0.14

(G)

0

0.04

0.91

0.30

0.01

a

Devices (A)-(D) were fabricated at pH 11; devices (E) and (F) were fabricated at pH 7. Reference device (G) is based on an ITO/C60 (40 nm)/BCP (7 nm)/Al structure without TSCuPc.

J-V analysis for basic solution processed devices (A)-(D) demonstrate a steep increase in VOC with increasing TSCuPc layer thickness from d = 8 to 30 nm. The best VOC obtained for the TSCuPc/C60 devices is 0.13 V higher than that typically achieved for standard CuPc/C60 devices (0.46 V).29 This gain in VOC can be explained by the increased ionization potential of the donor material caused by the presence of the electron withdrawing sulfonic acid groups. The HOMO of TSCuPc lies at -5.3 eV, which is 0.2 eV deeper than the HOMO of CuPc (-5.1 eV) (Figure 5c).23 For very thin TSCuPc layers, the substrate coverage is probably not uniform due to the crystalline network and C60 can contact through to the ITO substrate forming a bypass which causes a loss in voltage due to the lack of an energetically matching organic heterojunction. The extreme case of VOC loss is demonstrated in device (G), which is based only on C60 and has no Donor/Acceptor heterointerface. The JSC decreases slightly with thicker TSCuPc layers from 1.56 mA cm-2 for device (A) to 1.32 mA cm-2 for device (B). In general, the film thickness of the photoactive layer, namely the donor and acceptor, is a compromise between absorption and the limited exciton diffusion length of organic semiconductors. For thicker films, the quantum efficiency for exciton diffusion decreases and the series resistance (RS) increases. Although an increase in JSC with increasing thickness for such thin layers of only 8 nm is expected based on the higher absorbance, the slightly decreased JSC values suggest that a trade-off in the photocurrent generation limit is already reached for device (A). The JSC maximum at 8 nm TSCuPc thickness may correlate with a short estimated exciton diffusion length for this material. Although the exciton diffusion length of vapor deposited CuPc is reported to be between 10 and 68 nm, it is almost certainly much lower for TSCuPc.1,30-32 It is clear that in thicker layers, the additional absorbed light does not lead to a higher JSC. More importantly, a thicker TSCuPc layer increases the charge collection pathway which is coupled to increased resistive current losses. In contrast to CuPc, solution processed TSCuPc films are made of randomly oriented nanocrystals with a high density of exciton quenching grain boundaries, as seen in the UV-vis, XRD, and AFM studies reported earlier. Originating from the solution process in air-based water, the thin films also contain more impurities and counterions, which act as additional exciton quenchers, influencing both the bulk and interface properties and also leading to possible traces of oxygen and trapped water. The impurities almost certainly originate from the lower purity of the

original compound. In addition, the increased interplanar spacing with the bulky substituent groups is likely to have a detrimental influence on charge mobility as a result of the increased separation between the molecular stacks. Surprisingly, the EQE measurements of device (B), shown in Figure 6b, reveal a clear dominance of C60 as the main contributor to the photocurrent with an EQE of above 15% and almost no contribution from TSCuPc when compared to the absorption spectrum. This finding is further confirmed by the relatively high JSC of 0.91 mA cm-2 obtained for the device, based only on C60 (G), providing about two-thirds of the photocurrent without any donor-acceptor interface present. This leads to the conclusion that the main role of the TSCuPc layer is to provide a donor-acceptor heterointerface, with the relatively large VOC obtained reflecting the interface gap arising from the energy difference between the donor HOMO and acceptor LUMO. Furthermore, this interface enables successful charge transfer for excitons originating from the C60. This is also consistent with the observation that increased layer thicknesses do not result in any further enhancements in photocurrent. The main losses for the devices fabricated with thicker TSCuPc layers are reflected in the reduced FF values, primarily due to the increased RS. This problem could arise from the anisotropic charge mobility along the TSCuPc main crystal axis, which is horizontally aligned to the substrate, as seen in the AFM images (Figure 4). Another cause could be incomplete infiltration of the larger crystal network with monomer and smaller clusters, resulting in unwanted film vacancies. This device behavior can be seen in the well-pronounced kink or so-called double-diode behavior in the J-V curves under illumination (Figure 5a) as the film thickness increases from devices (A)-(D), indicating a charge extraction barrier which leads to charge accumulation. The devices fabricated at pH 7 (E and F) showed a generally poorer performance when compared to those processed from basic solution, with the lower JSC and VOC values, leading to a reduced PCE. As the film thicknesses of these devices are comparable to the ones spun from the same TSCuPc concentration at pH 11, the change in OPV performance is probably related to the change in morphology from smaller crystals, found for basic solution processed films, to larger ones (20 mg mL-1) based on the different dissolution behavior and film formation. The larger grain size for these devices with the elongated crystals aligning parallel to the substrate surface results in a rougher film with partly incomplete substrate coverage, leading to a disturbed D/A interface and therefore a reduced JSC and VOC.

4. CONCLUSIONS The characteristics of water-soluble TSCuPc in solution, thin film, and when incorporated as the donor layer in planar heterojunction OPV devices have been demonstrated. TSCuPc solution consists of dissolved monomer and partially suspended nanocrystals resulting in dense and defined crystal networks on the substrate surface after spin-coating. For optimized devices, a maximum PCE of 0.32% was obtained. This PCE is to our knowledge the highest demonstrated for OPV cells based on water-soluble molecular semiconductors. The TSCuPc devices also showed an improved VOC of up to 0.59 V, a 0.13 V improvement over standard CuPc/C60 devices. Despite this VOC, EQE measurements revealed a very low contribution to the photocurrent from the TSCuPc layer with the C60 acceptor 4920


The Journal of Physical Chemistry C being the main contributor. TSCuPc only serves as a favorable interface for efficient exciton dissociation from the photoactive C60 layer in the device. Devices fabricated with thicker TSCuPc layers suffer from several problems, including an increased RS with thicker TSCuPc layers leading to kinks in the J-V curves under illumination, a short exciton diffusion length compared to CuPc, and a grainy film morphology. The JSC reduction with increasing film thickness over a range from 8 to 30 nm does not follow a steep decline as expected if the device was purely exciton diffusion length limited. The increase in RS with thicker TSCuPc layers induces a carrier extraction barrier due to poor hole conductivity. As seen in the EQE, TSCuPc has almost no contribution to the photocurrent and an increase in film thickness only inhibits carrier extraction. The higher VOC was achieved with the larger TSCuPc HOMO-LUMO band gap compared to CuPc. The advantages of higher VOC and solubility in water through substitution by sulfonic acid functional groups are compromised by a lower charge mobility, lower exciton diffusion length, and a smaller FF with almost no contribution from TSCuPc to the photocurrent and therefore a reduced JSC. Devices from basic solution showed a generally higher performance than devices from pH neutral solution, which followed the same film thickness/performance trend. The main advantage of TSCuPc is its processability from aqueous solution, which demonstrates an important step towards greener, sustainable OPV device fabrication.

’ AUTHOR INFORMATION Corresponding Author

*Tel: þ44 (0)24 7652 8265. Fax: þ44 (0)24 7652 4112. E-mail: [email protected].

’ ACKNOWLEDGMENT The authors would like to thank Luke Rochford, Junliang Yang, and Ian Hancox for their help and useful discussions with the XRD and AFM analysis, respectively. The project was financially supported by BP Solar and the Engineering and Physical Sciences Research Council (EPSRC), U.K. Dr. Ross Hatton is supported by the Royal Academy of Engineering and the EPSRC via a Research Fellowship. Part of the equipment used in this research was obtained through Birmingham Science City: Innovative Uses for Advanced Materials in the Modern World (West Midlands Centre for Advanced Materials Project 2), with support from Advantage West Midlands (AWM) and part funded by the European Regional Development Fund (ERDF).

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Organic Photovoltaic Devices Based on Water-Soluble Copper

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