Rare Metals2018年第2期

Research progress in electron transport layer in perovskite solar cells

Gong-Ping Mao Wei Wang Sen Shao Xiao-Jun Sun Shi-An Chen Min-Hao Li Hua-Ming Li

School of Material Science and Engineering, Jiangsu University

School of Automotive and Traffic Engineering,Jiangsu University

Department of Photonics Engineering,Yuan Ze University

Institute for Energy Research of Jiangsu University

收稿日期：30 March 2017

基金：financially supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions (No.SZBF201437);A Funding of Jiangsu Innovation Program for Graduate Education (No.SJLX16_0429);

Research progress in electron transport layer in perovskite solar cells

Gong-Ping Mao Wei Wang Sen Shao Xiao-Jun Sun Shi-An Chen Min-Hao Li Hua-Ming Li

School of Material Science and Engineering, Jiangsu University

School of Automotive and Traffic Engineering,Jiangsu University

Department of Photonics Engineering,Yuan Ze University

Institute for Energy Research of Jiangsu University

Abstract：

Since perovskite solar cells appeared in 2009, its simple preparation process, high photoelectric conversion efficiency and the characteristic of low cost in preparation process let it become the hot spot of both at-home and abroad. Owing to the constant efforts of scientists, the conversion efficiency of perovskite solar cells is more than 20% now. Perovskite solar cells are mainly composed of conductive glass, electron transport layer and hole transport layer, perovskite layer and electrode parts. This paper will briefly introduce the working principle and working process about the electron transport layer of perovskite solar cells. The paper focuses on aspects such as material types(e.g., inorganic electron transport materials, organic small molecule electron transport materials, surface modified electron transport materials and doped electron transport materials), preparation technology of electron transport layer, the effects of electron transport layer on the photovoltaic performance of the devices, and the electron transport layer in the future research.

Keyword：

Perovskite solar cells; Electron transport layer; Material types; Preparation technology;

Author： Gong-Ping Mao,e-mail:maogp@ujs.edu.cn;

Received： 30 March 2017

1 Introduction

Most recently,the traditional energy is becoming deficient and environmental problems are increasingly extrusive [ 1, 2] .Because of these,the sustainable development of human is confronted with some challenges.As a new generation of clean energy,solar energy has very bright prospects for development [ 3, 4] .Perovskite solar cell can make clean energy which gradually becomes a part of people's life [ 5] .The efficiency of perovskite solar cells increased quickly due to the researchers'continuous works [ 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19] .Now,its efficiency can be compared with the traditional single-crystal silicon solar cells [ 20] .

In the research process of solar cells,the organic-inorganic metal halide perovskite materials were adopted in solar cells by Kojima et al. [ 21] and gained a photoelectric conversion efficiency of 3.8%.They created a precedent by using perovskite materials as absorption layer.In 2011,6.5%conversion efficiency of perovskite solar cells was fabricated by Park et al.,with the device by changing the size of perovskite materials grain (CH₃NH₃PbI₃) to improve its performance [ 22] .In 2013,the two-step deposition method was firstly adopted by Gratzel et al.to fabricate the solar cells,and the efficiency of this device is as high as 15% [ 23] .In the next year,the element Y was mixed to modify the structure of TiO₂ and improved photoelectric conversion efficiency to 19.3% [ 24] .In 2017,the argument of single-walled carbon nanotubes (SWCNT)into NiO can greatly increase the conduction of electrons was proposed by Liu et al. [ 25] .In order to resolve the problem of charge collection in hole transport layer-free perovskite solar cells,they used thin film of NiO/SWCNT to replace the conventional counter electrodes.The effects of concentration of CH₃NH₃PbI₃ on the thickness and morphology of perovskite layer in flexible perovskite solar cells were researched by Liu et al. [ 26] .In 2017,fabrication parameters of perovskite solar cells were also researched by Hatamvand et al. [ 27] .

As a new type of solar cells,perovskite is used in perovskite solar cells as light absorption layers [ 28, 29, 30, 31, 32] .The structure of perovskite solar cells can be pided into two types,mesoscopic structure and planar heterojunction structure.The structure of planar heterojunction has two types:positive structure and inversion structure (n-i-p and p-i-n) [ 33] .As seen in Fig.1,the order of positive structure is glass/fluorine doped tin oxide (FTO)/indium-tin oxide(ITO)/cathode/electronic transport layer/perovskite layer/-hole transport layer/anode.By contrast,inversion structure of the device has the reverse order.To inspire generated excitons,when the sunlight from the conductive glass enters the device,photons will be absorbed by perovskite when the incident photon energy is higher than the forbidden band width of perovskite absorption layer.Excitons separation phenomenon appears when they move to the interface of electron transport layer/perovskite absorption layer and perovskite absorption layer/hole transport layer,which will generate conduction band electrons and valence band holes.Finally,these electrons and holes will be injected into the electron transport layer and hole transport layer separately and absorbed by electrode.The device will work abroad if the outside of the device is connected to a load.Perovskite solar cells originate from dye-sensitized solar cells,and the similar structure can effectively reduce the compound of electrons and holes [ 34, 35] .With the aid of the favorable conditions,the performance will be improved drastically.The photoelectric conversion process is shown in Fig.2 [ 36] .

Fig.1 Positive structure and inverted structure of perovskite solar cells

Fig.2 Diagram of perovskite solar cells photoelectric conversion process [36]

2 Charge transport theory of electron transport layer in perovskite solar cells

Materials of electron transport layer are able to receive electronic carrier and can effectively transfer them.N-type semiconductor is usually used in electron transport materials,which can make the free electron concentration greater than hole concentration.Compared with perovskite materials,in order to receive and transfer electron into FTO from absorb layer,the conduction band should have smaller minimum value and electron transport layer should have high electron mobility.

In the sequential structures of planar heterojunction perovskite solar cells (n-i-p and p-i-n),the quality of two interfaces,electron transport layer/perovskite layer and perovskite layer/hole transport layer,affects the whole device photovoltaic performance.The research shows that the photovoltaic properties of planar heterojunction perovskite solar cells are related to the better energy level matching between them.To verify it,an experiment on the energy level matching to refer above view was researched by Singh et al. [ 37] .The result is that the energy level matching is advantageous to generate higher open-circuit photovoltage in p-i-n perovskite solar cells.Electron transport in the n-type inorganic semiconductor is mainly driven by the built-in electric field orientation drift and lattice scattering caused by thermal vibration [ 31] .Electron mobility can be expressed by Eq.(1):

where e is the elementary charge,τis the average free time and is the electron effective mass.The smaller the electron effective mass is,the higher the electron mobility is.Because of this,the semiconductor with smaller energy gap is widely used in the electron transport materials of perovskite solar cells.In addition,the electrical properties of inorganic semiconductor are related to its own impurities and defects.The donor and acceptor impurities can provide carrier and increase the conductivity;nondonor and acceptor impurities tend to produce composite center and shorten the non-equilibrium carrier lifetime;defects generally produce recombination center.Various impurities and defects on the carrier have a scattering function,with the migration rate decreasing,the conductivity decreases.In order to improve electron mobility,other elements can be mixed in inorganic semiconductor.

3 Effects of electron transport layer in perovskite solar cells

As shown in Fig.1,the adjacent interface (electron transport layer and light absorption layer) constitutes the selective contact [ 38] .Meeting the premise of suitable energy level matching,electrons can be extracted effectively in electron transport layer.Owing to the higher carrier mobility,electrons in the layer will be transported at a faster pace and absorbed by metal electrode finally.Furthermore,electron transport layer can effectively prevent hole migration in the direction of electron motion,so that the recombination rate of electrons and holes will be greatly reduced.As shown in Fig.3,electron transport materials were used as mesoscopic framework by Han et al. [ 39] .Using carbon instead of the tradition hole transport material (HTM) and Au electrode,they finally obtained a conversion efficiency of 10.64%.Mesoporous electron transport materials are conducive to the growth of perovskite crystal.Perovskite materials infiltrate in the interior of the mesoporous electron transport materials,so the transmission distance from n-type semiconductor to electrons is shortened obviously,and the recombination rate of electrons and holes could be reduced effectively.

The effectiveness and stability of the electron transport materials directly affect the quality of devices [ 40] .As mentioned above,the perovskite solar cells are made up by dye-sensitized cells evolved.Like in dye-sensitized cells,TiO₂ is used as the electron transport material in perovskite solar cells [ 41] .Although using TiO₂ in perovskite solar cells can bring a high efficiency,there are still some defects.In the sunlight,the device photovoltaic performance will decline rapidly due to its surface desorption of molecular oxygen [ 42] .In the present studies,the structure of electron transport layer is significant in perovskite solar cells,as opposed to hole transport layer structure is not indispensable.In 2013,the CH₃NH₃PbI₃/TiO₂ heterojunction perovskite solar cells with no hole transport layer were made by Abu Laban and Etgar [ 43] .CH₃NH₃PbI₃ was used as light absorption materials and hole transport materials;this was the no hole transport layer device to get the highest efficiency (e.g.,photoelectric conversion efficiency (PCE) of 8%and short-circuit photocurrent density (J_SC)of 18.8 mA·cm^-2),as shown in Fig.4 [ 43] .In 2014,the CH₃NH₃PbI₃ perovskite solar cells were made by Nazeeruddin et al.through the chemical bath deposition method depositing materials on FTO directly at the temperature of 70℃ [ 44] .With the absence of electron transport layer,the conversion efficiency obtained is only less than 2%.Then,the same method by using TiO₂ as electron transport layer was used by Nazeeruddin and others to get perovskite solar cells at low temperature and the device got the conversion efficiency of 13.7%.

Fig.3 Structure of solar cells uses electron transport materials as mesoscopic framework

4 Research status of electron transport materials in perovskite solar cells

4.1 Inorganic electron transport materials

Inorganic materials are used in perovskite solar cells as electron transport materials widely.With high electron mobility,they can effectively carry out the transmission of electrons.The nature of the material determines their strengths and weaknesses.Perovskite solar cells originated from previous dye-sensitized cells [ 45] ,and TiO₂ was used as mainly electron transport materials [ 28, 46, 47, 48, 49, 50, 51] .It can improve the separation and transmission efficiency of excitons due to the good energy level matching between TiO₂ and perovskite materials [ 52] .In order to improve its quality of electron transport,generally TiO₂ should go through the high temperature of 400-500℃sintering process.However,this also brings many adverse effects.Such a high sintering temperature is not conducive to the preparation process of the devices at low temperature.It is also bad for TiO₂ in the application of flexible base [ 31, 42] .People are continuously doing the research on electron transport materials after introducing mesoporous framework to perovskite solar cells.The nanometer framework of mesoporous TiO₂ is conducive to the growth of perovskite crystal,while the dense TiO₂ plays the role in electron transport.Anatase TiO₂ has a better quality as electron transport material [ 53, 54] .With the aid of ethanol and TiCl₄ aqueous solution preparation ion,the anatase TiO₂ was used to fabricate electron transport layer by twostep deposit method.The recombination rate of electrons and holes is effectively reduced,and the device got a photoelectric conversion efficiency of 12.5%.TiO₂nanoparticle can greatly increase the conduction of electrons,it also can reduce hysteresis effect,and let the cell get higher efficiency [ 55] .In some researches,short-length and high-density TiO₂ nanorod arrays (with the length of70 nm,the diameter of 20 nm) were used to increase photoelectric conversion efficiency [ 56, 57] .By using the seed crystal nucleation and templates to limit the growth of TiO₂ single crystal,the temperature can be controlled below 150℃by Snaith et al. [ 58] .The device's photoelectric conversion efficiency can be upped to 7.29%.Conings et al. [ 59] prepared ITO/TiO₂/CH₃NH₃PbI_3-xClx/Ag planar heterojunction solar cells and got a conversion efficiency of 13.6%under the low temperature (<135℃).An adapted annealing treatment was used in the fabrication process.According to their fabrication methods,they mixed titanium isopropoxide (TIP) and nitric acid;the average 6-nm-anatase TiO₂ nanoparticles were obtained after hydrolysis.This can make the annealing temperature of dense TiO₂ reduce to 135℃.The perovskite solar cells used TiO₂ as electron transport materials were fabricated by Yella et al. [ 44] by the method of chemical bath deposition.The conversion efficiency of 13.7%was got by optimizing the preparation technology.In Ref. [ 60] ,20-and 500-nm-TiO₂ slurries were prepared and many experiments using two different kinds of slurries with the mass ratios of 1:0,2:1,1:1,1:2,1:4 and 1:0 were carried out.The conclusion is that when the mass ratio of two kinds of slurry is 1:4,the device can achieve higher photovoltaic performance (e.g.,open-circuit photovoltage(V_OC) of (857±13) mV,J_SC of (20.5±0.29) mA·cm^-2,fill factor (FF) of (60.6±1.1)%and PCE of(10.67±0.15)%).In Refs. [ 61, 62] ,perovskite solar cells were fabricated by the method of impregnation and spray pyrolysis.Photovoltaic performance and effective spectral range of two kinds of devices were analyzed and compared,respectively,and they finally found that the spectrum of the two kinds of perovskite solar cells was increased by the method of spray pyrolysis.PCE increased from 10.2%to12.5%,open-circuit photo voltage increased to 0.948 V,and the fill factor increased from 60%to 69%(J_SC=19.1 mA·cm^-2).In addition,the method of spray pyrolysis can largely reduce the thickness of perovskite absorption layer and effectively reduce the leakage between the hole transport layer and the mesoporous TiO₂.

Fig.4 Structure of CH₃NH₃PbI₃/TiO₂ heterojunction solar cells with no hole transport layer (W_n,depletion width on n-side;W_p,depletion width on p-side) [43]

In addition to using metal oxide TiO₂ as electron transport material,many other materials can provide choice.ZnO as a metal oxide with high electron mobility is often used as electron transport material in perovskite solar cells [ 63, 64] .Compared with the traditional TiO₂,people can use ZnO to fabricate thin film with large area without sintering.In addition,the ZnO is considered as an ideal metal oxide semiconductor for its wide band gap(3.37 eV),a larger exciton binding energy (～60 meV) and the unique characteristics of photoelectric.Kumar et al.made the electron transport layer by the method of chemical bath deposition with ZnO nanorods and got a photoelectric conversion efficiency of 2.6%on PET flexible substrate [ 65] .Liu and Kelly [ 66] used ZnO nanoparticles film in perovskite solar cells as ZnO nanoparticles film on perovskite grain can serve as light scattering center and enhancing light absorption of the devices.As shown in Fig.5 [ 66] ,the perovskite solar cells were fabricated under the low-temperature process.The devices that they fabricated have got 15.7%and more than 10%photoelectric conversion efficiency,respectively,which are made on the glass substrate and PET flexible substrate.In Ref. [ 67] ,the ITO-PET/Gr/ZnO-QDs(APj et)/CH₃NH₃PbI₃/spiro-

OMeTA/Ag flexible perovskite solar cells were made.Compared with the traditional ZnO nanostructures,ZnO quantum dots with tunable band gap have a higher stability and conductivity in the same environment.With the prominent advantages,the devices got the photoelectric conversion efficiency of 9.73%.Device's structure is as shown in Fig.6.

WO3 has good stability and resistance to acid corrosion and has a higher mobility than TiO₂;WO₃ was used as the electron transport material in perovskite solar cells [ 68, 69] .But carriers in the interface of WO₃ and perovskite absorption layer are prone to compound and will reduce the short-circuit photocurrent density of the devices.

Fig.5 SEM images (a low and b high magnification) of surface of ITO/ZnO/CH₃NH₃PbI₃ film [66]

Fig.6 Structure of ZnO-QDs (APjet) in perovskite solar cells [67]

When using WO3 as the electron transport materials,the TiO₂ nanoparticles should be covered on its'surface,so as to improve the photovoltaic performance.Compared with TiO₂,SnO₂ has a lower conduction band edge,in theory;it is easier to let the electron from light absorption material into the conduction band of SnO₂ [ 70, 71] .During the research of Zhu et al.,mesoporous SnO₂ single crystals were used as an effective electron collector for perovskite solar cells [ 72] .They tested the performance of the devices under illumination of simulated solar AM 1.5 global light at 100 mW·cm^-2 and achieved a photoelectric conversion efficiency of 3.13%.Chen et al. [ 73] composed SnO₂crystals through low-temperature solution treatment (less than 100℃),and the ideal electron mobility was obtained.The low-temperature SnO₂-based electron selective contact for efficient and stable perovskite solar cells was researched by Song et al. [ 74] .Through comparing the illumination durability of the SnO₂-based and TiO₂-based perovskite solar cells under simulated sunlight illumination,they found that the devices with SnO₂-based electron selective contact have a better stability than the common devices.

4.2 Organic small molecule electron transport materials

In the process of making perovskite solar cells,organic small molecular materials have also been used to fabricate electron transport layer.They are easy to be processed and chemically modified to meet different requirements;however,they have a strong solubility in many organic compounds;it is easy to destroy the nanoscale electron transport layer.Nowadays,organic small molecule materials can be deposited in the surface of devices and have the ability to transport electrons.Common organic small molecule electron transport materials include fullerene and its derivatives.Most of them have high electron mobility,and the separation of excitons is implemented by the builtin electric field.Compared with TiO₂,fullerene and its derivatives can get rid of the disadvantages of high temperature sintering in the process of making electron transport layer.Wu et al. [ 75] used chemical bath (CB)solution of PC₇₁BM in the process of making solar cells by the method of spin coating (e.g.,speed of 1500 r·min^-1,time of 30 s).PCBM can adjust C₇₁ energy levels and improve the photoelectric conversion efficiency.Opencircuit photovoltage and fill factor of the device are increased obviously,and photoelectric conversion efficiency of 16.31%can be achieved.The photocurrent/photovoltage variation curve is shown in Fig.7 [ 75] .

Jeng et al. [ 76] used organic small molecular material fullerene C₆₀ as electron transport material,and the device got a photoelectric conversion efficiency of 1.6%.In their experiments,PCBM and ICBA were also put instead of fullerene C₆₀ to make electron transport layer,and photoelectric conversion efficiencies of 3.9%and 3.4%were got,respectively.The perovskite solar cells using C₆₀ as electron transport material were fabricated by Li et al. [ 77] .These researches indicated that the performance of devices depends on the external bias before and during measurements.They got an eightfold increase photoelectric conversion efficiency by appropriate optimization of the bias conditions (e.g.,PCE=8.55%,J_SC=20.19 mA·cm^-2and V_OC=0.902 V).In the process of Ref. [ 78] ,PCBM is creatively used as the electron transport material.By adopting the same low-temperature preparation process,they finally got the photoelectric conversion efficiency of10%on the glass substrate;the device used flexible substrate has gained a photoelectric conversion efficiency of more than 6%.You et al. [ 79] combined the method of liquid phase and also used PCBM as the electron transport material in CH₃NH₃PbI₃ perovskite solar cells under the low-temperature environment,and 11.5%and over 9%photoelectric conversion efficiency were gained in the glass substrate and a flexible substrate,respectively.In2015,the PC₆₁BM was used as electron transport materials in CH₃NH₃PbI₃ perovskite solar cells.Through optimizing CH₃NH₃PbI₃ crystal structure to make up for the defects of the crystalline grain,the photoelectric conversion efficiency is as high as 18% [ 80] .

Fig.7 Short-circuit photocurrent density and open-circuit photovolt-age changes in relationship [75]

Fig.8 Process SEM images of CH₃NH₃PbI₃ formed on SnO₂ thin film formation:a surface of mesoporous SnO₂ thin film,b surface of thin film after spin-coating PbI₂,and c surface topography of CH₃NH₃PbI₃ generation [83]

In March 2016,the heterojunction perovskite solar cells were fabricated by Chiang and Wu [ 81] under the condition of low temperature by two-step deposition method.Because PCBM has good ability of electron transmission,they put different amounts of PCBM (0 wt%-1 wt%) in the PbI₂ precursor solution in the process of making perovskite layer;the CH₃NH₃PbI₃/PCBM layers are made by spinning CH₃NH₃I on PbI₂-PCBM layer,and on this basis,they then used PCBM as electron transport material.Thereby,the efficiency of the electron transfer and the photovoltaic performance of the devices are effectively improved.The study has found that when the content of PCBM in PbI₂liquid precursor reached 0.1 wt%,these devices will have a higher photovoltaic performance (e.g.,V_OC=0.97 V,J_SC=20.2 mA·cm^-2,FF=82 and PCE=16.0%).

4.3 Surface modified electron transport materials

Surface modification of existing materials can improve the crystal surface structure,and the material will have better electron transfer ability.Many people have studied the surface structure of materials;the advantages and disadvantages of different materials in electron transport have been found.In Ref. [ 82] ,2-nm-TiO₂ deposition was put on the mesoporous TiO₂ framework to avoid the exciton composite in perovskite solar cells,and the photoelectric conversion efficiency is improved from the original 7.2%to 11.5%.Li [ 83] used SnO₂ as the electron transport materials.By optimizing the thickness of mesoporous SnO₂ thin film and using TiCl₄ treated on its surface,the perovskite solar cells have a higher short-circuit photocurrent density and open-circuit photovoltage (e.g.,J_SC=17.39 mA·cm^-2 and V_OC≈1 V).In addition,the photoelectric conversion efficiency of the devices is more than 10%.As shown in Fig.8 [ 83] ,CH₃NH₃PbI₃ is formed on the SnO₂ thin film.Table 1 [ 83] shows the effects of TiCl₄ aqueous used in the surface modification of SnO₂ on the properties of perovskite solar cells.

Seo et al. [ 84] reported that the perovskite solar cells were fabricated under the condition of low temperature by optimizing the thickness of the PCBM and inserting the LiF thin layer as hole blocking layer,gained a photoelectric conversion efficiency of 14.1%.In the process of their experiments,PCBM layers with different thicknesses corresponding to photovoltaic performance parameters of the devices are given in Table 2 [ 84] .Wojciechowski et al. [ 85] spread TiO₂ in TiAcAc ethanol solution,and anneal perovskite solar cells at 150℃by spin-coating method.It avoids the high temperature sintering processing of titanium dioxide,which makes the photoelectric conversion efficiency increase to 15.9%.In their experiments,TiAcAc was used as adhesive to splice TiO₂ nanoparticles,which makes the conductivity increase obviously.In Ref. [ 86] ,Zr was added to TiO₂ to fabricate the electron transport layer;they compared the two kinds of materials with Zr and without Zr during the process of making the devices.The results show that the devices with Zr have higher conversion efficiency due to the excellent physical properties of Zr.Compared with the devices without Zr and pyridine,the devices which used Zr-TiO₂ as electron transport materials have better performance (e.g.,V_OC=0.96 V,J_SC=14.9 mA·cm^-2,FF=69 and PCE=9.8%).

4.4 Doped electron transport materials

The electron transfer complex formed by doping electron transport material can significantly improve the efficiency of electron transport,but too much doping will decrease the mobility of the electrons and thus weaken the electron transport capacity.Mesoporous TiO₂ is used as nanometer framework in perovskite solar cells,which have wider range of its thickness.Owing to the low electron mobility of mesoporous TiO₂,the efficiency of electron transport from mesoporous TiO₂ to the electron transport layer is lower than that of the electron transport directly to the electron transport layer.There are many unknown defects in the surface of mesoporous TiO₂,which will have big effects when electrons inject into it.Therefore,the photovoltaic performance of the whole device will be affected seriously.A lot of researchers have prepared doped perovskite solar cells [ 87, 88] .In 2015,Zhu et al. [ 72] mingled TiO₂ and SnO₂ as electron transport material for perovskite solar cells.Through the experiments,they deposited SnO₂mesoporous single crystals paste from suspension in the ethanol and terpineol with the concentration of 5 wt%.Compared with the devices only used SnO₂ mesoporous single crystals as electron transport material,the devices with TiO₂-coated mesoporous single crystals have a higher photoelectric performance.SnO₂ mesoporous single crystals were used as an effective electron collector for perovskite solar cells.The devices got a photoelectric conversion efficiency of (7.31±1.33)%and a short-circuit photocurrent density of (16.72±0.44) mA.cm^-2.The performance of perovskite solar cells was improved by Yan et al. [ 89] by adding different ions in quaternary ammonium salt;the PCE of ITO/PEDOT:PSS/CH₃NH₃PbI₃/PCBM+salts/Ag perovskite solar cells increased from8.77%to 1 3.41%.

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Table 1 Effect of TiCl₄ aqueous used in surface modification of SnO₂ on properties of perovskite solar cells [83]

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Table 2 PCBM layer with different thicknesses corresponding to photoelectric performance parameters of device [84]

Mahmood et al. [ 90] fabricated ZnO/CH3NH3PbI3 and ZnO+Al/CH₃NH₃PbI₃ perovskite solar cells by the method of electronic spray.The method of electronic spray has faster deposition rate and higher efficiency than the traditional methods of chemical or physical deposition methods.In the experiment,the devices with pure ZnO thin films (400 nm) got photoelectric conversion efficiency of10.8%and the open-circuit photo voltage of the device which doped Al was improved from 1.01 to 1.04 V.Its photoelectric conversion efficiency also increased to12.0%.In 2017,according to the work in Ref. [ 91] ,Al was doped in ZnO as electron transport material to fabricate the solar cells.In the process of making high performance devices,ZnO and C₆₀ were combined as the electron transport material in the structure of perovskite solar cells [ 92] .Lai et al. [ 92] ,respectively,made some electron transport layers with different thicknesses (5 nm-C₆₀/ZnO,15 nm-C₆₀/ZnO and 30 nm-C₆₀/ZnO) of perovskite solar cells.They have found that the devices which have 15 nmC₆₀/ZnO electron transport layer have a higher photovoltaic performance (e.g.,J_SC=19.41 mA·cm^-2,V_OC=0.91 V,FF=62 and PCE=10.93%).In order to improve the photovoltaic performance of solar cells,themethod of mutual diffusion was adopted by Shao et al.to optimize the structure of ITO/PEDOT:PSS/CH₃NH₃PbI₃/PC₆₀BM/C₆₀/BCP/Al perovskite solar cells;the photoelectric conversion efficiency of the devices is as high as15.4%[].In addition,the efficiency of 85%of the devices is more than 14.5%.In 2014,PC₇₁BM was used as electron transport materials in perovskite solar cells.

Liao et al.mingled PC₆₁BM and Bis-C₆₀ to fabricate the solar cells [ 94] .They have researched the roles of alkyl halide additives in enhancing perovskite solar cells.In the experiments,PC₆₁BM and Bis-C₆₀ were sequentially spincoated at 1000 r·min^-1 for 60 s and 4000 r·min^-1 for 40 s,respectively.During their research,they achieved a photoelectric conversion efficiency of 13.09%.In 2011,Lee et al.made the perovskite quantum sensitized solar cells by the way of using Y-doped TiO₂ and gained a photoelectric conversion efficiency of 6.5%[].Yttrium ion radius and titanium ion radius are similar,which makes yttrium ion more easily to get into the TiO₂ lattice clearance,and yttrium ion energy level is close to the TiO₂ conduction band.Qin et al. [ 96] compared the perovskite solar cells which used TiO₂ as mesoporous framework and electron transport material with the cells which used mesoporous TiO₂-doped with 5 at%Y as framework and electron transport material.They found that the device doped with Y has better light absorption ability;the comparison between the two devices is shown in Fig.9 [ 96] .Photoelectric conversion efficiency also increased to 10%from less than 10%after these improvements in electron transport materials.According to the results in Ref. [ 97] ,the changing relations of photocurrent-photovoltage between the two kinds of perovskite solar cells were researched,which used Al₂O₃ and TiO₂ as electron transport materials,respectively (Fig.10, [ 97] ).Furthermore,the solar cells used the mixture (TiO₂ and Al₂O₃) gained an open-circuit photovoltage of 1.1 V and a conversion efficiency of10.9%.Because perovskite materials can permeate the frame of Al₂O₃ nanoparticles,it can improve the uniformity of the thin film to prevent leakage.In addition,due to the high lowest unoccupied molecular orbital (LUMO)energy level of Al₂O₃,electrons can move along the surface of the alumina and arrive in density TiO₂ electron transport layer.Stability of the devices processed by Al₂O₃has been improved obviously.Compared with ordinary devices,the CH₃NH₃PbI₃ perovskite materials can isolate moisture by the effect of Al₂O₃,so the moisture stability of perovskite materials increased obviously,which prolongs the service life of the device.Xiao et al. [ 98] reported that W-doped TiO₂ (less than 1000×10^-6) is conducive to the transmission and collection of electrons and improve the resistivity.

Fig.9 Light harvesting efficiency spectra of devices based on 0.5%Y-TiO₂ (black) and TiO₂ (red) [96]

Fig.10 Current-voltage characteristics measured under AM 1.5100 mW.cm^-2 simulated sunlight (solid) and in dark (dashed) for solid-state dye-sensitized solar cells with TiO₂ (red trace with crosses)and Al₂O₃.(black trace with circles) [97]

The study found that graphene has excellent properties of high conductivity,and its work function between FTO and TiO₂ can significantly improve the property of electron transport.Wang et al. [ 99] used graphene and TiO₂ as electron transport materials by the method of chemical bath deposition.It improves the short-circuit photocurrent and the fill factor of the devices obviously,and the photoelectric conversion efficiency of 15.6%is achieved.In2014,Nishino et al.let Sb₂S₃ layer growth on TiO₂ to fabricate the perovskite solar cells by the method of chemical bath deposition and got a photoelectric conversion efficiency of 5.24% [ 100] .On the one hand,compared with the traditional TiO₂ devices,the method can signally improve its stability:after 700 h,the efficiency of the devices which does not contain Sb₂S₃ is almost zero,and the device contains Sb₂S₃ after aging test still has some efficiency.Sb₂S₃ blocks the degradation of CH₃NH₃PbI₃perovskite materials and enables its long-term stability to exist,which improves the stability of the devices.On the other hand,as the barrier layer interface,Sb₂S₃ can greatly reduce the rate of exciton recombination,which provides guarantee for the conversion efficiency of the devices.According to the research in Ref. [ 101] ,in order to improve the electron transport rate and reduce the composite excitons,nanometer ZnO and organic small molecule materials(PCBM) were combined together;the nanometer ZnO layer (≈40 nm) is sprayed on the surface of PCBM layer(≈40 nm) to fabricate the electron transport layers which contain double molecular structure.Adopted by the devices,Trux-OMeTAD,MAPbI₃,PCBM and ZnO NPs have a good energy level matching,which enables the exciton to be better separated and transported.Eventually,the device got a conversion efficiency of 18.6%,an open-circuit photo voltage of 1.02 V,a short-circuit photocurrent density of 23.2 mA·cm^-2 and a fill factor of 79.From the perspective of improving the conversion efficiency,ultrathin graphene quantum dots (GQDs) layer was inserted into the position between perovskite and TiO₂ layer,and then the conversion efficiency increased from 8.8 1%to 10.15% [ 102] .From the view of transient absorption measurement,the enhancement of the devices can be attributed to the GQDs,leading to faster electronics extraction (90-106 ps).Referring to the devices which exclude GQDs,its electronic extraction rate stays in the range of 260-307 ps,which may eventually lead to the low photoelectric performance of the devices.Many scholars at home and abroad have studied the electron transport layer of perovskite solar cells.They have made many efficient devices by selecting different materials as the electron transport layer.Other types of device parameters are shown in Table 3.

5 Conclusions and prospects

As a new generation of solar cells,perovskite solar cells have the incomparable advantages,compared with many traditional solar cells.As an integral part of perovskite solar cells,electron transport layer can form the selectivity contact with perovskite absorption layer,which can effectively increase the extraction efficiency of electrons.The high electron mobility of electron transport materials makes the effective transport and provides guarantee for the quality of the devices.For electron transport materials research,at present,people mostly used inorganic materials,organic small molecule materials,surface modified materials and doped materials as raw electron transport materials for the perovskite solar cells.Now,the materials listed above still have many shortcomings:the low electron mobility,the bad energy level matching between electron transport materials and perovskite materials,and electron transport materials have the adverse effects on the stability of humidity and temperature.In the future,people should focus on the energy level matching between the materials,the mobility of the materials and so on.Developing the new materials can make up the shortages of the existing materials and thus improve the stability and photoelectric performance of the perovskite solar cells.

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Table 3 Perovskite solar cells with different structures corresponding to performance parameters of devices

Acknowledgements This study was financially supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions (No.SZBF201437) and A Funding of Jiangsu Innovation Program for Graduate Education (No.SJLX16＿0429).

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