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

### Recent Submissions

• #### Efficient bifacial monolithic perovskite/silicon tandem solar cells via bandgap engineering

(Nature Energy, Springer Science and Business Media LLC, 2021-01-11) [Article]
Bifacial monolithic perovskite/silicon tandem solar cells exploit albedo—the diffuse reflected light from the environment—to increase their performance above that of monofacial perovskite/silicon tandems. Here we report bifacial tandems with certified power conversion efficiencies >25% under monofacial AM1.5G 1 sun illumination that reach power-generation densities as high as ~26 mW cm–2 under outdoor testing. We investigated the perovskite bandgap required to attain optimized current matching under a variety of realistic illumination and albedo conditions. We then compared the properties of these bifacial tandems exposed to different albedos and provide energy yield calculations for two locations with different environmental conditions. Finally, we present a comparison of outdoor test fields of monofacial and bifacial perovskite/silicon tandems to demonstrate the added value of tandem bifaciality for locations with albedos of practical relevance.
• #### Dark Self-Healing Mediated Negative Photoconductivity of Lead-Free Cs3Bi2Cl9 Perovskite Single Crystal

(arXiv, 2021-01-07) [Preprint]
Halide perovskites are recently emerged as one of the frontline optoelectronic materials for device applications and have been extensively studied in past few years. Among these while, lead-based materials were most widely explored, investigation of optical properties of lead-free perovskites is limited. Being optically active, these materials were expected to show light-induced enhanced photoconductivity and the same was reported for lead halide perovskite single crystals. However, on contrary, herein, light-induced degradation of bismuth halide perovskite Cs3Bi2Cl9 single crystals is reported which was evidenced by negative photoconductivity with slow recovery. The femtosecond transient reflectance (fs-TR) spectroscopy studies further revealed these electronic transport properties were due to the formation of light-activated metastable trap states within the perovskite crystal. The figure of merits of Cs3Bi2Cl9 single-crystal detectors such as responsivity (17 mA/W), detectivity (6.23 X 10power 11 Jones) and the ratio of current in dark to light (~7160) was calculated and it is found that they are comparable or higher to reported perovskite single crystals based positive photodetectors. This observation for lead-free perovskite single crystals which were optically active but showed retroactive photocurrent on irradiation remained unique for such materials.
• #### Ion-exchange doped polymers at the degenerate limit: what limits conductivity at 100% doping efficiency?

(arXiv, 2021-01-05) [Preprint]
Doping of semiconducting polymers has seen a surge in research interest driven by emerging applications in sensing, bioelectronics and thermoelectrics. A recent breakthrough was a doping technique based on ion-exchange, which separates the redox and charge compensation steps of the doping process. The improved microstructural control this process allows enables us for the first time to systematically address a longstanding but still poorly understood question: what limits the electrical conductivity at high doping levels? Is it the formation of charge carrier traps in the Coulomb potentials of the counterions, or is it the structural disorder in the polymer lattice? Here, we apply ion-exchange doping to several classes of high mobility conjugated polymers and identify experimental conditions that achieve near 100% doping efficiency under degenerate conditions with nearly 1 charge per monomer. We demonstrate very high conductivities up to 1200 S/cm in semicrystalline polymer systems, and show that in this regime conductivity is poorly correlated with ionic size, but strongly correlated with paracrystalline disorder. This observation, backed by a detailed electronic structure model that incorporates ion-hole and hole-hole interactions and a carefully parameterized model of disorder, indicates that trapping by dopant ions is negligible, and that maximizing crystalline order is critical to improving conductivity.
• #### Ruddlesden–Popper-Phase Hybrid Halide Perovskite/Small-Molecule Organic Blend Memory Transistors

Controlling the morphology of metal halide perovskite layers during processing is critical for the manufacturing of optoelectronics. Here, a strategy to control the microstructure of solution-processed layered Ruddlesden-Popper-phase perovskite films based on phenethylammonium lead bromide ((PEA)<sub>2</sub> PbBr<sub>4</sub> ) is reported. The method relies on the addition of the organic semiconductor 2,7-dioctyl[1]benzothieno[3,2-b]benzothiophene (C<sub>8</sub> -BTBT) into the perovskite formulation, where it facilitates the formation of large, near-single-crystalline-quality platelet-like (PEA)<sub>2</sub> PbBr<sub>4</sub> domains overlaid by a ≈5-nm-thin C<sub>8</sub> -BTBT layer. Transistors with (PEA)<sub>2</sub> PbBr<sub>4</sub> /C<sub>8</sub> -BTBT channels exhibit an unexpectedly large hysteresis window between forward and return bias sweeps. Material and device analysis combined with theoretical calculations suggest that the C<sub>8</sub> -BTBT-rich phase acts as the hole-transporting channel, while the quantum wells in (PEA)<sub>2</sub> PbBr<sub>4</sub> act as the charge storage element where carriers from the channel are injected, stored, or extracted via tunneling. When tested as a non-volatile memory, the devices exhibit a record memory window (>180 V), a high erase/write channel current ratio (10<sup>4</sup> ), good data retention, and high endurance (>10<sup>4</sup> cycles). The results here highlight a new memory device concept for application in large-area electronics, while the growth technique can potentially be exploited for the development of other optoelectronic devices including solar cells, photodetectors, and light-emitting diodes.
• #### Molecular Doping of a Naphthalene Diimide–Bithiophene Copolymer and SWCNTs for n-Type Thermoelectric Composites

(ACS Applied Materials & Interfaces, American Chemical Society (ACS), 2020-12-29) [Article]
Molecular doping is a powerful tool to tune the thermoelectric (TE) properties of solution-processed semiconductors. In this work, we prepared a binary composite and effectively doped both of its constituents, that is, naphthalene diimide–bithiophene copolymers (PNDI2OD-T2) and single-walled carbon nanotubes (SWCNTs), by a 1H-benzimidazole derivative (N-DMBI). The doped composites show an n-type character and an in-plane TE figure of merit (ZT), exceeding the values obtained with the doped polymers. The use of SWCNTs consistently results in a higher σ with a maximum value above 102 S/cm, resulting in the highest power factor of 18.1 μW/mK2 for an SWCNT loading of 45.5 wt %. Furthermore, an SWCNT content up to 9 wt % does not compromise the low thermal conductivity of the polymer matrices, leading to a ZT value of 0.0045. The n-type composites show good solution processability and relatively stable Seebeck coefficients upon air exposure for 8 months.
• #### Light Propagation and Radiative Exciton Transport in Two-Dimensional Layered Perovskite Microwires

(ACS Photonics, American Chemical Society (ACS), 2020-12-29) [Article]
Layered quasi-two-dimensional perovskites are promising candidates for optoelectronic applications exhibiting excitons with high emission quantum yields, high stability, and ease of bandgap tunability. Here, we demonstrate a long-range (∼100 μm) exciton transfer in a layered perovskite structure (en)4Pb2Br9·3Br, with the ethylene diammonium (en) as a spacer that takes place via the reabsorption of emitted photons. Using the two-objectives setup, we directly map the spatiotemporal dynamics of photoluminescence (PL) from perovskite microwires that reveal a clear spectroscopic signature of photon recycling: the appearance of PL emission rise times and the corresponding elongation of the PL decay as a function of separation distance between the excitation and emission locations. We further show that a kinetic model based on the photon-mediated mechanism of the lateral exciton propagation indeed successfully describes all the salient features of the experimental data and gives an independent assessment of the radiative efficiency of the exciton recombination. Our demonstration points out the possibility of judiciously exploiting light management strategies for future high-performance optoelectronic devices with layered perovskite structures.
• #### N-type Rigid Semiconducting Polymers Bearing Oligo(Ethylene Glycol) Side Chains for High Performance Organic Electrochemical Transistors

(Angewandte Chemie, Wiley, 2020-12-24) [Article]
N-type conjugated polymers as the semiconducting component of organic electrochemical transistors (OECTs) are still undeveloped with respect to the p-type counterparts. Herein, we first report two rigid n-type conjugated polymers bearing oligo(ethylene glycol) (OEG) side chains, PgNaN and PgNgN, which demonstrated an essentially torsion-free π-conjugated backbone. The planarity and electron-deficient rigid structures enable the resulting polymers to achieve high electron mobility in an OECT device of up to 10 -3 cm 2 V -1 s -1 range, with a deep-lying lowest unoccupied molecular orbital (LUMO) energy level lower than -4.0 eV. Prominently, the polymers exhibited a high device performance with a maximum dimensionally normalized transconductance of 0.212 S cm -1 and the product of charge carrier mobility µ and volumetric capacitance C* of 0.662 ± 0.113 F cm -1 V -1 s -1 , which are among the highest in n-type conjugated polymers reported to date. Moreover, the polymers are synthesized via a metal-free aldol condensation polymerization, which is beneficial to their application in bioelectronics. Our work proves a new way for designing glycolated n-type conjugated polymers with low-lying LUMO and conformation-locked backbones for high performance OECTs.
• #### Acene Ring Size Optimization in Fused Lactam Polymers Enabling High n-Type Organic Thermoelectric Performance

(Journal of the American Chemical Society, American Chemical Society (ACS), 2020-12-22) [Article]
Three n-type fused lactam semiconducting polymers were synthesized for thermoelectric and transistor applications via a cheap, highly atom-efficient, and nontoxic transition-metal free aldol polycondensation. Energy level analysis of the three polymers demonstrated that reducing the central acene core size from two anthracenes (A-A), to mixed naphthalene–anthracene (A-N), and two naphthalene cores (N-N) resulted in progressively larger electron affinities, thereby suggesting an increasingly more favorable and efficient solution doping process when employing 4-(2,3-dihydro-1,3-dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzenamine (N-DMBI) as the dopant. Meanwhile, organic field effect transistor (OFET) mobility data showed the N-N and A-N polymers to feature the highest charge carrier mobilities, further highlighting the benefits of aryl core contraction to the electronic performance of the materials. Ultimately, the combination of these two factors resulted in N-N, A-N, and A-A to display power factors (PFs) of 3.2 μW m–1 K–2, 1.6 μW m–1 K–2, and 0.3 μW m–1 K–2, respectively, when doped with N-DMBI, whereby the PFs recorded for N-N and A-N are among the highest reported in the literature for n-type polymers. Importantly, the results reported in this study highlight that modulating the size of the central acene ring is a highly effective molecular design strategy to optimize the thermoelectric performance of conjugated polymers, thus also providing new insights into the molecular design guidelines for the next generation of high-performance n-type materials for thermoelectric applications.
• #### Fabrication of polymer infiltrated zirconia ceramic composite materials

(Molecular Crystals and Liquid Crystals, Informa UK Limited, 2020-12-18) [Article]
The aim of this study was to evaluate the microstructure of polymer infiltrated zirconia ceramics. Zpex smile from Tosoh company was used as a starting material. Sintered zirconia ceramics blocks were made through mold pressing. Then, the blocks were treated with the isostatic cool pressing method. To improve the mechanical properties, the sintering program at 1250 °C was used for all samples. Polymer-infiltrated zirconia ceramics were prepared by liquid monomer to infiltrate the porous zirconia ceramic networks before being polymerized. Subsequently, scanning electron microscopy (SEM) was used to examine the microstructure of the ceramic surface. Sintered zirconia ceramics blocks were successfully infiltrated with monomer. Mechanical properties can be improved due to a good combination of ceramic and monomer.
• #### Controlling the morphology of solution-processed CuIn(SSe)2 absorber layers by film thickness and annealing temperature

(Molecular Crystals and Liquid Crystals, Informa UK Limited, 2020-12-18) [Article]
We demonstrated the solution-processed CuIn(S,Se)2 solar cells using a diamine/dithiol cosolvent. Compared to the traditional CIGS materials made with the Selenization process, we fabricated CISSe films without the Selenization process. The crystallinity, optical properties, and morphology of the CIGSSe films were changed by annealing temperature and film thickness. The band gap energy were found approximately 1.5 eV with the different film thickness.
• #### [Cu23(PhSe)16(Ph3P)8(H)6]·BF4: Atomic-Level Insights into Cuboidal Polyhydrido Copper Nanoclusters and Their Quasi-simple Cubic Self-Assembly

(ACS Materials Letters, American Chemical Society (ACS), 2020-12-17) [Article]
Polyhydrido copper nanoclusters are an emerging class of nanomaterials. Unfortunately, insights into the structural evolution and structure–property relationship of such copper nanoclusters are scant, because of the difficulty of synthesizing and crystallizing nanoclusters with high nuclearity and new morphologies. Here, we report an anisotropic cuboidal polyhydrido copper nanocluster, [Cu23(PhSe)16(Ph3P)8(H)6]·BF4, with a distorted cuboctahedral Cu13 core stabilized by two square protecting motifs and six hydrides. The cuboidal nanoclusters self-assemble into a quasi-simple cubic packing pattern with perfect face-to-face contact of neighboring nanoclusters and interdigitation of intercluster surface ligands. Atomic-level observations reveal the crucial role that subtle synergies between nanocluster geometry and intercluster noncovalent interactions play in guiding nanocluster self-assembly. In addition, a comparison with previously reported analogous metal nanoclusters points to bulky monodentate phosphine ligands as a potent inducing agent for the formation of rectangular hexahedral nanoclusters. These findings have significant implications for the controllable synthesis of polyhedral nanomaterials and their superstructures.
• #### One-step Six-fold Cyanation of Benzothiadiazole Acceptor Units for Air-Stable High-Performance n-Type Organic Field-Effect Transistors

(Angewandte Chemie, Wiley, 2020-12-14) [Article]
We report a new high electron affinity acceptor end group for organic semiconductors, 2,1,3-benzothiadiazole-4,5,6-tricarbonitrile (TCNBT). An n-type organic semiconductor with an indacenodithiophene (IDT) core and TCNBT end groups was synthesized by a six-fold nucleophilic substitution with cyanides on a fluorinated precursor, itself prepared by a direct arylation approach. This one-step chemical modification was found to significantly impact the molecular properties: the fluorinated precursor, TFBT IDT, a poor ambipolar semiconductor, was converted into TCNBT IDT, a good n-type semiconductor. The highly electron-deficient end group TCNBT dramatically decreased the energy of the highest occupied and lowest unoccupied molecular orbitals (HOMO/LUMO) compared to the fluorinated analogue and improved the molecular orientation when utilized in n-type organic field-effect transistors (OFETs). Solution-processed OFETs based on TCNBT IDT exhibited a charge carrier mobility of up to µ e ≈ 0.15 cm 2 V -1 s -1 with excellent ambient stability for 100 hours, highlighting the benefits of the cyanated end group and the synthetic approach.
• #### Amphipathic Side Chain of a Conjugated Polymer Optimizes Dopant Location toward Efficient N-Type Organic Thermoelectrics.

(Advanced materials (Deerfield Beach, Fla.), Wiley, 2020-12-11) [Article]
There is no molecular strategy for selectively increasing the Seebeck coefficient without reducing the electrical conductivity for organic thermoelectrics. Here, it is reported that the use of amphipathic side chains in an n-type donor-acceptor copolymer can selectively increase the Seebeck coefficient and thus increase the power factor by a factor of ≈5. The amphipathic side chain contains an alkyl chain segment as a spacer between the polymer backbone and an ethylene glycol type chain segment. The use of this alkyl spacer does not only reduce the energetic disorder in the conjugated polymer film but can also properly control the dopant sites away from the backbone, which minimizes the adverse influence of counterions. As confirmed by kinetic Monte Carlo simulations with the host-dopant distance as the only variable, a reduced Coulombic interaction resulting from a larger host-dopant distance contributes to a higher Seebeck coefficient for a given electrical conductivity. Finally, an optimized power factor of 18 µW m$^{-1}$ K$^{-2}$ is achieved in the doped polymer film. This work provides a facile molecular strategy for selectively improving the Seebeck coefficient and opens up a new route for optimizing the dopant location toward realizing better n-type polymeric thermoelectrics.
• #### Interpreting time-resolved photoluminescence of perovskite materials.

(Physical chemistry chemical physics : PCCP, Royal Society of Chemistry (RSC), 2020-12-10) [Article]
Time-resolved photoluminescence (TRPL) spectroscopy is a powerful technique to investigate excited charge carrier recombinations in semiconductors and molecular systems. The analysis of the TRPL decays of many molecular systems (e.g. molecules and organic materials) is usually fairly straightfoward and can be fitted with an exponential function allowing extraction of the rate constants. Due to the non-excitonic nature of charge carriers in lead halide perovskite materials coupled with the presence of localised trap states in their band-gap, the TRPL of these materials is much more complicated to interpret. Here we discuss two models used in the literature to simulate charge carrier recombinations and TRPL in perovskites. These models consider the bimolecular nature of direct electron-hole recombination but differ in their treatment of trap-mediated recombination with one model describing trapping as a monomolecular process whereas the other as a bimolecular process between free carriers and the available trap states. In comparison, the classical analysis of perovskite TRPL decay curves (using a sum of exponentials) can lead to misinterpretation. Here we offer some recommendations for meaningful measurements of lead halide perovskite thin-films. The fluence dependence as well as charge carrier accumulation due to incomplete depopulation of all photoexcited carriers between consecutive excitation pulses are discussed for both models.
• #### Impact of Cation Multiplicity on Halide Perovskite Defect Densities and Solar Cell Voltages

(The Journal of Physical Chemistry C, American Chemical Society (ACS), 2020-12-08) [Article]
Metal-halide perovskites feature very low deep-defect densities, thereby enabling high operating voltages at the solar cell level. Here, by precise extraction of their absorption spectra, we find that the low deep-defect density is unaffected when cations such as Cs+ and Rb+ are added during the perovskite synthesis. By comparing single crystals and polycrystalline thin films of methylammonium lead iodide/bromide, we find these defects to be predominantly localized at surfaces and grain boundaries. Furthermore, generally, for the most important photovoltaic materials, we demonstrate a strong correlation between their Urbach energy and open-circuit voltage deficiency at the solar cell level. Through external quantum yield photoluminescence efficiency measurements, we explain these results as a consequence of nonradiative open-circuit voltage losses in the solar cell. Finally, we define practical power conversion efficiency limits of solar cells by taking into account the Urbach energy.
• #### Unraveling the New Role of an Ethylene Carbonate Solvation Shell in Rechargeable Metal Ion Batteries

(ACS Energy Letters, American Chemical Society (ACS), 2020-12-07) [Article]
Electrolytes play a critical role in controlling metal-ion battery performance. However, the molecular behavior of electrolyte components and their effects on electrodes are not fully understood. Herein, we present a new insight on the role of the most commonly used ethylene carbonate (EC) cosolvent both with the bulk and at the electrolyte-electrode interface. We have discovered a new phenomenon that contributes to stabilizing the electrolyte, besides the well-known roles of dissociating metal salt and forming a solid electrolyte interphase (SEI). As a paradigm, we confirm that EC can form an Li+–EC pair in a priority compared to other kinds of solvents (e.g., ethyl methyl carbonate) and then alter the Li+–solvent interactions in the electrolyte. The Li+–EC pair can dominate the desolvation structure at the electrode interface, therefore suppressing Li+–solvent decomposition due to the higher stability of Li+–EC. Our viewpoint is confirmed in different electrolytes for lithium, sodium, and potassium ion batteries, where the SEI is shown to be limited for stabilizing the electrode in the case of the less stable Li+–solvent pair. Our discovery provides a general explanation for the effect of EC and provides new guidelines for designing more reliable electrolytes for metal (ion) batteries.
• #### Efficient Visible-Light Driven Photothermal Conversion of CO 2 to Methane by Nickel Nanoparticles Supported on Barium Titanate

(Advanced Functional Materials, Wiley, 2020-12-04) [Article]
Solar-driven methanation represents a potentially cost-efficient and environmentally friendly route for the direct hydrogenation of CO2. Recently, photothermal catalysis, which involves the combination of both photochemical and thermochemical pathways, has emerged as a promising strategy for the production of solar fuels. For a photothermal catalyst to efficiently convert CO2 under illumination, in the absence of external heating, effective light harvesting, an excellent photothermal conversion and efficient active sites are required. Here, a new composite catalyst consisting of Ni nanoparticles supported on barium titanate that, under optimal reaction conditions, is able to hydrogenate CO2 to CH4 at nearly 100% selectivity with production rates as high as 103.7 mmol g–1 h–1 under both UV–visible and visible irradiation (production rate: 40.3 mmol g−1 h–1) is reported. Mechanistic studies suggest that reaction mostly proceeds through a nonthermal hot-electron-driven pathway, with a smaller thermal contribution.
• #### Device Performance of Emerging Photovoltaic Materials (Version 1)

(Advanced Energy Materials, Wiley, 2020-12-04) [Article]
Emerging photovoltaics (PVs) focus on a variety of applications complementing large scale electricity generation. Organic, dye-sensitized, and some perovskite solar cells are considered in building integration, greenhouses, wearable, and indoor applications, thereby motivating research on flexible, transparent, semitransparent, and multi-junction PVs. Nevertheless, it can be very time consuming to find or develop an up-to-date overview of the state-of-the-art performance for these systems and applications. Two important resources for recording research cells efficiencies are the National Renewable Energy Laboratory chart and the efficiency tables compiled biannually by Martin Green and colleagues. Both publications provide an effective coverage over the established technologies, bridging research and industry. An alternative approach is proposed here summarizing the best reports in the diverse research subjects for emerging PVs. Best performance parameters are provided as a function of the photovoltaic bandgap energy for each technology and application, and are put into perspective using, e.g., the Shockley–Queisser limit. In all cases, the reported data correspond to published and/or properly described certified results, with enough details provided for prospective data reproduction. Additionally, the stability test energy yield is included as an analysis parameter among state-of-the-art emerging PVs.
• #### Effects of Fluorination on Fused Ring Electron Acceptor for Active Layer Morphology, Exciton Dissociation, and Charge Recombination in Organic Solar Cells.

(ACS applied materials & interfaces, American Chemical Society (ACS), 2020-12-03) [Article]
Fluorination is one of the effective approaches to alter the organic semiconductor properties that impact the performance of the organic solar cells (OSCs). Positive effects of fluorination are also revealed in the application of fused ring electron acceptors (FREAs). However, in comparison with the efforts allocated to the material designs and power conversion efficiency enhancement, understanding on the excitons and charge carriers' behaviors in high-performing OSCs containing FREAs is limited. Herein, the impact of fluorine substituents on the active layer morphology, and therefore exciton dissociation, charge separation, and charge carriers' recombination processes are examined by fabricating OSCs with PTO2 as the donor and two FREAs, O-IDTT-IC and its fluorinated analogue O-IDTT-4FIC, as the acceptors. With the presence of O-IDTT-4FIC in the devices, it is found that the excitons dissociate more efficiently, and the activation energy required to split the excitons to free charge carriers is much lower; the charge carriers live longer and suffer less extent of trap-assisted recombination; the trap density is 1 order of magnitude lower than that of the nonfluorinated counterpart. Overall, these findings provide information about the complex impacts of FREA fluorination on efficiently performed OSCs.
• #### Microfluidics integrated n-type organic electrochemical transistor for metabolite sensing

(Sensors and Actuators, B: Chemical, Elsevier BV, 2020-12-01) [Article]
The organic electrochemical transistor (OECT) can translate biochemical binding events between a recognition unit and its analyte into an electrical signal. We present an OECT comprising an n-type (electron transporting) conjugated polymer-based channel and lateral gate electrode functionalized with the enzyme, glucose oxidase. The device is integrated with a microfluidic system for real-time glucose monitoring in a flow-through manner. The n-type polymer has direct electrical communication with glucose oxidase, allowing glucose detection while surpassing hydrogen peroxide production. The microfluidic-integrated OECT shows superior features compared to its microfluidic-free counterpart, including higher current and transconductance values as well as improved signal-to-noise (SNR) ratios, which enhances the sensor sensitivity and its detection limit. Thanks to the low noise endowed by the integrated microfluidics, the gate current changes upon metabolite recognition could be resolved, revealing that while the relative changes in gate and drain currents are similar, the drain current output has a higher SNR. This is the first demonstration of the integration of a microfluidic system with an n-type accumulation mode OECT for real-time enzymatic metabolite detection. The microfluidic-integrated design provides new insights into the mechanisms leading to high sensor sensitivities, crucial for the development of portable and autonomous lab-on-a-chip technologies.