Alkenyl Carboxylic Acid: Engineering the Nanomorphology in Polymer

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Alkenyl Carboxylic Acid: Engineering the Nanomorphology in Polymer-Polymer Solar Cells as Solvent Additive Yannan Zhang, Jianyu Yuan, Jianxia Sun, Guanqun Ding, Lu Han, Xufeng Ling, and Wanli Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b02075 • Publication Date (Web): 03 Apr 2017 Downloaded from http://pubs.acs.org on April 5, 2017

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ACS Applied Materials & Interfaces

Alkenyl Carboxylic Acid: Engineering the Nanomorphology in Polymer-Polymer Solar Cells as Solvent Additive Yannan Zhang, † Jianyu Yuan,*,† Jianxia Sun,† Guanqun Ding,† Lu Han,† Xufeng Ling† and Wanli Ma*,† †

Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University.

ABSTRACT: In this contribution, we have investigated a series of commercially available alkenyl carboxylic acid with different alkenyl chains length (trans-2-hexenoic acid (CA-6), trans-2-decenoic acid (CA-10), 9-tetradecenoic acid (CA-14)) for use as solvent additives in polymer-polymer nonfullerene solar cells. We systematically investigated their effect on the film absorption, morphology, carrier generation, transport and recombination in all-polymer solar cells. We revealed that these additives have significant impact on the aggregation of polymer acceptor, leading to improved phase segregation in the blend film. This in-depth understanding of the additives effect on the nanomorphology in all-polymer solar cell can help further boost the device performance. By using CA-10 with the optimal alkenyl chain length, we achieved fine phase separation, balanced charge transport and suppressed recombination in all-polymer solar cells. As a result, an optimal power conversion efficiency (PCE) of 5.71% was demonstrated which is over 50 % higher than that of the as cast device (PCE=3.71%) and slightly higher than that of devices with DIO treatment (PCE=5.68%). Compared with widely used DIO, these halogen-free alkenyl carboxylic acids have a more sustainable processing as well as better performance, which may make them more promising candidates for use as processing additives in organic nonfullerene solar cells.

KEWORDS: alkenyl carboxylic acid, processing additives, all-polymer solar cells, blend morphology, aggregation. 1. Introduction

Solution-processed thin-film solar cells made of conjugated organic materials are potential candidates for renewable energy generation. 1-2 The field of organic photovoltaic (OPV) has seen steady growth in efficiencies over the past 10 years,3-6 with record efficiencies for single- and multi-junction devices reaching 12%.7-8 Nearly all of the efficient OPV devices utilize a heterojunction consisting of a conjugated polymer or small molecule as the electron donor and a fullerene derivative (PCBM) as the electron acceptor.9-10 However, purifying fullerene derivatives normally requires specialized equipment such as high-performance liquid chromatography which limits their scalability and raises the cost.11-12 Moreover, most fullerene derivatives have weak absorption in visible and nearinfrared regions, resulting in inefficient utilization of solar radiation.13-15 In comparison, polymer acceptors have advantages like tunable chemical and electronic properties, as well as enhanced absorption and thermal stability.15 Solutionprocessed bulk hetero-junction (BHJ) polymer-polymer (allpolymer) solar cells16 have proven to be a viable alternative to the polymer/fullerene system, with reported power conversion efficiencies (PCE) now exceeding 8%.17 Quiet recently, conjugated polymers based on boron-nitrogen coordination bond have been developed and exhibit potential in all-polymer solar cell application. 18-19 However, the optimized PCE is still significantly lower than that of the polymer/fullerene or polymer/small molecules systems,8 which is considered related to the unoptimized morphology of current all-polymer system. Previous reports20-23 have demonstrated that the exciton

dissociation and charge transport process are strongly affected by the polymer/PCBM blend morphology. During the past decade, synergistic efforts in morphology optimization, including thermal annealing, solvent vapor annealing, most importantly, processing additive have efficiently increased light absorption, charge carrier mobility and suppressed charge recombination in BHJ blend film.24-27 Solar cell devices fabricated with solvent additive exhibit notably enhanced shortcircuit current density (Jsc), fill factor (FF), and accordingly enhanced PCEs. Compared with the conventional polymer/fullerene system, current all-polymer solar cells still suffer from low Jsc and FF. Reasons given for the lower Jsc and FF values include low electron mobility, limited exciton diffusion length of the acceptor polymer and unfavorable nanoscale morphology.28-31 In this means, optimizing film morphology with processing additive to improve all-polymer device performance may become an efficient and rapid strategy. However, such approach has received only limited attention in high-efficiency nonfullerene solar cells, especially all-polymer devices. To the best of our knowledge, the search of new solvent additives has not been systematically carried out for nonfullerene solar cells. The currently mostly used additive for all-polymer solar cells is 1,8-diiodooctane (DIO),32-33 which works effectively for fullerene-based organic solar cells (OSCs).34-35 However, DIO may not be the optimal additive for all-polymer solar cells since the interaction between DIO and the polymer acceptor may be different compared with that between DIO and fullerene. In addition, previous reports have proven that the halogen-contained DIO may be left in the active layers,36 which can’t meet the requirement for environ-

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mental-friendly processing. Moreover, the residual solvent additive may affect the stability of OSCs through photooxidation and thermally induced changes in the morphology of the active film. 37-41 Therefore, it is of importance to explore novel solvent additives with low-toxicity to effectively enhance the morphology of all-polymer system in a “greener” processing. And meanwhile we can achieve in-depth understanding of the additive effect on the nano-morphology of all-polymer solar cells. In this contribution, we have explored and investigated the application of commercially available alkenyl carboxylic acids (trans-2-hexenoic acid (CA-6), trans-2-decenoic acid (CA-10), 9-tetradecenoic acid (CA-14)) as processing additives in all-polymer solar cells. Halogen-free alkenyl carboxylic acid can be extracted from plants,42 and it also fits the concept of “green” solution process to fabricate solar cell devices. Besides, the halogen-free alkenyl carboxylic acid is more chemical stable relative to DIO, which may has less effect on the device stability. Polymer PTP8,43 reported in our previous work, was selected as the electron donor and a n-type polymer P(NDI2HD-T)44 was selected as the electron acceptor. PTP8/P(NDI2HD-T) all-polymer blend cast from neat solvent without any treatment exhibits a PCE of 3.71%, with corresponding open-circuit voltage (Voc) values of 0.99 V, a Jsc of 7.8 mA/cm2 and a FF of 48%. Significant improvements were observed after the introduction of DIO or alkenyl carboxylic acid as a solvent additive. Optimal PCEs of 4.65%, 5.71%, 4.31% and 5.68% were achieved for CA-6, CA-10, CA-14 and DIO, respectively. Among them, the enhancement of PCEs by using CA-10 were mainly due to the improved Jsc (7.8 mA/cm2 to 10.3 mA/cm2) and FF (48% to 57.7%). We then systemically investigated the influences of alkenyl carboxylic acids as solvent additives on charge generation, recombination, transport and blend nano-morphology of allpolymer solar cells. With optimal length of alkenyl chains, the additive CA-10 can improve the all-polymer blend morphology through enhancing the aggregation of polymer acceptor in the blend, leading to optimal phase separation and domain sizes. As a result, an increased Jsc and a largely enhanced PCE of 5.71% were finally achieved for the all-polymer solar cells. Compared with the widely used DIO, the halogen-free alkenyl carboxylic acids have abundant “green” resources and a more sustainable processing, which may make them potentially better candidates than DIO for use as processing additives in allpolymer solar cells. 2. Results and discussion 2.1 Molecular Structure and Properties The chemical structures of PTP8 and P(NDI2HD-T) can be found in Figure 1. As shown in Figure 1b, both polymer PTP8 and P(NDI2HD-T) exhibit broad absorption between 350-700 nm while the donor polymer has higher absorption coefficients in the visible region. Note that compared with fullerenes, the polymer acceptor P(NDI2HD-T) with an optical bandgap of ~1.78 eV shows greatly improved absorption. Meanwhile, the donor polymer PTP8 and acceptor polymer P(NDI2HD-T) forms ideal type-II heterojunction for efficient charge transfer in solar cells, as shown in Figure 1c. P(NDI2HD-T) has a low lying lowest unoccupied molecular orbital (LUMO) energy level of -3.9 eV, which provides a ~0.3 eV offset from the LUMO of PTP8 for electron transfer

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from donor to acceptor. Hole transfer from P(NDI2HD-T) to PTP8 may be slightly less efficient than that of electron transfer because of the smaller offset between the highest occupied molecular orbital (HOMO) of polymer donor and acceptor. In addition, In contrast to the widely used low bandgap n-type polymer P(NDI2OD-T2), P(NDI2HD-T) exhibit a more balanced crystallinity with that of the donor polymer PTP8,45 which is desired for further morphology optimization. Previous work 46-47 has proven that in polymer/PCBM solar cells the processing additive like DIO can selectively dissolve PCBM. In addition, the additives usually have higher boiling point than the host solvent. As a result, the PCBM molecules can remain certain mobility in the as-cast film, enabling finer control of the phase separation in the BHJ film.48-49 Therefore the criteria for processing additives in polymer/PCBM system is considered to be high boiling point and selective solubility towards one blend component.48 However, in all-polymer system, both donor and acceptor polymer exhibit similar properties. The mechanism of additive on this system may be different from that in polymer/PCBM system. So far, we noticed that there have only been a few investigations on the use of additive in all-polymer solar cells and the mechanism of additive governing the all-polymer blend morphology remains unclear.50-51 Alkenyl carboxylic acid as a processing additive in polymer/PCBM solar cells has been rarely reported.52 We further explored its potential as a “greener” solvent additive in all-polymer solar cells. As shown in Figure 1d, inspired by previous reports on DIO additives,53 we changed the length of the alkenyl chains of these processing additives in a wide range to adjust their effect on device morphology. The detailed boiling points of these alkenyl carboxylic acids can be found in Table S1 (Electronic Supplementary Information, ESI). 2.2 All-polymer Solar Cells Performance The all-polymer solar cell devices were fabricated using the conventional structure of ITO/PEDOT:PSS (40 nm)/ allpolymer blend/ Al (80 nm). As shown in Figure S1 and Table S2-S3 (ESI), we first carried out optimization of devices based on PTP8/P(NDI2HD-T) all-polymer blend. Device optimization included adjustments of processing solvents, donoracceptor blend ratios and active layer thickness. Chloroform was chosen as the processing solvent, the optimized D/A blend ratio was 1.5/1, and the optimal active layer thickness was ~70 nm. Then, alkenyl carboxylic acid and conventional DIO was introduced as solvent additives to further optimize the allpolymer solar cell devices. The optimized J-V curves of allpolymer solar cell devices fabricated from chloroform with/without (w/wo) optimal solvent additives are shown in Figure 2a, with the solar cell device parameters listed in Table 1. To better understand the additive effect on all-polymer solar cell performance, we systematically analysed the change of device parameters before and after the addition of additive. As shown in Figure 2a, the solar cell device without any additive (as-cast) exhibits a Voc of 0.99 V, a Jsc of 7.8 mA/cm2 and a FF of 48.0%. After adding 0.5% (v/v) CA-6 to the host solvent, the Jsc and FF are increased to 9.4 mA/cm2 and 54.4%, respectively, with an improved PCE of 4.65%. The devices obtained by adding 0.5% (v/v) CA-10 to the processing sol-


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vent yield a highest PCE of 5.71% with a Jsc of 10.3 mA/cm2, a Voc of 0.96 V and a FF of 57.7%. However, when adopting CA-14 with further increased alkenyl chain as the solvent additive, the PCE of solar cell devices starts to decrease to 4.31% with a Jsc of 9.6 mA/cm2, a Voc of 0.87 V and a FF of 51.6%. Analogous to the cases in polymer/fullerene system, this trend may be interpreted as insufficient or excessive enhancement of the phase separation in the BHJ film by additives with different boiling points 54. Additionally, their different dark J−V characteristics in Figure 2b show that the devices with additive exhibit lower leakage current under reverse bias compared to the as-cast device, indicating improved film quality. Since DIO is the current most popular additive in nonfullerene solar cells, we should also compare the effect of CA10 with that of DIO in enhancing the device performance. We obtain a maximum PCE of 5.68% for the device with DIO treatment, with a Jsc of 10.1 mA/cm2, a Voc of 0.96 V, and a FF of 58.6%. As shown in Table 1, when replacing the DIO with CA-10 as the processing additive, we obtain a slightly higher PCE for all-polymer solar cells. These results demonstrate that this halogen-free alkenyl carboxylic acid has the potential to be used as a novel and more efficient solvent additive in allpolymer solar cells. It is worth noting that the position of C=C double bond in CA-14 is different from that of the other two alkenyl carboxylic acids. In order to investigate the possible effect induced by the position of C=C double bond, we used trans-2-decenoic acid and 3-decenoic acid as the processing additives. As shown in Figure S2, these two alkenyl carboxylic acids have the same chemical formula, however, with different C=C bond position. The results show that the allpolymer solar cells using both additives show similar performance, which indicate that the position of C=C may not play an important role. In contrast, the length of alkenyl chain and the functional carboxyl group are more crucial factors influencing the performance of all polymer solar cells. To evaluate the photon response of PTP8/P(NDI2HD-T) all-polymer device and calibrate the Jsc data, external quantum efficiencies (EQE) of the devices w/wo additives were measured (Figure 2c). All these devices showed relatively high photo-conversion efficiency over the whole wavelength range of 350–700 nm, with the highest EQE values reaching 50–60% for devices using processing additive. In contrast, the monochromatic EQE values of as-cast device are only around ~45%, suggesting higher efficiency of charge extraction in solar cell devices using additive. The Jsc calculated by integrating the EQE curve with an AM 1.5G reference spectrum is within 5% error compared to the corresponding Jsc obtained from the J−V curves. 2.3 The Role of Additive in Charge Generation, Transport and Recombination In order to investigate the charge generation and extraction process in PTP8/P(NDI2HD-T) solar cells after using solvent additives. The dependence of the photocurrent density (Jph) on the effective voltage (Veff) was recorded under illumination at 100 mW/cm2 (Figure 2d). Jph is equal to JL –JD, where JL and JD are the measured current under illumination and in dark, respectively. Veff is equal to V0-Va where V0 is the voltage when Jph is zero and Va is the applied voltage.55 At low effective voltage below 0.5 V, the Jph of devices with CA-10 and DIO treatment increases drastically and reaches a plateau

at higher voltage, suggesting that free carriers are swept out efficiently. In comparison, the Jph of as-cast device and device with CA-6 and CA-14 treatment increases with a slower rate, indicating less efficient carrier extraction. Moreover, the saturated Jph of the as-cast device is lower than that of the device fabricated with additive. The higher Jph density at high applied voltage suggests that the carrier generation process in devices with additive treatment is more efficient than that in the ascast devices, which is in accordance with their corresponding higher Jsc values. There observations can be attributed to the decreased carrier geminate recombination after additive treatment. Space-charge-limited-current (SCLC) method was adopted to measure the charge carrier mobility, which is an important factor for carrier extraction. Note that both electron and hole mobilities are crucial to achieve efficient and balanced carrier transport. To investigate the charge transport properties of these all-polymer blend films, electron-only and hole-only diodes were fabricated and measured (Figure S3, ESI). As shown in Figure 3, the electron mobilities of the optimized PTP8/P(NDI2HD-T) blends were improved by adding either alkenyl carboxylic acid or DIO. In contrast, the hole mobilities of the PTP8/P(NDI2HD-T) blends are actually slightly decreased with the addition of alkenyl carboxylic acid as the processing additives. However, we notice that the devices hole mobility is usually higher than the electron mobility. The imbalanced carrier transport may hinder the charge transport and result in carrier recombination. We observe that the addition of alkenyl carboxylic acid can actually reduce the gap between the electron and hole mobility of the polymer blend, resulting in more balanced transport and thus reduced recombination. According to the previous report, not only the balanced electron and hole mobility but also the sufficient charge mobility contribute to a higher FF.56 The device treated with DIO exhibits the highest hole and electron mobilities and comparable µh/µe ratio, which then lead to a higher FF. We then carried out investigation on the effect of additive on the carrier recombination. After free charge carriers are generated, there is a subsequent competition between the collection of charge carriers at the electrodes and the non-geminate recombination.57-58 Non-geminate recombination is the recombination of free charge carriers that did not originate from the same absorption event and includes bimolecular as well as trapassisted recombination.59-60 If the rate of non-geminate recombination is too fast,56 losses can significantly limit both Jsc and FF. The dependence of Jsc on light intensity was measured to examine if bimolecular recombination might significantly limit the carrier transport and Jsc. In Jsc-light intensity measurements, the Jsc should follow a power law relationship as J ~ I α where α < 1.0 is indicative of the extent of bimolecular recombination in the film.61 As shown in Figure 4a, for all the optimized devices, Jsc scales almost linearly with light intensity (the value of α ranges from 0.97 to 0.99). The value of α indicates that bimolecular recombination is not a critical limiting factor for the performance of our all-polymer devices. Moreover, the addition of either CA-10 or DIO can suppress the bimolecular recombination more effectively than other additives. Meanwhile, we further measured the dependence of Voc on light intensity to study if trap-assisted recombination may play an important role (Figure 4b). In Voc-light intensity measurements, the Voc should follow a power law relationship as Voc ~ sI, where s > 1.0 kT/q is indicative of significant trapassisted recombination. Apparently, all the optimized devices



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exhibit s value higher than 1.0 kT/q, indicating significant trap-assisted recombination in our all-polymer devices. We can observe reduced recombination with the use of additives. Among them, CA-10 or DIO can suppress the trap-assisted recombination most effectively in all-polymer solar cell devices. In summary, these results suggest that additives of alkenyl carboxylic acids can reduce the both geminate and nongeminate recombination in these all-polymer devices, while CA-10 with the optimal alkenyl chain length outperforms other additives.

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blend solutions, the acceptor polymer P(NDI2HD-T) starts to aggregate in solution before film deposition, while the morphology of PTP8 has almost no change with the addition of additive. During the film deposition, the remaining additive in the film can further finely adjust the phase separation between PTP8 and P(NDI2HD-T) by selectively solubility. Therefore, in this work the effect of CA-10 on the control of blend morphology may mainly stem from its impact on the acceptor polymer P(NDI2HD-T). 2.5 The Role of Additive in All-Polymer Blend Morphology

2.4 The Role of Additive in Optical Properties We investigated the effect of additives on the optical properties of PTP8/P(NDI2HD-T) blend films. Figure 5 shows the UV-vis absorption spectra of the PTP8/P(NDI2HDT) thin films cast from solvent w/wo additive at optimal content. When the blend film is cast from pure solvent, we can observe distinct absorption peaks positioned in the ranges between 300 - 400 nm and 500 - 700 nm. Then the absorbance is enhanced after the addition of solvent additives. In particular, the blend film processed with CA-10 additive exhibits most pronounced enhancement in the whole absorption region. Such significant increase in the peak intensity indicates that the interactions of molecular inter-chains are most strong with the addition of CA-10, resulting in improved local structural order in the composite film. 62 In order to understand the effect arising from the additives, we investigated the change of absorption of polymer solutions with the concentration of added additives. The measurements were performed on dilute polymer solution in chloroform (0.01 mg/ml). Note that with the addition of more additive, the dilution effect becomes more apparent. As shown in Figure 6a, with gradually increased loading of CA-10 (from 0% to 30%, v/v), the absorption intensity of PTP8 solution is decreased as a result of dilution effect. However, the shape of the absorption spectra is kept the same, excluding the absorption of the additive. Meanwhile, we also measured the absorption of solution with the addition of extra 30 % CF, which has a similar absorption spectra with that of solution added with 30% additive. These results indicate the aggregation of donor PTP8 in solution has no significant change with the addition of additive into the host solvent. Likewise, we performed the same measurements on dilute P(NDI2HD-T) solutions. In contrast, as shown in Figure 6b, the absorption spectrum of acceptor polymer solution undergoes apparent change. When gradually increasing the loading of CA-10 into P(NDI2HD-T) solutions, we observe the absorption spectra starting to red-shift, and a new shoulder at ~600 nm emerges and is enhanced with the addition of more additive. The drastic absorption change of P(NDI2HD-T) indicates that CA-10 can cause strong aggregation of the polymer chains even in diluted solution.63 As shown in Figure 6c-6d, the effect of DIO on the aggregation of polymer in solution was also examined under the same condition for comparison. Interestingly, the absorption changes of both the donor and acceptor solution with DIO addition exhibit similar trend with those using CA10. From the additive dependent UV–vis measurements, we can conclude that the donor and acceptor exhibit notably different solubility in the additive of either CA-10 or DIO. Thus the additive can be used to selectively adjust the aggregation state of the polymers. When the additive is added into the

As we mentioned, the exciton dissociation and charge transport process are strongly affected by the active layer morphology in organic solar cells.64 Therefore, a better understanding of additive effect on morphological change is important to further enhance the performance of all-polymer solar cells. The microstructure of the PTP8/P(NDI2HD-T) blends were first investigated by atomic force microscopy (AFM) (Figure 7). By incorporating different additives into the all-polymer blend solutions, the corresponding blend films exhibit quite different topography features compared to the ascast blend film. We can observe that after the use of alkenyl carboxylic acid with different chain length, the film surface is getting rougher and the domain sizes become larger. Especially, the surface roughness changes from 0.77 nm for the as-cast film to 1.27 nm for the film using additive CA-14 with the longest chain length. In comparison, the blend film with CA10 treatment shows moderate domain sizes and a RMS value of 0.84 nm, indicating proper phase separation. Meanwhile, the all-polymer blend film cast from chloroform with optimal DIO exhibits similar AFM height morphology compared to blend film cast from chloroform with CA-10. The increased surface roughness and domain size in these optimized films may suggest properly enhanced phase segregation, leading to improved charge transport. In order to confirm the results, we also investigated the film morphology by using transmission electron microscopy (TEM). As shown in Figure 8, the as-cast PTP8/P(NDI2HD-T) blend film exhibits a certain extent of phase separation. However, we cannot observe well-developed donor-acceptor network structure. In addition, the small contrast in the image may suggest low phase purity in each domain. By adopting alkenyl carboxylic acids as processing additives, the contrast between domains is certainly increased and the network is more evident, which indicate enhanced phase segregation of the polymer blend. Especially for film processed with CA-14, we can observe huge domains resulted from excessive phase separation. In contrast, the blend film with CA-10 treatment shows finer domain sizes and better interconnected donor and acceptor network structure, which is beneficial for carrier separation and transport. All these TEM observations are consistent with the AFM images. Furthermore, the blend film with DIO was also characterized. In comparison with the blend film processed with CA-10, the one processed with DIO also shows clear network structure while the domains sizes are larger. This slightly enhanced phase segregation may result in better carrier transport, which is consistent with the carrier mobility measurement. Note that the electron mobility is increasing with the addition of additive which can be attributed to the effect of additive on the aggregation of polymer acceptor P(NDI2HD-T).


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AUTHOR INFORMATION

Conclusion

In order to search more efficient solvent additives for non-fullerene OPVs, we have successfully investigated a series of alkenyl carboxylic acid (CA-6, CA-10 and CA-14) for use as additives in all-polymer solar cells. Our results show that these novel halogen-free additives can fine-tune the film optical properties, improve charge generation, transport and suppress carrier recombination via optimizing the all-polymer BHJ morphology. By using PTP8 as the electron donor and NDI-based polymer P(NDI2HD-T) as the electron acceptor, the optimized solar cells using CA-10 with the optimal alkenyl chain length as the additive demonstrated an enhanced PCE of 5.71%, which is over 50 % higher than that of the as cast device (PCE=3.71%) and slightly higher than that of devices with DIO treatment (PCE=5.68%). Compared with widely used DIO, these halogen-free alkenyl carboxylic acids have a more sustainable processing as well as better performance, which may make them more promising candidates for use as processing additives in organic nonfullerene solar cells. 4. Experimental Section Materials: PTP8 and P(NDI2HD-T) were synthesized following the methods in previous works respectively. Alkenyl carboxylic acids and DIO were purchased from TCI. Unless otherwise stated, all chemicals were commonly commercially available products and used as received. Characterization: UV-vis-NIR spectra were recorded on a Perkin Elmer model Lambda 750. Tapping-mode AFM images were obtained with a Veeco Multimode V instrument. TEM images were performed on Tecnai G2 F20 S-Twin Transmission Electron Microscope. Solar Cells Device Fabrication and Characterization: Organic soar cells were fabricated with a structure of ITO/ PEDOT:PSS/active layer/Al. ITO coated glass substrates were cleaned sequentially with acetone, deionized water, isopropanol and acetone for 20 min each time, and then treated with UV/ozone for 30 min. A 40 nm PEDOT:PSS film was spincoated on ITO substrate at 4500 rpm and hot-annealing for 10 min for 150℃. Blend solution of PTP8/P(NDI2HD-T) with a 1.5/1 wt ratio was spin coated onto the PEDOT:PSS layer at 2500 rpm with or without 0.5% (v/v) additive. Before spin coating of the active layer, the solution should be kept heating at 40 °C for at least 6 h. The solvent was chloroform with concentration of 10 mg/ml. For device preparation, an aluminum (100 nm) as the cathode was deposited on the active layer in vacuum thermal evaporator. The area of each device is 7.25 mm2. The performance of all cells were tested under AM 1.5G light (Newport, Class AAA solar simulator, 94023A-U). The EQE was determined using a certified IPCE equipment (Zolix Instruments, Inc, SolarCellScan100).

ASSOCIATED CONTENT Supporting Information. The detailed boiling points of these alkenyl carboxylic acids, device optimization, device performance with two acids which have the same chemical formula but different C=C bond position, SCLC mobility measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author [email protected] (J. Yuan) [email protected] (W. Ma)

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT The author thanks the Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University. This work was supported by the National Key Research Projects (Grant No. 2016YFA0202402), the National Natural Science Foundation of China (Grant No. 61222401, No. 61674111). And we also acknowledge the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Figure 1. Chemical structure of PTP8 and P(NDI2HD-T) (a), thin film UV-vis absorption spectra of donor and acceptor materials (b), energy level diagram of all-polymer solar cell (c), chemical structure of additives including DIO, CA-6, CA-10 and CA-14 (d).

Figure 2. J-V curves of optimized all-polymer solar cell devices based on PTP8/P(NDI2HD-T) w/wo additive (a), dark current J-V curves (b), EQEs (c), photocurrent of the corresponding solar cell devices(d).


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Figure 3. The hole mobilities (µh) and electron mobilities (µe) of PTP8/P(NDI2HD-T) blend cast w/wo additives.

Figure 4. Jsc (a), Voc (b) dependence on illumination intensity for PTP8/P(NDI2HD-T) solar cell device without additive and with CA-6, CA-10, CA-14 and DIO as additives in symbols, together with linear fits in lines.


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Figure 5. UV-vis absorption spectra of PTP8/P(NDI2HD-T) film cast without additive and with CA-6, CA-10, CA-14 and DIO as solvent additives.

Figure 6. Solution absorption of PTP8 with different concentration of CA-10 (a), P(NDI2HD-T) with different concentration of CA-10 (b), PTP8 with different concentration of DIO (c), P(NDI2HD-T) with different concentration of DIO (d).


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Figure 7. AFM height images for PTP8/P(NDI2HD-T) blend films as-cast (a), with CA-6 (b), with CA-10 (c), with CA14 (d), with DIO (e).

Figure 8. TEM images for blend films as-cast (a), processed with CA-6 (b), with CA-10 (c), with CA-14 (d), with DIO (e).


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Table 1. Photovoltaic parameters of the PTP8/P(NDI2HD-T) solar cell devices cast without additive and with optimal additive concentration, including CA-6, CA-10, CA-14 and DIO.

a)

Processing condition

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%) Best (Ave.) a)

As cast

0.99

7.8

48.0

3.71(3.50)

+0.5% CA-6

0.91

9.4

54.4

4.65 (4.47)

+0.5% CA-10

0.96

10.3

57.7

5.71 (5.41)

+0.5% CA-14

0.87

9.6

51.6

4.31 (4.07)

+0.5% DIO

0.96

10.1

58.6

5.68 (5.35)

Average efficiency from 10 devices.


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Alkenyl Carboxylic Acid: Engineering the Nanomorphology in Polymer

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