The AM 1.5 G current density-voltage (JV
) characteristic of the GaAs n-i-p-i solar cell is shown in Figure
. The GaAs n-i-p-i cell recorded a short-circuit current density (Jsc
), open circuit voltage (Voc
), fill factor (FF) and efficiency (η
) of 16 mA/cm2
(accounting for grid coverage), 0.33 V, 54% and 2.2%, respectively. In our previous work, the best GaAs n-i-p-i device was annealed at 725°C and achieved Jsc
= 12.9 mA/cm2
= 0.19 V and η
= 0.87%, under the same illumination conditions [11
]. Though we have not established optimum annealing conditions for these devices, the results suggest that the devices which annealed at a higher temperature of 800°C will show an improvement in all figures of merit.
The AM 1.5 G J-V characteristics for GaAs and GaInNAs n-i-p-i solar cells.
The reflection losses have not been accounted for in calculating the current density; however, Jsc
= 16 mA/cm2
is still much lower than the >29 mA/cm2
recorded with conventional GaAs solar cells [12
Other recently demonstrated n-i-p-i devices show relatively high short-circuit currents and a disproportionately low Voc
and FF [8
]. The cause of the low Voc
is not yet fully established. Results show that the Voc
varies to some extent with the fabrication technique and the quality of the selective contacts. Ion implantation of the selective contact offers the possibility of fabricating contacts on the front and back of the device without shorting it as the implants can start deep in the device. However, the introduction of dislocations at ion-implanted interfaces possibly increases recombination diffusion dark current, consequently reducing Voc
. The re-grown contacts yield a slightly higher Voc
]. However, with the re-grown contacts, the easiest technique is to have both contacts at the top of the cell, which can pose a problem in growing tandem structures.
The spectral response of the GaAs n-i-p-i solar cell was obtained by illuminating the cell using a Bentham IL1 illuminator (Bentham Instruments Ltd., Reading, Berkshire, UK), followed by a Bentham M300 monochromator (Bentham Instruments Ltd.). The spectrum, shown in Figure
, is typical of GaAs solar cells with the spectral response staying almost constant between the short wavelength cut-off due to the bandgap of the window layer and the long wavelength cut-off due to the bandgap of the cell. This suggests that minority carriers generated at all depths in the device contribute to the photocurrent. This can be inferred to all the layers of the device contributing to the photogenerated current.
Spectral response of the GaAs n-i-p-i solar cell.
The spectral response profile of the GaInNAs n-i-p-i solar cell was taken using the same experimental conditions, and it is shown in Figure
. As expected, it extends to longer wavelengths of 1.1 μm corresponding to photon energies of approximately 1.1 eV. The dropping shape of the spectrum suggests that only the top layers contribute to the device current. This could be purely a fabrication problem which has no bearing in the material properties or the design. If the ion implants have not reached or been activated at the bottom layers, the device will collect carriers at the top layers (short wavelengths) while most of the carries at the bottom layers (long wavelengths) will recombine before reaching the vertical contacts, consequently leading to low short-circuit current density values. As a result, the device will behave like a thin cell which only absorbs shorter wavelength photons, and it is transparent to the longer wavelength ones. In relation with the J
curve, our GaInNAs device had Jsc
= 4.2 mA/cm2
= 0.19 V, as shown in Figure
. The very low value of Jsc
can be explained by the shape of the spectral response curve.
Spectral response of the GaInNAs n-i-p-i solar cell.