Positrons and Charge Carrier Trapping Centers in Halide Perovskites – TRAPPER

One novelty in TRAPPER is the main focus on vacancy type defects in HOIPs compounds and use of positron annihilation spectroscopy (PAS) to directly detect their open volume and determine their charge and nature. The PAS characterization is conducted for the HOIPs layers in the near-surface depth profiling mode (0= z(µm) = 1). TRAPPER develops a systematic approach to compare the characteristics of positron annihilation in fresh and treated HOIPs crystals, layers and photovoltaic device and to examine the relevance of positron behavior - within layers and at the surface/interface(s) of layers - to photovoltaic performance. One original feature is that the positron transport in the region close to HOIPs surfaces will be correlated to those of photo-carriers derived from SPV. A unique approach proposed by TRAPPER is to use high energy electron irradiation to control defects in HOIPs. The question is whether, depending on the history of HOIPs (preparation, aging, illumination, PV operation), the introduced defects affect positron capture, photo-excited carrier transport/recombination° and ion migration*: 2D-trSPV(10^-7-10^-2s°/>10^-1s*); ns-trPL (10-^7-10-^2s)°. Another unique aspect is the systematic characterization and processing of batches of reference layers for active layers prepared at the same time for solar cells (SCs). The close correlation between the performance of photovoltaic devices and the fundamental processes related to positron capture, photo-carrier capture and ion transport in the active layers (SCs) or their references is one of the TRAPPER highlight. This understanding is essential to efficiently guide the fabrication of SC devices with increased stability.

The 2020-2021 PAS data lead to conclude that positron capture and annihilation in HOIPs materials have unique and intriguing features. The recombination dynamics of charge carriers for HOIPs layers as derived from the 2020-2021 trSPV data are consistent with the time-resolved photoluminescence data reported in literature.

One future prospect in TRAPPER is to set-up different types of in-situ depth profile PAS to monitor the physical processes that control positron behaviour for various HOIPs treatments relevant to photovoltaïc operation. Another one is to develop a new acquisition mode in SPV adapted to the features observed in the first TRAPPER SPV data.

B. Grévin – invited – Implementation of pump-probe KPFM under UHV and application to third generation solar cells – Zurich Inst. SPM user meeting (virtual event) May 20th 2021
JPH2021 6ème Journées Pérovskites Halogénées - Sciencesconf.org;
jph2021.sciencesconf.org/data/Book_of_abstracts_JPH2021.pdf P. Aversa, M. Kim, V. Léger, H. Lee, D. Tondelier, O. Plantevin, J. Botsoa, P. Desgardin, J.E Bourrée, L. Liszkay, M. Dickmann, W. Egger, M.F. Barthe, B. Geffroy, C. Corbel; jph2021.sciencesconf.org/data/JPH2021_Programme_detaille_26032021.pdf C. Corbel

TRAPPER aims at understanding the nature of structural/chemical defects in halide organic-inorganic perovskites (HOIPS:APbX3) crystals, layers, photovoltaic (PV) devices & their impact on PV stability for finding solutions that improve it while maintaining or improving the power conversion efficiency. One key property for PV is that the photoexcited carriers have lifetimes long enough for transport to the (n/p) collecting electrodes in solar cells (SCs). One TRAPPER highlight is to couple an experimental & theoretical approach where nationally unique & internationally at the state of the art spectroscopies are used to characterize defects^, photocarrier°/ ion* transport (PAS^,fs°-tr2PPE,ns°-s*-2D-trSPV, ns°-trPL). The DFT calculations will provide information on the defect electronic structure, configuration stability, kinetics and expected values for defect signals experimentally derived, e.g. migration energy, PAS fingerprints.
TRAPPER will focus on new promising mixed APbX3 (e.g. nA/nX elements:=4/=2) for PV application. To gain insight into the mechanisms that control PV operation, the evolution of PV performance will be correlated to those of the defect & photo-carrier dynamics as a function of the conditions for APbX3 growth, ageing, e- irradiation (e.g. dark, sunlight, bias, atmosphere, temperature). The methodology is ambitious and realistic. Its potential is demonstrated by (2017-18) Partners results for prototypical CH3NH3PbI3 (MAPI) and emerging 3/4APbX3. Those show that it is most relevant to understand the nature, generation & role of the defects involved in the degradation process of PV performance in HOIPs.
The correlation between defect-carrier properties will be conducted in fresh/treated active layers (i) in p-i-n/n-i-p SCs, layers with interfaces relevant to different stages of the SCs fabrication, crystals for APbX3 that yield power conversion efficiency covering a wide range 8-20%. Partners have shown that ions migrate at room temperature in MAPI (i) under bias voltage in dark or (ii) under light. For (i), the ion type is directly determined as iodide. They also directly detected by using positron annihilation spectroscopy (PAS) that the vacancy-type population depend on the HOIPs type and history. This raises questions about the role of native defects associated to ionic migration in HOIPs and their impact on PV performance and long-term stability of device operation. For a given APbX3, PAS has also the advantage to indicate whether the e+ annihilation characteristics at the interfaces in layers or SCs evolve depending on preparation/treatment. So, PAS can discriminate which treatment affects the interfaces and whether defects are specifically generated in the interface vicinity.
One novelty in TRAPPER is the main focus on vacancy type defects with their direct detection, charge and nature identification by (PAS) neglected in HOIPs until now. Another one is the unique experimental approach where TRAPPER uses e-irradiation to control defects in HOIPs and investigates whether, depending on the HOIPs history (preparation, ageing, illumination, e- irradiation, PV operation), vacancy-type defects (PAS) affect the transport/recombination of photo-excited carriers° and ion migration*: fs-tr2PPE(10-13-10-9s)°; 2D-trSPV(10-7-10-2s°/>10-1s*); ns-trPL (10-7-10-2s)°. Another unique feature is the systematic characterization of layers prepared in the same sets as those used to fabricate SCs. The close correlation between PV device performance and fundamental processes related to positron, photo-carrier capture and ion transport in the SC active layers is one TRAPPER highlight. This understanding is key to efficiently guide the fabrication of SCs with increased stability.
Given the global economic challenge that lies in it, the stabilization of HOIPs based SCs performance will constitute a breakthrough for photovoltaic technology. The success of the TRAPPER project will have a strong international impact.