Inverted-type Perovskite Solar Cells Research Articles

Investigation of the incorporation of C60 into PC61BM to enhance the photovoltaic performance of inverted-type perovskite solar cells based on MAPbI3

Perovskite Solar Cells (PSCs) have managed to significantly capture attention by achieving efficiency values of 25.6% in a remarkably short period of around ten years. Each layer within the device plays a crucial role in the overall device efficiency when it comes to PSC production. PC61BM, a derivative of fullerene, is one of the most commonly used electron-transport layers (ETLs) in inverted-type PSCs. In this study, the improvement of the ETL was aimed by incorporating C60 into PC61BM, and the effects of the doped ETL on MAPbI3-based inverted-type PSCs were investigated. For inverted type PSCs which are fabricated under high humidity (40-60%) and room conditions (~25 °C), the power conversion efficiencies (PCEs) have boosted from 11.54% (for undoped PC61BM) to 13.40% (for C60-doped PC61BM). To comprehend the sources of improvement in the fabricated devices, a series of characterizations were carried out, including Current Density-Voltage (J-V), Hysteresis Factor (HF), Scanning Electron Microscope (SEM), and Atomic Force Microscope (AFM) measurements.

Using commercially available cost-effective Zn(II) phthalocyanine as hole-transporting material for inverted type perovskite solar cells and investigation of dopant effect

Commercially available, low-cost, and stable unsubstituted zinc phthalocyanine (ZnPc) was introduced as both dopant-free and doped hole-transporting material (HTM) in inverted type (p-i-n) perovskite solar cells (PSCs). A simple solution process was applied to the fabrication of PCSs with a device structure of ITO/HTM/CH3NH3Pbl3/PCBM/Al, in which ZnPc and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) were used as HTM, methylammonium lead iodide (MAPbI3) as an active layer, [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the electron transport material (ETM), and aluminum as the counter electrode. PEDOT:PSS based cell was used as a reference and a power conversion efficiency (PCE) of 14.3% was achieved. A PCE of 4.4% was achieved for dopant-free ZnPc and 7.7% for doped-ZnPc. Although lower efficiency than the reference was obtained for ZnPc-based cells, we observed that the stability of inverted cells drastically improved when ZnPcs were used as HTM instead of PEDOT-PSS.

Investigation of various commercial PEDOT:PSS (poly(3,4-ethylenedioxythiophene)polystyrene sulfonate) as a hole transport layer in lead iodide-based inverted planar perovskite solar cells

Inverted-type perovskite solar cells have drawn remarkable attention due to solution-processable, straightforward configuration, low-cost processing, and manufacturing at very high throughput, even on top of flexible materials. The hole transport material (HTM) plays a vital role to achieve high performance in inverted type of perovskite solar cells. Herein, we report on the effect of different commercial PEDOT:PSS such as PH 1000, PH 500, P VP AI, and P T2, on the performance of CH3NH3PbI3-based planar perovskite solar cells.

Controlled Removal of Surfactants from Double-Walled Carbon Nanotubes for Stronger p-Doping Effect and Its Demonstration in Perovskite Solar Cells.

Double-walled carbon nanotubes (DWNTs) have shown potential as promising alternatives to conventional transparent electrodes owing to their solution processability as well as high conductivity and transparency. However, their DC to optical conductivity ratio is limited by the surrounding surfactants that prevent the p-doping of the DWNTs. To maximize the doping effectiveness, the surfactants are removed from the DWNTs, with negligible damage to the nanotubes, by calcination in an Ar atmosphere. The effective removal of the surfactants is characterized by various analyses, and the results show that the optimal calcination temperature is 400°C. The conductivity of the DWNTs films improves when doped by triflic acid. While the conductivity increase of the surfactants-wrapped DWNT films is 31.9%, the conductivity increase of the surfactants-removed DWNT is found to be 59.7%. Using the surfactants-removed, p-doped, solution-processed transparent electrodes, inverted-type perovskite solar cells are fabricated, resulting in a power conversion efficiency of 17.7% without hysteresis. This work advances the application of DWNTs in transparent conductors, as the efficiency obtained is the highest value achieved to date for carbon nanotube electrode-based perovskite solar cells and solution-processable transparent electrode-based solar cells.

Synthesis and characterization of C60 and C70 acetylacetone monoadducts and study of their photochemical properties for potential application in solar cells

We report on the synthesis of C60 and C70 monoadducts at room temperature through the Bingel reaction; employing acetylacetone as ligand; in presence of DBU (1,8-Diazabicyclo [5.4.0] undec-7-ene), carbon tetrabromide (CBr4), and o-dichlorobenzene. Diacetylmethane-[C60-Ih]-fullerene-[5,6] and diacetylmethane-[C70-D5h]-fullerene-[5,6] monoadducts were obtained with yields of 69% and 44%, respectively. The products were purified by column chromatography (CC, on silica gel, using hexane, carbon disulfide, and chloroform as eluents at room temperature) and characterized by Nuclear Magnetic Resonance (1H and 13C), Fourier-Transform Infrared (FT-IR) and UV-Visible spectroscopies, Matrix-assisted Laser Desorption/Ionization-Time of Flight (MALDI-TOF) Mass spectrometry, Cyclic Voltammetry (CV), and Osteryoung Square Wave Voltammetry (OSWV). Both compounds showed irreversible reduction peaks controlled by diffusion, with LUMO energy levels of -3.09 eV, -3.13 eV for C60, and C70 monoadducts, respectively. These values are comparable with the -3.99 eV of PC61BM. The synthetized adducts were incorporated into inverted-type perovskite solar cells and were used as electron transporting materials (ETM) obtaining power conversion efficiencies (PCE) of 8.5% and 14.0% for the C60 and C70 monoadducts, respectively. When C60 is replaced by a lower symmetrical fullerene such as C70 an improved light absorption in the visible region is observed.

Unusual Hole Transfer Dynamics of the NiO Layer in Methylammonium Lead Tri-iodide Absorber Solar Cells.

Nickel oxides (NiO) as hole transport layers (HTLs) in inverted-type perovskite solar cells (PSCs) have been widely studied mainly because of their high stability under illumination. Increases in the power conversion efficiency (PCE) with NiO HTLs have been presented in numerous reports, although the photoluminescence (PL) quenching behavior does not coincide with the PCE increase. The dynamics of the charge carrier transport between the NiO HTLs and the organic-inorganic halide perovskite absorbers is not clearly understood yet and quite unusual, in contrast to organic/polymerics HTLs. We deposited NiO HTLs with precisely controlled thicknesses by atomic layer deposition (ALD) and studied their photovoltaic performances and hole transfer characteristics. Ground state bleaching (GSB) recovery was observed by ultrafast transient absorption spectroscopy (TAS), which suggested that backward hole injection occurred between the perovskites and NiO HTLs, so that the uncommon PL behaviors can be clearly explained. Backward hole injection from the NiO HTL to the perovskite absorber originated from their similar valence band (VB) energy positions. The thickness increase of the NiO HTLs induced VB sharing, which caused a red-shift of the photoinduced hole absorption spectrum in near-infrared (NIR) femtosecond TAS and a decrease in the PL intensity. Our studies on inorganic metal oxide transport layers, NiO in this work, with a thickness dependence and the comparison with organic layers provide a better understanding of the interfacial carrier dynamics in PSCs.

On the role of solution-processed bathocuproine in high-efficiency inverted perovskite solar cells

The s-shaped J-V curve of perovskite solar cells can be eliminated using the solvent penetration (SP) process with a PCBM spacer. The experimental results show that the viscosity of the solvent used in the SP process influences the shunt and series resistances simultaneously. In addition, the fill factor of PEDOT:PSS based perovskite solar cells can be largely increased from 40.3% to 70.7% when the bathocuproine (BCP) molecules are dissolved in IPA solvent during the SP process, which significantly increases the power conversion efficiency (PCE) by 98.9%. Besides, the PCE of perovskite solar cells increases from 13.56% to 20.29% when the PEDOT:PSS film is replaced by a P3CT-Na thin film. It is noted that the absorbance spectra, photoluminescence (PL) spectra, time-resolved PL, Raman spectra and X-ray diffraction patterns demonstrate that the BCP molecules penetrate into the PCBM thin film and the PCBM/perovskite interface during the SP process, which increases the electron mobility of the PCBM thin film and passivates the interfacial MA cations in perovskite thin film, respectively. In other words, the solution-processed BCP small molecules are vertically distributed across the PCBM thin film. Our results help completely understand the entire roles of the solution-processed BCP molecules in the high-performance inverted-type perovskite solar cells.

Ag modified bathocuproine:ZnO nanoparticles electron buffer layer based bifacial inverted-type perovskite solar cells

An Ag modified bathocuproine:ZnO nanoparticles (BCP:ZnO NPs) thin film was used as the buffer layer of inverted-type perovskite solar cells, which increased the bifacial power conversion efficiency from 6.82% to 15.50%. The surface-enhanced Raman scattering and absorbance spectra show that the Ag modified BCP small molecules can effectively passivate the surface oxygen defects of ZnO NPs and thereby increasing the crystallinity, which simultaneously increases the short-circuit current density (J SC ) and fill factor. It is noted that a high incident photon-to-electron conversion efficiency (IPCE) of 92% is achieved at the wavelength of 587 nm due to the constructive interference effect in the multilayer structure, which can be used to explain the high photocurrent generation in a semi-transparent solar cell. In addition, the light intensity-dependent experimental results of the bifacial perovskite solar demonstrates that the hole transportation is better than the electron transportation in the perovskite thin film. This concept can be readily used in the optimization of high-efficiency bifacial perovskite solar cells. • A bifacial PCE of 15.50% is achieved in the bifacial inverted-type MAPbI 3 solar cells. • Ag modified BCP:ZnO NPs thin film is used as the electron buffer layer of MAPbI3 solar cells. • The higher deposition rate of Ag results in the higher FF and PCE. • The asymmetric FF implies that the hole mobility is higher than electron mobility in MAPbI 3 thin films.

Influence of end groups variation of self assembled monolayers on performance of planar perovskite solar cells by interface regulation

We present the synthesis and application of new class of self-assembled monolayer molecules (SAMs) for acquiring feasible interfacial engineering in inverted type perovskite solar cells (PSCs). The proposed SAMs bearing different electron donating terminal groups have been utilized to tune the work function of indium tin oxide (ITO) electrodes. Fine-tuning of terminal groups of the SAMs allows us to compare relationship between molecular structures and device parameters. Moreover, ionic and hybrid nature of perovskite enables forming various chemical interactions with terminal groups of SAMs. Employed SAMs has resulted in permanent increase in work function of ITO, increase power conversion efficiency (PCE) of the cells and passivation of trap states at the interface between electrode and perovskite layer. The present study provides new insights into correlation between molecular engineering and solar cell performance through treating holistic comparison of synthesized molecules at the interface of ITO and perovskite layer.

Structural, optical and excitonic properties of urea grading doped CH3NH3PbI3 thin films and their application in inverted-type perovskite solar cells

—The structural, optical and excitonic characteristics of urea doped CH3NH3PbI3 (MAPbI3) multi-crystalline thin films were investigated by using the X-ray diffraction patterns, atomic-force microscopic images, absorbance spectra, photoluminescence (PL) spectra and Raman scattering spectra. The surface-sensitive Raman scattering spectra show that the urea small molecules are mainly distributed in the top region of the MAPbI3 thin films and thereby effectively passivating the electron-poor defects (interfacial MA cations) of MAPbI3 thin films. Besides, the thermal annealing temperature and the concentration of urea additive both strongly influence the formation of MAPbI3 thin films, which dominates the photovoltaic performance. The use of 5-wt% urea can increase the open-circuit voltage (VOC) and short-circuit current density (JSC) of the MAPbI3 solar cells from 0.88 V to 19.75 mA/cm2 to 0.94 V and 22.97 mA/cm2, respectively. In addition, the reduced current hysteresis in the J-V curves of the MAPbI3 solar cells can be explained as due to the effective defect passivation in the top region of the MAPbI3 thin film by the oxygen-donors of the urea small molecules.

Tetra and octa substituted Zn(II) and Cu(II) phthalocyanines: Synthesis, characterization and investigation as hole-transporting materials for inverted type-perovskite solar cells

Metallophthalocyanines (MPcs) are used as cost-effective and thermally stable hole-transporting materials (HTMs) in the production of perovskite solar cells (PSCs). Unlike the mostly used n-i-p type PSCs, there are a few studies based on p-i-n type PSC with pthalocyanine HTMs. Here, for the first time, both tetra and octa substituted zinc and copper phthalocyanines have been studied as hole transporting materials in the inverted-type (p-i-n) PSCs without using any dopant material. The developed tetra substituted ZnPc and CuPc possess four dichlorophenylthio groups, and the octa-substituted Pcs have eight dichlorophenylthio groups in their peripheral position which enhance their solubility in perovskite-friendly solvent and improved thin film quality. In the inverted solar cell (ITO/HTM/CH3NH3Pbl3/PCBM/Al), methylammonium lead iodide (MAPI) was used as the active layer and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the electron transport material. Devices based on tetra substituted zinc and copper Pcs, as dopant-free HTMs, indicated significant efficiency enhancement and cell stability compared to similar octa substituted Pcs. While the open-circuit voltage (Voc) for the reference PEDOT:PSS was measured 0.77 V, for tetra substituted CuPc was found to be 0.83 V. Due to higher Voc value, tetra substituted CuPc is an appropriate candidate that can replace expensive and unstable HTMs that are currently used in PSCs.

Performance enhancement of inverted perovskite solar cells through interface engineering by TPD based bidentate self-assembled monolayers

Perovskite solar cells (PSCs) have recently appeared as a promising photovoltaic technology and attracted great interest in both photovoltaic industry and academic community. Numerous active researches related to the material processing and operational aspects of device fabrication are under progress since PSCs have a great potential for attaining higher performance compared to that of other solar cell technologies. In particular, interfacial engineering is a crucial issue for obtaining high efficiency in solar cells where perovskite absorber layer is deposited between hole and electron transport layers. In inverted type architecture, PEDOT:PSS is used as both hole transport layer and surface modifier; but unfortunately, this material bears instability due to its acidic nature. Thus, self-assembled monolayers (SAMs) not only are considered as suitable alternative, but also their application is regarded as an efficient and cost effective method to modify electrode surface since it provides a robust and stable surface coverage. In this context, we have employed two novel N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) based SAM molecules to customize indium tin oxide (ITO) surface in inverted type PSCs. Furthermore, fine-tuning of spacer groups enables us to study device performance depending on molecular structure. This study proposes promising materials for anode interface engineering and provides a feasible approach for production of organic semiconductor based SAMs to achieve high performance PSCs.

Effect of UV exposure of ITO/PEDOT:PSS substrates on the performance of inverted-type perovskite solar cells

We have fabricated inverted-type perovskite solar cells employing CH3NH3PbI3−xClx perovskite in the form of ITO/PEDOT:PSS/Perovskite/PCBM/Al. The effects of UV radiation on ITO/PEDOT:PSS substrates have been investigated for perovskite solar cells. ITO/PEDOT:PSS substrates were exposed to UV radiation for 5 to 15 min. The perovskite solar cells fabricated on UV-irradiated ITO/PEDOT:PSS substrates exhibited better performance as compared to those fabricated on substrates employing non-treated PEDOT:PSS. We used photovoltaic characterization, UV–VIS absorption, sheet resistance, X-ray diffraction (XRD), scanning electron microscopy (SEM), fourier transform infrared spectroscopy (FTIR), and external quantum efficiency (EQE) characterization techniques to investigate the effect of UV irradiation of ITO/PEDOT:PSS substrates on the performance of perovskite solar cells. The devices fabricated on ITO/PEDOT:PSS (UV irradiated for 10 min) exhibited the best device performance of all. We achieved a 9% increase in the overall power conversion efficiency for the devices fabricated on substrates exposed to UV radiation.

Highly stable inverted methylammonium lead tri-iodide perovskite solar cells achieved by surface re-crystallization

A long-term operational stability over 1000 hours in the inverted type perovskite solar cells based on the MAPbI3 layer is demonstrated under ionic defect-free conditions.

Origins of the s-shape characteristic in J–V curve of inverted-type perovskite solar cells

Fullerene derivative thin films have been widely used in inverted-type perovskite solar cells as the electron transport layer (ETL) and hole blocking layer. However, the smooth contact at the interface between the hydrophobic [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and hydrophilic CH3NH3PbI3 (MAPbI3) thin film has not yet been completely understood. The contact at the PCBM/MAPbI3 interface strongly influences the photovoltaic performance. The photovoltaic devices were characterized by measuring the light intensity-dependent current density–voltage (J–V) curves and impedance spectra, which show that the contact at the PCBM/MAPbI3 interface simultaneously influences the shunt resistance (carrier recombination) and series resistance (interfacial contact). In addition, x-ray diffraction patterns, atomic force microscopic images, absorbance spectra and photoluminescence spectra were used to explore the contact at the PCBM/MAPbI3 interface. The experimental results show that the flat MAPbI3 thin film cannot be completely covered by a PCBM thin film and thereby results in the s-shape characteristic in the J–V curve of the resultant solar cells. The s-shaped J–V curve can be suppressed by increasing the crystallinity and surface roughness of the MAPbI3 thin film. With the use of an interface modification layer in between the PCBM thin film and Ag cathode, the power conversion efficiency of MAPbI3 solar cells can be increased from 10.50% to 13.71%.

Computational Design of Highly Efficient and Robust Hole Transport Layers in Perovskite Solar Cells

We investigated the effects of materials and the film thickness of hole transport layers (HTLs) on inverted type perovskite solar cells using optics-charge transport coupled simulations. Power conversion efficiencies (PCEs), and the variations in efficiency induced by the film thickness dispersion, were intensively studied, to compare potential HTLs candidates like NiOx, PEDOT:PSS, CuSCN, and CuI. The optimum thickness of the solar cell layers differed based on the chosen combination of HTL and perovskite. It is suggested that the optoelectronic properties of HTLs like band gap, extinction coefficient, and refractive index can be used to determine the best ideal efficiencies, and sensitivity to process fluctuation. CuSCN showed the most promising behaviors, in that it can produce over-25% PCEs, and the lowest efficiency dispersion for various HTL thickness conditions. The best performance by CuSCN can be ascribed to its having a proper refractive index with the perovskite layer, and wide band gap characteristic. NiOx and CuI showed PCEs comparable to the CuSCN, but their efficiencies were sensitive to the varying thickness of the HTL. PEDOT:PSS exhibited the lowest simulated PCEs due to its small band gap. Our study suggests the best HTL candidates for inverted type perovskite solar cells, and demonstrates the importance of sophisticated numerical material studies, and device design, when developing highly efficient and robust perovskite solar cells. Key words: perovskite solar cells, hole transport layers, simulation, optimization, inverted type

A solution-processed cobalt-doped nickel oxide for high efficiency inverted type perovskite solar cells

Optimal interfaces play an important role in determining the efficiency of perovskite solar cells. Despite nickel oxide is a promising hole transport material in perovskite solar cells, electrical conductivity and surface properties of pristine nickel oxide are not satisfactory for achieving high efficiency perovskite solar cells. Here, we demonstrate that cobalt doping significantly improves not only the electrical conductivity but also the interfaces of nickel oxide hole transport layer. As a result, the hole extraction, transport and collection properties are significantly improved in the device using cobalt-doped nickel oxide as a hole transport layer. Also, it reveals that cobalt-doped nickel oxide reduces the trap-sites of interfaces of perovskite/hole transport layer, leading to suppressed charge recombination in the devices. As a result, the devices using cobalt-doped nickel oxide achieve as high as 17.52% without severe hysteresis, which is attributed to better band alignment, superior hole extraction and decreased resistance, as compared to pristine nickel oxide-based devices.

Perovskite Solar Cells with Inorganic Electron- and Hole-Transport Layers Exhibiting Long-Term (≈500 h) Stability at 85 °C under Continuous 1 Sun Illumination in Ambient Air.

Despite the high power conversion efficiency (PCE) of perovskite solar cells (PSCs), poor long-term stability is one of the main obstacles preventing their commercialization. Several approaches to enhance the stability of PSCs have been proposed. However, an accelerating stability test of PSCs at high temperature under the operating conditions in ambient air remains still to be demonstrated. Herein, interface-engineered stable PSCs with inorganic charge-transport layers are shown. The highly conductive Al-doped ZnO films act as efficient electron-transporting layers as well as dense passivation layers. This layer prevents underneath perovskite from moisture contact, evaporation of components, and reaction with a metal electrode. Finally, inverted-type PSCs with inorganic charge-transport layers exhibit a PCE of 18.45% and retain 86.7% of the initial efficiency for 500 h under continuous 1 Sun illumination at 85 °C in ambient air with electrical biases (at maximum power point tracking).

Functional graded fullerene derivatives for improving the fill factor and device stability of inverted-type perovskite solar cells

A graded fullerene derivative thin film was used as a dual-functional electron transport layer (ETL) in CH3NH3PbI3 (MAPbI3) solar cells, to improve the fill factor (FF) and device stability. The graded ETL was made by mixing phenyl-C61-butyric acid methyl ester (PCBM) molecules and C60-diphenylmethanofullerene-oligoether (C60-DPM-OE) molecules using the spin-coating method. The formation of the graded ETLs can be due to the phase separation between hydrophobic PCBM and hydrophilic C60-DPM-OE, which was confirmed by XPS depth-profile analysis and an electron energy-loss spectroscope. Comprehensive studies were carried out to explore the characteristics of the graded ETLs in MAPbI3 solar cells, including the surface properties, electronic energy levels, molecular packing properties and energy transfer dynamics. The elimination of the s-shape in the current density–voltage curves results in an increase in the FF, which originates from the smooth contact between the C60-DPM-OE and hydrophilic MAPbI3 and the formation of the more ordered ETL. There was an improvement in device stability mainly due to the decrease in the photothermal induced morphology change of the graded ETLs fabricated from two fullerene derivatives with distinct hydrophilicity. Consequently, such a graded ETL provides dual-functional capabilities for the realization of stable high-performance MAPbI3 solar cells.

Carbon Nanotubes for Flexible Perovskite Solar Cells

Single-walled carbon nanotubes (SWNT), graphene, and fullerene (C60 and PCBM) would be very efficiently used in lead halide Perovskite solar cells. A film of SWNTs or graphene can be flexible and stretchable transparent-conductive layer. At the same time, this film can be carrier-selective layers, i.e., electron-blocking-layers or hole-blocking-layers. Based on our experiences of using nanotube films for CNT-Si solar cells and organic polymer solar cells [1,2], we have explored the application of SWNT films for organic-inorganic perovskite solar cells. We have demonstrated the replacement of ITO in inverted-type perovskite solar cells, SWNTs/PEDOT:PSS/CH3NH3PbI3/PCBM/Al [3]. The flexible application on polyethylene terephthalate (PET) is also demonstrated [3]. With the improved perovskite structure, a film of carbon nanotubes or graphene can be practical replacement of ITO for the flexible transparent electrode of inverted perovskite solar cells [4]. Replacement of electron-blocking-layer and metal electrode in normal-type perovskite solar cells is demonstrated as well. Those devices can have comparable PCE as the conventional design with organic electron-blocking layer and top metal electrode. The normal-type perovskite solar cell, composed of ITO/C60/CH3NH3PbI3/SWNTs, can achieve a PCE of 17 % with spiro-MeOTAD as dopant to SWNTs [5]. This structure with a perovskite layer sandwiched by C60 and SWNTs can lead to the solar cells without hysteresis and with much improved air-stability [5]. The effective passivation of the degradation of perovskite material by moisture can be achieved with C60 and SWNTs [5]. More recent configuration is using a film of SWNTs for both anode and cathode electrode [6]. The new physical understanding of band alignment must be discussed based on the demonstrations of efficient carrier selective layers with these dry-deposited SWNTs which are composed of metallic and semiconductor nanotubes. Part of this work was supported by JSPS KAKENHI Grant Numbers JP25107002 and JP15H05760.