Tobias M. Pletzer

Analysis of the Annual Performance of Bifacial Modules and Optimization Methods

Bifacial modules have the advantage of capturing sunlight from front as well as from rear surfaces, and therefore, they are able to produce larger amounts of energy, compared with standard (monofacial) modules. However, their performance depends on the spatial distribution of the irradiance incident on the rear module surface, which is strongly affected by several site-specific conditions, such as albedo, reflective surface size, module elevation, and tilt angle. In this study, we elaborate upon the individual and combined effects of these factors on the annual energy yield of stand-alone south-facing bifacial modules through simulations at two site locations with contrary climatic conditions. Following the optimization of the tilt angle of bifacial modules dependent on the site, albedo, and module elevation, we demonstrate that the annual energy yield of a bifacial module increases linearly with albedo, which shows a monotonically increasing but, in addition, saturating behavior versus reflective surface size, and increases up to a certain module elevation. Through the simultaneous consideration of these dependences, we suggest an optimal positioning of bifacial modules. Finally, we show that under these optimal conditions, bifacial modules can supply up to 25% more energy compared with standard modules.

Local Thermographic Efficiency Analysis of Multicrystalline and Cast-Mono Silicon Solar Cells

Local dark current contributions of multicrystalline (mc) and cast-mono silicon (Si) solar cells were evaluated by using dark lock-in thermography (DLIT) measurements. The goal of these investigations was to evaluate the influence of the contact metallization on the efficiency. Both materials are treated by nearly the same solar cell process. Because of a special print design, the current density caused by the front-side and rear-side silver metallization can be measured separately, and their influence on the overall cell performance can be estimated. We measure an increase of the local saturation current density J₀₁ by about 2.8 × 10^-12 to 6.7 × 10^-12A/cm² at the soldering pads of the solar cell, compared with about 6 × 10^-13 A/cm² in the undisturbed regions. In the region of the front-side busbars, J₀₁ increases locally by about 4.5 × 10^-13 to 17.6 × 10^-13A/cm². The local depletion region recombination current, as well as the ohmic shunt current, is not affected by the screen-printed contacts. The bulk and back surface recombination in mc-Si within regions with a low amount of defects are almost the same as in cast-mono material. However, in regions of recombination active bulk defects, a significant increase in the recombination activity at the front contact was observed.

Comparison of large area high ohmic emitter silicon solar cells with standard screen-printed contacts

Significant improvement in screen-printing technology and the advancement in commercial Ag pastes has attracted considerable attention of PV community due to the possibility to contact lowly doped emitters to boost cell efficiencies without need of complex processing steps and device structures (eg. selective emitter solar cell). In this paper, our focus is on high ohmic emitters (Rsheet ~ 100 Ω/sq.) formed using different approaches. These emitters possess either high doping level (Ns ~ 2.5 × 1020 cm-3) and are shallow in depth or a very low doping level (Ns ~ 1.6 × 1019 cm-3) and are deeper/Gaussian in nature. Thermal oxidation processes have been used to optimise such emitter profiles. Hence emitter formation and passivation was accomplished in a single step. By changing the oxidation temperature surface concentration significantly decreases while the emitter depth increases. The solar cells of this investigation were fabricated on large area p-type Cz-Si wafers and have been screen-printed with commercially available Ag paste at front and full area Al-BSF at rear. Despite the low doping levels, all solar cells found to have a very good ohmic contact (FF > 78%). The highest cell efficiency for this simple and low cost approach, featuring low temperature oxide (LTO) and SiNx stack at front is 18,33% with a Voc of 640 mV, Jsc of 37,03 mA/cm2 and a fill factor of 79,14%.

Contacting Moderately Doped Phosphorous Emitters of Silicon Solar Cells With Dopant Segregation During Nickel Silicidation

Emitter performance is crucial for the efficiency of solar cells. A further increase of efficiency requires reducing recombination losses while keeping parasitic resistances and shadowing at a minimum. In silicon solar cells, recombination can be lowered by decreasing the emitter doping concentration; however, this comes at the cost of increasing contact and sheet resistances. Dopant segregation during nickel silicidation resolves this tradeoff since it allows ohmic contact formation to emitters with almost arbitrary doping profile and concentration. Here, we elaborate on using dopant segregation during silicidation and demonstrate contact formation, as well as a substantial performance improvement of silicon solar cells exhibiting a lowly doped emitter with a peak doping concentration of 6 × 1018 cm-3. Furthermore, we show that the small achievable contact resistances in principle allow a strong reduction of the contact area. In turn, this enables lowering the shadowing while, at the same time, decreasing the sheet resistance due to a larger number of front contacts that can be placed close to each other. Using PC1D simulations, we study the solar cell performance as a function of the emitter dopant concentration estimating the achievable efficiency increase when employing dopant segregation.

Characterization of three-dimensional structures in silicon solar cells by spatially-resolved illuminated lock-in thermography

The continuous increase of solar cell efficiencies is related to a large extent to a number of device structure improvements such as the introduction of selective emitters, selective front surface fields (FSF) or local back surface fields (BSF). These structures allow even more complex solar cell designs like passivated emitter and rear contacts/locally diffused (PERC/PERL), metal/emitter wrap through (MWT/EWT) or interdigitated back contact (IBC) cells. All these designs exhibit three-dimensional (3D) doping profiles that are mostly processed with diffusion and are thus not directly visible. In this paper we present a method to characterize these non-visible emitters, FSF and BSF structures in a two-dimensional (2D) camera based spatially-resolved technique, which is based on the illuminated lock-in thermography (ILIT) and free carrier absorption/emission (FCA/FCE). While this technique, also referred to as carrier density imaging (CDI) [1] or infrared lifetime mapping (ILM) [2] and usually used to investigate the wafer material quality, has already been used to investigate phosphorus diffused selective emitter structures [5],[6], here we present to the best of our knowledge the first results of aluminum (Al) alloyed emitters, selective FSF and local Al-BSF structures measured with this method. All spatially-resolved structures were matched with their doping profiles measured with electrochemical capacitance voltage (ECV) measurements. The approach appears very suitable for an in-situ process control of structured emitters, FSFs and BSFs. In principle, patterns with dimensions down to 5 μm can be resolved due to the detected wavelengths (λ) of 3 - 5 μm. We present clearly visible structures with dimensions down to 120 μm on monocrystalline Czochralski silicon (Cz-Si) with a lateral resolution of 30 μm per pixel due to our camera setup. This is suitable for an automatic alignment during contact formation and offers the possibility of quantitative measurements.

Efficiency increase of lossy solar cells by laser post-processing and detailed analysis of the current-voltage characteristics

The current-voltage (I-V) characteristics of processed industrial multicrystalline silicon (mc-Si) solar cells usually vary within a narrow range (e.g. +/−1 % abs. efficiency). Nevertheless, occasionally cells with very low efficiencies are produced with loss mechanisms such as local shunts or areas with unusually high recombination. In this paper we investigated industrial mc-Si solar cells with exceptionally low efficiencies in detail by dark and illuminated I-V measurements followed by an analysis based on the two-diode-model to determine the dominant electrical loss mechanisms. Subsequently spatially-resolved lock-in thermography (LIT) measurements were used to localise defect positions in these solar cells. These defects were electrically isolated with a laser. The solar cells were characterised again using the same analysis methods. For shunted cells an efficiency gain of up to 2.4 % absolute was obtained and the shunts disappeared. A detailed investigation of I-V data before and after laser isolation followed by a parameter study based on a two-diode-model is presented.

Quantitative local current-voltage analysis with different spatially-resolved camera based techniques of silicon solar cells with cracks

The ongoing trend to decrease the wafer thickness in the fabrication of silicon solar cells increases the number of possible cracks. Therefore, an in-depth understanding of the influence of cracks on solar cells is necessary. In this work, we investigate silicon solar cells with cracks by employing three camera-based spatially-resolved techniques and use the data to quantitatively calculate local-current voltage parameters. Cracks mainly influence the recombination current density of a silicon solar cell. This fact is clearly shown by drastically increased recombination in the space charge region. This recombination reduces parameters like fill factor, open circuit voltage and the cell efficiency. The comparison of the solar cell data before and after the formation of cracks shows a 0.2 % absolute efficiency loss in the global current-voltage parameters and up to 1 % absolute efficiency loss in the local current-voltage analysis at the crack positions.

Acceleration of 3D numerical simulation of silicon solar cell using thread parallelism

We have investigated the potential to accelerate three-dimensional numeric simulation of silicon solar cell using thread parallelism. The device simulated is a rear side passivated cell with rear point contacts (PERC). The optical and electrical behaviour of the device was simulated with Sentaurus Device (formerly dessis). We show that the simulation run-time on a four socket Opteron 6168 machine is reduced down to 6% compared to the run-time without thread parallelism. Furthermore, limits of time reduction by varying the number of threads up to 48 are studied. Thereby, the number of threads for the optimum use of the hardware resources is determined.