Correlating the Polymorphism of Titanyl Phthalocyanine Thin Films

Letter pubs.acs.org/JPCL

Correlating the Polymorphism of Titanyl Phthalocyanine Thin Films with Solar Cell Performance Karolien Vasseur,†,‡ Barry P. Rand,*,† David Cheyns,† Kristiaan Temst,§ Ludo Froyen,‡ and Paul Heremans†,∥ †

Imec, Kapeldreef 75, B-3001 Heverlee, Belgium Department of Metallurgy and Materials Engineering, KU Leuven, Kasteelpark Arenberg 44, B-3001 Heverlee, Belgium § Instituut voor Kern- en Stralingsfysica, KU Leuven, Celestijnenlaan 200 D, B-3001 Heverlee, Belgium ∥ Department of Electrical Engineering, KU Leuven, Kasteelpark Arenberg 10, B-3001 Heverlee, Belgium ‡

S Supporting Information *

ABSTRACT: The structure of titanyl phthalocyanine (TiOPc) thin films is correlated with photovoltaic properties of planar heterojunction solar cells by pairing different TiOPc polymorph donor layers with C60 as an acceptor. Solvent annealing and the insertion of two different templating layers, namely 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) and CuI, prove to be effective methods to control the TiOPc thin film structure. The crystal phase of TiOPc thin films was identified by combining X-ray reflectivity (XRR) measurements with spectroscopic techniques, including absorption and micro-Raman measurements. Implementation of a donor layer with an absorption spectrum extending into the near-infrared (NIR) led to solar cells with external quantum efficiencies (EQEs) above 27% from λ = 600 − 890 nm, with the best device yielding a power conversion efficiency (PCE) of 2.6%. Our results highlight the need to understand the relationship between processing parameters and thin film structure, as these have important consequences on device performance. SECTION: Plasmonics, Optical Materials, and Hard Matter

T

II or Y has previously been demonstrated by applying postdeposition treatments such as thermal22−24 and solvent annealing,6,7,25,26 whereby the as-deposited film is exposed to heat or concentrated solvent vapors, respectively. The crystal structure of TiOPc thin films can also be influenced by (i) the growth conditions, including deposition rate and substrate temperature,27,28 (ii) the deposition technique,29,30 and (iii) the substrate.8,31−33 For solar cell applications, the growth of the NIR-active polymorph is of particular interest to maximize the short-circuit photocurrent density (JSC). In comparison to other nonplanar Pc's exhibiting NIR-sensitivity, TiOPc-based solar cells combine a high JSC with open-circuit voltages (VOC) exceeding 0.6 V.6,7,34 In this work, we compare the performance of planar heterojunction solar cells based on an amorphous, phase I, phase II, or phase Y TiOPc donor layer in combination with C60 as an acceptor material. The different TiOPc polymorphs are achieved by varying the substrate temperature (Tsub), solvent annealing as-deposited films, and employing two different surface modification layers, namely 1H,1H,2H,2Hperfluorodecyltrichlorosilane (FDTS) and CuI. These surface modification layers not only affect the crystal phase, but also

he high potential for organic photovoltaics lies in the compatibility of organic semiconductors with low-temperature and low-cost fabrication processes to enable large-area solar cell applications.1 To date, the maximal power conversion efficiency (PCE) of organic solar cells exceeds 10%,2 and this value is poised to increase even further, enabling this technology to enter the market. One approach to improve solar cell performance is to broaden the spectral overlap by combining solar cells that operate in different regions of the solar spectrum into a stacked structure.3,4 Since a significant part of the total solar photon flux is located at wavelengths λ > 800 nm, there is a need for well-performing donor materials that have an absorption spectrum extending into the nearinfrared (NIR). Among small-weight semiconductors, nonplanar phthalocyanines (Pc's) such as TiOPc are promising donor materials because of their high absorption coefficients and polymorphism that enables tuning of optoelectronic characteristics.5 Some polymorphs exhibit a packing arrangement that increases the π−π interaction between adjacent Pc rings, thereby causing a broadening of the Q-band spectrum toward the NIR.6−14 For TiOPc, the structure of four polymorphs has been described: a monoclinic phase I15 and phase C,16 a triclinic phase II,15 and another monoclinic phase Y.17 The latter two polymorphs are characterized by a redshifted absorption spectrum with a dominant peak at λ = 860 nm.18−21 The phase transition to the NIR-active TiOPc phase © 2012 American Chemical Society

Received: July 20, 2012 Accepted: August 13, 2012 Published: August 13, 2012 2395

dx.doi.org/10.1021/jz300993p | J. Phys. Chem. Lett. 2012, 3, 2395−2400

The Journal of Physical Chemistry Letters

Letter

influence the molecular orientation.35 Self-assembled monolayers (SAMs) such as FDTS lead to an edge-on orientation of TiOPc molecules,33 whereas CuI is reported to induce face-on orientation of copper36 and zinc Pc molecules.37 The combination of X-ray reflectivity (XRR) techniques with spectroscopic measurements, including absorption and microRaman spectroscopy, allows us to evaluate the out-of-plane ordering and the crystal structure of the various TiOPc thin films and to correlate this to photovoltaic performance. Figure 1 plots the XRR measurements of four different TiOPc thin films. For a 16 nm thick film deposited at Tsub = 25

preferential orientation of the TiOPc molecules when deposited on FDTS. The low energy surface characteristic for SAMs is known to favor a molecular orientation in which the direction of π−π overlap is parallel to the substrate surface.33 After solvent annealing (Figure 1d), the diffraction peak shifts to 2θ = 7.6°, consistent with a vertical periodicity of d = 11.6 Å. This value indicates that TiOPc molecules keep their preferred edge-on orientation despite the solvent treatment, but the latter could have induced a phase transition. The observed Bragg peak is superimposed on a peak resulting from constructive interference between the FDTS and SiO2 surfaces, as verified by an XRR measurement on the effective substrate. Consequently, the TiOPc diffraction peak might have shifted, thereby complicating quantitative analysis. To confirm the TiOPc peak position, we performed an off-specular XRD measurement by fixing the incidence angle at θ = 0.2° and scanning 2θ. These data are plotted in Figure S1, available in the Supporting Information. The measurement shows a large diffraction peak at 2θ = 7.6° and small diffraction peaks at 2θ = 25.4° and 28.6°. The positioning of these diffraction peaks corresponds to the calculated diffraction data from the phase II polymorph.19,33 We further evaluate the crystal structure and crystallinity of the TiOPc thin films with micro-Raman measurements. Coppedè and co-workers29 performed a systematic study of TiOPc layer structure by combining absorption data with micro-Raman measurements, which allowed them to correlate distinctive features in the Raman spectra to a specific TiOPc polymorph. They report that the amorphous character of a TiOPc layer can be evaluated by investigating the (i) TiO, (ii) isoindole, and (iii) pyrrole stretching mode, located around 950, 1450, and 1530 cm−1, respectively. Each of those peaks can consist of two components: the peak positioned at lower frequency is typical of a crystalline structure, whereas the higher frequency component is correlated to the amorphous character of the film. Figure 2 assembles the micro-Raman spectra for the four different TiOPc films that were investigated using XRR. Upon comparing the intensity ratio of the peaks at 1512 cm−1

Figure 1. The XRR measurements of vapor-deposited TiOPc thin films: (a) amorphous, (b) phase I, (c) phase Y, and (d) phase II. Deposition conditions and the applied solvent treatment for “d” are indicated in the figure.

°C on MoO3 (Figure 1a), Bragg peaks that originate from ordered vertical stacking of TiOPc molecules are absent. Despite the low degree of out-of-plane ordering in this layer, the finite size oscillations (at 2θ < 3°) are indicative of a limited thin-film roughness. Upon insertion of a CuI templating layer between the TiOPc thin film and MoO3 (Figure 1b), Bragg peaks emerge at 2θ = 13° and 26.1°; the latter peak is the most prominent and corresponds to an interplanar spacing of d = 3.4 Å. This vertical periodicity corresponds to the distance between two Pc-rings in a slipped stacking arrangement, thereby indicating a face-on orientation of the TiOPc molecules.31,38 The presence of the Bragg peak at 2θ = 13° suggests that the stacking unit is a bilayer composed of a convex and concave pair of TiOPc molecules.38 Upon comparing our measurements to reported diffraction data of the different TiOPc polymorphs,18,31 the diffraction peaks at 2θ = 13° and 26.1° can be assigned to phase I. Figure 1c corresponds to a TiOPc film of 18 nm thickness deposited at Tsub = 90 °C on top of an FDTSmodified SiO2 substrate, whereas Figure 1d shows the XRR measurement of the same film after solvent annealing in chloroform. Prior to solvent annealing (Figure 1c), a Bragg peak is visible at 2θ = 7°. This corresponds to a vertical periodicity of d = 12.4 Å, suggesting an edge-on arrangement of the TiOPc molecules. The peak could originate from the phase Y polymorph as its calculated diffraction spectrum exhibits a diffraction peak around 2θ = 7.3°.17 This diffraction peak is rather weak for a polycrystalline film, confirming the

Figure 2. Micro-Raman spectra of (a) amorphous, (b) phase I, (c) partial phase Y, and (d) phase II TiOPc thin films. 2396



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and 1530 cm−1 (pyrrole stretching), 940 cm−1 and 950 cm−1 (TiO stretching), and 1434 cm−1 and 1450 cm−1 (isoindole stretching), the micro-Raman spectra of the TiOPc film deposited at Tsub = 25 °C on MoO3 (Figure 2a) is indicative of a highly disordered film, consistent with the XRR data. However, weak Raman peaks around 1512 cm−1 and 940 cm−1 suggest the presence of a small crystalline fraction. On the CuI templating layer, the growth of the phase I polymorph is promoted according to the XRR data. The micro-Raman spectra (Figure 2b) are indicative of a film with relatively high crystallinity, as the peak at 950 cm−1 is absent, and the crystalline component of the pyrrole and isoindole stretching mode is significantly larger than their amorphous components. Furthermore, the peaks at 678 cm−1 and 750 cm−1 dominating the region below 1000 cm−1 is described as a typical signature of the phase I polymorph.29 The Raman spectrum of the TiOPc film deposited on FDTS at Tsub = 90 °C (Figure 2c) almost coincides with that of the amorphous film. Although XRR measurements suggested the presence of a crystalline fraction in the film, the absence of any higher order Bragg peak might relate to the partially amorphous character of the film. The micro-Raman measurement of the solvent annealed TiOPc film (Figure 2d) displays the highest crystalline character, as the pyrrole stretching peak is located at 1514 cm−1 and has no shoulder. Similarly, the TiO stretching peak comprises only a crystalline component, positioned at 940 cm−1. The peak at 680 cm−1 (macrocycle breathing) has an intensity comparable to that of the pyrrole stretching mode, and is followed by three peaks with similar intensities, at 752, 839, and 940 cm−1, which is a distinct feature of the phase II polymorph.29 As previously mentioned, a different crystal structure leads to different intermolecular interactions,5 which in turn influence optical transitions. This difference is reflected in the Q-band of the absorption spectrum, meaning that the absorption properties in the visible/NIR range are sensitively dependent on the crystalline structure.29 Amorphous TiOPc films display a doublet consisting of a dominant peak at λ = 720 nm and a shoulder at λ = 660 nm, whereas in the phase I polymorph, these peaks are red-shifted to λ = 760 nm and λ = 670 nm, respectively. The absorption features of phase II and phase Y are very similar, with the dominant features in the Q-band positioned at λ = 630 and λ = 850 nm. Figure 3 displays the absorption spectra of four distinct TiOPc layers deposited on

glass substrates. The absorption spectrum of the TiOPc film deposited at Tsub = 25 °C on MoO3 is commensurate with that of an amorphous film, consistent with the absence of Bragg peak in the XRR measurement. When grown on CuI, the TiOPc film is characterized by a very different absorption spectrum with features reminiscent of phase I.18,19,27 The high absorption strength can be correlated with the preferential faceon orientation of the TiOPc molecules,36,37 as indicated by the XRR measurements. The absorption spectrum of the solvent annealed TiOPc film coincides with the reported absorption spectrum of phase II.7,8,18,19 The Q-band in the absorption spectrum of the TiOPc film deposited at Tsub = 90 °C on FDTS exhibits a distinct shoulder around λ = 836 nm, suggesting the presence of a significant fraction of NIR-active polymorph in the film next to an amorphous part, giving rise to the absorption peak at λ = 725 nm. When comparing this spectrum to the reported absorption spectrum of TiOPc phase Y, there is a strong resemblance.18,19 This observation is in agreement with the assignment of the diffraction peak at 2θ = 7° to the phase Y form. To correlate TiOPc thin film structure with solar cell performance, the various TiOPc polymorphs are employed as donor layers in planar heterojunction solar cells with the structure: ITO/MoO3 (2 nm)/(CuI (1 nm) or FDTS)/TiOPc (16 or 18 nm)/C60 (45 nm)/BCP (10 nm)/Ag. Figure 4

Figure 4. Measured EQE spectrum for bilayer solar cells based on the four different TiOPc layers as donor layer and paired with C60 as acceptor.

depicts the external quantum efficiency (EQE) spectra of bilayer solar cells with the amorphous, phase I, phase II, or partial phase Y TiOPc thin films as donor layers. In these spectra, photocurrent contribution from C60 is generated between λ = 400 and 550 nm, and from TiOPc between λ = 550 and 1000 nm. All EQE spectra follow the absorption spectra of the constituent layers, indicating that the donor layer structure is homogeneous. The broadened absorption spectrum of phase II and phase Y polymorphs results in a significant improvement in the generated photocurrent when implemented in bilayer solar cell structures. For solar cells with phase II and phase Y donor layers, the EQE values are over 19% and 27% from λ = 600 to 890 nm, respectively, with peak values of 40% at λ = 855 nm and 25% at λ = 845 nm. Despite the narrower absorption window of the phase I TiOPc layer, the resulting solar cell exhibits a relatively high JSC due to EQE peak value of 33% at λ = 730 nm. This might be explained by the face-on orientation of the TiOPc molecules maximizing the

Figure 3. Absorption spectra of amorphous (blue solid line), phase I (gray dash-dotted line), phase II (red dashed line), and partial phase Y (green dotted line) TiOPc thin films. 2397



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improvement of JSC, and consequently solar cells based on the NIR-active phase II and phase Y polymorphs exhibit a better performance. For the solar cell with a phase II donor layer, a JSC of 7.9 mA/cm2 is combined with a high FF, thereby yielding an optimized device with a PCE of 2.6%.

absorption strength. We also note that the C60 signal is similar for all TiOPc/C60 heterojunctions except for the slightly higher EQE in the phase II TiOPc case. This could be related to multiple factors originating from differences in crystallinity, molecular orientation, and energy levels that can all have an impact on charge transfer.8,37 Table 1 summarizes the performance parameters JSC, open circuit voltage (VOC), fill factor (FF) and PCE from the solar

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EXPERIMENTAL METHODS Materials and Deposition Methods. The substrates used in this study are glass substrates with 80 nm thick prepatterned indium−tin-oxide (ITO, Kintec) and highly doped n++-type silicon wafers with thermally grown SiO2. Substrate cleaning consisted of sonication in detergent, deionized water, and acetone, followed by submersion in hot isopropanol. Finally, a 15 min ultraviolet-O3 treatment was applied. The organic materials, TiOPc (Sigma-Aldrich), C60 (SES research), and bathocuproine or BCP (C26H20N2, SigmaAldrich), were purified once using gradient sublimation, while Ag, MoO3 and CuI were used as received. All these layers are deposited in a high vacuum evaporator with a base pressure of ∼1 × 10−7 Torr. The deposition rate (rdep) of TiOPc was kept constant at 1 Å/s, as monitored by a quartz crystal microbalance. Substrate temperatures (Tsub) of 25 °C, without heating, and 90 °C were used. The growth of TiOPc was studied on top of (i) MoO3, (ii) FDTS, or (iii) CuI. On all samples, 2 nm of MoO3 was evaporated at a rate of 1 Å/s. On selected substrates, CuI was deposited in the same system at a rate of 0.2 Å/s, or FDTS was applied by vapor phase deposition in a home-built oven at 140 °C under reduced pressure. The molecular structure of FDTS is displayed in Figure S2. The solvent annealing treatment involved placing the as-deposited TiOPc film together with a vial of chloroform in a closed container for 5 h. For solar cell structures, subsequent layers (C60, BCP, Ag) were deposited in the same evaporator without breaking vacuum. No substrate heating was applied during deposition of these layers. The Ag cathode was evaporated through a shadow mask, defining an active area of 0.134 cm2. Thin Film Characterization. Micro-Raman and XRR measurements were performed on TiOPc films deposited onto Si/SiO2 substrates with the different templating layers. The XRR measurements were performed on a PANalytical X’Pert Pro Materials Research Diffractometer using Cu Kα radiation. In order to reduce interference and facilitate interpretation of the XRR measurements, no MoO3 layer was deposited in-between the FDTS-layer and the SiO2 substrate (cf. Figure 1c,d). MicroRaman spectra were obtained with a LabRAM HR system from Horiba equipped with a CCD detector and a He−Ne laser (λ = 633 nm emission). A grating of 600 grooves/mm was used, and the accuracy of the peak position was around 0.5 cm−1. Absorption spectra of the TiOPc films on glass substrates were measured between 300 and 1100 nm with a Shimadzu UV1601PC UV−visible spectrophotometer. Solar Cell Characterization. Current density−voltage characteristics of photovoltaic cells were measured in the dark and under simulated solar light, using a Keithley 2602 in combination with an Abet solar simulator, calibrated to produce 100 mW/cm2 AM1.5G illumination. In the EQE setup, light from Xe and quartz halogen lamps was coupled into a monochromator, and their intensities were calibrated with a Si photodiode. The light incident on the device was chopped, and the modulated current signal was detected with current− voltage and lock-in amplifiers.

Table 1. Performance Parameters of Planar Heterojunction Solar Cells from Figure 4, Measured under 1 sun AM1.5G Illumination TiOPc donor layer amorphous (14 nm) phase I (16 nm) phase II (18 nm) partial phase Y (18 nm)

JSC (mA/cm2)

VOC (V)

FF (%)

JSC,EQEa (mA/cm2)

PCEb (%)

4.6

0.57

54

4.7

1.4

6.9 8.7 9.2

0.6 0.54 0.46

44 61 46

7.0 7.9 9.1

1.8 2.6 1.9

a

JSC,EQE is the JSC expected by integration of the EQE spectrum over the AM1.5G solar spectrum. bThe PCE of the photovoltaic devices is calculated as [JSC,EQE·VOC·FF/(100 mW/cm2)].

cells of Figure 4 measured under calibrated 100 mW/cm2 AM1.5G illumination. Photovoltaic cells based on an amorphous TiOPc donor layer or phase I polymorph exhibit high VOC values, whereas solar cells based on the NIR-active polymorphs, namely, phase II and Y, are characterized by lower VOC values. This observation is in agreement with the work of Placencia et al.6 The significant variation in FF and VOC might be related to different energetics at the anode/TiOPc and TiOPc/C60 interfaces, which are impacted by the degree of crystallinity and crystal structure of the TiOPc donor layer, as well as by the molecular orientation. It is difficult to deconvolve the changes in FF and VOC from two aspects: the simple presence of the various templating layers and their impact on thin film structure. However, we feel that the structure of the TiOPc thin film, which has an influence on molecular orbitals, charge transport, and charge transfer, is the dominant mechanism reflected in our device comparison. The impact on JSC is, however, more straightforward, as it is dominated by the change in absorption of the various TiOPc thin films. For example, the solar cell based on the partial phase Y donor layer exhibits a JSC of over 9 mA/cm2, but has a limited PCE of 1.9% due to its low FF. The solar cell with the phase II TiOPc donor layer has a slightly lower JSC, but achieves a PCE of 2.6% as a result of an improved FF of 61%. In summary, the texture as well as the crystallinity and crystal phase of various TiOPc films was thoroughly investigated by combining XRR measurements with spectroscopic data. This information was correlated with the parameters enabling structural control, which allowed us to fabricate solar cells based on phase I, phase II, phase Y, or amorphous TiOPc donor layers. Solvent annealing was shown to induce the formation of the phase II polymorph, whereas changing the templating layer promoted the growth of a specific TiOPc phase and influenced the molecular orientation. On CuI, the phase I polymorph is obtained with the TiOPc molecules adopting a preferential face-on orientation, while on FDTS a partially crystalline phase Y is obtained with the TiOPc molecules exhibiting an edge-on arrangement. For photovoltaic devices, the high response in the NIR is crucial for the 2398



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ASSOCIATED CONTENT

S Supporting Information *

Figure S1 shows the off-specular XRD measurements of the TiOPc films deposited at Tsub = 90 °C onto an FDTS-modified SiO2 substrate before and after solvent annealing. Figure S2 shows the molecular structure of FDTS. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Electronic address: [email protected]. Phone: +32 16287780. Fax: +32 16 281097. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.V. acknowledges the Institute for the Promotion of Innovation through Science and Technology in Flanders (IWT-Vlaanderen) for financial support. K.T. acknowledges the KU Leuven GOA 09/006 research program. The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/20072013) under Grant No. 287818 of the X10D project.

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REFERENCES

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Correlating the Polymorphism of Titanyl Phthalocyanine Thin Films

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