• Energy Research

International audience

In this paper we focus on the understanding of the PtxCoy catalysts degradation in PEFC environments. A multiscale atomistic/kinetic model is derived providing new mechanistic insights on the impact of clusters nanostructure and operating conditions on PtxCoy materials durability. On the basis of ab initio (AI) data, we identify favorable pathways of the ORR on PtxCoy clusters and of the competitive Pt-Co dissolution in acidic media. The derived AI-kinetics is coupled to a description of the atomic reorganisation at the cluster level as function of the cumulated Pt and Co mass losses. This interfacial model is coupled with a transport microscale model of charges and O2 through the PEFC cathode, and simulation sensitivity studies to operating conditions and initial compositions/morphologies are performed. Experiments on DLIMOCVD-elaborated model electrodes are carried out by using RDE and half-cells: degradation structural changes are characterized by using TEM, XRD and XPS complementing the modeling studies.

Dynamics of the two LTO/sulfone-mixed electrolyte interfaces in a LTO/LTO symmetric cell.

Fabricating tin dioxide (SnO2) electron selective layers (ESL) by atomic layer deposition (ALD) can be of high interest for perovskite-based solar cell development since it offers a number of advantages over solution-based processes. However, ALD-grown SnO2 ESL has usually been reported to yield limited cell efficiency compared to solution-processed SnO2 ESL, without the causes being clearly identified. This is why we here conduct a thorough interface study using a set of complementary techniques. For this purpose, ALD-grown SnO2 thin films are characterized in a systematic comparison with reference solution-processed SnO2. Energetics analysis by ultraviolet photoelectron spectroscopy (UPS) points out an unfavorable band bending at the ALD-grown SnO2/perovskite interface. Chemical characterization by time-of-flight secondary ion mass spectroscopy (ToF-SIMS) and hard X-ray photoelectron spectroscopy (HAXPES) profiling unveils an unexpected lack of oxygen at the ALD-grown SnO2/perovskite interface, which may play a direct role in observed performance limitations.

Cu2ZnSnS4‐based solar cells suffer from limited power conversion efficiency (PCE) and relative small grain size compared to selenium containing absorbers. Introduction of Na in Cu2ZnSnS4 absorbers either during the synthesis or after this step is used to improve device performances and to determine whether its effect is based on structural properties improvement (grain size enhancement, better crystallization) or on opto‐electronic properties improvement (defect passivation). In both cases, presence of Na in the absorber notably improves current and voltage of the solar cells, but the effect is more pronounced when Na is present during synthesis. Quantum efficiency analysis shows that these improvements can be related to longer minority carrier diffusion length and reduced absorber/buffer interface recombination. Introducing Na in the process mostly leads to preferential (112) orientation of the crystal which is clearly correlated with better device performances. Otherwise, the performance limitation due to small grain size is discarded by the joint use of Sb and Na, which has a significant impact on grain size but does not affect solar cells efficiency.

AbstractKesterite‐based solar cells suffer from a large open‐circuit voltage deficit, which largely arises from carrier recombination at the buffer interface. In this study, we compare two strategies to passivate the absorber surface in order to fabricate devices with power conversion efficiency higher than 10% and an open‐circuit voltage deficit as low as 306 mV. These two strategies consist of annealing in air or performing a chemical etching of the absorbers before buffer deposition. They lead similarly to a significant reduction of the interface recombination but as well to a shortening of the minority carrier diffusion length from 1 μm to less than 500 nm. This latter effect limits the short‐circuit current and fill factor of the devices but is largely compensated by the open‐circuit voltage gain of more than 100 mV. For the absorber air annealing, which is the simplest solution to implement, absolute photoluminescence measurements reveal that the voltage gain is directly linked to a drop in the nonradiative losses in the absorber and to a small reduction of the band tailing. It is demonstrated that the removal of detrimental secondary phases at the surface of the absorber due to oxidation at elevated temperature and etching in the basic CdS solution is responsible for these improved opto‐electronic properties. On the contrary, the apparent Cu‐depletion observed after air annealing is totally recovered after the chemical bath and cannot be responsible for the improved performances.

Replacing the CdS buffer layer with a ZnSnO one in Cu2ZnSnS4-based solar cells allows both to improve the device performances and to avoid using toxic Cd. Additionally, using a sputtered buffer lay...