Controllable Spatial Configuration on Cathode Interface for Enhanced

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Surfaces, Interfaces, and Applications

Controllable Spatial Configuration on Cathode Interface for Enhanced Photovoltaic Performance and Device Stability Jiangsheng Li, Chenghao Duan, Ning Wang, Chengjie Zhao, Wei Han, Li Jiang, Jizheng Wang, Yingjie Zhao, Changshui Huang, and Tonggang Jiu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19429 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on April 30, 2018

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ACS Applied Materials & Interfaces

Controllable Spatial Configuration on Cathode Interface for Enhanced Photovoltaic Performance and Device Stability Jiangsheng Li†, Chenghao Duan†, Ning Wang†,§, Chengjie Zhao†, Wei Han‡, Li Jiang‡,§, Jizheng Wang‡,§, Yingjie Zhaoll, Changshui Huang†,§, Tonggang Jiu*†,§

†

Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of

Sciences, Qingdao, 266101, P. R. China. ‡

Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of

Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. §

University of Chinese Academy of Sciences, Beijing 100049, PR China.

ll

Qingdao University of Science and Technology, Qingdao 266042, P. R. China.

E-mail: [email protected]

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ABSTRACT:

Molecular structure of cathode interface modification materials can affect the surface morphology of active layer and key electron transfer processes occurring at interface of polymer solar cells in inverted structure mostly due to the change of molecular configuration. To investigate the effects of spatial configuration of cathode interfacial modification layer on PSCs device performances, we introduced two novel organic ionic salts (linear NS2 and 3D NS4) combined with ZnO film to fabricate highly efficient inverted solar cells. Both of organic ionic salts successfully decreased the surface traps of ZnO film and made its work function more compatible. Especially NS4 in three-dimensional configuration increased the electron mobility and extraction efficiency of interfacial film leading to a significant improvement of device performance. Power conversion efficiency (PCE) of 10.09% based on NS4 was achieved. Moreover, 3D interfacial modification could remain about 92% of its initial PCE over 160 days. It is proposed that 3D interfacial modification retards the element penetration-induced degradation without impeding the electron transfer from active layer to ZnO film, which significantly improves the device stability. This indicates that inserting three-dimensional organic ionic salt is an efficient strategy to enhance device performance.

KEYWORDS: controllable spatial configuration, organic ionic salts, ZnO, cathode buffer layer, polymer solar cells


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1. Introduction Polymer solar cells (PSCs) with inverted architecture have received great attention for their notable characteristics of long-term stability, ease of fabrication, solution-based processing, and the capability for large-area production at low temperatures.1-3 In this PSCs structure, the bottom electrodes are modified by interlayer materials, acting as cathode buffer layer (CBL). The change of interface molecular configuration exerts an effect on the interfacial morphology and electronic transport properties, consequently affects the device performance.4,5 Recently 3D configuration of dye molecular is found to play an influential role in the crucial processes of electron transfer and electron lifetime occurring at interface in dye sensitized solar cell.4,6-8 Therefore it’s a useful method to improve PSCs performance through spatial configuration design of the CBL molecules. However, the study of CBL’s molecular configurations in PSCs is still lacking, which confines its further specification and application. As a result, for the optimization of PSCs performance, the need to design a new molecular spatial configuration of CBL as well as further investigation on molecular spatial configuration and end group functionalization effects is urgent. ZnO is one of most appealing materials for CBL applications because of its attractive properties such as suitable energy levels, excellent optical transmittance, relatively high electron mobility, and easy to process with low cost techniques.9,10 However, without interface engineering, the surface defects of ZnO film can result in a bad interfacial contaction with active layer, consequently leading to a poor electron transfer and a high charge recombination.11,12 Thus, a large number of methods have



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been employed to optimize ZnO film and decrease the defects,9,10,13-17 for example, self assembly monolayer,18 doping ZnO CBLs and surface engineer.3,14,19 Among them, it is a useful and simple way via inserting an organic ionic salt between ZnO film and active layer.20 Organic ionic salt itself at the interfaces could retard energy loss in the processes of electron transfer.21-23 Those methods help enhance the quality of interconnection and improve the work function of CBL, further increasing the stability and performance of PSCs. However, few studies have focused on the effects of spatial configuration of modification molecular used for the ZnO film modification. In the present work, we introduce two organic ionic salts (linear NS2 and 3D NS4) with different spatial configuration combined with ZnO film as CBL to fabricate high performance inverted PSCs. The insertion of organic ionic salts between ZnO film and active layer successfully passivated the traps of ZnO surface and made work function more compatible. Especially a high power conversion efficiency (PCE) of 10.09% with 3D NS4 was achieved. Moreover, the stability of devices with 3D NS4 was improved. The device based on interfacial modification in 3D spatial configuration could remain about 92% of its initial PCE over 160 days and presented better device stability compared with linear interfacial molecular based device and ZnO-sole based device. To figure out the effects of different spatial configurations of CBL on devices, a series of measurements such as energy level property, surface morphology and the properties of photoluminescence were performed. Herein the active

layer

used

in

the

experiment

comprises

of

poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b`]dithiophene-alt-3-fluor


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othieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM).

2. Results and Discussion

Figure 1. (a) Schematic diagram of device fabrication. (b) The chemical structures of two organic ionic salts. (c) UPS curves of ZnO, ZnO/NS2 and ZnO/NS4 as well as (d) energy level of the components.

The device configuration with two organic ionic salts are shown in Figure 1(a), and the materials applied in the device are presented in Figure 1(b). Figure S1 shows the spectra of X-ray photoelectron spectroscopy (XPS) of different CBLs. The N 1s peaks



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of ZnO, ZnO/NS2 and ZnO/NS4 are exhibited in Figure S1 (a), and the atomic of N 1s with different cathode buffer layers is displayed in Figure S1 (a) inset. As can be seen, the atomic of N 1s is increased after the coating of NS2 and NS4, which indicates the existence of NS2 and NS4 on ZnO film. From Figure S1 (b), we can see that the Zn 2p3/2 peak appears at 1021.63 eV and 1021.66 eV for ZnO/NS2 and ZnO/NS4 , respectively, which is ~0.2 eV and ~0.17 eV shift to low binding energy for ZnO (1021.83 eV). The shift indicates that more Zn atoms bind with to O atoms.24-25 We propose the reason is that NS2 and NS4 works closely with ZnO to share a lone electron pair from oxygen of the sulfonic acid group. Due to the influence of the O-Zn bonding, NS2 was transformed into a two-dimensional configuration instead of a linear structure, and NS4 was distorted thus forming a 3D configuration. To understand the influence of different molecular configurations on the electronic property, we carried out ultraviolet photo-electron spectroscopy (UPS) measurement to study the energy levels differences of the three buffer layers. In UPS spectra of Figure 1(c), the left panel is Ecutoff obtained from the high binding energy cutoff, and ୌ୓୑୓ E୭୬ୣୱୣ୲ can be acquired in the right panel. The highest occupied molecular orbital

(HOMO) level can be worked out by the following equation: ୌ୓୑୓ EHOMO=hv-(E୭୬ୣୱୣ୲ -Ecutoff)

(1)

where hν is the incident photon energy (21.2 eV).26,27 Consequently, the HOMO levels for ZnO, ZnO/NS4, and ZnO/NS2 films are −7.65, −7.48, and −7.36 eV, respectively. As shown in Figure 2(d), the LUMO levels of ZnO, ZnO/NS4 and ZnO/NS2 can be worked out as -4.25 eV (ZnO), -4.08 eV (ZnO/NS4) and -3.96 eV(ZnO/NS2),


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obtained from UV curves (Figure S2). The increased LUMO levels revealed that an intermediate energy gradient between interlayer and active layer was formed by NS2 and NS4 at interface, which promoted the transfer of electron.28,29 In addition, work function (WF) is a crucial factor determining the minimum energy necessary to remove an electron from the material.30 For instance, electron density can not move into the metal from the organic layer if the WF of the organic layer is lower than that of the metal electrode, unless the Fermi levels is equilibrated.31 When energy of photons hv is known, the value of WF can be calculated by subtracting Ecutoff from hv. WF can be calculated by following formula:

WF=hv﹣Ecutoff

(2)

The WFs for ZnO, ZnO/NS4 and ZnO/NS2 are 3.63 eV, 3.46 eV and 3.4 eV, respectively. The reducing of work function of ZnO resulted in a downward shift of vacuum level, which can induce a large build-in potential and ultimately contribute to the enhancement of circuit voltage (Voc) as well as the electron collection.20 These findings confirm that the modification of organic ionic salts is in favor of the enhancement of the Voc and PCE of the PSCs. What’s more, a linear molecular structure is greatly beneficial to improve the Voc of the device.



Figure 2. Atomic force microscopy (AFM) images (2 µm × 2 µm) of (a) ZnO, (b) ZnO/NS2, (c) ZnO/NS4, active layer on top of (d) ZnO, (e) ZnO/NS2 and (f) ZnO/NS4.

As exhibited in Figure 2, AFM was performed to study the surface morphology change of ZnO nanoparticles and active layers affected by different organic ionic salts coating. The root-mean-square roughness (RMS) of ZnO film, ZnO/NS2 film and ZnO/NS4 film are 2.38 nm, 1.87 nm and 1.92nm, respectively. And the RMS of active layer films on ZnO, ZnO/NS2 and ZnO/NS4 films are 1.27nm, 1.18nm, and 1.25nm, respectively. We found that there was no significant difference but a slight decrease on the roughness among CBLs and active layers after the inserting of NS2 and NS4, which indicated that both of two spatial configurations of organic ionic salts have no influence on the roughness of the ZnO film as well as the active layer. Scanning electron microscopy (SEM) is also employed to examine the CBLs morphology, as shown in Figure S3 (Supporting


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Information). We can observe that all of pure ZnO layer, ZnO/NS2 layer and ZnO/NS4 layer formed dense films, which would be in favor for the collection of electrons and blocking of holes. For further study on the surface wettability, contact angle of water measurement was employed (Figure S4). The contact angles are 51.3º, 56.5º and 55.5º for ZnO, ZnO/NS2 and ZnO/NS4 interface layers, respectively. After coating with organic ionic salts, the contact angles of these films showed no obvious change, and no significant difference in surface wettability as well.



Figure 3. (a) Illuminated (b) dark current density-voltage (J-V) curves and (c) external quantum efficiency (EQE) curves of devices based on ZnO, ZnO/NS2 and ZnO/NS4 as CBL. (d) J-V plotted in the format ln (JL3/V2) versus (V/L)0.5 of the devices based on ITO/Al/ZnO/Al, ITO/Al/ZnO/NS2/Al and ITO/Al/ZnO/NS4/Al. Schematic illustration of the electron transport for (e) ZnO, (f) ZnO/NS2 and (g) ZnO/NS4 as CBLs respectively.


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To figure out the impact of two spatial configurations of organic ionic salts on the polymer solar cell performance, J-V curves are exhibited in Figure 3a and details are reported in Table S1 (Supporting Information). The device with ZnO as CBL exhibited a PCE of 9.27% (average 9.16%), open circuit voltage (Voc) of 0.790 V, short circuit current (Jsc) of 16.4 mA cm-2, and fill factor (FF) of 71.4%. After depositing the NS2 and NS4 layers onto the ZnO film, the device with ZnO/NS2 as CBL showed a PCE of 9.96% (average 9.68%), Voc of 0.796 V, Jsc of 17.0 mA cm-2 and FF of 73.5% while the device with ZnO/NS2 demonstrated a remarkably enhanced PCE of 10.09% (average 9.92%), Voc of 0.792 V, Jsc of 17.3 mA cm-2, FF of 73.7%. We found that both of two molecular spatial configurations show an effective contribution for the improvement in Jsc and PCE. Especially a three-dimensional organic ionic salt (NS4) exhibited more significant enhancement of Jsc and PCE. To examine the reproducibility, 14 devices with ZnO, ZnO/NS2 and ZnO/NS4 as CBL were examined under the same condition. The distribution of parameters are displayed in Figure S5 (Supporting Information). We can see that Jsc and PCE were enhanced obviously from devices with ZnO to that with ZnO/NS4. Meanwhile, the Voc and FF were also improved after the inserting of NS2 and NS4. It reveals that the devices with ZnO/NS2 and ZnO/NS4 bilayers as CBL demonstrate excellent reproducibility and favorable performance. To further investigate the electrical characteristics of different molecular spatial configurations of organic ionic salts, the J-V characteristics of the devices based on ZnO, ZnO/NS2 and ZnO/NS4 under dark condition are



depicted in Figure 3(b). Clearly, the device based on ZnO displayed a high current leakage in the reverse direction. The device based on ZnO/NS2 exhibited a lower current leakage while the device based on ZnO/NS4 showed a lowest current leakage duo to the remarkable diode characteristics which was beneficial to the extraction of electron.32 Figure 3(c) showed EQE spectra of the device with ZnO, ZnO/NS2 and ZnO/NS4 interlayers. It is observed that EQE of ZnO/NS2 based device was clearly enhanced at 450-710 nm compared to the device based on ZnO-only. The device based on ZnO/NS4 showed a significant improvement at 345-390 nm and 450-710nm compared with the device based on ZnO. The results are in excellent agreement with the Jsc exhibited above. To investigate the reason of Jsc enhancement, the measurement of electron mobility is performed.29,33 As shown in Figure 3(d), the results are plotted in the format ln(JL3/V2) versus (V/L)0.5, where J is the current density and L is the thickness of the ZnO or ZnO/NS2 or ZnO/NS4 layer. The lines are the fit to the respective experimental points. We can work out the electron mobility of ZnO based electron-only device was 8.52×10-5 cm2 V-1 s-1, which is close to the reported value 1.8 × 10-4 cm2 V-1 s-1.34 The electron mobility of devices with NS2 and NS4 are 2.25×10-4 and 9.11×10-4 cm2 V-1 s-1. The result shows an obvious influence of NS2 and NS4 modification on electron transport mobility. In particular, the insertion of 3D NS4 leads to a faster electron transport mobility compared with 2D NS2. It is proposed that three-dimensional organic ionic salt can make ZnO combined in a firmer way by forming a three-dimensional spatial configuration with ZnO film thus strengthening


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the contaction between interlayer and active layer, which opens a easier electron transfer passway, as shown in figure 3(e)-3(g). Therefore, the insertion of a three-dimensional configuration of organic ionic salt can enhance the Jsc more effectively compared with two-dimensional one formed by a linear molecule structure.

Figure 4(a) Optical transmittance curves for ITO/ZnO, ITO/ZnO/NS2 and ITO/ZnO/NS4. Figure 4(b) Photoluminescence spectra of devices with ZnO, ZnO/NS2 and ZnO/NS4. Figure 4(c) Time-resolved photoluminescence (TRPL) for ZnO, ZnO/NS2 and ZnO/NS4. Figure 4(d) Plotted in the format of photocurrent density (Jph) versus effective bias (Veff) of the devices basd on ZnO, ZnO/NS2 or ZnO/NS4. Figure 4(e) Electrochemical impedance spectroscopy (EIS) of devices based on ZnO, ZnO/NS2 and ZnO/NS4. Figure 4(f) Stability tests of devices based on



ZnO, ZnO/NS2 or ZnO/NS4. Figure 4(g) Schematic diagram of effect of two types of organic ionic salts. Figure 4(a) displayed the optical transmittance curves at wavelength range of 300-800 nm for ITO/ZnO, ITO/ZnO/NS2 and ITO/ZnO/NS4, respectively. All of the optical transmittance spectra showed over 80% average transmittance, which proved that both of NS2 and NS4 do no harm to the optical absorption of device as well as the light reaching active layer. To investigate the effect of NS2 and NS4 passivation on surface defects of ZnO, the steady-state photoluminescence (PL) measurements were investigated. We can see from Figure 4(b) that, there are two emission peaks (373nm and 535nm) existing on ZnO. The deep level emission of 535 nm was regarded as an evidence of the existence of defect states in ZnO.20 It should be noticed that the defect emission band (an emission band at 519 nm) decreased after the insertion of the layer of NS2 and NS4. The result indicates that the defects states were significantly reduced when inserting both of two organic ionic salts. Especially when NS4 in 3D configuration was coated onto the ZnO film, the surface defects of ZnO film was decreased more effectively. A lower trap results in an enhanced FF of the device by decreasing the possibility of carrier recombination at interface, so as to improve the PCE of device.35-37 We propose that ionic salts work closely with ZnO through sharing a lone electron pair of oxygen, passivating the defects of ZnO film. A three-dimensional geometry of organic ionic salt can be combined more firmly with the ZnO film by forming a three-dimensional geometrical structure consequently more effectively passivating the surface traps of ZnO film.


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To further confirm the charge transport improvement based on different molecular spatial configurations of organic ionic salts, the time resolved PL measurement was performed, as shown in Figure 4(c). The average carrier lifetime of ZnO film was 11.5 ns. And after NS2 and NS4 modification, the average carrier lifetime drops to 3.07 and 0.22ns, respectively. It was observed that the PL lifetime reduced significantly especially for 3D NS4 molecular configuration. The result confirms that NS2 and NS4 modifications contribute to faster charge transfer from active layer to CBL as compared to pure ZnO film.38,39 Moreover, 3D molecular spatial configuration achieves a more notable effect to improve electron transfer than 2D molecular configuration. This leads to an increased charge transport mobility and consequently improves the Jsc and PCE. To investigate the effect of different spatial configurations of CBL on the PSCs devices, the relationship between photocurrent density (Jph) and effective voltage (Veff) for the devices based on ZnO, ZnO/NS2 and ZnO/NS4 were plotted in Figure 4(d).40-42 As displayed in Figure 4(d), as the effective voltage increases, saturation photocurrent (Jsat) of device based on ZnO/NS2 and ZnO/NS4 reached earlier compared with that based on pure ZnO. As is well know, Jsat is relevent to maximum exciton generation rate (Gmax), which is is given by Jsat = qGmaxL ( q is the electronic charge and L is the thickness of active layer ). We can easily figure out that the Gmax of three devicesare almost equal (about 1.06×1028). Interestingly, exciton dissociation probability (P) determined by density Jph/Jsat is different. For instance, at 0.2 V of Veff, the value of P is 69.0% for the ZnO based PSCs whereas they are 81.6% and 84.1%



for ZnO/NS2 and ZnO/NS4 based devices, respectively. The increase of P suggests the decrease of exciton recombination rate. It indicates that charge recombinations have been suppressed after the insertion of NS2 and NS4 thus leading to better fill factor values.1,42 In addition, the result also indicates that three-dimensional spatial configuration of organic ionic salt can suppress the exciton recombination more effectively compared with two-dimensional one. EIS measurement was used to understand the interfacial properties in cells , as illustrated in Figure 4(e). The semicircle diameter in the plots, presents the charge transfer resistance (Rct), which is relevant to charge recombination and the leakage of current in CBL and active layer. Larger Rct is denoted as less charge recombination and the leakage of current occurring at interface.43-46 As displayed in Figure 4(e), the semicircle diameters raised obviously from device based on ZnO to that based on ZnO/NS2 and ZnO/NS4. The results indicates that the interfacial contact between ZnO film and active layer was improved dramatically by reducing charge recombination and the leakage of current. Therefore Jsc and FF were improved tremendously. Especially NS4 based device presented the largest diameters of the semicircles, which indicates that 3D molecular spatial configuration can achieve better inter-contact than 2D molecular configuration which consequently reduces current leakage and electron recombination more effectively. The result is consistent with the results of photoluminescence spectra and exciton dissociation probability, implying that the recombination of charge is really reduced after inserting NS2 and especially NS4 between ZnO film and active layer.


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The measurement of lifetime was conducted and showed in Figure 4 (f) to study the stability of PSCs based on different CBL. We can see that the PCE of devices with 3D interfacial modification remained 92.0% after 162 days storage which was far better than that based on pristine ZnO (remaining 81.6%). And the device based on NS2 modification retained the PCE of 90.6% and stayed stable as well, which indicated that inserting organic ionic salts especially 3D NS4 combined with ZnO film as CBL is a useful method to enhance the stability of PSCs. As certified above, a three-dimensional organic ionic salt can combine with ZnO in a firmer way by forming a three-dimensional spatial configuration with ZnO film, which successfully avoids the element permeating ultimately improving the stability of devices. The mechanism of electron transfer at the cathode interface based on two spatial configurations of organic ionic salts are presented on the basis of mentioned features. Figure 4(g) displayed that there are many defects existing on ZnO interface which serve as the centers of charges recombination, consequently blocking the transfer of charge and reducing FF of the PSCs, as demonstrated by PL spectra. After organic ionic salts coating, the defects of interface are remarkably passivated, which improves the transfer of electron and decreases the recombination of charge. At the same time, the energy levels are better matched, thus improving Voc and facilitating electron transfer between electrode and active layer. These findings result in the enhancement of Voc, Jsc, and FF so as to improve the performance of PSCs.

3. Conclusion



In this paper, we have designed and prepared two organic ionic salts in different spatial configurations combined with ZnO as CBL in PSCs. This allowed a study of the molecular spatial configuration effects on the optoelectronic properties of the resulting CBL. We found that controllable molecular spatial configuration can decrease the work function, resulting in a large build-in potential and consequently the improvement of the Voc and the collection of electron. Besides, it can increase charge transport mobility so as to improve Jsc. PL spectra revealed that molecular spatial configuration change can passivate the traps existing on ZnO interface, which decreased carrier recombinations at interface and consequently led to an improved Jsc and FF especially for 3D molecular spatial configuration. The measurement of exiton generation rate and electrochemical impedance spectroscopy further confirmed this conclusion. Stability tests indicated that inserting organic ionic salts especially 3D NS4 is a useful method to enhance the stability of device. Therefore, an optimized PCE of 10.09% was achieved for the devices based on ZnO/NS4 compared with PCEs of 9.96% and 9.27% for the devices based on ZnO/NS2 and pure ZnO. All of these results revealed that three-dimensional organic ionic salt configuration could effectively modify the cathode interface and improve the performance of PSCs devices. Therefore, we believe this work offers the enlightment for the design of molecular spatial configuration of CBL, which opens up a space of interfacial improvement and benefit for future device fabrication of PSCs. 4. Experimental section Experiment material, fabrications of device, measurements, as well as synthesis are


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provided in the Supporting Information. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: UV-vis absorption of ZnO, ZnO/NS2 and ZnO/NS4 on quartz, details of devices based on different CBLs, SEM images of ZnO, ZnO/NS2 and ZnO/NS4, details of distribution of devices with ZnO, ZnO/NS2 and ZnO/NS4, details of parameter of devices with different concentration (from 0.5 to 2 mg mL-1) based on ZnO/NS2 and ZnO/NS4 and Experimental section. AUTHOR INFORMATION *E-mail: [email protected] Notes The authors declare no competing financial interests. Acknowledgments The Project is financially supported by Natural Science Foundation of China (51672288), Major Basic Research Program of Shandong Natural Science Foundation (ZR2017ZB0313) and DICP QIBEBT (UN201705), Dalian National Laboratory For Clean Energy. This study was also supported by Youth Innovation Promotion Association (CAS). References (1)

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Controllable Spatial Configuration on Cathode Interface for Enhanced

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