Interfacial Charge Recombination Research Articles

Thiourea Interfacial Modification for Highly Efficient Planar Perovskite Solar Cells

Interfacial engineering is an efficient strategy for enhancing the performance of perovskite photovoltaic cells. Here, we present a novel method to modify the surface of TiO2 with thiourea. X-ray photoelectron spectroscopy indicates that thiourea treatment inactivates the TiO2 surface groups. Kelvin probe force microscopy and ultraviolet photoelectron spectra demonstrate an energy level downshift of the modified TiO2. Photoluminescence spectra reveal the reduced interface charge recombination and an improved charge transportation/extraction. Tafel and Nyquist plots elucidate a decrease of trap-state density and an increase of conductivity of the modified TiO2. Hence, the planar perovskite solar cell based on the TiO2 electron transport layer (ETL) modified with 0.1 M thiourea yields an efficiency of 19.18%, which is 7.5% larger than that of the device based on the pristine TiO2 ETL. The simple procedure and the significant performance improvement render this method promising.

Optoelectrical characterization of different fabricated donor substituted benzothiazole based sensitizers for efficient DSSCs

In the present work, we have synthesized three novel organic sensitizers, based on various donor groups which are in conjugation with similar bridge and acceptor moieties. We have studied the effect of donor groups with common bridge and anchoring group sensitizers on optical, electrochemical, dye sensitized solar cell’s performance and electron transfer interface properties as well. We have also represented systematically analysis of influence of donor groups in structural geometry, energy distribution and energy-transfer from HOMO to LUMO by density functional theory. The B2 sensitizer contains highly conjugated, strong electron withdrawing group and electron transfer form donor group to anchoring group happens much more rapid in it compared to other two sensitizers. However, B2 sensitizer has exhibited 6.23% efficiency under 1 sun light illumination. The influence of donor groups on device performance was clearly shown on IPCE spectrum as well. The studied effect of chenodeoxycholic acid (CDCA) in a sensitizer’s solution as a co-absorbent on the DSSCs performance and exhibited highest efficiency (6.68%). We found that co-absorbent can be hinder the sensitizer aggregation and rise electron injection yield and thus short circuit current (Jsc). The device interface kinetics was studied by electrochemical impedance spectroscopy and calculated electron lifetime, interface charge transfer resistance and recombination resistance by fitted equivalent circuit. Moreover, we have checked high performance device stability up to 700 h, under continuous 1 sun light illumination.

Dopamine-crosslinked TiO2/perovskite layer for efficient and photostable perovskite solar cells under full spectral continuous illumination

Even though TiO2 is the most widely used electron transport layer (ETL) in high-efficiency perovskite solar cells (PSCs), ultraviolet photocatalysis and existence of intrinsic oxygen vacancies result in interfacial charge recombination and poor long-term photo-stability for TiO2-based PSCs under full spectral continuous illumination. To solve the issues, here we report dopamine-capped TiO2 nanoparticles as ETL via chelating effect to improve interfacial binding with perovskite active layer. The introduction of dopamine can substantially reduce oxygen vacancies and suppress deep trap states within TiO2. In addition, the terminal amino groups in dopamine can passivate the uncoordinated Pb atoms and decrease the Pb-I/Br antisite defects on the interface of perovskite/TiO2. As an interfacial crosslinking agent, dopamine can not only reduce charge-accumulation and charge-recombination rate, but also increase charge-extraction efficiency at the TiO2 and perovskite interface. Based on the dopamine-capped TiO2 nanoparticles surface, the corresponding planner Cs0.05FA0.81MA0.14PbI2.55Br0.45 PSCs deliver a power conversion efficiency of nearly 21% with negligible hysteresis. Moreover, unencapsulated devices retain 80% of their initial performance after 1200 h operation under constant full-sun illumination in nitrogen atmosphere. Ideally, this chemical-bath-deposited dopamine-modified TiO2 provides an effective commercialized route for efficient and photostable planar PSCs.

Homoleptic d10 metal complexes containing ferrocenyl functionalized dithiocarbamates as sensitizers for TiO2 based dye-sensitized solar cells

Four synthesized d10 metal complexes, namely, ferrocenyl based pyridyl functionalized dithiocarbamates macro-cyclic metal-organic coordination polymers, [M(L)2]∞ (M = Zn(II), L = (N-ferrocenyl-methyl-N-pyridin-4-ylmethyl)dithiocarbamate Zn-2; Cd (II), L = (N-ferrocenyl-methyl-N-pyridin-3-ylmethyl) dithiocarbamate Cd-3 and dimers [M(L)2]2 (M = Zn(II), Hg(II), L = (N-ferrocenyl-methyl-N-pyridin-3-ylmethyl) dithiocarbamate, Zn-1 and Hg-4 were studied for their sensitization activities when anchored on TiO2 thin film electrode and employed as photoanode in dye-sensitized solar cells (DSSC). The absorption spectra of test dyes in dichloromethane solution indicated absorption in 400–500 nm range for all complexes, revealing the possibility of their use as photosensitizers for TiO2. The cyclic voltammetry (CV) demonstrated quasi-reversible behavior for all the complexes. All the dye sensitizers showed significant light harvesting properties with an exceptionally high light-to-electrical energy conversion efficiency (6.23%) shown by Zn-1 complex which was close to that obtained with standard Ru dye, N719 (6.96%) under similar experimental conditions. The better performance of Zn-1 complex compared to other studied dye sensitizers were attributed to its structural features. The extent of interfacial charge recombination and electron lifetime were evaluated by the electrochemical impedance spectroscopy (EIS).

Zn0.8Cd0.2S@PCBM Hybrid as an Efficient Electron Transport Layer for Air‐Processed p‐i‐n Planar Perovskite Solar Cells: Improvement of Interfacial Electron Transfer and Device Stability

In this study, an inorganic/organic hybrid, Zn0.8Cd0.2S nanoparticles (ZCS) embedded in [6,6]‐phenyl‐C61‐butyric acid methyl (PCBM), as an efficient electron transport layer (ETL) for air‐processed p‐i‐n perovskite solar cells (PSCs) has been demonstrated, and the doping effect and doping mechanism are systematically studied. As compared to PCBM, ZCS@PCBM ETL exhibits improved electron extraction at the perovskite/ETL interface, increased electron transportation within the ETL, enhanced charge collection efficiency, and suppressed interfacial charge recombination, resulting in significantly improved power conversion efficiency (PCE) from 14.41 to 17.18% by 19.2%. Interestingly, the ZCS nanoparticles can protect the perovskite layer from erosion by ambient moisture, and 82% of the initial PCE for the non‐encapsulated devices with ZCS@PCBM ETL is retained after 500 h storage in the atmosphere (humidity 30–60%) versus only 13% of the initial PCE for the PCBM ETL without ZCS doping.

Theoretical insights into co-sensitization mechanism in Zn-porphyrin and Y123 co-sensitized solar cells

Donor-π-acceptor YD2-o-C8, WW-6, and SM315 are among best-performing Zn-porphyrins for their outstanding light-harvesting properties. To reach the further enhancement of photovoltaic performance of the corresponding cells, co-sensitization of these typical porphyrins with those dyes possessing intense absorption in the green spectral region (typically, Y123) is necessary. In this work, using density functional theory (DFT) and time-dependent DFT (TD-DFT) approaches, co-sensitization mechanism in the above three typical Y123/Zn-porphyrin systems has been systematically investigated. Moreover, due to the excellent performance of dithienosilole (DTS) group in organic dyes, it is designed here to replace the benzothiadiazole (BTD) unit in SM315 to give a new Zn-porphyrin dye designated SM315-1. This specific modification of SM315 can effectively retard the interfacial charge recombination compared to that in YD2-o-C8 and WW-6 cells by reducing the contact between oxidized porphyrins and injected electrons in TiO2 substrate, and simultaneously improves the overall light harvesting ability of the co-sensitized film with the theoretical maximum limit of photocurrent to be 38.31 mA/cm2. The both favorable aspects suggest an attractive application of the fully novel Y123/SM315-1 co-sensitized film in dye-sensitized solar cell (DSSC).

Realization of 16.9% Efficiency on Nanowires Heterojunction Solar Cells with Dopant‐Free Contact for Bifacial Polarities

AbstractLow‐cost and efficient interfacial layer construction with the required charge selectivity and compatibility is necessary for nanostructured solar cells, and the proper integration of the interfacial layer with the light‐trapping system is required to improve the power conversion efficiency of the cell. Herein, low‐cost Si nanowires‐based solar cells with tunneling heterojunctions are developed by the deposition of MoOx and spin‐coating of Cs2CO3 as the carrier‐selective layers. The power conversion efficiency of 16.9% for a device of 4 cm2 in area is achieved by Si nanowires solar cells by the self‐assembly of ultra‐thin SiOx as the surface tunneling passivation layer. Self‐assembly is realized with an ultraviolet O3 treatment process at room temperature. Quasi‐steady‐state photoconductance, microwave‐detected photoconductance decay, and constant current–voltage measurements are used to characterize the passivation quality and tunneling transportation properties of the ultra‐thin SiOx layers. Interfacial charge recombination is suppressed and effective carrier tunneling properties are developed by the growth of ≈1.5 nm thick SiOx layers on the surfaces of the Si nanowires. This proposed Si nanowires solar cell architecture featuring tunneling heterojunctions achieves high performance and may be suitable for fabricating industrialized Si nanowires‐based photovoltaic devices through a cost‐effective, simple, and low‐temperature process.

Enhanced Performance of Perovskite Solar Cells by Using Ultrathin BaTiO3 Interface Modification

Efficiency promotion has been severely constrained by charge recombination in perovskite solar cells (PSCs). Interface modification has been proved to be an effective way to reduce the interfacial charge recombination. In this work, a mesoporous TiO2 (mp-TiO2) layer was modified by an ultrathin BaTiO3 layer to suppress charge recombination in PSCs. The ultrathin BaTiO3 modification layer was prepared by the spin coating method using a barium salt solution. The concentration of the barium salt solution was optimized, and the effect of the BaTiO3 modification layer on the performance of the cells was also investigated. The modification layer can not only successfully retard charge recombination but also effectively boost the rate of electron extraction at the interface, resulting in enhanced open-circuit voltage ( Voc), short circuit current density ( Jsc), and fill factor. Furthermore, the hysteresis of the PSCs was also significantly reduced after the modification. By optimizing and employing the BaTiO3 modification layer, the power conversion efficiency of the cells was increased from 16.13 to 17.87%.

High-efficiency dye-sensitized solar cells based on bilayer structured photoanode consisting of carbon nanofiber/TiO2 composites and Ag@TiO2 core-shell spheres

In this contribution, bilayer structured photoanode consisting of electron transport layer and light-scattering layer was elaborately designed for dye-sensitized solar cells (DSSCs). The electron transport layer was constructed by hierarchical carbon nanofiber/TiO2 (CF/TiO2) composites characterized by high electrical conductivity, while the light-scattering layer was assembled by Ag@TiO2 core-shell spheres with excellent light-scattering capacity. To demonstrate the potential application of such promising photoanode, the DSSC based on CF/TiO2-Ag@TiO2 bilayer structured photoanode was fabricated and characterized in detail. In stark contrast, the resultant DSSC exhibited a superior short-circuit photocurrent density of 16.51 mA cm−2, a prolonged electron lifetime of 93.7 ms, as well as a notable enhanced PCE up to 7.35%, which was about 41% higher than that of commercial P25-based DSSC. Along with the analysis of microstructure characteristics, light absorption/reflection spectra as well as electrochemical impedance spectroscopy, the significantly enhanced photovoltaic performance of DSSC based on the bilayer structured photoanode were believed to be related to the combined effect of sufficient dye loading endowed by the large specific surface area of photoanode materials, the outstanding light-harvesting capacity offered by the unique Ag@TiO2 light-scattering layer and the efficient electron transfer and/or negligible interfacial charge recombination benefited from the unique CF/TiO2 electron transport layer.

High-Efficiency Air-Stable Colloidal Quantum Dot Solar Cells Based on a Potassium-Doped ZnO Electron-Accepting Layer.

High-efficiency colloidal quantum dot (CQD) solar cells (CQDSCs) with improved air stability were developed by employing potassium-modified ZnO as an electron-accepting layer (EAL). The effective potassium modification was achievable by a simple treatment with a KOH solution of pristine ZnO films prepared by a low-temperature solution process. The resulting K-doped ZnO (ZnO-K) exhibited EAL properties superior to those of a pristine ZnO-EAL. The Fermi energy level of ZnO was upshifted, which increased the internal electric field and amplified the depletion region (i.e., charge drift) of the devices. The surface defects of ZnO were effectively passivated by K modification, which considerably suppressed interfacial charge recombination. The CQDSC based on ZnO-K achieved improved power conversion efficiency (PCE) of ≈10.75% ( VOC of 0.67 V, JSC of 23.89 mA cm-2, and fill factor of 0.68), whereas the CQDSC based on pristine ZnO showed PCE of 9.97%. Moreover, the suppressed surface defects of ZnO-K substantially improved long-term stability under air. The device using ZnO-K exhibited superior long-term air storage stability (96% retention after 90 days) compared to that using pristine ZnO (88% retention after 90 days). The ZnO-K-based device also exhibited improved photostability under air. Under continuous light illumination for 600 min, the ZnO-K-based device retained 96% of its initial PCE, whereas the pristine ZnO-based device retained only 67%.

Effect of SnO2 Nanoparticles on Band Gap Energy of x(SnO2)-y(Ag)-β-Carotene/FTO Thin Film

SnO2 shows higher electron mobility (100−200 cm2V−1s−1) than TiO2 (0.1-1.0 cm2V−1s−1), indicated that it has higher electron diffusion transport. As a result, it may minimize interfacial charge recombination losses to oxidized redox species in the electrolyte or solid-state hole transporter, thus enhancing solar cell device performance. Many researchers have done the synthesis of SnO2 by controlling morphologies and architectures to optimize solar cells performance. Unfortunately, it’s just view of them investigated the properties of the composite SnO2 with metallic materials and its potentials for solar cell devices. In this work, SnO2 nanoparticles, Ag nanoparticles, and β-Carotene were composited with a various mass portion of SnO2 nanoparticles to control the band gap energy and the other optoelectronic properties. The composites subsequently were deposited on FTO substrates using spin coater. The thin film of SnO2-Ag-β-Carotene/FTO composites were characterized by UV-Vis, FTIR spectroscopy, and XRD measurements. We found that the maximum direct band gap energy of 3.74 eV was reached by the sample 20, while the minimum direct band gap energy of 3.54 eV was achieved by sample 15. The maximum indirect band gap of 1.59 eV resulted in sample 20. On the other hand, the minimum value of the indirect band gap is 1.27 eV achieved by sample 25. Both direct and indirect Tauc plot analysis showed that there was no significant influence of SnO2 mass portion on the band gap energy of SnO2-Ag-β Carotene composite thin film. But it was a fact that each mass fraction composites had different band gap energy. This fact can be used to control the band gap energy to optimize the use of SnO2 composite as DSSC electron transporting electrode. The FTIR spectroscopy and XRD result were also detail examined in this work.

Bromine Doping as an Efficient Strategy to Reduce the Interfacial Defects in Hybrid Two-Dimensional/Three-Dimensional Stacking Perovskite Solar Cells.

Solar-to-electricity conversion efficiency, power conversion efficiency (PCE), and stability are two important aspects of perovskite solar cells (PSCs). However, both aspects are difficult to simultaneously enhance. In the recent two years, two-dimensional (2D)/three-dimensional (3D) stacking structure, designed by covering the 3D perovskite with a thin 2D perovskite capping layer, was reported to be a promising method to achieve both a higher PCE and improved stability simultaneously. However, when reducing the surface defects of 3D perovskite, the thin 2D capping layer itself may probably introduce additional interfacial defects in a 2D/3D stacking structure, which is thought to be able to trigger trap-assisted nonradiative recombination or ion migration. Thus, efforts should be paid to reduce the interfacial defects of 2D hybrid perovskite when serving as a modification layer in a 2D/3D stacking structure PSCs. Here, we demonstrate that bromine (Br) doping of the 2D perovskite capping layer is an efficient strategy to passivate interfacial defects robustly, by which the photoluminescence lifetime is enhanced notably, whereas the interfacial charge recombination is suppressed a lot. As a result, the PCE is enhanced from 18.01% (3D perovskite) to 20.07% (Br-doped 2D/3D perovskite) along with improved moisture stability.

Lithium-doped Cu2ZnSnS4 superstrate solar cells with 5% efficiency – An alternative to thin film kesterite photovoltaics

Cu2ZnSnS4 or kesterite was branded as “the dark horse of next-generation solar energy conversion” in a recent viewpoint article due to its advantageous inorganic, earth abundant, non-toxic composition over perovskites. It was also pointed out that an alternative device structure to thin-film configuration could be a key to unlock the full potential of kesterite photovoltaics. In response to this need herein a Cu2ZnSnS4 @TiO2 superstrate solar cell built via ethanol solution processing is described characterized by record efficiency of 5% and excellent stability. More specifically, lithium doping was found to promote kesterite grain growth and minimize interface charge recombination opening new prospects for kesterite photovoltaics.

Solar Cells: Enhancing the Performance of the Half Tin and Half Lead Perovskite Solar Cells by Suppression of the Bulk and Interfacial Charge Recombination (Adv. Mater. 35/2018)

Solar Cells: Enhancing the Performance of the Half Tin and Half Lead Perovskite Solar Cells by Suppression of the Bulk and Interfacial Charge Recombination (Adv. Mater. 35/2018)

9.13%-Efficiency and stable inorganic CsPbBr3 solar cells. Lead-free CsSnBr3-xIx quantum dots promote charge extraction

Interfacial charge recombination seriously drags efficiency enhancement of all-inorganic perovskite solar cells (PSCs) because of large energy-level differences. Although CsPbBr3-xIx light-harvesters markedly reduce interfacial charge recombination, the long-term stability of solar cell devices is still unsatisfactory to meet practical requirements. Here we present the improved charge extraction by setting an intermediated energy-level at CsPbBr3/carbon interface with colorful CsSnBr3-xIx quantum dots (QDs). The charge extraction is maximized by tuning Br:I ratio, yielding a power conversion efficiency as high as 9.13% for CsSnBr2I QDs-tailored CsPbBr3 solar cell. Moreover, all the devices show high stability in 80%RH or 80 °C over 720 h. The interfacial decoration of all-inorganic perovskite solar cells by lead-free perovskite QDs provides new opportunities of enhancing solar cell efficiency and reducing environmental impacts.

High efficiency non-fullerene organic solar cells without electron transporting layers enabled by Lewis base anion doping

Interfacial charge extraction and recombination play a critical role in photovoltaic characteristics of organic bulk heterojunction solar cells. Here we propose a methodology based on Lewis base n-doping with tetrabutylammonium acetate (TBAA) salts to boost charge extraction and resultant power conversion efficiencies (PCE) in OPVs with non-fullerene acceptors (NFA). Based on the PBDB-T:IT-M model system, we show that the TBAA doping via interfacial electron transfer is advantageous in maintaining the preferential intermolecular stacking and built-in potential necessitated for device operation compared to conventional bulk doping. Benefitting from the melioration of interfacial charge extraction and recombination, gains of photocurrent and fill factor are obtained in PBDB-T:IT-M solar cells. We found that the energy of charge transfer states after doping becomes slightly raised, which promotes the mitigation on radiative losses for open-circuit voltage. Furthermore, the described TBAA doping is found with the generality in modifying the device characteristics in non-fullerene based organic solar cells with different LUMO energies. Based on the PBDBT-2F:IT-4F BHJ blends, the solar cell doped with TBAA yields a PCE exceeding 13% without electron-transporting layer (ETL). This strategy allows for high efficiency ETL-free organic solar cells with potentials of reduced fabrication cost.

Enhancing the Performance of the Half Tin and Half Lead Perovskite Solar Cells by Suppression of the Bulk and Interfacial Charge Recombination.

In this article it is investigated how the hole extraction layer (HEL) influence the charge recombination and performance in half tin and half lead (FASn0.5 Pb0.5 I3 ) based solar cells (HPSCs). FASn0.5 Pb0.5 I3 film grown on PEDOT:PSS displays a large number of pin-holes and open grain boundaries, resulting in a high defect density and shunts in the perovskite film causing significant bulk and interfacial charge recombination in the HPSCs. By contrast, FASn0.5 Pb0.5 I3 films grown on PCP-Na, an anionic conjugated polymer, show compact and pin-hole free morphology over a large area, which effectively eliminates the shunts and trap states. Moreover, PCP-Na is characterized by a higher work function, which determines a favorable energy alignment at the anode interface, enhancing the charge extraction. Consequently, both the interfacial and bulk charge recombination in devices using PCP-Na HEL are considerably reduced giving rise to an overall improvement of all the device parameters. The HPSCs fabricated with this HEL display power conversion efficiency up to 16.27%, which is 40% higher than the efficiency of the control devices using PEDOT:PSS HEL (11.60%). Furthermore, PCP-Na as HEL offers superior performance in larger area devices compared to PEDOT:PSS.

Long lived-charge separation of ultrafast bimolecular electron transfer at PCE10 and fullerene interfaces

A profound understanding of the ultrafast interfacial charge transfer (CT), charge separation (CS), and charge recombination (CR) are paramount in enhancing the photoconversion efficiency (PCE) of the solar cell devices. Here, a combination of steady-state and femtosecond transient absorption spectroscopies with broadband capabilities are used to explore and decipher the photoinduced charge carrier dynamics at the PCE10/C60 interfaces. Our experimental results verified the efficient and ultrafast (sub-picosecond) photoinduced electron transfer (ET) from the PCE10 polymer to fullerene acceptor C60. Notably, the slow CR and fast ET make the current PCE10/C60 system ideal for its potential use in solar cells devices.

Low-Temperature Atomic Layer Deposition of Metal Oxide Layers for Perovskite Solar Cells with High Efficiency and Stability under Harsh Environmental Conditions

Rapid progress achieved on perovskite solar cells raises the expectation for their further development toward practical applications. Moisture sensitivity of perovskite materials is one of the major obstacles which limits the long-term durability of the perovskite solar cells, especially in outdoor operation where rainfall and water accumulation on the solar panels often occur. Micro/nanopinholes within the functional layers of the devices usually lead to water vapor penetration, thus subsequent decomposition of perovskites, and finally poor device performance and shortened operational lifetime. In this work, low-temperature atomic layer deposition (ALD) technique was utilized to incorporate pinhole-free metal oxide layers (TiO2 and Al2O3) into an inverted perovskite solar cell consisting of indium tin oxide/NiO/perovskite/PC61BM/TiO2/Ag. The interface properties between the inserted TiO2 layer and the perovskite layer were investigated by X-ray photoelectron spectroscopy. The results showed that TiO2 ALD fabrication process had made negligible degradation to the perovskite layer. The TiO2 layer can significantly reduce interfacial charge recombination loss, improve interfacial contact, and enhance water resistance. A maximum power conversion efficiency (PCE) of 18.3% was achieved for devices with TiO2 interface layers. A stacked Al2O3 encapsulation layer was designed and deposited on top of the devices to further improve device stability under harsh environmental conditions. The encapsulated devices with the best performance retained 97% of the initial PCE after being stored in ambient condition for a thousand hours. They also showed great water resistance, and no significant degradation in terms of PCE and photocurrent of the devices was observed after they were immersed in deionized water for as long as 2 h. Our approach offers a promising way of developing highly efficient and stable perovskite solar cells under real-world operational conditions.

Excitation Wavelength Dependent Interfacial Charge Transfer Dynamics in a CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> Perovskite Film

Elucidation of interfacial charge separation and recombination mechanisms is crucial to improve performance of organic-inorganic metal halide perovskite solar cells. Here, we have investigated influence of initially populated electron and hole potential levels in a perovskite conduction band (CB) and valence band (VB), respectively, by altering an excitation wavelength on interfacial charge separation and recombination dynamics in a CH3NH3PbI3 perovskite film sandwiched by a mesoporous TiO2 structure as an electron transport material (ETM) and a spiro-OMeTAD film as a hole transport material (HTM). Multi-phasic electron injection reactions are observed over <1.2 to several tens of nanoseconds, while most of holes are injected to a spiro-OMeTAD layer within the transient emission spectroscopy instrument response time (1.2 ns). In contrast, interfacial charge recombination rates are slower (from 5ms to 1.3 s) with the increase of the excitation wavelength. These kinetics suggest that as long as low excitation intensity is employed, e.g. 10 nJ/cm2 or 1 sun (100 mW/cm2), the APCE of ~100% can be expected at any excitation wavelength for the solar cells based on FTO/c-TiO2/m-TiO2/MAPbI3/OMeTAD films.