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.     

Degradation mechanisms in organic photovoltaic devices

In the present review, the main degradation mechanisms occurring in the different layer stacking (i.e. photoactive layer, electrode, encapsulation film, interconnection) of polymeric organic solar cells and modules are discussed. Bulk and interfacial, as well as chemical and physical degradation mechanisms are reviewed, as well as their implications and external or internal triggers. Decay in I-V curves in function of time is usually due to the combined action of sequential and interrelated mechanisms taking place at different locations of the device, at specific kinetics. This often makes the identification of specific root causes of degradation challenging in non-model systems. Additionally, constant development and refinement in terms of type and combination of materials and processes render the ranking of degradation mechanisms as a function of their probability of occurrence and their detection challenging.However, it clearly appears that for the overall stability of organic photovoltaic devices, the actual photoactive layer, as well as the properties of the barrier and substrate (e.g. cut of moisture and oxygen ingress, mechanical integrity), remain critical. Interfacial stability is also crucial, as a modest degradation at the level of an interface can quickly and significantly influence the overall device properties.     

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.     

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.     

复合自组装分子膜的摩擦特性研究

本文采用自组装技术制备了三氯十八硅烷(octadecyltrichorosilane 0TS)／3-胺基丙基-三甲氧基硅炕(3-amino-propyltrimethoxysilane APTMS)和APTMS／OTS复合自组装分子膜,在原子力／摩擦力显微镜上对薄膜的摩擦特性进行了测试,并与0TS和APTMS自组装分子膜(self-assembledmonolayers SAMs)进行了对比。结果表明,OTS／APTMS复合自组装分子膜因既保持了一定的键合强度叉增加了自组装分子的流动性,使其摩擦力显著降低。复合自组装分子膜的摩擦力随着载荷和滑动速度的增大而增大,这与自组装分子的受力响应和弛豫特性相关。合理地设计自组装分子膜可有效地减小摩擦。     

自组装FeOOH纳米膜的透射电镜分析

以氧化硅微球(SiO2)为基底,外侧功能团为巯基(SH)的自组装单层为模板,在铁盐浓度2.0 mmol/L、pH值2.05、水浴温度为70℃的Fe(NO3)3-HNO3低温液相反应体系中制备了FeOOH纳米薄膜.采用JEM-2100F型场发射高分辨透射电子显微镜对该薄膜的形成过程进行了分析,发现此实验条件下FeOOH纳米薄膜的形成经历了诱导时间(6 h)、晶核的形成或膜的初步形成(9 h)、晶核的生长或膜的增厚(12 h)三个阶段,证明了其形成过程是一个由自组装单层诱导成核再生长的表面成核过程.同时也表明水浴加热12 h后,该膜由一系列宽为10 nm,长为40 nm,垂直SiO2微球表面的纳米棒状针铁矿(α-FeOOH)组成.     

Ultra-thin alumina layer encapsulation of bulk heterojunction organic photovoltaics for enhanced device lifetime

Successful organic photovoltaic (OPV) device fabrication is contingent on selecting an effective encapsulation barrier layer to preserve device functionality by inhibiting atmosphere-induced degradation. In this work, ultra-thin AlO_x layers are deposited by atomic layer deposition (ALD) to encapsulate pre-fabricated OPV devices. A summary of ALD recipe effects (temperature, cycling time, and number of cycles) on AlO_x film growth and device longevity is presented. First, AlO_x film growth on the hydrophobic OPV surface is shown to occur by a 3D island growth mechanism with distinct nucleation and cluster growth regions before coalescence of a complete encapsulation layer with a thickness ⩾7 nm by 500 cycles. Encapsulated device performance testing further demonstrates that reducing ALD processing temperature to 100 °C minimizes OPV phase segregation and surface oxidation loss mechanisms as evidenced by improved short circuit current and fill factor retention when compared with the conventional 140–150 °C range. Ultra-thin AlO_x encapsulation by ALD provides significant device lifetime enhancement (∼30% device efficiency after 2000 h of air exposure), which is well beyond other ALD-based encapsulation works reported in the literature. Furthermore, the interfacial bonding strength at the OPV–AlO_x interface is shown to play a crucial role in determining film failure mode and therefore, directly impacts ultimate device lifetime.     

Photoisomerisation process of self-assembled monolayers of some novel azobenzenes†

Self-assembled monolayers (SAMs) of four azobenzene derivatives containing a sulphonamide linkage and a thiol group were deposited on both vacuum-evaporated and sputtered gold (Au) thin films. The optically induced switching effect on the surface potential U_s upon alternating irradiation with UV (centred at 360 nm) and visible (centred at 450 nm) light was investigated by the Kelvin probe technique. Changes in surface potential were in the range ΔU_s=4–70 mV for SAM samples, depending on the structure of the molecule. The dependence of surface potential change and decay time upon irradiation on the structure of the Au film was observed. Copyright © 2000 John Wiley & Sons, Ltd.     

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.     

Recent advances in two-dimensional photovoltaic devices

Two-dimensional (2D) materials have attracted tremendous interest in view of the outstanding optoelectronic properties, showing new possibilities for future photovoltaic devices toward high performance, high specific power and flexibility. In recent years, substantial works have focused on 2D photovoltaic devices, and great progress has been achieved. Here, we present the review of recent advances in 2D photovoltaic devices, focusing on 2D-material-based Schottky junctions, homojunctions, 2D−2D heterojunctions, 2D−3D heterojunctions, and bulk photovoltaic effect devices. Furthermore, advanced strategies for improving the photovoltaic performances are demonstrated in detail. Finally, conclusions and outlooks are delivered, providing a guideline for the further development of 2D photovoltaic devices.     

Large enhancement of hole injection in pentacene by modification of gold with conjugated self-assembled monolayers

Planar diodes with gold electrodes and a pentacene semiconductor layer were made and electrically characterized. The gold electrodes were modified with self-assembled monolayers of biphenylthiol and biphenylthiol substituted with a terminal fluorine atom (fluorobiphenylthiol). Atomic force microscopy of the pentacene layer reveals large morphological similarities when the film is grown on gold modified with either of the two kinds of self-assembled monolayer. This is at variance with the significant increase of the current observed when the gold electrodes are treated with fluorobiphelylthiol, with a bulk mobility rising up to 1 cm²/V s, while the treatment with biphelylthiol leads to a substantial decrease of the current. These results are interpreted in terms of adjustment of the injection barrier height as a result of the interface dipole induced by the self-assembled monolayer.     

Hystersis mechanism in perovskite photovoltaic devices and its potential application for multi-bit memory devices

The hysteresis in perovskites devices puzzled researchers because it was a big hurdle for device stability and the origin of it was still a riddle for people to solve. Here we reported our analysis in mechanism of the hysteresis based on the trap states in the perovskites film surface. We tried to explain the current hysteresis through the dynamic charge trapping–detrapping processes and the conclusion applied both in porous and planar structure devices. However, the proportion of deep traps and shallow traps are different in planar structure device and in porous structure device. Furthermore, we found perovskite devices has potentials of serving as memory devices due to the photocurrent hysteresis. The on/off ratio of memory based on perovskite can be higher than 60 and the write time was as low as 0.54 s as memory. It also had a very low read bias near 0 V. Moreover, the devices show multi-bit property and a multi-bit organic memory came forward as a novel application of perovskite devices.     

Adherence of self-assembled monolayers on gold and their effects for high-performance anisotropic conductive adhesives

To improve the electrical property of the anisotropic conductive adhesive (ACA) joints, self-assembled monolayer (SAM) compounds are introduced into the interface between the metal filler and the substrate bond pad. The formation of the SAM on gold and the thermal stability were investigated by measuring the contact angles of SAM compounds with a hydrophilic or hydrophobic tail groups such as octadecanethiol (ODT), mercpatoacetic acid (MAA), and 1,4-benzenedithiol (dithiol) on the Au surface. Epoxy resins with two different curing temperatures were used as polymer matrices of the ACA formulations. The SAM-treated ACA joints showed much lower resistance at the same applied current than nontreated joints, and the effect on the low curing temperature epoxy matrices was more significant.     

Photocarrier behavior in organic heterojunction photovoltaic cells

We have investigated the bias dependence of photocurrent in several organic heterojunction cells to elucidate the behavior of photogenerated charge carriers. Both the planar and planar-mixed heterojunction devices are shown to always have negative photocurrent even at large forward biases; this phenomena has been attributed to an increased driving force for carrier diffusion away from the heterointerface as the applied bias increases. In contrast, the drift current generally dominates in mixed heterojunction devices due to distributed nature of charge generation throughout the active layer, leading to a photocurrent that is highly dependent on the internal electric field. This dependence gives rise to the reversal of the photocurrent direction at high biases when compared to that at the short-circuit condition. However, the voltage yielding zero photocurrent shows appreciable wavelength dependence, which is strongly correlated to the detailed charge carrier generation profile within the active layer.     

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.     

The metal/organic monolayer interface in molecular electronic devices

The metal/molecules/metal is the basic device used to measure the electronic properties of organic molecules envisioned as the key components in molecular-scale devices (molecular diode, molecular wire, molecular memory, etc.). This review paper describes the main techniques used to fabricate a metal/molecules/metal device (or more generally electrode/molecules/electrode junctions, with electrodes made of metal or semiconductor). We discuss several problems encountered for the metallization of organic monolayers. The organic/electrode interface plays a strong role in the electronic properties of these molecular devices. We review some results on the relationships between the nature of the electrode/molecule interface (physisorbed or chemisorbed, evaporated metal electrode, mechanical contact, etc.) and the electronic transport properties of these molecular-scale devices. We also discuss the effects of symmetric versus asymmetric coupling of the two ends of the molecules with the electrodes.     

Uncertainty of the spectral mismatch correction factor in STC measurements on photovoltaic devices

Valuation of photovoltaic devices depends strongly on the measured power output of the device. This quantity is usually determined under artificial sunlight in production line measurement systems or industrial or research test labs. A practical calibration chain is realized essentially with measurements at solar simulators. The measurement conditions are defined in the IEC 60904 series of standards. An important part of the standard testing conditions is the definition of a specific spectral distribution of the sunlight (AM1.5 global). The inevitable deviations of the spectrum of artificial light sources from the standard spectrum have to be taken into account by a spectral mismatch factor. The uncertainty of this crucial correction is spectrally dependent, in most cases unknown and complex and inconvenient to evaluate. In this article a randomizing method is proposed which allows one to calculate the uncertainty of the mismatch factor from the uncertainties of the input parameters determined with high spectral resolution. Based on a range of different spectral responses of solar cells on the one hand and variations of the solar simulator spectral distribution on the other, we are able to generalize the results to a broad set of measurement configurations. A sensitivity analysis reveals the crucial wavelength regions and thus allows the systematic optimization of simulator spectra and selection of reference cells. Copyright © 2010 John Wiley & Sons, Ltd.     

Performance enhancement of diindenoperylene-based organic photovoltaic cells by nanocolumn-arrays

Crystalline and uniform nanocolumns of the organic semiconductor diindenoperylene (DIP) were fabricated by glancing-angle deposition and employed in organic photovoltaic cells (OPVCs) forming an interdigitated donor/acceptor heterojunction, with fullerene as electron acceptor. In comparison to reference bilayer devices the nanocolumn-based solar cells exhibit increased power conversion efficiency. Based on a comprehensive structural and morphological analysis, we identify three advantages of the interdigitated nanocolumn structures: (i) The active donor/acceptor interface area, crucial for exciton dissociation, is increased and the column diameter is in the range of the exciton diffusion length. (ii) The molecular orientation of DIP is such in the nanocolumns that light absorption is enhanced. (iii) The ubiquitous presence of vertical interfaces throughout nanocolumn-based devices is further beneficial to light absorption, as it fully compensates wavelength-dependent interference effects within the device structure. This work shows how the benefits of nanocolumns can go beyond simple interface area enlargement to improve the efficiency of OPVCs.     

Performance characteristics of pentacene-based organic photovoltaic cells

We demonstrate the maximum power conversion efficiency of 3.89% from organic photovoltaic cells using pentacene as a hole transport layer with PIN structure of ITO/poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS)-glycerol/pentacene/pentacene:C₆₀/C₆₀/BCP (bathocuproine)/Al under standard AM 1.5 illumination (100 mW/cm²). To achieve high power conversion efficiency, the optimization of thickness of pentacene and glycerol-doped poly(3,4-ethylenedioxy-thiophene)–poly(styrene sulfonate) (G-PEDOT:PSS) as well as pentacene:C₆₀ (1:1) thin film as an active layer was accomplished. Our results show that the PIN structure with enlarged interface between pentacene and C₆₀ thin films increases the power conversion efficiency of the devices than the PN devices. The morphology of pentacene thin film with various thicknesses and glycerol-doped PEDOT:PSS layers crucially affected the performance characteristics of pentacene-based photovoltaic cells.