Abstract
In the present study, random current fluctuations measured at different temperatures and for different illumination levels are used to understand the charge carrier kinetics in methylammonium lead iodide CH3NH3PbI3‐based perovskite solar cells. A model, combining trapping/detrapping, recombination mechanisms, and electron–phonon scattering, is formulated evidencing how the presence of shallow and deeper band tail states influences the solar cell recombination losses. At low temperatures, the observed cascade capture process indicates that the trapping of the charge carriers by shallow defects is phonon assisted directly followed by their recombination. By increasing the temperature, a phase modification of the CH3NH3PbI3 absorber layer occurs and for temperatures above the phase transition at about 160 K the capture of the charge carrier takes place in two steps. The electron is first captured by a shallow defect and then it can be either emitted or thermalize down to a deeper band tail state and recombines subsequently. This result reveals that in perovskite solar cells the recombination kinetics is strongly influenced by the electron–phonon interactions. A clear correlation between the morphological structure of the perovskite grains, the energy disorder of the defect states, and the device performance is demonstrated.
Highlights
In the present study, random current fluctuations measured at different ambipolar transport property and strong optical absorption coefficient.[1]
The observed cascade capture smaller bulk crystal defects compared to process indicates that the trapping of the charge carriers by shallow defects the organic semiconductors, has produced is phonon assisted directly followed by their recombination
At both the investigated temperatures, the total electron density population N0 is lower than what found in literature for the best performing CH3NH3PbI3 solar cell (η = 19%), i.e., 1016–1017 cm−3.[1]. This means that the occupation of the highest energy defect states in the Density of States (DOS) tail results to be severely limited by the presence of defects that influence the recombination process
Summary
The perovskite solar cell is composed of a CH3NH3PbI3 absorber layer sandwiched between the electron and hole transport layers. The illuminated J–V curves exhibit a low value of photocurrent hysteresis, when measuring the characteristics in forward and backward voltage directions This finding, already reported in literature, points out the usefulness of the PCBM material as efficient passivation layer for the large density of charge traps in the CH3NH3PbI3 material.[3]. I stands for intrinsic and corresponds to the CH3NH3PbI3 material, whereas n and p corresponds to the electron and hole transport layers (i.e., PCBM and PEDOT:PSS, respectively).[10] Several studies on the inverted perovskite solar cells (p–i–n) indicate the presence of a p–n heterojunction generated by electrical defects at the cathode, acting as p-type dopants within the absorber layer.[23,24] The value of the built-in voltage Vbi, which takes into account the bandgap discontinuity at the cathode interface, can be estimated by considering the linear part of the J–V curve at J = 0. A part of the recombination processes are trap-assisted by deep states related to the defects/impurities in the crystal bulk and/or along the grain boundaries.[4,26] Their occupation probability is governed by the Shockley–Read–Hall (SRH) statistics.[27]
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