Performance improvement of polymer solar cells by using a solution processible titanium chelate as cathode buffer layer Zhan’ao Tan, Chunhe Yang, Erjun Zhou, Xiang Wang, and Yongfang Li Citation: Applied Physics Letters 91, 023509 (2007); doi: 10.1063/1.2757125 View online: http://dx.doi.org/10.1063/1.2757125 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/91/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Performance improvement of inverted polymer solar cells by doping Au nanoparticles into TiO2 cathode buffer layer Appl. Phys. Lett. 103, 233303 (2013); 10.1063/1.4840319 TiOx/Al bilayer as cathode buffer layer for inverted organic solar cell Appl. Phys. Lett. 103, 173303 (2013); 10.1063/1.4826562 Cathode buffer layers based on vacuum and solution deposited poly(3,4-ethylenedioxythiophene) for efficient inverted organic solar cells Appl. Phys. Lett. 100, 183301 (2012); 10.1063/1.4709481 Submicron-scale manipulation of phase separation in organic solar cells Appl. Phys. Lett. 92, 023307 (2008); 10.1063/1.2835047 Influence of buffer layers on the performance of polymer solar cells Appl. Phys. Lett. 84, 3906 (2004); 10.1063/1.1739279
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APPLIED PHYSICS LETTERS 91, 023509 共2007兲
Performance improvement of polymer solar cells by using a solution processible titanium chelate as cathode buffer layer Zhan’ao Tan, Chunhe Yang, Erjun Zhou, Xiang Wang, and Yongfang Lia兲 Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100080, China
共Received 24 May 2007; accepted 18 June 2007; published online 13 July 2007兲 A solution processible titanium chelate, titanium 共diisopropoxide兲 bis 共2,4-pentanedionate兲 共TIPD兲, was used as the cathode buffer layer in the polymer solar cells 共PSCs兲 based on the blend of poly关2-methoxy-5-共2⬘-ethylhexyloxy兲-1,4-phenylenevinylene兴 and 关6,6兴-phenyl-C61-butyric acid methyl ester. Introducing TIPD buffer layer reduced the interface resistance between the active layer and Al electrode, leading to a lower device resistance. The power conversion efficiency of the PSC with TIPD buffer layer reached 2.52% under the illumination of AM1.5, 100 mW/ cm2, which is increased by 51.8% in comparison with that 共1.66%兲 of the device without TIPD buffer layer under the same experimental conditions. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2757125兴 Polymer solar cells 共PSCs兲 have received considerable attention in recent years because of their potential application for low-cost solar energy conversion.1 Since the bulkheterojunction PSC was reported by Yu et al.2 in 1995, power conversion efficiency of the PSCs has reached 5%.3–5 However, the PSCs are still far from competing with their inorganic counterparts regarding their efficiency and lifetime. The PSCs are composed of a photosensitive layer sandwiched between an indium tin oxide 共ITO兲 anode and a low work function metal cathode. The photosensitive layer is commonly a blend of a conjugated polymer as a donor and soluble C60 derivative 关6,6兴-phenyl C61-butyric acid methyl ester 共PCBM兲 as an acceptor. The interface structure between the active layer and the electrodes plays a key role in charge collection of the devices. There are lots of efforts on modifying the interface between the electrodes and the active layer by inserting a buffer layer.6 Poly共3,4-ethylene dioxythiophene兲:poly共styrene sulphonate兲 共PEDOT:PSS兲 is the most famous and well utilized buffer layer on modifying the interface between the ITO anode and the active layer.7 In comparison with the mature anode buffer layer, the cathode buffer layer is still a hot topic for the organic/polymer optoelectronic devices.5,8–11 A thin layer of LiF can dramatically improve the performance of PSCs.10,11 However, the LiF layer must be very thin 共typically less than 1 nm兲, since thicker layer was found to be detrimental to electron collection. Such thin layer of LiF is not easy to be controlled and unfavorable to fabricate on large area by vacuum evaporation. So introducing an efficient buffer layer through a simple method such as spin coating is desirable for reducing the cost and for large area fabrication of the PSCs. Recently, Kim et al.5 introduced a solution-based titanium oxide layer between the active layer and Al cathode as an optical spacer, the power conversion efficiency of the devices dramatically improved to 5%. However, the preparation processes of the TiOx layer involved exposure to air for hydrolysis, which could introduce water a兲
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and oxygen into the active layer of the devices. Here, we used a solution processible titanium chelate, titanium 共diisopropoxide兲 bis 共2,4-pentanedionate兲 共TIPD兲 共bought from Alfa Aesar兲 as the cathode buffer layer in the PSCs based on the blend of poly关2-methoxy-5共2⬘-ethylhexyloxy兲-1,4-phenylenevinylene兴 共MEH-PPV兲 and PCBM. The TIPD layer was spin coated from isopropanol solution on the active layer and then dried at 70 ° C for 30 min. The power conversion efficiency of the PSC with TIPD buffer layer reached 2.52% under the illumination of AM1.5, 100 mW/ cm2, which is increased by 51.8% in comparison with that 共1.66%兲 of the device without TIPD buffer layer under the same experimental conditions. Moreover, the TIPD buffer layer possesses the advantages of 共1兲 solution processability with protonic solvents, such as isopropanol and isooctanol, which facilitates the spin coating of the buffer layer on the polymer active layer because most conjugated polymers are insoluble in these protonic solvents; 共2兲 continuous fabrication following the formation of the active layer in dry box under inert atmosphere, which avoids exposure to air as the case of using TiOx as optical spacer5 关the oxygen and water in air could deteriorate the device performance of the PSCs 共Ref. 12兲兴; and 共3兲 low cost because TIPD is a cheap and commonly used material in paints and inks to improve the adhesion with substrate. Furthermore, the TIPD layer could increase the stability and lifetime of the PSCs because it can avoid the hot metal atom directly contacting the active layer under vacuum evaporation of the cathode metal and it can isolate the active layer of the devices from the surrounding atmosphere.13 The PSCs were fabricated with the traditional sandwich structure. PEDOT:PSS was spin coated from a PEDOT:PSS aqueous solution 共Bayer AG兲 on a precleaned ITO/glass substrate with a thickness of approximately 30 nm, as measured by Ambios Technology XP-2 surface profilometer, and was dried subsequently at 150 ° C in air for 10 min. The photosensitive blend layer was prepared by spin coating 共3000 rpm兲 the chlorobenzene solution of MEH-PPV and PCBM 共1:4 w/w兲 with the polymer concentration of 3 mg/ ml on the ITO/PEDOT:PSS electrode, and dried at 50 ° C for 20 min to improve the antisolvability, then cooled
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FIG. 2. 共Color online兲 ac impedance plots of the devices with and without TIPD cathode buffer layer in the dark at 0 V. Inset: equivalent RC circuit of the PSCs in the dark.
well as the device structure of the PSCs. The blend film of MEH-PPV:PCBM 共1:4 w/w兲 was used as active layer in the devices. The selection of MEH-PPV as the photovoltaic polymer is from the consideration that it is one of the most classical photovoltaic polymers and the coherence of TIPD isopropanol solution on the surface of MEH-PPV/PCBM blend film is good. The HOMO and LUMO of MEH-PPV,14 PCBM,15 and TIPD are schematically shown in Fig. 1共c兲. The energy level of TIPD was determined by electrochemical cyclic voltammetry. The LUMO energy level of TIPD FIG. 1. 共Color online兲 Layout of the polymer solar cell. 共a兲 Molecular struclocates in between the LUMO level of PCBM and the work tures of MEH-PPV, PCBM, and TIPD. 共b兲 Device structure consisting of an function of Al cathode; hence, it could help the electron col85 nm thick MEH-PPV:PCBM 共1:4 w/w兲 blend layer and a 25 nm TIPD lection of the cathode from the active layer.16 cathode buffer layer, sandwiched between ITO/PEDOT:PSS anode and an In order to examine the effect of TIPD buffer layer on aluminum top electrode. 共c兲 Schematic energy level diagram for the device. the interface resistance of the devices, the alternating current 共ac兲 impedance spectroscopy of the devices with and without it down to the room temperature. The thickness of the active TIPD buffer layer were measured in dark condition. Figure 2 layer is about 85 nm. Subsequently, a 3.75 wt % TIPD isoshows the ac impedance plots for the devices at 0 V in the propanol solution was spin cast on top of the active layer and frequency range of 10 Hz– 1 MHz. The impedance plots dried at 70 ° C for 30 min to remove the remained solvent. show standard semicircles similar to those of the polymer The thickness of TIPD is about 25 nm. Finally, a 150 nm light-emitting diodes.17 Therefore, the devices can be reprealuminum was thermally deposited on the top of the TIPD sented by an equivalent circuit of a resistance and a capacilayer or on the active layer in vacuum at a pressure of 5 tance in parallel, as shown in the inset of Fig. 2 and the ⫻ 10−5 Pa. The active area of the device is 4 mm2. These resistance R can be obtained from the diameter of the semiexperiments were carried out in a nitrogen-filled glovebox. circle on the “Re” axis. The R of the device without TIPD is Impedance spectroscopy was measured with a Zahner IM6e about 19 K⍀, while that of the device with TIPD is dramatiElectrochemical Workstation with an ac signal of 10 mV in cally decreased to 10.8 K⍀. These results indicate that the the frequency range of 10 Hz– 1 MHz. The current-voltage TIPD buffer layer reduced the interface resistance between 共J-V兲 measurement of the devices was conducted on a the active layer and Al electrode, leading to a lower device resistance. computer-controlled Keithley 236 source measure unit. A xeFigure 3共a兲 compares the IPCE spectra of the devices non lamp with AM1.5 filter was used as the white light with and without TIPD buffer layer. The device without source, and the optical power at the sample was TIPD buffer layer shows the typical spectral response of 100 mW/ cm2. The input photon to converted current effiMEH-PPV:PCBM composites with the maximum IPCE of ciency 共IPCE兲 was measured using a Keithley 2000 DMM 37% at about 510 nm.10,11 For the device with TIPD buffer coupled with WDG3 monochromator and 500 W xenon layer, the IPCE values increased obviously in the whole pholamp. The light intensity at each wavelength was calibrated tosensitive wavelength range and the maximum IPCE with a calibrated silicon photovoltaic cell. All these measurereached 44% at approximately 510 nm. The increase of the ments were performed under ambient atmosphere at room IPCE values could be benefited from the reduction of the temperature. device resistance with the TIPD buffer layer, as revealed in Figure 1 shows the chemical structures and electronic Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 204.121.6.3 On: Fri, 11 Nov 2016 the ac impedance plots in Fig. 2. energy levels of MEH-PPV, PCBM, and TIPD molecules, as 17:53:28
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open-circuit voltage 共Voc兲 of 0.84 V, short-circuit current density 共Jsc兲 of 4.29 mA/ cm2, fill factor 共FF兲 of 0.46, and power conversion efficiency 共PCE兲 of 1.66%, consistent with those reported in literatures.11 Interestingly, the Voc, Jsc, FF, and PCE of the PSC with TIPD buffer layer all increased and reached 0.87 V, 5.73 mA/ cm2, 0.51, and 2.52%, respectively, which are among the best values reported in literatures for the PSCs based on MEH-PPV and PCBM. By introducing TIPD buffer layer, the efficiency of the PSC increased by 51.8%. The performance improvement should be benefited from the lower device resistance and better electron collection of the device with TIPD cathode buffer layer. In conclusion, we demonstrated that TIPD can be used as an efficient cathode buffer layer for PSCs based on the blend of MEH-PPV and PCBM, and the buffer layer can be prepared conveniently by solution method. The insertion of TIPD layer improved the interface structure and reduced the interface resistance between the active layer and the metal cathode. The power conversion efficiency of the PSC with TIPD buffer layer reached 2.52% under the illumination of AM1.5, 100 mW/ cm2, which is increased by 51.8% in comparison with that 共1.66%兲 of the device without TIPD buffer layer under the same experimental conditions. In addition, the TIPD buffer layer could also improve the stability and lifetime of the devices. These results indicate that TIPD is a promising cathode buffer layer material for photovoltaic devices. This work was supported by NSFC 共Grant Nos. 20474069, 20421101, 20574078, and 50633050兲 and The Ministry of Science and Technology of China 共973 Project, No. 2002CB613404兲. S. Gunes, H. Neugebauer, and N. S. Sariciftci, Chem. Rev. 共Washington, D.C.兲 107, 1324 共2007兲. 2 G. Yu, J. Hummelen, F. Wudl, and A. J. Heeger, Science 270, 1789 共1995兲. 3 W. Ma, C. Yang, X. Gong, K. Lee, and A. J. Heeger, Adv. Funct. Mater. 15, 1617 共2005兲. 4 G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, and Y. Yang, Nat. Mater. 4, 864 共2005兲. 5 J. Y. Kim, S. H. Kim, H. H. Lee, K. Lee, W. Ma, X. Gong, and A. J. Heeger, Adv. Mater. 共Weinheim, Ger.兲 18, 572 共2006兲. 6 L. S. Roman, M. Berggren, and O. Inganas, Appl. Phys. Lett. 75, 3557 共1999兲. 7 F. L. Zhang, A. Gadisa, O. Inganäs, M. Svensson, and M. R. Andersson, Appl. Phys. Lett. 84, 3906 共2004兲. 8 S. E. Shaheen, C. J. Brabec, N. S. Sariciftci, F. Padinger, T. Fromherz, and J. C. Hummelen, Appl. Phys. Lett. 78, 841 共2001兲. 9 P. Peumans, A. Yakimov, and S. R. Forrest, J. Appl. Phys. 93, 3693 共2003兲. 10 C. J. Brabec, S. E. Shaheen, C. Winder, N. S. Sariciftci, and P. Denk, Appl. Phys. Lett. 80, 1288 共2002兲. 11 S. Alem, R. Bettignies, and J. M. Nunzi, Appl. Phys. Lett. 84, 2178 共2004兲. 12 Z. H. Kafafi and P. A. Lane, Proc. SPIE 5938, 211 共2005兲. 13 Y. Cao, G. Yu, I. D. Parker, and A. J. Heeger, J. Appl. Phys. 88, 3618 共2000兲. 14 J. H. Hou, C. H. Yang, J. Qiao, and Y. F. Li, Synth. Met. 150, 297 共2005兲. 15 C. J. Brabec, N. Cravino, D. Meissner, N. S. Sariciftci, T. Fromherz, M. T. Rispens, L. Sanchez, and J. C. Hummelen, Adv. Funct. Mater. 11, 374 共2001兲. 16 H. Hoppe, M. Niggemann, C. Winder, J. Kraut, R. Hiesgen, A. Hinsch, D. Meissner, and N. S. Sariciftci, Adv. Funct. Mater. 14, 1005 共2004兲. 17 Y. F. Li, J. Gao, G. Yu, Y. Cao, and A. J. Heeger, Chem. Phys. Lett. 287, 83 共1998兲. 1
FIG. 3. 共Color online兲 共a兲 Incident photon to converted current efficiency 共IPCE兲 spectra and 共b兲 J-V curves of the two PSCs with and without TIPD cathode buffer layer in the dark and under the illumination of AM15, 100 mW/ cm2. 共c兲 Linear scale of J-V curves of corresponding devices under the illumination of AM1.5, 100 mW/ cm2.
Figure 3共b兲 shows the current density-voltage 共J-V兲 curves of the PSCs with and without TIPD buffer layer in the dark and under the illumination of AM1.5, 100 mW/ cm2. For the J-V curves measured in the dark, the rectification ratio of the device without TIPD buffer layer is 6.11⫻ 102 at ±1.0 V, while that of the device with TIPD cathode buffer layer increased to 1.01⫻ 103. The higher rectification ratio of the PSC with TIPD buffer layer resulted from the higher injection current at the positive voltage of 1.0 V and lower leakage current at the negative voltages. Under the illumination of AM1.5, 100 mW/ cm2, the device with TIPD buffer layer showed improved photovoltaic performance 关see Fig. 3共c兲兴. The device without TIPD shows
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