Thin film solar cell materials

Applied Surface Science 70/71 (1993) 650-659 North-Holland

applied surface science

Thin film solar cell materials G.H. Bauer Institut fiir Physikalische Elektronik, Universitiit Stuttgart, Pfaffenwaldring 47, D 7000 Stuttgart 80, Germany Received 26 August 1992; accepted for publication 20 November 1992

Photovoltaic energy conversion is based upon the generation of charge carriers by absorption of light and on their local separation in semiconductor junctions. The necessary thickness of the absorbing layer has to correspond with the lengths across which photogenerated carriers are collected. With help of direct semiconductors, thicknesses of absorbers and devices can be reduced to 1 micrometer or less providing the entire spectrum of economic and energetic benefits of thin film technology. At present, primarily two classes of thin films are considered from which already at least laboratory cells with conversion efficiencies in the range of 12-15% were demonstrated: amorphous hydrogenated silicon (a-Si:H), and polycrystalline chalcogenide and chalcopyrite cells, like CdTe, or CuInSe2-based cells. However, additionally to the promising technological advantages of thin films some of their specific problems and limitations, such as metastable effects in a-Si:H, and the use of environmental hazardous metals like Cd have to be taken into account very carefully.

1. Introduction I n o r d e r to s u b s t i t u t e e x h a u s t i n g fossil fuels a n d m o r e o v e r to avoid a f u r t h e r i n c r e a s e o f envir o n m e n t a l p o l l u t i o n for f u t u r e e n e r g y supply t h e use o f r e n e w a b l e e n e r g y a n d in p a r t i c u l a r conversion of solar r a d i a t i o n is a must. P h o t o v o l t a i c e n e r g y c o n v e r s i o n is b a s e d u p o n (i) t h e a b s o r p tion o f p h o t o n s in s e m i c o n d u c t o r s a n d t h e acc o r d i n g g e n e r a t i o n o f free e l e c t r o n s a n d holes, which intermediately provide the storage of phot o n e n e r g i e s h u at t h e a m o u n t of p o t e n t i a l energy, w h i c h for e l e c t r o n s e.g. r e a d s ( E c - E v + k T ) , a n d (ii) t h e local s e p a r a t i o n o f t h e s e p h o t o g e n e r a t e d c a r r i e r s by g r a d i e n t s in q u a s i - F e r m i levels, i n t e r n a l e l e c t r i c fields, o r g r a d i e n t s in carrier concentration. P r e s e n t l y p h o t o v o l t a i c e n e r g y c o n v e r s i o n has a l r e a d y b e e n a p p l i e d in several p o w e r p l a n t s of u p to s o m e h u n d r e d s o f kWp ( p e a k p o w e r ins t a l l e d ) which n e a r l y exclusively consist o f single-, o r m u l t y c r i s t a l l i n e silicon solar cells. H o w e v e r , crystalline silicon as an i n d i r e c t s e m i c o n d u c t o r

with low a b s o r p t i o n coefficient r e q u i r e s film t h i c k n e s s e s for the a b s o r p t i o n of solar r a d i a t i o n o f at least 100 /xm. T r a d i t i o n a l l y available c-Si solar cell t h i c k n e s s e s d u e to also available Sit e c h n o l o g y a m o u n t to a b o u t 4 0 0 - 5 0 0 /xm, b e i n g p r e p a r e d via expensive p r o c e s s e s of high t e m p e r ature, e.g. for S i - m a t e r i a l p u r i f i c a t i o n ( e l e c t r o n i c g r a d e Si), a n d diffusion of d o p a n t s , as well as of m e c h a n i c a l t r e a t m e n t such as sawing of wavers, a n d w e l d i n g of i n t e r c o n n e c t i o n s of cells a n d ass e m b l i n g to m o d u l e s . Among these technological aspects that strongly e n t e r cost a n d price b a l a n c e o f c-Si cells, an e x t e n s i o n of p r o d u c t i o n r a t e s t o w a r d s tens or h u n d r e d s of M W / y e a r o r even G W / y e a r in ord e r to t o u c h t h e n e i g h b o r h o o d of few p e r c e n t s of r e g i o n a l o r n a t i o n a l e n e r g y d e m a n d , in t e r m s of availability of p r o d u c t i o n p l a n t s a n d i n v e s t m e n t cost, s e e m s to b e very unlikely. C o n s e q u e n t l y , thin film s e m i c o n d u c t o r s showing high a b s o r p t i o n coefficients ( a = 105-10 6 c m - 1 ) , a n d thus dem a n d i n g only thicknesses o f a b s o r b e r s o f few m i c r o m e t e r s o r even less, b e i n g d e p o s i t e d at low

0169-4332/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

G.H. Bauer / Thin film solar cell materials

substrate temperatures (T~ _< 450°C), suitable for n- and p-type doping, are promising candidates for large area, low cost photovoltaics.

651

30 ~ ~ _ ~20 g-

2. Selection criteria of thin film semiconductors

The criteria for the selection of photovoltaic thin film materials can be subdivided into physical (i-v), and technological (vi-x) ones, such as: (i) suitable optical band gap of 1.0 < Eg < 2.0 eV, (ii) high absorption coefficient a (hu > Eg) > 5

10 0

0.5 1.0 1.5 2.0 2.5 i

i

i

Zg [eV]

Fig. 2. Theoretical limit of photovoltaic solar energy conversion for air-mass-zero-radiation (AMO) versus band gap after ref. [1].

X 10 4 cm -1,

(iii) life t i m e / m o b i l i t y , respectively, drift lengths/diffusion lengths of minority carriers (Xs, L > 2/a), (iv) low surface recombination velocity ( o r / a n d suitability for passivation of surface states), (v) suitability for n- or p-type doping, (vi) availability of necessary elements, (vii) applicability for deposition on inexpensive substrates, (viii) reproducibility of deposition process, (ix) energy balance/energy pay back time, (x) safety issues during production and operation of devices. From optical absorption and band gaps of semiconductors, outlined in fig. 1 for the most relevant solar cells candidates, a theoretical limit for photovoltaic energy conversion has been cal-

106 CuInSe2 ~

CdTe

105

104

103 10 z

-S :

101 , 1.0

1.4

photon

1.8

energy

2.2

2.6

h u (eV)

Fig. 1. Spectral absorption coefficients of semiconductors.

culated, considering black-body-radiation energy and entropy fluxes to and from a receiver and introducing a threshold energy Eg for absorption/emission of photons according to electronic band-to-band transitions in semiconductors [1]. In fig. 2 theoretical efficiencies of non-concentrated solar radiation (AMO) versus optical band gap are drawn with various types of semiconductors included. Although the theoretical limit of the efficiency for solar energy conversion is rl < 0.85, in single junction solar cells only rt = 25% can be achieved. Obviously the lion's share of losses of about 60% has to be attributed to excess energy of photons ( h u - Eg) which as kinetic energy of "hot" electrons or holes thermalizes within 10-13 s to the band edges, as well as to photon energies too low to be absorbed since hv < Eg. In order to reduce these losses a further approach of spectrum splitting combines absorbers of different band gaps in optical series connection (tandem or multispectral structures). Thus, in fact, an increase in efficiency for photovoltaic energy conversion can be achieved [2] (fig. 3), provided, appropriate semiconductor band gaps can be tailored. There is a variety of amorphous and polycrystalline binary, ternary, or quaternary semiconductor thin films (alloys) meeting optical requirements for spectrum splitting. Amongst the attempts for the growth of thin crystalline silicon 30-50 Ixm and of Ill-V-compounds like GaAs, AIGaAs, or InP, which do not at all so far benefit from the advantages of thin film technology, such

G.H. Bauer / Thin film solar cell materials

652

600

>

~E

400

"-~ 2 0 0

0 .5

1.0

1.5

2.0

2.5

3.0

h~(eV) 50

o 40

30 o

20

f I

I

1 2 number

I

I

I

t

3 4 5 6 of s o l a r c e l l s

I

7

Fig. 3. Spectrum-splitting and m a x i m u m photovoltaic conversion efficiencies for single- and multi-junction solar cells after ref. [3].

as low temperature and large area deposition and low cost, presently hydrogenated amorphous silicon (a-Si:H) and its alloys, and polycrystalline chalcogenide or chalcopyrite thin films, particularly CdTe, and Cu(In, Ga)Se 2 are the most promising materials for single junction cells. Moreover, by alloying a-Si:H with Ge and C shrinking and widening of band gap in 1.0 < Eg < 3.0 eV has been achieved [3], and analogously in the quaternary system Cu(In, Ga)Se 2 a variation of Eg in 1.04 < Eg N 1.68 eV can be performed [41.

overcoordination of these networks, consisting of about 90% of exclusively tetrahedrally bounding Si and about 10% of hydrogen cause a certain amount of localized states in band tails (weak Si-Si-bonds) and some non-saturated, dangling bonds at midgap in the electronic density of states. Consequently transport of carriers is governed by interaction with band tail states (balance of trapping and phonon-assisted reemission), scattering lengths ( < 50 A [5]) and ambipolar diffusion lengths (200 nm [6]) are small, and recombination of electrons and holes is dominated by midgap defects with appropriate charge states. Doping of a-Si:H by incorporation of B or P during plasma enhanced gas phase deposition (PECVD) from silane by admixture of phosphine or diborane has been demonstrated successfully [7], and has been understood as electron exchange in a d o p a n t - d e fect reaction [8], despite in the early days 8 - Nrule was interpreted to prevent doping of those random networks [9]. While diffusion lengths in a-Si:H are smaller than the necessary thickness for light absorption (500 nm), solar cells are used to be heterojunction pin-diodes (high band gap p+-window) with an internal electric field supporting collection of photogenerated charges [10,11]. Best efficiencies of single barrier small area diodes after optimization of individual layers and process sequence deposited on glass substrates coated with a transparent conducting oxide front contact amount to 12-13% (fig. 4); efficiencies of large area ceils and modules up to 1000 cm 2 monolithically interconnected (fig. 5) have been improved over the years to close to 10% (see fig. 6).

3. Hydrogenated amorphous silicon solar cells

glass substrate

p+a-SiC:H (10nm) i n t r i n s i c a - S i : H ( 3J 5 0 - 4 5 0 n m ) /

3.1. General physical aspects of amorphous silicon As a consequence of the lack of translational symmetry in the disordered network of a-Si:H and due to strong electron-phonon-interaction selection rules for optical transitions are weakened and the optical absorption gets as high as in direct semiconductors. On the other hand the

light / t e x t u r e d TCO bt~ffer

(SnO~)

metal

(into) n+a_Si:H (15nm)

Fig. 4. Structure of high efficiency a-Si:H pin-thin film cell.

G.H. Bauer / Thin film solar cell materials metal back

653

contact

14

o

8

"~

6

•

•

AA

O 063 [] [-0 small area single jet. [
4

TCO f r o n t

contact

nrned, area single jet. [terns] iX hetero jet. + stacked • large area [100-1200crnz]

2

Fig. 5. Principle of monolithic interconncction of thin film cells to modules.

o 976

~ 1980 1985 calendar year

1990

Fig. 6. Cell and module efficiencies of a-Si:H pin diodes.

3.2. Metastability effects in amorphous silicon dangling bond conversion (defect-pool-model)

Since structural properties of a-Si:H as a glassy material in thermal equilibrium are temperature dependent, any departure from thermal equilibrium like changes in carrier densities by photogeneration or injection/depletion introduces changes in structural properties and consequently in optical and electronic properties as well. One of these changes observed in the early days has been the light induced increase of dangling bonds defects at midgap [12] which has recently been modeled quantitatively on the basis of a statistical mechanic's approach including weak-bond to

[13-15]. The influence of these metastable, reversible changes on solar cell parameters, simply spoken, cause an increase of defect density in the /-layer and accordingly higher space charge and more compensation of the internal electric field so that recombination of photogenerated carriers increases and their collection decreases; these effects get the more pronounced the lower the built-in-potential gets, say, the more the diode is operated close to open circuit voltage Voc. At

Table 1 Efficiencies of a-Si:H solar cells and modules [18,19] Initial efficiency (%) Single-junction technology Small-area cells Modules (933 cm 2) Modules (4700 cm 2 ) Modules (12 000 cm 2) (Light-induced degradation)

11.9 9.8 7.8 a) 6.2 (]5-30%)

Multi-junction technology Small-area triple junction Small-area dual junction (Si/SiGe) Small-area dual junction (Si/Si) Dual-junction modules (1200 cm 2, Si/Si) Dual-junction modules (3673 cm 2, Si/Si) Triple-junction modules (940 cm 2, S i / S i / S i G e ) Triple-junction modules (795 cm 2, S i / S i / S i G e )

13.3 12.4 11.9 9.7 7.4 9.0 9.7

a) Not confirmed by NREL. b) Active area efficiency.

b) b) a,b) a,b)

Stabilized efficiency (%)

4.7 a)

11.0 a,b) (600 h AM1) 9.7 (10.4 b)) 10.0 a,b) 8.6a, b) 6.3 7.2

654

G.H. Bauer / Thin film solar cell materials

most filling factors FF and open circuit voltages Voc and thus efficiencies r/ are affected by light induced changes, minor influences on short circuit current densities Jsc are observed. Although it has not been strictly proven experimentally, there is strong evidence that under illumination in films and in pin-diodes a steady state of the distribution and density of defects is established by which the efficiency of single barrier cells with traditional /-layer thicknesses of about 450 nm does not exceed 7-7.5%. 3.3. Stabilized cells and future concepts

Recent analyses of kinetics of defect generation and annealing, their energetic distribution in the gap in accordance with their charge state, and their saturation values strongly indicate metastability to be an intrinsic structural effect of a-Si:H [16,17], so that there are in principal no means to avoid them entirely, and even a further improvement of material stability keeping all other properties unchanged must be basically questioned. However, fortunately there are sufficient bona fide prospects to minimize the influence of these metastable effects on device parameters and on device function. As light induced defect generation is fed by energy supply from recombining carriers, a reduction of t h e / - l a y e r thickness across which carriers are collected more efficiently reduces effects of metastable defect creation, the more as in t h i n n e r / - l a y e r s as an additional feedback, higher electric fields are established; for the compensation of lower absorption of the first cell (top cell) in only about 100 nm thick absorber films a second pin diode (bottom cell) can be attached in optical series connection which consists either of a junction with the same bandgap (stacked cell) or of an even lower bandgap (tandem cell); the individual absorber thicknesses have to be properly adjusted for current matching considering particular absorption coefficients and solar spectral distribution [18-20]. The present state of the art of those types of stacked and tandem cells exclusively p r e p a r e d as two-terminal diodes, which means current matching (in all cells similar photocurrents have to be generated/collected) is summarized in table 1

t i

0

j

/rc0 al

--5

r~=

/~

a-Si:H pin-structure

IO-'a,or:'~"l

~)mV

j,q

.

..,._, - t O

o=

15

30"

(~ ~

i

. ~

-500

0

voltage

~ 500

,7=8 i~. i

1000

(mY)

Fig. 7. Novel structure of an ultrathin (100 nm), quasi-stable a-Si:H-pin diode, macroscopieally folded for improving red response after ref. [23]. [21] and indicates the option for high efficiencies and long term stability of a-Si:H based solar cells. The admixture of Ge to a-Si:H not only shrinks the optical band gap but also tends to reduces the susceptibility for light induced defect creation in a-SiGe:H films tremendously [22], so that aSiGe:H pin diodes can be expected to be much less affected by light soaking. A novel attempt to stabilize single-junction cells at high efficiencies (so far close to 9% [23]) is based upon a microscopic flat, macroscopically folded structure (fig. 7) in order to provide sufficient a b s o r p t i o n / g e n eration of charge carriers by multireflection, although the electronically active thickness of the absorber is only 100 nm: the electric parameters of pin-diodes of this thickness are known to keep unaffected by light soaking of the bulk material.

4. Polycristalline thin film solar cells 4.1. General aspects

A variety of direct polycristalline compound semiconductors exist with optical band gaps in the relevant range for photovoltaics (table 2). Commonly thin film solar cells are prepared as heterojunctions combining a high gap window semiconductor with a lower gap absorber in order to reduce surface recombination losses at the light entrance side. Window materials, typically n-type oxides with band gaps of Eg > 2.4 eV, like

655

G.H. Bauer / Thin film solar cell materials

Table 2 Photovoltaic relevant polycristalline thin films Semiconductor

Band gap (eV)

Conduction

FeS 2 FeSi 2 CuInSe 2 WeS2 InSe

0.8 0.9 1.04 1.3 1.3

p/n p p/n p/n p

CdTe CuInS2 CuGaSe 2 CdSe ZnTe

1.5 1.5 1.68 1.7 2.26

p/n p/n p n p

CdS (2.42 eV), ZnCdS (2.42-3.7 eV), ZnSe (2.8 eV), ZnO (3.2 eV), In203 (3.2 eV), In203SnO 2 (3.2 eV), or SnO 2 (3.3 eV), due to oxygen vacancies fit very well to p-type absorbers in which electrons with generally higher mobility and diffusion lengths than holes represent minority carriers. Combinations of absorbers and windows are selected considering furthermore low band discontinuities and tolerable densities of interface states which usually are introduced by differences in crystal orientation, lattice constants, and thermal expansion coefficients; availability of elements as well as suitability for large area deposition have to be met in addition. Depending on technological conditions, such as temperature profile during deposition, interdiffusion of elements, and need of buffer or blocking layers, front wall (back contact/absorber/window/front contact), or back wall cells (glass substrate/front contact/(=)window/absorber/back contact) are to be prepared; back wall cells on glass substrates benefit from already being half part encapsulated. Routinely layers close to the junction are less doped to improve carrier collection by an extended field zone, and additional thin interface and buffer layers (few nm) diminish the density of interface states between window and absorber resulting from lattice mismatch. The function of thin film heterojunction cells and in particular transport and recombination of carriers is strongly governed by grain boundaries, explicitly by their density and by the orientation

Fig. 8. SEM-picture of a polycristalline CdS/CulnSe2 heterojunction solar cell after ref. [24].

of crystals with respect to the particular current paths (figs. 8 and 9). 4.2. Technology and present state of the art

The deposition of today's photovoltaic relevant polycrystalline semiconductors, like Cu(Ga, In)(Se 2, $2), CdTe, CdSe, ZnTe, CdS, ZnO, several low temperature processes have been succesfully applied to diode preparation: vacuum evaporation/physical vapor deposition (VPD), sputtering deposition (SPD), spray-pyrolysis (SPL), electro-

m

~

contact

window I~

absorber ]ll]l]I[]]]l]][]]]i][]]]II]]]]]I[l]]]]]]] back contact

Fig. 9. Schematic transport and recombination of charge carriers in polycristalline heterojunctions. (1) photogenerated current; (2) interface recombination; (3) grain boundary recombination; (4) bulk recombination; (5) shunt current.

G.H. Bauer / Thin film solar cell materials

656

Table 3 Efficiencies of polycrystaUine CdTe-solar cells [26,30] Cell structure

Absorption

deposition

Voc (mV)

Jsc (mA/cm 2)

FF (%)

Efficiency (%)

Area (cm2)

SnO2/CdS/CdTe ITO/CdS/CdTe SnO2/CdS/CdTe SnOz/CdS/CdTe CdS/CdTe

CSVT ED SPL ED SC

843 720 783 800 797

25.1 27.9 25 24.3 21.1

75 65 67 67 67

15.8 13.1 12.3 13.0 11.3

1.05 0.02 0.31 0.02 1.02

CdS/CdTe (Zn, Cd)S/CdTe SnO2/CdTe SnOz/CdS/CdTe/ZnTe InzO3/CdTe

ALE SP CVST ED x-tal

804 870 663 767 892

23.8 23.6 28.1 22.4 21.6

73 61 56 70 75

14.0 12.6 10.5 11.2 14.4

0.12 0.3 4.0 1.07 0.02

CSVT: closed space vapor transport; ED: electrodeposition; SPL: spray pyrolysis; SC: scribing; ALE: atomic layer epitaxy.

chemical deposition/electrodeposition (ED), stacked-layer reactions (SLR), screen printing (SPR), chemical bath deposition (CBD) (see also tables 3 and 4), and very recently with lab cells based on both CdTe as well as on CulnSe 2 impressive improvements of efficiencies to over, or close to 15% have been demonstrated [24-27].

CdTe as congruently evaporating compound single elements show each higher vapor pressure than the compound - might be cooked by several methods and brought into ideal stoichiometric composition by subsequent annealing at 550 < T < 700°C providing presently highest thin film diode efficiencies of > 15% (table 3).

Table 4 Efficiencies of polycrystalline CulnSe 2 cells [24,31] Cell structure

Optical gap Eg (eV)

Voc (mV)

Jsc (mA/cm 2)

FF (%)

Efficiency (%)

Area (cm 2)

MgF2/ZnO/CdS / CulnSe 2 ZnO/CdS/CulnSe 2 ZnO/CdS/CulnSe 2 Z n O / B F b)/CulnSe2 MgF2/ZnO/CdS / Culn(S,Se) 2

1.02

513

40.4

72

14.8

0.33

1.02 a) 1.02 1.02 1.12

508 486 443 611

41.0 39.8 37.7 33.7

68 72 62 71

14.1 13.9 10.3 14.6

3.5 0.25 0.25 0.15

1.45 1.27

655 667

24.5 28.8

64 74

10.3 14.2

0.25 0.08

1.17

555

35.3

66

12.9

0.96

1.20

583

29.6

71

12.2

0.25

1.27

658

28.0

68

12.4

0.38

ZnO/CdS/Culn(Se)S 2 MgF2/ZnO/CdS / CulnSSe ZnO/ZnCdS/ Culn I _xGaxSe2 (x = 0.27) ZnO/CdS/ Culn I _xGa/Sez (x = 0.29) ZnO/CdS/ Culnl_xGaxSe 2 (x = 0.37)

All absorbers by PVD: physical vapor deposition. ~) Deposition method not pub)lished. b) Cd-free buffer layer (BF).

G.H. Bauer / Thin film solar cell materials

Regarding efficiency and stability the other promising candidate for polycrystalline thin film cells represents the chalcogenide/chalcopyrite system as a front wall cell with CdS/CuInSe 2 showing performances of 14.8% so far (see table 4). Alike band gap variation by alloying of a-Si:H, gap energies in the quaternary compound ln(Cu, Ga)Se 2 are precisely tunable within 1.04 Eg ~ 1.68 eV by the In/Ga-ratio and thus provide the basis for spectrum splitting tandem structures [4,27]. Spectral quantum yields and I V-curves of chalcopyrite cells with different band gaps in figs. 10 and 11 confirm the excellent suitability for multispectral photovoltaics.

5. Ill-V-compounds and thin crystalline silicon cells

III-V-compound semiconductors such as GaAs, GaAlAs InP, GalnP, GaSb due to direct optical transitions and high absorption coefficient in alliance with their appropriate band gaps represent excellent candidates for photovoltaic cells, provided they are deposited as monocrystalline,

657

0

_

_

~

-10 y=l

~-30

i

y=O.

-40

-50

0,0

0.2

0.4 v o l t a g e [V]

0.6

0.8

Fig. 10. /-V-characteristics of Cu(InGa)Se2-thin film cells with different G a / I n ratios.

high purity films with sufficient low dislocation density ( < 106 cm -1) and within the very low limits of local stoichiometry. These requirements, in particular to satisfy for high efficiency cells of about 18-22% (table 5) are presently only met by highly energy and cost intensive epitactic deposition methods and by growing films on substrates exhibiting nearly identical lattice constants and orientation such as Ill-V-materials or sophisticated multilayers that provide relaxation of struc-

Table 5 Efficiencies of llI-V-semiconductor solar cells [32] Cell type

Area (cm 2)

Voc (mV)

Jsc (mA/cm a)

FF (%)

Efficiency (%)

Thin films GaAs cell GaAs submodule GaAs stacked on CulnSe z

1011 4034 Four

27.6 6.6 Terminal

83.8 79.6

23.3 21.0 25.8

4.0 16.0 4.0

Epitaxial single junction AIGaAs/GaAs AIGaAs/GaAs(Ge) GalnP/GaAs(GaAs) GaAs(Si) InP(InP) homojunction

1022 1035 1038 891 878

28.2 27.6 28.7 25.5 29.3

87.1 85.3 86.4 77.7 85.4

25.1 24.3 25.7 17.6 21.9

4.0 4.0 0.2 0.3 4.0

82.5 83.4

28.7 a) 27.5 31.8 27.5 27.6 34.2 a) 31.0 a) 30.2

Concentrator cells GaAs single junction GalnAsP single junction (1.15 eV) InP/GalnAs monolith. (0.75 eV) GalnP/GaAs monolith. AIGaAs/GaAs monolith. GaAs/GaSb stacked tandem GaAs/Si stacked tandem G a A s / G a l n A s P stacked tandem

899 Three 2292 2403

Terminal 13.6 13.96

a) Recalibrating at Sandia will likely result in down-grading of efficiencies by 4%.

200.0 171.0 50.0 1.0 1.0 100.0 500.0 40.0

658

G.H. Bauer / Thin film solar cell materials

1.0

~

~' 0.6

.

.

~ ~-"

1-y~ayse2

y=l ",

m ~0.2 0.0

500

700 900 wavelength

llO0 [nm]

1300

Fig. 11. Spectral quantum yields of Cu(InGa)Se2-thin film cells with different G a / I n ratios.

tural properties [28]. The expensive components as well as the costly processing will evidently limit a potential terrestrial application to concentrating system. Crystalline Si with thicknesses of only few tens of micrometers (30-100 p~m) has been used for solar cells by increasing the poor absorption especially for long wavelengths by scattering surfaces [29]. Nonetheless, so far no real breakthrough in material preparation and economy, or in efficiencies has proven this option to be seriously considered for future applications.

MWp/year seems to be provided by thin film technology. Amorphous silicon in its early days has been considered too optimisticly; presently based on a sufficient understanding of metastability technological means are being developed to minimize consequences of metastability effects on solar cells. A stabilization of module efficiencies at 10% or above seems to be achievable. The biggest progress in efficiencies of thin film cells of close or more than 15% has been achieved with polycrystalline compound semiconductors. The reproducible technological processing of these cells, however, has not yet been proven, and it is worthwhile to mention that these impressive efficiencies have been realized with cells containing - and in the case of CdTe-diodes exclusively consisting of - environmentally hazardous elements. Amongst these presently highly recognized thin films, there remain lots of other potential candidates, which cannot be enumerated entirely, and which are already investigated, like p o r o u s / quantum-wire silicon, polycrystalline films, such as FeS2, FeSi2, AgGaSe2, CuGaSe2, CuGaTe 2, WSe 2, or quasi-crystals, and even C60-fullerenes.

6. Summary

References

Analogous to every technical commodity, also solar cells and the particular material from which they have been manufactured are keeping mutating and we certainly may assume not a single type of solar cell or respective material to emerge as only applicable device, rather on the contrary, different types of cells and modules depending on operation and on their individual temporal and technological maturity will enter into the market, provided photovoltaic power generation will counterbalance a considerable contribution of photovoltaic power to our future electrical energy demand. The announcements for c-Si-cell based modules of about 1 E C U / W p (peak Watt) was by far too optimistic, and today the best option for dumping costs, and for establishing payable manufacturing of tens, hundreds or thousands

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