Heterojunction Organic Photovoltaic Cells Research Articles

Impact of Alkyl-Based Side Chains in Conjugated Materials for Bulk Heterojunction Organic Photovoltaic Cells—A Review

The research for efficient organic materials organized in bulk heterojunction (BHJ) thin films for organic photovoltaics (OPVs) has shown a significant breakthrough in the past decade. Desired structural organization can be attained through various strategies. In this regard, the current review highlights tuning of alkyl chains introduced on molecular structures of active materials. The recent wide literature is classified based on the introduction of alkyl chains on polymers and small molecules used as donor and acceptor materials. The design of these materials, the morphological aspects of the active layers, and the performances of the related photovoltaic cells are detailed. A comprehensive discussion on chemical structures of the different material families considered, their modification by alkyl chains of various natures, and the morphological aspects are reported and tabulated.

Effect of active layer thickness on the charge recombination and dissociation in bulk heterojunction polymer solar cells under open circuit conditions

Bulk heterojunction organic photovoltaic cells (OPVs), which are based on anthracene-containing poly (p-phenylene-ethynylene)-alt-poly (p-phenylene-vinylene) (PPE-PPV) polymer denoted (AnE-PVstat) and phenyl C60 butyric acid methyl ester (PCBM). The active layer thickness was varied from 100 nm to 300 nm. The solar cell parameters and magnetoconductance (MC) effect were analyzed, and the results showed a positive MC effect (+MC) for active layer thicknesses of 100 nm, 150 nm, and 200 nm, while a negative MC effect (-MC) was observed for thicknesses of 250 nm and 300 nm. The Power conversion efficiency (PCE) of 4 ℅ was achieved for an active layer thickness of 200 nm. The triplet-doublet quenching (TQD) model was used to describe the MC effect, which is influenced by triplet-charge reactions on the current based on density matrix formalism using the Stochastic Liouville Equation. The dissociation and recombination rates of the T-D pairs increased with the thickness for thinner layers (100, 150, 200 nm) while decreased for thicker layers (250 nm and 300 nm).

Electronic and optical aspects of novel quinoxaline derivatives as electron donor materials for bulk heterojunction solar cells

In this paper, we design new forms of organic conjugated compounds-based quinoxaline derivatives. Specifically, we exploit density functional theory and time-dependent-density functional theory in order to study the structure, the optic, the electronic, the reorganization energy and the photovoltaic features of such new molecules. Particularly, all engineered compounds have a narrow band gap in the range of 0.696–0.721 eV, high oscillator frequency and good optical properties. Moreover, the PCBM is employed as an electron acceptor. Employing global reactivity descriptors, we demonstrate that the molecules can efficiently emit electrons into the PCBM and the electrons are attracted to PCBM from molecules. In addition, the results show an appropriate open circuit voltage in the range of 0.338–0.362 V. The proposed compounds exhibit excellent electron transport and charge conduction from the donor to the acceptor. These new molecules show potential properties to develop bulk heterojunction organic photovoltaic cells.

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.

Bulk heterojunction organic photovoltaic cells based on D–A type BODIPY small molecules as non-fullerene acceptors

The power conversion efficiency of PBDB-T:CF3-BDP-Cz is 13.07%, which is higher than that for PBDB-T:CF3-BDP-TPA, linked to the more appropriate nanoscale phase separation for exciton dissociation and charge transport.

Efficient planar heterojunction based on α-sexithiophene/fullerene through the use of MoO3/CuI anode buffer layer

In this work, planar heterojunction organic photovoltaic cells (PHJ-OPVs) based on alpha-sexithiophene (α-6T) and fullerene active materials were investigated. We have studied the effect of the hole transporting layer (HTL) on the performances of the PHJ-OPVs. The studied HTLs are molybdenum trioxide (MoO3), cuprous iodide CuI and MoO3 coupled with CuI (MoO3/CuI). The PHJ-OPVs were fabricated using the thermal evaporation/sublimation technique. The experimental study shows that double HTL MoO3/CuI, using 1.5 nm of CuI thickness, improves significantly the efficiency of the PHJ-OPVs compared with those with single HTLs. This enhancement is due to the fact that MoO3 contributes to the optimization of the holes collection and CuI causes significant influence on the nucleation and the growth of α-6T. CuI has led to spectacular modification of the structural properties of α-6T. When α-6T is deposited onto ITO/MoO3, the α-6T layers are amorphous, while they are crystallized into monoclinic crystalline structure with a laying down molecular orientation when they are deposited onto ITO/MoO3/CuI. The use of the double HTL and the optimization of the α-6T deposition parameters have led to an important improvement in the α-6T light absorption, roughness and the holes mobility. These findings participated in the significant enhancement of the PHJ-OPVs performances and their life time.

Applications of carbon nanotubes in different layers of P3HT: PCBM bulk heterojunction organic photovoltaic cells

Carbon nanotubes (CNTs) have been synthesized by a simple and efficient pyrolysis technique, and composites of CNTs with PEDOT:PSS, and with P3HT:PCBM blend have been prepared by a modest solution treatment. Bulk heterojunction organic photovoltaic cell (OPVC) has been fabricated by spin coating technique with configuration ITO/PEDOT:PSS/P3HT:PCBM/Al to serve as reference OPVC. Three more OPVCs have been fabricated by the same procedure, the first one with CNTs/PEDOT:PSS composite replacing PEDOT:PSS, second one with CNTs/P3HT:PCBM composite replacing P3HT:PCBM blend, and the third one with both replacements. The performances of all the OPVCs at room temperature have been compared and CNTs have been observed to cause extraordinary increment in the power conversion efficiency (PCE) as well as other electrical parameters of all the devices depending on the layer in which they have been incorporated. To extend the study for understanding further advantages of CNTs incorporation in fighting against degradation, all the fabricated OPVCs have been exposed to open atmosphere and their electrical properties have been measured at room temperature in regular intervals. Encouragingly positive outcomes have been attained, demonstrating not only a well maintained enhancement in the PCE, but also a considerably improved device stability. The best performing OPVC has achieved 24% more PCE and 67% more lifetime in open atmosphere than the reference device, which clearly does indicate the significance of applications of filler CNTs to improve not only the performance but also the stability of OPVCs.

On the physical and photo-electrical properties of organic photovoltaic cells based on 1,10-Phenanthroline and 5,10,15,20-Tetra(4-pyridyl)-21H,23H-porphine non-fullerene thin films

The active layer of the bulk heterojunction organic photovoltaic cells is typically based on a polymeric blend containing a donor polymer and a fullerene-based polymer electron acceptor. However, in the last decade non-fullerene polymer-based solar cells have seen a rapid progress, due to their extraordinary chemical, thermal and photo stability, and because of their ability to decrease the voltage loss. The conventional Organic Photovoltaic Cells (OPVs), structures based on either P3HT:PCBM bulk heterojunction (BHJ) polymeric blend or P3HT/PCBM bilayer heterojunction, have limited efficiency and stability, in principal due to the fullerene derivatives used as acceptor component. PCBM for example, as many other fullerene derivatives, exhibits some deficiencies, such as: limited chemical and energetic tunability, small range of absorption spectra, morphological instability and relatively high production costs. That is why in the last years, novel non-fullerene materials started to be used in order to overcome the above limits associated with the fullerene acceptors. In this paper two different non-fullerene acceptor materials have been tested as potential candidates for the efficient OPVs based on P3HT donor polymer as main absorber: a new conjugated polymer containing chelating ligands like 1,10-Phenanthroline (Phen), and a small-molecule acceptor like 5,10,15,20-Tetra(4-pyridyl)-21H,23H-porphine (TPyP). For both cell configurations a decrease of the voltage loss was obtained, and it was also observed that the thickness of the P3HT donor layer has an important contribution to the output power (Pmax) and fill factor (FF) of the OPV structures (FF and Pmax increased almost two times by decreasing the thickness of the P3HT donor layer).

Designing anisotropic microstructures with spectral density function

Materials’ microstructure is a critical aspect in design of functional materials as it links processing conditions to material performance. Previous efforts on microstructure mediated design mostly assume isotropy, which is not ideal for applications where material properties along specific directions must be tailored to meet performance requirements, such as those associated with transport phenomena. In this article, we propose an anisotropic microstructure design strategy that leverages Spectral Density Function (SDF) for rapid reconstruction of high resolution, two phase, isotropic or anisotropic microstructures in 2D and 3D. We demonstrate that SDF microstructure representation provides an intuitive method for quantifying anisotropy through a dimensionless scalar variable termed anisotropy index. The computational efficiency and low dimensional microstructure representation enabled by our method is demonstrated through an active layer design case study for Bulk Heterojunction Organic Photovoltaic Cells (OPVCs). Results indicate that optimized design, exhibiting strong anisotropy, outperforms isotropic active layer designs.

Real‐Time Study on Structure Formation and the Intercalation Process of Polymer: Fullerene Bulk Heterojunction Thin Films

Fullerene Intercalation In article number 1900508, Tobias Unruh and co-workers report on the formation process of a PC60BM:pBTTT-C14 active layer of a bulk heterojunction organic photovoltaic solar cell during evaporation of the solvent:additive mixture. By careful evaluation of the in-situ grazing incidence small-angle X-ray scattering measurements, a conclusive picture of the 5-step mechanism of fullerene intercalation and additive-tuned evolution of the nanoscopic film structure during the drying process could be obtained.

Real‐Time Study on Structure Formation and the Intercalation Process of Polymer: Fullerene Bulk Heterojunction Thin Films

Fullerene intercalation between the side chains of conjugated polymers has a detrimental impact on both charge separation and charge transport processes in bulk heterojunction (BHJ) organic photovoltaic cells (OPVs). In situ grazing incidence X‐ray scattering experiments allow to characterize the structure formation, drying kinetics, and intercalation in blends of phenyl‐c61‐butyric acid methyl ester (PC60BM) and poly(2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b]thiophene) named (pBTTT‐C14) from their 1,2‐orthodichlorobenzene (oDCB) solutions with different volume fractions of dodecanoic acid methyl ester (Me12) as a solvent additive. The structure formation process during evaporation of the solvent:additive mixture can be described by five periods, which are correlated to a multistep contraction of the lamellar stacking of the bimolecular crystals. The onset of crystallization is delayed by increasing the additive volume fraction in the coating solution leading to a promoted crystallinity. A conclusive picture of fullerene intercalation and additive‐tuned structural evolution during the drying of thin films of the polymer:fullerene BHJ blends will be presented.

Metallophthalocyanines in a ternary photoactive layer (P3HT:MPc:PC70BM) for bulk heterojunction solar cells

Novel bulk heterojunction organic photovoltaic solar cells have been fabricated by introducing a series of metallophthalocyanines into a photoactive blend of a poly(3-hexylthiophene-2,5-diyl) and [6,6]-phenyl C70 butyric acid methyl ester.

Lateral photovoltaic effect in ITO/PEDOT:PSS/MEH-PPV:PCBM/Al organic photovoltaic cells

Previous studies on lateral photovoltaic effect (LPE) mostly focused on the silicon-based inorganic semiconductor devices. Herein, we investigated the LPE in an bulk heterojunction organic photovoltaic (OPV) cell with ITO/PEDOT:PSS/MEH-PPV:PCBM/Al structure. The LPE exhibits a linear dependence on the laser spot position, with position sensitivity of 2.97 mV/mm (ITO) and 0.53 mV/mm (Al) at 300K. The transverse I–V curves in the dark and under AM1.5 illumination show the typical rectifying characteristic and the photovoltaic properties of OPV cells. With the increase of temperature, the lateral photovoltage increases accordingly. The mechanism of LPE in OPV devices was also discussed. It can be a kind of promising position-sensitive detectors for organic electronic applications.

New small organic molecules based on thieno[2,3-b]indole for efficient bulk heterojunction organic solar cells: a computational study

ABSTRACTThis study aims to evaluate the characteristics of novel organic D-π-A-π-D class small-molecules by using carefully the density functional theory, and time-dependent density functional theory calculations. Thedesigned sequence of (D-A) BHJ-1a to BHJ-4a in organic Bulk Heterojunction (BHJ) solar cells has been comprehensively analysed. Thiéno[2,3-b]indole (TI) has been used as donor, and Diketopyrrolopyrrole (DPP) as acceptor for all compounds. In order to improve the electronic, photovoltaic, and opticalproperties, we have substituted thiophene unit with furan, thieno[2,3-b]thiophene, thiazole and thiazolothiazole as π-bridge moieties. Thus, the result shows that the wise choice of the π-bridge units plays a significant role in improving Egap, producing a high bathochromic shift, and increasing VOC as well as a theoretical power conversion efficiency (PCE) over 7%. Interestingly, BHJ-4a with suitable π-bridge presents the optimal electronic properties with low band gap (1.870 eV) and high VOC (1.534 eV). Furthermore, we have modelled a Bulk heterojunction organic photovoltaic cells based on donor-PCBM complex in order to achieve the optimum Egap and VOC. Consequently, the obtained results provide a new way to design BHJ small molecule donors with higher power conversion efficiency.

Improved stability and performance of organic photovoltaic cells by application of carbon nanostructures and PEDOT:PSS composites as additional transparent electrodes

Conventional bulk heterojunction organic photovoltaic cell (OPVC) with regioregular poly[3-hexylthiophene-2,5-diyl] (P3HT): [6,6]-phenyl C61 butyric acid methyl ester (PCBM) blend as active layer has been fabricated with simple ITO/P3HT:PCBM/Al structure and its performance is compared with that of OPVC fabricated by incorporating an additional conducting layer of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) in between the active layer and ITO. An improvement of power conversion efficiency (PCE) has been observed from 2.716% to 2.733% due to the incorporation of PEDOT:PSS. The PCE has been further enhanced by replacing PEDOT:PSS with composites of carbon nanotubes (CNTs)/PEDOT:PSS and reduced graphene oxide (RGO)/PEDOT:PSS which have been prepared by simple solution method and the device parameters of all OPVCs are collated. A maximum PCE of 2.823% with corresponding short-circuit current density (Jsc) of 9.858 mA/cm2, open-circuit voltage (Voc) of 646.923 mV, and a fill factor (FF) of 44.263% has been achieved (at 25 °C) for the device with 5 wt% of CNTs in PEDOT:PSS. The performances of all fabricated OPVCs have been compared at different temperatures (25 °C, 30 °C, and 35 °C) and a declination in PCE is noted with increase in temperature. The stability of selected devices exposed to open atmosphere has been checked by measuring their parameters at different time intervals and it is observed that the degradation of materials reduces the performance of OPVCs. Under all different circumstances, OPVCs fabricated using the composites have been observed to perform better along with significant enhancement in PCE compared to OPVCs fabricated without using the composites.

DFT and TD-DFT design of small π-conjugated molecules with narrow band gap and high efficiency for organic solar cells

Five copolymers noted cop 1, cop 2, cop 3, cop 4 and cop 5 are theoretically designed. Each has a D–π–A architecture based on triarylamine used as a donor unit D for its high charge carrier mobility and its excellent stability, and five heterocyclic acceptors A with high carriers mobility. The aim is searching narrow band and high efficiency donor materials for organic photovoltaic cells. Using density functional theory DFT and time-dependent density functional theory TD-DFT, molecular orbitals, electronic and optical properties, exciton binding energies and charge transfer process are all predicted for all copolymers. The results show that all copolymers can be used with (PCBM) to design organic heterojunction photovoltaic cells. While comparing the various copolymers, we have noticed that cop 2 and cop 5 show the best optical properties, exciton dissociation and holes transport capability is more favorable for them, which improves short-circuit current (Jsc) and a fill factor (FF). Finally, the power conversion of composites involving cop 2/PCBM as well as cop 5/PCBM, respectively 8% and 9%, provides their efficiency. These results demonstrate that cop 2 and cop 5 will be two promising candidates for organic photovoltaic performance devices.

Optimizing the performance of the bulk heterojunction organic solar cells based on DFT simulations of their interfacial properties

Experimental studies suggest that the intermolecular interactions between polymers and fullerenes are critical to the design of efficient bulk heterojunction organic photovoltaic cells. However, a detailed understanding of these intermolecular interactions is still lacking. In this work, by correlating simulation data with experimentally determined efficiencies, we identify interfacial factors that can be used to enhance the performance of BHJ organic solar cells (OSCs). We employ dispersion corrected density functional theory method (B97D3) to investigate the properties of the interfacial region in various promising combinations of monomers (P3HT, PTB7, PCDTBT, PBDTTPD, PNT4T, PffBT4T, and PBTff4T) and fullerenes (PCBM and PC71BM) used in OSCs. We analyze the conformational structures and binding energies of these combinations, and obtain the electronic offsets of gas phase and interacting monomers and fullerenes. Our findings indicate that monomer/fullerene pairs that exhibit the highest experimentally determined PCEs (i.e. those containing PNT4T, Pff4TBT, and PBTff4T) have the following common characteristics: the lowest interfacial LUMO offset, the largest ratio of Voc (as determined by interfacial band gap) to monomer's energy gap, Eg, and a relatively high binding energy. We believe that selecting materials with these interfacial properties will lead to a development of OSCs with higher efficiencies.

Electrochemical Advances in Fluorinated/Chlorinated Boron Subphthalocyanines (BsubPcs) and Tetrabenzotriazacorroles (TBCS) Enabled By the Exploration of Their Base Chemistry and Their Application in Organic Voltaics

For some time, our group has been focused on the design, synthesis and application of derivatives of boron subphthalocyanines (BsubPcs). Our focal point has been and continues to be equally between the basic chemistry of BsubPcs and their application in organic electronic devices with a focus on two specific device applications, organic photovoltaics (OPVs)/solar cells and organic light emitting diodes (OLEDs). I will begin by presenting our efforts with regards to synthetic variants of BsubPcs and their application in planar heterojunction organic photovoltaic cells (OPVs, organic solar cells, OSCs).[1,2] After summarizing this work, I will then outline our very recent results. We have made recent observations that during the synthesis of peripherally fluorinated BsubPcs, through the reaction of tetrafluoro phthalonitrile and boron trichloride, that the resulting BsubPc has a significant portion of axial halide exchange yielding a mixture of Cl-F12BsubPc and F-F12BsubPc and separation of the mixture is not possible. The same scenario was observed for the synthesis of Cl-F6BsubPc. Therefore we developed a process to yield high purity F-F12BsubPc and F-F6BsubPc. Thereafter we revisited the electrochemistry of the peripherally fluorinated BsubPcs and their application in OPVs and compared results against peripherally chlorinated BsubPcs. Results will be presented. During past ECS meetings, we presented the electrochemistry and OPV application of peripherally chlorinated BsubPcs, specifically, BsubPcs with 6 and 12 peripheral chlorine atoms, Cl-Cl6BsubPc and Cl-Cl12BsubPc respectively. Recently we have developed a new chemical process that can yield a unique Cl-Cl9BsubPc and a Cl-Cl6BsubPc alternative. This developed chemistry will be presented and will also include an overview of the electrochemical and OPV performance characteristics that are dependent on the position and frequency of peripheral chlorine atoms. We have also recently shown that structural analogs to BsubPcs, boron subnaphthalocyanines (BsubNcs) are actually a mixed alloy composition of chlorinated materials designated as Cl-ClnBsubNcs. After establishing the correlation of electrochemical characteristics and OPV performance for Cl-ClnBsubNc we have recently shown that phenoxylated versions of ClnBsubNc are also applicable in BHJ OPVs also as electron accepting materials. More recently we have shown that Cl-ClnBsubNcs when applied as electron donating materials in OPVs have the same performance correlations to the amount of chlorination present and this will be presented. Finally, we have been exploring the chemistry that yields silicon tetrabenzotriazacorrole derivatives (SiTbcs). We have explored the chemistry using various sized axial ligands. The SiTbcs are formed from bis(hydroxy) silicon phthalocyanine ((OH)2-SiPc) via a simple reductive chemical process using magnesium metal and a chlorosilane in pyridine. The net change is the simple loss of 1 imide nitrogen atom yet the change in properties is significant including electrochemical and photophysical properties and these results will be presented. Coauthors and coinvestigators will be cited as appropriate throughout the presentation.

Enhancement in Open-Circuit Voltage in Organic Solar Cells by Using Ladder-Type Nonfullerene Acceptors.

The open-circuit voltage ( Voc) loss has always been a major factor in lowering power conversion efficiencies (PCEs) in bulk heterojunction organic photovoltaic cells (OPVs). A method to improve the Voc is indispensable to achieve high PCEs. In this paper, we investigated a series of perylene diimide-based ladder-type molecules as electron acceptors in nonfullerene OPVs. The D-A ladder-type structures described here lock our π-systems into a planar structure and eliminate bond twisting associated with linear conjugated systems. This enlarges the interface energy gap (Δ EDA), extends electronic delocalization, and hence improves the Voc. More importantly, these devices showed an increase in Voc without compromising either the Jsc or the FF. C5r exhibited a strong intermolecular interaction and a PCE value of 6.1%. Moreover, grazing-incident wide-angle X-ray scattering analysis and atomic force microscopy images suggested that our fused-ring acceptors showed a suitable domain size and uniform blend films, which were not affected by their rigid molecular structures.

Effects of neat C60 doping on the performance of bulk-heterojunction solar cells based on P3HT:PCBM

ABSTRACTWe explored the effect of neat C60 doping on the performance of bulk heterojunction (BHJ) organic photovoltaic cells (OPVs) based on poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61 butyric acid methyl ester (PCBM). The power conversion efficiencies (PCEs) were found to gradually decrease as the relative concentration of C60 increased. This PCE decrease is mainly due to the reduction in short-circuit current density (Jsc). Atomic force microscopy images revealed that C60-doped BHJ films consisted of unformed large P3HT crystals and many small PCBM aggregates. This morphology leads to a decrease in the effective P3HT conjugation length and an enhancement of absorption bleaching in neutral P3HT chains. These results explain well the blue-shift absorption spectra, the decrease of absorption coefficients, and the reduction in Jsc found in C60-doped BHJ OPVs.