Annealing-Induced Changes in the Molecular Orientation of Poly-3-hexylthiophene at Buried Interfaces

The molecular organization at interfaces of organic semiconducting materials plays a crucial role in the performance of organic photovoltaics and field effect transistors. Vibrational sum-frequency generation (VSFG) was used to characterize the molecular orientation at interfaces of regioregular poly-3-hexylthiophene (rrP3HT). Polarization-selected VSFG spectra of the CC stretch of the thiophene ring yield the orientation of the conjugated backbone of P3HT, which is directly relevant to the electronic properties at the interface. The molecular orientation at buried polymer–substrate interfaces was compared for films spin-cast on SiO₂ and AlO_X substrates, before and after thermal annealing at 145 °C. On SiO₂, annealing results in the thiophene rings adopting a more edge-on orientation, tilting away from the surface plane by Δθ = +(3–10)°. In contrast, an opposite change is observed for films deposited on AlO_<i>x</i>, Δθ = −(3–26)°, where annealing leads to a more face-on orientation of the thiophene rings of the polymer. Although subtle, such orientational changes may significantly affect the electron transfer rates across interfaces and hence the overall photovoltaic efficiency

Synergistically Enhanced Performance of Ultrathin Nanostructured Silicon Solar Cells Embedded in Plasmonically Assisted, Multispectral Luminescent Waveguides

Ultrathin silicon solar cells fabricated by anisotropic wet chemical etching of single-crystalline wafer materials represent an attractive materials platform that could provide many advantages for realizing high-performance, low-cost photovoltaics. However, their intrinsically limited photovoltaic performance arising from insufficient absorption of low-energy photons demands careful design of light management to maximize the efficiency and preserve the cost-effectiveness of solar cells. Herein we present an integrated flexible solar module of ultrathin, nanostructured silicon solar cells capable of simultaneously exploiting spectral upconversion and downshifting in conjunction with multispectral luminescent waveguides and a nanostructured plasmonic reflector to compensate for their weak optical absorption and enhance their performance. The 8 μm-thick silicon solar cells incorporating a hexagonally periodic nanostructured surface relief are surface-embedded in layered multispectral luminescent media containing organic dyes and NaYF₄:Yb³⁺,Er³⁺ nanocrystals as downshifting and upconverting luminophores, respectively, <i>via</i> printing-enabled deterministic materials assembly. The ultrathin nanostructured silicon microcells in the composite luminescent waveguide exhibit strongly augmented photocurrent (∼40.1 mA/cm²) and energy conversion efficiency (∼12.8%) than devices with only a single type of luminescent species, owing to the synergistic contributions from optical downshifting, plasmonically enhanced upconversion, and waveguided photon flux for optical concentration, where the short-circuit current density increased by ∼13.6 mA/cm² compared with microcells in a nonluminescent medium on a plain silver reflector under a confined illumination