Aqueous Solution Processed, Ultrathin ZnO Film with Low Conversion ...

Article pubs.acs.org/JPCC

Aqueous Solution Processed, Ultrathin ZnO Film with Low Conversion Temperature as the Electron Transport Layer in the Inverted Polymer Solar Cells Yawen Chen, Zhanhao Hu, Zhiming Zhong, Wen Shi, Junbiao Peng, Jian Wang,* and Yong Cao Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China S Supporting Information *

ABSTRACT: Ultrathin ZnO layer of ca. four nm thick deposited from an aqueous zinc oxide hydrate (ZnO·xH2O) solution is developed as the electron transport layer (ETL) in the inverted polymer solar cells (PSCs). Because of the low energy metal−ammine dissociation and hydroxide condensation/dehydration chemistry, a conversion temperature (TA) as low as 80 °C is achieved. With the active layer of poly(N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-3′,2′,1′-benzothiadiazole)) doped with [6,6]-phenyl C 7 1 butyric acid methyl ester (PCDTBT:PC71BM), the average power conversion efficiency of the devices is up to 6.48%, and the short circuit current density reaches 11.4 mA cm−2. In comparison, the devices with traditional sol−gel processed ZnO electron transport layer has a power conversion efficiency of 5.53% at an annealing temperature of 200 °C. The device performance improvement is attributed to the better charge transportation and smoother surface of the ZnO film deposited from ZnO·xH2O aqueous solution. The simple solution preparation and the facile device fabrication, combined with the low conversion temperature make the aqueous ZnO·xH2O one of the best candidates for ETL in the inverted PSCs fabricated on low cost, flexible substrates. temperature sensitive flexible substrates because the conversion temperature is often higher than 200 °C. Though solutionprocessed ZnO nanoparticles, which can be thermally converted to thin film at room temperature, emerged as an alternative,15,30,31 because of the unique physical characteristics of ZnO nanoparticles, they are unstable, and a ligand needs to be attached to the nanoparticle to avoid aggregation in solution. By using a precursor solution of carbon-free aqueous Zn(OH)x(NH3)y(2−x)+, the conversion temperature of solution-processed ZnO films was reduced to 150 °C.32−35 Unfortunately, the preparation of the ammine−hydroxo zinc complex requires several steps of purification to remove the counterions, which makes the preparation of the precursor solution complicated. In our contribution, an aqueous solution processed ZnO ETL with extremely low conversion temperature based on zinc oxide hydrate (ZnO·xH2O) has been successfully implemented in inverted PSCs. ZnO·xH2O possesses sufficient solubility in ammonia due to the crystal water. This probably leads to an ammine-hydroxo zinc complex similar to Zn(OH)x(NH3)y(2−x)+ in aqueous solution.36 Because of the low energy metal−ammine dissociation and hydroxide condensation/dehydration chemistry, a conversion temperature (TA) as

1. INTRODUCTION Recently, the bulk heterojunction (BHJ) polymer solar cells (PSCs) have attracted much attention in both academic institutions and industry due to their potentials in the application of lightweight, low cost, flexible, and renewable energy source.1−5 As the material synthetic chemistry and the device process progressively advance, the power conversion efficiency (PCE) of PSCs has exceeded 9% in a single junction cell.6−9 In a traditional device structure, the hole transport layer, the photoactive layer, and the top cathode are deposited sequentially on top of the transparent anode. The big challenge associated with such device structure is the long-term stability.10−13 To address the stability problem, inverted PSC device structure has been proposed and developed.14−27 In the inverted device structure, n-type metal oxides such as TiOx17,18 and ZnO19,20 are usually utilized as the electron transport layer (ETL) inserted between the transparent anode and the photoactive layer, to facilitate the electron extraction. Because of its low work function, high electron mobility, excellent optical transparency, and environmentally friendly nature, ZnO becomes the most widely used ETL in the inverted PSCs. Various techniques have been developed to deposit ZnO, such as atomic layer deposition, electrodeposition, spray pyrolysis, and sol−gel process,20,28,29 among which sol−gel process based on zinc acetate is the most popular. However, the sol−gel process is not compatible with inexpensive and © 2014 American Chemical Society

Received: June 30, 2014 Revised: August 25, 2014 Published: September 5, 2014 21819

dx.doi.org/10.1021/jp506463m | J. Phys. Chem. C 2014, 118, 21819−21825

The Journal of Physical Chemistry C

Article

Figure 1. (a) Device structure of the inverted PCDTBT:PC71BM solar cell. (b) Schematic illustrations of the energy level diagram of the device. (c) J−V characteristics of the solar cell devices with ZnO ETLs under AM 1.5G illumination. (d) J−V characteristics of the solar cell devices in dark.

low as 80 °C is achieved. With the photoactive layer of poly(N9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl3′,2′,1′-benzothiadiazole)):[6,6]-phenyl C71 butyric acid methyl ester (PCDTBT:PC71BM), an average PCE up to 6.48% is realized. In comparison, at a conversion temperature of 200 °C, the device with ZnO ETL deposited by sol−gel process based on zinc acetate has an average PCE of 5.53%. The device performance enhancement is attributed to the smoother ETL surface and better charge transportation of the ZnO ETL prepared by the aqueous solution. The low conversion temperature and the simple solution process make the aqueous ZnO·xH2O suitable in the inverted PSCs fabricated on low cost, flexible substrates.

solution was spin-coated on top of the substrate at 3000 rpm for 20 s, followed by annealing at 200 °C for 1 h in air. The thickness of the solid-state ZnO film was around 40 nm determined by a profilometer. The aqueous ZnO·xH2O solution was spin-coated on top of the substrate at 2000 rpm for 40 s, followed by 1 h annealing in air at difference temperatures, i.e., 60, 80, 100, 120, and 150 °C. The thicknesses of the ZnO films were about 4 nm determined by an ellipsometer. The solution of the photoactive layer was made by mixing PCDTBT and PC71BM with a 1:4 weight ratio first, then dissolving the mixture in a blended solvent of 1,2dichlorobenezene and chlorobenzene (3:1 by volume) to reach a concentration of 7 mg mL−1. The photoactive layer solution was spin-coated on top of the ZnO film at 1600 rpm for 40 s to achieve a thickness of around 70 nm. A thin layer of MoO3 (∼6 nm) followed by 100 nm of Al was deposited by thermal evaporation in a vacuum less than 3 × 10−6 Torr through a shadow mask to define the active area of 0.15 cm2. 2.3. Device Characterizations and Film Studies. The current density (J)−voltage (V) characteristics of the devices were measured using a Keithley 2400 Source Measure Unit and an Air Mass 1.5 Global (AM 1.5 G) solar simulator (SAN-EI corporation, XES-40S1 150 W, AAA class, Japan) with irradiation intensity of 100 mW cm−2. Tapping-mode atomic force microscopy (AFM) images were obtained through a NanoScope NS3A system (Digital Instruments). The XPS measurements were performed in a Kratos Analytical Axis Ultra DLD X-ray Photoelectron Spectroscope with a monochromatized Al Kα X-ray source (1486.6 eV) at a base pressure of 5 × 10−9 Torr. All recorded peaks were corrected for electrostatic effects by setting the C−C component of the C 1s peak to 284.8 eV.

2. EXPERIMENTAL SECTION 2.1. Materials. The ITO glasses were purchased from the China Southern Glass Holding Corp. PCDTBT and PC71BM were purchased from 1-Material Inc. All the solvents used in the study were purchased from Aldrich. MoO3 was purchased from Aldrich (99.9% purity). All chemicals and materials were purchased and used as received unless otherwise noted. The sol−gel ZnO precursor solution was prepared following the common route20 by dissolving 1 g of zinc acetate (Alfa Aesar, 99.98% purity) and 0.28 g of ethanolamine (Alfa Aesar, 99% purity) in 10 mL of 2-methoxyethanol (Alfa Aesar, 99% purity) under vigorous stirring. The aqueous ZnO precursor solution was prepared by dissolving ZnO·xH2O (Sigma-Aldrich, 97% purity) in ammonium hydroxide (Aladdin, ≥25% in H2O) with a concentration of 0.07 mmol mL−1. The solution was vigorously stirred, which eventually yielded a clear transparent Zn ammonium complex based solution. 2.2. Device Fabrication. ITO-coated glass substrates were first cleaned sequentially in an ultrasonic bath of acetone, isopropyl alcohol, detergent, deionized water, and isopropyl alcohol. After cleaning, the substrates were dried in an oven at 80 °C for 60 min under ambient condition prior to the precursor deposition. The sol−gel processed zinc acetate

3. RESULTS AND DISCUSSION The device structure of the inverted PCDTBT:PC71BM solar cells is schematically illustrated in Figure 1a. The energy level 21820



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Table 1. Device Performance of the Inverted PCDTBT:PC71BM Solar Cells with ZnO ETLs Deposited by Different Methods and at Different Annealing Temperatures device zinc acetate 200 °C ZnO·xH2O 60 °C ZnO·xH2O 80 °C ZnO·xH2O 100 °C ZnO·xH2O 120 °C ZnO·xH2O 150 °C

Jsc (mA cm−2) 10.4 10.1 11.4 11.5 11.5 11.5

± ± ± ± ± ±

0.06 0.46 0.02 0.06 0.06 0.05

Voc (V) 0.90 0.89 0.91 0.91 0.91 0.91

diagram of the inverted cell is depicted in Figure 1b. The J−V characteristics of the devices with the ZnO films deposited through different deposition conditions under AM 1.5 G irradiation (100 mW cm−2) and in dark are respectively shown in Figure 1c,d. In our experiment, 6 sets of 8 devices each were fabricated. The device performances are summarized in Table 1. Since the fabrication process of the reference device based on sol−gel processed zinc acetate solution followed the common route,20 the optimized ZnO film thickness of the reference device is about 40 nm. Because of the limitation of the sol−gel process, the reduction of the ZnO film thickness resulted in poor device performance. As a result, we kept the ZnO film thickness in the reference device as 40 nm to achieve the best device performance, while the ZnO film thickness in the device based on aqueous ZnO·xH2O solution is about 4 nm. To investigate the dependence of the device performance on the ZnO ETL thickness based on aqueous ZnO·xH2O solution, PSC devices with different ZnO film thickness were fabricated by depositing and annealing the aqueous ZnO·xH2O precursor several times before the deposition of the organic functional layers. The results are summarized in Table S1 in the Supporting Information. As the aqueous ZnO·xH2O ETL film thicknesses increase from 4 to 20 nm, the device performance does not change, which demonstrates that the improvement on the device performance of the aqueous ZnO· xH2O ETL compared to the sol−gel processed ZnO ETL is not due to the difference in film thickness. At 60 °C conversion temperature (TA), the performance of the device with aqueous ZnO·xH2O ETL is poor. The average PCE is only 3.38%, the fill factor (FF) is 37.4%, and the short circuit current density (JSC) is 10.1 mA cm−2. Increasing TA to 80 °C, the device performance is substantially enhanced with an average PCE of 6.48%, a FF of 62.4%, and a JSC of 11.4 mA cm−2. After further increasing TA to 150 °C, no further improvement on the device performance is observed. In comparison, the reference device with ZnO ETL based on sol− gel processed zinc acetate solution needs a conversion temperature of 200 °C. The device exhibits an average PCE of 5.53%, a FF of 59.3%, and a JSC of 10.4 mA cm−2. The increase of PCE from the zinc acetate device to the ZnO·xH2O device is mainly ascribed to JSC increasing from 10.4 to 11.4 mA cm−2. As shown in the dark J−V characteristics of the devices (Figure 1d), compared to the zinc acetate device, the ZnO· xH2O devices have a smaller leakage current and a higher rectification ratio except the device at TA of 60 °C, implying less charge recombination and more efficient charge extraction. Though the ZnO·xH2O device at TA of 60 °C has low leakage current, the current injection at high voltage is inferior to that of other devices. The J−V characteristics indicate that the aqueous ZnO·xH2O solution did not fully convert to ZnO at 60 °C.

± ± ± ± ± ±

0.01 0.01 0.01 0.01 0.01 0.01

FF (%) 59.3 37.4 62.3 62.3 62.4 62.2

± ± ± ± ± ±

1.23 2.83 0.46 0.54 0.47 0.51

PCE (%) 5.53 3.38 6.48 6.51 6.51 6.50

± ± ± ± ± ±

0.09 0.33 0.06 0.05 0.06 0.05

To verify the complete conversion from the solution into the solid film, the composition of the ZnO films deposited through different deposition conditions were characterized by XPS measurements. The XPS spectra of O 1s are shown in Figure 2.

Figure 2. O 1s XPS spectra of ZnO films deposited through different conditions.

The spectra exhibit asymmetric line shapes. The measured data were fitted by two Gaussian functions centered at 531.1 ± 0.1 and 532.7 ± 0.2 eV. The low binding energy peak is attributed to O atoms in the ZnO matrix, while the high binding energy peak is attributed to an oxygen-deficient component (Zn(OH)220 or Zn(OH)2(NH3)x37). At 60 °C, the spectrum is dominated by the high binding energy peak, showing that only a portion of the precursor is converted to ZnO. After TA increases to 80 °C, the low binding energy peak becomes dominant. Further increasing TA from 80 to 150 °C does not change the XPS spectra, showing that the ZnO·xH2O precursor completely converts to ZnO solid film at 80 °C. The sol−gel processed ZnO film annealed at 200 °C has the same XPS spectrum as the ZnO·xH2O converted film with TA at 80 °C or higher. The adverse impacts of the unconverted ZnO precursor on the electron transport was studied by electron-only devices with the structure of ITO/ZnO/PCDTBT:PC71BM/Ba/Al. The J− V characteristics are shown in Figure 3. Among all the devices, the ZnO·xH2O device at TA of 60 °C has the smallest electron current, which is expected due to the partial conversion of the precursor to the solid film. After the aqueous ZnO·xH2O precursor is completely converted to the solid ZnO at the temperature equal to or higher than 80 °C, the electron current increases by 1 order of magnitude. The field effect electron mobility of the ZnO solid film converted from ZnO·xH2O precursor at 100 °C has been demonstrated to be as high as 1 cm2 V−1 s−1.38 The high mobility is attributed to the carbonfree precursor route and its ability to form high-quality polycrystalline ZnO films.38 Moreover, the rapid, low-energy 21821



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were characterized by AFM. As show in Figure 4, smooth and uniform ZnO films are created by ZnO·xH2O precursor. The root-mean-square (RMS) roughness values are 1.36, 1.39, 1.40, 1.41, and 1.37 nm for TA at 60, 80, 100, 120, and 150 °C, respectively. Contrary to the ZnO films converted by the ZnO· xH2O precursor, the ZnO film deposited from zinc acetate has large grains and rough surface. The RMS roughness value is 2.46 nm. The dense and smooth ZnO films allow an intimate contact with the photoactive layer, while a rough surface may trap some voids leading to inferior contact.39 The smooth interface achieved by the ZnO·xH2O precursor facilitates the charges extracted from the active layer and transported through the ETL, thereby helping improve JSC and FF. Another source for the enhanced JSC is the total absorbed photons. By absorbing more photons, more excitons are created to be separated and transported out of the device, which will increase the JSC. The absorption spectra illustrated in Figure 5a show obvious difference between the zinc acetate and the ZnO·xH2O devices. For the zinc acetate device, the absorption in the 400−450 nm range is higher than that of ZnO·xH2O devices, while the absorption in the 470−600 nm range is lower. The absorption spectrum change is mainly caused by the ETL’s thickness. After the thickness of ZnO deposited by zinc acetate is reduced from 40 to 6 nm, the absorption spectra of all the devices are more or less the same as shown in Figure 5b. The dependence of the absorption spectrum on the device thickness is attributed to the microcavity effect.40,41 The total number of the absorbed photon was calculated by integrating the product of absorption and AM 1.5G spectra. On the basis of the devices’ absorption spectra (Figure 5a), the total absorbed photons are 1.04 × 1021 and 1.05 × 1021 for zinc acetate and ZnO·xH2O devices, respectively. The same total number of the absorbed photons

Figure 3. J−V characteristics of the electron-only devices with the device structure of ITO/ZnO/PCDTBT:PC71BM/Ba/Al.

kinetics of metal−ammine dissociation to form a dense oxide film leads to a relatively small, though nontrivial, defect density,32 which helps improve the mobility. Unfortunately, the ZnO solid film converted from sol−gel processed zinc acetate at 200 °C has a field effect electron mobility of only 4.0 × 10−3 cm2 V−1 s−1.20 As a result, the electron current of the zinc acetate device is much smaller than the fully converted ZnO· xH2O device. The higher the electron mobility, the higher the charge extraction efficiency, and the less charge recombination. Therefore, the ZnO·xH2O based ZnO ETL has an advantage on the electron transport and extraction, leading to higher JSC and better PCE. It is well-known that a smooth interface between the ETL and the active layer could reduce the interface defect states, which helps improve JSC and FF.39 The surface morphologies of ZnO films deposited through different deposition conditions

Figure 4. AFM images of the ZnO solid films deposited by (a) zinc acetate at 200 °C, (b) ZnO·xH2O at 60 °C, (c) ZnO·xH2O at 80 °C, (d) ZnO· xH2O at 100 °C, (e) ZnO·xH2O at 120 °C, and (f) ZnO·xH2O at 150 °C. 21822



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Figure 5. Absorption spectra of the inverted PCDTBT:PC71BM solar cells with (a) ZnO ETL layers deposited through different conditions and (b) ZnO ETL layers with different thicknesses.

completely converted to solid ZnO film. For zinc acetate devices, the low electron mobility and rough morphology of ZnO film reduce the short circuit current and the efficiency. The average PCE is 5.53% at an annealing temperature of 200 °C. The simple solution preparation and the facile device fabrication, combined with the low conversion temperature make the aqueous ZnO·xH2O one of the best candidates for ETL in the inverted PSCs fabricated on low cost, flexible substrates.

shows that the absorption makes little contribution to the improvement of Jsc in the ZnO·xH2O device. Efficient exciton dissociation can also help enhance the short circuit current. A large built-in potential induces a large internal electric field across the photoactive layer, which facilitates the charge separation. The built-in potentials of the devices were identified by plotting the photocurrent vs the operation voltage. The photocurrent Jph (Figure 6) was calculated by simply

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ASSOCIATED CONTENT

S Supporting Information *

Device performance of the PSC devices with different thickness of ZnO ETL based on the aqueous ZnO·xH2O. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(J.W.) Tel: +86 (20) 8711-4525. E-mail: [email protected]. cn. Notes

The authors declare no competing financial interest. Figure 6. Jph−V characteristics of PCDTBT:PC71BM solar cells under AM 1.5G illumination at 100 mW cm−2.

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subtracting the dark current (Figure 1d) from the total lightinduced current (Figure 1c).42,43 The built-in potential (Vbi) of all the devices is the same (0.96 V) except the ZnO·xH2O device (0.94 V) with ZnO precursor film annealed at 60 °C. The Vbi results are consistent with Voc values listed in Table 1. The same Vbi suggests that the high Jsc of the ZnO·xH2O devices is not caused by the improvement of the exciton dissociation efficiency.

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ACKNOWLEDGMENTS The authors are deeply grateful to the Ministry of Science and Technology (973 Program 2009CB623604 and 2009CB930604) and National Nature Science Foundation of China (61079116 and 51373057) for their financial supports. REFERENCES

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4. CONCLUSIONS Inverted polymer solar cells based on PCDTBT:PC71BM with ZnO electron transport layer deposited from the zinc oxide hydrate ZnO·xH2O aqueous solution exhibit higher power conversion efficiency and larger short circuit current density compared to the devices with ZnO deposited by the sol−gel processed zinc acetate. For ZnO·xH2O devices, the average power conversion efficiency reaches 6.48% with TA as low as 80 °C. Below 80 °C, the ZnO·xH2O aqueous precursor cannot be 21823



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