Role of electrode buffer layers in organic solar cells Takeaki Sakurai,1,2 Shenghao Wang,1 Susumu Toyoshima,1 Katsuhiro Akimoto1 1
Institute of Applied Physics, University of Tsukuba, Tsukuba, Japan 2 JST-PRESTO, Saitama, Japan
[email protected]
Abstract—A systematic study on the energy level alignment, electron doping, and exciton quenching at interfaces between bathocuproine (BCP) buffer layers and various types of metal cathodes in organic solar cells (OSCs) was carried out by performing ultraviolet photoelectron spectroscopy (UPS), electronic conductivity, and photoluminescence (PL) measurements. Suppression of energy losses, such as low contact resistance and reduction of exciton quenching, were found at the metal cathodes with the BCP buffer layers. Impact of buffer/cathode interface properties on the performance of OSCs is discussed in detail. Keywords-organic solar cells; buffer layer; energy level alignment; exciton quenching
I.
INTRODUCTION
An interaction at organic buffer layer/metal cathode interfaces influences its energy level alignment and carrier injection probability, hence it affects performance of OSCs. The electrical properties at organic buffer layer/metal cathode interfaces have been widely investigated recently [1]. However, the role of the buffer layer of OSCs is still unclear. In this study, we have investigated interactions between BCP (fig.1) buffer layers and metal cathodes to clarify the mechanisms for the improvement of the electrical properties of OSCs.
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III.
RESULTS AND DISCUSSION
A. Influence of metal cathodes on performances of OSCs Solar cells composed of ZnPc and C60 were fabricated with and without BCP buffer layers. The structure of the cell is given as ITO/PEDOT:PSS/ZnPc (5 nm)/ZnPc:C60 (1:1 codeposition; 15 nm)/C60 (30 nm)/buffer layer (BCP; 6 nm)/metal cathode (Al, MgAg, Ag, and In). The J-V characteristics of the cells under light irradiation are shown in fig. 2. The cell performances were remarkably enhanced by the insertion of BCP buffer layer, while the open circuit voltage (VOC) values were almost constant regardless of the sort of the metal cathodes. In principle, the performance of the cells must be affected by the Fermi level of the cathodes. In order to understand the relation of the cell performance with the BCP buffer layer, therefore, energy level offset at hetero-interfaces should be considered.
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Fig.1 Chemical structure of BCP II.
probe method, on the other hand, was applied for low resistive samples to subtract their contact resistance. The PL measurements were carried out by using a 532 nm laser as an excitation light source. The power of the laser is 10 mW.
EXPERIMENTAL
All samples were prepared in an ultrahigh-vacuum (UHV) system at room temperature using organic molecular beam deposition (OMBD) methods. The solar cells were fabricated using a combination of zinc phthalocyanine (ZnPc) and C60; the characteristics of these cells were determined using simulated solar light with an air mass of 1.5 (AM 1.5) under 80 mW/cm2 irradiation in vacuum. UPS measurements were carried out in a UHV analysis chamber at the BL11C at the KEK Photon Factory. The photon energy for UPS measurements was maintained constant at hν = 21.2 eV. Electrical conductivity measurements were performed in vacuum chamber using two- or four-point probe methods. The two-point probe method was applied for high resistive samples, i.e., undoped BCP layer. The four-point
Fig.2 The J-V characteristics of OSCs (a) without and (b) with BCP buffer layer under AM 1.5 light irradiation. The structure of the cell is given as ITO/PEDOT:PSS/ZnPc (5 nm)/ZnPc:C60 (1:1 co-deposition, 15 nm)/C60 (30 nm)/buffer layer (BCP; 6 nm)/metal cathode (Al, MgAg, Ag, and In). B. Energy level alignment at BCP interfaces Figure 3 shows the plots of the energy difference between the HOMO of BCP and the Fermi level of metal as a function of metal work function extracted from the UPS spectra. It was found that the position of HOMO level from metal Fermi level was almost constant with 3.7 eV for the case of Ca, Mg, Ag, whose work function were relatively low (Bardeen limit). Considering the HOMO-LUMO energy difference in BCP (3.5 eV), the energy position of the LUMO level almost accords with the Fermi level of these metals. Further, we observed new peaks at around 1 eV below the Fermi level which may be due to the formation of the interface states, as shown in fig. 4.
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2 dE HOMO − Fermi dΦ metal
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Metal work function (eV) Fig.3 Plots of the energy difference between the HOMO of BCP and the Fermi level of metal (EHOMO-Fermi) as a function of the metal work function. B C P 1.6 nm / Me ta l
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C. Conductivity of metal doped BCP Figure 6 shows the electronic conductivity of the Au-, Ag-, and Ca-doped BCP layers formed on Ag electrode patterned glass substrates as a function of the doping concentration. A clear correlation between their electronic structure and conductivity is observed. For the Ca- and Ag-doped BCP layer, the conductivity increases monotonously with increasing metal concentration, and these values are two orders of magnitude higher than those of the Au-doped BCP layer. The electric structures shown in fig. 4 suggest that the presence of gap states affects the enhancement of conductivity; that is, the electron transfer between adjacent gap states can take place easily via the BCP LUMO level, and it contributes to the carrier transport in BCP buffer layers.
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Fig.4 UPS spectra of thin BCP layers (1.6 nm) deposited on metal substrates.
C onduc tivity (S /m )
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Figure 5 shows (a)UPS spectra of C60(5 nm)/BCP(1.2 nm)/Ca hetero-structure and (b) its energy level diagram. The energy difference of LUMO between C60 and BCP is 0.06 eV; that is, electron injection barrier at C60/BCP/Ca hetero-structure is so small. The similar result (low injection barrier height) has already been observed for C60/BCP (0.6 nm)/Ag heterointerfaces [3]. Thus, BCP buffer layer shows superior properties for electron injection at metal cathodes of OSCs.
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Fig.6 Electronic conductivity of the Au-, Ag-, and Ca-doped BCP layers
Fig.5 (a) UPS spectra of C60/BCP/Ca heterostructure. (b) Schematic energy level diagram of C60/BCP/Ca heterostructure. These results suggest that electrons can through from LUMO of BCP to metal via interface state, that is, the barrier height for electrons is lowered by the interface state. For the specimens of BCP deposited on Cu, and Au, whose work function are relatively high, the position of the HOMO level from the Fermi level was varied depending on the work function of the metals, suggesting the increase of the barrier height at the interface (Schottky limit). No new peak was observed for the case of BCP on Cu and Au. From these results, it is considered that the interaction between BCP and metal depends on the work function of metal, and the close position of the LUMO level to the Fermi level is important to make an interaction between BCP and metal [2].
Fig.7 PL intensity of 30 nm C60 layer deposited onto bare glass, (10 nm) BCP/ glass and Ag-doped BCP (10 nm)/glass with various molar ratio of Ag to BCP. (inset) PL spectrum of 30 nm C60 layer deposited onto bare glass
D. Photoluminescence of C60 formed on BCP Figure 7 shows the PL intensity of 30 nm C60 layer deposited onto bare glass, (10 nm) BCP/ glass and Ag-doped BCP (10 nm)/glass with various molar ratio of Ag to BCP. When BCP layer or Ag-doped BCP with molar ratio of 1:1 is inserted between glass and C60, the PL intensities are nearly the same as the no BCP layer sample. As increasing the molar ratio of Ag to BCP, the peak intensity are weakened gradually. At heavy molar ratio, the Ag atoms will form Ag clusters. The Ag clusters will result in the C60/Ag interface when C60 layer is deposited on it, rendering recombination centers more likely and therefore resulting in the weak PL emission. Thus, the BCP buffer layer has a role of the suppression of exciton quenching at cathodes. IV.
which is observed when the metal work function is below 4.3 eV and formed due to the interaction between metals and the LUMO level. The electrical properties of metal-doped BCP layers are determined by the BCP/metal interaction; further, the density of gap states influences their electronic conductivity. Moreover, BCP buffer layer has important roles of suppression of exciton quenching at cathodes. Such improvements in optoelectronic properties at cathode interfaces by insertion of BCP buffer layer enhance performances of OSCs. ACKNOWLEDGMENT We are grateful to the Japanese Ministry of Education, Culture, Sports, Science and Technology for a Grant-in-Aid for Young Scientists (B) (No. 23760276).
CONCLUSIONS
We have investigated optoelectronic properties of BCP buffer layers in OSCs. The energy level alignment between BCP and metals was found to depend on the metal work function. For the metal work function below 4.3 eV, the position of HOMO level from metal Fermi level was fixed at 3.7 eV; that is, the energy position of the LUMO level almost accords with the Fermi level of these metals. The variations in the energy level alignment between BCP and metals is caused by the electron transfer from the metal Fermi level to gap states,
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