Temperature dependence of transport properties of spiro-MeOTAD as a hole transport material in solid-state dye-sensitized solar cells

Eliminating Resistance-Capacitance Coupling Shielding for Depicting the Defect Landscape in Perovskite Solar Cells by Capacitance Spectroscopy

Capacitance spectroscopy techniques have been widely utilized to evaluate the defect properties in perovskites, which contribute to the efficiency and operation stability development for perovskite solar cells (PSCs). Yet the interplay between the charge transporting layer (CTL) and the perovskite on the capacitance spectroscopy results is still unclear. Here, they show that a pseudo-trap-state capacitance signal is generated in thermal admittance spectroscopy (TAS) due to the enhanced resistance capacitance (RC) coupling caused by the carrier freeze-out of the CTL in PSCs, which could be discerned from the actual defect-induced trap state capacitance signal by tuning the series resistance of PSCs. By eliminating the RC coupling shielding effect on the defect-induced capacitance spectroscopy, it is obtain the actual defect density which is 4-folds lower than the pseudo-trap density, and the spatial distribution of defects in PSCs which reveals that the commonly adopted interface passivators can passivate the defects about 60 nm away from the decorated surface. It is further revealed that phenethylammonium ions (PEA+) possess a better passivation capability over octylammonium ions (OA+) due to the deeper passivation depth for PEA+ on the surface defects in perovskite films.

Organic Hole-Transport Layers for Efficient, Stable, and Scalable Inverted Perovskite Solar Cells

Hole-transporting layers (HTLs) are an essential component in inverted, p-i-n perovskite solar cells (PSCs) where they play a decisive role in extraction and transport of holes, surface passivation, perovskite crystallization, device stability, and cost. Currently, the exploration of efficient, stable, highly transparent and low-cost HTLs is of vital importance for propelling p-i-n PSCs toward commercialization. Compared to their inorganic counterparts, organic HTLs offer multiple advantages such as a tunable bandgap and energy level, easy synthesis and purification, solution processability, and overall low cost. Here, recent progress of organic HTLs, including conductive polymers, small molecules, and self-assembled monolayers, as utilized in inverted PSCs is systematically reviewed and summarized. Their molecular structure, hole-transport properties, energy levels, and relevant device properties and resulting performances are presented and analyzed. A summary of design principles and a future outlook toward highly efficient organic HTLs in inverted PSCs is proposed. This review aims to inspire further innovative development of novel organic HTLs for more efficient, stable, and scalable inverted PSCs.

Oxidized Spiro-OMeTAD: Investigation of Stability in Contact with Various Perovskite Compositions

The power conversion efficiency of perovskite solar cells (PSCs) has risen steadily in recent years; however, one important aspect of the puzzle remains to be solved-the long-term stability of the devices. We believe that understanding the underlying reasons for the observed instability and finding means to circumvent it is crucial for the future of this technology. Not only the perovskite itself but also other device components are susceptible to thermal degradation, including the materials comprising the hole-transporting layer. In particular, the performance-enhancing oxidized hole-transporting materials have attracted our attention as a potential weak component in the system. Therefore, we performed a series of experiments with oxidized spiro-OMeTAD to determine the stability of the material interfaced with five most popular perovskite compositions under thermal stress. It was found that oxidized spiro-OMeTAD is readily reduced to the neutral molecule upon interaction with all five perovskite compositions. Diffusion of iodide ions from the perovskite layer is the main cause for the reduction reaction which is greatly enhanced at elevated temperatures. The observed sensitivity of the oxidized spiro-OMeTAD to ion diffusion, especially at elevated temperatures, causes a decrease in the conductivity observed in the doped films of spiro-OMeTAD, and it also contributes significantly to a drop in the performance of PSCs operated under prolonged thermal stress.

Reduction of Hysteresis in Hybrid Perovskite Transistors by Solvent-Controlled Growth

The effect of crystallization process speed on the morphology of solution-processed methyl ammonium lead iodide (MAPbI₃) thin films is investigated. Crystallization speed is controlled by varying the number of annealing steps, temperature, and resting time between steps. The resting period allows solvent-controlled growth (SCG) in which crystallization progresses slowly via an intermediate phase—during which solvents slowly evaporate away from the films. SCG results in fewer residues, fewer pinholes, and larger grain sizes. Consequently, thin-film transistors with SCG MAPbI₃ exhibit smaller hysteresis in their current-voltage characteristics than those without, demonstrating the benefits of SCG toward hysteresis-free perovskite devices.

Temperature Coefficients of Perovskite Photovoltaics for Energy Yield Calculations

Temperature coefficients for maximum power (T _PCE), open circuit voltage (V _OC), and short circuit current (J _SC) are standard specifications included in data sheets for any commercially available photovoltaic module. To date, there has been little work on determining the T _PCE for perovskite photovoltaics (PV). We fabricate perovskite solar cells with a T _PCE of -0.08 rel %/°C and then disentangle the temperature-dependent effects of the perovskite absorber, contact layers, and interfaces by comparing different device architectures and using drift-diffusion modeling. A main factor contributing to the small T _PCE of perovskites is their low intrinsic carrier concentrations with respect to Si and GaAs, which can be explained by its wider band gap. We demonstrate that the unique increase in E _g with increasing temperatures seen for perovskites results in a reduction in J _SC but positively influences V _OC. The current limiting factors for the T _PCE in perovskite PV are identified to originate from interfacial effects.

Understanding the thermal degradation mechanism of perovskite solar cells via dielectric and noise measurements

Long term stability is a major obstacle to the success of perovskite solar cell (PSC) photovoltaic technology. PSC performance deteriorates significantly in the presence of humidity, oxygen and exposure to UV light and heat. Here the change in charge transport properties of PSC with temperature and the associated significant drop in device performance at high temperature have been investigated. The latter is shown to be primarily due to an increase in charge carrier recombination, which impacts the open-circuit voltage. To understand the pathway of temperature-induced degradation, low-frequency 1/f noise characteristics, and the capacitance-frequency, as well as capacitance-voltage characteristics have been investigated under various conditions. The results show that at high operating temperature accumulation of ions and charge carriers at the interface increase the surface recombination. Aging experiments at different temperatures show high stability of PSCs up to temperature <70 °C, but a drastic, irreversible degradation occurs at higher temperature (≥80 °C). Low-frequency 1/f noise study revealed that the magnitude of normalized noise in degraded perovskite solar cells is four orders of magnitude higher than the pristine device. This study shows the power of low-frequency noise measurement technique as a highly sensitive non-invasive tool to study the degradation mechanism of PSCs.

Designing Hole Transport Materials with High Hole Mobility and Outstanding Interface Properties for Perovskite Solar Cells

Organic-inorganic halide perovskite solar cells (PSCs) have attracted much attention due to their rapid increase in power conversion efficiencies (PCEs), and many efforts are devoted to further improving the PCEs. Designing highly efficient hole transport materials (HTMs) for PSCs may be one of the effective ways. Herein we theoretically designed three new HTMs (FDT-N, FDT-O, and FDT-S) by introducing a nitrogen-phenyl group, an oxygen atom, and a sulfur atom into the spiro core of an experimentally synthesized HTM (FDT), respectively. And then we performed quantum chemical calculation to study their application potential. The results show that the devices with FDT-O and FDT-S instead of FDT may have higher open circuit voltages owing to their lower highest occupied molecular orbital (HOMO) energy levels. Moreover, FDT-S exhibits the best hole transport performance among the studied HTMs, which may be due to the significant HOMO-HOMO overlap in the hole hopping path with the largest transfer integral. Furthermore, the results on interface properties indicate that introducing oxygen and sulfur atoms can enhance the MAPbI₃ /HTM interface interaction. The present work not only offers two promising HTMs (FDT-O and FDT-S) for PSCs but also provides theoretical help for subsequent research on HTMs.

Recycled Utilization of a Nanoporous Au Electrode for Reduced Fabrication Cost of Perovskite Solar Cells

Perovskite solar cells (PSCs) using metal electrodes have been regarded as promising candidates for next-generation photovoltaic devices because of their high efficiency, low fabrication temperature, and low cost potential. However, the complicated and rigorous thermal deposition process of metal contact electrodes remains a challenging issue for reducing the energy pay-back period in commercial PSCs, as the ubiquitous one-time use of a contact electrode wastes limited resources and pollutes the environment. Here, a nanoporous Au film electrode fabricated by a simple dry transfer process is introduced to replace the thermally evaporated Au electrode in PSCs. A high power conversion efficiency (PCE) of 19.0% is demonstrated in PSCs with the nanoporous Au film electrode. Moreover, the electrode is recycled more than 12 times to realize a further reduced fabrication cost of PSCs and noble metal materials consumption and to prevent environmental pollution. When the nanoporous Au electrode is applied to flexible PSCs, a PCE of 17.3% and superior bending durability of ≈98.5% after 1000 cycles of harsh bending tests are achieved. The nanoscale pores and the capability of the porous structure to impede crack generation and propagation enable the nanoporous Au electrode to be recycled and result in excellent bending durability.

α- and β-Substituted Metal-Free Phthalocyanines: Synthesis, Photophysical and Electrochemical Properties

Two novel phthalonitrile derivatives, bearing two hexyloxy groups and a benzodioxin (or a naphthodioxin) annulated ring, along with their corresponding metal-free phthalocyanines (H2Pc) were prepared. FT-IR, mass, electronic absorption, 1H NMR, and 13C NMR spectroscopy were employed for the characterization of all compounds. The effect of hexadeca substituents on the photophysical properties of metal-free Pcs was investigated. Photophysical properties of H2Pc were studied in tetrahydrofuran (THF). Fluorescent quantum yields of phthalocyanines (Pcs) were calculated and compared with the unsubstituted phthalocyanine. 1,4-Benzoquinone effectively quenched the fluorescence of these compounds in THF. Cyclic and square wave voltammetry methods were applied to metal-free phthalocyanines and Pc-centered oxidation and reduction processes were obtained.

A potassium thiocyanate additive for hysteresis elimination in highly efficient perovskite solar cells

Potassium thiocyanate as a cheap additive effectively eliminates the hysteresis effect of perovskite solar cells.

Influence of hole transport material/metal contact interface on perovskite solar cells

Interfaces have a significant impact on the performance of perovskite solar cells. This work investigated the influence of hole transport material/metal contact interface on photovoltaic behaviours of perovskite solar devices. Different hole material/metal contact interfaces were obtained by depositing the metal under different conditions. High incident kinetic energy metal particles were proved to penetrate and embed into the hole transport material. These isolated metal particles in hole transport materials capture holes and increase the apparent carrier transport resistance of the hole transport layer. Sample temperature was found to be of great significance in metal deposition. Since metal vapour has a high temperature, the deposition process accumulated a large amount of heat. The heat evaporated the additives in the hole transport layer and decreased the hole conductivity. On the other hand, high temperature may cause iodization of the metal contact.

Nanoarchitectures in dye-sensitized solar cells: metal oxides, oxide perovskites and carbon-based materials

Dye-sensitized solar cells (DSSCs) have aroused great interest and been regarded as a potential renewable energy resource among the third-generation solar cell technologies to fulfill the 21^st century global energy demand. DSSCs have notable advantages such as low cost, easy fabrication process and being eco-friendly in nature. The progress of DSSCs over the last 20 years has been nearly constant due to some limitations, like poor long-term stability, narrow absorption spectrum, charge carrier transportation and collection losses and poor charge transfer mechanism for regeneration of dye molecules. The main challenge for the scientific community is to improve the performance of DSSCs by using different approaches, like finding new electrode materials with suitable nanoarchitectures, dyes in composition with promising semiconductors and metal quantum dot fluorescent dyes, and cost-effective hole transporting materials (HTMs). This review focuses on DSSC photo-physics, which includes charge separation, effective transportation, collection and recombination processes. Different nanostructured materials, including metal oxides, oxide perovskites and carbon-based composites, have been studied for photoanodes, and counter electrodes, which are crucial to achieve DSSC devices with higher efficiency and better stability.

Efficient Carbon-Based CsPbBr3 Inorganic Perovskite Solar Cells by Using Cu-Phthalocyanine as Hole Transport Material

Metal halide perovskite solar cells (PSCs) have attracted extensive research interest for next-generation solution-processed photovoltaic devices because of their high solar-to-electric power conversion efficiency (PCE) and low fabrication cost. Although the world's best PSC successfully achieves a considerable PCE of over 20% within a very limited timeframe after intensive efforts, the stability, high cost, and up-scaling of PSCs still remain issues. Recently, inorganic perovskite material, CsPbBr₃, is emerging as a promising photo-sensitizer with excellent durability and thermal stability, but the efficiency is still embarrassing. In this work, we intend to address these issues by exploiting CsPbBr₃ as light absorber, accompanied by using Cu-phthalocyanine (CuPc) as hole transport material (HTM) and carbon as counter electrode. The optimal device acquires a decent PCE of 6.21%, over 60% higher than those of the HTM-free devices. The systematic characterization and analysis reveal a more effective charge transfer process and a suppressed charge recombination in PSCs after introducing CuPc as hole transfer layer. More importantly, our devices exhibit an outstanding durability and a promising thermal stability, making it rather meaningful in future fabrication and application of PSCs.

Effects of High Temperature and Thermal Cycling on the Performance of Perovskite Solar Cells: Acceleration of Charge Recombination and Deterioration of Charge Extraction

In this work, we investigated the effects of high operating temperature and thermal cycling on the photovoltaic (PV) performance of perovskite solar cells (PSCs) with a typical mesostructured (m)-TiO₂-CH₃NH₃PbI_3-xCl_x-spiro-OMeTAD architecture. After temperature-dependent grazing-incidence wide-angle X-ray scattering, in situ X-ray diffraction, and optical absorption experiments were carried out, the thermal durability of PSCs was tested by subjecting the devices to repetitive heating to 70 °C and cooling to room temperature (20 °C). An unexpected regenerative effect was observed after the first thermal cycle; the average power conversion efficiency (PCE) increased by approximately 10% in reference to the as-prepared device. This increase of PCE was attributed to the heating-induced improvement of the crystallinity and p doping in the hole transporter, spiro-OMeTAD, which promotes the efficient extraction of photogenerated carriers. However, further thermal cycles produced a detrimental effect on the PV performance of PSCs, with the short-circuit current and fill factor degrading faster than the open-circuit voltage. Similarly, the PV performance of PSCs degraded at high operation temperatures; both the short-circuit current and open-circuit voltage decreased with increasing temperature, but the temperature-dependent trend of the fill factor was the opposite. Our impedance spectroscopy analysis revealed a monotonous increase of the charge-transfer resistance and a concurrent decrease of the charge-recombination resistance with increasing temperature, indicating a high recombination of charge carriers. Our results revealed that both thermal cycling and high temperatures produce irreversible detrimental effects on the PSC performance because of the deteriorated interfacial photocarrier extraction. The present findings suggest that the development of robust charge transporters and proper interface engineering are critical for the deployment of perovskite PVs in harsh thermal environments.

Oxasmaragdyrins as New and Efficient Hole-Transporting Materials for High-Performance Perovskite Solar Cells

The high performance of the perovskite solar cells (PSCs) cannot be achieved without a layer of efficient hole-transporting materials (HTMs) to retard the charge recombination and transport the photogenerated hole to the counterelectrode. Herein, we report the use of boryl oxasmaragdyrins (SM01, SM09, and SM13), a family of aromatic core-modified expanded porphyrins, as efficient hole-transporting materials (HTMs) for perovskite solar cells (PSCs). These oxasmaragdyrins demonstrated complementary absorption spectra in the low-energy region, good redox reversibility, good thermal stability, suitable energy levels with CH₃NH₃PbI₃ perovskite, and high hole mobility. A remarkable power conversion efficiency of 16.5% (V_oc = 1.09 V, J_sc = 20.9 mA cm^-2, fill factor (FF) = 72%) is achieved using SM09 on the optimized PSCs device employing a planar structure, which is close to that of the state-of-the-art hole-transporting materials (HTMs), spiro-OMeTAD of 18.2% (V_oc = 1.07 V, J_sc = 22.9 mA cm^-2, FF = 74%). In contrast, a poor photovoltaic performance of PSCs using SM01 is observed due to the interactions of terminal carboxylic acid functional group with CH₃NH₃PbI₃.

Ultra-high open-circuit voltage of perovskite solar cells induced by nucleation thermodynamics on rough substrates

To obtain high performance CH₃NH₃PbI₃ perovskite solar cells, it is highly important to realise a high open-circuit voltage. Calculation results based on a modified diode model have indicated that a low bare ratio ϕ of the perovskite film is the most important factor determining the open-circuit voltage, where ϕ is defined as the ratio of the projection of the uncovered area of the perovskite film to the apparent area of the total substrate surface. To realise a low ϕ, we investigate the nucleation behaviour of crystals on rough substrates. The analysis results predict that, when CH₃NH₃PbI₃ is deposited on conventional transparent conductive oxide substrates such as fluorine-doped tin oxide, preferential heterogeneous nucleation will occur on the concave regions of the substrate; then, depending on the subsequent growth step, full coverage of the perovskite film at both the macroscopic and microscopic scales is realised. As a result, an ultra-high open-circuit voltage, i.e., 1.20 V, can be achieved in devices using the full coverage CH₃NH₃PbI₃ film. The thermodynamics theory of precipitation nucleation should shed light on solution engineering of thin films.

Interface Play between Perovskite and Hole Selective Layer on the Performance and Stability of Perovskite Solar Cells

Perovskite solar cells with variety of hole selective contacts such as 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD), poly(3-hexylthiophene-2,5-diyl), poly[bis(4-phenyl)(2,5,6-trimentlyphenyl)amine], 5,10,15-trihexyl-3,8,13-tris(4-methoxyphenyl)-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazole (HMPDI), and 2',7'-bis(bis(4-methoxyphenyl)amino)spiro[cyclopenta[2,1-b:3,4-b']dithiophene-4,9'-fluorene] were employed to elucidate its role at the interface of perovskite and metallic cathode. Microscopy images revealed Spiro-OMeTAD and HMPDI produce smoother and intimate contact between perovskite/hole transporting materials (HTM) interfaces among others evaluated here. This morphological feature appears to be connected with three fundamental facts: (1) hole injection to the HTM is much more efficient as evidenced by photoluminescence measurements, (2) recombination losses are less important as evidenced by intensity-modulated photovoltage spectroscopy and impedance spectroscopy measurements, and (3) fabricated solar cells are much more robust against degradation by moisture. Devices with higher open-circuit photovoltages are characterized by higher values of the recombination resistance extracted from the impedance data. The variation in device hysteresis behavior can be ascribed mainly due to the molecular interaction and the core of HTM employed. In all cases, this fact is related with a larger value of the low-frequency capacitance, which indicates that the HTM can induce specific slow processes of ion accumulation at the interface. Notably, these processes tend to slowly relax in time, as hysteresis is substantially reduced for aged devices.

Near-Infrared-Absorbing and Dopant-Free Heterocyclic Quinoid-Based Hole-Transporting Materials for Efficient Perovskite Solar Cells

New heterocyclic quinoid-based hole transporting materials (HTMs) with a rigid quinoid core [3,6-di(2H-imidazol-2-ylidene)cyclohexa-1,4-diene] have been synthesized. The new HTMs have good hole mobility (>10^-4  cm²  V^-1  s^-1 ) and very intense absorption in the near-infrared region extending to >800 nm. High performance perovskite solar cells can be fabricated using these HTMs without dopant. The best cell efficiency under simulated AM 1.5 G illumination reaches 12.22 %, which is comparable with that (12.58 %) using doped 2,2',7,7'-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD) as the HTM.

Efficiency Enhancement of Hybrid Perovskite Solar Cells with MEH-PPV Hole-Transporting Layers

In this study, hybrid perovskite solar cells are fabricated using poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and poly(3-hexylthiophene-2,5-diyl) (P3HT) as dopant-free hole-transporting materials (HTMs), and two solution processes (one- and two-step methods, respectively) for preparing methylammonium lead iodide perovskite. By optimizing the concentrations and solvents of MEH-PPV solutions, a power conversion efficiency of 9.65% with hysteresis-less performance is achieved, while the device with 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′spirobifluorene (Spiro-OMeTAD) doped with lithium salts and tert-butylpyridine (TBP) exhibits an efficiency of 13.38%. This result shows that non-doped MEH-PPV is a suitable, low-cost HTM for efficient polymer-based perovskite solar cells. The effect of different morphologies of methylammonium lead iodide perovskite on conversion efficiency is also investigated by incident photon-to-electron conversion efficiency (IPCE) curves and electrochemical impedance spectroscopy (EIS).

Bifunctional Heterocyclic Spiro Derivatives for Organic Optoelectronic Devices

A series of heterocyclic spiro derivatives has been successfully synthesized and characterized by photophysical and electrochemical studies. Taking advantage of their excellent hole-transporting properties, highly efficient small-molecular organic photovoltaic devices based on these heterocyclic compounds as donors with very low dopant concentrations have been prepared; particularly, a high open-circuit voltage of up to 1.10 V and a power conversion efficiency of up to 5.12% have been realized. In addition, most of these heterocyclic spiro derivatives are found to be highly emissive in solutions with photoluminescence quantum yields of up to 0.91, and high-performance deep-blue-emitting organic light-emitting diodes (OLEDs) have been achieved. Such devices exhibit a stable deep blue emission with CIE coordinates of (0.16, 0.04) and high external quantum efficiencies of up to 4.7%, which is one of the best values among the reported OLEDs with CIEy < 0.08.

Passivation of PbS Quantum Dot Surface with l-Glutathione in Solid-State Quantum-Dot-Sensitized Solar Cells

Surface oxidation of quantum dots (QDs) is one of the biggest challenges in quantum dot-sensitized solar cells (QDSCs), because it introduces surface states that enhance electron-hole recombination and degrade device performance. Protection of QDs from surface oxidation by passivating the surface with organic or inorganic layers can be one way to overcome this issue. In this study, solid-state QDSCs with a PbS QD absorber layer were prepared from thin mesoporous TiO2 layers by the successive ionic layer adsorption/reaction (SILAR) method. Spiro-OMeTAD was used as the organic p-type hole transporting material (HTM). The effects on the solar cell performance of passivating the surface of the PbS QDs with the tripeptide l-glutathione (GSH) were investigated. Current-voltage characteristics and external quantum efficiency measurements of the solar cell devices showed that GSH-treatment of the QD-sensitized TiO2 electrodes more than doubled the short circuit current and conversion efficiency. Impedance spectroscopy, intensity-modulated photovoltage and photocurrent spectroscopy analysis of the devices revealed that the enhancement in solar cell performance of the GSH-treated cells originates from improved charge injection from PbS QDs into the conduction band of TiO2. Time-resolved photoluminescence decay measurements show that passivation of the surface of QDs with GSH ligands increases the exciton lifetime in the QDs.

Laser-assisted crystallization of CH3NH3PbI3 films for efficient perovskite solar cells with a high open-circuit voltage

The pristine film of CH₃NH₃I·PbI₂·DMSO was crystallized via continuous laser irradiation at 450 nm.

Stable and Efficient Perovskite Solar Cells Based on Titania Nanotube Arrays

Highly ordered 1D TiO2 nanotube arrays are fabricated and applied as nanocontainers and electron transporting material in CH3 NH3 PbI3 perovskite solar cells. The optimized device shows a power conversion efficiency of 14.8%, and improved stability under an illumination of 100 mW cm(-2). This is the best result based on 1D TiO2 nanostructures so far.

FeCl3-Mediated Oxidative Spirocyclization of Difluorenylidene Diarylethanes Leading to Dispiro[fluorene-9,5'-indeno[2,1-a]indene-10',9''-fluorene]s

A novel FeCl3-mediated oxidative spirocyclization for construction of a new class of di-spirolinked π-conjugated molecules, dispiro[fluorene-9,5'-indeno[2,1-a]indene-10',9''-fluorene]s (DSFIIFs), has been reported. The combination of FeCl3 with FeO(OH) triggered an unprecedented double one-electron oxidation of difluorenylidene diarylethanes to afford the corresponding dispirocycles in high yields. The highest fluorescence quantum yield was up to 0.94 in solution. This protocol is also applicable to the synthesis of the non-spirolinked dihydroindenoindenes.

Enhanced charge collection with ultrathin AlOx electron blocking layer for hole-transporting material-free perovskite solar cell

An ultrathin AlOx layer has been deposited onto a CH3NH3PbI3 film using atomic layer deposition technology, to construct a metal-insulator-semiconductor (MIS) back contact for the hole-transporting material-free perovskite solar cell. By optimization of the ALD deposition cycles, the average power conversion efficiency (PCE) of the cell has been enhanced from 8.61% to 10.07% with a highest PCE of 11.10%. It is revealed that the improvement in cell performance with this MIS back contact is mainly attributed to the enhancement in charge collection resulting from the electron blocking effect of the AlOx layer.

Investigation of CH3NH3PbI3 degradation rates and mechanisms in controlled humidity environments using in situ techniques

Perovskite solar cells have rapidly advanced to the forefront of solution-processable photovoltaic devices, but the CH3NH3PbI3 semiconductor decomposes rapidly in moist air, limiting their commercial utility. In this work, we report a quantitative and systematic investigation of perovskite degradation processes. By carefully controlling the relative humidity of an environmental chamber and using in situ absorption spectroscopy and in situ grazing incidence X-ray diffraction to monitor phase changes in perovskite degradation process, we demonstrate the formation of a hydrated intermediate containing isolated PbI6(4-) octahedra as the first step of the degradation mechanism. We also show that the identity of the hole transport layer can have a dramatic impact on the stability of the underlying perovskite film, suggesting a route toward perovskite solar cells with long device lifetimes and a resistance to humidity.

The identification and characterization of defect states in hybrid organic–inorganic perovskite photovoltaics

Thin film photovoltaic cells based on hybrid halide perovskite absorbers have emerged as promising candidates for next generation photovoltaics.

Enhanced Hole Extraction in Perovskite Solar Cells Through Carbon Nanotubes

Here, we report the use of polymer-wrapped carbon nanotubes as a means to enhance charge extraction through undoped spiro-OMeTAD. With this approach a good solar cell performance is achieved without the implementation of conventional doping methods. We demonstrate that a stratified two-layer architecture of sequentially deposited layers of carbon nanotubes and spiro-OMeTAD, outperforms a conventional blend of the hole-conductor and the carbon nanotubes. We also provide insights into the mechanism of the rapid hole extraction observed in the two-layer approach.

Carbon nanotube/polymer composites as a highly stable hole collection layer in perovskite solar cells

Organic-inorganic perovskite solar cells have recently emerged at the forefront of photovoltaics research. Power conversion efficiencies have experienced an unprecedented increase to reported values exceeding 19% within just four years. With the focus mainly on efficiency, the aspect of stability has so far not been thoroughly addressed. In this paper, we identify thermal stability as a fundamental weak point of perovskite solar cells, and demonstrate an elegant approach to mitigating thermal degradation by replacing the organic hole transport material with polymer-functionalized single-walled carbon nanotubes (SWNTs) embedded in an insulating polymer matrix. With this composite structure, we achieve JV scanned power-conversion efficiencies of up to 15.3% with an average efficiency of 10 ± 2%. Moreover, we observe strong retardation in thermal degradation as compared to cells employing state-of-the-art organic hole-transporting materials. In addition, the resistance to water ingress is remarkably enhanced. These are critical developments for achieving long-term stability of high-efficiency perovskite solar cells.

Highly durable and efficient quantum dot-sensitized solar cells based on oligomer gel electrolytes

For stable quantum dot-sensitized solar cells, an oligomer-contained gel electrolyte was employed with a carbon-based counter electrode and a hierarchically shelled ZnO photoelectrode. Poly(ethylene glycol) dimethyl-ether (PEGDME) was added to the polysulfide electrolyte to enhance the stability of the methanol-based electrolyte. In addition, the nanocomposite gel electrolyte with fumed silica was used, which provided a solid three-dimensional network. A quantum-dot-modified ZnO nanowire photoanode enhanced the visible light harvesting, and a Pt/CNT-RGO counter electrode increased the catalytic activity. The oligomer gel electrolyte prevented the liquid electrolyte from leaking, and the carbon-based counter electrode retarded chemical poisoning at the counter electrode. The optimized cell exhibited 5.45% photoelectric conversion efficiency with long-term stability demonstrated over 5000 s operation time.

Recombination Study of Combined Halides (Cl, Br, I) Perovskite Solar Cells

We report on the preparation of a series of solution-processed perovskite solar cells based on methylammonium (MA) lead halide derivatives, MAPbX3, which show tunable optical properties depending on the nature and ratio of the halides employed (X = Cl, Br, and I). Devices have been prepared with different cell architecture, thin film, and mesoporous scaffold (TiO2 and Al2O3). We have analyzed different sample sets focusing on the characterization of the charge recombination by means of impedance spectroscopy (IS). On the one hand, our study discloses that the insertion of both Cl and Br in the perovskite lattice reduces the charge recombination rates in the light absorber film, thus determining the open circuit voltage (Voc) of the device. The samples prepared on a mesoporous Al2O3 electrode present lower charge recombination rates than those devices prepared on mesoporous TiO2. Furthermore, the addition of Br in the perovskite structure was demonstrated to improve slightly the lifetime of the devices; in fact, the efficiencies of all devices tested remained at least at the 80% of the initial value 1 month after their preparation. These results highlight the crucial role of the charge-recombination processes on the performance of the perovskite solar cells and pave the way for further progress on this field.

Role of the Selective Contacts in the Performance of Lead Halide Perovskite Solar Cells

The effect of electron- and hole-selective contacts in the final cell performance of hybrid lead halide perovskite, CH3NH3PbI3, solar cells has been systematically analyzed by impedance spectroscopy. Complete cells with compact TiO2 and spiro-OMeTAD as electron- and hole-selective contacts have been compared with incomplete cells without one or both selective contacts to highlight the specific role of each contact. It has been described how selective contacts contribute to enhance the cell FF and how the hole-selective contact is mainly responsible for the high Voc in this kind of device. We have determined that the recombination rate is mainly governed by the selective contacts. This fact has important implication for the future optimization of perovskite solar cells. Finally, we have developed a method to analyze the results obtained, and it has been applied for three different electron-selecting materials: TiO2, ZnO, and CdS.

General working principles of CH3NH3PbX3 perovskite solar cells

Organometal halide perovskite-based solar cells have recently realized large conversion efficiency over 15% showing great promise for a new large scale cost-competitive photovoltaic technology. Using impedance spectroscopy measurements we are able to separate the physical parameters of carrier transport and recombination in working devices of the two principal morphologies and compositions of perovskite solar cells, viz. compact thin films of CH3NH3PbI(3-x)Clx and CH3NH3PbI3 infiltrated on nanostructured TiO2. The results show nearly identical spectral characteristics indicating a unique photovoltaic operating mechanism that provides long diffusion lengths (1 μm). Carrier conductivity in both devices is closely matched, so that the most significant differences in performance are attributed to recombination rates. These results highlight the central role of the CH3NH3PbX3 semiconductor absorber in carrier collection and provide a new tool for improved optimization of perovskite solar cells. We report for the first time a measurement of the diffusion length in a nanostructured perovskite solar cell.

Impedance spectroscopic analysis of lead iodide perovskite-sensitized solid-state solar cells

Mesoscopic solid-state solar cells based on the inorganic-organic hybrid perovskite CH3NH3PbI3 in conjunction with the amorphous organic semiconductor spiro-MeOTAD as a hole transport material (HTM) are investigated using impedance spectroscopy (IS). A model to interpret the frequency response of these devices is established by expanding and elaborating on the existing models used for the liquid and solid-state dye-sensitized solar cells. Furthermore, the influence of changing the additive concentrations of tert-butylpyridine and LiTFSI in the HTM and varying the HTM overlayer thickness on top of the sub-micrometer thick TiO2 on the extracted IS parameters is investigated. The internal electrical processes of such devices are studied and correlated with the overall device performance. In particular, the features in the IS responses that are attributed to the ionic and electronic transport properties of the perovskite material and manifest as a slow response at low frequency and an additional RC element at intermediate frequency, respectively, are explored.

Yttrium-substituted nanocrystalline TiO₂ photoanodes for perovskite based heterojunction solar cells

We report the use of Y(3+)-substituted TiO2 (0.5%Y-TiO₂) in solid-state mesoscopic solar cells, consisting of CH₃NH₃PbI₃ as the light harvester and spiro-OMeTAD as the hole transport material. A power conversion efficiency of 11.2% under simulated AM 1.5 full sun illumination was measured. A 15% improvement in the short-circuit current density was obtained compared with pure TiO₂, due to the effect of Y(3+) on the dimensions of perovskite nanoparticles formed on the semiconductor surface, showing that the surface modification of the semiconductor is an effective way to improve the light harvesters' morphology and electron transfer properties in the solid-state mesoscopic solar cells.