Efficient Organic Photovoltaic Cells through Structural Modification of

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Efficient Organic Photovoltaic Cells through Structural Modification of Chloroaluminum Phthalocyanine/Fullerene Heterojunctions K. V. Chauhan, P. Sullivan, J. L. Yang, and T. S. Jones* Department of Chemistry, UniVersity of Warwick, CoVentry, CV4 7AL, United Kingdom ReceiVed: NoVember 06, 2009; ReVised Manuscript ReceiVed: December 23, 2009

The structure-function relationship of organic photovoltaic (OPV) cells based on the chloroaluminum phthalocyanine (ClAlPc)/fullerene (C60) planar heterojunction are explored. Optimization is achieved with the use of a molybdenum oxide (MoOx) and an underlying 3,4,9,10-perylene tetracarboxylic acid (PTCDA) interlayer at the hole extracting electrode, the latter acting as a structural template for the subsequent growth of the ClAlPc donor layer. OPV cells demonstrate power conversion efficiencies of 3.0% under simulated AM1.5G (air mass 1.5 global) illumination, with the short-circuit current (Jsc) showing an ∼25% improvement relative to a device without a templating layer. Results from X-ray diffraction and electronic absorption spectroscopy suggest an improved packing and crystallinity in the ClAlPc layer when deposited on the PTCDA template, which suggests an enhancement in charge transport through the film. External quantum efficiency measurements confirm an overall improvement in Jsc in the ClAlPc spectral region upon templating. The effect of the MoOx interlayer is to minimize losses in the open-circuit voltage and fill factor caused by significant band bending and pinning of the adjacent organic layer highest occupied molecular orbital levels to nonstoichiometric defect states in the near Fermi level region of MoOx. The results present an improved strategy for the development of higher-performance OPV cells based on small molecule heterojunctions. 1. Introduction Considerable progress to improve the performance of small molecule organic photovoltaic (OPV) cells has been achieved in recent years through variation of material sets to optimize energy level offsets and to maximize the short-circuit current (Jsc), open-circuit voltage (Voc), and fill factor (FF), since the power conversion efficiency (ηp) of the cell is proportional to these three terms.1-3 The performance of OPVs based on planar divalent phthalocyanines (e.g., CuPc and ZnPc) has been wellstudied, and these materials generally require codeposition with an acceptor material such as C60 to increase the Jsc and overcome limitations imposed by the short exciton diffusion lengths.4,5 An alternative method to overcome some of the restrictions imposed by the material sets and improve OPV cell performance was achieved by exploiting the structure-function relationship in perylene-phthalocyanine heterostructures.6-8 Sullivan et. al showed that a 60% improvement in the Jsc could be achieved by modifying the CuPc crystal orientation with the use of a very thin 3,4,9,10-perylenetetracarboxylic acid (PTCDA) interlayer at the hole extracting electrode, although there was no improvement in ηp due to the reduced Voc obtained.9 There has recently been a focus on the use of tri- and tetravalent phthalocyanine donor materials because they offer the possibility of extending the spectral response into the nearinfrared region of the electromagnetic spectrum.10,11 Furthermore, the electronic properties of these donor materials have been shown to be tunable by variation of the deposition conditions.2,12,13 Although extending the spectral response is clearly an important issue, this must be achieved without a reduction in the measured Voc, which would limit the overall ηp of the cell.14 * Corresponding author. Phone: +44(0)24 7652 8265. E-mail: [email protected].

Figure 1. Chemical structure of the main organic materials used for OPV cell fabrication.

In this paper, we demonstrate the beneficial effects of inserting an additional ultrathin PTCDA layer at the interface of the hole extracting electrode and the ClAlPc donor layer in a ClAlPc/ C60 heterojunction OPV cell (Figure 1). The organic semiconductor PTCDA, which is known to lie essentially flat on the surface of weakly interacting substrates,15 is used to template the orientation of the ClAlPc molecules through enhanced intermolecular interactions between the adjacent π-clouds of the PTCDA and ClAlPc molecules.16 Optimized cells, which also include a MoOx interlayer, result in a near 25% increase in the measured Jsc, with a minimal reduction in the Voc and FF and a subsequent increase in power conversion efficiency to 3.0% under simulated AM1.5G illumination.

10.1021/jp910601k  2010 American Chemical Society Published on Web 02/01/2010

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2. Experimental Methods Device Fabrication. Indium tin oxide (ITO)-coated glass substrates were used (Psiotec, 100 nm thick ITO, Rs < 15 Ω sq-1) after cleaning by ultrasonication in acetone, detergent, and isopropyl alcohol. The substrates were then exposed to an atmosphere of ozone under illumination by UV light to remove any remaining carbon residues. The basic OPV cell consisted of a 1 nm PTCDA (Aldrich) templating layer, a 20 nm ClAlPc (Acros Organics) donor layer, a 40 nm C60 (Nano-C Inc.) acceptor layer, and an 8 nm bathocuproine (BCP-Aldrich) exciton blocking layer. Occasionally, a 5 nm MoOx (Aldrich) layer was also deposited onto the ITO surface prior to deposition of the organic materials. The organic layers were grown by vacuum sublimation in a multisource Kurt J. Lesker Spectros organic molecular beam deposition system with a base pressure of about ∼2 × 10-8 mbar. All organic materials were purified by thermal gradient sublimation prior to deposition. Deposition rates were monitored using a series of quartz crystal microbalances, and all materials were thoroughly outgassed prior to film deposition. Deposition rates of 0.10, 0.02, 0.10, 0.05, and 0.10 nm s-1 were used for MoOx, PTCDA, ClAlPc, C60, and BCP, respectively. Aluminum electrodes were deposited in situ by evaporation through a shadow mask to a thickness of 100 nm, giving an active area of 0.16 cm2. Power conversion efficiencies were calculated under simulated AM1.5G solar illumination at ∼100 mW cm-2 using a Newport Oriel solar simulator with the intensity varied using neutral density filters. The intensity was measured using a Fraunhofer Institute of Solar Energy Systems calibrated monocrystalline silicon solar cell (PV Measurements, Inc.). The cells were sealed in an airtight sample holder in an inert N2 atmosphere. J-V curves were measured with a Keithley 2400 source meter, and the data were recorded using a LabView interface. External quantum efficiency (EQE) measurements were performed with light from the Newport Solar Simulator coupled into a PTI monochromator. Order-sorting optical cutoff filters were positioned at the monochromator input. The monochromatic light intensity was measured with a calibrated Si photodiode (Thorlabs). Light incident on the device was chopped into a square wave at 500 Hz, and the modulated current signal detection was performed by a current-voltage amplifier (Laser Component UK Ltd., DHPCA-100) and lock-in amplifier (Stanford Research SR 830). Electronic absorption spectra were measured using a PerkinElmer Lambda 25 spectrometer, and X-ray diffraction was performed using an X’Pert PRO (PANalytical) diffractometer with Cu KR radiation (λ ) 1.54056 Å). 3. Results 3.1. Electronic Absorption Spectroscopy. Figure 2 shows normalized electronic absorption spectra for pristine films of 20 nm ClAlPc, 6 nm PTCDA, and a 1 nm PTCDA/20 nm ClAlPc bilayer. The ClAlPc solution spectrum is also shown and highlights the Q-band with a monomer peak at 670 nm with vibronic progressions at shorter wavelengths. The pristine 20 nm ClAlPc film shows a maximum at 740 nm with shoulders either side centered at 670 and 770 nm, similar to previous reports.17 The shoulder at 670 nm correlates well with the absorption maximum from the solution spectrum and is most likely to arise from the presence of monomeric species, with the shoulder at 770 nm arising from higher-order aggregation.18,19 We also note the presence of an absorption band in the near-

Figure 2. Normalized electronic absorption spectra for pristine 6 nm PTCDA films (g), pristine 20 nm ClAlPc (0), ClAlPc in solution (O) and a 1 nm PTCDA/20 nm ClAlPc bilayer film (]). All films were grown on ITO substrates.

infrared region (∼825 nm), which has been previously wellcharacterized for ClAlPc in solution.20 The spectrum for the 1 nm PTCDA/20 nm ClAlPc bilayer shows a reduction in intensity of the shoulder at 670 nm, suggesting that the contribution from monomeric species has been reduced. This is accompanied by an increase in the intensity of the band at 770 nm. We believe the origin of this change in the bilayer film is a larger contribution from higher aggregates, which increases the intensity of the weaker peak at 770 nm in the pristine ClAlPc films. The dramatic rise in the intensity suggests a substantial increase in the aggregation and ordering of the molecules within the bilayer film. The change in the ClAlPc Q-band shape is not affected by the thickness of the PTCDA templating layer and is still observed for a 1-nm-thick underlying PTCDA film. 3.2. X-ray Diffraction (XRD). XRD traces for a pristine 70 nm ClAlPc film as well as films comprising 1 nm PTCDA/70 nm ClAlPc and 70 nm PTCDA/70 nm ClAlPc are shown in Figure 3. The trace for pristine PTCDA has also been included, and a peak is observed at 2θ ) 27.6° corresponding to diffraction from the (102) diffraction plane.21 The peak at 30.2° has been included as an ITO reference. The thickness of the ClAlPc layer was chosen to improve the signal-to-noise ratio in the XRD of ClAlPc, since in its pristine thin film form, it has been reported to be largely amorphous.10 A 70 nm pristine ClAlPc film showed very weak signals centered at 2θ ) 26.8°, suggesting very limited ordering. Furthermore, the film did not show any diffraction peaks in the 2θ ) 5-10° region, where a diffraction peak is typically observed for phthalocyanine films that adopt a “standing up” molecular configuration.22 On the basis of a previous structural characterization,23 this peak was indexed as the (014j) plane. Its presence suggests the formation of some ordering within the films, where ClAlPc molecules show short-range cofacial packing with the Cl atoms in adjacent molecules pointing toward each other.24,25 The increase in intensity of this peak after growth on the PTCDA layer reflects the improved crystallinity within the bilayer film. The trace for a 70 nm PTCDA/70 nm ClAlPc bilayer, which shows two welldefined peaks at 26.8° and 27.6°, confirms that the peak at 26.8° is induced by the underlying PTCDA. It should be noted that the (102) peak of PTCDA remains unaffected by subsequent growth of the ClAlPc layer, consistent with the results from electronic absorption spectroscopy, which

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Figure 3. Thin film XRD scans for (a) pristine 70 nm ClAlPc, (b) 1 nm PTCDA/70 nm ClAlPc, (c) 70 nm PTCDA/70 nm ClAlPc and (d) pristine 70 nm PTCDA, all grown on ITO substrates. The arrow illustrates an increase in film crystallinity on templating the ClAlPc with PTCDA. Each data set was normalized to the (222) plane of ITO at 30.2°.

show that the structure of the underlying PTCDA is not influenced by any interaction with the ClAlPc overlayer. On the basis of the above discussion, the schematic in Figure 4 outlines the proposed structural changes in the ClAlPc film upon templating and highlights the improved stacking and increased crystallinity in the PTCDA/ClAlPc bilayer. After templating, the ClAlPc molecules stack in a near-parallel, or “lying-down”, orientation with respect to the substrate. The (014j) plane is parallel to the substrate, and the phthalocyanine molecular plane is (001), which was obtained from the lattice structure and commercially available Mercury software. Using this, the software can be used to predict the angle between the two planes and was determined to be ∼14° (Figure 4b). This increased ordering and preferential stacking should be beneficial for improving OPV cell performance. 3.3. Device Performance. Figure 5 shows J-V curves under simulated AM1.5G illumination for OPV cells with the architectures highlighted in Table 1. The thickness of the PTCDA templating layer was fixed at 1 nm, with thicker layers resulting in a reduction of key cell characteristics. Table 1 summarizes the results obtained, and a clear increase in ηp was afforded by virtue of an increase in Jsc from 5.33 to 6.53 mA cm-2 through insertion of 5 nm MoOx and 1 nm PTCDA layers at the ITO/ ClAlPc interface. Control cells with the architecture ITO/20 nm ClAlPc/40 nm C60/8 nm BCP/Al and ITO/5 nm MoOx/20 nm ClAlPc/40 nm C60/8 nm BCP/Al have also been included. In the absence of the MoOx interlayer in the templated device, an ∼25% reduction in Voc was observed from 0.59 to 0.46 V, consistent with previous results for PTCDA/CuPc/C60 OPV cells.9 This was explained by the large barrier for the extraction of holes at the PTCDA/CuPc interface, which leads to an accumulation of charges within the device. Despite an increase in Jsc from 5.33 to 6.13 mA cm-2, the reduction in Voc and FF in the ClAlPc/C60 cells was enough to reduce the overall efficiency from 1.8 to 1.4%. There is, therefore, no net benefit of including the PTCDA layer alone in the ClAlPc/C60 cell. However, by insertion of a 5 nm MoOx layer at the ITO/PTCDA interface, the Voc decreased by a much smaller amount, and ηp improved from 2.6 to 3.0%. To ensure that the MoOx layer itself was not changing the structural or absorption properties of the PTCDA/ClAlPc

Chauhan et al. heterostructure, electronic absorption spectra and XRD data based on the ITO/MoOx/PTCDA /ClAlPc architecture were recorded, and no difference was found from those presented in Figures 2 and 3. Furthermore, the dark current characteristics for the templated cells were also checked (both with and without the MoOx interlayer) to ensure that the reduction in Voc was not due to enhanced leakage currents.26 No difference in the dark current behavior was observed.27 The external quantum efficiency for ClAlPc/C60 heterojunctions with and without a PTCDA templating layer on MoOx were recorded to confirm the improvements in Jsc. As expected, the general shape of the EQE spectra follow the absorption spectra with the EQE for the ClAlPc/C60 heterojunction peaking at ∼740 nm (17%), whereas the PTCDA/ClAlPc/C60 heterojunction peaks at ∼770 nm (22%). The increase in the EQE suggests that the improvement in Jsc is due to the induced structural changes in the ClAlPc layer, which are likely to enhance exciton diffusion to the ClAlPc/C60 heterojunction, thus facilitating improved charge carrier generation. Although several reports have commented on the improved cell performance with a MoOx layer in organic light emitting diode structures,28,29 the function in OPVs remains unclear. A common trait on the effect of MoOx appears to be the induction of a large vacuum level shift (∼2 eV) with both the ITO and the adjacent organic species.28 Furthermore, the large difference in work function between MoOx and the adjacent organic layers is likely to cause significant band-bending in ClAlPc and PTCDA.30,31 Band-bending across a junction would have the effect of forming a built-in field that enhances hole extraction from the HOMO levels of the organic materials that are pinned close to the Fermi level of MoOx. Strong band-bending effects have previously been observed to affect the interface electronic structure in PTCDA, with a difference in work function of 0.83 eV causing significant band-bending in thick PTCDA films.31 The difference in work function between MoOx and PTCDA is considerably larger than this (∼3 eV), so band-bending effects on an ultrathin PTCDA layer would be expected to be very large. We believe that the HOMO level of an ultrathin PTCDA layer is therefore pinned in the near-Fermi-level region of the MoOx. On the basis of the previous investigations of the MoOx/ ClAlPc interface,30 it is likely that the effects induced by MoOx would extend out beyond the thin 1 nm PTCDA to the ClAlPc interface and so overcome barriers (∆HOMOPTCDA - ∆HOMOClAlPc ) 1.3 eV) that exist for thicker films of PTCDA/ ClAlPc heterojunctions.32 This would avoid significant charge build-up at the hole-extracting electrode and prevent the reduction in Voc that is observed for the templated heterojunctions in the absence of the MoOx interlayer. The pinning of the HOMO levels in PTCDA and ClAlPc to the Fermi level of MoOx is likely to occur via a distribution of nonstoichiometric defect states (oxygen vacancies) situated close to the Fermi level in the bandgap of MoOx that have been observed with core-level X-ray photoelectron spectroscopy (XPS).33,34 The precise composition of the oxide layer has proved difficult due to the resolution limits in XPS, but our preliminary studies suggest x to be in the range 2.95-2.99. Furthermore, inverse photoemission spectroscopy measurements on MoOx have shown that the conduction band minimum lies just above the Fermi level, suggesting n-type behavior.35 This is also likely to assist in the extraction of holes and prevent a large reduction in Voc. On the basis of the cell data presented, we therefore propose the interfacial electronic energy level alignment in Figure 7. Detailed analysis of the electronic

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Figure 4. (a) Schematic of the proposed structural changes for the ClAlPc film after growth on PTCDA. The 1 nm PTCDA layer changes the molecular orientation into a “lying-down” configuration with closer ClAlPc-ClAlPc distances and dramatically increases the film crystallinity. (b) Representation of the single crystal data22 showing the orientation of the (014j) plane with respect to the substrate. The lattice parameters of ClAlPc are also highlighted.

Figure 5. Current density vs voltage (J-V) curves for the cells with the architecture ITO/ClAlPc/C60/BCP/Al (0), ITO/PTCDA/ClAlPc/C60/BCP/ Al(]), ITO/MoOx/ClAlPc/C60/BCP/Al (O), and ITO/MoOx/PTCDA/ClAlPc/C60/BCP/Al (g) under AM1.5G illumination. The arrows indicate an increase in Jsc and Voc upon insertion of the PTCDA and MoOx, respectively. The optimized cell architecture is also highlighted on the right.

TABLE 1: Short-Circuit Current, Open-Circuit Voltage, Fill Factor, and Power Conversion Efficiencies for Different OPV Cell Architectures Based on the Photoactive ClAlPc/C60 Heterojunction

Figure 6. External quantum efficiency data for the cell architectures ITO/MoOx/ClAlPc/C60/BCP/Al (0) and ITO/MoOx/PTCDA/ClAlPc/C60/ BCP/Al (O).

device architecture

Jsc, mA cm-2

Voc, V

FF

ηp, %

ITO/ClAlPc/C60/BCP/Al ITO/PTCDA/ClAlPc/ C60/BCP/Al ITO/MoOx/ClAlPc/ C60/BCP/Al ITO/MoOx/PTCDA/ ClAlPc/C60/BCP/Al

5.33 6.13

0.59 0.46

0.58 0.50

1.8 1.4

5.30

0.81

0.58

2.6

6.53

0.79

0.58

3.0

structure at these interfaces with photoemission experiments will be important to obtain an accurate determination of the exact band line up in these oxide-organic heterojunctions, and this will be the subject of a future investigation.

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Figure 7. Proposed energy level diagram in the near interface region for the heterostructure: ITO/MoOx/1 nm PTCDA/1-2 nm ClAlPc. A large shift in the vacuum level (VL) of the adjacent layers is predicted. The relative magnitude and direction of the VL shift at the PTCDA/ ClAlPc interface has been estimated from previous work.31 The large difference in the work function between the MoOx and the PTCDA/ ClAlPc causes band bending (Vb) in the near interface region.

4. Conclusions A nearly 25% increase in Jsc for an optimized ClAlPc/C60 OPV cell has been achieved through structural templating of ClAlPc with an underlying PTCDA layer. The increase in Jsc was achieved predominatly through an induced structural change, which improved stacking within the ClAlPc molecules. The maxima from the absorption spectra correspond to the spectral response from the EQE measurements for the cells under investigation. Insertion of a MoOx layer leads to further improvements in cell performance through the large difference in work function between MoOx and the organic materials, which causes significant band-bending and enhances the builtin field. The MoOx layer also acts to pin the HOMO levels of the adjacent organic semiconductors to defect states in the near Fermi level of the n-type MoOx interlayer, thus allowing for improved hole extraction. The MoOx interlayer minimizes losses in the Voc, and a power conversion efficiency of 3.0% was achieved. The optimized structure shows how the orientation of trivalent phthalocyanines can be tailored to improve Jsc while maintaining a high Voc and FF. The results also demonstrate how the thin film structure of photoactive organic materials can be tailored to improve the overall performance of organic solar cells. Acknowledgment. This work was supported by the Engineering and Physical Sciences Research Council (EPSRC), U.K., under the Supergen Excitonic Solar Cell Consortium. References and Notes (1) Mutolo, K. L.; Mayo, E. I.; Rand, B. P.; Forrest, S. R.; Thompson, M. E. J. Am. Chem. Soc. 2006, 128 (25), 8108.

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