Small Molecule Bulk Heterojunction Research Articles

Linear-Shaped Low-Bandgap Asymmetric Conjugated Donor Molecule for Fabrication of Bulk Heterojunction Small-Molecule Organic Solar Cells

A linear-shaped small organic molecule (E)-4-(5-(3,5-dimethoxy-styryl)thiophen-2-yl)-7-(5″-hexyl-[2,2':5',2″-terthiophen]-5-yl)benzo[c][1,2,5]thiadiazole (MBTR) comprising a benzothiadiazole (BTD) acceptor linked with the terminal donors bithiophene and dimethoxy vinylbenzene through a π-bridge thiophene was synthesized and analyzed. The MBTR efficiently tuned the thermal, absorption, and emission characteristics to enhance the molecular packing and aggregation behaviors in the solid state. The obtained optical bandgap of 1.86 eV and low-lying highest occupied molecular orbital (HOMO) level of -5.42 eV efficiently lowered the energy losses in the fabricated devices, thereby achieving enhanced photovoltaic performances. The optimized MBTR:PC71BM (1:2.5 w/w%) fullerene-based devices showed a maximum power conversion efficiency (PCE) of 7.05%, with an open-circuit voltage (VOC) of 0.943 V, short-circuit current density (JSC) of 12.63 mA/cm2, and fill factor (FF) of 59.2%. With the addition of 3% 1,8-diiodooctane (DIO), the PCE improved to 8.76% with a high VOC of 1.02 V, JSC of 13.78 mA/cm2, and FF of 62.3%, which are associated with improved charge transport at the donor/acceptor interfaces owing to the fibrous active layer morphology and favorable phase separation. These results demonstrate that the introduction of suitable donor/acceptor groups in molecular design and device engineering is an effective approach to enhancing the photovoltaic performances of organic solar cells.

Water‐Insensitive Electron Transport and Photoactive Layers for Improved Underwater Stability of Organic Photovoltaics

AbstractWater ingress is one of the major environmental stresses to cause the degradation of device performance in organic photovoltaic (OPVs) and is one of the major barriers impeding their commercialization. This work demonstrates that combining the use of a nanoparticle titanium dioxide (np‐TiO2) electron transporting layer (ETL) and an all‐polymer bulk heterojunction (BHJ) photoactive layer can endow the derived OPV with a much better water resistivity than the commonly employed zinc oxide (ZnO) ETL or polymer:small molecule BHJ blends. Polymer donors/acceptors are first shown to possess better water‐immersion than the small molecule counterparts. Hence, the all‐polymer blend exhibits the lowest absorbance losses after water immersion among the studied BHJ systems. Furthermore, the result reveals that tailoring the structure of the TiO2 ETL from planar to nanoparticles effectively strengthens the adhesion at the ETL/BHJ interface to prevent physical delamination. Finally, the np‐TiO2/all‐polymer blend (half‐cell) is demonstrated to have superior stability under water immersion, i.e., unchanged morphology and charge carrier transfer, as well as no efficiency changes in the complete cells. This work demonstrates the great potential of the all‐polymer blends and np‐TiO2 ETL for improving the durability of unencapsulated OPVs under high humidity environments and even water immersion.

Crystallinity and Molecular Packing of Small Molecules in Bulk-Heterojunction Organic Solar Cells

Crystallinity has played a major role in organic solar cells (OSCs). In small molecule (SM) bulk-heterojunction (BHJ) OSCs, the crystallinity and crystalline packing of SM donors have been shown to have a dramatic impact on the formation of an optimum microstructure leading to high-power conversion efficiency (PCE). Herein we describe how crystallinity differs from polymers to SMs, and how the packing habits of SMs (particularly donors) in active layers of BHJ devices can be described as following two different main modes: a single crystal-like and a liquid crystal-like packing type. This notion is reviewed from a chronological perspective, emphasising milestone donor structures and studies focusing on the crystallinity in SM-BHJ OSCs. This review intends to demonstrate that a shift towards a liquid crystalline-like packing can be identified throughout the history of SM-BHJ, and that this shift can be associated with an increase in overall PCE.

Vertical Miscibility of Bulk Heterojunction Films Contributes to High Photovoltaic Performance

AbstractThe power conversion efficiencies (PCEs) of organic photovoltaic cells employing donor (polymer):acceptor (small molecule) bulk heterojunction (BHJ) are being rapidly improved. The phase segregation along vertical (film‐depth) direction of the active layer, referring to vertical immiscibility between donor and acceptor, leads to vertically varied and nonlinearly distributed composition, optical and electronic properties. However, the correlation between vertical miscibility and photovoltaic performance is still confusing. Here, it is semi‐empirically found that such vertical variations induced by vertical immiscibility deteriorate PCE. Subsequently, using PM6:Y6‐based binary and ternary BHJ as examples, a combined statistical, experimental, and numerical investigation on the dependence of photovoltaic performance on vertical miscibility is reported. The vertical phase evolution of BHJ significantly depends on solvents and processing methods. As compared with other donor:acceptor systems, polymer:Y6 deposited from appropriate solvents could have the best vertical miscibility which is nearly independent on film‐depth, leading to a higher PCE. PM6:Y6:fullerene ternary blends also have a good and uniform vertical miscibility, forming spatially well‐mixed ternary BHJ. Consequently, under this design guideline, the optimized film‐depth‐dependent miscibility contributes to optimized vertical distribution of optical and electronic properties, leading to an optimized PCE 17.1% with a low sensitivity to fluctuation of film thickness.

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

Carbazole-based green and blue-BODIPY dyads and triads as donors for bulk heterojunction organic solar cells.

Two BODIPY derivatives with one (B2) and two (B3) carbazole moieties were synthesized and applied as electron-donor materials in organic photovoltaic cells (OPV). Their optical and electrochemical properties were systematically investigated. These BODIPY dyes exhibit excellent solubility in organic solvents and present high molar extinction coefficients (1.37-1.48 × 105 M-1 cm-1) in solutions with absorption maxima at 586 nm for mono-styryl groups and at 672 nm for di-styryl groups. The introduction of the styryl moieties results in a large bathochromic shift and a significant decrease in the HOMO-LUMO energy-gaps. The BODIPY dyes show relatively low HOMO energies ranging from -4.99 to -5.16 eV as determined from cyclic voltammetry measurements. Cyclic voltammetry measurements and theoretical calculations demonstrate that the frontier molecular orbital levels of these compounds match with those of PC71BM as the acceptor, supporting their application as donor materials in solution-processed small molecule bulk heterojunction (BHJ) organic solar cells. After the optimization of the active layer, B2:PC71BM and B3:PC71BM based organic solar cells showed an overall power conversion efficiency of 6.41% and 7.47%, respectively. The higher PCE of the B3-based OSC is ascribed to the more balanced charge transport and exciton dissociation, better crystallinity and molecular packing.

Understanding the loss mechanisms in high-performance solution-processed small molecule bulk heterojunction solar cells doped with a PFN impurity.

Contamination of the active layer with an impurity could result in significant degradation in the performance of bulk heterojunction (BHJ) solar cells as a result of enhancing the loss of the charge carriers via a trap-assisted recombination. In this study, PFN as an impurity was intentionally introduced to a BHJ solar cell composed of a high-performance solution-processed small molecule (p-DTS(FBTTh2)2 as a donor and PC60BM as an acceptor. The power conversion efficiency (PCE) of PFN doped devices degrades owing to the reduction of short-circuit current (Jsc) and fill factor (FF). At a low concentration, PFN mostly reduces the generation of charge carriers, whereas doubling the PFN concentration conversely affects both generation and collection of charge carriers. Charge carrier dynamics of devices has also been probed using photovoltage decay, time-resolved charge extraction (TRCE) and photoinduced charge extraction by linearly increasing voltage (photo-CELIV) before and after incorporation of PFN. The results reveal that traps introduced by PFN reduce the decay of charge carriers via bimolecular recombination, leading to a higher charge carrier density and photovoltage at long times under an open-circuit potential (Voc). However, under short-circuit (Jsc) conditions, traps considerably impede the collection of charge carriers causing the appearance of an S-shaped current density-voltage curve.

Minimizing geminate recombination losses in small-molecule-based organic solar cells

Geminate recombination rates are successfully predicted for series of small-molecule bulk heterojunction solar cells applying the Marcus–Levich–Jortner equation.

Comparison of the Solution and Vacuum-Processed Squaraine:Fullerene Small-Molecule Bulk Heterojunction Solar Cells.

Squaraine dyes have shown promising properties for high performance organic solar cells owing to their advantages of intense absorption and high absorption coefficients in the visible and near-infrared (NIR) regions. In this work, to directly compare the photovoltaic performance of solution- and vacuum-processed small-molecule bulk heterojunction (SMBHJ) solar cells, we employed a squaraine small molecular dye, 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (DIBSQ), as an electron donor combined with fullerene acceptors to fabricate SMBHJ cells either from solution or vacuum deposition process. The solution-processed SMBHJ cell possesses a power conversion efficiency (PCE) of ~4.3%, while the vacuum-processed cell provides a PCE of ~6.3%. Comparison of the device performance shows that the vacuum-processed SMBHJ cells possess higher short-circuit current density, fill factor and thus higher PCE than the solution-processed devices, which should be assigned to more efficient charge transport and charge extraction in the vacuum-processed SMBHJ cells. However, solution-processed SMBHJ cells demonstrate more pronounced temperature-dependent device performance and higher device stability. This study indicates the great potential of DIBSQ in photovoltaic application via both of solution and vacuum processing techniques.

Morphology of a Bulk Heterojunction Photovoltaic Cell with Low Donor Concentration.

Atomistic nonequilibrium molecular dynamics simulations have been used to model the morphology of small-molecule bulk heterojunction films formed by vapor deposition as used in organic photovoltaics. Films comprising C60 and 1, 5, 10, and 50 wt % of 1,1-bis[4-bis(4-methylphenyl)aminophenyl]cyclohexane (TAPC) were compared to films of neat C60. The simulations suggest that if holes can hop between donor molecules separated by as little as 1.2-1.5 nm, then a TAPC concentration of 5 wt % is sufficient to form a percolating donor network and facilitate charge extraction. The results provide an explanation for why low donor content organic photovoltaics can still have high efficiencies. In addition, the roughness, porosity, and crystallinity of the films were found to decrease with increasing TAPC content.

Investigation of S-shaped current-voltage characteristics in high-performance solution-processed small molecule bulk heterojunction solar cells

The appearance of an S-shaped current-voltage curve and its impact on the performance of solution-processed small molecule p-DTS(FBTTh2)2:PC70BM bulk heterojunction solar cell devices were investigated. The power conversion efficiency of the devices showing S-curve characteristics was almost half of what was expected for this combination of materials, which was due to a low fill factor (FF) and 20% lower short circuit current density. A normal diode-like current-voltage curve was retrieved when the PCBM content of the active layer was increased, with power conversion efficiency reaching 6%. Analysis of the current-voltage characteristics of the S-curve solar cell devices showed that the effective voltage of the devices throughout the active layer was diminished due to an energy barrier. As a result, the charge collection efficiency was significantly affected. In addition, smaller external quantum efficiency (EQE) values in the absorption range of the small molecule donor in the S-curve devices suggested that charge generation was also affected. Recombination dynamics of both the normal and S-curve devices under steady-state conditions were quite similar. However, photovoltage decay results showed a distinguished pattern of recombination at open circuit conditions in the S-curve devices. This suggested different recombination rates at the cathode and anode electrodes, which is possibly due to the different composition of the donor-acceptor film near the contacts.

BODIPY-based panchromatic chromophore for efficient organic solar cell

A novel donor-acceptor-donor (D-A-D) type chromophore, named BDP-dBDT, with a boron dipyrromethene (BODIPY) linked through alkynyl with two benzo [1,2-b:4,5-b’]dithiophene (BDT) terminal donors was designed and synthesized for solution-processed small molecule bulk heterojunction (BHJ) solar cell. The thermal and optochemical properties, BHJ morphology and solar cell performance were investigated. The new molecule shows panchromatic absorption with narrow optical band gap and high molar extinction coefficient. Moreover, the molecule displays deep highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. The BHJ morphology was systematically optimized by additives, thermal annealing (TA) and solvent vapor annealing (SVA). Processed by a combination of TA and SVA treatments, the BDP-dBDT:PC71BM device exhibits a short-circuit current density (Jsc) of 12.98 mA cm−2, and a power conversion efficiency (PCE) of 5.61%, which is one of the best values among the ever reported BODIPY based organic solar cells.

Robust Processing of Small-Molecule:Fullerene Organic Solar Cells via Use of Nucleating Agents

The power conversion efficiency (PCE) of small-molecule bulk heterojunction solar cells is highly sensitive to the “ink” formulation used to produce the photoactive layer. Here we demonstrate that the addition of nucleating agents renders device fabrication notably less susceptible to the ink composition, promising a route toward more robust processing of efficient devices over large areas and enabling more facile materials screening. We selected as a model system blends of 7,7-[4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b]dithiophene-2,6-diyl]bis[6-fluoro-4-(5-hexyl-[2,2-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole](p-DTS(FBTTh2)2) as the donor and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) as the acceptor because this is one of the small-molecule OPV blends with a device performance that is most sensitive to ink formulation, especially when used with the processing aid diiodooctane (DIO). Addition of DIO is essential to obtain high device performances; however, a notable increase in device perfor...

Performances and impedance spectroscopy of Small-molecule bulk heterojunction solar cells based on PtOEP: PCBM

Small-molecule bulk heterojunction (SBHJ) solar cells based on platinum octaethylporphyrin (PtOEP) as donor material and phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor were fabricated using spin coating techniques with weight ratios from 1:0.1 to 1:9. The formation of charge transfer complex CTC in the PtOEP: PCBM blend was specified from the redshift of the PtOEP absorption peak after blending with PCBM. The photovoltaic performance for PtOEP: PCBM blends were investigated using the external quantum efficiency (EQE) besides the current density–voltage (J–V) characteristics under illumination100 mW/cm2 (AM1.5G). The BHJ solar cell with PtOEP: PCBM ratio of 1:9 exhibited the best performance. The impedance spectroscopy (IS) was examined in the frequency range from 25 Hz to 1 MHz. The equivalent circuit model was evaluated in details to evaluate the impedance spectroscopy parameters. Dielectric constant \({\varepsilon ^\prime }\), dielectric loss \({\varepsilon ^{\prime \prime }}\) and dielectric modulus were included and discussed in terms of dielectric polarization processes. Dielectric modulus displays the non-Debye relaxation in PtOEP: PCBM BHJ solar cells.

T-Shaped Indan-1,3-dione derivatives as promising electron donors for bulk heterojunction small molecule solar cell

We report on the photovoltaic performance of novel T-Shaped Indan-1,3-dione derivatives as donors in a solution processed bulk heterojunction solar cells. Small molecule bulk heterojunction solar cells of these molecules with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) were fabricated and characterized. The preliminary characterization of these devices yielded a PCE of 0.24% and 0.33% for two separate derivatives. These low power conversion efficiencies were attributed to a high surface roughness with a large number of dewetting spots. Doping with 10% Polystyrene in the Indan-1,3-dione derivatives decreases surface roughness and dewetting spots thereby improving the efficiency of the devices. Efficiency of the devices was found as 0.39% and 0.51% for two derivatives after doping with polystyrene. The charge transfer mechanism was studied with photoluminescence quenching. The morphology and packing behavior of molecules were further studied using Atomic Force Microscopy (AFM) and X-ray diffraction (XRD).

Vinazene end-capped acceptor-donor-acceptor type small molecule for solution-processed organic solar cells

An acceptor-donor-acceptor (A-D-A) type molecule based on dioctyltertthiophene-benzo[1,2-b:4,5-b′]dithiophene-dioctyltertthiophene central donor and vinazene terminal acceptor was designed and synthesized for solution-processed small molecule bulk-heterojunction (BHJ) solar cells. The thermal and optochemical properties, BHJ morphology and solar cell performance were investigated. The BHJ morphology was systematically optimized by thermal annealing, solvent vapor annealing, and the use of solvent additives. Processed by a combination of thermal annealing and solvent vapor annealing treatments, V-BDT:PC71BM device showed an optimized PCE of 3.73% with a VOC of 0.89 V, an JSC of 6.88 mA cm−2 and a FF of 0.61.

Influence of fullerene-based acceptor materials on the performance of indacenodithiophene-cored small molecule bulk heterojunction organic solar cells

A model system is presented to investigate the influence of fullerene-based acceptor materials on the performance of small molecule organic solar cells (SM-OSCs). Two indacenodithiophene-(IDT)-cored small molecules and one diketopyrrolopyrrole-(DPP)-cored molecule are used as donor materials, and two most widely applicable fullerene-based acceptors, [6,6]-phenyl-C61(71)-butyric acid methyl esters (PC61(71)BM) are used as the acceptor material, respectively. The charge generation, transport, and recombination behaviors were studied under various irradiation intensities, and these parameters are relevant to device performance. The transient photocurrent and transient photovoltage operation are utilized to directly compare the photo-induced absorption, charge generation, and charge recombination properties of the corresponding blend films. Our results indicate that different fullerene derivatives not only affect the device performance with their different light absorption abilities but also have roles in charge generation and charge recombination, which greatly helps to understand how to select a better fullerene-based acceptor for the high-performance SM-OSCs.

Dithienobenzodithiophene-Based Small Molecule Organic Solar Cells with over 7% Efficiency via Additive- and Thermal-Annealing-Free Processing.

Here we introduce a novel small molecule based on dithienobenzodithiophene and rhodanine, DTBDT-Rho, developed to study the effect of the rhodanine substitutuent on small molecule bulk heterojunction (BHJ) solar cells. DTBDT-Rho possesses distinct crystalline characteristics, sufficient solubility in chlorinated solvents, and broad absorption properties. Therefore, solution-processed BHJ photovoltaic cells made with DTBDT-Rho:PC71BM blends showed an extremely high power conversion efficiency (PCE; 7.10%); notably, this PCE value was obtained without the use of additives or thermal treatments. To our knowledge, the PCE over 7% is a significantly powerful value among rhodanine-based small molecule BHJ solar cells without additives or thermal treatments.

High photo-current in solution processed organic solar cells based on a porphyrin core A-π-D-π-A as electron donor material

Two new conjugated acceptor-donor-acceptor (A-π-D-π-A) molecules with a porphyrin core linked by ethynylene bridges to two thiophene (1a) or thienylenevinylenethiophene (1b) units and both capped by N-ethylrhodanine have been synthesized. These compounds were used as the main electron donor moieties for bulk heterojunction small molecule organic solar cells (BHJ-SMOSC). The optimized devices, with PC71BM as the main electron acceptor molecule, show remarkable short circuit currents, up to 13.2 mA/cm2, an open circuit voltage of around 0.85 V, and power conversion efficiencies up to 4.3% under 100 W/cm2. The External Quantum Efficiency (EQE), Atomic Force Microscopy (AFM), hole mobility, Photo-Induced Charge Extraction (PICE) and Photo-Induced Transient Photo-Voltage (PIT-PV) were analyzed in devices based on 1a and 1b in order to account for differences in the final performance of the two molecules. The PIT-PV decays showed slower recombination kinetics for devices fabricated with 1b. Moreover, the EQE was greater for 1b and this is ascribed to the better nanomorphology, which allows better charge collection before carrier recombination takes place.

Solution-processed small-molecule organic solar cells based on diketopyrrolopyrrole and perlyene derivatives with coplanar structures

ABSTRACTA series of small-molecules (SMs) based on coplanar perylene unit coupled with diketopyrrolopyrrole chromophoric core exhibit broad absorption in the range of 500–800 nm and low bandgap energise of 1.6–1.7 eV. The power conversion efficiency approximately reach at 0.4% under 1.5 G illumination is achieved for organic solar cells based on a small-molecule bulk heterojunction system consisting of SMs as a donor and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as an acceptor.