Influence of the Acceptor on Electrical Performance and Charge Carrier Transport in Bulk Heterojunction Solar Cells with HXS‑1

Enhancing the open-circuit voltage (<i>V</i>_OC) is one way to increase the efficiency of organic solar cells. In cells with the polymer poly­(2-(5-(5,6-bis­(octyloxy)-4-(thiophen-2-yl)­benzo­[<i>c</i>]­[1,2,5]­thiadiazol-7-yl)­thiophen-2-yl)-9-octyl-9<i>H</i>-carbazole) (HXS-1) this can be achieved by replacing the acceptor [6,6]-phenyl-C71 butyric acid methyl ester (PC₇₁BM) with indene-C60-bis-adduct (ICBA). The lowest unoccupied molecular orbital (LUMO) of ICBA is located at a higher energy, which leads to an increase of <i>V</i>_OC from 0.86 to 1.05 V. However, the short-circuit current density (<i>J</i>_SC) and fill factor (<i>FF</i>) are significantly lower in HXS-1:ICBA cells when compared with HXS-1:PC₇₁BM cells, and thus the overall efficiency drops from almost 5% to 2.5%. Despite the smaller LUMO–LUMO offset between HXS-1 and ICBA, strong photoluminescence quenching as well as transient absorption studies indicate efficient and fast exciton dissociation in cells with either fullerene. The slope of the current–voltage characteristics of HXS-1:ICBA cells at short-circuit conditions and the lower dark current in forward direction suggest poor charge carrier transport. These findings were reproduced by a reduction of the electron mobility in electrical simulations. Furthermore, results from Suns-<i>V</i>_OC measurements reveal a dramatically increased transport resistance in cells with ICBA when compared with devices using PC₇₁BM. The observed effects could at least be partially due to a finer morphology in the HXS-1:ICBA layer, which is supported by AFM images

Significant Effect of Bromo Substituents on Nonlinear Optical Properties of Polymer and Chromophores

Four chromophores containing bromo substituents, a ployimide with bromo-containing chromophores, four reference functional polyimides, and fourteen reference chromophores were synthesized for studying effect of bromo substituents on nonlinear optical (NLO) properties of materials and chromophores. The results of hyper-Rayleigh scattering and UV−vis spectra show that static first molecular hyperpolarizability (β0) values of bromo-containing chromophores are 1.24−5.75 times as β0 of the corresponding chloro-containing chromophores (Hammett constants σ of chloro and bromo groups are same) without causing a visible shift of the absorption band to longer wavelength. UV−vis spectra and the results of Maker Fringe method show that the polyimide with chromophores containing bromo substituents exhibits a good optical transparency and a much higher macroscopic nonlinear optical coefficient (d33 = 20.1 pm/V) than the reference polyimides containing nitro (d33 = 9.6 pm/V) and cyano (d33 = 8.9 pm/V) groups in spite of nitro and cyano groups being strong electron acceptors. d33 of polyimide with chloro-containing chromophores is very small. Therefore, this paper suggests an effective strategy for improving the NLO properties of polymeric materials and chromophores without reducing optical transparency in designing NLO polymers and chromophores. On the basis of quantum chemistry calculations, the reasons of effect of bromo substituents on NLO properties of chromophores and materials were discussed

Highly Efficient Amorphous Zn₂SnO₄ Electron-Selective Layers Yielding over 20% Efficiency in FAMAPbI₃‑Based Planar Solar Cells

Amorphous Zn2SnO4 (am-ZTO) films with extreme surface uniformity, high electron mobility, and fewer charge traps were successfully developed by controlling the concentrations of 2-methoxyethanol solutions containing the 2:1 stoichiometric ratio of Zn to Sn. For the first time, we demonstrate that solution-processed am-ZTO thin films are highly efficient as an electron-selective layer (ESL) for mixed perovskite solar cells (PSCs). When am-ZTO ESLs were combined with bandgap-tuned FAMAPbI3 perovskites, a champion efficiency of 20.02% was achieved. In addition, devices based on am-ZTO showed a statistical reproducibility of 18.38 ± 0.61% compared to 15.85 ± 1.02% of the TiO2-based counterparts. This high efficiency is achieved by the significant increase in both the short-circuit current and open-circuit voltage owing to improved charge transport/extraction and recombination. Moreover, am-ZTO ESL-based devices show improved stability and reduced hysteresis, which is a promising result for future PSC research

Photoinduced Reduction of Manganese(III) <i>meso</i>-Tetrakis(1-methylpyridinium-4-yl)porphyrin at AT and GC Base Pairs

The photoreduction of water-soluble cationic manganese­(III) <i>meso</i>-tetrakis­(1-methylpyridium-4-yl)­porphyrin (Mn^III(TMPyP)⁴⁺) bound to a synthetic polynucleotide, either poly­[d­(A-T)₂] or poly­[d­(G-C)₂], was examined by conventional absorption and circular dichroism (CD) spectroscopy, transient absorption, and transient Raman spectroscopy. Upon binding, Mn^III(TMPyP)⁴⁺ produced a positive CD signal for both polynucleotides, suggesting external binding. In the poly­[d­(A-T)₂]–Mn^III(TMPyP)⁴⁺ adduct case, an interaction between the bound porphyrin was suggested. The transient absorption spectral features of Mn^III(TMPyP)⁴⁺ in the presence of poly­[d­(A-T)₂] and poly­[d­(G-C)₂] were similar to those of the photoreduced products, Mn^II(TMPyP)⁴⁺, whereas Mn^III(TMPyP)⁴⁺ in the absence of polynucleotides retained its oxidation state. This indicated that both poly­[d­(A-T)₂] and poly­[d­(G-C)₂] act as electron donors, resulting in photo-oxidized G and A bases. The transient Raman bands (ν₂ and ν₄) that were assigned to porphyrin macrocycles exhibited a large downshift of ∼25 cm^–1, indicating the photoreduction of Mn^III to Mn^II porphyrins when bound to both polynucleotides. The transient Raman bands for pyridine were enhanced significantly, suggesting that the rotation of peripheral groups for binding with polynucleotides is the major change in the geometry expected in the photoreduction process. These photoinduced changes do not appear to be affected by the binding mode of porphyrin

Effect of Surface Trap States on Photocatalytic Activity of Semiconductor Quantum Dots

Semiconductor quantum dots (QDs) are promising photocatalysts for water splitting due to the large specific area, but the influence of surface trap states on the photocatalytic activity of QDs is still not fully understood yet. To answer this question, CdSe QDs with the same morphology, diameter, crystal structure, and energy level are prepared following a hydrazine hydrate (N₂H₄) promoted synthesis strategy and conventional hydrothermal synthesis method. Through various characterizations and analysis, it is found that the conventional hydrothermal synthesized CdSe QDs (H-CdSe QDs) have a high concentration of Cd-involved shallow electron trap states, which seriously hinder the charge separation and transfer between CdSe and cocatalysts. In contrast, the N₂H₄ promoted synthesis strategy provides an energy-saving, low-cost, and facile pathway to eliminate the surface shallow electron traps, ensuring an efficient charge separation and H₂ production in CdSe QDs. As a result, the N₂H₄-promoted synthesized CdSe QDs (N-CdSe QDs) produce 44.5 mL (1998 μmol) H₂ in 7 h, roughly 1.6 times higher than that of H-CdSe QDs (27.5 mL, 1236 μmol). Because the surface trap states are widespread in semiconductor QDs, it is believed that our study provides valuable guidance on the design and preparation of QDs for photocatalysis

Photoinduced Charge Transfer in Donor–Acceptor (DA) Copolymer: Fullerene Bis-adduct Polymer Solar Cells

Polymer solar cells (PSCs) consisting of fullerene bis-adduct and poly­(3-hexylthiophene) (P3HT) blends have shown higher efficiencies than P3HT:phenyl C₆₁-butyric acid methyl ester (PCBM) devices, because of the high-lying lowest unoccupied molecular orbital (LUMO) level of the fullerene bis-adducts. In contrast, the use of fullerene bis-adducts in donor–acceptor (DA) copolymer systems typically causes a decrease in the device’s performance due to the decreased short-circuit current (<i>J</i>_SC) and the fill factor (FF). However, the reason for such poor performance in DA copolymer:fullerene bis-adduct blends is not fully understood. In this work, bulk-heterojunction (BHJ)-type PSCs composed of three different electron donors with four different electron acceptors were chosen and compared. The three electron donors were (1) poly­[(4,8-bis-(2-ethylhexyloxy)­benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene)-2,6-diyl-<i>alt</i>-(5-octylthieno­[3,4-<i>c</i>]­pyrrole-4,6-dione)-1,3-diyl] (PBDTTPD), (2) poly­[(4,8-bis-(2-ethylhexyloxy)benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene)-2,6-diyl-<i>alt</i>-(4-(2-ethylhexanoyl)-thieno­[3,4-<i>b</i>]­thiophene)-2,6-diyl] (PBDTTT-C), and (3) P3HT polymers. The four electron acceptors were (1) PCBM, (2) indene-C₆₀ monoadduct (ICMA), (3) indene-C₆₀ bis-adduct (ICBA), and (4) indene-C₆₀ tris-adduct (ICTA). To understand the difference in the performance of BHJ-type PSCs for the three different polymers in terms of the choice of fullerene acceptor, the structural, optical, and electrical properties of the blends were measured by the external quantum efficiency (EQE), photoluminescence, grazing incidence X-ray scattering, and transient absorption spectroscopy. We observed that while the molecular packing and optical properties cannot be the main reasons for the dramatic decrease in the PCE of the DA copolymers and ICBA, the value of the driving force for charge transfer (Δ<i><i>G</i></i>_CT) is a key parameter for determining the change in <i>J</i>_SC and device efficiency in the DA copolymer- and P3HT-based PSCs in terms of fullerene acceptor. The low EQE and <i>J</i>_SC in PBDTTPD and PBDTTT-C blended with ICBA and ICTA were attributed to an insufficient <i>Δ<i>G</i></i>_CT due to the higher LUMO levels of the fullerene multiadducts. Quantitative information on the efficiency of the charge transfer was obtained by comparing the polaron yield, lifetime, and exciton dissociation probability in the DA copolymer:fullerene acceptor films