Efficient Indium‐Doped TiOx Electron Transport Layers for High‐Performance Perovskite Solar Cells and Perovskite‐Silicon Tandems

In addition to a good perovskite light absorbing layer, the hole and electron transport layers play a crucial role in achieving high‐efficiency perovskite solar cells. Here, a simple, one‐step, solution‐based method is introduced for fabricating high quality indium‐doped titanium oxide electron transport layers. It is shown that indium‐doping improves both the conductivity of the transport layer and the band alignment at the ETL/perovskite interface compared to pure TiO₂, boosting the fill‐factor and voltage of perovskite cells. Using the optimized transport layers, a high steady‐state efficiency of 17.9% for CH₃NH₃PbI₃‐based cells and 19.3% for Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃‐based cells is demonstrated, corresponding to absolute efficiency gains of 4.4% and 1.2% respectively compared to TiO₂‐based control cells. In addition, a steady‐state efficiency of 16.6% for a semi‐transparent cell is reported and it is used to achieve a four‐terminal perovskite‐silicon tandem cell with a steady‐state efficiency of 24.5%.     

High‐Performance Flexible Perovskite Solar Cells via Precise Control of Electron Transport Layer

Flexible perovskite solar cells (f‐PSCs) have attracted great attention due to their promising commercial prospects. However, the performance of f‐PSCs is generally worse than that of their rigid counterparts. Herein, it is found that the unsatisfactory performance of planar heterojunction (PHJ) f‐PSCs can be attributed to the undesirable morphology of electron transport layer (ETL), which results from the rough surface of the flexible substrate. Precise control over the thickness and morphology of ETL tin dioxide (SnO₂) not only reduces the reflectance of the indium tin oxide (ITO) on polyethylene 2,6‐naphthalate (PEN) substrate and enhances photon collection, but also decreases the trap‐state densities of perovskite films and the charge transfer resistance, leading to a great enhancement of device performance. Consequently, the f‐PSCs, with a structure of PEN/ITO/SnO₂/perovskite/Spiro‐OMeTAD/Ag, exhibit a power conversion efficiency (PCE) up to 19.51% and a steady output of 19.01%. Furthermore, the f‐PSCs show a robust bending resistance and maintain about 95% of initial PCE after 6000 bending cycles at a bending radius of 8 mm, and they present an outstanding long‐term stability and retain about 90% of the initial performance after >1000 h storage in air (10% relative humidity) without encapsulation.     

Efficient and Hysteresis‐Free Perovskite Solar Cells Based on a Solution Processable Polar Fullerene Electron Transport Layer

Fullerene derivatives, which possess extraordinary geometric shapes and high electron affinity, have attracted significant attention for thin film technologies. This study demonstrates an important photovoltaic application using carboxyl‐functionalized carbon buckyballs, C60 pyrrolidine tris‐acid (CPTA), to fabricate electron transport layers (ETLs) that replace traditional metal oxide‐based ETLs in efficient and stable n‐i‐p‐structured planar perovskite solar cells (PSCs). The uniform CPTA film is covalently anchored onto the surface of indium tin oxide (ITO), significantly suppressing hysteresis and enhancing the flexural strength in the CPTA‐modified PSCs. Moreover, solution‐processable CPTA‐based ETLs also enable the fabrication of lightweight flexible PSCs. The maximum‐performing device structures composed of ITO/CPTA/CH₃NH₃PbI₃/2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐OMeTAD)/Au yield power conversion efficiencies of more than 18% on glass substrates and up to 17% on flexible substrates. These results indicate that the CPTA layers provide new opportunities for solution‐processed organic ETLs by substantially simplifying the procedure for fabricating PSCs for portable applications.     

SrTiO3/Al2O3‐Graphene Electron Transport Layer for Highly Stable and Efficient Composites‐Based Perovskite Solar Cells with 20.6% Efficiency

For practical use of perovskite solar cells (PSCs) the instability issues of devices, attributed to degradation of perovskite molecules by moisture, ions migration, and thermal‐ and light‐instability, have to be solved. Herein, highly efficient and stable PSCs based on perovskite/Ag‐reduced graphene oxide (Ag‐rGO) and mesoporous Al₂O₃/graphene (mp‐AG) composites are reported. The mp‐AG composite is conductive with one‐order of magnitude higher mobility than mp‐TiO₂ and used for electron transport layer (ETL). Compared to the mp‐TiO₂ ETL based cells, the champion device based on perovskite/Ag‐rGO and SrTiO₃/mp‐AG composites shows overall a best performance (i.e., V_OC = 1.057 V, J_SC = 25.75 mA cm^?2, fill factor (FF) = 75.63%, and power conversion efficiency (PCE) = 20.58%). More importantly, the champion device without encapsulation exhibits not only remarkable thermal‐ and photostability but also long‐term stability, retaining 97–99% of the initial values of photovoltaic parameters and sustaining ≈93% of initial PCE over 300 d under ambient conditions.     

Simultaneous Contact and Grain‐Boundary Passivation in Planar Perovskite Solar Cells Using SnO2‐KCl Composite Electron Transport Layer

The performance of perovskite solar cells is sensitive to detrimental defects, which are prone to accumulate at the interfaces and grain boundaries of bulk perovskite films. Defect passivation at each region will lead to reduced trap density and thus less nonradiative recombination loss. However, it is challenging to passivate defects at both the grain boundaries and the bottom charge transport layer/perovskite interface, mainly due to the solvent incompatibility and complexity in perovskite formation. Here SnO₂‐KCl composite electron transport layer (ETL) is utilized in planar perovskite solar cells to simultaneously passivate the defects at the ETL/perovskite interface and the grain boundaries of perovskite film. The K and Cl ions at the ETL/perovskite interface passivate the ETL/perovskite contact. Meanwhile, K ions from the ETL can diffuse through the perovskite film and passivate the grain boundaries. An enhancement of open‐circuit voltage from 1.077 to 1.137 V and a corresponding power conversion efficiency increasing from 20.2% to 22.2% are achieved for the devices using SnO₂‐KCl composite ETL. The composite ETL strategy reported herein provides an avenue for defect passivation to further increase the efficiency of perovskite solar cells.     

Simple,Robust, and Going More Efficient: Recent Advance on Electron Transport Layer‐Free Perovskite Solar Cells

Perovskite solar cells (PSCs) have shown great potential for photovoltaic applications with their unprecedented power conversion efficiency advancement. Such devices generally have a complex structure design with high temperature processed TiO₂ as the electron transport layer (ETL). Further careful design of device configuration to fully tap the potentials of perovskite materials is expected. Particularly, for the practical application of PSCs, it is crucial to simplify their device structures thus the associated manufacturing process and cost while maintaining their efficiency to be comparable with the conventional devices. But how simple is simple? ETL‐free PSCs promise the simplest structured, thus simple manufacturing processes and low cost large area PSCs in practical applications. They can also help the further exploration of the great potential of perovskite materials and understanding the working principle of PSCs. Within this review, the evolution of the PSC is outlined by discussing the recent advances in the simplification of device configuration and processes for cost effective, highly efficient, and robust PSCs, with a focus on ETL‐free PSCs. Their advancement, key issues, working mechanism, existing problems, and future performance enhancements. This review aims to promote the future development of low cost and robust ETL‐free PSCs toward more efficient power output.     

Amino‐Functionalized Conjugated Polymer as an Efficient Electron Transport Layer for High‐Performance Planar‐Heterojunction Perovskite Solar Cells

An amino‐functionalized copolymer with a conjugated backbone composed of fluorene, naphthalene diimide, and thiophene spacers (PFN‐2TNDI) is introduced as an alternative electron transport layer (ETL) to replace the commonly used [6,6]‐Phenyl‐C61‐butyric acid methyl ester (PCBM) in the p–i–n planar‐heterojunction organometal trihalide perovskite solar cells. A combination of characterizations including photoluminescence (PL), time‐resolved PL decay, Kelvin probe measurement, and impedance spectroscopy is used to study the interfacial effects induced by the new ETL. It is found that the amines on the polymer side chains not only can passivate the surface traps of perovskite to improve the electron extraction properties, they also can reduce the work function of the metal cathode by forming desired interfacial dipoles. With these dual functionalities, the resulted solar cells outperform those based on PCBM with power conversion efficiency (PCE) increased from 12.9% to 16.7% based on PFN‐2TNDI. In addition to the performance enhancement, it is also found that a wide range of thicknesses of the new ETL can be applied to produce high PCE devices owing to the good electron transport property of the polymer, which offers a better processing window for potential fabrication of perovskite solar cells using large‐area coating method.     

Impermeable Charge Transport Layers Enable Aqueous Processing on Top of Perovskite Solar Cells

Several applications of perovskite solar cells (PSCs) demand a semitransparent top electrode to afford top‐illumination or see‐through devices. Transparent conductive oxides, such as indium tin oxide (ITO), typically require postdeposition annealing at elevated temperatures, which would thermally decompose the perovskite. In contrast, silver nanowires (AgNWs) in dispersions of water would be a very attractive alternative that can be deposited at ambient conditions. Water is environmentally friendly without safety concerns associated with alcohols, such as flammability. Due to the notorious moisture sensitivity of lead‐halide perovskites, aqueous processing of functional layers, such as electrodes, on top of a perovskite device stack is elusive. Here, impermeable electron transport layers (ETLs) are shown to enable the deposition of semitransparent AgNW electrodes from green aqueous dispersions on top of the perovskite cell without damage. The polyvinylpyrrolidone (PVP) capping agent of the AgNWs is found to cause a work–function shift and an energy barrier between the AgNWs and the adjacent ETL. Thus, a high carrier density (≈10¹⁸ cm^?3) in the ETL is required to achieve well‐behaved J/V characteristics free of s‐shapes. Ultimately, semitransparent PSCs are demonstrated that provide an efficiency of 17.4%, which is the highest efficiency of semitransparent p‐i‐n perovskite solar cells with an AgNW top electrode.     

Efficient and Highly Air Stable Planar Inverted Perovskite Solar Cells with Reduced Graphene Oxide Doped PCBM Electron Transporting Layer

Reduced graphene oxide (rGO) is added in the [6,6]‐Phenyl‐C61‐butyric acid methyl ester (PCBM) electron transport layer (ETL) of planar inverted perovskite solar cells (PSCs), resulting in a power conversion efficiency (PCE) improvement of ≈12%, with a hysteresis‐free PCE of 14.5%, compared to 12.9% for the pristine PCBM based device. The universality of the method is demonstrated in PSCs based on CH₃NH₃PbI_3?x Cl_x and CH₃NH₃PbI₃ perovskites, deposited through one step and two step spin coating process, respectively. After a comprehensive spectroscopic characterization of both devices, it is clear that the introduction of rGO in PCBM ETL results in an important increase of the ETL conductivity, together with reduced series resistance and surface roughness. As a result, a significant photoluminescence quenching of such perovskite/ETL is observed, confirming the increased measured short circuit current density. Transient absorption measurements reveal that in the rGO‐based device, the relaxation process of the excited electrons is significantly faster compared to the reference, which implies that the charge injection rate is significantly faster for the first. Furthermore, the light soaking effect is significantly reduced. Finally, aging measurements reveal that the rGO stabilizes the ELT/perovskite interface, which results in the stabilization of perovskite crystal structure after prolonged illumination.     

Self‐Assembled 2D Perovskite Layers for Efficient Printable Solar Cells

2D organic–inorganic hybrid Ruddlesden–Popper perovskites have emerged recently as candidates for the light‐absorbing layer in solar cell technology due largely to their impressive operational stability compared with their 3D‐perovskite counterparts. The methods reported to date for the preparation of efficient 2D perovksite layers for solar cells involve a nonscalable spin‐coating step. In this work, a facile, spin‐coating‐free, directly scalable drop‐cast method is reported for depositing precursor solutions that self‐assemble into highly oriented, uniform 2D‐perovskite films in air, yielding perovskite solar cells with power conversion efficiencies (PCE) of up to 14.9% (certified PCE of 14.33% ± 0.34 at 0.078 cm²). This is the highest PCE to date for a solar cell with 2D‐perovskite layers fabricated by nonspin‐coating method. The PCEs of the cells display no evidence of degradation after storage in a nitrogen glovebox for more than 5 months. 2D‐perovskite layer deposition using a slot‐die process is also investigated for the first time. Perovskite solar cells fabricated using batch slot‐die coating on a glass substrate or R2R slot‐die coating on a flexible substrate produced PCEs of 12.5% and 8.0%, respectively.     

Efficient Perovskite Solar Cells over a Broad Temperature Window: The Role of the Charge Carrier Extraction

The mechanism behind the temperature dependence of the device performance in hybrid perovskite solar cells (HPSCs) is investigated systematically. The power conversion efficiency (PCE) of the reference cell using [60]PCBM as electron extraction layer (EEL) drops significantly from 11.9% at 295 K to 7% at 180 K. The deteriorated charge carrier extraction is found as the dominant factor causing this degradation. Temperature dependent spectroscopy and charge transport studies demonstrate that the poor electron transport in the [60]PCBM EEL at low temperature leads to inefficient charge carrier extraction. It is further demonstrated that the n‐type doping of [60]PCBM EEL or the use of an EEL (fulleropyrrolidine with a triethylene glycol monoethyl ether side chain) with higher electron transport capability is an effective strategy to achieve HPSCs working efficiently over a broad temperature range. The devices fabricated with these highly performing EELs have PCEs at 180 K of 16.7% and 18.2%, respectively. These results support the idea that the temperature dependence of the electron transport in the EELs limits the device performance in HPSCs, especially at lower temperatures and they also give directions toward further improvement of the PCE of HPSCs at realistic operating temperatures.     

A Printable Organic Electron Transport Layer for Low‐Temperature‐Processed,Hysteresis‐Free,and Stable Planar Perovskite Solar Cells

Despite recent breakthroughs in power conversion efficiencies (PCEs), which have resulted in PCEs exceeding 22%, perovskite solar cells (PSCs) still face serious drawbacks in terms of their printability, reliability, and stability. The most efficient PSC architecture, which is based on titanium dioxide as an electron transport layer, requires an extremely high‐temperature sintering process (≈500 °C), reveals hysterical discrepancies in the device measurement, and suffers from performance degradation under light illumination. These drawbacks hamper the practical development of PSCs fabricated via a printing process on flexible plastic substrates. Herein, an innovative method to fabricate low‐temperature‐processed, hysteresis‐free, and stable PSCs with a large area up to 1 cm² is demonstrated using a versatile organic nanocomposite that combines an electron acceptor and a surface modifier. This nanocomposite forms an ideal, self‐organized electron transport layer (ETL) via a spontaneous vertical phase separation, which leads to hysteresis‐free, planar heterojunction PSCs with stabilized PCEs of over 18%. In addition, the organic nanocomposite concept is successfully applied to the printing process, resulting in a PCE of over 17% in PSCs with printed ETLs.     

Improving the Performance and Stability of Inverted Planar Flexible Perovskite Solar Cells Employing a Novel NDI‐Based Polymer as the Electron Transport Layer

A new naphthalene diimide (NDI)‐based polymer with strong electron withdrawing dicyanothiophene (P(NDI2DT‐TTCN)) is developed as the electron transport layer (ETL) in place of the fullerene‐based ETL in inverted perovskite solar cells (Pero‐SCs). A combination of characterization techniques, including atomic force microscopy, scanning electron microscopy, grazing‐incidence wide‐angle X‐ray scattering, near‐edge X‐ray absorption fine‐structure spectroscopy, space‐charge‐limited current, electrochemical impedance spectroscopy, photoluminescence (PL), and time‐resolved PL decay, is used to demonstrate the interface phenomena between perovskite and P(NDI2DT‐TTCN) or [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM). It is found that P(NDI2DT‐TTCN) not only improves the electron extraction ability but also prevents ambient condition interference by forming a hydrophobic ETL surface. In addition, P(NDI2DT‐TTCN) has excellent mechanical stability compared to PCBM in flexible Pero‐SCs. With these improved functionalities, the performance of devices based on P(NDI2DT‐TTCN) significantly outperform those based on PCBM from 14.3 to 17.0%, which is the highest photovoltaic performance with negligible hysteresis in the field of polymeric ETLs.