Corrole-BODIPY Dyad as Small-Molecule Donor for Bulk

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Functional Inorganic Materials and Devices

Corrole-BODIPY Dyad as Small Molecule Donor for Bulk Heterojunction Solar Cell Ruchika Mishra, Biju Basumatary, Rahul Singhal, Ganesh D. Sharma, and Jeyaraman Sankar ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08519 • Publication Date (Web): 23 Aug 2018 Downloaded from http://pubs.acs.org on August 23, 2018

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ACS Applied Materials & Interfaces

Corrole-BODIPY Dyad as Small Molecule Donor for Bulk Heterojunction Solar Cell Ruchika Mishra,1 Biju Basumatary,1 Rahul Singhal,2 Ganesh D Sharma,*3 and Jeyaraman Sankar*1 1

Department of Chemistry, Indian Institute of Science Education and Research (IISER) Bhopal,

Bhopal Bypass Road, Bhauri, Bhopal, Madhya Pradesh , India - 462066 2

Department of Physics, Malviya National Institute of Technology, Jaipur, Rajasthan, India -

302031 3

Department of Physics, The LNM Institute of Information Technology (A Deemed University),

Jamdoil, Jaipur, Rajasthan, India - 302031 KEYWORDS. Organic solar cell, Corrole, BODIPY, low energy loss, Porphyrinoid.

ABSTRACT. Dyes based on charge-transfer (CT) characteristics are attractive candidates for organic photovoltaics due to their intense and broad absorption window. In these molecular frame works, electron-rich donor and electron-deficient acceptor are covalently linked to achieve an effective CT process. Corrole, a tetrapyrrolic congener of porphyrin, is an excellent example of an electron rich molecule with a large molar extinction coefficient. BODIPY, on the other hand, is a well-known electron-deficient bypyrrolic boron difluoride complex with intense

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absorption complimentary to the corrole. A combination of these two structural motifs should results in a dyad having wide absorption window, which will be suitable for OPVs. Herein, a corrole derivative has been envisaged as an efficient donor for solution processed bulk heterojunction solar cell with PC71BM as an acceptor for the first time. The current molecule exhibits broad absorption in the visible range in solution as well as in thin films, with high molar extinction coefficient and a low band gap of 1.79 eV. Frontier molecular orbital energy levels were found to be complimentary with those of the well-known acceptor PC71BM. The optimized devices based on Cor-BODIPY:PC71BM showcased high power conversion efficiency (PCE) of 6.6 % with Jsc = 11.46 mA/ cm2 , Voc = 0.90 V and FF = 0.61. A remarkable value of incident photon to current conversion efficiency (IPCE) of 61% has also been observed.

Introduction Organic photovoltaics have emerged as promising renewable energy source to address rising energy demand and environment related issues.1-6 Currently, power conversion efficiencies (PCEs) of solution processed bulk heterojunction(BHJ) solar cells have exceeded more than 11%, which is indicative of a bright future for these solar cells.7-19 So far, conjugated polymer donors along with fullerene and non-fullerene acceptors have indeed yielded the best PCEs of 13 %

20,21

and 14 %

22

for binary and ternary BHJ, respectively. However, polymers have certain

inherent demerits associated with their average molecular weight and polydispersity index. Therefore, it is essential to find an efficient, mechanically and electronically flexible, chemically robust and economically viable alternative to polymer-based BHJSCs. An ideal substitute should be, (i) readily accessible and processable to give uniform thin films, (ii) should offer architectural flexibility and (iii) should be robust to ensure durability. Nonetheless, there are several key


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requirements to be met by small molecules for utilization in BHJSCs such as broad and intense absorption, good solubility, high charge mobility, thermo-chemical stability and fairly strong intermolecular interactions etc. In this context, solution processable small molecules (SMs) are highly commended substitutes for polymers in bulk heterojunction solar cells (BHJSCs) due to the advantage of flexibility in design, low cost, defined molecular weight and easily tunable energy levels.23-31 Noteworthy to mention here that the PCEs of the newly developed SM-OSCs have reached 11.53 %, which is comparable to that of polymers.32 Due to the structural resemblance to the natural light harvesting antenna complexes, synthetic porphyrinoids are attractive candidates for photovoltaic applications where efficient light harvesting is the key step. They have large absorption coefficients and excellent excited state properties such as energy transfer and electron transfer due to their delocalized ߨ-circuit. Many synthetic models of porphyrinoids have been developed as photoactive materials for OPVs. In 2014, Peng and co-workers reported a PCE of about 7% from a small molecular donor based on Zn-porphyrin.33 Henceforth, efforts are being guided to identify and develop novel tetrapyrrolic pigments, which are structurally and functionally resemble the porphyrins. Corrole macrocycles are a class of contracted tetrapyrrolic systems, structurally similar to the corrin ligand in vitamin B12. Corroles are aromatic and have photophysical characteristics parallel to that of porphyrins.34-35 Also, their optical and electrochemical properties can be systematically tailored either by appropriate peripheral functionalization or varying metals at the core.36-40 Peripheral functionalization often involves introduction of conjugated chromophores that extend the ߨ- framework. This subsequently leads to the enhancement in solar light harvesting. Moreover, recent synthetic developments allow us to access these novel molecules in gram scales, much akin to porphyrins, along with much more structural flexibility. Another


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promising approach to maximize light harvesting is to use auxiliary chromophores onto the periphery. Such functionalized corroles have been successfully employed in many applications such as catalysis, sensors, biomedical imaging and also in dye sensitized solar cells.41-44 Surprisingly, unlike porphyrins, corroles have rarely been utilized for small molecule organic solar cells in spite of their excellent photophysical properties. In one of the earlier work, we have shown that BODIPYs (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) are one of the most suitable auxiliary pigments for corroles.44-45 They possess numerous complimentary properties such as excellent absorption in the visible region, high photochemical stability, high hole carrier mobility and flexible structure. BODIPYs can be tethered to the corrole periphery in a variety of ways and a complementary energy/electron transfer can be modulated. Taking cue from our previous studies on this class of molecules, we propose that such systems can be of potential use in BHJOSCs. Herein, we choose a judiciously designed Cor-BODIPY donor for small molecular bulk heterojunction solar cells.

This dyad is readily soluble in common organic solvents and

possesses strong intermolecular interactions in the solid state. Ga(III)corroles are structurally robust and have interesting photophysical features. Therefore they can be effective modules for organic photovoltaic investigations. Cor-BODIPY showed two strong absorption bands in thin films,centered at 432 nm and 516 nm corresponding to Soret band of corrole and BODIPY, respectively. The large molar extinction coefficient of this conjugate in thin films (0.92 x 105 M-1 at Soret band) further corroborates its suitability for OPV applications Moreover, the conjugate possesses a broad absorption in the 550- 640 nm wavelength region, corresponding to Q- bands. Cor-BODIPY, exhibits highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy level around -5.37 eV and -3.46 eV, respectively and these


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values are suitable for as donor component along with the PC71BM as acceptor (HOMO and LUMO energy levels are -6.10 eV and -4.2 eV, respectively), indicating that the HOMO offset and LUMO offset values are sufficient for the exciton dissociation and charge transfer for BHJ OSCs. After the optimization, i.e., the blend showed hole and electron mobilities of 1.13 x10-4 cm2/Vs and 2.45 x10-4 cm2/Vs respectively with electron to hole mobility ratio of 2.17. When combined with PC71BM as an acceptor, the dyad exhibited excellent photovoltaic characteristics after the optimization with a PCE of 6.60%, demonstrating the suitability of corrole derivatives in this direction. RESULTS AND DISCUSSIONS Experimental Section Synthesis: The molecule was synthesized as per the reported procedure by our group (Scheme 1).45 The dyad was further fully characterized by spectroscopic techniques. The details of synthesis and characterization are given in the electronic supporting information (ESI).


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Scheme 1. Synthesis of Cor-BODIPY dyad (4). (i) GaCl3 (10 equiv.),pyridine, reflux, 2h under Ar, (ii) DMF/POCl3 at 0 °C, DCM, RT, 10 min; H2O/K2CO3, overnight, (iii) TFA/CH2Cl2, Ar, 4h. (iv) DDQ/CH2Cl2, 30 min; DIPEA/BF3·OEt2, RT, 2h.[Note that after the reaction (ii), the second pyridine ligand was attached during column chromatography with the eluent containing pyridine (see ESI)]. Absorption and Emission Studies: The Cor-BODIPY dyad showed excellent solubility in common organic solvents. The ground state absorption spectrum was recorded in CHCl3 (Figure 1). Careful examination of the spectrum reveals three absorption bands in the range of 400-650 nm. The absorption maximum at ~420 nm is assigned to the Soret band while region between 560-650 nm well complies with the Q bands of corrole. Absorption maximum at 506 nm corresponds to the BODIPY component. The spectrum is simply an overlay of absorption of individual components i.e. corrole and BODIPY absorption, hinting at negligible ground state coupling . Also, the current dyad absorbs intensely in the whole visible region (390-640 nm) of solar spectrum unlike its individual chromophores.


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Figure 1. UV-Visible absorption spectrum of Cor-BODIPY in comparison to ref. BODIPY and Ga (III) Corrole (in CHCl3) . UV-Visible spectrum recorded on thin film (Figure 2) displayed a bathochromic shift in the absorption. Soret band for corrole moiety was observed at 432 nm with absorption maxima for BODIPY unit at 516 nm. Q bands were found to be red shifted in the range of 550-700 nm. The optical bandgap of the Cor-BODIPY estimated from the onset of the absorption spectra in thin film is about 1.79 eV. Eonset (Red) V

Eonset (Red)

HOMO

LUMO

λmaxabs

ε * 105

(eV)

(eV)

(Solution)

Lmol-1Cm-1

V -1.03

0.86

(nm)

-5.37

-3.46

419 (Soret)

1.6

506 (BOD)

0.76

560-650 (Q-bands) Table 1. Photophysical and electrochemical data of Cor-BODIPY. The dyad exhibited a corrole centered emission which is highly dependent on the polarity of the solvent. It is evident from the spectra (Figure S8, Table S1) that the molecule is highly


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emissive in CHCl3 but in high polar solvents like THF and DMSO, the fluorescence is quenched. This can be attributed to a possible photoinduced elctron transfer (PET) from corrole to BODIPY unit. 45

Figure 2. Comparison of UV-Visible Absorption spectrum of Cor-BODIPY in thin film and in solution. Computational and Electrochemical Investigations: For further inputs, frontier orbital contributions were taken from DFT computations. Theoretical calculations suggest that in Cor-BODIPY, the HOMO is mainly localized on corrole fragment while LUMO is delocalized onto the BODIPY which is evident from ground state absorption spectra lacking any donor-acceptor interaction (Figure 3). Electrochemical studies were carried out in dichloromethane with tetrabutylammonium hexafluorophosphate as the


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supporting electrolyte. Compared to BODIPY and corrole, the dyad displays a positive shift in reduction as well as oxidation potentials.45

Figure 3. Frontier orbitals as obtained from DFT studies at B3LYP/6-31G* (H, B, C, N, F) + LANL2DZ(Ga) level in vacuum and, energy levels as calculated from cyclic voltammetric studies. The HOMO and LUMO energy levels of the Cor-BODIPY deduced from the onset of oxidation and reduction potentials are -5.37 eV and -3.46 eV, respectively (Figure 3). The lower lying HOMO energy level may assist in obtaining higher Voc from the corresponding BHJ-OSC devices. On the other hand, better LUMO offset between Cor-BODIPY and PC71BM (higher


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than the threshold value of~0.3) is particularly beneficial for the efficient charge separation at D/A interfaces and transport in the BHJ active layer.46 Photoluminescence Measurements: In order to confirm the photo induced charge transfer in the active layer, steady state photoluminescence (PL) measurements were performed on the pristine Cor-BODIPY and its blend with PC71BM in thin film (Figure 4).

Figure 4. PL spectra of (a) pristine Cor-BODIPY, (b) as cast Cor-BODIPY:PC71BM and (c) SVA treated Cor-BODIPY:PC71BM thin films. When excited at 450 nm, a strong emission peak of Cor-BODIPY was observed at 624 nm. After blending with PC71BM, the PL peak is strongly quenched, indicating an efficient


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photo-induced charge separation in the blend active layer.

The PL quenching efficiency,

estimated from the ratio of integrated emission between the blend film and pristine Cor-BODIPY are 84 % and 93 % for as cast and SVA treated active layers respectively; validating more efficient charge separation in the SVA treated film. Photovoltaic Properties: In order to evaluate the photovoltaic properties of Cor-BODIPY, the BHJ OSCs were fabricated with the conventional configuration of ITO/PEDOT:PSS/Cor-BODIPY:PCBM/PFN/Al, where PFN (poly[9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene]) is used as cathode interlayer. The active layer was optimized by varying donor (Cor-BODIPY) to acceptor (PC71BM) weight ratio. The thickness of the as cast and SVA treated active layer is 90 ±5 nm. The details of the device fabrication are described in supplementary information.

(b) (a)

Figure 5. (a) Current-voltage characteristics under illumination and (b) IPCE spectra of the OSCs based on as cast and SVA treated Cor-BODIPY:PC71BM active layers.


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The current–voltage (J-V) characteristics under illumination and incident photon to current conversion efficiency (IPCE) spectra of the OSCs upon difference processing conditions are presented in figure 5. Corresponding photovoltaic data are summarized in Table 2. The devices based on as cast Cor-BODIPY: PC71BM with 1:1.5 blend ratio in active layer showed overall PCE of 2.54% with a short circuit current (Jsc) of 6.89 mA/cm2, an open circuit voltage (Voc) of 0.945 V and fill factor (FF) of 0.39. With the motive of enhancing the performance of the OSC, we have employed solvent vapour annealing technique (SVA) using THF solvent. This technique has been found to improve the PCE of the OSCs as reported earlier. 47-51 SVA treatment indeed put forth positive influence on the PCE of our devices. The current-voltage characteristics of the OSC based on SVA treated active layer is shown in Figure 5a and corresponding photovoltaic parameters are summarized in table 2. Active layer

Jsc

Voc (V)

FF

PCE (%)

(mA/cm2) CorBODIPY:PC71BM

BODIPY:PC71BM

μe

(cm2/Vs)

(cm2/Vs)

6.89

0.95

0.39

2.54

5.34

2.35

(±0.06)

(±0.02)

(±0.02)

(2.45±0.09)a

(±0.15)

(±0.09)

x10-5

x10-4

(as cast) Cor-

μh

11.46

0.915

0.63

6.60

1.13

2.45

(±0.07)

(±0.014)

(±0.03)

(6.53±0.07)a

(±0.11)

(±0.12)

x10-4

x10-4

(SVA)

Table 2. Photovoltaic parameters of the OSCs based on Cor-BODIPY:PC71BM active layers as cast and after SVA treatment. a average of eight individual devices


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After SVA treatment of the active layer for 40 s, the PCE of the OSC improved to 6.60 % with simultaneous increase in Jsc (11.46 mA/cm2) and FF (0.63) with an average PCE of 6.53% as calculated for an average over eight individual devices. IPCE spectra of the BHJOSCs based on Cor-BODIPY was recorded before (as cast) and after solvent annealing and is shown in figure 5b. From the figure it can be envisaged that incident photon to current conversion efficiency values are significantly improved after SVA treatment. The IPCE spectra closely resembles with the absorption spectra of the blend indicating that both Cor-BODIPY and PC71BM contribute to the photogeneration of excitons in the active layer and thereby photocurrent generation in the OSCs. The IPCE values for the OSC based on SVA treated active layers are much higher and broader as compared to as cast counterpart, indicating that photon to electron conversion is significantly enhanced. The devices fabricated without SVA exhibited IPCE of about 40 % for the range of 350 nm to 600 nm. However for SVA treated devices, IPCE was improved to 55% for the range of 350 nm to 550 nm. The maximum efficiency of 61 % was obtained in the region 600 nm to 720 nm. This is most probably responsible for the higher value of Jsc after SVA. The Jsc values calculated from the integration of IPCE spectra of the devices are 6.78 mA/cm2 and 11.34 mA/cm2 for OSCs based on as cast and SVA treated active layers, respectively. The Jsc values are consistent with those obtained from the J-V characteristics under illumination. Even though significant enhancement was observed in Jsc, FF and PCE, the Voc is slightly reduced after SVA treatment. This slight reduction in Voc can be accredited to either an enhanced crystalline ordering in electron donor or lowering in quasi-Fermi levels for electron and hole transport due to a depleted steady carrier density . 51-52


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In order to verify the effect of SVA on the charge transport in the active layers, we compared the hole and electron mobilities in hole only (ITO/PEDOT:PSS/active layer/Au) and electron only device (ITO/Al/active layer/Al), respectively before and after SVA treatment (Figure 6). The average values of hole and electron mobility (measured from five devices with different thickness) computed from space charge limited current (SCLC) method is displayed in table 2. The average value of hole mobility is considerably enhanced from 5.34 x10-5 cm2/Vs (as cast) to 1.13 x10-4 cm2/Vs after solvent vapor annealing.

(a)

(b)

Figure 6. Current–voltage characteristics of (a) hole only and (b) electron only devices based on as cast and SVA treated Cor-BODIPY:PC71BM thin films. However, average electron mobility did not show much improvement even after SVA and found to be slightly increased to 2.45 x10-4 cm2/Vs. The ratio of electron (μe) to hole mobility (μh) was decreased from 4.40 for as cast to 2.17 for SVA treated active layers, respectively. Consequently, the improved values of hole mobilities and reduced electron to hole mobility (μe / μh) ratio contribute to the enhancement in the charge transport within the active layer, hence, leading to the higher values of Jsc, FF and PCE.


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To understand the effect of SVA treatment on the charge generation, recombination and extraction properties in the OSCs, the variation of photocurrent (Jph) with effective voltage (Veff) was analyzed and shown in Figure 7. The Jph is given by Jph=JL-JD, where JL and JD are the current densities under illumination and in dark, respectively. Here, Veff is defined as the difference between Vo and Va, where V0 is the voltage at Jph = 0 and Va is the applied voltage. After SVA treatment, Jph increases rapidly at low voltages and shows plateau when Veff is around 0.63. It saturates at high Veff indicating that all the photogenerated excitons are already dissociated into free charge carriers and are efficiently collected by the electrodes. It also hints that internal electric field plays a minor role during the charge extraction.

Figure 7. Variation of Jph with Veff for OSCs based on as cast and SVA treated CorBODIPY:PC71BM thin films. Contrary to the above observation, for as cast devices, the Jph starts to saturate at much higher value of Veff. The ratio of the Jph to the saturation current density (Jphsat) i.e. Jph /Jphsat,


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under short circuit condition of the OSCs represents the exciton dissociation efficiency (Pdiss)

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and the values of Pdiss for the OSCs based on as cast and SVA treated active layers are about 0.87 and 0.92, respectively. This further corroborates that exciton dissociation is more efficient for SVA treated active layer OSC than as cast counterpart due to the better phase separation. Further, the Jph/Jphsat values at maximum power output corresponds to the charge transport and collection efficiency (Pcc) 54 and are 0.71 and 0.78 for as cast and SVA treated active layer OSC, respectively suggesting that charge transport and collection in the SVA treated OSCs is more efficient, substantiating the outcomes of mobility measurements. In general, at high value of Veff, the efficiencies of exciton dissociation and charge collection approaches to unity and the Jphsat mainly depends on the exciton generation rate (Gmax) which is expressed as Jphsat = qLGmax, where q is the elementary charge and L is the thickness of the active layer.

The Gmax is

calculated to be 6.9×1027 m-3 s-1 (Jphsat = 9.92 mA/cm2) and 7.9x1027 m-3s-1 (Jphsat= 11.31 mA/cm2) for the OSCs based on as cast and SVA active layers, respectively. The enhanced Gmax indicates that more excitons are generated in the active layer (particularly in the Cor-BODIPY donor) due to enhanced light absorption spectra after the SVA treatment (Figure 5b).


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(a)

(b)

Figure 8. Variation of (a) Jsc and (b) Voc with illumination intensity (Pin) for OSCs based on as cast and SVA treated Cor-BODIPY:PC71BM thin films. In order to get information about the difference in the recombination kinetics in the OSCs based on as cast and SVA treated active layers, we have measured the J-V characteristics at different illumination intensities55-56 and extracted the values of Jsc and Voc. The variation of the Jsc and Voc with the illumination intensity is displayed in the figures 8a and 8b, respectively. The extent of bimolecular recombination can be estimated from the dependency of Jsc on illumination γ

intensity and is described as J sc ∝ Pin . When the value of γ is unity, the bimolecular recombination to be negligible whereas the smaller values of γ indicate the competition between the recombination and charge extraction. The value of γ estimated from the Figure 8a for as cast OSC is 0.83, indicating significant bimolecular recombination losses. After SVA treatment of active layer, this value was increased to 0.94, suggesting that the bimolecular recombination is suppressed. The variation of Voc with Pin is shown in Figure 8b. When the slope of Voc vs Pin curve is about 1kT/q, where k is the Boltzmann’s constant, T is temperature in Kelvin and q is the


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electronic charge, the main loss mechanism is bimolecular recombination, whereas when the slope is equal to 2kT/q, related to Shockley-Read recombination attributed to trap-assisted recombination .57 For the devices based on Cor-BODIPY:PC71BM with and without SVA, the values of slopes are 1.23 kT/q and 1.41 kT/q, respectively, suggesting considerable trap-assisted recombination involved in the as cast device under open circuit condition, which is partially suppressed after SVA treatment.

Figure 9. TEM image of as cast and SVA treated Cor-BODIPY:PC71BM thin films. (Scale bar is 100 nm).


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Figure 10. AFM topography images of as cast and SVA treated Cor-BODIPY:PC71BM thin films. The surface morphology of the active layer plays crucial role in determining the overall PCE of the OSCs. We have investigated the surface morphology of the as cast and SVA treated active layers by transmission electron microscopy (TEM). As shown in the Figure 9, compared to as cast active layer, better phase separation was found for SVA treated counterpart, forming more bi-continuous interpenetrating networks, which is beneficial for charge transport efficiency, leading to improved Jsc and FF. As shown in the AFM images (Figure 10), the surface roughness of the active layer has been enhanced upon the SVA treatment (2.5 nm for as cast to 3.8 nm for SVA treated), indicating that the crystallization of active layer is improved upon the SVA treatment. This is beneficial for the charge transport thus leading the enhanced value of hole mobility.


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Figure 11. X-ray diffraction patterns of as cast and SVA treated Cor-BODIPY: PC71BM thin films. In order to explore the change in the crystallinity and molecular ordering after SVA treatment, the

morphology of

BHJ thin films were

investigated using X-ray diffraction

measurements of the as cast and SVA treated Cor-BODIPY:PC71BM active layers in thin film. As shown in figure 11, both the active layers exhibited a strong (100) diffraction peak at 2θ = 5.23° which corresponds to the lamellar distance of 1.82 nm. On the other hand, the (010) diffraction peak was observed at 2θ = 22.91 and 23.34° for as cast and SVA treated active layers, respectively corresponding to the π-π stacking distance of 0.38 nm and 0.34 nm, respectively. In addition to these two peaks, a weak diffraction peak around 2θ =18° is also observed in both films, which corresponds to PC71BM.58 This indicates that the after the SVA treatment, the CorBODIPY form a denser molecular packing than the as cast counterpart. Also, the SVA treated active layer showed stronger diffraction peaks corresponding to both (100) and (010), which indicates the enhancement in the degree of crystallinity after the SVA treatment. The higher crystalline behaviour and reduction in π-π stacking distance after SVA treatment may induce a


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better nanoscale phase separation which is beneficial for charge transport and collection.59 These results in turn account for the higher FF and improved PCE of the OSC. Conclusion In summary, current study introduces corrole derivatives as new and promising donor platform for BHJ solar cells.

The Cor-BODIPY derivative exhibited good photovoltaic

performance when used as a donor, employing PC71BM as an acceptor in solution processed small molecule OSCs. After the optimization of donor to acceptor weight ratio (1:1.5) and solvent vapour annealing time (40 s), the OSC showed overall PCE of 6.60 % which is significantly higher than that of as cast active layer. SVA treatment not only reduces the charge recombination but also helps in the formation of better nano-scale morphology of active layer, which in turn is essential for exciton generation and their dissociation into free charge carriers. The present outcomes suggest that corroles can be promising motifs in organic photovoltaic technology at low cost and will open up new avenues in the direction of developing highly efficient solar cells.

ASSOCIATED CONTENT Complete details of synthesis and characterization, additional spectra, computational details etc. are provide in ESI. Corresponding Authors * [email protected], [email protected]


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript Funding Sources RM thanks CREST-IISER Bhopal, MHRD-FAST, New Delhi for fellowship and BB & JS thank DST-SERB EMR/2016/005768 for funding. GDS and RS thank DST–SERI for funding. ACKNOWLEDGMENT We thank IISER Bhopal for providing infrastructure. RM thanks CREST-MHRD and BB thanks UGC, New Delhi for fellowship. ABBREVIATIONS OSC, organic Solar Cell, BHJ, Bulk Heterojunction Cell, OPVs Organic photovoltaics. REFERENCES (1) Brabec, C.; Scherf, U.; Dyakonov, V. Organic Photovoltaics: Materials, Device Physics, and Manufacturing Technologies. 2nd ed. Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2014. (2) Brabec, C.; Dyakonov, V.; Parisi, J.; Saritiftci, N. S. Organic Photovoltaics. Concepts and Applications. Springer: Berlin; Heidelberg, 2003. (3) Krebs, F. C.;Espinosa, N.; Hösel, M.; Søndergaard, R. R.; Jørgensen, M. Rise to Power – OPV-Based Solar Parks. Adv. Mater. 2013, 26, 29-39. (4) Service, R. F. Outlook Brightens for Plastic Solar Cells. Science 2011, 332, 293. (5) Heeger, A. J. Bulk Heterojunction Solar Cells: Understanding the Mechanism of Operation Adv. Mater. 2014, 26, 10-28. (6) Li, G.; Zhu, R.; Yang, Y. Polymer Solar Cells. Nat. Photonics 2012, 6, 153–161. (7) Yao, H.; Ye, L.;

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Corrole-BODIPY Dyad as Small-Molecule Donor for Bulk

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