Planar Heterojunction Perovskite Solar Cells Research Articles

Simulation studies on the electron transport layer based perovskite solar cell to achieve high photovoltaic efficiency

Perovskite solar cells have attracted the attention of the researchers in the last couple of years as a potential photovoltaic device. However, the use of expensive hole transport materials (HTM) in these devices often restricts their commercial adaptability. Thus exploring cost-effective, efficient HTL and ETL materials remain an important challenge to the researchers. In this work, simulation studies are carried out considering cupric oxide (CuO), a relatively inexpensive material as hole transport materials for planar heterojunction perovskite solar cells. The photo-voltaic performance of CuO based hole transport layer (HTL) has been estimated in combination with several electron transport materials (ETM) that include TiO2,SnO2,ZnO, CdS, ZnSe,PCBM and Cd1-xZnxS. Studies predict that among these materials, the Cd1-xZnxS electron transport layer (ETL) could be the most promising to result high photo-voltaic efficiency in combination to CuO based HTL. Also, the thickness and optical band gap of perovskite absorber are optimized in order to achieve maximum photo-voltaic efficiency. The cell efficiency of FTO / Cd1-xZnxS/CH3NH3PbI3/CuO/carbon structure is predicted 25.24% under optimized operational conditions with Voc, Jsc and Fill Factor of 1.1eV,26.32mA/cm2 and 87.14% respectively.

A Regularity‐Based Fullerene Interfacial Layer for Efficient and Stable Perovskite Solar Cells via Blade‐Coating

AbstractThe electron transport layer (ETL) plays a crucial part in extracting electron carriers while optimizing the interfacial contact of perovskite/electrode in planar heterojunction perovskite solar cells (PVSCs). Despite various ETLs being designed for efficient PVSCs, there exists hardly any research on the effect of molecular stacking order on device performance. Herein, poly(ethylene‐co‐vinyl acetate) (EVA) is employed as the [6,6]‐phenyl‐C61‐butyric acid methyl ester (PC61BM) solution additive. The strong binding energy between EVA with PC61BM promotes the molecular stacking order of ETLs, which alleviates the morphology inhomogeneity, possesses a matched energy level, blocks ion migration, and improves the water–oxygen barrier of perovskite devices. The blade‐coated MAPbI3‐based PVSCs achieve a power conversion efficiency (PCE) of 19.32% with positive reproducibility and negligible hysteresis, as well as maintain 90% and 80% of the initial PCE after storage under inert and ambient conditions (52% humidity) for 1500 h without encapsulation. This strategy also improves the champion PCE of CsFAMA‐based PVSCs to 20.33%. These findings demonstrate that the regulation of molecular stacking order is a valid approach to optimize interfacial charge‐carrier recombination in PVSCs, which meet the demand for high‐performance ETL in large‐area PVSCs and improve the upscaling of the fabrication technology toward practical applications.

Optimizing Vertical Crystallization for Efficient Perovskite Solar Cells by Buried Composite Layers

Planar‐heterojunction perovskite solar cells (PSCs) have experienced rapid evolution in recent years because of the low‐temperature processing, suitable alignment, and high mobility of the tin oxide buried contact layer. However, improper SnO2 surface states and poor crystallinity of the top perovskite films are still the main obstacles for the planar PSCs in which performance always lags behind their mesoporous counterparts. Herein, a new buried contact is reported by introducing graphitic carbon nitride (g‐C3N4) into the commonly used SnO2 which performs outstanding transmittance, conductivity, and surface states for a high‐quality electron‐transporting layer. Moreover, the vertical composition and crystallinity of the top perovskite film are manipulated by rich amino groups on the edge of the g‐C3N4 nanosheets which induce the prenucleation of the lead‐rich species at the buried interface. Benefiting from the high‐quality buried contacts and the optimized perovskite layers, the resultant PSCs achieve a champion efficiency of 21.5% with all photovoltaic parameters enhanced in comparison with their control counterparts (<20%).

Exploring low-temperature processed multifunctional HEPES-Au NSs-modified SnO2 for efficient planar perovskite solar cells

In the development of high-efficiency n-i-p planar heterojunction perovskite solar cells (PSCs), the development of dense electron-transporting layers (ETLs) with excellent electrical properties and hetero-interface quality is very important for promoting perovskite growth and carrier transport dynamics. Herein, we propose a simple and effective strategy to simultaneously improve the electronic properties and interface quality of a tin oxide (SnO2) ETL by introducing 2-hydroxyethyl modified gold nanostars (HEPES-Au NSs) into a commercial SnO2 colloidal dispersion in order to synthesize mixed ETLs at low temperatures (≤150 ℃). Based on the unique plasmonic structure of the HEPES-Au NSs, we demonstrate that the modified ETLs exhibit higher conductivity and greater efficiency in the extraction, transfer, and collection of photogenerated electrons than conventional SnO2 ETLs. Moreover, the chemical bond interaction between HEPES-Au NSs and the perovskite enhances the affinity of the SnO2/perovskite hetero-interface, which effectively improves the nucleation and crystallization kinetics of the perovskite, thus producing a high-quality absorber with high crystallinity and superior absorbance. Meanwhile, the 2-hydroxyethyl (HEPES) adsorbed onto the SnO2 surface effectively passivates perovskite-related trap states, suppressing the non-radiative recombination and leakage current, and ultimately improving the relative electronic properties and photovoltaic performance output of the modified device. The results show that the power conversion efficiency (PCE) of the modified PSCs is significantly better than that of the original SnO2-based planar devices, and the optimal PCE reached 21.13%, with negligible hysteresis. Additionally, owing to the multifunctional effects of SnO2 modified by HEPES-Au NSs, the functionalized devices without encapsulation also demonstrate reliable reproducibility and good stability, which shows great advantages in the development of high-efficiency flexible PSCs and monolithic tandem devices.

Interior/Interface Modification of Textured Perovskite for Enhanced Photovoltaic Outputs of Planar Solar Cells by an In Situ Growth Passivation Technology

To compensate for the photoelectric losses of planar heterojunction perovskite solar cells (PSCs), the development of high-quality textured absorbers with excellent light-harvesting ability and carrier extraction/transfer efficiency is of great significance to achieve a high-efficiency stable photovoltaic output. In this paper, we propose an in situ growth passivation technique to construct high-performance textured absorbers by adding a 2-amino-4-chlorophenol (AC) modifier consisting of multiple groups during the growth of textured perovskite. Initially, according to the Ostwald ripening mechanism, the strongly polar dimethylformamide (DMF) was used as the etchant to systematically study its synergistic effect on the morphology evolution, crystallization kinetics, light-trapping capability, and photovoltaic loss of textured absorbers. An appropriate amount of DMF induces formamidinium cations (FA+) to replace methylammonium cations (MA+) in the perovskite lattice while etching the absorber to form a texture configuration, which effectively broadens the spectral absorption range, thus greatly improving the light-trapping capacity and short-circuit current density of planar PSCs. In contrast, excess DMF deteriorates the device performance due to the excessive corrosion of the perovskite. Moreover, the introduction of the AC modifier is of great significance for passivating deep-level defects and accelerating the charge extraction/transfer. Owing to the electron-donating nature of the Lewis base, the hydroxyl groups with a higher electron density in AC molecules can better coordinate with Pb2+ ion defects, which effectively improves the crystallinity of the textured perovskite, thus suppressing the nonradiative recombination and ultimately improving the photovoltaic outputs of modified devices, particularly the fill factor and the open-circuit voltage. Thus, the photovoltaic performance of the AC-modified planar PSC is significantly better than that of the conventional textured device, with a reverse efficiency of 21.18% and forward efficiency of 20.77%. Owing to the synergistic effect of (1) the superior optical properties of the textured perovskite induced by DMF and (2) excellent charge dynamics driven by AC, the functionalized devices without encapsulation also exhibited good photovoltaic output stability and reproducibility. This work provides novel insights into the growth mechanism of textured absorbers and paves the way for more efficient and stable planar PSCs.

Solving the equivalent circuit of a planar heterojunction perovskite solar cell using Lambert W-function

Solar cell studies are enhanced by the availability of its equivalent circuits that can describe the electrical characteristics represented by their terminal current and voltage behavior under a particular operating condition. The device designation and its optimization can be facilitated theoretically via analytical solutions of its transcendental J-V equation corresponding to its equivalent circuit. We present an analytical solution of the transcendental J-V equation corresponding to a generalized equivalent circuit of a planar heterojunction perovskite solar cell (PSC). The simulated current-voltage curves, power-voltage plots and the calculated performance parameters of four PSCs confirm the validity of the proposed analytical solution.

(Invited) Novel Pyridine-functionalized Fullerene Derivatives toward the Efficient and Hysteresis-free Perovskite Solar Cells

Fullerene derivatives with strong electron-accepting abilities have been extensively used as electron transport layers (ETLs) of inverted (p–i–n) planar heterojunction perovskite solar cells (iPSCs); however the effect of the alkyl chains and a specific heteroatom site within the fullerene derivative on its electron transport properties has scarcely been studied. Herein,by employing pyridine carboxaldehydes bearing nitrogen sites within the pyridine moiety as a reactant, a series of novel pyridine-functionalized fullerene derivatives with variable nitrogen sites and alkyl chains length were synthesized facilely via a one-step 1, 3-dipolar cycloaddition reaction, and their molecular structures as well as molecular packing were unambiguously determined by X-ray single crystal diffraction. Despite that fullerene derivatives exhibit negligible difference in the molecular energy level and optical absorption, however, their electron transport properties are found to sensitively depend on the length of alkyl chains and nitrogen sites within the pyridine moiety. The C60-3-Py-based device shows the highest power conversion efficiency (PCE) of 17.57%. The discrepancy of the electron transport properties of fullerene derivatives is primarily due to the difference in the coordination interaction between the pyridine moiety and the Pb2+ ion of the MAPbI3 perovskite layer interaction between the pyridine moiety and the Pb2+ ion of the MAPbI3 perovskite layer.

(Invited) Solution-Processed Reorganization of Semiconductor Nanocrystals in Thin Films

Colloidal nanocrystals (NCs) have been attracted because of their strongly size-dependent luminous characteristics and solution processability. As prepared colloidal NCs covered with some bulky and long-chain organic ligands such as oleic acid, oleylamine (OAm), trioctylphosphine oxide, and so on are mostly dispersed in some solvents such as hexane, toluene, and so on. After preparation of thin films from the dispersed solutions of NCs, reduced inter NC distance (viz. NCs coming closer) with highly ordered alignment will lead to enhanced electronic coupling between adjacent NCs resulting in better wave function overlap and charge transport of the conducting solid materials.In this context, remarkable improvement of the alignment of the particles through solution-processed reorganization of AgInS2 quantum dots (QDs) covered with OAm by cast method was confirmed by bright-field TEM observation [1,2]. The interplanar spacing of the QDs was 0.33 nm, which corresponds to (112) crystal planes of the tetragonal phase of AgInS2.On the other hand, it was found that PbS colloidal QDs can be packed densely and infiltrated efficiently into the interrod space of vertically aligned ZnO nanowire arrays through convective deposition method, which is one of the meniscus coating techniques [3] as compared with spin-coating process, which brings many cracks and voids in the obtained thin films. As a result, a record power conversion efficiency of 9.92% has been demonstrated for ordered bulk heterojunction structured solar cells that are compatible with low-temperature and scalable manufacturing processes.While comparisons of perovskite CsPbBr3 NCs grafted on the surface of silica nanohelices as templates in dispersed state in toluene and dried state on quartz glass substrate revealed that the interparticle distance becomes shorter after the dryness by cast method from the dispersed state and the closely and helically packed NCs [4]. Tuning the conditions of solution-processed reorganization of CsPbBr3 NCs is crucial for reducing the inter NC distance with highly ordered alignment in the cast films with large circular dichroism and circularly polarized luminescence signals.On the basis of the above findings, reorganization of NCs with highly controlled crystal orientation and inter NC distance in thin films has deep influence on specific spectral characteristics and photovoltaic performances.[1] Kaewprajak, A.; Kumnorkaew, P.; Sagawa, T. Silver-indium-sulfide quantum dots in titanium dioxide as electron transport layer for highly efficient and stable perovskite solar cells. J. Mater. Sci.: Mater. Electron., 2019, 30, 4041-4055.[2] Kaewprajak, A.; Kumnorkaew, P.; Sagawa, T. Improved photovoltaic performance and device stability of planar heterojunction perovskite solar cells using TiO2 and TiO2 mixed with AgInS2 quantum dots as dual electron transport layers. Org. Electron., 2019, 69, 26-33.[3] Shi, G.; Kaewprajak, A.; Ling, X.; Hayakawa, A.; Zhou, S.; Song, B.; Kang, Y.-w.; Hayashi, T.; Altun, M. E.; Nakaya, M.; Liu, Z.; Wang, H.; Sagawa, T.; Ma, W. Finely interpenetrating bulk heterojunction structure for lead sulfide colloidal quantum dot solar cells by convective deposition. ACS Energy Lett., 2019, 4, 960-967.[4] Liu, P.; Chen, W.; Okazaki, Y.; Battie, Y.; Brocard, L.; Decossas, M.; Pouget, E.; M.-Buschbaum, P.; Kauffmann, B.; Pathan, S.; Sagawa, T.; Oda, R. Optically active perovskite CsPbBr3 nanocrystals helically arranged on inorganic silica nanohelices. Nano Lett., 2020, 20, 8453-8460.

High‐Performance Planar Heterojunction Perovskite Solar Cells Based on BaCl2 Additive and Power Conversion Efficiency of Over 21%

AbstractThe photoelectric conversion efficiency of perovskite solar cells (PSCs) has rapidly developed in the past decade. However, the development of PSCs still has many limitations: for instance, the accumulation of harmful defects on their surfaces and grain boundaries limit their performance. Here, PSCs with ITO/SnO2/FAxMA1−xPb(IyBr1−y)3/Spiro‐OMeTAD/Ag structure are fabricated under air conditions, meanwhile, the effect the different masses of BaCl2 (0, 10, 15, 20, and 25 mg) added to FAxMA1−xPb(IyBr1−y)3 perovskite precursor solution on the performance of PSCs is investigated. The results show that the best performance is obtained when 20 mg BaCl2 is added, and the performance parameters Jsc, Voc, FF of the PSCs (those with 20 mg BaCl2) are 24.43 mA cm−2, 1137 mV, and 75.82, respectively, and their power conversion efficiency is up to 21.06%, which is ≈25% higher than that of the reference devices (those without BaCl2). Test results, such as XRD, SEM, TRPL, SCLC, UV–Vis, show that right amount of BaCl2 additive can significantly improve the crystallinity, absorbance, and interface defect state of perovskite films. It can also increase the separation and mobility of charge carriers, thus greatly improving the performance of PSCs.

Binary-mixed organic electron transport layers for planar heterojunction perovskite solar cells with high efficiency and thermal reliability

The optimal electron transport layer (ETL) is a crucial component in the inverted perovskite solar cells (PSCs) for achieving high photovoltaic performance and device stability. We herein address the alloy ETL strategy, which incorporates a non-fullerene small molecule, ITIC-4F, into the fullerene-based ETL to optimize the electron-selective interfacial properties of CH3NH3PbI3-based inverted PSCs. The alloy ETL exhibited tailored energy level alignment at the interface of perovskite and ETL, so the electron-extraction/recombination behavior is greatly improved. Also, the hydrophobic surface of the alloy ETL effectively impedes not only moisture invasion but also morphological deterioration under continuous operation, so the resultant CH3NH3PbI3-based inverted device modified with the alloy ETL exhibited superior power conversion efficiency (PCE) up to 19.99%, with a negligible hysteresis. The alloy ETL-based PSCs also achieves 25.1% efficiency under indoor lighting condition, which promises a feasible design strategy of interfacial optimization in high-performance PSCs using organic semiconductors.

Solid-state reaction process for high-quality organometallic halide perovskite thin film

Recently, a hybrid perovskite material, ABX3 (A= Cs, CH3NH3, NH2CHNH2; B= Pb, Sn; X= Cl, Br, I), has received much attention as an active layer in new-generation solar cells. This material is usually fabricated with either a one-step or a two step process in solution. However, the surface morphology, nucleation rate and grain growth rate of the CH3NH3PbI3 perovskite light absorber prepared by the solution reaction process (SRP) are hard to control. Here, we show a fast solid-solid reaction process (SSRP), to fabricate ultraflat (roughness of approximately 12 nm) CH3NH3PbI3 perovskite thin films with large grain sizes (~947 nm). The SSRP simply involves directly contacting a lead iodide thin film (PbI2) with methylammonium iodide powder (CH3NH3I) without any chemical reagents at 120°C under a normal atmospheric environment. The SSRP reaction dynamics is investigated by an in situ heating scanning electron microscope (SEM) system. This innovative SSRP is an easy approach for the large-scale fabrication of planar heterojunction perovskite solar cells and allows us to demonstrate a power conversion efficiency of approximately 15.27% (active area of 0.16 cm2).

Boosting interfacial charge transfer by constructing rare earth–doped WOx nanorods/SnO2 hybrid electron transport layer for efficient perovskite solar cells

Recently，the highest power conversion efficiency (PCE) of perovskite solar cells (PSCs) is up to 25%. Interfacial engineering strategy is an efficient route for improving carrier extraction and transport in planar heterojunction PSCs. Herein, we use the rare earth (RE)-doped WOx nanorods/SnO2 hybrid films as the electron transport layer (ETL) to accurately modulate the interfacial charge dynamics for hysteresis-free, high-performance devices. The RE-WOx nanorods with SnO2 can improve the perovskite crystal quality and hinder electron-hole recombination by reducing the interface traps/defects between the ETL and perovskite. Meanwhile, owing to the excellent mobility and conductivity of RE-WOx nanorods and the well gradient band alignment, the improved electron extraction and transport at the interface of ETL and perovskite in PSCs are obtained compared with the SnO2 ETL. With these desirable properties, the optimized PSC device yields a high PCE of 21.42% with negligible hysteresis. Meanwhile, the stability of PSC devices could also be significantly boosted without any encapsulation. The introduction of RE-WOx nanorods as the interface modifier is a significant strategy to improve the interface property in the PSCs and opens up a novel and promising avenue for application in efficient and stable PSCs.

High Mobility Reactive Sputtered CuxO Thin Film for Highly Efficient and Stable Perovskite Solar Cells

Copper oxide (CuxO) films are considered to be an attractive hole-transporting material (HTM) in the inverted planar heterojunction perovskite solar cells due to their unique optoelectronic properties, including intrinsic p-type conductivity, high mobility, low-thermal emittance, and energy band level matching with the perovskite (PS) material. In this study, the potential of reactive sputtered CuxO thin films with a thickness of around 100 nm has been extensively investigated as a promising HTM for effective and stable perovskite solar cells. The as-deposited and annealed films have been characterized by using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Photoluminescence (PL), UV-Vis spectroscopy, and Hall-effect measurement techniques. The significant change in structural and optoelectronic properties has been observed as an impact of the thermal annealing process. The phase conversion from Cu2O to CuO, including grain size increment, was observed upon thermal annealing. The transmittance and optical bandgap were found to vary with the films’ crystallographic transformation. The predominant p-type conductivity and optimum annealing time for higher mobility have been confirmed from the Hall measurement. Films’ optoelectrical properties were implemented in the complete perovskite solar cell for numerical analysis. The simulation results show that a 40 min annealed CuxO film yields the highest efficiency of 22.56% with a maximum open-circuit voltage of 1.06 V.

In Situ Surface Fluorination of TiO2 Nanocrystals Reinforces Interface Binding of Perovskite Layer for Highly Efficient Solar Cells with Dramatically Enhanced Ultraviolet-Light Stability.

Low‐temperature solution‐processed TiO2 nanocrystals (LT‐TiO2) have been extensively applied as electron transport layer (ETL) of perovskite solar cells (PSCs). However, the low electron mobility, high density of electronic trap states, and considerable photocatalytic activity of TiO2 result in undesirable charge recombination at the ETL/perovskite interface and notorious instability of PSCs under ultraviolet (UV) light. Herein, LT‐TiO2 nanocrystals are in situ fluorinated via a simple nonhydrolytic method, affording formation of Ti─F bonds, and consequently increase electron mobility, decrease density of electronic trap states, and inhibit photocatalytic activity. Upon applying fluorinated TiO2 nanocrystals (F‐TiO2) as ETL, regular‐structure planar heterojunction PSC (PHJ‐PSC) achieves a champion power conversion efficiency (PCE) of 22.68%, which is among the highest PCEs for PHJ‐PSCs based on LT‐TiO2 ETLs. Flexible PHJ‐PSC devices based on F‐TiO2 ETL exhibit the best PCE of 18.26%, which is the highest value for TiO2‐based flexible devices. The bonded F atoms on the surface of TiO2 promote the formation of Pb─F bonds and hydrogen bonds between F− and FA/MA organic cations, reinforcing interface binding of perovskite layer with TiO2 ETL. This contributes to effective passivation of the surface trap states of perovskite film, resulting in enhancements of device efficiency and stability especially under UV light.

Cover Image

The cover image is based on the Research Article Enhanced fill factor for normal n-i-p planar heterojunction and mesoscopic perovskite solar cells using ruthenium-doped TiO2 electron transporting layer by Sawanta S. Mali et al., https://doi.org/10.1002/pip.3352.

Bilayer broadband antireflective coating to achieve planar heterojunction perovskite solar cells with 23.9% efficiency

Although perovskite solar cells (PSCs) have achieved encouraging efficiency, the photon loss at the substrate due to light reflection has not been well addressed. Light management is promising to reduce reflection loss and realize higher power conversion efficiency (PCE) of PSCs. Here, a bilayer antireflective coating (ARC) has been designed and coated onto the backside of the glass substrate of (FAPbI3) x -(MAPbBr3)1− x PSCs to enhance photon harvesting and consequently the device efficiency. The bottom layer of the bilayer ARC is made from a silica polymer and the top layer is made from the mixture of hexamethyldisiloxane-modified mesoporous silica nanoparticles and a fluorinated silica polymer. By adjusting the refractive index and the film thickness of each layer according to a two-layer model, enhanced glass transmittance in a broadband wavelength range can be reached, with the maximum transmittance increasing from ca. 90% to over 95%. With the bilayer ARC, the maximum short-circuit current density and PCE of (FAPbI3) x (MAPbBr3)1− x PSCs can be increased from 25.5 mA cm−2 and 22.7% to 26.5 mA cm−2 and 23.9% with negligible changes in fill factor and open-circuit voltage. This work presents a simple yet effective strategy to enhance the efficiency of solar cells employing bilayer antirefective coatings, which can be applied to other types of solar cells.

Thiophene Terminated Fullerene Derivatives for Interfacial Modification toward High Efficiency MAPbI3 Perovskite Solar Cells

Interface engineering has become a critical process to fabricate efficient and stable planar heterojunction perovskite solar cells (PSCs), as the degradation of perovskite, hysteresis, photo and th...

Enhanced Efficiency and Stability of Planar Perovskite Solar Cells Using a Dual Electron Transport Layer of Gold Nanoparticles Embedded in Anatase TiO2 Films.

Incorporating plasmonic nanostructures is a promising strategy to enhance both the optical and electrical characteristics of photovoltaic devices via more efficient harvesting of incident light. Herein, we report a facile fabrication scheme at low temperature for producing gold nanoparticles embedded in anatase TiO2 films, which can simultaneously improve the efficiency and stability of n–i–p planar heterojunction perovskite solar cells (PSCs). The PSCs based on rigid and flexible substrates with 0.2 wt % Au–TiO2/TiO2 dual electron transport layers (ETLs) achieved power conversion efficiencies up to 20.31 and 15.36%, superior to that of devices with TiO2 as a single ETL. Moreover, 0.2 wt % Au–TiO2/TiO2 devices demonstrated significant stability in light soaking, which is attributed to improved light absorption, low charge recombination loss, and enhanced carrier transport, and extraction with the plasmonic Au–TiO2/TiO2 dual ETL. The present work improves the practicability of high-performance and flexible PSCs by engineering the photogenerated carrier dynamics at the interface.

Fast Wetting of a Fullerene Capping Layer Improves the Efficiency and Scalability of Perovskite Solar Cells.

Fullerene derivatives, especially [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), have been widely applied as electron transport layers of inverted planar heterojunction perovskite solar cells (PSCs). However, the solution-processed PCBM capping layer suffers from limited surface wetting which hinders the improvement in efficiency and scalability of PSCs. Herein, we develop a facile hybrid solvent strategy that enables very fast wetting of the PCBM capping layer atop of the perovskite surface, leading to an improved interfacial contact and electron transport. The significantly enhanced wettability of the PCBM solution fulfilled through blending isopropyl alcohol into the commonly used chlorobenzene (CB) is attributed to the reduced surface tension while retaining viscosity. As a result, the electron mobility and electric conductivity of the PCBM capping layer increase by around two times, and the PSC devices exhibit the highest power conversion efficiency (PCE) of 19.92%, which is improved by ∼18% relative to that of the control device (16.78%). Importantly, this strategy is also applicable for other alcohols (ethanol and methanol) and CB blends. Moreover, the fast wetting approach enables us to deposit the PCBM capping layer using a facile drop-casting method, affording comparable PCEs to those obtained by the conventional spin-coating method, which is not achievable by using the conventional single solvent. This fast wetting PCBM capping layer also contributes to efficiency improvement of large-area (1 cm2) devices. These advances hold great potential for other scalable deposition methods such as blade-coating and slot-die coating.

High performance novel flexible perovskite solar cell based on a low-cost-processed ZnO:Co electron transport layer

In this work, high quality uniform and dense nanostructured cobalt-doped zinc oxide (ZnO:Co) films were used as electron-transport layers in CH3NH3PbI3-based planar heterojunction perovskite solar cells (PSCs) on a flexible conductive substrate. Highly photo catalytically active ZnO:Co films were prepared by a low cost hydrothermal process using the aqueous solution of zinc nitrate hexahydrate, hexamethylenete-tramine and cobalt (II) nitrate hexahydrate. ZnO:Co films were deposited on indium tin oxide (ITO) covered polyethylene terephthalate (PET) flexible substrates. The growth was controlled by maintaining the autoclave temperature at 150 °C for 4 h. The CH3NH3PbI3 layer was deposited on the ZnO:Co films by spin coating. Spiro-OMeTAD was employed as a hole-transporting material. The structural, morphology and optical properties of the grown ZnO nanostructures were characterized by X-ray diffraction (XRD), field-emission scanning electron microcopy (FESEM), energy-dispersive X-ray spectrometry (EDX), ultraviolet-visible (UV-Vis) and photoelectrochemical propriety. XRD spectra showed that both ZnO and ZnO:Co nanorods had a hexagonal wurtzite structure with a strong preferred orientation along the (002) plane. The surface morphology of films was studied by FESEM and showed that both the pure and Co-doped ZnO films had hexagonal shaped nanorods. In the steady state, the ZnO electrode gave a photocurrent density of about 1.5 mA/cm2. However, the Co-doped ZnO electrode showed a photocurrent density of about 6 mA/cm2, which is 4-fold higher than that of the ZnO electrode. Based on the above synthesized Co-doped ZnO films, the photovoltaic performance of PSCs was studied. The Co-doped ZnO layers had a significant impact on the photovoltaic conversion efficiency (PCE) of the PSCs. The latter was attributed to an efficient charge separation and transport due to the better coverage of perovskite on the nanostructured Co-doped ZnO films. As a result, the measured PCE under standard solar conditions (A M 1.5G, 100 mW/cm2) reached 7%. SCAPS-1D simulation was also performed to analyze the effect of the co-doped ZnO thin film on the corresponding solar cell performances.