Nonfullerene Polymer Solar Cells Reaching a 9.29% Efficiency Using

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Non fullerene Polymer Solar Cells Reaching a 9.29% Efficiency Using a BODIPY-thiophene Backboned Donor Material Léo Bucher, Nicolas Desbois, Pierre D Harvey, Claude P. Gros, Rajneesh Misra, and Ganesh D. Sharma ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00535 • Publication Date (Web): 15 Jun 2018 Downloaded from http://pubs.acs.org on June 22, 2018

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ACS Applied Energy Materials

Non-fullerene Polymer Solar Cells Reaching a 9.29% Efficiency Using a BODIPY-thiophene Backboned Donor Material Léo Bucher,[1,2] Nicolas Desbois,[1] Pierre D. Harvey,[2] Claude P. Gros,*[1] Rajneesh Misra,[3] Ganesh D. Sharma*[4] [1]

ICMUB (UMR CNRS 6302), Université de Bourgogne Franche-Comté - 9, Avenue Alain

Savary - BP 47870, 21078 Dijon Cedex, France. e-mail: [email protected] [2]

Department of Chemistry, Université de Sherbrooke, 2500, Boul. de l’Université, J1K 2R1

Sherbrooke, QC, Canada. [3]

Department of Chemistry, Indian Institute of Technology, Indore, MP, 452020, India

[4]

Department of Physics, LNM Institute of Information Technology, Rupa ki Nagal, Jamdoli,

Jaipur 302031, Rajasthan, India. e-mail: [email protected]

1 ACS Paragon Plus Environment

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ABSTRACT: A conjugated polymer donor containing BODIPY-thiophene dyads in the backbone, P(BdP-EHT), combined with a low bandgap non-fullerene acceptor (SMDPP) consisting of carbazole and diketopyrrolopyrrole units linked with a tetracyanobutadiene acceptor π-linker, was used to design bulk heterojunction polymer solar cells. After the optimization of the donor to acceptor weight ratio and solvent vapour annealing of the P(BdP-EHT):SMDPP active layer, the resulting polymer solar cell showed an overall power conversion efficiency of 9.29%, which is significantly higher than that for the polymer solar cell based on PC71BM (7.41%) processed under identical conditions. This improved power conversion efficiency is attributed to enhanced values of short circuit photocurrent and open circuit voltage, the better light harvesting efficiency of the P(BdP-EHT):SMDPP active layer in the near infrared region, and the higher Lowest Unoccupied Molecular Orbital (LUMO) energy level of the SMDPP as compared to PC71BM, combined. Moreover, energy loss in the device based on P(BdP-EHT):SMDPP active layer is significantly low (0.48 eV) as compared to P(BdP-EHT):PC71BM counterpart (0.78 eV). Since the P(BdP-EHT) consists of triple bond, a linker may be beneficial for the stability of the polymer solar cells.

KEYWORDS: organic photovoltaics, polymer solar cell, bulk heterojunction, non-fullerene acceptor, BODIPY, conjugated polymer


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1. INTRODUCTION Polymer solar cells (PSCs) based on bulk heterojunction (BHJ) active layers have attracted a significant attention for energy conversion technology due to their simple low-cost fabrication method, light weight, flexibility and large scale production.1-2 The BHJ active layer consists typically of physically intermixed polymers or small molecule electron donors and fullerene acceptors to create a solid state phase separation with the appropriate interfaces for charge generation and transport.3-6 After exhaustive investigations on the control of the morphology of the active layer via thermal/solvent vapor annealing, solvent additive, interface engineering and designs of new conjugated polymers or small molecule donors, the overall power conversion efficiency has reached 12-13% for single BHJ based PSCs using fullerene derivatives as acceptor.7-9 Although the fullerene derivatives feature a high electron mobility and good miscibility with most of the donor materials, their intrinsic drawbacks such as low absorption in the visible region of solar spectrum as well as high costs have restricted their applications in large scale production of stable PSCs.10-14 In addition to these above drawbacks, the PCE of the PSC based on fullerene acceptors is also limited due to their larger voltage loss and insufficient charge generation, since high driving force is required for charge separation.15 Over last three years, substantial efforts have been devoted to develop non-fullerene small-molecule acceptors (NFSMA) to overcome these drawbacks, taking advantage of their their distinctive properties such as strong absorption in visible to near infrared region of solar spectrum, easy tailoring of frontier energy levels and low cost of synthesis and purification.5, 16-17 Moreover, in addition to the high absorption coefficients of NFSMA, the energy levels can be easily adjusted to be compatible with the polymer donors and can also reduce production cost of PSCs.18-19 In 2015, Zhan and coworkers have reported the first example of SMA having an acceptor–donor–acceptor (A-D-A) molecular structure,20-21 after that many SMA have been developed21-26 and PCE over 12% based on BHJ PSCs have been reached in a span of two years.20-21,

27-29

Recently, Hou et al. have

reported a stimulating research work with a record high PCE of 13.1%,8 suggesting the sustainability of PSCs based on NFSMA. Moreover, compared to fullerene based PSCs, the decrease in voltage loss (Eloss) in NFSMA based PSCs is possible due to the small ∆HOMO or ∆LUMO values between donor and acceptor, and allows for a more efficient exciton dissociation and charge transfer in BHJ active layer.30 This feature would permit the most advantageous tradeoff between short-circuit current (Jsc) and open-circuit voltage (Voc) and



thus maximizing the overall PCE of the PSCs. Noteworthy, PCE of 14% have been recently reported for PSCs using a non-fullerene acceptor and PC71BM based on ternary active layer.31 In the case of PSCs, the matching rules between polymer donors and NFSMAs, following criteria should be adopted: (i) complementary absorption between donor and acceptor materials to enhance the light harvesting, (ii) matched frontier energy levels to ensure efficient charge transfer after exciton dissociation and to minimize the Eloss, and favorable morphology including high crystallinity and proper phase separation with appropriate domain size. In order to achieve high efficient PSCs, it is not only important to design the high performance NFSMA, but also to develop matching donor polymers. For this reason, large efforts have been devoted to the development of donor polymers, especially for NFSMA based PSCs. Moreover, in order to achieve high PCE of the devices, polymers need to exhibit intense and large absorption bands extending to the longer wavelength region of solar spectrum and also fulfill the requirements of appropriate Highest Occupied Molecular Orbital (HOMO)/Lowest Unoccupied Molecular Orbital (LUMO) energy levels for efficient charge separation and transport. In this respect, BODIPY represents a class of promising chromophores due to its large variety of structural modifications for the tailoring of the target polymers or small molecules.32 Incidentally, numerous BODIPY-based polymers have been synthesized in the past for applications taking advantage of their outstanding absorption and emission properties.33-37 This molecular unit has also been used several times in high charge carrier mobility materials for organic electronics.38-41 These properties inexorably placed BODIPY as a strong candidate for the design of novel electron donor materials for applications in organic solar cells. The first reports of solar cell designs using BODIPY moieties were made in early 2010’s and showed moderate although then promising PCEs up to 2.0% and 4.7% for polymer and small molecule donors, respectively.42-43 Nevertheless, very efficient BODIPY-based solar cells have been reported this last couple of years with impressive PCEs,38,

44-45

as high as 8.8% for polymers,46 showing that the BODIPY could

indeed play a major role as building block in efficient PSC’s. We now wish to report the synthesis of a new copolymer P(BdP-EHT) (Chart 1) bearing alternating electron-deficient BODIPY and electron-rich thiophene units linked by ethynyl bridge and its photophysical and electrochemical properties were investigated. The use of ethynyl bridges to connect BODIPY with thiophene is motivated by its electron withdrawing nature and by the cylindrical-like π-electron density that is known to lower the


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HOMO energy level of P(BdP-EHT). Moreover, ethynyl linkers are also recognized as one of the typical π-linker unit, because it could not only increase the rigidity of the molecules, but also potentially improve the molecular planarity, thus facilitating intra and intermolecular interactions47-48 and charge carrier mobility. We have used P(BdP-EHT) as donor for the design of solution processed PSCs in combination with a NFSMA denoted as SMDPP and compared the photovoltaic response with PC71BM based PSCs processed under identical experimental conditions. The structure of SMDPP is presented in Chart 1. The synthesis as well as physical, electrochemical and photovoltaic properties of this NFSMA have been previously reported.49-50 SMDPP have already been used as an efficient electron acceptor in PSCs and was chosen in the present work since it exhibits complementary absorption spectra with P(BdP-EHT) and well-matched HOMO (−5.58 eV) and LUMO (−3.83 eV) frontier energy levels. After the optimization of active layer morphology PSC based on P(BdPEHT):SMDPP showed overall PCE of 9.29% is significantly higher than PSC employing PC71BM acceptor (7.41%). Moreover, the energy loss for the PSC based on SMDPP is quite low (0.48 eV) as compared to the PC71BM counterpart (0.83 eV). To the best of our knowledge, this PCE of 9.29%, with low energy loss of 0.48 eV, is the highest among the PSC based on BODIPY based copolymer.

Chart 1: Chemical structures of P(BdP-EHT) donor and SMDPP acceptor

2. EXPERIMENTAL SECTION The BODIPY 151 and thiophene 252 building blocks have been synthesized according to previously reported procedures. All reagents were ordered from Sigma-Aldrich and used as received. The decomposition temperature of P(BdP-EHT) was evaluated from the TGA traces and corresponds to 5% of the weight loss of the sample. More experimental details about electrochemistry and fluorescence quantum yield measurements are given in the SI. The synthesis of P(BdP-EHT) and the details for the device fabrication and characterization are also given in the SI. 5 ACS Paragon Plus Environment


3. RESULTS AND DISCUSSION 3.1. Synthesis and characterization. P(BdP-EHT) material was synthesized using a pallado-catalyzed Sonogashira cross-coupling between bis-ethynyl BODIPY 1 and dibromothiophene 2 (as shown in Scheme 1). The experimental conditions used for this synthesis were the same as previously reported by us,46 using tetrakis(triphenylphosphine)palladium(0) along with copper(I) iodide and triethylamine in dry THF. The BODIPY 151 and thiophene 252 building blocks were prepared as described in the literature beforehand. The resulting dark blue polymer was isolated in 91% yield and identified by 1H NMR spectroscopy. P(BdP-EHT) featured a decomposition temperature of 358 °C (see Figure S1). This high thermal stability made this new material suitable for photovoltaic applications. The material was found very soluble in common organic solvents such as CH2Cl2, CHCl3 or THF. However, P(BdP-EHT) showed a quite low degree of polymerization according to GPC experiments with a Mn of 6000 g.mol-1. This result was surprising compared to other structurally-close polymers we synthesized in the past which exhibited Mn around 30000 g.mol-1.46,

53

It is worth noting that the only differences between these materials lie in the

nature of the alkyl chains and the use of the bromo-thiophene instead of the iodo derivative.

Scheme 1: Synthesis of the P(BdP-EHT) material.

3.2. Optical and electrochemical properties. The normalized absorption spectra of the P(BdP-EHT) in dilute chlorobenzene (CB) solution and as thin film cast from CB are presented in Figure 1. The absorption spectrum of P(BdP-EHT) in solution exhibits a strong absorption band ranging from 500 nm to 700 nm centered at 624 nm. This feature corresponds to the expected S0 → S1 transition of the BODIPY chromophore exhibiting an expanded π-conjugation. In addition to this band, a weaker absorption band (∼350-500 nm) is depicted, which agrees to the BODIPY-type S0 → S2 transition.54 The absorption spectrum of P(BdP-EHT) as thin film displays a minor red shift and broader profile compared to that 6 ACS Paragon Plus Environment

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observed in solution. This phenomenon is most probably indicative of some controlled structure in solid state, and may also be due to π-π interactions, which could also be conveniently favorable for exciton dissociation and charge generation in the resulting PSCs.55 The absorption onset of P(BdP-EHT) in thin film is positioned at 718 nm, which corresponds to optical bandgap of 1.73 eV. The use of an A-π-D (A = acceptor and D = donor) NFSMA, SMDPP, consisting of a carbazole (D) and a diketopyrrolopyrrole (A) units linked with tetracyanobutadiene acceptor π-linker, was made.49 The absorption spectrum of SMDPP in solid state is shown in Figure 1 and exhibits strong absorption band in the wavelength range 600 - 850 nm, complementary to P(BdP-EHT).

Figure 1: Normalized optical absorption spectra of P(BdP-EHT) in chlorobenzene solution (black trace) and as thin film (red trace) and SMDPP as film (blue trace).

Cyclic voltammetry (CV) was employed to determine the HOMO and LUMO energy levels of the P(BdP-EHT) (Figure 2a). The CV trace of P(BdP-EHT) exhibits one irreversible oxidation wave and one irreversible reduction signal both corresponding to the BODIPY core. HOMO and LUMO energy levels of P(BdP-EHT) are found around −5.45 eV and −3.69 eV, respectively. The energy level diagram of the P(BdP EHT), SMDPP and PC71BM are shown in Figure 2b. Noteworthy, this copolymer shows deep HOMO energy levels, which is normally preferred for good air stability and elevated Voc in PSCs. Moreover, the energy gaps of the LUMOD − LUMOA (= ∆LUMO, "D" means electron donor and "A" electron acceptor) and HOMOA − HOMOD (= ∆HOMO) for P(BdP-EHT):PC71BM and P(BdPEHT):SMDPP and blends are ~0.55 and 0.14 eV and ~0.46 and 0.13 eV, respectively. The values of ∆LUMO and ∆HOMO energy offsets between P(BdP-EHT) and SMDPP are generally too low for exciton dissociation to surmount the exciton binding energy (~0.3 eV).56 7 ACS Paragon Plus Environment


Although some reports reveal that the energy offsets below 0.3 eV is also sufficient for exciton dissociation and charge separation using non-fullerene acceptors.27, 30, 57-61

Figure 2: (a) CV trace of P(BdP-EHT) as thin film in 0.1 M TBAPF6 MeCN solution (scan rate = 50 mV.s-1) (b) Energy level diagram of donor and acceptor materials estimated from CV experiments, PC71BM frontier orbitals energy values are taken from literature62 In order to get information about the exciton dissociation and charge transfer in the P(BdP-EHT):SMDPP active layer with such low values of LUMO and HOMO offsets, we have measured the photoluminescence (PL) spectra of P(BdP-EHT), SMDPP and P(BdPEHT):SMDPP thin films (Figure 3). Indeed, over 88 and 91 % photo-luminescence (PL) quenching of SMDPP acceptor and P(BdP-EHT) donor, respectively was observed for the P(BdP-EHT):SMDPP film. The efficient PL quenching indicates that indeed efficient exciton dissociation and charge transfer occur between P(BdP-EHT) and SMDPP i.e. holes from SMDPP to P(BdP-EHT) and electrons from P(BdP-EHT) to SMDPP, even at low values of ∆ELUMO and ∆EHOMO for the P(BdP-EHT):SMDPP active layer.


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Figure 3: PL spectra of P(BdP-EHT) at ߣex = 620 nm (black trace), SMDPP at ߣex = 760 nm (blue trace), and P(BdP-EHT):SMDPP blend at ߣex = 620 nm (red trace) and ߣex = 760 nm (green trace) in thin films. 3.3.

Photovoltaic

properties.

The

conventional

PSCs

with

structure

of

ITO/PEDOT:PSS/active layer/PFN/Al were fabricated as described in supplementary information. The photovoltaic performance was optimized under different conditions by first adjusting the weight ratios between donor to acceptor in chloroform (CF) solution and then by solvent vapor annealing time. The PSC were built using the PC71BM acceptor under the identical

experimental

conditions

for

comparison

purposes.

The

current-voltage

characteristics of the optimized active layer (weight ratio of 1:2) processed with CF solution for both P(BdP-EHT):SMDPP and P(BdP-EHT):PC71BM are displayed in Figure 4a and their photovoltaic parameters are compiled in Table 1. Table 1: Photovoltaic parameters of the PSCs based on P(BdP-EHT):SMDPP and P(BdPEHT):PC71BM active layers processed under different conditions. Active layer P(BdP-EHT):PC71BM as cast P(BdP-EHT):SMDPP as cast P(BdP-EHT):PC71BM SVA P(BdP-EHT):SMDPP SVA a Average of 10 devices b

Jsc mA/cm2

Voc V

FF

PCE %

Rs Ω/cm2

Rsh Ω/cm2

7.14 (7.03)b

0.96

0.40

2.74 (2.61)a

24.06

578

8.96 (8.84)b

1.12

0.36

3.60 (3.52)a

22.01

625

12.16 (12.07)b 0.91

0.67

7.41 (7.29)a

12.17

845

13.48 (13.41)b 1.06

0.65

9.29 (9.17)a

10. 42

921

Estimated from the integration of IPCE spectra of the PSCs



The PSCs with as cast P(BdP-EHT):PC71BM and P(BdP-EHT):SMDPP active layers showed poor PCE’s of 2.74 % (with Jsc = 7.14 mA/cm2, Voc = 0.96 V and FF = 0.40) and 3.60 % (with Jsc = 8.96 mA/cm2, Voc = 1.12 V and FF = 0.36), respectively. After the solvent annealing of the active layer, the PCEs of the resulting PSC’s based on P(BdPEHT):PC71BM and P(BdP-EHT):SMDPP were largely enhanced up to 7.41 % (Jsc = 12.16 mA/cm2, Voc = 0.91 V and FF = 0.67) and 9.29 % (Jsc = 13.48 mA/cm2, Voc = 1.06 V and FF = 0.65), respectively. The increase in the PCE for the SMDPP based PSCs, irrespective of the processing conditions, is mainly attributed to the increase in Jsc and Voc, although the FF is slightly reduced as compared to the PC71BM based counterpart PSCs. The higher value of Voc may be due to the lower value of LUMO energy level of SMDPP (−3.83 eV) as compared to that for PC71BM (−4.10 eV), since the Voc of the BHJ organic solar cells is directly related HOMO-LUMO offset between donor and acceptor. It is noted that the difference between the HOMO of P(BdP-EHT) and LUMO of SMDPP is higher (1.62 eV) than that of between the HOMO of P(BdP-EHT) and LUMO of SMDPP (1.35 eV), leading to higher value of P(BdP-EHT):SMDPP based PSCs. The higher value of Jsc for NFSMA based device may be related to the broader absorption spectra of P(BdP-EHT):SMDPP (from 400 nm to 850 nm) as compared to that for P(BdP-EHT):PC71BM (300 - 700 nm) (Figure 5). Since the Jsc of the PSCs is highly depending on the light harvesting efficiency (LHE) of the corresponding active blend layers. The LHE of these two blend films with same thickness is estimated from following expression:63-64 −αd

∫ (1 − 10 ).S (λ )dλ LHE = ∫ S (λ ).dλ where S(λ) is the photon flux, i.e. number of photons available at a given wavelength in the AM1.5 G solar spectrum, α is the absorption coefficient, and d is the active layer thickness. Integrating the absorption spectra (Figure 5) from 350 nm to 850 nm, the values of LHE are 43.07 and 39.13 % for P(BdP-EHT):SMDPP and P(BdP-EHT):PC71BM, respectively. The higher value of LHE for P(BdP-EHT):SMDPP as compared to P(BdPEHT):PC71BM may explain the high value of Jsc for former device.


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Figure 4: (a) Current –voltage characteristics under illumination and (b) incident photon to current conversion efficiency (IPCE) spectra of the PSC based on P(BdP-EHT):SMDPP (red traces) and P(BdP-EHT):PC71BM (black traces) active layers as cast (dashed lines) and after SVA treatments (solid lines)

The IPCE spectra of the PSCs are shown in Figure 4b and are reminiscent to the absorption spectra of the corresponding active layers ( as shown in figure 5, indicating that both the donor and acceptor materials in the active layer contribute to the photocurrent generation. For PC71BM based PSC, the SVA treatment improves the IPCE values between 500 and 700 nm with maximum is located at 634 nm (61 %). The PSCs based on SMDPP exhibit an extended IPCE response up to 850 nm with maximum of 71 % at 680 nm, in comparison to PC71BM counterpart. Moreover after SVA treatment, the broad and high IPCEs over the entire range from 400 nm to 850 nm is consistent with the enhanced value of Jsc. Moreover, high values of IPCE in the range of 700 - 850 nm where the light absorption is mainly ascribed to the SMDPP, demonstrate an efficient hole transfer from SMDPP to P(BdP-EHT) even with low value of 0.12 eV for ∆HOMO. The Jsc values estimated from the integration of the IPCE spectra (Table 1) are somewhat smaller than the values obtained from the J-V measurements under illumination. Although the ∆LUMO and ∆HOMO offset between P(BdP-EHT) and SMDPP is significantly lower than that of the threshold value of 0.3 eV, the hole transfer from SMDPP to P(BdP-EHT) and electron transfer from P(BdPEHT) to SMDPP seems to be highly efficient. This hypothesis is established by the IPCE values of the optimal PSC in the wavelength range going from 400 nm to 900 nm and the PL quenching measurements as discussed earlier.



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Figure 5: Normalized optical absorption spectra of P(BdP-EHT):SMDPP (red trace) and P(BdP-EHT):PC71BM (black trace) in thin film Table 2: Effect of SVA on hole and electron mobility properties of the active layers based on P(BdP-EHT):SMDPP and P(BdP-EHT):PC71BM. Active layer

Hole mobility µh cm2/Vs

Electron mobility µe cm2/Vs

µe/µh

P(BdP-EHT):SMDPP (as cast)

6.34 x 10-5

1.82 x 10-4

2.87

P(BdP-EHT):PC71BM (as cast)

6.89 x 10-5

2.32 x 10-4

3.37

P(BdP-EHT):SMDPP (SVA)

1.17 x10-4

1.89 x 10-4

1.61

P(BdP-EHT):PC71BM (SVA)

1.12 x 10-5

2.38 x 10-4

2.12

The “as cast” and SVA treated SMDPP based PSCs exhibit a high Voc of 1.12 and 1.06 V with lower voltage loss (Eloss) of 0.42 and 0.48 eV, respectively, as compared to PC71BM based counterpart (0.73 to 0.78 eV for “as cast” and SVA). The Eloss is defined as Eloss = Eg − qVoc, where Eg is the optical band of the either donor or acceptor material which is low.56-57 The optical bandgap of SMDPP and P(BdP-EHT) was used for the estimation of Eloss for P(BdP-EHT):SMDPP and P(BdP-EHT):PC71BM based PSCs, respectively. The Eloss is generally defined by the loss in free energy when converting band edge photons to collectable charge carriers at open circuit.65-66 The dielectric constant of the P(BdP12 ACS Paragon Plus Environment

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EHT):SMDPP (4.21) and P(BdP-EHT):PC71BM (3.74) active layers was also measured at 1 kHz. The higher dielectric constant leads to the low exciton binding energy and may be associated with the low Eloss in the PSCs based on P(BdP-EHT):SMDPP. The high value of Voc and low Eloss in the devices based on SMDPP may be related to radiative recombinations as described by Vandewal et al. for polymer: fullerene based PSCs.67-68 As reported in the literature, the Eloss of the PSCs generally stems from three contributions, which include the unavoidable radiative loss (0.2 - 0.4 eV), and other recombination and non-radiative recombination losses. The main loss in the P(BdP-EHT):SMDPP is assumed to be the radiative one while in the case of P(BdP-EHT):PC71BM, in addition to radiative loss, the other losses are also contributing significantly. The Voc of the PSC can be defined by the following expression:69 A D − EHOMO − EbCTE ELUMO Voc = q A

D

where E LUMO and EHOMO are the LUMO and HOMO energy level of the acceptor CTE

and donor, respectively and Eb

is the binding energy of charge transfer exciton (CTE).

CTE is inversely proportional to the dielectric constant and thought to be the precursor of the free charge carriers. Since the dielectric constant of the P(BdP-EHT):SMDPP film is higher than that for the P(BdP-EHT):PC71BM film, indicating a lower value of CTE, therefore a small driving force is sufficient for hole transfer from SMDPP to P(BdP-EHT) and electron transfer from P(BdP-EHT) to SMDPP in the BHJ active layer to occur, which leads to a lower value of Eloss and a higher Voc. The difference in the PCE of the PSCs mainly lies on the different values of Jsc, Voc and FF. The high value of Voc for P(BdP-EHT):SMDPP has been discussed above already. The values of Jsc and FF mainly depend largely on the kinetics of exciton dissociation, charge collection and charge recombination. In order to gain information on the exciton generation and charge collection behavior in the PSCs based on SMDPP and PC71BM acceptors, the dependence of the photocurrent density (Jph) with effective voltage (Veff) was measured70 (Figure 6). Thus, the maximum exciton generation rate (Gmax) and exciton dissociation and charge collection efficiency (Pdiss) were evaluated. The Jph is the difference in the current densities under illumination and in dark, and Veff is defined as Vo − Vappl, where Vo is the voltage when Jph is zero and Vappl is the applied voltage. As illustrated in this figure 6, the Jph increases linearly with the applied voltage and saturate at higher values of Veff. In case of “as 13 ACS Paragon Plus Environment


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cast” devices, both based on SMDPP and PC71BM, the Jph showed a stronger dependence over the entire Veff range and was not fully saturated even at Veff = 1.8 V. This result suggests low efficiencies in either charge carrier generation or charge collection extraction or in both, thereby leading to a lower FF.71-72 However, for the PSCs based on the SVA treated active layers, Jph quickly reaches the saturation value of Veff = 0.5 V and 0.9 V, for P(BdPEHT):SMDPP and P(BdP-EHT):PC71BM, respectively, suggesting that after the excitons, due to the absorption of light, are dissociated into free charge carriers and subsequently collected efficiently at the respective electrodes. The Pdiss (i.e. Jsc/Jphsat) of the devices were estimated from the short circuit (Jsc) and saturation currents (Jphsat). The Pdiss values for the devices based on P(BdP-EHT):SMDPP (“as cast”), P(BdP-EHT):PC71BM (“as cast”), P(BdP-EHT):SMDPP (SVA) and P(BdP-EHT):PC71BM (SVA) are respectively 0.81, 0.83, 0.96 and 0.91. The Pdiss value increases significantly after SVA treatment for both SMDPP and PC71BM based PSCs. The highest Pdiss value for P(BdP-EHT):SMDPP (SVA) based device indicates that for this device both process of dissociation of photo-generated exciton into free charge carriers and charge extraction at the electrodes are more efficient than for other devices. Moreover, Pdiss is higher for the SMDPP based PSCs, irrespective of the processing conditions, indicating that a small driving force is needed for exciton dissociation. The Gmax values estimated according to Jphsat = qGmaxL, where q is the electronic charge and L is the thickness of the active layer. The Gmax values for P(BdP-EHT):SMDPP (“as cast”), P(BdP-EHT):PC71BM

(“as

cast”),

P(BdP-EHT):SMDPP

(SVA)

and

P(BdP-

EHT):PC71BM (SVA) are respectively 0.80 x 1028, 0.59 x 1028,1.05 x 1028 and 0.98 x 1028 m3 -1

s . This trend is reliable with the enhancement of Jsc and IPCE values, indicating the

presence of more efficient exciton generation and dissociation in the SMDPP based devices.


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Figure 6: Variation of photocurrent (Jph) with the effective voltage (Veff) for the PSC using different active layers.

The series (Rs) and shunt resistance (Rsh) of the PSCs are estimated from their respective J-V characteristics under illumination (reciprocal of the slopes at respectively Jsc and Voc, Table 1). For PSCs with PC71BM and SMDPP acceptors, the SVA treated devices display comparatively lower Rs and higher Rsh, in comparison to their “as cast” counterparts, confirming the improved diode characteristics after SVA treatment.73 The hole and electron mobility properties were measured from the dark J-V characteristics of hole and electron only devices (Figure 7a and 7b; only for the P(BdP-EHT):SMDPP), and fitting the data with space charge limited current model.74 Similar results were also obtained for P(BdPEHT):PC71BM. The hole (µh) and electron (µe) mobility properties are simultaneously increased for both devices based on PC71BM and SMDPP acceptors after SVA treatment and for optimized P(BdP-EHT):SMDPP and P(BdP-EHT):PC71BM (Table 2). Their values of µh/µe are 1.61 and 2.21, respectively, whereas for the “as cast” the µh/µe values are 2.87 and 3.37, respectively. A larger mobility values and more balanced mobility ratio (µh/µe) are beneficial for enhanced FF and PCE of the PSCs.



Figure 7: Dark current-voltage characteristics of (a) hole only and (b) electron only devices, respectively, for the P(BdP-EHT):SMDPP processed under different conditions

The variations of Jsc and Voc with illumination intensity (Pin) were measured to investigate the charge recombination processes in the PSCs (Figures 8a and 8b, respectively). γ

In Figure 8a, the Jsc related with Pin as J sc ∝ Pin if the electrodes are able to sweep out all the charge carriers, before recombination. When the slope γ is equal to unity, this means that all the charge carriers generated after the exciton dissociation are swept out by the electrode. Conversely when γ < 1, it implies the existence of bimolecular recombination. The γ values estimated from the slope of the variation of Jsc with Pin in logarithmic scales for the devices based on P(BdP-EHT):SMDPP (“as cast”), P(BdP-EHT):PC71BM (“as cast”), P(BdPEHT):SMDPP (SVA) and P(BdP-EHT):PC71BM (SVA) active layers are 0.83, 0.85, 0.94 and 0.96, respectively. These lower γ values for the “as cast” devices suggest higher bimolecular recombination under short circuit conditions. Concurrently, the γ values increase for the devices based on the SVA treated active layers, which indicate that the bimolecular recombination is appreciably suppressed. This is consistent with the fact that the related PSC’s exhibit balanced charge carrier mobility properties leading to an enhancement in FF and Jsc. The variation of Voc with Pin is shown in Figure 8b. When the slope of the Voc vs Pin trace is about equal to 1 kT/q, with k being the Boltzmann’s constant, T being temperature in kelvin and q being the electronic charge, the main loss mechanism is the bimolecular recombination. Concurrently, when the slope is equal to 2 kT/q, the loss mechanism is related to Shockley-Read recombination, which attributed to trap-assisted recombination.75-76 For the device based on P(BdP-EHT):PC71BM with and without SVA, the slopes are 1.28 kT/q and 1.45 kT/q, respectively, suggesting a significant trap-assisted recombination is involved in the 16 ACS Paragon Plus Environment

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“as cast” device under open circuit conditions, which is reasonably suppressed after SVA treatment. A slightly decreased slope of 1.31 kT/q is observed for the device based on “as cast” P(BdP-EHT):SMDPP film as compared to P(BdP-EHT):PC71BM counterpart, representing that bimolecular recombination is the dominating mechanism and trap assisted recombination is negligible. Noteworthy, after SVA treatment to P(BdP-EHT):SMDPP, the trap-assisted recombination is suppressed with a slope of 1.16 kT/q. The low recombination of free charge carriers should partly contribute to the low energy loss and increasing the Voc.

Figure 8: Variation of (a) Jsc and (b) Voc with illumination intensity Pin for the PSCs based on P(BdP-EHT):SMDPP and P(BdP-EHT):PC71BM active layers processed under different conditions.

4. CONCLUSION A new material, P(BdP-EHT), a sort of push-pull conjugated polymer with alternating BODIPY-thiophene dyads in the backbone was designed and synthesized. It was then used as an electron donor along with a carbazole-tetracyanobutadiene-diketopyrrolopyrrole (SMDPP) as an electron acceptor, exhibiting a D-π-A structure, for the fabrication of solution-processed BHJ polymer solar cells. The PSCs based on the optimized P(BdPEHT):SMDPP (1:2 donor to acceptor weight ratio and SVA treated active layer) provided a record high overall PCE of 9.29% of all BODIPY-based solar cells. This value is also higher than that for the P(BdP-EHT):PC71BM counterpart (7.41%) built under identical conditions. The improved PCE of P(BdP-EHT):SMDPP based PSCs may be attributed to the higher values of Jsc and Voc, which are related to a better light harvesting of photons in the longer wavelength region up to 900 nm and to a higher LUMO energy level of SMDPP as compared 17 ACS Paragon Plus Environment


to PC71BM. Furthermore, the energy loss for the P(BdP-EHT):SMDPP based PSCs is 0.48 eV, which is significantly lower than that for P(BdP-EHT):PC71BM (0.78 eV). This value of energy loss is among the lowest for PSCs based on conjugated polymers and DPP based NFSMA’s. This work highlights the growing interest for the BODIPY chromophore to design new donors, as well as the use of NFSMA, instead of PCBM acceptors, in order to extend the range of photons absorbed by the solar cell in the NIR, and so improve the PCEs. Since the P(BdP-EHT):SMDPP active layer showed relatively poor absorption in the higher energy wavelength region (< 500 nm), the PCE of the PSCs could be further improved by incorporation of a small amount of PC71BM as second acceptor (ternary blend active layer).31 Our research groups are currently working in this direction.

ASSOCIATED CONTENT Supporting Information. TGA traces and 1H NRM of P(BdP-EHT), absorption spectra in different solvents, emission and excitation spectra in solution (298 K and 77 K) and in thin film of P(BdP-EHT). Notes The authors declare that they have no conflict of interest.

ORCID Claude P. Gros: 0000-0002-6966-947X Ganesh D. Sharma: 0000-0002-1717-0116 Pierre D. Harvey: 0000-0002-6809-1629

ACKNOLEDGMENTS The “Centre National de la Recherche Scientifique” (ICMUB, UMR CNRS 6302) is gratefully thanked for financial support. L.B. also gratefully acknowledges the French Research Ministry for a PhD fellowship. Support was provided by the CNRS, the “Université de Bourgogne Franche-Comté”, the FEDER and the “Conseil Régional de Bourgogne” through the PARI II CDEA project. “Réalisé avec le soutien du Service de Coopération et d’Action Culturelle du Consulat Général de France à Québec” (Samuel de Champlain grant 65-101 (2015). GDS thanks to Department of Science and Technology (DST), Government


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of India for financial support. PDH thanks the Natural Sciences and Engineering Research Council of Canada for financial support.

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Nonfullerene Polymer Solar Cells Reaching a 9.29% Efficiency Using

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