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%.     

Enhanced performance of inverted perovskite solar cells using solution-processed carboxylic potassium salt as cathode buffer layer

Using a novel solution-processed carboxylic potassium salt (F-R-COOK) as cathode buffer layer (CBL), a power conversion efficiency (PCE) of 14.37% is obtained, which is more than 51% increase compared with that of the Ag-only device under similar fabrication conditions. The test result of single electron devices and Electrochemical impedance spectroscopy (EIS) measurements demonstrate that the interlayer decreases charge transport resistance. Ultraviolet photoelectron spectroscopy (UPS) measurements are used to study the interfacial effects induced by the new CBL. It is found that F-R-COOK can reduce the work function of the Ag electrode by forming desired interfacial dipoles. Our work indicates the promising applications of F-R-COOK based CBL in perovskite solar cells and may provide some insights into the design and synthesis of new interfacial materials to further improve the device performance.     

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.     

Polyacetylene-based polyelectrolyte as a universal interfacial layer for efficient inverted polymer solar cells

Here we report that poly(N-dodecyl-2-ethynylpyridiniumbromide) (PDEPB) interlayers between electron-collecting zinc oxide (ZnO) layers and bulk heterojunction (BHJ) layers act as a universal interfacial layer for improving the performances of inverted-type polymer:fullerene solar cells. Three different BHJ layers, poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM), poly[(4,8-bis(2-ethylhexyloxy)-benzo[1,2-b:4,5-b']dithiophene)-2,6-diyl-alt-(N-2-ethylhexylthieno[3,4-c]pyrrole-4,6-dione)-2,6-diyl]] (PBDTTPD):PC₆₁BM, and poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM), were employed so as to prove the role of the PDEPB interlayers. Results showed that the power conversion efficiency (PCE) of polymer:fullerene solar cells with the three different BHJ layers increased in the presence of the PDEPB interlayers prepared from 0.5 mg/ml solutions. The improved PCE was attributed to the conformal coating of the PDEPB layers on the ZnO layers (by atomic force microscopy measurement), lowered work functions of ZnO induced by the PDEPB layers (by Kelvin probe measurement), and reduced interface resistance (by impedance spectroscopy measurement), as supported by the noticeable change in the atom environments of both the ZnO and PDEPB layers (by X-ray photoelectron spectroscopy measurement).     

Zwitterionic ammonium and neutral amino molecules as cathode interlayer for inverted polymer solar cells

Cathode interlayer is essential to inverted bulk heterojunction polymer solar cells (PSCs). A series of zwitterionic ammonium and neutral amino organic molecules are introduced into inverted PSCs as cathode interlayer and power conversion efficiency (PCE) as high as 8.07% is demonstrated. Compared to the devices without interlayer, all the devices exhibit significant improvements of the device parameters by reducing the work function of indium tin oxide (ITO) cathode. It is striking that the devices with neutral amino molecules as interlayer exhibit remarkably higher PCEs than the devices with zwitterionic ammonium molecules as interlayer. We attribute the improved performance to the better photoactive morphology induced by the hydrophobic properties of the neutral amino derivatives through research of ultraviolet photoelectron spectroscopy, atomic force microscopy, and contact angle measurements. Interestingly, the PCEs of the inverted PSCs with cathode interlayer are positively correlated with the hydrophobic properties of the interlayer materials, since devices with neutral amino molecules or molecules with a more hydrophobic alkyl pendant (piperidine) as interlayer exhibit higher PCEs. These results pave the way to the design of effective cathode interlayer materials.     

Acid-functionalized fullerenes used as interfacial layer materials in inverted polymer solar cells

Two types of carboxylic acid functionalized fullerence derivatives, 4-(2-ethylhexyloxy)-[6,6]-phenyl C₆₁-butyric acid (p-EHO-PCBA) and bis-4-(2-ethylhexyloxy)-[6,6]-phenyl C₆₁-butyric acid (bis-p-EHO-PCBA), were synthesized and investigated as an interfacial layer for inverted polymer solar cells (iPSCs). The –COOH groups on the PCBAs chemisorb to inorganic metal oxide (TiO_X), generating fullerene-based self-assembled monolayers (FSAMs). The devices with the mono- and bis-FSAMs exhibited substantially lower series resistance (R_S) values of 2.10 Ω cm² and 1.46 Ω cm², compared to that (4.15 Ω cm²) of the unmodified device. The TiO_X films modified with mono- and bis-FSAMs showed higher contact angles of 50° and 91°, respectively, than that of the pristine TiO_X film (33°). The increased contact angles were attributed to the enhanced hydrophobicity, improving the wetting properties with the organic photoactive layer. In addition, a comparison of device characteristics with electroactive FSAMs and non-electroactive benzoic acid SAMs clearly indicates that the FSAMs may suggest an additional pathway for photo-induced charge transfer and charge collection to ITO. After surface modification with FSAMs, the short-circuit current density (J_SC) and fill factor (FF) values increased substantially. The iPSCs based on poly(5,6-bis(octyloxy)-4-(thiophen-2-l)benzo[c][1,2,5]thiadiazole) (PTBT) and [6,6]phenyl-C₆₁-butyric acid methyl ester (PCBM) as an active layer showed remarkably improved power conversion efficiency up to 5.13% through incorporation of the FSAMs-based interfacial layer.     

A low-cost and low-temperature processable zinc oxide-polyethylenimine (ZnO:PEI) nano-composite as cathode buffer layer for organic and perovskite solar cells

Nanocomposite buffer layer based on metal oxide and polymer is merging as a novel buffer layer for organic solar cells, which combines the high charge carrier mobility of metal oxide and good film formation properties of polymer. In this work, a nanocomposite of zinc oxide and a commercialized available polyethylenimine (PEI) was developed and used as the cathode buffer layer (CBL) for the inverted organic solar cells and p-i-n heterojunction perovskite solar cells. The cooperation of PEI in nano ZnO offers a good film forming ability of the composite material, which is an advantage in device fabrication. In addition, power conversion efficiency (PCE) of the ZnO:PEI CBL based device was also improved when compared to that of ZnO-only and PEI-only devices. The highest PCE of P3HT:PC₆₁BM and PTB7-Th:PC₆₁BM devices reached to 3.57% and 8.16%, respectively. More importantly, there is no obvious device performance loss with the increase of the layer thickness of ZnO:PEI CBL to 60 nm in organic solar cells, which is in contrast to the PEI based devices, whose device performance decreases dramatically when the PEI layer thickness is higher than 6 nm. Such a nano composite material is also applicable in inverted heterojunction perovskite solar cells. A PCE of 11.76% was achieved for the perovskite solar cell with a thick ZnO:PEI CBL (150 nm) CBL, which is around 1.71% higher than that of the reference cell without CBL, or with ZnO CBL. In addition, stability of the organic and perovskite solar cells having ZnO:PEI CBL was also found to be improved in comparison with that of PEI based device.     

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.     

多孔ZnO纳米花电子传输层对有机太阳能电池性能的优化

通过水热法制备了多孔、单晶结构的三维ZnO纳米花材料。研究了不同生长时间下(6h、9h和12h)的ZnO材料的形貌及光电性能。结果表明,反应9h的多孔ZnO纳米花材料具有较高透光率、低缺陷密度以及高载流子迁移率等优点,是较为理想的电子传输层材料。将这些材料应用于有机太阳能电池的制备,性能测试结果表明,以生长时间为9h的多孔ZnO纳米花材料作为电子传输层的器件性能最佳,与无ZnO修饰层的参比器件相比,其短路电流密度Jsc和光电转化效率(PCE)明显提高,分别达到了5.68mA/cm2和1.24%。     

An ammonia modified PEDOT: PSS for interfacial engineering in inverted planar perovskite solar cells

Poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) is one of the most widely used hole transport layers (HTL) in inverted perovskite solar cells (PSCs) due to its simple solution-processed ability, high transparency, and conductivity. However, PEDOT:PSS-based devices suffer a lower open-circuit voltage (V_oc) than devices with the conventional structure. To address this issue, we fabricated ammonia-modified PEDOT:PSS films by simply doping PEDOT:PSS solution with different ratio of ammonia. The acidity of PEDOT:PSS can be neutralized by the doped ammonia, which inhibits the ion-exchange reaction between PSS-H and CH₃NH₃I, thus retarding the reduction of the work function for PEDOT:PSS to some extent. As a result, a superior power conversion efficiency (PCE) of 15.5% was obtained for the device based on the ammonia-doped PEDOT:PSS HTL than that of the pristine PEDOT:PSS-based device. We ascribe the PCE enhancement to the increased V_oc and fill factor (FF), which is attributed not only to the better energy-level alignment between the ammonia-modified PEDOT:PSS film and perovskite layer but also to the increased grain size and crystallinity of perovskite film.     

In situ vapor-deposited parylene substrates for ultra-thin,lightweight organic solar cells

We fabricate the thinnest (1.3 μm) and lightest (3.6 g/m²) solar cells yet demonstrated, with weight-specific power exceeding 6 W/g, in order to illustrate the lower limits of substrate thickness and materials use achievable with a new processing paradigm. Our fabrication process uniquely starts with growth of an ultra-thin flexible polymer substrate in vacuum, followed by deposition of electrodes and photoactive layers in situ. With this process sequence, the entire cell—from transparent substrate to active layers to encapsulation—can be fabricated at room temperature without solvents and without breaking vacuum, avoiding exposure to dust and other contaminants, and minimizing damage risk associated with handling of thin substrates. We use in situ vapor-phase growth of smooth, transparent, and flexible parylene-C films to produce ultra-thin, lightweight molecular organic solar cells as thin as 2.3 μm including encapsulation with a second parylene-C film. These parylene-based devices exhibit power conversion efficiencies and fabrication yields comparable to glass-based cells. Flexible solar cells on parylene membranes can be seamlessly adhered to a variety of solid surfaces to provide additive solar power.     

Ga-doped ZnO as an electron transport layer for PffBT4T-2OD: PC70BM organic solar cells

Ga-doped ZnO(GZO) is investigated as an electron transport layer in organic solar cells based on a promising donor: acceptor system of poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3‴-di(2-octyldode-cyl)-2,2′; 5′,2″; -5″,2‴-quaterthio-phen-5,5‴-diyl)] (PffBT4T-2OD):phenyl-C71-butyric acid methyl ester (PC₇₀BM). With the inverted geometry having a configuration of ITO/GZO (40 nm)/PffBT4T-2OD:PC₇₀BM (270 nm)/MoO₃ (20 nm)/Al (100 nm), maximum power conversion efficiency (PCE) of 9.74% has been achieved, while it is limited at 8.72% for devices with undoped ZnO. Our study based on the structural, morphological, compositional, and electrical characterizations indicate that suggests enhanced device performance of the GZO-based devices resulted mainly from the improved electrical properties of Ga-ZnO thin films as compared to undoped ZnO.     

Flexible organic solar cells using an oxide/metal/oxide trilayer as transparent electrode

Organic solar cells (OSCs) have attracted much attention as a clean and renewable energy convention system, owning to the low-cost and easy-processing nature of organic semiconductors. While indium tin oxide (ITO) is commonly used in OSCs as the transparent conductive electrode, the rising cost of indium, the high temperature process and the poor flexibility of ITO, make it incompatible with large-scale roll-to-roll manufacture of OSCs. In this paper, the MoO₃/thin metal/MoO₃ trilayer structure was used to replace the ITO electrode in OSCs. The optical and electrical properties of the trilayer were shown to depend on the material and thickness of the intermediate metal layer. The maximum power conversion efficiency of up to 2.5% under simulated 1 sun AM 1.5 solar illumination was achieved for OSCs based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), compared to a maximum efficiency of 3.1% for the ITO-based devices. Moreover, due to the flexible nature of the trilayer structure, the OSCs with the trilayer electrode exhibited good mechanical flexibility. The efficiency of the flexible device was only reduced by ∼6% from its original performance after 500 bending cycles with a bending radius of 1.3 cm. Therefore, the performance of the ITO-free devices on rigid/flexible substrates suggests that this oxide/metal/oxide trilayer electrode is a promising ITO replacement in OSCs.     

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.     

基于CuBB为阴极缓冲层有机太阳能电池性能的研究

通过定向合成Cu(I)配合物,首次将 其作为阴极缓冲层引入到有机太阳能电池(OSCs)中。实验分析发现,OSCs的光电能量转换效 率(PCE)与CuBB层厚度紧密相关,在标准太阳 光照条件下,结构为ITO/CuPc (20nm)/C60(40nm)/CuBB (x m m)/Al (100nm)的器件PCE随着CuBB厚度的增加 先增大后变小,当厚为8nm 时PCE达到0.94%。器件性能提高的原因主要是CuBB具有良好 的电子迁移率,但厚度过大时则由于串联电阻增加及电子不能经阴极缓冲层传输而使性能降 低。     

Properties of inverted polymer solar cells based on novel small molecular electrolytes as the cathode buffer layer

Novel small-molecule electrolytes were designed and synthesized for use in the cathode interlayer in bulk-heterojunction polymer solar cells (PSCs). The synthesized materials consist of polar quaternary ammonium bromide with the addition of multiple hydroxyl groups, which are N,N,N,N,N,N-hexakis(2-hydroxyethyl)butane-1,4-diaminium bromide (C4) and N,N,N,N,N,N-hexakis(2-hydroxyethyl)hexane-1,6-diaminium bromide (C6). The materials generate a favorable interface dipole through the quaternary ammonium bromide. In addition, the multiple polar hydroxyl groups increased the interface dipole magnitude. The power conversion efficiency of the devices with the interlayer was up to 9.20% with a J_sc of 17.22 mA/cm², a V_oc of 0.75 V, and an FF of 71.3%. The PCE of devices with an interlayer show better long-term stability than a device without an interlayer. Our strategy shows that it is possible to enhance the efficiency of PSCs by simple approaches without complicated syntheses.     

阴极材料对有机太阳电池性能的影响

分别用Al、LiF/Al和Ca/Al制备了三种不同阴极材料的体相异质结有机太阳电池。对其光电特性进行了表征,分析了不同阴极材料对电池性能的影响机制。结果表明:所制备的有机太阳电池在10–1W/cm2辐照度的光照下,开路电压分别为0.419 3,0.565 0和0.591 1 V,能量转换效率分别为1.17%、2.06%和1.91%;采用LiF/Al层状阴极制备的有机太阳电池具有更高的能量转换效率;功函数愈低的材料做阴极,有机太阳电池的能量转换效率也愈高。     

Amphiphilic fullerene derivative as effective interfacial layer for inverted polymer solar cells

Amphiphilic fullerene derivative with poly(ethylene glycol) chain (C60-PEG) was applied as effective interfacial layer to improve the performance of inverted polymer solar cells. C60-PEG could not only be used as cathode buffer layer alone by replacing ZnO, but also be used as a self-assembled monolayer to modify ZnO. C60-PEG can tune energy level alignment and improve the interfacial compatibility between active layer and ITO or ZnO. Moreover, due to the strong interaction between ZnO nanoparticles and PEG chain, C60-PEG can passivate the surface defects and traps of ZnO, and facilitate the charge selective and dissociation. Consequently, inverted polymer solar cells based on thieno[3,4-b]thiophene/benzodithiophene (PTB7):[6,6]- phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) present a PCE of 6.6% by incorporating C60-PEG into as cathode buffer layer. Moreover, an improved PCE of 7.4% with good long-term stability in air were further achieved by using C60-PEG/ZnO interlayer. In this work, C60-PEG could be prepared by solution process at room temperature without additional annealing, which shows the potential in large-scale printed polymer solar cells.     

PI衬底柔性透明硅薄膜太阳能电池的制备及性能

利用硬质玻璃为载板,采用传统硅薄膜太阳能电池生产设备,在聚酰亚胺(PI)塑料薄膜衬底上沉积了B掺杂的ZnO(BZO)薄膜,并以此作为前电极制备了单节电池结构及多节串联一体结构的非晶硅(a-Si)太阳能电池;研究了PI衬底上BZO薄膜的光学及电学性能。结果表明,PI衬底上沉积BZO薄膜后在300~1 200 nm波长范围的透光率为76.63%,方块电阻19.7?/□。所制备的单节和多节串联一体结构的a-Si薄膜太阳能电池的转化效率分别达到6.45%和5.1%,封装后电池组件具有一定的透光性,透光率约达到30.2%。     

Solution-processed annealing-free ZnO nanoparticles for stable inverted organic solar cells

We report the development and application of high-quality zinc oxide nanoparticles (ZnO NPs) processed in air for stable inverted bulk heterojunction solar cells as an electron extraction layer (EEL). The ZnO NPs (average size ∼11 nm) were dispersed in chloroform and stabilized by propylamine (PA). We demonstrated that the ZnO NP dispersion with 4 vol.% of PA as stabilizer can be used in air directly and remains clear up to one month after preparation. Our inverted solar cells consisted of a blade-coated poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole (PCDTBT) and [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) (1: 4 by weight) active layer sandwiched between a ZnO electron extraction layer and a MoO₃/Ag anode. All solar cells with ZnO films fabricated in air using PA-stabilized ZnO dispersions prepared within a time window of one month exhibited power conversion efficiencies (PCE) above 4%. In contrast, if the ZnO film was prepared in air using regular un-stabilized ZnO NP dispersion, the PCE would drop to 0.2% due to poor film quality. More interestingly, X-ray photoelectron spectroscopy and nuclear magnetic resonance measurements indicated that the PA ligands were not covalently bonded to ZnO NPs and did not exist in the deposited ZnO films. The spin-cast ZnO thin films (without any thermal treatment) are insoluble in organic solvents and can be directly used as an EEL in solar cells. This feature is beneficial for fabricating organic solar cells on flexible polymer substrates. More importantly, our non-encapsulated inverted solar cells are highly stable with their PCEs remaining unchanged after being stored in air for 50 days.