Periodic Reporting for period 3 - SOPHY (The role of Softness in the Physics of Defects: Probing Buried Interfaces in Perovskites Optoelectronic Devices)
Période du rapport: 2021-09-01 au 2023-02-28
SOPHY wants to probe optoelectronic processes at buried interfaces in devices, at operating conditions. This is a target which has been long pursued in many fields of nanotechnology for the study of multi-junctions devices, which includes solar cells, light-emitting diodes (LEDs), electro-chemical and photo-catalytic devices. However, so far, direct probe has not been fully achieved. Information has been often built on indirect feedback1 obtained from studies employing independent techniques and from simplified model systems2 or on experimental data collected without the appropriate temporal/spatial resolution. Unfortunately, given the complexity of interfacial processes, such approaches are not immune from misinterpretation.
Perovskite based photovoltaic (PV) devices have recently made spectacular progress in terms of device efficiency, however, the reported values still belong to devices electrically unstable. Most of the advances so far have not been supported by an understanding of the basic physics, chemistry, and material science which is required for assessing the potential and limitations of the technology. Tellingly, the description of the nature of carrier in the semiconductor and of the physics of the related devices, in particular, the simple description of a perovskite based diode, is not available. This implies a lack of predictive design of the device, pivotal in the development of a reliable technology. There is no direct experimental method that measures the band diagram, and carrier and electric field distributions and evolution in a completed diode. Thus, the study of buried interfaces and the electrostatic of an heterojunction based diode, which will eventually determine the device performances, is a difficult task. For perovskite based devices it is even harder.
SOPHY overall objective is the development and exploitation of an experimental approach which will put together the advantages of optical spectroscopy and electronic spectroscopy and microscopy and will challenge their limitations when taken individually. Time-resolved (visible) pump- (UV)push photoelectron (Tr-PPE) spectroscopy set up will be coupled to a photoelectron microscope (PEEM). Here, this system will be developed to push, within the same experiment, both time (fs) and space resolution (< 50nm). It will map in space electronic structures and the related excitations and how they evolve in time over a large timescale (from femtoseconds to microseconds), in order to follow a wide set of cascade phenomena. Such approach, implemented for the first time on diodes’ cross-sections, will provide the possibility of studying the device at different operation conditions, the key to monitor the “real life” transformations.
The achievemnt of such target will allow the educated design of perovskite based optoelectronic design, boosting the development of a new technological platform for optoelectronics
Perovskite based photovoltaic (PV) devices have recently made spectacular progress in terms of device efficiency, however, the reported values still belong to devices electrically unstable. Most of the advances so far have not been supported by an understanding of the basic physics, chemistry, and material science which is required for assessing the potential and limitations of the technology. Tellingly, the description of the nature of carrier in the semiconductor and of the physics of the related devices, in particular, the simple description of a perovskite based diode, is not available. This implies a lack of predictive design of the device, pivotal in the development of a reliable technology. There is no direct experimental method that measures the band diagram, and carrier and electric field distributions and evolution in a completed diode. Thus, the study of buried interfaces and the electrostatic of an heterojunction based diode, which will eventually determine the device performances, is a difficult task. For perovskite based devices it is even harder.
SOPHY overall objective is the development and exploitation of an experimental approach which will put together the advantages of optical spectroscopy and electronic spectroscopy and microscopy and will challenge their limitations when taken individually. Time-resolved (visible) pump- (UV)push photoelectron (Tr-PPE) spectroscopy set up will be coupled to a photoelectron microscope (PEEM). Here, this system will be developed to push, within the same experiment, both time (fs) and space resolution (< 50nm). It will map in space electronic structures and the related excitations and how they evolve in time over a large timescale (from femtoseconds to microseconds), in order to follow a wide set of cascade phenomena. Such approach, implemented for the first time on diodes’ cross-sections, will provide the possibility of studying the device at different operation conditions, the key to monitor the “real life” transformations.
The achievemnt of such target will allow the educated design of perovskite based optoelectronic design, boosting the development of a new technological platform for optoelectronics
Since the beginning of the project the following work has been performed:
- Study of defect photochemistry in 3D perovskites based on lead and tin perovskites. In lead based semiconductors we have demonstrated that photo-instabilities related to light-induced formation and annihilation of defects acting as carrier trap states. We show that these phenomena coexist and compete. In particular, long-living carrier traps related to halide defects trigger photo-induced material transformations, which drive both processes. On short scales, defect annihilation can prevail over defect formation, which occurs on longer scales. Nevertheless, defect formation can be controlled by blocking under-coordinated surface sites, which act as a defect reservoir. The carrier recombination dynamics in Sn-based perovskite thin films are dominated by shallow trap states which are likely responsible for the notorious self-doping mechanism. In agreement, the time-resolved PL decay is monomolecular (about 4 ns lifetime) and independent of the excitation density over several orders of magnitude, which is a clear fingerprint of electronic doping. Such observations are in agreement with theoretically predictions, who show that an enhancement of the valence band when tin substitutes lead leads to the suppression of deep transitions associated with undercoordinated iodine defects that are typical of MAPbI3 For pristine lead-based thin film perovskite layers, the external PLQY at 1 Sun equivalent photon flux is typically <15%. In line with this, the measured sample, when embodied in solar cells, leads to devices with a power conversion efficiency of the order of 18%. The result is surprising when looking at the Sn-based thin film because it indicates that a similar, or even higher, fraction of photogenerated carrier recombine radiatively when compared to the more successful lead-based counterpart, despite their shorter lifetime. This suggests that the optoelectronic quality of state of the art pristine Sn based perovskite thin films is inherently good for the fabrication of solar cells with a higher open-circuit voltage and power conversion efficiency. Nevertheless, the device performances do not respond to such observation. Thus, re-designing the device architecture can be a promising way for boosting the performances.
- Prototypal 2D perovskites have been used as model system to investigate the role of structural deformation on the formation of defects. We found that increased intra‐octahedral distortion induces exciton localization processes and leads to formation of multiple photoinduced emissive colour centres. Ultimately, this leads to highly Stokes‐shifted, ultrabroad white light emission at room temperature. We have investigated doping of 2D perovskites with europium, either by direct substitutional doping with Eu3+ ions or by protection of Eu3+ in the form of the tetrakis -diketonate complex Eu(tta)4P(Ph)4, Eu(L), designing the carrier evolution path by designing the energetics of the semicondutor at molecular level. We have showed to expand the range of synthetic strategies for designing functional MHPs and sets the basis for further developments.
- development of a spatially resolved pump&push photo-electron spectroscopy set-up. The last year has been devoted toabuilding An ultraviolet (UV) photon source with the wavelength of 205nm, built as imaging light source for the photoemission electron microscope (Elmitec PEEM III). The setup is built using a femtosecond laser system (Pharos) with a tunable repetition rate of single shot to 1MHz and a center wavelength of 1030nm. this was needed to allow the probing of the electronci structure of materials with different bandgaps.
- Study of defect photochemistry in 3D perovskites based on lead and tin perovskites. In lead based semiconductors we have demonstrated that photo-instabilities related to light-induced formation and annihilation of defects acting as carrier trap states. We show that these phenomena coexist and compete. In particular, long-living carrier traps related to halide defects trigger photo-induced material transformations, which drive both processes. On short scales, defect annihilation can prevail over defect formation, which occurs on longer scales. Nevertheless, defect formation can be controlled by blocking under-coordinated surface sites, which act as a defect reservoir. The carrier recombination dynamics in Sn-based perovskite thin films are dominated by shallow trap states which are likely responsible for the notorious self-doping mechanism. In agreement, the time-resolved PL decay is monomolecular (about 4 ns lifetime) and independent of the excitation density over several orders of magnitude, which is a clear fingerprint of electronic doping. Such observations are in agreement with theoretically predictions, who show that an enhancement of the valence band when tin substitutes lead leads to the suppression of deep transitions associated with undercoordinated iodine defects that are typical of MAPbI3 For pristine lead-based thin film perovskite layers, the external PLQY at 1 Sun equivalent photon flux is typically <15%. In line with this, the measured sample, when embodied in solar cells, leads to devices with a power conversion efficiency of the order of 18%. The result is surprising when looking at the Sn-based thin film because it indicates that a similar, or even higher, fraction of photogenerated carrier recombine radiatively when compared to the more successful lead-based counterpart, despite their shorter lifetime. This suggests that the optoelectronic quality of state of the art pristine Sn based perovskite thin films is inherently good for the fabrication of solar cells with a higher open-circuit voltage and power conversion efficiency. Nevertheless, the device performances do not respond to such observation. Thus, re-designing the device architecture can be a promising way for boosting the performances.
- Prototypal 2D perovskites have been used as model system to investigate the role of structural deformation on the formation of defects. We found that increased intra‐octahedral distortion induces exciton localization processes and leads to formation of multiple photoinduced emissive colour centres. Ultimately, this leads to highly Stokes‐shifted, ultrabroad white light emission at room temperature. We have investigated doping of 2D perovskites with europium, either by direct substitutional doping with Eu3+ ions or by protection of Eu3+ in the form of the tetrakis -diketonate complex Eu(tta)4P(Ph)4, Eu(L), designing the carrier evolution path by designing the energetics of the semicondutor at molecular level. We have showed to expand the range of synthetic strategies for designing functional MHPs and sets the basis for further developments.
- development of a spatially resolved pump&push photo-electron spectroscopy set-up. The last year has been devoted toabuilding An ultraviolet (UV) photon source with the wavelength of 205nm, built as imaging light source for the photoemission electron microscope (Elmitec PEEM III). The setup is built using a femtosecond laser system (Pharos) with a tunable repetition rate of single shot to 1MHz and a center wavelength of 1030nm. this was needed to allow the probing of the electronci structure of materials with different bandgaps.
For the first time we have identified the nature of defects in lead based perovskite semiconductors. Such knowledge explain the origin of defect tolerance in perovkite semiconductors and also allowed us to explain the materials behaviour under light exposure. Importantly this has allowed the community to proceed thorugh educated strategy for the passivation and stabilization of lead halide perovskites.
We have also demosntrated for the first time that the main limitation for the employment of tin based perovskites is not the optoelectronic quality of the thin film, but the device architecture used.
Finally, with the development of the 11 eV fs line we will be able, for the first time, to map the evolution in time of the electronc structure in metal halide perovskite and of large library of materials
We have also demosntrated for the first time that the main limitation for the employment of tin based perovskites is not the optoelectronic quality of the thin film, but the device architecture used.
Finally, with the development of the 11 eV fs line we will be able, for the first time, to map the evolution in time of the electronc structure in metal halide perovskite and of large library of materials