Marc Burgelman

Modelling polycrystalline semiconductor solar cells

Abstract An overview is given of various electronic effects present in polycrystalline thin film solar cells, which do not occur in standard crystalline Si solar cells. It is explained how these effects are treated numerically in a numerical solar cell simulation tool, SCAPS, developed at the authors institute. The capabilities and limitations of SCAPS are discussed. Simulation examples of current–voltage, capacitance–voltage and capacitance–frequency characteristics are given. The agreement between the simulations and measurements is shown and discussed, both for CdTe and for Cu(In,Ga)Se 2 solar cells.

Effects of the Au/CdTe back contact on IV and CV characteristics of Au/CdTe/CdS/TCO solar cells

A simple analytical theory is presented to explain the measured roll over and cross over behaviour of the IV characteristics of thin film CdTe solar cells. It involves a classical description of the CdS/CdTe junction and the CdTe/back contact structure and is extended with a new description of minority carrier current in the CdTe contact region. This extension is crucial in describing the light dependence of the forward IV curves, and hence cross over. The same model also explains the measured CV curves. It is shown that analysis of the capacitance measurement can yield additional information about the doping density of CdTe in the vicinity of the contact. A relationship between the fill factor of the solar cell and the barrier height of the back contact is derived; this relation is useful as a new, practical criterion for the quality of the back contact. The results of this simple analytical model are confirmed by full numerical calculations of the dc and ac characteristics.

Model for electronic transport in Cu(In,Ga)Se2 solar cells

Temperature-dependent measurements of the current–voltage characteristics and of the junction admittance of ZnO/CdS/Cu(In,Ga)Se2 heterojunction solar cells are presented, together with numerical modelling of these experimental results. We explain the cross-over between dark and illuminated current–voltage characteristics currently observed for this type of device by the impact of the defect chalcopyrite layer at the surface of the Cu(In,Ga)Se2 absorber. Our model assumes an illumination-dependent voltage drop across a defect layer with a thickness of 15 nm to explain the cross-over. The voltage drop results from the electrical dipole made up of donor-like states at the interface between the defect layer and CdS and deep acceptor states in the defect layer itself. The illumination dependence of this voltage drop is explained by photogenerated holes trapped by the deep acceptor states in the defect layer. Numerical simulations have been carried out using the program SCAPS-1D in order to verify our model assumptions. From our model, indirect conclusions are derived concerning the maximum conduction band offsets between CdS and the defect layer and between CdS and ZnO. Copyright

On the CdS/CuInSe2 conduction band discontinuity

Recent calculations of the electron affinity difference between CdS and CuInSe2 indicate that the conduction band (CB) minimum of CuInSe2 is below the CB minimum of CdS. As a consequence, a spike occurs in the CB at the CdS/CuInSe2 interface. Such a spike is commonly considered as in conflict with good photovoltaic performance of heterojunction solar cells. It is outlined here that the simple assumption of thermionic emission across the junction can explain an unimpeded electron transport in the case of an n+p structure (n‐type window, p‐type absorber), even when a spike in the CB occurs.

Calculation of CIS and CdTe module efficiencies.

Abstract An accurate and fast method to calculate the efficiency of Cu(In,Ga)Se2 (CIGS) and CdTe thin-film solar modules is presented here. This comprises a new method to calculate the fill factor as a function of discrete and distributed series resistance, and of shunt conductance: a three-dimensional, third-order polynomial approximation is presented, and the expansion of the coefficients as a power series of 1/Voc is given. Analytical expressions are presented which fit experimental data of the optical absorption in ZnO as a function of its thickness or sheet resistance. Together with a calculation outline of the series and shunt effects of the module integration, this constitutes a practical module design tool. This is illustrated with results of dependence of module efficiency on cell length, window and absorber sheet resistance, interconnect contact resistance, “softness” of the cell I–V curve, and absorber material (CIGS or CdTe). Optimal or critical values for these parameters are given.

Defects in Cu(In, Ga) Se2 semiconductors and their role in the device performance of thin-film solar cells

This contribution is a summary of an international, interdisciplinary workshop dedicated to defects in chalcopyrite semiconductors and their relation to the device characteristics of thin-film solar cells, held on 3–5 June 1996 in Oberstdorf, Germany. Results of different characterization methods were brought together to identify common observations. The comparison of results from electrical defect spectroscopy and luminescence investigations confirmed the presence of energetic distributions of defects throughout the bandgap of chalcopyrite thin films. Electrical defect spectroscopy detects a defect about 280 meV above the valence band edge of Cu(In, Ga)Se2 regardless of the preparation conditions of the sample. In a solar cell the density of this defect depends on the operation conditions. This observation might be related to the migration of copper in an electric field, which occurs even at room temperature. Other defects appear to be related to processing or impurities. Photoluminescence decay measurements yield time constants of several nanoseconds under low injection conditions. Modelling of the current–voltage characteristics of Cu(In, Ga)Se2-based thin-film cells suggests that compensating acceptor states in the CdS or at the heterointerface are responsible for the frequently observed cross-overs between the dark and illuminated curves.© 1997 John Wiley & Sons, Ltd.

Investigation of defects by admittance spectroscopy measurements in poly (3-hexylthiophene):(6,6)-phenyl C61-butyric acid methyl ester organic solar cells degraded under air exposure

Electrical transport properties of poly (3-hexylthiophene) (P3HT) (6,6)-phenyl C61-butyric acid methyl ester (PCBM) solar cells, with and without encapsulation, have been investigated and analyzed using admittance spectroscopy and capacitance voltage measurements at different temperatures. The admittance spectroscopy clearly reveals two defect states with activation energies of 53 and 100 meV, and a concentration ten times higher in the unencapsulated sample. These defects seem to have a strong effect on the charge transport and the solar cell performance when they are present with a high concentration, since they lead to a decrease of the mobility and also the short-circuit current and the efficiency. The origin of these defects has been assigned to reaction of the blend with O2 which is also known to induce p-type doping in pure P3HT. In an attempt to understand the effect of these defects on the organic solar cell performance, modeling and simulation were carried out using the effective medium layer mo...

Modeling metastabilities in chalcopyrite-based thin film solar cells

Cu(In,Ga)Se2-based thin film solar cell devices exhibit metastable electrical behavior. This behavior is often ascribed to intrinsic defects that can change configuration accompanied by large lattice relaxations. We extended the thin film solar cell simulation software scaps to enable the simulation of the metastable behavior of this kind of defects. The statistics that are needed to describe metastable defects are discussed. The procedure that has been implemented is introduced, and special attention is paid to the convergence of the method for high defect densities. The model is demonstrated by simulating the effect of voltage induced metastabilities on the capacitance-voltage characteristics. Some of the features present in the measured apparent doping density profiles can be directly related to presence of metastable defects.

SCREEN PRINTED AND SINTERED CdTe-CdS SOLAR CELLS

Screen printed and sintered CdTe-CdS solar cells differ in some aspects from other CdTe cells: the layers are thicker, the fabrication temperatures higher, the grain size is usually better. A good morphology, a good junction behaviour and consequently a high open circuit voltage, and efficient doping of the CdTe are seen to be an inherent advantage of the screen printing and sintering technology. On the other hand, care has to be taken to avoid strong interdiffusion and optical transmission loss in the CdS window.

A theoretical model for the front region of cadmium telluride solar cells

Recently, the modelling investigation on thin film chalcogenide solar cells has been focused on the front region of the devices as a possible source of fill factor losses through a light dependent barrier modulation. We present a model based on a modulated barrier photodiode in series with the main junction, which consistently explains apparent quantum efficiency measurements and, light and dark IV characteristics of CdTe devices. This bulk barrier forms in the front region of the device as a consequence of an increased compensation of donors in CdS, which can induce crossover and in extreme cases rollover in the IV curves. The internal resistance due to this majority carrier barrier, which depends on the irradiance level, is an important limiting factor for cell efficiency, inherent to the cell structure. Numerical simulation gives consistent results with the observed measurement characteristics.