Search

32nd European Photovoltaic Solar Energy Conference and Exhibition; 580-582, This contribution presents our latest progress on 5x5 inch solar cells yielding a maximum efficiency = 22.0%. The cell process utilizes four laser irradiation steps for emitter and back surface doping as well as for the ablation of the rear passivation layer and structuring of the metallization. The improvement from the previously measured = 20.8% is achieved by busbar-tapering, improved cleaning and higher area utilization. Photolumines-cence proves the homogeneity and accuracy of all four laser processes. Electroluminescence illustrates the improve-ment achieved by the tapered of the busbars and suggests that even higher efficiencies will be possible.

31st European Photovoltaic Solar Energy Conference and Exhibition; 462-465, We present our latest results on interdigitated back contact (IBC) laser processed solar cells, achieving = 23.2% efficiency on an area A = 4 cm² (independently confirmed by Fraunhofer ISE CalLab) and = 20.8% on A = 156 cm² (in-house measurement). The solar cells process uses four novel laser irradiation steps and thus avoids complex photolithographic masking. Laser doping generates local emitter and back surface field using a pulsed green laser. Furthermore, an UV laser locally ablates the rear passivation stack before aluminum metallization. The UVlaser also structures the evaporated aluminum layer into the rear side finger and busbar metallization. A cost of ownership calculation shows that the simple processing of the laser IBC solar cells reduces the module production costs to 0.53 €/Wp while achieving high efficiencies.

27th European Photovoltaic Solar Energy Conference and Exhibition; 1954-1957, Advanced structures in solar cells, such as selective emitter or back contacted back junction solar cells, require different phosphorus doping levels. A phosphorus furnace diffusion creates the lowly doped region, needed for high lifetime between the contacts. The phosphosilicate glass deposited during the diffusion serves as precursor layer for laser doping. It leads to a high doping required for a good contact resistance between the metallization and the doped area. We pursue a trade-off between the low Auger recombination due to lowly doped regions and the low contact resistance due to sufficient phosphorus concentration under metallization. The decrease of the diffusion temperature, the introduction of a preoxidation prior to the diffusion, and the high POCl3/O2 ratio lead to high sheet resistances, even after the surface passivation by thermal oxidation, which redistributes the P-atoms. Subsequently, we analyze the effect of a light diffusion on the P-precursor (i.e. the phosphosilicate glass) that serves as additional dopant source for the laser doping of the highly doped areas. Surface saturation current density J0s down to J0s = 12 fA/cm² are obtained on the lowly doped region while contact resistance measurements of the highly laser doped areas give pc,LD = 3.6 mΩcm² on the same sample.

27th European Photovoltaic Solar Energy Conference and Exhibition; 1272-1277, This work applies computer modeling to optimize the structure of laser-processed ipv back-contact back-junction n-type c-Si solar cells. We employ a 2D numerical device simulator to investigate the conversion efficiency , the short circuit current density Jsc, the open circuit voltage Voc, and the fill factor FF of the cells with different profiles of laser doped emitter, laser doped back surface field (BSF), and furnace diffused front surface field (FSF). Additionally, by applying an analytical model, we investigate the influence of the finger line resistance on FF and for different pitches p. A pitch p = 1200 μm demonstrates the highest conversion efficiency = 23.4 % for the simulated 6” solar cell by experimentally achieved FSF, BSF, and emitter profiles, and a metallization with resistivity = 5 μΩcm.

26th European Photovoltaic Solar Energy Conference and Exhibition; 275-280, This work applies state of the art computer modelling to optimize the structure of fully-laser-processed ipv back-contact back-junction n-type c-Si solar cells. In this work we employ the software TCAD Silvaco® Atlas to investigate the conversion efficiency , the short circuit current density Jsc, the open circuit voltage Voc, and the fill factor FF of the cells with different pitch sizes p and different emitter widths wemitter. The simulation shows that p = 1400 μm and wemitter = 1300 μm demonstrate the best conversion efficiency = 25.6 % for the simulated solar cell with an optimized rear side design. We investigate the effect of the front- and rear side passivation on . The results show that the combination of a front surface field and a front side dielectric passivation layer has a strong effect on . Rear side field effect passivation of the base using laser doped back surface field has also strong passivation effect. The rear side dielectric passivation of the laser doped emitter decreases of the back-contact backjunction solar cell only for higher surface recombination velocities Semitter >103 cm/s.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 2028-2031, Reducing the inactive phosphorus concentration at heavily doped emitter surface reduces emitter surface recombination velocity. A new 60 Ω/sq emitter with less inactive phosphorus at the surface reduces emitter saturation current density by a factor two. The new 60 Ω/sq emitter shows a gain of +15 mV for VOC and +0.8 mA/cm2 for JSC when compared to a conventional 60 Ω/sq emitter. However, a reduced FF by Delta FF = –1.8 % results in no gain in conversion efficiency. Modelling of the emitter saturation current density shows a lower surface recombination velocity for the new emitter compared to the conventional emitter, thus explaining the higher blue response of the measured quantum efficiency.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 1510-1512, Screen printing is still the most cost effective technology for metallization used in the industrial solar cell production. We introduce an optimized drying process and test it with two different Ag pastes on large area solar cells. The new drying process consists of the pre-drying step with lower temperature Tpeak1 = 80 °C and a maindrying step with Tpeak2 = 220 °C. In the pre-drying step the wet paste warms up and adjusts in height without dispersing. During the main-drying step, the remaining solvents dry out of the lines. Silver lines dried with the optimized process show an improved homogeneity in height and their line resistance decreases by 6% compared to the reference lines with a not optimized drying process. Solar cells processed with the optimized drying results in increased fill factors for about 0.7% absolute.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 1923-1925, The emitter saturation current density of three emitters with sheet resistance e = 60, 80 and 100 /sq but with the same surface concentration are analyzed as a function of surface recombination velocity S. We found that 60, 80 and 100 /sq emitter sets an upper limit for open-circuit voltage of mc-Si solar cells at VOC = 640, 650 and 660 mV. The recombination takes mainly place at the emitter surface and within the thin heavily doped part. The thin heavily doped part contributes up to 95 % absolute to the recombination of the emitter bulk. Lifetime measurements of multicrystalline wafers with 60 /sq passivated by SiNx show an emitter saturation current density of J0e = 960 fA/cm2. This value is equivalent to an open circuit voltage of VOC = 627 mV which is close to the VOC = 625 mV of our finished mc-Si solar cells.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 1390-1392, The present contribution reports on a novel concept for an industrial screen printing line which implements three improved components: i) a high quality screen printer which prints two solar cells at the same time, thus increasing printing speed by 100 %, ii) a drying furnace with a paternoster principle, thus using much smaller foot print than conventional furnaces, iii) a firing furnace with a new transport principle, using quartz glass rolls instead of a metal conveyor belt, thus decreasing energy consumption and metal contamination. We designed and investigated this prototype of industrial screen printing line. Processing the same type of cells in our line gives similar, improved results when compared to a conventional industrial line. Latest processed and produced solar cells from ipe show in house measured efficiencies of = 17.2 % on mono crystalline solar cells and = 16.4 % on multi crystalline cells each with an industrial size of 156 x 156 mm2.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 2237-2241, This work shows an advanced method for front grid optimization. The theoretical results of the present method promises > + 0.4 % absolute efficiency gain for industrial fine-line printed Si solar cells compared to industrial front grid design. The optimal front grid design consisting of more than 85 fingers of width wf < 70 μm, and two busbars of width wb = 1.5 mm. The method at hand takes into account the change of the current and voltage at the maximum power point with the change of the solar cell front grid design. The present work simulates the solar cell efficiency for different front grid design.