Exploring charge behavior at the terminal surface and charge separation interface of organo-lead perovskite using in situ surface photovoltage spectroscopy.

The differences in charge separation and transfer processes between the 'non-charge-separation' terminal surface and perovskite/FTO 'charge-separation' interface have been studied using comparative lock-in amplifier-based SPV signals. The SPV phase vector model delves deeper into the direction of charge separation and trapping at the perovskite surface/interface.

Charge transport dynamics of a C6H4NH2CuBr2I/TiO2 heterojunction in aqueous solution under reverse bias.

The photocurrent output of the C6H4NH2CuBr2I/TiO2 heterojunction photoelectrode in an aqueous solution is super stable even after 30 000 s. However, the photocurrent is extremely weak. Intensity-modulated photocurrent spectroscopy revealed that the electron transfer in the C6H4NH2CuBr2I/TiO2 photoelectrode without bias is not sufficiently fast to compete with the charge recombination process due to the short diffusion length (∼23 nm), resulting in a low photocurrent. The charge separation and charge transfer efficiency in the bulk of C6H4NH2CuBr2I could be significantly improved under a small reverse electric field (Er), resulting in an enhanced photocurrent.

Quasi-Homojunction Organic Nonfullerene Photovoltaics Featuring Fundamentals Distinct from Bulk Heterojunctions.

In contrast to classical bulk heterojunction (BHJ) in organic solar cells (OSCs), the quasi-homojunction (QHJ) with extremely low donor content (≤10 wt.%) is unusual and generally yields much lower device efficiency. Here, representative polymer donors and nonfullerene acceptors are selected to fabricate QHJ OSCs, and a complete picture for the operation mechanisms of high-efficiency QHJ devices is illustrated. PTB7-Th:Y6 QHJ devices at donor:acceptor (D:A) ratios of 1:8 or 1:20 can achieve 95% or 64% of the efficiency obtained from its BHJ counterpart at the optimal D:A ratio of 1:1.2, respectively, whereas QHJ devices with other donors or acceptors suffer from rapid roll-off of efficiency when the donors are diluted. Through device physics and photophysics analyses, it is observed that a large portion of free charges can be intrinsically generated in the neat Y6 domains rather than at the D/A interface. Y6 also serves as an ambipolar transport channel, so that hole transport as also mainly through Y6 phase. The key role of PTB7-Th is primarily to reduce charge recombination, likely assisted by enhancing quadrupolar fields within Y6 itself, rather than the previously thought principal roles of light absorption, exciton splitting, and hole transport.

Simultaneously Enhancing Exciton/Charge Transport in Organic Solar Cells by an Organoboron Additive.

Efficient exciton diffusion and charge transport play a vital role in advancing the power conversion efficiency (PCE) of organic solar cells (OSCs). Here, a facile strategy is presented to simultaneously enhance exciton/charge transport of the widely studied PM6:Y6-based OSCs by employing highly emissive trans-bis(dimesitylboron)stilbene (BBS) as a solid additive. BBS transforms the emissive sites from a more H-type aggregate into a more J-type aggregate, which benefits the resonance energy transfer for PM6 exciton diffusion and energy transfer from PM6 to Y6. Transient gated photoluminescence spectroscopy measurements indicate that addition of BBS improves the exciton diffusion coefficient of PM6 and the dissociation of PM6 excitons in the PM6:Y6:BBS film. Transient absorption spectroscopy measurements confirm faster charge generation in PM6:Y6:BBS. Moreover, BBS helps improve Y6 crystallization, and current-sensing atomic force microscopy characterization reveals an improved charge-carrier diffusion length in PM6:Y6:BBS. Owing to the enhanced exciton diffusion, exciton dissociation, charge generation, and charge transport, as well as reduced charge recombination and energy loss, a higher PCE of 17.6% with simultaneously improved open-circuit voltage, short-circuit current density, and fill factor is achieved for the PM6:Y6:BBS devices compared to the devices without BBS (16.2%).

Electrochemically Prepared Polyaniline as an Alternative to Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) for Inverted Perovskite Solar Cells.

The goal of this work is to substitute the conventional high-cost poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) in inverted perovskite solar cells (PSCs) with an efficient and conducting polyaniline (PANI) polymer. The reported use of PANI in PSCs involves a chemical synthesis method which is prone to contamination with impurities as it requires several materials for polymerization and adhesion improvement with substrates, contributing to low device efficiencies. This work mitigates this issue using an electrochemical method that is low cost, less time consuming, and capable of producing thin films of PANI with excellent adhesion to substrates. Results demonstrated that the power conversion efficiency of the electrochemically synthesized PANI-based PSC is 16.94% versus 15.11% for the PEDOT:PSS-based device. It was observed that the work function of PANI was lower compared to that of PEDOT:PSS which decreased V OC but enhanced hole extraction at the hole transport layer/perovskite interface, thus increasing J SC. Doping electrolyte solution with lithium bis(trifluoromethanesulfonyl)imide LiTFSI increased the work function of PANI, thus increasing V OC from 0.87 to 0.93 V. This method enables simple and scalable synthesis of PANI as a competitive hole transport material to replace rather expensive PEDOT:PSS, thus enabling an important step toward low-cost inverted perovskite photovoltaic devices.

Roles of Organic Ligands in Ambient Stability of Layered Halide Perovskites.

The combination of organic ligands and inorganic Pb-I frameworks in layered perovskites has bestowed upon them high structural tunability and stability, while their microscopic degradation mechanism remains unclear. Here, we found the key role of ligands in intrinsic structural stability and the consequent morphological evolution in layered perovskites during long-term ambient aging based on (GA)(MA)nPbnI3n+1 (GA = guanidinium, = 4) and (BDA)(MA)n-1PbnI3n+1 (BDA = 1,4-butanediammonium, < n > = 4) perovskites. The BDA-based perovskites have a low intrinsic stability due to high crystal formation energy (ΔH), which are prone to hydration during ambient aging. We overserved changed crystal orientation from perpendicular to parallel, a delayed charge populating time from <1 ps to >50 ps, an inhibited carrier transfer kinetics between quantum wells, an increase of 0.9 µs of charge carrier transport time and a decrease of 1.2 µs of charge carrier lifetime in the BDA-based film during ambient aging, which accounts for a large power-conversion efficiency (PCE) loss (14.2% vs 11.2%). By contrast, the GA ligand increases the intrinsic structural stability of perovskites, which not only yields an initial PCE as high as 20.0% but also helps retain excellent optoelectronic properties during aging. Therefore, only a slight PCE loss (20.0% vs 19.1%) was observed. Our work reveals the key role of organic-inorganic interaction affecting the intrinsic structural stability and optoelectronic properties, and provides a theoretical basis for the future design of stable and efficient optoelectronic devices.

Mitigating Interfacial Mismatch between Lithium Metal and Garnet-Type Solid Electrolyte by Depositing Metal Nitride Lithiophilic Interlayer.

Solid-state lithium batteries are generally considered as the next-generation battery technology that benefits from inherent nonflammable solid electrolytes and safe harnessing of high-capacity lithium metal. Among various solid-electrolyte candidates, cubic garnet-type Li7La3Zr2O12 ceramics hold superiority due to their high ionic conductivity (10-3 to 10-4 S cm-1) and good chemical stability against lithium metal. However, practical deployment of solid-state batteries based on such garnet-type materials has been constrained by poor interfacing between lithium and garnet that displays high impedance and uneven current distribution. Herein, we propose a facile and effective strategy to significantly reduce this interfacial mismatch by modifying the surface of such garnet-type solid electrolyte with a thin layer of silicon nitride (Si3N4). This interfacial layer ensures an intimate contact with lithium due to its lithiophilic nature and formation of an intermediate lithium-metal alloy. The interfacial resistance experiences an exponential drop from 1197 to 84.5 Ω cm2. Lithium symmetrical cells with Si3N4-modified garnet exhibited low overpotential and long-term stable plating/stripping cycles at room temperature compared to bare garnet. Furthermore, a hybrid solid-state battery with Si3N4-modified garnet sandwiched between lithium metal anode and LiFePO4 cathode was demonstrated to operate with high cycling efficiency, excellent rate capability, and good electrochemical stability. This work represents a significant advancement toward use of garnet solid electrolytes in lithium metal batteries for the next-generation energy storage devices.

Synergistic Approach toward Erbium-Passivated Triple-Anion Organic-Free Perovskite Solar Cells with Excellent Performance for Agrivoltaics Application.

All-inorganic perovskite solar cells (IPSCs) have gained massive attention due to their less instability against common degradation factors (light, heat, and moisture) than their organic-inorganic hybrid counterparts. Inorganic perovskites bear a general formula of CsPbX3 (X = Cl, I, Br). The mixed halide CsPbIBr2 perovskite possesses an intermediate band gap of 2.03 eV with enhanced stability, which is still available for photovoltaic applications and the research focus of this work. We present a synergistic approach of pre-heated solution dropping with inorganic additive inclusion to deposit the organic-free triple anion CsPbIBr2 PSC. Erbium (Er)-passivated triple-anion CsI(PbBr2)0.97(ErCl3)0.03 IPSCs with inorganic carrier selective layers (CTLs), that is, organic-free, are fabricated with enhanced carrier diffusion length and crystalline grain size while lessening the grain boundaries near perovskite active layer (PAL)-bulk/carrier selective interfaces. As a result, the trap-state densities within the perovskite bulk were suppressed with stabilized CTL/PAL interfaces for smooth and enhanced carrier transportation. Therefore, for the first time, we contradict the common belief of VOC loss due to halide segregation, as a nice VOC of about 1.34 V is achieved for an organic-free IPSC through enriching initial radiative efficiency, even when halide segregation is present. The optimized organic-free IPSC yielded a power conversion efficiency of 11.61% and a stabilized power output of 10.72%, which provides the potential opportunity to integrate into agrivoltaics (AgV) projects.

Photoelectrochemical Application and Charge Transport Dynamics of a Water-Stable Organic-Inorganic Halide (C6H4NH2CuCl2I) Film in Aqueous Solution.

A water-stable thin film composed of C6H4NH2CuCl2I was fabricated using spin-coating precursor solutions that dissolved equimolar amounts of C6H4NH2I and CuCl2 in N,N-dimethylformamide. Photoelectrochemical characteristics show that the C6H4NH2CuCl2I film demonstrated a stable photocurrent (∼1 µA/cm2) in an aqueous solution under white light (11.5 mW/cm2) even after 3000 s, while exhibiting a photon-to-current efficiency of 0.093% under AM1.5 (100 mW/cm2) illumination. However, these values were significantly lower than those of the CH3NH3PbX3 (X = I, Cl) film in solid devices. The electron diffusion length L(e-) (373 nm) and hole diffusion length L(h+) (177 nm) in the C6H4NH2CuCl2I photoelectrode were significantly lower than those of CH3NH3PbX3, limiting the photoelectrochemical and photocatalysis performances. Moreover, L(h+) was shorter than L(e-) in the C6H4NH2CuCl2I photoelectrode, resulting in the hole-collecting efficiency [ηc(h+)] being lower than the electron-collecting efficiency [ηc(e-)]. A CuO interlayer was introduced as a hole transport layer for the C6H4NH2CuCl2I photoelectrode, which improved L(h+) and ηc(h+).

Grain Boundary Defect Passivation of Triple Cation Mixed Halide Perovskite with Hydrazine-Based Aromatic Iodide for Efficiency Improvement.

Perovskites have been unprecedented with a relatively sharp rise in power conversion efficiency in the last decade. However, the polycrystalline nature of the perovskite film makes it susceptible to surface and grain boundary defects, which significantly impedes its potential performance. Passivation of these defects has been an effective approach to further improve the photovoltaic performance of the perovskite solar cells. Here, we report the use of a novel hydrazine-based aromatic iodide salt or phenyl hydrazinium iodide (PHI) for secondary post treatment to passivate surface and grain boundary defects in triple cation mixed halide perovskite films. In particular, the PHI post treatment reduced current at the grain boundaries, facilitated an electron barrier, and reduced trap state density, indicating suppression of leakage pathways and charge recombination, thus passivating the grain boundaries. As a result, a significant enhancement in power conversion efficiency to 20.6% was obtained for the PHI-treated perovskite device in comparison to a control device with 17.4%.