As a renewable energy source, solar energy can be harvested and will be increasingly important in the inevitable transition from our current level of dependence on fossil fuels. One of the biggest challenges in photovoltaic technology is how to increase the photocurrent conversion efficiency. Since solar cells were developed several decades ago, consistent efforts have been made in exploring variables that influence optimal efficiency. In recent years, research into high-efficiency solar cells has accelerated with the rapid emergence of new materials and devices showing promising performance characteristics. To achieve further increases in the photocurrent conversion efficiency, it is necessary to choose a suitable material, fabricate a device with proper structure, and to understand the physics behind the materials and devices as a basis for developing new approaches to optimize the materials and device …