2D/3D Hybrid Cs₂AgBiBr₆ Double Perovskite Solar Cells: Improved Energy Level Alignment for Higher Contact‐Selectivity and Large Open Circuit Voltage

Abstract

Since their introduction in 2017, the efficiency of lead‐free halide perovskite solar cells based on Cs2AgBiBr6 has not exceeded 3%. The limiting bottlenecks are attributed to a low electron diffusion length, self‐trapping events and poor selectivity of the contacts, leading to large non‐radiative VOC losses. Here, 2D/3D hybrid double perovskites are introduced for the first time, using phenethyl ammonium as the constituting cation. The resulting solar cells show an increased efficiency of up to 2.5% for the champion cells and 2.03% on average, marking an improvement by 10% compared to the 3D reference on mesoporous TiO2. The effect is mainly due to a VOC improvement by up to 70 mV on average, yielding a maximum VOC of 1.18 V using different concentrations of phenethylammonium bromide. While these are among the highest reported VOC values for Cs2AgBiBr6 solar cells, the effect is attributed to a change in recombination behavior within the full device and a better selectivity at the interface toward the hole transporting material (HTM). This explanation is supported by voltage‐dependent external quantum efficiency, as well as photoelectron spectroscopy, revealing a better energy level alignment and thus a better hole‐extraction and improved electron blocking at the HTM interface.

Ionic Liquid‐Tuned Crystallization for Stable and Efficient Perovskite Solar Cells

“…The full width half maximum (FWHM) of the PL emission peak based on the BAAc-containing perovskite film was estimated to be 40.83, which was lower than that of the pristine case (44. 19). These optical changes should be attributable to the BAAc-assisted high-quality perovskite film with enhanced crystallinity.…”

“…Furthermore, the ultraviolet photoelectron spectroscopy (UPS) measurement was employed to disclose the work function (WF) and valence band maximum (E V ) of the pristine and the BAAcbased perovskite films. [19,20] The cutoff binding energy (E cutoff ), Fermi edge (E F, edge ), Fermi energy level (E F ), E V , conduction band minimum (E C ), and the difference between E F and E C (E F -E C ) extracted from UPS spectra (Figure 5a) were summarized in Table S4, Supporting Information. The optical bandgap was estimated to be almost the same value as 1.59 eV, extracted from the first derivative of the EQE as a function of photon energy (Figure S19, Supporting Information).…”

“…To reveal the charge-recombination behavior in the devices, the dependence of V OC on the light intensity ranging from 10 to 100 mW cm À2 was performed and shown in Figure 5d. The slope of nk B T/q in the linearly fitted V OC versus light intensity reflects the trap-assisted recombination in the devices, [19,22] where n represents the device ideal factor, k B is the Boltzmann's constant, q is the elementary charge, and T is the absolute temperature. The value of n is closer to 1, the trap-assisted recombination is more significantly suppressed.…”

Toward Understanding the Short‐Circuit Current Loss in Perovskite Solar Cells with 2D Passivation Layers

“…[14][15][16] For further interface modification, recent approaches employ materials that induce the growth of a quasi-2D perovskite on top of the 3D perovskite bulk, for example, n-butylammonium bromide (n-BABr), phenetylammonium iodide (PEAI), phenetylammonium chloride (PEACl), or 4-tert-butyl-benzylammonium iodide (tBBAI). [17][18][19][20][21] Using the aforementioned passivation strategies often means balancing on the small ridge between the desired performance increase and unwanted losses introduced by additional layers or additives, which is often manifested in a reduction of the short-circuit current density ( J SC ) of the solar cells. Amino acid additives can for example introduce a significant loss in J SC attributed to inefficient charge extraction.…”

Halide Double-Perovskite Semiconductors beyond Photovoltaics

“…So far, the PV community has mainly focused on Cs 2 AgBiBr 6 , which, despite efforts, still shows poor performance in solar cells (i.e., PCE < 3%). 20 This is in part due to the intrinsic limitation of weak sunlight absorption, because of the large (2.2 eV) and indirect bandgap of Cs 2 AgBiBr 6 . On the other hand, the performance of Cs 2 AgBiBr 6 has been more encouraging in other applications, such as X-ray detectors, 21 , 22 due to its strong X-ray absorption.…”