In the light dependent reactions light energy is absorbed by the photosytem II and an electron is released. This electron causes a process called photolysis to occur(H20-->2H++2e-+1/2O2). This is how O2 is released in the atmosphere. The electrons resulted from photolyisis enter the electron transport chain. In the electron transport chain using the energy in the elecrtons, hydrogen protons are pumped inside the thylakoid. Those protons accumulate to form an electrochemical gradient. That means the protons need to flow out, and they do through an enzyme called ATP-synthase which turns ADP and inorganic phosphate into ATP and water. In the electron transport chain, the electrons reach Photosystem I where NADP+ is reduced and becomes NADPH.
ATP and NADPH store the energy absorbed in the light dependent reactions. Those two molecules are needed when CO2 is fixated in Calvin's cycle to synthesize glucose.
Solar cells usually convert solar energy into electricity which could later be reserved. The efficiency of solar cells is still low, compared to other sources, with a conversion rate of 15% - 22%. This reduces the attractiveness of solar cells as a competitive source of energy. This also makes this form of energy expensive due to its inefficiency.
If this is for the scientific process unit, the answer is passing laws to prevent certain types of scientific research. I know because I had this question in biology.
