The Critical Role of Anode Work Function in Non-Fullerene Organic

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The Critical Role of Anode Work Function in NonFullerene Organic Solar Cells Unveiled by Counterionsize-controlled Self-doping Conjugated Polymers Yong Cui, Guoxiao Jia, Jie Zhu, Qian Kang, Huifeng Yao, Lili Lu, Bowei Xu, and Jianhui Hou Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04982 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018

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Chemistry of Materials

The Critical Role of Anode Work Function in Non-Fullerene Organic Solar Cells Unveiled by Counterion-size-controlled Self-doping Conjugated Polymers Yong Cui,†, ‡ Guoxiao Jia,§ Jie Zhu,∥ Qian Kang,⊥ Huifeng Yao,†, ‡ Lili Lu,†, ‡ Bowei Xu,*, † and Jianhui Hou*, †, ‡ †

State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, Insti‐ tute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.

‡

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

§

School of Chemistry and Biology Engineering, University of Science and Technology Beijing, Beijing 100083, China.

∥

Institute of Material Science and Engineering, Ocean University of China, Qingdao 266100, P. R. China.

⊥

Department of Chemistry, Northeast Normal University, Changchun, Jilin 130024.

ABSTRACT: In this work, we demonstrated an effective method to modulate the p‐type self‐doping effect as well as the molecular energy level of an anionic conjugated polymer, namely PCP‐x (x= H, Li, Na, K, Cs) by changing the counterions, and hence developed a series of anode interlayer materials for non‐fullerene organic solar cells (NF‐OSCs). With the de‐ creasing cation radius, self‐doping effect of PCP‐x can be enhanced and hence the work function (WF) of the PCP‐x‐ modified anodes can be tuned from 4.78 to 5.11 eV. The photovoltaic performance of NF‐OSCs based on J52‐2F:IT‐4F active layer was significantly affected by the WF of the anodes, and a PCE of 12.8% could be achieved by using the PCP‐H‐modified ITO anodes. Furthermore, the PCP‐x‐modified anodes were used to fabricate NF‐OSCs with different active layers, includ‐ ing PBDTTT‐E‐T:IEICO and PBDB‐T‐2F:IT‐4F. It was found that the energetic offsets between WF of the anodes and ioni‐ zation potential (IP) of the donors should be controlled below 0.1 eV to minimize the loss in photovoltaic performance. This work provides not only an effective method for synthesizing a series of p‐type interlayer materials with tunable WF but also an insight on the critical importance of developing anode interlayer materials with high WF.

INTRODUCTION Bulk‐heterojunction (BHJ) organic solar cell (OSC),1‐5 in which a photoactive layer composing of a blend film of electron donor and acceptor sandwiched between cathode and anode, has attracted extensive attention in the past decades.6‐9 In OSCs, the contact between the electrodes and photoactive layers can be optimized by employing in‐ terlayers so as to facilitate carrier collection. It has been well recognized that the development of the interlayer ma‐ terials has played a critical role in advancing the power conversion efficiencies (PCEs) of OSCs. In the past decades, many types of materials including metals with various work functions (WFs),10, 11 metal oxides,12‐15 conjugated polyelectrolytes16‐20 and organic compounds21‐23 have been successfully applied as the interlayers in OSCs. For the interlayer materials, the WF modulation is a critical issue, because the WFs of anode and cathode need to be tuned to match the ionization potentials (IPs) of the donors and the electron affinities (EAs) of the acceptors, respectively. For anode modification, the applications of the interlayers with relatively

higher WFs are helpful to improve hole collection and therefore to enhance the PCEs of the OSCs. For exam‐ ple, Marks et al. utilized NiO to modify the ITO anode and get a WF of 5.4 eV, by which VOC of the device could be improved by 14 mV;24 Heeger et al. employed CPEPh‐Na to get an optimized WF of 5.2 eV, which led to a relatively higher FF of 0.69.25 Recently, the rapid progress in non‐fullerene (NF) acceptors26‐31 provides great opportunities to improve the PCEs and also raises new requirements for the interlayer materials. In the view of energy level, in the highly efficient NF‐ OSCs, although the NF‐acceptors often have similar electron affinities (EAs) as the mostly used fullerene acceptors, the donors usually possess higher IPs than those of the donors used in the fullerene‐based coun‐ terparts.32‐35 In principle, the high IPs of the donor will enlarge the energy offsets at the anode/photoactive layer interface, giving rise to large interfacial barriers and hence affecting device performance. However, this topic has been seldom studied.

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Figure 1. (a) Molecular structures of PCP‐x. (b) Normalized absorption spectra of PCP‐x in solid film. (c) UPS spectra of the ITO electrode modified with PCP‐x. (d) Schematic illustration representing self‐doping effect. A series of devices based on the anode interlayer materi‐ effect can be enhanced, which greatly changes electronic als with gradually tuned WFs will be desired to investigate energy levels of PCP‐x. When PCP‐x are used to fabricate this interesting topic. Although many anode interlayer ma‐ the anode interlayers onto ITO, the WFs of the anodes can terials with different WFs are accessible, the diversity of be tuned from 4.78 to 5.11 eV. Then, we used the PCP‐x‐ their chemical structures may cause unpredicted influence modified ITO substrates to fabricate the NF‐OSCs based on photovoltaic performance that cannot be excluded from on a polymer donor (ploy[4‐(5‐(4,8‐bis(5‐(2‐ethylhexyl)‐4‐ fluorothiophen‐2‐yl)benzo[1,2‐b:4,5‐b']dithiophen‐2‐ the effect of their WFs. Recently, we reported a self‐doped yl)thiophen‐2‐yl)‐5,6‐difluoro‐2‐(2‐hexyldecyl)‐7‐(thio‐ p‐type conductive polymer, namely PCP‐Na, for making phen‐2‐yl)‐2H‐benzo[d][1,2,3]triazole], J52‐2F)38 and a the anode interlayer in a highly efficient NF‐OSC.36 In this polymer, the self‐doping effect is originated from the po‐ non‐fullerene acceptor (IT‐M),39 and found that photovol‐ larization of its conjugated backbone by counterion.37 We taic performance of the devices could be significantly af‐ fected by the WFs of the anodes. The PCP‐H‐modified ITO infer that the WF of this polymer can be tuned by varying has the highest WF and the corresponding device exhibits the counterion without changing its conjugated backbone an outstanding PCE of 12.8% with a high VOC of 0.96 V. and hence a set of anode interlayer materials with very sim‐ ilar chemical structures but different WFs may be obtained. However, with decrease of the WF, VOC of the devices are reduced. Furthermore, PCP‐x are used in the devices based In this work, we demonstrate an effective method to on the other two photoactive layers, and the results clearly modulate the p‐type self‐doping effect by changing the indicate that when the WF of the anode is similar to or counterions in an anionic conjugated polymer with alkyl‐ higher than ionization potential (IP) of the polymer donor, sulfonate side groups and obtain five interlayer materials, the five PCP‐x polymers are equally efficient serving as the namely PCP‐x (x= H, Li, Na, K, Cs). As expected, we found that with the decrease of the cation radius, the self‐doping

Figure 2. (a) EPR curves of interfacial materials. (b) I–V characteristics of electrical conductivity measurements.


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Table 1. Summary of work function, spin density and conductivity of interfacial materials relying on the water. Interfacial layer PCP‐H

Work function (eV)

Spin density (/mol)

Conductivity (S/cm)

5.11

/

/

PCP‐Na

5.03 4.98

3.04 × 1022 4.24 × 1022

1.31 × 10‐3 2.78 × 10‐3

PCP‐K

4.87

2.43 × 1022

2.12 × 10‐4

PCP‐Cs

4.78

2.31 × 1022

1.20 × 10‐8

PCP‐Li

Table 2. Devices parameters using different anode interfacial layer. The device structure is ITO/interfacial layer/J52‐2F:IT‐M/PFN‐Br/Al. Interfacial layer

VOC

JSC (mA/cm2)

Jcal (mA/cm2) a)

FF

PCE (%) b)

PCP‐H

0.96

18.4

18.2

0.730

12.8 (12.4 ± 0.3)

PCP‐Li

0.93

18.3

17.9

0.698

11.8 (11.2 ± 0.4)

PCP‐Na

0.91

18.0

17.7

0.672

11.0 (10.4 ± 0.4)

PCP‐K PCP‐Cs

0.87

17.5

17.1

0.654

9.99 (9.41 ± 0.43)

0.84

17.1

16.6

0.646

9.10 (8.46 ± 0.47)

a) J from the EQE curves. cal b) The average PCEs were obtained from more than 10 independent devices.

anode interlayer; however, if the WF is lower than the IP, a great drop in VOC will be observed.

RESULTS AND DISCUSSION The chemical structures of the five conjugated polymers PCP‐x are shown in Figure 1a. PCP‐Na, PCP‐K and PCP‐Cs are synthesized according to the reported method through palladium catalyzed Suzuki coupling reactions of bi‐ phenylborate and 4H‐cyclopenta‐[2,1‐b;3,4‐b’]‐dithio‐ phene (CPDT) units modified with different propyl sul‐ fonate group.36, 40 PCP‐Li is synthesized by dialyzing PCP‐ Na in 0.1 M LiCl methanol solution as lithium sulfonate cannot be directly introduced on the side chain of the CPDT unit by using our synthetic method because of the weak basicity nature of LiOH. PCP‐H was prepared by acidifying PCP‐Na with diluted hydrochloric acid. The de‐ tailed synthetic methods are provided in the Supporting Information (SI). The alkyl sulfonate groups endow the polymers with good water/methanol solubility except for PCP‐H which can only be dissolved in dimethyl sulfoxide (DMSO). The UV/Vis‐NIR absorption spectra of the five polymers as solid films are provided in Figure 1b. All the polymers exhibit similar absorption profiles with the absorption maxima at 478 nm, which are attributed to the π‐π* tran‐ sition of polymer backbone. The broad absorption tails from 540 to 900 nm with the peaks at approximately 805 nm are ascribed to polaronic transition (λpolaron),41‐43 reveal‐ ing the self‐doping effect of the five polymers. Moreover, the fraction of the polaronic transition to π‐π* transition gradually increases by the order of PCP‐Cs, ‐K, ‐Na, ‐Li and ‐H, indicating that the self‐doping effect is enhanced by re‐ ducing the radius of the counterions. In PCP‐x, the conju‐ gated backbones are polarized by the cations, leading to p‐

doping of the polymers. The counterion with smaller ra‐ dius has stronger electrostatic interaction with the poly‐ mer backbone, resulting in the higher p‐doping effect, and hence the p‐doping effects in these polymers follow the re‐ verse order as the cation radius. Ultraviolet photoelectron spectroscopy (UPS) measure‐ ment was carried out to evaluate the work function (WF) of the ITO electrodes modified by the polymers. As shown in Figure 1c, the WF of the ITO electrode modified with PCP‐H, ‐Li, ‐Na, ‐K and ‐Cs are 5.11, 5.03, 4.98, 4.87 and 4.78 eV, respectively, as determined by the Ecutoff at the lower binding energy side. Clearly, the evolution of the WF is consistent with the doping effect as observed in UV‐vis measurement. Electron spin resonance (EPR) spectroscopy was per‐ formed to investigate the polaron populations of the PCP‐ x in aqueous solution. Unfortunately, the spin density of PCP‐H could not be measured for comparison because of its poor solubility in water. As shown in Figure 2a, for the four solutions, the sharp and strong signals confirm the ex‐ istence of delocalized polaron in these polymers. The un‐ paired spin densities of the samples are determined by double integration of the ESR signals and found to be 3.04 × 1022, 4.24 × 1022, 2.43 × 1022 and 2.31 × 1022 for 1 mol sample of PCP‐Li, ‐Na, ‐K and ‐Cs, respectively. The variation in the polaron densities generally coincided with the counter‐ ion radius in PCP‐x except for PCP‐Li. We infer that the low polaron density in the PCP‐Li solution should be as‐ cribed to its low inoization effect in water, which is a com‐ mon nature of litium salts. Since the electrical conductivity of a conductive polymer is determined the population of polaron, the conductivities of the polymers were measured using the two‐point probe method under a bias voltage of 50 V. As shown in Figure 2d, the conductivities of PCP‐Li,



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Figure 3. (a) The energy level diagram of interlayers and polymer donors. (b) J‐V characteristics and (c) EQE spectra of J52‐ 2F:IT‐M devices modified with PCP‐H, PCP‐Li, PCP‐Na, PCP‐K and PCP‐Cs.

Figure 4. The dependence of device performance on the dE (dE = IP‐WF) of interfacial layer. PCP‐Na, PCP‐K and PCP‐Cs are estimated to be 1.31 × 10−3, 2.78 × 10−3, 2.12 × 10−4 and 1.2 × 10−8 S/cm, respectively. The conductivity is consistent with the EPR result. Atomic force microscopy (AFM) and contact angle (CA) measurements were carried out to investigate the rough‐ ness and wettability of the ITO surfaces modified with PCP‐x. As shown in Figure S9, all the PCP‐x‐modified ITO substrates exhibited similar surface morphology with root mean square roughness (Rq) values of ca. 3 nm, which are smoother than the bare ITO. The CA of chlorobenzene (CB) on the PCP‐x‐modified ITO surfaces is approximately 4.7o, indicating that the wettability of the surfaces are suitable for device fabrication. Moreover, surface energy for each PCP‐x‐modified ITO substrate was calculated with eth‐ ylene glycol and tricresyl phosphate liquid by using the Wu model.44, 45 As listed in Table S1, the surface energies of PCP‐x‐modified ITO substrates showed slight increase from PCP‐H to PCP‐Cs in the range of 47.5—50.2 mJ/m2.

The surface energy of the ITO/PCP‐x substrates is very similar with that of the ITO/PEDOT:PSS. We use the PCP‐x modified ITO substrates to make the OSCs with a device architecture of ITO/PCP‐x/photoactive layer/PFN‐Br/Al to evaluate the influence of the WF change on device performance. Here, the blend based on a polymer donor (J52‐2F) and a NF acceptor (IT‐M) is used as the photoactive material. All of the devices were pre‐ pared in parallel through the same procedure. For conven‐ ience, the devices modified by PCP‐H, ‐Li, ‐Na, ‐K and ‐Cs are named as D‐H, D‐Li, D‐Na, D‐K and D‐Cs, respectively, and the device modified by PEDOT:PSS was also fabricated as control. Figure 3b and c shows the J‐V curves of the OSCs under the illumination of AM 1.5 G, 100 mW/cm2 and Table 2 lists the corresponding photovoltaic parameters and also the statistical data. The OSC modified with PCP‐H exhib‐ ited the best photovoltaic performance with a PCE of 12.8%, a VOC of 0.96 V, JSC of 18.4 mA/cm2 and FF of 0.73, which


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are comparable to the control device. The external quan‐ tum efficiency (EQE) of the devices are provided in Figure 3c. The integral current density of this best performing de‐ vice is 18.2 mA/cm2, which is coincident with the value ob‐ tained from the J‐V measurements. Overall, these results indicate PCP‐H is a promising anode interlayer material for this NF‐OSC.

parison with the PEDOT:PSS‐modified device, all photo‐ voltaic parameters for the D‐H device drop simultaneously (see Table S1). The D‐Cs device gives a VOC of 0.61 V, a JSC of 19.3 mA/cm2 and a FF of 0.62, resulting in a PCE of 7.32%. Very clearly, for this blend film based on this high IP donor, the large energetic barrier for hole collection causes a sig‐ nificant drop in photovoltaic performance.

The IPs and EAs of the photoactive materials measured by UPS (see Figure S12) are schemed in Figure 3a along with the WFs of the interlayers. As shown, the WF of the PCP‐ H‐modified ITO is almost aligned with the IP of J52‐2F, while that of the PEDOT:PSS‐modified ITO has a higher WF. All of the photovoltaic parameters of these two de‐ vices are almost identical, implying the hole collection in them are equally efficient. However, for the other four de‐ vices, the VOC gradually dropped from 0.92 to 0.84 V with the decreased WFs of the anodes. As is known, when WF of the anode is lower than IP of the donor, an energetic barrier for hole collection will be formed, which leads to higher charge recombination and hence simultaneously depresses the VOC, JSC and FF.25, 46 As a result, for the D‐Cs device, a PCE of 9.1% was recorded, which is much lower that of the D‐H device. According to these photovoltaic re‐ sults, we can conclude that the WF of the anode should be higher than or at least aligned with the IP of the donor, and the enlarged barrier for hole collection will induce the ob‐ vious drops in all three photovoltaic parameters and lead to low PCE. To exclude the influence of active layer morphology on device performances, we carried out AFM measurements to investigate the morphologies of the active layer on different PCP‐x layers. As shown in Figure S11, the J52‐2F:IT‐M active layer on different PCP‐x interlayers exhibited very similar phase‐separated morphologies, indicating that PCP‐x layers have little influence on BHJ phase segregation in the active layer. Therefore, it is practically the energy barriers at the PCP‐ x/active layer interfaces to cause the degradation of device performances..

According to the photovoltaic characteristics of all of the devices, we can see the photovoltaic parameters are highly affected by the energetic barriers at the interfaces between the photoactive layers and anode. In order to clearly illus‐ trate the dependences of the photovoltaic parameters on the energetic barriers, we plot the VOC, JSC, FF and PCE as the function of the energetic offset (dE) between WF of the anodes and IP of the donors (dE = IP‐WF) in Figure 4. As shown, the parameters of the three photoactive layers are differently affected by the dE. For the PBDTTT‐E‐T:IEICO devices, the largest dE value is about 0.1 eV and further re‐ ducing the dE has no influence on all of the parameters. For the PBDB‐T‐2F:IT‐4F devices, all of the dE values are quite large (dE > 0.16 eV), and under such a situation, the VOC, JSC, FF and PCE are highly dependent on the dE values, i.e. these parameters sharply drop with the increase of the dE. For the J52‐2F:IT‐M devices, the parameters are also af‐ fected by the dE but the dependence is not as strong as that in the PBDB‐T‐2F:IT‐4F devices. Based on the above anal‐ ysis, we can make the summaries as following. When dE is larger than 0.1 eV, the energetic barrier for hole collection can induce significant loss in photovoltaic performance. The dependences on dE are different for the varied photo‐ active layers. Although a low dE should be helpful to obtain higher photovoltaic performance, further reducing the dE to below 0.1 eV or even lower as that in the PBDTTT‐E‐ T:IEICO device modified by PCP‐H does not make contri‐ bution for enhancing photovoltaic performance. Accord‐ ing to the Integer Charge‐Transfer (ICT) model,49‐52 when the anode WF is larger than the positive integer charge‐ transfer state (EICT+), the Fermi level is pinned to the EICT+ at the interface. Therefore, we propose that the WF at the PCP‐x/PBDTTT‐E‐T:IEICO interfaces are all equal to the EICT+ of PBDTTT‐E‐T. In the PBDTTT‐E‐T:IEICO‐based de‐ vices, Fermi‐level pinning of anodes to the EICT+ could elim‐ inate the energy barriers at the interfaces, which is benefi‐ cial for hole collection. Unfortunately, we failed to accu‐ rately evaluate the EICT+ value of polymer donors due to the great difficulty in preparing the ultra‐thin film of polymer donors on the PCP‐x layers. Future work will focus on the determination of the accurate EICT+ value of polymer do‐ nors and try to get an in‐depth understanding of the energy level alignment at the anode/active layer interface.

In order to verify the above conclusion, we firstly se‐ lected a photoactive layer based on PBDTTT‐E‐T:IEICO to make the devices based on the different ITO anodes by re‐ ferring the previous report.47 The polymer donor PBDBTT‐ E‐T has a IP of 4.86 eV, which is slight higher than WF of PCP‐Cs‐modified ITO and lower than those of the other four ITO anodes. As shown in Figure S13 and Table S2, all the PBDTTT‐E‐T:IEICO‐based devices exhibit similarly high PCEs over 10%, which are comparable with the results for the control device. In detail, the J‐V curves are almost overlapped, suggesting that the photovoltaic properties of the devices are independent to the WFs of the anodes. Furthermore, we also selected another photoactive layer based on PBDB‐T‐2F48:IT‐4F32 to make the devices based on the anodes. In this blend, the IP of the donor is 5.27 eV, which is higher than the WFs of all the ITO anodes, imply‐ ing the high energetic barriers for hole collection will be formed in these devices. For the PEDOT:PSS‐modified de‐ vice, a PCE of 13.3% was recorded, accompanying with a VOC of 0.83 V, a JSC of 21.1mA/cm2 and a FF of 0.76. In com‐

CONCLUSIONS In conclusion, we demonstrate an effective method to modulate p‐doping effect and synthesize a series of p‐ doped conjugated polymers, PCP‐x, for the application in OSCs as the anode interlayer materials. As these polymers are self‐doped due to the polarization effect of the cations, we are able to easily tune the p‐doping effect by changing the radius of the cations, i.e. smaller ion can induce



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stronger p‐doping effect. When these polymers are em‐ ployed as interlayer materials, WF of the ITO anodes can be tuned in the range from 4.78 to 5.11 eV. Based on the PCP‐H‐modified ITO anodes, the one with the highest WF, a highly efficient NF‐OSC with a PCE of 12.8% can be fab‐ ricated. Furthermore, based on these anodes with the grad‐ ually changed WFs, we are able to investigate the correla‐ tion between the photovoltaic performance of the NF‐ OSCs and the energetic offsets at the ITO anode/photoac‐ tive layer interfaces, and find that the energetic offset be‐ tween WF of the anodes and IP of the donors is a critical parameter for the device, and a dE below 0.1 eV is needed to minimize the loss in photovoltaic performance.

Spectral Response Measurement System QE‐R3011 (Enli Technology Co., Ltd, Taiwan). IP of samples were analyzed on Thermo Scientific ESCALab 250Xi using UPS. The gas discharge lamp was used for UPS, with helium gas admit‐ ted and the HeI (21.22eV) emission line employed. The he‐ lium pressure in the analysis chamber during analysis was about 3× 10‐8 mbar. The data were acquired with ‐10V bias. The conductivities of the PCP‐x were measured using the two‐point probe method according to previously pub‐ lished article.36

According to the developing trend in the field, the IPs of the donors used in the highly efficient NF‐OSCs are quite high and will be further elevated for pursuing high VOC. Although PEDOT:PSS seems a quite good material for ITO anode modification in NF‐OSCs, the WF of the PEDOT:PSS‐modified ITO will be not sufficiently high to match the IP of new donors. Overall, in this contribution, we not only report a method to prepare a series of p‐type interlayer materials with tunable WFs, but also demon‐ strate the critical importance of developing the anode in‐ terlayer materials with high WFs.

Supporting Information Synthetic route of PCP‐x, NMR and MS spectra for PCP‐x, photovoltaic data of device.

EXPERIMENTAL SECTION Device Fabrication. In this work, devices were fabri‐ cated with the conventional device structure of ITO/anode interlayer/photoactive layer/PFN‐Br/Al. For the anode in‐ terlayers, PEDOT:PSS(4083) was purchased from the CleviosTM, and was used without additional treatment. Ex‐ cept the PCP‐H was dissolved in DMSO, the rest of the PCP‐x was dissolved in the water. All the photoactive ma‐ terials PBDTTT‐E‐T:IEICO (D/A 1:1.25), J52‐2F:IT‐M (D/A 1:1) and PBDB‐T‐2F:IT‐4F (D/A 1:1) were dissolved in CB at the polymer concentration of 10 mg/ml. To dissolve the polymers fully, active layer solution PBDB‐T‐2F:IT‐4F was stirred at 40˚C, PBDTTT‐E‐T:IEICO and J52‐2F:IT‐M at 60˚C for 2 hours at least. Before spin coating, 1,8‐iodooc‐ tane (0.5%, v/v) was added to the active layer solution J52‐ 2F:IT‐M and PBDB‐T‐2F:IT‐4F. In addition, 1,8‐iodooctane with a volume ratio of 3% was added to PBDTTT‐E‐ T:IEICO. For the cathode interlayer, PFN‐Br was dissolved in methanol with the concentration of 0.5 mg/ml. The pro‐ cess of device fabrication is as follows: About 5 nm PCP‐x or 30 nm PEDOT:PSS layer was spin‐coated on the pre‐ cleaned ITO substrates and annealed at 150°C for 20 mins in the air. Subsequently, the substrates were transferred to the glove box. The active layer was spin‐coated on the in‐ terlayers and the films were treated with thermal annealing at 100˚C for 10 mins. The thickness of all active layers was controlled about 100 nm. PFN‐Br layer was spin‐coated on the top of all the active layers at 3000 rpm for 30 s. Finally, 100 nm Al layer was deposited under high vacuum (ca.3× 10‐4 Pa). Instruments and Measurements. The J–V curves of devices were measured suing a AAA solar simulator (XES‐ 70S1, SAN‐EI Electric Co., Ltd) under 100 mW/cm2 AM1.5G. The EQE spectrum was measured through the Solar Cell

ASSOCIATED CONTENT

AUTHOR INFORMATION Corresponding Author *E‐mail: [email protected]. *E‐mail: [email protected].

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The authors acknowledge the financial support from the National Natural Science Foundation of China (Grants 51673201, 21325419, 21504095) and the Chinese Academy of Sciences (Grant XDB12030200).

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9 ACS Paragon Plus Environment

The Critical Role of Anode Work Function in Non-Fullerene Organic

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