optoelectronics. [ 1,2 ] Advantages including reduced materials consumption, relaxed requirements of materials purity, and ability to form large-area devices on unlimited classes of module substrates make them particularly useful as building blocks for realizing high-effi ciency, low-cost photovoltaic systems. [3][4][5] The photovoltaic performance of ultrathin silicon solar cell is, however, inherently limited by incomplete absorption of longer wavelength photons near its bandgap. [ 6,7 ] While light trapping methods based on various diffractive and/ or refl ective optical elements can greatly help to improve the absorption of optically thin silicon, [8][9][10][11][12][13] complementary means to additionally capitalize on such low energy photons are desirable to further improve the performance of ultrathin silicon solar cells. In particular, spectral upconversion, a concept proposed for addressing the sub-bandgap transparency of solar cells, [14][15][16][17][18][19][20][21] is an attractive approach that provides an additional pathway to enhance the quantum effi ciency of above-bandgap longer wavelength photons by converting them into high energy photons that can be more strongly absorbed by the ultrathin silicon. One of key challenges for the practical application of spectral upconversion in photovoltaics (PVs), however, is that the intensity of natural sunlight in relevant wavelengths (i.e., near-infrared) is often too weak to yield meaningful effects of upconversion. [22][23][24] In this regard, recent advances in light manipulation using metallic nanostructures, A type of composite photovoltaic system that can improve the absorption of longer wavelength photons for ultrathin silicon solar cells is presented by synergistically exploiting spectral upconversion and plasmonic light manipulation under a reconfi gurable platform where individual module components can be independently optimized and strategically combined by printing-based deterministic materials assemblies. The ultrathin (≈8 µm) nanostructured silicon solar cells are embedded in a thin polymeric medium containingNaYF 4 :Yb 3+ ,Er 3+ nanocrystals, coated on a plasmonically engineered substrate that incorporates hybrid nanostructures of cylindrical nanoholes and truncated-cone-shaped nanoposts. Both excitation and emission processes of upconversion luminophores are signifi cantly enhanced by combined effects of surface plasmon resonance to amplify the light intensity at the excitation wavelength as well as to facilitate the far-fi eld outcoupling at the emission wavelengths, respectively. The performance of the integrated solar module is improved by ≈13% compared to devices on a nanostructured plasmonic substrate without luminophores due to collective contributions from plasmonically enhanced spectral upconversion, together with effects of waveguiding and fl uorescence of NaYF 4 :Yb 3+ ,Er 3+ . Detailed studies on optical properties of engineered plasmonic nanostructures and device performance in both experiments and numerical modeling provide quanti...

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 AlOX 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 x , 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.

Molecular orientation at the donor–acceptor interface plays a crucial role in determining the efficiency of organic semiconductor materials. We have used vibrational sum frequency generation spectroscopy to determine the orientation of poly-3-hexylthiophene (P3HT) at the planar buried interface with fullerene (C60). The thiophene rings of P3HT have been found to tilt significantly toward C60, making an average angle θ ≈ 49° ± 10° between the plane of the ring and the interface. Such tilt may be attributed to π–π stacking interactions between P3HT and C60 and may facilitate efficient charge transfer between donor and acceptor. Upon annealing, the thiophene rings tilt away from the interface by Δθ = 12–19°. This may be attributed to higher crystallinity of annealed P3HT that propagates all the way to the interface, resulting in more “edge-on” orientation, which is consistent with the observed red-shift by ∼6 cm–1 and spectral narrowing of the C=C stretch bands.

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 NaYF:Yb,Er nanocrystals as downshifting and upconverting luminophores, respectively, via printing-enabled deterministic materials assembly. The ultrathin nanostructured silicon microcells in the composite luminescent waveguide exhibit strongly augmented photocurrent (∼40.1 mA/cm) 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/cm compared with microcells in a nonluminescent medium on a plain silver reflector under a confined illumination.

Molecular organization of vapor-deposited organic molecules in the active layer of organic light-emitting diodes (OLEDs) has been a matter of great interest as it directly influences various optoelectronic properties and the overall performance of the devices. Contrary to the general assumption of isotropic molecular orientation in vacuum-deposited thin-film OLEDs, it is possible to achieve an anisotropic molecular distribution at or near the surface under controlled experimental conditions. In this study, we have used interface-specific vibrational sum frequency generation (VSFG) spectroscopy to determine the orientation of a low-molecular weight OLED material, 4,4′-bis­(N-carbazolyl)-1,1′-biphenyl (CBP), at free (air) and buried (CaF2) interfaces. VSFG spectra were measured at four different polarization combinations for five different thicknesses of the CBP film. The spectral shift and VSFG intensity changes with the film thickness can be accurately modeled by considering the optical interference effect of the signals coming from the CBP/air and CBP/CaF2 interfaces. A global fitting of the experimental spectra for all thicknesses along with theoretical simulations reveal that the long molecular axis of CBP is oriented at an angle of ∼58° (47–70°) from the surface normal at the air/CBP interface, whereas at the CBP/CaF2 interface, the angle is ∼48° (43–52°). Such a change in the angle (∼10°) suggests that the CBP molecule tends to orient more vertically (edge-on) at the buried CaF2 interface, which may be attributed to the intermolecular π–π stacking interaction between adjacent CBP molecules.