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The certified power conversion efficiency of state-of-the-art organic–inorganic hybrid perovskite solar cells has surpassed 25%, showing promising potential for commercialization. Compared with vol...

One of the organic components in the perovskite photo-absorber, the methylammonium cation, has been suggested to be a roadblock to the long-term operation of organic–inorganic hybrid perovskite-based solar cells. In this work we systematically explore the crystallographic and optical properties of the compositional space of mixed cation and mixed halide lead perovskites, where formamidinium (FA+) is gradually replaced by cesium (Cs+), and iodide (I−) is substituted by bromide (Br−), i.e., CsyFA1−yPb(BrxI1−x)3. Higher tolerance factors lead to more cubic structures, whereas lower tolerance factors lead to more orthorhombic structures. We find that while some correlation exists between the tolerance factor and structure, the tolerance factor does not provide a holistic understanding of whether or not a perovskite structure will fully form. By screening 26 solar cells with different compositions, our results show that Cs1/6FA5/6PbI3 delivers the highest efficiency and long-term stability among the I-rich compositions. This work sheds light on the fundamental structure–property relationships in the CsyFA1−yPb(BrxI1−x)3 compositional space, providing vital insight to the design of durable perovskite materials. Our approach provides a library of structural and optoelectronic information for this compositional space.

One of the organic component in the perovskite photo-absorber, the methylammonium cation, has been suggested to be a roadblock to long-term operation of organic-inorganic hybrid perovskite-based solar cells. Methylammonium-free perovskites thus represent a possible direction for more stable photo-absorbers that are also compatible with multijunction solar cells. However, most work on methylammonium-free perovskites involves cesium and formamidinium as the A-site cations, which are thermodynamically less stable than the methylammonium-based materials. In this work we systematically explore the crystallographic and optical properties of the compositional space of mixed cation and mixed halide lead perovskites, where formamidinium (FA+) is gradually replaced by cesium (Cs+), and iodide (I-) is substituted by bromide (Br-), i.e., CsyFA1–yPb(BrxI1–x)3. The crystal phases, which could be tuned by changing the tolerance factor for mixed perovskite alloys, are qualitatively determined and the composition–structure relationship is established in the CsyFA1–yPb(BrxI1–x)3 compositional space. We find that higher tolerance factors lead to more cubic structures, whereas lower tolerance factors lead to more orthorhombic. We also find that while some correlation exists between tolerance factor and structure, tolerance factor does not provide a holistic understanding of whether a perovskite structure will fully form. Given the wide range of bandgaps produced by this compositional space, an empirical expression is devised to predict the optical bandgap of CsyFA1–yPb(BrxI1–x)3 perovskites – which changes as a function of composition –, conducive to the design of absorbers with bandgaps tailor-made for specific tandem and single-junction applications. By screening 26 solar cells with different compositions, we find that Cs1/6FA5/6PbI3 delivers the highest efficiency and long-term stability among I-rich compositions. This work sheds light on the fundamental structure-property relationships in the CsyFA1–yPb(BrxI1–x)3 compositional space, providing vital insight to the design of durable perovskite materials. Our approach provides a library of structural and optoelectronic information of this compositional space.

The impact on device stability of the bulky cation-modified interfaces in halide perovskite solar cells is not well-understood. We demonstrate the thermal instability of the bulky cation interface layers used in some of the highest performing solar cells to date. X-ray photoelectron spectroscopy and synchrotron-based grazing incidence X-ray scattering measurements reveal significant changes under thermal stress in the chemical composition and structure at the surface of these films. The changes impact charge carrier dynamics and device operation, as shown in transient photoluminescence, excitation correlation spectroscopy, and solar cells. The type of cation used for passivation affects the extent of these changes, where long carbon chains provide more stable interfaces and thus longer durability (more than 1000 hrs at 55ºC). Such findings highlight that annealing the treated interfaces before characterization is critical to enable reliable reporting of performances and to drive the selection between different cations.

The impact on device stability of the bulky cation-modified interfaces in halide perovskite solar cells is not well-understood. We demonstrate the thermal instability of the bulky cation interface layers used in some of the highest performing solar cells to date. X-ray photoelectron spectroscopy and synchrotron-based grazing incidence X-ray scattering measurements reveal significant changes under thermal stress in the chemical composition and structure at the surface of these films. The changes impact charge carrier dynamics and device operation, as shown in transient photoluminescence, excitation correlation spectroscopy, and solar cells. The type of cation used for passivation affects the extent of these changes, where long carbon chains provide more stable interfaces and thus longer durability (more than 1000 hrs at 55oC). Such findings highlight that annealing the treated interfaces before characterization is critical to enable reliable reporting of performances and to drive the selection between different cations.

A small amount of alkali metals goes a long way toward improving hybrid perovskites for hot carrier solar cells., Successful implementation of hot carrier solar cells requires preserving high carrier temperature as carriers migrate through the active layer. Here, we demonstrated that addition of alkali cations in hybrid organic-inorganic lead halide perovskites led to substantially elevated carrier temperature, reduced threshold for phonon bottleneck, and enhanced hot carrier transport. The synergetic effects from the Rb, Cs, and K cations result in ~900 K increase in the effective carrier temperature at a carrier density around 1018 cm−3 with an excitation 1.45 eV above the bandgap. In the doped thin films, the protected hot carriers migrate 100 s of nanometers longer than the undoped sample as imaged by ultrafast microscopy. We attributed these improvements to the relaxation of lattice strain and passivation of halide vacancies by alkali cations based on x-ray structural characterizations and first principles calculations.

Organic-inorganic halide perovskites have been identified as favorable candidates for the next generation of photovoltaics. Adding alkali metal halides to perovskite films has been shown to be a viable option to improve the perovskite film quality and to modulate their fundamental properties. In this work, we perform optical and electron-beam based characterizations of mixed Sn/Pb based perovskite films to investigate the effect of the addition of the alkaline metal halide SrI2. By analyzing structural (X-ray diffraction), morphological (Scanning Electron Microscopy), optical (photoluminescence), and chemical properties (X-ray photoelectron spectroscopy), we show a complex interplay of effects upon addition of Sr2+ into the perovskite solution. Low concentrations of Sr2+ increases lattice strain, which hints at incorporation of the additive into the perovskite lattice and improves the film optoelectronic properties. As the additive concentration increases beyond 0.5 mol %, microstrain decreases. At concentrations >0.5 mol %, Sr2+ induces significant reduction of the average domain size, which impacts both structural and optical properties of the perovskite film.

Halide perovskites are a strong candidate for the next generation of photovoltaics. Chemical doping of halide perovskites is an established strategy to prepare the highest efficiency and most stable perovskite-based solar cells. In this study, we unveil the doping mechanism of halide perovskites using a series of alkaline earth metals. We find that low doping levels enable the incorporation of the dopant within the perovskite lattice, whereas high doping concentrations induce surface segregation. The threshold from low to high doping regime correlates to the size of the doping element. We show that the low doping regime results in a more n-type material, while the high doping regime induces a less n-type doping character. Our work provides a comprehensive picture of the unique doping mechanism of halide perovskites, which differs from classical semiconductors. We proved the effectiveness of the low doping regime for the first time, demonstrating highly efficient methylammonium lead iodide based solar cells in both n-i-p and p-i-n architectures.

Lead halide perovskites (LHPs) are promising semiconductors for optoelectronic applications. In LHP solar cells, the focus thus far has been mainly on compositional optimization of the MHP layer, without much understanding of the effects of compositional mixing on structure and texture. This is a serious gap in our knowledge because research has shown that texture underlies the mechanisms of charge carrier and ionic transport. Therefore, it is essential to understand the mechanisms that drive changes in texture in LHPs. This work examines the effect of moisture and composition on the structure and texture of LHPs and their impacts on optoelectronic properties. Exposure to moisture is shown to induce a crystallographic reorientation in the polycrystalline films, which is also dependent on the amount of organic cation material present at the surface. For films with an excess of organic halide, moisture was shown to induce texture in the (001) plane contributing to the enhancement of photocurrents and long-Term device stability. This work shows the importance of texture for the electronic properties of LHPs with a special emphasis on charge carrier extraction in optoelectronic devices.

Alkali addition in organic–inorganic perovskite has become the standard recipe for achieving solar cells with efficiencies exceeding 20%, but the mechanism is not well understood. We use non-contact carrier lifetime measurements, mobility measurements, and synchrotron-based x-ray characterization techniques to show that there is a unique benefit to adding hybrid perovskite samples with Rb and Cs simultaneously. When either Rb or Cs is added, charge carrier mobility increases with alkali concentration. Charge carrier lifetime benefits from alkali incorporation as well, but is optimized with only moderate concentration at 1%. When both Rb and Cs are introduced, however, the high mobility is maintained and the charge carrier lifetime increases considerably. Our results show that when incorporated alone, Rb and Cs have very similar roles in a perovskite crystal, but when co-added, halide distribution becomes homogenized correlating with improved charge transport properties.