Improved Rear Surface Passivation of Cu(In,Ga)Se$_{\bf 2}$ Solar Cells: A Combination of an Al$_{\bf 2}$O $_{\bf 3}$ Rear Surface Passivation Layer and Nanosized Local Rear Point Contacts

Abstract

An innovative rear contacting structure for copper indium gallium (di) selenide (CIGS) thin-film solar cells is developed in an industrially viable way and demonstrated in tangible devices. The idea stems from the silicon (Si) industry, where rear surface passivation layers are combined with micron-sized local point contacts to boost the open-circuit voltage (V O C ) and, hence, cell efficiency. However, compared with Si solar cells, CIGS solar cell minority carrier diffusion lengths are several orders lower in magnitude. Therefore, the proposed CIGS cell design reduces rear surface recombination by combining a rear surface passivation layer and nanosized local point contacts. Atomic layer deposition of Al 2 O 3 is used to passivate the CIGS surface and the formation of nanosphere-shaped precipitates in chemical bath deposition of CdS to generate nanosized point contact openings. The manufactured Al 2 O 3 rear surface passivated CIGS solar cells with nanosized local rear point contacts show a significant improvement in V O C compared with unpassivated reference cells. Index Terms-Al 2 O 3 , atomic layer deposition, copper indium gallium selenide (CIGS), Cu(In,Ga)Se 2 , Ga grading, nanosized, passivated emitter, passivated emitter and rear cell (PERC), photovoltaics, point contact openings rear locally diffused cell (PERL), rear surface passivation, Si, solar cells, thin film.

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

“…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].…”

“…www.pss-rapid.com known to be excellent diffusion barriers [9,10], and typically only 2% to 5% of their total area is opened [5]. Hence, the Al 2 O 3 passivation layer is a physical barrier for Na and consequently limits Na diffusion from the SLG to the CIGS absorber layer.…”

“…Hence, the Al 2 O 3 passivation layer is a physical barrier for Na and consequently limits Na diffusion from the SLG to the CIGS absorber layer. Therefore, additional Na provided by a NaF precursor layer is required for the rear passivated solar cells [4,5]. However, it occasionally happens that the NaF layer is too thin resulting in underperforming rear passivated cells, due to a lack of Na as apparent by J-V curves with characteristic "roll-over" effects [4,5].…”