Dual roles of MoO3-doped pentacene thin films as hole-extraction and multicharge-separation functions in pentacene/C60 heterojunction organic solar cells Yan-Hui Lou, Mei-Feng Xu, Zhao-Kui Wang, Shigeki Naka, Hiroyuki Okada et al. Citation: Appl. Phys. Lett. 102, 113305 (2013); doi: 10.1063/1.4798281 View online: http://dx.doi.org/10.1063/1.4798281 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v102/i11 Published by the AIP Publishing LLC.
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APPLIED PHYSICS LETTERS 102, 113305 (2013)
Dual roles of MoO3-doped pentacene thin films as hole-extraction and multicharge-separation functions in pentacene/C60 heterojunction organic solar cells Yan-Hui Lou,1,2 Mei-Feng Xu,1 Zhao-Kui Wang,1,3,a) Shigeki Naka,3 Hiroyuki Okada,3 and Liang-Sheng Liao1,b)
1 Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu 215123, China 2 School of Energy, Soochow University, Suzhou, Jiangsu 215123, China 3 Graduate School of Science and Technology, University of Toyama, 3190 Gofuku Toyama, Japan
(Received 14 January 2013; accepted 11 March 2013; published online 21 March 2013) The authors demonstrate a pentacene/C60 heterojunction organic solar cell utilizing MoO3-doped pentacene thin films as an interfacial layer at anode and a multicharge-separation layer between pentacene and C60, respectively. The short-circuit current density and the open-circuit voltage were improved simultaneously compared with the reference device, resulting in an improvement in power conversion efficiency from 0.97% to 2.29%. Absorption spectra measurement, surface morphology analysis, and interfacial evaluation at anode side in the hole-dominant devices were C 2013 American carried out to reveal the functions of MoO3-doped pentacene films in OSCs. V Institute of Physics. [http://dx.doi.org/10.1063/1.4798281]
Exceeding 10% power conversion efficiencies (PCE) of polymer organic solar cells (OSCs) has been achieved recently.1 However, the PCE in small molecular OSCs improves slowly owing to limited carrier mobility and short exciton diffusion length of representative donor (D) materials (i.e., metal phthalocyanine compounds).2,3 Pentacene is one of the most promising organic semiconductors with high mobility, longer exciton diffusion length, and maximum absorption peak at 670 nm, which makes it as a good donor in conjunction with C60 as acceptor (A) in OSCs.4–7 However, the highest occupied molecular orbital (HOMO) of pentacene restricts the open-circuit voltage (Voc), which is generally decided by the offset between the HOMO level of the donor and the lowest unoccupied molecular orbital (LUMO) level of the acceptor.8 Recently, a multicharge separation (MCS) interface structure, which is constructed by inserting a suitable interfacial material between the D/A heterojunction interface, is proposed because the Voc can be improved effectively.9–12 In addition, further understanding of the physical properties such as carrier charge behavior at metal/organic interfaces and charge separation process at interpenetrating network interfaces is still necessary for designing materials and choosing good cell structures with the goal of high performance OSCs.13–15 Inserting a suitable interfacial layer between the metal electrode and the active layer can align the energy levels and format carrier-selective contact for desired carrier extraction from the active layer to the electrodes.16–18 It is also reported that the electrical doping in organic materials is very efficient for resolving the issues of low conductivity and high carrier barriers in organic electronics.19–22 In particular, transition metal oxides (i.e., MoO3) doped organic semiconductor demonstrated special merits on improving device performance and stability.23–28 a)
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In this letter, we report the improved performance including Voc and short-circuit current density (Jsc) simultaneously, in a pentacene/C60 heterojunction OSC by using thin MoO3pentacene film as the anode interfacial layer and the MCS layer, respectively. Compared with non-doped device, PCE is improved from 0.97% to 2.29%. Absorption spectra measurement and interfacial property evaluation are carried out to clarify the origin of PCE improvement. A schematic device structure under investigation is shown in Fig. 1(a). In sequence, MoO3-doped pentacene (5 nm) as an anode interfacial layer, pure pentacene (PEN, 40 nm) as donor, MoO3-doped pentacene (5 nm) as a MCS layer, C60 (40 nm) as acceptor, bathocuproine (BCP) (10 nm) as an exciton blocking layer, and Ag (100 nm) as cathode were thermal evaporated onto ITO coated glass substrate at a base pressure of 2 106 Torr. The device (named as D) has a structure of ITO/PEN:MoO3 (5 nm)/PEN (45 nm)/ PEN:MoO3 (5 nm)/C60/BCP/Ag. MoO3-doped pentacene layers were deposited by co-evaporation under 60 C (substrate temperature) with separated source in optimized weight ratio of 1:4. A shadow metal mask was used for Ag evaporation, giving an active area of 0.04 cm2. By keeping the thicknesses of C60 (40 nm) and BCP (10 nm), devices with structure of A: ITO/PEN (50 nm)/C60/BCP/Ag, B: ITO/PEN (45 nm)/PEN:MoO3 (5 nm)/C60/BCP/Ag, and C: ITO/PEN:MoO3 (5 nm)/PEN (45 nm)/C60/BCP/Ag were also fabricated for comparison. The current density-voltage (J-V) characteristics were measured using a semiconductor parameter analyzer (HP 4155B). Figure 2 shows the J-V curves in four OSC devices under a simulated AM 1.5G spectrum illumination. The key cell parameters, including Jsc, Voc, fill factor (FF), and PCE of all devices are listed in Table I. Reference device A showed a lower PCE of 0.97% with Jsc ¼ 6.05 mA/cm2, Voc ¼ 0.38 V, and FF ¼ 43%. In device B (using MoO3-doped pentacene as a CMS layer), Voc was increased to 0.47 V and Jsc was slightly
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Lou et al.
Appl. Phys. Lett. 102, 113305 (2013) TABLE I. Performance summary (including Jsc, Voc, FF, and PCE) in pentacene/C60 heterojunction OSCs with and without different combination of MoO3-doped pentacene.
FIG. 1. (a) Schematic device structure under investigation. (b) The energy level of organic materials and work functions of electrodes.
increased to 6.88 mA/cm2, corresponding to an improved PCE of 1.58%. In device C (using MoO3-doped pentacene as an anode interfacial layer), Jsc was improved to 8.30 mA/cm2 while no large change in Voc was observed, and the device showed a PCE of 1.42%. Noticeably, in device D (using MoO3-doped pentacene as CMS and anode interfacial layers), a combined effect with increased Voc (0.52 V) and enhanced Jsc (9.31 mA/cm2) occurred, resulting in an improved PCE of 2.29%. By obvious difference on key parameters among four devices, the dual roles of MoO3-doped pentacene as chargeseparation and hole-extraction functions is confirmed. The increase of Voc in device D is mainly attributed to the energy level alignment between pentacene (donor) and
FIG. 2. Typical J-V characteristics in pentacene/C60 heterojunction OSC A, B, C, and D with and without different combination of MoO3-doped pentacene under a simulated AM 1.5G spectrum illumination.
Devices
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
A: PEN/C60 B: PEN/PEN:MoO3/C60 C: PEN:MoO3/PEN/C60 D: PEN:MoO3/PEN/PEN:MoO3/C60
0.38 0.47 0.41 0.52
6.05 6.85 8.30 9.31
0.41 0.42 0.48 0.47
0.97 1.58 1.42 2.29
C60 (acceptor) by the thin MoO3-doped pentacene CMS layer. Fig. 1(b) shows the energy level diagram of materials under investigation. The p-doping characteristics of pentacene by MoO3 incorporation have been confirmed by ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling microscopy studies, and an increase of HOMO level for MoO3-doped pentacene is observed compared with that of pure pentacene.25,26 The increased energy difference between the HOMO of MoO3-doped pentacene and the LUMO of C60 resulted in the increase in Voc, which also occurred in device B. Fig. 3 shows the atomic force microscopy (AFM) images of surface morphologies for pure pentacene (50 nm), pentacene (45 nm)/MoO3 (5 nm), and pentacene (45 nm)/pentacene:MoO3 (5 nm). For pure pentacene, a large surface roughness of 5.12 nm (in root-meansquare (rms)) was observed owing to the crystalline growth of pentacene. After covering a MoO3 film (5 nm) on the top of pentacene, rms lowered to 4.57 nm because of relatively smoothed surface morphology of MoO3 film. Noticeably, when covering a MoO3-doped pentacene (5 nm) film, a remarkable decrease in rms occurred, suggesting that the interface between pentacene and C60 could be effectively smoothed by covering a thin MoO3-doped pentacene film on top of pure pentacene. The relatively smoothed surface morphology makes it possible for the formation of a good interface contact between pentacene and C60.10,29 Large enhancement in Jsc from 6.05 mA/cm2 to 9.31 mA/cm2 was observed in device D. In general, Jsc is restrained by the cell external quantum efficiency, which is decided by the product of efficiencies in the following operating processes: (1) exciton generation by photon absorption, (2) exciton diffusion, (3) exciton dissociation by charge transfer, and (4) carrier collection at electrodes.30,31 The enhanced Jsc in the present case was mainly associated to the improved efficiencies of processes (1), (3), and (4). Process (2) associating with MoO3-doped pentacene CMS layer could be neglected because the thickness of CMS layer in device D is far smaller than the exciton diffusion length of pentacene (60 nm).5,6 Further experiments were carried out to validate the assumptions above mentioned. Fig. 4 shows the absorption spectra of pentacene, MoO3-doped pentacene, and MoO3 films on cleaned Corning glass with the same thickness 50 nm. A slight absorption enhancement in short wavelength region (
