Bulk Heterojunction Photovoltaic Cells Research Articles

Novel photoactive material and fabrication techniques for solar cells application: nanocellulose-based graphene oxide CdS composite

Abstract In recent times, solar energy has become one of the largest available sources of renewable energy at our disposal. However, the design of highly efficient solar cells is increasingly becoming crucial as there has been a surge for economically viable alternative energy sources with the lowest cost. Significant advances have been made through different routes to make photovoltaic (PV)/solar technologies economically viable, eco-friendly and consequently scalable. As a result, cellulose nanomaterials have become one of the emerging technologies in this regard because of the advantages of high-value bio-based nanostructured materials, such as their abundance and sustainability. Nanocellulose-based photoactive nanocomposite materials can be made by integrating conducting photoactive and electroconductive materials with hydrophilic biocompatible cellulose. Inorganic nanoparticles, such as graphene/reduced graphene oxide cadmium sulphide quantum dots, amongst others, can be introduced into the nanocellulose matrix and can be applied either as charge transporters or photoactive materials in different types of solar cells. Thus, in this review, we highlight the optoelectronic properties of different photoactive materials, particularly nanocellulose-based graphene nanocomposites; their efficiencies and drawbacks were X-rayed. The effect of doping each PV material on the PV performance is also discussed. It is anticipated that the novel material would result in a reduction in the cost of solar cells, jointly enhancing their efficacy in generating environmentally friendly electricity. Since the fabrication techniques and equipment play a crucial role in the development of solar cells, the fabrication techniques of bulk-heterojunction (BHJ) cells containing a nanocellulose-based graphene composite and case studies of already fabricated BHJ PV cells with nanocellulose-based graphene composite are discussed.

Study of a new composite based on SnO2 nanoparticles — P3HT:PC71BM co-polymer blend, used as potential absorber in bulk heterojunction photovoltaic cells

The properties of a novel P3HT:PC 71 BM:SnO 2 -nanoparticles (NPs) composite, used as potential absorber for Organic Photovoltaic Cells (OPCs), were investigated. The structural, morphological and optical characterizations of this composite demonstrated an enhancement of the ITO/PEDOT:PSS/P3HT:PC 71 BM: SnO 2 -NPs/LiF/Al ternary structure when compared to the ITO/PEDOT:PSS/ P3HT:PC 71 BM/LiF/Al binary one. This behavior is due, on one hand to the fact that SnO 2 -NPs behave as a secondary non-fullerene acceptor, and on the other hand to the superior electrical conductivity of the SnO 2 -NPs, very well dispersed in the polymeric blend as compared to other nanostructured inorganic materials. These properties led to a better photogeneration and transport of charge carriers, decreasing the series resistance, increasing the fill factor, and consequently the cell’s efficiency, too. The incorporation of NPs into ternary OPCs could represent a way to enhance their performances by improving the photogeneration of charge carriers, as well as their transport and collection to the electrodes. • Solar cells based on ternary P3HT:PC 71 BM:SnO 2 -nanoparticles compound were fabricated. • SnO 2 -nanoparticles addition allowed an extended absorption spectra at shorter wavelengths. • EQE increased by 50% in the 400-450 nm range, when compared to the binary system. • Current–voltage characteristics of the photovoltaic element revealed improved parameters.

Computational Study of New Small Molecules based Thiophene as Donor Materials for Bulk Heterojunction Photovoltaic Cells.

In this research work, we study the structural, optical, electronic, and photovoltaic properties of eight thiophene-based π-conjugated organic molecules using quantum methods namely time-dependent density functional theory. In particular, we identify the relationships between the chemical structure of these π-conjugated organic molecules and their optoelectronic properties. Moreover, we calculate and compare the highest energy occupied molecular orbital and lowest energy unoccupied molecular orbital energy levels of these compounds which act as donor with the ones of the acceptorphenyl-C61-butyric acid methyl ester. As a result, the investigated molecules show a low band gap, suitable open-circuit voltage and appropriate alignment energy level between the engineered donor molecules and the acceptor phenyl-C61-butyric acid methyl ester. This theoretical study shows that these new molecules have potential properties for the development of organic heterojunction photovoltaic cells.

New BODIPY derivatives with triarylamine and truxene substituents as donors for organic bulk heterojunction photovoltaic cells

We have designed two new BODIPY derivatives, denoted as 6a and 6b, substituted with truxene moiety and triphenylamine (TPA) unit groups and have investigated their optical and electrochemical properties. Dyes 6a and 6b were employed as donor along with PC71BM or Y6 as acceptor for the fabrication of binary and ternary organic solar cells. After optimization of the binary and ternary active layers, we have achieved over all power conversion efficiency (PCE) of 11.37 % and 13.32% for 6a:PC71BM:Y6 and 6b:PC71BM:Y6 ternary organic solar cells, respectively, which are higher than the binary organic solar cells based on PC71BM or Y6 acceptor. The higher power conversion efficiency for ternary OSC is possibly attributed to the better exciton generation rate, exciton dissociation and charge collection.

Efficient and thermally stable BHJ solar cells based on a soluble hydroxy-functionalized regioregular polydodecylthiophene

A new regioregular polythiophene derivative, called poly[3-(12-hydroxydodecyl)thiophene] (PT12OH), was synthesized by post-functionalizing its ω-brominated precursor poly[3-(12-bromododecyl)thiophene] (PT12Br) prepared using the Grignard metathesis route. Thanks to the optimal balance between hydrophilic and hydrophobic groups within its structure, PT12OH was highly soluble and easily filmable from common organic solvents allowing for its complete characterization. It also showed enhanced thermal properties, crystallinity, and self-assembling capabilities by the formation of strong inter- and intrachain hydrogen bonds. Bulk heterojunction photovoltaic cells with PT12OH and PC61BM showed a PCE of 4.83% and a remarkable over-time stability, offering good photoconversion efficiency even after 120 h of accelerated aging. Indeed, the PCE decrease was 34% for the hydroxylated polymer and 65% for its brominated precursor. It should also be pointed out that the enhanced thermal stability of PT12OH was achieved without resorting to any complex post-annealing photochemical, thermal, or chemical treatment and was thus directly ascribable to the polymer chemical structure. The simple and effective synthetic procedure, photovoltaic efficiency, and enhanced stability revealed the potential of PT12OH for large-scale organic solar cell applications.

Photoelectrochemistry of Poly‐3‐hexylthiophene and Stannum Chromium Bimetallic Nanoparticle Heterojunction Blend

AbstractIn the design and fabrication of photovoltaic cells, acceptors are as important as donors. Quite a large variety of donors exist while most common acceptors are fullerene based and these acceptors are limited in properties. Herein we report the design, optoelectronic and photoelectrochemical properties of three new stannum‐based bimetallic nanoparticles (SnTi, SnV and SnCr) combined with poly(3‐hexylthiophene)(P3HT) as electron donor material in the fabrication of bulk heterojunction photovoltaic cells. SnCr nanoparticles displayed excellent electrocatalytic and photoelectronic properties. Solar cell device constructed with P3HT as donor and SnCr as acceptor achieved an open circuit voltage ca. 1 V and efficiency 1.46 %.

Application of an inverse-design method for designing new branched thiophene oligomers for bulk-heterojunction solar cells

Using our recently developed theoretical inverse-design method (PooMa) a new series of branched oligothiophene molecules for photovoltaic donors are designed. PooMa uses a genetic algorithm to screen a huge pool of compounds combined with a fast electronic-structure method (Density-Functional Tight-Binding, DFTB) with reasonable accuracy. Here, we apply this inverse-design method to identify a set of 20 branched oligothiophene systems with promisingly high efficiencies by using a Quantitative Structure Property Relation (QSPR) model based on five electronic descriptors that describe the performance of organic solar cells. We consider a pool of oligomers that are modified by attaching to each of 7 different sites one out of 22 functional groups, i.e., a pool of 227 ≈ 2.5×109 molecules. Subsequently, density-functional-theory (DFT) and Time-Dependent-DFT (TD-DFT) calculations in the gas phase with a 6-31G(d,p) basis set have been carried through to give further information on the suggested oligomers. Bulk-heterojunction photovoltaic cells were designed with the suggested oligothiophenes as donors and Phenyl-C61-butyric acid methyl (PCBM) derivatives as acceptors. Conversion efficiencies of the designed photovoltaic cells were examined with the Scharber diagram model.

Bulk-heterojunction polymer photovoltaic cells manufactured using non-halogenated and non-aromatic solvent

The use of photovoltaic devices as converters of solar energy into electricity is a relevant technology to the global energy needs and, at the same time, it mitigates the negative impact on the environment caused by conventional sources. Although polymer solar cells are properly considered as clean and renewable source of electricity production, the processing involved in the manufacture of such devices still contains toxic elements. The formation of thin films of active layer in bulk-heterojunction organic solar cells, as example, usually uses halogenated solvents as chlorobenzene (CB), dichlorobenzene (DCB), or chloroform (CF), which represent certain risks of toxicity to the human body and to the environment. This work aims to show that it is possible to manufacture organic solar cells using less-toxic solvents, in special to the environment, whose final efficiency results are similar to those that have been processed by conventional halogenated solvents. With the aid of the Hansen solubility parameters (HSP), we identified non-halogenated and non-aromatic solvent that showed chemical compatibility with donor polymers and acceptor molecules used in the device's active layer. The fabricated devices were then characterized and their photovoltaic responses compared to the conventional equivalents.

Effect of insertion of bathocuproine buffer layer at grating-structured cathode–organic-layer interface in bulk-heterojunction solar cells

A grating-structured interface of a poly(3-hexylthiophene) (P3HT) and n-type [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)-based bulk-heterojunction (BHJ) photovoltaic (PV) cell was designed and fabricated to obtain a desirable thickness distribution of the deposited bathocuproine (BCP) buffer layer to efficiently utilize its potentials. As a master mold of the grating-structure, a commercially available recordable digital versatile disc (DVD-R) substrate was employed. The grating-structured surface of the P3HT:PCBM layer was successfully produced by duplication from a poly(dimethylsiloxane) secondary mold using the spin cast molding technique. From morphological observations of the grating-structured surface covered with vapor-deposited BCP, we roughly estimated the ratio of the BCP thickness at “walls” to that at “top” and “bottom” regions to be ∼0.5. The grating-type BHJ PV cell with a 5-nm-thick BCP layer exhibited the maximum power-conversion efficiency (ηp) of 3.51%. Compared with the conventional flat-type BHJ PV cell with a 20-nm-thick BCP layer, the performance of the grating-type BHJ PV cell with a 20-nm-thick BCP layer was remarkably improved, owing to the contribution of the wall side contact, which provides a lower-barrier path of the electrons toward the cathode through the thinner BCP layer.

Optimization and performance enhancement of conjugated polymer and fullerene derivatives based bulk heterojunction photovoltaic cells: weight ratios of donor and acceptor, anodic buffer layer thickness and heat treatment

In this research, we systematically investigated and optimized the performance of photovoltaic cells based on P3HT: PCBM in different conditions. The applied conditions to these cells include the change in the weight ratio of the donor (P3HT) to the acceptor (PCBM), the thickness of the anodic buffer layer (MoO3), and the annealing temperature of the active layer. An accurate correlation between these physical parameters is essential for the better performance of bulk heterojunction photovoltaic cells. At each step of the experiment, the power conversion efficiency, short-circuit current density, and fill factor increased. Laboratory evidence suggests that a cell fabricated from the same weight ratio of the donor and acceptor (1:1) has a better performance than other proportions. The power conversion efficiency in this case was 3.12%. In addition, the optimum thickness of 7 nm was determined for MoO3 as anodic buffer layer, in which the power conversion efficiency of the cell increased to 3.52%. By investigating the different annealing temperatures of the active layer to optimize the polymer photovoltaic cell, it was observed that at annealing temperature of 140 °C, the cell demonstrated better performance and the power conversion efficiency of the cell increased to 4.74%. Finally, the correlation between morphology changes of the active layer surface and the efficiency of the cell were investigated.

Simple BODIPY dyes as suitable electron-donors for organic bulk heterojunction photovoltaic cells

In this work we present the synthesis and characterization (including the report of two new crystal structures) of a series of meso-substituted boron-dipyrromethene (BODIPY) molecules. With an accessible synthesis, these simple BODIPYs have characteristics that par with those BODIPY's having more complex structures, what makes them very interesting materials for industrial production. The photovoltaic performance of the synthesized BODIPY's was evaluated in bulk heterojunction organic solar cells, where they were used as electron-donors. We found that the presence of electron-withdrawing groups has a detrimental effect on the photovoltaic performance, while the meso-free structure shows an improved efficiency in comparison with the meso-substituted BODIPYs. We also studied the origin of the meso-unsubstituted derivative, elucidating at least one of its formation processes, which can open a new way of BODIPY synthesis.

Synthesis, solar cell application, and biological study of vinyl substituted isophorone derivatives

In this study, isophorone derivatives, which are single step aldol products, were designed and synthesized. Our overview of the synthesis of solar cell application and biological applications of series vinyl substituted isophorone compounds is reported. All synthesized compounds were characterized both through experimental techniques and calculations with density functional theory. Additionally, we investigated the photophysical properties of small organic compounds for organic bulk heterojunction photovoltaic cells. The solar cell efficiency results were compared and compound 4 was found to be more efficient than the other compounds. It was found that the optical quantum yield values were very close. Isophorone derivatives showed better antioxidant activities, which were DPPH, metal chelating, reducing activity, and antimicrobial activities against Salmonella typhimurium, Klebsiella pneumonia, Candida albicans, Staphylococcus aureus, Bacillus subtilis, and Escherichia coli. Compound 7 (which is substituted quinoline) and compound 5 (which is substituted cyno) had the highest antioxidant effect. The 4-brom phenyl, 4-cyno phenyl were used against gram negative bacteria and the 4-ter-butyl phenyl group was indicated for gram positive bacteria and yeast.

Rational design of (D-A) copolymers towards high efficiency organic solar cells: DFT and TD-DFT study

In this work, we focus on designing a donor copolymer for the improvement of photovoltaic performance. Using density functional theory and time-dependent density functional theory, we investigated the electronic, optical and charge transfer properties of a series of new designed copolymers based on the reported copolymer Pa0 which is composed of a donor fluorene unit and an acceptor 4,7-dithien-2-yl-2,1,3-benzothiadiazole. We first obtained two copolymers Pb0 and Pc0 by replacing the benzothiadiazole (BTZ) with two different strong acceptors units to decrease the LUMO level of conjugated polymers. Then, we designed Pa1, Pb1 and Pc1 copolymers by adding a substituent methyl group to the thiophene spacer unit (T). Bulk-heterojunction photovoltaic cells were designed with the copolymers as the donors and PCBM as the acceptor. Our results show that the cells based on Pb1 and Pc1 have a suitable electronic structure with energy conversion efficiency exceeding 10%. Moreover, we used Marcus theory to evaluate the intermolecular charge transfer (inter-CT) and recombination (inter-CR) rates of these cells (copolymer/PCBM). The ratio Kinter-CT/Kinter-CR of Pc1/PCBM heterojunction is about 106 times higher than that of Pb1/PCBM which clearly reveals that the designed donor molecule Pc1 will be a promising candidate for high performance organic photovoltaic devices. Our strategy to design novel donor copolymers provides a theoretical guideline for further improving in electrical, optical properties and the efficiency of the photovoltaic device.

Mesoscopic investigation of the effect of MWCNT/rGO network on the performance of P3HT:PC60BM solar cells

Abstract Bulk heterojunction (BHJ) photovoltaic cells (PVCs) have been fabricated with a new active layer architecture by introducing 1-dimensional (1D) multi-walled carbon nanotubes (MWCNTs) and 2-dimensional (2D) reduced graphene oxide (rGO) in Poly[3-hexylthiophene-2,5-diyl] (P3HT): Phenyl-C61-butyric acid methyl ester (PC60BM) blends. The fabricated solar cells revealed a maximum short-circuit current density (Jsc) of ∼11.01 mA/cm2, open-circuit voltage (Voc) of ∼0.63 V and fill factor (FF) of ∼0.5 yielding an efficiency of ∼4.13%. The improved efficiency has been attributed to the efficient exciton dissociation at the junctions formed by CNTs and rGOs in addition to PC60BM with P3HT. The faster electron-transport by PC60BM and hole by CNT/rGO network in the blend that inhibits the carrier recombination contribute significantly towards the improved performance of the fabricated PVCs. The effect of MWCNTs:rGOs network on the mesoscopic phase distribution of P3HT and PC60BM have also been investigated.

Oligo(ethylene oxide) chains in fluorene bridge units of perylenediimide dimers as an efficient strategy for improving the photovoltaic performance in organic solar cells

Perylenediimides (PDIs) are among the most promising non-fullerene electron-acceptors for applications in organic photovoltaics (PV), providing a large scope for structural modifications. PDI-based dimers have shown the highest performances in PV devices owing to their characteristic twisted conformations that reduce their tendency to form large aggregates which are detrimental for PV operation. In this work, two fluorene-bridged perylenediimide (PDI) dimers with oligo(ethylene oxide) or alkyl chains anchored to the C9 position of the fluorene unit were synthesized and investigated in solution-processed organic bulk heterojunction (BHJ) photovoltaic cells as electron acceptors. The PDI dimer substituted with oligo(ethylene oxide) chains resulted in improved power conversion efficiencies (in 20%–53%) in solution processed bulk heterojunction (BHJ) cells with two different polymer donors, PTB7 and PffBT4T-2OD. Nevertheless, the replacement of alkyl chains by oligo(ethylene oxide) chains did not significantly affect the geometric characteristics of the PDI dimers and the optical and electrochemical properties were only marginally modified. Increased exciton dissociation and enhanced charge transport derived from a more densely packed π-π stacking in the solid state caused by ethylene oxide groups are pointed out as possible causes for the improved PV performance. The influence of 1,8-diiodooctane as solvent additive in the blend films was also investigated and allowed to further increase the efficiencies of the cells with PTB7. Overall, the results show that the simple replacement of alkyl chains by oligo(ethylene oxide) chains in PDI dimers is an efficient way to improve the PV performance without compromising the optoelectronic properties of the PDI acceptor.

Examining Structure–Property–Function Relationships in Thiophene, Selenophene, and Tellurophene Homopolymers

Heavy atom main group element-containing conjugated polymers have attracted increasing attention in recent years. The synthesis of these compounds is generally involved, and little is known about their optoelectronic device performance. Here we examine the relationship between polymer structure and optoelectronic behavior in a series of chalcogenophene homopolymers of thiophene, selenophene, and tellurophene with well-matched molecular weights, dispersity, and regioregularity. We employ fast and slow drying device preparations to study the effect of polymer–fullerene separation on charge separation and collection in canonical bulk heterojunction photovoltaic cells. In both preparations, increasing heteroatom size leads to larger proportions of finely mixed polymer–fullerene domains. Differences in polymer–fullerene separation between preparations result in the formation of optimal morphologies in selenophene and tellurophene devices with little impact on thiophene devices. We then use planar heterojunction devices to directly examine the effects of heteroatom substitution on charge transport and charge generation and find that in the absence of polymer–fullerene mixing, devices exhibit similar diode behavior. We further demonstrate that ultrafast decay pathways unique to heavy heteroatom-containing polymers are apparent in both planar and bulk heterojunctions and thus not dependent on polymer–fullerene mixing or polymer assembly. This work directly examines the role of heteroatom substitution in defining the photovoltaic performance of conjugated homopolymers. Through single-atom substitution we are able to significantly modify polymer assembly, mixing, and optoelectronic properties. Specific emphasis on tellurophene polymers reveals relationships between polymer structure and properties that are not apparent in more traditional light-atom chalcogenophenes such as thiophene and selenophene.

Morphology of a Bulk Heterojunction Photovoltaic Cell with Low Donor Concentration.

Atomistic nonequilibrium molecular dynamics simulations have been used to model the morphology of small-molecule bulk heterojunction films formed by vapor deposition as used in organic photovoltaics. Films comprising C60 and 1, 5, 10, and 50 wt % of 1,1-bis[4-bis(4-methylphenyl)aminophenyl]cyclohexane (TAPC) were compared to films of neat C60. The simulations suggest that if holes can hop between donor molecules separated by as little as 1.2-1.5 nm, then a TAPC concentration of 5 wt % is sufficient to form a percolating donor network and facilitate charge extraction. The results provide an explanation for why low donor content organic photovoltaics can still have high efficiencies. In addition, the roughness, porosity, and crystallinity of the films were found to decrease with increasing TAPC content.

Study on effect of substituents grafted on cyclopenta-fused anthracences as the cascade material for improving ternary bulk heterojunction photovoltaic cells

Searching alternatives to enhance the attainable conversion efficiency of organic photovoltaics is one of the most crucial issues toward renewable energy source. Here, the model ternary organic solar cells systems were designed to improve the performance of P3HT/PC61BM organic solar cells which promise a potential to large area and flexible fabrication, based on a group of cyclopent[hi]aceanthrylenes (CPAs) derivatives as the third cascade component in active layer of the device. In all of these solar cells, although all ternary structures containing a group of cyclopent[hi]aceanthrylene derivatives demonstrate the improvement of open-circuit voltage (VOC) compared to binary device of P3HT/PC61BM, the conversion efficiencies of these devices can not all be improved. This indicates the different influence on observable device metrics attributed by the different substituents around cyclopent[hi]aceanthrylene core. By extension, these results suggest that the ternary system used a simple one single active layer processing step can provide a potentially effective way to optimize the performance in BHJ solar cells.

Application of Conjugated Organic Polymers for Photovoltaic's: Review

Photovoltaic effect is the emergence of a voltage between electrodes attached to a solid or liquid system up on shining light on to this system. Conjugated polymer is a molecular entity whose structure is represented as a system of alternating single and double bonds which give rise to their semi-conductor properties. Conjugated polymers are used for photovoltaic devices because, intrinsically stable up on photoexcitation with visible light, High absorption cross-section for photon harvesting, Tunable band gap with in the entire visible spectral range and High yield of charge generation when mixed with electron acceptor materials. The important physical process in the energy conversion process that take place in polymers for photovoltaic cells are; Absorption of a photon of light by photoactive material and generation of excitons, diffusion of excitons in conjugated polymers, dissociation of charge carriers (electron-hole pair) at the donor-acceptor interface in to free carriers, transport of free carriers towards the electrodes, and extraction of the charge carriers at the respective electrode interfaces. The efficiency of converting solar to electrical energy by a solar cell depends on the band gap of the light absorbing semiconductor. Band gap (Eg) is the difference in energy between the HOMO and LUMO and there by the maximum amount of energy required for an excitation or is the energy difference between the edges of the conduction band and valence band. The power conversion efficiency is a function of band gap. For device architectures of conjugated polymer based photovoltaic cells; there are three types Single layer photovoltaic cell, Bilayer hetero junction photovoltaic cell and Bulk hetero junction Photovoltaic cell.

Solid-state showdown: Comparing the photovoltaic performance of amorphous and crystalline small-molecule diketopyrrolopyrrole acceptors

A novel glass-forming diketopyrrolpyrrole (DPP) derivative (DPP-glass) and a crystalline isoelectronic analogue were synthesized and their photophysical, electrochemical and photovoltaic properties were investigated. The compounds were processed into thin films that were further studied by Atomic force microscopy (AFM) and X-ray diffraction (XRD). Solution-processed inverted and regular bulk heterojunction photovoltaic cells were fabricated in different blend ratios with P3HT as donor. Both DPP derivatives show appreciable photovoltaic performance with power conversion efficiencies (PCE) ranging from 0.30 to 2.50%. The regular device with amorphous DPP-glass showed the highest efficiency (2.50%) in the blend ratio 1:2 whereas the device with crystalline DPP derivative shows an efficiency of 1.56% under simulated 1.5 solar irradiation of 100 mWcm−2. The better performance of DPP-glass in comparison to its crystalline analogue is attributed to a more uniform film morphology and fewer defects, leading to improved electron mobility.