= Solution
From the illuminated side downward, the p-type <gallium arsenide> cap provides a low-resistance route to the front metal contact. It should remain beneath the contact grid but be removed from the optical aperture to reduce parasitic absorption. The wider-<band gap> p-type <aluminium gallium arsenide> window transmits much of the useful light and reduces front-surface recombination of carriers in the adjacent absorber. The optical-window and reduced-recombination rationale has a primary experimental basis in early heteroface devices.<High-efficiency Ga1-xAlxAs-GaAs solar cells by Jerry M. Woodall and H. J. Hovel>[Applied Physics Letters 21 (1972), p. 379, transparency and surface-recombination rationale; p. 380, contacts and antireflection coating]{ref}
The p-type <gallium arsenide> emitter and n-type <gallium arsenide> base form the <p-n junction>. Absorbed photons generate <electrons> and <electron holes>; <diffusion> and the junction's <electric field> direct them toward the appropriate contacts. The n-type <aluminium gallium arsenide> back-surface-field layer provides a barrier to minority holes moving toward the substrate and reduces rear recombination. The n-type <gallium arsenide> substrate supports the epitaxial layers and conducts majority carriers to the rear contact.
The front metal must collect current with limited shading and the back metal provides the return contact. Interpreting the patterned hatched region as the optical antireflection coating, it reduces reflected light over the exposed window. \b[Carrier selectivity, low recombination, low series resistance and low shading must work together; the equilibrium junction field alone is not a source of electrical <energy>.]
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