Cs2AgBiBr6 has attracted much interest as a potential lead‐free alternative for perovskite solar cells. Although this material offers encouraging optoelectronic features, severe bottlenecks limit the performance of the resulting solar cells to a power conversion efficiency of below 3%. Here, the performance‐limiting factors of this material are investigated in full solar cells featuring various architectures. It is found that the photovoltaic parameters of Cs2AgBiBr6‐based solar cells strongly depend on the scan speed of the J/V measurements, suggesting a strong impact of ionic conductivity in the material. Moreover, a sign change of the photocurrent for bias voltages above 0.9 V during the measurement of the external quantum efficiency (EQE) is revealed, which can be explained by non‐selective contacts. The radiative loss of the VOC from sensitive subgap‐EQE measurements is calculated and it is revealed that the loss is caused by a low external luminescence yield and therefore a high non‐radiative recombination, supported by the first report of a strongly red shifted electroluminescence signal between 800 and 1000 nm. Altogether, these results point to a poor selectivity of the contacts and charge transport layers, caused by poor energy level alignment that can be overcome by optimizing the architecture of the solar cell.

with outstanding optoelectronic properties. [1] In 2009, these materials were introduced in solar cells and have since established a striking increase in performance, reaching over 22% in stateof-the-art devices. [2] Here, the perovskite absorber is sandwiched between two selective charge extraction layers, that transport the charges to the electrodes. [3] Although efficient inorganic hole transporting materials (HTMs) have been reported, [4] the most well-known HTMs are the organic materials 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′spirobifluorene (Spiro-OMeTAD) and polytriarylamine (PTAA). Alternatives that compete in performance have been published, [5][6][7] however, just like Spiro-OMeTAD and PTAA, most of these materials are synthesized in multistep synthetic procedures, involving (transition) metal catalyzed cross-coupling reactions, stringent reaction conditions and extensive product purification. This results in a relative high material cost, consequently leading to a significant contribution to the total device cost. [5,8,9] Additionally, the tedious synthesis hampers large State-of-the-art perovskite-based solar cells employ expensive, organic hole transporting materials (HTMs) such as Spiro-OMeTAD that, in turn, limits the commercialization of this promising technology. Herein an HTM (EDOT-Amide-TPA) is reported in which a functional amide-based backbone is introduced, which allows this material to be synthesized in a simple condensation reaction with an estimated cost of <$5 g −1 . When employed in perovskite solar cells, EDOT-Amide-TPA demonstrates stabilized power conversion efficiencies up to 20.0% and reproducibly outperforms Spiro-OMeTAD in direct comparisons. Time resolved microwave conductivity measurements indicate that the observed improvement originates from a faster hole injection rate from the perovskite to EDOT-Amide-TPA. Additionally, the devices exhibit an improved lifetime, which is assigned to the coordination of the amide bond to the Li-additive, offering a novel strategy to hamper the migration of additives. It is shown that, despite the lack of a conjugated backbone, the amide-based HTM can outperform state-of-the-art HTMs at a fraction of the cost, thereby providing a novel set of design strategies to develop new, low-cost HTMs.

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.

Lead-free double perovskites have recently attracted growing attention as possible alternatives to lead-based halide perovskites in photovoltaics and other optoelectronic applications. The most prominent compound Cs2AgBiBr6, however, presents issues such as a rather large and indirect band gap, high exciton binding energies, and poor charge carrier transport, especially in thin films. In order to address some of these challenges, we systematically modified the stoichiometry of the precursors used for the synthesis of thin films toward a BiBr3-deficient system. In combination with a stoichiometric excess of AgBr, we obtained highly oriented double perovskite thin films. These modifications directly boost the lifetime of the charge carriers up to 500 ns as observed by time-resolved photoluminescence spectroscopy. Moreover, time-resolved microwave conductivity studies revealed an increase of the charge carrier mobility from 3.5 to around ∼5 cm2/(V s). Solar cells comprising the modified films as planar active layers reached power conversion efficiency (PCE) values up to 1.11%, exceeding the stoichiometric reference film (∼0.97%), both on average and with champion cells. The results in this work underline the importance of controlling the nanomorphology of the bulk film. We anticipate that control of precursor stoichiometry will also offer a promising approach for enhancing the efficiency of other perovskite photovoltaic absorber materials and thin films.

A series of azomethine-based HTMs is synthesized using simple condensation chemistry. Their photovoltaic performance and moisture barrier properties are presented.