Uwe Zimmermann | Uppsala University (original) (raw)

Papers by Uwe Zimmermann

A computer model for Cu(In,Ga)Se 2 (CIGS) based, monolithic solar cell modules was derived and ca... more A computer model for Cu(In,Ga)Se 2 (CIGS) based, monolithic solar cell modules was derived and calibrated using measurement data from an actual mini-module with an aperture area of 80 cm 2 . Simulations using this model accurately reproduce the measurement results on mini-modules with different cell widths to within ±3% of the parameters V oc , I sc , fill-factor and efficiency. The optical and electrical properties of the transparent front-contact were obtained from quantum efficiency measurements on a series of solar cells with ZnO:Al layers of different thicknesses and incorporated into the model. Finally, computer simulations were used to optimize CIGS thin-film modules for the application in low-concentrating systems. Aperture area efficiencies exceeding 14% at up to 10x concentration are predicted by the simulations for CIGS thin-film modules with cell widths of w = 2 mm. These simulation results are supported by preliminary measurements.

Solar Energy Materials and Solar Cells, 2011

We investigate Cu(InGa)Se 2 thin films grown in multi-stage coevaporation processes and solar cel... more We investigate Cu(InGa)Se 2 thin films grown in multi-stage coevaporation processes and solar cells fabricated from such absorbers. Despite some interdiffusion during film growth, Ga/(Ga+In) gradients defined via evaporation-profile variations in the process are to a good part retained in the finished film. This indicates that the bandgap can be engineered in this type of process by varying the evaporation profiles, and consequently also that unintended profile variations should be noted and avoided. With front-side gradients the topmost part of many grains seems to be affected by a higher density of lattice defects due to the strong change of gallium content under copper-poor growth conditions. Electrically, both backside gradients and moderate front-side gradients are shown to yield an improvement of device efficiency. If a front-side gradient is too wide, though, it causes strong voltage-dependent collection and the fill factor is severely reduced. This is the author's version of a work that was accepted for publication in Solar Energy Materials and Solar Cells. Changes resulting from the publishing process, such as editing, corrections and structural formatting may not be reflected in this document. A definitive version was subsequently published in Solar Energy Materials and Solar Cells 95 (2011) pp. 721-726,

Electronic power devices made of silicon carbide promisesuperior performance over today's sil... more Electronic power devices made of silicon carbide promisesuperior performance over today's silicon devices due toinherent material properties. As a result of the material'swide band gap of 3.2eV, high thermal conductivity, itsmechanical and chemical stability and a high critical electricfield, 4H-silicon carbide devices have the potential to be usedat elevated temperatures and in harsh environments. Shortercarrier lifetimes and a reduction in the necessary width of thelow-doped drift zone in silicon carbide devices compared totheir silicon counterparts result in faster switching speedsand lower switching losses and thus in much more efficientpower devices.High-voltage 4H-silicon carbide diodes have been fabricatedin a newly developed processing sequence, using standardsilicon process equipment. Epitaxial layers grown by chemicalvapor deposition (CVD) on commercial 4H-silicon carbidesubstrates were used as starting material for both mesa-etchedepitaxial and implanted p+n-n+ planar diodes, Schottky diodesand merged pn-Schottky (MPS) diodes, together with additionaltest structures. The device metallization was optimized to givea low contact resistivity on implanted and epitaxial layers anda sufficiently high Schottky barrier with a singlemetallization scheme. Different high-field termination designshave been tested and breakdown voltages of up to 4 kV onimplanted, field-ring terminated diodes were achieved,corresponding to 80% of the critical electric field. A 5kVepitaxial diode design with a forward voltage drop of 3.5V at acurrent density of 100Acm-2 equipped with an implanted junctiontermination extension (JTE) was also fabricated.A new measurement setup was designed and built with thecapability of measuring current-voltage and capacitance-voltagecharacteristics of semiconductor devices at reverse biases upto 10kV. Together with these electrical measurements, theresults of other characterization techniques were used toidentify performance limiting defects in the fabricated siliconcarbide diodes. Increased forward voltage drop of bipolardevices during on-state operation was studied and it was shownthat the stacking faults causing forward degradation arevisible in scanning electron microscopy. With the help ofsynchrotron white-beam X-ray diffraction topographs (SWBXT),electron beam induced current (EBIC) and electroluminescencemeasurements of silicon carbide diodes, the role of screwdislocations as a dominant source of device failure in the formof localized microplasma breakdown was identified. Screwdislocations with and without open core have been found tocause a 20-80% reduction in the critical electric field of4H-silicon carbide diodes, both for low-voltage (150V) andhigh-voltage (~5kV) designs. While micropipes have almost beeneliminated from commercial silicon carbide material,closed-core screw dislocations are still abundant withdensities in the order of 10000cm-2 in state-of-the-art siliconcarbide epitaxial layers.

Materials Science Forum, 2002

High-temperature chemical vapour deposition (HTCVD) in a vertical chimney reactor was used to gro... more High-temperature chemical vapour deposition (HTCVD) in a vertical chimney reactor was used to grow thick low-doped epitaxial layers of 4H silicon carbide. These layers were used as drift layers in a combined process to manufacture both bipolar and unipolar high-voltage diodes. The resulting diodes were characterized electrically in order to gain knowledge about the electric quality of the HTCVD epitaxial layers to assess the high-voltage properties of this material.

Progress in Photovoltaics: Research and Applications, 2013

In this work, we investigate the effect of ageing Mo-coated substrates in a dry and N 2 flooded c... more In this work, we investigate the effect of ageing Mo-coated substrates in a dry and N 2 flooded cabinet. The influence was studied by preparing Cu(In,Ga)Se 2 solar cells and by comparing the electrical performance with devices where the Mo layer was not aged. The measurements used for this study were current-voltage (J-V), external quantum efficiency (EQE), secondary ion mass spectroscopy (SIMS) and capacitance-voltage (C-V). It was concluded that devices prepared with the aged Mo layer have, in average, an increase of 0.8% in efficiency compared with devices that had a fresh Mo layer. Devices with aged Mo exhibited a nominal increase of 12.5 mV of open circuit voltage, a decrease of 1.1 mA/cm À2 of short circuit current and a fill factor increase of 2.4%. Heat treatment of fresh Mo layers in oxygen atmosphere was also studied as an alternative to ageing and was shown to provide a similar effect to the aged device's performance.

The purpose of this investigation was to model the thermodynamic behavior of a Cu(In,Ga)Se2 sampl... more The purpose of this investigation was to model the thermodynamic behavior of a Cu(In,Ga)Se2 sample during the growth process and especially during the conversion from copper-rich to copper-poor material in a two-step process. Starting from a very simple model of a directly heated substrate the model was refined until it qualitatively and quantitatively explained the features observed in the real experiment. The results can be used to determine more accurate criteria for the endpoint detection in a real-world evaporation system.

IEEE Journal of Photovoltaics, 2013

Solar Energy Materials and Solar Cells, 2008

The use of laser scribed Cu(In,Ga)Se 2 as a high-speed and low-stress alternative to the standard... more The use of laser scribed Cu(In,Ga)Se 2 as a high-speed and low-stress alternative to the standard mechanical scribing has been investigated for thin-film PV modules on glass substrates. Laser scribe lines were smooth, narrow and straight compared to the chipped and jagged mechanically scribed lines. Modules fabricated using laser patterning have reached a 15% module efficiency and fill factors as high as 73%. Contact resistances were found to be in the range of 0:1 O cm for standard scribing and 0:4 O cm for the laser-formed contacts. In spite of the elevated contact resistance performance is comparable to conventionally processed modules and dry heat stability tests are encouraging.

Materials Science …, 2003

... in detail earlier [7]. The epitaxial layer was grown in a chimney reactor on a commercial 4H-... more ... in detail earlier [7]. The epitaxial layer was grown in a chimney reactor on a commercial 4H-SiC wafer (Cree, n ... The trapezoidal features seen in the SEM images (Figs. ... Further investigation of the stacking fault-like defects is necessary in order to understand their presence in the ...

Journal of applied …, 2003

[Journal of Applied Physics 93, 611 (2003)]. Uwe Zimmermann, John Österman, Dan Kuylenstierna, An... more [Journal of Applied Physics 93, 611 (2003)]. Uwe Zimmermann, John Österman, Dan Kuylenstierna, Anders Hallén, Andrey O. Konstantinov, William M. Vetter, Michael Dudley. Abstract. ... Square mesa diodes of and were formed by etching with molten potassium hydroxide. ...

IEEE journal of photovoltaics, Apr 18, 2013

In this paper, co-evaporation of Cu(In,Ga)Se2 (CIGS) in an inline single-stage process is used to... more In this paper, co-evaporation of Cu(In,Ga)Se2 (CIGS) in an inline single-stage process is used to fabricate solar cell devices with up to 18.6% conversion efﬁciency using a CdS buffer layer and 18.2% using a Zn1−xSnxOyCd-free buffer layer. Furthermore, a 15.6-cm2 mini-module, with 16.8% conversion efﬁciency,has been made with the same layer structure as the CdS baseline cells, showing that the uniformity is excellent. The cell results have been externally veriﬁed. The CIGS process is described in detail, and material characterization methods show that the CIGS layer exhibits a linear grading in the [Ga]/([Ga]+[In]) ratio, with an average [Ga]/([Ga]+[In]) value of 0.45. Standard processes for CdS as well as Cd-free alternative buffer layers are evaluated, and descriptions of the baseline process for the preparation of all other steps in the Angstrom Solar Center standard solar cell are given.

Chemical Engineering Science, 2014

This paper presents the elaboration of an experimentally validated model of a continuous cross-fl... more This paper presents the elaboration of an experimentally validated model of a continuous cross-flow atomic layer deposition (ALD) reactor with temporally separated precursor pulsing encoded in the Modelica language. For the experimental validation of the model, in situ quartz crystal microbalance (QCM) diagnostics was used to yield submonolayer resolution of mass deposition resulting from thin film growth of ZnO from Zn(C 2 H 5) 2 and H 2 O precursors. The ZnO ALD reaction intrinsic kinetic mechanism that was developed accounted for the temporal evolution of the equilibrium fractional surface concentrations of precursor adducts and their transition states for each half-reaction. This mechanism was incorporated into a rigorous model of reactor transport, which comprises isothermal compressible equations for the conservation of mass, mo

Materials Science Forum - MATER SCI FORUM, 2002

Page 1. Materials Science Forum Vols. 389-393 (2002) pp 1309-1312 © (2002) Trans Tech Publication... more Page 1. Materials Science Forum Vols. 389-393 (2002) pp 1309-1312 © (2002) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.389-393.1309 All rights reserved. No part of contents of this paper may ...

$Research paper thumbnail of Rear Contact Passivation for High Bandgap Cu(In, Ga)Se<formula> <tex>$_{2}$</tex> </formula> Solar Cells With a Flat Ga profile$

IEEE Journal of Photovoltaics

In this study, Cu(In, Ga)Se 2 solar cells with a high bandgap (1.31 eV) and a flat Ga profile ([G... more In this study, Cu(In, Ga)Se 2 solar cells with a high bandgap (1.31 eV) and a flat Ga profile ([Ga]/([Ga]+[In]) 0.60) were examined. For absorber layer thicknesses varying from 0.60 to 1.45 µm, the Mo rear contact of one set of samples was passivated with an ultrathin (27 nm) Al 2 O 3 layer with point contact openings, and compared with reference samples where the rear contact remained unpassivated. For the passivated samples, mainly large gains in the short-circuit current led to an up to 21% (relative) higher power conversion efficiency compared with unpassivated cells. The differences in temperature-dependent current voltage behavior between the passivated and the unpassivated samples and the thin and the thick samples can be explained by an oppositely poled secondary photodiode at the rear contact.

ABSTRACT The study of solar irradiance is of vital importance in order to quantify the fluctuatio... more ABSTRACT The study of solar irradiance is of vital importance in order to quantify the fluctuation that the transmission system needs to handle in case of large scale solar power integration. In this paper, early results from a low-cost solar irradiance logger network are presented. The main advantages of the solar logger are its low cost (~$50) and mobility. Our results confirm previous models of the station pair correlation of clear sky index step changes, with the modification that the isotropic correlation appears to decrease exponentially as a function of Δt^(-1/2) rather than Δt^(-1) as has been proposed earlier.

A new atomic layer deposition process was developed for deposition of Zn-Sn-O buffer layers for C... more A new atomic layer deposition process was developed for deposition of Zn-Sn-O buffer layers for Cu(In,Ga)Se 2 solar cells with tetrakis(dimethylamino) tin, Sn(N(CH 3) 2) 4 , diethyl zinc, Zn(C 2 H 5) 2 , and water, H 2 O. The new process gives good control of thickness and [Sn]/([Sn]+[Zn]) content of the films. The Zn-Sn-O films are amorphous as found by grazing incidence X-ray diffraction, have a high resistivity, show a lower density compared with ZnO and SnO x , and have a transmittance loss that is smeared out over a wide wavelength interval. Good solar cell performance was achieved for a [Sn]/([Sn]+[Zn]) content determined to be 0.15-0.21 by Rutherford backscattering. The champion solar cell with a Zn-Sn-O buffer layer had an efficiency of 15.3% (V oc =653mV, J sc (QE)=31.8mA/cm 2 , and FF=73.8%) compared with 15.1% (V oc =663mV, J sc (QE)=30.1mA/cm 2 , and FF=75.8%) of the best reference solar cell with a CdS buffer layer. There is a strong light-soaking effect that saturates after a few minutes for solar cells with Zn-Sn-O buffer layers after storage in the dark. Stability was tested by 1000h of dry heat storage in darkness at 85 C, where Zn-Sn-O buffer layers with a thickness of 76nm retained their initial value after a few minutes of light soaking.

ABSTRACT A computer model for Cu(In,Ga)Se 2 (CIGS) based, monolithic solar cell modules was deriv... more ABSTRACT A computer model for Cu(In,Ga)Se 2 (CIGS) based, monolithic solar cell modules was derived and calibrated using measurement data from an actual mini-module with an aperture area of 80 cm 2 . Simulations using this model accurately reproduce the measurement results on mini-modules with different cell widths to within ±3% of the parameters V oc , I sc , fill-factor and efficiency. The optical and electrical properties of the transparent front-contact were obtained from quantum efficiency measurements on a series of solar cells with ZnO:Al layers of different thicknesses and incorporated into the model. Finally, computer simulations were used to optimize CIGS thin-film modules for the application in low-concentrating systems. Aperture area efficiencies exceeding 14% at up to 10x concentration are predicted by the simulations for CIGS thin-film modules with cell widths of w = 2 mm. These simulation results are supported by preliminary measurements.

When solar modules are partially shaded they will be under conditions of partial reverse bias. To... more When solar modules are partially shaded they will be under conditions of partial reverse bias. To test and evaluate the effect of reverse bias, CIGS thin film PV modules were placed under extreme conditions of reverse stress. Stressing caused modules to exhibit visible "wormlike" damages. These damages were caused by hot spot activity during reverse stress. Local heating resulted in pore formation and forced the hot spot to move within the cell. This effect appeared to cause intermixing of the top ZnO layer with the CIGS absorber. Some phase segregation of an undetermined, Cu rich compound was also found near the back contact. Electrically, the observed damages caused local shunt conductance to increase resulting in irreversibly reduced module fill factors. Keywords: Cu(InGa)Se 2 , Modules, Stability 1 INTRODUCTION In recent years photovoltaic thin film technologies such as CdTe and CIGS have passed from promising laboratory performance to commercial introduction, taking c...

Materials Science Forum, 2003

ABSTRACT Device simulations of B implanted 4H-SiC p(+)nn(+) diodes were performed including trans... more ABSTRACT Device simulations of B implanted 4H-SiC p(+)nn(+) diodes were performed including transient trapping of carriers in the deep B level. Most traps become filled at forward current densities above 3000 A/cm(2) thus enabling conductivity modulation by a carrier plasma and a negative resistance in agreement with pulsed forward bias measurements. Reverse recovery measurements were performed to verify a significant conductivity modulation at 4000 A/cm(2) and the absence of this at 1000 A/cm(2) in agreement with simulations.