Bulk Heterojunction Organic Photovoltaics Research Articles

Effect of Additives and Annealing on the Performance of Nonfullerene‐Based Binary and Ternary Organic Photovoltaics

Fine tuning of blend morphology is a key factor that limits the performance of the bulk‐heterojunction organic photovoltaics (BHJ‐OPVs). Herein, the morphological control of the binary (PM6:Y7) and ternary (PM6:Y7:PC70BM) blends is conducted through 1‐chloronaphthalene (CN) solvent additive and thermal annealing (TA) treatment with respect to their influence on the photovoltaic performance. Moreover, a distinct study is accomplished on the optical and electronic properties of the treated and nontreated binary and ternary devices by external quantum efficiency measurements and impedance spectroscopy. The results indicate that these treatments affect the performance of the binary and ternary OPVs differently. Regarding the 2% CN addition, the current density of the binary devices is improved by ≈27%, whereas the fill factor of the ternary devices shows a pronounced increment of ≈22%. A contradictory behavior is exhibited by TA for the binary and ternary OPVs. The PCEs for binary devices (with/without CN) and 2% CN‐treated ternary ones are improved, while diminishing the PCEs of the ternary ones with 0% CN. Accordingly, the highest efficiencies of the binary and ternary OPVs are obtained due to the dual effect of 2% CN solvent additive along with the TA treatments.

Changes in Optimal Ternary Additive Loading when Processing Large Area Organic Photovoltaics by Spin‐ versus Blade‐Coating Methods

As we move toward roll‐to‐roll processing of organic photovoltaics (OPVs), it is important to validate the process and the ink formulations. A poly[[9‐(1‐octylnonyl)‐9Hcarbazole‐2,7‐diyl]‐2,5‐thiophenediyl‐2,1,3‐benzothiadiazole‐4,7 diyl2,5thiophenediyl]:[6,6]‐phenyl C71 butyric acid methyl ester (PCDTBT:PC71BM)‐based OPV with an increase in power conversion efficiency (PCE) from 4.6% up to 5.4% by incorporating bis(tri‐hexylsiloxy) silicon phthalocyanine ((3HS)2‐SiPc) NIR absorbing ternary additives on indium tin oxide (ITO)/polyethyleneterapthalate (PET) flexible substrate with active area of 1 cm2 under simulated AM 1.5G one sun irradiation. Maximum PCE of the ternary OPVs was obtained with 5 wt% of (3HS)2‐SiPc added when processing the active layer by spin coating, whereas the addition of 10 wt% of (3HS)2‐SiPc is required for the maximization of PCE when processing the films by blade coating. The importance of processing conditions when optimizing the concentration of ternary additives in bulk heterojunction (BHJ) OPVs is demonstrated.

Optoelectronic Properties of a Benzodithiophene-Based Organic Photovoltaic

Benzo[1,2-b:4,5-b′]dithiophene (BDT) contains electron donating groups and conjugates with acceptor material as a donor-acceptor type conjugated polymer, which can be applied in bulk heterojunction organic photovoltaics due to its high charge carrier mobility. In this study, we used Stille coupling polymerization to synthesize a BDT monomer with thiophene side-chains and copolymerized with dioctyl 2,5-dibromoterephthalate (DTP) to form a 2-D type donor-acceptor conjugated polymer, BDT-DTP. Thermal analyses indicated that BDT-DTP has a high thermal decomposition temperature of 342.07 °C (weight loss = 5%), showing excellent thermal stability. The active layer comprised of BDT-DTP:3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))−5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC) blended film in a OPV device could achieve a power conversion efficiency of 1.08%, open-circuit voltage of 0.88 V, short-circuit current density of 4.08 mA cm−2, and fill factor of 30.1%. This work demonstrates a novel structure of benzodithiophene-based donor material used in the photovoltaic application.

Non-fullerene organic photovoltaics based on thienopyrroledione comprising random copolymers; effect of alkyl chains

Two new random donor-acceptor (D-A) copolymers, signed as P1 and P2, were designed and synthesized. Electrochemical and spectroelectrochemical measurements were performed to investigate absorption, energy levels, electronic and optical band gaps for comparison. The polymers were used as donor polymers in the active layer to fabricate non-fullerene, bulk heterojunction (BHJ) organic photovoltaics (OPVs). Investigations were carried out through the conventional BHJ structure; ITO/PEDOT: PSS/Active Layer/LiF/Al, where active layer consists of 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene (ITIC) as the acceptor and thienopyrroledione containing donors. The device based on P1:ITIC(1:1) blend with a thickness of 161 nm gave the best performance with a power conversion efficiency (PCE) of 7.94%, an open-circuit voltage (VOC) of 0.86 V, a short-current density (JSC) of 18.45 mA cm−2 and a fill factor (FF) of 50.12%. The highest PCE obtained from P2 based organic solar cell is 1.96%. P2 exhibited low solubility attributed to the lack of alkyl groups enhancing polymer solubility, electronic properties, and photovoltaic performances. The research outputs exhibit that introduction of alkyl chains on the polymer backbone can enhance device performance.

Continuum Approach for Studying Morphological Deformations of Multiple-Phase Organic Photovoltaic Cells

Optimizing the properties of the mosaic 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 in studies of BHJ morphology, is the identification and utilization of a three-phase (donor/mixed/acceptor) BHJ, where the (intermediate) mixed-phase can inhibit morphological changes, such as phase separation. Using a continuum approach, we develop and analyze a model that undertakes the coupling between the spatiotemporal evolution of the material and charge dynamics along with charge transfer at the device electrodes. Starting from an ideal BHJ represented by stripes, we reveal and distinguish between generic mechanisms that alter the evolution of stripes: the bending (zigzag mode) and the pinching (cross-roll mode) of the donor/acceptor domains. We then emphasize that the latter instability mode, is notorious as it leads to the formation of disconnected domains and hence to loss of charge flux. Consequently, the analysis implies that donor-acceptor mixtures with higher mixing energy are more likely to develop pinching under charge-flux boundary conditions. We believe that these results provide a qualitative roadmap for morphological BHJ optimization, using mixed-phase composition and moreover, open new vistas to application involving three phase morphologies, such as ion-intercalated rechargeable batteries.

Investigating the Indoor Performance of Planar Heterojunction Based Organic Photovoltaics

Recently, the bulk heterojunction (BHJ)-based photoactive layer has been favored over the planar heterojunction (PHJ)-based photoactive layer for organic photovoltaics (OPVs), owing to the evitable thickness limit of the latter induced by the low mobility of charge carriers. However, under dim indoor light conditions, the PV performance becomes immune to the resistive components of the PHJ OPVs, such as a thick photoactive layer, thereby rendering them attractive for operating indoors. In this study, we devise and demonstrate indoor OPVs with a PHJ-based photoactive layer constituting poly(3-hexylthiophene-2,5-diyl) (P3HT) and indene-C60 bisadduct (ICBA). The performance of the PHJ OPVs was optimized by modulating the thickness of the ICBA layer under a 1000 lx light-emitting diode lamp. OPVs with a 65 nm-thick ICBA layer exhibited excellent indoor performance with a power conversion efficiency of 15.3 ± 0.3%, comparable to that of the reference BHJ OPVs (15.0 ± 0.2%). No noticeable performance degradation was observed by increasing the thickness of the PHJ-based photoactive layer; instead, a thick photoactive layer only enhanced the light absorption, resulting in excellent indoor performance with a higher photocurrent.

Effects of the photoactive layer properties and current transmission mechanism on optical and electrical characteristics of organic photovoltaic

The P3HT:PCBM is an important organic material as an active layer for bulk heterojunction (BHJ) organic photovoltaics (OPVs). However, in terms of photophysical properties, blend ratios of both materials and device design is an important parameter for OPVs devices. In this study, the P3HT:PCBM photoactive layer, blended in different proportions by mass (1:1, 1:0.8, 1:0.6) was coated on the PEDOT:PSS surface using the spin coater method for different devices. Finally, Al have been evaporated on the photoactive layer and the device structure completed. In other examples, 2 nm lithium fluoride (LiF) as buffer layer evaporated on the active layer for each blending ratio before metal. Optical absorption, AFM, XRD, Raman and XPS measurements were taken for each active layer and it was observed that the surface morphology and absorption characteristic of the active layer changed according to the mixing ratios. This change in active layer properties directly affected the device parameters. Electrical characterization of the six different devices performed with current density-voltage (J–V) measurement and by used a solar simulator under 100 mW/cm2 irradiation intensity. Depending on the increase in short circuit current density (Jsc) and open current voltage (Voc) values in the devices, the power conversion efficiency (PCE) of the OPVs obtained varied between 0.86% and 2.74%. This change can be attributed to the fact that the P3HT:PCBM photoactive layer forms a smoother surface morphology depending on the PCBM ratio and the series resistance decreases. Another reason for this change in PCE can be explained by the effective electron injection of the LiF layer, LiF/Al layer with lower work function and less carrier deposition at its metal-active layer interfaces. It result has been obtained that the active layer with a ratio of 1:0.8 has a more homogeneous surface morphology and the series resistance decreases, the presence of the LiF layer in the device structures increases the Jsc and Voc, accordingly the PCE increases to 2.74%. These results showed that BHJ OPV with high PCE can be obtained with the ideal mixing ratio and appropriate device design.

Towards Improving the Efficiency of Organic Solar Cells by Coarse-Grained Atomistic Modeling of Processing Dependent Morphologies

Solar energy conversion to electricity using organic semiconductor materials is a complex process due to the underlying transport physics of electrons and photons. Experimental characterizations of the 3-D morphology of bulk heterojunction organic photovoltaics are challenging; the poor contrast of the reconstructed morphology due to weak electronic scattering of organic molecules is a major impediment for microstructural imaging by electron microscopy. Thus, enhancing the power-conversion efficiency (PCE) of organic solar cells requires predictive design of both material and processing parameters. To this end, large-scale coarse-grained molecular simulations are needed to probe the morphology of the nanostructures, to develop process-structure-performance correlations that assist in fundamental understanding of the physical mechanisms, and to simultaneously aid in selection and optimization of design parameters. Here, we summarize the outcomes from high-performance coarse-grained molecular dynamics simulations that are employed to mimic solvent evaporation and thermal annealing of typical bulk heterojunction solar cell active layers. The latter consist of a blend of electron-donor and electron-acceptor materials. We extensively explore the dependence of PCE and the thermo-mechanical stability of the blend morphology on different solution processing conditions, and correlate the identified parameters with the dominant design variables and the microstructure. The simulations reveal that the composition of constituent donor and acceptor materials, respective molecular weights, polydispersity of donor polymer chains, and the thermal annealing temperature are the major parameters that significantly impact the morphology, thermo-mechanical stability, and subsequently the PCE of organic photovoltaics.

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.

Benzodithiophene-based wide-bandgap small-molecule donors for organic photovoltaics with large open-circuit voltages

Abstract We have developed new small-molecule electron donors, SMBDT-S and SMBDT-SF, based on benzo[1,2-b:4,5-b’]dithiophene (BDT) with thiophene side chains as a core unit and rhodanine end-capping units to enhance the open-circuit voltage (Voc), which is one of the factors limiting performance of small-molecule organic photovoltaics (OPVs). The SMBDT-S and SMBDT-SF donor molecules exhibited excellent thermal stability and high crystallinity. SMBDT-S was found to have a wide bandgap of 1.85 eV with the highest occupied molecular orbital (HOMO) energy level of − 5.56 eV. The fluorination in SMBDT-SF led to the same bandgap of 1.86 eV with an even lower-lying HOMO energy level of − 5.72 eV. Both SMBDT-S and SMBDT-SF have large thin-film absorption coefficients of (1.4–1.5) × 105 cm−1. Bulk heterojunction OPV devices based on pairing SMBDT-S and SMBDT-SF respectively with PC71BM electron acceptor showed Voc as high as 1.18 eV, which is among the highest Voc reported for all-small-molecule solar cells. These results demonstrate that introducing fluorine atoms and alkylthio side chains are effective strategies to downshift the HOMO energy level and enhance the Voc in OPVs.

V OC variation with different molecular weight fractions in highly efficient organic photovoltaic bulk heterojunctions

Tuning molecular weight fractions is one of the most important strategies for optimizing the morphology of bulk heterojunctions (BHJs) to obtain high-performance organic solar cells. Here, we investigate the effects of molecular weight fractions on the performance of state-of-the-art PM6:Y6 BHJ solar cells. A remarkable blueshift in the photoluminescence spectra can be observed with the introduction of the PM6 component. The electroluminescent and morphological analyses indicate that this observation can be attributed to increased charge-transfer state energy and an intermolecular interaction between donor and acceptor, leading to open circuit voltage (V OC) variations in the photovoltaic device. The transient absorption spectra confirm that the optimized donor:acceptor (D:A) ratio can suppress the generation of polarons and achieve high efficiency exciton dissociation. The results of atomic force microscopy and neutron reflectometry reveal that higher acceptor contents promote the formation of a desirable interpenetrating network and enable high fill factors and short-circuit currents to be obtained.

Self-Assembled Monolayer Enables Hole Transport Layer-Free Organic Solar Cells with 18% Efficiency and Improved Operational Stability

We report on bulk-heterojunction (BHJ) organic photovoltaics (OPVs) based on the self-assembled monolayer (SAM) 2PACz as a hole-selective interlayer functionalized directly onto the indium tin oxid...

Investigation of Hierarchical Structure Formation in Merocyanine Photovoltaics

Author(s): Murrey, TL; Hertel, D; Nowak, J; Bruker, R; Limbock, T; Neudorfl, J; Ruth, S; Schelter, J; Olthof, S; Radulescu, A; Moule, AJ; Meerholz, K | Abstract: Merocyanines (MCs) are a versatile class of small-molecule dyes. Their optoelectronic properties are easily tunable by chemically controlling their donor-acceptor strength, and their structural properties can be tuned by simple side-chain substitution. This manuscript demonstrates a novel series of MCs featuring an indoline donor with varying hydrocarbon side-chain length (from 6 to 12 carbons) and a tert-butyl-thiazole acceptor, labeled InTBT. Bulk heterojunction organic photovoltaics are fabricated with a [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) acceptor and characterized. Films composed of I8TBT:PCBM and I9TBT:PCBM produced the highest power conversion efficiency of 4.5%, which suggests that the morphology is optimized by controlling the side-chain length. Hierarchical structure formation in InTBT:PCBM films is studied using grazing incidence X-ray diffraction (GIXRD), small-angle neutron scattering (SANS), and atomic force microscopy (AFM). When mixed with PCBM, InTBTs with ≤8 side-chain carbons form pure crystalline domains, while InTBTs with ≥9 side-chain carbons mix well with PCBM. SANS demonstrates that increasing side-chain length increases the InTBT-rich domain size. In addition, a branched hexyl-dodecyl side-chain IHDTBT:PCBM film was studied and found to exhibit the worst-performance organic photovoltaic (OPV) device. The large-branched side chain inhibited mixing between IHDTBT and PCBM resulting in large segregated phases.

Ultrafast Charge Dynamics in Dilute-Donor versus Highly Intermixed TAPC:C60 Organic Solar Cell Blends.

Elucidating the interplay between film morphology, photophysics, and device performance of bulk heterojunction (BHJ) organic photovoltaics remains challenging. Here, we use the well-defined morphology of vapor-deposited di-[4-(N,N-di-p-tolyl-amino)-phenyl]cyclohexane (TAPC):C60 blends to address charge generation and recombination by transient ultrafast spectroscopy. We gain relevant new insights to the functioning of dilute-donor (5% TAPC) fullerene-based BHJs compared to molecularly intermixed systems (50% TAPC). First, we show that intermolecular charge-transfer (CT) excitons in the C60 clusters of dilute BHJs rapidly localize to Frenkel excitons prior to dissociating at the donor:acceptor interface. Thus, both Frenkel and CT excitons generate photocurrent over the entire fullerene absorption range. Second, we selectively monitor interfacial and bulk C60 clusters via their electro-absorption, demonstrating an energetic gradient that assists free charge generation. Third, we identify a fast (<1 ns) recombination channel, whereby free electrons recombine with trapped holes on isolated TAPC molecules. This can harm the performance of dilute solar cells, unless the electrons are rapidly extracted in efficient devices.

Modeling process–structure–property relationship in organic photovoltaics using a robust diffuse interface approach

Organic photovoltaics (OPVs) can potentially provide a cost-efficient means of harnessing solar energy. However, optimum OPV performance depends on understanding the process–structure–property (PSP) correlation in organic semiconductors. In the working of bulk-heterojunction OPVs, the morphology plays a crucial role in device performance. In order to understand PSP linkage, a theoretical framework has been developed. We first established process–structure correlations by generating a range of morphologies with various blend ratios of donor and acceptor organic semiconductors for various annealing periods. Second, we calculated the effective electronic properties corresponding to the simulated structures using a diffuse interface approach that is numerically more robust and straightforward than the classical sharp interface method. This novel framework, wherein both the process–structure and the structure–property relationship have been established using the diffuse interface approach, completes the theoretical PSP linkage, allowing the optimization of process parameters for device applications. The theoretical PSP linkage is then benchmarked qualitatively with experimental results on a model P3HT:PCBM system. We have been able to identify the morphological characteristics that maximize device performance. This work is carried out in the broad overview of the integrated computational materials engineering framework wherein the processing parameters are optimized by determining the process–structure–property relationships.

Porphyrins-based Materials as Efficient Donor in Organic Solar Cells

Twenty years after the discovery of bulk heterojunction solar cells (BHJSCs), porphyrins were underrepresented in the field of organic solar cells (OCSs)1 despite their high epsilon of 300 000 M-1cm-1 and their electron rich aromatic core, which make them a good electron donors. In 2012, a power conversion efficiency (PCE) of 8.08% for a BHJSC built with a small molecule containing a porphyrin core was reported.2 Then in 2017, a PCE of 6.44 % was reported for BHJSC constructed with a polymer containing a porphyrin inside the conjugated chain.3 These advances brought porphyrins at the center of attention in the field of OSCs. In 2019, using a planar non-fullerene acceptor and a porphyrin based-polymer, a 9.16% PCE was reported; a record for this type of dye.4 Both a high Voc of 1.01 V and a low Eloss of 0.45 eV, two BHJSC’s characteristics, make the porphyrin dyes competing with the most efficient donors of the field. Mechanistically speaking, like all donors porphyrin-based donors form interfacial charge transfer (CT) species with the acceptors where upon excitation, a charge separated (CS) state is produced efficiently. However, detailed investigations of the CT signature and photophysical traits of these species simply do exit due to the bulk nature of the materials. We designed a supramolecular porphyrin-graphene nanoribbon (GNR) model taking advantage of π-stacking effects between GNRs and anchoring pyrene groups. Upon selective excitation of the GNRs, the GNRs form excitons for which 95% are involved in an energy transfer with the π-stacked porphyrin-GNR CT species very efficiently (35%in 759 fs; 65% < 135 fs), then an electron transfer (charge separation) occurs from these excited CT species to the GNRs in the ps time scale. 1. Bucher, L.; Desbois, N.; Harvey, P.; Sharma, G.; Gros. C.; Porphyrins and BODIPY as Building Blocks for Efficient Donor Materials in Bulk Heterojunction Solar Cells. Solar RRL 2017, 1, 1700127.2. Huang, Y.; Li, L.; Peng, X.; Peng, J.; Cao, Y.; Solution processed small molecule bulk heterojunction organic photovoltaics based on a conjugated donor–acceptor porphyrin. J. Mater. Chem. 2012, 22, 21841-21844.3. Bucher, L.; Tanguy, L.; Fortin, D.; Desbois, N.; Harvey, P.; Sharma, G.; Gros, C.; A Very Low Band Gap Diketopyrrolopyrrole–Porphyrin Conjugated Polymer. Chempluschem 2017, 82, 625-630.4. Tanguy, L.; Malhotra, P.; Singh, S.; Brisard, G.; Sharma, G.; Harvey, P.; A 9.16% Power Conversion Efficiency Organic Solar Cell with a Porphyrin Conjugated Polymer Using a Nonfullerene Acceptor. ACS Appl. Mater. Interfaces 2019, 11, 31, 28078-28087. Figure 1

Alternative sequential deposition for optimization-free multi-component organic bulk heterojunctions

Multi-component (ternary, quaternary, and beyond) bulk heterojunction (BHJ) active layers are considered one of the most effective ways of increasing the performance of organic photovoltaics (OPVs). Herein, an alternative sequential deposition method for the development of efficient multi-component OPVs without the need for the complex optimization steps typical for blend parameters is discussed. Simple sequential spin-coating of two binary donor:acceptor blends was applied to produce modified quaternary BHJ layers, making full use of the optoelectronic advantages offered by the two binary blends and their optimized morphological features. A combination of theoretical and experimental analyses revealed the occurrence of molecular reorganization during sequential deposition which facilitated self-optimized molecular stratification and cascade energy level alignment. By using expanded universality test to account for the diverse BHJ systems, this sequential deposition methodology creates new opportunities as an alternative fabrication platform to secure high-performance multi-component BHJ OPVs for versatile (i.e., indoor and outdoor) irradiation environments while overcoming many of the drawbacks of conventional manufacturing procedures. • Alternative manufacturing process of organic photovoltaics (OPVs) is demonstrated. • Sequential spin-coating of two binary blends produces modified multi-component bulk heterojunction layers. • Performance is optimized for both indoor and outdoor light conditions without the need for fine-tuned blend optimization. • Sequential deposition is a universal platform to achieve optimization-free, high-performance multi-component OPVs.

Origin of Low Open-Circuit Voltage in Surfactant-Stabilized Organic-Nanoparticle-Based Solar Cells.

Organic-nanoparticle-based solar cells have drawn great attention due to their eco-friendly and environmentally friendly fabrication procedure. However, these surfactant-stabilized nanoparticles suffer open-circuit voltage loss due to charge trapping and poor extraction rate at the polymer cathode interface. Here, we have investigated the origin of voltage loss and charge trapping in surfactant-stabilized nanoparticle-based devices. Efficient organic photovoltaic (OPV) devices have been fabricated from an aqueous dispersion of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) nanoparticles stabilized by anionic surfactants. AC impedance spectroscopy has been used to understand the charge transport properties in the dark and in operando conditions. We have demonstrated the similarities in the charge transport properties, as well as photocarrier dynamics of the nanoparticle-based OPVs and the bulk heterojunction OPVs despite fundamental differences in their nanostructure morphology. This study emphasizes the possibility of fabricating highly efficient OPVs from organic nanoparticles by reducing surface defects and excess doping of the polymers.

Organic bulk heterojunction photovoltaics incorporating cyclopenteno[60]fullerene monoadducts as n‐type materials display superior power conversion efficiency than with PC61BM

AbstractOrganic bulk heterojunction photovoltaics, merely incorporating monoadducts of cyclopenteno[60]fullerenes (CPFs) as n‐type materials and P3HT as p‐type materials, display superior power conversion efficiency up to 4.6 ± 0.12%, superseding that with PC61BM/P3HT (3.8 ± 0.20%) for ca. 20%, under AM 1.5G irradiation―primarily attributed to the lack of homo‐conjugation on CPFs and their higher LUMO energy levels.

Squaraine dyes containing diphenylamine group: Effects of different type structures on material properties and organic photovoltaic performances

Four donor-acceptor-donor (D-A-D’) unsymmetrical squaraines (USQs) with different molecular skeletons (XZ-type and YZ-type), containing diphenylamine group with/without methoxy substituent were synthesized as donor materials in bulk-heterojunction (BHJ) organic photovoltaics (OPVs). The introduction of methoxy group in USQs has little different effects on the overall photovoltaic performance. Conversely, the different molecular skeleton types of the USQs have significant influence on their material properties and photovoltaic performances. Compared to BIDPSQ and BIDPOMeSQ with XZ-type molecular skeleton, IDPSQ and IDPOMeSQ with YZ-type molecular skeleton display closer solid-state packing, much lower highest occupied molecular orbital (HOMO) energy level, higher hole mobility and smaller phase separation domain size. Consequently, YZ-type USQs exhibit the most excellent performance with power conversion efficiency (PCE) of ~4%, which is approximately 300% higher than those of XZ-type USQs. Surprisingly, even though IDPSQ and IDPOMeSQ show wide band gaps, the corresponding devices still achieve a highest PCE of ~4%, which is comparable to or even higher than the PCEs of some reported SQ-based devices with low band gaps. These results indicate the YZ-type molecular skeleton and the electron-donating diphenylamine group are very promising to construct highly efficient squaraine donor materials.