Enhanced photovoltaic characteristics of solar cells based on n-type triphenodioxazine derivative

Polymer solar cells based on poly (2-methoxy-5-(2′-ethyl-hexyloxy)-1, 4-phenylene vinylene) (MEH-PPV):1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6)C61(PCBM):3, 10-di(trifluoromethane) triphenodioxazine (TFTD) was fabricated using spin coating technology. The absorption spectra of MEH-PPV: PCBM: TFTD film coated from chlorobenzene solution was broadened and showed higher intensity compared with that of the pure MEH-PPV. The TFTD acting as electron acceptors in combination with PCBM induced the photoluminescence (PL) quenching of MEH-PPV, which were associated with the photoinduced charge transfer characteristics of composite film. Further, a photovoltaic conversion efficiency up to 1.03% under 16.7 mW/cm² white light illumination was achieved in the MEH-PPV: PCBM: TFTD system.     

有机-金属钙钛矿太阳能电池微结构及载流子传输机制

采用有机-金属卤化物钙钛矿作为光吸收材料的固态太阳能电池取得了巨大的突破与快速的发展,电池效率已超过20%。对固态钙钛矿太阳能电池的微结构和激发载流子在电池内的产生与传输机制做了详细的综述与分析,指出了电池结构从介孔"敏化"结构开始,经历介观-超结构、平面异质结结构、无空穴传输层结构及无电子传输层结构的发展阶段。分析表明电池结构的发展主要取决于对有机-金属卤化物钙钛矿材料中激发载流子分离与传输机制的深入认识,利用既可传输电子又可传输空穴的有机-金属卤化物钙钛矿材料制备的介孔结构和异质结结构的电池都可实现高效率转化。     

Influence of TiO2 compact layer precursor on the performance of perovskite solar cells

The optimization of the hole-blocking layer in perovskite solar cells (PSC), typically based on TiO₂, is crucial, as it strongly affects the device performance. In this work, we thoroughly characterize the thickness, roughness, and crystal structure of a set of TiO₂ compact layers produced by spin coating of different precursor sols and correlate the choice of the TiO₂ precursor to the photovoltaic performance of the PSC. By replacing the commonly used titanium isopropoxide (TTIP) blocking layer precursor with titanium tetrachloride (TiCl₄), a clear enhancement in the PSC performance was observed, particularly in the hysteresis behavior and stability. The results from the morphological/structural analysis and transient photoluminescence studies clarify the different behavior of the compact layers in PSCs.     

Low temperature fabrication of formamidinium based perovskite solar cells with enhanced performance by chlorine incorporation

Recently formamidinium (FA) based perovskite solar cell was demonstrated to show high performance and better stability upon partial substitution of FA with Cs cation. However, the fabrication of device required high-temperature processing on TiO₂ electrode and thus limits the use of flexible polymeric substrates. Here, we present a low temperature approach for the fabrication of p-i-n perovskite solar cells based on Cs_0.15FA_0.85PbI₃. Furthermore, we investigated the effects of chlorine on the morphology and crystallinity of the perovskite films and the corresponding photovoltaic performance. Chlorine incorporation can significantly enlarge the size of grains and improve the crystallinity of perovskite films with full surface coverage. A best power conversion efficiency of 14.5% was realized for planar perovskite solar cells with negligible hysteresis and remarkable reproducibility.     

Impact of molecular weight on charge carrier dissociation in solar cells from a polyfluorene derivative

The effect of the molecular weight of poly[9,9-didecanefluorene-alt-(bis-thienylene) benzothiadiazole] (PF10TBT) on the photovoltaic performance of fullerene-based bulk heterojunction solar cells is investigated. An increase in molecular weight of two orders of magnitude results in a 30% increase of the short-circuit current and a rise of the fill factor from 0.45 to 0.63. Electron and hole transport are found to be virtually unaffected by changing molecular weight, which means that space-charge effects do not play a role in low molecular weight devices. Using optical modeling and numerical device simulations, we demonstrate that at low molecular weight the efficiency is mainly limited by a short lifetime of bound electron–hole pairs. This short lifetime prohibits efficient dissociation and is attributed to a deficiency in phase separation for low molecular weights.     

Origin of high fill factor in polymer solar cells from semiconducting polymer with moderate charge carrier mobility

The fill factor of polymer bulk heterojunction solar cells (PSCs), which is mainly governed by the processes of charge carrier generation, recombination, transport and extraction, and the competition between them in the device, is one of the most important parameters that determine the power conversion efficiency of the device. We show that the fill factor of PSCs based on thieno[3,4-b]-thiophene/benzodithiophene (PTB7):[6,6]-phenyl C₇₁-butyric acid methylester (PC₇₁BM) blend that only have moderate carrier mobilities for hole and electron transport, can be enhanced to 76% by reducing the thickness of the photoactive layer. A drift–diffusion simulation study showed that reduced charge recombination loss is mainly responsible for the improvement of FF, as a result of manipulating spatial distribution of charge carrier in the photoactive layer. Furthermore, the reduction of the active layer thickness also leads to enhanced built-in electric field across the active layer, therefore can facilitate efficient charge carrier transport and extraction. Finally, the dependence of FF on charge carrier mobility and transport balance is also investigated theoretically, revealing that an ultrahigh FF of 80–82% is feasible if the charge mobility is high enough (∼10⁻³–10⁻¹ cm²/V s).     

Effects of defect states on the performance of perovskite solar cells

We built an ideal perovskite solar cell model and investigated the effects of defect states on the solar cell''s performance. The verities of defect states with a different energy level in the band gap and those in the absorption layer CH₃NH₃PbI₃ (MAPbI₃), the interface between the buffer layer/MAPbI₃, and the interface between the hole transport material (HTM) and MAPbI₃, were studied. We have quantitatively analyzed these effects on perovskite solar cells'' performance parameters. They are open-circuit voltage, short-circuit current, fill factor, and photoelectric conversion efficiency. We found that the performances of perovskite solar cells change worse with defect state density increasing, but when defect state density is lower than 10¹⁶ cm^-3, the effects are small. Defect states in the absorption layer have much larger effects than those in the adjacent interface layers. The perovskite solar cells have better performance as its working temperature is reduced. When the thickness of MAPbI₃ is about 0.3 μm, perovskite solar cells show better comprehensive performance, while the thickness 0.05 μm for Spiro-OMeTAD is enough.     

In-situ investigation of interfacial effects on charge accumulation and extraction in organic solar cells based on transient photocurrent studies

In organic solar cells, the interfacial and bulk photovoltaic processes are typically coupled based on charge transport and accumulation. In this article, we demonstrated that the in situ transient photocurrent measurements can be a powerful approach to separately investigate the interfacial effects on interfacial and bulk photovoltaic process. Based on this method, the effects of interfacial dipoles on charge extraction, accumulation, and recombination are solely studied by comparing Ca and Al devices with standard architecture of ITO/PEDOT/P3HT:PCBM/cathode. We observe that stronger interfacial dipoles can significantly decrease the charge extraction time and consequently increase the charge extraction efficiency. More importantly, stronger interfacial dipoles can also decrease the charge accumulation within the bulk photovoltaic layer. Furthermore, our experimental results indicate that the bulk-accumulated charges can act as recombination centers under device-operating condition, resulting in the recombination loss in photogenerated carriers. Clearly, our studies of transient photocurrents elucidated the charge extraction, accumulation, and recombination in OSCs.     

Fused-ring acceptors based on quinoxaline unit for highly efficient single-junction organic solar cells with low charge recombination

Two non-fullerene small molecule acceptors (TFQ-F and TFQ-Cl) based on quinoxaline unit were designed and synthesized for efficient organic solar cells (OSCs). These two acceptors showed intense absorption up to 900 nm and high thermal stabilities with decomposition temperatures over 360 °C due to their fused-ring skeletons. TFQ-F and TFQ-Cl are the A-D-A′-D-A type acceptors (A/A′ for acceptor unit and D for donor unit). TFQ-F and TFQ-Cl have the same D-A′-D fragment, which was flanked with different ending groups. The effect of different ending groups on their photophysical properties, electrochemical behaviors, micro-structures and charge recombination properties of active layers, and device performance were investigated systematically. PM6 with the complementary absorption to the two acceptors was used as the donor material. The pristine PM6:TFQ-F blend films displayed the optimal morphologies as revealed by AFM and TEM measurement. Organic solar cells based on PM6:TFQ-Cl blend film showed high J_SC of 25.19 mA/cm² and PCE of 13.2%. The Voc, J_SC and PCE for PM6:TFQ-F film based device were 0.857 V, 23.70 mA/cm² and 13.51%, respectively. The dependence of V_OC/J_SC on various light intensities indicated that PM6:TFQ-F/Cl based device had low charge recombination.     

Effect of CdSe nanoparticles incorporation on the performance of P3OT organic photovoltaic cells

Photoluminescence and photovoltaic properties of P3OT:%CdSe nanocomposite films are investigated as a function of the mass concentration (wt%) of the CdSe nanoparticles (NPs) incorporated in the films. The incorporation of CdSe NPs produces a quenching of the photoluminescence and improves the performance of the nanocomposite solar cells. These effects are explained in terms of exciton dissociation and charge separation occurring at P3OT/CdSe interfaces within the Förster formalism, involving non-radiative energy transfer from the donor (P3OT) to the acceptor (CdSe NPs). An exciton quenching rate constant of 1.4×10⁻¹⁰ cm³ s⁻¹ is determined using the Stern–Volmer equation. In addition, scanning electron microscopy (SEM) images reveal that surface morphology is changed by CdSe NPs incorporation, in agreement with FTIR spectra. The current density–voltage (J–V) characteristics of ITO/P3OT:%CdSe/Al photovoltaic cells performed for different CdSe concentrations are also reported and indicate a significant improvement of the photovoltaic parameters cells, particularly, the conversion efficiency becomes 20 times greater than that of the cell based on pure polymer.     

The effect of external electric field on the performance of perovskite solar cells

Planar heterojunction perovskite solar cells were fabricated through a low temperature approach. We find that the device performance significantly depends on the external bias before and during measurements. By appropriate optimization of the bias conditions, we could achieve an 8-fold increase in the power conversion efficiency. The significant improvement in device performance might be caused by the ion motion in the perovskite under the external electric field.     

Effects of processing additives on nanoscale phase separation,crystallization and photovoltaic performance of solar cells based on mesogenic phthalocyanine

A study on the effects of processing additives on the nanoscale phase separation, crystallization, and photovoltaic performance of bulk heterojunction (BHJ) thin films made of 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C6PcH₂) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) via spin-casting for photovoltaic applications is reported. By incorporating various solvents as processing additives to a volume of a few percent, the separation of donor and acceptor phases in C6PcH₂:PCBM thin films, which discussed by taking the photoluminescence quenching, Davydov splitting at the Q-band of the absorbance spectra and the surface nanomorphology into consideration, is improved, and the crystallinity of the discotic C6PcH₂ molecules with hexagonal structures is reinforced. Photovoltaic cells with the optimum phase-separated BHJ materials and high crystallinity of the discotic C6PcH₂ molecules are demonstrated to have a power conversion efficiency of 4.2%.     

Grain structure control and greatly enhanced carrier transport by CH3NH3PbCl3 interlayer in two-step solution processed planar perovskite solar cells

We report that the use of a CH₃NH₃PbCl₃ interlayer onto the PEDOT:PSS layer in the two-step solution deposition of CH₃NH₃Pbl₃ for planar p-i-n type perovskite solar cells (PSCs) can lead to a dramatic enhancement of short-circuit current density (J_sc) by 52.8% from 13.07 mA cm⁻² to 19.98 mA cm⁻². While the absorption and thus the composition of the perovskite layers remain unchanged, Incident photon-to-current efficiency (IPCE) measurement results reveal much enhanced carrier transport, which in turn can be correlated to the larger and more columnar grain structure in the perovskite layer with the use of the CH₃NH₃PbCl₃ interlayer. On the other hand, the two-step solution processed perovskite layers without the CH₃NH₃PbCl₃ interlayer exhibit smaller and more cross-hatching grain structure and yield significantly smaller J_sc. Therefore our results revealed clearly that the insertion of CH₃NH₃PbCl₃ interlayer, which affects the nucleation dynamics, may control the grain structure of the two-step solution processed perovskite layers and improve dramatically the photovoltaic performance of the resultant planar p-i-n type PSCs. Our CH₃NH₃PbCl₃ interlayer may thus serve as an effective method for p-i-n PSCs to achieve high J_sc with thicker perovskite layer.     

Low temperature fabrication of hybrid solar cells using co-sensitizer of perovskite and lead sulfide nanoparticles

Our cost-effective approach for hybridizing methylammonium lead iodide and PbS nanoparticles at low temperature (≤100 °C) for photovoltaic devices is introduced. As employed into a perovskite based solar cell platform, effects of PbS on the device performance were investigated. Through experimental observations under simulated air-mass 1.5G illumination (irradiation intensity of 100 mWcm⁻²), the efficiency of a perovskite:PbS device is 11% higher than that of a pristine perovskite solar cell under the same fabrication conditions as a result of the broadened absorption range in the infrared region. The highest photovoltaic performance was observed at a PbS concentration of 2% with an open-circuit voltage, short-circuit current density, fill factor, and power-conversion efficiency of 0.557 V, 22.841 mA cm⁻², 0.55, and 6.99%, respectively. Furthermore, PbS NPs could induce hydrophobic modification of the perovskite surface, leading to an improvement of the device stability in the air. Finally, the low-temperature and cost-effective fabrication process of the hybrid solar cells is a good premise for developing flexible/stretchable cells as well as future optoelectronic devices.     

Improving the photovoltaic performance of co‐evaporated Cu2ZnSnS4 thin‐film solar cells by incorporation of sodium from NaF layers

We have investigated the influence of sodium (Na) on the properties of co‐evaporated Cu₂ZnSnS₄ (CZTS) layer microstructures and solar cells. The photovoltaic performance and diode properties were improved by incorporating Na from NaF layers into the CZTS layers, while Na had a negligible effect on the microstructural properties of the layer. The best cell fabricated by using an optimal CZTS layer (Cu/(Zn + Sn) = 0.70, Zn/Sn = 1.8) yielded an active area efficiency of 5.23%. The analysis of device properties suggests that charge‐carrier recombination at CZTS/CdS interface is suppressed by intentional Na incorporation from NaF layers. Copyright © 2016 John Wiley & Sons, Ltd.     

Experimental and simulation study for impact of different halides on the performance of planar perovskite solar cells

MeA-PbX₃ and MeA-PbI₂X (where MeA=CH₃NH₃; X=I, Br, Cl) systems have been synthesized using grinding processing. Plainly, the crystal structures of the perovskite materials were altered with the variation of the halide ions. Meanwhile, the band gap energy was enhanced from 1.5 eV for MeA-PbI₃ to 2.1 and 2.8 eV for MeA-PbBr₃ and MeA-PbCl₃ as a result of substitution by halide Br and Cl, respectively. The intensity peaks of different perovskite structures were confirmed by photoluminescence (PL). Furthermore, the following energy parameters, heat of formation, high occupied molecular orbital (HOMO) and low unoccupied molecular orbital (LUMO) were evaluated using hyperchem system software. Herein, we performed a device modeling and theoretical study on planar perovskite solar cells without a hole transporting material (HTM) using a solar cell simulation program (wxAMPS) as an update of the popular solar cell simulation tool (AMPS; Analysis of Microelectronic and Photonic Structures). Simulation and experimental design of MeA-PbX₃ and MeA-PbI₂X (where MeA=CH₃NH₃; X=I, Br, Cl) systems were investigated. The cells without HTM have been suggested to enhance the low cost and simple assembly of organic-inorganic lead halide perovskite based solar cells. MeA-PbBr₃ with HTM-free solar cells was achieved a high PCE of 13.96% in simulation program compared to 3.88% as experimental one.     

Ambipolar carrier transport properties of triphenylamine/dibenzofulvene derivative and its application for efficient n-i-p perovskite solar cells

Herein, the carrier transport properties of 9-(4-(diphenylamino)benzylidene)-N′,N′,N′,N′-tetraphenyl-9H-fluorene-2,7-diamine (YC-1) was firstly investigated with time-of-flight method. YC-1 exhibited ambipolar conductive behaviour, with hole and electron mobilities up to 10⁻⁴ cm²(Vs)⁻¹. We introduced YC-1 as a hole transporting layer (HTL) in a planar n-i-p perovskite solar cell. A device fabricated using YC-1 exhibited enhanced performance compared to the device fabricated using P3HT, demonstrating 14.91% power conversion efficiency (PCE) and low hysteresis with a hysteresis effect index (HEI) of 0.007. A comprehensive analysis of the J-V characteristics in hole-only devices revealed the improvement of the YC-1 based devices were mainly contributed to the higher conductivity and lower trap density, which were beneficial to the device performance. This work suggests the thermally evaporable triphenylamine/dibenzofulvene can be a good candidate for HTL of conventional perovskite solar cells.     

Analytical modeling of organic solar cells including monomolecular recombination and carrier generation calculated by optical transfer matrix method

An analytical model of organic solar cell has been developed including the effect of monomolecular recombination and charge carrier generation rate, simultaneously. The charge carrier generation rate, depending on position and wavelength, obtained from optical transfer matrix method, has been incorporated into electrical transport equation of carriers; this has led to combining optical and electrical phenomena into this model. Charge carrier generation rate profile has been investigated and included to develop the model. The proposed model addresses the propagation of light and the effects of optical phenomena like reflection and interference inside the device. Compared to previous models, this model is an improved version because of considering recombination mechanism and position and wavelength dependent generation rate simultaneously. This analytical model is useful for finding the performance of the organic solar cell device such as current-voltage relation, power-voltage relation, efficiency, etc. avoiding the complexities of numerical calculations. The proposed model has been validated by comparing the results obtained from the model with that of published experimental works. This model may help to analyse organic solar cells and optimize their parameters for improving the performance.     

Improved performance of nanoporous TiO2 film in dye-sensitized solar cells via ZrCl4 and TiCl4 surface co-modifications

In this study, nanoporous TiO₂ films were modified by a dip-coating process using a mixture aqueous solution of ZrCl₄ and TiCl₄ followed by calcination to prepare a photoanode for dye-sensitized solar cells. Compared with the control film modified with 0.04 mol L⁻¹ TiCl₄, the power conversion efficiency of the TiO₂ film modified with a mixed solution of 0.05 mol L⁻¹ ZrCl₄ and 0.04 mol L⁻¹ TiCl₄, was 18.67% higher because of the improved short circuit current (J_sc) and open circuit voltage (V_oc). The improvement in J_sc was due to the suppression of charge recombination, which was demonstrated by a series of measurements, including electrochemical impedance spectroscopy, monochromatic incident photon-to-electron conversion efficiency spectroscopy, and the open-circuit voltage decay technique. The Mott-Schottky measurement results indicated that the negative shift of a flat band led to the increased V_oc.     

Perylene diimide based all small-molecule organic solar cells: Impact of branched-alkyl side chains on solubility,photophysics, self-assembly,and photovoltaic parameters

Perylene diimide derivatives have been under intense investigation to replace fullerenes as the electron accepting component in organic photovoltaics, with molecular complexity continuing to grow. Simple alkyl-substituted perylene diimide monomers at the imide nitrogen position, however, have not been extensively investigated. Herein we demonstrate that subtle alkyl-substitutions at the imide-nitrogen position lead to significant changes in solubility, thin-film self-assembly and optical properties. When blended with a small-molecule donor to form all small-molecule, fullerene-free, solution processed organic solar cells, we show that the photovoltaic device performance and consistency can be tuned via alkyl-chain modifications. In addition we have simplified the device fabrication process by utilizing a silver cathode coupled with a small-molecule-ionic interlayer and achieved comparable performance to devices fabricated with a traditional Ca/Al cathode.