Impact of Na Dynamics at the Cu₂ZnSn(S,Se)₄/CdS Interface During Post Low Temperature Treatment of Absorbers

Abstract

Cu2SnZn(S,Se)4 (CZTSSe) solar cells based on earth abundant and nontoxic elements currently achieve efficiencies exceeding 12%. It has been reported that, to obtain high efficiency devices, a post thermal treatment of absorbers or devices at temperatures ranging between 150 and 400 °C (post low temperature treatment, PLTT) is advisable. Recent findings point toward a beneficial passivation of grain boundaries with SnOx or Cu-depleted surface and grain boundaries during the PLTT process, but no investigation regarding alkali doping is available, even though alkali dynamics, especially Na, are systematically reported to be crucial within the field. In this work, CZTSSe absorbers were subjected to the PLTT process under different temperatures, and solar cells were completed. We found surprisingly behavior in which efficiency decreased to nearly 0% at 200 °C during the PLTT process, being recovered or even improved at temperatures above 300 °C. This unusual behavior correlates well with the Na dynamics in the devices, especially with the in-depth distribution of Na in the active CZTSSe/CdS interface region, indicating the key importance of Na spatial distribution on device properties. We present an innovative model for Na dynamics supported by theoretical calculations and additional specially designed experiments to explain this behavior. After optimization of the PLTT process, a Se-rich CZTSSe solar cell with 8.3% efficiency was achieved.

Na‐Diffusion Enhanced p‐type Conductivity in Cu(In,Ga)Se2: A New Mechanism for Efficient Doping in Semiconductors

“…Second, unlike Na doping, K doping leads to the formation of a K-enriched layer on the CIGS surface, [14,46,52,53] which is water-insoluble and hinders the out-diffusion of K from the CIGS grain interiors. [55][56][57] Since Na is a fast diffusor in CZTSSe [58,59] and prefers replacing Cu when incorporated into the CZTSSe grains, [60,61] Na can also promote the formation of V Cu and thus increase the hole concentration in the CZTSSe grains, similar to the case in CIGS. We then conclude that K is not as efficient as Na in increasing the p-type conductivity of CIGS films, in good agreement with the experimental observations that K is not as efficient as Na in increasing the hole concentration in CIGS films.…”

Surface modification through air annealing Cu2ZnSn(S,Se)4 absorbers

“…Most work has been focused on annealing of the bare CZTSSe absorber before further processing, while improvements have also been observed when annealing the CZTSe/CdS stack [3], as well as the complete device stack [3]. The studies in which the heat treatment is performed in an inert N 2 atmosphere focus on changes of the Na content in the absorber [6,7] and in the CZTSSe/CdS buffer layer region [7]. It is argued that the annealing treatment can be used to obtain an appropriate level of Na [7].…”

“…The studies in which the heat treatment is performed in an inert N 2 atmosphere focus on changes of the Na content in the absorber [6,7] and in the CZTSSe/CdS buffer layer region [7]. It is argued that the annealing treatment can be used to obtain an appropriate level of Na [7]. In studies where the annealing treatment is performed in air, the focus is mainly directed to oxidation of grain boundaries [5,8].…”

“…It has been argued that a Zn-rich and Cu-poor surface is beneficial and leads to improved performance [3,5,9]. Most work on air annealing (AA) has been performed on CZTSSe or CZTSe, while only one study mentions the pure sulfide CZTS [7]. In this work we compare the effects of air annealing on pure sulfide CZTS and the selenium containing CZTSSe counterpart.…”

CZTSe solar cells developed on polymer substrates: Effects of low‐temperature processing

“…The second concern is the lack of alkalis in PI in contrast to conventional soda‐lime glass. Alkalis (especially Na and K) have been observed to play a fundamental role in the synthesis of high quality CIGS and kesterite absorbers through many different mechanisms like crystal growth enhancement, grain boundary passivation along with an increment of carrier concentration that result in a drastic enhancement of solar cell performance . This work endeavours to tackle both issues.…”