Novel cost-effective acceptor:P3HT based organic solar cells exhibiting the highest ever reported industrial readiness factor

Novel acceptor enhances the industrial readiness of solution based organic solar cells for low-cost electricity production.


Introduction
For Organic solar cells (OSCs) to become economically viable, an important element to consider would be the production cost of the end product, i.e. electricity [1][2][3][4] . This is mainly influenced by the cost of manufacture, power conversion efficiency (PCE), stability of the produced OSCs, and installation & maintenance (which will not be further evaluated, herein) 2 . The cost of manufacture has two main inputs: materials and processing. It is well-known that processing has to be conducted using techniques which are roll-to-roll compatible ， as high-throughput is a necessity for reducing the processing cost, such techniques include slot-die coating, flexographic printing, screen printing, and to some extent thin-film vacuum deposition methods [5][6][7][8][9] . In contrast, the cost of materials is a more debatable subject as a trade-off between cost of materials and the PCE of the produced OSCs are generally present. As record performances have only been achieved with highly complex molecules requiring a multitude of synthetic steps and limiting the overall yield, thereby increasing the material costs [10][11][12][13] . Whereas P3HT is generally considered as the ideal donor (D) candidate for upscaling of OSCs with its simple synthesis, high hole mobility and high stability towards photo-oxidation. However, OSCs prepared with such simple low-cost materials as P3HT:PCBM commonly exhibit performance limitations, due to reduced photon absorption and large energy loss. These performance limitations have been overcome/improved by the implementation of non-fullerene acceptors (NFA) 14,15 for devices paired with P3HT with Voc as high as 1.22 V 16 and the extended absorption from ~650 nm to ~850 nm, leading to PCEs surpassing 6 % when O-IDTBR 17 and P3HT:SF(DPPB) 4 18-21 were chosen. While somewhat simplified electron acceptors such as DFPCBR still reaches efficiencies of 5.25 %. 22 The highest efficiency achieved for P3HT based devices were reported recently by Xu et al. therein they presented an impressive device efficiency of up to 8.25 % 23 , however, this efficiency was only achieved through time-dependent ink modifications which definitely reduces the scalability of the ink system. Therefore, P3HT:O-IDTBR is still viewed as the most likely candidate for upscaling, even though the material system achieved an impressive PCE, later studies of the industrial figure of merit or industrial readiness (i-FOM) revealed that this material combination only scored 0.17 whereas 0.7 has been hypothesized as the required value 11 . The i-FOM score is deducted from the relationship among device efficiency, stability, and synthetic complexity, revealing that simpler molecules with a reduced synthetic complexity could reduce the gap to the required i-FOM.
Herein we present two approaches for simplifying O-IDTBR to improve the obtained i-FOM value. Initially the benzothiadiazole was omitted to a avoid large number of synthetic steps, thereby reducing the synthetic complexity, moreover the electron donating core was substituted with less fused moieties to either adopt cost-effective starting materials or reduce their synthetic complexity 24 even further and also to evaluate the effect of a core degree of fusing. The molecules presented herein are 4,9-dihydro-4,4,9,9-

Physical properties
The chemical structure of the four novel electron acceptors (A1-A4) can be seen in Figure 1A. The core molecules were synthesized according to literature [25][26][27][28] after which Knoevenagel condensation between 3-ethylrhodanine and the dialdehyde functionalized core molecules were utilized to obtain the designed electron acceptors with a yield of 71.6 %, 50.0 %, 37.8 %, and 60.1 % for A1, A2, A3, and A4, respectively, further synthetic details can be seen in supporting information as well as molecular characterization (NMR Figure S7 and MS Figure S8). The planarity of the prepared molecules was investigated through density functional theory (DFT) as seen in Figure 1B, A1 was calculated to be essentially planar whereas both A2 and A3 appears less planar due to their non-fused core, e.g., for A2, the non-fused half of the molecule has a steric twist disrupting planarity and conjugation. The introduction of the ether bond into the fused part of the core molecule seems to create an additional steric twist within A3, resulting in a slight twisting of the fused part of the molecule. The calculations for A4 also displayed a dominantly planar molecule with just the slightest steric twisting. The increased planarity of A1 promotes an increase in conjugation length which in turn manifested in a red-shift of the absorption onset of 33 nm when compared to A2 ( Figure 1C and Table 1). Moreover, A1 also displays an enhanced J-aggregation strength (a relative enhancement in the primary absorption peak (first peak/plateau from the right in the absorption spectra) compared to the secondary absorption peak) which is associated with a reduced voltage loss [29][30][31] . The film absorption ( Figure 1D) for all four materials exhibit red-shifts in their onset in a range of 29 -42 nm, however, whereas A1 and A4 presents a λ Max red-shift of 16 nm , A2 and A3 have slight blue-shifts in λ Max of 13 and 17 nm, respectively, which implies a hinderance in π-π stacking likely due to the steric twists within the molecules 32,33 . The absorbance spectra of blended films in a P3HT:Acceptor of 1:1 can be found in Figure S1 supplementary information. Cyclic voltammetry (CV) (supplementary information Figure S2) of the thin films shows that A1-A3 have very similar highest occupied molecular orbital (HOMO) values around -5.6 eV whereas A4 possess a slightly lower one with -5.7 eV. As for the lowest unoccupied molecular orbital (LUMO), A1, A3, and A4 present very similar values in the range of -3.55 eV to -3.59 eV while A2 displaying a slightly higher one of -3.41 eV. The LUMO level of P3HT was measured as comparison to be -2.74 eV which would allow for sufficient energetic offset for optimum energy transfer between the donor and the acceptor(s). These acceptors are therefore all suitable for the preparation of OSCs pairing with P3HT as a donor.

Photovoltaic properties
Solar cells were prepared with P3HT as the donor and A1 -A4 as the acceptor, respectively, due to their matching energy levels ( Figure 2A) and low synthetic complexity. Devices were prepared in a normal geometry with a device stack of glass/ITO/PEDOT:PSS Al4083/Active layer/ZnO/Ag as displayed in Figure 2B, similar to previously reported. 34 All material combinations were optimized for annealing conditions (see supporting information), D : A ratio ( Figure 3A, C, E, and G), and active layer film thickness ( Figure 3B, D, F, and H), the specific photovoltaic characteristics can be found in supplementary information Figure S3 -S6 for A1 -A4, respectively. The optimized J-V curve for the four material combinations can be found in Figure 2C with the corresponding photovoltaic characteristics summarized in Table 2.   Figure 3A, C, E, and G).
Interestingly, such optimized ratio switches unfavourably towards higher quantity of acceptor as the core becomes less fused and thereby obtains more degrees of freedom. Thinner films were generally found to be favourable, with only P3HT:A1 devices being improved upon increasing the film thickness from around 55 nm to 70 nm. The average optimized performances were found to be 5.39 %, 3.37 %, 3.21 % and 1.00 % for devices from P3HT:A1, A2, A3, and A4, respectively, as seen in Figure 3B, D, F, H. The discovered limitations in film thicknesses could be ascribed to low charge carrier mobilities presented herein and sufficient to ensure efficient dissociation (LUMO offsets significantly larger than the hypothesized 0.3 eV). It is therefore more likely that the J SC for these devices, especially P3HT:A4, is hindered by a non-optimal phase-separation allowing excitons to be recombined prior to dissociation. Free carrier extraction has been discussed above with P3HT:A2-A4 exhibiting a vastly imbalanced carrier mobility.

33.2
In order to do a proper quantification of the difference between A1 and O-IDTBR (and other novel materials), the previously published standardized i-FOM was applied for A1 and compared with that for O-IDTBR. 11 The i-FOM is given by a relationship amongst device efficiency, stability after 200 h of light exposure, and synthetic complexity (SC) as presented in Equation 1.     Upscaling of P3HT:A1 to large area slot-die coated ITO-free organic solar cells. have previously been utilized with P3HT:O-IDTBR with great success. 38,39 However, from the obtained results herein ( Figure 5B), it can be seen that only DIO or o-DCB with PCE of 1.1 % enhances the device efficiency above the CB:CF base solvent mixture. As the material combination have proven to be highly thickness dependent, a series of device with active layer thicknesses ranging from 70-220 nm were prepared. As illustrated in Figure 5C, 100 nm is found to be the optimized active layer thickness with slight decrease in efficiency of approximately 10 % for the a bit thinner one (70 nm), and with a plateau of 70 % of the optimum for devices with active layer above 150 nm . The underlying reasons for the relatively low device performance were investigated through a thermal annealing test as can be seen in Table S4 in the supplementary information, 140° C appears to be too high a temperature as the device efficiency only decreases with extended annealing time. Therefore, the annealing temperature was lowered to 120° C for both pre-and post-annealing of the prepared devices for the following experiments. The ink composition was optimized firstly through varying the o-DCB concentration, giving a peak efficiency of 3.58 % at 20 % o-DCB with low molecular weight P3HT (20 kDa). While high molecular weight P3HT (60 kDa) only allows a hero efficiency of 2.68 % as seen in Figure 5D. Secondly through variation of chloroform content in the inks as displayed in Figure 5E, a chloroform concentration of 20 % appears to be optimum with a slight device efficiency increase from 10 % to 20 % before decreasing again at 30 % chloroform in the ink mixture. From Figure 5E it is also clearly evident that devices with ethanol based ZnO have superior performance over acetone based ones, likely do to with solubilities as A1 exhibits slight soluble in acetone, leading to a depletion of A1 in the top part of the active layer and a mixed ETLdue to the deposition of acetone based ZnO. In recent literatures, a multitude of minor deposition variations have proven highly efficient for the sake of enhancing the efficiency of slot-die coated OSCs devices. Initially, Na et al. presented an impressive efficiency optimization from approximately 2.5 % to around 8 % through controlling the substrate and slot-die coating-head temperatures during active layer deposition. 40 Implementing this approach did, however, not have a positive influence on our prepared devices, as we observed a decrease in device performance from around 2.5 % to 1.2 % with increased substrate temperature from 60 to 80° C, and a further temperature increase to 100° C did not appear to change the performance further, as seen in Figure 6A. Meng et al. published another approach, in which they significantly decreased the concentration of the ink, illustrating that the obtained morphology can much better emulate spin-coating with a PCE around 9.5 %. 41 Meanwhile this approach did not result in improved device efficiencies for the system presented herein, as seen in Figure 6B, no differences were observed in the device efficiency when diluting the active layer ink from 12.6 mg mL -1 to 6.3 mg mL -1 .

Conclusion
In this work, we present four novel small molecule non-fullerene acceptors with four different coremolecules (4,9-dihydro-4,4,9,9-  ethylrhodanine. These molecules were designed as synthetically simplified alternatives to O-IDTBR as acceptors for the best cost-effective donor polymer P3HT. As a consequence of the simplified structure of A1-A4, their optical absorbance has a greater overlap with P3HT than previously reported O-IDTBR. The optical band gaps of the acceptor presented herein are between 1.95 eV and 2.05 eV, due to their difference in core rigidity allowing A2 to twist thereby disrupting the molecular packing.