PAPER REFERENCE NUMBER: E-III/P13
Porous silicon influence on surface recombination in solar cells G.Kopitkovas1, I.Mikulskas* 1, J.Vaitkus1, K.Grigoras2, A.Šimkienë2, R.Tomašiûnas1 1
Institute of Materials Science & Applied Research, Vilnius University, 2054 Vilnius, Lithuania 2 Institute of Semiconductor Physics, Lithuanian Academy of Science, 2000 Vilnius, Lithuania
Abstract Laser-induced transient grating measurements were performed to monitor porous silicon influence on surface recombination of highly doped emitter in solar cells. Different depth regions of the emitter were excited. Using a kinetic model, which includes carrier diffusion and recombination at the surface and in the bulk, surface recombination velocity was evaluated. From the analysis, we conclude that porous silicon passivates the surface of emitter, i.e. reduces the rate of surface recombination. Keywords: solar cell, porous silicon, surface recombination velocity.
1. Introduction Porous silicon (PS) became attractive for solar cell researchers, mainly because of its high total area and low light reflection [1]. PS layer formed onto the emitter improves the device performance also due to passivation of the surface [2]. Solar cells usually incorporate a shallow and highly doped emitter, which causes additional defects and increases carrier recombination at the surface. The aim of present work is to complete our previous report on p-n+ junctions, obtained by the phosphorus doping from silicon oxide films deposited by a spin-on technique [3], and to discuss new experimental results within a framework of a kinetic model, which enables us to extract more precise the values of recombination rate at the porous silicon - crystalline silicon interface. Bulk and surface recombination parameters were monitored by laser induced transient grating technique (LIG) [4].
2. Experimental procedure and kinetic model Starting material of the prepared cells was (100) oriented p-type (1 Ω×cm, CZ) silicon wafers of 480 µm thickness with polished surfaces. They were sliced to 2×3 cm2 cells. The samples were rinsed for *
Corresponding author: tel: +370-2-769503; fax: +370-2-767313; e-mail:
[email protected] 1
10 min. in boiling dimethylformamide and in acetone, then cleaned by ethanol. The native SiO2 layer was removed by pure HF. For doping tetraethoxysilane solution mixed with H3PO4 at a volume ratio 1:1 was applied by spin-on technique. The diffusion was performed by annealing for 30 min. at 950 °C in argon environment followed by a slow cooling down. The phosphorus concentration at the surface was about 2×1020 cm-3. After emitter formation, the top surface was passivated with a porous silicon layer, formed by electrochemical etching in HF:H2O (1:1) solution for some tens of seconds at 30 mA/cm2 [5]. In the LIG technique, two coherent laser pulses interfere in the sample creating a transient photocarrier of the grating (grating period Λ) and the diffraction of a third time-delayed pulse on it monitors the washing out the grating in time. The short wavelength excitation of a mode-locked YAG:Nd3+ laser (0.53 µm and 0.355 µm), which is more strongly absorbed in crystalline silicon, was employed in order to create charge carriers just at the surface area. Pumping intensity used was Ip = 6 MW/cm2. The LIG diffraction efficiency η, defined as η = Ipr/Ip0 was measured. Ipr and Ip0 are the intensities of diffracted and probe pulse (1.06 µm), respectively. All experiments were performed at room temperature. We suppose that within the duration of pump pulses, free electron-hole pairs with spatially modulated density N(x,y,t) are created in the core of the emitter (x-axis is perpendicular to the sample surface and y-axis - parallel to the sample surface and corresponds to the grating alteration direction). Temporal and spatial evolution of the photoexcited carrier density is described by a following rate equation ∂ N(x, y , t) ∂ 2 N (x , y , t ) ∂ 2 N (x , y , t ) N ( x , y , t ) × + D × − − C × N ( x, y, t ) 3 = G +D ay ax 2 ∂t τd ∂x ∂y 2
with boundary conditions
S ∂N( 0, y ,t) × N (0, y, t ) , = ∂x Dax
N(x, y, 0 ) = 0 ,
(1)
∂N(x = d, y, t) = 0, ∂x
where G means the generation rate, Dax (Day) is the ambipolar diffusion coefficient of photocarrier for the x (y) direction, C is the Auger recombination coefficient, τd is the linear recombination time, S is the surface recombination velocity. In the case of crystalline silicon, we have neglected the bimolecular recombination term. η(t) ∼ N2(t), which links the measured quantity η(t) with the solution of the Eq.(1). We accept that Dax = Day = Da.
4. Experimental results and discussion First, we have started with the “small period grating” case. The LIG’s were excited by 355 nm -1 wavelength and measured for several values of Λ = 2.37, 3.1, 5.34 and 9.01 µm. Values of τl = =τl-1+Da×4π2/Λ2 were obtained from the linear part of grating decay. Thus, from the slope τl-1 against 4π2/Λ2 we have extracted the value of ambipolar diffusion coefficient Da ~ 7 cm2s-1. This value of Da is in good agreement with the generally anticipated diffusion coefficient for crystalline Si [6]. The second and most important “large period grating” case (Λ = 20 µm), when studying photocarrier diffusion in the x direction was performed at two different excitation wavelengths. The second and third laser harmonics used for pumping formed corresponding gratings of 1 µm and 0.1 µm thickness 2
(absorption coefficient α of ~ 104 cm-1 and ~ 105 cm-1 for crystalline silicon [7], respectively). When taking into account the shallow emitter, the 0.1 µm LIG’s were more efficient for interpreting the experimental results than the 1 µm LIG’s. In the latter case, the excitation depth exceeds the p-n+ junction and gives rise to an additional photocarrier redistribution processes just at the depletion region. Formation of an internal electric field by photocarriers in the junction, typical for solar cell function, would diminish the diffusion from surface. This was confirmed by our model where the calculated curves fit satisfactorily to the experimental points of the 0.1 µm LIG's, but fail to follow the experimental points of the 1 µm case. Parameters entering Eq.(1) are N0, R, τL, τd, t0, α, Da, C and S. The only free parameters are, however, Da and S. N0 is proportional to the total number of photocarriers created during the pump pulse and corresponds to average non-equilibrium concentration 1.5×1019 cm-3, τd = 1 µs as a fastest lifetime limit for bulk silicon [8], τL = 40 ps is the laser pulse duration, t0 = 50 ps and C = 4×10-31 cm6⋅s-1 [8]. Reflectivities R = 0.58 (at 0.355 µm), R = 0.35 (at 0.532 µm) are known for silicon. 1
+
p-n
η, a.u.
η, a.u.
1
-1
10
+
p - n (PS)
-1
10
5
S=1.6-2.2 x 10 cm/s
5
S=1.3-1.7 x 10 cm/s
-2
10
0
-2
10
100 200 300 400 500 600 700 Delay time, ps
0
100 200 300 400 500 600 700 Delay time, ps
Fig. 1. The grating dynamics measured in the sample before (left) and after (right) PS layer formation at two different excitation quantum energies (open circles - 2.34 eV, filled - 3.51 eV). The points are experimental values, the lines are theoretical modeling according to Eq.(1). Each filled area corresponds to a series of kinetics calculated with intermediate values of surface recombination value S (presented in each figure). Relative high S values in a range of 1.6 - 2.2×105 cm/s were obtained for samples with the already formed emitter, i.e. the stage just before porous silicon formation. These values correspond well with the results presented by King and co-workers for both n-type [9] and p-type [10] highly doped silicon, and, also with the very recent results obtained by Cuevas et al [11] on phosphorus diffused n-type silicon with S values in the range of 2×105 cm/s. Nevertheless, our main attention was focused on the surface recombination velocity modification in the emitter with porous silicon. We’ve got values 1.3 – 1.7×105 cm/s, which show that porous silicon is diminishing the surface recombination rate. This tendency is in line with our previous expectations made on similar samples [3]. We would not discuss just the evaluated surface recombination rate values as calibrated for a porous silicon – crystalline silicon interface, because they are dependant of the porosity factor and crystalline silicon doping. Though, one can seen that the calculated curves are quite sensitive to the S variation.
3
We consider PS as a partially interconnected network of Si nanocrystallites with typical dimensions of 3 ÷ 5 nm. It is well known that the interior of the nanocrystallites preserves the silicon crystalline structure, but there is a lot of localized states (tail states) near the band edges of PS, originating probably from perturbed bonds near to the surface. So, the interpretation of the surface recombination reduction at the emitter surface covered by porous silicon could be threefold. First, a part of natural defects at the surface can be removed while etching. Second, surface passivation by a hydrogen coverage of the surface bonds can take place. Third, a lot of localized states formed in the PS are efficiently capturing carriers [12-14], thus, acting as a repulsion barrier for the photoexcited carrier. In conclusion, LIG experiments performed at room temperature with a picosecond time resolution on samples typical for successive operations of solar cell technology revealed a tendency of surface recombination velocity decrease on the emitter after porous silicon formation. And this due to modified trap distribution at the porous silicon-crystalline silicon interface. Our observations are based on a model involving carrier recombination (diffusion) at (from) the surface and in the bulk of emitter.
Acknowledgments This work was supported in part by NATO Linkage grant HTECH.LG972051 and by a grant of the Lithuanian State Foundation for Science and Studies.
References 1. L. Stalmanis, W. Laureys, K. Said, M. Caymax, J. Poortmans, J. Nijs, R. Mertens, S. Strelke, C. Levy-Clement, L. Ventura, A. Slaoui, G. Bremond, A. Daami, A. Laugier, Proceedings of the 14th European Photovoltaic Solar Energy Conference, Barcelona, Spain, June 30 - July 4, 1997 (Published by H.S. Stephens & Associates, Bedford, UK, 1997), p. 2484-2487. 2. V. Paèebutas, K. Grigoras, A. Krotkus, Physica Scripta T69 (1997) 255. 3. K. Grigoras, A. Major, I. Ðimkienë, E. Gaubas, Semicond. Sci. Technol. 13 (1998) 517. 4. H.J. Eichler, P. Günter, D.W. Pohl. Laser-Induced Dynamics Gratings, Springer, Berlin, 1986, p.256. 5. K. Grigoras, V. Paèebutas, Rev. Sci. Instrum. 67 (1996) 2337. 6. See, e.g., J. Linnros, V. Grivickas, Phys. Rev. B 50 (1994) 16943. 7. See, e.g., V. Grivickas, P. Basmaji, Thin Solid Films 235 (1993) 234. 8. See, e.g., V. Grivickas, M. Willander, in: R. Hull (Ed.), Properties of Crystalline Silicon, EMIS Datareviews Series No. 20, Chapter 13.5, INSPEC, London, 1999, pp. 710-717. 9. R.R. King, R.A. Sinton, R.M. Swanson, IEEE Trans. Electron Devices 37 (1990) 365. 10. R.R. King, R.M. Swanson, IEEE Trans. Electron Devices 38 (1991) 1399. 11. A. Cuevas, P.A. Basore, G. Giroult-Matlakowski, C. Dubois, J. Appl. Phys. 80 (1996) 3370. 12. R. Tomasiunas, I. Pelant, J. Kocka, P. Knapek, R. Levy, P. Gilliot, J.B. Grun, B. Hönerlage, J. Appl. Phys. 79(5) (1996) 2481. 13. P.M. Fauchet et al., in: P.F. Barbara, W.H. Knox, G.A. Mourou, A.H. Zewail (Eds.), Ultrafast Phenomena IX, Springer, New York, 1994, p. 283. 14. J. von Behren, K.B. Ucer, L. Tsybeskov, J.V. Vandyshev, P.M. Fauchet, J. Vac. Sci. Technol. B 13 (1995) 1225.
4
PAPER REFERENCE NUMBER: E-III/P13
Set of figures
Figure captions:
Fig. 1. The grating dynamics measured in the sample before (left) and after (right) PS layer formation at two different excitation quantum energies (open circles - 2.34 eV, filled - 3.51 eV). The points are experimental values, the lines are theoretical modeling according to Eq.(1). Each filled area corresponds to a series of kinetics calculated with intermediate values of surface recombination value S (presented in each figure).
5
PAPER REFERENCE NUMBER: E-III/P13
10
1
+
p-n
η, a.u.
η, a.u.
1
-1
+
p - n (PS)
-1
10
5
S=1.6-2.2 x 10 cm/s 10
5
S=1.3-1.7 x 10 cm/s
-2
-2
0
10
100 200 300 400 500 600 700 Delay time, ps
Fig.1
6
0
100 200 300 400 500 600 700 Delay time, ps
