Diffusion

The final section of this chapter forms the background forChapter 6. Starting with the general laws of diffusion, the special conditions related to exciton diffusion within NCs will be derived. Based thereon, two concepts for the measurement of the exciton diffusion length will be introduced.

2.5.1 General Laws of Diffusion

Generally, diffusion denotes the movement of particles from a region of higher concentration towards a region of lower concentration.¹⁴⁷This process could for example involve atoms or molecules within a closed system. The basic equations for diffusion date back to the nineteenth century and were established by Adolf Fick.¹⁴⁸For the three-dimensional case, his first law states

J(r) = −𝐷∇𝑐(r) , (2.57)

whereJ(r)denotes the diffusion flux vector that defines the direction and magnitude of the correspond-ing particle flow at positionr.Equation 2.57shows thatJ(r)is directly proportional to the gradient of the concentration distribution∇𝑐(r)with a factor𝐷which is known as the diffusion coefficient. This means that a given initial gradient in concentration is the driving force for diffusion leading to the movement of the respective particles. No external force is involved in this process, it is rather referred to as a “random walk”.¹⁴⁹The temporal evolution of the concentration is considered in Fick’s second law which states

𝜕𝑐(r, 𝑡)

𝜕𝑡 = 𝐷∇²𝑐(r, 𝑡) . (2.58)

Here and in the following, the diffusion coefficient𝐷is assumed to be time-independent.

2.5.2 Exciton Diffusion in Nanocrystals

An important question for this work was, which role diffusion plays for excitons within individual LHP NCs or their assemblies. In contrast to other particles, excitons first have to be generated and also have a limited lifetime𝑇₁until their decay. Equation 2.58has to be adapted accordingly. The corresponding following equation restricts itself to the representation of diffusion along one dimension, the z-direction in this case.

𝜕𝑛_ex(𝑧, 𝑡)

𝜕𝑡 = 𝑛⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟₀exp(−𝛼𝑧)

generation

+ 𝐷𝜕𝑛_ex(𝑧, 𝑡)

𝜕𝑧²

⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

diffusion

− 𝑛_ex(𝑧, 𝑡) 𝑇₁

⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟⏟

extinction

(2.59)

Obviously, the equivalent to the concentration𝑐inEquation 2.58is the exciton density𝑛_ex. Additionally, the adapted formula includes a term attributed to exciton generation which is assumed to occur instantaneously and is based on the Beer-Lambert law considering an incident light wave in z-direction.^150,151Finally, the last term accounts for the extinction of excitons, i.e., the decay with lifetime 𝑇₁.

48 2.5 Diffusion

x,y x,y

z z

nex(x,y)

n_ex(z)

nex(x,y)

n_ex(z) (i)

(ii)

(i) (ii)

t = 0 t = t1

Figure 2.21: Exciton Diffusion in Nanocrystals.A laser pulse (green) hits the surface of a LHP NC film (i) at normal incidence. Excitons (red) are generated close to the surface within this film due to absorption of the photons. Therefore, the initial distribution of excitons𝑛_ex(𝑡 = 0)is basically determined by the Gaussian profile of the excitation beam in the xy-plane and by the exponential decay of the Beer-Lambert law along the z-axis. Through diffusion the distribution of excitons spreads out such that some excitons potentially reach the adjacent layer of material (ii) at a time𝑡₁> 0before their decay.

Generally, one can distinguish between two types of diffusion in the NC systems investigated through-out the course of this work. The first case usually applies to NCs which are relatively large in at least one dimension, i.e., the length in this dimension should be much greater than the exciton Bohr radius 𝑎_ex. Within such a NC, an exciton is basically free to move in the non-confined directions as long as it is not bound to a defect. The second case plays a role for densely packed small NCs, for example a film of nanocubes. In such cases an exciton is usually confined to one NC, but has the possibility to transfer to an adjacent NC via Förster resonance energy transfer (FRET),¹⁵²also referred to as exciton hopping in this specific case of exciton transport.^153,154

Figure 2.21illustrates the diffusion occurring in a NC ensemble right after the initial excitation with a laser pulse. Here, the pulse propagates towards the NC film at normal incidence. Hence, the Gaussian shaped beam profile of the excitation laser determines the initial density of excitons in the lateral directions𝑛_ex(𝑥, 𝑦, 𝑡 = 0). Moreover, as mentioned above,𝑛_ex(𝑧, 𝑡 = 0)is given by the Beer-Lambert law. Starting from this situation — depicted in the left panel ofFigure 2.21— the spreading of excitons throughout the NC film may be described byEquation 2.59and the respective versions of this equation for the x and y direction. The right panel of the figure shows the exciton distribution after a certain time𝑡₁> 0. In agreement with the laws of diffusion, the exciton density is strongly reduced in the initial excitation spot, while excitons are distributed over a larger volume.

2.5.3 Determination of the Exciton Diffusion Length

To compare exciton diffusion within several systems, it is best to assign a value to each of these systems that determines the average distance an exciton travels during its lifetime: the exciton diffusion length, described by

𝐿_D= √2 𝐷 𝑇1=

√2 𝜇𝑘_B𝑇

𝑒 𝑇₁, (2.60)

exhibiting a square-root dependence on the product of the diffusion coefficient𝐷and the average lifetime of an exciton𝑇₁ within the pure material of the examined film.^155,156In some parts of the literature, the factor2is dropped, but here the mathematically rigorous convention is used, such that

𝐿_Ddenotes the minimum net displacement achieved in one dimension by1/𝑒of the starting exciton population.¹⁵⁷Moreover,Equation 2.60reveals the proportionality of the diffusion coefficient𝐷to the exciton mobility𝜇and the temperature𝑇. Large electron-hole diffusion lengths up to around 1 µm — mainly due to long lifetimes — have been reported for bulk films of organometal trihalide perovskites at room temperature and are also one of the main reasons for the success of this material type in the recent development of solar cells.¹⁵⁸In this work NC assemblies were investigated to find out how the exciton diffusion properties differ compared to a bulk film. To this end, two approaches were applied to study the exciton diffusion length in such LHP NC systems, as explained briefly in the following.

Photoluminescence Mapping

The concept of photoluminescence mapping is based on diffusion along the lateral direction parallel to the film surface. In the scheme ofFigure 2.21this corresponds to diffusion within the xy-plane.

For this approach to be accurate, the film thickness should be as uniform as possible. In particular, it should not contain any empty spots void of NCs. Moreover, the supporting layer underneath the NC film — i.e., layer (ii) inFigure 2.21— should be an exciton blocking material in this scenario.

The Gaussian profile of the exciton distribution𝑛_ex(𝑥, 𝑦, 𝑡)can be studied by observing the in- and outgoing light waves from this film on a substrate. First, the ingoing wave, i.e., the excitation laser beam, determines the exciton distribution𝑛_ex(𝑥, 𝑦, 𝑡 = 0). Therefore, the laser beam and𝑛_ex(𝑥, 𝑦, 𝑡 = 0) exhibit the same Gaussian profile such that the FWHM of this distribution (𝐹 𝑊 𝐻 𝑀_Laser) may be obtained by mapping the surface of the film while only measuring the reflected excitation beam. Then, the emitted wave shall be observed, i.e., the photoluminescence (PL) of the NC film which occurs at a longer wavelength as compared to the excitation. Not all exciton recombination processes exhibit a radiative nature, however, the magnitude of PL originating from a certain point is directly proportional to the local exciton density𝑛_ex(𝑥, 𝑦). It does not even matter if the excitation light source is a pulsed or continuous wave laser when mapping a time-integrated PL distribution along the surface. The averaged PL map will represent the Gaussian profile corresponding to the average exciton lifetime𝑇₁ such that𝐹 𝑊 𝐻 𝑀_PL= 𝐹 𝑊 𝐻 𝑀_PL(𝑇₁)may be extracted from this distribution.¹⁵⁹

Obtaining 𝐹 𝑊 𝐻 𝑀_Laser and 𝐹 𝑊 𝐻 𝑀_PL enables the calculation of the standard deviation for both Gaussian profiles according to

𝜎 = 𝐹 𝑊 𝐻 𝑀

2 √2 ln(2) (2.61)

such that𝜎_Laserand𝜎_PLmay be obtained. Finally then, the exciton diffusion length

𝐿_D= √𝜎^PL2 − 𝜎_Laser² (2.62)

may be calculated using the aforementioned standard deviation values. In the above description, identical diffusion along the x and y axes has been assumed. In NCs or NC assemblies that are not fully symmetric along the xy-plane, the behavior will likely deviate. InChapter 6this will be shown and discussed in greater detail.

50 2.5 Diffusion Photoluminescence Quenching

In contrast to photoluminescence mapping, the second approach to determine the exciton diffusion length𝐿_Dis based on an exciton quenching layer adjacent to the NC film. Such a quenching layer could for example consist of a fullerene such as C₆₀¹⁵⁵or the fullerene derivative phenyl‐C₆₁‐butyric acid methyl ester (PCBM)²²— materials that are generally known to be strong electron acceptors.¹⁶⁰ If an exciton reaches the interface to the adjacent layer via diffusion, the exciton will be dissociated and the resulting free electron will transfer to layer (ii) whereas the free hole stays in layer (i) (see Figure 2.21). Since this e-h pair is no longer available for radiative recombination, this method is referred to as photoluminescence quenching. This possibility of surface quenching represents an additional way for an exciton to decay.^161,162In the previous case without a quenching layer, the only way for an exciton to decay was recombination (radiatively or nonradiatively), occuring at a rate 𝑘_re= 1/𝑇₁as described in the previous sections. The additional path for exciton decay is referred to as charge transfer and consists of diffusion towards the interface and subsequent exciton dissociation.

Therefore, the rate for the charge transfer process is𝑘_ct= 𝑘_diff+𝑘_diss. Consequently, the overall exciton decay within such a system of two layers may then be described by

𝑘_total = 𝑘_re+ 𝑘_ct= 1 𝑇₁ + 1

𝑇_ct = 1

𝑇_total . (2.63)

Moreover, two boundary conditions may be defined forEquation 2.59.¹⁵⁷All coordinates with𝑧 = 0 represent the surface of the NC film and the ones with𝑧 = 𝐿represent the interface between layer (i) and (ii) (cf. Figure 2.21). That is, the NC film thickness is given by𝐿. There is no exciton movement out of the surface of the film into the air at𝑧 = 0, therefore the first boundary condition is

𝜕𝑛_ex(𝑧 = 0, 𝑡)

𝜕𝑧 = 0 . (2.64)

Moreover, assuming perfect quenching, the exciton population decays completely at the interface, i.e.,

𝑛_ex(𝑧 = 𝐿, 𝑡) = 0 . (2.65)

Considering these boundary conditions in combination withEquation 2.59andEquation 2.63, it can be shown that the overall exciton lifetime in the presence of a quenching layer𝑇_totalmay be estimated as

𝑇_total ≈ 𝑇₁(1 + 𝜋² 8 (𝐿_D

𝐿 )

2

)

−1

. (2.66)

This estimation is reasonably accurate as long as𝛼𝐿 < 1, where𝛼is the absorption coefficient of the NC film.¹⁵⁷ Finally, solvingEquation 2.66for the exciton diffusion length leads to

𝐿_D ≈ 2 𝐿

𝜋 √2 ( 𝑇₁

𝑇_total − 1) . (2.67)

Hence,𝐿_Dmay be calculated from the exciton lifetimes with and without an adjacent quenching layer (𝑇_totaland𝑇₁, respectively) if the NC layer thickness𝐿is known.

Materials and Methods 3

Having laid the ground with fundamental explanations concerning the experiments conducted in the course of this thesis, the following chapter serves the purpose of introducing the respective employed materials and methods. Firstly, a typical perovskite NC synthesis is described, followed by an introduction to the different NC compositions and morphologies that may be achieved. Next, the three lasers used most commonly in the experiments for this thesis shall be explored with special emphasis on the optical nonlinear processes occurring within these systems. It goes without saying that none of the presented work would have been possible without a well-functioning, stable and reliable light source — in this case, pulsed lasers whose maintenance and optimization represented a key part of my efforts. In the final section of this chapter, various spectroscopic experiments that were used to study the light-matter interaction in perovskite NCs will be explained in detail.

51

52 3.1 Perovskite Nanocrystal Synthesis and Variety

Im Dokument Exciton dynamics in lead halide perovskite nanocrystals: recombination, dephasing and diffusion (Seite 65-70)