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Halide perovskite materials as light harvesters for solar energy conversion Chao Ran Dong , Yue Wang , Kan Zhang , Haibo Zeng PII: DOI: Reference:
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Please cite this article as: Chao Ran Dong , Yue Wang , Kan Zhang , Haibo Zeng , Halide perovskite materials as light harvesters for solar energy conversion, EnergyChem (2020), doi: https://doi.org/10.1016/j.enchem.2020.100026
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Highlights Recent progress in halide perovskite materials for solar energy conversion is summarized. Atom, crystal and device engineering for enhancing solar harvesting is highlighted. Challenges and future development in solar to fuel conversion are pointed out.
Halide perovskite materials as light harvesters for solar energy conversion Chao Ran Dong, Yue Wang,* Kan Zhang and Haibo Zeng* Institute of Optoelectronics & Nanomaterials, MIIT Key Laboratory of Advanced Display Material and Devices, College of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China *Correspondence:
[email protected] (Y. W.),
[email protected] (H. Z.)
Abstract Due to the unsustainable fossil fuels, those conventional energy sources are diminishing and getting expensive. Since sun can provide insolation levels of 150–300 watts/m², or 3.5–7.0 kWh/m² per day in most of the world's population live in areas, efficient utilization of the enormous energy source is continued to pursue. Solar energy can be usually harvested in a couple of different ways, among which harvesting solar energy by photon absorption in band gap materials and the subsequent collection of photo-induced charge carrier has been actively explored as promising strategy to store the abundant energy source. Halide perovskite materials, having optically high absorption characteristics and balanced charge transport properties, is considered a most potential light harvester to photovoltaics, as well as for solar energy conversion. Compared to charge transport, light harvesting capability is a must for high conversion efficiency of solar energy. In this review, we summarize the recent research progress in enhancing and modulating light harvesting capability of halide perovskite for PV devices and solar to fuel conversion from the perspectives of atomic level, crystal film level and device level, respectively. Keywords: halide perovskite, solar cell, light harvesting, solar to fuel
1. Introduction Photovoltaics (PV) provides the conversion of light quanta, photon, into direct current electricity through band gap materials. Typically, the energy conversion starts with the light-harvesting that is being absorbed by electrons in the valance band, causing excited electrons to transfer to a high energy state, and thus become free to move across the conduction band. In 1839, French physicist Alexandre-Edmond Becquerel discovered the PV effect, ever since, mainly semiconductor materials have been utilized to generate PV effect in solid-state electronic devices. Lately, halide perovskite materials have dragged the attention of scientific and industrial communities by being excellent light harvester materials. It is attributed due to the basic structure and unique properties of the halide perovskite materials in favor of being a remarkable photosensitizer. Since the first successful application of halide perovskite as a photosensitizer in solar cell,1 several innovative strategies have emerged in order to enhance the light-harvesting performance of PV devices (Scheme 1).
Scheme 1. The timeline of the light harvesting enhancement strategy in PSCs.
H Halide perovskite materials were first reported in 1893, and have brought rapid growth in electronics in the year 2009 after being used a light absorber in solar cells.1 Thereafter, researches have been dedicated to extending its constituent‟s element library since the beginning for further advancement in light-harvesting. So far, halide perovskite with formula ABX3 has been widely studied. It consists of methylammonium (MA), formamidinium (FA), cesium (Cs) and rubidium (Rb) at cation A sites, lead (Pb)/tin (Sn) at cation B sites, and halide (Cl, Br, I) components at anion X sites, respectively.2,7-14 Importantly, several studies showed that change in the constituents elements leads to notable variation in the crystal structure and hence electronic-optical properties of halide perovskite materials. Thus, the simple tunable optical properties enable marvelous and wide applications in optoelectronic field.15-26 The crystal structure of ABX3 shows a 3D structure with cation A located at eight corners of the cubic cell
and a corner-sharing octahedral network of BX6 (Figure 1a). The unique structure can be diversified by composition with other earth-abundant elements (Figure 1b). To further estimate the crystallographic properties of halide perovskite, researchers proposed a unique and reliable method by defining a tolerance factor (t) and octahedral factor (μ). Tolerance factor “t” is defined as the ratio of the distance A-X and the distance B-X in an idealized solid-sphere model. The “t” as follows: t=(Ra+Rx)/(RB+RX)1/2, where RA, RB and RX are the ionic radii of the corresponding ions. The octahedral factor “μ” is calculated as RB/RX (Figure 1c). For a high-symmetry cubic 3D perovskite, the ideal “t” should be in the range between 0.813 and 1.107. However, low-dimensional derivatives (2D: layered, 1D: chain-like, 0D: isolated) can be generated when the observed value of “t” would be beyond this range.27-32 The variation in the external factors such as temperature9 and illumination33,34 and a slight change in the inner composition causes a drastic change in the value of “t” and “u”. Subsequently, B-X octahedral will get the tilt, resulting in a less phase symmetry like from cubic phase to tetragonal, orthorhombic, monoclinic phase as well as stability issues may arise.35-37
Figure 1. a) Cubic perovskite crystal structure. Reproduced with permission.38 Copyright 2014, Nature Publishing Group. b) Atom fraction of elements used for different types of solar cells. Reproduced with permission.39 Copyright 2018, American Association for the Advancement of Science. c) Calculated t and μ factors for 12 halide perovskites.38 d) The absorption coefficient of CH3NH3PbI3.38 e) UV-vis absorption spectra from the CH3NH3PbI3/PC61BM bilayer and its constituents with AM 1.5G spectral irradiance included for comparison. Reproduced with permission.40 Copyright 2014, Royal Society of Chemistry.
Specifically, halide perovskite materials show interesting optical and electronic properties such as high absorption coefficient40 (Figure 1d), high carrier mobility (about 7.5 cm2V-1S-1 for electrons and 12.5-66 cm2V-1S-1 for holes due to the small effective mass of charge carrier41), low exciton binding energy42 and long diffusion length.43 More importantly, the simple tuning of composition for halide perovskite materials can alter its bandgap ranging from 1.1 eV to 3 eV. 44 By considering methylammonium lead indium (MAPbI3) as an example, bandgap can be tuned at about 1.55 eV-1.62 eV that promises the full absorption spectrum in theory (Figure 1e). As per
Shockley-Queisser‟s theory, the bandgap of MAPbI3 even close to the ideal value of 1.42 eV for single-junction cells.44,46 Until now, various facile approaches have been utilized to synthesize the halide perovskite thin film, which exhibits huge competitive edges on PV cells. These tunable inherent properties of the halide perovskite materials made it an excellent light harvester as compared to other semiconductors in photovoltaics. The successful incorporation of halide perovskite materials in a solar cell exhibited a power conversion efficiencies (PCEs) of 3.9% in 2009.1 Also, certified PCEs beyond 25% has been achieved recently from National Renewable Energy Laboratory (NREL). It‟s worth to note that the light-harvesting, as one of the significant sections in PV cell, is always on priority from a modification perspective.47 Two aspects of benefits can be acquired from the advanced light-harvesting in halide perovskite solar cells (PSCs). Firstly, high light-harvesting efficiency represents that less photosensitizer material is needed to fulfill the light absorption requirement in solar cell i.e. cost-effective fabrication. Secondly, considering the real working conditions of halide perovskite thin film in PSCs, the modulation in light-harvesting enhancement is needed to recover from the scarcity in the light absorption caused by device architecture. In step forward, the strategies to enhance the light-harvesting in PSCs can be investigated in terms of both inherent properties and operational conditions of perovskite thin film, comprehensively. Based on the above-mentioned considerations, we will explain the strategies for enhancement in the light-harvesting in PSCs from different points of view. For example, atomic level, crystal film and device configuration, accuracy of the bandgap engineering, optical engineering, and light modulation in perovskite crystal film in the device. Furthermore, as an important extent light harvester, both PSCs48-66 and halide perovskites powder67-72 have drawn lots of attentions in last few years for direct solar to fuel (STF) synthesis.
2. Light harvesting enhancement in halide perovskite solar cells In this study, the modification in the halide perovskite materials to PV device architecture which includes design from the atomic level to formation of the crystal film, PSC device design, and strategies for enhancement in light-harvesting mechanisms will be discussed. From the perspective of atom level analysis, precise bandgap engineering of the perovskite materials for PSCs will be described. Additionally, optical engineering of perovskite crystal film in PSCs, including light trapping-structure and plasmonic resonance effect will be discussed comprehensively. Light modulation in the device configuration including facile implementation in optical design on the substrate, electrode and interface mechanisms will be discussed.
Moreover, the construction of the tandem solar cells will be further explored (Scheme 2).
Scheme 2. Light harvesting enhancement in PSCs from three levels.
2.1 Atom-level: Band engineering of halide perovskite for light absorption enhancement. As for PSCs, one of the most important photoelectronic properties of perovskite material is the optical bandgap, which directly decides the absorbance range and upper limit of the open-circuit voltage of the cell. In the following part, the development routes for bandgap engineering in halide perovskite-based PV devices will be organized from several aspects. For example, in order to extend the absorption spectrum in solar cells, insightful and systematic research on the substitution of three different ion elements in perovskite structure
2, 6, 73-76
as well
as optimal combination of these components for maximum light absorption in the solar cell will be explored.77 It is notable to claim that the rational design is important in the investigation of prototypical perovskite material with suitable bandgap near around the ideal range for the photovoltaic according to the Schockley-Queisser analysis. Based on this consideration, Giustino et al proposed a combination of geometric opinions and first-principles calculations. The demonstrated that modulation of the optical bandgap can be attained by altering the cations in A-PbI3 network while retaining the highly desirable features such as the long lifetime of carriers. The optical band gap can be tuned over the almost one electron-volt by taking consideration of the Pb-I-Pb bond angles in PbI6 network. The bond angle can be tailored because of the different sizes of cations in the common network of PbI6, though without disturbing the metal-halide chemistry (Figure 2a and b). In particular, researchers explored around 22 structures including 18 hypothetical compounds that are not reported practically to date (Figure 2c and d).26
Figure 2. a) Two-dimensional map of the DFT band gap of the Platonic model of PbI 3-based perovskites as a function of the apical and equatorial metal–halide–metal bond angles. b) Tuning band gap of the metal-halide perovskites via the steric size of cation. c) Calculated tolerance factors for all the perovskite considered on the work. d) Measured absorbance spectra of the dark phase of Rb xCs1-xPbI3 perovskite thin films with different x. Blue shift of absorption onset is supposed to the more pronounced octahedral tilt in PbI3 network, as illustrated in the inset. Reproduced with permission.28 Copyright 2014, Nature Publishing Group.
Apart from the bond angle contribution between cation and metal-halide octahedral network, the size of cation may also cause the formation of insulating barriers between PbI4 layers. it resulted in the two dimensional (2D) structure and exhibited wider bandgaps.78,6, 79 Whereas, the cation with a small atomic radii, such as methylammonium (MA+) and formamidinium (FA+) can form
3D
perovskite
and
exhibited
smaller
bandgap.
Gratzel‟s
group
reported
a
mixed-organic-anion halide perovskite with composition of (MA)x(FA)1-xPbI3 (x=0-1) as the photosensitizer for mesoscopic solar cells for the first time. The bandgap of MAPbI3 decreased by substitution of MA with FA cations gradually. The reported MA0.6FA0.4PbI3 exhibited superior performance to the other compositions owing to a greater light-harvesting and yielded 14.9% PCE under the AM 1.5G simulated solar spectrum.6
Figure 3. a) UV−vis absorption spectra of FTO/bl-TiO2/mp-TiO2/ MAPb(I1−xBrx)3/Au cells measured using an integral sphere. b) Photographs of 3D TiO2/MAPb(I1−xBrx)3 bilayer nanocomposites on FTO glass substrates. c) A quadratic relationship of the bandgaps of MAPb(I 1−xBrx)3 as a function of Br composition x. d) Power conversion efficiencies of the heterojunction solar cells as a function of Br composition x. e) Photocurrent density−voltage (J−V) characteristics of the heterojunction solar cells. f) Corresponding external quantum efficiency (EQE) spectra. Reproduced with permission.2 Copyright 2013, American Chemical Society.
Except for cations, the halide anions also directly affect the electronic state of perovskite, which further modifies the optical bandgap of materials. Seok‟s group synthesized solar cell using MAPb(I1-xBrx)3 as light-harvester on mesoporous TiO2 substrate, which showed the tunable bandgap covering almost the entire visible spectrum.10 As shown in Figure 3a, the absorption spectra showed a wider distribution between 786 nm of MAPbI3 and 544 nm of MAPbBr3, which correspond to obvious color changes in perovskite thin film (Figure 3b). The variation in bandgap with Br component was estimated from the onset absorption edge (Figure 3c), exhibiting an almost linear relationship between bandgap and composition, as can be seen in Figure 3d. When the value of x is increased from 0 to 1, the open-circuit voltages increase from 0.87 V to 1.13 V while the current densities are decreased from 18 mA cm-2 to 5 mA cm-2 (Figure 3e). The external quantum efficiency spectra also matched with the current density (Figure 3f). The solar cell fabricated from x=0 to 0.2 exhibited an average of more than 10% of PCE under AM 1.5 radiation. Furthermore, an alternating of both A cation and X anion to achieve a synergistic effect in bandgap engineering is considered. Snaith‟s group reported formamidinium lead bromide-iodide mixed halide perovskites (FAPbIyBr3-y) and explored the bandgap tunability. First, they investigated the effect of the variation in size of cation A on the optical properties of the perovskite by synthesizing perovskite based on MA, FA, Cs (Figure 4a). It turns out that the increment in the radius of cation expanded the lattice, causing decreased bandgap. A redshift in absorption onset was observed, delivering the smallest bandgap of 1.48 eV which is smaller than
1.57 eV for commonly MAPbI3 (Figure 4b). With the successive substitution between halide anion, the crystal phase of perovskite transferred from a cubic (for y<0.5) to a tetragonal (y>0.7) crystal structure, as shown in (Figure 4c). By describing tetragonal as a „pseudocubic‟ structure with lattice parameter comparatively larger than that of cubic lattices, researchers observed that a large pseudocubic lattice parameter results in a narrower bandgap and providing a whole range of colored perovskites, as shown in Figure 4(d, e). Due to the narrowed bandgap with wider spectrum absorption, FAPbI3-based solar cells exhibited the photocurrent up to 840 nm (Figure 4f). With reduced bandgap of 1.48 eV when y equals 1, the cell allows absorption of photons over a greater proportion of the solar spectrum which is closer to the ideal bandgap for a single-junction solar cell and the high PCE up to 14.2% can be obtained (Figure 4g).75
Figure 4. a) The ABX3 perovskite crystal structure and atomic structure of three A site cations. b) UV-Vis spectra for the APbI3 perovskites formed. c) XRD spectra of FAPbIyBr3-y films. d) Corresponding steady-state photoluminescence spectra for the same films. (c) Photographs of the FAPbI yBr3-y perovskite films with y increasing from 0 to 1 (left to right). e) UV-vis absorbance of the FAPbIyBr3-y perovskite with varying y. f) External quantum efficiency spectrum of a representative FAPbI3 solar cell. g) Current– voltage characteristics of FAPbIyBr3-y planar heterojunction solar cells. Reproduced with permission. 75 Copyright 2014, Royal Society of Chemistry.
On the other hand, the substitution of Pb ion at B site has been drawing considerable attention and mainly motivated by the purpose of the heavy metal-free photoelectrical devices. Also, the tolerance factor and structure of octahedral BX6 is expected to have an influence on the intrinsic electronic properties. Kanatzidis et al. have been explored a series of hybrid metal perovskites, demonstrating that solid solutions between the Sn and Pb perovskite compounds are able to span the optical bandgap range from 1.1 eV to 1.7 eV throughout composition range.9
Similar work has been reported by Ogimi et al,8 suggesting that Sn-Pb perovskite could be a potential choice for lower bandgap solar cells. Hao et al. completely substituted Pb by Sn, the resulted halide perovskite MASnI3 with a narrowest band gap of 1.1 eV and showed a panchromatic light absorption with whole visible and infrared spectrum up to 1050 nm.80 From the perspective of thermal stability along with the slightly broadened absorption region of FA-based perovskite, Liu et al. fabricated planar perovskite heterojunction solar cells by using FAPb1-xSnxI3 materials as a light-harvester. The mixed metal FAPb1-xSnxI3 exhibited a narrow bandgap partly due to the smaller ionic radii of Sn2+ as compared to Pb2+, leading to light-harvesting enhancement and higher Jsc as well. It resulted an extended absorption spectrum up to 965nm corresponding to bandgap of the 1.28 eV at x equals to 0.5.76 In addition to the bandgap adjustment based on the ion substitution, Seok‟s group reported a promising bilayer solar cell architecture using MAPbBr3 and FAPbI3 as a combination (FAPbI3)1-x(MAPbBr3)x for light-harvester. The UV-vis absorption spectra showed a systematic shift in the absorption band edge towards the shorter wavelengths as MAPbBr3 content was increased, causing a reduction in Jsc due to reduced light-harvesting efficiency. Additionally, an increment in the open circuit voltage can be attributed to the widening of bandgap due to the high value of x. Along with the simultaneous enhancement of Jsc, FF and Voc, the PCE of (FAPbI3)1-x(MAPbBr3)x combination exhibited a maximum value of 17.7% at x=0.15.77 We have discussed the responsible mechanisms for enhancement in light-harvesting in halide PSCs from the perspective of bandgap engineering. The optical bandgap for light-harvesting is a crucial factor for attaining high PCE and can be tuned precisely through the substitution of cation A or B, and anion X. As the above discussed example of BX6 network, cation interchange in halide perovskite induced variation in bond angle and so the alternation in bandgap. In other way, the tuning of the bandgap achieved through variation in halide anion is attributed to a strong dependence of electronic energies on the effective exciton mass.81 In other words, the interchange in ions resulting in the change of optical properties which clarified the strong dependence of varying properties between composition design of materials, features of internal structure, and properties change. However, in the case of FAPbX3, the materials would present an amorphous phase with high levels of energetic disorder and low absorption during the substitution of iodide with bromide.82 It should be noted that an efficient modulation in light-harvesting is needed besides the atomic level designing/modification in halide perovskite materials.
2.2 Crystal-level: Modulation of perovskite crystal film for light absorption enhancement. As the photosensitizer layer in the PSCs, the perovskite crystal film directly decides the light
absorption ability of cells. In the following part, the strategies applied in perovskite crystal film for the effective modulation in the light-harvesting are addressed. The formation of light-trapping structures and the implication of plasmonic resonance effect are foremost factors to affect the light-harvesting in perovskites. In the beginning, the impact of crystal structure on the light absorption is introduced. From above discussion, it is confirmed that bandgap engineering of halide perovskite is one of the most effective way to enhance the light-harvesting in PSCs via extending the absorption range. Nevertheless, alternation in the composition is also an effective way to modulate the optical absorption range of perovskite film. Petrozza‟s group investigated the relationship between the optical properties and structural features of the MAPbI3 crystal film.83 Raman spectrum indicated that an ordered arrangement of cations could grow on the flat substrate in MAPbI3 crystallites, while cation arrangement would be more disordered in at the rough substrate.84 Furthermore, it is supposed that both the displacement of the organic cation and its interaction with the inorganic counterpart affect the electronic properties of the compound.83,85,86 For example, Angelis et al. demonstrated that the structure-induced enhancement of spin-orbit coupling could be mediated by the interplay of organic cations size and hydrogen bonds. 86 As a result, a clear shift in UV-vis absorption (Figure 5a and b) was observed when MAPbI3 film was deposited on the flat and rough substrate, respectively.
Figure 5. a) b) Normalized optical absorption (at 700 nm) for the “meso” MAPbI3 (red solid line) and the flat (black solid line) MAPbI3 films and corresponding photoluminescence spectra (dashed lines). Reproduced with permission. 83 Copyright 2016, American Chemical Society. c) Schematic illustration of the direct, below-bandgap transitions and absorption coefficient of methylammonium lead triiodide (MAPbI3) from a polycrystalline film. d) EQE of single-crystal solar cells and polycrystalline thin-film
solar cells. e) The calculated absorption of MAPbI3 films with different thickness. f) external quantum efficiency (EQE) curves and integrated current density of the optimal single-crystal solar cells using methylammonium lead triiodide thin single crystal with different thickness . Reproduced with permission.87 Copyright 2017, Nature Publishing Group.
Another way to expand the absorption spectrum of PSCs was investigated by Huang‟s group. They investigated the efficient utilization of the below-bandgap absorption of perovskites materials, attributed to the transition to the indirect-bandgap absorption with a bandgap of ~60 meV or smaller than that of the direct bandgap (Figure 5c).83 Under the consideration of the low absorption coefficient due to below-bandgap in perovskite required a absorber film of thickness of several micrometers. Therefore, the thickness of perovskite film thus needs to be over several micrometers in a single crystal film, which is beyond the carrier diffusion length in polycrystalline film. (Figure 5d). As can be seen in Figure 5e, the thickness-dependent absorption of MAPbI3 film and device efficiency curve is summarized which exhibiting the significant expansion in the absorption range towards the near-infrared region with absorption edge to 850 nm. However, other key parameters such as Voc shows the opposite variation trend with respect to the thickness of film as compared to the Jsc. These studies confirmed that by incorporating an optimal absorber thickness of the perovskite single crystal film, a high EQE (Figure 5f) and PCE values can be obtained.87 In another way, the upconversion process also provides a feasible approach to expand the absorption range in PSCs via utilizing the near-infrared photons which cannot be absorbed in general.88 Enhanced light absorption and improvement of Jsc in PSCs have been attained via converting near-infrared photons into high-energy photons.89,90 However, the upconversion efficiency is limited due to the strict generation condition of upconverting luminescence which can only be realized under high-power excitation such as laser irradiation.88 It is a fundamental consensus that the micromorphology of the photosensitizer materials shows a considerable impact on the interaction of sunlight and photosensitizer.90 Hence, construction of the photonic crystal with a periodic structure at the scale similar to the wavelength of sun light can be an effective way to enhance the interaction between light and matter which is supposed to lead the enhancement in the light absorption reactions in photosensitizer.6 Initially, a well-defined crystal film with opal-based morphology to enhance the light absorption has been one of the most used structure via constructing topologically light trapping cavity.91-95 Song et al. took a simple surface-imprinted method to fabricate whispering-gallery (WG) structured perovskite film which boosts the efficiency of solar cell from 15.3% to 19.8% (Figure 6a). The WG structured perovskite crystal film showed in Figure 6b exhibiting uniform thickness and high quality. The grain size in the vertical direction is comparable to the perovskite film thickness. The absorbance of WG-structured perovskite film is better than that of the pristine, which is
attributed to both light-trapping structure and high-quality formation of the perovskite thin film, as shown in Figure 6c. Moreover, the applied WG-like structure can decrease light reflection and increase LHE significantly, as shown in Figure 6d. Chen‟s group fabricated and explored 2D inverse-opal (IOP) structured perovskite films with various chemical ingredients and applied in the solar cells, as shown in Figure 6e.95 The continuous, uniform and honeycomb-like structures of the 2D IOP films can be ascertained by the AFM measurement in Figure 6f. The tunability of absorption onsets and bandgaps observed in the UV-vis measurement indicates the feasibility of energy level adjustment of IOP films. As shown in Figure 6g, the difference in Jsc is further analyzed by the incident photon-to-electron conversion efficiency (IPCE) spectra of the IOP film based PSCs. The lower value of Voc for the IOP-500 based SC compared to that of the IOP-1000 SC device. It is attributed to the smaller crystal size in the IOP-500 film that induces more non-radiative charge recombination at grain boundaries.94 Although, the construction of the photonic crystal structure in perovskite film enhanced light-harvesting efficiency, though the fabricated solar cell might suffer from an additional problem such as lower Voc. It is because of a short circuit contact across the charge carrier layer to the unsolid perovskite layer.96 Besides, the low fraction of the absorbent layer also contributes to inferior device performance.90 Therefore, further efforts are needed to optimize the balance between color and photocurrent output. It is worthy to mention that the photonic crystal morphology of perovskite film has been widely applied in the field of the photodetector to enhance the light-harvesting. By incorporating light-trapping strategies, a considerable improvement in performance has been achieved.97 So far, the opal-related morphology of the photonic crystal film has been one of the most common well-defined structure to enhance the light absorption via constructing topological light trapping cavity.92,98-100
Figure 6. a) Schematic of WG structured perovskite film and light trapping setup with WG-like structure. b) AFM image and line profile of WG-structured perovskite film. c) Absorption spectra for experiment and calculation. d) Reflectance and Light harvesting efficiency (LHE) for experiment and calculation. Reproduced with permission.95 Copyright 2018, Elsevier. e) Schematic diagrams of the preparation process for 2D IOP photonic films. f) AFM images with traces of height profiles of IOP-500, IOP-1000, and IOP-2000 MAPbI3 films. From left to right. g) Left: J−V characteristics of the SCs based on the IOP-500 and IOP-1000 MAPbI3 films for the forward bias to short circuit scan. The insets show the digital photos of the IOP-500 and IOP-1000 based SCs from both sides. Right: IPCE spectra of the SCs based on the IOP-500 and IOP-1000 MAPbI3 films. Reproduced with permission.94 Copyright 2016, American Chemical Society.
As for light-harvesting enhancement, application of surface plasmonic resonance (SPR) effect has been proved as an effective and facile route to enhance the interaction of light and photosensitizer.101-105 SPR effect can be generated via incorporating a plasmonic metal nanostructure in the semiconductor absorber layer. It assists the light absorption in the absorber by photonic enhancement i.e. frequency of incident light matches the frequency of exciting electrons at the interface between metal and dielectric material.102,106 The applications of the SPR effect in PSCs have been drawn attentions since the beginning.107-110 Snaith‟s group embedded core-shell nanoparticle (Ag@TiO2) in perovskite solar cell and demonstrated a PCE of 16.3% with a significant improvement in Jsc (Figure 7a), while no notable enhancement in light absorption was observed. (Figure 7b). They proposed a mechanism and related theoretical model to explain the improvement in Jsc i.e. highly polarizable metallic nanoparticles increase the optical path length by acting as antennas for light reemitted from radiative recombination of electron-hole pair, shown in the Figure 7c.111 Earlier, they had concluded that the improvement
of Jsc is attributed to the incorporation of metal nanoparticles, resulting in the enhanced generation of free charge carriers and reduced exciton binding energy.5 Furthermore, Míguez‟s group unveiled the influence of three type plasmonic metals such as gold, silver and aluminum on the light absorption enhancement in the embedded MAPbI3 crystal film by using finite-difference time-domain (FDTD) method.112 The maximum absorption enhancement described as a function of perovskite film thickness and estimated to serve as an upper limit as ∫Amax (Figure 7d). For a series of MAPbI3 films with different thickness, the integrated solar absorption, ∫Ap, was calculated by combined Ap = 1 − RT − TT, o and equation: ∫Ap = ∫A (λ)·AM1.5D (λ) dλ (Figure 7e). Where, RT and TT are the calculated total reflectance and the total transmittance. On the basis of ∫Amax and ∫Ap, a significant enhancement in light absorption of the perovskite film via surface plasmonic effect was observed, as shown in Figure 7f. A MAPbI3 film with a thickness of 300 nm embedded with the silver particles can absorb solar radiation as much as its pristine counterpart with a thickness larger than 1 μm. Furthermore, simulations of absorptance spectra of 500 nm thick MAPbI3 film decorated with gold rods, gold stars and silver cubes have been explored to investigate the impact of shape of particles (Figure 7g). On contrary to what expected, it turned out that the effect of sharper features around which light efficiently localized was depressed by the concentration of poor optical field and provided an absorption enhancement comparable to the attained from round-shape particles. In the meantime, it was pointed out that the case of rod-shape was specific because the effective absorption is strongly dependent on the orientation of their long axis with respect to the incident beam. It means that vertically oriented rods parallel to the incident beam would significantly enhance the light absorption. In the final, the consequences of shielding the metal particle with dielectric coatings have been assessed (Figure 7h). It showed that although light harvesting by perovskite film was always lies on the higher side when uncoated particles were used. Still now, the coated particles provided a great deal of reinforcement effect in light absorption and extra protection is required due to mediocre thickness of the coating layer. In addition, the same group also presented a detailed theoretical analysis about the effect of incorporating plasmonic gold nanoparticles on the optical absorption of organic-inorganic halide perovskite films. Furthermore, plasmonic near-field enhancement and effect of scattering as a function of particle size, concentration and dimer formation have been analyzed to develop the rules that maximize sunlight harvesting via perovskite. In conclusion, they found that the incorporation of the gold particles in perovskite film resulted in an enhancement of 6% to 12% in sunlight absorption. Besides, the absorption would be maximum when plasmonic near-field and light scattering effects were adequately balanced.110
Figure 7. a) Light harvesting efficiency of the photoactive layer. b) IPCE for respective devices. c) The proposed mechanism within a perovskite film. Reproduced with permission.111 Copyright 2015, Wiley-VCH. d) Calculated absorptance spectra of MAPbI3 perovskite films of different thickness e) Normalized solar absorption versus MAPbI3 film thickness, Schematic of the modeled perovskite unit cell in inset. (f) Normalized solar absorption for a 300 nm MAPbI3 film containing metal nanospheres g) The contour plot in panel d represents the spatial distribution of the normalized electric field intensity at λ = 750 nm. h) Normalized solar absorption of a 500 nm MAPI film embedding gold spheres silica shell. Reproduced with permission.112 Copyright 2016, American Chemical Society
Lately, halide perovskite nanocrystals based PSCs have received a lot of attention due to its superior optical properties which overcome the Shockley-Queisser limit. Luther et al. demonstrated PSCs with abrupt compositional change throughout the perovskite film via layer-by-layer deposition of perovskite quantum dots (QDs).113 The abrupt composition changes create internal heterojunction, which benefits the charge separation at the internal interface and improves the photocarrier harvesting. The PV device with PCE of 15.52% showed increased value of Jsc, leading to more photogenerated charge carriers that can be harvested in the nanocrystals-based film. In early report, This research group also developed an AX (A=cation; X=anion) treatment strategy to further modify the already-high mobility of CsPbI3 QD films and demonstrated that the CsPbI3 nanocrystal solar cell exceeds 80% of the Shockley-Queisser limit.114 In addition, hot-carrier solar cells can also overcome the Schottky-Queisser limit by harvesting excessive energy from hot carriers. Sum et al. demonstrated efficient hot-electrons extraction (up to ~ 83%) within 1 ps by an energy-selective electron acceptor layer from
surface-treated perovskite nanocrystals thin films.115 On the other hand, in order to promote the electrical coupling between QDs effective removal of the insulating ligands is necessary, however, it will generate stability issues due to the ionic bonding character of the perovskite QDs. Based on that consideration, Yang et al. proposed a post-synthetic process for effective controlling of the density of insulating ligand on FAPbI3 QDs and demonstrated a PCE of 8.38% with superior stability to those of bulk FAPbI3 devices.116 In summary, at the crystalline level, enhancement in the light-harvesting of PSCs mainly focuses on the enhanced interaction between light and perovskite thin crystal film. Furthermore, other part of a solar cell, such as charge carrier transporting layer (CTL), electrode and substrate and their development, also play an important role in light-harvesting. Specifically, the restrictions will deviate more often when the proposed design is not performed on the delicate perovskite film.
2.2.3 Device-level: Device configuration modulation for light absorption enhancement. As we mentioned before, light trapping engineering on the delicate perovskite thin film in solar cell is undesirable to enhance the device performance.90,96 Based on that consideration, this part will introduce the light-harvesting manipulation from device level, focusing on the optical design on the substrate, electrode and electron transport layer (ETL) in the PSCs. In addition, the implication of the tandem solar cells will also be addressed which provides an effective way to boost light utilization efficiency and photovoltaic performance. Light scattering is one of the most effective methods to improve the light harvesting efficiency which can be realized via the deliberate optical design on device architecture.117 Park‟s group reported the synthesis of silicon master and nanopatterned mp-TiO2 layers with conical shaped moth-eye structures and applied those structure in PSCs, as shown in Figure 8a and b. Due to the structural anti-reflective property of the moth-eye nanostructure on the interface between perovskite layer and mp-TiO2 layer, the optical enhancement leads to 5.3% improvement of Jsc and 10.4% for PCE. To further investigate the effect of moth-eye TiO2 on optical enhancement, researchers measured the reflectance and absorbance of the films without Au counter electrode in the wavelength ranging from 350 to 800 nm. Besides, the light absorbance measurement in Figure 8e and the light harvest efficiency in Figure 8f further confirmed the light harvesting enhancement in moth-eye TiO2. Furthermore, the absorption profile inside the perovskite layer was analyzed via a 3D FDTD method, as shown in Figure 8g. It indicated that the absorption is enhanced at both the boundary and center in the moth-eye TiO2 structure. As a result, better higher Jsc (Figure 8h) and better EQE (Figure 8i) were achieved compared to the
flat TiO2 counterpart.118 Except for the ETL, efforts have been devoted in preparing the textured substrates for higher light absorption. Chen‟s group fabricate highly ordered hexagonal-tiled ITO substrate using the PS-coate substrate. The hexagonal-tailed ITO substrates could largely change the path of light transmission and enhance the light absorption, attaining improvement 7.2% in Jsc and 7.4% in PCE compared to traditional ITO substrate.119 Except for the light modulation from the substrate and the ETL, considering the synergistic effect between functional layer in the device, researcher tended to improve the efficiency of both light harvesting and charge transporting by adjusting perovskite/HTM interface into a rough interface via controlling the growth parameter of film formation only.120 Similarly, a random nanotexturing of HTL/Au electrode was applied in the back of planar perovskite solar cells via simply transfer without using any lithography or nanoimprinting technique. The strong backscattering and more effective absorption can be caused by large contrast of refractive index at the back interface and excitation of a surface plasmon, respectively.121
Figure 8. a) Schematic illustrations of preparing silicon master b) Patterning mesoporous (mp)-TiO2 layer with conical shaped moth-eye structures. c) Reflectance spectra of the FTO/blocking layer (bl-TiO2)/mp-TiO2 layout with and without moth-eye structure. d) The reflectance of the FTO/bl-TiO2/mp-TiO2/perovskite configuration with and without moth-eye structure. e) Absorbance, and f) Light harvesting efficiency (LHE) of the solar cell with and without moth-eye structure. g) The spatial profile of the optical absorption per unit volume at a wavelength 620 nm (h) Current–voltage curves of the best performing perovskite solar cells for flat TiO2 and moth-eye TiO2. i) EQE spectra for flat TiO2 and moth-eye TiO2, along with integrated Jsc calculated based on EQE data. Reproduced with permission.118 Copyright 2016, Wiley-VCH.
Although light trapping effect in PSCs can be achieved through the construction of well-designed optical structure as we discussed before, most of the schemes need the precise engineering in nanoscale structure, which in contents hinder the large-area solar cell fabrication.
In this circumstance, desirable controllability and reproducibility soft nanoimprinting technique to incorporate photonic structures into PSCs shows great potential in industrial large-scale production. Tang et al reported a scalable and convenient light-trapping method by combined moth-eye nanostructures with the metal back electrode via soft nanoimprint lithography using prepatterned polydimethylsiloxane (PDMS) as pattern mold, which is fabricated through the low-temperature solution process, as illustrated in Figure 9a. Both experimental and theoretical conclusions indicated that the moth-eye nanostructure is more beneficial to the light absorption enhancement (Figure 9b) and EQE (Figure 9c) than the conventional periodic gratings due to the broadband response to solar spectrum with polarization independence. Besides, an observable performance improvement in Jsc up to 14.3% (Figure 9d) was achieved comparing to the flat architecture without sacrificing the other electrical properties, which means this facile and cost-effective light trapping method is compatible with solution-processed PSCs. The IQE curves (Figure 9e) of the grating and moth-eye PSCs are almost identical to that of the flat one, which are consistent with the almost identical dark current characteristics of these three PSCs, indicating that the enhancements of EQE and PCE are mainly attributed to the absorption enhancement. In conclusion, the light absorption enhancement is mainly attributed to the synergistic effect of broadband light scattering and surface plasmonic resonance excited by the antenna like corrugated metal back electrode.122
Figure 9. a) Schematic of the fabrication process of nanopatterned PSCs. b) The absorption spectra of the
devices with and without pattern extracted from the reflection spectra. c) EQE spectra of PSCs. d) Current density versus voltage curves of flat, grating, and moth-eye patterned PSCs. e) Internal quantum efficiency (IQE) of the devices. Reproduced with permission.122 Copyright 2017, Wiley-VCH.
In addition, the implication of the tandem solar cell can be a facile way to boost the light absorption efficiency and power conversion efficiency.3,46,82,123-126 Integrating wide bandgap semiconductor based PV cell serving as a top cell with narrow bandgap semiconductor based PV cell serving as bottom cell is expected to tremendously improve the light harvesting efficiency due to the efficient multiple utilization of photons with different energy, which leads to a total PCE efficiency beyond the Shockley-Quiesser limits of single junction solar cells. However, the integrating multiple junctions of traditional solar cells require to counterbalance both band gap and high efficiency in each component, which is hard to modulate in traditional PV cell. In this regarding, PSCs show tremendous advantages in component depending bandgaps to optimize the bandgap engineering for constructing multiple junctions. Besides, the huge amounts of researches have been explored a plenty of facile fabrication methods towards high quality halide perovskite thin film and loosen the restriction of substrate selection, which is supposed to be an serious problem in the integration of commercial silicon PV cells with other efficient PV cells such as CIGS, CdTe, ect.46 Even though, there are still some difficulties remain.127,128 One is that the optimal efficiency of perovskite/Si tandem PVs is still hindered by non-ideal absorber band gaps of perovskite PV cells. The other one is to construction all-perovskite based solar cells.
Figure 10. a) Ultraviolet-visible absorbance spectra of films of FAPb[I( 1-x)Brx]3 and FA0.83Cs0.17Pb[I(1-x)Brx]3. b) I-V curve for the best perovskite devices fabricated. c) EQE spectrum measured in short-circuit (JSC) configuration for perovskite cell and the SHJ cell. Reproduced with permission.82 Copyright 2018, American Association for the Advancement of Science. d) Schematics showing 2T and 4T tandem perovskite solar cell concepts. e) Scanning electron micrograph of the 2T perovskite tandem. f) Scanned current-voltage of the two-terminal perovskite-perovskite tandem, the 1.2-eV solar cell, and the ITO-capped 1.8 eV solar cell. g) External quantum efficiency spectra for the sub cells. h) The stabilized power output for the 2T perovskite solar cell and the mechanically stacked tandem under AM1.5G illumination. Reproduced with permission.126 Copyright 2016, American Association for the Advancement of Science.
Snaith et al explored a FA-based perovskite with the ideal band gap for tandem PVs coupling with commercial Si cells. Structurally stable mixed-halide perovskite with different optical band gap was achieved by varying halide composition, as shown in Figure 10a. Single PV cell based on FA0.83Cs0.17Pb(I0.6Br0.4)3 delivered a Jsc of 19.4 mA cm-2, a Voc of 1.2V and a PCE of 17.1%. By integrating semitransparent perovskite cells in front of Si heterojunction (SHJ) cell, tandem solar cell demonstrated the highest PCE up to 25.2% if the highest measured FA0.83Cs0.17Pb(I0.6Br0.4)3 cell was used (Figure 10b). Spectral response also confirmed that the wider band gap of the perovskite solar cell integrates give 19.2 mA cm-2, which is in agreement to the measured Jsc (Figure 10c).82
In another way, although the design of multiple junctions of PV cells boost the PCEs, the first kind of commercial iterations of perovskite PVs would likely be as an “add-on” to silicon PVs. Besides, an all-perovskite-based tandem cell could deliver lower fabrication costs. However, the band gap of Pb-based halide perovskite cannot be tuned to below 1.48 eV while most effective tandem devices would require a rear cell with band gap of 0.9 to 1.2 eV. On above consideration, Snaith et al demonstrated a 14.8% efficient perovskite solar cell based on FA0.75Cs0.25Pb0.5Sn0.5I3 with band gap around 1.2 eV. Furthermore, it was combined with another solar cell based on FA0.83Cs0.17Pb(I0.5Br0.5)3 perovskite with band gap 1.8 eV to demonstrated current-matched monolithic all-perovskite two-terminal tandem solar cells (Figure 10d and e) with Voc large than 1.65 V (Figure 10f). The external quantum efficiency (EQE) measurements showed that two sub cells are fairly matched (Figure 10g), with current limiting from the wide gap sub cell. Furthermore, researchers fabricated 20.3% efficiency all-perovskite four terminal tandems
solar
cell
using
a
15.8%
FA0.83Cs0.17Pb(I0.83Br0.17)3
cell
to
filter
14.8%
FA0.75Cs0.25Sn0.5Pb0.5I3 cell. In conclusion, by combining FA0.75Cs0.25Pb0.5Sn0.5I3 cell and FA0.83Cs0.17Pb(I0.5Br0.5)3 cell, the best PCE was up to 17% and 20.3% was for 2T tandem device and 4T tandem device, respectively (Figure 10h).126
3. Halide perovskites being photosensitizer in photocatalysis Halide perovskites based photovoltaic device exhibit huge potential for solar to fuel (STF) conversion, attributing to its larger Voc compared to other PV cells4,129-131 (above 1 V for PSCs and 0.7 V for Silicon, CIGS and CdTe solar cells for single junction), accompanied with the satisfied Jsc, which are both essential to yield high STF efficiency.4,132,133 Furthermore, sustainable and inexpensive raw material of halide perovskite shows advantages in large scale production compared to expensive Ⅴ-Ⅲ light absorber.59 In the following part, we will concentrate on halide perovskite-based PEC system after the brief introduction of determined factors of STF efficiency. Researches of halide perovskite serving as powder photocatalyst for STF conversion will not be under description in this paper.67,70,71 As shown in Figure 11a, the real operation current Jop of PEC/PV cell relies on the intersection between J-V curve of photoelectrode and PV cell. Large Voc and high Jop for PV cell have advantages to generate a high operation current, which turns into high STF efficiency in a PEC/PV cell. As discussed before, PCSs get its advantages in Voc, which exhibits huge advantages in coupling various classical photoelectrode materials.55,57,61,62 The light trapping strategies have been achieved considerable STF efficiency improvement in PEC/PV system based on traditional semiconductor materials which is supposed to play an intriguing role in
perovskite-based PEC system.48-66,106 Meanwhile, as for the part of photoelectrode, small onset potential Vonset and large slope of photoanode benefit to achieve high system Jpl.134,135
Figure11 a) Typical J–V curve of a photoanode (deep yellow) and a PV cell (red). Reproduced with permission.134 Copyright 2019, Wiley-VCH. b) Theoretical analysis of the STH efficiency. c) Schematic diagram. DSA, the dimensionally stable anode. d) Energy potential diagram. e) AM 1.5G photon flux and the maximum electron flux that the CIGS photocathodes can generate behind the perovskite solar cells. f) J–V curves of the CH3NH3PbBr3-based solar cell and CIGS photocathode behind CH3NH3PbBr3 solar cell. Reproduced with permission.61 Copyright 2015, Wiley-VCH.
With the same purpose of constructing tandem solar cells to improve the light harvesting efficiency as discussed before.136-140 Researches about the integration of heterojunction PV cells with reasonable band gap paring have also expanded the direction of perovskite PV cells severing in the PEC system. Shen‟ group synthesized a novel core/shell TiO2@BiVO4 photoanode and combined it with single perovskite PV cell, simultaneously improve the light absorption and charge separation.57 Matthew‟s group develop a low-energy photo transparent Cu2O photocathode as first absorber coupling with hybrid perovskite MAxFA1-xPbI3 PV cell and investigate the influence of device architecture on the balance between performance and transparency, achieving a highest STH efficiency up to 2.5%.62 It has been proved that an optimal pairing of two different bang gap absorbers contributes to higher efficiency. Most perovskite-based PEC system put perovskite in back absorber because of its smaller band gap compared to another classic photocatalyst light absorber. However, theoretical studies of the optimal band gap paring for two absorbers based on the Shockley–Queisser limit explored a new direction (Figure 11b).61 By
pairing two absorbers with band gap of 1.6–1.8 eV and 1.1 eV band gap absorbers in a stacked configuration, considering overpotential for both electrodes are 0.5 V, photovoltage produced by each cell can still provide excessive energy for the electrolysis and improving water splitting efficiencies. Based on above consideration, Michael‟s group demonstrated a panchromatic tandem water splitting device comprising a semitransparent and high open circuit voltage perovskite solar cell (MAPbBr3, Voc=1.4 V) and a CIGS (band gap=1.1 eV) photocathode (Figure 11c and d). The maximum current densities that the photocathode can possibly reach are around 10.7 and 15.9 mA cm−2 when CH3NH3PbI3 and CH3NH3PbBr3 are used as the top cell, respectively (Figure 11e). The tandem device could achieve an solar to hydrogen (STH) conversion efficiency of 6% while using CH3NH3PbBr3 as top cell, which is the highest value obtained so far for a PEC/PV water splitting device using only one single-junction solar cell as the bias source under one sun illumination. (Figure 11f). Compared to the deliberate band gap alignment between halide perovskite and other different semiconductor, the PV-electrolyte system that using junctions of halide perovskite cells directly provides current bias voltage to photocatalyst system shows much advantages in both fabrication convenience and STF efficiency.4,50 Although a lot of ideal band gap of perovskite materials for tandem PEC/PV system are difficult to explored due to the phase transformation of perovskite,75,141,142 the breakthrough in material synthesis will surely make a great advancement in PEC configuration design. Besides, the improvement of light absorption efficiency of perovskite-based PEC could be inspired from its PV counterparts as we mentioned above. For example, a multiple thin film with alternative refractive can serve as a selective mirror to enhance light trapping inside the PEC cell.143
4. Conclusion Halide perovskite-based photovoltaics cell have been achieved impressive progress since its emergence. With its unique advantages in photoelectronic properties as well as its earth abundant compositions, PSCs march from a new star in laboratory towards competitive candidates for the next generation of commercial solar cells. In this article, instead of some popular issues such as stability and Pb-free in PV cells, we focus on the advanced light harvesting of perovskite materials. From atom level to device level, the popular modulations in light harvesting enhancement can be split into various categories, mainly including the extending of absorption range via band gap engineering (atom level); usage of plasmonic resonance enhancement effect (crystal film level); light trapping (crystal film level and device level); and arrangement of light absorb via tandem absorber devices (device level). Bandgap engineering of perovskites materials
relies on the facile interchange of composition.75 Meantime, the crystallinity of perovskite crystal thin film also affects the light absorption range inside PSCs.83 Systemic theoretical research work on SPR effect on perovskite thin film can serve as a guideline.112 Construction of light trapping structure developed from the precise modulation on perovskite film to the design of light-functional substrate,93 stepping further towards large-scale industrial production.122 Last but not the least, the deliberate design in tandem PCSs not only deliver a lower fabrication costs, but also yielding a tremendous efficiency boosting.126 Furthermore, the tandem devices exhibit large potential in photocatalysis. By optimal pairing of perovskite photovoltaics and widely studied semiconductor light absorber such as BiVO4 or CIGS, the enhancement in light utilization will boost the STF efficiency.51 In all, hierarchic presentation of light manipulation strategies in perovskite based photovoltaic devices provides an organized summary in advanced light harvesting, which is expected to inspire the future researches.
ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Jiangsu Province (No. BK20190446) and the NSFC (No. 11904172).
AUTHOR CONTRIBUTIONS Chao Ran Dong: Writing-original draft. Yue Wang: Supervision. Kan Zhang: Conceptualization, Supervision, Writing – review & editing. Haibo Zeng: Supervision.
CONFLICT OF INTEREST The authors declare no conflict of interest.
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Author biographies
Chao Ran Dong is a Ph.D. candidate in Prof. Zeng‟s group. His current research focus on the solar to energy conversion and electrochemical energy storage, especially H2O2 production.
Yue Wang received Ph.D. degree in physics from Nanyang Technological University. He is currently working at School of Materials Science and Engineering in Nanjing University of Science and Technology. His research interests include the optical spectroscopy, semiconductor photophysics and optically pumped
lasers
Kan Zhang obtained his Ph.D. degree in SKKU Advanced Institute of Nano-technology (SAINT) from Sungkyunkwan University, Korea. He is currently working at School of Materials Science and Engineering in Nanjing University of Science and Technology. His research interests involve synthesis and modification of metal oxide and sulfide for solar energy conversion and electrochemical energy storage.
Haibo Zeng obtained his Ph.D. in Material Physics from Institute of Solid State Physics in Chinese Academy of Sciences in 2006. Following visiting scholar at University of Karlsruhe (with Professor Claus Kling Shirn and Professor Heinz Kalt) and then postdoctoral work at National Institute for Materials Science (with Professor Yoshio Bando and Professor Dmitri Golberg), he joined the faculty at Nanjing University of Science and Technology in 2011 and initiated the Institute of
Optoelectronics & Nanomaterials in 2013. His research interests are focused on the exploratory design of semiconducting nanocrystals and 2D crystals, with an emphasis on optoelectronic applications.
Graphic abstract The review introduced some popular strategies for the light harvesting enhancement in the halide perovskite solar cells from different levels, including band gap engineering in atom level; modulation of crystal structural, plasmonic resonance, optical structure in crystal level and optical design in device level.