Formerly the "Solar and Photovoltaic Engineering Research Center (SPERC)"

### Recent Submissions

• #### Lithium-Ion Desolvation Induced by Nitrate Additives Reveals New Insights into High Performance Lithium Batteries

(Advanced Functional Materials, Wiley, 2021-04-02) [Article]
Electrolyte additives have been widely used to address critical issues in current metal (ion) battery technologies. While their functions as solid electrolyte interface forming agents are reasonably well-understood, their interactions in the liquid electrolyte environment remain rather elusive. This lack of knowledge represents a significant bottleneck that hinders the development of improved electrolyte systems. Here, the key role of additives in promoting cation (e.g., Li+) desolvation is unraveled. In particular, nitrate anions (NO3−) are found to incorporate into the solvation shells, change the local environment of cations (e.g., Li+) as well as their coordination in the electrolytes. The combination of these effects leads to effective Li+ desolvation and enhanced battery performance. Remarkably, the inexpensive NaNO3 can successfully substitute the widely used LiNO3 offering superior long-term stability of Li+ (de-)intercalation at the graphite anode and suppressed polysulfide shuttle effect at the sulfur cathode, while enhancing the performance of lithium–sulfur full batteries (initial capacity of 1153 mAh g−1 at 0.25C) with Coulombic efficiency of ≈100% over 300 cycles. This work provides important new insights into the unexplored effects of additives and paves the way to developing improved electrolytes for electrochemical energy storage applications.
• #### Theory-Guided Synthesis of Highly Luminescent Colloidal Cesium Tin Halide Perovskite Nanocrystals

(Journal of the American Chemical Society, American Chemical Society (ACS), 2021-04-01) [Article]
The synthesis of highly luminescent colloidal CsSnX<sub>3</sub> (X = halogen) perovskite nanocrystals (NCs) remains a long-standing challenge due to the lack of a fundamental understanding of how to rationally suppress the formation of structural defects that significantly influence the radiative carrier recombination processes. Here, we develop a theory-guided, general synthetic concept for highly luminescent CsSnX<sub>3</sub> NCs. Guided by density functional theory calculations and molecular dynamics simulations, we predict that, although there is an opposing trend in the chemical potential-dependent formation energies of various defects, highly luminescent CsSnI<sub>3</sub> NCs with narrow emission could be obtained through decreasing the density of tin vacancies. We then develop a colloidal synthesis strategy that allows for rational fine-tuning of the reactant ratio in a wide range but still leads to the formation of CsSnI<sub>3</sub> NCs. By judiciously adopting a tin-rich reaction condition, we obtain narrow-band-emissive CsSnI<sub>3</sub> NCs with a record emission quantum yield of 18.4%, which is over 50 times larger than those previously reported. Systematic surface-state characterizations reveal that these NCs possess a Cs/I-lean surface and are capped with a low density of organic ligands, making them an excellent candidate for optoelectronic devices without any postsynthesis ligand management. We showcase the generalizability of our concept by further demonstrating the synthesis of highly luminescent CsSnI<sub>2.5</sub>Br<sub>0.5</sub> and CsSnI<sub>2.25</sub>Br<sub>0.75</sub> NCs. Our findings not only highlight the value of computation in guiding the synthesis of high-quality colloidal perovskite NCs but also could stimulate intense efforts on tin-based perovskite NCs and accelerate their potential applications in a range of high-performance optoelectronic devices.
• #### Microfluidic Integrated Organic Electrochemical Transistor with a Nanoporous Membrane for Amyloid-β Detection

(ACS Nano, American Chemical Society (ACS), 2021-03-30) [Article]
Alzheimer’s disease (AD) is a neurodegenerative disorder associated with a severe loss in thinking, learning, and memory functions of the brain. To date, no specific treatment has been proven to cure AD, with the early diagnosis being vital for mitigating symptoms. A common pathological change found in AD-affected brains is the accumulation of a protein named amyloid-β (Aβ) into plaques. In this work, we developed a micron-scale organic electrochemical transistor (OECT) integrated with a microfluidic platform for the label-free detection of Aβ aggregates in human serum. The OECT channel–electrolyte interface was covered with a nanoporous membrane functionalized with Congo red (CR) molecules showing a strong affinity for Aβ aggregates. Each aggregate binding to the CR-membrane modulated the vertical ion flow toward the channel, changing the transistor characteristics. Thus, the device performance was not limited by the solution ionic strength nor did it rely on Faradaic reactions or conformational changes of bioreceptors. The high transconductance of the OECT, the precise porosity of the membrane, and the compactness endowed by the microfluidic enabled the Aβ aggregate detection over eight orders of magnitude wide concentration range (femtomolar–nanomolar) in 1 μL of human serum samples. We expanded the operation modes of our transistors using different channel materials and found that the accumulation-mode OECTs displayed the lowest power consumption and highest sensitivities. Ultimately, these robust, low-power, sensitive, and miniaturized microfluidic sensors helped to develop point-of-care tools for the early diagnosis of AD.
• #### Challenges to the Success of Commercial Organic Photovoltaic Products

(Advanced Energy Materials, Wiley, 2021-03-24) [Article]
Recent advances in the development of organic photovoltaic (OPV) materials has led to significant improvements in device performance; now closing in on the 20% efficiency threshold. Despite these improvements in performance, the commercial viability of organic photovoltaic products remains elusive. In this perspective, the current limitations of high performing blends are uncovered, particularly focusing on the industrial upscaling considerations of these materials, such as synthetic scalability, active layer processing, and device stability. Moreover, a simplified metric, namely, the scalability factor (SF), is introduced to evaluate the scale-up potential of specific OPV materials and blends thereof. Of the most popular molecular design strategies investigated in recent times, it is found that the use of Y-series nonfullerene acceptors (NFAs) and synthetically simple materials, such as PTQ-10 and ternary blends, are most effective at maximizing the efficiency without negatively impacting the SF. Furthermore, the improvements that are needed, in terms of device processability and stability, are considered for industrial scale-up and final product application. Finally, an outlook of organic photovoltaics is provided both from a perspective of important research avenues and applications that can be exploited.
• #### Engineering Band-Type Alignment in CsPbBr 3 Perovskite-Based Artificial Multiple Quantum Wells

Semiconductor heterostructures of multiple quantum wells (MQWs) have major applications in optoelectronics. However, for halide perovskites—the leading class of emerging semiconductors—building a variety of bandgap alignments (i.e., band-types) in MQWs is not yet realized owing to the limitations of the current set of used barrier materials. Here, artificial perovskite-based MQWs using 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole), tris-(8-hydroxyquinoline)aluminum, and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline as quantum barrier materials are introduced. The structures of three different five-stacked perovskite-based MQWs each exhibiting a different band offset with CsPbBr3 in the conduction and valence bands, resulting in a variety of MQW band alignments, i.e., type-I or type-II structures, are shown. Transient absorption spectroscopy reveals the disparity in charge carrier dynamics between type-I and type-II MQWs. Photodiodes of each type of perovskite artificial MQWs show entirely different carrier behaviors and photoresponse characteristics. Compared with bulk perovskite devices, type-II MQW photodiodes demonstrate a more than tenfold increase in the rectification ratio. The findings open new opportunities for producing halide-perovskite-based quantum devices by bandgap engineering using simple quantum barrier considerations.
• #### [Cu 15 (PPh 3 ) 6 (PET) 13 ] 2+ : a Copper Nanocluster with Crystallization Enhanced Photoluminescence

(Small, Wiley, 2021-03-19) [Article]
Due to their atomically precise structure, photoluminescent copper nanoclusters (Cu NCs) have emerged as promising materials in both fundamental studies and technological applications, such as bio-imaging, cell labeling, phototherapy, and photo-activated catalysis. In this work, a facile strategy is reported for the synthesis of a novel Cu NCs coprotected by thiolate and phosphine ligands, formulated as [Cu<sub>15</sub> (PPh<sub>3</sub> )<sub>6</sub> (PET)<sub>13</sub> ]<sup>2+</sup> , which exhibits bright emission in the near-infrared (NIR) region (≈720 nm) and crystallization-induced emission enhancement (CIEE) phenomenon. Single crystal X-ray crystallography shows that the NC possesses an extraordinary distorted trigonal antiprismatic Cu<sub>6</sub> core and a, unique among metal clusters, "tri-blade fan"-like structure. An in-depth structural investigation of the ligand shell combined with density functional theory calculations reveal that the extended CH···π and π-π intermolecular ligand interactions significantly restrict the intramolecular rotations and vibrations and, thus, are a major reason for the CIEE phenomena. This study provides a strategy for the controllable synthesis of structurally defined Cu NCs with NIR luminescence, which enables essential insights into the origins of their optical properties.
• #### Adjusting the energy of interfacial states in organic photovoltaics for maximum efficiency

(Nature Communications, Springer Science and Business Media LLC, 2021-03-19) [Article]
AbstractA critical bottleneck for improving the performance of organic solar cells (OSC) is minimising non-radiative losses in the interfacial charge-transfer (CT) state via the formation of hybrid energetic states. This requires small energetic offsets often detrimental for high external quantum efficiency (EQE). Here, we obtain OSC with both non-radiative voltage losses (0.24 V) and photocurrent losses (EQE > 80%) simultaneously minimised. The interfacial CT states separate into free carriers with ≈40-ps time constant. We combine device and spectroscopic data to model the thermodynamics of charge separation and extraction, revealing that the relatively high performance of the devices arises from an optimal adjustment of the CT state energy, which determines how the available overall driving force is efficiently used to maximize both exciton splitting and charge separation. The model proposed is universal for donor:acceptor (D:A) with low driving forces and predicts which D:A will benefit from a morphology optimization for highly efficient OSC.
• #### Mixed Conduction in an N-Type Organic Semiconductor in the Absence of Hydrophilic Side-Chains

(Advanced Functional Materials, Wiley, 2021-03-18) [Article]
Organic electrochemical transistors (OECTs) are the building blocks of biosensors, neuromorphic devices, and complementary circuits. One rule in the materials design for OECTs is the inclusion of a hydrophilic component in the chemical structure to enable ion transport in the film. Here, it is shown that the ladder-type, side-chain free polymer poly(benzimidazobenzophenanthroline) (BBL) performs significantly better in OECTs than the donor–acceptor type copolymer bearing hydrophilic ethylene glycol side chains (P-90). A combination of electrochemical techniques reveals that BBL exhibits a more efficient ion-to-electron coupling and higher OECT mobility than P-90. In situ atomic force microscopy scans evidence that BBL, which swells negligibly in electrolytes, undergoes a drastic and permanent change in morphology upon electrochemical doping. In contrast, P-90 substantially swells when immersed in electrolytes and shows moderate morphology changes induced by dopant ions. Ex situ grazing incidence wide-angle X-ray scattering suggests that the particular packing of BBL crystallites is minimally affected after doping, in contrast to P-90. BBL's ability to show exceptional mixed transport is due to the crystallites’ connectivity, which resists water uptake. This side chain-free route for the design of mixed conductors could bring the n-type OECT performance closer to the bar set by their p-type counterparts.
• #### Impact of Photoluminescence Reabsorption in Metal-Halide Perovskite Solar Cells

(Solar RRL, Wiley, 2021-03-17) [Article]
The precise quantification of the impact of photoluminescence reabsorption (PLr) in metal-halide perovskite solar cells (PSCs) has remained challenging. Here, the PLr effect is examined by combined time-resolved photoluminescence (TRPL) spectroscopy and time-resolved terahertz spectroscopy (TRTS) and a model is proposed which relates both the PLr and non-radiative recombination rate (k$_{nr}$) to the quasi-Fermi-level-splitting (QFLS). PLr is shown to be beneficial for the QFLS when k$_{nr}$ is below a critical value of ∽7×10$^5$ s$^{−1}$; at high k$_{nr}$ PLr is detrimental to the QFLS. By incorporating PLr into a two-diode model that allows extraction of the effective k$_{nr}$, the series resistance (r$_s$), and the shunt resistance (r$_{sh}$) in PSCs, we find that neglecting PLr overestimates the effective k$_{nr}$, while it does not affect the value of r$_s$ and r$_{sh}$. Our findings provide insight into the impact of the PLr effect on metal-halide PSCs.
• #### Successes and Challenges of Core/Shell Lead Halide Perovskite Nanocrystals

(ACS Energy Letters, American Chemical Society (ACS), 2021-03-17) [Article]
Newly emerging perovskite nanocrystals (NCs) have shown a huge potential to be utilized in a gamut of optoelectronic devices due to their superior photoluminescence quantum yield (PLQY), tunable emission wavelength, and facile synthesis protocols at low cost. Despite the enormous progress made in synthetic protocol development, their poor stability against environmental stressors remains a major shortcoming that significantly restricts their practical applications and future commercialization. Of particular interest, core/shell NC engineering has fueled significant progress not only to improve the luminescent properties, reduce exciton recombination, suppress non-radiative recombination, and enhance the charge carrier transport but also, perhaps more importantly, to improve the semiconductor materials’ stability under harsh environmental conditions. Accordingly, this architecture represents a promising avenue to alleviate the stability issue and, therefore, could push the devices’ operational stability and performance forward. In this Focus Review, we explore the successes and challenges of recently reported perovskite core/shell heterostructures and summarize the synthesis methods, the photophysics after shelling, the theoretical approaches, and the applications. Finally, we conclude with a discussion of new opportunities and suggestions to push this research area a step forward.
• #### [Ag9(1,2-BDT)6]3–: How Square-Pyramidal Building Blocks Self-Assemble into the Smallest Silver Nanocluster

(Inorganic Chemistry, American Chemical Society (ACS), 2021-03-17) [Article]
The emerging promise of few-atom metal catalysts has driven the need for developing metal nanoclusters (NCs) with ultrasmall core size. However, the preparation of metal NCs with single-digit metallic atoms and atomic precision is a major challenge for materials chemists, particularly for Ag, where the structure of such NCs remains unknown. In this study, we developed a shape-controlled synthesis strategy based on an isomeric dithiol ligand to yield the smallest crystallized Ag NC to date: [Ag<sub>9</sub>(1,2-BDT)<sub>6</sub>]<sup>3-</sup> (1,2-BDT = 1,2-benzenedithiolate). The NC's crystal structure reveals the self-assembly of two Ag square pyramids through preferential pyramidal vertex sharing of a single metallic Ag atom, while all other Ag atoms are incorporated in a motif with thiolate ligands, resulting in an elongated body-centered Ag<sub>9</sub> skeleton. Steric hindrance and arrangement of the dithiolated ligands on the surface favor the formation of an anisotropic shape. Time-dependent density functional theory based calculations reproduce the experimental optical absorption features and identify the molecular orbitals responsible for the electronic transitions. Our findings will open new avenues for the design of novel single-digit metal NCs with directional self-assembled building blocks.
• #### Temperature-Modulated Doping at Polymer Semiconductor Interfaces

(ACS Applied Electronic Materials, American Chemical Society (ACS), 2021-03-12) [Article]
Understanding doping in polymer semiconductors has important implications for the development of organic electronic devices. This study reports a detailed investigation of the doping of the poly(3-hexylthiophene) (P3HT)/Nafion bilayer interfaces commonly used in organic biosensors. A combination of UV–visible spectroscopy, dynamic secondary ion mass spectrometry (d-SIMS), dynamic mechanical thermal analysis, and electrical device characterization reveals that the doping of P3HT increases with annealing temperature, and this increase is associated with thermally activated interdiffusion of the P3HT and Nafion. First-principles modeling of d-SIMS depth profiling data demonstrates that the diffusivity coefficient is a strong function of the molar concentration, resulting in a discrete intermixed region at the P3HT/Nafion interface that grows with increasing annealing temperature. Correlating the electrical conductance measurements with the diffusion model provides a detailed model for the temperature-modulated doping that occurs in P3HT/Nafion bilayers. Point-of-care testing has created a market for low-cost sensor technology, with printed organic electronic sensors well positioned to meet this demand, and this article constitutes a detailed study of the doping mechanism underlying such future platforms for the development of sensing technologies based on organic semiconductors.
• #### Free-standing nanopaper electrode for all-printed super-flexible perovskite solar cells

(Research Square, 2021-03-10) [Preprint]
The development of all-printed, flexible solar cells of high efficiency and ultra-low weight will offer advancements for new market entrants. Herein, we report the design and fabrication of all-printed in ambient air, super-flexible perovskite solar cells with approaching 20% power conversion efficiency and extremely low weight of 5.1 g m-2 leading to an unprecedented power-to-weight ratio of 38,470 W Kg-1. This performance advance was achieved through the design of a highly transparent and conducting nanopaper used as a free-standing bottom electrode (FSBE). The FSBE consisted of cellulose nanocrystals grafted with semi-metallic super-reduced polyoxometalate clusters that enabled high conductivities up to 18 S cm-1 combined with transparency >96%. It also acted as a conformal barrier preventing performance loss upon heating at 95 oC under continuous illumination in inert environment; and strong resistance to decomposition when immersed in a mild citric acid water solution for 100 days, which we further exploit to demonstrate full device recyclability. The inherent flexibility of cellulose nanocrystals enabled remarkable flexibility of these cells under 2,000 repeated bending and folding cycles and mechanical strength upon extensive strain up to 20%. Notably, the nanopaper remained unaffected for strains up to 60%. These findings open the door for efficient and lightweight solar cells with a low environmental impact.
• #### Implication of polymeric template agent on the formation process of hybrid halide perovskite film

(Nanotechnology, IOP Publishing, 2021-03-10) [Article]
The use of polymeric additives supporting the growth of hybrid halide perovskites has proven to be a successful approach aiming at high quality active layers targeting optoelectronic exploitation. A detailed description of the complex process involving the self-assembly of the precursors into the perovskite crystallites in presence of the polymer is, however, still missing. Here we take starch:CH3NH3PbI3 (MAPbI3) as example of highly performing composite, both in solar cells and light emitting diodes, and study the film formation process through differential scanning calorimetry and in situ time-resolved grazing incidence wide-angle X-ray scattering, performed during spin coating. These measurements reveal that starch beneficially influences the nucleation and growth of the perovskite precursor phase, leading to improved structural properties of the resulting film which turns into higher stability towards environmental conditions.
• #### Twisted Bodipy Derivative: Intersystem Crossing, Electron Spin Polarization and Application As a Novel Photodynamic Therapy Reagent

(Physical Chemistry Chemical Physics, Royal Society of Chemistry (RSC), 2021-03-10) [Article]
The photophysical properties of a heavy atom-free Bodipy derivative with twisted π-conjugation framework were studied. Efficient intersystem crossing (ISC. Quantum yield: 56%) and exceptionally long-lived triplet state wereobserved (4.5 ms. In solid polymer film matrix; the intrinsic triplet state lifetime in fluid solution is 197.5 μs). Time-resolved electron paramagnetic resonance (TREPR) spectroscopy and DFT computations confirmed the delocalization of triplet state on whole twisted π-conjugation framework, and the zero-field-splitting (ZFS) D parameter is D = −69.5 mT, which is smaller than the 2,6-diiodoBodipy (D = − 104.6 mT). The electron spin polarization (ESP) phase pattern of the triplet state TREPR spectrum of the twisted Bodipy is (a, a, e, a, e, e), which is different from that of 2,6-diiodoBodipy (e, e, e, a, a, a), indicates the electron spin selectivity of the ISC of the twisted structure is different from that of the spin orbital coupling effect. The computed spin-orbit coupling matrix elements (0.154 - 1.964 cm−1), together with the matched energy of the S1/Tn states, the ISC was proposed to be via S1-->T2/T3. The computational results are in agreement with the TREPR results on the electron spin selectivity (the overpopulation of the TY sublevel of the T1 state). The advantage of long-lived triplet state of twisted Bodipy was demonstrated by the efficient singlet oxygen (1O2) photosensitizing (quantum yield = 50.0%) even under severe hypoxia atmosphere (pO2 = 0.2%, v/v). High light toxicity (EC50 = 1.0 μM) and low dark toxicity (EC50 = 78.5 μM) was observed for the twisted Bodipy, cellular studies demonstrate its potential as a novel potent heavy atom-free photodynamic therapy (PDT) reagent.
• #### Gentle Materials Need Gentle Fabrication: Encapsulation of Perovskites by Gas-Phase Alumina Deposition.

(The journal of physical chemistry letters, American Chemical Society (ACS), 2021-03-03) [Article]
Metal halide perovskites have attracted tremendous attention as promising materials for future-generation optoelectronic devices. Despite their outstanding optical and transport properties, the lack of environmental and operational stability remains a major practical challenge. One of the promising stabilization avenues is metal oxide encapsulation via atomic layer deposition (ALD); however, the unavoidable reaction of metal precursors with the perovskite surface in conventional ALD leads to degradation and restructuring of the perovskites’ surfaces. This Perspective highlights the development of a modified gas-phase ALD technique for alumina encapsulation that not only prevents perovskites’ degradation but also significantly improves their optical properties and air stability. The correlation between precise atomic interactions at the perovskite–metal oxide interface with the dramatically enhanced optical properties is supported by density functional theory calculations, which also underlines the widespread applicability of this gentle technique for a variety of perovskite nanostructures unbarring potential opportunities offered by combination of these approaches.
• #### High-Gain Chemically Gated Organic Electrochemical Transistor

(Advanced Functional Materials, Wiley, 2021-03-03) [Article]
Organic electrochemical transistors (OECTs) have exhibited promising performance as transducers and amplifiers of low potentials due to their exceptional transconductance, enabled by the volumetric charging of organic mixed ionic/electronic conductors (OMIECs) employed as the channel material. OECT performance in aqueous electrolytes as well as the OMIECs’ redox activity has spurred a myriad of studies employing OECTs as chemical transducers. However, the OECT's large (potentiometrically derived) transconductance is not fully leveraged in common approaches that directly conduct chemical reactions amperometrically within the OECT electrolyte with direct charge transfer between the analyte and the OMIEC, which results in sub-unity transduction of gate to drain current. Hence, amperometric OECTs do not truly display current gains in the traditional sense, falling short of the expected transistor performance. This study demonstrates an alternative device architecture that separates chemical transduction and amplification processes on two different electrochemical cells. This approach fully utilizes the OECT's large transconductance to achieve current gains of 103 and current modulations of four orders of magnitude. This transduction mechanism represents a general approach enabling high-gain chemical OECT transducers.