Recent advancement in inorganic-organic electron transport layers in perovskite solar cell: current status and future outlook

Organic-inorganic lead halide perovskite solar cells have captured significant attention in recent years due to low processing costs and unprecedented development in power conversion efficiency (PCE). It has appeared from 2009 with PCE of 3.8% to being claimed more than 25.2% PCE in a very short span of time, showing their future prospective toward the fabrication of less expensive and stable solar cells. The incredible advancement in this technology encourages at one end, whereas several hurdles restricting its complete utilization for commercial purposes at another end. Although the selection of perovskite structure is limited with planar and mesoporous electron transport layers (ETLs), but identification of appropriate ETLs necessitates excellent effort to improve the surface morphology of absorber and obtain enhanced PCE with higher stability. In the present review, we have investigated various inorganic-organic ETLs with different device configurations of PSCs, primarily focusing on crystallization and morphology control techniques of ETL thin films. Numerous strategies such as surface functionalization, doping, and addition of interfacial layer are adopted for ETLs, and their effect on device efficiency, performance, and hysteresis is also discussed in detail. Additionally, designs of PSCs with different device configurations are discussed as well, providing future guidelines for significant progress in PSCs structure with different ETLs.     

TiO_2 nanoparticle-based electron transport layer with improved wettability for efficient planar-heterojunction perovskite solar cell

The electron transport layer(ETL) plays an important role in planar heterojunction perovskite solar cell(PSCs),by affecting the light-harvesting, electron injection and transportation processes, and especially the crystallization of perovskite absorber. In this work, we utilized a commercial TKD-TiO_2 nanoparticle with a small diameter of 6 nm for the first time to prepare a compact ETL by spin coating. The packing of small-size particles endowed TKD-TiO_2 ETL an appropriate surface-wettability, which is beneficial to the crystallization of perovskite deposited via solution-processed method. The uniform and high-transmittance TKD-TiO_2 films were successfully incorporated into PSCs as ETLs. Further careful optimization of ETL thickness gave birth to a highest power conversion efficiency of 11.0%, which was much higher than that of PSC using an ETL with the same thickness made by spray pyrolysis. This TKD-TiO_2 provided a universal solar material suitable for the further large-scale production of PSCs. The excellent morphology and the convenient preparation method of TKD-TiO_2 film gave it an extensive application in photovoltaic devices.     

低成本富勒烯衍生物电子传输层在钙钛矿太阳能电池的应用

Organic-inorganic hybrid perovskite solar cells (PSCs) have attracted significant attention owing to their high absorption coefficient and ambipolar charge transport properties. With only several years of development, the power conversion efficiency (PCE) has increased from 3.8% to 22.7%. In general, PSCs have two types of structural architecture: mesoporous and planar. The latter possesses higher potential for commercialization due to its simpler structure and fabrication process, especially the inverted planar structure, which possesses negligible hysteresis. In an inverted PSC, the electron transport materials (ETM) are deposited on a perovskite film. Only a few ETMs can be used for inverted PSCs as the perovskite film is easily damaged by the solvent used to dissolve the ETM. Furthermore, the energy levels of the ETM should be well aligned with that of the perovskites. Normally it is difficult to use inorganic ETMs as they require high temperatures for the annealing process to improve the electron conductivity; the perovskite film cannot sustain these high temperatures. To date, the fullerene derivative, [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM), is the most commonly used organic ETM for high efficiency inverted planar PSCs. However, the high manufacturing cost due to its complex synthesis retards the industrialization of the PSCs. Here, we introduce a fullerene pyrrolidine derivative, N-methyl-2-pentyl-[60]fullerene pyrrolidine (NMPFP), synthesized via the Prato reaction of C₆₀ directly with cheap hexanal and sarcosine. Then the NMPFP electron transport layer (ETL) was prepared by a simple solution process. The properties of the resulting NMPFP ETLs were characterized using UV-Vis absorption spectroscopy, cyclic voltammetry measurements, atomic force microscopy, and conductivity test. From the results of the UV-Vis absorption spectroscopy and cyclic voltammetry measurements, the LUMO level of NMPFP ETL was calculated to be 0.2 eV higher than that of the PCBM ETL. This contributes to a higher open-circuit photovoltage. In addition, the NMPFP film presented higher conductivity than the PCBM film. Thus, the photo-generated charge carriers in the perovskite films should be transported more efficiently to the NMPFP electron transport layer (ETL) than to the PCBM ETL. This was confirmed by the results of the steady-state photoluminescence spectroscopy. Finally, the NMPFP as an alternative low-cost ETL was employed in an inverted planar PSC to evaluate the device performance. The device made with the NMPFP ETL yielded an efficiency of 13.83% with negligible hysteresis, which is comparable to the PCBM counterpart devices. Moreover, since stability is another important parameter retarding the commercialization of PSCs, the stability of the PCBM and NMPFP base PSCs were investigated and compared. It was found that the NMPFP devices possessed significantly improved stability due to the higher hydrophobicity of the NMPFP. In conclusion, this research demonstrates that NMPFP is a promising ETL to replace PCBM for the industrialization of cheap, efficient and stable inverted planar PSCs.     

ZnO/ZnS core-shell composites for low-temperature-processed perovskite solar cells

Electron transport layers(ETLs)in perovskite solar cells(PSCs)are a key factor to determine the photovoltaic performance.Herein,we demonstrate preparation of Zn O/ZnS core-shell composites through directly synthesizing ZnS on the ZnO nanoparticles in solution.We confirmed the formation of ZnO/ZnS core-shell composites by the uses of X-ray diffraction patterns and the Fourier transform infrared spectroscopy.ZnO/ZnS composites exhibit much homogeneous surface morphology as compared with the bare Zn O as revealed in the scanning electronic microscopy.Moreover,the upper shift of conduction band level upon composition of the Zn O/Zn S film results in a better alignment of energy level,which facilitates cascade charge extraction and thus improves the current density of perovskite solar cell.The shift of conduction band also improves the voltage of the PSCs.The photoluminescence(PL)spectroscopies measured in both steady and transient states were carried out to characterize the charge extraction at the interface between CH_3NH_3PbI_3and the electron transport layers of either ZnO or ZnO/ZnS composite.The ZnO/ZnS composite can more efficiently quench the PL signal of perovskite absorber than bare Zn O resulting in enhanced photocurrent generation in PSCs.     

Bowl-Assisted Ball Assembly for Solvent-Processing the C60 Electron Transport Layer of High-Performance Inverted Perovskite Solar Cell

Pristine fullerene C₆₀ is an excellent electron transport material for state-of-the-art inverted structure perovskite solar cells (PSCs), but its low solubility leaves thermal evaporation as the only method for depositing it into a high-quality electron transport layer (ETL). To address this problem, we herein introduce a highly soluble bowl-shaped additive, corannulene, to assist in C₆₀-assembly into a smooth and compact film through the favorable bowl-ball interaction. Our results show that not only corannulene can dramatically enhance the film formability of C₆₀, it also plays a critical role in forming C₆₀-corannulene (CC) supramolecular species and in boosting intermolecular electron transport dynamics in the ETL. This strategy has allowed CC devices to deliver high power conversion efficiencies up to 21.69 %, which is the highest value among the PSCs based on the solution-processed-C₆₀ (SP-C₆₀) ETL. Moreover, the stability of the CC device is far superior to that of the C₆₀-only device because corannulene can retard and curb the spontaneous aggregation of C₆₀. This work establishes the bowl-assisted ball assembly strategy for developing low-cost and efficient SP-C₆₀ ETLs with high promise for fully-SP PSCs.     

钙钛矿太阳能电池电子传输层的制备及应用

目前,有机-无机杂化钙钛矿太阳能电池(PSC)的器件效率已经超过25％.电子传输层作为PSC中的重要组成部分在提取和传输光生电子,阻挡空穴,修饰界面,调节界面能级和减少电荷复合等方面起着关键作用.无机n型材料,例如TiO2、ZnO、SnO2和其他金属氧化物材料具有成本低和稳定性好的特点,经常在传统PSC中被用作电子传输...     

Designing a Perylene Diimide/Fullerene Hybrid as Effective Electron Transporting Material in Inverted Perovskite Solar Cells with Enhanced Efficiency and Stability

Electron transport materials (ETM) play an important role in the improvement of efficiency and stability for inverted perovskite solar cells (PSCs). This work reports an efficient ETM, named PDI‐C₆₀, by the combination of perylene diimide (PDI) and fullerene. Compared to the traditional PCBM, this strategy endows PDI‐C₆₀ with slightly shallower energy level and higher electron mobility. As a result, the device based on PDI‐C₆₀ as electron transport layer (ETL) achieves high power conversion efficiency (PCE) of 18.6 %, which is significantly higher than those of the control devices of PCBM (16.6 %) and PDI (13.8 %). The high PCE of the PDI‐C₆₀‐based device can be attributed to the more matching energy level with the perovskite, more efficient charge extraction, transport, and reduced recombination rate. To the best of our knowledge, the PCE of 18.6 % is the highest value in the PSCs using PDI derivatives as ETLs. Moreover, the device with PDI‐C₆₀ as ETL exhibits better device stability due to the stronger hydrophobic properties of PDI‐C₆₀. The strategy using the PDI/fullerene hybrid provides insights for future molecular design of the efficient ETM for the inverted PSCs.     

Self-doped n-type small molecular electron transport materials for high-performance organic solar cells

Two naphthalene diimide (NDI) and perylene diimide (PDI) based n-type water/alcohol soluble small molecules (NFN and PFP) are designed and utilized as electron transport layers (ETLs) for organic solar cells (OSCs). NFN and PFP are synthesized by using Sonogashira coupling from alkynyl modified fluorene with mono-bromo substituted NDI and PDI. Density functional theory study results of NFN and PFP show that they possess excellent planarity due to the employment of triple bonds as connection units. Moreover, it was shown by electron paramagnetic resonance study that both NFN and PFP possess obvious self-doping behaviors, which may effectively enhance their charge transporting capability as ETLs in OSCs. Power conversion efficiencies of 8.59% and 9.80% can be achieved for OSCs with NFN and PFP as ETLs, respectively. The higher power conversion efficiency (PCE) of PFP based photovoltaic device is originated from the stronger doping property and higher mobility of PFP.     

Mixed-phase Mesoporous TiO2 Film for High Efficiency Perovskite Solar Cells

Mesoporous scaffold structures have played great roles in halide perovskite solar cells(PSCs),due to the excellent photovoltaic performance and commercial perspective of mesoporous PSCs.Here,we reported a mixed-phase TiO2 mesoporous film as an efficient electron transport layer(ETL)for mesoporous perovskite solar cells.Due to the improved crystal phase,fihn thickness and nanopartMe size of TiO2 layer,which were controlled by varying the one-step hydrothermal reaction time and annealing time,the PSCs exhibited an outstanding short circuit photocurrent density of 25.27 mA/cm^2,and a maximum power conversion efficiency(PCE)of 19.87%.It is found that the ultra-high Jsc attributes to the excellent film quality,light capturing and excellent electron transport ability of mixed-phase TiO2 mesoporous film.The results indicate that mix-phase mesoporous metal oxide fihns could be a promising candidate for producing effective ETLs and high efficiency PSCs.     

SnO2双层电子传输层对钙钛矿太阳能电池界面电荷传输的影响

为了改善基于SnO₂电子传输层的钙钛矿太阳能电池的界面电荷传输特性和迟滞现象,我们采用低温溶液处理工艺制备了4种不同类型的SnO₂电子传输层用于钙钛矿太阳能电池,包括由SnCl₄·5H₂O溶胶-凝胶层(Cl₄-SnO₂)、SnCl₂·2H₂O溶胶-凝胶层(Cl₂-SnO₂)和SnO₂纳米颗粒层(NP-SnO₂)与SnO₂胶体层(Col-SnO₂)两两相互作用形成的同质结SnO₂双层电子传输层和Col-SnO₂单层电子传输层;并系统研究了不同SnO₂双层电子传输层对器件光电性能和迟滞现象的影响。通过扫描电镜(SEM)、X射线衍射(XRD)、稳态光致发光(PL)、电化学阻抗(EIS)和稳定性测试等表征证实,在Col-SnO...     

Eliminating Hysteresis of Perovskite Solar Cells with Hollow TiO2 Mesoporous Electron Transport Layer

Current density-voltage(J-V) hysteresis issue caused by unbalanced charge transport has greatly limited the improvement of power conversion efficiency(PCE) of halide perovskite solar cells(PSCs). Herein, hollow TiO₂ mesoporous electron transport layer(ETL) was used to fabricate PSCs. The structure-dependent charge collection as well as its effect on PCE and hysteresis impactor(HI) of PSC were investigated. The results demonstrate that TiO₂ hollow spheres in a size of around 50 nm (HS-50) can form a high quality perovskite/ETL interface with a less trap density. Moreover, the hollow TiO₂ with the thin shell can help promote the extraction of electrons from perovskite layer to ETL, so as to reduce the charge accumulation and recombination at the perovskite/ETL interface and alleviate the hysteresis behavior. As a result, PSCs with HS-50 TiO₂ delivered a champion PCE of 16.81% with a small HI of 0.0297, indicating a better performance than the commercial P25(PCE of 15.87%, HI of 0.2571).     

A mini review: Constructing perovskite p-n homojunction solar cells

Organic metal halide perovskite materials have excellent photoelectric properties, and the power conversion efficiency(PCE) of the perovskite solar cells(PSCs) has increased from 3.8% to more than 25%. In the development of PSCs, innovative architectures were being proposed constantly. However, the use of the electron transport layer(ETL) and hole transport layer(HTL) increases manufacturing costs and process complexity. Perovskite material has ambipolar charge transport characteristics, so it c...     

An Azaacene Derivative as Promising Electron‐Transport Layer for Inverted Perovskite Solar Cells

It is highly desirable to develop novel n‐type organic small molecules as an efficient electron‐transport layer (ETL) for the replacement of PCBM to obtain high‐performance metal‐oxide‐free, solution‐processed inverted perovskite solar cells (PSCs) because this type of solar cells with a low‐temperature and solution‐based process would make their fabrication more feasible and practical. In this research, the new azaacene QCAPZ has been synthesized and employed as non‐fullerene ETL material for inverted PSCs through a solution‐based process without the need for additional dopants or additives. The as‐fabricated inverted PSCs show a power conversion efficiency up to 10.26 %. Our results clearly suggest that larger azaacenes could be promising electron‐transport materials to achieve high‐performance solution‐processed inverted PSCs.     

Suppressing Interfacial Charge Recombination in Electron-Transport-Layer-Free Perovskite Solar Cells to Give an Efficiency Exceeding 21 %

The performances of electron-transport-layer (ETL)-free perovskite solar cells (PSCs) are still inferior to ETL-containing devices. This is mainly due to severe interfacial charge recombination occurring at the transparent conducting oxide (TCO)/perovskite interface, where the photo-injected electrons in the TCO can travel back to recombine with holes in the perovskite layer. Herein, we demonstrate for the first time that a non-annealed, insulating, amorphous metal oxyhydroxide, atomic-scale thin interlayer (ca. 3 nm) between the TCO and perovskite facilitates electron tunneling and suppresses the interfacial charge recombination. This largely reduced the interfacial charge recombination loss and achieved a record efficiency of 21.1 % for n-i-p structured ETL-free PSCs, outperforming their ETL-containing metal oxide counterparts (18.7 %), as well as narrowing the efficiency gap with high-efficiency PSCs employing highly crystalline TiO₂ ETLs.     

In-situ fabrication of dual porous titanium dioxide films as anode for carbon cathode based perovskite solar cell

We develop a dual porous(DP) TiO_2 film for the electron transporting layer(ETL) in carbon cathode based perovskite solar cells(C-PSCs). The DP TiO_2 film was synthesized via a facile PS-templated method with the thickness being controlled by the spin-coating speed. It was found that there is an optimum DP TiO_2 film thickness for achieving an effective ETL, a suitable perovskite/TiO_2 interface, an efficient light harvester and thus a high performance C-PSC. In particular, such a DP TiO_2 film can act as a scaffold for complete-filling of the pores with perovskite and for forming high-quality perovskite crystals that are seamlessly interfaced with Ti_O2 to enhance interfacial charge injection. Leveraging the unique advantages of DPTiO2 ETL, together with a dense-packed and pinhole-free TiO_2 compact layer, PCE of the C-PSCs has reached 9.81% with good stability.     

Supramolecular Coordination of Pb2+ Defects in Hybrid Lead Halide Perovskite Films Using Truxene Derivatives as Lewis Base Interlayers

Truxene derivatives, due to their molecular structure and properties, are good candidates for the passivation of defects when deposited onto hybrid lead halide perovskite thin films. Moreover, their semiconductor characteristics can be tailored through the modification of their chemical structure, which allows-upon light irradiation- the interfacial charge transfer between the perovskite film and the truxene molecules. In this work, we analysed the use of the molecules as surface passivation agents and their use in complete functional solar cells. We observed that these molecules reduce the non-radiative carrier recombination dynamics in the perovskite thin film through the supramolecular complex formation between the Truxene molecule and the Pb²⁺ defects at the perovskite surface. Interestingly, this supramolecular complexation neither affect the carrier recombination kinetics nor the carriers collection but induced noticeable hysteresis on the photocurrent vs voltage curves of the solar cells under 1 sun illumination.     

Bending-stability Interfacial Layer as Dual Electron Transport Layer for Flexible Organic Photovoltaics

The flexibility of organic photovoltaics (OPVs) has attracted worldwide attention in recent years.To realize the bending-stability of OPVs,it is necessary to put forward the bending-stability of interracial layer.A novel bendable composite is explored and successfully applied as an electron transport layer (ETL) for fully-flexible OPVs.We incorporated poly(vinylpyrrolidone)(PVP) into conjugated electrolytes (CPE) to composite a bendable ETL for high-performance OPVs devices.Fortunately,the devices based on PVP-modified CPE exhibited better device performances and more excellent mechanical properties of bendability.The fullerene-free OPVs based on PM6:IT-4F with CPE@PVP as ETLs yield the best power conversion efficiency (PCE) of 13.42％.Moreover,a satisfying efficiency of 12.59％ has been obtained for the fully-flexible OPVs.As far as we know,this is one of the highest PCE for fully-flexible OPV based PM6:IT-4F system.More importantly,the flexible OPVs devices can retain more than 80％ of its initial efficiency after 5000 bending cycles.Furthermore,among various curvature radii,the mechanical properties of the device based on CPE@PVP are superior to those of the device based on bare CPE as ETL.These findings indicate that the functional flexibility of CPE as a cathode interfacial layer is an effective strategy to fabricate high-performance flexible devices in the near future.     

Compact layer influence on hysteresis effect in organic–inorganic hybrid perovskite solar cells

Organic–inorganic hybrid perovskite solar cells have attracted great attention due to their high power conversion efficiency and low cost. However, an anomalous hysteresis effect exists in the perovskite solar cells, especially with TiO₂ as the n-type electron extraction layer. In this communication, we prepare two kinds of TiO₂ compact layers using Atomic Layer Deposition (ALD) and Spin-Coating (SC) methods and compare their influences on the hysteresis effect. By efficiency comparison and AC impedance spectroscopy study, we find that the thickness and morphology of compact layer have a significant influence on the hysteresis effect. Compared to the SC approach, the ALD prepared compact layer is ultra-thin with uniform morphology and shows small interfacial capacitance and large recombination resistance, meaning reduced interfacial charge accumulation and accelerated electron transport, which would relieve the hysteresis effect.     

Unconventional Route to Oxygen‐Vacancy‐Enabled Highly Efficient Electron Extraction and Transport in Perovskite Solar Cells

The ability to effectively transfer photoexcited electrons and holes is an important endeavor toward achieving high‐efficiency solar energy conversion. Now, a simple yet robust acid‐treatment strategy is used to judiciously create an amorphous TiO₂ buffer layer intimately situated on the anatase TiO₂ surface as an electron‐transport layer (ETL) for efficient electron transport. The facile acid treatment is capable of weakening the bonding of zigzag octahedral chains in anatase TiO₂, thereby shortening staggered octahedron chains to form an amorphous buffer layer on the anatase TiO₂ surface. Such amorphous TiO₂‐coated ETL possesses an increased electron density owing to the presence of oxygen vacancies, leading to efficient electron transfer from perovskite to TiO₂. Compared to pristine TiO₂‐based devices, the perovskite solar cells (PSCs) with acid‐treated TiO₂ ETL exhibit an enhanced short‐circuit current and power conversion efficiency.     

Dual-Interface Engineering in Perovskite Solar Cells with 2D Carbides

Passivating the interfaces between the perovskite and charge transport layers is crucial for enhancing the power conversion efficiency (PCE) and stability in perovskite solar cells (PSCs). Here we report a dual-interface engineering approach to improving the performance of FA_0.85MA_0.15Pb(I_0.95Br_0.05)₃-based PSCs by incorporating Ti₃C₂Cl_x Nano-MXene and o-TB-GDY nanographdiyne (NanoGDY) into the electron transport layer (ETL)/perovskite and perovskite/ hole transport layer (HTL) interfaces, respectively. The dual-interface passivation simultaneously suppresses non-radiative recombination and promotes carrier extraction by forming the Pb−Cl chemical bond and strong coordination of π-electron conjugation with undercoordinated Pb defects. The resulting perovskite film has an ultralong carrier lifetime exceeding 10 μs and an enlarged crystal size exceeding 2.5 μm. A maximum PCE of 24.86 % is realized, with an open-circuit voltage of 1.20 V. Unencapsulated cells retain 92 % of their initial efficiency after 1464 hours in ambient air and 80 % after 1002 hours of thermal stability test at 85 °C.