Using Low Temperature Photoluminescence

molecules Article

Using Low Temperature Photoluminescence Spectroscopy to Investigate CH3NH3PbI3 Hybrid Perovskite Degradation Khaoula Jemli 1,2 , Hiba Diab 1 , Ferdinand Lédée 1,2 , Gaelle Trippé-Allard 1 , Damien Garrot 3 , Bernard Geffroy 4 , Jean-Sébastien Lauret 1 , Pierre Audebert 2, * and Emmanuelle Deleporte 1, * 1

2 3

4

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Laboratoire Aimé Cotton, Ecole Normale Supérieure de Cachan, CNRS, Université Paris-Sud, Université Paris-Saclay, Bât 505 Campus d’Orsay, 91405 Orsay, France; [email protected] (K.J.); [email protected] (H.D.); [email protected] (F.L.); [email protected] (G.T.-A.); [email protected] (J.-S.L.) Laboratoire de Photophysique et Photochimie Supramoléculaires et Macromoléculaires de l’Ecole Normale Supérieure de Cachan, 61 Avenue du Président Wilson, 94235 Cachan, France Groupe d’Etudes de la Matière Condensée (GEMaC), CNRS, Université de Versailles Saint-Quentin-en-Yvelines, Université Paris-Saclay, 45 Avenue des Etats-Unis, 78035 Versailles Cedex, France; [email protected] Laboratory of Innovation in Surface Chemistry and Nanosciences, NIMBE, CEA, CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France; [email protected] Correspondence: [email protected] (P.A.); [email protected] (E.D.); Tel.: +33-147-405-313 (P.A.); +33-169-352-024 (E.D.)

Academic Editor: J.A.A.W. Elemans Received: 25 May 2016; Accepted: 30 June 2016; Published: 8 July 2016

Abstract: Investigating the stability and evaluating the quality of the CH3 NH3 PbI3 perovskite structures is quite critical both to the design and fabrication of high-performance perovskite devices and to fundamental studies of the photophysics of the excitons. In particular, it is known that, under ambient conditions, CH3 NH3 PbI3 degrades producing some PbI2 . We show here that low temperature Photoluminescence (PL) spectroscopy is a powerful tool to detect PbI2 traces in hybrid perovskite layers and single crystals. Because PL spectroscopy is a signal detection method on a black background, small PbI2 traces can be detected, when other methods currently used at room temperature fail. Our study highlights the extremely high stability of the single crystals compared to the thin layers and defects and grain boundaries are thought to play an important role in the degradation mechanism. Keywords: CH3 NH3 PbI3 ; hybrid perovskite; solar cells; optoelectronics; degradation; photoluminescence

1. Introduction The easy synthesis by low-cost technologies of layered hybrid organic-inorganic perovskites appeared has been known for less than 30 years, and it has become more and more attractive in the optoelectronics field [1–10]. A scientific breakthrough was achieved in the photovoltaic domain in 2009 with the 3D hybrid perovskite CH3 NH3 PbI3 , thanks to several remarkable properties such as ambipolar transport properties, electron-hole diffusion lengths exceeding 1 micrometer [11,12], high mobilities, large spectral absorption from the near-infrared range [13], and efficient charge separation [14,15]. Huge and rapid progress has been made in a short period of time since 2012 [16–27], leading to the recent record of 22.1% for the power conversion efficiency [28]. Despite this runaway success, some critical points remain, such as the environmental problem of the lead use [29,30], the poor

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photostability Molecules 2016, 21,[31] 885 and the sensitivity to atmospheric moisture [32–40]. These points have absolutely 2 of 13 to be addressed to promote the use of hybrid perovskites in optoelectronic devices, such as solar cells, to be addressed to promote the use of hybrid perovskites in optoelectronic devices, such as solar cells, to be addressed to promote the use of hybrid perovskites in optoelectronic devices, such as solar cells, at at aa large large scale. scale. at a large scale. It It is is now now established established that that the the problem problem of of the the aging aging of of the the solar solar cells cells is is largely largely due due to to the the aging aging of of It is now established that the problem of the aging of the solar cells is largely due to the aging of the CH NH PbI (called MAPI hereafter) layer itself [21]. The degradation of the MAPI perovskite the CH33NH33PbI33(called MAPI hereafter) layer itself [21]. The degradation of the MAPI perovskite the CHmoisture 3NH3PbI3 (called MAPI hereafter) layer itself [21]. The degradation of the MAPI due Niu and and coworkers coworkers haveperovskite proposed due to to moisture has has been been particularly particularly studied studied recently recently [32,36–40]. [32,36–40]. Niu have proposed due to moisture has been particularly studiedinrecently [32,36–40]. Niu and coworkers have proposed aa mechanism for the degradation of MAPI, which the final products of the degradation mechanism for the degradation of MAPI, in which the final products of the degradation are are PbI PbI22 aand mechanism for the degradation of MAPI, in which the final products of the degradation are PbI2 I [36]. 2 and I2 [36]. and IFigure 2 [36]. shows aa photograph photograph of of aa 800 800nm-thick nm-thickMAPI MAPIlayer layerdeposited depositedbybyspin-coating spin-coatingonona Figure 11 shows Figure 1 shows a photograph of a 800the nm-thick MAPI layer deposited by spin-coating on a aquartz quartz substrate. Just after the deposition, sample appears grey, but only 4 days later it it appears substrate. Just after the deposition, the sample appears grey, but only 4 days later appears quartz substrate. Just after the deposition, the sample appears grey, but only 4 days later it appears completely inside the the layer layer [34]. [34]. On On the the contrary, contrary, Figure Figure 22 shows shows completely yellow yellow due due to to the the apparition apparition of of PbI PbI22 inside completely yellowofdue to the apparition of PbI2crystal inside aged the layer [34]. On the contrary, Figure 2 shows the photography a millimeter sized MAPI six months, and no trace of PbI can the photography of a millimeter sized MAPI crystal aged six months, and no trace of PbI22 can be be the photography of a millimeter sized MAPI aged six to months, and no trace of PbI2 can be detected Nevertheless, it is is crystal very important check detected by by the the naked naked eye. eye. Nevertheless, it very important to check if if traces traces of of PbI PbI22 exist exist in in this this detected by theeven naked these eye. Nevertheless, it is very because important is to check ifbelieved traces of PbI2hydrates exist in this crystal crystal or or not, not, even if if these traces traces are are very very small, small, because it it is usually usually believed that that hydrates are are crystal or not, even if these traces are very small, because it is usually believed that hydrates are harmful layers are probably harmful to to the the transport transport properties, properties, as as insulating insulating layers are probably formed formed at at the the frontier frontier between between harmful to the transport properties, as insulating layers are probably formed at the frontier between grain grain boundaries boundaries [38]. [38]. grain boundaries [38].

Figure 1. 1. Photography of of a 800 nm nm thick CH CH3NH3PbI layer deposited by spin-coating on a quartz quartz Figure PbI333 layer layer deposited deposited by by spin-coating spin-coating on on aa Figure 1. Photography Photography of aa 800 800 nm thick thick CH33NH NH33PbI quartz substrate, just after the deposition (t = 0) and 4 days after the deposition (t = 4 days). substrate, just 0 day) 4 days the deposition = 4 days). substrate, just after after the the deposition deposition (t (t = = 0) and 4and days afterafter the deposition (t = 4(tdays).

Figure 2. Photography of a millimeter sized MAPI crystal, 6 months after its growth. Figure 2. Photography of a millimeter sized MAPI crystal, 6 months after its growth. Figure 2. Photography of a millimeter sized MAPI crystal, 6 months after its growth.

Characterization means currently used to detect the presence of PbI2 are absorption spectroscopy Characterization means currently used to detect the presence of PbI2 are absorption spectroscopy and X-diffraction analysis performed at room temperature [31,32,34–37]. Recently, other methods Characterization meansperformed currently used to detect the presence of PbI2 areRecently, absorption spectroscopy and X-diffraction analysis at room temperature [31,32,34–37]. other methods have been proposed: ellipsometry [38] and in situ electrical resistance measurement [39]. We propose and X-diffraction analysis performed atand room Recently, other methods have have been proposed: ellipsometry [38] in temperature situ electrical[31,32,34–37]. resistance measurement [39]. We propose here the use of photoluminescence (PL) spectroscopy at low temperature as a powerful tool to detect been the proposed: ellipsometry [38] and in spectroscopy situ electricalat resistance measurement [39]. We tool propose here here use of photoluminescence (PL) low temperature as a powerful to detect the use presence of PbI2 traces and (PL) to finely characterize the temperature aging of MAPI layers and crystals using this the of photoluminescence spectroscopy at low as a powerful tool to detect the the presence of PbI2 traces and to finely characterize the aging of MAPI layers and crystals using this technique.ofInPbI thetraces particular case of MAPI crystals,the X-diffraction and absorption spectroscopies are presence and to finely characterize aging of MAPI layers and crystals using this 2 particular case of MAPI crystals, X-diffraction and absorption spectroscopies are technique. In the inadequate to detect small PbI 2 traces, so the only way to efficiently manage to this goal, without technique. In particular case of MAPI crystals, X-diffraction and absorption spectroscopies are inadequate to the detect small PbI 2 traces, so the only way to efficiently manage to this goal, without destroying or changing the PbI nature of the sample, is to use low temperature PL spectroscopy, which inadequate to detect small traces, the only efficiently manage this goal, without 2 destroying or changing the nature of thesosample, is way to usetolow temperature PL to spectroscopy, which is an in situor characterization method. destroying changing the nature of the sample, is to use low temperature PL spectroscopy, which is is an in situ characterization method. an in situ characterization method.

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2. Results and Discussion 2. Results and Discussion

2. Results and Discussion

2.1. Optical Properties of Thin Layers 2.1. Optical Properties of Thin Layers 2.1. Optical Properties of Thin Layers Figure 3 shows the absorption front of the 800 nm thick MAPI layer and its photoluminescence Figure 3 shows thethe absorption ofofthe 800 nmthick thickMAPI MAPI layer and photoluminescence its photoluminescence Figure 3 shows absorptionfront front the 800deposition nm layer (PL) spectrum at room temperature just after the at time tand = 0itsday. The layer presents a (PL) spectrum at room temperature just after the deposition at time t = 0. The layer presents a PL (PL) spectrum at at room temperature just after theand deposition at time 0. Thecoherent layer presents PL spectra PL spectrum centered 1.64 eV. The absorption PL spectra aret =quite with athe spectrum centered at 1.64 eV.eV.The PL spectra spectraare arequite quite coherent the spectra spectrum centered at 1.64 Theabsorption absorption and and PL coherent withwith the spectra commonly seen in the literature [21]. commonly seenseen in the literature [21]. commonly in the literature [21].

Figure 3. Absorption spectrum (blue curve) and photoluminescence spectrum (black curve) of a

Figure 3. Absorption spectrum (blue curve) and photoluminescence spectrum (black curve) of 800 nm thick MAPI layer at room temperature, taken just after the deposition by spin-coating (t = 0). a 800 nm thick MAPI layer atis room taken justinset after the deposition by spin-coating The3. excitation wavelength 3.815 eVtemperature, (325 nm,and HeCdphotoluminescence laser). The exhibits the molecular structure Figure Absorption spectrum (blue curve) spectrum (black curve) of a (t =800 0 day). The excitation wavelength is 3.815 eV (325 nm, HeCd laser). The inset exhibits the molecular of CH 3 NH 3 PbI 3 (MAPI). nm thick MAPI layer at room temperature, taken just after the deposition by spin-coating (t = 0). structure of CH3wavelength NH3 PbI3 (MAPI). The excitation is 3.815 eV (325 nm, HeCd laser). The inset exhibits the molecular structure

Figure 4a shows the evolution of the absorption front of the MAPI thin layer at room of CH3NH3PbI3 (MAPI). temperature after 4 days: the absorption front disappears when the perovskite degrades. After 4 days, Figure 4a shows front of the MAPI thintolayer at roomoftemperature a feature aroundthe 2.5evolution eV appearsof inthe the absorption absorption spectrum, corresponding the presence PbI2. Figure 4a shows the evolution of the absorption front of the MAPI thin ata room after 4 days: the absorption frontfeatures disappears the perovskite 4layer days, feature In a correlated way, two new appear when in the PL spectrum at lowdegrades. temperatureAfter shown in Figure temperature after 4 days: the absorption front disappears when the perovskite degrades. After 4 days, PL signalinbetween 1.9 eV and 2.2 eV and a narrow PL peaks around 2.4presence eV. These features around 4b: 2.5a broad eV appears the absorption spectrum, corresponding to the of PbI 2 . In a a feature around 2.5 eV appears in theobtained absorption spectrum, corresponding to the presence of PbI2. are way, quite similar to the PL spectra 15spectrum K of 60 µm 2 layers produced by the spray correlated two new features appear in theatPL atPbI low temperature shown in Figure 4b: In a correlatedtechnique way, two[41]. newWe featuresdeposited appear ina the lowbytemperature in Figure 200 PL nmspectrum thick PbI2 at layer spin-coating shown on a quartz a broad pyrolysis PL signal between 1.9 eVhave and 2.2 eV and a narrow PL peaks around 2.4 eV. These features are substrate, registered PLeV spectrum K (excitation wavelength at 325 2.4 nm,eV. HeCd laser). 4b: a broad PL and signal betweenits1.9 and 2.2 at eV10and a narrow PL peaks around These features quite similar to the PLthe spectra obtained atPbI 152 K of 60 µm2.3–2.6 PbI2 eV layers produced by the spray pyrolysis Figure 4c shows PL spectrum of the layer in the range, superimposed to the MAPI are quite similar to the PL spectra obtained at 15 K of 60 µm PbI2 layers produced by the spray technique We have a 200 thickatPbI by spin-coating on2 luminescence a quartz substrate, and 2 layer PL [41]. spectrum at t =[41]. 4deposited days. the nm PL peak 2.4 nm eV isthick characteristic of the pyrolysis technique WeClearly, have deposited a 200 PbI2 layer by PbI spin-coating on and a quartz registered its PL spectrum at 10 K (excitation can be taken as the signature of PbI 2 presence.wavelength at 325 nm, HeCd laser). Figure 4c shows substrate, and registered its PL spectrum at 10 K (excitation wavelength at 325 nm, HeCd laser).

theFigure PL spectrum thePL PbI eVin range, superimposed to the MAPI PL 2 layer in 4c showsofthe spectrum ofthe the 2.3–2.6 PbI2 layer the 2.3–2.6 eV range, superimposed to spectrum the MAPI at t =PL 4 days. Clearly, PL peak at 2.4the eVPL is peak characteristic ofcharacteristic the PbI2 luminescence and can be taken spectrum at t =the 4 days. Clearly, at 2.4 eV is of the PbI2 luminescence and as thecan signature of PbI presence. be taken as the2 signature of PbI2 presence.

Figure 4. Cont.

Figure 4. 4. Cont. Figure Cont.

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Figure Absorption at room room temperature (a); and (b) PL spectra at low temperature 10 (excitation Figure 4. 4. Absorption Absorption at at temperature (a); (a); and and (b) (b) PL PL spectra spectra at at low low temperature temperature 10 10 K K (excitation (excitation Figure 4. room temperature K at 3.815 eV for the PL) of aa 800 nm thick MAPI layer, at tt = 0 (blue day (blue line) and tdays = 4 days (black at 3.815 eV for the PL) of 800 nm thick MAPI layer, at = 0 line) and t = 4 (black line).line). The at 3.815 eV for the PL) of a 800 nm thick MAPI layer, at t = 0 (blue line) and t = 4 days (black line). The The bold line shows the spectra for MAPI* sample obtained with the powdered PbI . The PL spectra 2 bold line line shows shows the the spectra spectra for for MAPI* MAPI* sample sample obtained obtained with with the the powdered powdered PbI PbI22.. The The PL PL spectra spectra have have bold have been normalized sothe thatmaximum the maximum PL ateV 1.64 eV(c) is PL 1; (c) PL spectra atin10the K in the 2.3–2.6 eV been normalized so that PL at 1.64 is 1; spectra at 10K 2.3–2.6 eV range been normalized so that the maximum PL at 1.64 eV is 1; (c) PL spectra at 10K in the 2.3–2.6 eV range range of MAPI layer at t = 4 days and PbI . The PL spectra have been normalized so that the maximum of MAPI MAPI layer layer at at tt == 44 days days and and PbI PbI22.. The The2 PL PL spectra spectra have have been been normalized normalized so so that that the the maximum maximum PL PL of PL at 2.4 eV is 1. at 2.4 eV is 1. at 2.4 eV is 1.

This experimental ofof PbI could be be alsoalso used to determine rapidly and experimentalmethod methodofof ofdetection detection of PbI traces could be also used to determine determine rapidly 2 traces This experimental method detection PbI 22 traces could used to rapidly easily if some PbI clusters remain after a two-step deposition method, such as the one developed and easily if some PbI 2 clusters remain after a two-step deposition method, such as the one developed 2 and easily if some PbI2 clusters remain after a two-step deposition method, such as the one developed by Wu Wu et et al. al. [35], [35], for for example. example. It can also also outline outline the the influence influence of of the the precursors: precursors: for by Wu et al. [35], for example. It can can also outline the influence of the precursors: for example, example, here here we study influence of the texture of the PbI which is used to prepare the CH NH PbI layers. the influence of the texture of the PbI 2 which is used to prepare the CH 3 NH 3 PbI 3 layers. Let 2 3 3 3 we study the influence of the texture of the PbI2 which is used to prepare the CH3NH3PbI3 layers. Let Let us compare the MAPI sample prepared PbI , in forms of grains, and the MAPI* sample prepared us compare the MAPI sample prepared PbI 2 , in forms of grains, and the MAPI* sample prepared with us compare the MAPI sample prepared PbI2, in 2forms of grains, and the MAPI* sample prepared with with powdered PbI2 (same deposition method the same parameters and same thickness fortwo the powdered PbI22 (same (same deposition method with with the same same parameters and same same thickness for the the two powdered PbI deposition method with the parameters and thickness for two samples). Figure 4b shows the PL spectrum of these two samples just after the deposition samples). Figure 4b shows the PL spectrum of these two samples just after the deposition (t = 0). It samples). Figure 4b shows the PL spectrum of these two samples just after the deposition (t = 0). It (t = 0 day). It can be seen that we can detect PbI traces in MAPI* sample and no PbI traces in the can be seen that we can detect PbI 2 traces in MAPI* sample and no PbI 2 traces in the MAPI sample, 2 2 can be seen that we can detect PbI2 traces in MAPI* sample and no PbI2 traces in the MAPI sample, MAPI sample, showing that the conditioning PbI2 origin and conditioning is important obtain showing that the the choice of of thethe PbIchoice originofand and conditioning is important important to obtain obtain PbI22-free -freetosamples samples showing that choice the PbI 22 origin is to PbI PbI -free samples at t = 0 day. In the following samples presented in this paper, we will use grains of at t = 0. In the following samples presented in this paper, we will use grains of PbI 2 for the preparation. 2 at t = 0. In the following samples presented in this paper, we will use grains of PbI2 for the preparation. PbI for the preparation. Moreover, observing the Figure 5 exhibiting the photography of the MAPI* Moreover, observing the Figure 5 exhibiting the photography of the MAPI* sample, it has to be 2 Moreover, observing the Figure 5 exhibiting the photography of the MAPI* sample, it has to be sample, it has to be pointed that there is no evidence, looking naked eyes, ofof pointed out that there is no noout evidence, looking with naked naked eyes,with of the the presence ofthe PbIpresence traces of in pointed out that there is evidence, looking with eyes, of presence PbI 22 traces in PbI traces in the sample. the sample. the 2sample.

Figure 5. 5. Photograph Photograph of of the the MAPI* MAPI* thin thin layer layer at at tt == 00 day. day. Figure Figure 5. Photograph of the MAPI* thin layer at t = 0 day.

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In order to apply this method of using the PL spectroscopy at low temperature to detect traces of PbI2 ,In weorder also to studied the aging ofofMAPI layers, protectedatfrom atmospheretomoisture or ambient apply this method usingthin the PL spectroscopy low temperature detect traces of light, as also a great number of papers consider the important these two moisture parameters. Firstly, we PbI2, we studied the aging of MAPI thin layers, protected role fromofatmosphere or ambient protect MAPI layer of with a 200 nm thick polymethylmetacrylate (PMMA) layerFirstly, deposited light, asthe a great number papers consider the important role of these two parameters. we by protect the MAPI a 200thick nm thick polymethylmetacrylate (PMMA) layer deposited spinspin-coating overlayer the with 800 nm MAPI layer: the deposition parameters of the 4%bywt PMMA ˝ coating over the 800 are nm1000 thickrpm MAPI deposition of theThe 4% wt PMMA solutionsample in solution in toluene forlayer: 40 s,the annealed at 95parameters C for 20 min. PMMA protected toluene are 1000 rpm for 40 s, annealed at 95 °C for 20photographs min. The PMMA protected sample will beat called will be called MAPI-PMMA. Figure 6a,b show the of the MAPI-PMMA layer t = 0 day MAPI-PMMA. 6a,b show photographs the MAPI-PMMA = 0 and = 7 days and t = 7 days Figure respectively: thethe sample doesn’t of appear yellow, evenlayer afterat7 tdays. To tcheck if there respectively: the sample doesn’t appear yellow, even after 7 days. To check if there are some PbI traces, are some PbI2 traces, we have performed the PL spectrum at low temperature: in Figure2 6c, no PbI2 we signal have performed the PL spectrum at low 6c, no PbI2 PL can beindetected, PL can be detected, so that we cantemperature: affirm that in noFigure PbI2 formation hassignal occurred this sample so that we can affirm that no PbI2 formation has occurred in this sample after 7 days. We have done the after 7 days. We have done the experiment up to 30 days and no PbI2 traces have been detected. As a experiment up to 30 days and no PbI2 traces have been detected. As a consequence, we can conclude consequence, we can conclude the coating of the perovskite with a 200 nm PMMA layer is efficient to the coating of the perovskite with a 200 nm PMMA layer is efficient to protect from the degradation protect from the degradation of the sample due to the atmospheric moisture. We have also protected of the sample due to the atmospheric moisture. We have also protected an identical MAPI layer with an identical MAPI layer with a 300 nm thick Spiro-OMeTAD layer, which is a Hole Transport Layer a 300 nm thick Spiro-OMeTAD layer, which is a Hole Transport Layer commonly used in hybrid commonly used in hybrid perovskite solar cells. After 17 days of aging in ambient atmosphere, we can perovskite solar cells. After 17 days of aging in ambient atmosphere, we can conclude that no PbI2 conclude that has no PbI has been detected. 2 luminescence luminescence been detected.

Figure 6. t =t 0=day; (b)(b) t = t7 =days of a of 800a nm MAPIMAPI layer layer overcoated with a with Figure 6. Photographs Photographsatat(a)(a) 0 day; 7 days 800 thick nm thick overcoated 200 nm thick PMMA layer (called MAPI-PMMA); (c) PL spectra (excitation at 3.815 eV) in the 2.3–2.6 a 200 nm thick PMMA layer (called MAPI-PMMA); (c) PL spectra (excitation at 3.815 eV) in the eV range for MAPI-PMMA at t = 0 and MAPI-PMMA at t = 7 days, MAPI t = 4 days is recalled here. 2.3–2.6 eV range for MAPI-PMMA at t = 0 day and MAPI-PMMA at t = 7 days, MAPI t = 4 days The PL spectra have been normalized so that the maximum PL at 1.64 eV is 1 (I1.64 eV = 1). is recalled here. The PL spectra have been normalized so that the maximum PL at 1.64 eV is 1 (I1.64 eV = 1).

Secondly, we tried to evaluate the role of light exposition. For this, we studied the degradation of unprotected samples in the ambient light, or in the dark: we thus prepared a 800 nm thick MAPI we tried to evaluate role of light this, we studied layerSecondly, on a quartz substrate, and cutthe the sample in exposition. two pieces,For respectively stored the in adegradation dark box of unprotected samples in the ambient light, or in the dark: we thus prepared a 800 nm thick MAPI layer (MAPI’-dark), and in a transparent box (MAPI’). It can be seen in Figure 7 that after 4 days, the sample on a quartz sample in2two pieces,atrespectively stored in asample dark box (MAPI’-dark), stored in thesubstrate, black boxand hascut notthe aged (no PbI detected), the difference of the stored in the and in a transparent box (MAPI’). It can be seen in Figure 7 that after 4 days, the sample stored in the transparent box. the above PL spectra in2 Figures 6c and 7cdifference have beenof normalized eV = 1), the transparent ratio of the box. blackAs box has not aged (no PbI detected), at the the sample(I1.64 stored in the PL intensity at 2.4 eV and the PL intensity at 1.64 eV: I 2.4 eV /I 1.64 eV , is directly related to the As the above PL spectra in Figures 6c and 7c have been normalized (I1.64 eV = 1),quantity the ratioofof the theintensity degradedathybrid 8 shows theeV: values of/I this each considered sample. PL 2.4 eVperovskite. and the PLFigure intensity at 1.64 I2.4 eV directly related to the quantity 1.64ratio eV , isfor Onthe thisdegraded kind of diagram, one can readFigure easily 8the evolution of the degradation of the MAPI layers over of hybrid perovskite. shows the values of this ratio for each considered sample. time; when the degradation level is small, the PbI 2 amount generated is also small and very divided, On this kind of diagram, one can read easily the evolution of the degradation of the MAPI layers and therefore the reabsorption (by PbI 2) can be neglected and the measurements can be considered over time; when the degradation level is small, the PbI2 amount generated is also small and very as sincere; at high degradation rates this is accurate any longer, intrinsic error divided, and therefore the reabsorptionprobably (by PbI2not ) can be neglected andand thesome measurements can be might exist, however the result still display a good approximation. We read synthetically that PMMA considered as sincere; at high degradation rates this is probably not accurate any longer, and some

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intrinsic Moleculeserror 2016, might 21, 885 exist, however the result still display a good approximation. We read synthetically 6 of 13 that PMMA and Spiro-OMeTAD layers able the to protect the MAPI layerthe and that the mechanism and Spiro-OMeTAD layers are able to are protect MAPI layer and that mechanism which is and Spiro-OMeTAD are able of to protect thethe MAPI that mechanism which is which is involved in thelayers degradation MAPI and appearance PbI2 the is by activated involved in the degradation of MAPI and the appearance oflayer PbI2 and is of activated light. by light. involved in the degradation of MAPI and the appearance of PbI2 is activated by light.

Figure 7. Photographs of (a) a 800 nm thick MAPI layer stored in a dark box at t = 4 days; (b) a 800 Figure 7. Photographs of (a) a 800 nm thick MAPI layer stored in a dark box at t = 4 days; Figure 7. Photographs ofin(a)a transparent a 800 nm thick a dark(excitation box at t = at 4 days; (b) ain800 thick MAPI layer stored box,MAPI at t = 5layer days;stored (c) PLin spectra 3.815 eV) the (b) a 800 thick MAPI layer stored in a transparent box, at t = 5 days; (c) PL spectra (excitation at thick MAPI layer stored in a transparent box, at t = 5 days; (c) PL spectra (excitation at 3.815 eV) 2.3–2.6 eV range for MAPI layer in the dark box at t = 4 days (MAPI’-dark), MAPI layer in in the the 3.815 eV) eV in the 2.3–2.6 eV range for layer box(MAPI’-dark), at t = 4 days (MAPI’-dark), 2.3–2.6 range for MAPI in MAPI the boxinatthe t =dark 4been days MAPI layer in MAPI the transparent box (MAPI’) at tlayer = 5 days. Thedark PL spectra have normalized so that the maximum PL layer in the transparent box (MAPI’) at t = 5 days. The PL spectra have been normalized so that the transparent (MAPI’) at 1.64 eV isbox 1 (I1.64 eV = 1).at t = 5 days. The PL spectra have been normalized so that the maximum PL maximum PL at 1.64 eV is 1 (I = 1). 1.64 eV at 1.64 eV is 1 (I1.64 eV = 1).

Figure 8. Values of the ratio (I2.4 eV/I1.64 eV) for each considered samples. Figure Values the ratio(I(I2.4 eV/I /I1.64 eV) for each considered samples. Figure 8. 8. Values ofof the ratio 2.4 eV 1.64 eV ) for each considered samples.

2.2. Optical Properties and Stability of Millimeter Sized Crystals 2.2. Optical Properties and Stability of Millimeter Sized Crystals 2.2. Optical Properties and of Millimeterimage SizedofCrystals Figure 9a shows theStability optical microscopy a millimeter sized MAPI crystal, taken 6 months Figure 9a shows the optical microscopy image ofstorage a millimeter MAPI crystal, taken 6 months after its growth. No precautions were taken for the of thissized sample during these 6taken months: the Figure 9a shows the optical microscopy image of a millimeter sized MAPI crystal, 6 months after its growth. No precautions were taken for the storage of this sample during these 6 months: the sample has been stored in a transparent box, and left under ambient conditions. Although the growth after its growth. No precautions were taken for the storage of this sample during these 6 months: the sample has been stored a transparent box, left time, underand ambient conditions. Although the growth of this crystal has beenindone in solution forand a long despite the absence of precautions for sample has beenhas stored indone a transparent and lefttime, under ambient conditions. Although the growth of crystal been in detectable solutionbox, for long despite the absence precautions for thethis storage, no PbI 2 traces are bya naked eye. and Figure 9b shows the PL of spectrum at room of the thisstorage, crystal hasPbI been doneare in detectable solution for anaked long time, and despite the the absence of precautions traces Figure 9b shows PL spectrum at roomfor temperatureno of the 2millimeter sized crystalbyshown ineye. Figure 9a, under different excitation conditions: thetemperature storage, noofPbI traces are detectable by naked eye. Figure 9b shows the PL spectrum at room the2 millimeter sized shown in Figure 9a,aunder excitation conditions: a continuous excitation at 3.815 eVcrystal (325 nm) obtained with HeCddifferent laser and a quasi-continuous temperature of the millimeter sized shown in Figure 9a, under different conditions: aexcitation continuous at nm) 3.815 eVcrystal (325 nm) obtained with a HeCd laser andFor aexcitation quasi-continuous at excitation 3.1 eV (400 with a doubled-frequency pulsed Ti-Sa laser. both considered a continuous excitation at 3.815 eV (325 nm) obtained with a HeCd laser and a quasi-continuous excitation 3.1 eV (400 nm) withofa the doubled-frequency pulsed Ti-Sa atlaser. For both considered excitation at conditions, the emission crystal was recorded centered the same wavelength than excitation atconditions, 3.1previously eV (400 withMAPI doubled-frequency Ti-Sa laser. For both considered excitation thenm) emission ofa the crystal wasIn recorded the same wavelength than the one of the studied thin layers. orderpulsed tocentered check if at PbI 2 traces can be detected in excitation conditions, the emission of the crystal was recorded centered at the same wavelength than the one of the previously studied MAPI thin layers. In order to check if PbI 2 traces can be detected in this monocrystal, the PL spectrum has been done at low temperature: in Figure 10, some extremely this monocrystal, the PL spectrum has been done at low temperature: in Figure 10, some extremely

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the one of the previously studied MAPI thin layers. In order to check if PbI2 traces can be detected in Molecules 2016, 21,the 885 PL spectrum has been done at low temperature: in Figure 10, some extremely 7 of 13 this monocrystal, 2016, 21,2885 7 of 13some smallMolecules traces of PbI can be detected. In a crystal, it is difficult to extract quantitative data because small traces of PbI2 can be detected. In a crystal, it is difficult to extract quantitative data because some reabsorption of the PbI2 luminescence can occur, but at least, we can affirm that small PbI2 traces are reabsorption the PbI2 luminescence can occur, but at least, we can affirm that small PbI2 traces are small traces ofofPbI present in the sample.2 can be detected. In a crystal, it is difficult to extract quantitative data because some present in the sample. reabsorption of the PbI2 luminescence can occur, but at least, we can affirm that small PbI2 traces are present in the sample.

Figure 9. (a) Microscopy image of a millimeter sized MAPI crystal; (b) PL spectrum at room temperature

Figure 9. (a) Microscopy image of a millimeter sized MAPI crystal; (b) PL spectrum at room temperature of this MAPI crystal, taken six of months after its growth, under continuous excitation at Eexc = 3.815 eV 9. crystal, (a) Microscopy a millimeter MAPI crystal; (b) PL spectrum at room of thisFigure MAPI taken image six months after itssized growth, under continuous excitation attemperature Eexc = 3.815 eV (λ exc = 325 nm, HeCd laser) (red line) and quasi-continuous excitation at Eexc = 3.1 eV (λexc = 400 nm) of this MAPI crystal, taken six months after its growth, under continuous excitation at Eexc = 3.815 eV (λexc with = 325a nm, HeCd laser) (red line)pulsed and quasi-continuous excitation at Eexc = 3.1 eV (λexc = 400 nm) 80 MHz doubled-frequency Ti-Sa laser (black line). (λexc = 325 nm, HeCd laser) (red line) and quasi-continuous excitation at Eexc = 3.1 eV (λexc = 400 nm) with a 80 MHz doubled-frequency pulsed Ti-Sa laser (black line). with a 80 MHz doubled-frequency pulsed Ti-Sa laser (black line).

Figure 10. PL spectrum of the millimeter sized MAPI crystal at 10 K, taken six months after its growth under at 3.1 eVsized (400 nm) with a 80 MHz pulsed laser. Figurequasi-continuous 10. PL spectrum excitation of the millimeter MAPI crystal at 10doubled-frequency K, taken six months after Ti-Sa its growth

Figure 10. PL spectrum of the millimeter sized MAPI crystal at 10 K, taken six months after its under quasi-continuous excitation at 3.1 eV (400 nm) with a 80 MHz doubled-frequency pulsed Ti-Sa laser. growth under quasi-continuous excitation 3.1 eV (400 nm) with a 80 MHz Such small PbI2 traces could not be at observed by X-diffraction. Thesedoubled-frequency traces couldn’t bepulsed either detected from thePbI absorption spectrum. from 3 and 4a, the order magnitude of the Ti-Sa laser. Such small 2 traces could not beIndeed, observed byFigures X-diffraction. These tracesofcouldn’t be either −1 that is to say that the optical MAPI absorption evaluatedIndeed, in the 2.4 eVFigures range: 3750 detected from the coefficient absorptionisspectrum. from 3 andcm 4a, ,the order of magnitude of the density for a crystal having a is thickness of 2inmm be 750, making impossible weak −1, that is to MAPIsmall absorption evaluated thewill 2.4 eV 3750 cm to detect say that the signals optical Such PbI2 coefficient traces could not be observed by range: X-diffraction. These traces couldn’t be either coming from PbI2 absorption. The low temperature PL is therefore here the onlytomean toweak detectsignals these density for a crystal having a thickness of 2 mm will be 750, making impossible detect detected from the absorption spectrum. Indeed, from Figures 3 and 4a, the order of magnitude of the weak PbI 2 traces. coming from PbI2 absorption. The low temperature PL is therefore here´the only mean to detect these 1 , that MAPI absorption coefficient is evaluated indetect the 2.4weak eV range: cmit is is to say thatmethod the optical spectroscopy is a powerful tool to signals 3750 because a signal detection weakPL PbI 2 traces. density for a crystal having a thickness of 2 mm will be 750, making impossible to detect weak signals requiring a black background: in thetool range of wavelengths where the sample showsdetection no emission, the PL spectroscopy is a powerful to detect weak signals because it is a signal method coming from PbI absorption. The low temperature PL is therefore here the only mean to detect emission is strictly zero, as range a consequence, if a small signal for certain 2 requiring signal a black background: in the of wavelengths where emission the sample showsexists no emission, thethese weakwavelengths, PbI2 traces. it will be possible the detector in order to detect emission signal is strictly zero,toasincrease a consequence, if asensitivity small emission signal existstheforemission certain signal. It is quite impossible to do that in an absorption experiment: in the range ofdetection wavelengths PL spectroscopy is a powerful tool to detect weak signals because it is a signal method wavelengths, it will be possible to increase the detector sensitivity in order to detect the emission where the sample shows no absorption, the incident light is completely transmitted, so a high signal requiring black background: thethat range of absorption wavelengths where the sample no emission, signal.a It is quite impossible in to do in an experiment: in the rangeshows of wavelengths arrives in signal the detector. If no a small exists at light a certain wavelength, the signal incident be where the sample absorption, incident transmitted, soexists alight highwill signal the emission isshows strictly zero,absorption as a the consequence, ifisacompletely small emission for certain nearly completely transmitted, as a consequence it will be impossible to play with the detector arrives in the detector. If a small absorption exists at a certain wavelength, the incident light will be

wavelengths, it will be possible to increase the detector sensitivity in order to detect the emission nearly completely transmitted, as a consequence it will be impossible to play with the detector

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signal. It is quite impossible to do that in an absorption experiment: in the range of wavelengths where the sample shows no absorption, the incident light is completely transmitted, so a high signal arrives in the detector. If a small absorption exists at a certain wavelength, the incident light will be nearly completely transmitted, as a consequence it will be impossible to play with the detector sensitivity Molecules 2016, 21, 885 8 of 13 to Molecules 885 absorption signal among the large transmitted signal. It is the reason why 8 of 13PL detect the 2016, very21,weak sensitivity to very weak absorption signal spectroscopy among the large transmitted the spectroscopy is detect much the more sensitive than absorption to detect weaksignal. signalsItinis solids. sensitivity detect the very is weak absorption signal than among the largespectroscopy transmitted signal. It weak is the reason whyto PL spectroscopy much more sensitive absorption detect Additionally, the low temperature PL spectroscopy can be performed in situ, in theto cryostat. In fact, reason in why PL spectroscopy isthe much more sensitive absorption toindetect weak signals solids. Additionally, low temperature PLthan spectroscopy canspectroscopy beare performed situ, in the in order to optimize the solar cells performances, advanced optical studies necessary: for example, signals in solids. Additionally, the low temperature PL spectroscopy can be performed in situ, in the cryostat. In fact, in order to optimize the solar cells performances, advanced optical studies are it is crucial to understand the nature and the dynamics of the relaxation mechanisms involved in the cryostat. In fact, in order to optimize the solar cells performances, advanced optical studies are necessary: for example, it is crucial to important understandtothe nature and the dynamics of theexcitons, relaxation trapping of excitons on defects, it is also study some behaviors of the free such necessary: forinvolved example,initthe is crucial to of understand thedefects, nature and oftothe relaxation trapping excitonstime-resolved on itPL is the alsodynamics important study some as mechanisms their diffusion length. To perform these studies, experiments at low temperature mechanisms involved in the such trapping of excitons defects, it is alsothese important study some behaviors of the free excitons, as their diffusionon length. To perform studies,to time-resolved arebehaviors necessaryof[42,43]. Inexcitons, this case, theas method we propose hereToallows to check, in a simple way and in the at free such their diffusion length. perform these studies, PL experiments low temperature are necessary [42,43]. In this case, the method we time-resolved propose here situ, the presence of PbI traces. 2 temperature are necessary [42,43]. In this case, the method we propose here PL experiments at alow allows to check, in simple way and in situ, the presence of PbI2 traces. allows to check, in a simple way and in situ, the presence of PbI2 traces. 3. Discussion on MAPI Degradation Mechanisms 3. Discussion on MAPI Degradation Mechanisms 3. The Discussion on MAPI Degradation Mechanisms experiments performed on MAPI thin layers showed that PMMA and Spiro-OMeTAD layers The experiments performed on MAPI thin layers showed that PMMA and Spiro-OMeTAD are bothThe able to protect the MAPI layer MAPI and that thelayers mechanism which is involved inSpiro-OMeTAD the degradation thin that PMMA and layers areexperiments both able toperformed protect theon MAPI layer and thatshowed the mechanism which is involved in the of MAPI and the appearance of PbI activated by light. As we have demonstrated in the case 2D layers are both able to protect the MAPI layer and that the mechanism which is involved in of the 2 is degradation of MAPI and the appearance of PbI2 is activated by light. As we have demonstrated in hybrid perovskite [44], the primary processof inPbI perovskite degradation is the (minor) evolution ofinthe degradation of MAPI and the appearance 2 is activated by light. As we have demonstrated the case of 2D hybrid perovskite [44], the primary process in perovskite degradation is the (minor) the case ofof2D hybrid perovskite [44], state the primary in“Pb perovskite degradation the (minor) photogenerated charge transfer excited “Pb+excited . . process . I˝ ”state in the material, I2isgenerating which goesI2off. +…I°” ingenerating evolution the photogenerated charge transfer the material, + +…I°” in the material, generating I2 evolution of the photogenerated charge transfer excited state “Pb Thewhich fate of the transient very unstable “Pb ” species is not clear, but it is likely that it reacts in air with goes off. The fate of the transient very unstable “Pb+” species is not clear, but it is likely that it ´but ´ likely which goes off. The fate of the transient very unstable “Pb+” species is not clear, it is that it . . . thereacts strongest easily diffusing oxidizing reagent, that is oxygen. ROS species (O , HO ) can 2 2 species (Othen in air with the strongest easily diffusing oxidizing reagent, that is oxygen. ROS 2−, − reacts in air with the strongest easily diffusing oxidizing reagent, that is oxygen. ROS species (O 2 be HO formed, which oxidize iodides, in theiodides, end generate hydroxide ions. hydroxide These hydroxide 2−…) can theninbeturn formed, which in turnand oxidize and in the end generate ions., HO 2−…) can then be formed, which in turn oxidize iodides, and in the end generate hydroxide ions. ions deprotonate ammoniums a slower process the solid matrix), low boiling These hydroxidethe ions deprotonate(likely the ammoniums (likely ain slower process in theand solidthe matrix), and These hydroxide ions deprotonate the ammoniums (likely a slower process in the solid matrix), methylamine also methylamine goes off. In the end, only is left. equations, theIn picture can be summarized the low boiling also goes off.PbI In 2the end,In only PbI2 is left. equations, the picture and can as thesummarized low boiling as methylamine also goes off. In in the end, only PbI2): 2 is left. In equations, the picture can (overall stoichiometry only in the circled global Schemes 1 and be (overall stoichiometry only the circled global Schemes 1 and 2): be summarized as (overall stoichiometry only in the circled global Schemes 1 and 2):

Scheme 1. Degradation processes susceptible to occur upon the photochemical degradation of MAPI, Scheme 1. 1. Degradation processes susceptible to upon the thephotochemical photochemicaldegradation degradation of MAPI, Scheme Degradation susceptible to occur occur upon MAPI, either involving simplyprocesses water (top Scheme) or oxygen (bottom Scheme). ROS stands for of Reactive either involving simply water (top Scheme) or oxygen (bottom Scheme). ROS ROSstands standsfor forReactive Reactive either involving simply water (top Scheme) or oxygen (bottom Scheme). Oxygen Species. Oxygen Species. Oxygen Species.

Scheme 2. Alternative degradation mechanism of MAPI with production of hydrogen. Scheme Alternativedegradation degradationmechanism mechanism of Scheme 2. 2. Alternative of MAPI MAPI with withproduction productionofofhydrogen. hydrogen.

This mechanism is consistent with the work of Haque and co-workers who argued that under This mechanism is consistent with the work Haque and co-workers that under illumination, photoexcited electrons would react of with molecular oxygen towho formargued superoxide (O2−) illumination, photoexcited electrons would react with molecular oxygen to form superoxide (O2−) that would subsequently decompose the perovskite [45]. On the other hand, Christians et al. [37] have that would subsequently decompose the perovskite [45]. On the other hand,air Christians al. [37] convincingly demonstrated that formation of a perovskite hydrate in moist occurred,etand thathave this convincingly demonstrated that formation of a perovskite hydrate in moist air occurred, and that was linked to the fast photodegradation of the material afterwards. In that case, the water presentthis in was linked to the fast photodegradation of the material afterwards. In that case, the water present in

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This mechanism is consistent with the work of Haque and co-workers who argued that under illumination, photoexcited electrons would react with molecular oxygen to form superoxide (O2 ´ ) that2016, would decompose the perovskite [45]. On the other hand, Christians et al. [37] have9 of 13 Molecules 21, subsequently 885 convincingly demonstrated that formation of a perovskite hydrate in moist air occurred, and that this was linked to thewell fast photodegradation of the which material afterwards. that case, the water present in the structure might be the reducing agent, would triggerInan alternative mechanism, a little the structure might well be the reducing agent, which would trigger an alternative mechanism, a little slower and producing dihydrogen instead of water. slower producing dihydrogen of water. Lastly,and it should be noticed thatinstead the PMMA covered MAPI layer is slowly photodegraded, since Lastly, it should be noticed that the PMMA covered MAPI layer is slowly photodegraded, since PMMA completely protects from moisture, but not from oxygen; in addition, nobody has never PMMA completely protects from moisture, but not from oxygen; in addition, nobody has never reported the production of hydrogen gas, which tends to suggest that the second mechanism would reported the production of hydrogen gas, which tends to suggest that the second mechanism would be be a aminor one. Finally, the two mechanisms can exist concomitantly and present synergy. Even if minor one. Finally, the two mechanisms can exist concomitantly and present synergy. Even if water water is not the active in the degradation it help might helpofthe diffusion of both is not the active agentagent in the degradation process, itprocess, might well thewell diffusion both oxygen and oxygen and hydroxide ions to the accelerate the overall process. hydroxide ions to accelerate overall process. From the the experiments onon MAPI highlightisisthe theextremely extremely high stability ofcrystals the crystals From experiments MAPIcrystals, crystals, the the highlight high stability of the compared to the thin layers. crystal,an anintimate intimate contact is is much more difficult thanthan in in compared to the thin layers. InIn a acrystal, contactwith withwater water much more difficult a thin layer insertion waterwould wouldprobably probably provoke breakdown). OnOn the the other hand, a thin layer (the(the insertion of of water provokeaacrystal crystal breakdown). other hand, Figure 11 shows a Scanning Electron Microscopy (SEM) image of a MAPI thin layer, showing the large Figure 11 shows a Scanning Electron Microscopy (SEM) image of a MAPI thin layer, showing the that cancan be in withwith waterwater and above all the all great of grain boundaries. largesurface surface that becontact in contact and above thenumber great number of grain boundaries.

Figure 11. Scanning Electron Microscopy (SEM) image of an 800 thick MAPI thin layer deposited by Figure 11. Scanning Electron Microscopy (SEM) image of an 800 thick MAPI thin layer deposited by spin-coating on aonquartz substrate. spin-coating a quartz substrate.

As our millimeter crystals are single crystals presenting a low density of defects and likely have As our millimeter crystals are single crystals presenting a low density of defects and likely have no grain boundaries, wewe conclude that these playan animportant important role in no grain boundaries, conclude that thesedefects defectsand andgrain grain boundaries boundaries play role the degradation mechanism and that growing very high quality perovskite films in solar cells is in the degradation mechanism and that growing very high quality perovskite films in solar cells is therefore necessary to to avoid degradation. isquite quitecoherent coherent with recent interpretations. therefore necessary avoid degradation.This This finding finding is with recent interpretations. In their studies about degradationmechanisms mechanisms of MAPI and crystals, Leguy et al.et[38] In their studies about degradation MAPIininthin thinlayers layers and crystals, Leguy al. [38] suggested a high degree penetrationof of water water molecules thin films, arising probably suggested a high degree ofofpenetration moleculesinto intoMAPI MAPI thin films, arising probably the diffusion water moleculesalong along the the grain et et al. al. [39][39] have studied MAPI fromfrom the diffusion of of water molecules grainboundaries. boundaries.LuLu have studied MAPI thin layers deposited by two different methods, one of this method producing higher quality thin thin layers deposited by two different methods, one of this method producing higher quality thin layers: they observe a more important interaction with moisture in the layers containing more defects, layers: they observe a more important interaction with moisture in the layers containing more defects, providing in their opinion more active sites for the adsorption of water molecules. providing in their opinion more active sites for the adsorption of water molecules. 4. Materials and Methods 4.1. Preparation of Perovskite Precursors First of all, methylammonium iodide CH3NH3I (called MAI hereafter) is synthetized by reacting CH3NH2 (30 mL, 2 M in methanol, Sigma Aldrich, St. Louis, MO, USA) and aqueous solution of iodic

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4. Materials and Methods 4.1. Preparation of Perovskite Precursors First of all, methylammonium iodide CH3 NH3 I (called MAI hereafter) is synthetized by reacting CH3 NH2 (30 mL, 2 M in methanol, Sigma Aldrich, St. Louis, MO, USA) and aqueous solution of iodic acid (HI, 34.4 mL 57% in water, Sigma Aldrich) in a 250 mL round-bottom flask, stirred at 0 ˝ C for 4 h. The precipitate is recovered after evaporation at 60 ˝ C for 1 h. Then the MAI is dissolved in ethanol and recrystallized three times from diethyl ether. The final product is dried at 60 ˝ C in vacuum oven for 24 h. To prepare the perovskite CH3 NH3 PbI3 (MAPI), the synthesized CH3 NH3 I (0.5 g) and lead iodide (PbI2 , 1.47 g of grains from Sigma Aldrich 99.99% for all the samples except for the sample called MAPI*, for which powdered Sigma-Aldrich 99.99% PbI2 was used) are mixed in γ-butyrolactone (4.47 mL, Sigma Aldrich reagent plus > 99%) (stoichiometric amounts) and stirred at 60 ˝ C for 12 h. The obtained solution is homogeneous with high transparency in the visible region, and remains totally clear when it is stored during a long time (more than one month), that is to say that no PbI2 is formed inside the solution. 4.2. Deposition of Perovskite Layer UV quartz slides (from Neyco, Vanves, France) are used for deposition. They are cleaned sequentially with acetone, ethanol and propanol in an ultrasonic bath (15 min for each step). Finally the slides are immerded in 1 M KOH in ethanol for 15 min. Then the slides are rinsed with distilled water and dried by a nitrogen gas flow. MAPI films are deposited on these quartz substrates by spin-coating, adjusting the acceleration and duration of the acceleration to obtain the desired thickness. After the deposition, the samples are annealed at 120 ˝ C for 10 min. We focus the paper on layers of thickness 800 nm as the thickness of the MAPI layers in the solar cells is typically several hundred of nm. These layers are obtained with spin-coater parameters of 2000 rpm for 10 s, the thickness has been checked by AFM measurements. 4.3. Preparation of Millimeter Sized Crystals On the basis of articles recently published [46–49], we have grown millimeter sized MAPI crystals. Methylammonium iodide (0.78 g, 5 mmol) and lead iodide (2.30 g, 5 mmol) were dissolved in GBL (5 mL) at 60 ˝ C. The yellow solution (2 mL) was placed in a vial and heated at 120 ˝ C during one to four hours depending on the desired crystal size. The solution could be heated in a hot plate as well as an oil bath. 5. Conclusions Investigating the stability and evaluating the quality of the perovskite structures is quite critical both for the design and fabrication of high-performance perovskite devices and for fundamental studies of the photophysics of the excitons. We have shown that low temperature PL spectroscopy is a powerful tool to detect PbI2 traces in hybrid perovskite layers and crystals. Because PL spectroscopy is a signal detection method on a black background, small PbI2 traces can be detected, when other methods, currently used at room temperature, fail. Additionally, in the context of advanced optical studies of the excitons, PL spectroscopy allows to check in situ the presence of PbI2 in samples in a very simple way. In particular, this method is quite convenient for the study of the crystal aging and quality, as X-diffraction and absorption spectroscopies are inadequate to detect the presence of PbI2 traces in this case. This method has been applied to study MAPI thin layers and single crystals. The processes involved in the degradation likely involve dioxygen, or water, but more likely the two together, with mutual enhancement, the water helping oxygen, the strongest oxidant, to reach reactive zones localized on defects. Our study highlights the extremely high stability of the single crystals compared to the thin layers. So defects and grain boundaries are thought to play an important role in the degradation

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mechanism. As a consequence, growing very high quality perovskite films in solar cells is quite necessary to avoid degradation and increase the resistance to moisture. Acknowledgments: This work is supported by DIM Nano-K (grant PEROVOLT) and by Institut d’Alembert of Ecole Normale Supérieure de Cachan. This work has received funding from the European Union’s Horizon 2020 research and innovation programme under the grant agreement N˝ 687008. The information and views set out in this paper are those of the author(s) and do not necessarily reflect the official opinion of the European Union. Neither the European Union institutions and bodies nor any person acting on their behalf may be held responsible for the use which may be made of the information contained herein. Author Contributions: K.J., H.D., F.L., D.G. and J.-S.L. performed the experiments. G.T.-A., K.J. and F.L. synthesized the thin layers and the crystals. D.G., B.G., J.-S.L., E.D. and P.A. analyzed the data. E.D. and P.A. wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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