Numerical simulation of the effect of the free carrier motilities on light-soaked a-Si:H p-i-n solar cell

L. Ayat; A. F.  Bouhdjar; AF. Meftah; N. Sengouga

doi:10.1088/1674-4926/36/7/074002

J. Semicond. > 2015, Volume 36 > Issue 7 > 074002, doi: 10.1088/1674-4926/36/7/074002

SEMICONDUCTOR DEVICES

Numerical simulation of the effect of the free carrier motilities on light-soaked a-Si:H p-i-n solar cell

L. Ayat¹, A. F. Bouhdjar², AF. Meftah² and N. Sengouga²

+ Author Affiliations

Corresponding author: L. Ayat, E-mail: AF. Meftah: af_mef@yahoo.fr

Abstract: Using a previous model, which was developed to describe the light-induced creation of the defect density in the a-Si:H gap states, we present in this work a computer simulation of the a-Si:H p-i-n solar cell behavior under continuous illumination. We have considered the simple case of a monochromatic light beam nonuniformly absorbed. As a consequence of this light-absorption profile, the increase of the dangling bond density is assumed to be inhomogeneous over the intrinsic layer (i-layer). We investigate the internal variable profiles during illumination to understand in more detail the changes resulting from the light-induced degradation effect. Changes in the cell external parameters including the open circuit voltage, V_oc, the short circuit current density, J_sc, the fill factor, FF, and the maximum power density, P_max, are also presented. This shows, in addition, the free carrier mobility influence. The obtained results show that V_oc seems to be the less affected parameter by the light-induced increase of the dangling bond density. Moreover, its degradation is very weak-sensitive to the free carrier mobility. Finally, the free hole mobility effect is found to be more important than that of electrons in the improvement of the solar cell performance.

Key words: a-Si:H, Staebler-Wronski effect, defect pool model, p-i-n

References

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Fig. 1. Cross section of (a) SOI CMOS NFET and (b) standard CMOS NFET.

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Fig. 2. Light-induced increase of the defect state density in the gap of the a-Si:H.

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Fig. 3. 3D variation of the defect state density.

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Fig. 4. Dangling bond concentration profiles.

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Fig. 5. $n$ and $p$ density profiles, under short circuit condition.

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Fig. 6. $n \cdot p$ product profile, under short circuit condition.

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Fig. 7. (a) Recombination rate profiles for tail bands, $U_{\rm c}$, $U_{\rm v}$ and dangling bonds $U_{\rm db}^+$, $U_{\rm db}^0$, in annealed state. (b) Trapped charge density profiles in band tails, $p_{\rm t}$, $n_{\rm t}$, and dangling bonds $D^+$, $D^0$, $D^-$, in annealed state.

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Fig. 8. (a) Recombination rate profiles for tail bands, $U_{\rm c}$, $U_{\rm v}$ and dangling bonds $U_{\rm db}^+ $, $U_{\rm db}^0$, in degraded state. (b) Trapped charge density profiles in band tails, $p_{\rm t}$, $n_{\rm t}$, and dangling bonds $D^+$, $D^0$, $D^-$, in degraded state.

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Fig. 9. Gap structure under short circuit condition in (a) annealed state and (b) degraded state.

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Fig. 10. Electric potential profile, under short circuit condition.

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Fig. 11. Electric field profile, under short circuit condition.

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Fig. 12. Space charge profile, under short circuit condition.

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Fig. 13. Electron and hole current density profiles.

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Fig. 14. (a) Drift- and diffusion-electron current density profiles in annealed state. (b) Drift- and diffusion-hole current density profiles in annealed state. (c) Drift- and diffusion-electron current density profiles in degraded state. (d) Drift- and diffusion-hole current density profiles in degraded state.

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Fig. 15. Electron and hole current density profiles.

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Fig. 16. $P$-$V$ characteristic degradation.

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Fig. 17. Cell photo-parameters versus $N_{\rm d}(t)/N_{\rm d}(0)$ ratio. (a) $J_{\rm sc}$, (b) $V_{\rm oc}$, (c) FF, and (d) $P_{\rm max}$.

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Fig. 18. Influence of free electron and hole mobilities on degradation of (a) $J_{\rm sc}$, (b) $V_{\rm oc}$, (c) FF, and (d) $P_{\rm max}$.

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Table 1. Density of states parameters.

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Table 2. Simulation parameters of the a-Si:H p-i-n cell. $C_{\rm nt}$, $C_{\rm ct} $, $C_{\rm nd}$ and $C_{\rm cd} $, are the capture coefficients of the neutral and charged states in band tails and dangling bonds, respectively. $N_{\rm a}$ and $N_{\rm d}$ are, respectively, the p- and n-layer doping densities.

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Received: 15 July 2014 Revised: Online: Published: 01 July 2015

Using a previous model, which was developed to describe the light-induced creation of the defect density in the a-Si:H gap states, we present in this work a computer simulation of the a-Si:H p-i-n solar cell behavior under continuous illumination. We have considered the simple case of a monochromatic light beam nonuniformly absorbed. As a consequence of this light-absorption profile, the increase of the dangling bond density is assumed to be inhomogeneous over the intrinsic layer (i-layer). We investigate the internal variable profiles during illumination to understand in more detail the changes resulting from the light-induced degradation effect. Changes in the cell external parameters including the open circuit voltage, V_oc, the short circuit current density, J_sc, the fill factor, FF, and the maximum power density, P_max, are also presented. This shows, in addition, the free carrier mobility influence. The obtained results show that V_oc seems to be the less affected parameter by the light-induced increase of the dangling bond density. Moreover, its degradation is very weak-sensitive to the free carrier mobility. Finally, the free hole mobility effect is found to be more important than that of electrons in the improvement of the solar cell performance.

Numerical simulation of the effect of the free carrier motilities on light-soaked a-Si:H p-i-n solar cell

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