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Photoluminescence energy and open circuit voltage in microcrystalline silicon solar cells
Thin Solid Films 451 – 452 (2004) 285–289
Photoluminescence energy and open circuit voltage in microcrystalline silicon solar cells T. Merdzhanovaa,b,*, R. Cariusa, S. Kleina, F. Fingera, D. Dimova-Malinovskab b
a ¨ Photovoltaik, Forschungszentrum Julich ¨ ¨ Institut fur GmbH, 52425 Julich, Germany Central Laboratory for Solar Energy and New Energy Sources, Bulgarian Academy of Science, 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria
Abstract The interrelation of photoluminescence (PL) properties and open circuit voltage (Voc ) of thin film p-i-n microcrystalline silicon (mc-Si:H) solar cells with efficiencies above 9% was studied. Hot wire chemical vapour deposition (HW-CVD) technique was used for the preparation of intrinsic layer at various silane concentrations (SC). The effect of the tail states on the splitting of the quasi-Fermi energies and carrier distributions was studied by monitoring Voc and PL energy as a function of temperature and optical generation rate (go ). An increase of the PL energy and Voc is observed for (i) increasing SC, (ii) increasing go and (iii) decreasing temperature. It is also found that (i) Voc and PL energy are limited by the band tail states and (ii) short circuit current density drops at low temperatures. The latter effect is attributed to a reduced carrier extraction due to trapping of the carriers in band tail states and subsequent recombination. It is proposed that increasing SC leads to a reduction of the density of band tail states due to structural relaxation of the mc-Si:H network by the presence of hydrogen or hydrogenated amorphous silicon. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Photoluminescence; Hot-wire CVD; Solar cells; Silicon
1. Introduction Microcrystalline silicon (mc-Si:H) thin films deposited near the transition from crystalline to amorphous growth have attracted a great interest because the best performance mc-Si:H solar cells were obtained by using such material. Previous works on mc-Si:H solar cells with intrinsic (i-) layers prepared by plasma enhanced chemical vapour deposition (PECVD) and HW-CVD have shown increasing open circuit voltage (Voc) with increasing silane concentration (SC) w1,2x. It was also found that the PL band, which is located below the band gap of crystalline silicon, shifts to higher energies with increasing SC, i.e. with decreasing crystalline volume fraction w3,4x. This PL band was attributed either to radiative recombination at defects in the crystalline phase or to transitions between states in the grain boundary region w5–7x. Here, we assume that the PL originates from transitions between localized band tail *Corresponding author. Tel.: q49-2461-61-2851; fax: q49-246161-3735. E-mail address: [email protected] (T. Merdzhanova).
states similar to those in a-Si:H w8,9x. In this paper, we provide a systematic study of the influence of the SC on the PL properties and Voc of very high quality mcSi:H solar cells in order to get insight into the mechanisms that limit Voc in these solar cells. The emphasis is on the relationship between Voc, i.e. the splitting of the quasi-Fermi energies and PL peak energy, i.e. the carrier distributions in the band tails as a function of temperature and optical generation rates. 2. Experiment Photoluminescence (PL) spectra were measured using a modulation technique in order to eliminate spurious PL signals from the glass, the transparent conductive oxide (TCO) and the p-layer. We applied an external voltage to the solar cells using an HP 8116A pulse generator. Upon the change of the applied voltage from 0 V to Voc a change of the PL signal was detected with lock-in technique. The short circuit current (Isc) was monitored with a digital oscilloscope (Le Croy 9400). The contact area was typically 1 mm2. The maximum PL signal is obtained at Voc where full carrier recombi-
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.11.052
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T. Merdzhanova et al. / Thin Solid Films 451 – 452 (2004) 285–289
Fig. 1. Modulated PL spectra of solar cells with i-layers prepared by HW-CVD with different SC measured at 150 K. Note the logarithmic intensity scale and tentatively corrected interference fringes for solar cells with 4% and 5% SC.
nation and no extraction in the i-layer are expected. The minimum PL signal is obtained at Isc – full carrier extraction and no recombination. By using this technique only relevant carriers are probed. For the measurements the samples were mounted in a continuous flow helium cryostat, providing temperatures from 10–300 K and excited by 632.8 nm line of a He–Ne laser with a maximum power density of 5.9 Wycm2. The PL spectra were analysed with an FTIR spectrometer (Bruker FS66v) in step scan mode and detected with LN2-cooled Ge detector. A Keithley 2400 source meter was used for the Voc and Isc measurements. The optical generation rate was varied by three orders of magnitude by neutral density glass filters. The investigated mc-Si:H solar cells were prepared at a substrate temperature of Tss185 8C in p-i-n deposition sequence on textured ZnO substrates with a ZnOyAg back reflector. The intrinsic layer was deposited by HW-CVD under a variation of the dilution of the process gas silane in hydrogen (SCswSiH4 x y wSiH4qH2x) at a deposition pressure (Pd) of 3.5–5 Pa. The i-layer was between 0.85 and 1.3 mm thick. Details of the deposition parameters are published elsewhere w2,10x. 3. Results and discussion 3.1. Silane concentration In Fig. 1, modulated PL spectra obtained at 150 K for solar cells with different silane concentration SC (3–5.6%) are shown. For the lowest SC, the PL spectra reveal a broad emission band with the maximum at about 0.86 eV (mc-Si-band) and a half-width of about 0.14 eV, attributed to the microcrystalline phase. To
demonstrate the shift of the mc-Si-band towards higher energies with increasing SC (up to 0.91 eV), and the absence of any contribution at 1.35 eV originating from an amorphous phase, a logarithmic intensity scale was chosen. A good correlation between the PL peak energy and microstructure, i.e. a decrease of PL energy with increasing crystalline volume fraction is found in agreement with the findings for undoped mc-Si:H thin films w3x. Interference fringes modify the shape of the PL spectra for solar cells with SC between 4% and 5.2%. The indicated open circles and dashed line refer to tentatively corrected spectra. The quantum efficiency of the PL and the width of the PL band are very similar for all solar cells. We explain the above observations as follows: with increasing SC, the PL band (mc-band) shifts to higher energy whereas the PL intensity and the width are hardly affected. As the PL energy is well below the band gap of c-Si and various experiments indicate localised states below the gap the increase of the PL peak energy is attributed to reduction of the density of band tail states. This results in a shift of the carrier distributions to higher energy. Strain and interface states are possible sources for these band tail states. We propose that the reduction of the density of band tail states is due to structural relaxation of the mc-Si:H network by the incorporation of hydrogen or amorphous hydrogenated silicon. Fig. 2a,b illustrate the dependences of open circuit voltage Voc and PL energy (Epl) on the SC at different temperatures. Increasing SC from 3% to 5.6% causes a continuous increase of Voc. Such an increase of Voc with increasing SC is observed, irrespective of the preparation method w1,2x. Here, we obtain a higher Voc at 300 K compared to the values measured under AM1.5 illumination w2x due to the high optical generation rate (1.2=1022 cmy3 sy1) used. An increase of Voc of approximately 90 mV upon change of SC from 3% to 5.6% is found for all temperatures (125–300 K). A similar but weaker shift is observed for the PL peak energy, i.e. an increase of ;50 meV with increasing SC, for all temperatures (see Fig. 2b). 3.2. Temperature dependence Fig. 3 shows the effect of the temperature (T) on the short circuit current density ( jsc) for the investigated series of solar cells. The sample with 5.2% SC has been omitted for clarity. The higher value for the current density of approximately 100 mAycm2 than usually measured at AM1.5 illumination results from the high generation rate used in the present study. At high temperatures ()225 K) jsc is almost constant for all devices indicating efficient carrier extraction. A decreasing current density with decreasing temperature is observed below 125 K for the solar cells with SCF5% but for the solar cell with SCs5.6% it starts to decrease
T. Merdzhanova et al. / Thin Solid Films 451 – 452 (2004) 285–289
at ;200 K and decreases gradually to 5 mAycm2 at 10 K. The sample with 3% SC shows a sharper drop of the current density and could not be measured at temperatures lower than 60 K. It is tempting to identify the carrier transport as the limiting factor at low temperatures. The strongly reduced carrier extraction at low T seems to be linked to the observed limit of the continuous increase of the Voc shown in Fig. 4a. In this figure a systematic study of the Voc as a function of the temperature for the same sample series as in Fig. 3 is shown. With decreasing temperature, first a strong increase of the Voc is found, with a weaker increase towards lower T. For the solar cell with SCs3% the Voc decreases at approximately 150 K. For the highest SC (5.6%) Voc shows the highest value (;961 mV) at 10 K and bends over at lowest temperatures. We explain the limitation of Voc and the strongly reduced carrier extraction at low temperatures by the existence of band tail states, which limit the shift of the quasi-Fermi energies towards higher energies and leads to a short diffusion or drift length of the carriers. In Fig. 4b, the energy of the PL band is presented as a function of the temperature. At high temperature the PL energies are much larger than Voc while the shift is much weaker compared to the shift of Voc. This can be interpreted as follows: we distinguish between the probability for the occupation of states in the conduction and valence band tail, respectively, given by quasi-Fermi energies and the transition (recombination) of carriers between ‘real states’. The difference between quasiFermi energies for the electron and holes determines Voc and the recombination between the two carrier distributions determines the photoluminescence transitions. Therefore the difference in the increase of the Epl and Voc with decreasing temperature is assigned to the difference of the shift of the carrier distributions in
287
Fig. 3. Influence of the measurement temperature on the short circuit current density jsc obtained at maximum optical generation rate go (1.2=1022 cmy3 sy1) for the solar cells with 3%-SC-5.6%.
the band tails and the shift of the quasi-Fermi levels to higher energies. At low temperatures the quasi-Fermi energies are close to the band edges, i.e. close to or within the carrier distributions and the difference between Epl and Voc is small. 3.3. Optical generation rate The optical generation rate go (cmy3 sy1), i.e. the number of the generated electron-hole pairs per unit volume, was calculated by taking into account the optical absorption coefficient ao (1.4=104 cmy1) at 632.8 nm. The temperature dependence of ao has been neglected for simplicity. Fig. 5 shows the influence of
Fig. 2. Dependence of the open circuit voltage Voc (a) and photoluminescence peak energy Epl (b) on the silane concentration at indicated temperatures for investigated series of solar cells. Note the same energy scale for Voc and Epl.
T. Merdzhanova et al. / Thin Solid Films 451 – 452 (2004) 285–289
288
the optical generation rate on the Voc over three orders of magnitude on a logarithmic scale in a wide temperature range (10–300 K). We present the results for the solar cell with 5.6% SC as an example since the others solar cells studied exhibited a similar behaviour. This material contains a high amorphous volume fraction and shows reversible light induced degradation w10x. The arrow in Fig. 5 corresponds to the observed recovering process after annealing at 160 8C for 1 h. At room temperature, a logarithmic dependence of the Voc on go, i.e., (DVoc;ln go) was found for high optical generation rates. This is expected, according to Eq. (1) if the short circuit current density jsc, which is proportional to go, is much larger than the dark saturation current density jo.
Vocs
E kT B jsc ØlnC q1F D jo G e
(1)
With decreasing temperature (down to 100 K) the logarithmic dependence of the Voc on go is maintained and only the slope changes. A deviation from the trend, i.e. an enhancement of the Voc by 30 mV for a change of the go-rate by two orders of magnitude and subsequent slight decrease was observed at 10 K. A deviation from the logarithmic dependence of the Voc on the go was found also at low generation rate (1.5=1019 cmy3 sy1) for all temperatures. This indicates limitations by the jo and recombination via defects. The influence of jo can be reduced with an increase of the photo generated charge carriers or a decrease of the temperature, respectively. In contrast to the behaviour of Voc, the PL energy is found to be proportional to the logarithm of the generation rate (Epl;log go) for all temperatures investigated (between 100 and 175 K, data not shown).
Fig. 5. Open circuit voltage as a function of the logarithm of optical generation rate and temperature for the solar cell with SCs5.6%.
4. Summary In summary, we have studied the relationship between PL energy and open circuit voltage Voc in mc-Si:H solar cells. A systematic study of the temperature dependencies of Voc, PL energy and jsc revealed: (i) with increasing temperature a strong decrease of Voc due to continues shift of the quasi-Fermi energies to lower energies and a weak decrease of PL energy due to weak change of the carrier distributions in the band tails; (ii) a low temperature limit for the increase of the Voc and PL energy possibly related to a drop of current density. Trapping in band tail states is suggested as a possible reason for short diffusion or drift length leading to the observed jsc behaviour. The increase of the PL energy
Fig. 4. Open circuit voltage Voc (a) and photoluminescence peak energy Epl (b) as a function of the temperature for the same devices as in Fig. 3. Note the same energy scale for Voc and Epl.
T. Merdzhanova et al. / Thin Solid Films 451 – 452 (2004) 285–289
and Voc for increasing SC and generation rate is attributed to the shift of the carrier distributions and thus the quasi-Fermi levels to higher energies. We suggest that the density of band tail states is reduced by improved lattice relaxation because of presence of hydrogen or hydrogenated amorphous silicon. Acknowledgments ¨ The authors especially thank Markus Hulsbeck and Josef Klomfass for the contributions to this publication. References w1x O. Vetterl, R. Carius, L. Houben, C. Scholten, M. Luysberg, A. Lambertz, F Finger, H. Wagner, Mater. Res. Soc. Symp. Proc. 609 (2000) A15.2.
289
w2x S. Klein, F. Finger, R. Carius, B. Rech, L. Houben, M. Luysberg, M. Stutzmann, Mater. Res. Soc. Symp. Proc. 715 (2002) A26.2. w3x R. Carius, T. Merdzhanova, F. Finger, S. Klein, O. Vetterl, J. Mater. Sci.-Mater. Electron., Kluwer Academic Publishers, 2003 in print. w4x D. Han, G. Yue, J.D. Lorentzen, J. Lin, H. Habuchi, Q. Wang, J. Appl. Phys. 87 (2000) 1882. w5x P.K. Bhat, G. Diprose, T.M. Searle, I.G. Austin, P.G. LeComber, W.E. Spear, Physica B 117–118 (1983) 917. w6x A.U. Savchouk, S. Ostapenko, G. Nowak, J. Lagowski, L. Jastrzebski, Appl. Phys. Lett. 67 (1995) 82. w7x G. Yue, J.D. Lorentzen, J. Lin, D. Han, Q. Wang, Appl. Phys. Lett. 75 (1999) 492. w8x R.A. Street, Adv. Phys. 30 (1981) 593. w9x R. Carius, T. Merdzhanova, F. Finger, Mater. Res. Soc. Symp. Proc. 762 (2003) A4.2. w10x S. Klein, F. Finger, R. Carius, T. Dylla, B. Rech, M. Grimm, L. Houben, M. Stutzmann, Thin Solid Films 430 (2003) 202.
Photoluminescence energy and open circuit voltage in microcrystalline silicon solar cells T. Merdzhanovaa,b,*, R. Cariusa, S. Kleina, F. Fingera, D. Dimova-Malinovskab b
a ¨ Photovoltaik, Forschungszentrum Julich ¨ ¨ Institut fur GmbH, 52425 Julich, Germany Central Laboratory for Solar Energy and New Energy Sources, Bulgarian Academy of Science, 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria
Abstract The interrelation of photoluminescence (PL) properties and open circuit voltage (Voc ) of thin film p-i-n microcrystalline silicon (mc-Si:H) solar cells with efficiencies above 9% was studied. Hot wire chemical vapour deposition (HW-CVD) technique was used for the preparation of intrinsic layer at various silane concentrations (SC). The effect of the tail states on the splitting of the quasi-Fermi energies and carrier distributions was studied by monitoring Voc and PL energy as a function of temperature and optical generation rate (go ). An increase of the PL energy and Voc is observed for (i) increasing SC, (ii) increasing go and (iii) decreasing temperature. It is also found that (i) Voc and PL energy are limited by the band tail states and (ii) short circuit current density drops at low temperatures. The latter effect is attributed to a reduced carrier extraction due to trapping of the carriers in band tail states and subsequent recombination. It is proposed that increasing SC leads to a reduction of the density of band tail states due to structural relaxation of the mc-Si:H network by the presence of hydrogen or hydrogenated amorphous silicon. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Photoluminescence; Hot-wire CVD; Solar cells; Silicon
1. Introduction Microcrystalline silicon (mc-Si:H) thin films deposited near the transition from crystalline to amorphous growth have attracted a great interest because the best performance mc-Si:H solar cells were obtained by using such material. Previous works on mc-Si:H solar cells with intrinsic (i-) layers prepared by plasma enhanced chemical vapour deposition (PECVD) and HW-CVD have shown increasing open circuit voltage (Voc) with increasing silane concentration (SC) w1,2x. It was also found that the PL band, which is located below the band gap of crystalline silicon, shifts to higher energies with increasing SC, i.e. with decreasing crystalline volume fraction w3,4x. This PL band was attributed either to radiative recombination at defects in the crystalline phase or to transitions between states in the grain boundary region w5–7x. Here, we assume that the PL originates from transitions between localized band tail *Corresponding author. Tel.: q49-2461-61-2851; fax: q49-246161-3735. E-mail address: [email protected] (T. Merdzhanova).
states similar to those in a-Si:H w8,9x. In this paper, we provide a systematic study of the influence of the SC on the PL properties and Voc of very high quality mcSi:H solar cells in order to get insight into the mechanisms that limit Voc in these solar cells. The emphasis is on the relationship between Voc, i.e. the splitting of the quasi-Fermi energies and PL peak energy, i.e. the carrier distributions in the band tails as a function of temperature and optical generation rates. 2. Experiment Photoluminescence (PL) spectra were measured using a modulation technique in order to eliminate spurious PL signals from the glass, the transparent conductive oxide (TCO) and the p-layer. We applied an external voltage to the solar cells using an HP 8116A pulse generator. Upon the change of the applied voltage from 0 V to Voc a change of the PL signal was detected with lock-in technique. The short circuit current (Isc) was monitored with a digital oscilloscope (Le Croy 9400). The contact area was typically 1 mm2. The maximum PL signal is obtained at Voc where full carrier recombi-
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.11.052
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T. Merdzhanova et al. / Thin Solid Films 451 – 452 (2004) 285–289
Fig. 1. Modulated PL spectra of solar cells with i-layers prepared by HW-CVD with different SC measured at 150 K. Note the logarithmic intensity scale and tentatively corrected interference fringes for solar cells with 4% and 5% SC.
nation and no extraction in the i-layer are expected. The minimum PL signal is obtained at Isc – full carrier extraction and no recombination. By using this technique only relevant carriers are probed. For the measurements the samples were mounted in a continuous flow helium cryostat, providing temperatures from 10–300 K and excited by 632.8 nm line of a He–Ne laser with a maximum power density of 5.9 Wycm2. The PL spectra were analysed with an FTIR spectrometer (Bruker FS66v) in step scan mode and detected with LN2-cooled Ge detector. A Keithley 2400 source meter was used for the Voc and Isc measurements. The optical generation rate was varied by three orders of magnitude by neutral density glass filters. The investigated mc-Si:H solar cells were prepared at a substrate temperature of Tss185 8C in p-i-n deposition sequence on textured ZnO substrates with a ZnOyAg back reflector. The intrinsic layer was deposited by HW-CVD under a variation of the dilution of the process gas silane in hydrogen (SCswSiH4 x y wSiH4qH2x) at a deposition pressure (Pd) of 3.5–5 Pa. The i-layer was between 0.85 and 1.3 mm thick. Details of the deposition parameters are published elsewhere w2,10x. 3. Results and discussion 3.1. Silane concentration In Fig. 1, modulated PL spectra obtained at 150 K for solar cells with different silane concentration SC (3–5.6%) are shown. For the lowest SC, the PL spectra reveal a broad emission band with the maximum at about 0.86 eV (mc-Si-band) and a half-width of about 0.14 eV, attributed to the microcrystalline phase. To
demonstrate the shift of the mc-Si-band towards higher energies with increasing SC (up to 0.91 eV), and the absence of any contribution at 1.35 eV originating from an amorphous phase, a logarithmic intensity scale was chosen. A good correlation between the PL peak energy and microstructure, i.e. a decrease of PL energy with increasing crystalline volume fraction is found in agreement with the findings for undoped mc-Si:H thin films w3x. Interference fringes modify the shape of the PL spectra for solar cells with SC between 4% and 5.2%. The indicated open circles and dashed line refer to tentatively corrected spectra. The quantum efficiency of the PL and the width of the PL band are very similar for all solar cells. We explain the above observations as follows: with increasing SC, the PL band (mc-band) shifts to higher energy whereas the PL intensity and the width are hardly affected. As the PL energy is well below the band gap of c-Si and various experiments indicate localised states below the gap the increase of the PL peak energy is attributed to reduction of the density of band tail states. This results in a shift of the carrier distributions to higher energy. Strain and interface states are possible sources for these band tail states. We propose that the reduction of the density of band tail states is due to structural relaxation of the mc-Si:H network by the incorporation of hydrogen or amorphous hydrogenated silicon. Fig. 2a,b illustrate the dependences of open circuit voltage Voc and PL energy (Epl) on the SC at different temperatures. Increasing SC from 3% to 5.6% causes a continuous increase of Voc. Such an increase of Voc with increasing SC is observed, irrespective of the preparation method w1,2x. Here, we obtain a higher Voc at 300 K compared to the values measured under AM1.5 illumination w2x due to the high optical generation rate (1.2=1022 cmy3 sy1) used. An increase of Voc of approximately 90 mV upon change of SC from 3% to 5.6% is found for all temperatures (125–300 K). A similar but weaker shift is observed for the PL peak energy, i.e. an increase of ;50 meV with increasing SC, for all temperatures (see Fig. 2b). 3.2. Temperature dependence Fig. 3 shows the effect of the temperature (T) on the short circuit current density ( jsc) for the investigated series of solar cells. The sample with 5.2% SC has been omitted for clarity. The higher value for the current density of approximately 100 mAycm2 than usually measured at AM1.5 illumination results from the high generation rate used in the present study. At high temperatures ()225 K) jsc is almost constant for all devices indicating efficient carrier extraction. A decreasing current density with decreasing temperature is observed below 125 K for the solar cells with SCF5% but for the solar cell with SCs5.6% it starts to decrease
T. Merdzhanova et al. / Thin Solid Films 451 – 452 (2004) 285–289
at ;200 K and decreases gradually to 5 mAycm2 at 10 K. The sample with 3% SC shows a sharper drop of the current density and could not be measured at temperatures lower than 60 K. It is tempting to identify the carrier transport as the limiting factor at low temperatures. The strongly reduced carrier extraction at low T seems to be linked to the observed limit of the continuous increase of the Voc shown in Fig. 4a. In this figure a systematic study of the Voc as a function of the temperature for the same sample series as in Fig. 3 is shown. With decreasing temperature, first a strong increase of the Voc is found, with a weaker increase towards lower T. For the solar cell with SCs3% the Voc decreases at approximately 150 K. For the highest SC (5.6%) Voc shows the highest value (;961 mV) at 10 K and bends over at lowest temperatures. We explain the limitation of Voc and the strongly reduced carrier extraction at low temperatures by the existence of band tail states, which limit the shift of the quasi-Fermi energies towards higher energies and leads to a short diffusion or drift length of the carriers. In Fig. 4b, the energy of the PL band is presented as a function of the temperature. At high temperature the PL energies are much larger than Voc while the shift is much weaker compared to the shift of Voc. This can be interpreted as follows: we distinguish between the probability for the occupation of states in the conduction and valence band tail, respectively, given by quasi-Fermi energies and the transition (recombination) of carriers between ‘real states’. The difference between quasiFermi energies for the electron and holes determines Voc and the recombination between the two carrier distributions determines the photoluminescence transitions. Therefore the difference in the increase of the Epl and Voc with decreasing temperature is assigned to the difference of the shift of the carrier distributions in
287
Fig. 3. Influence of the measurement temperature on the short circuit current density jsc obtained at maximum optical generation rate go (1.2=1022 cmy3 sy1) for the solar cells with 3%-SC-5.6%.
the band tails and the shift of the quasi-Fermi levels to higher energies. At low temperatures the quasi-Fermi energies are close to the band edges, i.e. close to or within the carrier distributions and the difference between Epl and Voc is small. 3.3. Optical generation rate The optical generation rate go (cmy3 sy1), i.e. the number of the generated electron-hole pairs per unit volume, was calculated by taking into account the optical absorption coefficient ao (1.4=104 cmy1) at 632.8 nm. The temperature dependence of ao has been neglected for simplicity. Fig. 5 shows the influence of
Fig. 2. Dependence of the open circuit voltage Voc (a) and photoluminescence peak energy Epl (b) on the silane concentration at indicated temperatures for investigated series of solar cells. Note the same energy scale for Voc and Epl.
T. Merdzhanova et al. / Thin Solid Films 451 – 452 (2004) 285–289
288
the optical generation rate on the Voc over three orders of magnitude on a logarithmic scale in a wide temperature range (10–300 K). We present the results for the solar cell with 5.6% SC as an example since the others solar cells studied exhibited a similar behaviour. This material contains a high amorphous volume fraction and shows reversible light induced degradation w10x. The arrow in Fig. 5 corresponds to the observed recovering process after annealing at 160 8C for 1 h. At room temperature, a logarithmic dependence of the Voc on go, i.e., (DVoc;ln go) was found for high optical generation rates. This is expected, according to Eq. (1) if the short circuit current density jsc, which is proportional to go, is much larger than the dark saturation current density jo.
Vocs
E kT B jsc ØlnC q1F D jo G e
(1)
With decreasing temperature (down to 100 K) the logarithmic dependence of the Voc on go is maintained and only the slope changes. A deviation from the trend, i.e. an enhancement of the Voc by 30 mV for a change of the go-rate by two orders of magnitude and subsequent slight decrease was observed at 10 K. A deviation from the logarithmic dependence of the Voc on the go was found also at low generation rate (1.5=1019 cmy3 sy1) for all temperatures. This indicates limitations by the jo and recombination via defects. The influence of jo can be reduced with an increase of the photo generated charge carriers or a decrease of the temperature, respectively. In contrast to the behaviour of Voc, the PL energy is found to be proportional to the logarithm of the generation rate (Epl;log go) for all temperatures investigated (between 100 and 175 K, data not shown).
Fig. 5. Open circuit voltage as a function of the logarithm of optical generation rate and temperature for the solar cell with SCs5.6%.
4. Summary In summary, we have studied the relationship between PL energy and open circuit voltage Voc in mc-Si:H solar cells. A systematic study of the temperature dependencies of Voc, PL energy and jsc revealed: (i) with increasing temperature a strong decrease of Voc due to continues shift of the quasi-Fermi energies to lower energies and a weak decrease of PL energy due to weak change of the carrier distributions in the band tails; (ii) a low temperature limit for the increase of the Voc and PL energy possibly related to a drop of current density. Trapping in band tail states is suggested as a possible reason for short diffusion or drift length leading to the observed jsc behaviour. The increase of the PL energy
Fig. 4. Open circuit voltage Voc (a) and photoluminescence peak energy Epl (b) as a function of the temperature for the same devices as in Fig. 3. Note the same energy scale for Voc and Epl.
T. Merdzhanova et al. / Thin Solid Films 451 – 452 (2004) 285–289
and Voc for increasing SC and generation rate is attributed to the shift of the carrier distributions and thus the quasi-Fermi levels to higher energies. We suggest that the density of band tail states is reduced by improved lattice relaxation because of presence of hydrogen or hydrogenated amorphous silicon. Acknowledgments ¨ The authors especially thank Markus Hulsbeck and Josef Klomfass for the contributions to this publication. References w1x O. Vetterl, R. Carius, L. Houben, C. Scholten, M. Luysberg, A. Lambertz, F Finger, H. Wagner, Mater. Res. Soc. Symp. Proc. 609 (2000) A15.2.
289
w2x S. Klein, F. Finger, R. Carius, B. Rech, L. Houben, M. Luysberg, M. Stutzmann, Mater. Res. Soc. Symp. Proc. 715 (2002) A26.2. w3x R. Carius, T. Merdzhanova, F. Finger, S. Klein, O. Vetterl, J. Mater. Sci.-Mater. Electron., Kluwer Academic Publishers, 2003 in print. w4x D. Han, G. Yue, J.D. Lorentzen, J. Lin, H. Habuchi, Q. Wang, J. Appl. Phys. 87 (2000) 1882. w5x P.K. Bhat, G. Diprose, T.M. Searle, I.G. Austin, P.G. LeComber, W.E. Spear, Physica B 117–118 (1983) 917. w6x A.U. Savchouk, S. Ostapenko, G. Nowak, J. Lagowski, L. Jastrzebski, Appl. Phys. Lett. 67 (1995) 82. w7x G. Yue, J.D. Lorentzen, J. Lin, D. Han, Q. Wang, Appl. Phys. Lett. 75 (1999) 492. w8x R.A. Street, Adv. Phys. 30 (1981) 593. w9x R. Carius, T. Merdzhanova, F. Finger, Mater. Res. Soc. Symp. Proc. 762 (2003) A4.2. w10x S. Klein, F. Finger, R. Carius, T. Dylla, B. Rech, M. Grimm, L. Houben, M. Stutzmann, Thin Solid Films 430 (2003) 202.