Thesis (Master)--İzmir Institute Of Technology, İzmir, 2005. / Keywords: Photovoltaic, Solar cells. Includes bibliographical references (leaves. 100-104).

Dissertation (Ph.D.)--University of Toledo, 2008. / Typescript. "Submitted as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Physics." Bibliography: leaves 97-100.

Bulk heterojunction (BHJ) organic photovoltaic (OPV) is one of the most promising techniques to generate electricity with advantages of flexibility, solution processing and capability for large area device fabrication. Although the power conversion efficiency (PCE) of BHJ solar cells has already achieved over 13%, there are still problems remain to be solved. This thesis presents the binary and ternary organic BHJ devices with alternative donor:acceptor (D:A) ratios, and the charge transport properties and electronic interactions in their BHJ films. In a high performance BHJ solar cell, the commonly optimized D:A weight ratio is about 1:x, where x is commonly in excess of 1.5, when PC71BM is used as the acceptor. We demonstrated how to achieve high PCEs of BHJ solar cells by enriching the D:A weight ratios. The PCEs of the re-optimized cells were improved for the PTB7:PC71BM, PCDTBT:PC71BM, PDTSTPD:PC71BM devices. Current-voltage (JV) and admittance spectroscopy (AS) measurements indicate enhanced hole mobilities for the polymer-rich BHJs based on PTB7, PCDTBT, and PDTSTPD. At the same time, although the relative weight ratio of PC71BM is reduced, the electron mobilities are maintained due to the dispersion of fullerene domains by increased DIO concentrations. The active layer thickness of most optimized BHJ solar cells is about 100nm. The thin active layer is unfavorable for optical absorption and film coating. We employed a ternary strategy to address this problem, and the thick-film BHJ devices can retain 90% PCEs of their optimized thin-film devices. Three model systems were studied, involving PTB7:PC71BM, PTB7-Th:PC71BM and P3HT:PCBM BHJs. Into these BHJs, a ternary component, p-DTS(fbtth2)2 (DTS) is introduced. With DTS, the corresponding thick film devices have significantly improved PCEs. The ternary component DTS improves hole mobility and reduces sub-bandgap trap states. Both observations are well correlated with improved FFs of the ternary BHJ cells. Photothermal deflection spectroscopy (PDS) and 1H nuclear magnetic resonance (1H NMR) results indicate that DTS behaves as conducting bridges in between two neighboring polymer segments. Most lab-based BHJ solar cells are optimized by their power conversion efficiencies (PCEs). We challenge this conventional view by showing that BHJ cells using fullerene acceptors should be optimized by their fill-factors (FFs). With the optimized-FF approach, BHJ cells tend to have higher fullerene content when compared to the BHJ cells that are optimized by PCEs. The FF-optimized BHJ cells have slightly reduced PCEs (due to smaller Jscs) compared to the PCE-optimized cells. Yet, FF-optimized cells enjoy a much better thermal stability. We demonstrate that these FF-optimized BHJs possess better-balanced electron-to-hole mobility ratios due to weakly field-dependent electron mobilities. The improved mobility ratio suppresses carrier recombination. Our results suggest that BHJ cells optimized by their PCEs should be meta-stable, and other D:A ratios should be considered for practical BHJ cell development.

Thesis (M.S.)--Tennessee Technological University, 2009. / Title from title page screen (viewed on July 26, 2010). Includes bibliographical references.

The semiconducting sulphosalt Wittichenite has been identified as a possible absorber material for thin film photovoltaic devices. It has the chemical formula Cu3BiS3 and its component elements are those of low toxicity and high abundance making it a very attractive prospect for photovoltaic devices. The copper bismuth sulphur material system is not very well understood and information on it limited to a few small regions. To aid understanding of this system a pseudo-binary phase diagram along the CuS-Bi join of the Cu-Bi-S ternary phase diagram was constructed by making bulk samples of various compositions along the join and analysing them using X-ray diffraction and differential scanning calorimetry. This join was chosen because is crosses the point at which Cu3BiS3 would be expected to occur due to its stoichiometry. The CuS-Bi phase diagram shows Cu3BiS3 forms across a wide compositional range but is mixed with either bismuth metal or copper sulphides depending on composition. Films of Cu3BiS3 were made using sputtered copper and bismuth films annealed in a sulphur atmosphere and thermal co- evaporation of copper sulphide and bismuth.

Organic semiconducting materials have been attracted considerable attention as a promising technology for the next generation flexible electronic devices, such as solar cells and field-effect transistors because of their advantages of low-cost, structural versatility and flexibility. Many organic semiconducting materials have been developed in recent years. In this thesis, four pi-conjugated building blocks based on benzodithiophene and quinoxalinedithienothiophene were applied to develop novel photovoltaic materials, including donor-acceptor alternating copolymers as a donor material for polymer solar cells, photosensitizers for dye sensitized solar cells, small molecule hole transporting materials for perovskite solar cells and small molecule acceptors for organic solar cells. A comprehensive review of current development of organic photovoltaic materials was presented in Chapter 1. In Chapter 2, a series of D-A copolymers (PBB-n) based on 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole and 4,5-bis((2-ethylhexyl)oxy)benzo[2,1-b:3,4-b'] dithiophene attached with different solubilizing side-chains were designed, synthesised and characterized. In general, PBB-n polymers showed good absorption in the region of visible light and UV region, indicating such polymers are a promising light harvester. Also, PBB-n exhibited suitable energy levels, suggesting that they could be applied as the donor materials in polymer solar cells. PBB-n also exhibited various extent of aggregation behaviour. Chapter 3 described syntheses and the fluorination effect of two series of fluoro-substituted PBB-n copolymers, namely PfBB-n and PffBB-n on optical, electrochemical, and optoelectronic properties. Among them, PfBB-n series was characterized with photovoltaic performance. The champion devices fabricated from PfBB-12 showed a PCE as high as 9.7%, with a Voc of 0.92 V, a Jsc of 16.60 mA/cm-2 and a FF of 63.49%. Cells fabricated from other PfBB-n copolymers also exhibited good PV performance with PCE ranging from 7.4 - 8.5%. For PffBB-n polymers, temperature-dependent aggregation behaviour was exploited by modulating the coating temperature during device fabrication. PSC devices based on PffBB-n exhibited good PV performance with PCE ranging from 7.4% to 9.9%. Among which, PffBB-14 provided the most promising PV performance with PCE of 9.9%, a Voc of 0.92 V, a Jsc of 16.8 mA/cm-2 and a FF of 64.36%. Electron deficient conjugated structure was seldom used as the π-bridge in metal-free photosensitizers. In Chapter 4, four novel organic photosensitizers, namely QC5-m and PC5-n were designed with an electron deficient π-bridge. Typical sandwich-structured DSSCs based on the newly developed photosensitizers exhibited promising photovoltaic performance with PCE ranging from 5.23 - 7.77 %, with a maximum Jsc as high as 15.63 mA cm-2. These results suggest that the use of electron deficient π-bridge provides alternative approach to construct efficient organic photosensitizers. Chapter 5 and Chapter 6 described the design, synthesis and investigation of novel hole-transporting materials and electron acceptor materials based on benzo[2,1-b:3,4-b']dithiophene-4,5-dione derived building blocks as potential organic photovoltaic materials for solar cell applications. Keywords: organic photovoltaic materials, photosensitizers, polymer solar cell, electron acceptor, hole-transporting materials.

This thesis is an investigation of the photophysical processes that occur in organic photovoltaic blends in the time between light being absorbed and free charges being generated. The purpose of all solar cells is to generate a photocurrent. The free charges, as they flow out of the device, make up the photocurrent, so understanding the processes by which they are created is vitally important to organic photovoltaic research. The main experimental method used was time-resolved fluorescence spectroscopy. This technique was used to probe the exciton population with respect to time for a variety of blends of organic semiconductors, including the high performance photovoltaic materials PCDTBT, PTB7, C71-PCBM and P3HT. The main goal of the work was to characterise the exciton diffusion lengths of these materials by developing a technique called volume quenching. Volume quenching involves blending a small quantity of quenching material into a thin film of semiconducting material. These introduced quenching sites render excitons unemissive on contact. Thus, from the drop in fluorescence compared with the ‘unquenched' material, it was possible to work out what proportion of the initial excitons have encountered a quenching site in the blends. The results can then be fitted to quantify how diffusive the excitons are -i.e. how far they move. By looking at the rate constant of the quenching process and how it varies with respect to time, quencher concentration and quencher type, it was possible to generate a wealth of additional information, not just about exciton diffusion, but about all the inter-related processes that contribute to exciton harvesting. These processes included the measurement of long-range energy transfer from the donor to the acceptor, electron transfer at the interface with the acceptor and the understanding of nanomorphology of donor-acceptor heterojunctions.

Thesis (Ph.D)--Electrical and Computer Engineering, Georgia Institute of Technology, 2008. / Committee Chair: Honsberg, Christiana; Committee Co-Chair: Ferguson, Ian; Committee Member: Citrin, David; Committee Member: Klein, Benjamin; Committee Member: Rohatgi, Ajeet; Committee Member: Snyder, Robert. Part of the SMARTech Electronic Thesis and Dissertation Collection.