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.

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.

Metal halide perovskite photodiodes (PPDs) offer high responsivity and broad spectral sensitivity, making them attractive for low-cost visible and near-infrared sensing. A significant challenge in achieving high detectivity in PPDs is lowering the dark current density (JD) and noise current (in). This is commonly accomplished using charge-blocking layers to reduce charge injection. By analyzing the temperature dependence of JD for lead-tin based PPDs with different bandgaps and electron-blocking layers (EBL), we demonstrate that while EBLs eliminate electron injection, they facilitate undesired thermal charge generation at the EBL-perovskite interface. The interfacial energy offset between the EBL and the perovskite determines the magnitude and activation energy of JD. By increasing this offset we realized a PPD with ultralow JD and in of 5 × 10−8 mA cm−2 and 2 × 10−14 A Hz−1/2, respectively, and wavelength sensitivity up to 1050 nm, establishing a new design principle to maximize detectivity in perovskite photodiodes.

Organic bulk heterojunction photodiodes (OPDs) attract attention for sensing and imaging. Their detectivity is typically limited by a substantial reverse bias dark current density (Jd). Recently, using thermal admittance or spectral photocurrent measurements, Jd has been attributed to thermal charge generation mediated by mid‐gap states. Here, the temperature dependence of Jd in state‐of‐the‐art OPDs is reported with Jd down to 10−9 mA cm−2 at −0.5 V bias. For a variety of donor‐acceptor bulk‐heterojunction blends it is found that the thermal activation energy of Jd is lower than the effective bandgap of the blends, by ca. 0.3 to 0.5 eV, but higher than expected for mid‐gap states. Ultra‐sensitive sub‐bandgap photocurrent spectroscopy reveals that the minimum photon energy for optical charge generation in OPDs correlates with the dark current thermal activation energy. The dark current in OPDs is attributed to thermal charge generation at the donor‐acceptor interface mediated by intra‐gap states near the band edges.

Defects in perovskite solar cells are known to affect the performance, but their precise nature, location, and role remain to be firmly established. Here, we present highly sensitive measurements of the sub-bandgap photocurrent to investigate defect states in perovskite solar cells. At least two defect states can be identified in p-i-n perovskite solar cells that employ a polytriarylamine hole transport layer and a fullerene electron transport layer. By comparing devices with opaque and semi-transparent back contacts, we demonstrate the large effect of optical interference on the magnitude and peak position in the sub-bandgap external quantum efficiency (EQE) in perovskite solar cells. Optical simulations reveal that defects localized near the interfaces are responsible for the measured photocurrents. Using optical spacers of different lengths and a mirror on top of a semi-transparent device, allows for the precise manipulation of the optical interference. By comparing experimental and simulated EQE spectra, we show that sub-bandgap defects in p-i-n devices are located near the perovskite-fullerene interface.