A red LED is made from a semiconductor of band gap 1.9 eV, and a blue LED from a semiconductor of…

An LED gives light of wavelength $\lambda = \dfrac{hc}{E_g}$, with $hc = 1240\ \text{eV nm}$, and needs a series resistor $R = (V_s - V_f)/I$. A photodiode with a share $\eta$ of photons counted gives $I = \eta P/E$ for light of power $P$ and photon energy $E$ in eV. A solar cell gives $P_{max} = FF \times V_{oc} I_{sc}$ and has the efficiency $\dfrac{P_{max}}{G A}$.