Organic Photovoltaic Solar Cells Research Articles

Synthesis and band gap determination for 4-phenyl-7-(5-(thiazol-2-yl) thien-2-yl)benzo[c][1,2,5]thiadiazole, a new polyheterocyclic molecule with potential applications as a semiconductor in organic photovoltaic solar cells

Organic photovoltaic (OPV) solar cells are a promising emerging technology for sustainable energy generation. In these cells, the absorbing layer consists of a repeated series of aromatic organic rings with a system of π delocalized electrons, a feature that allows these materials to absorb sunlight and facilitate electron transfer. In the present work, a new tetrameric molecule with high structural complexity, 4-phenyl-7-(5-(thiazol-2-yl) thien-2-yl)benzo[c][1,2,5]thiadiazole, was synthesized by a convergent pathway. The final step of the synthetic route was the Suzuki coupling of two new dimeric molecules, obtained from commercially available raw materials such as thiophene and o-phenylenediamine. The band gap of the final compound was found to be 2.5 eV using an optical methodology and 2.38 eV using an electrochemical methodology. The results suggest that this tetrameric molecule has potential for application in OPV cells.

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.

Intramolecular and Intermolecular Interaction Switching in the Aggregates of Perylene Diimide Trimer: Effect of Hydrophobicity

The research on perylene diimide (PDI) aggregates effectively promotes their applications in organic photovoltaic solar cells and fluorescent sensors. In this paper, a PDI fabricated with three peripheral PDI units (N, N'-bis(6-undecyl) perylene-3,4,9,10-bis(dicarboximide)) is investigated. The trimer shows different absorption and fluorescence properties due to hydrophobicity when dissolved in the mixed solvent of tetrahydrofuran (THF) and water. Through comprehensive analysis of the fluorescence lifetime and transient absorption spectroscopic results, we concluded that the trimer underwent different excited state kinetic pathways with different concentrations of water in THF. When dissolved in pure THF solvent, both the intramolecular charge-transfer and excimer states are formed. When the water concentration increases from 0 to 50% (v/v), the formation time of the excimer state and its structural relaxation time are prolonged, illustrating the arising of the intermolecular excimer state. It is interesting to determine that the probability of the intramolecular charge-transfer pathway will first decrease and then increase as the speed of intermolecular excimer formation slows down. The two inflection points appear when the water concentration is above 10% and 40%. The results not only highlight the importance of hydrophobicity on the aggregate properties of PDI multimers but also guide the further design of PDI-based organic photovoltaic solar cells.

End group modulation of A-D-A type small donor molecules for DTP based organic photovoltaic solar cells: a DFT approach.

Here, five new acceptor-donor-acceptor (A-D-A) type small donor molecules C1-C5, have been designed based on the central D unit, dithieno[3,2-b:2',3'-d]pyrrole (DTP). Besides, five different A units, viz. 1,1-dicyano-methylene-5,6-dimethyl-3-indanone, 1,1-dicyano-methylene-5,6-difluoro-3-indanone, 1,1-dicyano-methylene-5,6-dichloro-3-indanone, 1,1-dicyano-methylene-5-nitro-3-indanone, and 1,1-dicyano-methylene-5,6-diamino-3-indanone are selected for these designed compounds C1-C5, respectively. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods have been employed to study the influence of different A units on the geometric, electronic, optical, charge transport and photovoltaic properties of the designed donor molecules. The results reveal that the performance of the designed donor molecules have been improved on attachment of the strong electron withdrawing A units. The observed reorganization energy (λ) values infer the electron donating nature of the designed compounds. Moreover, the absorption properties of the designed compounds manifest that compound C4 possesses the high values of maximum wavelength (λmax) in both gas and solvent phases. The properties of the D/A blends reveal that all designed blends C1-C5/C60-CN, have the capacity to promote charge carrier separation at the D/A interface. Further, the photovoltaic performance of the D/A complexes also reveal that complex C4/C60-CN, with a theoretical PCE of 18%, can be considered as the most promising candidate for application in OSCs.

Polysulfone/Chitosan membranes with inorganic nanoparticles

Photovoltaic solar energy has been widely used as an alternative energy source. Composite materials combine the properties of organic polymers, including flexibility and easier processing, with those of inorganic compounds, such as thermal and chemical stability. These characteristics are advantageous to produce materials for organic solar cells. This work aimed to synthesize by casting method and characterize the structure, morphology, chemical, and thermal properties of composite membranes of polysulfone and chitosan (PSF/CS) incorporated with different masses of nickel-zinc ferrite nanoparticles (NZFN) and magnetite nanoparticles (MN). PSF/CS composite membranes incorporated with inorganic nanoparticles were uniform and transparent, evidencing a good dispersion of the nanoparticles and the homogeneity of the synthesized material. The addition of inorganic nanoparticles increases the stability and efficiency of organic substrates making them suitable for different applications in renewable energy systems, such as organic photovoltaic solar cells.

(Invited) Probing Charge Carrier Dynamics in Porphyrin-Based Bulk Heterojunction Thin Films with Time-Resolved Terahertz Spectroscopy

Organic photovoltaic (OPV) solar cells have attracted much attention in recent years because of the low production cost and ease of device fabrication. However, the power conversion efficiency is still low compared to the silicon and perovskite solar cells so that further research is necessary to improve their performance. In particular, understanding of the detailed mechanisms of charge carrier generation, recombination and carrier transport process is very important for developing design principles of new compounds for OPVs. Until now, polymer-based compounds have been widely used for p-type semiconductors. However, it is not easy to obtain information on how the microscopic structure of bulk heterojunction (BHJ) blend of donor and acceptor materials affect the charge carrier dynamics because one cannot easily control the molecular weight and conformational distributions of polymer compounds. On the other hand, small molecule-based semiconductors have several unique advantages such well-defined chemical structures, ease of purification and small batch-to-batch variations.Tetrabenzoporphyrin (BP) has been frequently used as a small-molecule organic semiconductor. Recently, Yamada and coworkers synthesized diketopyrrolopyrrole-linked tetrabenzoporphyrin (DPP-BP) conjugates that have linear alkyl groups on the DPP moieties whose chain length is defined as n, namely, Cn-DPP-BP (n=4, 6, 8, or 10). For DPP-BP based solar cells, they demonstrated that alkyl chain length affects the power conversion efficiency. Therefore, it is very interesting to see how the local BHJ structures of DPP-BP based thin films affect the initial step of the charge carrier dynamics because such studies are difficult to perform for polymer-based systems. We can take advantage of small molecule-based systems to study the relationship between the local structure and the charge carrier dynamics.In this work, we used time-resolved terahertz (THz) spectroscopy to investigate the charge carrier dynamics of DPP-BP BHJ thin films blended with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). Time-resolved THz spectroscopy is a unique method to quantify charge carrier mobility with very fast time resolution (less than picosecond). This spectroscopy reflects the charge carrier dynamics in the local spatial region because of the high-frequency nature of the THz electric field. We chose C4-DPP-BP:PC61BM and C10-DPP-BP:PC61BM thin films for the THz measurements. In the previous study, it was found that C4-DPP-BP molecule prefers to take a face-on geometry on the substrate. A grain size of the aggregates was around 100 nm or smaller which were measured by atomic force microscopy. In such a structure, one can expect that charge transport takes place very efficiently to the electrode. Actually, power conversion efficiency is 5.2 % which is highest among a series of DPP-BP chromophores. On the other hand, C10-DPP-BP adopts an edge-on geometry on the substrate and forms micrometer-sized aggregates. In this structure, molecular stacking is oriented perpendicular to the electrode so that power conversion efficiency decreases to 0.19%.In contrast to the other measurements, the THz spectroscopy has sensitivity to in-plane motion of the charge carriers with respect to the substrate, we expect to observe higher in-plane mobility of the charge carriers in the C10-DPP-BP:PC61BM BHJ films compared to C4-DPP-BP based films. In contrast to our intuitive expectation, we found that the product of the carrier mobility and the yield of the photo-generated charge carriers is similar to each other. We consider that the similarity of the charge carrier dynamics results from the high-frequency nature of the local mobility of the polarons because most of the polarons are confined in less than a 10-nm length scale. Furthermore, the lowest limit of the carrier mobility obtained from our results is about 1.0 cm2V-1s-1 when the quantum yield of the charge carrier generation is assumed to be unity. This value is much higher than those obtained by space-charge-limited current method. We expect that charge separation occurs very efficiently when the local mobility of the charge carrier is high.In this contribution, we will present importance of the local mobility of the charge carriers and difference of charge transport between short and long length scales in DPP-BP based solar cells.

Microscopic Structures, Dynamics, and Spin Configuration of the Charge Carriers in Organic Photovoltaic Solar Cells Studied by Advanced Time-Resolved Spectroscopic Methods.

Organic photovoltaics (OPVs) are promising solutions for renewable energy and sustainable technologies and have attracted much attention in recent years. Two types of organic semiconductors are used as donor materials to fabricate OPV cells. One type is a photoconductive polymer, and the other type is a small-molecule-based compound. The discovery of a bulk-heterojunction (BHJ) structure using a mixture of p- and n-type organic semiconductors has dramatically increased the power conversion efficiency (PCE) of OPV cells. In this feature article, we review our recent studies on organic BHJ thin films and OPVs by using advanced time-resolved spectroscopic techniques. Two topics regarding the microscopic behaviors of the charge carriers are discussed. The first topic is focused on how to quantify the local mobility of the charge carriers. Here, we discuss charge carrier dynamics in diketopyrrolopyrrole-linked tetrabenzoporphyrin (DPP-BP) BHJ thin films studied by time-resolved terahertz spectroscopy on a subpicosecond to several tens of picoseconds time scale and by transient photocurrent measurements on a microsecond time scale. The second topic concerns the spin configuration and interaction of the electron and hole of the polaron pairs in polymer-based BHJ thin films and OPV cells studied by the time-resolved electron paramagnetic resonance method, time-resolved simultaneous optical and electrical detection, and measurement of the magnetoconductance effect.

Photon harvesting and light trapping in pentacene and PTCDI-C13H27 for organic solar cell application

The deep understanding of optical and spectroscopic properties of organic semiconductors is essential for designing the organic photovoltaic solar cells (OSCs) and estimating their prospect performance. In this work, the optoelectronic properties of pentacene and PTCDI-C13H27 conjugated molecules were estimated using density function theory (DFT) calculations. Hence, the parameters of energy gaps, charge transfer (CT), open circuit voltage (VOC), reorganization energy and light harvesting efficiency (LHE) have been extracted using the DFT at B3LYP/6-311G(d,p) level. The results predicted that pentacene and PTCDI-C13H27 molecules possess an efficient broad-band photon harvesting over the visible spectrum with absorption maximum at 643 and 514.5 nm respectively. Moreover, the light trapping inside the pentacene and PTCDI-C13H27 layers integrated in a proposed OSC have been studied using 3D finite difference time domain (FDTD) model. The optimum thickness of the OSC layers were evaluated based on their absorption capability.

Outdoor Performance of Organic Photovoltaics: Comparative Analysis

Organic photovoltaic (OPV) solar cells represent an emerging and promising solution for low-cost clean energy production. Being flexible and semi-transparent and having significant advantages over conventional PV technologies, OPV modules represent an innovative solution even in applications that cannot be based on traditional PV systems. However, relatively low efficiencies, poor long-term stability, and thermal issues have so far prevented the commercialization of this technology. This paper describes two outdoor experimental campaigns that compared the operation of OPV modules with traditional PV modules—in particular crystalline silicon and copper–indium–selenium (CIS)—and assessed the OPV modules’ power generation potential in vertical installation and facing towards the cardinal directions.

Optoelectrical Properties and the Study of Thickness and Annealing in Poly-3-hexylthiophene Based ITO Free Organic Solar Cells with TiO2 and MoO3 as Transport Layers

Inverted organic photovoltaic solar cells were fabricated with the configuration of FTO/TiO2/P3HT:PC61BM/MoO3/Ag. Besides, the influence of transport layers, titanium dioxide and molybdenum trioxide, on the performance of solar cells were investigated. These compounds showed excellent optical (around 80% for molybdenum trioxide and 95% for titanium dioxide), electrical (like charge carrier density of 3.3 x1015 cm-3 and 2.5 x1014 cm-3 for titanium and molybdenum, respectively) and structural (anatase and amorphous hexagonal phase for titanium and molybdenum, respectively) properties to be used as transport layers. Also the influence of the thickness of the electron transport layer is studied, as well as the thickness, temperature and heat treatment time of the active layer. The correct selection of TiO2’s thickness (70 nm) and active layer’s thickness (250 nm) and annealing (at 100 degrees for 8 minutes) can increase the power conversion efficiency. Moreover, the cell fabricated with transport layers and the best conditions found showed a maximum efficiency of 3.3%, which indicates that the titanium dioxide and molybdenum trioxide played a determining role in the solar cell performance.

Organic Nanostructured Materials for Sustainable Application in Next Generation Solar Cells

Meeting our current energy demands requires a reliable and efficient renewable energy source that will bring balance between power generation and energy consumption. Organic photovoltaic cells (OPVs), perovskite solar cells and dye-sensitized solar cells (DSSCs) are among the next-generation technologies that are progressing as potential sustainable renewable energy sources. Since the discoveries of highly conductive organic charge-transfer compounds in the 1950s, organic semiconductors have captured attention. Organic photovoltaic solar cells possess key characteristics ideal for emerging next-generation technologies such as being nontoxic, abundant, an inexpensive nanomaterial with ease of production, including production under ambient conditions. In this review article, we discuss recent methods developed towards improving the stability and average efficiency of nanostructured materials in OPVs aimed at sustainable agriculture and improve land-use efficiency. A comprehensive overview on developing cost-effective and user-friendly organic solar cells to contribute towards improved environmental stability is provided. We also summarize recent advances in the synthetic methods used to produce nanostructured active absorber layers of OPVs with improved efficiencies to supply the energy required towards ending poverty and protecting the planet.

Performance efficiency of an organic solar cell FTO:PTB7:PC70BM free of ITO and its degradation

The expensive ITO has widely been used as an electrode for fabricating organic photovoltaic solar cells (OPV), which turns out to be an obstacle to the commercialization of solar cells. Therefore, in this work, we aim to replace the ITO with fluorine-doped tin oxide (FTO), which is 40 times cheaper than ITO. Besides, titanium dioxide is used as an electron transport layer instead of zinc oxide (ZnO). The active layer comprises the conductive polymer PTB7 as a donor and the fullerene derivative of PC70BM as an acceptor. The inverted OPV is fabricated with FTO/TiO2/PTB7:PC70BM/MoO3/Ag device structure. The fabricated OPV showed a power conversion efficiency (PCE) of 0.48%. Therefore, to enhance the PCE of the OPV, the device parameters such as the thickness of each layer, thermal treatment of the active layer and the OPV are optimized. As a result, the PCE is greatly enhanced from 0.48% to 2.88%. Furthermore, the open-circuit voltage, short circuit current density, and fill factor are significantly improved. The degradation efficiency measurements are reported. Hence, this research demonstrates a new pathway towards enhancing the OPV with stable, non-toxic and low-cost materials.

An attention-driven long short-term memory network for high throughput virtual screening of organic photovoltaic candidate molecules

Organic Photovoltaic (OPV) Solar Cells are a rapidly developing technology with promising capabilities over leading renewable energy sources. Screening methods for determining promising donor and acceptor molecules to augment the efficiencies of such cells can be substantially accelerated through deep learning. Textual descriptors, specifically Simplified Molecular Input Line Entry System (SMILES), are utilized as network inputs, while quantum-chemical calculations based on Density Function Theory (DFT) provide chemically-accurate targets for training and testing. We present a Long Short-Term Memory (LSTM) based network which uses a self-attention mechanism and a robust data augmentation routine to predict several OPV optoelectronic properties (e.g. highest occupied molecular orbital and lowest unoccupied molecular orbital). The LSTM cells, coupled with self-attention, learn the successive ordering and pairing of SMILES characters while attending to certain salient constituents of the molecule, which produce a robust understanding of the molecular graph. The Harvard Clean Energy Project (CEP) and National Renewable Energy Laboratory (NREL) OPV datasets are used for this study. The CEP dataset portion which we use contains ~1.2E6 candidate donor molecules with their respective DFT-computed properties, whereas the NREL OPV dataset possesses ~9.1E4 samples. Compared to contemporary graph-based model selections, our network reduces the MAE overall considered optoelectronic properties on the CEP and NREL OPV datasets by an average of 21.23% and 10.06% respectively. Furthermore, we demonstrate that our model generalizes well to the pharmaceutical drug discovery focused ZINC-250k dataset, reducing the MAE across all properties by an average of 28.2% from the current state-of-the-art model.

Influence of the Active Layer Structure on the Photovoltaic Performance of Water-Soluble Polythiophene-Based Solar Cells.

A new side-chain C60-fullerene functionalized thiophene copolymer bearing tributylphosphine-substituted hexylic lateral groups was successfully synthesized by means of a fast and effective post-polymerization reaction on a regioregular ω-alkylbrominated polymeric precursor. The growth of the polymeric intermediate was followed by NMR spectrometry in order to determine the most convenient reaction time. The obtained copolymer was soluble in water and polar solvents and was used as a photoactive layer in single-material organic photovoltaic (OPV) solar cells. The copolymer photovoltaic efficiency was compared with that of an OPV cell containing a water-soluble polythiophenic homopolymer, functionalized with the same tributylphosphine-substituted hexylic side chains, in a blend with a water-soluble C60-fullerene derivative. The use of a water-soluble double-cable copolymer made it possible to enhance the control on the nanomorphology of the active blend, thus reducing phase-segregation phenomena, as well as the macroscale separation between the electron acceptor and donor components. Indeed, the power conversion efficiency of OPV cells based on a single material was higher than that obtained with the classical architecture, involving the presence of two distinct ED and EA materials (PCE: 3.11% vs. 2.29%, respectively). Moreover, the synthetic procedure adopted to obtain single material-based cells is more straightforward and easier than that used for the preparation of the homopolymer-based BHJ solar cell, thus making it possible to completely avoid the long synthetic pathway which is required to prepare water-soluble fullerene derivatives.

Studies of photoexcitations in polymer/non-fullerene blend for high-efficiency organic solar cells

We studied the photoexcitations properties in the blend of PBDB-T-SF, a donor π-conjugated copolymer, and ITIC-2F, a non-fullerene acceptor molecule, which is used as an active layer in high-performance organic photovoltaic (OPV) solar cells. We used several steady-state spectroscopies such as photoinduced absorption (PIA), photoluminescence (PL), magneto-photoinduced absorption (MPA), and magneto-PL in the pristine and blend films. The PIA spectra of the pristine copolymer and acceptor films contain a photoinduced absorption (PA) band due to triplet excitons, which is confirmed by the MPA(B) response, whereas the PIA spectrum of the copolymer/molecule blend contains several PA bands due to polaron pairs at the copolymer chain/molecule interfaces. Interestingly, when exciting the PIA spectrum of the blend at two different photon energies, namely, ℏω≈2.39 eV that preferentially excites the copolymer chains and ℏω≈1.58 eV that excites only the acceptor molecules, we found that the PIA spectra are the same. This shows that charge photogeneration is possible when exciting either the copolymer chains or the acceptor molecules, which might explain the high power conversion efficiency of OPV cells based on this blend. We also found from the MPA(B) responses of the pristine copolymer and acceptor films and their blend that the back reaction of the photogenerated polaron pairs into the copolymer forming triplet excitons is absent, further explaining the high power conversion efficiency of this blend as the active layer in OPV solar cells.

Singlet Fission

There is no doubt that solar energy will play an increasingly key role in the future of humankind. The ability to harvest, store, and utilize energy from the sun, however, poses a number of significant challenges that need to be met and overcome to efficiently exploit this resource. The past decades have seen significant efforts in the development of new materials, improved solar cell design, and increased device efficiencies. Organic photovoltaic solar cells (OPVs) have the potential to make a crucial contribution to our energy future through, for example, the possibility of low-cost device formation and the use of earth abundant elements. Realizing the potential of OPVs, however, requires that both the efficiencies and long-term stabilities be increased. This special collection in ChemPhotoChem describes a variety of approaches toward using singlet fission as a basis for an improved scheme of solar energy capture and conversion. These articles and a minireview provide exciting insight into new molecules, theoretical analyses, and mechanistic schemes at the forefront of research efforts to harness the potential of singlet fission. Why highlight singlet fission in a special collection of papers? The upper thermodynamic limit for single-junction solar cells is just above 30 %, due largely to the fact that excess energy from the absorbed photons is lost to heat rather that used for charge generation. To overcome this limit, creative and innovative strategies, such as singlet fission, are necessary. Singlet fission is a spin-allowed process in which, upon photoexcitation, the initial singlet state spontaneously splits into a pair of correlated triplet states. Current models suggest that solar cells based on singlet fission could achieve efficiencies of nearly 50 %. Singlet fission in systems based on organic molecules has been known for over 40 years, but only recently has it been embraced as a realistic scheme for OPVs. In the past decade or so, many significant advances have been made with respect to optimizing molecular materials, incorporation of materials into proof-of-concept devices, and understanding the mechanism of singlet fission. Nevertheless, challenges remain, including the ability to effectively “harvest” and convert triplet-excited states formed by singlet fission into useable forms of energy as well as the development of materials and devices that feature long-term stability in OPVs. The design of stable new molecules for singlet fission will undoubtedly play an important role toward solving these changes, including for example, electron-accepting and radical-based systems to complement the use of electronic donating acenes. We hope that this special collection will provide both a window into our current understanding of singlet fission for OPVs as well as provide inspiration toward providing answers for challenges that remain in the field. Rik R. Tykwinski is Professor and Chair of Chemistry at the University of Alberta (Canada). As a physical organic chemist, his research interests center on the synthesis of molecules that model the carbon allotrope carbyne and structure–function studies of carbon-rich conjugated systems. When not in the office, he is most likely entertaining his two sons or out on his mountain bike. Dirk M. Guldi is Professor of Chemistry at the University of Erlangen (Germany). His research interests are centered around light- and charge-management in solar energy conversion schemes using nano-carbons. While doing a full-time academic job, he fits in tri-training into a full and fulfilled life.

Low Band Gap Conjugated Semiconducting Polymers

AbstractImportant parameters of an organic semiconductor material are the electronic band gap (Eg) and the position of highest occupied and lowest unoccupied bands versus vacuum. These bands are called valence and conduction band for inorganic semiconductors. For organic semiconductors the bands defining the band gap are often called highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). One advantage of semiconducting polymers is the ability to tune the band gap and the position of HOMO and LUMO levels by molecular chemical design. The organic photovoltaic solar cells need absorbers with a smaller bandgap to maximize the power conversion efficiency of these devices. There are several chemical strategies to synthesize low band gap polymers for optoelectronic applications. In this manuscript, an updated overview on the current status of these low band gap conjugated polymers will be given. The design principles of low band gap polymers, the properties of the resulting materials, and important applications and devices realized with this material class will be briefly discussed.

Modeling and optimization of organic tandem photovoltaic solar cells using silver nanowires as electrode and interconnecting layer

Much research has been conducted to improve the performance of photovoltaic solar cells. Transparent conductive film and interconnection layers have a significant impact on the performance of photovoltaic cells. In this work, we analyze the experimental results obtained on tandem organic photovoltaic solar cells with simple inverted structures using silver nanowires AgNW as transparent conductive electrode (TE) and as interconnection layer (ICL) between PEDOT: PSS and ZnO. This type of contact leads to a strong ohmic contact in both sub-cells having P3HT: ICBA as the lower active layer and having PTB7: PC71BM (1: 1.5) as the upper active layer with a good complement of the absorption spectrum. To study the advantages of using AgNWs as an interconnection layer (PEDOT: PSS/AgNWs/ZnO) in tandem photovoltaic solar cells and as an anode and its impact on the performance of these organic cells, we have simulated the electrical characteristics obtained by these tandem organic photovoltaic cells using an equivalent circuit model. This model is based on a single diode model with five photovoltaic parameters. We therefore extracted all the physical parameters of the illuminated photovoltaic cell from its experimental characteristics (J–V), such as the diode saturation current density (J0), the series and shunt resistors (RS, RSh), the ideality factor (n) and the photogenerated current density (JPh). For this we have solved the analytical equations of the current density using Newton Raphson's method. The equations are derived from the single diode equivalent circuit proposed to simulate the measured current density as a function of the voltage of the manufactured tandem type organic solar cells. A good agreement was obtained between the theoretical model and the experimental electrical characteristics. This confirms that the use of AgNWs between PEDOT: PSS and ZnO as an interconnection layer in reverse geometry of these tandem devices, has improved the efficiency (PCE = 9.24%) and is proving to be an efficient recombination layer for tandem organic photovoltaic solar cells.

Theoretical study of the open circuit voltage decay on Organic Photovoltaic (OPV) solar cells based on space radiation ionizing damage

Organic photovoltaic (OPV) solar cells have progressed quite significantly as an affordable energy technology, with high-throughput roll-to-roll solution processing driving down costs to the point of competitiveness with current technologies. They potentially offer significant advantages over classical inorganic semiconductor cells; specifically, down costs, lightness, flexibility and controlled donor-acceptor film composition. Our specific interest is based on the applicability of organic photovoltaics cells for use in space based solar panels. The present work is a theoretical study of ionizing radiation effects in the organic photovoltaic structure P3HT: PCBM for total accumulated doses up to 1kGy (SiO2). We find that the open circuit voltage (Voc) varies with the accumulation of irradiation; however, other parameters such as relaxation time, short circuit current, and charge carrier density remain to first order constant. At the interface, the energetic mismatch of the molecular orbitals provides enough driving force to split the exciton in order to create free charge carriers (an electron (e-) and the corresponding hole (h+)). This is consistent with observations on preirradiation cases that depend directly on the Voc, due to carriers and quasi states; this leads to a linear recombination according to the Dose Damage Displacement (Dd) and Non-Ionizing Energy Loss (NIEL). Finally, we conclude that the organic photovoltaics will survive in a space environment up to 1 kGy (SiO2), contrary to popular belief that organics would be radiation “soft.”

Improving P3HT:PCBM absorber layers by blending TiO2/CdS nanocomposites for application in photovoltaic solar cells

In this work we propose the use of the semiconducting TiO2/CdS nanocomposite (NC) to improve the optical and electrical properties of the P3HT:PCBM polymeric system employed as the absorber layer in organic photovoltaic solar cells. Therefore, we report a methodology for obtaining the TiO2/CdS-NC, its incorporation into precursor solutions containing P3HT and PCBM polymers, and the fabrication of hybrid P3HT:PCBM:TiO2/CdS-NC absorber layers. The effect of the mass ratio between the TiO2/CdS-NC and the polymeric system was studied. XRD measurements conducted on the TiO2/CdS-NC showed that the TiO2 component has a tetragonal crystalline structure (anatase phase) with a nanocrystalline size of 15.8 nm, while the CdS component has the hexagonal close-packed crystalline structure with a nanocrystalline size of 14.9 nm. XPS analysis confirmed the formation of stoichiometric TiO2 and CdS compounds. The formation of any other compound comprising the elements Ti, O, Cd and S was not found, however, elements Cl, Br, N and C were observed at trace levels, which come from the capping agent that was used to obtain the nanocomposite. Measurements on the hybrid P3HT:PCBM:TiO2/CdS-NC absorber layers through UV-vis spectroscopy showed a shift on the absorption edge and an improved light absorption for wavelengths below 550 nm. The four-point probe measurements showed a decrease of the electrical resistivity from 8.5 × 108 Ω-cm in pristine P3HT:PCBM to a minimum of 4 × 105 Ω-cm in the hybrid absorber layers. Both the light absorption and the electrical resistivity are modulated by the mass ratio between the TiO2/CdS-NC and the P3HT:PCBM polymeric system.