Unit of an EWT Si solar cell (not drawn to scale). The simulation domain comprises one quarter of this unit (upper left).
Fabrication sequence of the EWT solar cells presented in this paper.
Measured effective excess carrier lifetime of the cell (open circles), the SRH lifetime in the bulk (closed circles), and a calculation of by means of Eqs. (1), (2a)–(2c), and (3), taking into account the lifetime limitation by the boron-oxygen complexes (line).
Top panels: measured characteristics under 1 sun illumination (shifted by ), under dark conditions, and the values obtained under various illumination intensities of two EWT cells with the denoted base resistivity. Center panels: measured local ideality factors of the above characteristics. Bottom panels: lumped series resistance, extracted from the 1 sun and curves.
Simulated electron densities in the EWT solar cells. The domain is split into two regions at the location where the electron density in the base region is maximal, to visualize the demarcation area. This area divides the front collecting from the rear collecting part of the base, as indicated by the filled-in arrows in the upper left panel. The top and bottom rows show the results for a base resistivity of 0.5 and , respectively. The three columns represent the operation conditions at short-circuit, at 530 mV and at open-circuit, respectively. Also indicated are the electric potentials along the emitter and in the base of the solar cell. The values of denote the voltage drop between the external voltage at the rear emitter contact and the front emitter (near the via and at the midpoint between neighboring vias). The values of denote the voltage drop between the external voltage at the rear base contact and the base near the via.
Simulated recombination losses (top panels) and local ideality factors of the recombination characteristics (bottom panels) of the EWT cells with a base resistivity of (left) or (right). The curves are shifted to the first quadrant by the generation current-density to represent the total recombination losses (top panels), which can be directly compared to the main recombination losses: by means of SRH in the front and rear collection regions, and Auger in the front emitter. To obtain the local ideality factors, the curves are shifted to the first quadrant by the short-circuit current-density . Also shown is the current-density flowing from the front to the back of the cell through the EWT-via-emitter and the external voltages (marked as black crosses) at which the demarcation area in Fig. 5 begins to shift.
Equivalent circuit of resistivities experienced by the electrons flowing through the front or rear collecting part in the EWT cell, where represent the generalized minority carrier resistivity.
Calculated resistivity of the front and rear collecting current path in an EWT cell, shown in Fig. 7, with a base resistivity of (black) or (red) and for two different EWT-via sheet resistances of 50 and , as a function of . The two crosses “1” and “2” correspond to the values in Fig. 6.
Cell parameters of an EWT and a corresponding FJ cell design, simulated in three dimensions as a function of (using ) which is essentially , for a base resistivity of and , respectively.
Geometrical EWT solar cell parameters used in this study.
The measured parameters of the characteristics of two cells made with different wafer resistivities under standard conditions (halogen lamp approximating the am1.5g spectrum, ).
Models and parameters used in the numerical model.
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