Synthesis of a Fully Conjugated Phthalocyanine-Diketopyrrolopyrrole-Phthalocyanine Triad as Low Band Gap Donor in Small Molecule Bulk Heterojunction Solar Cells

We describe the synthesis and photovoltaic properties of a fully conjugated phthalocyanine-diketopyrrolopyrrole-phthalocyanine triad (ZnPc-DPP-ZnPc), which presents strong visible absorption from 400 to 900 nm. The synthesis of these phthalocyanine fully conjugated with diketopyrrolopyrrole provides access to a new family of low band gap materials (1.57 eV). Organic solar cells (OSCs) employing BHJ ZnPc-DPP-ZnPc:PC 70 BM ﬁlms using MoO 3 as anodic interfacial layer (IFL) show a power conversion e ﬃ ciency of 1.04%. The power conversion efficiency decreases considerably using as IFL PEDOT:PSS as a consequence of the protonation of the ZnPc.


Introduction
Although most commercial photovoltaic devices are made from silicon, organic solar cells (OSCs) are a promising alternative that can potentially provide light-weight, low-cost and flexibility advantages. 1 Despite polymer-based donor materials are one of the most important components in organic bulk heterojunction (BHJ) as donors in the active layer, 2 small molecule-based donor materials are an interesting alternative offering several promising advantages over their polymeric counterparts such as monodispersity and well-defined structure, easy purification, and better reproducibility (less batch-to-batch variation). 3 Recently, single-molecule BHJ OSCs are approaching the results of conjugated polymers with power conversion efficiencies (PCEs) exceeding 7%. 4 However, this type of materials present other challenges such as generation of high quality films in which interconnectivity between the domains is adequate and the selection of the interfacial layer (IFL) is key to obtain optimum efficiencies.
On the other hand, devices based on both polymer and small molecule absorbers still miss a great part of the red and infrared region of the solar spectrum ultimately limiting their achievable efficiency. Indeed, to date examples in which contribution to the photocurrent takes place up to 900 nm (band gap <1.6 eV) with driving force enough to provide efficient charge separation are rare. 2b Phthalocyanines (Pcs) and diketopyrrolopyrroles (DPPs) are two of the most extensively investigated molecules in organic photovoltaics because of their outstanding chemical and physical properties. Pcs are porphyrin analogues with, generally, planar structure and high conjugation of the central ring (18 π electrons). They present interesting properties for application in photovoltaic cells, especially strong absorbance in the visible and near infrared radiation region and high thermal and chemical stabilities. 6 Few examples are reported till now where soluble Pc derivatives are blended with acceptor materials to form BHJ active layers. 7 Record efficiencies using Pcs up to 1.6% in solution-processed BHJs have been described by Torres and Bäuerle using RuPcs axially functionalized with pyridyl-dendritic oligothienylenes blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC 71 BM). 8 On the other hand, DPPs possess exceptional absorbance properties in the 500-600 nm, strong fluorescent performance besides a great stability. A common feature of the DPP-based materials is their relatively high oxidation potential, leading to high-energy charge separated states when combined with fullerenes and correspondingly high voltages are obtained in solar cells. For these reasons DPPs, either as polymers or as single molecules, are being investigated as active layer on different OSCs devices. 9 Nguyen and co-workers reported DPP-containing single-molecule donors that achieved up to 4.8% PCEs with PC 70 BM in solutionprocessed BHJ devices. 10 The combination of the exceptional absorption characteristics of both Pcs and DPPs could provide highly light-absorbing single molecule donor materials in a broad visible-near infrared spectral region. Furthermore, in a molecule that combine two units of Pc covalently attached to the opposite positions of DPP unit, the strong donor character of the phthalocyanine will enhance the push-pull character of the molecule in a centrosymmetric configuration in order to increase intermolecular interaction and to reduce energetic disorder for charge transport. 11 Here, we describe for the first time the synthesis of a strong light-harvester donor which presents strong visible absorption from 400 to 900 nm ready to be used in small-molecule BHJ OSCs: a fully conjugated molecule that combines two planar aromatic zinc phthalocyanine units with the rigid π-conjugated DPP core, ZnPc-DPP-ZnPc 1 (Chart 1). Moreover, we have also investigated the influence of the nature of the anodic IFL in the device performance using either PEDOT:PSS or MoO 3 as IFL in solution processed BHJ devices.
Characterization. As expected, ZnPc-DPP-ZnPc 1 is soluble in common organic solvents such as CHCl 3 , THF, toluene, and chlorobenzene. Broad signals were obtained for 1 in 1 H-RMN even using a coordinating solvent as THF-d 8 due to the high π-π stacking and also to the mixture of regioisomers.
The UV-vis spectra of the triad 1, ( t Bu) 4 ZnPc and T 2 DPP in CHCl 3 as solvent are represented in Figure 1a and their data included in Table S1. The existence of orbital overlap among the units in 1 leads to a strong interaction in the ground state, as reflected by the dramatic broadening and shift of the Q-band, wich reaches the near IR (Figure 1a). The UV-vis absorption spectrum of ZnPc-DPP-ZnPc 1 was also studied in different solvents, THF and CHCl 3 , and in solid state (Figure 1b) detecting important differences. The spectrum in THF shows a fine structure with maxima at 350 nm( Soret band), 674 and 729 nm (Q1 and Q2 bands, attributed to the existence of regioisomers) and a shoulder at 605 nm (from the DPP unit). This well-resolved spectrum is a consequence of the coordination of the solvent (THF) to the Zn ions, thus preventing aggregation in the diluted solution. A more coarse and broad UV-vis spectrum with lower molar extinction coefficients is obtained in chloroform due to the π-π aggregation effect, resulting in a high absorption from 500 to 900 nm. The Soret band hardly moves two nanometers (352 nm) to the red, the shoulder appears at 617 nm, the Q 1 band undergoes a small bathochromic shift (from 674 nm in THF to 683 nm), the Q 2 band shifts to the blue 2 nm (727 nm) and one additional band appears at 768 nm. The broadest spectrum was obtained in solid state; the Q band has an amplitude of about 400 nm covering part of the near infrared till 1000 nm.  ( t Bu) 4 ZnPc revealed a fluorescence band located at 689 nm in CHCl 3 solution (Fig. S6a). T 2 DPP also shows a very high fluorescence in CHCl 3 solution at 564 and 609 nm (Fig. S6b).
However, fluorescence is totally quenched in 1, which could be attributed to intramolecular energy and/or electron transfer processes from the ZnPc to the DPP moiety (Table S1).
Cyclic voltammetric studies performed on a THF solution of ZnPc-DPP-ZnPc 1 containing 0.1 M Bu 4 NPF 6 as supporting electrolyte, showed three oxidation peaks at 0.1, 0.3 and 0.5 V and two reduction peaks at -1.5 and -1.7 V vs Fc/Fc + . The oxidation peaks are positively shifted comparing to the first oxidation of the ( t Bu) 4 ZnPc (0.5 V) and the first and second oxidation peaks of the T 2 DPP reference compound (0.6 V and 1.1 V) revealing a strong coupling among all the moieties in the ground state. This coupling is also observed comparing the more negative shift of the reduction peaks of the triad with the first reduction potential of the ( t Bu) 4 ZnPc (-1.2 V) and the first reduction potencial of the T 2 DPP (-1.6 V) (Figure 2 and Table S1).
The energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of ZnPc-DPP-ZnPc 1 have been determined to be approximately -4.90 and -3.33 eV, respectively, calculated from the electrochemical oxidation and reduction potentials. The HOMO and LUMO energy values suggest that the frontier molecular orbitals line up favourably with those of common fullerene acceptors, as PC 70 BM, which has HOMO and LUMO levels of −6.0 and −4.0 eV, respectively, to generate useful V oc in BHJ solar cells (Table S1). Solar Cell Performance. Photovoltaic devices were fabricated using the general architecture ITO/IFL/ZnPc-DPP-ZnPc 1:PC 70 BM/Ca/Al.
Poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonic acid) (PEDOT:PSS) was used as IFL anode. As it can be observed, there is a mismatch in the energy level alignment at the anode interface and an energy barrier to extract holes (see Figure S7). However, for preliminary results and considering there is no availability of HILs with adequate HOMO levels, we tested the material in this configuration. A series of studies were conducted to optimize the power conversion efficiencies (PCE).
The influence of the ZnPc-DPP-ZnPc 1:PC 70 BM ratio was examined by looking at different compositions in chloroform. A detailed device fabrication process is described as supporting information. We found that a 1.5:1 ratio provided maximum efficiencies of 0.47 % using PEDOT:PSS as IFL. For this combination, a photocurrent (J sc ) of 6 mA/cm 2 was obtained indicating that charge separation with the acceptor material occurs readily. Open circuit voltage (V oc ) and fill factor (FF) were low, seriously limiting the device performance. Considering that other families of small molecule donors are very sensitive to the selection of the anode IFL, we also examined the use of MoO 3 as IFL. 14 For this system, efficiencies could be doubled to obtain 1.04% with relatively high V oc for such a low band gap material and a short circuit current (J sc ) of 5.0 mA/cm 2 . However, the FF remains low (38%). Typical current-voltage characteristics and external quantum efficiency (EQE) plots are shown in Figures 3 and the resulting photovoltaic parameters are summarized in Table  1. The EQE plot clearly shows that the contribution to the photocurrent extends up to 900 nm.
Morphology measurements of ZnPc-DPP-ZnPC:PC 70 BM using chloroform as solvent have been evaluated by Atomic Force Microscopy (AFM) (See Fig S8). The film is smooth with RMS roughness of 1.7 nm 2 . Two different domains are visible, dark spots in a light matrix. It is possible that domains are not clearly interconnected indicating that the morphology is not optimum. However, AFM does provide information at the surface level and better interconnectivity could be possible in the bulk of the active layer. At this stage it appears that further material structural and device processing optimizations are needed. Nevertheless, these initial results are highly encouraging.  protonate basic nitrogens of the active layer materials which hinders the transfer of holes from the active layer to the anode. 4c In order to test this possibility for ZnPc-DPP-ZnPc 1, we measured the absorbance profile as a function of the concentration of trifluoroacetic acid and the resulting absorption profiles (Figure 4) showed significant changes immediately upon acid addition, namely new low energy transitions, confirming that the chromophore backbone is influenced by the protonation. This effect saturates when the TFA concentration is ten times greater than that of ZnPc-DPP-ZnPc 1. Therefore, the use of PEDOT:PSS is not recommended for future device optimization for materials with similar combination of structural moieties.

Conclusions
ZnPc-DPP-ZnPc 1 is the first example of a family of materials that present great potential to produce high efficiency BHJs due to its exceptional harvesting capability towards the near infrared and adequate electron donor character. When mixed with PC 70 BM and sandwiched between selective electrodes, efficiencies reached 1.04% when MoO 3 is used as IFL. We have shown that the interaction with the anode IFL is critical, as for other small molecule absorbers, and needs to be taken into account for further development. Thus, the use of PEDOT:PSS leads to degradation of the active layer interface and the efficiency can be doubled with the use of MoO 3 .

MATERIAL AND METHODS
General methods. Solvents and reagents were obtained from commercial source and used as received. Column chromatography: SiO2 (40-63 μm) TLC plates coated with SiO2 60F254 were visualized by UV light. NMR spectra were recorded at 25°C using a Bruker AC300 spectrometer. The solvents for spectroscopic studies were of spectroscopic grade and used as received. UV-vis spectra were measured with a Helios Gamma spectrophotometer. IR spectra were measured with Nicolet Impact 400D spectrophotometer. High resolution mass spectra were obtained from a Bruker Microflex LRF20 matrixassisted laser desorption/ionization time of flight (MALDI-TOF) using dithranol as matrix. Melting points were measured with Melting Point Apparatus SMP3.

OPV DEVICE FABRICATION
Patterned ITO-coated glass substrates with a resistivity of 10Ω/cm 2 and thickness were cleaned by sequential sonication at 50 °C in soap/DI water, DI water, methanol, isopropanol, and acetone for 30 min. ITO substrates were then treated for 30 min in a UV/O3 oven (Jelight Co.). Next, either PEDOT:PSS or MoO 3 was deposited. PEDOT:PSS (Clevios P VP Al 4083) was spun-cast at 5000 rpm for 30 sec and subsequently annealed at 150°C for 15 min in air. Samples were then transferred to a N 2 -filled glove box and an additional drying step was carried out at 100°C for 10 min to remove traces of water. MoO 3 (Sigma-Aldrich, 99.995%) was thermally deposited at a pressure of 1.0 × 10-6 Torr at a rate of 0.1 Å/s. Prior to active layer deposition, the MoO3 films were transferred to air for ~2 min. Active layer solutions containing donor materials and PC 71 BM were formulated inside the glove box with optimum ratios (wt:wt): ZnPc-DPP-ZnPc 1:PC 71 BM (1.5:1.0 in chloroform; 7 mg/mL). Active layer solutions were stirred for ~1 h at 45°C. The active layer solution was spun-cast between 2000 rpm for 1 min after passing the solution through a 0.22 µm PTFE filter to afford active layers between 80 nm. To finish device fabrication, Ca (5.0 nm)/Ag(100 nm) were thermally evaporated, sequentially, at a base pressure of ~1.0 × 10-6 Torr. The top Ca/Ag electrodes were then encapsulated with UV-curable epoxy and a glass slide before testing. Each substrate had 4 cells with a defined average area of 0.25 cm 2 .