Inverted Solar Cells Research Articles

Highly efficient and stable inverted planar solar cells from (FAI)x(MABr)1−xPbI2 perovskites

We report highly efficient and stable inverted planar lead mixed-halide (Br, I) perovskite solar cells with a configuration of ITO/poly(3-bromothiophene)/(FA)y(MA)1−yPbBrxI3−x/C60/BCP/Ag (FA: HC(NH2)2+; MA: CH3NH3+). We found that small changes in the composition of (FA)y(MA)1−yPbBrxI3−x have big impact on the material properties and device performance. Appropriate Br-doping enlarges MAPbBrxI3−x's bandgaps and prolongs the life of the excited charge carrier, which leads to higher device open-circuit voltage (VOC). Replacing “MA” with “FA” extends the absorption of (FA)y(MA)1−yPbBrxI3−x which compensates the JSC loss in MAPbBrxI3−x from Br-doping. The optimized perovskite film with a composite of FA0.8MA0.2PbBr0.2I2.8 shows a lifetime of 670ns and a photoelectric response to 830nm, resulting in an enhanced JSC of 22.2mAcm−2, a high FF of 0.80, and an efficiency of 18.1%. In addition, the inverted device based on FA0.8MA0.2PbBr0.2I2.8 showed long-term stability with 80% efficiency remained after 4 months in a glovebox without encapsulation. Our results demonstrate highly efficient and stable inverted planar perovskite solar cells can be achieved by optimizing absorber material composition, which offer a reference for their applications in flexible or tandem solar cells.

A life cycle assessment of perovskite/silicon tandem solar cells

AbstractGiven the rapid progress in perovskite solar cells in recent years, perovskite/silicon (Si) tandem structure has been proposed to be a potentially cost‐effective improvement on Si solar cells because of its higher efficiency at a minimal additional cost. As part of the evaluation, it is important to conduct a life cycle assessment on such technology in order to guide research efforts towards cell designs with minimum environmental impacts. Here, we carry out a life cycle assessment to assess global warming, human toxicity, freshwater eutrophication and ecotoxicity and abiotic depletion potential impacts and energy payback time associated with three perovskite/Si tandem cell structures using silver (Ag), gold (Au) and aluminium (Al) as top electrodes compared with p–n junction and hetero‐junction with intrinsic inverted layer Si solar cells. It was found that the replacement of the metal electrode with indium tin oxide/metal grid in the tandem cell reduces the environmental impacts significantly compared with the perovskite cell. For all the impacts assessed, we conclude that the perovskite/Si tandem using Al as top electrode has better environmental outcomes, including energy payback time, when compared with the other tandem structures studied. Use of Al in preference to noble metals for contacts, Si p–n junction in preference to intrinsic inverted layer and the avoidance of 2,20,7,70‐tetrakis(N,N‐di‐p‐methoxyphenylamine)9,90‐spirobifluorene (Spiro‐OMeTAD) are environmentally beneficial. The key result found of this work is that the most important factor for the better environmental impacts of these tandem solar cells is the transparency and electrical conductivity of the perovskite layer after it fails. Copyright © 2017 John Wiley & Sons, Ltd.

Interfacial electronic structures revealed at the rubrene/CH3NH3PbI3 interface.

The electronic structures of rubrene films deposited on CH3NH3PbI3 perovskite have been investigated using in situ ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). It was found that rubrene molecules interacted weakly with the perovskite substrate. Due to charge redistribution at their interface, a downward 'band bending'-like energy shift of ∼0.3 eV and an upward band bending of ∼0.1 eV were identified at the upper rubrene side and the CH3NH3PbI3 substrate side, respectively. After the energy level alignment was established at the rubrene/CH3NH3PbI3 interface, its highest occupied molecular orbital (HOMO)-valence band maximum (VBM) offset was found to be as low as ∼0.1 eV favoring the hole extraction with its lowest unoccupied molecular orbital (LUMO)-conduction band minimum (CBM) offset as large as ∼1.4 eV effectively blocking the undesired electron transfer from perovskite to rubrene. As a demonstration, simple inverted planar solar cell devices incorporating rubrene and rubrene/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole transport layers (HTLs) were fabricated in this work and yielded a champion power conversion efficiency of 8.76% and 13.52%, respectively. Thus, the present work suggests that a rubrene thin film could serve as a promising hole transport layer for efficient perovskite-based solar cells.

Inverted metamorphic triple-junction solar cell and its radiation hardness for space applications

In recent years, with the development of solar cell technology, the conversion efficiency of the lattice-matched Ga0.51In0.49P/In0.01Ga0.99As/Ge triple-junction solar cell has achieved 30% under AM0 spectrum. As is well known, it is difficult to further improve the efficiency due to the limited bandgap combination. Therefore, an inverted metamorphic triple-junction solar cell is designed by replacing the Ge subcell with a 1.0 eV InGaAs subcell. The efficiency could be increased with the open-circuit voltage increasing, while the short circuit current maintains a similar value.#br#In this paper, the inverted metamorphic GaInP/GaAs/In0.3Ga0.7As triple-junction solar cells are grown on 4-inch GaAs substrates via metal organic chemical vapor deposition. Optimizing the epitaxy process, AlInGaAs graded buffer shows nearly 100% relaxation by the reciprocal space mapping of the high-resolution X-ray diffraction and low average threading dislocation density~5.4×106/cm2 evaluated from the cathodoluminescence image. Finally, the inverted metamorphic triple-junction solar cell with 24 cm2 area shows a conversion efficiency of 32% with an open-circuit voltage of 3.045 V and a short-circuit current of 404.5 mA under one sun, AM0 spectrum, 25℃ conditions, which is 5% higher than the lattice-matched GaInP/InGaAs/Ge triple-junction solar cell. Under 1 MeV electron irradiation test, the degradations of the external quantum efficiency and I-V characteristics of inverted metamorphic triple-junction solar cell are exhibited each as a function of fluence, and finally the end-of-life efficiency is 27.2% with a degradation of 15% under 1×1015/cm2 fluence. More experiments mainly focusing on the lattice quality of AlInGaAs graded buffer and the structure of In0.3Ga0.7As subcell, will be carried out to improve the efficiency and enhance the radiation hardness.

Analyses of the effect of mismatch on the performance of inverted GaInP/InxGa1-xAs/InyGa1-yAs triple-junction solar cells

The traditional lattice matched GaInP/(In) GaAs/Ge triple-junction (3J) solar cell has no much room to enhance its practical achievable conversion efficiency because of its inappropriate ensemble of bandgap energies. According to the P-N junction formation mechanism and the close equilibrium condition, we explore a series of computational codes in the framework of MATLAB to simulate and optimize the inverted structure of series-connected 3J solar cells with a fixed top bandgap of 1.90 eV on GaAs substrate. In this paper, structural optimization is conducted in the real device design, because the realistic (QE) is closely related to a set of material parameters in the subcell, i.e., the absorbtion coefficient of material, subcell thickness, minority carrier diffusion length, surface recombination velocity, etc. The results indicate improved inverted 3J solar cells with nearly optimized bandgaps of 1.90, 1.38, and 0.94 eV, by utilizing two independently lattice-mismatches (0.17% and 2.36% misfit respectively) to the GaAs substrate. A theoretical efficiency of 51.25% at 500 suns is demonstrated with this inverted design with the optimal thickness (4 m GaInP top and 3.1 m InGaAs middle). By contrast, the efficiency with the infinite thickness of subcells is reduced by 1%, which is mainly attributed to the effect of minority carrier recombination on Jsc. Exactly speaking, if photo-generated carriers make a contribution to Jsc, they must be collected effectively by the P-N junction before recombining. A new model is proposed based on the effect of dislocation on the metamorphic structure properties by regarding dislocation as minority-carrier recombination center. Our calculation indicates that threading dislocations density in the middle junction is approximate to 1.70105 cm-2 when dislocations in the gradient buffer layer are neglected. The theoretical efficiency is increased by 0.3% compared with the inverted design containing a single metamorphic junction. As a result, based on the two metamorphic combinations, a solar cell with an area of 30.25 mm2 is prepared. The efficiency of the designed cell with two lattice-mismatched junctions reaches 40.01% at 500 suns (AM1.5D, 38.4 W/cm2, 25℃), which is 0.4% higher than that of the single metamorphic junction 3J solar cell.

Efficient and Versatile Interconnection Layer by Solvent Treatment of PEDOT:PSS Interlayer for Air‐Processed Organic Tandem Solar Cells

A robust and efficient interconnection layer (ICL) based on low conductive poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) (PEDOT:PSS) and ZnO nanoparticles is implemented in a solution‐air‐processed, blade coated inverted polymer tandem solar cell. The commercial PEDOT:PSS (Heraeus Clevios P VP Al 4083) is modified with a fluorosurfactant to allow its deposition onto any hydrophobic active layer surface. However, this method alters the electrical and energetic properties of the PEDOT:PSS thus affecting the interface with the ZnO layer, responsible for an inefficient ICL and poorly performing tandem devices. The ohmic contact at the PEDOT:PSS/ZnO interface is successfully optimized through a simple solvent treatment of the PEDOT:PSS film surface, leading to tandem devices with power conversion efficiencies (PCEs) improved by more than 50% compared to the untreated reference system. The reported method is an easy and versatile approach to optimize the functionality of the PEDOT:PSS/ZnO ICL of inverted multijunction devices, applicable onto any organic active layer and compatible with a roll‐to‐roll production line.

A Universal Deposition Protocol for Planar Heterojunction Solar Cells with High Efficiency Based on Hybrid Lead Halide Perovskite Families.

A robust and expedient gas quenching method is developed for the solution deposition of hybrid perovskite thin films. The method offers a reliable standard practice for the fabrication of a non-exhaustive variety of perovskites exhibiting excellent film morphology and commensurate high performance in both regular and inverted structured solar cell architectures.

Solvent effects on the morphology and stability of PTB7:PCBM based solar cells

The ability to fabricate highly efficient organic photovoltaic devices should be combined with an adequate device lifetime, as well as with reduced environmental impact of the manufacture process. Herein PTB7:PCBM-based inverted solar cells were fabricated using different solvents, and their morphology and stability were characterized. The solvents used were the halogenated chlorobenzene (CB) and the non-halogenated ortho-xylene (XY), which is expected to be less environmentally hazardous. While the PV performance was comparable for both cell types, morphology characterization showed a solvent-dependent vertical stratification with PTB7-enriched top surface in CB-films and PCBM-rich surface in XY-films. The cell performances were investigated following different stressing conditions to determine their relative stability and degradation mechanisms. Stressing of encapsulated cells demonstrated the effect of photolysis on the cell stability and showed enhanced degradation of XY-cells compared to CB-cells, possibly related to residual XY in the active film. Stressing of non-encapsulated cells showed the effect of ambient on the cell stability, and corroborated the enhanced XY-cell sensitivity to UV illumination. We suggest that thorough removal of solvent traces, as well as UV filtering, would improve XY-cells’ stability, and allow its use in large-scale cell manufacture while eliminating halogenated solvents from the process.

Luminescent cathode buffer layer for enhanced power conversion efficiency and stability of bulk-heterojunction solar cells

We report high photovoltaic efficiency of over 9% in solution-processed, small-molecule (SPSM) 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c]1,2,5]thiadiazole) p-DTS(FBTTh2)2:[6-6]-phenyl C70 butyric acid methyl ester (PC70BM) blend based inverted BHJ solar cell by incorporating luminescent zinc oxide doped with sodium (ZnO:Na) quantum dots (QD) (l-ZnO) as a cathode buffer layer (CBL) in inverted bulk-heterojunction (BHJ) solar cells for the first time. The l-ZnO absorbs ultraviolet (UV) light and down-converts it to visible light. The l-ZnO layer's emission overlaps significantly with the absorption of p-DTS(FBTTh2)2, leading to an enhanced absorption by p-DTS(FBTTh2)2. This resulted in a significant enhancement of photo-current from 15.4 to 17.27 mA/cm2 and efficiency from 8% to 9.2% for ZnO and l-ZnO based devices, respectively. This is among one of the highest efficiency values reported so far in the case of SPSM based single junction BHJ solar cells. The luminescent ZnO layer also protects the active layer from UV-induced degradation as solar cells show high stability under constant solar light illumination retaining more than 90% (∼28 h) of its initial efficiency, whereas BHJ solar cells without the luminescent ZnO layer degraded to ∼50% of its initial value under same conditions. Since ZnO is an essential part of inverted organic solar cells, the luminescent l-ZnO CBL has great potential in inverted organic solar cells.

Efficiency enhancement of ZnO based inverted BHJ solar cells via interface engineering using C70 modifier

The interface quality of ZnO and the photoactive polymer blend is of utmost importance in the performance of organic-inorganic hybrid photovoltaic devices. The chemically prepared ZnO electron transporting layer often produce surfaces unacceptable for efficient electron extraction and understate the photovoltaic performance. Herein, we propose a facile interfacial modification technique to enhance the charge collection efficiency of ZnO cathode electrode by efficiently bridging the superficial troughs and ridges of ZnO with the photoactive PCDTBT: PC71BM polymer blend. The investigations show that vacuum sublimated C70 interlayer efficiently fills the gaps between ZnO and the polymer blend reducing accumulation of the charges at the interface and thus minimizing the recombination probability. It also plays a very crucial role in passivating ZnO electrode against interfacial traps due to adsorbed chemical species. The inclusion of C70 interlayer into the devices led to a substantial increase in device performance with PCE reaching close to 4%, an increment by a factor of 2 compared to the control devices. Our investigations aim towards showing the efficacy of C70 small molecule in significantly enhancing the PCE of ZnO based BHJ solar cells fabricated and measured in ambient conditions rather than setting benchmark efficiency for the configured device. However, better performances for the devices are conceivable by performing the fabrication and measurement in controlled inert atmosphere.

Enhance the light-harvesting capability of the ITO-free inverted small molecule solar cell by ZnO nanorods.

The ITO-free inverted SMPV1:PC71BM solar cells with an Al doped ZnO (AZO) transparent electrodes are fabricated. The AZO thin film prepared by pulsed laser deposition (PLD) technique exhibits high transmission (>85%) and low sheet resistance (~30 Ω/sq) and the power conversion efficiency (PCE) of devices based on AZO electrode can reach around 4%. To further enhance the light harvesting of the absorption layer of solar cells, ZnO nanorods interlayer is grown on the AZO layer before the deposition the active layer. The absorption spectrums of devices under various conditions are also simulated by RCWA method to identify the optical saturation length of the ZnO nanorods. The PCE of ITO-free inverted small molecule solar cell improved with ZnO nanorods can reach 6.6%.

Dialkylthio benzo[1,2-b:4,5-b′]difuran polymer for efficient organic photovoltaics with solvent treatment in active layers

The facile synthesis of dialkylthio substituted benzo[1,2-b:4,5-b′]difuran unit via a straightforward palladium-catalyzed carbon–sulfur bond formation reaction is reported. A novel donor–acceptor alternating copolymer was further developed by Stille coupling. The optical and electrochemical properties of the polymer have been carefully studied. The photovoltaic performance by blending the copolymer with [6,6]-phenyl-C71-butyric acid methyl ester were explored in both conventional and inverted solar cell devices. Solvent additive and vapor treatment exhibit positive effects on the device performance. The best devices delivered a power conversion efficiency of 5.48%, with an active layer processed from chloroform solutions with 1,8-diiodooctane (3%) and 30 s tetrahydrofuran vapor treatment.

Hybrid solar cells using solution-processed TiO2/Sb2S3 bilayer as electron transport layer

Efficient inverted hybrid solar cells using poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) bulk heterojunction as active layers and TiO2/Sb2S3 bilayers as electron transport layers (ETL) have been prepared in ambient air. The microstructural, photoelectronic, and electrochemical characteristics on these cells have been investigated. Photoluminescence measurement indicates that the exciton dissociation in TiO2/Sb2S3 is more effective than that in ITO/Sb2S3 structure. The Nyquist plots obtained from electrochemical impedance spectroscopy implies that charge recombination has been suppressed in TiO2/Sb2S3 with respect to Sb2S3 structure. In addition, the TiO2/Sb2S3 bilayer structure exhibits significantly enhanced external quantum efficiency in the visible region. The cell structure consisting of ITO/TiO2/Sb2S3/P3HT:PCBM/PEDOT:PSS/Ag with the TiO2/Sb2S3 bilayer synthesized under the optimized conditions gains an average efficiency of 3.52%, which is enhanced for ∼28.5% and ∼209% referring to the cells only with the TiO2 layer and Sb2S3 layer as the ETL, respectively.

Indium-free organic thin-film solar cells using a plasmonic electrode

We propose a new kind of organic solar cell (OSC) that substitutes the standard indium tin oxide (ITO) electrode with a silver layer with randomly arranged circular nanoholes (plasmonic electrode). The quasi-random structure in the silver layer efficiently converts wideband incident light into surface plasmon polaritons propagating along the surface of the silver film. In this way, the converted surface plasmon polaritons enhance light absorption in the active layer. We describe in detail the fabrication process we used and we give a thorough report of the resulting optical characteristics and performances. Although the transmittance of the plasmonic electrode is approximately one-third of that of the ITO electrodes, the power conversion efficiency of the OSCs with our plasmonic electrode is comparable to that of conventional inverted solar cells using ITO electrodes. Moreover, the obtained incident photon to current efficiency was better than that of the inverted solar cells in the wavelength regions around 400 nm and over 620 nm.

(Invited) Prisitine C60 and C70 fullerenes As Electron Transport Materials for Perovskite Solar Cells

The photovoltaic community is currently experiencing the revolution of the solar cells based on perovskite materials. Since the first reports based on a solid hole-transporting layer appeared, a great variety of organic molecules have been employed as electron or hole-selective contacts in regular and/or inverted perovskite-based solar cells (PSCs) (Figure 1 A and C, respectively). Fullerene and their derivatives are playing a very important role as electron-selective contact (ESC) in inverted solar cells as well as in self-assembled monolayers of fullerene derivatives on TiO2 in regular devices. Moreover, the interest of fullerenes in PSCs has been increased by the recent report of Snaith’s group, which describes the successful incorporation of [60]fullerene instead of the usual TiO2 as ESC in regular PSCs architectures (Figure 1B). In the present comunication, careful photovoltaic studies following a well-described experimental protocol have been carried out with PSC fabricated with [70]fullerene, [60]fullerene and [60]fullerene derivatives as Electron Transport Material in regular perovskite solar cell architecture, in order to study their suitability as ETM for PSC. The obtained results indicate that the use of these materials as ET materials in PSC allow the obtention of highly efficient devices (≥10%), due to the introduction of the "fullerene saturation approach". Figure 1

Design Flexibility of Ultrahigh Efficiency Four-Junction Inverted Metamorphic Solar Cells

The inverted metamorphic solar cell has highly tunable bandgaps, in part due to the metamorphic subcells. Using phosphide-based compositionally graded buffers, we show a wide variety of GaInAs solar cells, ranging in bandgap from 1.2 to 0.7 eV. These metamorphic subcells are all high quality and can be used for a wide variety of multijunction designs. GaInAs solar cells with 0.70 eV bandgaps are developed using an InAsP buffer that extends beyond the InP lattice constant, allowing access to an additional 2 mA/cm2 of photocurrent at AM1.5D and 25 °C. This subcell is implemented into a four-junction inverted metamorphic solar cell combined with an appropriate antireflective coating, which increases the series-connected multijunction current by 0.5 mA/cm2 with respect to designs using 0.74-eV GaInAs. However, the optimal design depends on the spectrum and operating temperature. We show how the device flexibility can be used to fine-tune the design for various spectra in order to maximize energy yield for a given operating condition. One-sun devices achieve 35.3 ± 1.2% efficiency under the AM0 spectra and 37.8 ± 1.2% efficiency under the global spectra at 25 °C. Concentrator devices designed for elevated operating temperature achieve 45.6 ± 2.3% peak efficiency under 690× the direct spectrum and 45.2 ± 2.3% efficiency at 1000× and 25 °C. Device optimization is performed for the direct spectrum on 1-sun devices with 2% shadowing, which achieve 39.8 ± 1.2% efficiency under the direct spectrum at 1 sun, highlighting the excellent performance and bandgap tunability of the four-junction inverted metamorphic solar cell.

Morphological control and performance improvement of organic photovoltaic layer of roll-to-roll coated polymer solar cells

We systematically investigated the thermal effects of drying process as well as thermal annealing on morphology and performance of inverted polymer solar cells (PSCs) fabricated by roll-to-roll (R2R) slot-die coating. Power conversion efficiency (PCE) of PSCs layer is strongly influenced by bulk heterojunction (BHJ) active layer. Compared to the conventional annealing process, we demonstrate that the drying process for transforming the wet active layer into solid-state film (an early polymer crystallization step) in the R2R process is a critical control of morphological evolution to the desired structure. We have demonstrated that a significant improvement in performance of inverted PSCs can be achieved by tuning the parameters in the drying process prior to the annealing process. This study provides the mechanistic understanding of how the drying and annealing processes affect the morphological evolution regarding to the crystallinity, BHJ structure and interface between layers. Moreover, with respect to environmental concerns, halogen-free are carried out as an alternative solvent to halogenated solvents. We successfully incorporate halogen-free solvent, o-xylene, with an elevated drying temperature for improving the performance of R2R slot-die coated inverted solar cells.

Influence of ZnO:Al, MoO3 and PEDOT:PSS on efficiency in standard and inverted polymer solar cells based on polyazomethine and poly(3-hexylthiophene)

This paper is devoted to the development of standard and inverted polymer solar cells based on polyazomethine (PAZ-Car-TPA), poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). We analyzed the influence of: (i) PEDOT:PSS or MoO3 as a hole transporting layer in standard devices, (ii) aluminum doping level (from 0 to 3.7%) in ZnO (obtained by atomic layer deposition, ALD) applied as an electron transporting layer in inverted solar cells and (iii) the method applied to obtain ZnO layer (ALD or sol-gel) on its photovoltaic properties. The best device configuration is ITO/AZO (3% Al)/P3HT:PCBM/MoO3/Ag which exhibits a power conversion efficiency of 1.51% under about 100mW/cm2 AM 1.5G simulated solar emission. Devices were additionally tested by electrochemical impedance spectroscopy. HOMO-LUMO levels of PAZ-Car-TPA and its mixture with HCl, H2SO4, p-toluenesulfonic acid, PEDOT:PSS and water were analyzed by cyclic voltammetry and quantum mechanical calculations using Density Functional Theory method.

Squaraine based solution processed inverted bulk heterojunction solar cells processed in air.

Inverted bulk heterojunction solar cells based on low temperature solution processed squaraine (SQ) and [6,6]-phenyl C71 butyric acid methyl-ester (PC71BM) with varying blend ratios were made in air. An optimized bulk heterojunction device of SQ and PC71BM (with a blend ratio of 1 : 6) showed a power conversion efficiency (PCE) of 2.45% with an incident photon to current conversion efficiency of 65% at 680 nm and a spectral window extending to the NIR region. The devices also showed an enhanced PCE value of 4.12% upon continuous illumination from an AM1.5G light source of intensity 1 Sun. The intensity dependent photocurrent studies showed a monomolecular recombination mechanism in the photovoltaic device performance. The device stored in air showed reasonable stability for a period of one month.

Electron-transporting third component modifying cathode for simplified inverted ternary blend solar cells

Electron transporting materials widely used in organic light-emitting diodes, such as Bphen, BCP and TPBI, are used as a third component to serve as a cathode buffer layer for fabricating simplified inverted ternary blend polymer solar cells without an additional cathode-modifying interlayer.