Currently, one of the main limitations in ultrathin Cu(In, Ga)Se $_2$ (CIGS) solar cells are the optical losses, since the absorber layer is thinner than the light optical path. Hence, light management, including rear optical reflection, and light trapping is needed. In this paper, we focus on increasing the rear optical reflection. For this, a novel structure based on having a metal interlayer in between the Mo rear contact and the rear passivation layer is presented. In total, eight different metallic interlayers are compared. For the whole series, the passivation layer is aluminum oxide (Al $_2$ O $_3$ ). The interlayers are used to enhance the reflectivity of the rear contact and thereby increasing the amount of light reflected back into the absorber. In order to understand the effects of the interlayer in the solar cell performance both from optical and/or electrical point of view, optical simulations were performed together with fabrication and electrical measurements. Optical simulations results are compared with current density–voltage (J–V) behavior and external quantum efficiency measurements. A detailed comparison between all the interlayers is done, in order to identify the material with the greatest potential to be used as a rear reflective layer for ultrathin CIGS solar cells and to establish fabrication challenges. The Ti-W alloy is a promising a rear reflective layer since it provides solar cells with light to power conversion efficiency values of 9.9%, which is 2.2% (abs) higher than the passivated ultrathin sample and 3.7% (abs) higher than the unpassivated ultrathin reference sample.

A work function study based on the onset shift (i.e., following low energy cut-off) of secondary electron spectra has been used for the last four decades to monitor the deposition and adsorption in real time, measure the dipole momentum and polarizability of the surface layer, and determine the lateral distribution of the work function. In this work, we show that the onset shift depends on both the coverage of adsorbed species that change the work function and the size of low work function patches. Additionally, the extraction field, which is always applied in these measurements, may also influence the onset shift. Numerical calculations of the potential distributions above different non-uniform surfaces were performed in order to quantitatively determine each of these influences. Depending on the patch size, we define three measurement regimes in which the onset position is related to either the surface average of the work function (small patches), the minimum local work function (large patches), or a value in-between (intermediate size patches). Experimental data have corroborated these findings and demonstrated that manipulating the extraction field intensity enables transition between the measurement regimes. Typical misinterpretations due to neglecting the patch size contribution and the surface non-uniformity to the onset shift are analyzed. Additionally, possible application of work function study for determination of the growth mode in the case of submonolayer deposition was discussed.