Perovskite Solar Cells (PSC) have attracted great attention due to the high efficiencies achieved in the past few years (up to 24.2 %). Perovskite semiconductors show excellent light absorption and large charge-carrier mobilities. In addition, device fabrication is low cost and easily up-scalable. However, the current density-voltage curve (J-V) shows hysteresis and devices suffer from stability issues which are still poorly understood. Among all perovskite materials, mixed-cation lead mixed-halide PSC have become very popular due to their high efficiencies and reasonably good stabilities1,2. On the other hand, Impedance Spectroscopy (IS) is a very valuable non-destructive technique to obtain information about dynamical mechanisms occurring both in the bulk and at the interfaces3 . In this work, J-V curves and the impedance response have been measured for CsFAPbIBr-based PSC from 1 Hz up to 1 MHz, under different illumination levels (from 0.06 mW/cm2 to 100 mW/cm2 ) both at 0 V (short circuit) and at Voc (open circuit). Impedance spectra show two significant arcs, associated to different recombination and charge accumulation mechanisms. IS data have been fitted to a circuital model that consists of a low-frequency RCPE subcircuit in series with a high frequency resistance, all shunted with a high-frequency capacitance. Dependence of the circuital parameters with Voc and Isc will be discussed.
Impedance Spectroscopy (IS) is a non-destructive characterization technique that has been extensively applied to different electronic devices, such as LEDs, photodiodes and solar cells. This technique provides access to valuable information about dynamical mechanisms (minority carrier recombination, diffusion, etc.) taking place in the different layers of the device. Besides, material and device parameters, such as dielectric constant, built-in potential, and carrier mobilities can be extracted. Impedance spectra results from applying a small AC signal over a steady DC bias and measuring the resulting small AC current over a frequency range, typically from 1 Hz to 1 MHz, 𝑍(jω) = VAC/1AC. The Nyquist plot of the complex impedance (imaginary part vs real part) generally presents one or more features (mainly semicircles), depending on the number of mechanisms governing the device. Fitting an electrical equivalent circuit to the complete impedance spectra provides parameters related to each feature (resistance and capacitance). In this work, IS has been used to characterize organic and perovskite solar cells (OSCs and PSCs, respectively). Measurements have been performed in dark and under illumination conditions at different bias (from 0 V to VOC). A simple circuital model containing two resistances and two capacitances has been used to fit the measured IS spectra. The interpretation of extracted circuital parameters and its relationship with the physical model of the device will be discussed.
The 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 (RS and RP 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 RS, and ideality
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 RS, RP
and η. 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):
(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
Values extracted for RS range from 142 Ω to 273 Ω, values for RP range from 25 kΩ to 331 kΩ. Ideality factor ranges
from 5 to 17.
In this work, 4x4 organic light emitting diode passive matrices based on new poli(2,7-fluorene phenylidene) (PFP) derivatives have been developed. The fabrication process has involved spin-cast heterostructures that improve charge carrier injection, processing of devices by means of photolithography, together with metallic contact evaporation. Electroluminescent diodes using different polymer derivatives as active layer, and different geometries, have been fabricated and compared. Electrical characterization was carried out in terms of pulsed current-voltage (I-V) measurements. Dependence of the threshold voltage on active material and structural parameters is obtained from the I-V curves, yielding values from 10 V to 25 V. Electroluminescence spectra recorded from the new PFP based devices, as well as commercial polymers, are in good agreement with similar devices found in literature. Finally, experimental data have been fitted using a theoretical model considering several injection and transport mechanisms, including thermionic, reverse, tunnelling, ohmic and space charge limited currents.