Transferring organic photovoltaics (OPV) from the laboratory into economically feasible products, requires the fabrication of modules, a series of connected single cells. During this transition, there is typically a drastic decrease in power conversion efficiency (PCE). This thesis reports on the design, fabrication, and characterization of state-of-the-art, high-performance organic photovoltaic modules with a novel geometry that composed of unit cells with alternating electrical polarities. Such configuration is realized by exclusive patterning of the interlayers and electrodes and avoids patterning of the photoactive layer. With this novel architecture, area losses of photovoltaic module can be significantly reduced compared with the conventional configurations. The processing of this new solar cell module is also compatible with large area processing techniques such as slot-die coating. This thesis reports on 4-cell and 8-cell modules, wherein the measured fill-factors (FF) and PCE of the constituent sub-cells and of the modules are almost identical. The 4-cell module, with a total area of 0.8 cm2, exhibits an open-circuit voltage (VOC) of 3.15 V, a short circuit-current density (JSC) of 2.3 mA/cm2 and a FF of 0.69, yielding a PCE of 5.01%. The 8-cell module, with a total area of 1.6 cm2, exhibits a VOC of 6.39 V, a JSC of 1.2 mA/cm2 and a FF of 0.63, yielding a PCE of 5.06%. Similar PCE values between 4-cell and 8-cell module is a demonstration of scalability of this novel geometry without compromising the efficiency.

In this thesis CIGS absorber layers have been deposited by gas flow sputtering with an extra supply of selenium, a method that displays promise for large scale production because of its one-step sputtering route which deposits low energy particles in a high deposition rate. In this thesis a method was developed where selenium was added to the sputtering process inside the sputter chamber in a controllable manner and in larger amount than done in previous projects. A total of five samples were manufactured with altered evaporation temperatures and an extra supply of selenium which then were finalized to solar cells using the standard baseline process of the Ångström solar center. The characteristics of the CIGS layer and solar cells were analyzed by XRF, IV- and QE measurements. A cell with a conversion efficiency of 11.6 %, Jsc of 27.9 mA/cm2, Voc of 0.63 V and fill factor of 66.2 % was obtained on a 0.5 cm2 cell area without an antireflective coating. All samples contained cells with obtained efficiencies above 10 %, but over the whole samples the efficiencies varied considerably. The samples that were deposited with moderately large selenium evaporation provided the highest efficiencies with a relatively good homogeneity over the substrate. Results show a deficiency of copper in the CIGS films compared to the target composition. The copper content was lower than 70 % expressed in Cu/(Ga+In), which probably resulted in a low diffusion length for electrons, leading to limited cell efficiencies.  Through the duration of the thesis issues that concerned the power supply- and the controls to the substrate heaters as well as the control of the evaporation temperature during the depositions arose that required problem solving and needs to be resolved for the future progression of this work. The conclusions drawn from this thesis are that it is possible to vary the temperature of the selenium source and thereby control the amount of selenium that evaporates during the deposition process even though there is a lot of additional heating in the sputter chamber from both the substrate heaters and the sputter source which could affect the ability to control the amount of selenium being evaporated. That the most likely reason for the limited efficiencies is due to the low copper content in the CIGS films and that a larger amount of evaporated selenium compared to previous work did not result in higher obtained efficiencies.

This dissertation presents novel applications of ligand-capped II-VI semiconductor nanocrystals (i.e. CdSe and CdTe).Hybrid polymer-nanocrystal thin films were prepared using a bottom-up electrochemical crosslinking method, where thiophene-functionalized CdSe NCs were wired to electron-rich 3,4-dioxy-substituded thiophene polymers. Both nanocomposite and effective monolayer (EML) films were achieved by controlling monomer feed ratios during the crosslinking steps. These hybrid thin films showed enhanced photoelectrochemical current efficiencies with a variety of solution acceptor molecules compared to polymer control films, which was due to sensitization by the CdSe NCs. The electronic structure of the polymer played a critical role in the potential (doping) dependent hole capture efficiency from photoexcited CdSe NCs. Furthermore, photocurrent efficiencies were correlated with nanocrystal size, which was a direct product of frontier orbital energy shifting due to quantum confinement effects.All-inorganic CdTe-CdSe nanocrystal solar cells were fabricated by a facile layer-by-layer procedure. A low-temperature sintering strategy was utilized to electronically couple the nanocrystal thin films, which maintained the individual electronic properties of the nanocrystals. The electrical characteristics of these solar cells displayed predictable trends in open circuit voltage with varying CdSe NC diameter.Novel CdSe NC diffraction gratings were prepared by a facile microcontact molding procedure. These transmission gratings showed exceptionally high diffraction efficiencies that were dependent on optimum grating morphologies and the refractive index contrast provided by the nanocrystals, which was size-dependent. These films also showed promise as coupling gratings for internal reflection elements.

Solution processed solar cells offer the promise of a low cost solution to global energy concerns. Colloidal quantum dots are one material that can be easily synthesized in and deposited from solution. These nanoparticles also offer the unique ability to select the desired optical and electrical characteristics, all within the same materials system, through small variations in their physical dimensions. These materials, unfortunately, are not without their limitations. To date, films made from colloidal quantum dots exhibit limited mobilities and short minority diffusion lengths. These limitations imply that simple device structures may not be sufficient to make an efficient solar cell. Here we show that through clever manipulation of the geometric and energetic structures, we can utilize the size-tunability of CQDs while masking their poor electrical characteristics. We further outline the physical mechanisms present within these architectures, namely the utilization of a distributed built-in electric field to extract current through drift rather than diffusion. These architectures have consequently exceeded the performance of legacy architectures such as the Schottky cell. Finally, we discuss some of the limiting modes within these architectures and within CQD films in general including the impact of surface traps and polydispersity in CQD populations. Through the development of these novel architectures, the power conversion efficiency of CQD solar cells has increased from ~3.5% to 7.4%; the highest efficiencies yet reported for colloidal quantum dot solar cells.

Surface plasmon is a collective oscillation behavior of electrons in metal nanoparticle induced by the excitation of incident light, which can create an enhanced localized electric field near the surface of metal nanoparticle. To date, metal nanoparticle surface plasmon resonances have been extensively studied in the photoluminescence domain; little work however was devoted to electroluminescent and photovoltaic research. In this thesis, as a fundamental study we firstly investigated surface plasmon enhanced europium complex luminescence and obtained an improved understanding of the importance of optical spacer in metal enhanced fluorescence phenomenon. Under this guideline, we incorporated metal NPs into organic light emitting diodes (OLED) and organic solar cells, by means of thermal evaporation and wet chemistry. Metal nanoparticles are demonstrated to enhance the efficiency of both OLEDs and solar cells only under tailored device architecture. The surface plasmon enhanced local electric field plays an important and comprehensive role in enhancing device performance. In Alq3 based OLED we observed increased charge carrier injection by depositing Ag nanoparticles underneath the Al cathode; in Ir(ppy)3 based OLED we gained enhanced luminous efficiency via doping silica functionalized Ag nanoparticles into emitting layer; in P3HT based organic polymer solar cell we noticed an increased polymer absorption by incorporating Ag nanoparticles over the active layer. On the other hand, adverse effects such as metal nanoparticle induced charge carrier recombination and light extinction are also observed. The study of surface plasmon effects in organic optoelectronic devices reveals interesting surface plasmon features and permits to optimize optoelectronic devices from a novel point of view. / Thesis (Ph.D, Chemistry) -- Queen's University, 2012-01-05 17:22:40.074

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CIGS thin film solar cells usually include a thin layer of intrinsic zinc oxide (i-ZnO) deposited on a CdS buffer layer by sputtering. However an interest has grown in using chemical vapor deposition (CVD) instead. Hence, the aim of this thesis was to study how well CVD i-ZnO performs on a CdS buffer layer in a CIGS solar cell and how the properties of the layer can be controlled when using a hot-wall CVD reactor with diethylzinc and water as precursors. The process was characterized through depositions on glass substrates and was then successfully implemented in solar cell devices. The main influences of temperature, thickness and precursor flows on resistivity, optical band gap and film structure were mapped out. The analysis methods used included X-ray diffraction (XRD), X-ray fluorescence (XRF), four point probe resistivity measurements, mechanical profilometry and absorption spectrophotometry. In addition, the solar cell devices were characterized using external quantum efficiency (EQE) and current-voltage (IV) measurements. It was found that the CVD process was sensitive to the condition of the CdS surface, which resulted in a large distribution of shunted cells when grown on aged CdS. Unexpected trends in open-circuit voltage and fill factor were found. Both these factors increased when the growth temperature was decreased, resulting in higher conversion efficiencies. Compared to i-ZnO deposited by an in-house baseline sputtering process, the CVD process resulted in cells with higher short-circuit current due to higher EQE in the short-wavelength region. It was shown that the CVD process used is capable of producing solar cell devices whose performances contest those of cells manufactured with sputtered i-ZnO.

As part of the SUNRISE project (Semiconductors Using Nanostructures for Record Increases in Solar-cell Efficiency), high efficiency, III-V semiconductor, quantum-dot-enhanced, triple-junction solar cells designed and manufactured by Cyrium Technologies Inc. were integrated into OPEL Solar, MK-I, Fresnel-lens-based, 550x concentrating modules carried on a dual axis tracker. Over its first year of operation 1.8 MWh of AC electrical energy was exported to the grid. Measurements of the direct and indirect components of the insolation, as well as the spectral irradiance of light incident on the demonstrator in Ottawa, Canada are presented. The system efficiency is measured and compared to that predicted by a system model to identify loss mechanisms so that they can be minimized in future deployments.

A novel approach to the design of multi-junction solar cells on silicon substrates for 1-sun applications is described. Models for device simulation including porous silicon layers are presented. A silicon bottom subcell is formed by diffusion of dopants into a silicon wafer. The top of the wafer is porosified to create a compliant layer, and a III-V buffer layer is then grown epitaxially, followed by middle and top subcells. Due to the resistivity of the porous material, these designs are best suited to high efficiency 1-sun applications. Numerical simulations of a multi-junction solar cell incorporating a porous silicon compliant membrane indicate an efficiency of 30.7% under AM1.5G, 1-sun for low threading dislocation densities (TDD), decreasing to 23.7% for a TDD of 10^7 cm^-2.