Structures, Materials,and Scale INTRODUCTION
The key material in a solar cell is the absorber. These materials are capable of absorption-caused excited states produced by photons with energies in the photon-rich range of the solar spectrum （Fig. 1.1）. The resulting excited states must be mobile; i.e., free electron-hole pairs, which can be separated, or excitons, which can be disassociated into free electrons and free holes and separated. Absorber materials can be organic or inorganic semiconductors, dye molecules, or quantum dots, which are the manmade inorganic particle equivalent of dye molecules. In some configurations, the absorption and separation are both accomplished in the same
material. In these cases there is a region in the absorber with a built-in electric field designed to break symmetry thereby forcing electrons in one direction and holes in the other. We will term this region a junction. In other configurations a second material is used with the absorber to set up the symmetry-breaking region. In this case the symmetry-breaking may be accomplished with an electric field, effective fields, or both. We will also refer to such regions as junctions.
Besides the absorber and any junction-forming materials, there is a supporting cast of other material components in a solar cell structure. They may include materials that block one carrier while supporting transport of the other to help make one direction of carrier motion different from the other. These materials obviously augment the symmetry-breaking region. Ideally, they are either hole transport-electron blocking layer （HT-EBL） or electron transport-hole blocking layer （ET-HBL） materials. Another important component is the antireflection material. Antireflection materials serve as an optical impedance matching medium; i.e., they are used to couple light efficiently into the solar cell structure. Plasmonic （metals） or photonic （insulators, semiconductors, metals） materials may also be employed as optical components for controlling scattering, reflection, interference, and diffraction thereby dictating the optical electric field strength and thus the absorption distribution in the absorber. Conducting materials provide the ohmic （ideally no voltage drop） contacts of the cell electrodes and the grids needed for carrying the current to the outside world. These materials must produce minimal electrical and optical loss. Contact materials may be metals or transparent conducting oxides （TCOs）. The latter type of contact material provides low electrical resistance yet allows the transmission of light into a cell. Finally, protection of the overall device, while not hindering optical coupling, is the function of encapsulating materials.
While the materials system for a solar cell can be involved, the core of the cell operation is the absorber and the symmetry-breaking region, the ”charge separation engine.“ Of course, for power to be drawn from a solar cell, contacts are needed to allow current to leave. The core plus contacts is the simplest solar cell structure. Everything else is added to enhance performance.
We just discussed that the creation of the symmetry-breaking region is done by building in an electric field, effective fields, or both, and no mention was made of diffusion. We will demonstrate in concrete terms in this chapter that diffusion is relatively unimportant in charge separation and photovoltaic action. We will see it can be very important in charge collection. After we establish this point, we will then turn to the question of how solar cell structures are achieved in practice. We will examine the criteria for selecting an absorber, and then will focus on length scales. our primary questions about scale are: Are there natural length scales that occur in photovoltaic materials and structures and, if so, what are they, and do they lie in the or nano- or microscale regime, or both.
BASIC STRUCTURES FOR PHOTOVOLTAIC ACTION
We determined in Section 2.5 that the mathematical system of equations describing the physics of solar cells points to （1） the presence of an electric field, （2） the presence of effective force fields （changes with position in affinities or equivalently in the LUMo or Homo levels and densities of states）， and （3） diffusion, as the three possible causes of photovoltaic action. To examine the relative importance of these potential mechanisms, we will look at a series of very basic structures, each designed to focus on one of these possibilities.