Jarla Thiesbrummel

Abstract:

All-perovskite tandem solar cells provide high power conversion efficiency at a low cost^1-4. Rapid efficiency improvement in small-area (<0.1 cm²) tandem solar cells has been primarily driven by advances in low-bandgap (approximately 1.25 eV) perovskite bottom subcells^5-7. However, unsolved issues remain for wide-bandgap (> 1.75 eV) perovskite top subcells⁸, which at present have large voltage and fill factor losses, particularly for large-area (>1 cm²) tandem solar cells. Here we develop a self-assembled monolayer of (4-(7H-dibenzo[c,g]carbazol-7-yl)butyl)phosphonic acid as a hole-selective layer for wide-bandgap perovskite solar cells, which facilitates subsequent growth of high-quality wide-bandgap perovskite over a large area with suppressed interfacial non-radiative recombination, enabling efficient hole extraction. By integrating (4-(7H-dibenzo[c,g]carbazol-7-yl)butyl)phosphonic acid in devices, we demonstrate a high open-circuit voltage (V_OC) of 1.31 V in a 1.77-eV perovskite solar cell, corresponding to a very low V_OC deficit of 0.46 V (with respect to the bandgap). With these wide-bandgap perovskite subcells, we report 27.0% (26.4% certified stabilized) monolithic all-perovskite tandem solar cells with an aperture area of 1.044 cm². The certified tandem cell shows an outstanding combination of a high V_OC of 2.12 V and a fill factor of 82.6%. Our demonstration of the large-area tandem solar cells with high certified efficiency is a key step towards scaling up all-perovskite tandem photovoltaic technology.

Abstract:

Mobile ions in perovskite photovoltaic devices can hinder performance and cause degradation by impeding charge extraction and screening the internal field. Accurately quantifying mobile ion densities remains a challenge and is a highly debated topic. We assess the suitability of several experimental methodologies for determining mobile ion densities by using drift-diffusion simulations. We found that charge extraction by linearly increasing voltage (CELIV) underestimates ion density, but bias-assisted charge extraction (BACE) can accurately reproduce ionic lower than the electrode charge. A modified Mott-Schottky (MS) analysis at low frequencies can provide ion density values for high excess ionic densities, typical for perovskites. The most significant contribution to capacitance originates from the ionic depletion layer rather than the accumulation layer. Using low-frequency MS analysis, we also demonstrate light-induced generation of mobile ions. These methods enable accurate tracking of ionic densities during device aging and a deeper understanding of ionic losses.

Abstract:

In this work, we couple theoretical and experimental approaches to understand and reduce the losses of wide bandgap Br-rich perovskite pin devices at open-circuit voltage (V_OC) and short-circuit current (J_SC) conditions. A mismatch between the internal quasi-Fermi level splitting (QFLS) and the external V_OC is detrimental for these devices. We demonstrate that modifying the perovskite top-surface with guanidinium-Br and imidazolium-Br forms a low-dimensional perovskite phase at the n-interface, suppressing the QFLS-V_OC mismatch, and boosting the V_OC. Concurrently, the use of an ionic interlayer or a self-assembled monolayer at the p-interface reduces the inferred field screening induced by mobile ions at J_SC, promoting charge extraction and raising the J_SC. The combination of the n- and p-type optimizations allows us to approach the thermodynamic potential of the perovskite absorber layer, resulting in 1 cm² devices with performance parameters of V_OCs up to 1.29 V, fill factors above 80% and J_SCs up to 17 mA/cm², in addition to a thermal stability T₈₀ lifetime of more than 3500 h at 85 °C.