Nanomorphology dependence of the environmental stability of organic solar cells

Previous studies have reported contradictory effects of small-molecule acceptors on the environmental stability of polymer:small-molecule blends, with one showing that a small-molecule acceptor stabilizes and another showing that it destabilizes the polymer donor. In this work, to investigate the origin of these contradictory results, the effects of the nanomorphologies of small-molecule acceptors on the environmental stability of polymer:small-molecule blends are demonstrated. Investigations on the environmental stabilities of polymer:fullerene blends of poly[[4, 8-bis[(2-ethylhexyl)oxy]benzo[1, 2-b:4, 5-b′]dithiophene-2, 6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3, 4-b]thiophenediyl]] (PTB7):phenyl-C₆₁-butyric acid methyl ester (PCBM) with contrasting nanomorphologies of PCBM reveal that dispersed PCBM in a mixed phase is the critical factor that causes triplet-mediated singlet oxygen generation and, hence, the severe photooxidation of PTB7, whereas an aggregated PCBM phase stabilizes PTB7 by reducing the formation of PTB7 triplet excitons. In addition, the photooxidation of PTB7 substantially degrades hole transport in the PTB7:PCBM blends by destroying the crystalline PTB7 phases within the films; this effect is strongly correlated with the efficiency losses of the PTB7:PCBM organic solar cells. These conclusions are also extended to polymer:nonfullerene blends of PTB7:ITIC and PTB7:Y6, thereby confirming the generality of this phenomenon for polymer:small-molecule organic solar cells

Defect-Stabilized Tin-Based Perovskite Solar Cells Enabled by Multifunctional Molecular Additives

Sn-based perovskite solar cells (Sn-PSCs) are the most viable replacements for Pb-based PSCs. However, the facile oxidation of Sn2+ and a high defect density on the surfaces and grain boundaries in Sn-PSCs complicate the task of attaining highly stable Sn-PSCs. Here, both surfaces and grain boundaries are passivated using a fulleropyrrolidine with a triethylene glycol monoethyl ether side chain (PTEG-1) as a multifunctional molecular additive for the first time. The ether group and fullerene group in PTEG-1 interact with Sn2+ and I-, respectively, thereby inhibiting the formation of Sn4+ and I3-. This multifunctional passivation suppresses nonradiative recombination and improves the stability of Sn-PSCs. As a result, Sn-PSCs with encapsulation retain 65% of their initial power conversion efficiency after 1000 h of light illumination under ambient conditions. Our results provide a guideline for the future design of multifunctional molecules with functional groups that enable the fabrication of stable Sn-PSCs.11Nsciescopu

Tuning the LUMO levels of non-fullerene acceptors via extension of pi-conjugated cores for organic solar cells

We investigate how extending the length of a pi-conjugated central A ' core in Acceptor-Donor-Acceptor '-Donor-Acceptor (A-D-A '-D-A)-type non-fused-ring electron acceptors (NFREAs) affects their energy level and the miscibility of donor polymer:NFREA organic solar cells (OSCs). The extended pi-conjugated central A ' core increases their lowest unoccupied molecular orbital (LUMO) energy level. We designed and synthesized three NFREAs: BT-4F, which has a benzothiadiazole (BT) core; DTBT-4F, which has two fused thiophene rings on the BT core; and BTST-4F, which has two fused thienothiophene rings on the BT core. In these NFREAs, the LUMO level is upshifted in the order of increasing length of the extended pi-conjugated core. The open-circuit voltage (V-OC) is related to the gap between the LUMO of the NFREAs and the highest occupied molecular orbital (HOMO) of the donor polymer, thus, the V-OC increased from 0.66 V in a BT-4F-based OSC to 0.81 and 0.84 V in OSCs based on DTBT-4F and BTST-4F, respectively. The PBDB-T donor polymer is more miscible with BTST-4F than with BT-4F and DTBT-4F. The BTST-4F-based blends have a narrower light-absorption range than the PBDB-T:BT-4F blends; however, because of the miscible morphologies of the PBDB-T:BTST-4F blends, the BTST-4F-based devices exhibit a comparable short-circuit current density (J(SC)) to that of BT-4F-based devices. These results indicate that the introduction of extended pi-conjugation of the central core in A-D-A '-D-A-type NFREAs could be an effective method to increase the V-OC of NFREA-based devices while maintaining a decent J(SC). These results further indicate that introducing an extended pi-conjugated central core is a promising design strategy to achieve highly efficient OSCs based on NFREAs.11Nsciescopu

Boundary Migration Induced Plasticity during Recrytallization and Growth under Applied Stress

In general, plastic strain occurs over a certain stress, called yield stress. However, it has been reported that the permanent strain could happen during boundary migrating even under the extremely slight externally applied stress. In this study, we performed dilatometry experiments under the various compressive stresses and measured the amount of recrystallization and growth induced permanent strain. A new empirical constitutive equation was suggested to describe the recrystallization and growth induced plasticity. This equation was verified by comparing the calculated values with dilatometric experimental data under the various compressive stresses.11sciescopu

Surface Stabilization of a Formamidinium Perovskite Solar Cell Using Quaternary Ammonium Salt

Dimensionality engineering is an effective approach to improve the stability and power conversion efficiency (PCE) of perovskite solar cells (PSCs). A two-dimensional (2D) perovskite assembled from bulky organic cations to cover the surface of three-dimensional (3D) perovskite can repel ambient moisture and suppress ion migration across the perovskite film. This work demonstrates how the thermal stability of the bulky organic cation of a 2D perovskite affects the crystallinity of the perovskite and the optoelectrical properties of perovskite solar cells. Structural analysis of (FAPbI(3))(0.95)(MAPbBr(3))(0.05) (FA = formamidinium ion, MA = methylammonium ion) mixed with a series of bulky cations shows a clear correlation between the structure of the bulky cations and the formation of surface defects in the resultant perovskite films. An organic cation with primary ammonium structure is vulnerable to a deprotonation reaction under typical perovskite-film processing conditions. Decomposition of the bulky cations results in structural defects such as iodide vacancies and metallic lead clusters at the surface of the perovskite film; these defects lead to a nonradiative recombination loss of charge carriers and to severe ion migration during operation of the device. In contrast, a bulky organic cation with a quaternary ammonium structure exhibits superior thermal stability and results in substantially fewer structural defects at the surface of the perovskite film. As a result, the corresponding PSC exhibits the PCE of 21.6% in a reverse current-voltage scan and a stabilized PCE of 20.1% with an excellent lifetime exceeding 1000 h for the encapsulated device under continuous illumination.11Nsciescopu

Elucidating the photoluminescence-enhancement mechanism in a push-pull conjugated polymer induced by hot-electron injection from gold nanoparticles

Understanding the photophysical interactions between the components in organic-inorganic nanocomposites is a key factor for their efficient application in optoelectronic devices. In particular, the photophysical study of nanocomposites based on organic conjugated polymers is rare. We investigated the effect of surface plasmon resonance (SPR) of gold nanoparticles (Au NPs) on the photoluminescence (PL) property of a push-pull conjugated polymer (PBDB-T). We prepared the hybrid system by incorporating poly(3-hexylthiophene)-stabilized Au NPs (P3HT-Au NPs) into PBDB-T. The enhanced and blueshifted PL was observed in the hybrid system compared to PL in a neat PBDB-T system, indicating that the P3HT chains attached to the Au NPs suppressed charge-transfer from PBDB-T to the Au NPs and relayed the hot electrons to PBDB-T (the band-filling effect). This photophysical phenomenon limited the auto-dissociation of PBDB-T eadtons. Thus, the radiative recombination of the excitons occurred more in our hybrid system than in the neat system. (C) 2021 Chinese Laser Press11Ysciescopu

Augmented Photoluminescence in a Conjugated Polymer by the Incorporation of CdSe/CdS Quantum Dots

We investigated the photophysical interactions between CdSe/CdS quantum dots (QDs) and a conjugated polymer (CP, P3HT). The photoluminescence intensity of P3HT in the QDs/P3HT hybrid system is significantly enhanced compared to that of the neat P3HT system. We found via transient absorption spectroscopy that the energy level differences at the interfaces between P3HT and QDs resulted in delayed relaxation dynamics of the P3HT singlet (S1) excitons and suppressed polaron formation. Thus, the radiative recombination of the S1 excitons occurs frequently in the hybrid system than in the neat P3HT system. Our findings on the CP-based hybrid system may provide important information to improve the efficiencies of optoelectronic devices, such as organic light-emitting diodes.11Nsciescopu

Intrachain Delocalization Effect of Charge Carriers on the Charge-Transfer State Dynamics in Organic Solar Cells

We studied the charge-generation mechanism in low-bandgap polymer (P4TNTz-2F)-fullerene bulk heterojunction (BHJ) organic solar cells (OSCs) using transient absorption (TA) spectroscopy. The highly crystalline nanowire structure of P4TNTz-2F in a blend film prepared with chlorobenzene (CB) and 1,8-diiodooctane (DIO) induced more long-lived charge carriers than those in a blend film prepared with CB only. Pump-wavelength-dependent TA data revealed that the increased charge-delocalization by the intrachain ordering of P4TNTz-2F in the blend film prepared with CB/DIO is the key factor to increasing the OSC efficiency. The intrachain charge-delocalization increased the charge-transfer (CT) state lifetime and suppressed geminate recombination losses, resulting in the efficient dissociation of CT states into free carriers. Our findings provide new insights into the excited-state dynamics study of BHJ blends, which can serve as a good guide for the development of novel OSC materials.11Nsciescopu