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Förster Resonance Energy Transfer in Thick Photoactive Layer of Benzodithiophene-Diketopyrrolopyrrole based Ternary Blend Organic Solar Cell AISWARYA ABHISEK MOHAPATRA, Vincent Kim, Boregowda Puttaraju, Aditya Sadhanala, Xuechen Jiao, Christopher R. McNeill, Richard H. Friend, and Satish Patil ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00896 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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ACS Applied Energy Materials

Förster Resonance Energy Transfer in Thick Photoactive Layer of BenzodithiopheneDiketopyrrolopyrrole based Ternary Blend Organic Solar Cell Aiswarya Abhisek Mohapatra,a Vincent Kim,b Boregowda Puttaraju,a Aditya Sadhanala,b Xuechen Jiao,c Christopher R. McNeill,c Richard H. Friend,b and Satish Patila* a

b

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, 560012, India

Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge, CB3 0HE, United Kingdom c

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

KEYWORDS: Organic photovoltaics, ternary blend organic solar cells, Förster resonance energy transfer, diketopyrrolopyrrole, organic small molecule

ACS Paragon Plus Environment

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ABSTRACT

The use of a ternary blend is a promising strategy to enhance the power conversion efficiency of organic solar cells. However, an active layer thickness of ~ 100 nm is typically required to achieve optimized performance in ternary blend organic solar cells. The efficiency of thicker ternary blend film is limited by the low exciton diffusion length and charge carrier mobility of organic semiconductors, which leads to significant energy loss. In this work, we have employed a thick layer (~ 300 nm) of ternary blend, featuring a donor-acceptor type diketopyrrolopyrrole (2DPP-BDT) based small molecule along with P3HT and PC71BM and established the role of Förster resonance energy transfer (FRET) to improve the power conversion efficiency (PCE). A dramatic enhancement (27%) in PCE was observed for the ternary blend organic solar cell compared to the binary blend solar cell containing P3HT:PC71BM as active layer. The performance enhancement is attributed to the extended light absorption by the ternary blend photoactive layer, which emphasizes the contribution of 2DPP-BDT to harvest photons in nearIR region of the solar spectrum. FRET between P3HT and 2DPP-BDT is found to be crucial in the exciton dissociation process. Steady-state and transient absorption spectroscopy unambiguously established the role of FRET to enhance the device performance. This work highlights the significance of FRET to improve the performance of ternary blend organic solar cells.


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1. INTRODUCTION Among emerging photovoltaic technologies, organic solar cells (OSCs) promise to be low cost and non-toxic.1 OSCs can be fabricated on flexible supports with commonly used methods in plastic manufacturing such as roll-to-roll printing with a small carbon footprint.2,3 A typical OSC relies on the concept of bulk heterojunction (BHJ) where π-conjugated polymer as donor (D) is blended with fullerene derivatives as acceptor (A) in a nanoscale network. The arrangement of BHJ mediates efficient charge separation by providing a large D/A interfacial surface area which overcomes the short exciton diffusion length.4 In BHJ, self-organization of donor and acceptor influences the efficiency of OSCs and, so far, a systematic method to control the nanostructures of BHJ has not been developed. The morphology of BHJ is controlled by various parameters such as thermal annealing, blend composition, solvent viscosity and additives.5 Despite these challenges, the efficiencies of OSCs have steeply increased owing to the development of narrow band-gap semiconducting polymers based on the concept of D-A.6 The high charge carrier mobility of these polymers along with the ability to harvest photons near-IR have led to the improvement in power conversion efficiency of OSCs beyond 10%.7 To maximize the efficiency further and harvest photons in the near-IR region of the solar spectrum, various attempts have been made such as the synthesis of non-fullerene acceptors,8,9 ternary blend solar cells10–14 and hybrid inorganic-organic solar cells.15,16 Among these several strategies, the concept of ternary blend organic solar cells (TBSCs) which contain three organic semiconductors in a single layer has emerged as a promising method to realize the benefit of tandem solar cells in single junction.17 In a tandem solar cell, two cells with complementary absorption spectra are stacked on top of each other to effectively harvest the photons from the visible to near-IR region of the solar spectrum.18 Since the two cells are


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stacked in series, the sum of the open circuit voltages (VOC) of the individual cells is realized in tandem solar cells. However, the complexity of tandem device fabrication forestalls the path towards commercialization. Unlike tandem solar cells, a TBSC incorporates a third component in a single junction to broaden the absorption spectrum and hence simplifies device fabrication.19 Due to the synergistic effect of the third component, a large VOC, suppressed trap-limited recombination, and enhanced light-absorption can be realized in TBSCs.20,21 Regardless of the debate on the origin of open circuit voltage in TBSCs,22,23 many attempts have been made to enhance the VOC. For example, Thompson and co-workers observed a gradual dependence of VOC by systematically modifying the ratio of the third component in TBSCs.24 Yan et al. demonstrated substantial increase in power conversion efficiency by addition of high mobility conjugated polymer as third component in TBSCs.25 In TBSCs, the built-in chemical potential difference between the donor(s) and the acceptor(s) drives the charge separation, simultaneously a parallel FRET process operates along with charge transfer at the heterojunction if necessary conditions are fulfilled. There are few reports of ternary and quaternary blends where FRET between different components in the active layer enhances the PCE significantly as compared to their binary counterparts.26–29 Although PCEs as high as 12% have been achieved for TBSCs in lab-scale,10,11 translating the same efficiencies into large scale roll-to-roll printed solar cells is limited by a low active layer thickness (typically ̴ 100nm) for optimized blends.30,31 In order to realize the full potential of ternary blend photovoltaics, it is important to utilize thicker photoactive layer to achieve upscaling potential of organic solar cells.32 However, very little progress to date has been made combining the virtue of FRET in a thick active layer of ternary blend to realize significant improvement in PCE.


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Figure 1. Chemical structures and frontier molecular orbital energy levels of the three components used in the ternary blend. In view of these considerations, we employed a thicker photoactive layer of ternary blend, consisting of poly(3-hexylthiophene) (P3HT) as one of the donor materials, [6,6]-Phenyl-C71butyric acid methyl ester (PC71BM) as acceptor and the third component, 6,6'-(5,5'-(4,8-bis(5-(2ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(thiophene-5,2-diyl)) bis(2,5-bis(2-octyldodecyl)-3-(5-phenylthiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione) (2DPP-BDT). The chemical structures of the three components are shown in Figure 1. FRET between P3HT and 2DPP-BDT is shown to enhance the PCE of the ternary blend by 27% as compared to that of P3HT:PC71BM binary blend. Decrease in the singlet exciton lifetime of P3HT upon addition of 2DPP-BDT (obtained using time correlated single photon counting measurements) is observed confirming FRET between the two species. Additionally, transient absorption spectroscopy measurements also show clear signatures of FRET between P3HT and 2DPP-BDT. The energy transfer efficiency was monitored by systematic addition of 2DPP-BDT in the blend and was found to be maximum for 1:1 ratio of P3HT to 2DPP-BDT. 2. RESULTS AND DISCUSSION 2DPP-BDT is a diketopyrrolopyrrole-based oligomer and known to exhibit large molar extinction coefficient (Figure S4, SI), suitable energy levels and strong tendency to form J-


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aagregates.33 Based on these properties, we employed 2DPP-BDT as third component in present studies. Synthetic procedures and structural characterization details of 2DPP-BDT are available in the supporting information (SI). The frontier orbital energy levels of the material were obtained by combining UPS data (Figure S5, SI) with solid state UV-visible absorption spectra (Figure S6, SI). The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels were found to be -5.0 eV and -3.4 eV, respectively.

Figure 2. (a) The overlap of emission of P3HT and absorption of 2DPP-BDT which shows the possibility of energy transfer; (b) PL spectra of blend excited at P3HT excitation wavelength showing sensitized emission of 2DPP-BDT upon increase in P3HT concentration, the PL of neat 2DPP-BDT is plotted for comparison; (c) more than 90% quenching of PL of P3HT upon addition of 2DPP-BDT showing efficient charge/energy transfer; (d) wider spectral coverage of


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the ternary blend compared to the binary ones. All these measurements were carried out in solid state at ambient conditions. The UV-visible spectrum of 2DPP-BDT in solution shows maximum absorption at 647 nm which corresponds to a π-π* (HOMO-LUMO) transition (Figure S6, SI). The thin film absorption spectrum is red shifted compared to that of the solution with extended spectral coverage to near-IR region. The emergence of a new peak at 710 nm is attributed to formation of J-aggregates.34,35 The molecule also exhibits large extinction coefficient ~105 cm-1 with an absorption maximum at 647 nm in thin film. The UV-visible spectrum of 2DPP-BDT and emission spectrum of P3HT is shown in Figure 2a. The overlap of the PL spectrum of P3HT and the absorption spectrum of 2DPP-BDT in solid state clearly suggests possibility of FRET between P3HT (FRET donor) and 2DPP-BDT (FRET acceptor). To confirm it further, we conducted a systematic study by varying the ratio of P3HT in 2DPP-BDT. We observed sensitized emission at 767 nm in P3HT:2DPP-BDT blend film by selective excitation of P3HT at 554 nm. The emission at 767 nm corresponds to 2DPP-BDT (Figure 2b) and systematic enhancement of the emission by increasing the concentration of P3HT confirms FRET between the two species. Similarly, PL quenching of P3HT upon addition of 2DPP-BDT further supports FRET between these two materials (Figure 2c). The quenching was maximum (96%) for 1:1 ratio of P3HT to 2DPP-BDT. PL quenching (~52%) of 2DPP-BDT was also observed upon addition of PC71BM in equal ratio (Figure S7, SI). Therefore, the charge transfer is energetically allowed between P3HT and PC71BM as well as 2DPP-BDT and PC71BM. Simultaneously, energy transfer is also clearly seen between P3HT and 2DPP-BDT. These photophysical events in ternary blend leads to efficient exciton dissociation. The energy level diagram of the three components is shown in Figure 1. The absorption of P3HT was


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complemented by the absorption of 2DPP-BDT in the longer wavelength region which extends the spectral coverage of the ternary blend up to 750 nm, as compared to that of the two binary blends as shown in Figure 2d. The Förster distance, R0, at which the energy-transfer efficiency is 50%, is calculated to be 18.2 Å. The detailed calculations are shown in the SI. To establish the FRET between P3HT and 2DPP-BDT pair, we examined the photophysical processes using transient optical absorption (TA) spectroscopy. The ultrafast time-resolved method allows us to simultaneously probe processes occurring in both materials when they are blended together in a film. TA spectroscopy was performed on a film of pristine P3HT, pristine 2DPP-BDT, and a P3HT:2DPP-BDT blend with 50 weight % of 2DPP-BDT. Thin films were excited with a non-collinear optical parametric amplifier (NOPA) that generated broad-band (~100 nm) excitation pulses centered at 525 nm, allowing a time resolution of 50 fs through chirped mirror compression. The laser fluence was set to 6 µJ cm-2, which is within the linear response of P3HT.28 At 525 nm, we selectively excite P3HT so the signal from 2DPP-BDT we observe is due to FRET from P3HT to 2DPP-BDT. The pristine P3HT film contains features previously reported.28 A positive signal at 620 nm represents the ground state bleach (GSB), a negative signal at 650 nm is the photoinduced absorption (PIA) of the excited states, and the positive signal occurring at 710 nm is due to the stimulated emission (SE) of P3HT (Figure 3a). In the pristine 2DPP-BDT film, we observe a positive signal with two peaks at 650 nm and 710 nm representing the GSB. Beyond 750 nm lies a negative signal due to the PIA of the excited states of 2DPP-BDT (Figure 3b).


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Figure 3. Transient absorption spectra of (a) pure P3HT film (6 µJ cm-2), (b) pure 2DPP-BDT film (12 µJ cm-2), and (c) P3HT:2DPP-BDT 1:1 film (15 µJ cm-2) using 525 nm pump pulse, a vertical line across 1 ps is drawn to compare the features before and after. Selected time intervals of these transient absorption spectra are presented in the supporting information (Figure S9). (d) normalized photoluminescence intensities of FRET donor P3HT in a pristine P3HT film and three P3HT:2DPP-BDT blend films of various mixing ratios. PL was measured with timecorrelated single photon counting (TCSPC). Faster decay profiles of the blends as compared to that of P3HT shows energy transfer between the two (the inset). These features also appear in the P3HT:2DPP-BDT 1:1 blend (Figure 3c). Before 1 ps, the signal for this blend contains the GSB of P3HT at 620 nm, the PIA of excited states of P3HT,


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and the stimulated emission of P3HT. After 1 ps, we begin to see the positive GSB signal of 2DPP-BDT increase, which is distinctly shown by the signal at 650 nm changing from negative to positive. At 710 nm, a peak of the GSB of 2DPP-BDT overlaps with the SE feature of P3HT. Because P3HT is selectively excited at 525 nm, much of the signal of 2DPP-BDT is most likely due to energy transfer from P3HT to 2DPP-BDT. A small portion of the 2DPP-BDT might be directly excited, but any signal from 2DPP-BDT is completely overshadowed by the signal of P3HT at times before 1 ps, so it is unlikely that there is a large amount of direct excitation of 2DPP-BDT, which is verified from low absorption cross-section of 2DPP-BDT at the excitation wavelength as evident from the UV-visible absorption spectra (Figure S8, SI). Selected time slices of the transient absorption maps from Figure 3 are shown in the supporting information (Figure S9, SI). Since FRET offers an alternative non-radiative pathway from the donor to the acceptor, increase in concentration of the FRET acceptor will decrease the excited-state lifetime of the FRET donor.36 We measured the excited state lifetime of the donor (P3HT) by observing the time-resolved PL of blends of various donor-acceptor (P3HT:2DPP-BDT) mixing ratios. The longer lifetime component of the excited state was monitored by using time-correlated single photon counting (TCSPC) measurements after the excitation of films at 470 nm. As shown in Figure 3d, we observed that the excited state lifetime of the pure P3HT film (τ = 594 ps) is longer than that of the blends. As we increase the concentration of 2DPP-BDT, we observed decrease in the excited state lifetime of P3HT with a blend ratio of P3HT:2DPP-BDT 1:0.2 (τ = 521 ps), 1:0.5 blend (τ = 463 ps) and further in the P3HT:2DPP-BDT 1:1 blend (τ = 433 ps), thereby confirming the FRET from P3HT to the 2DPP-BDT within few ps. The energy transfer efficiency between P3HT and 2DPP-BDT was calculated by using following equation.36


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1

Where, τDA is the fluorescence lifetime of the donor in donor-acceptor blend and τD is the fluorescence lifetime of the donor in the pristine sample. Using this equation, the P3HT:2DPPBDT 1:0.2, 1:0.5, and 1:1 blends have energy transfer efficiencies of 12%, 22%, and 27% respectively, indicating energy transfer between P3HT:2DPP-BDT system. FRET between P3HT and 2DPP-BDT leads to efficient exciton dissociation and improved device performance as compared to binary blend.

Figure 4. (a) the inverted device architecture used for fabricating all the devices; (b) frontier energy level diagram depicting charge transfer and energy transfer processes upon solar irradiation; (c) PDS spectra of the ternary blend with and without 1 volume % 1chloronaphthalene, emergence of a charge transfer state can be clearly seen in case of sample containing additive; (d) current density-voltage characteristics of the three blends under standard


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AM 1.5G illumination; (e) external quantum efficiency spectra of the three blends. For ternary blend, the shoulder near 710 nm shows the contribution of 2DPP-BDT in generating photocurrent. Organic solar cells were fabricated with an inverted architecture (glass/indium tin oxide (ITO)/ZnO/active layer/MoO3/Ag) using the ternary blend as active layer material (Figure 4a). Binary blend solar cells having P3HT:PC71BM and 2DPP-BDT:PC71BM as active layers were also fabricated as control devices using similar device geometry. Frontier molecular orbital energy level diagram shown in Figure 4b schematically depicts the photophysical processes occurring in the device upon irradiation. The thickness of optimized active layer in ternary solar cell is found to be ~307 nm, measured using Dektak surface profilometer. To see the effect of thickness on device performance, we fabricated TBSCs with thinner active layers by varying the spin-speeds. We observed that thinner active layers, resulted in low current density and thereby poorer power conversion efficiencies (Table S1, SI). The compared J-V curves for the best devices of different active layer thickness is shown in Figure S10, SI. Thicker active layer is one of the desired aspects for roll-to-roll processing of OSCs. Maintaining high fill factor and current density for thicker devices suggests better charge generation and reduced charge recombination losses.37 J-V characteristics of the ternary blend along with binary blends devices of P3HT:PC71BM and 2DPP-BDT: PC71BM are compared in Figure 4d. Device parameters along with the optimum active layer thickness for the three optimized blends are listed in Table 1. We observed ~ 27% increase in PCE for the ternary blend OSC, which has 1 volume % 1-chloronaphthalene (1-CN) as solvent additive, as compared to P3HT:PC71BM solar cell. It is well established in literature that, the solvent additives enhances crystallization of a particular domain by selectively dissolving the other component in blend and plays important


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role in optimized domain formation to increase the power conversion efficiency.38,39 We attribute the enhancement in PCE to increased interfacial area between the donor and acceptor molecules. To resolve and clearly show charge transfer event at heterojunction, we have performed photothermal deflection spectroscopy (PDS). PDS is a highly sensitive absorption measurement technique capable of measuring 4-5 orders of magnitude dynamic range of absorption data. The increase in the charge-transfer (CT) state absorption after addition of solvent additive is evident from the PDS spectra (Figure 4c). We speculate that, the absence of this peak in other binary blends is possibly due to the lower D-A interfacial absorption cross-section as compared to the samples with additive that increase the D-A interfacial absorption cross-section. The empirical relation for OPVs40: Eloss = Eg - qVoc Where, Eg is the lowest bandgap among the donor or acceptors, Voc is the open circuit voltage, q is the charge of electron and Eloss is the photon energy loss in the device. For our ternary blends the lowest bandgap among the three components is 1.6 eV. The Voc is 0.61 V as shown in Table 1. Hence, the energy loss is 0.99 eV. For comparison purpose, the P3HT:PC61BM cell has Eloss of 1.09 eV. Table 1. Photovoltaic characteristics of binary and ternary blend solar cells* Active layer

Thickness

(weight ratio)

(nm)

2DPP-BDT: PC71BM (1:1)

62

Jsc (mA/cm2)

Voc (V)

Fill factor (%)

PCE (%)

Average

Best

Average

Best

Average

Best

Average

Best

0.79 ± 0.01

0.80

2.4 ± 0.03

2.45

53.5 ± 2.3

57.7

1.01 ± 0.06

1.14


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P3HT: PC71BM (1.5:1)

60

0.57 ± 0.01

0.59

8.8 ± 0.44

8.45

59.3 ± 4.1

65.0

2.98 ± 0.25

3.23

P3HT: 2DPP-BDT: PC71BM (0.8:0.2:1) (1 vol % 1-CN)

307

0.59 ± 0.02

0.61

11.9 ± 0.70

11.88

54.5 ± 2.3

56.7

3.80 ± 0.20

4.11

* The statistics in this table are based on 10 individual devices

The VOC for the ternary blend was found to be closer to that of the P3HT:PC71BM device rather than 2DPP-BDT:PC71BM device. By varying the ratio of P3HT to 2DPP-BDT in the ternary blend, we observed that the VOC remains almost constant (Table S2, SI). These results suggest that the HOMO and LUMO levels of P3HT and PC71BM, rather than that of 2DPP-BDT and PC71BM, dictate the VOC of the ternary blend. The external quantum efficiency (EQE) spectra of the three blends are compared in Figure 4e. It corroborates well with the absorption spectra of the blends as shown in Figure 2d. As the percentage of 2DPP-BDT is merely 20% in the optimized blend, its contribution towards total EQE is less but well evident from the spectra in the NIR region. The device parameters of the ternary blend with and without additive are shown in Table S4, SI. The additive improved the crystallinity of active layer, discussed in detail by GIWAXS studies in subsequent section of the manuscript. To find the origin of loss mechanism, we investigated recombination dynamics by measuring the dependence of Jsc on light intensity.41 For PCBM based solar cells, the short-circuit current density, J α Iα, where I refers to intensity of incident light and α is the exponent. The logarithmic plot of JSC dependence on light intensity is shown in Figure S11(a), and the slope, α is found to be 0.97. These results imply that monomolecular recombination is strongly dominated with partial bimolecular recombination. At open circuit conditions, the current is zero; causing all the photo-generated charge carriers to recombine within the ternary blend active layer. If the slope of


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VOC vs. ln(I) is close to 2(kBT/q), i.e. the ideality factor n approaches to 2, then Shockley-ReadHall (SRH) type, trap-assisted recombination dominates.42 As shown in Figure S11(b), the slope is 2.38(kBT/q) and reveals that the SRH type recombination in the ternary blend as a dominant loss mechanism. This could be due to presence of interfacial trap states in the film, acting as reservoir for one-type of carrier and thereby trapping the opposite charge carriers resulting in non-radiative recombination loss. The variation in current density-voltage characteristics with varying light intensity can be seen in the Figure S12, SI.

Figure 5. AFM height images of (a) P3HT:PC71BM; (b) 2DPP-BDT:PC71BM; ternary blend (c) without 1 vol% 1-CN; (d) with 1 vol% 1-CN. The morphology of the active layer was investigated using atomic force microscopy (AFM). As shown in Figure 5(a-d), the root mean square roughness (Rq) of the ternary blend active layer without 1 volume % 1-CN was found to be 3.59 nm which is intermediate between that of 2DPP-


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BDT:PC71BM (4.47 nm) and P3HT:PC71BM (2.09 nm). It is noteworthy that the ternary blend with 1 volume % 1-CN exhibit larger domains with Rq of 4.11 nm (Figure 5d). AFM results reveal that, the additive improved finer mixing of the three components with well-distributed larger domains and this morphology is beneficial to the formation of interpenetrating network for exciton dissociation.

Figure 6: 2D GIWAXS patterns for (a) pure 2DPP-BDT thin film; (b) thermally annealed P3HT:2DPP-BDT thin film; (c) thermally annealed P3HT:PC71BM thin film; (d) as-cast 2DPP-


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BDT:PC71BM thin film and thermally annealed P3HT:2DPP-BDT:PC71BM ternary thin film processed (e) without and (f) with 1-CN additive. The subscript D denotes 2DPP-BDT and P denotes P3HT We further investigated the morphology of ternary blend by transmission electron microscopy (TEM). The binary blends have a discontinuous network of domains (Figure S13: a, b, SI), whereas the ternary blend film with additive exhibits evenly distributed domains across the film (Figure S13: c, d, SI). This optimized morphology promotes charge carrier transport in the vertical direction leading to high PCE. To further study molecular orientation and the crystallization/aggregation of binary and ternary blend films, we performed grazing incidence wide-angle X-ray scattering (GIWAXS) measurements. As demonstrated from the 2D GIWAXS patterns in Figure 6 and Figure S14, SI, thin films composed of different component materials exhibit different diffraction patterns. The neat film of P3HT exhibit an edge-on stacking of crystallites (Figure S14, SI) with features consistent with previous reports.43 The simultaneous occurrence of out-of-plane (OOP) lamellar diffraction (h00) and OOP π-π diffraction (010) for the neat film of 2DPP-BDT (Figure 6a) is suggestive of a rolling-log distribution adopted by the 2DPP-BDT crystallites.44 The lamellar stacking peaks of 2DPP-BDT appear at lower q (higher d-spacing) than P3HT (consistent with the longer side chains of 2DPP-BDT) enabling for distinguishing between P3HT and 2DPP-BDT crystallites. From the GIWAXS pattern of the thermally annealed P3HT:2DPP-BDT binary thin film (Figure 6b) the well-defined multi-order lamellar stacking diffraction from both P3HT and 2DPP-BDT indicates that the component materials have strong tendency to form highly ordered crystallites along the lamellar stacking direction, indicating a high interaction parameter χ

45

where the degree of intermixing between P3HT and 2DPP-BDT is limited. Compared to the


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GIWAXS patterns of pure P3HT and pure 2DPP-BDT, the reduced orientational ordering (higher mosaicity) of P3HT π-π stacking, combined with the disappearance of 2DPP-BDT π-π diffraction, suggests that the intermolecular interaction between P3HT and 2DPP-BDT is dominantly along the π-π direction, which should promote FRET due to the short π-π stacking distance (~3.8 Å). The GIWAXS pattern of the optimized P3HT:PC71BM blend is shown in Figure 6c, with welldefined P3HT OOP lamellar and in-plane (IP) π-π diffraction peaks, along with the appearance of sharp PC71BM diffraction rings. The formation of a speckled diffraction pattern along each individual PC71BM diffraction ring is suggestive of large grains of PC71BM crystallites within the thin film produced by thermal annealing. When blended with PC71BM, 2DPP-BDT shows much reduced crystallinity of 2DPP-BDT (Figure 6d) evidenced by the loss of higher order lamellar diffraction compared to the pure 2DPP-BDT thin film. Meanwhile, the orientational variation of 2DPP-BDT π-π diffraction from predominately OOP in the pure thin film to IP in the 2DPP-BDT:PC71BM blend, combined with the persisted OOP lamellar diffraction suggests that the 2DPP-BDT crystallites adopt edge-on preferential orientation with respect to the substrate in the blend. Lastly, the clear appearance of diffraction from each of component materials in the ternary blend processed without (Figure 6e) and with 1-CN additive (Figure 6f) points out that all three components are able to form phase-separated domains in the ternary blends. Of particular interest is the observation of the considerably enhanced diffraction from 2DPP-BDT relative to P3HT in the ternary thin films processed with 1-CN additive (Figure S15, SI). The improved crystallinity of 2DPP-BDT with the incorporation of 1-CN additive may be attributed to the selective interaction between 1-CN additive and 2DPP-BDT molecules. More


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detailed examination and comparison of the diffraction features can be found from the reduced 1D diffraction profiles in the SI (Figure S16, SI).

3. CONCLUSION In summary, a detailed study to establish FRET between two donor components in the ternary blend solar cell is presented. The transient absorption data of P3HT:2DPP-BDT 1:1 blend clearly showed the GSB of 2DPP-BDT at 710 nm although it was not directly excited, which suggests FRET between the two molecules. The decrease in lifetime of the excited state donor P3HT upon addition of 2DPP-BDT is in accordance with Förster theory. Lower FRET efficiencies suggest that P3HT and 2DPP-BDT domains in the optimized thin-films lie far apart and increased amount of non-radiative recombination as confirmed from the intensity dependent J-V studies. Quantification of the average distance between P3HT and 2DPP-BDT in the optimized blend film is now a subject of investigation. The enhancement of ~ 27% in PCE strongly suggests that 2DPP-BDT contributed to the photocurrent generation through efficient charge and energy transfer with other components. The SRH type trap-assisted charge carrier recombination was found to be one of the major loss mechanisms, potentially caused by the trap-limited disorders present in the active layer due to higher thickness. Our results demonstrate that the thicker active layers in TBSCs can be utilized to enhance the efficiency of organic solar cells and considered as a promising avenue for roll-to-roll production.


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ASSOCIATED CONTENT Materials and methods, synthetic scheme and characterization of the 2DPP-BDT oligomer, details of FRET distance calculation; device fabrication and characterization details, photothermal deflection spectra of blends, TEM images and GIWAXS data. This material is available free of charge via the Internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] URL: http://oesscu.in/ Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT A.A.M. thanks Ms. Baodan Zhao for her help in intensity-dependent photocurrent and photovoltage measurements. A.A.M. thanks Indian Institute of Science, Bangalore for Senior Research Fellowship. V.K. acknowledges funding and support from Gates Cambridge Trust. A.S. and R.H.F. acknowledge the support from EPSRC and Indo-UK APEX phase II project. S.P. thanks Department of Science and Technology, India for Swarnajayanti fellowship and funding through Indo-UK APEX-II Program.


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