Characterization of Planar Lead Halide Perovskite Solar Cells by Impedance Spectroscopy, Open-Circuit Photovoltage Decay, and Intensity-Modulated Photovoltage/Photocurrent Spectroscopy

Abstract

Thin film lead halide perovskite cells, where the perovskite layer is deposited directly onto a flat titania blocking layer, have reached AM 1.5 efficiencies of over 15%, showing that the mesoporous scaffold used in early types of perovskite solar cells is not essential. We used a variety of techniques to gain a better understanding of thin film perovskite cells prepared by a solution-based method. Twelve cells were studied, which showed AM 1.5 efficiencies of ∼11%. The properties of the cells were investigated using impedance spectroscopy, intensity-modulated photovoltage spectroscopy (IMVS), intensity-modulated photocurrent spectroscopy (IMPS), and open-circuit photovoltage decay (OCVD). Despite the fact that all 12 cells were prepared at the same time under nominally identical conditions, their behavior fell into two distinct groups. One half of the cells exhibited ideality factors of m ≈ 2.5, and the other half showed ideality factors of m ≈ 5. Impedance spectroscopy carried out under illumination at open circuit for a range of intensities showed that the cell capacitance was dominated by the geometric capacitance of the perovskite layer rather than the chemical or diffusion capacitance due to photogenerated carriers. The voltage dependence of the recombination resistance gave ideality factors similar to those derived from the intensity dependence of the open-circuit voltage. The IMVS time constant was determined by the product of the geometric capacitance and the recombination resistance. The two types of cells gave very different OCVD responses. The cells with m ≈ 2.5 showed a persistent photovoltage effect that was absent in the case of the cells with higher ideality factors. The IMPS responses provide evidence of minor efficiency losses by recombination under short-circuit conditions.

Charge‐Carrier Balance for Highly Efficient Inverted Planar Heterojunction Perovskite Solar Cells

“…[29] As shown in Figure 5c, the Voc of the balanced device had a slower photovoltage decay than the reference, suggesting a lower charge carrier recombination rate and, thus, more balanced charge carrier transport. [41] Suppressed charge carrier recombination was also confirmed by the dark current measurements, [31] as shown in Figure 5d. The dark current densities of the balanced devices were almost two orders of magnitude lower than those of the reference ones.…”

“…This could reduce energy loss during the charge carrier collection, thus boost the device photocurrent and PCE. [40,41] We also conducted open-circuit photovoltage decay measurements to understand the balanced charge carrier transport. Here, we monitored the device Voc as a function of time starting from the illuminated steady-state equilibrium to the dark equilibrium.…”

The Role of Rubidium in Multiple‐Cation‐Based High‐Efficiency Perovskite Solar Cells

“…Finally, the low frequency spectrum which is due to the impedance of perovskite material has been assigned to Z LF , which is combination of recombination resistance and chemical capacitance in parallel. [31][32][33] The complete set of IS spectra obtained for both devices follows the same trend, while the magnitude of various resistive and capacitive components is different (SI). The plot in Figure 3a and 3b corresponds to the zero and moderate forward bias (below knee voltage, which is 850 mV for the device A and 900 mV for the device B) for both devices.…”

Reduction in the Interfacial Trap Density of Mechanochemically Synthesized MAPbI3

“…The capacitance that corresponds to ionic and electronic accumulation as a function of frequency can be obtained from the imaginary impedance and is shown in Figure 3 (c) (d) 13 whereas the peak at low frequency ~1 Hz has been associated with the ionic movement. 43,44,45 From the high frequency response centered at ~10 4 Hz with weak bias dependence, it could be expected that the transport and diffusion across the absorber layer or absorber/HTM interface in the studied devices are similar and cannot limit their performance. Thus, it becomes important to recognize the physical origin of high frequency spectra before assigning to the transport phenomena; thereby taking the values of R S and τ HF (from high frequency intercept of IS), the obtained capacitance equivalent to geometrical capacitance (C g ) signifies τ HF to be dominated by and 0.52 eV in MAPbI 3 (m) devices can be ascribed to the movement of iodine vacancies, as was observed by Eames et al 49 We find that the defect density at deeper defect levels mostly located on the surface of perovskite absorber layer is nearly an order of magnitude lower in MAPbI 3 (m) based device.…”