DOE OSTI2015
"The first step towards building a high-efficiency solar cell is to develop an absorber with few recombination-active defects. Many photovoltaic technologies have already achieved this (monocrystalline Si, III-V materials grown on lattice-matched substrates, perovskites, polycrystalline CdTe and CIGS); those that have not (a-Si:H, organics) have been limited to low open-circuit voltage. The second step is to develop contacts that both inhibit surface recombination and allow for low-resistance collection of either only electrons or only holes. For most photovoltaic technologies, this step is both more difficult and less explored than the first, and we are unaware of a prescribed methodology for selecting materials for contacts to solar cells. We elucidate a unified, conceptual understanding of contacts within which existing contacting schemes can be interpreted and future contacting schemes can be imagined. Whereas a split of the quasi-Fermi levels of holes and electrons is required in the absorber of any solar cell to generate a voltage, carriers are eventually collected through a metallic wire in which no such quasi-Fermi-level split exists. We define a contact to be all layers between the bulk of the absorber and the recombination-active interface through which carriers are extracted. The quasi-Fermi levels must necessarily collapse at this interface, and thus the transition between maximal quasi-Fermi-level splitting (in the absorber) and no splitting occurs entirely in the contact. Depending on the solar cell architecture, the contact will usually extend from the surface of the absorber to the surface of a metal or transparent conductive oxide layer, and may include deposited or diffused doped layers (e.g., as in crystalline and thin-film Si cells) and heterostructure buffer layers (e.g., the CdS layer in a CdTe device). We further define a passivating contact as one that enables high quasi-Fermi-level splitting in the absorber (large “internal” voltage), where “high” is relative to the splitting dictated by bulk recombination. Finally, we define a carrier-selective contact as one that enables a high “external” voltage measured across the contacts, where “high” is relative to the internal voltage. With these definitions, passivating contacts are those that allow only electrons, only holes, or neither electrons or holes to transport from the absorber to any position in the contact that has recombination-active defects. An excellent example of this is a SiNx layer on a Si wafer: the layer removes dangling bonds at the wafer surface (where there are both electrons and holes) and does not allow either carrier type to travel to its outermost surface, where there are undoubtedly defects. Carrier-selective contacts are then passivating contacts that also allow for low-impedance flow of either electrons or holes (but not both) to the recombination-active, extracting interface. The most common example is a heavily doped layer that establishes an electric field at the absorber surface, which then “filters” the carriers that may pass to the contact according to the sign of their charge.