12.5% Flexible Nonfullerene Solar Cells by
Passivating the Chemical Interaction Between the
Active Layer and Polymer Interfacial Layer
Sixing Xiong, Lin Hu, Lu Hu, Lulu Sun, Fei Qin, Xianjie Liu, Mats Fahlman and Yinhua Zhou
The self-archived postprint version of this journal article is available at Linköping University Institutional Repository (DiVA):
http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-159292
N.B.: When citing this work, cite the original publication.
Xiong, S., Hu, L., Hu, Lu, Sun, L., Qin, F., Liu, X., Fahlman, M., Zhou, Y., (2019), 12.5% Flexible Nonfullerene Solar Cells by Passivating the Chemical Interaction Between the Active Layer and Polymer Interfacial Layer, Advanced Materials, 31(22), 1806616.
https://doi.org/10.1002/adma.201806616
Original publication available at:
https://doi.org/10.1002/adma.201806616
Copyright: Wiley (12 months)
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12.5% flexible non-fullerene solar cells by passivating the chemical interaction between the active layer and polymer interfacial layer
Sixing Xiong,† Lin Hu,† Lu Hu,† Lulu Sun,Fei Qin, Xianjie Liu, Mats Fahlman, and Yinhua Zhou*
S. X. Xiong, L. Hu, L Hu, L. L. Sun,F. Qin, and Prof. Y. H. Zhou*
Wuhan National Laboratory for Optoelectronics, and School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China
Dr. X. J. Liu and Prof. M. Fahlman,
Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-58183 Linköping, Sweden
†These authors equally contribute to this work.
*Corresponding author. E-mail: yh_zhou@hust.edu.cn
Nonfullerene (NF) organic solar cells (OSCs) have been attracting significant attention in the past several years. It is still challenging to achieve high-performance flexible NF OSCs. NF acceptors are chemically reactive and tend to react with the temperature-processed low-work function (low-WF) interfacial layers, such as polyethylenimine ethoxylated (PEIE), which leads to the “S” shape in the current-density characteristics of the cells. In this work, the chemical interaction between the NF active layer and the polymer interfacial layer of PEIE by increasing its protonation is deactivated. The PEIE processed from aqueous solution shows more protonated N+ than that processed from isopropanol solution, observed from X-ray photoelectron spectroscopy. NF solar cells (active layer: PCE-10:IEICO-4F) with the protonated PEIE interfacial layer show an efficiency of 13.2%, which is higher than the reference cells with a ZnO interlayer (12.6%). More importantly, the protonated PEIE interfacial layer processed from aqueous solution does not require a further thermal annealing treatment (only processing at room temperature). The room-temperature processing and effective WF reduction enable the demonstration of high-performance (12.5%) flexible NF OSCs.
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Organic solar cells (OSCs) have been attracting considerable interest due to their advantages of easy processing and good mechanical flexibility.[1-6] Over the past several decades, the electron acceptors of bulk-heterojunction OSCs were typically fullerenes and their derivatives, such as [6,6]-phenyl-C61 (or C71)-butyric acid methyl ester (PC61BM or PC71BM). In the fullerene acceptor-containing active layers, an energy offset of approximately 0.3 eV is required between the lowest unoccupied molecular orbital levels of the donor and the fullerene acceptor for efficient charge separation.[7-9] The required energy offset results in large voltage loss and therefore limits the theoretical power conversion efficiency (PCE) of the OSCs.[10–12] To date, the reported highest PCE of fullerene acceptor-containing OSCs (single junction) is approximately 12%.[13,14]
Recently, the emergence of nonfullerene (NF) acceptors breaks this limit. The required energy offset can be nearly 0 eV, which could significantly reduce the energy loss and enhance the theoretical limit of PCE.[15–17] The reported PCE of binary OSCs based on NF active layers (hereafter referred to as NF OSCs) reaches over 14% (single junction), which is higher than that of OSCs based on fullerene acceptors.[18–20] Though PCE of the NF solar cells was enhanced rapidly on rigid glass substrates, development of flexible NF OSCs lags behind. To date, the reported highest PCE of flexible NF OSCs is approximately 10%.[21,22] The bottleneck for high-PCE flexible NF OSCs is the development of efficient low-temperature (even room-temperature) processable interfacial layers for the high-performance NF active layers. Low-temperature-processed n-type oxides have been used as the electron-transporting layer (ETL) for flexible OSCs.[23–28] However, flexible NF solar cells with the oxides as the ETL have not exhibited high efficiency to date.[21,29]
NF acceptors have different chemical structure from the fullerenes. State-of-the-art high-performance NF acceptors are members of the ITIC families that contain acceptor– donor– acceptor (A-D-A) moieties with intramolecular charge transfer.[30–33] The NF acceptors’ chemical activities are different from the fullerenes. The developed interfaces for fullerene-based active layers might not be applicable for the NF acceptors. For example, polyethylenimine (PEI) and poly-ethylenimine ethoxylated (PEIE) perform very efficiently as low-temperature-processed interfacial layers for fullerene-based flexible solar cells and tandem solar cells.[34–39] It is also compatible with printing techniques.[40–43] However, PEI or PEIE could react with the NF acceptors. This reaction destroys the chemical structure and original intramolecular charge transfer in the ITIC and results in poor solar cell performance.[44] This limits the application of PEI or PEIE as low-temperature-processed interfacial layers for high-performance NF active layers. Therefore, it is desirable to properly redesign low-temperature-processed interfacial layers for NF active layer to realize high-performance flexible OSCs.
In this work, we introduce a strategy of increasing the protonation of PEIE to passivate the chemical interaction between the PEIE and the NF acceptors. Different polar protic solvents (isopropanol, ethanol, methanol, and water) were used to process the PEIE to tune the protonation. Carbon dioxide (CO2), acetic and phosphoric acids were added into the aqueous PEIE solution to further increase its protonation. As a result, the protonated PEIE can work efficiently as a low-work function (low-WF) interfacial layer for NF active layers. Importantly, it can be processed at room temperature without additional thermal annealing, which is
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desirable to fabricate flexible solar cells. Flexible NF solar cells exhibit a PCE of 12.5%, which is the highest efficiency compared to the reports in the literature.
Figure 1. a) Chemical structures of PCE10, IEICO-4F, and PEIE; b) photograph of IEICO-4F and IEICO-4F mixed with PEIE solutions; c) absorbance spectra of the IEICO-4F and IEICO-4F mixed with PEIE solution (in CB).
Figure 1a shows the chemical structures of PCE-10 and IEICO-4F that are used as the donor and acceptor for the active layer, respectively, and the interfacial layer material PEIE. The active layer of PCE-10:IEICO-4F mixture exhibits strong absorption up to 1000 nm (Figure S1, Supporting Information). OSCs based on a PCE-10:IEICO-4F active layer fabricated on glass exhibit a low energy loss approximately 0.5 eV and a high short-circuit current density (JSC) of approximately 27 mA cm−2.[45,46] This active layer does not require thermal treatment for
optimum photovoltaic performance. These merits enable the active layer of PCE-10:IEICO-4F to be suitable for high-performance flexible solar cells. When PEIE is used as the low-temperature-processed interfacial layer, a chemical reaction occurs between the PEIE and the IEICO-4F. Figure 1b depicts the pristine IEICO-4F solution and IEICO-4F mixed PEIE solution dissolved in chlorobenzene (CB). The IEICO-4F exhibits a blue color, while the mixture is brown. Figure 1c shows the absorbance spectra of the two solutions. The pristine IEICO-4F solution shows strong absorption in the region of 600–900 nm with a peak at approximately 750 nm. After mixing with PEIE, the main absorption band (600–900 nm) disappears and a new absorption band centered at approximately 450 nm appears. This suggests that the chemical reaction between the PEIE and NF acceptor IEICO-4F changes the original electronic structure
(a)
PCE10 IEICO-4F PEIE
IEICO-4F + PEIE IEICO-4F 300 450 600 750 900 0.00 0.04 0.08 0.12 0.16 IEICO-4F + PEIE IEICO-4F Absorba nce (a. u.) Wavelength (nm) (b) (c)
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of the IEICO-4F. We have also mixed PEIE with other NFAs, ITIC and IT-M (Figure S2, Supporting Information). The both solutions show a color change from blue to yellow after PEIE is added. The main absorption bands show a hypsochromic shift. This phenomenon is similar to that observed for the IEICO-4F. PEIE is also added into the donor polymer PCE10 solution. Compared with the pristine PCE10 solution, there is no color change or hypsochromic shift of the absorption (Figure S3, Supporting Information). This observation suggests that the PEIE reacts with the NF acceptors instead of the donor.
To elucidate the chemical reaction between the NF acceptors and PEIE, we employed ethanolamine as a model compound to react with IEICO-4F in dichloromethane. From the 1H nuclear magnetic resonance (NMR) in Figure S4 in the Supporting Information, the broad singlet ≈d = 8.73 ppm can be assigned to the proton in the C=C linkage that links the donor (D) and acceptor (A) moieties in the IEICO-4F. After introducing 1.0 eq of ethanolamine, the resonance at 8.73 ppm diminishes and a series of other resonances (≈d = 4.08, 3.85, and 3.72 ppm) emerges. The amine reacts as a nucleophile with the C=C linkage moiety through the Michael addition reaction.[47] After further adding the ethanolamine to 8.0 eq in the NMR tube, the solution color turns to yellow. The corresponding 1H NMR implies the loss of both terminal acceptor moieties in the IEICO-4F. The mass spectrum in Figure S5 in the Supporting Information verifies this observation. The pristine IEICO-4F delivers an m/z of 1806. After introducing 1.0 eq of ethanolamine, the emerging fragment at m/z of 1868 indicates that the ethanolamine reacted with the IEICO-4F through an addition reaction. After further increasing the ratio of ethanolamine to 8.0 eq, the emerging fragment at m/z of 1470 could presumably be attributed to the addition of ethanolamine to both the C=C linkage moieties and the loss of both terminal acceptor moieties (Figure S5, Supporting Information).
-0.2 0.0 0.2 0.4 0.6 0.8 -30 -20 -10 0 10 Light Dark Curre n t (m A/c m 2 ) Voltage (V) VOC= 0.69 V JSC= 25.6 mA/cm2 FF = 0.53 PCE = 9.4% With i-PEIE (b) (a) ITO
i-, e-, m-, or a-PEIE
Ag PCE-10:IEICO-4F MoO3 glass -0.2 0.0 0.2 0.4 0.6 0.8 -30 -20 -10 0 10 Light Dark Curre n t (m A/c m 2 ) Voltage (V) VOC= 0.70 V JSC= 27.2 mA/cm2 FF = 0.69 PCE = 13.2% With a-PEIE (e) -0.2 0.0 0.2 0.4 0.6 0.8 -30 -20 -10 0 10 Light Dark Curre n t (m A/c m 2 ) Voltage (V) (d) With m-PEIE VOC= 0.69 V JSC= 25.1 mA/cm2 FF = 0.57 PCE = 9.97% -0.2 0.0 0.2 0.4 0.6 0.8 -30 -20 -10 0 10 Light Dark Curre n t (m A/c m 2 ) Voltage (V) (c) With e-PEIE VOC= 0.69V JSC= 25.1 mA/cm2 FF = 0.56 PCE = 9.78%
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Figure 2. a) Device structure of OSCs; i-, e-, m-, and a-PEIE denote PEIE processed from
isopropanol, ethanol, methanol, and aqueous solutions, respectively; J–V characteristics of the NF OSCs with different interfacial layers: b) with i-PEIE; c) with e-PEIE; d) with m-PEIE; e) with a-PEIE.
The reaction between the PEIE and the NF active layer is detrimental to the photovoltaic performance. Figure 2a shows the device structure of OSCs with the PEIE interfacial layer and PCE-10:IEICO-4F active layer on a glass substrate. When PEIE is processed from isopropanol (denoted as i-PEIE), an “S” shape is observed in the current-density–voltage (J–V) characteristics under 100 mW cm−2 AM 1.5 illumination. The device shows an open-circuit
voltage (VOC) of 0.69 V, a JSC of 25.6 mA cm−2, and a fill factor (FF) of 0.53, yielding a PCE of
9.4% (Figure 2b). Photovoltaic data are summarized in Table 1. To passivate the detrimental reaction, we employ other protic solvents (ethanol, methanol, water) to process the PEIE to increase the protonation. The PEIE processed from ethanol, methanol and water are denoted as e-PEIE, m-PEIE, and a-PEIE, respectively.
Table 1. Photovoltaic data for NF OSCs on rigid glass and flexible PET substrates with different
electron-collecting interfacial layers. The device structure is: substrate/ITO/interfacial layer (ZnO or PEIE)/ PCE-10:IEICO-4F/MoO3/Ag. ZnO is used as the reference interfacial layer.
Comparing with i-PEIE, the OSCs with e-PEIE and m-PEIE show slightly higher performance (Figure 2c,d): VOC = 0.69 V, JSC = 25.1 mA cm−2, FF = 0.56, and PCE = 9.78% for e-PEIE, and
VOC = 0.69 V, JSC = 25.1 mA cm−2, FF = 0.57, and PCE = 9.97% for m-PEIE. Although the OSCs
with e-PEIE and m-PEIE show improved performance compared to the OSC with i-PEIE, the PCE is still not as high as the reference OSCs with ZnO as the electron-collecting layer. As shown in Figure S6 in the Supporting Information, the reference OSCs show VOC = 0.70 V, JSC = 26.5 mA cm−2, FF = 0.67, and PCE = 12.6%. The lower FF is the main reason for the poorer
performance, which is associated with the “S” shape due to the poor charge collection at the interface of the active layer/electrode.
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Figure 3. a) XPS results of ITO/a-PEIE (upper panel) and ITO/i-PEIE (lower panel); b) ultraviolet
photoelectron spectroscopy results of ITO/a-PEIE and ITO/i-PEIE.
Surprisingly, when PEIE is processed from aqueous solution (denoted as a-PEIE), the OSCs show high performance (Figure 2e): VOC = 0.70 V, JSC = 27.3 mA cm-2, FF = 0.69, and PCE = 13.2% (Table 1). The performance is even higher than the reference OSCs with ZnO as the electron-collecting layer. Compared with the reference cells, the higher PCE of OSCs with a-PEIE arises from the enhanced JSC and FF. The higher JSC is attributed to the improved transmittance of a-PEIE compared to the ZnO layer (Figure S7, Supporting Information). The higher FF for the OSCs with a-PEIE is very encouraging. This finding suggests that electrons can be very efficiently collected at the interface of the a-PEIE/active layer. PEIE, which is widely used in fullerene OSCs, can work well in the NF OSCs after the protonation. Figure S8 in the Supporting Information shows the mixture of IEICO-4F with a-PEIE and i-PEIE, respectively. The color of the IEICO-4F does not change after adding the a-PEIE solution while adding i-PEIE solution changes the color of IEICO-4F and the absorption spectra. The results suggest that the reaction between a-PEIE and IEICO-4F was inhibited by protonating the amine group. The protonation strategy of PEIE is also effective for a PBDB-T:ITIC-based NF solar cell. After the i-PEIE is changed to a-PEIE, the PCE of the cell (glass/ITO/PEIE/PBDB-T:ITIC/MoO3/Ag) performance increases from 6.8% (VOC = 0.88 V, JSC = 14.8 mA cm-2, FF = 0.53) to 9.0% (VOC = 0.90 V, JSC = 16.1 mA cm-2, FF = 0.62, Figure S9, Supporting Information). For these polar protic solvents, their dielectric constant increases from 17.9 (isopropanol), 24.5 (ethanol), 32.7 (methanol) to 80.1 (water). Their dipole moments increase from 1.66 (isopropanol), 1.69 (ethanol), 1.70 (methanol) to 1.85 (water). The increased dipole moments of the protic solvents tend to promote the protonation of PEIE. We performed X-ray
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photoelectron spectroscopy (XPS) measurements on the samples of ITO/a-PEIE and ITO/i-PEIE. The isopropanol and water have the smallest and largest dipole moments, respectively, among these four protic solvents. Figure 3a shows the highresolution XPS spectra of the ITO/a-PEIE and ITO/i-ITO/a-PEIE. The features centered at 400 and 401 eV are associated with neutral amine nitrogen with different chemical environments in PEIE, whereas the feature at 402.75 eV is associated with nitrogen in protonated amine groups.[48,49] Compared to ITO/i-PEIE, ITO/a-PEIE shows the stronger N 1s intensity at 402.75 eV, which means more amine gets protonated in the aqueous solution. This finding confirms that the polar protic solvent with a larger dielectric constant and dipole moment more easily induces PEIE to become protonated. Furthermore, the WF of ITO/i-PEIE and ITO/a-PEIE samples was measured by ultraviolet photoemission spectroscopy (UPS). As shown in Figure 3b, the binding energy cutoff (Ecutoff) values of ITO/i-PEIE and ITO/a-PEIE samples are 17.30 and 17.34 eV, respectively. Accordingly, the calculated WF of ITO/ i-PEIE and ITO/a-PEIE samples are 3.94 and 3.90 eV, respectively. In addition, the WF values of ITO/i-PEIE and ITO/a-PEIE were also independently measured by a Kelvin probe, where the values are 4.24 and 4.25 eV, respectively. In both techniques, the WF values of the two samples are quite similar and within the measurement errors of the respective techniques. The negligible discrepancy in WF suggests that the WF is not the major reason leading to the large difference of PCE between devices with i-PEIE and a-PEIE. The passivation of the chemical activity of PEIE via the protonation is the key point for efficient charge collection.
The protonation is associated with the pH value of the solvent. We measured the pH values of the deionized (DI) water that is used for processing PEIE. The pH value of the DI water is 5.79. The acidic character might be due to the adsorption of CO2. To remove the CO2, the water was boiled. The pH value changed to 6.93, almost neutral. To check if more protons in water would increase the device performance, CO2 and different acids (acetic and phosphoric acids) were added to the PEIE aqueous solutions. The pH values changed to 4.09, 3.99, and 4.08, respectively. After adding 0.1 wt% PEIE to these different water solutions, their pH values became 8.73 (DI water), 9.36 (boiled water), 5.36 (with CO2), 7.53 (acetic acid), 8.17 (phosphoric acid), respectively. These pH values are summarized in Table S1 in the Supporting Information. PEIE processed from water samples with different pH values (with the addition of acids) was used in the solar cells to investigate its influence on the photovoltaic performance with the device structure shown in Figure 2a. Figure S10 in the Supporting Information shows the J–V characteristics of the corresponding devices. The OSCs with the PEIE processed from boiled water or acidic water show comparable performance to the cells with a-PEIE processed from DI water. Therefore, water as the processing solvent of PEIE is critical to the solar cell performance, whereas the tuning of the proton concentrations via boiling method or adding acids into water has little influence on the device performance. The protonation with the stronger acid (phosphoric acid) yields better device stability than the CO2-PEIE interface (Figure S11, Supporting Information). This is probably because the stronger acids shift the equilibrium (between proto-nation and base) toward protonation.
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Figure 4. a) Photograph of a fabricated flexible NF OSC with a-PEIE interface; b) J–V characteristics and c) EQE of the flexible solar cell.
PEIE processed from aqueous solution is the optimal condition for the NF solar cells, as the chemical reaction between PEIE and the NF active layer is passivated. Furthermore, the a-PEIE does not require additional thermal annealing after coating. Based on the optimization, we fabricated flexible NF OSCs on PET/ITO substrates with PEIE processed from aqueous solution. A photograph of the flexible device is shown in Figure 4a, and the J–V characteristics are shown in Figure 4b. The flexible device exhibits a VOC of 0.69 V, JSC of 27.2 mA cm−2, and
FF of 66.4%, yielding a PCE of 12.5%. The external quantum efficiency (EQE) for the device is shown in Figure 4c. Table S2 in the Supporting Information summarizes the PCE of flexible OSCs reported in the literature. The PCE of 12.5% is the highest efficiency for the flexible OSCs reported in the literature, which is ascribed to the effectiveness of PEIE for electron collection and simple processing at room temperature. The cell also shows good mechanical flexibility, maintaining 90% of the initial PCE after 2000 bending cycles with a bending radius of 5 mm (Figure S12, Supporting Information).
In this report, we describe a strategy of protonation that can effectively passivate the detrimental chemical reaction between PEIE and NF acceptors. Water is the optimal processing solvent for PEIE compared to the typically used polar protic alcohol solvents (isopropanol, ethanol, and methanol). The OSCs with PEIE processed from alcohol solvents feature PCE values lower than 10%, and an “S” shape is observed in the J–V characteristics under illumination. In contrast, solar cells with PEIE processed from aqueous solution show a PCE of 13.2%, which is higher than the PCE (12.6%) of the reference solar cells with ZnO as the electron-collecting layer. Flexible NF solar cells have been demonstrated based on the room-temperature-processed PEIE interfacial layer with a PCE of 12.5%, which is the highest efficiency for the flexible OSCs in the literature. This study for the first time represents the widely used PEIE in fullerene solar cells can work very efficiently in NF OSCs. The new strategy of protonation to solve the compatibility between the amine-based polymer and the NF acceptor will considerably promote the development of NF flexible and printable solar cells. With the protonation strategy, the interface between the NF active layer and the bottom electrode can efficiently collect electrons. Next, the interface between the top electrode and active layer is to be studied for realizing fully printed NF OSCs. When the evaporated MoO3
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HTL is replaced by printable poly(3,4-eth ylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), the devices with ZnO (Figure S13a, Supporting Information) and a-PEIE (Figure S13b, Supporting Information) both show poor performance with an “S” shape in the J–V characteristics. Physical or chemical interactions occur when PEDOT:PSS is deposited on top of the active NF layer, which needs to be studied and deactivated.
Experimental Section
Materials: Indium tin oxide (ITO)-coated glass substrates were purchased from CSG Holding Co., Ltd, Shenzhen. ITO-coated PET substrates for flexible solar cells were purchased from Kaivo Optoelectronic Tech Co., Ltd, Zhuhai. PEIE and 1-chloronaphthalene (CN) were purchased from Sigma-Aldrich. PCE-10 and IEICO-4F were purchased from Organtecsolar Materials, Beijing. Inc. Zinc acetate dehydrates (99%) and ethanolamine (98%) were purchased from Sigma-Aldrich. All chemicals were used as received without further purification.
Device Fabrication: ITO glass substrates were cleaned stepwise in ultrasonic baths of detergent in DI water, DI water, acetone, and 2-propanol for 30 min. The PET substrates were wiped by cotton swab dipped in ethanol. Then, the ITO substrates were treated by air plasma for 3 min. PEIE solution was prepared by dissolving PEIE in isopropanol, ethanol, methanol, and water at a weight concentration of 0.1 wt%. Thickness of PEIE is about 10 nm, measured by spectroscopic ellipsometry. The ZnO precursor solution was prepared by dissolving 0.4 g of zinc acetate dehydrate and 0.112 g of ethanolamine in 4 mL of 2-methoxyethanol. PEIE solution was spin-coated on top of ITO glass at 3500 rpm for 40 s without any postannealing treatment. ZnO precursor solution was spin-coated on top of ITO glass at 3500 rpm for 40 s following a thermal annealing at 200 °C for 30 min in air. After the deposition, the substrate was transferred to the N2-filled glovebox. Subsequently, the NF active layer of PCE-10:IEICO-4F (1:1.5, weight ratio) was prepared by spin-coating from a solution with the total concentration of 20 mg mL−1 in a mixed CB and CN (96:4, volume ratio) solution on
glass/ITO/PEIE (or ZnO) or PEN/ ITO/PEIE substrates at 2500 rpm for 40 s. Finally, MoO3/Ag electrodes were deposited using a Mini-spectros (Kurt J. Lesker) system at a base pressure of 2 × 10−7 Torr. The effective device area was determined to be 4.1 mm2.
Device and Sample Characterization: The absorption and transmittance were measured on a spectrophotometer (UV-3600, Shimadzu Scientific Instruments). The J–V characteristics were measured inside a N2-filled glove box using a Keithley 2400 source meter controlled by the LabVIEW program in the dark and under illumination (AM 1.5, 100 mW cm2). Light intensity was calibrated using a silicon photodetector (Newport 818-UV). The EQE test was performed by a standard system using a 150 W xenon lamp (Oriel) fitted with a monochromator (Cornerstone 74 004) as a monochromatic light source. The NMR spectra were collected on a Bruker Ascend 400 MHZ NMR spectrometer using deuterated dichloromethane as the solvent and tetramethylsilane (δ = 0) as the internal standard. The detection was operated in the negative ion mode. The LC-MSD analysis was performed on an Agilent 1100 LC-MS Trap XCT System (Agilent Technologies, USA) using an APCI in the negative ion mode. The pH value was measured by a Mettler Toledo FE20 system. The WFs of a-PEIE and i-PEIE were determined by a Kelvin probe (KP020). The XPS and UPS experiments were carried out using a Scienta ESCA 200 spectrometer. The measurements were performed in the analysis chamber at a base pressure of 10−10 mbar using monochromatic Al (Kα) X-rays at hν = 1486.6 eV and He I radiation
10 Acknowledgements
S.X.X., L.H., and L.H. equally contributed to this work. The work was supported by the National Natural Science Foundation of China (Grant No. 51773072, 21474035), the Recruitment Program of Global Youth Experts, the HUST Innovation Research Fund (Grant No. 2016JCTD111, 2017KFKJXX012), the Science and Technology Program of Hubei Province (2017AHB040), and the China Postdoctoral Science Foundation (2016M602289). M.F. and X.J.L. acknowledge support from the Swedish Research Council (project Grant No. 2016-05498). The authors also would like to thank the Analytical and Testing Center of Huazhong University of Science and Technology for providing the facilities to conduct the characterization. The authors acknowledge Prof. Qinye Bao from East China Normal University for the help on the XPS measurements, and Prof. Zhong’an Li from Huazhong University of Science and Technology for his fruitful discussion regarding the reaction mechanism between the amine and NF acceptors.
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S1
Supporting Information
12.5% flexible non-fullerene solar cells by passivating the chemical interaction between the active layer and polymer interfacial layer
Sixing Xiong,† Lin Hu,† Lu Hu,† Lulu Sun,Fei Qin, Xianjie Liu, Mats Fahlman, and Yinhua Zhou*
S. X. Xiong, L. Hu, L. Hu, L. L. Sun,F. Qin, and Prof. Y. H. Zhou*
Wuhan National Laboratory for Optoelectronics, and School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China
Dr. X. J. Liu and Prof. M. Fahlman,
Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-58183 Linköping, Sweden
†These authors equally contribute to this work.
S2
S3
Figure S2 Nonfullerene acceptors chemically react with PEIE. (a) Chemical structure of ITIC;
(b) picture of pristine ITIC and ITIC mixed with PEIE solutions; (c) absorbance spectra of ITIC and ITIC mixed with PEIE solutions; (d) chemical structure of IT-M; (e) picture of pristine IT-M and IT-M mixed with PEIE solutions; (f) absorbance spectra of IT-M and IT-M mixed with PEIE solutions.
ITIC +PEIE ITIC IT-M +PEIE IT-M (a) (b) (d) (c) ITIC IT-M (e) (f)
S4
Figure S3 (a) Picture; and (b) absorbance spectra of PCE-10 and PCE-10 mixed with PEIE
solutions.
PCE-10 +PEIE PCE-10
S5
Figure S4 1H NMR spectra of pristine IEICO-4F (top), IEICO-4F after adding 1.0 eq (middle) and 8.0 eq ethanol amine (bottom), respectively.
a b c c e d f a b g g d IEICO-4F e,f
IEICO-4F + ethanol amine (1.0 eq)
S6
Figure S5. Mass spectra of pristine IEICO-4F (top), IEICO-4F mixed with 1.0 eq (middle)
and 8.0 eq ethanol amine (bottom), respectively. IEICO-4F
IEICO-4F + ethanol amine (1.0 eq)
S7
Figure S6 (a) Device structure; (b) J-V characteristics of reference organic solar cells with
ZnO as the electron-collecting layer.
ITO ZnO Ag PCE-10:IEICO-4F MoO3 glass (a) (b) VOC= 0.70V J SC= 26.5 mA/cm2 FF = 0.67 PCE = 12.6%
S8
Figure S7. Transmittance spectra of PEIE and ZnO films on quartz substrates.
PEIE ZnO
S9
Figure S8 (a) Picture; and (b) absorbance spectra of of IEICO-4F, IEICO-4F + a-PEIE and
IEICO-4F + i-PEIE solutions, respectively. IEICO-4F
+ a-PEIE
IEICO-4F IEICO-4F
+ i-PEIE
S10
Figure S9. J-V characteristics of PBDB-T:ITIC-based non-fullerene organic solar cells with
a- or i-PEIE (device structure: glass/ITO/a- or i-PEIE/PBDB-T:ITIC/MoO3/Ag).
S11
Figure S10. J-V characteristics of the PCE10:IEICO-4F-based non-fullerene organic solar
cells with PEIE processed from different aqueous solutions: (a) boiled water; (b) deionized
water with CO2; (c) boiled water with acetic acid; (d) boiled water with phosphoric acid.
(a) (b) (c) (d) VOC= 0.70 V JSC= 26.4 mA/cm2 FF = 0.68 PCE = 12.5% VOC= 0.70 V JSC= 26.4 mA/cm2 FF = 0.69 PCE = 12.7% VOC= 0.70V JSC= 26.9 mA/cm2 FF = 0.69 PCE = 12.9% VOC= 0.69 V JSC= 26.6 mA/cm2 FF = 0.68 PCE = 12.5%
S12
Figure S11. Evolution of photovoltaic parameters (VOC, JSC, FF and PCE) of
PCE-10:IEICO-4F solar cells with PEIE processed from different aqueous solutions (phosphoric acid, acetic acid, and CO2, respectively) as a function of illumination time (under
continuous AM1.5 illumination).
(a) (b)
S13
Figure S12. Normalized VOC, JSC, FF and PCE of PCE10:IEICO-4F-based flexible solar cell
S14
Figure S13. J-V characteristics of non-fullerene organic solar cells with solution-processed
PEDOT:PSS as hole-collecting layer on top of the active layer: (a) with ZnO; (b) with a-PEIE as the interfacial layer.
VOC= 0.67 V JSC= 26.2 mA/cm2 FF = 0.55 PCE = 9.7% Glass/ITO/ZnO/PCE-10:IEICO-4F/PEDOT:PSS/Ag V OC= 0.67 V J SC= 26.3 mA/cm2 FF = 0.54 PCE = 9.5% (a) (b) Glass/ITO/a-PEIE/PCE-10:IEICO-4F/PEDOT:PSS/Ag
S15
Table S1. pH values of different aqueous conditions with and without 0.1 wt.% PEIE.
pH1-pH5 denotes five independent measurements.
pH1 pH2 pH3 pH4 pH5 pH(Average) Boiled water 6.96 6.95 6.88 46.93 6.91 6.93 PEIE solution (Boiled water) 9.34 9.37 9.33 9.36 9.39 9.36 DI water 5.83 5.78 5.77 5.77 5.80 5.79 PEIE solution (DI water) 8.73 8.73 8.74 8.73 8.72 8.73 DI water + CO2 4.11 4.07 4.09 4.06 4.10 4.09 PEIE solution (DI water + CO2) 5.38 5.33 5.35 5.37 5.39 5.36
S16
Table S2. Summary of high-performance flexible OSCs reported in the literature.
VOC (V) JSC (mA/cm2) FF PCE (%) Year
0.87 11 0.64 6.04 2014[1] 0.72 14.1 0.69 7.1 2014[2] 0.84 6.6 0.6 3.3 2015[3] 0.74 12.8 0.65 6.17 2015[4] 0.77 14.22 0.7 7.7 2015[5] 0.79 16.94 0.74 9.9 2015[6] 0.81 18.25 0.7 10.4 2015[7] 0.8 12.92 0.52 5.38 2016[8] 0.74 15.4 0.62 7.1 2016[9] 0.76 17.4 0.64 8.75 2017[10] 0.73 15.5 0.58 6.57 2018[11] 0.93 15.49 0.7 10.12 2018[12] 0.9 16.94 0.66 10.02 2018[13] 0.69 27.3 0.66 12.55 This work
S17
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