Interrelation between crystal packing and small-molecule organic solar cell performance

X-ray investigations on single crystals of a series of terminally dicyanovinyl-substituted quaterthiophenes and co-evaporated blend layers with C(60) give insight into molecular packing behavior and morphology, which are crucial parameters in the field of organic electronics. Structural characteristics on various levels and length scales are correlated with the photovoltaic performance of bulk heterojunction small-molecule organic solar cells.     

Investigation of post-thermal annealing-induced enhancement in photovoltaic performance for squaraine-based organic solar cells

In this work, we have systematically investigated the post-thermal annealing-induced enhancement in photovoltaic performance of a 2,4-bis[4-(N, N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (DIBSQ)/C₆₀ planar heterojunction (PHJ) organic solar cells (OSCs). An increased power conversion efficiency (PCE) of 3.28% has been realized from a DIBSQ/C₆₀ device with thermal annealing at 100 °C for 4 min, which is about 33% enhancement compared with that of the as-cast device. The improvement of the device performance may be mainly ascribed to the crystallinity of the DIBSQ film with post-thermal annealing, which will change the DIBSQ donor and C₆₀ acceptor interface from PHJ to hybrid planar-mixed heterojunction. This new donor–acceptor heterojunction structure will significantly improve the charge separation and charge collection efficiency, as well as the open circuit voltage (V_oc) of the device, leading to an enhanced PCE. This work provides an effective strategy to improve the photovoltaic performance of SQ-based OSCs.     

Influence of chemically p-type doped active organic semiconductor on the film thickness versus performance trend in cyanine/C60 bilayer solar cells

Simple bilayer organic solar cells rely on very thin coated films that allow for effective light absorption and charge carrier transport away from the heterojunction at the same time. However, thin films are difficult to coat on rough substrates or over large areas, resulting in adverse shorting and low device fabrication yield. Chemical p-type doping of organic semiconductors can reduce Ohmic losses in thicker transport layers through increased conductivity. By using a Co(III) complex as chemical dopant, we studied doped cyanine dye/C₆₀ bilayer solar cell performance for increasing dye film thickness. For films thicker than 50 nm, doping increased the power conversion efficiency by more than 30%. At the same time, the yield of working cells increased to 80%. We addressed the fate of the doped cyanine dye, and found no influence of doping on solar cell long term stability.     

Influence of chemically p-type doped active organic semiconductor on the film thickness versus performance trend in cyanine/C60 bilayer solar cells

Abstract

Simple bilayer organic solar cells rely on very thin coated films that allow for effective light absorption and charge carrier transport away from the heterojunction at the same time. However, thin films are difficult to coat on rough substrates or over large areas, resulting in adverse shorting and low device fabrication yield. Chemical p-type doping of organic semiconductors can reduce Ohmic losses in thicker transport layers through increased conductivity. By using a Co(III) complex as chemical dopant, we studied doped cyanine dye/C₆₀ bilayer solar cell performance for increasing dye film thickness. For films thicker than 50 nm, doping increased the power conversion efficiency by more than 30%. At the same time, the yield of working cells increased to 80%. We addressed the fate of the doped cyanine dye, and found no influence of doping on solar cell long term stability.     

Independent control of bulk and interfacial morphologies of small molecular weight organic heterojunction solar cells

We demonstrate that solvent vapor annealing of small molecular weight organic heterojunctions can be used to independently control the interface and bulk thin-film morphologies, thereby modifying charge transport and exciton dissociation in these structures. As an example, we anneal diphenyl-functionalized squaraine (DPSQ)/C(60) heterojunctions before or after the deposition of C(60). Solvent vapor annealing of DPSQ before C(60) deposition results in molecular order at the heterointerface. Organic photovoltaics based on this process have reduced open circuit voltages and power conversion efficiencies relative to as-cast devices. In contrast, annealing following C(60) deposition locks in interface disorder found in unannealed junctions while improving order in the thin-film bulk. This results in an increase in short circuit current by >30% while maintaining the open circuit voltage of the as-cast heterojunction device. These results are analyzed in terms of recombination dynamics at excitonic heterojunctions and demonstrate that the optimal organic photovoltaic morphology is characterized by interfacial disorder to minimize polaron-pair recombination, while improved crystallinity in the bulk increases exciton and charge transport efficiency in the active region.     

Controlled growth of a molecular bulk heterojunction photovoltaic cell.

The power conversion efficiency of organic photovoltaic cells has increased with the introduction of the donor-acceptor heterojunction that serves to dissociate strongly bound photogenerated excitons. Further efficiency increases have been achieved in both polymer and small-molecular-mass organic photovoltaic cells through the use of the bulk heterojunction (BHJ), where the distance an exciton must diffuse from its generation to its dissociation site is reduced in an interpenetrating network of the donor and acceptor materials. However, the random distribution of donor and acceptor materials in such structures can lead to charge trapping at bottlenecks and cul-de-sacs in the conducting pathways to the electrodes. Here, we present a method for growing crystalline organic films into a controlled bulk heterojunction; that is, the positions and orientations of donor and acceptor materials are determined during growth by organic vapour-phase deposition (OVPD), eliminating contorted and resistive conducting pathways while maximizing the interface area. This results in a substantial increase in power conversion efficiency compared with the best values obtained by 'random' small-molecular-weight BHJ solar cells formed by high-temperature annealing, or planar double heterojunction photovoltaic cells using the same archetypal materials systems.     

Organic photovoltaic cell employing organic heterojunction as buffer layer

Hexadecafluorophthalocyaninatocopper (F₁₆CuPc)/zinc phthalocyanine (ZnPc) heterojunction layer has been used as buffer layer in organic photovoltaic (OPV) cells based on ZnPc and C₆₀. The F₁₆CuPc/ZnPc heterojunction with highly conductive property decreased the contact resistance between the indium-tin-oxide anode and the organic layer. As a result, the short-circuit current density and fill factor were increased, and the power-conversion efficiency was improved by over 60%. Therefore, the method provides an effective path to improve the performance of OPV cells.     

Insights into photovoltaic properties of ternary organic solar cells from phase diagrams

The efficiency of ternary organic solar cells relies on the spontaneous establishment of a nanostructured network of donor and acceptor phases during film formation. A fundamental understanding of phase composition and arrangement and correlations to photovoltaic device parameters is of utmost relevance for both science and technology. We demonstrate a general approach to understanding solar cell behavior from simple thermodynamic principles. For two ternary blend systems we construct and model phase diagrams. Details of EQE and solar cell parameters can be understood from the phase behavior. Our blend system is composed of PC₇₀BM, PBDTTT-C and a near-infrared absorbing cyanine dye. Cyanine dyes are accompanied by counterions, which, in a first approximation, do not change the photophysical properties of the dye, but strongly influence the morphology of the ternary blend. We argue that counterion dissociation is responsible for different mixing behavior. For the dye with a hexafluorophosphate counterion a hierarchical morphology develops, the dye phase separates on a large scale from PC₇₀BM and cannot contribute to photocurrent. Differently, a cyanine dye with a TRISPHAT counterion shows partial miscibility with PC₇₀BM. A large two-phase region dictated by the PC₇₀BM: PBDTTT-C mixture is present and the dye greatly contributes to the short-circuit current.     

Ternary semitransparent organic solar cells with a laminated top electrode

Abstract

Tinted and colour-neutral semitransparent organic photovoltaic elements are of interest for building-integrated applications in windows, on glass roofs or on facades. We demonstrate a semitransparent organic photovoltaic cell with a dry-laminated top electrode that achieves a uniform average visible transmittance of 51% and a power conversion efficiency of 3%. The photo-active material is based on a majority blend composed of a visibly absorbing donor polymer and a fullerene acceptor, to which a selective near-infrared absorbing cyanine dye is added as a minority component. Our results show that organic ternary blends are attractive for the fabrication of semitransparent solar cells in general, because a guest component with a complementary absorption can compensate for the inevitably reduced current generation capability of a high-performing binary blend when applied as a thin, semitransparent film.     

C60 fullerene nanocolumns--polythiophene heterojunctions for inverted organic photovoltaic cells

Inverted organic photovoltaic cells have been fabricated based on vertical C(60) nanocolumns filled with spin-coated poly[3-(4-carboxybutyl)thiophene-2,5-diyl] (P3CBT). These C(60) nanocolumns were prepared via glancing angle deposition (GLAD), an efficient synthetic approach that controls the morphology of the resulting film, including intercolumn spacing, nanostructure shapes, and overall film thickness, among others. Intercolumn spacing was tuned to better match the expected P3CBT exciton diffusion length while simultaneously increasing heterointerface area. Due to observed in situ dissolution of the C(60) nanocolumns in solvents typically used to spin-coat polythiophene-based polymers (i.e., chloroform and chlorobenzene), the carboxylic acid-substituted polythiophene, P3CBT, was used as it is soluble in dimethyl sulfoxide (DMSO), a solvent that did not affect the structure of the GLAD-produced C(60) nanostructures. Preservation of the C(60) nanocolumnar structure in the presence of DMSO, with and without P3CBT, was verified by absorbance spectroscopy and SEM imaging. Incorporating these nanostructured C(60)/P3CBT films into photovoltaic devices on indium tin oxide (ITO) showed that the engineered nanomorphology yielded a 5-fold increase in short-circuit current and a power conversion efficiency (PCE) increase from (0.2 ± 0.03)% to (0.8 ± 0.2)% when compared to a planar device. When compared to a standard bulk heterojunction (BHJ) device based upon the same materials, the C(60)-GLAD device outperformed fully solution-processed bulk heterojunctions, which were observed to have PCEs of (0.49 ± 0.03)%.     

Structural Defects Control the Energy Level Alignment at Organic/Organic Interfaces

The dependence of the energy level alignment (ELA) on structural defects at an organic/organic heterojunction (OOH) of perfluoropentacene (PFP)‐on‐diindenoperylene (DIP) was investigated using X‐ray scattering and ultraviolet photoelectron spectroscopy. The density of structural defects near the interface between the PFP and DIP layers was varied by changing the growth temperature of the DIP film. A direct relationship was found between the defect density and the ELA at the OOH; the ELA together with the change in the electrostatic potential (quasi‐interface dipole layer) at the OOH varies systematically with the defect density near the interface. This indicates that a key factor affecting the ELA is the electrostatic potential change across the OOH interface, which is produced by electron transfer from DIP occupied gap states to PFP unoccupied gap states. These gap states originate from the defects and are effectively controlled by adjusting the growth conditions of the organic films. As a result, the ELA at OOH interfaces can be controlled by the density of structural defect, which is important for organic devices employing OOHs, such as organic photovoltaic cells.     

Polymer–Fullerene Bulk‐Heterojunction Solar Cells

Solution‐processed bulk heterojunction organic photovoltaic (OPV) devices have gained serious attention during the last few years and are established as one of the leading next generation photovoltaic technologies for low cost power production. This article reviews the OPV development highlights of the last two decades, and summarizes the key milestones that have brought the technology to today’s efficiency performance of over 7%. An outlook is presented on what will be required to drive this young photovoltaic technology towards the next major milestone, a 10% power conversion efficiency, considered by many to represent the efficiency at which OPV can be adopted in wide‐spread applications. With first products already entering the market, sufficient lifetime for the intended application becomes more and more critical, and the status of OPV stability as well as the current understanding of degradation mechanisms will be reviewed in the second part of this article.     

Vertical concentration gradients in bulk heterojunction solar cells induced by differential material solubility

Highly soluble fullerene derivatives (HSFD) and a low soluble polymer (LSP) were investigated as modifiers of the active layer morphology in conventional P3HT/PCBM bulk heterojunction solar cells. The observed changes in photovoltaic and electrical characteristics of the devices after addition of one or two modifiers suggest that they induced favourable vertical phase separation in the blends simply due to different solubilities of the components. In particular, HSFD is supposed to accumulate at the top of the film serving as a hole-blocking interlayer at the cathode/active layer interface. On the contrary, LSP seems to form electron-blocking buffer layer at the bottom of the device at the active layer/anode interface. Thus, the differential material solubility was suggested as a tool for adjustment of vertical morphology of organic bulk heterojunction solar cells.     

聚苯胺太阳电池的研究

本文利用聚苯胺和聚苯胺复合材料与n型硅制备了有机p-n异质结太阳电池,对电池的伏安特性、开路电压、短路电流和稳定性进行了表征和分析,对电池的机理进行了探讨,分析了影响聚苯胺p-n异质结电池特性的各种因素,得到了一些很有意义的结果.     

Fabrication and characteristics of fullerene-perylene dyad based organic photovoltaic cell

Fullerene is an acceptor material which is used most usually in organic photovoltaic cell. By the way, the reduction of electron mobility and the phase separation of conducting polymer and fullerene in the actual bulk heterojunction photovoltaic cell limit further improvement of device performance. In order to overcome the problems, fabrication of hybrid planar mixed heterojunction cells and synthesis of donor-acceptor dyad have been studied. In the present work, we have synthesized fullerene-perylene dyad to improve the fullerene based photovoltaic cell. In order to explore the properties of the synthesized material, the measurements of absorption spectrum and energy level were carried out. We have investigated the energy conversion efficiency of organic photovoltaic cell consisting of ITO/PEDOT-PSS/MEH-PPV:fullerene-perylene dyad/Al.     

Optically tunable intramolecular charge transfer dyes for vacuum deposited bulk heterojunction solar cells

We present the tunability of the photophysical and electrochemical properties of a series of intramolecular charge transfer compounds by facile molecular design and synthesis. The photovoltaic performances based on these sublimable materials and C(60) bulk heterojunction cells are compared and reported. The structural modification of the charge transfer dyes altered not only the electronic properties, but also the morphology of the bulk heterojunction thin films, as revealed by AFM and SEM studies. Addition of PEDOT:PSS between the ITO and the photoactive layer improved the hole injection from the photosensitizer into the anode, and the overall power conversion efficiency is also enhanced.     

Charge Transfer in Fullerene-Conducting Polymer Compositex: Electronic and Excitonic Properties

C₆₀ doping into conducting polymer with highly extended π-electron system in the main chain induces remarkable quenching of photoluminescence in conducting polymer and drastic enhancement of photoconductivity. These results can be explained in terms of photo-induced charge transfer between conducting polymer and C₆₀. That is, photoexcited excitons or exciton-polarons on conducting polymer are effectively dissociated at C₆₀ molecules transferring electrons to C₆₀. Photoexcitation of C₆₀ results in the transfer of hole from C₆₀ to conducting polymer. These novel C₆₀ doping effects have been observed not only in conducting polymers with non-degenerated ground state structures but also those with degenerated ground state structure such as di-substituted acetylene polymers with solitonic electronic systems.

Highly effective photo-induced charge transfer has been also observed in conducting polymer/C₆₀ heterojunctions, which are interpreted as donor (D)-acceptor (A) photocell. Based on this finding we have demonstrated an organic photovoltaic cell with D-A double heterojunction, Al/C₆₀/OEP/conducting polymer/TTO, in which OEP is octaethylporphine as an light absorbing antenna molecule. Novel characteristics have also been observed in various other junction devices utilizing C₆₀ doped conducting polymer.

Granular and multiphase superconductivity has been found in C₆₀-conducting polymer-alkali metal composites.

Effect of other type of fullerenes such as C₇₀, modified C₆₀ and C₆₀ polymers, and also effect of C₆₀ doping in polysilanes and their derivatives have also been studied.     

The Bulk Heterojunction in Organic Photovoltaic,Photodetector, and Photocatalytic Applications

Organic semiconductors require an energetic offset in order to photogenerate free charge carriers efficiently, owing to their inability to effectively screen charges. This is vitally important in order to achieve high power conversion efficiencies in organic solar cells. Early heterojunction-based solar cells were limited to relatively modest efficiencies (<4%) owing to limitations such as poor exciton dissociation, limited photon harvesting, and high recombination losses. The development of the bulk heterojunction (BHJ) has significantly overcome these issues, resulting in dramatic improvements in organic photovoltaic performance, now exceeding 18% power conversion efficiencies. Here, the design and engineering strategies used to develop the optimal bulk heterojunction for solar-cell, photodetector, and photocatalytic applications are discussed. Additionally, the thermodynamic driving forces in the creation and stability of the bulk heterojunction are presented, along with underlying photophysics in these blends. Finally, new opportunities to apply the knowledge accrued from BHJ solar cells to generate free charges for use in promising new applications are discussed.     

Tailoring organic heterojunction interfaces in bilayer polymer photovoltaic devices

In an ideal model, a p-n junction is formed by two stacked slabs of semiconductors. Although the construction of actual devices is generally more complex, we show that such a simple method can in fact be applied to the formation of organic heterojunctions. Two films of the organic semiconductors poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) can be connected by a simple film-transfer method without disturbing their flat surfaces. Each film can further be modified with a surface-segregated monolayer to tune the strength and direction of the surface dipole moment. Using this method, we fabricated bilayer organic photovoltaic devices with interfacial dipole moments that were selected to align the energy levels at the heterojunction. The open-circuit voltages of the P3HT/PCBM devices could be tuned over a wide range between 0.3 and 0.95 V, indicating that, even if the same combination of bulk materials is used, the interfacial properties drastically alter the performance of organic photovoltaic devices.     

Efficient Self‐Driven Photodetectors Featuring a Mixed‐Dimensional van der Waals Heterojunction Formed from a CdS Nanowire and a MoTe2 Flake

Heterojunctions formed from low‐dimensional materials can result in photovoltaic and photodetection devices displaying exceptional physical properties and excellent performance. Herein, a mixed‐dimensional van der Waals (vdW) heterojunction comprising a 1D n‐type Ga‐doped CdS nanowire and a 2D p‐type MoTe₂ flake is demonstrated; the corresponding photovoltaic device exhibits an outstanding conversion efficiency of 15.01% under illumination with white light at 650 µW cm^?2. A potential difference of 80 meV measured, using Kelvin probe force microscopy, at the CdS–MoTe₂ interface confirms the separation and accumulation of photoexcited carriers upon illumination. Moreover, the photodetection characteristics of the vdW heterojunction device at zero bias reveal a rapid response time (<50 ms) and a photoresponsivity that are linearly proportional to the power density of the light. Interestingly, the response of the vdW heterojunction device is negligible when illuminated at 580 nm; this exceptional behavior is presumably due to the rapid rate of recombination of the photoexcited carriers of MoTe₂. Such mixed‐dimensional vdW heterojunctions appear to be novel design elements for efficient photovoltaic and self‐driven photodetection devices.