Engineering vertical morphology with nanoparticulate organic photovoltaic devices

Sequential deposition of monolayers, composed of nanoparticles with varied donor-acceptor concentration ratios, has allowed the fabrication of organic photovoltaic (OPV) active layers with engineered vertical morphology. The performance of polymer-polymer poly(9,9-dioctylfluorene-co-bis-N,N-phenyl-1,4-phenylenediamine):poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (PFB:F8BT) and polymer-fullerene poly(3-hexylthiophene):phenyl C61 butyric acid methyl ester (P3HT:PCBM) nanoparticulate (NP), graded nanoparticulate (GNP) and bulk heterojunction (BHJ) OPV devices have been studied. For both material systems the highest device V_OC is observed for the graded structure. Furthermore, thermal treatments can be used to alleviate parasitic series resistance in the GNP devices, thus improving device J_SC and efficiency. Overall, this work shows that the nanoparticle approach provides a new experimental lever for morphology control in OPV devices.     

Design and synthesis of soluble dibenzosuberane-substituted fullerene derivatives for bulk-heterojunction polymer solar cells

Two new dibenzosuberane-substituted fullerene derivatives, dibenzosuberane-C₆₀ mono-adduct (DBSCMA) and bis-adduct (DBSCBA) were synthesized using a classical cyclopropanation reaction via a tosylhydrazone route for application as acceptor materials in polymer solar cells (PSCs). DBSCBA shows good solubility in common organic solvents and both derivatives were characterized by ¹HNMR, ¹³C NMR, MALD-TOF, elemental analysis and UV–vis absorption measurements. The shift of fullerene energy levels induced by the dibenzosuberane substitution was investigated by using theoretical simulations and ultraviolet photoelectron spectroscopy. Bulk-heterojunction PSCs based on poly (3-hexylthiophene) (P3HT) and dibenzosuberane-C₆₀ derivatives were fabricated and optimized by adjusting the donor/acceptor ratio and using thermal annealing and solvent additive. The morphologies of the active layers processed under different conditions were also examined by atomic force microscopy. When tested under an illumination of AM 1.5 G at 100 mW/cm², the highest power conversion efficiency of the devices using DBSCBA is 3.70% which is superior to that of conventional P3HT:PCBM devices.     

Degradation of self-assembled monolayers in organic photovoltaic devices

Self-assembled monolayers (SAMs) based on n-octylphosphonic acid (C8PA) and 1H,1H,2H,2H-perfluorooctanephosphonic acid (PFOPA) were investigated for application as an anode buffer layer in C₆₀-based organic photovoltaic (OPV) devices. We found that the degradation of the OPV efficiency with respect to air exposure was significantly reduced with the perfluorinated PFOPA compared to the aliphatic C8PA. We attribute the OPV degradation to moisture diffusion from the top aluminum electrode and the lowering of the anode work function as a result of hydrolysis of the SAM buffer layer.     

Electronic structure of fullerene derivatives in organic photovoltaics

The electronic structures of the fullerene derivatives [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), [6,6]-diphenyl C₆₂ bis (butyric acid methyl ester) (bisPCBM), C₇₀, [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₀BM), [6,6]-phenyl-C₆₁-butyric acid butyl ester (PCBB), [6,6]-phenyl-C₆₁-butyric acid octyl ester (PCBO), [6,6]-thienyl-C₆₁-butyric acid methyl ester (TCBM), and indene-C₆₀ bisadduct (ICBA), which are frequently used as n-type materials in organic photovoltaics, were studied by ultraviolet photoelectron spectroscopy and inverse photoemission spectroscopy. We also performed molecular orbital calculation based on density functional theory to understand the experimental results. The electronic structures near the energy gap of the compounds were found to be governed predominately by the fullerene backbone. The side chains also affected the electronic structures of the compounds. The ionization energy and electron affinity were strongly affected by the number of carbons and functional groups in the side chain.     

Partitioning of the organic layers for the fabrication of high efficiency organic photovoltaic devices

Lateral partitioning of hole extraction layer with insulating walls improved the power conversion efficiency of organic photovoltaic device. When the conductivity of the hole extraction layer is low, no improvement is obtained by partitioning. However, when the conductivity is high, a significant improvement was obtained in the partitioned cells, showing the estimated power conversion efficiency of 4.58% compared to the 3.54% of the single cell structure. This improvement, carefully corrected by masking at measurement, could be explained by the reduction of series resistance. Although accurate estimation of device area at partitioned device might be difficult, its effectiveness on the properties of large area organic photovoltaic device is clear, as shown in the result of 1 cm²-size cell with the same manner.     

Integrating nanostructured electrodes in organic photovoltaic devices for enhancing near-infrared photoresponse

We introduce a simple methodology to integrate prefabricated nanostructured-electrodes in solution-processed organic photovoltaic (OPV) devices. The tailored “photonic electrode” nanostructure is used for light management in the device and for hole collection. This approach opens up new possibilities for designing photonically active structures that can enhance the absorption of sub-bandgap photons in the active layer. We discuss the design, fabrication and characterization of photonic electrodes, and the methodology for integrating them to OPV devices using a simple lamination technique. We demonstrate theoretically and experimentally that OPV devices using photonic electrodes show a factor of ca. 5 enhancement in external quantum efficiency (EQE) in the near infrared region. We use simulations to trace this observed efficiency enhancement to surface plasmon polariton modes in the nanostructure.     

Molecular engineering to improve carrier lifetimes for organic photovoltaic devices with thick active layers

The morphology of the bulk heterojunction absorber layer in an organic photovoltaic (OPV) device has a profound effect on the electrical properties and efficiency of the device. Previous work has consistently demonstrated that the solubilizing side-chains of the donor material affect these properties and device performance in a non-trivial way. Here, using Time-Resolved Microwave Conductivity (TRMC), we show by direct measurements of carrier lifetimes that the choice of side chains can also make a substantial difference in photocarrier dynamics. We have previously demonstrated a correlation between peak photoconductance measured by TRMC and device efficiencies; here, we demonstrate that TRMC photocarrier dynamics have an important bearing on device performance in a case study of devices made from donor materials with linear vs. branched side-chains and with variable active layer thicknesses. We use Grazing-Incidence Wide Angle X-ray Scattering to elucidate the cause of the different carrier lifetimes as a function of different aggregation behavior in the polymers. Ultimately, the results help establish TRMC as a technique for screening OPV donor materials whose devices maintain performance in thick active layers (>250 nm) designed to improve light harvesting, film reproducibility, and ease of processing.     

Dispersion control of Ag nanoparticles in bulk-heterojunction for efficient organic photovoltaic devices

This research focuses on the effect of different capping agents on Ag nanoparticles (NPs), for the improved efficiency of organic photovoltaic cells. Ag NPs were produced by solution chemistry of the polyol process, and then successfully capped with oleylamine (OA), polyvinylpyrrolidone (PVP), or thiol terminated polystyrene (PS-SH), as proven by FT-IR spectra. These Ag NPs with different capping agents were finally embedded in the photoactive layer of poly(3-hexylthiophene):6,6-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) bulk heterojunction solar cells. Because of the presence of a suitable capping agent that prevents aggregation, the dispersity of the Ag NPs in organic solvent was significantly improved, in the sequence of OA, PVP, and PS-SH. The photovoltaic cells exhibit increased performance from 3.11% to 3.49%, at an optimized blend ratio of Ag NPs (2.5 wt%) capped with PS-SH. This enhancement is mainly attributed to the improved short circuit current (increased from 8.49 mA/cm² to 9.29 mA/cm²) and extinction with effective light scattering, caused by improved dispersion of the Ag NPs in BHJ films, through reducing unwanted particle aggregation.     

Electronic and chemical properties of ZnO in inverted organic photovoltaic devices

Photo-conversion efficiency of inverted polymer solar cells incorporating pulsed laser deposited ZnO electron transport layer have been found to significantly increase from 0.8% to up to 3.3% as the film thickness increased from 4 nm to 100 nm. While the ZnO film thickness was found to have little influence on the morphology of the resultant ZnO films, the band structure of ZnO was found to evolve only for films of thickness 25 nm or more and this was accompanied by a significant reduction of 0.4 eV in the workfunction. The films became more oxygen deficient with increased thickness, as found from X-ray photoelectron spectroscopy (XPS) and valence band XPS (VBXPS). We attribute the strong dependence of device performance to the zinc to oxygen stoichiometry within the ZnO layers, leading to improvement in the band structure of ZnO with increased thickness.     

Photogenerated charge carrier transport and recombination in polyfluorene/fullerene bilayer and blend photovoltaic devices

Using extraction of photogenerated charge carriers by linearly increasing voltage (photo-CELIV), we investigated two key transport parameters in photovoltaic materials based on the donor APFO-3 and acceptor PCBM: the mobility and lifetime of photogenerated charge carriers, in bilayers of varying geometry and in blends with various acceptors loading. We find that mobility depends strongly on delay time for shorter delay time in all devices. The observed recombination kinetics is found to be monomolecular. The mean lifetime of charge carriers is 2–3 μs in blends and is slightly greater than 4 μs in bilayer devices. In addition, the implications of mobility and lifetime values on the collection efficiency of the devices are presented.     

Understanding the effect of triplet sensitizers in organic photovoltaic devices

The effect of PtOEP as a dopant on the performance of MEH-PPV/C₆₀ photovoltaic devices was studied. Bilayer heterojunction devices with various compositions and layer structures were used to determine the possible pathways by which the photogeneration efficiency is enhanced. A key finding is that photocurrent generation enhancement always occurs in the MEH-PPV absorption region, regardless of the PtOEP dopant concentration or the MEH-PPV layer thickness. This result suggests that the presence of PtOEP in the donor MEH-PPV layer is primarily responsible for increasing the triplet exciton diffusion length of MEH-PPV by acting as a triplet sensitizer, rather than as an additional absorber for direct photogeneration. Values obtained from simulation show that the enhancement of exciton diffusion length of MEH-PPV can be more than a factor of 2 with optimal PtOEP concentrations. Further support for the role of PtOEP as a triplet sensitizer in MEH-PPV was obtained in experiments incorporating a blocking layer between MEH-PPV and C₆₀, whereby the various exciton transfer processes can be differentiated.     

Reversible photoinduced bi-state polymer solar cells based on fullerene derivatives with azobenzene groups

[6,6]-Phenyl-C61-butyric acid-4′-hydroxyl-azobenzene ester (PCBAb) was synthesized and used as the acceptor in the fabrication of reversible UV–VIS response bi-state polymer solar cells (PSCs) based on the photoinduced cis–trans isomerization of PCBAb. The device can be switched between “active” and “sleep” by the irradiation of UV and visible light, respectively. The active device has a PCE of 2.0%. With UV irradiation, the device goes to “sleep” with a lowered PCE (0.4%), and simultaneously decreased J_sc, V_oc and FF, while after visible light treatment, the device is made “active” again. The mechanism of the bi-state process involves the different electron mobilities of the isomers.     

Tetrasubstituted adamantane derivatives with arylamine groups: Solution-processable hole-transporting and host materials with high triplet energy and good thermal stability for organic light-emitting devices

Four new host/hole-transporting materials, namely 4,4′,4″,4‴-(adamantane-1,3,5,7-tetrayl)tetrakis(N,N-diphenylaniline) (4TPA-Ad, 1),4,4′,4″,4‴-(adamantane-1,3,5,7-tetrayl)tetrakis(N,N-di-p-tolylaniline) (4MTPA-Ad, 2), 1,3,5,7-tetrakis(4-(9H-carbazol-9-yl)phenyl)adamantane (4Cz-Ad, 3) and 1,3,5,7-tetrakis(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)adamantane (4tBuCz-Ad, 4), were designed and synthesized by incorporating four electron-donating arylamine units into the rigid adamantane skeleton via a simple C–N coupling reaction. Their thermal, photophysical and electrochemical properties were investigated. The molecular design endows the materials with high triplet energies of ∼3.0 eV, good solution processability, high thermal stability and appropriate HOMO levels. Two types of electroluminescent devices using 1–4 as hole-transporting or host materials were fabricated. The device based on 2 as solution-processed hole-transporting material and tris(quinolin-8-yloxy)aluminum as an emitter revealed a maximum current efficiency of 4.2 cd A⁻¹, which was comparable with the TAPC-based control device. The sky-blue device employing 2 as solution-processed host material and 4,6-(difluorophenyl)pyridine-N,C^2′)picolinate (FIrpic) as an emitter showed a maximum current efficiency of 16.6 cd A⁻¹ with Commission Internationale de I’Eclairage (CIE) coordinates of (0.16, 0.32).     

Synthesis and electroluminescent properties of highly efficient anthracene derivatives with bulky side groups

Three new asymmetric light emitting organic compounds were synthesized with diphenylamine or triphenylamine side groups; 10-(3,5-diphenylphenyl)-N,N-diphenylanthracen-9-amine (MADa), 4-(10-(3,5-diphenylphenyl)anthracen-9-yl)-N,N-diphenylaniline (MATa), and 4-(10-(3′,5′-diphenylbiphenyl-4-yl)anthracen-9-yl)-N,N-diphenylaniline (TATa). MATa and TATa had a PL_max at 463 nm in the blue region, and MADa had a PL_max at 498 nm. MADa and MATa had T_g values greater than 120 °C, and TATa had a T_g of 139 °C. EL devices containing the synthesized compounds were fabricated in the configuration: ITO/4,4′,4′′-tris(N-(2-naphthyl)-N-phenyl-amino)-triphenylamine (2-TNATA) (60 nm)/N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) (15 nm)/MADa or MATa or TATa or 9,10-di(2′-naphthyl)anthracene (MADN) (30 nm)/8-hydroxyquinoline aluminum (Alq₃) (30 nm)/LiF (1 nm)/Al (200 nm). The efficiency and color coordinate values (respectively) were 10.3 cd/A and (0.199, 0.152; bluish-green) for the MADa device, 4.67 cd/A and (0.151, 0.177) for the MATa device, and 6.07 cd/A and (0.149, 0.177) for the TATa device. The TATa device had a high external quantum efficiency (EQE) of 6.19%, and its luminance and power efficiencies and life-time were more than twice those of the MADN device.     

Al掺杂ZnO纳米棒的性能研究及其在太阳能电池中的应用

通过水热法制备了不同质量分数(0%,0.5%,1.0%和1.5%)的Al 3+掺杂ZnO纳米棒,扫描电镜(SEM)、X射线衍射(XRD)、紫外-可见(UV-vis)吸收光谱等测试结果表明,通过这种方法得到了较为规整的ZnO纳米阵列,结晶良好、具有明显的c轴生长取向;掺杂浓度的增加对产物的形貌和晶体结构产生了明显的影响。通过瞬态光谱和面电阻测试发现,Al 3+掺杂提高了ZnO传导电子的能力。将Al 3+掺杂的ZnO纳米棒同时作为电极与电子传输层,应用于有机太阳能电池器件中,在低浓度(0.5at.%)掺杂时得到最佳的器件性能,相比于未掺杂的ZnO纳米棒,短路电流提高了30%,光电转化效率提高了50%。     

Carbazole-based small molecules for vacuum-deposited organic photovoltaic devices with open-circuit voltage exceeding 1 V

Fabrication of vacuum deposited small molecules organic solar cell with open-circuit voltage (V_oc) exceeding 1 V is crucial in advancing the applications of organic photovoltaics (OPVs). Here, a novel carbazole-based donor-π bridge-acceptor (D-π-A) of p-type material (F-series) in combination with fullerene derivative C₆₀ or C₇₀ as n-type material for bulk-heterojunction OPVs with the structure of ITO/MoO₃ (15 nm)/F-series donor: C₆₀ or C₇₀ (40 or 80 nm)/BCP (7 nm)/Ag (120 nm) have been proposed. The vacuum deposited small molecules OPV with the donor layer consisting of F1 combined with the electron acceptor C70 exhibits a high power conversion efficiency (PCE) of 4.93%. The higher PCE of the OPV is attributed to the large V_oc value of 1.02 V. The analysis of photophysical properties using a time-dependent density functional theory model and the B3LYP functional corroborates the experimental results and provides the evidence on increasing the V_oc of OPVs.     

Electrical aspects of photovoltaic devices based on bi-layer organic semiconducting materials

Here we have investigated the opto-electrical properties of bi-layer sandwich devices based on polymer/C₆₀ hetero junctions. In this type of device the polymer acts as an electron donor to the second layers the C₆₀ which plays the role of an electron acceptor.     

Solvent-dependent morphology of thermally converted copper phthalocyanine for solution-processed small molecule organic photovoltaic devices

We fabricated copper phthalocyanine (CuPc)-based organic photovoltaic (OPV) devices via a solution process. CuPc was obtained through thermal conversion of its precursor CuPc(OMe)₂, which has excellent solubility in various organic solvents, on a substrate. Solvent-dependent performance of the resulting devices was observed, which could be explained by considering film morphology. Using a 1:2 (wt/wt) mixture of chlorobenzene and chloroform produced a p–n type OPV device with a power conversion efficiency of 1.35% under 1 sun simulated AM1.5G solar illumination.     

Control over fibril width via different solubility additives for diketopyrrolopyrrole-based photovoltaic devices

Control over polymeric bulk heterojunction (BHJ) morphology is one of the key factors in obtaining high-efficiency devices. The domain size influence on device performance is widely considered critical. In this paper, the fibril width of 3,6-bis-(thiophen-2-yl)-N,N′-bis(2-octyl-1-dodecyl)-1,4-dioxo-pyrrolo[3,4-c]pyrrole and thieno[3,2-b]thiophene (PDBT-TT):[6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) blend thin film was adjusted by different processing additives. By decreasing the solubility of PDBT-TT in different additives, the fibril width can be decreased from 65.7 nm to 14.8 nm. It is possible that the PDBT-TT seed-crystallite nuclei concentration is higher in the relatively low solubility solvents than that in the relatively high solubility solvents, thus leading to the formation of narrower fibrils. The PDBT-TT/PC71BM narrow fibrillar interpenetrating network structure was beneficial to exciton separation and charge transport processes. As a result, the solar cell with the narrowest fibril width has a higher short circuit current (J_sc) and fill factor (FF), thus achieving optimized device performance from less than 1% to 4.75%.     

Series resistance in organic bulk-heterojunction solar devices: Modulating carrier transport with fullerene electron traps

Series resistance is one of the key parameters affecting the performance of organic photovoltaic devices. Several electronic mechanisms arising from different structures within the solar cell can contribute to increasing it. We focus on the series resistance origin by altering the acceptor transport properties trough the incorporation of fullerene traps located at energies below the transporting electron levels. Indene-C₆₀ bisadduct as acceptor molecule blended with poly(3-hexylthiophene) forms the active layer in which small amounts of [6,6]-phenyl-C₆₁-butyric acid methyl ester have been added as trapping sites. A complete analysis of the impedance response has allowed identifying bulk transport resistive circuit elements in the high-frequency part of the spectra. Series resistance is observed to be dependent on the concentration of fullerene traps, thus indicating a connection between bulk transport processes and resistive elements. By comparing different contacts it has been discarded that outer cathode interfaces influence the series resistance experimentally extracted from impedance spectroscopy.