Molecular helices as electron acceptors in high-performance bulk heterojunction solar cells

Despite numerous organic semiconducting materials synthesized for organic photovoltaics in the past decade, fullerenes are widely used as electron acceptors in highly efficient bulk-heterojunction solar cells. None of the non-fullerene bulk heterojunction solar cells have achieved efficiencies as high as fullerene-based solar cells. Design principles for fullerene-free acceptors remain unclear in the field. Here we report examples of helical molecular semiconductors as electron acceptors that are on par with fullerene derivatives in efficient solar cells. We achieved an 8.3% power conversion efficiency in a solar cell, which is a record high for non-fullerene bulk heterojunctions. Femtosecond transient absorption spectroscopy revealed both electron and hole transfer processes at the donor−acceptor interfaces. Atomic force microscopy reveals a mesh-like network of acceptors with pores that are tens of nanometres in diameter for efficient exciton separation and charge transport. This study describes a new motif for designing highly efficient acceptors for organic solar cells

UNDERSTANDING THE EFFECTS OF STRUCTURAL MODIFICATION ON THE ELECTRICAL AND OPTICAL PROPERTIES OF POLYMER-ACID-TEMPLATED CONDUCTING POLYMERS

Conducting polymers exhibit electrical conductivities approaching those of metals while maintaining the chemical, mechanical, and processing attributes of plastics. Complexing conducting polymers with polymer acids yields electrically conducting particles that are water dispersible. Polyaniline that is complexed with poly(2-acrylamido-2methyl-1-propane sulfonic acid), PANI-PAAMPSA, and poly(3,4-ethylenedioxythiophene) complexed with poly(styrene sulfonate), PEDOT:PSS, are the primary conducting polymers studied in this thesis. We examined the effects of structural modification on the electrical and optical properties of conducting polymers as active components in flexible sensors, thermoelectric generators, and electrochromic windows. The processing-structure-function relationships developed in this thesis can guide the synthesis of new materials and the development of new processing routes to access next-generation conducting polymers with tailored properties. Conducting polymers are inherently piezoresistive; we showed that both the magnitude and polarity of the piezoresistive response of conducting polymers can be tuned by their crystallinity, particulate nature, and the compositional distribution in thin films, either at the onset of synthesis or with processing. Such tunability allowed us to attain a high gauge factor for strain sensing and a near-zero gauge factor for thermo-/chemo-resistive sensing applications with a single class of material. Additionally, we enhanced the stability of the piezoresistive response of PANI-PAAMPSA and PEDOT:PSS to humidity and strain rate via structural modification. We also elucidated the correlations between the structure and thermoelectric properties of conducting polymers. The thermoelectric power factor of conducting polymer complexes is a strong function of both ion and hole/electron conductivities and is therefore very sensitive to processing-induced structural changes. We leveraged this sensitivity to decouple the electrical conductivity and the Seebeck coefficient of PANI-PAAMPSA and PEDOT:PSS, and increased their power factor by two orders of magnitude. PANI-PAAMPSA and PEDOT:PSS can be combined in a single electrochromic window (ECW) as the electrochromic and charge-balancing layers, respectively. Their optical complementarity enhances the switching contrast of the ECWs without reducing the transparency of the clear state. We quantified the stability and kinetics of electrochromic switching, and showed that PANI-PAAMPSA/PEDOT:PSS-based ECWs can be powered by near-UV organic solar cells to regulate the transmission of visible light and near-infrared heat in a self-powered smart window prototype

Tuning the Magnitude and the Polarity of the Piezoresistive Response of Polyaniline through Structural Control

We demonstrate the tunability of both the polarity and the magnitude of the piezoresistive response of polyaniline that is template-synthesized on poly­(2-acrylamido-2-methyl-1-propanesulfonic acid), PANI–PAAMPSA, by altering the template molecular weight. Piezoresistivity is quantified by gauge factor, a unitless parameter that relates changes in electrical resistance to applied strain. The gauge factor of PANI–PAAMPSA decreases linearly and becomes negative with decreasing PAAMPSA molecular weight. The polarity of PANI–PAAMPSA’s gauge factor is determined by macroscopic connectivity across thin films. PANI–PAAMPSA thin films comprise electrostatically stabilized particles whose size is determined at the onset of synthesis. An increase in the interparticle spacing with applied strain results in a positive gauge factor. The presence of PANI crystallites increases connectivity between particles; these samples instead exhibit a negative gauge factor whereby the resistance <i>decreases</i> with increasing strain. The tunability of the piezoresistive response of these conducting polymers allows their utilization in a broad range of flexible electronics applications, including thermo- and chemoresistive sensors and strain gauges

Formation of Organic Alloys in Ternary-Blend Solar Cells with Two Acceptors Having Energy-Level Offsets Exceeding 0.4 eV

Recent studies demonstrated that with proper selection of chemically compatible constituents the open-circuit voltage (<i>V</i>_oc) of ternary-blend solar cells can be tuned across the composition window of the active layer. In this study, we probed the limit of the offset between the lowest unoccupied molecular orbital (LUMO) energy levels of the two acceptors in ternary blends containing one donor and two acceptors. We demonstrate, for the first time, that ternary-blend active layers with two acceptors having an energy-level difference between their LUMO levels exceeding 0.4 eV can still result in solar cells exhibiting composition-dependent open-circuit voltage (<i>V</i>_oc). Our results suggest strong electronic interactions between the acceptors, with the electron wave function delocalized over multiple molecules. These findings have broadened the library of possible candidates for active layers of ternary-blend solar cells with tunable <i>V</i>_oc and established guidelines for the design of next-generation materials for efficient performance of such devices

Spin-Forbidden Excitation Enables Infrared Photoredox Catalysis

We describe a new family of catalysts that undergo direct ground state singlet to excited state triplet excitation with IR light, leading to photoredox catalysis without the energy waste associated with intersystem crossing. The finding allows a mole scale reaction in batch using infrared irradiation

Tuning Morphology and Melting Temperature in Polyethylene Films by MAPLE

The control of structure and thermal stability in semicrystalline polymer films remains an important challenge in applications ranging from solar energy devices to packaging films. Here, we demonstrate the ability to dramatically alter the morphology and melting temperature (<i>T</i>_m) of low-molecular-weight linear polyethylene (PE) by employing an innovative vapor-assisted deposition process termed matrix assisted pulsed laser evaporation (MAPLE). We report the ability to tune <i>T</i>_m of PE films by 20 °C by simply adjusting the deposition temperature during MAPLE processing. This unique capability stems from the ability of MAPLE to exploit confined crystallization during thin film growth. In addition, we demonstrate the ability to exploit MAPLE to design PE films that exhibit the same <i>T</i>_m as their melt-crystallized analogues but have an ∼25% higher degree of crystallinity. Our investigation offers new insights into how confinement effects in polymer crystallization can be utilized in the emerging field of polymer film fabrication by MAPLE to control structure and key material properties of semicrystalline polymer films

Contorted Hexabenzocoronenes with Extended Heterocyclic Moieties Improve Visible-Light Absorption and Performance in Organic Solar Cells

The large band gaps of existing contorted hexabenzocoronene derivatives severely limit visible-light absorption, restricting the photocurrents generated by solar cells utilizing contorted hexabenzocoronene (cHBC). To decrease the band gap and improve the light-harvesting properties, we synthesized cHBC derivatives having extended heterocyclic moieties as peripheral substituents. Tetrabenzofuranyldibenzocoronene (cTBFDBC) and tetrabenzothienodibenzocoronene (cTBTDBC) both exhibit broader absorption of the solar spectrum compared to cHBC, with peak absorbances on the order of 10⁵ cm^–1 in the near-ultraviolet and in the visible. Planar-heterojunction organic solar cells comprising cTBFDBC or cTBTDBC as the donor and C₇₀ as the acceptor surpass those having cHBC in photocurrent generation and power-conversion efficiency. Interestingly, devices containing cTBFDBC/C₇₀ exhibit the highest photocurrents despite cTBTDBC having the smallest band gap of the three cHBC derivatives. X-ray reflectivity of the active layers indicates a rougher donor–acceptor interface when cTBFDBC is employed instead of the other two donors. Consistent with this observation, internal quantum efficiency spectra suggest improved charge transfer at the donor–acceptor interface when cTBFDBCas opposed to cTBTDBC or cHBCis used as the donor

P(III) vs. P(V): A P(V) Reagent for Thiophosphoramidate Linkages and Application to An Asymmetric Synthesis of a Cyclic Dinucleotide STING Agonist

A highly stereoselective synthesis of a cyclic dinucleotide (CDN) STING agonist containing two chiral thiophosphoramidate linkages is described. These rare, yet key functional groups were, for the first time, installed efficiently and with high diastereoselectivity using a specially designed P(V) reagent. By utilizing this strategy, the CDN was prepared in greater than sixteen-fold higher yield than the prior P(III) approach, with fewer hazardous reagents and chromatographic purifications