Heterojunction solar cell with passivated emitter surface

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In general, the invention pertains to the fabrication and characterization of a heterojunction solar cell with an efficiency greater than 25% (one sun, AM 1.
5).
The present invention addresses the problem of the limited efficiency of conventional GaAs solar cells attributed to the quality of the GaAs emitter layer and the fact that the high recombination velocity at the GaAs surface (even when passivated with AlGaAs) reduces the V.
sub.
oc, J.
sub.
sc, and fill factor of these solar cell devices.
A schematic of a heterojunction GaInP/GaAs solar cell having an AR coating is shown in FIG.
1.
A passivating layer of p-GaInP.
sub.
2 (p-Ga.
sub.
0.
5 In.
sub.
0.
5 P) is coated on the GaAs substrate.
The structure is grown in a vertical, air-cooled reactor at one atmosphere using organometallic chemical vapor deposition (OMCVD), the detailed aspects of which are described in J.
M.
Olson and A.
Kibbler, J.
Crystal Growth, 77, 182 (1986), incorporated herein by reference.
The Group III source gases were trimethylindium, trimethylgallium and trimethylaluminum; the Group V source gases were arsine and phosphine.
The dopant sources were diethylzinc and hydrogen selenide.
Generally, the device described here was grown at a temperature T.
sub.
g =700.
degree.
C.
The alGaInP layers were grown at a rate of R.
sub.
g =80-100 nm/minute and a ratio of phosphine partial pressure to trimethylaluminum plus trimethylgallium plus trimethylindium partial pressure (called the V/III ratio) of 30.
For the GaAs layers, R.
sub.
g =120-150 nm/minute and V/III=35.
Also, the absorber was doped with Zn to a nominal level of 0.
5-4.
times.
10.
sup.
17 cm.
sup.
-3, and the emitter layer was doped with Se at about 10.
sup.
18 cm.
sup.
-3.
However, the optoelectronic properties and photovoltaic quality of the materials set forth above are complex and coupled functions of T.
sub.
g, V/III, dopant type and concentration, substrate quality, and the precise composition of the AlGaInP alloy.
See S.
R.
Kurtz, J