Point-contact passivation layers have been proven beneficial in most solar cells (SCs). However, the latest theoretical simulations suggested that a high back-contact recombination velocity S b can also be beneficial in ultrathin CIGSe (Cu­(In,Ga)­Se2) SCs if they have a relatively high back potential barrier height E h. SCAPS simulations predicted that a high S b will deteriorate the SC efficiency Eff when E h is in the range of 0–0.17 eV (Ohmic contact). Yet, when E h is greater than 0.17 eV (Schottky contact), a high S b can also diminish the current limitation arising from the back Schottky diode since it has a reverse direction to the main p–n junction. Therefore, a high S b can support the carriers in passing the Schottky barrier via recombination, thus enhancing the cell performance. This work aims to verify the simulation prediction in practical experiments. To achieve different S b values, we fabricate SiO2 passivation layers with point contacts of various dimensions by nanosphere lithography. The passivation effects are studied comparatively on Mo and ITO (In2O3:Sn) back contacts. The emphasis is on E h, which is marginal for Mo but acts Schottky-like on ITO. We show that for Mo-based solar cells, the E h is trivial; hence, a high S b (without SiO2 passivation) deteriorates the efficiency. In contrast, on ITO, the reference sample without SiO2 shows less current limitation than the passivated ones, implying that a high S b improves the efficiency. Comparing the differences of SiO2 on Mo and ITO back contacts in experiments, with the contrasting behavior of S b on Ohmic and Schottky contacts in simulation, we conclude that E h decides about the role of S b in ultrathin CIGSe SCs. These findings deepen the understanding of the Schottky back contact and pave the way for future optimization of bifacial semitransparent ultrathin CIGSe SCs.

Herein, an optoelectrical model is presented for copper indium gallium diselenide (CIGSe) solar cells in COMSOL Multiphysics, capable of multidimensional simulations, and it is applied to ultrathin (500 nm absorber thickness) solar cells. First, the modeling approach is shown. Special attention is paid to back contact materials, interface states, and defect application and their impact on the current–voltage (J–V) characteristics. To address whether the back contact is Schottky or Ohmic, the influence of the Schottky barrier height, recombination velocity, and interface states is shown. Then, the additional application of an acceptor defect gradient at the absorber back and a donor defect density distribution at the p–n junction is investigated. The results of these parameter adjustments are discussed, and the trends are shown to enable fast fitting of experimental J–V curves. Finally, the results are compared to the experimental J–V curves for indium tin oxide and Mo back contact, and challenges encountered are discussed while fitting. The optoelectrical model for CIGSe solar cells, established in two dimensions here, paves the way for comprehensively describing 2D and 3D solar cell structures, e.g., nanotextured or microsolar cells, as well as for considering different absorber thicknesses.