Optical absorption and photoluminescence spectroscopy

C H A P T E R

3 Optical absorption and photoluminescence spectroscopy Mojtaba Abdi-Jalebi1, M. Ibrahim Dar2, Aditya Sadhanala1, Erik M.J. Johansson3 and Meysam Pazoki4 1

Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge, United Kingdom 2Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, E´cole Polytechnique Fe´de´rale de Lausanne, Lausanne, Switzerland 3Department of ˚ ngstro¨m laboratory, Uppsala University, Uppsala, Sweden 4Department of Chemistry, A ˚ ngstro¨m laboratory, Uppsala University, Engineering Sciences, Solid State Physics, A Uppsala, Sweden

3.1 Introduction Optical absorption and photoluminescence spectroscopy are important tools for studying semiconductors and electronic devices because they are non-destructive and nonintrusive. In particular, for a semiconductor film within a solar cell device, it is substantially important to measure the bandgap and estimate the absorbed light in order to determine the theoretical limit for the power conversion efficiency and photovoltaic parameters of the subsequent solar cells. In addition, determination and analysis of luminescence in a semiconductor is key to understand the origin of losses in optoelectronic devices.

3.2 Optical absorption spectroscopy Absorption of a photo-absorber semiconductor is an important aspect for its application in an optoelectronic device as it determine the optical bandgap, which is key to calculate the theoretical limit for various parameters of a semiconductor device. Metal-halide perovskite has a direct bandgap and therefore thin film based perovskite is suffice to efficiently absorb the light while in indirect bandgap based semiconductors such silicon another of magnitude thicker films is required to absorb same amount of light. Characterization Techniques for Perovskite Solar Cell Materials DOI: https://doi.org/10.1016/B978-0-12-814727-6.00003-7

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3. Optical absorption and photoluminescence spectroscopy

3.3 Steady state UV
Vis
NIR spectroscopy Ultraviolet
visible
near-infrared spectroscopy (UV
Vis
NIR) refers to absorption spectroscopy in the ultraviolet
visible
near-infrared spectral region. Absorption spectroscopy fluorescence/photoluminescence spectroscopy are complementary in nature wherein, the transitions from excited state to ground state results in photoluminescence and the reverse
transition from ground state to excited state due/leads to absorption of photons. The absorption is quantified by the Beer
Lambert law:
 A 5 log10 I=I0 (3.1) where A is absorbance, I0 is the intensity of the incident light, and I is the transmitted intensity (Fig. 3.1). All the parameters are for a given wavelength of light and when we scan all the wavelengths from UV to visible to near IR, a complete absorbance spectra of the perovskite semiconductor can be obtained. Transmittance ‘T’ is given by I/I0 and is expressed in % as %T, when measuring the transmitted light and the absorbance is then given by:   %T A 5 log10ðTÞ 5 log10 (3.2) 100% When measuring reflectance, the spectrophotometer measures I the intensity of light reflected from a sample, with reference to a reference sample reflection I0 which is generally obtained by using a white reflective sample coated with Barium Sulfate (BaSO4). Herein, I/I0 is defined as the reflectance ‘R’ and is usually represented as %R. Due to the crystalline to polycrystalline nature of the perovskite samples in solar cell devices, the perovskite samples do show significant reflection and scattering leading to errors in simple UV
Vis measurement. In this case one should use an integrating sphere based UV
Vis
NIR measurement that accounts for the total scattering and reflection providing the correct contribution from of absorption.

3.3.1 Photothermal deflection spectroscopy (PDS) Photothermal Deflection Spectroscopy (PDS) technique is a highly sensitive absorption measurement technique that can probe absorption coefficients down to 1 cm21 or an absorbance level of down to 1025 [1], which corresponds to a 4
5 orders of magnitude FIGURE 3.1 Schematic of UV
Vis
NIR technique showing incident light intensity of I0 and transmitted intensity I through the sample upon absorption.

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3.3 Steady state UV
Vis
NIR spectroscopy

FIGURE 3.2 Schematic of (A) transverse and (B) collinear PDS techniques.

dynamic sensitivity range. Such high sensitivity is achieved at ambient temperatures by using the principles of mirage effect. Jackson et al. first demonstrated the PDS technique in 1981 [2]. This technique was originally used for the detection of defect states in inorganic semiconductors like a-Si, GaAs, InGaAsP, etc [3
5]. PDS techniques are mainly of two types: transverse and collinear PDS techniques respectively (Fig. 3.2) [2]. Both systems have a tunable monochromatic excitation source (or heating beam) and a fixed wavelength continuous wave (CW) probe laser source used for probing the absorption in the sample for a given wavelength of excitation. In the collinear PDS system the excitation and the probe laser sources go through the sample and are arranged in such a way that they overlap each other while in the sample. Whereas, in the case of transverse PDS configuration, the excitation source is normal to the plane of the sample and the probe laser is perpendicular to the excitation beam path and parallel to the sample surface grazing it. Furthermore, the excitation beam and the probe beam overlap each other within the Rayleigh range of interaction. The deflection of the probe beam upon creation of a mirage near the sample surface (due to non-radiative relaxation of excited species created upon absorption of light) causes the deflection of the probe laser beam that can be measured using a photodetector. By scanning through different wavelengths, one can measure the absorption spectra of the sample. Standard absorption measurement techniques like UV
Vis
NIR spectrometers that usually measure absorption in the transmission mode, are plagued with errors due to various optical effects like
light scattering, reflection, and interference, which limits the sensitivity. The PDS however is immune to any optical effect because of the working principle of the measurement is on the basis of absorption induced heating effect in the sample, due to non-radiative relaxation of the excited species. It is notable that the UV
Vis
NIR spectrometers with an integrating sphere and a reference has ability to suppress the optical effects errors though in most of the laboratories a simplified set-up is used which suffers from the aforementioned optical effect errors. The measurement range depends on the emission spectrum of the lamp used, for example
an ozone free Xenon lamp would facilitate the PDS setup to measure absorption in the wavelength range of 380
2100 nm. The samples used for the PDS measurements mostly involve thin-films coated on quartz substrates. Fig. 3.3 shows the comparison between UV
Vis
NIR and PDS technique where a CH3NH3PbI3 perovskite thin-film is measured using these two techniques. Optical effects

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3. Optical absorption and photoluminescence spectroscopy

FIGURE 3.3 Comparison between the UV
Vis
NIR and PDS technique
absorption spectra of a rough CH3NH3PbI3 thin-film. Linear fit (dash line) on the PDS absorption spectra is used to extract the Urbach energy (see Section 2.4.2).

like scattering and reflection affect the UV
Vis
NIR measurements while, the PDS is minimally affected by these effects and hence, is capable of measuring the band-tails down to absorbance of 1025 i.e. five orders of magnitude dynamic range as compared to standard UV
Vis
NIR technique. Considering a 1D formalism
probe laser beam deflection angle (φ) is given by [6]:   s dn dTðz; tÞ (3.3) φ5 n dT dz where, n 5 refractive index of the fluid in which the sample is immersed, gradient, s 5 interaction path length.

dT dz 5 temperature

Rosencwaig and Gersho have given a detailed description of the sample thickness ‘l’, absorption coefficient ‘α’, thermal diffusion coefficients of the sample (μs), and measurement frequencies on the deflection signal [7,8]. There are few important considerations to keep in mind while measuring using the PDS; (1) absorbance is directly proportional to the magnitude of the PDS signal for an optically transparent sample i.e. where the optical penetration depth is more than the sample thickness. (2) PDS signal saturates for thermally thin samples (l , μs) at αl 5 1. (3) PDS signal saturates for thermally thick samples (1l , α , μ1 ) at αμs 5 1. s

3.3.2 Estimation of the bandgap As mentioned earlier, estimation of the bandgap is substantially important in any optical absorption measurements. There are a number of main methods of bandgap calculation using absorption data obtained using UV
Vis
NIR or PDS or any other absorption measurement technique as described below.

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3.3 Steady state UV
Vis
NIR spectroscopy

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3.3.2.1 Simple calculation Band gap energy; E 5

hc λ

(3.4)

where, Planks constant h 5 6.626 3 10234 Js Speed of light c 5 3 3 108 m/s λ is the wavelength of light (the onset in the UV
Vis spectrum) in meters 3.3.2.2 Tauc plots Herein, we first have to extract the absorption coefficient “α” data from the absorbance/transmission data measured from UV
Vis
NIR or PDS or any other absorption measurement technique including the Diffuse Reflectant spectroscopy (DRS) that measures reflection spectra. The absorption coefficient is given by the following equation:
n A hv2Eg α5 (3.5) hv 1

1

(αhv)n 5 An hv 2 A1=n Eg

(3.6)

where, A is absorbance (obtained from transmittance T or reflectance R), hv is the photon energy represented in eV units 5 1240/(incident wavelength in nm) h i 1 1 If you plot a graph of (αhvÞn against hv, then the hv axis intercept (αhvÞn 5 0 , of the linear slope fitted to the linear section of the plot will give the bandgap Eg. Wherein: n 5 12 for direct bandgap semiconductors, n 5 2 for indirect bandgap semiconductors, n 5 3 for semiconductors with direct forbidden transitions, and n 5 4 for semiconductors with indirect forbidden transitions. Most of the photoabsorber films implemented in the highly efficient perovskite devices have direct band gap. It is notable that derivative method is a rudimentary way of obtaining the bandgap. In this method, we use the absorption spectra obtained from various techniques and take the derivative of the spectra and the position where the maximum intensity peak position obtained in the derivative spectra gives the bandgap.

3.3.3 Near band edge trap states The near bandgap defects arise due to various reasons no limited to the
sample processing conditions, crystallinity, structural disorder, defects, doping, etc. These near band edge defects play a major role in affecting the charge transport and other optoelectronic properties that can determine the quality of a semiconductor device. An empirical way of quantifying these near band edge is by estimation of Urbach energy. Using PDS data or any other sensitive absorption measurement technique that can measure sub-bandgap

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3. Optical absorption and photoluminescence spectroscopy

absorption one can estimate an empirical parameter known as “Urbach energy” (see Fig. 3.3) annotated as ‘EU’ defined as:   E A 5 A0  exp (3.7) EU 1  ½  dE lnðAÞ

EU 5
d

(3.8)

The Urbach energy EU can be calculated from the inverse slope of the linear fit to the Urbach tail (or Urbach front) in the absorption spectra plotted on a natural logarithmic scale (as shown in Fig. 3.3). It has a unit of energy (eV) per decade however, generally only the energy unit (eV) is explicitly specified. Franz Urbach first conceived this in 1953 [9]. Furthermore, temperature dependent absorption measurements done by Knox showed the direct proportionality and correlation between the Urbach energy and thermal energy (kT, where k is Boltzmann constant and T is temperature in Kelvin) [10], leading to a conclusion that transitions below the bandgap are mainly due to phonons [10]. However, Urbach tail can also be caused by other mechanisms, which induce potential fluctuations that could result in local energy level and hence bandgap variation in the semiconductors. Hence, various theoretical models used to describe the Urbach energy rule [11
15]. However, there is no unified theory yet that describes the origin of Urbach energy which, can be best described as an empirical parameter. Similar to PDS there are several other measurement techniques that are capable of measuring absorption in a sensitive way. Many of these techniques depend on the measurement of photocurrent and hence are limited to measuring contribution due to only exciton species that can result in a free charge pair and doped samples that contain a fixed ion and a free opposite charge does not show up in such measurements [16]. However, PDS measures contribution due to all such species as shown in the Fig. 3.4. Herein, the perovskite samples containing tin (Sn) show intrinsic doping and have free charge carriers that cannot be extracted by doing electrical measurements and hence, electrical measurements lead to misleading absorption data that lacks contribution due to doping that is captured accurately using PDS. However, both electrical and PDS show similar conclusions for undoped samples like lead only perovskite CH3NH3PbI3. Therefore, it is important to choose the right sensitive absorption measurement as the findings have significant impact on the analysis obtained about the resultant semiconductor quality and their optoelectronic properties [17
19]. The PDS technique as a powerful and ultra sensitive absorption tool is extensively used to measure the near band edge trap states of various semiconductors. In particular, improving the quality of titanium dioxide as a key semiconductor for the electron transport layer in perovskite solar cell [20] and dye-sensitized solar cell [21,22] architectures and providing an interface with the minimum defect density have been studied using the PDS technique. In Fig. 3.5A and B, the absorption spectra near the band-edge of TiO2 has significantly changed upon two different treatments on the titania mesoporous layer (mp-TiO2), TiCl4 treatment [20] and lithium treatement [23], respectively, where the subgap level as well as the Urbach energy of the titania reduced substantially.

Characterization Techniques for Perovskite Solar Cell Materials

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Vis
NIR spectroscopy

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FIGURE 3.4 (A
C) shows the normalized PDS and the photocurrent (EQE) spectra of the CH3NH3PbI3, CH3NH3Pb0.4 Sn0.6I3, and CH3NH3 Pb0.2Sn0.8I3 perovskites respectively. The shaded region indicates the difference between EQE and PDS spectra that signifies the states that do not cause charge generation. The estimate Urbach energies obtained from PDS and EQE based measurements is stated for each sample. Reproduced from B. Zhao, M. Abdi-Jalebi, M. Tabachnyk, H. Glass, V.S. Kamboj, W. Nie, et al., High open circuit voltages in tin-rich low-bandgap perovskites based planar heterojunction photovoltaics, Adv. Mater. 29 (2) (2016) 1604744 [24].

These treatment also enhances the quality of the perovskite capping layer providing a better interface between mp-titania and perovskite (Fig. 3.5C). Furthermore, addition of various modifier to the top surface of mp-TiO2 can indeed passivated the intra bandgap states within TiO2 and provides a superior interface with the perovskite top layer as it is evident from Fig. 3.5D.

Characterization Techniques for Perovskite Solar Cell Materials

FIGURE 3.5 (A) PDS absorbance spectra of mp-TiO2 films, pristine and TiCl4 post-treated. The inset shows the corresponding Urbach energies. (B) Absorbance spectra of pristine (black curve) and Li-treated (green curve) mp-TiO2. (C) Absorbance spectra of FAPbBr3 perovskite deposited on pristine (black curve) and Li-treated (blue curve) mp-TiO2. The inset shows the corresponding Urbach energies. (D) PDS absorption spectra of MAPbI3 films on TiO2 with different modifiers deposited on quartz glass. The inset shows PDS absorption spectra of TiO2 only and its modified analogs. (A) Reproduced from M. Abdi-Jalebi, M.I. Dar, A. Sadhanala, S.P. Senanayak, F. Giordano, S.M. Zakeeruddin, et al., Impact of a mesoporous titania
perovskite interface on the performance of hybrid organic
inorganic perovskite solar cells, J. Phys. Chem. Lett. 7 (16) (2016) 3264
3269, Copyright 2016 American Chemical Society. (C) Reproduced from N. Arora, M.I. Dar, M. Abdi-Jalebi, F. Giordano, N. Pellet, G. Jacopin, et al., Intrinsic and extrinsicstability of formamidinium lead bromide perovskite solar cells yielding high photovoltage, Nano Lett. 16 (11) (2016) 7155
7162, Copyright 2016 American Chemical Society. (D) Reprinted with permission from K.K. Wong, A. Fakharuddin, P. Ehrenreich, T. Deckert, M. Abdi-Jalebi, R.H. Friend, et al., Interface-dependent radiative and nonradiative recombination in perovskite solar cells, J. Phys. Chem. C 122 (20) (2018) 10691
10698 [25], Copyright 2018 American Chemical Society.

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Vis
NIR spectroscopy

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3.3.4 Absorption properties of metal-halide perovskites Metal-halide perovskites have in general strong light absorption properties. This is very important for highly efficient solar cell devices, since a strong light absorption enable the use of a thin photoactive layer in the solar cell to harvest the solar light. After light absorption in the thin photoactive layer, the photogenerated charges only needs to travel a short distance to reach the charge extraction layers, which reduces the recombination of the photo-generated charges. In comparison to other solar cell materials, the metal-halide perovskite shows a much stronger light absorption than silicon, and similar light absorption as other thin film solar cell materials, such as CIGS, CdTe and GaAs (see Fig. 3.6) [26]. In addition to the strong light absorption for the metal-halide perovskites, they also show a rather sharp optical absorption edge close to the reported band gap energy of the material (see Fig. 3.7) [27]. By using photo-thermal deflection spectroscopy (PDS) and Fourier transform photocurrent spectroscopy (FTPS), it was observed that the optical absorption edge is sharp and a small Urbach energy of 15 meV reported for CH3NH3PbI3 [28]. In PDS, the sample is immersed in a liquid and the sample is illuminated with different wavelengths of light. When light is absorbed by the sample, the light induced temperature change in the liquid affects the deflection of a laser beam in the liquid, and the change in deflection can be used to detect the light absorption spectrum of the sample. However, in FTPS the photocurrent between two contacts connected to the material is measured at a bias voltage. The results from the two methods (PDS and FTPS) were similar, and the absorption edge was found to be exponential over four orders of magnitude [27]. The small Urbach energy suggests a low degree of structural disorder, and no optically detectable deep band gap states [29]. The sharp absorption edge and small Urbach energy may be connected to the high open-circuit voltage obtained for the CH3NH3PbI3 based solar cell and an empirical trend shows lower voltage losses in solar cell materials with low Urbach energies [27,30].

FIGURE 3.6 (A) The absorption coefficients for a number of solar cell materials. (B) The real and imaginary parts of the dielectric constant for CH3NH3PbI3 at 300 K, as a function of frequency. Reprinted by permission from Springer Nature M.A. Green, A. Ho-Baillie, H.J. Snaith, The emergence of perovskite solar cells, Nat. Photonics 8 (7) (2014) 506
514.

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3. Optical absorption and photoluminescence spectroscopy

FIGURE 3.7 PDS, Fourier transform photocurrent spectroscopy (FTPS) and 1-RspecularTspecular spectra for CH3NH3PbI3 perovskite. All spectra were measured at room temperature. Adopted with permission from S. De Wolf, J. Holovsky, S.-J. Moon, P. Lo¨per, B. Niesen, M. Ledinsky, et al., Organometallic halide perovskites: sharp optical absorption edge and its relation to photovoltaic performance, J. Phys. Chem. Lett. (2014) In Press, 140305122150008, copy right 2014 American Chemical Society.

3.3.5 Light absorption process in metal-halide perovskites The top valence band in CH3NH3PbI3 is mainly composed of halide p-orbitals mixed with a smaller contribution of lead states, and the conduction band is mainly composed of lead p-orbitals mixed with a smaller amount of halide states [31]. Light absorption near the band gap energy is therefore mainly a transition between the halide p-orbitals (mixed with lead orbitals) in the valence band to lead p-orbitals (mixed with halide orbitals) in the conduction band. The optical transition is therefore mainly including the lead halide part of the material and the organic cations are not involved in light absorption close to the band gap energy. However, by changing the organic cation, the Pb
I octahedrals will be affected, which affects the band gap energy. For example by changing the organic cation from methylammonium (MA) to formamidinium (FA), the crystal structure of the perovskite moves from tetragonal to quasi cubic [32]. This structural change also results in a change of the optical properties with a lower band gap value for the perovskite with formamidinium cations. The structural change s can be explained by different sizes of the organic cations and electrostatic interactions between the cations and negatively charged inorganic Pb
I matrix. The changes in the Pb
I matrix includes changes in bond length, bond angles as well as octahedral tiltings that modifies the electronic structure and therefore changes the band gap value. The changes in the band gap due to differences in Pb
I bond angles from different Pb
I octahedral tilting in the structure was theoretically explained by changes in the Pb
I bonds and the spin-orbit coupling (see Fig. 3.8) [33]. A number of different cations have been inserted in the perovskite structure, which show that the cation affects the light absorption properties [34,35]. One important reason for investigating different cations in the perovskite structure is also to obtain more stable perovskite materials. Therefore, also inorganic cations such as Cesium have been introduced in the perovskite structure to obtain more stable perovskites [36]. Due to the changes in the P
I matrix, the light absorption is also affected, which is important for the solar cell properties.

Characterization Techniques for Perovskite Solar Cell Materials

FIGURE 3.8 (A) Calculated total energy by SR- and SOC-DFT as a function of the α-dihedral angle. Zero energy is set at α 5 0. (B) Band gap (Eg) calculated by SR- and SOC-DFT and the difference between them for CsPbI3. (C) Partial density of states for lead (solid lines) and iodide (dashed lines), for variation of the tilting angle α in CsPbI3. (D) Calculated average effective electron mass (me, dashed line) and hole mass (mh, solid line) calculated by SOC-DFT for the CsPbI3, varying the tilting angle α. Adopted from A. Amat, E. Mosconi, E. Ronca, C. Quarti, P. Umari, M.K. Nazeeruddin, et al., Cation-induced band-gap tuning in organohalide perovskites: interplay of spin-orbit coupling and octahedra tilting, Nano Lett. (2014).

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3.3.6 Excitons in metal-halide perovskites For solar cell devices, the excited electron needs to be separated from the positive hole to be able to produce a photocurrent. In a perovskite solar cell device, the perovskite is sandwiched between an electron extraction layer and a hole extraction layer, and understanding of the exciton (bound excited electron and hole pair) and its separation into free charges is essential. In the first perovskite solar cell devices it was not clear if the charge separation occurred at the interfaces with the electron extraction layer (or hole extraction layer) or within the perovskite material itself. Experimental observations of the exciton binding energy and the photoconductivity later on showed that the charge separation can occur directly in the perovskite material at room temperature, giving a photocurrent directly in the perovskite layer [37
39]. By investigating the magnetic field dependence of the light absorption, it was possible to determine the exciton binding energy to be around 16 meV at 4 K, which is comparable to conventional III-V semiconductors with similar band gap energy, and at room temperature the exciton binding energy in the perovskite is even lower and the photo-induced carriers are therefore essential free carriers (Fig. 3.9) [37,38].

3.3.7 Tuning of the light absorption spectrum via chemical modifications in metal-halide perovskite One important property of the metal-halide perovskites is the tunability of the band gap. This is important in solar cells in order to either obtain a band gap close to the optimal for a single junction solar cell, or to obtain a band gap for optimal performance in tandem or multi junction solar cells. The band gap of CH3NH3PbI3 can be tuned towards larger band gap energy by mixing iodide with bromide, see Fig. 3.10 [40]. The pure bromide perovskite, CH3NH3PbBr3, has a band gap energy around 2.3 eV, which is significantly higher compared to the iodide based perovskite with a band gap around 1.6 eV. The shift in band gap energy is obtained due to changes in the valence band structure, where the halide is dominating the electronic structure [41]. In the iodide-based perovskite, the valence band is shifted towards lower binding energies compared to the valence band for the bromide-based perovskite. The mixed iodide and bromide perovskites have band gaps between the pure iodide and bromide perovskites, and by choosing a ratio of iodide and bromide the band gap can be tuned to obtain a specific value, see Fig. 3.10. The light absorption of lead based perovskites can be tuned by changing the halide or the cations, as discussed above. On the other hand, the divalent cation (e.g. lead) may be replaced in the structure by other metal ions. It was early suggested that tin, Sn, can replace lead in the perovskite structure for efficient solar cells [18,19]. The tin based perovskites has a red shifted light absorption edge compared to the lead based perovskites. This makes it possible to extend the absorption range into the near IR-region for perovskites, which for lead perovskites is limited to around 800 nm. Furthermore, other metal ions have been suggested to replace lead, and bismuth halides has shown to be a low toxic alternative to the lead based perovskites for solar cells [42]. The bismuth halides have larger band gap than the lead halide based perovskites, and for Cs3Bi2I9, the band gap is around 2 eV [43]. This material has zero dimensional electronic properties, and the optical properties are not significantly affected by the cations and the band gap is indirect.

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Vis
NIR spectroscopy

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FIGURE 3.9

(A) Optical density (Log(1/transmission)) spectra recorded in the presence of a single pulse of the incoming magnetic field. To enable comparison the spectra are offset. (B, C) Ratios of the transmission in magnetic field T(B) to that measured at zero field. (B) The 2 s absorption at lower fields and (C) at higher fields. Reprinted by permission from Springer Nature A. Miyata, A. Mitioglu, P. Plochocka, O. Portugall, J.T.-W. Wang, S.D. Stranks, et al., Direct measurement of the exciton binding energy and effective masses for charge carriers in organicinorganic tri-halide perovskites, Nat. Phys. 11 (7) (2015) 582
587.

Although the band gap is indirect, the light absorption is high, which makes the material interesting for photovoltaic applications. Combinations of bismuth and silver halides have also shown interesting optical and photovoltaic properties, and by combining bismuth and silver halides, double perovskite materials can be formed [44]. Moreover, other structures can form for mixed bismuth and silver halides, with a range of optical properties, interesting for solar cells [45]. In addition to bismuth and silver halides, copper- and germaniumbased perovskites have shown interesting light absorption properties for photovoltaic applications [46]. The light absorption of the metal-halide perovskites therefore in general

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FIGURE 3.10 Band gap of perovskite absorber as a function of the nature of halide content. (A) UV
Vis absorption spectra of FTO/bl-TiO2/mp-TiO2/ MAPb (I1
xBrx)3/Au solar cells measured using an integral sphere. (B) Photographs of 3D TiO2/MAPb(I1
xBrx)3 bilayer nanocomposites on FTO glass substrates. (C) A quadratic relationship of the band-gaps of MAPb(I1
xBrx)3 as a function of Br composition (x). Reprinted with permission from J.H. Noh, S.H. Im, J.H. Heo, T.N. Mandal, S.Il. Seok, Chemical management for colorful, efficient, and stable inorganic
organic hybrid nanostructured solar cells, Nano Lett. 13 (4) (2013) 1764
1769, copy right 2013 American chemical Society.

seem to be rather strong, and many new metal-halide perovskite materials interesting for solar cell application with different light absorption spectra can still be expected to be discovered.

3.4 Photoluminescence spectroscopy Emission characteristics of perovskite absorbers provide us with sufficient evidence to understand high performance shown by perovskite optoelectronic devices. The perovskite semiconductors absorb light over a wide range, depending on their bandgaps, generating charge carriers, which relax towards the conduction band minimum and valence band maximum before producing strong emission via radiative recombination (Fig. 3.11). For example, the iodide-based perovskites exhibit a very broad spectral response, i.e. absorb photons of different energies, generating majorly free charge carriers, which

Characterization Techniques for Perovskite Solar Cell Materials

3.4 Photoluminescence spectroscopy

63

FIGURE 3.11 (A) Absorption and emission characteristics of MAPbI3 perovskite, and (B) the calculated band structure of MAPbI3 perovskite. Adopted with permission from J. Even, Pedestrian guide to symmetry properties of the reference cubic structure of 3D all-inorganic and hybrid perovskites, J. Phys. Chem. Lett. (2015) [47], 2015 American Chemical society.

recombine around the top of the valence band and bottom of the conduction band. Such a band-to-band relaxation is radiative in nature, leading to a bright and narrow emission. Depending on the excitation source, one could study either photoluminescence, electroluminescence or cathodoluminescence. As the photovoltaic applications involve the absorption of photons from the sun spectrum, the investigation of photoluminescence features allows us to understand the photo-physical processes occurring within the operational perovskite solar cells.

3.4.1 Processes involved in photoluminescence In a solar cell device, after the photo excitation, the generated charge carriers in conduction band (CB) and valence band (VB) of perovskite build a special charge density profile within the film which depends on the extinction coefficient at each specific wavelength. Depending on the working condition of the device, part of the carriers would be collected at charge selective contacts and part of them would remain stationary at the film to build up a certain Fermi level. The latter is under a balance in between partial charge generation and total recombination rates (R 5 G); At open circuit condition the total generation rate equals the recombination rate. The recombination processes can be classified into the radiative and non-radiative recombination. If the charge carriers are not separated and collected at contacts, after a certain time
called carrier life time
they will recombine; in the case of radiative recombination one electron-hole pair emits a photon instead which can be detected in the PL spectra. In the PL spectroscopy, the film is excited by photons normally with a higher energy than the band gap and the emission spectrum would be recorded. Special care should be taken into account for the stability of the air and humidity sensitive samples, i.e. by measuring the spectrum at inert atmosphere or using protection layers, tuning the right

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excitation intensity and time and checking the absorption spectrum of the film before and after the measurement. The intensity of PL peak at band edges is indicative of radiative recombination of charge carriers and thus an evaluation of charge recombination within the film and also the charge collection at contacts can be obtained from that. The former/ later can be evaluated from the time resolved PL measurements of perovskite absorber versus time with/without the presence of charge selective contacts [48]. PL spectrum contains information about the band edge and presence of excitonic peaks [49], presence of trap states, and the degree of electron-phonon interactions [50]. Exciton binding energy can be estimated from the temperature dependent PL spectrum as well [51,52]. These parameters are of crucial importance for characterization and optimization of solar cell devices
see Chapter 6. The schematic of the recorded emission in the PL spectrum are illustrated in Fig. 3.12 and can be due to the direct recombination of electron holes at the band edge (process 2), or before the cooling (process 1), exciton radiative dissociation (process 3), or from the trap states (process 4). Process 5 is indicative of carrier cooling. In the following sections, several examples of PL spectroscopy for perovskite solar cell materials are shown. On the other note, the intra-band gap trap states within the bandgap of the material is one of the main source of the PL quenching via trapping the electron or holes, or making new bands in the PL spectrum related to the energy of the traps [53,54]. The observed traps in the metal-halide perovskite films can also be photo-induced where the effect is reversible and can be heeled in dark or in the presence of oxygen [54]. Density and energy of trap states can be studied by temperature dependent PL spectroscopy [53]. In the following sections, we present examples of PL spectroscopy for studying different photovoltaic processes within the perovskite solar cell device.

3.4.2 Diffusion length and carrier lifetime In the absence of a charge selective layer, the recombination of free electrons and holes is mainly responsible for the time dependent PL spectra. However, in the presence of the charge selective layer, diffusion and collection of the charge carriers towards the contacts interfere and compete with electron-hole recombination and changes the estimated PL decay times. The decay time of the PL peak to 1/e of its initial value is considered as a fair FIGURE 3.12 An illustration of fundamental processes involved in the PL emission of perovskite film (left) and a typical PL spectrum of perovskite MAPbI3 (right).

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65

evaluation of the corresponding lifetimes. For example PL decay time of perovskite by itself represents the free electron-hole recombination time or as the so-called electron life time (τrec) within the device. Moreover the PL decay time (τ) in the presence of charge extraction layers has contributions from both charge transport (to extraction layers
τtrans) and charge recombination (τrec) that can be phrased as: 1 1 1 5 1 τ τ trans τ rec

(3.9)

For a photo-absorber to work properly, the collection time should be much faster than the recombination time and otherwise the distinguishing the contributions of two processes would be hard to be evaluated. In the highly efficient devices, charge transport time is significantly faster than recombination time and it is plausible to correlate the PL decay time with the charge transport time. Furthermore, Einstein relation can be used to calculate the corresponding charge diffusion length from the carrier transport time and the film thickness. The carrier lifetime measured in different regimes i.e. different fluences (Fig. 3.13) or temperatures gives fundamental insights about the recombination kinetics in the perovskite thin films.

3.4.3 Photon recycling in metal-halide perovskites The radiative recombination can be reabsorbed in the perovskite film and be implemented for the generation of charge carriers [56]. From the PL spectrum of the perovskite film with different physical distances of excitation and the PL detection probes, Pazos-Quton et al shown that the Beer
lambert’s law of photon absorption expected for the perovskite FIGURE 3.13 Time-resolved PL spectra of MAPbI3 in different fluences. Represented from T. Handa, D.M. Tex, A. Shimazaki, A. Wakamiya, Y.Kanemitsu, Charge injection mechanism at heterointerfaces in CH3NH3PbI3 perovskite solar cells revealed by simultaneous time-resolved photoluminescence and photocurrent measurements, J. Phys. Chem. Lett. (2017) [55]. (Ref DOI: 10.1021/ acs.jpclett.6b02847).

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3. Optical absorption and photoluminescence spectroscopy

FIGURE 3.14

Experimentally measured emission map for different distances between the (A) excitation and detection probes and (B) the Beer
Lambert prediction. Adopted with permission from L.M. Pazos-Outon, M. Szumilo, R. Lamboll, J.M. Richter, M.Crespo-Quesada, M. Abdi-Jalebi, et al., Photon recycling in lead iodide perovskite solar cells, Science 351 (6280) (2016) 1430
1433.

film is different from the experimentally measured PL spectra (shown in Fig. 3.14) [56]. This phenomenon is considered as a factor that can further supports higher efficiencies; however, the net effect in a 300 nm perovskite film is not significant. The photon recycling effect is the origin of huge reduction in the external photoluminescence quantum yield (PLQE) from the internal yield as reported for metal halide perovskite [57]. This statement was confirmed further via the substantial enhancement in the PLQE of the deposited perovskite thin films on textured substrates to a high value of 57% compare to only 15% in planar film (Fig. 3.15) [57]. This significant increase in PLQE is related to efficient light out-coupling in perovskite films deposited on textured substrates. Therefore, an efficient strategy to enhance the conversion efficiencies in the both solar cells and light emitting diodes (LEDs) is to use textured active layer to maximize the PLQE of the halide perovskite layer in the device.

3.4.4 Exciton binding energy and excitonic peaks Exciton binding energy is an important parameter for photovoltaic performance of the device, indicating how good the charge separation happens in the excited state. With a low exciton binding energy, in the range of thermal energy 25 meV for lead halide perovskites, there would not be any binding between the photo-generated holes and electrons, which ease the selective charge collection at contacts. The most widely used method for determination of exciton binding energy is the fitting of the near band edge absorption spectrum with Elliot’s theory (Fig. 3.16) [52]. Temperature dependent PL quenching can also determine the exciton binding energy as reported in Ref. [51]. Excitonic peaks show up as sharp peaks in the absorption spectrum near the band edge. The perovskite phases that show high photovoltaic performance with a charge separation efficiency close to 1, such as CH3NH3PbI3, show no such peaks in the absorption spectrum. However, MaPbBr3 or the alloyed perovskites with high Br constituents can

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FIGURE 3.15 (A) Sketch illustrating the improved sunlight in-coupling of textured solar cells. In a planar film, light will leave the film at the second interface. In a textured film, light undergoes about 30 total internal reflections and can therefore travel significantly longer in the active layer. (B) Measured external PLQEs of CH3NH3PbI32xClx films when deposited on a textured (circles) and planar (squares) substrate under CW excitation, together with computed PLQEs for different photon escape probabilities ηesc. While we were estimating the escape probability for the planar film to be 12.7%, we determine an escape probability of about 50% for the textured substrate, yielding an external PLQE of 57%. Adopted from J.M. Richter, M. Abdi-Jalebi, A. Sadhanala, M. Tabachnyk, J.P.H. Rivett, L.M. Pazos-Outo´n, et al., Enhancing photoluminescence yields in lead halide perovskites by photon recycling and light out-coupling, Nat. Commun. 7 (2016) 13941. FIGURE 3.16 Temperature-dependent integrated PL intensity of the CH3NH3PbI3 film under excitation of a 532 nm continuous-wave laser beam. The solid line is the best fit based on the figure adopted from S. Sun, T. Salim, N. Mathews, M. Duchamp, C. Boothroyd, G. Xing, et al., The origin of high efficiency in low-temperature solution processable bilayer organometal halide hybrid solar cells, Energy Environ. Sci. 7 (1) (2014) 399
407, published by The Royal Society of Chemistry.

show the exciton absorption peak in the UV
Vis spectrum [49]. These excitons show the fingerprints at similar wavelengths in the PL spectra as well. Fig. 3.17 compares PL and absorption spectra of two different mixed perovskite phases.

3.4.5 Tunability and stability of PL in alloyed perovskites The wavelength of photoluminescence can be systematically tuned by changing the composition of the perovskite absorber material exhibiting the general formula ABX3. In

Characterization Techniques for Perovskite Solar Cell Materials

FIGURE 3.17 PL spectra of alloyed perovskite thin films with two different compositions. Reproduced with permission from J.T. Jacobsson, J.P. Correa Baena, M. Pazoki, M. Saliba, K. Schenk, M. Gra¨tzel, et al., Exploration of the compositional space for mixed lead halogen perovskites for high efficiency devices, Energy Environ. Sci. 9 (2016) 1706
1724, Royal Society of Chemistry.

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principle and as clarified in Chapter 1, by changing A-cation, B-cation and/or halide (X) or using a mixture, the desired composition can be formulated. For example, by substituting MA1 with FA1, photoluminescence in the range of 780
850 nm can be obtained from APbI3 (MA1 or/and FA1). Similarly, using a mixture of halides seems to have a larger impact on the bandgap as compared to using a mixed A-cation formulation (Fig. 3.18) [40,49,58]. For example, by changing the iodide-bromide ratio in MAPbI3-xBrx, the emission wavelength can be finely tuned from 550 nm to 780 nm. On the other hand, a complete substitution of MA1 with FA1 from MAPbBr3 exhibits a minimal effect on the bandgap as the λmax of photoluminescence shifts by 10 nm from 560 nm for FAPbBr3 to 550 nm to MAPbBr3 [59]. This kind of flexibility in bandgap tuning by modifying the precursor concentrations in chemical solutions is rather unique for perovskites compared to other thin film solar cell technologies like silicon and CIGS which need expensive techniques with much less flexibility in the obtained bandgap. The state of the art perovskite composition that shows the highest efficiencies as well as the best reported stabilities, composed from a mixed composition, i.e. different halides and monovalent cations within the perovskite lattice. However, under the light illumination, a photo-induced phase segregation has been observed by which i.e. a mixed perovskite composed from I and Br halogens can be decomposed to a iodine reach and bromine reach perovskite regions (Fig. 3.19) [60]. Such phase segregations can be understood by a PL mapping technique in which PL spectrum from different regions of the film can be recorded and therefore fingerprints of iodine- or bromine- rich regions in comparison to the mixed-phase can be detected. This type of phase segregation is reported to be reversible after keeping the film long enough at dark [61]. A similar behavior has been observed from PL mapping data in pure MAPbI3 in which the iodine anions can diffuse towards the grain boundaries under light and come back after keeping in the dark in a process called photo-induced ionic movement [62] that has been suggested by stark spectroscopy

FIGURE 3.18 (A) Tuning the PL of alloyed perovskite thin films via altering the type of halide and monovalent cation and the ratios. (B) Macro images of the perovskite films deposited on glass corresponding to the PL spectra. Adopted from J.T. Jacobsson, J.P. Correa Baena, M. Pazoki, M. Saliba, K. Schenk, M. Gra¨tzel, et al., Exploration of the compositional space for mixed lead halogen perovskites for high efficiency devices, Energy Environ. Sci. 9 (2016) 1706
1724.

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3. Optical absorption and photoluminescence spectroscopy

FIGURE 3.19 Photo-induced segregation of mixed halide based perovskites. Adopted from E.T. Hoke, D.J. Slotcavage, E.R. Dohner, A.R. Bowring, H.I. Karunadasa, M.D. McGehee, Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics, Chem. Sci. 6 (1) (2015) 613
617; M.C. Brennan, S. Draguta, P.V. Kamat, M. Kuno, Light-induced anion phase segregation in mixed halide perovskites, ACS Energy Lett. (2018) [64,65], Published by The Royal Society of Chemistry (right) and American Chemical Society (left).

as well [63]. Moreover, a pure tetragonal perovskite encounter phase transitions to cubic and orthrombic at different temperatures that accompanied by peak shifts or additional peaks at PL spectrum. The precise tenability of emission wavelength achieved by mixing iodide and bromide generates an enormous amount of interest, primarily for light emission applications, however, the segregation of iodide-rich and bromide-rich perovskite domains under continuous illumination had eluded various application of these mixed-halide compositions (Fig. 3.19) [64]. This photo-induced segregation also imposed restrictions on the content on bromide present in iodide-based perovskite crystal structure. Furthermore, the existence of substantial parasitic non-radiative recombination in perovskite thin films and when interfaced into devices, is one of the main losses in the performance of perovskite-based devices that preventing them to reach their efficiency limit [66]. Recently, a novel approach known as potassium passivation is developed by AbdiJalebi et al. as an effective way to not only inhibit the photo-induced phase segregation and bandgap instability of alloyed perovskite materials but also leads to stabilized high luminesce yield (Fig. 3.20) [67]. In the potassium passivation, by filling the halide vacancies in the perovskite crystal structure via introducing excess halide and decorating the hybrid grains with K1 where the potassium draws out the Br from the perovskite lattice and lock the excess halide in the grain boundaries, the photo-induced segregation of mixed iodide-bromide compositions is completely mitigated [67]. Furthermore, the mechanistic insights gained through a combination of techniques including experiments and theory establish that energetically favorable segregation of iodide dominant domains induces the formation of phase mixture. The main criteria in an optoelectronic device to reach the efficiency limit is to maximize the luminescence quantum yield and inhibit any non-radiative losses in the active layer in particular when it is interfaced with the device electrodes (see Chapter 6 for the detailed description). There have been several approaches to enhance the PLQE of the perovskite films via ligand passivation where the transport properties were effected by adding the organic molecule into the perovskite layer [68]. However, using the potassium passivation

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71

FIGURE 3.20 (A) Stabilization of PLQE and (B
D) mitigation of photo-induced instability of perovskites thin films via potassium passivation. (E) Schematic showing the mechanism of the passivation approach. In all the panels, x, represent the fraction potassium added to the precursor solution of perovskites. Adopted from M. AbdiJalebi, Z. Andaji-Garmaroudi, S. Cacovich, C. Stavrakas, B. Philippe, J.M. Richter, etal., Maximizing and stabilizing luminescence from halide perovskites with potassium passivation, Nature 555 (7697) (2018) 497
501.

FIGURE 3.21 Enhanced (A) time-resolved photoluminescence lifetime and (B) external PLQE of the perovskite layer interfaced with device electrodes (n-type: TiO2/perovskite, p-type: perovskite/Spiro OMeTAD and Stack: TiO2/perovskite/Spiro OMeTAD). Adopted from M. Abdi-Jalebi, Z. Andaji-Garmaroudi, S. Cacovich, C. Stavrakas, B. Philippe, J.M. Richter, et al., Maximizing and stabilizing luminescence from halide perovskites with potassium passivation, Nature 555 (7697) (2018) 497
501.

approach, Abdi-Jalebi et al. able to reach significantly enhanced PL lifetime and high PLQE in the device stack (e.g. 15%) without perturbing the charge transport properties of the perovskite films (Fig. 3.21).

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3. Optical absorption and photoluminescence spectroscopy

3.4.6 Impact of perovskite crystalline quality, fluence and charge extraction layer on PL In addition to the radiative recombination, the charge-carrier dynamics also involves other processes, which are non-radiative in nature, such as, trap-assisted recombination also known as Shockley
Read
Hall (SRH) recombination and higher order recombinations, such as Auger recombination. The SRH recombination can be controlled by improving the crystal quality through minimizing the trap-state density within the perovskite material. These traps or defects can be classified into two major categories; (1) deep-traps, and (2) surface traps. The origin of deep traps is majorly associated with the halide vacancies or interstitial ions, whereas surface traps arise from the unsaturated bonds present at the grain boundaries and/or from the impurity phases interfacing with the absorber layer. Overall, the nature and amount of defects directly influence the kinetics of the charge carrier recombination thus the contribution from the corresponding recombination constants (Fig. 3.22) [69]. The PL fluence alters the charge carrier density, which directly influences the kinetics of recombination. For example, the contribution from Auger recombination becomes significant only under high influences when the charge carrier density exceeds 1017 cm23 (Fig. 3.23).

FIGURE 3.22 (A) Steady-state absorption, (B) time-integrated and (C
D) time-resolved emission dynamics as a function of the excitonic quality of FAPbBr3 films. FA (1) film deposition involves DMF solvent, FA (2) film deposition involves a mixture of DMF and DMSO solvent. Adopted from N. Arora, M.I. Dar, M. Hezam, W. Tress, G. Jacopin, T. Moehl, et al., Photovoltaic and amplified spontaneous emission studies of high-quality formamidinium lead bromide perovskite films, Adv. Funct. Mater. (2016).

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FIGURE 3.23 Charge carrier dynamics as a function of fluences in MAPbI3 films recorded at 300 K. (A) Fluence-dependent time-resolved PL of the tetragonal phase and (B) charge carrier lifetime (t10) recorded in the tetragonal phase at 300 K decreases with increasing fluence (t10, time at which the maximum PL intensity decreases by a factor of 10). Adopted from M.I. Dar, G. Jacopin, S. Meloni, A. Mattoni, N. Arora, A. Boziki, et al., Origin of unusual bandgap shift and dual emission in organic-inorganic lead halide perovskites. Sci. Adv. 2 (10) (2016).

Under low fluences, the trapping of charges at various defect sites plays a critical role, which can have a noticeable effect on the overall performances of the solar cells and light emitting devices [50,70]. The analytical expressions for kinetic of different recombination processes in the perovskite material are presented in Chapter 6. The recombination of charge carriers occurring within the perovskite absorber layer can be influenced by the presence of selective contacts including electron and hole transporting layers [71]. This is primarily due to a decrease in the initial density of electrons or holes present in the absorber layer when interfaced, respectively, with electron and hole transporting layers. Fig. 3.9A and B displays the effect of HOMO position on the hole extraction property of HTM. By increasing the energy gap between the valence band of perovskite layer and the HOMO of HTM, the extraction of holes from the former layer becomes relatively more rapid as evident from the PL quenching and time-resolved PL studies (Fig. 3.24A and B). Similarly, by improving the interfacial properties between the absorber layer and ETL, the extraction of electrons from the perovskite layer becomes relatively more efficient as shown by steady-state and time-resolved PL studies (Fig. 3.24C and D) [23]. As mentioned in previous Section 4.3.2, time resolved PL in the presence/ absence of charge extraction layers can be implemented to study the transport and diffusion processes in the perovskite film.

3.4.7 Temperature dependent PL in metal halide perovskite In addition to the fluence, various photo-physical processes show a strong dependence on the temperature as the chemical bonding between the constituent atoms or ions gets affected. Consequently, the lattice constant, lattice vibrations, and electron-phonon interactions etc. vary and influence the bandgap and charge-carrier dynamics. Contrary to

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3. Optical absorption and photoluminescence spectroscopy

FIGURE 3.24 Steady-state and time-resolved photoluminescence studies demonstrating the effect of interfaces on the charge extraction processes across various interfaces. (A and B) Adopted from N. Arora, S. Orlandi, M.I. Dar, S. Aghazada, G.Jacopin, M. Cavazzini, et al., High open-circuit voltage: fabrication of formamidinium lead bromide perovskite solar cells using fluorene-dithiophene derivatives as hole-transporting materials, ACS Energy Lett. (2016); (C and D) N. Arora, M.I. Dar, M. Abdi-Jalebi, F. Giordano, N. Pellet, G. Jacopin, et al., Intrinsic and extrinsic stability of formamidinium lead bromide perovskite solar cells yielding high photovoltage, Nano Lett. 16 (11) (2016) 7155
7162.

conventional semiconductors, the band gap of perovskite materials increases with increasing temperature. Such an unusual behavior cannot be explained using the Varshni model, according to which the bandgap should decrease with increasing temperature due to the dilatation of the lattice [72]. Although the similar phenomenon of lattice expansion occurs in the perovskite materials at higher temperatures, the origin of the unusual bandgap shift can be explained by evoking the antibonding nature of the valence and conduction band orbitals (Fig. 3.25) [50]. Another interesting feature shown by the perovskite emitters at low temperature ( . 120 K) is the dual emission, which is well distinct and well evident in MA-cation based lead halide perovskites. The classical molecular dynamics (CMD) shows that at low temperature, e.g., .150 K for MAPbI3, the organic cation can exhibit two different orientations; (1) perfectly antiparallel (ordered), and (2) random (disordered) alignment. The former is denoted as the ordered orthorhombic domain yielding emission around 750 nm at 15 K, whereas the latter gives rise to the disordered orthorhombic domain yielding emission

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3.4 Photoluminescence spectroscopy

FIGURE 3.25 (A) Normalized photoluminescence as a function of temperature in MAPbI3 recorded from 15 to 300 K. (B) Central energy of the emission peaks corresponding to MA-ordered (blue color, top plot) and MA-disordered orthorhombic domains (red symbols, bottom plot) and tetragonal phase (red symbols, bottom plot, T . 150 K) of CH3NH3PbI3 as a function of temperature. Adopted from M.I. Dar, G. Jacopin, S. Meloni, A. Mattoni, N. Arora, A. Boziki, et al., Origin of unusual bandgap shift and dual emission in organic-inorganic lead halide perovskites, Sci. Adv. 2 (10) (2016). (A)

(B) 1.8

(d)

(c)

1.75

100 K

(a)

300 K

(b)

Eg (eV)

1.7 1.65 1.6

Ortho ordered Ortho disordered Tetra

1.55

(e) 1.5

6.25

a*

6.3 a (Å)

6.35

6.4

Temperature

FIGURE 3.26 Classical molecular dynamics simulations. (A) Snapshots extracted from the simulations at (a) 100 K and (b) 300 K. Panels (c), (d), and (e) show the configurations of the MA-ordered and MA-disordered orthorhombic and the tetragonal domains, respectively. (B) Bandgap as a function of the pseudo-cubic lattice parameter (a) for the MA-ordered (square symbols) and MA-disordered (circle symbols) orthorhombic systems and the tetragonal system (rhombus symbols). Adopted from M.I. Dar, G. Jacopin, S. Meloni, A. Mattoni, N. Arora, A. Boziki, et al., Origin of unusual bandgap shift and dual emission in organic-inorganic lead halide perovskites, Sci. Adv. 2 (10) (2016).

around 788 nm, which is redshifted when compared to ordered orthorhombic domain (Fig. 3.26). Such a redshift could be explained by evoking the dipole moment of the organic cation, producing a local Stark-like effect, which is stronger in case of ordered phases than in disordered ones. In addition, due to the higher dipole moment of MA1 than FA1, the band gap difference between ordered and disordered systems is B85 meV,

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allowing to resolve the emission peaks. In case of an FA-based system, this energy difference turns out to be quite small, i.e., 20 meV, which apparently makes it difficult to resolve the emission features associated with ordered or disordered domains. Interestingly, the energy difference seems to be invariant of the nature of halide anion as substitution of all iodides with bromides from the MAPbI3 system does not influence the band gap difference between ordered and disordered systems, which remained constant, i.e., B85 meV.

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