Highly efficient inverted organic solar cells using

Highly efficient inverted organic solar cells using amino acid modified indium tin oxide as cathode Aiyuan Li, Riming Nie, Xianyu Deng, Huaixin Wei, Shizhao Zheng, Yanqing Li, Jianxin Tang, and King-Young Wong Citation: Applied Physics Letters 104, 123303 (2014); doi: 10.1063/1.4870096 View online: http://dx.doi.org/10.1063/1.4870096 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/104/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A water-processable organic electron-selective layer for solution-processed inverted organic solar cells Appl. Phys. Lett. 104, 053304 (2014); 10.1063/1.4864622 Self-assembled monolayer as an interfacial modification material for highly efficient and air-stable inverted organic solar cells Appl. Phys. Lett. 102, 143303 (2013); 10.1063/1.4802086 Polyvinylpyrrolidone-modified indium tin oxide as an electron-collecting electrode for inverted polymer solar cells Appl. Phys. Lett. 101, 073303 (2012); 10.1063/1.4745772 Enhanced hole injection in organic electroluminescent device with an additional oxygen-rich indium–tin–oxide sublayer J. Vac. Sci. Technol. B 22, 758 (2004); 10.1116/1.1688352 A photoelectron spectroscopy study on the indium tin oxide treatment by acids and bases Appl. Phys. Lett. 74, 880 (1999); 10.1063/1.123397

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APPLIED PHYSICS LETTERS 104, 123303 (2014)

Highly efficient inverted organic solar cells using amino acid modified indium tin oxide as cathode Aiyuan Li,1 Riming Nie,1 Xianyu Deng,1,a) Huaixin Wei,2 Shizhao Zheng,3 Yanqing Li,2 Jianxin Tang,2 and King-Young Wong3

1 Research Center for Advanced Functional Materials and Devices, Shenzhen Key Laboratory of Advanced Materials, School of Materials Science and Engineering, Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, People’s Republic of China 2 Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou 215123, People’s Republic of China 3 Department of Physics, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, People’s Republic of China

(Received 26 February 2014; accepted 17 March 2014; published online 27 March 2014) In this paper, we report that highly efficient inverted organic solar cells were achieved by modifying the surface of indium tin oxide (ITO) using an amino acid, Serine (Ser). With the modification of the ITO surface, device efficiency was significantly enhanced from 0.63% to 4.17%, accompanied with an open circuit voltage (Voc) that was enhanced from 0.30 V to 0.55 V. Ultraviolet and X-ray photoelectron spectroscopy studies indicate that the work function reduction induced by the amino acid modification resulting in the decreased barrier height at the ITO/organic C 2014 AIP Publishing LLC. interface played a crucial role in the enhanced performances. V [http://dx.doi.org/10.1063/1.4870096] Organic material based solar cells have attracted much attention because of their ease of fabrication, and they are being suitable for roll-to-roll printable devices. Inverted organic solar cells, where the bottom metal oxide electrode of indium tin oxide (ITO) is used as a cathode, has become more and more important because they are more promising in both high efficiency and long term stability compared to the traditional forward structure.1–4 One key problem for ITO as a cathode is its high work function (about 4.7 eV) which is largely mismatched with the energy level of organic active materials, and this leads to poor device performance. To obtain high efficiency inverted devices, several methods have been employed to cover the shortage. One of the methods is the insertion of an alkali salt, a n-type Metal oxide, or a doped organic molecule layer between the ITO and the active layer.5–11 The other is the modification of the ITO surface with a neutral insulating polymer, conjugated polymer electrolyte, zwitterions, and self-assembled molecular layers.12–16 However, we note that previous studies less employed naturally occurring biomaterials to modify the ITO in inverted organic solar cells. Because the natural compounds and bio-inspired materials have various advantages that they are environmentally friendly, healthy, and continuously productive, in this paper, we report on the use of serine (Ser) in organic inverted solar cells. ITO was modified with Ser by a hydrothermal reaction process. The performance of the inverted poly (3-hexylthiophene-2, 5-diyl) and [6, 6]phenyl C61-butyric acid methyl ester (P3HT: PCBM) based solar cells with the modified ITO was studied. The modification of ITO was executed by the following process: ITO substrates were first cleaned in ultrasonic acetone, detergent, deionized water and isopropyl alcohol for 10 min each. The cleaned ITO substrates were dried in a dry box and then treated by UV-ozone for 15 min. The a)

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UV-ozone treated ITO substrates were put into a hydrothermal reaction kettle that contained a 2 mg/ml Serine solution with methanol as a solvent. An ITO substrate in a reaction kettle with pure methanol was also investigated for comparison. The reaction kettle was then placed in a sealed dry box with a temperature from 60 C to 120 C for 2 h. The treated ITO was rinsed with orderly pure water and alcohol to remove physically absorbed molecules before surface measurement or device fabrication. The inverted solar cells were fabricated on the ITO substrates through the above treatment. An active layer of thickness of about 250 nm was produced by spin-coating a solution of P3HT: PCBM (1:1) dissolved in chlorobenzene on top of the ITO substrates. Right after the spin-coating of the active layer solution, the wet sample underwent a solvent annealing by covering the sample using a petri dish, so that the solvent was slowly evaporated by a period of more than 60 min. After the sample was completely dried, it was annealed at 110 C for 10 min. Finally, a MoO3 buffer layer with a thickness of 5 nm and an Al anode layer with a thickness of 100 nm were deposited on top of the active layer by thermal evaporation in a vacuum system with a pressure of 10 6 millibars. The typical active area of the devices fabricated in this study was defined to be about 0.12 cm2 by using a shadow mask during the evaporation of the cathode. The current-voltage (I-V) characteristics were recorded using a Keithley 2400 source meter under an illumination of simulated AM1.5 sunlight with an intensity of 100 mW/cm2 provided by a solar simulator whose intensity was calibrated by a standard single-crystal Si photovoltaic cell. A Newport Oriel 91150V PV cell was used as a reference. Ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS) experiments were performed in an ultrahigh vacuum surface analysis system. UPS measurements were performed with an unfiltered HeI (21.22 eV) gas discharge lamp with a total instrumental energy resolution of 100 meV. XPS

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measurements were carried out using a monochromatic Al Ka source (1486.6 eV). Fig. 1 shows the I-V characteristics of the inverted solar cells. To look for the optimal conditions of the device performance, different treatment temperatures of the ITO in Ser solution were used for the modification. The performances of the devices are summarized in Table I. It can be seen that the efficiency of the devices with Ser modification were significantly enhanced from 0.63% to as much as 4.17%. The enhancement was due to the increases in the open circuit voltage (Voc), short-circuit current (Jsc), and the fill factor (FF), with their values enhanced from 0.30 to 0.60 V, 8.53 to 12.32 mA/cm2, and 25% to 62%, respectively. The series resistance (Rs) of the Ser device also significantly decreased from 26 to 6.08 X cm2. The large increases of the Isc are thereby attributed to the decrease of the Rs, which in turn is believed to have been arisen from the improvement of the ohmic contact at the interfaces of the ITO electrodes modified with Ser. To investigate the original mechanics of the improvement of the ohmic contact at the interfaces and the enhancement of the device performance, the UPS measurements were carried out. We compared the work functions of bare and Ser-treated ITO which are shown in the inset of Fig. 2(a). It exhibited that the ITO with Ser had a work function reduction about 0.8 eV (from 4.6 eV to 3.8 eV). The lowered work function of ITO is beneficial for enhancing the electron selectivity and the built-in potential in organic electronic devices.17,18 On the other hand, the decreased work function of ITO could also offer better energy level alignment with the lowest unoccupied molecular orbital (LUMO) levels of the materials in the organic active layer, which

FIG. 1. The chemical structure of Ser and the schematic device structure of the inverted solar cells (a). Current density versus voltage (J-V) characteristics of the devices with bare ITO and ITO with modification of Ser (b).

Appl. Phys. Lett. 104, 123303 (2014) TABLE I. Effects of the temperature of the Ser solution used for modifying ITO on the averageda performance parameters of the inverted organic solar cells. The device with bare ITO as cathode is also given for reference.

Temperature

Voc (V)

Jsc (mA/cm2)

FF (%)

Efficiency (%)

Rs (X cm2)

Rsh (X cm2)

Bare ITO 60 C 80 C 100 C 120 C

0.30 0.45 0.55 0.55 0.55

8.5 10.7 10.8 12.3 11.7

25 38 62 62 57

0.63 1.83 3.68 4.17 3.67

26.0 18.5 5.26 6.08 7.13

36.1 232.6 909.1 666.6 711.1

a

The averaged value was calculated from ten devices.

would thereby facilitate electron transfer from the organic layer to the ITO electrode. To investigate the improvement of the energy match, we characterized the UPS of an ultra thin PCBM and an ultra thin P3HT spin-coated on top of the ITO surface with or without Ser. Fig. 2(a) shows the comparison of the UPS in these samples. The spectrum of both P3HT and PCBM film is essentially similar with that reported in the literatures.19–22 The left side of the Fig. 2(a) shows that, with Ser modification, both P3HT and PCBM samples have a 0.3 eV shift of the cutoff of high binding energy to a higher level, which corresponds to the dropped vacuum level caused by interface dipoles. The right of the Fig. 2(a) displays the molecular orbital feature of the P3HT and the PCBM. With the modification of Ser, the highest

FIG. 2. Normalized UPS spectra of P3HT and PCBM spin-coated on bare ITO and ITO modified with Ser, and for inset, normalized UPS spectra of bare ITO and ITO modified with Ser (a). Schematic energy-level alignment based UPS measurements (b).


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FIG. 3. Survey of the XPS profiles acquired from the bare ITO and the Ser-treated ITO (a). High-resolution profiles of C1s obtained from ITO and Au substrates with Ser treatment and a wash with pure water and alcohol (b).

occupied molecular orbital (HOMO) level was 1.1 eV for the P3HT and 0.3 eV for the PCBM that shifted to a higher binding energy region. These results reveal that the Ser modification made the LUMO of organic materials close to the Femi level of the ITO electrode. To clearly show the contribution, an energy level diagram of the ITO and organic materials is shown in Fig. 2(b). It shows that both the LUMO of the P3HT and the LUMO of PCBM were dropped near the Femi level on condition of the ITO electrodes modified with Ser, which resulted in the reduction of the barrier height between the ITO and the LUMO level of the organic active layer. This will improve an ohmic contact at the cathode interface and facilitate the electron collections. To disclose the nature of the adsorption of the amino acid on the surface of the ITO substrate, an XPS measurement was carried out. Fig. 3(a) displays the survey of the XPS profiles acquired from the bare ITO and the Ser-treated ITO which were synchronously prepared with the method introduced in this study. The two samples both underwent a final washing with pure water before the measurement. From Fig. 3(a), it can be seen that the XPS spectra from the sample with Ser has an additional peak at around 402 eV corresponding to the N1s peak, which clearly demonstrated the presence of Ser on the surface of the ITO. Fig. 3(b) shows the high-resolution profiles of C1s obtained from ITO after the treatment in Ser solution and a wash with pure water and alcohol. There was an obvious additional shoulder peak at about 289 eV related to the –COO– on the ITO substrate. These phenomena imply that the Ser was adsorbed on the ITO surface with more strong binding force. This can be explained by the reason that the –COOH of the amino acid is chemically bonded to the ITO surface by reaction with the –OH on the surface of metal oxides. The adsorbed amino acids on the surface of the ITO in this way produced interface dipoles made from negative carboxylic groups and positive amine groups. Thus, surface dipoles produced by the amino acid molecules, as shown in the structure of Fig. 1(a), led to the work function reduction of the ITO. In summary, we have demonstrated highly efficient inverted P3HT: PCBM-based solar cells using amino acid

modified ITO as the cathode. The modification of the ITO with Ser made the power conversion efficiency of the inverted solar cells increase from 0.63% to 4.17%. This enhancement was attributed to the enhanced electron selectivity of the ITO electrode and then increased the ohmic contact between the ITO and the organic active layer, which were originally raised from the work function reduction of the ITO induced by surface dipoles of the amino acid molecules. Amino acids, which are nutrients and are naturally occurring in biological bodies, lead to a low cost and healthy way for the fabrication of organic solar cells. This work was financially supported by the Natural Scientific Research Innovation Foundation in Harbin Institute of Technology (HIT.NSRIF, 2009142), the Shenzhen Research Foundation Project (Nos. JC201005260119A, JC201105160573A, and JC201104220174A). 1

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