Lead-Tin Perovskites for Narrow Bandgap Photovoltaics

To meet the rising demand of sustainable and carbon-neutral energy systems, renewable energy technologies are being intensively investigated and deployed at larger scales. Among these, the use of solar energy has gained widespread adaption, primarily through silicon-based photovoltaics. However, further improvements in efficiency and versatility in substrate choice remain essential to meet the growing energy needs, especially in densely populated areas. Metal-halide perovskites have emerged as promising materials for photovoltaics because of their favorable optoelectronic properties, low-cost fabrication, and compatibility with lightweight and flexible substrates. Their ionic nature enables facile compositional engineering, allowing for tuning the bandgap across a wide spectral range. This dissertation focuses on lead-tin halide perovskites, a particularly interesting class of semiconductors because their bandgap depends in a non-linear way on the lead to tin metal-ion ratio. This enables reducing the bandgap to 1.25 eV, i.e., below the bandgap that can be achieved with only lead or only tin metal ions. Chapter 1 gives an introduction into the field of metal-halide perovskites, focusing on the challenges around lead-tin and mixed-halide systems. The inclusion of tin in lead perovskites introduces significant challenges such as rapid oxidation of Sn2+ to Sn4+, which leads to degradation and inhomogeneous crystallization of the perovskite. Additives are commonly used to increase the performance of metal-halide perovskite solar cells, but detailed information on the origin of the beneficial outcome is often lacking. In Chapter 2, the effect of glycine hydrochloride as an additive to the precursor solution is investigated. By combining the characterization of the photovoltaic performance and stability under illumination, with determining the quasi-Fermi level splitting (QFLS), time-resolved microwave conductivity, and morphological and elemental analysis a comprehensive insight is obtained. Glycine hydrochloride is able to retard the oxidation in the precursor solution and at low concentrations it improves the grain size distribution and crystallization of the perovskite, causing a smoother and more compact layer, reducing non-radiative recombination, and enhancing the lifetime of photogenerated charges. These improve the photovoltaic performance and have a positive effect on stability. By determining the QFLS on perovskite layers without and with charge transport layers it is found that glycine hydrochloride primarily improves the bulk of the perovskite layer and does not contribute significantly to passivation of the interfaces of the perovskite with either the hole or electron transport layer (ETL). Besides the bulk, also the perovskite/ETL interface is known to cause high non-radiative recombination losses, thereby lowering the open-circuit voltage (VOC) of the solar cells. Interface passivation can reduce these losses, and improve the power conversion efficiency (PCE). A wide variety of molecules has been successfully applied in literature, resulting in a gain in VOC. However, this gain is often associated with a loss of stability. In Chapter 3, we systematically investigate the structure-property relationships of alkane ammonium iodide interface passivators with different alkane linker lengths, different substitution, and varying the number of ammonium groups for the narrow bandgap perovskite composition. The impact of these passivators on interfacial recombination, charge extraction, and device stability is studied by tracking the QFLS of the intrinsic perovskite and of perovskite/C₆₀ bilayers, together with recording the VOC of complete devices over one month. All tested passivators reduce the non-radiative recombination losses at the perovskite/C₆₀ interface, but the extent to which this translates into improved photovoltaic performance strongly depends on the molecular structure. Short-chain primary diammonium ion passivators provide the most favorable balance between effective passivation and charge extraction, yielding QFLS values that closely match the device VOC. In contrast, extended and branched multi-ammonium ion passivators improve photovoltage stability but impede charge carrier transport, leading to reduced fill factors (FF) and short-circuit current densities (JSC). Combining smaller and larger passivators lifts the trade-off between efficiency and stability, and enables simultaneous enhancement of QFLS, VOC, FF, and operational stability. ETLs are essential in perovskite solar cells to easily extract electrons from the perovskite layer towards the contact and to repel holes to prevent charge recombination. The most efficient lead-tin perovskite solar cells use Buckminsterfullerene (C60) as ETL, despite the deep trap states that are known to form at the perovskite/C60 interface. In Chapter 4, properties such as charge extraction, non-radiative recombination losses, and device stability are analyzed by means of absolute photoluminescence, transient photovoltage, and device performance using fullerene derivatives as ETL. While all fullerene derivatives evaluated resulted in reduced non-radiative recombination, the hysteresis in the current density – voltage characteristics due to mobile ions increased and the stability of the solar cells under continuous operation was compromised compared to cells that use C60 as ETL. A newly synthesized fullerene derivative that incorporates a functional group to interact with perovskite, offers the perspective of an increased VOC without affecting the stability when used as an interlayer between the perovskite and C60 layers. In Chapter 5, an amorphous-silicon (a-Si) solar cell is combined with a narrow bandgap perovskite solar cell to fabricate a first a-Si/perovskite thin-film monolithic tandem cell. The main challenge in fabricating this tandem cell is the recombination junction which must be transparent to light and allow for the loss-free recombination of electrons and holes from adjacent sub-cells. Hence, an Ohmic contact must be established between the electron collecting contact of the a-Si sub-cell and the hole collecting layer of the perovskite sub-cell. First, in an explorative study, the wetting of the aqueous poly(3,4-ethylenedioxy¬thiophene):poly(styrene sulfonate) (PEDOT:PSS) dispersion on the a-Si sub-cell was improved to enable conformal coverage of the PEDOT:PSS hole-transporting layer. The strong s-shape behavior observed in the current density – voltage characteristics of the tandem cell required modifying the recombination junction. An ultrathin gold layer (1 nm) was found to improve the Ohmic character of the recombination junction and reduce the s-shape and hysteresis. The champion device achieved a PCE of 7.1% and VOC of 1.5 V, improving the performance over the corresponding single-junction a-Si devices. Incorporating bromide into metal-iodide perovskites is a commonly used approach for widening the bandgap of lead-halide perovskites. In Chapter 6, mixing of iodide and bromide is explored in narrow bandgap lead-tin perovskites to create a Cs0.1FA0.6MA0.3Pb0.5Sn0.5I2.5Br0.5 perovskite composition, achieving the optimal bandgap of 1.34 eV for single-junction solar cells. Introducing bromide into the precursor solution, markedly influenced film formation and resulted in singular 40 μm-sized perovskite crystals. Supported by in-situ absorption measurements, it is found that the delay time between starting the spin-coating of the perovskite precursor and depositing the antisolvent is key in controlling the film morphology. By drastically reducing this delay time, homogenous nucleation is induced and smooth closed films are obtained. The perovskite did not show signs of light-induced halide segregation during prolonged illumination. Using ammonium thiocyanate (NH4SCN) as additive in the precursor solution, the grain size could be further controlled. In solar cells, NH4SCN improved reproducibility and decreased hysteresis is observed. Applying passivation to reduce non-radiative recombination at the perovskite-ETL interface and optimizing the device configuration results in a PCE of 19.0%. This is among the highest for perovskites in the 1.3 − 1.4 eV bandgap range reported to date. All-perovskite multi-junction solar cells offer a compelling pathway towards PCEs beyond the single-junction limit, yet their further development is constrained by the instability of the wide-bandgap mixed-halide lead perovskite top-cell. Mixed iodide-bromide perovskites suffer from light-induced halide segregation, leading to performance losses. Partial substitution of lead with tin has been found as an effective strategy to suppress phase segregation and ion migration, but the crystallization behavior and optoelectronic quality remain insufficiently understood. In Chapter 7, a systematic investigation of mixed-metal, mixed-halide Cs0.1FA0.6MA0.3Pb0.5Sn0.5I3−xBrx perovskite compositions is presented in which the bromide concentration was increased in steps of x = 0.5 to go from a bandgap of 1.26 to 2.07 eV. Through a combination of in-situ absorption spectroscopy, morphological analysis, and photoluminescence characterization, we found that from x = 1.5 onwards, the rate of formation of the perovskite phase during thermal annealing becomes noticeable slower and that initially phases with wider bandgap are formed. Simultaneously, this is the onset for surface defects and reduced photoluminescence. Finally, in Chapter 8, an outlook with recommendations based on the preceding chapters is given. With these results, the thesis provides further insight into lead-tin perovskites and the exploration of new compositions. By optimizing the crystallization behavior of the perovskite and analyzing the device stack, efficient devices were fabricated, underscoring the potential of mixed lead-tin perovskites for next-generation photovoltaic technologies.