Exploration of the compositional space for mixed lead halogen perovskites for high efficiency solar cells

Lead halide perovskites have attracted considerable interest as photoabsorbers in PV-applications over the last few years. The most studied perovskite material achieving high photovoltaic performance has been methyl ammonium lead iodide, CH3NH3PbI3. Recently the highest solar cell efficiencies have, however, been achieved with mixed perovskites where iodide and methyl ammonium partially have been replaced by bromide and formamidinium. In this work, the mixed perovskites were explored in a systematic way by manufacturing devices where both iodide and methyl ammonium were gradually replaced by bromide and formamidinium. The absorption and the emission behavior as well as the crystallographic properties were explored for the perovskites in this compositional space. The band gaps as well as the crystallographic structures were extracted. Small changes in the composition of the perovskite were found to have a large impact on the properties of the materials and the device performance. In the investigated compositional space, cell efficiencies, for example, vary from a few percent up to 20.7%. From the perspective of applications, exchanging iodide with bromide is especially interesting as it allows tuning of the band gap from 1.5 to 2.3 eV. This is highly beneficial for tandem applications, and an empirical expression for the band gap as a function of composition was determined. Exchanging a small amount of iodide with bromide is found to be highly beneficial, whereas a larger amount of bromide in the perovskite was found to cause intense sub band gap photoemission with detrimental results for the device performance. This could be caused by the formation of a small amount of an iodide rich phase with a lower band gap, even though such a phase was not observed in diffraction experiments. This shows that stabilizing the mixed perovskites will be an important task in order to get the bromide rich perovskites, which has a higher band gap, to reach the same high performance obtained with the best compositions.

Goldschmidt's Rules and Strontium Replacement in Lead Halogen Perovskite Solar Cells: Theory and Preliminary Experiments on CH3NH3SrI3

Ulf Anders Hagfeldt, Tor Jesper Jacobsson

During the past few years, organic lead halogen perovskites have emerged as a class of highly promising solar cell materials, with certified solar cell efficiencies now surpassing 20%. Concerns have, however, been raised about the possible environmental and legalization problems associated with a new solar cell technology based on a water-soluble lead compound. Replacing lead in the perovskite structure: with a less toxic element, without degrading the favorable photo physical properties, would therefore be of interest. In this paper, the possibility of replacing lead with other metal ions is explored by following the replacement rules of Goldschmidt together with additional quantum mechanical considerations. This analysis provides a conceptual toolbox toward replacing lead, as well as additional insights into the photo physics of the metal halogen perovskites. This approach is exemplified by focusing on strontium in particular, which is nontoxic and relatively inexpensive. The ionic radius of Sr2+ and Pb2+ are almost identical, suggesting an exchange could be made without affecting the crystal structure. Couple cluster calculations on the metal ions and their halogen salts give the bonding patterns to be sufficiently similar and density functional theory (DFT) revealed the strontium perovskite, CH3NH3SrI3, to be a stable phase, despite the difference in electronegativity between lead and strontium. This is further supported by the existence of binary PM, and SrI2 compounds and the beneficial formation energy of the strontium perovskite. The electronic properties of both CH3NH3SrI3 and CH3NH3PbI3 were simulated and compared, revealing a higher degree of ionic interaction in the metal halogen bound in the strontium perovskite. This is a consequence of the lower electronegativity of strontium, which, together with the lack of d-orbitals in the Valence of Sr2+, results, in a higher band gap. The band gap for the strontium perovskite was estimated to 3.6 eV, which unfortunately is too high for an efficient photo absorber. Initial investigations on experimental synthesis of the strontium perovskite, using wet chemical methods, revealed it to be harder to produce than the lead perovskite This is explained as a:consequence of different bonding patterns in the metal iodine salts, which obstruct the methylammonium intercalation pathway utilized for forming the perovskite. Vapor phase methods are instead suggested as more promising synthesis routes.

American Chemical Society2015

Lead-Free MA(2)CuCl(x)Br(4-x), Hybrid Perovskites

Michael Graetzel, Nripan Mathews

Despite their extremely good performance in solar cells with efficiencies approaching 20% and the emerging application for light-emitting devices, organic-inorganic lead halide perovskites suffer from high content of toxic, polluting, and bioaccumulative Pb, which may eventually hamper their commercialization. Here, we present the synthesis of two-dimensional (2D) Cu-based hybrid perovskites and study their optoelectronic properties to investigate their potential application in solar cells and light-emitting devices, providing a new environmental-friendly alternative to Pb. The series (CH3NH3)(2)CuClxBr4-x, was studied in detail, with the role of Cl found to be essential for stabilization. By exploiting the additional Cu d-d transitions and appropriately tuning the Br/C1 ratio, which affects ligand-to-metal charge transfer transitions, the optical absorption in this series of compounds can be extended to the near-infrared for optimal spectral overlap with the solar irradiance. In situ formation of Cu+ ions was found to be responsible for the green photoluminescence of this material set. Processing conditions for integrating Cu-based perovskites into photovoltaic device architectures, as well as the factors currently limiting photovoltaic performance, are discussed: among them, we identified the combination of low absorption coefficient and heavy mass of the holes as main limitations for the solar cell efficiency. To the best of our knowledge, this is the first demonstration of the potential of 2D copper perovskite as light harvesters and lays the foundation for further development of perovskite based on transition metals as alternative lead-free materials. Appropriate molecular design will be necessary to improve the material's properties and solar cell performance filling the gap with the state-of-the-art Pb-based perovskite devices.

American Chemical Society2016

Unreacted PbI2 as a Double-Edged Sword for Enhancing the Performance of Perovskite Solar Cells

Marine Eva Fedora Bouduban, Juan-Pablo Correa-Baena, Ulf Anders Hagfeldt, Elham Halvani Anaraki, Tor Jesper Jacobsson, Jacques-Edouard Moser, Kurt Schenk, Joël Teuscher, Wolfgang Richard Tress

Lead halide perovskites have over the past few years attracted considerable interest as photo absorbers in PV applications with record efficiencies now reaching 22%. It has recently been found that not only the composition but also the precise stoichiometry is important for the device performance. Recent reports have, for example, demonstrated small amount of PbI2 in the perovskite films to be beneficial for the overall performance of both the standard perovskite, CH3NH3PbI3, as well as for the mixed perovskites (CH3NH3)x(CH(NH2)2)(1–x)PbBryI(3–y). In this work a broad range of characterization techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), photo electron spectroscopy (PES), transient absorption spectroscopy (TAS), UV–vis, electroluminescence (EL), photoluminescence (PL), and confocal PL mapping have been used to further understand the importance of remnant PbI2 in perovskite solar cells. Our best devices were over 18% efficient, and had in line with previous results a small amount of excess PbI2. For the PbI2-deficient samples, the photocurrent dropped, which could be attributed to accumulation of organic species at the grain boundaries, low charge carrier mobility, and decreased electron injection into the TiO2. The PbI2-deficient compositions did, however, also have advantages. The record Voc was as high as 1.20 V and was found in PbI2-deficient samples. This was correlated with high crystal quality, longer charge carrier lifetimes, and high PL yields and was rationalized as a consequence of the dynamics of the perovskite formation. We further found the ion migration to be obstructed in the PbI2-deficient samples, which decreased the JV hysteresis and increased the photostability. PbI2-deficient synthesis conditions can thus be used to deposit perovskites with excellent crystal quality but with the downside of grain boundaries enriched in organic species, which act as a barrier toward current transport. Exploring ways to tune the synthesis conditions to give the high crystal quality obtained under PbI2-poor condition while maintaining the favorable grain boundary characteristics obtained under PbI2-rich conditions would thus be a strategy toward more efficiency devices.

American Chemical Society2016

Goldschmidt's Rules and Strontium Replacement in Lead Halogen Perovskite Solar Cells: Theory and Preliminary Experiments on CH3NH3SrI3

Ulf Anders Hagfeldt, Tor Jesper Jacobsson

American Chemical Society2015

Unreacted PbI2 as a Double-Edged Sword for Enhancing the Performance of Perovskite Solar Cells

Marine Eva Fedora Bouduban, Juan-Pablo Correa-Baena, Ulf Anders Hagfeldt, Elham Halvani Anaraki, Tor Jesper Jacobsson, Jacques-Edouard Moser, Kurt Schenk, Joël Teuscher, Wolfgang Richard Tress

American Chemical Society2016