Employing Si solar cell technology to increase efficiency of ultra‐thin Cu(In,Ga)Se₂ solar cells

Abstract

Reducing absorber layer thickness below 500 nm in regular Cu(In,Ga)Se2 (CIGS) solar cells decreases cell efficiency considerably, as both short-circuit current and open-circuit voltage are reduced because of incomplete absorption and high Mo/CIGS rear interface recombination. In this work, an innovative rear cell design is developed to avoid both effects: a highly reflective rear surface passivation layer with nano-sized local point contact openings is employed to enhance rear internal reflection and decrease the rear surface recombination velocity significantly, as compared with a standard Mo/CIGS rear interface. The formation of nano-sphere shaped precipitates in chemical bath deposition of CdS is used to generate nano-sized point contact openings. Evaporation of MgF2 coated with a thin atomic layer deposited Al2O3 layer, or direct current magnetron sputtering of Al2O3 are used as rear surface passivation layers. Rear internal reflection is enhanced substantially by the increased thickness of the passivation layer, and also the rear surface recombination velocity is reduced at the Al2O3/CIGS rear interface. (MgF2/)Al2O3 rear surface passivated ultra-thin CIGS solar cells are fabricated, showing an increase in short circuit current and open circuit voltage compared to unpassivated reference cells with equivalent CIGS thickness. Accordingly, average solar cell efficiencies of 13.5% are realized for 385 nm thick CIGS absorber layers, compared with 9.1% efficiency for the corresponding unpassivated reference cells.

Potential‐induced optimization of ultra‐thin rear surface passivated CIGS solar cells

“…As-finished, higher V OC and J SC can be seen in Table 2 for the Al 2 O 3 rear passivated cells. This can be explained by an increase in rear surface passivation and rear internal reflectance thanks to the passivation layer, as already discussed in [6]. However, the FF of these passivated cells is lower than is the case for the reference cells.…”

“…Hence, Na deficiency will occur if the contacting area is too small. As the formation of openings in the passivation layer by removal of nanoparticles is a statistical process step, it can indeed result in too small (this work) or sufficient (as reported in [6]) contacting area.…”

“…As shown in [4][5][6], these ultrathin ungraded CIGS absorber layers with uniform low Ga concentration are exemplary characterization devices to assess rear surface passivation and rear internal reflection. Next, the buffer layer is deposited using a standard CBD [4][5][6].…”

“…Next, the buffer layer is deposited using a standard CBD [4][5][6]. Note that the unpassivated reference cells have the same processing sequence, but without the formation of the advanced rear passivation structure (steps 3-5 in Table 1).…”

“…On the other hand, the Al 2 O 3 rear surface passivation leads to a reduction in interface recombination at the rear of CIGS solar cells (rear-contact/absorber-layer). It consists of an Al 2 O 3 film with nano-sized point openings between the rear contact and the CIGS absorber layer, which results in a reduction of interface recombination by chemical (a reduction in interface trap density) and field-effect passivation (a reduction in surface minority charge carrier concentration), while the point openings allow for contacting [4][5][6].…”