Low-Temperature Solution-Processed Thiophene-Sulfur-Doped

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Low-Temperature Solution-Processed Thiophene-SulfurDoped Planar ZnO Nanorods as Electron Transporting Layers for Enhanced Performance of Organic Solar Cells Swapnil B. Ambade, Rohan B. Ambade, Sushil S. Bagde, Seung Hun Eom, Rajaram Sakharam Mane, Won Suk Shin, and Soo-Hyoung Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10843 • Publication Date (Web): 28 Dec 2016 Downloaded from http://pubs.acs.org on December 29, 2016

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ACS Applied Materials & Interfaces

Low-Temperature Solution-Processed Thiophene-Sulfur-Doped Planar ZnO Nanorods as Electron Transporting Layers for Enhanced Performance of Organic Solar Cells Swapnil B. Ambade, †, # Rohan B. Ambade, †, # Sushil S. Bagde, † Seung Hun Eom, †, § Rajaram S. Mane, ‡ Won Suk Shin*, ± and Soo-Hyoung Lee*, † †

School of Semiconductor and Chemical Engineering, Chonbuk National University, 567 Baekje-daero, Jeonju, 54896, Republic of Korea. E-mail: [email protected] ; Fax: +82-063270-2306; Tel: +82-063-270-2435 §

Division of Advanced Materials, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeong-ro, Yuseong-gu, Daejeon 34114, Republic of Korea. ‡

Centre for Nanomaterials & Energy Devices, School of Physical Sciences, SRTM, University, 431606, Nanded, India. ±

Energy Materials Research Centre, Korea Research Institute of Chemical Technology, Daejeon, Korea. E-mail: [email protected]

KEYWORDS inverted organic solar cells, electron transporting layer,

thiophene-sulfur doped-ZnO,

morphology, work function.

ACS Paragon Plus Environment

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ABSTRACT

1-D ZnO represents a fascinating class of nanostructures that are significant to optoelectronics. In this work, we investigated the use of an eco-friendly, metal free in situ doping through a pure thiophene-sulfur (S) on low temperature processed ( R > R, suggesting highest growth rate along the c-axis [0001]. Thus, the reaction involving the synthesis of Thi-S-PZnRs results in the 1D nanostructure that is sharp and long enough.19 Since the growth conditions largely control the aspect ratio of nanostructures, typically, with an increase in thiophene concentration, the aspect ratio decreased. However, such nanostructures were found to have a lesser crystalline feature which obviously was not too effective as cathode buffers (data not shown). Figure 2c-d show TEM images of ETLs (PZnRs, Thi-S-PZnRs). The TEM image clearly reveals that PZnRs (Figure 2c) are shorter in lengths as compared to Thi-SPZnRs (Figure 2d). The HRTEM analysis presented in Figure 2e-f confirms the lattice spacing of 0.36 nm and 0.513 nm for PZnRs and Thi-S-ZnRs, respectively, along the (002) c-axis direction. The crystalline nature of Thi-S-PZnRs compared to PZnRs is evident from the corresponding fast Fourier transform (FFT) images presented in the inset of Figure 2e, f. Thus, improved crystallinity is observed after Thi-S doping in ZnRs, which is consistent with XRD data. Some


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distortions (yellow marks) were caused into the ZnO lattice by incorporation of S, ascribed to greater Bohr radius in S than O.

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The energy dispersive X-ray spectroscopy (EDS)

microanalysis presented in Figure 3 confirms the presence of Zn and O in PZnR sample and Zn, O and S atoms in Thi-S-PZnRs. Figure 4 shows FT-IR spectra of ETLs (PZnRs and Thi-S-PZnRs). The peaks at 1450, 1588, 1150 cm-1 corresponding respectively to C=C,

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C-H stretching and bending in the

thiophene ring36 are detected in the FT-IR spectrum of Thi-S-PZnRs. The characteristics peaks related to C-S stretching in the thiophene ring are observed at 980, 695, 872-879 cm-1. The bands concerning aromatic ring stretching in thiophene and in-phase bending vibration37-40 are found at 1065 and 781 cm-1, respectively. The above peaks are found only in Thi-S-PZnRs and not in PZnRs. The prominent band ascribed to Zn-O stretching vibration in ZnO at ~500 cm-1 is present in both the spectra. Also, the band related to O-H stretching in ZnO is observed for both the ETLs around 3484 cm-1 though a minor shift is seen in the case of Thi-S-PZnRs. The difference in chemical states of Thi-S-PZnRs compared to PZnRs was investigated using X-ray photoelectron spectroscopy (XPS) (calibrated with C 1s to 284.6 eV) performed on samples of PZnRs and Thi-S-PZnRs. The XPS survey spectra in Figure 5 shows strong signatures of Zn and O in both the samples indicating the presence of Zn, O, while in Thi-SPZnRs, an additional weaker peak corresponding to S 2p core level appears. The high-resolution XPS peaks corresponding to Zn, O and S, are depicted in Figure 6. The peaks located respectively at 1043, 1020, 529, 164 eV are attributions of O 1s, Zn 2p, and S 2p electronic states.20, 30, 41-43 The Zn 2p region (Figure 6a) reveals two typical peaks for Zn 2p3/2 and 2p1/2 binding energies in PZnRs and Thi-S-PZnRs. For PZnRs, the Zn 2p3/2 and Zn 2p1/2 binding energies are ~22.97 eV apart while, on the incorporation of S-atom into PZnRs, this difference


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increased to 23.02 eV, indicating higher binding energy shift on S-doping. This shift can be assigned to the difference in electronegativity as well as the size of S and O. The three deconvoluted asymmetric O1s peaks in Figure 6b-c denotes that the surface of PZnRs and Thi-SPZnRs possess several oxygen species. The low binding energy O1s peak at 529.38 eV ascribed to O ions in lattice oxygen in wurtzite structure of ZnRs (Figure 6b) is found to be shifted by 0.4 eV on thiophene-S doping (Figure 6c). The O1s binding energy centered at 531.06 eV for PZnRs, which is assigned to O-deficient ZnOx region, shifts by ~0.25 eV in Thi-S-PZnRs. The peak at high binding energy (532.66 eV) ascribed to loosely bound oxygen such as absorbed O2 on the surface of PZnR also shifts by ~0.20 eV after Thi-S-doping on PZnRs.20, 44 The shift in the binding energy of all the three O1s peaks could be attributed to the higher electronegativity of S (2.58 eV) compared to Zn+2 (1.65 eV). The elemental S, which was undetected in the XRD, is apparent in XPS spectra. The deconvoluted S2p peaks corresponding to S2p3/2 and S2p1/2 appear at binding energies of ~164.4 and 165.7 eV and are in good agreement with the position of Thi-S caused by spin-orbital coupling.30 Further, the peak at 164.4 eV is ascribed to O-Zn-S binding energy. The peaks at lower binding energies i.e. ~162 eV (thiolate, SH), ~163 (Zn-S) and at higher binding energies i.e. ~167-169 eV (oxidized S) are not detected.30, 45-47 The absence of any additional peak for S2p clearly suggests that S atom is in its neutral state. The S2p XPS spectrum indicates that there is no impurity peak and that S atoms are incorporated into the ZnRs. In our experience, such sulfur doping via pure thiophene in PZnRs is an unprecedented one. The structure of Thi-S-PZnRs was determined using Raman analysis (Figure 7). The Raman peaks at 570.47 cm-1 & 1086.44 cm-1 in ZnRs sample denote the first and second order longitudinal optical phonons LO1 and LO2, respectively (Figure 7a). In comparison to ZnRs,


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LO1 and LO2 peaks of Thi-S-PZnRs sample appear to be red shifted by 14.4 and 11.85 cm-1 respectively, which is indeed related to doping effects. Moreover, overtone modes of higher order denoted by bands highlighted in green, orange, and gray (Figure 7b) in Thi-S-PZnRs sample are additionally detected. These peaks typically correspond to C’=C, C=Cβ, and C-C’ stretching vibrational modes of the thiophene, respectively.48 This strongly suggests that Sdoping in PZnRs is by the means of Thi-S configuration. The J-V characteristics of the iOSCs incorporating undoped and doped PZnRs as ETLs under AM 1.5 G illuminations and in the dark are summarized in Table 1 and illustrated in Figure 8. The iOSCs with ETL of PZnRs yielded a PCE of 2.38% owing to open-circuit voltage (Voc) of 0.55 V, short-circuit current density (Jsc) of 8.01 mA cm-2, and fill factor (ff) of 0.54. After modification of S-doping on ZnRs ETL, a significant change in Jsc is observed. The iOSCs of S-doped PZnRs exhibited an enhancement in Jsc, Voc, ff, and PCE to 10.81 mA cm-2, 0.61 V, 0.55, and 3.68%, respectively. The difference in the iOSCs device performance after S-doping on PZnRs was mainly from an increase in Jsc and Voc. To investigate the versatility of S-doped PZnR ETLs, we fabricated iOSC devices for the photoactive system of PTB7-Th: PCBM, that yielded a promising PCE of 8.15% compared to 6.50% exhibited by the ETLs of pristine PZnRs. The Jsc is determined by a series of photo-electronic events namely absorption of light, exciton dissociation, efficient charge transport to and collection by the charge collecting electrodes. To understand the exact mechanism of enhancement in Jsc, a detailed analysis of various factors was carried out. External quantum efficiency (EQE) spectra presented in Figure 8d clearly indicate higher EQE (~70%) in the iOSCs of Thi-S-PZnRs compared to iOSCs of PZnRs (~51%) at 550 nm wavelength for a photoactive layer of P3HT: PC60BM. A similar trend in EQE enhancement is


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seen for the iOSCs comprising BHJ of PTB7-Th-PC71BM (Figure 8e). Apparently, EQE values in the lower wavelength region corresponding to absorption of PZnRs is higher in the case of Thi-S-PZnRs, suggesting that contribution to photocurrent by an ETL is certainly more in Thi-SPZnRs and thus has much greater capabilities as an ETL. To confirm the charge dissociation in both the devices, photoluminescence (PL) quenching (Figure 8f) was carried out. For this experiment, P3HT was used as the representative acceptor to investigate the PL quenching.49 The PL intensity of S-doped ZnRs integrated device is significantly lower compared to undoped ZnRs, indicating more efficient charge transfer in iOSCs of Thi-S-PZnRs. The PL quenching clearly shows that S-doped ZnRs could provide additional charge separating interface at its boundary with P3HT: PCBM, ultimately leading to enhanced PCE. For iOSCs, the conductivity of ETL is crucial for exciton dissociation and effective charge collection. To see the effect on the electrical properties of Thi-S-doped PZnRs, electrical conductivity was measured using Hall Effect analysis (based on Van der Pauw method). Hall analyses (presented in Table 2) confirm n-type conductivity in PZnRs and Thi-SPZnRs samples. While the Hall electron mobilities were not too different in both the ETLs, the electrical conductivity of Thi-S-PZnRs was found to be significantly increased after S-doping. Strikingly, the electrical conductivity in doped PZnRs increased nearly by four orders compared to undoped ones, ascribed to increment in charge carrier density incorporated by S-doping. The charge carrier concentration was found to increase by ~32% for Thi-S-PZnRs. Moreover, the increase in crystallization and orientation of Thi-S-PZnRs resulted in the decreased resistivity. Thus, increased conductivity certainly contributed to an enhancement in Jsc value.


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The nanoscale conductivity in both ETLs was probed using Conductive atomic force microscopy (C-AFM). The corresponding topography and current-electron current images are shown in Figure 9. Herein, C-AFM was measured in forward bias (+2V), where electrons are injected from the AFM tip to the ETL.50-52 In the forward bias, bright region corresponds to dominant electron conduction pathways. Figure 9a shows the C-AFM for the PZnRs, which shows weaker currents in the PZnR ETL. Furthermore, the C-AFM carried out on the PZnR coated with BHJ photoactive, showed the non-uniform distribution of current (Figure 9b). After the modification of PZnRs by S-doping, the magnitude and distribution of current and significantly increased both in the case of Thi-S-PZnR ETL as well as photoactive coated ETL (Figure 9c-d). The increase in the bright regions over the entire surface scan on Thi-S-PZnRs sample indicates that electronic conduction is orders of magnitude higher as well as uniform after S-doping on PZnRs. The conduction pathways were found to be too weak in the films of ZnRs ETL. It is not too surprising since the crystallinity of ZnRs was not as high as expected, owing to low temperature annealing (170 oC). The films of Thi-S-PZnRs were also annealed at 170 oC. Despite such low-temperature annealing, the crystallinity of Thi-S-PZnRs was quite high, which is ascribed to controlled S-doping of ZnRs. Moreover, the addition of impurities by S-doping increases the carrier concentration, which eventually increases the conductivity. Increased conductivity obviously results in higher photocurrents in iOSCs of Thi-S-PZnRs. Additionally, series resistance (Rs) is significantly affected by the electrical conductivity. Poor electrical conductivity at the ETL/photoactive BHJ interface increased Rs that typically led to a decrease in Jsc. The values of Rs and shunt resistance (Rsh) of the iOSCs incorporating PZnRs and Thi-S-PZnRs as ETLs, calculated from the dark J-V measurement curves (Figure 9b) are


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presented in Table 1. As the ff of iOSCs is determined by Rs and Rsh directly, a lower Rs and higher Rsh in the iOSCs of Thi-S-PZnRs is consistent with increased ff and higher photocurrent density compared to iOSCs of PZnRs. Thus, a higher forward bias current along with lower leakage in the devices of Thi-S-PZnRs suggests that Thi-S-PZnRs must have better charge selectivity over undoped PZnRs. The increase in the forward bias current gives evidence that the oxygen interstitials, which acts as electron traps in ZnRs, could be suppressed by S-doping in ZnRs. The interfacial morphological properties have a significant influence on the performance; the surface morphology of ETL plays an important role for in achieving intimate contact of ETL with the hydrophobic BHJ active layers. From the AFM images of ETLs (Figure 10a, c), the root mean square (RMS) roughness for PZnRs and Thi-S-PZnRs was 12 and 6 nm, respectively. On coating the photoactive layer over the ETLs, the corresponding RMS values were found to be 1.48 and 0.91 nm for ZnRs and Thi-S-PZnRs, respectively (Figure 10 b, d). It is thus clear that the Thi-S-PZnRs showed the smoothest surface morphology meaning, improved interfacial compatibility between the metal oxide and polymer blend and hence, reduced contact resistance which eventually has improved the charge collection efficiency.53 To identify the mechanism for the enhanced device performance, the surface contact angle of ETLs were measured. A strong relationship between ETL surface contact angle and device performance is observed. The surface contact angle of ZnRs was 35.8o, while that for ThiS-PZnRs was 51.3o (Figure 11). This clearly reveals that the surface properties have been changed for Thi-S-PZnRs. The surface contact angle reflects the surface energy, and in the present case, Thi-S-PZnRs has lowered the surface energy of ZnRs. A lower surface energy of


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ETLs is beneficial to achieve good interfacial contact between ETLs/photoactive layers. Improved interfacial contact resulted in higher photocurrents owing to reduced charge trapping. The presence of buffer layers like ETLs and the hole transporting layer (HTL) in OSCs affects the WF of the electrode. The WF ultimately influences the ohmic contact at electrodes. The electronic properties (energy levels, WF) of ETLs were studied using ultraviolet photoelectron spectroscopy (UPS). The Thi-S-PZnRs film showed a reduced WF (4.01 eV) compared to that of PZnRs film (4.22 eV) (Figure 12). The exact energy levels (Valence band maximum, VBM and conduction band minimum, CBM) of each ETL were determined from the respective binding energies (Figure S1, Table S1). The high binding-energy cut off (Ecutoff) and the onset (Eonset) relative to Fermi level (EF) of both the ETLs are shown in Figure S1a and b respectively. The energy band diagram presented in Scheme 1b suggests that the conduction band (CB) of PZnRs is upshifted on S-doping i.e. there is a decrease of the WF. The decreased WF of ETL facilitated an easy electron transport to the cathode through improved ohmic contact. Our J-V analysis revealed that iOSCs of Thi-S-PZnRs exhibited greater Voc values, which is consistent with the upshifted WF of Thi-S-PZnRs ETL. As the WF decreased, the CB of Thi-SPZnRs was aligned closer to the LUMO of PC60BM acceptor. Thus, higher Voc in iOSCs of ThiS-PZnRs is justified. The electrical properties were further examined by means of EIS analysis and were used to gain insight into the improvement of device characteristics before and after S-doping on ZnRs. The EIS is a powerful technique to obtain information about the interfacial charge transfer processes where recombinations usually occur at the ETL-photoactive layer interface, affecting the PCE.54-56 Figure 13a shows Nyquist plots of iOSCs measured under simulated 1 sun illumination at an individual open circuit voltage condition using two electrode system. The


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equivalent circuit57 used to fit Nyquist plot is shown in Figure 13b. Two distinct semicircles in the Nyquist plot correspond to involvements of varying charge transfer mechanisms. The charge recombination arising from electron transfer at the interface of ETL and photoactive BHJ is represented by the semicircle in the high-frequency range.54 As observed from the Nyquist plots illustrated in Figure 13a, the iOSCs device of Thi-S-PZnRs shows smaller radius, meaning low resistances. The Rs reduces significantly from 20 (for ZnRs) to 3 Ω cm2 (Thi-S-PZnRs). The reduced Rs value complements J-V characteristics for the iOSCs of PZnRs and Thi-S-PZnRs.

Conclusions In conclusion, an eco-friendly, metal-free, low-temperature solution processed Thi-S doped PZnRs were explored as ETLs for iOSCs. The XRD, TEM, HRTEM, FTIR and XPS studies clearly show that the thiophene-S atom is incorporated on/into PZnRs. The C-AFM and Hall measurements reveal that Thi-S-PZnRs showed increased current homogeneity at the ETLphotoactive interface as a result of increased conductivity. Moreover, EIS result shows reduced electron-hole recombination and efficient charge collection for Thi-S-PZnRs ETL. The iOSCs based on PTB7-Th: PCBM and P3HT: PCBM BHJ photoactive layer containing Thi-S-PZnRs as ETL exhibited PCEs of 8.15% and 3.68%, respectively under simulated AM 1.5G, 100 mW cm−2 illumination. The magnificent enhancement in PCE value compared with iOSCs of undoped PZnRs (PCE= 6.5% and 2.38%) ETL is attributed to a combination of desirable energy alignment, morphological modification, increased conductivity and the overall doping effect. This enhancement in PCE is observed to be driven mainly through an improved Jsc which is assigned to the increased exciton dissociation along with charge extraction, while; better ohmic


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contact, low leakage, and reduced charge recombination resulted in increased Voc. This simple doping technique for device engineering is found to be an effective method for increasing the photovoltaic performance of OSCs and has a potential application in flexible optoelectronics, owing to low-temperature processing. Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org/ at DOI: UPS Analysis, Tables S1-S3.

Author Contributions These authors contributed equally.

#

ACKNOWLEDGEMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future planning (2015R1A2A2A01004404). This research was also supported by a grant from the Program of the Korea Research Institute of Chemical Technology (KRICT), Republic of Korea.

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Scheme 1 (a) Device structure of iOSCs with PZnRs and Thi-S-PZnRs as ETLs, (b) Energy level diagram of various components of iOSCs used in this work.


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Figure 1. (a) XRD patterns of ETLs, and (b) comparison of their (002) peaks.


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Figure 2. FE-SEM images of (a) PZnRs, and (b) Thi-S-PZnRs. TEM and HR-TEM images of the (c-e) PZnRs and (d-f) Thi-S-PZnRs.


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Figure. 3 EDS spectra and elemental mapping of (a-b) PZnRs, and (c-d) Thi-S-PZnRs.


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Figure. 4 FT-IR spectra of PZnRs and Thi-S-PZnRs.


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Figure 5. XPS survey scans of PZnRs and Thi-S-PZnRs.


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Figure 6. XPS spectra (a) Zn 2p, (b) O 1s of PZnRs, (c) O 1s of Thi-S-PZnRs and (d) (c) S 2p of Thi-S-PZnRs.


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Figure 7. (a) Low, and (b) high order Raman modes of PZnRs and Thi-S-PZnRs.


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Figure 8. Photovoltaic performance of PZnRs and Thi-S-PZnRs in (a) light, and (b) dark for P3HT: PC60BM BHJ (c) light for PBT7-Th: PC71BM BHJ, (d) EQE spectra for P3HT: PC60BM BHJ, (e) EQE spectra for PBT7-Th: PC71BM BHJ and (f) PL quenching spectra for P3HT: PC60BM BHJ.


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Figure 9. (a) C-AFM dark electron current images of (a) PZnRs, (b) ZnO NRs with P3HT:PCBM, (c) Thi-S-PZnRs and (d) Thi-S-PZnRs with P3HT:PCBM (scan areas of 3 µm x 3 µm) corresponding C-AFM maps at +2V in forward bias.


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Figure 10. AFM topographic images of ETLs and photoactive blends over ETLs for (a-b) PZnRs, (c-d) Thi-S-PZnRs.


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Figure 11. Contact angle measurements on (a) PZnRs and (b) Thi-S-PZnRs.


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PZnRs Thi-S-PZnRs

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ϕ = 4.22 eV ϕ = 4.01 eV

3.6

3.8

4.0

4.2

4.4

4.6

4.8

5.0

Kinetic Energy (eV) Figure 12. The WF of PZnRs and Thi-S-PZnRs determined by UPS studies with He (hν=21.2 eV) source.


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Figure 13. EIS spectra (a) Nyquist plot and (b) Equivalent circuit of P3HT: PC60BM based iOSCs.


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Table 1. Photovoltaic parameters of the iOSCs based on PZnRs and Thi-S-PZnRs as the ETL under 1.5 G illuminations.

Jsc ETLs

ZnO NRs

Thi-SZnO NRs

a

Voc

ff

PCEmax

Rs

Rsh

(PCEavga)

Donor (mA cm-2)

(V)

(%)

(%)

(Ω cm2)

(Ω cm2)

P3HT

8.01

0.55

54

2.38 (2.35)

8.66

837

PTB7Th

13.73

0.78

60.47

6.5 (6.42)

P3HT

10.81

0.61

55

3.68 (3.6)

1.78

2867

PTB7Th

14.96

0.79

68.95

8.15 (8.05)

PCEavg was obtained for more than 30 iOSCs devices.


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Table 2. Electrical properties of iOSCs based on PZnRs and Thi-S-PZnRs as ETLs. Hall electron Mobility

Resistivity

Conductivity

Carrier Density

*10-5

*103

* 1021

(Ω-cm)

(S/cm)

(cm-3)

(cm2V-1 s-1)

ZnO NRs

93.45

1.07

3.29

35.13

Thi-S-ZnO NRs

17.83

5.6

4.35

44.79

ETLs


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Table of Content (TOC)


41

Low-Temperature Solution-Processed Thiophene-Sulfur-Doped

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