Inverted Bulk Heterojunction Research Articles

Critical Impact of Hole Transporting Layers and Back Electrode on the Stability of Flexible Organic Photovoltaic Module

Properties of hole transporting layers (HTLs) and back electrode are very critical to the stability of inverted bulk heterojunction organic photovoltaic (OPV) modules. Here, various deposition methods for back electrodes and materials of HTLs are examined by applying to inverted organic solar cells with a structure of indium tin oxide/ZnO/photoactive layer/poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/Ag. The experiment is performed on encapsulated modules with flexible barrier films under accelerated conditions. The OPV modules with screen‐printed Ag electrodes are shown to be electrically unstable with a reduction of the current density under damp heat condition at 85 °C/85% RH. Optical images for the active layer/PEDOT:PSS interface reveal that a reaction between the solvent from the Ag electrode and the underlying layers is the major cause for the degradation. In comparison with materials of the HTLs, the PEDOT:PSS layer shows low stability compared to the MoO3 layer under the accelerated conditions. Unusual chemical changes in the PEDOT:PSS film are observed through X‐ray photoelectron spectroscopy and this is further addressed by correlating the stability of the OPV devices.

Thin indium tin oxide nanoparticle films as hole transport layer in inverted organic solar cells

Indium tin oxide nanoparticles (ITO-NPs) were used as hole transport layer (HTL) in doctor-bladed inverted bulk-heterojunction organic solar cells. After the non-aqueous synthesis of ITO nanoparticles, a customized stabilization strategy was established. As result, ethylenediamine, a stabilizer with a low boiling point was used for an easy removal of the remaining organic compounds in the ITO-NP layer by a short annealing step at 120°C. Furthermore, different post-processing treatments (thermal annealing and plasma treatment) on the ITO-NP layer were investigated to improve the electrical parameters of the solar cell. Using poly(3-hexylthiophen) as photoactive polymer, power conversion efficiencies of 3% were achieved with the incorporation of ITO-NPs as HTL, being almost as good as the efficiencies of the reference system where poly(3,4-ethylenedioxythiophene) was used as HTL.

Luminescent cathode buffer layer for enhanced power conversion efficiency and stability of bulk-heterojunction solar cells

We report high photovoltaic efficiency of over 9% in solution-processed, small-molecule (SPSM) 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c]1,2,5]thiadiazole) p-DTS(FBTTh2)2:[6-6]-phenyl C70 butyric acid methyl ester (PC70BM) blend based inverted BHJ solar cell by incorporating luminescent zinc oxide doped with sodium (ZnO:Na) quantum dots (QD) (l-ZnO) as a cathode buffer layer (CBL) in inverted bulk-heterojunction (BHJ) solar cells for the first time. The l-ZnO absorbs ultraviolet (UV) light and down-converts it to visible light. The l-ZnO layer's emission overlaps significantly with the absorption of p-DTS(FBTTh2)2, leading to an enhanced absorption by p-DTS(FBTTh2)2. This resulted in a significant enhancement of photo-current from 15.4 to 17.27 mA/cm2 and efficiency from 8% to 9.2% for ZnO and l-ZnO based devices, respectively. This is among one of the highest efficiency values reported so far in the case of SPSM based single junction BHJ solar cells. The luminescent ZnO layer also protects the active layer from UV-induced degradation as solar cells show high stability under constant solar light illumination retaining more than 90% (∼28 h) of its initial efficiency, whereas BHJ solar cells without the luminescent ZnO layer degraded to ∼50% of its initial value under same conditions. Since ZnO is an essential part of inverted organic solar cells, the luminescent l-ZnO CBL has great potential in inverted organic solar cells.

Heterojunction Solar Cells: Remarkably High Conversion Efficiency of Inverted Bulk Heterojunction Solar Cells: From Ultrafast Laser Spectroscopy and Electron Microscopy to Device Fabrication and Optimization (Adv. Energy Mater. 11/2016)

Heterojunction Solar Cells: Remarkably High Conversion Efficiency of Inverted Bulk Heterojunction Solar Cells: From Ultrafast Laser Spectroscopy and Electron Microscopy to Device Fabrication and Optimization (Adv. Energy Mater. 11/2016)

Multi‐Length Scaled Silver Nanowire Grid for Application in Efficient Organic Solar Cells

Transparent conducting electrodes (TCEs) with multi‐length scaled structure are promising candidates as a potential replacement for indium tin oxide (ITO). In this work, multi‐length scaled silver nanowire (AgNW) grids are demonstrated as TCEs for organic solar cells. The multi‐length scale silver nanowire grids are prepared by top‐down patterning using a neutral vapor etching process. Patterning AgNW film into multi‐length scale grid structures could improve the optical transmittance and enhance the use of incident photons. Based on these multi‐length scale AgNW grids, inverted bulk heterojunction polymer solar cells with power conversion efficiency up to 9.02% are fabricated, which are higher than that based on the original AgNW films and comparable to that based on ITO.

Enhancement of the photoelectric performance in inverted bulk heterojunction solid solar cell with inorganic nanocrystals

Nanocrystal/polymer solid solar cells have the advantages of low-cost, simple process, and flexible manufacture. In this work, ternary solid solar cells based on FeS2 and PbS nanocrystals exhibited photovoltaic conversion efficiency of 3.0% and 3.1%, respectively. As a kind of semiconductor with optical absorption in the visible and near-infrared regions, FeS2 nanocrystals matched well with the solar radiation spectrum. Furthermore, PbS Nanocrystals could increase the number of electrons, due to its multiple exciton effect. Additionally, the FeS2 nanocrystals solar cells showed high stability, with 83.3% of its initial efficiency remained after 15weeks of exposure in air, and kept good stable performance at 20–80°C. The photovoltaic conversion efficiency fluctuation magnitudes were also found to be smaller than quantum-dot sensitized solar cell under the same conditions.

Remarkably High Conversion Efficiency of Inverted Bulk Heterojunction Solar Cells: From Ultrafast Laser Spectroscopy and Electron Microscopy to Device Fabrication and Optimization

In organic donor–acceptor systems, ultrafast interfacial charge transfer (CT), charge separation (CS), and charge recombination (CR) are key determinants of the overall performance of photovoltaic devices. However, a profound understanding of these photophysical processes at device interfaces remains superficial, creating a major bottleneck that circumvents advancements and the optimization of these solar cells. Here, results from time‐resolved laser spectroscopy and high‐resolution electron microscopy are examined to provide the fundamental information necessary to fabricate and optimize organic solar cell devices. In real time, CT and CS are monitored at the interface between three fullerene acceptors (FAs) (PC71BM, PC61BM, and IC60BA) and the PTB7‐Th donor polymer. Femtosecond transient absorption (fs‐TA) data demonstrates that photoinduced electron transfer from the PTB7‐Th polymer to each FA occurs on the sub‐picosecond time scale, leading to the formation of long‐lived radical ions. It is also found that the power conversion efficiency improves from 2% in IC60BA‐based solar cells to >9% in PC71BM‐based devices, in support of our time‐resolved results. The insights reported in this manuscript provide a clear understanding of the key variables involved at the device interface, paving the way for the exploitation of efficient CS and subsequently improving the photoconversion efficiency.

Interfacial Energy Alignment at the ITO/Ultra-Thin Electron Selective Dielectric Layer Interface and Its Effect on the Efficiency of Bulk-Heterojunction Organic Solar Cells.

We have investigated the photovoltaic properties of an inverted bulk heterojunction (BHJ) cell in a device with an indium-tin-oxide (ITO)/electron selective layer (ESL)/P3HT:PCBM active layer/MoOx/Ag multilayered structure. The insertion of only single layer of poly(diallyl-dimethyl-ammonium chloride) (PDDA) cationic polymer film (or poly(ethyleneimine) (PEI) polymeric interfacial dipole layer) and titanium oxide nanosheet (TN) films as an ESL effectively improved cell performance. Abnormal S-shaped curves were observed in the inverted BHJ cells owing to the contact resistance across the ITO/active layer interface and the ITO/PDDA/TN/active layer interface. The series resistance across the ITO/ESL interface in the inverted BHJ cell was successfully reduced using an interfacial layer with a positively charged surface potential with respect to ITO base electrode. The positive dipole in PEI and the electronic charge phenomena at the electrophoretic deposited TN (ED-TN) films on ITO contributed to the reduction of the contact resistance at the electrode interface. The surface potential measurement revealed that the energy alignment by the transfer of electronic charges from the ED-TN to the base electrodes. The insertion of the ESL with a large positive surface potential reduced the potential barrier for the electron injection at ITO/TN interface and it improved the photovoltaic properties of the inverted cell with an ITO/TN/active layer/MoOx/Ag structure.

Syntheses, Charge Separation, and Inverted Bulk Heterojunction Solar Cell Application of Phenothiazine-Fullerene Dyads.

A series of phenothiazine-fulleropyrrolidine (PTZ-C60) dyads having fullerene either at the C-3 aromatic ring position or at the N-position of phenothiazine macrocycle were newly synthesized and characterized. Photoinduced electron transfer leading to PTZ(•+)-C60(•-) charge-separated species was established from studies involving femtosecond transient absorption spectroscopy. Because of the close proximity of the donor and acceptor entities, the C-3 ring substituted PTZ-C60 dyads revealed faster charge separation and charge recombination processes than that observed in the dyad functionalized through the N-position. Next, inverted organic bulk heterojunction (BHJ) solar cells were constructed using the dyads in place of traditionally used [6,6]-phenyl-C61- butyric acid methyl ester (PCBM) and an additional electron donor material poly(3-hexylthiophene) (P3HT). The performance of the C-3 ring substituted PTZ-C60 dyad having a polyethylene glycol substituent produced a power conversion efficiency of 3.5% under inverted bulk heterojunction (BHJ) configuration. This was attributed to optimal BHJ morphology between the polymer and the dyad, which was further promoted by the efficient intramolecular charge separation and relatively slow charge recombination promoted by the dyad within the BHJ structure. The present finding demonstrate PTZ-C60 dyads as being good prospective materials for building organic photovoltaic devices.

(Z)-(Thienylmethylene)oxindole-Based Polymers for High-Performance Solar Cells

An oxindole-based monomer, (Z)-3-(thiophen-2-yl-methylene)indolin-2-one (TEI) has been synthesized. The hemi-isoindigo, TEI was polymerized separately with bis(trimethylstannyl) functionalized thiophene, bis(alkoxy)benzodithiophene, and bis(alkylthienyl)benzodithiophene to form polymers PTEI-T, PTEI-BDTO, and PTEI-BDTT, respectively. These new conjugated polymers showed low-lying HOMO energy levels (−5.26 to −5.42 eV), suitable LUMO energy levels (−3.63 to −3.66 eV), strong absorption in visible region and extended to near-IR. The inverted bulk heterojunction (BHJ) polymer solar cells based on the new polymers were fabricated and tested. The solar cell devices based on the blend of PTEI-T:[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) achieved a short-circuit current density (Jsc) value of 13.4 mA cm–2, a fill factor (FF) of 0.65, an open circuit voltage (Voc) of 0.85 V, and a power conversion efficiency (PCE) of 7.32%. The high performance solar cell devices were realized with the polymer which had ...

Bulk heterojunction perovskite–PCBM solar cells with high fill factor

An inverted bulk heterojunction perovskite–PCBM solar cell with a high fill factor of 0.82 and a power conversion efficiency of up to 16.0% was fabricated by a low-temperature two-step solution process. The cells exhibit no significant photocurrent hysteresis and their high short-circuit current density, fill factor and efficiency are attributed to the advantageous properties of the active layer, such as its high conductivity and the improved mobility and diffusion length of charge carriers. In particular, PCBM plays a critical role in improving the quality of the light-absorbing layer by filling pinholes and vacancies between perovskite grains, resulting in a film with large grains and fewer grain boundaries. Bulk heterojunction perovskite solar cells with a high fill factor are reported.

Polypyridyl complexes as electron transporting materials for inverted bulk heterojunction solar cells: the metal center effect

The fundamental science behind the design of organic photovoltaic (OPV) cells lies in the formation of energy level gradients for efficient charge separation and collection.

Squaraine based solution processed inverted bulk heterojunction solar cells processed in air.

Inverted bulk heterojunction solar cells based on low temperature solution processed squaraine (SQ) and [6,6]-phenyl C71 butyric acid methyl-ester (PC71BM) with varying blend ratios were made in air. An optimized bulk heterojunction device of SQ and PC71BM (with a blend ratio of 1 : 6) showed a power conversion efficiency (PCE) of 2.45% with an incident photon to current conversion efficiency of 65% at 680 nm and a spectral window extending to the NIR region. The devices also showed an enhanced PCE value of 4.12% upon continuous illumination from an AM1.5G light source of intensity 1 Sun. The intensity dependent photocurrent studies showed a monomolecular recombination mechanism in the photovoltaic device performance. The device stored in air showed reasonable stability for a period of one month.

Photoactive area modification in bulk heterojunction organic solar cells using optimization of electrochemically synthesized ZnO nanorods

In this work, ZnO nanorod arrays grown by an electrochemical deposition method are investigated. The crucial parameters of length, diameter, and density of the nanorods are optimized over the synthesize process and nanorods growth time. Crystalline structure, morphologies, and optical properties of ZnO nanorod arrays are studied by different techniques such as x-ray diffraction, scanning electron microscope, atomic force microscope, and UV–visible transmission spectra. The ZnO nanorod arrays are employed in an inverted bulk heterojunction organic solar cell of Poly (3-hexylthiophene):[6-6] Phenyl-(6) butyric acid methyl ester to introduce more surface contact between the electron transporter layer and the active layer. Our results show that the deposition time is a very important factor to achieve the aligned and uniform ZnO nanorods with suitable surface density which is required for effective infiltration of active area into the ZnO nanorod spacing and make a maximum interfacial surface contact for electron collection, as overgrowing causes nanorods to be too dense and thick and results in high resistance and lower visible light transmittance. By optimizing the thickness of the active layer on top of ZnO nanorods, an improved efficiency of 3.17% with a high FF beyond 60% was achieved.

The Impact of Grain Alignment of the Electron Transporting Layer on the Performance of Inverted Bulk Heterojunction Solar Cells.

This report presents a new strategy for improving solar cell power conversion efficiencies (PCEs) through grain alignment and morphology control of the ZnO electron transport layer (ETL) prepared by radio frequency (RF) magnetron sputtering. The systematic control over the ETL's grain alignment and thickness is shown, by varying the deposition pressure and operating substrate temperature during the deposition. Notably, a high PCE of 6.9%, short circuit current density (J(sc)) of 12.8 mA cm(-2), open circuit voltage (V(oc)) of 910 mV, and fill factor of 59% are demonstrated using the poly(benzo[1,2-b:4,5-b']dithiophene-thieno[3,4-c]pyrrole-4,6-dione):[6,6]-phenyl-C(71) -butyric acid methyl ester polymer blend with ETLs prepared at room temperature exhibiting oriented and aligned rod-like ZnO grains. Increasing the deposition temperature during the ZnO sputtering induces morphological cleavage of the rod-like ZnO grains and therefore reduced conductivity from 7.2 × 10(-13) to ≈1.7 × 10(-14) S m(-1) and PCE from 6.9% to 4.28%. An investigation of the charge carrier dynamics by femtosecond (fs) transient absorption spectroscopy with broadband capability reveals clear evidence of faster carrier recombination for a ZnO layer deposited at higher temperature, which is consistent with the conductivity and device performance.

Photoelectric Memory Effect in an Organic Bulk Heterojunction Device

We report on a memory effect observed in an inverted bulk heterojunction organic photovoltaic device, where the electron-collecting electrode is ZnO nanowires grown on an indium tin oxide (ITO) substrate. The device presented a unique response to light by switching from a rectifying behavior in the dark to a resistive response under illumination. After cessation of light, the device slowly stabilized back to its rectifying nature after ∼270 min, introducing a volatile photoelectric memory effect. The reversibility of the response is verified through multiple cycles of light exposure and placing the device in the dark. The device is also illuminated with different light intensities to study the photovoltaic response through I–V characterization. It is found that the time constant associated with the transition between the rectifying and resistive characteristics is independent from the light intensity. Further study revealed that there is a hysteresis loop in the I–V curve in the dark, but the loop vanished in the resistive mode under illumination. A mechanism based on oxygen absorption–desorption has been suggested to explain the observed effect. Such a memory effect can be used in various optoelectronic devices to save the optical information for an extended time.

Nanoimprinted ZnO and ZnO Quantum Dots Embedded SiO2 Layers for Inverted Bulk Heterojunction Solar Cells

Nanoimprinted ZnO and ZnO Quantum Dots Embedded SiO₂ Layers for Inverted Bulk Heterojunction Solar Cells

Influence of annealing treatments on solution-processed ZnO film deposited on ITO substrate as electron transport layer for inverted polymer solar cells

In this work we studied the influence of the annealing treatments of a sol–gel derived ZnO electron transport layer deposited on ITO substrate, on the performances of inverted bulk heterojunction polymer solar cells using a blend of poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b;4,5-b′]dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiopene)-2,6-diyl] and [6,6]-phenyl C71 butyric acid methyl ester. Since the annealing treatments needed to complete the formation of the solution-processed ZnO film can modify the underlying ITO electrode, we analyzed the performance of the fabricated cells in terms of the properties of ITO and ZnO films. We found a linear relationship between the sheet resistance of the ITO layer and the series resistance of the corresponding device, which strongly influences the fill factor. The best power conversion efficiency (7%) under simulated AM 1.5G illumination of 100mW/cm2 was achieved for the polymer solar cell fabricated using a ZnO film annealed at 150°C for only 5min. Higher annealing temperatures and times increase the sheet resistance of the ITO worsening the device performances.

(Invited) Development of Printed Flexible Organic Solar Panels, Field Effect Transistors, and Logic Circuits on PET Substrates

In this talk, I will introduce the research activities on the development of printed flexible organic solar panels, field effect transistors, and pMOS based logic circuits under NRC’s Printable Electronics Flagship Program. I will report the development of polycarbazole-based, printed organic solar panels and the application of a printable, air-stable, and annealing-free zinc oxide nanoparticle (ZnO NP) solution in the fabrication of inverted bulk heterojunction solar cells. The as-coated ZnO thin films are insoluble in organic solvents and can be directly used as an electron extraction layer in solar cells. The process is R2R compatible. Our non-encapsulated inverted solar cells are highly stable with their PCEs remaining unchanged after being stored in air for more than 50 days. The development of inkjet-printed pMOS inverters and logic gates on flexible substrates will be also presented. The fabrication of inkjet-printed OTFTs has achieved a yield of 97%. Different types of logic gates, inverters, and ring oscillators have been successfully fabricated by using these basic pMOS devices.

Novel medium band gap conjugated polymers based on naphtho[1,2-c:5,6-c]bis[1,2,3]triazole for polymer solar cells

Two new donor–acceptor type of conjugated polymers derived from naphtho[1,2-c:5,6-c]bis(2-octyl-[1,2,3]triazole) and dithieno[3,2-b:2′,3′-d]silole were prepared by Stille polymerization. The thermal, electrochemical and photophysical properties of these copolymers were studied. Both polymers have excellent thermal stability with decomposition temperature up to 430 °C. The copolymers show medium band gap of ∼1.8 eV. Inverted bulk heterojunction solar cells in which these copolymers are used as electron donors in combination with PC61BM as the electron acceptor were fabricated. Under AM 1.5 G illumination at 100 mW/cm2, device based on P1:PC61BM as the photoactive layer displays a moderate power conversion efficiency of 2.58% with an open-circuit voltage of 0.83 V, a short circuit current of 5.53 mA/cm2, and a fill factor of 56.13%. Our work indicates that naphtho[1,2-c:5,6-c]bis(2-octyl-[1,2,3]triazole) can be a promising electron-deficient building block for designing donor–acceptor type of polymeric materials.