Microsecond Carrier Lifetimes, Controlled p-Doping, and Enhanced Air Stability in Low-Bandgap Metal Halide Perovskites

Abstract

Mixed lead–tin halide perovskites have sufficiently low bandgaps (∼1.2 eV) to be promising absorbers for perovskite–perovskite tandem solar cells. Previous reports on lead–tin perovskites have typically shown poor optoelectronic properties compared to neat lead counterparts: short photoluminescence lifetimes (<100 ns) and low photoluminescence quantum efficiencies (<1%). Here, we obtain films with carrier lifetimes exceeding 1 μs and, through addition of small quantities of zinc iodide to the precursor solutions, photoluminescence quantum efficiencies under solar illumination intensities of 2.5%. The zinc additives also substantially enhance the film stability in air, and we use cross-sectional chemical mapping to show that this enhanced stability is because of a reduction in tin-rich clusters. By fabricating field-effect transistors, we observe that the introduction of zinc results in controlled p-doping. Finally, we show that zinc additives also enhance power conversion efficiencies and the stability of solar cells. Our results demonstrate substantially improved low-bandgap perovskites for solar cells and versatile electronic applications.

Understanding the Film Formation Kinetics of Sequential Deposited Narrow‐Bandgap Pb–Sn Hybrid Perovskite Films

“…Improving the homogeneity of the perovskite film is of critical importance to the electrical properties such as defect density and carrier lifetime. [ 24,45 ] To investigate the phase distribution of Pb–Sn hybrid perovskite films using sequential deposition method, we carried out a detailed microstructure analysis. First, surface X‐ray photoelectron spectroscopy (XPS) reveals that the Sn/Pb atomic ratio matches the composition used in precursor solutions.…”

“…The decrease in FF can result from the uncontrolled p‐doping induced by Sn 2+ oxidation to Sn 4+ (Figure S14, Supporting Information), which significantly reduces the charge carrier diffusion length of Sn‐based perovskite films. [ 8,45 ] While the PSC with 60% Sn still shows a PCE of 13.7%, the performance quickly deteriorates upon further increasing Sn‐content ( x ≥ 0.75). We expect that the rough Sn‐rich perovskite films (Figure S8, Supporting Information) introduce shunting pathways and thereby more recombination losses in the solar cells.…”

Controlling the Growth Kinetics and Optoelectronic Properties of 2D/3D Lead–Tin Perovskite Heterojunctions

“…In that system, the large‐bandgap 2D surface layer is sufficiently thin that charge injection to the contacts can still occur, though the layer still passivates trap states at the surfaces and grain boundaries of the 3D material. The self‐assembled heterostructures presented in our work here could be optimized for application to low bandgap Pb:Sn 3D perovskite solar cells, relevant as bottom cells in tandem structures, which to date are even more hindered by nonradiative losses and stability issues than their pure‐lead counterparts . In another example, the most efficient light‐emitting diodes (LEDs) employing bulk perovskite films utilize 2D/3D mixed systems that allow an efficient cascade of injected carriers from the wider bandgap 2D materials to the lower bandgap 3D materials, leading to high luminescence efficiencies in the 3D layer .…”

The Development Of Long Timescale Terahertz Spectroscopy Techniques To Measure Lifetimes Of Photovoltaic Materials

“…Perovskite photovoltaic devices have demonstrated efficiencies of up to 25.2% in lab conditions [49] and typically have lifetimes ranging from tens of nanoseconds to microseconds [50]. Perovskites with short lifetimes (tens of ns) can be resolved with TRTS techniques [51], however perovskite lifetimes on the order of microseconds cannot be calculated with TRTS [40] and have been resolved with PL techniques [50]. Moreover, perovskite semiconductors can be fabricated using wet chemistry techniques.…”