Neat C₇₀-based bulk-heterojunction polymer solar cells with excellent acceptor dispersion

Direct Investigation of Excited C60 Dianion and Its Intramolecular Electron Transfer Behaviors

For the first time, the dynamics of excited fullerene dianions and associated intramolecular electron transfer (ET) were directly investigated by using femtosecond pump-probe laser flash photolysis on selectively reduced C60, pyrrolidino[60]fullerene (C60H), and dyads including C60-naphthalenediimide (NDI) and C60-pyromellitimide (PI). Upon near-infrared laser excitation, the excited dianion of C60 or C60H displayed two states with lifetimes of less than one and several tens of ps, attributed to prompt internal conversion from the theoretically predicted Sn state. Furthermore, the ET processes from the excited C602- in dyad molecules, including C602--NDI•- and C602--PI•-, were confirmed with varied ET rate constants due to the difference in the driving force for ET. The current findings provide a clear description of the hitherto uncharted excited-state and photoinduced ET characteristics of fullerene dianions, paving the way for photochemical studies of excited multi-ions (excited multi-polarons) and their application in organic semiconducting materials.

The effect of PbS quantum dots on molecular dynamics and conductivity of PTB7:PC71BM bulk heterojunction as revealed by dielectric spectroscopy

A ternary photovoltaic blend containing the PTB7 donor component, the PC71BM acceptor component, and colloidal quantum dots of lead sulfide (PbS QDs) was investigated using broadband dielectric spectroscopy. In the dielectric loss spectrum of PTB7:PC71BM:PbS QDs, γ- and β-relaxation processes in PTB7 were recognized and analyzed in terms of Arrhenius-type equations. To elucidate the effect of PbS QDs on molecular dynamics of PTB7, the activation energies of both processes were evaluated and compared with those obtained for the binary PTB7:PC71BM blend. Using the CELIV method, the charge carrier mobility was estimated. The PbS QD incorporation into the binary blend was shown to decrease both electron and hole mobility in the ternary PTB7:PC71BM:PbS QD blend. For evaluating the charge carrier lifetime in the ternary blend, the Cole-Cole diagrams derived from the dc conductivity data were plotted. The charge carrier lifetime was found to be much less than the hole extraction time, thus providing effective accumulation of charge carries at the electrodes in the ternary blend under investigation.

Phosphorene Nanoribbon-Augmented Optoelectronics for Enhanced Hole Extraction

Phosphorene nanoribbons (PNRs) have been widely predicted to exhibit a range of superlative functional properties; however, because they have only recently been isolated, these properties are yet to be shown to translate to improved performance in any application. PNRs show particular promise for optoelectronics, given their predicted high exciton binding energies, tunable bandgaps, and ultrahigh hole mobilities. Here, we verify the theorized enhanced hole mobility in both solar cells and space-charge-limited-current devices, demonstrating the potential for PNRs improving hole extraction in universal optoelectronic applications. Specifically, PNRs are demonstrated to act as an effective charge-selective interlayer by enhancing hole extraction from polycrystalline methylammonium lead iodide (MAPbI₃) perovskite to the poly(triarylamine) semiconductor. Introducing PNRs at the hole-transport/MAPbI₃ interface achieves fill factors above 0.83 and efficiencies exceeding 21% for planar p-i-n (inverted) perovskite solar cells (PSCs). Such efficiencies are typically only reported for single-crystalline MAPbI₃-based inverted PSCs. Methylammonium-free PSCs also benefit from a PNR interlayer, verifying applicability to architectures incorporating mixed perovskite absorber layers. Device photoluminescence and transient absorption spectroscopy are used to demonstrate that the presence of the PNRs drives more effective carrier extraction. Isolation of the PNRs in space-charge-limited-current hole-only devices improves both hole mobility and conductivity, demonstrating applicability beyond PSCs. This work provides primary experimental evidence that the predicted superlative functional properties of PNRs indeed translate to improved optoelectronic performance.

Suppressing the Surface Recombination and Tuning the Open-Circuit Voltage of Polymer/Fullerene Solar Cells by Implementing an Aggregative Ternary Compound

In this work, we present a novel small molecule based on dithienylthienothiadiazole units (named SM1) acting as an efficient component in ternary blend organic solar cells to modify the hole extraction at the interface. Our findings show that the SM1 suppresses the surface recombination and enhances the open-circuit voltage ( V_oc). By introducing SM1 in a host system composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl- C₆₁-butyric acid methyl ester (PCBM), we obtained V_oc values of up to 0.75 V and fill factors larger than 70% for the ternary blends. As a consequence, the power conversion efficiency is improved by about 30% compared to P3HT:PCBM binary devices. Interestingly, external quantum efficiency and absorption spectra in the near-infrared region do not show any contribution of SM1 in dried films. Instead, the addition of the small molecule improves the V_oc by reducing the surface recombination losses. To shed light on the recombination processes, we carried out Fourier-transform photocurrent spectroscopy and impedance spectroscopy measurements. This work shows that the ternary concept can also have functionalities other than photosensitization and can even act as a morphology-directing agent or an interface modifier.

From PCBM-Polymer to Low-Cost and Thermally Stable C60/C70-Polymer Solar Cells: The Role of Molecular Structure, Crystallinity, and Morphology Control

The adoption of neat-fullerene (C60 and C70) in polymer solar cells offers opportunities to develop cost-effective and thermally stable devices. Here, through rational side-chain engineering of low optical-gap polymer poly(benzodithiophene-furan-diketopyrrolopyrrole)s (PBDs), we demonstrated for the first time a polymer/C70 blend exhibited higher efficiency (best 6.1%) compare to their polymer/[70]PCBM (best 5.7%) counterparts, and the best efficiency is at the front of efficient polymer/neat-fullerene solar cells. More importantly, we first demonstrated the morphology optimization methodology for solution-processed polymer/neat-fullerene blends in order to reduce the strong crystallization and aggregation of neat-fullerene molecules. In comparison with previous work, these results can provide not only material design strategy but also fundamental difference between polymer/neat-fullerene and polymer/PCBM blend morphology, which allow us better understanding of how to choose proper materials and optimize blend morphology in polymer/neat-fullerene based device to deliver higher photovoltaic performance.

A family of solution-processable macrocyclic and open-chain oligothiophenes with atropoisomeric scaffolds: structural and electronic features for potential energy applications

Cyclic and linear inherently chiral oligothiophenes have been employed as donors in bulk heterojunction solar cells with fullerenes as acceptors.

Investigation of the hole transport characterization and mechanisms in co-evaporated organic semiconductor mixtures

The hole transport characteristics in small molecule semiconductor mixtures of HAT-CN : NPB, HAT-CN : TAPC and HAT-CN : CBP in the ratio of 2 : 1 have been investigated by admittance spectroscopy measurements.

Toward Efficient Thick Active PTB7 Photovoltaic Layers Using Diphenyl Ether as a Solvent Additive

The development of thick organic photovoltaics (OPV) could increase absorption in the active layer and ease manufacturing constraints in large-scale solar panel production. However, the efficiencies of most low-bandgap OPVs decrease substantially when the active layers exceed ∼100 nm in thickness (because of low crystallinity and a short exciton diffusion length). Herein, we report the use of solvent additive diphenyl ether (DPE) that facilitates the fabrication of thick (180 nm) active layers and triples the power conversion efficiency (PCE) of conventional thienothiophene-co-benzodithiophene polymer (PTB7)-based OPVs from 1.75 to 6.19%. These results demonstrate a PCE 20% higher than those of conventional (PTB7)-based OPV devices using 1,8-diiodooctane. Morphology studies reveal that DPE promotes the formation of nanofibrillar networks and ordered packing of PTB7 in the active layer that facilitate charge transport over longer distances. We further demonstrate that DPE improves the fill factor and photocurrent collection by enhancing the overall optical absorption, reducing the series resistance, and suppressing bimolecular recombination.

Blend films of an amorphous conjugated polymer and a thermal precursor fullerene: effects of annealing temperatures on film structures and photovoltaic properties

Thermal precursor approach using C₆₀–9-methylanthracene adducts enables blend film formations with bulk heterojunction structures containing highly aggregating pristine C₆₀ molecules in combination with an amorphous conjugated polymer, PCDTBT.