Humidity and Moisture Degradation of Perovskite Material in Solar Cells: Effects on Efficiency

Humidity is the concentration of water vapor in air and moisture is the relatively small amount of water in a material. The two both have positive and negative influences on the development of perovskite solar cells. It is becoming increasingly clear that the effects of humidity will be a key factor to drive the commercialization of this promising new solar cell technology. The interface amid MAPbI3 and water vapor has been explored and studied by different researchers through optical absorption spectrometry, morphological and crystallographic studies. These studies have helped to demystify the complex interactions going on in the system. A particular study reported the possibility of a perovskite molecule forming a hydrate compound when exposed to H2O vapor in the dark coupled with its dissolution of MAPbI3. Some researchers have said that to improve the stability of this device in moisture. The device must be studied as a whole system rather than studying just the individual components. This review paper explores the works done on perovskite stability highlighting how humidity and moisture affect both the preparation and performance of perovskite material and perovskite solar cell.


Introduction
Recently, different kinds of perovskite solar cell (PSCs) material have been explored attributable to the promising power conversion efficiency perovskite materials and devices are showing and their affordable production costs [1]. There has been a rapid rise in the photovoltaic potential of perovskites within just 7 years. However, the challenge perovskite  [2]. A major source of degradation for perovskite is through the decomposition of chemical constituents of the material in unfavorable environmental conditions. Possible solutions for this problem have been adopted such as encapsulation techniques for the organic material. Still, the long-term solution can only be discovering new perovskite materials and better fabrication techniques that will aid stability [3]. This new hybrid perovskite structure ABX 3 (usually A=CH 3 NH 3 (MA) or NH 2 CHNH 2 (FA); B=Pb or Sn; X= Cl, Br, I or mixed halides) that has achieved as high as 22.1% power conversion efficiency with low production costs and as a result of their optical and electronic properties have also experienced drawbacks such as voltage hysteresis, light soaking, ionic migration, heavy metal lead and long term stability [4].
Humidity is the concentration of water vapor in the air and relative humidity is used to refer to the amount of humidity as a percentage. Moisture is the relatively small amount of water diffused in a material. The reason for instability of perovskite is the hygroscopic character of the organic ammonium starting material and metal salts solubility in unfavorable environments [5]. This unfavorable condition is usually during high humidity and moisture content in the atmosphere. Other possible causes of degradation are thermal decomposition,bias-induced ion migration, light-induced trap state formation and phase transition [6][7], [8]. Several recent studies have highlighted the degradation mechanisms of perovskite both intrinsically and extrinsically for various structures and materials and have suggested some ways to improve stability [9]. Faming and Mingzhen reported that the perovskite materialself-stabilitycan be improved through mixed halide and cation doping, finding inorganic charge transport materials that are moisture-resistant and modifications of PSCs interfaces can be explored to mitigate the instability of perovskite materials in humid and moist environments [1], [10]. Moisture is said to have a positive impact on MAPbI 3 at the annealing stage, but its effects in other stages and different perovskite films remain uncertain [11][12]. It was reported that moisture harmed the nucleation of perovskite films during spin coating while enhancing the crystal growth at the annealing stage for organicinorganic lead perovskite films by Gao et al [13].

Experimental
A study was performed to revealhow humidity affects the preparation of perovskite films.
To achieve this, we first distinguish how humidity affects the deposition step and test by using water vapor to control relative humidity. This was carried out to determine the dissolution and recrystallization process of Cs 2 SnI 6 . The vapor pressure was increased at a constant rate of 20pa/min to 1000pa (Relative Humidity= 58.5%) and subsequently, at a rate of 10pa/min to 1660pa. At constant pressure, crystal dissolution was allowed to occur for 20min. Before the pressure was reduced to 715pa at a constant rate of 10pa/min [16]. Xu et al.stated that the presence of moisture in perovskite films under 60% relative humidity is not problematicfor the production of very efficient perovskite solar cells [11]. Christian et al.
studiedhow MAPbI 3 and H 2 O vapor would interact without illumination and discovered the formation of (MA) 4 PbI 6 .2H 2 O. This changed the crystal structure of the material and effectively reduced the perovskite material light absorption potential [17]. According to Habisreutinger et al., irreversible decomposition of the dihydrate phase occurs faster in perovskite cells stored under illumination than the ones stored in dark conditions [18].

Results and discussion
Perovskite solar cells kept at 90% humidity showed a fast reduction from 12% to <0.7% in photovoltaic performance within just three days. Some cells kept at 50% and 0% humidity had better stability with a less than 5% loss in photovoltaic performance within 21 days [17]. Wang et al. reported in 2016 that the stability of perovskite should not be focused on perovskite material alone or the factors affecting the perovskite material. Rather efforts should be made to understand how the device as a whole system interacts regarding stability [19]. Also, they reported that perovskite film absorption edge  [25]. The β-cyclodextrin (β-CD) supramolecular macrocycle that was used helped improve the ambient condition stability of PSCs and consequently the photovoltaic performance. Furthermore, the supramolecular macrocycle β-CB also enhanced the crystalline characteristics of the perovskite films, therefore, protecting the perovskite films under from moisture defects [23]. An efficiency of 20.09% was achieved by adding this β-CD material to the perovskite film.
It was discovered that from 0%-10% relative humidity the performance of the perovskite degradation [28]. When perovskite material is exposed to H 2 O it acts as a As the perovskite is further exposed to humidity the perovskite structure takes up another water molecule and therefore a new structure is formed with two molecules of water. When the perovskite structure is in the dihydrate phase the excess water may cause the MA + to dissolve which creates an unalterable degradation mechanism of the perovskite molecule as shown in the equation above [31]. This is also in agreement with the theory of acid-base reaction proposed by Frost et al. as a likely route for the decomposition of perovskite material with moisture present and under illumination [29].
Planar methylammonium lead iodide PSCswere studied for stability at high temperature and humidity using encapsulation technique and it was discovered that encapsulated devices had decent performance at room temperature and zero humidity but then degradation was observed at higher temperature and humidity. The stability in humidity and different length of time as shown in the figure below. The result showed no substantial degradation at <50% RH, but beyond that MAPbI 3 began to degrade, with the color change observed (dark brown to yellow). Furthermore, the composites were stored at 35% RH while the MAPbI 3 composites were exposed for a day at 55% RH.
The patterns prove that at high humidity the perovskite experiences rapid hydration as seen in (d) and confirmed in (e).  Figure 3. Shows the power conversion efficiency for a heterojunction MAPb(I 1-x Br x ) 3 (x=0, 0.06, 0.20, 0.29). At 35% RH and subsequent exposure to 55% RH for performance variation analysis [34].

Conclusion
In conclusion, humidity and moist environments hurt perovskite solar cell devices performance by 0.2% and 0.12% respectively. This has been proven as detailed by all the papers review herein. This effect humidity has on perovskite can be managed and handled properly if regulatory actions are considered when fabricating these solar cell devices. With the current technology available and present knowledge about perovskite, it is best to keep explore better perovskite materials that can handle high humidity conditions. This coupled with improved fabrication techniques can be very necessary to improve the effects of humidity on perovskites.