Tandem solar cells that pair silicon with a metal halide perovskite are a promising option for surpassing the single-cell efficiency limit. We report a monolithic perovskite/silicon tandem with a certified power conversion efficiency of 29.15%. The perovskite absorber, with a bandgap of 1.68 electron volts, remained phase-stable under illumination through a combination of fast hole extraction and minimized nonradiative recombination at the hole-selective interface. These features were made possible by a self-assembled, methyl-substituted carbazole monolayer as the hole-selective layer in the perovskite cell. The accelerated hole extraction was linked to a low ideality factor of 1.26 and single-junction fill factors of up to 84%, while enabling a tandem open-circuit voltage of as high as 1.92 volts. In air, without encapsulation, a tandem retained 95% of its initial efficiency after 300 hours of operation.

We introduce new hole-selective contacts for next-generation perovskite photovoltaics and point to design paths for molecular engineering of perfect interfaces.

The unprecedented emergence of perovskite‐based solar cells (PSCs) has been accompanied by an intensive search of suitable materials for charge‐selective contacts. For the first time a hole‐transporting self‐assembled monolayer (SAM) as the dopant‐free hole‐selective contact in p–i–n PSCs is used and a power conversion efficiency of up to 17.8% with average fill factor close to 80% and undetectable parasitic absorption is demonstrated. SAM formation is achieved by simply immersing the substrate into a solution of a novel molecule V1036 that binds to the indium tin oxide surface due to its phosphonic anchoring group. The SAM and its modifications are further characterized by Fourier‐transform infrared and vibrational sum‐frequency generation spectroscopy. In addition, photoelectron spectroscopy in air is used for measuring the ionization potential of the studied SAMs. This novel approach is also suitable for achieving a conformal coverage of large‐area and/or textured substrates with minimal material consumption and can potentially be extended to serve as a model system for substrate‐based perovskite nucleation and passivation control. Further gains in efficiency can be expected upon SAM optimization by means of molecular and compositional engineering.

We demonstrate a monolithic perovskite/CIGS tandem solar cell with a certified power conversion efficiency (PCE) of 24.2%. The tandem solar cell still exhibits photocurrent mismatch between the subcells; thus optical simulations are used to determine the optimal device stack. Results reveal a high optical potential with the optimized device reaching a short-circuit current density of 19.9 mA cm −2 and 32% PCE based on semiempirical material properties. To evaluate its energy yield, we first determine the CIGS temperature coefficient, which is at −0.38% K −1 notably higher than the one from the perovskite subcell (−0.22% K −1 ), favoring perovskite in the field operation at elevated cell temperatures. Both single-junction cells, however, are significantly outperformed by the combined tandem device. The enhancement in energy output is more than 50% in the case of CIGS single-junction device. The results demonstrate the high potential of perovskite/CIGS tandem solar cells, for which we describe optical guidelines toward 30% PCE.

The vast majority of the hole transporting materials require the use of chemical doping as an essential step for preparation of efficient perovskite solar cells. An oxidized organic hole-transporting material, obtained during a doping procedure, could potentially be one of the weak links in the device composition. It is not uncommon for the solar cell to heat up under summer sun; therefore, all device components must possess some degree of resistance to repetitive thermal stress. In the current publication, a series of oxidized hole-transporting materials have been synthesized and their long-term stability investigated. During thermal stability testing of the films, kept at 100 °C under an inert atmosphere, it was observed that oxidized HTMs start to degrade and partly revert to original unoxidized material. It is known that oxidized HTM, formed during doping, is responsible for the increased conductivity and ultimately for better efficiency of hole extraction process in the PSC device; therefore, observed instability of the oxidized HTMs in the thin films at elevated temperatures could be one of the causes of drop in conductivity reported for the doped spiro-OMeTAD. It could also potentially be one of the reasons why perovskite solar cells lose their efficiency under prolonged thermal stress.