This title appears in the Scientific Report :
2005
Please use the identifier:
http://hdl.handle.net/2128/528 in citations.
Microcrystalline silicon films and solar cells investigated by photoluminescence spectroscopy
Microcrystalline silicon films and solar cells investigated by photoluminescence spectroscopy
A systematic investigation on photoluminescence (PL) properties of microcrystalline silicon ($\mu$c-Si :H) films with structural composition changing from highly crystalline to predominantly amorphous is presented. The samples were prepared by PECVD and HWCVD with different silane concentration in h...
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Personal Name(s): | Merdzhanova, Tsvetelina (Corresponding author) |
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Contributing Institute: |
Institut für Photovoltaik; IPV |
Imprint: |
Jülich
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
2005
|
Physical Description: |
X, 137 S. |
Dissertation Note: |
Academy of Science, Sofia, Diss., 2004 |
ISBN: |
3-89336-401-3 |
Document Type: |
Book Dissertation / PhD Thesis |
Research Program: |
Photovoltaik |
Series Title: |
Schriften des Forschungszentrums Jülich. Reihe Energietechnik / Energy Technology
41 |
Subject (ZB): | |
Link: |
OpenAccess |
Publikationsportal JuSER |
A systematic investigation on photoluminescence (PL) properties of microcrystalline silicon ($\mu$c-Si :H) films with structural composition changing from highly crystalline to predominantly amorphous is presented. The samples were prepared by PECVD and HWCVD with different silane concentration in hydrogen (SC). By using photoluminescence in combination with Raman spectroscopy the relationship between electronic properties and the microstructure of the material is studied. The PL spectra of gc-Si :H reveal a rather broad ($\thicksim$0.13eV) featureless band at about 1 eV (`$\mu$c'-Si-band) . In mixed phase material of crystalline and amorphous regions, a band at about 1.3eV with halfwidth of about 0.3eV is found in addition to `$\mu$c'-Si-band, which is attributed to the amorphous phase (`a'-Si-band). Similarly to amorphous silicon, the `$\mu$c'-Si-band is assigned to recombination between electrons and holes in band tail states. An additional PL band centred at about 0.7eV with halfwidth slightly broader than the `$\mu$c'-Si-band is observed only for films prepared at high substrate temperature and it is preliminarily assigned to defect-related transitions as in polycrystalline silicon. With decreasing crystalline volume fraction, the `$\mu$c'-Si-band shifts continuously to higher energies for all $\mu$c-Si :H films but the linewidth of the PL spectra is almost unaffected. This is valid for all deposition conditions investigated. The results are interpreted, assuming decease of the density of band tail states with decreasing crystalline volume fraction. The reason for the band tails and their reduction is not clear but strain might play a critical role and hydrogen or hydrogenated amorphous silicon might be effective for strain reduction. By applying the `carrier thermalization model' developed for a-Si:H the slope, E$_{o}$, of the conduction band tail states are derived from temperature dependence of the PL intensity quenching (E$_{o}$ $\approx$ 0.035eV) and from the shift of the PL peak energy (E $\approx$ 0.022eV). The reason for this discrepancy is not clear yet, but the simple model assuming that only one type of carriers are involved in the process of thermal excitation might be too simple for ~$\mu$c-Si :H. By using a new technique, namely voltage modulated PL on solar cells, information on the carrier distributions is obtained. It relates the splitting of the quasi-Fermi-levels of electrons and holes and the respective excess carrier distributions via open circuit voltage V$_{oc}$ and the PL energy. This relationship is studied as a function of SC, temperature and optical generation rate go. An increase of the PL energy and V$_{oc}$ is found for (i) increasing SC, (ii) increasing go and (iii) decreasing temperature. It is suggested that the reason in all cases is the shift of the distribution of electrons and holes to higher energies. We propose that with increasing SC, the density of band tail states is reduced and the carrier distributions shift to higher energies as a result of an increasing generation rate and increasing carrier lifetime with decreasing temperature, respectively. The shift of quasi-Fermi levels to higher energies is always accompanied by a weak shift of carrier distributions in the band tails. It is concluded that the maximum achievable V_ and the PL peak energy are determined by the band tail states. At low temperature, a strongly reduced carrier extraction is observed, which indicates reduced a drift or a diffusion length. Multiple trapping processes in the band tail states are suggested as the reason for this. A simple model is proposed to simulate PL spectra and V$_{oc}$ in $\mu$c-Si:H solar cells as a function of temperature, based on carrier distributions in quasi-equilibrium conditions. In the model is assumed symmetric density of states distributions for electrons and holes in the conduction and the valence band tail states. The best agreement between the model calculations and experimental results for two solar cells with different structural properties was obtained by using a E$_{o}$ $\approx$ 0.03eV for the slope of both exponential band tail states, which fits reasonably well with E$_{o}$ $\approx$ 0.035eV from the temperature dependence of the luminescence intensity. |