Structural phase transformations for common hybrid perovskites. The table was taken from  with permission.
Owing to its excellent light harvesting, high-charge carrier mobility, and long electron- and hole-transport lengths, organic–inorganic lead halide perovskite solar cells have attracted enormous attention recently under the urgent demands of green energy with environmental friendliness. Although various photovoltaic architectures based on alkylammonum lead halides have been fabricated and have achieved impressive power conversion efficiencies (PCEs), there are still several issues that need to be further addressed and solved properly, for example, the requirement of facile fabrication procedure, the chemical stability of perovskite films, and the environmental friendliness. Herein, we review the recent experimental progress on the external doping of hybrid perovskite devices by organics and metals, which demonstrate the tuning of optical absorption gap and the enhancement of both devices’ stability and performance. Doping at varying layers in the perovskite films was discovered to contribute differently to the improvement of the hybrid organic–inorganic electronics. In the end, prospective was also made on the development of hybrid organic–inorganic devices.
- hybrid organic electronics
- perovskite solar cells
- external doping
- device performance
Depletion of fossil fuels and the shortage of energy has become one of the most serious problems nowadays, which strongly restricts the sustainable development in our society. To achieve a sustainable society, we need methods of converting energy from other resources, such as wind and sunlight. Among the proposed approaches, photoelectrochemical devices offer the promise of solar fuel production through artificial photosynthesis and led to considerable development in numerous areas related to photovoltaic cells and electronics. Thin-film solar cells, such as dye-sensitized solar cells, organic photovoltaics, and colloidal nanocrystal solar cells, can be assembled with low-cost materials and manufactured with cost-effective methods and are considered very promising renewable energy technologies. Especially, hybrid organic–inorganic perovskites based on the metal halides have emerged as one class of promising light-harvesting materials because of their exceptional properties such as direct band gaps, large absorption coefficients, and high carrier mobility. Meanwhile, the organic–inorganic perovskite solar cell also have many other advantages compared to the conventional solar cells, such as easy solution process, low processing cost, extremely high power conversion efficiency which reached over 20% recently. All these merits have granted perovskite a promising candidate for the next-generation solar cells.
Although great success in the use perovskite solar cells has been witnessed over the past few years, there are still several bottlenecks which limit significantly the wide deployment of its outdoor application: for example, the poor stability of the perovskite film and the charge transport layers, dangers and health risks arising from toxic and harmful element due to the usage of lead and other heavy metal atoms. Based on these urgent demands and concerns, the improvement of perovskite electronics has been explored and demonstrated. Interestingly, doping of perovskite electronics seems to be an efficient way to enhance both the stability and performance of perovskite electronics, which is at moment classified into several approaches: doping into the electron transport layer, doping into the perovskite films, and doping into the hole transport layer. In this chapter, we review the progress of in situ fabrication and characterization of organic–inorganic hybrid electronics by external doping and propose some of them.
2. Structure and performance of perovskite electronics
Since 2009, Kojima and his coworkers used CH3NH3PbBr3 and CH3NH3PbI3 as sensitizers in solar cell; great progress has been made for such kind of solar cells not only due to the fact that the power conversion efficiency (PCE) is increased from 3.8 to 20.8% [1,2], but also that it has high absorption characteristics, appropriate direct band gaps, high carrier mobility, long charge carrier diffusion length, low cost, and easy fabrication processes . All these excellent performances are mostly originated from the
The sandwich-like structures of the organic–inorganic hybrid perovskite devices are shown in Figure 1b. Mesoporous structure and planar structure (Figure 1b) are the so-called positive structure, while the inverted structure is presented in Figure 1c with the electron transport layer (ETL) and the hole transport layer (HTL) upside down as compared to the regular perovskite structure device. As discovered, the inverted structure can reduce J-V hysteresis and is more stable due to the hydrophobic PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) and its easy fabrication. With the thin perovskite layer placed between the HTL and the ETL under illumination, voltage/current is formed by generating and directionally moving the excitons (holes and electrons) after overcoming the energy barrier of the band gap. Electron–hole pairs are generated almost instantaneously after photo excitation and dissociated in the time scale of several ps, followed by the formation of high mobile charges in the neat perovskite . After the generation and separation of excitions, the holes and electrons diffuse to the opposite site of the sensitizer to reach electrodes forming electrical current; however, not all of the holes and electrons can get to the electrode and contribute to the power generation, since some of them will be captured by the defects or recombined with other electrons/holes on the way to electrodes. The routes of carrier transmission are illustrated in Figure 2.
With various cations/ions in the perovskite structure, there are different lattice parameters. However, it is not always suitable to have random combinations of cation A, B, and anion X. The crystallographic stability and probable structure can be evaluated by considering the Goldschmidt's tolerance factor
|PSC||Phase||Temperature (K)||Structure||Space group||Lattice parameter (Å)|
||a=6.3115 b=6.3115 c=6.3161|
||a=8.849 b=8.849 c=12.642|
||a=5.673 b=5.628 c=11.182|
|γ||<172.9||Orthorhombic||P2221||a=5.673 b=5.628 c=11.182|
||a=7.979 b=8.580 c=11.849|
||a=6.2302 b=6.2302 c=6.2316|
||a=8.7577 b=8.7577 c=12.429|
||a=8.9817 b=8.9817 c=11.006|
|β||150||Trigonal||P3||a=17.791 b=17.791 c=10.091|
The thickness of this sensitized layer is usually limited to be around several hundred nanometers concerning the fact that holes and electrons will be recombined quickly if the film thickness is thicker than the diffusion length of charge carriers. Therefore, we simply consider the diffusion length of excitions which depend not only on the amount but also on the concentration of precursors [12–14]. As known, the nature of ambipolar of the perovskite material leads to the transport of both electrons and holes . For example, the trioxide absorber CH3NH3PbI3 has the electron–hole diffusion length of several hundred nanometers and a relatively long life time .
Even though hybrid perovskite films are expected to have such appealing properties, only the elaborate design of every counterpart of the whole device can make the performance of hybrid perovskite devices as great as possible. For example, the band level alignment of different layers and materials in devices should be considered carefully. As summarized from literatures, the energy bands of ETL must satisfy the following conditions : (1) Its conduction band (CB) must lie under the CB of the active perovskite layer to extract electrons which reach the interfaces afterward. (2) Its valence band (VB) must lie much under the VB of the perovskite to reject the holes. (3) The electron affinity of the ETL must be greater than that of the perovskite. (4) The VB of ETL should have large difference compared to the VB of perovskite in order to reject holes, so the ETL usually has wide band gap. (5) The electron mobility in ETL must be sufficiently high. Similar requirements can also be deduced for HTL. The values of VB and CB of some commonly used materials are listed in Figure 3.
The exciton property, electrical property, and the energy band of various ETL/HTL materials have been discussed so far. Moreover, optical properties will also be illustrated in this section. The mostly used technique to explore the optical property is ultraviolet-visible (UV) and photoluminescence (PL) spectroscopy. Different UV/PL spectra are shown in Figure 4a and b [18, 19]. The absorption edges of perovskite films in Figure 4 clearly indicate that hybrid perovskite devices have high absorption factor at visible and near-infrared range, which just fit well to the solar spectrum. Figure 5 shows the complementarities between the Si-based solar cell and the hybrid perovskite device as they have opposite external quantum efficiency (EQE). Furthermore, the internal quantum efficiency (IQE) has been reported to reach 100% through optimizing the perovskite device , which inspires the researchers that tandem solar cells based on perovskite, and Si is very interesting if the fabrication technology can be improved to use the sunlight more effectively.
3. Doping into different layers and the influence on devices
3.1. Doping into the electron transport layer
As discovered, the boundaries and defects existing in ETL generally result in the recombination of electrons and holes associated with pinhole/cracks between ETL and substrate and will therefore lead into the decline of PCE and stability. Consequently, proposed materials for ETL should own high electron affinity, excellent surface morphology, and effective hole-blocking properties. From this point of view, visible improvement should be realized by properly doping the ETL for the electron transport property, the hole blocking ability, since external doping usually rearranges the band alignment due to the pining of Fermi level, which may influence the electron transport property, hole blocking ability, for example, in the manner of removing deep electronic traps and sub-band states, enhance the carrier’s life time and the film’s conductivity by several orders of magnitude. Meanwhile, the doping into ETL can also enhance the film morphology by removing defects or filling the gap in between islands in the film. Nevertheless, the band gap of ETL itself is also modified by doping due to the change of band structures. In the following, mostly used electron transport materials are described in detail.
Rutile TiO2 nanostructures on fluorine-doped tin oxide (FTO) substrates are interesting building blocks for solar cells (as shown in Figure 7a). Positive structure and FTO substrate are always adopted together for the electron transport material, since FTO can be annealed up to 450℃ when rutile TiO2 is formed. The properties of large band gap, suitable band edge for charge injection and extraction, long lifetime of excited charge carriers, exceptional resistance to photo corrosion, non-toxicity, and low cost have made TiO2 a popular material for solar energy applications. In spite of this, pure TiO2 is not ideal for ETL which need to be doped to achieve n-type characters for higher electron extraction.
The scale of the pure TiO2 nanopaticles which results in the highest PCE in organic–inorganic devices is about 50 nm . The crafts for the fabrication of compact TiO2 layer (spin coating, spray pyrolysis, sol-gel methods, magnetron sputtering) have developed rapidly, and one effective way is to decorate TiO2 in order to improve the device’s performance. For example, Mg-doped TiO2 can increase the open-circuit voltages (
On the contrary, the size of the particles used as mesoporous TiO2 is very small (around 15 nm) and can be easily controlled by tuning the concentration, pH of the reactants, and the annealing temperature. As also discovered, reduced graphene oxide/mesoporous TiO2 nanocomposite as scaffold material could improve the electron collection efficiency, enhance the
Compared to TiO2, ZnO (Figure 7b) owing to high electron collection as ETL is more environment friendly as it does not need to be sintered as high as to 450℃ to become rutile crystal and grows very fast by solution process . Furthermore, less chance of recombination of carriers would occur in ZnO because its conductivity is several orders of magnitude higher than that of TiO2 [32–34]. As known, ZnO is an important II–VI semiconductor with a direct band gap of 3.4 eV and an exciton binding energy of 60 meV at room temperature. Moreover, ZnO has excellent optical properties such as a high infrared reflectivity and transparency in the visible spectrum. In addition, it is abundant in nature and inexpensive . However, ZnO accelerates the degradation of perovskite layer into PbI2 when it is thermally annealed . Similar to TiO2, ZnO also suffers from photo corrosion which reduces the performance and stability .
As reported, Al-doped ZnO lifted the PCE from 10.8 to 12.0% by optimizing the thickness of ETL and smoothing the interface . Though fewer reports have been found for the doping of ZnO till date, it is proved gradually to be an excellent candidate with external doping to replace TiO2 and to solve the problems existing in TiO2. In a word, metal atoms are always chosen to be the doping materials in the doping of either TiO2 or ZnO.
PCBM (Figure 7c) is one of the fullerene derivatives. Its excellent properties of high electron affinity and transmission make it extremely popular in organic solar cells. PCBM is widely used in inverted perovskite structure accompanied with indium tin oxide (ITO) substrate.
PCBM doped with 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzoimidazole (DMBI) results in n-doped ETL which significantly increases the
3.2. Doping into the perovskite layer
3.2.1. Doping at site A
The cation at site A has no direct contribution to the properties of electron injection or transport. The B–X octahedra and A are relatively independent owing to the fact that there is almost no overlap between the electron clouds of A and B-X due to the large unit cell . However, the radius of ion A affects the symmetry of the perovskite structure which in turn affects the electron’s properties [46–48]. As summarized from literatures, site A is always occupied by Cs, Rb, methylammonium (MA), formamidinium (FA), or ethylammonium (EA). The first study about mixed cations (MA)
3.2.2. Doping at site B
The ideal element which suits the organic–inorganic hybrid perovskite structure at site B is Pb, and one can easily refer to the same group elements, for example, Ge or Sn. Javier Navas and his coworkers tried to dope/replace Pb2+ with Sn2+, Sr2+, Cd2+, and Ca2+ and revealed the development of the properties of crystalline phase, band structure (Figure 9a), and emission and optical characteristics. Non-covalent interaction (NCI) analysis complemented with the results from electron localization functions (ELFs) indicated that the strength of the dopant-I interaction follows the order with Cd-I > Sn-I ≈ Pb-I > Sr-I for the tetragonal structure . The system of MASn1−xPbxI3 was explored in detail at different ratios of Sn and Pb (Figure 9b) [18, 56], and optimized hybrid devices were fabricated afterward [57, 58]. In the end, it is worth pointing out that Pb is rather toxic, and environmental friendly materials are urgently needed to replace lead at site B. As far as explored, no other stable element has been found which can fully replace Pb without downgrading the hybrid perovskite devices’ performance. Consequently, doping at site B seems to be the dominant approach to resolve the above-mentioned challenges.
3.2.3. Doping at site X
Site X, usually occupied by halogen atoms, typically was iodine at beginning and was attempted with doping of Cl afterward which encouragingly claimed the improvement of both diffusion length and carrier lifetime [14, 58]. Doped MAPbI3−
Apart from the doping of Cl, Br has also been tried to partly replace I at the site X. In fact, bromide has been effectively used to tune the band gap of perovskite films. UV and PL spectrum are shown for various doping of Br from 0 to 100% in CH3NH3PbI3 perovskite films in Figure 10c and d . For the synthesis the perovskite CH3NH3PbI3−xBrx, doping the MAI and PbI2 solution with MABr can change the band gap, which overlaps the whole usable solar spectrum for perovskite solar cells and shifts the XRD peak due to the different ratio of MABr and MAI . The calculation for mixed halide has also been done using first principle calculations. For example, Herz et al. studied the charge carrier dynamics and mobility in formamidinium lead-mixed-halide perovskites and found that the auger recombination constant exists in the mixed halide perovskite system which strongly depends on the value of y in FAPb(Br
Another choice of doping at the site X could be the utilization of anion. MAPbI3−x(BF4)x has a band gap of 1.5 eV slightly lower than that of MAPbI3, and the absorption wavelength is around 827 nm, where the sunlight spectrum has stronger irradiance compared to the absorption wavelength of 800 nm for MAPbI3. MAPbI3−x(BF4)x was proved to own good properties such as good electrical conductivity and high photo response . MAPb(SCN)2I was demonstrated to show high stability in moisture environment compared to MAPbI3, and the fabrication process is rather similar .
3.2.4. Mixed doping
The MAPbI3–PEOXA precursor was used as the sensitizer of hybrid perovskite solar cell to improve the interface morphology of the perovskite-polymer films and to reduce the undesired contact between PEDOT:PSS and PCBM layers for the minimized shunting path at the device . It can be concluded now that the main point for doping hybrid perovskite device is that the proper parameters of
3.3. HTL doping
NiO (as shown in Figure 11a) is a relatively rare material which has a nature of p-type semiconductor in both perovskite device and dye-sensitized solar cells, so it is a promising candidate for hybrid devices. The NiOx thin film is very compact without pinholes which can block the electrons very effectively . Therefore, it is expected that NiOx doping can improve the properties of hybrid organic inorganic devices. For example, Cu-doped NiOx increases the electron transmission significantly and enhances the PCE from 8.9 to 15.4% in the inverted structured solar cell . Afterward, low-temperature process for the doping of Cu has been reported . NixMg1–xO was also used as HTL and led to large-area and stable devices with higher PCE . Therefore, we can say that the NiOx will replace the organic HTL due to its commercial future.
Moreover, graphene oxide (GO) was proved to be a promising candidate as HTL, owing to the enhanced crystallization, high surface coverage ratio, and preferred in-plane orientation of the (110) plane of perovskite film on top. Efficient hole extraction and the enhanced stability of hybrid devices were demonstrated in the GO-based inverted devices, as reported by our group  and other studies . CuI is another inorganic material used as HTL which is a p-type semiconductor and has a matched band structure with the perovskite films .
3.3.2. Spiro-MeOTAD and PEDOT:PPS
Spiro-MeOTAD (shown in Figure 11b) was also used as HTL as an alternate which supports great PCE for the fabricated devices [86, 87]. However, all of the Spiro-MeOTAD based layers need to be doped because of the nature of such material with bad charge transmission. For instance, Spiro-OMeTAD doped with iodide-reduced graphene oxide has increased the stability of corresponding device and reduced the cost of fabrication. Spiro-MeOTAD116 was doped with Co(III) complex as a p-type dopant for the HTL to ensure a sufficient conductivity and low series resistance [88, 89]. The n-type doped Spiro-OMeTAD functionalized as the pinhole-free HTL has a proper band structure which matches well the band of perovskite film .
Moreover, dopant-free PBDTTT-C polymer , dopant-free spiro-CPDT , dopant-free TPB , and dopant-free linear acene derivative were also utilized as the hole transport material . These materials are used as HTL for their appealing properties to replace Spiro-OMeTAD.
PEDOT: PPS (shown in Figure 11c) is always designed as the HTL in inverted hybrid perovskite devices, but its acidic characteristics severely threaten long-term stability and performance . Thus, the substitute was explored and found and, for example, CPE-K which contributes to a better performance of the related device , is gradually used as HTL. In fact, graphene can not only be used in ETL but also in HTL. PEDOT: PPS doped with graphene quantum rings (GQRs) ensures the efficient hole extraction in hybrid solar cells and relatively high PCE. Hydrophilic graphene oxide doping in PEDOT: PSS composited films demonstrated a PCE of 1.8 times higher than that of the hybrid electronics based on the pristine PEDOT: PSS buffer layer . Besides, PTAA (Poly[bis(4-phenyl)(2,5,6-trimentlyphenyl)amine, P3HT(poly(3-hexylthiophene)), PCPDTBT (Poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)]), VB-DAAF (vinylbenzyl 9,9-diarylfluorene-based triaryldiamine), etc. are also used as HTL for their cheap cost, easy synthesis, and better performance. Actually, the fabricated devices showed higher PCE and better performance than the commonly used PEDOT:PPS and Spiro-MeOTAD materials [98–101].
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