Correlation between Polymer Structure and Polymer:Fullerene Blend Morphology and Its Implications for High Performance Polymer Solar Cells

Abstract

We synthesized four polymers (pT3DPP-HD, pT3DPP-OD, pT2TTDPP-HD, and pT2TTDPP-OD) and characterized their photovoltaic properties as a function of the backbone planarity, alkyl side chain length, and film morphology. The polymers were donor–acceptor type low-band-gap (1.2–1.3 eV) polymers employing terthiophene (T3) or thiophene–thieno[3,2-b]thiophene–thiophene (T2TT) as the donor and 2,5-bis(2-hexyldecyl)pyrrolo[3,4-c]pyrrole-1,4-(2H,5H)-dione (DPP-HD) or 2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4-(2H,5H)-dione (DPP-OD) as the acceptor. The T2TT moiety in the polymer backbone is more planar than the T3; the OD moiety as the alkyl side chain ensured a higher solubility than the HD moiety. Polymer solar cells (PSCs) were fabricated, and their properties were characterized. The photoactive layer consisted of one of the four polymers and one of the fullerene derivatives (PC70BM or PC60BM). For a given fullerene derivative, the PCEs prepared with each of the four polymers were ordered according to pT3DPP-OD, pT2TTDPP-HD, pT3DPP-HD, and pT2TTDPP-OD. Studies on the morphologies of the polymer:fullerene layers revealed that the pT3DPP-OD:PC70BM blend exhibited an optimal degree of phase separation between the polymer and the fullerene, while retaining a high degree of interconnectivity, thereby yielding the highest PCE measured in this series. By contrast, the pT2TTDPP-OD:fullerene yielded the lowest PCE because of too high crystalline fibrous polymer domains. In conclusion, we demonstrate that minute variations in the polymer chemical structure strongly affects both (i) the nanoscale miscibility between the polymers and fullerenes and (ii) the interconnectivity of the polymer chains, and these properties are tightly correlated with the solar cell performance.

Ultrafast Spectroscopic Study of Donor–Acceptor Benzodithiophene Light Harvesting Organic Conjugated Polymers

“…Broad absorption in the visible spectrum is crucial for generation of higher free carrier concentrations which The Journal of Physical Chemistry C drastically impacts the performance of light harvesting conjugated polymers. 48 More charge carriers result in more conversion of photons into electrical work. The results from this study suggest that the donor−acceptor conjugated polymers with the stronger electron-withdrawing monomers will be able to harvest a wide spectrum of photons, which will result in more charge carrier generation which will increase the efficiency of the solar cell.…”

Improving the Photovoltaic Performance of Polymer Solar Cells Based on Furan-Flanked Diketopyrrolopyrrole Copolymers via Tuning the Alkyl Side Chain

“…The strong electron deficient character of the DPP endows the CPs with an absorption in the near-infrared and ambipolar charge transport in organic field effect transistors with good mobilities for holes and electrons, which is favorable for the highly efficient PSCs. − In addition, in the process of CPs’ design, the effect of solubilizing alkyl chain should not be ignored. Type, length, and position of the alkyl side chains on a CP backbone can significantly influence the processability, morphology, and transport channel formation in CP/fullerene blend film, which determine the PSC’s performance. − For donor–acceptor (D–A) CPs including dithieno­[2,3- b ;7,6- b ]­carbazole (DTC) and DPP, the alkyl chain on the DTC unit has a strong impact on the film morphology of CP/PC 71 BM blends. Severe phase separation was found for polymers containing branched alkyl chains, whereas the CPs with straight alkyl chains formed uniform films featuring fine phase separation .…”

Enhancement of photovoltaic performance by two-step dissolution processed photoactive blend in polymer solar cells

“…The morphology of the photoactive layer can be optimized towards better uniformity, crystallinity, and enhanced D/A interface area by engineering the solution process of the PSCs [15][16][17][18], which includes the selection of a proper solvent with high solubility of the polymer [19,20], using mixed solvent systems [21][22][23], and combining the additive and solvent [24]. Besides the above methods, approaches like solution heating [25], solution freeze-drying [26], and the poor solvent inducing method [27] have also been used to regulate the morphology and structure of the photoactive layer.…”