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The glass transition temperature (Tg) of a conjugated polymer can be used to predict its morphological stability and mechanical properties. Despite the importance of this parameter in applications from organic solar cells to wearable electronics, it is not easy to measure. The Tg is often too weak to detect using conventional differential scanning calorimetry (DSC). Alternative methods—e.g., variable temperature ellipsometry—require specialized equipment. This paper describes a technique for measuring the Tg of thin films of semicrystalline conjugated polymers using only a hot plate and an ultraviolet–visible (UV–vis) spectrometer. UV–vis spectroscopy is used to measure changes in the absorption spectrum due to molecular-scale rearrangement of polymers when heated past Tg, corresponding to the onset of the formation of photophysical aggregates. A deviation metric, defined as the sum of the squared deviation in absorbance between as-cast and annealed films, is used to quantify shifts in the absorption spec...

The ways in which organic solar cells (OSCs) are measured and characterized are inefficient: many substrates must be coated with expensive or otherwise precious materials to test the effects of a single variable in processing. This serial, sample-by-sample approach also takes significant amounts of time on the part of the researcher. Combinatorial approaches to research OSCs generally do not permit microstructural characterization on the actual films from which photovoltaic measurements were made, or they require specialized equipment that is not widely available. This paper describes the formation of one- and two-dimensional gradients in morphology and thickness. Gradients in morphology are formed using gradient annealing, and gradients in thickness are formed using asymmetric spin coating. Use of a liquid metal top electrode, eutectic gallium–indium (EGaIn), allows reversible contact with the organic semiconductor film. Reversibility of contact permits subsequent characterization of the specific areas o...

An organic solar cell based on a bulk heterojunction (BHJ) of a polymer and a methanofullerene ([60]PCBM or [70]PCBM) exhibits a complex morphology that controls both its photovoltaic and mechanical compliance (robustness, flexibility, and stretchability). Methanofullerenes are excellent electron acceptors; however, they have relatively high cost and production energy (in the purest samples) compared to other small-molecule semiconductors. Moreover, [60]PCBM and [70]PCBM-typical of van der Waals solids-can be stiff and brittle. Stiffness and brittleness may lower the yield of working modules in roll-to-roll manufacturing, shorten the lifetime against mechanical failure in outdoor conditions, and jeopardize wearable and portable applications that demand stretchability or extreme flexibility. This paper tests the hypothesis that technical grade PCBM (incompletely separated but otherwise pure blends containing >= 90% [60]PCBM or [70]PCBM) could lower the cost of manufacturing organic solar cells while simultaneously increasing their mechanical stability. Measurements of the tensile modulus of five methanofullerene samples, technical grades and 99% grades of both [60]PCBM and [70]PCBM, and a 1:1 mixture [60]PCBM and [70]PCBM, along with their blends with regioregular poly(3-hexylthiophene) (P3HT), lead to two important conclusions: (1) films of pure [70]PCBM are approximately five times more compliant than films of pure [60]PCBM; BHJ films with [70]PCBM are also more compliant than those with [60]PCBM. (2) BHJ films comprising technical grades of [60]PCBM and [70]PCBM are approximately two to four times more compliant than are films made using 99% grades. Tensile modulus is found to be an excellent predictor of brittleness: BHJs produced with technical grade methanofullerene accommodate strains 1.4-2.2 times greater than those produced with 99% grades. The smallest range of stretchability was found for BHJs with 99% [60]PCBM (fracture at 3.5% strain), while the greatest was found for technical grade [70]PCBM (11.5% strain). Mechanical properties are correlated to the microstructures of the blended films informed from analyses of UV-vis spectra using the weakly interacting H-aggregate model. Photovoltaic measurements show that solar cells made with technical grade [70]PCBM have similar efficiencies to those made with higher-grade material but with decreased cost and increased mechanical robustness.

This Perspective describes electronic materials whose molecular structure permits extreme deformation without the loss of electronic function. This approach—“molecularly stretchable” electronics—is complementary to the highly successful approaches enabled by stretchable composite materials. We begin by identifying three general types of stretchable electronic materials: (1) random composites of rigid structures sitting atop or dispersed in an elastic matrix, (2) deterministic composites of patterned serpentine, wavy, or fractal structures on stretchable substrates, and (3) molecular materials—noncomposite conductors and semiconductors—that accommodate strain intrinsically by the rational design of their chemical structures. We then identify a short-term and a long-term goal of intrinsically stretchable organic electronics: the short-term goal is improving the mechanical stability of devices for which commercialization seems inevitable; the long-term goal is enabling of electronic devices in which every co...