Bernard Wenger

Abstract:

We present the synthesis of formamidinium lead tribromide (FAPbBr3) perovskite nanocrystals through a phosphine oxide route, where in comparison to more traditional syntheses oleylamine is replaced with trioctylphosphine oxide (TOPO). This route has previously been shown to be successful for the inorganic cesium lead tribromide perovskite nanocrystals. We examine the interactions between the precursors via nuclear magnetic resonance spectroscopy (NMR). We confirm the existence of an interaction between FA-oleate and TOPO and use this to guide the optimization of our synthesis. When the reaction is conducted at room temperature, we observe the formation of nanoparticles with high photoluminescence quantum yield (PLQY, ∼70%) at 2.39 eV (518 nm) with little ripening or size defocusing over time. Although we obtain narrow emission peaks, the crystals are irregular in shape—a testament to the impact of the FA-oleate:TOPO interaction. Despite a drop in PLQY in the washed solutions, films made maintain a high PLQY of ∼50% at 2.33 eV (532 nm), which is fortuitously the ideal wavelength for the green emission channel in displays, and we demonstrate 532 nm electroluminescence in light-emitting diodes with an EQE of 3.7%.

Abstract:

Longevity has been a long-standing concern for hybrid perovskite photovoltaics. We demonstrate high-resilience positive-intrinsic-negative perovskite solar cells by incorporating a piperidiniumbased ionic-compound into the formamidinium-cesium lead-trihalide perovskite absorber. With the band gap tuned to be well suited for perovskite-on-silicon tandem cells, this piperidinium additive enhances the open-circuit voltage and cell efficiency. This additive also retards compositional segregation into impurity phases and pinhole formation in the perovskite absorber layer during aggressive aging. Under full-spectrum simulated sunlight in ambient atmosphere, our Confidential unencapsulated and encapsulated cells retain 80% and 95% of their peak and “post-burn-in” efficiencies for 1010 and 1200 hours at 60 and 85 degree Celsius, respectively. Our analysis reveals detailed degradation routes that contribute to the failure of aged cells.

Abstract:

Longevity has been a long-standing concern for hybrid perovskite photovoltaics. We demonstrate high-resilience positive-intrinsic-negative perovskite solar cells by incorporating a piperidinium-based ionic compound into the formamidinium-cesium lead-trihalide perovskite absorber. With the bandgap tuned to be well suited for perovskite-on-silicon tandem cells, this piperidinium additive enhances the open-circuit voltage and cell efficiency. This additive also retards compositional segregation into impurity phases and pinhole formation in the perovskite absorber layer during aggressive aging. Under full-spectrum simulated sunlight in ambient atmosphere, our unencapsulated and encapsulated cells retain 80 and 95% of their peak and post-burn-in efficiencies for 1010 and 1200 hours at 60° and 85°C, respectively. Our analysis reveals detailed degradation routes that contribute to the failure of aged cells.

Abstract:

Light-emitting diodes (LEDs) made from metal halide perovskites have demonstrated external electroluminescent quantum efficiencies (EQEEL) in excess of 20%. However, their poor operational stability, resulting in lifetimes of only tens to hundreds of hours, needs to be dramatically improved prior to commercial use. There is little consensus in the community upon which factors limit the stability of these devices. Here, we investigate the role played by ammonium cations on the operational stability. We vary the amount of phenylethylammonium bromide, a widely used alkylammonium salt, that we add to a precursor solution of CsPbBr3 and track changes in stability and EQEEL. We find that while phenylethylammonium bromide is beneficial in achieving high efficiency, it is highly detrimental to operational stability. We investigate material properties and electronic characteristics before and after degradation and find that both a reduction in the radiative efficiency of the emitter and significant changes in current–voltage characteristics explain the orders of magnitude drop in the EQEEL, which we attribute to increased ionic mobility. Our results suggest that engineering new contacts and further investigation into materials with lower ionic mobility should yield much improved stability of perovskite LEDs.

Abstract:

Current designs for all-perovskite multi-junction solar cells require mixed-metal Pb-Sn compositions to achieve narrower band gaps than are possible with their neat Pb counterparts. The lower band gap range achievable with mixed-metal Pb-Sn perovskites also encompasses the 1.3 to 1.4 eV range that is theoretically ideal for maximising the efficiency of single-junction devices. Here we examine the optoelectronic quality and photovoltaic performance of the ((HC(NH2)2)0.83Cs0.17)(Pb1-ySny)I3 family of perovskite materials across the full range of achievable band gaps by substituting between 0.001% and 70% of the Pb content with Sn. We reveal that a compositional range of "defectiveness"exists when Sn comprises between 0.5% and 20% of the metal content, but that the optoelectronic quality is restored for Sn content between 30-50%. When only 1% of Pb content is replaced by Sn, we find that photoconductivity, photoluminescence lifetime, and photoluminescence quantum efficiency are reduced by at least an order of magnitude, which reveals that a small concentration of Sn incorporation produces trap sites that promote non-radiative recombination in the material and limit photovoltaic performance. While these observations suggest that band gaps between 1.35 and 1.5 eV are unlikely to be useful for optoelectronic applications without countermeasures to improve material quality, highly efficient narrower band gap absorber materials are possible at or below 1.33 eV. Through optimising single-junction photovoltaic devices with Sn compositions of 30% and 50%, we respectively demonstrate a 17.6% efficient solar cell with an ideal single-junction band gap of 1.33 eV and an 18.1% efficient low band gap device suitable for the bottom absorber in all-perovskite multi-junction cells.