Erin M. Sanehira

Quantum dot–induced phase stabilization of α-CsPbI3 perovskite for high-efficiency photovoltaics

Maintaining a stable phase For solar cell applications, all-inorganic perovskite phases could be more stable than those containing organic cations. But the band gaps of the former, which determine the electrical conductivity of these materials, are not well matched to the solar spectrum. The cubic structure of CsPbI3 is an exception, but it is stable in bulk only at high temperatures. Swarnkar et al. show that surfactant-coated α-CsPbI3 quantum dots are stable at ambient conditions and have tunable band gaps in the visible range. Thin films of these materials can be made by spin coating with an antisolvent technique to minimize surfactant loss. When used in solar cells, these films have efficiencies exceeding 10%, making them promising for light harvesting or for LEDs. Science, this issue p. 92 The cubic crystalline phase of CsPbI3, which has a more favorable band gap for solar cells, is stabilized as a nanomaterial. We show nanoscale phase stabilization of CsPbI3 quantum dots (QDs) to low temperatures that can be used as the active component of efficient optoelectronic devices. CsPbI3 is an all-inorganic analog to the hybrid organic cation halide perovskites, but the cubic phase of bulk CsPbI3 (α-CsPbI3)—the variant with desirable band gap—is only stable at high temperatures. We describe the formation of α-CsPbI3 QD films that are phase-stable for months in ambient air. The films exhibit long-range electronic transport and were used to fabricate colloidal perovskite QD photovoltaic cells with an open-circuit voltage of 1.23 volts and efficiency of 10.77%. These devices also function as light-emitting diodes with low turn-on voltage and tunable emission.

Heterogeneous Charge Carrier Dynamics in Organic–Inorganic Hybrid Materials: Nanoscale Lateral and Depth-Dependent Variation of Recombination Rates in Methylammonium Lead Halide Perovskite Thin Films

We reveal substantial luminescence yield heterogeneity among individual subdiffraction grains of high-performing methylammonium lead halide perovskite films by using high-resolution cathodoluminescence microscopy. Using considerably lower accelerating voltages than is conventional in scanning electron microscopy, we image the electron beam-induced luminescence of the films and statistically characterize the depth-dependent role of defects that promote nonradiative recombination losses. The highest variability in the luminescence intensity is observed at the exposed grain surfaces, which we attribute to surface defects. By probing deeper into the film, it appears that bulk defects are more homogeneously distributed. By identifying the origin and variability of a surface-specific loss mechanism that deleteriously impacts device efficiency, we suggest that producing films homogeneously composed of the highest-luminescence grains found in this study could result in a dramatic improvement of overall device efficiency. We also show that although cathodoluminescence microscopy is generally used only to image inorganic materials it can be a powerful tool to investigate radiative and nonradiative charge carrier recombination on the nanoscale in organic-inorganic hybrid materials.

Structural and chemical evolution of methylammonium lead halide perovskites during thermal processing from solution

Following the prominent success of CH3NH3PbI3 in photovoltaics and other optoelectronic applications, focus has been placed on better understanding perovskite crystallization from precursor and intermediate phases in order to facilitate improved crystallinity often desirable for advancing optoelectronic properties. Understanding of stability and degradation is also of critical importance as these materials seek commercial applications. In this study, we investigate the evolution of perovskites formed from targeted precursor chemistries by correlating in situ temperature-dependent X-ray diffraction, thermogravimetric analysis, and mass spectral analysis of the evolved species. This suite of analyses reveals important precursor composition-induced variations in the processes underpinning perovskite formation and degradation. The addition of Cl− leads to widely different precursor evolution and perovskite formation kinetics, and results in significant changes to the degradation mechanism, including suppression of crystalline PbI2 formation and modification of the thermal stability of the perovskite phase. This work highlights the role of perovskite precursor chemistry in both its formation and degradation.

High-Performance Flexible Perovskite Solar Cells on Ultrathin Glass: Implications of the TCO

For halide perovskite solar cells (PSCs) to fulfill their vast potential for combining low-cost, high efficiency, and high throughput production they must be scaled using a truly transformative method, such as roll-to-roll processing. Bringing this reality closer to fruition, the present work demonstrates flexible perovskite solar cells with 18.1% power conversion efficiency on flexible Willow Glass substrates. We highlight the importance of the transparent conductive oxide (TCO) layers on device performance by studying various TCOs. While tin-doped indium oxide (ITO) and indium zinc oxide (IZO) based PSC devices demonstrate high photovoltaic performances, aluminum-doped zinc oxide (AZO) based devices underperformed in all device parameters. Analysis of X-ray photoemission spectroscopy data shows that the stoichiometry of the perovskite film surface changes dramatically when it is fabricated on AZO, demonstrating the importance of the substrate in perovskite film formation.

Highly stable cesium lead iodide perovskite quantum dot light-emitting diodes

Recently, all-inorganic perovskites such as CsPbBr3 and CsPbI3, have emerged as promising materials for light-emitting applications. While encouraging performance has been demonstrated, the stability issue of the red-emitting CsPbI3 is still a major concern due to its small tolerance factor. Here we report a highly stable CsPbI3 quantum dot (QD) light-emitting diode (LED) with red emission fabricated using an improved purification approach. The device achieved decent external quantum efficiency (EQE) of 0.21% at a bias of 6 V and outstanding operational stability, with a L 70 lifetime (EL intensity decreases to 70% of starting value) of 16 h and 1.5 h under a constant driving voltage of 5 V and 6 V (maximum EQE operation) respectively. Furthermore, the device can work under a higher voltage of 7 V (maximum luminance operation) and retain 50% of its initial EL intensity after 500 s. These findings demonstrate the promise of CsPbI3 QDs for stable red LEDs, and suggest the feasibility for electrically pumped perovskite lasers with further device optimizations.Recently, all-inorganic perovskites such as CsPbBr3 and CsPbI3, have emerged as promising materials for light-emitting applications. While encouraging performance has been demonstrated, the stability issue of the red-emitting CsPbI3 is still a major concern due to its small tolerance factor. Here we report a highly stable CsPbI3 quantum dot LED with red emission fabricated using an improved purification approach. The device achieved decent external quantum efficiency (EQE) of 0.21 % at a bias of 6 V and outstanding operational stability, with a L70 lifetime (EL intensity decreases to 70% of starting value) of 16 h and 1.5 h under a constant driving voltage of 5 V and 6 V (maximum EQE operation) respectively. Furthermore, the device can work under a higher voltage of 7 V (maximum luminance operation) and retain 50% of its initial EL intensity after 500 s. These findings demonstrate the promise of CsPbI3 quantum dots for stable red LEDs, and suggest the feasibility for electrically pumped perovskite lasers with further device optimizations.

Targeted Ligand Exchange Chemistry on Cesium Lead Halide Perovskite Quantum Dots for High-Efficiency Photovoltaics

The ability to manipulate quantum dot (QD) surfaces is foundational to their technological deployment. Surface manipulation of metal halide perovskite (MHP) QDs has proven particularly challenging in comparison to that of more established inorganic materials due to dynamic surface species and low material formation energy; most conventional methods of chemical manipulation targeted at the MHP QD surface will result in transformation or dissolution of the MHP crystal. In previous work, we have demonstrated record-efficiency QD solar cells (QDSCs) based on ligand-exchange procedures that electronically couple MHP QDs yet maintain their nanocrystalline size, which stabilizes the corner-sharing structure of the constituent PbI64- octahedra with optoelectronic properties optimal for solar energy conversion. In this work, we employ a variety of spectroscopic techniques to develop a molecular-level understanding of the MHP QD surface chemistry in this system. We individually target both the anionic (oleate) and cationic (oleylammonium) ligands. We find that atmospheric moisture aids the process by hydrolysis of methyl acetate to generate acetic acid and methanol. Acetic acid then replaces native oleate ligands to yield QD surface-bound acetate and free oleic acid. The native oleylammonium ligands remain throughout this film deposition process and are exchanged during a final treatment step employing smaller cations-namely, formamidinium. This final treatment has a narrow processing window; initial treatment at this stage leads to a more strongly coupled QD regime followed by transformation into a bulk MHP film after longer treatment. These insights provide chemical understanding to the deposition of high-quality, electronically coupled MHP QD films that maintain both quantum confinement and their crystalline phase and attain high photovoltaic performance.

Non-toxic, colloidal ZnS-AgInS 2 nanoparticles for organic-inorganic hybrid photovoltaics

Non-toxic, colloidal ZnS-AgInS2 nanoparticles are synthesized and characterized for organic-inorganic hybrid photovoltaic applications. The optical properties of these particles are easily tuned by changing the chemical composition of the nanoparticle. Additionally, the photoluminescence quantum yield of 37.5% suggests this material is a promising candidate for optoelectronic devices. A comparison of the photoluminescence spectra of ZnS-AgInS2 nanoparticle and poly-3(hexylthiophene) blends to poly-3(hexylthiophene) neat indicates charge transfer between the nanoparticle and the polymer occurs in solution. Photovoltaic devices were fabricated using blends of ZnS-AgInS2 nanoparticles, poly-3(hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester, yielding an average power conversion efficiency of 2.3%.