Effect of External Bias on Nongeminate Recombination in Polythiophene/Methanofullerene Organic Solar Cells

Much recent literature suggests that nongeminate recombination is the loss mechanism that predominantly determines the bias dependence of the photocurrent in efficient organic solar cells. Here, we report a new experimental technique based on measuring the quasi-steady-state current–voltage characteristics during illumination by two pulsed lasers and observing how the current–voltage characteristics change as a function of the time delay between the two pulsed lasers. This technique unequivocally demonstrates a bias dependence of nongeminate recombination and reveals the dwell time of charge carriers in a photovoltaic device. We relate the results of our pulsed experiment to devices under solar illumination and find that the reduction of the charge carrier dwell time with increasing internal electric field explains the observed bias dependence of the device photocurrent under constant illumination and consequently affects the fill factor of high-performance organic solar cells

Effect of Nongeminate Recombination on Fill Factor in Polythiophene/Methanofullerene Organic Solar Cells

A key factor in solar cell efficiency is the dependence of the photocurrent on applied bias. With respect to organic solar cells, it is often suggested that this factor is governed by the field dependence of charge-transfer state separation. Here, we demonstrate that this is not the case in benchmark polythiophene/methanofullerene solar cells. By examining the temperature and light intensity dependence of the current−voltage characteristics, we determine that (1) the majority of free charge generation is not dependent on the field or temperature and (2) the competition between extraction and recombination of free charges principally determines the dependence of photocurrent on bias. These results are confirmed by direct observation of the temperature dependence of charge separation and recombination using transient absorption spectroscopy and highlight that in order to achieve optimal fill factors in organic solar cells, minimizing free carrier recombination is an important consideration

Trap-Free Hot Carrier Relaxation in Lead–Halide Perovskite Films

Photovoltaic devices that employ lead–halide perovskites as photoactive materials exhibit power conversion efficiencies of 22%. One of the potential routes to go beyond the current efficiencies is to extract charge carriers that carry excess energy, that is, nonrelaxed or “hot” carriers, before relaxation to the band minima is completed. Lead–halide perovskites have been demonstrated to exhibit hot-carrier relaxation times exceeding 100 ps for both single- and polycrystalline samples. Here, we demonstrate, using a combined time-resolved photoluminescence and transient absorption study supported by basic modeling of the dynamics, that the decay of the high-energy part of the photoluminescence occurs on a time scale (∼100 ps) very similar to the repopulation of the band minima when excited with a photon energy larger than 2.6 eV. The similarity between the two time scales indicates that the depopulation of hot states occurs without transient trapping of electrons or holes

The Effect of Solvent Additive on the Charge Generation and Photovoltaic Performance of a Solution-Processed Small Molecule:Perylene Diimide Bulk Heterojunction Solar Cell

The photovoltaic performance and charge generation dynamics in thin film bulk heterojunction organic photovoltaic (BHJ OPV) devices comprising the small molecule donor 7,7′-(4,4-bis­(2-ethylhexyl)-4H-silolo­[3,2-b:4,5-b′]­dithiophene-2,6-diyl)­bis­(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)­benzo­[c]­[1,2,5]­thiadiazole) (p-DTS­(FBTTh₂)₂) and a perylene diimide (PDI) electron acceptor are investigated with and without the processing additive 1,8-diiodooctane (DIO). UV–vis absorption spectroscopy indicates that the use of DIO during processing increases the structural order of both p-DTS­(FBTTh₂)₂ and PDI compared to films cast from chlorobenzene alone. Excitation intensity dependent broadband vis–NIR transient absorption pump–probe experiments over a dynamic range from 100 fs to 100 μs reveal that, in blends processed without DIO, essentially none of the interfacial charge transfer states generated after exciton dissociation at the donor–acceptor interface split into spatially separated charge carriers. In contrast, in blends processed with 0.4 vol% DIO, geminate recombination is significantly reduced, and spatially separated charge carriers are generated. It appears that the drastic increase in the power conversion efficiency in p-DTS­(FBTTh₂)₂:PDI BHJ OPV devices upon the use of DIO, from 0.13% to 3.1%, is a consequence of the increased solid state order of both p-DTS­(FBTTh₂)₂ and PDI, which leads to a significant improvement of the exciton dissociation efficiency and makes this system among the most efficient non-fullerene BHJ organic solar cells to date

Improved Morphology and Efficiency of n–i–p Planar Perovskite Solar Cells by Processing with Glycol Ether Additives

Planar perovskite solar cells can be prepared without high-temperature processing steps typically associated with mesoporous device architectures; however, their efficiency has been lower, and producing high-quality perovskite films in planar devices has been challenging. Here, we report a modified two-step interdiffusion protocol suitable to preparing pinhole-free perovskite films with greatly improved morphology. This is achieved by simple addition of small amounts of glycol ethers to the preparation protocol. We unravel the impact the glycol ethers have on the perovskite film formation using in situ ultraviolet–visible absorbance and grazing incidence wide-angle X-ray scattering experiments. From these experiments we conclude that addition of glycol ethers changes the lead iodide to perovskite conversion dynamics and enhances the conversion efficiency, resulting in more compact polycrystalline films, and it creates micrometer-sized perovskite crystals vertically aligned across the photoactive layer. Consequently, the average photovoltaic performance increases from 13.5% to 15.9%, and reproduciability is enhanced, specifically when 2-methoxyethanol is used as the additive

Impact of Nonfullerene Acceptor Core Structure on the Photophysics and Efficiency of Polymer Solar Cells

Small-molecule “nonfullerene” acceptors are promising alternatives to fullerene (PC61/71BM) derivatives often used in bulk heterojunction (BHJ) organic solar cells; yet, the efficiency-limiting processes and their dependence on the acceptor structure are not clearly understood. Here, we investigate the impact of the acceptor core structure (cyclopenta-[2,1-b:3,4-b′]­dithiophene (CDT) versus indacenodithiophene (IDTT)) of malononitrile (BM)-terminated acceptors, namely CDTBM and IDTTBM, on the photophysical characteristics of BHJ solar cells. Using PCE10 as donor polymer, the IDTT-based acceptor achieves power conversion efficiencies (8.4%) that are higher than those of the CDT-based acceptor (5.6%) because of a concurrent increase in short-circuit current and open-circuit voltage. Using (ultra)­fast transient spectroscopy we demonstrate that reduced geminate recombination in PCE10:IDTTBM blends is the reason for the difference in short-circuit currents. External quantum efficiency measurements indicate that the higher energy of interfacial charge-transfer states observed for the IDTT-based acceptor blends is the origin of the higher open-circuit voltage

Efficiency-Limiting Processes in Low-Bandgap Polymer:Perylene Diimide Photovoltaic Blends

The charge generation and recombination processes following photoexcitation of a low-bandgap polymer:perylene diimide photovoltaic blend are investigated by transient absorption pump–probe spectroscopy covering a dynamic range from femto- to microseconds to get insight into the efficiency-limiting photophysical processes. The photoinduced electron transfer from the polymer to the perylene acceptor takes up to several tens of picoseconds, and its efficiency is only half of that in a polymer:fullerene blend. This reduces the short-circuit current. Time-delayed collection field experiments reveal that the subsequent charge separation is strongly field-dependent, limiting the fill factor and lowering the short-circuit current in polymer:PDI devices. Upon excitation of the acceptor in the low-bandgap polymer blend, the PDI exciton undergoes charge transfer on a time scale of several tens of picoseconds. However, a significant fraction of the charges generated at the interface are quickly lost because of fast geminate recombination. This reduces the short-circuit current even further, leading to a scenario in which only around 25% of the initial photoexcitations generate free charges that can potentially contribute to the photocurrent. In summary, the key photophysical limitations of perylene diimide as an acceptor in low-bandgap polymer blends appear at the interface between the materials, with the kinetics of both charge generation and separation inhibited as compared to that of fullerenes

Electron-Exchange-Assisted Photon Energy Up-Conversion in Thin Films of π-Conjugated Polymeric Composites

The mechanism of triplet–triplet annihilation (TTA)-induced up-converted (UC) delayed luminescence is studied in two different binary organic systems consisting of platinum(II) octaethyl porphyrin (PtOEP) mixed with either poly(fluorene) (PF26) or ladder-type pentaphenylene (L5Ph). Cyclic voltammetry and differential pulse voltammetry are employed for estimating the ionization potentials of PtOEP and L5Ph. Delayed luminescence spectroscopy sets the energy of the lowest excited triplet state of L5Ph 0.20 eV higher than the triplet state of PtOEP (1.90 eV). The different phosphorescence PtOEP lifetime indicates differences in PtOEP aggregation in the polymer matrices. The presented results propose that the difference in the relative intensities of the delayed UC luminescence is determined by the difference between the ionization potentials of PtOEP and the polymer matrix. In the solid state, the electric-field-induced quenching of the delayed L5Ph UC luminescence suggests the formation of an intermediate charge-transfer state after the TTA within the PtOEP domains

Effect of Charge Transfer in Magnetic-Plasmonic Au@MO_<i>x</i> (M = Mn, Fe) Heterodimers on the Kinetics of Nanocrystal Formation

Heteronanoparticles represent a new class of nanomaterials exhibiting multifunctional and collective properties, which could find applications in medical imaging and therapy, catalysis, photovoltaics, and electronics. This present work demonstrates the intrinsic heteroepitaxial linkage in heterodimer nanoparticles to enable interaction of the individual components across their interface. It revealed distinct differences between Au@MnO and Au@Fe₃O₄ regarding the synthetic procedure and growth kinetics, as well as the properties to be altered by the variation of the electronic structure of the metal oxides. The chemically related metal oxides differ concerning their band gap; while MnO is a Mott-Hubbard insulator with a large band gap, Fe₃O₄ is a semimetal with thermally activated conductivity. The fluorescence dynamics indicate a prolonged relaxation time (>2 ns) for electrons of the conduction band of the Au nanoparticles after interfacing to Fe₃O₄. Here, the semiconductor is not depleted and forms an ohmic contact to the Au domain. In contrast, the fluorescence dynamics and ESCA of Au@MnO affirmed the weak interaction with the electrons of the Au domain, where the junction behaves as a Schottky barrier

Synthesis of Functional Block Copolymers Carrying One Poly(<i>p</i>‑phenylenevinylene) and One Nonconjugated Block in a Facile One-Pot Procedure

Block copolymers composed of a MEH–PPV block and a nonconjugated functional block (molecular weights between 5 and 90 kg/mol) were synthesized in a facile one-pot procedure via ROMP. This one-pot procedure permits the synthesis of numerous block copolymers with little effort. Amphiphilic block copolymers were obtained via incorporation of oxanorbornene carrying a PEG side chain as well as via postpolymerization modification of a reactive ester carrying norbornene derivative with methoxy­poly­(ethylene glycol)­amine. These amphiphilic block copolymers can be self-assembled into micelles exhibiting different sizes (60–95 nm), morphologies (micelles or fused, caterpillar-like micelles), and optical properties depending on the polymer composition and the micellization procedure. Furthermore, the reactive ester carrying block copolymers enabled the introduction of anchor groups which facilitated the preparation of nanocomposites with CdSe/CdZnS core–shell QDs. The obtained composites were studied using time-resolved photoluminescence measurements. The results revealed an increased interaction based on an accelerated decay of the QD emission for composites as compared to the mixture of the QDs with unfunctionalized polymers