Renaud Varache

Band bending and determination of band offsets in amorphous/crystalline silicon heterostructures from planar conductance measurements

An analytical model for the calculation of the band bending in amorphous/crystalline silicon (a-Si:H/c-Si) heterojunctions is presented and validated by comparison with full numerical simulations. The influence of the various structure properties and parameters, such as the density of states in bulk a-Si:H or at interface defects, the position of the Fermi level in a-Si:H, the temperature dependence of band gaps, is investigated. Significant band offsets imply the presence of a strong inverted layer at the c-Si surface of both (p)a-Si:H/(n)c-Si and (n)a-Si:H/(p)c-Si structures, forming two-dimensional hole and electron gases, respectively. This leads to high sheet carrier densities that have been evidenced from planar conductance measurements. Experimental data obtained on samples coming from various research institutes are analyzed with our model in order to extract the band offsets. We find that the valence band offset ranges between 0.32 and 0.42 eV with an average value at 0.36 eV; the conduction band...

Insertion of a thin highly doped crystalline layer in silicon heterojunction solar cells: Simulation and perspectives towards a highly efficient cell concept

An emerging cell concept based on silicon heterojunctions called hetero-homojunction is investigated by means of numerical simulations. Compared to the usual amorphous/crystalline silicon (a-Si:H/c-Si) heterojunction architecture, the hetero-homojunction cell contains an additional thin and highly doped (p+)- or (n+)- c-Si layer at the front or back (i)a-Si:H/(n)c-Si interface, respectively. In this paper, we show the dependence of solar cell performance on the additional heavily doped c-Si layer parameters (thickness and doping) and a-Si:H/c-Si interface properties. Insertion of the (p+)c-Si improves the cell power conversion efficiency by almost 1% absolute and lowers its sensitivity to a-Si:H/c-Si interface defects. Improved field effect passivation leading to higher open circuit voltage and fill factor is evidenced and the added layer is optimized with regard to hetero-homojunction cell efficiency. The (n+)c-Si layer addition also decreases the recombination rate at the back hetero-interface but does ...

Light induced changes in the amorphous—crystalline silicon heterointerface

The photostability of the amorphous—crystalline silicon heterointerface is investigated. It is revealed that the metastability of hydrogenated amorphous silicon (a-Si:H) causes significant light induced changes in the heterointerface. Unlike bulk a-Si:H, the photostability of the heterointerface is not controlled by the microstructural properties of a-Si:H but rather by the initial heterointerface properties. Interfaces that initially have low interface defect density show the greatest degradation while those that initially have high interface defect density actually show light-induced improvement. It is shown that the degree of light induced change in the interface defect density is linearly proportional to the natural logarithm of the initial interface defect density. Further, it is revealed that the kinetics of light-induced change in the heterointerface defect density can be faster or slower than light-induced changes in bulk a-Si:H films depending on the initial properties of the heterointerface. Lig...

Solar cells with gallium phosphide/silicon heterojunction

One of the limitations of current amorphous silicon/crystalline silicon heterojunction solar cells is electrical and optical losses in the front transparent conductive oxide and amorphous silicon layers that limit the short circuit current. We propose to grow a thin (5 to 20 nm) crystalline Gallium Phosphide (GaP) by epitaxy on silicon to form a more transparent and more conducting emitter in place of the front amorphous silicon layers. We show that a transparent conducting oxide (TCO) is still necessary to laterally collect the current with thin GaP emitter. Larger contact resistance of GaP/TCO increases the series resistance compared to amorphous silicon. With the current process, losses in the IR region associated with silicon degradation during the surface preparation preceding GaP deposition counterbalance the gain from the UV region. A first cell efficiency of 9% has been obtained on ∼5×5 cm2 polished samples.