We have utilized a commercially available metal-organic precursor to develop a new, low-temperature, solution-processed molybdenum oxide (MoO x ) hole-collection layer (HCL) for organic photovoltaic (OPV) devices that is compatible with high-throughput roll-to-roll manufacturing. Thermogravimetric analysis indicates complete decomposition of the metal-organic precursor by 115 C in air. Acetonitrile solutions spin-cast in a N 2 atmosphere and annealed in air yield continuous thin films of MoO x . Ultraviolet, inverse, and X-ray photoemission spectroscopies confirm the formation of MoO x and, along with Kelvin probe measurements, provide detailed information about the energetics of the MoO x thin films. Incorporation of these films into conventional architecture bulk heterojunction OPV devices with poly(3-hexylthiophene) and [6,6]-phenyl-C 61 butyric acid methyl ester afford comparable power conversion efficiencies to those obtained with the industry-standard material for hole injection and collection: poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). The MoO x HCL devices exhibit slightly reduced open circuit voltages and short circuit current densities with respect to the PEDOT:PSS HCL devices, likely due in part to charge recombination at Mo 5+ gap states in the MoO x HCL, and demonstrate enhanced fill factors due to reduced series resistance in the MoO x HCL.

Conjugated alternating copolymers were designed with low optical band gaps for organic photovoltaic (OPV) applications by considering quinoid resonance stabilization. Copolymers of thienoisoindoledione (TID) and benzodithiophene (BDT) had appreciably lower band gaps (by ∼0.4 eV) than copolymers of thienopyrroledione (TPD) and BDT. In addition to intramolecular charge transfer stabilization (i.e., the “push-pull” effect), the former copolymer’s quinoid resonance structure is stabilized by a gain in aromatic resonance energy in the isoindole unit. Additionally, the HOMO levels of the copolymers could be tuned with chemical modifications to the BDT monomer, resulting in open circuit voltages of greater than 1 V in photovoltaic devices. Despite the optimized band gap, TID containing polymers displayed lower photoconductance, as determined by time-resolved microwave conductivity, and decreased device efficiency (2.1% vs 4.8%) as compared with TPD analogues. These results were partially attributed to morphology, as computational modeling suggests TID copolymers have a twisted backbone, and X-ray diffraction data indicate the polymer films do not form ordered domains, whereas TPD copolymers are considerably more planar and are shown to form partially ordered domains.

In this work, we use the time-resolved microwave conductivity (TRMC) technique to study the dynamics of charge carrier generation in sequentially deposited conjugated polymer/fullerene layers. These layers are either fully solution-processed, using orthogonal solvents for the layers of the polymer poly(3-hexylthiophene) (P3HT) and the fullerene phenyl-C 61 -butyric acid methyl ester (PCBM), or prepared by thermally evaporating a C 60 layer onto P3HT films. Our work is motivated by the remarkable efficiency of organic photovoltaic (OPV) devices using a sequentially processed P3HT/PCBM active layer. Here we use an electrodeless photoconductivity probe, so we can photoexcite the sample either through the polymer or the fullerene layer. We use samples with extremely thick P3HT films (2.4 μm) and show that excitation from either side of both as-cast and thermally annealed sample yields virtually identical results, consistent with mixing of the PCBM into the polymer film. We also compare solution-deposited samples to samples made by thermally evaporating C 60 on P3HT, and find that we can distinguish between charge generation in bulk-P3HT and at the polymer/ fullerene interface. We show that, despite their morphological differences, the carrier dynamics in the sequentially processed samples resemble those of mixed, bulk heterojunction (BHJ) systems. All of this is consistent with the idea that PCBM readily mixes into the P3HT film in sequentially deposited P3HT/PCBM samples, although the total amount of fullerene mixed into the P3HT appears to be less than that typically used in an optimized BHJ. Finally, we discuss the implications for OPV device architectures prepared by sequential deposition from solution.

artificial intelligence/machine learning (AI/ML), cloud-based services, telemedicine, and autonomous vehicles, as well as, demands from remote work due to the COVID-19 pandemic. To keep pace with demand for optical and wireless communications, there is an urgent need for electro-optic modulators with large bandwidths, high power-efficiency, and micrometer-scale footprints that enable dense chip-scale integration with complementary metal-oxide-semiconductor electronics. [1][2][3][4] Significant improvements in device performance have been made by silicon-organic hybrid [5][6][7][8][9][10][11][12][13][14] and plasmonicorganic hybrid. [1,[15][16][17][18][19][20][21][22][23][24][25][26][27] Pockels effect modulators, which have proven to be effective photonic platforms for both analog and digital applications.Achieving groundbreaking performance requires synergistic innovation from rational design of organic electrooptic (OEO) materials to device engineering and advancements in communication systems. [20,[28][29][30] As the active component for the Pockels effect, OEO materials, using conjugated π-electron systems, deliver large EO coefficients > 300 pm V −1 (>10× lithium niobate), low dielectric constant (static ε < 7), and femtosecond (<30 fs) response times. [28] Large EO coefficients (r 33 ) of organic material require a combination of high chromophore hyperpolarizability (β), electric field poling-induced acentric order of the chromophores (), and chromophore number density (ρ N ). Furthermore, the EO modulator operating voltage is strongly dependent on the refractive index of the material (n) at the operational wavelength: The half-wave voltage-length parameter V π L for an EO modulator is inversely proportional to n 3 r 33 . Furthermore, n increases with β and ρ N . [28,29] The past two decades of research and development have demonstrated many effective molecular engineering strategies to increase ρ N (increasing ρ N by eliminating the inactive polymer host, while maintaining good poling efficiency). These strategies include side-chain engineering, site-isolation, self-assembly, multi-chromophore dendrimers, chromophore blending, and control of chromophore shape. [31][32][33][34][35][36][37][38][39][40][41] These strategies, led to materials with enhanced r 33 (300-550 pm V −1 at 1310 nm wavelength), [31,35,36,[42][43][44][45][46] which is a large improvement over lithium niobate (≈30 pm V −1 ), High performance organic electro-optic (OEO) materials enable ultrahigh bandwidth, small footprint, and extremely low drive voltage in silicon-organic hybrid and plasmonic-organic hybrid photonic devices. However, practical OEO materials under device-relevant conditions are generally limited to performance of ≈300 pm V −1 (10× the EO response of lithium niobate). By means of theory-guided design, a new series of OEO chromophores is demonstrated, based on strong bis(4-dialkylaminophenyl)phenylamino electron donating groups, capable of EO coefficients (r 33 ) in excess of 1000 pm V −1 . Density functi...

Application of bis(propylenedioxythiophene) (bis(ProDOT)) π-conjugated bridges bearing alkyl or aryl substituents in electro-optic (EO) chromophores is presented. A series of three bis(ProDOT)-based chromophores and a bithiophene-based control chromophore were prepared and fully characterized with regard to EO applications. The highly planar bis(ProDOT) bridge results in slightly larger (∼10%) molecular hyperpolarizability ( ) values as compared to the bithiophene bridge, as measured by hyperRayleigh scattering at a variety of wavelengths. In amorphous polycarbonate guest-host films, however, the bulky substituents on the bis(ProDOT) bridge result in significantly larger (∼70%) poling-induced EO coefficient (r 33 ) values, as measured by simple reflection ellipsometry at 1310 nm. This can be attributed to a roughly 2-fold enhancement in poling efficiency due to reduced intermolecular dipole-dipole interactions. This chromophore architecture also exhibits excellent temporal alignment stability and photochemical stability as compared to benchmark AJL8, FTC, and CLD chromophore systems. Incorporation of the strong CF 3 -phenyl-substituted tricyano-furan (TCF) acceptor into a bis(ProDOT)-based chromophore resulted in a zzz (-2ω;ω,ω) value at 1907 nm of 5700 ( 400 × 10 -30 esu and an r 33 value of 69 ( 14 pm/V at 32.8 total chromophore weight %.