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PbS colloidal quantum dots (CQDs) are a promising material class for near-infrared optoelectronics. 

Abstract The design of polymeric semiconductors exhibiting high electrical conductivity (σ) and thermoelectric power factor (PF) will be vital for flexible large‐area electronics. In this work, four polymers based on diketopyrrolopyrrole (DPP), 2,3‐dihydrothieno[3,4‐b][1,4]dioxine (EDOT), thieno[3,2‐b]thiophene (TT), and 3, 3′‐bis (2‐(2‐(2‐methoxyethoxy) ethoxy) ethoxy)‐2, 2′‐bithiophene (MEET) are investigated as side‐chains, with the MEET polymers newly synthesized for this study. These polymers are systematically doped with tetrafluorotetracyanoquinodimethane ( F₄TCNQ), CF3SO3H, and the synthesized dopant Cp(CN)₃‐(COOMe)₃, differing in geometry and electron affinity. The DPP‐EDOT‐based polymer containing MEET as side‐chains exhibits the highest conductivity (σ) ≈700 S cm−1 in this series with the acidic dopant (CF₃SO₃H). This polymer also shows the lowest oxidation potential by cyclic voltammetry (CV), the strongest intermolecular interactions evidenced by differential scanning calorimetry (DSC), and has the most oxygen‐based functionality for possible hydrogen bonding and ionic screening. Other polymers exhibit high σ ≈300–500 S cm−1 and power factor up to 300 µW m⁻¹K⁻². The mechanism of conductivity is predominantly electronic, as validated by time‐dependent conductance studies and transient thermo voltage monitoring over time, including for those doped with the acid. These materials maintain significant thermal stability and air stability over ≈6 weeks. Density functional theory calculations reveal molecular geometries and inform about frontier energy levels. Raman spectroscopy, in conjunction with scanning electron microscopy (SEM‐EDS) and x‐ray diffraction, provides insight into the solid‐state microstructure and degree of phase separation of the doped polymer films. Infrared spectroscopy enables this study to further quantify the degree of charge transfer from polymer to dopant. 

Colloidal quantum dots are a promising candidate material for solar energy generation because of their band gap tunability and solution-based processing flexibility. However, conventional colloidal quantum dot solar cell fabrication techniques are still limited by their lack of scalability, environment conditions, and difficult installation scenarios. Here, we develop spray-casting manufacturing methods for fabricating thin film solar cells, discuss the trade-off between conductivity and transmittance in transparent contact materials, and demonstrate the feasibility of spray-casting colloidal quantum dot layers. This work on flexible manufacturing methods paves the way for installing solar energy devices in a variety of novel scenarios. 

Colloidal quantum dots are a promising candidate material for thin film solar cells due to their size-dependent band gap tunability and solution-based processing flexibility. Spray-casting technology has the potential to reduce the strict environmental requirements associated with traditional fabrication procedures for colloidal quantum dot solar cells, potentially enabling installation-site solar cell fabrication. Here, we demonstrate spray-casting of silver nanowire electrodes and zinc oxide electron transport layers, demonstrate their use in colloidal quantum dot solar cells, analyze the existing challenges in current spray-casting procedures, and outline a path to producing fully spray-cast colloidal quantum dot solar cells. 

Spectral selectivity is of interest for many photovoltaic applications, such as in multijunction and transparent solar cells, where wavelength-selectivity of the photoactive material is necessary. We investigate using artificial photonic band engineering as a method for achieving spectral selectivity in an absorbing material such as PbS CQD thin films. Using FDTD simulations, we find that a CQD-based photonic crystal (CQD-PC) is able to maintain its photonic band structure, including the existence of a reduced photonic density of states, in the presence of weak material absorption. This shows that CQD-PCs are a promising material for photovoltaic applications that require spectral selectivity. 

The most common solution for achieving arbitrary spectral selectivity in optoelectronic devices is adding external filters. Here we propose using semiconductor thin film photonic crystals with relevant photonic bands that fall within the absorbing frequency range of the material for spectral selectivity. Optical simulations show that the in-plane photonic bands couple strongly to normal-incidence external fields, inducing tunable resonance features in the out-of-plane transmission and reflection spectra. Experimentally, we fabricate a proof-of-principle photonic structure with enhanced visible transparency, consisting of a self-assembled polystyrene bead array infiltrated with colloidal quantum dots, showing promise for multijunction and transparent photovoltaics. 

Abstract Efficient doping of polymer semiconductors is required for high conductivity and efficient thermoelectric performance. Lewis acids, e.g., B(C₆F₅)₃, have been widely employed as dopants, but the mechanism is not fully understood. 1:1 “Wheland type” or zwitterionic complexes of B(C₆F₅)₃are created with small conjugated molecules 3,6‐bis(5‐(7‐(5‐methylthiophen‐2‐yl)‐2,3‐dihydrothieno[3,4‐b][1,4]dioxin‐5‐yl)thiophen‐2‐yl)‐2,5‐dioctyl‐2,5‐dihydropyrrolo[3,4‐c]pyrrole‐1,4‐dione [oligo_DPP(EDOT)₂] and 3,6‐bis(5''‐methyl‐[2,2':5',2''‐terthiophen]‐5‐yl)‐2,5‐dioctyl‐2,5‐dihydropyrrolo[3,4‐c]pyrrole‐1,4‐dione [oligo_DPP(Th)₂]. Using a wide variety of experimental and computational approaches, the doping ability of these Wheland Complexes with B(C₆F₅)₃are characterized for five novel diketopyrrolopyrrole‐ethylenedioxythiophene (DPP‐EDOT)‐based conjugated polymers. The electrical properties are a strong function of the specific conjugated molecule constituting the adduct, rather than acidic protons generated via hydrolysis of B(C₆F₅)₃, serving as the oxidant. It is highly probable that certain repeat units/segments form adduct structures inp‐type conjugated polymers which act as intermediates for conjugated polymer doping. Electronic and optical properties are consistent with the increase in hole‐donating ability of polymers with their cumulative donor strengths. The doped film of polymer (DPP(EDOT)₂‐(EDOT)₂) exhibits exceptionally good thermal and air‐storage stability. The highest conductivities, ≈300 and ≈200 S cm⁻¹, are achieved for DPP(EDOT)₂‐(EDOT)₂doped with B(C₆F₅)₃and its Wheland complexes.