Solar Cell Design Principles

Solar Cell design is a key process for producing highly efficient Solar cells with high quality and low cost. Solar cell design process includes the specification of the parameters of solar cell structure for the purpose of maximizing efficiency, within a certain set of limitations. These limitations will be determined by the conditions and working environment that solar cells are produced in. As an example, the solar cells produced in a commercial environment in which the goal is the production of a competitively priced solar cell, the cost for fabricating a specific solar cell structure is one of the main considerations. Yet, solar cells produced in a research environment in which the goal is the production of a highest possible efficiency for a laboratory-type cell, thus maximizing efficiency instead of cost, is then from the main considerations.  

Evolution of silicon solar cell efficiency

Theoretically, the solar cell efficiency for photovoltaic conversion is higher than 86.8% .Yet, the 86.8% number uses in depth balance calculations and does not explain implementation of device. For solar cells made from silicone, a more accurate efficiency under the conditions of one sun operation is around 29%. The current maximum measured efficiency for a silicon solar cell is 24.7% under normal conditions. The gap between the higher theoretical efficiencies and the actual efficiencies measured from terrestrial solar cells is based on two elements. The first one is that the theoretical efficiency calculations assume that energy from every photon is used optimally, that no photons are unabsorbed and that every photon is absorbed in a material with a band gap equal to the energy of the photon. Theoretically this can be achieved by modeling an infinite stack of solar cells of variable band gap materials, each one only absorbing the photons exactly corresponding to its band gap. The second one is that the higher theoretical efficiency assumes a high concentration ratio. With the assumption that the temperature and the resistive effects do not govern in the concentrator solar cell, the increase of the light intensity leads to increase in the short circuit current. Since the V_oc  (open circuit voltage) also relies on the short circuit current, V_oc increases as well with light level. In addition because the maximum fill factor increases with V_oc, the possible maximum FF increases as well with concentration. The additional V_oc and FF increases with concentration which allows for higher efficiencies in concentrators.  

Solar Cell design principles

In the design process of single junction solar cells, the principles for having maximum cell efficiency are:

1. Increasing the quantity of light collected by cells that is converted into carriers
2. Increasing the collection rate of light generated carriers by p-n junction
3. Minimizing the forward bias dark current
4. Extracting the cell current with no resistive losses.