Early research in the 1960s by several European groups found that the photovoltaic effect could be achieved through extrinsic, two-photon excitation using impurity levels rather than requiring photon energies greater than the bandgap. This extrinsic photovoltaic effect was demonstrated in semiconductors like GaP, AlP, SiC, Si, Ge, GaAs, CdS, and ZnS. The effect was explained by optical excitation transferring carriers between impurity levels and bands. Thiessen later outlined the conditions needed for generating both minority and majority carriers via optical excitation to impurity levels. This extrinsic effect could potentially extend the spectral range of solar cells but would require materials with wider bandgaps than silicon

1. Early Non-Conventional Photovoltaics in Europe
EU - Russian Workshop
St. Petersburg Nov. 2003
Klaus Thiessen, GOS e.V. Berlin-Adlershof

2. Abstract
• For photovoltaics in p-n structures it is necessary to
have both types of excess carriers - electrons and
holes (minority and majority carriers), unlike
photoconductivity requiring only one type of carriers,
independent of its sign.
• That is the reason why for many years people
believed that to obtain a photovoltaic effect at p-n
junctions it is necessary to have photon energies
equal to or exceeding the energy gap (hν>Eg).
• Only in 1955, Walter Schottky in a response to a
lecture by R.Wiesner pointed out that, in principle, it
should also be possible to observe a photo-emf at p-n
junctions by excitation by means of two photons using
an impurity level, if that level is located near the
middle of the energy gap
•

3. In the sixties several groups in
Europe published results on the
extrinsic photovoltaic effect in
different semiconductors
and on the
ampification of intrinsic pv by
additional light in the extrinsic range.

4. • PV by extrinsic excitation
H.G.Grimmeiss, H.Koelmans. Phil.Res.Rep. 15, 290 (1960) GaP
H.G.Grimmeiss, W.Kischio, A.Rabenau. J.Phys.Chem.Solids 16, 302 (1960) AlP
G.Jungk, K.Thiessen. pss 1, K127 (1961) SiC
K.Thiessen, G.Jungk. pss 2, 473 (1962) SiC
F.M.Berkovski, S.M.Ryvkin. FTT 4, 366 (1962) Si
G.Jungk, K.Thiessen, F.Witt. pss 3, 735 (1963) SiC
F.M.Berkovski, R.S.Kasymova, S.M.Ryvkin. FTT 5, 524 (1963) Ge
A.A.Gutkin, D.N.Nasledov, W.E.Sedov. FTT 5, 1138 (1963) GaAs
G.Jungk, H.Menniger, K.Thiessen. pss 6, 241 (1964) SiC
W.Palz, W.Ruppel. pss 6, K161 (1964) CdS
W.Palz. Thesis, Univ. Karlsruhe, (1965) CdS
W.Palz, W.Ruppel. pss 15, 665 (1966) CdS
H.Friedrich, K.Thiessen. Proc.Int.Conf.Luminescence (Budapest)
II, 210 (1966) ZnS
H.Friedrich, L.A.Sysoev, K.Thiessen. pss 26 107 (1968) ZnS
A.A.Gutkin, E.M.Magerramov, D.N.Nasledov, V.E.Sedov. (1969) GaAs

5. • These results will be discussed and the
general conditions for an extrinsic
Photovoltaic effect, without Schottky´s
restriction, will be given.
K.Thiessen. pss 2,1260 (1962)

6. Grimmeiss et al. 1960, GaP
intrinsic
extrinsic

7. Grimmeiss et al. (1962), GaP

8. Grimmeiss et al. 1960, AlP

9. Jungk, Thiessen 1961, SiC
intrinsicextrinsic
Photon-energy
Photo-emf (arb.units)

10. Jungk,Thiessen, 1961
Possible explanation of the extrinsic photovoltage
- impurity level in the forbidden zone
- impurity band in the forbidden zone

11. Thiessen, Jungk 1962, SiC
Curve 1:
extrinsic photo-emf
Curve 2 and 3:
Luminescence (mea-
sured by Kholuyanov)

12. Friedrich, Sysoev, Thiessen 1968, ZnS (Cu)
Photocond.
PV

13. Gutkin, Magerramov, Nasledov, Sedov, 1969,
1969
GaAs (S), 293 K

14. Amplification of intrinsic pv by additional
long wavelength illumination
Several groups investigated the influence of an additional steady
state illumination in different ranges of wavelength of the spectral
distribution, measured by chopped light.
Hence we measured the increase of the diffusion length by
recharging the recombination (excitation) levels.
Conclusion now:
the efficiency of solar cells seems to come from both, the
intrinsic and the extrinsic generation
It may be possible to increase the efficiency of pv-cells by
suitable (deep level) doping

15. Palz, Ruppel 1963, CdS
Extrinsic Photovoltage
Extrinsic Photoconduct.

16. Jungk, Menniger, Thiessen. 1964, Si
isc (arb.units), curve 1: without add.light
curve 3: with light through Si-filter; right: amplification

17. thermal transitions, thermodynamic equlibrium
p0
n0

18. Optical electron-hole pair excitation
(Band-Band)
δn
δp
hν > Eg

19. Optical excitation,
from valence band to impurity level Ej
δp

20. Optical excitation,
from Impurity level Ej to conduction band
δn

21. Two step optical excitation
δp
δn

22. Recombination,
electron capture by impurity level Ej

23. Recombination,
hole capture by impurity level Ej

24. Thiessen, 1962, conditions for the optical generation
of minority and majority carriers

25. Thiessen, 1962, conditions for the optical generation
of minority and majority carriers

26. K.Thiessen, G.Jungk. pss 2, 473 (1962)
Extrinsic Photovoltaic Effect in SiC
The measurements on extrinsic photo-emf demonstrate that it
should in principle be possible by suitable doping to extend the
spectral range of p-n-photocells.

27. Conclusion
• If scientists, working in semiconductor physics and those working in
luminescence already in the fifties would speak in the same
language, we would much earlier investigate the extrinsic
photovoltaic effect, because the Anti-Stokes-Luminescence was
already well understood by a two step optical generation.
• Unfortunately the same impurity levels, providing two step
excitation, are at the same time recombination centers. Increasing
the concentration of these impurity centers could lead to a decrease
of the PV in the intrinsic range of excitation.
• The extrinsic PV should be at room temperature high enough for
potential use in solar cells only in wide gap semiconductors. Silicon
is to my opinion not the material of choice for an extension of the
spectral range to longer wavelengths.
• The amplification of intrinsic PV by means of an influence of
simultaneously longer wavelength illumination should be discussed
further.

28. 100th Birthday of Max Planck, Leipzig, 1958
A.F.Ioffe M.v.Laue

29. 100th Birthday of Max Planck, Leipzig 1958