Thesis (Ph. D.)--Rutgers University, 2008. / "Graduate Program in Physics and Astronomy." Includes bibliographical references (p. 153-161).

No description available.

Dye-sensitised solar cells (DSCs) have the potential to be a low cost solar cell candidate due to the relatively low cost of materials and ease of processing. Also, unlike traditional silicon solar cells, DSCs can be lightweight and flexible, and perform well in diffuse sunlight and indoors which make them an extremely attractive prospect. This thesis investigates the time intensive heating stages associated with the fabrication of a DSC which are currently a bottleneck for translating this technology from the laboratory to an industrial scale. In addition some steps associated with the fabrication of a DSC share similarity to other technologies so these methods could be extremely applicable and versatile. Near infrared (NIR) radiative heating was used here to drastically reduce the heating times associated with DSC fabrication steps. NIR heating involves the absorption of NIR photons by the free electrons of an infrared absorbing substrate which releases thermal energy rapidly. NIR radiation has previously been used for the heating of metallic substrates but this is the first time it has been used to heat glass based substrates, which significantly broadens the potential applications of NIR heating. Upon 12.5 s of NIR exposure FTO and ITO coated glass reached significantly high temperatures, temperatures corresponding well to those required for the DSC heating steps. NIR radiation was used to sinter TiO2 working electrodes and thermally platinise counter electrodes on FTO glass in 12.5 s, 144 times faster than the conventional oven heating of 30 minutes. When assembled into DSC devices these electrodes performed identically to their oven equivalents. When combined with a faster dyeing process this enabled the overall laboratory manufacturing time of a DSC to be reduced from 123 min to 5 min with no compromise in efficiency which is an extremely promising step for the viability of DSC commercialisation.

This research work was undertaken to solve an industrial problem related to roll-to- roll production of dye-sensitised solar cells (DSCs). It is possible to manufacture DSCs in a roll-to-roll production line on a sheet metal such as titanium. However, DSCs produced in such a way are not commercially viable due to the use of expensive titanium metal. Therefore, the intention behind this work was to utilize a cheap sheet metal such as ECCS (electro chrome coated steel) to manufacture DSCs in a roll-to-roll production facility of TATA steel Europe, as this project was funded by them. Unfortunately, ECCS corrodes in the I[-]/I[3-] redox electrolyte present in a DSC therefore, to protect ECCS from the corrosion whilst using it as a DSC substrate was the real challenging task in this research. In order to solve this problem high temperature resistant polyimide based coatings were developed which can be used to coat ECCS substrates whilst maintaining excellent dimensional stability at the DSC processing temperatures. Such coatings were electrically conducting which helped preserve the electrical conductivity of the underlying metallic substrate. Electrically conductive polyimides were developed by simply blending conductive fillers such as carbon materials and titanium nitride. It was initially thought that carbon/polyimide based coatings would be suitable for this application. However, severe interfacial charge recombination and poor reflectivity made carbon/PI coatings inferior compared to the TiN/PI coatings. TiN/PI coatings performed well but poor reflectivity produced low current outputs. Moreover, TiN/PI was found to reduce the catalytic activity of thermally deposited platinum therefore it was not useful as a counter electrode material. As a solution to these problems, TiN and carbon materials based hybrid coatings were developed. Hybrid coatings did perform efficiently in terms of overall PV performance but due to poor reflectivity, such coatings also produced low J[sc] values. However, counter electrodes prepared using hybrid coating demonstrated excellent PV performance with thermally deposited platinum. Furthermore, TCO (transparent conducting oxide) free glass substrates can also be used to manufacture low-cost PV devices when coated with these conductive coatings.

No description available.

The molecular design, synthesis and spectroscopic characterization of a series of ruthenium(II), metal-free and platinum(II) photosensitizers were discussed. The applications of some of these compounds in dye-sensitized solar were also outlined. To start with, a brief overview on the background of dye-sensitized solar cells (DSSCs) was presented in Chapter 1. In Chapter 2, a series of new thiocyanate-free ruthenium(II) cyclometalated complexes with different ligands were successfully synthesized and some of them were fully characterized by spectroscopic and computational methods. The nature of cyclometalating ligands effectively tunes the properties of the metal complexes and the resulting DSSC performance. In Chapter 3, new di-anchoring organic dyes have been synthesized and characterized. This molecular design strategy can significantly enhance the . value because this successfully inhibits the undesirable charge combination and prolongs the electron lifetime. The discoveries open up a new avenue to the evolution of organic sensitizers and the optimization of bridged di-anchoring dyes for highly efficient co-adsorbent-free DSSCs. In Chapter 4, a series of new thiophene-free platinum sensitizers for the application of DSSCs was developed. Four unsymmetrical platinum(II) di-acetylide complexes containing phenothiazine moiety with different donor units were designed and synthesized. These photosensitizers were fully characterized by spectroscopic as well as computational studies and also successfully employed in DSSC fabrication. These findings provided positive evidence that platinum-acetylide complexes have a great potential and prospect for the use as promising metal-based photosensitizers in DSSC applications. Finally, Chapters 5 and 6 present the concluding remarks and the experimental details of the work described in Chapters 2–4.

Στη παρούσα διπλωματική εργασία, μελετήθηκαν ευαισθητοποιημένες ηλεκτροχημικές ηλιακές κυψελίδες. Ιδιαίτερη έμφαση δόθηκε στη βελτιστοποίηση κύριων χαρακτηριστικών των κυψελίδων αυτών, όπως είναι η απόδοση και ο χρόνος ζωής των ηλεκτρονίων. Για το σκοπό αυτό, παρασκευάστηκαν ηλεκτροχημικές κυψελίδες οι οποίες περιείχαν λεπτά υμένια ημιαγωγού TiO2. Δοκιμάστηκαν διαφορετικά υλικά κατά τη σύνθεση της πάστας TiO2 (ΗNO3, CH3COCH2COCH3, H3PO4) καθώς και διαφορετικοί τρόποι παρασκευής των υμενίων αυτών (doctor blade, spin coating), με στόχο πάντα την επίτευξη υψηλότερης απόδοσης. Έγινε σύγκριση των αποτελεσμάτων έτσι ώστε να προσδιοριστούν οι διάφοροι παράγοντες στους οποίους οφείλονται τα προβλήματα στη λειτουργία των κυψελίδων αυτών. Για την ευαισθητοποίηση των παραπάνω υμενίων δοκιμάστηκε η χρωστική ρουθηνίου Ν3. Επίσης, στο αρχικό στάδιο των πειραμάτων, τα υμένια TiO2 των κυψελίδων χαρακτηρίστηκαν και ως προς τη μεταβατική φωτοαγωγιμότητά τους για μια επιπλέον ένδειξη της συμπεριφοράς τους στο κενό και στον αέρα. Η μέτρηση της μεταβατικής φωτοαγωγιμότητας έγινε, επίσης, και σε διαφορετικά πάχη υμενίων με στόχο τον καθορισμό του βέλτιστου πάχους. Δείγματα που περιείχαν HNO3 διαπιστώθηκε πως είχαν υψηλότερη απόδοση εν αντιθέσει με δείγματα που περιείχαν H3PO4 τα οποία είχαν χαμηλότερη απόδοση. Ο χρόνος ζωής ενός ηλεκτρονίου φαίνεται να διατηρείται σε υψηλά επίπεδα σε δείγματα που περιείχαν ΗΝΟ3, εν αντιθέσει με άλλα δείγματα στα οποία ο χρόνος ζωής είτε είχε ραγδαία μείωση εξαρχής (Η3PO4), ή είχε ασυνήθιστη μείωση και αύξηση στη δεύτερη και τρίτη μέτρηση (CH3COCH2COCH3). Η προσθήκη CH3COCH2COCH3 στην παρασκευή των υμενίων είχε ως αποτέλεσμα υμένια με υψηλότερη μεταβατική φωτοαγωγιμότητα τόσο στο κενό, όσο και στον αέρα, όμως, χωρίς ιδιαίτερη σταθερότητα. Αντίθετα, η προσθήκη HNO3 παρουσίασε ομαλότερη συμπεριφορά και σαφώς υψηλότερη μεταβατική φωτοαγωγιμότητα στα υμένια σε σύγκριση με υμένια που περιείχαν H3PO4, τα οποία ίσως και λόγω υψηλής ηλεκτραρνητικότητας της φωσφορικής ρίζας σε σύγκριση με αυτή της νιτρικής ρίζας, φάνηκαν ιδιαιτέρως ασθενή. Τα δείγματα παρουσίαζαν μείωση της απόδοσης σε συνάρτηση με το χρόνο. Επιπλέον, το γενικό συμπέρασμα για το πάχος ήταν πως πρέπει να κυμαίνεται γύρω στα 10μm για την αποδοτικότερη λειτουργία μιας ηλεκτροχημικής κυψελίδας. Το υμένιο τέτοιων κυψελίδων αποδείχτηκε πως έχει τη βέλτιστη μεταβατική φωτοαγωγιμότητα στο κενό και στον αέρα, επίσης. Επιπρόσθετα, η σύνδεση των ηλεκτροδίων ομοεπίπεδα (coplanar) σε σχέση με τη την σύνδεση τους ως σάντουιτς δίνει καλύτερα αποτελέσματα και μάλιστα με διαφορά μεγέθους τεσσάρων τάξεων, όσον αφορά τη μεταβατική φωτοαγωγιμότητα τόσο στο κενό, όσο και στον αέρα. Τέλος, η μέθοδος εναπόθεσης spin-coating ενδείκνυται για τα συγκεκριμένα πειράματα, λόγω επίτευξης υμενίων ιδιαιτέρως ομοιόμορφων και με επαναληψιμότητα. Λόγω επίτευξης μικρού πάχους υμενίων, ενδείκνυνται οι πολλαπλές επιστρώσεις για μεγαλύτερο πάχος των υμενίων. Συνίσταται, επίσης, εναλλακτικά είτε πυκνότερη πάστα, είτε λιγότερα δευτερόλεπτα περιστροφής, ή συνδυασμός αυτών των δύο παραμέτρων. Αντιθέτως, η μέθοδος doctor blade οδήγησε σε υμένια που δεν ήταν τόσο ομοιόμορφα και αυτό το γεγονός είχε αντίκτυπο και στη λειτουργία της κυψελίδας. / In the present thesis, dye-sensitized solar cells were studied. Particular emphasis was placed on optimizing the main characteristics of these cells, such as the efficiency and the lifetime of electrons. For this purpose, dye-sensitized solar cells were prepared containing thin films of semiconductor TiO2. Different materials were tested during the composition of the paste TiO2 (ΗNO3, CH3COCH2COCH3, H3PO4) as well as different ways of preparation of these films (doctor blade, spin coating), always aiming to achieve higher performance. Also, the results compared in order to identify individual factors behind the problems in the functioning of these cells. For the sensitization of these films, the ruthenium dye N3 was tested. Also, at the initial stage of the experiments, the TiO2 films of the cells were characterized in terms of their transient photoconductivity for an additional indication of the behavior in vacuum and in air. The measurement of the transient photoconductivity was taken place also at different film thicknesses in order to determine the optimal thickness. Samples containing HNO3 found to have higher performance unlike samples containing H3PO4 which had a lower performance. The lifetime of an electron appears to persist at high levels in samples containing HNO3, in contrast with other samples in which the lifetime was either rapidly reducing from the beginning (H3PO4), or had unusual decrease and increase in the second and third measurement (CH3COCH2COCH3). Adding CH3COCH2COCH3 in the preparation of the films had as a result films with higher transient photoconductivity both in vacuum and in air, but without particular stability. In contrast, the addition of HNO3 showed smoother behavior and clearly higher transient photoconductivity in films in comparison with films containing H3PO4, which perhaps due to the high electronegativity of phosphate in comparison with that of the nitrate anions, seemed particularly weak. The samples showed a reduction as a function of the time. Moreover, the overall conclusion for the thickness was that it should range around 10μm in order to achieve the most efficient operation of an electrochemical cell. It has proved that the film of such cells has the most optimum transient photoconductivity in vacuum and in air, too. Additionally, the coplanar way of connecting electrodes in relation with the “sandwich” way of connecting electrodes gives better results, with difference of four grades size concerning the transient photoconductivity both in vacuum and in air. Finally, the method of spin-coating deposition is indicated for these experiments, achieving films particularly uniform and highly reproducible. Since films are thin, multiple coatings are appropriate for thicker films. Also, it is recommended, alternatively, either denser paste or fewer seconds of rotation, or a combination of these two parameters. In contrast, doctor blade method leaded to films that are not uniform at all and this fact had an impact on the function of the cell.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

The main goal of this thesis is to investigate and develop the physics of dye-based photovoltaic physics through molecular and photonic routes. Numerous photovoltaics devices have been fabricated through the course of this work to study their characteristics, performance, physical composition and action. The relative youth of the field of dye-based optoelectronics provides extensive scope for new research which provides fascinating opportunities in terms of physical processes.This thesis is divided into two main projects; exploring the adaption of conventional dye-sensitized solar cells via starkly different routes which serendipitously culminated in striking similarities at their conclusion. The first route is through incorporating spectrally complementary dye molecules with the intention of extending the range of light absorption of the final devices. This initial aim was achieved and was then furthered by the realisation of an unexpected and unprecedented energy transfer process occurring, imparting enhanced photocurrent generation in both the near-IR and visibile region. The second route involves investigating the effect on dye-sensitized solar cell physics and performance of the inclusion of metallic nanoparticles with the expectation of inducing plasmonic interactions. Novel systems were designed and implemented, devices were made which display significant performance enhancement, attributed to plasmonic coupling into the dyes and thereby increasing photocapture. A number of other investigations are documented whose current completion does not sufficiently warrant independent chapters but whose scientific interest is evident.

No description available.