Performance Of Organic Photovoltaic Devices Research Articles

Photovoltaic Performance of Block Copolymer Devices Is Independent of the Crystalline Texture in the Active Layer

The electronic properties of organic semiconductors are strongly influenced by intermolecular packing. When cast as thin films, crystalline π-conjugated molecules are strongly textured, potentially leading to anisotropic charge transport. Consequently, it is hypothesized that the orientation of crystallites in the active layer plays an important role in charge extraction and organic photovoltaic device performance. Here we demonstrate orientation control of molecular packing from mostly face-on to edge-on configurations in the active layer of P3HT-b-PFTBT block copolymer photovoltaics using 1-chloronaphthalene as a solvent additive. The effect of molecular orientations in P3HT crystals on charge transport and solar cell performance is examined. We find that optimized photovoltaic device performance is independent of the crystalline texture of P3HT. Our observations provide further insights into the molecular organization required for efficient charge transport and overall device efficiencies. The dominant...

Controlled growth of ZnPc nanostructures via heat assisted solvent vapour treatment method and application in photovoltaic devices

We demonstrated a facile and well controlled way for surface nanostructuring of Zinc Phthalocyanine (ZnPc) thin films. Investigations have been carried out on the structural and optical properties of as deposited and acetone vapour treated ZnPc thin films. Furthermore, organic photovoltaic (OPV) device has been also fabricated employing plane/nanostructured-ZnPc films as electron donor and [6,6]-phenyl C61 butyric acid methyl ester as the electron acceptor layer in conventional bilayer device architectures. OPV device performance has been compared with devices with planer and nanostructured-ZnPc thin films. Three times improvement in power conversion efficiency (η %) has been observed in nanostructured-ZnPc treated for 20 s acetone treatment.

Long-Range Energy Transfer and Singlet-Exciton Migration in Working Organic Light-Emitting Diodes

The motion of excitons in organic semiconductors is critical to the performance of organic light-emitting diodes (OLEDs) and organic photovoltaic devices (OPVs). The authors quantify the contributions of energy transfer and diffusion to the migration of excitons through the thin organic film in an OLED. Their results emphasize that an exciton's $e\phantom{\rule{0}{0ex}}f\phantom{\rule{0}{0ex}}f\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}v\phantom{\rule{0}{0ex}}e$ and $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c$ diffusion lengths can differ strongly, under some conditions. This thorough, comprehensive study will inform rational device design, particularly of white OLEDs for solid-state lighting.

Effects of Processing Solvent on the Photophysics and Nanomorphology of Poly(3-butyl-thiophene) Nanowires:PCBM Blends.

We report the effect of the processing solvent on the nanoscale morphology and photophysical dynamics of poly(3-butyl-thiophene) nanowires (P3BT-nw). P3BT-nw assembled in ortho-dichlorobenzene (ODCB) show higher crystallization and a longer conjugation length with increased exciton delocalization compared with those assembled in chlorobenzene (CB). It is proposed that this solvent effect is associated with the higher ordered structures formed from ODCB solution state. Charge-transfer dynamics and phase separation for P3BT-nw:PCBM blends were investigated by ultrafast fluorescence techniques. The more efficient fluorescence quenching observed in P3BT-nw:PCBM blend films processed from ODCB suggests that there is intimate contact between P3BT-nw and PCBM that facilitates charge transfer. The superior performance of organic photovoltaic devices based on P3BT-nw:PCBM bulk heterojunctions processed using ODCB is attributed to the higher crystallization of P3BT-nw, optimized phase separation, and more efficient charge transfer from P3BT-nw to PCBM.

Rapid and Checkable Electrical Post-Treatment Method for Organic Photovoltaic Devices.

Post-treatment processes improve the performance of organic photovoltaic devices by changing the microscopic morphology and configuration of the vertical phase separation in the active layer. Thermal annealing and solvent vapor (or chemical) treatment processes have been extensively used to improve the performance of bulk-heterojunction (BHJ) organic photovoltaic (OPV) devices. In this work we introduce a new post-treatment process which we apply only electrical voltage to the BHJ-OPV devices. We used the commercially available P3HT [Poly(3-hexylthiophene)] and PC61BM (Phenyl-C61-Butyric acid Methyl ester) photovoltaic materials as donor and acceptor, respectively. We monitored the voltage and current applied to the device to check for when the post-treatment process had been completed. This electrical treatment process is simpler and faster than other post-treatment methods, and the performance of the electrically treated solar cell is comparable to that of a reference (thermally annealed) device. Our results indicate that the proposed treatment process can be used efficiently to fabricate high-performance BHJ-OPV devices.

The molecular regioregularity induced morphological evolution of polymer blend thin films

The polymer regioregularity has a dramatic effect on device performance of organic photovoltaics in terms of material crystallinity and phase separated morphology. In this study, we investigated the molecular regioregularity induced morphological evolution of P3HT and PF12TBT blend thin films processed by solvents with different solubility and boiling point. After cast by good solvents chloroform (CF) and o-dichlorobenzene (oDCB), the regioregular (RR) and regiorandom (RA)-P3HT/PF12TBT blend films showed the similar phase separated morphology, namely fast solidification induced unsharp morphology for CF blend films and long-range demixing via spinodal decomposition within extending drying process for oDCB blend films, while the domain size for RA blend films were larger than those of RR blend films which could be attributed to easily disentanglement and diffusion of RA-P3HT molecules facilitated by smaller aggregation size in RA blend solutions. In addition, the morphology of RR blend films exhibited micro change while the RA blend films developed into long-range phase separation after thermal annealing for CF blend films which also confirmed the molecular diffusion induced morphological evolution. For marginal solvents p-xylene (XY), the RR P3HT showed obvious molecular aggregation in XY solution due to the intensive solvent–polymer interaction (Δδ) and high regioregularity and therefore the value of aggregation size (Ra) for RR-P3HT is as three times as that of RA-P3HT. After cast from XY solution, the RA blend films displayed droplet and bicontinuous morphology based on various blend ratios via spinodal decomposition. Meanwhile, the nanofiber morphology could be observed when the weight fraction of RR-P3HT beyond 10% in RR blend films, which resulting from the continuous growth of P3HT aggregations formed anterior of the phase separation in the film solidification process.

Improved performance of organic photovoltaic devices by doping F4TCNQ onto solution-processed graphene as a hole transport layer

Interfacial engineering is crucial for the stability and efficiency of organic solar cells. PEDOT:PSS, which has been widely used as a hole transport layer, has stability issues when exposed to air because of its acidic and hygroscopic nature. Herein, we investigated the electrical properties of reduced graphene oxide covered with an F4TCNQ interfacial layer as an alternative and its effect on the photovoltaic performance. Using an array of charge transport, spectroscopic and imaging techniques we found that the reduced graphene oxide film is efficiently hole-doped through an interfacial charge transfer, which enhances its electrical properties and favorably modifies its work function. Consequently, the open-circuit voltage and fill factor of solar cells incorporating such films are improved. P3HT might also be hole-doped by F4TCNQ, due to the formation of an intermixed interfacial layer, resulting in an increase of power conversion efficiency.

The mixed alloyed chemical composition of chloro-(chloro)n-boron subnaphthalocyanines dictates their physical properties and performance in organic photovoltaic devices

We have determined that chloro-boron subnaphthalocyanine (Cl-BsubNc) is a mixture of products with random amounts of chlorination in the bay position. We have developed chemical processes to varying the amount of chlorination.

Charge generation in organic photovoltaics: a review of theory and computation

One of the most important factors in determining organic photovoltaic device performance is the efficiency of exciton dissociation and charge separation at donor/acceptor heterojunctions. This review discusses theoretical and computational approaches to modeling this crucial process of charge generation.

Molecular Modeling of Interfaces between Hole Transport and Active Layers in Flexible Organic Electronic Devices

Molecular modeling methods are used to understand the interfacial properties between the hole-transport and active layers in organic photovoltaic (OPV) devices. The hole-transport layer (HTL) consists of a blend of poly(styrene-sulfonate) and poly(3,4-ethylenedioxythiophene) (PEDOT:PSS), whereas the active layer (AL) consists of a blend of poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (P3HT:PCBM). Simulation results on the HTL confirm the interpenetrating lamellar structure with alternating PSS and PEDOT domains as observed in experiments. In addition, interfacial results show high PCBM interactions with the HTL, which result in PCBM migration to the HTL surface. The observed PCBM concentration profile is discussed from the perspective of attractive interactions, and it is shown that these interactions are governed by the side chain of PCBM. Calculations also suggest that OPV device performance could be improved by, for example, increasing the number of benzene rings and backbone −CH2– groups in the PCBM side chain, which would be expected to reduce PCBM concentration at the HTL surface. The results yield important insights into molecular interactions associated with the HTL and AL interfaces that contribute to final device morphology and thus provide guidelines toward materials design approaches for optimized device performance.

Analysis of Charge Transport and Device Performance in Organic Photovoltaic Devices with Active Layers of Self-Assembled Nanospheres

Nanoparticle (NP) assemblies are particularly appealing as active layers of organic photovoltaic (OPV) devices because their aqueous synthesis reduces the usage of chlorinated solvents and because the nanoparticle size, ratio, and internal structure can be controlled precisely. Understanding quantitatively the effects of active layer nanostructure on charge carrier transport in NP-assembly-based OPV devices is crucial in order to optimize device performance. Toward this end, in this study, we report results of numerical simulations of electron and hole transport in OPV devices with active layers consisting of P3HT:PCBM nanoparticle assemblies based on deterministic charge carrier transport models. The models account for the dynamics of bounded electron–hole pairs and free charge carriers, as well as trapped charge carrier kinetics, self-consistently with the electric field distribution in the device active layer. The models reproduce both transient photocurrents from time-of-flight (TOF) experiments and s...

Comparison of conventional and inverted organic photovoltaic devices with controlled illumination area and extraction layers

The impact of device architecture on organic photovoltaic device performance has been problematic to quantify due to extraction layer materials and device testing issues. In particular, published reports have used different pairs of hole and electron extraction layer materials for conventional versus inverted devices. The origins of the large apparent discrepancy in device J–V characteristics become difficult to understand, arising from differences in built-in potentials and in the extraction layers’ conductivities that can affect collection of stray current outside of device area. Here, we show that by using an identical pair of hole and electron extraction layers, and by precisely defining illuminated device area with a shadow mask, most of the differences in the performance of conventional and inverted P3HT:PCBM organic photovoltaic devices are eliminated. The remaining difference in short-circuit current density and EQE spectral shape can be explained primarily by the ratio of hole and electron mobility that leads to more efficient carrier extraction in the inverted devices, with a minor contribution from unintentional p-type doping of the active layer that influences the drift field profile.

The effect of residual palladium catalyst on the performance and stability of PCDTBT:PC70BM organic solar cells

Palladium (Pd) is commonly used as a catalyst in the polymerisation of conjugated polymers such as poly[N-9′-heptadecanyl-2,7-carbozole-alt-5,5-(4′,7′-di-2-thenyl-2′,1′,3′-benzothiadiazole)] (PCDTBT). Here we explore the effect of residual catalyst on the performance of organic photovoltaic devices (OPVs) based on a PCDTBT:fullerene thin-film blend. We find that as the relative concentration of Pd increases, the power conversion efficiency of the PV is reduced, dropping from 4.55% to 2.42% as the Pd concentration was increased to 2570 ppm (relative to that of the PCDTBT). This reduction in efficiency resulted primarily from a reduction in PV fill factor and shunt-resistance, indicating the presence of current-shunts within the device. Using optical microscopy, laser beam induced current mapping and scanning electron microscopy, we are able to demonstrate that such current shunts are associated with micron-sized aggregates of Pd-containing nanoparticles. We show that the presence of high concentrations of Pd within a PCDTBT OPV contribute to a larger drop in efficiency during the initial ‘burn-in’ period.

Effects of Contact-Induced Doping on the Behaviors of Organic Photovoltaic Devices.

Substrates can significantly affect the electronic properties of organic semiconductors. In this paper, we report the effects of contact-induced doping, arising from charge transfer between a high work function hole extraction layer (HEL) and the organic active layer, on organic photovoltaic device performance. Employing a high work function HEL is found to increase doping in the active layer and decrease photocurrent. Combined experimental and modeling investigations reveal that higher doping increases polaron-exciton quenching and carrier recombination within the field-free region. Consequently, there exists an optimal HEL work function that enables a large built-in field while keeping the active layer doping low. This value is found to be ~0.4 eV larger than the pinning level of the active layer material. These understandings establish a criterion for optimal design of the HEL when adapting a new active layer system and can shed light on optimizing performance in other organic electronic devices.

Optimization of organic photovoltaic device performance via exciton generation profile adjustment

We analyzed the conversion performance of conventional organic photovoltaic (OPV) and inverted organic photovoltaic (IOPV) devices with an active layer of polymer, PTB7: PC70BM. We computed the current density–voltage (JV) curves, short-circuit current density (Jsc), open-circuit voltage (Voc), maximum current density (Jmax), maximum power density (Pmax), and fill-factor (FF) under various scenarios. We employed the one-dimensional optical transfer matrix theory to calculate the light intensity that was then used as the input at the active layer for optical carrier generation. Then we obtained electrical performance parameters from the JV curves plotted by solving Poisson and charge transport equations. The effects of adjusting the exciton generation profile by tuning the active layer width and optical spacer thickness under 100 mW·cm−2 air mass 1.5 global (AM 1.5G) illumination are also analyzed. In addition, the effect on the conversion performance by using different electron and hole mobility relations in the polymers composing the active layer is computed. To identify the optimal performance, we proposed an exciton generation profile that maintains a constant amplitude when shifted through the active layer. Subsequently, by adjusting the active layer width, optical spacer thickness, and electron and hole mobility, we found that the OPV structure achieved performance characteristics previously reported only for IOPV structures.

Drift-Diffusion Modeling of the Effects of Structural Disorder and Carrier Mobility on the Performance of Organic Photovoltaic Devices

Solar cells based on organic polymers could serve as a cheap, renewable energy source, but their adoption has been hampered by poor performance due to the disorder inherent to their constituent materials. The authors present a simple one-dimensional model to investigate the deleterious effects of structural disorder on bulk heterojunction solar cells. Little work has been done on this aspect of photovoltaic design, and this study is bound to influence not just device engineering, but also more sophisticated models for finer assessment.

ITO surface modification for inverted organic photovoltaics

The work function (WF) of indium-tin-oxide (ITO) substrates plays an important role on the inverted organic photovoltaic device performance. And electrode engineering has been a useful method to facilitate carrier extraction or charge collection to enhance organic photovoltaic (OPV) performance. By using self-assembly technique, we have deposited poly(dimethyl diallylammonium chloride) (PDDA) layers onto ITO coated glass substrates. The results indicate that the surface WF of ITO is reduced by about 0.3 eV after PDDA modification, which is attributed to the modulation in electron affinity. In addition, the surface roughness of ITO substrate became smaller after PDDA modification. These modified ITO substrates can be applied to fabricate inverted OPVs, in which ITO works as the cathode to collect electrons. As a result, the photovoltaic performance of inverted OPV is substantially improved, mainly reflecting on the increase of short circuit current density.

Effect of size and morphology of Au nanostructures on boosting performance of organic photovoltaic devices: Plasmonic and non-plasmonic effects

We fabricated organic photovoltaic (OPV) devices containing various Au nanostructures mixed with hole-collecting buffer layer. The presence of the Au nanostructures results in enhancement of the external quantum efficiencies (EQE) at dissimilar wavelengths of visible light, which can be attributed to the modulated plasmonic absorption frequency of the Au nanostructures. In addition to this plasmonic effect induced by visible light absorption, an increase in the EQE was also found upon UV excitation, which can be attributed to scattering effects induced by Au particles. The optical response pattern of organic photovoltaic devices can be modulated in a wide range of visible and UV wavelengths, by controlling sizes and shapes of the Au nanostructures.

Plasmonic Bulk Heterojunction Solar Cells: The Role of Nanoparticle Ligand Coating

There has been a lot of interest regarding the influence of active-layer-incorporated plasmonic nanoparticles (NPs) in the performance of bulk heterojunction organic photovoltaic (OPV) devices, while both an increase and decrease in performance have been reported. In this paper, following a systematic approach, we demonstrate strong evidence of the critical importance of the NPs’ ligand shell on the device performance. In particular, it is argued that the plasmonic effect accountable for the performance enhancement takes place only in the case in which the NP’s core is in direct contact with the active layer polymer donor. This can be achieved with the utilization of either ligand-free NPs or NPs terminated with the same polymer donor as the active layer. Using this concept we achieved an enhanced efficiency of 7.16% in OPV devices incorporating the poly(3-hexylthiophene-2,5-diyl) (P3HT):indene-C60 bisadduct (ICBA) active layer. On the contrary, devices with ligand-terminated Au NPs show lower performance...

Improving performance and lifetime of small-molecule organic photovoltaic devices by using bathocuproine-fullerene cathodic layer.

In this study, we compared the use of neat bathocuproine (BCP) and BCP:C60 mixed buffer layers in chloroboron subphthalocyanine (SubPc)/C60 bilayer organic photovoltaic (OPV) devices and analyzed their influence on device performance. Replacing the conventional BCP with BCP:C60 enabled manipulating the optical field distribution for optimizing the optical properties of the devices. Estimation of the interfacial barrier indicated that the insertion of the BCP:C60 between the C60 and electrode can effectively reduce the barrier for electrons and enhance electron collection at the electrode. Temperature-dependent measurements of the OPV devices performed to calculate the barrier height at the SubPc/C60 interface suggested that band bending was larger when the BCP:C60 buffer layer was used, reflecting increased exciton dissociation efficiency. In addition, the device lifetime was considerably improved when the BCP:C60 buffer layer was used. The device performance was stabilized after the photodegradation of the active layers, thereby increasing the device lifetime compared with the use of the neat BCP buffer layer. Atomic force microscopy images showed that the neat BCP was easily crystallized and could degrade the cathodic interface, whereas the blend of C60 and BCP suppressed the crystallization of BCP. Therefore, the optimal buffer layer improved both the device performance and the device lifetime.