Photothermal spectroscopy on multicrystalline silicon solar cell materials by dual sampling method in atomic force microscopy

In order to investigate nonradiative recombination properties in a multicrystalline Si solar cell, we have performed photothermal (PT) measurement by dual sampling method in atomic force microscopy with a Si piezoresistive cantilever operated under near-infrared light excitation, and observed the distribution of the PT signal around some grain boundaries (GBs). From the PT signal image taken under the light at a photon energy of 1.57 eV which is higher than the bandgap of Si (1.12 eV), we found that there exist some regions, exhibiting enhancement of the PT signal, near GB. In comparison with the surface potential distribution observed by Kelvin probe force microscopy, we also found good correspondence between the regions where the PT signal was enhanced and where the surface potential for electron was lowered. We interpret these results as follows: (i) segregation of donor-like impurities or defects occurs in the vicinity of the grain boundary, which lowers the potential for electron, and consequently the photo-generated electrons are attracted towards the segregated region, and (ii) those photocarriers frequently and nonradiatively recombine via impurity or defect levels, which enhances the PT signal there. In addition, we observed the enhancement of the PT signals near certain GBs under the light at a photon energy of 0.78 eV, which is below the Si bandgap. We deduce that this region include a lot of dislocations and/or heavy-metal impurities.