Nonfused Ring Electron Acceptors for Ternary Polymer Solar Cells with Low Energy Loss and Efficiency Over 18

Ternary polymer solar cells(PSCs) have been identified as an effective approach to improving power conversion efficiency (PCE) of binary PSCs. However, most of the third component, especially Y-series non-fullerene acceptors, is a fused ring acceptor which often requires a rather tedious synthesis and the use of hazardous organostannane reagents. In this work, two nonfused ring acceptors IOEH-4F and IOEH-N2F are synthesized by a green synthetic route and incorporated into PM6:Y6 blend. Encouragingly, the IOEH-4F and IOEH-N2F-based ternary PSCs exhibited more efficient charge transfer, exciton separation, and lower energy loss than PM6:Y6-based PSCs. And the IOEH-4F and IOEH-N2F-based ternary PSCs achieved an impressive PCE of 17.80% and 18.13%, respectively, which are higher than that of PM6:Y6 based PSCs (16.18%). Notably, these PCE values are also the highest PCEs for ternary PSCs including non-fused ring acceptors. Importantly, even when the IOEH-N2F:Y6 ratios increased from 0.05:1.2 to 0.50:1.2, the PCE of IOEH-N2F-based ternary PSCs (16.70%) are still higher than that of PM6:Y6 based PSCs, indicating the great potential for cost saving. It is believed that the findings will help the design of better nonfused ring acceptors and the optimization of corresponding ternary PSCs with cost-saving advantage.

Enhanced Photovoltaic Properties of Y6 Derivatives with Asymmetric Terminal Groups: A Theoretical Insight

Y6 derivatives with asymmetric terminal groups have attracted considerable attention in recent years. However, the effects of the asymmetric modification of terminal groups on the photovoltaic performance of Y6 derivatives are not well understood yet. Therefore, we designed a series of Y6-based acceptors with asymmetric terminal groups by endowing them with various electron-withdrawing abilities and different conjugated rings to conduct systematic research. The electron-withdrawing ability of the Y6-D1 terminal group (substituted by IC-2F and IC-2NO2 terminals) is strongest, followed by Y6 (substituted by two same IC-2F terminals), Y6-D2 (substituted by IC-2F and 2-(4-oxo-4,5-dihydro-6H-cyclopenta[b]thiophen-6-ylidene)malononitrile terminals), Y6-D4 (substituted by IC-2F and indene ring), and Y6-D3 (substituted by IC-2F and thiazole ring). Computed results show that A-A stacking is the main molecular packing mode of Y6 and four other asymmetric Y6 derivatives. The ratios of A-A stacking face-on configuration of Y6-D1, Y6-D2, Y6-D3, Y6-D4, and Y6 are 51.6%, 55.0%, 43.5%, 59.3%, and 62.4%, respectively. Except for Y6-D1 substituted by the IC-2F and IC-2NO2 (the strongest electron-withdrawing capacity) terminal groups, the other three asymmetric molecules are mainly electron-transporting and can therefore act as acceptors. The open-circuit voltages of organic solar cells (OSCs) based on Y6-D2, Y6-D3, and Y6-D4, except for Y6-D1, may be higher than those of OSCs based on the Y6 acceptor because of their higher energy levels of lowest unoccupied molecular orbital (LUMO). PM6/Y6-D3 and PM6/Y6-D4 have better light absorption properties than PM6/Y6 due to their higher total oscillator strength. These results indicate that Y6-D3 and Y6-D4 can be employed as good acceptors.

Enhancing the Photovoltaic Properties via Incorporation of Selenophene Units in Organic Chromophores with A2-π2-A1-π1-A2 Configuration: A DFT-Based Exploration

Currently, polymer organic solar cells (POSCs) are widely utilized due to their significant application, such as low-cost power conversion efficiencies (PCEs). Therefore, we designed a series of photovoltaic materials (D1, D2, D3, D5 and D7) by the incorporation of selenophene units (n = 1-7) as π₁-spacers by considering the importance of POSCs. Density functional theory (DFT) calculations were accomplished at MPW1PW91/6-311G (d, p) functional to explore the impact of additional selenophene units on the photovoltaic behavior of the above-mentioned compounds. A comparative analysis was conducted for designed compounds and reference compounds (D1). Reduction in energy gaps (∆E = 2.399 - 2.064 eV) with broader absorption wavelength (λ_max = 655.480 - 728.376 nm) in chloroform along with larger charge transference rate was studied with the addition of selenophene units as compared to D1. A significantly higher exciton dissociation rate was studied as lower values of binding energy (E_b = 0.508 - 0.362 eV) were noted in derivatives than in the reference (E_b = 0.526 eV). Moreover, transition density matrix (TDM) and density of state (DOS) data also supported the efficient charge transition origination from HOMOs to LUMOs. Open circuit voltage (V_oc) was also calculated for all the aforesaid compounds to check the efficiency, and significant results were seen (1.633-1.549 V). All the analyses supported our compounds as efficient POSCs materials with significant efficacy. These compounds might encourage the experimental researchers to synthesize them due to proficient photovoltaic materials.

The Impact of TPA Auxiliary Donor and the π-Linkers on the Performance of Newly Designed Dye-Sensitized Solar Cells: Computational Investigation

The efficiency of the newly designed dye-sensitized solar cells (DSSCs) containing triphenylamine, diphenylamine (TPA), phenothiazine, and phenoxazine as donors and triazine, phenyl with D₁-D₂-π-linker-π-(A)₂ architecture has been investigated using density functional theory (DFT) and time-dependent (TD-DFT) methods. These methods were used to investigate the geometrical structures, electronic properties, absorption, photovoltaic properties, and chemical reactivity. Furthermore, the calculated results indicate that different architectures can modify the energy levels of HOMO and LUMO and reduce the energy gap. The absorption undergoes a redshift displacement. This work aims at calculating the structural geometries and the electronic and optical properties of the designed dyes. Furthermore, the dye adsorption characteristics, such as the optoelectronic properties and the adsorption energies in the TiO₂ clusters, were calculated with counterpoise correction and discussed.

Surface Passivation Using 2D Perovskites toward Efficient and Stable Perovskite Solar Cells

3D perovskite solar cells (PSCs) have shown great promise for use in next-generation photovoltaic devices. However, some challenges need to be addressed before their commercial production, such as enormous defects formed on the surface, which result in severe SRH recombination, and inadequate material interplay between the composition, leading to thermal-, moisture-, and light-induced degradation. 2D perovskites, in which the organic layer functions as a protective barrier to block the erosion of moisture or ions, have recently emerged and attracted increasing attention because they exhibit significant robustness. Inspired by this, surface passivation by employing 2D perovskites deposited on the top of 3D counterparts has triggered a new wave of research to simultaneously achieve higher efficiency and stability. Herein, we exploited a vast amount of literature to comprehensively summarize the recent progress on 2D/3D heterostructure PSCs using surface passivation. The review begins with an introduction of the crystal structure, followed by the advantages of the combination of 2D and 3D perovskites. Then, the surface passivation strategies, optoelectronic properties, enhanced stability, and photovoltaic performance of 2D/3D PSCs are systematically discussed. Finally, the perspectives of passivation techniques using 2D perovskites to offer insight into further improved photovoltaic performance in the future are proposed.

Designing Potential Donor Materials Based on DRCN5T with Halogen Substitutions: A DFT/TDDFT Study

Experimental researchers have found that the organic solar cell (OSC) based on DRCN5T (an oligothiophene) possesses excellent power conversion efficiency (PCE) of 10.1%. However, to date, there have been few studies about halogenation of DRCN5T, and its effects on photovoltaic properties of halogenated DRCN5T are still not clear. In the present work, we first perform benchmark calculations and effectively reproduce experimental results. Then, eight halogenated DRCN5T molecules are designed and investigated theoretically by using density functional theory (DFT) and time-dependent DFT. The dipole moments, frontier molecular orbital energies, absorption spectra, exciton binding energy (E_b), singlet–triplet energy gap (ΔE_ST), and electrostatic potential (ESP) of these molecules, and the estimated open circuit voltages (V_OCs) of the OSCs with PC₇₁BM as acceptor are presented. We find that (1) generally, halogen substitutions would increase V_OC; (2) E_b rises with more fluorine substitutions, but for Cl and Br substitutions, E_b increases firstly and then drops; (3) ΔE_ST keeps increasing with more halogen substitutions; (4) except for Br substitutions, the averaged ESP arises along with more halogen substitutions; (5) the absorption strength of UV–Vis spectra of DRCN5T2F, DRCN5T4F, DRCN5T6F, and DRCN5T2Cl in the visible region is enhanced with respect to DRCN5T. Based on these results, overall, DRCN5T2Cl, DRCN5T4F, and DRCN5T6F may be promising donors.

2D Hybrid Halide Perovskites: Structure, Properties, and Applications in Solar Cells

2D metal-halide perovskites have attracted intense research interest due to superior long-term stability under ambient environments. Compared to their 3D analog, the alternate arrangement of organic and inorganic layers leads to forming a multilayer quantum well (MQW), which endows 2D perovskites with anisotropic optoelectronic properties. In addition, the spacer layer functions as a hydrophobic barrier to effectively prevent 2D perovskite films from ion migration and moisture penetrating, thus realizing outstanding stability. Recently, 2D perovskites have been widely developed with abundant species. The stunning photovoltaic performance with the coexistence of long-term stability and high-power conversion efficiency (PCE) has been realized in 2D perovskite solar cells (PSCs), which paves an avenue for commercialization of PSCs. This review begins with an introduction of crystal structure and crystallization kinetics to illustrate the unique layer characters in 2D perovskites. Then, electron structure, excitons, dielectric confinement, and intrinsic stability properties are discussed in detail. Next, the photovoltaic performance based on recent Ruddlesden-Popper (RP), Dion-Jacobson (DJ), and alternating cations in the interlayer (ACI) phase 2D-PSCs is comprehensively summarized. Finally, the confronting challenges and strategies toward structural design and optoelectronic studies of 2D perovskites are proposed to offer insight into the advanced underlying properties of this family of materials.

Isomeric Pyrene-Porphyrins for Efficient Dye-Sensitized Solar Cells: An Unexpected Enhancement of the Photovoltaic Performance upon Structural Modification

Four pyrene-porphyrins were synthesized to study the isomer effect on the photovoltaic performance of dye-sensitized solar cells. One of these porphyrins is conjugated with a terminal pyrene, whereas the other three are each attached with a pyrene bearing an extra donor group. According to the positions of the extra donor and porphyrin core on pyrene, the 1,6-, 1,8-, and 2,7-isomers were compared for their fundamental and photovoltaic properties. For fundamental properties, UV-visible absorption, fluorescence emission, electrochemistry, and DFT calculations were carried out. For photovoltaic measurements, the seemingly inferior 1,8-isomer outperforms others with an overall efficiency of 10.30% under one-sun irradiation. Superior photovoltaic performance of the 1,8-isomeric dye may be related to the so-called umbrella effect. The findings of this work may provide insight into isomeric dye design for future applications.

Boosting Photovoltaic Properties and Intrinsic Stability for MA-Based Perovskite Solar Cells by Incorporating 1,1,1-Trimethylhydrazinium Cation

The most critical reason for limiting the extensive study and promotion of MA-based perovskites is their intrinsic instability when compared to FA-based perovskites. Therefore, it is necessary to develop a simple and effective method to improve their intrinsic stability. Herein, the 1,1,1-trimethylhydrazinium cation (TMH⁺) was first introduced into MAPbI₃ to fabricate high-performance mixed-cation perovskite solar cells (PSCs) with an enhanced power conversion efficiency (PCE) of 19.86%, which benefits by the improved crystallization and morphology of films. On the one hand, the slightly large size of TMH⁺ is complementary to the low tolerance factor of MAPbI₃ and then enhances the structure stability. On the other hand, the presence of methyl groups in TMH⁺ is beneficial to promote the hydrophobicity of MA-based perovskite. More importantly, the hydrazinium group can effectively inhibit the production of Pb⁰ in perovskites, which is the initial stage of degradation. As a result, the intrinsic stability of PSCs has been observably boosted. After aging at 45 ± 5% RH for 1800 h and 85 °C for 200 h, the unencapsulated PSCs retained 77 and 79% of initial PCE, respectively. This work provides a new design for the selection of suitable cations with special structures and chemical groups to enhance the moisture resistance and intrinsic stability of MA-based perovskite at the source of degradation.

Device Characteristics of Band gap Tailored 10.04% Efficient CZTSSe Solar Cells Sprayed from Water-Based Solution

A diverse S/(S + Se) ratio of Cu₂ZnSn(S_x,Se_1-x)₄ (CZTSSe) thin-film solar cells is derived by the water-based spray pyrolysis approach. By fine-tuning the S/(S + Se) ratio, base CZTSSe device efficiency has significantly improved from 7.02 to 10.04% by minimizing the V_oc deficit up to 616 mV and increasing fill factor (FF) from 56.42 to 62.38%. As the S/(S + Se) ratio was increased from 0 to 0.4, surface compactness was observed to be improved with slightly decreased grain size, which increased shunt resistance and resultantly increased FF. However, when S-alloying was more than S/(S + Se) = 0.4, grain size decreased too much and had a detrimental effect on device performance. To deeply understand the role effect of the S/(S + Se) ratio, detailed spectroscopic analysis is performed with admittance spectroscopy, temperature-dependent current-voltage characteristic (J-V-T), time-resolved photoluminescence, and Raman depth profiling. Experimental results revealed that the different power conversion efficiency limiting factors were developed with various S/(S + Se) ratios. High density of deep defect states generated with the S/(S + Se) ≥ 40% content and larger conduction band offset observed with red kink were formed in the Se/(S + Se) ≥ 80% content. Hence, in order to get the high-efficient CZTSSe solar cell, fine tuning of the S/(S + Se) ratio is necessary.

Fuse the π-Bridge to Acceptor Moiety of Donor-π-Acceptor Conjugated Polymer: Enabling an All-Round Enhancement in Photovoltaic Parameters of Nonfullerene Organic Solar Cells

The D-π-A conjugated polymers with a benzotriazole (BTz) unit as the A moiety have been intensively investigated as donor materials in nonfullerene solar cells. However, these BTz even the fluorinated-BTz constructed D-π-A polymers mostly suffered from upward highest occupied molecular orbital (HOMO) energy levels, leading to inferior open-circuit voltage (V_OC) and efficiencies in the fabricated solar cells. Herein, we explored a new approach in response to this issue via the strategy of π-bridge fusion to A moiety. As a result, the medium band gap D-π-A polymer PY2 was evolved into wide band gap D-A polymer PY1 with fused-DTBTz as the new A moiety, accompanied with a greatly declined HOMO energy level by 0.26 eV, a remarkable blue-shifted absorption onset by about 51 nm, and concurrently moderately enhanced face-on stacking orientations in neat polymer and donor/acceptor blend films. The synergetic optimizations in energy level, absorption characteristic and molecular stacking feature via the π-bridge fusion design witness an all-round improvement in photovoltaic parameters including the focused V_OC, short-circuit current density (J_SC), and fill factor (FF), with narrow band gap ITIC as the acceptor material. Specifically, the PY1-based solar cells produce an optimal power conversion efficiency (PCE) of 12.49%, with superior V_OC of 0.94 V, J_SC of 18.46 mA cm^-2, and FF of 0.72, significantly surpassing those of PY2-based optimal device with a PCE of 7.39%, V_OC of 0.77 V, J_SC of 14.54 mA cm^-2, and FF of 0.66 and even the reported classical fluorinated-BTz based polymer J51 (V_OC of 0.82 V, PCE of 9.26%). Promisingly, there is a huge room for improvement in photovoltaic properties with rational fluorination or chlorination of the fused-DTBTz unit or the D moiety of the D-A polymers.

Influences of Non-fullerene Acceptor Fluorination on Three-Dimensional Morphology and Photovoltaic Properties of Organic Solar Cells

Fluorination of conjugated molecules has been established as an effective structural modification strategy to influence properties and has attracted extensive attention in organic solar cells (OSCs). Here, we have investigated optoelectronic and photovoltaic property changes of OSCs made of polymer donors with the non-fullerene acceptors (NFAs) ITIC and IEICO and their fluorinated counterparts IT-4F and IEICO-4F. Device studies show that fluorinated NFAs lead to reduced V_oc but increased J_sc and fill-factor (FF), and therefore, the ultimate influence to efficiency depends on the compensation of V_oc loss and gains of J_sc and FF. Fluorination lowers energy levels of NFAs, reduces their electronic band gaps, and red-shifts the absorption spectra. The impact of fluorination on the molecular order depends on the specific NFA, and the conversion of ITIC to IT-4F reduces the structural order, which can be reversed after blending with the donor PBDB-T. Contrastingly, IEICO-4F presents stronger π-π stacking after fluorination from IEICO, and this is further strengthened after blending with the donor PTB7-Th. The photovoltaic blends universally present a donor-rich surface region which can promote charge transport and collection toward the anode in inverted OSCs. The fluorination of NFAs, however, reduces the fraction of donors in this donor-rich region, consequently encouraging the intermixing of donor/acceptor for efficient charge generation.

Adjusting Aggregation Modes and Photophysical and Photovoltaic Properties of Diketopyrrolopyrrole-Based Small Molecules by Introducing B←N Bonds

The packing mode of small-molecular semiconductors in thin films is an important factor that controls the performance of their optoelectronic devices. Designing and changing the packing mode by molecular engineering is challenging. Three structurally related diketopyrrolopyrrole (DPP)-based compounds were synthesized to study the effect of replacing C-C bonds by isoelectronic dipolar B←N bonds. By replacing one of the bridging C-C bonds on the peripheral fluorene units of the DPP molecules by a coordinative B←N bond and changing the B←N bond orientation, the optical absorption, fluorescence, and excited-state lifetime of the compounds can be tuned. The substitution alters the preferential aggregation of the molecules in the solid state from H-type (for C-C) to J-type (for B←N). Introducing B←N bonds thus provides a subtle way of controlling the packing mode. The photovoltaic properties of the compounds were evaluated in bulk heterojunctions with a fullerene acceptor and showed moderate performance as a consequence of suboptimal morphologies, bimolecular recombination, and triplet-state formation.

Structural and Solar Cell Properties of a Ag-Containing Cu2ZnSnS4 Thin Film Derived from Spray Pyrolysis

A silver (Ag)-incorporated kesterite Cu₂ZnSnS₄ (CZTS) thin film was fabricated by a facile spray pyrolysis method. Crystallographic analyses indicated successful incorporation of various amounts of Ag up to a Ag/(Ag + Cu) ratio of ca. 0.1 into the crystal lattice of CZTS in a homogeneous manner without formation of other impurity compounds. From the results of morphological investigations, Ag-incorporated films had larger crystal grains than the CZTS film. The sample with a relatively low Ag content (Ag/(Ag + Cu) of ca. 0.02) had a compact morphology without appreciable voids and pinholes. However, an increase in the Ag content in the CZTS film (Ag/(Ag + Cu) ca. 0.10) induced the formation of a large number of pinholes. As can be expected from these morphological properties, the best sunlight conversion efficiency was obtained by the solar cell based on the film with Ag/(Ag + Cu) of ca. 0.02. Electrostructural analyses of the devices suggested that the Ag-incorporated film in the device achieved reduction in the amounts of unfavorable copper on zinc antisite defects compared to the bare CZTS film. Moreover, the use of a Ag-incorporated film improved band alignment at the CdS(buffer)-CZTS interface. These alterations should also contribute to enhancement of device properties.

Development of Spiro[cyclopenta[1,2-b:5,4-b']dithiophene-4,9'-fluorene]-Based A-π-D-π-A Small Molecules with Different Acceptor Units for Efficient Organic Solar Cells

Three acceptor-π-donor-π-acceptor (A-π-D-π-A) small molecules (STFYT, STFRDN, and STFRCN) with spiro[cyclopenta[1,2-b:5,4-b']dithiophene-4,9'-fluorene] (STF) as the central donor unit, terthiophene as the π-conjugated bridge, indenedione, 3-ethylrhodanine, or 2-(1,1-dicyanomethylene)rhodanine as the acceptor unit are designed, synthesized, and characterized as electron donor materials in solution-processing organic solar cells (OSCs). The effects of the spiro STF-based central core and different acceptors on the molecular configuration, absorption properties, electronic energy levels, carrier transport properties, the morphology of active layers, and photovoltaic properties are investigated in detail. The three molecules exhibit desirable physicochemical features: wide absorption bands (300-850 nm) and high molar absorption coefficients (4.82 × 10⁴ to 7.56 × 10⁴ M^-1 cm^-1) and relatively low HOMO levels (-5.15 to -5.38 eV). Density functional theory calculations reveal that the spiro STF central core benefits to reduce the steric hindrance effect between the central donor block and terthiophene bridge and suppress excessive intermolecular aggregations. The optimized OSCs based on these molecules deliver power conversion efficiencies (PCEs) of 6.68%, 3.30%, and 4.33% for STFYT, STFRDN, and STFRCN, respectively. The higher PCE of STFYT-based OSCs should be ascribed to its better absorption ability, higher and balanced hole and electron mobilities, and superior active layer morphology as compared to the other two compounds. So far, this is the first example of developing the A-π-D-π-A type small molecules with a spiro central donor core for high-performance OSC applications. Meanwhile, these results demonstrate that using spiro central block to construct A-π-D-π-A molecule is an alternative and effective strategy for achieving high-performance small molecule donor materials.

Impact of Precursor Compositions on the Structural and Photovoltaic Properties of Spray-Deposited Cu2 ZnSnS4 Thin Films

Pure sulfide Cu2 ZnSnS4 thin films were fabricated on Mo-coated glass substrates by facile spray deposition of aqueous precursor solutions containing Cu(NO3 )2 , Zn(NO3 )2 , Sn(CH3 SO3 )2 , and thiourea followed by annealing at 600 °C. When a precursor solution containing a stoichiometric composition of Cu, Zn, and Sn was used, the resulting Cu2 ZnSnS4 thin film contained a Cu2-x S impurity phase owing to the evaporation of Sn components during the annealing process. The Cu2-x S impurity in the Cu2 ZnSnS4 thin film was removed by reducing the concentration of Cu in the precursor solution. This resulted in an improvement of the structural features (i.e., grain sizes and compactness) as well as the electric properties such as acceptor densities, the nature of the acceptor defects, and carrier lifetimes. A solar cell based on the Cu2 ZnSnS4 film with an empirically optimal composition showed conversion efficiency of 8.1 %. The value achieved was one of the best efficiencies of Cu2 ZnSnS4 -based cells derived from a non-vacuum process.

Demonstration of ultrarapid interfacial formation of 1D fullerene nanorods with photovoltaic properties

We demonstrate ultrarapid interfacial formation of one-dimensional (1D) single-crystalline fullerene C60 nanorods at room temperature in 5 s. The nanorods of ∼ 11 μm in length and ∼ 215 nm in diameter are developed in a hexagonal close-pack crystal structure, contrary to the cubic crystal structure of pristine C60. Vibrational and electronic spectroscopy provide strong evidence that the nanorods are a van der Waals solid, as evidenced from the preservation of the electronic structure of the C60 molecules within the rods. Steady state optical spectroscopy reveals a dominance of charge transfer excitonic transitions in the nanorods. A significant enhancement of photogenerated charge carriers is observed in the nanorods in comparison to pristine C60, revealing the effect of shape on the photovoltaic properties. Due to their ultrarapid, large-scale, room-temperature synthesis with single-crystalline structure and excellent optoelectronic properties, the nanorods are expected to be promising for photosensitive devices applications.