Alternative energy sources such as solar energy have attracted an extensive interest in the petroleum shortage era. Among solar cells, dye-sensitized solar cell (DSSC) attracts the attention of widespread research teams because of the easy-production process, low cost, and good photon-to-electron conversion efficiency. In this study, both UV and acid solution such as HCl are used to improve the efficiency of DSSC. The UV illumination can eliminate organic contaminates on TiO2 by photocatalysis and enhance the adsorption of dye molecules. Meanwhile, the coordination mode between TiO2 and dye could be changed and lower the electron transportation. If the HCl solution is used after UV illumination, the coordination mode can be preserved. Moreover, H+ from HCl can attract the COO¡Ð anchoring group of dye by electrostatic force. It further increases the adsorption of dye and improves the DSSC efficiency. The coordination mode was measured by Fourier-transform infrared spectrometer (FTIR). The internal resistance was measured by electrical impedance spectroscopy (EIS). The chemical properties were characterized by X-ray photoelectron spectroscopy (XPS). The light absorbance was measured by ultraviolet-visible spectroscopy (UV-Vis). The morphology was observed by field emission scanning electron microscope (FE-SEM). The performance of the cells was measured by a semiconductor device analyzer. In our results, the conversion efficiency was improved from 6.29% of untreated one to 6.71 and 7.39% for UV and UV + HCl treated ones.

In this research, gel-state and quasi-solid state dye-sensitized solar cells (DSSCs) were fabricated with polydimethylbenzimidazole(PDMBI) as the polymer electrolyte. These devices are stable under room light in air, even without encapsulation. The energy conversion efficiency of gel-state cells was drastically increased around 200% after the device worked. We propose that appropriately aggregated PDMBI in electrolyte layer could provide pathways which would facilitate the diffusion of ion through the electrolyte. Moreover, this arrangement induces it an ion exchange reaction which could lead to the promotion of the diffusion rate between iodide species. An optimized device performs well with a power conversion efficiency of 4.98% under air-mass 1.5 global (AM 1.5G) illumination. For the fabrication of quasi-solid state dye-sensitized solar cells, we immersed a few liquid electrolyte to improve electrical contact between TiO2 porous layer and PDMBI layer. The quasi-solid state cell efficiency fabricated with PDMBI as electrolyte was 2.26%. Furthermore, our device architecture is performing well because of the good band alignment among TiO2, dye, and PDMBI. In this research, we have successfully demonstrated gel-state and quasi-solid state dye-sensitized solar cells comprising PDMBI as electrolyte.

Wide band gap metal oxides have recently become one of the most investigated materials in surface science. Among these metal oxides especially TiO2 attracts great interest, because of its wide range applications, low cost, biocompatibility and ease of analysis by all experimental techniques. The usage of TiO2 as a component in solar cell technology is one of the most investigated applications of TiO2 . The wide band gap of TiO2 renders it inecient for isolated use in solar cells. TiO2 surface are therefore coated with a dye in order to increase eciency. This type of solar cells are called dye sensitized solar cells . The eciency of dye sensitized solar cells is directly related with the absorbed light portion of the entire solar spectrum by the dye molecule. Inspite of the early dyes, recent dye molcules, which are called wider wavelength response dye molecules, can absorb a larger portion of entire solar spectrum. Thus, the eciency of dye sensitized solar cells is increased by a considerably amount. In this thesis the electronic structure of organic rings, which are the fundamental components of the dye molecules, adsorbed on anatase (001) surface is analyzed using density functionaltheory. The main goal is to obtain a trend in the electronic structure of the system as a function of increasing ring number. Electronic structure analysis is conducted through band structure and density of states calculations. Results are presented and discussed in the framework of dye sensitized solar cells theory.

Thesis (Ph. D.)--Rutgers University, 2009. / "Graduate Program in Materials Science and Engineering." Includes bibliographical references (p. 164-172).

The dye sensitized solar cell (DSSC) is a promising low-cost technology alternative to conventional solar cell in certain applications. A DSSC is a photo-electrochemical photovoltaic device, mainly composed of a working electrode, a dye sensitized semiconductor layer, an electrolyte and a counter electrode. Sunlight excites the dye, producing electrons and holes that can be transported by the semiconductor and electrolyte to the external circuit, converting the sunlight into an electrical current. A material that could be useful for DSSCs is the nanoscale cupric oxide, which can act as a p-type semiconductor and has interesting properties such as low thermal emittance and relatively good electrical properties. The goal of this project was to synthesize and characterize CuO nanoparticles using three different methods and look into each products potential use and efficiency in DSSCs. The particles were synthesized using two different solution based chemical precipitation methods and a flame spray pyrolysis method, yielding nanostructures with different compositions, structures and sizes ranging from ~20 to 1000 nm. The nanoparticle powder synthesized by the flame spray pyrolysis route was tested as semiconductor layer in the working electrode of the DSSC. Current-voltage measurements presented low solar conversion efficiencies with a reversed current, meaning that the cupric oxide cells did not work in a desirable way. Further studies of the cupric oxide synthesis and its suitability in DSSCs are needed to increases the future possibilities for gaining well working p-type DSSCs with higher efficiencies.

In this dissertation, the multiple facets of benzodithiophene (BDT) units are explored, with a focus on understanding how the isomerism of the BDT structure affects the macroscopic properties of the oligomeric and polymeric materials created. First, the story focuses on an overview of the BDT synthons and their applications in organic electronics. A straightforward synthesis of BDT and its derivatization to seven π-conjugated building blocks and seven polymers is presented. Then, symmetric (donor)2-acceptor (D2-A) dye architecture for application in dye-sensitized solar cells are investigated. Two isomeric systems are studied, and the branched sensitizers show a greater incident photon-to-current efficiency than the linear dyes. The nature of the accepting core is also varied between dibenzophenazine to dithienophenazine. The sensitizer with the weakest accepting core displays the best photovoltaic performance, due to an increase in the open-circuit voltage of ~100 mV caused by the favorable shift of the metal oxide conduction band. Lastly, a study of the donating building blocks in these (D2-A) sensitizers demonstrates that increasing the number of donor units from two to six thiophene moiety doubles the solar cell performance, due to the improvement of the light harvesting ability.

The pure organic dye sensitized solar cells (DSSCs) were studied and a new organic dye of donor-π-2acceptors (D-π-2A configuration) was fabricated. This dye, denoted as B2, was investigated and applied in DSSCs. Density functional theory (DFT) was used to examine the electronic distribution of the frontier orbitals of the B2 dye. It was found that intramolecular charge transfer (ICT) between the donor moieties and acceptor moieties of the B2 dye may take place under photo irradiation. The LUMO, LUMO+1 and LUMO+2 of B2 are all distributed on the acceptor moieties and this is very helpful to enhance the intramolecular electron transfer from the donor moieties to the acceptor moieties, which will consequently promote the chance of electron injection into the semiconductor. DSSCs based on B2 demonstrated an power conversion efficiency of 3.62 %. This efficiency value is approximately half of the power conversion efficiency of DSSCs based on N719 (7.69 %) under the same conditions. Femtosecond transient absorption and nanosecond transient absorption (TA), and time-correlated single photon counting (TCSPC) technique were applied to examine the electron transfer processes occurring on the surface of B2/TiO2. B2 dye has life time of the excited states three orders in magnitude shorter than that of N719. The electron injection time from excited B2 to TiO2 is also three orders in magnitude shorter than that from excited N719 to TiO2. It was revealed that the delocalized electrons of π → π* transition for both the B2 dye and the N719 dye could be further guided into the semiconductor, while such injection processes may not happen for the localized electrons in π → π* transition of these dyes. The nanosecond transient absorption and transient emission spectroscopy of the ruthenium bipyridyl sensitizer N719 in different solvents were studied. Three kinds of ZnO nanoparticles were utilized to study the electron transfer process taking place on the interface of N719/ZnO with and without electrolyte by Time-Correlated Single Photon Counting (TCSPC) technique, TA and transient emission spectroscopy. Isopropanol was found to stabilize the singlet excited state of N719 and a related emission band centered at 460 nm was observed in nanosecond time scale. It was revealed that the electrolyte has a significant impact upon the electron transfer dynamics on the N719/ZnO interface. In the absence of electrolyte, the electron transfer process on the N719/ZnO interface is dependent upon the depth of defects in ZnO nanoparticles. Conversely, in the presence of electrolyte, the impact of ZnO defects upon the electron transfer process is eliminated and the effective electron injection happens from the excited states of N719 to ZnO, in spite of the ZnO particle sizes. The polymer based solar cells were studied and a polymer incorporated with a pyrenylcarbazole pendant was synthesized and applied in the functionalization of multi-wall carbon nanotubes (MWCNT) by noncovalent π-π interaction. The polymer/MWCNT hybrids were isolated and examined. The strong interaction between the polymer and MWCNT in a 1,1,2,2-tetrachloroethane (TCE) solution was investigated. The emission spectra demonstrated an effective quenching of emission from the polymer by the MWCNT. DFT calculations showed an electron delocalization phenomenon between the pyrene and carbazole moieties. The LUMO of the polymer is mainly located on the pyrene moiety while the LUMO+1 of the polymer is predominantly positioned on the carbazole moiety. The electronic transition of LUMO+1→LUMO results in intramolecular charge transfer (ICT) from the carbazole moieties to the pyrene moieties. Femtosecond TA determined the characteristic TA feature of the excited states, which are contributed from both the pyrene and carbazole moieties. The excited state lifetime of the polymer was calculated to be 659 ps and the photo excited electrons can inject into the MWCNT very fast on a time scale of 420 fs. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Dye-sensitized solar cells (DSSCs) are photoelectrochemical cells that offer efficient and potentially economical alternative to conventional solar electricity production technologies. DSSCs belong to the third generation of solar cells and offer several advantages over the solid-state junction solar cells. They utilize materials, such as titanium dioxide that are inexpensive and abundant relative to those used in conventional solar cells. Moreover, DSSCs can be fabricated with simple and scalable manufacturing processes. Finally, in DSSCs, photon absorption and charge-carrier transport are undertaken by different materials, namely molecular dyes and wide band gap semiconductors, respectively. Unlike conventional solar cells, no compromise is necessary between decreasing the band gap for visible light absorption and increasing the band gap to resist photocorrosion. For successful commercialization, a photovoltaic system incorporating DSSCs must operate reliably under a wide range of solar irradiance and operating temperatures. This experimental study reports the fabrication and characterization of the performance of a DSSC as a function of irradiance and operating temperature. The prototyped DSSCs had (i) nanocrystalline titanium(IV) dioxide, TiO₂, photoanode, (ii) platinum thin film cathode, and (iii) acetonitrile based liquid electrolyte. The photoanodes were sensitized with N-749 dye. The current-voltage characteristics of the DSSCs were measured at operating temperatures from 5 to 50° C and under 500, 1000, and 1500 W m⁻² irradiance. The open circuit voltage, V[subscript oc], decreased linearly with increasing temperature and had positive, logarithmic relation with irradiance. At temperatures lower than 15° C and 1500 W m⁻² irradiance, short circuit current density, J[subscript sc], was limited by the diffusion of I₃ in the electrolyte and increased with increasing temperature. At temperatures lower than 15° C and lower irradiance, J[subscript sc] increased with increasing temperature due to electron density limited recombination of electrons injected into the TiO₂ conduction band. At higher temperatures, the recombination was dominant over diffusion and J[scubscript sc] decreased with increasing temperature. Moreover, J[subscript sc] increased linearly with increasing irradiance. The DSSC photoconversion efficiency did not vary appreciably at temperatures lower than 15° C but decreased with increasing temperature. Finally, the DSSC efficiency increased with increasing irradiance. There was no indication of significant coupling effect of irradiance and temperature on DSSC efficiency. This study reports for the first time the coupling between irradiance and thermal effects on the operation of DSSCs. The results reported in this study can be used in recovering kinetic and transport properties that can be used in modeling and optimization of DSSCs. / text

Στην εργασία αυτή πραγματοποιήθηκε μια βιβλιογραφική ανασκόπηση σχετικά με τις ευαισθητοποιημένες ηλεκτροχημικές κυψελίδες και τα οργανικά φ/β, τις αρχές λειτουργίας τους,τα υλικά τους και τους τρόπους παρασκευής τους. Παρασκευάσαμε ηλεκτροχημικές κυψελίδες χρησιμοποιώντας διαφορετικές χρωστικές και αντιηλεκτρόδια. Όλες οι κυψελίδες δοκιμάστηκαν κάτω από διαφορετικές συνθήκες για την εύρεση των παραγόντων που επηρεάζουν την απόδοσή τους. Παράγοντες όπως το πάχος και η τραχύτητα του υμενίου, διαφορετικές χρωστικές και αντιηλεκτρόδια κτλ βελτιστοποιήθηκαν για την αύξηση τησ απόδοσης. Επίσης εξετάστηκε η σταθερότητα των κυψελίδων αυτών με το χρόνο. Τέλος εξετάστηκαν και εξηγήθηκαν οι παράγοντες για τη χαμηλή απόδοση των κυψελίδων. / In this study we make a comprehensive literature review on dye-sensitized nanostructured and organic solar cells, their operating principles, materials as well as their manufacturing methods. We have fabricated dye – sensitized electrochemical solar cells using different dyes and counter electrodes. All solar cells were tested under different conditions in order to find the factors limiting their efficiency. Factors like film thickness and roughness, different dyes and counter electrodes etc were changed in order to improve efficiency. The stability of these solar cells were also evaluated for different time intervals. The efficiency was rather low and the reasons for this are examined and explained thorougηly.

The work presented here report findings in (1) infrared detectors based on p-GaAs/AlGaAs heterojunctions, (2) J and H aggregate sensitized heterojunctions for solar cell and photon detection applications, (3) heterojunctions sensitized with quantum dots as low cost solar energy conversion devices and near infrared photodetectors. (1)A GaAs/AlGaAs based structure with a graded AlGaAs barrier is found to demonstrate a photovoltaic responsivity of ~ 30mA/W (~ 450mV/W) at the wavelength of 1.8 mm at 300K. Additionally the graded barrier has enhanced the photoconductive response at 78 K, showing a responsivity of ~ 80mA/W with a D*=1.4×108 Jones under 1V bias at 2.7 mm wavelength. This is an approximately 25 times improvement compared to the flat barrier detector structure, probably due to the improved carrier transport, and low recapture rate in the graded barrier structure. However, these graded barrier devices did not indicate a photoresponse with photoconductive mode at 300K due to high shot noise. Additionally, two generation-recombination noise components and a 1/f noise component were identified. A series of GaAs/AlGaAs multilayer hetero-junction structures were tested as thermal detectors. A superlattice structure containing 57% Al fraction in the barrier and 3 × 1018 cm-3 p-doped GaAs emitter showed the highest responsivity as a thermal detector with a TCR of ~ 4% K-1, at 300K. (2)The photovoltaic properties of heterojunctions with J-/ H- aggregated dye films sandwiched between n– and p-type semiconductors were investigated for potential application as solar cells and IR detectors. Films of cationic dye Rhodamine-B-thiocyanate adsorbed on Cu2O substrate are found to form organized dye layers by self-assembled J- aggregation, resulting in large red-shifts in the photo -response. Additionally, cells sensitized with a pentamethine cyanine dye exhibited a broad spectral response originating from J- and H-aggregates. The photocurrent is produced by exciton transport over relatively long distances with significant hole-mobility as well as direct sensitized injection at the first interface. (3) A ZnO/PbS-QD/Dye heterostructure had enhanced efficiency compared to ZnO/Dye heterostructure as a solar cell. Furthermore, a ZnO/PbS-QD structure has demonstrated UV and NIR responses with 4×105V/W (390 nm) and 5.5×105 V/W (750 nm) under 1V bias at 300K.