Publications Search Results

Now showing 1 - 10 of 38
  • Publication
    The Challenge of Measuring Busbarless Solar cells and the Impact on Cell-to-Module Losses
    ( 2020)
    Rauer, M.
    ;
    Krieg, A.
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    Pfreundt, A.
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    Mittag, M.
    ;
    Pingel, S.
    The realistic measurement of solar cells is key for the whole PV industry, as accurate information about cell power is one of the most important aspects in solar cell purchase and PV module design. The omission of busbars introduces new challenges to the current-voltage measurement of solar cells, since contact to every single grid finger has to be established with independent current and voltage contacts. It is not just the shadow correction of the measurement unit that needs to be carried out more laboriously, but also the contacting of the front metal grid, which is more critical because of the high resistivity of the grid fingers. The position of the voltage sensing contact and the number of current contacts can thus have a noticeable impact on the measured performance of busbarless solar cells. Measured efficiencies are highly dependent on the contacting schemes used in different measurement systems, as these vary in contact number and sensing configuration. Two different main approaches for measuring busbarless solar cells have evolved, representing either realistic or idealized application of the cells in the module. The pros and cons of both approaches are discussed in detail in this paper. Realistic measurement conditions lead to efficiencies which best predict module performance, but are hard to realize and require knowledge about the subsequent module design. Although not their primary purpose, the use of idealized measurement conditions can make it easier to achieve record cell efficiencies, but with the disadvantage of limited comparability with busbar-based solar cell concepts. Idealized conditions can moreover lead to hidden losses in performance of the solar cells, related to the application in a module, which in turn causes inflated cell-to-module (CTM) losses. If solar cells are bought in terms of $/Wp and modules are sold likewise, the economic implications arising from the different measurement configurations have to be considered. Whichever approach is used for the measurement of busbarless solar cells, full disclosure of the measurement configuration is absolutely essential.
  • Publication
    Comprehensive Evaluation of IEC Measurement Procedures for Bifacial Solar Cells and Modules
    ( 2020)
    Rauer, M.
    ;
    Schmid, A.
    ;
    Guo, F.
    ;
    Neuberger, F.
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    Gebhardt, P.
    ;
    Hohl-Ebinger, J.
    The IEC technical specification (TS) 60904-1-2 has been published recently in order to establish a standardized way of evaluating bifacial solar cells and modules. Two different methods for indoor measurements of the current-voltage characteristics of bifacial devices have been reported, which are based on simultaneous front and rear illumination (referred to as bifacial method) or on illumination at elevated irradiance levels from front only (referred to as equivalent irradiance (GE) method). In this study, the measurement procedures and input parameters specified by the IEC TS for the bifacial and GE methods are analysed in detail. This particularly applies to the bifaciality coefficients Isc and Pmpp, which are the ratios of rear to front Isc and Pmpp, respectively, and are used for the calculation of the equivalent irradiance GE. It is shown that the bifaciality coefficients have to be selected carefully to accurately determine the bifacial low-light performance with the GE method. Evaluating the bifaciality coefficients as proposed by the IEC TS for nonlinear solar cells can lead to deviations between the bifacial and GE methods of up to several percent in the parameters BiFi, PmppBiFi10% and PmppBiFi20% -standardized measures for the power gain of the bifacial device caused by additional rear irradiance. In addition, differences between bifacial and GE methods can also arise for bifacial solar modules with partial rear shading. By comparing measurements with GE and bifacial methods for these modules, it is shown that following the IEC procedure can lead to errors in BiFi of more than 18 % and in PmppBiFi20% of more than 2 %. An alternative approach of evaluating bifaciality coefficients is therefore proposed in this study: By applying only the bifaciality coefficient Isc, which is physically the most meaningful coefficient, and by evaluating Isc at the actual front irradiance of the measurement, the agreement between the bifacial and GE methods can be considerably improved. A criterion for the applicability of the GE method using Isc is derived by systematically varying the rear shading fraction of bifacial modules. It is thus shown that both methods can be applied in good agreement, if amendments to the IEC TS are made.
  • Publication
    Results of the Bifacial PV Cells and PV Modules Power Measurement Round Robin Activity of the PV-Enerate Project
    ( 2020)
    Koutsourakis, G.
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    Rauer, M.
    ;
    Schmid, A.
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    Bellenda, G.
    ;
    Betts, T.
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    Blakesley, J.
    ;
    Bliss, M.
    ;
    Bonilla Castro, J.
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    Bothe, K.
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    Dittmann, S.
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    Lopez-Garcia, J.
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    Herrmann, W.
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    Hinken, D.
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    Kenny, R.
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    Molinero, R.
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    Pavanello, D.
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    Riechelmann, S.
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    Sträter, H.
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    Vegas, A.
    ;
    Winter, S.
    Bifacial Photovoltaic (PV) technology is expected to acquire an increased market share of the solar PV industry in the future, due to the ability of bifacial PV modules to collect light from both the front and rear side, increasing the energy yield for a specific available area for a PV system. Accurate measurement procedures for bifacial products result in more accurate energy yield estimates, consequently improving the bankability of bifacial PV technology. This work aims to evaluate the procedures for the measurement of electrical power and bifacial parameters of bifacial solar devices, concerning to their applicability in calibration laboratories and production line environments. An intercomparison for bifacial PV cells and modules is carried out between nine different test laboratories across Europe, as part of the PV-Enerate project. The measurement procedures as described in IEC TS 60904-1-2:2019 are followed among the different test laboratories, to evaluate the applicability of these procedures. Three different types of bifacial PV cells and three different types of bifacial PV modules are measured. The intercomparison involves measurements with systems using both single and double-sided illumination conditions. The uncertainty budgets and systematic differences that these procedures result in between different laboratories are determined and discussed. Specific common mistakes are reported and improvements to the IEC TS 60904-1-2:2019 are proposed as a result of this intercomparison activity.
  • Publication
    Contacting of Busbarless Solar Cells for Accurate I-V Measurements
    ( 2020)
    Bothe, K.
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    Kruse, C.
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    Hinken, D.
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    Brendel, R.
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    Rauer, M.
    ;
    Hohl-Ebinger, J.
    In recent years, solar cell development has undergone a major change in metallization layout. In their most radical form, busbarless solar cells completely omit the busbar and leave the fingers as solely contacting area. Consequently, characterization and calibration laboratories were forced to develop new contacting units. At the same time, the question of the correct arrangement of current and sense contacts arises. To perform accurate and precise measurements of the current-voltage characteristic of busbarless solar cells, we transfer the well-established concept of busbar-resistance neglecting contacting to the measurement of busbarless cells. The result is a universally valid gridresistance neglecting contacting scheme, which provides the same fill factor as one would get if one had contacted the entire metallized area of the solar cell. We demonstrate that a variety of contacting schemes are able to determine this fill factor if the sensing contact is correctly placed. We provide experimental evidence of consistent results for a contacting with 12 contact bars at ISFH CalTeC and 30 wires at Fraunhofer ISE CalLab, respectively. For the fill factor of 15 silicon solar cells with finger line resistances ranging from 0.6 to 12 /cm we show that the En-values between both calibration laboratories are well below 1, demonstrating a very good agreement within the accompanied measurement uncertainty.
  • Publication
    Accurate Measurements of Busbarless Silicon Solar Cells
    ( 2019)
    Rauer, M.
    ;
    Kordelos, K.
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    Krieg, A.
    ;
    Hohl-Ebinger, J.
  • Publication
    IV-Measurements of Bifacial Solar Cells in an Inline Solar Simulator with Double-Sided Illumination
    ( 2018)
    Krieg, A.
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    Greulich, J.
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    Ramspeck, K.
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    Dzafic, D.
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    Wöhrle, N.
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    Rauer, M.
    ;
    Rein, S.
    Bifacial solar cells allow higher output power densities due to double-sided power generation compared to monofacial cells. An important step is the accurate measurement of the current-voltage (IV) characteristics of the cells under realistic conditions. The most accurate option to measure a bifacial solar cell is to illuminate the cells simultaneously from both sides. A new method for irradiance and short-circuit current correction for bifacial solar cells suitable for inline measurements in mass production is presented, where the crosstalk during a single-sided measurement and deviations of the ratio of the nominal to the measured irradiance at double-sided illumination measurements will be corrected by taking the bifaciality coefficient into account. This reduces typical measurement errors for the short-circuit current Isc below 0.1%. The simple approach, adding the front and the rear side irradiance directly to calculate the bifacial irradiance, shows Isc errors of more than 0.5% in typical measuring conditions and should not be used when the solar cell is double-sided illuminated.
  • Publication
    Quantitative theoretical and experimental analysis of alloying from screen-printed aluminum pastes on silicon surfaces
    ( 2018)
    Rauer, M.
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    Schmiga, C.
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    Glatthaar, M.
    ;
    Glunz, S.W.
    In this study, we present detailed theoretical and experimental investigations on full-area alloying from screen-printed aluminum pastes on silicon surfaces for solar cell applications. We introduce a simple analytical model for the description of the alloying process derived from existing models for evaporated Al layers, which we adapt to printed Al pastes. Thereby, we particularly account for the recrystallization of Si within the paste particles, which we refer to as parasitic Si recrystallization. Applying our model, we demonstrate good accordance of calculated with measured eutectic layer thicknesses. We show that the model can be versatilely used to investigate screen-printed Al-alloyed contacts in detail: We demonstrate that the latent heat of the Al paste significantly influences the alloying process. Thus, the effective peak temperature of the alloying process can be several 100 °C below the set peak temperature of the firing furnace. By combining calculations of the effective peak temperature with measurements of the Al doping concentration, we determine a parameterization of the solid solubility of Al in Si down to the eutectic temperature of 577 °C. Our investigations therefore provide improved understanding of alloying from printed Al pastes and enable the specific optimization of Al-alloyed contacts.
  • Publication
    Theoretical and experimental investigation of aluminum-boron codoping of silicon
    ( 2016)
    Rauer, M.
    ;
    Schmiga, C.
    ;
    Raugewitz, A.
    ;
    Glatthaar, M.
    ;
    Glunz, S.W.
    We present a detailed study on aluminum-boron codoping of silicon by alloying from screen-printed aluminum pastes containing boron additives (Al-B pastes). We derive an analytical model for the formation of the Al-B acceptor profiles by quantitatively describing (i) the composition of the Al-B-Si melt and (ii) the incorporation of Al and B acceptor atoms into the recrystallizing Si lattice. We show that measured Al-B dopant profiles can be excellently described by this model, which therefore offers a straightforward method for the comprehensive investigation of alloying from Al-B pastes. The formation of a characteristic kink in the Al-B dopant profile curve can thus be ascribed to the exhaustion of the B additive dissolution during alloying. By intentionally adding elemental B powder to an Al paste, we demonstrate that only a low percentage of the B powder actually dissolves into the melt. We show that this incomplete dissolution of the B additive strongly affects the recombination characteristics of Al-B-p(+) regions and, thus, is an important element of alloying from Al-B pastes. This study therefore provides improved understanding of aluminum-boron codoping of silicon.
  • Publication
    Adapted parameterization of incomplete ionization in aluminum-doped silicon and impact on numerical device simulation
    ( 2015)
    Steinkemper, H.
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    Rauer, M.
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    Altermatt, P.
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    Heinz, F.D.
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    Schmiga, C.
    ;
    Hermle, M.
    The amount of incomplete ionization of aluminum-doped silicon is measured at room temperature by comparing electrochemical capacitance-voltage measurements with micro Raman spectroscopy. It is shown that commonly used parameterizations significantly underestimate the effect of incomplete ionization in Al doped Si. Based on the experimental data, we propose new parameter values for the parameterization of incomplete ionization given in Altermatt et al., J. Appl. Phys. 100, 113715 (2006). Using these new values, the saturation current density J0,pþ of the Al-alloyed region of a standard silicon solar cell is determined by means of numerical device modeling. It is shown that the parameterization influences J0,pþ significantly. Additionally, the weakening effect of incomplete ionization on band gap narrowing (BGN) should be taken into account in modeling that aims to predict device behavior after changes made to the Al-alloyed region.
  • Publication
    Alloying from Screen-printed Aluminum Pastes for Silicon Solar Cell Applications
    ( 2015)
    Rauer, M.
    Alloying from screen-printed aluminum pastes on silicon is a simple, reliable and cost-effective metallization technique, which is standardly applied for the rear contact formation of silicon solar cells in today's production lines. Despite its long history and widespread utilization, however, there have still been open questions on the fundamentals of the alloying process, on the interaction of paste additives with the Al-Si system during alloying, and on the recombination characteristics of Al-alloyed contacts. This work has therefore dealt with investigating and quantifying alloying from screen-printed Al pastes on Si for solar cell applications in detail. The main achievements of this work are summarized in brief in the following: (1) Derivation of a comprehensive analytical model for alloying from screen-printed Al pastes which allows for the accurate calculation of the Al contact structure and the doping profiles of the Al-doped p+ Si (Al-p+) regions for a broad range of printing and firing conditions. (see chapter 2) (2) Detailed theoretical and experimental investigation of the recombination characteristics of Al-p+ regions with and without surface passivation. The influence of the printing and firing conditions on the recombination characteristics of the Al-p+ regions has been clarified and optimal conditions have been determined. Implied open-circuit voltages of up to 651 mV for non-passivated and 699 mV for surface-passivated Al-p+ regions have been realized. (see chapter 3) (3) Deduction of a comprehensive analytical model for alloying from screen-printed Al pastes containing boron additives. This model enables the precise calculation of the acceptor profiles of the Al- and B-co-doped p+ Si (Al-B-p+) regions and the investigation and optimization of their recombination characteristics. Excellent implied open-circuit voltages of 665 mV have been achieved for full-area Al-B-p+ regions without surface passivation for the optimal effective B percentage of 0.03 wt%. (see chapter 4) (4) Investigation of the structural and electrical properties of local contacts formed by full-area screen-printing and firing of Al pastes on locally opened dielectric layers. By intentionally adding Si to the Al paste, the recombination characteristics of the local contacts were improved, so that the effective rear surface recombination velocity was reduced down to less than a third of the value of a conventional Al paste. (see chapter 5) (5) Application of these results to improve the conversion efficiencies of n-type Si solar cells with screen-printed full-area or local Al-p+ rear emitters. Nickel plating has been demonstrated to be a promising technique for the front contact formation of these solar cells. Etching of highly phosphorus-doped n+ Si regions was implemented as an industrially feasible approach for the formation of the n+ front surface fields. Conversion efficiencies in the range of the highest values so far reported have been achieved. (see chapter 6)