Multiphoton Microscopy of Ï-Conjugated ... - ACS Publications

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Multiphoton Microscopy of π‑Conjugated Copolymers and Copolymer/Fullerene Blends for Organic Photovoltaic Applications Shai R. Vardeny,† Sangita Baniya,‡ Benjamin Cromey,† Khanh Kieu,† Nasser Peyghambarian,† and Z. Valy Vardeny*,‡ †

College of Optical Sciences, University of Arizona, Tucson, Arizona 85721, United States Department of Physics & Astronomy, University of Utah, Salt Lake City, Utah 84112, United States

‡

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ABSTRACT: Organic photovoltaic (OPV) cells based on πconjugated copolymer/fullerene blends are devices with the highest power conversion efficiencies within the class of organic semiconductors. Although a number of image microscopies have been applied to films of π-conjugated copolymers and their fullerene blends, seldom have they been able to detect microscopic defects in the blend films. We have applied multiphoton microscopy (MPM) using a 65 fs laser at 1.56 μm for spectroscopy and mapping of films of various π-conjugated copolymers and their fullerene blends. All pristine copolymer films have shown third harmonic generation (THG) and two-photon or three-photon photoluminescence that could be used for mapping the films with micrometer spatial resolution. Since the fullerenes have much weaker THG efficiency than those of the copolymers, we could readily map the copolymer/ fullerene blend films that showed interpenetrating micron-sized grains of the two constituents. In addition, we also found second harmonic generation from various micron-sized defects in the films that are formed during film deposition or light illumination at ambient conditions, which do not possess inversion symmetry. The MPM method is therefore beneficial for organic films and devices for investigating the properties and growth of copolymer/fullerene blends for OPV applications. KEYWORDS: OPV cells, MPM method, copolymer/fullerene blends, THG efficiency

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INTRODUCTION The development of organic photovoltaic (OPV) solar cell technology1−3 and in particular, PV solar cells based on πconjugated polymers, is driven by the potential to harness solar energy cheaply, since the devices may be fabricated using simple roll-to-roll printing methods. The principal active layer in polymer OPV solar cells has been a blend of two organic components, namely, the π-conjugated polymers as electron donors and fullerene molecules such as phenyl-C61-butyric acid methyl ester (PCBM) as electron acceptors.4−7 The main polymer used for OPV has been regioregular polythiophene that absorbs in the visible range of the solar spectrum.8 Recently, π-conjugated polymers have been replaced by πconjugated block copolymers of which repeating units consist of alternating donor (D) and acceptor (A) moieties, known as DA-copolymers.2,9−15 This chain architecture substantially reduces the optical gap, and thus the DA-copolymers absorb more in the near infrared, catching a larger fraction of the solar spectrum. The power conversion efficiencies (PCE) of organic solar cells based on DA-copolymers as donor materials and fullerene molecules as acceptors have exceeded 12%.14 In particular, OPV cells based on fullerene blends with the DAcopolymer PffBT4T, namely, poly[(5,6-difluoro-2,1,3-benzo© XXXX American Chemical Society

thiadiazol-4,7-diyl)-alt-(3,3000-di(2-octyldodecyl)2,20;50,200;500,2000-quaterthiophen-5,5000-diyl)] (see Figure 1), have reached a PCE of up to 10.8%.15 We focus our present optical studies on films of pristine DA-copolymers with low-to-moderate energy gaps and their blends with PCBM fullerenes. A common design for a solar cell, called bulk heterojunction (BHJ), consists of an active layer of phase-separated blend of copolymer donors and fullerene acceptors. The donor/ acceptor blend is usually deposited onto a transparent conductive oxide film that acts as the anode. A buffer layer, used to block electron (holes), is placed between the active layer and cathode (anode). An exciton is generated when a photon with energy that is equal or above the highest occupied molecular orbital−lowest unoccupied molecular orbital energy gap of the donor strikes the active layer. With the proper chemical potential, the exciton diffuses to the interface of the donor and acceptor grains in the blend film. A potential is applied to the junction and the electron−hole pair is separated; Received: July 8, 2018 Accepted: August 27, 2018

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DOI: 10.1021/acsami.8b11378 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Absorption and photoluminescence (PL) spectra of three π-conjugated copolymers. Panels (A), (B), and (C) show the optical density and PL spectra of the three π-conjugated DA-copolymers, fluorinated benzotriazole (FTAZ), polythieno[3,4-b]thiophene-alt-benzodithiophene (PTB7), and PffBT4T, respectively. The copolymer backbone structures are given in each inset. Note the two moieties in each copolymer chain that serve as intrachain donor and acceptor, respectively.

In addition, we also note that these different techniques can seldom detect defects in the organic blend films. This is because the chemical composition does not change at a native defect site. Moreover, nanoscopic techniques are too fine to detect microscopic defects. In the present work, we introduce a novel scanning technique for mapping films of pristine copolymers and their blend with PCBM fullerenes based on nonlinear optical (NLO) response of the blend constituents, called multiphoton microscopy (MPM). In this technique, we use the different third-order NLO coefficients (χ(3)) such as third harmonic generation (THG) of the copolymer and PCBM, respectively, to map the blend film with micron resolution. Furthermore, we use the fact that the second harmonic generation (SHG) associated with χ(2) occurs in πconjugated copolymers only in defected local sites that lack inversion symmetry, to map microscopic defects in the film. We have observed static defects in the pristine copolymer films and in OPV blend films that are created during film deposition, and we have also detected the formation of defects in the blend film upon exposing it to prolonged illumination at ambient conditions. The ability to detect micron-sized defects may be of interest to the PV community in general and OPV community in particular. For example, one may be able to monitor the formation of the well-known Stabler−Wronski defects in a-Si:H solar cells51,52 and the ion migration in solar cells based on hybrid organic−inorganic perovskites.53 Here, we focus on films of three DA-copolymers and their blends with fullerene PCBM molecules. The backbone structure of the three copolymers is shown in Figure 1. The copolymers are FTAZ, which is polybenzodithiophene fluorinated benzotriazole (PBnDT-FTAZ) with a band gap of ∼2.01 eV, and two other copolymers, both with band gap of ∼1.7 eV, which are polythienothiophene benzodithiophene 7 (PTB7) and PffBT4T-2OD. The latter two copolymer blends with PCBM form the active layers of some of the most efficient OPV solar cells to date.15 As seen, each DA-copolymer chain is composed of two moieties with different electron affinities and thus may serve as an intrachain donor and acceptor that may help exciton ionization into free-charged polarons.54,55 The details of film preparation are given in the Materials and Methods section. We have used sapphire substrates since they show the least amount of NLO emission, such as THG, no SHG, and lack of fluorescence or PL emission, which would have added a background signal to the MPM scans (see Section S1 for Sapphire MPM mapping and power dependence measurements).

the hole travels toward the anode whereas the electron travels toward the cathode. The advantage of BHJ is that the donor− acceptor interface is appropriately phase-separated so the probability that the charges encounter the correct interface within the exciton diffusion length is high.1 The OPV research community has established a direct correlation of the critical performance of OPV devices to that of the topography and chemical morphology of (co)polymers and/or their fullerene blends.16,17 A number of mapping methods have been utilized to analyze the nanoscale structure of these films and try to obtain a more fundamental physical understanding behind the increase in PCE for said topography and morphology. The basic mapping methods (and their variants) utilized for these analysis are scanning electron microscopy (SEM), transmission electron microscopy, and atomic force microscopy (AFM). The length scales and the phase relation between the blend constituents are observed, such as “polymer rich” and “fullerene rich” domains. Studying the phase-separation between these two domains has been correlated to increased device performance with the proper (co)polymer/fullerene proportions.18−37 Recently, work is being conducted on correlating the performance of nonfullerene blends to morphology.37 To take into account the extremely limited lateral, chemical, and crystal order information of the material with the above methods, other spectroscopic and mapping methods must be correlated to probe the finer physical interactions of the material. Some of these methods include dynamic time-of-flight secondary ion mass spectrometry18,38,39 and Kelvin probe force microscopy19 for lateral composition analysis. For a deeper understanding of the chemical composition and crystallinity, scanning transmission X-ray microscopy is used.40−43 AFM has been combined with Raman spectral mapping and fluorescence imaging to study the topographical and lateral information of patterned geometries.44 Resonance Raman spectroscopic imaging was combined with AFM to introduce a physical probe in identifying morphology-dependent variations of intraand interchain interactions and order in P3HT/PCBM blend.45 Grazing incidence X-ray scattering represents a visualization and understanding of the crystal structure, order, and orientation in the crystalline regions of the films.25,27,34,46−50 Most of these techniques, however, are limited to about 10 nm from the surface of the film; they are mostly surface analysis techniques. Furthermore, they ignore the light−matter interaction of the molecular structure and thus must usually be combined with spectroscopy. B


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Figure 2. MPM imaging setup and typical MPM spectra: (A) the MPM imaging apparatus, where each optical component is denoted. The detailed description is given in the Materials and Methods section. Typical MPM spectra of (B) silicon wafer and (C) GaAs samples. The THG, SHG, and TP-PL emission bands are denoted. Note that the MPM spectrum of GaAs shows SHG whereas Si does not. The multiple peaks seen in the THG and SHG emission bands are due to the broad spectrum of the mode-locked laser source.

Figure 1). The PL was excited with a laser diode at 530 nm, collected, dispersed by a monochromator, and detected by a Si photodetector. The PL is a blessing for MPM since it is possible to use it for mapping via NLO-based process such as two-photon-photoluminescence (TP-PL) or three-photon-PL (ThP-PL).

For measuring the MPM spectrum upon illumination, the copolymer films were exposed to prolonged illumination by a 100 W Tungsten-Halogen incandescent lamp for approximately 1 h at an intensity of ∼500 W/cm2. Each copolymer film shows a pronounced PL band with a shifted onset compared to that of the absorption spectrum (see C


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Figure 3. MPM spectroscopy of three copolymers. (A), (C), and (E) are the MPM spectra of FTAZ, PTB7, and PffBT4T, respectively. The emission bands THG and nonlinear PL (TP-PL and ThP-PL) are denoted. (B), (D), and (F) are the THG (left axis) and nonlinear PL emission (right axis) strengths vs the laser excitation intensity of the respective three copolymers shown in the left panels.

The multiphoton images (MPI) of the copolymer films were taken using a customized multiphoton microscope (Figure 2), where the setup and experimental procedures have been extensively detailed and may be found in the literature56−58 (see Materials and Methods). In brief, for excitation, we used an erbium-doped femtosecond mode-locked laser operating at 1560 nm, having pulse duration ∼65 fs, at an average power up

to 80 mW, with an ∼8.5 MHz repetition rate; hence, the estimated peak power was ∼145 kW with ∼9 nJ pulse energy.59 The laser light was focused on the sample onto a ∼1 μm2 spot size, and a half-wave plate (HWP) and linear polarizer were placed in front of a beam-splitter (BS) for power attenuation. A rotatable 870 nm dichroic mirror lets the excitation reflection from the sample pass and diverts the D


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ACS Applied Materials & Interfaces resulting THG, SHG, or TP-PL/ThP-PL onto the photomultiplier tubes (PMTs). A 560 nm dichroic mirror is used to further split the THG to one PMT and the SHG/PL to another PMT. The resulting emission is also coupled into the spectrometer (Ocean Optics QE65000) by rotating the 870 nm dichroic and feeding the light into a pigtailed collecting lens. Placing various bandpass (BP) filters in front of the PMTs allows mapping of different wavelengths of interest. For demonstrating the MPM capabilities, we present in Figure 2 (lower panels) two MPM spectra measured on a Si wafer and GaAs sample, respectively. As clearly seen, both show a THG emission band at ∼520 nm, since the third-order nonlinearity χ(3) is allowed in any material regardless of its symmetry group. However, only the GaAs sample shows a strong SHG emission band at ∼780 nm, since χ(2) is allowed only in crystals that lack inversion symmetry (such as GaAs, which belongs to the Td symmetry group that does not contain the inversion symmetry operation). This is a virtue of the MPM technique, which we use below to identify defects in the copolymer films. In addition, GaAs has a PL band at ∼880 nm that can also be used in MPM mapping, because it is generated via two-photon absorption (χ(3) process). Since the copolymers used here show relatively strong PL emission, we may use it as well in MPM.

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RESULTS AND DISCUSSION Figure 3 (left panels) shows typical MPM spectra of the three copolymers that we studied here. Each copolymer shows a Table 1. Best Fitting Parameters for the THG Intensity in the MPM Spectrum vs the Excitation Intensity Using Equation 1 IS (mW)

compound FTAZ PTB7 PffBT4T FTAZ/PCBM PTB7/PCBM PffBT4T/PCBM PCBM sapphire

150 267 213 187 169 223 782 5000

± ± ± ± ± ± ± ±

10 30 23 10 23 34 91 2600

Table 2. Same as in Table 1 but for the PL Emission Band in the MPM Spectraa compound

n

FTAZ PTB7 PffBT4T FTAZ/PCBM PTB7/PCBM PffBT4T/PCBM

3 2 2/3 2 2 2/3

IS (mW) 159 112 215 415 300 215

± ± ± ± ± ±

11 7 24 44 110 30

Figure 4. Ratio r of the THG and nonlinear PL emission intensities as a function of the excitation intensity (or power) for the three copolymers (A) FTAZ, (B) PTB7, and (C) PffBT4T of Figure 3.

a

The exponent n is the nonlinear optical order of the nonlinear PL process.

strong THG band at ∼520 nm, followed by a PL band that is specific to each copolymer, as demonstrated above in Figure 1. Since the laser photon energy (∼0.8 eV) cannot excite the copolymers’ PL bands, it is obvious that the PL emission is due to two- or three-photon absorption in the film. To check whether the PL band in each copolymer is due to TP-PL or ThP-PL, we conducted the PL laser excitation intensity (IL)

dependence compared to the excitation dependence of the THG emission, which is a three-photon process. Indeed, we could fit the THG intensity, I dependence using IL3 function that undergoes saturation at high power, as follows I(IL) = a × IL n(1 − (IL / IS)) E

(1) DOI: 10.1021/acsami.8b11378 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. MPM images of pristine FTAZ and FTAZ/PCBM blend films. (A) The MPM image of pristine FTAZ, where the separate signals are chosen with representative colors. Green represents the signal from THG, and red represents the signal from nonlinear PL emission, respectively. The insets show the PL (left) and THG (right) filtered region. (B) The MPM image of FTAZ/PCBM film, where green represents THG coming from the copolymer film and black shows the PCBM grains. The inset is a zoomed image of the interpenetrating FTAZ/PCBM micron-sized grains. The PL layer is omitted everywhere. The scale bar in each picture is denoted.

The saturation levels IS are given for each copolymer in Table 1 for the THG and Table 2 for the nonlinear PL emissions. As seen in Figure 3 (right panels), the IL dependence of the nonlinear PL emission follows that of the THG for the copolymers FTAZ and PffBT4T but not for PTB7, where the nonlinear PL emission does not follow that of the THG emission. This is nicely demonstrated in Figure 4, where we plot the ratio r = ITHG/IPL vs IL. As seen, r remains constant with IL for FTAZ, verifying that the nonlinear PL in this copolymer is ThP-PL type. This is to be expected, since the PL in this copolymer peaks at ∼700 nm, which is not reachable by twophoton absorption at 780 nm. However, the ratio r shows a linear dependence on IL for PTB7, indicating that the nonlinear PL in this copolymer is of TP-PL type; this is in agreement with the peak in the PL spectrum at ∼800 nm, which allows excitation by a two-photon NLO process. Surprisingly, r in PffBT4T has a complex IL dependence. At low IL, it shows a linear increase that saturates and becomes constant at high IL. This indicates a TP-PL process at low IL and a ThP-PL at high IL, which probably occurs since the ThPPL process takes over. It is interesting to note that this situation does not happen in PTB7 copolymer. Figure 5 shows MPM mapping of FTAZ and FTAZ/PCBM blend. The other copolymers and their blends show similar results. Figure 5A shows that although the THG emission occurs everywhere on the film, the ThP-PL is larger at preferred areas of the film so that its emission intensity is in fact larger than that of the THG emission. This may be explained if we assume that there are radiative recombination centers on the copolymer chains of which PL is the preferred recombination channel. This is caused by the copolymer film inhomogeneity, which may also influence the large width of the PL bands in the copolymers (Figure 1). Figure 5B shows a typical MPM mapping of FTAZ/PCBM blend. Since the χ(3)

coefficient is larger in the copolymer chain than in PCBM molecules (see Section S2), probably because of the quasi-onedimensional morphology of the chains, a contrast in the THG intensity is generated. However, in comparison to the copolymer films, we found that the THG of the fullerene molecules do not saturate at high IL (Figure S3A). This indicates that the THG contrast between the two blend constituents worsens at high IL (see MPI in Figure S4). At relatively smaller IL, the THG contrast results in an efficient mapping of the blend. Figure 5B demonstrates that the film is composed of interpenetrating grains of FTAZ and PCBM of about 1−2 μm, which is ideal for exciton separation, and separate charge transport of the resulting electrons and holes to the cathode and anode, respectively. To strengthen the claim that the contrast is a result of the FTAZ and PCBM constituents, the same mapping with PL included is shown in Section S3. Furthermore, in Section S4, two well-known and established imaging techniques, namely, AFM and SEM, were conducted on FTAZ/PCBM blend to better show that the phase-separated matrix of Figure 5B is a result of FTAZ copolymer and PCBM grains. Figures 6 and 7 demonstrate the unique advantage of MPM mapping compared to other mapping methods described in the literature. Namely, the MPM spectroscopic technique is able to pinpoint defects in the film that lack inversion symmetry, a feature sorely lacking in other mapping methods. Figure 6 identifies a defect in a PTB7 film that shows relatively strong SHG emission. There are other such defects in the film, but their SHG emission is weaker. Such a defect cannot originate from a single copolymer chain, as for example from a radical or break in the chains, because such local defects do not necessarily break the inversion symmetry of the chain. We believe that the native defect showing SHG must indicate several polymer chains that are intertwined together. This type of structure may lack inversion symmetry and would be large F


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capability to detect such light-induced defects, which might be created even under encapsulating conditions upon illumination for longer periods of time.

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CONCLUSIONS We have presented a novel method of mapping films of three π-conjugated copolymers and their blend with fullerene molecules that are used in OPV applications. The method is based on multiple-photon emission, such as THG, SHG, and PL induced by NLO mechanisms such as two-photon and three-photon processes. The MPM is capable of observing the separate microsized grains of the copolymer and fullerene in D−A blends, which is highly needed for OPV applications. Since the copolymer chain structure possesses inversion symmetry, there is no SHG from the pristine copolymer and copolymer/PCBM blend, unless a micron-sized defect breaks the inversion symmetry. On the basis of this principle, we have shown that SHG observed in MPI is capable of detecting native defects born during film deposition, as well as induced defects by prolonged illumination. The latter is especially important for OPV solar cell applications since long exposure times to light may lead to device degradation that can be readily monitored in situ by MPM.

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MATERIALS AND METHODS

Film Deposition. PTB7. Polythieno[3,4-b]thiophene-alt-benzodithiophene (PTB7) copolymer powder was purchased from Solarmer and used without further purification. Thick films of PTB7 were spin cast at a speed of 800 rpm or drop cast from solution in 1,2dichlorobenzene (ODCB) at a concentration of 10 mg/mL onto sapphire. For the PTB7/PCBM blend, we used a solution of PTB7 and PC60BM (1:1.5 in weight) with the same concentration in ODCB, which was stirred in nitrogen atmosphere overnight. PffBT4T. Poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt(3,3000-di(2-octyldodecyl)- 2,20;50,200;500,2000-quaterthiophen5,5000-diyl)] (PffBT4T) copolymer powder was purchased from Solarmer and used without further purification. Thick films of PffBT4T were spin cast at a speed of 800 rpm or drop cast from solution in 1,2-dichlorobenzene (ODCB) and chlorobenzene (CB) [ODCB/CB (1:1)] at a concentration of 10 mg/mL onto sapphire. For the PffBT4T/PCBM blend, we used a solution of PffBT4T and PC70BM (1:1.4 in weight) (which gives the best PCE performance) with the same concentration in ODCB/CB solvent, which was stirred in nitrogen atmosphere overnight. PBnDT-FTAZ. Polybenzodithiophene fluorinated benzotriazole (PBnDT-FTAZ) copolymer powder was synthesized in Prof. Wei You’s group at North Carolina University. Thick copolymer films were spin cast at a speed of 800 rpm or drop cast from solution in trichlorobenzene at a concentration of 12 mg/mL onto sapphire. PBnDT-FTAZ copolymers were blended with PC60BM (1:2 in weight) in trichlorobenzene solvent at a concentration of 12 mg/mL. Multiphoton Microscope Setup. The multiphoton images (MPI) of the π-conjugated copolymers for this study were taken using a customized multiphoton microscope presented in Figure 2, where the variational setups and experimental procedures have been detailed elsewhere.56−58 An erbium-doped femtosecond mode-locked laser, using a carbon-nanotube saturable absorber59 at 1560 nm, full width at half-maximum ∼65 fs pulses, linearly polarized, and operated at an ∼8.5 MHz repetition rate, was used as the excitation source. The average power was measured at 80 mW, with an estimated peak power of 145 kW and 9.4 nJ energy/pulse. In this setup, the excitation light beam is coupled to a polarizationmaintaining fiber that is terminated to an FC/APC connector. The connector is attached to an FC/APC adapter collimator, where the light is launched onto a two-dimensional galvanometer system in which the laser spot is rasterized across the sample. A half-wave plate (HWP) and linear polarizer are placed in front of a beam-splitter (BS)

Figure 6. MPM image of a PTB7 film that contains defects. (A) The MPM image of a pristine PTB7 film. The green color represents the THG band mapping; the red dots are defects that show SHG emission. (B) The MPM spectrum of the main defect shown in (A), which shows both THG and SHG emission bands.

enough to show SHG that is more intense than the THG emission band. Another important demonstration is the ability of MPM imaging to detect defects in the copolymer/fullerene blends that are created upon prolonged illumination; these type of defects may lead to device degradation.60−62 Such conditions are important for OPV solar cells, since the cell would be exposed to sun illumination for many hours. In Figure 7 (see Figure S7 for a clear image of PTB7/PCBM phase-separated matrix via Multimode AFM), we show MPM mappings of a copolymer/fullerene blend film, before and after illumination with light collected from a 100 W Tungsten-Halogen incandescent lamp for 1 h at an intensity of ∼500 W/cm2 at ambient conditions. It is clearly seen that there are “lightinduced defects” formed in the PTB7/PCBM blend film upon prolonged illumination. We note, however, that OPV solar cells are usually encapsulated to better withstand exposure to light at ambient. Nonetheless, we show here the MPM G


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Figure 7. MPM of PTB7/PCBM blend under prolonged illumination. (A) Before illumination, showing the fine interpenetrating BHS of the copolymer film and fullerene microsized grains. The green color represents THG from the copolymer film; the black is from the fullerene grain. TPPL is omitted. (B) After illumination for 1 h by 100 W incandescent light at ∼500 W/cm2, showing the formation of a micron-sized defect. (C) The MPM spectrum before illumination that shows THG and TP-PL emission bands. (D) The MPM spectrum from the light-induced defect seen in (B) that shows THG and SHG but lacks TP-PL emission. optical component in the beam path while the ∼5% reflection was fed into a reference detector so that the lock-in may operate in external mode at the repetition rate of ∼8.5 MHz. This allows for detection of small signals resulting from low IL such as 0.5 mW. The power dependence measurements were conducted by rotating the HWP from 0 to 45° and attenuating the laser from 1 to 55 mW. A power meter was utilized to translate the rotation in degrees to power input of the laser (i.e., 18° → 35 mW). The time constant (TC) of the lock-in was set at 200 ms order 3 for input powers below 5 mW and 1 μs order 1 for input powers above 5 mW. The BP filters were removed for power dependence measurements and PL mapping. The beam spot was rastered such that an idle time of 10 μs per pixel was attained at an area of 250 × 250 μm2 (1000 × 1000 pixels) using a 20× 0.75 NA objective for a total scan time of 10 s. The TC was set at 1 μs order 1 for fast scan speeds. The spectrum of the SHG spot found in PTB7 for Figure 6 was obtained by first scanning the area and using the user interface of the inhouse-built multiphoton microscope LabVIEW program to convert the pixel coordinates of the point of interest to the proper

for power attenuation, which was needed for the IL dependency measurements. The Keplerian telescope set up after the galvanometers expand the laser beam, so the back aperture of the objective is filled such that the full numerical aperture (NA) may be used and the beam properly focused to a focal spot with an optimized size. A rotatable 870 nm dichroic mirror lets the excitation reflection from the sample pass and diverts the resulting THG, SHG, and/or nonlinear PL due to two- or three-photon absorption toward the PMT branch. A 560 nm dichroic mirror is used to further split the THG to one PMT and the SHG/PL to another PMT. Bandpass (BP) filters at 517/20 nm for THG and 780/12 nm for SHG or PL are placed in front of the PMTs to further focus on the wavelength of interest. The resulting emissions may be coupled into the spectrometer (Ocean Optics QE65000) by rotating the 870 nm dichroic mirror and feeding the light into the attached pigtailed collecting lens. The largest deviation of this setup from previous iterations is the replacement of the preamplifiers (Stanford Research System SR570) with a lock-in amplifier (Zurich UHFLI). A BS was placed as the first H


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Solar Cells by Processing with Alkane Dithiols. Nat. Mater. 2007, 6, 497−500. (7) He, F.; Yu, L. How Far Can Polymer Solar Cells Go? In Need of a Synergistic Approach. J. Phys. Chem. Lett. 2011, 2, 3102−3113. (8) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. Thermally Stable, Efficient Polymer Solar Cells with Nanoscale Control of the Interpenetrating Network Morphology. Adv. Funct. Mater. 2005, 15, 1617−1622. (9) Zhang, Y.; Hau, S. K.; Yip, H. L.; Sun, Y.; Acton, O.; Jen, A. K. Y. Efficient Polymer Solar Cells Based on the Copolymers of Benzodithiophene and Thienopyrroledione. Chem. Mater. 2010, 22, 2696−2698. (10) Chen, W.; Xu, T.; He, F.; Wang, W.; Wang, C.; Strzalka, J.; Liu, Y.; Wen, J.; Miller, D. J.; Chen, J.; et al. Hierarchical Nanomorphologies Promote Exciton Dissociation in Polymer/Fullerene Bulk Heterojunction Solar Cells. Nano Lett. 2011, 11, 3707−3713. (11) Zhou, H.; Zhang, Y.; Seifter, J.; Collins, S. D.; Luo, C.; Bazan, G. C.; Nguyen, T. Q.; Heeger, A. J. High-Efficiency Polymer Solar Cells Enhanced by Solvent Treatment. Adv. Mater. 2013, 25, 1646− 1652. (12) You, J.; Dou, L.; Yoshimura, K.; Kato, T.; Ohya, K.; Moriarty, T.; Emery, K.; Chen, C. C.; Gao, J.; Li, G.; et al. A Polymer Tandem Solar Cell with 10.6% Power Conversion Efficiency. Nat. Commun. 2013, 4, No. 1446. (13) Kularatne, R. S.; Magurudeniya, H. D.; Sista, P.; Biewer, M. C.; Stefan, M. C. Donor-Acceptor Semiconducting Polymers for Organic Solar Cells. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 743−768. (14) Hawks, S. A.; Deledalle, F.; Yao, J.; Rebois, D. G.; Li, G.; Nelson, J.; Yang, Y.; Kirchartz, T.; Durrant, J. R. Relating Recombination, Density of States, and Device Performance in an Efficient Polymer:Fullerene Organic Solar Cell Blend. Adv. Energy Mater. 2013, 3, 1201−1209. (15) Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H. Aggregation and Morphology Control Enables Multiple Cases of High-Efficiency Polymer Solar Cells. Nat. Commun. 2014, 5, No. 5293. (16) Coffey, D. C.; Reid, O. G.; Rodovsky, D. B.; Bartholomew, G. P.; Ginger, D. S. Mapping Local Photocurrents in Polymer/Fullerene Solar Cells with Photoconductive Atomic Force Microscopy. Nano Lett. 2007, 7, 738−744. (17) DeLongchamp, D. M.; Kline, R. J.; Herzing, A. Nanoscale Structure Measurements for Polymer-Fullerene Photovoltaics. Energy Environ. Sci. 2012, 5, 5980−5993. (18) Van Duren, J. K. J.; Yang, X.; Loos, J.; Bulle-Lieuwma, C. W. T.; Sieval, A. B.; Hummelen, J. C.; Janssen, R. A. J. Relating the Morphology of Poly(p-Phenylene Vinylene)/Methanofullerene Blends to Solar-Cell Performance. Adv. Funct. Mater. 2004, 14, 425−434. (19) Hoppe, H.; Sariciftci, N. S. Morphology of Polymer/Fullerene Bulk Heterojunction Solar Cells. J. Mater. Chem. 2006, 16, 45−61. (20) Yang, X.; Loos, J. Toward High-Performance Polymer Solar Cells: The Importance of Morphology Control. Macromolecules; American Chemical Society, 2007; pp 1353−1362. (21) Dante, M.; Peet, J.; Nguyen, T. Q. Nanoscale Charge Transport and Internal Structure of Bulk Heterojunction Conjugated Polymer/ Fullerene Solar Cells by Scanning Probe Microscopy. J. Phys. Chem. C 2008, 112, 7241−7249. (22) van Bavel, S.; Sourty, E.; de With, G.; Veenstra, S.; Loos, J. Three-Dimensional Nanoscale Organization of Polymer Solar Cells. J. Mater. Chem. 2009, 19, 5388−5393. (23) Ayzner, A. L.; Tassone, C. J.; Tolbert, S. H.; Schwartz, B. J. Reappraising the Need for Bulk Heterojunctions in Polymer Fullerene Photovoltaics: The Role of Carrier Transport in AllSolution-Processed P3HT/PCBM Bilayer Solar Cells. J. Phys. Chem. C 2009, 113, 20050−20060. (24) Pfannmöller, M.; Flügge, H.; Benner, G.; Wacker, I.; Sommer, C.; Hanselmann, M.; Schmale, S.; Schmidt, H.; Hamprecht, F. A.; Rabe, T.; et al. Visualizing a Homogeneous Blend in Bulk

galvanometer voltages such that the laser spot is directed to the physical spot. The 870 nm dichroic is then rotated and the spectrum taken. The BP filters were placed in front of the PMTs to observe spots or regions of SHG in the films. All scanned images and spectra were taken at a power input of 11 mW unless otherwise stated.

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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/acsami.8b11378. Power dependence of a sapphire substrate and PCBM, power ratio of FTAZ and sapphire, power ratio of FTAZ and PCBM, MPI of a sapphire substrate, MPI of FTAZ/ PCBM blend at higher powers, MPI of FTAZ/PCBM with PL included, SEM and AFM of FTAZ/PCBM, AFM of PTB7/PCBM, extra curve fitting parameters (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shai R. Vardeny: 0000-0002-9679-8488 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work at the University of Utah was supported by the AFOSR grant FA9550-16-1-0207; the work at the University of Arizona was supported in part by the Space Exploration and Optical Solutions Technology Research Initiative Fund. Support for the multiphoton microscope work came from a National Science Foundation Graduate Research Fellowship under DGE-1143953 and NSF ECCS under Grant #1610048. We are grateful to Prof. Wei You for supplying the PBnDTFTAZ powder. SEM and AFM measurements were performed at the Surface Analysis Laboratory of the University of Utah. All Multimode AFM images and data were collected in the W.M. Keck Center for nanoscale imaging in the Department of Chemistry and Biochemistry at the University of Arizona. This instrument purchase was supported by the Arizona Technology and Research Initiative Fund (A.R.S.§15-1648).

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REFERENCES

(1) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. High-Efficiency Solution Processable Polymer Photovoltaic Cells by Self-Organization of Polymer Blends. Nat. Mater. 2005, 4, 864−868. (2) Chen, H.-Y. Y.; Hou, J. H.; Zhang, S. Q.; Liang, Y. Y.; Yang, G. W.; Yang, Y.; Yu, L. P.; Wu, Y.; Li, G. Polymer Solar Cells with Enhanced Open-Circuit Voltage and Efficiency. Nat. Photonics 2009, 3, 649−653. (3) Guo, X.; Zhou, N.; Lou, S. J.; Smith, J.; Tice, D. B.; Hennek, J. W.; Ortiz, R. P.; Navarrete, J. T. L.; Li, S.; Strzalka, J.; et al. Polymer Solar Cells with Enhanced Fill Factors. Nat. Photonics 2013, 7, 825− 833. (4) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Photoinduced Electron Transfer from a Conducting Polymer to Buckminsterfullerene. Science 1992, 258, 1474−1476. (5) Brabec, C. J. Organic Photovoltaics: Technology and Market. Sol. Energy Mater. Sol. Cells 2004, 83, 273−292. (6) Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.; Bazan, G. C. Efficiency Enhancement in Low-Bandgap Polymer I


Research Article

ACS Applied Materials & Interfaces Heterojunction Polymer Solar Cells by Analytical Electron Microscopy. Nano Lett. 2011, 11, 3099−3107. (25) Kozub, D. R.; Vakhshouri, K.; Orme, L. M.; Wang, C.; Hexemer, A.; Gomez, E. D. Polymer Crystallization of Partially Miscible Polythiophene/Fullerene Mixtures Controls Morphology. Macromolecules 2011, 44, 5722−5726. (26) Hedley, G. J.; Ward, A. J.; Alekseev, A.; Howells, C. T.; Martins, E. R.; Serrano, L. A.; Cooke, G.; Ruseckas, A.; Samuel, I. D. W. Determining the Optimum Morphology in High-Performance Polymer-Fullerene Organic Photovoltaic Cells. Nat. Commun. 2013, 4, No. 2867. (27) Kesava, S. V.; Fei, Z.; Rimshaw, A. D.; Wang, C.; Hexemer, A.; Asbury, J. B.; Heeney, M.; Gomez, E. D. Domain Compositions and Fullerene Aggregation Govern Charge Photogeneration in Polymer/ Fullerene Solar Cells. Adv. Energy Mater. 2014, 4, No. 1400116. (28) Alekseev, A.; Hedley, G. J.; Al-Afeef, A.; Ageev, O. A.; Samuel, I. D. W. Morphology and Local Electrical Properties of PTB7:PC 71 BM Blends. J. Mater. Chem. A 2015, 3, 8706−8714. (29) Masters, R. C.; Pearson, A. J.; Glen, T. S.; Sasam, F. C.; Li, L.; Dapor, M.; Donald, A. M.; Lidzey, D. G.; Rodenburg, C. SubNanometre Resolution Imaging of Polymer-Fullerene Photovoltaic Blends Using Energy-Filtered Scanning Electron Microscopy. Nat. Commun. 2015, 6, No. 6928. (30) Dyck, O.; Hu, S.; Das, S.; Keum, J.; Xiao, K.; Khomami, B.; Duscher, G. Quantitative Phase Fraction Detection in Organic Photovoltaic Materials through EELS Imaging. Polymers 2015, 7, 2446−2460. (31) Roehling, J. D.; Baran, D.; Sit, J.; Kassar, T.; Ameri, T.; Unruh, T.; Brabec, C. J.; Moulé, A. J. Nanoscale Morphology of PTB7 Based Organic Photovoltaics as a Function of Fullerene Size. Sci. Rep. 2016, 6, No. 30915. (32) Zhao, F.; Li, Y.; Wang, Z.; Yang, Y.; Wang, Z.; He, G.; Zhang, J.; Jiang, L.; Wang, T.; Wei, Z.; et al. Combining Energy Transfer and Optimized Morphology for Highly Efficient Ternary Polymer Solar Cells. Adv. Energy Mater. 2017, 7, No. 1602552. (33) Zhang, Y.; Parnell, A. J.; Pontecchiani, F.; Cooper, J. F. K.; Thompson, R. L.; Jones, R. A. L.; King, S. M.; Lidzey, D. G.; Bernardo, G. Understanding and Controlling Morphology Evolution via DIO Plasticization in PffBT4T-2OD/PC71BM Devices. Sci. Rep. 2017, 7, No. 44269. (34) Lin, Y.; Zhao, F.; Wu, Y.; Chen, K.; Xia, Y.; Li, G.; Prasad, S. K. K.; Zhu, J.; Huo, L.; Bin, H.; et al. Mapping Polymer Donors toward High-Efficiency Fullerene Free Organic Solar Cells. Adv. Mater. 2017, 29, No. 1604155. (35) Yao, H.; Li, Y.; Hu, H.; Chow, P. C. Y.; Chen, S.; Zhao, J.; Li, Z.; Carpenter, J. H.; Lai, J. Y. L.; Yang, G.; et al. A Facile Method to Fine-Tune Polymer Aggregation Properties and Blend Morphology of Polymer Solar Cells Using Donor Polymers with Randomly Distributed Alkyl Chains. Adv. Energy Mater. 2018, 8, No. 1701895. (36) Yan, Y.; Li, W.; Cai, F.; Cai, J.; Huang, Z.; Gurney, R. S.; Liu, D.; Lidzey, D. G.; Pearson, A. J.; Wang, T. Correlating Nanoscale Morphology with Device Performance in Conventional and Inverted PffBT4T-2OD:PC 71 BM Polymer Solar Cells. ACS Appl. Energy Mater. 2018, 1, 3505−3512. (37) Singh, R.; Suranagi, S. R.; Lee, J.; Lee, H.; Kim, M.; Cho, K. Unraveling the Efficiency-Limiting Morphological Issues of the Perylene Diimide-Based Non-Fullerene Organic Solar Cells. Sci. Rep. 2018, 8, No. 2849. (38) Treat, N. D.; Varotto, A.; Takacs, C. J.; Batara, N.; Al-Hashimi, M.; Heeney, M. J.; Heeger, A. J.; Wudl, F.; Hawker, C. J.; Chabinyc, M. L. Polymer-Fullerene Miscibility: A Metric for Screening New Materials for High-Performance Organic Solar Cells. J. Am. Chem. Soc. 2012, 134, 15869−15879. (39) Schroeder, B. C.; Li, Z.; Brady, M. A.; Faria, G. C.; Ashraf, R. S.; Takacs, C. J.; Cowart, J. S.; Duong, D. T.; Chiu, K. H.; Tan, C. H.; et al. Enhancing Fullerene-Based Solar Cell Lifetimes by Addition of a Fullerene Dumbbell. Angew. Chem., Int. Ed. 2014, 53, 12870−12875. (40) McNeill, C. R.; Watts, B.; Thomsen, L.; Belcher, W. J.; Kilcoyne, A. L. D.; Greenham, N. C.; Dastoor, P. C. X-Ray

Spectromicroscopy of Polymer/Fullerene Composites: Quantitative Chemical Mapping. Small 2006, 2, 1432−1435. (41) Collins, B. A.; Gann, E.; Guignard, L.; He, X.; McNeill, C. R.; Ade, H. Molecular Miscibility of Polymer-Fullerene Blends. J. Phys. Chem. Lett. 2010, 1, 3160−3166. (42) Pedersen, E. B. L.; Tromholt, T.; Madsen, M. V.; Böttiger, A. P. L.; Weigand, M.; Krebs, F. C.; Andreasen, J. W. Spatial Degradation Mapping and Component-Wise Degradation Tracking in PolymerFullerene Blends. J. Mater. Chem. C 2014, 2, 5176−5182. (43) Watts, B.; Belcher, W. J.; Thomsen, L.; Ade, H.; Dastoor, P. C. A Quantitative Study of PCBM Diffusion during Annealing of P3HT: PCBM Blend Films. Macromolecules 2009, 42, 8392−8397. (44) Park, L. Y.; Munro, A. M.; Ginger, D. S. Controlling Film Morphology in Conjugated Polymer:Fullerene Blends with Surface Patterning. J. Am. Chem. Soc. 2008, 130, 15916−15926. (45) Gao, Y.; Grey, J. K. Resonance Chemical Imaging of Polythiophene/Fullerene Photovoltaic Thin Films: Mapping Morphology-Dependent Aggregated and Unaggregated CC Species. J. Am. Chem. Soc. 2009, 131, 9654−9662. (46) Müller-Buschbaum, P. The Active Layer Morphology of Organic Solar Cells Probed with Grazing Incidence Scattering Techniques. Adv. Mater. 2014, 26, 7692−7709. (47) Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H. Aggregation and Morphology Control Enables Multiple Cases of High-Efficiency Polymer Solar Cells. Nat. Commun. 2014, 5, No. 5293. (48) Verploegen, E.; Mondal, R.; Bettinger, C. J.; Sok, S.; Toney, M. F.; Bao, Z. Effects of Thermal Annealing upon the Morphology of Polymer-Fullerene Blends. Adv. Funct. Mater. 2010, 20, 3519−3529. (49) Lilliu, S.; Agostinelli, T.; Pires, E.; Hampton, M.; Nelson, J.; MacDonald, J. E. Dynamics of Crystallization and Disorder during Annealing of P3HT/PCBM Bulk Heterojunctions. Macromolecules 2011, 44, 2725−2734. (50) Gomez, E. D.; Barteau, K. P.; Wang, H.; Toney, M. F.; Loo, Y.L. Correlating the Scattered Intensities of P3HT and PCBM to the Current Densities of Polymer Solar Cells. Chem. Commun. 2011, 47, 436−438. (51) Staebler, D. L.; Wronski, C. R. Reversible Conductivity Changes in Discharge-Produced Amorphous Si. Appl. Phys. Lett. 1977, 31, 292−294. (52) Stuckelberger, M.; Despeisse, M.; Bugnon, G.; Schüttauf, J. W.; Haug, F. J.; Ballif, C. Comparison of Amorphous Silicon Absorber Materials: Light-Induced Degradation and Solar Cell Efficiency. J. Appl. Phys. 2013, 114, No. 154509. (53) Eames, C.; Frost, J. M.; Barnes, P. R. F.; O’Regan, B. C.; Walsh, A.; Islam, M. S. Ionic Transport in Hybrid Lead Iodide Perovskite Solar Cells. Nat. Commun. 2015, 6, No. 7497. (54) Aryanpour, K.; Dutta, T.; Huynh, U. N. V.; Vardeny, Z. V.; Mazumdar, S. Theory of Primary Photoexcitations in Donor-Acceptor Copolymers. Phys. Rev. Lett. 2015, 115, No. 267401. (55) Huynh, U. N. V.; Basel, T. P.; Ehrenfreund, E.; Li, G.; Yang, Y.; Mazumdar, S.; Vardeny, Z. V. Transient Magnetophotoinduced Absorption Studies of Photoexcitations in π -Conjugated DonorAcceptor Copolymers. Phys. Rev. Lett. 2017, 119, No. 017401. (56) Kieu, K.; Mehravar, S.; Gowda, R.; Norwood, R. A.; Peyghambarian, N. Label-Free Multi-Photon Imaging Using a Compact Femtosecond Fiber Laser Mode-Locked by Carbon Nanotube Saturable Absorber. Biomed. Opt. Express 2013, 4, 2187− 2195. (57) Säynätjoki, A.; Karvonen, L.; Riikonen, J.; Kim, W.; Mehravar, S.; Norwood, R. A.; Peyghambarian, N.; Lipsanen, H.; Kieu, K. Rapid Large-Area Multiphoton Microscopy for Characterization of Graphene. ACS Nano 2013, 7, 8441−8446. (58) Cromey, B.; McDaniel, A.; Matsunaga, T.; Vagner, J.; Kieu, K. Q.; Banerjee, B. Pancreatic Cancer Cell Detection by Targeted Lipid Microbubbles and Multiphoton Imaging. J. Biomed. Opt. 2018, 23, No. 046501. J


Research Article

ACS Applied Materials & Interfaces (59) Kieu, K.; Jones, R. J.; Peyghambarian, N. Generation of FewCycle Pulses from an Amplified Carbon Nanotube Mode-Locked Fiber Laser System. IEEE Photon. Technol. Lett. 2010, 22, 1521−1523. (60) Jørgensen, M.; Norrman, K.; Krebs, F. C. Stability/Degradation of Polymer Solar Cells. Sol. Energy Mater. Sol. Cells 2008, 686−714. (61) Reese, M. O.; Nardes, A. M.; Rupert, B. L.; Larsen, R. E.; Olson, D. C.; Lloyd, M. T.; Shaheen, S. E.; Ginley, D. S.; Rumbles, G.; Kopidakis, N. Photoinduced Degradation of Polymer and PolymerFullerene Active Layers: Experiment and Theory. Adv. Funct. Mater. 2010, 20, 3476−3483. (62) Grossiord, N.; Kroon, J. M.; Andriessen, R.; Blom, P. W. M. Degradation Mechanisms in Organic Photovoltaic Devices. Org. Electron. 2012, 13, 432−456.

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