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Perovskite Solar Cells

Interfaces in Perovskite Solar Cells Jiangjian Shi, Xin Xu, Dongmei Li, and Qingbo Meng*

From the Contents 1. Introduction ..............................................2 2. General Principles of the Semiconductor Interfaces in the Cell ..................................2 3. Atomic and Electronic Structures of the Interfaces in the Cell .................................. 5 4. Interfacial Charge Dynamics in the Cell ...................................................6 5. Interface Engineering in the Cells ..............11 6. Conclusion and Future Prospects ..............13

small 2015, DOI: 10.1002/smll.201403534

The interfacial atomic and electronic structures, charge transfer processes, and interface engineering in perovskite solar cells are discussed in this review. An effective heterojunction is found to exist at the window/ perovskite absorber interface, contributing to the relatively fast extraction of free electrons. Moreover, the high photovoltage in this cell can be attributed to slow interfacial charge recombination due to the outstanding material and interfacial electronic properties. However, some fundamental questions including the interfacial atomic and electronic structures and the interface stability need to be further clarified. Designing and engineering the interfaces are also important for the next-stage development of this cell.

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1. Introduction Semiconductor photovoltaic science has been developed over tens of years to solve increasingly serious energy and environment issues.[1,2] Among photovoltaics, crystal silicon solar cells have received wide attention due to their high efficiency and mature manufacturing technology. To further decrease the cost and environmental pollution, efforts toward finding new materials and new technologies for low-cost solar cells are ongoing. Currently, thin-film solar cells with Cu(In, Ga) Se2 and CdTe as light absorbers also exhibit highly efficient photoelectric conversion capacity,[3,4] comparable to multicrystalline silicon solar cells.[5] In addition, some new semiconductor solar cells, organic thin-film heterojunction solar cells, and photoelectrochemical cells have been developed, such as sensitized solar cells and organic polymer solar cells.[6–8] Although these new-generation solar cells can hardly compete with the silicon solar cells due to their cost, immature large-scale production technology, and so on, related research into material and device design, charge generation, transfer, and storage principles in multiple systems have promoted communication between different photoelectricity subjects, such as photovoltaics, photochemistry, photocatalysis, and light emission. Perovskite organic metal halides, which were investigated as transistor materials for decades,[9] were employed to sensitize TiO2 in 2009,[10] starting its miracle journey as a light harvester for the widely known perovskite solar cell. In a short five years, the perovskite solar cell was developed from a liquid junction to a solid type,[11] and the power-conversion efficiency (PCE) was promoted from 3.8% to a certified 20.1%,[5] standing with the multicrystalline silicon solar cell. On the way to high efficiency, much effort has been made regarding perovskite film deposition,[12] structure optimization, and interface manipulation.[13] In addition, a number of valuable works have also focused on clarifying the charge processes,[14–19] which is beneficial for understanding the working principles of the cell. So far, varied perovskite materials including CH3NH3PbX3 (X = I and Br), CH(NH2)2PbI3, CH3NH3SnI3, CsSnI3, and their solid solutions,[20] varied deposition methods for the films, and different device structures including mesoporous,[11] planar,[13] and hole transport material-free[21–23] have already been developed. In a solar cell, charge generation, collection, and transport layers are stacked together, producing several interfaces and interfacial regions. Besides the layers themselves, the interfaces also have a significant influence on the charge processes in the cell.[2] The photo-induced free carriers must transfer across the interfaces to be collected, and the charge recombination usually occurs at the interfaces due to interfacial defects and the specific charge distribution. Thus, for a highefficiency solar cell, aside from having appropriate materials, device structures, and a good film quality, interfaces are the key. In the crystal silicon and Cu(In, Ga)Se2 solar cells, a front p–n junction, metal–semiconductor contacts are carefully manipulated by controlling the compositions or introducing doping, interfacial buffers, and passivation layers.[2,24,25] Although the perovskite solar cell has developed rapidly in a short few years, one can find that the development of this

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cell is mainly based on the improvement in the perovskite film quality and device structure. From a long-term perspective, the interfaces in the perovskite solar cell are also important, and worthy of more investigation. For this purpose, the general principles of the semiconductor heterojunction and interfaces will firstly be presented in Section 2. The interfaces in the perovskite solar cell, including the atomic and electronic structures (Section 3), interfacial charge dynamics (Section 4), and interface engineering (Section 5) are then discussed in this review. We hope that this review can show the importance of the interfaces in the cell, bring some new lucid understanding of the charge processes to researchers, and stimulate research on interface engineering for the cell. For clarity, the structure and interfaces of the perovskite solar cell, as well as the cross-sectional scanning electron microscopy (SEM) image of a typical mesoscopic cell are shown in Figure 1, where the charge-transport pathways are depicted with arrows. For comparison, the structure of the silicon solar cell is presented as well. In the perovskite solar cell, the n-type compact and mesoporous window layers (e.g., TiO2 or ZnO), perovskite absorber, hole transport material (HTM, not included in the HTM-free cell), and the metal electrode (e.g., Au or Ag) are sequentially deposited onto the transparent conducting substrate (e.g., FTO or ITO), making up a complete solar cell. As can be seen, four visible interfaces usually exist in the cells. The charge is extracted at the window/absorber and the absorber/p+ layer interfaces, and is collected through the front and back contact interfaces by vertical and lateral transport. Moreover, the perovskite solar cell has almost the same structure as the silicon solar cell, although an organic HTM layer is used.

2. General Principles of the Semiconductor Interfaces in the Cell 2.1. Semiconductor Junction Theory for the Solar Cell The maturity and application of semiconductor theory stimulated the appearance and rapid development of solar cells. Although some photo-electrochemistry concepts have been introduced in recent years, semiconductor theory is still the most widely recognized and still guides the design of solar cells. The perovskite solar cell has been proven to be a free

J. Shi, X. Xu, D. Li, Q. Meng Key Laboratory for Renewable Energy Chinese Academy of Sciences Beijing 100190, PR China E-mail: [email protected] J. Shi, X. Xu, D. Li, Q. Meng Beijing Key Laboratory for New Energy Materials and Devices Beijing 100190, PR China J. Shi, X. Xu, D. Li, Q. Meng Institute of Physics Chinese Academy of Sciences Beijing 100190, PR China DOI: 10.1002/smll.201403534

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carrier-based device,[26] where charge transport and separation are carried out more like a heterojunction solar cell.[17] Thus, for a better understanding of the charge processes, especially interfacial charge transfer and the driving force for charge transport in the cell, semiconductor junction theory, including interfacial energy band structures and charge transfer properties, will be firstly shown in this section. According to the Anderson model and thermal equilibrium theory,[27–29] when two semiconductors contact directly, forming a whole system, the free carriers will diffuse spontaneously, yielding a unified Fermi energy level and energyband bending. This diffusion and equilibrium will produce a charge depletion region with a built-in electric field, which is usually defined as a junction. The junction is also in the scope of interfaces we are focusing on in this review. Aside from the same semiconducting material with different Fermi energy levels such as p- and n-type silicon, the contact between different semiconducting materials or between a semiconductor and a conductor can also produce junctions. The interfacial energy-band diagrams of the p–n homojunction, p–n heterojunction with a positive or negative conducting band offset (ΔEC) and a Schottky junction are shown in Figure 2. As can be seen, a smooth energy-band bending can be obtained in the p–n homojunction, but an energy cliff or notch usually appears at the interface of a heterojunction due to the band offset. The charge transfer through the interfacial junction is supposed to significantly influence the voltage output of a solar cell. For a homo- or heterojunction, the charge transfer characteristics can be written as qVD − ΔEC ⎞ ⎡ qV ⎞ ⎤ J = J 00 exp ⎛⎜ − × exp ⎛⎜ − 1⎥ , ⎝ ⎝ AKT ⎠⎟ AKT ⎠⎟ ⎢⎣ ⎦

(1)

where J00 is the current density constant determined by the carrier density, diffusion length, and diffusion coefficient in the semiconductors, A is the ideality factor, K is the Boltzmann constant, T is the thermodynamic temperature, q is the elementary charge, and VD is the electrostatic potential of the junction. The charge-transfer properties of the junction will be further discussed in the Section 4.

Jiangjian Shi is a PhD candidate in the Institute of Physic, Chinese Academy of Sciences (IOP, CAS), majoring in condensed matter physics. He received his BS degree in photophysics from Southeast University in 2012. He then joined Prof. Q. Meng's group, investigating photovoltaic devices. Recently, he has focused on the junction nature, charge transport, and carrier control in perovskite heterojunction solar cells.

Qingbo Meng received his BS and PhD degrees from Jilin University and Changchun Institute of Applied Chemistry, CAS, in 1987 and 1997, respectively. He worked in the IOP, CAS, from 1997 to 1999 as a postdoctoral fellow. From 1999 to 2002, he worked as a research fellow of the Japan Science and Technology (STA), Kanagawa Academy of Science and Technology (KAST), and the University of Tokyo. Since 2002, he has worked as a full professor in IOP, CAS. His research interests focus on photoelectrical and photochemical devices and self-assembling photonic crystals for light control.

2.2. General Charge Processes in the Cell The junction is mainly based on the thermal equilibrium theory, where the Fermi energy level is unified in the whole system and the built-in electric field gives a force to separate free charges in the depletion region.[2,29] When light is absorbed by a solar cell with a junction, the thermal equilibrium will be broken, and charge transport and separation processes occur. The charge processes in a solar cell under illumination determine the performance of this device. Figure 3 shows the general charge generation, transport, extraction, and recombination processes in a working solar cell. When a photon is absorbed by the absorber in the cell,

Figure 1. Structure and interfaces of a) silicon and b) perovskite solar cells, and c) cross-sectional scanning electron microscopy (SEM) image of a typical perovskite solar cell. The arrows depict the charge transport pathways. small 2015, DOI: 10.1002/smll.201403534

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Figure 2. Interfacial energy band diagrams of the a) p–n homojunction, p–n heterojunction with b) a positive or c) a negative conducting band offset (ΔEC), and d) a Schottky junction.

a couple of free electrons and holes are then excited. Before being extracted, the free carriers have to first transport into the absorber by diffusion or migration. During transport, the photo-induced carriers can be scattered or recombined due to crystal boundaries and defects, leading to charge loss. The minority electrons are extracted through the window/ absorber interface with the electrostatic force in the depletion region. Then, the electrons will transport along the window layer and the external electric circuit, reaching the back electrode of the cell. To keep charge neutralization, electrons in the back electrode will inject into the valence band of the absorber through the absorber/back electrode interface, and recombine with the majority carriers. For high performance, it is expected that all the photoinduced carriers can export the external load. However, in any solar cell, charge loss due to recombination can not be completely avoided. As the dashed arrows show in Figure 3, charge recombination can occur in various places in bodies and interfaces of the cell. For multicrystal, nanocrystal, and amorphous semiconductors, numerous bodies and boundary defects exist, resulting in recombination. Here, we focus more on the recombination from interfacial defects and interfacial charge transfer under bias. The homojunction is usually constructed with ion implantation or diffusion,[24] producing few defects in the junction interface. For the heterojunction, however, the window or absorber is post-deposited onto the substrate with a solid, vapor, or solution method. Defects due to lattice mismatch, uncoordinated electrons, and thermal vibrations usually exist in this interface,[28,29] as the dashed lines at interfaces depict in Figure 3. The absorber/back electrode interface is also a high-recombination region due to its relatively high concentration of minority carriers and interfacial defects. Meanwhile, when working under bias, interfacial charge transfer can also cause charge losses, lowering the photocurrent,

Figure 3. Schematic diagram of charge generation, transport, extraction, and recombination in a semiconductor solar cell, where the solid arrows depict charge generation, transport and transfer under bias, dashed arrows depict recombination and dashed lines at interfaces depict defects.

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which is closely related to the charge-transfer characteristics as represented in Equation (1). Moreover, free carriers can accumulate at the cliffs and notches existing at interfaces, leading to serious recombination. Thus, the output of the cell is strongly influenced by interfacial defects and chargetransfer properties, which need be considered.

2.3. Carrier Distribution in the Solar Cell In general, a solar cell can be simplified as a physical model consisting of a carrier source and two boundaries, as shown in Figure 4a, where the carrier source supplies the free carriers after light absorption and the boundaries determine the carrier distribution and transport. In a semiconductor, the distribution of free carriers can be described by a steady-state diffusion equation derived from charge conservation (taking the 1D electron as an example),[29] Dn

∂ 2 n( x) n − n0 + Gn ( x ) − = 0, τn ∂x 2

(2)

where Dn is the carrier diffusion coefficient of the semiconductor, Gn(x) is the carrier generation rate, n0 is the carrier density at thermal equilibrium and τn is the carrier lifetime. The carrier density or extraction velocity at the two boundaries determines the solution of this equation and the carrier distribution. In a heterojunction solar cell, photo-induced electrons in the absorber are extracted into the window

Figure 4. Schematic diagrams for a) the simplified model of a solar cell with a carrier source and two boundaries, and a carrier distribution of b) electrons and c) holes at the front and back boundaries, respectively.

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layer through the interfacial depleted electric field. The boundary condition of electron extraction is represented by the interfacial current density, that is, j = qDn∂n/∂x. Figure 4b gives the schematic diagram of the electron distribution with different interfacial current densities as the boundary condition. As can be seen, the interfacial carrier density will increase if the charge-extraction velocity decreases from lines 1 to 3, which may enhance interfacial charge recombination, Figure 5. Possible interfacial atomic structures at the perovskite CH3NH3PbI3/TiO2 interface, leading to carrier losses. To the holes, a where a) iodine atoms are coordinated with titanium atoms, or b) lead atoms are coordinated similar carrier distribution also exists at with oxygen atoms. the boundary B if an extraction occurs, as shown in Figure 4c. Besides diffusion, the fast migration of free carriers in the observation on the atomic scale and the complicated theoretdepleted region sets the boundary conditions. When the car- ical calculations. Fortunately, F. De Angelis and his colleagues riers transport in the depleted electric field, the current den- attempted to address this issue with both experiments and sity is j = σE + qDn∂n/∂x, where σ is the conductivity and E first-principle calculations.[30,31] is strength of the electric field. When E is large enough, free A first-derivative Stark effect was found in the TiO2/ electrons or holes can be extracted quickly, carrier accumula- CH3NH3PbI3 heterojunction, which was attributed to the tion at interfaces is avoided, and the quantum efficiency of existence of a static dipole moment at this interface. Thus, the cell is enhanced. Thus, charge transport in the solar cell an interfacial atomic structure was proposed, and theoretical is determined by the carrier distribution and electric field. calculations based on density functional theory (DFT) were However, carrier distribution and transport in the perovskite also made. The major exposed plane in the anatase TiO2 is solar cell may be more complicated than the ideal model, the (101) slab, where oxygen atoms are on the surface, and since the cell exists on the nanometer scale and the diffusion under-coordinated titanium atoms expose a little below, as length of the minority carrier is comparable to the thickness shown in Figure 5. For the perovskite CH3NH3PbI3, iodine of the cell.[14] In spite of this, the basic principles of semicon- atoms were thought to be exposed at the surface. Thus, the ductor physics are still applicable to this cell.[17,21,22] interactions between iodine atoms and titanium atoms may dominate the atomic structure at the TiO2/CH3NH3PbI3 interface, as shown in Figure 5a. In this case, charge displacement occurs, leading to a strong interfacial polarization 3. Atomic and Electronic Structures of the and a static dipole moment. Based on this result, it is found Interfaces in the Cell that the existence of chlorine atoms at this interface can As discussed in Section 2, the photo-induced carriers have to enhance the interfacial interaction, improving the binding first transport across the interfaces in the cell, and charge loss between TiO2 and CH3NH3Pb(I, Cl)3 and the interfacial usually occurs due to interfacial defects. These defects mainly polarization. However, whether this interface polarization come from lattice mismatches, which cause active under-coor- has any influence on the charge transport across this interdinated electrons and dislocations at the hetero-interfaces. To face is still under investigation. Another possible interfacial avoid charge loss, deposition of the heterojunction is always atomic structure may also exist in this system, where lead carefully controlled, and some film-deposition methods on atoms are coordinated with the oxygen atoms, as shown in the scale of atoms have also been developed, such as mole- Figure 5b, since Coulomb interactions and strong binding cular beam epitaxy (MBE), metalorganic chemical vapor between lead and oxygen atoms were observed in the PbI2 deposition (MOCVD) and atomic layer deposition (ALD) systems.[32] Due to the complexity of this system, it is still premature technologies. A thorough understanding of the interfacial atomic structure and interactions is more fundamental, which to confirm the interfacial atomic structure. A mixed structure could give a theoretical guide for experimental manipula- may also exist, because of the low-temperature film depotion. In the perovskite solar cell, the free charges are mainly sition process, which is far from being at a thermodynamic extracted through the window/absorber interface. The per- equilibrium. Aside from the atomic structure and its influence ovskite absorber, a crystal film with its own long-range-order on interfacial charge transport, another crucial concern also lattice structure, is directly deposited onto the window layer needs to be addressed, that is, whether this interfacial atomic via a low-temperature process without enough thermal relax- structure and the binding between the window and absorber ation. Thus, the interfacial structure in this cell may be more can remain stable under working conditions, with a temperacomplicated than that of the conventional semiconductor ture of about 350 K, a high electric field, and incident highcell or the molecule absorption-based dye-sensitized solar energy photons.[33] Compared to TiO2, the ZnO was observed cell. Little work has been reported on the atomic structure to bind more poorly with the perovskite CH3NH3PbI3 due to of this interface, mainly due to the difficulties in experimental a large lattice mismatch and chemical reactions.[34] Thus, the small 2015, DOI: 10.1002/smll.201403534

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interfacial stability is still an unresolved issue in the perovskite solar cell. The atomic structure at the absorber/HTM can also significantly influence cell performance, although the HTM usually exhibits an amorphous phase. In the Sb2S3 solar cell, it is found that an effective interaction between the functional moieties in the HTM and the Sb atoms can facilitate the charge transport at this interface.[35] Due to this fact, the polytriarylamine HTM cannot exhibit high performances in the Sb2S3 solar cell, but shows a remarkably high efficiency in the perovskite solar cell.[36] Until now, most HTMs for high-efficient perovskite solar cells have been based on the phenylamine moiety structure. Thus, a selective interaction between the perovskite CH3NH3PbI3 and HTM may exist, significantly influencing the charge injection efficiency and recombination.[37] Recently, the attachment of methoxy onto the surface of the perovskite CH3NH3PbI3 has been studied with first principle calculations, and it was found that methoxy in the HTM prefers to locate in the interstices of four-corner-sharing octahedra and cannot remain stable on the PbI2 surface.[38] These results regarding the interfacial atomic structure may help to modify the interface and design new HTMs for this cell. Besides the atomic structure, the interfacial electronic and energy structures can directly determine the charge extraction and collection processes in the cell.[29] X-ray and UV photoelectron spectra have been applied to clarify and depict the interfacial energy level alignment in the perovskite solar cell,[39–41] the results of which are shown in Figure 6. An energy difference of about 2.1 eV is observed between the valence band maxima (VBM) of TiO2 and CH3NH3PbI3, which gives an energy level difference of about 0.4 eV in the conduction band minimum (CBM) at this interface.[39] This energy level difference may be the motivation for free charge extraction. Interestingly, a bending of the highest occupied molecular orbital (HOMO) energy level was observed at the CH3NH3PbI3/HTM interface,[40] as shown in Figure 6. This energy level bending may result from selective electronic interactions or local charge transfer at this interface, which is not beneficial for smooth carrier transport across this interface, and may result in charge accumulation. More interestingly, it is found that the band position of the perovskite CH3NH3PbI3 can be influenced by the carrier type of the substrates.[41] When deposited on n-type substrates, the energy difference between the Fermi energy level (EF) and the VBM (EV) is much larger than that when deposited on p-type substrates. Meanwhile, the working function of CH3NH3PbI3 also changed with the substrate. Usually, the value of EF-EV is a constant in the neutral region in a junction, but can be influenced in the depletion region.[29] Thus, this experimental result may imply energy-band bending at the depletion region when the absorber CH3NH3PbI3 contacts with the substrates. As a brief summary, the interfacial atomic and electronic structures in perovskite solar cells have already been investigated through experiment and theoretical calculations. However, to further understand the working principles and charge processes in the cell, more systematical and rigorous works are still needed due to the complicated device system.

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Figure 6. Possible interfacial electronic structure measured with photoelectron spectra. Reproduced with permission.[39,40] Copyright 2014, ACS and RSC, respectively.

4. Interfacial Charge Dynamics in the Cell As discussed in Section 2, the photo-induced carriers have to experience several processes including generation, transport, extraction, and collection, where more or less charge loss exists due to recombination and transfer under bias. Electricity is produced only when the charge generated in the cell is put out and flows through the external circuit. Thus, it is of importance to have a look at the charge processes in the cell. For a cell on the scale of nanometers, the role of the interface becomes more significant in determining the charge dynamics by influencing the boundary conditions. These charge processes and dynamics in the perovskite solar cell have been investigated by steady-state,[22] transient,[14–16,42] small-perturbation,[43–45] and real-space electronics measurements.[17,19,46,47] In the photon–electron interaction process, excitons can usually be excited when absorbing photons. Excitons with large binding energies will lead to the poor separation of an electron and hole, lowering the charge collection rate of the cell. Fortunately, it has been demonstrated that the exciton binding energy in the perovskite organic lead iodide is smaller than 50 meV,[26,48] and radiative recombination in this material is dominated by the free carriers,[26] which allows the theory of free-carrier transport to be applied to discuss the charge processes in this cell.

4.1. Interfacial Electric Field and Charge Extraction To clarify the working process, space-resolved current and potential measurements were carried out to investigate the interfacial junction and charge-collection properties in the perovskite solar cell. Figure 7 shows the structure of the widely used mesoporous perovskite solar cell and the schematic diagram of the space-resolved measurement results. Kelvin-probe force microscopy results revealed that a continuous electric potential exists cross the cell, indicating space-charge regions in the cell.[19,46] Moreover, a large and continuous drop of the contact potential difference (CPD) appears at the TiO2/CH3NH3PbI3 interface, indicating an effective electric field in the width of hundreds of nanometers. However, such a drop at the CH3NH3PbI3/HTM interface is much slighter, indicating a much weaker electric field. Carefully referring to the experimental results,[19] a simplified distribution of the vacuum energy cross the cell is shown in Figure 7b. With this distribution, the electric field in the

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boundary condition for free-electron distribution and transport in this cell, and the free electrons can not accumulate at the mesoporous TiO2/CH3NH3PbI3 interface. Recently, ion migration was reported to occur in the perovskite absorber driven by an electric field with a strength comparable to that of the depletion field.[49] Although this ion migration makes it possible for a perovskite homojunction, whether the perovskite absorber and the window/absorber interface can stay stable under the depletion field is still a question in this case, since the ion migration may decay the charge depletion and the heterojunction. Aside from the space-resolved technology, the interfacial depletion capacitance properties of the cell were investigated by Mott–Schottky measurements.[21,46,50] In a heterojunction, the depletion capacitance can be written as[29] Figure 7. a) Structure of the mesoporous perovskite solar cell, b) schematic diagram of the vacuum energy distribution across the cell, carefully referring to experimental results from real-space observations of electric potential,[19] and c) the electric field derived from the vacuum energy distribution.

cell can also be derived. As in Figure 7c, an electrostatic field exists at the TiO2/CH3NH3PbI3 interface, which is beneficial for the fast extraction of free electrons and the effective blocking of free holes. Unexpectedly, no effective electric field at the CH3NH3PbI3/HTM interface was observed in this measurement, which may be due to the small difference in the Fermi energy levels of CH3NH3PbI3 and HTM. To further analyze the electric field in the cell quantitatively, the CPD results[19] and the electric field derived from it are shown in Figure 8. As can be seen, a large proportion of the mesoporous TiO2/CH3NH3PbI3 mixed region shows a flat CPD signal, indicating little space charge in this region. The CPD signal drops continuously by about 0.2 V in the CH3NH3PbI3 absorber close to the TiO2/CH3NH3PbI3 interface, which indicates that the depletion region with space charge is mainly located in the perovskite absorber region. This drop of potential on the scale of nanometers results in a large electrostatic field of about 106 V m−1. As has been discussed in Section 2, this large field constructs a perfect 0.00 4 -1

Electric field (10 V m )

CH3NH3PbI3 -0.05

3

CPD (V)

6

-0.10 mp-TiO2/CH3NH3PbI3 -0.15

2

-0.20 1

Electrostatic field

-0.25 0.65

0.70

0.75

0.80

0

Position (µm) Figure 8. CPD results for HTM-free perovskite solar cells and the electrostatic field derived from the CPD signal. Reproduced with permission.[19] Copyright 2014, NPG. small 2015, DOI: 10.1002/smll.201403534

(

)

1 1 1 1 ⎛ V − V − 2 KT ⎞ , = + ⎜ D q ⎟⎠ C 2 q ε 1N A ε 2 N D ⎝ where C is the capacitance, ε1 and ε2 are the dielectric constants of p-type and n-type semiconductors, respectively, and NA and ND are the hole and electron densities, respectively. Usually, in an efficient heterojunction solar cell, ND>>NA,[46] and a wide depleted region in the absorber is obtained for carrier collection. In this case, the widely used Mott–Schottky model is derived to be 1 2 ⎛ 2 KT ⎞ , = VD − V − q ⎠ C 2 qε 1 N A ⎝

(3)

which has been applied to calculate the hole density in perovskite CH3NH3PbI3 films. Typical Mott–Schottky curves of the n-window/perovskite absorber heterojunctions are shown in Figure 9. As can be seen, typical linear regions exist in these curves, agreeing with Equation (3). From the slope of the linear region, the hole density in the perovskite CH3NH3PbI3 is calculated to be about 1015–1016 cm−3. With this result, the maximum depleted electric field is estimated to be ≈106 V m−1, perfectly agreeing with the CPD result shown in Figure 8. Meanwhile, the width of the depletion region (WD) in the perovskite absorber is calculated to exceed 500 nm if the absorber is thick enough. The WD is comparable to the thickness of the cell, implying that the free carriers can migrate quickly and be extracted into the n-window layer. Moreover, we found that the junction nature can be adjusted by changing the carrier properties of the perovskite absorber (Figure 9a) or of the n-window layers (Figure 9b), which provides an approach to control the charge storage and extraction characteristics in this cell. For example, when changing the immersion concentration of CH3NH3I in the two-step deposition process, the hole density of the CH3NH3PbI3 as well as the VD can be obviously influenced, as shown in Figure 9a. For the ZnObased heterojunction, when increasing the electron density of the ZnO by doping, the depleted VD can also be enhanced, as shown in Figure 9b. Thus, it can be concluded that an effective depleted electric field exists at the window/absorber interface, forming a

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(a)

(b) 2.5 -2 4

2.0 1.5

14

1 0

ZnO/CH3NH3PbI3

1.0

2

4 14

2

2

3

1/C (10 cm F )

TiO2/CH3NH3PbI3

-2

1/C (10 cm F )

4

0.2

0.4

0.6

0.8

Voltage (V)

0.5 0.0 0.0

0.2

0.4

0.6

0.8

Voltage (V)

Figure 9. Mott–Schottky curves of the a) TiO2/CH3NH3PbI3 and b) ZnO/CH3NH3PbI3 heterojunctions with different experimental conditions.

heterojunction and contributing to the fast extraction of the free electrons in the cell. However, whether a depleted electric field exists at the absorber/HTM interface still needs to be clarified. If not, whether the perovskite solar cell is a typical single heterojunction or a p–i–n solar cell has to be further clarified. Moreover, why it is also possible to quickly extract holes at this interface is still unclear. A fast extraction and separation of the photo-induced carriers is necessary to enhance charge collection and suppress charge loss. Until now, the charge extraction dynamics at the TiO2/perovskite interface has been clearly clarified with transient spectroscopy and real space current measurements. A schematic diagram of the interactions between the incident photons and the electrons in the perovskite absorber is shown in Figure 10a. When absorbing the photons, the electrons are excited from the valence band (VB) to the conduction band (CB) by direct transition. Usually, multiple VBs or CBs may exist, and a fast relaxation of photo-induced carriers between them is supposed to occur. Experiments found that multiple photo-induced absorption (PA) at 480 nm (VB2) and 760 nm (VB1) exists in the CH3NH3PbI3 absorber, and a fast relaxation of the holes from VB2 to VB1 occurs on a time scale of about 0.4 ps.[14] When the perovskite absorber is interconnected with a carrier extraction layer, the free carriers can be quickly extracted. It is found that hundreds of

picoseconds are needed for the electrons and holes to be extracted into the TiO2 (PCBM) and Spiro–OMeTAD layer, respectively,[14] as shown in Table 1. Transient terahertz spectroscopy and microwave conductivity have also been applied to investigate carrier generation, separation, and recombination dynamics in the CH3NH3PbI3 absorber.[16] Measurements on a time scale of sub-picoseconds found that the free carriers are generated within 2 ps after absorbing the photons. Moreover, the existence of the TiO2 extraction layer can accelerate exciton separation due to the existence of a depleted electric field in the perovskite absorber. In a word, the charge generation and separation processes in perovskite solar cells are ultrafast in comparison to the free-carrier lifetime, ensuring a high carrier-collection efficiency.[42] However, remaining PbI2 between the perovskite absorber and the TiO2 was found, which may slower the charge extraction,[51] though some work proposed that the existence of the PbI2 can passivate the surface defects of CH3NH3PbI3.[52] Electron beam-induced current (EBIC) was also applied to investigate charge collection in the cell.[17,47] The effects of interfacial charge collection and carrier transport can be found from the EBIC results. As in Figure 10d, two EBIC peaks appear close to the TiO2/CH3NH3PbI3 and CH3NH3PbI3/HTM interfaces. The EBIC results are determined by the boundary conditions, carrier diffusion length,

Figure 10. Schematic diagrams of a) the interactions between photons and electrons, b) experimental results with transient photo-induced absorption (PA) and photoluminescence (PL), c) transient terahertz spectroscopy, and d) the EBIC signal of the cell. The EBIC signal is reproduced with permission.[17] Copyright 2014, NPG.[47]

8 www.small-journal.com

© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

small 2015, DOI: 10.1002/smll.201403534

www.MaterialsViews.com Table 1. Time parameters of charge dynamics in dye- and quantum dot (QD)-sensitized, Cu(In, Ga)Se2, and perovskite solar cells. Absorber lifetime [ns]

Quenching lifetime [ns]

Extraction time [ps]

Extraction rate [s−1]

Dye/TiO2[53]

25

Interfaces in perovskite solar cells.

The interfacial atomic and electronic structures, charge transfer processes, and interface engineering in perovskite solar cells are discussed in this...
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