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Cite This: Chem. Mater. 2018, 30, 1078−1084

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*,†,‡

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State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, Institute 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, P. R. China S Supporting Information *

ABSTRACT: In this work, we demonstrated an effective method to modulate the ptype 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 nonfullerene organic solar cells (NFOSCs). With the decreasing 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-xmodified anodes were used to fabricate NF-OSCs with different active layers, including PBDTTT-E-T:IEICO and PBDB-T2F:IT-4F. It was found that the energetic offsets between WF of the anodes and ionization 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.

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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 interlayers so as to facilitate carrier collection. It has been well recognized that the development of the interlayer materials 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 example, © 2018 American Chemical Society

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 nonfullerene (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 NFOSCs, 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 counterparts.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. A series of devices based on the anode interlayer materials with gradually tuned WFs will be desired to investigate this Received: November 29, 2017 Revised: January 17, 2018 Published: January 17, 2018 1078

DOI: 10.1021/acs.chemmater.7b04982 Chem. Mater. 2018, 30, 1078−1084

Article

Chemistry of Materials

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.

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interesting topic. Although many anode interlayer materials with different WFs are accessible, the diversity of their chemical structures may cause unpredicted influence on photovoltaic performance that cannot be excluded from the effect of their WFs. Recently, we reported a self-doped p-type conductive polymer, namely PCP-Na, for making the anode interlayer in a highly efficient NF-OSC.36 In this polymer, the self-doping effect is originated from the polarization of its conjugated backbone by counterion.37 We infer that the WF of this polymer can be tuned by varying the counterion without changing its conjugated backbone and hence a set of anode interlayer materials with very similar chemical structures but different WFs may be obtained. In this work, we demonstrate an effective method to modulate the p-type self-doping effect by changing the counterions in an anionic conjugated polymer with alkylsulfonate side groups and obtain five interlayer materials, namely PCP-x (x = H, Li, Na, K, Cs). As expected, we found that with the decrease of the cation radius, the self-doping effect can be enhanced, which greatly changes electronic energy levels of PCP-x. When PCP-x are used to fabricate the anode interlayers onto ITO, the WFs of the anodes can be tuned from 4.78 to 5.11 eV. Then, we used the PCP-x-modified ITO substrates to fabricate the NF-OSCs based on a polymer donor (ploy[4-(5(4,8-bis(5-(2-ethylhexyl)-4-fluorothiophen-2-yl)benzo[1,2b:4,5-b′]dithiophen-2-yl)thiophen-2-yl)-5,6-difluoro-2-(2-hexyldecyl)-7-(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole], J52− 2F)38 and a nonfullerene acceptor (IT-M),39 and found that photovoltaic performance of the devices could be significantly affected by the WFs of the anodes. The PCP-H-modified ITO has the highest WF and the corresponding device exhibits an outstanding PCE of 12.8% with a high VOC of 0.96 V. However, with decrease of the WF, VOC of the devices are reduced. Furthermore, PCP-x are used in the devices based on the other two photoactive layers, and the results clearly indicate that when the WF of the anode is similar to or higher than ionization potential (IP) of the polymer donor, the five PCP-x polymers are equally efficient serving as the 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 biphenylborate and 4H-cyclopenta-[2,1-b;3,4-b′]-dithiophene (CPDT) units modified with different propyl sulfonate 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 detailed 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* transition 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 revealing 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 reducing the radius of the counterions. In PCP-x, the conjugated backbones are polarized by the cations, leading to p-doping of the polymers. The counterion with smaller radius has stronger electrostatic interaction with the polymer backbone, resulting in the higher p-doping effect, and hence the p-doping effects in these polymers follow the reverse order as the cation radius. Ultraviolet photoelectron spectroscopy (UPS) measurement 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. 1079

DOI: 10.1021/acs.chemmater.7b04982 Chem. Mater. 2018, 30, 1078−1084

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Chemistry of Materials

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

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.

modified ITO surfaces is approximately 4.7°, 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 ethylene glycol and tricresyl phosphate liquid by using the Wu model.44,45 As listed in SI 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 to 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 prepared in parallel through the same procedure. For convenience, 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 show the J−V curves of the OSCs under the illumination of AM 1.5 G, 100 mW/cm2 and Table 1 and Table 2 lists the corresponding photovoltaic parameters and also the statistical data. The OSC modified with PCP-H exhibited 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 are comparable to the control device. The external quantum efficiency (EQE) of the devices are provided in Figure 3c. The integral current density of this best performing device is 18.2 mA/cm2, which is coincident with the value obtained from the

Clearly, the evolution of the WF is consistent with the doping effect as observed in UV−vis measurement. Electron spin resonance (EPR) spectroscopy was performed 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 existence of delocalized polaron in these polymers. The unpaired 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 counterion radius in PCP-x except for PCP-Li. We infer that the low polaron density in the PCP-Li solution should be ascribed to its low inoization effect in water, which is a common 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, 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 roughness and wettability of the ITO surfaces modified with PCP-x. As shown in SI 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-x1080

DOI: 10.1021/acs.chemmater.7b04982 Chem. Mater. 2018, 30, 1078−1084

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Chemistry of Materials

the photovoltaic parameters of these two devices are almost identical, implying the hole collection in them are equally efficient. However, for the other four devices, 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 results, 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 obvious 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 SI 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.. In order to verify the above conclusion, we first selected a photoactive layer based on PBDTTT-E-T:IEICO to make the devices based on the different ITO anodes by referring 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 SI Figure S13 and Table S2, all the PBDTTT-ET: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

Table 1. Summary of Work Function, Spin Density and Conductivity of Interfacial Materials Relying on the Water interfacial layer

work function (eV)

PCP-H PCP-Li PCP-Na PCP-K PCP-Cs

5.11 5.03 4.98 4.87 4.78

spin density (/mol) 3.04 4.24 2.43 2.31

× × × ×

conductivity (S/ cm)

1022 1022 1022 1022

1.31 2.78 2.12 1.20

× × × ×

10−3 10−3 10−4 10−8

Table 2. Devices Parameters Using Different Anode Interfacial Layera interfacial layer

VOC

JSC (mA/cm2)

Jcal (mA/cm2)b

FF

PCP-H

0.96

18.4

18.2

0.730

PCP-Li

0.93

18.3

17.9

0.698

PCP-Na

0.91

18.0

17.7

0.672

PCP-K

0.87

17.5

17.1

0.654

PCP-Cs

0.84

17.1

16.6

0.646

PCE (%)c 12.8 (12.4 11.8 (11.2 11.0 (10.4 9.99 (9.41 9.10 (8.46

± 0.3) ± 0.4) ± 0.4) ± 0.43) ± 0.47)

a

The device structure is ITO/interfacial layer/J52-2F:IT-M/PFN-Br/ Al. bJcal from the EQE curves. cThe average PCEs were obtained from more than 10 independent devices.

J−V measurements. Overall, these results indicate PCP-H is a promising anode interlayer material for this NF-OSC. The IPs and EAs of the photoactive materials measured by UPS (see SI Figure S12) are schemed in Figure 3a along with the WFs of the interlayers. As shown, the WF of the PCP-Hmodified ITO is almost aligned with the IP of J52−2F, while that of the PEDOT:PSS-modified ITO has a higher WF. All of

Figure 4. Dependence of device performance on the dE (dE = IP-WF) of interfacial layer. 1081

DOI: 10.1021/acs.chemmater.7b04982 Chem. Mater. 2018, 30, 1078−1084

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Chemistry of Materials

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, that is, smaller ion can induce stronger p-doping effect. When these polymers are employed 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 fabricated. Furthermore, based on these anodes with the gradually changed WFs, we are able to investigate the correlation between the photovoltaic performance of the NFOSCs and the energetic offsets at the ITO anode/photoactive layer interfaces, and find that the energetic offset between 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. 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:PSSmodified 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 demonstrate the critical importance of developing the anode interlayer materials with high WFs.

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-2F:48IT-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, implying the high energetic barriers for hole collection will be formed in these devices. For the PEDOT:PSS-modified device, a PCE of 13.3% was recorded, accompanying with a VOC of 0.83 V, a JSC of 21.1 mA/cm2 and a FF of 0.76. In comparison with the PEDOT:PSS-modified device, all photovoltaic parameters for the D-H device drop simultaneously (see SI 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 significant drop in photovoltaic performance. 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 illustrate 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 reducing the dE has no influence on all of the parameters. For the PBDB-T-2F:IT4F 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 affected by the dE but the dependence is not as strong as that in the PBDB-T-2F:IT-4F devices. Based on the above analysis, 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 photoactive 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-ET:IEICO device modified by PCP-H does not make contribution for enhancing photovoltaic performance. According 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 devices, Fermi-level pinning of anodes to the EICT+ could eliminate the energy barriers at the interfaces, which is beneficial for hole collection. Unfortunately, we failed to accurately evaluate the EICT+ value of polymer donors due to the great difficulty in preparing the ultrathin film of polymer donors on the PCP-x layers. Future work will focus on the determination of the accurate EICT+ value of polymer donors and try to get an indepth understanding of the energy level alignment at the anode/active layer interface.

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EXPERIMENTAL SECTION

Device Fabrication. In this work, devices were fabricated with the conventional device structure of ITO/anode interlayer/photoactive layer/PFN-Br/Al. For the anode interlayers, PEDOT:PSS(4083) was purchased from the Clevios, and was used without additional treatment. Except the PCP-H was dissolved in DMSO, the rest of the PCP-x was dissolved in the water. All the photoactive materials 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-ET:IEICO and J52−2F:IT-M at 60 °C for 2 h at least. Before spin coating, 1,8-iodooctane (0.5%, v/v) was added to the active layer solution J52−2F:IT-M and PBDB-T-2F:IT-4F. In addition, 1,8iodooctane with a volume ratio of 3% was added to PBDTTT-ET:IEICO. For the cathode interlayer, PFN-Br was dissolved in methanol with the concentration of 0.5 mg/mL. The process of device fabrication is as follows: About 5 nm PCP-x or 30 nm PEDOT:PSS layer was spin-coated on the precleaned ITO substrates and annealed at 150 °C for 20 min in the air. Subsequently, the substrates were transferred to the glovebox. The active layer was spin-coated on the interlayers and the films were treated with thermal annealing at 100 °C for 10 min. 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 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 admitted and the HeI (21.22 eV) emission line employed. The helium pressure in the analysis chamber during analysis was about 3× 10−8 mbar. The data were acquired with −10 V bias. The conductivities of the PCP-x were measured using the two-point probe method according to previously published article.36

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CONCLUSIONS In conclusion, we demonstrate an effective method to modulate p-doping effect and synthesize a series of p-doped conjugated 1082

DOI: 10.1021/acs.chemmater.7b04982 Chem. Mater. 2018, 30, 1078−1084

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Chemistry of Materials

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04982. Synthetic routes of PCP-x, NMR, and MS spectra for PCP-x, photovoltaic data of device (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Huifeng Yao: 0000-0003-2814-4850 Bowei Xu: 0000-0001-6467-6147 Jianhui Hou: 0000-0002-2105-6922 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We 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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REFERENCES

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