Flexible Photovoltaic Devices Research Articles

A solution processed Ag-nanowires/C60 composite top electrode for efficient and translucent perovskite solar cells

All solution-processed semi-transparent perovskite solar cells (PSCs) with high efficiency were fabricated with Ag nanowire (AgNW) top electrode directly coated on the organic electron transport layer (ETL). A unique AgNW/ETL interface morphology was attained by the fabrication of the AgNW-C60 composite electrode through a sequential spin-coating method. The C60 filler spin-coated on the AgNW network engendered a high contact area and favourable energy level alignment at the interface between the ETL and AgNW. As a result, devices based on AgNW-C60 electrode far outperformed devices with pristine AgNW electrodes. By adjusting the thickness of the AgNW film, the AgNW-C60 devices attained a PCE of 11.02%. The incorporation of C60 into the AgNW electrode also led to remarkable stability improvement. The low-temperature solution-processed and semi-transparent AgNW-C60 top electrode is also applicable to other PSC structures with various interfacial layers, thus showing a promising path towards fully-printable, flexible and translucent photovoltaic devices.

Breakdown of the electron delocalization in hexagonal borophene toward tunable energy gap

The absence of energy gap (Eg) in borophene polymorphs, where multi-center bonds favor electron delocalization and form metallic phases, restricts their potential applications in microelectronic and optoelectronic industry. Herein, we propose to incorporate chalcogenide atoms to breakdown the electron delocalization in hexagonal borophene and introduce an Eg. Based on first principle calculation, we report a group of semiconducting boron chalcogenide monolayers, denoted as B2X (X = O, Se and Te), with an indirect Eg of 2.84, 0.97 and 1.04 eV under HSE06 levels, respectively. Moreover, the introduced Eg can be modulated by external strains (±6%) affordable by substrates, with electronic effective mass being comparable or even lower than those of MoS2 along certain directions. Furthermore, B2X monolayers exhibit desirable light absorption properties under visible light, which can be improved by stacking bilayers. The current study presents an alternative route to introduce the Eg in borophene, endows them with new vitality and highlights their promise in flexible electronics, optoelectronics and photovoltaic devices.

Photonic-structured TCO front contacts yielding optical and electrically enhanced thin-film solar cells

Wavelength-structured transparent conductive oxide (TCO) electrodes are highly promising to improve both the optical and electrical performance of photovoltaic (PV) devices, due to wave-optical light-trapping (LT) effects and higher TCO volume without increasing optical losses.Herein we present a complete study of the benefits of microstructured IZO contacts applied on amorphous-silicon (a-Si) thin film solar cells. The IZO LT structures were integrated by an innovative colloidal lithography process on the front contact of the cells, resulting in enhancements of 26.7% in photocurrent, with respect to planar reference cells, when using an ultra-thin (30 nm) flat IZO layer between the LT structures and the a-Si absorber. However, the best efficiency enhancement (23.1%) was attained with an optimized thickness of 190 nm for this layer, due to a more favorable combination of optical and electrical gains.In view of the application of this LT strategy in flexible PV devices operating under bending, the angular response of the cells was studied for 0-90° incidence angles. This showed that the LT enhancements are generally higher at oblique incidence, reaching 53.2% and 52%, respectively in photocurrent and efficiency, at ± 70° angles with the optimized flat IZO thickness of 190 nm; and 52.2% in efficiency at ± 40° with the ultra-thin thickness of 30 nm. These results are among the highest gains reported thus far for LT-enhanced thin film solar cells.

Toward ultra-thin and full functional perovskite solar cells by broadband light scattering management and efficient interfacial modification

Decreasing the thickness of perovskite absorption layer is significant for obtaining semi-transparent, tandem and flexible photovoltaic devices with high performance. However, the light harvesting would be influenced due to the limited absorption. Introducing plasmonic metal nanoparticles into perovskite solar cells (PSCs) is regarded as a promising approach on the development of ultra-thin PSCs. In this work, gold nanostars (Au NSs) with strong local electromagnetic field and SnO2 surface modification layer were successfully incorporated into ultra-thin PSCs. This design increased light harvesting and carrier transporting. Furthermore, we systematically investigated the plasmon effect of Au NSs on the power conversion efficiency (PCE) of the PSCs with different thicknesses of the perovskite layer and observed that the deceased thickness of perovskite layer led to more intense PCE improvement as the same Au NSs were introduced. The ultra-thin PSCs with 250-nm perovskite layer show a PCE improvement from 15.34 to 18.50% after Au NSs/SnO2 modification, accounting for a maximum enhancement factor of 20.60%. Simultaneously, the modified PSCs with 400-nm perovskite layer exhibit a best reported PCE of 20.06% for the plasmonic PSCs, with an enhanced factor of 13.70% and enhanced light stability. More importantly, flexible, large area and semi-transparent PSCs were also achieved. The flexible devices show more than 90% of their initial PCE values after 100 bending cycles. Our result promises a facile way for developing efficient, flexible and semi-transparent PSCs with ultra-thin absorption layer.

Tunable visible-light excitonic absorption and high photoconversion efficiency in two-dimensional group-VI monolayer materials

Recent discovery of tellurene, a two-dimensional (2D) allotrope of tellurium (Te), has effectively extended the realm of 2D materials to group VI. In this paper, we have proceeded to determine the electronic and excitonic optical properties of 2D tellurene and selenene, together with stable monolayer ${\mathrm{TeSe}}_{2}$ in this group, by means of the GW-Bethe-Salpeter equation approach. We find that these materials stabilized in $1\mathrm{T}\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$-like structure appear to be indirect semiconductors and possess ultrahigh isotropic carrier mobilities, which outperform their 2D ${\mathrm{MoS}}_{2}$ counterpart. The exciton is found to be strongly bound and thermally stable at room temperature with a binding energy of 0.37, 0.53, and 0.54 eV, respectively, for the three types of monolayers. Strikingly, we find that the optical gaps of monolayer selenene and ${\mathrm{TeSe}}_{2}$ lie in the ideal energy window of $1.0--1.5\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ corresponding to the maximum photoconversion efficiency. The number of layers in the stack further provides an effective means to tune the absorption edge across a wide energy range. These characteristics combined with the strong optical absorbance in the whole visible-light region make both 2D materials promising in the applications of efficient, ultrathin, and flexible photovoltaic devices with upper bounds to the conversion efficiency that rivals organic and dye-sensitized devices.

On the origin of open-circuit voltage losses in flexible n-i-p perovskite solar cells

ABSTRACT The possibility to manufacture perovskite solar cells (PSCs) at low temperatures paves the way to flexible and lightweight photovoltaic (PV) devices manufactured via high-throughput roll-to-roll processes. In order to achieve higher power conversion efficiencies, it is necessary to approach the radiative limit via suppression of non-radiative recombination losses. Herein, we performed a systematic voltage loss analysis for a typical low-temperature processed, flexible PSC in n-i-p configuration using vacuum deposited C60 as electron transport layer (ETL) and two-step hybrid vacuum-solution deposition for CH3NH3PbI3 perovskite absorber. We identified the ETL/absorber interface as a bottleneck in relation to non-radiative recombination losses, the quasi-Fermi level splitting (QFLS) decreases from ~1.23 eV for the bare absorber, just ~90 meV below the radiative limit, to ~1.10 eV when C60 is used as ETL. To effectively mitigate these voltage losses, we investigated different interfacial modifications via vacuum deposited interlayers (BCP, B4PyMPM, 3TPYMB, and LiF). An improvement in QFLS of ~30–40 meV is observed after interlayer deposition and confirmed by comparable improvements in the open-circuit voltage after implementation of these interfacial modifications in flexible PSCs. Further investigations on absorber/hole transport layer (HTL) interface point out the detrimental role of dopants in Spiro-OMeTAD film (widely employed HTL in the community) as recombination centers upon oxidation and light exposure.

Two-Dimensional/Three-Dimensional Schottky Junction Photovoltaic Devices Realized by the Direct CVD Growth of vdW 2D PtSe2 Layers on Silicon.

Two-dimensional (2D) platinum diselenide (PtSe2) layers are a new class of near-atom-thick 2D crystals in a van der Waals-assembled structure similar to previously explored many other 2D transition-metal dichalcogenides (2D TMDs). They exhibit distinct advantages over conventional 2D TMDs for electronics and optoelectronics applications such as metallic-to-semiconducting transition, decently high carrier mobility, and low growth temperature. Despite such superiority, much of their electrical properties have remained mostly unexplored, leaving their full technological potential far from being realized. Herein, we report 2D/three-dimensional Schottky junction devices based on vertically aligned metallic 2D PtSe2 layers integrated on Si wafers. We directly grew 2D PtSe2 layers of controlled orientation and carrier transport characteristics via a low-temperature chemical vapor deposition process and investigated 2D PtSe2/Si Schottky junction properties. We unveiled a comprehensive set of material parameters, which decisively confirm the presence of excellent Schottky junctions, i.e., high-current rectification, small ideality factor, and temperature-dependent variation of Schottky barrier heights. Moreover, we observed strong photovoltaic effects in the 2D PtSe2/Si Schottky junction devices and extended them to realize flexible photovoltaic devices. This study is believed to significantly broaden the versatility of 2D PtSe2 layers in practical and futuristic electronic devices.

Solar Cells: Quantifying Quasi‐Fermi Level Splitting and Mapping its Heterogeneity in Atomically Thin Transition Metal Dichalcogenides (Adv. Mater. 25/2019)

Two-dimensional atomically thin transition metal dichalcogenides (TMDs) show a remarkable light–matter interaction and atomic thickness. In article number 1900522, Mike Tebyetekerwa, Yuerui Lu, Daniel Macdonald, Hieu T. Nguyen, and co-workers quantify the possible maximum open-circuit voltages, one of the most important photovoltaic parameters, of monolayer TMD-based solar cells. The values they determine reveal the potential of atomically thin TMDs for high-voltage, ultralight, flexible, and eye-transparent future photovoltaic devices.

A simple and robust route toward flexible CIGS photovoltaic devices on polymer substrates: Atomic level microstructural analysis and local opto-electronic investigation

In this work, copper indium gallium selenide (Cu(In,Ga)Se2; CIGS) absorbers were grown on polyimide (PI)/molybdenum substrates by a three-stage co-evaporation process at various temperatures, film formation was systemically studied using various advanced characterization methods such as transmission electron microscopy, micro-Raman spectroscopy, Kelvin probe force microscopy, and atom probe tomography. The CIGS films on PI were found to exhibit considerable physical and electrical variations with respect to the process temperature of three-stage co-evaporation. In particular, when the process temperature reached 400 °C, the CIGS absorber on PI began to exhibit controlled microstructure and intergrain properties. By adjusting the microstructure and intergrain properties of the absorber films by means of the process temperature of three-stage co-evaporation, flexible CIGS solar cells on PI with an efficiency of 16.7% (with anti-refection coating) were achieved.

Highly efficient flexible organic photovoltaics using quasi-amorphous ZnO/Ag/ZnO transparent electrodes for indoor applications

We report the indoor performance of flexible organic photovoltaic devices utilizing quasi-amorphous ZnO/Ag/ZnO as the transparent conducting electrode. A ZnO/Ag/ZnO electrode with specific thickness values of 40/9/50 nm provides excellent transparent conducting electrode properties with transmittances up to 92% in the visible region, a sheet resistance of 4.8 Ω/sq, and a root-mean squared surface roughness value of 2.1 nm. In addition, the micro-cavity effect and quasi-amorphous structural properties of the ZnO/Ag/ZnO electrode allow further enhanced light absorption and mechanical stability, respectively. Poly (3-hexylthiophene):indene-C60 bisadduct photoactive layer-based inverted organic photovoltaics with the ZnO/Ag/ZnO (40/9/50 nm) electrode yield an averaged power-conversion efficiency of 12.3% under a light-emitting diode lamp with a luminance of 500 lux, which is 20% greater than the power-conversion efficiency value of the reference organic photovoltaics with an indium tin oxide electrode. Furthermore, the same organic photovoltaics on flexible polyethylene terephthalate substrates exhibit excellent mechanical stability (i.e., 92% of the initial power-conversion efficiency value is maintained even after 400 bending cycles with a bending radius of 9.55 mm), with averaged power-conversion efficiency values of 10.2% under the 500-lux light-emitting diode.

Graphene for Flexible Photovoltaic Devices

Flexible photovoltaic devices (FPD’s) are emerging as next-generation technology in photovoltaic research. FPD’s have attracted great research attention because of their broad potential applications especially in wearable devices, portable electronics, integrated textiles, unmanned aerial vehicles, transportation, and military etc. The existing technologies have evolved over the years, improving efficiency and performance of photovoltaic devices. However, these technologies mostly rely on rigid electrodes that are brittle, costly and chemically unstable. For FPD’s to become practical, new materials that offer inherent flexibility without compromising on mechanical and optical properties must be the focus. Researchers have made significant advances over the past decade towards developing various aspects of FPD’s to improve its optical transmittance, mechanical stability, chemical stability etc. Graphene is increasingly been recognized as an excellent material for flexible photovoltaic devices because of its unique optical, electrical and mechanical properties. The prospects of introducing an inexpensive and abundant carbon-based material such as graphene in making flexible, low-cost, transparent PV cells cannot be over emphasized. However, the method to synthesize graphene to achieve the best performance is still complicated. This paper presents a brief overview of recent developments made in flexible photovoltaic devices using graphene.

Rapid, Chemical-Free Generation of Optically Scattering Structures in Poly(ethylene terephthalate) Using a CO2 Laser for Lightweight and Flexible Photovoltaic Applications

Highly light scattering structures have been generated in a poly(ethylene terephthalate) (PET) film using a CO2 laser. The haze, and in some cases the transparency, of the PET films have been improved by varying the processing parameters of the laser (namely, scanning velocity, laser output power, and spacing between processed tracks). When compared with the unprocessed PET, the haze has improved from an average value of 3.26% to a peak of 55.42%, which equates to an absolute improvement of 52.16% or a 17-fold increase. In addition to the optical properties, the surfaces have been characterised using optical microscopy and mapped with an optical profilometer. Key surface parameters that equate to the amount and structure of surface roughness and features have been analysed. The CO2 laser generates microstructures at high speed, without affecting the bulk properties of the material, and is inherently a chemical-free process making it particularly applicable for use in industry, fitting well with the high-throughput, roll to roll processes associated with the production of flexible organic photovoltaic devices.

Solution‐Processed Metal Oxide Nanocrystals as Carrier Transport Layers in Organic and Perovskite Solar Cells

AbstractThere has been rapid progress in solution‐processed organic solar cells (OSCs) and perovskite solar cells (PVSCs) toward low‐cost and high‐throughput photovoltaic technology. Carrier (electron and hole) transport layers (CTLs) play a critical role in boosting their efficiency and long‐time stability. Solution‐processed metal oxide nanocrystals (SMONCs) as a promising CTL candidate, featuring robust process conditions, low‐cost, tunable optoelectronic properties, and intrinsic stability, offer unique advantages for realizing cost‐effective, high‐performance, large‐area, and mechanically flexible photovoltaic devices. In this review, the recent development of SMONC‐based CTLs in OSCs and PVSCs is summarized. This paper starts with the discussion of synthesis approaches of SMONCs. Then, a broad range of SMONC‐based CTLs, including hole transport layers and electron transport layers, are reviewed, in which an emphasis is placed on the improvement of the efficiency and device stability. Finally, for the better understanding of the challenges and opportunities on SMONC‐based CTLs, several strategies and perspectives are outlined.

High-performance and flexible photodetectors based on chemical vapor deposition grown two-dimensional In2Se3 nanosheets

Two-dimensional (2D) In2Se3 with unique optical and electrical properties has great potential in next generation optoelectronics and multilevel phase-change memories. Here, for the first time, we report high-performance rigid and flexible photodetectors based on chemical vapor deposition (CVD) grown 2D In2Se3. Both rigid and flexible 2D In2Se3 photodetectors show a broadband response range from ultraviolet (254 nm) to visible light (700 nm). High photoresponsivities of 578 and 363 A · W−1 are achieved using rigid and flexible 2D In2Se3 photodetectors, respectively, under 700 nm light illumination, which are higher than those of photodetectors based on mechanically exfoliated 2D In2Se3 and physical vapor deposition grown 2D In2Se3. Furthermore, flexible 2D In2Se3 photodetectors show good mechanical durability and photoresponse stability under repeated bending tests. A high and stable photoresponse provides an opportunity for CVD-grown 2D In2Se3 applications in flexible optoelectronic and photovoltaic devices.

A Smart Flexible Solid State Photovoltaic Device with Interfacial Cooling Recovery Feature through Thermoreversible Polymer Gel Electrolyte.

Quasi-solid-state dye-sensitized solar cells (DSSCs) fabricated with lightweight flexible substrates have a great potential in wearable electronic devices for in situ powering. However, the poor lifespan of these DSSCs limits their practical application. Strong mechanical stresses involved in practical applications cause breakage of the electrode/electrolyte interface in the DSSCs greatly affecting their performance and lifetime. Here, a mechanically robust, low-cost, long-lasting, and environment-friendly quasi-solid-state DSSC using a smart thermoreversible water-based polymer gel electrolyte with self-healing characteristics at a low temperature (below 0 °C) is demonstrated. When the performance of the flexible DSSC is hindered by strong mechanical stresses (i.e., from multiple bending/twisting/shrinking actions), a simple cooling treatment can regenerate the electrode/electrolyte interface and recover the performance close to the initial level. A performance recovery as high as 94% is proven possible even after 300 cycles of 90° bending. To the best of our knowledge, this is the first aqueous DSSC device with self-healing behavior, using a smart thermoreversible polymer gel electrolyte, which provides a new perspective in flexible wearable solid-state photovoltaic devices.

Fabrication and characterization of graphene based silicon Schottky solar cell

Graphene, which is mechanically flexible, electrically conductive, and optically nearly transparent, is a promising contact material in flexible electronics and photovoltaic devices. In the present investigation, methods to obtain graphene film on silicon wafer and fabricate a graphene based Schottky solar cell have been presented. Scrutiny of atomic force microscopy (AFM) and Raman spectroscopy elucidates the formation of thin layer graphene on silicon. Further, I-V (current-voltage) characteristics of Schottky junction (graphene/silicon junction) have also been studied and presented.

Recent Advance in Solution-Processed Organic Interlayers for High-Performance Planar Perovskite Solar Cells.

Planar heterojunction perovskite solar cells (PSCs) provide great potential for fabricating high‐efficiency, low‐cost, large‐area, and flexible photovoltaic devices. In planar PSCs, a perovskite absorber is sandwiched between hole and electron transport materials. The charge‐transporting interlayers play an important role in enhancing charge extraction, transport, and collection. Organic interlayers including small molecules and polymers offer great advantages for their tunable chemical/electronic structures and low‐temperature solution processibility. Here, recent progress of organic interlayers in planar heterojunction PSCs is discussed, and the effect of chemical structures on device performance is also illuminated. Finally, the main challenges in developing planar heterojunction PSCs based on organic interlayers are identified, and strategies for enhancing the device performance are also proposed.

Improving on the interparticle connection for performance enhancement of flexible quantum dot sensitized solar cells

Flexible photovoltaic devices have wide applications in many unique areas. However, the interparticle connection of photo-electrode in flexible photovoltaic devices still needs to be improved. Here, TiO2 slurries have been prepared by a three-step process, which are employed to prepare the photo-anode for flexible CuInS2 quantum dot sensitized solar cells (QDSSCs). 5 nm-sized TiO2 nanoparticles and hydrochloric acid (HCl) have been introduced in 20 nm-sized TiO2 based photo-electrodes under TiO2/HCl mass ratio of 3:1, which exhibits excellent photovoltaic performance and charge transfer property. Due to the interparticle connection enhancement by suitable content of 5 nm-sized TiO2 nanoparticles, photo-anodes with the 20/5-TiO2 nanoparticles mass ratio of 3:1 exhibits the better optical absorption property and photovoltaic performance of flexible QDSSCs. Moreover, there has maintained 98% photovoltaic conversion efficiency of flexible QDSSCs with the seriously curving condition, which can be considered the great foreground in future applications.

Implementation of graphene as hole transport electrode in flexible CIGS solar cells fabricated on Cu foil

Graphene has great potential to be used as electrode in many opto-electronic devices, owing to its superior optical and electrical properties. Graphene films have been employed as window electrode in various thin film solar cells. However, to date graphene film has not been used as hole transport electrode, particularly in CIGS solar cell. In this work, we have demonstrated a novel structure for graphene-based flexible CIGS solar cell, in which graphene film on flexible Cu foil was implemented as hole transport electrode. CIGS solar cells were directly fabricated on the chemical vapor deposited graphene film on Cu foil, without any transfer process. Several techniques, including Raman spectroscopy, X-ray diffraction, scanning electron microscopy, external quantum efficiency and J-V characteristics under illumination, have been used to investigate the device performance. The graphene-based device displayed power conversion efficiency of 9.91 ± 0.89% with a fill factor of 64.75 ± 7.34%, which are substantially higher compared to reference cell fabricated using conventional Mo/stainless steel electrode. High open circuit voltage together with substantially large fill factor is primarily responsible for high cell efficiency of graphene/Cu foil based device. This study provides a plausible implication of graphene as hole transport electrode in flexible CIGS photovoltaic devices.

All‐Carbon‐Electrode‐Based Endurable Flexible Perovskite Solar Cells

AbstractEndured, low‐cost, and high‐performance flexible perovskite solar cells (PSCs) featuring lightweight and mechanical flexibility have attracted tremendous attention for portable power source applications. However, flexible PSCs typically use expensive and fragile indium–tin oxide as transparent anode and high‐vacuum processed noble metal as cathode, resulting in dramatic performance degradation after continuous bending or thermal stress. Here, all‐carbon‐electrode‐based flexible PSCs are fabricated employing graphene as transparent anode and carbon nanotubes as cathode. All‐carbon‐electrode‐based flexible devices with and without spiro‐OMeTAD (2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene) hole conductor achieve power conversion efficiencies (PCEs) of 11.9% and 8.4%, respectively. The flexible carbon‐electrode‐based solar cells demonstrate superior robustness against mechanical deformation in comparison with their counterparts fabricated on flexible indium–tin oxide substrates. Moreover, all carbon‐electrode‐based flexible PSCs also show significantly enhanced stability compared to the flexible devices with gold and silver cathodes under continuous light soaking or 60 °C thermal stress in air, retaining over 90% of their original PCEs after 1000 h. The promising durability and stability highlight that flexible PSCs are fully compatible with carbon materials and pave the way toward the realization of rollable and low‐cost flexible perovskite photovoltaic devices.