Metal Oxide Electron Transport Layer Research Articles

Compact Layer‐Free Perovskite Solar Cells with Low‐Temperature UVO Photochemical Annealed Mesoporous TiO2 Layers

In recent years, the field of perovskite solar cells (PSCs) has seen rapid development, with most high‐efficiency devices incorporating dense titanium dioxide (TiO2) barrier layers and mesoporous TiO2 layers to enhance selective electron transport. However, the manufacturing process requires high‐temperature sintering steps above 450 °C, leading to significant energy consumption. In addition, this requirement greatly limits the potential applications of PSCs in the field of flexible electronics. This study introduces a new method for preparing dense‐layer‐free mesoporous PSCs using low‐temperature UVO annealing of m‐TiO2. UVO annealing effectively removes residual organic components from m‐TiO2 precursor films, enhances the conductivity and wettability of the films, thereby reducing carrier recombination and improving the performance of PSCs. Research has shown that PSC with a 40 min UVO annealed m‐TiO2 layer exhibits a final photoelectric conversion efficiency of 17.79%, comparable to devices with traditional high‐temperature annealed m‐TiO2 PSCs. In addition, after 7 days at room temperature and ambient humidity, the unpackaged device maintains a maximum conversion efficiency of 84%. These findings indicate that UVO light annealing is a feasible alternative to high‐temperature annealing, providing a simpler, more cost‐effective, and energy‐saving method for the preparation of metal oxide electron transport layer in PSCs.

Metal chalcogenide electron extraction layers for nip-type tin-based perovskite solar cells

Tin-based perovskite solar cells have garnered attention for their biocompatibility, narrow bandgap, and long thermal carrier lifetime. However, nip-type tin-based perovskite solar cells have underperformed largely due to the indiscriminate use of metal oxide electron transport layers originally designed for nip-type lead-based perovskite solar cells. Here, we reveal that this underperformance is caused by oxygen vacancies and deeper energy levels in metal oxide. To address these issues, we propose a metal chalcogenide electron transport layer, specifically Sn(S0.92Se0.08)2, which circumvents the oxygen molecules desorption and impedes the Sn2+ oxidation. As a result, tin-based perovskite solar cells with Sn(S0.92Se0.08)2 demonstrate a VOC increase from 0.48 – 0.73 V and a power conversion efficiency boost from 6.98 – 11.78%. Additionally, these cells exhibit improved stability, retaining over 95% of their initial efficiency after 1632 h. Our findings showcase metal chalcogenides as promising candidates for future nip-type tin-based perovskite solar cell applications.

Chemical Bath Deposited Antimony Oxide Thin Films for Efficient Perovskite Solar Cells.

The metal oxide electron transport layers (ETLs) of n-i-p perovskite solar cells (PSCs) are dominated by TiO2 and SnO2, while the efficacy of the other metal oxide ETLs still lags far behind. Herein, an emerging, economical, and environmentally friendly metal oxide, antimony oxide (Sb2Ox, x = 2.17), prepared by chemical bath deposition is reported as an alternative ETL for PSCs. The deposited Sb2Ox film is amorphous and very thin (∼10 nm) but conformal on rough fluorine-doped tin oxide substrates, showing matched energy levels, efficient electron extraction, and then reduced nonradiative recombination in PSCs. The champion PSC based on the Sb2Ox ETL delivers an impressive power conversion efficiency of 24.7% under one sun illumination, which represents the state-of-the-art performance of all metal oxide ETL-based PSCs. Additionally, the Sb2Ox-based devices show improved operational and thermal stability compared to their SnO2-based counterparts. Armed with these findings, we believe this work offers an optional ETL for perovskites-based optoelectronic devices.

Recent Advances in Metal Oxide Electron Transport Layers for Enhancing the Performance of Perovskite Solar Cells.

Perovskite solar cells (PSCs) have attracted considerable interest owing to their low processing costs and high efficiency. A crucial component of these devices is the electron transport layer (ETL), which plays a key role in extracting and transmitting light-induced electrons, modifying interfaces, and adjusting surface energy levels. This minimizes charge recombination in PSCs, a critical factor in their performance. Among the various ETL materials, titanium dioxide (TiO2) and tin dioxide (SnO2) stand out due to their excellent electron mobility, suitable band alignment, high transparency, and stability. TiO2 is widely used because of its appropriate conduction band position, easy fabrication, and favorable charge extraction properties. SnO2, on the other hand, offers higher electron mobility, better stability under UV illumination, and lower processing temperatures, making it a promising alternative. This paper summarizes the latest advancements in the research of electron transport materials, including material selection and a discussion of electron collection. Additionally, it examines doping techniques that enhance electron mobility and surface modification technologies that improve interface quality and reduce recombination. The impact of these parameters on the performance and passivation behavior of PSCs is also examined. Technological advancements in the ETL, especially those involving TiO2 and SnO2, are currently a prominent research direction for achieving high-efficiency PSCs. This review covers the current state and future directions in ETL research for PSCs, highlighting the crucial role of TiO2 and SnO2 in enhancing device performance.

Dopamine Sulfonate Ligand-Assisted TiO2 Deposition Enabling Highly Efficient and Stable Perovskite Solar Cells.

The metal oxide electron transport layers (ETLs) with flat morphology and high electrical quality are essential to manufacture highly efficient perovskite solar cells (PSCs), in which the regulation of the metal oxide deposition process plays a crucial role. Herein, a judiciously designed dopamine sulfonate (DS) ligand-assisted deposition of titanium dioxide (TiO2) films approach is implemented based on electrostatic repulsion and steric hindrance of assembled ligands to improve colloidal nanoparticles dispersity in precursor and effectively inhibit their aggregation, which could enable obtaining smooth topography of TiO2 films and initiating growth of top high-quality perovskite films. Furthermore, sulfonate bridges bonded on the perovskite buried layer that is beneficial to form better buried interface contact and accelerate electron extraction. As a result, the PSCs employing DS/TiO2 ETLs exhibit the best power conversion efficiency of 24.53% with impressive storage stability and operation stability, i.e., remaining more than 88% of their initial efficiency upon storage N2 glovebox without encapsulation over 4000 h, and the efficiency does not attenuate significantly under maximum power point for 60 h.

Tuning Surface Defect States in Sputtered Titanium Oxide Electron Transport Layers for Enhanced Stability of Organic Photovoltaics.

Nonfullerene acceptors (NFAs) have dramatically improved the power conversion efficiency (PCE) of organic photovoltaics (OPV) in recent years; however, their device stability currently remains a bottleneck for further technological progress. Photocatalytic decomposition of nonfullerene acceptor molecules at metal oxide electron transport layer (ETL) interfaces has in several recent reports been demonstrated as one of the main degradation mechanisms for these high-performing OPV devices. While some routes for mitigating such degradation effects have been proposed, e.g., through a second layer integrated on the ETL surface, no clear strategy that complies with device scale-up and application requirements has been presented to date. In this work, it is demonstrated that the development of sputtered titanium oxide layers as ETLs in nonfullerene acceptor based OPV can lead to significantly enhanced device lifetimes. This is achieved by tuning the concentration of defect states at the oxide surface, via the reactive sputtering process, to mitigate the photocatalytic decomposition of NFA molecules at the metal oxide interlayers. Reduced defect state formation at the oxide surface is confirmed through X-ray photoelectron spectroscopy (XPS) studies, while the reduced photocatalytic decomposition of nonfullerene acceptor molecules is confirmed via optical spectroscopy investigations. The PBDB-T:ITIC organic solar cells show power conversion efficiencies of around 10% and significantly enhanced photostability. This is achieved through a reactive sputtering process that is fully scalable and industry compatible.

Mathematical Modeling and Optimization of Highly Efficient Nontoxic All-Inorganic CsSnGeI3-Based Perovskite Solar Cells with Oxide and Kesterite Charge Transport Layers.

Despite exceptional optoelectronic properties and rapidly increasing efficiency of perovskite solar cells (PSCs), the issues of toxicity and device instability have hampered the commercialization of this renewable energy technology. Lead (Pb) being the main culprit creates major environmental risks and therefore must be replaced with a nontoxic material such as tin (Sn), germanium (Ge), etc. Moreover, replacing organic cations in the perovskite's ABX3 structure with inorganic ones like cesium (Cs) helps aid the stability issues. This study uses six different kesterite-based hole transport layers (HTLs) and three different metal oxide-based electron transport layers (ETLs) to numerically simulate and optimize all-inorganic CsSnGeI3 PSCs. Metal oxide ETLs are used in this study due to their large band gap, while kesterite HTLs are used due to their excellent conductive properties. All of the simulations are performed under standard testing conditions. A total of 18 novel planar (n-i-p) PSCs are modeled by the combination of various charge transport layers (CTLs), and the device optimization was done to enhance the power conversion efficiencies (PCEs) of the PSCs. Furthermore, the effect of CTLs on the energy band alignment, electric field, quantum efficiency, light absorption, and recombination rate is analyzed. Additionally, a detailed analysis of the impact of defect density (Nt), interface defects (ETL/Perv, Perv/HTL), temperature, and work function on the functionality of 18 different CsSnGeI3-based PSCs is conducted. The simulation findings demonstrate that SnO2/CsSnGeI3/CNTS is the most efficient optimized PSC among all of the simulated structures, with a PCE of 27.33%, Jsc of 28.04 mA/cm2, FF of 85%, and Voc of 1.14 V.

Ground-state electron transfer in all-polymer donor:acceptor blends enables aqueous processing of water-insoluble conjugated polymers

Water-based conductive inks are vital for the sustainable manufacturing and widespread adoption of organic electronic devices. Traditional methods to produce waterborne conductive polymers involve modifying their backbone with hydrophilic side chains or using surfactants to form and stabilize aqueous nanoparticle dispersions. However, these chemical approaches are not always feasible and can lead to poor material/device performance. Here, we demonstrate that ground-state electron transfer (GSET) between donor and acceptor polymers allows the processing of water-insoluble polymers from water. This approach enables macromolecular charge-transfer salts with 10,000× higher electrical conductivities than pristine polymers, low work function, and excellent thermal/solvent stability. These waterborne conductive films have technological implications for realizing high-performance organic solar cells, with efficiency and stability superior to conventional metal oxide electron transport layers, and organic electrochemical neurons with biorealistic firing frequency. Our findings demonstrate that GSET offers a promising avenue to develop water-based conductive inks for various applications in organic electronics.

Vitamin C for Photo-Stable Non-fullerene-acceptor-Based Organic Solar Cells.

The recent advent of the new class of organic molecules, the so-called non-fullerene acceptors, has resulted in skyrocketing power conversion efficiencies of organic solar cells. However, rapid degradation occurs under illumination, particularly when photocatalytic metal oxide electron transport layers are used in these devices. We introduced vitamin C (ascorbic acid) into the organic solar cells as a photostabilizer and systematically studied its photostabilizing effect on inverted PBDB-T:IT-4F devices. The presence of vitamin C as an antioxidant layer between the ZnO electron transport layer and the photoactive layer strongly suppressed the photocatalytic effect of ZnO that induces NFA photodegradation. Upon 96 h of exposure to AM 1.5G 1 Sun irradiation, the reference devices lost 64% of their initial efficiency, while those containing vitamin C lost only 38%. The UV-visible absorption, impedance spectroscopy, and light-dependent voltage and current measurements reveal that vitamin C reduces the photobleaching of NFA molecules and suppresses the charge recombination. This simple approach using a low-cost, naturally occurring antioxidant, provides an efficient strategy for improving photostability of organic semiconductor-based devices.

Laser Derived Electron Transport Layers with Embedded p-n Heterointerfaces Enabling Planar Perovskite Solar Cells with Efficiency over 25.

Electron transport layers (ETLs) with pronounced electron conducting capability are essential for high performance planar perovskite photovoltaics, with the great challenge being that the most widely used metal oxide ETLs unfortunately have intrinsically low carrier mobility. Herein is demonstrated that by simply addressing the carrier loss at particle boundaries of TiO2 ETLs, through embedding in ETL p-n heterointerfaces, the electron mobility of the ETLs can be boosted by three orders of magnitude. Such embedding is encouragingly favorable for both inhibiting the formation of rutile phase TiO2 in ETL, and initiating the growth of high-quality perovskite films with less defect states. By virtue of these merits, creation of formamidinium lead iodide perovskite solar cells (PSCs) with a champion efficiency of 25.05% is achieved, setting a new benchmark for planar PSCs employing TiO2 ETLs. Unencapsulated PSCs deliver much-improved environmental stability, i.e., more than 80% of their initial efficiency after 9000 h of air storage under RH of 40%, and over 90% of their initial efficiency at maximum power point under continuous illumination for 500 h. Further work exploring other p-type nanocrystals for embedding warrants the proposed strategy as a universal alternative for addressing the low-carrier mobility of metal oxide based ETLs.

Highly Efficient and Stable Perovskite Solar Cells Incorporating Chlorinated-Tin Oxide Electron Transport Layers Prepared via Coordination of Diacyl-Metal Cations

The surface passivation of metal oxide electron-transport layers (ETLs) is a powerful strategy for realizing high-performance perovskite solar cells. The surface properties of ETLs strongly influence carrier injection and transfer dynamics; therefore, control of the carrier trap density is crucial. Semiconducting small molecules are considered suitable materials for surface passivation. However, they are expensive and vulnerable to humid atmospheres. Ultrathin polymer layers can have poor surface coverage owing to their spinodal dewetting. In this study, we employ adipoyl chloride as an organic ligand for the chemically robust and efficient surface passivation of SnO2 ETLs. Through the strong coordination of diacyl-metal cations, acyl groups are adsorbed onto the SnO2 surfaces, and the density of oxygen vacancies is significantly reduced. Furthermore, the changing surface properties of the SnO2 ETLs also contribute to the improvement of perovskite morphologies. The deeper energy levels and reduced defect density of the ETLs promote electron injection and transfer at the perovskite and ETL interface. The enlarged perovskite grains are accompanied by improved electron mobility and reduced grain-boundary density. The resulting power conversion efficiency (PCE) increases from 19.36 to 21.41%. The normalized PCE is retained at 90.4% of the initial value for 720 h under 1 sun illumination without the encapsulation of the devices.

Amorphous F‐doped TiOx Caulked SnO2 Electron Transport Layer for Flexible Perovskite Solar Cells with Efficiency Exceeding 22.5%

AbstractFlexible perovskite solar cells (f‐PSCs) show great promise in portable‐power applications (e.g., chargers, drones) and low‐cost, scalable productions (e.g., roll‐to‐roll). However, in conventional n–i–p architecture f‐PSCs, the low‐temperature processed metal oxide electron transport layers (ETLs) usually suffer from high resistance and severe defects that limit the power conversion efficiency (PCE) improvement of f‐PSCs. Besides the enhancement in the mobility of metal oxide and passivation for perovskite/ETL interfacial defects reported in previous literature, herein, the electron transport loss between the metal oxide nanocrystallines within the ETL is studied by introducing an amorphous F‐doped TiOx (F‐TiOx) caulked crystalline SnO2 composite ETL. The F‐TiOx in this novel composite ETL acts as an interstitial medium between adjacent SnO2 nanocrystallines, which can provide more electron transport channels, effectively passivate oxygen vacancies, and optimize the energy level arrangement, thus significantly enhancing the electron mobility of ETL and reducing the charge transport losses. The composite ETL‐based f‐PSCs achieve a high PCE of 22.70% and good operational stability. Furthermore, a moderate roughness of the composite ETL endows f‐PSCs with superior mechanical reliability by virtue of a strong coupling at the ETL/perovskite interface, by which the f‐PSCs can maintain 82.11% of their initial PCE after 4000 bending cycles.

Understanding the Space-Charge Layer in SnO2 for Enhanced Electron Extraction in Hybrid Perovskite Solar Cells.

Tin oxide (SnO2) has been widely used as an n-type metal oxide electron transport layer in perovskite solar cells (PSCs) owing to its superior electrical and optical properties and low-temperature synthesis process. In particular, the interfacial effect between indium tin oxide (ITO) and SnO2 is an important parameter that controls the charge transport properties and device performance of the PSCs. Therefore, understanding the interfacial effect of ITO/SnO2 and its role in PSCs is crucial, but it is not studied intensively. Herein, we investigated the space-charge effect at the interface of ITO/SnO2 using transfer length measurement and conductive atomic force microscopy as a function of SnO2 thickness. Moreover, optical, morphologic, and device measurements were performed to determine the optimal SnO2 thickness for PSCs. The space-charge effect was identified in ITO/SnO2 when the SnO2 layer was very thin due to electron depletion near the interface. Interestingly, a critical kink point was observed at approximately 10 nm SnO2 thickness, indicating the electron depletion and weak charge transfer behavior of the device. Thus, a thickness around 20 nm was favorable for the best PSC performance because charge transport behavior in the thin SnO2 layer was depressed by electron depletion. However, when the thickness of SnO2 exceeded 50 nm, the device performance deteriorated due to increased series resistance. This study provides a strategy to tune the electron transport layer and boost the charge transport behavior in PSCs, making important contributions to optimizing SnO2-based PSCs.

Solution-Processed Cu:SnO2 as an Efficient Electron Transport Layer for Fabrication of Low-Temperature Planar Perovskite Solar Cell Under Ambient Conditions

Rapid growth in photovoltaic performance of lead-halide-based perovskite solar cells (PSCs) has made them a potential candidate for emerging solar technology. The perovskite material itself can be manufactured at low temperatures, which makes it attractive for low-cost manufacturing. However, PSCs utilizing a low-temperature-processed metal-oxide electron transport layer have exhibited lower device performance due to less conductivity. The inclusion of metal dopant in this layer is a cheap and desirable approach that improves electrical conductivity and enables facilities for better energy band alignment that plays a crucial role in the obtained photovoltaic performance. In this work, a low-temperature solution-processed copper (Cu)-doped tin oxide (SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) is demonstrated as an effective electron transport layer, in which, Cu was simply introduced into the SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> solution. An optimal Cu concentration of 3.0 mol% exhibited 11.29% power conversion efficiency (PCE) with 73.38% fill factor (FF), whereas the PSCs utilizing undoped SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> resulted in 8.32% PCE and 59.9% FF when processed in ambient conditions with an average relative humidity of 62%. Our photovoltaic results indicated low-temperature-processed Cu-doped SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> as a potential candidate for device performance, which is also compatible with high throughput and cost-effective manufacturing of PSCs through automation.

Highly-efficient perovskite solar cells based on controllable oxygen defects in tin oxide electron transport layers

Defects regulation of metal oxide electron transport layers (ETLs) such as SnO2 is an effective strategy for improving the power conversion efficiency (PCE). However, it is difficult to control the surface defects of SnO2 using low boiling point solvents, thereby limiting the performance of the perovskite solar cells (PSCs). For this, we prepared high-quality SnO2 films by a low-temperature solution method, and found that solvent had an obvious influence on the energy level and oxygen vacancy within SnO2. The SnO2 electron transport layer was prepared by isopropanol (I–SnO2) dimethoxyethanol (D-SnO2), respectively. Comparing with I–SnO2, D-SnO2 shows excellent photoelectric properties. The perovskite cells based on D-SnO2 exhibited the optimal performance, and the efficiency increased to 20.35% from 18.63% of the device based on I–SnO2.

Formation of Metal Cation/Oxidized Pyridine Complexes‐Based Bifunctional Interfacial Layer for Fabrication of Highly Efficient and Reproducible Perovskite Solar Cells

Interfacial engineering of perovskite solar cells (PSCs) has been a core process for enhancing their power conversion efficiencies (PCEs) and environmental stability. Particularly, polymeric passivation of a metal oxide–electron transport layer (ETL) not only leads to a reduction in its surface defects but also improves the morphology of perovskite. However, the dissolution of the passivation layers by dimethylformamide (DMF) in a perovskite solution can cause a significant drop in the PCE and reproducibility of devices. Herein, oxidized poly(4‐vinylpyridine) (O‐P4VP) is used as a bifunctional passivation layer. The O‐P4VP layers are completely insoluble in DMF, which accompanies the highly reproducible deposition of perovskites without damaging the layers. Furthermore, based on metal–pyridine complexation, oxygen vacancies on the surface of the SnO2 ETLs are passivated with the O‐P4VP layers, and the perovskite grains become enlarged by the controlled nucleation and growth rate. The synergetic effects of interfacial passivation present deeper energy levels of the ETL and a prolonged photoluminescence lifetime of the perovskites. The resulting PCE jumps from 19.05% to 21.11% with respect to the pristine device, and the value is retained for 720 h under 1 sun illumination.

Performance enhancement through optimization of metal oxide electron transport layer in hybrid solar cell

This work deals with analysis and optimization of PEDOT:PSS-Si-SnO2 hybrid solar cell performance. The effect of interface defect and temperature on such device are analysed and calibrated with the experimental work in the literature. The performance of such solar cell is boosted using SnO2 as back side electron bypass layer and considerable optimization is done. The outcome of this work indicates suitable range for electron affinity of such bypass layer within 3.9 eV to 4.7 eV. Beyond these, the interface recombination current limits the open circuit voltage and efficiency is degraded. The energetic position of the interface defect state at the Si-SnO2 also affects the efficiency in such device. Moreover, allowed work function of back contact material for optimum performance in such device finds in the range 4 eV to 4.7 eV.

Bias-Dependent Dynamics of Degradation and Recovery in Perovskite Solar Cells

Degradation of perovskite solar cells (PSCs) is often found to be partially or fully reversible when the cells are allowed to recover in the dark. Unlike the dynamics of degradation, knowledge about the dynamics of PSC cell recovery is very limited. Here, we demonstrate that the PSC recovery strongly depends on the electrical bias conditions during the light-induced degradation and that it can be manipulated by applying an external electrical bias during the recovery phase. Investigation of the recovery dynamics allows us to analyze the degradation mechanisms in detail. More specifically, we aged a mixed-cation mixed-halide PSC with a n-i-p structure under illumination in open-circuit (OC) or short-circuit (SC) conditions, and periodically measured their characteristics during the recovery. PSCs aged in SC degrade faster and fully recover after the light is switched off, while the performance of the cells aged in OC does not recover but instead further decreases after the light is switched off (“drop-in-dark” effect). With the use of transient photoluminescence, secondary ion mass spectrometry, and drift-diffusion-based simulations, we hypothesize that extrinsic ion migration causes the drop-in-dark effect, by forming an electron extraction barrier at the metal oxide electron transport layer. The applied bias alleviates this effect. Our results are relevant for gaining a deeper understanding of the multiple degradation mechanisms present in perovskite solar cells, and for finding a practical way to assist their recovery.

Perspective on Predominant Metal Oxide Charge Transporting Materials for High-Performance Perovskite Solar Cells

Nowadays, the power conversion efficiency of organometallic mixed halide perovskite solar cells (PSCs) is beyond 25%. To fabricate highly efficient and stable PSCs, the performance of metal oxide charge transport layers (CTLs) is one of the key factors. The CTLs are employed in PSCs to separate the electrons and holes generated in the perovskite active layer, suppressing the charge recombination rate so that the charge collection efficiency can be increased at their respective electrodes. In general, engineering of metal oxide electron transport layers (ETLs) is found to be dominated in the research community to boost the performance of PSCs due to the resilient features of ETLs such as excellent electronic properties, high resistance to thermal temperature and moisture, ensuring good device stability as well as their high versatility in material preparation. The metal oxide hole transport layers in PSCs are recently intensively studied. The performance of PSCs is found to be very promising by using optimized hole transport materials. This review concisely discusses the evolution of some prevalent metal oxide charge transport materials (CTMs) including TiO2, SnO2, and NiOx, which are able to yield high-performance PSCs. The article begins with introducing the development trend of PSCs using different types of CTLs, pointing out the important criteria for metal oxides being effective CTLs, and then a variety of preparation methods for CTLs as employed by the community for high-performance PSCs are discussed. Finally, the challenges and prospects for future research direction toward scalable metal oxide CTM-based PSCs are delineated.

Metal oxide electron transport materials for perovskite solar cells: a review

Solar electricity is an unlimited source of sustainable fuels, yet the efficiency of solar cells is limited. The efficiency of perovskite solar cells improved from 3.9% to reach 25.5% in just a few years. Perovskite solar cells are actually viewed as promising by comparison with dye-sensitized solar cells, organic solar cells, and the traditional solar cells made of silicon, GaAs, copper indium gallium selenide (CIGS), and CdTe. Here, we review bare and doped metal oxide electron transport layers in the perovskite solar cells. Charge transfer layers have been found essential to control the performance of perovskite solar cells by tuning carrier extraction, transportation, and recombination. Both electron and hole transport layers should be used for charge separation and transport. TiO2 and 2,2′,7,7′-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene are considered as the best electron and hole transport layers. Metal oxide materials, either bare or doped with different metals, are stable, cheap, and effective.