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Structural templating of chloro-aluminum phthalocyanine layers for planar and bulk heterojunction organic solar cells
Organic Electronics 12 (2011) 2131–2139
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Structural templating of chloro-aluminum phthalocyanine layers for planar and bulk heterojunction organic solar cells Bregt Verreet a,b,⇑, Robert Müller a, Barry P. Rand a, Karolien Vasseur a,b, Paul Heremans a,b a b
Imec, Polymer and Molecular Electronics, Kapeldreef 75, B-3001 Leuven, Heverlee, Belgium ESAT, Katholieke Universiteit Leuven, Kasteelpark Arenberg 10, B-3001 Leuven, Heverlee, Belgium
a r t i c l e
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Article history: Received 25 July 2011 Received in revised form 29 August 2011 Accepted 29 August 2011 Available online 22 September 2011 Keywords: Organic solar cell Chloro-aluminum phthalocyanine FDTS Structural templating Bulk heterojunction
a b s t r a c t Chloro-aluminum phthalocyanine (ClAlPc) film growth on 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) and MoO3 is studied and correlated to ClAlPc/C60 solar cell performance for both planar and bulk heterojunction (HJ) architectures. On top of unheated substrates, ClAlPc films grow amorphous independent of the substrate surface. When heated to 105 °C, ClAlPc grows with a face-on orientation on MoO3, with a crystalline phase I-like absorption profile. On FDTS, the film is optically characterized as phase II, and adopts an edge-on orientation. Implemented in planar HJ cells, the latter films show a substantially higher current compared to the other growth conditions, leading to 3% efficient cells. This current increase is investigated with spectral response and reflectivity measurements, and is found to be related to a more efficient exciton dissociation. Next, ClAlPc and C60 are co-evaporated on FDTS and MoO3 modified ITO substrates to fabricate bulk HJ devices. Notably, we find that when a thin pure ‘‘templating’’ layer of ClAlPc is grown first, the subsequently grown ClAlPc:C60 bulk HJ propagates the templating effect, and films show a higher crystallinity than without this templating layer, with higher fill factors as a result. On MoO3, this approach yields efficiencies above 4%. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Progress [1] in the power conversion efficiency of organic photovoltaic cells (OPVCs) has generated significant interest for the generation of low-cost, renewable energy. The first critical improvement of OPVC device architecture was the introduction of the donor/acceptor (D/A) interface [2] that functions as a dissociation site for strongly bound excitons that are the product of light absorption in an organic semiconductor. The next major development was the bulk heterojunction (BHJ) architecture, whereby an interpenetrating D/A network ensures that a high percentage of excitons are able to diffuse towards a nearby D/A interface [3,4]. In solution-processed solar cells, such a BHJ is usually made by mixing donor and acceptor in one ⇑ Corresponding author at: Imec, Polymer and Molecular Electronics, Kapeldreef 75, B-3001 Leuven, Heverlee, Belgium. Tel.: +32 16 28 19 80. E-mail address: [email protected] (B. Verreet). 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.08.031
solution prior to deposition. In small-molecule evaporated OPVCs, BHJs are typically formed via co-evaporation of the donor and acceptor materials. The solar cells investigated here are based on chloroaluminum phthalocyanine (ClAlPc)/fullerene C60 as a D/A system. This combination has shown promise as it produces a high open-circuit voltage (Voc) of up to 0.85 V with respect to its near infrared absorption edge of 1.5 eV [5,6]. However, ClAlPc/C60 cells seem very sensitive to growth conditions, as varying the ClAlPc evaporation rate from 0.1–1.5 Å s1 leads to a variation in Voc from 0.49–0.71 V. Later, it was found that the deposition rate affects the molecular orientation of the ClAlPc molecules relative to the indium tin oxide (ITO) substrate, and that this in turn modifies the energy level alignment at the organic heterojunction interface [7]. Another effective way to control the growth is by modifying the substrate surface properties. To template the growth of subsequent ClAlPc layers [8,9], an interlayer composed of 3,4,9,10-perylene tetracarboxylic
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acid (PTCDA) has been used at the anode, whereby the improved stacking of the ClAlPc molecules led to an increase in short-circuit current density Jsc of 25% [10]. For organic transistors, the use of self-assembled monolayers (SAMs) is a well-established technique for the control of the growth [11]. Using this approach, the molecular orientation of ClAlPc molecules could be changed from face-on (parallel to the substrate) to edge-on (normal to the substrate) by treating a SiO2 substrate with octadecyltrichlorosilane [12]. For OPVCs, SAMs have been used to treat the anode, though here the aim has typically been to adjust the ITO work function, rather than adjusting the growth of subsequent layers [13–16]. To obtain a BHJ that combines good transport properties with efficient exciton separation, the control of morphology and growth is even more critical. For example, the co-evaporation of a planar phthalocyanine (Pc) and C60 at room temperature will typically lead to an amorphous film [17,18]. Compared to the pure CuPc layer these amorphous films have poor hole mobilities, limiting cell efficiency. By applying an elevated substrate temperature (Tsub) during Pc:C60 deposition, the efficiency can be improved compared to deposition at room temperature [19]. Also here, the growth surface can have a large impact on the solar cell performance, with films grown on 3,4-polyethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS) showing a different morphology and lower efficiency compared to devices with films grown directly on ITO [20]. For ClAlPc, Li et al. showed that the surface area of the films could be increased by using oblique angle vacuum deposition [21]. This resulted in an increase in Jsc in their ClAlPc/C60 cells, thereby showing that ClAlPc cells could be improved with a bulk heterojunction approach. First promising results for co-evaporated ClAlPc:C70 BHJ cells have already been reported [22], where Tsub was shown to have a strong impact on the growth of the blend, with nanocrystal formation at an elevated temperature from 370–390 K. Consequently, Jsc and FF were enhanced significantly, resulting in a power conversion efficiency of 4.1%. In this article, we will further investigate the growth of ClAlPc for BHJ solar cells. We will focus on the use of structural templating layers, an approach which has already proven successful in planar heterojunction cells, but has not been thoroughly investigated for their role to improve BHJ-based cells. The two different surfaces we compare as templating layers are molybdenum oxide (MoO3) and MoO3 treated with a SAM of 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS). First, pure ClAlPc films were grown on top of these surfaces, and the influence of Tsub on the film properties was investigated. Planar HJ solar cells were then produced from these pure films to probe how the different growth properties affect the ensuing solar cell performance. Next, we proceed by growing co-evaporated ClAlPc:C60 films at elevated Tsub in order to grow crystalline BHJ layers. Here, we observe a significant device performance improvement depending on the presence of an initial pure ClAlPc templating layer underneath the BHJ layer. Bulk heterojunction solar cells produced with these blends all show increased current with respect to the planar HJ devices, with power conversion efficiencies (g) greater than 4%.
2. Experimental Glass substrates coated with ITO (Kintec, 85 nm, sheet resistance <20 X h1) arevv cleaned by subsequent ultrasonic treatment in detergent, de-ionized water, acetone, and iso-propanol, for 5 min each, followed by an ultraviolet-ozone treatment for 900 s. On the samples for film characterization, a 25 nm thick layer of PEDOT:PSS (HC Starck Clevios P) was deposited. Prior to FDTS surface modification, 10 nm of MoO3 is evaporated at a rate of 1 Å s1 and pressure of 107 Torr in a vacuum chamber. Then FDTS is applied by vapor phase deposition in a home-built oven under reduced pressure (140 °C, 30 min.). The MoO3 (550 °C), ClAlPc (350 °C), C60 (380 °C) and bathocuproine (BCP) (150 °C) materials are evaporated at a rate of 1 Å s1 in a vacuum chamber at a pressure of 107 Torr. The mixed ClAlPc:C60 (1:1) layer was formed by keeping both rates constant at 0.5 Å s1. Deposition rates are determined from calibration of the film thickness, measured ex-situ by spectroscopic ellipsometry (Sopra GESP-5). All organic materials are purified before loading into the chamber by thermal gradient sublimation. Tsub was elevated with heat lamps, calibrated by thermocouple measurements. Before subsequent layers were deposited on the heated layer, samples were cooled to 40 °C. The cathode, Ag (107 Torr; 1.5 Å s1), is deposited through a shadow mask, without breaking vacuum. X-ray reflectivity (XRR) measurements were performed on a PANalytical X’Pert Pro Materials Research Diffractometer using Cu Ka radiation. An integration time of 7 s per 2h of 0.01° was used for scanning h/2h. Absorptance spectra are measured with a Shimadzu UV-1601PC. The surface morphology was studied with atomic force microscopy (AFM) using a Picoscan PicoSPM LE scanning probe in tapping mode. Static contact angle measurements were performed on MoO3 and ClAlPc films with an OCA-20 contact angle meter from DataPhysics Instruments GmbH with SCA20 software. Electrical characterization of the solar cells is performed with a Keithley 2602 measurement unit. Cells are measured in the dark (see supplementary information) and under 100 mW cm2 AM1.5G simulated solar illumination. Solar illumination is applied using an Abet solar simulator, calibrated with a Fraunhofer certified photovoltaic cell. For all studied device architectures, 6 cells with active areas of 0.134 cm2 were measured, where the best performing (highest power conversion efficiency) is reported here. A commercial set-up (Bentham) is used to measure the external quantum efficiency (EQE). Light from a Xe arc lamp (300–670 nm) and a quartz halogen lamp (670–900 nm) is chopped, coupled into a monochromator and aimed at the device. The resulting current is sent through a Bentham477 current pre-amplifier, then arriving in the Bentham485 lock-in amplifier. Calibration is done with a certificated Si cell. The integration of these EQEs over the solar spectrum [23] is listed in Tables 1 and 2 as JEQE. The same optics and measurement setup is used with a DTR6 integrating sphere to determine the reflection.
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Table 1 Jsc, Voc, FF and efficiency g measured for 100 mW cm2 AM1.5G simulated solar illumination. JEQE is the current density as calculated by integration of the EQE over the AM1.5G solar spectrum. Structure
Jsc (mA cm2)
JEQE (mA cm2)
VOC (V)
FF (%)
g (%)
(a) MoO3/ClAlPc (RT)/C60/BCP/Ag (b) MoO3/FDTS/ClAlPc (RT)/C60/BCP/Ag (c) MoO3/ClAlPc (105 °C)/C60/BCP/Ag (d) MoO3/FDTS/ClAlPc (105 °C)/C60 /BCP/Ag
4.9 6.3 5.3 8.9
4.7 6.3 5.0 8.7
0.80 0.70 0.72 0.53
62 52 49 65
2.4 2.3 1.9 3.0
Table 2 Jsc, Voc, FF and efficiency g measured for 100 mW cm2 AM1.5G simulated solar illumination. JEQE is the current density as calculated by integration of the EQE over the AM1.5G solar spectrum. Structure
Jsc (mA cm2)
JEQE (mA cm2)
Voc (V)
FF (%)
g (%)
(e) MoO3/ClAlPc:C60 (105 °C)/C60/BCP/Ag (f) MoO3/FDTS/ClAlPc:C60 (105 °C)/C60/BCP/Ag (g) MoO3/ClAlPc (105 °C)/ClAlPc:C60 (105 °C)/C60/BCP/Ag (h) MoO3/FDTS/ClAlPc (105 °C)/ClAlPc:C60 (105 °C)/C60/BCP/Ag
9.1 10.8 10.7 10.6
7.9 9.0 9.3 9.6
0.78 0.58 0.74 0.62
32 49 61 54
2.2 3.1 4.8 3.5
3. Results and discussion 3.1. Effect of substrate surface and temperature on planar HJ cells 3.1.1. Characterization of pure ClAlPc films Four samples are prepared to study the influence of temperature and substrate surface on the film growth of ClAlPc: a) ITO/25 nm PEDOT:PSS/5 nm MoO3/50 nm ClAlPc (Tsub = room temperature (RT)) b) ITO/25 nm PEDOT:PSS/5 nm MoO3/FDTS/50 nm ClAlPc (Tsub = RT) c) ITO/25 nm PEDOT:PSS/5 nm MoO3/50 nm ClAlPc (Tsub = 105 °C) d) ITO/25 nm PEDOT:PSS/5 nm MoO3/FDTS/50 nm ClAlPc (Tsub = 105 °C) To study the orientation of the ClAlPc molecules with respect to the substrate, an X-ray reflectivity (XRR) measurement is performed. The PEDOT:PSS in the structures functions as a flattening layer on top of ITO in order to get a well-defined XRR signal. For ClAlPc grown at RT, only peaks at 2h = 21.2° (not shown) and 2h = 30.1° are detected, corresponding to peaks of the ITO substrate (Fig. 1a and b). The absence of ClAlPc peaks suggests the films have very limited crystalline order and are likely amorphous. At Tsub = 105 °C, the ClAlPc films exhibit an increased crystallinity. For the films on MoO3, (Fig. 1c) a peak at 2h = 27.1° (d = 3.3 Å) corresponds to diffraction from the (0 0 1) plane of phase I ClAlPc [24,12], indicating that the ClAlPc molecules are parallel to the substrate (face-on orientation). This can be expected when the substrate-molecule interactions are strong [25]. In order to quantify these interactions, contact angle measurements were performed with four different liquids (de-ionized water, ethylene glycol, diiodomethane and decane) in order to calculate the polar and dispersive component of the surface energy by applying the Owens–Wendt–Rabel–Kaelble method [26]. Due to the rather apolar nature of organic
Fig. 1. X-ray reflectivity measurements of (a) ITO/PEDOT:PSS/MoO3/ ClAlPc (RT); (b) ITO/PEDOT:PSS/MoO3/FDTS/ClAlPc (RT); (c) ITO/PEDOT:PSS/MoO3/ClAlPc (105 °C); (d) ITO/PEDOT:PSS/MoO3/FDTS/ClAlPc (105 °C). A constant offset is added between the graphs for visual clarity; in between 2h = 10° and 2h = 25° no ClAlPc peak was observed, hence the break.
semiconductors, it is mainly the dispersive component that can be correlated to the substrate-molecule interaction. For ClAlPc a dispersive component of 30.2 mN/m was derived, compared to 25.5 mN/m for MoO3, thereby indicating that ClAlPc-MoO3 interactions are significant. For FDTS, a peak at 2h = 7.4° (d = 11.9 Å) implies that the ClAlPc molecules adopt an edge-on orientation (Fig. 1d). Molecule–molecule interactions are stronger than substrate-molecule interactions in this case (dispersive component of the surface energy is 11.2 mN/M for FDTS [26]), favoring a growth where the ClAlPc molecules are facing each other rather than the substrate. Due to the fact that different phases of ClAlPc have distinct absorption features [27], the absorption of the pure thin films is also characterized. The ClAlPc films grown at RT both show a very similar quite featureless absorptance profile, with the Q-band peaking at a wavelength of k = 760 nm, consistent with an amorphous film growth (Fig. 2a and b) [28]. When grown at 105 °C, the films show
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quite different absorptance profiles, suggesting that not only orientation of the molecules is influenced by the substrate, but that the crystalline phase of the ClAlPc is also substrate-dependent. On MoO3, the Q-band absorption maximum shifts slightly to k = 770 nm, while a more pronounced shoulder appears at k = 716 nm (Fig. 2c). This absorption profile is very similar to that of phase I of titanyl phthalocyanine (TiOPc) [27]. In phase I, the phthalocyanine molecules are slightly distorted, thereby lifting the doubly degenerate excited state and splitting it into two absorption bands whose transition moments are orthogonal [27]. On FDTS, the Q-band peak undergoes a very pronounced red-shift to k = 824 nm, with a profile that was previously identified to belong to phase II (Fig. 2d), a phase with an even stronger molecular distortion. This phase is further distinguished by very close p-p atomic contacts and a highly polarized excited state [28]. Topographical AFM images reveal rather smooth films when grown at RT, with root mean square (rms) roughness values of 2.6 nm on MoO3 (Fig. 3a) and 3.9 nm on FDTS (Fig. 3b). As a consequence, the ratio of the surface area and the projected area are close to one, as expressed by the folding ratios (FR) of FRMoO3,RT = 1.03 and FRFDTS,RT = 1.11, respectively. At 105 °C, rougher films with larger crystallites are observed. On MoO3, the crystallites have a pyramidal structure, with a high roughness of rms = 35.6 nm and an increased surface area with FRMoO3,105 °C = 1.38 (Fig. 3c). In order to study the initial stages of film growth, a sample with a thinner ClAlPc thickness of 5 nm was characterized (Fig. 3c bis, rms = 13.4 nm). Here, ClAlPc was found to grow in an island (i.e., Volmer– Weber) mode, as described before for ClAlPc growth on SiO2, whereby ClAlPc adatoms aggregate in discrete islands that preferentially expand in the vertical direction [12]. On FDTS, the crystal shapes look more disk-like, while the 50 nm ClAlPc film is slightly smoother than on MoO3 with rms = 14.7 nm and FRFDTS,105 °C = 1.21 (Fig. 3d). In Fig. 3d bis, a 5 nm thick ClAlPc film grown in the same conditions covers almost the whole substrate (rms = 4.8 nm). This suggests that this growth mode can be described as the Stranski–Krastanov or layer-island growth mode [12]. In
Fig. 2. UV–Vis absorptance of (a) ITO/PEDOT:PSS/MoO3/ClAlPc (RT): black solid curves; (b) ITO/PEDOT:PSS/MoO3/FDTS/ClAlPc (RT): grey solid curves; (c) ITO/PEDOT:PSS/MoO3/ClAlPc (105 °C): black dashed curves; (d) ITO/PEDOT:PSS/MoO3/FDTS/ClAlPc (105 °C): grey dotted curves.
fact, upon close inspection of Fig. 3d bis, molecular planes can even be discerned. In summary: at RT, ClAlPc grows as a conformal, amorphous film independent of the substrate surface. When the substrate is heated to 105 °C, the grown films are rougher and more crystalline in nature. On MoO3, the ClAlPc molecules will be face-on and in phase I, whereas on FDTS, molecules adept an edge-on and phase II orientation. 3.1.2. Characterization of ClAlPc/C60 planar HJ solar cells In order to study whether the investigated film properties have an influence on solar cell performance, the following devices are produced: a) ITO/5 nm MoO3/20 nm ClAlPc (Tsub = RT)/40 nm C60/ 10 nm BCP/100 nm Ag b) ITO/5 nm MoO3/FDTS/20 nm ClAlPc (Tsub = RT)/ 40 nm C60/10 nm BCP/100 nm Ag c) ITO/5 nm MoO3/20 nm ClAlPc (Tsub = 105 °C)/40 nm C60/10 nm BCP/100 nm Ag d) ITO/5 nm MoO3/FDTS/20 nm ClAlPc (Tsub = 105 °C)/ 40 nm C60/10 nm BCP/100 nm Ag When comparing the solar cells grown at RT, it can be noticed that MoO3 devices show slightly higher Voc and fill factor (Voc = 0.8 V and FF = 62%) compared to FDTS devices (Voc = 0.7 V and FF = 52%) (c.f. Fig. 4 and Table 1). This can best be explained by the fact that MoO3 forms a better electrical contact with ClAlPc than FDTS, due to a better alignment between the work function of the electrode and the ClAlPc highest occupied molecular orbital (HOMO) [14,29]. The samples on FDTS also show a higher Jsc of 6.3 mA cm2 than those on MoO3 (Jsc = 4.9 mA cm2). We suspect that the MoO3 contact dissociates or quenches excitons [30,31]. The FDTS would then act as an exciton blocking layer [32], thereby hindering the loss of excitons at the contact. The solar cells on MoO3 exhibit a lower Voc (Voc = 0.72 V) and FF (FF = 49%) when the samples were heated during ClAlPc deposition (c.f. Table 1c). Here it is not clear whether this lower performance is related to the phase change from amorphous to phase I, or to the roughening of the donor layer (see Fig. 3c), which causes more shunting currents (see suppl. Info). On FDTS, the heating results in a dramatic reduction of Voc to 0.53 V. This shift we attribute to the phase change from amorphous ClAlPc to phase II. It was previously reported by Alloway et al. that ClAlPc grown edge-on on perylenetetracarboxylicdianhydride-N,N0 -bis (butyl)imide (C4-PTCDI) has a lower ionization potential (EHOMO = 5.1 eV) than ClAlPc with face-on orientation on PTCDA (EHOMO = 5.4 eV) [33]. This shift in ionization potential was then attributed to a speculated phase-change. As it is well established that Voc is strongly correlated to the difference in HOMO of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor [34–36], such an energy shift can explain the lower Voc observed here. Most striking is the current density increase to Jsc = 8.9 mA/cm2. Together with a FF of 65%, this gives rise to a considerable planar HJ cell efficiency of g = 3%. In order to get better insight as to the origin of this photocurrent increase,
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Fig. 3. Topographic AFM images (2500 2500 nm). (a), (b), (c) and (d) labeled as in Figs. 1 and 2. (c bis) and (d bis) are similar samples as (c) and (d), but with only 5 nm ClAlPc instead of 50 nm ClAlPc.
spectral response and reflectivity measurements are performed. When we compare the external quantum efficiency (EQE) spectra of the heated ClAlPc cells on MoO3 and FDTS (Fig. 5c and d respectively), we can see that two effects lead to a higher Jsc on FDTS. First: the red-shifted absorption profile on FDTS allows the cell to harvest a broader portion of the solar spectrum, and second: the EQE of the cells on FDTS is overall higher. A similar effect has been observed before for solvent annealed, phase II TiOPc. [37] The EQE can be further analyzed with the following formula [38]:
EQEðk; VÞ ¼ gA ðkÞgED gCT ðVÞgCC ðVÞ
ð1Þ
Here, gA is the absorption efficiency. The reflectivity of the device shown in Fig. 5 (dashed and dotted curves) represents gA: all light that is not reflected (100R, in %) is absorbed in either the organic layers, the substrate, or the electrodes. Therefore, gA alone cannot explain the increased EQE on FDTS, as the absorption on FDTS around k = 700 nm is lower than that on MoO3. Exciton diffusion efficiency (gED) expresses how many excitons reach the D/A interface before recombination. This can typically be improved by either increasing the D/A surface or by increasing the exciton diffusion length. According to the AFM data (Fig. 3) the D/A area should be higher on MoO3 (FRMoO3,105 °C = 1.38) than on FDTS (FRFDTS,105 °C = 1.21), so according to this effect one would rather expect higher Jsc
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the different phase and the different orientation of the ClAlPc molecules in the cells. 3.2. Effect of substrate surface and temperature on bulk heterojunction cells 3.2.1. Characterization of co-evaporated ClAlPc:C60 films To investigate how co-evaporated ClAlPc:C60 blends grow on different substrates, the following structures were fabricated:
Fig. 4. Current density–voltage (J–V) curves for 100 mW cm2 AM1.5G simulated solar illumination, of (a) ITO/MoO3/ClAlPc (RT)/C60/BCP/Ag: black solid curves; (b) ITO/MoO3/FDTS/ClAlPc (RT) /C60/BCP/Ag: grey solid curves; (c) ITO/MoO3/ClAlPc (105 °C) /C60/BCP/Ag: black dashed curves; (d) ITO/MoO3/FDTS/ClAlPc (105 °C) /C60/BCP/Ag: grey dotted curves.
Fig. 5. Measured external quantum efficiency (EQE) and absorption spectra (100R, in %) of devices on MoO3 (EQE: black solid curves, 100R: dashed curve) or on FDTS (EQE: grey solid curves, 100R: dotted curve), with ClAlPc processed with the substrate at room temperature (top panel) or at 105 °C (bottom panel).
on MoO3. Moreover, an increase of the exciton diffusion length in ClAlPc does not explain why the EQE signal of C60 (the peak around k = 460 nm) also increases on FDTS. When charges are dissociated, they still need to reach an electrode and they will do so with a charge collection efficiency gCC. For gCC to be able to explain the device trends, however, one would expect this to be correlated with the FF of the devices, and for the devices at RT this seems not to be the case. There, the device on FDTS shows the highest EQE (Fig. 5a and b), while it has at the same time a lower FF than the device on MoO3 (Table 1a and b). Following this process of elimination, it seems that the increased EQE on FDTS is most likely related to a more efficient charge transfer process (gCT) at the D/A interface. The difference in gCT on MoO3 and FDTS is likely related to both
a) ITO/25 nm PEDOT:PSS/5 nm MoO3/100 nm ClAlPc:C60 (Tsub = 105 °C) b) ITO/25 nm PEDOT:PSS/5 nm MoO3/FDTS/100 nm ClAlPc:C60 (Tsub = 105 °C) c) ITO/25 nm PEDOT:PSS/5 nm MoO3/5 nm ClAlPc (Tsub = 105 °C)/100 nm ClAlPc:C60 (Tsub = 105 °C) d) ITO/25 nm PEDOT:PSS/5 nm MoO3/FDTS/5 nm ClAlPc (Tsub = 105 °C)/100 nm ClAlPc:C60 (Tsub = 105 °C) Here, all the ClAlPc:C60 blends (ratio 1:1) are evaporated at 105 °C as we are aiming to grow crystalline, phase-separated BHJ layers. For structures (g) and (h), a 5 nm ClAlPc templating layer was grown before blend deposition, to see whether such a film can influence the growth of the overlying blend. The XRR measurements show that the ClAlPc component in the blend still grows preferentially face-on on MoO3 and edge-on on FDTS (Fig. 6e and f). However, the intensity of the peaks at, respectively, 2h = 27.1° and 2h = 7.4°, is lower for the blended layers than for the pure layers of Fig. 1. When the 5 nm pure ClAlPc layer is grown before blend deposition, XRR signals become more pronounced, especially for the samples on FDTS. As the increase in signal is too strong to come from the 5 nm film itself, we ascribe this to a templating effect. The pure ClAlPc layer grows as a crystalline seed layer, which then templates the growth of the blend towards a similar preferred crystalline orientation [39]. The absorptance spectra of the blends on MoO3 (Fig. 7e and g) more closely resemble that of the pure
Fig. 6. X-ray reflectivity measurements of (e) ITO/PEDOT:PSS/MoO3/ ClAlPc:C60 (105 °C); (f) ITO/PEDOT:PSS/MoO3/FDTS/ClAlPc:C60 (105 °C); (g) ITO/PEDOT:PSS/MoO3/ClAlPc (105 °C)/ClAlPc:C60 (105 °C); (h) ITO/ PEDOT:PSS/MoO3/FDTS/ClAlPc (105 °C)/ClAlPc:C60 (105 °C).
B. Verreet et al. / Organic Electronics 12 (2011) 2131–2139
Fig. 7. UV–Vis absorptance of (e) ITO/PEDOT:PSS/MoO3/ClAlPc:C60 (105 °C): black solid curves; (f) ITO/PEDOT:PSS/MoO3/FDTS/ClAlPc:C60 (105 °C): grey solid curves; (g) ITO/PEDOT:PSS/MoO3/ClAlPc (105 °C)/ ClAlPc:C60 (105 °C): black dashed curves; (h) ITO/PEDOT:PSS/MoO3/FDTS/ ClAlPc (105 °C)/ClAlPc:C60 (105 °C): grey dotted curves.
film (Fig. 2c), when a templating layer is applied. So, in addition to a preferred orientation, the templating layer can also help to achieve a certain ClAlPc phase in the blend. On FDTS (Fig. 7f and h), both films show a redshifted Q-band absorption shoulder at k > 800 nm. These absorption signals are not as pronounced as the signal for a pure ClAlPc film on FDTS (Fig. 2d), but also here the templating layer helps to reinforce this red-shifted absorption feature.
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According to AFM images, growing ClAlPc:C60 blends directly on MoO3 results in very rough films with rms = 51.4 nm (Fig. 8e). Islands 0.5 lm in size are formed, and the space in between the islands is filled with smaller crystallites. These islands have a more irregular shape than those of the 50 nm pure ClAlPc film grown under similar conditions (Fig. 3c). As already shown in Fig. 3c bis, the templating layer consists of crystallites with a height of about 40 nm and width of 100–200 nm, spread over the surface. When the ClAlPc:C60 blend is grown on top of this layer, they form islands with a very similar spacing as the template crystallites (Fig. 8g). This hints that the pure ClAlPc crystallites of the templating layer form nucleation sites, around which the subsequently deposited ClAlPc:C60 blend grows. This results in smaller, more closely spaced ClAlPc:C60 islands compared to the film without a templating layer. Images of the blends grown on FDTS, with and without a templating layer, are shown in Fig. 8f and h. Also here, it can be seen that the templating layer (Fig. 3d bis) has an impact on the growth of the blend layer, although in this case the use of a templating layer leads to larger islands. 3.2.2. Characterization of bulk heterojunction devices The co-evaporated films studied above were then incorporated into bulk heterojunction solar cells with the following structures: a) ITO/5 nm MoO3/100 nm ClAlPc:C60 (Tsub = 105 °C)/ 40 nm C60/10 nm BCP/100 nm Ag
Fig. 8. Topographic AFM images (2500 2500 nm). (e), (f), (g) and (h) labeled as in Figs. 6 and 7.
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b) ITO/5 nm MoO3/FDTS/100 nm ClAlPc:C60 (Tsub = 105 °C)/40 nm C60/10 nm BCP/100 nm Ag c) ITO/5 nm MoO3/5 nm ClAlPc (Tsub = 105 °C)/100 nm ClAlPc:C60 (Tsub = 105 °C)/40 nm C60/10 nm BCP/ 100 nm Ag d) ITO/5 nm MoO3/FDTS/5 nm ClAlPc (Tsub = 105 °C)/ 100 nm ClAlPc:C60 (Tsub = 105 °C)/40 nm C60/10 nm BCP/100 nm Ag All of the produced bulk heterojunction cells show an increased Jsc compared to the planar HJ cells (Fig. 9 and Table 2). The planar HJ cells were partially limited by exciton diffusion, but within a bulk heterojunction the generated excitons do not have to diffuse as far to reach a D/A junction. The EQE spectra and reflectivity data (plotted as 100R (%)) are shown in Fig. 10. These spectra make clear that the current of all these devices is generated over a wide wavelength region, ranging from k = 300–800 nm. Reflectivity data show that near k = 700 nm, about 90% of incident light gets absorbed, while both the EQE and ‘‘100R’’ spectra are becoming relatively featureless. This indicates that absorption is nearing saturation. But while about 90% of the light gets absorbed in the devices, maximum EQE is only 40%. This indicates that even in the BHJ devices, internal quantum efficiency is still quite low, leaving room for possible future enhancement. Integration of the EQE over the AM1.5G solar spectrum is listed in Table 2 as JEQE. Differences in between Jsc and JEQE can be attributed to spectral mismatch between the solar simulator and the ideal AM1.5G spectrum. Both for devices on MoO3 and on FDTS, the use of a 5 nm ClAlPc templating layer results in an improved FF. (On MoO3: FFno template = 32% versus FFtemplated = 61%; On FDTS: FFno template = 49% versus FFtemplated = 54%). This improved FF is ascribed to the higher crystallinity of the templated films, and hence better morphology and transport. Unlike planar HJ cells, the templated bulk heterojunction structures can combine relatively high Jsc with decent FFs. But similar to the planar HJ cells, the BHJ cells on FDTS show a decreased Voc. As a result, highest performance is on MoO3, where the best cell reached a power conversion
Fig. 9. Current density–voltage (J–V) curves for 100 mW cm2 AM1.5G simulated solar illumination, of (e) ITO/MoO3/ClAlPc:C60 (105 °C)/C60/ BCP/Ag: black solid curves; (f) ITO/MoO3/FDTS/ClAlPc:C60 (105 °C)/C60/ BCP/Ag: grey solid curves; (g) ITO/MoO3/ClAlPc (105 °C)/ClAlPc:C60 (105 °C)/C60/BCP/Ag: black dashed curves; (h) ITO/MoO3/FDTS/ClAlPc (105 °C)/ClAlPc:C60 (105 °C)/C60/BCP/Ag: grey dotted curves.
Fig. 10. Measured external quantum efficiency (EQE) and absorption spectra (100R, in %) of bulk heterojunctions on MoO3 (EQE: black solid curves, 100R: dashed curve) or FDTS (EQE: grey solid curves, 100R: dotted curve), with ClAlPc processed directly on the substrate (top panel) or on top of a ClAlPc templating layer (bottom panel).
of g = 4.8%. When this value is corrected for spectral mismatch, a value of gcorrected = (JEQE FF Voc)/Plight = 4.2% is obtained. 4. Conclusion In conclusion, we have investigated ClAlPc films grown on either FDTS or MoO3. At RT, the ClAlPc films grow amorphous, independent of the surface. When heated to 105 °C, ClAlPc grows face-on on MoO3, with a phase I-like absorption profile. On FDTS, the film is characterized as edge-on, phase II. These films are then used to fabricate planar HJ cells. On FDTS, the induced phase is accompanied by a lower Voc, but also by a higher current, leading to 3% efficient planar HJ cells. The higher current on FDTS compared to MoO3 is believed to be caused by two effects: the FDTS surface would quench fewer excitons than the MoO3 surface, and the exciton charge transfer would be more efficient for phase II ClAlPc. Next, ClAlPc and C60 are co-evaporated on FDTS and MoO3 substrates. In order to achieve an enhanced crystallinity, a thin pure ‘‘templating’’ layer of ClAlPc proved advantageous. When the films were used in bulk heterojunction cells, the more crystalline films grown on top of a templating layer showed higher FF than devices without templating layers. The use of the co-evaporated bulk heterojunctions ensured high photocurrents, independent of the growth surface. Here, the higher Voc of devices on MoO3 was beneficial, with devices showing efficiencies greater than 4% as a result. Acknowledgements The research leading to these results has received funding from the European Community’s Seventh Framework
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Structural templating of chloro-aluminum phthalocyanine layers for planar and bulk heterojunction organic solar cells Bregt Verreet a,b,⇑, Robert Müller a, Barry P. Rand a, Karolien Vasseur a,b, Paul Heremans a,b a b
Imec, Polymer and Molecular Electronics, Kapeldreef 75, B-3001 Leuven, Heverlee, Belgium ESAT, Katholieke Universiteit Leuven, Kasteelpark Arenberg 10, B-3001 Leuven, Heverlee, Belgium
a r t i c l e
i n f o
Article history: Received 25 July 2011 Received in revised form 29 August 2011 Accepted 29 August 2011 Available online 22 September 2011 Keywords: Organic solar cell Chloro-aluminum phthalocyanine FDTS Structural templating Bulk heterojunction
a b s t r a c t Chloro-aluminum phthalocyanine (ClAlPc) film growth on 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) and MoO3 is studied and correlated to ClAlPc/C60 solar cell performance for both planar and bulk heterojunction (HJ) architectures. On top of unheated substrates, ClAlPc films grow amorphous independent of the substrate surface. When heated to 105 °C, ClAlPc grows with a face-on orientation on MoO3, with a crystalline phase I-like absorption profile. On FDTS, the film is optically characterized as phase II, and adopts an edge-on orientation. Implemented in planar HJ cells, the latter films show a substantially higher current compared to the other growth conditions, leading to 3% efficient cells. This current increase is investigated with spectral response and reflectivity measurements, and is found to be related to a more efficient exciton dissociation. Next, ClAlPc and C60 are co-evaporated on FDTS and MoO3 modified ITO substrates to fabricate bulk HJ devices. Notably, we find that when a thin pure ‘‘templating’’ layer of ClAlPc is grown first, the subsequently grown ClAlPc:C60 bulk HJ propagates the templating effect, and films show a higher crystallinity than without this templating layer, with higher fill factors as a result. On MoO3, this approach yields efficiencies above 4%. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Progress [1] in the power conversion efficiency of organic photovoltaic cells (OPVCs) has generated significant interest for the generation of low-cost, renewable energy. The first critical improvement of OPVC device architecture was the introduction of the donor/acceptor (D/A) interface [2] that functions as a dissociation site for strongly bound excitons that are the product of light absorption in an organic semiconductor. The next major development was the bulk heterojunction (BHJ) architecture, whereby an interpenetrating D/A network ensures that a high percentage of excitons are able to diffuse towards a nearby D/A interface [3,4]. In solution-processed solar cells, such a BHJ is usually made by mixing donor and acceptor in one ⇑ Corresponding author at: Imec, Polymer and Molecular Electronics, Kapeldreef 75, B-3001 Leuven, Heverlee, Belgium. Tel.: +32 16 28 19 80. E-mail address: [email protected] (B. Verreet). 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.08.031
solution prior to deposition. In small-molecule evaporated OPVCs, BHJs are typically formed via co-evaporation of the donor and acceptor materials. The solar cells investigated here are based on chloroaluminum phthalocyanine (ClAlPc)/fullerene C60 as a D/A system. This combination has shown promise as it produces a high open-circuit voltage (Voc) of up to 0.85 V with respect to its near infrared absorption edge of 1.5 eV [5,6]. However, ClAlPc/C60 cells seem very sensitive to growth conditions, as varying the ClAlPc evaporation rate from 0.1–1.5 Å s1 leads to a variation in Voc from 0.49–0.71 V. Later, it was found that the deposition rate affects the molecular orientation of the ClAlPc molecules relative to the indium tin oxide (ITO) substrate, and that this in turn modifies the energy level alignment at the organic heterojunction interface [7]. Another effective way to control the growth is by modifying the substrate surface properties. To template the growth of subsequent ClAlPc layers [8,9], an interlayer composed of 3,4,9,10-perylene tetracarboxylic
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acid (PTCDA) has been used at the anode, whereby the improved stacking of the ClAlPc molecules led to an increase in short-circuit current density Jsc of 25% [10]. For organic transistors, the use of self-assembled monolayers (SAMs) is a well-established technique for the control of the growth [11]. Using this approach, the molecular orientation of ClAlPc molecules could be changed from face-on (parallel to the substrate) to edge-on (normal to the substrate) by treating a SiO2 substrate with octadecyltrichlorosilane [12]. For OPVCs, SAMs have been used to treat the anode, though here the aim has typically been to adjust the ITO work function, rather than adjusting the growth of subsequent layers [13–16]. To obtain a BHJ that combines good transport properties with efficient exciton separation, the control of morphology and growth is even more critical. For example, the co-evaporation of a planar phthalocyanine (Pc) and C60 at room temperature will typically lead to an amorphous film [17,18]. Compared to the pure CuPc layer these amorphous films have poor hole mobilities, limiting cell efficiency. By applying an elevated substrate temperature (Tsub) during Pc:C60 deposition, the efficiency can be improved compared to deposition at room temperature [19]. Also here, the growth surface can have a large impact on the solar cell performance, with films grown on 3,4-polyethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS) showing a different morphology and lower efficiency compared to devices with films grown directly on ITO [20]. For ClAlPc, Li et al. showed that the surface area of the films could be increased by using oblique angle vacuum deposition [21]. This resulted in an increase in Jsc in their ClAlPc/C60 cells, thereby showing that ClAlPc cells could be improved with a bulk heterojunction approach. First promising results for co-evaporated ClAlPc:C70 BHJ cells have already been reported [22], where Tsub was shown to have a strong impact on the growth of the blend, with nanocrystal formation at an elevated temperature from 370–390 K. Consequently, Jsc and FF were enhanced significantly, resulting in a power conversion efficiency of 4.1%. In this article, we will further investigate the growth of ClAlPc for BHJ solar cells. We will focus on the use of structural templating layers, an approach which has already proven successful in planar heterojunction cells, but has not been thoroughly investigated for their role to improve BHJ-based cells. The two different surfaces we compare as templating layers are molybdenum oxide (MoO3) and MoO3 treated with a SAM of 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS). First, pure ClAlPc films were grown on top of these surfaces, and the influence of Tsub on the film properties was investigated. Planar HJ solar cells were then produced from these pure films to probe how the different growth properties affect the ensuing solar cell performance. Next, we proceed by growing co-evaporated ClAlPc:C60 films at elevated Tsub in order to grow crystalline BHJ layers. Here, we observe a significant device performance improvement depending on the presence of an initial pure ClAlPc templating layer underneath the BHJ layer. Bulk heterojunction solar cells produced with these blends all show increased current with respect to the planar HJ devices, with power conversion efficiencies (g) greater than 4%.
2. Experimental Glass substrates coated with ITO (Kintec, 85 nm, sheet resistance <20 X h1) arevv cleaned by subsequent ultrasonic treatment in detergent, de-ionized water, acetone, and iso-propanol, for 5 min each, followed by an ultraviolet-ozone treatment for 900 s. On the samples for film characterization, a 25 nm thick layer of PEDOT:PSS (HC Starck Clevios P) was deposited. Prior to FDTS surface modification, 10 nm of MoO3 is evaporated at a rate of 1 Å s1 and pressure of 107 Torr in a vacuum chamber. Then FDTS is applied by vapor phase deposition in a home-built oven under reduced pressure (140 °C, 30 min.). The MoO3 (550 °C), ClAlPc (350 °C), C60 (380 °C) and bathocuproine (BCP) (150 °C) materials are evaporated at a rate of 1 Å s1 in a vacuum chamber at a pressure of 107 Torr. The mixed ClAlPc:C60 (1:1) layer was formed by keeping both rates constant at 0.5 Å s1. Deposition rates are determined from calibration of the film thickness, measured ex-situ by spectroscopic ellipsometry (Sopra GESP-5). All organic materials are purified before loading into the chamber by thermal gradient sublimation. Tsub was elevated with heat lamps, calibrated by thermocouple measurements. Before subsequent layers were deposited on the heated layer, samples were cooled to 40 °C. The cathode, Ag (107 Torr; 1.5 Å s1), is deposited through a shadow mask, without breaking vacuum. X-ray reflectivity (XRR) measurements were performed on a PANalytical X’Pert Pro Materials Research Diffractometer using Cu Ka radiation. An integration time of 7 s per 2h of 0.01° was used for scanning h/2h. Absorptance spectra are measured with a Shimadzu UV-1601PC. The surface morphology was studied with atomic force microscopy (AFM) using a Picoscan PicoSPM LE scanning probe in tapping mode. Static contact angle measurements were performed on MoO3 and ClAlPc films with an OCA-20 contact angle meter from DataPhysics Instruments GmbH with SCA20 software. Electrical characterization of the solar cells is performed with a Keithley 2602 measurement unit. Cells are measured in the dark (see supplementary information) and under 100 mW cm2 AM1.5G simulated solar illumination. Solar illumination is applied using an Abet solar simulator, calibrated with a Fraunhofer certified photovoltaic cell. For all studied device architectures, 6 cells with active areas of 0.134 cm2 were measured, where the best performing (highest power conversion efficiency) is reported here. A commercial set-up (Bentham) is used to measure the external quantum efficiency (EQE). Light from a Xe arc lamp (300–670 nm) and a quartz halogen lamp (670–900 nm) is chopped, coupled into a monochromator and aimed at the device. The resulting current is sent through a Bentham477 current pre-amplifier, then arriving in the Bentham485 lock-in amplifier. Calibration is done with a certificated Si cell. The integration of these EQEs over the solar spectrum [23] is listed in Tables 1 and 2 as JEQE. The same optics and measurement setup is used with a DTR6 integrating sphere to determine the reflection.
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Table 1 Jsc, Voc, FF and efficiency g measured for 100 mW cm2 AM1.5G simulated solar illumination. JEQE is the current density as calculated by integration of the EQE over the AM1.5G solar spectrum. Structure
Jsc (mA cm2)
JEQE (mA cm2)
VOC (V)
FF (%)
g (%)
(a) MoO3/ClAlPc (RT)/C60/BCP/Ag (b) MoO3/FDTS/ClAlPc (RT)/C60/BCP/Ag (c) MoO3/ClAlPc (105 °C)/C60/BCP/Ag (d) MoO3/FDTS/ClAlPc (105 °C)/C60 /BCP/Ag
4.9 6.3 5.3 8.9
4.7 6.3 5.0 8.7
0.80 0.70 0.72 0.53
62 52 49 65
2.4 2.3 1.9 3.0
Table 2 Jsc, Voc, FF and efficiency g measured for 100 mW cm2 AM1.5G simulated solar illumination. JEQE is the current density as calculated by integration of the EQE over the AM1.5G solar spectrum. Structure
Jsc (mA cm2)
JEQE (mA cm2)
Voc (V)
FF (%)
g (%)
(e) MoO3/ClAlPc:C60 (105 °C)/C60/BCP/Ag (f) MoO3/FDTS/ClAlPc:C60 (105 °C)/C60/BCP/Ag (g) MoO3/ClAlPc (105 °C)/ClAlPc:C60 (105 °C)/C60/BCP/Ag (h) MoO3/FDTS/ClAlPc (105 °C)/ClAlPc:C60 (105 °C)/C60/BCP/Ag
9.1 10.8 10.7 10.6
7.9 9.0 9.3 9.6
0.78 0.58 0.74 0.62
32 49 61 54
2.2 3.1 4.8 3.5
3. Results and discussion 3.1. Effect of substrate surface and temperature on planar HJ cells 3.1.1. Characterization of pure ClAlPc films Four samples are prepared to study the influence of temperature and substrate surface on the film growth of ClAlPc: a) ITO/25 nm PEDOT:PSS/5 nm MoO3/50 nm ClAlPc (Tsub = room temperature (RT)) b) ITO/25 nm PEDOT:PSS/5 nm MoO3/FDTS/50 nm ClAlPc (Tsub = RT) c) ITO/25 nm PEDOT:PSS/5 nm MoO3/50 nm ClAlPc (Tsub = 105 °C) d) ITO/25 nm PEDOT:PSS/5 nm MoO3/FDTS/50 nm ClAlPc (Tsub = 105 °C) To study the orientation of the ClAlPc molecules with respect to the substrate, an X-ray reflectivity (XRR) measurement is performed. The PEDOT:PSS in the structures functions as a flattening layer on top of ITO in order to get a well-defined XRR signal. For ClAlPc grown at RT, only peaks at 2h = 21.2° (not shown) and 2h = 30.1° are detected, corresponding to peaks of the ITO substrate (Fig. 1a and b). The absence of ClAlPc peaks suggests the films have very limited crystalline order and are likely amorphous. At Tsub = 105 °C, the ClAlPc films exhibit an increased crystallinity. For the films on MoO3, (Fig. 1c) a peak at 2h = 27.1° (d = 3.3 Å) corresponds to diffraction from the (0 0 1) plane of phase I ClAlPc [24,12], indicating that the ClAlPc molecules are parallel to the substrate (face-on orientation). This can be expected when the substrate-molecule interactions are strong [25]. In order to quantify these interactions, contact angle measurements were performed with four different liquids (de-ionized water, ethylene glycol, diiodomethane and decane) in order to calculate the polar and dispersive component of the surface energy by applying the Owens–Wendt–Rabel–Kaelble method [26]. Due to the rather apolar nature of organic
Fig. 1. X-ray reflectivity measurements of (a) ITO/PEDOT:PSS/MoO3/ ClAlPc (RT); (b) ITO/PEDOT:PSS/MoO3/FDTS/ClAlPc (RT); (c) ITO/PEDOT:PSS/MoO3/ClAlPc (105 °C); (d) ITO/PEDOT:PSS/MoO3/FDTS/ClAlPc (105 °C). A constant offset is added between the graphs for visual clarity; in between 2h = 10° and 2h = 25° no ClAlPc peak was observed, hence the break.
semiconductors, it is mainly the dispersive component that can be correlated to the substrate-molecule interaction. For ClAlPc a dispersive component of 30.2 mN/m was derived, compared to 25.5 mN/m for MoO3, thereby indicating that ClAlPc-MoO3 interactions are significant. For FDTS, a peak at 2h = 7.4° (d = 11.9 Å) implies that the ClAlPc molecules adopt an edge-on orientation (Fig. 1d). Molecule–molecule interactions are stronger than substrate-molecule interactions in this case (dispersive component of the surface energy is 11.2 mN/M for FDTS [26]), favoring a growth where the ClAlPc molecules are facing each other rather than the substrate. Due to the fact that different phases of ClAlPc have distinct absorption features [27], the absorption of the pure thin films is also characterized. The ClAlPc films grown at RT both show a very similar quite featureless absorptance profile, with the Q-band peaking at a wavelength of k = 760 nm, consistent with an amorphous film growth (Fig. 2a and b) [28]. When grown at 105 °C, the films show
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quite different absorptance profiles, suggesting that not only orientation of the molecules is influenced by the substrate, but that the crystalline phase of the ClAlPc is also substrate-dependent. On MoO3, the Q-band absorption maximum shifts slightly to k = 770 nm, while a more pronounced shoulder appears at k = 716 nm (Fig. 2c). This absorption profile is very similar to that of phase I of titanyl phthalocyanine (TiOPc) [27]. In phase I, the phthalocyanine molecules are slightly distorted, thereby lifting the doubly degenerate excited state and splitting it into two absorption bands whose transition moments are orthogonal [27]. On FDTS, the Q-band peak undergoes a very pronounced red-shift to k = 824 nm, with a profile that was previously identified to belong to phase II (Fig. 2d), a phase with an even stronger molecular distortion. This phase is further distinguished by very close p-p atomic contacts and a highly polarized excited state [28]. Topographical AFM images reveal rather smooth films when grown at RT, with root mean square (rms) roughness values of 2.6 nm on MoO3 (Fig. 3a) and 3.9 nm on FDTS (Fig. 3b). As a consequence, the ratio of the surface area and the projected area are close to one, as expressed by the folding ratios (FR) of FRMoO3,RT = 1.03 and FRFDTS,RT = 1.11, respectively. At 105 °C, rougher films with larger crystallites are observed. On MoO3, the crystallites have a pyramidal structure, with a high roughness of rms = 35.6 nm and an increased surface area with FRMoO3,105 °C = 1.38 (Fig. 3c). In order to study the initial stages of film growth, a sample with a thinner ClAlPc thickness of 5 nm was characterized (Fig. 3c bis, rms = 13.4 nm). Here, ClAlPc was found to grow in an island (i.e., Volmer– Weber) mode, as described before for ClAlPc growth on SiO2, whereby ClAlPc adatoms aggregate in discrete islands that preferentially expand in the vertical direction [12]. On FDTS, the crystal shapes look more disk-like, while the 50 nm ClAlPc film is slightly smoother than on MoO3 with rms = 14.7 nm and FRFDTS,105 °C = 1.21 (Fig. 3d). In Fig. 3d bis, a 5 nm thick ClAlPc film grown in the same conditions covers almost the whole substrate (rms = 4.8 nm). This suggests that this growth mode can be described as the Stranski–Krastanov or layer-island growth mode [12]. In
Fig. 2. UV–Vis absorptance of (a) ITO/PEDOT:PSS/MoO3/ClAlPc (RT): black solid curves; (b) ITO/PEDOT:PSS/MoO3/FDTS/ClAlPc (RT): grey solid curves; (c) ITO/PEDOT:PSS/MoO3/ClAlPc (105 °C): black dashed curves; (d) ITO/PEDOT:PSS/MoO3/FDTS/ClAlPc (105 °C): grey dotted curves.
fact, upon close inspection of Fig. 3d bis, molecular planes can even be discerned. In summary: at RT, ClAlPc grows as a conformal, amorphous film independent of the substrate surface. When the substrate is heated to 105 °C, the grown films are rougher and more crystalline in nature. On MoO3, the ClAlPc molecules will be face-on and in phase I, whereas on FDTS, molecules adept an edge-on and phase II orientation. 3.1.2. Characterization of ClAlPc/C60 planar HJ solar cells In order to study whether the investigated film properties have an influence on solar cell performance, the following devices are produced: a) ITO/5 nm MoO3/20 nm ClAlPc (Tsub = RT)/40 nm C60/ 10 nm BCP/100 nm Ag b) ITO/5 nm MoO3/FDTS/20 nm ClAlPc (Tsub = RT)/ 40 nm C60/10 nm BCP/100 nm Ag c) ITO/5 nm MoO3/20 nm ClAlPc (Tsub = 105 °C)/40 nm C60/10 nm BCP/100 nm Ag d) ITO/5 nm MoO3/FDTS/20 nm ClAlPc (Tsub = 105 °C)/ 40 nm C60/10 nm BCP/100 nm Ag When comparing the solar cells grown at RT, it can be noticed that MoO3 devices show slightly higher Voc and fill factor (Voc = 0.8 V and FF = 62%) compared to FDTS devices (Voc = 0.7 V and FF = 52%) (c.f. Fig. 4 and Table 1). This can best be explained by the fact that MoO3 forms a better electrical contact with ClAlPc than FDTS, due to a better alignment between the work function of the electrode and the ClAlPc highest occupied molecular orbital (HOMO) [14,29]. The samples on FDTS also show a higher Jsc of 6.3 mA cm2 than those on MoO3 (Jsc = 4.9 mA cm2). We suspect that the MoO3 contact dissociates or quenches excitons [30,31]. The FDTS would then act as an exciton blocking layer [32], thereby hindering the loss of excitons at the contact. The solar cells on MoO3 exhibit a lower Voc (Voc = 0.72 V) and FF (FF = 49%) when the samples were heated during ClAlPc deposition (c.f. Table 1c). Here it is not clear whether this lower performance is related to the phase change from amorphous to phase I, or to the roughening of the donor layer (see Fig. 3c), which causes more shunting currents (see suppl. Info). On FDTS, the heating results in a dramatic reduction of Voc to 0.53 V. This shift we attribute to the phase change from amorphous ClAlPc to phase II. It was previously reported by Alloway et al. that ClAlPc grown edge-on on perylenetetracarboxylicdianhydride-N,N0 -bis (butyl)imide (C4-PTCDI) has a lower ionization potential (EHOMO = 5.1 eV) than ClAlPc with face-on orientation on PTCDA (EHOMO = 5.4 eV) [33]. This shift in ionization potential was then attributed to a speculated phase-change. As it is well established that Voc is strongly correlated to the difference in HOMO of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor [34–36], such an energy shift can explain the lower Voc observed here. Most striking is the current density increase to Jsc = 8.9 mA/cm2. Together with a FF of 65%, this gives rise to a considerable planar HJ cell efficiency of g = 3%. In order to get better insight as to the origin of this photocurrent increase,
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Fig. 3. Topographic AFM images (2500 2500 nm). (a), (b), (c) and (d) labeled as in Figs. 1 and 2. (c bis) and (d bis) are similar samples as (c) and (d), but with only 5 nm ClAlPc instead of 50 nm ClAlPc.
spectral response and reflectivity measurements are performed. When we compare the external quantum efficiency (EQE) spectra of the heated ClAlPc cells on MoO3 and FDTS (Fig. 5c and d respectively), we can see that two effects lead to a higher Jsc on FDTS. First: the red-shifted absorption profile on FDTS allows the cell to harvest a broader portion of the solar spectrum, and second: the EQE of the cells on FDTS is overall higher. A similar effect has been observed before for solvent annealed, phase II TiOPc. [37] The EQE can be further analyzed with the following formula [38]:
EQEðk; VÞ ¼ gA ðkÞgED gCT ðVÞgCC ðVÞ
ð1Þ
Here, gA is the absorption efficiency. The reflectivity of the device shown in Fig. 5 (dashed and dotted curves) represents gA: all light that is not reflected (100R, in %) is absorbed in either the organic layers, the substrate, or the electrodes. Therefore, gA alone cannot explain the increased EQE on FDTS, as the absorption on FDTS around k = 700 nm is lower than that on MoO3. Exciton diffusion efficiency (gED) expresses how many excitons reach the D/A interface before recombination. This can typically be improved by either increasing the D/A surface or by increasing the exciton diffusion length. According to the AFM data (Fig. 3) the D/A area should be higher on MoO3 (FRMoO3,105 °C = 1.38) than on FDTS (FRFDTS,105 °C = 1.21), so according to this effect one would rather expect higher Jsc
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the different phase and the different orientation of the ClAlPc molecules in the cells. 3.2. Effect of substrate surface and temperature on bulk heterojunction cells 3.2.1. Characterization of co-evaporated ClAlPc:C60 films To investigate how co-evaporated ClAlPc:C60 blends grow on different substrates, the following structures were fabricated:
Fig. 4. Current density–voltage (J–V) curves for 100 mW cm2 AM1.5G simulated solar illumination, of (a) ITO/MoO3/ClAlPc (RT)/C60/BCP/Ag: black solid curves; (b) ITO/MoO3/FDTS/ClAlPc (RT) /C60/BCP/Ag: grey solid curves; (c) ITO/MoO3/ClAlPc (105 °C) /C60/BCP/Ag: black dashed curves; (d) ITO/MoO3/FDTS/ClAlPc (105 °C) /C60/BCP/Ag: grey dotted curves.
Fig. 5. Measured external quantum efficiency (EQE) and absorption spectra (100R, in %) of devices on MoO3 (EQE: black solid curves, 100R: dashed curve) or on FDTS (EQE: grey solid curves, 100R: dotted curve), with ClAlPc processed with the substrate at room temperature (top panel) or at 105 °C (bottom panel).
on MoO3. Moreover, an increase of the exciton diffusion length in ClAlPc does not explain why the EQE signal of C60 (the peak around k = 460 nm) also increases on FDTS. When charges are dissociated, they still need to reach an electrode and they will do so with a charge collection efficiency gCC. For gCC to be able to explain the device trends, however, one would expect this to be correlated with the FF of the devices, and for the devices at RT this seems not to be the case. There, the device on FDTS shows the highest EQE (Fig. 5a and b), while it has at the same time a lower FF than the device on MoO3 (Table 1a and b). Following this process of elimination, it seems that the increased EQE on FDTS is most likely related to a more efficient charge transfer process (gCT) at the D/A interface. The difference in gCT on MoO3 and FDTS is likely related to both
a) ITO/25 nm PEDOT:PSS/5 nm MoO3/100 nm ClAlPc:C60 (Tsub = 105 °C) b) ITO/25 nm PEDOT:PSS/5 nm MoO3/FDTS/100 nm ClAlPc:C60 (Tsub = 105 °C) c) ITO/25 nm PEDOT:PSS/5 nm MoO3/5 nm ClAlPc (Tsub = 105 °C)/100 nm ClAlPc:C60 (Tsub = 105 °C) d) ITO/25 nm PEDOT:PSS/5 nm MoO3/FDTS/5 nm ClAlPc (Tsub = 105 °C)/100 nm ClAlPc:C60 (Tsub = 105 °C) Here, all the ClAlPc:C60 blends (ratio 1:1) are evaporated at 105 °C as we are aiming to grow crystalline, phase-separated BHJ layers. For structures (g) and (h), a 5 nm ClAlPc templating layer was grown before blend deposition, to see whether such a film can influence the growth of the overlying blend. The XRR measurements show that the ClAlPc component in the blend still grows preferentially face-on on MoO3 and edge-on on FDTS (Fig. 6e and f). However, the intensity of the peaks at, respectively, 2h = 27.1° and 2h = 7.4°, is lower for the blended layers than for the pure layers of Fig. 1. When the 5 nm pure ClAlPc layer is grown before blend deposition, XRR signals become more pronounced, especially for the samples on FDTS. As the increase in signal is too strong to come from the 5 nm film itself, we ascribe this to a templating effect. The pure ClAlPc layer grows as a crystalline seed layer, which then templates the growth of the blend towards a similar preferred crystalline orientation [39]. The absorptance spectra of the blends on MoO3 (Fig. 7e and g) more closely resemble that of the pure
Fig. 6. X-ray reflectivity measurements of (e) ITO/PEDOT:PSS/MoO3/ ClAlPc:C60 (105 °C); (f) ITO/PEDOT:PSS/MoO3/FDTS/ClAlPc:C60 (105 °C); (g) ITO/PEDOT:PSS/MoO3/ClAlPc (105 °C)/ClAlPc:C60 (105 °C); (h) ITO/ PEDOT:PSS/MoO3/FDTS/ClAlPc (105 °C)/ClAlPc:C60 (105 °C).
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Fig. 7. UV–Vis absorptance of (e) ITO/PEDOT:PSS/MoO3/ClAlPc:C60 (105 °C): black solid curves; (f) ITO/PEDOT:PSS/MoO3/FDTS/ClAlPc:C60 (105 °C): grey solid curves; (g) ITO/PEDOT:PSS/MoO3/ClAlPc (105 °C)/ ClAlPc:C60 (105 °C): black dashed curves; (h) ITO/PEDOT:PSS/MoO3/FDTS/ ClAlPc (105 °C)/ClAlPc:C60 (105 °C): grey dotted curves.
film (Fig. 2c), when a templating layer is applied. So, in addition to a preferred orientation, the templating layer can also help to achieve a certain ClAlPc phase in the blend. On FDTS (Fig. 7f and h), both films show a redshifted Q-band absorption shoulder at k > 800 nm. These absorption signals are not as pronounced as the signal for a pure ClAlPc film on FDTS (Fig. 2d), but also here the templating layer helps to reinforce this red-shifted absorption feature.
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According to AFM images, growing ClAlPc:C60 blends directly on MoO3 results in very rough films with rms = 51.4 nm (Fig. 8e). Islands 0.5 lm in size are formed, and the space in between the islands is filled with smaller crystallites. These islands have a more irregular shape than those of the 50 nm pure ClAlPc film grown under similar conditions (Fig. 3c). As already shown in Fig. 3c bis, the templating layer consists of crystallites with a height of about 40 nm and width of 100–200 nm, spread over the surface. When the ClAlPc:C60 blend is grown on top of this layer, they form islands with a very similar spacing as the template crystallites (Fig. 8g). This hints that the pure ClAlPc crystallites of the templating layer form nucleation sites, around which the subsequently deposited ClAlPc:C60 blend grows. This results in smaller, more closely spaced ClAlPc:C60 islands compared to the film without a templating layer. Images of the blends grown on FDTS, with and without a templating layer, are shown in Fig. 8f and h. Also here, it can be seen that the templating layer (Fig. 3d bis) has an impact on the growth of the blend layer, although in this case the use of a templating layer leads to larger islands. 3.2.2. Characterization of bulk heterojunction devices The co-evaporated films studied above were then incorporated into bulk heterojunction solar cells with the following structures: a) ITO/5 nm MoO3/100 nm ClAlPc:C60 (Tsub = 105 °C)/ 40 nm C60/10 nm BCP/100 nm Ag
Fig. 8. Topographic AFM images (2500 2500 nm). (e), (f), (g) and (h) labeled as in Figs. 6 and 7.
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b) ITO/5 nm MoO3/FDTS/100 nm ClAlPc:C60 (Tsub = 105 °C)/40 nm C60/10 nm BCP/100 nm Ag c) ITO/5 nm MoO3/5 nm ClAlPc (Tsub = 105 °C)/100 nm ClAlPc:C60 (Tsub = 105 °C)/40 nm C60/10 nm BCP/ 100 nm Ag d) ITO/5 nm MoO3/FDTS/5 nm ClAlPc (Tsub = 105 °C)/ 100 nm ClAlPc:C60 (Tsub = 105 °C)/40 nm C60/10 nm BCP/100 nm Ag All of the produced bulk heterojunction cells show an increased Jsc compared to the planar HJ cells (Fig. 9 and Table 2). The planar HJ cells were partially limited by exciton diffusion, but within a bulk heterojunction the generated excitons do not have to diffuse as far to reach a D/A junction. The EQE spectra and reflectivity data (plotted as 100R (%)) are shown in Fig. 10. These spectra make clear that the current of all these devices is generated over a wide wavelength region, ranging from k = 300–800 nm. Reflectivity data show that near k = 700 nm, about 90% of incident light gets absorbed, while both the EQE and ‘‘100R’’ spectra are becoming relatively featureless. This indicates that absorption is nearing saturation. But while about 90% of the light gets absorbed in the devices, maximum EQE is only 40%. This indicates that even in the BHJ devices, internal quantum efficiency is still quite low, leaving room for possible future enhancement. Integration of the EQE over the AM1.5G solar spectrum is listed in Table 2 as JEQE. Differences in between Jsc and JEQE can be attributed to spectral mismatch between the solar simulator and the ideal AM1.5G spectrum. Both for devices on MoO3 and on FDTS, the use of a 5 nm ClAlPc templating layer results in an improved FF. (On MoO3: FFno template = 32% versus FFtemplated = 61%; On FDTS: FFno template = 49% versus FFtemplated = 54%). This improved FF is ascribed to the higher crystallinity of the templated films, and hence better morphology and transport. Unlike planar HJ cells, the templated bulk heterojunction structures can combine relatively high Jsc with decent FFs. But similar to the planar HJ cells, the BHJ cells on FDTS show a decreased Voc. As a result, highest performance is on MoO3, where the best cell reached a power conversion
Fig. 9. Current density–voltage (J–V) curves for 100 mW cm2 AM1.5G simulated solar illumination, of (e) ITO/MoO3/ClAlPc:C60 (105 °C)/C60/ BCP/Ag: black solid curves; (f) ITO/MoO3/FDTS/ClAlPc:C60 (105 °C)/C60/ BCP/Ag: grey solid curves; (g) ITO/MoO3/ClAlPc (105 °C)/ClAlPc:C60 (105 °C)/C60/BCP/Ag: black dashed curves; (h) ITO/MoO3/FDTS/ClAlPc (105 °C)/ClAlPc:C60 (105 °C)/C60/BCP/Ag: grey dotted curves.
Fig. 10. Measured external quantum efficiency (EQE) and absorption spectra (100R, in %) of bulk heterojunctions on MoO3 (EQE: black solid curves, 100R: dashed curve) or FDTS (EQE: grey solid curves, 100R: dotted curve), with ClAlPc processed directly on the substrate (top panel) or on top of a ClAlPc templating layer (bottom panel).
of g = 4.8%. When this value is corrected for spectral mismatch, a value of gcorrected = (JEQE FF Voc)/Plight = 4.2% is obtained. 4. Conclusion In conclusion, we have investigated ClAlPc films grown on either FDTS or MoO3. At RT, the ClAlPc films grow amorphous, independent of the surface. When heated to 105 °C, ClAlPc grows face-on on MoO3, with a phase I-like absorption profile. On FDTS, the film is characterized as edge-on, phase II. These films are then used to fabricate planar HJ cells. On FDTS, the induced phase is accompanied by a lower Voc, but also by a higher current, leading to 3% efficient planar HJ cells. The higher current on FDTS compared to MoO3 is believed to be caused by two effects: the FDTS surface would quench fewer excitons than the MoO3 surface, and the exciton charge transfer would be more efficient for phase II ClAlPc. Next, ClAlPc and C60 are co-evaporated on FDTS and MoO3 substrates. In order to achieve an enhanced crystallinity, a thin pure ‘‘templating’’ layer of ClAlPc proved advantageous. When the films were used in bulk heterojunction cells, the more crystalline films grown on top of a templating layer showed higher FF than devices without templating layers. The use of the co-evaporated bulk heterojunctions ensured high photocurrents, independent of the growth surface. Here, the higher Voc of devices on MoO3 was beneficial, with devices showing efficiencies greater than 4% as a result. Acknowledgements The research leading to these results has received funding from the European Community’s Seventh Framework
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