Quantifying Squaraine Phase Separation, Aggregate Populations, and Solar Cell Performance with a Correlated Multimethod Approach

Abstract

Both the nature of the excited state and the morphology of the active layer combine to greatly influence the efficiency in all-small-molecule organic photovoltaic devices. Hence, squaraines, for which synthesis is simple, reproducible, and low-cost, will be favored active layer materials for commercialization when their higher device efficiencies can be correlated with phase separation, miscibility, and crystallinity. Here, multiple measurement methods are used to provide a self-consistent framework that connects active layer morphology, excited-state populations, and device efficiency. Thin-film thickness is accurately measured with atomic force microscopy, which leads to the extinction coefficient for absorption. Spectroscopy is subsequently used to measure populations of electronic states as squaraines are blended with varying amounts of [6,6]-phenyl C 61 butyric acid methyl ester (PCBM). The populations interchange between such observed states as monomers in dilute mixtures, dimers in concentrated mixtures, and chargetransfer H-aggregates in pure/crystalline regions. The measured efficiency of devices is shown to correlate with these changing populations. A model is presented, which allows for incorporation of DBSQ(OH) 2 molecules within PCBM interstices. The model also accounts for phase separation in as-cast squaraine/PCBM mixtures for concentrations of squaraine above the eutectic point. The increased efficiency achieved through changing excited-state populations informs a process such that devices can be manufactured to have their highest efficiency when their thermodynamically stable state is burnt into the morphology while the device is in use.

Directional Exciton Diffusion, Measured by Subpicosecond Transient Absorption as an Explanation for Squaraine Solar Cell Performance

“…The dashed line spectra of as-cast pure DBSQ­(OH) 2 and DHSQ­(OH) 2 thin films are also, respectively, shown in Figure a and b. As has been described in prior work, , the shoulder at around 600 nm for DBSQ­(OH) 2 represents the spectroscopic contribution of dimer H-aggregates (DHA), , which are closely coupled but randomly oriented pairs of squaraines with attributes of H-aggregation. As PCBM is introduced that coupling is reduced and the 600 nm peak falls away, as demonstrated by the solid line spectrum for DBSQ­(OH) 2 , wherein the peak at 680 nm dominates, which originates from well-mixed monomer squaraines.…”

“…Yet, efficiency increases have recently been observed for devices with extensive CT-HA formation when concomitant phase separation was actively limited. 50,54 Above all, it is necessary to provide a more encompassing and improved understanding of the energy transfer properties of CT-HAs in the development of OPV devices. A significant amount of work has already been done on the energy transfer properties of both Squaraine monomers and squaraine DHAs.…”

“…However, commercial viability of squaraine-based devices depends on device efficiency, which has room for improvement, and an optimized thermodynamic equilibrium morphology . Through modification of SQ side-chain substituents, the thermodynamics of mixing with a fullerene can be dramatically altered without substantial changes to energy levels resulting from that functionalization. − This means that a variety of different morphologies are possible both through the formation of a temperature sensitive kinetic equilibrium during spin-casting and through the thermodynamically driven phase separation or mixing which occurs over time. This is extremely powerful for OPV research, allowing for evaluation of a range of morphologies, spanning extremely well blended to highly phase separated structures.…”

“…With SQ phase separation from fullerene acceptors, to form large domains alongside CT-HA formation, lower device efficiency has been registered, − although this domain enlargement occurs in parallel to CT-HA formation, and so the relative contribution of each factor to efficiency loss is hard to quantify conclusively. Yet, efficiency increases have recently been observed for devices with extensive CT-HA formation when concomitant phase separation was actively limited. , Above all, it is necessary to provide a more encompassing and improved understanding of the energy transfer properties of CT-HAs in the development of OPV devices.…”

Azulene‐Containing Bis(squaraine) Dyes: Design, Synthesis and Aggregation Behaviors

“…[7] The aggregation behavior can be modulated under the combined effect of intermolecular forces, such as dispersion force, electrostatic force and π-electron interactions. [8] For example, the performance parameters of molecular materials can be influenced through thermal treatment, [9] changes of solvent environment [10] or the application of external force. [11] Compared with popular aromatic rings used in constructing squaraines, azulene is a kind of nonbenzenoid aromatic hydrocarbon with remarkable dipole moment (1.08 D) and unique physicochemical properties, which is originated from the fusion of an electron-rich five-membered ring and an electrondeficient seven-membered ring and its non-mirror symmetrical frontier orbitals.…”