Symposium ES16 : Perovskite Photovoltaics and Optoelectronics

Perovskite solar cells promise to yield efficiencies beyond 30% by further improving the quality of the materials and devices. Electronic defect passivation, and suppression of detrimental charge-carrier recombination at the different device interfaces has been used as a strategy to achieve high performance perovskite solar cells.
In this presentation, I will discuss the role of electronic defects and how these can be passivated to improve charge-carrier lifetimes and to achieve high open-circuit voltages. I will discuss the characterization of 2D and 3D defects, such as grain boundaries, crystal surface defects, and precipitate formation within the films, by synchrotron-based techniques. The importance of interfaces and their contribution to detrimental recombination will also be discussed. As a result of these contributions to better understanding 2D and 3D defects, the perovskite solar cell field has been able to improve device performance. Albeit the rapid improvements in performance, there is still a need to improve these defects to push these solar cells beyond the current state-of-the-art.

Non-radiative recombination loss specially at the interfaces is a big challenge in the perovskite solar cells (PSCs) and affect the performance, stability, reproducibility of devices. Engineering the interfacial regions of the PSCs using an interface layer or additives is an effective strategy to address this issue. Our new findings report on a major breakthrough in the PSCs research. We discovered amazingly effective ways to mitigate the radiationless recombination of charge carriers at the interfaces of the perovskite with either electron transporting layer (ETL) or the hole transporting layer (HTL). In terms of ETL modification, we treat the surface of ETLs in the PSCs using SnO₂ and graphene, results in drastically improved the charge transfer properties, retarded the recombination rate and reduced the unwanted interfacial reactions at the interfaces with perovskite film. Moreover, we found that incorporation of the molecular modulators such as adamantanes either at the interface of perovskite with HTL or into the HTL solution prevents non-radiative recombination drastically, resulting in a highly efficient and stable PSC. Based on these techniques, we achieved PSCs (for both planar and mesoporous structures) with electric to power conversion efficiency (PCE) of ~22%, an open circuit voltage of up to 1245 mV, external electroluminescence yield of 2.5% and a great operational stability, which are the records for triple A-cation PSCs with respect to the band gap (1.61 eV). Our proposed approaches open up a promising route for fabrication of cost-effective solar cell and pave the way for the development and commercialization of the PSCs.

Understanding defects in perovskite films is key to improving device performance and stability. It has been hypothesized that vacancy defects enable ion migration¹, which has been implicated to cause current-voltage hysteresis and long-term material degradation. Additionally, defects at the surface of the perovskite film may affect the interfaces in a device, and interface engineering is seen as an important avenue for improving device performance. Scanning tunneling microscopy (STM) offers the ability to probe the surface of OHPs with atomic resolution, including resolving individual vacancy defects.^2-4 Scanning the same area multiple times allows for observation of dynamic events. Here, multiple types of defects were resolved and dynamic ion migration to and from the surface was imaged at the atomic scale. DFT calculations indicate vacancy defects are MABr vacancies and that vacancy defects at the surface of the film change the local work function, which has important implications for energy level alignment and charge transfer between layers in a photovoltaic device.

References
¹ J. Azpiroz, E. Mosconi, J. Bisquert and F. De Angelis, Eng. Environ. Sci. 8, 2118 (2015).
² L. She, M. Liu, D. Zhong, ACS Nano. 10, 1126 (2016).
² R. Ohmann, L.K. Ono, H.S. Kim, H. Lin, M.V. Lee, Y. Li, N.G. Park, Y.B. Qi, J. Am. Chem. Soc. 137, 16049 (2015).
⁴ Y. Liu, K. Palotas, X. Yuan, T. Hou, H. Lin, Y. Li and S.T. Lee, ACS Nano. 11 2060 (2017).

Abstract. Organic and inorganic cation alloying on the A-site in mixed halide perovskites solar cells has enabled remarkable improvements in efficiency and environmental stability. However, the added compositional complexity may lead to undesired phase segregation that detracts from optoelectronic performance. To clarify this balancing act, we assess the nanoscale chemical and electronic impacts of alkali-metal cation addition by means of synchrotron-based nanoprobe X-ray fluorescence and induced current. We find that the halide distribution homogenizes upon the addition of CsI and RbI precursors for films prepared with stoichiometric or excess lead halide. The halide homogenization coincides with long-lived charge carrier decays and spatially homogenous carrier dynamics visualized by ultrafast microscopy. At the same time, we observe Rb-rich clusters that phase separate within the film. We identify these Rb aggregates as recombination active sites using X-ray and E-beam induced current microscopy. Based on our microscopy investigations upon careful tuning of precursor stoichiometry, we will share quantitative precursor design strategies for preparing high-performance alloyed perovskite films that take advantage of the beneficial effects of alkali cations on homogenizing the lead-halide electronic backbone while minimizing secondary phase formation. These insights provide a more comprehensive understanding of the role of alkali cations in alloyed perovskite and indicate directions to further improve the performance and stability of perovskite optoelectronic devices.

Grain boundaries (GBs) are the most prominent microstructural features that play significant roles in determining the physical properties and photovoltaic functions of halide perovskite (HP) thin films. While enormous effort has been devoted to modifying the HP GBs and making them benign, the microstructures in these modified/functionalized HP thin films have been somehow random and/or uncertain. Herein, we demonstrate several unique chemical approaches to functionalize the HP GBs in a continuous, precisely-controlled manner. The key to the unprecedent success of the confocal functionalization is the strong molecular interaction between HPs grains and functionalizing agents. Microscopic characterization methods including analytical transmission electron microcopy have been employed to confirm the microstructures in our HP thin films. Combined experimental and theoretical studies have showed that the confocal functionalization of HP GBs not only leads to electronic passivation of defects, but also prevents HP grains from moisture/oxygen ingression and unfavorable phase transformation. As a result, highly efficient and stable perovskite solar cells are demonstrated. The concept of continously functionalization of the HP GBs is paving the way for developing higher-performance perovskite solar cells of the future.

We are developing effectively transparent superstrates for perovskite solar cells. These superstrates incorporate triangular-shaped grid fingers with thin transparent conductors, which mitigate shading losses associated with conventional grid fingers and/or thick TCO layers. We have investigated various types of polymers for use as superstrates in both rigid (glass-backed) and flexible applications. We have developed an electroplating process to fabricate the triangular gridlines, which improves their conductivity vs. prior silver ink printing approaches. We will present a summary of perovskite solar cells that were fabricated on these effectively transparent superstrates, including a discussion of their photovoltaic efficiency, spectral response, and LBIC mapping. The results demonstrate that effectively transparent superstrates enable an improvement in the short-circuit current density and fill factor for large-area perovskite solar cells.
Perovskite solar cells are of great interest due to their potential for low cost and high performance. One of the challenges to attaining high photovoltaic conversion efficiency, particularly for large-area cells, is the tradeoff between the optical and electrical performance of the top contact. Because perovskite absorbers and selective electrode materials provide very little lateral conductivity for current collection, a transparent conductive oxide (TCO) such as indium tin oxide (ITO) must be used for the front contact. However, TCOs offer a tradeoff between transparency and conductivity, resulting in a compromise between short-circuit current density (due to optical losses) and fill factor (due to resistive losses) for solar cells. A solution is to increase the density of the grid fingers such that thinner TCOs can be used; however, this increases the shading losses.
Recently, a method to produce effectively transparent front contact grids has been described (Adv. Optical Mater. 4 (10), 1470-1474 (2016); Photovoltaic Specialists Conference (PVSC) IEEE 43rd, 3612-3615, (2016); Sustainable Energy and Fuels, 1 (3), 593-598, (2017)). This approach yields a relatively dense array of high-aspect-ratio, triangular-shaped front contact fingers, in which light striking the metal is reflected towards the cell. Our current work pertains to the application of this technology to perovskite solar cells.

Solution processable hybrid organic-inorganic perovskites such as CH₃NH₃PbI_3-zCl_z have attracted enormous fundamental and applied interest because of their outstanding optoelectronic properties. There is considerable interest in establishing methods to control perovskite film morphology, for example, using micropatterning. Here, hydrophilic poly(N-vinylformamide)-based swellable colloidal sponge-like particles were dispersed in perovskite precursor solution which was then spin coated to deposit CH₃NH₃PbI_3-zCl_z films for the first time. Remarkably, the CH₃NH₃PbI_3-zCl_z films formed disordered inverse opal (DIO) films. Our unique approach enables micropatterning of the perovskite capping layer in a single step. The CH₃NH₃PbI_3-zCl_z pore wall thickness is shown to be controlled by the concentration of sponge-like colloid particles used. The particles not only caused CH₃NH₃PbI_3-zCl_z to be more efficiently deposited but also increased light absorption and photoluminescence intensity. Results from demonstration solar cells constructed containing the DIO CH₃NH₃PbI_3-zCl_z films are discussed. A mechanism for DIO film formation is also presented[1]. We also present recent results obtained by extending this approach to prepare semi-transparent perovskite solar cells.

[1] Dokhan et al, Phys. Chem. Chem. Phys., DOI: 10.1039/c8cp05148h.

The efficiency of perovskite solar cells has skyrocketed from 3.8% to 23.3% in the past few years. For the best MAPbI₃ perovskites, the open-circuit voltage deficit, defined as the difference between bandgap and open-circuit voltage (V_oc), is only 0.37 V, approaching other best technologies such as GaAs [1]. However, similar to III-V materials, higher bandgap perovskites tend to have larger V_oc deficits. For example, for 1.64-eV and 1.7-eV perovskites, the best V_oc deficits are 0.49 V and 0.51 V, respectively [2]. This voltage loss limits the performance of wide-bandgap perovskites, and their applications on, for instance, tandem solar cells.
The V_oc of photovoltaic devices is governed, hierarchically, by recombination in the absorber and the carrier-selective contacts of the devices. Grain engineering has been proven to be an effective way to reduce defect density and, thus, suppress recombination to enhance the V_oc [2][3]. In our work, we focus on exploring different carrier-selective contact materials for perovskites.
With inverted architecture, we have replaced PTAA hole contact with boron-doped a-Si:H material. Our first device shows a V_oc of 1.02 V on a 1.6-eV CsMAFABr perovskite, and 0.92 V on a 1.67-eV perovskite. These numbers are very close to the PTAA control devices with V_oc of ~1.1 V. Although the FF is rather poor due to shunts and possible large series resistance due to band misalignment. We are in the process of making new devices without shunts, and characterizing the band alignment between our a-Si:H hole contact and perovskite absorber. By the time we present, we believe we’ll have such information. In addition to the current-voltage measurement, we will use surface photovoltage and XPS/UPS etc technique to obtain the band information, and also use a Suns-V_oc tool that we developed to probe the series-resistance-free fill factor of the device with a-Si:H contacts. We’ll also further employ phosphorous-doped a-Si:H as an electron contact to perovskite, and perform a through analysis on that as well.

To maximize the scalable deployment of perovskite PV, we investigate low-temperature organic monolayer-based morphologies to chemically and electrically glue perovskites to their contacting phases while maintaining a “soft”, flexible interface and carrier selectivity. We have covalently grafted vertically oriented perylene dianhydride overlayers on TiO₂ through and imide linkage to a surface-bound silane. Secondary functionalization with a diamine to the terminal anhydride yields A-type cation moieties that serve as a chemical hook for perovskite deposition. UPS-quantified HOMO and LUMO levels of perylene diimide molecules aligned to facilitate electron transport to the TiO₂ substrate while blocking holes. Reflection-Absorption Infrared Spectroscopy (RAIRS) and X-ray photoelectron spectroscopy revealed a high coverage of vertically oriented perylene diimides. Perylene interfacial monolayers yield improved energy conversion relative to TiO₂/perovskite interfaces that we ascribe to enhanced interfacial carrier selectivity and transport, and reduced recombination. We discuss the present results in the context of other morphologies relevant to tandem photovoltaics.

Perovskite solar cells are surprising the photovoltaic community as unconventional behaviors have been reported. In most of the cases the origin of these behaviors is not completely understood and also, very important for the final application, how them influence the final performance of the device. On the other hand Impedance spectroscopy is a non-destructive characterization technique that can help in the understanding of these devices. Impedance spectroscopy is a characterization method in the frequency domain that allows to decouple physical processes with different characteristic times at the working conditions i.e. under illumination and applied bias. Despite the huge potentiality of this technique for the characterization of perovskite solar cell a complete model of impedance for this kind of cells applicable in all the conditions and configuration has been elusive for the moment. Undoubtedly, the combined action of electron and holes and ions in perovskite solar cell is at the base of the complex behavior observed in this kind of devices. In this talk we compare the well know dye sensitized solar cells with the perovskite solar cells highlighting similarities and differences. In addition the interest of impedance characterization of different types of perovskite solar cells is discussed.

Mobile ions in hybrid perovskite semiconductors introduce a new degree of freedom to electronic devices suggesting applications beyond photovoltaics. An intuitive device model describing the interplay between ionic and electronic charge transfer is needed to unlock the full potential of the technology. We describe the perovskite-contact interfaces as transistors which couple ionic charge redistribution to energetic barriers controlling electronic injection and recombination. This reveals an amplification factor between the out of phase electronic current and the ionic current. Our findings suggest a strategy to design thin film electronic components with large, tuneable, capacitor-like and inductor-like characteristics. The resulting simple equivalent circuit model, which we verified with time-dependent drift-diffusion simulations of measured impedance spectra, allows a general description and interpretation of perovskite solar cell behaviour.

arXiv:1805.06446

Photo-induced phase separation has been reported for a range of mixed-halide perovskites, which limits the available band-gap energies for photovoltaic applications [1,2]. An enhanced understanding of the phase separation mechanism is essential to rationalize limitations and design stable perovskite semiconductors. Up to now, phase separation and segregation has been detected by means of X-ray diffraction (XRD), photoluminescence (PL) and cathodoluminescence experiments [3,4].
During electron microscope experiments, the electron beam may cause changes in halide perovskites [4, 5] and we here show that phase separation can be induced by electron beam irradiation of the sample. Inorganic CsPb(Br_xI_1-x)₃ thin films were deposited by spin-coating and the phase separation investigated in-situ by using transmission electron microscopy (TEM). By this approach, it was possible to directly monitor the phase separation of CsPb(Br_0.5I_0.5)₃ thin films on the nanoscale into CsPbI₃ and probably PbBr₂ domains. We discuss the comparability of the interactions of electrons and photons with the halide-perovskite thin film and correlate the in-situ TEM with XRD and PL measurements of the same samples. Furthermore, we present different approaches of sample preparation for this in-situ TEM investigation.


[1] E. T. Hoke et al. “Reversible photo-induced trap formation in mixed- halide hybrid perovskites for photovoltaics”, Chem. Sci.,2015, 6, 613
[2] E. L. Unger et al. “Roadmap and roadblocks for the band gap tunability of metal halide perovskites,” J. Mater. Chem. A,2017, 5, 11401
[3] Barker et. al “Defect-Assisted Photoinduced Halide Segregation in Mixed-Halide Perovskite Thin Films”, ACS Energy Lett. 2017, 2, 1416−1424
[4] W. Li, et al. “Phase Segregation Enhanced Ion Movement in Efficient Inorganic CsPbIBr2 Solar Cells”, Adv. Energy Mater. 2017, 7, 1700946
[5] Z. Dang et al. “In Situ Transmission Electron Microscopy Study of Electron Beam-Induced Transformations in Colloidal Cesium Lead Halide Perovskite Nanocrystals”, ACS Nano 2017, 11, 2124−2132

In recent years, the organic-inorganic hybrid perovskite has gained an increasing research interest in academia for applications in thin film solar cells, due to rapidly increased efficiency (from 3.8 to 23.3% within a decade) [1], high absorption coefficient [2], low-cost fabrication process, and material availability [3]. Among the hybrid perovskites, MAPbI₃ (CH₃NH₃PbI₃) based solar cells have shown high power conversion efficiencies but with several obstacles such as thermal instability, hysteresis loss at room temperature. Therefore, the commercialization of these solar cells is still a challenge. Understanding and resolving these issues necessitate the investigation of the sample at the atomic scale to determine the underlying fundamental processes.
Here, we present the growth and experimental characterization of thin MAPbI₃ films on Au (111) under ultra-high vacuum conditions (UHV=1x10^-10 Torr). The thin films were prepared by vacuum evaporation of the precursor molecules MAI and PbI₂ with a thickness of a few monolayers (approx. 4 nm). We characterize the sample with scanning tunneling microscopy (STM), and X-ray photoelectron spectroscopy (XPS), obtaining information about the atomic structure, and chemical composition. For the electronic properties analysis, we used ultraviolet photoemission spectroscopy (UPS), and inverse photoemission spectroscopy (IPES). Our study will provide the basis for further understanding ion incorporation and stability at the atomic scale.

Mixed halide lead perovskites offer a useful strategy for continuous tuning of semiconductor bandgap. Based on their tunable optical properties these mixed halides are being considered as attractive candidates to develop single junction and multijunction tandem solar cells. The ease of halide ion exchange property poses a problem to create a tandem structure with layers of metal halide perovskites of different compositions. In order to keep the lead halide perovskite nanocrystals intact without undergoing exchange of halide ions and retain the original band structure one needs to suppress the halide ion migration across the nanocrystals. We have now successfully achieved this task by capping CsPbBr₃ and CsPbI₃ nanocrystals with PbSO₄-Oleate. Absorption measurements show that the nanocrystal assemblies maintain their identity as either CsPbBr₃ or CsPbI₃, for several days. Furthermore, we have electrophoretically deposited these assemblies as hierarchical structures on electrode surfaces and employ them in light emitting devices. The effectiveness of PbSO₄-Oleate capping of lead halide perovskite nanocrystals offers new opportunities to overcome the challenges of halide ion exchange and aid towards the tandem design of perovskite light harvesting assemblies.

Metal halide perovskites have sparked tremendous enthusiasm in the photovoltaic community with their promising performances. Currently holding a record power conversion efficiency of 23.3%, this exciting material class is approaching the mighty Si-powerhouse, concurrently opening encouraging avenues in (all-) perovsk