a-SiGe:H Alloy and its Application to Tandem-Type Solar Cell
- PDF / 388,796 Bytes
- 8 Pages / 420.48 x 639 pts Page_size
- 46 Downloads / 176 Views
a-SiGe:H ALLOY AND ITS APPLICATION TO TANDEM-TYPE SOLAR CELL Y. YUKIMO AND M. AIGA LSI R&D Laboratory, Mitsubishi Electric Corp., 4-1, Mizuhara, Itami 664, Japan ABSTRACT Amorphous SiGe:H alloy is the key material in achieving high conversion efficiency with tandem-type amorphous silicon alloy solar cells. Status and issues for this key material are discussed, and efforts made to irprove it are reviewed to obtain directions for higher quality a-SiGe:H alloys. An application of the iniproved alloy to tndeM-type solar cell to achieve 9.6% efficiency for the cell size of 100 an is reported. INTRODUCTION The use of a-SiGe:H alloy for tandem-type solar cells was proposed in 1979 by Marfaing(1). In the same year, real multijunction cells were fabricated using a-Si:H by Okamoto et al (2). Their basic concept is that, by building a multijunction structure, in which the optical band gap energy, Egopt, varies one over the other, each active layer can be thin enough so that the space charge region can spread over the entire thickness, where the charge collection is efficient. Photons near or below Egopt are not thoroughly absorbed within such a thin layer, and are transmitted to the lower gap junction where the lower energy photons are absorbed. With a-SiGex:H alloy, Egopt varies from 1.1 eV(x=1) to 1.85 eV(x=0) (3), which is suitable for the above structure. Since single junction cells of a-Si:H absorb photons above 1.7 eV, a-SiGe:H alloy is expected to absorb photons of longer wavelength than 700 nm for which Egopt of 1.5 eV or less is desirable. In the following sections, we discuss properties of a-SiGe:H alloy to be improved for higher solar cell efficiencies, efforts made for preparation of high quality a-SiGe:H films are reviewed, and finally an application of improved alloy films to tandemtype solar cells is reported. PROPERTIES OF a-SiGe:H AND ISSUES For high efficiency tandem-type solar cells, films having Egopt: 1.5 eV are necessary. However, spin density of a-SiGe :H films increases rapidly with increasing x from around x=0.4 x(4) which corresponds to Egopt =1.5 eV. From photoconductivity and capacitance study similar degradation of photoelectric properties of the alloy was reported by Huang et al(5). They found that for xZ 0.4,a hump in the density of gap states around 1.2 eV above the valence band edge appears. This hump may be associated with Ge dangling bonds. Increased gap states indicate that the space charge region in a-SiGe :H(x=0.5) pin junction is so narrow that the collection of photocarriers created by weakly absorbed(long wavelength) light is inefficient. In fact, a large reverse voltage is necessary to saturate the long wavelength response as shown in Fig.1. They are so-called Horn curves, developed by Osaka University
Mat. Res. Soc. Symp. Proc. Vol. 70. 1986 Materials Research Society
494
1.2 -\ -D S:D.14 o \
1.0
----
14
-5i.0 V
10%!.
LuO.8
" ...
Fig. 1 Normalized collection efficiencies as a function of absorption coefficient for applied voltages of -5 V and 0.4V and for different diluti
Data Loading...
