Browsing by Subject "CIGS"
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Item Colloidal nanocrystal assemblies : self-organization, properties, and applications in photovoltaics(2011-12) Goodfellow, Brian William; Korgel, Brian Allan, 1969-; Chelikowsky, James R; Dodabalapur, Ananth; Ekerdt, John G; Vanden Bout, David AColloidal nanocrystal assemblies offer an attractive opportunity for designer metamaterials. The ability to permute chemical composition, size, shape, and arrangement of nanocrystals leads to an astounding number of unique materials properties that find use in an extensive array of applications---ranging from solar cells to medicine. However, to take full advantage of these materials in useful applications, the nature of their assembly and their behavior under external stimuli must be well understood. Additionally, the assembly of colloidal nanocrystals into thin films provides a promising pathway to the solution-processing of inorganic materials that are prohibitively too expensive and/or difficult to deposit by conventional methods. Nanocrystal superlattices (NCSLs) of sterically stabilized nanocrystals were assembled by slow evaporation of colloidal dispersions on various substrates. Detailed analysis of the NCSL structures was carried out using transmission and scanning electron microscopy (TEM and SEM) and small-angle x-ray scattering (SAXS). Body-centered cubic (bcc) NCSLs, in particular, were studied in detail and ligand packing frustration was proposed as a significant driving force for their assembly. The behavior of NCSLs was also studied by SAXS under mild heating and solvent vapor exposure revealing several remarkable order-order, order-disorder, and amorphous-crystalline structural transitions. Colloidal Cu(In [subscript 1-x] Ga [subscript x])Se₂ (CIGS) nanocrystals were synthesized by arrested precipitation and formulated into inks. These inks were spray deposited into thin films under ambient conditions to serve as the active light absorbing material in printed low-cost photovoltaic (PV) devices. These devices, which were fabricated without the need for high temperature processes, have achieved power conversion efficiencies above 3 % under AM1.5 illumination. While the efficiencies of these devices are still too low for commercial viability, this work does provide a proof of concept that reasonable efficient solar cells can be created with a low-cost printable process using nanocrystal inks. Since high temperatures are not used to form the light-absorbing layer, nanocrystal-based solar cells were built on flexible light weight plastic substrates. The main obstacle to achieving high power conversation efficiencies was found to be the ability to extract the photo induced charge carriers. Nanocrystal films suffer from poor transport that leads to high recombination rates in thicker films. To date, the best efficiencies have been achieved with thin light absorber layers that only absorb a fraction of the incident light.Item Hybrid Photovolvoltaic Devices Based on Nanocrystals and Conducting Metallopolymers Using the Seeded Growth Method(2011-08) Huynh, Uyen Nguyen Phuong; Holliday, Bradley J.; Jones, Richard A.Described herein are two projects focusing on developing and investigating two types of nanoparticles (NPs) grown by the seeded growth method from a conducting metallopolymer for photovoltaic (PV) applications. Core/shell CdS/ZnS NPs are proven to resist the photo-oxidation of PV devices, while CuInxGa(1-x)Se2 (CIGS) NPs are expected to optimize the efficiency of PV devices.Item Methods development and measurements for understanding morphological effects on electronic and optical properties in solution processable photovoltaic materials(2012-12) Ostrowski, David Paul; Vanden Bout, David A.; Rossky, Peter J; Holliday, Bradley J; Korgel, Brian A; Dodabalapur, Ananth JThe effects of morphology on electronic and optical properties in solution processable photovoltaic (PV) materials have been studied through two different approaches. One approach, scanning photocurrent (PC) and photoluminescence (PL) microscopy, involved mapping PC generation and PL in functional PV devices on the length scale of around 250-500 nm. Additionally, local diode characteristics were studied from regions of interest in the PV through local voltage-dependent photocurrent (LVPC) measurements. In a PV made from a Copper Indium Gallium Selenide (CIGS) nanocrystal (NC) "ink", two morphological features were found to cause the spatial heterogeneity in PC generation. Cadmium Sulfide (CdS) aggregates lowered PC generation by blocking incident light to the photoactive layer, and cracks in the CIGS-NC film enhanced PC generation through improved charge carrier extraction. LVPC measurements showed all regions to have similar diode characteristics with the main difference being the PC generated at zero bias voltage. For another PV made from a donor/acceptor blend of poly(9,9-dioctylfluorene-co-bis-N,N-(4-butylphenyl)-bis-N,Nphenyl- 1,4-phenylenediamine (PFB) and poly(9,9-dioctylfluorene-co-benzothiadiazole)(F8BT), two incident laser wavelengths were used to selectively illuminate only one or both polymers. The results showed that when F8BT is illuminated, the PFB-rich regions produced the most PC and when both polymers are illuminated (but mostly PFB), the F8BT-rich regions produce the most PC; showing PC generation is more affective when less absorber material is present in the morphology. The other approach to study morphological effects on PV properties was to fabricate particles that mimicked morphological variations known to occur in solution-processable PVs. Through solution processing of an oligothiophene molecule, a range of weakly coupled H-aggregate particles were made. These particles, identifiable by shape, were shown to have a varying degree of energetic disorder (as gauged by the 0-0 vibronic band intensity in the emission spectrum), despite all particles showing a similarly high degree of molecular order from fluorescence dichroism (FD) measurements. A trend was observed correlating a decrease in energetic disorder with an increase in the local contact potential (LCP) difference as measured with Kelvin probe force microscopy (KPFM). The LCP difference was found to range by 70 mV between particles of moderate to low energetic disorder.Item Morphological effects of organic and inorganic semiconducting materials by scanning probe microscopy(2012-12) Glaz, Micah Sivan; Vanden Bout, David A.; Webb, Lauren J.; Zhu, Xiaoyang; Holliday, Bradley J.; Korgel, Brian A.Solution deposition of thin film photovoltaic materials leads to large variations in the morphological and chemical compositions of the film. In order to improve device functionality, it is important to understand how morphology and chemical composition affects charge generation, separation, and collection. This PhD work will first study bulk methods in order to characterize materials in solution and films. The results are then correlated with microscopy studies examining morphology. Other methods used in this PhD work will directly couple spectra and microscopy. Microscopic regions of such films and devices can be illuminated using scanning confocal microscopy or near-field scanning optical microscopy (NSOM), which allows for one to directly probe regions of the film at or below the optical diffraction limit. By scanning the sample over a fixed laser spot we can simultaneously create image maps of the topographical, electrical and optical properties. This technique, known as laser beam induced current (LBIC) allows one to directly probe a local area of a device with 100-300nm resolution. Along with bulk device efficiency studies, near field and confocal data of inorganic and organic materials are investigated. These include devices fabricated with a blend of P3HT (poly[3-hexylthiophene]) and perylene diimide derivatives, and Cu(InxGa1-x)Se2 [CIGS] nanoparticle devices. Finally, we use a new device architecture, a lateral organic photovoltaic (LOPV) in order to spatially resolve transport in functional organic devices.