Synthesis and Nanoarchitectonics of Novel Squaraine Derivatives for Organic Photovoltaic Devices

Necessary advancements in the area of organic photovoltaic (OPV) devices include the upgrade of power conversion efficiencies (PCE) and stability. One answer to these demands lies in the research into new absorbers. Here, we focus on the development of new small molecule absorbers from the group of squaraines (SQs). These modular absorbers can be applied as donors in organic solar cells and have the ability to utilize a broad range of solar radiation if blended with suitable acceptors. In order to allow for the compatibility and favorable organization of donor and acceptor in the absorber layer, we intend to optimize the structure of the SQ by varying the groups attached to the squaric acid core. For that purpose, we accordingly developed a well-suited synthesis route. The novel alkyl- and benzyl-substituted aryl aminosquaraines were synthesized through an improved and eco-friendly procedure. Special emphasis was placed on optimizing the amination reaction to obtain initial precursors in the synthesis of squaraine, avoiding hitherto common catalytic processes. All SQ precursors and SQ products were completely described. The derived SQs were additionally characterized in thin-film configuration using cyclic voltammetry and UV-VIS spectroscopy and then processed to prepare self-standing bulk heterojunction (BHJ) thin films in conjunction with fullerene-based electron acceptors, which were characterized via profilometry. The comparison between SQ and BHJ solutions and thin films, using atomic force microscopy and UV-VIS spectroscopy, revealed differences in susceptibility for the organization and orientation of the constituting domains.

Comparison of the Solution and Vacuum-Processed Squaraine:Fullerene Small-Molecule Bulk Heterojunction Solar Cells

Squaraine dyes have shown promising properties for high performance organic solar cells owing to their advantages of intense absorption and high absorption coefficients in the visible and near-infrared (NIR) regions. In this work, to directly compare the photovoltaic performance of solution- and vacuum-processed small-molecule bulk heterojunction (SMBHJ) solar cells, we employed a squaraine small molecular dye, 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (DIBSQ), as an electron donor combined with fullerene acceptors to fabricate SMBHJ cells either from solution or vacuum deposition process. The solution-processed SMBHJ cell possesses a power conversion efficiency (PCE) of ~4.3%, while the vacuum-processed cell provides a PCE of ~6.3%. Comparison of the device performance shows that the vacuum-processed SMBHJ cells possess higher short-circuit current density, fill factor and thus higher PCE than the solution-processed devices, which should be assigned to more efficient charge transport and charge extraction in the vacuum-processed SMBHJ cells. However, solution-processed SMBHJ cells demonstrate more pronounced temperature-dependent device performance and higher device stability. This study indicates the great potential of DIBSQ in photovoltaic application via both of solution and vacuum processing techniques.

Colorful Squaraines Dyes for Efficient Solution-Processed All Small-Molecule Semitransparent Organic Solar Cells

Semitransparent organic solar cells (ST-OSCs) exhibit highly promising applications to develop integrated photovoltaics and power-generating windows. However, the development of ST-OSCs is significantly lagging behind opaque OSCs, especially for all small-molecule ST-OSCs. Here, four unique squaraines dyes (IDPSQ, SQ-BP, D-BDT-SQ, and AzUSQ) were successfully used as donors in ST-OSCs, whose colors can be tuned from purple to blue, green, and dark green, respectively. While using ultrathin Ag as a transparent electrode, the ST-OSCs fabricated using IDPSQ:PC₇₁BM, SQ-BP:PC₇₁BM, D-BDT-SQ:PC₇₁BM, and AzUSQ:PC₇₁BM yield power conversion efficiencies (PCEs) of 2.96, 4.36, 4.91, and 1.71%, respectively, and their colors are purple, cyan, brown, and light brown, respectively. Compared to their opaque OSCs (PCEs of 3.95, 5.45, 5.84, and 1.91%, respectively), the reduction in the PCEs are as low as 25, 20, 16, and 10%, respectively. Furthermore, each of these ST-OSCs exhibit good average visible transmittance (AVT) of 25-30%, favorable CIE color coordinates, and a color rendering index (CRI) of 71-97%. Finally, by changing the thickness of the Ag electrode, an impressive PCE of 4.9% along with an AVT of 25% and a CRI of 97% can be obtained in D-BDT-SQ:PC₇₁BM-based ST-OSCs, which is the highest PCE among all small-molecule ST-OSCs.

Optimized synthesis of π-extended squaraine dyes relevant to organic electronics by direct (hetero)arylation and Sonogashira coupling reactions

This study reports on the synthesis and characterization of four molecular π-extended squaraine compounds relevant to the field of organic electronics.

Unsymmetrical squaraines with new linkage manner for high-performance solution-processed small-molecule organic photovoltaic cells

Two unsymmetrical squaraines were employed as donors for high performance BHJ-OPV devices with J_sc > 13 mA cm⁻² and PCE > 5%.

Asymmetrical Squaraines Bearing Fluorine-Substituted Indoline Moieties for High-Performance Solution-Processed Small-Molecule Organic Solar Cells

Two novel asymmetrical squaraines based on the indoline unit, ASQ-5-F and ASQ-5-DF, with one and two fluorine substituents, have been developed to investigate the effect of fluorine substituted on small-molecule bulk-heterojunction (BHJ) organic solar cells (OSCs). In comparison with non-fluorine-substituted ASQ-5, both fluorine-substituted ASQ-5-F and ASQ-5-DF possess analogous absorption band gaps but 0.05 and 0.10 eV lowered highest occupied molecular orbital (HOMO) energy levels, respectively. Single-crystal analysis exhibits that ASQ-5-DF shows more desirable intermolecular packing patterns for the hole-carrier collection than ASQ-5 does; hence, higher hole mobility could be acquired. Therefore, solution-processed small-molecule BHJ OSCs fabricated with ASQ-5-F/PC71BM and ASQ-5-DF/PC71BM blends exhibit extremely higher power conversion efficiency (PCE; 5.0% and 6.0%, respectively) than that of ASQ-5/PC71BM (4.5%). The much improved PCE could be attributed to the simultaneously enhanced Voc, Jsc, and FF relative to those of the ASQ-5-based device. To our knowledge, this is the highest PCE (6.0%) among squaraine-based solution-processed BHJ OSCs and the highest PCE in OSCs based on the fluorinated donor segment of small molecules.

Improved performances of PCDTBT:PC71BM BHJ solar cells through incorporating small molecule donor

Incorporating SQ-BP into a PCDTBT:PC₇₁BM host blend not only broadens the absorption spectrum but also decreases the energy loss of excited PCDTBT.

Small molecular thienoquinoidal dyes as electron donors for solution processable organic photovoltaic cells

Two small molecular quinoidal thiophene dyes, featuring low molecular weight, high molar extinction coefficient, and narrow band-gap, have been synthesized as donors for organic photovoltaic cells, giving a best power conversion efficiency of 5.12%.