Organic Solar Cells Based on Non-Fullerene Low Molecular Weight Organic Semiconductor Molecules

The development of narrow bandgap A-D-A- and ADA'DA-type non-fullerene small molecule acceptors (NFSMAs) along with small molecule donors (SMDs) have led to significant progress in all-small molecule organic solar cells. Remarkable power conversion efficiencies, nearing the range of 17-18 %, have been realized. These efficiency values are on par with those achieved in OSCs based on polymeric donors. The commercial application of organic photovoltaic technology requires the design of more efficient organic conjugated small molecule donors and acceptors. In recent years the precise tuning of optoelectronic properties in small molecule donors and acceptors has attracted considerable attention and has contributed greatly to the advancement of all-SM-OSCs. Several reviews have been published in this field, but the focus of this review concerns the advances in research on OSCs using SMDs and NFSMAs from 2018 to the present. The review covers the progress made in binary and ternary OSCs, the effects of solid additives on the performance of all-SM-OSCs, and the recently developed layer-by-layer deposition method for these OSCs. Finally, we present our perspectives and a concise outlook on further advances in all-SM-OSCs for their commercial application.

Understanding photochemical degradation mechanisms in photoactive layer materials for organic solar cells

Over the past decades, the field of organic solar cells (OSCs) has witnessed a significant evolution in materials chemistry, which has resulted in a remarkable enhancement of device performance, achieving efficiencies of over 19%. The photoactive layer materials in OSCs play a crucial role in light absorption, charge generation, transport and stability. To facilitate the scale-up of OSCs, it is imperative to address the photostability of these electron acceptor and donor materials, as their photochemical degradation process remains a challenge during the photo-to-electric conversion. In this review, we present an overview of the development of electron acceptor and donor materials, emphasizing the crucial aspects of their chemical stability behavior that are linked to the photostability of OSCs. Throughout each section, we highlight the photochemical degradation pathways for electron acceptor and donor materials, and their link to device degradation. We also discuss the existing interdisciplinary challenges and obstacles that impede the development of photostable materials. Finally, we offer insights into strategies aimed at enhancing photochemical stability and discuss future directions for developing photostable photo-active layers, facilitating the commercialization of OSCs.

Green Energy by Hydrogen Production from Water Splitting, Water Oxidation Catalysis and Acceptorless Dehydrogenative Coupling

In this review, we want to explain how the burning of fossil fuels is pushing us towards green energy. Actually, for a long time, we have believed that everything is profitable, that resources are unlimited and there are no consequences. However, the reality is often disappointing. The use of non-renewable resources, the excessive waste production and the abandonment of the task of recycling has created a fragile thread that, once broken, may never restore itself. Metaphors aside, we are talking about our planet, the Earth, and its unique ability to host life, including ourselves. Our world has its balance; when the wind erodes a mountain, a beach appears, or when a fire devastates an area, eventually new life emerges from the ashes. However, humans have been distorting this balance for decades. Our evolving way of living has increased the number of resources that each person consumes, whether food, shelter, or energy; we have overworked everything to exhaustion. Scientists worldwide have already said actively and passively that we are facing one of the biggest problems ever: climate change. This is unsustainable and we must try to revert it, or, if we are too late, slow it down as much as possible. To make this happen, there are many possible methods. In this review, we investigate catalysts for using water as an energy source, or, instead of water, alcohols. On the other hand, the recycling of gases such as CO2 and N2O is also addressed, but we also observe non-catalytic means of generating energy through solar cell production.

Influence of P3HT:PCBM Ratio on Thermal and Transport Properties of Bulk Heterojunction Solar Cells

The influence of P3HT:PCBM ratio on thermal and transport properties of solar cells were determined by photothermal beam deflection spectrometry, which is advantageous tool for non-destructively study of bulk heterojunction layers of organic solar cells. P3HT:PCBM layers of different P3HT:PCBM ratios were deposited on top of PEDOT:PSS/ITO layers which were included in organic bulk-heterojunction solar cells. The thermal diffusivity, energy gap and charge carrier lifetime were measured at different illumination conditions and with a different P3HT:PCBM ratios. As expected, it was found that the energy band gap depends on the P3HT:PCBM ratio. Thermal diffusivity is decreasing, while charge carrier lifetime is increasing with PCBM concentration. Energy band gap was found to be independent on illumination intensity, while thermal diffusivity was increasing and carrier lifetime was decreasing with illumination intensity. The carrier lifetime exhibits qualitatively similar dependence on the PCBM concentration when compared to the open-circuit voltage of operating solar cells under AM1.5 illumination. BDS and standard I-V measurement yielded comparable results arguing that the former is suitable for characterization of organic solar cells.

In Situ Optical Spectroscopy Demonstrates the Effect of Solvent Additive in the Formation of All-Polymer Solar Cells

1-Chloronaphthalene (CN) has been a common solvent additive in both fullerene- and nonfullerene-based organic solar cells. In spite of this, its working mechanism is seldom investigated, in particular, during the drying process of bulk heterojunctions composed of a donor:acceptor mixture. In this work, the role of CN in all-polymer solar cells is investigated by in situ spectroscopies and ex situ characterization of blade-coated PBDB-T:PF5-Y5 blends. Our results suggest that the added CN promotes self-aggregation of polymer donor PBDB-T during the drying process of the blend film, resulting in enhanced crystallinity and hole mobility, which contribute to the increased fill factor and improved performance of PBDB-T:PF5-Y5 solar cells. Besides, the nonradiative energy loss of the corresponding device is also reduced by the addition of CN, corresponding to a slightly increased open-circuit voltage. Overall, our observations deepen our understanding of the drying dynamics, which may guide further development of all-polymer solar cells.

Covalently linked benzothiadiazole-fullerene adducts for organic optoelectronic devices: synthesis and characterization

Fullerene adducts have attracted attention in a variety of applications including organic optoelectronic devices. In this regard, we have designed a covalently linked donor-acceptor dyad comprising a fluorobenzothiadiazole-thiophene (BTF₂-Th) unit with the electron acceptor fullerene in an Acceptor-Donor-Acceptor (A-D-A) molecular arrangement. We synthesized and characterized two new covalently bonded benzothiadiazole-based fullerene molecules, mono-adduct, 7 (benzothiadiazole : PC₆₁BM = 1 : 1, anchored terminally via esterification reaction) and multi-adduct, 10-I (benzothiadiazole : PC₆₁BM = n : 1, where n ≥ 1, attached directly to the fullerene core via the Prato reaction) using different synthetic strategies. A broadening of the UV-visible spectra of the modified fullerene derivative with strong absorption from 350 to 500 nm and at low wavelengths is observed as compared to PC₆₁BM. A suitable bandgap, good electronic conductivity, and appreciable solubility in solvents suggest their utility in optoelectronic devices. The mono-adduct 7 showed two-order higher electron mobility as compared to bis-adduct 10-I due to retention of extended conjugation in fullerene, as in the case of PC₆₁BM. Experimentally determined optical properties and energy levels of the fullerene adducts were found to be in good agreement and supported by theoretical calculations. The presence of BTF₂ affects the ground state dipole moments as well as the absorption strengths, most noticeable in the case of two attached BTF₂ moieties. The HOMO and LUMO levels are found to be localized on the fullerene cage with the extension of the HOMO to the BTF₂ unit more and the same is noticed in ground state dipole moment in the side-chain functionalized structure. Such structural arrangement provides easy charge transfer between acceptor and donor units to allow a concomitant effect of favorable optoelectronic properties, energy levels of the frontier orbitals, effective exciton dissociation, and charge transport which may reduce processing complexity to advance single material-based future optoelectronic devices.

Computational Evolution of High-Performing Unfused Non-Fullerene Acceptors for Organic Solar Cells

Materials optimization for organic solar cells (OSCs) is a highly active field, with many approaches using empirical experimental synthesis, computational brute-force approaches to screen candidates in a given subset of chemical space, or generative machine learning methods which often require significant training sets. While these methods may find high-performing materials, they can be inefficient and time-consuming. Genetic algorithms (GAs) are an alternative approach, allowing for the "virtual synthesis" of molecules and a prediction of their ``fitness' for some property, with new candidates suggested based on good characteristics of previously generated molecules. In this work, a GA is used to discover high-performing unfused non-fullerene acceptors (NFAs) based on an empirical prediction of power conversion efficiency (PCE) and provides design rules for future work. The electron withdrawing/donating strength, as well as the sequence and symmetry of those units are examined. The utilization of a GA over a brute force approach resulted in speedups up to $1.8 \times 10^{12}$. New types of units not frequently seen in OSCs are suggested, and in total 5,426 NFAs are discovered with the GA. Of these, 1,087 NFAs are predicted to have a PCE greater than 18\%, which is roughly the current record efficiency. While the symmetry of the sequence showed no correlation with PCE, analysis of the sequence arrangement revealed that higher performance can be achieved with a donor core and acceptor end groups. Future NFA designs should consider this strategy as an alternative to the current A-D-A$'$-D-A architecture.

In Situ Optical Studies on Morphology Formation in Organic Photovoltaic Blends

The efficiency of bulk heterojunction (BHJ) based organic solar cells is highly dependent on the morphology of the blend film, which is a result of a fine interplay between donor, acceptor, and solvent during the film drying. In this work, a versatile set-up of in situ spectroscopies is used to follow the morphology evolution during blade coating of three iconic BHJ systems, including polymer:fullerene, polymer:nonfullerene small molecule, and polymer:polymer. the drying and photoluminescence quenching dynamics are systematically study during the film formation of both pristine and BHJ films, which indicate that the component with higher molecular weight dominates the blend film formation and the final morphology. Furthermore, Time-resolved photoluminescence, which is employed for the first time as an in situ method for such drying studies, allows to quantitatively determine the extent of dynamic and static quenching, as well as the relative change of quantum yield during film formation. This work contributes to a fundamental understanding of microstructure formation during the processing of different blend films. The presented setup is considered to be an important tool for the future development of blend inks for solution-cast organic or hybrid electronics.

Control over π-π stacking of heteroheptacene-based nonfullerene acceptors for 16% efficiency polymer solar cells

Nonfullerene acceptors are being investigated for use in polymer solar cells (PSCs), with their advantages of extending the absorption range, reducing the energy loss and therefore enhancing the power conversion efficiency (PCE). However, to further boost the PCE, mobilities of these nonfullerene acceptors should be improved. For nonfullerene acceptors, the π-π stacking distance between cofacially stacked molecules significantly affects their mobility. Here, we demonstrate a strategy to increase the mobility of heteroheptacene-based nonfullerene acceptors by reducing their π-π stacking distances via control over the bulkiness of lateral side chains. Incorporation of 2-butyloctyl substituents into the nonfullerene acceptor (M36) leads to an increased mobility with a reduced π-π stacking distance of 3.45 Å. Consequently, M36 affords an enhanced PCE of 16%, which is the highest among all acceptor-donor-acceptor-type nonfullerene acceptors to date. This strategy of control over the bulkiness of side chains on nonfullerene acceptors should aid the development of more efficient PSCs.

Efficient Low-Cost All-Flexible Microcavity Semitransparent Polymer Solar Cells Enabled by Polymer Flexible One-Dimensional Photonic Crystals

High-efficiency low-cost all-flexible microcavity semitransparent polymer solar cells (STPSCs) are comparatively studied in both fullerene- and nonfullerene-based systems with the structure strategy, employing polymer flexible one-dimensional photonic crystals (F-1DPCs), as well as indium tin oxide-free UV/ozone plasma-treated polymer/ultrathin metal and PEDOT:PSS transparent electrodes. Based on the reasonable optimization of electrical and optical characteristics in the device, the maximum power conversion efficiency with the use of polymer F-1DPCs can be greatly improved by 24-27% compared to the respective control devices. The improvement in J_SC is comprehensively discussed, which is mainly ascribed to the enhancement of effective photon absorption in the device. Although color tunability of fullerene and nonfullerene all-flexible STPSCs can be easily achieved with the use of different photonic band gaps of polymer F-1DPCs, the CIE coordinates of nonfullerene STPSCs differ a lot from the original light source compared to the fullerene ones because of a high absorption coefficient in a narrow wavelength region. This work presents an easy and effective microcavity device strategy incorporated with different elements and demonstrates a new sketch of structure-absorption-performance relationships for fullerene- versus nonfullerene-based all-flexible STPSCs, which is compatible with low-cost roll-to-roll manufacturing and surely has a diversity of potential applications to better meet specific needs.

Structural and electronic properties of a CN fullerene with N = 20, 60, 80, 180, and 240

In this paper, the structural and electronic properties of a C_N fullerene with N = 20, 60, 80, 180, and 240 have been investigated using a sp³ tight-binding model. The analytical expressions for the calculation of the total number of carbon atoms, hexagons, pentagons, and bonds found within the geometrical structure of a C_N fullerene have been developed and verified using the simulation, therefore proving the validation of both the simulation and analytical results. The simulation results show that the total number of carbon atoms within fullerene is equal to the value of N and the total number of hexagons, pentagons, and bonds within the structure of a fullerene increases with the increase in the value of N. Further, the electronic properties of these fullerenes have been identified with the help of their energy level diagrams obtained using the simulation. It has been observed that the C₂₀ and C₈₀ fullerenes are metallic because of their zero band gaps while the C₆₀ fullerene is an insulator with a very wide band gap of 5 eV whereas the C₁₈₀ and C₂₄₀ fullerenes are semiconducting with band gaps of 1.43 eV and 1.05 eV, respectively. Finally, it has been observed from these studies that the metallic fullerenes are best suited for interconnects and the semiconducting fullerenes are bested suited as a channel material for designing high-performance nanoelectronic devices.

A Comparative Study on Hole Transfer Inversely Correlated with Driving Force in Two Non-Fullerene Organic Solar Cells

We report a faster rate of hole transfer under a smaller Δ_HOMO in a comparative study of two group organic solar cells (OSCs) consisting of IT-4F as an acceptor and PBDBT and PBDBT-SF as donors. In the OSCs based on PBDBT-SF:IT-4F, a higher short-circuit current (J_SC) was observed with a Δ_HOMO of 0.31 eV compared to a lower J_SC in PBDBT:IT-4F OSCs with a larger Δ_HOMO (0.45 eV). Intensive investigation indicates that the rate of transfer of a hole from IT-4F to PBDBT-SF or PBDBT is inversely proportional to the Δ_HOMO between IT-4F and donors. The larger J_SC in the PBDBT-SF:IT-4F device is attributed to a synergy of faster hole transfer, slower recombination, and rapid charge extraction enabled by desired morphology and balanced charge carrier mobilities with PBDBT-SF, suggesting that under a sufficiently high Δ_HOMO, comprehensive considerations of the transport, film morphology, and energy levels are needed when designing new materials for high-performance OSCs.

Limitations and Perspectives on Triplet-Material-Based Organic Photovoltaic Devices

Organic photovoltaic cells (OPVs) have attracted broad attention and become a very energetic field after the emergence of nonfullerene acceptors. Long-lifetime triplet excitons are expected to be good candidates for efficiently harvesting a photocurrent. Parallel with the development of OPVs based on singlet materials (S-OPVs), the potential of triplet materials as photoactive layers has been explored. However, so far, OPVs employing triplet materials in a bulk heterojunction have not exhibited better performance than S-OPVs. Here, the recent progress of representative OPVs based on triplet materials (T-OPVs) is briefly summarized. Based on that, the performance limitations of T-OPVs are analyzed. The shortage of desired triplet materials with favorable optoelectronic properties for OPVs, the tradeoff between long lifetime and high binding energy of triplet excitons, as well as the low charge mobility in most triplet materials are crucial issues restraining the efficiencies of T-OPVs. To overcome these limitations, first, novel materials with desired optoelectronic properties are urgently demanded; second, systematic investigation on the contribution and dynamics of triplet excitons in T-OPVs is necessary; third, close multidisciplinary collaboration is required, as proved by the development of S-OPVs.

Singlet/triplet exciton dissociation and charge recombination in donor-acceptor ThQs-C60 /PDIxCN2 complexes

The donor-acceptor interface plays a critically important role in determining the power conversion efficiency of organic solar cells via controlling charge separation (CS) and recombination (CR) processes. Here, we combine the electronic structure calculations with electron transfer rate theory to clarify the CS and CR processes in ThQs-C₆₀ /PDIxCN₂ donor-acceptor complexes. The results reveal that in ThQs-PDIxCN₂ the CS comes from both the dissociations of photo-induced singlet exciton and singlet fission-induced triplet exciton with a high efficiency, whereas in ThQs-C₆₀ only the singlet exciton dissociation can take place because the triplet exciton lies below the charge-transfer exciton. However, very high CR rates in ThQs-PDIxCN₂ obliterate the benefit of fast CS, inversely leading to the ThQ-C₆₀ complex with a better cell efficiency. The present results are consistent with experimental observation and may furnish a possible patten to improve the overall conversion efficiency. © 2018 Wiley Periodicals, Inc.

In Situ Synthesis of Ternary Block Copolymer/Homopolymer Blends for Organic Photovoltaics

A detailed investigation of in situ-synthesized all-conjugated block copolymer (BCP) compatibilized ternary blends containing poly(3-hexylthiophene) (P3HT) and poly{[ N, N'-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dibcarboximide)-2,6-diyl]- alt-5,5'-(2,2'-bithiophene)} (PNDIT2) as donor and acceptor polymers, respectively, is presented. Both polymers are incompatible and show strong segregation in blends, which renders compatibilization with their corresponding BCPs promising to enable nanometer-phase-separated structures suitable for excitonic devices. Here, we synthesize a ternary block copolymer/homopolymer blend system and investigate the phase behavior as a function of block copolymer molecular weight and different annealing conditions. The device performance decreases on increasing annealing temperatures. To understand this effect, morphological investigations including atomic force microscopy, high-resolution transmission electron microscopy (HR-TEM), and grazing incidence wide- and small-angle X-ray scattering (GIWAXS/GISAXS) are carried out. On comparing domain sizes of pristine and compatibilized blends obtained from GISAXS, a weak compatibilization effect appears to take place for the in situ-synthesized ternary systems. The effect of thermal annealing is most prevalent for all samples, which, for the highest annealing temperature above the melting point of PNDIT2 (310 °C), ultimately leads to a change from the face-on to edge-on orientation of PNDIT2, as seen in GIWAXS. This effect dominates and decreases all photovoltaic parameters, irrespective of whether a pristine or compatibilized blend is used.

Interaction of polyhydroxy fullerenes with ferrihydrite: adsorption and aggregation

The rapid development of nanoscience and nanotechnology, with thousands types of nanomaterials being produced, will lead to various environmental impacts. Thus, understanding the behaviors and fate of these nanomaterials is essential. This study focused on the interaction between polyhydroxy fullerenes (PHF) and ferrihydrite (Fh), a widespread iron (oxyhydr)oxide nanomineral and geosorbent. Our results showed that PHF were effectively adsorbed by Fh. The adsorption isotherm fitted the D-R model well, with an adsorption capacity of 67.1mg/g. The adsorption mean free energy of 10.72kJ/mol suggested that PHF were chemisorbed on Fh. An increase in the solution pH and a decrease of the Fh surface zeta potential were observed after the adsorption of PHF on Fh; moreover, increasing initial solution pH led to a reduction of adsorption. The Fourier transform infrared spectra detected a red shift of C-O stretching from 1075 to 1062cm^-1 and a decrease of Fe-O bending, implying the interaction between PHF oxygenic functional groups and Fh surface hydroxyls. On the other hand, PHF affected the aggregation and reactivity of Fh by changing its surface physicochemical properties. Aggregation of PHF and Fh with individual particle sizes increasing from 2nm to larger than 5nm was measured by atomic force microscopy. The uniform distribution of C and Fe suggested that the aggregates of Fh were possibly bridged by PHF. Our results indicated that the interaction between PHF and Fh could evidently influence the migration of PHF, as well as the aggregation and reactivity of Fh.

Polymer solar cells based on D–A low bandgap copolymers containing fluorinated side chains of thiadiazoloquinoxaline acceptor and benzodithiophene donor units

The increase in the fluorine atoms in the copolymer improves the power conversion efficiency of the polymer solar cell.

Dual Function of UV/Ozone Plasma-Treated Polymer in Polymer/Metal Hybrid Electrodes and Semitransparent Polymer Solar Cells

In this work, high-performance inverted indium tin oxide (ITO)-free semitransparent polymer solar cells are comprehensively investigated using a novel polymer/metal hybrid transparent electrode. The electrical and optical characteristics of hybrid electrodes are significantly enhanced by introducing UV/ozone plasma treatment on the polymer poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN), which is functioned as both a seed layer for ultrathin Ag metal electrode and an optical spacer for transparent devices. The optimized sheet resistance of PFN/Ag (12 nm) hybrid electrode is only half of the commercial ITO (9.4 vs 20.0 Ω sq^-1) and the high wavelength-dependent reflectance of hybrid electrode helps to increase the ITO-free device short-circuit current density. Furthermore, the interface property between PFN and ultrathin Ag is analyzed in detail and the optical field distribution is calculated for comparison. A high power conversion efficiency of 5.02%, which is increased by 35% compared to that of the ITO-based device, is achieved in the ITO-free semitransparent device in conjunction with an excellent average visible transmittance above 28% that is higher than the benchmark of 25% for power-generating window, indicating its great potential in building integrated photovoltaic systems in the future. Furthermore, the strategy is successfully developed for other polymer systems, suggesting the universal applicability for plastic electronics.

Regulating Molecular Aggregations of Polymers via Ternary Copolymerization Strategy for Efficient Solar Cells

For many high-performance photovoltaic materials in polymer solar cells (PSCs), the active layers usually need to be spin-coated at high temperature due to the strong intermolecular aggregation of donor polymers, which is unfavorable in device repeatability and large-scale PSC printing. In this work, we adopted a ternary copolymerization strategy to regulate polymer solubility and molecular aggregation. A series of D-A₁-D-A₂ random polymers based on different acceptors, strong electron-withdrawing unit ester substituted thieno[3,4-b]thiophene (TT-E), and highly planar dithiazole linked TT-E (DTzTT) were constructed to realize the regulation of molecular aggregation and simplification of device fabrication. The results showed that as the relative proportion of TT-E segment in the backbone increased, the absorption evidently red-shifted with a gradually decreased aggregation in solution, eventually leading to the active layers that can be fabricated at low temperature. Furthermore, due to the excellent phase separation and low recombination, the optimized solar cells based on the terpolymer P1 containing 30% of TT-E segment exhibit high power conversion efficiency (PCE) of 9.09% with a significantly enhanced fill factor up to 72.86%. Encouragingly, the photovoltaic performance is insensitive to the fabrication temperature of the active layer, and it still could maintain high PCE of 8.82%, even at room temperature. This work not only develops the highly efficient photovoltaic materials for low temperature processed PSCs through ternary copolymerization strategy but also preliminarily constructs the relationship between aggregation and photovoltaic performance.

Stacking Sequence and Acceptor Dependence of Photocurrent Spectra and Photovoltage in Organic Two-Junction Devices

Both single-junction and tandem organic photovoltaic cells have been well developed. A tandem cell contains two junctions with a charge recombination layer (CRL) inserted between the two junctions. So far, there is no detailed report on how the device will perform that contains two junctions but without a CRL in between. In this work, we report the photocurrent spectra and photovoltage output of the devices that contains two bulk-heterojunctions (BHJ) stacked directly on top of each other without a CRL. The top active layer is prepared by transfer printing. The photocurrent response spectra and photovoltage are found to be sensitive to stacking sequence and the selection of electron acceptors. The open-circuit voltage of the devices (up to 1.09 V) can be higher than the devices containing either junction layer. The new phenomenon in the new device architecture increases the versatility of the optoelectronic devices based on organic semiconductors.

Applications of Fluorogens with Rotor Structures in Solar Cells

Solar cells are devices that convert light energy into electricity. To drive greater adoption of solar cell technologies, higher cell efficiencies and reductions in manufacturing cost are necessary. Fluorogens containing rotor structures may be helpful in addressing some of these challenges due to their unique twisted structures and photophysics. In this review, we discuss the applications of rotor-containing molecules as dyes for luminescent down-shifting layers and luminescent solar concentrators, where their aggregation-induced emission properties and large Stokes shifts are highly desirable. We also discuss the applications of molecules containing rotors in third-generation solar cell technologies, namely dye-sensitized solar cells and organic photovoltaics, where the twisted 3-dimensional rotor structures are used primarily for aggregation control. Finally, we discuss perspectives on the future role of molecules containing rotor structures in solar cell technologies.

Aqueous Nanoparticle Polymer Solar Cells: Effects of Surfactant Concentration and Processing on Device Performance

Polymer solar cells based on PDPP5T and PCBM as donor and acceptor materials, respectively, were processed from aqueous nanoparticle dispersions. Careful monitoring and optimization of the concentration of free and surface-bound surfactants in the dispersion, by measuring the conductivity and ζ-potential, is essential to avoid aggregation of nanoparticles at low concentration and dewetting of the film at high concentration. The surfactant concentration is crucial for creating reproducible processing conditions that aid in further developing aqueous nanoparticle processed solar cells. In addition, the effects of adding ethanol, of aging the dispersion, and of replacing [60]PCBM with [70]PCBM to enhance light absorption were studied. The highest power conversion efficiencies (PCEs) obtained are 2.0% for [60]PCBM and 2.4% for [70]PCBM-based devices. These PCEs are limited by bimolecular recombination of photogenerated charges. Cryo-TEM reveals that the two components phase separate in the nanoparticles, forming a PCBM-rich core and a PDPP5T-rich shell and causing a nonoptimal film morphology.

Enhanced Davydov Splitting in Crystals of a Perylene Diimide Derivative

We report the polarized absorption spectra of high-quality, thin crystals of a perylene diimide (PDI) species with branched side chains (B2). The absorption spectrum shows exemplary polarization-dependent H-like and J-like aggregate behavior upon orthogonal excitation, with a sizable Davydov splitting (DS) of 1230 cm^-1 and peak to peak splitting of 3040 cm^-1. The experimental results are compared to theoretical calculations with remarkable agreement. The theoretical analysis of the polarized absorption spectra shows evidence of a high degree of intermolecular charge transfer, which, along with Coulombic coupling, conspires to create the unprecedented DS for this family of dye molecules. The large polarization dependence of the electronic spectra is attributed to the unique twisted crystal structure, in which a substantial rotational displacement exists between neighboring chromophores within a π-stack. These results highlight the strong sensitivity of the Davydov splitting to intermolecular geometry in PDI systems.

Performance limitations in thieno[3,4-c]pyrrole-4,6-dione-based polymer:ITIC solar cells

Three thieno[3,4-c]pyrrole-4,6-dione-based conjugated polymers were applied in non-fullerene solar cells, in which the polymer PTPDBDT provided a high photovoltage but a low quantum efficiency. This was caused by the large phase separation in the bulk-heterojunction as confirmed by systematic studies.