Magnetic, Optical and Dielectric Effects on Photovoltaic Processes in Organic Solar Cells

Organic bulk heterojunction photovoltaics have attracted extensive attention during the past decade due to the global energy crisis, and it had been nominated as one of the most promising substitution for the next generation of green energy. Organic Photovoltaics, also named as “plastic solar cells”, have many advantages including super-low cost, flexibility, and compatibility with the ink printing fabrication technique, etc. Although the photovoltaic efficiency of the organic bulk heterojunction is still not as high as that of the inorganic ones, its great potential makes it the most promising solar cells in the future. In this dissertation, Chapter 1 presents a basic introduction to the concepts of conjugated polymers, the widely utilized materials in photovoltaic devices, and the fundamental device physics. Meanwhile, some basic spintronics was also discussed in this chapter. Finally, the peer publications review is briefly discussed in order to cover the academic progress in this field. Chapter 2 and Chapter 3 systematically study the origin of open circuit voltage in organic photovoltaics. Chapter 4 and Chapter 5 study the magnetic field effect on photocurrent change of bulk heterojunction and double layer photovoltaics, respectively. Chapter 6 focuses on the “intra-molecular” interaction effect on internal photovoltaic processes in new low band gap materials based on magnetic field effect and photoassisted dielectric response techniques. Finally, Chapter 7 gives a short conclusion for the entire dissertation

Hybrid quantum dot-tin disulfide field-effect transistors with improved photocurrent and spectral responsivity

We report an improved photosensitivity in few-layer tin disulfide (SnS2) field-effect transistors (FETs) following doping with CdSe/ZnS core/shell quantum dots (QDs). The hybrid QD-SnS2 FET devices achieve more than 500 percent increase in the photocurrent response compared with the starting SnS2-only FET device and a spectral responsivity reaching over 650 A/W at 400 nm wavelength. The negligible electrical conductance in a control QD-only FET device suggests that energy transfer between QDs and SnS2 is the main mechanism responsible for the sensitization effect, which is consistent with the strong spectral overlap between QD photoluminescence and SnS2 optical absorption as well as the large nominal donor-acceptor interspacing between QD core and SnS2. We also find an enhanced charge carrier mobility in hybrid QD-SnS2 FETs which we attribute to a reduced contact Schottky barrier width due to an elevated background charge carrier density

Spin Radical Enhanced Magnetocapacitance Effect in Intermolecular Excited States

This article reports the magnetocapacitance effect (MFC) based on both pristine polymer MEH-PPV and its composite system doped with spin radicals (6R-BDTSCSB). We observed that a photoexcitation leads to a significant positive MFC in the pristine MEH-PPV. Moreover, we found that a low doping of spin radicals in polymer MEH-PPV causes a significant change on the MFC signal: an amplitude increase and a line-shape narrowing under light illumination at room temperature. However, no MFC signal was observed under dark conditions in either the pristine MEH-PPV or the radical-doped MEH-PPV. Furthermore, the magnitude increase and line-shape narrowing caused by the doped spin radicals are very similar to the phenomena induced by increasing the photoexcitation intensity. Our studies suggest that the MFC is essentially originated from the intermolecular excited states, namely, intermolecular electron–hole pairs, generated by a photoexcitation in the MEH-PPV. More importantly, by comparing the effects of spin radicals and electrically polar molecules on the MFC magnitude and line shape, we concluded that the doped spin radicals can have the spin interaction with intermolecular excited states and consequently affect the internal spin-exchange interaction within intermolecular excited states in the development of MFC. Clearly, our experimental results indicate that dispersing spin radicals forms a convenient method to enhance the magnetocapacitance effect in organic semiconducting materials

Nonradiative Energy Transfer from Individual CdSe/ZnS Quantum Dots to Single-Layer and Few-Layer Tin Disulfide

The combination of zero-dimensional (0D) colloidal CdSe/ZnS quantum dots with tin disulfide (SnS₂), a two-dimensional (2D)-layered metal dichalcogenide, results in 0D–2D hybrids with enhanced light absorption properties. These 0D–2D hybrids, when exposed to light, exhibit intrahybrid nonradiative energy transfer from photoexcited CdSe/ZnS quantum dots to SnS₂. Using single nanocrystal spectroscopy, we find that the rate for energy transfer in 0D–2D hybrids increases with added number of SnS₂ layers, a positive manifestation toward the potential functionality of such 2D-based hybrids in applications such as photovoltaics and photon sensing

Thick-Shell CuInS₂/ZnS Quantum Dots with Suppressed “Blinking” and Narrow Single-Particle Emission Line Widths

Quantum dots (QDs) of ternary I–III–VI₂ compounds such as CuInS₂ and CuInSe₂ have been actively investigated as heavy-metal-free alternatives to cadmium- and lead-containing semiconductor nanomaterials. One serious limitation of these nanostructures, however, is a large photoluminescence (PL) line width (typically >300 meV), the origin of which is still not fully understood. It remains even unclear whether the observed broadening results from considerable sample heterogeneities (due, e.g., to size polydispersity) or is an unavoidable intrinsic property of individual QDs. Here, we answer this question by conducting single-particle measurements on a new type of CuInS₂ (CIS) QDs with an especially thick ZnS shell. These QDs show a greatly enhanced photostability compared to core-only or thin-shell samples and, importantly, exhibit a strongly suppressed PL blinking at the single-dot level. Spectrally resolved measurements reveal that the single-dot, room-temperature PL line width is much narrower (down to ∼60 meV) than that of the ensemble samples. To explain this distinction, we invoke a model wherein PL from CIS QDs arises from radiative recombination of a delocalized band-edge electron and a localized hole residing on a Cu-related defect and also account for the effects of electron–hole Coulomb coupling. We show that random positioning of the emitting center in the QD can lead to more than 300 meV variation in the PL energy, which represents at least one of the reasons for large PL broadening of the ensemble samples. These results suggest that in addition to narrowing size dispersion, future efforts on tightening the emission spectra of these QDs might also attempt decreasing the “positional” heterogeneity of the emitting centers

Using Perovskite Nanoparticles as Halide Reservoirs in Catalysis and as Spectrochemical Probes of Ions in Solution

The ability of cesium lead halide (CsPbX₃; X = Cl^–, Br^–, I^–) perovskite nanoparticles (P-NPs) to participate in halide exchange reactions, to catalyze Finkelstein organohalide substitution reactions, and to colorimetrically monitor chemical reactions and detect anions in real time is described. With the use of tetraoctylammonium halide salts as a starting point, halide exchange with the P-NPs was performed to calibrate reactivity, stability, and extent of ion exchange. The exchange of CsPbI₃ with Cl^– or Br^– causes a significant blue-shift in absorption and photoluminescence, whereas reacting I^– with CsPbBr₃ causes a red-shift of similar magnitudes. With the high local halide concentrations and the facile nature of halide exchange in mind, we then explored the ability of P-NPs to catalyze organohalide exchange in Finkelstein like reactions. Results indicate that the P-NPs serve as excellent halide reservoirs for substitution of organohalides in nonpolar media, leading to not only different organohalide products, but also a complementary color change over the course of the reaction, which can be used to monitor kinetics in a precise manner. The merits of using P-NP as spectrochemical probes for real time assaying is then expanded to other anions which can react with, or result in unique, classes of perovskites

Frameshift Deletion by Sulfolobus solfataricus P2 DNA Polymerase Dpo4 T239W Is Selective for Purines and Involves Normal Conformational Change Followed by Slow Phosphodiester Bond Formation*

The human DNA polymerase κ homolog Sulfolobus solfataricus DNA polymerase IV (Dpo4) produces “−1” frameshift deletions while copying unmodified DNA and, more frequently, when bypassing DNA adducts. As judged by steady-state kinetics and mass spectrometry, bypass of purine template bases to produce these deletions occurred rarely but with 10-fold higher frequency than with pyrimidines. The DNA adduct 1,N2-etheno-2′-deoxyguanosine, with a larger stacking surface than canonical purines, showed the highest frequency of formation of −1 frameshift deletions. Dpo4 T239W, a mutant we had previously shown to produce fluorescence changes attributed to conformational change following dNTP binding opposite cognate bases (Beckman, J. W., Wang, Q., and Guengerich, F. P. (2008) J. Biol. Chem. 283, 36711–36723), reported similar conformational changes when the incoming dNTP complemented the base following a templating purine base or bulky adduct (i.e. the “+1” base). However, in all mispairing cases, phosphodiester bond formation was inefficient. The frequency of −1 frameshift events and the associated conformational changes were not dependent on the context of the remainder of the sequence. Collectively, our results support a mechanism for −1 frameshift deletions by Dpo4 that involves formation of active complexes via a favorable conformational change that skips the templating base, without causing slippage or flipping out of the base, to incorporate a complementary residue opposite the +1 base, in a mechanism previously termed “dNTP-stabilized incorporation.” The driving force is attributed to be the stacking potential between the templating base and the incoming dNTP base