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Improvement of Hybrid Halide Perovskites

Today, inventing enhanced or new sources of energy has become one of the most important tasks of modern engineering science. In this endeavour, particular focus is being placed on developing more efficient and stable perovskite solar cells as they are cheaper and easier to produce than current commercial silicon-based solar cells. Perovskites are a type of solar cell which comprises a perovskite-structured compound, consisting of a hybrid organic–inorganic lead or tin halide-based material which acts as the light-harvesting active layer, such as methylammonium lead halides and all-inorganic caesium lead halide. However, there are still issues concerning the efficiency and reliability of perovskites which need to be solved before they can find large-scale application.

In a new study, researchers from the University of Missouri have now (2022) discovered a new method of enhancing the stability and performance of a particular type of perovskites. In collaboration with scientists from the University of Western Cape in South Africa and physicists at the U.S. Department of Energy’s (DOE) Argonne National Laboratory, they have developed a new type of hybrid perovskites. These are a combination of organic and inorganic semiconducting materials which are intended to form the basis of new solar cells or other electronic devices.  

The main achievement of this study consists in improving the methods for making lead halide perovskites. Previously, these thin-film perovskites were made using liquid processing with the help of solvents, which made the films susceptible to degradation when exposed to air. Also, in the course of the prior manufacturing process, one of its molecules experiences a change to its structure, which causes performance limitations in real-world operating conditions. 

To confirm the molecular structure of the perovskite the scientists used material X-ray diffraction measurements at Argonne’s Advanced Photon Source (APS), a DOE Office of Science user facility. This provided them with the opportunity to evaluate the possibilities of this functional material. They found that preventing the phase change seems to be the key to ensure improved device performance. Therefore, through maintaining a stable structure throughout the operating temperature window, they managed to develop an improved and potentially useful perovskite. 

In view of the energy crisis, improving the efficiency of perovskite solar cells has become an important task. In 2014, current–voltage (I–V) characteristics of CH3NH3PbI3 perovskite solar cells were researched using a time-dependent current response with stepwise sweeping of the bias voltage. As opposed to crystalline Si solar cells, the perovskite solar cells exhibited time-dependent current response at a given bias voltage. The current was increased with time and became steady at forward scan from short-circuit to open-circuit. On the other hand, it was witnessed to decay and saturate with time at reverse scan from open-circuit to short-circuit. Time-dependent current response eventually caused I–V hysteresis, the deviation of the contact angle from its theoretical value, depending on the scan direction and the scan rate. The crystal size of CH3NH3PbI3 and the mesoporous TiO2 (mp-TiO2) film were found to influence I–V hysteresis. The I–V hysteresis decreased as crystal size increased and in the presence of mp-TiO2. The capacitance at low frequency (0.1 to 1 Hz) was witnessed to decrease as the size of perovskite and mp-TiO2 layer thickness increased. This suggested that the origin of hysteresis depended on the capacitive characteristic of CH3NH3PbI3 and the degree of hysteresis was strongly dependent on perovskite crystal size and mesoporous TiO2 layer.

Image: SEM images of CH3NH3PbI3 perovskite grown in two-step spin coating procedure with different CH3NH3I concentration of (a) 41.94, (b) 52.42, and (c) 62.91 mM, leading to average dimension of 440, 170, and 130 nm, respectively. I–V curves measured at FS (solid line) and RS (dashed line) for the perovskite solar cell employing CH3NH3PbI3 with size of (d) 440, (e) 170, and (f) 130 nm. The voltage settling time was 200 ms, and light intensity was AM 1.5G one sun (100 mW/cm2)

Source: Hui-Seon Kim, Nam-Gyu Park/ Parameters Affecting I–V Hysteresis of CH3NH3PbI3 Perovskite Solar Cells: Effects of Perovskite Crystal Size and Mesoporous TiO2 Layer/ The Journal of Physical Chemistry Letters Volume 5, Issue 17, September 4, 2014/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

In 2018, based on first-principles calculations, scientists analysed the effects of an external electric field (E) applied along the [111] direction of the orthorhombic perovskite CH3NH3PbI3, on its electronic structure and optical properties. The results showed that the electric field strength had an effect on the band gap (Eg) of CH3NH3PbI3 (MAPbI3, MA = CH3NH3). The energy difference between the two peaks closest to the Fermi level (the thermodynamic work required to add one electron to the body) in the density of states diagram was witnessed to decrease when increasing electric field strength was applied along the [111] direction. This indicated that the covalent character actually increased between A-sites cations and I-sites anions. Both the cell volume and the final energy showed the same increasing trend. The absorption peaks were seen to move toward the visible-frequency range, with the optimal band gap of 1.1–1.45 eV and E = 0.04–0.06 eV/Å/e.

Image: Role of an external electric field on hybrid halide perovskite CH3NH3PbI3 band gaps

Source: Denghui Ji, Mula Na, Shuling Wang, Hong Zhang, Kun Zhu, CongMin Zhang & Xiuling Li/ Role of an external electric field on hybrid halide perovskite CH3NH3PbI3 band gaps/ Scientific Reports volume 8, Article number: 12492 (2018), 21 August 2018/ Open Access This is an Open Access article is distributed under the terms of the
Creative Commons Attribution 4.0 International (CC BY 4.0)

There are many advantages associated with hybrid perovskite solar cells: With the new technique, the researchers were able to prevent the phase change in the hybrid perovskite solar cell and held the affected molecule in a stable structure throughout a large temperature range. Moreover, the new technique rendered the perovskite air stable, making it appropriate for potential solar cell application. 

The study is one of the first to look at the growth method itself with the aim to boost the final performance of a device. In many cases, perovskites solar cells have become as efficient as silicon-based solar cells. Also, they are also much more versatile than silicon and can be used for a broad range of applications. There is hope that we will soon be able to harness their full potential in a broad range of applications.