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International Journal of Photoenergy
Volume 2013 (2013), Article ID 346818, 6 pages
Organic Photovoltaic Cells Based on PbPc Nanocolumns Prepared by Glancing Angle Deposition
Key Laboratory of Luminescence and Optical Information, Ministry of Education, Beijing Jiaotong University, Beijing 100044, China
Received 24 January 2013; Accepted 14 March 2013
Academic Editor: Sudhakar Shet
Copyright © 2013 Yang Liu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Organic small material lead phthalocyanine (PbPc) nanocolumns were prepared via glancing angle deposition (GLAD) on indium tin oxide (ITO) coated glass substrates. Organic electron acceptor materials fullerene (C60) was evaporated onto the nanocolumn PbPc thin films to prepare heterojunction structure ITO/PbPc/C60/Bphen/Al organic photovoltaic cells (OPVs). It is worthwhile to mention that C60 molecules firstly fill the voids between PbPc nanocolumns and then form impact C60 layer. The interpenetrating electron donor/acceptor structure effectively enhances interface between electron donor and electron acceptor, which is beneficial to exciton dissociation. The short circuit current density (Jsc) of organic photovoltaic devices (OPVs) based on PbPc nanocolumn was increased from 1.19 mA/cm2 to 1.74 mA/cm2, which should be attributed to the increase of interface between donor and acceptor. The effect of illumination intensity on the performance of OPVs was investigated by controlling the distance between light source and sample, and the Jsc of two kind of OPVs was increased along with the increase of illumination intensity.
Organic photovoltaic devices (OPVs) are potential candidates for light weight, low-cost solar energy conversion, and ease of processing [1–4]. Typical OPVs are composed of a donor (p-type semiconductor) and an acceptor (n-type semiconductor) as active layers. Photo-generated excitons are dissociated into free holes and electrons at the interface of donor-acceptor, the free charge carriers are collected by the electrodes, respectively. Two kinds of architectures with this D-A interface were developed: planar heterojunction (PHJ) and bulk heterojunction (BHJ) [5, 6]. Tang firstly reported PHJ devices based on CuPc/C60 as the active layer, which was considered as a milestone of OPVs development . The efficiency of exciton dissociation of the PHJ cell is decreased due to the limit of the D-A interface. Some photo-generated excitons in the active layers maybe quenched when travelling a distance longer than the diffusion length to the D-A interface. The exciton dissociation efficiency could be increased in the BHJ cells, where the donor and acceptor form an interpenetrating network of structures which shortens the excitons’ route to the D-A interface. However, random distribution of donor and acceptor materials in the blend film may lead to lower charge collection due to the so-called dead ends in the conducting paths and long conducting distance from the active layers to the electrodes [4, 8]. Therefore, the key point to enhance the PCE is to form the structure which provides high interface area and gold charge transport routes in the active layer. Knorr and Hoffman firstly discovered a new kind of evaporation technology, named as glancing angle deposition (GLAD). The morphology and microstructure of this kind of films prepared by GLAD could be easily controlled by changing the substrate rotating speed, the evaporation speed, the distance between substrate and evaporation source, the angle between incoming incidence flux, and the substrate surface [9–13]. Van Dijken et al. applied the GLAD technology to fabricate donor layer and then spin-coated acceptor solution onto the nanostructure donor layer for achieving a mixed heterojunction OPVs, which shows a better performance than the common bilayer heterojunction OPV [14–16]. Also this approach has been used to fabricate nanocolumn structural thin films for indium tin oxide (ITO) electrodes  and electron acceptor material C60 [18, 19], providing large interfacial area for exciton extraction and charge carrier collection. Hoppe et al. pointed out that only 30% of the photons in the whole solar spectrum have energy higher than 1.9 eV . Lead phthalocyanine (PbPc), as a promising small molecule donor material, has attracted more and more attention due to its near infrared absorption . Organic electron acceptor materials fullerene (C60) is a proper matching for PbPc to form the heterojunction.
In this paper, two kinds of OPVs with different morphology PbPc films as the donor layer and C60 films as the acceptor layer were fabricated and measured under different illumination intensities. The OPVs with PbPc films prepared by GLAD show better performance compared with the OPVs based on PbPc films prepared under normal conditions. The underlying reason for the improvement of OPVs performance was discussed from the effect of PbPc film morphology on the absorption spectra and donor/acceptor interfaces.
2. Experimental Details
Indium tin oxide (ITO) coated glass substrates with a sheet resistance of 15 Ω/□ (purchased from Shenzhen Jinghua Displays Co. Ltd.) were cleaned with detergent, deionized water and ethanol successively in ultrasonic baths for 15 min. All substrates were dried by nitrogen gas and were treated by UV-ozone for 10 min to improve the work function of ITO. Electron donor material PbPc (purchased from Jilin Optical and Electronic Materials Co., Ltd.) was deposited on the ITO substrates via GLAD. A computer-controlled stepper motor was used to control rotation of substrate about the substrate surface normal. The angle between the molecular flux incidence and the substrate normal was set to 75° and 0°. The substrate rotation rate was controlled about 6 rounds per minute (rpm). The deposition rate was about 0.1 ~ 0.3 nm/s, which was monitored by quartz-crystal oscillator monitor. Electron acceptor material C60 (purchased from Alfa Aesar) films and hole blocking material 4,7-diphenyl-1, 10-phenanthroline (Bphen, purchased from Jilin OLED Material Tech Co., Ltd) films were deposited by thermal evaporation under 4 × 10−4 Pa vacuum conditions successively. Al cathode of 100 nm thickness was thermally evaporated on the hole blocking layer through shadow masks. The active area of the cells was 3 × 3 mm2. The schematic diagram of GLAD installation is shown in Figure 1(a) (left side): the angle between substrate normal and the incoming particle flux is defined as α, and the angle between PbPc nanocolumns and substrate surface normal is defined as β. The schematic process of GLAD is presented in Figure 1(a) (right side): (i) a few PbPc particles arrive at the substrate at first and form some islands on the surface, (ii) the former islands block the incoming particles resulting in selective growth along the island due to the shadow effect, (iii) formed column structure. The schematic diagram of PbPc nanocolumn films and OPVs are shown in Figures 1(b) and 1(c). The energy level alignment of used materials is shown in Figure 1(d).
Morphology of PbPc films was investigated by a Hitachi S-4800 scanning electron microscope (SEM) with a secondary electron detector. The absorption spectra of films were measured via Shimadzu UV-3101PC UV-VIS. Current-voltage (I-V) curves of fabricated OPVs were measured by Keithley 4200 source measure unit under 100 mW/cm2 illumination or dark conditions.
3. Results and Discussions
The absorption spectra of PbPc films under different preparation conditions were measured and are shown in Figure 2(a). The absorption spectrum of PbPc film prepared under normal conditions (titled angle is 0°) shows two broader absorption peaks, one at around 515 nm and the other at 350 nm, which accords with previously observed results . The absorption spectrum of PbPc thin films prepared by GLAD with the tilted angle 75° is similar with that of PbPc thin film prepared under normal conditions. In order to investigate the effect of film prepared conditions on its absorption performance, the normalized absorption spectra are show in Figure 2(b). It is worthwhile to mention that PbPc thin films prepared by GLAD have slightly strong absorption from 400 nm to 480 nm, comparing with that of PbPc films prepared under normal conditions, which should be attributed to the increased of OPVs (as shown in Figure 3).
Two kinds of bilayer heterojunction OPVs with different structural PbPc films as the electron donor layers were prepared, Cells A: ITO/PbPc (40 nm, prepared under normal conditions)/C60 (50 nm)/Bphen (8 nm)/Al (100 nm), Cells B: ITO/PbPc (40 nm, prepared by GLAD)/C60 (50 nm)/Bphen (8 nm)/Al (100 nm). The 8 nm Bphen layer was used as hole-blocking layer and protection layer for the active layer during metal deposition. The J-V characteristics of two kinds of OPVs were measured at 100 mW/m2 illumination and in dark conditions, as shown in Figure 3. The of Cells B is about 1.5 times larger than that of Cells A, which may be attributed to the increase of absorption and donor/acceptor interface induced by PbPc nanocolumn structure prepared by GLAD. The open-circuit voltage () of the Cells B is about 0.34 V and lowers than that (0.4 V) of Cells A, which may be induced by the leakage current of Cells B resulting from the interface between PbPc layer and ITO. According to the dark J-V characteristics curves of two kinds of cells, it is apparent that both kinds of cells have good blocking characteristics in the reverse bias region, the Cells B have a lower break-cover voltage in forward bias region. The power conversion efficiency (PCE) of Cells B is about 0.21%, which is larger than that (0.16%) of Cells A. The key parameters, including , , FF and PCE of the two kinds of OPV cells are summarized in Table 1.
In order to further investigate the effect of PbPc film morphology on the performance of OPVs, the J-V characteristic curves were measured under different illumination intensities. Figure 4(a) shows the J-V characteristic curves of Cells A; Figure 4(b) shows the J-V characteristic curves of Cells B. Both kinds of OPVs also show similar behaviors dependent on the illumination intensity. The is increased along with the increase of illumination intensity, and keeps constant under different illumination intensities. It is known that photongenerated excitons strongly depend on the incident photon numbers, which have enough energy to excite active layer materials [23, 24]. The increased should be well understood due to more photo-generated excitons under higher illumination intensity. It is known that is determined by the difference between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor . However, the large input power also increases the active layer temperature, which may induce exciton quenched in organic materials . The exciton dissociation and quenching are two competitive processes, which codetermine PCE of OPVs. According to the data summarized in Tables 2 and 3, the PCE of OPVs is decreased along with the increase of illumination intensity. It means that more photo-generated excitons were quenched under higher illumination intensity, which results in the decrease of PCE. The key parameters of two kinds of OPVs are summarized in Tables 2 and 3.
From the variation of two kinds of cells’ photovoltaic characteristic of OPVs, PbPc films prepared by GLAD have a positive effect on the performance of OPVs. In order to clarify the positive effect, the morphology of PbPc films prepared by normal conditions and GLAD were investigated by SEM, as shown in Figure 5. From the SEM images of Figures 5(a) and 5(b), the surface of PbPc films prepared by GLAD shows some small holes, and PbPc films prepared under normal conditions look more smooth. According to Figures 5(c) and 5(d), the surface roughness of PbPc films prepared by GLAD should be larger than that of PbPc films prepared by normal condition. The similar experimental phenomenon was observed in our pervious experimental results [27, 28]. The dynamic growth process of PbPc films is described by shadow effect and atomic surface diffusion. Shadow effect implies that one given point on the surface can receive fewer particles than other points, because nearby surface features block some of the incoming particles, which results in tilted nanocolumns on the substrates . Atomic scale fluctuations inevitably exist on the nominally smooth surfaces. PbPc particles arrive at the top of the surface and will migrate to other points due to their kinetic energy, leading to a smooth surface. The competition between these two factors strongly determines the morphological evolution of the growing surface. The schematic growth process is shown in Figure 1(b).
During the glancing angle deposition process, the angle () between PbPc nanocolumns and substrate surface normal strongly depends on the angle () between substrate surface normal and the incoming particle flux. The angle is not simply proportional to the cosine of the tilted angle α, which is given by the following formulas: (i) tangent rule (tan, °) described by line 1 in Figure 6 and (ii) cosine rule (°), described by line 2 in Figure 6 . In this work, α was set to 75°, therefore column angle was measured by formula (II) resulting in 45°. In terms of the thickness of PbPc film, the actual thickness of PbPc film is defined as , and the evaporation thickness monitored by quartz-crystal oscillator monitor is . The relationship of the two parameters could be expressed as , described by line 3 in Figure 6. In the Cells A, the thicknesses of the PbPc films prepared under normal conditions are 40 nm. For keeping PbPc films thickness constant in the two kinds of Cells, the evaporation thickness () monitored by quartz-crystal oscillator monitor was controlled about 56.5 nm, which could keep the factual PbPc films thickness () at 40 nm. The PbPc films prepared by GLAD show nanocolumn structures, which could increase the interfaces with C60, resulting in more exciton dissociation. The PbPc nanocolumn also provides an effective charge carrier transporting road, which is beneficial to the charge carrier collection by their individual cathodes.
Two kinds of OPVs based on different morphology PbPc film prepared under normal conditions or glancing angle deposition were fabricated; the OPVs based on PbPc nanocolumn prepared by GLAD obtain a larger PCE compared with OPVs based on PbPc film prepared under normal conditions. The main contribution for the increase of PCE should be due to the enhanced absorption and enlarger interface between PbPc and C60 layer. The effect of illumination intensity on the performance of OPVs was investigated by controlling the distance between light source and sample, the of two kind of OPVs was increased along with the increase of illumination intensity.
The authors express their thanks to the Beijing Natural Science Foundation (2122050) and Basic Research Foundation of the Central Universities (2013JBZ004). The authors also thank the supports on using synchrotron radiation light from the Beijing Synchrotron Radiation Facility. F. J. Zhang thanks the support from the State Key Laboratory of Catalysis and the Key Laboratory of Photochemical Conversion and Optoelectronic Materials, TIPC, CAS.
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