Electron and hole partial resistances: a framework to understand passivation, conductivity, and selectivity of solar cell contacts
Arthur Onno1, Christopher Chen1, Priyaranga Koswatta2, Mathieu Boccard3, Zachary C. Holman1
1Arizona State University, Tempe, AZ, United States
/2Intel Corporation, Hillsboro, OR, United States
/3École Polytechnique Fédérale de Lausanne, Neuchâtel, Switzerland

Although carrier selectivity is recognized as a requirement for optimal contact performance – alongside passivation and conductivity – a common definition and a common metric for the selectivity of a given contact structure are only now emerging. In order to elucidate the interplay of these three contact properties and their influence on the performance of the overall device, we propose the concept of partial specific contact resistances. For a given contact structure, we thus define distinct specific contact resistances to electrons and to holes. From these voltage-dependent resistances we can reconstruct the cells implied, pseudo, and actual J-V curves and define metrics for each of the aforementioned contact properties: the sum of the resistances is a metric for passivation, their ratio a metric for selectivity, and the majority-carrier contact resistance a metric for conductivity. Using PC1D, we validate our model by simulating 10,500 cases of homojunction contacts to c-Si absorbers, varying the thickness, doping density, electron mobility, and hole mobility of the electron contact; the hole contact and the absorber being otherwise assumed perfect. Although, in that instance, we demonstrate our theory on a simple model system, the framework is more general and can easily be applied to a wide range of absorbers and more complex contact structures, such as heterostructures and tunneling contacts. A major result is that, in the case of fixed contact fraction (e.g. full-area contacts), selectivity is necessary but not sufficient to ensure optimal efficiency. Conversely, combined passivation and conductivity imply selectivity and, together, they are necessary and sufficient to ensure maximal efficiency. Selectivity is, thus, not a relevant metric for fixed-area contacts. However, when the contact fraction can be optimize to scale the electron and hole partial specific resistances in concert, selectivity drives the cell efficiency.