Voltage Of Silicon Solar Cells Research Articles

Influence of the Recombination Parameters at the Si/SiO2Interface on the Ideality of the Dark Current of High Efficiency Silicon Solar Cells

Analytical study of surface recombination at the Si/SiO 2 interface is carried out in order to set the optimum surface conditions that result in minimum dark base current and maximum open circuit voltage in silicon solar cells. Recombination centers are assumed to form a continuum rather than to be at a single energy level in the energy gap. It is shown that the presence of a hump in the dark I-V characteristics of high efficiency PERL cells is due to the dark current transition from a high surface recombination regime at low voltage to a low surface recombination regime at high voltage. Successful fitting of reported dark I-V characteristics of a typical PERL cell is obtained with several possible combinations of surface parameters including equal electron and hole capture cross sections.

Publisher's Note: “Photoluminescence imaging for determining the spatially resolved implied open circuit voltage of silicon solar cells” [J. Appl. Phys. 115, 044901 (2014)

Publisher's Note: “Photoluminescence imaging for determining the spatially resolved implied open circuit voltage of silicon solar cells” [J. Appl. Phys. <b>115</b>, 044901 (2014)

Photoluminescence imaging for determining the spatially resolved implied open circuit voltage of silicon solar cells

Photoluminescence imaging has widely been used as a characterisation tool for the development of silicon solar cells. However, photoluminescence images typically only give qualitative information due to the presence of an unknown calibration constant. In this work, quasi-steady-state photoconductance measurements on partially processed solar cells and I-V measurements on finished solar cells are used to determine the calibration constants to yield spatially resolved implied open circuit voltage images. This technique is then applied to determine the implied open circuit voltage of laser doped selective emitter solar cells at various stages of cell fabrication after the formation of the full area aluminium back surface field when other characterisation techniques such as photoconductance cannot be used.

Spatially Resolved Determination of Junction Voltage of Silicon Solar Cells

For the extraction of spatially resolved solar cell parameters, a variety of methods has been introduced in the past years. Nearly all methods use the fact that the local luminescence intensity can be calibrated to local junction voltage. The different methods however use different approaches for the calibration. In this work we discuss the different approaches used throughout literature. We investigate the key assumption of two approaches which assumes that there are no lateral junction voltage gradients at sufficiently low excitation conditions. Our investigation is based on circuit simulations and experimental results on an example cell. We give an explanation for the remaining contrasts in luminescence images taken at these low excitation conditions. A third approach which does not rely on the key assumption is discussed with respect to measurement time in comparison to the other approaches. Finally the role of the short circuit current is discussed and a modification to the approach as known from literature is proposed.

Fermi degeneracy and its effects on the open-circuit voltage of silicon solar cells

A theoretical explanation of the high open-circuit voltage (Voc) in n+-p silicon solar cells despite the heavy doping effects (such as Auger recombination and rigid band-gap narrowing) is given. It is shown that, contrary to recent expectations, when the surface impurity concentration is Ns ≥2×1020 cm−3 the open-circuit voltage is approximately the same as that for surface passivated cells with Ns ≤2×1019 cm−3. Furthermore, in the case of high values of Ns the open-circuit voltage becomes almost independent of the surface recombination velocity. Our model shows that these facts are due to the Fermi degeneracy and the internal electric field. Therefore, high concentrations of activated donors should be looked forward to for obtaining high-efficiency n+-p solar cells.

The effects of phosphorus precipitation on the open-circuit voltage in N +/P silicon solar cells

We have found that the emitter recombination current, which limits the open-circuit voltage in silicon solar cells with heavily phosphorus-doped shallow emitters, depends on the junction fabrication procedures. In fact, the physical properties which influence the emitter recombination current include not only the surface recombination velocity (which can be controlled by appropriate surface passivation techniques and optimized front metal coverage factors) but also the bulk emitter lifetime. It is shown that the bulk emitter lifetime depends on the presence of phosphide precipitates, and it is this precipitation phenomenon that causes reduced values of the open-circuit voltage. Junction fabrication procedures which result in the suppression of the precipitation phenomenon and experimental data which demonstrate the importance of this effect are presented and discussed.

Silicon solar cells with reduced temperature sensitivity

As the open-circuit voltage of silicon solar cells continues to improve, one resulting advantage, not Widely appreciated, is reduced temperature sensitivity of device performance. Recently a new type of silicon solar cell has been described which has resulted in a significant increase in open circuit voltage. Experimental results are described for these devices which demonstrate the lowest temperature sensitivity ever reported for silicon cells under unconcentrated sunlight. With further improvements in voltage, it should be possible to approach the relative temperature insensitivity of some high performance GaAs devices.

Influence of the junction area to edge area ratio on the open-circuit voltage of silicon solar cells

This work shows experimentally the decrease in open-circuit voltage produced by edge recombination in silicon solar cells. The effect is related to the edge area to junction area ratio.

Limitations on the open-circuit voltage imposed by P+ and N+ regions in silicon solar cells

It is shown theoretically and experimentally that the emitter recombination current, which limits the open-circuit voltage of silicon solar cells, can be more easily suppressed in P+N cells than in N+P cells. This result is due to fundamental effects that occur in heavily doped silicon: degeneracy of the majority charge carriers, Auger recombination, and energy-band-gap narrowing. Cell designs to suppress the emitter current are discussed, and experimental data supporting our theoretical analysis are presented.

On the deionization of impurities as an explanation for excess intrinsic carrier density in heavily doped silicon

The recent models of Heasell and Popovic are discussed from the unifying viewpoint that both use the deionization of impurities to try to explain the excess intrinsic carrier density (or deficit effective doping concentration) experimentally observed in heavily doped silicon. The conclusion is that neither of these models can explain the experimental data. Thus neither satisfactorily replaces the interpretation given earlier that attributes the experimental findings to energy-bandgap narrowing. From the standpoint of device analysis, this conclusion bears on the models used to explain the experimental result that the common-emitter current gain in silicon bipolar transistors and the open-circuit voltage in silicon solar cells are often much lower than predicted by classical theory.

Theoretical effects of surface diffused region lifetime models on silicon solar cells

A computer simulation of silicon solar cells has indicated that the combination of band gap reduction due to heavy doping and certain spatial forms of lifetime dependence can combine to form severe limitations to the open circuit voltage of silicon solar cells. The interaction of these effects tends to shift the active region of the diffused surface layer away from the injecting junction resulting in an increase in the current density injected into the surface region. Reductions in open circuit voltage as great as 10% over models which do not include these effects can be seen.