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Critical role of Pendant Group Substitution on the Performance of Efficient All-polymer Solar Cells Kedar D. Deshmukh, Shyamal K. K. Prasad, Naresh Chandrasekaran, Amelia C. Y. Liu, Eliot Gann, Lars Thomsen, Dinesh Kabra, Justin M Hodgkiss, and Christopher R. McNeill Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 23 Dec 2016 Downloaded from http://pubs.acs.org on December 25, 2016

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Chemistry of Materials

Critical role of Pendant Group Substitution on the Performance of Efficient All-polymer Solar Cells Kedar D. Deshmukh,† Shyamal K. K. Prasad,‡ Naresh Chandrasekaran,§,|| Amelia C. Y. Liu,⊥,# Eliot Gann,†,∇ Lars Thomsen,∇ Dinesh Kabra,§ Justin M. Hodgkiss,‡ and Christopher R. McNeill*,† †

Department of Materials Engineering Science and Engineering, Monash University, Clayton, Victoria 3800, Australia.

‡

MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, New Zealand. §

Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai, 400076, India.

||

IITB-Monash Research Academy, IIT Bombay, Mumbai, 400076, India.

⊥

Monash Centre for Electron Microscopy, Monash University, Clayton, Victoria 3800, Australia. School of Physics and Astronomy, Monash University, Clayton, Victoria 3800, Australia.

# ∇

Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria, 3168, Australia.

ABSTRACT: Most high-performance all-polymer solar cell systems employ donor polymers with side-groups containing bulky aromatic units. The rationale behind the use of bulky sidegroups in efficient all-polymer systems however is not well understood. In this study, we investigate the doubling of power conversion efficiency in all-polymer solar cells that occurs when substituting the pendant oxygen group in polymer donor PTB7 for thiophene. Specifically, polymer blends using either PTB7 or PTB7-Th as donor with P(NDI2OD-T2) as acceptor are compared. We comprehensively examine the photophysics, morphology and device physics of these two systems and find that PTB7-Th:P(NDI2OD-T2) blends have suppressed geminate recombination and improved charge collection efficiencies compared to PTB7:P(NDI2OD-T2) blends. While the switching of oxygen for thiophene does not have a dramatic effect on blend morphology, the bulky side-group in PTB7-Th helps to destabilize the interfacial charge transfer state, with the five-fold higher hole mobility of PTB7-Th also resulting in improved charge collection.

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INTRODUCTION The efficiency of single-junction polymer solar cells utilizing fullerene acceptors has recently crossed 11% via tuning of aggregation with polymer chemistry.1, 2 While the highest efficiency polymer solar cells still utilize fullerene-based acceptors, there has been renewed interested in so called ‘all-polymer’ solar cells (APSCs) that utilize a polymer acceptor instead of fullerene acceptor. Although APSCs have been around for 20 years since the bulk heterojunction concept was first conceived,3, 4 over the course of time the efficiency of the APSCs has traditionally lagged that of polymer/fullerene solar cells due to a lack of synthetic effort in development of polymer acceptors, poor morphology control and a lack of focus on the specific and sustained development of all-polymer systems.5, 6 This neglect belies the fact the use of a polymer acceptor can confer multiple benefits over fullerene acceptors including improved and tunable optical absorption, tunable energy levels, greater synthetic adaptability, mechanical flexibility and better control over viscosity during roll-to-roll processing.7-10 The lack of suitable ‘n-type’ acceptor polymers with sufficient electron affinity and electron mobility has recently been overcome with the development of low-band gap donor/acceptor co-polymers such as P(NDI2OD-T2).11-16 The issue of non-optimal morphologies has also been overcome with finely-intermixed blends achieved via control of molecular weight, aggregation, the use of solvent additives, and new synthetic strategies such as core and side-chain engineering.11, 12, 17-26 Recently, the strategy of pairing low bandgap donor polymers with low bandgap acceptor polymers with complementary absorption27 has seen a rapid increase in the power conversion efficiency of APSCs from ~ 3% in 2012 to over 8% in 2016.28 Indeed, efficient APSCs are now able to combine high fill factors (up to 0.7) with a high current density.28 A common feature of high efficiency APSCs is the use of donors with bulky pendant units such as thiophene or phenyl units in the polymer side-chains.29-31 A dramatic example of the efficacy of such bulky side-groups is the comparison of solar cells efficiencies achieved using 2 ACS Paragon Plus Environment

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either PTB7 or PTB7-Th as acceptor. PTB7 and PTB7-Th are identical polymers except for the substitution of thiophene for oxygen in the pendant sidechains attached to the benzodithiophene units (see Figure 1 for chemical structures). While solar cells based on PTB7:P(NDI2OD-T2) blends have shown efficiencies of up to 2.7 %,32 PTB7Th:P(NDI2OD-T2) blends have shown efficiencies of up 5.7 %.33 Thus the simple replacement of oxygen for thiophene appears to result in a doubling of solar cell efficiency. In contrast, polymer/fullerene cells made with either PTB7 or PTB7-Th as donor and PC71BM as acceptor show efficiencies of ~ 9.2 % and 10% respectively.34, 35 In this study we investigate the origin of the doubling of efficiency in PTB7Th:P(NDI2OD-T2) blends compared to PTB7:P(NDI2OD-T2) blends. Each system is carefully optimized using a common batch of acceptor polymer, and accounting for differences in the molecular weight of the donor polymers. Then, the various processes which lead to generation of electrical power in the solar cells are systematically examined. Typically, photocurrent generation proceeds via the absorption of photons to produce excitons, the diffusion of such excitons to a donor/acceptor interface to form interfacial electron-hole pairs (also known as charge transfer (CT) states), the separation of these interfacial electron-hole pairs to form free charges and lastly the transport of free charges via drift to the electrodes through phase separated but percolating domains.36, 37 Competing with these various processes required for charge collection are various recombination mechanisms including exciton recombination (the fate of excitons that don’t reach a donor/acceptor interface during their lifetime), geminate recombination (the fate of interfacial electron-hole pairs that are not successfully separated), bimolecular recombination and trap-assisted recombination.38 Here, in order to separately assess these different stages of photocurrent generation and their associated recombination mechanisms, a range of analytical tools are employed, namely optical spectroscopy to assess light absorption, photoluminescence quenching to assess exciton dissociation efficiency, transient absorption spectroscopy to 3 ACS Paragon Plus Environment


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assess the dynamics of charge generation and separation, transient photocurrent analysis to assess charge transport kinetics and space-charge limited current (SCLC) measurements to assess charge mobility. As the efficiency of charge generation, separation and collection processes can be intimately related to the thin-film nano-morphology of the polymer blend, a range of advanced microstructural characterization tools have also been employed to characterize morphology. In particular transmission electron microscopy (TEM) is combined with synchrotron-based techniques such as grazing-incidence wide-angle X-ray scattering (GIWAXS) to probe different critical aspects of morphology from molecular packing on the nanometer scale to the size of phase-separated domains on the length scale of tens to hundreds of nanometers.39-41 These combined studies covering photophysics, device physics and thinfilm microstructure enable identification of the underlying reasons why PTB7-Th outperforms PTB7 providing guidance for future materials development. EXPERIMENTAL SECTION Materials. P(NDI2OD-T2) was purchased from Polyera Corporation (Activink N2200) with Mn= 41 kDa, Ð = 3 (batch no.CZH-XIV-65-22). PTB7-Th (PCE-10, batch no.YY8162) and PTB7 (batch no.YY7248) were both purchased from 1-Materials Inc. with Mn =25 kDa, Ð = 3.6 and Mn = 44 kDa, Ð = 4.3 respectively. Polyethyleneimine ethoxylated (PEIE) and Zinc acetate dihydrite were purchased from Sigma Aldrich. Device fabrication. Devices were fabricated with an ITO/ZnO/PEIE/PTB7-TH or PTB7:P(NDI2ODT2)/MoOx/Ag geometry. The ITO patterned glass substrate (Luminescence Technology Corp. 15 Ω/□ resistance) was first cleaned in acetone for 10 min in an ultrasonic bath, followed by cleaning with isopropyl alcohol. This wet cleaning step was followed by oxygen plasma treatment for 10 min. Approximately 10 nm of zinc oxide was then deposited by spin-coating a precursor solution of zinc acetate dihydrate (0.1 M in 2-methoxy methanol with 1:1 zinc–amine ratio) at 2000 rpm and curing for 15 min in air at 200 °C. A very thin 4 ACS Paragon Plus Environment

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layer of PEIE was then spin coated on the ZnO film from a 0.4 wt% solution in 2methoxyethanol and cured at 100 °C for 15 min. This double layer of ZnO + PEIE was found to be more effective than a single layer of either material. After transferring the substrates to a Nitrogen glovebox the active layer was then spin coated from chlorobenzene for the PTB7Th:P(NDI2OD-T2) blend with a 1:1 weight ratio, or from p-Xylene for the PTB7 P(NDI2ODT2) blend with a 1:1.5 weight ratio. 15 nm of molybdenum oxide was then deposited on top of the active layer by thermal evaporation in vacuum followed by 100 nm of Ag. The devices were encapsulated using glass and epoxy adhesive before removal from the glove box for testing. Device characterization. Device performance under simulated sunlight was performed using a Photo Emission Tech. model SS50AAA solar simulator with the current–voltage curves measured with a Keithley 2635 source meter. The intensity of the solar simulator was set using a calibrated silicon reference cell with a KG3 glass filter (PV Measurements). The output of the solar simulator was characterized with a spectroradiometer (PV Measurements) with a spectral mismatch of 1.01 to 1.03 was determined for PTB7/PTB7-Th:P(NDI2OD-T2) cells. External quantum efficiency (EQE) was measured as a function of wavelength by dispersing light from a tungsten filament (Newport 250 W QTH) through a monochromators (Oriel Cornerstone 130) with a spot size smaller than the device active area. Light intensity of less than 1 mW/cm2 was used with short-circuit current recorded using a Keithley 2635 source meter. The system was calibrated by placing a calibrated photodiode (Thorlabs FDS100CAL) in the device under test position and referencing the intensity measured to that of another silicon photodiode. Steady-state

optical

spectroscopy.

Ultraviolet-Visible

(UV-Vis)

absorption

spectroscopy was performed with use of an Agilent 8453 UV-Visible spectrophotometer. Photoluminescence spectroscopy and photoluminescence quenching measurements were performed with use of a Horiba Jobin Yvon Fluorolog spectro fluorometer. 5 ACS Paragon Plus Environment


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Transient absorption spectroscopy. A detailed description of the system used to carry out the TA spectroscopy can be found elsewhere.42, 43 Broadband probe pulses spanning the visible to near-IR were generated by focussing the 800 nm fundamental laser pulses onto a 3 mm YAG crystal, and pump pulses were generated in a TOPAS-C (light conversion). All data is read out shot-to-shot, whereby differential transmission (ΔT/T) spectra are obtained by comparing alternate probe shots collected with and without the pump, which is chopped at half of the 3 kHz laser frequency. Differential transmission values greater than 2.5 standard deviations away from the mean at each wavelength were removed prior to averaging. This process was completed at each time delay after averaging approximately 2-4 scans to give the final time and wavelength dependent TA surface. Excitation fluences were kept to below a few µJ/cm2 to suppress bimolecular processes, unless intensity dependent effects were being specifically probed. TA surfaces were decomposed into spectral and kinetic components by bilinear decomposition. Singular value decomposition was first used to identify the number of spectral components required. These solutions were then rotated using iterative least-squares fitting such that the exciton spectra match a spectral mask from the neat polymer data, and the kinetic traces were non-negative. Space-charge-limited current (SCLC) measurements. Contacts for hole-only and electron-only devices were carefully selected to ensure efficient injection of the desired carrier and effective blocking of the other carrier. For each material three thicknesses were spin coated to ensure that the current density scaled with the film thickness and that the current is not injection limited. For the hole-only devices, a configuration of ITO/MoOx/Active layer/Au was used. The MoOx layer was 15 nm thick and the Au layer was 30 nm with both deposited via thermal evaporation in vacuo. For electron only devices a configuration of ITO/ZnO/Active layer/Cs2CO3/Al was adopted. The ZnO interlayer was prepared in the same manner as outlined for solar cell preparation. The Cs2CO3 layer 1 nm 6 ACS Paragon Plus Environment

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thick along with 50 nm Al was thermally evaporated on top. Note that different interlayers were used for SCLC devices compared to photovoltaic devices as SCLC devices requires the efficient injection of one charge carrier type and the blocking of the other charge carrier type to ensure space-charge limited current. The fact that the current measured in SCLC devices is space-charge limited implies that the derived mobility values are characteristic of bulk mobility of the measured layers. Space-charge limited current measurements were made on encapsulated devices with current-voltage curves acquired using a Keithley 2635 source meter. All measurements, unless otherwise stated, were performed in forward bias, where the ITO side of the device was biased positively. Measurements were made in the absence of illumination by placing the devices in a dark chamber. The data was analyzed in OriginPro 8 and Matlab by fitting of current voltage curves to the Murgatroyd equation. 44 Transient photocurrent measurements. Transient photocurrent measurements were made by recording the short-circuit current response of devices to a 200 µs square pulse from an LED on an Agilent Technologies InfiniiVision DSO-X 3032A digital oscilloscope (DOSC). Illumination was provided by a Kingbright L-7104VGC-H green led (525 nm wavelength) driven by an Agilent 3522A function generator. Cells were connected in series with the 50 Ω terminated input of the oscilloscope. Light intensity was varied using a set of neutral density filters. Microscopy. TEM measurements were performed at the Monash Centre for Electron Microscopy with a JEOL JEM 2100F FEGTEM operated at 200 kV in bright-field mode using a small (20 µm) objective aperture to enhance mass-thickness contrast with a constant defocus of -10 µm to enhance phase contrast from the phase boundaries. AFM measurements were performed at the Melbourne Centre for Nanofabrication on a Veeco Nanoscope V microscope using ScanAsyst mode.



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Near Edge X-ray Absorption Fine Structure Spectroscopy. NEXAFS Measurements were performed at the Soft X-ray spectroscopy beamline Australian Synchrotron.45 Data was acquired using total electron yield (TEY) mode where the current flowing to neutralize the sample following photoemission is used measure the strength of X-ray absorption. Data were normalized to the incident photon flux using the ‘‘stable monitor method’’ with further details regarding normalization and analysis provided elsewhere.46 For determination of surface composition blend spectra acquired at 55° were fitted to neat reference spectra using singlevalue decomposition. Average tilt angles were determined by measuring angle-dependent NEXAFS spectra and fitting to the expression:47

1 1 = [1 + 3 − 13 − 1] 3 2

Where; I is the area of the π* manifold, θ is the X-ray incidence angle and γ is the average tilt angle of the carbon 1s to π* transition dipole moment. Data were analyzed using the QANT software package.48 Grazing Incidence Wide Angle X-ray Scattering. GIWAXS measurements were performed at the SAXS/WAXS beamline at the Australian Synchrotron.49 9 keV photons were used with data taken with a grazing angle of 0.15° that was above the critical angle of the polymer films but below the critical angle of the substrate. Data was acquired with a Dectris Pilatus 1M detector. Three 1 s exposures were taken with different lateral positions of the detector with these images stiched to remove the gaps between the detector elements. Sampleto-detector distance was calibrated with a silver behenate sample. Data were analzyed using a modified version of the Nika software packing in IgorPro.50

RESULTS AND DISCUSSION Device Performance. The chemical structures of the polymers used in this study and their frontier orbital energies are shown in Figure 1. PTB7 – which is a part of the PTB family 8 ACS Paragon Plus Environment

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– is based on alternating ester substituted thienothiophene and benzodithiophene units and was first reported by Liang et al. in 2010.51 PTB7-Th (also known as PBDTTT-EFT) was also developed initially for use with fullerene acceptors.52,

53

The substitution of oxygen for

thiophene in PTB polymers results in a lowering of the HOMO and LUMO, a red-shifting of the absorption and an improvement in hole mobility.54 P(NDI2OD-T2) (also known as Polyera ActivInkTM N2200] is one of the most widely used polymer acceptors with excellent electron mobility and good absorption in the near IR region.22, 29, 55, 56 In order to ensure a fair side-by-side comparison of PTB7:P(NDI2OD-T2) and PTB7Th:P(NDI2OD-T2) blends, each system was carefully optimized using an inverted device architecture. PTB7-Th-based devices were found to perform slightly better in standard geometry but for a better comparison we have compared devices with the same interlayers and electrodes. For PTB7:P(NDI2OD-T2) blends, a best power conversion efficiency of 2.2 % was achieved with a 1:1.5 weight ratio and p-xylene as the solvent. For PTB7Th:P(NDI2OD-T2) blends a best power conversion efficiency of 4.5 % was achieved using a 1:1 donor:acceptor ratio with chlorobenzene as solvent. We justify the use of different solvents and weight ratios for the two different systems as we are interested in understanding what is limiting the performance of each system, and hence we need to study each system in its optimized state. While the molecular weights for the PTB7 and PTB7-Th batches were different (Mn = 44 kDa for PTB7 and Mn = 25 kDa for PTB7-Th), the performance of PTB7based cells was optimized using a range of different PTB7 batches with molecular weight varying between Mn = 21 kDa to Mn = 89 kDa (see Figure S1). A molecular weight of with different molecular weights was available ensuring that the performance of Mn = 44 kDa was found to optimize the PTB7:P(NDI2OD-T2) system. Thus the lower performance of the PTB7:P(NDI2OD-T2) system is not a result of molecular weight differences between the PTB7-based cell and the PTB7-Th-based cell. The active layer thickness employed for each blend was comparable for optimized devices. 9 ACS Paragon Plus Environment


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Figure 2 presents the device data J-V characteristics and EQE spectra for the PTB7 and PTB7-Th based devices after optimization. Despite having a lower lying HOMO, both cells have similar VOC with a value of 0.78 V for the PTB7-based cell and 0.80 V for the PTB7-Thbased cell. The major differences in performance lie in the JSC and fill factor (FF) values with the PTB7-based cell having JSC = 5.8 mA/cm2 and FF = 0.43 compared to JSC = 10.8 mA/cm2 and FF = 0.50 for the PTB7-based cell. The significantly improved JSC and FF of the PTB7Th device results in it achieving a power conversion efficiency more than double that of the PTB7 device, confirming the results previously reported in the literature. The EQE spectra of the two cells (Figure 2 (b)) show that the PTB7-Th cell has a higher EQE across the visible to NIR spectral range, with a peak EQE of 52% compared to a peak EQE of 29% for the PTB7 cell. The cell parameters for the two different types of device are summarized in Table 1. Optical properties and light absorption. Having established the optimum conditions for device fabrication, the various properties of the active layers as prepared for optimized cells are compared. Figure 3 presents the optical absorption spectra of the neat polymers and their blends. P(NDI2OD-T2) has a broad near IR charge-transfer band from 500 to 850 nm and a higher energy peak at ~ 400 nm corresponding to the π → π* transition. PTB7 has an absorption edge at about 750 nm while PTB7-Th has its absorption onset at about 790 nm as a result of its lower band-gap. The lowest energy vibronic peak for PTB7 occurs at 675 nm with the lowest energy vibronic peak for PTB7-Th occurring at 715 nm. The blend spectra largely resemble a linear combination of the neat spectra with the spectral features of the neat materials reflected in their blends. With the lower band-gap of PBT7-Th compared to PTB7 one may expect improved light harvesting by the PTB7-Th:P(NDI2OD-T2) system compared to the PTB7:P(NDI2OD-T2) system. In order to quantify differences in light absorption, an integrating sphere has been used to measure percentage-absorption in completed devices across the visible to NIR range using a reflection geometry, Figure 4. Measuring absorption in this way also accounts for 10 ACS Paragon Plus Environment

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optical interference effects. Both devices have similar peak %-absorption values with up to 90 % of the light absorbed by the device stack between 600 and 700 nm. Only slight differences in the absorption profiles of the two devices are evident between 700 and 800 nm. To quantify how these differences may impact the overall JSC achievable, these %-absorption profiles were integrated with the AM1.5G spectrum to calculate the maximum JSC achievable assuming all absorbed photons were collected as photocurrent. The two cells have very similar theoretically maximum JSC with a value of 20.1 mA/cm2 for the PTB7 device and a value of 20.2 mA/cm2 for the PTB7-Th device. The similar JSC values can be understood by the PTB7 blend having higher absorption between 750 nm and 800 nm offsetting the lower absorption between 700 nm and 750 nm. The higher acceptor content in the PTB7:P(NDI2OD-T2) cell (weight ratio of 1:1.5 compared to weight ratio of 1:1 used for the PTB7-Th:P(NDI2OD-T2) cell) can be understood in terms of compensating for the lower bandgap of PTB7 compared to PTB7-Th. It is evident from Figure 4 and the analysis above that there are negligible differences in the amount of light absorbed by each cell type. Thus the reason(s) for the stark difference in JSC must be sought elsewhere. Photophysics. Steady-state photoluminescence (PL) measurements have been performed on blends to assess the efficiency of exciton dissociation. Figure S2 in the supporting information shows the PL spectra of blends taken with excitation at 532 nm, 625 nm and 715 nm. These data have been corrected for detector response and for differences in the number of photons absorbed by each of the blends at these excitation wavelengths. The neat donors exhibit emission from about 700 nm to past 850 nm with a characteristic peak at 770 nm for PTB7-Th and 800 nm for PTB7. P(NDI2OD-T2) has much lower emission from 800 nm to past 1100 nm.20 Due to the low emission from P(NDI2OD-T2) it is difficult to infer information regarding exciton dissociation efficiency in the P(NDI2OD-T2) phase. Table 2 summarizes the PL quenching data for the blends for different excitation wavelengths. For both 532 nm and 625 nm excitation PL quenching efficiencies of over 98% are achieved for 11 ACS Paragon Plus Environment


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both blends indicating efficient dissociation of excitons on the donor phase. For 715 nm excitation a slightly lower efficiency of 96.8 % is seen for the PTB7 blend with the PL quenching efficiency of the PTB7-Th blend remaining above 98%. The PL quenching data thus indicate similar levels of exciton dissociation efficiency for the two blends. In order to compare charge generation and recombination kinetics in the two polymer blend systems, transient absorption (TA) spectroscopy was performed. The probed spectral range - spanning the visible to near infrared - allows excitons and charges to be distinguished and quantitatively accounted for from hundreds of femtoseconds to several nanoseconds. The full TA surfaces (Figures S3, S5, Supporting Information) were analysed by a bilinear decomposition to extract spectra (Figure S7) and associated kinetics of excitons and charges (Figures S8, S9), with minimal residuals (Figures S4, S6). In both polymer blend films, the series of TA spectra shown in Figure 5 (a) and (b) can be decomposed into either three or four spectra, which are shown in Figure S7. One spectrum for each blend represents excitons in the donor polymer, which absorbs 58 % of the 700-nm excitation in the PTB7 blend and 70 % in the PTB7-Th blend. The spectra assigned to excitons contribute substantially to the 100 fs spectra in Figures 5 (a) and 5 (b) and match those of neat films of donor polymers (also shown), featuring a ground state bleach above 1.6 eV, photoinduced absorption peaked around 0.8 - 1.0 eV, and characteristic stimulated emission around 1.4 - 1.6 eV. The spectra shown on longer timescales can be assigned to charges, specifically hole polarons in the donor polymer (optical signatures of the acceptor are comparatively much weaker).20 These polaron spectra retain the ground state bleach, but lack stimulated emission, and exhibit a dynamically red-shifting photoinduced absorption peak near 1.1 eV, which is why either two or three spectral components are required to account for charges in the bilinear decompositions for the PTB7 and PTB7-Th blends, respectively. This spectral evolution can be assigned to charges occupying different structural regions of the polymers. In other systems, a similar spectral shift has been attributed to charges migrating to 12 ACS Paragon Plus Environment

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lower energy sites,42 and in some cases to triplet formation.57, 58 In this case, triplet formation is excluded because the spectra do not match triplet reference spectra obtained through sensitizing films of the donor polymers with a platinum porphyrin chromophore (Figure S10). The overall charge generation and recombination dynamics for both blends are shown in Figure 5 (c). This plot is obtained by adding the kinetics of all charge components, using the integrated ground state bleach between 1.65 - 2.2 eV for the PTB7 blend and 1.65 - 2.0 eV for the PTB7-Th blend to internally reference the intensities of all three components, and noting that the high PL quenching efficiencies justify normalizing the peak charge yields to unity. In the PTB7-Th:P(NDI2OD-T2) blend, approximately half of photoexcitations promptly generate charges within the 200 fs instrument response, reflecting excitations that are close to a donor:acceptor interface. A slower phase of charge generation via exciton diffusion then proceeds on the picosecond timescale. Although the weaker optical signatures of the P(NDI2OD-T2) acceptor component mean that the measurement mostly probes excitations in the PTB7-Th donor polymer, we can still conclude the picosecond exciton diffusion kinetics is dominated by the PTB7-Th donor phase; slow hole transfer from P(NDI2OD-T2) to PTB7Th would increase the total population of PTB7-Th-based excitations (excitons + charges), however this quantity only grows by approximately 10% on the picosecond timescale with 700 nm excitation (Figure S8). Charges build up to a maximum (approximately unity) yield at approximately 300 ps, commensurate with the decay of the remaining PTB7-Th excitons (Figure S8). In the PTB7:P(NDI2OD-T2) blend, a higher fraction (~70%) of charges are generated promptly, and the remaining diffusive component is also faster, with the charge population peaking at approximately 20 ps. However, fast charge photogeneration does not necessarily translate to efficient charge extraction; finely intermixed blends (both polymer:polymer

and

polymer:fullerene)

frequently

suffer

from

severe

geminate

recombination losses when their morphology18, 59 or interfacial electronic structures60 restrict charges from escaping the domains in which they were generated. The ratio of prompt and 13 ACS Paragon Plus Environment


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delayed charge generation reflects the balance of intermixed and pure polymer phases, with the PTB7-Th blend apparently having a lower fraction of intermixed regions. This balance is likely an important factor for driving long-range charge separation and suppressing charge recombination. Charges generated in intermixed phases may be driven apart into pure regions, provided that the intermixed phase is not so large that charges remain trapped there.42, 61-63 In the PTB7-Th:P(NDI2OD-T2) blend, half of the photogenerated charges recombine within three nanoseconds. Charge recombination is slightly accelerated at higher excitation fluence (still kept sufficiently low to avoid exciton annihilation), which points to the onset of bimolecular charge recombination (Figure S11). Counter intuitively, this rapid bimolecular recombination is consistent with a high performing system; rapid bimolecular recombination under comparatively high excitation densities from pulsed excitation points towards free charges that are highly mobile – at least locally - even though their recombination can still be slower than charge extraction under weaker solar fluences. In the PTB7:P(NDI2OD-T2) blend, recombination is faster at comparable fluences; half of the charges recombine within approximately 1 ns. However, the similar, if not slightly weaker intensity dependence of recombination in the PTB7:P(NDI2OD-T2) blend (Figure S12) reflects a greater impact of geminate, rather than bimolecular recombination, which is detrimental to the photovoltaic performance of the PTB7-based devices. Similar charge generation and recombination behaviour is seen when changing the excitation wavelength to 600 nm or 400 nm. The main difference is that 400 nm excitation results in a fraction of diffusion limited charge generation from excitation of the P(NDI2OD-T2) acceptor component (Figures S11, S12). Charge transport and collection. Space-Charge Limited Current (SCLC) measurements have been performed to assess the mobilities of the neat materials and the two blends. For hole-only devices, a configuration of ITO/MoOx/polymer/Au was adopted while for electrononly devices a configuration of ITO/ZnO/polymer/Cs2CO3/Al was used. Three different polymer film thicknesses were used for each material to ensure that the current density scaled 14 ACS Paragon Plus Environment

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with the film thickness and that the current was not injection limited. Charge carrier mobility was calculated by fitting the measured J-V curves to the Murgatroyd equation which in turn is derived from the Mott-gurney equation describing space-charge limited current.44 The Murgatroyd equation takes into account the Poole-Frenkel type field-dependence of mobility which is commonly observed in organic semiconductors. Examples of measured and fitted JV curves are presented in the supporting information (Figure S13). Table 3 summarizes the fitted mobility, Poole-Frenkel exponent (γ) along with the dielectric constant used (determined from CELIV measurements) for each material studied are presented. Note that in general, negative γ values are fitted indicating a negative field-dependence of mobility which has been observed before for semiconducting polymers and can be understood in terms of the dominance of spatial disorder over energetic disorder.64 The neat PTB7-Th film was processed from chlorobenzene as for PTB7-Th:P(NDI2OD-T2) blends while the neat PTB7 film was processed from p-xylene as used for PTB7-P(NDI2OD-T2) blends. Data is presented for P(NDI2OD-T2) processed from both chlorobenzene and p-xylene. Considering the hole mobility first, the neat PTB7 film is found to exhibit a hole mobility of 3.1×10-4 cm2/Vs, while the neat PTB7-Th film is found to have a fivefold higher hole mobility of 1.7×10-3 cm2/Vs. These values match the neat film mobility values in the literature12, 33 consistent with the observation that replacing oxygen with thiophene increases the hole mobility of PTB polymers.54 In the blends, the PTB7-Th blend shows a hole mobility of 5.5×10-4 cm2/Vs which is an order of a magnitude higher than the PTB7 blend (5.8×10-5 cm2/Vs). In electrononly devices, neat P(NDI2OD-T2) was found to exhibit a mobility of 3.1 × 10-3 cm2/Vs when processed from p-xylene and 1.7 × 10-3 cm2/Vs when processed in chlorobenzene. In blends, a higher electron mobility is seen for the PTB7 blend (1.2 × 10-3 cm2/Vs) compared to the PTB7-Th (5.8 × 10-4 cm2/Vs) blend. The observation of a higher electron mobility in the PTB7 blend may be explained by the different solvent used for the two blends, and the fact 15 ACS Paragon Plus Environment


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that the PTB7 blend has a slightly higher P(NDI2OD-T2) content. Comparing the two blends, the PTB7-Th:P(NDI2OD-T2) has well-matched hole and electron mobilities at ~ 5 to 6 × 10-4 cm2/Vs while the PTB7:P(NDI2OD-T2) blend has unbalanced mobilities with the hole mobility being twenty times lower than the electron mobility. The hole and electron blend mobility values reported here in general match previously reported values.12, 33, 65 Steady-state measurements such as J-V characterization and external quantum efficiency measurements suffer from the disadvantage of being unable to easily disentangle the various processes that are simultaneously occurring in a solar cell. Time-dependent measurements can provide additional insight in particular with regard to the nature of recombination.66,

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Figure 6 presents transient photocurrent measurements of PTB7 and PTB7-Th based devices, showing the short-circuit response of devices to a 200 µs square light pulse of varying intensity.68 Devices are kept in the dark before and after each pulse with the current traces in Figure 6 showing the turn-on and turn-off dynamics with sufficient time between pulses such that there is no effect of the pulse frequency on the current traces. Observing the response from such a set-up allows turn-on and turn-off dynamics to be studied and in particular how long it takes for the device to reach a steady state after turn-on and after turn-off. Examining the light-intensity dependence of the shape of these curves reveals information about nonlinear processes such as trapping and de-trapping.68, 69 In Figure 6 (b), the PTB7-Th blend shows fast turn on and turn-off dynamics with very little change in the shape of the transient photocurrent curves with light intensity up to 1 sun equivalent. Following turn off and after the fast initial decay, there is a persistent photocurrent tail that is easier to see in the plot of normalizes traces on a log-current scale (see Figure S15 in the supporting information). The relative area under this photocurrent tail decreases with increasing light intensity which can be explained by the slower transport of less mobile carriers in states closer to the band edge (at higher intensities a larger fraction of photoinduced carriers are in mobile states that exit the device quickly).66 In contrast, the 16 ACS Paragon Plus Environment

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PTB7 blend (Figure 6 (a)) exhibits a distinct over-shoot in the in the current turn-on after an initial sharp rise. The severity of this photocurrent overshoot increases with increasing light intensity. Furthermore the photo-current tail in the PTB7 blend is significantly longer than in PTB7 devices and shows a stronger dependence on light intensity, indicating a greater density of less mobile states.66 The initial fast rise and fast decay in both PTB7 and PTB7-Th devices can be explained in terms of fast free-carrier transport while the slower dynamics that are particularly evident in the PTB7 device can be explained by slower trapping and de-trapping processes. After an initial fast turn-on, it takes tens of microseconds (or longer) for the trap population to reach equilibrium for a given light intensity.68 Similarly, after turn off the slower carriers take much longer to detrap and exit the device after the more mobile carriers are quickly swept out.

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The decrease in photocurrent after the initial fast rise in the PTB7-blend is explained by trapmediated recombination. Recombination may occur either by an explicit trap assisted recombination mechanism such as Shockley-Read-Hall70 or via enhanced bimolecular recombination with the buildup of space charge leading to a redistribution of the local electric field that results in increased and overlapping electron and hole distributions.66 In the case of field-dependent charge separation, a redistributed local electric field can also result in reduced charge separation efficiencies.66 In any case it is clear that the slower transport of charge in the PTB7 device facilitates recombination that competes with charge collection. Interestingly it is hole transport that is limiting the performance of the PTB7 device (the PTB7 device has a 20-fold lower hole mobility compared to electron mobility) with previous APSCs being limited by electron trapping rather than hole trapping.68 To further understand the recombination mechanisms at short-circuit; the steady-state current density as a function of light intensity is plotted and fitted to a power law JSC ∝ Iα, Figure 7 (a), where α is the slope of the lines on the log-log plot. The value of α thus parameterizes the severity of intensity-dependent recombination. For the PTB7-Th cell a 17 ACS Paragon Plus Environment


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power law exponent of α = 1.0 is observed over the range of intensities from 1 mW/cm2 to 100 mW/cm2 indicating a lack of intensity-dependent recombination at short-circuit. For the PTB7 cell in contrast a power law exponent of α = 0.90 is found for intensities above 10 mW/cm2. The data of Figure 7 (a) is plotted in Figure 7 (b) in terms of external quantum efficiency vs. light intensity which provides a clearer way of distinguishing and characterizing light intensity-dependent recombination.71 As expected for a power law exponent of α = 1.0 the PTB7-Th cell maintains a constant EQE with light intensity. In contrast the normalized EQE of the PTB7 cell decreases slowly at first (below 10 mW/cm2) and then decreases more dramatically between 10 mW/cm2 and 100 mW/cm2. The shape of the EQE vs. light intensity curve suggests that this light-intensity dependent recombination is occurring predominantly via bimolecular recombination rather than Shockley-Read-Hall (SRH) recombination.71 A decrease in EQE caused by SRH is expected at lower light intensities which then plateaus out at higher intensities.71 In contrast bimolecular recombination exhibits increased severity with increasing light intensity consistent with what is seen here for the PTB7 cell. Of course, charge trapping leading to the buildup of space charge and bimolecular recombination are linked, as shown previously by numerical simulations.66 Thus the slower extraction of holes in the PTB7 cell whether by charge trapping or a lower effective mobility is causing significant non-geminate recombination in the PTB7 cell compared to the PTB7-Th cell. (Note that while the TA results show fast bimolecular recombination in the PTB7-Th blend, these measurements are performed essentially at open-circuit voltage conditions with no extraction of charge possible; thus the high mobility of charges in the PTB7-Th system promote fast bimolecular recombination at open circuit but efficient charge extraction at short circuit.) In explaining the very different JSC values in the two cells it appears that the recombination of separated charges en route to the electrodes is not sufficient to explain the large difference in JSCs recorded by the two cells. At low light intensities (such as under which the EQE spectra were measured) there is still a significant difference in quantum 18 ACS Paragon Plus Environment

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efficiency (29 % max. EQE for the PTB7 cell compared to 52 % max. EQE for the PTB7-Th cell) that is not easily explained by the trap-mediated recombination effects seen. Indeed, while the photocurrent overshoot observed for the PTB7 device appears significant this recombination only accounts for a ~ 15 % decrease in JSC at 1 sun conditions. Thus intensity independent recombination, namely geminate recombination, appears to be the dominant mechanism explaining the stark contrast in operational JSC, consistent with the TA spectroscopy results. Certainly charge transport effects are likely to be influencing cell FF with the lower FF of the PTB7 cell likely to be caused in part by the lower hole mobility in the PTB7 cell compared to the PTB7-Th cell. Field-dependent charge separation however is also expected to be influencing the low measured FFs which will be discussed later. Morphology. To gauge the effect of the change in side-group substituent on thin-film morphology, we have studied the microstructure of the two blends thoroughly using a combination of various techniques. Atomic Force Microscopy (AFM) is employed to image the surface topography along with Transmission Electron Microscopy (TEM) to image the bulk morphology. The synchrotron based Grazing Incidence Wide-Angle X-ray Scattering (GIWAXS) and Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy were employed to examine the film crystallinity and the composition of the top and buried interfaces.39 Figure 8 presents AFM images of the top surface of each of blend along with TEM images taken at 60k magnification. AFM of the surface of the neat polymer P(NDI2OD-T2) shows a distinct fibrillar structure when processed from both p-xylene and chlorobenzene (Figure S16). In contrast, the surface of neat PTB7 and neat PTB7-Th films do not show any particularly distinct features. The AFM images of both the blends exhibit fibrillar features although they are not as distinct as for the neat P(NDI2OD-T2) films. This observation suggests a mixed composition on the top surface with the growth of P(NDI2OD-T2) fibrils at the surface suppressed by the presence of the other polymer. Furthermore the AFM images of 19 ACS Paragon Plus Environment


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the two blends are relatively smooth and indicate a lack distinct phase-separated features. TEM images of the two blends indicate relative well-mixed nanomorphologies with a lack of coarse phase separation seen in other all-polymer systems.72, 73 There are some larger-sized enclosed dark features of the order of tens of nanometers across which could be due to the excess stoichiometric P(NDI2OD-T2) in the blend however the majority of the morphology appears the be well mixed consistent with the PL quenching that indicates that the vast majority of excitons are able to diffuse to and dissociate at a donor/acceptor interface Surface sensitive NEXAFS measurements were performed to study the chemical composition of the top surface and buried interface of the blend. The substrate/film interface was revealed by spin-coating onto a thick, uncured layer of PEIE layer and immediately floating off onto deionized water. The floated film was then picked up on a conductive Si substrate with the previously buried interface now at the top. Total electron yield (TEY) mode was used with a surface sensitivity of ~ 3 nm with the surface composition of blends determined by fitting blend spectra with a linear combination of the neat spectra (see figure S17). The average tilt angle of the C1s to π* transition dipole moment was calculated from the dichroism observed in angle-dependent NEXAFS spectra (please refer to Figure S18 in supporting information).74 Table 4 presents the surface composition values determined from fitting the NEXAFS spectra taken of the blends. In both blends, the top and bottom interfaces are rich in P(NDI2OD-T2). The PTB7 blend has a top surface composition of 79 % P(NDI2OD-T2) with a value of 69 % for the PTB7-Th blend. At the bottom surface the PTB7 blend has 67 % P(NDI2OD-T2) with a value of 88 % P(NDI2OD-T2) for the PTB7-Th blend (see Figure S17). The NEXAFS data corroborates very well with the AFM data, where characteristic P(NDI2OD-T2) fibrils can be seen at the top surface. The PTB7-Th blend has a significantly higher fraction of P(NDI2OD-T2) at the bottom interface compared to the PTB7 blend, however in both cases there is sufficient P(NDI2OD-T2) content to facilitate electron 20 ACS Paragon Plus Environment

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collection in the inverted geometry. Similarly, while the top interface is enriched with P(NDI2OD-T2) there should still be sufficient donor present to aid hole collection. The angle dependent NEXAFS analysis of polymer tilt angles at these interfaces (summarized in table 5) show both donor and acceptor chains exhibit on average a more face-on orientation at the bottom interface, with average tilt angles for the C 1s to π* transition dipole moment of ~ 40°. (An angle of less than 45° corresponds to a preferential face-on orientation, with 0° being perfectly face-on and an angle greater than 45° corresponds to a preferential edge-on orientation, with 90° being perfectly edge-on.) At the top surface the acceptor polymer exhibits a slightly edge-on orientation in both blends, with the donor polymer again being more face-on. Figure 9 presents the two-dimensional GIWAXS images of the two blends. GIWAXS images of neat films are provided in the supporting information (Figure S19) for comparison. Also shown in Figure 9 are one-dimensional line profiles taken along the in-plane (IP) and out-of-plane (OOP) scattering directions along with pole figures of the alkyl stacking (100) peak of the two blends. Both neat films and blends show well-defined scattering features indicating a semi-crystalline morphology consistent with previous reports.55, 75, 76 Both neat and blend films are characterized by alkyl stacking (100) peaks at ~ 0.26 Å-1 found largely in plane and π-π stacking (010) peaks at ~ 1.7 Å-1 found largely out of plane, indicating that the polymers are predominantly packing ‘face-on’ to the substrate. Backbone repeat peaks are also observed for both the polymer blends characteristic of well-ordered polymer blends. Qualitatively the blend GIWAXS images resemble a linear combination of the neat GIWAXS images suggesting that the polymers are packing in a similar fashion in the blend to in neat films. Table 6 presents the results of peak fitting of OOP π-π stacking and IP alkyl stacking peaks. Since the donor and acceptor polymers have similar alkyl stacking and π-π stacking peaks it is hard to separately assess the properties of donor and acceptor crystallites in the 21 ACS Paragon Plus Environment


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blend. The fitted d-spacings therefore are averaged over the two populations (donor and acceptor) with the coherence length not being truly reflective of crystal sizes due to overlapping (100) peaks. These numbers do however enable a more quantitative comparison for the crystalline properties of the two blends. Both the blends show similar d-spacings and coherence lengths based on the combined (100) donor and acceptor peaks. The PTB7 and PTB7-Th blends also have similar d-spacings and coherence lengths for the composite π-π stacking peak. We have also quantified backbone orientation with respect to the substrate using Herman’s orientation parameter, S, listed in Table 6. The Herman’s S parameter is calculated based on the χ-dependence of the (100) peak intensity (Figure 9 (d)) using the following equation:77

χ cos χ sin χdχ 1 = 3 − 1! 2 χ sin χdχ (1)

S runs from 1 for perfectly edge-on crystallites to -0.5 for perfectly face-on crystallites. All films (both neat and blends) exhibit highly face-on orientations with values ranging from S = -0.29 for neat PTB7 to -0.37 for neat PTB7-Th. Both blends have similar S parameters of -031 for PTB7:P(NDI2OD-T2) and -0.32 for PTB7-Th:P(NDI2OD-T2). From the above extensive morphological analysis it is evident that there are only very subtle differences in the two blends, with no key microstructural or morphological markers found that could easily explain the stark differences in cell efficiency. There may be subtle differences in domain purity and size as suggested by the TA measurements, however these are hard to quantify with TEM and GIWAXS. (Resonant soft X-ray scattering has been attempted

however

poor

materials

contrast

and

parasitic

scattering

hampered

characterization.) Interestingly the fact that similar crystalline features are seen in the blend films such as the fibrillar nature of the surface and the crystalline properties measured by 22 ACS Paragon Plus Environment

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GIWAXS suggests that the polymers do not phase separate before crystallizing. Studies on other semicrystalline polymer systems indicate that crystallization of polymer chains suppresses phase separation templating the creation of nanoscale domains for efficient device operation.78 Discussion. From the results presented above, the dominant effect of switching oxygen for thiophene in the donor polymer is the suppression of geminate recombination. While the TA results suggest that the PTB7:P(NDI2OD-T2) blend is less phase pure, the TEM and GIWAXS results combined with the PL quenching results found that the morphologies of the two blends are very similar, characterized by nanoscale phase separation and similar levels of crystallinity. Thus while a slightly coarser morphology or more purer domains could contribute to the improved charge separation in the PTB7-Th:P(NDI2OD-T2) blend, differences in interfacial electronic structure are likely to be significant. In particular Holcombe et al. have shown that for solar cell systems employing either poly(3hexylthiophene) (P3HT) or the more bulky poly[3-(4-n-octyl)-phenylthiophene] (POPT) as donor, POPT-based blends out-performed P3HT-based blends by almost two-fold for a range of acceptors, both polymeric and molecular.79 The conclusion of Holcombe et al. was that the bulky side chain of the POPT donor served to destabilize the thermally relaxed interfacial charge state via increase physical separation of donor and acceptor molecules. Such a conclusion is consistent with the results presented here, although it is the first time that such an effect is prominent in high performance, low-bandgap all-polymer solar cells. This destabilisation of interfacial electron-hole pairs may also lead to an improvement in FF particularly given that interfacial charge separation is likely to be field-dependent given the relatively low FFs observed. The improved charge transport properties of PTB7-Th over PTB7 has effect on the kinetics of subsequent charge collection, but this effect is secondary to the suppression of geminate recombination in determining the overall solar cell performance. Interestingly, the benefit of steric effects appears to trump the influence of the dielectric 23 ACS Paragon Plus Environment


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constant. Increasing the dielectric constant of organic semiconductor materials has been identified as a promising route to increase the efficiency of organic photovoltaic cells.80 From our CELIV measurements PTB7 is found to have a higher relative dielectric constant of εr = 4.3 compared to εr = 4.0 for PTB7-Th. The larger εr for PTB7 can be understood in terms of the higher polarizability of the alkoxy side chain compared to thiophene containing side chain. A promising future strategy therefore could be the use of bulky and highly polarizable side-chains to benefit from steric and dielectric effects.

CONCLUSIONS We have presented a thorough analysis of the effect of introducing bulky side chains on the performance, morphology and photophysics of low-bandgap all-polymer solar cells. From our investigations the most significant effect of switching PTB7 for PTB7-Th is a suppression of geminate recombination at short time scales, explaining the dramatic increase in shortcircuit current at both low and high intensities. The increased hole mobility of PTB7-Th compared to PTB7 also results in improved charge collection with PTB7-based cells exhibiting pronounced trapping effects at short-circuit. Morphological analysis by TEM, GIWAXS and NEXAFS spectroscopy found little different in the morphology and microstructure of PTB7:P(NDI2OD-T2) and PTB7-Th:P(NDI2OD-T2) blends, with both blends exhibiting a nanoscale semicrystalline morphology. The main effect of the bulkier side group is therefore on the conformation of chains at the donor/acceptor interface, with a greater interfacial separation donor and acceptor chains in the PTB7-Th:P(NDI2OD-T2) blend serving to destabilize the charge transfer state, in line with the conclusions of Holcombe et al.79

ASSOCIATED CONTENT


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Supporting Information. Device performance of PTB7:P(NDI2OD-T2) cells as a function of PTB7 molecular weight; photoluminescence data; supporting transient absorption spectroscopy data; SCLC data; CELIV data; additional transient photocurrent graphs; AFM images of neat films; NEXAFS spectroscopy data; GIWAXS data of neat films; This material is available free of charge via the Internet at http://pubs.acs.org.



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AUTHOR INFORMATION Corresponding Author * Email: [email protected]

ACKNOWLEDGMENT This work was performed in part on the Soft X-ray and SAXS/WAXS beamlines at the Australian Synchrotron, Victoria, Australia. This work was also performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). The authors acknowledge use of facilities within the Monash Centre for Electron Microscopy. CRM acknowledges support from the Australian Research Council (FT100100275). JMH and SKKP acknowledge support from a Rutherford Discovery Fellowship to JMH.


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Figure 1. Chemical structures of PTB7, PTB7-Th, P(NDI2OD-T2) and frontier energy levels. 51, 53


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Figure 2. Device performance of optimized PTB7:P(NDI2OD-T2) and PTB7-Th:P(NDI2ODT2) bulk heterojunction devices: (a) Current voltage characteristics; (b) EQE spectra.



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Figure 3. Ultraviolet-visible and near IR absorption spectra of the neat polymer films and their blends: (a) PTB7, P(NDI2OD-T2) and PTB7:P(NDI2OD-T2) processed from p-xylene; (b) PTB7-Th, P(NDI2OD-T2) and PTB7-Th: P(NDI2OD-T2) processed from chlorobenzene.


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Figure 4. Percentage-absorption spectra of optimized PTB7:P(NDI2OD-T2) and PTB7Th:P(NDI2OD-T2) devices acquired in an integrating sphere.



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Figure 5 (a) Transient absorption spectra of a PTB7-Th:P(NDI2OD-T2) blend film at the times indicated after excitation at 700 nm (0.61 µJ/cm2), along with a TA spectrum of the neat PTB7-Th donor polymer. Spectra are normalized according to the peak signal for the earliest spectrum. (b) Transient absorption spectra of a PTB7:P(NDI2OD-T2) blend film at the times indicated after excitation at 700 nm (1.4 µJ/cm2), along with a TA spectrum of the neat PTB7 donor polymer. (c) Kinetics of charge generation and recombination for the two blends, extracted from a bilinear decomposition of exciton and charge populations from the same measurements shown in parts (a) and (b). Further details about data processing are provided in the Experimental Section.


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Figure 6. Short-circuit transient photocurrent curves of optimized PTB7:P(NDI2OD-T2), (a), and PTB7-Th:P(NDI2OD-T2), (b), cells in response to a 200 µs square pulse of varying intensity.



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Figure 7. (a) Dependence of steady-state short circuit current on light intensity. The data is fitted to JSC ∝ Iα. (b) Linear-log plot of normalized EQE vs. light intensity.


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Figure 8. AFM images (a, c) and TEM images, (b, d) of PTB7:PNDI2OD-T2 (a, b) and PTB7-Th:PNDI2OD-T2 (c, d) blends.



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Figure 9. Two-dimensional GIWAXS images of optimized PTB7:P(NDI2OD-T2), (a), and PTB7-Th:P(NDI2OD-T2), (b). blends. Part (c) shows one-dimensional scattering profiles taken along the in-plane (IP) and out-of-plane (OOP) scattering directions. Part (d) presents the pole figures of the (100) lamellar stacking peaks.


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Table 1. Summary of device performance parameters for optimized PTB7:P(NDI2OD-T2) and PTB7-Th:P(NDI2OD-T2) bulk heterojunction devices. Blend

JSC [mA/cm2]

FF

VOC [V]

PCE [%]

Max. EQE [%]

PTB7:P(NDI2OD-T2)

5.8±0.2

0.43±0.1

0.78

2.1± 0.2

29

PTB7-Th: P(NDI2OD-T2)

10.8±0.2

0.50±0.2

0.80

4.4 ± 0.3

52

Table 2. Percentage photoluminescence quenching values for both the blends for different excitation wavelengths. Blend

532 nm

625 nm

715 nm

PTB7:P(NDI2OD-T2)

98.7 %

98.6 %

96.8 %

PTB7-Th:P(NDI2OD-T2)

98.8 %

98.5 %

98.3 %

Table 3. Space-charge limited current hole (µhole) and electron (µelectron) mobilities of neat polymers and blends with their respective relative dielectric constants, ε. Layer

µhole [cm2/Vs]

γhole

PTB7

3.1 × 10-4 -3

PTB7-Th

1.6 × 10

µelectron [cm2/Vs]

γelectron

ε

-5.5 × 10-4

N/A

N/A

4.27 ± 0.09

-4

N/A

-5.0 × 10

N/A -3

3.99 ± 0.08 -3

P(NDI2OD-T2) from XY

N/A

N/A

3.1 × 10

P(NDI2OD-T2) from CB

N/A

N/A

1.7 × 10-3

-1.4 × 10-3

3.83 ± 0.07

-3

-4

4.03 ± 0.09

-1.7 × 10-4

3.92 ± 0.05

-5

PTB7:P(NDI2OD-T2)

5.8 × 10


5.5 × 10-4

-2.9 × 10

-4

-3.5 × 10-4

1.2 × 10

-1.4 × 10

-2.4 × 10

5.8 × 10-4

-

Table 4. Average composition of the top and bottom interfaces of PTB7:P(NDI2OD-T2) and PTB7-Th:P(NDI2OD-T2) blends measured by TEY NEXAFS spectroscopy. Blend

Wt.% of P(NDI2OD-T2) at top surface

Wt.% of P(NDI2OD-T2) at bottom surface

PTB7:P(NDI2OD-T2)

79 ± 1

69 ± 1


68 ± 1

88 ± 2



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Page 42 of 43

Table 5. Data of the average tilt angles of C 1S to π* in the polymers in the blends. Blend

Top Surface

Bottom Surface

Donor

Acceptor

Average

Donor

Acceptor

Average

PTB7:P(NDI2OD-T2)

40.1 ± 0.9°

54.2 ± 0.4°

50.6 ± 0.4°

40.4 ± 0.9°

40.4 ± 0.6°

40.4 ± 0.6°


37.5 ± 1.0°

46.5 ± 0.5°

42.8 ± 0.5°

37.1 ± 1.8°

40.4 ± 0.9°

38.8 ± 0.4°

Table 6. Summary of parameter extracted from line-cut profiles and polar plot for neat polymers and the blends Layer

In-plane (100) peak

Out-of-plane (010) peak

S-parameter

d-spacing [nm]

Coherence Length [nm]

d-spacing [nm]

Coherence Length [nm]

PTB7

2.02 ± 0.02

3.26 ± 0.01

0.37 ± 0.01

1.61 ± 0.01

-0.37

PTB7-Th

2.35 ± 0.01

7.91 ± 0.01

0.39 ± 0.01

2.54 ± 0.01

-0.29

P(NDI2OD-T2) from XY

2.44 ± 0.01

22.3 ± 0.01

0.37 ± 0.01

4.22 ±0.03

-0.34

P(NDI2OD-T2) from CB

2.43 ± 0.02

16.1 ± 0.03

0.38 ± 0.02

3.57 ±0.02

-0.34

PTB7:P(NDI2OD-T2)

2.36 ± 0.02

10.0 ± 0.02

3.37 ± 0.01

2.08 ± 0.01

-0.31


2.37 ± 0.01

9.47 ± 0.01

0.37 ± 0.01

2.23 ± 0.01

-0.32


Page 43 of 43

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