Diketopyrrolopyrrole linked porphyrin dimers for visible-near-infrared photoresponsive nonfullerene organic solar cells

The binding of a porphyrin dimer donor (ETDPP-2P) and a non-fullerene acceptor IT-M leads to a visible-near-infrared photoresponse in all small molecule organic solar cells with a power conversion efficiency of 5.69%.


Introduction
Solution-processed organic solar cells (OSCs) have eventually been considered as one of the most promising next-generation photovoltaics due to their lightweight, optimistic flexibility, and low production cost. [1][2][3][4][5] Over the past few decades, the power conversion efficiencies (PCEs) of OSCs have exceeded 14% for binary layers consisting of polymer-based donors and fullerene derivative acceptors. [6][7][8][9] On the other hand, nonfullerene small-molecule (NFSM) acceptors have also attracted early interest to overcome the disadvantages of fullerene derivatives such as purification techniques, high-cost synthetic procedures, and unfavourable morphology. 10,11 Impressively, the PCEs of NF-OSCs have enhanced rapidly over 15%, owing to their complementary absorption, low energy loss, adequate and balanced electron mobility, and flexible molecular structures. [12][13][14][15][16][17] Among the various candidates, A-D-A structural SM acceptors built from fused indacenodithiophene (IDT) 18 rings and their analogues, such as ITIC, 17 IEIC, 19 IT-4F, 20 IT-M, 21 and IDIC 22 (see abbreviations in the ESI †), have shown the most promising performance in NF-OSCs because of their intensive near-infrared (NIR) absorption, easily processable capability, well-defined crystallinity, and ideal bandgaps. 8,23,24 However, SM donors sporadically succeeded with several advantages such as high crystallinity, rapid aggregation pathways, controllable energy levels, etc., and as a result, assisting the significant enhancement of PCEs in recent years. [25][26][27][28][29][30][31][32][33][34][35][36][37][38] However, the combination of a SM donor and a SM acceptor has been a bottleneck hindering the performance so far due to their high crystallinity and unfavourable phase separation and morphologies. 25,39 Recently, Wei et al. reported an A-p-D-p-A structural narrow bandgap SM donor based on a larger coplanar core DTBDT and achieved the highest PCE of 14.34% at that time for binary all SM OSCs by optimizing their hierarchical morphologies. 35 This significant success has led to an enhanced performance of all SM donors and acceptors which are comparable to that of NF-OSCs based on polymer donors. However, there are still very limited SM donors developed with the combination of NF acceptors for all SM-OSCs and those are with a fully responsive spectrum in the visible-NIR region.
On the other side, porphyrins have been extensively explored as a SM donor or acceptor in OPVs with impressive performances. 40,41 Over the past several years, we and other groups have steadily worked for the development of porphyrin SM electron donors in OSCs due to their excellent super p-conjugation properties, tremendous light-harvesting capabilities, exceptional charge transport mobilities, and easy processability with fullerene acceptors, as a result, achieving high PCEs up to 10%. [42][43][44][45][46][47][48][49][50][51][52][53][54][55] One major disadvantage of porphyrin derivatives for OSCs is that they usually show a deep valley between the intense Soret band (400-500 nm) and Q-bands (650-800 nm). 45,56 It is still challenging to design a porphyrin molecule with a full spectrum from 300 nm to 900 nm. Recently, ternary OSCs were proposed to be used as the third component as an absorption complementary donor or acceptor to achieve a full absorption spectrum. [57][58][59] For example, we have recently constructed a ternary OSC using a narrow bandgap porphyrin (DPP-2TTP) and an absorption complementary wide-bandgap SMs (DR3TBDTTF) as donors, and PCBM as the acceptor. Impressively, the photon-current conversion of the device crossed over UV-visible and NIR regions, and as a result, a PCE of 11% was recorded. 45 At the same time, Jang et al., 60 Jen et al., 29 and Wan et al. 61 have succeeded without using a third component to construct a binary layer system of porphyrin donors and NF acceptors for all SM-OSCs. However, only a very few binary layer systems featuring porphyrin donors and NFSM acceptors have been reported so far for OSCs. It is very challenging to construct porphyrins with NIR light absorbing nature and compensate the valley between the Soret and Q-bands by a NFSM acceptor. Thus, we are motivated to develop NIR porphyrin-based donors and investigate their compatibility with NFSM acceptors targeting for high-performance all SM-OSCs in the absence of a third component.
In this study, we designed and synthesized two new porphyrinbased SMs as ETDPP-2P and TDPP-2P with A 2 -p-D-A 1 -D-p-A 2 architecture, in which vertically meso-alkyl-substituted porphyrin (D) dimers are linked by thiophene-flanked diketopyrrolopyrrole (A 1 = TDPP) and ethynylene-thiophene-flanked diketopyrrolopyrrole (A 1 = ETDPP), respectively, and further horizontally p-extended via a p-bridge of ethynylene-phenylene and end-capped with electron-deficient 3-ethylrhodanine (A 2 ). As compared with TDPP-2P, ETDPP-2P shows a longer p-conjugated planar structure with an exceptionally red-shifted absorption over 1000 nm. To achieve panchromatic light-harvesting, SM acceptor IT-M with a perfect complementary absorption to ETDPP-2P was selected for all SM binary OSCs, resulting in a PCE of 5.69%. And a lower PCE of 4.12% was recorded for the binary OSC based on the TDPP-2P:IT-M active layer due to the less light harvesting in the NIR region.

Results and discussion
The synthesis of the intermediates 3 and 6 is described in the ESI † in Scheme S1. Briefly, ETDPP-2P and TDPP-2P (Scheme 1) can be prepared through Sonogashira/Stille coupling and Knoevenagel condensation reactions. Both porphyrin SMs are well soluble in most organic solvents for solution processing OSCs. Moreover, all the new compounds were characterized by 1 H NMR, 13 C NMR, and MALDI-TOF techniques (ESI †).  Table 1. Both the compounds majorly display two broad and intense bands. The bands at ca. 390-560 nm for TDPP-2P and at ca. 390-570 nm for ETDPP-2P correspond to the Soret band. Similarly, the peaks at ca. 590-725 nm for TDPP-2P and at ca. 620-900 nm for ETDPP-2P are attributed to the Q-band absorption. Compared with TDPP-2P, ETDPP-2P with ethynylene conjugated DPP exhibits very broad and redshifted (175 nm) Q-band absorption over the NIR region because the elongated p-conjugation enhances the strong intramolecular charge transfer (ICT) interactions within the porphyrin molecules. However, the thin films of both compounds exhibit much red-shifted absorption spectra than those in solution as shown in Fig. 1b.  In contrast, ETDPP-2P shows a significant broad peak of Q-band over 1000 nm with a shoulder peak at 760 nm. This could be ascribed to the extended p-conjugation of ETDPP-2P possessing stronger intermolecular p-p stacking within the donor molecules. 62,63 On the other hand, the absorption spectra of the IT-M thin-film disclose an optical response from 470 to 830 nm with a shoulder peak at ca. 640 nm.

Electrochemical properties
The redox potentials of ETDPP-2P and TDPP-2P were calculated by performing the cyclic voltammetry (CV) measurements in CHCl 3 solution (Fig. S1, ESI †) and the corresponding data are noted in Table 1. The onset oxidation potentials were used to calculate the highest occupied molecular orbitals (HOMOs), and the lowest unoccupied molecular orbitals (LUMOs) were then determined according to E LUMO = E HOMO + E opt g eV. The HOMO/LUMO energy levels of ETDPP-2P and TDPP-2P were approximated to be À5.12/ À3.72 eV and À5.09/À3.37 eV, respectively. Obviously, the ethynylene linkage between porphyrin and TDPP led to a more planar A 2 -p-D-A 1 -D-p-A 2 structure of ETDPP-2P with the elongated p-conjugation, as well as the deeper HOMO and LUMO levels. Fig. 1d presents the energy level diagrams of TDPP-2P, ETDPP-2P, and IT-M molecules, respectively. The compound ETDPP-2P shows perfect alignment energy levels with the IT-M acceptor to transfer hole and electrons when compared to the TDPP-2P compound due to the low lying HOMO and LUMO levels.

Photovoltaic properties
The photovoltaic performance of ETDPP-2P and TDPP-2P compounds as donors with IT-M acceptors was evaluated for the conventional device structure of ITO/PEDOT:PSS/donor:acceptor/ ZrAcac/Al under simulated AM 1.5 G illumination at 100 mW cm À2 . The optimal blend films were processed from chloroform (CF) as the solvent and 1,8-diiodooctane (DIO) as an additive (99.5 : 0.5 v/v) in a weight ratio of donor : acceptor = 1 : 0.6 (w/w). Furthermore, the films underwent TA treatment at 150 1C for about 15 min. Fig. 2a shows the current density-voltage ( J-V) curves of the devices and the key parameters are summarized in Table 2. The optimal device ETDPP-2P:IT-M exhibits an open circuit voltage (V oc ) of 0.84 V, along with a short circuit current ( J sc ) of 11.05 mA cm À2 , and a fill factor (FF) of 61.3%, thus resulting in a promised maximum PCE of 5.69%. On the other hand, the device TDPP-2P:IT-M shows a comparatively lower performance with a V oc of 0.79 V, a J sc of 8.96 mA cm À2 , and a FF of 58.2%, corresponding to a PCE of 4.12%. The overall better photovoltaic performance of the device ETDPP-2P:IT-M could be attributed to efficient charge dissociation, charge transfer, and well-ordered molecular packing morphology of the active layer (vide infra). Fig. 2b shows the external quantum efficiency (EQE) curves of the optimized devices for photocurrent generation. Notably, the device ETDPP-2P:IT-M reaches a broad photo-response range from 300-910 nm with a higher maximum EQE (EQE max ) value of 45% at ca. 700 nm. However, the device TDPP-2P:IT-M shows a comparatively lower photo-response from 300-780 nm with an EQE max value of 45% at ca. 620 nm. The calculated current density values ( J sc EQE ) are found to be 10.71 mA cm À2 and 8.68 mA cm À2 for ETDPP-2P:IT-M and TDPP-2P:IT-M, respectively. These results are consistent with those collected from J-V measurements with a little deviation, as shown in Table 2a. The higher photo-response and EQE value of the ETDPP-2P:IT-M device indicates the sufficient exciton separation, charge transport, and collection (vide infra). Fig. 3a shows the photocurrent density ( J ph ) ( J ph = J L À J D ) for ETDPP-2P and TDPP-2P devices, where J L and J D are the current density of the devices under illumination and dark, respectively, as a function of effective voltage V eff (V eff = V 0 À V), where V 0 is the voltage at J ph = 0. The J sat is the saturation current density which is equal to J ph when V eff Z 2.0 V. 64,65 Thus, the charge collection probabilities P(E,T) can be characterized by using the ratio J ph /J sat . Under the optimized conditions and maximal power output, the calculated ratios are found to be 0.92% for ETDPP-2P:IT-M and 0.83% for TDPP-2P:IT-M. The high P(E,T) value of ETDPP-2P:IT-M shows that the device has more efficient exciton dissociation and charge collection efficiency compared to the TDPP-2P:IT-M device.

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Furthermore, the J sc and V oc measurements were performed under different light intensities (P light ) to identify the charge recombination mechanisms of these devices. Fig. 3b shows the relationship of J sc and (P light ) which can be described as J sc p (P light ) g , 66 where g is the bimolecular recombination exponential factor. The recorded g values were found to be 0.91 for ETDPP-2P:IT-M and 0.88 for TDPP-2P:IT-M. When the g value is close to 1, indicating that bimolecular recombination in the ETDPP-2P:IT-M device is negligible. 67 On the other hand, to analyse the trap-assisted recombination of these devices, as shown in Fig. S2 (ESI †) the plot of V oc against the light intensity (P light ) is constructed in log scales, which can be expressed as V oc p (SkT/q)ln(P light ), using the slope equation where k is Boltzmann's constant, T is the temperature, and q is the elementary charge. Generally, the S parameter value is in the range of 1 to 2. When S 41, it indicates the existence of trapassisted recombination and when S = 1, it indicates that it is free of trap-assisted recombination. The recorded S values are found to be 1.36 (kT/q) for ETDPP-2P:IT-M and 1.51 (kT/q) for TDPP-2P:IT-M. Both the devices show trap-assisted recombination, however, the device ETDPP-2P:IT-M has less extent of traps than the device of TDPP-2P:IT-M, leading to an improved FF of 61.3% and a V oc of 0.84 V.
In addition, the charge mobilities of the devices were measured by using the space charge limited current (SCLC) method. The optimal device structure of glass/ITO/PEDOT:PSS/ donor:acceptor/MoO 3 /Au was used for the hole only device and glass/Al/donor:acceptor/Al was used for the electron only device.
The hole and electron mobilities (m h and m e ) were calculated from the slopes of J 1/2 -V curves as shown in Fig. 4, respectively, and the data are summarized in Table 2. The m h mobilities of the blend films were fitted to be 7.16 Â 10 À5 cm 2 V À1 s À1 for ETDPP-2P:IT-M and 3.64 Â 10 À5 cm 2 V À1 s À1 for TDPP-2P: IT-M. And the m e mobilities were recorded to be 5.83 Â 10 À5 cm 2 V À1 s À1 for ETDPP-2P:IT-M and 1.47 Â 10 À5 cm 2 V À1 s À1 for TDPP-2P:IT-M. The balanced m e /m h mobility ratio was calculated to be 0.81 for ETDPP-2P:IT-M and 0.40 for TDPP-2P:IT-M. The higher and more balanced mobilities of the ETDPP-2P:IT-M blend film contribute to its high J sc and FF values.
The morphology of the active layers was measured by using trapping-mode atomic force microscopy (AFM). As illustrated in Fig. 5a and b, the height images of ETDPP-2P:IT-M and    TDPP-2P:IT-M exhibit a smooth surface morphology with a root-mean-square (RMS) roughness of 2.01 nm and 2.46 nm, respectively. The lower RMS of the ETDPP-2P:IT-M film would be beneficial to construct bi-continuous interpenetrating networks when compared to that of the TDPP-2P:IT-M counterpart. This is essential to promote the exciton dissociation and charge transport, as discussed previously, that enables relatively high J sc and FF values. On the other hand, the phase images in Fig. 5c and d show a more well-ordered morphology of the donor and acceptor interlayers in the blend ETDPP-2P:IT-M film, which facilitates the efficient charge dissociation and transfer in the device.

Conclusions
In summary, we have designed and synthesized two new porphyrin dimers TDPP-2P and ETDPP-2P constructed from the dimeric porphyrin core linked by TDPP and diethynyl substituted TDPP, respectively. Due to the longer p-conjugation and more planar structure, the ETDPP-2P solid film exhibits a stronger and much red-shifted absorption spectrum ranging from 300 to 1000 nm, whereas TDPP-2P shows a narrower absorption from 300 to 750 nm due to its less p-conjugation and planarity. And the deep absorption valley between the Soret band and Q-band of the ETDPP-2P film could be perfectly compensated by the SM acceptor IT-M. Consequently, the optimal device based on the active layer of ETDPP-2P:IT-M possesses a panchromatic photoresponse in the range of 300-900 nm and a PCE of 5.69% with a V oc of 0.84 V, a J sc of 11.05 mA cm À2 and an FF of 61.3%. In contrast, the device based on the active layer of TDPP-2P:IT-M shows a lower PCE of 4.12% with a V oc of 0.79 V, a J sc of 8.96 mA cm À2 , and an FF of 58.2%. To date, the molecular behaviours and photovoltaic properties of a porphyrin donor with a NFSM acceptor are still underexplored; thus, our study can offer a crucial guidance for the development of porphyrin-based all small molecule organic solar cells.

Conflicts of interest
There are no conflicts to declare.