Electron Selective Layer Research Articles

UV‐Inert ZnTiO3 Electron Selective Layer for Photostable Perovskite Solar Cells

AbstractAlthough planar‐structured perovskite solar cells (PSCs) have power conversion efficiencies exceeding 24%, the poor photostability, especially with ultraviolet irradiance (UV) severely limits commercial application. The most commonly‐used TiO2 electron selective layer has a strong photocatalytic effect on perovskite/TiO2 interface when TiO2 is excited by UV light. Here a UV‐inert ZnTiO3 is reported as the electron selective layer in planar PSCs. ZnTiO3 is a perovskite‐structured semiconductor with excellent chemical stability and poor photocatalysis. Solar cells are fabricated with a structure of indium doped tin oxide (ITO)/ZnTiO3/Cs0.05FA0.81MA0.14PbI2.55Br0.45/Sprio‐MeOTAD/Au. The champion device exhibits a stabilized power conversion efficiency of 19.8% with improved photostability. The device holds 90% of its initial efficiency after 100 h of UV soaking (365 nm, 8 mW cm−2), compared with 55% for TiO2‐based devices. This work provides a new class of electron selective materials with excellent UV stability in perovskite solar cell applications.

Mechanically Stacked, Two-Terminal Graphene-Based Perovskite/Silicon Tandem Solar Cell with a Stabilized Efficiency of 25.9%

Perovskite/silicon tandem solar cells represent an attractive strategy to push the market-leading crystalline silicon technology beyond its theoretical limit, maintaining low module costs. Record-high efficiency silicon heterojunction cells with a micrometre-sized pyramid textured front-surface hinder high-temperature and low-cost solution processing of the top-cell in a monolithic architecture. We present a mechanically stacked two-terminal perovskite/silicon tandem device, allowing independent fabrication and optimization of the sub-cells, subsequently coupled by contacting the back-electrode of the mesoscopic perovskite top-cell with the texturized and metalized front-contact of the silicon bottom-cell. Our champion device exhibits a stabilized efficiency of 25.9% over a 1.43 cm2 active area, achieved by optically engineering the hole-selective layer/rear-contact structure to minimize the optical losses, and improving the electrical performance with a graphene-doped mesoporous electron-selective layer. This represents a simple path toward fabricating tandem devices overcoming the limit of single junction solar cells and with a competitive levelized cost of energy.

Electrochemical growth of Y doped ZnO nanorods for use in inverted type organic solar cells as electron transport layer

In this study, Yttrium (Y) doped and undoped Zinc oxide (ZnO) nanorods (NRs) were synthesized by the electrochemical method on indium tin oxide (ITO) substrate. Y doped and undoped ZnO-NRs were used as an electron selective layer in inverted type organic solar cell structure. Effect of Y doping in ZnO was investigated via x-ray diffraction (XRD) and scanning electron microscopy (SEM). Poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) was used as an active polymer blend. Inverted type organic solar cells were fabricated with the configuration ‘ITO/Y:ZnO-NRs/P3HT:PCBM/Ag’. Active layer was coated on ZnO-NRs by using drop-casting method to completely fill the gaps between the ZnO-NRs. Short circuit current density of the solar cells was increased from 10.6 to 16.4 mA cm−2 by using 3% Y doped ZnO-NRs as electron selective layer, where as the solar cell efficiency was changed from 2.35% to 3.87%. It was experimentally found that charge injection and selection in the Zn-ONRs were improved by doping the ZnO-NRs with Y atoms.

SnO2/Mg combination electron selective transport layer for Si heterojunction solar cells

In this paper, SnO2 prepared by sol-gel method was spin-coated on textured n-type crystalline silicon (c-Si) wafers to replace phosphorus doped n-type hydrogenated amorphous silicon (a-Si:H) in novel Si heterojunction (SHJ) solar cells. Good coverage of the sol-gel SnO2 layer on pyramidal textured silicon substrate was achieved. It was demonstrated that a low-work-function metal electrode is necessary for the dopant-free electron transport layer. The SnO2/Mg combination layer was determined to have a low-work-function feature and exhibit a synergistic effect in promoting the selective transport of carriers. A combination passivation layer containing intrinsic amorphous silicon (a-Si:H(i)) and SiOx which was formed from UV-O3 photo oxidation of the a-Si:H(i) surface for a short time was used to passivate the interface between the SnO2 and the c-Si. The new type a-Si:H/SiOx/SnO2/Mg contact is effective to achieve not only a low contact resistivity but also good passivation properties. Finally, a power conversion efficiency (PCE) of 18.6% and an open circuit voltage of 695 mV was achieved on a novel SHJ solar cell with an a-Si:H(i)/SiOx combination passivation layer and a SnO2/Mg combination electron selective transport layer.

Effects of sputtering power of SnO2 electron selective layer on perovskite solar cells

Efficient perovskite solar cell (PSC) with SnO2 electron selective layer (ESL) prepared by radio frequency magnetron sputtering method at room-temperature has been realized. In this work, we systematically discussed the effect of sputtering power and surface roughness of SnO2 ESL on the photoelectric performance of planar PSC. This research shows that PSC based on SnO2 ESL sputtered at the low power of 60 W exhibits the better photoelectric properties than that based on SnO2 ESL sputtered at the high power of 120 W. The lower sputtering power can decrease the surface roughness of SnO2 ESL and improve the contact interface between perovskite layer and ESL, leading to the reduced carrier recombination and enhanced charge transfer property of planar PSC. Thus, with the decreased sputtering power from 120 to 60 W, the photoelectric conversion efficiency of the corresponding PSC increased from 8.19 to 12.58%.

Gradient Sn-Doped Heteroepitaxial Film of Faceted Rutile TiO2 as an Electron Selective Layer for Efficient Perovskite Solar Cells.

The high-efficiency photocarrier collection at the interfaces plays an important role in improving the performance of perovskite solar cells (PSCs) because the photocarrier effective diffusion lengths in the lead halide perovskite absorbers usually surpass the incident depths of light. Developing the electron selective layer (ESL) that has good interfaces with photoactive perovskite and current collector layer-like fluorine-doped tin oxide (FTO) is actively pursued. Here, an unusual dense film of faceted rutile TiO2 single crystals with a gradient of the Sn4+ dopant grown heteroepitaxially on the FTO layer is obtained by a hydrothermal route and subsequent thermal treatment. Owing to the global features including low concentration of defects, atomically smooth coherent interface with FTO, and gradient doping-induced built-in electric field to promote the collection of photoelectrons in it, an optimal PSC with such a film as the ESL exhibits an efficiency of 17.2% with an open-circuit voltage of 1.1 V and fill factor of 76.1%, which are among the highest values of the PSCs with rutile TiO2 films as ESLs.

Electron/Hole-Selective Interfaces in Perovskite Photovoltaics: Electrochemical Studies

Though the first perovskite solar cell (PSC) reported in 2009, was an electrochemical (liquid-junction) device, there were only few electrochemical studies during the next decade1. This paper will show, that electrochemistry can, nevertheless, provide inputs for the development of state-of-art PSCs’ components. The vectorial charge-transfer over several interfaces in PSCs requires right alignment of conduction band minima (CBM) at the electron selective layer terminal and valence band maxima (VBM) at the hole-selective terminal, respectively. Furthermore the electron/hole selective layers should be compact (pinhole-free) to avoid recombination. All these properties can be characterized electrochemically. The electron-selective layer is usually made from TiO2 or SnO2 2. Amorphous ALD-made SnO2 or TiO2 films are pinhole-free for thicknesses down to several nm, but amorphous and crystalline ALD SnO2 films substantially differ in their conduction band positions. The energy of CBM is usually measured by optical spectra (giving the band gap) and photoelectron spectra (XPS, UPS) providing VBM. Electrochemical alternative is the measurement of the flatband potential, which yields CBM, too. However, there is a considerable controversy between the electrochemical and vacuum (XPS, UPS) techniques concerning the position of CBM in TiO2 (anatase, rutile, including the crystals with distinguished facets). A refined electrochemical analysis of single crystal TiO2 electrodes (anatase, rutile, brookite) together with vacuum and near-ambient pressure XPS studies as well as theoretical (DFT) modelling points at the effect of interface influencing the CBM positions. Even 1-2 monolayers of water cause the relevant CBM shifts, which could explain these conflicts. Similar tasks are addressed at the positive (hole-selective) terminal of PSC. A promising hole-conductor to replace spiro-OMeTAD is CuSCN, particularly if it is interfaced to reduced graphene oxide (rGO). The natural p-doping of CuSCN is demonstrated by both Hall-effect and by Mott-Schottky plots in aqueous electrolyte solution. The corresponding flatband potentials (in V vs. Ag/AgCl) varied with the substrate type as follows: 0.12 (CuSCN@FTO), 0.08 (CuSCN@Au), -0.02 (CuSCN@glass-like-carbon) and 0.00 V (CuSCN@rGO). The acceptor concentrations determined from electrochemical impedance spectroscopy are by orders of magnitude larger than those from electrical conductivity and Hall effect. Raman spectra confirm that thiocyanate is the dominating structural motif over the isomeric isothiocyanate. In-situ Raman spectroelectrochemistry discloses substrate-specific intensity changes upon electrochemical charging. The blocking function of CuSCN is tested by a newly designed redox-probe, Ru(NH3)6 3+/2+. It has not only the appropriate redox potential for investigation of the CuSCN films, but also avoids complications of the standard ‘ferrocyanide test’ which is normally used for this purpose. L. Kavan, Curr. Opinion Electrochem., 11, 122 (2018).L. Kavan, Catal. Today, DOI: 10.1016/j.cattod.2018.10.065.

Photoelectrochemical Hydrogen Generation Using a Bis-Chelating Nickel PNP Catalyst Tethered to Band-Edge Modified Si(111)|TiO2

We report the generation of a novel “bis-bidentate” chelating nickel phosphine electrocatalyst functionalized with pendant amines. Two PNP appended donor moieties are tethered to a single surface-tethering anchor, with the intention of leveraging the chelate effect to increase the kinetic stability of the bound nickel ion upon surface immobilization. The carboxylic acid moiety of this bis-bidentate PNP catalyst allows for adsorption to band-edge modified p-silicon(111) photoelectrodes coated with amorphous TiO2 for photogeneration of dihydrogen under illumination and negative applied bias. The synthesis of this ligand also allows for variability in metal oxide binding groups and phosphine R-groups. The band-edge modification of p-silicon(111) is achieved with a mixed monolayer of phenyl-3,5-bismethoxytriethylene glycol and methyl units, which also enhances electron transport through the interface. An ultrathin film (20Å) of amorphous titania further passivates the semiconductor from the generation of surface defects, acts as an electron selective layer, and allows for adsorption of the metal complex to the electrode. Cyclic voltammetry and XPS were used in conjunction to estimate catalyst loading, evaluate photoelectrochemical performance of these devices, and investigate different mechanisms of activity loss such as via loss of the nickel or desorption of the ligand. Different modes of device construction, such as via adsorption of the PNP ligand to the TiO2-coated photoelectrodes followed by treatment with a solution of [Ni(MeCN)6]·2ClO4 and via deposition of the fully constructed, metalated complex were evaluated for differences in Ni loading and device performance. Both the adsorbed catalyst as well as non-specifically adsorbed Ni2+ on TiO2 were shown to be active for proton reduction, albeit at different potentials. Efforts were therefore made to isolate the activity of the nickel phosphine catalyst and prevent convolution of the electrochemical response.

15% Efficiency Ultrathin Silicon Solar Cells with Fluorine-Doped Titanium Oxide and Chemically Tailored Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) as Asymmetric Heterocontact.

In order to achieve a high performance-to-cost ratio to photovoltaic devices, the development of crystalline silicon (c-Si) solar cells with thinner substrates and simpler fabrication routes is an important step. Thin-film heterojunction solar cells (HSCs) with dopant-free and carrier-selective configurations look like ideal candidates in this respect. Here, we investigated the application of n-type silicon/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HSCs on periodic nanopyramid textured, ultrathin c-Si (∼25 μm) substrates. A fluorine-doped titanium oxide film was used as an electron-selective passivating layer showing excellent interfacial passivation (surface recombination velocity ∼10 cm/s) and contact property (contact resistivity ∼20 mΩ/cm2). A high efficiency of 15.10% was finally realized by optimizing the interfacial recombination and series resistance at both the front and rear sides, showing a promising strategy to fabricate high-performance ultrathin c-Si HSCs with a simple and low-temperature procedure.

Chlorine capped SnO2 quantum-dots modified TiO2 electron selective layer to enhance the performance of planar perovskite solar cells

SnO2 quantum dots (QDs) ended with chlorine ions are introduced at the interface of spin-coated TiO2 electron selective layer (ESL)/perovskite to fill the pinholes in the layer and passivate the trapping defects. As a result of the increased interface electron collection and reduced bulk recombination, the planar perovskite solar cell with the QDs modified ESL gives the large power conversion efficiency enhancement from 14.9% to 17.3% and greatly improved stability under the continuous light irradiation.

Seeding-method-processed anatase TiO2 film at low temperature for efficient planar perovskite solar cell

Electron-selective layers (ESLs) processed at low temperature play a vital role for planar perovskite solar cells (PSCs), especially for flexible devices. In this scheme, a novel strategy namely seeding-method is developed to prepare highly-compact anatase TiO2 ESLs at dramatically low temperature of 150 °C. This method allows the preparation of anatase TiO2 via hydrolyzation of Titanium diisopropoxide bis (acetylacetonate) (TiAcAc) induced by TiO2 crystal seed. Seeding-method permits the hydrolyzation of TiAcAc occurs at just 70 °C to obtain anatase TiO2, which is greatly reduced compared with normal procedure where 500 °C is desired. Accordingly, the acquired anatase TiO2 reacted with hydrochloric acid to prepare Cl-doped TiO2 (Cl@TiO2). Highly-compact anatase TiO2 ESLs were formed by spin-coating Cl@TiO2 onto FTO and then annealed at 150 °C. Power conversion efficiency of planar PSCs based on anatase Cl@TiO2 ESLs was 15.73%. Seeding-method processed at low temperature can be an effective way to advance the development of flexible perovskite photovoltaic.

Photovoltaics literature survey (no. 149)

Photovoltaics literature survey (no. 149)

Ionic Effect Enhances Light Emission and the Photovoltage of Methylammonium Lead Bromide Perovskite Solar Cells by Reduced Surface Recombination

The achievement of optimal power conversion efficiencies (PCEs) in wide-band-gap perovskite solar cells (PSCs) is delayed by photovoltage losses associated with poor understanding of recombination dynamics. In this work, we use high-quality methylammonium lead bromide perovskite solar cells with selective contacts treated with lithium-containing additives to investigate recombination mechanisms. By comparison of the photovoltaic performance of devices we confirm that the presence of this additive in the electron selective layer (ESL) significantly increases the open-circuit potential to values of 1.58 V. Impedance spectroscopy coupled with electroluminescence and photoluminescence analysis reveals that lithium ions present at the mesoporous TiO2 layer dramatically enhances the radiative recombination in the perovskite by reduction of undesired nonradiative and surface recombination pathways. This work highlights that the employ of additives helps to modify the electronic charge distribution at the metal o...

Facilitating electron collection of organic photovoltaics by passivating trap states and tailoring work function

The low-temperature solution-processed ZnO normally has intrinsic oxygen vacancies and various surface defects such as organic residues and surface adsorbates. When ZnO film is used in organic solar cells as electron selective layer, these defects will serve as bimolecular recombination centers for light induced charge carriers and trap electrons, leading to an unsatisfied device performance. Herein, we demonstrate an effective way to employ a simple combination of UV-ozone (UVO) and dipole treatment to passivate the ZnO layer, and the electronic property of ZnO layer is great improved upon eliminating the residual hydrocarbons and passivating surface traps. The reengineered uniform ZnO films play a major role on the facilitated charge transfer and decreased contact resistance, resulting in an enhanced device efficiency. The optimized ZnO transport layer also regulates the upper active layer depositing and morphology, and consequently increases the dissociation of photo-excitons and suppresses charge recombination.

Improved power conversion efficiency of silicon nanowire solar cells based on transition metal oxides

An n-type Si nanowire (SiNW) solar cell based on transition metal oxides V2O5 and TiO2 has been fabricated. The cell is demonstrated in the ITO/V2O5/n-SiNWs/TiO2/Al heterojunction structure, in which the V2O5 and TiO2 work as the hole- and the electron-selective layers, respectively. Parameters of cells with different lengths of nanowires have been investigated. Owing to the excellent light trapping effect of nanowire arrays and superior carrier transporting properties of thin V2O5 and TiO2 layers, an improved power conversion efficiency of 12.7% has been achieved in a cell with 1.45-μm-long nanowires. Due to the large work function difference between V2O5 and n-Si, a build-in potential of 0.75 V was obtained from the capacitance-voltage measurement, which implies the possibility of achieving a large open circuit voltage for these nanowire solar cells. The efficiency loss is mainly caused by the recombination via the defects on the unpassivated surface of SiNWs. By solving this problem, the cell efficiency may be improved further and realize the application in PV industries.

A straightforward chemical approach for excellent In2S3 electron transport layer for high-efficiency perovskite solar cells.

Perovskite solar cells (PSCs) have attracted significant attention in recent years owing to some of their advantages: high-efficiency, low cost and ease of fabrication. In perovskite photovoltaic devices, charge transport layers play a vital role for selectively extracting and transporting photo-generated electrons and holes to opposite electrodes. Therefore, it is very important to prepare high-quality charge transport layers using simple processes at low cost. As reported, In2S-based electron selective layers display excellent performance including high solar-cell efficiency and negligible hysteresis. In this study, a simple chemical method was developed to prepare In2S3 thin films as the electron selective layers in organic–inorganic hybrid perovskite photovoltaic devices to shorten the fabrication time and simplify the technology, which can provide a new avenue for a low-cost and solution-processed method. By optimizing the preparation conditions, it was demonstrated that In2S3 thin film prepared using our straightforward chemical approach have higher electron extraction efficiency and comparable efficiency compared with archetypical TiO2 as the electron transport layer (ETL) in perovskite photovoltaic device.

PbTiO3 as Electron‐Selective Layer for High‐Efficiency Perovskite Solar Cells: Enhanced Electron Extraction via Tunable Ferroelectric Polarization

AbstractPbTiO3 (PTO) is explored as a versatile and tunable electron‐selective layer (ESL) for perovskite solar cells. To demonstrate effectiveness of PTO for electron–hole separation and charge transfer, perovskite solar cells are designed and fabricated in the laboratory with the PTO as the ESL. The cells achieve a power conversion efficiency (PCE) of ≈12.28% upon preliminary optimization. It is found that the PTO ferroelectric layer can not only increase the PCE, but also tune the photocurrent via tuning PTO's ferroelectric polarization. Moreover, to understand the physical mechanism underlying the carrier transport by the ferroelectric polarization, the electronic structure of PTO/CH3NH3PbI3 heterostructure is computed using the first‐principles methods, for which the triplet state is used to simulate charge transfer in the heterostructure. It is shown that the synergistic effect of type II band alignment and the specific ferroelectric polarization direction provide the effective extraction of electrons from the light absorber, while minimize recombination of photogenerated electron–hole pairs. Overall, the ferroelectric PTO is a promising and tunable ESL for optimizing electron transport in the perovskite solar cells. The design offers a different strategy for altering direction of carrier transport in solar cells.

High efficiency flexible perovskite solar cells using SnO2/graphene electron selective layer and silver nanowires electrode

Ultrafine silver (Ag) nanowires dispersed in a mixed solution of EMIMBF4 and water were spin-coated onto polyethylene terephthalate as the flexible electrode of perovskite solar cells (PVSCs). The weakly oxidized graphene nanosheets (GNs) were incorporated into SnO2 (SnO2/GNs) to enhance the electron mobility and flexibility of the electron selective layer (ESL). The deposited C60-self-assembled monolayer (C60-SAM) between the ESL and the perovskite can inhibit charge recombination. Herein, the improved surface photovoltage responses can be attributed to the synergistic effect of C60-SAM and SnO2/GN ESL under zero and applied electric field. The best-performing PVSC has achieved a power conversion efficiency (PCE) of 13.36%, a Voc of 1.10 V, a Jsc of 18.39 mA cm−2, and a fill factor (FF) of 0.66 under a reverse voltage scan (the corresponding PCE of 12.81%, Voc of 1.10 V, Jsc of 18.19 mA cm−2, and FF of 0.64 under a forward voltage scan), indicating a negligible hysteresis. The EMIMBF4 can improve the dispersivity and intrinsic contacts between Ag nanowires. The C60-SAM will passivate the charge trap states of the perovskite, and the SnO2/GNs can promote electron transport. The PVSC from a low-temperature solution process is compatible with roll-to-roll manufacturing, and the intrinsic charge dynamics was also explored.

Modulation-doped ZnO as high performance electron-selective layer for efficient silicon heterojunction solar cells

It has been a long-time dream for photovoltaic (PV) researchers to achieve high performance silicon (Si) heterojunction solar cells (SCs) relying only on low-temperature and solution-based processes. A newly emerging PV technique of carrier-selective contacts, which comprise high performance hole- and electron-selective layers (HSL and ESL) for both polarities on Si substrate, is highly expected to meet this dream. Here, we report a precise control of the doping level in solution-processed ZnO thin films and their successful application as ESLs for organic/Si heterojunction SCs. We show that addition of Li in ZnO (Li-ZnO) allows for the accurate tuning of its work function from 4.15 to 3.85 eV. In case of Si/ZnO/Al heterocontact, presence of the Li-ZnO layer with optimized Li concentration results in a remarkable enhancement in extraction of electrons, showing by a big reduction of contact resistivity (ρc) from 100 to 18 mΩ cm2. When used in combination with an interfacial passivation layer of intrinsic amorphous silicon (a-Si:H), the bilayer ESL of a-Si:H/Li-ZnO helps to yield an open-circuit voltage (Voc) of 643 mV and an efficiency up to 15.1%, both remarkable for an organic/Si SC fabricated using solution-based processes. Progress in tuning the Li-ZnO properties to yield improved interfaces between Si and rear-sided electrode may open up a new direction of exploring more functional materials for high performance ESLs via effectively solution-based method.

The Role of Surface Recombination on the Performance of Perovskite Solar Cells: Effect of Morphology and Crystalline Phase of TiO2 Contact

AbstractHerein, the preparation of 1D TiO2 nanocolumnar films grown by plasma‐enhanced chemical vapor deposition is reported as the electron selective layer (ESL) for perovskite solar devices. The impact of the ESL architecture (1D and 3D morphologies) and the nanocrystalline phase (anatase and amorphous) is analyzed. For anatase structures, similar power conversion efficiencies are achieved using an ESL either the 1D nanocolumns or the classical 3D nanoparticle film. However, lower power conversion efficiencies and different optoelectronic properties are found for perovskite devices based on amorphous 1D films. The use of amorphous TiO2 as electron selective contact produces a bump in the reverse scan of the current–voltage curve as well as an additional electronic signal, detected by impedance spectroscopy measurements. The dependence of this additional signal on the optical excitation wavelength used in the IS experiments suggests that it stems from an interfacial process. Calculations using a drift‐diffusion model which explicitly considers the selective contacts reproduces qualitatively the main features observed experimentally. These results demonstrate that for a solar cell in which the contact is working properly the open‐circuit photovoltage is mainly determined by bulk recombination, whereas the introduction of a “bad contact” shifts the balance to surface recombination.