Electrical scanning probe microscopy approaches to investigate solar cell junctions and devices

Abstract

C-AFM and KPFM techniques have been applied to investigate advanced junctions that are currently involved in highly efficient silicon solar cells. Our first study focuses on silicon heterojunctions and notably hydrogenated amorphous silicon (a-Si:H)/crystalline silicon (c-Si) P/n or N/p heterostructures which band bending at the interface forms a 2D channel. This conductive channel was indeed evidenced for the first time by cross-sectional investigations by C-AFM confirming the analysis of macroscopic planar conductance measurements. A second example of nanoscale characterization concerns the passivating selective contacts consisting in a thin silicon oxide (SiOx) layer between the c-Si and a highly doped polysilicon (poly-Si) layer. The electrical carrier transport is here not limited by the oxide layer and it is assumed that tunnelling through the oxide and/or the presence of pinholes are the main competitive mechanisms. For this specific heterostructure KPFM reveals local surface potential drops of 15-30 mV, which do not exist on samples without SiOx. These potential drops suggest the presence of pinholes that are formed during the poly-Si annealing process performed in the range of 700-900 °C. Finally, in a third study, we concentrate on p-in radial junction (RJ) silicon nanowire (SiNW) devices that are investigated under illumination by KPFM, in the so-called surface photovoltage (SPV) technique. This work focuses on the possibility of extracting the open-circuit voltage (VOC) on single isolated SiNW RJ by local SPV measurements using different AFM tip shapes and illumination directions in order to minimize shadowing effects.

Nanoscale Study of the Hole-Selective Passivating Contacts with High Thermal Budget Using C-AFM Tomography

“…Among these, the oxide tunneling model and oxide breakup model are the most prominent. , Recently, a more elaborate investigation with temperature-dependent current–voltage , and transmission line measurements suggests that the actual transport through the interfacial SiO x is probably a combination of these models, and it depends on the used oxide density and thickness. While the interface SiO x layer was closely studied in the past, reports on the charge-carrier transport mechanism through the rest of the passivating stack are scarce. − …”

Observation of photovoltaic effect within locally doped silicon nanojunctions using conductive probe AFM

“…In particular, in the field of PV, local scale C-AFM investigations (in dark and illuminated conditions) have demonstrated added value in understanding charge transport and in improving the performance of the final device. Numerous C-AFM studies have been carried out on different solar cell configurations (planar, nanowires, cross-section, …) and in particular, on the layers or structures of which they are composed [26][27][28][29][30][31][32]. In this work, the C-AFM method will be used to directly probe the photovoltaic properties of nanojunctions formed in a c-Si wafer.…”