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Optimization of protocrystalline silicon p-type layers for amorphous silicon n–i–p solar cells
Journal of Non-Crystalline Solids 338–340 (2004) 694–697 www.elsevier.com/locate/jnoncrysol
Optimization of protocrystalline silicon p-type layers for amorphous silicon n–i–p solar cells G.M. Ferreira, Chi Chen, R.J. Koval, J.M. Pearce, C.R. Wronski, R.W. Collins
*
Materials Research Laboratory and Department of Physics, Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA
Abstract Real time spectroscopic ellipsometry has been applied to develop deposition phase diagrams for p-type hydrogenated silicon (Si:H) films prepared at low temperature (200 °C) by rf plasma-enhanced chemical vapor deposition using gas mixtures of SiH4 , H2 , and BF3 . These diagrams depict the regimes of accumulated thickness and H2 -dilution ratio R ¼ ½H2 =½SiH4 within which p-type amorphous Si:H [a-Si:H], mixed-phase Si:H [(a + lc)-Si:H], and single-phase microcrystalline Si:H [lc-Si:H] films are obtained in depositions on R ¼ 0 a-Si:H surfaces. The performance of n–i–p solar cells incorporating p-layers deposited under the same conditions as those used in the phase diagram development has been correlated with the deduced p-layer characteristics. This correlation demonstrates that Voc is maximized when the highest possible R value is used for the p-layer while ensuring single-phase amorphous film growth throughout its thickness. Ó 2004 Elsevier B.V. All rights reserved. PACS: 84.60.Jt; 81.05.Gc; 81.15.Gh; 81.40.Tv
1. Introduction Hydrogenated amorphous silicon (a-Si:H) n–i–p solar cells fabricated by rf plasma-enhanced chemical vapor deposition (PECVD) exhibit high open circuit voltages ðVoc Þ when low temperatures (T 50–200 °C) and high hydrogen-to-silane flow ratios (typically R ½H2 = ½SiH4 50–200) are used in the fabrication of the ptype, boron-doped layers [1–3]. It was proposed that such optimum top-contact p-layers are microcrystalline Si:H (lc-Si:H) and that this material yields overall improved cell performance through an increase in the built-in potential of the top junction as well as through decreases in series resistance and optical absorption losses [1]. More recent studies of Si:H film growth employing real time spectroscopic ellipsometry (RTSE) to extract deposition phase diagrams have demonstrated that the properties of films prepared at high R depend sensitively
*
Corresponding author. Tel.: +1-814 863 1880; fax: +1-814 865 2326. E-mail address: [email protected] (R.W. Collins). 0022-3093/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.03.062
on both the substrate and the accumulated thickness [4,5]. Thus, an explanation of the high Voc for n–i–p solar cells in terms of a lc-Si:H p-layer is not compelling unless characterization is performed on materials in the actual device configuration or in structures simulating this configuration [6]. In the present study, the problem of Si:H p-layer optimization has been reconsidered from the viewpoint of the phase diagram for p-layers deposited using similar conditions of substrate, thickness, and R as are used for devices. Ex situ SE results in the device configuration support the validity of the phase diagram interpretation. 2. Experimental details The undoped and p-type doped Si:H layers studied for phase diagram development were deposited on R ¼ 0 a-Si:H using single-chamber rf PECVD and were measured in real time using a rotating-compensator multichannel ellipsometer [7]. The substrate temperature T , rf plasma power P , and SiH4 flow F were set at T ¼ 200 °C, P ¼ 0:8 W/cm2 , and F ¼ 1 stand. cm3 /min, respectively. In addition, the H2 -dilution ratio R, doping gas ratio D, and total pressure ptot were set within
G.M. Ferreira et al. / Journal of Non-Crystalline Solids 338–340 (2004) 694–697
the ranges R 50–200, D ¼ ½BF3 =½SiH4 0–0:2, and ptot 0:3–0:9 Torr, respectively. For these Si:H layers, P was an order of magnitude higher than for the underlying R ¼ 0 a-Si:H films. In previous p-layer deposition studies [8], the underlying R ¼ 0 a-Si:H surface was subjected to a 2 min H2 -plasma treatment before p-layer growth to promote the nucleation of the lc-Si:H phase; however, this step was omitted in the present phase diagram studies. State-of-the-art n–i–p solar cells for correlation studies with the phase diagram were fabricated in a multichamber rf PECVD system on Cr-coated glass held at 200 °C. The i-layers of these cells were prepared to a using R ¼ 10 and thus were amorthickness of 4000 A phous throughout this thickness. The p-layers were using R values within prepared to a thickness of 200 A the range of 50–200, and D ¼ ½BF3 =½SiH4 was set at either 0.1 or 0.2. Other deposition conditions for the players were the same as those used in single-chamber deposition. For one of the n–i–p solar cells, an initial H2 plasma treatment of the underlying R ¼ 10 i-layer was performed prior to p-layer deposition at R ¼ 200 and D ¼ 0:2. As described in previous work, this two-step process led to immediate nucleation and growth of a single-phase lc-Si:H p-layer [8]. Finally, previous studies have found excellent agreement between deposition phase diagrams obtained by RTSE for single-chamber deposition and those obtained by ex situ SE and atomic force microscopy for multichamber deposition, applying similar process parameters in each case [9].
3. Results Fig. 1 shows superimposed deposition phase diagrams deduced by RTSE for undoped ðD ¼ 0Þ and p-type doped ðD ¼ 0:2Þ Si:H on freshly-deposited R ¼ 0 a-Si:H, a structure intended for correlation with n–i–p solar cell performance. In fact, since both R ¼ 0 and 10 layers are single-phase a-Si:H, the use of R ¼ 10 for the ilayer of the solar cell does not invalidate this correlation. The deposition phase diagram for the p-type doped layer with D ¼ 0:2 and 100 6 R 6 200 in Fig. 1 includes both the amorphous-to-(mixed-phase) Si:H transition thickness [designated a ! ða þ lcÞ; filled squares, solid line] and the (mixed-phase)-to-(single-phase) microcrystalline Si:H transition thickness [designated ða þ lcÞ ! lc; (open squares, broken line)]. This diagram shows that for R ¼ 100, the film grows initially as a-Si:H and crystallites begin to nucleate from the amorphous phase after a bulk layer thickness of 900 A. The resulting mixed-phase ða þ lcÞ-Si:H continues to is reached. grow until the final film thickness of 1500 A The open symbol and the upward arrow for R ¼ 100 indicates that the transition to single-phase lc-Si:H must occur for thicknesses greater than the designated 1500
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Fig. 1. Superimposed deposition phase diagrams for undoped (circles) and p-type doped (squares) Si:H layers prepared with D ¼ ½BF3 = ½SiH4 ¼ 0 and 0.2, respectively, on R ¼ 0 a-Si:H substrate films held at 200 °C. The (filled symbols, solid lines) and the (open symbols, dashed lines) identify the a ! ða þ lcÞ and ða þ lcÞ ! lc transitions, respectively. The (up, down) arrows indicate that the transition occurs (above, below) the identified thickness values.
In contrast, at R ¼ 200, the transition from predomA. inantly a-Si:H to mixed-phase ða þ lcÞ-Si:H occurs at an and the transition to accumulated thickness of 20 A, It should be noted single phase lc-Si:H occurs at 200 A. that an initial H2 -plasma treatment of the underlying a mixed phase Si:H film completely eliminates this 200 A region [8]. The deposition phase diagram in Fig. 1 for the undoped Si:H layers (D ¼ 0, but otherwise identical deposition conditions as the D ¼ 0:2 series) prepared with 50 6 R 6 150 includes both a ! ða þ lcÞ (filled circles, solid line) and ða þ lcÞ ! lc (open circles, broken line) transitions. In fact, the downward pointing arrows on all points of the former transition line indicate that mixedphase ða þ lcÞ-Si:H nucleates immediately from the underlying R ¼ 0 a-Si:H for the full range of R. It is clear from the overall results of Fig. 1 that the transition lines for the p-type Si:H with D ¼ 0:2 are shifted to much higher R and thickness compared to those for undoped Si:H. This effect is attributed to B incorporation that strongly suppresses crystallite nucleation [8]. Fig. 2 shows Voc values as a function of the H2 -dilution ratio R (open squares) for a series of a-Si:H n–i–p D ¼ 0:2 p-layers solar cells, incorporating 200 A deposited under the same conditions as those of Fig. 1. Also shown in Fig. 2 is a result obtained applying an initial H2 -plasma treatment of the i-layer prior to deposition of a p-layer with R ¼ 200 and D ¼ 0:2 (cross). Finally, results are also shown for a second series of cells incorporating p-layers prepared with a lower gas phase doping level of D ¼ 0:1 (solid circles).
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Fig. 2. Open-circuit voltages of a-Si:H n–i–p solar cells incorporating thick R ¼ 10 i-layers and 200 A thick p-layers deposited with 4000 A variable R, plotted along the abscissa. Two different gas phase doping levels D ¼ ½BF3 =½SiH4 were employed for the p-layers, D ¼ 0:1 (closed circles) and D ¼ 0:2 (open squares). For the result given by the cross, D ¼ 0:2, and the underlying i-layer was exposed to a H2 plasma for 2 min in order to enhance p-layer microcrystallite nucleation.
A comparison of the D ¼ 0:2 results in Fig. 2 with the p-layers phase diagram of Fig. 1, suggests that the 200 A with R 6 150 are amorphous throughout their thickness. However, because such films would eventually evolve to lc-Si:H, given sufficient thickness, they are classified as ‘protocrystalline’ [4]. For the p-layer with R ¼ 175, Fig. 1 predicts mixed-phase ða þ lcÞ-Si:H within the top 50 of the 200 A p-layer, and for the one with R ¼ 200, A Fig. 1 predicts ða þ lcÞ-Si:H throughout much of its thickness. To verify the connection with the phase diagram, Fig. 3 shows the dielectric functions of the D ¼ 0:2 p-layers prepared with R ¼ 150 and 200 and measured in the actual device configuration using ex situ
Fig. 4. Annealed state J –V characteristics for the optimum a-Si:H n–i– p solar cell of Fig. 2 prepared with an R ¼ 150 protocrystalline Si:H player (closed squares), and for an a-Si:H n–i–p cell with an R ¼ 200 single-phase lc-Si:H p-layer (open circles). In the latter p-layer process, the underlying i-layer was exposed to a H2 plasma for 2 min in order to enhance p-layer microcrystallite nucleation.
SE. Also shown in the lower panels of Fig. 3 are the second derivatives of the dielectric functions. These latter spectra identify the band structure critical points associated with the lc-Si:H phase only in the R ¼ 200 film, as would be expected if the phase diagram of Fig. 1 was relevant. Finally in Fig. 4, annealed state J –V characteristics measured under AM1.5 illumination are shown for two solar cells from Fig. 2, one incorporating the protocrystalline Si:H p-layer deposited with R ¼ 150 and D ¼ 0:2 that maximizes Voc , and the other incorporating the single-phase lc-Si:H p-layer deposited with R ¼ 200 and D ¼ 0:2 after an initial H2 -plasma treatment of the i-layer.
4. Discussion
Fig. 3. Real and imaginary parts of the dielectric functions (e1 , e2 ), as well as their second derivatives (eV2 ) for: (a), (b) an R ¼ 150 protocrystalline Si:H p-layer and (c), (d) an R ¼ 200 ða þ lcÞ-Si:H p-layer, both prepared with D ¼ 0:2. These results were obtained by ex situ SE in reflection from the p-layer surface of the n–i–p solar cell.
A comparison of the phase diagram in Fig. 1 for D ¼ 0:2 with the corresponding solar cell results in Fig. 2, supported by the direct optical measurements of the cell p-layers in Fig. 3, shows that the optimum p-layer deposition, i.e. the one that maximizes Voc , is clearly within the protocrystalline Si:H growth regime. In fact, the p-layer should be deposited at the maximum R value that can be sustained without crossing the a ! ða þ lcÞ transition boundary throughout the desired thickness. The continuous increase in Voc with increasing R for R 6 150 is attributed to the widening of the p-layer mobility gap without a decrease in material quality, in contrast to the behavior for a-Si1x Cx :H p-layers, for example, in which case gap widening is achieved by increasing the C content. In addition, with increasing p-layer R in Fig. 2, an increasing flux of plasma atomic H may also enter the i-layer in the initial stage of p-layer deposition, widening
G.M. Ferreira et al. / Journal of Non-Crystalline Solids 338–340 (2004) 694–697
the i-layer gap and narrowing its band tail near the i/p interface. The continuous decrease in Voc with increasing R for the n–i–p solar cells incorporating p-layers deposited at the lower doping level of D ¼ 0:1 is of interest. A comparison of the two phase diagrams in Fig. 1 would lead one to expect that this effect arises from a shift to lower R in the a ! ða þ lcÞ transition with the decrease in D from 0.2 to 0.1, such that all p-layers above R ¼ 50 are mixed-phase ða þ lcÞ-Si:H with an increasing volume fraction of microcrystallinity. This expectation cannot be valid, however, since ex situ SE measurements of the R ¼ 100 and D ¼ 0:1 p-layer of Fig. 2 in the actual n–i–p cell configuration show that it is fully amorphous with similar optical characteristics as the R ¼ 150 player in Fig. 3(a), (b). Instead, the decrease in Voc at least for 50 6 R 6 100 is due to a decrease in the conductivity and an increase in the conductivity activation energy with increasing R due to a reduction in the effectiveness of doping in the protocrystalline growth regime. Such a conclusion is supported by direct conductivity mea p-layers with D ¼ 0:1 in selected i/p surements of 200 A structures. Based on this behavior, one can also conclude that even higher Voc values than the optimum in Fig. 2 may be possible in depositions that combine higher R and D values than 150 and 0.2, respectively.
5. Summary In a re-examination of optimum p-layer fabrication for a-Si:H-based n–i–p solar cells, it is concluded that the incorporation of single-phase lc-Si:H p-layers or even mixed phase ða þ lcÞ-Si:H p-layers is detrimental to Voc and overall cell performance. In fact, the optimization principle for p-layers is the same as that established earlier for i-layers that led to the concept of protocrystallinity [4], namely, that the H2 -dilution ratio
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should be maximized while remaining completely within the amorphous growth regime throughout the deposition. To apply this principle and obtain a maximum in Voc versus R, the gas phase doping level must be set at a sufficiently high value to ensure an adequate Fermi level shift under the high R protocrystalline growth conditions. Evidence obtained to date indicates that these overall conclusions also apply to the p-layers of p-i–n solar cells; however, additional studies are needed for confirmation in this configuration [9].
Acknowledgements This research was supported by NREL Subcontracts XAF-8-17619-22, NDJ-1-30630-01, and AAD-9-18-66809 and by NSF Grant DMR-0137240.
References [1] S. Guha, J. Yang, P. Nath, M. Hack, Appl. Phys. Lett. 49 (1986) 218. [2] A. Banerjee, Solar Energ. Mater. Solar Cells 36 (1995) 295. [3] X. Deng, S.J. Jones, T. Liu, M. Izu, S.R. Ovshinsky, K. Hoffman, Mater. Res. Soc. Symp. Proc. 467 (1997) 795. [4] J. Koh, Y. Lee, H. Fujiwara, C.R. Wronski, R.W. Collins, Appl. Phys. Lett. 73 (1998) 1526. [5] J. Koh, A.S. Ferlauto, P.I. Rovira, C.R. Wronski, R.W. Collins, Appl. Phys. Lett. 75 (1999) 2286. [6] R.J. Koval, C. Chen, G.M. Ferreira, A.S. Ferlauto, J.M. Pearce, P.I. Rovira, C.R. Wronski, R.W. Collins, Appl. Phys. Lett. 81 (2002) 1258. [7] R.W. Collins, J. Koh, H. Fujiwara, P.I. Rovira, A.S. Ferlauto, J.A. Zapien, C.R. Wronski, R. Messier, Appl. Surf. Sci. 154 (2000) 217. [8] J. Koh, H. Fujiwara, R.J. Koval, C.R. Wronski, R.W. Collins, J. Appl. Phys. 85 (1999) 4141. [9] R.W. Collins, A.S. Ferlauto, G.M. Ferreira, C. Chen, J. Koh, R.J. Koval, Y. Lee, J.M. Pearce, C.R. Wronski, Solar Energ. Mater. Solar Cells 78 (2003) 143.
Optimization of protocrystalline silicon p-type layers for amorphous silicon n–i–p solar cells G.M. Ferreira, Chi Chen, R.J. Koval, J.M. Pearce, C.R. Wronski, R.W. Collins
*
Materials Research Laboratory and Department of Physics, Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA
Abstract Real time spectroscopic ellipsometry has been applied to develop deposition phase diagrams for p-type hydrogenated silicon (Si:H) films prepared at low temperature (200 °C) by rf plasma-enhanced chemical vapor deposition using gas mixtures of SiH4 , H2 , and BF3 . These diagrams depict the regimes of accumulated thickness and H2 -dilution ratio R ¼ ½H2 =½SiH4 within which p-type amorphous Si:H [a-Si:H], mixed-phase Si:H [(a + lc)-Si:H], and single-phase microcrystalline Si:H [lc-Si:H] films are obtained in depositions on R ¼ 0 a-Si:H surfaces. The performance of n–i–p solar cells incorporating p-layers deposited under the same conditions as those used in the phase diagram development has been correlated with the deduced p-layer characteristics. This correlation demonstrates that Voc is maximized when the highest possible R value is used for the p-layer while ensuring single-phase amorphous film growth throughout its thickness. Ó 2004 Elsevier B.V. All rights reserved. PACS: 84.60.Jt; 81.05.Gc; 81.15.Gh; 81.40.Tv
1. Introduction Hydrogenated amorphous silicon (a-Si:H) n–i–p solar cells fabricated by rf plasma-enhanced chemical vapor deposition (PECVD) exhibit high open circuit voltages ðVoc Þ when low temperatures (T 50–200 °C) and high hydrogen-to-silane flow ratios (typically R ½H2 = ½SiH4 50–200) are used in the fabrication of the ptype, boron-doped layers [1–3]. It was proposed that such optimum top-contact p-layers are microcrystalline Si:H (lc-Si:H) and that this material yields overall improved cell performance through an increase in the built-in potential of the top junction as well as through decreases in series resistance and optical absorption losses [1]. More recent studies of Si:H film growth employing real time spectroscopic ellipsometry (RTSE) to extract deposition phase diagrams have demonstrated that the properties of films prepared at high R depend sensitively
*
Corresponding author. Tel.: +1-814 863 1880; fax: +1-814 865 2326. E-mail address: [email protected] (R.W. Collins). 0022-3093/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.03.062
on both the substrate and the accumulated thickness [4,5]. Thus, an explanation of the high Voc for n–i–p solar cells in terms of a lc-Si:H p-layer is not compelling unless characterization is performed on materials in the actual device configuration or in structures simulating this configuration [6]. In the present study, the problem of Si:H p-layer optimization has been reconsidered from the viewpoint of the phase diagram for p-layers deposited using similar conditions of substrate, thickness, and R as are used for devices. Ex situ SE results in the device configuration support the validity of the phase diagram interpretation. 2. Experimental details The undoped and p-type doped Si:H layers studied for phase diagram development were deposited on R ¼ 0 a-Si:H using single-chamber rf PECVD and were measured in real time using a rotating-compensator multichannel ellipsometer [7]. The substrate temperature T , rf plasma power P , and SiH4 flow F were set at T ¼ 200 °C, P ¼ 0:8 W/cm2 , and F ¼ 1 stand. cm3 /min, respectively. In addition, the H2 -dilution ratio R, doping gas ratio D, and total pressure ptot were set within
G.M. Ferreira et al. / Journal of Non-Crystalline Solids 338–340 (2004) 694–697
the ranges R 50–200, D ¼ ½BF3 =½SiH4 0–0:2, and ptot 0:3–0:9 Torr, respectively. For these Si:H layers, P was an order of magnitude higher than for the underlying R ¼ 0 a-Si:H films. In previous p-layer deposition studies [8], the underlying R ¼ 0 a-Si:H surface was subjected to a 2 min H2 -plasma treatment before p-layer growth to promote the nucleation of the lc-Si:H phase; however, this step was omitted in the present phase diagram studies. State-of-the-art n–i–p solar cells for correlation studies with the phase diagram were fabricated in a multichamber rf PECVD system on Cr-coated glass held at 200 °C. The i-layers of these cells were prepared to a using R ¼ 10 and thus were amorthickness of 4000 A phous throughout this thickness. The p-layers were using R values within prepared to a thickness of 200 A the range of 50–200, and D ¼ ½BF3 =½SiH4 was set at either 0.1 or 0.2. Other deposition conditions for the players were the same as those used in single-chamber deposition. For one of the n–i–p solar cells, an initial H2 plasma treatment of the underlying R ¼ 10 i-layer was performed prior to p-layer deposition at R ¼ 200 and D ¼ 0:2. As described in previous work, this two-step process led to immediate nucleation and growth of a single-phase lc-Si:H p-layer [8]. Finally, previous studies have found excellent agreement between deposition phase diagrams obtained by RTSE for single-chamber deposition and those obtained by ex situ SE and atomic force microscopy for multichamber deposition, applying similar process parameters in each case [9].
3. Results Fig. 1 shows superimposed deposition phase diagrams deduced by RTSE for undoped ðD ¼ 0Þ and p-type doped ðD ¼ 0:2Þ Si:H on freshly-deposited R ¼ 0 a-Si:H, a structure intended for correlation with n–i–p solar cell performance. In fact, since both R ¼ 0 and 10 layers are single-phase a-Si:H, the use of R ¼ 10 for the ilayer of the solar cell does not invalidate this correlation. The deposition phase diagram for the p-type doped layer with D ¼ 0:2 and 100 6 R 6 200 in Fig. 1 includes both the amorphous-to-(mixed-phase) Si:H transition thickness [designated a ! ða þ lcÞ; filled squares, solid line] and the (mixed-phase)-to-(single-phase) microcrystalline Si:H transition thickness [designated ða þ lcÞ ! lc; (open squares, broken line)]. This diagram shows that for R ¼ 100, the film grows initially as a-Si:H and crystallites begin to nucleate from the amorphous phase after a bulk layer thickness of 900 A. The resulting mixed-phase ða þ lcÞ-Si:H continues to is reached. grow until the final film thickness of 1500 A The open symbol and the upward arrow for R ¼ 100 indicates that the transition to single-phase lc-Si:H must occur for thicknesses greater than the designated 1500
695
Fig. 1. Superimposed deposition phase diagrams for undoped (circles) and p-type doped (squares) Si:H layers prepared with D ¼ ½BF3 = ½SiH4 ¼ 0 and 0.2, respectively, on R ¼ 0 a-Si:H substrate films held at 200 °C. The (filled symbols, solid lines) and the (open symbols, dashed lines) identify the a ! ða þ lcÞ and ða þ lcÞ ! lc transitions, respectively. The (up, down) arrows indicate that the transition occurs (above, below) the identified thickness values.
In contrast, at R ¼ 200, the transition from predomA. inantly a-Si:H to mixed-phase ða þ lcÞ-Si:H occurs at an and the transition to accumulated thickness of 20 A, It should be noted single phase lc-Si:H occurs at 200 A. that an initial H2 -plasma treatment of the underlying a mixed phase Si:H film completely eliminates this 200 A region [8]. The deposition phase diagram in Fig. 1 for the undoped Si:H layers (D ¼ 0, but otherwise identical deposition conditions as the D ¼ 0:2 series) prepared with 50 6 R 6 150 includes both a ! ða þ lcÞ (filled circles, solid line) and ða þ lcÞ ! lc (open circles, broken line) transitions. In fact, the downward pointing arrows on all points of the former transition line indicate that mixedphase ða þ lcÞ-Si:H nucleates immediately from the underlying R ¼ 0 a-Si:H for the full range of R. It is clear from the overall results of Fig. 1 that the transition lines for the p-type Si:H with D ¼ 0:2 are shifted to much higher R and thickness compared to those for undoped Si:H. This effect is attributed to B incorporation that strongly suppresses crystallite nucleation [8]. Fig. 2 shows Voc values as a function of the H2 -dilution ratio R (open squares) for a series of a-Si:H n–i–p D ¼ 0:2 p-layers solar cells, incorporating 200 A deposited under the same conditions as those of Fig. 1. Also shown in Fig. 2 is a result obtained applying an initial H2 -plasma treatment of the i-layer prior to deposition of a p-layer with R ¼ 200 and D ¼ 0:2 (cross). Finally, results are also shown for a second series of cells incorporating p-layers prepared with a lower gas phase doping level of D ¼ 0:1 (solid circles).
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Fig. 2. Open-circuit voltages of a-Si:H n–i–p solar cells incorporating thick R ¼ 10 i-layers and 200 A thick p-layers deposited with 4000 A variable R, plotted along the abscissa. Two different gas phase doping levels D ¼ ½BF3 =½SiH4 were employed for the p-layers, D ¼ 0:1 (closed circles) and D ¼ 0:2 (open squares). For the result given by the cross, D ¼ 0:2, and the underlying i-layer was exposed to a H2 plasma for 2 min in order to enhance p-layer microcrystallite nucleation.
A comparison of the D ¼ 0:2 results in Fig. 2 with the p-layers phase diagram of Fig. 1, suggests that the 200 A with R 6 150 are amorphous throughout their thickness. However, because such films would eventually evolve to lc-Si:H, given sufficient thickness, they are classified as ‘protocrystalline’ [4]. For the p-layer with R ¼ 175, Fig. 1 predicts mixed-phase ða þ lcÞ-Si:H within the top 50 of the 200 A p-layer, and for the one with R ¼ 200, A Fig. 1 predicts ða þ lcÞ-Si:H throughout much of its thickness. To verify the connection with the phase diagram, Fig. 3 shows the dielectric functions of the D ¼ 0:2 p-layers prepared with R ¼ 150 and 200 and measured in the actual device configuration using ex situ
Fig. 4. Annealed state J –V characteristics for the optimum a-Si:H n–i– p solar cell of Fig. 2 prepared with an R ¼ 150 protocrystalline Si:H player (closed squares), and for an a-Si:H n–i–p cell with an R ¼ 200 single-phase lc-Si:H p-layer (open circles). In the latter p-layer process, the underlying i-layer was exposed to a H2 plasma for 2 min in order to enhance p-layer microcrystallite nucleation.
SE. Also shown in the lower panels of Fig. 3 are the second derivatives of the dielectric functions. These latter spectra identify the band structure critical points associated with the lc-Si:H phase only in the R ¼ 200 film, as would be expected if the phase diagram of Fig. 1 was relevant. Finally in Fig. 4, annealed state J –V characteristics measured under AM1.5 illumination are shown for two solar cells from Fig. 2, one incorporating the protocrystalline Si:H p-layer deposited with R ¼ 150 and D ¼ 0:2 that maximizes Voc , and the other incorporating the single-phase lc-Si:H p-layer deposited with R ¼ 200 and D ¼ 0:2 after an initial H2 -plasma treatment of the i-layer.
4. Discussion
Fig. 3. Real and imaginary parts of the dielectric functions (e1 , e2 ), as well as their second derivatives (eV2 ) for: (a), (b) an R ¼ 150 protocrystalline Si:H p-layer and (c), (d) an R ¼ 200 ða þ lcÞ-Si:H p-layer, both prepared with D ¼ 0:2. These results were obtained by ex situ SE in reflection from the p-layer surface of the n–i–p solar cell.
A comparison of the phase diagram in Fig. 1 for D ¼ 0:2 with the corresponding solar cell results in Fig. 2, supported by the direct optical measurements of the cell p-layers in Fig. 3, shows that the optimum p-layer deposition, i.e. the one that maximizes Voc , is clearly within the protocrystalline Si:H growth regime. In fact, the p-layer should be deposited at the maximum R value that can be sustained without crossing the a ! ða þ lcÞ transition boundary throughout the desired thickness. The continuous increase in Voc with increasing R for R 6 150 is attributed to the widening of the p-layer mobility gap without a decrease in material quality, in contrast to the behavior for a-Si1x Cx :H p-layers, for example, in which case gap widening is achieved by increasing the C content. In addition, with increasing p-layer R in Fig. 2, an increasing flux of plasma atomic H may also enter the i-layer in the initial stage of p-layer deposition, widening
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the i-layer gap and narrowing its band tail near the i/p interface. The continuous decrease in Voc with increasing R for the n–i–p solar cells incorporating p-layers deposited at the lower doping level of D ¼ 0:1 is of interest. A comparison of the two phase diagrams in Fig. 1 would lead one to expect that this effect arises from a shift to lower R in the a ! ða þ lcÞ transition with the decrease in D from 0.2 to 0.1, such that all p-layers above R ¼ 50 are mixed-phase ða þ lcÞ-Si:H with an increasing volume fraction of microcrystallinity. This expectation cannot be valid, however, since ex situ SE measurements of the R ¼ 100 and D ¼ 0:1 p-layer of Fig. 2 in the actual n–i–p cell configuration show that it is fully amorphous with similar optical characteristics as the R ¼ 150 player in Fig. 3(a), (b). Instead, the decrease in Voc at least for 50 6 R 6 100 is due to a decrease in the conductivity and an increase in the conductivity activation energy with increasing R due to a reduction in the effectiveness of doping in the protocrystalline growth regime. Such a conclusion is supported by direct conductivity mea p-layers with D ¼ 0:1 in selected i/p surements of 200 A structures. Based on this behavior, one can also conclude that even higher Voc values than the optimum in Fig. 2 may be possible in depositions that combine higher R and D values than 150 and 0.2, respectively.
5. Summary In a re-examination of optimum p-layer fabrication for a-Si:H-based n–i–p solar cells, it is concluded that the incorporation of single-phase lc-Si:H p-layers or even mixed phase ða þ lcÞ-Si:H p-layers is detrimental to Voc and overall cell performance. In fact, the optimization principle for p-layers is the same as that established earlier for i-layers that led to the concept of protocrystallinity [4], namely, that the H2 -dilution ratio
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should be maximized while remaining completely within the amorphous growth regime throughout the deposition. To apply this principle and obtain a maximum in Voc versus R, the gas phase doping level must be set at a sufficiently high value to ensure an adequate Fermi level shift under the high R protocrystalline growth conditions. Evidence obtained to date indicates that these overall conclusions also apply to the p-layers of p-i–n solar cells; however, additional studies are needed for confirmation in this configuration [9].
Acknowledgements This research was supported by NREL Subcontracts XAF-8-17619-22, NDJ-1-30630-01, and AAD-9-18-66809 and by NSF Grant DMR-0137240.
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