Interfacial Charge Recombination Research Articles

Improvement in open circuit voltage of solar cells with zinc sulfate-modified photoelectrodes

ABSTRACTWe report on the effects of surface-modified TiO2 on the open-circuit voltage (Voc) of dye-sensitized solar cells (DSSCs). The TiO2 films were immersed into an aqueous zinc sulfate (ZS) solution to give ZS-modified photoelectrodes. The DSSCs with ZS-modified photoelectrodes showed a considerable improvement in Voc, compared to that of the control device with pristine TiO2 layer. By monitoring open-circuit voltage decay, electrochemical impedance and dark current, it was revealed that the modification of the TiO2 surface by ZS led to a longer electron lifetime due to a reduction of interfacial charge recombination between photo-injected electrons and I−3 ions, and thereby inducing the increase in Voc.

Sensitizers for Aqueous-Based Solar Cells.

Aqueous dye-sensitized solar cells (DSSCs) are attractive due to their sustainability, the use of water as a safe solvent for the redox mediators, and their possible applications in photoelectrochemical water splitting. However, the higher tendency of dye leaching by water and the lower wettability of dye molecules are two major obstacles that need to be tackled for future applications of aqueous DSSCs. Sensitizers designed for aqueous DSSCs are discussed based on their functions, such as modification of the molecular skeleton and the anchoring group for better stability against dye leaching by water, and the incorporation of hydrophilic entities into the dye molecule or the addition of a surfactant to the system to increase the wettability of the dye for more facile dye regeneration. Surface treatment of the photoanode to deter dye leaching or improve the wettability of the dye molecule is also discussed. Redox mediators designed for aqueous DSSCs are also discussed. The review also includes quantum-dot-sensitized solar cells, with a focus on improvements in QD loading and suppression of interfacial charge recombination at the photoanode.

Engineering Processes at the Interface of p‐Semiconductor for Enhancing the Open Circuit Voltage in p‐Type Dye‐Sensitized Solar Cells

To prevent the interfacial charge recombination between injected holes in the valence band and the redox mediator in the electrolyte in p‐type dye sensitized solar cells (p‐DSSC) the passivation of the recombination sites by organic insulator chenodeoxycholic acid (CDCA) layer is critically investigated in this study. Rather than classical coating of the semiconductor's surface by simultaneous co‐adsorption of CDCA during the dyeing step, two other methods are investigated. The first consists in dissolving CDCA in the electrolyte, while the second consists in spin coating an ethanol solution of CDCA onto the already dyed photocathode. In this study, different sensitizers, electrolytes, and p‐SCs, (NiO, CuGaO2) are explored. Analysis of the current/voltage curves and electrochemical impedance spectroscopy provides evidence that the role of the CDCA layer is to create a physical barrier to prevent the approach of the redox mediator from the NiO surface and consequently raise the open circuit voltage (Voc). The important finding of this study is the demonstration that the Voc in p‐DSSC is heavily limited by interfacial charge recombination and that higher Voc values much above 100 mV and as high as 500 mV can be attained with conventional materials (NiO) if this deleterious side reaction can be suppressed or diminished.

Probing single-molecule electron-hole transfer dynamics at a molecule-NiO semiconductor nanocrystalline interface.

Interfacial charge transfer dynamics in dye-sensitized NiO nanoparticles are being investigated for photocathodes in p-type dye-sensitized solar cells. In the photoreaction, after fast electron transfer from NiO to a molecule, the recombination of the hole in the nanoparticles with the electron in a reduced molecule plays an important role in the charge separation process and solar energy harvesting. Nevertheless, knowledge of the interfacial charge recombination (CR) rate and its mechanism is still limited due to the complex photoinduced electron and hole dynamics and lack of characterization of the inhomogeneity of the dynamics. Here, we report our work on probing interfacial charge recombination dynamics in Zn(ii)-5,10,15,20-tetra(3-carboxyphenyl)porphyrin (m-ZnTCPP) dye-sensitized NiO nanoparticles by correlating single-molecule fluorescence blinking dynamics with charge transfer dynamics using single-molecule photon-stamping spectroscopy. The correlated analyses of single-molecule fluorescence intensity, lifetime, and blinking reveal the intrinsic distribution and temporal fluctuation of interfacial charge transfer reactivity, which are closely related to site-specific molecular interactions and dynamics.

First Principle Modelling of Materials and Processes in Dye-Sensitized Photoanodes for Solar Energy and Solar Fuels

In the context of solar energy exploitation, dye-sensitized solar cells and dye-sensitized photoelectrosynthetic cells offer the promise of low-cost sunlight conversion and storage, respectively. In this perspective we discuss the main successes and limitations of modern computational methodologies, ranging from hybrid and long-range corrected density functionals, GW approaches and multi-reference perturbation theories, in describing the electronic and optical properties of isolated components and complex interfaces relevant to these devices. While computational modelling has had a crucial role in the development of the dye-sensitized solar cells technology, the theoretical characterization of the interface structure and interfacial processes in water splitting devices is still at its infancy, especially concerning the electron and hole transfer phenomena. Quantitative analysis of interfacial charge separation and recombination reactions in multiple metal-oxide/dye/catalyst heterointerfaces, thus, undoubtedly represents the compelling challenge in the field of modern computational material science.

Effective suppression of interfacial charge recombination by a 12-crown-4 substituent on a double-anchored organic sensitizer and rotating disk electrochemical evidence

A crown-ether-substituted double-anchored organic dye suppressed charge recombination with iodide and cobalt redox mediators to reach the efficiencies of 10.12% (under 1 sun) and 11.17% (under 0.1 sun).

Reduction of charge recombination in PbS colloidal quantum dot solar cells at the quantum dot/ZnO interface by inserting a MgZnO buffer layer

Colloidal quantum dot solar cells with an improved performance are reported by employing MgZnO as a buffer layer with tunable electronic energy levels in the solar cells to reduce interfacial charge recombination and hence improve charge collection.

Excited-State and Charge Carrier Dynamics in a High-Photovoltage and Thermostable Dye-Sensitized Solar Cell

Apart from the harvesting of more infrared solar photons for a higher photocurrent, improving the photovoltage and thermal stability of a dye-sensitized solar cell are the other two key challenging issues for its performance enhancement. Herein we report a metal-free donor–acceptor dye (SC-4) characteristic of a triphenylamine-dihexylbithiophene electron donor and a benzothiadiazole-benzoic acid electron acceptor. This organic dye can be utilized for the fabrication of sensitized titania solar cells exhibiting excellent photovoltages of 1005 and 825 mV, respectively, when a volatile tris(1,10-phenanthroline)cobalt-based electrolyte and a nonvolatile iodine-based ionic liquid composite electrolyte are applied, respectively. With respect to the control dye C239 using the traditional electron acceptor cyanoacrylic acid, dye SC-4 displays not only an enhanced photovoltage owing to slower interfacial charge recombination but also an improved stability of photocurrent and efficiency even under a long-term therm...

Analysis of Photoinduced Carrier Recombination Kinetics in Flat and Mesoporous Lead Perovskite Solar Cells

In this work, we analyze the carrier recombination kinetics and the associated charge carrier density in methylammonium lead iodide perovskite (MAPI) solar cells that use mesoporous TiO2 as selective contact (m-MAPI) and flat solar cells (without the mesoporous TiO2, f-MAPI), which are the most common device architectures for perovskite solar cells. The use of PIT-PV (photoinduced transient photovoltage) and L-TAS (laser transient absorption spectroscopy) showed that for devices that cannot reach efficiencies close to 19% there is a slow component of the photovoltage decay that corresponds to a charge recombination pathway for carrier losses responsible for the lower device efficiency. Moreover, we have also identified a primary interfacial charge recombination pathway for carrier losses that is common in all devices studied, independent of their efficiency or their device structure, which we have associated with the recombination reaction between electrons in the perovskite and holes in the organic semic...

Effect of anchoring groups on N-annulated perylene-based sensitizers for dye-sensitized solar cells and photocatalytic H2 evolution

In this work, two new D-A-π-A perylene dyes featuring with cyanoacrylic acid and 2-(1,1-dicyanomethylene) rhodamine (DCRD) as the electron acceptor/anchoring group, respectively, in combination with a N-annulated perylene electron donor and a quinoxaline auxiliary acceptor were investigated for application in dye-sensitized solar cells (DSCs) and dye-sensitized photocatalytic hydrogen production. The results showed that the sensitizer with cyanoacrylic acid (YFT-1) based DSCs showed a higher conversion efficiency of 7.58% under AM1.5 solar conditions. However, it was interesting that the sensitizer with DCRD (YFT-2) sensitized Pt/TiO2 photocatalysts displayed a higher rate of H2 evolution of 83.5 μmol h−1 under visible-light irradiation with wavelength between 420 nm and 780 nm. This may be ascribed to less interface charge recombination and more effective interfacial electron transfer between photo-excited YFT-2 and TiO2 in aqueous solution. This study demonstrates that YFT-2 containing DCRD anchoring group is a promising candidate to achieve high and stable activity of H2 production.

TiO2-ZnS Cascade Electron Transport Layer for Efficient Formamidinium Tin Iodide Perovskite Solar Cells.

Achieving high open-circuit voltage (Voc) for tin-based perovskite solar cells is challenging. Here, we demonstrate that a ZnS interfacial layer can improve the Voc and photovoltaic performance of formamidinium tin iodide (FASnI3) perovskite solar cells. The TiO2-ZnS electron transporting layer (ETL) with cascade conduction band structure can effectively reduce the interfacial charge recombination and facilitate electron transfer. Our best-performing FASnI3 perovskite solar cell using the cascaded TiO2-ZnS ETL has achieved a power conversion efficiency of 5.27%, with a higher Voc of 0.380 V, a short-circuit current density of 23.09 mA cm-2, and a fill factor of 60.01%. The cascade structure is further validated with a TiO2-CdS ETL. Our results suggest a new approach for further improving the performance of tin-based perovskite solar cells with a higher Voc.

Band-structure tailoring and surface passivation for highly efficient near-infrared responsive PbS quantum dot photovoltaics

Abstract PbS is a promising light harvester for near-infrared (NIR) responsive quantum dot (QD) photovoltaics due to its narrow bulk band gap (0.41 eV) and large exciton Bohr radius (18 nm). However, the relatively low conduction band (CB) and high-density surface defects of PbS as two major drawbacks for its use in solar cells severely hamper the photovoltaic performance enhancement. In this work, a modified solution-based successive ionic layer adsorption and reaction (SILAR) utilizing mixed cationic precursors of Pb2+ and Cd2+ is explored, and such a scheme offers two benefits, band-structure tailoring and surface passivation. In-situ deposited CdS suppresses the excessive growth of PbS in the mesopores, thereby facilitating the favorable electron injection from PbS to TiO2 in view of the up-shifted CB level of QDs; the intimate interpenetration of two sulfides with each other leads to superior passivation of trap state defects on PbS, which suppresses the interfacial charge recombination. With the construction of photovoltaics based on such a hybrid (Pb,Cd)S/CdS configuration, impressive power conversion efficiency up to 4.08% has been reached, outperforming that of the conventional PbS/CdS pattern (2.95%). This work highlights the great importance of band-structure tailoring and surface passivation for constructing highly efficient PbS QD photovoltaics.

Improving the efficiency of dye-sensitized solar cells by photoanode surface modifications

Dye-sensitized solar cells (DSSCs) provide a promising alternative solar cell technology because of their high efficiency, environmental friendliness, easy fabrication, and low cost. Power conversion efficiency is an important parameter to measure the performance of DSSCs, but the severe charge recombination that occurs at the photoanode hinders the future improvement of power conversion efficiency. Therefore, one of the key goals for achieving high efficiency is to reduce the energy loss caused by the unwanted charge recombination at various interfaces. From this perspective, surface modification of the photoanode is the simplest method among the various approaches available in the literature for enhancing the performance of DSSCs by inhibiting the interfacial charge recombination. After some brief notes on DSSCs, in this review, we present a comprehensive discussion on surface modifications of different photoanodes that have been adopted in the literature not only for reducing recombination but also for enhancing light harvesting. Depending on the electrode materials, we discuss surface modifications of binary oxides such as TiO2 and ZnO and ternary oxides, including Zn2SnO4, SrSnO3, and BaSnO3. We also talk about methods of surface modification and the materials suitable for surface treatment. Finally, we end with a brief future outlook of DSSCs.

MOF-199: A simple strategy for improvement of crystallinity and photovoltaic property

In this study, thin films of MOF-199 were prepared using a Layer-by-Layer deposition approach and applied as sensitizing materials in TiO2-based solar cells. Our results indicate that by introducing 3,4-dihydroxybenzoic acid at the interface of TiO2-MOF via a simple surface modification step, TiO2 surface roughness is reduced and texture quality of MOF-199 polycrystalline film enhances along 〈111〉 crystallographic direction while the overall conversion efficiency of the cell increases as well. The improvement in Jsc could be a consequence of enhanced crystallinity, reduced surface roughness and enhanced light-harvesting properties of surface-modified photoanode. Dark I–V characteristics of the cells also indicate that the interfacial charge recombination at TiO2-MOF interface is inhibited upon introducing DHBA thus, Voc is improved.

Increased open‐circuit voltage of ZnO nanowire/PbS quantum dot bulk heterojunction solar cells with solution‐deposited Mg(OH)2 interlayer

An ultrathin Mg(OH)2 layer was solution‐deposited onto the ZnO nanowires to solve the problem of interfacial charge recombination, caused by the increase of interfacial area in bulk heterojunction (BHJ) PbS colloidal quantum dot solar cells (CQDSCs). This Mg(OH)2 interlayer efficiently passivated the surface defects of ZnO nanowires and provided tunnel barrier at ZnO/PbS interface. As a result, the charge recombination at ZnO/PbS interface was largely suppressed, proved by the significantly elongated electron lifetime and the increased open‐circuit voltage of the Mg(OH)2‐involved BHJ CQDSCs. Careful thickness optimization of Mg(OH)2 interlayer finally brought a ∼33% increase in Voc and ∼25% improvement in power conversion efficiency.

Enhanced Conversion Efficiencies in Dye-Sensitized Solar Cells Achieved through Self-Assembled Platinum(II) Metallacages.

Two-component self-assembly supramolecular coordination complexes with particular photo-physical property, wherein unique donors are combined with a single metal acceptor, can be utilized for many applications including in photo-devices. In this communication, we described the synthesis and characterization of two-component self-assembly supramolecular coordination complexes (SCCs) bearing triazine and porphyrin faces with promising light-harvesting properties. These complexes were obtained from the self-assembly of a 90° Pt(II) acceptor with 2,4,6-tris(4-pyridyl)-1,3,5-triazine (TPyT) or 5,10,15,20-Tetra(4-pyridyl)-21H,23H-porphine (TPyP). The greatly improved conversion efficiencies of the dye-sensitized TiO2 solar cells were 6.79 and 6.08 respectively, while these SCCs were introduced into the TiO2 nanoparticle film photoanodes. In addition, the open circuit voltage (Voc) of dye-sensitized solar cells was also increased to 0.769 and 0.768 V, which could be ascribed to the inhibited interfacial charge recombination due to the addition of SCCs.

Power Conversion Efficiency and Device Stability Improvement of Inverted Perovskite Solar Cells by Using a ZnO:PFN Composite Cathode Buffer Layer.

We have demonstrated in this article that both power conversion efficiency (PCE) and performance stability of inverted planar heterojunction perovskite solar cells can be improved by using a ZnO:PFN nanocomposite (PFN: poly[(9,9-bis(3'-(N,N-dimethylamion)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl)-fluorene]) as the cathode buffer layer (CBL). This nanocomposite could form a compact and defect-less CBL film on the perovskite/PC61BM surface (PC61BM: phenyl-C61-butyric acid methyl ester). In addition, the high conductivity of the nanocomposite layer makes it works well at a layer thickness of 150 nm. Both advantages of the composite layer are helpful in reducing interface charge recombination and improving device performance. The power conversion efficiency (PCE) of the best ZnO:PFN CBL based device was measured to be 12.76%, which is higher than that of device without CBL (9.00%), or device with ZnO (7.93%) or PFN (11.30%) as the cathode buffer layer. In addition, the long-term stability is improved by using ZnO:PFN composite cathode buffer layer when compare to that of the reference cells. Almost no degradation of open circuit voltage (VOC) and fill factor (FF) was found for the device having ZnO:PFN, suggesting that ZnO:PFN is able to stabilize the interface property and consequently improve the solar cell performance stability.

Hydrophobic Hole-Transporting Materials Incorporating Multiple Thiophene Cores with Long Alkyl Chains for Efficient Perovskite Solar Cells

Two hydrophobic hole transporting materials (HTMs) carrying multiple thiophene cores of 5,7-bis(3-hexylthiophen-2-yl)-2,3-dihydrothieno[3,4-b][1,4]dioxine (for SP-01) or 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene (for SP-02) and long alkyl substituents have been prepared through facile routes with high yields, together with the hole-transporting molecule 2,5-bis(4,4′-bis(methoxyphenyl)aminophen-4′′-yl)-3,4-ethylene dioxythiophene (H101) as reference compound. The structure-property relationships of these HTMs for perovskite solar cells have been systematically investigated. It is found that introducing hydrophobic alkyl chains into HTMs can retard interfacial charge recombination and, specifically, improve stability owing to the enhanced moisture resistance. Perovskite solar cells based on SP-01 give an overall conversion efficiency of 12.37% without, and 13.91% with the addition of FK102, respectively. These values are better than that of H101 (11.59%) and comparable to that of the standard spiro-OMeTAD (12.95%). Moreover, the devices based on hydrophobic HTMs SP-01 show better stability under ∼40% humid atmosphere than those based on H101 and spiro-OMeTAD, which paves the way for developing new HTMs combining high efficiency, low cost, and excellent stability features by appropriate structural modifications.

Phenothiazine organic dyes with the cone-shaped structural: Broad photoresponse and limitations on short-circuit photocurrent density in dye-sensitized solar cells

Three novel phenothiazine organic dyes (WR11-13) with the cone-shaped structural have been developed for dye-sensitized solar cells applications. The effect of electron donor alternation on the optical, electrochemical and the photovoltaic properties is systematically investigated. The results indicate that benzo[b]thiophene is a promising electron donor for organic dyes. With a cone-shaped electron donor, these dyes possess an attractively broad photo-response ranging but shows relatively lower short-circuit photocurrent density than that of the reference dye WR7 (a rod-shaped dye). The origin of photocurrent limitation has been investigated in terms of electron injection, dye regeneration and interfacial charge recombination, with the latter found to be the governing factor. The present result shows that molecules able to control the charge recombination in DSSCs would be structures of interest in the design of highly efficient phenothiazine dyes.

Oriented rutile TiO2 nanorod arrays for efficient quantum dot-sensitized solar cells with extremely high open-circuit voltage

TiO2 nanoparticles are typically employed to construct the porous films for quantum dot-sensitized solar cells (QDSCs). However, undesirable interface charge recombination at grain boundaries would hinder the efficient electron transport to the conducting substrate, giving rise to the decline of open-circuit voltage (Voc). In this work, vertically aligned architectures of oriented one-dimensional (1D) TiO2 nanorod arrays hydrothermally grown on substrates pave a way in designing highly efficient QDSCs with efficient radial-directional charge transport. SEM, TEM, XRD, and Raman spectroscopy were employed to characterize the as-prepared TiO2 nanorods, showing the rutile phase with single-crystalline structure. The homogeneous deposition of CdS/CdSe QDs on the surface of TiO2 nanorods has been achieved by in-situ grown strategies (i.e., successive ionic layer absorption and reaction, and chemical bath deposition). An extremely high Voc value up to 0.77V has been achieved for CdS/CdSe QDSCs based on the well-ordered 1D nanorod arrays. To the best of our knowledge, it is the highest Voc reported for TiO2-based QDSCs. Dependencies of photovoltaic performance, optical absorption, and interfacial charge behavior on the length of nanorods were systematically investigated. A 1.7μm nanorod-array photoelectrode-based QDSC delivers a remarkable power conversion efficiency up to 3.57% under simulated AM 1.5 100mWcm−2 illumination, attributed to the balance of competition between the increase of QD loading and suppression of interfacial recombination. This work highlights the combination of QDs with high absorption coefficient 1D architectures possessing efficient charge transport for constructing high efficiency solar cells.