Early Non-Conventional Photovoltaics in Europe

EU - Russian WorkshopSt. Petersburg Nov. 2003

Klaus Thiessen, GOS e.V. Berlin-Adlershof

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

•

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

• PV by extrinsic excitation

H.G.Grimmeiss, H.Koelmans. Phil.Res.Rep. 15, 290 (1960) GaPH.G.Grimmeiss, W.Kischio, A.Rabenau. J.Phys.Chem.Solids 16, 302 (1960) AlPG.Jungk, K.Thiessen. pss 1, K127 (1961) SiCK.Thiessen, G.Jungk. pss 2, 473 (1962) SiCF.M.Berkovski, S.M.Ryvkin. FTT 4, 366 (1962) SiG.Jungk, K.Thiessen, F.Witt. pss 3, 735 (1963) SiCF.M.Berkovski, R.S.Kasymova, S.M.Ryvkin. FTT 5, 524 (1963) GeA.A.Gutkin, D.N.Nasledov, W.E.Sedov. FTT 5, 1138 (1963) GaAsG.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) CdSW.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) ZnSA.A.Gutkin, E.M.Magerramov, D.N.Nasledov, V.E.Sedov. (1969) GaAs

• 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)

Grimmeiss et al. 1960, GaP

intrinsic

extrinsic

Grimmeiss et al. (1962), GaP

Grimmeiss et al. 1960, AlP

Jungk, Thiessen 1961, SiC

intrinsicextrinsic

Photon-energy

Photo-emf (arb.units)

Jungk,Thiessen, 1961

Possible explanation of the extrinsic photovoltage

- impurity level in the forbidden zone

- impurity band in the forbidden zone

Thiessen, Jungk 1962, SiC

Curve 1:

extrinsic photo-emf

Curve 2 and 3:

Luminescence (mea-sured by Kholuyanov)

Friedrich, Sysoev, Thiessen 1968, ZnS (Cu)

Photocond.

PV

Gutkin, Magerramov, Nasledov, Sedov, 1969, 1969

GaAs (S), 293 K

Amplification of intrinsic pv by additionallong 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

Palz, Ruppel 1963, CdS

Extrinsic Photovoltage

Extrinsic Photoconduct.

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

thermal transitions, thermodynamic equlibrium

p0

n0

Optical electron-hole pair excitation(Band-Band)

δn

δp

hν > Eg

Optical excitation, from valence band to impurity level Ej

δp

Optical excitation,from Impurity level Ej to conduction band

δn

Two step optical excitation

δp

δn

Recombination,electron capture by impurity level Ej

Recombination,hole capture by impurity level Ej

Thiessen, 1962, conditions for the optical generationof minority and majority carriers

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

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.

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.

100th Birthday of Max Planck, Leipzig, 1958

A.F.Ioffe M.v.Laue

100th Birthday of Max Planck, Leipzig 1958