# Opal-Like Photonic Structuring of Perovskite Solar Cells Using a Genetic Algorithm Approach
## 1 Introduction  
Crystalline Si solar cells are the most abundant technology on the photovoltaic (PV) market due to their high power conversion efficiency (PCE). The best research cell efficiency for crystalline Si to date is attributed to Amonix Inc., with a PCE of 27.6% [1]. However, due to the market being highly competitive, different routes are being investigated to cut production costs. One of them is thin film technology, which drastically reduces the amount of absorbing material. A major drawback of this approach is the high photon penetration depth around the optical bandgap of the semiconducting material, which is located at 1.08 \(\upmu\)m for Si [2]. In 2014, Solexel manufactured a thin film crystalline Si solar cell with a PCE of 21.2% [1], with this value being 6.4% lower than Amonix's device. Light management inside thin film PV technologies is consequently necessary in order to enhance the amount of light entering and eventually being absorbed within the cell. Surface texturing [3; 4; 5; 6], anti-reflective coatings [7], and use of plasmonics [8; 9] or photonic crystals [10] are some of the methods explored in the recent years in order to improve the efficiency of PV devices and increase the amount of light captured inside the photo-active layer. Our attention is focused on photonic crystals, since they enable the excitation of quasi-guided resonances, allowing an enhanced electromagnetic (EM) field concentration inside the dedicated absorbing layer [11; 12; 13; 14; 15]. These kinds of phenomena intrinsically have a very narrow bandwidth. Adjusting the geometrical properties of the photonic crystal enables the position of the resonance wavelength to be tailored within the desired range, and can thereforehelp in solving the high photon penetration depth issue occurring around the bandgap wavelength. A peculiar characteristic of quasi-guided modes is their ability to couple with external light [16]. These modes drastically affect the transmission and reflection of light within the slab, resulting in complex resonant line shapes, which are described well by the Fano interference process [17]. This concept refers to an interaction process between resonant and broadband scattering processes. Another strategy to enhance the PCE is to use a more absorbent PV material than Si, with a higher optical absorption coefficient, and consequently a shorter photon penetration depth. Among the emerging PV technologies, one can cite dye-sensitized solar cells (record PCE of 12.3% [1]), organic cells (record PCE of 17.4% [1]), and quantum dots cells (record PCE of 16.6% [1]), among others. The fastest growing emerging technology involves organic-inorganic perovskite solar cells (PSCs); starting from 2.2% PCE [18] in 2006, they reached 25.2% in 2019 [1]. Their high optical absorption coefficient, high diffusion length, and low fabrication costs are indubitable advantages drawing the attention of the PV community to these unique materials [19; 20; 21; 22]. From a degradation point of view, several studies point out the fact that the structure of methylammonium lead triiodide (\(CH_{3}NH_{3}Pbl_{3}\)), the photo-active component used in most of the PSCs studied lately, is stable against spontaneous degradation, as long as the cells remain isolated from environmental factors, such as temperature, humidity, oxygen, and intense solar irradiation. The use of filters and encapsulation strategies for the assembly of PSCs usually protects the perovskite material from these troublemakers. However, if this is not the case, the degradation of \(CH_{3}NH_{3}Pbl_{3}\) into \(Pbl_{2}\) and \(CH_{3}NH_{3}I\) can be significant, especially in presence of water. Then, the perovskite stability has been shown to be enhanced by substituting iodine with bromine or chlorine. However, the associated increase of the band gap, often exceeding 2 eV, can be detrimental in terms of accessible light absorption and consecutive power conversion [23; 24].