Nanoscale

Published on 05 June 2014. Downloaded by University of Chicago on 28/10/2014 13:36:52.

COMMUNICATION

Cite this: Nanoscale, 2014, 6, 8585

Received 21st May 2014 Accepted 2nd June 2014 DOI: 10.1039/c4nr02780a

View Article Online View Journal | View Issue

High-efficiency inverted organic solar cells with polyethylene oxide-modified Zn-doped TiO2 as an interfacial electron transport layer† M. Thambidurai,‡*a Jun Young Kim,‡a Youngjun Ko,a Hyung-jun Song,a Hyeonwoo Shin,a Jiyun Song,a Yeonkyung Lee,a N. Muthukumarasamy,b Dhayalan Velauthapillaic and Changhee Lee*a

www.rsc.org/nanoscale

High efficiency inverted organic solar cells are fabricated using the PTB7:PC71BM polymer by incorporating Zn-doped TiO2 (ZTO) and 0.05 wt% PEO:ZTO as interfacial electron transport layers. The 0.05 wt% PEO-modified ZTO device shows a significantly increased power conversion efficiency (PCE) of 8.10%, compared to that of the ZTO (7.67%) device.

Organic solar cells (OSCs) have attracted considerable attention as a promising alternative for producing clean and renewable energy because of their potential for the large area fabrication process, lightweight, exibility, and inexpensiveness.1–5 Polymer:fullerene bulk heterojunction (BHJ) solar cells have recently reached power conversion efficiencies (PCEs) over 9% in single junction devices.6 Signicant efforts have been made to improve the power conversion efficiency (PCE) of OSCs through modications in interlayer and device architectures.7,8 One of the disadvantages of OSCs is their low chemical stability, which is due to the oxidation of their interfaces by oxygen and water and the photodegradation of the active layers.9,10 In order to improve their stability, various methods have been employed. Recently, an OSC with an inverted structure was developed, which was shown to be more stable than conventional solar cells. In most conventional OSCs, a PEDOT:PSS hole transport layer and a low-work-function metal electrode (Ca or Ba) are generally used. However, the strong acidic nature of PEDOT:PSS a

Department of Electrical and Computer Engineering, Global Frontier Center for Multiscale Energy Systems, Seoul National University, Seoul 151-744, Republic of Korea. E-mail: [email protected]; phy_thambi@rediffmail.com; Fax: +82-2877-6668; Tel: +82-2880-9093

b

Department of Physics, Coimbatore Institute of Technology, Coimbatore 641 014, India. E-mail: [email protected]; Fax: +91-0422-2575020; Tel: +9194429-54202

c Department of Engineering, University College of Bergen, Bergen, Norway. E-mail: [email protected]; Fax: +47-5558-7030; Tel: +47-5558-7711

† Electronic supplementary information (ESI) available: Experimental part, UPS spectra, absorption spectra, XPS spectra, J–V characteristics, IPCE spectra, AFM, and PL spectra. See DOI: 10.1039/c4nr02780a ‡ These authors contributed equally to this work.

This journal is © The Royal Society of Chemistry 2014

leads to degradation of the ITO electrode, which imposes a large problem with respect to device stability and reproducibility of solar cells. In particular, the low-work-function metal is susceptible to degradation by oxygen and water vapor.11–14 In addition, optical loss due to absorption of the PEDOT:PSS lm is signicant, approaching 10% in the visible range and even higher in the near infrared region.15 Some reports have shown that PEDOT:PSS is also an inefficient electron-blocking layer, reducing the efficiency of electronic devices through electron leakage to the anode electrode.16,17 For these reasons, alternative high work function electrodes have been explored and demonstrated by replacing PEDOT:PSS with transition metaloxides such as molybdenum oxide (MoO3),vanadium oxide (V2O5), tungsten oxide (WO3), and nickel oxide (NiO).18–21 One key challenge in making efficient inverted organic solar cells (IOSCs) lies in the electron transport layer (ETL). This layer should satisfy several criteria, such as high transparency, high electron affinity energy, and efficiency to collect the electrons. A number of studies have been carried out in IOSCs using the TiO2 lm as an ETL, owing to its superior air stability when compared to other semiconductor oxides. In order to increase the device performance of inverted solar cells, several groups have studied the use of the metal doped ntype buffer layer. The metal doping is an effective procedure to modify the grain size, orientation, conductivity and could greatly inuence the optical and electrical properties of the ntype buffer layer.22,23 In recent years, some researchers have suggested that doping the TiO2 lm with metal ions could be a promising approach to improve the electron transfer in the TiO2 electrode. Zn-doped TiO2 (ZTO) has some merits due to its high transparency, low material cost, more stability and small lattice mismatch upon doping compared with those of other doped TiO2 lms. Zn has been investigated as a n-type dopant for TiO2 because it can occupy some of the Ti sites in the TiO2 crystal. To enhance the electron transport properties in the TiO2 lms, Zn was used in this work as a doping impurity. The efficiency of solar cells has been found to be improved on using a ZTO interfacial layer. Herein, we prepared the TiO2 and ZTO ETL on

Nanoscale, 2014, 6, 8585–8589 | 8585

View Article Online

Published on 05 June 2014. Downloaded by University of Chicago on 28/10/2014 13:36:52.

Nanoscale

an ITO electrode by solution processing using titanium (IV) butoxide, 2-methoxyethanol, acetylacetone, and zinc acetate dihydrate, and then the lms were subjected to thermal annealing at 500  C for 1 hour in air. The solution processed TiO2 and ZTO layers are highly transparent in the visible range and show effective electron transport properties. In this communication, we report about the development of a generally applicable ETL by polyethylene oxide (PEO) modication of the TiO2 and ZTO surface. The IOSCs composed of thieno[3,4-b]thiophene/benzodithiophene (PTB7):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) blend show considerably improved device performance when PEO:TiO2 and PEO:ZTO electron transport layers (ETLs) formed by the spin coating technique are used. Especially, we have achieved an efficiency of 7.51% and 8.10% for IOSCs based on PTB7:PC71BM by using PEO:TiO2 and PEO:ZTO lms as the ETLs, respectively. The inverted device structure, molecular structures of PTB7, PC71BM, and the energy levels of the component materials are shown in Fig. 1. The energy levels of PTB7,6,24 PC71BM,6 and MoO3 (ref. 25) were taken from the literature. The work function of TiO2, PEO:TiO2, ZTO and PEO:ZTO lms was measured by ultraviolet photoelectron spectroscopy (UPS), and the band gap (Eg) obtained from the UV absorption spectra is shown in Fig. S1 and S2 and Table S1.† When light irradiates the photoactive layer of the solar cells through an ITO electrode, the active layer will absorb photons to produce excitons and the excitons will diffuse towards and dissociate at the PTB7:PC71BM interface into electrons in the lowest unoccupied molecular orbital (LUMO) of the acceptor PC71BM and holes in the highest occupied molecular orbital (HOMO) of the donor PTB7. Since

(a) The device architecture of the inverted organic solar cell. The molecular structure of (b) PTB7 and (c) PC71BM. (d) Energy level diagram of the component materials used in device fabrication. Fig. 1

8586 | Nanoscale, 2014, 6, 8585–8589

Communication

the LUMO level of TiO2 (4.46 eV) is the same as that of PC71BM, electrons can easily transport to the ITO electrode through TiO2. As the HOMO level (7.66 eV) of TiO2 is much lower than that (5.15 eV) of PTB7, it blocks the hole transport from PTB7 to the ITO electrode. The HOMO level (5.3 eV) of MoO3 is very close to the HOMO level (5.15 eV) of PTB7, and this facilitates the hole transfer to the Al electrode through MoO3. The relatively high-lying LUMO (2.3 eV) of MoO3 prevents the electron transfer from PC71BM to the Al electrode. In order to study the composition of TiO2 and ZTO lms, the X-ray photoelectron spectroscopy (XPS) measurements were carried out. The XPS spectra of TiO2 and ZTO lms are shown in Fig. S3.† The binding energies of ZTO are 457.02 eV, 462.77 eV and 528.21 eV for Ti2p3/2, Ti2p1/2 and O1s respectively. These binding energies are found to be shied towards the higher energy side compared to those of TiO2 (456.6, 462.42 and 527.85 eV for Ti2p3/2, Ti2p1/2 and O1s, respectively). This result suggests that zinc is incorporated into the TiO2 lattice. Fig. S3c shows the peaks at 1019.89 eV and 1042.86 eV corresponding to the Zn2p3/2 and Zn2p1/2 binding energy levels of zinc.† The chemical constituents present in the sample according to the XPS results are Ti ¼ 24.32 at.%, and O ¼ 75.68 at.% for TiO2, Ti ¼ 23.50 at.%, O ¼ 74.82 at.%, Zn ¼ 1.68 at.% for ZTO. Fig. 2a shows the absorption spectra of PTB7:PC71BM, TiO2/ PTB7:PC71BM and ZTO/PTB7:PC71BM lms. The PTB7:PC71BM blend lm exhibits a strong absorption covering a range from 300 to about 800 nm. It is observed that TiO2/PTB7:PC71BM and ZTO/PTB7:PC71BM show strong absorption from 450 to 650 nm when compared to the PTB7:PC71BM lm. Moreover, it could also be seen that the Zn dopant improved the optical absorption performance of the TiO2 lm. Absorption spectra of TiO2 and ZTO lms are shown in Fig. S4.† Absorption spectra of ZTO show that the absorption edge is slightly shied towards the shorter wavelength (blue shi) when compared to that of TiO2.

Fig. 2 (a) Absorption spectra of PTB7:PC71BM, TiO2/PTB7:PC71BM and ZTO/PTB7:PC71BM films. (b) Illuminated J–V, (c) dark J–V characteristics and (d) IPCE spectra of PTB7:PC71BM based IOSCs with ETLs of TiO2 and PEO:TiO2.

This journal is © The Royal Society of Chemistry 2014

View Article Online

Published on 05 June 2014. Downloaded by University of Chicago on 28/10/2014 13:36:52.

Communication

Nanoscale

This shi towards shorter wavelengths indicates the increase of band gap on Zn doping. It is mainly attributed to the Burstein– Moss effect, since the absorption edge of a degenerate semiconductor is shied to shorter wavelengths with increasing carrier concentration.26 It is well known that the electrical properties of the interfacial layers are very important for the performance of the devices because they will affect charge transport at the interface. The resistivity of TiO2 and ZTO thin lms is 0.401  103 U cm and 0.352  103 U cm, respectively. The resistivity value of ZTO is small when compared to that of TiO2. The lower resistivity of the lm aer doping with the optimum Zn content can be explained by the fact that the Zn atom substitutes the Ti atom in the crystal lattice and behaves as a donor and induces native n-type conductivity in the TiO2 lm. The mobility has been measured by the bottom-gate top-contact thin lm transistor (TFT) method. The mobility is found to be 3.15  104 cm2 V1 s1 and 3.20  104 cm2 V1 s1 for TiO2 and ZTO (Fig. S5†) thin lms respectively. The current density–voltage (J–V) characteristics of the PTB7:PC71BM based IOSCs fabricated using various PEO:TiO2 ETLs are shown in Fig. 2b and S6a† and these have been measured under 100 mW cm2 air mass 1.5 global (AM1.5G) illumination. The solar cell parameters, short circuit current density (Jsc), open circuit voltage (Voc), ll factor (FF), power conversion efficiency (PCE), series resistance (Rs) and shunt resistance (Rshunt), are summarized in Table 1. As can be seen, the reference device using TiO2 shows a Jsc of 14.85 mA cm2, a Voc of 0.76 V, a FF of 62.22% and a PCE of 6.98%. The device performance was considerably improved when PEO-modied TiO2 was used as the ETL. The highest efficiency was achieved when 0.05 wt% PEO-modied TiO2 was used as the ETL, which exhibited a Jsc of 15.51 mA cm2, a Voc of 0.76 V, a FF of 63.34% and a PCE of 7.51%. However, as the PEO concentration was further increased, the device performance decreased. When the PEO concentration was increased to 0.3 wt%, the device showed a reduced Jsc of 15.40 mA cm2 and a reduced FF of 61.14%, giving a PCE of 7.06%. We calculated the series resistance (Rs) and shunt resistance (Rshunt) from the J–V curve. The reference TiO2 device shows a Rs of 10.28 U cm2 and a Rshunt of 451.61 U cm2. The 0.05 wt% PEO:TiO2 device exhibited the smallest Rs of 9.22 U cm2 and the highest Rshunt of 1198.16 U cm2. The

Table 1

reduced Rs is favorable for charge collection and the increased Rshunt indicates reduced leakage current as well as restrained recombination loss of carriers, which explains the improved FF and Jsc. When the PEO concentration is increased to 0.3 wt%, the Rs of the device increases to 9.46 U cm2, which renders a large energy barrier for charge collection. In this case, many carriers recombine before they are collected at the electrodes. Therefore, small FF and Jsc can be expected when charge collection is inefficient due to the presence of large Rs. The dark J–V characteristics shown in Fig. 2c and S6b† show that the 0.05 wt% PEO:TiO2 device exhibited excellent diode characteristics and lower leakage current under reverse bias than the device based on TiO2. The incident photon to charge carrier efficiency (IPCE) spectra of PTB7:PC71BM based IOSCs are shown in Fig. 2d and S6b.† It is observed that the 0.05 wt% PEO:TiO2 based device showed a maximum IPCE of 78%, which is higher than 72% of the TiO2 device. The Jsc values have been calculated using IPCE data with an AM1.5G reference spectrum. Jsc is found to be 14.51 and 15.20 mA cm2 for TiO2 and 0.05 wt% PEO:TiO2 respectively. These values match well with those obtained from J–V measurement. The J–V characteristics of the PTB7:PC71BM based IOSCs fabricated using ETLs of ZTO and PEO-modied ZTO are shown in Fig. 3a and S7a† and Table 1. A similar variation in the performance of the PEO:ZTO based device was observed as previous PEO:TiO2 based devices. The 0.05 wt% PEO-modied ZTO device shows a considerably improved Jsc of 16.14 mA cm2 and a FF of 65.83% as compared to the reference device, which shows a Jsc of 15.61 mA cm2 and a FF of 64.78%. Consequently, the device using a 0.05 wt% PEO-modied ZTO layer shows a

Fig. 3 (a) J–V characteristics and (b) IPCE spectra of PTB7:PC71BM based IOSCs with ETLs of ZTO and PEO:ZTO.

Performance parameters of the devices based on PTB:PC71BM blends using PEO:TiO2 and PEO:ZTO ETLs

ETLs

Active layer

Jsc [mA cm2]

Voc [V]

FF [%]

PCE [%]

Rs [U cm2]

Rshunt [U cm2]

TiO2 0.01 wt% PEO:TiO2 0.05 wt% PEO:TiO2 0.1 wt% PEO:TiO2 0.3 wt% PEO:TiO2 ZTO 0.01 wt% PEO:ZTO 0.05 wt% PEO:ZTO 0.1 wt% PEO:ZTO 0.3 wt% PEO:ZTO

PTB7:PC71BM

14.85 15.43 15.51 15.47 15.40 15.61 15.60 16.14 16.02 15.46

0.76 0.76 0.76 0.76 0.75 0.76 0.76 0.76 0.76 0.76

62.22 62.80 63.34 62.60 61.14 64.78 65.80 65.83 62.35 60.06

6.98 7.32 7.51 7.31 7.06 7.67 7.79 8.10 7.54 7.13

10.28 9.39 9.22 9.42 9.46 8.33 6.84 6.63 7.88 8.71

451.61 989.90 1198.16 1026.28 853.46 633.63 1015.10 1376.59 1115.64 997.90

This journal is © The Royal Society of Chemistry 2014

Nanoscale, 2014, 6, 8585–8589 | 8587

View Article Online

Published on 05 June 2014. Downloaded by University of Chicago on 28/10/2014 13:36:52.

Nanoscale

considerably improved PCE of 8.10% as compared to 7.67% of the device with the ZTO layer. The IPCE spectra shown in Fig. 3b and S7c† conrm the improved Jsc of the device based on the PEO-modied ZTO layer. The device-performance stability data of TiO2, 0.05 wt% PEO:TiO2, ZTO and 0.05 wt% PEO:ZTO based devices are shown in Fig. S8.† We observed that TiO2 and 0.05 wt% PEO:TiO2 based devices almost retain their original efficiency even aer 30 days under ambient conditions with reduction of PCE from 6.98 to 6.72% and 7.51 to 7.30%, while for the device fabricated using ZTO and 0.05 wt% PEO:ZTO ETL the PCE decreased from 7.67 to 7.32% and 8.10 to 7.82% under the same conditions. This clearly shows that TiO2, PEO:TiO2, ZTO and PEO:ZTO based devices exhibit good stability. To investigate the effect of PEO modication on surface defects of TiO2 lms, photoluminescence (PL) spectra were recorded for various PEO modied TiO2 lms and are shown in Fig. S9.† As can be seen, two emission peaks were observed. The narrow emission band at 372 nm is attributed to radiative annihilation of excitons, while the intense broad emission around 440 nm is assigned to the trap emission. It shows clearly that the defect emission is considerably restrained by surface modication of TiO2 by PEO, indicating reduced surface traps in TiO2 aer PEO surface modication. It is proposed that PEO species coordinate with TiO2 by sharing a lone electron pair of oxygen in the PEO backbone, which effectively passivates the surface traps of TiO2. The ll-up of surface electron traps of TiO2 favors the improvement in the conductivity of the TiO2 interlayer, leading to the reduced Rs of the devices. Moreover, the reduction of traps can decrease the possibility of trap-assisted interfacial recombination of carriers and consequently increase the Rshunt of devices. Both reasons contribute to the improved Jsc and FF for the PEO-modied TiO2 based devices when the PEO concentration is less than 0.3 wt%. Fig. S10† shows the PL spectra of PEO:ZTO lms. A similar variation in the PL spectra of PEO:ZTO lms was observed as previous PEO:TiO2 lms. Another factor inuencing the performance of these devices is the interfacial morphology properties. The surface roughness of TiO2, PEO:TiO2, ZTO and PEO:ZTO lms was investigated by atomic force microscopy (AFM). As shown in Fig. 4a and b, the root-mean-square (RMS) surface roughness is found to be 0.34 nm

Fig. 4 AFM images of (a) TiO2, (b) 0.05 wt% PEO:TiO2, (c) ZTO and (d) 0.05 wt% PEO:ZTO.

8588 | Nanoscale, 2014, 6, 8585–8589

Communication

and 2.80 nm for TiO2 and 0.05 wt% PEO:TiO2 respectively. As can be seen from Fig. 4c and d, uniform ZTO and 0.05 wt% PEO:ZTO lms composed of small grains are observed. The surface roughness is found to be 0.22 nm and 0.41 nm for ZTO and 0.05 wt% PEO:ZTO respectively. However, when the PEO concentration is further increased to 0.3 wt%, a rough surface is observed in the TiO2 and ZTO lms (Fig. S11 and S12†). The variation in the surface morphology may be due to the change in the surface properties of TiO2 and ZTO lms aer PEO modication. In conclusion, high efficiency IOSCs fabricated with solution-processed PEO:TiO2 and PEO:ZTO lms as ETLs have been demonstrated. It is found that PEO-modication of TiO2 or the ZTO lm surface can effectively passivate the surface traps present in TiO2 or ZTO, and suppress the recombination loss of carriers, reduce the series resistance and improve the electrical coupling of the TiO2/active layer or the ZTO/active layer. For PEO:TiO2 and PEO:ZTO based IOSCs, the PCEs are signicantly improved from the reference 6.98% and 7.51% to 7.67% and 8.10%. This work was supported by the Global Frontier R&D Program of the Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future, Korea (2011-0031561) and the Human Resources Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (no. 20124010203170). This work was also supported by the BK21 Plus program.

Notes and references 1 C. J. Brabec, N. S. Saricici and J. C. Hummelen, Adv. Funct. Mater., 2001, 11, 15. 2 K. M. Coakley and M. D. McGehee, Chem. Mater., 2004, 16, 4533. 3 S. Gunes, H. Neugebauer and N. S. Saricici, Chem. Rev., 2007, 107, 1324. 4 J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T. Q. Nguyen, M. Dante and A. J. Heeger, Science, 2007, 317, 222. 5 G. Li, R. Zhu and Y. Yang, Nat. Photonics, 2012, 6, 153. 6 Z. He, C. Zhong, S. Su, M. Xu, H. Wu and Y. Cao, Nat. Photonics, 2012, 6, 591. 7 J. Y. Kim, S. Noh, Y. M. Nam, J. Y. Kim, J. Roh, M. Park, J. J. Amsden, D. Y. Yoon, C. Lee and W. H. Jo, ACS Appl. Mater. Interfaces, 2011, 3, 4279. 8 J. You, C.-C. Chen, L. Dou, S. Murase, H.-S. Duan, S. A. Hawks, T. Xu, H. J. Son, L. Yu, G. Li and Y. Yang, Adv. Mater, 2012, 24, 5267. 9 Z. Tan, W. Zhang, Z. Zhang, D. Qian, Y. Huang, J. Hou and Y. Li, Adv. Mater., 2012, 24, 1476. 10 Z. Lin, C. Jiang, C. Zhu and J. Zhang, ACS Appl. Mater. Interfaces, 2013, 5, 713. 11 K. Norrman, M. V. Madsen, S. A. Gevorgyan, F. C. Krebs and J. Am, Chem. Soc., 2010, 132, 16883. 12 A. K. K. Kyaw, X. W. Sun, C. Y. Jiang, G. Q. Lo, D. W. Zhao and D. L. Kwong, Appl. Phys. Lett., 2008, 93, 221107.

This journal is © The Royal Society of Chemistry 2014

View Article Online

Published on 05 June 2014. Downloaded by University of Chicago on 28/10/2014 13:36:52.

Communication

13 H. Yan, P. Lee, N. R. Armstrong, A. Graham, G. A. Evmenenko, P. Dutta and T. J. Marks, J. Am. Chem. Soc., 2005, 127, 3172. 14 M. Jorgensen, K. Norrman and F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2008, 92, 686. 15 S. C. J. Meskers, J. K. J. van Duan and R. A. J. Janssen, Adv. Funct. Mater., 2003, 13, 805. 16 Y. H. Kim, S.-H. Lee, J. Noh and S.-H. Han, Thin Solid Films, 2006, 510, 305. 17 K. Yao, L. Chen, X. Chen and Y. Chen, Chem. Mater., 2013, 25(6), 897. 18 C. Girotto, E. Voroshazi, D. Cheyns, P. Heremans and B. P. Rand, ACS Appl. Mater. Interfaces, 2011, 3, 3244. 19 C.-P. Chen, Y.-D. Chen and S.-C. Chuang, Adv. Mater., 2011, 23, 3859.

This journal is © The Royal Society of Chemistry 2014

Nanoscale

20 T. Stubhan, N. Li, N. A. Luechinger, S. C. Halim, G. J. Matt and C. J. Brabec, Adv. Energy Mater., 2012, 2, 1433. 21 J. R. Manders, S.-W. Tsang, M. J. Hartel, T.-H. Lai, S. Chen, C. M. Amb, J. R. Reynolds and F. So, Adv. Funct. Mater., 2013, 23, 2993. 22 J. You, C.-C. Chen, L. Dou, S. Murase, H.-S. Duan, S. A. Hawks, T. Xu, H. J. Son, L. Yu, G. Li and Y. Yang, Adv. Mater., 2012, 24, 5267. 23 H. Karaagac, E. Yengel and M. S. Islam, J. Alloys Compd., 2012, 521, 155. 24 Y. Liang, Z. Xu, J. Xia, S.-T. Tsai, Y. Wu, G. Li, C. Ray and L. Yu, Adv. Mater., 2010, 22, E135. 25 C.-W. Chu, S.-H. Li, C.-W. Chen, V. Shrotriya and Y. Yang, Appl. Phys. Lett., 2005, 87, 193508. 26 E. Burstein, Phys. Rev., 1954, 93, 632–633.

Nanoscale, 2014, 6, 8585–8589 | 8589

High-efficiency inverted organic solar cells with polyethylene oxide-modified Zn-doped TiO2 as an interfacial electron transport layer.

High efficiency inverted organic solar cells are fabricated using the PTB7:PC71BM polymer by incorporating Zn-doped TiO2 (ZTO) and 0.05 wt% PEO:ZTO as...
659KB Sizes 0 Downloads 5 Views