Tuning the Organic Solar Cell Performance of Acceptor 2,6-Dialkylaminonaphthalene Diimides by Varying a Linker between the Imide Nitrogen and a Thiophene Group

Core-substituted naphthalene diimides (NDI) are promising candidates as acceptors for organic solar cells. To study their structure–property relationships, a series of 2,6-dialkylamino-NDI compounds with various substituents were synthesized, characterized, and tested in bulk heterojunction solar cells by blending with regioregular poly­(3-hexylthiophene) (P3HT). The imide substituents consisted of a linker connected to a thiophene group, where the linker was phenyl, methyl, or ethyl. The core substituents were cyclohexylamino or 2-ethylhexylamino. While the various substituents had little effect on the optoelectronic properties in solution, they strongly affected device performance and blend morphology. Under the conditions studied, the best performance was obtained with the methyl linker combined with the cyclohexylamino core substituent, with a power conversion efficiency of 0.48% and a high open circuit voltage of 0.97 V. For blends of P3HT with modified NDI non-fullerene acceptors, the methyl linker promoted larger phase-separated domains than the ethyl or phenyl linkers. DFT calculations showed that the linker determines the orientation of the thiophene conjugated plane with respect to the NDI conjugated plane. That angle was 114°, 45°–61°, and 8° for the methyl, phenyl, and ethyl linkers, respectively. Using thiophene at the end of the imide substituent adds a unique dimension to tune morphology and influence the molecular heterojunction between donor and acceptor

Synthesis of Perfluoroalkyl End-Functionalized Poly(3-hexylthiophene) and the Effect of Fluorinated End Groups on Solar Cell Performance

A series of well-defined perfluoroalkyl end-functionalized poly­(3-hexylthiophenes) (P3HT) were synthesized by Stille coupling of stannylated 2-perfluoralkylthiophene with the bromine end of P3HT. The length of the perfluoroalkyl end group was varied from −C₄F₁₃ to −C₈F₁₇. These polymers were fully characterized and tested in bulk heterojunction solar cells with phenyl-C₆₁-butyric acid methyl ester (PCBM) as the acceptor. Performance of the solar cells was highest for the unmodified P3HT and decreased as the length of the perfluoroalkyl end increased. The most affected device parameters were the short-circuit current density (<i>J</i>_sc) and series resistance, pointing to lower charge carrier mobility and poor morphology as the cause for the lower performance. While the morphology of blends did not significantly change with perfluoroalkyl end modification, analysis of blended films by energy-filtered transmission electron microscopy (EF-TEM) revealed wider P3HT domains, consistent with the perfluorinated end groups segregating to the edge or exterior of P3HT domains, causing two domains to join. This study demonstrates that the perfluoroalkyl end group can be detrimental to polymer solar cell device performance, and further work toward understanding the interface between the donor and acceptor phases is required to fully understand this effect

Direct Conversion of Hydride- to Siloxane-Terminated Silicon Quantum Dots

Peripheral surface functionalization of hydride-terminated silicon quantum dots (SiQD) is necessary in order to minimize their oxidation/aggregation and allow for solution processability. Historically thermal hydrosilylation addition of alkenes and alkynes across the Si–H surface to form Si–C bonds has been the primary method to achieve this. Here we demonstrate a mild alternative approach to functionalize hydride-terminated SiQDs using bulky silanols in the presence of free-radical initiators to form stable siloxane (∼Si–O–SiR₃) surfaces with hydrogen gas as a byproduct. This offers an alternative to existing methods of forming siloxane surfaces that require corrosive Si–Cl based chemistry with HCl byproducts. A 52 nm blue shift in the photoluminescent spectra of siloxane versus alkyl-functionalized SiQDs is observed that we explain using computational theory. Model compound synthesis of silane and silsesquioxane analogues is used to optimize surface chemistry and elucidate reaction mechanisms. Thorough characterization on the extent of siloxane surface coverage is provided using FTIR and XPS. TEM is used to demonstrate SiQD size and integrity after surface chemistry and product isolation