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Enhanced efficiency and stability of polymer solar cells with TiO2 nanoparticles buffer layer
Organic Electronics 15 (2014) 835–843
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Enhanced efficiency and stability of polymer solar cells with TiO2 nanoparticles buffer layer Jian Xiong a,b, Bingchu Yang a,b,⇑, Conghua Zhou a,b,⇑, Junliang Yang a,b,⇑, Haichao Duan a,b, Wenlong Huang a,b, Xiang Zhang a,b, Xingda Xia a,b, Lei Zhang a,b, Han Huang a,b, Yongli Gao a,b,c a Institute of Super-microstructure and Ultrafast Process in Advanced Materials, School of Physics and Electronics, Central South University, Changsha, Hunan 410083, China b Hunan Key Laboratory for Super-microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha, Hunan 410083, China c Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA
a r t i c l e
i n f o
Article history: Received 7 October 2013 Received in revised form 20 December 2013 Accepted 31 January 2014 Available online 13 February 2014 Keywords: TiO2 nanoparticles Buffer layer Polymer solar cells Stability
a b s t r a c t TiO2 sols synthesized with a facile solution-based method were used as a buffer layer between the active layer and the cathode Al in conventional structure polymer solar cells (PSCs). Using transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray diffraction (XRD) and atomic force microscopy (AFM), the morphological and crystallographic properties of synthesized TiO2 nanoparticles (TiO2 NPs) as well as the buffer layer were studied in detail. It was observed that by increasing H2O in the process of peptization both the crystallinity and particle size of TiO2 NPs were enhanced, while the particles in sol showed a narrower size distribution conformed by dynamic light scattering. Inserting TiO2 NPs as a buffer layer in conventional structure PSCs, both the power conversion efficiency (PCE) and stability were improved dramatically. PSCs based on the structure of ITO/PEDOT:PSS/P3HT:PCBM/TiO2 NPs/Al showed the short-circuit current (Jsc) of 12.83 mA/cm2 and the PCE of 4.24%, which were improved by 31% and 37%, respectively comparing with the reference devices without a TiO2 buffer layer. The stability measurement showed that PSC devices with a TiO2 NPs buffer layer could retain 80% of the original PCEs after exposed in air for 200 h, much better than the devices without such a buffer layer. The effect can be attributed to the protection by the buffer layer against oxygen and H2O diffusion into the active layers. The observations indicate that TiO2 NPs synthesized by facile solution-based method have great potential applications in PSCs, especially for large-area printed PSCs. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Polymer solar cells (PSCs) show great potentials as renewable energy sources due to their advantages, such ⇑ Corresponding authors at: Institute of Super-microstructure and Ultrafast Process in Advanced Materials, School of Physics and Electronics, Central South University, Changsha, Hunan 410083, China. Tel.: +86 731 88879525. E-mail addresses: [email protected] (B. Yang), chzhou@csu. edu.cn (C. Zhou), [email protected] (J. Yang). http://dx.doi.org/10.1016/j.orgel.2014.01.024 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
as low cost, light, flexible, semitransparent [1–4]. Polymer/fullerene bulk heterojunctions (BHJ) are commonly used in PSCs for obtaining high performance devices [5,6]. The power conversion efficiency (PCE) of state-ofthe-art PSCs based on BHJ has been reported over 10.0% [7,8]. The lifetime or stability is regarded as another important issue for the commercialization of PSCs [9]. Unlike inorganic counterparts, PSC devices are sensitive to moisture and oxygen, and the performance degrades dramatically when exposed to air. For improving the PCE and
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lifetime of PSC devices, one efficient method is to insert an inorganic buffer layer between the electrode and the active layer. It was suggested that such buffer layer could modify the organic/electrode interface and change the chemical nature of the interface. Moreover, it could prevent the diffusion of electrode atoms, oxygen, and water into the active layer of the device [9–12]. A variety of inorganic salts and n-type inorganic semiconductor materials, for example, LiF [9], CsCO3 [13], ZnO [14], deposited by vacuum evaporation or solution process, have been reported as the buffer layer to improve the PCE or lifetime of PSCs. TiOx is another material that has been used in PSCs as a buffer layer [15–19]. The TiOx can act as optical spacer, efficient electron transport or hole blocking layer (ETL/ HBL), as well as shielding and scavenging layer, which enhance the PCEs of the PSC devices and prevent the permeation of moisture and oxygen into the active layer. However, TiOx was found to be thermally unstable when treated in post-annealing processes at temperatures over 80 °C limiting its application in PSCs [18]. In addition, the preparation of TiOx involves sol–gel synthesis of precursors, followed by hydrolysis in air after deposition, which complicates the device fabrication and risks the degradation of PSC active materials. To overcome these shortcomings, TiO2 nanoparticles (NPs) seem to be a proper choice because of the thermally stability and the role as efficient ETL/HBL [20–22]. However, the air stability of devices with a TiO2 NPs layer is not encouraging and rarely reported. Lee et al. pointed out that the oxygen/water protection and scavenging capability originate primarily from the oxygen deficiencies of TiO2 film [17]. Salim et al. reported the t80 (time for the PCE decays to 80% of initial measured value) of the PSCs is less than 1 h based on the structure of ITO/ PEDOT:PSS/P3HT:PCBM/TiO2 NPs/Al [22]. Those observations indicate that a device with pure TiO2 NPs cannot have good air stability. In this paper, we report our work in developing a facile solution-based method to synthesize TiO2 sols, which avoid the in-air hydrolysis [15–17], centrifugation [20,21] and autoclave process [22]. The synthesized TiO2 NPs with this new method possess the advantages of both TiOx and TiO2 NPs synthesized with other methods when used as a buffer layer between the Al electrode and active layer. Using a conventional structure of ITO/PEDOT:PSS/ P3HT:PCBM/TiO2 NPs/Al, we demonstrate that both the PCE and stability of PSC devices with the TiO2 NPs buffer layer were improved dramatically, showing great potential applications in other PSC devices and large-area printed PSC devices.
environment for easier peptization, while acetic acid is used to improve dispersion of nanoparticles in the sols. Secondly, tetrabutyl titanate (TTBT, Ti(OBu)4, 24 mL) was added slowly into the above mixture under vigorously stirring. Following, two types of precursor gels (70 ml) were prepared, in which one was slowly added 3 ml distilled H2O while another was slowly added 7 ml distilled H2O. It was found that no gel formed as 3 ml H2O was slowly added but white gel formed quickly when 7 ml H2O was added. The former was diluted with organic solvent 2methoxyethanol (CH3OCH2CH2OH), while the latter was dissolved with organic solvent (2-methoxyethanol) and H2O, respectively. Those three types of sols are named as O3–TiO2, O7–TiO2, H7–TiO2, respectively. These sols were further stirred at 40 °C until they became transparent again. All samples were stirred at 80 °C for several hours and the concentration of Ti ions in the solution was kept 0.47 M. TiO2 NPs were achieved in such sols. Finally, the sols were diluted in ethanol (CH3CH2OH) for fabricating PSCs, in which the concentration of the Ti ions was 0.016 M.
2.2. Device fabrication Fig. 1 shows the structure of PSCs. The patterned indium tin oxide (ITO) coated glass were used as the substrate, which was ultrasonically cleaned in acetone, detergents, distilled water and isopropyl alcohol for 15 min respectively, then dried by N2 flow and treated by ozone for 15 min. The PEDOT: PSS layer (Baytron, PVP AI 4083) with thickness of about 40 nm was spin-coated (5000 rpm) onto patterned ITO substrate and dried on hot plate at 150 °C for 10 min. The fabrication of active layer was performed in glove box (both H2O and O2 < 1.0 ppm). Poly (3-hexylthiophene) (P3HT, Sigma–Aldrich) and fullerene derivative [6, 6]-phenyl C61-butyric acid methyl ester (PCBM, American Dye Source, Inc.) were dissolved in 1,2-dichlorobenzene (DCB) at the ratio of 1.0:0.8 wt% with a concentration of 27 mg/ml. The active solution was spin-coated on PEDOT:PSS-coated ITO glass at speed of 1000 rpm for 30 s. The spin-coated film was kept in a covered petri dish and dried slowly for 2 h in glove box. Annealing at 60 °C was performed to remove residual solvents. Then TiO2 sols as-synthesized above
2. Experimental details 2.1. TiO2 sols synthesis TiO2 sols were synthesized by developing a solutionbased method. The synthesis route is shown in Fig. S1 in Supporting Information. Firstly, acidic solution was prepared by dissolving nitric acid (HNO3, 0.6 mL) and acetic acid (CH3COOH, 5.4 mL) in n-butanol (CH3CH2CH2OH, 40 mL). Nitric acid is introduced to provide a strong acidic
Fig. 1. Schematic structure of the PSC device.
J. Xiong et al. / Organic Electronics 15 (2014) 835–843
were spin-coated on the active layer. A 100 nm Al electrode was deposited on top of TiO2 NPs films by thermal evaporation under the vacuum of 2 106 mbar. Finally all devices were annealed at 150 °C for 10 min in glove box. The device active area is 0.12 cm2. For comparison, devices without TiO2 NPs buffer layer were also prepared using similar procedures.
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gate the air-stability. Monochromatic incident photon-toelectron conversion efficiency (IPCE) of the devices was performed by AC mode with a monochromator-calibrated wavelength control (BEIJING 7-STAR OPTICAL INSTRUMENTS CO., LTD.). A calibrated Si photodiode was used as the reference device for the counting of incident photons. 3. Results and discussion
2.3. Characterization and testing 3.1. Morphological and crystallographic properties of TiO2 NPs Morphological and crystallographic properties of TiO2 NPs in sols were characterized by transmission electron microscopy (TEM, JEM-2100F, Japan), selected area electron diffraction (SAED) and X-ray diffraction (XRD, D8 Advance, Bruker AXS, Germany). The TEM samples were prepared by casting a drop of sol on copper grids on which thin carbon layer was loaded. The size of TiO2 NPs was evaluated based on TEM images, in which 20 isolated particles were chosen to give a mean particle size. While the XRD powder samples were prepared by drying sols at 150 °C. Particle-size distribution of TiO2 NPs in the sols was monitored by dynamic light scattering (DLS, Zetasizer nano ZS, Malvern, England). The Ti–O–Ti bonds were analysized by Fourier transform infrared spectrum (Nicolet, America). The absorption of the device was measured by optical system (Zolix, Beijing). The work function of TiO2 NPs was measured by ultraviolet photoelectron spectroscopy system (Specs, Germany). The morphology of thin film was characterized by atomic force microscopy (Agilent Technologies 5500 AFM/SPM System, USA) with tapping-mode. Current density–voltage (J–V) characteristics of the devices were measured by digital sourcemeter (Keithley, model 2420). A solar simulator (91160s, Newport, AM 1.5G) was used for PCE testing. Light intensity was 100 mW/cm2 calibrated by a standard silicon solar cell. The device was exposed in air up to 200 h to investi-
Morphological properties of particles in TiO2 sols are shown in Fig. 2. The rough statistical mean size of TiO2 NPs was evaluated based on TEM images. For O3–TiO2 sol, the sphere-like small particles with the size of 2.5 nm are observed (Fig. 2a), and the particles are amorphous since no Debye–Sherrer diffraction rings are seen in the SAED image, as shown in the inset of Fig. 2a. For O7–TiO2 sol, 3.6 nm oval-like crystallites appear and weak Debye– Sherrer diffraction rings could be observed, as shown in the inset of Fig. 2b, suggesting the formation of TiO2 crystallites. For H7–TiO2 sol, the size of TiO2 NPs is further enlarged to 5.5 nm, and the crystallinity is dramatically improved since the strong (1 0 1), (0 0 4), (2 0 0), (1 0 4), (2 0 4) Debye–Sherrer diffraction rings could be observed clearly (inset of Fig. 2c), corresponding to TiO2 NPs anatase phase according to the PDF card No.78-2486. The results suggest that the appropriate excessive H2O for the process of peptization is beneficial to the growth and crystallinity of TiO2 NPs. The pure H2O solvent can further enhance this effect. In Fig. 2a–c, some aggregations can be observed as well for three sols, especially for H7–TiO2 sol. It is well known that once the alkoxide meets with water, the hydrolysis–condensation reactions would happen, as presented by Eqs. (1) and (2). The addition of the acetic acid supports the third reaction, as shown in Eq. (3).
Fig. 2. TEM images of TiO2 particles in different sols: (a) O3–TiO2; (b) O7–TiO2; (c) H7–TiO2; (d) schematic diagram of the Ti–O–Ti chain binding to TiO2 NPs.
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Ti½OR4 þ vH OH ! Ti½OR4v ½OHv þ vH OR
ð1Þ
(101)
) ½Ti O Tin
ð2Þ
Ti OH þ HO CO CH3 ! Ti O CO CH3 þ H OH
ð3Þ
After the peptization, there are many ‘‘Ti–O–Ti’’ chains. As shown in Fig. S2 in Supporting Information, fourier transform infrared spectrum suggests that the absorption bands at 1360 cm1 can be clearly distinguished for O3– TiO2 NPs and O7–TiO2 NPs sols, which corresponds to the absorption of the Ti–O–Ti chains [18]. The large ‘‘Ti–O– Ti’’ chains support the formation of three-dimensional framework through hydrolysis–condensation. The sol losses its mobility and becomes the gel. On the other hand, the gel could be broken under violent stirring and the sol is formed again. Through the sol–gel–sol process, it prompts mostly of amorphous Ti to be fully hydrolyzed. At a higher temperature, the fully-hydrolyzed chains contribute to the formation of the TiO2 crystallites, while the chains without fully-hydrolyzation are disadvantageous to form TiO2 crystallites. It suggests that the more the H2O in the gel, the more the ‘‘Ti–O–Ti’’ chains consumed to form TiO2 crystallites. By trial and error, 3 ml H2O could barely make the sol transform to the gel while 7 ml H2O could change the sol to the gel quickly. However, more H2O (>7 ml) was disadvantageous to the formation of the gel. In contrary, it destroyed the stability of the sol since the different solvent polarity in organic solvents. In the H2O solvent, the chains can be further hydrolyzed. The crystallinity and particle size of TiO2 NPs can be enhanced with the increase of H2O in the gel. Meanwhile, some chains probably remain in the sol since TTBT is less hydrolysable. The addition of the acetic acid into the sol is helpful to form Ti–O–CO– CH3, which is more difficult to hydrolyze and benefits the existence of chains in the sol. Hydroxyl groups (–OH) remaining in the chains and on the TiO2 NPs were confirmed in previous studies [23,24]. Some of the ‘‘Ti–O–Ti’’ chains in sols are probably bound on the surface of TiO2 NPs by hydrolysis–condensation reactions (Fig. 2d). These chains can act as binders by further hydrolysis–condensation reactions, linking TiO2 NPs each other and leading to the formation of cluster and aggregation. Therefore, the lower concentration of the binders results in a better dispersion and less aggregation. XRD patterns of different TiO2 powder obtained by drying solvent in oven at 150 °C are shown in Fig. 3. The powder originating from O3–TiO2 sol shows no clear peaks, suggesting the particles are smaller nucleuses with lower crystallinity. The powers obtained by other two sols, especially for the H7–TiO2 sol, show the obvious diffraction peaks indexed as (1 0 1), (0 0 4), (2 0 0), (1 0 5)/(2 1 1) based on the anatase phase. These results coincide with the SAED studies discussed above. Meanwhile, The Sherrer’s equation was used to calculate the average crystallites size of nanocrystallites with 2.7 nm, 3.3 nm, 4.3 nm respectively based on the 2h-FWHM of diffraction peak (1 0 1) from
Intensity (a.u.)
Ti OH þ HO Ti ! Ti O Ti þH OH (004) (200)
(211) (105) (204)
(220) (215)
c b a
20
30
40
50
60
70
80
2θ ( Ο ) Fig. 3. XRD patterns of the TiO2 NPs originated from three kinds of sols: curve a, O3–TiO2; curve b, O7–TiO2; curve c, H7–TiO2. The position and relatively intensity of diffraction peak of pure anatase TiO2 were indicated by green lines according to PDF card No.78-2486.
the XRD results. These values based on the Sherrer’s calculation can basically match with the statistical results by the TEM images. Aggregation is unavoidable in a sol system. For their applications in PSCs, a buffer layer is usually less than 50 nm. Heavy aggregation in the sols could affect the microstructure as well as the compactness of thin film, and finally influence the PSCs performance. The dispersion of particle size in sols is monitored using the DLS technique, as shown in Fig. 4. For O3–TiO2 sol, the size distribution can be divided into three main parts: 1–3 nm, 5–10 nm, and 80–110 nm (Fig. 4a). While for the O7–TiO2 and H7–TiO2 sols, the size distribution is greatly narrowed. The size range in both sols can be divided into two parts: one main part is from 3 nm to 36 nm, another part is >80 nm. Based on the TEM and XRD studies, the mean size of single particle is 2.5 nm, 3.6 nm, 5.5 nm for the O3–TiO2, O7–TiO2 and H7–TiO2 sol respectively, which are partly in the main distribution range (Fig. 4). The main distribution range attributes to single particle and aggregations, while the larger size parts (>80 nm) ascribe to the amorphous [Ti] [23]. Comparing the results from the DLS studies, it can be found that the particle dispersibility is improved with the increment of H2O in sols, probably attributing to the decreased of ‘‘Ti–O–Ti’’ chains.
3.2. Polymer solar cells with TiO2 NPs Fig. 5 shows the AFM images of the P3HT:PCBM blend films with and without TiO2 NPs layer. The value of rootmean-square (RMS) surface roughness of P3HT:PCBM is 9.81 nm for 4 4 lm2 (Fig. 5a), coinciding with the previous reported by slow growth process [25]. The image of the active layer coated by O3–TiO2 sol is shown in Fig. 5b, and the RMS value drops to 3.67 nm. According to the DLS and TEM results, there are many smaller TiO2 NPs and some ‘‘Ti–O–Ti’’ chains in O3–TiO2 sol. When the TiO2 sol is spin-coated onto the active layer, the small particles and ‘‘Ti–O–Ti’’ chains penetrate into the grooves and connect closely, resulting in the dense and smooth thin films after
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18
20 18 16 14 12 10 8 6 4 2 0
16
(a)
(b)
14
intensity (%)
intensity (%)
J. Xiong et al. / Organic Electronics 15 (2014) 835–843
12 10 8 6 4 2 0
1
10
100
1
10
intensity (%)
size (nm)
100
size (nm)
20 18 16 14 12 10 8 6 4 2 0
(c)
1
10
100
size (nm) Fig. 4. The particle size distribution of three kinds of TiO2 sols obtained by DLS technique: (a) O3–TiO2; (b) O7–TiO2; (c) H7–TiO2.
annealing. Fig. 5c is the AFM image of TiO2 NPs layer prepared from the O7–TiO2 sol, the film surface is relatively smooth (RMS = 4.8 nm) and some larger aggregation are observed, coinciding well with the previous TEM results. However, the TiO2 NPs layer prepared from the H7–TiO2 sol becomes very rough (RMS = 9.1 nm), attributing to the larger particle size and the reduction of ‘‘Ti–O–Ti’’ chains. The illuminated current density–voltage (J–V) characteristics of devices with and without TiO2 NPs buffer layer are shown in Fig. 6, and the detailed device parameters are summarized in Table 1. The PSC reference device exhibits a PCE of 3.09% with a short-circuit current (Jsc) of 9.79 mA/ cm2, open circuit voltage (Voc) of 0.59 V and fill factor (FF) of 0.53, which are similar to the results from many other research groups [21,26,27]. The PCE of device with TiO2 NPs buffer layer prepared from O3–TiO2 sol is similar with the reference device. Both Voc and Jsc are reduced, dropping from 0.59 V to 0.52 V and from 9.79 mA/cm2 to 9.58 mA/cm2, respectively. But the PCE is compensated with a higher FF (60%). The reason for a decreased Voc is apparently attributed to the fact that some TTBT or Ti–O– CO–CH3 (or both of them) did not hydrolyze entirely in the buffer layer due to the less hydrolysis [19]. The Voc could increase during the exposure in air, as discussed below. The device with TiO2 NPs buffer layer prepared from O7–TiO2 sol shows excellent performance characteristics with a PCE of 4.24%, Jsc of 12.69 mA/cm2, Voc of 0.58 V and FF of 0.58. Device with TiO2 NPs buffer layer prepared from H7–TiO2 sol is also improved and lead to a result with a PCE of 4.12%, Jsc of 12.83 mA/cm2, Voc of 0.57 V and FF of 0.56. Comparing with the reference device, the PCEs of de-
vices with TiO2 NPs prepared from the later two sols are remarkably improved by 37% and 33%, respectively. It is noticeable that the Jsc reaches over 12.5 mA/cm2, which is higher than the conventional structure devices with TiOx or TiO2 NPs buffer layer (9–11.2 mA/cm2) [15–18,20,22]. However, the Jsc of the device with a buffer layer of O7– TiO2NPs or H7–TiO2NPs calculated from the integration of IPCE spectra is smaller than the Jsc by measurement (Table 1). Both the illumination intensity of simulator and the device area are double checked. Hence they probably relate with the concentration of free carrier in TiO2 NPs under the different test conditions for the IPCE and J–V measurements. The higher the carrier concentration, the better the conductivity. The Jsc can reach a maximum when treated with 5–10 min pre-illumination under 100 mW/cm2 during the measurement. The light intensities are different in the measurements of the IPCE and J–V, resulting in a difference in the carrier concentration of TiO2 NPs, and accordingly the different in the calculated and measured Jsc. This phenomenon was also reported previously [21]. The disagreement between the calculated and measured Jsc for the devices without TiO2 NPs attributed to the quick degradation when exposed in air. The UPS measurement suggested that the work function of O3–TiO2NPs, O7–TiO2NPs, H7–TiO2NPs are 3.9 eV, 4.4 eV, 4.5 eV, respectively, as shown in Fig. S3 in Supporting Information. The positions of work function are helpful to the transform of the electrons from the PCBM to the cathode (Al), although there is a very small energy barrier [28]. Especially, the carrier mobility of crystallites is much better than that of amorphous particles [29]. On the other
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Fig. 5. AFM images of P3HT:PCBM thin films without TiO2 NPs buffer layer (a) and with TiO2 NPs buffer layer from the sols (b) O3–TiO2, (c) O7–TiO2, (d) H7– TiO2.
Jsc (mA/cm2)
4 2
without buffer layer
0
with O3-TiO 2 NPs
-2
with O7-TiO 2 NPs
-4
with H7-TiO 2 NPs
-6 -8 -10 -12 -14 -0.4
-0.2
0.0
0.2
0.4
0.6
0.8
Voltage (V) Fig. 6. J–V curves obtained under illumination of 1.5 G at 100 mW/cm2 for PSC devices with and without TiO2 NPs buffer layer. (Black square – PSC device without TiO2 NPs buffer layer; red circle – PSC device with TiO2 NPs buffer layer prepared from O3–TiO2 sol; blue up-triangle – PSC device with TiO2 NPs buffer layer prepared from O7–TiO2 sol; dark-cyan down-triangle – PSC device with TiO2 NPs buffer layer prepared from H7– TiO2 sol. Similarly thereafter). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
hand, the highest-occupied molecular orbital (HOMO) level of TiO2 are larger than that of the P3HT, resulting in
the prevention of hole accumulation at interface between the active layer and the Al, which leads to reduced interfacial charge recombination. Meanwhile, the absorption of the device in reflection geometry was enhanced with TiO2 NPs buffer layer, as shown in Fig. S4 in Supporting Information. It suggests the function of optical spacer of TiO2 NPs buffer layer in the PSC devices. Hence, the improved Jsc in the devices results from the functions of TiO2 NPs buffer layer played as the ETL, the HBL, as well as the optical spacer layer. The TiOx could exist in buffer layer originating from the ‘‘Ti–O–Ti’’ chains. The content of TiOx decreases and the crystallinity of TiO2 NPs are enhanced in the buffer layer prepared by sols with the increment of H2O. It is reason why the Jsc of device increases gradually with the TiO2 NPs buffer layer prepared from O3–TiO2, O7–TiO2 and H7–TiO2 sols. Comparing the performance of the devices in details with different buffer layer, the FF inversely related to the RMS of TiO2 NPs coated on P3HT:PCBM. It suggests that the smoother TiO2 buffer layer surfaces are probably beneficial to the deposition of higher quality Al electrode and form an ohmic contact between the layers. Meanwhile, it is also confirmed that some TiOx (in form of ‘‘Ti–O–Ti’’ chain) in film can act as a binder and filler
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J. Xiong et al. / Organic Electronics 15 (2014) 835–843 Table 1 Performance parameters of conventional structure PSC devices prepared without and with TiO2 NPs buffer layer from different sols. PCE (%)
Jsc (mA/cm2)
Calculated Jsca (mA/cm2)
Voc (V)
FF
Without TiO2 With O3–TiO2 NPs With O7–TiO2 NPs With H7–TiO2 NPs
3.09 3.04 4.24 4.12
9.79 9.58 12.69 12.83
9 9.48 10.44 10.68
0.59 0.52 0.58 0.57
53 60 58 56
The Jsc was calculated from the integration of IPCE spectra. It is smaller than the measured Jsc and the detailed discussion is shown in the text.
between TiO2 NPs, ensuring a close connection between the buffer layer and the active layer as well as the particles. It is helpful for improving the structure of buffer layer and decreasing the RMS of active layer. Most of the reported PSCs with TiOx showed poor thermal stability. It was found that the post-annealing of the devices with TIP-based TiOx buffer layer leads the decrease of the PCE from 2.76% to 0.93% [18]. The main reason for the degradation is that the post-annealing process would change the TIP-based TiOx film morphology, which possibly deteriorated the interface between the Al electrode and the active layer, and accordingly degrade the performance of PSC devices. This kind of degradation was not observed in the devices with three types of TiO2 NPs buffer layer including the ‘‘Ti–O–Ti’’ chains in our study. It confirms that the newly developed TiO2 NPs have a better thermal stability than the conventional TiOx. Actually, the thermally stable oxide compound used in PSCs as a buffer layer is very important, since the post-annealing can enhance the formation of C–O–Al [30] and Ti–O–Al [31], which can improve the interface adhesion and yield better device performance. Fig. 7 shows the IPCE spectra of the PSC devices with and without TiO2 NPs buffer layer. It can be seen from the diagram that the IPCE of device with TiO2 NPs buffer layer prepared from O3–TiO2 sol is improved notably at rang of 430–570 nm, while the IPCE of devices with TiO2 NPs prepared from O7–TiO2 and H7–TiO2 sol increase in a much broader wave range from 380 to 600 nm. It indicates that the TiO2 NPs layer act as the efficient ETL and HBL as well as the optical spacer layer in PSC devices. 3.3. Stability of polymer solar cells with TiO2 NPs Although the TiOx is thermally unstable when the temperature is over 80 °C, it can improve the air-stability of PSC devices because its role acting as a shielding and scavenging layer to prevents the intrusion of oxygen and humidity into the electronically active polymers. These properties originate principally from the oxygen deficiencies in the TiOx [17]. However, no report shows that the stability of PSCs could be obviously improved using the TiO2 NPs. Here the ‘‘Ti–O–Ti’’ chains are bound to the TiO2 NPs surface and acts as the binder link of the nanoparticles. At the same time, the ‘‘Ti–O–Ti’’ chains also exist in sols. When those sols are deposited onto the active layer, it will form a compact TiO2 NPs buffer layer blended with TiOx after annealing. The buffer layer probably achieves higher air-stability devices. For studying the influence of the TiO2 NPs buffer layer on the air-stability of the devices, the aging test was carried out for the devices exposed in the air for 200 h.
70 60 50
IPCE (%)
a
Devices
40 30
without buffer layer with O3-TiO 2 NPs
20
with O7-TiO 2 NPs
10 0 300
with H7-TiO 2 NPs 400
500
600
700
Wavelength (nm) Fig. 7. The IPCE spectra of the devices without and with TiO2 NPs buffer layer from different sols.
Fig. 8 shows the normalized PCE, Jsc, Voc and FF values of devices with and without buffer layer during the aging test. The device with TiO2 NPs buffer layer shows better air-stability than that without one, as seen in Fig. 8a. After 200 h, the ultimate PCE of the reference device drop to 29% of initial value, while the PCE of the devices with TiO2 NPs buffer layer prepared from O3–TiO2, O7–TiO2, H7–TiO2 drop to 80%, 64%, 75% of their initial values, respectively. It can be comparable with the inverted structural PSCs based on P3HT:PCBM [32,33]. Comparing the performance parameters, the main degradation is the Jsc, showing almost the same degradation trends with the PCE. From Fig. 8c, it is found the Voc of both devices with TiO2 NPs buffer layer prepared from O3–TiO2 and O7–TiO2 increases during 0–84 h. The former one is more obvious which increases from 0.525 V to 0.581 V, and the later one is from 0.578 V to 0.588 V. These effects attribute to the further hydrolysis of TTBT or Ti–O–CO–CH3 (or both) when exposed in air. After 84 h, the hydrolysis process completes and the Voc reaches the peaks. As shown in Fig. 8d, the FF of the device with TiO2 NPs buffer layer prepared from O7–TiO2 and the reference device declined more obviously than the device with TiO2 NPs buffer layer prepared from O3–TiO2 and H7–TiO2. This decline can be ascribed to the aggregation in film (as shown in Fig. 2c), affecting the buffer layer surface and influencing the adhesion between the buffer layer and the Al electrode. It is the reason leading a worse air stability than the device with buffer layer prepared from H7–TiO2 sol. It is known that the typical decay curves can be divided into two stages: the initial decay is attributed to interfacial degradation while the longer time scale decay is due to the intrinsic or oxidation driven degradation of the bulk active
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(a)
0.8 0.6 0.4
(b)
1.0
Normalized Jsc
Normalized PCE
1.0
0.2
0.8 0.6 0.4 0.2 0.0
0.0 0
50
100
150
200
0
50
Normalized FF
Normalized Voc
1.05 1.02 0.99 0.96
1.0 0.9 0.8 without buffer layer
0.7
with O3-TiO2 NPs with O7-TiO2 NPs
0.6
with H7-TiO2 NPs
0.5 100
200
(d)
1.1
1.08
50
150
1.2
(c)
1.11
0
100
Time (h)
Time (h)
150
200
0
50
Time (h)
100
150
200
Time (h)
Fig. 8. The degradation of normalized performance parameters of (a) PCE, (b) Jsc, (c) Voc, (d) FF during 200 h for the PSC devices without and with TiO2 NPs buffer layer from different sols.
layers [9]. At the first 3 h exposed in air, the reference device shows linearly drops to 62% of the initial value, but this effect is distinctly eliminated in the device with TiO2 NPs buffer layer. The interesting observation can be simplified as that the buffer layer plays an important role in improving the organic/electrode interface and preventing the interfacial degradation efficiently. At the longer term, the rates of the degradation are also slowed down by TiO2 NPs buffer layer. It is confirmed that the TiO2 NPs buffer layer with existence of TiOx can also act as a shielding and scavenging layer for preventing the intrusion of oxygen and humidity into the electronically active polymers. 4. Conclusion The pure anatase phase TiO2 NPs spreading in sols have been synthesized using a new simple solution-based method. Both the crystallinity and particle size of TiO2 NPs were enhanced while the distribution of particle size in sol was narrowed, as of the content of H2O increases for the process of peptization, especially the effect is further enhanced when the H2O acts as the solvent. The TiO2 NPs buffer layer can act as efficient electron extraction and hole blocking layer as well as optical spacer layer. Under one sun simulated AM1.5G illumination, the PCE of 4.24% was achieved based on the conventional structure device ITO/ PEDOT:PSS/P3HT:PCBM/TiO2 NPs/Al, which is ca. 37% higher than the devices without TiO2 NPs buffer layer. Meanwhile, the Jsc is improved to as high as 12.83 mA/cm2. Those TiO2 NPs buffer layer acts as a shielding and scav-
enging layer for preventing the intrusion of oxygen and humidity into the electronically active polymers. The devices with TiO2 NPs buffer layer are very stable, and the PCE only drop by about 20% after 200 h in air without encapsulation, which is much more stable than other conventional structural PSCs and comparable with the inverted structural PSCs based on P3HT:PCBM system. This kind of TiO2 NPs shows the potential applications in high-performance and long-life time PSC devices as well as in printed electronics. Acknowledgments This work was supported in part by the National Natural Science Foundation of China (NSFC, 61172047), the National Research Foundation for the Doctoral Program of Higher Education of China (20110162110059), and the Innovation fund for PhD students of Central South University (2013ZZTS010). J.L. Yang acknowledges the support of the NSFC (51203192), the Hunan Provincial Natural Science Foundation of China (13JJ4019), and the Program for New Century Excellent Talents in University (NCET-130598). Y.L. Gao acknowledges the support of NSFC (51173205, 11334014). Appendix A. Supplementary material Supplementary material associated with this article can be found, in the online version, at http://dx.doi.org/ 10.1016/j.orgel.2014.01.024.
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Enhanced efficiency and stability of polymer solar cells with TiO2 nanoparticles buffer layer Jian Xiong a,b, Bingchu Yang a,b,⇑, Conghua Zhou a,b,⇑, Junliang Yang a,b,⇑, Haichao Duan a,b, Wenlong Huang a,b, Xiang Zhang a,b, Xingda Xia a,b, Lei Zhang a,b, Han Huang a,b, Yongli Gao a,b,c a Institute of Super-microstructure and Ultrafast Process in Advanced Materials, School of Physics and Electronics, Central South University, Changsha, Hunan 410083, China b Hunan Key Laboratory for Super-microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha, Hunan 410083, China c Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA
a r t i c l e
i n f o
Article history: Received 7 October 2013 Received in revised form 20 December 2013 Accepted 31 January 2014 Available online 13 February 2014 Keywords: TiO2 nanoparticles Buffer layer Polymer solar cells Stability
a b s t r a c t TiO2 sols synthesized with a facile solution-based method were used as a buffer layer between the active layer and the cathode Al in conventional structure polymer solar cells (PSCs). Using transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray diffraction (XRD) and atomic force microscopy (AFM), the morphological and crystallographic properties of synthesized TiO2 nanoparticles (TiO2 NPs) as well as the buffer layer were studied in detail. It was observed that by increasing H2O in the process of peptization both the crystallinity and particle size of TiO2 NPs were enhanced, while the particles in sol showed a narrower size distribution conformed by dynamic light scattering. Inserting TiO2 NPs as a buffer layer in conventional structure PSCs, both the power conversion efficiency (PCE) and stability were improved dramatically. PSCs based on the structure of ITO/PEDOT:PSS/P3HT:PCBM/TiO2 NPs/Al showed the short-circuit current (Jsc) of 12.83 mA/cm2 and the PCE of 4.24%, which were improved by 31% and 37%, respectively comparing with the reference devices without a TiO2 buffer layer. The stability measurement showed that PSC devices with a TiO2 NPs buffer layer could retain 80% of the original PCEs after exposed in air for 200 h, much better than the devices without such a buffer layer. The effect can be attributed to the protection by the buffer layer against oxygen and H2O diffusion into the active layers. The observations indicate that TiO2 NPs synthesized by facile solution-based method have great potential applications in PSCs, especially for large-area printed PSCs. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Polymer solar cells (PSCs) show great potentials as renewable energy sources due to their advantages, such ⇑ Corresponding authors at: Institute of Super-microstructure and Ultrafast Process in Advanced Materials, School of Physics and Electronics, Central South University, Changsha, Hunan 410083, China. Tel.: +86 731 88879525. E-mail addresses: [email protected] (B. Yang), chzhou@csu. edu.cn (C. Zhou), [email protected] (J. Yang). http://dx.doi.org/10.1016/j.orgel.2014.01.024 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
as low cost, light, flexible, semitransparent [1–4]. Polymer/fullerene bulk heterojunctions (BHJ) are commonly used in PSCs for obtaining high performance devices [5,6]. The power conversion efficiency (PCE) of state-ofthe-art PSCs based on BHJ has been reported over 10.0% [7,8]. The lifetime or stability is regarded as another important issue for the commercialization of PSCs [9]. Unlike inorganic counterparts, PSC devices are sensitive to moisture and oxygen, and the performance degrades dramatically when exposed to air. For improving the PCE and
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lifetime of PSC devices, one efficient method is to insert an inorganic buffer layer between the electrode and the active layer. It was suggested that such buffer layer could modify the organic/electrode interface and change the chemical nature of the interface. Moreover, it could prevent the diffusion of electrode atoms, oxygen, and water into the active layer of the device [9–12]. A variety of inorganic salts and n-type inorganic semiconductor materials, for example, LiF [9], CsCO3 [13], ZnO [14], deposited by vacuum evaporation or solution process, have been reported as the buffer layer to improve the PCE or lifetime of PSCs. TiOx is another material that has been used in PSCs as a buffer layer [15–19]. The TiOx can act as optical spacer, efficient electron transport or hole blocking layer (ETL/ HBL), as well as shielding and scavenging layer, which enhance the PCEs of the PSC devices and prevent the permeation of moisture and oxygen into the active layer. However, TiOx was found to be thermally unstable when treated in post-annealing processes at temperatures over 80 °C limiting its application in PSCs [18]. In addition, the preparation of TiOx involves sol–gel synthesis of precursors, followed by hydrolysis in air after deposition, which complicates the device fabrication and risks the degradation of PSC active materials. To overcome these shortcomings, TiO2 nanoparticles (NPs) seem to be a proper choice because of the thermally stability and the role as efficient ETL/HBL [20–22]. However, the air stability of devices with a TiO2 NPs layer is not encouraging and rarely reported. Lee et al. pointed out that the oxygen/water protection and scavenging capability originate primarily from the oxygen deficiencies of TiO2 film [17]. Salim et al. reported the t80 (time for the PCE decays to 80% of initial measured value) of the PSCs is less than 1 h based on the structure of ITO/ PEDOT:PSS/P3HT:PCBM/TiO2 NPs/Al [22]. Those observations indicate that a device with pure TiO2 NPs cannot have good air stability. In this paper, we report our work in developing a facile solution-based method to synthesize TiO2 sols, which avoid the in-air hydrolysis [15–17], centrifugation [20,21] and autoclave process [22]. The synthesized TiO2 NPs with this new method possess the advantages of both TiOx and TiO2 NPs synthesized with other methods when used as a buffer layer between the Al electrode and active layer. Using a conventional structure of ITO/PEDOT:PSS/ P3HT:PCBM/TiO2 NPs/Al, we demonstrate that both the PCE and stability of PSC devices with the TiO2 NPs buffer layer were improved dramatically, showing great potential applications in other PSC devices and large-area printed PSC devices.
environment for easier peptization, while acetic acid is used to improve dispersion of nanoparticles in the sols. Secondly, tetrabutyl titanate (TTBT, Ti(OBu)4, 24 mL) was added slowly into the above mixture under vigorously stirring. Following, two types of precursor gels (70 ml) were prepared, in which one was slowly added 3 ml distilled H2O while another was slowly added 7 ml distilled H2O. It was found that no gel formed as 3 ml H2O was slowly added but white gel formed quickly when 7 ml H2O was added. The former was diluted with organic solvent 2methoxyethanol (CH3OCH2CH2OH), while the latter was dissolved with organic solvent (2-methoxyethanol) and H2O, respectively. Those three types of sols are named as O3–TiO2, O7–TiO2, H7–TiO2, respectively. These sols were further stirred at 40 °C until they became transparent again. All samples were stirred at 80 °C for several hours and the concentration of Ti ions in the solution was kept 0.47 M. TiO2 NPs were achieved in such sols. Finally, the sols were diluted in ethanol (CH3CH2OH) for fabricating PSCs, in which the concentration of the Ti ions was 0.016 M.
2.2. Device fabrication Fig. 1 shows the structure of PSCs. The patterned indium tin oxide (ITO) coated glass were used as the substrate, which was ultrasonically cleaned in acetone, detergents, distilled water and isopropyl alcohol for 15 min respectively, then dried by N2 flow and treated by ozone for 15 min. The PEDOT: PSS layer (Baytron, PVP AI 4083) with thickness of about 40 nm was spin-coated (5000 rpm) onto patterned ITO substrate and dried on hot plate at 150 °C for 10 min. The fabrication of active layer was performed in glove box (both H2O and O2 < 1.0 ppm). Poly (3-hexylthiophene) (P3HT, Sigma–Aldrich) and fullerene derivative [6, 6]-phenyl C61-butyric acid methyl ester (PCBM, American Dye Source, Inc.) were dissolved in 1,2-dichlorobenzene (DCB) at the ratio of 1.0:0.8 wt% with a concentration of 27 mg/ml. The active solution was spin-coated on PEDOT:PSS-coated ITO glass at speed of 1000 rpm for 30 s. The spin-coated film was kept in a covered petri dish and dried slowly for 2 h in glove box. Annealing at 60 °C was performed to remove residual solvents. Then TiO2 sols as-synthesized above
2. Experimental details 2.1. TiO2 sols synthesis TiO2 sols were synthesized by developing a solutionbased method. The synthesis route is shown in Fig. S1 in Supporting Information. Firstly, acidic solution was prepared by dissolving nitric acid (HNO3, 0.6 mL) and acetic acid (CH3COOH, 5.4 mL) in n-butanol (CH3CH2CH2OH, 40 mL). Nitric acid is introduced to provide a strong acidic
Fig. 1. Schematic structure of the PSC device.
J. Xiong et al. / Organic Electronics 15 (2014) 835–843
were spin-coated on the active layer. A 100 nm Al electrode was deposited on top of TiO2 NPs films by thermal evaporation under the vacuum of 2 106 mbar. Finally all devices were annealed at 150 °C for 10 min in glove box. The device active area is 0.12 cm2. For comparison, devices without TiO2 NPs buffer layer were also prepared using similar procedures.
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gate the air-stability. Monochromatic incident photon-toelectron conversion efficiency (IPCE) of the devices was performed by AC mode with a monochromator-calibrated wavelength control (BEIJING 7-STAR OPTICAL INSTRUMENTS CO., LTD.). A calibrated Si photodiode was used as the reference device for the counting of incident photons. 3. Results and discussion
2.3. Characterization and testing 3.1. Morphological and crystallographic properties of TiO2 NPs Morphological and crystallographic properties of TiO2 NPs in sols were characterized by transmission electron microscopy (TEM, JEM-2100F, Japan), selected area electron diffraction (SAED) and X-ray diffraction (XRD, D8 Advance, Bruker AXS, Germany). The TEM samples were prepared by casting a drop of sol on copper grids on which thin carbon layer was loaded. The size of TiO2 NPs was evaluated based on TEM images, in which 20 isolated particles were chosen to give a mean particle size. While the XRD powder samples were prepared by drying sols at 150 °C. Particle-size distribution of TiO2 NPs in the sols was monitored by dynamic light scattering (DLS, Zetasizer nano ZS, Malvern, England). The Ti–O–Ti bonds were analysized by Fourier transform infrared spectrum (Nicolet, America). The absorption of the device was measured by optical system (Zolix, Beijing). The work function of TiO2 NPs was measured by ultraviolet photoelectron spectroscopy system (Specs, Germany). The morphology of thin film was characterized by atomic force microscopy (Agilent Technologies 5500 AFM/SPM System, USA) with tapping-mode. Current density–voltage (J–V) characteristics of the devices were measured by digital sourcemeter (Keithley, model 2420). A solar simulator (91160s, Newport, AM 1.5G) was used for PCE testing. Light intensity was 100 mW/cm2 calibrated by a standard silicon solar cell. The device was exposed in air up to 200 h to investi-
Morphological properties of particles in TiO2 sols are shown in Fig. 2. The rough statistical mean size of TiO2 NPs was evaluated based on TEM images. For O3–TiO2 sol, the sphere-like small particles with the size of 2.5 nm are observed (Fig. 2a), and the particles are amorphous since no Debye–Sherrer diffraction rings are seen in the SAED image, as shown in the inset of Fig. 2a. For O7–TiO2 sol, 3.6 nm oval-like crystallites appear and weak Debye– Sherrer diffraction rings could be observed, as shown in the inset of Fig. 2b, suggesting the formation of TiO2 crystallites. For H7–TiO2 sol, the size of TiO2 NPs is further enlarged to 5.5 nm, and the crystallinity is dramatically improved since the strong (1 0 1), (0 0 4), (2 0 0), (1 0 4), (2 0 4) Debye–Sherrer diffraction rings could be observed clearly (inset of Fig. 2c), corresponding to TiO2 NPs anatase phase according to the PDF card No.78-2486. The results suggest that the appropriate excessive H2O for the process of peptization is beneficial to the growth and crystallinity of TiO2 NPs. The pure H2O solvent can further enhance this effect. In Fig. 2a–c, some aggregations can be observed as well for three sols, especially for H7–TiO2 sol. It is well known that once the alkoxide meets with water, the hydrolysis–condensation reactions would happen, as presented by Eqs. (1) and (2). The addition of the acetic acid supports the third reaction, as shown in Eq. (3).
Fig. 2. TEM images of TiO2 particles in different sols: (a) O3–TiO2; (b) O7–TiO2; (c) H7–TiO2; (d) schematic diagram of the Ti–O–Ti chain binding to TiO2 NPs.
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Ti½OR4 þ vH OH ! Ti½OR4v ½OHv þ vH OR
ð1Þ
(101)
) ½Ti O Tin
ð2Þ
Ti OH þ HO CO CH3 ! Ti O CO CH3 þ H OH
ð3Þ
After the peptization, there are many ‘‘Ti–O–Ti’’ chains. As shown in Fig. S2 in Supporting Information, fourier transform infrared spectrum suggests that the absorption bands at 1360 cm1 can be clearly distinguished for O3– TiO2 NPs and O7–TiO2 NPs sols, which corresponds to the absorption of the Ti–O–Ti chains [18]. The large ‘‘Ti–O– Ti’’ chains support the formation of three-dimensional framework through hydrolysis–condensation. The sol losses its mobility and becomes the gel. On the other hand, the gel could be broken under violent stirring and the sol is formed again. Through the sol–gel–sol process, it prompts mostly of amorphous Ti to be fully hydrolyzed. At a higher temperature, the fully-hydrolyzed chains contribute to the formation of the TiO2 crystallites, while the chains without fully-hydrolyzation are disadvantageous to form TiO2 crystallites. It suggests that the more the H2O in the gel, the more the ‘‘Ti–O–Ti’’ chains consumed to form TiO2 crystallites. By trial and error, 3 ml H2O could barely make the sol transform to the gel while 7 ml H2O could change the sol to the gel quickly. However, more H2O (>7 ml) was disadvantageous to the formation of the gel. In contrary, it destroyed the stability of the sol since the different solvent polarity in organic solvents. In the H2O solvent, the chains can be further hydrolyzed. The crystallinity and particle size of TiO2 NPs can be enhanced with the increase of H2O in the gel. Meanwhile, some chains probably remain in the sol since TTBT is less hydrolysable. The addition of the acetic acid into the sol is helpful to form Ti–O–CO– CH3, which is more difficult to hydrolyze and benefits the existence of chains in the sol. Hydroxyl groups (–OH) remaining in the chains and on the TiO2 NPs were confirmed in previous studies [23,24]. Some of the ‘‘Ti–O–Ti’’ chains in sols are probably bound on the surface of TiO2 NPs by hydrolysis–condensation reactions (Fig. 2d). These chains can act as binders by further hydrolysis–condensation reactions, linking TiO2 NPs each other and leading to the formation of cluster and aggregation. Therefore, the lower concentration of the binders results in a better dispersion and less aggregation. XRD patterns of different TiO2 powder obtained by drying solvent in oven at 150 °C are shown in Fig. 3. The powder originating from O3–TiO2 sol shows no clear peaks, suggesting the particles are smaller nucleuses with lower crystallinity. The powers obtained by other two sols, especially for the H7–TiO2 sol, show the obvious diffraction peaks indexed as (1 0 1), (0 0 4), (2 0 0), (1 0 5)/(2 1 1) based on the anatase phase. These results coincide with the SAED studies discussed above. Meanwhile, The Sherrer’s equation was used to calculate the average crystallites size of nanocrystallites with 2.7 nm, 3.3 nm, 4.3 nm respectively based on the 2h-FWHM of diffraction peak (1 0 1) from
Intensity (a.u.)
Ti OH þ HO Ti ! Ti O Ti þH OH (004) (200)
(211) (105) (204)
(220) (215)
c b a
20
30
40
50
60
70
80
2θ ( Ο ) Fig. 3. XRD patterns of the TiO2 NPs originated from three kinds of sols: curve a, O3–TiO2; curve b, O7–TiO2; curve c, H7–TiO2. The position and relatively intensity of diffraction peak of pure anatase TiO2 were indicated by green lines according to PDF card No.78-2486.
the XRD results. These values based on the Sherrer’s calculation can basically match with the statistical results by the TEM images. Aggregation is unavoidable in a sol system. For their applications in PSCs, a buffer layer is usually less than 50 nm. Heavy aggregation in the sols could affect the microstructure as well as the compactness of thin film, and finally influence the PSCs performance. The dispersion of particle size in sols is monitored using the DLS technique, as shown in Fig. 4. For O3–TiO2 sol, the size distribution can be divided into three main parts: 1–3 nm, 5–10 nm, and 80–110 nm (Fig. 4a). While for the O7–TiO2 and H7–TiO2 sols, the size distribution is greatly narrowed. The size range in both sols can be divided into two parts: one main part is from 3 nm to 36 nm, another part is >80 nm. Based on the TEM and XRD studies, the mean size of single particle is 2.5 nm, 3.6 nm, 5.5 nm for the O3–TiO2, O7–TiO2 and H7–TiO2 sol respectively, which are partly in the main distribution range (Fig. 4). The main distribution range attributes to single particle and aggregations, while the larger size parts (>80 nm) ascribe to the amorphous [Ti] [23]. Comparing the results from the DLS studies, it can be found that the particle dispersibility is improved with the increment of H2O in sols, probably attributing to the decreased of ‘‘Ti–O–Ti’’ chains.
3.2. Polymer solar cells with TiO2 NPs Fig. 5 shows the AFM images of the P3HT:PCBM blend films with and without TiO2 NPs layer. The value of rootmean-square (RMS) surface roughness of P3HT:PCBM is 9.81 nm for 4 4 lm2 (Fig. 5a), coinciding with the previous reported by slow growth process [25]. The image of the active layer coated by O3–TiO2 sol is shown in Fig. 5b, and the RMS value drops to 3.67 nm. According to the DLS and TEM results, there are many smaller TiO2 NPs and some ‘‘Ti–O–Ti’’ chains in O3–TiO2 sol. When the TiO2 sol is spin-coated onto the active layer, the small particles and ‘‘Ti–O–Ti’’ chains penetrate into the grooves and connect closely, resulting in the dense and smooth thin films after
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18
20 18 16 14 12 10 8 6 4 2 0
16
(a)
(b)
14
intensity (%)
intensity (%)
J. Xiong et al. / Organic Electronics 15 (2014) 835–843
12 10 8 6 4 2 0
1
10
100
1
10
intensity (%)
size (nm)
100
size (nm)
20 18 16 14 12 10 8 6 4 2 0
(c)
1
10
100
size (nm) Fig. 4. The particle size distribution of three kinds of TiO2 sols obtained by DLS technique: (a) O3–TiO2; (b) O7–TiO2; (c) H7–TiO2.
annealing. Fig. 5c is the AFM image of TiO2 NPs layer prepared from the O7–TiO2 sol, the film surface is relatively smooth (RMS = 4.8 nm) and some larger aggregation are observed, coinciding well with the previous TEM results. However, the TiO2 NPs layer prepared from the H7–TiO2 sol becomes very rough (RMS = 9.1 nm), attributing to the larger particle size and the reduction of ‘‘Ti–O–Ti’’ chains. The illuminated current density–voltage (J–V) characteristics of devices with and without TiO2 NPs buffer layer are shown in Fig. 6, and the detailed device parameters are summarized in Table 1. The PSC reference device exhibits a PCE of 3.09% with a short-circuit current (Jsc) of 9.79 mA/ cm2, open circuit voltage (Voc) of 0.59 V and fill factor (FF) of 0.53, which are similar to the results from many other research groups [21,26,27]. The PCE of device with TiO2 NPs buffer layer prepared from O3–TiO2 sol is similar with the reference device. Both Voc and Jsc are reduced, dropping from 0.59 V to 0.52 V and from 9.79 mA/cm2 to 9.58 mA/cm2, respectively. But the PCE is compensated with a higher FF (60%). The reason for a decreased Voc is apparently attributed to the fact that some TTBT or Ti–O– CO–CH3 (or both of them) did not hydrolyze entirely in the buffer layer due to the less hydrolysis [19]. The Voc could increase during the exposure in air, as discussed below. The device with TiO2 NPs buffer layer prepared from O7–TiO2 sol shows excellent performance characteristics with a PCE of 4.24%, Jsc of 12.69 mA/cm2, Voc of 0.58 V and FF of 0.58. Device with TiO2 NPs buffer layer prepared from H7–TiO2 sol is also improved and lead to a result with a PCE of 4.12%, Jsc of 12.83 mA/cm2, Voc of 0.57 V and FF of 0.56. Comparing with the reference device, the PCEs of de-
vices with TiO2 NPs prepared from the later two sols are remarkably improved by 37% and 33%, respectively. It is noticeable that the Jsc reaches over 12.5 mA/cm2, which is higher than the conventional structure devices with TiOx or TiO2 NPs buffer layer (9–11.2 mA/cm2) [15–18,20,22]. However, the Jsc of the device with a buffer layer of O7– TiO2NPs or H7–TiO2NPs calculated from the integration of IPCE spectra is smaller than the Jsc by measurement (Table 1). Both the illumination intensity of simulator and the device area are double checked. Hence they probably relate with the concentration of free carrier in TiO2 NPs under the different test conditions for the IPCE and J–V measurements. The higher the carrier concentration, the better the conductivity. The Jsc can reach a maximum when treated with 5–10 min pre-illumination under 100 mW/cm2 during the measurement. The light intensities are different in the measurements of the IPCE and J–V, resulting in a difference in the carrier concentration of TiO2 NPs, and accordingly the different in the calculated and measured Jsc. This phenomenon was also reported previously [21]. The disagreement between the calculated and measured Jsc for the devices without TiO2 NPs attributed to the quick degradation when exposed in air. The UPS measurement suggested that the work function of O3–TiO2NPs, O7–TiO2NPs, H7–TiO2NPs are 3.9 eV, 4.4 eV, 4.5 eV, respectively, as shown in Fig. S3 in Supporting Information. The positions of work function are helpful to the transform of the electrons from the PCBM to the cathode (Al), although there is a very small energy barrier [28]. Especially, the carrier mobility of crystallites is much better than that of amorphous particles [29]. On the other
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Fig. 5. AFM images of P3HT:PCBM thin films without TiO2 NPs buffer layer (a) and with TiO2 NPs buffer layer from the sols (b) O3–TiO2, (c) O7–TiO2, (d) H7– TiO2.
Jsc (mA/cm2)
4 2
without buffer layer
0
with O3-TiO 2 NPs
-2
with O7-TiO 2 NPs
-4
with H7-TiO 2 NPs
-6 -8 -10 -12 -14 -0.4
-0.2
0.0
0.2
0.4
0.6
0.8
Voltage (V) Fig. 6. J–V curves obtained under illumination of 1.5 G at 100 mW/cm2 for PSC devices with and without TiO2 NPs buffer layer. (Black square – PSC device without TiO2 NPs buffer layer; red circle – PSC device with TiO2 NPs buffer layer prepared from O3–TiO2 sol; blue up-triangle – PSC device with TiO2 NPs buffer layer prepared from O7–TiO2 sol; dark-cyan down-triangle – PSC device with TiO2 NPs buffer layer prepared from H7– TiO2 sol. Similarly thereafter). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
hand, the highest-occupied molecular orbital (HOMO) level of TiO2 are larger than that of the P3HT, resulting in
the prevention of hole accumulation at interface between the active layer and the Al, which leads to reduced interfacial charge recombination. Meanwhile, the absorption of the device in reflection geometry was enhanced with TiO2 NPs buffer layer, as shown in Fig. S4 in Supporting Information. It suggests the function of optical spacer of TiO2 NPs buffer layer in the PSC devices. Hence, the improved Jsc in the devices results from the functions of TiO2 NPs buffer layer played as the ETL, the HBL, as well as the optical spacer layer. The TiOx could exist in buffer layer originating from the ‘‘Ti–O–Ti’’ chains. The content of TiOx decreases and the crystallinity of TiO2 NPs are enhanced in the buffer layer prepared by sols with the increment of H2O. It is reason why the Jsc of device increases gradually with the TiO2 NPs buffer layer prepared from O3–TiO2, O7–TiO2 and H7–TiO2 sols. Comparing the performance of the devices in details with different buffer layer, the FF inversely related to the RMS of TiO2 NPs coated on P3HT:PCBM. It suggests that the smoother TiO2 buffer layer surfaces are probably beneficial to the deposition of higher quality Al electrode and form an ohmic contact between the layers. Meanwhile, it is also confirmed that some TiOx (in form of ‘‘Ti–O–Ti’’ chain) in film can act as a binder and filler
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J. Xiong et al. / Organic Electronics 15 (2014) 835–843 Table 1 Performance parameters of conventional structure PSC devices prepared without and with TiO2 NPs buffer layer from different sols. PCE (%)
Jsc (mA/cm2)
Calculated Jsca (mA/cm2)
Voc (V)
FF
Without TiO2 With O3–TiO2 NPs With O7–TiO2 NPs With H7–TiO2 NPs
3.09 3.04 4.24 4.12
9.79 9.58 12.69 12.83
9 9.48 10.44 10.68
0.59 0.52 0.58 0.57
53 60 58 56
The Jsc was calculated from the integration of IPCE spectra. It is smaller than the measured Jsc and the detailed discussion is shown in the text.
between TiO2 NPs, ensuring a close connection between the buffer layer and the active layer as well as the particles. It is helpful for improving the structure of buffer layer and decreasing the RMS of active layer. Most of the reported PSCs with TiOx showed poor thermal stability. It was found that the post-annealing of the devices with TIP-based TiOx buffer layer leads the decrease of the PCE from 2.76% to 0.93% [18]. The main reason for the degradation is that the post-annealing process would change the TIP-based TiOx film morphology, which possibly deteriorated the interface between the Al electrode and the active layer, and accordingly degrade the performance of PSC devices. This kind of degradation was not observed in the devices with three types of TiO2 NPs buffer layer including the ‘‘Ti–O–Ti’’ chains in our study. It confirms that the newly developed TiO2 NPs have a better thermal stability than the conventional TiOx. Actually, the thermally stable oxide compound used in PSCs as a buffer layer is very important, since the post-annealing can enhance the formation of C–O–Al [30] and Ti–O–Al [31], which can improve the interface adhesion and yield better device performance. Fig. 7 shows the IPCE spectra of the PSC devices with and without TiO2 NPs buffer layer. It can be seen from the diagram that the IPCE of device with TiO2 NPs buffer layer prepared from O3–TiO2 sol is improved notably at rang of 430–570 nm, while the IPCE of devices with TiO2 NPs prepared from O7–TiO2 and H7–TiO2 sol increase in a much broader wave range from 380 to 600 nm. It indicates that the TiO2 NPs layer act as the efficient ETL and HBL as well as the optical spacer layer in PSC devices. 3.3. Stability of polymer solar cells with TiO2 NPs Although the TiOx is thermally unstable when the temperature is over 80 °C, it can improve the air-stability of PSC devices because its role acting as a shielding and scavenging layer to prevents the intrusion of oxygen and humidity into the electronically active polymers. These properties originate principally from the oxygen deficiencies in the TiOx [17]. However, no report shows that the stability of PSCs could be obviously improved using the TiO2 NPs. Here the ‘‘Ti–O–Ti’’ chains are bound to the TiO2 NPs surface and acts as the binder link of the nanoparticles. At the same time, the ‘‘Ti–O–Ti’’ chains also exist in sols. When those sols are deposited onto the active layer, it will form a compact TiO2 NPs buffer layer blended with TiOx after annealing. The buffer layer probably achieves higher air-stability devices. For studying the influence of the TiO2 NPs buffer layer on the air-stability of the devices, the aging test was carried out for the devices exposed in the air for 200 h.
70 60 50
IPCE (%)
a
Devices
40 30
without buffer layer with O3-TiO 2 NPs
20
with O7-TiO 2 NPs
10 0 300
with H7-TiO 2 NPs 400
500
600
700
Wavelength (nm) Fig. 7. The IPCE spectra of the devices without and with TiO2 NPs buffer layer from different sols.
Fig. 8 shows the normalized PCE, Jsc, Voc and FF values of devices with and without buffer layer during the aging test. The device with TiO2 NPs buffer layer shows better air-stability than that without one, as seen in Fig. 8a. After 200 h, the ultimate PCE of the reference device drop to 29% of initial value, while the PCE of the devices with TiO2 NPs buffer layer prepared from O3–TiO2, O7–TiO2, H7–TiO2 drop to 80%, 64%, 75% of their initial values, respectively. It can be comparable with the inverted structural PSCs based on P3HT:PCBM [32,33]. Comparing the performance parameters, the main degradation is the Jsc, showing almost the same degradation trends with the PCE. From Fig. 8c, it is found the Voc of both devices with TiO2 NPs buffer layer prepared from O3–TiO2 and O7–TiO2 increases during 0–84 h. The former one is more obvious which increases from 0.525 V to 0.581 V, and the later one is from 0.578 V to 0.588 V. These effects attribute to the further hydrolysis of TTBT or Ti–O–CO–CH3 (or both) when exposed in air. After 84 h, the hydrolysis process completes and the Voc reaches the peaks. As shown in Fig. 8d, the FF of the device with TiO2 NPs buffer layer prepared from O7–TiO2 and the reference device declined more obviously than the device with TiO2 NPs buffer layer prepared from O3–TiO2 and H7–TiO2. This decline can be ascribed to the aggregation in film (as shown in Fig. 2c), affecting the buffer layer surface and influencing the adhesion between the buffer layer and the Al electrode. It is the reason leading a worse air stability than the device with buffer layer prepared from H7–TiO2 sol. It is known that the typical decay curves can be divided into two stages: the initial decay is attributed to interfacial degradation while the longer time scale decay is due to the intrinsic or oxidation driven degradation of the bulk active
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J. Xiong et al. / Organic Electronics 15 (2014) 835–843
(a)
0.8 0.6 0.4
(b)
1.0
Normalized Jsc
Normalized PCE
1.0
0.2
0.8 0.6 0.4 0.2 0.0
0.0 0
50
100
150
200
0
50
Normalized FF
Normalized Voc
1.05 1.02 0.99 0.96
1.0 0.9 0.8 without buffer layer
0.7
with O3-TiO2 NPs with O7-TiO2 NPs
0.6
with H7-TiO2 NPs
0.5 100
200
(d)
1.1
1.08
50
150
1.2
(c)
1.11
0
100
Time (h)
Time (h)
150
200
0
50
Time (h)
100
150
200
Time (h)
Fig. 8. The degradation of normalized performance parameters of (a) PCE, (b) Jsc, (c) Voc, (d) FF during 200 h for the PSC devices without and with TiO2 NPs buffer layer from different sols.
layers [9]. At the first 3 h exposed in air, the reference device shows linearly drops to 62% of the initial value, but this effect is distinctly eliminated in the device with TiO2 NPs buffer layer. The interesting observation can be simplified as that the buffer layer plays an important role in improving the organic/electrode interface and preventing the interfacial degradation efficiently. At the longer term, the rates of the degradation are also slowed down by TiO2 NPs buffer layer. It is confirmed that the TiO2 NPs buffer layer with existence of TiOx can also act as a shielding and scavenging layer for preventing the intrusion of oxygen and humidity into the electronically active polymers. 4. Conclusion The pure anatase phase TiO2 NPs spreading in sols have been synthesized using a new simple solution-based method. Both the crystallinity and particle size of TiO2 NPs were enhanced while the distribution of particle size in sol was narrowed, as of the content of H2O increases for the process of peptization, especially the effect is further enhanced when the H2O acts as the solvent. The TiO2 NPs buffer layer can act as efficient electron extraction and hole blocking layer as well as optical spacer layer. Under one sun simulated AM1.5G illumination, the PCE of 4.24% was achieved based on the conventional structure device ITO/ PEDOT:PSS/P3HT:PCBM/TiO2 NPs/Al, which is ca. 37% higher than the devices without TiO2 NPs buffer layer. Meanwhile, the Jsc is improved to as high as 12.83 mA/cm2. Those TiO2 NPs buffer layer acts as a shielding and scav-
enging layer for preventing the intrusion of oxygen and humidity into the electronically active polymers. The devices with TiO2 NPs buffer layer are very stable, and the PCE only drop by about 20% after 200 h in air without encapsulation, which is much more stable than other conventional structural PSCs and comparable with the inverted structural PSCs based on P3HT:PCBM system. This kind of TiO2 NPs shows the potential applications in high-performance and long-life time PSC devices as well as in printed electronics. Acknowledgments This work was supported in part by the National Natural Science Foundation of China (NSFC, 61172047), the National Research Foundation for the Doctoral Program of Higher Education of China (20110162110059), and the Innovation fund for PhD students of Central South University (2013ZZTS010). J.L. Yang acknowledges the support of the NSFC (51203192), the Hunan Provincial Natural Science Foundation of China (13JJ4019), and the Program for New Century Excellent Talents in University (NCET-130598). Y.L. Gao acknowledges the support of NSFC (51173205, 11334014). Appendix A. Supplementary material Supplementary material associated with this article can be found, in the online version, at http://dx.doi.org/ 10.1016/j.orgel.2014.01.024.
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