Planar Structure Perovskite Solar Cells Research Articles

Ultraviolet-light induced H+ doping in polymer hole transport material for highly efficient perovskite solar cells

Poly(N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine) (PTPD) is an appropriate hole transporting material (HTM) in perovskite solar cells (PSCs) due to its simple fabrication process. However, the relatively low conductivity of PTPD compared with traditional inorganic HTMs limits its application in highly efficient PSCs. Here, we employ an ultraviolet (UV)-light induced H+ doping strategy to improve the conductivity of PTPD hole transporting layer (HTL) for high-performance n-i-p planar structure PSCs. The released proton (H+) from 4-isopropyl-4′-methyldiphenyliodonium tetrakis-(pentafluorophenyl-borate) (DPI-TPFB) induced by UV-light irradiation can coordinate with PTPD to form PTPD-NH+, which dramatically enhances the hole carrier transport and extraction. As a result, a champion power conversion efficiency (PCE) of 21.38% is achieved in PSCs with H+ doped PTPD as HTL, which is much higher than that (17.63%) of the PSCs with pristine PTPD HTL. Furthermore, the stability of the PSCs in humid air is significantly improved after adopting H+ doped PTPD to replace pristine PTPD, owing to the increased hydrophobicity of PTPD HTL.

Progress and Challenges of SnO2 Electron Transport Layer for Perovskite Solar Cells: A Critical Review

Organic inorganic halide perovskites have drawn great attention in the past decade, due to their superior photovoltaic performance with an efficiency over 25%. For planar heterojunction structure perovskite solar cells (PSCs) tin oxide based electron transport layers (ETLs) have become one of the most suitable candidates to replace titanium oxide to make flexible devices because of their low‐temperature processing. The deposition techniques of SnO2 can be categorized into chemical deposition, such as sol‐gel, chemical bathing or atomic layer deposition, and physical deposition, such as thermal evaporation or sputtering. Depending on the deposition technique, defects, and morphology in the SnO2 layer may vary drastically, leading to poor performance of PSCs. In this review, we have provided a comprehensive picture of the recent progress and challenges of different SnO2 ETL deposition techniques and the performance of PSC devices. The additional modifications on SnO2 mentioned in this article are also effective ways to eliminate the intrinsic defects in the film. The drawbacks and benefits of SnO2 ETLs and the corresponding actions for this are also discussed. We hope this review will help with the comprehensive understanding of the relationship between the property of SnO2 and its structure of ETLs to enhance PSCs' performance.

Alkali Metal Chloride-Doped Water-Based TiO2 for Efficient and Stable Planar Perovskite Photovoltaics Exceeding 23% Efficiency.

TiO2 is one of the most broadly employed electron transport materials in n-i-p structure perovskite solar cells (PSCs). Low-temperature non-hydrolyzed sol-gel method is developed to prepare TiO2 in order to simplify the fabrication process and match with the planar structure PSCs. Conventional low-temperature TiO2 film using organic solvents as dispersants makes direct doping challenging due to limited solubility. Here, a newly developed water-based TiO2 solution is directly doped with different alkali chlorides, resulting in better conductivity, compatible energy level matching, and enhanced charge extraction in terms of electron transport layer (ETL) for PSCs. As a result, a power conversion efficiency of 23.15% is achieved based on NaCl-doped TiO2 with competitive storage stability and light stability. The water-based TiO2 ETL for more general doping of various solutes opens up a new avenue for environmental-friendly manufacturing superior ETL toward high-efficiency and stable perovskite photovoltaic devices.

A Mixed Heterojunction Layer for High Performance and Stability P-I-N-Based Perovskite Solar Cells

Perovskite solar cells (PSCs) have attracted a lot of attention due to high efficiency and low-cost fabrication processing. For planar structure PSCs, the electron transport layer (ETL) plays an important role to enhance the electron extraction capacity, device stability and balance the energy level between perovskite materials and metal electrode, which can effectively optimize the performance of PSCs. However, traditional ETL materials are not comfortable for commercialization of PSCs due to their inherent characteristics like strong aggregate, relatively low electrical conductivity, and poor ohmic contact with the metal electrodes. Here, a mixed heterojunction layer (MHL) combined an amphiphilic polymer material of poly[(9, 9-bis(3′-(N, N-dimethylamino)propyl)-2, 7-fluo-rene)-alt-2, 7-(9, 9-dioctylfluorene)] with conventional ETL materials [6, 6] phenyl C61-butyric acid methyl ester (PC61BM) was developed to optimize the interface characteristics between perovskite and metal electrode. Atomic force microscopy, the space charge limited current, photoluminescence measurements, and impedance spectroscopy have been measured to reveal the interface contact, electronic, and optical properties of the MHL modified PSCs device. As a result, the power conversion efficiency (PCE) of PSCs with MHL has dramatically increased from 15.17% to 18.39%. More importantly, the stability of the MHL based PSCs was improved significantly, with reduced PCE degradation (from 29% to 18%) after 160 h under ambient conditions. This research provides a simple but effective method to improve the efficiency and stability of PSCs.

Methylammonium chloride as an interface modificator for planar-structure perovskite solar cells with a high open circuit voltage of 1.19V

Recently, the mixed-cation perovskites have been widely used in high-performance perovskite solar cells due to its excellent photoelectric properties. However, the mixed precursor-based method, particularly the complicated composition engineering, will cause phase transitional strain, low phase crystallinity and phase instability. Hence, interface modification engineering is very necessary to improve the phase crystallinity and stability, and then to optimize the performance of the photovoltaic devices. Herein, an interfacial engineering strategy is developed to enlarge the grain size and enhance the crystallinity of perovskite film through inserting a methylammonium chloride (MACl) layer between the SnO2 electron transport layer and perovskite layer. Therefore, the carrier extraction and transport are accelerated and the interfacial recombination is suppressed. Consequently, the optimal MACl-modified devices achieve a very high open circuit voltage of 1.19 V and a power conversion efficiency of 19.20% with the negligible hysteresis and enhanced stability in ambient condition, showing great superiority to the control devices without any modification.

Multifunctional Polymer-Regulated SnO2 Nanocrystals Enhance Interface Contact for Efficient and Stable Planar Perovskite Solar Cells.

Perovskite solar cells (PSCs) have rapidly developed and achieved power conversion efficiencies of over 20% with diverse technical routes. Particularly, planar-structured PSCs can be fabricated with low-temperature (≤150 °C) solution-based processes, which is energy efficient and compatible with flexible substrates. Here, the efficiency and stability of planar PSCs are enhanced by improving the interface contact between the SnO2 electron-transport layer (ETL) and the perovskite layer. A biological polymer (heparin potassium, HP) is introduced to regulate the arrangement of SnO2 nanocrystals, and induce vertically aligned crystal growth of perovskites on top. Correspondingly, SnO2 -HP-based devices can demonstrate an average efficiency of 23.03% on rigid substrates with enhanced open-circuit voltage (VOC ) of 1.162 V and high reproducibility. Attributed to the strengthened interface binding, the devices obtain high operational stability, retaining 97% of their initial performance (power conversion efficiency, PCE > 22%) after 1000 h operation at their maximum power point under 1 sun illumination. Besides, the HP-modified SnO2 ETL exhibits promising potential for application in flexible and large-area devices.

Interfacial defects passivation using fullerene-polymer mixing layer for planar-structure perovskite solar cells with negligible hysteresis

Recently, planar-structure perovskite solar cells processed by low- temperature solution method have attracted tremendous attention. However, the interfacial carrier recombination limits the further improvement in efficiency and causes serious hysteresis. Therefore, the effective interface passivation techniques are necessary to further improve the efficiency of perovskite solar cells (PSCs). In this work, we demonstrate that an ultrathin mixture layer of PMMA:PCBM with the optimization ratio of 1:2 can effectively passivate defects at the electron transport layer/perovskite interface, thus significantly inhibiting the interfacial carrier recombination, contributing to the improved open-circuit voltage (Voc) and negligible hysteresis effect. The PMMA: PCBM mixture passivation layer elevates the Voc of triple-cation perovskite (Cs0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3) solar cells by 40 mV, with champion device achieving a Voc of 1.17 V. The maximum efficiency of the passivated PSCs is 18.63% with negligible hysteresis. Moreover, the interfacial passivation method improves the long-term stability of PSCs, which retains 82% initial performance after over 768 h of storage at room temperature. This passivation approach successfully provides an avenue for further improving the efficiency and stability of PSCs.

Enhanced carrier separation efficiency and performance in planar-structure perovskite solar cells through an interfacial modifying layer of ultrathin mesoporous TiO2

The structure of interface between electron transport layer (ETL) and perovskite material is crucially important for a perovskite solar cell (PSC), especially for its carrier separation efficiency and hysteresis effect. Here, an ultrathin mesoporous TiO2 (thin-m-TiO2) layer is inserted to the interface between the compact TiO2 and the perovskite film in a planar PSC, serving as an interfacial modifying layer. The results show that the insertion of thin-m-TiO2 greatly improves the compactness and the grain size of perovskite films. Moreover, compared with traditional planar and mesoporous PSCs, the cells with an interfacial modifying layer exhibit improved performance. An average photoelectric conversion efficiency of 18.5% and a hysteresis coefficient as low as 4.5% are achieved. It is found that the interfacial modifying layer can not only enhance the carrier separation efficiency, but also maintain a high electron transport capacity. This work provides a feasible and facile strategy for developing high-performance PSCs.

High-performance perovskite solar cells based on passivating interfacial and intergranular defects

Planar-structure perovskite solar cells have attracted more and more attention, because their simple and low-temperature preparation processing. However, the performance of perovskite solar cells is currently limited by defect-induced recombination at interfaces between perovskite and charge transport layers. In this work, a filmy poly methyl methacrylate (PMMA) layer introduced in Perovskite/Spiro-OMeTAD interface to passivate the interfacial and interganular defects, by which a high open-circuit voltage (1.18 V) is acquired, and the optimal device shows a steady-state power conversion efficiency of 20.5% and negligible hysteresis. Femtosecond transient absorption measurement confirms a significant reduction in non-radiative recombination for passivated devices. Mott-Schottky measurement indicates improved flat band potential and carrier density in passivated devices, consisting with the increased voltage. In addition, PMMA film can protect perovskite film from moisture and oxygen erosion. The unsealed device still maintains 95% of the initial efficiency under ambient conditions with 60% relative humidity for one month. This approach solves one of the main limitations of interfacial recombination and shows its potential to improve the performance of perovskite solar cells in the future.

Improving the power conversion efficiency of perovskite solar cells by adding carbon quantum dots

High-quality perovskite films are the key factor in manufacturing high-performance devices. In this work, we for the first time use carbon quantum dots (CQDs) as additive in the methylammonium iodide solution for high-quality CH3NH3PbI3 (MAPbI3) films. Appropriate concentration of CQDs (0.04 mg ml−1) can passivate the crystal defects, improve the crystallinity, increase the grain size and reduce the grain boundary of MAPbI3 film. Various characterization results show that CQDs additive can reduce trap-state density, decrease carrier recombination and improve photoelectric performance. Compared with the pristine device, the average power conversion efficiency (PCE) of CQD-modified MAPbI3 device was increased from 15.8 ± 0.37% to 18.81 ± 0.45%, and the maximum PCE of champion device was increased from 16.21 to 19.17%. In this study, we propose a simple method for the preparation of stable and efficient inverted planar structure perovskite solar cells using CQD additives in a two-step spin-coated deposition method.

UV‐Inert ZnTiO3 Electron Selective Layer for Photostable Perovskite Solar Cells

AbstractAlthough planar‐structured perovskite solar cells (PSCs) have power conversion efficiencies exceeding 24%, the poor photostability, especially with ultraviolet irradiance (UV) severely limits commercial application. The most commonly‐used TiO2 electron selective layer has a strong photocatalytic effect on perovskite/TiO2 interface when TiO2 is excited by UV light. Here a UV‐inert ZnTiO3 is reported as the electron selective layer in planar PSCs. ZnTiO3 is a perovskite‐structured semiconductor with excellent chemical stability and poor photocatalysis. Solar cells are fabricated with a structure of indium doped tin oxide (ITO)/ZnTiO3/Cs0.05FA0.81MA0.14PbI2.55Br0.45/Sprio‐MeOTAD/Au. The champion device exhibits a stabilized power conversion efficiency of 19.8% with improved photostability. The device holds 90% of its initial efficiency after 100 h of UV soaking (365 nm, 8 mW cm−2), compared with 55% for TiO2‐based devices. This work provides a new class of electron selective materials with excellent UV stability in perovskite solar cell applications.

Surface‐Modified Metallic Ti3C2Tx MXene as Electron Transport Layer for Planar Heterojunction Perovskite Solar Cells

AbstractMXenes are a large and rapidly expanding family of 2D materials that, owing to their unique optoelectronic properties and tunable surface termination, find a wide range of applications including energy storage and energy conversion. In this work, Ti3C2Tx MXene nanosheets are applied as a novel type of electron transport layer (ETL) in low‐temperature processed planar‐structured perovskite solar cells (PSCs). Interestingly, simple UV‐ozone treatment of the metallic Ti3C2Tx that increases the surface TiO bonds without any change in its bulk properties such as high electron mobility improves its suitability as an ETL. Improved electron transfer and suppressed recombination at the ETL/perovskite interface results in augmentation of the power conversion efficiency (PCE) from 5.00% in the case of Ti3C2Tx without UV‐ozone treatment to the champion PCE of 17.17%, achieved using the Ti3C2Tx film after 30 min of UV‐ozone treatment. As the first report on the use of pure MXene layer as an ETL in PSCs, this work shows the great potential of MXenes to be used in PSCs and displays their promise for applications in photovoltaic technology in general.

Enhancing Planar Perovskite Solar Cell Performance by Doping Zr in TiOx Layer.

In this work, a Zr-doped TiOx layer is introduced into p-i-n planar-structure perovskite solar cells via a simple sol-gel method. The prepared device configuration is ITO/PEDOT:PSS/CH₃NH₃PbIxCl3-x/PC60BM/Zr-TiOx/Al. The performance of the perovskite solar cells was investigated through measurements of the current-voltage characteristics, impedance analysis, and photocurrent decay. A power conversion efficiency of 15.43% was measured under 1 sun condition; this level of efficiency is 27% higher than that of the device with a Zr-free TiOx layer. Additionally, the incorporation of Zr into the TiOx layer improved the carrier extraction as a result of the oxygen vacancy trap passivation. Lastly, the trap passivation of the Zr-TiOx film was confirmed by analyzing the photoluminescence spectrum.

Achieving High Open-Circuit Voltage on Planar Perovskite Solar Cells via Chlorine-Doped Tin Oxide Electron Transport Layers.

The open-circuit voltage deficit is one of the main limiting factors for the further performance improvement in planar structured perovskite solar cells. In this work, we elaborately develop chlorine binding on the surface of tin oxide electron transport layer for a high open-circuit voltage device (1.195 V). The chlorine passivation on SnO2 not only effectively mitigates the interfacial charge recombination between SnO2 and perovskite but also enhances the binding of chlorine with lead at the SnO2/perovskite interface. The chlorine-passivated SnO2 electron transport layer exhibits a better energy alignment with the perovskite layer and an improved electron mobility, which will promote efficient electron transfer at the interface. In addition, the elevated Fermi level of SnO2 electron transport layer increases carrier extraction and suppresses interfacial recombination, which is responsible for the open-circuit voltage enhancement. Planar perovskite solar cells with chlorine-passivated SnO2 exhibit a higher open-circuit voltage of 1.195 V than that of reference ones (1.135 V) for a lower band gap of 1.58 eV perovskite absorbers, which achieve a power conversion efficiency of 20% with negligible hysteresis.

Design of an Inorganic Mesoporous Hole-Transporting Layer for Highly Efficient and Stable Inverted Perovskite Solar Cells.

The unstable feature of the widely employed organic hole-transporting materials (HTMs) (e.g., spiro-MeOTAD) significantly limits the practical application of perovskite solar cells (PSCs). Therefore, it is desirable to design new structured PSCs with stable HTMs presenting excellent carrier extraction and transfer properties. This work demonstrates a new inverted PSC configuration. The new PSC has a graded band alignment and bilayered inorganic HTMs (i.e., compact NiOx and mesoporous CuGaO2 ). In comparison with planar-structured PSCs, the mesoporous CuGaO2 can effectively extract holes from perovskite due to the increased contact area of the perovskite/HTM. The graded energy alignment constructed in the ultrathin compact NiOx , mesoporous CuGaO2 , and perovskite can facilitate carrier transfer and depress charge recombination. As a result, the champion device based on the newly designed mesoscopic PSCs yields a stabilized efficiency of ≈20%, which is considered one of the best results for inverted PSCs with inorganic HTMs. Additionally, the unencapsulated PSC device retains more than 80% of its original efficiency when subjected to thermal aging at 85 °C for 1000 h in a nitrogen atmosphere, thus demonstrating superior thermal stability of the device. This study may pave a new avenue to rational design of highly efficient and stable PSCs.

Improving photovoltaic performance of inverted planar structure perovskite solar cells via introducing photogenerated dipoles in the electron transport layer

Organic-inorganic hybrid perovskites have attracted great attentions for photovoltaic applications due to impressive energy conversion efficiency and low cost fabrication. This article reports the effect of photogenerated dipoles in the electron transport layer (ETL) on the photovoltaic actions in perovskite solar cells (PSCs). A nonfullerene material ITIC with optically induced dipoles was doped into PCBM acting as ETL in inverted planar structure PSCs (ITO/PEDOT:PSS/MAPbI3-xClx/ETL/PEI/Ag). The power conversion efficiency (PCE) of PSCs increases from 13.48% to 14.52% with introducing ITIC into ETL. The capacitance measurements indicate that optically induced dipoles can decrease interfacial charge accumulation. Furthermore, impedance spectroscopy suggests the charge recombination as well as ionic migration are reduced with ITIC doped ETL. Steady and transient PL spectra reveal that the electron extraction is more efficient with optically induced dipoles in the ETL. These results provide unique design of ETL in perovskite solar cells.

Detailed Investigation of Evaporated Perovskite Absorbers with High Crystal Quality on Different Substrates.

Dual-source vapor-phase deposition enables low-temperature fabrication of high-performance planar structure perovskite (CH3NH3PbI3) solar cells (PSCs), applicable in tandem devices or for industrial production with high homogeneity. Herein, we report low-temperature fabrication of high-efficiency PSCs by dual-source vapor-phase deposition and significance of TiO2 surface modification with [6,6]-phenyl C61 butyric acid methyl ester (PCBM) on cell performance. Co-evaporation of PbI2 and CH3NH3I, as confirmed by X-ray diffraction and high-resolution transmission electron microscopy analyses, results in CH3NH3PbI3 layers with a well-crystallized tetragonal phase formed on both TiO2 and TiO2/PCBM electron-transport layers (ETLs). The devices with PCBM interlayer between TiO2 and CH3NH3PbI3 showed remarkably higher performance than those with TiO2 only, which was attributed to enhance charge extraction and reduced recombination at the TiO2/PCBM/CH3NH3PbI3 interface. The devices composed of evaporated CH3NH3PbI3 on top of the TiO2/PCBM and [2,2',7,7'-tetrakis( N, N-di- p-methoxyphenyl-amine)-9,9'-spirobifluorene] (Spiro-OMeTAD) as hole-transport material demonstrated power conversion efficiencies of 17.1% (reverse scan) and 13.4% (forward scan) with stabilized efficiency of over 16%, which is, to the best of our knowledge, the highest efficiency reported for evaporated perovskite solar cells using low-temperature fabrication method involving compact TiO2 layer as ETL. Furthermore, we show that this process can be used to deposit a CH3NH3PbI3 layer on top of a textured silicon substrate, which is the first step for preparing perovskite-silicon tandem devices with enhanced antireflection and light-trapping properties.

Exposed the mechanism of lead chloride dopant for high efficiency planar-structure perovskite solar cells

Although the PbCl2 has been exploited to improve the performance of perovskite solar cells, the mechanism of enhancement by the introducing PbCl2 is not fully understudied. In present work, we used the PbCl2 as a dopant to improve the perovskite film quality, we found the crystalline and grain size of perovskite film both increased with 10% molar ratio of PbCl2. The in situ time-resolved grazing incidence wide-angle X-ray scattering measurements revealed that more pre-nucleated suspended in the solution when employed the PbCl2 dopant, leading to high crystalline for perovskite film. An incorporation of the space charge limit current, photoluminescence, time-resolved photoluminescence measurements, we uncover the function of PbCl2 in reduced trap density, decreased charge recombination and enhanced carrier lifetime. As a result, the efficiency of planar-structure perovskite solar cells based on PbCl2 enhanced to as high as 19.79% with reduced hysteresis, increased by ∼23% compared to the device without PbCl2 (16.05%). We believe that this work would provide a fundamental prospective for mixed perovskite and promote its application in perovskite photovoltaics.

Balancing transformation and dissolution–crystallization for pure phase CH3NH3PbI3 growth and its effect on photovoltaic performance in planar-structure perovskite solar cells

In situ transformation and dissolution–crystallization mechanisms play a competing role in determining the characteristics of perovskite films that greatly affect the device performance of perovskite solar cells in the sequential two-step process. Herein, we develop a facile solution engineering to balance the transformation from PbI2 to CH3NH3PbI3 and dissolution–crystallization of CH3NH3PbI3 crystal growth, producing pure phase CH3NH3PbI3 crystals for high-efficient planar-structure solar cells. Low concentration of CH3NH3I in a mixed solvent of isopropanol/cyclohexane with low polarity is applied to suppress dissolution–crystallization (Ostwald ripening growth) of perovskite, while increases the transformation time from PbI2 to CH3NH3PbI3. Combination of porous PbI2 and temperature-assistance effectively promote the transformation from PbI2 to CH3NH3PbI3 and reduce the time of Ostwald ripening growth of perovskite. This solution engineering reconciles the complete PbI2 transformation and dissolution–crystallization of CH3NH3PbI3, resulting in a pure phase perovskite without any residual PbI2 in a short time. This strategy exemplified here can serve in the design and development of more sophisticated perovskites based on planar-structure applications without mesoporous TiO2 scaffold.

Synergistic improvement of perovskite film quality for efficient solar cells via multiple chloride salt additives

Perovskite crystal film quality is critical for obtaining efficient perovskite solar cells. Anti-solvent processing was used for fast crystallization of perovskite precursor film, which can form dense perovskite film. However, the crystals from this method are usually small due to the fast crystal growth process, which could lead to grain boundary recombination. Here, element chloride is introduced to enhance the perovskite layer crystallinity via slowing down the perovskite crystallization process by simultaneous introduction of methylammounium chloride (MACl) and cesium chloride (CsCl) into precursor solution. As a result, we achieve high quality of pin-hole free perovskite film with large crystal size. A power conversion efficiency of 21.55% with free of hysteresis of the device is obtained, which is among the highest efficiency of planar structure perovskite solar cells.