Quantification of nano- and mesoscale phase separation and relation to donor and acceptor quantum efficiency, J(sc), and FF in polymer:fullerene solar cells

Ternary Blend Organic Solar Cells: Understanding the Morphology from Recent Progress

Ternary blend organic solar cells (TB-OSCs) incorporating multiple donor and/or acceptor materials into the active layer have emerged as a promising strategy to simultaneously improve the overall device parameters for realizing higher performances than binary devices. Whereas introducing multiple materials also results in a more complicated morphology than their binary blend counterparts. Understanding the morphology is crucially important for further improving the device performance of TB-OSC. This review introduces the solubility and miscibility parameters that affect the morphology of ternary blends. Then, this review summarizes the recent processes of morphology study on ternary blends from the aspects of molecular crystallinity, molecular packing orientation, domain size and purity, directly observation of morphology, vertical phase separation as well as morphological stability. Finally, summary and prospects of TB-OSCs are concluded.

Blending Donors with Different Molecular Weights: An Efficient Strategy to Resolve the Conflict between Coherence Length and Intermixed Phase in Polymer/Nonfullerene Solar Cells

Long coherence lengths (CLs) of crystals and proper intermixed phase amount guarantee charge transport and exciton dissociate efficiently, which is crucial for organic solar cells (OSCs) to achieve high device performance. However, extending CLs usually reduces the intermixed phase, leading to an insufficient interface for exciton dissociation. Herein, a strategy using a binary polymer with different molecular weights as donor is employed, that is, poly(3-hexylthiophene-2,5-diyl) (P3HT) with high (P3HT-H) and low (P3HT-L) molecular weight are blended as donor, and (5Z,5'Z)-5,5'-(((4,4,9,9-tetraoctyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(benzo[c][1,2,5]thiadiazole-7,4-diyl))bis(methanylylidene))bis(3-ethyl-2-thioxothiazolidin-4-one) (O-IDTBR) is used as acceptor. In kinetics, the entanglements of P3HT-H are relieved due to the higher molecular diffusivity of P3HT-L. In thermodynamics, the miscibility of P3HT-L/O-IDTBR, P3HT-H/O-IDTBR, and P3HT-L/P3HT-H blends increases in turn. Hence, P3HT forms a more ordered structure with longer CLs after adding P3HT-L, which also drives O-IDTBR dispersed in P3HT crystalline regions diffuse to the O-IDTBR crystalline regions to further self-organize. Consequently, the CLs of both P3HT and O-IDTBR are extended, while keeping the intermixed phase amount proper. The optimized microstructure boosts device performance from 7.03% to 7.80%, which is one of the highest values reported for P3HT/O-IDTBR blends. This is a novel way to solve the conflict mentioned above, which may provide guidance to finely regulating the morphology of the active layer.

Custom Synthesis of ZnO Nanowires for Efficient Ambient Air-Processed Solar Cells

Nanostructuration of solar cells is an interesting approach to improve the photovoltaic conversion efficiency (PCE). This work aims at developing architectured 3D hybrid photovoltaic solar cells using ZnO nanowires (ZnONWs) as the electron transport layer (ETL) and nanocollectors of electrons within the active layer (AL). ZnONWs have been synthesized using a hydrothermal process with a meticulous control of the morphology. The AL of solar cells is elaborated using ZnONWs interpenetrated with a bulk heterojunction composed of donor (π-conjugate low band gap polymer: PBDD4T-2F)/acceptor (fullerene derivate: PC71BM) materials. An ideal interpenetrating ZnONW-D/A system with predefined specific morphological characteristics (length, diameter, and inter-ZnONW distances) was designed and successfully realized. The 3D architectures based on dense ZnONW arrays covered with conformal coatings of AL result in an increased amount of the ETL/AL interface, enhanced light absorption, and improved charge collection efficiency. For AL/ZnONW assembly, spin-coating at 100 °C was found to be the best. Other parameters were also optimized such as the D/A ratio and the pre/post-treatments achieving the optimal device with a D/A ratio of 1.25/1 and methanol treated on ZnONWs before and after the deposition of AL. A PCE of 7.7% (1.4 times better than that of the 2D cells) is achieved. The improvement of the performances with the 3D architecture results from both of: (i) the enhancement of the ZnO/AL surface interface (1 μm2/μm2 for the 2D structure to 6.6 μm2/μm2 for the 3D architecture), (ii) the presence of ZnONWs inside the AL, which behave as numerous nanocollectors (∼60 ZnONW/μm2) of electrons in the depth of the AL. This result validates the efficiency of the concept of nanotexturing of substrates, the method of solar cell assembly based on the nano-textured surface, the chosen morphological characteristics of the nanotexture, and the selected photoactive organic materials.

Intermolecular Interaction Control Enables Co-optimization of Efficiency, Deformability, Mechanical and Thermal Stability of Stretchable Organic Solar Cells

Promoting efficiency, deformability, and life expectancy of stretchable organic solar cells (OSCs) have always been key concerns that researchers are committed to solving. However, how to improve them simultaneously remains challenging, as morphology parameters, such as ordered molecular arrangement, beneficial for highly efficient devices actually limits mechanical stability and deformability. In this study, the unfavorable trade-off among these properties has been reconciled in an all-polymer model system utilizing a mechanically deformable guest component. The success of this strategy stems from introducing a highly ductile component without compromising the pristine optimized morphology. Preferable interaction between two donors can maintain the fiber-like structure while enhancing the photocurrent to improve efficiency. Morphology evolution detected via grazing incidence X-ray scattering and in situ UV-vis absorption spectra during stretching have verified the critical role of strengthened interaction on stabilizing morphology against external forces. The strengthened interaction also benefits thermal stability, enabling the ternary films with small efficiency degradation after heating 1500 h under 80 °C. This work highlights the effect of morphology evolution on mechanical stability and provides new insights from the view of intermolecular interaction to fabricate highly efficient, stable, and stretchable/wearable OSCs.

Bending and pinching of three-phase stripes: From secondary instabilities to morphological deformations in organic photovoltaics

Optimizing the properties of the mosaic nanoscale morphology of bulk heterojunction (BHJ) organic photovoltaics (OPV) is not only challenging technologically but also intriguing from the mechanistic point of view. Among the recent breakthroughs is the identification and utilization of a three-phase (donor-mixed-acceptor) BHJ, where the (intermediate) mixed phase can inhibit mesoscale morphological changes, such as phase separation. Using a mean-field approach, we reveal and distinguish between generic mechanisms that alter, through transverse instabilities, the evolution of stripes: the bending (zigzag mode) and the pinching (cross-roll mode) of the donor-acceptor domains. The results are summarized in a parameter plane spanned by the mixing energy and illumination, and show that donor-acceptor mixtures with higher mixing energy are more likely to develop pinching under charge-flux boundary conditions. The latter is notorious as it leads to the formation of disconnected domains and hence to loss of charge flux. We believe that these results provide a qualitative road map for BHJ optimization, using mixed-phase composition and, therefore, an essential step toward long-lasting OPV. More broadly, the results are also of relevance to study the coexistence of multiple-phase domains in material science, such as in ion-intercalated rechargeable batteries.

Single particle dual plasmonic effect for efficient organic solar cells

Incorporating localized surface plasmon resonance (LSPR) into organic solar cells (OSCs) is a popular method for improving the power conversion efficiency (PCE) by introducing better light absorption. In this work, we designed a one-pot synthesis of Ag@SiO2@AuNPs dual plasmons and observed an immense increase in light absorption over a wide range of wavelengths. Ag@SiO2 plays the main role in enhancing light absorption near the ultraviolet band. The silica shell can also further enhance the LSP resonance effect and prevent recombination on the surface of AgNPs. The AuNPs on the Ag@SiO2 shell exhibited strong broad visible-light absorption due to LSP resonance and decreased light reflectance. By utilizing Ag@SiO2@AuNPs, we could enhance the light absorption and photoinduced charge generation, thereby increasing the device PCE to 8.57% and Jsc to 17.67 mA cm−2, which can be attributed to the enhanced optical properties. Meanwhile, devices without LSPR nanoparticles and Ag@SiO2 LSPR only showed PCEs of 7.36% and 8.18%, respectively.

Significance of thermodynamic interaction parameters in guiding the optimization of polymer:nonfullerene solar cells

Polymer solar cells (PSCs) based on polymer donors and nonfullerene small molecule acceptors are a very attractive technology for solar energy conversion, and their performance is heavily determined by film morphology. It is of considerable interest to reveal instructive morphology-performance relationships of these blends. This feature article discusses the recent advances in analysing the morphology formation of nonfullerene PSCs with an effective polymer thermodynamic quantity, i.e., Flory-Huggins interaction parameter χ. In particular, guidelines of high and low χ systems are summarized. The fundamental understanding of χ and its correlations to film morphology and photovoltaic device parameters is of utmost relevance for providing essential material design criteria, establishing suitable morphology processing guidelines, and thus advancing the practical applications of PSCs based on nonfullerene acceptors.

Interfacial and Bulk Nanostructures Control Loss of Charges in Organic Solar Cells

Organic solar cells (OSCs) have emerged as one promising sustainable energy resource since the introduction of state-of-the-art bulk heterojunction (BHJ) device structure in early 1990s. Impressively developed molecular design methodologies in the past decade have led researchers toward utilizing more suitable pairs of low (p-type) and high (n-type) electron affinity organic semiconducting materials. Among other attributes, versatile absorption capabilities of these materials highlight their favorable utilization in a single layer BHJ structure. Interaction of these verstile organic materials may lead to explicit interfaces, phase distributions, and crystalline nanostructures. Structural characterization techniques involving soft and hard X-rays have enabled us to measure these morphology parameters quantitatively including their string correlation with photovoltaic (PV) parameters. Favorable processing techniques have been adopted to realize auspicious interfacial areas and charge percolations in bulk toward efficient short circuit current (J_SC) and fill factor (FF) values. Collaborative efforts in the fields of chemical structure design of materials, device characterization, and engineering have pushed the power conversion efficiencies (PCEs) of OSCs to 16%. However, the single layer BHJ structure still requires further optimizations for the extension of their PCEs toward the theoretical limit. Maximum utilization of solar energy by organic blend films is the key to match their potential with inorganic/perovskite solar cells. Having comparable J_SC and FF values in organic versus inorganic photovoltaic devices, open circuit voltage (V_OC) is the only PV parameter limiting the development of OSCs in comparison to their inorganic competitors. This is due to unfavorable competition between rates of charge generation and recombination. Loss of charges during these generation and recombination processes account for the energy loss of the device, ranging from 0.6 to 1.0 V in state-of-the-art OSCs. Highly efficient (14-16%) single layer BHJ devices usually suffer from high energy loss with V_OC limited to 0.9 V. Comparatively, devices with reported V_OC > 0.9 V suffer from poor J_SC and FF values due to unfavorable interfacial ordering and bulk crystalline nanostructures. First part of the Account will address the charge losses during their transfer (interfacial losses) and influential role of interfacial nanostructures in controlling them toward efficient J_SC and V_OC values. Later, we will discuss losses during exciton diffusion and free charge transport (bulk losses) toward limited charge extraction. We will debate about the role of donor/acceptor nanostructures in correlation with influential photophysics studies to control these losses in small molecule (SM) acceptor based devices. We search for exaggerated crystalline phases of SM acceptor in competition with polymer donor to realize balanced and more efficient charge percolations. These improved diffusion and transport bulk nanostructures will suppress nonradiative (NR) pathways and bulk charge losses toward simultaneous enhancement of FF and V_OC values. Favorable interfacial and bulk morphology will drive efficient diffusion, transfer, transport, and extraction of charges in organic blend films. This Account will guide chemists and engineers to optimize chemical structure design and blend film nanostructures toward suppressed energy loss of OSCs.

Fine Optimization of Morphology Evolution Kinetics with Binary Additives for Efficient Non-Fullerene Organic Solar Cells

The power conversion efficiency of polymer solar cells (PSCs) is strongly affected by active layer morphology. Here, two solvent additives (ODT: octance-1,8-dithiol; DIO: 1,8-diiodooctane) are used to optimize the bulk heterojunction morphology of FTAZ:ITIC-Th based PSCs and ≈11% efficiency is obtained, which is 10% higher than the untreated device. Based on the morphological characterizations, the influence of binary solvent additives on manipulating molecular packing and phase separation of blend films is successfully revealed. More importantly, in situ grazing incidence wide-angle X-ray scattering characterization is adopted to explore the crucial role played by these two solvent additives at different stages of the film-forming process, that is, ODT influences the initial stage of the film-forming process, while DIO later establishes the ultimate photoactive film formation. Due to the impacts of two additives at different film processing stages, an optimal ratio of ODT:DIO (0.375%:0.125%) is obtained, which helps in realizing the optimized morphology.

Constructing Desired Vertical Component Distribution Within a PBDB-T:ITIC-M Photoactive Layer via Fine-Tuning the Surface Free Energy of a Titanium Chelate Cathode Buffer Layer

Rationally controlling the vertical component distribution within a photoactive layer is crucial for efficient polymer solar cells (PSCs). Herein, fine-tuning the surface free energy (SFE) of the titanium(IV) oxide bis(2,4-pentanedionate) (TOPD) cathode buffer layer is proposed to achieve a desired perpendicular component distribution for the PBDB-T:ITIC-M photoactive layer. The Owens-Wendt method is adopted to precisely calculate the SFE of TOPD film jointly based on the water contact angle and the diiodomethane contact angle. We find that the SFE of TOPD film increases as the annealing temperature rises, and the subtle SFE change causes the profound vertical component distribution within the bulk region of PBDB-T:ITIC-M. The results of secondary-ion mass spectroscopy visibly demonstrate that the TOPD film with an SFE of 48.71 mJ/cm2, which is very close to that of the ITIC film (43.98 mJ/cm2), tends to form desired vertical component distribution. Consequently, compared with conventional bulk heterojunction devices, the power conversion efficiency increases from 9.00 to 10.20% benefiting from the short circuit current density increase from 14.76 to 16.88 mA/cm2. Our findings confirm that the SFE adjustment is an effective way of constructing the desired vertical component distribution and therefore achieving high-efficiency PSCs.

Balancing Crystal Size in Small-Molecule Nonfullerene Solar Cells through Fine-Tuning the Film-Forming Kinetics to Fabricate Interpenetrating Network

The nanoscale interpenetrating network of active layer plays a key role in determining the exciton dissociation and charge transport in all small-molecule nonfullerene solar cells (AS-NFSCs). However, fabricating interpenetrating networks in all small-molecule blends remains a critical hurdle due to the uncontrolled crystallization behavior of small molecules. In this study, we proposed that the balanced crystal size between the donor and the acceptor is an essential prerequisite to construct optimal interpenetrating networks. We also provided a solvent additive strategy to reduce the gap of crystal size between the donor and the acceptor in S-TR:ITIC all small-molecule blend system through manipulating the solution state and film-forming kinetics. As a result, the crystal size of S-TR decreased and the crystal size of ITIC increased, leading to nanoscale interpenetrating networks. This optimized morphology improved the exciton dissociation efficiency and suppressed the bimolecular recombination, achieving almost double power conversion efficiency compared to the reference device. This work demonstrates that manipulation of the balanced crystal size of donor and acceptor may be a key to further boost the efficiency of AS-NFSCs.

Naphtho[1,2-c:5,6-c']bis[1,2,5]thiadiazole-Based Nonfullerene Acceptors: Effect of Substituents on the Thiophene Unit on Properties and Photovoltaic Characteristics

The development of new electron-accepting π-conjugated systems for application as nonfullerene acceptors in organic solar cells (OSCs) is urgently needed. Although π-conjugated systems based on naphtho[1,2-c:5,6-c']bis[1,2,5]thiadiazole (NTz) and naphthalimide (Np) as central and terminal units, respectively, represent possible candidates for nonfullerene acceptors, our knowledge of the structure-property-device performance relationship of these compounds remains limited. We report herein on an investigation of the effect of the substituents on the thiophene (T) linker between NTz and Np on the properties and photovoltaic performance. The photophysical and physicochemical measurements showed that the absorption behavior as well as frontier-orbital energy levels can be fine-tuned by the choice of the substituent on the thiophene rings. Bulk-heterojunction-type OSCs based on these acceptors under blending with poly(3-hexylthiophene) as a donor showed various power conversion efficiencies, ranging from 0.26 to 2.14%. The substituents on the thiophene rings also have a significant influence on the blend film properties, which explain the differences in the short-circuit current densities and fill factors in the OSCs. These results indicate the importance of molecular design in preparing nonfullerene acceptors with NTz and Np units in terms of tuning both the molecular properties of the materials and donor-acceptor interface engineering in the blended films.

Molecular Engineering Strategy for High Efficiency Fullerene-Free Organic Solar Cells Using Conjugated 1,8-Naphthalimide and Fluorenone Building Blocks

We designed, synthesized, and characterized a series of novel electron deficient small molecule nonfullerene acceptors based on 1,8-naphthalimide (NAI) and 9-fluorenone (FN) with different branched alkyl chains using various techniques. These molecules are based on an acceptor-donor-acceptor-donor-acceptor (A1-D-A2-D-A1) molecular design configuration with NAI as the end-capping acceptor (A1), FN as electron-withdrawing central (A2) group, and thiophene ring as a donor (D) unit. These materials are named as NAI-FN-NAI (BO) and NAI-FN-NAI (HD) where BO and HD represent butyloctyl and hexyldecyl alkyl groups, respectively. To further modify energy levels of these materials, we converted the weak electron withdrawing ketonic group (C═O) attached to the FN moiety of NAI-FN-NAI (BO) to a stronger electron withdrawing cyano group (C≡N) to obtain the compound NAI-FCN-NAI (BO) by keeping the same alkyl chain. The optical, electrochemical, and thermal properties of the new acceptors were studied. The materials exhibited higher to medium band gaps, low lowest unoccupied molecular orbital (LUMO) energy levels, and highly thermally stable properties. Organic solar cell devices employing conventional poly(3-hexylthiophene) (P3HT) a donor polymer and the newly designed small molecules as the acceptor were investigated. Among all new materials, organic solar cell devices based on NAI-FN-NAI (BO) as an acceptor exhibit the highest performance with an open circuit voltage (V_OC) of 0.88 V, a short-circuit current density (J_SC) of 9.1 mAcm^-2, a fill factor (FF) of 45%, and an overall power conversion efficiency (PCE) of 3.6%. This is the first report of 9-fluorenone based nonfullerene acceptor with P3HT donor in organic solar cell devices with such a promising performance.

Revelation of Interfacial Energetics in Organic Multiheterojunctions

Efficient charge generation via exciton dissociation in organic bulk heterojunctions necessitates donor-acceptor interfaces, e.g., between a conjugated polymer and a fullerene derivative. Furthermore, aggregation and corresponding structural order of polymer and fullerene domains result in energetic relaxations of molecular energy levels toward smaller energy gaps as compared to the situation for amorphous phases existing in homogeneously intermixed polymer:fullerene blends. Here it is shown that these molecular energy level shifts are reflected in interfacial charge transfer (CT) transitions and depending on the existence of disordered or ordered interfacial domains. It can be done so by systematically controlling the order at the donor-acceptor interface via ternary blending of semicrystalline and amorphous model polymers with a fullerene acceptor. These variations in interfacial domain order are probed with luminescence spectroscopy, yielding various transition energies due to activation of different recombination channels at the interface. Finally, it is shown that via this analysis the energy landscape at the organic heterojunction interface can be obtained.

Panchromatic Sequentially Cast Ternary Polymer Solar Cells

A sequential-casting ternary method is developed to create stratified bulk heterojunction (BHJ) solar cells, in which the two BHJ layers are spin cast sequentially without the need of adopting a middle electrode and orthogonal solvents. This method is found to be particularly useful for polymers that form a mechanically alloyed morphology due to the high degree of miscibility in the blend.

Mapping Polymer Donors toward High-Efficiency Fullerene Free Organic Solar Cells

Five polymer donors with distinct chemical structures and different electronic properties are surveyed in a planar and narrow-bandgap fused-ring electron acceptor (IDIC)-based organic solar cells, which exhibit power conversion efficiencies of up to 11%.

Impact of Fullerene Mixing Behavior on the Microstructure, Photophysics, and Device Performance of Polymer/Fullerene Solar Cells

Here, a comprehensive study of the influence of polymer:fullerene mixing behavior on the performance, thin-film microstructure, photophysics, and device physics of polymer solar cells is presented. In particular, blends of the donor polymer PBDTTT-EFT with the acceptor PC₇₁BM that exhibit power conversion efficiencies over 9% are investigated. Through tuning of the fullerene concentration in PBDTTT-EFT:PC₇₁BM blends, the impact of fullerene mixing behavior is systematically investigated via a combination of synchrotron-based X-ray scattering and spectroscopy techniques. The impact of fullerene loading on photophysics and device physics is further explored with steady-state photoluminescence measurements, ultrafast transient absorption spectroscopy, and transient photovoltage measurements. In the low fullerene concentration regime (<50 wt %), most fullerene molecules are dispersed in the polymer matrix, resulting in severe geminate and nongeminate recombination due to a lack of pure fullerene aggregates and percolating pathways for charge separation and transport. In the high fullerene concentration regime (>70 wt %), large fullerene domains result in incomplete PC₇₁BM exciton harvesting with the presence of fullerene molecules also disrupting the molecular packing of polymer crystallites. The optimum fullerene concentration of ∼60-67 wt % balances the requirements of charge generation and charge collection. These findings demonstrate that controlling the fullerene concentration in the mixed phase and optimizing the balance between pure and mixed phases are critical for maximizing the efficiency of highly mixed polymer/fullerene solar cells.

Enhanced Molecular Packing of a Conjugated Polymer with High Organic Thermoelectric Power Factor

The detailed relationship between film morphology and the performance of solution processed n-type organic thermoelectric (TE) devices is investigated. It is interesting to find that the better ordered molecular packing of n-type polymer can be achieved by adding a small fraction of dopant molecules, which is not observed before. The better ordered structure will be favorable for the charge carrier mobility. Meanwhile, dopant molecules improve free carrier concentration via doping reaction. As a result, a significantly enhanced electrical conductivity (12 S cm(-1)) and power factor (25.5 μW m(-1) K(-2)) of TE devices are obtained. Furthermore, the phase separation of conjugated polymer/dopants is observed for the first time with resonant soft X-ray scattering. Our results indicate that the miscibility of conjugated polymers and dopants plays an important role on controlling the morphology and doping efficiency of TE devices.

Unraveling the Fundamental Mechanisms of Solvent-Additive-Induced Optimization of Power Conversion Efficiencies in Organic Photovoltaic Devices

The realization of controllable morphologies of bulk heterojunctions (BHJ) in organic photovoltaics (OPVs) is one of the key factors enabling high-efficiency devices. We provide new insights into the fundamental mechanisms essential for the optimization of power conversion efficiencies (PCEs) with additive processing to PBDTTT-CF:PC71BM system. We have studied the underlying mechanisms by monitoring the 3D nanostructural modifications in BHJs and correlated the modifications with the optical analysis and theoretical modeling of charge transport. Our results demonstrate profound effects of diiodooctane (DIO) on morphology and charge transport in the active layers. For small amounts of DIO (<3 vol %), DIO promotes the formation of a well-mixed donor-acceptor compact film and augments charge transfer and PCE. In contrast, for large amounts of DIO (>3 vol %), DIO facilitates a loosely packed mixed morphology with large clusters of PC71BM, leading to deterioration in PCE. Theoretical modeling of charge transport reveals that DIO increases the mobility of electrons and holes (the charge carriers) by affecting the energetic disorder and electric field dependence of the mobility. Our findings show the implications of phase separation and carrier transport pathways to achieve optimal device performances.

Probing Ternary Solvent Effect in High V(oc) Polymer Solar Cells Using Advanced AFM Techniques

This work describes a simple method to develop a high V(oc) low band gap PSCs. In addition, two new atomic force microscopy (AFM)-based nanoscale characterization techniques to study the surface morphology and physical properties of the structured active layer are introduced. With the help of ternary solvent processing of the active layer and C60 buffer layer, a bulk heterojunction PSC with V(oc) more than 0.9 V and conversion efficiency 7.5% is developed. In order to understand the fundamental properties of the materials ruling the performance of the PSCs tested, AFM-based nanoscale characterization techniques including Pulsed-Force-Mode AFM (PFM-AFM) and Mode-Synthesizing AFM (MSAFM) are introduced. Interestingly, MSAFM exhibits high sensitivity for direct visualization of the donor-acceptor phases in the active layer of the PSCs. Finally, conductive-AFM (cAFM) studies reveal local variations in conductivity in the donor and acceptor phases as well as a significant increase in photocurrent in the PTB7:ICBA sample obtained with the ternary solvent processing.

Alcohol-soluble interfacial fluorenes for inverted polymer solar cells: sequence induced spatial conformation dipole moment

Three fluorene-based alcohol-soluble organic small molecule electrolytes (SMEs) with different conjugated backbones, namely, TFTN-Br, FTFN-Br and FTTFN-Br, were designed as cathode interfacial layers for inverted polymer solar cells (i-PSCs). The insertion of SMEs to the ITO/active layer interfaces effectively lowered the energy barrier for electron transport and improved the inherent compatibility between the hydrophilic ITO and hydrophobic active layers. Due to these advantages, the device based on poly(3-hexylthiophene) (P3HT):(6,6)-phenyl-C61 butyric acid methyl ester (PC61BM) with TFTN-Br as the cathode interfacial layer achieved an improved power conversion efficiency (PCE) of 3.8%, which is a 26% improvement when compared to the standard device comprising ZnO cathode interfacial layers (PCE = 3.0%). Devices with FTFN-Br and FTTFN-Br also showed an improved PCE of 3.1% and 3.5%, respectively. The variation in device performance enhancement was found to be primarily correlated with the different conformation of their assembly onto the electrode caused by the joint sequence of the polar group of the SMEs, consequently impacting the dipole moment and interface morphology. In addition, introducing SMEs as the cathode interfacial layer also produced devices with long-term stability.

Stability of organic solar cells: challenges and strategies

This review highlights the factors limiting the stability of organic solar cells and recent developments in strategies to increase the stability of organic solar cells.

An oligothiophene chromophore with a macrocyclic side chain: synthesis, morphology, charge transport, and photovoltaic performance

Molecular chromophores tend to form crystals beyond nanometer sizes upon thermal aging. A novel ring-protection structure has shown promise to solve morphological stability problem of solution-processed small molecule solar cell devices.

A comparative study of a fluorene-based non-fullerene electron acceptor and PC61BM in an organic solar cell at a quantum chemical level

A comparison of the OSC efficiency between PC61BM and non-fullerene small molecule acceptor-FENIDT was studied by microscopic analysis based on the DFT/TDDFT calculation.

Understanding the Impact of Hierarchical Nanostructure in Ternary Organic Solar Cells

Ternary organic solar cells (OSCs), which blend two donors and fullerene derivatives with different absorption ranges, are a promising potential strategy for high-power conversion efficiencies (PCEs). In this study, inverted ternary OSCs are fabricated by blending a highly crystalline small molecule BDT-3T-CNCOO in a low band gap polymer PBDTTT-C-T:PC71BM. As the small molecule is introduced, the overall PCEs increase from 7.60% to 8.58%. The morphologies of ternary blends are studied by combining transmission electron microscopy and X-ray scattering techniques at different length scales. Hierarchical phase separation is revealed in the ternary blend, which is composed of domains with sizes of ≈88, ≈50, and ≈20 nm, respectively. The hierarchical phase separation balances the charge separation and transport in ternary OSCs. As a result, the fill factors of the devices significantly improve from 58.4% to 71.6%. Thus, ternary blends show higher hole mobility and higher fill factor than binary blends, which demonstrates a facile strategy to increase the performance of OSCs.

Enhancing the Photovoltaic Performance by Tuning the Morphology of Polymer:PC₇₁BM Blends with a Commercially Available Nucleating Agent

The use of a commercially available nucleating agent as the additive for the fabrication of polymer:PC71BM-based active layers by solution-processing can greatly enhance the power conversion efficiency (PCE) of bulk heterojunction polymer solar cells (BHJ PSCs). The enhancement of device performance is mainly due to the addition of nucleating agent, which is able to regulate the drying process of the active layer and decrease the oversized domain size of conjugated polymers. Via this effective strategy to optimize the film morphology, the designed device exhibits an enhancement as great as 30.8%.

Peculiarity of Two Thermodynamically-Stable Morphologies and Their Impact on the Efficiency of Small Molecule Bulk Heterojunction Solar Cells

Structural characteristics of the active layers in organic photovoltaic (OPV) devices play a critical role in charge generation, separation and transport. Here we report on morphology and structural control of p-DTS(FBTTh₂)₂:PC₇₁BM films by means of thermal annealing and 1,8-diiodooctane (DIO) solvent additive processing, and correlate it to the device performance. By combining surface imaging with nanoscale depth-sensitive neutron reflectometry (NR) and X-ray diffraction, three-dimensional morphologies of the films are reconstituted with information extending length scales from nanometers to microns. DIO promotes the formation of a well-mixed donor-acceptor vertical phase morphology with a large population of small p-DTS(FBTTh₂)₂ nanocrystals arranged in an elongated domain network of the film, thereby enhancing the device performance. In contrast, films without DIO exhibit three-sublayer vertical phase morphology with phase separation in agglomerated domains. Our findings are supported by thermodynamic description based on the Flory-Huggins theory with quantitative evaluation of pairwise interaction parameters that explain the morphological changes resulting from thermal and solvent treatments. Our study reveals that vertical phase morphology of small-molecule based OPVs is significantly different from polymer-based systems. The significant enhancement of morphology and information obtained from theoretical modeling may aid in developing an optimized morphology to enhance device performance for OPVs.

A Large-Bandgap Conjugated Polymer for Versatile Photovoltaic Applications with High Performance

A new copolymer PM6 based on fluorothienyl-substituted benzodithiophene is synthesized and characterized. The inverted polymer solar cells based on PM6 exhibit excellent performance with Voc of 0.98 V and power conversion efficiency (PCE) of 9.2% for a thin-film thickness of 75 nm. Furthermore, the single-junction semitransparent device shows a high PCE of 5.7%.

Conjugated Polymer-Small Molecule Alloy Leads to High Efficient Ternary Organic Solar Cells

Ternary organic solar cells are promising candidates for bulk heterojunction solar cells; however, improving the power conversion efficiency (PCE) is quite challenging because the ternary system is complicated on phase separation behavior. In this study, a ternary organic solar cell (OSC) with two donors, including one polymer (PTB7-Th), one small molecule (p-DTS(FBTTH2)2), and one acceptor (PC71BM), is fabricated. We propose the two donors in the ternary blend forms an alloy. A notable averaged PCE of 10.5% for ternary OSC is obtained due to the improvement of the fill factor (FF) and the short-circuit current density (J(sc)), and the open-circuit voltage (V(oc)) does not pin to the smaller V(oc) of the corresponding binary blends. A highly ordered face-on orientation of polymer molecules is obtained due to the formation of an alloy structure, which facilitates the enhancement of charge separation and transport and the reduction of charge recombination. This work indicates that a high crystallinity and the face-on orientation of polymers could be obtained by forming alloy with two miscible donors, thus paving a way to largely enhance the PCE of OSCs by using the ternary blend strategy.

Adding Amorphous Content to Highly Crystalline Polymer Nanowire Solar Cells Increases Performance

Polymer solar cells are fabricated with systematic variation of the phase purity. Photovoltaic tests demonstrate that devices with ca. 10% of mixed phases outperform pure-phase devices. Photophysical studies reveal the effects of mixed phase on charge generation and recombination. These results show a promising strategy for the optimization of organic electronic materials.

Importance of domain purity and molecular packing in efficient solution-processed small-molecule solar cells

Connections are delineated between solar-cell performance, charge-carrier mobilities, and morphology in a highperformance molecular solar cell. The observations show that maximizing the relative phase purity and structural order while simultaneously limiting the domain size may be essential for achieving optimal solar-cell performances in solution-processed small-molecule solar cells .

Sequential deposition: optimization of solvent swelling for high-performance polymer solar cells

Organic solar cells based on a typical DPP polymer were systematically optimized by a solvent swelling assisted sequential deposition process. We investigated the influence of solvent swelling on the morphology and structure order of the swollen film and the resultant device performance. Morphological and structural characterization confirmed the realization of ideal bulk heterojunctions using a suitable swelling solvent. A trilayered morphology was also found with the conjugated polymer concentrated bottom layer, PC71BM concentrated top layer, and interpenetrated networks of donor and acceptor in the middle by solvent swelling instead of thermal annealing in the sequential solution processing method. We proposed a simple strategy to optimize the sequential deposition fabricated devices by tuning the concentration of the PC71BM solution instead of thermal annealing. The best device showed a PCE of 7.59% with a Voc of 0.61 V, Jsc of 17.95 mA/cm(2), and FF of 69.6%, which is the highest reported efficiency for devices fabricated by a sequential processing method and among the best results for DPP polymers.

Rational selection of solvents and fine tuning of morphologies toward highly efficient polymer solar cells fabricated using green solvents

The polymer solar cells fabricated by the green solvent (anisole/diphenyl ether) exhibited an outstanding PCE of 8.37%.

Unique insight into phase separation in polymer solar cells from their electric characteristics

A simple approach is proposed to investigate donor/acceptor phase separation based on the J–V curves of corresponding electron-only devices under forward or revise bias.

Optimization of charge carrier transport balance for performance improvement of PDPP3T-based polymer solar cells prepared using a hot solution

Using a hot solution may prove to be an effective method to improve the charge carrier transport for high performance PSCs.

Interplay of solvent additive concentration and active layer thickness on the performance of small molecule solar cells

A relationship between solvent additive concentration and active layer thickness in small-molecule solar cells is investigated. Specifically, the additive concentration must scale with the amount of semiconductor material and not as absolute concentration in solution. Devices with a wide range of active layers with thickness up to 200 nm can readily achieve efficiencies close to 6% when the right concentration of additive is used.