Alkali Salt-Doped Highly Transparent and ... - ACS Publications

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Alkali Salt-Doped Highly Transparent and Thickness-Insensitive Electron-Transport Layer for High-Performance Polymer Solar Cell Rongguo Xu,† Kai Zhang,*,† Xi Liu,† Yaocheng Jin,† Xiao-Fang Jiang,† Qing-Hua Xu,‡ Fei Huang,*,† and Yong Cao† †

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China ‡ Department of Chemistry, National University of Singapore, 117543 Singapore S Supporting Information *

ABSTRACT: Solution-processable highly transparent and thickness-insensitive hybrid electron-transport layer (ETL) with enhanced electron-extraction and electron-transport properties for high-performance polymer solar cell was reported. With the incorporation of Cs2CO3 into the poly[(9,9-bis(6′-((N,N-diethyl)-N-ethylammonium)-hexyl)2,7-fluorene)-alt-1,4-diphenylsulfide]dibromide (PF6NPSBr) ETL, the power conversion efficiency (PCE) of resulted polymer solar cells (PSCs) was significantly enhanced due to the favorable interfacial contact, energy-level alignment, and thus facile electron transport in the PSC device. These organic− inorganic hybrid ETLs also exhibited high transparency and high electron mobility. All of these combined properties ensured us to design novel thickness-insensitive ETLs that avoid the parasitic absorption of ETL itself simultaneously. With the conventional device structure with poly{4,8-bis[5-(2-ethylhexyl)thiophen-2-yl]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-3-fluoro-2-[(2ethylhexyl)carbonyl]thieno[3,4-b]thiophene-4,6-diyl} (PTB7-Th) as a donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an acceptor, devices with hybrid ETLs exhibited PCE of 8.30−9.45% within a wide range of ETL thickness. A notable PCE of 10.78% was achieved with the thick active layer poly(2,5-thiophene-alt-5,5′-(5,10-bis(4-(2-octyldodecyl)thiophen-2-yl)naphtho[1,2-c:5,6-c′]bis([1,2,5]thiadiazole)) (PTNT812):PC71BM. These findings indicated that doping alkali salt into the organic interfacial materials can be a promising strategy to design highly efficient and thickness-insensitive ETL, which may be suitable for large-area PSC modules device fabrication with roll-to-roll printing technique. KEYWORDS: alkali salt, highly transparent, thickness-insensitive, electron-transport layer, polymer solar cell

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INTRODUCTION Bulk-heterojunction polymer solar cells (PSCs) based on a blend of donor and acceptor have drawn much attention due to their potential as renewable energy source, with the advantages of lightweight, low cost, and solution processability.1−3 Over the past few years, much progress had been achieved via the design of new active layer materials,4−9 interface engineering,10−13 and new processing methods,14−16 with power conversion efficiencies (PCEs) exceeding 13%.17 These achievements have driven the development of roll-to-roll printing processes for large-area PSC modules. However, most of the commonly used thickness-sensitive materials that are processed by spin coating in lab showed low compatibility with roll-to-roll printing.18 This can be ascribed to that roll-toroll printing often results in film thickness with tens of nanometers variation.19,20 To meet the technical requirements of roll-to-roll printing processes, new materials and device fabrication processes that are suitable for large-area modules should be developed. Currently, plenty of efforts have been devoted to the development of active materials that can work efficiently within a wide range of thickness variation, e.g., © XXXX American Chemical Society

several kinds of active layer materials have been designed and shown considerably high efficiency with thickness of 300−1000 nm,4,5,21−25 which provides an effective pathway for roll-to-roll printing. However, in terms of interfacial materials, there is still a long way to cover before making it compatible with roll-toroll printing processes. Interfacial materials between the active layer and electrodes have been proven to efficiently improve the contacts between cathodes and active layers, increase electron extraction, suppress bimolecular recombination, and eventually improve efficiencies of PSCs.26 Thus, inserting appropriate interfacial materials was proved to be an efficient way to improve PSC performance. Several kinds of interfacial materials have been reported, e.g., transition-metal oxides (NiO,27,28 TiO2,29−31 ZnO,32−34 etc.), alkali-metal oxides (Li2CO3,35 Cs2CO3,36−38 etc.), nonconjugated polymers (polyethylenimine,39,40 polyethylenimine ethoxylated,41,42 etc.), and conjugated polymers Received: November 9, 2017 Accepted: December 22, 2017

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DOI: 10.1021/acsami.7b17076 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Scheme 1. (a) Device Figuration of Conventional PSC; (b) Chemical Structures of PF6NPSBr and the Relevant Components of the Active Layer

(poly(3,4-ethylenedioxythiophene) (PEDOT),43,44 poly[(9,9bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9dioctylfluorene)] (PFN),45−47 etc.). However, different from the spin-coat process, ideal interfacial materials compatible with roll-to-roll printing should be easy to be fabricated by lowtemperature solution-processing and work efficiently within a wide range of thicknesses without significantly affecting the performance of the PSCs. Unfortunately, there are few interfacial materials that possess such combined properties. Although recent works on ZnO doping demonstrated that it can work efficiently within a wide thickness range,12,48 the hightemperature thermal annealing and photo instability of ZnO still hinder its practical application in large-area device fabrication. Water/alcohol-soluble conjugated polymers (WSCPs), which consist of conjugated main chains and polar/ionic side chains, have been successfully used as cathodic interfacial materials in optoelectronic devices due to their unique advantages.46,47,49,50 One of the most representative WSCPs, poly[(9,9-bis(3′-(N,Ndimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN),45 have been widely used as the electron-transport layer (ETL) in organic light-emitting diode devices51 and PSC devices.52,53 However, due to their relatively low electron mobility, these WSCPs can work efficiently only when they keep extremely thin, typically less than 10 nm. To enhance the thickness tolerance of the ETL materials, groups like naphthalene diimide (NDI) 13,54 and perylene diimide (PDI)49,55−57 with planar structures and high electron affinity were introduced to synthesize thickness-insensitive ETL materials. These NDI- or PDI-based main chains combined with the polar functional side chains endow the polymers with good electron mobility, excellent solubility in polar solvents, and efficient cathode modification ability. All of these specific properties bring them potential application as thick ETLs without significantly sacrificing the performance of the PSCs. As an alternative way to molecular design, n-doping has proven to be an efficient method to fabricate highly conductive and thickness-insensitive ETL.58 In this article, we report a new strategy for achieving thickness-insensitive cathode interlayer by doping alkali salt Cs2CO3 into water/alcohol-soluble nonconjugated polymer, poly[(9,9-bis(6′-((N,N-diethyl)-N-ethyl-

ammonium)-hexyl)-2,7-fluorene)-alt-1,4-diphenylsulfide]dibromide (PF6NPSBr, Scheme 1b).59 The limited conjugation of the polymer endows it with weak optical absorption and high transparency in the visible region. Incorporation of Cs2CO3 into the ETL changes the surface morphology of ETL and forms a good interfacial contact between ETL and metal cathode, leading to facile electron transport in the PSC device. This inorganic−organic hybrid ETL exhibited excellent electrode work function (WF) modification ability, which is an essential feature for the ETL to fabricate high-performance PSCs. Using an active layer of poly{4,8-bis[5-(2-ethylhexyl)thiophen-2-yl]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophene-4,6diyl} (PTB7-Th):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), a significant improvement from 8.73 to 9.45% was achieved with the optimum doping concentration of 10.0 wt % Cs2CO3 in the PF6NPSBr ETL. More interestingly, the introduction of Cs2CO3 could also greatly improve the electron mobility of the PF6NPSBr ETL, which motivated the fabrication of high-performance PSCs with thick ETLs. Prominent PCEs as high as 8.30−9.45% were achieved within a wide range of thickness of Cs2CO3-doped PF6NPSBr ETL, much higher than that of undoped ETL. We also tested the compatibility of the hybrid ETLs with thick active layer system, poly(2,5-thiophene-alt-5,5′-(5,10-bis(4-(2-octyldodecyl)thiophen-2-yl)naphtho[1,2-c:5,6-c′]bis([1,2,5]thiadiazole)) (PTNT812):PC71BM.60 Our results suggest that alkali doping is an efficient strategy to achieve thickness-insensitive cathode interlayer without significantly sacrificing the performance. This inorganic−organic hybrid ETL is a promising candidate to serve as the ETL for roll-to-roll-processed large-area PSC modules.

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RESULTS AND DISCUSSION Device Performance of PSCs. To investigate the effects of introducing Cs2CO3 into the PF6NPSBr ETL, PSC devices with conventional configuration of indium tin oxide (ITO)/ PEDOT:polystyrene sulfonate (PSS)/PTB7-Th:PC71BM/ PF6NPSBr:x%-Cs2CO3 ETL/Ag were fabricated, as shown in Scheme 1a. The thickness of the ETLs was fixed at about 30 nm by solution-processing the blend of PF6NPSBr and Cs2CO3, B


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Table 1. Photovoltaic Performance of PSCs with PF6NPSBr ETLs (30 nm) Doped with Different Concentrations of Cs2CO3 under AM 1.5 G Irradiation (100 mW/cm2) Jsc (mA/cm2)

Voc (V) 0 wt % 5.0 wt % 7.5 wt % 10.0 wt % 12.5 wt % 15.0 wt % Cs2CO3

0.743 0.760 0.783 0.783 0.766 0.764 0.709

± ± ± ± ± ± ±

0.001 0.004 0.002 0.002 0.008 0.007 0.008

14.83 14.85 14.87 15.10 14.62 13.43 15.53

± ± ± ± ± ± ±

FF (%) 0.10 0.01 0.24 0.21 0.09 0.09 0.05

62.44 65.32 67.50 71.18 57.89 47.15 62.63

PCE (%) ± ± ± ± ± ± ±

0.67 0.56 0.13 0.14 1.04 0.61 0.68

PCE (best)

6.88 7.37 7.86 8.41 6.48 4.84 6.89

± ± ± ± ± ± ±

0.13 0.03 0.09 0.15 0.14 0.07 0.07

6.97 7.39 7.92 8.52 6.58 4.90 6.94

devices with PF6NPSBr ETL without Cs2CO3 doping exhibited a best PCE of 6.97% with an open-circuit voltage (Voc) of 0.744 V, a short-circuit current density (Jsc) of 14.90 mA/cm2, and a fill factor (FF) of 62.91%. The performance of the PSCs slightly changed when 5 wt % Cs2CO3 was incorporated into PF6NPSBr ETL. With the increase of Cs2CO3 in the ETL, the performance of PSCs continuously enhanced in certain scope and reached to a best PCE of 8.52% when the concentration of the Cs2CO3 increased to 10.0 wt %. Compared to the control devices without Cs2CO3 dopant, performance of devices with 10 wt % Cs2CO3-doped ETL showed 22% improvement, mainly originated from the increase of Voc and FF, from 0.744 to 0.784 V and 62.91 to 71.29%, respectively. Further increasing the concentration of the Cs2CO3 resulted decreased performance, that is, PCEs dropped to 6.59 and 4.90% with concentrations of 12.5 and 15.0 wt %, respectively. As a contrast, PSCs with solution-processed pure Cs2CO3 as the ETL were also fabricated, which exhibited a best PCE of 6.94%. Morphology Characterization. To study the influence of Cs2CO3 on the morphology of PF6NPSBr ETL film, atomic force microscopy (AFM) and scanning electron microscopy (SEM) measurements were performed. Figure 2 shows topographical height images of solution-processed PF6NPSBr

where the concentrations of the Cs2CO3 in the ETL were 5.0, 7.5, 10.0, 12.5, and 15 wt %. The photovoltaic performances under AM 1.5 G illumination at 100 mW/cm2 are summarized in Table 1 and Figure 1. The PTB7-Th:PC71BM-based control

Figure 1. J−V curves of PSCs with solution-processed PF6NPSBr ETLs (30 nm) doped with different concentrations of Cs2CO3.

Figure 2. AFM topographical height images (5 μm × 5 μm) of solution-processed PF6NPSBr ETL films doped with different concentrations of Cs2CO3: (a) 0 wt %, rms = 4.47 nm; (b) 5.0 wt %, rms = 3.68 nm; (c) 7.5 wt %, rms = 2.27 nm; (d) 10.0 wt %, rms = 2.22 nm; (e) 12.5 wt %, rms = 3.64 nm; (f) 15.0 wt %, rms = 6.98 nm. C


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Figure 3. J−V curves of the single-carrier devices with PF6NPSBr:Cs2CO3 ETLs: (a) electron-dominated devices (ITO/Al/PTB7-Th:PC71BM/ PF6NPSBr:x%-Cs2CO3 ETL/Ca/Al); (b) hole-dominated devices (ITO/PEDOT/PTB7-Th:PC71BM//PF6NPSBr:x%-Cs2CO3 ETL/MoO3/Al); (c) electron-only devices (ITO/Al/ETL/Al).

Figure 4. (a) Transient photocurrent of PSCs with Cs2CO3-doped PF6NPSBr ETLs; (b) ultraviolet photoelectron spectroscopy (UPS) images around secondary electron cutoff for the PF6NPSBr ETLs doped with different concentrations of Cs2CO3; (c) energy-level diagram of devices.

that the incorporation of Cs2CO3 smoothes the surface of the ETLs and forms a moderate interfacial contact between ETL and metal cathode, thus facilitating charge extraction in PSCs. On the other hand, further increasing Cs2CO3 concentration to 12.5 and 15.0 wt % resulted in rough surfaces with rms roughness values of 3.64 and 6.98 nm, respectively (Figure 2e,f). With these high doping concentrations, self-aggregations of Cs2CO3 dopant appeared and led to clear phase separation. As we can see in the SEM images in Figure S1, the ETL with high (15.0 wt %) doping concentration arose many islands due to the aggregation of Cs2CO3, which explained the observed decrease in device performance for ETLs with excessive dopant. A significantly decreased charge extraction and charge transport

ETLs with different concentrations of Cs2CO3. We observed that PF6NPSBr film without Cs2CO3 doping showed a relatively rugged morphology with a root-mean-square (rms) roughness of 4.47 nm (Figure 2a), whereas the Cs2CO3-doped PF6NPSBr ETLs exhibited a significant change in surface morphology. PF6NPSBr ETLs with 5.0 and 7.5 wt % Cs2CO3 showed smoother surfaces (rms values = 3.68 and 2.27 nm, Figure 2b,c) compared to the PF6NPSBr ETL without doping, possibly because Cs2CO3 dopant filled the hollows of the PF6NPSBr film and turned it into a uniform ETL film. With the Cs2CO3 concentration increased to 10.0 wt %, the film exhibited the minimal roughness with rms value of 2.22 nm (Figure 2d). According to the surface morphology variation with different Cs2CO3 concentrations in the ETLs, we suggest D


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time of 0.20 μs. This result verified that Cs2CO3 doping in the ETL indeed improved the charge-extraction property of the PSC. As the concentration of Cs2CO3 increased to 10.0 wt %, the devices showed the shortest extraction time of 0.12 μs, and this result was consistent with the highest PCE in the PSC devices. Therefore, the introduction of Cs2CO3 into PF6NPSBr ETL increased the charge-extraction rate and thus reduced charge recombination, which was the major reason for the improved FF in the corresponding PSC devices.62,63 Certainly, the devices with excessive doping (15.0 wt %) had the longest charge-extraction time (0.24 μs) and the lowest PCE due to the inferior morphology in the solution-processed PF6NPSBr:Cs2CO3 blend ETLs, which interrupted charge extraction during the operation of the devices. UPS Measurements. The WF variation of Ag electrode being coated with PF6NPSBr ETLs with different concentrations of Cs2CO3 was studied by ultraviolet photoelectron spectroscopy (UPS). As shown in Figure 4b, the WF of the pristine Ag electrode was found to be 4.65 eV, calculated from the secondary electron cutoff of the spectra. The cutoff energy position of Ag electrode shifted to a lower kinetic energy when PF6NPSBr ETLs were coated on the electrode and showed WFs of 4.20, 4.11, 3.90, and 3.85 eV for 0, 5.0, 10.0, and 15.0 wt % Cs2CO3 doped in the ETL, respectively. The WF of the electrode decreased significantly when Cs2CO3 was introduced into PF6NPSBr, leading to a better energy-lever alignment to PC71BM. Because the lowest unoccupied molecular orbital of PC71BM is about −3.91 eV (Figure 4c), the energy level of the PF6NPSBr ETL doped with 10.0 wt % Cs2CO3 matched ideally with that of PC71BM. The ideal energy-level alignment and better contact reduced the energy barrier between the active layer and electrode, which facilitated electron extraction and transport from the active layer to the cathode. The WF variation caused by the Cs2CO3 doping can also be used to explain the improved Voc of the PSCs with Cs2CO3-doped PF6NPSBr ETLs.64 Ultraviolet−Visible (UV−Vis) Transmission Measurement. Ultraviolet−visible (UV−vis) transmission spectra of the Cs2CO3-doped PF6NPSBr ETLs (30 nm) are shown in Figure S2. For comparison, the transparency of Cs2CO3 film was studied as well. Because Cs2CO3 possessed very low absorption in the UV−vis range, the PF6NPSBr:Cs2CO3 blend film showed a similar transparency to the undoped PF6NPSBr ETL. All of the PF6NPSBr:Cs2CO3 films possessed high transparency of over 90% above 400 nm, which is essential for the ETL to avoid the parasitic absorption and thereby maximize the light absorption of active layer in the PSCs. As we mentioned above, the PF6NPSBr:Cs2CO3 hybrid ETLs not only reach an ideal interfacial contact between ETL and metal cathode, but also exhibit the capability of tuning the WF of electrode and efficient charge extraction and transport in PSC. These combined properties are beneficial to promote the manufacture of the high-efficiency PSCs with thicknessinsensitive ETL. Highly Transparent and Thickness-Insensitive ETL. The dependence of the device performance on the thickness of PF6NPSBr:Cs2CO3 ETLs was investigated. The device structure remained to be ITO/PEDOT:PSS/PTB7Th:PC71BM/PF6NPSBr:Cs2CO3 ETL/Ag. The thickness of the ETLs was controlled by varying the concentration of solutions and determined by the Beer−Lambert law (Figure S3). As a contrast, we first examined the effect of varying thickness of undoped PF6NPSBr ETL on the performance of

can be suspected when a separate Cs2CO3 phase emerges in the ETL because Cs2CO3 is an insulator in nature. Charge-Extraction and Charge-Transport Properties. To investigate the charge-extraction and charge-transport properties of Cs2CO3-doped ETL in PSCs, two kinds of single-carrier devices based on the electron-dominated device (ITO/Al/PTB7-Th:PC71BM/PF6NPSBr:x%-Cs2CO3 ETL/ Ca/Al) and the hole-dominated device (ITO/PEDOT/PTB7Th:PC71BM/PF6NPSBr:x%-Cs2CO3 ETL/MoO3/Al) were fabricated. As shown in Figure 3a, the current density of the electron-dominated devices with 5.0% Cs 2 CO 3 -doped PF6NPSBr ETLs was higher than that of the devices with undoped ETLs, which meant that the electron-transport properties of the PF6NPSBr film were indeed significantly enhanced upon doping. As shown by the highest current density of electron-dominated devices, the 10 wt % Cs2CO3doped ETL exhibited the most efficient electron-transport ability. However, further increasing the Cs2CO3 doping concentration resulted in decreased current density. This meant that the electron transport would be suppressed by excessive doping, presumably due to the self-aggregation of the redundant Cs2CO3 in the ETL revealed by the morphology measurement. This trend matched well with the performance tendency of PSCs with Cs2CO3-doped ETLs. The J−V characteristics of the hole-dominated devices containing the PF6NPSBr ETL doped with Cs2CO3 are shown in Figure 3b. The current density of the hole-dominated devices with PF6NPSBr ETL significantly decreased as the doping concentration increased. This implied that the hole transport in the PF6NPSBr ETL was restrained upon doping. All of these results clearly indicated that Cs2CO3-doped PF6NPSBr ETLs possess enhanced electron-transport and hole-blocking ability compared to the undoped ETL, resulting in significantly improved device performance. To gain more insight into the charge-transport behavior of Cs2CO3-doped PF6NPSBr, we used space-charge-limited current (SCLC) measurement to quantitatively investigate the electron-transport properties of 10.0 wt % Cs2CO3-doped PF6NPSBr ETL. Electron-only devices based on the architectures of ITO/Al/ETL/Al were prepared, and electron mobilities of the ETLs were extracted by fitting the data using the SCLC model.61 As shown in Figure 3c, the electron-only device with the undoped PF6NPSBr ETL exhibited relatively low current density with an extracted electron mobility of 2.5 × 10−6 cm2/V s. After 10.0 wt % Cs2CO3 doped in the PF6NPSBr ETL, the device showed much higher current density with an electron mobility of 1.63 × 10−4 cm2/V s, which was 2 orders of magnitude higher than the undoped ETL. This result indicated that Cs2CO3 doping is an effective method to improve electron mobility, which can reduce the bulk resistance of devices with thick ETLs and is crucial to achieve thickness-insensitive PF6NPSBr:Cs2CO3 hybrid ETL. Transient Photocurrent (TPC) Measurements. To further characterize the charge-extraction process in the PSC devices, we used the transient photocurrent (TPC) measurements to study the competition between carrier sweep-out and recombination during the operation of the devices. As illustrated in Figure 4a, the transient photocurrent of the PSCs with PF6NPSBr ETLs doped with different concentrations of Cs2CO3 was measured at 0 bias. The devices with PF6NPSBr ETLs without Cs2CO3 doping showed a chargeextraction time of 0.23 μs, whereas those with 5.0 wt % Cs2CO3-doped ETL had a slightly decreased charge-extraction E


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Figure 5. J−V curves of PSCs with (a) undoped PF6NPSBr ETL; (b) PF6NPSBr:Cs2CO3 hybrid ETL in various thicknesses; (c) PCE and FF of PSCs with ETLs in various thicknesses; (d) EQE of PSC devices with ETLs in various thicknesses; (e) Jph vs Veff curves of PSC devices with ETLs in various thicknesses.

Table 2. Photovoltaic Performance of PSCs with Undoped and Doped PF6NPSBr ETL in Various Thickness under AM 1.5 G Irradiation (100 mW/cm2) ETL PF6NPSBr

PF6NPSBr:Cs2CO3

thickness (nm) 5 11 19 33 50 5 10 22 31 52

Jsc (mA/cm2)

Voc (V) 0.760 0.743 0.740 0.729 0.728 0.779 0.779 0.780 0.780 0.781

± ± ± ± ± ± ± ± ± ±

0.003 0.007 0.006 0.002 0.003 0.004 0.003 0.003 0.001 0.001

16.61 16.82 15.72 14.86 14.22 16.36 16.19 15.92 15.74 15.05

± ± ± ± ± ± ± ± ± ±

0.17 0.20 0.03 0.45 0.03 0.09 0.06 0.13 0.13 0.09

FF (%) 69.03 66.41 65.44 64.45 59.69 73.97 73.94 72.82 71.90 70.02

PCE (%) ± ± ± ± ± ± ± ± ± ±

0.76 0.74 0.57 0.65 0.33 0.21 0.39 0.57 0.65 0.78

8.71 8.31 7.62 6.98 6.17 9.43 9.32 9.05 8.83 8.23

± ± ± ± ± ± ± ± ± ±

PCE (best) 0.05 0.09 0.04 0.21 0.08 0.03 0.02 0.04 0.02 0.07

8.73 8.40 7.66 7.18 6.26 9.45 9.34 9.07 8.85 8.30

With further increase of thickness to about 52 nm, the PCE still maintained a high value of 8.30%. Compared to the control devices with undoped PF6NPSBr ETL, the PSCs with hybrid ETLs exhibited invariable Voc and slightly decreased Jsc and FF when the thickness of ETLs increased. This can be attributed to the enhanced charge extraction and charge transport and suppressed charge recombination, as we discussed above. All of these results clearly indicated that doping Cs2CO3 into the PF6NPSBr ETL can make it work efficiently in a broad range of thickness without significantly sacrificing the performance. The external quantum efficiency (EQE) curves of the PSCs with ETLs of various thicknesses are shown in Figure 5d. The integrated Jsc values obtained from the EQE curves were 16.05 and 13.71 mA/cm2 for the devices with 5 and 50 nm PF6NPSBr ETL, respectively. The main reason for the

the PTB7-Th:PC71BM-based PSCs. As shown in Figure 5a and Table 2, the devices with undoped PF6NPSBr ETLs showed relatively low PCE from 8.73% (∼5 nm ETL) to 6.26% (∼50 nm ETL) when the thickness of ETL was varied. The main drawback of increasing the ETL thickness is that it increases the bulk resistance of the ETL, resulting in the unfavorable charge extraction in the PSC devices. This is the major reason for the sharply decreased FF from 69.03% (∼5 nm ETL) to 59.69% (∼50 nm ETL) in the PSCs with undoped PF6NPSBr ETLs (Figure 5c).65 In comparison, after the doping of 10.0 wt % Cs2CO3 into the PF6NPSBr ETL, the PSC devices showed significantly improved performance, as summarized in Figure 5b and Table 2. The PSC devices with PF6NPSBr:Cs2CO3 hybrid ETLs (∼5 nm) exhibited the best PCE of 9.45% with an Voc of 0.776 V, a Jsc of 16.43 mA/cm2, and an FF of 74.12%. F


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most efficient PSC devices are 0.758 V, 19.93 mA/cm2, and 71.35%, respectively, yielding a best PCE of 10.78% with a 5 nm PF6NPSBr:Cs2CO3 ETL. When thickness of the hybrid ETL was increased to 31 nm, the PCE still maintained at about 10.0%, due to the good charge mobility of the PF6NPSBr:Cs2CO3 hybrid ETL. Further increasing the thickness of ETL to 52 nm resulted in a slight decline in PCE of 9.64%. This is a considerably high efficiency for the PSCs with thick active layer over 250 nm and thick cathodic interlayer over 50 nm, which is particularly attractive in developing high-performance large-area PSC modules.

decreased EQE value with thick PF6NPSBr ETL can be the increased resistance of the ETL. After incorporating Cs2CO3 in the PF6NPSBr ETLs, the device showed similar EQE curve with integrated Jsc value of 15.95 mA/cm2 for the thin (∼5 nm) ETL, whereas the EQE values enhanced in the region from 400 to 750 nm with a Jsc value of 14.55 mA/cm2 for thick (∼52 nm) ETL compared to the undoped PF6NPSBr ETL. These results suggest that PSCs with both undoped and doped thin PF6NPSBr ETL exhibit efficient charge collection, whereas the Cs2CO3-doped ETLs work more efficiently under the thickETL condition. To gain more insight into exciton dissociation processes of the PSCs with PF6NPSBr:Cs2CO3 ETLs in various thicknesses, the differences in the photocurrent density (Jph) and effective voltage (Veff) of the devices were analyzed, as shown in Figure 5e. Photocurrent density (Jph) is defined as Jph = JL − JD, where JL and JD are the current densities under illumination and dark conditions, respectively. Effective voltage (Veff) is defined as Veff = V0 − Va, where V0 is the voltage when Jph = 0 and Va is the applied bias voltage.66 It is assumed that all of the photogenerated excitons were dissociated into free carriers and collected by electrodes as Veff exceeded 1 V. With this assumption, the saturation current density (Jsat) will only be limited by the total amount of incident photons, and the maximum generation rate (Gmax) of the bound exciton pairs per unit volume of the PSCs can be obtained from the equation Jsat = qLGmax, where q is the electronic charge and L is the thickness of the active layer in the device.67 The Gmax values of the devices were 1.05 × 1028 and 1.06 × 1028 m−3 s−1 for undoped and doped PF6NPSBr thin ETLs, respectively. Because Gmax values mainly related to the absorption of the incident photons in the active layer, the similar Gmax values indicated that the photon absorption of the active layers almost unchanged after adding Cs2CO3 in the ETL, which was consistent with the results of the UV−vis transmission measurement. When the thickness of ETL increased to about 50 nm, the Gmax values slightly reduced to 9.43 × 1027 and 9.49 × 1027 m−3 s−1 for the devices with undoped and doped PF6NPSBr thick ETLs, respectively. These results suggested that the photon absorption of the active layer for the thick ETL tinily reduced in an acceptable range mainly due to the high transparency of the PF6NPSBr:Cs2CO3 ETL. In addition, in the low-Veff region, Jph of the device with Cs2CO3-doped PF6NPSBr thick ETL was higher than that with undoped PF6NPSBr thick ETL, whereas the device showed similar curves for thin ETLs. The results showed that the incorporation of Cs2CO3 in the PF6NPSBr ETL facilitated the charge dissociation and extraction of the PSCs, especially under thick ETL condition. This property explains the steady FF and PCE of the PSCs with PF6NPSBr:Cs2CO3 hybrid ETL in a range of thickness. Compatibility with the PTNT812:PC71BM System. Recently, PSCs with relatively thick active layers have been shown to be an efficient way to improve device performance and to be compatible with roll-to-roll printing processes. To test the applicability of the PF6NPSBr:Cs2CO3 hybrid ETL in the thick active layer, we fabricated PSCs with PTNT812:PC71BM as active layer60 and 10.0 wt % Cs2CO3doped PF6NPSBr as ETL. The device structure was ITO/ PEDOT:PSS/PTNT812:PC71BM/PF6NPSBr:Cs2CO3 ETL/ Ag, and the photovoltaic parameters under AM 1.5 G illumination at 100 mW/cm2 are summarized in Table S1 and Figure S4. The device parameters Voc, Jsc, and FF of the

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CONCLUSIONS

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ASSOCIATED CONTENT

We have shown a new strategy of achieving efficient and thickness-insensitive ETLs by doping alkali-metal salt into organic interfacial materials to fabricate high-performance PSCs. AFM images demonstrated that doping Cs2CO3 into PF6NPSBr can smooth the surface of ETL and provide a facile interfacial contact with electrode. The charge transport and TPC measurement indicated that introduction of Cs2CO3 into ETL can increase the charge-extraction rate and thus reduce charge recombination. UPS measurement revealed that the electrode WF tuning ability of PF6NPSBr can be enhanced by the incorporation of Cs2CO3. UV−vis transmission and SCLC measurement also indicated that the PF6NPSBr:Cs2CO3 hybrid ETL possesses both high transparency and high electron mobility. These combined properties brought new insights into the design of novel thickness-insensitive ETLs that avoid the parasitic absorption of ETL itself simultaneously. Subsequently, the hybrid ETL has been successfully applied in the PTB7Th:PC71BM-based PSCs in various thicknesses with the best PCE of 9.45%. Moreover, the PF6NPSBr:Cs2CO3 hybrid ETLs were also compatible with thick active layer system PTNT812:PC71BM with an optimal PCE of 10.78%, and a prominent PCE of 9.64% with an ETL thickness of 52 nm was achieved. The success of the hybrid ETL indicates that incorporating Cs2CO3 in the organic interfacial materials can be particularly useful in developing highly transparent, thickness-insensitive ETL for high-performance PSC. These special properties also make them potential candidates as cathodic interlayers in high-performance PSC modules with roll-to-roll printing processes.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b17076. Experiment section, including preparation of materials, fabrication of devices, instruments and characterization; SEM images; UV−vis transmission spectra; J−V curves and photovoltaic performance of PTNT812:PC71BM system (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.Z.). *E-mail: [email protected] (F.H.). ORCID

Qing-Hua Xu: 0000-0002-4153-0767 Fei Huang: 0000-0001-9665-6642 G


Research Article

ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by the Natural Science Foundation of China (Nos. 21634004, 21490573, and 91633301), the Ministry of Science and Technology (No. 2014CB643501), and the Science and Technology Program of Guangzhou, China (Nos. 201707020019 and 201607020010).

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