• Energy Research

We show how molecular doping can be implemented to improve the performance of solution processed bulk heterojunction solar cells based on a low-bandgap polymer mixed with a fullerene derivative. The molecular dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) is introduced into blends of poly[2,6(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b0]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) via co-solution in a range of concentrations from 0% to 1%. We demonstrate that the hole conductivity and mobility increase with doping concentration using field-effect measurements. Photoinduced absorption (PIA) spectroscopy reveals that the polaron density in the blends increases with doping. We show that the open circuit voltage and short circuit current of the corresponding solar cells can be improved by doping at 0.5%, resulting in improved power conversion efficiencies. The increase in performance is discussed in terms of trap filling due to the increased carrier density, and reduced recombination correlated to the improvement in mobility.

Variations in local strain, defect densities, and composition of hybrid metal-halide perovskites have been reported to create heterogeneous energy landscapes in thin films, which impact charge-carrier diffusion and recombination dynamics. Here, we employ one- and two-photon transient absorption spectroscopy to selectively probe the dynamics of charge carriers from surface and bulk regions of methylammonium lead bromide thin films. Differences in the transient absorption spectra indicate that an energy gradient of approximately 100 meV is formed between the higher band-gap surface and lower band-gap bulk regions. Thus, during their lifetime, photoexcited carriers move away from the surface to recombine in the bulk, where our experiments detect long-lived charge populations despite the significant band splitting that has conventionally been assumed to inhibit efficient radiative recombination. Supported by first-principles calculations, we demonstrate that bright emission can still arise from the bulk with states that occupy a wide range of momenta in the vicinity of the band extrema, which show strong dipole transitions. Our results report that photoexcitations in the hybrid perovskites avoid defect-rich surface regions, and that particularly strong emission is generated from accumulated excitation populations in the bulk. Published by the American Physical Society 2024

The study of the photophysical properties of organic-metallic lead halide perovskites, which demonstrate excellent photovoltaic performance in devices with electron- and hole-accepting layers, helps to understand their charge photogeneration and recombination mechanism and unravels their potential for other optoelectronic applications. We report surprisingly high photoluminescence (PL) quantum efficiencies, up to 70%, in these solution-processed crystalline films. We find that photoexcitation in the pristine CH3NH3PbI3-xClx perovskite results in free charge carrier formation within 1 ps and that these free charge carriers undergo bimolecular recombination on time scales of 10s to 100s of ns. To exemplify the high luminescence yield of the CH3NH3PbI3-xClx perovskite, we construct and demonstrate the operation of an optically pumped vertical cavity laser comprising a layer of perovskite between a dielectric mirror and evaporated gold top mirrors. These long carrier lifetimes together with exceptionally high luminescence yield are unprecedented in such simply prepared inorganic semiconductors, and we note that these properties are ideally suited for photovoltaic diode operation.

Elucidating and quantifying the effects of doping on halide perovskites using lithium ion batteries.

Using circularly polarized broadband transient absorption, time-resolved circular photoluminescence, and transient Faraday rotation spectroscopy, we report that spin-dependent interactions have a significant impact on exciton energies and spin depolarization times in layered Ruddlesden-Popper hybrid metal-halide perovskites. In BA2FAPb2I7, we report that room-temperature spin lifetimes are largest (3.2 ps) at a carrier density of ∼1017 cm-3 with increasing depolarization rates at higher exciton densities. This indicates that many-body interactions reduce spin-lifetimes and outcompete the effect of D'yakonov-Perel precessional relaxation that has been previously reported at lower carrier densities. We further observe a dynamic circular dichroism that arises from a photoinduced polarization in the exciton distribution between total angular momentum states. Our findings provide fundamental and application relevant insights into the spin-dependent exciton-exciton interactions in layered hybrid perovskites.

Electron/hole traps related to interstitial iodine defects show the typical features of iodine photo-electrochemistry, inducing MAPbI3 defect tolerance.

Les mesures de photocourant à température ambiante dans des dispositifs de pérovskite inorganique-organique bidimensionnels (2D) révèlent que les excitons contribuent fortement aux photocourants malgré le fait qu'ils possèdent des énergies de liaison plus de 10 fois supérieures aux énergies thermiques. Le 2PbI4 de type p (C6H9C2H4NH3)libère des photoporteurs au niveau des contacts métalliques en aluminium Schottky, mais l'incorporation de couches de transport d'électrons et de trous augmente les photocourants extraits de 100 fois. Un gain supplémentaire de 10 fois est trouvé lorsque les nanoparticules de TiO2 sont directement intégrées dans les couches de pérovskite, bien que les couches semi-conductrices d'excitons 2D ne soient pas significativement perturbées. Ces résultats montrent que des matériaux excitoniques forts peuvent être utiles en tant que matériaux photovoltaïques malgré des énergies de liaison d'excitons élevées et suggèrent des mécanismes pour mieux comprendre les propriétés photovoltaïques des pérovskites tridimensionnelles apparentées. Las mediciones de fotocorriente a temperatura ambiente en dispositivos bidimensionales (2D) de perovskita inorgánica-orgánica revelan que los excitones contribuyen fuertemente a las fotocorrientes a pesar de poseer energías de unión más de 10 veces mayores que las energías térmicas. El tipo p (C6H9C2H4NH3)2PbI4 libera fotoportadores en los contactos metálicos de aluminio Schottky, pero la incorporación de capas de transporte de electrones y huecos mejora las fotocorrientes extraídas en 100 veces. Se encuentra una ganancia adicional de 10 veces cuando las nanopartículas de TiO2 se integran directamente en las capas de perovskita, aunque las capas semiconductoras de excitones 2D no se alteran significativamente. Estos resultados muestran que los materiales excitónicos fuertes pueden ser útiles como materiales fotovoltaicos a pesar de las altas energías de unión de excitones y sugieren mecanismos para comprender mejor las propiedades fotovoltaicas de las perovskitas tridimensionales relacionadas. Room-temperature photocurrent measurements in two-dimensional (2D) inorganic–organic perovskite devices reveal that excitons strongly contribute to the photocurrents despite possessing binding energies over 10 times larger than the thermal energies. The p-type (C6H9C2H4NH3)2PbI4 liberates photocarriers at metallic Schottky aluminum contacts, but incorporating electron- and hole-transport layers enhances the extracted photocurrents by 100-fold. A further 10-fold gain is found when TiO2 nanoparticles are directly integrated into the perovskite layers, although the 2D exciton semiconducting layers are not significantly disrupted. These results show that strong excitonic materials may be useful as photovoltaic materials despite high exciton binding energies and suggest mechanisms to better understand the photovoltaic properties of the related three-dimensional perovskites. تكشف قياسات درجة حرارة الغرفة الضوئية في أجهزة البيروفسكايت العضوية غير العضوية ثنائية الأبعاد (2D) أن الإكسيتون يساهم بقوة في الأجسام الضوئية على الرغم من امتلاكه طاقات ربط أكبر بعشر مرات من الطاقات الحرارية. يحرر النوع p (C6H9C2H4NH3) 2PbI4 الناقلات الضوئية عند تلامسات شوتكي المعدنية للألمنيوم، ولكن دمج طبقات نقل الإلكترون والثقب يعزز المركبات الضوئية المستخرجة بمقدار 100 ضعف. تم العثور على كسب إضافي بمقدار 10 أضعاف عندما يتم دمج جسيمات TiO2 النانوية مباشرة في طبقات البيروفسكايت، على الرغم من أن الطبقات شبه الموصلة للإكسيتون ثنائية الأبعاد لا تتعطل بشكل كبير. تظهر هذه النتائج أن المواد الاستثارية القوية قد تكون مفيدة كمواد كهروضوئية على الرغم من طاقات ربط الإكسيتون العالية وتقترح آليات لفهم الخصائص الكهروضوئية للبيروفسكايتات ثلاثية الأبعاد ذات الصلة بشكل أفضل.