Organic Solar Cells Based on Non-fullerene Small-Molecule Acceptors: Impact of Substituent Position

Abstract

•Molecular dynamics simulations provide detailed access to packing patterns in blends•Calculations of Flory-Huggins parameters rationalize donor/acceptor mixing in blends•DFT calculations detail the impact of packing patterns on electronic properties•Our computational methodology elucidates the impact of substitution positions Recent enhancements in the efficiencies of organic solar cells have come from efforts to optimize the pairs of polymer donors and non-fullerene small-molecule acceptors (SMAs) forming the active layer. In particular, functional groups introduced at the SMA edges allow the fine tuning of the electronic and morphological properties of the blends. However, it remains experimentally challenging to characterize the origin of the impact of the precise substitution positions at the molecular scale. Here, a combination of molecular dynamics simulations and long-range corrected density functional theory calculations enables a detailed molecular description of the relationships among substitution position, packing pattern, and electronic properties in representative blends, as well as a comprehensive rationalization of recent experimental data. Our methodology is expected to translate well to other polymer/SMA systems. Molecular engineering of non-fullerene small-molecule acceptors (SMAs) plays a key role in enhancing the performance of organic solar cells. An effective strategy is to introduce functional groups into SMA end groups to tune the electronic and morphological properties of polymer/SMA blends. Here, molecular dynamics simulations and long-range corrected density functional theory calculations are combined to examine the impact of the position of methoxy substitution in the SMA end groups. As representative systems, blends of the IT-OM small-molecule acceptor with the PBDB-T polymer donor are explored; three different positions of the methoxy substitution of the IT-OM end groups are examined. By considering intermolecular mixing and packing, the energetic distribution of the charge-transfer electronic states, the exciton-dissociation and non-radiative recombination processes, and the electron-transfer rates among adjacent acceptors, we provide a comprehensive molecular-scale rationalization of the significant experimental variations in device performance for PBDB-T/IT-OM-based solar cells as a function of methoxy position. Molecular engineering of non-fullerene small-molecule acceptors (SMAs) plays a key role in enhancing the performance of organic solar cells. An effective strategy is to introduce functional groups into SMA end groups to tune the electronic and morphological properties of polymer/SMA blends. Here, molecular dynamics simulations and long-range corrected density functional theory calculations are combined to examine the impact of the position of methoxy substitution in the SMA end groups. As representative systems, blends of the IT-OM small-molecule acceptor with the PBDB-T polymer donor are explored; three different positions of the methoxy substitution of the IT-OM end groups are examined. By considering intermolecular mixing and packing, the energetic distribution of the charge-transfer electronic states, the exciton-dissociation and non-radiative recombination processes, and the electron-transfer rates among adjacent acceptors, we provide a comprehensive molecular-scale rationalization of the significant experimental variations in device performance for PBDB-T/IT-OM-based solar cells as a function of methoxy position. For bulk heterojunction (BHJ) organic solar cells (OSCs), recent impressive enhancements in their power-conversion efficiencies (PCEs) have emerged from the use of non-fullerene small-molecule acceptors (SMAs) as electron-accepting materials;1Lin Y. Wang J. Zhang Z.-G. Bai H. Li Y. Zhu D. Zhan X. An electron acceptor challenging fullerenes for efficient polymer solar cells.Adv. Mater. 2015; 27: 1170-1174Crossref PubMed Scopus (2835) Google Scholar for instance, PCEs of over 16% have been achieved for single-junction BHJ OSCs2Cui Y. Yao H. Zhang J. Zhang T. Wang Y. Hong L. Xian K. Xu B. Zhang S. Peng J. et al.Over 16% efficiency organic photovoltaic cells enabled by a chlorinated acceptor with increased open-circuit voltages.Nat. Commun. 2019; 10: 2515Crossref PubMed Scopus (1245) Google Scholar and of over 17% for tandem cells.3Meng L. Zhang Y. Wan X. Li C. Zhang X. Wang Y. Ke X. Xiao Z. Ding L. Xia R. et al.Organic and solution-processed tandem solar cells with 17.3% efficiency.Science. 2018; 361: 1094-1098Crossref PubMed Scopus (2031) Google Scholar In comparison with their fullerene-based counterparts, a key feature of efficient OSCs based on non-fullerene SMAs is the reduced non-radiative voltage losses.2Cui Y. Yao H. Zhang J. Zhang T. Wang Y. Hong L. Xian K. Xu B. Zhang S. Peng J. et al.Over 16% efficiency organic photovoltaic cells enabled by a chlorinated acceptor with increased open-circuit voltages.Nat. Commun. 2019; 10: 2515Crossref PubMed Scopus (1245) Google Scholar,4Baran D. Kirchartz T. Wheeler S. Dimitrov S. Abdelsamine M. Gorman J. Ashraf R.S. Holliday S. Wadsworth A. Gasparini N. et al.Reduced voltage losses yield 10% efficient fullerene free organic solar cells with >1 V open circuit voltages.Energy Environ. Sci. 2016; 9: 3783-3793Crossref PubMed Google Scholar,5Qian D. Zheng Z. Yao H. Tress W. Hopper T.R. Chen S. Li S. Liu J. Chen S. Zhang J. et al.Design rules for minimizing voltage losses in high-efficiency organic solar cells.Nat. Mater. 2018; 17: 703-709Crossref PubMed Scopus (553) Google Scholar Indeed, much attention has been given to the molecular optimization of pairs of polymer donors and non-fullerene SMAs to minimize the energetic offset between either the ionization energies or the electron affinities of the donor and acceptor components, which increases the energy of the interfacial charge-transfer (CT) states, hybridizes the local-exciton and CT states, and contributes toward decreasing the voltage losses.5Qian D. Zheng Z. Yao H. Tress W. Hopper T.R. Chen S. Li S. Liu J. Chen S. Zhang J. et al.Design rules for minimizing voltage losses in high-efficiency organic solar cells.Nat. Mater. 2018; 17: 703-709Crossref PubMed Scopus (553) Google Scholar In the case of non-fullerene SMAs, a major focus has often been on the structural modification of the prominent ITIC molecule (i.e., 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene), which displays a so-called acceptor-donor-acceptor (A-D-A) structure prone to intramolecular CT.1Lin Y. Wang J. Zhang Z.-G. Bai H. Li Y. Zhu D. Zhan X. An electron acceptor challenging fullerenes for efficient polymer solar cells.Adv. Mater. 2015; 27: 1170-1174Crossref PubMed Scopus (2835) Google Scholar Modification of its electron-rich indacenodithieno[3,2-b]thiophene core,2Cui Y. Yao H. Zhang J. Zhang T. Wang Y. Hong L. Xian K. Xu B. Zhang S. Peng J. et al.Over 16% efficiency organic photovoltaic cells enabled by a chlorinated acceptor with increased open-circuit voltages.Nat. 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In particular, introducing functional groups such as electron-donating methyl or methoxy groups or electron-withdrawing fluorine or chlorine atoms on the benzene terminal groups has proved to be a facile and effective strategy to optimize the electronic properties.2Cui Y. Yao H. Zhang J. Zhang T. Wang Y. Hong L. Xian K. Xu B. Zhang S. Peng J. et al.Over 16% efficiency organic photovoltaic cells enabled by a chlorinated acceptor with increased open-circuit voltages.Nat. Commun. 2019; 10: 2515Crossref PubMed Scopus (1245) Google Scholar,3Meng L. Zhang Y. Wan X. Li C. Zhang X. Wang Y. Ke X. Xiao Z. Ding L. Xia R. et al.Organic and solution-processed tandem solar cells with 17.3% efficiency.Science. 2018; 361: 1094-1098Crossref PubMed Scopus (2031) Google Scholar,5Qian D. Zheng Z. Yao H. Tress W. Hopper T.R. Chen S. Li S. Liu J. Chen S. Zhang J. et al.Design rules for minimizing voltage losses in high-efficiency organic solar cells.Nat. Mater. 2018; 17: 703-709Crossref PubMed Scopus (553) Google Scholar,10Yao H. Cui Y. Yu R. Gao B. Zhang H. Hou J. Design, synthesis, and photovoltaic characterization of a small molecular acceptor with an ultra-narrow band gap.Angew. Chem. Int. Ed. 2017; 56: 3045-3049Crossref PubMed Scopus (656) Google Scholar, 11Wang J. Zhang J. Xiao Y. Xiao T. Zhu R. Yan C. Fu Y. Lu G. Lu X. Marder S.R. et al.Effect of isomerization on high-performance nonfullerene electron acceptors.J. Am. Chem. Soc. 2018; 140: 9140-9147Crossref PubMed Scopus (330) Google Scholar, 12Yuan J. Zhang Y. Zhou L. Zhang G. Yip H.-L. Lau T.-K. Lu X. Zhu C. Peng H. Johnson P.A. et al.Single-junction organic solar cell with over 15% efficiency using fused-ring acceptor with electron-deficient core.Joule. 2019; 3: 1140-1151Abstract Full Text Full Text PDF Scopus (3135) Google Scholar, 13Fan B. Zhang D. Li M. Zhong W. Zeng Z. Ying L. Huang F. Cao Y. Achieving over 16% efficiency for single-junction organic solar cells.Sci. China Chem. 2019; 62: 746-752Crossref Scopus (776) Google Scholar, 14Li S. Ye L. Zhao W. Zhang S. Mukherjee S. Ade H. Hou J. Energy-level modulation of small-molecule electron acceptors to achieve over 12% efficiency in polymer solar cells.Adv. Mater. 2016; 28: 9423-9429Crossref PubMed Scopus (1246) Google Scholar, 15Li S. Ye L. Zhao W. Zhang S. Ade H. Hou J. Significant influence of the methoxyl substitution position on optoelectronic properties and molecular packing of small-molecule electron acceptors for photovoltaic cells.Adv. Energy Mater. 2017; 7: 1700183Crossref Scopus (163) Google Scholar,17Zhao W. Li S. Yao H. Zhang S. Zhang Y. Yang B. Hou J. Molecular optimization enables over 13% efficiency in organic solar cells.J. Am. Chem. Soc. 2017; 139: 7148-7151Crossref PubMed Scopus (2308) Google Scholar, 18Li W. Ye L. Li S. Yao H. Ade H. Hou J. A high-efficiency organic solar cell enabled by the strong intramolecular electron push-pull effect of the nonfullerene acceptor.Adv. Mater. 2018; 30: 1707170Crossref PubMed Scopus (341) Google Scholar, 19Zhang H. Yao H. Hou J. Zhu J. Zhang J. Li W. Yu R. Gao B. Zhang S. Hou J. Over 14% efficiency in organic solar cells enabled by chlorinated nonfullerene small-molecule acceptors.Adv. Mater. 2018; 30: 1800613Crossref PubMed Scopus (602) Google Scholar, 20Zhang S. Qin Y. Zhu J. Hou J. Over 14% efficiency in polymer solar cells enabled by a chlorinated polymer donor.Adv. Mater. 2018; 30: 1800868Crossref PubMed Scopus (934) Google Scholar, 21Li Y. Lin J.-D. Che X. Qu Y. Liu F. Liao L.-S. Forrest S.R. High efficiency near-infrared and semitransparent non-fullerene acceptor organic photovoltaic cells.J. Am. Chem. Soc. 2017; 139: 17114-17119Crossref PubMed Scopus (342) Google Scholar, 22Che X. Li Y. Qu Y. Forrest S.R. High fabrication yield organic tandem photovoltaics combining vacuum- and solution-processed subcells with 15% efficiency.Nat. Energy. 2018; 3: 422-427Crossref Scopus (425) Google Scholar, 23Kan B. Feng H. Yao H. Chang M. Wan X. Li C. Hou J. Chen Y. A chlorinated low-bandgap small-molecule acceptor for organic solar cells with 14.1% efficiency and low energy loss.Sci. China Chem. 2018; 61: 1307-1313Crossref Scopus (199) Google Scholar End-group substitution also plays an important role in determining the morphological properties of polymer/SMA blends. This holds especially true when the four central, out-of-plane 4-hexylphenyl side chains impede any compact arrangement around the cores, such that intermolecular packing is expected to take place essentially through the end groups.14Li S. Ye L. Zhao W. Zhang S. Mukherjee S. Ade H. Hou J. Energy-level modulation of small-molecule electron acceptors to achieve over 12% efficiency in polymer solar cells.Adv. Mater. 2016; 28: 9423-9429Crossref PubMed Scopus (1246) Google Scholar,15Li S. Ye L. Zhao W. Zhang S. Ade H. Hou J. Significant influence of the methoxyl substitution position on optoelectronic properties and molecular packing of small-molecule electron acceptors for photovoltaic cells.Adv. Energy Mater. 2017; 7: 1700183Crossref Scopus (163) Google Scholar,18Li W. Ye L. Li S. Yao H. Ade H. Hou J. A high-efficiency organic solar cell enabled by the strong intramolecular electron push-pull effect of the nonfullerene acceptor.Adv. Mater. 2018; 30: 1707170Crossref PubMed Scopus (341) Google Scholar,20Zhang S. Qin Y. Zhu J. Hou J. Over 14% efficiency in polymer solar cells enabled by a chlorinated polymer donor.Adv. Mater. 2018; 30: 1800868Crossref PubMed Scopus (934) Google Scholar,27Wang T. Brédas J.-L. Nonfullerene small-molecule acceptors for organic photovoltaics: understanding the impact of methoxy substitution position on molecular packing and electron-transfer properties.Adv. Funct. Mater. 2019; 29: 1806845Crossref Scopus (19) Google Scholar Since the ITIC benzene end group provides four available substitution positions, it is of interest to examine the relationships among the substitution positions of functional groups as well as the impact on the electronic and morphological properties of resulting polymer/SMA blends and, ultimately, the OSC device efficiencies. Developing such an understanding will aid in the rational optimization of both the electronic and morphological properties and, thus, in further improvements in OSC efficiencies. Hou and co-workers recently synthesized four isomers (referred to as IT-OM-1, IT-OM-2, IT-OM-3, and IT-OM-4, see Figure 1) by introducing a single electron-donating methoxy group on the different substitution positions of the ITIC benzene end group. These authors then investigated the methoxy position-global morphology-device performance relationships in PBDB-T/IT-OM-based BHJ OSCs, where PBDB-T (poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophen)-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c′]dithiophene)-4,8-dione)]) acts as the polymer donor (Figure 1).15Li S. Ye L. Zhao W. Zhang S. Ade H. Hou J. Significant influence of the methoxyl substitution position on optoelectronic properties and molecular packing of small-molecule electron acceptors for photovoltaic cells.Adv. Energy Mater. 2017; 7: 1700183Crossref Scopus (163) Google Scholar Interestingly, varying the methoxy position was seen to have very significant impact on the global morphology and device performance. For instance, the PBDB-T/IT-OM-4 blend exhibits a much larger extent of phase separation as compared with the rather homogeneous morphology found in the other three blends. Also, in going from IT-OM-1 to IT-OM-2, IT-OM-3, and IT-OM-4, the BHJ OSCs yield open-circuit voltages (VOC) of 1.01, 0.93, 0.97, and 0.96 V, short-circuit currents (JSC) of 12.31, 17.53, 16.38, and 14.69 mA cm−2, and fill factors (FFs) of 0.51, 0.73, 0.68, and 0.56, leading to PCEs of 6.3%, 11.9%, 10.8%, and 7.9%, respectively.15Li S. Ye L. Zhao W. Zhang S. Ade H. Hou J. Significant influence of the methoxyl substitution position on optoelectronic properties and molecular packing of small-molecule electron acceptors for photovoltaic cells.Adv. Energy Mater. 2017; 7: 1700183Crossref Scopus (163) Google Scholar However, it remains experimentally challenging to characterize the molecular-scale impact of the methoxy position on the local morphology (i.e., intermolecular mixing and packing) and the electronic properties of the blends. In this work, we consider the PBDB-T/IT-OM-1, PBDB-T/IT-OM-2, and PBDB-T/IT-OM-4 blends as representative systems, given that they produce the largest variations among the group. All-atom molecular dynamics (MD) simulations allow us to examine, at the molecular scale, the impact of the methoxy position on the intermolecular mixing and packing in these blends. The mixing is then analyzed by evaluating the Flory-Huggins interaction parameters and the intermolecular interaction energies. Via long-range corrected density functional theory (DFT) calculations, the intermolecular packing is then correlated with the electronic properties, including the energetic distribution of the CT states, the exciton-dissociation rates from the local-exciton (LE) states to the CT states, the non-radiative recombination rates from the CT states to the ground state and related voltage losses, and the electron-transfer rates among neighboring IT-OM molecules. Experimental investigations on the global morphologies of the PBDB-T/IT-OM blends (via atomic force microscopy on the surface and transmission electron microscopy) indicate that the PBDB-T/IT-OM-4 blend presents a large extent of phase separation with domain sizes of ∼200–300 nm, in contrast to the rather homogeneous morphology of the PBDB-T/IT-OM-(1,2) blends.15Li S. Ye L. Zhao W. Zhang S. Ade H. Hou J. Significant influence of the methoxyl substitution position on optoelectronic properties and molecular packing of small-molecule electron acceptors for photovoltaic cells.Adv. Energy Mater. 2017; 7: 1700183Crossref Scopus (163) Google Scholar While the limitations of all-atom MD simulations in terms of both system sizes and time scales prevent a full computational description of the global morphological characteristics of the blends, it is expected that the degree of PBDB-T/IT-OM mixing present in the simulated blends can correlate with the extent of phase separation measured experimentally. Thus, as a first step, we chose to analyze the impact of the methoxy position on the PBDB-T/IT-OM mixing in the PBDB-T/IT-OM-(1,2,4) blends. Importantly, the comparison with the experimental findings can in turn validate our MD simulation procedures. Figure 2 displays the radial distribution functions (RDFs), g(r), for PBDB-T backbones versus IT-OM backbones in the PBDB-T/IT-OM-(1,2,4) blends. (Refer to Wang et al.28Wang T. Chen X.-K. Ashokan A. Zheng Z. Ravva M.K. Brédas J.-L. Bulk Heterojunction solar cells: impact of minor structural modifications to the polymer backbone on the polymer-fullerene mixing and packing and on the fullerene-fullerene connecting network.Adv. Funky. Mater. 2018; 28: 1705868Crossref Scopus (31) Google Scholar for a more detailed description and assessment of these RDFs; here, we recall that a higher g(r) peak indicates a larger packing density at a given distance r.) It is seen from Figure 2 that in going from IT-OM-1 to IT-OM-2 and to IT-OM-4, the g(r) peak values (around ∼5 Å) follow the order IT-OM-1 ≈ IT-OM-2 > IT-OM-4, which points to a larger degree of PBDB-T/IT-OM mixing in the PBDB-T/IT-OM-(1,2) blends than in PBDB-T/IT-OM-4. To obtain a more quantitative description of the differences, we extracted all the PBDB-T/IT-OM pairs from the simulated blends, where a pair is defined as consisting of a PBDB-T backbone and an IT-OM backbone directly interacting within a distance of ∼5 Å of each other (the cutoff of intermolecular distance along the π-π interaction direction is based on the first-peak position present in the RDFs, see Figure 2). In the IT-OM-1, IT-OM-2, and IT-OM-4 cases, 181, 183, and 159 such pairs are produced out of a total of 239 IT-OM molecules; on average, one molecule is involved in 0.76, 0.77, and 0.67 pairs, respectively, which confirms the RDF findings. Considering that a larger degree of mixing corresponds to a smaller extent of phase separation, our findings are fully consistent with the experimental observations (since the extent of phase separation in the three blends is observed to follow the order IT-OM-1 ≈ IT-OM-2 < IT-OM-4). We note that the lesser degree of mixing in the PBDB-T/IT-OM-4 blend leading to large (∼200–300 nm) domain sizes15Li S. Ye L. Zhao W. Zhang S. Ade H. Hou J. Significant influence of the methoxyl substitution position on optoelectronic properties and molecular packing of small-molecule electron acceptors for photovoltaic cells.Adv. Energy Mater. 2017; 7: 1700183Crossref Scopus (163) Google Scholar can be detrimental to exciton diffusion to the PBDB-T/IT-OM interfaces; the consequence is a lower JSC value in the PBDB-T/IT-OM-4 OSC. From a thermodynamics standpoint, the degree of mixing between two components (here, polymer donor and SMA) depends on the Gibbs free energy change, ΔGmix, accompanying mixing at constant temperature and pressure:ΔGmix = ΔHmix ‒ TΔSmix,(Equation 1) where T denotes the temperature; ΔGmix, ΔHmix, and ΔSmix are the Gibbs free energy of mixing, enthalpy of mixing, and entropy of mixing, respectively; and ΔGmix > 0 means that mixing is unfavorable and phase separation occurs. The Flory-Huggins interaction parameter, χ, is known as a fundamental metric of the degree of mixing and is related to ΔGmix via the relation29Flory P.J. Principles of Polymer Chemistry. Cornell University, 1953Google ScholarΔGmixnRT=(1−ϕ)ln(1−ϕ)+ϕrlnϕ+(1−ϕ)ϕχ,(Equation 2) where R denotes the ideal gas constant; r, the number of repeat units of polymer donor; n, the total number of moles (with n = nacceptor + ndonorr); and ϕ, the volume fraction of polymer donor. Thus, ΔGmix increases (becomes less negative or more positive) as χ increases. When ΔGmix is positive, a larger χ value tends to bring a smaller degree of mixing (i.e., a larger extent of phase separation), provided the system can overcome kinetic barriers and ultimately achieve the equilibrium state. To better understand the impact of the methoxy position on the PBDB-T/IT-OM mixing, we examined the temperature-dependent χ values for the three blends (for details see Supplemental Information and Figure S5). As shown in Figure 3, χ values increase as T decreases; at the experimental annealing temperature (423.15 K or 150°C),15Li S. Ye L. Zhao W. Zhang S. Ade H. Hou J. Significant influence of the methoxyl substitution position on optoelectronic properties and molecular packing of small-molecule electron acceptors for photovoltaic cells.Adv. Energy Mater. 2017; 7: 1700183Crossref Scopus (163) Google Scholar the χ values are calculated to be 1.66, 3.54, and 5.23 for the PBDB-T/IT-OM-(2,1,4) blends, respectively. They are all larger than the threshold of ∼1.59 at which ΔGmix = 0; thus, phase separation occurs in all three blends. Another important finding from Figure 3 is that the χ value for the PBDB-T/IT-OM-4 blend is significantly larger than those for PBDB-T/IT-OM-(1,2), which supports the smaller degree of mixing and