Electricity power is normally generated by burning fossil fuels, having detrimental impacts on the environment and will be depleted. One of the most appropriate ways to solve the foreseeable world's energy crisis is to utilize the power of the sun. Solar cells, using photovoltaic effect, are of wide interest as they can convert solar energy to electricity. Chalcopyrite based thin-film solar cell is considered as the low-cost and high-efficiency solar cells. One of the most important chalcopyrite compounds for photovoltaic application is Cu(In, Ga)Se2. Fabricating the solar cells on flexible substrates is intriguing, as it can be applied to the roll-to-roll process with the ability to reduce production cost of the solar cells. Consequently, the purpose of the book is to provide the overview for accomplishing the good physical properties and suitable double [Ga]/([Ga]+[In])-grading profiles of Cu(In, Ga)Se2 absorbers on flexible stainless steel substrates for the thin-film solar cells with high conversion efficiency (>15%).