05 Fakultät Informatik, Elektrotechnik und Informationstechnik
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/6
Browse
5 results
Search Results
Item Open Access Quantifizierende Elektrolumineszenz für Silizium-Solarzellen und -module(2019) Kropp, Timo; Werner, Jürgen H. (Prof. Dr. rer. nat. habil.)Diese Arbeit präsentiert zwei neue Messmethoden auf Basis der Elektrolumineszenz zur Charakterisierung von Solarzellen und -modulen. Beide Methoden nutzen die Strominjektion, um ein Lumineszenzbild zu quantifizieren. Der Unterschied zwischen den Methoden besteht in der zeitlichen Variation der Strominjektion bzw. Stromextraktion. Bei der gepulsten Strominjektion sowie -extraktion hängt der zeitliche Verlauf der resultierenden Elektrolumineszenz von der effektiven Ladungsträgerlebensdauer in der untersuchten Solarzelle ab. Die eingeführte analytische Beschreibung der normierten periodischen Intensitätsdifferenz zwischen zwei unterschiedlich strommodulierten Lumineszenzbildern ist unabhängig von der Belichtungszeit der Bildaufnahme. Bei der zeitlich konstanten Strominjektion ist die Amplitude der Lumineszenzintensität zusätzlich durch den lokalen Serienwiderstand bzw. Parallelwiderstand einer Solarzelle bestimmt. Die zweite entwickelte Methode dieser Arbeit ist in der Lage, Leistungsverluste von Photovoltaikmodulen durch mechanische Defekte sowie potentialinduzierte Degradation anhand eines einzelnen Lumineszenzbildes quantitativ zu bewerten. Der durch einen Defekt hervorgerufene Leistungsverlust gegenüber der ursprünglich nach dem Datenblatt verfügbaren Leistung wird präzise vorhergesagt.Item Open Access a-Si:H/c-Si heterojunction front- and back contacts for silicon solar cells with p-type base(2010) Rostan, Philipp Johannes; Werner, Jürgen H. (Prof. Dr. rer. nat. habil.)This thesis reports on low temperature amorphous silicon back and front contacts for high-efficiency crystalline silicon solar cells with a p-type base. The back contact uses a sequence of intrinsic amorphous (i-a-Si:H) and boron doped microcrystalline (p-μc-Si:H) silicon layers fabricated by Plasma Enhanced Chemical Vapor Deposition (PECVD) and a magnetron sputtered ZnO:Al layer. The back contact is finished by evaporating Al onto the ZnO:Al and altogether prepared at a maximum temperature of 220 °C. Analysis of the electronic transport of mobile charge carriers at the back contact shows that the two high-efficiency requirements low back contact series resistance and high quality c-Si surface passivation are in strong contradiction to each other, thus difficult to achieve at the same time. The preparation of resistance- and effective lifetime samples allows one to investigate both requirements independently. Analysis of the majority charge carrier transport on complete Al/ZnO:Al/a-Si:H/c-Si back contact structures derives the resistive properties. Measurements of the effective minority carrier lifetime on a-Si:H coated wafers determines the back contact surface passivation quality. Both high-efficiency solar cell requirements together are analyzed in complete photovoltaic devices where the back contact series resistance mainly affects the fill factor and the back contact passivation quality mainly affects the open circuit voltage. The best cell equipped with a diffused emitter with random texture and a full-area a-Si:H/c-Si back contact has an independently confirmed efficiency η = 21.0 % with an open circuit voltage Voc = 681 mV and a fill factor FF = 78.7 % on an area of 1 cm². An alternative concept that uses a simplified a-Si:H layer sequence combined with Al-point contacts yields a confirmed efficiency η = 19.3 % with an open circuit voltage Voc = 655 mV and a fill factor FF = 79.5 % on an area of 2 cm². Analysis of the internal quantum efficiency shows that both types of back contacts lead to effective diffusion lengths in excess of 600 μm. An extended fill factor analysis shows that fill factor limitations for the full-area a-Si:H/c-Si contacts result from non-ideal diode behavior, ascribed to the injection dependence of the heterojunction interface recombination velocity. Analysis of the external quantum efficiency under back side illumination with different bias light intensities delivers the effective surface recombination Seff(Φ) in dependance of the illumination intensity Φ. The front contact (emitter) uses a sequence of intrinsic and phosphorous doped amorphous silicon layers together with a ZnO:Al or a SnO2:In layer and an Al front contact grid. The emitter is prepared at a maximum temperature of 220 °C. Measurements of the minority carrier lifetime on symmetric i/n-a-Si:H coated wafers judge the emitter passivation quality. The best solar cells that use a thermal oxide back side passivation with Al-point contacts and flat a-Si:H emitters have open circuit voltages up to 683 mV and efficiencies up to 17.4 %. The efficiency of such devices is limited by a low short circuit current due to the flat front side. Using the same back contact structure with random pyramid textured wafer front sides and a-Si:H emitters yields open circuit voltages up to 660 mV and efficiencies up to 18.5 %, sofar limited by a relatively low fill factor FF ≤ 74.3 %. Analysis of the external quantum efficiency underlines the excellent surface passivation properties of the amorphous emitter. Combining both, amorphous front- and back contacts yields p-type heterojunction solar cells completely fabricated at temperatures below 220 °C. The best devices reach an open circuit voltage Voc = 678 mV and an efficiency η = 18.1 % with random textured wafers, limited by low fill factors FF ∼ 75 %. Besides the cell fabrication and characterization, this thesis reveals that the inherent a-Si:H/c-Si band offset distribution with a low conduction band offset and a large valence band offset is disadvantageous for p-c-Si heterojuntion solar cells if compared to their n-c-Si counterparts. A calculation of the saturation current densities of the cell's emitter, bulk and back contact demonstrates that the n-a-Si:H/p-c-Si emitter suffers from a low built-in potential. Modelling of the back contact based on the charge carrier transport equations shows that the insertion of an i-a-Si:H layer with a thickness d ≥ 3 nm (that is mandatory for a high surface passivation quality) leads to a series resistance that is critical for usage in a solar cell. The model mainly ascribes the high back contact resistance to the large valence band offset at the heterojunction.Item Open Access Jahresenergieerträge unterschiedlicher Photovoltaik-Technologien bei verschiedenen klimatischen Bedingungen(2010) Zinßer, Bastian; Werner, Jürgen H. (Prof. Dr. rer. nat. habil.)Es ist sowohl für Ingenieure als auch für Investoren sehr wichtig zu wissen, welche Energiemenge E [kWh] eine Photovoltaik(PV)-Anlage im realen Betrieb ins Stromnetz einspeist. Hintergrund ist meist die Frage nach den Kosten für den Solarstrom in €ct/kWh. Das Datenblatt gibt den Wirkungsgrad von PV-Modulen meist nur für Standardtestbedingungen im Labor an. In der Praxis verursachen erhöhte Modultemperaturen T, schwächere Einstrahlung G und ein anderes Spektrum des Lichts Abweichungen vom Wirkungsgrad unter Standardtestbedingungen; letztere kommen im realen Betrieb in Deutschland praktisch nie vor. In sonnigeren, südlichen Ländern variieren die Betriebsbedingungen noch stärker als in Deutschland, wodurch die Auswirkungen solcher Variationen auf den Jahresenergieertrag E_Jahr dort größer sind. Zur Klärung der Frage, welche PV-Technologie unter welchen klimatischen Bedingungen den höchsten Jahresenergieertrag E_Jahr erzeugt, wurden im Rahmen dieser Dissertation dreizehn verschiedene PV-Systeme unterschiedlicher Technologie in Stuttgart, Nikosia und Kairo aufgebaut und mit einer umfangreichen Messtechnik für Wetter- und Systemdaten ausgestattet. Dabei kommen sowohl mono- und multikristallines Silizium (Si) als auch verschiedene Dünnschicht-Technologien (amorphes-Si, Cu(InGa)Se2 (CIGS) und CdTe) zum Einsatz. Diese Arbeit untersucht mehrere Möglichkeiten, den Jahresenergieertrag einer PV-Anlage im Voraus zu bestimmen und vergleicht die modellierten Erträge mit den tatsächlich gemessenen. Die Felddatenauswertung bestätigt die in der Literatur oft genannten, besseren Temperaturkoeffizienten der Dünnschicht-Technologien. Die HIT-Technologie zeigt ein besseres Schwachlichtverhalten gegenüber den übrigen kristallinen Si-Technologien, die alle ein ähnliches Schwachlichtverhalten zeigen. Die CIGS-Technologie weist im Feld ein zum Labor entgegengesetztes Schwachlichtverhalten auf. Im Feld zeigen die amorphen Si- und CdTe-Technologien ein deutlich günstigeres Schwach-lichtverhalten als die kristallinen Si-Module. Den größten Einfluss auf die Bestimmung des Jahresenergieertrages E_Jahr hat, neben der örtlichen Einstrahlung G und Verschmutzung, die Toleranz der Nominalleistung P_STC der PV-Module. Nimmt man eine übliche Toleranz von ±2 bis 6% mit einem zusätzlichen Fehler von ±2% bei der Energiemessung an, so können sich zwei PV-Systeme um bis zu 8 bis 16% im Jahresenergieertrag E_Jahr unterscheiden, ohne dass dies auf die PV-Technologie zurückgeführt werden kann. Die PV-Anlagen erzeugen in Stuttgart im langjährigen Mittel einen Jahresenergieertrag E_Jahr von ca. 1000 kWh/kWp. In Nikosia ist der Ertrag mit ca. 1650 kWh/kWp um 65% größer. In Kairo beträgt der Jahresenergieertrag E_Jahr aufgrund starker Verschmutzung durch Sandstaub lediglich ca. 1300 kWh/kWp. Nach zwei Monaten vermindert der Staub die Leistung um 25%, so dass in Kairo eine regelmäßige Reinigung der Module unerlässlich ist. Als wesentliches Ergebnis dieser Arbeit lässt sich feststellen, dass die vom Hersteller angegebene Nominalleistung P_STC mit ihren Toleranzen, neben der Verschmutzung, den größten Einfluss auf den normierten Jahresenergieertrag E_Jahr einer Photovoltaikanlage hat. Die Effekte durch ein besseres Temperatur- und/oder besonders durch das Schwachlichtverhalten gehen bisher meist in den Toleranzen der Nominalleistung P_STC unter. Dennoch zeigt der Technologievergleich, dass die meisten Dünnschicht-Module und die HIT-Technologie ein besseres Temperatur- und Schwachlichtverhalten aufweisen und an wärmeren Standorten zu höheren Erträgen tendieren. Sobald exaktere Nominalleistungsbestimmungen möglich sind, werden die in dieser Arbeit entwickelten Methoden die Unterschiede im Temperatur- und Schwachlichtverhalten deutlich besser analysieren können.Item Open Access Fluorescent materials for silicon solar cells(2012) Prönneke, Liv; Werner, Jürgen H. (Prof. Dr. rer. nat. habil.)Photovoltaic systems with fluorescent collectors use the conversion and concentration of solar photons to increase solar cell efficiencies. Fluorescent dye in a dielectric plate absorbs incoming rays and emits spatially randomized photons with a lower energy range. The acrylic plate then guides part of the emitted spectrum to the collector side surfaces due to total internal reflection. Conventional research therefore applies solar cells to the side surfaces. This work analyzes the efficiency enhancement due to fluorescent collectors on top of solar cells which promises an easier technological handling. The first part of this work uses a Monte-Carlo simulation to model photovoltaic systems with fluorescent collectors and photonic structures. The results allow the comparison between side- and bottom-mounted solar cells. Examining the systems in the radiative limit achieves maximum theoretical limits. In each system, the photon collection probability depends strongly on the scaling of cell size and distance. The side-mounted solar cells perform better for larger scales, but for small scales bottom-mounted solar cells achieve equally high efficiencies. Consideration of non-radiative loss mechanisms and the application of a photonic structure also leads to the result that the application of solar cells to the collector back side needs careful scaling but performs as good as side-mounted solar cells. The second part presents the results of five experiments which analyze basic mechanisms in the fluorescent collector. Additionally, the experiments explore the benefits of fluorescent material in photovoltaic modules. i) The reabsorption experiment directs photons from an LED with wavelength 406 nm onto the collector top surface. A camera under the collector photographs photons which leave the back side. These photons are reabsorbed at least once. An analytical description extracts the reabsorption coefficient a = 0.021 1/mm from the camera picture. ii) Light beam induced current (LBIC) measurements on an amorphous silicon solar cell show that a fluorescent collector on top increases the collected current by 7%. The additional application of a photonic structure enhances the current by 95%. An analytical description of the absorption and emission processes in the collector using the reabsorption coefficient determined in the first experiment predicts the line-scans gained in the LBIC measurements. Therefore, the reabsorption measurement is sufficient enough to predict the collection performance of photovoltaic systems with fluorescent collectors without performing long LBIC-measurements. iii) Outdoor experiments compare mono crystalline silicon (c-Si) solar cells in acrylic troughs with and without fluorescent collectors on top. Fluorescent distribution added to the geometrical concentration decreases the current gain if limited to the trough aperture. A five times larger fluorescent collecting plate leads to a current gain enhancement by at least 50% compared to the limited aperture. This shows the advantage of fluorescent concentration. Achieving an increased current gain with geometrical concentration requires a new trough and more solar cell material. The experiments also show another advantage: Fluorescent collectors concentrate photons independent of their angle. Thus, photovoltaic systems using fluorescent concentration perform best even without tracking. iv) Two parallel connected c-Si solar cells under a fluorescent plate achieve an electrical output power P = 189 mW. The same set-up with an undoped acrylic plate on top gains P = 125 mW. By varying the cell distance this experiment additionally points out that the activation of surrounding photovoltaic inactive area is crucial to compensate losses directly above the solar cell. v) The last experiment avoids unfavorable losses by applying fluorescent dye to only the optical inactive cell connectors of an industrial c-Si solar cell encapsulated under glass. The fluorescent dye covering the white painted connector distributes incoming photons at all angles. The glass-air surface guides distributed photons onto the solar cell via total internal reflection. Derived with LBIC and Quantum Efficiency measurements, the efficiency of the solar cell increases from 16.0% to 16.2%. In conclusion, this work not only finds a new characterization method for the fluorescent concentration. Additionally, it presents that applying fluorescent dye on top of photovoltaic solar modules increase efficiencies under careful consideration of the scaling.Item Open Access Characterization of a laser doping process for crystalline silicon solar cells(2010) Ametowobla, Mawuli Francis; Werner, Jürgen H. (Prof. Dr. rer. nat. habil.)This thesis characterizes a process for the laser based formation of highly doped layers on crystalline silicon, which was developed at Institut für Physikalische Elektronik (ipe), University of Stuttgart. A first step analyzes silicon, which is laser irradiated without being doped at the same time. This approach allows for separating the effects of the laser process itself from the influence of employed doping precursors. The irradiation leads to a signicant reduction of the minority carrier lifetime in the processed silicon. A thorough characterization of suchlike treated samples shows the incorporation of the impurities oxygen, carbon and nitrogen, up to concentration levels of C = 1019/cm^3. At the same time, an n-type doping occurs within the irradiated surface layers. The doping concentrations are in the range ND = 10^17/cm^3. In spite of the numerously employed characterization methods, a complete identification of the mechanism, which causes the lifetime reduction, is not achieved. However, there exists the strong indication that the incorporated impurities, which lead to the n-type doping, are at the same time responsible for the lifetime degradation. Due to the low thickness d < 500 nm of the laser melted layers, defect induced local lifetimes t_SRH < 10 ns are required to explain the measured effective lifetimes. The examination of laser irradiated high effciency n-type emitters on p-type silicon substrates, the emitters being pre-fabricated by phosphorous furnace diffusion, yields information about the lifetime SRH of laser induced defects inside the emitters. The values found for t_SRH are in the range of 3 ns < t_SRH < 7 ns. These values are in accordance to the results found for irradiated, undiffused samples. The use of optimized laser parameters allows for obtaining very low emitter saturation current densities J0e = 45 fA/cm^2 after the irradiation. An experimental overview over various liquid and sputtered phosphorous precursors reveals strongly differing J0e values of samples, fabricated with different precursors. Considering the laser induced defects allows for predicting the potential for J0e and the open circuit voltage Voc of solar cells, fabricated with the best precursor, to J0e < 100 fA/cm^2 and Voc = 680 mV. This prediction points out that in the vast majority of cases, the defects, induced by the laser treatment itself, do not limit the performance of emitters, fabricated with the ipe process. Solar cells, which were produced in the course of this work, exhibit significantly lower open circuit voltages of maximum Voc = 635mV. In addition, these cells often exhibit low fill factors FF < 70 % and effciencies < 15 %. However, this lower performance is not in contrast to the predicted Voc potential. An analysis shows that mainly technological problems, leading to doping inhomogeneities, cause the lower efficiencies. An additional characterization of aluminium as a precursor material for p-type emitters on n-type silicon substrates shows exceptionally high doping concentrations ND > 10^21/cm^3. Corresponding emitter saturation current densities reach extremely high values of J0e > 10^11 A/cm^2. Consequently, the open circuit voltage Voc of solar cells, fabricated with aluminium doped emitters, is limited to Voc = 550 mV and their effciency to 7 %.