Performance Of Organic Photovoltaic Cells Research Articles

Asymmetrical impurity effect of C70:C60 mixture in thick small molecule organic photovoltaic cells

Fullerenes, especially C70 and C60, and their derivatives are popular acceptor materials for many organic semiconductor devices, including organic photovoltaic cells. One of the main impurities in C70 is C60, and intentional mixture of C60 and C70, or their derivatives, which have been utilized for maximizing the performance of organic photovoltaic cells or perovskite solar cells. However, the influence of impurities in C70 on the device characteristics has not been fully clarified when incorporating C70 into organic devices. To further clarify the relationship between the fullerene mixture and impurities, the effect of impurities in fullerenes on thick photovoltaic cells was evaluated, and a strange asymmetrical impurity effect of the C60:C70 mixture was demonstrated by using C70 with various purities as an acceptor material and zinc phthalocyanine (ZnPc) as a donor material. It was found that C60, C78, and C84 impurities in C70 caused carrier recombination during electron transport, but the same effect of the C70 impurity in C60 was much smaller. Additionally, the impurity effect of higher fullerenes on C70 was clearly larger than that of C60. The effect of higher fullerene impurities can be explained by electron trapping due to their deep lowest unoccupied molecular orbital (LUMO) levels. For the small difference of LUMO energy between C70 and C60, however, it is suggested that the shallow trapping of C70 impurities does not largely inhibit the transport of photocarriers by C60, while the low energy barrier of C60 impurities largely inhibits the transport of photocarriers by C70. For clear comparison, the photoelectrical conversion layers were crystallized using co-evaporant induced crystallization. An effective purification method for C70 using sublimation purification, which depends on the C70 concentration in the sublimation source, is also reported.

Organic laser power converter for efficient wireless micro power transfer

Wireless power transfer with collimated power transmission and efficient conversion provides an alternative charging mode for off-grid and portable micro-power electronics. However, charging micro-power electronics with low photon flux can be challenging for current laser power converters. Here we show laser power converters with organic photovoltaic cells with good performance for application in laser wireless power transfer. The laser selection strategy is established and the upper limit of efficiency is proposed. The organic laser power converters exhibit a 36.2% efficiency at a 660 nm laser with a photon flux of 9.5 mW cm−2 and achieve wireless micro power transfer with an output of 0.5 W on a 2 meter scale. This work shows the good performance of organic photovoltaic cells in constructing organic laser power converters and provides a potential solution for the wireless power transfer of micro-power electronics.

Photovoltaic characteristics of organic heterocyclic 2,9-dimethyl quinacridone in different solvents using DFT approach

In this work, efforts had been made to develop the photovoltaic utility of 2,9-dimethyl quinacridone (2,9-DMQA) using density functional theory (DFT). Initially, the structural analysis was done using the molecule's ground state geometry to study the molecule's structural characteristics. In particular, the global reactivity descriptors, density of states (DOS) analysis, molecular electrostatic potential (MEP), contour plot, and, ionization potential surface, dipole moment, Mulliken, and natural charge distribution, were computed and analyzed to identify the nucleophilic and electrophilic sites of the molecule. Considering N – H the donor and C = O the acceptor group, the intramolecular charge transfer (ICT) was established. Hirshfeld surfaces and fingerprint plots were also determined to explore intermolecular interactions. The electronic transitions were analyzed using the simulated absorption and emission spectra. To better understand the role of solvents in inducing the photovoltaic property, the light-harvesting efficiency of the molecules was computed in the considered solvents. The title molecule provided light harvesting efficiency in order of chloroform > ethanol > cyclohexane. Other photovoltaic properties were also computed to predict the power conversion efficiency of the molecule. Consequently, the title molecule offers a strong charge conduction ability and effective electron transport from the donor to the acceptor. The obtained results were favorable enough to test experimentally 2,9-DMQA which could be used as a photosensitizer in dye-synthesized solar cells and could enhance the performance of organic photovoltaic cells.

Accessing Deep Traps Distribution in FeS2‐Doped Organic Photovoltaics

Although doping has the potential to improve the performance of organic photovoltaic cells (OPVc), doping effects on charge transport, recombination, and energetic disorder are still obscure. Doping has two opposing effects: on the one hand, dopant ions create more trap centers, while free dopant‐induced charges fill deep states, potentially providing better performance. The optimum amount of dopants can considerably improve the performance of OPVc. Herein, the energetic distribution of trap states in P3HT: PC71BM‐based OPVc doped with iron pyrite nanocubes (NCs) is reported. Using the reverse bias transient photocurrent (TPC) measurement, the energetic trap distributions with different doping conditions are studied. The photovoltaic characteristics and TPC phenomena of the OPVc greatly improve through doping. Variations in trap distributions with doping levels are analyzed to interpret the obtained trap density of states profiles. The light‐dependent current–voltage characteristics help to identify the presence of a less trap‐assisted recombination process in the optimum device. This study highlights the mechanism for performance improvement in devices with optimal doping of iron pyrite NCs.

Investigation of the different possible energy band structure configurations for planar heterojunction organic solar cells

We present a study of performances of organic photovoltaic cells (OPVs) using a wide variety of small molecules. These OPVs use two or three molecules to form binary or ternary cells, i.e. consisting of two or three flat stacked layers. Different ternary OPVs designs for efficient light harvesting and energy band structure combinations are studied. Molecules have been chosen to study all possible energy band structure alignments in ternary configuration. We discuss various parameters limiting their efficiency, such as carrier mobility, organic layer morphology, energy band structure alignment. Special attention has been paid to understand the device behavior. It is shown that, if light absorption domains, carriers mobility, energy band structure alignment are decisive for achieving performing OPVs, the geometry of the molecules is also decisive. In ternary OPVs, the third organic layer, intercalated between the electron donor and the electron acceptor, must serve as a bridge in the energy levels of the outer layers, which permits an energetic cascade effect. For this effect to be positive it is necessary that this intermediate layer facilitates creation and transport of charge carriers, but also that it allows transport of energy, all leading to an improvement of the performance of the OPVs.

Effect of the interfacial materials on the performance of organic photovoltaic cells

There are two different architectures for organic solar cells: standard and inverted. They differ according to the role of the lower electrode as a cathode or an anode. In the standard structure, the holes go towards the ITO electrode while in the reverse structure the holes go towards the ITO electrode. Interface materials are located between the active layer and the electrodes. The advantages of these layers is the alignment of energy levels between the active layer, the electrodes and these interface materials in order to facilitate transport of charges to the electrodes and to minimize recombination.The objective of this paper is to obtain by simulation, the electrical characteristics and the behavior of a photovoltaic system by using General-Purpose Photovoltaic Device Model GPVDM software, this according to the variations of the parameters of the model. The comparison of the simulation results of the different structures allowed as choosing an optimized structure with suitable interface materials. The optimized cell is ITO/AZO (40 nm)/P3HT:PCBM (200 nm)/Cu2O (20 nm)/Ag, with the following photovoltaic parameters: PCE = 5%, VOC = 0.6 V, JSC = 12 mA/cm2 and FF = 67.53%.

Ink formulation for organic photovoltaic active layers using non-halogenated main solvent for blade coating process

Large area processing of organic photovoltaic cells and modules is a key step towards commercialization. At the same time, the scale-up of fabrication entails careful evaluation and selection of solvents, and optimization of processing parameters. It has been proven that using proper processing additives in polymer:PCBM bulk-heterojunction solution is one of the most effective strategies to improve the performance of organic photovoltaic cells. Here we report a new transition solvent strategy of using non-halogenated solvents to fabricate PDTSTPD: PC71BM based active layers. This strategy uses a combination of miscible good main solvent such as 1,2,4-trimethylbenzene and coadditives such as diphenyl ether (DPE) and 1-methylnaphtalene (MeN) for middle molecular weight PDTSTPD, or 1,8-diiodooctane (DIO) and MeN for high molecular weight PDTSTPD to tune the nano-scale morphology of the active layer during the film drying process. Inverted organic photovoltaic cells with an active area of 1cm2 on PET substrate fabricated by a blade coating process using the formulated ink have demonstrated a power conversion efficiency up to 5 %.

IR thermographic camera as useful and smart tool to analyse defects in organic solar cells

The main goal of this paper was to show that organic layers prepared for organic solar cells construction can be partially characterized by IR thermographic camera to detect dysfunctionalities of the active layer. Moreover, we used IR thermographic camera to check thermal and electrical behaviour of created simple solar cells to reduce time and cost in global production of solar cells. As model system organic compounds such as PTB7 polymer and small molecule PC71BM as commercial components and newly synthesized imine PV-BLJ-SC11 there were used, to construct simple devices. Full Text: PDF ReferencesK. D. G. I. Jayawardena, L. J. Rozanski, C. A. Mills, M. J. Beliatis, N. A. Nismy, S. R. P. Silva, "Inorganics-in-Organics’: recent developments and outlook for 4G polymer solar cells", Nanoscale, 2013, 5, 8411-8427. CrossRef Y. Lin, X. Zhan, "Oligomer Molecules for Efficient Organic Photovoltaics", Accounts of Chemical Research, 2016, 49, 175-183. CrossRef R. Steim, S. A. Choulis, P. Schilinsky, U. Lemmer, C. J. Brabec, "Formation and impact of hot spots on the performance of organic photovoltaic cells", Applied Physics Letters, 2009, 94, 043304. CrossRef J. Hepp, F. Machui, H. J. Egelhaaf, C. J. Brabec, A. Vetter, "Automatized analysis of IR‐images of photovoltaic modules and its use for quality control of solar cells", Energy Science and Engineering. 2016, 4, 363-371. CrossRef H. Hoppe, J. Bachmann, B. Muhsin, K. H. Drüe, I. Riedel, G. Gobsch, C. Buerhop-Lutz, C. J. Brabec, V. Dyakonov, "Quality control of polymer solar modules by lock-in thermography", Journal of Applied Physics, 2010, 107, 014505. CrossRef J. Bachmann, C. Buerhop-Lutz, C. Deibel, I. Riedel, H. Hoppe, C. J. Brabec, V. Dyakonov, "Organic solar cells characterized by dark lock-in thermography", Solar Energy Materials and Solar Cells, 2010, 94, 642-647. CrossRef A. Różycka, K. A. Bogdanowicz, N. Górska, J. Rysz, M. Marzec, A. Iwan, R. Pich, A. Januszko, "Influence of TiO2 Nanoparticles on Liquid Crystalline, Structural and Electrochemical Properties of (8Z)-N-(4-((Z)-(4-pentylphenylimino)methyl)benzylidene)-4-pentylbenzenamine", Materials, 2019, 12, 1097. CrossRef M. L. Petrus, R. K. M. Bouwer, U. Lafont, S. Athanasopoulos, N. C. Greenham, T. J. Dingemans, "Small-molecule azomethines: organic photovoltaics via Schiff base condensation chemistry", Journal of Materials Chemistry A, 2014, 2, 9474-9477. CrossRef K. A. Bogdanowicz, A. Iwan, "Symmetrical Imines with Trirphenylamine Core and the Method of Obtaining Thereof.", March 11th 2019, patent application P.429044. DirectLink K. P. Korona, T. Korona, D. Rutkowska-Zbik, S. Grankowska-Ciechanowicz, A. Iwan, M. Kamińska, "Polyazomethine as a component of solar cells-theoretical and optical study", Journal of Physics and Chemistry of Solids, 2015, 86, 186-193. CrossRef

Metal–organic framework nanosheets for enhanced performance of organic photovoltaic cells

Metal–organic framework nanosheets (MONs) are incorporated into the active layer of bulk heterojunction polymer–fullerene solar cells for the first time, resulting in an almost doubling of power conversion efficiency.

COMPARISON OF PERFORMANCES OF ORGANIC PHOTOVOLTAIC CELLS USING SubPc AS CENTRAL AMBIPOLAR LAYER IN TERNARY STRUCTURES AND AS ELECTRON ACCEPTOR IN BINARY STRUCTURES

We compare the performances of organic photovoltaic cells (OPVCs) based on binary and ternary planar heterojunctions. The organic active layers are pentathiophene (5T), subphthalocyanine (SubPc) and fullerene (C60). SubPc being an ambipolar material we used it either as electron acceptor in binary OPVCs or as central layer in ternary cells in order to increase the efficiency of OPVCs using 5T as electron donor. So, the different OPVC configurations were 5T/C60, 5T/SubPc and 5T/SubPc/C60. The effect of the different organic layer thicknesses on the device performances was studied. In order to understand the behavior of the different OPVC configurations, we proceeded with a morphological study. The influence of the high roughness of the 5T layer on the OPVCs performances is discussed. The best OPVCs performances are obtained with the binary structure 5T/SubPc. Its maximum efficiency corresponds to an increase of 50% compared to the OPVC based on the couple 5T/C60. External Quantum Efficiency measurements show that both layers participate to the current generation. The efficiency increase is mainly due to the increase of the open circuit voltage ([Formula: see text]). In the case of ternary OPVCs, [Formula: see text] is limited by the band structure of 5T and C60, moreover, the efficiency is also limited by the poor charge collection efficiency of the ternary structure and the series resistance of the three stacked organic layers.

Elongated Nanodomains and Molecular Intermixing Induced Doping in Organic Photovoltaic Active Layers with Electric Field Treatment

The effects of the electric-field-assisted annealing on the bulk heterojunction nano-morphology in the P3HT/PCBM active layer of the organic photovoltaic cells (OPVCs) are presented here. It was widely accepted that the electric-field-assisted annealing will facilitate the P3HT, the polar polymer, to be better crystalline to enhance the charge mobility, hence the improvement of the OPVC performance. The influences on the nano-morphology of the electron donor and accepter domains are not well understood. Here, using the cross-sectional scanning tunneling microscopy and spectroscopy (XSTM/S), the electric-field-assisted annealing treatment is found to influence the molecular domains to be elongated with the orientation near the direction of the external electric field. The elongation of the molecular domains is believed to facilitate the domain percolation, which causes higher charge mobility, hence the higher short-circuit current density (Jsc). On the other hand, it was also observed that the electronic properties of the P3HT-rich and PCBM-rich domains in the electric-field-assisted annealed samples showed smaller energy band gaps and smaller molecular orbital offset between the two domains, which is argued to decrease the open circuit voltage (Voc) and negatively impact the OPVC performance. Based on the X-ray diffraction (XRD) and small angle X-ray scattering (SAXS) results, the altered electronic properties are argued to be due to the molecular intermixing induced doping effects. These results point out competing factors affecting the OPVC performance with the electric-field-assisted annealing treatment.

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.

Near-field enhanced performance of organic photovoltaic cells

Metal nano-particles (NPs) have been successfully synthesized by the reduction of Ag@Cu2O with the aid of surfactant PVP and excessive reducer ascorbic acid in ambient conditions. The composition and structures of the NPs were characterized by scanning and tunnelling microscopies (SEM and TEM). The synthesized nano-particles were employed as solar absorber in thin film organic photovoltaic (OPV) with the view to exploit the effect of local surface plasmon resonance (LSPR) of the metals. As a consequence, substantial improvement on the major solar cell parameters were measured by the incorporation of Ag@Cu NPs in the P3HT:PCBM based photo-active medium. The power conversion efficiency of the solar cell increased by 103% compared to those devices fabricated without metal nano-particles. The observed balanced charge transport, in the current experiment, is attributed to presence of metal nano-particles in OPV devices which are responsible for near field enhanced photons scattering in the medium.

A Spectral Density Function Approach for Active Layer Design of Organic Photovoltaic Cells

Organic photovoltaic cells (OPVCs), having received significant attention over the last decade, are yet to be established as viable alternatives to conventional solar cells due to their low power conversion efficiency (PCE). Complex interactions of several phenomena coupled with the lack of understanding regarding the influence of fabrication conditions and nanostructure morphology have been major barriers to realizing higher PCE. To this end, we propose a computational microstructure design framework for designing the active layer of P3HT:PCBM based OPVCs conforming to the bulk heterojunction (BHJ) architecture. The framework pivots around the spectral density function (SDF), a frequency space microstructure characterization, and reconstruction methodology, for microstructure design representation. We validate the applicability of SDF for representing the active layer morphology in OPVCs using images of the nanostructure obtained by cross-sectional scanning tunneling microscopy and spectroscopy (XSTM/S). SDF enables a low-dimensional microstructural representation that is crucial in formulating a parametric-based microstructure optimization scheme. A level-cut Gaussian random field (GRF, governed by SDF) technique is used to generate reconstructions that serve as representative volume elements (RVEs) for structure–performance simulations. A novel structure–performance (SP) simulation approach is developed using a physics-based performance metric, incident photon to converted electron (IPCE) ratio, to account for the impact of microstructural features on OPVC performance. Finally, a SDF-based computational IPCE optimization study incorporating only three design variables results in 36.75% increase in IPCE, underlining the efficacy of the proposed design framework.

Combined effects of carriers charge mobility and electrodes work function on the performances of polymer/fullerene P3HT:PCBM based organic photovoltaic solar cell

This paper reports a study on the effect of different parameters such as charge carriers mobility, electrodes work function, energy gap, series as well as shunt resistances on the performances of an organic photovoltaic cell based on polymer/fullerene P3HT: PCBM. Thus, numerical simulations have been investigated on ITO/PEDOT:PSS/P3HT: PCBM/LiF/Al structure with Analysis of Microelectronic and Photonic Structures the simulation one dimension (AMPS-1D) and General-Purpose Photovoltaic Device Model (GPVDM) softwares. The results show that the optimum efficiency is obtained for electron and hole motility values of 10−4 cm2 V−1 s−1and 2 × 10−4 cm2 V−1 s−1, respectively. Moreover, we reported that the ohmic contact for both anode and cathode electrodes of the investigated device remains very important to get the maximum efficiency. Furthermore, when the gap energy increases, the efficiency is considerably improved, and reach's a value of about 5.421%. In addition, in this present work, the complimentary between the both softwares AMPS and GPVDM is also well illustrated as an original modeling approach to investigate the performances of organic photovoltaic cells.

Identifying Molecular Orientation in a Bulk Heterojunction Film by Infrared Reflection Absorption Spectroscopy.

The molecular orientation of organic molecules of zinc phthalocyanine (ZnPc) in single-component films on copper iodide (CuI) substrates can be controlled to achieve a molecular orientation lying flat on the substrate (flat-on) owing to π–d orbital interactions between the ZnPc molecules and the CuI. A 3-fold enhancement in the performance of organic photovoltaic cells has been reported by introducing a CuI interlayer between a ZnPc:fullerene (C60) bulk heterojunction (BHJ) film and the substrate. However, the mechanism underpinning the resultant solar cell performance enhancement was unclear. Herein, we report on the results of using in situ reflection absorption spectroscopy measurements during the vacuum deposition of coevaporated ZnPc:C60 BHJ films on various substrates to investigate the ZnPc molecular orientation. Our results revealed that the flat-on molecular orientation of ZnPc molecules in ZnPc:C60 BHJ films on CuI interlayers and flat-on ZnPc substrates can be successfully identified via the strong π–π interactions between the BHJ film and the substrate. The π–π interactions between individual ZnPc molecules are stronger than the π–d interactions between ZnPc molecules and CuI in coevaporated ZnPc:C60 films, as is evident from the molecular orientation of ZnPc, as determined by in situ reflection absorption spectroscopy. Our findings demonstrate that precisely controlling the molecular orientations of the films could enhance organic photovoltaic (OPV) performance. The present work provides important insights that will enable the design of higher performance OPV cells.

The Influence of Deposition Rates on Properties of AlPcCl Thin Films and on the Performance of Planar Organic Solar Cells

The performance of organic photovoltaic cells depends on different properties of the organic materials, such as carrier mobility, band structure, morphology, and crystallinity. For AlPcCl, it is shown that these properties are dependent on the layer deposition rates. As the deposition rate increases, the surface roughness of AlPcCl layers decreases. In contrast, the crystallinity of the layers increases as the deposition rate decreases, thereby increasing the hole mobility value. These effects are opposed: the former effect allows higher open circuit voltages to be obtained as the deposition rate increases, whereas the latter increases the short circuit current as the deposition rate decreases. Therefore, there is a trade‐off between open and short circuit current values, and it is necessary to find the best deposition rate for AlPcCl. Here, it is shown that the highest mobility carriers in layers deposited slowly allow the use of thicker AlPcCl layers, which permits acceptable open circuit voltage values and optimum photovoltaic cell efficiencies to be obtained. This results in an optimum efficiency value of 3.97% for a 26‐nm‐thick AlPcCl film that is deposited at 0.02–0.03 nm s−1.

Effect of optical intensity distribution on device performances of PTB7-Th:PC71BM-based organic photovoltaic cells

Abstract In order to realize high performance of organic photovoltaic cells (OPVs), it is indispensable to optimize device structures according to composing materials. In particular, the optical density distribution in stacked layers is one of important factors to enhance the optical absorption within the photoactive layer, resulting in the improved photoconversion efficiency. In this study, we optimized the film thickness of the photoactive layer as a fundamental way to control the optical density distribution. The influence of the photoactive layer thickness on photovoltaic performances of OPVs was first investigated using, poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6diyl-alt-(4-(2-ethylhexyl)-3-uorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] and [6,6]-phenyl C 71 -butyric acid methyl ester blend films in normal and inverted OPV structures, respectively. The short circuit current density (J sc ) showed a peak at the thickness at around 100–120 nm and 80–100 nm in normal and inverted OPVs, respectively. In addition, J sc of inverted OPVs were higher than those of normal OPVs for all the thicknesses. The FF decreased continuously with increasing the thickness in both device structures. The fill factor of normal OPV decreased with increasing thickness more slowly than that of the inverted OPV at the thickness of over than 200 nm. Furthermore, we simulated the optical density distribution in the stacked layers using the algorithm utilized matrix formulas based on Abeles's formulas. The simulated optical intensity distribution as a function of the position and the wavelength is in a good agreement with the experimental spectra of external quantum efficiency. The result indicates a close correspondence between the optical density distribution and the photovoltaic performance. At the optimum photoactive layer thickness of 116 nm in the normal and 76 nm in the inverted OPVs, we obtained the highest PCEs of 9.25 and 10.4%, respectively.

Magnetic field enhancement of organic photovoltaic cells performance

Charge separation is a critical process for achieving high efficiencies in organic photovoltaic cells. The initial tightly bound excitonic electron-hole pair has to dissociate fast enough in order to avoid photocurrent generation and thus power conversion efficiency loss via geminate recombination. Such process takes place assisted by transitional states that lie between the initial exciton and the free charge state. Due to spin conservation rules these intermediate charge transfer states typically have singlet character. Here we propose a donor-acceptor model for a generic organic photovoltaic cell in which the process of charge separation is modulated by a magnetic field which tunes the energy levels. The impact of a magnetic field is to intensify the generation of charge transfer states with triplet character via inter-system crossing. As the ground state of the system has singlet character, triplet states are recombination-protected, thus leading to a higher probability of successful charge separation. Using the open quantum systems formalism we demonstrate that the population of triplet charge transfer states grows in the presence of a magnetic field, and discuss the impact on carrier population and hence photocurrent, highlighting its potential as a tool for research on charge transfer kinetics in this complex systems.

Improved performance of organic photovoltaic cells with PTB7-Th:PC71 BM by optimized solvent evaporation time in electrospray deposition

As poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b; 4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] has good potential as a low-band gap donor polymer for organic photovoltaic cells (OPVs), we investigated the optimized electrospray deposition condition for realizing suitable polymer ordering and/or crystallite size by controlling the solvent evaporation time. Previous studies on the electrospray process have mainly focused on novel device structure owing to its unique characteristic of small droplet size, which is less than 1 μm. However, in this research, we investigated the spontaneous formation of interpenetrating continuous networks of the donor- and acceptor-rich domains of solvent evaporation during the electrospray process. By evaluating the ultraviolet–visible absorption spectrum, Raman spectroscopy, and direction of polymer ordering, it was shown that the polymer-stacking condition was not influenced by solvent evaporation time, even though poly(3-hexylthiophene-2,5-diyl) along the face-on direction was well stacked under the slow solvent evaporation condition. In contrast, the crystallite size, which was estimated from the full width at half maximum X-ray diffraction pattern, increased as the solvent evaporation time increased. This means that the crystalline grain spontaneously grew in the droplet and that the large crystalline grain was formed during the slow evaporation condition. Furthermore, the photovoltaic performance trend was the same as the performance trend of the crystallite size and were increased with increasing solvent evaporation time for both polymers. Therefore, the crystalline grain size was a dominant factor in determining the photovoltaic performance. Additionally, the crystalline grain size could be controlled by the solvent evaporation time. Finally, by optimizing the active-layer thickness, the highest photoconversion efficiency of 8.6% was achieved. This is the highest value of an electrospray-based device. These results indicate that the solvent evaporation time is an important factor in determining the crystallite size of an organic thin film, which directly affects the photoconversion efficiency of OPVs.