**2.1. Electron injection from QD to MO**

and at lower cost. This has led to conceptualization and development of hybrid solar cells, a structure where an organic molecule is usedas light absorber andthen attached to a metal oxide (MO). The earliest of this type is the dye‐sensitized solar cell also known as the Graetzel‐type solar cell. Upon light excitation, the dye generate excitons wherein electrons are injected into metaloxide(usuallyTiO2)whileholestraverseinliquidiodineelectrolyte.Thisfieldhasbranched outto many material substitutions,two of which will be discussed in this chapter. On one hand, organic dyes were replaced by quantum dots, whose absorption spectra highly depends on its miniscule size, have now reached an overall power conversion efficiency (PCE) of 11.3% accordingtothesolar cellefficiencychartoftheNationalRenewableEnergyLaboratory(NREL), USA. On the other hand, organo‐metal halide perovskite solar cells adopt a perovskite crystal structure (usually orthorhombic), but has organic molecules, e.g., methyl ammonium, within its unit cell. Since its first discovery about 6 years ago, it has become one of the most serious

In this chapter, we present the ultrafast charge dynamics of these materials from the subpico‐ second (ps) to a nanosecond (ns) time scale. Using transient spectroscopy techniques, the evolution of the charges from photoexcitation to recombination will be discussed. This chapter is divided into two main sections. First, results on quantum dot (QD) sensitized solar cell will be presented. This will include the electron injection in an *n*‐type metal oxide (MO) acceptor as well as the hole injection to a p‐type MO. The influence of single layer and multiple layers of QD on excitation transfer will be also examined. The second section will focus on the nature of photogenerated charges in organo‐metal halide perovskite (OMHP), mobility and lifetime of charge carriers, and the mechanism and time scale of charge injection from perovskite

Semiconductor nanocrystals, so‐called quantum dots (QD), are confined quantum objects whose optoelectronic properties are dependent on their sizes [1]. Due to recent progress in chemical solution processing techniques for synthesis of colloidal QDs [2], it has attracted increasing attention on its fundamental properties [3] and applications [4]. The QDs can be utilized as imaging markers [5], as building blocks in light‐emitting diode devices, [6] lasers and light harvesters in solar cells devices [7, 8]. Particularly, the potential application in photovoltaic devices has become the focus of the field over the past decade. One of the reasons for this is the possibility of breaking the Shockley‐Queisser thermodynamic limit of single junction solar cells via multiple exciton generation (MEG) [9] or hot electron transfer (HET) [10]. Besides, the efficiencies of colloidal QD‐based solar cells have been rapidly improving [11]. In this work, we would only discuss colloidal QDs and would refer it simply as QD. The recent progress in the understanding of the photo‐induced dynamic processes in QD‐based solar cell components is summarized in this section. Electron injection dynamics in QD—metal oxide (MO) composites is investigated followed by the studies of hole transfer dynamics and trapping. We also analyze excitation transfer in the films of QDs. The article is mainly based

competitors of silicon solar cells having a PCE 22.1% (NREL).

**2. Dynamics of charge carriers in QD‐sensitized solar cells**

material to organic electrodes.

118 Nanostructured Solar Cells

Electron injection from QD to MO is a process responsible for charge separation in the vast majority of QD‐sensitized solar cells (the so‐called *n*‐type solar cells). The first study can be found already in the 1990s [12]. The idea of using QD‐MO heterojunction in solar cells was directly inspired by the study of dye‐sensitized solar cells. As a key factor in photovoltaic process that would greatly determine its efficiency, electron injection in this system has been widely studied [13–19]. Time‐resolved spectroscopy techniques are commonly used method‐ ology to analyze electron injection dynamics including transient absorption (TA) or time‐ resolved photoluminescence (TRPL). However, combining such experiments with other spectroscopic techniques would be more useful to obtain more thorough picture of the overall injection process. The typical systems to study in QD‐MO hetero‐junction are CdSe QDs attached to a suitable wide band‐gap MO (i.e., TiO2 or ZnO) [20]. Generally speaking, the electron population in the QDs plays a domination role in the TA signal of CdSe QDs in the visible region [20]. Thus, one can easily distinguish the difference between electron and hole dynamics. Other than the CdSe, the PbS and PbSe QDs are also widely studied [21–23]. The band gap of the lead chalcogenide QDs is much narrower allowing it to harvest more photons over the whole sunlight spectrum. The narrower band gap is also favorable in studying the MEG process [24]. It should be noted that in the lead chalcogenide QDs, the transient absorption signal have features from both electron and hole dynamics. Therefore, a more careful identification of the spectral features including both interband and intraband transi‐ tions are highly needed [21].

By using the TA and TRPL measurements, it is possible to track down the density of mean electron population in QDs after excitation [20]. Decrease in the population, however, does not imply electron injection. For instance the electron trapping results in such decay as well. Identification of the electron injection itself can therefore become a complicated issue. The conventional techniques (visible TA or TRPL) can only be used to probe the electron population in QDs. However, one can also monitor electron population in MO by combining other spectroscopy techniques to directly show electron transfer. Blackburn et al. [19] provided a good example in QD‐MO system. In that work, they tracked the population in MO using the TA in far IR region (around 5 μm wavelength) and it can therefore be used to detect the arrival of electrons. However, QDs have features in this spectral region as well. It is therefore necessary to correctly normalize and subtract QD‐MO and QD signals to extract the true injection kinetics. Another suitable probe for the electron injection is a terahertz (THz) light source, which also has been applied previously on the dye‐sensitized MOs. Absorbance in THz wave is correlated to the change in photoconductivity, and therefore we can use it to probe the evolution of the mobile charges in the system. In QDs, the charges are highly localized with rather low THz absorption. However, due to the relatively large mobility of the electrons in MO, the THz absorption would be much larger (especially for ZnO) [25, 26]. In this scenario, we utilized THz spectroscopy to probe the electron injection from QD to MO. By observing simultaneous depopulation of electrons in QDs (probed by visible TA) with population of mobile charges in the system (probed by THz) one can directly show the electron transfer process and estimate the transfer rate [25]. This is illustrated in **Figure 1(b)**.

**Figure 1.** (A) Comparison of slow TA decay kinetics for bare CdSe QDs in solution (dotted line) and significantly faster decay kinetics of CdSe QD‐ZnO system (solid line) indicates electron injection. (B) Overlapping decays of TA (cyan line) and THz kinetics (black line) demonstrating the electron transfer from QD to MO. (C) Model of the THz kinetics to exclude the possible hetereogeneous injection and confirm the injection via CTS (red line). Figures are reproduced from Ref. [25]. Copyright American Chemical Society, 2012.

In QDs, one cannot ignore the effect of electron‐hole Coulomb interaction— for example, the exciton binding energy in conventional CdSe QDs can reach hundreds of meV, which is much larger than their bulk value [27]. Such Coulomb interaction prevents electron injection process [16]. In dye‐sensitized or polymer solar cells with the similar high exciton binding energy, a so‐called charge transfer state (CTS) has been observed [28]. This CTS formation indicates the build‐up of an electron‐cation bound complex after electron injection. The further movement of the injected electrons would follow the dissociation of such complex. The CTS is therefore important to the charge separation and collection. We have reported this CTS formation in the CdSe QD‐ZnO system whose details can be found in Ref. [25]. The combination of THz and TA spectroscopy provided here is a direct evidence of that type charge transfer. A two‐ component dynamics can be observed in both pump‐probe and the THz spectroscopy. The explanation that assumes injection of the electrons from two classes of QDs (the so‐called heterogeneous injection, HI), cannot simultaneously reproduce both the transient kinetics. In constrast, the injection of electrons via a CTS can fully explain both TA and THz dynamics see **Figure 1(c)**. The formation of the CTS greatly depends on the binding energy. Therefore, in PbS QDs with low exciton binding energy, the CTS would be negligible which explains the faster injection process [21–23].

#### **2.2. Hole injection in** *p***‐type solar cells**

In *n*‐type solar cells, the hole transfer from QDs to liquid electrolyte is usually 2–3 orders of magnitude slower than the corresponding electron injection [29]. Thus, the hole dynamics becomes the limiting factor for the photo‐conversion efficiency [30]. To circumvent this, *p*‐type solar cells are becoming popular, wherein QDs are attached to *p*‐type MOs, e.g., NiO where holes are injected and extracted at the electrodes [31]. The lifetime and the pathway of the photo‐generated holes are therefore essential for the photon‐to‐current conversion efficiency. It has been recently reported that the photon‐to‐current conversion efficiency is relatively low, about 17% only [32]. In this section, we give an overview of our studies on the hole dynamics in QDs, specifically the hole trapping process and the injection to *p*‐type MOs.

simultaneous depopulation of electrons in QDs (probed by visible TA) with population of mobile charges in the system (probed by THz) one can directly show the electron transfer

**Figure 1.** (A) Comparison of slow TA decay kinetics for bare CdSe QDs in solution (dotted line) and significantly faster decay kinetics of CdSe QD‐ZnO system (solid line) indicates electron injection. (B) Overlapping decays of TA (cyan line) and THz kinetics (black line) demonstrating the electron transfer from QD to MO. (C) Model of the THz kinetics to exclude the possible hetereogeneous injection and confirm the injection via CTS (red line). Figures are reproduced

In QDs, one cannot ignore the effect of electron‐hole Coulomb interaction— for example, the exciton binding energy in conventional CdSe QDs can reach hundreds of meV, which is much larger than their bulk value [27]. Such Coulomb interaction prevents electron injection process [16]. In dye‐sensitized or polymer solar cells with the similar high exciton binding energy, a so‐called charge transfer state (CTS) has been observed [28]. This CTS formation indicates the build‐up of an electron‐cation bound complex after electron injection. The further movement of the injected electrons would follow the dissociation of such complex. The CTS is therefore important to the charge separation and collection. We have reported this CTS formation in the CdSe QD‐ZnO system whose details can be found in Ref. [25]. The combination of THz and TA spectroscopy provided here is a direct evidence of that type charge transfer. A two‐ component dynamics can be observed in both pump‐probe and the THz spectroscopy. The explanation that assumes injection of the electrons from two classes of QDs (the so‐called heterogeneous injection, HI), cannot simultaneously reproduce both the transient kinetics. In constrast, the injection of electrons via a CTS can fully explain both TA and THz dynamics see **Figure 1(c)**. The formation of the CTS greatly depends on the binding energy. Therefore, in PbS QDs with low exciton binding energy, the CTS would be negligible which explains the

In *n*‐type solar cells, the hole transfer from QDs to liquid electrolyte is usually 2–3 orders of magnitude slower than the corresponding electron injection [29]. Thus, the hole dynamics

from Ref. [25]. Copyright American Chemical Society, 2012.

120 Nanostructured Solar Cells

faster injection process [21–23].

**2.2. Hole injection in** *p***‐type solar cells**

process and estimate the transfer rate [25]. This is illustrated in **Figure 1(b)**.

To rigorously confirm the hole injection rates, complementary analysis using both TRPL and TA is necessary. It should be noted that the DOS of conduction band at the band‐edge transition of CdSe QDs is significantly smaller than the DOS of valence band. Moreover, the hole states are much more closely spaced in QDs [9, 33]. Therefore, the signal of TA bleaching is dominated by the electron filling while hole contribute much less. Moreover, the PL measurements are sensitive to both charge carriers. It is therefore through TA kinetics that we are able to ascertain or preclude the role of electron depopulation in PL quenching as shown in **Figure 2** [34].

**Figure 2.** (a) PL decay kinetics trace of CdSe QDs after continuous laser excitation. The inset shows the evolution of the steady‐state PL intensity. (b) TA kinetics at the band‐edge bleach and PL decay of CdSe QDs with different capping agent attached to QD. Figures are reproduced from Ref. [34]. Copyright American Chemical Society, 2012.

In *p*‐type dye‐sensitized solar cells, the hole injection to MO can occur if it is energetically favorable [35, 36]. In QD‐sensitized solar cells, the hole injection would also be restricted by the fast hole trapping. However, such hole trapping is likely to be greatly self‐passivated. One example is the self‐passivation by continuous light soaking. The effect is commonly explained by surface passivation induced by the chemical changes of the QD surface during photoirra‐ diation [37].

Compared with electron injection in *n*‐type MOs, the hole injection in *p*‐type MOs such as NiO turns out to be much slower reaching hundreds of picoseconds that is due to the weaker electronic coupling, heavier effective mass of holes and less driving force for the charge transfer [37]. The driving force of QD is changed depending on its size, which means it also influences the holes' injection rate. The influence does not come from the difference in energy levels as the valence band of QDs tends to be pinned when attached to MOs [38]. The difference in the driving forces, mainly originate from the size‐dependent Coulomb energy wherein larger driving force is found in larger QDs since Coulomb coupling is weaker.

#### **2.3. Excitation transfer**

Due to large exciton binding energy in CdSe QDs, initial photo‐generated charge species behave like excitons [39]. The motion of such excitons between the QDs occurs via Förster resonant energy transfer (FRET), which is essential for the function of optical devices with densely packed QD films including light emitters and solar cells [40–42].

In QD‐sensitized MOs, it is conventionally believed that electron injection only occurs from QDs directly attached to the MO surface while multilayer QD attachment would hinder the electron collection process. However, in the system of QD‐MO with multiple layer QDs attachement, it is found the excitation transfer (Förster energy transfer) also occurs within the aggregates of QDs [41]. This transfer process can be traced in TA as an additional long‐lived (5 ns) excited states depopulation (see **Figure 3**). Such energy transfer has also been reported in tandem‐layered cadmium chalcogenide QD solar cells, which can be an effective comple‐ ment to improve the solar cell efficiency [40].

**Figure 3.** Energy transfer between indirectly attached QDs in QD‐MO photoanodes. (a) TA kinetics of QD‐ZnO NWs with different sensitization times. The inset illustrates the long‐lived TA component (red dots) extracted from the expo‐ nential fit of the kinetics. The consistence between the TA amplitude and the number of indirectly attached QDs indi‐ cates indirect exciton depopulation with ns time scale. (b) Main photoinduced processes, including direct electron injection and energy. Figures are reproduced from Ref. [41]. Copyright American Chemical Society, 2012.

#### **2.4. Outlook**

QDs provide great opportunities for the development of optoelectronic devices. Moreover, the booming of QD solar cell research will surely put forward photo‐induced dynamics questions waiting to be revealed in the future. For example, the band alignment assembly in lead chalcogenide QD devices currently holds the highest record of solar cell efficiency. The general principles of the device have been revealed but the atomistic details are still to be understood. The conventional spectroscopic techniques used in investigating the photo‐induced dynamics are usually very different from the solar illumination conditions. These include the excitation intensity, the loading of circuit, medium conditions, etc. Systematic studies of charge carries dynamics under real solar cell functioning conditions may provide more useful reference for device application. Another issue of future research is to utilizing the "green" elements in the materials to replace the toxic Cd and Pb, which are overwhelmingly used in recent studies. All in all, many challenges remain to be faced before the QD solar cells can be considered as a real viable solar technology. However, recent advances in QD research give ground for optimism.
