Polymers are with low carrier mobility. If polymer solar cells are to exhibit high power conversion efficiencies, their carrier mobilities must be improved. Metallic NPs are promising materials for use in polymer solar cells because of their high conductivities. In this work, we studied the carrier transport characteristic of metallic nanoparticle blending into polymers. We blended Pt nanoparticles (Pt NPs) and Pd nanoparticles (Pd NPs) into polymers to improve carrier mobility, and enhance the power conversion efficiency of the polymer solar cell. P3HT was used as a donor material because of its high stability and with high absorption in visible light. PCBM was used as a acceptor material because of its high stability and with high electron transportation. We blended modified Pt NPs and Pd NPs into the P3HT:PCBM active layer, with the device configurations of ITO/PEDOT:PSS/P3HT:PCBM: Pt NPs/Al and ITO/PEDOT:PSS/P3HT:PCBM:Pd NPs/Al, respectively polymer solar cells measured was under AM 1.5G 100mW/cm2 illumination. When we blended Pt NPs into the active layer, the open-circuit remained 0.64V, the short-circuit current increased from 6.67mA/cm2 to 9mA/cm2, the power conversion efficiency increased from 1.96% to 3.08%. When we blended Pd NPs into the active layer, the open-circuit remained 0.62V, the short-circuit current increased from 6.33mA/cm2 to 7.33mA/cm2, the power conversion efficiency increased from 1.7% to 2.48%. The enhanced efficiency originated from the increased carrier mobility of the active layer when the Pt NPs or Pd NPs were present.

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.

My research on metal oxide electron-harvesting interlayers for organic solar cells was focused as three interrelated projects in this dissertation: i) development of a chemical vapor deposition (CVD) system for titanium dioxide (TiO₂) film; ii) an electrochemical methodology to evaluate ZnO thin film charge (hole) blocking ability; iii) the effects of plasma modifications on sol-gel ZnO and sol-gel ZnO/organic (active layer) interfaces. In i), we showed that nanoscale (12-36 nm) CVD TiO₂ film deposited at 210 °C from our system obtains properties of conformal growth with ITO substrate, superior hole blocking ability, stoichiometric metal to oxide ratio, and close energetic alignment with electron acceptors, e.g., fullerenes. The introduction of CVD TiO₂ film as an electron transport layer (ETL) into organic solar cell significantly improves its J-V characteristics compared to bare ITO electrode. The optimum TiO₂ thickness in the OPV device applications was found to be 24 nm with a high fill factor (0.58) and power conversion efficiency (3.7%) obtained. In ii), simple electrochemical methods, i.e., cyclic voltammetry, impedance spectroscopy have been used to evaluate sol-gel derived ZnO (sg-ZnO) and sputtered ZnO (sp-ZnO) porosity and pinhole density. We showed that sg-ZnO with high surface area porous structure allows the probe molecules and poly-thiophene (P3HT) thin layer to direct contact ITO substrate, whereas sp-ZnO with dense structural property efficiently eliminates the probe molecule diffusion and the penetration of P3HT layer to ITO substrate. This electrochemical property difference also directly reflects on the device shunt resistance (Rp), where we observed larger leakage current for the devices using sg-ZnO than that of devices using sp-ZnO. We envision these simple electrochemical characterizations can be applied into other similar metal oxide interlayers as well as on flexible TCO substrates, in which pinholes and physical imperfections, e.g., cracking may occur after multiple bending processes. In iii), we demonstrated low power (10.5 watts) radio frequency (RF) O₂ and Ar plasma treatments have significant impacts on sg-ZnO near-surface chemical compositions, which in turn influence the onset potential of sg-ZnO electron injection from the underlying ITO substrate and its energetic alignment with electron acceptors, e.g., C₆₀. Using UPS, we found the presence of localized mid-gap states near the Fermi-level (Ef) of sg-ZnO, which induces the most favorable band bending and the largest vacuum level shift due to significant electron transfer from sg-ZnO to C₆₀. As a result, the resultant solar cells show the best device performance. Upon the plasma treatments, the passivation effects eliminate the mid-gap state. Therefore, we observed less degree of band bending at ZnO/C₆₀ interface and poorer device performance for the plasma treated sg-ZnO. The study demonstrates the importance of oxide/organics interface in operations of organic solar cells and provides a modification method to tune surface properties of oxide materials which can apparently be applied in other organic electronic devices, e.g., field effect transistors (FETs), organic light emitting diodes (OLEDs), etc.

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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Excitonic photovoltaic devices, including organic, hybrid organic/inorganic, and dye-sensitized solar cells, are attractive alternatives to conventional inorganic solar cells due to their potential for low cost and low temperature solution-based processing on flexible substrates in large scale. Though encouraging, they are currently limited by the efficiency from not yet optimized structural and material parameters and poor overall knowledge regarding the fundamental details. This dissertation aims to achieve improved performance of hybrid solar cells by enhancing material property and designing new device architecture. The study begins with the addition of XD-grade single-walled carbon nanotube (XDSWNT) into poly(3-hexylthiophene) (P3HT) to improve the current density. By having a weight ratio of XDSWNT and P3HT equaled to 0.1:1, short-circuit current was quadrupled from 0.12 mA cm-2 to 0.48 mA cm-2 and solar cell efficiency was tripled from 0.023% to 0.07%, compared to devices with pure P3HT as a hole transport material. Secondly, a significant improvement in device efficiency with 250 nm long ZnO nanorod arrays as photoanodes has been achieved by filling the interstitial voids of the nanorod arrays with ZnO nanoparticles. The overall power conversion efficiency increased from 0.13% for a nanorod-only device to 0.34% for a device with combined nanoparticles and nanorod arrays. The higher device efficiency in solid-state DSSCs with hybrid nanorod/nanoparticle photoanodes is originated from both large surface area provided by nanoparticles for dye adsorption and efficient charge transport provided by the nanorod arrays to reduce the recombinations of photogenerated carriers. Followed by the novel layer-by-layer self-assembly deposition process, the hybrid photoanode study was extended to the longer ZnO nanorod arrays. The best performance, 0.64%, was achieved when the thickness of the photoanodes equaled to 1.2 ?m. Finally, the photovoltaic devices were modified by adding ZnO nanoarpticles into P3HT to increase interfacial area between ZnO and P3HT. The efficiency was enhanced from 0.18% to 0.45% when the ZnO nanorod arrays were 625 nm in length. Our successful design of the device morphology significantly contributes to the performance of solid-state hybrid solar cells.

Easy manufacturing, light weight and inexpensive materials are the key qualities of organic solar cells that makes them a highly researched area. To make organic solar cells adequate for the market, the efficiency of power conversion has to increase further, and the lifetime of organic solar cells has to improve. Avoiding recombination losses is a piece in the puzzle that can make organic solar cells more efficient. Organic solar cells with two different hole transport layers were therefore examined by I-V measurements. It was found that the organic solar cell with MoO3 as the HTL possesses a higher current density in both the reverse region and forward region. The higher current density in both regions points towards a less successful blocking of electrons travelling to the anode (reverse region) and a better ability to transport holes from the active layer to the anode. Insight to different state of recombination was also found from the slope values in the Voc and Jsc as a function of light intensity plots. It was concluded that both solar cells experience a dominant monomolecular recombination under short circuit condition and evolved into bimolecular recombination under open circuit condition. However, the cell with CuSCN showed a more dominant bimolecular recombination, which was shown from a slope closer to one unity kT/q in the Voc as a function of light intensity plot.

As one of the most promising solar cell technologies, organic solar cells have unique superiorities distinct from inorganic counterparts, such as semitransparency, flexibility and solution-processability, as well as tunable photophysical properties, which originate from the structural verstailities of organic semiconductors. A major breakthrough in OSCs was the exploration of novel non-fullerene electron acceptor (NFAs): In comparison with traditional fullerene derivative acceptors, NFA possesses several advantages, such as low synthesis cost, tunable absorption range and adjustable energetic level, which effectively provides a wide light-harvesting window with low energetic loss. In recent decades, fused-ring electron acceptors (FREAs) have obtained an irreplaceable status in the development of OSCs. However, there are still initial drawbacks to FREA-based devices including: 1: the degree of molecular packing and the corresponding impact on device performance, which has not been studied in depth; 2: the feasibility of approaches for controlling the bulk heterojunction morphology of the film, which also has not been systemic researched; 3: the presence of bulk (geminate and non-geminate) and surface recombination which significantly affects the efficiency and stability of working devices. In this thesis, I took the above three issues as my main doctoral research subjects. In the first part of the thesis, I shine light onto the strength of π-conjugated backbones in FREA molecular structures, which strongly affect the intramolecular interaction. Herein, two FREA with different conjugated framework (IDT core vs IDTT core) are synthesized and employed as electron acceptors in OSCs. A significantly enhanced power conversion efficiency of 11.2% is obtained from IDTTIC-based devices in comparison with that of IDTIC-based devices (5.6%). After considering the electron-donating part in FREA molecules, I also study the effect of the terminal unit, which has a strong relationship with the intramolecular charge transfer effect and intermolecular interactions. Solvent additives are another powerful strategy to further improve the photovoltaic efficiency. 1-chloronaphthalene (CN) was found to be useful in the PTB7-Th:IEICO-4F system, which show a PCE improvement from 9.5% to 12.8%. Furthermore, by utilizing a small molecule donor, BIT-4F-T, as a third component, an optimum PCE of 14.0% is achieved in the devices based on PTB7-Th:IEICO-4F.

This thesis is dedicated to studies of the excited-state dynamics in organic molecular systems for solar energy conversion by employing time-resolved experimental techniques. Organic photovoltaic (OPV) devices have received significant attention in the past decade and reaching record high power conversion efficiencies (PCE) above 17%. An essential step towards reaching the predicted PCE limit of 25.5% is to develop a comprehensive picture of the photophysical processes, specifically the loss processes, in OPV devices. It is the aim of this thesis to investigate and understand the fate of excited-states in organic electron donor/acceptor systems by ultrafast spectroscopic techniques, specifically, to reveal the interplay between energy and charge transfer processes. The first part deals with the identification of different polymorphs in a diketopyrrolopyrrole-based (DPP) polymer. Applying time-resolved photoluminescence (TRPL) measurements to the polymer dissolved in different solvent mixtures and using multivariate curve resolution (MCR) to deconvolute the ground-state absorption spectra reveals the co-existence of an amorphous (α) and two semi-crystalline (β1 and β2) polymer phases. The OPV device performance is shown to increase by the additional absorption of the β2 phase. The second part compares the efficiency of direct and energy transfer-mediated charge generation in prototypical donor-acceptor dyads that use as the electron donor triangulene derivatives chemically linked to the electron acceptor perylenediimide (PDI) block via oligophenylene spacers of different lengths. Charge generation efficiencies are found to be similar and increase with the donor-acceptor spatial separation. A combination of transient absorption (TA) measurements and computation of the dyad’s excited-state landscape revealed the presence of “optically-dark” excited-states that are populated by ultrafast donor-acceptor energy transfer prior to hole (back) transfer. The last part of the dissertation uses TRPL, TA, and time-delayed collection field (TDCF) measurements alongside MCR analysis to provide a comprehensive analysis of the yield of individual photophysical processes in OPV devices. A systematic methodology is proposed and tested on two all-polymer BHJ devices used as model systems. The experimental findings are supported by successful simulation of the solar cells’ JV characteristics using the spectroscopically-determined kinetic parameters. More generally, this approach can be used to quantify efficiency-limiting processes in other donor-acceptor BHJs.

Abstract: Even though improvements in the efficiency of organic solar cells encouraged the commercialization of this technology in the past two decades, the stability of organic solar cells is still an active area of research. The effect of photo-oxidative degradation on the performance of organic solar cell devices is significant. One way to lower the rate of photooxidation degradation is by preventing oxygen molecules from reaching the active layer of organic solar cells. This could be achieved by fabricating the devices in an inert environment in the absence of oxygen. Once the devices are fabricated, they would be encapsulated in a transparent material.1, 2 Even though this is a viable solution, there are two main issues. First, it was shown that oxygen molecules could diffuse through the encapsulating material and degrade the devices.3 Second, implementing this solution would increase the fabrication cost of these devices, which would make this solution commercially unfeasible compared to other solar cell technologies.3 Speller and his colleges reported a possible mechanism of the photo-oxidative degradation and showed a relationship between the rate of degradation and LUMO energy levels of electron acceptor molecule4. In this thesis, we report the photo-oxidative degradation rate of O-IDTBR and O-IDTBR-(C3N2)2. The later electron acceptor is analogous to O-IDTBR with deeper LUMO by 0.1 eV. After four hours of constant irradiation from a 1-sun intensity xenon solar simulator, the maximum UV-Vis absorbance of O-IDTBR is reduced by 12% relative to O-IDTBR-(C3N2)2. Lower absolute degradation rates were observed when 1-sun LED solar simulator was used compare to xenon solar simulator.