Informacije MIDEM 23(1993)4, Ljubljana UDK 621.3:(53+54+621+66), ISSN0352-9045 
TANDEM AMORPHOUS SILICON SOLAR CELLS 
J. Furlan, W. Kusian*, G. Conte** 
Faculty of Electrical and Computer Engineering, Ljubljana, Slovenia 
*Siemens AG, Corporate Research and Development, Muenchen, Germany 
**ENEA, Centro Ricerche Fotovoltaiche, Portici-Napoli, Italy 
INVITEO PAPER 
21 st International Conference on Microelectronics, MIEL'93 
29th Symposium on Devices and Materials, SO'93 
September 29 - October 1, 1993, Bled, SLOVENIA 
Key words: photovoltaic energy, solar cells, tandem cells, amorphous silicon, state of the art, conversion efficiency, stability, series tandem, parallel 
tandem, fabrication 
Abstract - Recent progress in the field of amorphous silicon tandem solar cells is reviewed. The advantages of series connected tandem cells with 
reg ard to single junction amorphous solar cells are discussed. Partitioning a thicker celi into two or three thinner cells results in higher inherent 
built-in electric fields in the cells with better conversion efficiency and stability. A new approach is described, utilizing parallel connection of cells in 
tandem structure to additionally improve efficiency, stability and sensitivity to thickness tolerances. The essential similarities and differences between 
series and parallel tandem celi arrangements are discussed and the present status of tandem cells fabrication is presented. 
Tandemske amorfne silicij eve sončne celice 
Ključne besede: energija fotonapetostna, celice sončne, celice tandemske, silicij amorfni, stanje razvoja, izkoristek konverzijski, stabilnost, tandem 
serijski, tandem paralelni, izdelava 
Povzetek: Prikazani so dosežki s področja amorfnih silicijevih tandemskih celic. Opisane so prednosti serijskih tandemskih celic proti eni sami 
amorfni silicijevi sončni celici. Razdelitev debelejše celice v dve ali tri tanjše vodi do večjih vgrajenih električnih polj v posameznih celicah in k boljšim 
izkoristkom pretvorbe in večji stabilnosti delovanja. Opisan je novi pristop z vzporedno vezavo posameznih celic, pri kateri je pričakovati dodatno 
izboljšanje izkoristka, stabilnosti in neobčutljivosti na tolerance debelin celic. Prikazane so glavne podobnosti in razlike med serijsko in paraleino 
vezavo ter trenutno stanje tehnologije izdelave tandemskih celic. 
INTRODUCTION 
The majority of photovoltaic energy conversion devices 
on the market today are silicon solar celis, including 
most efficient but expensive single crystalline devices, 
less expensive recrystallized metallurgical silicon de-
vices, as well as low cost thin film solar cells from 
amorphous silicon which received a great attention dur-
ing the last decade as one of the most prospective 
materials for low cost solar celis. 
Amorphous silicon (a-Si) can be inexpensively de-
posited on large substrates of various materials and 
shapes. During its deposition it needs to be treated to 
relatively low temperatures in the range of 200-300°C. 
Due to high light absorption of a-Si, the a-Si solar celi 
thickness can be very small, typically less than 1 ~lm. 
The main limitation in obtaining high solar celi conver-
sion efficiency are poor transport properties of charge 
carriers in amorphous silicon. The photogenerated elec-
trons and holes which make up the collected solar celi 
photocurrent have very short diffusion lengths which are 
much too short to provide effective collection of these 
259 
carriers. Therefore, the a-Si celi has to be very thin and 
an aiding electric field is built inside of the celi to assist 
more effective charge separation and collection. On the 
other hand, the celi should be sufficiently thick in order 
to provide asuitable absorption of light flux. These 
opposing criteria directed to the idea of tandem a-Si 
solar celi structures where the individual cells are de-
posited sequentially forming a multilayer solar celi struc-
ture in which the unit cells are optically and electrically 
connected in series 11 l. Since the collected photocurrent 
must be the same in all unit cells, each consecutive celi 
in a series tandem structure must be thicker than the 
preceding one. 
A parallel connected tandem a-Si celi structure in which 
the individual cells are optically connected in series and 
electrically in parallel has been proposed recently 12/. 
This configuration which is presently under intensive 
investigations is hoped to outperform its series tandem 
counterpart with respect to conversion efficiency and 
stability promising al so lower sensitivity of solar celi 
efficiency on nonuniform a-Si deposition rate in large 
area solar celi modules 13/. 
J.Furlan, W.Kusian, G.Conte: Tandem Amorphous Silicon 
Solar Cells 
SERIES TANDEM CELL STRUCTURE 
ln series tandem structures two or three cells are opti-
cally and electrically connected in series, as shown in 
Fig. 1 11/. The front p+in+ celi is covered by a transparent 
conductive oxide (TCO) layer providing electric contact 
to the p+ layer of this celI. The metal on n+ layer is serving 
for electric contact to the back p+in+ celI. Junctions n+p+ 
interfacing consecutive unit cells are acting as ohmic 
contacts, electrically interconnecting succeeding p+in+ 
cells. 
The individual cells in such serially stacked devices can 
use the same amorphous material in all unit cells. Such 
structure which affords a sufficient absorption of photon 
flux and enables a satisfactory collection of photogener-
ated carriers is regarded as one of the most practical 
solutions. The fact that the electric current in a series 
connected tandem celi is the same in all unit cells 
demands a greater thickness of each consecutive celI. 
Consequently, a practical stacked a-Si celi structure 
consists of only two cells. A triple tandem structure would 
require an unacceptably thin front and thick back unit 
celI. Most frequently these double stacked cells are 
fabricated by depositing thin layers of TCO, amorphous 
TCO 
glass 
Fig. 1: The basic series tandem structure. 
silicon and metal on a glass substrate, as shown sche-
matically in Fig. 2. The p+ and n+ layers are indispens-
able for creating the bui It-in electric field in i-Iayers 
assisting charge carrier separation and collection. Since 
the diffusion lengths in highly doped a-Si are extremely 
short, these dead layers must be very thin, usually in the 
range between 15 and 30nm. 
Even a very thin p+ layer of the front celi parasitically 
absorbs a large amount of photons at short wave-
lenghts. For this reason the energy gap of the front p+ 
layer is increased to about 2eV by mixing amorphous 
silicon with carbon, resulting in a sort of optical window. 
ln spite of a wider band-gap a relatively large part of light 
flux is absorbed in this so-called p+ window layer, as 
shown in Figs. 3.a and b, representing the calculated 
portions of the absorbed sunlight flux in specific layers 
of a double stacked tandem celi for two sets of solar celi 
thicknesses, given in Table I. It can be noticed from these 
plots that the absorbed fluxes in i-Iayers of both cells are 
matched when the top celi is much thinner tllan the 
bottom celI. 
260 
Fig. 2: 
Informacije MIDEM 23(1993)4, str. 259-263 
N P N P 
1st celi 2nd celi I 3rd celi 
Schematic presentation of double-stacked 
a-Sila-Si series tandem solar celi. 
The absorbed photons in both i-Iayers generate elec-
trons and holes which are collected at opposite elec-
trodes. The effectiveness of this charge collection great-
Iy depends on the density of localized states in the gap 
of a-Si. Higher is the density of states in the i-Iayer, 
greater is the decrease of the built-in electric field and 
higher are thermal recombinations of photogenerated 
carriers, resulting in a decreased collection efficiency. 
The so-called Staebler-Wronski effect (SW effect), 
which originates from increased states density in the 
gap during a strong illumination, is the main cause of 
unstable operation of a-Si celis. When decreasing the 
thicknesses of individual cells in a-Si tandem structures, 
the built-in electric field in i-Iayers increases, and the 
solar celi degradation due to SW effect is reduced. 
Table I Thicknesses of thin layers selected for calcula-
tion of absorptance 
Thickness (nm) 
layer(s) case (a) case (b) 
Teo 200 200 
P1 15 15 
h 30 470 
N1 15 15 
P1/h/N1 60 500 
P2 15 15 
12 370 570 
N2 15 15 
P2/12/N2 400 600 
The initial efficiency of double stacked a-Si/a-Si cells has 
been obtained around 11 % 14/. However, after light 
induced degradation the stabilized efficiencies of 8% 
were obtained in large area modules. 
The second kind of series tandem celi structure consists 
of two or three p+in+ amorphous cells having different 
band-gaps. This so-called multi-gap amorphous solar 
celi uses the principle of spectral splitting. As shown in 
Flo 4. the incident light enters the first celi made from a 
wide band-gap material, absorbing only the blue portion 
Informacije MI DEM 23(1993)4, str. 259-263 
1.0 
Q) 0.8 12 () 
>=1 
cd 
0.6 -;-J 
o., 
H 
O 
ul 0.4 ..o 
cd 
0.2 
0.0 
400 600 800 
wavelength [nm] 
1000 
J.Furlan, W.Kusian, G.Conte: Tandem Amorphous Silicon 
Solar Cells 
1.0 
0.8 
0.6 
0.4 
0.2 
0.0 
400 600 800 1000 
wavelength [nm] 
Fig. 3: Absorptance of sunlight in variaus layers of a double-stacked a-Si/a-Si solar celi in cases of an unmatched (a) and 
a matched (b) structure. 
of the solar spectrum and producing inherently high 
open-circuit voltage. Higher wavelength light passes 
through the first celi without being absorbed and gener-
ates charge carriers in amorphous material of the sec-
ond celi having smaller band-gap. The rest of the long 
wavelength flux which passes also the second celi is 
finally absorbed in the third celi made from the smallest 
band-gap material. A typical three celi configuration uses 
amorphous silicon-carbon, amorphous silicon, and 
amorphous silicon-germanium as semiconductor ma-
terials in such tandem configuration. As in case of double 
stacked a-Si/a-Si tandem structure the photogenerated 
currents in all cells in series must be equal, so that the 
thicknesses of individual cells are tai lo red with regard to 
their light absorption properties and carrier collection 
capabilities. 
Fig. 4: 
(a) 
(b) 
MUlti-gap stackeci solar celi structure (a) with its 
band-gap representation (b). 
261 
Triple celi modules have been fabricated with initial 
conversion efficiencies between 10 and 11 %, which 
degraded for less than 20% after extended light soaking 
15/. 
PARALLEL TANDEM CELL STRUCTURE 
A new a-Si tandem solar celi structure has been pro-
posed recently in which individual cells are optically 
connected in series and electrically in parallel 12/' This 
structure, shown schematically in Fig. 5, consists of two 
or more a-Si cells stacked in the sequence pin-nip-pin-
... etc. At each interface between consecutive cells the 
electric contact to both joining cells is provided by a thin 
transparent conductive layer. 
ln contrast with series tandem structure in which the 
current passing all unit cells is the same, the parallel 
tandem arrangement requires equal photovoltages in all 
cells. This voltage, being in the vicinity of the open-circuit 
voltage, does not vary strongly with different light gener-
ated and collected charge carrier densities. This is be-
cause the voltage at the maximum celi collection effi-
ciency markedly depends on strongly varying dark cur-
rent and much less on gradually changing photocurrent 
component. This suggests that all unit cells in a parallel 
tandem arrangement must have equal dark charac-
teristics, demanding the same amorphous semiconduc-
tor for all unit cel Is. 
Intensive studies and experiments are currently carried 
on in fabricating a double stacked parallel tandem struc-
ture, and computer simulation of parallel tandem celi has 
also been initiated 13/. Computer modelling of a-Si tan-
dem cells has indicated that the expected sensitivity of 
a parlllel tandem arrangement on the thickness vari-
8ti0 rl '; of individual cells is much lower than in case of 
S811\'S tandem strLJcture. As an example, Fig. 6 shows 
J.Furlan, W.Kusian, G.Conte: Tandem Amorphous Silicon 
Solar Cells 
I 
N 
p 
Fig,5: Parallel tandem a-Si celi eonneetion. 
the computed conversion efficiency of parallel and 
series tandem cells having a fixed bottom celi thickness 
and a varying top celi thickness. It can be noticed that in 
contrast with series tandem celi the thickness of the front 
celi in parallel connection can vary broadly not affecting 
strongly the conversion efficiency. Since the thickness 
of unit cells in parallel tandem connection is not critical 
it can be expected that this tandem arrangement will 
outperform the series tandem structure by having a 
smaller decrease of conversion efficiency due to nonuni-
form a-Si deposition rate in large area solar celi mo-
J r metal 
P i n 
P i 
n 
-, r TeO 
glass 
I r metal 
I I 
1 r TeO 
I r 
1 r Teo 
gia ss 
{ 
Informacije MIDEM 23(1993)4, str. 259-263 
1 .) ." 
(jOOnm , 
11 lo ------~ -:--. ~~--~~: ==-~=~~~ .~~ 
i Lj ()(IIlm //_:::-:::: i 
1.".., /V-- 400nm I 
10 [. ~,~o"\.<~ boUom cclI thickncsses 1 
; Yi 40() und 600 nm 
8
9 1 .:;/'\ ___ .__ parallej tandem . ~ i '\ _ _ _ _ _ scries tandem 
7 L \\600nm J-
o 
Fig. 6: 
\\ 1 
100 200 300 400 500 
top cclI ~_hickness [nm] 
Series and paral/el tandem celi effieieney as 
runetion of top celi thieknesses. 
600 
dules. Considering the fact that all unit cells in a parallel 
tandem structure can be made equally thin it can be 
expected that a structure consisting of multiple thin cells 
in parallel connection will behave very stable after strong 
illumination, having a very weak SW effect. 
FABRICATION OF TANDEM CELL STRUCTURES 
A variety of methods, such as chemical vapor deposition 
and sputtering, could be used to deposit a-Si based solar 
cells. However, the only technique which produced ac-
I I 
) 
I I (a) 
\ 
I I 
Pi n I I 
1 I 
pin I I 
1 I (b) 
i 
Fig. 7: Cross-seetional view with laser serihe pattern of double sta(,vDri,c.pries eonneetion (a) and parallel conneciion (b). 
262 
Informacije MI DEM 23(1993)4, str. 259-263 
ceptable undoped and doped amorphous silicon layers 
is plasma enhanced chemical vapor deposition or short-
Iy glow-discharge process. The existing reactors can 
produ ce today the monolithic modules with active areas 
greater than 40cm x 1 OOcm where the individual cells 
are patterned and interconnected into panels. The deli-
neation of the monolitic pattern can be achieved by 
photolitography, mechanical abrasion, lift-off techniques 
or laser scribing. The most attractive approach for large 
area photovoltaic modules is that of laser scribing pro-
cess, where laser cuts off the layers that have been 
deposited on the substrate. The typical laser scribe 
pattern of a two celi series tandem connection is shown 
in Fig. 7.a. The same patterning pfi.nciple can be used 
for delineation of parallel tandem arrangement, shown 
in Fig. 7.b. Multiple celi panels can be made as exten-
sions of cross sectional presentations in Figs. 7.a and 
b. 
CONCLUSIONS 
Tandem amorphous solar celi consisting of serially con-
nected unit cells presents an improvement of electrical 
characteristics when compared to those of single amor-
phous silicon celI. The partition of a single celi into two 
or three cells connected in series results in smaller 
thickness of each unit celi while maintaining long enough 
path for photons to be moderately absorbed. The con-
sequence ofthinner intrinsic layers of individual cells are 
higher built-in electric field strengths with an improved 
collection efficiency of photogenerated carriers. Higher 
internal electric field causes also smaller celi instability 
due to Staebler-Wronski effect. The concept of parallel 
connected stacked solar celi represents a meaningful 
alternative to the conventional series connected ar-
rangement. The parallel connected structure promises 
a higher conversion efficiency and a smaller sensitivity 
to unit celi thickness variations and will seemingly suffer 
a lower Staebler-Wronski effect. 
263 
J.Furlan, W.Kusian, G.Conte: Tandem Amorphous Silicon 
Solar Cells 
ACKNOWLEDGEMENT 
The work on parallel tandem celi structure is supported 
by the Commission of the European Communities, Pro-
gramme Joule II. 
REFERENCES: 
1. Y. Hamakawa, H. Okamoto, Y. Nitta, A new type ol amorphous 
silicon photovoltaic celi generating more than 2V, Appl. Phys. LeU. 
35 (1979) 187. 
2. J. Furlan, Parallel connected tandem a-Si celi structure, Proc. 6th 
Int. Photovoltaic Science and Engineering Coni. (1992) 287. 
3. W. Kusian, J. Furlan, G. Conte, The pin/TCO/nip a-Si:H solar mo-
dule, submitted to 1 ih European Photovoltaic Solar Energy Coni. 
(1994). 
4. Y. Ichikawa, Recent progress in stability ol a-Si solar celis, Proc. 
5th Int Photovoltaic Science and Engineering Coni. (1990) 617. 
5. R. R. Arya, A. Catalano, M. Bennett, J. Newton, B. Fieselmann, 
L. Yang, Y. M. Li, R. D'Aiello, Triple injection, triple bandgap, a-Si:H 
alloy based devices lor stable performance, Proc. 11th Photovoltaic 
Solar Energy Conf. (1992) 199. 
J. Furlan 
Faculty of Electrical and Computer Engineering, 
Tržaška 28,61000 Ljubljana, Slovenia 
tel. +38661 123 1121 
fax. +386 61 264 990 
W. Kus/an 
Siemens AG, Corporate Research and Development 
Munchen, Germany 
G. Conte 
ENEA, Centra Ricerche Fotovoltaiche, 
Portici-Napoli, Italy 
Prispelo (Arrlved): 10.12.93 Sprejeto (Accepted): 23.12.93