Surface morphology and grain structure are seen by scanning electron microscopy (SEM). The films used in devices have grain diameters on the order of 1 pm, but the grain size and morphology can vary greatly, depending on fabrication method and conditions.
The composition changes gradually from bulk to the surface of the films due to Cu migration (Klein et al. 2007). The surface Fermi level approaches the conduction band due to surface defects, adsorbates or junction formation. Reduced Cu concentration has then been interpreted as the formation of Cu vacancies, which becomes more favorable with higher Fermi level (Zhang et al. 1997). There can also be a band bending induced by surface charges that drive electro-migrating Cu into the bulk, leaving the surface depleted of Cu (Herberholz et al. 1999). This depletion is stopped when the composition is that of CuIn3Se5, since further depletion requires a structural change of the material. Electro-migration of Cu in CuInSe2 has been correlated with type conversion of the chalcopyrite (Gartsman et al. 1997). Copper diffusion into the bulk is reduced when oxides form on the surface as the material is exposed to air. The surface oxidation is enhanced by Na (Ruckh et al. 1996a). Compounds that have been identified for extended oxidation include In2O3, Ga2O3, SeOx, andNa2CO3 (Kylner 1999).
Post-treatment in air, typically at 200 °C, completes Cu(InGa)Se2 solar cell. After CdS or (CdZn)S is vacuum-deposited to form the junction, annealing were extended for several hours for optimization (Mickelsen and Chen 1981; Damaskinos et al. 1987). Oxygen passivates the selenium vacancies on the surface of grains (Ca – hen and Noufi 1989). This indicates that the donor-type selenium vacancy act as recombination center. The positive charge of these donor-type defect reduces the effective hole concentration, hence the inter-grain carrier transport is impeded. When oxygen substitutes the missing selenium, this impediment is canceled.
The beneficial effect of Na on the performance of Cu(InGa)Se2 solar cells, however, lacks a complete explanation. It could be a catalytic effect of Na on oxidation, by enhanced dissociation of molecular oxygen into atomic oxygen, that makes the passivation of VSe more effective (Kronik et al. 1998). This is consistent with the observation that Na and O are predominantly found at the grain boundaries rather than in the bulk of the grains in CuInSe2 (Niles et al. 1999). However, with little composition difference between the interior and the boundary (Boyd and Thompson 1980; Kessler et al. 2005).
The insensitivity of Cu(InGa)Se2 solar cells is to grain size and morphology indicates that there is no significant recombination loss at grain boundaries. Even polycrystalline Cu(InGa)Te2 solar cells outperform single-crystalline solar cells, and may suggest that grain boundaries are actually beneficial for solar cells.