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Interfacial modification via boronic acid functionalized self-assembled monolayers for efficient inverted polymer solar cells
Materials Science in Semiconductor Processing 107 (2020) 104860
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Interfacial modification via boronic acid functionalized self-assembled monolayers for efficient inverted polymer solar cells Çisem Kırbıyık a, *, Mustafa Can b, Mahmut Kus¸ c a
Department of Chemical Engineering, Konya Technical University, 42075, Turkey Department of Engineering Sciences, Izmir Katip Celebi University, 35620, Turkey c Instıtute of Energy Technologies, Gebze Technical University, Turkey b
A R T I C L E I N F O
A B S T R A C T
Keywords: Self-assembled monolayer Inverted polymer solar cells Blocking TiO2 Modification
In this study, we describe that the interface modification of blocking TiO2 (bl-TiO2) based polymer solar cells (PSCs) by self-assembled monolayer (SAM) can enhance the entire device performance. Three different fluorineterminated boronic acid SAMs were applied to modify bl-TiO2 layer in inverted PSCs. It was determined that surface traps could be passivated and the energy level matching could be improved by introducing of fluorineterminated boronic acid SAMs. The modifying with SAMs enhance the charge transport properties of bl-TiO2 layer. The morphological, optical and electrical characterizations were carried out for each solar cell fabricated. PSCs fabricated with 3,5-fluorophenylboronic acid (2-F) SAM modified bl-TiO2 resulted a power conversion efficiency (PCE) of 4.4%, which is a significant increase (~90%) compared with non-modified device. These underrated SAM molecules and their easy application to electronic devices provides an opportunity to investigate their applications.
1. Introduction Bulk heterojunction (BHJ) polymer solar cells (PSCs) are suitable substitute for traditional photovoltaic technologies due to their unique properties such as rapid progress of power conversion efficiency (PCE), low-cost fabrication process, thin and light-weight configurations and mechanical flexibility [1]. The PSCs can be classified in two architec tures depending on the charge flow direction: the conventional and inverted architecture, in which the photoactive layer is located between hole and electron transport layers [2]. In a PSC, the photoactive layer typically consists of the blending of an electron donating semiconductor polymer and an electron accepting fullerene derivative, forming a BHJ structure [3]. Due to some undesirable issues such as low work function metal electrodes or acidic corrosive materials used for conventional (p-i-n) architecture, the inverted (n-i-p) device architecture have been receiving incremental attention [4]. Despite the all advantages, there are still some problems to overcome. The PCE of inverted devices fabricated depends on a number of limiting factors, including charge transport properties, trap states at interfaces and migration, dissocia tion, and recombination of excitons [5]. These limiting factors are mainly influenced by the layer morphologies of PSCs. To reduce the effect of these limiting factors, interface modifications play an important
role to improve the photovoltaic characteristics, that is, the surface modification may improve carrier transport properties at the interfaces well as the upper layer morphologies [6]. Up to now, several metal oxides and inorganic salts have been widely investigated as an electron transport layer (ETL) in inverted PSCs. Among the other materials, TiO2 has been noted as an excellent ETL in many studies due to its high transparency, wide band gap blocking holes and chemical stability. However, TiO2 possesses higher trap density compared with other ETL materials like SnO2 [7]. Therefore, morpho logical and electrical properties of TiO2 ETL layer still need to be improved. As well known, the inverted PSCs are suitable to enhance the interfacial contact and the optical electric field [8]. Several techniques such as doping chemistry [9,10], morphology [11] or surface modifi cation [12] and so on have been reported to improve the injection and selection of charges. Especially, surface modification by self-assembled monolayers (SAMs) has been widely studied for the modification of the surface states and band structure to form a good ohmic contact and reported to be a feasible method on different electronic devices [13,14]. SAMs have been having a growing interest as an attractive compo nent of electronic devices due to their large multi-functionality. The SAMs are used to improve the quality of conducting metals or trans parent metal oxides. The surface-active head group of acidic SAMs binds
* Corresponding author. E-mail address: [email protected] (Ç. Kırbıyık). https://doi.org/10.1016/j.mssp.2019.104860 Received 24 September 2019; Received in revised form 9 November 2019; Accepted 19 November 2019 Available online 23 November 2019 1369-8001/© 2019 Published by Elsevier Ltd.
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(200 nm)/MoO3 (10 nm)/Ag (60 nm), the higher PCE was achieved compared to non-modified device structure. The PCE increased from 2.3% to 4.4% after modification with 2-F SAM, which could be attrib uted to improved electron extraction between photoactive and electron transport layers. This study proves the using of fluorophenylboronic acid SAMs for improved efficiency in PSCs and their promising potential in large scale application. 2. Experimental details 2.1. Preparation of bl-TiO2 layers The etched FTO substrates (1.5 cm � 1.5 cm) were sequentially cleaned by ultrasonic treatment with diluted detergent solution (Hell manex III, Hellma Analytics), acetone, isopropyl alcohol, and distilled water, each step lasted for 10 min. After drying with a nitrogen stream, the cleaned substrates were treated by plasma system (15 min, O2 plasma) to remove organic residues. The preparation of blocking TiO2 (bl-TiO2) layer solution was per formed as follows: 3 mL of titanium (IV) isopropoxide (Sigma-Aldrich, 99.9%) was added in 9 mL of absolute ethanol, and at the same time 2 mL of acetyl acetone (Sigma-Aldrich, 99.5%) was added in 6 mL of ab solute ethanol in another vial. After adding the second solution to first solution drop by drop, the solution resulted was aged for 24 h and diluted with absolute ethanol in ratio 1:10. The diluted bl-TiO2 solution was sequentially spin coated at 1500 rpm for 20 s and 2000 rpm for 20 s. The coated substrates were sintered at 450 � C for 30 min.
Scheme 1. Diversity of boronic acid applications.
to specific substrate sites. For example, the most common bonding to specific substrates are thiol/metals, acids/metal oxides and silane/SiO2 [15]. Although, it is worth to note that several studies have been re ported that SAMs can be utilized to improve the performance of different electronic devices such as organic thin-film transistors [16], organic light-emitting diodes [17], there have been very limited study to use of in PSCs, even, there are far fewer number of utilizing in PSCs. In recent years, significant improvements have been made in the utilizing of boronic acids in medical therapeutics, catalysis, separation and material science (Scheme 1) [18]. In this framework, we investi gated the fabrication of highly efficient PSCs by introducing of ultra-thin fluorine-terminated boronic acid derivatives SAMs. In our previous work, we studied the impact of boronic acid SAMs with different func tional groups (-OCH3) for the modification of blocking TiO2 surface in PSCs. It has been shown that the modification of TiO2 layer by boronic acid functionalized SAMs has positive effects on morphological and photovoltaic characteristics of PSCs. The molecular structure of 1-F (4-fluorophenylboronic acid), 2-F (3,5-fluorophenylboronic acid) and 3-F (3,4,5-fluorophenylboronic acid) SAMs utilized in this study are presented in Fig. 1 and, to the best of our knowledge, these SAMs were studied on the inverted PSCs for the first time. These molecules allow to investigate the behaviour of SAMs on photovoltaic performance because of their small and simple structure. Fluorine-terminated boronic acid derivatives SAMs can promote the electron transport properties since the presence of π-conjugated structure on phenyl backbone and boronic acid group can be easily bonded to bl-TiO2 surface chemically. In case of all modified device structure of FTO/bl-TiO2 (90 nm)/SAM/P3HT:PCBM
2.2. Preparation of SAM modified bl-TiO2 layers For the preparation of SAM modified bl-TiO2 layers, 1 mM of SAM solutions in dimethyl sulfoxide (DMSO) were firstly prepared by dis solving certain amount of 1-F, 2-F and 3-F SAMs. The prepared bl-TiO2 layers were immersed into SAM solutions at room temperature for 48 h to attach SAMs covalently. After being removed from the SAM solutions, the modified surfaces were successively rinsed with DMSO and acetone to remove the possible absorbed molecules. Finally, the substrates were dried with nitrogen gun to eliminate the residual solvents. 2.3. Device fabrication and characterization The active layer was deposited by spin coating. Typically, the active layer solution in chloroform:chlorobenzene (1:1) was prepared by dis solving P3HT (Sigma-Aldrich) and PCBM (Lumtec) at the ratio of 1.0:0.6 wt% with a concentration of 40 mg/mL. The solution was magnetically stirred for 24 h at 70 � C followed by filtering through a polytetrafluoroethylene (PTFE) syringe filter with pore size of 0.45 μm. The prepared active layer solution was spin coated onto bl-TiO2 layers at 1000 rpm for 1 min. Then, the coated surfaces were immediately annealed at 160 � C for 10 min. Finally, 10 nm MoO3 and 80 nm silver was thermally evaporated under 10 6 bar vacuum with a mask (with
Fig. 1. The molecular structure of SAMs used in this study. 2
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Materials Science in Semiconductor Processing 107 (2020) 104860
Fig. 2. The inverted architecture of bl-TiO2 based PSCs and corresponding energy diagram.
Fig. 3. Water droplet contact angle measurement on non-modified and modified bl-TiO2 surfaces with 1-F, 2-F and 3-F SAMs.
Fig. 4. (a) XPS survey spectrum and XPS deconvolutions of (b) O 1s, (c) Ti 2p, and (d) C 1s core level for 2-F modified bl-TiO2. 3
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Fig. 5. (a) XRD pattern and (b) UV–Vis absorption spectra of non-modified and modified bl-TiO2 surfaces.
0.09 cm2 active area) to complete device fabrication process. The morphological analysis was conducted by an atomic force mi croscope (AFM, NT-MDT AFM NTEGRA Solaris) in “tapping” mode. The wettability properties of non-modified and SAM modified bl-TiO2 sur faces were characterized by sessile water drop contact angle measure ments (Kruss Easy Drop). Ultraviolet–visible (UV–vis) absorption spectra of modified surfaces were recorded in the 300–800 nm wave length by Biochrom Libra S22 UV–vis spectrophotometer. X-ray diffraction (XRD) patterns were recorded Bruker D8 Advance diffrac tometer. X-ray photoelectron spectroscopy (XPS) characterization of 2-F modified bl-TiO2 layer were carried out by SPECS EA 300 with an aluminium anode. Fourier transform infrared spectra were measured with the range of wavenumber from 400 to 4000 cm 1 by Bruker Vertex 70 FTIR spectroscopy. Photocurrent density-voltage (J-V) characteris tics of devices fabricated without any encapsulation were measured under N2 atmosphere using a Keithley 2400 Source under dark or illu mination conditions (AM 1.5 G, 100 mW cm 2) provided by a solar simulator (ATLAS) glove-box integrated.
modified surface. The increased hydrophobicity may enhance the spreading of photoactive layer solution and interfacial contact due to the relatively hydrophobic nature of BHJ layer [19]. XPS is one of most common used technique to determine the bonded SAM molecules onto surface after modification. Fig. 4 shows the survey and high resolution XPS scan spectra of 2-F modified bl-TiO2. Fig. 4 (a) shows the characteristic peaks of bl-TiO2 layer and the O 1s, F 1s (a peak at 686.1 eV) [20] and C 1s core levels of organic molecule. In Fig. 4 (b), the peaks at 528.0 and 531.1 eV can be assigned to oxygen in rutile TiO2 [21]. As shown in Fig. 4 (c), the Ti 2p1/2 and Ti 2p3/2 peaks are located at 462.5 and 456.8 eV [22,23], respectively. In Fig. 4 (d), the C 1s spectrum can be fitted to two characteristic peaks at 285.3 and 283.1 eV, which can be assigned C–O bonds and C¼C bonds. The crystal structures of non-modified and modified bl-TiO2 thin films were characterized with XRD method. The pattern obtained is seen in Fig. 5 (a). It is obviously seen that the diffraction peaks belong to tetragonal rutile phase of TiO2 and there is no other phase or impurities detected [24]. Notably, after modification, the intensity of the XRD diffraction peaks decreased, which could be attributed to the acids/metal oxides bonding on the surfaces. It can be said that the crystalline quality of SAM modified bl-TiO2 layer decreases. To investigate the effect of SAMs on optical properties, UV–visible absorption measurements were carried out and Fig. 5 (b) illustrates the UV–Vis absorbance spectra of bl-TiO2 surfaces scanned in the range of 300 nm–800 nm. It is clear that the presence of SAM layer does not affect the absorbance of the bl-TiO2 layer in both UV and Visible region. For better understanding of the influence of interface modification on the device performances, the optical absorbance data was plotted as a function of photon energy (Fig. 6). The majority of researchers have
3. Results and discussion The solar cell structure and corresponding energy diagram are given in Fig. 2. To check the covering homogeneity of SAMs onto bl-TiO2, the contact angle measurements of water droplets on modified and nonmodified surfaces were performed and the average results are given in Fig. 3. As seen, the contact angle (CA) of varies from 34� to 53� and the modified bl-TiO2 surfaces show higher hydrophobicity in compared to non-modified surface. It could be inferred that SAMs successfully covered bl-TiO2 surfaces. The highest hydrophobicity is observed on 2-F
Fig. 6. (a) Direct band gap and (b) indirect band gap estimation from UV–Vis absorption data. 4
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gabs of bl-TiO2 can be easily tuned by introducing of fluorine-terminated boronic acid SAMs. The morphology of active layer is one the most important parameter to improve the electron and hole mobility, thus, the photovoltaic pa rameters are strongly associated with the morphological property of [26]. The growth of upper layer can be influenced and control by the chemical structure of modification layer. Fig. 7 shows the AFM images of modified bl-TiO2 surfaces and the average roughness (Ra) values are given in Table 2. The images demonstrate that the Ra values of non-modified bl-TiO2, 1-F/bl-TiO2, 2-F/bl-TiO2, and 3-F/bl-TiO2 sur faces were 17.1, 16.3, 14.9, and 13.7, respectively. The reduced Ra values could be attributed to the reduced number of pin holes and cracks on the bl-TiO2 surface. It reduces the density of trap states, which pro motes the photovoltaic stability [27]. The decreasing of Ra values of ETL may also help to improve the active layer morphology. The AFM images of P3HT:PCBM coated modified bl-TiO2 surfaces are given in Fig. 8. The surface properties of active layer seem to be influenced by the number and the position of substituent on the SAMs (Table 2). It can be said that SAMs lead to form an ordered upper layer structure via
Table 1 Direct and indirect bandgap transitions for non-modified and modified c-TiO2 surfaces. Surface
Bandgap (eV)
bl-TiO2/FTO 1-F/bl-TiO2/FTO 2-F/bl-TiO2/FTO 3-F/bl-TiO2/FTO
Direct
Indirect
3.788 3.823 3.846 3.806
3.568 3.603 3.619 3.590
reported that rutile phase of TiO2 has both direct and indirect band gaps [25]. Therefore, direct and indirect band gaps of non-modified and modified surfaces was calculated from optical absorption spectra. Fig. 6 (a) shows (αhν)2 vs. hν plot for direct band gap calculation, whereas Fig. 6 (b) shows (αhν)1/2 vs. hν plot for indirect band gap calculation, where α is the absorption coefficient and hν is the photon energy. The band gap values are calculated through the extrapolating to the X-axis. The values calculated are given in Table 1. The direct and indirect band gaps of non-modified bl-TiO2 surfaces were found to be 3.79 and 3.57 eV. After the modification of bl-TiO2 surfaces with 1-F, 2-F and 3-F SAMs, the direct band gaps increased to be 3.82, 3.85 and 3.81 eV, respectively, whereas indirect band gap increased to be 3.60, 3.62 and 3.59 eV, respectively. Increased energy gap could be beneficial for the improving of electron transport properties since lower values is to be responsible to lost a lot of electrons generating in the form of heat. As seen in Fig. 1, SAMs have different number of substituent and they give different permanent dipole moment magnitude and direction. Hence, the electrical properties of bl-TiO2 layer modified with SAMs strongly depend on the number of substituent. It can be said that the energy band
Table 2 The calculated values of water contact angle, work function change and average roughness differences between unmodified and modified c-TiO2 surfaces with different SAMs. Surface
Ra (nm)
Surface
Ra (nm)
bl-TiO2/FTO 1-F/bl-TiO2/FTO 2-F/bl-TiO2/FTO 3-F/bl-TiO2/FTO
17.1 16.3 14.9 13.7
P3HT:PCBM/bl-TiO2/FTO P3HT:PCBM/1-F/bl-TiO2/FTO P3HT:PCBM/2-F/bl-TiO2/FTO P3HT:PCBM/3-F/bl-TiO2/FTO
5.54 1.18 1.07 2.69
Fig. 7. AFM images of (a) non-modified bl-TiO2, (b) 1-F modified bl-TiO2, (c) 2-F modified bl-TiO2, and (d) 3-F modified bl-TiO2 surfaces. 5
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Fig. 8. AFM images of P3HT:PCBM layer on top of (a) non-modified bl-TiO2, (b) 1-F modified bl-TiO2, (c) 2-F modified bl-TiO2, and (d) 3-F modified blTiO2 surfaces.
Fig. 9. J-V curves of PSCs non-modified and modified with SAMs under (a) illuminated and (b) dark conditions.
surface-attaching chemically onto blocking TiO2 (bl-TiO2) layer at mo lecular level. It is known that the active layer morphology affected the charge transport properties and more homogenous surface increases the interfacial contact and benefits PCE [28]. Fig. 9 (a) shows the J-V curves of inverted PSCs based on modified with SAMs and non-modified bl-TiO2 layers to evaluate the photovoltaic characteristics as a function of modification layer. The performance characteristics extracted from J-V curves under illumination are sum marized in Table 3. For the non-modified device, photovoltaic charac teristics were observed with Jsc of 10.56 mA cm 2, Voc of 0.53 V and FF
of 41.6%, resulting in PCS 2.33%. This efficiency is approximately comparable with reported efficiencies of P3HT:PCBM based inverted PSCs, as seen in Table 4 [9,29–34]. In case of the modification through all SAMs, the device perfor mances present a notable increasing. Especially, the modification of blTiO2 layer with 2-F SAM yields a remarkable enhancement in overall device performance where Jsc increases to 16.51 mA cm 2, Voc to 0.58 V and FF to 45.7%, resulting in PSC 4.36%. The improved performance of modified PSCs is correlated to surface hydrophobicity and interfacial contact, thus, improved electrical properties. The Jsc and Voc of the 6
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Table 3 Summary of device parameters extracted from illuminated J-V curves presented in Fig. 9. Device FTO/bl-TiO2/P3HT: PCBM/MoO3/Ag FTO/bl-TiO2/1-F/P3HT: PCBM/MoO3/Ag FTO/bl-TiO2/2-F/P3HT: PCBM/MoO3/Ag FTO/bl-TiO2/3-F/P3HT: PCBM/MoO3/Ag
Voc (V)
Jsc (mA/ cm2)
FF (%)
PCE (%)
Rs (Ohms)
0.532
10.555
41.57
2.334
49.86
0.561
12.481
46.26
3.239
46.94
0.579
16.512
45.67
4.359
44.92
0.571
11.369
51.83
3.365
46.19
Table 5 The ideality factor (n) and dark saturation current density (Jo) extracted from dark J-V curves.
Voc (V)
Jsc (mA/ cm2)
FF (%)
Efficiency (%)
Ref.
ITO/TiO2/P3HT:PCBM/ PEDOT:PSS/Ag ITO/NP TiO2/TiO2/P3HT: PCBM/PEDOT:PSS/Ag ITO/TiO2/P3HT:PCBM/ V2O5/Ag FTO/c-TiO2/m-TiO2/P3HT: PCBM/Ag FTO/TiO2/P3HT:PCBM/ V2O5/Ag ITO/NP TiO2/P3HT:PCBM/ PEDOT:PSS/Ag ITO/TiO2/P3HT:PCBM/ MoO3/Al
0.58
6.38
55.4
2.06
[29]
0.59
9.89
47.00
2.72
[30]
0.53
8.75
36.65
1.70
[9]
0.54
6.30
38.06
1.30
[31]
0.56
8.61
43.59
2.13
[32]
0.55
8.54
62.08
2.88
[33]
0.60
6.54
37.00
1.45
[34]
Jo
FTO/bl-TiO2/P3HT:PCBM/MoO3/Ag FTO/bl-TiO2/1-F/P3HT:PCBM/MoO3/Ag FTO/bl-TiO2/2-F/P3HT:PCBM/MoO3/Ag FTO/bl-TiO2/3-F/P3HT:PCBM/MoO3/Ag
1.39 1.20 3.16 1.02
n � 10 � 10 � 10 � 10
3 3 4 3
2.488 2.144 2.140 2.143
conductivity. To study the charge transport properties in the dark, we extracted the reverse saturation current density (Jo) and the ideality factor (n) from dark J-V curves on logarithmic plot seen in Fig. 9 (b). Shockley-ReadHall equation, Eq. (1), was used to identify these parameter [42]. � � � � qV J ¼ Jo exp 1 (1) n kB T
Table 4 The comparison of photovoltaic parameters of reported similar device configu ration to non-doped device configuration used in this study. Device configuration
Device
where J is the current density, q is the electron charge, V is the applied voltage, kB is Boltzmann’s constant, and T is temperature. The Jo and n values, which are extracted from the intercept and the slope of dark J-V curves, are shown in Table 5. The decreased values of Jo in the PSCs modified with SAMs suggest the decreased back charge recombination leading to improved photovoltaic performance. Also, n values decreased after the modification, which is in agreement with the findings of series resistance calculation. 4. Conclusions Herein, we investigated the SAM modification of bl-TiO2 layer and the interface morphology of each layer of PSCs. This study demonstrates that charge transport properties in photoactive blend is intensively depend on the morphological and optical properties of bottom layer, which is bl-TiO2 in our configuration. It was found that better interfacial contact and reduced trap state density can be formed when different fluorine-terminated boronic acid SAMs is applied between bl-TiO2 and photoactive layer. Subsequently, we investigated the impact of the number and the position of substituent on the SAMs on photovoltaic characteristics. The PSC modified with 2-F SAM resulted the highest efficiency (4.4%) along with other enhanced photovoltaic characteris tics. This improvement was achieved thanks to enhanced charge trans port properties and band gap overlap at the interface between electron transport and photoactive layer. The results obtained in this study encourage the utilize of interfacial SAM modification on inverted PSCs to achieve higher efficiency.
modified device are enhanced in the order of Jsc,2-F > Jsc,1-F > Jsc,3-F > Jsc, non-modified and in the order of Voc,2-F > Voc,3-F > Voc,1-F > Voc, nonmodified, respectively. The enhanced Jsc values could be attributed to increased electron injection efficiencies and it is supported by the observation of optical absorption results [35]. The interfacial engi neering can lead to an enhancement in the photon-harvesting properties, which results an increase in the Jsc values of modified devices [36]. The FF values are improved from 42% to 52% after SAM modification. This improvement shows that the modified bl-TiO2 provides better interfacial contact for active layer. Also, the energy level matching at the interface between bl-TiO2 and active layer can enhance the both Voc and FF values [37]. Even though, the introduction of fluorine substitute into para- and meta-positions of boronic acid SAMs increase the photovoltaic charac teristics, it was clearly seen that substitution of fluorine in only both meta-positions of SAMs show much more positive effect on the charge injection efficiency. The series resistance (Rs) can be also used to investigate the inter facial contact between the photoactive and electron transport layers. As seen in Table 3, Rs in the 2-F modified device is 44.9 Ω, whereas Rs in the non-modified device presented a higher value (49.9 Ω). This reduced Rs values for all SAM modified devices are coherent with the other photovoltaic parameters suggesting the improved interfacial contact [38]. Additionally, it can be said that lower values of Rs suggest the reduced carrier recombination after SAM modification, which is an important result to obtain highly efficient PSCs [39]. As well known, the FF value that can be obtained is strongly related to the Rs of devices prepared. The high Rs values arise the poor ohmic contact at the inter face which causes to contact resistance and charge transfer rate losses [40]. The high FF values of devices fabricated in this study show that the rough surface of bl-TiO2 layer prevents photogenerated carriers at the donor-acceptor interface from reaching the electrode [41]. The increasing FF values after modification could be attributed to reduced series resistance due to the improved the interlayer contact and
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to thank to Abdullah Kantar and Mert Temür undergraduate students for their contribution. The authors would also like to thank the Selçuk University Advanced Technology Research and Application Centre for the facilities and technical assistance. References [1] Z. Gao, L. Guo, Y. Sun, W. Qu, T. Yang, B. Li, J. Li, L. Duan, Passivating ZnO with a naphthalimide-Schiff base as electron transport layer for inverted polymer solar cells, Org. Electron. 67 (2019) 232–236. [2] J.-L. Lan, Z. Liang, Y.-H. Yang, F.S. Ohuchi, S.A. Jenekhe, G. Cao, The effect of SrTiO3:ZnO as cathodic buffer layer for inverted polymer solar cells, Nano Energy 4 (2014) 140–149.
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Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp
Interfacial modification via boronic acid functionalized self-assembled monolayers for efficient inverted polymer solar cells Çisem Kırbıyık a, *, Mustafa Can b, Mahmut Kus¸ c a
Department of Chemical Engineering, Konya Technical University, 42075, Turkey Department of Engineering Sciences, Izmir Katip Celebi University, 35620, Turkey c Instıtute of Energy Technologies, Gebze Technical University, Turkey b
A R T I C L E I N F O
A B S T R A C T
Keywords: Self-assembled monolayer Inverted polymer solar cells Blocking TiO2 Modification
In this study, we describe that the interface modification of blocking TiO2 (bl-TiO2) based polymer solar cells (PSCs) by self-assembled monolayer (SAM) can enhance the entire device performance. Three different fluorineterminated boronic acid SAMs were applied to modify bl-TiO2 layer in inverted PSCs. It was determined that surface traps could be passivated and the energy level matching could be improved by introducing of fluorineterminated boronic acid SAMs. The modifying with SAMs enhance the charge transport properties of bl-TiO2 layer. The morphological, optical and electrical characterizations were carried out for each solar cell fabricated. PSCs fabricated with 3,5-fluorophenylboronic acid (2-F) SAM modified bl-TiO2 resulted a power conversion efficiency (PCE) of 4.4%, which is a significant increase (~90%) compared with non-modified device. These underrated SAM molecules and their easy application to electronic devices provides an opportunity to investigate their applications.
1. Introduction Bulk heterojunction (BHJ) polymer solar cells (PSCs) are suitable substitute for traditional photovoltaic technologies due to their unique properties such as rapid progress of power conversion efficiency (PCE), low-cost fabrication process, thin and light-weight configurations and mechanical flexibility [1]. The PSCs can be classified in two architec tures depending on the charge flow direction: the conventional and inverted architecture, in which the photoactive layer is located between hole and electron transport layers [2]. In a PSC, the photoactive layer typically consists of the blending of an electron donating semiconductor polymer and an electron accepting fullerene derivative, forming a BHJ structure [3]. Due to some undesirable issues such as low work function metal electrodes or acidic corrosive materials used for conventional (p-i-n) architecture, the inverted (n-i-p) device architecture have been receiving incremental attention [4]. Despite the all advantages, there are still some problems to overcome. The PCE of inverted devices fabricated depends on a number of limiting factors, including charge transport properties, trap states at interfaces and migration, dissocia tion, and recombination of excitons [5]. These limiting factors are mainly influenced by the layer morphologies of PSCs. To reduce the effect of these limiting factors, interface modifications play an important
role to improve the photovoltaic characteristics, that is, the surface modification may improve carrier transport properties at the interfaces well as the upper layer morphologies [6]. Up to now, several metal oxides and inorganic salts have been widely investigated as an electron transport layer (ETL) in inverted PSCs. Among the other materials, TiO2 has been noted as an excellent ETL in many studies due to its high transparency, wide band gap blocking holes and chemical stability. However, TiO2 possesses higher trap density compared with other ETL materials like SnO2 [7]. Therefore, morpho logical and electrical properties of TiO2 ETL layer still need to be improved. As well known, the inverted PSCs are suitable to enhance the interfacial contact and the optical electric field [8]. Several techniques such as doping chemistry [9,10], morphology [11] or surface modifi cation [12] and so on have been reported to improve the injection and selection of charges. Especially, surface modification by self-assembled monolayers (SAMs) has been widely studied for the modification of the surface states and band structure to form a good ohmic contact and reported to be a feasible method on different electronic devices [13,14]. SAMs have been having a growing interest as an attractive compo nent of electronic devices due to their large multi-functionality. The SAMs are used to improve the quality of conducting metals or trans parent metal oxides. The surface-active head group of acidic SAMs binds
* Corresponding author. E-mail address: [email protected] (Ç. Kırbıyık). https://doi.org/10.1016/j.mssp.2019.104860 Received 24 September 2019; Received in revised form 9 November 2019; Accepted 19 November 2019 Available online 23 November 2019 1369-8001/© 2019 Published by Elsevier Ltd.
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(200 nm)/MoO3 (10 nm)/Ag (60 nm), the higher PCE was achieved compared to non-modified device structure. The PCE increased from 2.3% to 4.4% after modification with 2-F SAM, which could be attrib uted to improved electron extraction between photoactive and electron transport layers. This study proves the using of fluorophenylboronic acid SAMs for improved efficiency in PSCs and their promising potential in large scale application. 2. Experimental details 2.1. Preparation of bl-TiO2 layers The etched FTO substrates (1.5 cm � 1.5 cm) were sequentially cleaned by ultrasonic treatment with diluted detergent solution (Hell manex III, Hellma Analytics), acetone, isopropyl alcohol, and distilled water, each step lasted for 10 min. After drying with a nitrogen stream, the cleaned substrates were treated by plasma system (15 min, O2 plasma) to remove organic residues. The preparation of blocking TiO2 (bl-TiO2) layer solution was per formed as follows: 3 mL of titanium (IV) isopropoxide (Sigma-Aldrich, 99.9%) was added in 9 mL of absolute ethanol, and at the same time 2 mL of acetyl acetone (Sigma-Aldrich, 99.5%) was added in 6 mL of ab solute ethanol in another vial. After adding the second solution to first solution drop by drop, the solution resulted was aged for 24 h and diluted with absolute ethanol in ratio 1:10. The diluted bl-TiO2 solution was sequentially spin coated at 1500 rpm for 20 s and 2000 rpm for 20 s. The coated substrates were sintered at 450 � C for 30 min.
Scheme 1. Diversity of boronic acid applications.
to specific substrate sites. For example, the most common bonding to specific substrates are thiol/metals, acids/metal oxides and silane/SiO2 [15]. Although, it is worth to note that several studies have been re ported that SAMs can be utilized to improve the performance of different electronic devices such as organic thin-film transistors [16], organic light-emitting diodes [17], there have been very limited study to use of in PSCs, even, there are far fewer number of utilizing in PSCs. In recent years, significant improvements have been made in the utilizing of boronic acids in medical therapeutics, catalysis, separation and material science (Scheme 1) [18]. In this framework, we investi gated the fabrication of highly efficient PSCs by introducing of ultra-thin fluorine-terminated boronic acid derivatives SAMs. In our previous work, we studied the impact of boronic acid SAMs with different func tional groups (-OCH3) for the modification of blocking TiO2 surface in PSCs. It has been shown that the modification of TiO2 layer by boronic acid functionalized SAMs has positive effects on morphological and photovoltaic characteristics of PSCs. The molecular structure of 1-F (4-fluorophenylboronic acid), 2-F (3,5-fluorophenylboronic acid) and 3-F (3,4,5-fluorophenylboronic acid) SAMs utilized in this study are presented in Fig. 1 and, to the best of our knowledge, these SAMs were studied on the inverted PSCs for the first time. These molecules allow to investigate the behaviour of SAMs on photovoltaic performance because of their small and simple structure. Fluorine-terminated boronic acid derivatives SAMs can promote the electron transport properties since the presence of π-conjugated structure on phenyl backbone and boronic acid group can be easily bonded to bl-TiO2 surface chemically. In case of all modified device structure of FTO/bl-TiO2 (90 nm)/SAM/P3HT:PCBM
2.2. Preparation of SAM modified bl-TiO2 layers For the preparation of SAM modified bl-TiO2 layers, 1 mM of SAM solutions in dimethyl sulfoxide (DMSO) were firstly prepared by dis solving certain amount of 1-F, 2-F and 3-F SAMs. The prepared bl-TiO2 layers were immersed into SAM solutions at room temperature for 48 h to attach SAMs covalently. After being removed from the SAM solutions, the modified surfaces were successively rinsed with DMSO and acetone to remove the possible absorbed molecules. Finally, the substrates were dried with nitrogen gun to eliminate the residual solvents. 2.3. Device fabrication and characterization The active layer was deposited by spin coating. Typically, the active layer solution in chloroform:chlorobenzene (1:1) was prepared by dis solving P3HT (Sigma-Aldrich) and PCBM (Lumtec) at the ratio of 1.0:0.6 wt% with a concentration of 40 mg/mL. The solution was magnetically stirred for 24 h at 70 � C followed by filtering through a polytetrafluoroethylene (PTFE) syringe filter with pore size of 0.45 μm. The prepared active layer solution was spin coated onto bl-TiO2 layers at 1000 rpm for 1 min. Then, the coated surfaces were immediately annealed at 160 � C for 10 min. Finally, 10 nm MoO3 and 80 nm silver was thermally evaporated under 10 6 bar vacuum with a mask (with
Fig. 1. The molecular structure of SAMs used in this study. 2
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Fig. 2. The inverted architecture of bl-TiO2 based PSCs and corresponding energy diagram.
Fig. 3. Water droplet contact angle measurement on non-modified and modified bl-TiO2 surfaces with 1-F, 2-F and 3-F SAMs.
Fig. 4. (a) XPS survey spectrum and XPS deconvolutions of (b) O 1s, (c) Ti 2p, and (d) C 1s core level for 2-F modified bl-TiO2. 3
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Fig. 5. (a) XRD pattern and (b) UV–Vis absorption spectra of non-modified and modified bl-TiO2 surfaces.
0.09 cm2 active area) to complete device fabrication process. The morphological analysis was conducted by an atomic force mi croscope (AFM, NT-MDT AFM NTEGRA Solaris) in “tapping” mode. The wettability properties of non-modified and SAM modified bl-TiO2 sur faces were characterized by sessile water drop contact angle measure ments (Kruss Easy Drop). Ultraviolet–visible (UV–vis) absorption spectra of modified surfaces were recorded in the 300–800 nm wave length by Biochrom Libra S22 UV–vis spectrophotometer. X-ray diffraction (XRD) patterns were recorded Bruker D8 Advance diffrac tometer. X-ray photoelectron spectroscopy (XPS) characterization of 2-F modified bl-TiO2 layer were carried out by SPECS EA 300 with an aluminium anode. Fourier transform infrared spectra were measured with the range of wavenumber from 400 to 4000 cm 1 by Bruker Vertex 70 FTIR spectroscopy. Photocurrent density-voltage (J-V) characteris tics of devices fabricated without any encapsulation were measured under N2 atmosphere using a Keithley 2400 Source under dark or illu mination conditions (AM 1.5 G, 100 mW cm 2) provided by a solar simulator (ATLAS) glove-box integrated.
modified surface. The increased hydrophobicity may enhance the spreading of photoactive layer solution and interfacial contact due to the relatively hydrophobic nature of BHJ layer [19]. XPS is one of most common used technique to determine the bonded SAM molecules onto surface after modification. Fig. 4 shows the survey and high resolution XPS scan spectra of 2-F modified bl-TiO2. Fig. 4 (a) shows the characteristic peaks of bl-TiO2 layer and the O 1s, F 1s (a peak at 686.1 eV) [20] and C 1s core levels of organic molecule. In Fig. 4 (b), the peaks at 528.0 and 531.1 eV can be assigned to oxygen in rutile TiO2 [21]. As shown in Fig. 4 (c), the Ti 2p1/2 and Ti 2p3/2 peaks are located at 462.5 and 456.8 eV [22,23], respectively. In Fig. 4 (d), the C 1s spectrum can be fitted to two characteristic peaks at 285.3 and 283.1 eV, which can be assigned C–O bonds and C¼C bonds. The crystal structures of non-modified and modified bl-TiO2 thin films were characterized with XRD method. The pattern obtained is seen in Fig. 5 (a). It is obviously seen that the diffraction peaks belong to tetragonal rutile phase of TiO2 and there is no other phase or impurities detected [24]. Notably, after modification, the intensity of the XRD diffraction peaks decreased, which could be attributed to the acids/metal oxides bonding on the surfaces. It can be said that the crystalline quality of SAM modified bl-TiO2 layer decreases. To investigate the effect of SAMs on optical properties, UV–visible absorption measurements were carried out and Fig. 5 (b) illustrates the UV–Vis absorbance spectra of bl-TiO2 surfaces scanned in the range of 300 nm–800 nm. It is clear that the presence of SAM layer does not affect the absorbance of the bl-TiO2 layer in both UV and Visible region. For better understanding of the influence of interface modification on the device performances, the optical absorbance data was plotted as a function of photon energy (Fig. 6). The majority of researchers have
3. Results and discussion The solar cell structure and corresponding energy diagram are given in Fig. 2. To check the covering homogeneity of SAMs onto bl-TiO2, the contact angle measurements of water droplets on modified and nonmodified surfaces were performed and the average results are given in Fig. 3. As seen, the contact angle (CA) of varies from 34� to 53� and the modified bl-TiO2 surfaces show higher hydrophobicity in compared to non-modified surface. It could be inferred that SAMs successfully covered bl-TiO2 surfaces. The highest hydrophobicity is observed on 2-F
Fig. 6. (a) Direct band gap and (b) indirect band gap estimation from UV–Vis absorption data. 4
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gabs of bl-TiO2 can be easily tuned by introducing of fluorine-terminated boronic acid SAMs. The morphology of active layer is one the most important parameter to improve the electron and hole mobility, thus, the photovoltaic pa rameters are strongly associated with the morphological property of [26]. The growth of upper layer can be influenced and control by the chemical structure of modification layer. Fig. 7 shows the AFM images of modified bl-TiO2 surfaces and the average roughness (Ra) values are given in Table 2. The images demonstrate that the Ra values of non-modified bl-TiO2, 1-F/bl-TiO2, 2-F/bl-TiO2, and 3-F/bl-TiO2 sur faces were 17.1, 16.3, 14.9, and 13.7, respectively. The reduced Ra values could be attributed to the reduced number of pin holes and cracks on the bl-TiO2 surface. It reduces the density of trap states, which pro motes the photovoltaic stability [27]. The decreasing of Ra values of ETL may also help to improve the active layer morphology. The AFM images of P3HT:PCBM coated modified bl-TiO2 surfaces are given in Fig. 8. The surface properties of active layer seem to be influenced by the number and the position of substituent on the SAMs (Table 2). It can be said that SAMs lead to form an ordered upper layer structure via
Table 1 Direct and indirect bandgap transitions for non-modified and modified c-TiO2 surfaces. Surface
Bandgap (eV)
bl-TiO2/FTO 1-F/bl-TiO2/FTO 2-F/bl-TiO2/FTO 3-F/bl-TiO2/FTO
Direct
Indirect
3.788 3.823 3.846 3.806
3.568 3.603 3.619 3.590
reported that rutile phase of TiO2 has both direct and indirect band gaps [25]. Therefore, direct and indirect band gaps of non-modified and modified surfaces was calculated from optical absorption spectra. Fig. 6 (a) shows (αhν)2 vs. hν plot for direct band gap calculation, whereas Fig. 6 (b) shows (αhν)1/2 vs. hν plot for indirect band gap calculation, where α is the absorption coefficient and hν is the photon energy. The band gap values are calculated through the extrapolating to the X-axis. The values calculated are given in Table 1. The direct and indirect band gaps of non-modified bl-TiO2 surfaces were found to be 3.79 and 3.57 eV. After the modification of bl-TiO2 surfaces with 1-F, 2-F and 3-F SAMs, the direct band gaps increased to be 3.82, 3.85 and 3.81 eV, respectively, whereas indirect band gap increased to be 3.60, 3.62 and 3.59 eV, respectively. Increased energy gap could be beneficial for the improving of electron transport properties since lower values is to be responsible to lost a lot of electrons generating in the form of heat. As seen in Fig. 1, SAMs have different number of substituent and they give different permanent dipole moment magnitude and direction. Hence, the electrical properties of bl-TiO2 layer modified with SAMs strongly depend on the number of substituent. It can be said that the energy band
Table 2 The calculated values of water contact angle, work function change and average roughness differences between unmodified and modified c-TiO2 surfaces with different SAMs. Surface
Ra (nm)
Surface
Ra (nm)
bl-TiO2/FTO 1-F/bl-TiO2/FTO 2-F/bl-TiO2/FTO 3-F/bl-TiO2/FTO
17.1 16.3 14.9 13.7
P3HT:PCBM/bl-TiO2/FTO P3HT:PCBM/1-F/bl-TiO2/FTO P3HT:PCBM/2-F/bl-TiO2/FTO P3HT:PCBM/3-F/bl-TiO2/FTO
5.54 1.18 1.07 2.69
Fig. 7. AFM images of (a) non-modified bl-TiO2, (b) 1-F modified bl-TiO2, (c) 2-F modified bl-TiO2, and (d) 3-F modified bl-TiO2 surfaces. 5
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Fig. 8. AFM images of P3HT:PCBM layer on top of (a) non-modified bl-TiO2, (b) 1-F modified bl-TiO2, (c) 2-F modified bl-TiO2, and (d) 3-F modified blTiO2 surfaces.
Fig. 9. J-V curves of PSCs non-modified and modified with SAMs under (a) illuminated and (b) dark conditions.
surface-attaching chemically onto blocking TiO2 (bl-TiO2) layer at mo lecular level. It is known that the active layer morphology affected the charge transport properties and more homogenous surface increases the interfacial contact and benefits PCE [28]. Fig. 9 (a) shows the J-V curves of inverted PSCs based on modified with SAMs and non-modified bl-TiO2 layers to evaluate the photovoltaic characteristics as a function of modification layer. The performance characteristics extracted from J-V curves under illumination are sum marized in Table 3. For the non-modified device, photovoltaic charac teristics were observed with Jsc of 10.56 mA cm 2, Voc of 0.53 V and FF
of 41.6%, resulting in PCS 2.33%. This efficiency is approximately comparable with reported efficiencies of P3HT:PCBM based inverted PSCs, as seen in Table 4 [9,29–34]. In case of the modification through all SAMs, the device perfor mances present a notable increasing. Especially, the modification of blTiO2 layer with 2-F SAM yields a remarkable enhancement in overall device performance where Jsc increases to 16.51 mA cm 2, Voc to 0.58 V and FF to 45.7%, resulting in PSC 4.36%. The improved performance of modified PSCs is correlated to surface hydrophobicity and interfacial contact, thus, improved electrical properties. The Jsc and Voc of the 6
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Materials Science in Semiconductor Processing 107 (2020) 104860
Table 3 Summary of device parameters extracted from illuminated J-V curves presented in Fig. 9. Device FTO/bl-TiO2/P3HT: PCBM/MoO3/Ag FTO/bl-TiO2/1-F/P3HT: PCBM/MoO3/Ag FTO/bl-TiO2/2-F/P3HT: PCBM/MoO3/Ag FTO/bl-TiO2/3-F/P3HT: PCBM/MoO3/Ag
Voc (V)
Jsc (mA/ cm2)
FF (%)
PCE (%)
Rs (Ohms)
0.532
10.555
41.57
2.334
49.86
0.561
12.481
46.26
3.239
46.94
0.579
16.512
45.67
4.359
44.92
0.571
11.369
51.83
3.365
46.19
Table 5 The ideality factor (n) and dark saturation current density (Jo) extracted from dark J-V curves.
Voc (V)
Jsc (mA/ cm2)
FF (%)
Efficiency (%)
Ref.
ITO/TiO2/P3HT:PCBM/ PEDOT:PSS/Ag ITO/NP TiO2/TiO2/P3HT: PCBM/PEDOT:PSS/Ag ITO/TiO2/P3HT:PCBM/ V2O5/Ag FTO/c-TiO2/m-TiO2/P3HT: PCBM/Ag FTO/TiO2/P3HT:PCBM/ V2O5/Ag ITO/NP TiO2/P3HT:PCBM/ PEDOT:PSS/Ag ITO/TiO2/P3HT:PCBM/ MoO3/Al
0.58
6.38
55.4
2.06
[29]
0.59
9.89
47.00
2.72
[30]
0.53
8.75
36.65
1.70
[9]
0.54
6.30
38.06
1.30
[31]
0.56
8.61
43.59
2.13
[32]
0.55
8.54
62.08
2.88
[33]
0.60
6.54
37.00
1.45
[34]
Jo
FTO/bl-TiO2/P3HT:PCBM/MoO3/Ag FTO/bl-TiO2/1-F/P3HT:PCBM/MoO3/Ag FTO/bl-TiO2/2-F/P3HT:PCBM/MoO3/Ag FTO/bl-TiO2/3-F/P3HT:PCBM/MoO3/Ag
1.39 1.20 3.16 1.02
n � 10 � 10 � 10 � 10
3 3 4 3
2.488 2.144 2.140 2.143
conductivity. To study the charge transport properties in the dark, we extracted the reverse saturation current density (Jo) and the ideality factor (n) from dark J-V curves on logarithmic plot seen in Fig. 9 (b). Shockley-ReadHall equation, Eq. (1), was used to identify these parameter [42]. � � � � qV J ¼ Jo exp 1 (1) n kB T
Table 4 The comparison of photovoltaic parameters of reported similar device configu ration to non-doped device configuration used in this study. Device configuration
Device
where J is the current density, q is the electron charge, V is the applied voltage, kB is Boltzmann’s constant, and T is temperature. The Jo and n values, which are extracted from the intercept and the slope of dark J-V curves, are shown in Table 5. The decreased values of Jo in the PSCs modified with SAMs suggest the decreased back charge recombination leading to improved photovoltaic performance. Also, n values decreased after the modification, which is in agreement with the findings of series resistance calculation. 4. Conclusions Herein, we investigated the SAM modification of bl-TiO2 layer and the interface morphology of each layer of PSCs. This study demonstrates that charge transport properties in photoactive blend is intensively depend on the morphological and optical properties of bottom layer, which is bl-TiO2 in our configuration. It was found that better interfacial contact and reduced trap state density can be formed when different fluorine-terminated boronic acid SAMs is applied between bl-TiO2 and photoactive layer. Subsequently, we investigated the impact of the number and the position of substituent on the SAMs on photovoltaic characteristics. The PSC modified with 2-F SAM resulted the highest efficiency (4.4%) along with other enhanced photovoltaic characteris tics. This improvement was achieved thanks to enhanced charge trans port properties and band gap overlap at the interface between electron transport and photoactive layer. The results obtained in this study encourage the utilize of interfacial SAM modification on inverted PSCs to achieve higher efficiency.
modified device are enhanced in the order of Jsc,2-F > Jsc,1-F > Jsc,3-F > Jsc, non-modified and in the order of Voc,2-F > Voc,3-F > Voc,1-F > Voc, nonmodified, respectively. The enhanced Jsc values could be attributed to increased electron injection efficiencies and it is supported by the observation of optical absorption results [35]. The interfacial engi neering can lead to an enhancement in the photon-harvesting properties, which results an increase in the Jsc values of modified devices [36]. The FF values are improved from 42% to 52% after SAM modification. This improvement shows that the modified bl-TiO2 provides better interfacial contact for active layer. Also, the energy level matching at the interface between bl-TiO2 and active layer can enhance the both Voc and FF values [37]. Even though, the introduction of fluorine substitute into para- and meta-positions of boronic acid SAMs increase the photovoltaic charac teristics, it was clearly seen that substitution of fluorine in only both meta-positions of SAMs show much more positive effect on the charge injection efficiency. The series resistance (Rs) can be also used to investigate the inter facial contact between the photoactive and electron transport layers. As seen in Table 3, Rs in the 2-F modified device is 44.9 Ω, whereas Rs in the non-modified device presented a higher value (49.9 Ω). This reduced Rs values for all SAM modified devices are coherent with the other photovoltaic parameters suggesting the improved interfacial contact [38]. Additionally, it can be said that lower values of Rs suggest the reduced carrier recombination after SAM modification, which is an important result to obtain highly efficient PSCs [39]. As well known, the FF value that can be obtained is strongly related to the Rs of devices prepared. The high Rs values arise the poor ohmic contact at the inter face which causes to contact resistance and charge transfer rate losses [40]. The high FF values of devices fabricated in this study show that the rough surface of bl-TiO2 layer prevents photogenerated carriers at the donor-acceptor interface from reaching the electrode [41]. The increasing FF values after modification could be attributed to reduced series resistance due to the improved the interlayer contact and
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to thank to Abdullah Kantar and Mert Temür undergraduate students for their contribution. The authors would also like to thank the Selçuk University Advanced Technology Research and Application Centre for the facilities and technical assistance. References [1] Z. Gao, L. Guo, Y. Sun, W. Qu, T. Yang, B. Li, J. Li, L. Duan, Passivating ZnO with a naphthalimide-Schiff base as electron transport layer for inverted polymer solar cells, Org. Electron. 67 (2019) 232–236. [2] J.-L. Lan, Z. Liang, Y.-H. Yang, F.S. Ohuchi, S.A. Jenekhe, G. Cao, The effect of SrTiO3:ZnO as cathodic buffer layer for inverted polymer solar cells, Nano Energy 4 (2014) 140–149.
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