Glancing Angle Deposition of Copper Iodide Nanocrystals for Efficient

Jul 16, 2012 - JST-PRESTO, Japan Science and Technology Agency (JST), Saitama, Japan. § Research Center ... We report a simple method to achieve effi...
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Glancing Angle Deposition of Copper Iodide Nanocrystals for Efficient Organic Photovoltaics Ying Zhou,*,† Tetsuya Taima,*,†,‡ Tetsuhiko Miyadera,‡,§ Toshihiro Yamanari,§ Michinori Kitamura,∥ Kazuhiro Nakatsu,∥ and Yuji Yoshida§ †

Research Center for Sustainable Energy and Technology, Kanazawa University, Kanazawa, Japan JST-PRESTO, Japan Science and Technology Agency (JST), Saitama, Japan § Research Center for Photovoltaic Technologies, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan ∥ Sumika Chemical Analysis Service, Ltd., Tsukuba, Japan ‡

S Supporting Information *

ABSTRACT: We report a simple method to achieve efficient nanostructured organic photovoltaics via patterning copper iodide (CuI) nanocrystals on indium tin oxide by glancing angle deposition. The strong interfacial interaction between zinc phthalocyanine (ZnPc) and CuI leads to the formation of nanopillar arrays with lying-down molecular order, which greatly improve light absorption and surface roughness for exciton dissociation. Optimized ZnPc/C60 bilayer cell has a power conversion efficiency of 4.0 ± 0.1%, which is about 3-fold larger than that of conventional planar cell. KEYWORDS: Organic photovoltaics, copper iodide nanocrystals, heterojunction, glancing angle deposition, vacuum evaporation

T

MoO3) or conducting polymer (PEDOT/PSS) are used to facilitate the hole extraction. However, on these surfaces, molecules (i.e., metal phthlocyanine) exhibit a standing-up geometry due to their weak interfacial interaction. The strong interfacial coupling between the surface electronic states of CuI and molecular π-orbitals leads to lying-down (π−π) molecular crystalline order, which is highly favored in organic optelectronics.24 Large improvements have been achieved in the performances (especially photocurrent) of OPV by inserting a thin CuI film into ITO/organic interface.25 On the other hand, GLAD is a popular method to prepare nanostructured organic17−20,26 and inorganic films.27 This technique is also attracting much interest in dye-sensitized and organic−inorganic hybrid photovoltaics.28−30 In this Letter, we investigate the GLAD of CuI NCs and its influences on organic thin films for OPV applications. The strong interaction between CuI NCs and organic molecules dominates the following growth of organic thin films, whose growth on ITO is suppressed, accordingly. Such unique selective growth directly leads to the formation of organic nanopillar arrays with lyingdown molecular order, resulting in a 3-fold increase in PCE from 1.4 ± 0.2 to 4.0 ± 0.1%. GLAD is a physical vapor deposition based on large oblique angle (α) between incident vapor flux and substrate normal.26 Figure 1 shows the growth schematics of CuI NCs and

he power conversion efficiency (PCE) of organic photovoltaic (OPV) cells has been significantly increased, since the introduction of donor/acceptor interface and following bulk-heterojunction (BHJ).1−8 The ideal BHJ structure includes an interpenetrating network of phaseseparated donor and acceptor domains, which can provide large interface area for dissociation of excitons into free charge carriers and continuous pathways for charge carrier transport from photoactive layer to corresponding electrodes.9 Many efforts have been made to optimize the nanoscale morphology of BHJ using solution-processed polymers10 or coevaporated small molecules.11,12 Unfortunately, isolated regions are usually formed, resulting in the formation of charge trapping sites and a poor charge transport efficiency. Recent extensive studies have been carried out on constructing nanostructured organic films, especially three-dimensional nanopillar arrays by nanoimprinting,13,14 organic vapor phase deposition (OVPD),15,16 glancing angle deposition (GLAD),17−20 solvent treatment,21,22 and molecularly seeding growth.23 These approaches provide larger interface area and greatly improve the cell efficiency over conventional planar cells. Although there is a lot of room for the improvement in performances compared with present BHJ cells, these progresses arouse further development of nanostructured cell architecture. Here, we demonstrate an effective method for growing nanostructured crystalline organic films to realize highly efficient BHJ; that is, patterning copper iodide (CuI) nanocrystals (NCs) on indium−tin-oxide (ITO) coated glass by GLAD. Generally, metal-oxide semiconductors (NiO, © 2012 American Chemical Society

Received: May 6, 2012 Revised: June 26, 2012 Published: July 16, 2012 4146

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Figure 1. (a) Schematic illustrations of the growths of CuI nanocrystals and ZnPc nanostructures. AFM images of CuI evaporated on ITO at different deposition incidence angles: (b) α = 0°, (c) α = 45°, and (d) α = 70°. Heights are 20 nm for (b) and 40 nm for (c,d). AFM images of 40 nm ZnPc films grown on different CuI surfaces prepared at (e) α = 0°, (f) α = 45°, (g) α = 70°. Height: all 30 nm. Scale bar: all 200 nm.

Figure 2. SEM images of 40 nm ZnPc thin films on (a) CuI thin film (α = 0°), and (b,c) CuI nanocrystals (α = 70°). (d,e) Cross-sectional brightfield TEM images of ITO/CuI nanocystals (α = 70°)/ZnPc. Al was deposited as a protective layer.

is a dominant factor during the film growth. Greater number of CuI is grown onto the nuclei, while film growth around the shadowed area where vapor flux cannot directly reach is suppressed, resulting in a Volmer−Weber island growth mode. In addition, increasing α enhances the initial nucleation and shadowing effect, since smaller defects can act as seeds. Interestingly, Figure 1d shows heartlike crystals (Supporting Information, Figure S3) with better uniformity and smaller size for α = 70°. The apex of heartlike crystals is oriented toward the incident vapor, indicating that growth of crystals is enhanced toward incident vapor, and suppressed at shadowed side. CuI plays an important role in the growth of ZnPc.

following zinc phthalocyanine (ZnPc), and their atomic force microscopy (AFM) images. The thickness (t, Supporting Information, Figure S1) of CuI is 3 nm. The growth of CuI is strongly dependent on the α. Figure 1b shows a surface morphology being similar to ITO (Supporting Information, Figure S2), indicating that a continuous CuI thin film is formed for α = 0°. Typical GLAD nanopillar structures appear when α is increased to above 45°, as shown in Figure 1c,d. At the initial step of GLAD process, the nucleation of CuI is enhanced at the edges of ITO structural defects (hills, voids), which act as seeds. These ITO defects with various characteristics lead to a variation in the size and shape of CuI nuclei. Shadowing effect 4147

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Figure 3. (a) XRD patterns of 3 nm CuI with different deposition angles (α = 0° and α = 70°) and 40 nm ZnPc films on the surfaces of bare and CuI-coated ITO. (b) Absorbance of 40 nm ZnPc thin films on bare ITO, CuI thin film (α = 0°) and CuI nanocrystals (α = 70°) coated ITO substrates.

Figure 4. (a) J−V characteristics, (b) PCE and JSC, and (c) IPCE of ITO/CuI (or no)/ZnPc (40 nm)/C60 (50 nm)/BCP (8 nm)/Al cells using CuI deposited with different deposition angles (t = 3 nm). (d) J−V characteristics, (e) PCE and JSC, and (f) IPCE of the cells using CuI deposited with different thicknesses (α = 70°).

Smooth surface is observed in ZnPc on CuI thin film (α = 0°) (Figure 1e), while ZnPc nanopillar arrays appear for CuI NCs (α > 45°). The average grain sizes of ZnPc are 85 (α = 45°) and 45 nm (α = 70°), respectively, and the average peak-tovalley height values of both samples are about 25 nm, which is 60% of film thickness. The root-mean-square (rms) roughness values of ZnPc thin films on CuI for α = 0, 45, and 70° are 1.94, 4.82, and 4.64 nm, respectively. Apparently, nanostructured ZnPc films provide larger surface area (about twice) over conventional planar film. The growth characteristics of CuI NCs and ZnPc thin films are further investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The top-view SEM images confirm the formations of a dense and continuous ZnPc film on CuI thin film (α = 0°) (Figure 2a) and a nanostructured ZnPc film with an average diameter of around 40 nm on CuI NCs (α = 70°) (Figure 2b). Note that the cross-sectional SEM image (Figure 2c) shows some isolated nanopillar structures throughout film thickness. Bright-field TEM images identify two CuI NCs, which indicate two typical growth characteristics of CuI NCs. In Figure 2d, the CuI NC

grown on a rough ITO area exhibits a height of 14 nm and a width of 18 nm, while in Figure 2e, the CuI NC grown on a relatively smoother area exhibits a height of 9 nm and a width of 36 nm with an atomically smooth surface. It indicates that the growth of CuI along c-axis is enhanced at rough area of ITO because of the shadowing effects. Thus, the structural defects on ITO surface become important to grow vertical CuI NCs. On the other hand, the energy-filtered TEM image clearly shows a thickness variation of ZnPc films grown on CuI and bare ITO (Supporting Information, Figure S4). It suggests that growth of ZnPc is preferred on CuI surface, and correspondingly its growth is suppressed on ITO surface. We infer that more ZnPc molecules can be absorbed on the CuI surface to form nuclei and grow domains, rather than on bare ITO surface, because of the significantly stronger interface force between CuI and ZnPc. Moreover, as described in previous studies,17−20 typical GLAD of phthalocyanine films usually leads to the formation of tilted nanopillars toward incident molecular flux. Following growth of C60 will further develop the voids because ballistic shadowed regions cannot receive any C60 molecules. Here, tilted pillar morphology is avoided by 4148

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which is in the range of mostly reported values in ZnPc/C60 or CuPc/C60 planar heterojunction cells.15,25,34 The best cell using CuI (t = 3.5 nm, α = 70°) exhibits a high PCE of 4.0 ± 0.1%, which is 3-fold larger than that of planar reference cell. Such bilayer-processed cell is much more efficient than reported coevaporated CuPc/C60 BHJ (p-i-n) cell (PCE = 2.55%) using CuI thin film.25 Especially, with introducing CuI NCs, phaseseparated cell architecture with high-quality crystalline ZnPc greatly improves the carrier transfer efficiency, resulting in a high fill factor (FF) of 0.65. The effects of growth condition of CuI on detailed performances and incident photon-to-current conversion efficiency (IPCE) are investigated (Figure 4). Note that the improvement in PCE is consistent with the trends in shortcircuit current (JSC). Figure 4c,f shows the wavelength dependence of JSC, where IPCE peaks centered at wavelengths of λ = 615 and λ = 720 nm correspond to ZnPc absorption region, while the peak at λ = 430 nm corresponds to C60 absorption region. For the OPV cell with CuI film (α = 0°), the IPCE peak at λ = 615 nm is 34%, which is 45% larger than that of reference cell (without CuI), while their IPCE spectra in C60 absorption region are similar. As shown in Figure 1d, ZnPc exhibits a smooth surface on CuI (α = 0°), resulting in a similar interface area to that of ZnPc grown on bare ITO. Thus, the increase in JSC is mainly attributed to the improved absorption (Figure 3b) of ZnPc with lying-down molecular geometry. On the other hand, IPCE at both of ZnPc and C60 absorption region is significantly increased with α, because introducing CuI NCs further improves the absorption (Figure 3) and roughness of ZnPc. A further 50% increase in IPCE spectra from 22 to 31% at λ = 430 nm and from 34 to 53% at λ = 615 nm is observed for α = 70°, compared to the planar cell for α = 0°. To clarify the origin of the improvement, internal quantum efficiency (IQE) is calculated from the ratio of IPCE and spectral absorption, as shown in Figure 5. Being similar to

vertically evaporating ZnPc molecules on CuI NCs, whose titled angles are negligible, as shown in Figure 2. X-ray diffraction (XRD) patterns of ZnPc films are shown in Figure 3a. The diffraction peak at 2θ = 25.5° indicates that GLAD improves the crystalline growth of γ-CuI. Two ZnPc diffraction peaks centered at 2θ = 6.9 and 27.8°, correspond to standing-up (d = 12.9 Å) and lying-down (d = 3.2 Å) crystalline order, respectively. ZnPc films exhibit standing-up molecular order on bare ITO, amorphous state on CuI thin film (α = 0°), and lying-down molecular order on CuI NCs (α = 70°), respectively. Supporting Information, Figure S5 shows the epitaxial growth of ZnPc on crystalline CuI. It is believed that similar growth occurs on the atomically smooth surface of CuI NCs (Figure 2e), resulting in a high-quality crystalline growth of ZnPc. Moreover, introduction of CuI NCs not only provides higher carrier mobility but also enables greater light capture, as shown in Figure 3b. There are two possible reasons should be considered for the improved light absorption: (1) light scattering resulting from nanostructured surfaces of ZnPc or CuI (will be discussed below) and (2) lying-down molecular order. It has been reported that GLAD of ZnPc pillar arrays with standing-up order cannot improve the light absorption.19,20 Thus, the molecular orientation plays a crucial role in the improvement. It is known that the probability of absorption is proportional to the square of a scalar product between the absorption transition moment and the electrical field vector. Extensive theoretical and experimental researches31−33 show that for molecules like ZnPc whose π-orbitals are antisymmetric with respect to the molecular plane, the absorption intensity is maximum when the molecular planes is perpendicular to the electrical vector. Compared with other techniques previously reported,13−23 GLAD of CuI NCs allows us to construct nanostructured ZnPc films with desired molecular order and to prepare OPV cells without vacuum break to avoid surface contamination. The OPV cells were grown on ITO substrate, which was precleaned by oxygen plasma. The t and α for depositing CuI were varied from 2 to 5 nm and 0 to 80°, respectively. After deposition of CuI, ZnPc (40 nm) and C60 (50 nm) were evaporated, respectively. The current−voltage (J−V) characteristics under 1 sun AM 1.5 G (100 mW/cm2) are shown in Figure 4a,d. The OPV parameters are summarized in Table 1. The reference cell (without CuI) exhibits a PCE of 1.4 ± 0.2%, Table 1. Summary of Cell Performances of ITO/CuI(or without)/ZnPc (40 nm)/C60 (50 nm)/BCP/Al under 1 sun

Figure 5. Internal quantum efficiency (IQE) spectra of the cells prepared on CuI thin film (α = 0°) and CuI nanocrystals (α = 70°).

CuI α (o) 0 0 30 45 60 70 80 70

thickness (nm) 0 3

2 2.5 3.5 4 5

PCE (%)

VOC (V)

JSC (mA/cm2)

FF

IPCEMAX (%)

± ± ± ± ± ± ± ± ± ± ± ±

0.52 0.55 0.52 0.56 0.56 0.56 0.52 0.54 0.56 0.57 0.56 0.56

5.0 6.6 8.8 10.7 10.8 11.0 9.3 9.0 10.5 10.9 10.3 10.0

± ± ± ± ± ± ± ± ± ± ± ±

0.55 0.60 0.59 0.61 0.62 0.63 0.60 0.63 0.63 0.65 0.65 0.65

23 34 41 51 51 53 48 43 50 51 49 47

1.4 2.1 2.7 3.7 3.7 3.9 2.9 3.0 3.7 4.0 3.7 3.6

0.2 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.1 0.1 0.1 0.2

0.4 0.2 0.2 0.2 0.2 0.2 0.5 0.4 0.2 0.2 0.2 0.3

IPCE, the IQE spectra throughout λ = 400 to 800 nm is increased with α. Especially, for α = 70°, the IQE at λ = 430 and 630 nm are 58 and 69%, both of which are about 40% higher than 43 and 49% (for α = 0°), respectively. Since IQE represents the photocurrent generation efficiency from absorbed light, the 40% increase should be attributed to increased interface area. For the optimum conditions of CuI (α = 70°, t = 3−3.5 nm), ZnPc grains have an average diameter of 40 nm (Figure 2), which is within the exciton diffusion length of ZnPc (10−50 nm).15 It implies that photogenerated excitons can be more efficiently dissociated with nanostructured ZnPc. Moreover, another 10% increase in JSC is consistent with the 4149

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Figure 6. The chemical structures, AFM images (on ITO and CuI/ITO surfaces), and J−V characteristics of (a) ClAlPc, (b) SubPc, and (c) DBP.

Table 2. Summary of Cell Performances of ITO/CuI/Donors/C60 (50 nm)/BCP/Al under 1 sun donor/C60 (50 nm) ClAlPc (30 nm) SubPc (25 nm) DBP (20 nm)

without CuI CuI (α = 70°) without CuI CuI (α = 70°) without CuI CuI (α = 70°)

PCE (%)

VOC (V)

± ± ± ± ± ±

0.36 0.45 0.84 0.93 0.73 0.90

0.8 2.3 1.8 2.6 1.8 3.8

0.1 0.1 0.1 0.1 0.1 0.1

JSC (mA/cm2)

FF

IPCEMAX (%)

± ± ± ± ± ±

0.55 0.58 0.46 0.54 0.58 0.66

25 42 38 40 41 55

4.0 9.1 4.9 5.2 4.3 6.3

0.2 0.2 0.2 0.2 0.2 0.2

from 0.55 to 0.65. It is clear that efficient filling the voids with C60 is becoming crucial to achieve further improvements. For comparison of ZnPc/C60 heterojunction, several cells were fabricated using various donor materials, including aluminum phthalocyanine chloride (ClAlPc)35,36 boron subphthalocyanine chloride (SubPc),37,38 and dibenzotetraphenylperiflanthene (DBP).39,40 They represent nonplanar phthalocyanines and nonphthalocyanine, respectively, which exhibit very different electronic structure and crystalline order. Figure 6 shows their chemical structures, AFM images and the J−V characteristics of the cells based on ITO/CuI(or without)/ donors/C60 (50 nm)/BCP (8 nm)/Al, respectively. The growth parameters for CuI were kept with (α = 70° and t = 3.5 nm). The detailed performances are summarized in Table 2. All the films grown on bare ITO show smooth surface morphologies. Being similar to ZnPc film, introduction of CuI NCs leads to the formation of nanopillar arrays for ClAlPc and DBP films. Note that nanostructured ClAlPc and DBP films greatly improve the OPV performances. Particularly, JSC is increased from 4.0 ± 0.2 to 9.1 ± 0.2 mA/cm2 for ClAlPc, and from 4.3 ± 0.2 to 6.3 ± 0.2 mA/cm2 for DBP, which results in three- and 2-fold increases in PCEs from 0.7 ± 0.1 to 2.3 ± 0.1% for ClAlPc and from 1.8 ± 0.1 to 3.8 ± 0.1% for DBP, respectively. On the other hand, no clear morphological variation is observed for SubPc thin films grown on bare ITO and CuI NCs (see Figure 5b), and therefore no improvement in JSC in almost whole absorption region is observed (Supporting Information, Figure S7). Moreover, it implies

further improved light absorption (Figure 3b). Similar improvement (50% increase in JSC) has been reported in nanostructured donors with standing-up molecular order, where general vacuum evaporation was utilized to grow C60.18 Moreover, the increase in JSC reached 120% for nanostructured CuPc with standing-up molecular order, when the voids were efficiently filled with acceptor by introducing an organic phase-vapor deposition (OPVD) technology.15 To better understand the D/A interface, the actual cell using CuI (t = 3 nm, α = 70°) was investigated by TEM, as shown in Supporting Information, Figure S6. The TEM images show the nanostructured ZnPc film and a few of large C60 crystalline domains following ZnPc protrusions. On the other hand, some bright-contrast spots are observed in ZnPc film, indicating the existence of voids or other porous structures (lower material density). Here, because of the limited surface diffusion of C60 molecules and shadowing effects resulting from ZnPc protrusions, apparently, the voids in ZnPc films are not thoroughly followed by C60. Thus, there is a lot of room for the improvement of present OPV cells. It is concluded that there are three possible reasons for the 3-fold increase in PCE: (1) lying-down molecular order enables greater light capture, resulting in a 60% increase in JSC; (2) ZnPc nanopillar arrays provide larger D/A interface area for exciton dissociation over conventional planar heterojunction, resulting in a further 40% increase in JSC; (3) crystalline ZnPc and C60 pathways enable more efficient charge transfer, resulting in a 20% increase in FF 4150

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CuI thin film, cell characteristics. This material is available free of charge via the Internet at http://pubs.acs.org.

that the CuI NCs plays a negligible role in the improvement in light absorption. Apparently the interfacial interaction between CuI and SubPc is not strong enough to enhance the molecular absorption/desorption ratio on CuI surface. Randomly absorbed SubPc molecules lead to uniformly distributed nuclei on both of ITO and CuI. With increasing the film thickness, the rough substrate has less influence on film growth, resulting in a continuous layer with smooth surface morphology. It is evidence that the strong interaction plays an important role in the selective growth and surface roughness. The increases in VOC and FF indicate that CuI NCs also enable efficient hole extraction because of a better alignment between the work function of ITO and HOMO of donor materials.25 In conclusion, we demonstrate that patterning CuI nanocrystals on ITO via glancing angle deposition enables a unique selective growth of ZnPc on CuI/ITO surface, resulting in the formation of ZnPc nanopillar arrays with lying-down (π−π) molecular order. These nanostructured ZnPc not only provide larger ZnPc/C60 interface area but also enable greater light capture, which significantly facilitate the photocurrent generation and charge carrier transfer. A 3-fold increase in power conversion efficiency from 1.4 ± 0.2 to 4.0 ± 0.1% is achieved in bilayer-processed ZnPc/C60 cells. Moreover, other bilayer cells using various donor materials are also greatly improved by this method. The results indicate that it is a promising route to fabricate highly efficient small-molecule photovoltaic cells. Experimental Methods. All the samples including OPV cells were fabricated on commercial indium tin oxide (ITO) patterned glass, which was pretreated by oxygen plasma for 20 min. ZnPc, ClAlPc, and SubPc were further purified three times prior to use. All the materials were evaporated under a deposition pressure of about 5 × 10−5 Pa. The thickness and growth rate were monitored with a quartz crystal oscillator. The cell structure was ITO/CuI (or without)/donor/C60 (50 nm)/ BCP (8 nm)/Al (100 nm) with an area of 0.04 cm2. The thicknesses of donor materials were 40 nm for ZnPc, 30 nm for ClAlPc, 25 nm for SubPc, and 20 nm for DBP, respectively. The growth rates were kept at 0.1 Å/s for organic materials and CuI, and 1 Å/s for Al, respectively. The entire cell preparations as well as the electrical measurements were performed at room temperature without air exposure (vacuum or N2 gas atmosphere). The J−V characteristics of the cells were measured under simulated AM 1.5 G solar illumination with a Keithley 2400 Digital Source Meter. Incident power was calibrated by using a standard silicon photovoltaic to match 1sun intensity (100 mW/cm2). The error bars represent the standard derivation from measurement taken from at least two separate cells. IPCE curves were collected using a xenon lamp, integrated with a computer controlled monochromator. Source power spectrum was measured using a calibrated silicon photodiode. To calculate the IQE, the absolute reflectance of actual cell was measured with an integrating sphere system. AFM analyses were performed dynamic mode. XRD (SmartLab) measurements were performed using 45 kV Cu Kα resource. SEM images were taken with a Hitachi S-4800 field emission SEM. TEM investigation was performed using an FEI Titan80-300, where the sample was prepared by focused ion beam in an SII SMI3050 system.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (Y.Z.) [email protected]; (T.T.) taima@se. kanazawa-u.ac.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Precursory Research for Embryonic Science and Technology (PRESTO) program from the Japan Science and Technology Agency (JST) and “Nanotechnology Network Japan” MEXT, Japan, at AIST Nano-Processing Facility.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Additional information regarding thickness description, ITO surface morphology, structural characterizations of ZnPc on 4151

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dx.doi.org/10.1021/nl301709x | Nano Lett. 2012, 12, 4146−4152