Examinando por Autor "Pozo, Gonzalo del"

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Evolution with annealing of solar cell parameters modeling the S-shape of the current-voltage characteristic
(Elsevier Science, 2012) Pozo, Gonzalo del; Romero, Beatriz; Arredondo, B
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, R-P2. 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 R-P2, which diminishes the reverse diode effect on the J-V curve.
Exact analytical solution of a two diode circuit model for organic solar cells showing S-shape using Lambert W-functions
(Exact analytical solution of a two diode circuit model for organic solar cells showing S-shape using Lambert W-functions, 2012) Romero, Beatriz; Pozo, Gonzalo del; Arredondo, Belén
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 (R-P2) 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 (I-02) and R-P2 have been performed in order to study the dependence of S-shape with these parameters.
Extraction of circuital parameters of organic solar cells using the exact solution based on Lambert W-function
(SPIE, 2012) Pozo, Gonzalo del; Romero, Beatriz; Arredondo, Belén
he electrical behavior of organic solar cell (OSC) has been analyzed using a simple circuital model consisting on an ideal diode together with a series and parallel resistances (R-S and R-P respectively). Applying Kirchhoff's Laws to the circuit leads to a transcendental equation that can be solved numerically without approximations using the Lambert W function. Theoretical expression has been fitted to experimental current-voltage (I-V) curves under forward bias, obtaining fairly accurate values for the electrical parameters. This model has been validated comparing the extracted parameters for dark and illumination conditions of different devices. Results show good agreement for R-S, and ideality factor eta. Electrical parameters obtained in this work are also compared to those ones extracted using an approximated method often employed by other authors (1). We conclude that approximated method leads to reasonable good values for R-S, R-P and. However, in the case of Rp the voltage range chosen to fit the data with the exact method must be constrained to the fourth quadrant, where the role of parallel resistance is more critical. To validate the model, a bunch of organic solar cells with structure ITO/ poly(3,4-ethylenedioxythiophene)-poly (4-styrene sulfonate (PEDOT:PSS)/ poly(3-hexylthiophene) (P3HT): 1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6) C61 (PCBM)/Al has been fabricated in inert atmosphere. Different active layers were deposited varying the P3HT: PCBM ratio (1:0.64, 1:1, 1:1.55) and the active layer thickness ( ranging from 100 to 280 nm). Devices are encapsulated inside the glove-box prior its characterization outside the glove-box. Electro optical characterization has been performed with a halogen lamp. Values extracted for RS range from 142 Omega to 273 Omega, values for R-P range from 25 k Omega to 331 k Omega. Ideality factor ranges from 5 to 17.
High-bandwidth organic photodetector analyzed by impedance spectroscopy
(IEEE, 2012) Arredondo, Belén; Dios, Cristina de; Vergaz, Ricardo; Pozo, Gonzalo del; Romero, Beatriz
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 (fc) can be estimated with the parameters extracted from this circuit obtaining good agreement between measured and calculated fc.
Influence of solvent additive on the performance and aging behavior of non-fullerene organic solar cells
(Elsevier, 2022) Arredondo, Belén; Pérez-Martínez, José Carlos; Muñoz-Díaz, Laura; López-González, Maria del Carmen; Martín-Martín, Diego; Pozo, Gonzalo del; Hernández-Balaguera, Enrique; Romero, Beatriz; Lamminaho, Jani; Turkovic, Vida; Madsen, Morten
The performance of organic solar cells has improved significantly in recent years due to the use of non-fullerene acceptors (NFA). While processing additives are typically added to the active layer blends to enhance device performance in NFA organic solar cells, their impact on device degradation remains unclear. In this work we have compared the performance, in pristine and degraded state, between air-processed slot-die coated NFA ITOfree organic solar cells with and without the processing additive DIO, using a structure of PET/Ag/ZnO/PBDB-T: ITIC/FHC PEDOT:PSS. We observed an improvement in the power conversion efficiency of the devices when adding DIO, from 4.03% up to 4.97%. The evolution of the performance for both devices under ISOS-L1 life testing protocol reveals that the drop in efficiency is mainly due to a decay of JSC for both cells. In the short time scale the efficiency of non-DIO cells decays faster than the DIO cells, whereas in the long time scale the efficiency of non-DIO cells tends to stabilize sooner. Carrier mobilities estimated from impedance measurements decrease with time at similar rate for both degraded samples. Besides, DIO devices present a steep increase of the series resistance with time causing a decrease of the FF and thus of the efficiency. Moreover, in both degraded devices, the open-circuit voltage saturates with increasing illumination intensity. Numerical simulations reveal that a reduced anode work function of 5 eV is needed to fit experimental data.
Temperature behaviour of mixed-cation mixed-halide perovskite solar cells. Analysis of recombination mechanisms and ion migration
(Elsevier, 2023) López-González, Mari Carmen; Pozo, Gonzalo del; Arredondo, Belén; Delgado, Silvia; Martín-Martín, Diego; García-Pardo, Marina; Romero, Beatriz
In our study, we show that compositional engineering of the “A” site cation of ABX3 perovskite structure formed by a mix of organic and inorganic cations is an effective route to improve the thermal stability of perovskite solar cells (PSCs). In this work, mixed-cation mixed-halide PSCs have been fabricated and characterized with temperature, from 253 up to 333 K. The active layer based on CsRbFAMAPb(IBr)3 results in a more stable device compared to standard MAPbI3 devices. Electrical characterization reveals a decrease of the solar cell parameters with temperature. Using Impedance Spectroscopy (IS) characterization, we have estimated an activation energy for the halide ion migration of 0.63 ± 0.08 eV, an ion diffusion coefficient of 10− 14 cm2 s − 1 , and a defect density of 7.27⋅1015 cm− 3 . To our knowledge, this is the first time that these parameters have been calculated in CsRbFAMAPb(IBr)3 based devices, resulting in improved values compared to MAPbI3 devices. The worsening of device performance for temperatures above 300 K is attributed to a decrease of the spiro-OMeTAD conductivity and the degradation of the perovskite/spiro-OMeTAD interface. It is shown that for low temperatures (from 253 to 323 K), Shockley-Red-Hall (SRH) recombination in the bulk governs, while for temperatures above 323 K the increase in surface recombination becomes dominant due to the presence of non-selective contacts. Numerical simulations using SILVACO ATLAS corroborate the role of SRH in the perovskite active layer for low and medium temperatures, and the crucial influence of spiro-OMeTAD transport properties in the device performance parameters.