Inverted Device Configuration Research Articles

Influence of the charge transport layers on charge extraction and interface recombination in quasi-two-dimensional perovskite solar cells

The identification of electronic processes at the charge-selective contact buried interface is very important for photovoltaic research. The main loss of perovskite solar cell (PeSCs) is generally bound up with its charge transfer layer. Especially, the current record for the highest power conversion efficiency of quasi-two-dimensional (quasi-2D) PeSCs is achieved by inverted device configurations, compared with the efficiency of upright structures. This study investigated, the carrier recombination and charge extraction in quasi-2D PeSCs by leveraging scanning probe microscope technology, steady-state photoluminescence (PL) measurements, and time-resolved PL spectroscopy. The built-in potential in quasi-2D bulk perovskite can be regarded as a budget to hinder energy loss in inverted device configurations. Interface photogenerated recombination in quasi-2D PeSCs can be fully comprehended only when the complete device is under consideration. Our work underlines the significance of considering restructuring loss from the perspective of the complete device instead of individual layers or interfaces in quasi-2D PeSCs.

Anion‐Induced Catalytic Reaction in a Solution‐Processed Molybdenum Oxide for Efficient Inverted Ternary Organic Photovoltaics

AbstractSolution‐processed transition metal oxides (TMOs) prepared from complex ion precursors are developed as promising scalable interfacial layers for non‐fullerene organic photovoltaics (OPVs); however, challenges remain in achieving defect‐free and highly oriented metal‐oxygen networks without post‐deposition treatments due to the presence of residual organic metal‐binding ligands in films. Herein, the novel strategy that the problematic organic metal‐binding ligands in TMO precursors can be successfully eliminated by an anion‐induced catalytic reaction (ACR) at room temperature is demonstrated, in which the low‐level anions induce electron redistribution and instability of TMO precursors, expediting binding ligand removal during the hydrolysis reaction. The subsequent condensation process facilitates a dimensionally confined and continuous metal‐oxygen network with a 20‐fold increase in electrical conductivity (from 8.4 × 10−4 to 1.8 × 10−2 S m−1) and superior work function tunability (from 5.1 to 5.3 eV) compared to the pristine film. The ACR‐derived TMO thin film on top of a ternary PBDB‐TF:Y6:PC71BM photoactive layer enables an inverted device configuration with improved efficiency of 17.6%, as well as enhanced stability over 70% of the initial efficiency for up to 100 h AM 1.5G illumination.

Manipulation of Zinc Oxide with Zirconium Doping for Efficient Inverted Organic Solar Cells.

Solution-processed zinc oxide (ZnO) is one of the widely used electron transporting layers (ETLs) for organic solar cells (OSCs). However, low optical transparency along with thickness-sensitivity of ZnO ETL constrains the improvement of photovoltaic performance and large-scale fabrication compatibility. To resolve these issues, zirconium (Zr) doping is applied to tailor the optoelectronic and morphological properties of ZnO layer. This approach not only improves light transmittance with the suppressed parasitic absorption, but also provides an optimized surface morphology for enhancing charge extraction property and reducing potential of charge trap-assisted recombination. By using ZnO:Zr as ETL in inverted device configuration, the maximum power conversion efficiency (PCE) of PM6:Y6:PC71 BM solar cell devices is up to 17.2%, which makes an enhancement of 9.55% compared to ZnO-based devices (15.7%). As the thickness of ZnO:Zr ETL increases to ≈60nm, the presence of the lower parasitic absorption together with uniform surface morphology can help photovoltaic performance maintain above 15%, which is beyond the performance of the pristine ZnO-based device achieving only 11.9%. Such superiority of ZnO:Zr ETL is also validated by a series of well-known BHJ systems, where in comparison with the devices based on pristine ZnO ETL, a better photovoltaic performance from ZnO:Zr device can be achieved.

Recent progress in inverted perovskite solar cells employing nickel oxide (NiOx) as a hole transport materials

Perovskite solar cells (PSCs) have made phenomenal improvement in the power conversion efficiency from 3.8% in 2009 to 25.2% in 2020. Even though TiO2 continues to be the most used electron transport layer in PSCs, research in the field showed that n- type SnO2 when used on the transparent front electrode with conventional structure (n-i-p) has the highest device efficiency. However, their scope for further commercial application is limited due to the need for high temperature process and the comparatively higher J-V hysteresis in conventional (n-i-p) structure. Therefore, inverted device configuration (p-i-n) using NiOx as a hole transport material (HTM) has drawn considerable interest in the past several years. This is primarily due to their excellent chemical stability, smaller processing temperature for device fabrication, and good transmittance compared to organic HTMs. Deposition of NiOx hole transport material on the FTO/ITO substrate is an efficient way to improve the photo physics of manufacturing devices. It has shown impressive outcomes in inverted PSCs. Recent advances in the preparation of NiOx HTMs are detailed in this article. The impact of NiOx deposition on FTO/ITO substrate by different methods and their effect on electrical and optical properties was carefully studied. We also address the benefits and existing issues of using NiOx HTMs in PSCs and aim to foresee future success by incorporating them in high-efficiency inverted perovskite solar cell architecture.

Optimizing Lignosulfonic Acid-Grafted Polyaniline as a Hole-Transport Layer for Inverted CH3NH3PbI3 Perovskite Solar Cells.

A conducting polymer of lignosulfonic acid-grafted, polyaniline-doped camphorsulfonic acid (LS-PANI-CSA), created via a low-temperature solution process, has been explored as an efficient hole-transport layer (HTL) for inverted single cation–anion CH3NH3PbI3 perovskite solar cells. The performance of the solar cell was optimized in this study by tuning the morphology and work function of LS-PANI-CSA films using dimethylsulfoxide (DMSO) as a solvent in treatment. Results showed that DMSO washing enhanced the electronic properties of the LS-PANI-CSA film and increased its hydrophobicity, which is very important for perovskite growth. The perovskite active layer deposited onto the DMSO-treated LS-PANI-CSA layer had higher crystallinity with large grain sizes (>5 μm), more uniform and complete surface coverage, and very low pinhole density and PbI2 residues compared to untreated LS-PANI-CSA. These enhancements result in higher device performance and stability. Using DMSO-treated LS-PANI-CSA as an HTL at 15 nm of thickness, a maximum 10.8% power conversion efficiency was obtained in ITO/LS-PANI-CSA/MAPbI3/PCBM/BCP/Ag inverted-device configurations. This was a significant improvement compared to 5.18% for devices based on untreated LS-PANI-CSA and a slight improvement over PEDOT:PSS-based devices with 9.48%. Furthermore, the perovskite based on treated LS-PANI-CSA showed the higher stability compared to both untreated LS-PANI-CSA and PEDOT:PSS HTL-based devices.

Effect of solvent vapour annealing on bismuth triiodide film for photovoltaic applications and its optoelectronic properties

The annealing ambient conditions affect the morphology and optoelectronic quality of bismuth triiodide film and hence impact on the photovoltaic device parameters.

Dual-functional cathode buffer layer for power conversion efficiency enhancement of bulk-heterojunction solar cells

Abstract Here, we report a luminescent cathode buffer layer (CBL) for power conversion efficiency (PCE) enhancement of organic solar cells (OSCs). ZnO doped with Aluminum (Al) and Europium (Eu) was cast as CBL by a solution-processed method. CBL serves a dual purpose by acting both as a spectral conversion and an electron transporting layer. The luminescent ZnO:Al,Eu CBL layer has broad absorption spanning the ultraviolet (300–400 nm) spectrum, contributing to absorption enhancement. Moreover, the emission of CBL layer overlaps with the absorption of poly [N -9′-heptadecanyl-2,7-carbazole- alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) polymer thus elevating the overall absorption of the active layer and improving the photocurrent. With this ZnO:Al,Eu CBL in the inverted device configuration (ITO/CBL/active layer/MoOx/Al), an enhanced power conversion efficiency (PCE) of 6.9% was obtained while the device with pristine ZnO as CBL showed PCE of 5.9%. A blend of PCDTBT donor and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) acceptor was used as an active layer in both the cases. In ZnO:Al,Eu CBL layer, Al doping improves the conductivity, while Eu doping significantly enhances the emission in the visible region by down-shifting the incoming solar UV light to the visible range which overlaps with the absorption of PCDTBT polymer resulting in energy transfer and improved overall device efficiency. The findings of the study show the significance of luminescent ZnO:Al,Eu nanoparticle CBL in enhancing the performance of organic solar cells.

Solution‐Processed Tin Oxide‐PEDOT:PSS Interconnecting Layers for Efficient Inverted and Conventional Tandem Polymer Solar Cells

Tin oxide nanoparticles are employed as an electron transporting layer in solution‐processed polymer solar cells. Tin oxide based devices yield excellent performance and can interchangeably be used in conventional and inverted device configurations. In combination with poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) as a hole transporting layer, tin oxide forms an effective interconnecting layer (ICL) for tandem solar cells. Conventional and inverted tandem cells with this ICL provide efficiencies up to 10.4% in good agreement with optical‐electrical modeling simulations. The critical advantage of tin oxide in an ICL in a conventional tandem structure over the commonly used zinc oxide is that the latter requires the use of a pH‐neutral formulation of PEDOT:PSS to fabricate the ICL, limiting the open‐circuit voltage (VOC) because of its low work function. The SnO2/PEDOT:PSS ICL, on the other hand, provides a nearly loss‐free VOC.

A Cu3PS4 nanoparticle hole selective layer for efficient inverted perovskite solar cells

Cu3PS4 nanoparticles are used as a new inorganic hole selective layer (HSL) to fabricate efficient perovskite thin-film solar cells in the inverted device configuration.

Effects of solvent vapour annealing on the performances of benzo[1,2-b:4,5-b′]dithiophene and 4,7-di(4-hexyl-thiophen-2-yl)-5,6-difluorine-2,1,3-benzothiadiazole-based alternating polymer solar cells with different configurations

Solvent vapour annealing (SVA) treatments have been proven to be an efficient approach to optimize the morphology of the active layer in bulk heterojunction polymer solar cells. For an alternating polymer based on dioctylthiophene substituted benzo[1,2-b:4,5-b′]dithiophene and 4,7-di(4-hexyl-thiophen-2-yl)-5,6-difluorine-2,1,3-benzothiadiazole (PBDTT-DFDTBT), enhanced power conversion efficiency (PCE) from 7.65% to 8.06% was achieved in regular device configuration as ITO/PEDOT:PSS/PBDTT-DFDTBT:phenyl-C71-butyric acid methyl ester (PC71BM)/poly(9,9-bis(3-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluoren)(PFN)/Al after tetrahydrofuran (THF) SVA treatment. As for the inverted device configuration of ITO/ZnO/PFN/PBDTT-DFDTBT:PC71BM/MoO3/Al with THF SVA post-treatment, simultaneously decreased open-circuit voltage (VOC) and short-circuit current density (JSC) were obtained, giving rise to a reduced PCE of 6.60%. We observed increased absorption intensity, improved hole/electron mobility and phase purity for the THF treated blend films, implying the improvement of the photovoltaic performances. However, the THF SVA-induced upper migration of the PC71BM in the active layer and the resulting trap-induced recombination are ascribed to the suppressed photovoltaic parameters of inverted device relative to the regular one.

Solution processed graphene as electron transport layer for bulk heterojunction based devices

The exceptional electrical properties of the graphene offer many opportunities for the performance improvement in the devices. In this study, the importance of incorporation of solution processed graphene as an electron transport layer (ETL) in bulk heterojunction (BHJ) devices is elucidated by measuring the current-voltage characteristics under dark and illumination conditions. Raman spectra P3HT:PCBM blend thin films display the same modes as of pristine P3HT. The UV/Vis analysis confirms the formation of P3HT:PCBM blend. The three different devices of architectures: standard structure, standard structure with ETL and inverted structure with ETL, comprising active layer of as-prepared P3HT:PCBM blends are fabricated and compared. Various Schottky diode parameters like ideality factor, series resistance, and barrier height are determined from current-voltage curves using Cheung and Cheung approach. The incorporation of graphene ETL in the device shows the enhancement in photocurrent resulting from the better extraction and improved charge collection efficiency. The photovoltaic effect is also observed for inverted device configuration. The charge transport analysis of the devices show overall power law trend, i.e. Ohmic current at low voltage followed by space charge limited conduction in existence of discrete trap levels at higher voltage range.

The Impact of Device Polarity on the Performance of Polymer–Fullerene Solar Cells

AbstractDiketopyrrolopyrrole (DPP)‐conjugated polymers are a versatile class of semiconductors for application in organic solar cells because of their tunable optoelectronic properties. A record power conversion efficiency (PCE) of 9.4% was recently achieved for DPP polymers, but further improvements are required to reach true efficiency limits. Using five DPP polymers with different chemical structures and molecular weights, the device performance of polymer:fullerene solar cells is systematically optimized by considering device polarity, morphology, and light absorption. The polymer solubility is found to have a significant effect on the optimal device polarity. Soluble polymers show a 10–25% increase in PCE in inverted device configurations, while the device performance is independent of device polarity for less soluble DPP derivatives. The difference seems related to the polymer to fullerene weight ratio at the ZnO interface in inverted devices, which is higher for more soluble DPP polymers. Optimization of the nature of the cosolvent to narrow the fibril width of polymers in the blends toward the exciton diffusion length enhances charge generation. Additionally, the use of a retroreflective foil increases absorption of light. Combined, the effects afford a PCE of 9.6%, among the highest for DPP‐based polymer solar cells.

Organic solar cells on Al electroded opaque substrates: Assessing the need of ZnO as electron transport layer

Interfacial buffer layers such as electron and hole transporting layers are crucial in maximizing power conversion efficiencies of organic solar cell (OSC) devices. In this manuscript, we studied the role of solution processed ZnO as electron transport layer in devices with Aluminum (Al) cathode on opaque substrates in inverted device configuration where Al and Gold (Au) are used as cathode and anode, ZnO and MoO3 are used as electron and hole transport layers respectively whilst P3HT:PC60BM is used as active layer (ActL). The results show that a thin layer of Aluminum Oxide (Al2O3) present on thermally evaporated Al cathode acts as an electron transport layer (ETL) when ZnO is absent, with surface states present on Al2O3 layer aiding the electron transport to cathode. The presence of ZnO as ETL between Al and active layer leads to a decrease in the efficiency of solar cell from 2.9% to 1.9%, attributed to the trapping of electrons at Al/ZnO interface due to unfavourable band alignment created by the presence of composite Al2O3/ZnO layer, implying that an additional ETL is not required when Al is used as a bottom electrode and could be an added advantage due to reduced processing steps.

Electron injection and interfacial trap passivation in solution-processed organic light-emitting diodes using a polymer zwitterion interlayer

Abstract Here we describe the use of a polymer zwitterion as a solution-processable material that serves as the key component of the electron injection layer (EIL) in solution processed organic light-emitting diodes (OLEDs). Poly(sulfobetaine methacrylate) (PSBMA) was employed in both regular and inverted device configurations as a work-function modifier for Al and ZnO cathodes, respectively. For both architectures, PSBMA significantly improved the OLED performance when compared to reference devices without EIL in terms of turn-on voltage and luminance. In inverted devices, PSBMA showed a passivation effect on ZnO surface trap states, producing better performing and more stable devices.

Alternating polymers based on fluorinated alkoxyphenyl-substituted benzo[1,2-b:4,5-b′]dithiophene and isoindigo derivatives for polymer solar cells

To effectively modulate the molecular energy levels of isoindigo-based polymers through side-chain modification on the donor segments, three alkoxyphenyl-modified benzo[1,2-b:4,5-b′]dithiophene (BDT) were synthesized and used as electronic-donor units, which were copolymerized with the electron-withdrawing segment 2-ethylhexyl-substituted isoindigo (IID) to construct donor-acceptor (D-A) polymers. The obtained three polymers are named as P-IID, mF-IID and oF-IID, which possess 4-(2-octyldodecaneoxy)-phenyl, 2-fluoro-4-(2-octyldodecaneoxy)-phenyl and 3-fluoro-4-(2-octyldodecaneoxy)-phenyl side-chains in the BDT unit, respectively. All polymers display broad absorption regions with the film absorption edges of 759 nm (P-IID), 742 nm (mF-IID) and 726 nm (oF-IID). The fluorination on the phenyl side-chains of BDT unit results in obvious blue-shift in the absorption spectra, especially for the ortho-position fluorinated polymer oF-IID because of the enlarged dihedral angles between the phenyl rings and the BDT backbones. Gradient reduction in the highest occupied molecular orbital (HOMO) energy levels are observed for the three polymers, with HOMO values of −5.35 eV for P-IID, −5.48 eV for mF-IID and −5.53 eV for oF-IID, separately. As the result of the HOMO energy levels, polymer solar cells (PSCs) with inverted device configuration by blending the polymers with [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM) show continually increased open circuit voltage (Voc) from 0.84 V for P-IID to 0.88 V (mF-IID) and then a notable value of 0.96 V for oF-IID. However, the fluorination leads to intensively reduced short-circuit current density (Jsc) of the PSCs, 9.68 mA cm−2 for P-IID, 5.61 mA cm−2 for mF-IID and a very low value of 1.61 mA cm−2 for oF-IID, respectively. Consequently, moderate power conversion efficiency (PCE) of 5.23% is achieved for the non-fluorinated polymer P-IID as well as a decreased value of 2.50% for the meta-fluorinated polymer and a very low PCE of 0.93% for the ortho-fluorinated polymer. These results imply that although fluorine substituted side-chain in the donor unit can effectively deepen the HOMO energy levels of the polymers to obtain enlarged Voc, the absorption losses of the fluorinations caused low Jsc should be taken into consideration to design high efficient electron-donating alternating polymers for polymer solar cells.

Efficient bifacial semitransparent perovskite solar cells with silver thin film electrode

Bifacial semi-transparent perovskite solar cells (PSCs) are highly attractive for improving the efficiency of tandem solar cells and extending bifacial photovoltaic applications. However, the further improvement of bifacial performance has always been limited by the semitransparent electrode. Here we report an efficient triple-layer cathode of polyethyleneimine ethoxylate/silver/molybdenum oxide (PEIE/Ag/MoOx) for hysteresis-free planar PSCs with an inverted device configuration. Firstly, the optimal thickness of Ag electrode (11nm) is achieved by theoretical calculation, and the fabricated PSC achieves the PCE of 8.04% at indium-tin-oxide (ITO) side and 5.24% at Ag film side. Then, by introducing the MoOx (10nm) optical coupling layer and PEIE (10nm) interfacial modifying layer, the resulted PSC shows an obviously increased PCEs of 13.55% (ITO side) and 8.41% (Ag side), which is 60% higher than that of Ag only PSCs. The optimized PSCs has a structure of Glass/ITO/Poly (3,4-ethylenedioxythiophene): poly(styrenesulfonate)/Methylammonium-Lead-Iodide (about 300nm)/Phenyl-C61-butyric-acid-methyl-ester/PEIE/Ag/MoOx. It is thought that the enhanced Ag film transmittance by high-refractive-index MoOx and electron extraction ability by low-work-function PEIE contribute to the improved cathode property and bifacial PSC performance. Moreover, when a back-reflector is employed, the PCE of device could be further improved to 14.50% (ITO side) and 11.37% (Ag side).

Optimization of the power conversion efficiency in high bandgap pyridopyridinedithiophene-based conjugated polymers for organic photovoltaics by the random terpolymer approach

We report that the organic photovoltaic (OPV) performance of wide band gap pyridopyridinedithiophene-based conjugated polymers can be significantly improved by employing the random terpolymer approach for the development of new pyridopyridinedithiophene-based conjugated polymers. This is demonstrated by the synthesis of the alternating copolymer (P1) consisting of 3,3′-difluoro-2,2′-bithiophene and pyridopyridinedithiophene and the random terpolymer (P2) containing pyridopyridinedithiophene 3,3′-difluoro-2,2′-bithiophene and thiophene. OPV devices fabricated by P1 and P2 in combination with PC61BM and PC71BM in an inverted device configuration exhibited power conversion efficiencies (PCEs) of 1.5% and 4.0%, respectively. We identified that the main reason for the enhanced performance of the OPV devices based on the P2 random copolymer was the improved morphology (miscibility) between P2 and PCBM as compared to P1. More specifically, atomic force microscopy (AFM) and scanning electron microscopy (SEM) studies revealed that the P1 based films showed rougher surface with clear crystallization/precipitation of the polymer chains even after the addition of chloronaphthalene (CN) to the chloroform processing solvent which significantly limited the short circuit current density (JSC), fill factor (FF) and overall performance of the prepared photovoltaic devices. On the other hand, P2 based films showed better miscibility with the acceptor particularly when processed using 5% CN containing chloroform solvent giving a respectable improvement in the PCE of the photovoltaic devices.

Synthesis and photovoltaic properties of donor–acceptor conjugated polymers based on 4,7-dithienyl-2,1,3-benzothiadiazole functionalized silole

Three new donor–acceptor (D–A) type of conjugated polymers, with 1,1-diheptyl-3,4-diphenyl-2,5-bis(4,7-dithienyl-2,1,3-benzothiadiazole)-silole as acceptor unit, were synthesized through palladium-catalyzed Stille polymerization. The resulting polymers exhibit a broad absorption in the range from 350 to 850nm and have excellent thermal stability with decomposition temperature up to 400°C. The HOMO energy levels of the polymers determined from CV are lower than −5.1eV, so these polymers are promising donor materials for polymer solar cells (PSCs). Bulk-heterojunction (BHJ) PSCs based on an inverted device configuration of ITO/PFN/PDTBDT-HSTBT:PC61BM/MoO3/Ag showed good photovoltaic performance with power conversion efficiency of 3.24%.

Structure-induced resonant tail-state regime absorption in polymer: fullerene bulk-heterojunction solar cells

In this work, we present resonant tail-state regime absorption enhanced organic photovoltaics. We combine periodically structured ${\mathrm{TiO}}_{2}$ bottom electrodes with P3HT-PCBM bulk-heterojunction solar cells in an inverted device configuration. The wavelength-scale patterns are transferred to the electron-selective bottom electrodes via direct laser interference patterning, a fast method compatible with roll-to-roll processing. Spectroscopic and optoelectronic device measurements suggest polarization-dependent absorption enhancement along with photocurrent generation unambiguously originating from the population of tail states. We discuss the effects underlying these absorption patterns with the help of electromagnetic simulations using the discontinuous Galerkin time domain method. For this, we focus on the total absorption spectra along with spatially resolved power loss densities. Our simulations stress the tunability of the absorption resonances towards arbitrary wavelength regions.

Transition Metal-Oxide Free Perovskite Solar Cells Enabled by a New Organic Charge Transport Layer.

Various electron and hole transport layers have been used to develop high-efficiency perovskite solar cells. To achieve low-temperature solution processing of perovskite solar cells, organic n-type materials are employed to replace the metal oxide electron transport layer (ETL). Although PCBM (phenyl-C61-butyric acid methyl ester) has been widely used for this application, its morphological instability in films (i.e., aggregation) is detrimental. Herein, we demonstrate the synthesis of a new fullerene derivative (isobenzofulvene-C60-epoxide, IBF-Ep) that serves as an electron transporting material for methylammonium mixed lead halide-based perovskite (CH3NH3PbI(3-x)Cl(x)) solar cells, both in the normal and inverted device configurations. We demonstrate that IBF-Ep has superior morphological stability compared to the conventional acceptor, PCBM. IBF-Ep provides higher photovoltaic device performance as compared to PCBM (6.9% vs 2.5% in the normal and 9.0% vs 5.3% in the inverted device configuration). Moreover, IBF-Ep devices show superior tolerance to high humidity (90%) in air. By reaching power conversion efficiencies up to 9.0% for the inverted devices with IBF-Ep as the ETL, we demonstrate the potential of this new material as an alternative to metal oxides for perovskite solar cells processed in air.