PCBM-based Devices Research Articles

Improved Performance and Stability of Inverted Planar Perovskite Solar Cells Using Fulleropyrrolidine Layers.

Inverted planar structure perovskite solar cells (PSCs), due to their low-temperature precessing and lack of hysteretic problems, are attracting increased attention by researchers around the world. Fullerene derivatives are the most widely used electron transport materials (ETMs) in inverted planar perovskite solar cells, especially [6,6]-phenyl-C61-butyric acid methylester (PC61BM), which exhibits very good performance. However, to the best of our knowledge, the influence of adducts on fullerene-based PSCs performance has not been fully explored to date. In this work, two fullerene derivatives, 2,5-(dimethyl ester) C60 fulleropyrrolidine (DMEC60) and the analogous C70 derivative (DMEC70), were synthesized in high yield via a 1,3-dipolar cycloaddition reaction at room temperature and incorporated into CH3NH3PbI3 perovskite solar cells as electron transport materials. Possibly because the attached pyrrolidine ester groups are able to coordinate with the perovskite layer, the devices based on DMEC60 and DMEC70 achieved power conversion efficiencies (PCE) of 15.2% and 16.4%, respectively. Not only were both devices' efficiencies higher than those based on PC61BM and PC71BM, but their stabilities were also higher than those for PCBM-based devices. The results suggest that DMEC60 and DMEC70 are better alternatives than PC61BM and PC71BM for the ETMs in PSCs.

Regio- and stereo-selective intermolecular [2+2] cycloaddition of allenol esters with C60leading to alkylidenecyclobutane-annulated fullerenes

The intermolecular [2+2] cycloaddition of allenol esters, which were in situ generated by Pt-catalyzed 1,3-acyloxy migration of propargylic esters, with C60 proceeded regio- and stereo-selectively to give a novel class of alkylidenecyclobutane-annulated fullerenes. The cyclobutane-annulated fullerene derivatives have high-lying LUMO levels, which gave a high open-circuit voltage in organic solar cell applications. The observed high electron mobility provided a good fill factor compared with the PCBM-based devices.

Enhancing Efficiency of Organic Bulkheterojunction Solar Cells by Using 1,8-Diiodooctane as Processing Additive

Organic solar cells (OSCs) are attractive as an alternative to inorganic devices for their easy fabrication and solution-processability. A major and unsolved problem with bulk heterojunction devices remains the optimization of the network morphology. Here, we discuss the influence of the 1,8-diiodooctane (DIO) solvent additive on the efficiency of OSCs and show that by selectively controlling the crystallization of the organic material, the power conversion efficiency (PCE) can be increased by about 30%. For P3HT:PCBM-based devices, the power conversion efficiency (PCE) was increased from 3.7% to 4.9% for PCPDTBT:P3HT:PCBM-based devices from 3.2% to 4.1%. This improvement is due to the higher I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</sub> , which is in agreement with the higher external quantum efficiency (EQE) observed on the devices fabricated with DIO. We correlate this to an increase of the surface roughness observed with atomic force microscopy (AFM) analysis. We demonstrate that the effect of the DIO additive is equivalent to a high-temperature thermal annealing.

Enhancing fullerene-based solar cell lifetimes by addition of a fullerene dumbbell.

Cost-effective, solution-processable organic photovoltaics (OPV) present an interesting alternative to inorganic silicon-based solar cells. However, one of the major remaining challenges of OPV devices is their lack of long-term operational stability, especially at elevated temperatures. The synthesis of a fullerene dumbbell and its use as an additive in the active layer of a PCDTBT:PCBM-based OPV device is reported. The addition of only 20 % of this novel fullerene not only leads to improved device efficiencies, but more importantly also to a dramatic increase in morphological stability under simulated operating conditions. Dynamic secondary ion mass spectrometry (DSIMS) and TEM are used, amongst other techniques, to elucidate the origins of the improved morphological stability.

An energy-harvesting scheme employing CuGaSe2 quantum dot-modified ZnO buffer layers for drastic conversion efficiency enhancement in inorganic–organic hybrid solar cells

We demonstrated a promising route to enhance the performance of inverted organic photovoltaic (OPV) devices by the incorporation of CuGaSe2 (CGS) quantum dots (QDs) into the ZnO buffer layer of P3HT:PCBM-based devices. The modification of QDs provides better band alignment between the organic/cathode interface, improves ZnO crystal quality, and increases photon absorption, leading to more effective carrier transport/collection. By employing this energy-harvesting scheme, short-circuit current density, open-circuit voltage, and fill factor of the OPV device after CGS QD modification are improved by 9.43%, 7.02% and 6.31%, respectively, giving rise to a 23.8% enhancement in the power conversion efficiency.

Polymer Triplet Energy Levels Need Not Limit Photocurrent Collection in Organic Solar Cells

We study charge recombination via triplet excited states in donor/acceptor organic solar cells and find that, contrary to intuition, high internal quantum efficiency (IQE) can be obtained in polymer/fullerene blend devices even when the polymer triplet state is significantly lower in energy than the intermolecular charge transfer (CT) state. Our model donor system comprises the copolymer PIDT-PhanQ: poly(indacenodithiophene-co-phenanthro[9,10-b]quinoxaline), which when blended with phenyl-C(71)-butyric acid methyl ester (PC(71)BM) is capable of achieving power conversion efficiencies of 6.0% and IQE ≈ 90%, despite the fact that the polymer triplet state lies 300 meV below the interfacial CT state. However, as we push the open circuit voltage (V(OC)) higher by tailoring the fullerene reduction potential, we observe signatures of a new recombination loss process near V(OC) = 1.0 V that we do not observe for PCBM-based devices. Using photoinduced absorption and photoluminescence spectroscopy, we show that a new recombination path opens via the fullerene triplet manifold as the energy of the lowest CT state approaches the energy of the fullerene triplet. This pathway appears active even in cases where direct recombination via the polymer triplet remains thermodynamically accessible. These results suggest that kinetics, as opposed to thermodynamics, can dominate recombination via triplet excitons in these blends and that optimization of charge separation and kinetic suppression of charge recombination may be fruitful paths for the next generation of panchromatic organic solar cell materials with high V(OC) and J(SC).

Donor–Acceptor Alternating Copolymer Based on Thermally Converted Isothianaphthene Dimer and Thiazolothiazole Subunits

A novel donor–acceptor-conjugated polymer PBITT consisting of isothianaphthene (ITN) dimer donor unit and thiazolothiazole acceptor unit was synthesized by thermal conversion method. First, a soluble precursor polymer with an alternating main chain structure of bicyclo[2.2.2]octadiene (BCOD)-fused thiophene dimer and benzodithiophene (PPBITT) was synthesized by palladium(0)-catalyzed Stille coupling reaction. The BCOD moiety underwent the retro-Diels–Alder reaction by the thermal treatment of a red PPBITT film to afford a dark blue film of PBITT that was insoluble in any organic solvents. The optical bandgap of PBITT (1.3 eV) became significantly narrow compared with that of PPBITT (2.1 eV) due to the stabilized quinoid resonance structure of the PBITT main chain. The field-effect hole mobility (μh) of PBITT was determined to be 2.2 × 10–4 cm2 V–1 s–1 with on–off ratio (Ion/Ioff) of 2.5 × 102, whereas the corresponding PPBITT-based device did not show any p- and n-type response. Organic photovoltaic (OPV) devices were fabricated based on the bulk heterojunction film of the polymers and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The device with the PBITT:PCBM film exhibited higher short-circuit current and lower open-circuit voltage than those of the PPBITT:PCBM-based device, resulting in the comparable power conversion efficiency (∼0.3%). These results obtained here will provide fundamental information on the design of thermally induced donor–acceptor alternating polymers for organic electronics.

Analysis of photocurrents in lateral-geometry organic bulk heterojunction devices

Lateral-geometry organic bulk heterojunction (BHJ) devices resemble field-effect transistors without a gate and are useful for probing electrical characteristics of BHJ blends. The lateral structure allows physical access to the BHJ during device operation and lends itself well to device modelling. Photocurrent-voltage data from P3HT:PCBM-based lateral devices were found to possess features of space-charge limited (SCL) extraction. Numerical simulations indeed suggest a physical picture of SCL operation, effectively dividing the device into a photogenerative zone and a photoconducting zone. Based on these simulations, an analytical model was created to describe SCL photocurrents that account for channel lengths and bimolecular recombination.

Synthesis of low bandgap polymers based on thienoquinodimethane units and their applications in bulk heterojunction solar cells

A non-fused ring building block of an electron-rich quinoid structure, 2,5-thienoquinodimethane, has been synthesized and used in the synthesis of novel donor (D)–acceptor (A) type low bandgap polymers for the first time. Namely, 2,5-thienoquinodimethane with 4-(tert-butyl)phenyl or 4-(octyloxy)phenyl side chain as a solubilizing group was copolymerized with an electron-deficient diketopyrrolopyrrole subunit (PQD1 and PQD2, respectively). These polymer films exhibited broad and intense absorption bands in the region of 400–1000 nm. Photovoltaic devices with active layers consisting of PQD1 or PQD2 with [6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM) revealed a broad photoresponse range covering from 400 to 1000 nm, whereas the power conversion efficiencies (η) were found to be moderate (1.44% for PQD1 and 0.96% for PQD2) under the illumination of AM 1.5G, 100 mW cm−2. The superior η value of the PQD1:[70]PCBM-based device relative to the PQD2:[70]PCBM-based device can be attributed to the more favorable phase separation nanostructure in the active layer as well as the higher crystallinity of PQD1 than PQD2. These results provide valuable, basic guidelines for rational designs of quinoidal heterole-based low bandgap polymers for high performance organic solar cells.

The effect of side-chain length on regioregular poly[3-(4-n-alkyl)phenylthiophene]/PCBM and ICBA polymer solar cells

The photovoltaic, charge transport, light absorption and morphology properties of a series of poly[3-(4-n-alkyl)phenylthiophene] (PAPT):acceptor (phenyl-C61-butyric acid methyl ester (PCBM) or indene-C60 bisadduct (ICBA)) blend films are reported as a function of alkyl side-chain length. Regioregular poly[3-(4-n-butyl)phenylthiophene] (PBPT), poly[3-(4-n-hexyl)phenylthiophene] (PHPT), poly[3-(4-n-octyl)phenylthiophene] (POPT), and poly[3-(4-n-decyl)phenylthiophene] (PDPT) were successfully synthesized by a modified Grignard metathesis (GRIM) polymerization method. The effects of the alkyl side-chain length on the optical, electrochemical and structural properties of the polymers were carefully investigated to establish a relationship between the molecular structure and device function. Bulk heterojunction solar cells made of PAPTs blended with either PCBM or ICBA showed a strong photovoltaic property dependence on the alkyl side-chain length. Among all PAPTs used in this study, the best performance was observed in PHPT-based devices. This is the first report of the use of PHPT in polymer solar cells. In addition, PAPT devices blended with ICBA generally showed higher open-circuit voltages (Voc) and power conversion efficiencies than PCBM-based devices. For example, a PHPT:ICBA photovoltaic device showed a Voc of 0.79 V and a power conversion efficiency of 3.7%, which is the highest performance reported thus far using PAPT polymers. While the optical properties of PAPT/electron acceptor films exhibit the strongest effect on the short-circuit current of the devices, a balance between the optical, electrical and morphological properties of blended films caused by the alkyl side-chain length determines the overall photovoltaic performance of these devices. Therefore, our work provides a fundamental understanding of the molecular structure–device function relationship, especially with respect to changes in the alkyl side-chain length in conjugated polymers.

Dihydronaphthyl-based [60]fullerene bisadducts for efficient and stable polymer solar cells

Dihydronaphthyl-based [60]fullerene bisadduct derivative, NC(60)BA, was synthesized at mild temperature in high yield. NC(60)BA not only possesses a LUMO energy level 0.16 eV higher than PC(61)BM but also has amorphous nature that can overcome thermal-driven crystallization. The fabricated P3HT:NC(60)BA-based polymer solar cells exhibit superior photovoltaic performance and thermal stability compared to PC(61)BM-based devices under the same conditions.

A survey of electron-deficient pentacenes as acceptors in polymer bulk heterojunction solar cells

We have prepared, characterized and surveyed device performance for a series of electron deficient pentacenes for use as acceptors in polymer bulk heterojunction solar cells, using P3HT as the donor material. All of the materials reported here behaved as acceptors, and variations in the position and nature of the electron-withdrawing group on the pentacene core allowed tuning of device open-circuit voltage. Photocurrent was strongly correlated with the pentacene crystal packing motif; materials with 2D π-stacking interactions performed poorly compared with materials exhibiting 1D π-stacking interactions. The best pentacene acceptors gave repeatable device efficiency in excess of 1.2%, compared with 3.5% exhibited for PCBM-based devices.

Influence of the Intermediate Density-of-States Occupancy on Open-Circuit Voltage of Bulk Heterojunction Solar Cells with Different Fullerene Acceptors

Electron density of states (DOS) and recombination kinetics of bulk heterojunction solar cells consisting of a poly(3-hexylthiophene) (P3HT) donor and two fullerene acceptors, either [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) or 4,4′-dihexyloxydiphenylmethano[60]fullerene (DPM6), have been determined by means of impedance spectroscopy. The observed difference of 125 mV in the output open-circuit voltage is attributed to significant differences of the occupancy of the DOS in both fullerenes. Whereas DPM6 exhibits a full occupation of the electronic band, occupancy is restricted to the tail of the DOS in the case of PCBM-based devices, implying a higher rise of the Fermi level in the DPM6 fullerene. Carrier lifetime describes a negative exponential dependence on the open-circuit voltage, exhibiting values on the microsecond scale at 1 sun illumination.

Enhanced Performance of Bulk Heterojunction Solar Cells Using Novel Alternating Phenylenevinylene Copolymers of Low Band Gap with Cyanovinylene 4-Nitrophenyls

Two novel low band gap soluble copolymers, P1 and P2, were synthesized and characterized. P1 consisted of alternating dihexyloxyphenylene and α-[[4-(diphenylamino)phenyl]methylene]-4-nitro-benzeneacetonitrile. P2 consisted of alternating dihexyloxyphenylene and α,α′-[(1,4-phenylene)dimethylidyne]bis(αZ,α′Z)-4-nitrobenzeneacetonitrile. These copolymers showed broad absorption curves with long-wavelength absorption maximum around 620 nm and optical band of 1.68 and 1.64 eV for P1 and P2, respectively. Both P1 and P2 were blended with PCBM to study the photovoltaic response of bulk heterojunction (BHJ) solar cells. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of both P1 and P2 are well aligned with those of PCBM acceptor. This allows efficient photoinduced charge transfer and high open circuit voltage, leading to an overall power conversion efficiency (PCE) of 3.15% and 2.60% for the as cast P1:PCBM- and P2:PCBM-based devices, respectively. The PCE of the devices has been further improved up to 4.06% and 3.35% for the devices based on thermally annealed P1:PCBM and P2:PCBM blends, respectively.

Novel Low Band Gap Small Molecule and Phenylenevinylene Copolymer with Cyanovinylene 4-Nitrophenyl Segments: Synthesis and Application for Efficient Bulk Heterojunction Solar Cells

A novel star-shaped small monomer SM containing a 1,3,5-triazine core and arms with terminal cyanovinylene 4-nitrophenyls was synthesized. Moreover, an alternating p-phenylenevinylene copolymer P containing thiophene with cyanovinylene 4-nitrophenyl side segments was synthesized by Heck coupling. Both SM and P showed broad absorption spectra with long-wavelength maximum at 630−648 nm, which for P is attributable to an intramolecular charge transfer. The optical band gap was 1.57 eV for SM and 1.70 eV for P. Both SM and P were blended with PCBM to study the donor−acceptor interactions on the blend film morphology and device characteristics of organic bulk heterojunction solar cells. A combination of characterization techniques including X-ray diffraction and optical topographical images were used to investigate the film morphology. The HOMO and LUMO levels of both SM and P are well-aligned with those of the PCBM acceptor, allowing efficient electron transfer and suitable open circuit voltage, leading to overall power conversion efficiencies (PCEs) of 2.53 and 1.43% for SM:PCBM and P:PCBM-based devices, respectively. The thermal annealing leads to suitable phase separation due to the increase in crystallinity of donor material and material distribution so that highly effective bulk heterojunction morphologies are obtained which further increases the PCE up to 3.82% and 2.37% for SM:PCBM and P:PCBM-based devices, respectively. These results are preliminary based on the illumination without using a solar simulator.

Regioregularity Effects in Poly(3-hexylthiophene):PCBM-Based Solar Cells Incorporating Acid-Doped Polyaniline Nanotubes as an Interfacial Layer

We adopted the and Grignard metathesis (McCullough) methods to synthesize three poly(3-hexylthiophene)s (P3HTs) exhibiting different degrees of regioregularity and then blended them with [6,6]-phenyl--butyric acid methyl ester (PCBM) to obtain bulk heterojunction phases on the top of an acid-doped polyaniline nanotube (a-PANINT) interfacial layer. From integration of NMR spectra, we determined that the three P3HTs had head-to-tail coupling contents of 67, 81, and 96%, respectively. The photovoltaic (PV) performance of P3HT:PCBM-based devices fabricated without the a-PANINT interfacial layer increased as the regioregularity of the P3HT increased. The presence of the a-PANINT interfacial layer resulted in improved PV performances of the P3HT:PCBM-based devices. This improvement in the PV performance resulted from the highly conductive, controlled one-dimensional tubular nanoscale morphology of the annealed a-PANINT interfacial layer, which mediated the efficient migration of photogenerated holes to the buffer layer and suppressed exciton recombination.