Graded Absorption Layers in Bulk Heterojunction Organic Solar Cells

Apr 16, 2013 - Bulk heterojunction organic solar cells with a vertical variation of the C60:ZnPc composition within the absorption layer have been fab...
3 downloads 16 Views 317KB Size
Download PDF
Article pubs.acs.org/JPCC

Graded Absorption Layers in Bulk Heterojunction Organic Solar Cells Beatrice Beyer, Richard Pfeifer, Johannes K. Zettler, Olaf R. Hild, and Karl Leo* Fraunhofer Research Institution for Organics, Materials and Electronic Devices Dresden (COMEDD), Maria-Reiche-Straße 2, 01109 Dresden, Germany S Supporting Information *

ABSTRACT: Bulk heterojunction organic solar cells with a vertical variation of the C60:ZnPc composition within the absorption layer have been fabricated. The resulting gradient layer has been characterized by UV/vis transmission and reflection spectroscopy. Depending on the mixing strategy, the formation of higher aggregated ZnPc species can be initialized earlier in a blended thin film. The gradient strength has been varied and its influence on the solar cell performance has been determined. A variation of the absorption layer thickness has been carried out to investigate a possible charge carrier transportation improvement. In order to explain the positive effect of a graded structure within the absorption layer, detailed voltage-dependent spectral response measurements have been performed. It is shown that not only the absorption behavior of the cell is improved, but also the charge carrier transportation properties.

■

INTRODUCTION Organic solar cells (OSC) have drawn much attention in the past years due to their many advantages, such as manufacturability on lightweight, flexible substrates, the possibility to produce transparent modules due to the thin stack architecture, and the adjustment of the desired absorption profile by selecting the appropriate absorbing compound. The first efficient thin-film organic solar cell was fabricated by Tang in 1985, reaching an efficiency of 0.95%.1 Recently, OSC have much improved with reaching an efficiency value of almost 11%, and their introduction in the mass market is becoming likely.2 This improvement was based on active research in academia and industry resulting in complex device architectures comprising the introduction of functional layers, such as charge carrier injection layers, charge carrier transport layers, exciton blocking layers, absorption layers, or the concept of tandem devices. Furthermore, postprocessing steps such as annealing were found to increase the efficiency due to morphology changes.3,4 In order to improve the charge generation by efficient exciton dissociation, the donor (D) and acceptor (A) material of the absorption layers are deposited together within one layer, forming a bulk heterojunction (BHJ) which results in an enhanced interface surface between the two materials.5 The challenge of this approach lies in balancing the gains resulting from the improved exciton dissociation with the losses due to the intrinsically lower charge carrier mobility of these layers compared to neat thin films.6,7 This has been investigated by Rand et al. by determining the hole and electron mobility for differently assembled copper phthalocyanine:fullerene (CuPc:C60) blended thin films. They pointed out that the charge carrier mobility decreases for holes (or electrons, respectively) with enhancing the material concentration of the © 2013 American Chemical Society

acceptor (or donor, respectively). Furthermore, the hole mobility was much more affected by concentration variations than the electron mobility. It is generally agreed that an optimum nanoscale morphology with a balanced interfacial area and continuous pathways are necessary. Although considerable effort has been put into morphology optimization, less work has been performed on absorber layers with controlled compositional changes. Because of the dependency on the charge carrier mobility with regard to the blend composition, a gradient-like structure within the absorption layer should result in cumulative charge carrier collection efficiency toward the electrodes and an improved chemical potential energy gradient.8 Additionally, if the concentration of a lower energy absorbing material such as zinc phthalocyanine (ZnPc) is higher at the opposite side of the reflecting electrode, the graded constitution is supposed to enhance the photocurrent because of the optical field distribution. The interference maximum for short wavelengths is closer to the reflecting aluminum cathode than for longer wavelengths. The existence of a donor−acceptor gradient has already been proposed for several polymer-based solar cells. They have shown better performance compared with their uniform counterparts. The graded composition of the absorptive layers has been realized by appropriate solvent modification,9,10 annealing,3,11 or thermally induced interdiffusion of two films.10,12,13 For small molecule based devices, little work has been done in order to investigate the positive influence of the gradient-like structure. The simplest graded structure has been Received: November 6, 2012 Revised: April 11, 2013 Published: April 16, 2013 9537

dx.doi.org/10.1021/jp3109732 | J. Phys. Chem. C 2013, 117, 9537−9542

The Journal of Physical Chemistry C

Article

2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquino-dimethane (F4TCNQ) and Cs2CO3, respectively.

realized many times without calling it a gradient: Herein, a mixed D:A layer is positioned between a neat donor and acceptor material layer,14 or an interlayer with intermediate, but similar transport properties for holes and electrons is inserted between the donor and acceptor.15 An example where both approaches are realized within one solar cell is described by Takahashi and co-workers.16 Sullivan et al. and Heutz et al. have gone a step further. They have prepared several samples including a device where the constitution of the blended absorption layer has been varied in three steps (CuPc:C60 with 75:25, 50:50, 25:75) surrounding them by neat CuPc and C60.17,18 Pandey and Holmes have intensively studied the impact of “zero end point” gradients starting with 100% of the donor and ending exclusively with the acceptor.19 The increase in JSC and FF has been attributed to an improved charge collection efficiency driving the charges to the appropriate electrode. This has also been confirmed by the work of Chen and co-workers.20 Tress et al. on the other side have used driftdiffusion simulations showing that the gradient helps to reduce recombination processes which is in particular beneficial for hybrid planar/bulk heterojunction structures. Efficiency improvements have mainly been related to optical effects.21 In this contribution, we show that both effects are significant for the cell architecture investigated here. After a detailed optical characterization of the absorption layer has been performed, the positive influence of a gradient absorption layer structure on the solar cell performance is demonstrated by the variation of the gradient strength and the absorption layer thickness.

■

RESULTS AND DISCUSSION Optical Characterization. In the experiments, the deposition rate of the donor (acceptor) has been continuously decreased (increased). The gradient-like structure has also been proven by reflection spectroscopy to exclude a rearrangement of the film during or after evaporation or the inaccuracy of processing. Figure 1 shows the measured reflection spectrum of

Figure 1. Measured reflection spectra of graded (50 → 33% ZnPc, black square) and homogeneously processed (41.5% ZnPc, red circle) C60:ZnPc blended thin films and their simulated analogs (dotted for EMA model, dashed for graded model). The lined curves represent the difference between the two simulated spectra.

■

a graded (50 → 33%, referred to ZnPc) and uniform (41.5% referred to ZnPc), 80 nm thick C60:ZnPc mixed layer deposited on an 85 °C tempered substrate. The spectra have been fitted with the software WVASE32 (J. A. Woollam Co., Inc.) considering a homogeneous (using the effective-mediumapproximation EMA) and a gradient-like (using a graded model) distribution based upon the optical properties of C60 and ZnPc determined from neat films prepared under comparable conditions. Due to the dependency of Q+/Q described below, the oscillator strength has been introduced as a fitting parameter in addition to the film thickness d and the layer composition or gradient, respectively. For the homogeneously prepared samples both models are suitable to describe the measurement. The red dotted line in Figure 1 shows a negligible difference of the two simulated spectra. The graded model results in a fitted gradient of (41 → 38.5%, MSE of 0.81) which is consistent with a homogeneous layer within the expected fitting error range (MSE of EMA fit was 0.92). However, with a graded sample, the two fitting models result in significantly different simulated spectra, as the homogeneous model cannot account for the graded layer composition (MSE of 2.30). The best fit using the graded model results in a gradient of 49 → 30% (MSE of 0.80), which is in the range of the proposed contents according to the deposition process. Furthermore, discriminate morphological features were not observed for a graded and uniform sample according to AFM measurements on an area of 5 × 5 μm2 recorded at different stages during growth of the absorption layer. However, the determination of the transmission behavior revealed that a gradient-like composition with a higher initial ZnPc concentration has supported the formation of higher aggregated ZnPc species. Therefore, for sample preparation for graded and homogeneously processed samples consisting of ITO covered

EXPERIMENTAL SECTION Solar cells were fabricated by consecutive (co)evaporation of organic materials in a custom-made organic material beam deposition tool (Bestec, Germany) at a pressure of 10−8−10−7 mbar and a substrate temperature of 85 °C. The gradient was realized by a continuous, automatic modification of the donor and acceptor material rate. To ensure comparability between graded and homogenously processed layers, special effort was placed on the total material content. The variation due to the processing method (graded vs uniform) of the total content of a material in a layer was below 1%. Precleaned indium tin oxide (ITO) coated glass (Luminescence Technologies) was used as substrate. J−V measurements (Keithley 2400 source meter, Kepco power supply) were recorded in dark conditions and under AM1.5 and 100 mW/cm2 illumination (KHS Technical lighting) at 25 °C. The optical constants were determined by transmission and reflection spectroscopy (Solid Spec, Shimadzu) and by subsequent data fitting with the software WVASE32 (J.A.Woollam Co., Inc.). Spectral response curve data were recorded with a spectrofluorometer (Horiba Jobin Yvon, Fluoromax-4) calibrated with a Si photodiode and a Keithley 2400 source meter. We used a p-i-n device architecture in order to provide ohmic contacts to both electrodes.22 The active layer (ZnPc:C60, materials were twice sublimed prior to utilization) was embedded between a 50 nm thick p-doped hole transport layer (HTL) of 2,2′,7,7′-tetra(m-tolyl-phenylamino)-9,9′-spirobifluorene (spiro-TTB) and a 20 nm thick blocking layer of C60 (BL), followed by an n-doped electron transporting layer of 15 nm C60 (ETL) and a cathode of 200 nm aluminum. For processing reasons, p- and n-doping was done employing the proprietary molecular dopant NDP9 and NDN26 (Novaled AG, Germany), which could be replaced by the open materials 9538

dx.doi.org/10.1021/jp3109732 | J. Phys. Chem. C 2013, 117, 9537−9542

The Journal of Physical Chemistry C

Article

glass, the p-HTL (50 nm) and C60:ZnPc have been stopped at different C60:ZnPc layer thicknesses ranging from 30 to 180 nm. This resulted in graded samples with an increasing gradient with rising layer thickness. The concentration of the homogeneously prepared samples was independent of the layer thickness. The corresponding absorbance spectra are depicted in Figure 2. The ratio of the peak heights at 610 nm

Table 1. Key Parameters of Various Solar Cells in Dependence on the Gradient Strengtha structure, content of ZnPc

VOC/mV

JSC/mAcm−2

FF/%

η/%

39 → 33% 36% 50 → 33% 41.5% 66 → 33% 50%

539 531 526 512 511 505

5.93 5.96 7.86 7.44 7.73 6.19

44.5 46.3 51.4 47.0 58.3 52.6

1.42 1.45 2.14 1.80 2.21 1.65

a

Glass/130 nm ITO/50 nm p-doped spiro-TTB/35 nm C60:ZnPc/20 nm C60/15 nm n-doped C60/200 nm Al. The substrate temperature during deposition was set to 85 °C. The total amount of ZnPc is equal both for graded as well as uniform absorption layers.

Figure 2. Absorbance spectra of graded (top, left) and homogeneously processed (top, right) C60:ZnPc mixed thin films deposited on an ITO surface covered with the p-HTL (50 nm) in dependence on the C60:ZnPc layer thickness (from black, 30 nm, to orange, 180 nm). The bottom plot shows the absorbance ratios of Q+/Q-peaks for the graded and uniform films with growing layer thicknesses of the C60:ZnPc film (from black, 30 nm, to orange, 180 nm). In graded samples, the concentration of higher aggregates increases faster than for uniform samples.

Figure 3. J−V curves of graded (filled symbols) and uniform (open symbols) organic solar cell under illumination (line + symbol) and in the dark (line) in dependence on the absorption layer composition.

The introduction of a gradient-like structure in the absorption layer has directly affected the performance of the solar cells. Whereas the positive contribution of a small gradient (39 → 33% vs 36% ZnPc) can be hardly separated from the reference sample, it becomes obvious after the graded structure is sufficiently pronounced. The improvement on the short circuit current (JSC) and the fill factor (FF) of graded samples with regard to their homogeneous analogues correlates with the strength of the gradient, and hence results in the highest efficiency at a graded absorption layer with a 66 → 33% ZnPc distribution. In principle, the graded samples are subject to the same physical relationships as their reference samples with modifying the blend constitution. VOC lowers with increasing the total content of ZnPc, which has also been reported elsewhere.29 This drop in VOC has been related to different polarization energies in the mixtures, which results in a shift of the HOMO level from ZnPc.30 Comparing the VOC between the graded and uniform samples, it seems that graded samples always show slightly higher values. Since the increase is in the range of deviations due to processing fluctuation and measurements, it is difficult to conclude here a significant trend. However, numerical simulations predicted an increase of around 10 mV for comparable graded samples.21 The higher the total content of ZnPc, the lower the series resistance, which can clearly be seen in the fourth quadrant of Figure 3. Furthermore, the series resistance of all graded samples lowers with the gradient strength. This can be attributed to regions within the absorption layer with higher charge carrier mobility, in particular for holes since a higher sensitivity has been reported for holes than for electrons in phthalocyanine:fullerene blends.6 The improvement becomes significant for 66 → 33% graded samples. The parallel resistance is affected neither by the total content of ZnPc in

(Q+, corresponding to aggregated molecules with cofacial arrangement) and 690 nm (Q, claimed to be related to monomeric species) indicates the composition of ZnPc within the blended layer.23−28 The bottom part of Figure 2 shows the ratios of Q+/Q with growing layer thickness. Initially, the content of higher aggregated ZnPc species is almost equal for both sample types. However, with rising thickness, the concentration of the higher aggregates rises which is originated to boundary surface effects. In graded samples, the concentration of higher aggregates increases faster than that for uniform films and saturates at a certain point. For uniform samples, this saturation ratio is reached about 90 nm later. Hence, with a gradient-like structure, the formation of higher aggregated ZnPc has been accelerated. This higher aggregation grade may result in a comparatively higher hole mobility on the ZnPc sites, which will be beneficial for the device performance due to facilitated charge carrier extraction. Organic Solar Cells. In order to investigate the influence of the gradient strength on the solar cell performance, the gradients have been varied by continuously increasing the initial concentration of ZnPc, but keeping the total thickness and amount in the absorption layer constant with respect to the uniform reference sample. The corresponding key values of the resulting gradient-like samples and their homogeneous analogs are summarized in Table 1. The J−V characteristics in the dark and under illumination are presented in Figure 3. 9539

dx.doi.org/10.1021/jp3109732 | J. Phys. Chem. C 2013, 117, 9537−9542

The Journal of Physical Chemistry C

Article

circuit condition, the internal electrical field decreases. Because bimolecular recombination increases and the charge separation efficiency declines, the spectral response signal becomes weaker. Brenner et al. used the shape variations in his voltage dependent EQE spectra because of its sensitivity to film morphology and microstructure.31 For this purpose, three sample pairs have been chosen for a detailed investigation with the following C60:ZnPc structure: 50 nm with total content of 41.5%, 35 nm with 41.5%, and 35 nm with 50% of ZnPc within the absorption layer. As an example, the recorded curves for the last sample pair is depicted in Figure 5. An increase in the

the absorption layer nor by the layer architecture. Therefore, the FF reaches its maximum with a graded cell at a total content of 50% ZnPc, whereas JSC reaches its maximum with a graded sample at 50 → 33% ZnPc. After the positive influence of the gradient strength has been demonstrated, the absorption layer thickness has been varied from 35 to 60 nm. Figure 4 presents the corresponding J−V

Figure 4. J−V curves of graded (filled symbols) and uniform (open symbols) organic solar cell under illumination (line + symbol) and in the dark (line) in dependence on the absorption layer thickness.

curves of the solar cells in the dark and under illumination. The key values are summarized in Table 2. VOC is unaffected by the Table 2. Key Parameters of Various Solar Cells in Dependence on the Thickness of Graded and Homogeneously Processed Absorption Layersa structure

dAL/ nm

dBL/dETL/ nm

VOC/ mV

JSC/ mAcm−2

FF/%

η/%

graded uniform graded uniform graded uniform

35 35 50 50 60 60

20/15 20/15 5/15 5/15 5/5 5/5

517 515 513 511 507 508

9.55 9.62 9.86 9.72 10.10 9.73

55.2 53.8 50.5 46.2 49.2 45.0

2.71 2.66 2.56 2.30 2.51 2.24

Figure 5. Spectral response measurements of organic solar cells with graded (66 → 33% ZnPc, top) and uniform (50% ZnPc, bottom) absorption layers (35 nm) in dependence on the applied bias.

a

Glass/130 nm ITO/50 nm p-doped spiro-TTB/C60:ZnPc/C60/ndoped C60/200 nm Al. The substrate temperature during deposition has been set to 85 °C. The gradient-like structure has been realized with 50 → 33% ZnPc and has been referenced with 41.5% ZnPc content.

applied bias leads to a reduction of the measured photocurrent. For better comparison, the lower subplot shows the curves normalized to the first peak at 340 nm. For homogeneously processed samples (lower plots), the normalized shape of the spectra remains unchanged with rising bias voltages up to 400 mV. Only a bias voltage close to Voc leads to a slight change. For the graded sample on the other hand, a different behavior appears. The normalized spectra reveal a decrease at the C60 related bands and an increase for the ZnPc attributed bands. Because of the distribution of the incident light within the absorption layer, most of the blue light is absorbed by C60 close to the cathode, where also the concentration of that molecule is higher. The red light is more absorbed by ZnPc close to the anode. Furthermore, the hole mobility is in a C60:ZnPc blend lower than for electrons. Holes that are generated by excitons originating from C60 related absorption in the blue region of light have to move for a long distance with a slowly rising mobility toward the anode. Thus, many holes recombine with electrons before reaching the transport layers. On the other hand, the current originating from ZnPc increases, which is a result of normalization at 340 nm where both ZnPc and C60 related excitons contribute to that peak. This has also been described by Tress and co-workers.21 We conclude that the positive influence of a graded structure originates from the

thickness modification and the introduction of a gradient of 50 → 33%, JSC rises with the layer thickness due to the enhanced absorption and, thus, charge carrier generation, whereas the FF drops with increasing the absorption layer thickness due to the higher charge carrier recombination probability. Furthermore, the parallel resistance increases with the absorption layer thickness which can be assigned to the modification of the blocking and electron transport layers in order to keep the overall device thickness constant. Hence, the maximum value for the efficiency has been obtained by the 35 nm thin layer sample with 2.71%. Nevertheless, with respect to JSC and FF, all graded samples show a better performance than their uniform analogues. Moreover, the difference between the graded and uniform sample increases with greater absorption layer thickness. Electrical Characterization. In order to study the influence of a gradient-like structure on the absorption layers, voltage dependent spectral response (SR) measurements have been recorded. When the applied voltage reaches the open 9540

dx.doi.org/10.1021/jp3109732 | J. Phys. Chem. C 2013, 117, 9537−9542

The Journal of Physical Chemistry C

Article

and absorption layer thicknesses. The results show that a gradient at a particular strength, which is far beyond the composition fluctuations during processing, increases the efficiency. The improvement is attributed to a better matching absorption profile and at a specific strength to an increase of the charge carrier mobility, which compensates the rising recombination probability. It is assumed that these results can also be transferred to other donor:acceptor systems, but have to be investigated for each system itself. Furthermore, although realizing a graded structure has been more time-consuming when using point sources for evaporation than for uniform layers, this principle can easily be integrated in mass production, where inline sources are used for deposition.

better light incoupling adjusted to the absorption profile of the materials and its distribution in the absorption layer. In order to prove this, the relative decrease of the photocurrent signal from 0 to 400 mV applied bias has been determined and is summarized in Table 3. Starting with the 35 nm sample with a Table 3. Overview of the Spectra Decay for Graded and Uniform OSC in Dependence on the Composition with a Bias Difference from 0 to 400 mV composition (ZnPc)

dAL/nm

decrease/% (0−400 mV)

50 → 33% 41.5% 50 → 33% 41.5% 66 → 33% 50%

50 50 35 35 35 35

−29.2 −29.0 −26.2 −23.6 −20.8 −30.0

■

ASSOCIATED CONTENT

S Supporting Information *

Absolute and normalized bias dependent spectral response measurements for organic solar cells with 50→33% or 41.5% ZnPc in 35 and 50 nm thick absorption layers. This material is available free of charge via the Internet at http://pubs.acs.org.

gradient of 50 → 33%, the relative decrease of the photocurrent is for the graded sample larger than that for the uniform one, which means that the aforementioned recombination of the holes is higher for graded cells than for uniform samples. An increase of the thickness from 35 to 50 nm shows that both structures behave similarly. As expected, the absolute value of the decrease is higher, resulting from the enhanced recombination since the charge carriers have to pass a longer distance through the thicker absorption layer. For both types, the recombination extent is similar, which means that the increased recombination for graded cells is compensated. Since the improved absorption for graded samples is for all sample pairs in the same range, the higher number of generated excitons can not be used as the only explanation. With a closer look to the sample pair with the stronger gradient 66 → 33% (35 nm thick), the difference between the relative decline is remarkable. While the current drops only by 21% for a graded sample, the current falls by 30% for the uniform sample. Whereas the recombination has increased for uniform samples, it has dropped for graded cells. Recombination can be reduced by increasing the mobility of the charge carriers, which is supposed to be the case in graded layers. Since this property is supposed to be more pronounced for 66 → 33% than for 50 → 33% devices, the charge transport improvement is measurable by the stronger gradient sample. The positive influence of a graded absorption layer at low gradient strengths is thus attributed to the matched absorption profile for a mixed system. After the gradient strength overcomes a certain level, the mobility improvement due to the varied mixing ratio of hole transporting donor and electron transporting acceptor contributes significantly to the increased photocurrent. Resulting from that, a high concentration of the donor material at the p-side and of the acceptor material at the n-side within a solar cell enables the compensation of the charge carrier mobility loss after the intermixing with a counter material, as it is the case for blended absorption layers.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +49351 8823 145. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Parts of this work have been funded by BMBF within the project OPEG partner. Lieselotte Ilg is kindly acknowledged for her support with the SR measurements and Prof. Dieter Wöhrle for discussion.

■

REFERENCES

(1) Tang, C. W. Two-Layer Organic Photovoltaic Cell. Appl. Phys. Lett. 1985, 48, 183−185. (2) Tandem solar cell by Heliatek with an efficiency of 10.7% on an area of 1.1 cm2, certified at SGS, press release, 2012. (3) Pfützner, S.; Meiss, J.; Petrich, A.; Riede, M.; Leo, K. Thick C60:ZnPc Bulk Heterojunction Solar Cells with Improved Performance by Film Deposition on Heated Substrates. Appl. Phys. Lett. 2009, 94, 253303. (4) Heutz, S.; Sullivan, P.; Sanderson, B. M.; Schultes, S. M.; Jones, T. S. Influence of Molecular Architecture and Intermixing on Photovoltaic, Morphological and Spectroscopic Properties of CuPcC60 Heterojunctions. Sol. Energy Mater. Sol. Cells 2004, 83, 229−245. (5) Kumar, A.; Li, G.; Hong, Z.; Yang, Y. High Efficiency Polymer Solar Cells with Vertically Modulated Nanoscale Morphology. Nanotechnology 2009, 20, 165202. (6) Rand, B. R.; Xue, J.; Uchida, S.; Forrest, S. R. Mixed DonorAcceptor Molecular Heterojunctions for Photovoltaic Applications. I. Material Properties. J. Appl. Phys. 2005, 98, 124902. (7) Xue, J.; Rand, B. P.; Uchida, S.; Forrest, S. R. Mixed DonorAcceptor Molecular Heterojunctions for Photovoltaic Applications. Ii. Device Performance. J. Appl. Phys. 2005, 98, 124903. (8) Gregg, B. A. Excitonic Solar Cells. J. Phys. Chem. B. 2003, 107, 4688−4698. (9) Wang, D. H.; Lee, H. K.; Choi, D.-G.; Park, J. H.; Park, O. Solution-Processable Polymer Solar Cells from a Poly(3-hexylthiophene)/[6,6]-Phenyl C61-Butyric Acidmethyl Ester Concentration Graded Bilayers. Appl. Phys. Lett. 2009, 95, 043505. (10) Loiudice, A.; Rizzo, A.; Latini, G.; Nobile, C.; de Giorgi, M.; Gigli, G. Graded Vertical Phase Separation of Donor/Acceptor Species for Polymer Solar Cells. Sol. Energy Mater. Sol. Cells 2012, 100, 147.

■

CONCLUSION A gradient-like structure has been implemented in a C60:ZnPc blended absorption layer in order to improve the solar cell performance. The graded absorption layer has been characterized by reflection and absorbance measurements. It turned out that a graded structure accelerates the formation of higher ZnPc aggregates in a blended film. Organic solar cells have been successfully fabricated with different gradient strengths 9541

dx.doi.org/10.1021/jp3109732 | J. Phys. Chem. C 2013, 117, 9537−9542

The Journal of Physical Chemistry C

Article

(11) Kim, Y.; Choulis, S. A.; Nelson, J.; Bradley, D. C.; Cook, S.; Durrant, J. R. Device Annealing Effect in Organic Solar Cells with Blends of Regioregular Poly(3-hexylthiophene) and Soluble Fullerene. Appl. Phys. Lett. 2005, 86, 063502. (12) Kaur, M.; Gopal, A.; Davis, R. M.; Heflin, J. R. Concentration Gradient P3HT/PCBM Photovoltaic Devices Fabricated by Thermal Interdiffusion of Separately Spin-Cast Organic Layers. Sol. Energy Mater. Sol. Cells 2009, 83, 1779−1784. (13) Drees, M.; Davis, R. M.; Heflin, J. R. Improved Morphology of Polymer-Fullerene Photovoltaic Devices with Thermally Induced Concentration Gradients. J. Appl. Phys. 2005, 97, 036103. (14) Hiramoto, M.; Fujiwara, H.; Yokoyama, M. Three-Layered Organic Solar Cell with a Photoactive Interlayer of Codeposited Pigments. Appl. Phys. Lett. 1991, 58, 1062−1064. (15) Yoshino, K.; Tada, K.; Fujii, A.; Conwell, E. M.; Zakhidov, A. A. Novel Photovoltaic Devices Based on Donor-Acceptor Molecular and Conducting Polymer Systems. IEEE Trans. Electron Devices 1997, 44, 1315−1324. (16) Takahashi, K.; Kuraya, N.; Yamaguchi, T.; Momura, T.; Murata, K. Three-Layer Organic Solar Cell with High-Power Conversion Efficiency of 3.5%. Sol. Energy Mater. Sol. Cells 2000, 61, 403−416. (17) Sullivan, P.; Heutz, S.; Schultes, S. M.; Jones, T. S. Influence of Codeposition on the Performance of CuPc-C60 Heterojunction Photovoltaic Devices. Appl. Phys. Lett. 2004, 84, 1210−1212. (18) Heutz, S.; Sullivan, P.; Sanderson, B. M.; Schultes, S. M.; Jones, T. S. Influence of Molecular Architecture and Intermixing on the Photovoltaic, Morphological and Spectroscopic Properties of CuPcC60 Heterojunctions. Sol. Energy Mater. Sol. Cells 2004, 83, 229−245. (19) Pandey, R.; Holmes, R. J. Graded Donor-Acceptor Heterojunctions for Efficient Organic Photovoltaic Cells. Adv. Mater. 2010, 22, 5301−5305. (20) Chen, L.; Tang, Y.; Fan, X.; Zhang, C.; Chu, Z.; Wang, D.; Zou, D. Improvement of the Efficiency of CuPc/C60-Based Photovoltaic Cells Using a Multistepped Structure. Org. Electron. 2009, 10, 724− 728. (21) Tress, W.; Leo, K.; Riede, M. Effect of Concentration Gradients in ZnPc:C60 Bulk Heterojunction Organic Solar Cells. Sol. Energy Mater. Sol. Cells 2011, 95, 2981−2986. (22) Maennig, B.; Gebeyehu, D.; Simon, P.; Kozlowski, F.; Werner, A.; Li, F.; Grundmann, S.; Sonntag, S.; Koch, M.; Leo, K.; et al. Organic p-i-n Solar Cells. Appl. Phys. A: Mater. Sci. Process. 2004, 79, 1−14. (23) Kment, S.; Kluson, P.; Dobrek, M.; Kuzel, R.; Gregora, I.; Kohout, M.; Hubicka, Z. Preparation of Thin Phthalocyanine Layers and Their Structural and Absoprtion Properties. Thin Solid Films 2009, 517, 5274−5279. (24) Senthilarasu, S.; Sathyamoorthy, R.; Lalitha, S.; Subbarayan, A.; Natarajan, K. Thermally Evaporated ZnPc Thin Films − Band Gap Dependence on Thickness. Sol. Energy Mater. Sol. Cells 2004, 82, 179− 186. (25) Keil, C.; Tsaryova, O.; Lapok, L.; Himcinschi, C.; Wöhrle, D.; Hild, O. R.; Zahn, D. R. T.; Gorun, S. M.; Schlettwein, D. Growth and Characterization of Thin Films Prepared from PerfluoroisopropylSubstituted Perfluorophthalocyanines. Thin Solid Films 2009, 517, 4379−4384. (26) Leznoff, C. C.; Lever, A. B. P. Phthalocyanines: Properties and Applications; VHC: New York, 1996. (27) Zanfolim, A. A.; Volpati, D.; Olivati, C. A.; Job, A. E.; Constantino, C. J. L. Structural and Electric-Optical Properties of Zinc Phthalocyanine Evaporated Thin Films: Temperature and Thickness Effects. J. Phys. Chem. C 2010, 114, 12290−12299. (28) Ray, D.; Furno, M.; Sibert-Henze, E.; Leo, K.; Riede, M. Quantitative Estimation of Electronic Quality of Zinc Phthalocyanine Thin Films. Phys. Rev. B 2011, 84, 075214. (29) Tress, W.; Pfuetzner, S.; Leo, K.; Riede, M. Open Circuit Voltage and IV Curve Shape of ZnPc:C60 Solar Cells with Varied Mixing Ratio and Hole Transport Layer. J. Photonics Energy 2011, 1, 011114.

(30) Park, S. H.; Jeong, J. G.; Kim, H.-J.; Park, S. H.; Cho, M.-H.; Cho, S. W.; Yi, Y.; Heo, M. Y.; Sohn, H. The Electronic Structure of C60/ZnPc Interface for Organic Photovoltaic Device with Blended Layer Architecture. Appl. Phys. Lett. 2010, 96, 013302. (31) Brenner, T. J. K.; Li, Y.; McNeill, C. R. Phase-Dependent Photocurrent Generation in Polymer/Fullerene Bulk Heterojunction Solar Cells. J. Phys. Chem. C 2011, 115, 22075−22083.

9542

dx.doi.org/10.1021/jp3109732 | J. Phys. Chem. C 2013, 117, 9537−9542