Oxide Electron Transport Layer Research Articles

Role of interface engineering in amorphous InGaZnO ETL for non-fullerene organic solar cells

Amorphous metal-oxide semiconductors have been developed as prospective substitutes for organic and silicon compounds, and their applications have been expanded due to their superior mobility in amorphous phase. As a result, this study demonstrated a facile approach to amend the interface quality by oxygen vacancy engineering. Here, we compared the performance of optimized oxygen-deficient (Od) and oxygen-rich (Or) amorphous indium–gallium–zinc oxide (a-IGZO) electron transport layers (ETL) for non-fullerene organic solar cells (OSCs). The ETL was fabricated using rf magnetron sputtering by changing oxygen contents. The power conversion efficiency (PCE) of Or device is 5.1 %, which was boosted to 6.6 % for Od device. The device with Od a-IGZO ETL outperforms the Or device, as the oxygen vacancies in Od device aid as electron donors, which deliver free electron carriers to IGZO film. These free electron carriers promote electron concentration, device mobility and deteriorate the resistivity, which facilitate charge transport and collection towards the electrode. Furthermore, devices improved their stability, as device with Od a-IGZO ETL retained almost 80 % of its initial PCE in ambient conditions. Consequently, interfacial engineering has played a crucial role in bringing OSC towards commercialization as it has substantial influence on charge collection and transport properties in devices.

Highly conducting Al-doped zinc oxide electron transport layer for all-inorganic perovskite solar cells: An experimental and simulation study

The need for an excellent electron transport layer (ETL) material is immensely significant for higher conversion efficiency of perovskite solar cells (PSCs). This work centers on the exceptional performance achieved by the Al-doped ZnO (AZO) ETL with reference to undoped ZnO (ZnO). Both doped and undoped ETL materials were synthesized and characterized to determine their structural, morphological, and optical behavior. In this work, solar cell performance of two similar CsPbIBr2-based PSCs with different ETLs namely FTO/ZnO/CsPbIBr2/NiO (Cell #1) and FTO/AZO/CsPbIBr2/NiO (Cell #2) are investigated through numerical simulation method. The obtained characteristics affirms that Cell #2 offers sufficient better performance parameters (Jsc, Voc, FF, and η) against Cell #1 with respect to variation in CsPbIBr2 PVT, ETL and HTL thickness. The adverse effect on J-V characteristics caused by controllable introduction of defect density (Nt) in PVT layer is negligible for Cell #2 compared to its ZnO counterpart. Further, the contribution of AZO ETL layer on cell's performance is evaluated in terms of variation in respective ETL and HTL thickness. The investigation reveals that Cell #2 offers a desirable η of 19.46% at electron capture cross section of 2 × 10−15 cm2 while Cell #1 delivers only 12.48%.

Rational Design of a Chemical Bath Deposition Based Tin Oxide Electron-Transport Layer for Perovskite Photovoltaics.

Chemical bath deposition (CBD) is widely used to deposit tin oxide (SnOx ) as an electron-transport layer in perovskite solar cells (PSCs). The conventional recipe uses thioglycolic acid (TGA) to facilitate attachments of SnOx particles onto the substrate. However, nonvolatile TGA is reported to harm the operational stability of PSCs. In this work, a volatile oxalic acid (OA) is introduced as an alternative to TGA. OA, a dicarboxylic acid, functions as a chemical linker for the nucleation and attachment of particles to the substrate in the chemical bath. Moreover, OA can be readily removed through thermal annealing followed by a mild H2 O2 treatment, as shown by FTIR measurements. Synergistically, the mild H2 O2 treatment selectively oxidizes the surface of the SnOx layer, minimizing nonradiative interface carrier recombination. EELS (electron-energy-loss spectroscopy) confirms that the SnOx surface is dominated by Sn4+ , while the bulk is a mixture of Sn2+ and Sn4+ . This rational design of a CBD SnOx layer leads to devices with T85 ≈1500h, a significant improvement over the TGA-based device with T80 ≈250h. The champion device reached a power conversion efficiency of 24.6%. This work offers a rationale for optimizing the complex parameter space of CBD SnOx to achieve efficient and stable PSCs.

Photoexcitation-induced passivation of SnO2 thin film for efficient perovskite solar cells.

A high-quality tin oxide electron transport layer (ETL) is a key common factor to achieve high-performance perovskite solar cells (PSCs). However, the conventional annealing technique to prepare high-quality ETLs by continuous heating under near-equilibrium conditions requires high temperatures and a long fabrication time. Alternatively, we present a non-equilibrium, photoexcitation-induced passivation technique that uses multiple ultrashort laser pulses. The ultrafast photoexcitation and following electron-electron and electron-phonon scattering processes induce ultrafast annealing to efficiently passivate surface and bulk defects, and improve the crystallinity of SnO2, resulting in suppressing the carrier recombination and facilitating the charge transport between the ETL and perovskite interface. By rapidly scanning the laser beam, the annealing time is reduced to several minutes, which is much more efficient compared with conventional thermal annealing. To demonstrate the university and scalability of this technique, typical antisolvent and antisolvent-free processed hybrid organic-inorganic metal halide PSCs have been fabricated and achieved the power conversion efficiency (PCE) of 24.14% and 22.75% respectively, and a 12-square-centimeter module antisolvent-free processed perovskite solar module achieves a PCE of 20.26%, with significantly enhanced performance both in PCE and stability. This study establishes a new approach towards the commercialization of efficient low-temperature manufacturing of PSCs.

Development of MnxCo0.015Zn1-xO electron transport layer for perovskite solar cells: Experimental and simulation study

The electron transport layer (ETL) plays an important role in the performance and stability enhancement of perovskite solar cells (PSCs). Zinc oxide is considered a cost-effective and low-temperature processed ETL for PSCs, which offers decent mobility. However, the interface reaction occurs via a reaction of zinc oxide with the organic-inorganic metal hybrid halide perovskite absorber, which results in the degradation of the PSCs. To overcome this issue, metal doping was carried out. However, metal doping causes a reduction in the transmittance of the zinc oxide ETL layer because of high absorption. Thus, the overall performance of the PSCs was reduced due to the lessening of the photogenerated current. In this regard metal co-doping (Mn and Co) was done to improve the electrical as well as electronic properties. Furthermore, the influence of Mn doping on the structural, optical, and photoluminescence properties of MnxCo0.015Zn1-xO (x = 0, 0.002, 0.008, and 0.012) hexagonal nanosheets was investigated. X-ray diffraction results revealed that the inter-planar spacing, cell parameters, unit cell volume, lattice strain, and dislocation density were maximum for the Mn content of 0.2%. The acquired values of the optical band gap (Eg) are 3.39, 3.52, 3.58, and 3.18 eV for x = 0, 0.2%, 0.8%, and 1.2%, respectively. The corresponding absorption coefficient is reduced with increasing Mn content. The obtained refractive indices are 1.437, 1.402, 1.428, and 1.426 for x = 0, 0.2, 0.8, and 1.2%, respectively. The above results confirm that the MnxCo0.015Zn1-xO with x = 0.8% provides the lowest absorption and the highest value of bandgap. Thus, it can be the most suitable ETL for high-performance PSCs. The simulation results reveal that via applying MCZO ETL, the efficiency can be achieved over 23%.

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.

Fabrication of perovskite solar cells with PCE of 21.84% in open air by bottom-up defect passivation and stress releasement

The archetypical n-type semiconductor material, tin oxide (SnO2), is common and well-known electron transport layer (ETL) material in regular perovskite solar cells (PSCs) due to the superior electron mobility and suitable energy level alignment with perovskite. However, plentiful detrimental defects concentrated at perovskite/SnO2 ETLs interface and unfavorable residual stress within perovskite films, compromise and dampen the photovoltaic performance and shelf stability of PSCs. Herein, alkali metal salts (i.e., 2, 2′-biquinoline-4, 4′-dicarboxylic acid alkali metal salts (BCA-2H, BCA-2Na, and BCA-2K)) are employed as pre-buried modifiers in SnO2 films to achieve a holistic amelioration of ETLs, perovskite photoactive layer, and their interface heterojunction. The BCA modification facilitates crystal orientation and crystalline of SnO2 nanocrystals, instrumental in decreasing trap state densities within functional layers, releasing interfacial residual tensile strain, and upgrading perovskite film performance. In comparison to BCA-2H- and BCA-2Na-doped ETLs, the BCA-2K decoration corroborate better defect passivation and stress release effect originating from preferable interface enhancement and desirable surface chemical properties. Consequently, the PSCs based on SnO2-BCA-2K ETL fabricated under open-air conditions deliver the champion efficiency of 21.84% accompanied by negligible hysteresis. Moreover, the unpacked devices maintain 81.2% of its original value after 1200 h storage under an ambient condition of 55 ± 5% (relative humidity, RH) and 25 ± 5 ℃ with open-circuit conditions.

Enhancing the performance of the perovskite solar cells by modifying the SnO2 electron transport layer

The tin oxide (SnO2) electron transport layer (ETL) plays a vital role in the photo-conversion efficiency (PCE) and stability of organic-inorganic perovskite solar cells (PSCs). However, SnO2 ETL-induced defects such as hydroxyl groups, oxygen vacancies, exposed Sn atoms, and dangling bonds hinder device performance. In this study, rubidium chloride (RbCl) has been used to modify the SnO2 ETL. Perovskite film formed on the RbCl-modified SnO2 ETL exhibits improved crystallinity with enlarged grain size and reduced grain boundaries and enhanced optical absorption. The Hall-effect measurements indicate the improved carrier mobility, and the dark J-V curve shows the increment of electrical conductivity for the RbCl-modified SnO2 ETL. X-ray photoelectron spectroscopy (XPS) results demonstrate the surface defects passivation of the perovskite layer by modifying the SnO2 ETL. A champion PCE of 19.35% has been achieved for the RbCl-modified SnO2 ETL-based devices with improved stability, while the control devices with unmodified SnO2ETL show a PCE of 17.18%.

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.

Molecule‐Assisted Chemical Bath Deposition of Tin Oxide Electron Transport Layers in Perovskite Solar Cells

Chemical bath deposition (CBD) is a widely used approach to deposit the tin oxide (SnO2) electron transport layer (ETL) in the perovskite solar cell (PSC). However, the defect states in the CBD‐resulted SnO2 ETLs limit the electron extraction/transport from the perovskite to the ETL and lead to poor PSC performance. Herein, ethylenediaminetetraacetic acid dipotassium (EDTA‐2K) is used as an additive in the CBD precursor of the SnO2 ETL, which results in the chelation of Sn2+ and EDTA during the hydrolysis process. This strategy decreases the concentration of free Sn2+ in the precursor for hydrolysis and slows down the CBD process, thus attributes to the decreased surface defect states as well as the enhanced conductivity of the ETL. As a result, the EDTA‐2K additive makes the CBD SnO2 ETL with efficient electron extraction and transporting capability. The champion device achieves a power conversion efficiency (PCE) of 21.87%, which is significantly higher than that of the pristine CBD SnO2‐based device (20.25%). In addition, the device with an active area of 1.21 cm2 achieves a high PCE of 19.23%. This strategy makes the CBD SnO2 an excellent ETL candidate for the development of low‐cost and large‐scale PSCs.

Potassium hexacyanoferrate(III): A promising additive for perovskite precursors in carbon-based perovskite solar cells

Perovskite solar cells (PSCs) that use carbon electrodes and low-temperature processed electron transport layers (ETLs) show great promise in meeting global energy needs at an affordable price. Our current research is focused on the development of carbon-based perovskite solar cells (PSCs) that incorporate low-temperature titanium oxide electron transport layers (ETLs) to enhance their photovoltaic performance. In our approach, we introduce potassium hexacyanoferrate(III) material into the pre-solution of the MAPbI3 perovskite to fabricate a light-harvesting layer. This promising technique has shown great potential in improving the overall performance of PSCs. This additive reduces the formation of non-radiative recombination centers, resulting in a perovskite layer that is desirable and free from defects. Furthermore, photovoltaic devices based on potassium hexacyanoferrate(III) exhibit reduced transfer resistance, leading to faster charge transfer at the interfaces of TiO2/perovskite and perovskite/carbon electrodes. As a result, the efficiency of PSCs can be improved by up to 14.89%, which is significantly higher than the recorded efficiency of unmodified PSCs at 12.05%. In addition, PSCs based on potassium hexacyanoferrate show greater stability in ambient air compared to their unmodified counterparts.

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.

Spray‐Deposited, Virus‐Templated SnO2 Mesoporous Electron Transport Layer for High‐Efficiency, Sequential‐Deposited Perovskite Solar Cells

In recent years, researchers have developed spray deposition technology to fabricate tin oxide electron transport layer (ETL) with the aim of manufacturing high‐efficiency, large‐area perovskite solar cell (PSC). However, the power conversion efficiency (PCE) of PSC based on sprayed SnO2 ETL remains inferior to that of the spin‐coated SnO2 ETL. Herein, the combined use of spray deposition and genetically engineered M13 bacteriophages for the deposition of M13‐SnO2 biohybrid ETL over large‐area (62.5 cm2) substrates is demonstrated. The spray‐deposited M13‐SnO2 ETLs exhibit mesoporous morphologies with >85% transmittance in UV–vis region. Through the use of M13‐SnO2 ETL, the sequential‐deposited PSCs achieve a maximum PCE of ≈22.1%. The improved performance of the PSC is attributable to the mesoporous morphology of M13‐SnO2 ETL that facilitates the growth of larger perovskite grains. The PSCs based on M13‐SnO2 ETLs also display highly consistent photovoltaic performance which manifests the excellent scalability of the spraying process. Furthermore, M13‐SnO2‐based PSCs exhibit higher ambient stability compared to the SnO2‐based PSCs, showing that the use of M13 bacteriophage is incredibly beneficial to both the efficiency and stability of PSCs.

Interface engineering of Tin(IV) oxide electron transport layer with Yttrium(III) fluoride for efficient carbon-based perovskite solar cells

Carbon-based perovskite solar cells (C–PSCs) are the most promising technology to meet global energy demands at a reasonable price. In the current study, to assemble efficient and stable C–PSCs, the interface between the tin oxide electron transport layer and the mixed-cation perovskite layer was tailored by coating yttrium fluoride (YF3) material over the tin oxide layer. Results show that YF3–SnO2 ETL produces a more suitable substrate for perovskite fabrication, causing the formation of a perovskite layer with enlarged grains and a lower PbI2 residual. In addition, the results indicate that YF3 treatment could boost the conductivity of SnO2, reduce defects in the mixed-cation perovskite layer, and suppress nonradiative recombination in the C–PSCs. YF3-based interface engineering brought champion C–PSCs with an efficiency of 14.01% under simulated illumination of 100 mW cm−2, which was considerably higher than the 11.58% recorded for control C–PSCs. After 168 days of aging, the unsealed, modified C-PSC devices can demonstrate superior ambient air stability behavior conditions and retain 95% of their initial performance.

Active SnO2 Crystal Planes Enable Efficient and Ultra‐Bendable n‐i‐p Perovskite Solar Cells with Record Certificated Power Conversion Efficiency

AbstractThe tin (IV) oxide (SnO2) electron transport layer (ETL) has been widely employed to fabricate high‐performance perovskite solar cells (PSCs). It has been reported that carbon quantum dots (CQDs) can be used to enhance electron mobility of SnO2. However, an in‐depth understanding of the driving force in this process is still lacking. Here, a high‐angle annular dark‐field scanning transmission electron microscope (HAADF‐STEM) is employed, for the first time, to reveal the SnO2 crystal face changes with one new type of CQD doping. Synchrotron‐based grazing incidence wide‐angle X‐ray scattering (GIWAXS) can penetrate the flexible substrate to detect the buried region of the perovskite layer, showing the crystallinity and phase purity of the perovskite are significantly improved with CQD‐modified SnO2. The flexible n‐i‐p PSCs delivers a power conversion efficiency (PCE) up to 23.57% (22.75%, certificated), which is one of the highest values for single‐junction n‐i‐p flexible PSCs. The corresponding n‐i‐p flexible modules achieve a PCE of 17.79% with aperture area ~ 24 cm2. Furthermore, the flexible PSCs show excellent stability, preserving ≈95% of their initial efficiency after 1200 h under 40% relative humidity and 1‐sun light irradiation at 25 °C, and maintained > 90% of initial efficiency after 2500 bending cycles at a bending radius of 6 mm.

Buried-in interface with two-terminal functional groups for perovskite-based photovoltaic solar cells

Interface plays an important role in perovskite solar cells. Herein, a functional molecular with two-terminal donor groups was deposited between the tin oxide (SnO2) electron transport layer and halide perovskite to induce the perovskite crystal growth and passivate defects at the interface. It is found that isonicotinohydrazide (INHA) can anchor Pb2+ cluster in precursor and promote uniform nucleation, which helps to adjust the crystal growth in perovskite films. As well, more analysis shows that interfacial modification can greatly reduce trap defects and therefore facilitate photogenerated carrier-transferring. The efficient electron transfer and reduced interface traps correlate well with the corresponding fill factors and open-circuit voltages (VOC) of working devices. The resulting perovskite solar cell exhibits striking improvements to reach the champion efficiency of 21.12%. The long-term stability is also significantly enhanced by comparing to the pristine devices. This work highlights the effect of INHA/perovskite interfacial interaction and provides a multi-functional passivation strategy for further perfecting perovskite films.

Reduced hysteresis and enhanced air stability of low-temperature processed carbon-based perovskite solar cells by surface modification

Low temperature processed carbon-based perovskite solar cells (C-PSCs) have gained great interest because of low cost and ease of fabrication. By replacing the Au electrode with carbon, stable solar cells suited for mass-production process can be made. However, power conversion efficiencies (PCEs) of C–PSCs still lag behind that of PSCs with Au contact.Here we explore low temperature (≤150 °C) processed C-PSCs with, where a two-step method is used to prepare mixed-ion lead perovskite films, with tin oxide (SnO2) electron transport layer, poly(3-hexylthiophene-2,5-diyl) (P3HT) hole transport layer and carbon electrode, resulting in devices with a PCE of 14.0%. Moreover, hexyl trimethylammonium bromide (HTAB) was introduced to improve the interface between perovskite and P3HT. Perovskite grains were remarkably enlarged into micrometer-size and defects were reduced. As a result, a champion PCE of 16.1% was obtained, mainly due to enhanced fill factor from 0.67 to 0.73. The interface modification by HTAB molecule is an effective way to passivate the perovskite defects and facilitate the carrier transport at the perovskite/HTL interface. Unencapsulated devices showed excellent stability over 1500 h stored under ambient air (relative humidity ∼ 50 ± 10%).

Cage Polyamine Molecule Hexamethylenetetramine as Additives for Improving Performance of Perovskite Solar Cells

As a rising star in the photovoltaic field, perovskite solar cells (PSCs) have attracted extensive attention. However, the industrialization development of PSCs still faces the dual challenges of improving efficiency and stability. Herein, a cage polyamine molecule hexamethylenetetramine (HMTA) as additive is incorporated into the tin oxide electron transport layer (ETL). The introduction of HMTA improves the electron extraction ability of SnO2 and optimizes the energy‐level alignment of function layers. The modification of HMTA also passivates the uncoordinated Pb2+ in the perovskite and reduces the oxygen vacancy defects of SnO2, which alleviate trap‐state density and carrier nonradiative recombination. As a result, the PSC based on the SnO2 ETL modified with HMTA achieves a high power conversion efficiency of 22.37% along with good stability. The research results demonstrate a typical case of using cage polyamines to improve the performance and stability of PSCs.

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.

Modification of SnO2 Electron Transport Layer in Perovskite Solar Cells.

Rapid development of the device performance of organic-inorganic lead halide perovskite solar cells (PSCs) are emerging as a promising photovoltaic technology. Current world-record efficiency of PSCs is based on tin oxide (SnO2) electron transport layers (ETLs), which are capable of being processed at low temperatures and possess high carrier mobilities with appropriate energy- band alignment and high optical transmittance. Modification of SnO2 has been intensely investigated by various approaches to tailor its conductivity, band alignment, defects, morphology, and interface properties. This review article organizes recent developments of modifying SnO2 ETLs to PSC advancement using surface and bulk modifications, while concentrating on photovoltaic (PV) device performance and long-term stability. Future outlooks for SnO2 ETLs in PSC research and obstacles remaining for commercialization are also discussed.