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High performance inverted polymer solar cells using ultrathin atomic layer deposited TiO2 films
Synthetic Metals 207 (2015) 31–34
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
High performance inverted polymer solar cells using ultrathin atomic layer deposited TiO2 films Hyun-Soo Choa , Nara Shinb , Kyungkon Kimc, BongSoo Kimd,* , Do-Heyoung Kima,** a School of Applied Chemical Engineering and Research Institute for Catalysis, College of Engineering, Chonnam National University, 300 Youngbong-dong, GwangJu 500-757, Republic of Korea b Department of Chemistry, Yonsei University, 50 Yeonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea c Department of Chemistry and Nano Science, Ewha Womans University, 52 Ewhayeodae-gil, Seodaemun-gu, Seoul 120-750, Republic of Korea d Department of Science Education, Ewha Womans University, 52 Ewhayeodae-gil, Seodaemun-gu, Seoul 120-750, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 March 2015 Received in revised form 23 May 2015 Accepted 1 June 2015 Available online xxx
Photovoltaic properties of inverted type polymer solar cells (PSCs) using ultrathin TiO2 layers as an adjuvant electron collecting layer were investigated. To prepare the ultrathin (2.5 nm) TiO2 layers on top of TiO2 nanoparticles, atomic layer deposition (ALD) process was conducted at 125, 175, and 200 C. The addition of ALD TiO2 on nanoparticulated TiO2 effectively enhanced the photovoltaic performances of inverted organic solar cells. The inverted PSC device with the thin 200 C-ALD TiO2 layers showed the highest power conversion efficiency of 3.50%, which is an enhancement of approximately 30% compared to the cells without the ALD TiO2 layer (PCE = 2.72%). This work demonstrates that the ALD process plays a critical role in the enhancement of electron extraction efficiency with treating the surface defects of the TiO2 nanoparticles. ã2015 Elsevier B.V. All rights reserved.
Keywords: Inverted photovoltaic devices Atomic layer deposition Power conversion efficiency Electron extraction layer Titanium nanoparticles
1. Introduction Polymer solar cells (PSCs) have attracted considerable attention in recent years due to their promising characteristics such as low production cost, simple structure, light weight and flexibility with flexible substrates [1–4]. In general, conventional PSC consist of a bottom indium tin oxide(ITO) anode, a poly(3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) anode interfacial layer, a bulk-heterojunction (BHJ) active layer consisting of conjugated polymer donor and fullerene acceptor, and low work function metal cathode such as aluminum or calcium. However, poor stability of the conventional type of PSC in air is considered as a major drawback [5,6]. The facile oxidation of the low work function metal and high acidity of PEDOT:PSS on ITO are considered to be major factors influencing the poor stability of the cells. Recently, the use of inverted structure, where an n-type metal oxide film is used as the electron collecting layer and a high work function metal is used as a hole collecting layer, have demonstrated improved air stability [7–11]. However, the power
* Corresponding author. Tel.: +82 2 3277 5954. ** Corresponding author. Tel.: +82 62 530 1894. E-mail addresses: [email protected] (B. Kim), [email protected] (D.-H. Kim). http://dx.doi.org/10.1016/j.synthmet.2015.06.001 0379-6779/ ã 2015 Elsevier B.V. All rights reserved.
conversion efficiency (PCE) of the inverted PSC is often lower than that of the conventional PSC, and thus, the development of PSC materials and device architectures are still being investigated for further improvements of cell performance. One of the key factors affecting the efficiency of the inverted PSC is the electron collecting layer, which should enhance electron extraction while suppressing charge recombination. TiO2 has been considered as a promising electron collecting material due to its good electron transport properties and excellent air stability compared to other n-type semiconductor oxides [1,12–17]. A number of studies have used TiO2 as an electron collecting layer in the inverted PSC. Spin-coated nanoparticulated (NP) TiO2 is commonly used as an electron collecting layer [1,14–17]. However, the connectivity of the spincoated TiO2 film is poor, resulting in low PCEs due to a high interfacial resistance between the TiO2 layer and the neighboring layers. Therefore, more efficient methodology is required to improve the interconnection of TiO2 nanoparticles coated on ITO for electron collecting layer. Atomic layer deposition (ALD) is highly suitable for this purpose due to its production of uniform and highly conformal films on uneven surfaces [18,19]. In this work, we evaluate the ALD processes for the improvement of connectivity of the spin coated NP TiO2, and also for its ability to passivate the surface defects of NP TiO2 layer, resulting in an improvement of PCE.
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2. Experimental The inverted polymer solar cells manufactured in this study were prepared on indium tin oxide (ITO)-coated glass substrates. The ITO coated substrates (15 V/&) were cleaned using acetone, isopropyl alcohol, and again with acetone for 10 min each in an ultrasonic bath. The cleaned substrates were then dried in an oven at 60 C for at least 2 h. After that, TiO2 nanoparticles (5 nm) were dispersed in ethanol with a concentration of 0.4 wt%, and were spin coated onto the ITO substrate followed by drying at 60 C for 1 h in air to obtain a 20 nm thick TiO2 layer. On the top of the NP TiO2 layer, TiO2 ALD was carried out using a reaction system of tetrakisdimethyl-amido titanium (TDMAT) and H2O2 at 1 Torr. The TDMAT and H2O2 (50 wt%) were purchased from UP Chemicals, Inc. and Sigma-Aldrich, respectively. Both precursors were used without any further purification. Discrete introduction of the TDMAT and H2O2 pulses with an intermediate Ar purging step were used and one deposition cycle consisted of 0.5 s of TDMAT pluse, 10 s of argon purge, 0.5 s of H2O2 pulse, and 10 s of argon purge. The TDMAT and H2O2 were introduced to the reactor from containers, held at 45 C and 35 C, respectively. The deposition temperature was varied from 125 to 200 C, but the thickness of the ALD processed TiO2 films was fixed to about 2.5 nm by controlling the number of the ALD cycles. The thicknesses of the ALD processed TiO2 films was estimated using X-ray reflection (XRR) and transmission electron microscopy (TEM). Also, the surface roughness of films was evaluated using atomic force microscopy (AFM). Following the ALD TiO2 formation, a poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) solution of weight ratio 1:0.6 in chlorobenzene was spin-coated on the ALD TiO2 layer at 500 rpm for 35 s to form a 200 nm thick active layer. The active layers were then dried for 10 min in air, and poly (ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS, Plextronics AQ1300) solution, diluted with t-butanol by 1:1 vol%, was then spin-coated onto the active layer at 4000 rpm for 60 s, and dried at 80 C for 10 min in an oven, achieving a thickness of 80 nm. Finally, a 100 nm thickness of Ag was deposited by thermal evaporation after annealing at 150 C for 10 min in the same thermal evaporator. As a reference sample, other structure of Ag/PEDOT:PSS/P3HT:PCBM/ NP TiO2/ ITO/ glass was prepared without an ALD TiO2 layer. Current density versus voltage (J–V) characteristic were recorded on a Keithley model 2400 source measuring unit. A class-A solar simulator with a 150 W Xenon lamp (Newport) equipped with a KG-3 filter served as a light source. Its light intensity was adjusted to AM 1.5 G 1 sun light intensity, using a NREL-calibrated mono Si solar cell. External quantum efficiency (EQE) was measured as a function of wavelength from 300 to 800 nm on incident photo-to-current conversion equipment (PV measurement Inc.). Calibration was performed using a silicon photodiode G425, which is NIST-calibrated as a standard.
Fig. 1. (a) Thickness of TiO2 film as a function of ALD cycles determined from XRR (b) Cross-sectional TEM of 2.5 nm thick ALD TiO2 deposited on Si substrate.
0.039 0.001 nm/cycle and 63 ALD cycles was run to prepare a very thin (2.5 nm) film. An ultrathin ALD TiO2 films (2.5 nm) was used for this study because the slow deposition rates of ALD processes, typically <0.2 nm per cycle, is considered as a major drawback for practical applications of ALD [20]. As a 2.5 nm ALD TiO2 film is too thin to measure using XRR, a few samples were analyzed with a cross-sectional HRTEM to confirm a thickness of 2.5 nm. Fig. 1(b) shows a typical HRTEM image of a sample where the ALD TiO2 layer was deposited on Si wafers.
3. Results and discussion It is well known that ALD process offers excellent thickness uniformity over large areas and conformality of the deposited films [19]. A precise thickness control in the ALD process can be achieved by a linear relationship between film thickness and the number of cycles. We first verified the linear dependence of the film thickness on the number of cycles under our ALD experimental conditions. After running ALD cycles, the film thickness was estimated using XRR (Supplementary data, Fig. S1). Fig. 1(a) shows a linear relationship between the film thickness and the number of cycles, indicating that the deposition conditions used in this work are of the ALD-type growth mode. The estimated film growth rate of a
Fig. 2. (a) J–V characteristics of the inverted solar cells; spin-coated TiO2 nanoparticles only (black), 2.5 nm ALD TiO2 deposited at 125 C (red), 175 C (blue), and 200 C (green) on the spin coated TiO2 nanoparticles (b) Optical transmittance of the TiO2 electron collecting layers; spin coated TiO2 nanoparticles only (black), 2.5 nm ALD TiO2 deposited at 125 C (red), 175 C (blue), and 200 C (green) on the spin coated TiO2 nanoparticles. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
H.-S. Cho et al. / Synthetic Metals 207 (2015) 31–34 Table 1 Photovoltaic parameters of the inverted photovoltaic cells and surface roughness of the spin coated TiO2 nano particles without and with the ALD TiO2 layer as a function of ALD temperature. Voc (V) Without ALD TiO2 125 C ALD TiO2 175 C ALD TiO2 200 C ALD TiO2
Jsc (mA/cm2) FF
0.59 9.89 0.59 10.11 0.61 10.12 0.61 10.19
0.47 0.51 0.56 0.56
PCE (%)
Rs Rsh RMS (V cm2) (V cm2) (nm)
2.72 109 3.06 99.6 3.44 77.0 3.50 51.2
1370 2310 4900 5690
8.94 8.42 7.94 7.34
PSC devices with an inverted structure of Ag/PEDOT:PSS/P3HT: PCBM/ALD TiO2/NP TiO2/ITO glass were fabricated with 2.5 nm thick ALD TiO2 layers that were deposited at 125, 175, and 200 C. Fig. 2(a) shows the J–V characteristics of the inverted solar cells with and without 2.5 nm thick ALD TiO2, measured under 100 mW cm 2 illumination (AM 1.5 G). Table 1 summarizes their photovoltaic parameters. The reference device without the ALD TiO2 layer shows an open circuit voltage (Voc) of 0.59 V, shortcircuit current density (Jsc) of 9.89 mA/cm2, fill-factor (FF) of 0.47, and a power conversion efficiency (PCE) of 2.72%. Compared to the reference device, the device performance is considerably improved when the ALD TiO2 layer was added on the NP TiO2 layer as the electron transport layer and the device PCEs increased with an increasing ALD temperature. The highest PCE was achieved from the 200 C-ALD TiO2 layer, which displayed a Voc of 0.61 V, a Jsc of
33
10.19 mA/cm2, a FF of 0.56, and a PCE of 3.50%. EQE spectra (Fig. S2) showed that the EQE values in the device incorporating the ALD TiO2 layer were higher for the reference device in all the wavelength range. Thus, the ALD process was effective to improve all the parameters, especially Jsc and FF. To find origins of why ALD TiO2-inserted devices performed better compared to the reference device, we investigated optical transmittance and surface roughness of the surface of the ALD TiO2/TiO2 NP layer. Fig. 2(b) shows the transmittance of the TiO2 films on ITO. All the tested samples show nearly the same transmittance, suggesting that the transmittance can be ruled out for the enhanced Jsc. Next, we examined the surface of the ALD TiO2/NP TiO2 layer using AFM. Fig. 3 shows AFM images with the root mean square (RMS) roughnesses of the surfaces of the TiO2 NP layer and the ALD TiO2/TiO2 NP layers at deposited at (b) 125 C, (c) 175 C, and (d) 200 C on top of the TiO2 NP layer. The surface roughness slightly decreased with the addition of the ALD TiO2 layer, which demonstrates an advantageous feature of conformal surface coating of the ALD process. Importantly, this smoother surface suggests that the flatter ALD TiO2/NP TiO2 layer would facilitate the more intimate contact with the photoactive layer, allowing the more efficient electron extraction. In this manner, the surface flattening contributed in some degree to the increased Jsc. However, because the device PCEs were more significantly improved than we may expect from the surface morphology difference, we considered two factors of (i) interconnection
Fig. 3. AFM images of the surface of (a) the TiO2 NP layer and the ALD TiO2 layers deposited at (b) 125 C, (c) 175 C, and (d) 200 C on top of the TiO2 NP layer.
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H.-S. Cho et al. / Synthetic Metals 207 (2015) 31–34
between TiO2 nanoparticles and (ii) surface defect passivation of the NP TiO2 layer contacting the photoactive materials. It is wellknown that a high level of defects in nanoparticles results in low PCEs in the nanoparticle based inverted cells [21–23]. It is also important to examine series resistance (Rs) and shunt resistance (Rsh) carefully, because the Rs is related to the ohmic loss in the entire device structure, including contact resistance at the interfaces and the bulk resistance of the interfacial layers and the photoactive layer, and the Rsh reveals the loss of charge carriers due to the current leakage pathways and recombination of charges in the bulk or at the interface. [24] When the ALD TiO2 layer is added to the NP TiO2 layer in the inverted PSC, the distance of electron transport from the photoactive layer to the electrode increased a bit. Thus, it can be expected that electron transport might be deterred so that Jsc and FF would be decreased. However, Jsc values increased with the usage of ALD TiO2 and more importantly, Rss were significantly decreased, up to 50% from 109 to 51.2 V cm2 (see Table 1 as well). This opposite trend implies that even if the thickness increased slightly, electron transport through the ALD TiO2/NP TiO2 layer was considerably improved, which suggests that the interconnection between TiO2 nanoparticles were improved. In addition, it was reported by us and others that a NP TiO2 layer can be an efficient hole blocking layer as well as an electron extraction layer [1,17]. We presume that there must exist surface defects between the TiO2 nanoparticles as well as on the TiO2 nanoparticle surface and these defects are potentially a carrier recombination site [20]. Therefore, it is of great importance to treat surface defects in the TiO2 NP layer, which would prevent carrier recombination and plays better as a hole blocking layer. In this study, we observed that (i) compared to the reference device, the device incorporating 200 C-ALD TiO2/NP TiO2 layer showed improved Voc from 0.59 to 0.61 V, (ii) it showed more than 4 times increased Rsh from 1370 to 5690 V cm2 (see Table 1), and (iii) EQE values were enhanced over all the light absorption range, as mentioned above. These results suggest that the ALD TiO2 coating, indeed, cured surface defects of TiO2 NP layer and reduced current leakage, which more effectively reduced the carrier recombination loss at the interface between the photoactive layer and the electrode by reducing recombination sites on the TiO2 surface. Conclusively, the combination of the improved electron transport and surface-defect passivation by the ALD process resulted in all the enhanced photovoltaic parameters. 4. Conclusions In summary, ultrathin ALD TiO2 layers were used as an adjuvant electron collecting and transporting layer of the inverted PSCs. The usage of ALD TiO2 on top of the NP TiO2 layer significantly improved the photovoltaic performances of inverted organic solar cells from the PCE of 2.72 to 3.50%. Based on the analysis of the photovoltaic parameters and the surface roughness, we found that the enhanced performance of inverted PSCs was attributed mainly to the improved electron transport and reduced defect sites in the
TiO2 layer via defect passivation of TiO2 nanoparticles as well as the improved interfaces between the photoactive layer and the TiO2 layer. Therefore, we demonstrate that the ALD process effectively treated the surface defects in the NP TiO2 layer and improved the connection between TiO2 nanoparticles. Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of Korea, funded by the Ministry of Education, Science and Technology, Korea (NRF-2013R1A1A4A01013486). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. synthmet.2015.06.001. References [1] J. You, C.-C. Chen, L. Dou, S. Murase, H.-S. Duan, S.A. Hawks, T. Xu, H.J. Son, L. Yu, G. Li, Y. Yang, Adv. Mater. 24 (2012) 5267–5272. [2] Y. Liu, J. Zhao, Z. Li, C. Mu, W. Ma, H. Hu, K. Jiang, H. Lin, H. Ade, H. Yan, Nat. Commun. 5 (2014) . [3] Y. Liu, C. Mu, K. Jiang, J. Zhao, Y. Li, L. Zhang, Z. Li, J.Y.L. Lai, H. Hu, T. Ma, R. Hu, D. Yu, X. Huang, B.Z. Tang, H. Yan, Adv. Mater. 27 (2015) 1015–1020. [4] H. Kang, M.A. Uddin, C. Lee, K.-H. Kim, T.L. Nguyen, W. Lee, Y. Li, C. Wang, H.Y. Woo, B.J. Kim, J. Am. Chem. Soc. 137 (2015) 2359–2365. [5] C.E. Small, S. Chen, J. Subbiah, C.M. Amb, S.-W. Tsang, T.-H. Lai, J.R. Reynolds, F. So, Nat. Photonics 6 (2012) 115–120. [6] C.-P. Chen, Y.-D. Chen, S.-C. Chuang, Adv. Mater. 23 (2011) 3859–3863. [7] S.K. Hau, H.-L. Yip, N.S. Baek, J. Zou, K. O’Malley, A.K.-Y. Jen, Appl. Phys. Lett. 92 (2008) 253301. [8] L.-M. Chen, Z. Hong, G. Li, Y. Yang, Adv. Mater. 21 (2009) 1434–1449. [9] Z. Xu, L.-M. Chen, G. Yang, C.-H. Huang, J. Hou, Y. Wu, G. Li, C.-S. Hsu, Y. Yang, Adv. Funct. Mater. 19 (2009) 1227–1234. [10] S.K. Hau, H.-L. Yip, A.K.Y. Jen, Polym. Rev. 50 (2010) 474–510. [11] B. Zhang, D.-H. Lee, H. Chae, C. Park, S. Cho, Korean J. Chem. Eng. 27 (2010) 999–1002. [12] M. Wang, X. Wang, Sol. Energy Mater. Sol. Cells 92 (2008) 766–771. [13] B. Ecker, H.-J. Egelhaaf, R. Steim, J. Parisi, E. von Hauff, J. Phys. Chem. C 116 (2012) 16333–16337. [14] J. Liu, S. Shao, B. Meng, G. Fang, Z. Xie, L. Wang, X. Li, Appl. Phys. Lett. 100 (2012) 213906. [15] Z. Lin, C. Jiang, C. Zhu, J. Zhang, ACS Appl. Mater. Interfaces 5 (2013) 713–718. [16] S. Chambon, E. Destouesse, B. Pavageau, L. Hirsch, G. Wantz, J. Appl. Phys. 112 (2012) 094503. [17] Y. Suh, N. Lu, S.H. Lee, W.-S. Chung, K. Kim, B. Kim, M.J. Ko, M.J. Kim, ACS Appl. Mater. Interfaces 4 (2012) 5118–5124. [18] Y.-W. Kim, D.-H. Kim, Korean J. Chem. Eng. 29 (2012) 969–973. [19] R. Pheamhom, C. Sunwoo, D.-H. Kim, J. Vac. Sci. Technol. A 24 (2006) 1535–1539. [20] D. Hausmann, J. Becker, S. Wang, R.G. Gordon, Science 298 (2002) 402–406. [21] P.E. Stallworth, J.B. d'Espinose de la Caillerie, J. Maquet, F. Babonneau, J.F. Guillemoles, M. Powalla, V. Lyakovitskaya, M. Yakushev, B. Tomlinson, Thin Solid Films 387 (2001) 235–238. [22] S. Chen, C.E. Small, C.M. Amb, J. Subbiah, T. -h. Lai, S.-W. Tsang, J.R. Manders, J.R. Reynolds, F. So, Adv. Energy Mater. 2 (2012) 1333–1337. [23] J. You, M.-F. Lo, W. Liu, T.-W. Ng, S.-L. Lai, P. Wang, C.-S. Lee, J. Mater. Chem. 22 (2012) 5107–5113. [24] M.-S. Kim, B.-G. Kim, J. Kim, ACS Appl. Mater. Interfaces 1 (2009) 1264–1269.
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
High performance inverted polymer solar cells using ultrathin atomic layer deposited TiO2 films Hyun-Soo Choa , Nara Shinb , Kyungkon Kimc, BongSoo Kimd,* , Do-Heyoung Kima,** a School of Applied Chemical Engineering and Research Institute for Catalysis, College of Engineering, Chonnam National University, 300 Youngbong-dong, GwangJu 500-757, Republic of Korea b Department of Chemistry, Yonsei University, 50 Yeonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea c Department of Chemistry and Nano Science, Ewha Womans University, 52 Ewhayeodae-gil, Seodaemun-gu, Seoul 120-750, Republic of Korea d Department of Science Education, Ewha Womans University, 52 Ewhayeodae-gil, Seodaemun-gu, Seoul 120-750, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 March 2015 Received in revised form 23 May 2015 Accepted 1 June 2015 Available online xxx
Photovoltaic properties of inverted type polymer solar cells (PSCs) using ultrathin TiO2 layers as an adjuvant electron collecting layer were investigated. To prepare the ultrathin (2.5 nm) TiO2 layers on top of TiO2 nanoparticles, atomic layer deposition (ALD) process was conducted at 125, 175, and 200 C. The addition of ALD TiO2 on nanoparticulated TiO2 effectively enhanced the photovoltaic performances of inverted organic solar cells. The inverted PSC device with the thin 200 C-ALD TiO2 layers showed the highest power conversion efficiency of 3.50%, which is an enhancement of approximately 30% compared to the cells without the ALD TiO2 layer (PCE = 2.72%). This work demonstrates that the ALD process plays a critical role in the enhancement of electron extraction efficiency with treating the surface defects of the TiO2 nanoparticles. ã2015 Elsevier B.V. All rights reserved.
Keywords: Inverted photovoltaic devices Atomic layer deposition Power conversion efficiency Electron extraction layer Titanium nanoparticles
1. Introduction Polymer solar cells (PSCs) have attracted considerable attention in recent years due to their promising characteristics such as low production cost, simple structure, light weight and flexibility with flexible substrates [1–4]. In general, conventional PSC consist of a bottom indium tin oxide(ITO) anode, a poly(3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) anode interfacial layer, a bulk-heterojunction (BHJ) active layer consisting of conjugated polymer donor and fullerene acceptor, and low work function metal cathode such as aluminum or calcium. However, poor stability of the conventional type of PSC in air is considered as a major drawback [5,6]. The facile oxidation of the low work function metal and high acidity of PEDOT:PSS on ITO are considered to be major factors influencing the poor stability of the cells. Recently, the use of inverted structure, where an n-type metal oxide film is used as the electron collecting layer and a high work function metal is used as a hole collecting layer, have demonstrated improved air stability [7–11]. However, the power
* Corresponding author. Tel.: +82 2 3277 5954. ** Corresponding author. Tel.: +82 62 530 1894. E-mail addresses: [email protected] (B. Kim), [email protected] (D.-H. Kim). http://dx.doi.org/10.1016/j.synthmet.2015.06.001 0379-6779/ ã 2015 Elsevier B.V. All rights reserved.
conversion efficiency (PCE) of the inverted PSC is often lower than that of the conventional PSC, and thus, the development of PSC materials and device architectures are still being investigated for further improvements of cell performance. One of the key factors affecting the efficiency of the inverted PSC is the electron collecting layer, which should enhance electron extraction while suppressing charge recombination. TiO2 has been considered as a promising electron collecting material due to its good electron transport properties and excellent air stability compared to other n-type semiconductor oxides [1,12–17]. A number of studies have used TiO2 as an electron collecting layer in the inverted PSC. Spin-coated nanoparticulated (NP) TiO2 is commonly used as an electron collecting layer [1,14–17]. However, the connectivity of the spincoated TiO2 film is poor, resulting in low PCEs due to a high interfacial resistance between the TiO2 layer and the neighboring layers. Therefore, more efficient methodology is required to improve the interconnection of TiO2 nanoparticles coated on ITO for electron collecting layer. Atomic layer deposition (ALD) is highly suitable for this purpose due to its production of uniform and highly conformal films on uneven surfaces [18,19]. In this work, we evaluate the ALD processes for the improvement of connectivity of the spin coated NP TiO2, and also for its ability to passivate the surface defects of NP TiO2 layer, resulting in an improvement of PCE.
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H.-S. Cho et al. / Synthetic Metals 207 (2015) 31–34
2. Experimental The inverted polymer solar cells manufactured in this study were prepared on indium tin oxide (ITO)-coated glass substrates. The ITO coated substrates (15 V/&) were cleaned using acetone, isopropyl alcohol, and again with acetone for 10 min each in an ultrasonic bath. The cleaned substrates were then dried in an oven at 60 C for at least 2 h. After that, TiO2 nanoparticles (5 nm) were dispersed in ethanol with a concentration of 0.4 wt%, and were spin coated onto the ITO substrate followed by drying at 60 C for 1 h in air to obtain a 20 nm thick TiO2 layer. On the top of the NP TiO2 layer, TiO2 ALD was carried out using a reaction system of tetrakisdimethyl-amido titanium (TDMAT) and H2O2 at 1 Torr. The TDMAT and H2O2 (50 wt%) were purchased from UP Chemicals, Inc. and Sigma-Aldrich, respectively. Both precursors were used without any further purification. Discrete introduction of the TDMAT and H2O2 pulses with an intermediate Ar purging step were used and one deposition cycle consisted of 0.5 s of TDMAT pluse, 10 s of argon purge, 0.5 s of H2O2 pulse, and 10 s of argon purge. The TDMAT and H2O2 were introduced to the reactor from containers, held at 45 C and 35 C, respectively. The deposition temperature was varied from 125 to 200 C, but the thickness of the ALD processed TiO2 films was fixed to about 2.5 nm by controlling the number of the ALD cycles. The thicknesses of the ALD processed TiO2 films was estimated using X-ray reflection (XRR) and transmission electron microscopy (TEM). Also, the surface roughness of films was evaluated using atomic force microscopy (AFM). Following the ALD TiO2 formation, a poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) solution of weight ratio 1:0.6 in chlorobenzene was spin-coated on the ALD TiO2 layer at 500 rpm for 35 s to form a 200 nm thick active layer. The active layers were then dried for 10 min in air, and poly (ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS, Plextronics AQ1300) solution, diluted with t-butanol by 1:1 vol%, was then spin-coated onto the active layer at 4000 rpm for 60 s, and dried at 80 C for 10 min in an oven, achieving a thickness of 80 nm. Finally, a 100 nm thickness of Ag was deposited by thermal evaporation after annealing at 150 C for 10 min in the same thermal evaporator. As a reference sample, other structure of Ag/PEDOT:PSS/P3HT:PCBM/ NP TiO2/ ITO/ glass was prepared without an ALD TiO2 layer. Current density versus voltage (J–V) characteristic were recorded on a Keithley model 2400 source measuring unit. A class-A solar simulator with a 150 W Xenon lamp (Newport) equipped with a KG-3 filter served as a light source. Its light intensity was adjusted to AM 1.5 G 1 sun light intensity, using a NREL-calibrated mono Si solar cell. External quantum efficiency (EQE) was measured as a function of wavelength from 300 to 800 nm on incident photo-to-current conversion equipment (PV measurement Inc.). Calibration was performed using a silicon photodiode G425, which is NIST-calibrated as a standard.
Fig. 1. (a) Thickness of TiO2 film as a function of ALD cycles determined from XRR (b) Cross-sectional TEM of 2.5 nm thick ALD TiO2 deposited on Si substrate.
0.039 0.001 nm/cycle and 63 ALD cycles was run to prepare a very thin (2.5 nm) film. An ultrathin ALD TiO2 films (2.5 nm) was used for this study because the slow deposition rates of ALD processes, typically <0.2 nm per cycle, is considered as a major drawback for practical applications of ALD [20]. As a 2.5 nm ALD TiO2 film is too thin to measure using XRR, a few samples were analyzed with a cross-sectional HRTEM to confirm a thickness of 2.5 nm. Fig. 1(b) shows a typical HRTEM image of a sample where the ALD TiO2 layer was deposited on Si wafers.
3. Results and discussion It is well known that ALD process offers excellent thickness uniformity over large areas and conformality of the deposited films [19]. A precise thickness control in the ALD process can be achieved by a linear relationship between film thickness and the number of cycles. We first verified the linear dependence of the film thickness on the number of cycles under our ALD experimental conditions. After running ALD cycles, the film thickness was estimated using XRR (Supplementary data, Fig. S1). Fig. 1(a) shows a linear relationship between the film thickness and the number of cycles, indicating that the deposition conditions used in this work are of the ALD-type growth mode. The estimated film growth rate of a
Fig. 2. (a) J–V characteristics of the inverted solar cells; spin-coated TiO2 nanoparticles only (black), 2.5 nm ALD TiO2 deposited at 125 C (red), 175 C (blue), and 200 C (green) on the spin coated TiO2 nanoparticles (b) Optical transmittance of the TiO2 electron collecting layers; spin coated TiO2 nanoparticles only (black), 2.5 nm ALD TiO2 deposited at 125 C (red), 175 C (blue), and 200 C (green) on the spin coated TiO2 nanoparticles. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
H.-S. Cho et al. / Synthetic Metals 207 (2015) 31–34 Table 1 Photovoltaic parameters of the inverted photovoltaic cells and surface roughness of the spin coated TiO2 nano particles without and with the ALD TiO2 layer as a function of ALD temperature. Voc (V) Without ALD TiO2 125 C ALD TiO2 175 C ALD TiO2 200 C ALD TiO2
Jsc (mA/cm2) FF
0.59 9.89 0.59 10.11 0.61 10.12 0.61 10.19
0.47 0.51 0.56 0.56
PCE (%)
Rs Rsh RMS (V cm2) (V cm2) (nm)
2.72 109 3.06 99.6 3.44 77.0 3.50 51.2
1370 2310 4900 5690
8.94 8.42 7.94 7.34
PSC devices with an inverted structure of Ag/PEDOT:PSS/P3HT: PCBM/ALD TiO2/NP TiO2/ITO glass were fabricated with 2.5 nm thick ALD TiO2 layers that were deposited at 125, 175, and 200 C. Fig. 2(a) shows the J–V characteristics of the inverted solar cells with and without 2.5 nm thick ALD TiO2, measured under 100 mW cm 2 illumination (AM 1.5 G). Table 1 summarizes their photovoltaic parameters. The reference device without the ALD TiO2 layer shows an open circuit voltage (Voc) of 0.59 V, shortcircuit current density (Jsc) of 9.89 mA/cm2, fill-factor (FF) of 0.47, and a power conversion efficiency (PCE) of 2.72%. Compared to the reference device, the device performance is considerably improved when the ALD TiO2 layer was added on the NP TiO2 layer as the electron transport layer and the device PCEs increased with an increasing ALD temperature. The highest PCE was achieved from the 200 C-ALD TiO2 layer, which displayed a Voc of 0.61 V, a Jsc of
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10.19 mA/cm2, a FF of 0.56, and a PCE of 3.50%. EQE spectra (Fig. S2) showed that the EQE values in the device incorporating the ALD TiO2 layer were higher for the reference device in all the wavelength range. Thus, the ALD process was effective to improve all the parameters, especially Jsc and FF. To find origins of why ALD TiO2-inserted devices performed better compared to the reference device, we investigated optical transmittance and surface roughness of the surface of the ALD TiO2/TiO2 NP layer. Fig. 2(b) shows the transmittance of the TiO2 films on ITO. All the tested samples show nearly the same transmittance, suggesting that the transmittance can be ruled out for the enhanced Jsc. Next, we examined the surface of the ALD TiO2/NP TiO2 layer using AFM. Fig. 3 shows AFM images with the root mean square (RMS) roughnesses of the surfaces of the TiO2 NP layer and the ALD TiO2/TiO2 NP layers at deposited at (b) 125 C, (c) 175 C, and (d) 200 C on top of the TiO2 NP layer. The surface roughness slightly decreased with the addition of the ALD TiO2 layer, which demonstrates an advantageous feature of conformal surface coating of the ALD process. Importantly, this smoother surface suggests that the flatter ALD TiO2/NP TiO2 layer would facilitate the more intimate contact with the photoactive layer, allowing the more efficient electron extraction. In this manner, the surface flattening contributed in some degree to the increased Jsc. However, because the device PCEs were more significantly improved than we may expect from the surface morphology difference, we considered two factors of (i) interconnection
Fig. 3. AFM images of the surface of (a) the TiO2 NP layer and the ALD TiO2 layers deposited at (b) 125 C, (c) 175 C, and (d) 200 C on top of the TiO2 NP layer.
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between TiO2 nanoparticles and (ii) surface defect passivation of the NP TiO2 layer contacting the photoactive materials. It is wellknown that a high level of defects in nanoparticles results in low PCEs in the nanoparticle based inverted cells [21–23]. It is also important to examine series resistance (Rs) and shunt resistance (Rsh) carefully, because the Rs is related to the ohmic loss in the entire device structure, including contact resistance at the interfaces and the bulk resistance of the interfacial layers and the photoactive layer, and the Rsh reveals the loss of charge carriers due to the current leakage pathways and recombination of charges in the bulk or at the interface. [24] When the ALD TiO2 layer is added to the NP TiO2 layer in the inverted PSC, the distance of electron transport from the photoactive layer to the electrode increased a bit. Thus, it can be expected that electron transport might be deterred so that Jsc and FF would be decreased. However, Jsc values increased with the usage of ALD TiO2 and more importantly, Rss were significantly decreased, up to 50% from 109 to 51.2 V cm2 (see Table 1 as well). This opposite trend implies that even if the thickness increased slightly, electron transport through the ALD TiO2/NP TiO2 layer was considerably improved, which suggests that the interconnection between TiO2 nanoparticles were improved. In addition, it was reported by us and others that a NP TiO2 layer can be an efficient hole blocking layer as well as an electron extraction layer [1,17]. We presume that there must exist surface defects between the TiO2 nanoparticles as well as on the TiO2 nanoparticle surface and these defects are potentially a carrier recombination site [20]. Therefore, it is of great importance to treat surface defects in the TiO2 NP layer, which would prevent carrier recombination and plays better as a hole blocking layer. In this study, we observed that (i) compared to the reference device, the device incorporating 200 C-ALD TiO2/NP TiO2 layer showed improved Voc from 0.59 to 0.61 V, (ii) it showed more than 4 times increased Rsh from 1370 to 5690 V cm2 (see Table 1), and (iii) EQE values were enhanced over all the light absorption range, as mentioned above. These results suggest that the ALD TiO2 coating, indeed, cured surface defects of TiO2 NP layer and reduced current leakage, which more effectively reduced the carrier recombination loss at the interface between the photoactive layer and the electrode by reducing recombination sites on the TiO2 surface. Conclusively, the combination of the improved electron transport and surface-defect passivation by the ALD process resulted in all the enhanced photovoltaic parameters. 4. Conclusions In summary, ultrathin ALD TiO2 layers were used as an adjuvant electron collecting and transporting layer of the inverted PSCs. The usage of ALD TiO2 on top of the NP TiO2 layer significantly improved the photovoltaic performances of inverted organic solar cells from the PCE of 2.72 to 3.50%. Based on the analysis of the photovoltaic parameters and the surface roughness, we found that the enhanced performance of inverted PSCs was attributed mainly to the improved electron transport and reduced defect sites in the
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