Il
Jeon
a,
James W.
Ryan
a,
Tafu
Nakazaki
b,
Kee Sheng
Yeo
a,
Yuichi
Negishi
b and
Yutaka
Matsuo
*a
aDepartment of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. E-mail: matsuo@chem.s.u-tokyo.ac.jp
bDepartment of Applied Chemistry, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan
First published on 12th September 2014
Highly stable ZnO nanoparticles (NPs) for use in organic solar cells (OSCs) have been synthesized in 2-aminoethanol, which acts as both a stabilizing ligand and a solvent. The ZnO NP films do not require thermal annealing and applying them in inverted P3HT:mix-PCBM OSCs fabricated almost entirely under ambient conditions show efficiencies >3%. We find that thermally annealing the ZnO NP films does not give rise to any significant differences in device performance up to 150 °C. Annealing the films at higher temperatures leads to reduced short-circuit current densities and fill factors due to the removal of 2-aminoethanol from the NP surface, as evidenced by X-ray photoelectron spectroscopy. Furthermore, to confirm that the post-annealing of P3HT:PCBM devices at 150 °C does not affect our results, we fabricated inverted small molecule OSCs using a squaraine donor and PC71BM acceptor that only requires a low-temperature thermal annealing step (70 °C). No substantial differences between annealed and non-annealed devices were observed, which demonstrates the applicability of these new ZnO NPs in flexible OSCs.
Once the solution was cooled down, it was centrifuged, which led to a precipitate that contained ZnO NPs with diameters in excess of 100 nm and a pale yellow supernatant that provided well dispersed ZnO NPs with a mean diameter of 2.5 nm as determined by transmission electron microscopy (see ESI Fig. S1-1†). The concentration of the solution obtained was 0.24 mg ml−1 and stable for at least 6 months, which, considering that no additional stabilizing ligands were required in addition to the 2-aminoethanol solvent molecules, is an interesting and important finding.
The as-obtained supernatant solution was then used in fabricating OSCs by spin-coating it on indium tin oxide (ITO) patterned glass substrates. Following spin-coating, the films were then subjected to thermal annealing at temperatures ranging from room temperature (i.e. no annealing) to 210 °C. The annealing time was fixed at 1 h in each case. The transmittance of the ZnO thin films on glass/ITO substrates are shown in Fig. 1 and shows a decrease in transmittance with annealing temperature. Non-annealed ZnO NP films show the highest transmittance, with a maximum transmittance of 85.2% at 520 nm.
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Fig. 1 Transmittance spectra of ZnO NP films on glass/ITO substrates annealed at different temperatures. |
P3HT:mix-PCBM active layers were then deposited, followed by a PEDOT:PSS hole transport layer and Au electrode. Note that all layers, except for gold, were deposited in air. Furthermore, devices were also annealed and measured in air, without the addition of an encapsulation layer, demonstrating the good stability of these inverted devices. The current–voltage (J–V) curves of the OSCs are shown in Fig. 2 and Table 1 summarizes the key figures of merit. Remarkably, devices processed without subjecting the ZnO layer to any thermal annealing have the highest power conversion efficiency (PCE, η) of 3.1% due to the higher fill factor (FF) compared with the annealed devices. Although, the difference between the devices with non-annealed ZnO films and the devices annealed up to 120 °C do not in fact differ considerably. There is, however, a significant difference in η when the annealing temperature exceeds 120 °C, where the FF decreases with increasing temperature as does the short-circuit current density (JSC), with a significant drop observed when annealing exceeds 150 °C. This decrease in FF arises from an increase in series resistance (RS) as well as the shunt resistance (RSh), with values decreasing with annealing temperature (see Table 1). S-shaped curves are observed for devices employing ZnO films annealed at 210 °C, which further decreases the FF. The origin of the S-shaped kink in the J–V curve is not entirely clear but probably arises from ZnO inducing a barrier to charge injection or extraction.11 Considering that 2-aminoethanol should have been removed from the ZnO NP surface due to the high annealing temperature (at least for the NPs on the ZnO surface), we would expect the NPs to have a high concentration of trap states that may reduce the conductivity of the ZnO film as well as act as trap-sites for free charge carriers generated in the active layer. All devices displayed a short UV-activation time, i.e. light soaking, but there was no noticeable trend between the activation time and the annealing conditions. Reference P3HT:mix-PCBM inverted devices were also fabricated for comparison using a standard sol–gel method to deposit the ZnO film (thermally annealed at 200 °C),4b which shows slightly higher but not significantly different performance (η = 3.2%), demonstrating the high applicability of our ZnO NP approach in OSCs. We have fabricated devices using ZnO NPs synthesized in ethylene glycol and found that they do not perform as well as ZnO NPs in 2-aminoethanol. (see ESI Fig. S3†) We attribute this to the larger diameter of the ZnO NPs (4.09 nm on average) that also display a larger size deviation than the ZnO NP in 2-aminoethanol (see also ESI Fig. S1-2†).
Annealing temperature | V OC V | J SC mA cm−2 | FF | η% | R S Ω cm2 | R Sh Ω cm2 |
---|---|---|---|---|---|---|
a Sol–gel reference devices. | ||||||
NA | 0.61 | 9.08 | 0.56 | 3.1 | 8.29 | 1660 |
120 °C | 0.62 | 9.13 | 0.53 | 3.0 | 8.43 | 1540 |
150 °C | 0.62 | 8.65 | 0.53 | 2.9 | 8.35 | 1270 |
180 °C | 0.61 | 7.98 | 0.45 | 2.2 | 9.77 | 1220 |
210 °C | 0.59 | 6.91 | 0.34 | 1.4 | 20.11 | 1120 |
200 °Ca | 0.60 | 9.42 | 0.56 | 3.2 | 5.71 | 2230 |
Investigating the effect of thermal annealing on the thin film properties of ZnO, we found that the topography, as measured by atomic force microscopy (AFM) on ITO/Glass substrates, did not change significantly with rms roughness values between 5 and 6 nm (Fig. 3). Non-annealed films had the roughest topography and it can be seen that upon annealing at 120 °C, the roughness decreases and then increases with temperature. The changes are subtle, however, and are probably not significant enough to explain the large changes in the J–V characteristics.
The surface chemistry of the ZnO films was then probed using X-ray photoelectron spectroscopy (XPS). Fig. 4 shows the peaks of ZnO films corresponding to (a) Zn 2p3/2 and (b) N 1s core levels (refer to the ESI† for the wide-scan images of each film). A positive shift in the binding energy (BE) of ∼1 eV is observed for both Zn 2p3/2 and N 1s peaks when the films are annealed at 150 °C and above. This increase in BE for both Zn and N indicates that 2-aminoethanol is cleaved from the surface of the ZnO NPs. Furthermore, the intensity of the 2-aminoethanol peaks is seen to decrease with annealing temperature, even for the films annealed at 120 °C, which is much lower than the boiling point of 2-aminoethanol (170 °C). The ∼1 eV increase in BE for both Zn and N does not exactly reflect the trend observed in the J–V curves above, as the device employing a ZnO film annealed at 150 °C still has relatively high efficiency even though 2-aminoethanol has been cleaved from the ZnO surface. However, when the films are annealed at temperatures above the boiling point of 2-aminoethanol we do see a significant decrease in the performance. These results suggest that the presence of 2-aminoethanol is important. Indeed, a recent study on a polar solvent treatment of rippled-ZnO films by Song and co-workers has shown that treating the metal oxide film with a solution of 2-methoxyethanol and just 1% 2-aminoethanol enhances the electrical contact between the ZnO and the LUMO of PCBM.12
The spectra for the O 1s core level are shown in Fig. 5, which correspond to the O in ZnO. We observe an asymmetric peak for each sample that was deconvoluted using curve-fitting methods into two peaks centered at 529.7 ± 0.5 eV (O1) and 531.1 ± 0.5 eV (O2) that correspond to O2− ions of the Zn–O bond for the ZnO wurtzite structure and oxygen deficient component of ZnO where O2− ions correspond to the hydroxyl groups, respectively.13 The ratio of O1:
O2 is seen to change with annealing temperature. O1 is slightly more dominant in the non-annealed ZnO film (O1
:
O2 = 55
:
45). Upon annealing at 120 °C in air, O2 is the slightly more dominant species (48
:
52). However, the percentage of O2 recorded for films heated at higher temperatures proceeds to decrease with temperature. For films annealed at 210 °C the O1
:
O2 ratio is 60
:
40. It is difficult to correlate these data with the experimental J–V curves, as one would expect a decrease of hydroxyl groups to correlate with a decrease in the number of trap states and thus better performance, particularly concerning the FF. The opposite trend until 120 °C is observed, however, due to the removal of 2-aminoethanol. It could be possible that the OH group of 2-aminoethanol is also partially responsible for the peaks centered at 531 eV.
While a clear trend in device performance with ZnO annealing temperature is observed, it must be remembered that the device undergoes a post-annealing process at 150 °C to improve the active layer morphology, which may anneal ZnO in the process and affect the surface chemistry of the ZnO NPs. Therefore, to minimize the influence of the device annealing temperature, we fabricated inverted small molecule organic solar cells (SMOSCs) that only require the active layer to be annealed at 70 °C based on a squaraine donor, 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ)14 and a PC71BM acceptor.15 Active layers were prepared by spin-coating a 20 mg ml−1 solution of SQ:PC71BM in chloroform; the donor:
acceptor ratio was 1
:
5 (see the Experimental section for a detailed description of device fabrication). Fig. 6 shows the J–V curves recorded under standard 1 Sun illumination conditions for devices using non-annealed ZnO NP films as well as films annealed at 120 °C and 150 °C. We observed very little difference between the device characteristics, with PCEs of approximately 3% (see Table 2). While the FF for each device is low, it does not differ significantly to what is reported for normal-architecture SQ:PC71BM devices, which generally have FFs between 40 and 45%.15,16 We expect that through optimizing device fabrication, higher PCE can be achieved. However, the main focus here is simply to demonstrate that the ZnO NP films do not require high temperature thermal annealing for efficient OSCs, which is clearly evident in the J–V curves in Fig. 6.
Annealing temperature | V OC V | J SC mA cm−2 | FF | η% |
---|---|---|---|---|
NA | 0.84 | 9.74 | 0.36 | 3.0 |
120 °C | 0.82 | 10.41 | 0.36 | 3.0 |
150 °C | 0.82 | 9.96 | 0.38 | 3.1 |
For the chemical synthesis of ZnO nanoparticles NPs in ethylene glycol, zinc(II) acetate, Zn(CH3COO)2 (Wako, 99%) and ethylene glycol, C2H4(OH)2 (Wako, 99.0%) were used. The ZnO NPs were synthesized in the same manner as the ZnO NPs in 2-aminoethanol as aforementioned.
P3HT:mix-PCBM devices were fabricated in the following manner. A poly(3-hexylthiophene) (P3HT, regioregular, Sigma Aldrich Chemical Co., Inc.) and mix-PCBM (Frontier Carbon Co., Nanom spectra E124) solution with a donor:
acceptor ratio of 5
:
3 and a concentration of 40 mg ml−1 was prepared in ortho-dichlorobenzene (anhydrous, 99%, Sigma Aldrich Chemical Co., Inc.). The solution was left stirring for 2 h at 65 °C. The solution was then spin-coated on the ZnO layers at a speed of 850 rpm for 60 s to give films of approximately 300 nm. The films were then immediately placed in a Petri-dish for 40 minutes to allow slow evaporation of the solvent. A poly-(3,4-ethylenedioxythiophene)-polystyrenesulfonic acid (PEDOT:PSS) dispersion in water (Clevios P VP, Heraeus Precious Metals GmbH & Co.) containing 0.5 wt% polyoxyethylene(6) tridecyl ether (Sigma Aldrich Chemical Co., Inc.) was spin-coated on top of the active layer to form the hole transport layer with a 30 nm thickness. An approximately 200 nm thick Au layer was thermally evaporated at a pressure of 3 × 10−3 Pa, with the use of a shadow mask, which defined the device active area as 1 cm2. Finally, the devices were heated on a hot plate in air at a temperature of 150 °C for 5 min. All processes, except for Au deposition, were performed in air and the devices were not encapsulated.
SQ:PC71BM devices were fabricated on both ZnO NP and sol–gel films from a 20 mg ml−1 CHCl3 solution, where the SQ:PC71BM weight ratio was 1:
5. The solution was spin-coated at 3000 rpm for 60 s inside a N2 filled glovebox and the films were then annealed at 70 °C for 10 min. Following annealing, the films were transferred, without exposure to air, to a vacuum chamber, where MoO3 (8 nm) and Au (100 nm) were deposited by thermal evaporation. Devices were encapsulated before measuring their J–V curves. SQ was purchased from Sigma Aldrich and recrystallized by the slow diffusion of MeOH in a saturated solution of SQ in chloroform, following a previously reported method.14 PC71BM was bought from Solenne and was used as received.
Footnote |
† Electronic supplementary information (ESI) available: TEM images of ZnO NPs, wide-scan XPS spectra for each sample, and device performance of ZnO in ethylene glycol based solar cells. See DOI: 10.1039/c4ta04595e |
This journal is © The Royal Society of Chemistry 2014 |