Performance Enhancement of Ternary Polymer Solar Cells Induced by

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Performance Enhancement of Ternary Polymer Solar Cells Induced by F4TCNQ Doping Shuwen Yu, Qing Yang, Wei Yu, Jing Zhang, Junxue Liu, Shengye Jin, Xin Guo, and Can Li Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b02520 • Publication Date (Web): 22 Aug 2019 Downloaded from pubs.acs.org on August 23, 2019

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Chemistry of Materials

Performance Enhancement of Ternary Polymer Solar Cells Induced by F4TCNQ Doping Shuwen Yu,† Qing Yang,†,‡ Wei Yu,† Jing Zhang, †,‡ Junxue Liu,† Shengye Jin,† Xin Guo†,* and Can Li†,* † State

Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences; Dalian National Laboratory for Clean Energy, Dalian 116023, China. ‡ University of Chinese Academy of Sciences, Beijing 100049, China ABSTRACT: Molecular electrical doping has been proved as an effective strategy to improve the performance of traditional binary polymer solar cells (PSCs). However, its effectiveness is not demonstrated yet in ternary PSCs which have recently emerged as a promising technique for organic photovoltaics. In this report, we investigate tetrafluorotetracyanoquinodimethane (F4TCNQ) doping of a ternary PSC composed of one donor polymer and dual acceptors. It is found that ion-pair forms between the dopant and donor polymer, leading to the increases of conductivity, hole density, and hole mobility. By employing a low doping concentration of 0.1%, the power conversion efficiency of the ternary PSC is improved, mainly contributed by enhancements of short-circuit current density (Jsc) and fill factor (FF). Investigation of charge transport processes and kinetics reveals that the Jsc and FF improvements are ascribed to more efficient charge extraction, restrained non-geminate charge recombination and optimized microstructure in doped ternary PSCs.

Introduction Ternary polymer solar cells (PSCs) have emerged as a promising organic photovoltaic technology in the past few years, with the power conversion efficiency (PCE) over 14%, contributed by the rapid advance of non-fullerene acceptors.1-10 The ternary PSCs can potentially provide higher efficiencies because three-component absorbers can produce better complementary absorption overlapping with the solar spectrum, solve the problem of energy mismatch in comparison to binary devices, and regulate the morphology of blending films.1-6 Despite this, some issues still exist in such a ternary system impeding the further improvement of PCE. For instance, increasing the number of photons absorbed by the active layer does not inevitably guarantee an improved photocurrent. One of the reasons is the severe internal photocurrent loss caused by charge recombination in the processes of charge separation and transportation due to the complexity of donor-acceptor (DA) interfaces in the bulk. Therefore, reducing the photocurrent losses caused by charge recombination and improving the charge transport properties of the ternary active layer are desired to further increase the PCE of ternary PSCs.11-13 Recently, molecular doping into binary D-A blends to enhance their electrical property and thus the device performance has drawn great attention.14-18 For instance, tetrafluorotetracyanoquinodimethane (F4TCNQ) used for p-type doping was able to improve the PCEs of PSCs by the increase of either short circuit current density (Jsc) or fill

factor (FF).19, 20 The F4TCNQ doping with a low concentration could contribute to increased background charge carrier density and suppressed charge recombination via trap-states filling.19-21 Notably, F4TCNQ has also been used as an additive in non-fullerene PSCs, where it has been reported to affect phase purity, while not being incorporated into the device itself in appreciable quantities.18 Shang et al illustrated that the effect of F4TCNQ doping strongly correlated to its character in bulk heterojunction devices.21 When acting as Coulomb traps it was detrimental to PCE; in contrast, when filling trap-states prior to forming new traps, it would yield a positive effect on PCE. Obviously, many factors can influence the doping efficiency and the dopant’s function in polymers,22 and there is still a lack of understanding on how molecular electrical doping contributes to an enhancement of highperformance PSCs, especially for ternary blend systems. Therefore, it is worthy of investigating the molecular electrical doping in ternary PSCs to further understand the doping effects and to enhance the device performance. In this work, we explore the F4TCNQ doping into a ternary PSC based on a donor polymer poly[(2,6-(4,8-bis(5-(2ethylhexyl) thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene) -co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)-benzo [1,2-c:4,5-c′]dithiophene-4,8-dione)] (PBDB-T) and two acceptors [6,6]-Phenyl-C71-butyric acid methyl ester (PC71BM) and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5, 5,11,11-tetrakis(4-hexylphenyl)-dithie -no[2,3-d:2’,3’-d’]-s-indaceno[1, 2-b:5,6-b’]dithiophene (IT-

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Figure 1. Schematic diagrams of material energy levels and chemical structures a) molecular p-type doping as F4TCNQ in PBDB-T and energy levels of PBDB-T, F4TCNQ, ITIC and PC71BM. Semi-arrows: electron spin configuration, blue and black lines represent HUMO and LUMO levels respectively. b) FTIR absorption spectra of diagnostic cyano-vibrational stretching signal and polaronic absorption in long wavenumber range in pristine and doped films.

IC) (Figure 1). Ion-pair formation is observed between the dopant and the donor polymer, which proves an effective electrical doping, leading to improved conductivity and hole mobility of the doped blend film. Thereafter, a PCE enhancement from 8.86% to 10.08% for the ternary PSC is obtained due to increases of both Jsc and FF. Importantly, the enhanced charge extraction and suppressed non-geminate charge recombination in doped devices account for the improved Jsc and FF, as revealed by the investigation of charge transport kinetics. Results and discussion The initial perception of F4TCNQ doping in PBDB-T assumes the ion-pair formation (Figure 1a), analogue to the observation in widely studied F4TCNQ doped P3HT.21-25 Different from the P3HT, the PBDB-T is a D-A copolymer composed of a strong electron donor moiety BDT and an electron acceptor moiety BDD. Therefore, the doping effect highly depends on where the F4TCNQ resides at and whether it can effectively occupy an electron from the PBDB-T. While the F4TCNQ resides close to the BDT moiety, it can get an electron to be ionized, forming the ion-pair; and otherwise, the F4TCNQ may contribute in other ways.26, 27 To that end, we explored the possible doping modus via UVVis absorption spectroscopy on PBDB-T films with variable F4TCNQ doping amounts (Figure S1a). With the increase of doping concentration (molar ratio, e.g. 0.1% is given as one F4TCNQ molecule per thousand BDB-T repeat units), the absorption intensity of intra-chain π-π* transition gradually decreases28, attributed to the interaction between F4TCNQ and BDT moiety in the PBDB-T backbone. In addition, a new absorption band in the near infrared region (NIR) centered at 1.35 eV (Figure S1b) appears after doping, resulting from the superposition of ionized F4TCNQ and polymer polarons.29, 30 In order to gain insight into doping effect in the solid film, we conducted Fourier Transform Infrared (FTIR) spectroscopy. Figure 1b shows that a characteristic absorption band peaking at 2227 cm-1 assigned to the cyano-vibrational stretching shifts to 2196 cm-1 after the F4TCNQ doping, which indicates the negatively charged state of F4TCNQ.31-33 In addition, a new broad absorption

band in F4TCNQ doped PBDB-T film appears in the infrared region (3250-3750 cm-1), contributed by the dopinginduced polaron absorbance.21, 23, 26, 34 Under the conditions used for processing the films in this study, the data support the proposed ion-pair formation. Furthermore, we confirmed the doping influence on the film microstructure with grazing-incident wide-angle X-ray scatting (GIWAXS). As shown in Figure S3, a strong (100) peak at 0.32 Å-1 in both non-doped and doped PBDB-T films in the out-of-plane direction (qz) reveals a good polymer crystallinity. Moreover, a slightly face-on preferential orientation is observed by indication of the (010) peak at 1.72 Å-1. The π-π stacking distance in both films is 3.66 Å-1,35 implying that the 0.1% F4TCNQ doping does not interrupt the π-π stacking in pure film, and that the intercalation of F4TCNQ in PBDB-T is not along the polymer backbone. The coherence length for the doped film is 27.1 Å-1, slightly larger than that for the non-doped film 26.4 Å-1. Therefore, we assume that the dopant preferentially intercalates between polymer side chains, which has been observed in previous work.27 Then we investigated the influence of F4TCNQ doping on the performance of binary PSCs with PBDB-T as a donor (Figure S4). The Jsc improvement is achieved when employing PC71BM as a single acceptor and the FF increase is found when employing ITIC as a single acceptor, respectively (Table S1). Interestingly, when we apply the doping to ternary PSCs with the PBDB-T as a donor and the PC71BM and ITIC as dual acceptors, the Jsc and FF are both improved at an optimized doping ratio of 0.1% (Figure S6a) compared to the control one. Meanwhile, the open-circuit voltage (Voc) keeps similar to that of the latter (Figure 2a and Table 1), consequently yielding a PCE over 10% (representing a 14% enhancement). External quantum efficiency (EQE) measurement displays that the spectral response of the doped device is enhanced in a wide visible regime and the integral Jsc of control and doped devices are both close to their results from the J-V measurement (Figure 2b). Compared to the control device, the better diode rectification feature of doped device agrees with the smaller series resistance (Rs) and the relatively larger shunt resistance (Rsh) extracted and the relatively larger shunt


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Chemistry of Materials

Figure 2. Characteristics of non-doped and 0.1% doped devices. a) Current density versus voltage (J-V) curves measured under 100 mW cm-2 illumination, inset: histogram of PCE for 30 samples of 0.1% doped devices; b) EQE curves and integral current density of corresponding devices; c) Nyquist plots of electrochemical impedance spectra measured at the Voc condition in the dark; d) Mott-Schottky plot of capacitance versus voltage measured in the dark; e) typical photocurrent versus light density at the maximum power point curve; f) light intensity dependence of Voc.

Table 1. Device parameters of control and 0.1% doped ternary PSCs under illumination of AM 1.5G (100 mW cm-2). The statistical average PCE is obtained from 30 samples fabricated under identical conditions. Device Voc (V) Jsc (mA cm-2) FF PCE (%) PCEavg (%) Rs (Ω) Rsh (kΩ) control 0.1% doped

0.876 0.893

15.10 16.12

0.67 0.70

(Rsh) extracted from electrochemical impedance spectroscopy (Figure 2c) and the J-V measurement in dark (Figure S6d). Since there is no obvious additional absorption in visible regime (Figure S1) by doping, the improved Jsc can be attributed to enhanced sweep-out charge carriers resulting from the F4TCNQ doping. The background charge density p0 for control and doped devices are summarized in Table 2, which are determined by MottSchottky analysis (Figure 2d). The higher charge density in doped device agrees with the assumption that the intercalated dopants prefer to increase the population of free charges and delocalized polarons, donating to fill the band tail states (the Gaussian density of states distribution) and trap states prior to introducing new trap states. In order to assure this assumption, we firstly investigate the doping effect on charge collection behavior by checking the dependence of the photocurrent density (Jph) on the effective voltage (Veff).36 The Jph saturates nearly at the same Veff , indicating no remarkable differences in charge collection to electrodes (Figure S7a). The value of Jph saturation determines the maximum number of photogenerated free charge carriers transported to and collected by electrodes. The number of charge carrier in the doped device is slightly larger than that in the control device, meaning that more charges can be finally collected by electrodes in the former. The Jsc change depending on the

8.86 10.08

8.59±0.21 9.94±0.14

90.90 26.36

17.76 24.56

incident light intensity (Plight) was examined to understand the doping effect on charge transport (Figure S7b).37 Through fitting the data with power law Jsc ~Pinα, the α of control and doped devices is 1.00 and 1.03, respectively, both close to 1, suggesting that no additional recombination path formed by doping. The photocurrent at the maximum power point (Jmpp) versus light intensity was studied (Figure 2e), and the slope of Jmpp increase for the doped device is larger than that for the non-doped device, suggesting that photocurrent loss induced by non-geminate recombination is reduced in the doped device. On the other hand, we investigated the light intensity dependence of Voc with the universal rule (Figure 2f). When the geminate recombination domains, the slope is about 2 kBT/e; when the non-geminate recombination is in charge, the slope is approximately kBT/e. According to the linear fit, the slope of the doped device is 1.11 kBT/e, smaller than 1.27 kBT/e of the control device. These results suggest that the nongeminate recombination in the doped device is noticeably suppressed in comparison to that in the control device. Therefore, we believe that a more efficient charge carrier extraction in 0.1% doped device leads to a higher Jsc. the PCE of PSCs. In previous reports, the charge carrier mobility was regarded as one of the important parameters affecting the FF; generally, a more balanced electron and hole mobility in bulk heterojunction can result in a better FF.38-41 Recently,



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Figure 3. Transient absorption spectra at indicated delay times under an excitation at 2.43 eV with 7 μJ cm-2/pulse, a) pristine PBDBT film; b) decay kinetics of PBDB-T singlet exciton in non-doped and 0.1% doped device-film, probed at 1.03 eV; c) non-doped devicefilm; d) 0.1% doped active layer film; e) decay kinetics of polaron absorption probed at 1.33 eV; thick lines are double exponential fits of the kinetics; f) fluence dependent TA spectra probed at 1.33 eV for non-doped device-film.

Table 2. Charge carrier density Po and mobility of non-doped and 0.1% doped ternary PSCs. devices Po (×1016 cm-3) μh (×10-4 cm2 V-1 s-1) μe (×10-4 cm2 V-1 s-1) Non-doped device 0.96 5.03±0.31 1.30±0.25 0.1% doped device 6.72 9.22±0.44 1.69±0.22 researchers claimed that this was not a universal regulation to all PSCs since balanced charge carrier mobility did not guarantee a high FF.42-44 In our case, balanced mobility is not a critical factor to obtain a high FF either. The hole and electron mobilities were extracted from single charge carrier devices (Table 2 and Figure S8), indicating that the hole mobility of 0.1% doped device increases but electron mobility nearly keeps constant. Consequently, the initial unbalanced hole and electron mobility (μh/μe) in the control device is further enlarged after doping. However, the case is that the FF of 0.1% doped device is higher than that of the control device. Therefore, besides balanced mobility, we can propose that enhanced charge carrier density, charge mobility, and conductivity (Figure S9) also benefit to improve the FF due to the faster charges extraction.45 To figure out the increase of FF in the doped device, we performed femtosecond transient absorption (TA) measurements on non-doped and doped films. Considering the materials absorption features (Figure S1), all samples are excited with pump pulses of 2.43 eV to assure creation of predominant excitons in the donor polymer. Figure 3a shows the absorption centering at ~1.03 eV assigned to singlet excitons in the polymer, which is consistent with previous works. 46, 47 The singlet exciton decay kinetics of non-doped and doped device-films are nearly identical (Figure 3b) and detail fitting parameters can be found in Table S5. The short life-time reveals efficient exciton separation to free charges or charge transfer either in the

μh/μe 3.85 5.45

non-doped or doped device-films. In the TA spectra of blend films, there are two distinctive absorption features centered at ~1.33 eV and ~1.03 eV. Besides the singlet exciton absorption (~1.03 eV), the peak of ~1.33 eV can be attributed to excited-state absorption of polymer polarons, which has been isolated by the multivariate curve resolution alternating least square analysis (MCR-ALS).46, 48 Their decay kinetics are plotted in Figure 3e and fitted with biexponential function (Table S6). We obtain a weighted decay lifetime for the non-doped film of 248 ps; and correspondingly, the decay lifetime for the doped film is 507 ps, twice longer than that of the non-doped film. Furthermore, we find that the polaron decays are excitation fluence dependent (Figure 3f), mainly attributed to the nongeminate recombination. It thus tells that the longer decay lifetime of polarons presents weaker non-geminate recombination, which can be ascribed to the cooperation of a larger population of delocalized polarons and suppressed trap-assisted recombination via trap-filling.19, 49, 50 Overall, we confirm that non-geminate recombination in the ternary active layer is noticeably restrained by doping, which favours improving the FF since its loss is mainly induced by charge recombination under light stress.45, 49 In view of the long life-time of charges in the devices, we conducted transient photovoltage (TPV) measurement to further confirm the suppression of non-geminate recombination by doping in the device (Figure S11a). The photovoltage for the doped device decays much slower in


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Chemistry of Materials suppression of non-geminate recombination. Our results illustrate that appropriate controlled molecular doping is a promising strategy to effectively promote the performance of ternary polymer solar cells.

Experimental section

Figure 4. 2D GIWAXS patterns for (a) non-doped and (b) 0.1% doped ternary blend films, respectively. In-plane (c) and outof-plane (d) line-cut profiles at incident angle of 0.13 for nondoped and 0.1% doped ternary blend films.

comparison to that for the non-doped device, suggesting a suppressed charge recombination in the doped device. Furthermore, the charge carrier life-time decreases as the background light intensity increases (Figure S11b), which is consistent with the conclusion from TA results. Contribution of microstructure change to device performance improvement for the doped ternary film is also considered. Figure 4 shows 2D GIWAXS patterns and the corresponding out-of-plane and in-plane (qxy) line-cut profiles of the non-doped and doped blend films. The presence of (100) peak in the in-plane direction at 0.29 Å-1 for both films suggests a good lamellar stacking (Figure 4c). The d-spacing of lamellar stacking for non-doped and doped blend films is 21.05 and 21.12 Å-1, respectively. The slight increase of lamellar stacking spacing in doped film can be induced by intercalation of F4TCNQ into polymer side chains.27 The (010) peak in the out-of-plane direction at 1.69 Å-1 proves the face-on preferential orientation in the blend films (Figure 4d). The π-π stacking distance for nondoped and doped blend films increases to 3.72 Å-1 and 3.73 Å-1 in comparison to that (3.66 Å-1) of pure PBDB-T film, respectively, and the corresponding coherence length is 22.89 Å-1 and 23.92 Å-1.35 The larger coherence length of doped blend film turns out to be beneficial to improvement of charge transport and reduction of non-geminate recombination.51-54 Conclusions In summary, a molecular electrical doping strategy using the F4TCNQ has been applied to a ternary PSC for improved device performance. The ion-pair formation between F4TCNQ and PBDB-T indicated the effective p-type doping, resulting in the enhanced conductivity and hole mobility. Molecular packing keeps unchanged under the conditions of 0.1% F4TCNQ doping, and only coherence length for doped blend films increases slightly. Utilization of 0.1% F4TCNQ doped into the ternary PSC improved the PCE from 8.86% to 10.08% via enhancing Jsc and FF together, primarily contributed by the efficient charge carrier extraction and

Device fabrication. In this work, all devices employed a conventional device structure of ITO/PEDOT: PSS (AI4083)/active layer/Ca/Al. ITO coated glass substrates (AimCore Technology Co., Ltd, sheet resistance < 10 Ω/sq.) were cleaned for 30 min by an ultrasonic bath in acetone, isopropanol and ethanol subsequently. Ultraviolet-ozone treatment for 20 min was applied to improve the wetting of the substrate by an aqueous suspension of the intrinsically conducting polymer PEDOT: PSS. Deionized water diluted PEDOT: PSS was prepared by spin coating at 3000 r.p.m. and sequentially annealed at 160 oC under ambient conditions for 20 min to desorb residual water. PBDB-T as donor and ITIC together with PC71BM as acceptors were dissolved in chlorobenzene with D/A weight ratio of 1:1 (PBDB-T concentration: 12 mg/mL), and the molar ratio of ITIC and PC71BM is 2:1. A half hour before active layer deposition, 1.25 vol% 1,8-diiodooctane as additive and F4TCNQ as p-dopant were added in active layer solution. The device active area was ~0.04 cm2 for all solar cells discussed in this work and active layer thickness is ~100 nm. Devices were completed by depositing 15 nm thick calcium and 100 nm aluminum atop active layer as a cathode in a high vacuum chamber (1×10-4 Pa). Instruments and Measurements. The J-V characteristics were measured in-situ in dark and under an illumination intensity of 100 mW cm-2 calibrated by a standard silicon solar cell with a KG5 filter with a Keithley 2400 SourceMeter® to bring spectral mismatch to unity. The corresponding EQEs were characterized using QE-R3011 EQE measurement system (Enli Technology Co. Ltd.) equipped with a standard silicon diode. Light-intensity dependent J-V curves were obtained by tuning the light source power and calibrated with the current of the silicon solar cell with a KG5 filter. The sample morphology was investigated with a Bruker Metrology Nanoscope Ⅲ -D atomic force microscope (AFM). Grazing incident wide-angle X-ray scattering measurement was carried out at Shanghai Synchrotron Radiation Facility (SSRF) with the incident photon energy of 10 keV (wavelength of 1.2398 angstrom) at an incident angle 0.13o and an exposure time of 60 s. Absorption spectra were recorded with a Cary 5000 UVVis-NIR spectrophotometer. Fourier-transform infrared spectra (FTIR) were collected using micro-ATR mode with Broker Optics Hyperion 3000. The femtosecond TA spectra were recorded by an ultrafast transient absorption spectrometer (Time-Tech Spectra, femtoTA100) based on a regenerative amplified Ti: sapphire laser system from Coherent (800 nm, 35 fs, 6 mJ/pulse, and 1 kHz repetition rate), nonlinear frequency mixing techniques and the Helios spectrometer S3. The film thickness was determined by a profilometer (Bruker). Impedance analysis was implemented on the same PSCs by using CIMPS-4 system (ZAHNER PP211) controlled by a Thales measurement software. The mottschottky analysis was carried out on a PARSTAT 2273 workstation (Princeton Applied Research) in dark. The electron and hole mobilities were measured using the SCLC method by fitting the J-V curves with a space charge limited theory. The conductivity was measured between interdigitated ITO contacts with a Keithley 4200 SourceMeter®.



ASSOCIATED CONTENT Supporting Information. The supporting Information is available free of charge via the Internet at http://pubs.acs.org.” UV-Vis absorption, FTIR data, J-V data of binary solar cells, morphology data, optimized device performance data, light intensity dependence data, conductivity data, TPV data

AUTHOR INFORMATION Corresponding Author * Email: [email protected] (X.G.) * Email: [email protected] (C.L.)

Author Contributions All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Nos. 51773204 and 61604152), and the China Postdoctoral Science Foundation (No. 2016M591461). X.G. acknowledges the support from the “Thousand Talents Program for Young Scholars” of China. The authors would like to thank the Shanghai Synchrotron Radiation Facility for providing the beamtime for GIWAXS measurements.

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