Multi-Sulfur-Annulated Fused Perylene Diimides ... - ACS Publications

Article Cite This: ACS Appl. Energy Mater. 2019, 2, 3805−3814

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Multi-Sulfur-Annulated Fused Perylene Diimides for Organic Solar Cells with Low Open-Circuit Voltage Loss Xiangchun Li,†,‡ Hengbin Wang,† Hidenori Nakayama,†,§ Zitang Wei,∥ Julia A. Schneider,† Kyle Clark,∥ Wen-Yong Lai,*,‡ Wei Huang,‡ John G. Labram,† Javier Read de Alaniz,†,∥ Michael L. Chabinyc,*,†,⊥ Fred Wudl,*,†,∥,⊥ and Yonghao Zheng#

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†

Mitsubishi Center for Advanced Materials, ∥Department of Chemistry & Biochemistry, and ⊥Materials Department, University of California, Santa Barbara, California 93106-5050, United States ‡ Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing University of Posts & Telecommunications, Nanjing 210023, P. R. China § Electronics Materials and New Energy Laboratory, Mitsubishi Chemical Corporation, Yokohama R&D Center 1000, Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan # School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China S Supporting Information *

ABSTRACT: Nonfullerene acceptors are important for increasing the power conversion efficiency of organic bulk heterojunction (BHJ) solar cells. The number of wellperforming nonfullerene acceptors is still relatively limited compared to the wide range of donor materials making it difficult to find donor−acceptor pairs with matching optical and electronic properties. We report the synthesis of three sulfur-annulated fused perylene diimide-based compounds (2PDI-2S, 2PDI-3S, and 2PDI-4S) that have varying electron affinity. BHJ solar cells with these acceptors and the donor polymer PTB7-Th have solar power conversion efficiencies above 5% with relatively high fill factors (>60%) for nonfullerene acceptors. The origin of the performance of the BHJs was studied using a combination of physical and optoelectronic characterization methods. X-ray scattering revealed that the domains of 2PDI-nS acceptors are disordered in neat films and in BHJs. The carrier mobility was highest for 2PDI-4S leading to the highest fill factor for the series of acceptors. The open-circuit voltage loss was modeled using two approaches and was found to be low for the series relative to many BHJs. This work demonstrates the utility of fused-sulfur atoms to tune the electron affinity of this class of nonfullerene acceptors. KEYWORDS: organic solar cell, nonfullerene acceptor, organic electronics, charge-transfer state, X-ray scattering

1. INTRODUCTION Bulk heterojunction (BHJ) organic solar cells are processable using roll-to-roll methods at potentially low cost, making them an attractive solution for energy harvesting.1−4 BHJs are blends of electron-donating and electron-accepting materials that form a bicontinuous morphology with nanoscale phase separation. The nanoscale phase separation allows for efficient charge generation and extraction.5 The highest-performance BHJs currently comprise blends of polymeric donors with molecular acceptors; the power conversion efficiency (PCE) at 1 Sun illumination of small-area single-junction BHJ solar cells has increased to more than 10%,6−11 with reports of single junction BHJs at a certified PCE of 14.9%12 and tandem cells of BHJs exceeding 17%.13 The potential upper limits of the performance of BHJ solar cells have not been achieved experimentally and require improvements in the molecular design of both the © 2019 American Chemical Society

donor and acceptor along with improved control of selfassembly of the nanostructured BHJ.2,4,14 Here we report the synthesis and characterization of a series of nonfullerene acceptors (NFAs) based on a core with two fused perylene diimide units with sulfur atoms fused at the bay position (referred to by 2PDI-nS, where n equals the number of sulfur atoms). These materials have tunable electron affinities (EA) that are controlled by the number of sulfur atoms fused to the PDI units. These NFAs exhibit good fill factors (FF) and low loss to the open-circuit voltage (Voc) in BHJs solar cells with the donor polymer, poly([2,6′-4,8-di(5ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene]{3-fluoroReceived: March 6, 2019 Accepted: April 22, 2019 Published: May 6, 2019 3805

DOI: 10.1021/acsaem.9b00492 ACS Appl. Energy Mater. 2019, 2, 3805−3814

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ACS Applied Energy Materials

Figure 1. Chemical structures of several PDI-based nonfullerene acceptors and their reported PCE with varying donor polymers (SdiPBI-S,24 hPDI4,25 SF-PDI2,26 SdiPBI-Se,27 TPB,28and TPH-Se29).

Figure 2. Chemical structures of 2PDI, 2PDI-2S, 2PDI-3S, and 2PDI-4S.

acceptor, where generation and recombination occurs, plays a key role.14,19−21 The design of acceptors, electron-transporting materials, for BHJs has historically been difficult. The first efficient BHJs were formed with fullerenes due to their efficient and fast charge transfer (CT) with donor materials and glassy structure that allows nanoscale phase separation.1 However, synthetic methods to tune the electronic levels of fullerenes are limited, leading to the inability to control energetic losses. The development of NFAs, both small molecules and polymers, has led to new advances.2,3 NFAs have the advantages of increased optical absorption and ready chemical tuning of their electronic structure leading to PCE above 14% for BHJ solar cells.12 Rylene diimides are widely investigated as NFAs due to their good electron-accepting ability, high electron mobility, and substitutions that modify the frontier molecular orbital levels.2,22,23 Early studies of perylene diimide (PDI) acceptors

2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7-Th). These acceptors demonstrate that it is possible to tune the EA of the NFA without sacrificing low energetic losses. The power conversion efficiency of organic solar cells depends on their short-circuit current (Jsc), open-circuit voltage (Voc), and fill factor (FF).14,15 While design rules exist to maximize the Jsc of BHJs by tuning their optical absorption, it is more challenging to optimize the FF and Voc relative to their limits for a given BHJ. In the past decade, the FF of organic BHJ solar cells has reached over 75% for singlejunction cells.16,17 High carrier mobility of the donor and acceptor in blends is recognized as a critical factor in this gain,18 along with electrode layers that do not cause parasitic series resistance.14 The limit of the Voc of BHJs is still under investigation with clear evidence that the energetics of the interfacial charge-transfer state between the donor and 3806


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ACS Applied Energy Materials Scheme 1. Synthetic Route of 2PDI-2S, 2PDI-3S, and 2PDI-4S

revealed efficient charge transfer with donors but large-scale phase separation due to fast crystallization of the small molecule. Many of the most successful perylene diimide NFAs have a molecular design that inhibits crystallization by either linking multiple PDI groups to a central core or by fusing the PDI at the bay position (Figure 1). The introduction of heteroatoms to PDI core (Figure 1) allows for further tuning of the electronic levels by modifying the conjugated core.18 Herein, we report the synthesis and characterization of sulfur-annulated fused-PDI molecules 2PDI-2S, 2PDI-3S, and 2PDI-4S (Figure 2). 2PDI has previously been reported to be an efficient acceptor in BHJ solar cells.30 Sulfur atoms were substituted at the bay position of the double PDI core to modify the electron affinity (EA) by changing the contribution to the π-system through a single sulfur substitution on each side to addition of two sulfur atoms on each side. This modification does not substantially change the size or the gross geometry of the molecule, yet it allows the EA to be tuned. The series of compounds was studied as acceptors in BHJs with PBT7-Th as a donor polymer. These acceptors all have good FF with low losses to Voc relative to many BHJs.

tunity to study the effect of sulfur atoms in PDI-based acceptors.31−34 All the compounds remain readily soluble in common organic solvents at room temperature. The effect of multiple sulfur atom annulation is discernible in the UV−vis absorption spectra of 2PDI, 2PDI-2S, 2PDI-3S, and 2PDI-4S (Figure 3). Like 2PDI, all the compounds exhibit

2. RESULTS AND DISCUSSION 2.1. Synthesis and Electronic Properties of SulfurFused PDIs. The synthetic route for compounds 2PDI-2S, 2PDI-3S, and 2PDI-4S is shown in Scheme 1. Details of the synthetic methods are given in the Supporting Information (Scheme S1 and Figures S1−S6), and we summarize the results here. The parent 2PDI was easily nitrated to form dinitro-2PDI using fuming nitric acid in dry CH2Cl2 at room temperature. Reacting Dinitro-2PDI with sulfur powder in boiling N-methylpyrrolidone (NMP) gave a mixture of multisulfur fused PDIs, identified as 2PDI-2S, 2PDI-3S, and 2PDI-4S, in excellent yields. The ratio of these products was dependent on the reaction time, and we note that compounds 2PDI-3S and 2PDI-4S could be converted to 2PDI-2S with longer heating (>90 min). This formation of three possible derivatives of sulfur annulation on fused PDI in a one-pot reaction is unprecedented, representing an excellent oppor-

Figure 3. Normalized UV−vis absorption spectra of 2PDI, 2PDI-2S, 2PDI-3S, and 2PDI-4S measured in a dichloromethane solution.

strong absorption bands near 400 nm, indicative of their fused core. With sulfur annulation, however, comes a weakening in the typical split absorption bands of the perylene core, expected between 450 and 550 nm. The observed broadening of the optical onsets as a result of the introduction of increasing sulfur atoms signifies changes to the rigid perylene core and a resulting narrowing of the optical gap (Table 1). All of the 2PDI-nS compounds display multiple fully reversible reductions and EA values that vary with the number of annulated sulfur atoms (Table 1, Table S1, and Figure S7). The EA of 2PDI-2S extracted from the E1/2 in the cyclic voltammogram is 3.7 eV, slightly lower than the reported value for parent 2PDI. As we increase the number of sulfur atoms, however, the EA increases to 4.0 eV for 2PDI-4S. All the 3807


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HOMO−1 energy levels (∼0.4 eV). This effect is seen to a much smaller extent in the lowest unoccupied molecular orbital (LUMO) and LUMO+1 energy levels of the molecules. It is worth noting that, because 2PDI-4S has symmetrical delocalized HOMO and LUMO orbitals that cover both the PDI core and the sulfur atoms, it likely has the greatest HOMO/LUMO orbitals overlap. The optimized geometries of the molecules show a significant twisting in the central core of ∼20°, which likely prevents aggregation and contributes to the molecules’ solubility (Figure S8). 2.2. Sulfur-Fused PDIs as Nonfullerene Acceptors in BHJs. 2.2.1. Performance of BHJ Solar Cells with SulfurFused PDIs. To assess the performance of the 2PDI-nS series as NFAs, we made BHJ solar cells using a commonly used donor polymer with an appropriately matched EA and IE. We chose PTB7-Th as the donor polymer because of its optical gap of 1.58 eV (785 nm), IE of ∼5.24 eV, and EA smaller than ∼3.7 eV.35 This donor polymer has also been widely examined using fullerene-based acceptors and NFAs.36−40 Photovoltaic properties of the 2PDI-2S, 2PDI-3S, and 2PDI-4S as NFAs in BHJs were investigated with an inverted device configuration of indium tin oxide (ITO)/ZnO/PTB7-Th:2PDI-nS/MoO3/ Ag (Table 2). The performance of solar cells as a function of

Table 1. Optoelectronic Properties of 2PDI, 2PDI-2S, 2PDI-3S, and 2PDI-4S 2PDI 2PDI-2S 2PDI-3S 2PDI-4S

λonset (nm)

Eg,opt (eV)

EAa (eV)

estimated IEb (eV)

572 574 644 680

2.17 2.16 1.93 1.82

3.8 3.7 3.9 4.0

6.0 5.9 5.8 5.8

a

Obtained by cyclic voltammetry in dichloromethane from E1/2 vs Fc/Fc+ assuming a redox potential of 4.80 eV. bIE estimated from the EA determined by CV and the optical gap (Eg,opt) in solution with an uncertainty of the exciton binding energy.

compounds can easily accept multiple electrons, and the voltage offsets between the first and second reductions give a clear picture of the molecules’ symmetry. The symmetrical 2PDI-2S and 2PDI-4S both have closely spaced first and second reductions (0.16 and 0.17 V, respectively), while in 2PDI-3S a 0.22 V difference is observed between the first and second reductions. The addition of multiple sulfur atoms to the 2PDI core did not lead to significant changes in the estimated ionization energies (IE) of ∼5.8 eV, based on an estimate using the optical gaps. Hence, the 2PDI-nS series provides a means to rationally alter the EA to study their behavior as nonfullerene acceptors in BHJs. As shown by cyclic voltammetry, the electronic properties of 2PDI-2S, 2PDI-3S, and 2PDI-4S are significantly influenced by whether one or two sulfur atoms is annulated into the PDI core. Density functional theory (DFT) calculations at the B3LYP/6-31G* level of theory were performed on all of the molecules to visualize the effects of the sulfur atoms on the frontier energy levels (Figure 4, Figure S8, and Tables S2−S4).

Table 2. Summary of Device Parameters of PTB7-Th:PDInS Solar Cells with an Inverted Device Structure of ITO/ ZnO/PTB7-Th:2PDI-nS/MoO3/Ag 2PDI-2S 2PDI-3S 2PDI-4S

Voc (V)

Jsc (mA/cm2)

FF (%)

PCEa (%)

0.863 ± 0.005 0.698 ± 0.003 0.639 ± 0.003

12.9 ± 0.3 11.4 ± 0.1 12.2 ± 0.4

62 ± 1 68 ± 1 70 ± 1

6.9 ± 0.1 5.4 ± 0.1 5.5 ± 0.1

a

Average PCE values were calculated from six devices fabricated using the same conditions.

solvent and processing conditions was examined (see Supporting Information and Figure S10); the best results were found for a mixture of PTB7-Th and 2PDI-nS with a w/ w ratio of 1:2 dissolved in chlorobenzene (1% diiodooctane) with a total concentration of 20 mg mL−1 (Figure 5a). The current density−voltage (J−V) characteristics of the primary devices are shown in Figure 5b. The open-circuit voltage (VOC) of PTB7-Th:2PDI-2S was highest in the series at 0.86 V with decreasing values for increasing number of sulfur atoms in line with the EA of the compounds (Table 2). The shortcircuit currents (JSC) in the BHJs were all comparable with changes near ∼550 nm due to the blue-shifted absorbance of 2PDI-2S relative to 2PDI-3S and 2PDI-4S that separates its absorbance from PTB7-Th (Figure 5c). The external quantum efficiencies (EQEs) of all the three devices are over 50% for much of their absorbance range with integrated Jsc of 11.7, 10.6, and 11.1 mA/cm2 for 2PDI-2S, 2PDI-3S, and 2PDI-4S, respectively, for 1 sun illumination and are within ∼10% of the J−V characteristics under simulated solar illumination. The performance of the 2PDI-nS acceptors with PTB7-Th can be compared to other acceptors. Solar cells of BHJs of PTB7-Th:PC71BM made with the same batch of polymer had a Jsc ≈ 17 mA/cm2 and PCE of 9.2%; therefore, the 2PDI-nS underperform PC71BM with the current device structure and processing conditions (Table S5). All three BHJs have FF of more than 60%, with PTB7-Th:2PDI-4S having an FF of 70%; these values are, however, comparable to reported perylene diimide molecular acceptors.2 We also fabricated PTB7-

Figure 4. Frontier molecular orbitals and HOMO−LUMO gaps as calculated by DFT at the B3LYP/6-31G* level of theory.

Addition of one sulfur atom on each side of the compound modifies the aromatic ring, essentially donating electron density into the core with little highest occupied molecular orbital (HOMO) electron probability density located on the sulfur atoms themselves, as seen in 2PDI-2S. With two sulfur atoms in the ring, that aromaticity is lost, and the sulfur atoms are purely electron-rich substituents commanding significant electron density (see 2PDI-3S or 2PDI-4S). On the one hand, this imparts very symmetrical electron densities to 2PDI-2S and 2PDI-4S as exemplified by small difference (less than 0.2 eV) in the HOMO and HOMO−1 energy levels. 2PDI-3S, on the other hand, shows HOMO electron density predominantly on the side of the compound with two sulfur atoms and therefore has a larger energy difference between HOMO and 3808


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Figure 5. (a) Absorption spectra of PTB7-Th:2PDI-nS on a glass/ITO/ZnO stack. (black) 2PDI-2S, (red) 2PDI-3S,(blue) 2PDI-4S. (b) J−V curves for PTB7-Th: 2PDI-2S, 2PDI-3S, and 2PDI-4S solar cells. (c) EQE spectra of PTB7-Th: 2PDI-2S, 2PDI-3S, and 2PDI-4S devices.

Figure 6. 2D GIWAXS from neat films on ZnO of (a) 2PDI-2S, (b) 2PDI-3S, (c) 2PDI-4S, and BHJ blend films of PTB7-Th with (d) 2PDI-2S, (e) 2PDI-3S, (f) 2PDI-4S.

sulfur atoms is ∼15 Å, which is larger than the observed spacing, particularly with the addition of a van der Waals separation of molecules. The scattering from the BHJ films (cast using the same conditions as the solar cells) has features from 2PDI-nS and PTB7-Th as expected for a phase-separated blend (Figure 6d−f and Figure S12). PTB7-Th takes a face-on orientation based on the out-of-plane scattering near the qz axis at q ≈ 1.6 Å−1 assigned to a π−π staking. Similar to the neat films, there is no apparent preferential orientation of the acceptors in the BHJs. Overall the intensity of the GIWAXS is relatively weak, indicating that all of the blends are not highly crystalline. To reveal the domain size in the BHJs, we used resonant soft X-ray scattering (RSoXS), in a transmission geometry. To ensure the morphologies of the BHJ blend films are consistent among different characterization methods, the films for the RSoXS studies were first deposited on ZnO/ITO/glass and then transferred onto silicon nitride windows. Two-dimensional (2D) RSoXS patterns were measured by transmission through the films and reduced in one-dimensional (1D) I−q

Th:2PDI solar cells and found comparable performance to 2PDI-2S (PCE ≈ 7%) with slightly higher Jsc and comparable FFs (Table S6), but the smaller EA of 2PDI-2S increases the Voc relative to 2PDI. To better understand the origin of the performance, we performed a combination of physical characterization methods on thin films cast in the same way as the BHJ layer in the solar cells. 2.2.2. Morphology of 2PDI-nS BHJs. We examined the local order in neat films of 2PDI-nS and their BHJs with PBT7-Th using X-ray scattering. Grazing incidence wide-angle X-ray scattering (GIWAXS) of neat films of 2PDI-nS (Figure 6) revealed scattering features with no preferential orientation. The widths of the peaks are relatively broad (full width at halfmaximum (fwhm) ≈ 0.06 Å−1) indicating significant structural disorder and comparable to other fused perylene diimide acceptors (Figure S11).25,30 The three compounds all share a prominent scattering feature near q = 0.45 Å−1, a spacing of ∼14 Å that is comparable to ∼0.5 of the distance from the end of one alkyl chain to the other across the core of the molecule (∼28.2 Å by DFT). The other molecular dimension across the 3809


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thicknesses studied (∼50 to 60 nm) (Figure S14). The slope of the Voc with respect to light intensity yields an ideality factor (n) greater than 1 for the devices, suggesting that trap-assisted recombination occurs in the devices (Figure S14). To understand the role of the 2PDI-nS on the behavior of the BHJs, we examined their electron mobility in blends. We determined the electron mobilities with SCLC on electron-only devices with comparable electrode structures to the solar cells and with thin-film transistors (TFTs). Under an assumption of ε = 3.8, we obtained SCLC mobilities of 1.3 × 10−5, 1.7 × 10−5, and 2.4 × 10−5 cm2/(V s) for 2PDI-2S, 2PDI-3S, and 2PDI-4S, respectively (Figure S15). These values are comparable to reports for other fused perylene diimide acceptors.25,30 The J−V characteristics of 2PDI-3S and 2DPI-4S show a clear transition from the ohmic to SCLC regime. In contrast, the J−V characteristic for 2PDI-2S has a curvature that suggests the presence of a trap level. The EA of the 2PDI-2S is ∼0.2 eV higher than that of 2PDI-3S and 2PDI-4S and is close to the electron trap level observed in many materials.47 In contrast to the SCLC results, electron mobilities in thin-film transistors of pristine films of the 2PDInS series show significant differences. We found that the electron mobility in TFTs of 2PDI-4S was highest at 7 × 10−2 cm2/(V s), while the electron mobilities of 2PDI-2S and 2PDI-3S were ∼1 × 10−4 and 4.5 × 10−4 cm2/(V s), respectively (Figures S16 and S17). While the mobility in TFTs is measured at higher carrier density than in organic photovoltaics (OPVs), the results suggested that 2PDI-4S will have a higher carrier mobility than 2PDI-2S and 2PDI-3S in neat films. We also examined the photoconductivity and lifetime of carriers in BHJs of 2PDI-4S using time-resolved microwave conductivity (TRMC). TRMC reveals the photoconductivity of the blend that can be converted into a figure-of-merit: ϕΣμ, that is, the product of the charge-generation yield ϕ and the sum of electron and hole mobilities (∑μ = μe + μh).48 For organic semiconductors with relatively high exciton binding energies relative to kbT, ϕ is less than 1, indicating that Σμ = μe + μh is a lower bound to the sum of the carrier mobilities. TRMC reveals that (ϕ∑μ)max is 0.1 cm2/(V s) at incident fluence of ∼1 × 1014 photons/cm2 for PTB7-Th:2PDI-4S BHJs (Figure S18). In our system, we were unable to measure a signal for BHJs of 2PDI-2S or 2PDI-3S, suggesting lower values of (ϕ∑μ)max well-below 1 × 10−3 cm2/(V s). Such low values are consistent with the mobilities from SCLC measurements, but the high figure-of-merit for 2PDI-4S is not. Because TRMC is a high-frequency measurement, it is likely probing transport within domains of 2PDI-4S, whereas the SCLC measurement provides transport through multiple domains within the BHJ.49 The relatively higher mobility by TRMC is consistent with the observation from TFTs that 2PDI-4S has the highest mobility of the three compounds and has the highest fill factor in BHJs. 2.2.4. 2PDI-nS Acceptors Have Low Voc Loss. One of the most significant concerns for the performance of BHJ solar cells is the energy lost from the incident photons to the Voc.15,50,51 Several methods have been discussed to describe the energy loss in BHJ solar cells.50 The simplest metric of the energy loss is given by the difference between the linear onset of the optical gap and the Voc (eq 1).52 A widely used metric compares the Voc to the energies of CT states determined by sensitive EQE and electroluminescence measurements.19,50,53 The loss, qΔVoc, is the difference in the energy of the CT state,

plots by azimuthally averaging the intensity. We scanned the incident photon energy from 280.0 eV (well-away from the Xray absorption of the two materials) to 290.0 eV with a step of 0.1 eV and found that scattering was most prominent at 284.0 eV, similar to literature results for BHJs with PTB7-Th with other acceptors.38,39,41 The scattering peaks near q = 0.015 Å−1 (corresponding to ∼40 nm) with the BHJ of 2PDI-4S have a smaller q feature than the others (Figure 7). Atomic force

Figure 7. RSoXS of the BHJ blend films PTB7-Th:2PDI-nS. Lorentzcorrected scattering profiles (log−log scale) at incident energy of 284.0 eV. Black, 2PDI-2S; red, 2PDI-3S; blue 2PDI-4S.

microscopy (AFM) images show that the surfaces are all relatively smooth with height variation of ±5 nm with features of size 30−60 nm that agree with the length scale from RSoXS (Figure S13). Similar domain sizes have been observed for other twisted perylene diimide-based NFAs.41−43 The EQE near the absorption edge of PTB7-Th is comparable at ∼50% for the three compounds, indicating that the domain sizes in the three BHJs lead to similar charge generation and extraction near short-circuit conditions. The lower Jsc relative to other BHJs with PTB7-Th is likely due to the use of thin (∼50−60 nm) BHJs that limit the absorption of light. The integrated scattering intensity (ISI), the integral of Iq2, has been used as a metric of the purity of domains and thereby the FF of the BHJ.43,44 The ISI at the incident energy is actually lowest for the 2PDI-4S BHJ, but it has the highest FF. We also point out that the interpretation of ISI as a metric of miscibility relies on the assumption of isotropic three-dimensional (3D) morphology,45 which is unlikely to be true in many cases. 2.2.3. Charge Transport. The fill factors of the BHJs of 2PDI-nS are relatively high for perylene diimide acceptors.2 Many nonfullerene acceptors, including those that form highly efficient cells, have low mobility, for example, values of ∼1 × 10−4 cm2/(V s) have been reported for 2,2′-[[6,6,12,12tetrakis(4-hexylphenyl)-6,12-dihydrodithieno[2,3-d:2′,3′-d′]-sindaceno[1,2-b:5,6-b′]dithiophene-2,8-diyl]bis[methylidyne(3oxo-1H-indene-2,1(3H)-diylidene)]]bis[propanedinitrile] (ITIC) based on space charge limited current (SCLC) measurements.46 The dependence of Jsc on light intensity of the three BHJs reveal that charge extraction is efficient at the 3810


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ACS Applied Energy Materials ECT, observed by fitting the tail of the EQE measurement to a Marcus-type model, and the Voc (eq 2).19,20,50 With PC61BM as an acceptor in BHJs, many donors lead to qΔVoc of 0.6 eV.15,20 An alternative method defines a loss using the Shockely-Quiesser (S-Q) limit that is commonly used for inorganic solar cells.26,54 This method partitions the energy loss into factors relative to the optical gap and nonradiative losses and uses a fit to the tail of the EQE of the BHJ to extract the radiative and nonradiative components.26,54 The most critical of these is the nonradiative loss qΔVnr, which is the difference in the voltage expected from integration of the absorption of the BHJ Voc,rad and the observed Voc (eq 3).20,26,54 An increase in the width of the band tail leads to larger losses in both models. E loss = Eoptical − qVoc

(1)

qΔVoc = ECT − qVoc

(2)

qΔVnr = qVoc,rad − qVoc

(3)

using the S-Q method that separates the loss into radiative and nonradiative components (eq 3).26,54 The observed values of qΔVnr are ∼0.3 eV for all three compounds (Table 3), which is relatively small for polymer BHJs.26,54 Table 3. Open-Circuit Voltage Loss Characteristics of PTB7-Th:PDI-nS Acceptor Solar Cells 2PDI-2S 2PDI-3S 2PDI-4S

qVoc (eV)

Eloss (eV)

ECT (eV)

qΔVoc (eV)

qΔVnr (eV)

0.86 0.70 0.64

0.71 0.87 0.93

1.37 1.22 1.15

0.53 0.52 0.51

0.31 0.32 0.33

We also tested the Voc loss using a different donor, a copolymer of cyclopentadithiophene, pyridyl[2,1,3]thiadiazole, and indacenodithiophene (PIPCP), which is known to have a low Voc loss with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) of ∼0.5 V.56 BHJs formed with PIPCP as a donor and with 2PDI-3S as an acceptor (due to its well-matched EA) had relatively low performance (PCE ≈ 0.8%) with peak EQE of ∼0.1 (Figures S19−S21 and Table S7). Extraction of the energetic losses of these solar cells gives Eloss = 0.4 eV and qΔVnr = 0.31 eV, comparable losses to the metrics for BHJs with PTB7-Th as the donor. These results suggest that the low loss observed with PTB7-Th are not unique and are related to the properties of the acceptor. The 2PDI-nS acceptors have varying EAs that can be matched to the energy levels of donor polymers and result in BHJs with low Voc loss. A surprising feature of these results is that all three acceptors in the 2PDI-nS series have losses with PTB7-Th that are below a recent report of an apparent lower limit of ECT and ΔVnr, as a function of ECT.20 This limit was determined based on a series of dilute donor BHJs with a C60 acceptor. The mechanism was attributed to intramolecular vibrations of the donors, particularly C−H modes. Here the donor has such modes in the backbone, whereas less of a contribution is expected from the acceptor due to the large conjugated structure. Whether the origin of the low loss is due to reduced interfacial area relative to other BHJs is difficult to determine quantitatively; the domain sizes from RSoXS (∼40 nm) are comparable to other BHJs.57 Further studies such as temperature-dependent behavior and electroluminescence are needed to reveal the origin of the loss in these systems.3,51,55,58 The ability to examine the origin of losses in BHJ solar cells should be greatly improved by systematic studies to disentangle whether molecular design or disorder in BHJ blends dominates the energetic loss.

We find that the 2PDI-nS acceptors all exhibit low Voc losses with respect to the interface states.19,55 The optical gap of the BHJ is set by the absorption of PTB7-Th, which we set at 1.58 eV. The Eloss varies with the acceptors, as expected due to the shift in EA, but it is still relatively low for 2PDI-2S (0.7 eV).52 The BHJs all exhibit a tail of states due to the CT state between PTB7-Th and 2PDI-nS that provides another metric for the energy loss. The ECT for the PTB7-Th:2PDI-nS BHJs varies systematically with the EA of the acceptor (from 1.15 to 1.37 eV), as observed by the shift of a low-energy feature in EQE measurements (Figure 8). These shifts are consistent

3. CONCLUSION In summary, we have designed and synthesized three new sulfur-fused fused PDI compounds, namely, 2PDI-2S, 2PDI3S, and 2PDI-4S, as nonfullerene acceptors for organic solar cells. BHJs with PTB7-Th as a donor with these acceptors exhibit good performance, with PCEs up to 6.9% and FFs up to 70%. These results demonstrate the utility of the twisted core design that inhibits formation of large ordered domains that limits charge generation in planar PDI NFAs. The low Jsc relative to other nonplanar PDIs can be attributed to the use of thin layers that do not absorb all of the incident light; alternative processing methods for thicker cells that maintain the phase-separated domain sizes of ∼40 nm could lead to improvements in performance. The energetic losses to the open-circuit voltage in these BHJs were found to be relatively

Figure 8. EQE (solid lines) of the BHJs with 2PDI-2S (gold), 2PDI3S (green), and 2PDI-4S (blue) showing fits to Marcus-type model for the interfacial CT state (dashed lines).

with the changes in EA for the acceptors with a fixed donor, PBT7-Th. With these values of ECT, the loss, qΔVoc defined by eq 2, is ∼0.52 V for all of the acceptors in the 2PDI-nS series, which is lower than the typical value of 0.6 eV observed in a variety of BHJs.14,19,20 More accurate values for this can be obtained with comparison of electroluminescence,19 but we do not anticipate a substantial shift because of the clear feature in the EQE spectrum and the expected systematic shift between acceptors. As an alternative, we also examined the Voc loss 3811


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small across the whole series of acceptors. These results demonstrate that the EA of an NFA can be rationally tuned while maintaining low energetic losses. NFAs with tunable energetics should help to elucidate the characteristics of the CT state between donors and acceptors in BHJs that can improve their PCE.19,20,51

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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/acsaem.9b00492.

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Synthetic details and NMR spectra; density functional theory calculations; cyclic voltammetry; solar cell characteristics; X-ray scattering; microwave conductivity data (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (W.-Y.L.) *E-mail: [email protected]. (F.W.) *E-mail: [email protected]. (M.L.C.) ORCID

Wen-Yong Lai: 0000-0003-2381-1570 John G. Labram: 0000-0001-6562-9895 Javier Read de Alaniz: 0000-0003-2770-9477 Michael L. Chabinyc: 0000-0003-4641-3508 Yonghao Zheng: 0000-0001-5102-5214 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS X.L., W.-Y.L., and W.H. acknowledge financial support by the National Key Basic Research Program of China (973 Program, 2014CB648300), the National Natural Science Foundation of China (21422402, 21674050, 61136003), and China Scholarship Council (201508320254). J.G.L. gratefully acknowledges Virgil Elings and Betty Elings Wells for financial support through the Elings Fellowship Awards. Access to Advanced Light Source at Lawrence Berkeley Laboratory was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DESC-0012541. The research reported here also made use of the shared facilities of the UCSB MRSEC (NSF DMR 1720256), a member of the Materials Research Facilities Network (www.mrfn.org).

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