Robert I. Frank

Measurement of minority carrier lifetime in solar cells from photo-induced open-circuit voltage decay

We present an experimental technique for determining the excess minority carrier lifetime within the base region of p-n junction solar cells. The procedure is to forward-bias the solar cell with a flash from a stroboscope and then to monitor the decay of the open-circuit voltage. Results are given for conventional horizontal-junction devices, as well as for vertical single- and multijunction solar cells. Lifetimes obtained with this technique are compared with those obtained from a method based on open-circuit voltage decay following the abrupt termination of a forward current, and with results obtained from a traveling light spot measurement of base minority carrier diffusion length in vertical-junction solar cells, from which the lifetime can be inferred. It is found that the forward current method does not yield a reliable lifetime estimate.

Performance of a new high‐intensity silicon solar cell

A new silicon solar cell, designed to have improved electrical, optical, and thermal transfer characteristics at very high incident light intensities, has been fabricated and provides experimental verification of the basic design concepts. The AM1 efficiency for nonoptimized cells is 12.8% at 25 °C. At 300 suns the efficiency increases to 19%. It is shown that efficiencies of over 25% are possible for this type of cell in a more‐optimized form at intensities of ∼500–1000 suns.

Open‐circuit voltage of vertical‐junction photovoltaic devices at high intensity

Vertical single‐junction silicon photovoltaic cells show a steady increase in open‐circuit voltage with increasing incident light intensity of approximately 0.1 V per decade of intensity, up to approximately 100 W/cm2 (∼1000 suns). Voltages as high as 0.76 V have been observed at 25 °C with no apparent saturation of voltage at high intensity. Measurements are presented for cells of various base doping levels. An efficiency of 19.1% has been observed at 76 W/cm2 and 25 °C using an unfiltered xenon short‐arc lamp for a nonoptimized cell.

New analysis of a high‐voltage vertical multijunction solar cell

We present an analysis and a model for a vertical multijunction solar cell with covering lens, for which a 21% efficiency has been predicted previously. It is shown that the maximum conversion efficiency is in fact about 0.3%. The present analysis confirms our belief that back‐to‐back diodes should be designed into a solar cell only with great caution.