Jean-Paul Kleider

Si doped GaP layers grown on Si wafers by low temperature PE-ALD

Low-temperature plasma enhanced atomic layer deposition (PE-ALD) was successfully used to grow silicon (Si) doped amorphous and microcrystalline gallium phosphide (GaP) layers onto p-type Si wafers for the fabrication of n-GaP/p-Si heterojunction solar cells. PE-ALD was realized at 380u2009°C with continuous H2 plasma discharge and the alternate use of phosphine and trimethylgallium as sources of P and Ga atoms, respectively. The layers were doped with silicon thanks to silane (SiH4) diluted in H2 that was introduced as a separated step. High SiH4 dilution in H2 (0.1%) allows us to deposit stoichiometric GaP layers. Hall measurements performed on the GaP:Si/p-Si structures reveal the presence of an n-type layer with a sheet electron density of 6–10u2009×u20091013u2009cm−2 and an electron mobility of 13–25u2009cm2 V−1u2009s−1 at 300u2009K. This is associated with the formation of a strong inversion layer in the p-Si substrate due to strong band bending at the GaP/Si interface. GaP:Si/p-Si heterostructures exhibit a clear photovoltaic effect, with the performance being currently limited by the poor quality of the p-Si wafers and reflection losses at the GaP surface. This opens interesting perspectives for Si doped GaP deposited by PE-ALD for the fabrication of p-Si based heterojunction solar cells.

Two-Dimensional Simulations of Interdigitated Back Contact Silicon Heterojunctions Solar Cells

Interdigitated back contact silicon heterojunction (IBC-SiHJ) solar cells that combine the amorphous silicon/crystalline silicon (a-Si:H/c-Si) heterojunction- and interdigitated back contact (IBC) concepts are very promising in order to reach the highest one-junction efficiencies. In this chapter, a comparative two-dimensional simulation study has been done on the IBC-SiHJ structure based on n-type and p-type crystalline silicon by varying the values of the following parameters: minority carrier lifetime in c-Si, c-Si thickness, c-Si doping concentration, surface recombination velocity, density of defect states at the a-Si:H/c-Si hetero-interface and rear side geometry. The influence of these parameters has been tested by generating the current-voltage characteristics under illumination. Results indicate that the key parameters to achieve high efficiency are high c-Si substrate quality, low surface recombination velocity especially at the front surface, and a low recombining a-Si:H/c-Si interface. The width of the gap region (spacing between the back-surface field (BSF) and the emitter) must be kept as small as possible to avoid recombination of minority carriers in the bulk c-Si. For IBC-SiHJ based on n-type c-Si, the optimum geometry corresponds to a minimum size BSF region and a maximum size emitter region while for IBC-SiHJ based on p-type c-Si a BSF width equivalent to around 30% of the pitch is an optimum.

Defect properties of InGaAsN layers grown as sub-monolayer digital alloys by molecular beam epitaxy

The defect properties of InGaAsN dilute nitrides grown as sub-monolayer digital alloys (SDAs) by molecular beam epitaxy for photovoltaic application were studied by space charge capacitance spectroscopy. Alloys of i-InGaAsN (Egu2009=u20091.03u2009eV) were lattice-matched grown on GaAs wafers as a superlattice of InAs/GaAsN with one monolayer of InAs (<0.5u2009nm) between wide GaAsN (7–12u2009nm) layers as active layers in single-junction solar cells. Low p-type background doping was demonstrated at room temperature in samples with InGaAsN layers 900u2009nm and 1200u2009nm thick (less 1u2009×u20091015u2009cm−3). According to admittance spectroscopy and deep-level transient spectroscopy measurements, the SDA approach leads to defect-free growth up to a thickness of 900u2009nm. An increase in thickness to 1200u2009nm leads to the formation of non-radiative recombination centers with an activation energy of 0.5u2009eV (NTu2009=u20098.4u2009×u20091014u2009cm−3) and a shallow defect level at 0.20u2009eV. The last one leads to the appearance of additional doping, but its concentration i...