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 performances 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 data set of over 850 individual CZTS solar cells. By reducing the absorber thickness from above 1000 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.
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.
ABSTRACT
The control and removal of secondary phases is one of the major challenges for the development of Cu2ZnSn(S,Se)4 (CZTSSe)-based solar cells. Although etching processes have been developed for Cu(S,Se), Zn(S,Se), and CuSn(S,Se) secondary phases, so far very little attention has been given to the role of Sn(S,Se). In this paper, we report a chemical route using a yellow (NH4)2S solution to effectively remove Sn(S,Se). We found that Sn(S,Se) can form on the surface either because of stoichiometric deviation or by condensation. After etching, the efficiency of devices typically increases between 20 and 65% relative to the before etch efficiencies. We achieved a maximum 5.9% efficiency in Se-rich CZTSSe-based devices. It is confirmed that this feature is related not only to the removal of Sn(S,Se) but also to the unexpected passivation of the surface. We propose a phenomenological model for this passivation, which may open new perspectives for the development of CZTSSe-based solar cells.
ABSTRACT
Cu2ZnSnSe4 kesterite compounds are some of the most promising materials for low-cost thin-film photovoltaics. However, the synthesis of absorbers for high-performing devices is still a complex issue. So far, the best devices rely on absorbers grown in a Zn-rich and Cu-poor environment. These off-stoichiometric conditions favor the presence of a ZnSe secondary phase, which has been proved to be highly detrimental for device performance. Therefore, an effective method for the selective removal of this phase is important. Previous attempts to remove this phase by using acidic etching or highly toxic organic compounds have been reported but so far with moderate impact on device performance. Herein, a new oxidizing route to ensure efficient removal of ZnSe is presented based on treatment with a mixture of an oxidizing agent and a mineral acid followed by treatment in an aqueous Na2S solution. Three different oxidizing agents were tested: H2O2, KMnO4, and K2Cr2O7, combined with different concentrations of H2SO4. With all of these agents Se(2-) from the ZnSe surface phase is selectively oxidized to Se(0), forming an elemental Se phase, which is removed with the subsequent etching in Na2S. Using KMnO4 in a H2SO4-based medium, a large improvement on the conversion efficiency of the devices is observed, related to an improvement of all the optoelectronic parameters of the cells. Improvement of short-circuit current density (J(sc)) and series resistance is directly related to the selective etching of the ZnSe surface phase, which has a demonstrated current-blocking effect. In addition, a significant improvement of open-circuit voltage (V(oc)), shunt resistance (R(sh)), and fill factor (FF) are attributed to a passivation effect of the kesterite absorber surface resulting from the chemical processes, an effect that likely leads to a reduction of nonradiative-recombination states density and a subsequent improvement of the p-n junction.
ABSTRACT
Pentenary Cu2ZnSn(S(y)Se(1-y))4 (kesterite) photovoltaic absorbers are synthesized by a one-step annealing process from copper-poor and zinc-rich precursor metallic stacks prepared by direct-current magnetron sputtering deposition. Depending on the chalcogen source--mixtures of sulfur and selenium powders, or selenium disulfide--as well as the annealing temperature and pressure, this simple methodology permits the tuning of the absorber composition from sulfur-rich to selenium-rich in one single annealing process. The impact of the thermal treatment variables on chalcogenide incorporation is investigated. The effect of the S/(S+Se) compositional ratio on the structural and morphological properties of the as-grown films, and the optoelectronic parameters of solar cells fabricated using these absorber films is studied. Using this single-step sulfo-selenization method, pentenary kesterite-based devices with conversion efficiencies up to 4.4 % are obtained.
