Modulation of PEDOT:PSS pH for Efficient Inverted Perovskite Solar

Nov 2, 2016 - Search; Citation; Subject .... Modulation of PEDOT:PSS pH for Efficient Inverted Perovskite Solar Cells with Reduced Potential Loss and ...
0 downloads 0 Views 7MB Size
Download PDF
Research Article www.acsami.org

Modulation of PEDOT:PSS pH for Efficient Inverted Perovskite Solar Cells with Reduced Potential Loss and Enhanced Stability Qin Wang,†,‡ Chu-Chen Chueh,† Morteza Eslamian,*,‡ and Alex K.-Y. Jen*,† †

Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98105, United States University of MichiganShanghai Jiao Tong University Joint Institute, Shanghai 200240, China

‡

S Supporting Information *

ABSTRACT: Inverted p-i-n perovskite solar cells (PVSCs) using PEDOT:PSS as the hole-transporting layer (HTL) is one of the most widely adopted device structures thus far due to its facile processability and good compatibility for high throughput manufacturing processes. However, most of the PEDOT:PSS-based CH3NH3PbI3 PVSCs reported to date suffered an inferior open-circuit voltage (VOC) (0.88−0.95 V) compared to that (1.05−1.12 V) obtained for common CH3NH3PbI3 PVSCs, revealing a severe potential loss issue. Herein, we describe a simple method to alleviate this problem by tuning the pH value of PEDOT:PSS with a mild base, imidazole. Accompanied by the pH modulation, the blended imidazole concurrently tailors the surface texture and electronic properties of PEDOT:PSS to promote the quality and crystallization of the perovskite film deposited on top of it and enable better energy-level alignment at this corresponding interface. Consequently, the PVSC using this modified PEDOT:PSS HTL yields an enhanced power conversion efficiency (PCE) of 15.7% with an enlarged VOC of 1.06 V and improved long-term stability. These outperform the pristine device showing a PCE of 12.7% with a much smaller VOC of 0.88 V and unsatisfactory environmental stability. KEYWORDS: PEDOT:PSS, pH, potential loss, environmental stability, perovskite solar cells

1. INTRODUCTION With the astonishing progress made in photovoltaic performance, organic−inorganic hybrid perovskite based solar cells (PVSCs) have been considered as a viable candidate for next generation photovoltaics.1 As identified in the literature, the hybrid perovskites possess exceptional semiconducting properties inherent to inorganic materials, such as intense light harvesting capability,2 small exciton binding energy (∼kT),3 long carrier diffusion length (100−1000 nm),4 and high charge carrier mobility5 while holding the simple solution processability at a mild temperature (∼120 °C) inherent to organic materials.6 In principle, the corresponding bandgap (Eg) of the photoactive material determines the upper limit of the achievable open-circuit voltage (VOC). According to the Shockley−Queisser theory,7 there exists intrinsically thermodynamic constraints for materials to result in a minimum potential loss of 250−300 mV, making the achievable VOC always lower than the Eg of the photoactive absorber. However, the nonradiative charge recombination within the photoactive materials and associated interfacial barrier in the stratified devices8−10 also contribute additional potential losses to the device, further reducing the resultant VOC. Therefore, it is critical to pursue material systems with minimal potential loss for developing high-performance solar cell devices. Encouragingly, PVSC has been shown to have the potential of achieving low potential loss ( n-PEDOT:PSS > a-PEDOT:PSS, suggesting better hole transfer efficiency at the b-PEDOT:PSS/perovskite interface than the others, which is beneficial for achieving highperformance PVSCs. This enhancement observed for bPEDOT:PSS might be due to the aligned energy levels and interactions between itself and MAPbI3.

influence of PEDOT:PSS in different pH levels on the structures and characteristics of perovskite deposited on top of it. The morphology of the deposited perovskite films were first examined and all the films showed compact surface morphology with decent surface coverage as presented in Figure 4a−c. Nevertheless, the perovskite film deposited on aPEDOT:PSS exhibits few pinholes (red circles in Figure 4a), which might increase the risk of device shorting.20,46 Considering that all the studied perovksite films have similar thickness, the resembling UV−vis absorption spectra shown in Figure S4 thus indicate their similar morphology and film quality. However, despite the similar compact morphology, the grain size for each film is slightly different. Illustrated in Figure 4d is the in-plane grain size distribution of the light green region shown in Figure 4a−c. The average grain sizes of the perovskite film deposited on a- and n-PEDOT:PSS are 246 and 221 nm, respectively, while the size of the film grown on b-PEDOT:PSS increases to 297 nm. The increase in crystal grain size can be attributed to the lower surface energy of b-PEDOT:PSS (Figure S1), which promotes the vertical crystal growth and 32072

DOI: 10.1021/acsami.6b11757 ACS Appl. Mater. Interfaces 2016, 8, 32068−32076

Research Article

ACS Applied Materials & Interfaces

Figure 6. Normalized (a) PCE, (b) Voc, (c) FF, and (d) Jsc of PSCs based on a-PEDOT:PSS, n-PEDOT:PSS, and b-PEDOT:PSS thin films over 32 devices as a function of storage time in ambient condition (air) with 20% relative humidity.

2.3. Device Performance of Derived PVSCs. To realize the efficacy of different PEDOT:PSS serving as the HTL in PVSC, an inverted planar device consisting of ITO/studied PEDOT:PSS/MAPbI3/PCBM/bis-C60/Ag12 was fabricated as illustrated in Figure 1. The fabrication details are described in the Experimental Section. A cross-sectional SEM of the fabricated device excluding the cathode was displayed in the inset of Figure 5a, showing a discrete multilayer structure with obvious interfaces. The thickness of the studied PEDOT:PSS, MAPbI3, and PCBM films is estimated to be 30, 260, and 80 nm, respectively. The current density−voltage (J−V) curves of the fabricated PVSCs were tested under AM 1.5G illumination at 100 mW/ cm2 (Figure 5a) and their corresponding device parameters such as VOC, short-circuit current (JSC), and fill factor (FF) are listed in Table 1. As can be clearly seen, the b-PEDOT:PSS PVSC delivered the best PCE of 15.7% (under reverse scan) among the studied PVSCs (the PCE under reverse scan for the a- and n-PEDOT:PSS devices is 12.7% and 12.4%, respectively). It represents an impressive 20% enhancement in PCE compared to the regular PEDOT:PSS PVSC, wherein the significant increase of VOC is the major contributing factor. The VOC of the device increases from 0.88 to 1.06 V when the regular a-PEDOT:PSS HTL is replaced by b-PEDOT:PSS HTL, demonstrating a much reduced potential loss of the fabricated device. This diminished potential loss can be attributed to the better energy-level alignment at b-PEDOT:PSS/MAPbI3 interface

(Figure 3b). Originally, the WF of regular a-PEDOT:PSS is 4.91 eV, possessing a large energy level offset (0.52 eV) with MAPbI3. However, after hybridizing with imidazole, the WF of PEDOT:PSS increases and thereby curtails the energy level offset at this corresponding interface, leading to the improved VOC. As introduced earlier, the associated ionic interactions22 and the inferior pervoskite crystallization at PEDOT:PSS/ pervoskite interface23 might also play critical roles in causing the potential loss observed in PEDOT:PSS-based inverted PVSC. Therefore, we suspect the promoted crystallization of perovskite on b-PEDOT:PSS and its modified surface texture also contribute to the reduced potential loss. Besides the increased VOC, the JSC is also increased from 18.2 mA/cm2 (a-PEDOT:PSS) to 19.1 mA/cm2 (b-PEDOT:PSS). The external quantum efficiency (EQE) spectra shown in Figure 5b illustrates the higher photon response of bPEDOT:PSS PVSC in the range between 600 and 800 nm, which can be attributed to the higher transmittance of bPEDOT:PSS in this region (Figure 2b) arising from the imidazole-induced redistribution of polaron/bipolaron as discussed previously. Table 1 compares the JSC calculated from EQE and the JSC obtained in J−V curve, for which a very good agreement is observed. Figure S6 presents the device performance of the champion b-PEDOT:PSS PVSC under different scan directions at a scan rate of 0.1 V s−1. As can be seen, same PCE of 15.7% was obtained regardless of the scan direction, revealing its negligible hysteresis. To further confirm the reliability of the as-prepared 32073

DOI: 10.1021/acsami.6b11757 ACS Appl. Mater. Interfaces 2016, 8, 32068−32076

Research Article

ACS Applied Materials & Interfaces

3. CONCLUSIONS In summary, a simple and generally applicable method is developed to minimize the potential loss commonly encountered in the inverted PEDOT:PSS-based PVSCs by modulating the pH value of PEDTO:PSS. Imidazole is employed as a dedoping additive to tune the pH value of PEDOT:PSS. It was found that the blended imidazole concurrently modulates the surface texture and electrical properties of PEDOT:PSS to not only promote the quality and crystallization of the perovskite film grown on top of it but also enable better energy-level alignment at this interface. Consequently, the PVSC using the basic PEDOT:PSS as HTL can yield an enhanced PCE of 15.7% with an enlarged VOC of 1.06 V, outperforming the pristine device with a PCE of 12.7% and a much smaller VOC of 0.88 V. More importantly, the basic PEDOT:PSS can suppress the indium diffusion into the active layer to significantly enhance the enviornmental stability of its derived device, which can maintains 75% of its original PCE and 95% of original VOC after 14-day storage in ambient condition with a controlled 20% relative humidity, which is superior to the pristine device that degraded quickly to 38% of its original PCE under the same condition.

devices, we measured the stabilized PCE at a bias around the maximum power point voltage (0.75 V for a-PEDOT:PSS, 0.76 V for n-PEDOT:PSS, and 0.88 V for b-PEDOT:PSS), and the data were plotted in Figure 5c, where steady-state PCEs consistent with the value obtained in J−V curves were achieved, affirming the accuracy of the performance. The PCE histograms of the studied PVSCs with Gaussian fitting are displayed in Figure 5d. It apparently unveils the enhanced PCE of the b-PEDOT:PSS device, in which an average PCE of 14.0% over 32 devices is showed (Table 1). Besides, 60% of the fabricated b-PEDOT:PSS devices can yield a PCE larger than 14%, substantiating the reproducibility of the results. Moreover, a summary of the state-of-the-art inverted PEDOT:PSS based PVSCs from this work and the literature is provided in Figure S7 and Table S1 to highlight the significance of reduced potential loss demonstrated in this work. 2.4. Ambient Stability of PVSCs Using Different PEDOT:PSS. Inferior ambient stability of PVSCs has been identified as a critical issue to hinder its practical applications and commercialization.47 To study the ambient stability of the studied PVSCs using different PEDOT:PSS HTLs, all devices were put in a drybox attached with a hygrometer to monitor the humidity, in which CaO powders were used to maintain a 20% relative humidity without encapsulation in the dark environment and then tested in the glovebox under the condition of one-sun illumination every 24 h. The device performance parameters versus storage time are recorded in Figure 6a−d, respectively. Although the inferior stability of PVSC is partly attributed to the degradation of perovskite layer and the corrosion of the metal electrode, a striking difference in degradation speed of the PVSCs using different PEDOT:PSS HTLs is clearly observed. The PCE of a pristine a-PEDOT:PSS device quickly decreased to 38% of its original PCE after 2 weeks. In contrast, the n- and b-PEDOT:PSS devices showed much higher ambient stability, especially for the b-PEDOT:PSS cell that maintains 75% of its original PCE and 95% of original Voc after 2 weeks. The reason behind the declined PCEs of all devices mainly stems from the fast decrease of JSC, which is relevant to the degradation of CTLs or the perovskite layer. This result is similar to previous works in the OPV field that showed the same phenomenon of JSC decrease.37 A difference in the device degradation rate is thus stronly correlated to the empolyed PEDOT:PSS HTLs. In OPVs, one of the degradation mechanisms induced by PEDOT:PSS has been identified to originate from the corrosion of ITO by its acidity48 providing the constituent PSS− is a strong acid and can react with In2O3. As a result, the dissociated indium ions can diffuse into active layer and deteriorate the stability of the device.37 Bearing this in mind, we examined the XPS data of the PEDOT:PSS/ITO samples after 3 days (Figure S8). As summarized in Table S2, the atomic concentration of indium in a-, n-, and b-PEDOT:PSS are calculated to be around 14.35%, 0.26%, and 0.09%, respectively. This clearly shows that the diffusion of indium into PEDOT:PSS strongly depends on the pH value of PEDOT:PSS. For a-PEDOT:PSS PVSC, the acidity nature of PEDOT:PSS accelerates the indium diffusion to incur gap states at associated interfaces as the charge trapping sites to reduce the extracted photocurrent.49−51 The indium contamination52 is apparently inhibited while using bPEDOT:PSS, leading to the much improved ambient stability as observed.

4. EXPERIMENTAL SECTION 4.1. Materials. Methylammonium iodide was synthesized according to the reported literature.53 PEDOT:PSS (4083) was purchased from Heraeus Company. Bis-C60 was synthesized based on our previous work.12 All the other materials used in the experiment were bought from Sigma-Aldrich, U.S.A. without any further purification. 4.2. Preparation of PEDOT:PSS with Different pH. The PEDOT:PSS solutions with different pH were prepared by adding different weight ratios of imidazole and then stirred overnight. For aPEDOT:PSS (acidic), n-PEDOT:PSS (neutral) and b-PEDOT:PSS (base), the weight ratio of imidazole was 0, 0.5, and 5 wt %, respectively. 4.3. Solar Cell Fabrication. ITO glass substrates were cleaned in detergent, deionized water, acetone, and isopropanol (IPA), respectively, for 10 min, via ultrasonication before use. UV-Ozone was used to treat the substrates for 15 min. PEDOT:PSS solution with different pH filtered by nylon filter (0.45 μm) was spin-coated on these cleaned substrates at the speed of 5000 rpm for 30 s, followed by annealing at 150 °C for 15 min in air. The substrates coated with different PEDOT:PSS were taken into a nitrogen atmosphere glovebox for perovskite deposition. The MAPbI3 solution was obtained by adding 1 M MAI with 1 M lead(II) iodide (PbI2) in 700 μL γ-butyrolactone and 300 μL dimethyl sulfoxide. The filtered MAPbI3 precursor solution (0.45 μm PTFE membrane) was then spin-coated onto different modified PEDOT:PSS thin films at the speed of 1000 rpm for 15 s and 4000 rpm for 45 s. After 20 s had passed in the second stage of spin coating, 500 μL dry toluene was dropped onto the rotating film. The as-spun film was then annealed at 100 °C for 10 min. Next, filtered 20 mg mL−1 PCBM in chlorobenzene solution was spin-coated onto the MAPbI3 layer at the speed of 1500 rpm for 60 s. Afterward, filtered bis-C60 solution (2 mg mL−1 in IPA) was spin-coated onto the PCBM at the speed of 3000 rpm for 60 s. Finally, a 150-nm thick top silver electrode was thermally evaporated under high vacuum (around 1 × 10−7 Torr). The device area was 3.14 mm2, defined by an iron-made mask with nine apertures. 4.4. Characterization. The pH values of modified PEDOT:PSS solutions were measured by Oakton pH 700 Benchtop Meter. Conductivity of PEDOT:PSS with different pH was measured by constructing ITO/prepared PEDOT:PSS/Ag device. AFM images were obtained under tapping mode using a Veeco multimode instrument with a Nanoscope III controller. UV−vis spectra were recorded using a Varian Caty5000 spectrophotometer. XPS spectra and secondary electron cutoff were measured using a PHI Versaprobe 32074

DOI: 10.1021/acsami.6b11757 ACS Appl. Mater. Interfaces 2016, 8, 32068−32076

Research Article

ACS Applied Materials & Interfaces system with an Al Kα X-ray source and the beam size is 100 μm. All the samples were measured under ultrahigh vacuum of around 10−10 Torr. The work function of studied PEDOT:PSS films was obtained from UPS. XRD patterns were characterized by an X-ray diffractometer (Bruker D8 Discover). SEM images were obtained by FEI corporation. The steady-state PL spectra were obtained by Horiba Fluorolog FL-3 and the excitation wavelength is set at 550 nm for the perovskite films on glass substrates. For the J−V measurements, the light intensity is calibrated through a standard silicon photodiode detector mounted with a KG-5 filter that is calibrated with the standard cell of the National Renewable Energy Laboratory (NREL). All the J−V curves were obtained using a Keithley 2400 source meter. The EQE spectra were obtained by setting up a monochromatic incident photon-to-electron device which contains a 450 W xenon lamp (Oriel), a monochromator, a chopper (400 Hz), and a lock-in amplifier (SR 830, Sanford Research Corp). A calibrated Si photodiode (J115711-1 Si detector) is used for calibration.

■

(6) Williams, S. T.; Rajagopal, A.; Chueh, C. C.; Jen, A. K. Current Challenges and Prospective Research for Upscaling Hybrid Perovskite Photovoltaics. J. Phys. Chem. Lett. 2016, 7, 811−819. (7) Shockley, W.; Queisser, H. J. Detailed Balance Limit of Efficiency of P-N Junction Solar Cells. J. Appl. Phys. 1961, 32, 510−519. (8) Chueh, C.-C.; Li, C.-Z.; Jen, A. K. Y. Recent Progress and Perspective in Solution-Processed Interfacial Materials for Efficient and Stable Polymer and Organometal Perovskite Solar Cells. Energy Environ. Sci. 2015, 8, 1160−1189. (9) Cowan, S. R.; Roy, A.; Heeger, A. J. Recombination in PolymerFullerene Bulk Heterojunction Solar Cells. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 245207. (10) Elumalai, N. K.; Uddin, A. Open Circuit Voltage of Organic Solar Cells: An in-Depth Review. Energy Environ. Sci. 2016, 9, 391− 410. (11) Kim, H.; Lim, K.-G.; Lee, T.-W. Planar Heterojunction Organometal Halide Perovskite Solar Cells: Roles of Interfacial Layers. Energy Environ. Sci. 2016, 9, 12−30. (12) Liang, P.-W.; Chueh, C.-C.; Williams, S. T.; Jen, A. K. Y. Roles of Fullerene-Based Interlayers in Enhancing the Performance of Organometal Perovskite Thin-Film Solar Cells. Adv. Energy Mater. 2015, 5, 1402321. (13) Kaltenbrunner, M.; Adam, G.; Glowacki, E. D.; Drack, M.; Schwodiauer, R.; Leonat, L.; Apaydin, D. H.; Groiss, H.; Scharber, M. C.; White, M. S.; Sariciftci, N. S.; Bauer, S. Flexible High Power-PerWeight Perovskite Solar Cells with Chromium Oxide-Metal Contacts for Improved Stability in Air. Nat. Mater. 2015, 14, 1032−1039. (14) Malinkiewicz, O.; Yella, A.; Lee, Y. H.; Espallargas, G. M.; Graetzel, M.; Nazeeruddin, M. K.; Bolink, H. J. Perovskite Solar Cells Employing Organic Charge-Transport Layers. Nat. Photonics 2013, 8, 128−132. (15) Xue, Q.; Chen, G.; Liu, M.; Xiao, J.; Chen, Z.; Hu, Z.; Jiang, X.F.; Zhang, B.; Huang, F.; Yang, W.; Yip, H.-L.; Cao, Y. Improving Film Formation and Photovoltage of Highly Efficient Inverted-Type Perovskite Solar Cells through the Incorporation of New Polymeric Hole Selective Layers. Adv. Energy Mater. 2016, 6, 1502021. (16) Lim, K.-G.; Ahn, S.; Kim, Y.-H.; Qi, Y.; Lee, T.-W. Universal Energy Level Tailoring of Self-Organized Hole Extraction Layers in Organic Solar Cells and Organic−Inorganic Hybrid Perovskite Solar Cells. Energy Environ. Sci. 2016, 9, 932−939. (17) Lim, K.-G.; Ahn, S.; Kim, H.; Choi, M.-R.; Huh, D. H.; Lee, T.W. Self-Doped Conducting Polymer as a Hole-Extraction Layer in Organic-Inorganic Hybrid Perovskite Solar Cells. Adv. Mater. Interfaces 2016, 3, 1500678. (18) Huang, X.; Wang, K.; Yi, C.; Meng, T.; Gong, X. Efficient Perovskite Hybrid Solar Cells by Highly Electrical Conductive Pedot:Pss Hole Transport Layer. Adv. Energy Mater. 2016, 6, 1501773. (19) Zhao, D.; Sexton, M.; Park, H.-Y.; Baure, G.; Nino, J. C.; So, F. High-Efficiency Solution-Processed Planar Perovskite Solar Cells with a Polymer Hole Transport Layer. Adv. Energy Mater. 2015, 5, 1401855. (20) Zhang, H.; Cheng, J.; Lin, F.; He, H.; Mao, J.; Wong, K. S.; Jen, A. K.; Choy, W. C. Pinhole-Free and Surface-Nanostructured Niox Film by Room-Temperature Solution Process for High-Performance Flexible Perovskite Solar Cells with Good Stability and Reproducibility. ACS Nano 2016, 10, 1503−1511. (21) Lee, T.-W.; Chung, Y. Control of the Surface Composition of a Conducting-Polymer Complex Film to Tune the Work Function. Adv. Funct. Mater. 2008, 18, 2246−2252. (22) Zhang, Y.; Liu, M.; Eperon, G. E.; Leijtens, T. C.; McMeekin, D.; Saliba, M.; Zhang, W.; de Bastiani, M.; Petrozza, A.; Herz, L. M.; Johnston, M. B.; Lin, H.; Snaith, H. J. Charge Selective Contacts, Mobile Ions and Anomalous Hysteresis in Organic−Inorganic Perovskite Solar Cells. Mater. Horiz. 2015, 2, 315−322. (23) Bi, C.; Wang, Q.; Shao, Y.; Yuan, Y.; Xiao, Z.; Huang, J. NonWetting Surface-Driven High-Aspect-Ratio Crystalline Grain Growth for Efficient Hybrid Perovskite Solar Cells. Nat. Commun. 2015, 6, 7747. (24) Yan, W.; Li, Y.; Li, Y.; Ye, S.; Liu, Z.; Wang, S.; Bian, Z.; Huang, C. High-Performance Hybrid Perovskite Solar Cells with Open Circuit

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b11757. Surface energy, optical property, morphology of different PEDOT:PSS films, relative device performance, and additional references (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.E.). *E-mail: [email protected] (A.K.-Y.J.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was partially supported by the Office of Naval Research (N00014-14-1-0246), the National Science Foundation (DMR-1608279), the Department of Energy SunShot (DE-EE0006710), and the Asian Office of Aerospace R&D (FA2386-15-1-4106). Q.W. thanks the financial support from the China Scholarship Council (CSC N 201506230080). A.K.Y.J. is thankful for financial support from the Boeing−Johnson Foundation. The authors would like to thank Fatemeh Zabihi, University of MichiganShanghai Jiao Tong University Joint Institute, for performing the XPS and UPS analyses.

■

REFERENCES

(1) Polman, A.; Knight, M.; Garnett, E. C.; Ehrler, B.; Sinke, W. C. Photovoltaic Materials: Present Efficiencies and Future Challenges. Science 2016, 352, aad4424. (2) Ziang, X.; Shifeng, L.; Laixiang, Q.; Shuping, P.; Wei, W.; Yu, Y.; Li, Y.; Zhijian, C.; Shufeng, W.; Honglin, D.; Minghui, Y.; Qin, G. G. Refractive Index and Extinction Coefficient of CH3NH3PbI3 Studied by Spectroscopic Ellipsometry. Opt. Mater. Express 2015, 5, 29−43. (3) Miyata, A.; Mitioglu, A.; Plochocka, P.; Portugall, O.; Wang, J. T.W.; Stranks, S. D.; Snaith, H. J.; Nicholas, R. J. Direct Measurement of the Exciton Binding Energy and Effective Masses for Charge Carriers in Organic-Inorganic Tri-Halide Perovskites. Nat. Phys. 2015, 11, 582−587. (4) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in An Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341− 344. (5) Gratzel, M. The Light and Shade of Perovskite Solar Cells. Nat. Mater. 2014, 13, 838−842. 32075

DOI: 10.1021/acsami.6b11757 ACS Appl. Mater. Interfaces 2016, 8, 32068−32076

Research Article

ACS Applied Materials & Interfaces Voltage Dependence on Hole-Transporting Materials. Nano Energy 2015, 16, 428−437. (25) Kim, J. H.; Liang, P. W.; Williams, S. T.; Cho, N.; Chueh, C. C.; Glaz, M. S.; Ginger, D. S.; Jen, A. K. High-Performance and Environmentally Stable Planar Heterojunction Perovskite Solar Cells Based on a Solution-Processed Copper-Doped Nickel Oxide HoleTransporting Layer. Adv. Mater. 2015, 27, 695−701. (26) Wang, Z. K.; Gong, X.; Li, M.; Hu, Y.; Wang, J. M.; Ma, H.; Liao, L. S. Induced Crystallization of Perovskites by a Perylene Underlayer for High-Performance Solar Cells. ACS Nano 2016, 10, 5479−5489. (27) Hou, F.; Su, Z.; Jin, F.; Yan, X.; Wang, L.; Zhao, H.; Zhu, J.; Chu, B.; Li, W. Efficient and Stable Planar Heterojunction Perovskite Solar Cells with an Moo3/Pedot:Pss Hole Transporting Layer. Nanoscale 2015, 7, 9427−9432. (28) Yan, W.; Ye, S.; Li, Y.; Sun, W.; Rao, H.; Liu, Z.; Bian, Z.; Huang, C. Hole-Transporting Materials in Inverted Planar Perovskite Solar Cells. Adv. Energy Mater. 2016, 6, 1600474. (29) Yip, H.-L.; Jen, A. K. Y. Recent Advances in Solution-Processed Interfacial Materials for Efficient and Stable Polymer Solar Cells. Energy Environ. Sci. 2012, 5, 5994−6011. (30) Lim, K.-G.; Kim, H. B.; Jeong, J.; Kim, H.; Kim, J. Y.; Lee, T.-W. Boosting the Power Conversion Efficiency of Perovskite Solar Cells Using Self-Organized Polymeric Hole Extraction Layers with High Work Function. Adv. Mater. 2014, 26, 6461−6466. (31) Tsai, T.-C.; Chang, H.-C.; Chen, C.-H.; Huang, Y.-C.; Whang, W.-T. A Facile Dedoping Approach for Effectively Tuning Thermoelectricity and Acidity of Pedot:Pss Films. Org. Electron. 2014, 15, 641−645. (32) Tehrani, P.; Kanciurzewska, A.; Crispin, X.; Robinson, N.; Fahlman, M.; Berggren, M. The Effect of Ph on the Electrochemical over-Oxidation in Pedot:Pss Films. Solid State Ionics 2007, 177, 3521− 3527. (33) Chen, S.; Song, L.; Tao, Z.; Shao, X.; Huang, Y.; Cui, Q.; Guo, X. Neutral-Ph Pedot:Pss as over-Coating Layer for Stable Silver Nanowire Flexible Transparent Conductive Films. Org. Electron. 2014, 15, 3654−3659. (34) Bhatnagar, A.; Sharma, P.; Kumar, N. A Review on “Imidazoles”: Their Chemistry and Pharmacological Potentials. Int. J. PharmTech Res. 2011, 3, 268−282. (35) Kim, S.; Kim, S. Y.; Chung, M. H.; Kim, J.; Kim, J. H. A OneStep Roll-to-Roll Process of Stable Agnw/Pedot:Pss Solution Using Imidazole as a Mild Base for Highly Conductive and Transparent Films: Optimizations and Mechanisms. J. Mater. Chem. C 2015, 3, 5859−5868. (36) Cho, A.; Kim, S.; Kim, S.; Cho, W.; Park, C.; Kim, F. S.; Kim, J. H. Influence of Imidazole-Based Acidity Control of Pedot:Pss on Its Electrical Properties and Environmental Stability. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1530−1536. (37) Meng, Y.; Hu, Z.; Ai, N.; Jiang, Z.; Wang, J.; Peng, J.; Cao, Y. Improving the Stability of Bulk Heterojunction Solar Cells by Incorporating Ph-Neutral Pedot:Pss as the Hole Transport Layer. ACS Appl. Mater. Interfaces 2014, 6, 5122−5129. (38) Gilot, J.; Wienk, M. M.; Janssen, R. A. Optimizing Polymer Tandem Solar Cells. Adv. Mater. 2010, 22, E67−71. (39) Sheng Hsiung, C.; Chien-Hung, C.; Feng-Sheng, K.; Chuen-Lin, T.; Chun-Guey, W. Unraveling the Enhanced Electrical Conductivity of Pedot:Pss Thin Films for Ito-Free Organic Photovoltaics. IEEE Photonics J. 2014, 6, 1−7. (40) de Kok, M. M.; Buechel, M.; Vulto, S. I. E.; van de Weijer, P.; Meulenkamp, E. A.; de Winter, S. H. P. M.; Mank, A. J. G.; Vorstenbosch, H. J. M.; Weijtens, C. H. L.; van Elsbergen, V. Modification of Pedot:Pss as Hole Injection Layer in Polymer Leds. Phys. Status Solidi 2004, 201, 1342−1359. (41) Crispin, X.; Jakobsson, F. L. E.; Crispin, A.; Grim, P. C. M.; Andersson, P.; Volodin, A.; van Haesendonck, C.; Van der Auweraer, M.; Salaneck, W. R.; Berggren, M. The Origin of the High Conductivity of Poly(3,4-Ethylenedioxythiophene)−Poly-

(Styrenesulfonate) (Pedot−Pss) Plastic Electrodes. Chem. Mater. 2006, 18, 4354−4360. (42) Hwang, J.; Schwendeman, I.; Ihas, B.; Clark, R.; Cornick, M.; Nikolou, M.; Argun, A.; Reynolds, J.; Tanner, D. In Situ Measurements of the Optical Absorption of Dioxythiophene-Based Conjugated Polymers. Phys. Rev. B 2011, 83, 195121. (43) Greczynski, G.; Kugler, T.; Keil, M.; Osikowicz, W.; Fahlman, M.; Salaneck, W. R. Photoelectron Spectroscopy of Thin Films of Pedot−Pss Conjugated Polymer Blend: A Mini-Review and Some New Results. J. Electron Spectrosc. Relat. Phenom. 2001, 121, 1−17. (44) Timpanaro, S.; Kemerink, M.; Touwslager, F. J.; De Kok, M. M.; Schrader, S. Morphology and Conductivity of Pedot/Pss Films Studied by Scanning−Tunneling Microscopy. Chem. Phys. Lett. 2004, 394, 339−343. (45) Williams, S. T.; Chueh, C. C.; Jen, A. K. Navigating OrganoLead Halide Perovskite Phase Space Via Nucleation Kinetics toward a Deeper Understanding of Perovskite Phase Transformations and Structure-Property Relationships. Small 2015, 11, 3088−3096. (46) Liang, P. W.; Liao, C. Y.; Chueh, C. C.; Zuo, F.; Williams, S. T.; Xin, X. K.; Lin, J.; Jen, A. K. Additive Enhanced Crystallization of Solution-Processed Perovskite for Highly Efficient Planar-Heterojunction Solar Cells. Adv. Mater. 2014, 26, 3748−3754. (47) Zhu, Z.; Chueh, C.-C.; Lin, F.; Jen, A. K. Y. Enhanced Ambient Stability of Efficient Perovskite Solar Cells by Employing a Modified Fullerene Cathode Interlayer. Adv. Sci. 2016, 3, 1600027. (48) Jørgensen, M.; Norrman, K.; Krebs, F. C. Stability/Degradation of Polymer Solar Cells. Sol. Energy Mater. Sol. Cells 2008, 92, 686−714. (49) Williams, S. T.; Rajagopal, A.; Chueh, C.-C.; Jen, A. K. Current Challenges and Prospective Research for Upscaling Hybrid Perovskite Photovoltaics. J. Phys. Chem. Lett. 2016, 7, 811−819. (50) Lee, S. T.; Gao, Z. Q.; Hung, L. S. Metal Diffusion from Electrodes in Organic Light-Emitting Diodes. Appl. Phys. Lett. 1999, 75, 1404−1406. (51) Wong, K. W.; Yip, H. L.; Luo, Y.; Wong, K. Y.; Lau, W. M.; Low, K. H.; Chow, H. F.; Gao, Z. Q.; Yeung, W. L.; Chang, C. C. Blocking Reactions between Indium-Tin Oxide and Poly (3,4-Ethylene Dioxythiophene):Poly(Styrene Sulphonate) with a Self-Assembly Monolayer. Appl. Phys. Lett. 2002, 80, 2788−2790. (52) Schlatmann, A. R.; Floet, D. W.; Hilberer, A.; Garten, F.; Smulders, P. J. M.; Klapwijk, T. M.; Hadziioannou, G. Indium Contamination from the Indium−Tin−Oxide Electrode in Polymer Light-Emitting Diodes. Appl. Phys. Lett. 1996, 69, 1764−1766. (53) Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316−319.

32076

DOI: 10.1021/acsami.6b11757 ACS Appl. Mater. Interfaces 2016, 8, 32068−32076