Organic and Hybrid Organic-Inorganic Semiconductors: Advances and Applications

Session 3B: 12:30 PM – 2:45 PM Pacific Time on Friday, August 21.

Carolin Sutter-Fella*, Yi Liu

Hybrid and organic solar cells made significant advances in terms of efficiency and stability. Advancements are often related to better understanding and control of synthetic conditions to make superior materials. In parallel, this research domain has shown synergistic effects between the two fields and both present fascinating opportunities and challenges to further exploit unique properties of these materials. Topics of this symposium include photophysics, interfaces, defects, instabilities, and device applications from experimental and computational perspectives. We want to bring together the interdisciplinary research community to discuss recent advances of organic and hybrid semiconductors and to stimulate collaborative efforts.

Symposium Schedule:

Symposium Abstracts:

12:30 PM

Understanding and Designing Interfaces Defects and Crystallographic Orientations in Metal Halide Perovskites

Prof. Juan-Pablo Correa-Baena
Materials Science & Engineering, Georgia Tech

Perovskite solar cells promise to yield efficiencies beyond 30% by further improving the quality of the materials and devices. Electronic defect passivation, and suppression of detrimental charge-carrier recombination at the different device interfaces has been used as a strategy to achieve high performance perovskite solar cells. The impact of crystallographic orientations on charge carrier transport and ionic movement have not been thoroughly explored in metal halide perovskites. In this presentation, I will discuss the role of electronic defects and how these can be passivated to improve charge-carrier lifetimes and to achieve high open-circuit voltages. I will discuss the characterization of 2D and 3D defects, such as grain boundaries, crystal surface defects, and precipitate formation within the films, by synchrotron-based techniques. The importance of interfaces and their contribution to detrimental recombination will also be discussed. As a result of these contributions to better understanding 2D and 3D defects, the perovskite solar cell field has been able to improve device performance. In addition, I will discuss the role of crystallographic orientations on solar cell performance and long term durability. Despite the rapid improvements in performance, there is still a need to improve defects and design better polycrystalline thin films to push these solar cells beyond the current state-of-the-art.

12:55 PM

Improving Efficiency and Stability of Perovskite Solar Cells Enabled by A Near-Infrared-Absorbing Moisture Barrier

Dr. Qin Hu
Materials Sciences Division, Berkeley Lab

Simultaneously improving device efficiency and stability is the most important issue in perovskite solar cells (PSCs) research. Here, we strategically introduce a multi-functional interface layer (MFIL) with integrated roles of: 1) electron transport, 2) moisture barrier, 3) near-infrared photocurrent enhancement, 4) trap passivation and 5) ion migration suppression to enhance the device performance. The narrow-band gap non-fullerene acceptor, Y6, was screened out to replace the most commonly used PCBM, in the inverted PSCs. A significantly improved power conversion efficiency of 21.0% was achieved, along with a remarkable stability (up to 1700 hours) without encapsulation under various external stimuli (light/heat/moisture). Furthermore, systematic studies of the molecular orientation/passivation and the charge carrier dynamics at the perovskite/MFIL interface were presented. These results offer deep insights for designing advanced interlayers, and establish the correlations between molecular orientation, interface molecular bonding, trap state density, non-radiation recombination, and the device performance.

1:00 PM

Tackling Instabilities in Halide Perovskites from the Macro to the Nanoscale

Prof. Marina Leite
Materials Science & Engineering, UC Davis

We identify the individual and combined effects of extrinsic (humidity and oxygen) and intrinsic (light, bias, and temperature) stressors on halide perovskite materials by implementing macro- and nanoscopic measurements under environmentally controlled conditions, including in situ photoluminescence (PL) and scanning probe microscopy. We clarify the influence of distinct humidity levels on the charge carrier recombination in Cs_xFA_1-xPb(I_yBr_1-y)₃ perovskites through in situ PL, where we temporally and spectrally measure light emission within loops of critical relative humidity (rH) levels. Our results demonstrate that the Cs/Br ratio strongly affects the spectral stability of light emission hysteresis, as well as its extent. The photo-emission dynamics in metal halide perovskites with both I and Br is also interrogated by environmental PL, where we find that the presence of Br suppresses hysteresis. In the realm of nanoscale measurements, we unfold the spatial variations in open-circuit voltage in multi-cation perovskites by mapping ion motion in real-time and with spatial resolution < 50 nm. To pursue optimal 'rest' and 'recovery' conditions for photovoltaic stable operation, we propose a machine learning approach based on supervised learning combined with in situ PL, which will be discussed in detail.

1:25 PM

Real-time Investigation of Crystallization Pathways of Room-temperature-Processed Organo-Metal-Halide Perovskites

Dr. Maged Abdelsamie
Materials Sciences Division, Berkeley Lab & SSRL, SLAC, Stanford University
Coauthors: Tianyang Li¹, Finn Babbe², Michael F. Toney³, Qiwei Han¹, Carolin M. Sutter-Fell², David B. Mitzi¹, Junwei Xu^3
1Duke University; ²Berkeley Lab; ³SSRL, SLAC, Stanford University

Perovskite solar cells (PSCs) have gained tremendous attention as potential materials for photovoltaics due to their high efficiencies approaching the best silicon solar cells and their compatibility with low-cost low-temperature fabrication methods (such as solution processing). Several additive-processing approaches have been used to control the perovskite film crystallization process and tune the microstructure of perovskite films. Recently, thiocyanate containing additives, such as MASCN (MA = methylammonium), have been used successfully for manipulating perovskite microstructure and obtaining highly efficient room-temperature perovskite processing. Nevertheless, the mechanisms of perovskite formation and crystallization pathways involved in MASCN-additive processing approach are far from clear. Here, we investigate the perovskite formation during spin coating and the subsequent drying process using in situ x-ray scattering and photoluminescence, aiming at revealing the mechanisms of additive-assisted- perovskite-formation. Time-resolved monitoring of the perovskite thin film processing reveals the formation of intermediate precursor phases with ordered solvent-solute complexes, whereas perovskite formation is dominated by a sol-gel process. We show that the addition of MASCN to precursor solution induces large precursor aggregates in solution, promotes the formation of perovskite crystals in addition to intermediate solvent-complex phases during spin coating, and facilitates the dissociation of these intermediate phases leading to further growth of the perovskite crystals during the subsequent N₂-drying.

Our findings reveal that the nature of the precursor phases and their formation/dissociation dynamics have an impact on the extent of nucleation and growth of perovskite phase affecting the microstructure of the perovskite film. These findings provide in-depth understanding of the mechanism of room-temperature- additive-assisted-perovskite-processing and should guide further development towards more facile room-temperature perovskite processing.

1:35 PM

Two-Dimensional Halide Perovskite Lateral Epitaxial Heterostructures

Prof. Letian Dou
Chemical Engineering, Purdue University

Atomically sharp epitaxial heterostructures based on oxide perovskites, III-V, II-VI, and transition metal dichalcogenides semiconductors form the foundation of modern electronics and optoelectronics. As an emerging family of tunable semiconductor materials with exceptional optical and electronic properties, halide perovskites are attractive for applications in next-generation solution-processed solar cells, LEDs, photo/radiation detectors, lasers, etc. The inherently soft crystal lattice allows for greater tolerance to lattice mismatch, making them promising for heterostructure formation and semiconductor integration. However, epitaxial growth of atomically sharp heterostructures of halide perovskites have not been achieved so far owing to two critical challenges. First, the fast intrinsic ion mobility in these materials leads to interdiffusion and large junction widths. Second, poor chemical stability in these materials leads to decomposition of prior layers during the fabrication of the subsequent layers. In fact, the facile ionic motion and poor stability are limiting the commercialization of any halide perovskite-based electronic devices. Therefore, understanding the origins of the instability and identifying effective approaches to suppress ion motion are of great significance and urgency.

In this talk, I will present an effective strategy to substantially inhibit in-plane ion diffusion in two-dimensional (2D) halide perovskites via incorporation of rigid p-conjugated organic ligands. For the first time, we demonstrate highly stable and widely tunable lateral epitaxial heterostructures, multi-heterostructures, and superlattices of 2D halide perovskites via a solution-phase synthetic strategy. Near atomically sharp interfaces and epitaxial growth are revealed from low dose aberration-corrected high-resolution transmission electron microscopy characterizations. Molecular dynamics simulations reveal that the suppressed halide anion diffusivity is attributed to a combination of reduced heterostructure disorder and larger vacancy formation energies for 2D perovskites with conjugated ligands. These findings suggest critical fundamental insights regarding the immobilization and stabilization of halide perovskite semiconductor materials and provide a new materials platform for complex and molecularly thin superlattices, devices, and integrated circuits.

2:00 PM

Time-resolved cathodoluminescence imaging of Mn²⁺ luminescence in Mn-doped CsPbCl₃

Rebecca Wai
Chemistry, UC Berkeley
Coauthors: Naomi Ginsberg¹, Namrata Ramesh¹, Frank Ogletree², Steve Zeltmann^1,2, Shaul Aloni², Jonathan Raybin^1
1UC Berkeley; ²Berkeley Lab

Doping is a common method for introducing new properties to materials. Mn²⁺ doping in lead halide perovskites has been demonstrated to give rise to orange dopant emission in addition to the perovskite emission. Using time-resolved cathodoluminescence (TRCL) imaging, we measure the pixel-by-pixel cathodoluminescence decay of Mn²⁺ dopants in cesium lead chloride perovskite microplates at the Molecular Foundry. This measurement generates a spatially resolved map of the excited state decay dynamics of the Mn²⁺ dopant, which we use to suggest an explanation for enhanced Mn²⁺ emission near the surface of the microplate. In the TRCL, we measure a biexponential decay with 0.2 ms and 1 ms lifetime components. Near the surface of the microplate, the contribution from the 1 ms lifetime component increases, while the lifetimes remain the same. This implies that the population of excited Mn²⁺ is higher near the surface without a corresponding increase in Mn²⁺ concentration. We suggest that this arises due to the increased probability of carrier recombination at a Mn²⁺ dopant near the surface, possibly enabled by an increased concentration of traps. One possible implication of this hypothesis is that nanocrystals may be a preferable sample form-factor for enhanced Mn²⁺ emission because of their large amount of surface relative to other possible geometries.

2:05 PM

Changing the Dimensions of Organic Metal Halide Semiconductors

Prof. Michael Chabinyc
Materials, UC Santa Barbara

Hybrid organic metal halides, such as CH₃NH₃PbI₃, have garnered significant attention because they are earth-abundant, solution-processable materials that can be used to form solar cells with high power conversion efficiency (> 20%). Despite their performance in lab-based devices, there are significant questions about the phase behavior of these materials and their resulting properties. For example, an interesting feature of these materials is the ability to form layered Ruddlesden-Popper phases with quantum confinement by judicious choice of mixed organic cations. We will present our work on understanding the electronic properties of 3D and 2D Pb- and Bi-based systems using techniques such as time-resolved microwave conductivity. We will also describe our efforts to characterize and control the phase behavior in thin films of layered Ruddlesden-Popper compounds, (CH₃(CH₂)₃NH₃)₂(CH₃NH₃)_n-1Pb_nI_3n+1 (n = 1, 2, 3, 4). Despite the structural disorder apparent from quantitative analysis of grazing incidence X-ray scattering and electron microscopy, these materials surprisingly still have sharp band edges. Routes to control the phase purity of these materials during growth from solution will be presented that enable control of their optoelectronic properties.