Observation and Study of Wavelength Dependence of the Open-Circuit Voltage in a-Si:H Based p-i-n Solar Cells
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OBSERVATION AND STUDY OF WAVELENGTH DEPENDENCE OF THE OPEN-CIRCUIT VOLTAGE IN a-Si:H BASED p-i-n SOLAR CELLS M. GORN, N. KNIFFLER, AND G. WINTERLING Energy and Process Technology Group, Messerschmitt-BblkowBlohm GmbH, P.O. Box 80 11 09, D-8000 M~nchen 80, Federal Republic of Germany ABSTRACT The spectral dependence of Voc is studied in a-Si:H based p-i-n junctions by using chopped monochromatic light. A dispersion AVoc = Voc(blue) - Voc(red) is observed which depends systematically on a low level doping of the central layer. While Voc is largest and positive in phosphorous doped cells it is negative in boron doped cells. Voc is found to be enhanced by an additional white bias light. The dispersion data are discussed by considering the contributions to Voc arising from drift in an inhomogeneous field and from carrier diffusion. 1. EXPERIMENTAL TCO/p (SiCx:H) i - n/Al solar cells have been made by rf-plasma decomposition of SiH4 . The special feature of these cells was a light doping of the cental layer (thickness around 5000 A) by adding either PH 3 or B 2 H6 to SiH4 in the range between 0 and 1 vppm. The spectral variation of the current-voltage curves were studied by using chopped quasimonochromatic (aX= 5nm) light with an intensity in the range of 10-4 Watt/cm2 and an additional white bias light of 60mW/cm 2 . The signal current at the chopping frequency of 333 Hz was recorded with an lock-in amplifier. When changing the wavelength the intensity of the chopped light was adjusted such as to keep the signal current level constant at zero bias voltage. Then the signal current was measured with increasing forward bias yielding Voc (X). 2.
RESULTS
Light doping effects in p-i-n solar cells have been studied primarily in the context of cell efficiency and the spectral dependence of the fill factor. Thestresults have described elsewere 1i. In the present paper we report on observations of the spectral dependence of Voc. Dispersion denotes the difference Voc
=
Voc(blue)-Voc(red)
(2.1)
with typically \ = 400nm and 600nm, respectively. We had several series, also from different reactors which apart from some details at lower doping levels, clearly exhibit remarkable agreement in the following features:
Mat. Res. Soc. Symp. Proc. Vo4.70- 1986 Materials Research Society
538
1) positive (figs. 1 2) negative >.75 ppm
dispersion in undoped and phosphorus-doped cells a, b, and c) dispersion in boron doped cells with doping level B2 H6 (fig. 1 d) 2
current density
lttA/cm ]
50
ld .5
Ub
Volt' v/
-25
b l/
50
,-50
.5
b
I /
[Voit-
Fig. 1: Spectral I-V curves (forward bias): full line 420 nm, broken line 600 nm (a)
50
-50
/
/
50.5
/ /
c c/
central
layer undoped (,? 8%)
(b) .5 ppm PH3 (c) as (b) i- weak bias light (d) .75 ppm B2H6
Ub b Vol
In addition, bias light is found to strongly affect the dispersion of Voc in some cases. Voc (X) is plotted versus c , optical absorption, in fig. 2 for various bias light intensities and light doping levels. The main features are: a) phosphorus doped central laye
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