NIR absorbing D–π–A–π–D structured diketopyrrolopyrrole–dithiafulvalene based small molecule for solution processed organic solar cells

Abstract

A new low band gap small molecule DPP–DTF with a D–π–A–π–D molecular structure composed of a dithiafulvalene (DTF) donor and a diketopyrrolopyrrole (DPP) acceptor was synthesized and tested for organic solar cells. Using the DPP–DTF small molecule as an electron donor, and PC₇₁BM as an acceptor a high power conversion efficiency (PCE) of 4.3% was achieved.

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Organic photovoltaics (OPVs) have drawn immense attention in the field of energy demanding global services owing to their cheap production costs, large area and lightweight flexible panels and viability for easy solution processing methods.¹ Although polymers are popular with a high power conversion efficiency (PCE) of ∼ 10%,² small molecules have unique advantages such as well defined molecular structures with high purity, intermolecular arrangements known by crystallographic analysis, easy synthesis and purification methods, better batch-to-batch reproducibility, relatively high charge carrier mobility and a high open circuit voltage (V_OC).³ Recently small molecules also reached the bench mark PCE of 10%, but it requires further boosting for commercial applications.⁴ However, small molecules suffer with poor film formation and morphology which can be minimized by incorporating long alkyl chains into the design and annealing the films, respectively.⁵ So far using a molecule with a push–pull architecture is the most efficient method to increase the PCE due to its narrow band gap and easy tuning of the HOMO and LUMO energy levels by simple modification of the donors (D) and acceptors (A).⁶ Recently, a variety of designs such as D–π–A, D–A–D, D–π–D, A–π–A and D–π–A–π–D were proven as high potential designs for high efficiency with linear, trigonal and X-shaped geometries.³

The major challenge in achieving a higher PCE for OPVs is the increase of the short-circuit current density (J_SC) which can be improved by introducing strong electron withdrawing chromophores into the molecular design leading to longer wavelength absorption, while the V_OC and fill factor (ff) can be improved by interfacial engineering, morphology control of the device and selecting an efficient cathode layer.^5,7 In this context, diketopyrrolopyrrole (DPP) has created a range of OPVs with good efficiency owing to its NIR absorption, high molar extinction coefficients, thermal and photochemical stability, good charge transport properties and high co-planarity.⁸ DPP is mainly used as an acceptor end group or linking unit in the centre with architectures A–D–A or D–A–D and achieves a PCE of ∼ 7%⁹ which needs to be improved for commercial OPV applications by introducing suitable donors. Several small molecules have been reported with DPP in a D–A–D design with various donor units such as oligothiophene, triphenylamine, benzofuran and benzothiophene (BDT).¹⁰ Also, DPP has been used as an end group unit in an A–D–A design with several conjugate donor/linkers such as oligothiophene, carbazole, phenothiazine, dithienopyran, BDT, [2,2]paracyclophane, naphthodithiophene (NDT) and indacenodithiophene and it was found that electron rich units improved the donor–acceptor interactions leading to longer absorption and then higher J_SC.^9,11 Recently, Li and co-workers achieved promising efficiency with a high V_OC by developing benzothiadiazole–triphenylamine based small molecules.¹² Incorporation of an alkyl chain majorly influences the solubility, crystallization, molecular interactions and morphology and it depends on the nature of the alkyl chain like length, size, branching and symmetry.¹³ Yang et al. explored dithiafulvalene (DTF) as a donor unit for DSSCs and achieved a highest PCE of 8.3% due to its unique optoelectronic properties and effective charge separation but it was not used for OPVs; its alkyl chains also provide the aforementioned benefits for OPV applications.¹⁴

In this context, we have designed and synthesized DPP–DTF (Scheme 1) by combining a DPP acceptor, a DTF donor and an oligothiophene π-linker in a D–π–A–π–D architecture to enhance the optical and photovoltaic properties. This small molecule displayed longer absorption wavelengths with suitable HOMO and LUMO energy levels with PC₇₀BM to construct blends efficiently for achieving a high PCE. Interestingly, DPP–DTF has a very high thermal stability up to 290 °C when compared to reported DPP-based compounds. The synthesis strategy used to prepare DPP–DTF is demonstrated in Scheme 1 and the synthesis details are given in the ESI.†DPP–DTF was synthesized in good yield via the Wittig–Horner reaction of DPP dialdehyde¹⁵ and DTF in the presence of P(OEt)₃.

Scheme 1 Synthesis scheme to prepare DPP–DTF.

Fig. 1 illustrates the absorption spectra of DPP–DTF in dichloromethane (DCM), relevant data are compiled in Table 1, and it shows mainly two peaks. The high intensity peak in the lower energy region at 674 nm (ε = 84 400 M⁻¹ cm⁻¹) is attributed to a charge transfer transition from the DTF donor to the DPP acceptor. The lower absorption peak at 380 nm arises from a localized π–π* transition of the conjugate back bone of the molecule. For DPP–DTF, an additional band appeared at 771 nm for the film which is attributed to strong π–π interactions in the solid state packing of the film. Absorption spectra of the blends with different ratios of PC₇₀BM (Fig. 1) are red shifted and cover the broad region of 300–900 nm with tailing up to 1100 nm. New peaks appeared in the 300–500 nm range, originating from fullerene, and their intensity increased with increasing concentration of PC₇₀BM.⁹ In DCM, DPP–DTF is moderately emitting in the NIR region at 740 nm. The photoluminescence (PL) is red shifted from polar to non-polar solvents while there is no change for the absorption spectra, which clearly indicates that the excited state is more polar when stabilized by polar solvents compared to the ground state (Fig. 2(a)).

Fig. 1 Absorption spectra of DPP–DTF in DCM, film and blended films with the addition of PCBM (1.0, 1.15 and 2.0).

Table 1 Optical properties of DPP–DTF recorded in DCM

λ abs (nm) (ε × 10^3 M^−1 cm^−1)	λ em (nm)	E oxd ^a (V)	E red ^a (V)	HOMO^b (eV)	LUMO^c (eV)	E 0–0 ^d (eV)
a Redox potentials versus ferrocene as an external standard. b Deduced from the oxidation potential using the formula HOMO = 4.8 + Eox. c Deduced using the formula LUMO = 4.8 + Ered. d Calculated from the HOMO and LUMO gap.
675 (84.4), 438 (23.5), 389 (35.4)	752	0.02 (63), 0.13 (76), 0.51 (64)	−1.55 (119)	4.82	3.25	1.57

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Fig. 2 (a) PL spectra of DPP–DTF in different solvents and (b) PL quenching in ODCB solution with increasing concentration of PC₇₀BM (×10⁻⁵); inset is showing Stern–Volmer quenching plot.

In order to know about the charge separation and photo-induced electron transfer from DPP–DTF to PC₇₀BM, the PL of DPP–DTF was measured in ortho-dichlorobenzene (ODCB) with increasing concentrations of PC₇₀BM (Fig. 2(b)). The PL intensity of DPP–DTF decreased with increasing concentration of PC₇₀BM, which clearly indicates the efficient photo-induced electron transfer to PC₇₀BM.¹⁶ The PL emission of the blended film was quenched with a value of 94% efficiency relative to pure DPP–DTF in solution. Stern–Volmer quenching constants were calculated using the equation F⁰/F = 1 + K_SV[PC₇₀BM], where F⁰ and F are emission intensities in the absence and presence of PC₇₀BM, respectively and K_SV is the Stern–Volmer quenching constant. A high K_SV value of 7235 M⁻¹ obtained for the DPP–DTF donor indicates that it has a high binding affinity towards the quencher i.e. PC₇₀BM.

In order to scrutinize the redox properties of DPP–DTF, we measured the cyclic voltammogram in DCM (shown in ESI†) and it showed one quasi-reversible reduction peak attributed to the reduction of the DPP unit and multiple oxidation peaks corresponding to the removal of an electron from the DTF and oligo-thiophene units, respectively. The HOMO and LUMO energy levels of DPP–DTF are 4.82 eV and 3.25 eV vs. vacuum, respectively. The LUMO–LUMO offset of DPP–DTF and PC₇₀BM is 0.75 eV, which is enough for efficient exciton dissociation and charge separation at the donor–acceptor interface.^1,3

TDDFT calculations performed with the most reliable method for D–A compounds, named as BMK¹⁷/DGDZVP¹⁸ using the PCM¹⁹ model (DCM phase), showed the HOMO is located on the whole molecule and the LUMO is mainly located on DPP (Fig. 3(b)), which clearly indicates that the charge transfer transition arises from the DTF donor to the DPP acceptor. The computed absorption wavelengths closely match the experimental values in DCM (Table S1, ESI†).

Fig. 3 Device configuration based on DPP–DTF (a) and its energy level diagram (b).

OPVs based on DPP–DTF with a configuration of ITO/MoO_x/DPP–DTF:PC₇₀BM/Al were fabricated with an active layer (100 nm) of the DPP–DTF donor and the PC₇₀BM acceptor in different ratios, the detailed device fabrication procedure is given in the ESI† and the preliminary results are compiled in Table 2. J–V curves and EQE of the devices based on DPP–DTF:PC₇₀BM are shown in Fig. 4(a) and (b), respectively. Devices based on blend ratios of 1 : 1 and 1 : 1.5 show EQEs > 40% in the region of 320–710 nm and the latter one has a better EQE with tailing up to 850 nm, while the blend with a 1 : 2 ratio has a high EQE in the UV region but poor EQE in the visible region and it is consistent with the absorption spectra recorded for the thin film. The device composed of the DPP–DTF:PC₇₀BM blend in a weight ratio of 1 : 1.5 gave the highest PCE of 4.30% with a J_SC of 12.24 mA cm⁻², V_OC of 0.69 V and ff of 0.51 attributed to its highest J_SC when compared to the blends with various ratios of PC₇₀BM (1 : 1 and 1 : 2). The ratio of D and A in the active layer strongly influences the J_SC and V_OC.⁵ The increase of the acceptor ratio decreased the J_SC and V_OC and revealed that the optimum ratio for D and A is 1 : 1.5.^9,11g The J_SC calculated from the integration of the EQE spectrum is in good agreement (ca. 2% error) with the J_SC obtained from the J–V curve.

Table 2 Photovoltaic performances of DPP–DTF blended with PC₇₀BM in different ratios

Active layer (DPP–DTF:PC70BM)	V OC (V)	J SC (mA cm^−2)	ff	PCE (%)
1 : 1	0.69	10.69	0.47	3.50
1 : 1.5	0.69	12.24	0.51	4.30
1 : 2	0.64	9.93	0.43	2.70

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Fig. 4 J–V curves (a) and EQE (b) of the BHJ using DPP–DTF blended with PC₇₀BM.

To gain a deeper understanding about the morphology of the active layer (DPP–DTF:PC₇₀BM) made with various ratios, which closely affect the PCE of OPVs, we investigated and compared the morphologies using atomic force microscopy (AFM) (Fig. 5). The root-mean-square (RMS) roughness of the blends (DPP–DTF:PC₇₀BM) increases in the order of 1 : 1.5 (1.3 nm) < 1 : 1 (1.9 nm) < 1 : 2 (2.6 nm), clearly indicating that the D and A ratio affects the morphology and domain size of the blend. The blend DPP–DTF:PC₇₀BM (1 : 1.5) might form effective bi-continuous D–A nano-scale networks which favor high J_SC and PCE compared to other blends, attributed to generated excitons in the small domain of the blend efficiently reaching the D–A interface.²⁰

Fig. 5 AFM images of the DPP–DTF:PC₇₀BM active layer made with various ratios.

In conclusion, we have designed and synthesized a D–π–A–π–D type small molecule (DPP–DTF) composed of a diketopyrrolopyrrole acceptor and a dithiafulvalene donor, which showed intense absorption bands in the visible and NIR regions in solution spectra and showed a broad absorption spectrum for blends with PC₇₀BM in different ratios. A device based on DPP–DTF showed a preliminary PCE of 4.3% for a blend ratio of 1 : 1.5 with PC₇₀BM. We are further studying the optimization of blending in various solvents and with cathode alteration to improve the PCE, since the optimization of these two processes strongly influences the photovoltaic performance.⁴

The authors gratefully acknowledge the financial support from CSIR-TAPSUN (NWP-0054). KN thanks CSIR for providing the senior research fellowship. SPS thanks Dr Ramanuj Narayan and Dr L. Giribabu for helpful discussions.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5cc07435e

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