50 nm sized spherical TiO2 nanocrystals for highly efficient mesoscopic perovskite solar cells

Emerging Trends in Electron Transport Layer Development for Stable and Efficient Perovskite Solar Cells

Perovskite solar cells (PSCs) stand at the forefront of photovoltaic research, with current efficiencies surpassing 26.1%. This review critically examines the role of electron transport materials (ETMs) in enhancing the performance and longevity of PSCs. It presents an integrated overview of recent advancements in ETMs, like TiO2, ZnO, SnO2, fullerenes, non-fullerene polymers, and small molecules. Critical challenges are regulated grain structure, defect passivation techniques, energy level alignment, and interfacial engineering. Furthermore, the review highlights innovative materials that promise to redefine charge transport in PSCs. A detailed comparison of state-of-the-art ETMs elucidates their effectiveness in different perovskite systems. This review endeavors to inform the strategic enhancement and development of n-type electron transport layers (ETLs), delineating a pathway toward the realization of PSCs with superior efficiency and stability for potential commercial deployment.

Selective Control of Novel TiO2 Nanorods: Excellent Building Blocks for the Electron Transport Layer of Mesoscopic Perovskite Solar Cells

Novel TiO2 nanorods (NRs) with various lengths of 70-200 nm and uniform widths of 46-48 nm are selectively synthesized by a solvothermal reaction under a basic environment. The length of TiO2 NRs is reproducibly tuned by varying the concentration of tetramethylammonium hydroxide (TMAH), while the NRs in the pure anatase phase are grown in the [001] direction, caused by the preferential binding affinity of TMAH to the TiO2 (101) facet. TiO2 NRs of various lengths are then applied to form the electron transporting layer (ETL) of mesoscopic perovskite solar cells (PSCs). We found that PSC devices with NRs exhibit superior photovoltaic (PV) performance to those with conventional 46 nm-sized TiO2 nanoparticles (NP46). Particularly, the PSC with TiO2 NRs of 110 nm length (NR110) exhibits the optimum PV conversion efficiency (PCE): the average PCE is 22.64% with a VOC of 1.137 V, a JSC of 24.60 mA·cm-2, and a FF of 80.96%, while the champion PCE is 23.18%. In addition, the PSC with NR110 (PSC-NR110) reveals significantly improved long-term stability in air with a relative humidity of 40-50%. In 1000 h, its PCE is reduced by only 9% whereas that of PSC with NP46 decreases by 25%. The PSC properties analyzed by impedance spectroscopy and J-V curve measurements under dark conditions and at various light intensities provide evidence that PSC-NR110 has fewer defects and shows significantly reduced charge recombination. We discuss the advantages of NR structures in preparing the ETL of PSC devices and also explain why the charge recombination is suppressed.

Formation of Highly Efficient Perovskite Solar Cells by Applying Li-Doped CuSCN Hole Conductor and Interface Treatment

Li-doped CuSCN films of various compositions were applied as hole-transporting material (HTM) for mesoscopic perovskite solar cells (PSCs). Those films of ~60 nm thickness, spin-coated on the perovskite layer, exhibit significantly higher crystallinity and hole mobility compared with the pristine CuSCN films. Among them, 0.33% Li-doped CuSCN (Li0.33:CuSCN) shows the best performance as the HTM of mesoscopic PSC. Furthermore, by depositing a slight amount of PCPDTBT over the Li0.33:CuSCN layer, the VOC was increased to 1.075 V, resulting in an average PCE of 20.24% and 20.65% for the champion device. These PCE and VOC values are comparable to those of PSC using spiro-OMETAD (PCE: 20.61%, VOC: 1.089 V). Such a remarkable increase can be attributed to the penetration of the PCPDTBT polymer into the grain boundaries of the Li0.33:CuSCN film, and to the interface with the perovskite layer, leading to the removal of defects on the perovskite surface by paving the non-contacting parts, as well as to the tight interconnection of the Li0.33:CuSCN grains. The PSC device with Li0.33:CuSCN showed a high long-term stability similar to that with bare CuSCN, and the introduction of PCPDTBT onto the perovskite/Li0.33:CuSCN further improved device stability, exhibiting 94% of the initial PCE after 100 days.

Metal Oxide Compact Electron Transport Layer Modification for Efficient and Stable Perovskite Solar Cells

Perovskite solar cells (PSCs) have appeared as a promising design for next-generation thin-film photovoltaics because of their cost-efficient fabrication processes and excellent optoelectronic properties. However, PSCs containing a metal oxide compact layer (CL) suffer from poor long-term stability and performance. The quality of the underlying substrate strongly influences the growth of the perovskite layer. In turn, the perovskite film quality directly affects the efficiency and stability of the resultant PSCs. Thus, substrate modification with metal oxide CLs to produce highly efficient and stable PSCs has drawn attention. In this review, metal oxide-based electron transport layers (ETLs) used in PSCs and their systemic modification are reviewed. The roles of ETLs in the design and fabrication of efficient and stable PSCs are also discussed. This review will guide the further development of perovskite films with larger grains, higher crystallinity, and more homogeneous morphology, which correlate to higher stable PSC performance. The challenges and future research directions for PSCs containing compact ETLs are also described with the goal of improving their sustainability to reach new heights of clean energy production.

Added value recyclability of glass fiber waste as photo-oxidation catalyst for toxic cytostatic micropollutants

There is an increased interest in recycling valuable waste materials for usage in procedures with high added values. Silica microparticles are involved in the processes of catalysis, separation, immobilization of complexants, biologically active compounds, and different nanospecies, responding to restrictive requirements for selectivity of various chemical and biochemical processes. This paper presents the surface modification of accessible and dimensionally controlled recycled silica microfiber with titanium dioxide. Strong base species in organic solvents: methoxide, ethoxide, propoxide, and potassium butoxide in corresponding alcohol, activated the glass microfibres with 12-13 µm diameter. In the photo-oxidation process of a toxic micro-pollutant, cyclophosphamide, the new composite material successfully proved photocatalytic effectiveness. The present work fulfills simultaneously two specific objectives related to the efforts directed towards a sustainable environment and circular economy: recycling of optical glass microfibers resulted as waste from the industry, and their usage for the photo-oxidation of highly toxic emerging micro-pollutants.

Enhanced Organic and Perovskite Solar Cell Performance through Modification of the Electron-Selective Contact with a Bodipy-Porphyrin Dyad

Photovoltaic devices based on organic semiconductors and organo-metal halide perovskites have not yet reached the theoretically predicted power conversion efficiencies while they still exhibit poor environmental stability. Interfacial engineering using suitable materials has been recognized as an attractive approach to tackle the above issues. We introduce here a zinc porphyrin-triazine-bodipy donor-π bridge-acceptor dye as a universal electron transfer mediator in both organic and perovskite solar cells. Thanks to its "push-pull" character, this dye enhances electron transfer from the absorber layer toward the electron-selective contact, thus improving the device's photocurrent and efficiency. The direct result is more than 10% average power conversion efficiency enhancement in both fullerene-based (from 8.65 to 9.80%) and non-fullerene-based (from 7.71 to 8.73%) organic solar cells as well as in perovskite ones (from 14.56 to 15.67%), proving the universality of our approach. Concurrently, by forming a hydrophobic network on the surface of metal oxide substrates, it improves the nanomorphology of the photoactive overlayer and contributes to efficiency stabilization. The fabricated devices of both kinds preserved more than 85% of their efficiency upon exposure to ambient conditions for more than 600 h without any encapsulation.

A Cryogenic Process for Antisolvent-Free High-Performance Perovskite Solar Cells

A cryogenic process is introduced to control the crystallization of perovskite layers, eliminating the need for the use of environmentally harmful antisolvents. This process enables decoupling of the nucleation and the crystallization phases by inhibiting chemical reactions in as-cast precursor films rapidly cooled down by immersion in liquid nitrogen. The cooling is followed by blow-drying with nitrogen gas, which induces uniform precipitation of precursors due to the supersaturation of precursors in the residual solvents at very low temperature, while at the same time enhancing the evaporation of the residual solvents and preventing the ordered precursors/perovskite from redissolving into the residual solvents. Using the proposed techniques, the crystallization process can be initiated after the formation of a uniform precursor seed layer. The process is generally applicable to improve the performance of solar cells using perovskite films with different compositions, as demonstrated on three different types of mixed halide perovskites. A champion power conversion efficiency (PCE) of 21.4% with open-circuit voltage (V_OC ) = 1.14 V, short-circuit current density ( J_SC ) = 23.5 mA cm^-2 , and fill factor (FF) = 0.80 is achieved using the proposed cryogenic process.

Precision excimer laser annealed Ga-doped ZnO electron transport layers for perovskite solar cells

Organic-inorganic hybrid perovskite solar cells (PSCs) continue to attract considerable attention due to their excellent photovoltaic performance and low cost. In order to realize the fabrication of PSCs on temperature-sensitive substrates, low-temperature processing of all the components in the device is required, however, the majority of the high-performance PSCs rely on the electron transport layers (ETLs) processed at high temperatures. Herein, we apply excimer laser annealing (ELA) to treat ETLs (Ga-doped ZnO, GZO) at room temperature. A synergetic improvement in optical transparency and electrical conductivity is achieved after ELA treatment, which in turn improves light absorption, enhances electron injection, and depresses charge recombination. Devices fabricated with ELA treated GZO ETL acheived a power conversion efficiency (PCE) of 13.68%, higher than that of the PSCs utilizing GZO with conventional high-temperature annealing (12.96%). Thus, ELA is a promising technique for annealing ETLs at room temperature to produce efficient PSCs on both rigid and flexible substrates.

Improved efficient perovskite solar cells based on Ta-doped TiO2 nanorod arrays

Organometal halide perovskite solar cells (PSCs) are nowadays regarded as a rising star in photovoltaics. In particular, PSCs incorporating oriented TiO₂ nanorod (NR) arrays as the electron transport layer (ETL) have attracted significant attention owing to TiO₂ NR's superior electron transport abilities and its potential in long-term stable PSCs. In addition to improve the electron-transport ability of TiO₂ NRs, the tuning of the band alignments between the TiO₂ NR array and the perovskite layer is also crucial for achieving efficient solar cells. This work describes a facile, one-step, solvothermal method for the preparation of tantalum (Ta) doped TiO₂ NR arrays for efficient PSCs. It is shown that the trace doping with Ta tunes the electronic structure of TiO₂ NRs by a synergistic effect involving the lower 5d orbitals of the doped Ta⁵⁺ ions and the reduced oxygen vacancies. The synergistic tuning of the electronic structure improves the band alignment at the TiO₂ NR/perovskite interface and boosts the short-circuit current and the fill factor. By using the optimized doped TiO₂ NR array as the ETL, a record efficiency of 19.11% was achieved, which is the highest among one-dimensional-array based PSCs.

Silicotungstate, a Potential Electron Transporting Layer for Low-Temperature Perovskite Solar Cells

Thin films of a heteropolytungstate, lithium silicotungstate (Li₄SiW₁₂O₄₀, termed Li-ST), prepared by a solution process at low temperature, were successfully applied as electron transporting layer (ETL) of planar-type perovskite solar cells (PSCs). Dense and uniform Li-ST films were prepared on FTO glass by depositing a thin Li-ST buffer layer, followed by coating of a main Li-ST layer. The film thickness was controlled by varying the number of coating cycles, consisting of spin-coating and thermal treatment at 150 °C. In particular, by employing 60 nm-thick Li-ST layer obtained by two cycles of coating, the fabricated CH₃NH₃PbI₃ PSC device demonstrates the photovoltaic conversion efficiency (PCE) of 14.26% with J_SC of 22.16 mA cm^-2, V_OC of 0.993 mV and FF of 64.81%. The obtained PCE is significantly higher than that of the PSC employing a TiO₂ layer processed at the same temperature (PCE = 12.27%). Spectroscopic analyses by time-resolved photoluminescence and pulsed light-induced transient measurement of photocurrent indicate that the Li-ST layer collects electrons from CH₃NH₃PbI₃ more efficiently and also exhibits longer electron lifetime than the TiO₂ layer thermally treated at 150 °C. Thus, Li-ST is considered to be a promising ETL material that can be applied for the fabrication of flexible PSC devices.

Carbon Nanotubes in TiO2 Nanofiber Photoelectrodes for High-Performance Perovskite Solar Cells

1D semiconducting oxides are unique structures that have been widely used for photovoltaic (PV) devices due to their capability to provide a direct pathway for charge transport. In addition, carbon nanotubes (CNTs) have played multifunctional roles in a range of PV cells because of their fascinating properties. Herein, the influence of CNTs on the PV performance of 1D titanium dioxide nanofiber (TiO2 NF) photoelectrode perovskite solar cells (PSCs) is systematically explored. Among the different types of CNTs, single-walled CNTs (SWCNTs) incorporated in the TiO2 NF photoelectrode PSCs show a significant enhancement (≈40%) in the power conversion efficiency (PCE) as compared to control cells. SWCNTs incorporated in TiO2 NFs provide a fast electron transfer within the photoelectrode, resulting in an increase in the short-circuit current (Jsc) value. On the basis of our theoretical calculations, the improved open-circuit voltage (Voc) of the cells can be attributed to a shift in energy level of the photoelectrodes after the introduction of SWCNTs. Furthermore, it is found that the incorporation of SWCNTs into TiO2 NFs reduces the hysteresis effect and improves the stability of the PSC devices. In this study, the best performing PSC device constructed with SWCNT structures achieves a PCE of 14.03%.

Continuous Size Tuning of Monodispersed ZnO Nanoparticles and Its Size Effect on the Performance of Perovskite Solar Cells

ZnO has been demonstrated to be a promising candidate to fabricate high efficiency perovskite solar cells (PSCs) in terms of its better electron extraction and transport properties. However, the inability of synthesis of ZnO nanoparticles (NPs) with minimal surface defects and agglomeration remains a great challenge hindering the fabrication of highly efficient PSCs. In this work, highly crystalline and agglomeration-free ZnO NPs with controlled size were synthesized through a facile solvothermal method. Such ZnO NPs were applied in the fabrication of meso-structured PSCs. The solar cells with ∼40 nm ZnO NPs exhibit the highest power conversion efficiency (PCE) of 15.92%. Steady-state and time-resolved photoluminescence measurements revealed the faster injection and lower charge recombination at the interface of ∼40 nm ZnO NPs and perovskite, resulting in significantly enhanced J_SC and V_OC.

Mesostructured perovskite solar cells based on highly ordered TiO2 network scaffold via anodization of Ti thin film

An anodized TiO₂ interconnected network was fabricated and utilized as a mesoporous scaffold and electron transporter in perovskite solar cells. By modifying the synthesis parameters, the morphological features of the interconnected TiO₂ nanostructures can be widely tuned and precisely controlled. The functional properties of the anodized TiO₂ network are found to be severely influenced by morphology as well as the extent of oxidation. The device with the optimized TiO₂ network exhibits superior electron extraction and transferability, resulting in conspicuous enhancement of the photocurrent and power conversion efficiency (PCE). This work proposes a promising and facile method for improving the performance of perovskite solar cells.

Electrosprayed TiO2 nanoporous hemispheres for enhanced electron transport and device performance of formamidinium based perovskite solar cells

Titanium dioxide (TiO₂) nanoporous hemispheres (NHSs) with a radius of ∼200 nm are fabricated by electrospraying a hydrothermally synthesized TiO₂ nanoparticle (NP) suspension solution. The resulting TiO₂ NHSs are highly porous, which are beneficial to the infiltration of perovskites and provide a larger contact area, as building blocks to construct a mesoporous TiO₂ layer for FA_0.81MA_0.15Pb(I_0.836Br_0.15)₃ based perovskite solar cells (PSCs). By varying the TiO₂ NHS collecting period (15 s, 30 s, 60 s and 90 s) during the electrospraying process, the performance of PSCs changes with different TiO₂ NHS distribution densities. The optimized PSC employing TiO₂ NHSs (60 s) exhibits a photovoltaic conversion efficiency (PCE) as high as 19.3% with a J_sc of 23.8 mA cm^-2, a V_oc of 1.14 V and a FF of 0.71. Furthermore, the PSC possesses a reproducible PCE value with little hysteresis in its current density-voltage (J-V) curves. The small perturbation transient photovoltage (TPV) measurement reveals a longer free carrier lifetime within the TiO₂ NHS based PSC than that in the TiO₂ NP based PSC, and the time of flight (TOF) photoconductivity measurement shows that charge mobilities in this system are also enhanced. These characteristics make TiO₂ NHSs a promising electron transport material for efficient photovoltaic devices.

Current status of electron transport layers in perovskite solar cells: materials and properties

Methyl ammonium lead halide-based hybrid perovskite solar cells (PSCs) have been intensively studied in recent years because of their high efficiency and low processing costs.

Vertically aligned nanostructured TiO2 photoelectrodes for high efficiency perovskite solar cells via a block copolymer template approach

We fabricated perovskite solar cells with enhanced device efficiency based on vertically oriented TiO2 nanostructures using a nanoporous template of block copolymers (BCPs). The dimension and shape controllability of the nanopores of the BCP template allowed for the construction of one-dimensional (1-D) TiO2 nanorods and two-dimensional (2-D) TiO2 nanowalls. The TiO2 nanorod-based perovskite solar cells showed a more efficient charge separation and a lower charge recombination, leading to better performance compared to TiO2 nanowall-based solar cells. The best solar cells employing 1-D TiO2 nanorods showed an efficiency of 15.5% with VOC = 1.02 V, JSC = 20.0 mA cm(-2) and fill factor = 76.1%. Thus, TiO2 nanostructures fabricated from BCP nanotemplates could be applied to the preparation of electron transport layers for improving the efficiency of perovskite solar cells.

TiO2 Sub-microsphere Film as Scaffold Layer for Efficient Perovskite Solar Cells

TiO2 sub-microspheres composed of anatase granular-like nanocrystallines with an average diameter ∼250 nm are synthesized using sol-gel method and employed as the scaffold layer for efficient mesocopic perovskite solar cells. Compared with mesoporous TiO2 films composed of ∼18 nm nanoparticles, the sub-microsphere films show superior light-trapping characteristics and significantly improve the light-harvesting capability of the solar cells. In addition, the charge-transport performance is also dramatically improved according to the transient photocurrent decay despite there being no significant difference in the perovskite layer surface morphology. As a result, an average power conversion efficiency of 15% with a highly uniform distribution is achieved for the solar cells with TiO2 sub-microsphere films, 12% higher than those with TiO2 nanoparticle films. The combination of light-harvesting capability and fast charge transfer make the TiO2 sub-microsphere film a good candidate as the scaffold layer for efficient perovskite solar cells.

Mechanism of biphasic charge recombination and accumulation in TiO2mesoporous structured perovskite solar cells

The charge recombination process is dominated by perovskite or mesoporous TiO₂owing to different charge populations at various energy levels.

Low cost and solution-processable zinc phthalocyanine as alternative hole transport material for perovskite solar cells

A solution-processable and dopant-free ZnPc(tBu)₄was used a potential cost-effective substitute for the expensive HTMs containing multifold dopants used in the current PSCs.

Use of feedback system control in optimizing chemical combinations to synthesize nanoparticles with desired characteristics

Defining desired chemical combinations through the FSC method.

Improved photovoltaic performance of mesoporous perovskite solar cells with hydrogenated TiO2: prolonged photoelectron lifetime and high separation efficiency of photoinduced charge

Hydrogenated titanium dioxide (H-TiO₂) nanocrystals and nanorods (H-TNRs) are successfully synthesized and employed as electron transfer materials in mesoscopic perovskite solar cells (PSCs).