Stefaan Degrave

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.

Key Aspects of CdTe/CdS Solar Cells

Recent developments in the following areas are briefly reviewed: a) the electrical structure of grain boundaries in CdTe absorbers, b) impurities and non-stoichiometry in CdTe solar cells and c) use of Sb 2 Te 3 in contacts to CdTe. Nominally identical solar cells fabricated using 99.999% pure CdTe feedstock from two different suppliers were compared. Differences in the photovoltaic response and absorber grain size were correlated with the purity of the feedstock, the purer material giving the higher V oc , FF and efficiency, and larger grain size. Quantum efficiency and C-V measurements indicated that the performance differences are most likely to result from reduced doping at the back contact surface in the less pure sample. A quantitative SIMS study of Sb-Te contacts to CdTe reveals that annealing in air at 400 °C causes an influx of Sb and O into the absorber layer. Free energy calculations indicate that this is driven by the preferential reaction of O with Sb compared to CdTe oxidation.

DLTS and admittance measurements on CdS/CdTe solar cells

Abstract Deep level states can affect the characteristics of thin film solar cells both by their charge and by their recombination. Hence, full solar cell characterisation should include deep states. Deep level transient spectroscopy (DLTS) and admittance spectroscopy (AS) measurements ( Y ( f , T )) have been made on a series of CdTe/CdS thin film solar cells. Majority and minority trap DLTS was performed between 5 and 330 K revealing semi-shallow to mid-gap traps, depending on the sample. The DLTS results are compared with those of AS in the 100–320 K range at frequencies from 100 Hz to 1 MHz. The energy range probed with the AS is narrower than that of the DLTS method. Both methods give comparable results in the energy range where they overlap. The results are compared with literature data. The difference between cells which received a CdCl 2 treatment in air and those which received the treatment in vacuum is discussed.

Indications for presence and importance of interface states in CdTe/CdS solar cells

Abstract Quantum efficiency measurements under forward voltage bias conditions often show a large response in the wavelength region from 350 to 500 nm. It is seen that the light induced changes do not only cause a higher current in the illuminated case than in the dark case at forward bias (the situation which is explained in earlier work), but also often on the contrary a lower current at moderate forward bias (the current direction can be deduced from the phase shift between input signal and response signal). This second situation can be studied in detail by comparing the I – V curves in dark to the ones illuminated with the monochromatic light under consideration. Also the dependence of the response on the illumination intensity can be clearly shown in these measurements. A model for this situation is built up, making use of traps at the CdTe/CdS interface and extending a previously presented model. This model shows that the quantum efficiency measurement is a sensitive method to determine the presence of interface states.

Modelling of polycrystalline thin film solar cells: new features in SCAPS version 2.3

Modern characterisation of thin film solar cells is based on a variety of standard and advanced (opto-) electrical experiments. In the case of complicated structures, like most of the polycrystalline thin film solar cells, interpretation of such experiments is almost impossible without the help of simulation programs. In many cases numerical simulation of a thin film solar cell can lead to better insight into the details of physical operation. It can even give new insights to improve the technology. The numerical simulation tool SCAPS is developed at our lab to deal with polycrystalline thin film solar cells. The program is able to simulate standard experiments like I(V) and QE(/spl lambda/) as well as more advanced measurements like C(V) and C(f). The program is now in a state of extension to transient analysis; it will be able to simulate current and capacitance in function of time. Different methods to solve the time dependent problem and first results and observations of transient behaviour are treated in this article.

Importance of electron current in p-type CdTe in CdS/CdTe thin film solar cells at forward bias

In previous work [1,2], we modeled the cross-over of the I-V curves of thin film CdS/CdTe solar cells in terms of an electron (minority carrier) current in the vicinity of the back contact. In this work, we focus on the necessary extension of this analytical model based on a series of measurement results. Especially the wavelength and voltage dependence of the current at forward bias is illustrated in these measurements. The various possible causes for this kind of behavior are discussed and modeled. The extensions to the previous model, needed to describe the voltage and wavelength dependent behavior of I-V curves of real CdTe/CdS solar cells, are proposed.

Modeling the cross-over of the I-V characteristics of thin film CdTe solar cells

The I-V curves of high efficiency CdTe thin film solar cells show a kink in the forward region (roll-over) this fact is well studied and ascribed to a contact barrier e.g. at the back contact of the CdTe layer. Also, light and dark I-V characteristics intersect (cross-over), due to an increase of the contact saturation current under illumination. We observed on many occasions that a fan of several I-V curves all intersect at one single point in the first quadrant (forward current and voltage), when some parameter is varied, like illumination intensity or minimal changes of processing conditions. We ascribe this to the contribution of electron minority current at the CdTe back contact Schottky barrier. We show how this can explain the one-point-intersection. The validity of the assumptions are discussed with theoretical considerations, numerical simulations and experiments.

Importance of air ambient during CdCl/sub 2/ treatment of thin film CdTe solar cells studied through temperature dependent admittance spectroscopy

CdCl/sub 2/ treatment is a crucial step in the production of high efficiency CdTe solar cells. Thin-film CdS/CdTe solar cells have been prepared by ANTEC GmbH (Germany) both in vacuum and in air ambient. We compared these series of samples using the admittance spectroscopy (AS) technique. The presence of air increases remarkably the shallow acceptor concentration in the CdTe, confirming earlier studies. A broad band of deep defects is still found in all samples (independent of the activation). We also illustrate how the presence of a Schottky back contact to the CdTe solar cells introduces a capacitance step in the AS measurements, enabling the determination of the barrier height.

Optimisation of CISCuT cell design

Abstract The special features of the CISCuT material require cell concepts that differ from conventional CuInS2 solar cells. For the n-type CISCuT bulk material, a p-type buffer is required. In this paper, the currently used p-type Cu(O,S) buffer layer is replaced by the higher bandgap material CuI. On 10-mm2 cells the efficiency profit made by the replacement of Cu(O,S) with CuI is demonstrated. For larger cell areas, two different cell configurations are described. In the first concept, a ZnO:Al window layer is deposited on top of the CuI buffer layer. The second concept uses a metal grid as a front contact. Theoretical considerations point out the consequences of the given concepts for the CISCuT cell performance. For the Cu/CISCuT/CuI/ZnO:Al layer configuration, an additional tunnel junction is expected at the interface between the p-type CuI and the n-type ZnO:Al. In the metal grid concept, CuI doping with iodine is required to lower the sheet resistance of the layer. Some precautions against iodine out-diffusion have to be taken. For the window layer configuration on an area of 400 mm2, a cell performance of 5.2% was obtained.