Mesoscopic Perovskite Solar Cells Research Articles

Improving the Performance of Perovskite Solar Cells via a Novel Additive of N,1‐Fluoroformamidinium Iodide with Electron‐Withdrawing Fluorine Group

AbstractAdditives are widely adopted for efficient perovskite solar cells (PSCs), and proper additive design contributes a lot to PSCs’ various breakthroughs. Herein, a novel additive of N,1‐fluoroformamidinium iodide (F‐FAI), whose cation replaces one amino group in guanidinium (GA+) with electron‐withdrawing fluorine group, is synthesized and applied as the additive for PSCs. The electron‐withdrawing effect of fluorine promotes the molecular polarity of N,1‐fluoroformamidine (F‐FA), enhancing the interaction of N,1‐fluoroformamidinium (F‐FA+) with MAPbI3. Compared with the nonpolar GA+, F‐FA+ improves the crystallinity, passivates the defect, and downshifts the Fermi level of MAPbI3 more significantly. The charge transfer and built‐in field in printable triple mesoscopic PSCs are therefore enhanced. Moreover, charge transport in MAPbI3 is also promoted by F‐FAI. With these benefits, a power conversion efficiency of 17.01% for printable triple mesoscopic PSCs with improved open‐circuit voltage and fill factor is obtained with the addition of F‐FAI, superior to the efficiency of 15.24% for those devices with guanidinium iodide additives.

Improving Hole‐Conductor‐Free Fully Printable Mesoscopic Perovskite Solar Cells’ Performance with Enhanced Open‐Circuit Voltage via the Octyltrimethylammonium Chloride Additive

Hole‐conductor‐free fully printable mesoscopic perovskite solar cells (MPSCs) based on mp‐TiO2/mp‐ZrO2/carbon triple mesoscopic layers are competitive candidates among various rapidly developed PSCs for future photovoltaic applications due to the characteristics of low‐cost, easy upscaling, and superior stability. However, the open‐circuit voltage (VOC) loss in printable MPSCs is relatively large compared to that in conventional PSCs, deteriorating the power conversion efficiency (PCE). Herein, the VOC loss is reduced by the octyltrimethylammonium chloride (OTAC) additive. OTAC is found to upshift the Fermi level of TiO2 and passivate trap states in bulk MAPbI3 perovskite, thus optimizing the energy‐level alignment of the TiO2/perovskite heterojunction and suppressing nonradiative recombination in devices. As a result, MPSCs deliver the highest PCE of 16.53% with an improved VOC of 1007 mV. The work demonstrates a facile strategy to reduce the VOC loss in printable MPSCs by simultaneously optimizing the energy‐level alignment and suppressing nonradiative recombination.

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.

Improved photovoltage of printable perovskite solar cells via Nb5+ doped SnO2 compact layer

The state-of-the-art perovskite solar cells (PSCs) with SnO2 electron transporting material (ETL) layer displays the probability of conquering the low electron mobility and serious leakage current loss of the TiO2 ETL layer in photoelectronic devices. The rapid development of SnO2 ETL layer has brought perovskite efficiencies >20%. However, high density of defect states and voltage loss of high temperature SnO2 are still latent impediment for the long-term stability and hysteresis effect of photovoltaics. Herein, Nb5+ doped SnO2 with deeper energy level is utilized as a compact ETL for printable mesoscopic PSCs. It promotes carrier concentration increase caused by n-type doping, assists Fermi energy level and conduction band minimum to move the deeper energy level, and significantly reduces interface carrier recombination, thus increasing the photovoltage of the device. As a result, the use of Nb5+ doped SnO2 brings high photovoltage of 0.92 V, which is 40 mV higher than that of 0.88 V for device based on SnO2 compact layer. The resulting PSCs displays outstanding efficiency of 13.53%, which contains an ∼10% improvements compared to those without Nb5+ doping. Our study emphasizes the significance of element doping for compact layer and lays the groundwork for high efficiency PSCs.

Solution-processed ITO nanoparticles as hole-selective electrodes for mesoscopic lead-free perovskite solar cells

All-solution-processable ITO nanoparticulate electrodes were developed to replace carbon electrodes in mesoscopic hybrid tin-based PSCs to attain a record efficiency of 5.4%.

3D graphene nanosheets from plastic waste for highly efficient HTM free perovskite solar cells.

Herein, we report the first time application of waste plastic derived 3D graphene nanosheets (GNs) for hole transport material (HTM) free perovskite solar cells (PSCs), where 3D GNs have been employed as an electrode dopant material in monolithic carbon electrode based mesoscopic PSCs. Waste plastics were upcycled into high-quality 3D GNs by using two-step pyrolysis processes, where, a nickel (99.99%) metal mesh was taken as the catalytic and degradation template to get an acid free route for the synthesis of 3D GNs. Raman spectroscopy, HRTEM analysis and XRD analysis show the presence of 1–2 graphene layers within the 3D GNs. Further, the optical band gap study has also been performed to analyze the applicability of 3D GNs for PSCs. The optimized device with 3D GNs shows a power conversion efficiency (PCE) of 12.40%, whereas the carbon-based control device shows a PCE of 11.04%. Further, all other device parameters such as short circuit current (Jsc), open circuit voltage (Voc) and fill factor (FF) have been improved with the addition of 3D GNs. The performance enhancement in 3D GN doped HTM free PSC solar cells is attributed to the enhancement in conductivity and reduced recombination within the device. Further, the photocurrent study shows that the 3D GN device shows better performance as compared to the reference device due to the larger diffusion current. Thus, the upcycling of waste plastics into 3D GNs and their exploitation for application in energy conversion show an effective and potential way to convert waste into energy.

A 2D Model for Interfacial Recombination in Mesoscopic Perovskite Solar Cells with Printed Back Contact

A physical model to explain the 2D charge recombination in mesoscopic graphite‐based perovskite solar cells (PSCs) having a highly selective front electrode and a nonselective back electrode is presented. Steady‐state photovoltage and photoluminescence (PL) as well as transient PL are studied for a wide range of device configurations, providing insights in the interface recombination at the front and back contact, namely, the mesoporous titania (m‐TiO2) and the graphite layer. Combining experimental evidence with the first 2D simulation of a perovskite solar cell, it is found that the characteristic thick absorber layer of mesoscopic graphite‐based PSC is a necessity to enhance the photovoltage. This is because the interface recombination at the back contact is a diffusion‐limited process. The electrode spacing should, however, not be enhanced by increasing the perovskite/m‐TiO2 thickness as this increases surface recombination losses at this interface. The study determines design rules for the optimal geometry of the mesoporous layers and helps to identify the limiting recombination pathways for an improvement of future device architectures.

Low-temperature fabrication of carbon-electrode based, hole-conductor-free and mesoscopic perovskite solar cells with power conversion efficiency > 12% and storage-stability > 220 days

A low-temperature fabrication routine is developed for hole-conductor-free and mesoscopic perovskite solar cells using a TiO2 nanoparticle-binding carbon electrode as the top electrode. Vacuum treatment is adopted to help the infiltration and formation processes of the organic–inorganic hybrid perovskite crystallites. It is observed that such treatment not only condenses the mesoporous skeleton and improves film conductance of the carbon electrode but also makes the perovskite crystallites grow in the core part of the mesoporous skeleton. As such, the extraction process of photogenerated charge carriers is accelerated due to the strengthened interfacial contact between the perovskite crystallites and the skeleton. Accordingly, the photo-to-electric power conversion efficiency of the low-temperature devices is upgraded from 7.38 (±1.40)% to 10.17 (±0.86)% (optimized at 12.29%, AM 1.5 G, 100 mW/cm2). In addition, prolonged stability is observed. Due to the condensed device structure, storage stability of 225 days has been achieved in ambient air (with relative humidity of about 40–60%), even without encapsulation. The proposed strategy is helpful in further reducing the production cost.

Improved Pore-Filling and Passivation of Defects in Hole-Conductor-Free, Fully Printable Mesoscopic Perovskite Solar Cells Based on d-Sorbitol Hexaacetate-Modified MAPbI3.

For hole-conductor-free, fully printable mesoscopic perovskite solar cells (MPSCs), it is difficult to achieve free and efficient diffusion of perovskite precursors in micron-scale porous structures. Thus, the wettability of the perovskite precursor is one of the most crucial factors that determine the performance of MPSCs. Here, d-sorbitol hexaacetate (DSHA) is introduced as an additive for fabricating hole-conductor-free, fully printable MPSCs based on methylammonium lead iodide (MAPbI3). The fabricated MPSCs exhibited an efficiency of 14.33%. Moreover, the influence of DSHA on the optical properties, morphology, and filling of perovskite in the MPSCs has been systematically investigated. The results revealed that DSHA effectively optimized the morphology, improved the pore-filling, and passivated the defects of perovskite films. Remarkably, the unencapsulated MPSCs retained 93% of their original power-conversion efficiency (PCE) after 45 days of storage in air with humidity of 50 ± 5%.

Graphene‐Based Materials in Planar Perovskite Solar Cells

Metal halide perovskite solar cells (PSCs) have recently become the most promising new‐generation solar cells, with a breathtaking growth of efficiency from 3.8% to 25.2% in just one decade. Scientists have abandoned the traditional high‐temperature‐processed mesoscopic layer of the initial mesoscopic PSCs in designing and manufacturing planar PSCs. This new feature endows planar PSCs with possibilities of low‐temperature processibility and large‐scale production. Nevertheless, the advancement of planar PSCs remains limited by two bottlenecks: charge loss and device degradation. To address these two issues, researchers have adopted graphene‐based materials, which demonstrate tremendous potentials due to their superb optical transparency, outstanding carrier mobility, remarkable electrical conductivity, and superior physicochemical stability. Defects inside films and at interfaces are regulated by graphene, thereby contributing to more efficient charge extraction and suppressed charge recombination. The graphene protective layer enhances the moisture and heat stability of planar PSCs, thereby extending the lifetime of devices. Herein, the typical synthesis methods of graphene and the recent applications of graphene in planar PSCs are summarized and discussed. Furthermore, concluding perspectives on current challenges and the future development of graphene in planar PSCs are proposed.

Efficient Bifacial Passivation Enables Printable Mesoscopic Perovskite Solar Cells with Improved Photovoltage and Fill Factor

Surface defects, which mediate nonradiative recombination, are detrimental to both the photovoltaic performance and stability of perovskite solar cells (PSCs). Improving photovoltage and fill factor (FF) in screen‐printed mesoporous PSCs is a major challenge for approaching the power conversion efficiency (PCE) of the planar configured devices. Herein, a novel bifacial passivation strategy which simultaneously suppresses deep trap states within TiO2 and perovskite through interaction between functional groups and defects is demonstrated for fully printable mesoscopic PSCs. The application of monoethanolamine (MEA) treatment to TiO2 surface not only reduces the energy barrier between TiO2 and perovskite for accelerating the charge transfer but also passivates the uncoordinated Pb defects on the perovskite interface. Due to the synergistic effect of charge extraction promotion and trap passivation, the fabricated PSCs deliver a champion PCE of 15.5% with an enhanced Voc of 0.94 V and FF of 70.4% compared with PSCs without MEA passivation, and the device maintains 97% of its topmost PCE after 240 h under constant simulated solar illumination in air atmosphere. This investigation helps exploit new approaches for defect passivation to further improve both the efficiency and stability of printable mesoscopic PSCs.

Enhanced efficiency and reduced hysteresis by TiO2 modification in high-performance perovskite solar cells

Recent research has focused on increasing the open-circuit voltage (VOC) and current density (JSC) of perovskite solar cells (PSCs) by introducing p- or n-type dopants with higher electronegativity than Ti into the TiO2 electron transport layer (ETL). However, these kinds of dopant create undesired charge traps that hinder charge transport through TiO2. Therefore, the improvement in VOC is often accompanied by an undesired current density–voltage (J–V) hysteresis problem. Herein, we demonstrate that the introduction of 4-chlorobenzoic acid (4-ClBA-TiO2) dopant into TiO2 not only overcomes the J–V hysteresis issue but also increases the VOC, JSC and power conversion efficiency (PCE) in mesoscopic PSCs. Also, the material shown the better device performance compared to the state-of-art ETL, with the 4-CLBA-TiO2. We speculate that the interaction between the 4-ClBA and the perovskite interface is more selective for electrons. Further the 4-ClBA-TiO2 electron mobility has been improved 1.9 times compared with TiO2. As a result, for mesoscopic PSCs, the doping of 4-ClBA-TiO2 increases the efficiency from 18.23% to 20.22%, while the hysteresis is largely reduced from 20.2% to 1.5%. To the best of our knowledge, this is the first report using the spin coating blocking layer to achieve over 20%. Thus, we believe that this approach will be an effective design strategy capable of enhancing the performance of PSCs with less hysteresis.

Enhanced photovoltaic performance and reduced hysteresis in hole-conductor-free, printable mesoscopic perovskite solar cells based on melamine hydroiodide modified MAPbI3

The fabrication of high-performance, hole-conductor-free, printable mesoscopic perovskite solar cells (PMPSCs) is still a major challenge because it is difficult to realize the free diffusion of precursors throughout the micrometer-scale porous structure. Here, a facile and efficient additive manufacturing technique is proposed with melamine hydroiodide (MLAI) as an additional material. We systematically investigated the impact of MLAI on the crystallization, morphology, and defects of the perovskite film and the performance of the PMPSCs. The results revealed that the MLAI effectively optimized the morphology of perovskite and improved the pore-filling as well as passivated the defects of the perovskite film, thus hysteresis was reduced and the performance of the devices was enhanced. Based on the addition of MLAI, the power conversion efficiency (PCE) was obtained as 13.86%, which was 18.5% higher than that of the additive-free device. The additive engineering technique exhibits immense potential for large-scale production and application of PMPSCs.

Influence of precursor concentration on printable mesoscopic perovskite solar cells.

Over the last decade, the power conversion efficiency of hybrid organic-inorganic perovskite solar cells (PSCs) has increased dramatically from 3.8% to 25.2%. This rapid progress has been possible due to the accurate control of the morphology and crystallinity of solution-processed perovskites, which are significantly affected by the concentration of the precursor used. This study explores the influence of precursor concentrations on the performance of printable hole-conductor-free mesoscopic PSCs via a simple one-step drop-coating method. The results reveal that lower concentrations lead to larger grains with inferior pore filling, while higher concentrations result in smaller grains with improved pore filling. Among concentrations ranging from 0.24-1.20 M1), devices based on a moderate strength of 0.70 M were confirmed to exhibit the best efficiency at 16.32%.

Determination of the nonradiative conversion efficiency of lead mixed-halide perovskites using optical and photothermal spectroscopy.

Mixed-halide organic-inorganic hybrid perovskites are considered promising light-absorbing materials in the development of solar cells related to the obtained high-power conversion efficiency. Current efforts are focused on the study of the energy-conversion mechanisms, where the nonradiative recombination pathway is the least explored. In this work, a combination of optical and photoacoustic spectroscopies is used to determine the visible spectral light-into-heat conversion efficiency of lead-based mixed-halide organic-inorganic hybrid perovskites in a semicomplete n-i-p mesoscopic perovskite solar cell (PSC). A remarkable average conversion efficiency of about 87% has been found for the nonradiative combination in the perovskite, with the estimated composition ${{\rm FA}_{0.71}}{{\rm MA}_{0.29}}{{\rm PbI}_{2.9}}{{\rm Br}_{0.1}}$FA0.71MA0.29PbI2.9Br0.1 in the wavelength range of 400 to 800 nm. As a result, 13% of the incident light is transformed in radiative recombination processes and/or photodegradation of the material. Furthermore, the extinction coefficient and refractive index of the material are reported, and it was found that the optical constants and the optical absorption in the short-wavelength range are significantly smaller than previously reported for${{\rm MAPbI}_3}$MAPbI3.

Suppressing Shallow Defect of Printable Mesoscopic Perovskite Solar Cells with a N719@TiO2 Inorganic–Organic Core–Shell Structured Additive

Shallow defects are one of the energy states that trap photoexcited electrons leading to charge recombination and limit the increase in the photocurrent of perovskite solar cells (PSCs). Due to the large perovskite thickness and uncontrollable crystallization processes, suppressing shallow defects, especially methylamine (MA) vacancies, has become a key challenge for fully printable PSCs. Herein, nano‐TiO2 is unprecedentedly used to load the commercial dye N719, forming N719@TiO2 nanoparticles, which crucially improves the passivation effect of MA vacancies on the surface of perovskite and charge extraction, by the unbounded carboxyl group of N719 as a shell on the surface of TiO2. Meanwhile, the core TiO2 serves as a centre to bind the dyes, assisting the perovskite crystallization and enhancing the passivation effect. It is found that the charge extraction increases to 1.8007 × 10−9 C for the devices based on N719@TiO2 from 1.5507 × 10−9 C for the control group. Simultaneously, the short‐circuit current density (Jsc) is significantly enhanced to 23.58 mA cm−2 in the device containing N719@TiO2 over that of the control device (21.95 mA cm−2). This opens up a novel pathway to reduce shallow defects in PSCs via organic passivator with carboxyl anchoring group loaded on n‐type semiconductors (nano‐TiO2).

Impact of the implementation of a mesoscopic TiO2 film from a low-temperature method on the performance and degradation of hybrid perovskite solar cells

High efficiencies of over 20% have been reported in the literature for both planar and mesoscopic hybrid perovskite solar cells, and the preferred configuration for scale-up and commercialization is still a matter of debate. The mesoscopic configuration generally requires a high-temperature processing step, which limits applications and makes the process less cost-effective. We have used low-temperature (LT) processing (≤120 °C) to fabricate high-efficiency planar and mesoscopic TiO2-based hybrid perovskite solar cells with comparable performance, highlighted by a champion LT mesoscopic solar cell with 16.2% efficiency. Photovoltaic efficiencies of 14–16% have been achieved for a mesoporous film thickness ranging from 120 to 480 nm by fine-tuning the precursor solution chemistry. The presence of the LT mesoporous layer improves the preservation of performance under conditions of relative humidity of 60%, especially under illumination. Impedance spectroscopy illustrates a similarity of the locus and kinetics of the recombination processes for both configurations. However, inductive loops usually related to ion migration are observed showing different characteristics between both configurations, pointing to the previously suggested correlation between ion migration and degradation. These results suggest that the beneficial role of a mesoporous TiO2 layer might be the stabilization of harmful defects at the perovskite/electron extraction layer interface, and indicate that interface engineering is critical to achieving improved long-term performance.

Ionic liquid doped organic hole transporting material for efficient and stable perovskite solar cells

As a hole transporting material (HTM), N2,N2,N2′,N2′,N7,N7,N7′,N7′-octakis (4-methoxyphenyl) spiro [fluorene-9,9′-xanthene]-2,2′,7,7′-tetraamine (X60) in mesoscopic perovskite solar cells (PSCs) has been widely utilized for substitution of the 2,2′,7,7′-tetrakis (N,N-di-p-methoxyphenylamine)-9,9′-spiro-bi-fluorene (spiro-OMeTAD). In this study, we have introduced an ionic liquid N-butyl-N'-(4-pyridylheptyl) imidazolium bis (trifluoromethane) sulfonamide (BuPyIm-TFSI) as a p-dopant to increase the hole conductivity and stability of the X60 based perovskite solar cells. As a result, based on the different concentrations of BuPyIm-TFSI in mesoscopic PSCs, the optimal condition (4.85 mM) showed the best power conversion efficiency (PCE) of 14.65%, which is extremely higher than the device without BuPyIm-TFSI. Moreover, the device based on X60: BuPyIm-TFSI composite HTM at ambient conditions with humidity of ~40% exhibited good PSCs performance with the long-term stability of 840 h. Hence, the use of BuPyIm-TFSI as a p-dopant for X60 played a significant role in enhancing the electrical properties, stability and efficiency of PSCs.

Carbon-based, novel triple cation mesoscopic perovskite solar cell fabricated entirely under ambient air conditions

In the past, various reports on Perovskite Solar Cells (PSCs) have presented significant results. However, due to stability issues, scale-up drawbacks and high fabrication costs, scientists turned their focus on PSCs with a simpler structure, based on carbon electrodes. In this work, we report a carbon-based PSC, embedding a novel, triple cation perovskite, created after combining Methylammonium Iodide (MAI), 5-Aminovaleric Acid Iodide (5-AVAI) and Lead(II) Iodide (PbI2) along with Phenethylammonium Iodide (PEAI) salt. In our Hole Transport Layer (HTL) free device, the perovskite was infiltrated through the conductive porous carbon cathode layer, which is deposited on top of compact/mesoporous TiO2 (c-TiO2/mp-TiO2) and mesoporous insulating ZrO2 (mp-ZrO2) layers, with all processes being carried out under ambient conditions and high relative humidity (40–60%). Finally, the perovskite films (5-AVA)x(MA)1-xPbI3 and (5-AVA)x(PEA)x(MA)1-xPbI3+x are optically/structurally characterized and the electrical performance of the corresponding devices is examined.

Life cycle assessment of hole transport free planar–mesoscopic perovskite solar cells

Organo-metal lead halide perovskite solar cells (PSCs) attract attention due to their low cost and high power conversion efficiency. Some weak points of this technology are short lifetime, instability, and expensive metal electrode deposition. Eliminating the unstable hole transport layer (HTL) and using carbon-based materials as the counter electrode would address both. In this work, we present a cradle-to-gate life cycle assessment of two HTL-free PSC designs, which use solution phase deposition to achieve mesoscopic and planar structures. Environmental impacts of producing 1 m2 PSCs are converted to impacts per kWh electricity generation assuming 5 years of operational lifetime. We find that major impacts come from fluorine doped tin oxide (FTO) glass patterning due to the electricity consumption of FTO patterning and glass cleaning processes. Even though the electricity consumption when manufacturing both PSCs is similar, their different efficiencies make the environmental impacts per kWh of electricity higher for the mesoscopic PSC than for the planar PSC. Energy payback time values of planar PSCs and mesoscopic PSCs are 0.58 and 0.74 years, respectively, and these values are shorter than those of commercial first and second generation solar cells. However, the global warming potential (GWP) values of planar and mesoscopic PSCs are 75 and 94 g CO2-eq/kWh, respectively, and these values are still higher than those of commercial solar cells. To reach the GWP of commercial cells, the operational lifetime would have to be 8 and 10 years for planar and mesoscopic PSCs, respectively.