Non-Fullerene Organic Solar Cells with 6.1% Efficiency through Fine

Dec 10, 2014 - solvent for solvent vapor annealing (SVA), the volume ratio of the additive versus the host solvent for SVA, and the time for SVA. Thro...
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Non-Fullerene Organic Solar Cells with 6.1% Efficiency through FineTuning Parameters of the Film-Forming Process Xin Zhang, Chuanlang Zhan,* and Jiannian Yao* Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China S Supporting Information *

ABSTRACT: One of the key issues limiting the efficiency of non-fullerene organic solar cells (NF-SCs) is the low electron mobility and strong recombination loss. In this paper, we report an approach of fine-tuning the parameters relative to the film-forming kinetics to increase the power conversion efficiency, which significantly improved from 1.4 up to 6.1%. The film-forming process was judiciously optimized by carefully manipulating the following four parameters: the additive content during film processing, the volume of the host solvent for solvent vapor annealing (SVA), the volume ratio of the additive versus the host solvent for SVA, and the time for SVA. Through such controls, the photocurrent dramatically increased from 5.40 to 12.83 mA/cm2 and the fill factor from 32.61 to 56.43% as a result of the reduction of the monomolecular and bimolecular loss and the improvement of the electron mobility. These improvements in the electric properties are associated with the reconstruction of the film morphology, i.e., solvent annealing of the ascast active film leads to the improvement of the phase segregation and the consequent enhancement of the self-aggregation of the blend donor and acceptor molecules in the solar cell active film.

1. INTRODUCTION A non-fullerene solar cell (NF-SC) employs an N-type organic molecule (small or polymeric) as the non-fullerene acceptor (A), which replaces the fullerene one in organic solar cells and blends with the polymer or small-molecule donor (D) as the photoactive layer to harvest the solar energy.1−3 With respect to the fullerene, organic acceptors are richer in source and more easily amenable in both the optical spectrum and frontier molecular orbitals. To date, several types of small-molecule acceptors have been reported from different organic semiconductor moieties, such as those based on vinazene,4,5 dicyansubstituted quinacridone,6 fluoranthene-fused imide,7,8 9,9′bifluorenylidene,9,10 electron-deficient pentacene,11 diketopyrrolopyrrole (DPP),12,13 naphthalene diimide (NDI),14−16 and perylene diimide (PDI).17−26 Some of these moieties, such as PDI,27−29 NDI,30,31 DPP,32 and others,33 have been selected as the electron-accepting units to synthesize polymer acceptors, accompanied with appropriate electron-donor units. Recently, the power conversion efficiency (PCE) of the state-of-the-art solution-processed NF-SCs has been improved to 4−5% by using small-molecule34−36 or polymer37−40 blend acceptors. Very recently, a PCE of 5.9% was realized from a smallmolecule acceptor by using an inverted cell structure with a fullerene self-assembled monolayer (C60-SAM) on the ZnO electron-selective layer.41 During the submission and revision of this paper, PCEs of 6.1%42 and 6.3%43 using two different small-molecule acceptors have been reported. Another PCE value of 6.4% was released by Polyera Company with a polymer acceptor, so-called PNDI2OD-T2.3 A PCE of 8.4% from a © XXXX American Chemical Society

vacuum-deposition planar-heterojunction cell has been recently reported.44 In this paper, we present a unique polymer:small molecule (D:A) system of PBDTTT-C-T45 and bis-PDI-T-EG22 (Figure 1a) and report a high-performance solution-processed NF-SC with an average PCE of 6.0% (highest 6.08%), which was achieved by judiciously controlling the parameters relative to the film-forming kinetics. This efficiency was achieved from a conventional single-junction solar cell with a structure of ITO/ PEDOT:PSS/donor:acceptor/Ca/Al (Figure 1b), where PEDOT:PSS is poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate). One striking difference between a fullerene and a smallmolecule acceptor involves their molecular shapes: The spherical shape of the fullerene greatly enhances phase segregation between the blend donor and fullerene acceptor, forming charge-separation- and transportation-favorable nanoscale interpenetrating networks, which can greatly improve the electric properties of the cell device. In contrast, the performance of a small-molecule-acceptor-based NF-SC is largely restricted by the lower electron mobility and recombination losses.25,34−36,41,43 The twisted conformation of a PDI dimer such as bis-PDI-TEG is a hindrance to phase segregation of the blend donor and acceptor37 and also to aggregation of the dimeric acceptor Received: October 8, 2014 Revised: December 8, 2014

A

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of DIO versus o-DCB that is preadded into the lid-covered Petri dish with a constant total volume of 7.5 μL. These four parameters were optimized by following the normal SVA process in two steps: Step I was the formation of the as-cast active film via spin-coating. The DIO content, [DIO], was varied from 0 to 5% (v/v). The as-cast active film should contain residue solvents, i.e., high-boiling point o-DCB (Tb = 180.4 °C) and DIO (Tb = 332.5 °C). Step II was the vapor annealing of the as-cast active film in an enclosed space by using a selected solvent. In this step, the as-cast active film was put into the lid-covered Petri dish as quickly as possible (within 1 s) after the spin-coating was finished. The annealing solvent with a selected volume was preadded 5−10 min prior to the ascast active film. Because the as-cast active film should involve both o-DCB and DIO, vapor annealing of the host only and that of the mixture of the host and additive solvent were considered separately in step II. First, the vapor annealing from the host solvent only was performed. Vo‑DCB was carefully controlled from 0 to 20 mL by the volume of o-DCB preadded. [DIO] was varied from 0 to 5%. The time for SVA was practically maintained for 10 h with the Petri dish fully covered. Table S1 in the Supporting Information lists the corresponding photovoltaic data. One can see that the best cell appears in such a trend: a lower [DIO] needs a higher Vo‑DCB to give the best performance. For example, as [DIO] increased from 1 to 1.5 and 2%, the Vo‑DCB generating the best performance decreased from 15 to 7.5 and 5 μL. The best PCEs were 5.06, 5.69, and 5.41%, respectively. At a higher [DIO] level, for example, 5%, a PCE of 4.94% was obtained when 5 μL of the host solvent was used for SVA. When the dish was only half-covered and the annealing time was 12 h, the best PCE was 4.07%, which is the exact value we previously obtained under the same conditions.34 After optimization of [DIO] and Vo‑DCB, the time for SVA was checked using the optimum values of [DIO] and Vo‑DCB (1.5% and 7.5 μL, respectively), since this combination of the parameters yielded the highest PCE of 5.69%. As shown in Table S2 in the Supporting Information, the best performance occurred at a moderate time period of t = 8 h. Shorter and longer time periods both led to a decrease of the PCE, in particular a rapid deterioration when the annealing time was shortened. After the optimum t was realized, [DIO]′ was then optimized while [DIO] was kept at 1.5% and t was kept constant at 8 h. In the case of the optimization of [DIO]′, the volume ratio of the additive versus the host solvent was cautiously controlled by keeping the total volume of o-DCB and DIO as a constant (i.e., Vo‑DCB+DIO = 7.5 μL) while their volume ratio [DIO]′ was varied from 0% to 30%. Table S3 in the Supporting Information collects the relative photovoltaic data obtained, from which one can see that the best performance was obtained with [DIO]′ = 15%. Figure 2a gives the current density−voltage (J−V) curves of the best cells obtained under the four optimal conditions A−D, which are defined in the caption of Figure 2. Condition A was performed as a control to see how having the dish open or covered affected the performance. Table 1 lists the relative photovoltaic data. From condition A to B, C, and D, the shortcircuit current density (Jsc) increased from 5.40 to 10.35, 12.09, and 12.83 mA/cm2, respectively, the fill factor (FF) changed from 32.61 to 45.04, 57.64, and 56.43%, respectively, while the open-circuit voltage (Voc) remained nearly constant at 0.82−

Figure 1. (a) Molecular structures of the donor and the acceptor. (b) Structure of the solar cell employed in this work. (c) Normalized UV− vis absorption spectra of the pure donor and acceptor film. (d) Diagram of the electron transfer and hole transfer, respectively, for the usage of the solar energy harvested by the donor and acceptor phases.

molecules.34 Finding ways to improve the phase segregation and to optimize the aggregation of the dimeric acceptor is urgent for the realization of efficient NF-SCs. Solvent-vapor annealing (SVA) is a method to put the as-cast active film in the vapor of a solvent to reconstruct the film morphology. It has been found to have positive effects to improve the cell performance of fullerene-based polymer or small-molecule solar cells.46−48 We present herein the significant enhancement of the electric performance of the NF-SC via the vapor annealing of the host and the mixed host/ additive solvent, respectively. Specifically, we perform the fine control of the parameters relative to the film-forming kinetics and systemically show its effect on the cell performance.

2. RESULTS AND DISCUSSION As displayed in Figure 1c, the absorption spectrum of the acceptor is complementary to that of the donor, yielding a wide spectral coverage extending from 300 to 800 nm, beneficial for the usage of the visible and near-infrared photons of the solar emission. The energies of the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular (HOMO) of the donor are −3.25 and −5.11 eV, and those of the acceptor are −3.84 and −5.65 eV, respectively. This D:A pair have balanced energy offsets, i.e., ΔELUMO ≈ ΔEHOMO (0.59 vs 0.54 eV) (Figure 1d). The film-forming kinetics is optimized by carefully controlling the following four parameters: (1) [DIO] (% v/ v), the additive content of 1,8-diiodooctane (DIO) over the host solvent, 1,2-dichlorobenzene (o-DCB); (2) Vo‑DCB, the volume of the host solvent o-DCB that is preadded into a selected lid-covered Petri dish with an inner volume of 42.4 cm3; (3) t, the time for SVA; and (4) [DIO]′, the volume ratio B

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harvesting the excitons generated by the donor and the nonfullerene acceptor phases, respectively. The efficient charge separation is evidenced by the fluorescence spectrum, in which the photoluminescence from the acceptor and that from the donor are both quenched nearly completely in the solar cell blends prepared under each condition (Figure 3).

Figure 2. (a) J−V curves and (b) EQE spectra of the best cells obtained under four optimal conditions: (A) [DIO] = 1.5%, Vo‑DCB = 0 μL, and t = 8 h, with the lid fully open; (B) [DIO] = 1.5%, Vo‑DCB = 0 μL, and t = 8 h; (C) [DIO] = 1.5%, Vo‑DCB = 7.5 μL, and t = 8 h; and (D) [DIO] = 1.5%, Vo‑DCB+DIO = 7.5 μL, t = 8 h, and [DIO]′ = 15%. Under conditions B−D, the dish was fully covered.

0.86 V. The PCE rose from 1.44 to 4.01, 5.85, and 6.08%, respectively. Apparently, the change from condition A to B led to dramatic increases in Jsc by 1.92-fold and FF by 1.38-fold. Vapor annealing with the host solvent improved FF and Jsc, which increased by 28% and 17%, respectively, while SVA with the mixed solvent involving the additive slightly improved Jsc by 6.1% but had little effect on FF. As we mentioned above, the ascast active film contains both the host and additive residue, while the evaporation speed of the residue solvent in the as-cast active film is related to the density (or vapor pressure) of this solvent in the gaseous phase of an enclosed space. The evaporation rate of the residue host and additive solvent in the as-cast active film can be thus lowered by putting the as-cast active film either in an enclosed space without any solvent preadded inside or, even more, in an enclosed space with the gaseous vapor of the host solvent (or a mixture with the additive solvent) compared with putting it in an open area. The comparisons of the cell parameters obtained under the four processing conditions clearly indicate that lowering the evaporation rate of the residue host solvent is much more important for improvement of the cell performance. Figure 2b displays the external quantum efficiency (EQE) spectra of the four best cells. The current densities integrated from the EQE spectra are comparable to those measured from the corresponding J−V curves within 10% error. The EQE response from each cell covers a wide wavelength range from 300 to 800 nm. The responses around 550 nm are mainly contributed by the blend acceptor and those around 710 nm are principally from the donor. The response around 550 nm is comparable to or even higher than that around 710 nm under each condition. The EQE data indicate that both the donor and the acceptor efficiently contribute to the photon-to-electron conversion. The balanced energy offsets of ΔELUMO ≈ ΔEHOMO (Figure 1d) offer efficient electron and hole transfer for

Figure 3. Fluorescence spectra of the pure acceptor (a) and donor (b) films and the solar cell blends prepared under conditions A−D. The excitation wavelengths were 540 nm for the acceptor and 707 nm for the donor.

The scale of Jsc relates to the recombination loss. The recombination mechanism was studied by measuring lightpower-dependent J−V curves with the incident light intensity changing from 20 to 100 mW/cm2. At short circuit, the recombination loss is described using a power-law dependence of Jsc on the light intensity, Jsc ∝ Pα (Figure 4a).49 The α values

Figure 4. Light-intensity dependence of (a) Jsc and (b) Voc obtained from the optimal cells under conditions A, B, C, and D.

from the fits were 0.924, 0.935, 0.950, and 0.960 under conditions A, B, C, and D, respectively. All of these values are close to unity, indicating that the monomolecular contribution dominates the recombination loss, and the increase in α reveals that the bimolecular contribution decreases upon selective annealing. At open circuit, the recombination mechanism is reflected by formula Voc ∝ nkBT/(q ln P) (Figure 4b), where kB, T, and q are the Boltzmann constant, absolute temperature, and elementary charge, respectively.14 The values of n from the fits

Table 1. Photovoltaic Parameters of the Optimal Cells under AM 1.5G Illumination of 100 mW/cm2 cond.a A B C D

Voc (V)b 0.82 0.86 0.84 0.84

(0.82 (0.86 (0.84 (0.84

± ± ± ±

0.01) 0.01) 0.01) 0.01)

Jsc (mA/cm2)b 5.40 10.35 12.09 12.83

(5.11 ± 0.25) (10.19 ± 0.20) (12.14 ± 0.13) (12.54 ± 0.17)

FF (%)b 32.61 45.04 57.64 56.43

(31.42 (44.25 (56.88 (56.67

± ± ± ±

PCE (%)b 1.4) 0.7) 1.0) 2.1)

1.44 4.01 5.85 6.08

(1.28 (3.95 (5.80 (6.00

± ± ± ±

0.11) 0.05) 0.04) 0.06)

Rsc [Ω cm2]

Rpc [kΩ cm2]

μhd

μed

81.2 24.6 11.1 11.8

0.09 0.30 0.56 0.52

4.31 7.36 9.70 10.30

0.49 1.21 5.07 6.06

a

Conditions are defined in the caption of Figure 2. bThe best values are given, followed by the averages and variances in parentheses, calculated from 10 devices. cSeries (Rs) and parallel (Rp) resistances estimated from the best cells. dHole (μh) and electron (μe) mobilities in units of 10−3 cm2 V−1 s−1. C

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for the top and bottom surfaces. RD/A was always larger than unity in the top surface and decreased from 7.69 to 5.00, 4.35, and 4.00 as the condition varied from A to B, C, and D, respectively. However, following this order of the conditions, the value of RD/A in the bottom surface was generally lower than unity and increased from 0.31 to 0.37, 0.39, and 0.39. An RD/A value larger than unity indicates the formation of a donorrich surface, while a value lower than unity suggests the formation of an acceptor-rich surface. The formation of the donor-rich top surface and the acceptor-rich bottom surface is consistent with our previous observations from other PDIdimer-based NF-SCs.35 The increase in the acceptor weight percentage in the donor-rich top surface is beneficial for the extraction of electrons from the top contact, and the increase in the donor abundance in the acceptor-rich bottom surface is helpful for extraction of holes from the bottom contact of the conventional cell. Accordingly, the favorable top and bottom contacts are responsible for the enhancement in Jsc after treatment of the active layer with SVA. Figure 7 presents typical transmission electron microscopy (TEM) images of the 1:1 (w/w) solar cell blend films fabricated under the optimal conditions A−D. Under condition A, the phase segregation is negligible, and it becomes visible upon annealing with the lid fully covered (condition B). SVA using 7.5 μL of o-DCB for 8 h (condition C) yields nanoscale phase segregation, and both white fibers and dark domains with sizes of ca. 20 nm can be seen clearly. Annealing under condition D gives larger dark domains with a bigger size of ca. 40 nm, and the white fibers form clusters in which the fibril size is ca. 30 nm. Apparently, slowing the evaporation of the residue solvent by putting the as-cast solar cell blend into an enclosed space (without any solvent preadded inside) and further vapor annealing using the host and further the mixture of the host and additive solvent produce a more and more contrasted white/ black image, meaning that solvent annealing is helpful for the phase segregation and aggregation of the blend polymer and dimer. This is consistent with the increases in the hole and electron mobilities, the reduction of the recombination loss, and the increase in Jsc and FF in going from condition A to conditions B, C, and then D. In the two-dimensional (2D) grazing-incidence X-ray diffraction (GIXRD) images, the pure polymer film gives a weak greenish-yellow ring of intensity (Figure 8a), while the pristine dimer film shows an intense yellow ring of intensity (Figure 8b). Both the (100) diffractions from the pure polymer and dimer are localized around q = 0.3 Å−1 because the polymer size along the side-chain direction (2.0 nm) is close to the size of the dimer (2.0 nm).34 As displayed in Figure 8c−f, each solar cell blend exhibits an intense yellow arch of intensity in the qz direction, and the intensity of the (100) signal becomes stronger and stronger as the condition goes from A to B, C, and then D, again suggesting that treatment of the solar cell blend with SVA helps to improve the phase segregation and consequent self-aggregation of the blend acceptor (donor), which is in line with the observations from the TEM images. Since the amorphous polymer gives a much weaker intensity of the diffraction than the dimer, the dimer likely adopts a more ordered packing than the polymer. The black domains are thus assumed to be mainly formed from the dimer aggregates because the ordered dimer aggregates show relatively higher electron scattering density than the amorphous donor aggregates.35,50,51

were 1.43, 1.25, 1.15, and 1.02, respectively. Voc exhibits much less light-power dependence in the cell treated upon annealing, indicating a reduction in the monomolecular contribution, which evolves in the bimolecular recombination loss at open circuit. Taken together, the results show that upon annealing from conditions A to D, reduction of both the monomolecular and bimolecular losses results in the photocurrent enhancement. To see the carrier transport properties, both the electron and hole mobilities were measured with the space-charge-limited current (SCLC) method. The measurements were conducted on device structures of ITO/TIPD/blend/Al (electron-only) and ITO/PEDOT:PSS/blend/Au (hole-only). Figure 5 shows

Figure 5. Plots of ln(JL3/V2) vs (V/L)0.5 obtained from the (a) electron-only and (b) hole-only devices for estimation of the electron and hole mobilities of the solar cell blends prepared under conditions A, B, C and D.

the plots of ln(JL3/V2) versus (V/L)0.5 extracted from the experimental data, and Table 1 gives the estimated electron and hole mobilities. The hole mobility of the solar cell blend was always higher than the electron mobility under the same condition, indicating that the electron mobility is a factor that limits the photocurrent. Following the same trend of the increase in Jsc, the electron mobility increased from 0.49 to 1.21, 5.07, and 6.06 × 10−3 cm2 V−1 s−1 as the condition varied from A to B, C, and D, respectively, and also the hole mobility increased in the same order as the electron mobility. Upon SVA (conditions C and D), the electron and hole mobilities became relatively balanced, accounting for the increase in FF. The D/A weight ratios, RD/A, in the top and bottom surfaces of the solar cell blends were measured using X-ray photoelectron spectroscopy (XPS) by following our previously reported procedure.35 Figure 6 displays the estimated values

Figure 6. D/A compositions in the top and bottom surfaces of the solar cell blends prepared under conditions A, B, C and D. The D/A weight ratios (RD/A) were estimated by following the methods given in ref 35. D

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Figure 7. TEM images of the solar cell blends fabricated under the optimal conditions (a) A, (b) B, (c) C, and (d) D.

Figure 9. 1D GIXRD data for the solar cell blends fabricated under conditions A, B, C, and D (a) in the qz (out-of-plane) direction and (b) in the qxy (in-plane) direction.

Figure S1 in the Supporting Information and Figure 10 show the absorption spectra of the pure donor and pure acceptor

Figure 8. 2D GIXRD images of the (a) pure donor and (b) pure acceptor films and the solar cell blends fabricated under conditions (c) A, (d) B, (e) C, and (f) D.

Figure 10. Normalized UV−vis absorption spectra of the solar cell blends prepared under conditions A, B, C and D. Each spectrum was measured using the ITO/PEDOT:PSS substrate as the reference for subtraction.

To gain further insight into the change in the film structures upon annealing, one-dimensional (1D) GIXRD experiments were performed on the four blends. As shown in Figure 9, the (100) diffraction peak in the out-of-plane direction becomes clearer and clearer. Simultaneously, the (100) peak in the inplane direction moves to a smaller 2θ value, with the d100 value shifting from 2.2 to 2.4, 2.6, and 2.7 nm, and the (200) peak becomes detectable and also moves to a smaller 2θ value. In all, the 1D GIXRD data indicate the rearrangement of the packing of the donor and acceptor molecules as the active film was treated upon SVA, going from condition A to conditions B, C, and then D. This is in line with the observations from the TEM and 2D GIXRD data.

films and the solar cell blends prepared under the four conditions. For both the pure donor and pure acceptor films, the absorption spectra are identical to each other obtained under each of the four conditions (Figure S1). Also, the fluorescence spectra from the four conditions almost sit on each other (Figure 3). However, slight differences are seen from the blends (Figure 10), again suggesting that changes related to the phase segregation and consequent self-aggregation of the donor and acceptor molecules are associated with the solvent annealing treatments. The variations in the blend absorption spectra involve the following: (1) The absorption in the wavelength region of 470−700 nm from condition B becomes stronger than that from condition A. As we showed previously, E

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lammonium hexafluorophosphate (Bu4NPF6) in dichloromethane (DCM) at a scan speed of 0.1 V/s. A Pt wire and Ag/AgCl were used as the counter and reference electrodes, respectively. The concentration of the dimer was 1 × 10−4 mol/L in chromatographicaly pure DCM. Absorption spectra were measured on a Hitachi U-3010 UV−vis spectrophotometer. Fluorescence spectra were measured using a Hitachi F-4500 spectrometer. All of the spectroscopic measurements were carried out at room temperature. The film spectra were obtained in transmission mode. The thickness of the solid films was measured using a Dektak Profilometer. Sample Preparation and Characterization. Film samples for measurements of absorption and fluorescence spectra, XPS, and TEM were all prepared atop precoated ITO/PEDOT:PSS substrates via the spin-coating method. The pure donor film was spin-cast from a 20 mg/mL o-DCB solution and the pure acceptor film was spin-coated from a 10 mg/mL chloroform solution. All of the blend films were prepared by spin-casting from a 40 mg/mL o-DCB solution in which the dimer and the polymer were mixed in a 1:1 weight ratio with a DIO content of 1.5%. For XPS, the data for the top surface were directly collected from the top of the blend film, whereas those in the buried surface were obtained by first floating the blend film on water and then transferring it onto a silicon substrate with the buried surface upward. For TEM, the films were obtained by transferring the floated blends from the water onto the 200 mesh Cu grid. For the measurements of the absorption and fluorescence spectra, the films atop the ITO/PEDOT:PSS substrates were measured directly. For GIXRD, the blend films were prepared on silicon/PEDOT:PSS substrates and then used directly. For each of characterizations, four blends were used and prepared under the optimal conditions A, B, C, and D. Fabrication and Characterization of Organic Solar Cells. Solar cell devices with a typical configuration of ITO/PEDOT:PSS/ active layer/Ca/Al were fabricated as follows: The ITO glass was precleaned with deionized water, CMOS-grade acetone, and isopropanol in turn for 15 min. The organic residues were further removed by treatment with UV/ozone for 1 h. Then the ITO glass was modified by spin-coating the PEDOT:PSS layer (30 nm) onto it. After this was dried in oven at 150 °C for 15 min, the active layer was spincoated onto the ITO/PEDOT:PSS surface with a blend solution of bis-PDI-T-EG and PBDTTT-C-T (40 mg/mL in o-DCB). The DIO content, the volume of o-DCB, the volume ratio of DIO versus o-DCB in the lid-covered Petri dish, and the time for SVA were finecontrolled. The Ca (20 nm) and Al (80 nm) electrodes were then thermally evaporated onto the active layer under a vacuum of 1 × 10−6 Torr. The active area of the devices was 0.06 cm2, and the thicknesses of the active films were ∼100 nm. The devices were characterized in a nitrogen atmosphere under AM 1.5 G simulated solar illumination at 100 mW/cm2 using a xenon-lamp-based solar simulator (AAA grade, XES-70S1). The J−V measurements on the devices were conducted on a computer-controlled Keithley 2400 source measure unit. The EQE measurements were performed in air using a QE-R3011 instrument (Enli Technology Co. Ltd., Taiwan) with a scan increment of 10 nm per point. Mobility Measurements by the SCLC Method. The hole-only devices were fabricated with a configuration of ITO/PEDOT:PSS/ blend (200−250 nm)/Au. The electron-only devices were fabricated with a configuration of ITO/titanium diisopropoxide bis(2,4pentanedionate)50 (TIPD) (20 nm)/blend (150 nm)/Al (100 nm). Since the HOMO and LUMO energy levels of TIPD are −3.91 and −6.0 eV, respectively, it could be used to fabricate the electron-only SCLC device. The TIPD buffer layer was prepared by spin-coating a 3.5 wt % solution of TIPD in isopropanol onto the precleaned ITO substrate followed by baking at 150 °C for 10 min to convert TIPD into TOPD.52 Subsequently, the blend was spin-coated onto it under the same condition as for preparation of the optimal solar cell. The Au or Al layer was thermally deposited on the top of the blend in vacuum. The Au layer was deposited at a low speed (0.1 Å/s) to avoid the penetration of Au atoms into the active layer. The Al layer was deposited at a speed of 1 Å/s. The hole and electron mobilities were extracted by fitting the J−V curves using the equation J = 9/8εε0μV2/

the absorption around 645 nm becomes relatively stronger as the acceptor transitions from the dilute solution (molecular state) to the pure film, where the dimer molecules form aggregates, while the 654 nm absorption becomes stronger relative to that of the 706 nm peak when the donor goes from the dilute solution (molecular state) to the pure film (aggregate state).34 This spectral increase thus indicates enhanced selfaggregation of the donor and acceptor because more donor and acceptor molecules are phase-segregated in going from condition A to condition B. (2) Upon annealing with the host solvent only or the mixture of the host and additive (conditions C and D, respectively), the absorption between 640 and 700 nm is even stronger than that from condition B and one peak shifts from 643 to 650 nm, also as a result of the selfaggregation of more donor and acceptor molecules because of the even better phase segregation. The change in the solar cell blends in going from condition A to conditions B and C/D agrees well with the TEM and GIXRD data, and all of the characterization data, including that from XPS, demonstrate that the reconstruction of the film morphology is responsible for the improvement in the electric performance of the solar cell upon fine-tuning of the parameters of the film-forming process.

3. CONCLUSIONS In summary, we have presented a detailed approach of finetuning the parameters relative to the film-forming kinetics to readily improve the performance of non-fullerene organic solar cells. Annealing of the as-cast active film in the gaseous vapor of the host and a mixture of the host and additive solvents, respectively, gave PCEs of 5.85 and 6.08%, both of which were obtained from conventional single-junction solar cells. These values are significantly enhanced with respect to the value of 1.44% without any treatments of solvent annealing. The improvement in the cell performance is in line with the increase in the electron mobility and the reduced recombination loss upon solvent vapor annealing as a result of the reconstruction of the film morphology, i.e., that the fine-tuning of the parameters relative to the film-forming process improves the phase segregation and consequently enhances the selfaggregation of the donor and acceptor molecules in the solar cell blend. These findings are helpful for the further advance of non-fullerene organic solar cells, particularly in the case of improvement of the device performance via process optimization, which plays a central role in the realization of highperformance non-fullerene organic solar cells. 4. EXPERIMENTAL SECTION Materials and Instruments. The PBDTTT-C-T polymer was purchased from Solarmer Company. Both 1D and 2D GIXRD data were measured at the Diffuse X-ray Scattering (1W1A) experimental station at the Beijing Synchrotron Radiation Facility (BSRF). The authors gratefully acknowledge the assistance of the scientists at the 1W1A station during the experiments. TEM measurements were performed on a JEM-2011F transmission electron microscope operated at 200 kV. XPS experiments were performed on a Thermo Scientific ESCALab 250Xi spectrometer using 200 W monochromatized Al Kα radiation. The 500 μm X-ray spot was used for XPS analysis. The base pressure in the analysis chamber was about 3 × 10−10 mbar. Typically, the hydrocarbon C 1s line at 284.8 eV from adventitious carbon was used for energy referencing. The D/A weight ratios were estimated from the XPS data by following our previous procedure.35 Cyclic voltammetry was performed using a Zahner IM6e electrochemical workstation in a 0.1 mol/L solution of tetrabutyF

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L3 exp[0.89(V/E0L)0.5],53−56 in which ε is the dielectric constant of the polymer (here, 3 was used), ε0 is the permittivity of vacuum (8.85419 × 10−12 C V−1 m−1), μ is the zero-field mobility, E0 is the characteristic field, J is the current density, L is the film thickness, and V = Vappl − Vbi, where Vappl is the applied potential and Vbi is the builtin potential that results from the difference in the work functions of the anode and the cathode (in the hole-only device, Vbi = 0.2 V, and in the electron-only device, Vbi = 0 V). The hole and electron mobilities of the solar cell blends were deduced from the intercept value of ln(9εε0μ/8) by linearly plotting ln(JL3/V2) versus (V/L)0.5, as shown in Figure 5.

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(16) Wang, X.; Huang, J. H.; Niu, Z. X.; Sun, Y. X.; Zhan, C. L. Tetrahedron 2014, 70, 4726. (17) Schmidt-Mende, L.; Fechtenkötter, A.; Müllen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119. (18) Rajaram, S.; Armstrong, P. B.; Kim, B. J.; Fréchet, J. M. J. Chem. Mater. 2009, 21, 1775. (19) Sharenko, A.; Proctor, C. M.; van der Poll, T. S.; Henson, Z. B.; Nguyen, T.-Q.; Bazan, G. C. Adv. Mater. 2013, 25, 4403. (20) Singh, R.; Aluicio-Sarduy, E.; Kan, Z.; Ye, T.; MacKenzie, R. C. I.; Keivanidis, P. E. J. Mater. Chem. A 2014, 2, 14348. (21) Yan, Q.; Zhou, Y.; Zheng, Y.-Q.; Pei, J.; Zhao, D. H. Chem. Sci. 2013, 4, 4389. (22) Lu, Z. H.; Zhang, X.; Zhan, C. L.; Jiang, B.; Zhang, X. L.; Chen, L. L.; Yao, J. N. Phys. Chem. Chem. Phys. 2013, 15, 11375. (23) Jiang, B.; Zhang, X.; Zhan, C. L.; Lu, Z. H.; Huang, J. H.; Ding, X. L.; He, S. G.; Yao, J. N. Polym. Chem. 2013, 4, 4631. (24) Jiang, W.; Ye, L.; Li, X. G.; Xiao, C. Y.; Tan, F.; Zhao, W. C.; Hou, J. H.; Wang, Z. H. Chem. Commun. 2014, 50, 1024. (25) Shivanna, R.; Shoaee, S.; Dimitrov, S.; Kandappa, S. K.; Rajaram, S.; Durrant, J. R.; Narayan, K. S. Energy Environ. Sci. 2014, 7, 435. (26) Lin, Y. Z.; Wang, Y.; Wang, J.; Hou, J. H.; Li, Y.; Zhu, D.; Zhan, X. W. Adv. Mater. 2014, 26, 5137. (27) Cheng, P.; Ye, L.; Zhao, X. G.; Hou, J. H.; Li, Y. F.; Zhan, X. W. Energy Environ. Sci. 2014, 7, 1351. (28) Zhou, Y.; Yan, Q. F.; Zheng, Y.-Q.; Wang, J.-Y.; Zhao, D. H.; Pei, J. J. Mater. Chem. A 2013, 1, 6609. (29) Zhou, E.; Cong, J.; Wei, Q.; Tajima, K.; Yang, C.; Hashimoto, K. Angew. Chem., Int. Ed. 2011, 50, 2799. (30) Zhou, N.; Lin, H.; Lou, S. J.; Yu, X.; Guo, P.; Manley, E. F.; Loser, S.; Hartnett, P.; Huang, H.; Wasielewski, M. R.; Chen, L. X.; Chang, R. P. H.; Facchetti, A.; Marks, T. J. Adv. Energy Mater. 2014, 4, No. 1300785. (31) Earmme, T.; Hwang, Y.-J.; Murari, N. M.; Subramaniyan, S.; Jenekhe, S. A. J. Am. Chem. Soc. 2013, 135, 14960. (32) Li, W. W.; Roelofs, W. S. C.; Turbiez, M.; Wienk, M. M.; Janssen, R. A. J. Adv. Mater. 2014, 26, 3304. (33) Jung, I. H.; Lo, W. Y.; Jang, J.; Chen, W.; Zhao, D. L.; Landry, E. S.; Lu, L. Y.; Talapin, D. V.; Yu, L. P. Chem. Mater. 2014, 26, 3450. (34) Zhang, X.; Lu, Z. H.; Ye, L.; Zhan, C. L.; Hou, J. H.; Zhang, S. L.; Jiang, B.; Zhao, Y.; Huang, J. H.; Zhang, S. Q.; Liu, Y.; Shi, Q.; Liu, Y. Q.; Yao, J. N. Adv. Mater. 2013, 25, 5791. (35) Lu, Z. H.; Jiang, B.; Zhang, X.; Tang, A. L.; Chen, L. L.; Zhan, C. L.; Yao, J. N. Chem. Mater. 2014, 26, 2907. (36) Li, H.; Earmme, T.; Ren, G.; Saeki, A.; Yoshikawa, S.; Murari, N. M.; Subramaniyan, S.; Crane, M. J.; Seki, S.; Jenekhe, S. A. J. Am. Chem. Soc. 2014, 136, 14589. (37) Zhou, Y.; Kurosawa, T.; Ma, W.; Guo, Y.; Fang, L.; Vandewal, K.; Diao, Y.; Wang, C.; Yan, Q.; Reinspach, J.; Mei, J.; Appleton, A. L.; Koleilat, G. I.; Gao, Y.; Mannsfeld, S. C. B.; Salleo, A.; Ade, H.; Zhao, D.; Bao, Z. Adv. Mater. 2014, 26, 3767. (38) Mori, D.; Benten, H.; Okada, I.; Ohkita, H.; Ito, S. Adv. Energy Mater. 2014, 4, No. 1301006. (39) Earmme, T.; Hwang, Y.-J.; Subramaniyan, S.; Jenekhe, S. A. Adv. Mater. 2014, 26, 6080. (40) Mu, C.; Liu, P.; Ma, W.; Jiang, K.; Zhao, J.; Zhang, K.; Chen, Z.; Wei, Z.; Yi, Y.; Wang, J.; Yang, S.; Huang, F.; Facchetti, A.; Ade, H.; Yan, H. Adv. Mater. 2014, 26, 7224. (41) Zang, Y.; Li, C.-Z.; Chueh, C.-C.; Williams, S. T.; Jiang, W.; Wang, Z. H.; Yu, J.-S.; Jen, A. K. Y. Adv. Mater. 2014, 26, 5708. (42) Zhong, Y.; Trinh, M. T.; Chen, R. S.; Wang, W.; Khlyabich, P. P.; Kumar, B.; Xu, Q. Z.; Nam, C.-Y.; Sfeir, M. Y.; Black, C.; Steigerwald, M. L.; Loo, Y.-L.; Xiao, S. X.; Ng, F.; Zhu, X.-Y.; Nuckolls, C. J. Am. Chem. Soc. 2014, 136, 15215. (43) Zhao, J.; Li, Y.; Lin, H.; Liu, Y.; Jiang, K.; Mu, C.; Ma, T.; Lai, J. Y. L.; Hu, H.; Yu, D.; Yan, H. Energy Environ. Sci. 2014, DOI: 10.1039/C4EE02990A. (44) Cnops, K.; Rand, B. P.; Cheyns, D.; Verreet, B.; Empl, M. A.; Heremans, P. Nat. Commun. 2014, 5, No. 3406.

ASSOCIATED CONTENT

* Supporting Information S

Film absorption, surface XPS, and photovoltaic data optimized under different conditions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.Z.). *E-mail:[email protected] (J.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the Beijing Synchrotron Radiation Facility (BSRF) for the GIXRD measurements and the assistance of the scientists there. This work was funded by the National Natural Science Foundation of China (91433202, 21327805, 91227112, and 21221002), the Chinese Academy of Sciences (XDB12010200), and the Ministry of Science and Technology of the People’s Republic of China (973 Project, 2011CB808400 and 2012YQ120060).

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REFERENCES

(1) Anthony, J. E. Chem. Mater. 2011, 23, 583. (2) Eftaiha, A. F.; Sun, J.-P.; Hill, I. G.; Welch, G. C. J. Mater. Chem. A 2014, 2, 1201. (3) Facchetti, A. Mater. Today 2013, 16, 123. (4) Woo, C. H.; Holcombe, T. W.; Unruh, D. A.; Sellinger, A.; Fréchet, J. M. J. Chem. Mater. 2010, 22, 1673. (5) Bloking, J. T.; Han, X.; Higgs, A. T.; Kastrop, J. P.; Pandey, L.; Norton, J. E.; Risko, C.; Chen, C. E.; Brédas, J.-L.; McGehee, M. D.; Sellinger, A. Chem. Mater. 2011, 23, 5484. (6) Zhou, T. L.; Jia, T.; Kang, B. N.; Li, F. H.; Fahlman, M.; Wang, Y. Adv. Energy Mater. 2011, 1, 431. (7) Zhou, Y.; Ding, L.; Shi, K.; Dai, Y.-Z.; Ai, N.; Wang, J.; Pei, J. Adv. Mater. 2012, 24, 957. (8) Zhou, Y.; Dai, Y.-Z.; Zheng, Y.-Q.; Wang, X.-Y.; Wang, J.-Y.; Pei, J. Chem. Commun. 2013, 49, 5802. (9) Brunetti, F. G.; Gong, X.; Tong, M.; Heeger, A. J.; Wudl, F. Angew. Chem., Int. Ed. 2010, 49, 532. (10) Kim, H. U.; Kim, J.-H.; Suh, H.; Kwak, J.; Kim, D.; Grimsdale, A. C.; Yoon, S. C.; Hwang, D.-H. Chem. Commun. 2013, 49, 10950. (11) Shu, Y.; Lim, Y.-F.; Li, Z.; Purushothaman, B.; Hallani, R.; Kim, J. E.; Parkin, S. R.; Malliaras, G. G.; Anthony, J. E. Chem. Sci. 2011, 2, 363. (12) Lin, Y. Z.; Cheng, P.; Li, Y. F.; Zhan, X. W. Chem. Commun. 2012, 48, 4773. (13) Sonar, P.; Ng, G.-M.; Lin, T. T.; Dodabalapur, A.; Chen, Z.-K. J. Mater. Chem. 2010, 20, 3626. (14) Ren, G.; Ahmed, E.; Jenekhe, S. A. Adv. Energy Mater. 2011, 1, 946. (15) Ahmed, E.; Ren, G.; Kim, F. S.; Hollenbeck, E. C.; Jenekhe, S. A. Chem. Mater. 2011, 23, 4563. G

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Chemistry of Materials

Article

(45) Huo, L. J.; Zhang, S. Q.; Guo, X.; Xu, F.; Li, Y. F.; Hou, J. H. Angew. Chem., Int. Ed. 2011, 50, 9697. (46) Tang, H.; Lu, G.; Li, L.; Li, J.; Wang, Y.; Yang, X. J. Mater. Chem. 2010, 20, 683. (47) Wessendorf, C. D.; Schulz, G. L.; Mishra, A.; Kar, P.; Ata, I.; Weidelener, M.; Urdanpilleta, M.; Hanisch, J.; Mena-Osteritz, E.; Lindén, M.; Ahlswede, E.; Bäuerle, P. Adv. Energy Mater. 2014, 4, No. 1400266. (48) Kan, B.; Zhang, Q.; Li, M.; Wan, X.; Ni, W.; Long, G.; Wang, Y.; Yang, X.; Feng, H.; Chen, Y. J. Am. Chem. Soc. 2014, 136, 15529. (49) Cowan, S. R.; Roy, A.; Heeger, A. J. Phys. Rev. B 2010, 82, No. 245207. (50) Liu, F.; Gu, Y.; Wang, C.; Zhao, W.; Chen, D.; Briseno, A. L.; Russell, T. P. Adv. Mater. 2012, 24, 3947. (51) Yang, X.; Loos, J.; Veenstra, S. C.; Verhees, W. J.; Wienk, M. M.; Kroon, J. M.; Michels, M. A.; Janssen, R. A. Nano Lett. 2005, 5, 579. (52) Tan, Z. A.; Zhang, W. Q.; Zhang, Z. G.; Qian, D. P.; Huang, Y.; Hou, J. H.; Li, Y. F. Adv. Mater. 2012, 24, 1476. (53) Murgatroyd, P. N. J. Phys. D: Appl. Phys. 1970, 3, 151. (54) Campbell, A. J.; Bradley, D. D. C.; Lidzey, D. G. J. Appl. Phys. 1997, 82, 6326. (55) Malliaras, G. G.; Salem, J. R.; Brock, P. J.; Scott, C. Phys. Rev. B 1998, 58, 13411. (56) An, Z.; Yu, J.; Jones, S. C.; Barlow, S.; Yoo, S.; Domercq, B.; Prins, P.; Siebbeles, L. D. A.; Kippelen, B.; Marder, S. R. Adv. Mater. 2005, 17, 2580.

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