Solution-processed Bulk-heterojunction Organic Solar Cells Research Articles

The critical role of temperature-dependent mobilities in determining the open-circuit voltage of bulk-heterojunction organic solar cells

While solution-processed bulk-heterojunction organic solar cells (OSCs) continue to attract attention as their efficiencies approach 20%, the physical origin of the non-radiative energy loss in OSCs remains under debate. Understanding the temperature dependence of open-circuit voltage (VOC) is thus important because it provides unique insights into the origin of energy loss. Herein, we simulate the VOC vs T relation of PTB7-Th:PC71BM bulk-heterojunction OSCs within the range of 160–295 K by incorporating experimentally measured temperature-dependent mobilities into the drift-diffusion model, assuming bimolecular recombination as the primary recombination mechanism. Significantly, we find that the temperature dependence of VOC can only be correctly reproduced by the model when the temperature dependence of the carrier mobilities is taken into account. The effect of the Langevin reduction coefficient on the temperature dependence of VOC is also investigated.

Benzoselenadiazole‐core asymmetric D‐A‐A small molecule for solution processed bulk heterojunction organic solar cells

The present work is accounted on the designing of a new and efficient asymmetric organic chromophore, named 4-(3,5-bis [trifluoromethyl] phenyl)-7-(5′-hexyl-[2,2′-bithiophen]-5-yl)-benzo[c [1, 2, 5]selenadiazole, (RT-BSe-F), based on benzoselenadiazole central acceptor building blocks. The acceptor unit of 3,5-bis (trifluoromethyl) benzene and donor unit of alkyl bithiophene attached with benzoselenadiazole central unit showed large impacts on the optical and electrochemical properties. The reasonable optical band gap of ~2.02 eV and HOMO of −5.33 eV were obtained by RT-BSe-F chromophore due to a strong electron accepting nature of fluorine based compound. With 3,5-bis(trifluoromethyl) benzene unit, the absorption of RT-BSe-F chromophore was considerably increased to higher wavelength which might enhance the crystallinity of thin film with high hole mobility. RT-BSe-F chromophore was effectively applied to fabricate the solution-processed bulk-heterojunction organic solar cells (BHJ-OSCs) and attained a high power conversion efficiency (PCE) of ~3.75% accompanying high JSC of ~12.56 mA cm−2, FF of ~0.42 and VOC of ~0.71 V. The obtained high PCE might be associated to a high surface energy of TiO2 layer as buffer and the use of high mobility organic RT-BSe-F chromophore.

A symmetric benzoselenadiazole based D–A–D small molecule for solution processed bulk-heterojunction organic solar cells

Benzoselenadiazole as the central acceptor unit with terminal donor unit of n-hexylbithiophene was chosen to design the symmetric donor-acceptor-donor (D–A–D) configuration based small organic chromophore, 4,7-bis(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]selenadiazole, RTh-BSe-ThR, for the fabrication of solution-processed bulk-heterojunction organic solar cells (BHJ-OSCs). RTh-BSe-ThR chromophore showed assuring solubility in common organic solvents and exhibited wider absorption in the visible region with an optical band gap (Egopt) of ∼1.87eV. Benzoselenadiazole unit with two n-hexylbithiophene units exhibited an excellent electrochemical behavior with the highest occupied molecular orbital (HOMO) of −5.38eV and the lowest unoccupied molecular orbital (LUMO) of −3.51eV. The fabricated BHJ-OSCs with RTh-BSe-ThR:PC61BM (1:3, w/w) blend thin film displayed a power conversion efficiency (PCE) of ∼3.46% and a high value of short circuit current density (JSC) of ∼11.20mA/cm2. The enhancement in the photovoltaic parameters might be attributed to the significant improvements in the interfacial area of bulk heterojunction which might enhance the light harvesting property and the shunt resistance of the blend thin films.

Synthesis and characterization of new asymmetric thieno[3,4-b]pyrazine-based D−π−A−A type small molecular donors with near-infrared absorption and their photovoltaic applications

Abstract Two new asymmetric D−π−A1−A2 configured small molecules named PTP and TTP bearing thieno[3,4-b]pyrazine (TP) unit as the A1 group have been designed and synthesized for solution-processed bulk-heterojunction organic solar cells (OSCs). These new dipolar molecular donors not only exhibit strong near-IR absorption but pack into antiparallel dimers, in which PTP shows a coexistence of coplanar and cofacial antiparallel dimers configuration, while TTP reveals a coplanar centrosymmetric dimer structure. Solution-processed OSCs for PTP blended with pristine C70 achieves a promising power conversion efficiency (PCE) of 3.40% along with an open-circuit voltage (VOC) of 0.64 V, a short-circuit current density (JSC) of 9.83 mA cm−2 and a fill factor (FF) of 54.0%, which is the highest efficiency to date among the TP-based materials. Conversely, the optimized TTP:C70 solar cell delivers a VOC of 0.48 V, a JSC of 6.02 mA cm−2 and a FF of 43.3%, corresponding to a moderate PCE of 1.25%.

Improvement of organic solar cell performance via the incorporation of a MgO cathode interlayer fabricated by a reaction of thermally deposited Mg with MoO3

The use of magnesium oxide (MgO), which was formed by a reaction of thermally deposited magnesium with molybdenum trioxide, as a cathode interlayer material in vacuum-processed, planar pn-heterojunction organic solar cells (OSCs) using copper phthalocyanine and C70 and bulk-heterojunction (BHJ) OSCs using tris[(5-phenylthiophen-2-yl)phenyl]amine and C70 reduced the series resistance, thus improving the fill factor and power conversion efficiency (PCE). On the other hand, the use of the MgO cathode interlayer in solution-processed BHJ OSCs using poly(3-hexylthiophene) and [6,6]-phenyl-C71-butyric acid methyl ester improved the open-circuit voltage, thus improving the PCE. It is speculated that the improved performance is attributed to the high hole blocking ability of the MgO layer due to the deep-lying valence band level of MgO.

ITO with embedded silver grids as transparent conductive electrodes for large area organic solar cells

In this work, development of semi-transparent electrodes for efficient large area organic solar cells (OSCs) has been demonstrated. Electron beam evaporated silver grids were embedded in commercially available ITO coatings on glass, through a standard negative photolithography process, in order to improve the conductivity of planar ITO substrates. The fabricated electrodes with embedded line and square patterned Ag grids reduced the sheet resistance of ITO by 25% and 40%, respectively, showing optical transmittance drops of less than 6% within the complete visible light spectrum for both patterns. Solution processed bulk heterojunction OSCs based on PTB7:[70]PCBM were fabricated on top of these electrodes with cell areas of 4.38 cm2, and the performance of these OSCs was compared to reference cells fabricated on pure ITO electrodes. The Fill Factor (FF) of the large-scale OSCs fabricated on ITO with embedded Ag grids was enhanced by 18% for the line grids pattern and 30% for the square grids pattern compared to that of the reference OSCs. The increase in the FF was directly correlated to the decrease in the series resistance of the OSCs. The maximum power conversion efficiency (PCE) of the OSCs was measured to be 4.34%, which is 23% higher than the PCE of the reference OSCs. As the presented method does not involve high temperature processing, it could be considered a general approach for development of large area organic electronics on solvent resistant, flexible substrates.

Synthesis of Bioinspired Curcuminoid Small Molecules for Solution-Processed Organic Solar Cells with High Open-Circuit Voltage

Borondifluoride complexes of curcuminoid derivatives end-capped with triphenylamine groups were designed for solution-processed bulk-heterojunction organic solar cells. They were obtained very simply in a one-pot synthesis from cheap building blocks. Compared to push–pull systems based on borondifluoride complexes of hydroxychalcones, curcuminoids present the donor–acceptor–donor electronic structure and exhibit significantly improved chemical and thermal stability and photovoltaic performance. Indeed, power conversion efficiency up to 4.14% and high open-circuit voltage over 1.0 V have been achieved using PC61BM as acceptor.

Cyclopentadithiophene-based co-oligomers for solution-processed organic solar cells

A new family of low band-gap co-oligomers based on 4H-cyclopenta[2,1-b:3,4-b']dithiophene (CPDT) and thieno[3,2-b]thiophene (TT) units as central electron-donor cores has been synthesized and characterized for use as electron-donor materials in solution-processed bulk-heterojunction organic solar cells. An in-depth study into the role played by the hexyl chains linked to the thienylenevinylene-based π-bridge has been carried out. Power conversion efficiencies (PCE) of up to 4% and external quantum efficiencies as high as 50% have been achieved. Experiments carried out after solvent vapour annealing (SVA) as a post-treatment led to a doubling of the fill factor (FF) and PCE.

Conjugated D-A porphyrin dimers for solution-processed bulk-heterojunction organic solar cells.

Three conjugated D-A porphyrin dimers (DPP-ZnP-E)2, (DPP-ZnP-E)2-2T and (DPP-ZnP-E)2-Ph linked with diethynylene, diethynylene-dithiophene and diethynylene-phenylene have been developed for bulk heterojunction solar cells with high power conversion efficiencies of 4.50%, 5.50% and 6.42%, respectively, when blended with PC61BM as the electron acceptor material.

Edge-to-face stacking non-fullerene small molecule acceptor for bulk heterojunction solar cells

Synthesis of a novel small molecule accepter with perylenediimide linked through thiophene ring with triphenylamine is described. Introducing thiophene unit leads to extending the molecular absorption spectrum to the range of 330 nm–700 nm. The molecule showed the lowest unoccupied molecular orbital level (LUMO) of −3.80 eV and the highest occupied molecular orbital level (HOMO) of −5.60 eV. The X-ray Diffraction (XRD) spectra confirmed that the molecule self-assemble in the solid phase by edge-to-face aromatic interaction, and the addition of 1, 8-diiodooctane in the solvent can adjust its stacking, and thus improve the crystallinity and electron mobility. Solution-processed bulk-heterojunction organic solar cells constructed using the small molecule as acceptor and the polymer of PBDTTT-C-T as donor showed the best efficiency of 1.92%.

Improved efficiency of solution-processed bulk-heterojunction organic solar cells and planar-heterojunction perovskite solar cells with efficient hole-extracting Si nanocrystals

Si nanocrystals for efficient hole extraction in solution-processed BHJ OSCs and PHJ PrSCs led highly enhanced PCEs by ~11% and ~23% respectively, compared with those without Si nanocrystals.

Oligomer Molecules for Efficient Organic Photovoltaics.

Solar cells, a renewable, clean energy technology that efficiently converts sunlight into electricity, are a promising long-term solution for energy and environmental problems caused by a mass of production and the use of fossil fuels. Solution-processed organic solar cells (OSCs) have attracted much attention in the past few years because of several advantages, including easy fabrication, low cost, lightweight, and flexibility. Now, OSCs exhibit power conversion efficiencies (PCEs) of over 10%. In the early stage of OSCs, vapor-deposited organic dye materials were first used in bilayer heterojunction devices in the 1980s, and then, solution-processed polymers were introduced in bulk heterojunction (BHJ) devices. Relative to polymers, vapor-deposited small molecules offer potential advantages, such as a defined molecular structure, definite molecular weight, easy purification, mass-scale production, and good batch-to-batch reproducibility. However, the limited solubility and high crystallinity of vapor-deposited small molecules are unfavorable for use in solution-processed BHJ OSCs. Conversely, polymers have good solution-processing and film-forming properties and are easily processed into flexible devices, whereas their polydispersity of molecular weights and difficulty in purification results in batch to batch variation, which may hamper performance reproducibility and commercialization. Oligomer molecules (OMs) are monodisperse big molecules with intermediate molecular weights (generally in the thousands), and their sizes are between those of small molecules (generally with molecular weights <1000) and polymers (generally with molecular weights >10000). OMs not only overcome shortcomings of both vapor-deposited small molecules and solution-processed polymers, but also combine their advantages, such as defined molecular structure, definite molecular weight, easy purification, mass-scale production, good batch-to-batch reproducibility, good solution processability, and film-forming properties. Therefore, OMs are a good choice for solution-processed reproducible OSCs toward scalable commercialized applications. Considerable efforts have been dedicated to developing new OM electron donors and electron acceptors for OSCs. So far, the highest PCEs of solution-processed OSCs based on OM donors and acceptors are 9-10% and 6-7%, respectively. OM materials have become promising alternatives to polymer and/or fullerene materials for efficient and stable OSCs. In this Account, we present a brief survey of the recent developments in solution-processable OM electron donors and acceptors and their application in OSCs. Rational design of OMs with star- and linear-shaped structures based on triphenylamine, benzodithiophene, and indacenodithiophene units and their impacts on device performance are discussed. Structure-property relationships are also proposed. Furthermore, the remaining challenges and the key research directions in the near future are also addressed. In the next years, an interdisciplinary approach involving novel OM materials, especially electron acceptor materials, accurate morphology optimization, and advanced device technologies will probably bring high-efficiency and stable OSCs to final commercialization.

Furan-bridged thiazolo [5,4-d]thiazole based D–π–A–π–D type linear chromophore for solution-processed bulk-heterojunction organic solar cells

Novel furan-bridged thiazolo[5,4-d]thiazole based π-conjugated organic chromophore (RFTzR) was formulated and utilized for the fabrication of solution-processed small molecule organic solar cells (SMOSCs).

Solution-processed bulk-heterojunction organic solar cells employing Ir complexes as electron donors

To explore enhancing photocurrent in organic solar cells (OSCs) via harvesting triplet excitons, two novel bicycloiridium complexes (R1 and R2) are designed and synthesized. Conventional bulk-heterojunction triplet OSCs are solution processed using R1 or R2 as sole electron donors and phenyl-C71-butyric acid methyl ester (PC71BM) as the electron acceptor. A decent short circuit current (Jsc) of 6.5 mA cm−2 is achieved though the overlap between the absorption spectrum (with ∼550 nm absorption onset) of R2 and the solar flux is relatively small. With an open circuit voltage of 0.74 V and a fill factor of 0.42, an encouraging power conversion efficiency of 2.0% is achieved in the OSCs based on R2 and PC71BM without any processing additives and post-treatments. Our preliminary result demonstrates the possibility of utilizing Ir complexes as sole electron donors in OSCs, which extends available soluble small molecules for OSCs.

Synthesis and photovoltaic properties of a star-shaped molecule based on a triphenylamine core and branched terthiophene end groups

A new star-shaped donor-acceptor molecule has been synthesized for application as the donor material in solution-processed bulk-heterojunction organic solar cells (OSCs). The molecule consists of a triphenylamine (TPA) unit as the core and a donor unit with three arms containing benzo[1,2,5]thiadiazole (BT) acceptor units and 5,5″-dihexyl-2,2′:3′,2″-terthiophene (tTh) end groups. The molecule, denoted S(TPA-BT-tTh), exhibits a broad absorption band in the wavelength range 300–650 nm and high hole mobility of 1.1×10−4 cm2 V−1 s−1. An OSC device based on S(TPA-BT-tTh) as donor and [6,6]-phenyl C71-butyric acid methyl ester (PC70BM) as the acceptor (1:3, w/w) exhibited a power conversion efficiency of 2.28% with a short circuit current density of 6.39 mA/cm2 under illumination of AM.1.5, 100 mW/cm2.

A star-shaped oligothiophene with triphenylamine as core and octyl cyanoacetate as end groups for solution-processed organic solar cells

A new star-shaped D–π–A molecule with triphenylamine (TPA) as core and donor unit, octyl cyanoacetate (CA) as end group and acceptor unit, and 2,2′-bithiophene vinylene (bTV) as π bridge, S(TPA-bTV-CA) was designed and synthesized for the application as donor materials in solution-processed bulk-heterojunction organic solar cells (OSCs). The compound is soluble in common organic solvents. The thermal, optical and electrochemical properties of the star molecule were studied. The OSC devices were fabricated by spin-coating the blend solution of the molecule as donor and PC70BM as acceptor (1:3, w/w). The OSC device based on S(TPA-bTV-CA)/PC70BM demonstrated a high open circuit voltage of 0.91V, a short circuit current density of 4.64mA/cm2, a fill factor (FF) of 50%, corresponding to a power conversion efficiency of 2.1%, under the illumination of AM 1.5, 100mW/cm2.

An Easily Accessible Donor−π-Acceptor-Conjugated Small Molecule from a 4,8-Dialkoxybenzo[1,2-b:4,5-b′]dithiophene Unit for Efficient Solution-Processed Organic Solar Cells

A new donor-acceptor-conjugated organic small molecule, BDT(TBT)(2), comprised of benzo[1,2-b:4,5-b']dithiophene and 2,1,3-benzothiadiazole units was designed and synthesized. The small molecule BDT(TBT)(2) in its thin film showed an absorption band in the range of 300-700 nm with an absorption edge at 650 nm and an optical band gap of 1.90 eV. As estimated from the cyclic voltammetry measurements, the HOMO and LUMO energy levels of BDT(TBT)(2) were -5.44 and -3.37 eV, respectively. The spin-coated thin film of BDT(TBT)(2) exhibited p-channel output characteristics with a hole mobility of 2.7 × 10(-6). BDT(TBT)(2), when explored as an electron-donor material in solution-processed bulk-heterojunction organic solar cells in conjunction with a PC(71)BM acceptor with an active layer thickness of 50-55 nm, generated a power conversion efficiency (PCE) of 1.18%. A more impressive PCE of ~2.9% with a short-circuit current density (J(sc)) of 7.94 mA cm(-2) and an open-circuit voltage (V(oc)) of 0.89 V was achieved when the active layer of the cell was annealed at higher temperature (~180 °C).

Solution-processable star-shaped photovoltaic organic molecule with triphenylamine core and thieno[3,2-b]thiophene–dicyanovinyl arms

A new solution-processable star-shaped D–π–A molecule with triphenylamine (TPA) as core and donor unit, dicyanovinyl (DCN) as end group and acceptor unit, and 3,6-dihexyl-thieno[3,2-b]thiophene (DHT) as π bridge, S(TPA–DHT–DCN) was synthesized for the application as donor material in solution-processed bulk-heterojunction organic solar cells (OSCs). The compound exhibits broad absorption in the visible region with suitable energy levels, which are desirable for application as a donor material in organic solar cells. The OSC devices based on S(TPA–DHT–DCN) as the donor and PC71BM as the acceptor (1:2, w/w) exhibited power conversion efficiency (PCE) of 2.87%, with high open circuit voltage (Voc) of 0.96V, short circuit current density (Jsc) of 6.80mA/cm2, and fill factor (FF) of 43.5%, under the illumination of AM.1.5, 100mW/cm2. The Voc of 0.96V for S(TPA–DHT–DCN) is among the top values for the solution-processed molecular-based OSCs reported so far.

Solution-Processable Star-Shaped Molecules with Triphenylamine Core and Dicyanovinyl Endgroups for Organic Solar Cells

Two new star-shaped D−π−A molecules with triphenylamine (TPA) as core and donor unit, dicyanovinyl (DCN) as end group and acceptor unit, and 4,4′-dihexyl-2,2′-bithiophene (bT) or 4,4′-dihexyl-2,2′-bithiophene vinylene (bTV) as π bridge, S(TPA-bT-DCN) and S(TPA-bTV-DCN), were synthesized for the application as donor materials in solution-processed bulk-heterojunction organic solar cells (OSCs). The two compounds are soluble in common organic solvents, because of the three-dimensional structure of the TPA unit and the two hexyl side chains on the bithiophene unit. S(TPA-bTV-DCN) film shows a broad absorption band from 360 to 750 nm. Absorption edge of S(TPA-bTV-DCN) film is red-shifted by ca. 78 nm than that of S(TPA-bT-DCN) film, benefitted from the vinylene bridges between TPA and bithiophene units in S(TPA-bTV-DCN). Power conversion efficiency (PCE) of the solution-processed bulk-heterojunction OSC based on a blend of S(TPA-bTV-DCN) and [6,6]-phenyl-C71-butyric acid methyl ester (1:2, w/w) reached 3.0% with a short circuit current density of 7.76 mA/cm2 and an open circuit voltage of 0.88 V, under the illumination of AM.1.5, 100 mW/cm2. In comparison, PCE of the OSC based on S(TPA-bT-DCN) as donor is 1.4% under the same experimental conditions. The PCE of 3.0% for S(TPA-bTV-DCN) is among the top values for the solution-processed molecule-based OSCs reported so far.