Optical determination of Shockley-Read-Hall and interface recombination currents in hybrid perovskites

Valerio Sarritzu,Francesco Quochi,Giovanni Bongiovanni,Nicola Sestu,Daniela Marongiu,Michele Saba,Sofia Masi,Andrea Mura,Xueqing Chang,Aurora Rizzo,Silvia Colella

doi:10.1038/srep44629

Abstract

Metal-halide perovskite solar cells rival the best inorganic solar cells in power conversion efficiency, providing the outlook for efficient, cheap devices. In order for the technology to mature and approach the ideal Shockley-Queissier efficiency, experimental tools are needed to diagnose what processes limit performances, beyond simply measuring electrical characteristics often affected by parasitic effects and difficult to interpret. Here we study the microscopic origin of recombination currents causing photoconversion losses with an all-optical technique, measuring the electron-hole free energy as a function of the exciting light intensity. Our method allows assessing the ideality factor and breaks down the electron-hole recombination current into bulk defect and interface contributions, providing an estimate of the limit photoconversion efficiency, without any real charge current flowing through the device. We identify Shockley-Read-Hall recombination as the main decay process in insulated perovskite layers and quantify the additional performance degradation due to interface recombination in heterojunctions.

Highlights

Six decades after the demonstration of the first silicon photovoltaic cell[1], only a limited number of semiconductors enable single junction photovoltaic devices with power conversion efficiencies (PCEs) exceeding 20%2
Instead of the I-V curves, we studied the free energy of the electron-hole pairs (μ) as a function of the intensity of the exciting light (Iex), namely the μ-Iex characteristics
To provide an expression for μ in terms of measurable optical quantities, we consider Kirchhoff ’s law of radiation, which represents the detailed balance between emission and absorption, generalized by Würfel to account for non-equilibrium electron and hole populations[37]:

Summary

Introduction

Six decades after the demonstration of the first silicon photovoltaic cell[1], only a limited number of semiconductors enable single junction photovoltaic devices with power conversion efficiencies (PCEs) exceeding 20%2. Photovoltaic devices operate as non-ideal current generators in which a fraction of photogenerated electrons and holes recombines inside the cell, feeding the internal diode current instead of being injected into the external load[23]. The external electroluminescence quantum yield (EQYEL) quantifies the amount of radiative recombination with respect to non-radiative losses; the best solar cells show the highest EQYEL (10−3–10−1) and operate at a voltage as close as possible to the semiconductor band gap—up to 0.78 Eg in GaAs24 and very recently 0.76 Eg in HP8,20,25–27. Trap levels close to the ETL (HTL) are filled (empty), no trapping of electrons (holes) by mid-gap states is thereby possible, leading to negligible RSRH,e(h) values. Information on which annihilation process dominates can be inferred from the ideality factor m of the diode current[28,29], typically extracted by fitting the charge current-voltage (I-V) characteristics. It is difficult to identify from the electrical characterization what recombination processes limit the photoconversion efficiency, and to elaborate an informed strategy to improve the devices

Methods

Results

Discussion

Conclusion