by using both conventional and reverse cell manufacturing technologies. ... 1. direct cells n ÷ p p÷ where the p+ has been obtained by printed aluminium;.
SolarEmm~ Matehals and Solar C~ls
ELSEVIER
Solar Energy Materials and Solar Cells 43 (1996) 183-191
Comparison between normal and reverse thin crystalline silicon solar cells A. Benati a M.A. Butturi a C. Capperdoni a M.C. Carotta a G. Martinelli ~'*, M. Merli a L. Passari a G. Sartori a R. Van Steenwinkel h, G.M. Youssef c a INFM-Physics Department, Ferrara University, 12-Via Paradiso, 44100 Ferrara, Italy b Commission of the European Community, J.R.C. Ispra Establishment, 21020 lspra, Italy c Physics Department, Faculty of Science, Ain Shams University, Abbassia, Cairo, Egypt
Abstract The newly developed ingot growing techniques, as the three-grain and the columnar multigrain ingot processes, are now offering the possibility of slicing thinner wafers ( < 100 ~m). In this paper we present the results obtained on p type large area (_> 100 cm 2) and 100 ~m thick wafers by using both conventional and reverse cell manufacturing technologies. The conventional cells are provided with aluminium or boron BSF plus screen-printed silver mirror or a silver-aluminium net; the reverse cells have a FSF and the deep back junction completely covered by a screen-printed or CVD silver layer. The constructing parameters have been chosen on the base of one and two dimensions modeling and both raw material and devices have been completely characterized. This work shows that very thin wafers do not introduce serious problems for the conventional manufacturing of solar cells. The efficiencies of the normal and of the reverse cells are found to be comparable and are of the same order than those of thicker cells, however at a significant lower cost. The main obtained result has to be related to the demonstration of a cell manufacturing feasibility starting from very thin wafers. Keywords: Silicon; Thin cell
1. Introduction The present use of PV energy is extremely limited by costs, estimated as four times higher than those of conventional sources. It is therefore crucial to increase the
* Corresponding author. 0927-0248/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSDI 0927-0248(95)00165-4
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A. Benati et al. / Solar Energy Materials and Solar Cells 43 (1996) 183-191
efficiency cost ratio. In a previous paper [1] we have shown that monocrystalline reverse silicon solar cells with efficiencies comparable to those of conventional devices can be obtained if the wafer thickness is lower than 200 Ixm and the base lifetime longer than 30 txs. A recent 3D modeling of multicrystalline solar cells [2] also reached the conclusion that thin reverse cells could be a valid alternative. The newly developed ingot growing techniques, as the three-grain [3] and the columnar multigrain ingot processes, are now offering the possibility of slicing thinner wafers (___ 100 Ixm) making the use of reverse cells still more attractive. The aim of the present work is to demonstrate the feasibility of large area-thin wafer technology for large scale application and to compare the performances of conventional cells with those of the reverse ones. Two main aspects, one physical and one technological, must be considered in the manufacturing of reverse solar ceils: • it is essential to have a low surface recombination velocity on the front side; this can be obtained with a very good passivation or with a shallow front surface field which is normally easier to obtain; • the back side junction depth can be quite large without affecting the cell quality and further the back surface can be silvered completely thus providing a lower series resistance a higher shunt resistance and a better reflectivity. It should be mentioned however that the wafers obtained from a three grain or polycristalline ingot are not suitable for texturing since the required (100) orientation is not realized. On the other hand the experience with single crystalline ingots has shown that handling thin wafers is difficult. The limited experience in the field of the reverse cell suggests that significant improvements can be achieved to make this kind of technology a valid alternative for equivalent efficiencies. The numerical simulation were performed with a finite difference method both in the 1D and in the 2D cases. The discretisation of the basic equations was achieved according to the method proposed by D.L. Scharfetter and H.K. Gummel [4]. As a result, a discretised Poisson equation and two discretised cartier continuity equation are defined on each node of the mesh and the complete system of algebraic equations is solved with the Newton method in ID case and with the Newton-SLOR method in the 2D case. All simulations were made at 300K corresponding to a band gap of 1.124 eV. The high doping effect and the associated band gap narrowing are taken into account through a position dependent effective intrinsic concentration according to an expression given by J.W. Slotboom [5].
2. Experimental The area of the cells is 100 cm 2. The wafers have been cut out from a three grain ingot; they have a thickness of 100 txm and a 5 I I . cm p type bulk resistivity. The lifetime, measured by PCD and PEM [6], ranges from 10 to 15 txs corresponding to a diffusion length of the order of 200 txm. The cell manufacturing procedure is similar to the one used in [1]. We prepared the following runs: 1. direct cells n ÷ p p÷ where the p+ has been obtained by printed aluminium; 2. direct cells n ÷ p p÷ (boron) with silver-aluminium back coverage;
A. Benati et al, / Solar Energy Materials and Solar Cells 43 (1996) 183-191
185
Table l Run
Jsc (mA/cm2)
Voc (mV)
FF (%)
~ (%)
1 2 3 4
29.0 30.0 28.5 30.0
590 580 575 575
76 75 77 77
12.4 12.5 12.3 12.9
3. reverse cells p - p n ÷ with a printed silver back coverage (a reflectance over 80% has been proven [7]). Front grid by printed silver-aluminium; 4. reverse cells p÷ p n ÷ with a T i - P d - A g evaporated back coverage; front grid by printed silver-aluminium. Part of the cells have a TiO 2 A.R.C deposited by APCVD and part a printed TiO 2 based paste. The correspondent results are reported in Table 1 (equipments available at Physics Department of Ferrara University and calibrated at J.R.C-Ispra (VA) Italy were used): Eight cells were manufactured for each run. In the table we reported the mean value of the efficiency corresponding to the different runs. The junction depth and the width of the boron p+ regions were measured with an oxidation profiler. The junction depth is 0.6 I~m for all runs; for the cells of run 2 the width of the p+ boron region is 0.4 Ixm while for the reverse cells (runs 3 and 4) it is 0.15 p.m. In both cases we calculated that
0.6 - ~ - Reverse (30us)
o.s
Jsc---2g.04n~
- . - R . , , . (.,o=) . N - - , , oo=)
.ff#" J ' J ,J".l..
IX
o.4
1
i
0.3
0.2
0.1
0
300
400
500
600
700
800
900
1000
1100
1200
Wavelength [nm]
Fig. 1. ID spectral responsivities of conventional and reverse cell under the above described characteristics.
A. Benati et al. / Solar Energy Materials and Solar Cells 43 (1996) 183-191
186
Table 2 Jsc, Voc, Eff for two bulk carrier densities (Na = 3.1015 c m - 3 and Na = 1.1016 c m - 3) without and with bsf (Na 0 = 1 • 1020 cm-3). The data are calculated in 1D simulation with rbu]k = 10 p,s S(cm/s)
102 lO 3 104 105
bsf
Jsc ( m A / c m 2 )
yes no yes no yes no yes no
Voc (mY)
Eff(%)
Na = 3.1015
N a = 1 • 1016
N a = 3 . 1 0 ]5
N a = 1 • 1016
Na = 3.1015
Na = 1 •
cm-3
cm-3
cm-3
cm-3
cm-3
cm-3
30.44 30.38 30.44 29.95 30.40 28.69 30.21 28.09
30.20 30.14 30.19 29.66 30.06 28.39 29.55 27.83
606.7 604.3 606.5 592.5 605.0 573.3 599.2 567.3
630.0 628.2 629.6 619.4 626,3 603.4 617.3 598.7
15.20 15.13 15.20 14.58 15.15 13.00 14.92 12.24
15.83 15.75 15.81 15.26 15.65 14.17 15.15 13.76
1016
the measured front-surface reflectivities are lowering the short circuit current by about 12% in respect to the ideal case of zero reflectivity. 3. Numerical simulations Except where specified differently the cells used for the simulations have the following characteristics: 35
|
30 ~
...........
~.
\. --"
2. - ~ 3 .
"
3.
E 20 --*-- FSF:0.15um(
E. ,=~
15
FSF:1..~umtaOus) I
10
no FSF(3Ous) I --'--FSF:O.15um(10us)I ==e--FSF:I.5Oum(lOus)[ --~--no FSF(IOus) I
0 1.00E+O0
.................
I
.................
1.00E+02
~,
I
.................
1.00E+04
i
.................
1.00E+06
1.00E+08
$f [cm/$] Fig. 2, 1D short-circuit density current against surface recombination velocity with and without FSF.
A. Benati et al. / Solar Energy Materials and Solar Cells 43 (1996) 183-191 15
1412 ¢
I
--o--Normal (10us)
o---.
~
_
o.
10
E Ul 8
- I I - Reverse (lOus) I --e-- Reverse (30us) I
6
4
. . . .
0
r
. . . .
50
i
. . . .
100
I
. . . .
150
I
. . . .
200
~
. . . .
250
:
300
.... 350
T h l c k n e u IF.m] Fig. 3. 1D efficiencies of normal and reverse cells as a function of thickness.
0.54
0.62
0.6
~0.58 >. 0.56
- 0 - - NOn'hal (lOus) ]
Normal(30us)I --e-Reverse(10us)I --e--Reverse(30us)J
0.54
0.52
0.5 0
50
100
150
200
250
300
3.50
Thickneu [Fm] Fig. 4. 1D open-circuit voltage against thickness for normal and reverse cells.
187
A. Benati et al, / Solar Energy Materials and Solar Cells 43 (1996) 183-191
188
°
°
i,, -,"
19
17 15 13 11 , '
,
0
50
100
150
200
250
300
350
T h l c k n e u [imt]
Fig. 5. 1D short-circuit density current against thick hess for normal and reverse cells.
16
, - e - . Dw=70um(30us) 15
...,e-- Dw=100um(30us) - - . - - Dw=130um(30us) "~'~.
,&
Dw=70um(10us)
J. Dw=lOOum(1Ous)
14
~ , - - Dw=130um(10us)
¢: 13 _e u
S
Ill 12
11
10
0
i
i
0.2
0.4
i
0.6 FSF ~
i
~
f
i
0.8
1
12
14
1.6
[pm]
Fig. 6. 1D efficiency against FSF width for 70 Izm and 100 p,m thicknesses.
189
A. Benati et al./ Solar Energy Materials and Solar Cells 43 (1996) 183-191
type n ÷ p p ÷ total device thickness: 100 I~m junction depth: 0.6 p~m; p÷ depth: 0.15 ixm bulk concentration 3 • 10 ]5 cm -3 Surface donor concentration: 1 • 10 20 cm -3 Surface acceptor concentration: 1 • 10 20 cm -3 Gaussian doping profiles front and back surface recombination velocities: 5 . 1 0 4 cm//s base lifetimes: 10 ixs and 30 Ixs front reflection coefficient: 6% back reflection coefficient: 70% multireflections no surface charge front-surface coverage: 12% The normal cell simulations have been done with a p÷ thickness of 0.15 p,m instead o f 0,4 wm as experimentally determined. For normal cells however this fact has only very little effects on the simulations results. Fig. 1 shows the responsivities of conventional and reverse cells for two base lifetimes.
0,035
0,03
0,025
E u
Sf,~e3~ / s
.
