Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Temperature dependency of excitonic effective mass and charge carrier conduction mechanism in CH3NH3PbI3−xClx thin films

Abstract

In this paper we explain the temperature dependence of excitonic effective mass and charge carrier conduction mechanism occurs in CH3NH3PbI3−xClx thin films prepared by chemical dip coating (CDC), spray pyrolysis (Spray) and repeated dipping-withdrawing (Dipping). Hall Effect study confirmed that prepared CH3NH3PbI3−xClx samples are p-type semiconductor having carrier concentration of the order of ~ 1016 cm−3. The charge carrier mobility, mean free path and mean free life time were found to decrease with increasing temperature due to polaronic effect. The excitonic effective mass is estimated to (0.090–0.196)me and excitonic binding energy (15–33) meV, well consistent with Wannier-Mott hydrogenic model and the nature of exciton is likely to be Mott-Wannier type. From electrical measurement, it was observed that charge carrier conduction in CH3NH3PbI3−xClx is governed by migration of \({\mathrm{I}}^{-}\) and CH3N \({\mathrm{H}}_{3}^{+}\) vacancies and vacancy-assisted diffusion processes depending on temperature.

Introduction

Now-a-days, methylammonium lead halide perovskites (MLHP) are regarded worldwide as an emerging material class in the field of photovoltaic and optoelectronics. The typically chemical formula of MLHP is AMX3, where A is the organic/inorganic cation (CH3NH3), M is a heavy metal cation (Pb) and X is halide anions (I). When X site is replaced by another halogen element such as chlorine (Cl) into AMX3, it converts to mixed halide perovskites CH3NH3PbI3−xClx. This mixed halide perovskites exhibit semiconducting nature with some exciting properties for photovoltaic and optoelectronic device applications. The single-junction solar cell architecture, based on CH3NH3PbI3−xClx perovskite, shows power conversion efficiency of 25.5%1. Besides, CH3NH3PbI3−xClx thin film shows multi-colored emission2,3,4,5, second harmonic generation or frequency doubling5, bimolecular recombination6, trap-assist recombination7, strong tunable interband transitions8, and ambipolar charge transport9. However, mixed halide perovskite suffers a compositional instability due to halide (I, Br) segregation10. But a very small addition of Cl into (I/Br) lattice restrains the halide phase segregation, improves the charge carrier mobility, carrier lifetime and stability of solar cells without changing the crystallographic domain size11. These properties are directly correlated with the temperature dependent charge transport behavior in CH3NH3PbI3−xClx and expected to improve device configuration compared to (I/Br) composition.

Up to now, multiple approaches such as transient terahertz (THz) spectroscopy6,12, combination of time-resolved terahertz with optical transient reflection spectroscopy (TRTS-TR)13, photoluminescence quenching (PLQ)14, time resolved electro-absorption spectroscopy15, time-of-flight (TOF) photoconductivity16 and Hall effect measurement17 have been carried out to explore the transport properties of lead halide perovskites. These pioneer studies reveal the presence of non-Langevin charge carriers6, strong back-scattering on free carrier dynamics12, existence of higher hole mobility compared to electron mobility13, large electron–hole diffusion lengths (˃1 μm)14, electric field assisted charge carrier seperation15, grain size dependent mobility16, and influence of self-doping in carrier concentration17. Despite the advancement in deep-seated perceptive in transport behavior, a lot of issues such as charge-carrier type, the mechanisms for restraining carrier mobility, origin of electron–hole recombination rates, donor or acceptor ionization energy, position of Fermi level, effective mass of charge-carriers, grain boundary barrier height, carrier mean free path-life time in CH3NH3PbI3−xClx still perplexes the researchers. It is well known that the mixed halide perovskite undergoes a phase transition from orthorhombic-tetragonal-cubic at 160 K and in between (315–330) K, respectively18,19. However, the study of charge carrier dynamics is limited to tetragonal phase while it is also important to explore such properties for cubic phase above room temperature (RT).

Recently, it is reported that charge carries generate self-induced rotation of the organic part (CH3N \({\mathrm{H}}_{3}^{+}\)), which polarizes lead halide perovskite lattice to form quasi-particle polarons20,21,22,23,24,25. The conception of polaron is used to explain intrinsic electrical properties because it describes the interaction between charge carriers and phonons. The interaction of charge carriers with the organic part has significant influence on the effective mass, temperature and magnetic field dependent carrier transport and electrical conductivity of the mixed halide perovskite material.

In our previous work we report the characterization of nano-crystalline CH3NH3PbI3−xClx thin films prepared on glass substrate by chemical dipping-withdrawing (CDC)2, spray pyrolysis (Spray)5 and repeated dipping-withdrawing (Dipping)3 techniques in an ambient atmosphere. Our study confirmed the presence of tetragonal and mixed cubic-tetragonal phase in CH3NH3PbI3−xClx thin film. The film morphology was composed with spherical, rod, cuboid and polyhedral like crystal grains of sizes 100 nm to 2 μm2,3,5. However, this article concentrates on the charge carrier transport of CH3NH3PbI3−xClx thin films using Hall Effect and dc-electrical conductivity measurement as a function of temperature, ranging from RT to 378 K. The carrier concentration, mobility, excitonic effective mass, excitonic binding energy, Fermi level, grain boundary barrier height and mean free life time of charge carriers are calculated from Hall Effect study. Furthermore, activation energies in CH3NH3PbI3−xClx thin films are exposed from electrical study. The effect of polarons on charge carrier dynamics and possible carrier conduction mechanism in CH3NH3PbI3−xClx thin films has been discussed. Therefore, in-depth study of charge carrier transport and conduction mechanism above RT for CH3NH3PbI3−xClx may provide a broader impact in the arena of perovskite research.

Results

Temperature dependent Hall effect study

Temperature dependent Hall voltage measurement is done at a constant magnetic field of 9.815 KG and the variation of Hall voltage is shown in Fig. 1. From the Hall Effect study Hall mobility (µH) and carrier concentration (nc) have also been calculated.

Figure 1
figure 1

Hall voltage of CH3NH3PbI3−xClx. Variation of Hall voltage at constant magnetic field of 9.815 KG with temperature for CH3NH3PbI3−xClx thin films using Hall measurement setup.

From Fig. 1 it is seen that the Hall voltage is positive and increased up to a certain temperature after which it decreases all through the measured temperature range for all samples. The positive sign of Hall voltage indicates that CH3NH3PbI3−xClx is a p-type material. It is reported that a lot of Pb, CH3NH3, I and Cl vacancies present in the films17,26,27 as because precursor solution was prepared by dissolving CH3NH3I and PbCl2 at a molar ratio 3:1. However, Pb and CH3NH3 vacancies play the vital role for p-type conductivity of prepared CH3NH3PbI3−xClx thin films because of the lower formation energy of Pb and CH3NH3 compared to halogen vacancies.

The temperature dependent carrier concentration (left scale) and mobility (right scale) for (a) CDC, (b) spray and (c) dipping deposite CH3NH3PbI3−xClx thin films are shown in Fig. 2. The carrier concentration and mobility of dipping deposited sample is found almost half of the CDC and spray deposited samples. However, RT mobilities are found higher for all our samples than previously reported works6,14,16,19,28,29,30,31,32. Table 1 shows a comparison of RT mobilities for perovskite CH3NH3PbI3−xClx obtained in different studies. It is reported that the breaking of inversion symmetry generates Rashba effect33 which enhance longitudinal optical phonon scattering by displacing the organic part or Pb-halogen bending or stretching and limits the increase of charge-carrier mobility at room temperature.

Figure 2
figure 2

Charge carrier transport of CH3NH3PbI3−xClx. Variation of carrier concentration (left scale) and mobility (right scale) with temperature at a constant magnetic field of 9.815 KG for (a) CDC, (b) spray and (c) dipping deposited CH3NH3PbI3−xClx thin films.

Table 1 The RT Hall carrier concentration (nc) and mobility (μH) of CDC, spray and dipping deposited CH3NH3PbI3−xClx thin films.

The carrier concentration and mobility can be affected by both magnetic field and temperature but the effect of temperature is dominant. In this study, the carrier concentration of all CH3NH3PbI3−xClx samples is found to increase exponentially (Fig. 2) with increasing temperature but mobility shows completely an opposite trend. Such behavior can be explained as: the presence of Pb, CH3NH3 and halogen vacancies in mixed halide perovskite creates free dipolar polarons by inducing a rotational re-organization of organic CH3NH3 dipoles20; such polarons can migrate between the inorganic site of perovskite structure and lower the electron–hole recombination. Furthermore, sub-lattice thermal vibrations of inorganic octahedral have also an effect on carrier concentration and mobility in perovskite samples. Both the reorientation of organic site and sub-lattice thermal vibration enhance the localized charge carriers which introduce a dynamic force for the movement of charges with increasing temperature. Therefore, a polarization potential of polaron originated by enhanced localization suppresses carrier velocity and reduce the mobility of CH3NH3PbI3−xClx samples. Such a decrease of mobility with increasing temperature is reported19 measured by ultrafast spectroscopic method at temperature range -200 °C to 100 °C.

Effective mass, excitonic binding energy and Fermi level calculation

The acceptor ionization energy, Fermi level and effective mass of charge carriers have been calculated from the temperature dependent Hall voltage measurement. The temperature dependent carrier concentration in the presence of a constant magnetic field can be represented by following equations,

$$ {\text{n}}_{{\text{c}}} = \left( {{\text{p}}_{0} {\text{N}}_{{\text{a}}} } \right)^{\frac{1}{2}} \exp \left( { - \frac{{{\text{E}}_{{\text{a}}} }}{{2{\text{K}}_{{\text{B}}} {\text{T}}}}} \right) $$
(1)

and

$$ {\text{p}}_{0} = \, 2\left( {\frac{{2\pi {\text{m}}_{{\text{h}}}^{*} {\text{K}}_{{\text{B}}} {\text{T}}}}{{\hbar^{2} }}} \right)^{\frac{3}{2}} $$

where Ea is the acceptor ionization energy, Na is the acceptor density, \({\mathrm{m}}_{\mathrm{h}}^{*}\) is the effective mass of hole, T is the absolute temperature, KB is the Boltzmann constant and ħ is the reduced Planck constant. The value of Ea can be obtained from the slope of ln \(\left({\mathrm{n}}_{\mathrm{c}}{\mathrm{T}}^{-\frac{3}{4}}\right)\) vs. 1/T graph (Fig. 3). From figure it is clear that the variation of carrier concentration with temperature can be defined by two discrete temperature regions (TR): TR-I (298–343) K and TR-II (344–378) K. In TR-II, the values of ln \(\left({\mathrm{n}}_{\mathrm{c}}{\mathrm{T}}^{-\frac{3}{4}}\right)\) decrease linearly with inverse temperature, whereas, in TR-I, it decreases monotonically with lower slope compared to TR-II. The effective mass of hole \({\mathrm{m}}_{\mathrm{h}}^{*}\) can be found out using Ea as,

$$ {\text{E}}_{{\text{a}}} = \frac{{{\text{e}}^{4} {\text{m}}_{{\text{h}}}^{*} }}{{2\left( {4\pi \varepsilon \varepsilon_{0 } \hbar } \right)^{2} }} $$
(2)
Figure 3
figure 3

Diagram for calculating acceptor ionization energy. ln(ncT−3/4) vs inverse temperature graph for CDC, spray and dipping deposited CH3NH3PbI3−xClx thin films.

where ε is the dielectric constant of the material and ε0 is the permittivity of free space. Several research groups used dielectric constant ε = 5.6 to 25.7 (for high to low frequency) and excitonic effective mass μ* = 0.1me for calculating excitonic binding energy of perovskite. However, in this study ε = 9 has been used for calculating \({\mathrm{m}}_{\mathrm{h}}^{*}\) according to hydrogenic model35. The values of \({\mathrm{m}}_{\mathrm{h}}^{*}\) are estimated to 0.16me, 0.42me, 0.38me (me is the rest mass of electron) in the TR-I and 2.16me, 2.23me, 3.04me in the TR-II for CDC, spray and dipping deposited samples, respectively. The estimated \({\mathrm{m}}_{\mathrm{h}}^{*}\) is found in good agreement with recently reported values36,37. Taking the effective mass of electron \({\mathrm{m}}_{\mathrm{e}}^{*}\) = 0.21me from theoretical calculations38, the excitonic effective mass μ* is estimated to (0.090–0.196)me using equation, 1/μ* = 1/\({\mathrm{m}}_{\mathrm{e}}^{*}\) + 1/\({\mathrm{m}}_{\mathrm{h}}^{*}\) and tabulated in Table 2. The estimated μ* in TR-I is close to ~ 0.104me35, found for tetragonal phase (≥ 145 K). However, μ* in TR-II is found high because of non-parabolic nature of valance bands in CH3NH3PbI3−xClx. Theoretical study39 suggested that lead vacancies diminish anti-bonding atomic orbital overlap resulting in flatten of valance bands. Moreover, polaronic effect40 and strong hydrogen bonding due to van-der-Waals interactions39 may also increase μ*.

Table 2 The acceptor ionization energy (Ea), acceptor density (Na), Fermi level (EF), excitonic reduced effective mass (μ*) and excitonic binding energy (Eb) of charge carrier for CDC, spray and dipping deposited CH3NH3PbI3−xClx thin films for TR-I (298–343) K and TR-II (344–378) K.

The excitonic binding energy, Eb of all perovskite samples has been calculated using the equation,

$$ {\text{E}}_{{\text{b}}} = \frac{{{\text{m}}_{{\text{e}}} {\text{e}}^{4} }}{{2\left( {4\pi \epsilon _{0} \varepsilon \hbar } \right)^{2} }}\frac{{\mu ^{*} }}{{{\text{m}}_{{\text{e}}} }}\left( {{\text{eV}}} \right){\text{ }} \approx {\text{ }}13.56\frac{{\mu ^{*} }}{{{\text{m}}_{{\text{e}}} }}\frac{1}{{\varepsilon ^{2} }}\left( {{\text{eV}}} \right) $$
(3)

The excitonic binding energy is found (15–24) meV for TR-I and (32–33) meV for TR-II, which is well consistent according to Wannier–Mott hydrogenic model. This variation of Eb in TR-I and TR-II is due to temperature dependent polaronic effect of CH3NH3PbI3−xClx. The extending radius of the lowest bound state r* is calculated to ascertain the nature of excitons in CH3NH3PbI3−xClx using the equation, r* = ε (me*) rb, where, rb is the Bohr radius. The r* is estimated to 5.29 nm, 3.40 nm, 3.52 nm in TR-I and 2.49 nm, 2.48 nm, 2.43 nm in TR-II for CDC, spray and dipping deposited samples. The value of r* is larger than the lattice constants (either cubic or tetragonal) of CH3NH3PbI3−xClx indicating that the exciton is weak and likely to be Mott–Wannier type.

The position of Fermi level EF can be determined by knowing the value of Ea, Na and \({m}_{h}^{*}\) of the following equation

$$ {\text{E}}_{{\text{F}}} = - \frac{1}{2}{\text{E}}_{{\text{a}}} + { }\frac{1}{2}\left( {{\text{K}}_{{\text{B}}} {\text{T}}} \right){\text{ln}}\left[ {\frac{{{\text{N}}_{{\text{a}}} {\text{h}}^{3} }}{{2\left( {2{\pi m}_{{\text{h}}}^{*} {\text{K}}_{{\text{B}}} {\text{T}}} \right)^{\frac{3}{2}} }}} \right] $$
(4)

The values of Ea, Na and EF for both higher and lower temperature regions are tabulated in Table 2. The parameters nc and Na are not equal but vary with temperature and film processing methods as well, indicates CH3NH3PbI3−xClx is very sensitive to environment, chemical composition, growth parameters etc. The Fermi energy is found negative means EF lies below the acceptor level.

Grain boundary parameters and grain size calculations

The transport properties of a polycrystalline semiconductor are generally influenced by grain boundary effect. According to grain boundary trapping model41, the trapping states create a depletion region in the grain and a potential barrier at the interface. In a semiconductor sample the relation between charge carrier mobility and grain boundary barrier height can be expressed as

$$ \mu_{{\text{h}}} = \, \mu_{0} {\text{T}}^{{ - \frac{1}{2}}} {\text{exp}}\left( {\frac{{ - \phi_{{\text{b}}} }}{{{\text{K}}_{{\text{B}}} {\text{T}}}}} \right){\text{ with}} $$
(5)
$$ \mu_{0} = \frac{{{{e\zeta }}}}{{\left( {8{\pi m}_{{\text{h}}}^{*} {\text{K}}_{{\text{B}}} } \right)^{\frac{1}{2}} }}, $$

where, ϕb is the grain boundary barrier height and \(\upzeta \) is the grain size. The slop of ln(μhT1/2) vs 1/T graph (Fig. 4) gives the barrier height ϕb and intercept will provide grain size \(\upzeta .\)

Figure 4
figure 4

Figure for calculating grain boundary parameters. ln (μh T1/2) vs 1/T graph for CDC, spray and dipping deposited CH3NH3PbI3−xClx thin films.

The barrier height and grain size of all samples have been estimated for two different temperature regions (TR-I, TR-II) and tabulated in Table 3. From table it is clearly seen that the formation of bigger grain size is favorable at low temperature (TR-I) for mixed halide perovskites.

Table 3 Grain boundary barrier height (ϕb), grain size (\(\zeta \)), mean free path (Lm), mean free life time (τm) and activation energies (ΔE) for CDC, spray and dipping deposited CH3NH3PbI3−xClx thin films.

Mean free path and mean free time calculations

According to conventional Drude-Sommerfeld model, the carrier mean free path (Lm) and mean free time (τm) in CH3NH3PbI3−xClx thin films can be calculated from the following equations

$$ {\text{L}}_{{\text{m}}} = {\upmu }_{{\text{H}}} \frac{{(3{\text{m}}_{{\text{h}}}^{*} {\text{K}}_{{\text{B}}} {\text{T}})^{\frac{1}{2}} }}{{\text{e}}} $$
(6)
$$ \tau_{{\text{m}}} = \frac{{{\upmu }_{{\text{H}}} {\text{m}}_{{\text{h}}}^{*} }}{{\text{e}}} $$
(7)

The variation of Lm and τm with temperature for CDC, spray and dipping deposited CH3NH3PbI3−xClx thin films are given in Fig. 5.

Figure 5
figure 5

Mean free path and mean free time of CH3NH3PbI3−xClx. Mean free path and mean free time at different temperature region for (a) CDC, (b) spray and (c) dipping deposited CH3NH3PbI3−xClx thin films.

From figures it is seen that both Lm and τm are lower at TR-I compared to TR-II. The magnitudes of Lm and τm decrease with the increase of temperature except a sudden jump at around 340 K. The polaronic effect originated from rotational re-organization of CH3NH3 dipoles enhances trap assisted recombination with increasing temperature which reduces the carrier mean free path and mean free life time.

In the mixed halide perovskite, the transition from tetragonal to cubic phase occurs at temperature ~ 330 K19,42 due to the tilted inorganic PbI6 octahedral and disparity of organic CH3N \({\mathrm{H}}_{3}^{+}\) rotation. Such phase transition can modify the physical properties of mixed halide perovskites, though it does not cause remarkable change in optical and photovoltaic properties43. Recently, tetragonal to cubic phase transition is associated with the relative strength of the metal-halogen and hydrogen bonds as reported44. In our study, during the temperature dependent Hall measurement we have applied a magnetic field of ~ 1 T. This magnetic energy shifted the tetragonal to cubic phase transition at slightly higher temperature at (340–344) K.

Conduction mechanisms and activation energies

The electrical conduction process in CH3NH3PbI3−xClx is rather complex. It is strongly dependent on temperature, vacancies or interstitial defects and activation energies of the materials. In general, CH3NH3PbI3−xClx includes Pb, CH3NH3 and halogen vacancies as suggested from theoretical and experimental studies45,46. The low energy vacancies create a rotational re-organization of organic CH3NH3 dipoles resulting in free dipolar polarons in mixed halide perovskite20. Eames et al.45 proposed that these vacancies migrate from one site to the neighboring site due to low activation energy. They found activation energies involved 0.58 eV and 0.84 eV for migrating \({\mathrm{I}}^{-}\) and CH3N \({\mathrm{H}}_{3}^{+}\) vacancies, respectively. Futscher et al.47 also reported that the predicted activation energies for migration of \({\mathrm{I}}^{-}\) and CH3N \({\mathrm{H}}_{3}^{+}\) are (0.08–0.58) eV and (0.46–1.12) eV, respectively. Besides, grain boundary effect has an influence on carrier transport and the activation energies (0.18–0.27) eV were predicted for different grain size above 260 K48. In Fig. 6, it is clearly seen that there are three temperature regions where carriers are conducted through different conduction mechanisms. For further clarification, the activation energy was also calculated by Arrhenius equation which contains three separate activation energy terms,

$$ {\upsigma }_{{{\text{dc}}}} = {\upsigma }_{{\text{I}}} {\text{exp }}\left( {\frac{{ - \Delta {\text{E}}_{{\text{I}}} }}{{{\text{K}}_{{\text{B}}} {\text{T}}}}} \right) + {{ \upsigma }}_{{{\text{II}}}} {\text{exp }}\left( {\frac{{ - \Delta {\text{E}}_{{{\text{II}}}} }}{{{\text{K}}_{{\text{B}}} {\text{T}}}}} \right) + {\upsigma }_{{{\text{III}}}} {\text{exp }}\left( {\frac{{ - \Delta {\text{E}}_{{{\text{III}}}} }}{{{\text{K}}_{{\text{B}}} {\text{T}}}}} \right) $$
(8)
Figure 6
figure 6

Electrical properties of CH3NH3PbI3−xClx. Temperature dependent dc-electrical conductivity of CH3NH3PbI3−xClx thin films showing three distinct regions. Inset shows the variation of lnσ with 1000/T graph for calculating activation energy.

where, KB is the Boltzmann constant, T is the absolute temperature, \({\upsigma }_{\mathrm{I}},\) \({\upsigma }_{\mathrm{II}}\) and \({\upsigma }_{\mathrm{III}}\) are pre-exponential factors and ΔEI, ΔEII and ΔEIII are activation energies for region I, II and III, respectively. From the slope of lnσ vs 1000/T plot (inset of Fig. 6), the activation energies were calculated and tabulated in Table 3.

From Fig. 6, it is seen that dc-electrical conductivity (σdc) decreases with increasing temperature in region I (294–317) K. The initial decrease is caused by the strong interactions in between organic–inorganic part and rotational disorder of methylammonium cation within perovskite structure. In addition, longitudinal optical phonon scattering may also decrease the conductivity with temperature in region I. The activation energy ΔEI (Table 3) is found negative in region I, suggesting the Fermi level is pinned to the valance band of all samples. In region II (318–357) K, the dc-electrical conductivity increase slowly with increasing temperature and ΔEII = 0.44, 0.51, 0.47 eV for CDC, spray and dipping deposited samples respectively, which indicates the migration of \({\mathrm{I}}^{-}\) vacancies to PbI6 octahedron edge45,46,47. In region III (358–378) K, σdc increases rapidly and ΔEIII ˃ ΔEII. The activation energies ΔEIII are found close to 0.84 eV for CDC and spray deposited and ΔEIII = 0.57 eV for dipping deposited samples. The rapid increase of σdc in region III occurs because of diffusion or migration of CH3N \({\mathrm{H}}_{3}^{+}\) in addition to \({\mathrm{I}}^{-}\) vacancies to PbI6 octahedra or neighbor central vacant site45,46,47. Therefore, the conduction processes are attributed to the migration of \({\mathrm{I}}^{-}\) and CH3N \({\mathrm{H}}_{3}^{+}\) suggesting the existence of ionic transport in CH3NH3PbI3−xClx. Moreover, the concentrations of charge carriers (majority holes) which are activated with acceptor ionization energy (Ea) favorable for the transport across grain boundaries of CH3NH3PbI3−xClx samples.

Discussion

In summary, the charge carrier conduction mechanism of CH3NH3PbI3−xClx thin films have been evaluated as a function of temperature ranging from room temperature to 378 K. Hall measurement study confirms that the prepared CH3NH3PbI3−xClx films are p-type semiconductor with carrier concentration of the order of ~ 1016 cm−3. The carrier mobility found to decrease with increasing temperature due to polaronic effect. At RT, the mobilities for CDC, spray and dipping deposited samples are 46, 49 and 20 cm2V−1 s−1, respectively. The excitonic effective mass (μ*) is estimated to be (0.090–0.196)me and excitonic binding energy (Eb) varies from (15–33) meV, suggesting Mott-Wannier type exciton. Additionally, the grain boundary barrier height, mean free path and mean free life time of charge carriers have also been evaluated. Electrical study reveals that multi-conduction processes are involved in CH3NH3PbI3−xClx, which is governed by the migration of vacancies, vacancy-assisted diffusion of mobile ions and accumulation of holes across grain boundaries.

Methods

Synthesis

Lead halide perovskite solution for fabricating CH3NH3PbI3−xClx thin films was prepared using the following procedure. Methylamine (CH3NH2) solution was mixed with hydroiodic acid and ethanol at room temperature. The solution was perturbed continuously by stirring with a magnetic stirrer at a constant speed in an ice bath for 2 h. The color of CH3NH3I solution transforms from red to transparent and heated at 100 °C for 4 h in a furnace to achieve white-colored CH3NH3I powder. Finally, CH3NH3I and lead(II) chloride powders were dissolved in anhydrous N,Ndimethylformamide at a molar ratio 3:1 to achieve a mixed halide perovskite precursor solution.

CH3NH3PbI3−xClx thin films have been fabricated at ambient atmosphere by three different methods. These methods are (a) Chemical dip-coating (CDC) (b) Spray pyrolysis (spray) and (c) Repeated dipping-withdrawing (dipping). The details of preparing CH3NH3PbI3−xClx solution and different steps which have been made in these methods to deposit CH3NH3PbI3−xClx thin films have been reported elsewhere2,3,5.

Characterization

Temperature dependent Hall voltage was measured at constant magnetic field of 9.815 KG using the conventional van-der-Pauw method. The magnetic field used for the study of Hall Effect was provided by electromagnets designed and produced by Newport instruments Ltd. England. The circuit arrangement for temperature dependent Hall Effect measurement is shown in Fig. 1S. A dc voltage from the power supply unit was applied in order to flow the sample current through the specimen. Current and voltages were measured by a digital electrometer and a digital multi-meter. A voltage stabilizer was used so that a steady supply of current without fluctuation was maintained through the magnet. A special designed heater was used to vary temperature. The variation of temperature is controlled by a variac and measured by a chromel–alumel thermo-couple connected with a digital multi-meter.

The resistivity of all samples was measured by van-der-Pauw method. Experimental setup of the van-dar-Pauw’s specimen to measure the resistivity with varying temperature is shown in Fig. 2S. The voltage and current of all samples were measured for different temperatures. The sample is fixed to a sample holder which is placed on a specially designed heater to vary temperature. The conductivity and activation energies of prepared samples have been calculated using resistivity data. The experimental details of temperature dependent Hall Effect measurement and resistivity measurement are shown in supplementary section.

References

  1. NREL efficiency chart. Available online: https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies.20200104.pdf.

  2. Karim, A. M. M. T. et al. Multi-color excitonic emissions in chemical dip-coated organolead mixed-halide perovskite. Chem. Sel. 3, 6525–6530 (2018).

    Google Scholar 

  3. Karim, A. M. M. T. et al. Solution-processed mixed halide CH3NH3PbI3xClx thin films prepared by repeated dip coating. J. Mater. Sci. 54, 11818–11826 (2019).

    ADS  CAS  Article  Google Scholar 

  4. Karim, A. M. M. T., Hossain, M. S. & Khan, M. K. R. Coexistence of cubic and tetragonal phase with colored excitonic emissions in mixed halide Perovskite CH3NH3PbI3xClx thin film. J. Phys. Conf. Ser. 1412, 132016 (2020).

    Article  Google Scholar 

  5. Karim, A. M. M. T. et al. Structural, opto-electronic characteristics and second harmonic generation of solution processed CH3NH3PbI3xClx thin film prepared by spray pyrolysis. Mater. Sci. Eng. B 54, 11818–11826 (2020).

    Google Scholar 

  6. Wehrenfennig, C., Eperon, G. E., Johnston, M. B., Snaith, H. J. & Herz, L. M. High charge carrier mobilities and lifetimes in organolead trihalide perovskites. Adv. Mater. 26, 1584–1589 (2014).

    CAS  PubMed  Article  Google Scholar 

  7. Jin, H. et al. It’s a trap! On the nature of localised states and charge trapping in lead halide perovskites. Mater. Horiz. 7, 397–410 (2020).

    CAS  Article  Google Scholar 

  8. Polushkin, A. S., Tiguntseva, E. Y., Pushkarev, A. P. & Makarov, S. V. Single-particle perovskite lasers: From material properties to cavity design. Nano Photon. 9, 599–610 (2020).

    CAS  Google Scholar 

  9. Srivastava, P. & Bag, M. Elucidating tuneable ambipolar charge transport and field induced bleaching at CH3NH3PbI3/electrolyte interface. Phys. Chem. Chem. Phys. 22, 11062–11074 (2020).

    CAS  PubMed  Article  Google Scholar 

  10. Hoke, E. T. et al. Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics. Chem. Sci. 6, 613–617 (2015).

    CAS  PubMed  Article  Google Scholar 

  11. Xu, J. et al. Triple-halide wide-band gap perovskites with suppressed phase segregation for efficient tandems. Science 367, 1097–1104 (2020).

    ADS  CAS  PubMed  Article  Google Scholar 

  12. La-O-Vorakiat, C. et al. Elucidating the role of disorder and free-carrier recombination kinetics in CH3NH3PbI3 perovskite films. Nat. Commun. 7, 11054 (2016).

    ADS  CAS  PubMed  PubMed Central  Article  Google Scholar 

  13. Zhai, Y. et al. Individual electron and hole mobilities in lead-halide perovskites revealed by noncontact methods. ACS Energy Lett. 5, 47–55 (2020).

    CAS  Article  Google Scholar 

  14. Stranks, S. D. et al. Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber. Science 342, 341 (2013).

    ADS  CAS  PubMed  Article  Google Scholar 

  15. Paraecattil, A. A., Jonghe-Risse, J. D., Pranculis, V., Teuscher, J. & Moser, J.-E. Dynamics of photocarrier separation in MAPbI3 perovskite multigrain films under a quasistatic electric field. J. Phys. Chem. C 120, 19595–19602 (2016).

    CAS  Article  Google Scholar 

  16. Emin, S., Pavlica, E., Okuyucu, H., Valant, M. & Bratina, G. Charge carrier transport in polycrystalline CH3NH3PbI3 perovskite thin films in a lateral direction characterized by time-of-flight photoconductivity. Mater. Chem. Phys. 220, 182–189 (2018).

    CAS  Article  Google Scholar 

  17. Wang, Q. et al. Qualifying composition dependent p and n self-doping in CH3NH3PbI3. Appl. Phys. Lett. 105, 163508 (2014).

    ADS  Article  CAS  Google Scholar 

  18. Baikie, T. et al. Synthesis and crystal chemistry of the hybrid perovskite (CH3NH3)PbI3 for solid-state sensitised solar cell applications. J. Mater. Chem. A 1, 5628–5641 (2013).

    CAS  Article  Google Scholar 

  19. Milot, R. L., Eperon, G. E., Snaith, H. J., Johnston, M. B. & Herz, L. M. Temperature-dependent charge-carrier dynamics in CH3NH3PbI3 perovskite thin films. Adv. Funct. Mater. 25, 6218–6227 (2015).

    CAS  Article  Google Scholar 

  20. Chen, Y. et al. Extended carrier lifetimes and diffusion in hybrid perovskites revealed by Hall effect and photoconductivity measurements. Nat. Commun. 7, 12253 (2016).

    ADS  CAS  PubMed  PubMed Central  Article  Google Scholar 

  21. Wiktor, J., Ambrosio, F. & Pasquarello, A. Mechanism suppressing charge recombination at iodine defects in CH3NH3PbI3 by polaron formation. J. Mater. Chem. A 6, 16863–16867 (2018).

    CAS  Article  Google Scholar 

  22. Meggiolaro, D., Ambrosio, F., Mosconi, E., Mahata, A. & Angelis, F. D. Polarons in metal halide perovskites. Adv. Energy Mater. 10, 1902748 (2019).

    Article  CAS  Google Scholar 

  23. Zheng, F. & Wang, L.-W. Large polaron formation and its effect on electron transport in hybrid perovskite. Energy Environ. Sci. 12, 1219–1230 (2019).

    CAS  Article  Google Scholar 

  24. Baranowski, M. & Plochocka, P. Excitons in metal-halide perovskites. Adv. Energy Mater. 10, 1903659 (2020).

    CAS  Article  Google Scholar 

  25. Jin, L., Qian, Y., Zhang, Y., Bowen, M. & Ding, S. Polaronic transport in CH3NH3PbI3 single crystals. J. Mater. Sci. Mater. Electron. 31, 1945–1950 (2020).

    CAS  Article  Google Scholar 

  26. Yin, W-J., Shi, T. & Yan, Y. Unusual defect physics in CH3NH3PbI3 perovskite solar cell absorber. Appl. Phys. Lett. 104, 063903 (2014).

  27. Frolova, L. A., Dremova, N. N. & Troshin, P. A. The chemical origin of the p-type and n-type doping effects in the hybrid methylammonium-lead iodide (MAPbI3) perovskite solar cells. Chem. Commun. 51, 14917 (2015).

    CAS  Article  Google Scholar 

  28. Wehrenfennig, C., Liu, M., Snaith, H. J., Johnston, M. B. & Herz, L. M. Charge-carrier dynamics in vapour-deposited films of the organolead halide perovskite CH3NH3PbI3−xClx. Energy Environ. Sci. 7, 2269–2275 (2014).

    CAS  Article  Google Scholar 

  29. Karakus, M. et al. Phonon-electron scattering limits free charge mobility in methylammonium lead iodide perovskites. J. Phys. Chem. Lett. 6, 4991–4996 (2015).

    CAS  PubMed  Article  Google Scholar 

  30. Hutter, E. M., Eperon, G. E., Stranks, S. D. & Savenije, T. J. Charge carriers in planar and meso-structured organic-inorganic perovskites: Mobilities, lifetimes, and concentrations of trap states. J. Phys. Chem. Lett. 6, 3082–3090 (2015).

    CAS  PubMed  Article  Google Scholar 

  31. Ponseca, C. S. Jr. et al. Organometal halide perovskite solar cell materials rationalized: Ultrafast charge generation, high and microsecond-long balanced mobilities, and slow recombination. J. Am. Chem. Soc. 136, 5189–5192 (2014).

    CAS  PubMed  Article  Google Scholar 

  32. Reid, O. G., Yang, M., Kopidakis, N., Zhu, K. & Rumbles, G. Grain-size limited mobility in methylammonium lead iodide perovskite thin-films. ACS Energy Lett. 1, 561–565 (2016).

    CAS  Article  Google Scholar 

  33. Stranks, S. D. & Plochocka, P. The influence of the Rashba effect. Nat. Mater. 17, 377–384 (2018).

    ADS  Article  CAS  Google Scholar 

  34. Dong, Q. et al. Electron-hole diffusion lengths > 175 μm in solution-grown CH3NH3PbI3 single crystals. Science 347, 967–970 (2015).

    ADS  CAS  PubMed  Article  Google Scholar 

  35. Miyata, A. et al. Direct measurement of the exciton binding energy and effective masses for charge carriers in organic–inorganic tri-halide perovskites. Nat. Phys. 11, 582–587 (2015).

    CAS  Article  Google Scholar 

  36. Yu, C.-J. Advances in modelling and simulation of halide perovskites for solar cell applications. J. Phys. Energy 1, 022001 (2019).

    ADS  CAS  Article  Google Scholar 

  37. Davies, C. L. et al. Bimolecular recombination in methylammonium lead triiodide perovskite is an inverse absorption process. Nat. Commun. 9, 293 (2018).

    ADS  PubMed  PubMed Central  Article  CAS  Google Scholar 

  38. Menendez-Proupin, E., Palacios, P., Wahnon, P. & Conesa, J. C. Self-consistent relativistic band structure of the CH3NH3PbI3 perovskite. Phys. Rev. B 90, 045207 (2014).

    ADS  CAS  Article  Google Scholar 

  39. Ashari-Astani, N. et al. Computational characterization of the dependence of halide perovskite effective masses on chemical composition and structure. J. Phys. Chem. C. 121, 23886–23895 (2017).

    CAS  Article  Google Scholar 

  40. Feynman, R. P. Slow electrons in a polar crystal. Phys. Rev. 97, 660 (1955).

    ADS  CAS  MATH  Article  Google Scholar 

  41. Kazmerski, L. L. Polycrystalline and Amorphous Thin Films and Devices (Academic Press, 1980).

    Google Scholar 

  42. Ong, K. P., Goh, T. W., Xu, Q. & Huan, A. Mechanical origin of the structural phase transition in methylammonium lead iodide CH3NH3PbI3. J. Phys. Chem. Lett. 6, 681–685 (2015).

    CAS  PubMed  Article  Google Scholar 

  43. Quarti, C. et al. Structural and optical properties of methylammonium lead iodide across the tetragonal to cubic phase transition: Implications for perovskite solar cells. Energy Environ. Sci. 9, 155–163 (2016).

    CAS  Article  Google Scholar 

  44. Jiang, J., Liu, F., Tranca, I., Shen, Q. & Tao, S. Atomistic and electronic origin of phase instability of metal halide perovskites. ACS Appl. Energy Mater. 3, 11548–11558 (2020).

    CAS  Article  Google Scholar 

  45. Eames, C. et al. Ionic transport in hybrid lead iodide perovskite solar cells. Nat. Commun. 6, 7497 (2015).

    ADS  CAS  PubMed  Article  Google Scholar 

  46. Frost, J. M. & Walsh, A. What is moving in hybrid halide perovskite solar cells?. Acc. Chem. Res. 49, 528–535 (2016).

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  47. Futscher, M. H. et al. Quantification of ion migration in CH3NH3PbI3 perovskite solar cells by transient capacitance measurements. Mater. Horiz. 6, 1497–1503 (2019).

    CAS  Article  Google Scholar 

  48. Khassaf, H., Yadavalli, S. K., Zhou, Y., Padture, N. P. & Kingon, A. I. Effect of grain boundaries on charge transport in methylammonium lead iodide perovskite thin films. J. Phys. Chem. C 123, 5321–5325 (2019).

    CAS  Article  Google Scholar 

Download references

Author information

Authors and Affiliations

Authors

Contributions

A.M.M.T.K. designed the project, conducted all measurements, analyzed the data and wrote the manuscript. M.K.R.K. conceives and designed the project, analyzed the data, wrote the manuscript and supervised the whole work. M.S.H. designed and supervised the project. All authors reviewed the manuscript.

Corresponding authors

Correspondence to A. M. M. Tanveer Karim or M. K. R. Khan.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary Information

Rights and permissions

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Karim, A.M.M.T., Khan, M.K.R. & Hossain, M.S. Temperature dependency of excitonic effective mass and charge carrier conduction mechanism in CH3NH3PbI3−xClx thin films. Sci Rep 11, 10772 (2021). https://doi.org/10.1038/s41598-021-90247-x

Download citation

  • Received:

  • Accepted:

  • Published:

  • DOI: https://doi.org/10.1038/s41598-021-90247-x

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.

Search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing