The Effect of the Alkyl Chains of the Alkylammonium Pesudohalide Additives on the Performance of Dion Jacobson Perovskite Solar Cells.

Dion-Jacobson perovskite (DJP) films suffer from the high structural disorder and non-compact morphology, leading to inefficient and unstable solar cells (SCs). Here, how the alkyl chains of alkylammonium pseudohalide additives including methylammonium thiocyanate (MASCN) and ethylammonium thiocyanate (EASCN), and propylammonium thiocyanate (PASCN), impact the microstructures, optoelectronic properties and the performance of the solar cells is investigated. These additives substantially improve the structural order and the morphology of the DJP films, yielding more efficient and stable solar cells than the control device. They behave quite differently in modifying the morphological features. Particularly, EASCN outstands the additives in terms of the superior morphology, which is compact and uniform and consists of the largest flaky grains. Consequently, the corresponding device delivers a power conversion efficiency (PCE) of 15.27% and maintains ≈86% of the initial PCE after aging in the air for 182 h. Conversely, MASCN as an additive produces uneven DJP film and the device maintains only 46% of the initial PCE. PASCN as an additive produces the finest grains in the DJP film, and the corresponding device yields a PCE of 11.95%. From the economical point of view, it costs 0.0025 yuan per device for the EASCN additive, allowing for cost-effective perovskite solar cells.

Impact of backbone fluorination on nanoscale morphology and excitonic coupling in polythiophenes.

Fluorination represents an important strategy in developing high-performance conjugated polymers for photovoltaic applications. Here, we use regioregular poly(3-ethylhexylthiophene) (P3EHT) and poly(3-ethylhexyl-4-fluorothiophene) (F-P3EHT) as simplified model materials, using single-molecule/aggregate spectroscopy and molecular dynamic simulations, to elucidate the impacts of backbone fluorination on morphology and excitonic coupling on the molecular scale. Despite its high regioregularity, regioregular P3EHT exhibits a rather broad distribution in polymer chain conformation due to the strong steric hindrance of bulky ethylhexyl side chains. This conformational variability results in disordered interchain morphology even between a few chains, prohibiting long-range effective interchain coupling. In stark contrast, the experimental and molecular dynamic calculations reveal that backbone fluorination of F-P3EHT leads to an extended rod-like single-chain conformation and hence highly ordered interchain packing in aggregates. Surprisingly, the ordered and close interchain packing in F-P3EHT does not lead to strong excitonic coupling between the chains but rather to dominant intrachain excitonic coupling that greatly reduces the molecular energetic heterogeneity.

Hybridization of Local Exciton and Charge-Transfer States Reduces Nonradiative Voltage Losses in Organic Solar Cells.

A number of recent studies have shown that the nonradiative voltage losses in organic solar cells can be suppressed in systems with low energetic offsets between donor and acceptor molecular states, but the physical reasons underpinning this remain unclear. Here, we present a systematic study of 18 different donor/acceptor blends to determine the effect that energetic offset has on both radiative and nonradiative recombination of the charge-transfer (CT) state. We find that, for certain blends, low offsets result in hybridization between charge-transfer and lowest donor or acceptor exciton states, which leads to a strong suppression in the nonradiative voltage loss to values as low as 0.23 V associated with an increase in the luminescence of the CT state. Further, we extend a two-state CT-state recombination model to include the interaction between CT and first excited states, which allows us to explain the low nonradiative voltage losses as an increase in the effective CT to ground state oscillator strength due to the intensity borrowing mechanism. We show that low nonradiative voltage losses can be achieved in material combinations with a strong electronic coupling between CT and first excited states and where the lower band gap material has a high oscillator strength for transitions from the excited state to the ground state. Finally, from our model we propose that achieving very low nonradiative voltage losses may come at a cost of higher overall recombination rates, which may help to explain the generally lower FF and EQE of highly hybridized systems.

Alkylated Selenophene-Based Ladder-Type Monomers via a Facile Route for High-Performance Thin-Film Transistor Applications.

We report the synthesis of two new selenophene-containing ladder-type monomers, cyclopentadiselenophene (CPDS) and indacenodiselenophene (IDSe), via a 2-fold and 4-fold Pd-catalyzed coupling with a 1,1-diborylmethane derivative. Copolymers with benzothiadiazole were prepared in high yield by Suzuki polymerization to afford materials which exhibited excellent solubility in a range of nonchlorinated solvents. The CPDS copolymer exhibited a band gap of just 1.18 eV, which is among the lowest reported for donor-acceptor polymers. Thin-film transistors were fabricated using environmentally benign, nonchlorinated solvents, with the CPDS and IDSe copolymers exhibiting hole mobility up to 0.15 and 6.4 cm2 V-1 s-1, respectively. This high performance was achieved without the undesirable peak in mobility often observed at low gate voltages due to parasitic contact resistance.

Alternating 5,5-Dimethylcyclopentadiene and Diketopyrrolopyrrole Copolymer Prepared at Room Temperature for High Performance Organic Thin-Film Transistors.

We report that the inclusion of nonaromatic 5,5-dimethylcyclopentadiene monomer into a conjugated backbone is an attractive strategy to high performance semiconducting polymers. The use of this monomer enables a room temperature Suzuki copolymerization with a diketopyrrolopyrrole comonomer to afford a highly soluble, high molecular weight material. The resulting low band gap polymer exhibits excellent photo and thermal stability, and despite a large π-π stacking distance of 4.26 Å, it demonstrates excellent performance in thin-film transistor devices.

2D coherent charge transport in highly ordered conducting polymers doped by solid state diffusion.

Doping is one of the most important methods to control charge carrier concentration in semiconductors. Ideally, the introduction of dopants should not perturb the ordered microstructure of the semiconducting host. In some systems, such as modulation-doped inorganic semiconductors or molecular charge transfer crystals, this can be achieved by spatially separating the dopants from the charge transport pathways. However, in conducting polymers, dopants tend to be randomly distributed within the conjugated polymer, and as a result the transport properties are strongly affected by the resulting structural and electronic disorder. Here, we show that in the highly ordered lamellar microstructure of a regioregular thiophene-based conjugated polymer, a small-molecule p-type dopant can be incorporated by solid state diffusion into the layers of solubilizing side chains without disrupting the conjugated layers. In contrast to more disordered systems, this allows us to observe coherent, free-electron-like charge transport properties, including a nearly ideal Hall effect in a wide temperature range, a positive magnetoconductance due to weak localization and the Pauli paramagnetic spin susceptibility.

Effect of a heavy heteroatom on triplet formation and interactions in single conjugated polymer molecules and aggregates.

Triplet formation and interactions with emissive singlet excitons are investigated in poly(3-hexylselenophene) (P3HS) using single molecule spectroscopy. P3HS is a heavy atom analog of the more commonly studied poly(3-hexylthiophene) (P3HT), a benchmark polymer for solar cells. P3HS tends to aggregate strongly which necessitates dilution to ultra-low levels within a solid inert host in order to resolve photophysical responses of single chains. Fluorescence excitation intensity modulation is performed on isolated P3HS chains using a sequence of rectangular pulses of varying intensities to probe the presence of spin-forbidden triplet excitons. Triplet population dynamics originating from singlet-triplet and triplet-triplet interactions appear as quenching of the initial fluorescence intensity to steady-state levels on characteristic time scales of ∼1-10 µs. Over 80% of all molecules studied display significant fluorescence intensity modulation (quenching depths >50%) indicative of efficient intersystem crossing and large triplet occupancies. Because triplets are highly localized and singlet-triplet and triplet-triplet annihilation rate constants are comparable to those of intersystem crossing, multiple triplets are present at any given time on single P3HS chains. Triplet lifetimes were estimated to be ∼4 µs (upper limit) determined from recovery to the ground electronic singlet state in the absence of light and, surprisingly, triplets vanish at the onset of P3HS aggregation. This result was unexpected since P3HS triplet formation takes place on time scales <30 ps making this process competitive with most accessible non-radiative deactivation pathways.

Comparing blends and blocks: Synthesis of partially fluorinated diblock polythiophene copolymers to investigate the thermal stability of optical and morphological properties.

The microstructure of the active blend layer has been shown to be a critically important factor in the performance of organic solar devices. Block copolymers provide a potentially interesting avenue for controlling this active layer microstructure in solar cell blends. Here we explore the impact of backbone fluorination in block copolymers of poly(3-octyl-4-fluorothiophene)s and poly(3-octylthiophene) (F-P3OT-b-P3OT). Two block co-polymers with varying block lengths were prepared via sequential monomer addition under Kumada catalyst transfer polymerisation (KCTP) conditions. We compare the behavior of the block copolymer to that of the corresponding homopolymer blends. In both types of system, we find the fluorinated segments tend to dominate the UV-visible absorption and molecular vibrational spectral features, as well as the thermal behavior. In the block copolymer case, non-fluorinated segments appear to slightly frustrate the aggregation of the more fluorinated block. However, in situ temperature dependent Raman spectroscopy shows that the intramolecular order is more thermally stable in the block copolymer than in the corresponding blend, suggesting that such materials may be interesting for enhanced thermal stability of organic photovoltaic active layers based on similar systems.

Influence of Backbone Fluorination in Regioregular Poly(3-alkyl-4-fluoro)thiophenes.

We report two strategies toward the synthesis of 3-alkyl-4-fluorothiophenes containing straight (hexyl and octyl) and branched (2-ethylhexyl) alkyl groups. We demonstrate that treatment of the dibrominated monomer with 1 equiv of alkyl Grignard reagent leads to the formation of a single regioisomer as a result of the pronounced directing effect of the fluorine group. Polymerization of the resulting species affords highly regioregular poly(3-alkyl-4-fluoro)thiophenes. Comparison of their properties to those of the analogous non-fluorinated polymers shows that backbone fluorination leads to an increase in the polymer ionization potential without a significant change in optical band gap. Fluorination also results in an enhanced tendency to aggregate in solution, which is ascribed to a more co-planar backbone on the basis of Raman and DFT calculations. Average charge carrier mobilities in field-effect transistors are found to increase by up to a factor of 5 for the fluorinated polymers.

A close look at charge generation in polymer:fullerene blends with microstructure control.

We reveal some of the key mechanisms during charge generation in polymer:fullerene blends exploiting our well-defined understanding of the microstructures obtained in pBTTT:PCBM systems via processing with fatty acid methyl ester additives. Based on ultrafast transient absorption, electro-absorption, and fluorescence up-conversion spectroscopy, we find that exciton diffusion through relatively phase-pure polymer or fullerene domains limits the rate of electron and hole transfer, while prompt charge separation occurs in regions where the polymer and fullerene are molecularly intermixed (such as the co-crystal phase where fullerenes intercalate between polymer chains in pBTTT:PCBM). We moreover confirm the importance of neat domains, which are essential to prevent geminate recombination of bound electron-hole pairs. Most interestingly, using an electro-absorption (Stark effect) signature, we directly visualize the migration of holes from intermixed to neat regions, which occurs on the subpicosecond time scale. This ultrafast transport is likely sustained by high local mobility (possibly along chains extending from the co-crystal phase to neat regions) and by an energy cascade driving the holes toward the neat domains.

Direct Correlation of Charge Transfer Absorption with Molecular Donor:Acceptor Interfacial Area via Photothermal Deflection Spectroscopy.

Here we show that the charge transfer (CT) absorption signal in bulk-heterojunction solar cell blends, measured by photothermal deflection spectroscopy, is directly proportional to the density of molecular donor:acceptor interfaces. Since the optical transitions from the ground state to the interfacial CT state are weakly allowed at photon energies below the optical gap of both the donor and acceptor, we can exploit the use of this sensitive linear absorption spectroscopy for such quantification. Moreover, we determine the absolute molar extinction coefficient of the CT transition for an archetypical polymer:fullerene interface. The latter is ∼100 times lower than the extinction coefficient of the donor chromophore involved, allowing us to experimentally estimate the transition dipole moment as 0.3 D and the electronic coupling between the ground and CT states to be on the order of 30 meV.

Influence of side-chain regiochemistry on the transistor performance of high-mobility, all-donor polymers.

Three novel polythiophene isomers are reported whereby the only difference in structure relates to the regiochemistry of the solubilizing side chains on the backbone. This is demonstrated to have a significant impact on the optoelectronic properties of the polymers and their propensity to aggregate in solution. These differences are rationalized on the basis of differences in backbone torsion. The polymer with the largest effective conjugation length is demonstrated to exhibit the highest field-effect mobility, with peak values up to 4.6 cm(2) V(-1) s(-1).

Fused dithienogermolodithiophene low band gap polymers for high-performance organic solar cells without processing additives.

We report the synthesis of a novel ladder-type fused ring donor, dithienogermolodithiophene, in which two thieno[3,2-b]thiophene units are held coplanar by a bridging dialkyl germanium. Polymerization of this extended monomer with N-octylthienopyrrolodione by Stille polycondensation afforded a polymer, pDTTG-TPD, with an optical band gap of 1.75 eV combined with a high ionization potential. Bulk heterojunction solar cells based upon pDTTG-TPD:PC(71)BM blends afforded efficiencies up to 7.2% without the need for thermal annealing or processing additives.

Modulating the Alkylation Position on Terminal Thiophene Ring of Naphtho[2,3-b:6,7-b'] Bithieno[2,3-d] Thiophene (NBTT) for High-Performance Organic Optoelectronic Devices.

Several thiophene terminated thienoacenes with high mobilities in organic thin-film transistors (OTFTs) have been reported; however, the structure-property relationship of thiophene terminated thienoacenes was unclear, especially the impact of α or ß position substitution of terminal thiophene ring on molecular packing and physicochemical properties. Here, we report the synthesis and characterization of a six-ring-fused naphtho[2,3-b:6,7-b'] bithieno[2,3-d] thiophene (NBTT) and its derivatives 2,8-dioctyl-naphtho[2,3-b:6,7-b'] bithieno [2,3-d] thiophene (2,8-C8NBTT) and 3,9-dioctyl-naphtho[2,3-b:6,7-b'] bithieno [2,3-d] thiophene (3,9-C8NBTT). It is found that the alkylation on terminal thiophene ring can effectively tune the molecular stacking from a cofacial herringbone stacking mode (NBTT) to layer-by-layer packing (2,8-C8NBTT and 3,9-C8NBTT). Impressively, a hopping to "band-like" charge transport mechanism evolution of vacuum deposited films is realized by modulating the alkylation position on the terminal thiophene rings. As a result, the OTFTs based on 2,8-C8NBTT characterized by a "band-like" transport presents the highest mobility of 3.58 cm2 V-1 s-1 together with a remarkably high current on/off ratio around 109. Furthermore, organic phototransistors (OPTs) based on 2,8-C8NBTT thin film also exhibits higher photosensitivity (P) of 2.0 × 108, photoresponsivity (R) of 3.3 × 103 A W-1, and detectivity (D*) of 1.3 × 1016 Jones than those based on NBTT and 3,9-C8NBTT.

Near-Infrared Acceptors with Imide-Containing End Groups for Organic Solar Cells.

Near-infrared electron acceptors for organic solar cells (OSCs) mostly contain electron-withdrawing 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC) end groups, which can be modified by but limited to phenyl, thienyl, and naphthyl units with halogenated, methyl, and methyloxy substitution. In this work, we employed an imide-containing unit to construct a new IC end group, based on which a series of new electron acceptors were synthesized. The strong electron-deficient nature of imide units enables the new acceptors to show efficient intramolecular charge transfer and hence red-shifted absorption spectra compared to their IC counterparts. These new electron acceptors were applied to OSCs, providing efficiencies of over 17% with a low voltage loss of 0.52 eV. These results demonstrate that the new imide-containing end groups are promising fragments for the construction of near-infrared electron acceptors for high-performance OSCs.

p-Type Conjugated Polymers Containing Electron-Deficient Pentacyclic Azepinedione.

Bisthienoazepinedione (BTA) has been reported for constructing high-performing p-type conjugated polymers in organic electronics, but the ring extended version of BTA is not well explored. In this work, we report a new synthesis of a key building block to the ring expanded electron-deficient pentacyclic azepinedione (BTTA). Three copolymers of BTAA with benzodithiophene substituted by different side chains are prepared. These polymers exhibit similar energy levels and optical absorption in solution and solid state, while significant differences are revealed in their film morphologies and behavior in transistor and photovoltaic devices. The best-performing polymers in transistor devices contained alkylthienyl side chains on the BDT unit (pBDT-BTTA-2 and pBDT-BTTA-3) and demonstrated maximum saturation hole mobilities of 0.027 and 0.017 cm2 V-1 s-1. Blends of these polymers with PC71BM exhibited a best photovoltaic efficiency of 6.78% for pBDT-BTTA-3-based devices. Changing to a low band gap non-fullerene acceptor (BTP-eC9) resulted in improved efficiency of up to 13.5%. Our results are among the best device performances for BTA and BTTA-based p-type polymers and highlight the versatile applications of this electron-deficient BTTA unit.

Observing the On-Site Generation of Excitons and Charges by Low-Temperature Spectroscopy.

Understanding the relation between phase morphology and physical processes in polymer blends is the key to the fabrication of reproducible and reliable polymer optoelectronic devices. In this work, taking the advantage of low-temperature spectroscopy, we have observed the on-site generation of excitons and long-lived charges in different phase morphology polymer/fullerene blends. Probing at 10K, the photo-generated species are localized to where they are generated. We found that the generation of excitons and long-lived charges is highly influenced by the local molecular phase morphology. We further demonstrated that although the influence of phase morphology is localized to the place that excitons and long-lived charges are generated, this influence can persist over sub-millisecond timescales. Thus, we believe that the fate of excitons and long-lived charges is determined by the location at which they are generated, which can in turn be controlled precisely by molecular phase morphology.

Reconciling models of interfacial state kinetics and device performance in organic solar cells: impact of the energy offsets on the power conversion efficiency.

Achieving the simultaneous increases in the open circuit voltage (V oc), short circuit current (J sc) and fill factor (FF) necessary to further increase the power conversion efficiency (PCE) of organic photovoltaics (OPV) requires a unified understanding of how molecular and device parameters affect all three characteristics. In this contribution, we introduce a framework that for the first time combines different models that have been used separately to describe the different steps of the charge generation and collection processes in OPV devices: a semi-classical rate model for charge recombination processes in OPV devices, zero-dimensional kinetic models for the photogeneration process and exciton dissociation and one-dimensional semiconductor device models. Using this unified multi-scale model in conjunction with experimental techniques (time-resolved absorption spectroscopy, steady-state and transient optoelectronic measurements) that probe the various steps involved in charge generation we can shed light on how the energy offsets in a series of polymer: non-fullerene devices affect the charge carrier generation, collection, and recombination properties of the devices. We find that changing the energy levels of the donor significantly affects not only the transition rates between local-exciton (LE) and charge-transfer (CT) states, but also significantly changes the transition rates between CT and charge-separated (CS) states, challenging the commonly accepted picture of charge generation and recombination. These results show that in order to obtain an accurate picture of charge generation in OPV devices, a variety of different experimental techniques under different conditions in conjunction with a comprehensive model of processes occurring at different time-scales are required.

Multibranched aliphatic side chains for π-conjugated polymers with a high density of 'unshielded' aromatics.

The synthesis of strongly solubilising multibranched aliphatic side chains for π-conjugated polymers is reported. The solubilising capability of the side chains and their effect on the polymer properties are studied on the example of copolymers composed of up to six unsubstituted, 'unshielded' thiophene units per side chain-substituted naphthalene diimide unit.

Tunable Control of the Hydrophilicity and Wettability of Conjugated Polymers by a Postpolymerization Modification Approach.

A facile method to prepare hydrophilic polymers by a postpolymerization nucleophillic aromatic substitution reaction of fluoride on an emissive conjugated polymer (CP) backbone is reported. Quantitative functionalization by a series of monofunctionalized ethylene glycol oligomers, from dimer to hexamer, as well as with high molecular weight polyethylene glycol is demonstrated. The length of the ethylene glycol sidechains is shown to have a direct impact on the surface wettability of the polymer, as well as its solubility in polar solvents. However, the energetics and band gap of the CPs remain essentially constant. This method therefore allows an easy way to modulate the wettability and solubility of CP materials for a diverse series of applications.