Persistent Double-Layer Formation in Kesterite Solar Cells: A Critical Review

Abstract

In kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cell research, an asymmetric crystallization profile is often obtained after annealing, resulting in a bilayered -or double-layered -CZTSSe absorber. So far, only segregated pieces of research exist to characterize the appearance of this double layer, its formation dynamics and its effect on the performance of devices. In this work, we review the existing research on double-layered kesterites and evaluate the different mechanisms proposed. Using a cosputtering-based approach, we show that the two layers can differ significantly in morphology, composition and optoelectronic properties, and complement the results with a large statistical dataset of over 850 individual CZTS solar cells. By reducing the absorber thickness from above 1000 nm to 300 nm, we show that the double layer segregation is alleviated. In turn, we see a progressive improvement in the device performance for lower thickness, which alone would be inconsistent with the well-known case of ultrathin CIGS solar cells. We therefore attribute the improvements to the reduced double-layer occurrence, and find that the double layer limits the efficiency of our devices to below 7%. By comparing the results with CZTS grown on monocrystalline Si substrates, without a native Na supply, we show that the alkali metal supply does not determine the double layer formation, but merely reduces the threshold for its occurrence. Instead, we propose that the main formation mechanism is the early migration of Cu to the surface during annealing and formation of Cu2-xS phases, in a self-regulating process akin to the Kirkendall effect. Finally, we comment on the generality of the mechanism proposed, by comparing our results to other synthesis routes, including our own in-house results from solution processing and pulsed laser deposition of sulfide-and oxide-based targets. We find that although the double layer occurrence largely depends on the kesterite synthesis route, the common factors determining the double layer occurrence appear to be the presence of metallic Cu and/or a chalcogen deficiency in the precursor matrix. We suggest that understanding the limitations imposed by the double layer dynamics could prove useful to pave the way for breaking the 13% efficiency barrier for this technology.

Ag Incorporation with Controlled Grain Growth Enables 12.5% Efficient Kesterite Solar Cell with Open Circuit Voltage Reached 64.2% Shockley–Queisser Limit

“…The bi‐ or tri‐layer structure is commonly observed in CZTSSe absorbers because of different reasons. [ 26,45 ] In our case, the layered structure is due to unique grain growth mechanism. From our recent report, precursor film fabricated from Sn 4+ ‐based DMSO solution follows a top–down and bottom–up bidirectional grain growth and it may form tri‐ or bi‐layer structure depending on processing time.…”

Large‐Grain Spanning Monolayer Cu2ZnSnSe4Thin‐Film Solar Cells Grown from Metal Precursor

“…[ 6–10 ] Besides the abundant cation‐disordering defects that lead to short minority carrier lifetime and prevalent potential fluctuations, [ 2,11–13 ] the performance of CZTSSe solar cells is also plagued by the unfavorable morphology of the CZTSSe absorber with presence of small grains, voids, and secondary phases at the bottom of the absorber layer. [ 14,15 ] These undesirable structures are believed to be responsible for the poor carrier transport near the back contact, leading to considerable nonradiative recombination in this region and a significant increase of series resistance ( R S ) due to the enhanced scattering of hole carriers. [ 8,14,16 ] In a word, CZTSSe absorber with a large‐grain spanning monolayer morphology is still one of the desired but challenging fundamental requirements for high‐performance CZTSSe solar cells.…”

Enhancing the Photovoltaic Performance of Cu2ZnSn(S,Se)4 Solar Cells with Ba Trace Doping: Large Chemical Mismatch Cation Incorporation

“…However, both samples exhibit a double-layered structure, which is composed of the upper layer of compact large grains and the bottom layer of loose small grains, causing the higher series resistance R S and deteriorating fill factor (FF). [44,45] The EDS in-depth compositional elements mapping shows that there is no element segregation and Ba is uniformly distributed in the bulk of absorber, accompanied by Cu, Zn, Sn, and Se.…”