Highly efficient inverted rapid-drying blade-coated organic solar cells

Efficient inverted bulk-heterojunction (BHJ) poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM) organic solar cells fabricated by rapid-drying blade-coated were demonstrated. Optimized self-organization interpenetration networks and donor/acceptor domain sizes were obtained while maintaining the smooth surface morphology. By integrating with low-temperature-processed sol-gel ZnO electron extraction layer, power conversion efficiency (PCE) up to 4.4% under AM1.5G 1 sun illumination is achieved, compared to fast drying but low efficiency (1.2%) and high efficiency but with long-time solvent annealing treatment (4.3%) control cells deposited by spin coating in chlorobenzene (CB) and 1,2-dichlorobenzene (DCB) solution, respectively. The novel deposition technique reveals a promising process for highly efficient, high throughput, stable morphology organic solar cells fabrication.     

Cathodic multilayer transparent electrodes for ITO-free inverted organic solar cells

We demonstrate cathodic multilayer transparent electrodes based on a ZnS/Ag/TiO_x (ZAT) structure for ITO-free inverted organic solar cells. A quality solution-based TiO_x layer is adopted as an inner dielectric layer to modify the effective work function of Ag, ensuring the ZAT electrode works as a cathode. The effect of the TiO_x layer is seen on the open-circuit voltage of a solar cell incorporating this layer, increasing to 900 mV from 600 mV in the case of a cell with a bare Ag layer for a bulk-heterojunction of poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C₇₀-butyric acid methyl ester (PCBM70). The results of a joint theoretical and experimental study indicate that the photocurrent of a ZAT-based solar cell can be significantly enhanced by carefully balancing the optical-spacer and cavity-resonance effects, both of which are modulated by the thickness of the WO₃ layer used as a hole-collection layer at the top anode side. ZAT-based inverted solar cells with an optimized structure exhibit a power conversion efficiency as high as 5.1%, which is comparable to that of the ITO-based equivalent.     

Spontaneous growth by sol-gel process of low temperature ZnO as cathode buffer layer in flexible inverted organic solar cells

In this study, the sol–gel method was employed to prepare zinc oxide (ZnO) thin films as cathode buffer layers for inverted organic solar cells (IOSCs). We used a low temperature sol-gel process for the synthesis of ZnO thin films, in which the molar ratio of zinc acetate dihydrate (ZAD) to ethanolamine (MEA) was varied; subsequently, using the thin films, we successfully fabricated inverted solar cells on flexible plastic substrates. A ZnO sol–gel was first prepared by dissolving ZAD and MEA in ethylene glycol monomethyl ether (EGME). The molar ratios of ZAD to MEA were set as 1:1.2, 1:1, and 1:0.8, and we investigated the characteristics of the resulting ZnO thin films. We investigated the optical transmittance, surface roughness, and surface morphology of the films. Then, we discussed the reasons about the improvement of the device efficiency. The devices were fabricated using the ZnO thin films as cathode buffer layers. The results indicated that the morphology of the thin films prepared using the ZAD to MEA ratios of 1:1 and 1:0.8 changed to a rippled nanostructure after two-step annealing. The PCE was enhanced because of the higher light absorption in the active layer caused by the nanostructure. The structure of the inverted device was ITO/ZnO/P3HT:PC₆₁BM/MoO₃/Ag. The short-circuit current densities (8.59 mA/cm² and 8.34 mA/cm²) of the devices with films prepared using the ZAD to MEA ratios of 1:1 and 1:0.8 ratios, respectively, and annealed at 125 °C were higher than that of the device containing the ZnO thin film that was annealed at 150 °C. Inverted solar cells with ZnO films that were prepared using the ZAD to MEA ratios of 1:1 and 1:0.8 and annealed at 125 °C exhibited PCEs of 3.38% and 3.30%, respectively. More than that, PCEs of the flexible device can reach up to 1.53%.     

Merocyanines for vacuum-deposited small-molecule organic solar cells

We report here synthesis and photovoltaic properties of three merocyanines dyes (DPPT, DTPT, 1-NPPT) which are functionalized with electron withdrawing thiazolidenemalononitrile and electron rich diarylamine functionalities. It is found that structural feature of the diarylamino groups has a profound effect on the physical properties such as the absorption spectrum, oxidation potential, and HOMO/LUMO energy levels. The compound DTPT containing a better electron-donating ditolyl group, exhibits red-shifted absorption with relatively higher molar extinction coefficient, indicating its better light-harvesting ability. Hole mobility of these compounds is found to be strongly dependent on the various intermolecular interactions. Interestingly, single crystal structures reveal that the crystal packing motifs are rather closely related to the observed hole mobility in a trend of DPPT > DTPT > 1-NPPT. Vacuum-processed small-molecule organic solar cells were fabricated using the title merocyanines as p-type materials (donor) in combination with fullerene (C₆₀ or C₇₀) as n-type material (acceptor) with various device configurations. Among them, the DPPT-based devices outperform the devices based on DTPT and 1-NPPT. The power conversion efficiency (PCE) of DPPT-based device was improved from 1.55% of a BHJ device to 2.63% of a PMHJ device and 3.52% of a PMHJ device without the thin donor layer.     

Highly conductive Zinc-Tin-Oxide buffer layer for inverted polymer solar cells

Thin-films of Zinc Tin Oxide (ZTO) with an extremely high charge carrier mobility and superior optical transmittance are synthesized using a simple solution method. These ZTO films have been systematically studied for the application in inverted polymer solar cells (PSCs). The Hall effects measurements show that the charge mobility of the ZTO semiconductor is over 16.5 cm².V⁻¹.S⁻¹, which is the highest mobility value ever reported for oxide buffer made by using solution process. By applying the ZTO buffer layer in the inverted PSCs of P3HT:PC₆₁BM, the power conversion efficiency of the device is 30% higher than that of the devices made with other common buffer layers such as ZnO and TiO₂. Light intensity-dependent JV studies and PL measurements also indicate that ZTO buffer layer reduces surface recombination. This work demonstrates that the solution-synthesized ZTO is a promising new buffer layer with superior electron extraction capability for the solar cells.     

Stability of diketopyrrolopyrrole small-molecule inverted organic solar cells

Small-molecule DPP(TBFu)₂-based inverted organic solar cells were fabricated and their stability investigated. The effects of thermal annealing and solvent annealing on device performance and stability were compared. To increase the stability, mix-PCBM (PC₆₁BM and its C₇₀ analogue), which is reported to give higher device stability, was also included. Solvent-annealed devices showed the highest power conversion efficiency (PCE) of 4.62%, whereas thermally annealed devices showed a PCE of 3.94%. After the aging process, which involved thermal stress and exposure to air, thermally annealed and mix-PCBM devices retained a PCE of 3%, whereas solvent-annealed devices had a much lower PCE of 1.7%. Therefore, our results show that in the long-term stability perspective, thermal annealing is better than solvent annealing, and mix-PCBM is better than PC₆₁BM in the case of DPP(TBFu)₂. We fabricated small-molecule inverted organic solar cells that retain their performance in air for 3 weeks without encapsulation.     

Efficiency enhancement of ZnO based inverted BHJ solar cells via interface engineering using C70 modifier

The interface quality of ZnO and the photoactive polymer blend is of utmost importance in the performance of organic-inorganic hybrid photovoltaic devices. The chemically prepared ZnO electron transporting layer often produce surfaces unacceptable for efficient electron extraction and understate the photovoltaic performance. Herein, we propose a facile interfacial modification technique to enhance the charge collection efficiency of ZnO cathode electrode by efficiently bridging the superficial troughs and ridges of ZnO with the photoactive PCDTBT: PC₇₁BM polymer blend. The investigations show that vacuum sublimated C₇₀ interlayer efficiently fills the gaps between ZnO and the polymer blend reducing accumulation of the charges at the interface and thus minimizing the recombination probability. It also plays a very crucial role in passivating ZnO electrode against interfacial traps due to adsorbed chemical species. The inclusion of C₇₀ interlayer into the devices led to a substantial increase in device performance with PCE reaching close to 4%, an increment by a factor of 2 compared to the control devices. Our investigations aim towards showing the efficacy of C₇₀ small molecule in significantly enhancing the PCE of ZnO based BHJ solar cells fabricated and measured in ambient conditions rather than setting benchmark efficiency for the configured device. However, better performances for the devices are conceivable by performing the fabrication and measurement in controlled inert atmosphere.     

Efficient and stable inverted polymer solar cells using TiO2 nanoparticles and analysized by Mott-Schottky capacitance

The performance of both inverted and conventional polymer solar cells (PSCs) were examined with a low-temperature, solution-processed synthesized TiO₂ nanoparticles (TiO₂ NPs) as the electron extraction layer. The performance of inverted PSCs based on P3HT:PCBM bulk-heterojunction with a TiO₂ NPs layer was dramatically improved and the highest power conversion efficiency (PCE) of 4.56% was achieved via 24 h exposure in air, which is one of the highest PCEs for P3HT:PCBM bulk-heterojunction PSCs using TiO₂ as electron extraction layer. Meanwhile, the performance of inverted PSCs was superior to regular PSCs. Mott-Schottky capacitance analysis was carried out for both inverted and regular PSCs to obtain the built-in potential, the depletion width, as well as the doping level of the active layer, which all support the performance improvement of PSCs devices with inverted structure. In addition, inverted PSCs show excellent stability in air without encapsulation. The PCE can retain 87% of its original values after 400 h exposure in air, which is much better than that of regular PSCs. The results indicate that solution-processed TiO₂ NPs shows great potential applications in the fabrication of highly efficient and stable inverted PSCs as well as large-area, flexible printed PSCs.     

Solution-processed vanadium oxide as an anode interlayer for inverted polymer solar cells hybridized with ZnO nanorods

Solution-processed vanadium oxide (V₂O₅) as an anode interlayer is introduced between the organic layer and the Ag electrode for improving the performance of the low-cost inverted polymer solar cells hybridized with ZnO nanorods. Our investigations indicate that the solution-processed V₂O₅ interlayer as an electron-blocking layer can effectively prevent the leakage current at the organic/Ag interface. The power conversion efficiency is improved from 2.5% to 3.56% by the introduction of the V₂O₅ interlayer. The V₂O₅ interlayer also serves as an optical spacer to enhance light absorption, and thereby increases the photocurrent. Compared to the vacuum-deposited techniques, the fabrication of the solution-processed V₂O₅ interlayer is simple and effective. The solution-based approach makes it attractive for applications to mass production and potentially printed organic electronics.     

Efficient inverted polymer solar cells incorporating doped organic electron transporting layer

An efficient inverted polymer solar cell is enabled by incorporating an n-type doped wide-gap organic electron transporting layer (ETL) between the indium tin oxide cathode and the photoactive layer for electron extraction. The ETL is formed by a thermal-deposited cesium carbonate-doped 4,7-diphenyl-1,10-phenanthroline (Cs₂CO₃:BPhen) layer. The cell response parameters critically depended on the doping concentration and film thickness of the Cs₂CO₃:BPhen ETL. Inverted polymer solar cell with an optimized Cs₂CO₃:BPhen ETL exhibits a power conversion efficiency of 4.12% as compared to 1.34% for the device with a pristine BPhen ETL. The enhanced performance in the inverted device is associated with the favorable energy level alignment between Cs₂CO₃:BPhen and the electron-acceptor material, as well as increased conductivity in the doped organic ETL for electron extraction. The method reported here provides a facile approach to optimize the performance of inverted polymer solar cells in terms of easy control of film morphology, chemical composition, conductivity at low processing temperature, as well as compatibility with fabrication on flexible substrates.     

ZnO纳米颗粒表面缺陷对有机太阳能电池性能的影响

用温度控制ZnO纳米 颗粒粒径的大小,研究了颗粒粒径对表面缺陷的影响。由透射电镜(TEM)、紫外-吸收光谱 和荧光光谱测试表明,随着反应温度升高,ZnO纳米颗粒的尺寸增加,比表面积显著下降, 表面缺陷的体密度降低。将不同反应温 度下的ZnO纳米颗粒应用于ITO/ZnO/P3HT:PCBM/MoO₃/Ag结构的有机太阳能电池中,进一 步研究了缺陷对电池性能的影 响。实验结果表明,60℃下ZnO纳米颗粒薄膜作为电子传输层的器件 效果最好,电池效率可以达到3.05%。 这表明在一定范围内,ZnO纳米颗粒越大,缺陷密度越低,越有利于器件中电子的传输从而 提高太阳能电池器件的短路电流密度和光电转化效率。     

正置倒置异质结有机小分子太阳能电池

以MoO3为阳极修饰层,以Rubrene/C60为活性层,制备了正置和倒置异质结有机小分子太阳能电池。实验结果表明倒置器件的开路电压Voc、短路电流密度Jsc、填充因子FF和功率转换效率η比正置结构的器件分别提高了34%、20%、25%和102%。当插入BCP阴极缓冲层后,阻挡了热的Al原子对C60层的破坏,对倒置器件的性能没有明显的影响,但却显著改善了正置器件的性能,并分析了MoO3和BCP对倒置和正置器件的作用。     

Small molecules based on bithiazole for solution-processed organic solar cells

A series of donor-acceptor-donor small molecules (1-3) with bithiazole as acceptor unit, triphenylamine as donor unit and thiophene with different number (0, 1, 2) as bridge were synthesized by palladium(0)-catalyzed Suzuki or Stille coupling reactions. The thermal, optical, electrochemical, charge transport, and photovoltaic properties of these small molecules were examined. All compounds exhibit excellent thermal stability with decomposition temperatures (5% weight loss) over 390 °C in nitrogen atmosphere. As increasing the number of thiophene and π-conjugation length of molecule, the absorption maximum in film red shifts from 406 to 498 nm, the extinction coefficient increases from 1.35 × 10⁴ to 7.66 × 10⁴ M⁻¹ cm⁻¹, and the optical band gap decreases from 2.6 to 2.0 eV. The electron-donating thiophene and bithiophene in compounds 2 and 3 up-shift HOMO energy level from −5.42 (1) to −5.24 eV (2) or −5.22 eV (3), and down-shift LUMO energy level from −2.48 (1) to −2.84 eV (2) or −2.81 eV (3). The hole mobility of compound 3 is up to 3.6 × 10⁻⁴ cm² V⁻¹ s⁻¹, which is one order of magnitude higher than that of compound 2, but compound 1 shows no field-effect transistor performance. Solution-processed bulk heterojunction organic solar cells based on 1-3:PC₇₁BM (1:4, w/w) blend films exhibit increasing power conversion efficiency (up to 2.61%) as increasing thiophene unit number.     

Improved efficiency of solution processed small molecules organic solar cells using thermal annealing

A solution processable A-D-A-D-A structure small molecule DCAEH5TBT using a BT unit as the core has been designed and synthesized for application in BHJ solar cells. The device employing DCAEH5TBT/PC₆₁BM as active layer shows PCE of 2.43% without any post treatment. After thermal annealing (150 °C, 10 min), the PCE of this molecule based device increased to 3.07%, with J_sc of 7.10 mA/cm², V_oc of 0.78 V and FF of 55.4%, which indicates that high performance of solution processed small molecule based solar cells can be achieved using thermal annealing by carefully design molecule structure.     

Progress towards fully spray-coated semitransparent inverted organic solar cells with a silver nanowire electrode

We demonstrated a fully spray-coated semitransparent organic solar cell, from the lowermost organic layer to the uppermost top electrode. The fabricated devices based on a poly (3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) are semitransparent (∼70% transparency at long wavelength beyond 650 nm), fully spray-coated from organic layer to top electrode, highly efficient (∼80% of that of a device with a conventional metal electrode).     

Enhanced photovoltaic performance in inverted polymer solar cells using Li ion doped ZnO cathode buffer layer

We have proposed an approach to improve the photovoltaic performance of inverted polymer solar cells (i-PSC) using lithium ion doped ZnO (LiZnO) as cathode buffer layer (CBL). The LiZnO CBL was prepared using the diffusion technique, performed by inducing the Li ion of 8-hydroxyquinolatolithium (Liq) to diffuse into ZnO film through annealing the bi-layer ZnO/Liq film. Doping concentration of Li ion was controlled by using various thickness of Liq film and annealing temperature. Based on LiZnO CBL, the poly (3-hexylthiophene) [6,6]:-phenyl C61-butyric acid methyl ester (P3HT:PCBM) i-PSC device possessed a optimal power conversion efficiency (PCE) of 4.07%, which was 30% improved than that of the device with neat ZnO as CBL. The enhancement of the device performance could be attributed to the enhanced electron mobility and better band matching of the LiZnO CBL. Our finding indicates that the LiZnO film fabricated with relatively low temperature treatment has great potential for high-performance i-PSCs.     

Study on the thermal stability of Ga-doped ZnO thin film: A transparent conductive layer for dye-sensitized TiO2 nanoparticles based solar cells

Generally, optoelectronic devices are fabricated at a high temperature. So the stability of properties for transparent conductive oxide (TCO) films at such a high temperature must be excellent. In the paper, we investigated the thermal stability of Ga-doped ZnO (GZO) transparent conductive films which were heated in air at a high temperature up to 500 °C for 30 min. After heating in air at 500 °C for 30 min, the lowest sheet resistance value for the GZO film grown at 300 °C increased from 5.5 Ω/sq to 8.3 Ω/sq, which is lower than 10 Ω/sq. The average transmittance in the visible light of all the GZO films is over 90%, and the highest transmittance is as high as 96%, which is not influenced by heating. However, the transmittance in the near-infrared (NIR) region for the GZO film grown at 350 °C increases significantly after heating. And the grain size of the GZO film grown at 350 °C after annealing at 500 °C for 30 min is the biggest. Then dye-sensitized TiO2 NPs based solar cells were fabricated on the GZO film grown at 350 °C (which exhibits the highest transmittance in NIR region after heating at 500 °C for 30 min) and 300 °C (which exhibits the lowest sheet resistance after heating at 500 °C for 30 min). The dye-sensitized solar cell (DSSC) fabricated on the GZO film grown at 350 °C exhibits superior conversion efficiency. Therefore, transparent conductive glass applying in DSSCs must have a low sheet resistance, a high transmittance in the ultraviolet–visible–infrared region and an excellent surface microstructure.     

Power conversion efficiency enhancement of organic solar cells by addition of gold nanostars,nanorods, and nanospheres

The effects of gold (Au) nanoparticles (NPs) with different morphologies (star, rod, sphere) incorporated into buffer layer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), of polymer-based organic solar cells (OSCs) were investigated. Solar cells having gold nanoparticles exhibited significant improvement in device efficiency relative to the reference device. The observed improvement is most likely due to the surface plasmon and enhanced light reflection and scattering properties of the Au NPs. The power conversion efficiency (PCE) is increased ca. 29% with Au nanostars, ca. 14% with Au nanorods and 11% with Au nanospheres compared to the device with no Au NP (reference device). Au nanostars provide the strongest contribution to the efficiency among all NP morphologies studied as they have large size, sharp features, and strongest localized surface plasmon resonance effect associate with their morphology.     

Magnetoconductance of polymer–fullerene bulk heterojunction solar cells

The organic magnetoconductance (MC) effects in poly(3-hexylthiophene): [6,6]-phenyl-C61-butyricacid methylester based bulk heterojunction solar cells were studied in dark and under illumination. The correlations between the MC and current character were revealed in this study. Results show that the dark current always exhibits a negative MC whereas a sign change in MC under illumination occurs at the bias around the open circuit voltage V_oc. We suggest that the positive MC in photocurrent is due to the field dependent conversion of singlet electron–hole pairs to triplet states and the negative MC is associated with space charge limited current with traps. Other possible mechanisms about the magnetoconductance effects are also discussed.     

Morphological modification induced by external electric field during solution process of organic solar cells

An electric field was externally applied on the poly(3-hexylthiophene)/[6,6]-phenyl C₆₁ butyric acid methyl ester (P3HT:PCBM) blend film during the fabrication of the bulk-heterojunction (BHJ) solar cells to induce morphological modification. It influences the vertical ratio of P3HT:PCBM molecules. Because the field is applied externally to the device, its direction can be altered. When the electric field of 5.0 × 10⁵ V m⁻¹ was applied with the specific direction, it formed a P3HT-rich and rougher surface, compared with that of pristine active layer, to improve the performance of the inverted polymer solar cells. Hence, the current density was improved from 9.15 mA cm⁻² to 9.83 mA cm⁻², and power conversion efficiency increased from 3.16% to 3.51%. This finding provides guidance for morphology engineering in organic materials for higher power conversion efficiency of organic solar cells.