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Efficient polymer solar cells with a high open-circuit voltage containing a DPP-based non–fullerene acceptor core end-capped with rhodanine units Maria Privado, Virginia Cuesta, Pilar de la Cruz, Mukhamed L. Keshtov, Rahul Singhal, Ganesh D. Sharmad, and Fernando Langa ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15717 • Publication Date (Web): 13 Mar 2017 Downloaded from http://pubs.acs.org on March 13, 2017

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ACS Applied Materials & Interfaces

Efficient Polymer Solar Cells with a High OpenCircuit Voltage Containing a DPP-Based Non– Fullerene Acceptor Core End-Capped with Rhodanine Units María Privado,† Virginia Cuesta,† Pilar de la Cruz, † Mukhamed. L Keshtov,‡ Rahul Singhal,§ Ganesh D. Sharmad*,§,∥ and Fernando Langa*,†. †

Universidad de Castilla-La Mancha. Institute of Nanoscience, Nanotechnology and Molecular

Materials (INAMOL), Campus de la Fábrica de Armas, 45071-Toledo. Spain. ‡

A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,

Vavilova Str., 28, Moscow 119991, Russia §

∥

Department of Physics, MNIT, Jaipur (Raj.) 302031, India Department of Physics, The LNM Institute of Information Technology (Deemed University),

Rupa ki Nagal, Jamdoli, Jaipur (Raj.) 302031, India. KEYWORDS: Organic Solar Cells, BHJ, non-fullerene acceptor, diketopyrrolopyrrole, rhodanine.

ACS Paragon Plus Environment

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ABSTRACT: Herein we report the synthesis of a novel A-D-A-D-A non-fullerene small molecule acceptor (NFSMA) bearing a DPP acceptor central core coupled to terminal rhodanine acceptors via a thiophene donor linker (denoted as MPU1) for use in non-fullerene polymer solar cells (PSCs). This NFSMA exhibits a narrow optical bandgap (1.48 eV), strong absorption in the 600-800 nm wavelength region of the solar spectrum and a lowest unoccupied energy level of 3.99 eV. When using the mixture of a medium bandgap D-A copolymer P (1.75 eV) as donor and MPU1 as acceptor, the blend film showed a broad absorption profile covering from 400 nm to 850 nm, beneficial for light harvesting efficiency of the resulted polymer solar cell (PSC). After optimization of the donor-to-acceptor weight ratios and concentration of solvent additive, the P:MPU1-based PSC exhibited a PCE of 7.52% (Jsc= 12.37 mA/cm2, Voc = 0.98 V and FF= 0.62), which is much higher than that for P3HT:MPU1-based device (2.16%) prepared under identical conditions. The higher value for the P:MPU1- relative to the P3HT:MPU1-based device is related to the low energy loss and more balanced charge transport in the device based on the P donor. These results indicate that alteration of the absorption spectra and electrochemical energy levels of non-fullerene acceptors, and appropriate selection of the polymer donor with complementary absorption profile, is a promising means for further boosting the performance of PSCs.

1. INTRODUCTION Organic solar cells based on an active layer consists of a mixture of electron donor and acceptor materials are a promising and cost-effective option present photovoltaic technology based silicon due to their several advantages, including low cost of fabrication, light weight and flexibility.1-7 The power conversion efficiencies (PCEs) of polymer solar cells (PSCs) based on single bulk heterojunction (BHJ) active layers consisting of conjugated polymers and fullerene


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derivatives electron donor and acceptor, respectively, have been reported so far in the range of 911%.8-13 In most of the highly efficient BHJ PSCs, reported so far employ fullerene derivatives (PC61BM and PC71BM) as electron acceptor. However, PSCs based on fullerene acceptors suffer from relatively large energy loss (generally more than 0.6 eV), thus limiting further improvements in their PCE.14,15 Additionally, fullerene-based acceptors have many other drawbacks, such as a limited absorption profile in visible region of the solar spectrum, high production costs, poor bandgap and energy level tunability and morphological instability.16-18 As novel electron acceptors with broad and strong absorption spectra and appropriate energy levels are needed to overcome these problems and allow the commercialization of OSCs, increasing efforts have been devoted to developing non-fullerene acceptor materials.19-29 As a result, the PCE of PSCs based on non-fullerene acceptors has been significantly improved to around 912%, thereby approaching the best values obtained by their fullerene counterparts.30-37 In order to achieve high PCEs with organic BHJ solar cells, a broad absorption profile for the active layer is prerequisite. In PSC, the photoactive layer comprises of conjugated copolymer and non-fullerene small molecule acceptor (NFSMA), each have comparatively narrow absorption spectra, thus meaning that the donor and acceptor materials employed in this layer should have complementary absorption to each others so that the active layer could efficiently absorb more photons and thereby create more excitons. The photogenerated excitons in BHJ PSCs dissociate into free charge carriers, with the dissociation sequence playing an important function in the kinetics of photon to current generation kinetics.38 After the exciton dissociation in the BHJ active layer, the electrons jump from the lowest unoccupied molecular orbital (LUMO) energy level of the donor to the LUMO energy level of the acceptor, whereas holes jump from the highest occupied molecular orbital (HOMO) energy level of the acceptor to the


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HOMO energy level of the donor. As such, the energy loss during the exciton dissociation process must be minimised so that the LUMO and HOMO offsets between donor and acceptor are large enough to provide a driving force.39,40 Organic semiconducting materials based on Diketopyrrolopyrrole (DPP) have been shown to be promising donors for OSCs due to numerous attractive properties of the DPP units,41,42 particularly their excellent photochemical stability, strong light absorption and easy synthetic modification. As some DPP-based materials exhibit electron mobility greater 1cm2/Vs and the polar amide group of the DPP unit is strongly electron withdrawing, numerous DPPbased materials have also been investigated as acceptors for OSCs,43-47 with overall PCEs of 3.17% with a high Voc being achieved when these materials are used as small molecule acceptors along with P3HT as donor in the BHJ active layer of OSCs.48 For example, Jo et al. used small molecules containing benzothiadiazole and DPP as electron acceptors along with poly[thieno(3,4-b’)benzothiophene] (PTB) in OSCs to obtain an overall PCE of about 5.0%.49 3-Ethylrhodanine has been used as an electron acceptor flanking group to cap the ends of molecules. Indeed, rhodanine derivatives are commonly used in organic chemistry to create strong push-pull molecules, and small molecules containing rhodanine derivatives as end capping groups have previously been used as a donor in OSCs.50-52 However, there are few reports of small molecules based on rhodanine-based electron-withdrawing units as acceptors for OSCs.53-57 Moreover, rhodanine has an electron-withdrawing character at its periphery as a result of the ketone and thioketone groups, and it can be used to further control solubility by changing the alkyl amide chain. We used an ethyl group to restrain the hydrogen bonding associated with non-alkylated rhodanine to design our molecule, thereby increasing the solubility in common organic solvents while avoiding overly bulky alkyl groups on the periphery of the molecule.


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In this communication, we report the design and synthesis of an A-D-A-D-A NFSMA comprising a DPP acceptor unit as core linked to a rhodanine acceptor via a thiophene donor linker (denoted as MPU1) and investigate its optical and electrochemical properties. The resulting acceptor MPU1 exhibits a low bandgap (1.48 eV) and strong absorption in the 600-800 nm region of solar spectrum with a high LUMO energy level of 3.99 eV. In order to absorb light efficiently in a wide absorption range from 350 to 800 nm to generate sufficient charge carriers, we used a medium D-A conjugated polymer i.e. poly(fluorobenzotriazole-dithienosilole) denoted as P58 (chemical structure shown in scheme 1) as donor material. P3HT was used as a donor for comparison. The main absorption of P is in the wavelength region 350-700 nm, thereby complementing well with the absorption spectrum of MPU1.

Scheme 1 Chemical structures of MPU1, P3HT and P.


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After optimization of the donor-to-acceptor weight ratio and concentration of solvent additive, the PSC based on P:MPU1 showed an overall PCE of 7.52 % (Jsc =12.37 mA/cm2, Voc = 0.98 V and FF= 0.62), which is superior than that for P3HT:MPU1 (PCE=2.16 % with Jsc =6.54 mA/cm2, Voc = 0.60 V and FF = 0.55). These results may be useful to develop more efficient NFSMA based on A-D-A-D-A conjugated system in future and also confirm that combination with suitable donor polymers is of essential importance for development of high-performance non-fullerene PSCs.

2. EXPERIMENTAL SECTION Synthetic details and characterization of MPU1 are described in the Supporting Information. Device fabrication and characterization. The PSCs were fabricated in an ITO/P3HT or P:MPU1/PFN/Al configuration. Thus, indium tin oxide (ITO)-coated glass substrates were cleaned sequentially with deionized water, acetone, ethanol and iso-propanol in an ultrasound bath for 10 min each and then dried. The PEDOT:PSS solution was spin-coated onto the clean ITO substrates at 2000 rpm followed by annealing at 120° C for 15 min in ambient condition to get thickness of 40 nm. Mixtures of P and MPU1 were dissolved in chloroform (CF) with different D/A weight ratios of 1:05, 1:1, 1:1.5 and 1:2. The concentration of the mixture was kept constant at 16 mg/mL. The mixed solution was spin-coated onto the top of the PEDOT:PSS layer at 2500 rpm for 60 s and then dried at 40° C for 10 min. For the solvent additives, 3 v% of DIO was mixed into the host CF solvent. The thickness of the active layer is about 90 ±5 nm. A methanol (containing 5 v% AcOH as additive) solution of PFN with a concentration of 1.5 mg/mL was then spin-coated on top of the active layer at 3000 rpm for 30s to get thickness of 15


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nm, since PFN may reduce the work function of cathode leading the efficient electron extraction. At last, an Al top electrode was deposited onto the top of the PFN buffer layer by thermal evaporation at a base pressure of 2.0x10-5 Pa. The effective area of the devices was about 16 mm2. Devices, based on the P3HT donor (optimized weight ratio of 1:2), were also manufactured under identical conditions for comparison. All devices were characterized as described in our earlier communication.57 3. RESULTS AND DISCUSSION Synthesis, characterization and thermal properties. The new non-fullerene acceptor MPU1 was obtained in 40% yield by a double Knoevenagel condensation between the bis-aldehyde 159 and N-ethylrhodanine (Scheme 2). Compound 1 was synthesized in improved yield by modifying the previously described procedure.

Scheme 2 Synthesis of the new non-fullerene acceptor MPU1. MPU1 was characterized by 1H,

13

C NMR and FT-IR spectroscopies and MALDI-MS

(see Supporting Information for full analytical and spectroscopic data). The 1H NMR spectra (Figure S1) shows a singlet at 7.93 ppm assigned to the proton of the double bond between the Thiophene-DPP fragment and N-ethylrhodanine moieties, two doublets at 9.20 and 7.56 ppm (J


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= 4.3 Hz) corresponding to the thiophene rings and all signals corresponding to the alkyl chains. A signal at 190.9 ppm, corresponding to the C=S moiety, is observed in the 13C NMR spectrum (Figure S2). The FTIR spectrum of MPU1 is shown in Figure S3. The mass spectrum of MPU1 shows the molecular ion peak at m/z866.21 amu (Figure S4). Compound MPU1 presented good thermal stability with a decomposition temperature (Td) of 375.0°C (at 5% weight loss) and the DSC analysis showed a high melting transition temperature (310ºC) (Figure S5), thus making this non-fullerene acceptor suitable for photovoltaic applications. Theoretical Calculations. Theoretical calculations were carried out by means of density functional theory (DFT) at the B3LYP 6-31G* level in vacuo, using Gaussian 09W, in order to determine the most stable conformation and the energies of the frontier molecular orbitals.

Figure 1. Molecular orbitals for MPU1. The DPP-based derivative MPU1 presents an almost planar structure (Figure S6) in which the calculated dihedral angles for the conjugated system are less than 3° and thus allowing an effective conjugation throughout the conjugated system. Consequently, both the HOMO and LUMO coefficients are spread over the whole π-system (Figure 1).


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According with the theoretical energy levels of HOMO (-5.46 eV) and LUMO (-3.46 eV), the theoretical HOMO/LUMO gap of MPU1 (1.82 eV) is the same as that obtained from electrochemical measurements (vide infra). The difference between the theoretical LUMO energy of MPU1 (-3.64 eV) and that of polymer P is 0.3 eV. This value is in the range of the theoretical minimum offset (0.3-0.4 eV) necessary to ensure efficient exciton dissociation at the donor/acceptor interface of OSCs.60 Optical and electrochemical properties. The normalized optical absorption spectra of MPU1 were investigated in both chloroform (CF) solution and thin film and are shown in Figure 2. MPU1 exhibits a broad and strong absorption in the wavelength region 400-750 nm in CF solution, with two absorption bands. The absorption band around 400-500 nm is due to π-π* transitions whereas the absorption band at longer wavelength (500-750 nm) centred at 702 nm (log ε = 4.76), is attributed to intramolecular charge transfer (ICT) between donor and acceptor units in the molecular backbone. The absorption band corresponding to the ICT in the absorption spectra of MPU1 is red-shifted (∼46 nm) with respects to solution absorption, which may be due to its solid state packing. The optical bandgap estimated from the absorption edge of the absorption spectrum for the thin film is 1.48 eV (Egopt = 1240/λonset (eV)). Figure 2 also shows the absorption spectra of P3HT and the conjugated D-A polymer P in thin film. Copolymer P shows an ICT absorption peak at 592 nm (molar extinction coefficient of 2.3 x104 M-1 cm-1), with a vibronic shoulder at around 640 nm and an optical bandgap of 1.75 eV.58 It can be seen from Figure 3 that MPU1 shows complementary absorption spectra for P3HT or P, thus indicating that both the donor (P or P3HT) and acceptor contribute to exciton generation and therefore the photocurrent.


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Figure 2. Optical absorption spectra forMPU1 (solution), MPU1 (film), P3HT (film) and P (film) The HOMO and LUMO energy levels for MPU1 estimated from the onset oxidation and onset reduction potentials in the cyclic voltammograms (Figure S7 and S8), are -5.81 and -3.99 eV, respectively (Figure 3). The LUMO energy level for MPU1 is similar to that for PC71BM (3.99 eV and -4.00 eV respectively), thus being beneficial for the high Voc of the resultant device. We have used P3HT and a D-A conjugated polymer (P) as donors in this study. The HOMO/LUMO energy levels of P3HT and P are -5.08/-2.96 and -5.31/-3.33 eV, respectively.


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Figure 3. HOMO and LUMO energy levels for MPU1, P and PC71BM. It should be noted that the LUMO offset between MPU1 and P or P3HT is about 0.66/1.03eV, which is favourable for efficient exciton dissociation and electron transfer from P to MPU1. Since MPU1 also contributes to exciton and photocurrent generation, the transfer of hole from acceptor to donor has to be careful taken into account.61-63 The HOMO offset between P or P3HT and MPU1 is 0.50/0.73 eV, which is higher than the empirical value of 0.3 eV, thus indicating that hole transfer from P or P3HT to MPU1 is efficient in the resulting device. Photovoltaic properties. Motivated by the complementary absorption spectra and suitable matched molecular energy levels between P and MPU1, the PSCs were fabricated with a conventional ITO/PEDOT:PSS/ /active layer/PFN/Al device structure in which PEDOT:PSS and PFN are used as anode and cathode interlayers, respectively. The active layer is consists of either P:MPU1 or P3HT:MPU1. The device fabrication conditions were optimized by varying the donor-to-acceptor weight ratio (1:05, 1:1 and 1:1.5) and concentration of solvent (1, 2, 3 and 4 v%) DIO additives. The photovoltaic parameters for the devices (P:MPU1 active layers) using


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the different donor to acceptor weight ratio using chloroform solvent and employing different concentration of DIO additive for 1:1 weight ratio are summarized in Table S1 and S2 (supplementary information). We found that the maximum PCEs were obtained by employing DIO (3 v%) /chloroform as processing solvent and a D/A weight ratio of 1:1 for both blends.

Table 1 Photovoltaic parameters for the devices Donor

Jsc (mA/cm2)

Voc (V)

FF

PCE /PCEaver (%)

Jscc (mA/cm2)

P3HTa

4.22 (±0.14)

0.64 (±0.02)

0.42 (±0.01)

1.14 (1.08)

4.14

Pa

8.36 (±0.11)

1.04 (±0.03)

0.48 4.17(4.09) (±0.015)

8.23

P3HTb

6.54 (±0.13)

0.60 (±0.02)

0.55 2.16(2.07) (±0.012)

6.42

Pb

12.37 (±0.16)

a

0.98 0.62 7.52(7.45) (±0.03) (±0.016)

12.26

CF cast; bDIO/CF; cEstimated from the IPCE spectra; PCEaver is the average for 8 devices.

Figure 4a shows the J-V characteristics of the devices obtained using the optimal weight ratio for the active layers of 1:1 and processed using chloroform as solvent; the photovoltaic data are summarised in Table 1. The device containing a P:MPU1 active layer exhibits a PCE of 4.17% (Jsc = 8.36 mA/cm2, Voc =1.04 V and FF =0.48), is much higher than that for the P3HT:MPU1 active layer (PCE =1.14% with Jsc=4.22 mA/cm2, Voc=0.64 V, FF= 0.42). The device based on P:PC71BM exhibits PCE of 3.87 %.58


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Figure 4. (a) Current –voltage (J-V) and (b) IPCE spectra for the devices based on P3HT:MPU1 and P:MPU1 processed under different conditions.


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The J-V characteristics of the OSCs containing optimised active layers and processed with the solvent additive (i.e. DIO/CF) under illumination (AM1.5, 100 mW/cm2) are shown in Figure 4a. Use of the DIO solvent additive increases the PCE of the devices based on P:MPU1 and P3HT:MPU1 to 7.52% (Jsc= 12.37 mA/cm2, Voc = 0.98 V and FF= 0.62) and 2.16% (Jsc =6.54 mA/cm2, Voc = 0.60 V and FF = 0.55), respectively. The higher value of Voc for PSCs based on P may be related to the deeper HOMO energy level of P as compared to P3HT. The device based on P:PC71BM processed under same condition showed PCE of 7.66 %,58 indicating that MPU1 has a potential to replace the PC71BM for PSCs. The IPCE spectra of these devices (shown in Figure 4b) closely resemble the absorption spectra of the corresponding donor and acceptor employed in the active layer (Figure 5), thus indicating that both the donor (P3HT or P) and acceptor (MPU1) contribute to both exciton generation and the photocurrent. As shown in Figure 4b, the OSCs based on P3HT:MPU1 exhibit very low IPCE values over the whole wavelength region from 350 to 850 nm with highest value of 42 %, whereas the IPCE values for the OSCs based on P:MPU1 are higher (with highest value of about 60 %) and broader than those for P3HT:MPU1-based devices. The Jsc values calculated from integration of the IPCE spectra (Table 1) agree well with those obtained from the J-V characteristics under illumination. The higher values of Jsc and IPCE for P:MPU1 may be related to absorption efficiency of the corresponding active layer. The absorption spectra of the P:MPU1 and P3HT:MPU1 are shown in Figure 5. It can be seen from this figure that P:MPU1 showed stronger absorption in the wavelength region from 430 nm to 770 nm than that for P3HT:MPU1 counterpart. The absorption efficiency (ηA) for both the blend films with same thickness is estimated from following expression:64


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ηA =

−αd − ( 1 10 ).S (λ )dλ ∫

∫ S (λ ).dπ

where S(λ) is the photon flux, i.e. number of photons obtainable at particular wavelength (λ) in AM1.5 G solar spectrum, ∝ is the absorption coefficient, and d is the thickness of the active layer. The values of ηA estimated from about expression is about 39.34 % and 33.21 % for P:MPU1 and P3HT:MPU1 active layers, respectively, when integrated from 450 nm to 800 nm. Thus, the number of photogenerated excitons is higher for P:MPU1 film than P3HT:MPU1 counterpart due to the larger overlap between the absorption spectra of P:MPU1 and solar spectrum, confirms the higher value of Jsc for the OSC based on P:MPU1.

Figure 5 Normalized absorption spectra of P:MPU1 and P3HT:MPU1 blended thin films


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The photoluminescence (PL) spectroscopy was employed to get information about the exciton dissociation in the blended P:MPU1 thin film and the PL spectra is shown in Figure 6. The pristine P and MPU1 showed PL, with peak around 706 nm and 744 nm, respectively, when excitation wavelength is 520 nm and 702 nm, respectively. However, P:MPU1 thin film showed significant PL quenching either excited at 520 nm or 702 nm, suggesting that the exciton generated either in P or MPU1 are efficiently dissociated at D-A interfaces present in the active layer. Similar PL quenching was observed for P3HT:MPU1 blended films. Thus, the PL quenching indicates that MPU1 can accept electrons efficiently from P donor.

Figure 6 Normalized photoluminescence spectra of pristine P, MPU1 and P:MPU1 thin films

The photovoltaic parameters of the OSCs based on P3HT:MPU1 differ from those of their P:MPU1 counterpart manufactured under the same conditions. In order to gain further


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information about this difference, the charge-generation and -extraction mechanism was investigated by varying the photocurrent (Jph) with respect to the effective voltage (Veff) for the devices, as shown in Figure 7. Jph is defined as Jph =JL – JD, where JL and JD are the current densities under illumination and in the dark, respectively, whereas Veff = Vo – Vappl, where Vo is the voltage when Jph is zero and Vappl is the applied voltage. Since Veff corresponds to the internal electric field in the BHJ, it influences both charge carrier transport and extraction65-67 and may be related to the poor morphology of the P3HT:MPU1 active layer on the nanoscale.

Figure 7. Variation of Jph with Veff for the devices based on an active layer containing P3HT:MPU1 and P:MPU1 and processed using DIO/CF.


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It can be seen from Figure 7 that Jph increases linearly with Veff in the low voltage region and then becomes saturated at a sufficiently high value of Veff. We have estimated the values of Gmax to compare the exciton-dissociation, bimolecular-recombination and charge-generation processes in the OSCs based on P3HT:MPU1 and P:MPU1. We assume that all the photogenerated excitons dissociate into free charge carriers and contribute to photocurrent generation in the high Veff region (saturation region), and Gmax is defined as Gmax=Jphsat/qL, where Jphsat is the saturation photocurrent density, q is the electronic charge and L is the thickness of the active layer. The values of Gmax and Jphsat for the P:MPU1-based device (1.02x1028 m-3s-1, Jphsat = 13.90 mA/cm2) are higher than those for the P3HT:MPU1-based device (6.77x1027 m-3s-1, Jphsat = 9.21 mA/cm2). The exciton dissociation probability (Pc) for the devices was also estimated using the equation Pc =Jsc/Jphsat. If Pc is 100% then the geminate recombination loss of charge carriers is negligible. Since only a fraction of photogenerated excitons are able to dissociate into free charge carriers at the donor/acceptor interface, this also depends on the morphology of the active layer used in the devices. Moreover, as shown in Figure 7, the photocurrent of the P3HT:MPU1-based device does not reach Jphsat even at a bias of 2.8 V, thus indicating that charge recombination is occurring in this device. The photocurrent for the P:MPU1 device starts to saturate at low bias voltages and reaches full saturation at 2.8 V, thus demonstrating that the photogenerated excitons in this device are fully dissociated and that free charge carriers are effectively transported within the active layer and then collected by the anode and cathode electrodes. The values of Pc for P:MPU1 and P3HT:MPU1 are 89% and 71%, respectively. The lower value for P3HT:MPU1 strongly indicates that a large fraction of excitons recombine before charge separation.


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Bimolecular recombination in the active layer was assessed by determining the dependence of Jsc on illumination intensity (Pin) (Figure 8). Since exciton quenching in the BHJ active layer is closely related to the charge recombination kinetics, the dependence of J-V characteristics on light intensity can be used to probe the main recombination mechanism, which may affect device performance. It has been reported that the correlation between Jsc and Pin can be expressed by Jsc ∝ Pinγ, where the slope γ should be unity when bimolecular recombination is negligible (maximum charge carrier swept out towards the respective electrodes after exciton dissociation), whereas the value of γ less than 1 indicates the existence of some degree of bimolecular recombination.68,69 As can be seen from Figure 6, the logarithmic plots of Jsc vs. Pin give γ values of 0.94 and 0.89 for the devices based on P:MPU1 and P3HT:MPU1, respectively. This indicates that, in the P:MPU1-based device, the charge carriers are efficiently swept out, with negligible bimolecular recombination when compared to the P3HT:MPU1-based device.70

Figure 8. Variation of Jsc with illumination intensity for the devices containing an active layer based on P3HT:MPU1 and P:MPU1 and processed using DIO/CF.


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In general, the energy loss (Eloss), which is defined as Eloss =Eg –qVoc, is also an important factor when interpreting the difference in the photovoltaic performance of PSCs containing P and P3HT, particularly in terms of Voc. Eg is determined by the smaller value of donor or acceptor materials and, herein, refers to the optical bandgap of MPU1, which is smaller than that for either P3HT or P. The most efficient PSCs based on fullerene acceptors have Eloss values ranging from 0.7 to 0.8 eV,71-73 which are much higher than those for highly efficient solar cells based on perovskites (less than 0.5 eV). The lower value of Eloss for P:MPU1 (0.50 eV) is significantly lower than that for P3HT:MPU1 (0.84 eV), may also be related to the higher value of Voc for the P-based device. In order to get information about the difference in the PCEs for solar cells based on P and P3HT using MPU1 as acceptor, the charge-transport properties in the solvent additive blended films were investigated since a balance between hole and electron mobility is essential when designing high PSCs given that it is related to recombination processes. The holes and electrons were measured using the space charge limited current (SCLC) method together with the J-V characteristics of devices in the dark, using hole/electron only devices as discussed in the experimental section and shown in Figure 9. The hole mobilities for both P:MPU1 and P3HT:MPU1 films increased upon addition of DIO due to the high degree of ordering of chains in either P3HT or P. However, the hole mobility in P:MPU1 is higher than that for the P3HT:MPU1 blend film, which may be attributed to the much more crystalline nature of P compared to that of P3HT. We have also measured the electron mobility of the pristine MPU1 and found to be 1.64 x10-4 cm2/Vs.


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Figure 9. Dark current–voltage characteristics for the devices based on optimized P3HT:MPU1 and P:MPU1 active layers: (a) hole-only devices; (b) electron-only devices. As can be seen from Figure 9, the hole and electron mobilities of P:MPU1 are estimated as 5.11x10-5 and 8.83 x10-5 cm2/Vs, respectively, whereas the corresponding values for P3HT:MPU1 are 2.58 x10-5 and 8.15 x10-5 cm2/Vs, respectively, thus indicating a more balanced


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charge transport (µe/µh = 1.73) for P:MPU1 than for P3HT:MPU1 (µe/µh = 3.16). High as well as balanced between the hole and electron mobilities are crucial for reducing bimolecular recombination. As a result, high values of Jsc and FF were obtained for the P:MPU1-based PSC. The FF of these devices is still low may be due the fact that the µe/µh is higher than unity. TEMs image were recorded in order to obtain information regarding the morphology of the active layers when spin-cast from DIO/CF solution (Figure 10). It can be seen from Figure 10 that P3HT:MPU1 shows a uniform surface morphology with small domain size, whereas the P:MPU1 film forms slightly higher size of domains thereby favouring exciton dissociation and charge transport and resulting in high values for FF, Jsc and PCE.

Figure 10. TEM images for (a) P3HT:MPU1 and (b) P:MPU1 thin films cast from DIO/CF. Scale bar: 200 nm. The difference in the crystallinity and degree of molecular ordering of P and P3HT have been investigated by the X-ray diffraction (XRD) patterns of these polymers in their thin film form and shown in Figure 11. The P cast from CF showed two pronounced diffraction peaks at


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2θ =4.26° and 2θ=23.38° corresponding to (100) and (010) diffraction with laminar packing with d-spacing of 2.13 nm and π-π stacking with d-spacing of 0.32 nm, respectively. However, P3HT showed a laminar packing with d-spacing of 1.98 nm (100) at 2θ=4.96° and (010) π-π stacking (2θ=23.04°) with d-spacing of 0.37 nm. Moreover, the diffraction peaks are more intense for P as compared to P3HT. These results indicate that P exhibits more ordered crystalline structure with more compact π-π stacking than P3HT. The high crystalline nature of the P may be related to the fluorine substituents in the benzothriazole of P backbone, which may induce intra- and inter-chain dipole –dipole interaction. The more ordered and high crystalline nature of P may be one of the reasons for high hole mobility and thereby high Jsc for the P:MPU1 based OSCs.

Figure 11. XRD patterns of P3HT and P thin films cast from chloroform solution.


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4. CONCLUSION We have designed and synthesised a solution-processable, small molecule non-fullerene acceptor (MPU1) bearing DPP as central core acceptor unit and rhodanine as end-capping acceptor units linked via a thiophene donor linker. This design leads to a broader absorption in the range 600-800 nm, with a maximum absorption at 748 nm in thin film and an optical bandgap of 1.48 eV. The HOMO and LUMO energy levels of MPU1 are -5.81 and -3.99 eV, respectively, which is compatible with either P3HT or the D-A medium bandgap copolymer P and indicates that efficient photo-induced charge transfer is favourable when blended with either P3HT or P. Photoluminescence measurements also confirm that MPU1 has an excellent electron-accepting ability and also the efficient charge transfer from the MPU1 to the donor P in the active layer consist of P and MPU1. After optimization of the BHJ active layer (donor-toacceptor weight ratio and solvent additive concentration), the devices based on P3HT:MPU1 (1:2) and P:MPU1 (1:1) exhibit PCE values of 4.09% and 7.52%, respectively. The higher PCE for P:MPU1 is related to the broader absorption profile of the active layer, low energy loss and balanced charge transport. The development of such non-fullerene acceptors with broader absorption spectra may lead to the development of high-performance and cost-effective solutionprocessed organic solar cells.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXX


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Experimental details, 1H NMR, 13C NMR, FT-IR and MALDI-TOF spectra, TGA and DSC analysis, theoretical calculations and electrochemical studies for MPU1 (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. ORCID María Privado: 0000-0002-4519-5011 Virginia Cuesta: 0000-0003-2466-404X Pilar de la Cruz: 0000-0002-6307-6729 Rahul Singhal: 0000-0002-0337-3545 Ganesh D. Sharma: 0000-0002-1717-0116 Fernando Langa: 0000-0002-7694-7722 NOTES The authors declare no competing financial interest. ACKNOWLEDGMENT We are grateful for financial support from MINECO, Spain (CTQ2013-48252-P) and the Junta de Comunidades de Castilla-La Mancha, Spain (PEII-2014-014-P). VC would like to thank the Ministerio de Educación, Cultura y Deporte of Spain for an FPU grant.


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Table of Contents


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Scheme 1 Chemical structures of MPU1, P3HT and P 102x99mm (300 x 300 DPI)


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Scheme 2 Synthesis of the new non-fullerene acceptor MPU1 77x31mm (300 x 300 DPI)



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Figure 1. Molecular orbitals for MPU1 319x116mm (96 x 96 DPI)


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Figure 2. Optical absorption spectra forMPU1 (solution), MPU1 (film), P3HT (film) and P (film) 352x239mm (96 x 96 DPI)



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Figure 3. HOMO and LUMO energy levels for MPU1, P and PC71BM 286x190mm (96 x 96 DPI)


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Figure 4. (a) J-V and (b) IPCE spectra for the devices based on P3HT:MPU1 and P:MPU1 processed under different conditions 131x172mm (96 x 96 DPI)



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Figure 5 Normalized absorption spectra of P:MPU1 and P3HT:MPU1 blended thin films 341x297mm (96 x 96 DPI)


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Figure 6 Normalized photoluminescence spectra of pristine P, MPU1 and P:MPU1 thin films 459x297mm (96 x 96 DPI)



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Figure 7. Variation of Jph with Veff for the devices based on an active layer containing P3HT:MPU1 and P:MPU1 and processed using DIO/CF 358x297mm (96 x 96 DPI)


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Figure 8. Variation of Jsc with illumination intensity for the devices containing an active layer based on P3HT:MPU1 and P:MPU1 and processed using DIO/CF 358x297mm (96 x 96 DPI)



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Figure 9. Dark current–voltage characteristics for the devices based on optimized P3HT:MPU1 and P:MPU1 active layers: (a) hole-only devices; (b) electron-only devices 174x297mm (96 x 96 DPI)


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Figure 10. TEM images for (a) P3HT:MPU1 and (b) P:MPU1 thin films cast from DIO/CF. Scale bar: 200 nm 182x91mm (96 x 96 DPI)



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new figure 11 240x178mm (150 x 150 DPI)


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Efficient Polymer Solar Cells with High Open ... - ACS Publications

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