In this chapter the effect of using a semiconductor finger structure on illuminated IV characteristic parameters was analyzed by means of distributed circuit simulations.
The results were compared to the ones calculated by the area weighted mean IV characteristic neglecting series resistance effects. It turned out that the short circuit current density can well be estimated by the area weighted mean IV characteristic, even if series resistance effects are neglected. Also the open circuit voltage can be approximated by the area weighted mean IV characteristic under disregard of series
resistance effects, even though the area weighted mean overestimates the open circuit voltage slightly. In case of low series resistance effects even the current density at maximum power point is approximated very well using the area weighted mean IV characteristic. But as the voltage at maximum power is highly influenced by series resistance effects it is not possible to approximate it by the area weighted mean IV characteristic under disregard of series resistance effects. As the calculation of the effective series resistances of solar cells with semiconductor fingers is a two dimensional problem and therefore not to solve straight forward distributed circuit simulations are a useful tool to predict the efficiencies, which can be expected of such cell structures.
In the distributed circuit simulations presented in this chapter different solar cell parameters were varied and two different contact patterns were analyzed. It turned out that in the case of the analyzed structures a contact pattern, which contacts the whole metal finger can result in an efficiency gain compared to a solar cell without semiconductor fingers, while a contact pattern, which only contacts the regions above the semiconductor fingers results in a reduction in cell efficiency in most cases. This is probably due to an increase of current crowding effects in case of such low contact areas. Furthermore the difference in emitter sheet resistance between semiconductor finger region and not-semiconductor finger region has to be adequate to result in an efficiency gain. According to the simulations semiconductor fingers on a solar cell with emitter sheet resistance of 150 Ohm/sq in the not-semiconductor finger region do not promise any efficiency gain compared to a similar solar cell without semiconductor fingers while semiconductor fingers on a solar cell with 350 Ohm/sq emitter sheet resistance can improve the solar cell efficiency.
It has to be kept in mind that all predictions presented in this chapter depend on the specific solar cell structures, which were used in the simulations. Comparisons to experimental results will be performed in the future.
8 Analysis of solar cells with shoulders in the dark IV characteristic
Some of the solar cells, which are produced at Fraunhofer ISE using photolithography technique, have a dark IV characteristic with a shoulder.
Due to this shoulder the dark IV characteristics of the affected solar cells are not adjustable by the two diode model. This behavior gives rise to the analyses presented in this chapter, whose aim is twofold:
On the one hand, it is analyzed in which way defects, which might occur in the regarded solar cells, may influence the according dark IV characteristics. Thereto distributed circuit simulations are used. This aspect is complemented by a literature review, which gives an overview of publications, which are concerned with effects, whose influence on the dark IV characteristic goes beyond the one described by the one or two diode model.
On the other hand the affected solar cells are analyzed using different measurement techniques to narrow down possible reasons for the observed shoulder in the dark IV characteristics.
8.1 Introduction
Fig. 8.1 shows the dark IV characteristics of selected solar cells, which were produced in one batch in the Fraunhofer ISE clean room using photolithography technique.
Some of the depicted dark IV characteristics attract attention as they have a shoulder, which makes it impossible to adjust the two diode model (chapter 2.5.5) to these characteristics satisfactory. It is remarkable that the shoulder does not necessarily affect the fill factor as some solar cells with and without shoulder reach fill factors of more than 82%.
In the following chapter a review of existing theories about possible reasons for dark IV characteristics, which do not obey the two diode model, is given. This is followed by a short description of the structure of the analyzed solar cells, of which some are characterized by a dark IV characteristic with a shoulder. Afterwards measurement results of solar cells with dark IV characteristic with and without shoulder are compared. The analyzed solar cells were produced in the same batch. The aim is to find differences between the solar cells with dark IV characteristic with and without shoulder, which can be attributed to cause the shoulder. At last distributed circuit
simulations are used to analyze the effect of defects, which might occur in the affected solar cells, on the associated dark IV characteristics and local voltage maps.
Fig. 8.1: Dark IV characteristics of selected solar cells produced in one batch in the Fraunhofer ISE clean room using photolithography technique. The dark IV characteristics, which were not adjustable by the two diode model, are depicted by closed symbols. The dark IV characteristic of solar cell 12.7 is adjustable by the two diode model (open circles) using n2=2.1. The solar cell serves as reference solar cell in this chapter.