No description available.

No description available.

The cost of photovoltaic electricity needs to be significantly reduced in order to achieve a high electricity market penetration. Thin-film solar cells have good potential to achieve such cost savings though (i) large-area deposition onto low-cost foreign substrates, (ii) more streamlined processing, (iii) monolithic cell interconnection, and very efficient use of the expensive semiconductor material. Polycrystalline silicon (poly-Si) on glass is a promising technology for the cost-effective large volume production of PV modules since it (i) makes use of an abundant raw material, (ii) is non-toxic, (iii) does not suffer from light-induced degradation, and (iv) does not rely on TCO layers. Usually plasma enhanced chemical vapour deposition (PECVD) is used for the layer formation. This thesis explores the use of e-beam evaporation as deposition method since it is potentially much faster and cheaper than PECVD. The resulting solar cells are referred to as EVA (from EVAporation). Two inherent shunting mechanisms in EVA cells are demonstrated to be shunting through sub-micron sized pinholes when the back electrode is deposited and shunting between the emitter and the absorber layer at the glass-side electrode. Through the improved understanding of these shunting mechanisms it was possible to develop a suitable metallisation scheme for EVA cells using an aligned deposition of emitter and back surface field line contacts and a specially developed shunt mitigation etching technique. For the first time appreciable efficiencies of up to 5.2% were demonstrated on this material. It was also shown that only very lightly doped absorber layers can lead to the required high short-circuit currents in EVA cells. The resulting cells are currently completely limited by space charge region recombination occurring with comparatively low ideality factors of only ~ 1.4 This thesis also demonstrates the usefulness of Jsc-Suns measurements and investigates optical loss mechanisms in the current devices. Advanced modelling of distributed series resistance effects, influencing Suns-Voc, m-Voc and Jsc-Suns curves, is employed. PC1D modelling is used to extract relevant device parameters. In this work it was found that the diffusion length in the best EVA cells is longer than the absorber layer and that insufficient light trapping is currently the major hurdle to higher cell efficiencies. From the obtained results it can be concluded that EVA solar cells are promising candidates for the low-cost and high-volume production of solar modules.

The cost of photovoltaic electricity needs to be significantly reduced in order to achieve a high electricity market penetration. Thin-film solar cells have good potential to achieve such cost savings though (i) large-area deposition onto low-cost foreign substrates, (ii) more streamlined processing, (iii) monolithic cell interconnection, and very efficient use of the expensive semiconductor material. Polycrystalline silicon (poly-Si) on glass is a promising technology for the cost-effective large volume production of PV modules since it (i) makes use of an abundant raw material, (ii) is non-toxic, (iii) does not suffer from light-induced degradation, and (iv) does not rely on TCO layers. Usually plasma enhanced chemical vapour deposition (PECVD) is used for the layer formation. This thesis explores the use of e-beam evaporation as deposition method since it is potentially much faster and cheaper than PECVD. The resulting solar cells are referred to as EVA (from EVAporation). Two inherent shunting mechanisms in EVA cells are demonstrated to be shunting through sub-micron sized pinholes when the back electrode is deposited and shunting between the emitter and the absorber layer at the glass-side electrode. Through the improved understanding of these shunting mechanisms it was possible to develop a suitable metallisation scheme for EVA cells using an aligned deposition of emitter and back surface field line contacts and a specially developed shunt mitigation etching technique. For the first time appreciable efficiencies of up to 5.2% were demonstrated on this material. It was also shown that only very lightly doped absorber layers can lead to the required high short-circuit currents in EVA cells. The resulting cells are currently completely limited by space charge region recombination occurring with comparatively low ideality factors of only ~ 1.4 This thesis also demonstrates the usefulness of Jsc-Suns measurements and investigates optical loss mechanisms in the current devices. Advanced modelling of distributed series resistance effects, influencing Suns-Voc, m-Voc and Jsc-Suns curves, is employed. PC1D modelling is used to extract relevant device parameters. In this work it was found that the diffusion length in the best EVA cells is longer than the absorber layer and that insufficient light trapping is currently the major hurdle to higher cell efficiencies. From the obtained results it can be concluded that EVA solar cells are promising candidates for the low-cost and high-volume production of solar modules.

Silicon interposers with TSVs (through-silicon-vias) have been developed in single-crystalline silicon wafer to achieve the high I/O (Input/Output) density. However, single-crystalline silicon interposers suffer a few problems such as cost, electrical performance and reliability. To overcome these shortcomings, an entirely different approach using polycrystalline silicon interposers with thick polymer liners are proposed by Georgia Tech Packaging Research Center, aiming to achieve lower cost silicon interposers with high performance and reliability. The objective of this research is to explore and demonstrate thin polycrystalline silicon as a suitable interposer material to achieve high performance and high reliability TPVs (through-package-vias) in polycrystalline silicon materials with lower cost. Three fundamental challenges were defined, including: 1) low resistivity of the polycrystalline silicon, resulting in high electrical loss; 2) reliability problems resulting from CTE (coefficient of thermal expansion) mismatch between silicon and Cu, and 3) handling and processing of thin silicon panels. A three-dimensional EM (electromagnetic) model was developed to simulate insertion loss and crosstalk of TPVs and compared with TSVs. It has been shown thick polymer liner is effective in addressing the fundamental challenge of low resistivity for the polycrystalline silicon material, leading to better electrical performance of TPVs than TSVs. Parametric studies indicate that thicker sidewall liners result in better electrical performance. A two-dimensional axisymmetric model was established to simulate the first principal stresses in silicon and shear stresses in TPV under thermal cycling. TPVs with thick polymer liners present both smaller principal stresses and shear stresses than TSVs due to the low modulus of polymer. Parametric studies suggest that sidewall liners act as stress buffers and thicker liners result in better mechanical performance. Design guidelines based on simulation results were used in TPV demonstration and test vehicle fabrication. Fracture strength of polycrystalline silicon panel has been fundamentally studied with four-point bending tool and Weibull plot. Surface polymer liners on both sides were introduced to improve the handling of thin silicon panels. Quantitative study showed higher characteristic fracture strength for the panel with surface liners than raw silicon panel. Low cost and double-side processes have been developed for TPV fabrication including UV (ultraviolet) lasers for TPV formation, double laser method for liner fabrication and electroless Cu for seed formation. Key steps and mechanisms for aforementioned processes were summarized and discussed. Polycrystalline silicon interposers with TPVs and up to four metal RDLs (re-distribution layers) were designed, fabricated and characterized. Measurement results showed low insertion loss for both TPVs and CPW (co-planar waveguide) transmission lines. Good model to hardware correlation was also observed. Reliability test vehicles of polycrystalline silicon interposers were also designed and fabricated for thermal cycling test. TPVs survived 4000 cycles without significant resistance changes. SEM imaging on the cross-section of the samples confirmed no Cu or silicon cracking. Magnified images around corner also suggested good adhesion at Cu/liner and silicon/liner interfaces.

Στη συγκεκριμένη διπλωματική εργασία μελετάται η δομή και η λειτουργία φωτοβολταϊκών πλαισίων πολυκρυσταλλικού πυριτίου. Για το σκοπό αυτό είναι απαραίτητη τόσο η θεωρητική μελέτη και περιγραφή της λειτουργίας τους, όσο και η πειραματική προσέγγιση του θέματος. Αρχικά, παρουσιάζεται η δομή των ηλιακών κυττάρων, βασικών δομικών μονάδων των φωτοβολταϊκών πλαισίων. Στην αρχή δίνεται μια περιεκτική περιγραφή της λειτουργίας της p-n επαφής ώστε στη συνέχεια να γίνει ευκολότερα κατανοητός ο τρόπος που επιδρά το φωτοβολταϊκό φαινόμενο στη δομή αυτή. Αναλύονται ορισμένα μεγέθη που χρησιμοποιούνται στην επιστήμη των φωτοβολταϊκών καθώς και οι συνθήκες στις οποίες μετρώνται παγκοσμίως τα ηλιακά κύτταρα/πλαίσια προκειμένου να υπάρχει ένα κοινό μέτρο σύγκρισης. Η μελέτη ολοκληρώνεται με την πειραματική διαδικασία. Σε αυτήν αναλύονται διεξοδικά οι παράγοντες που επηρεάζουν τη λειτουργία των φωτοβολταϊκών σε μια πραγματική εγκατάσταση. Οι παράγοντες αυτοί είναι κυρίως περιβαλλοντικοί (ηλιοφάνεια, θερμοκρασία). Υπάρχουν εξίσου σημαντικοί παράγοντες, όμως, που είναι δυνατό να μεταβληθούν από τον άνθρωπο με τρόπο ώστε να επιτευχθεί η βέλτιστη απόδοση. Πιο συγκεκριμένα, η γωνία κλίσης του πλαισίου και η περίπτωση της σκίασής του είναι απαραίτητο να ληφθούν σοβαρά υπόψη. Τα συμπεράσματα που προέκυψαν έχουν άμεση εφαρμογή στον υπολογισμό και τη μελέτη μιας πραγματικής εγκατάστασης μιας και δίνουν συγκεκριμένες αριθμητικές τιμές. Για παράδειγμα η συνολική αποδιδόμενη ετήσια ενέργεια από ένα πλαίσιο 80Wp τοποθετημένο με γωνία κλίσης ίση με τη βέλτιστη (380) στην περιοχή της Πάτρας είναι 172.038kWh. Αξιοσημείωτη είναι και η σύγκριση των πειραματικών τιμών ισχύος και συντελεστή απόδοσης με τις τιμές του κατασκευαστή και αυτές που προέκυψαν μέσω μοντελοποίησης (σε υπολογιστή με τα προγράμματα Matlab και Mathematica). Αυτές είναι: η μετρούμενη ισχύς του πλαισίου 73,15Watt και συντελεστής απόδοσης η=11,5%, ισχύς διδόμενη από τον κατασκευαστή 81,71Watt με συντελεστή απόδοσης η=12,6% και ισχύς η που προκύπτει από τη μοντελοποίηση 89,95Watt με συντελεστή απόδοσης η=14,2%. Η αιτιολόγηση αυτών των διαφορετικών τιμών ισχύος και των αντιστοίχων συντελεστών απόδοσης αποδίδεται στην αδυναμία να πετύχουμε ίδιες συνθήκες διεξαγωγής αυτών των μετρήσεων καθώς και στην σε σειρά και παράλληλη αντίσταση που δεν είναι δυνατόν να εκτιμηθούν ακριβώς για να εισαχθούν στο μοντέλο. / This thesis studies the structure and performance of polycrystalline silicon solar modules. To do this, not only a theoretical study of their operation is necessary, but also an experimental approach of the subject. In a first step, we can see the structure of polycrystalline silicon solar cells, which compose the solar panels. It was thought useful to give a compact description of the –p-n junction function. This way, the photovoltaic phenomenon’s impact on –p-n junction’s function can be easier understood. Along with the theoretical part, there is an analysis of some values that are used to the photovoltaic science along with the conditions that are used in order to have a common meter of comparison. A study, however, would not be completed if the experimental process did not exist. In this process the factors that influence the photovoltaic operation are analyzed at great length in a real installation. These factors are mainly environmental (sunlight, temperature). There are equally important factors, however, which are possible to be altered in such a way to achieve the most optimal performance. Specifically, the tilt angle and the shading case of the solar module are essential to be taken seriously into consideration. The results can be directly used for the calculation of a real installation and are very helpful as they are quite specific. For example, the total output power with a tilt angle of 380 at Patras-Greece is 172.038kWh. It is also remarkable to have a comparison between the experimental data, the data given by the constructor and the data gathered through simulation method. The values for the output power are 73,15Watt (η=11,5%), 81,71Watt (η=12,6%) and 89,95Watt (η=14,2%) respectively. The explanation of different power factor and output power comes from the inability to achieve exactly the same experimental conditions. Moreover, the shunt and series resistances cannot be calculated exactly due to lack of constructor’s data details.

Στην παρούσα διπλωματική εργασία μελετάται η λειτουργία φωτοβολταϊκών πλαισίων πολυκρυσταλλικού πυριτίου και συγκεκριμένα γίνεται εξαγωγή των dc παραμέτρων (ηλεκτρικά χαρακτηριστικά) του συγκεκριμένου τύπου φωτοβολταϊκού πλαισίου υπό φωτισμό σε πραγματικές συνθήκες λειτουργίας. Αρχικά παρατίθεται η θεωρία για τα ηλιακά κύτταρα που είναι η βασική δομική μονάδα του φωτοβολταϊκού πλαισίου. Για να μπορεί να γίνει εύκολα αντιληπτό σε όλους το φωτοβολταϊκό φαινόμενο, δίδεται μια διεξοδική περιγραφή της λειτουργίας της p-n επαφής. Παράλληλα, μελετώνται κάποια προβλήματα που μπορεί να προκύψουν στις φωτοβολταϊκές εγκαταστάσεις και οι πιθανοί τρόποι σύνδεσης μιας οικιακής εγκατάστασης. Στην συνέχεια ακολουθεί το πειραματικό μέρος. Σε αυτό παρουσιάζεται μια σειρά μετρήσεων που διεξήχθησαν κατά την διάρκεια του ακαδημαικού έτους 2008-2009 και η χάραξη γραφικών παραστάσεων μέσω των οποίων καταδεικνύεται πόσο επηρεάζεται η λειτουργία των φωτοβολταϊκών πλαισίων από κάποιους περιβαλλοντικούς παράγοντες σε διαφορετικές εποχές. Τα συμπεράσματα που προέκυψαν από την πειραματική διαδικασία παρέχουν γνώση για την μελέτη μιας πραγματικής εγκατάστασης στην περιοχή της Πάτρας. Το πιο σημαντικό αποτέλεσμα είναι η ετήσια συνολική αποδιδόμενη ενέργεια από το πλαίσιο πολυκρυσταλλικού πυριτίου των 80W σε κλίση 38 μοίρες στην περιοχή της Πάτρας που είναι 98 KWh/έτος. Στη μελέτη αυτή περιλαμβάνεται και μια πρόταση εγκατάστασης φωτοβολταϊκών στην ταράτσα της φοιτητικής εστίας του Πανεπιστημίου Πατρών. Λαμβάνοντας υπόψιν μας τις ενεργειακές ανάγκες της φοιτητικής εστίας, έγινε προσπάθεια να βρεθεί μια λύση τοποθέτησης των φωτοβολταϊκών πλαισίων, έτσι ώστε το σύστημά μας να είναι λειτουργικό καλύπτωντας όσο το δυνατό τις ανάγκες για ηλεκτρική ενέργεια. Θεωρείται όμως αναγκαίο να αναφερθεί οτι οι πειραματικές μετρήσεις στις οποίες στηρίζεται η μελέτη αυτή συνιστούν ένα μέρος μόνο των μετρήσεων οι οποίες απαιτούνται για την πλήρη ανάπτυξη και διερεύνηση του θέματος που πραγματεύεται η παρούσα Διπλωματική εργασία. / The present thesis studies the operation of polycrystalline silicon solar modules and concretely the export of dc parameters (electric characteristics) of the particular type of solar module under real conditions of operation. Initially, the thesis mentions the theory of the solar cells that are the basic structural unit of solar modules. In order the photovoltaic phenomenon, to be easier understood to all people, an extensive description of operation of p-n junctions is given. At the same time, certain problems which come out in photovoltaic installations are studied and the likely ways of connection of domestic installation. Then, the experimental part follows. At this part, a group of measurements are presented which were made at the duration of academic year 2008-2009 and the mapping out of graphic representations, which shows the influence of the operation of solar modules from certain environmental factors per season. The results that came out from the experimental process constitute knowledge for the study of a real installation at the region of Patras-Greece. The most important result is the annual total energy yield from polycrystalline silicon solar module of 80W αt tilt angle 38 degrees at Patras being 98 KWh/year. It is included in this study an indicative photovoltaic installation at the flat-roof of students‟ housing of University of Patras. Knowing the energy needs of students‟ housing, we try to find a solution of placement of solar modules, so our system to be functional as long as possible for the needs for electric energy. It is however necessary to mention that the experimental measurements on which this study is built constitute a part only of the measurements required for the full development and investigation of the issue addressed by this thesis.

The goal of this research was to examine the relationship between dislocation density and in-plane residual stress in edge-defined film-fed growth (EFG) silicon wafers. Previous research has shown models for linking dislocation density and residual stress based on temperature gradient parameters during crystal growth. Residual stress and dislocation density have a positive relationship for wafers with very low dislocation density such as Cz wafers. There has been limited success in experimental verifications of residual stress for EFG wafers, without any reference to dislocation density. No model of stress relaxation has been verified experimentally in post production wafers. A model that assumes stress relaxation and links residual stress and dislocation density without growth parameters will be introduced here. Dislocation density and predominant grain orientation of EFG wafers have been measured by the means of chemical etching/optical microscope and x-ray diffraction, respectively. The results have been compared to the residual stress obtained by a near infrared transmission polariscope. A model was established to explain the results linking dislocation density and residual stress in a randomly selected EFG wafer.

This report determines the influence of spectral beam splitting on the temperature, maximum power and efficiency of a polycrystalline silicon cell under concentrated light. The PV cell was exposed to wavelengths ranging between 450 nm – 1000 nm. It was found that spectral beam splitting results in a temperature 11 °C lower than the PV cell that was exposed to the full spectrum after one hour. Additionally, it was also found that spectral beam splitting improves the efficiency of the PV cell by 2.1% at 980 W·m-2 and cell temperature of 25 °C. A study into the effect of light intensity on the efficiency showed that the efficiency increases between 580 W·m-2 – 680 W·m-2, after which the efficiency decreases up to 1380 W·m-2. Furthermore, it was found that the reason for the decrease in the efficiency was the decrease in the fill factor which is caused by the decrease in the shunt resistance. A comparison between the PV cell under the filtered spectrum and the full spectrum, showed that the PV cell exhibits a similar trend in efficiency as light intensity increases. However, the efficiency difference is initially at approximately 3% between 580 W·m-2 and 780 W·m-2, thereafter, the efficiency difference decreases to approximately 2 %. Based on these results, it has been recommended that further research be carried out to understand how wavelengths influence the band gaps of PV cells as the light intensity increases. / Dissertation (MEng)--University of Pretoria, 2018. / Chemical Engineering / MEng / Unrestricted

No description available.