6.1 Research background and motivation for the work
All polymer solar cells (all-PSCs), composed of binary blends of polymeric donor and acceptor, offer tunable optical and electrochemical properties as well as unique mechanical ductility for the application in portable and wearable devices.[1] While intensive efforts have focused on maximizing the all-PSCs photovoltaic efficiencies through the design of new materials and optimal combinations of donor and acceptor components in blends, yielding impressive power conversion efficiencies (PCEs) over 7%
within a brief period, only limited studies lie in realizing the intrinsic potential of its photoactive layer in mechanical properties.200-201
Being hindered by the contradictory material design approaches for high photovoltaic properties versus robust mechanical resilience, it is not a trivial endeavor to simultaneously possessing both two attributes in a constructed bulk-heterojunction (BHJ) blend film. The efficient BHJ morphology for charge separation/transport comprises with nanoscale phase separation and highly crystalline molecular packing, yet these characteristics were commonly accompanied by significant stiffness and brittleness behaviors, leading to the film fracture even under a very limited strain.17, 202 Likewise, in most cases, reducing the blending components crystallinity through structure modification induces better mechanical ductility but serious PCE degradation, due to the increased amorphous fractions in solid states growing charge recombination possibilities.203 Therefore, to surmount such incompatibilities between the photovoltaic and mechanical properties of the photoactive layer in one all-PSC remains a challenge.
Here, we developed a simple, high-viscosity linear polydimethylsiloxane with phenethyl partially substituted side chains (PDMS-S), as a macromolecular additive, to embed in the high-performance TQ-F:P(NDI2OD-T2) matrix140 via one-step solution processing, actualizing robust active layers with retaining excellent photovoltaic efficiencies in all-PSCs (Figure 6.1a). The resulting optimal FTQ:N2200:PDMS-S10 blend film exhibits not only superior toughness values of up to 9.67 MJ m-3 with a sufficient higher elongation at break of 50.92%, a prominent lower elastic modulus of only 0.54 GPa, but also a comparable PCE of 6.87% with greater fill factor (FF) of 65.96% in comparison of that in the PDMS-S-free blend film. Serving as a robust skeleton within the entangler donor and acceptor polymeric chains, the percolated networks of the PDMS-S successfully relieved external mechanical stress as well as preserved effective charge transporting channels. Thus, our work opens a new processing route to produce highly efficient and stretchable photoactive layers for portable and wearable all-PSCs.
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The high-viscosity additive PDMS-S was synthesized from trimethylsilyl terminated poly(dimethylsiloxane-co-methylhydrosiloxane) using Karstedt catalyzed hydrosilylation reaction at 80 °C for two days. The good solubility of PDMS-S in common organic solvents allows for one-step solution processing with the TQ-F:P(NDI2OD-T2) matrix. The blend films incorporated with various amounts (0, 10, 20, and 50 wt%) of PDMS-S relative to the total weight of the donor and acceptor are denoted as TQ-F:P(NDI2OD-T2), TQ-F:P(NDI2OD-T2):PDMS-S10, TQ-F:P(NDI2OD-T2):PDMS- S20, TQ-F:P(NDI2OD-T2):PDMS-S50, respectively.
Figure 6.1 (a) Chemical structures of the macromolecular additive (PDMS-S), the donor (TQ-F), and the acceptor (P(NDI2OD-T2)). (b) The film-on-water tensile test system. (c) The strain-stress curves and (d) corresponding elastic modulus and integrated toughness values of the (i) TQ-F:P(NDI2OD-T2), (ii) TQ-F:P(NDI2OD-T2):PDMS-S10, (iii) TQ-F:P(NDI2OD-T2):PDMS-S20, (iv) TQ-F:P(NDI2OD-T2):PDMS-S50 blend films. (e) Optical microscopy images of the TQ-F:P(NDI2OD-T2):PDMS-S10 blend film under different strains.
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A pseudo free-standing tensile test204 was performed to directly assess the intrinsic mechanical properties of the four blend films (Figure 6.1b). The elastic modulus, the elongation at break, and integrated toughness of the original TQ-F:P(NDI2OD-T2) film were 0.75 GPa, 32.56%, and 6.90 MJ m-3, respectively. The addition of 10 wt% PDMS-S into the TQ-F:P(NDI2OD-T2) matrix retains high tensile strength (24.62 MPa) but significantly lowers the elastic modulus to 0.54 GPa with greatly enhanced elongation at break of 50.92%, producing the highest toughness value of 9.67 MJ m-3 among the films studied (Figure 6.1c), which demonstrates crucial advantages for stretchable electronic application. Further increasing the PDMS-S fraction results in monotonically decreased elastic modulus and tensile strength; however, the elongation at break increases to its peak value of 53.15% in the TQ- F:P(NDI2OD-T2):PDMS-S20 film and then falls to 42.96% in the TQ-F:P(NDI2OD-T2):PDMS-S50 one, leading to overall declined toughness values (Figure 6.1d). The variations in mechanical properties are closely correlated with the evolutive microstructures caused by the different amounts of PDMS-S embedded (Figure 6.2a). Especially, in the optimal TQ-F:P(NDI2OD-T2):PDMS-S10 film, a well intermixed bulk morphology with percolated PDMS-S networks was observed, which primely accounts for it excellent mechanical compliance.
Besides, conventional all-PSCs with the configuration of indium tin oxide (ITO)/poly(3,4- ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)/TQ-F:P(NDI2OD-T2):PDMS- Sx/perylenediimide functionalized with amino N-oxide (PDINO)/Al were fabricated to evaluate the effect of PDMS-S on device performances. All active layers were optimized to be around 110 nm for equal comparisons. As shown in the current density-voltage (J-V) curves (Figure 6.2c), the TQ- F:P(NDI2OD-T2) reference cell obtained a decent PCE of 7.12% and an FF of 62.33%, consistent with the values reported. Upon adding PDMS-S, the short-circuit current density (Jsc) parameter drops gradually with increasing the PDMS-S fraction while maintaining similar open-circuit voltage (Voc) values in devices. Interestingly, adding 10 wt% PDMS-S provides a superior FF of 65.96% to offset the decline in Jsc (12.40 mA cm-2), yielding a comparable high PCE of 6.87% to the reference one. However, with further raise of the PDMS-S content in the matrix, a severe decrease in the FF was observed (Figure 6.2d). The improved FF of TQ-F:P(NDI2OD-T2):PDMS-S10 cell suggests that inclusion of the proper amount of PDMS-S offers fine-tune crystallinity and nanostructured blend to suppress bimolecular recombination. Moreover, the preferable face-on oriented molecular packing in FTQ:N2200:PDMS-S10 film (Figure 6.2b), as evidenced by the invisible (h00) lamellar diffraction in the out-of-plane direction in contrast to the intensified (100) diffraction in the in-plane direction, benefits the charge transfer through active layer to electrodes, which is favorable for photovoltaic application. In considering the above findings, one can speculate that the formation of finer
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interpenetrating networks with face-on orientated molecular packing is the key factor for achieving simultaneous high mechanical endurance and photovoltaic performance in all-PSCs.
Figure 6.2 (a) The TEM images, (b) close-ups of the (100) diffraction patterns in the GIWAXS and (c) corresponding current density-voltage (J-V) curves of (i) TQ-F:P(NDI2OD-T2), (ii) TQ-F:P(NDI2OD-T2):PDMS- S10, (iii) TQ-F:P(NDI2OD-T2):PDMS-S20, (iv) TQ-F:P(NDI2OD-T2):PDMS-S50 blend films. (d) PDMS-S content dependence of FF and PCE parameters in devices.
- 94 - 6.3 Conclusion
In summary, we developed a high-viscosity PDMS-S additive to help construct intrinsically stretchable BHJ layers with a much-enhanced elongation at break over 50%, while maintaining decent photovoltaic efficiencies comparable to the original one in all-PSCs. Our study not only paves a simple and robust approach to manufacture efficient ductile films but also provides guidelines for the design of active layers to overcome the reverse relationship between the photovoltaic and mechanical properties of organic semiconductors.
#Manuscript Preparation
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