Publications Search Results

Now showing 1 - 10 of 44
  • Publication
    All-screen-printed back-contact back-junction silicon solar cells with aluminum-alloyed emitter and demonstration of interconnection of point-shaped metalized contacts
    ( 2015)
    Woehl, R.
    ;
    Rüdiger, M.
    ;
    Biro, D.
    ;
    Wilde, J.
    In the following, high-efficiency back-contact back-junction silicon solar cells with aluminum-alloyed emitter are described. First, the theoretical background for the cell concept is explained. To that purpose, the bulk lifetime and the front surface field characteristics are considered. Three different process sequences for the phosphorus-diffused profiles on the front and back surfaces are depicted: One exhibits a shallow field, and two sequences have deeper, driven-in profiles. For realizing high efficiencies, such cell structures must meet several prerequisites, such as firing-stable front and rear passivations, and functional small screen-printed Al structures. Furthermore, it must be possible to create contacts on the Si surfaces using the driven-in P-profiles. With such a structure, cell efficiencies of 20.0% are reached. An analysis of the series resistance and area-weighted recombination is performed. The results are compared with the measured cell parameters. Two-dimensional simulations show the efficiency potential when decreasing the width of the backside field and when a cell structure, which would inhibit a passivated aluminum-alloyed p+-emitter, is created. Also, an advanced concept is demonstrated where a point array of both polarities on the cell backside is interconnected externally on module level. To that purpose, the cell is soldered to a printed wiring circuit board by using a reflow soldering process.
  • Publication
    Upconverter silicon solar cell devices for efficient utilization of sub-band-gap photons under concentrated solar radiation
    ( 2014)
    Fischer, S.
    ;
    Ivaturi, A.
    ;
    Fröhlich, B.
    ;
    Rüdiger, M.
    ;
    Richter, A.
    ;
    Krämer, K.
    ;
    Richards, B.
    ;
    Goldschmidt, J.
    Upconversion (UC) of sub-band-gap photons has the potential to increase the efficiency of solar cells significantly. We realized an upconverter solar cell device, by attaching an upconverter layer of v-NaYF4 doped with 25% Er3+ embedded in the polymer perfluorocyclobutyl to the rear side of a bifacial silicon solar cell. We determined the external quantum efficiency of such upconverter solar cell devices under broad-band sub-band-gap excitation. Under consideration of spectral mismatch, we calculated the expected increase of the short-circuit current density due to UC under the air mass 1.5 global illumination. We determined an enhancement of 2.2 mA/cm2 for a spectral excitation band ranging from 1450 to 1600 nm and a comparatively low solar concentration of 78 suns. Subsequently, a system of concentrator lens and upconverter solar cell device was characterized with a solar simulator. We determined an increase of the short-circuit current density due to UC of sub-band-gap photons of 13.1 mA/cm2 under a concentration of 210 suns. This corresponds to a potential relative increase of the solar cell efficiency of 0.19%.
  • Publication
    Bifacial n-type silicon solar cells for upconversion applications
    ( 2014)
    Rüdiger, M.
    ;
    Fischer, S.
    ;
    Frank, J.
    ;
    Ivaturi, A.
    ;
    Richards, B.S.
    ;
    Krämer, K.W.
    ;
    Hermle, M.
    ;
    Goldschmidt, J.C.
    Upconversion of sub-band-gap photons has the potential to increase the efficiency of solar cells significantly, but requires modification of the solar cells. In this paper, we present a calculation framework to assess the efficiency of a combined bifacial silicon solar cell upconverter device, which is then used to optimize the solar cell׳s front and rear side anti-reflection coatings. Our calculations show that an upconverter can increase the efficiency of an optimized solar cell by 3.0% relative. Subsequently, planar bifacial n-type silicon solar cells were fabricated with optimized anti-reflection coatings. An upconversion layer - containing the upconverter phosphor v-NaY0.8Er0.2F4 embedded in the polymer perfluorocyclobutyl - was attached to the rear side of the solar cells and an external quantum efficiency arising from the upconversion of sub-band-gap photons of 1.69% was measured under 1508 nm monochromatic excitation with an irradiance of 1091 W/m2. This corresponds to a value of 0.15 (W/cm2)−1 when normalized to the irradiance, constituting a five-fold increase compared to the previously best published normalized values that were achieved without optimized solar cells.
  • Publication
    Numerical current density loss analysis of industrially relevant crystalline silicon solar cell concepts
    ( 2014)
    Rüdiger, M.
    ;
    Steinkemper, H.
    ;
    Hermle, M.
    ;
    Glunz, S.W.
    In order to improve the solar cell efficiency of the solar cell concept of interest most efficiently, it is essential to analyze its loss mechanisms. Numerical simulations are used to calculate the individual solar cell loss mechanisms. The calculated current losses can be used to identify the main losses and can give recommendation for the next improvement steps. We present a method for a current density loss analysis of crystalline silicon solar cells based on 2-D and 3-D numerical simulations. With the presented method, a collection of relevant crystalline silicon solar cell concepts are investigated. By determining the main loss mechanisms of each cell concept, the limitations and the potential of the different concepts are pointed out.
  • Publication
    Parameterization of free carrier absorption in highly doped silicon for solar cells
    ( 2013)
    Rüdiger, M.
    ;
    Greulich, J.
    ;
    Richter, A.
    ;
    Hermle, M.
    Free carrier absorption (FCA) is a parasitic absorption process in highly doped silicon that might significantly reduce the amount of photons, potentially generating electron-hole pairs. Existing FCA parameterizations are mostly setup by evaluating absorption data in the range lambda >= 4m. If applied in the wavelength range lambda =1.0-2.0 m, including the relevant range for silicon solar cells, most parameterizations are not appropriate to describe FCA accurately. In this paper, new parameters are presented using optical simulation on the base of experimental reflection data to enhance the quantification of FCA losses in the considered wavelength range.
  • Publication
    Incomplete ionization of aluminum in silicon and its effect on accurate determination of doping profiles
    ( 2013)
    Rauer, M.
    ;
    Rüdiger, M.
    ;
    Schmiga, C.
    ;
    Strutzberg, H.
    ;
    Bähr, M.
    ;
    Glatthaar, M.
    ;
    Glunz, S.W.
    We present a detailed study on incomplete ionization (i.i.) of aluminum acceptors in highly aluminum-doped p(+) silicon formed by alloying from screen-printed Al pastes. We apply electrochemical capacitance-voltage (ECV) and secondary ion mass spectrometry (SIMS) measurements to detect the Al doping profiles and discuss key aspects necessary for a precise determination of the profiles. The excellent accordance of ECV- and SIMS-measured acceptor profile curves allows for the accurate investigation of Al acceptor ionization. We review the physics of i.i. and verify a simple quantitative model for incomplete Al acceptor ionization by comparing measured and calculated sheet-resistances of Al-doped p(+) Si surfaces. We thus show that the electrically active Al doping concentration is nearly two times lower than the total Al concentration, so that i.i. of Al acceptors has to be considered for the correct description of highly Al-doped p(+) Si regions. Therefore, our results allow for an improved quantitative analysis of n- and p-type silicon solar cells with Al-alloyed p(+) rear emitter or back surface field, respectively.
  • Publication
    Passivation layers for indoor solar cells at low irradiation intensities
    ( 2012)
    Rühle, K.
    ;
    Rauer, M.
    ;
    Rüdiger, M.
    ;
    Giesecke, J.
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    Niewelt, T.
    ;
    Schmiga, C.
    ;
    Glunz, S.W.
    ;
    Kasemann, M.
    The passivation mechanisms and qualities of Al2O3, SiNx, SiO2 and a-Si:H(i) on p- and n-type silicon are investigated by quasi-steady-state photoluminescence measurements. This technique allows effective lifetime measurements in an extremely large injection range between 1010 cm-3 and 1017 cm-3. The measurements are discussed focusing on injections below 1012 cm-3 in order to determine the most effective passivation layer for solar cells arranged for indoor applications. Fixed negative charges in the passivation layer cause field-effect passivation due to band bending leading to either accumulation or inversion at the passivation layer/silicon interface. Accumulation causes a stable passivation quality at low level injection. Inversion leads to effective lifetime losses similar to the losses in the space charge region. On p-type silicon the most effective surface passivation at low injections is provided by Al2O3 or a-Si:H(i). The n-type silicon samples passivated with a-Si:H(i) show the best effective lifetimes. SiNx and SiO2 show lifetimes one order of magnitude below a-Si:H(i). Al2O3 on n-type is the most effective passivation at high injections around 1015 cm-3. Due to inversion losses at low level injections the passivation quality decreases more than two orders of magnitude for injections around 1010 cm-3.
  • Publication
    Broad Range Injection-Dependent Minority Carrier Lifetime from Photoluminescence
    ( 2012)
    Giesecke, J.
    ;
    Niewelt, T.
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    Rüdiger, M.
    ;
    Rauer, M.
    ;
    Schubert, M.
    ;
    Warta, W.
  • Publication
    Diffractive gratings for crystalline silicon solar cells - optimum parameters and loss mechanisms
    ( 2012)
    Peters, M.
    ;
    Rüdiger, M.
    ;
    Hauser, H.
    ;
    Hermle, M.
    ;
    Bläsi, B.
    In this paper, we present guidelines for the design of backside gratings for crystalline silicon solar cells. We use a specially developed method based on a combination of rigorous 3D wave optical simulations and detailed semiconductor device modeling. We also present experimental results of fabricated structures. Simulation-based optimizations of grating period and depth d of a binary grating and calculations of the optical and electrical characteristics of solar cells with optimized gratings are shown. The investigated solar cell setup features a thickness of dbulk=40m and a flat front surface. For this setup, we show a maximum increase in short-circuit current density of jSC=1.8mA/cm2 corresponding to an efficiency enhancement of 1% absolute. Furthermore, we investigate different loss mechanisms: (i) an increased rear surface recombination velocity S0,b because of an altered surface caused by the introduction of the grating and (ii) absorption in the aluminum backsi de reflector. We analyze the trade-off point between gain due to improved optical properties and loss due to corrupted electrical properties. We find that, increasing the efficiency by 1% absolute due to improved light trapping, the maximum tolerable recombination velocity is S0,b(max)=5.2×103cm/s. From simulations and measurements, we conclude that structuring of the aluminum backside reflector should be avoided because of parasitic absorption. Adding a dielectric buffer layer between silicon and the structured aluminum, absorption losses can be tuned. We find that for a planar reflector, the thickness of a SiO2 buffer layer should exceed dSiO2=120nm.
  • Publication
    Numerical analysis of locally contacted rear surface passivated silicon solar cells
    ( 2012)
    Rüdiger, M.
    ;
    Hermle, M.
    We present a numerical simulation study on the optimization of locally contacted rear surface passivated p-Si solar cells considering float-zone and Czochralski-grown bulk material, latter by taking different amounts of oxygen concentrations into account and, thus, varying the quality of the bulk material. The conversion efficiency potential is figured out by a broad variation of the nominal base resistivity, thickness and rear contact distance of the solar cell. To focus on the bulk and rear side recombination properties, the front side contact and emitter properties have been idealized to avoid recombination losses in this region. It turns out that high level injection effects playing a major role in the description of high resistivity bulk materials.