57Fe charge states in MC-Si solar cells under light illumination after GeV-implantation of 57Mn

Abstract

Immediately after the GeV-implantation of 57 Mn nuclei produced by the RIKEN-RIBF facility, 57 Mn/ 57 Fe Mössbauer spectra in Si-solar cells are measured under light illumination. Comparing with the spectrum of p-type multi-crystalline-Si, the broad spectra of the solar cell under operation can be analyzed as a superposition of interstitial and substitutional Fe components with different charge states. The charge states of Fe impurities are created by the excess carrier injection followed by a directional carrier flow in the p-n junction. The present results provide us a possibility to clarify the carrier trapping process at the Fe impurities in Si-solar cells.

Mössbauer Spectroscopy on Fe Impurities in Si Materials

“…The isomer shift, which is proportional to the electron density at the nucleus, corresponds to the energy shift of the nuclear level for a Mössbauer component on a Doppler velocity scale of mm/s against the spectral centre of α-Fe at room temperature. The data have been obtained in different Mössbauer experiments on 57 Fe impurities, which were differently introduced into Si samples, and subsequently measured at different temperatures: (1) deposition at room temperature, and diffusion and measurements at high temperatures up to 1273K [11,14,15], (2) highly energetic implantation of the mother isotope of 57 Mn/ 57 Fe into Si [12,13,17], and (3) deposition and diffusion, and measurements at room temperature [16,[18][19][20]. All the results can be analysed by the new model based on the interstitial Fe with neutral, +1, and + 2 states (defect associated), in addition to substitutional Fe with neutral, possibly +1 and -1 states.…”

“…Mössbauer spectroscopy appears to be ideal to characterize Fe impurities in Si crystal [6][7][8][9][10]. Recently, we have performed a series of the experimental investigations [10][11][12][13][14][15][16][17][18][19][20] using Mössbauer spectroscopy on Fe impurities in p-type and n-type Si materials such as single crystal Si, multi-crystal line (mc) Si wafers, and even mc-Si solar cells, the results are appealing that we need a new picture for Fe impurities in Si materials: the Fe impurities in the Si matrix exist not only on the interstitial sites with Fe int 0/+ and Fe int + -Bpair, but also on a higher charge state of Fe int 2+ associated with defects as well as on the substitutional sites with Fe sub 0 and Fe sub -. Furthermore, these components are found to transform each other in the Mössbauer spectra by changing experimental conditions such as temperature, external voltage, electron or light irradiation, and external stress as well as by changing the Fermi level, carriers and their concentrations, and the device structures in the vicinity of 57 Fe probes.…”

Direct Observations of Fe Impurities in Si with Different Fermi Levels by Mössbauer Spectroscopy

“…Interstitial Fe atoms have been observed in our Mössbauer spectroscopic studies [7][8][9]: we have interpreted that the interstitial Fe int 0 and Fe int + components with neutral and positively charged states, respectively, correspond to the isomer shifts of 0.40(3) and 0.80(3) mm/s. There was, however, other assignment by Gunnlaugsson et al [5], who considered the isomer shifts of Fe int 0 and Fe int + to the values of 0.776 (10) and 0.72(2) mm/s, respectively.…”

“…Set-up (A) (1.0 mm/s), based on theoretical calculation of the isomer shift [11] and the analysis of 57 Mn implantation experiments both on a mc-Si wafer and on a mc-Si solar cell [7].…”

“…Substitutional iron atoms (Fe sub ) were not detected experimentally by standard evaluation techniques. In addition to the interstitial component, however, Mössbauer spectroscopy often found a component, which could be assigned to ''substitutional Fe'' after implantation [3][4][5][6][7] as well as after diffusion annealing [8].…”