= Solution
\b[The useful bandgap depends on the illumination spectrum, not only on whether the device is indoors or outdoors.] The stipulated $1.45\,\mathrm{eV}$ gap gives a long-wavelength absorption threshold near $855\,\mathrm{nm}$; $1.7\,\mathrm{eV}$ gives about $729\,\mathrm{nm}$ using $\lambda_g\simeq1240/E_g$ in nanometers.
Outdoor sunlight contains substantial visible and near-infrared <power>, so cadmium telluride can use a broader portion of that spectrum while retaining a useful output <voltage>. Its direct-gap absorption permits thin absorbers and large-area thin-film manufacture. Challenges include contacts and interfaces, collection and <recombination>, material supply and managing cadmium-containing material through fabrication and end-of-life handling.
Many indoor lamps concentrate their emission in the visible and provide little near-infrared <power>. A wider-gap absorber can then trade fewer lost useful photons for a higher attainable <voltage> and a smaller thermal dark-current penalty. Hydrogenated amorphous <silicon> has strong visible absorption, permits low-temperature large-area deposition and can suit low-power indoor devices. Disorder and defects restrict transport, and light-induced degradation and <passivation> stability must be controlled. Its larger gap forfeits more of the outdoor red/near-infrared spectrum.
These are suitability arguments, not guaranteed efficiencies. Indoor lamp spectra differ, and shunt leakage and <recombination> become particularly important at low light. Bandgap alone does not establish which finished device produces more <power> in every room.
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