E. López-Fraguas, B. Arredondo, C. Vega-Colado, G. del Pozo, M. Najafi, D. Martín-Martín, Y. Galagan, J. M. Sánchez-Pena, R. Vergaz and B. Romero
Visible Light Communication system using an organic emitter and a perovskite photodetector
Organic Electronics, 73, 292-298, 2019, https://doi.org/10.1016/j.orgel.2019.06.028
Abstract:
The past few years have seen a great increase in the development of Visible Light Communication systems (VLC), mainly triggered by the wide variety of situations they can be used in. These communication systems have traditionally employed inorganic light emitters and photodetectors. In this work, we present a VLC system using an organic emitter and a perovskite photodetector, both fabricated using low cost processing techniques. Perovskite devices have been widely studied as photovoltaics cells since they have achieved great efficiencies, and, in the recent years, there is also an intense research of these devices as photodetectors. In this work, we have fabricated and characterized a perovskite photodetector with layer structure ITO/PTAA/Perovskite/PCBM/BCP/Cu, and integrated it in a visible communication system to successfully link an audio signal.
URL: https://www.sciencedirect.com/science/article/pii/S1566119919303234
C. Vega-Colado, B. Arredondo, J. C. Torres, E. López-Fraguas, R. Vergaz, D. Martín-Martín, G. del Pozo, B. Romero, P. Apilo, X. Quintana, M. A. Geday, C. de Dios and J. M. Sánchez-Peña
An all-organic flexible visible light communication system
Sensors, 18, 3045, 2018, https://doi.org/10.3390/s18093045
Abstract: Visible light communication systems can be used in a wide variety of applications, from driving to home automation. The use of wearables can increase the potential applications in indoor systems to send and receive specific and customized information. We have designed and developed a fully organic and flexible Visible Light Communication system using a flexible OLED, a flexible P3HT:PCBM-based organic photodiode (OPD) and flexible PCBs for the emitter and receiver conditioning circuits. We have fabricated and characterized the I-V curve, modulation response and impedance of the flexible OPD. As emitter we have used a commercial flexible organic luminaire with dimensions 99 × 99 × 0.88 mm, and we have characterized its modulation response. All the devices show frequency responses that allow operation over 40 kHz, thus enabling the transmission of high quality audio. Finally, we integrated the emitter and receiver components and its electronic drivers, to build an all-organic flexible VLC system capable of transmitting an audio file in real-time, as a proof of concept of the indoor capabilities of such a system.
B. Arredondo, B. Romero, M. J. Beliatis, G. del Pozo, D. Martín-Martín, J. C. Blakesley, G. Dibb, F. C. Krebs, S.A. Gevorgyan and F. A. Castro
Solar Energy Materials and Solar Cells, 176, 397-404, 2018, https://doi.org/10.1016/j.solmat.2017.10.028
Abstract: In this work we study the degradation of roll-coated flexible inverted organic solar cells in different atmospheres. We demonstrate that impedance spectroscopy is a powerful tool for elucidating degradation mechanisms; it is used here to distinguish the different degradation mechanisms due to water and oxygen. Identical cells were exposed to different accelerated degradation environments using water only, oxygen only, and both water and oxygen simultaneously, all of them enhanced with UV light. The photocurrent is dramatically reduced in the oxygen-degraded samples. Impedance measurements indicate that this phenomenon is attributed to defects introduced by absorption of oxygen, which results in an increase of the acceptor impurity (NA) at the cathode interface obtained from a Mott-Schottky analysis. Simultaneously, at the anode interface where PEDOT:PSS is not shielded by the substrate, the nature of degradation differs for the water and oxygen degraded samples. While oxygen + UV light decreases the conductivity of the PEDOT:PSS layer, water + UV light changes the PEDOT:PSS work function inducing a depletion region at the anode.
URL: https://www.sciencedirect.com/science/article/pii/S0927024817305974
B. Romero, G. del Pozo, B. Arredondo, D. Martín-Martín , María P. Ruiz Gordoa, A. Pickering, A. Pérez-Rodríguez, E. Barrena and F. J. García- Sánchez
IEEE Transactions on electron devices, 64, 11, 2017.
Abstract: We explain how to obtain closed-form analytic solutions from the set of equations that describe the three-diode lumped-parameter equivalent circuit model proposed by Mazhari [1] to portray the undesirable S-shape often observed in I-V characteristics of illuminated organic solar cells (OSCs), and occasionally seen in other types of solar cells. This allows quick extraction of the model’s parameter values by directly fitting the resulting closed-form solution to the cell’s measured I-V data. Such mathematical simplification of the extraction procedure facilitates individually studying the effect of each parameter on the illuminated OSC I-V characteristics, and thus on its power generation capacity. We illustrate application of the direct extraction procedure to measured I-V characteristics of an experimental OSC, which exhibits the illumination intensity-dependent S-shapes. The usefulness of the analytic solution to assess the effect of the model parameters is further corroborated by graphically illustrating the progression of a series of hypothetical synthetic I-V characteristics generated by this analytic solution using gradually changing the parameter values. Analysis of the results, in this case, indicates that activation of the diode that represents recombination is the key factor responsible for the emergence of the illuminated I-V curve’s S-shape.
G. del Pozo, B. Arredondo, B. Romero, G. Susanna and F. Brunetti
Solar Energy, 144, 105-110, 2017.
Abstract: In this work PTB7:[70]PCBM (Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b] thiophenediyl]]:[6,6]-Phenyl-C71-butyric acid methyl-ester ([70])) based solar cells with PEIE (Polyethylenimine, 80% Ethoxylated) as electron transport layer (ETL) have been characterized along two months using impedance spectroscopy (IS). The experimental data were fitted using an electrical circuit and the time evolution of circuital parameters was analyzed. At high voltages, the capacitance associated to the PEIE layer decreases within the first days of measurements while the resistance increases during the same period. The flat band voltage, Vfb, obtained from a Mott-Schottky analysis, decreases with measuring time around 0.1 V, suggesting a deterioration of PEIE interface that no longer improves the ITO work-function. Carrier mobility decreases from 2.2 × 10−3 cm2/Vs to 2.4 × 10−4 cm2/Vs within the first few days of measurements.
URL: https://www.sciencedirect.com/science/article/pii/S0038092X17300129
B. Arredondo, M. B. Martín-López, B. Romero, R. Vergaz, P. Romero-Gómez and J. Martorell
Solar Energy Materials and Solar Cells, 144, 422-428, 2016, https://doi.org/10.1016/j.solmat.2015.09.050
Abstract:
We have used impedance spectroscopy technique to monitor degradation mechanisms in organic solar cells based on a blend of PTB7:PC71BM. We have measured the impedance of the cell on a periodical basis for almost four months, and experimental data have been modeled using three different circuits. The evolution of the circuital parameters gives information about the device dynamical mechanisms. We have observed at high voltages a low frequency feature that is more pronounced along days of measurement. This low frequency arc has been associated to charge accumulation that is related to a worsening of charge extraction through the contacts. The simultaneous increase of recombination and low frequency resistances at high voltages (around Voc) results in a decrease of the fill factor and therefore of the efficiency.
URL: https://www.sciencedirect.com/science/article/pii/S0927024815004833
C. Cuerva, J. A. Campo, M. Cano, B. Arredondo, B. Romero, E. Otón and J. M. Otón
New Journal of Chemistry, 11, 8467-8473, 2015.
Abstract:
The photoluminescence and mechanochromic behaviour of the bis(3-(3,5-bis(dodecyloxy)phenyl)-(5-pyridin-2-yl)pyrazolate)platinum(II) complex PT12, selected from a series of Pt(II) compounds with N,N′-pyridylpyrazolate ligands, has been investigated. The electroluminescence properties of polymer OLEDs based on PT12-doped polyfluorene (PFO) are also described. Addition of PT12 induces the formation of the PFO β-phase, a much more ordered phase with enhanced colour stability at high bias and high photoluminescence quantum efficiency. Additionally, the characteristic blue emission of PFO is red-shifted for PT12 concentrations equal to or higher than 3%. Depending on the dopant concentration and the applied current, the colour coordinates of these polymer OLEDs undergo a broad shift in the CIE colour space. In addition, through the strategic use of keto defect sites, white-emitting devices can be fabricated with a very small addition of PT12.
URL: https://pubs.rsc.org/en/content/articlelanding/2015/nj/c5nj01875g
L. F. Wu, S. Casado, B. Romero, J. M. Otón, J. Morgado, C. Muller, R. Xia and J. Cabanillas-González
Macromolecules, 48, 442-428, 2015, https://doi.org/10.1021/acs.macromol.5b02111
Abstract: Regioregular poly(3-hexylthiophene), (rr-P3HT), is widely regarded as an archetypical semiconducting electron-donor polymer in bulk heterojunction solar cells, mainly due to its notable exciton and charge transport properties. Conversely, its use in photonic/lighting devices received little interest owing to its low emission quantum yield in the solid state, related to its large tendency to self-organize into lamellar domains with highly nonemissive character. We present experimental evidence pointing to a large miscibility of P3HT in poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT), accompanied by mutual π–π interactions. Concomitant to this effect, the emission properties of P3HT:F8BT blends, which are attributed to heterogeneous interchain complexes, experience a significant improvement, with photoluminescence quantum efficiency (PLQE) values approaching 25% (almost a 7-fold efficiency enhancement with respect to neat/aggregated rr-P3HT). Our results open up new prospects for improved photonic properties through appropriate control of interchain interactions.
P. Romero-Gómez, R. Betancur, A. Martínez-Otero, X. Elias, M. Mariano, B. Romero, B. Arredondo, R. Vergaz and J. Martorell
Enhanced stability in semi-transparent PTB7/PC71BM photovoltaic cells
Solar Energy Materials and Solar Cells, 137, 44-49, 2015, https://doi.org/10.1016/j.solmat.2015.01.026
Abstract:
We studied the performance over time of opaque and semi-transparent PTB7:PC71BM bulk hetero-junction solar cells. For unsealed inverted configuration cells we observe that when the isolation from the environment is improved, the degradation observed is dominated by one single exponential decay. We demonstrate that a dielectric multilayer stack of approximately 550 nm provides an isolation that increases the lifetime of the cell close to ten times. In that event the fill factor appears to be the PV parameter dominating cell degradation resulting from a decrease in the shunt resistance. An Impedance analysis we performed indicates that a Warburg element, attributed to the presence of slowly moving charges such as heavy ions, must be included in the description of the experimental data. The contribution from such element increases as the cell degrades in good agreement with a degradation dominated by the corrosive effects from external agents reaching the active layer of the device.
URL: https://www.sciencedirect.com/science/article/pii/S0927024815000409
B. Romero, G. del Pozo, B. Arredondo, J. P. Reinhardt, M. Sessler and U. Würfel
Circuital model validation for S-shaped organic solar cells by means of impedance spectroscopy
IEEE Journal of Photovoltaics, 5, 234-237, 2015, doi: 10.1109/JPHOTOV.2014.2362978
Abstract: Organic solar cells (OSCs) based on Poly[N-9′-heptadecanyl-2,7-carbazole-alt-5, 5-(4′, 7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT):1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6) C61 (PCBM) have been electrically dc and ac characterized. Device structure is ITO/Poly(3,4-ethylenedioxy-wthiophene)-poly(4-styrene sulfonate) (PEDOT:PSS)/PCDTBT:PCBM/C60MA/Al. Devices show the so-called S-shaped current-voltage curve that has been modeled with a circuit including two diodes in forward and reverse bias. Impedance measurements show one semicircle at low bias ( V <; 0.4 V) and two semicircles at ( V <; 0.4 V). Results reveal that the high-frequency semicircle is associated with the forward diode-like behavior of the dc circuit, while the low-frequency semicircle is associated with the reverse diode that causes the S-shape. The analytical solution of the dc two-diode circuital model has been fitted to the experimental I-V, and dc circuital parameters have been obtained. The ac equivalent circuit was derived from the dc circuit, and small-signal parameters have been calculated from the fits to the experimental Cole-Cole.
B. Romero, G. del Pozo, E. Destouesse, S. Chambon and B. Arredondo
Organic Electronics, 15, 3546-3551, 2014, https://doi.org/10.1016/j.orgel.2014.09.033
Abstract: In this work light activation phenomenon in inverted bulk heterojunction (BHJ) organic solar cells (OSC) has been electrically modelled with a two-diode equivalent circuit. OSC are based on poly(3-hexylthiophene) (P3HT): 1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6) C61 (PCBM) with a titanium oxide (TiOx) sublayer. Current–voltage (I–V) characteristics show a highly pronounced S-shape that is gradually removed during light activation process. The circuit used to model I–V curves includes two diodes in forward and reverse bias together with two parallel resistances, RP1 and RP2. The parallel of the reverse bias diode and its corresponding resistance RP2 models the electrical behaviour of the TiOx interlayer. This interlayer has been thermally treated at different temperatures, from 80 °C up to 180 °C, reducing the activation time from 400 s for unbaked devices down to 30 s for devices annealed at temperatures higher than 80 °C. The S-shape shown in the I–V characteristic is completely removed after a few minutes of white-light illumination. I–V curves recorded during the activation process have been fitted with the analytical solution of the two-diode circuit based on W-Lambert function. A decrease of the subcircuit 2 equivalent resistance has been found to be the cause of S-shape removal. This resistance diminishing is in good agreement with the increase of TiOx conductance with baking temperature and white-light exposure time found by other authors.
URL: https://www.sciencedirect.com/science/article/pii/S156611991400425X
B. Arredondo, B. Romero, G. del Pozo, M. Sessler, C. Veit and U. Würfel
Impedance spectroscopy analysis of small molecule solution processed organic solar cell
Solar Energy Materials and Solar Cells, 128, 351-356, 2014, https://doi.org/10.1016/j.solmat.2014.05.050
Abstract: In this paper we study the transport-recombination mechanisms using impedance spectroscopy of organic solar cells (OSC) based on a blend of a small molecule, 7,7′-(4,4-bis(2ethylhexyl)-4H-silolo[3,2-b:4,5-b′] dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5] thiadiazole) (DTS(FBTTh2)2) and 1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6) C70 (PC70BM). We fabricate a cell with structure ITO/Poly(3,4-ethylenedioxythiophene)-poly(4-styrene sulfonate (PEDOT:PSS))/DTS(FBTTh2)2:PC70BM/Ca/Al that exhibits Jsc=10.2 mA/cm2, Voc=0.816 V and FF=65% resulting in a PCE=5.4%. We model the impedance behavior using two circuital models, the parallel R-CPE and the transmission line model proposed by Belmonte et al. [1]. We compared the results to those obtained for OSC based on a standard poly(3-hexylthiophene) (P3HT): 1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6) C61 (PC60BM) blend with structure ITO/PEDOT:PSS/P3HT:PC60BM/LiF/Al. We find that in the case of the small molecule based OSC diffusion dominates over recombination for this thickness, L=125 nm, even at high frequencies. We calculate the effective carrier lifetimes and mobilities for both structures using both models. Average electron mobility calculated for the small molecule cell is around 4–6.4×10−3 cm2/Vs, slightly higher than that obtained for the standard blend which is around 2×10−3 cm2/Vs.
URL: https://www.sciencedirect.com/science/article/pii/S092702481400316X
B. Arredondo, B. Romero, J. M. Sánchez-Peña, A. Fernández-Pacheco, E. Alonso, R. Vergaz and C. de Dios
Visible light communication system using an organic bulk heterojunction photodetector
Sensors, 13, 12266-12276, 2013, https://doi.org/10.3390/s130912266
Abstract: A visible light communication (VLC) system using an organic bulk heterojunction photodetector (OPD) is presented. The system has been successfully proven indoors with an audio signal. The emitter consists of three commercial high-power white LEDs connected in parallel. The receiver is based on an organic photodetector having as active layer a blend of poly(3-hexylthiophene) (P3HT) and phenyl C61-butyric acid methyl ester (PCBM). The OPD is opto-electrically characterized, showing a responsivity of 0.18 A/W and a modulation response of 790 kHz at −6 V.
B. Arredondo, C. de Dios, R. Vergaz, A. R. Criado, B. Romero, B. Zimmermann and U. Würfel
Organic Electronics, 14, 2484-2490, 2013, https://doi.org/10.1016/j.orgel.2013.06.018
Abstract: We report on the fabrication of Indium Tin Oxide (ITO)-free inverted organic bulk heterojunction (BHJ) photodetectors of poly(3-hexylthiophene) (P3HT): 1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6) C61 (PCBM). The final inverted device structure is Cr/Al/Cr/P3HT:PCBM/poly-3,4-ethylenedioxythiophene:poly-styrenesulfonate (PEDOT:PSS)/Ag (Zimmermann et al., 2009). The device is top-absorbing with the light entering through the hole contact grid. We have fabricated standard devices with structure ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al in order to carry out a comparison study. Inverted photodetectors show slightly higher quantum efficiency and responsivity compared to standard devices. Frequency responses at different bias voltages were measured showing a maximum −3 dB cut-off frequency of 780 kHz and 700 kHz at −3 V for the standard and inverted structures respectively. Parameters extracted from the fit of a circuital model to the impedance spectroscopy measurements were used to estimate the photodiode cut-off frequency as function of bias.
URL: https://www.sciencedirect.com/science/article/pii/S1566119913003017
B. Arredondo, C. de Dios, R. Vergaz, G. del Pozo and B. Romero
High-bandwidth organic photodetector analyzed by impedance spectroscopy
IEEE Photonic Technology Letters, 24, 1868-1871, 2012, 10.1109/LPT.2012.2217488
Abstract: An organic bulk heterojunction photodetector (OPD) is fabricated on the basis of a blend of poly(3-hexyl thiophene):1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6) C61. The OPD responsivity is calculated from current density-voltage characteristic (J-V) under green-LED illumination obtaining 1 A/W. Frequency response at different biases is measured, which shows a high-3-dB cutoff frequency of 343 kHz at -3 V . Impedance measurements are carried out at different reverse biases, and the results are fitted with a small signal equivalent circuit. The device cutoff frequency (f c ) can be estimated with the parameters extracted from this circuit obtaining good agreement between measured and calculated (f c ).
G. Del Pozo, B. Romero and B. Arredondo
Solar Energy Materials and Solar Cells, 104, 81-86, 2012, https://doi.org/10.1016/j.solmat.2012.04.0
Abstract: In this work we use a modified equivalent circuit to simulate the S-shape found in current density–voltage (J–V) characteristics of organic solar cells (OSC) based on P3HT:PCBM. The modification of the traditional model includes a second diode opposite to the main one together with a parallel resistance, RP2. The transcendental equation derived from the circuit is solved exactly without approximations. The S-shape (so called kink), usually attributed to a poor quality of polymer/cathode interface, can be removed after three slow annealing treatments. The evolution of the J–V curves with the annealing process has been quantified by extracting the equivalent circuit parameters of each curve (pristine and annealed) by fitting the model to the experimental data. Results show that kink annihilation is mostly due to a strong decrease of RP2, which diminishes the reverse diode effect on the J–V curve.
URL: https://www.sciencedirect.com/science/article/pii/S0927024812002243
B. Romero, G. Del Pozo and B. Arredondo
Solar Energy, 86, 3026-3029, 2012, https://doi.org/10.1016/j.solener.2012.07.010
Abstract: Organic solar cells (OSCs) often show a kink, also called S-shape, in the current–voltage (I–V) characteristics, that has been attributed to different physical phenomena such as poor quality of cathode-active layer interface or unbalance charge carrier mobilities. This non-ideal behaviour can be electrically modelled including a second diode, in reverse bias, together with an extra shunt resistance (RP2) in the traditional solar cell equivalent circuit. In this paper, we solve without approximations the transcendental equation system derived from this modified circuit. We have obtained an analytical expression for I–V curves decoupling the voltage drop in each diode using Lambert W function. This expression has been fitted to experimental data in order to obtain circuital parameters. Simulations varying saturation current of reverse diode (I02) and RP2 have been performed in order to study the dependence of S-shape with these parameters.
URL: https://www.sciencedirect.com/science/article/pii/S0038092X12002526