Promising Power Conversion Efficiency Research Articles

Asymmetric medium bandgap copolymers and narrow bandgap small-molecule acceptor with over 7% efficiency

To further investigate non-fullerene polymer solar cells based-thieno[2,3-f]benzofuran (TBF) polymers, we designed and synthesized two medium bandgap TBF-based polymers, namely TBF-BDD and TBF-BT, containing alkoxyphenyl substituted TBF electron-donor unit, 1,3-bis(thiophen-2-yl)-5,7-bis(2-ethylhexyl)benzo-[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD) and 4,7-di(thiophen-2-yl)-5,6-dioctyloxybenzo[c][1,2,5]thiadiazole (BT) electron-acceptor segment, respectively. When blended with ITIC (a n-type small molecule acceptor), two polymer:ITIC blends possess better complementary absorption than the absorption with PC71BM. The properties, including charge mobilities, and morphologies have been intensively investigated. The polymer solar cells based on TBF-BDD:ITIC and TBF-BT:ITIC (1:1,w/w) exhibited promising power conversion efficiencies (PCEs) of 7.13% and 7.03%, respectively, under the illumination of AM 1.5 G, 100 mW/cm2, which are higher in comparison with fullerene-based devices. Up to now, these PCEs are the highest among TBF-based polymer photovoltaic devices.

Synthesis of Tantalum-Doped Tin Oxide Thin Films by Magnetron Sputtering for Photovoltaic Applications

Tantalum-doped tin oxide transparent conductive thin films were deposited on glass substrates by radio frequency and direct current reactive magnetron co-sputtering methods in an argon and oxygen environment. Optimization of the thin films for photovoltaic applications was performed using a dimensionless figure of merit by combining electrical and transparency properties. The optimized thin film showed a weight-averaged transmittance of 83%, a band gap value of 3.2 eV, resistivity of 5.2 × 10−3 Ω cm, and bulk carrier concentration of 1.2 × 1020 cm−3. The lowest resistivity among all films was 2.1 × 10−3 Ω cm, corresponding to a weight-averaged transmittance of 62%. The optimized deposition condition was carried out by co-sputtering on heated substrates at 270°C. Thin films deposited under this optimized condition were applied for our perovskite solar cells, and demonstrated promising power conversion efficiency.

Two new medium bandgap asymmetric copolymers based on thieno[2,3-f]benzofuran for efficient organic solar cells

Two new medium bandgap, alkoxyphenyl substituted thieno[2,3-f]benzofuran (TBFPO) and alkoxyl substituted thieno[2,3-f]benzofuran (TBFP)-based polymers were designed, synthesized and applied in polymer solar cells (PSCs), namely, TBFPO-BDD and TBFO-BDD, respectively. Their thermal, optical, electronic properties and photovoltaic performances were systematically investigated. The PSCs prepared using o-dichlorobenzene as solvent, with the single layer device structure of ITO/PEDOT:PSS/TBFPO-BDD:PC71BM (1:1; w/w)/perylene diimide (PDI) derivative (PDINO)/Al, exhibited a promising power conversion efficiency (PCE) of 6.58% under the illumination of AM 1.5G, 100 mW/cm2, without any post-treatment. While the devices based on TBFO-BDD showed an enhanced PCE of 3.07% with 1 vol% 1,8-diodooctane (DIO) as additive. This study demonstrates that TBFPO-BDD is a promising polymer for organic electronics.

Thienopyrroledione and benzodithiophene/thiophene-based random terpolymer for polymer solar cells with improved fill factor

A new random conjugated terpolymer, coded as LGC-D054, was designed and synthesized. It is composed of N-alkylthieno[3,4-c]pyrrole-4,6-dione (TPD) as an electron-accepting unit, and benzodithiophene (BDT) and terthiophene as electron-donating units into the polymer backbone. The optical, electrochemical, and photovoltaic properties of the terpolymer were investigated. The polymer displayed an optical bandgap of 1.81 eV, with a highest occupied molecular orbital (HOMO) level of −5.25 eV. The bulk-heterojunction (BHJ) polymer solar cell (PSC) with the blend film based on LGC-D054:PC71BM exhibited a power conversion efficiency (PCE) of 4.46% with the short circuit current density (Jsc) of 8.71 mA/cm2 and a relatively high fill factor (FF) of 61.21%. After introducing 1,8-diiodooctane (DIO), BHJ film showed better phase separation and larger domain size. Furthermore, this optimized morphology promotes improved hole and electron mobilities, leading to a promising PCE of 5.08% with improved Jsc of 9.08 mA/cm2 and high FF of 66.02%.

Efficient planar n-i-p type heterojunction flexible perovskite solar cells with sputtered TiO2 electron transporting layers.

The development of hybrid organo-lead trihalide perovskite solar cells (PSCs) comprising an electron transporting layer (ETL), a perovskite light absorber and a hole transporting layer (HTL) has received significant attention for their potential in efficient PSCs. However, the preparation of a compact and uniform ETL and the formation of a uniform light absorber layer suffer from a high temperature processing treatment and the formation of unwanted perovskite islands, respectively. A low temperature/room temperature processed ETL is one of the best options for the fabrication of flexible PSCs. In the present work, we report the implementation of a room temperature processed compact TiO2 ETL and the synthesis of extremely uniform flexible planar PSCs based on methylammonium lead mixed halides MAPb(I1-xBrx)3 (x = 0.1) via RF-magnetron sputtering and a toluene dripping treatment, respectively. The compact TiO2 ETLs with different thicknesses (30 to 100 nm) were directly deposited on a flexible PET coated ITO substrate by varying the RF-sputtering time and used for the fabrication of flexible PSCs. The photovoltaic properties revealed that flexible PSC performance is strongly dependent on the TiO2 ETL thickness. The open circuit voltage (VOC) and fill factor (FF) are directly proportional to the TiO2 ETL thickness while the 50 nm thick TiO2 ETL shows the highest current density (JSC) of 20.77 mA cm-2. Our controlled results reveal that the room temperature RF-magnetron sputtered 50 nm-thick TiO2 ETL photoelectrode exhibits a power conversion efficiency (PCE) in excess of 15%. The use of room temperature synthesis of the compact TiO2 ETL by RF magnetron sputtering results in an enhancement of the device performance for cells prepared on flexible substrates. The champion flexible planar PSC based on this architecture exhibited a promising power conversion efficiency as high as 15.88%, featuring a high FF of 0.69 and VOC of 1.108 V with a negligible hysteresis under AM 1.5 G illumination. Furthermore, the mechanical bending stability revealed that the fabricated devices show stable PCE up to 200 bending cycles. The interface properties revealed that the 50 nm thick TiO2 ETL provides superior charge injection characteristics and low internal resistance. The present work provides a simplistic and reliable approach for the fabrication of highly efficient stable flexible perovskite solar cells.

High-efficiency perovskite solar cells prepared by using a sandwich structure MAI-PbI2-MAI precursor film.

Two-step deposition has been widely used in the perovskite layer preparation for perovskite solar cells due to its attractive morphology controllability. However, the limited diffusivity of CH3NH3I (MAI) might cause some PbI2 to remain in the perovskite film. The residual PbI2 in the perovskite film would lead to inferior performance of devices, such as, low power conversion efficiency (PCE), poor reproducibility and weak air stability. In this work, we developed a sandwich structure MAI-PbI2-MAI precursor film to prepare a PbI2-free CH3NH3PbI3 perovskite film. In comparison to the two-step approach, the MAI-PbI2-MAI precursor film with a typical sandwich structure formed a uniform and pinhole-free perovskite film without any PbI2 residue, which could significantly improve the performance of the devices. Moreover, the bottom MAI layer of the MAI-PbI2-MAI precursor film could improve the interfacial contact of the porous TiO2 layer, leading to the promotion of the charge transfer and reduction of the recombination rate. Therefore, the devices fabricated from the sandwich structure MAI-PbI2-MAI precursor films showed dramatic improvements of open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF) and PCE. As a result, a promising PCE of 17.8% with good long-term air stability was achieved for the MAI-PbI2-MAI precursor film based PSC, which is better than that prepared by a two-step approach.

Fabrication of selenization-free superstrate-type CuInS2 solar cells based on all-spin-coated layers

Today manufacturing of high efficiency chalcogenide thin film solar cells is based on high cost vacuum-based deposition processes at high temperature (>500 °C) and in chalcogen -containing atmosphere. In this paper, we introduce a simple vacuum-free and selenization-free, solution processing for fabricating a superstrate-type CuInS2 (CIS) solar cell. The absorber, buffer and blocking layers were all deposited by spin coating of molecular precursor inks. We demonstrate the deposition of In2S3 buffer layer by sol-gel spin casting for the first time. The rapid sintering process of CIS layer was carried out at 250 °C that is considered a very low temperature in CIGS thin-film technologies. A novel molecular-ink route to deposit In2S3 type buffer layer is presented. For the back contact we employed carbon, deposited by simple knife coating method. Different parameters including type of buffer, thickness of absorber layer, CIS and In2S3 annealing temperature and morphology were optimized. Our air stable simple device structure consisting of <Glass/FTO/TiO2/In2S3/CIS/Carbon > showed promising power conversion efficiency (PCE) of 2.67%.

High‐Performance Non‐Fullerene Organic Solar Cells Based on a Pair of Medium Band Gap Polymer Donor and Perylene Bisimide Derivative Acceptor

Herein, high performance solution‐processed non‐fullerene organic solar cells employing a medium band gap conjugated polymer (PBDTTS‐TTffBT) and a bay‐linked perylene bisimide derivative (di‐PBI) as an electron donor and acceptor, respectively, have been demonstrated. Owing to the complementary absorption range and well‐matched energy levels, PBDTTS‐TTffBT and di‐PBI have great potential for constructing high‐performance non‐fullerene organic solar cells. The facile control of blend film morphology by careful choice of a processing solvent achieves a promising power conversion efficiency of the device up to 6.51% with high open‐circuit voltage (0.92 V), short circuit current density (10.89 mA cm−2), and a fill factor (0.65). Furthermore, the blend film exhibits high thermal stability, retaining &gt;80% of its initial efficiency at 100 °C up to 120 min. This work demonstrates that the medium band gap is a promising candidate as a donor material for constructing high‐performance non‐fullerene organic solar cells. image

Fluoro-substituted low band gap polymers based on isoindigo for air-stable polymer solar cells with high open circuit voltages

Two low band gap polymers (PIDT2FBT and PIDT4FBZ), which are composed of fluorinated elctron donor units (dithienyldifluorobenzothiadiazole (DT2FBT) and dithienyltetrafluorobenzene (DT4FBZ)) and a isoindigo electron accepting unit were implemented in a photoactive layer to invesitigate thier photovoltaic performance. Both low band gap polymers possess exhibit low-lying energy levels and extended light absorption range, which are essential to generate high open circuit voltage and short circuit current density of polymer solar cell. In addition, the blend with PC71BM formed affirmative bulk heterojunction mrophology, resulting in promising power conversion efficiency up to 4.1% with high openc circuit voltage exceeding 1.1 V. Furthremore, the low-lying energy levels of the polymers leaded to good ambient stability of the devices up to 250 h, enabling PIDT2FBT and PIDT4FBZ to be promising candidates for photoactive layer of ambient stable polymer solar cells.

Head-to-Head Linkage Containing Bithiophene-Based Polymeric Semiconductors for Highly Efficient Polymer Solar Cells.

Narrow bandgap (1.37-1.46 eV) polymers incorporating a head-to-head linkage containing 3-alkoxy-3'-alkyl-2,2'-bithiophene are synthesized. The head-to-head linkage enables polymers with sufficient solubility and the noncovalent sulfur-oxygen interaction affords polymers with high degree of backbone planarity and film ordering. When integrated into polymer solar cells, the polymers show a promising power conversion efficiency approaching 10%.

ZnO Nanostructures for Dye-Sensitized Solar Cells Using the TEMPO+ /TEMPO Redox Mediator and Ruthenium(II) Photosensitizers with 1,2,3-Triazole-Derived Ligands.

A series of thiocyanate-free bis(tridentate) ruthenium(II) complexes incorporating 1,2,3-triazole-derived NNN-, NCN-, and CNC-coordinating ligands has been employed for sensitizing ZnO photoanodes for dye-sensitized solar cells (DSSCs). Additionally, the first use of the TEMPO+ /TEMPO (2,2,6,6-tetramethyl-piperidine-1-oxyl) redox mediator as a surrogate for the I3 - /I- redox couple in ZnO nanostructured DSSCs is presented. Compared with I3 - /I- -based electrolytes, shorter charge lifetimes and diffusion lengths were determined for the TEMPO+ /TEMPO-based electrolyte. Nonetheless, similar power conversion efficiencies (PCEs) were achieved with both electrolytes for the RuNCN and RuCNC complexes, whereas higher PCEs are enabled by the iodine-free electrolyte in case of RuNNN. The combination of the molecular sensitizers and the TEMPO-based electrolyte exhibits relatively high external quantum efficiency (EQE) and promising PCEs, ranging from 4.48 to 1.47 %, which are-in part-comparable to that of ZnO-DSSCs with the benchmark N749 black dye. The TEMPO-based electrolyte also exhibits less absorption compared with its I3 - /I- counterpart, a favorable feature for enhancing the light harvesting ability of the photoanode. Furthermore, the results show the effect of the dye-sensitization procedure on the PCE values: The use of ethanol as the solvent compared with methanol increases the DSSC's efficiency, which is attributed to improved chemisorption of the sensitizer onto the ZnO surface.

High performance of Mn-doped CdSe quantum dot sensitized solar cells based on the vertical ZnO nanorod arrays

Abstract Doping transition metal ions Mn2+ to semiconductor quantum dots (QDs) are extremely interesting for the development of photovoltaic devices. Quantum dot sensitized solar cells (QDSCs) are able to show promising power conversion efficiencies (PCE) by employing Mn2+ doped QDs. Herein we achieve effective CdS/Mn CdSe/ZnS QDs co-sensitized vertical ZnO nanorod arrays film that provides an appreciable enhancement in photovoltaic performance. The measured PCE of the solar cells with Mn2+ doped CdSe QDs is 4.14%, which is higher than the efficiency of 2.91% for the solar cells without Mn2+ or a ∼42% increase. The improvement in PCE is ascribed to a higher open-circuit voltage (Voc = 0.74 V) and a superior short-circuit current density (Jsc = 12.6 mA cm−2) with the introduction of Mn2+ into CdSe QDs. The enhancement seen with Mn2+ doped CdSe QDs are investigated and explained by the fact that the enhanced light absorption and reduced charge recombination by the formation of Mn CdSe passivation layer covering the QDs.

Double-layered TiO2 anodes from nanorods and nanoparticles for dye-sensitized solar cells

Double-layered TiO2 photoanodes consisting of bottom nanoparticles and top nanorods are synthesized for dye-sensitized solar cells (DSSCs). The cell performances can be maximized by optimizing the thickness of TiO2 nanoparticle layer, and the preliminary results demonstrate that a promising power conversion efficiency of 8.66% is determined on the device with 10µm-TiO2 nanoparticles/nanorods anode, yielding an 30.2% enhancement in cell efficiency in comparison to pristine TiO2 nanoparticle based solar cell.

Small molecules based on tetrazine unit for efficient performance solution-processed organic solar cells

Two D2-A-D1-A-D2 type small molecules using Benzo[1,2-b:4,5-b′]dithiophene (BDT) as central building block, tetrazine (Tz) as an electron accepting moiety, bithiophene or terthiophene as end donor units were synthesized. The optical and electrochemical properties of synthesized molecules indicate that these molecules can absorb sunlight in a broad spectral range from 300 to 700nm and show suitable energy level for efficient exciton dissociation. The introduction of tetrazine unit in these molecules effectively reduces the highest occupied molecular orbital (HOMO) energy levels of these molecules, thus increases air stability of these molecules as well as the open circuit voltage (Voc) in photovoltaic devices. Bulk heterojunction organic solar cells were fabricated by blending of these molecules with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) on indium tin oxide/glass substrates. This work afford promising power conversion efficiencies over 5.0%, with a large Voc of 0.98V, measured under air mass 1.5 global irradiation of 100mW/cm2.

The effect of mesoporous TiO2 pore size on the performance of solid-state dye sensitized solar cells based on photoelectrochemically polymerized Poly(3,4-ethylenedioxythiophene) hole conductor

Photoelectrochemical polymerization of poly(3,4-ethylenedioxythiphene) (PEDOT) has recently been introduced and widely investigated for fabrication of the hole transporting material (HTM) in highly efficient solid state dye sensitized solar cells (sDSCs). In this work, the effects of the surface area and pore size of TiO2 film were for the first time investigated in the sDSCs employing the in-situ polymerizated PEDOT HTM. Three different varieties of mesoporous TiO2 particles with controllable surface area and pore size were synthesized through the basic route in order to study the corresponding sDSC photovoltaic performances. It was found that the pore size plays an important role in the kinetics of the photoelectrochemical polymerization (PEP) process and the formation of the PEDOT capping layer. Larger pore sizes provided a more favourable pathway for the precursor diffusion through the mesoporous pores during the PEP process, which contributed towards a more efficient PEP. However, the interfacial contact area between the formed polymer and the dyes on the surface of TiO2 particle would be lower in the case of larger pore sizes, which consequently caused a less efficient dye regeneration process. Electronic diffusion on the other hand was improved for larger particle sizes. Employing an organic dye LEG4 and the self-made TiO2 with an optimal pore size of 25nm and particle size of 24nm, the sDSCs showed a promising power conversion efficiency (PCE) of 5.2%, higher than 4.5% for the commercial TiO2 Dyesol DSL-30. By measuring the dye regeneration yield and the kinetics through photoinduced absorption, it was observed that the homemade TiO2 based device had more efficient dye regeneration compared to the Dyesol based device, which could result from the better interfacial contact between the PEDOT and the dye. This work provides important information on the effect of meso-pore size on sDSCs and points to the necessity of further photoanode optimization toward the enhancement of the PCE of polymeric hole conductor-based DSCs.

Enhanced Efficiency and Stability of Inverted Perovskite Solar Cells Using Highly Crystalline SnO2 Nanocrystals as the Robust Electron-Transporting Layer.

Highly crystalline SnO2 is demonstrated to serve as a stable and robust electron-transporting layer for high-performance perovskite solar cells. Benefiting from its high crystallinity, the relatively thick SnO2 electron-transporting layer (≈120 nm) provides a respectable electron-transporting property to yield a promising power conversion efficiency (PCE)(18.8%) Over 90% of the initial PCE can be retained after 30 d storage in ambient with ≈70% relative humidity.

1,1,4,4-Tetracyanobuta-1,3-diene Substituted Diketopyrrolopyrroles: An Acceptor for Solution Processable Organic Bulk Heterojunction Solar Cells

Two small molecules composed of 1,1,4,4-tetracyanobuta-1,3-diene substituted diketopyrrolopyrroles (DPPs) denoted as DPP5 and DPP6 were synthesized and their photophysical and electrochemical properties were investigated. The frontier molecular orbitals based on empirical relation between cyclic voltammetry redox potentials, experimental IP, and EA energies indicate that these two small molecules can be used as an electron acceptor for the polymer bulk heterojunction solar cells. The BHJ solar cells combined with a low band gap D–A copolymer P as an electron donor exhibits promising power conversion efficiency of 3.90% and 4.95%, with DPP5 and DPP6, respectively, after the optimization of active layers, indicating that these small molecules based on DPPs can be the alternative as an electron acceptor to replace fullerene, leading to the low-cost solution-processed polymer solar cells.

Solution-Grown Monocrystalline Hybrid Perovskite Films for Hole-Transporter-Free Solar Cells.

High-quality perovskite monocrystalline films are successfully grown through cavitation-triggered asymmetric crystallization. These films enable a simple cell structure, ITO/CH3 NH3 PbBr3 /Au, with near 100% internal quantum efficiency, promising power conversion efficiencies (PCEs) >5%, and superior stability for prototype cells. Furthermore, the monocrystalline devices using a hole-transporter-free structure yield PCEs ≈6.5%, the highest among other similar-structured CH3 NH3 PbBr3 solar cells to date.

Facile synthesis of CuInS2 nanocrystals “photovoltaic ink” via hot-injection strategy under ambient environment

In this letter, oil-dispersed CuInS2 nanocrystals (CIS NCs) “photovoltaic ink” were successfully prepared using hot injection strategy under ambient environment, and different size of nanocrystals could be tailored through extending the reaction time. Series of analytical means were applied to characterize the obtained samples. The relative results revealed that the “copper rich” CIS NCs with a good monodispersity had tetragonal chalcopyrite structure; the size distribution of the samples tended to be widened along with extending reaction time, and the spectral absorption could also be regulated to cover most visible region. Finally, the obtained CIS NCs were sprayed as absorber in photovoltaic devices with the conventional configuration of (Glass/Mo/CIS/CdS/i-ZnO/AZO), and the best device exhibited promising power conversion efficiency of ~1.53% under AM 1.5g illumination.

ZnO nanorods assisted Ni1.1Pt and Co3.9Pt alloy microtube counter electrodes for efficient dye-sensitized solar cells

Pt counter electrode (CE) has been a burden for the commercialization of liquid-junction dye-sensitized solar cells (DSSCs). In searching for cost-effective CEs without sacrificing electrochemical activities, here we report the experimental realization of ZnO nanorods assisted Ni1.1Pt and Co3.9Pt alloy microtube (MT) CEs by combining a simple electrodeposition deposition on aligned ZnO nanorods and a galvanic displacement reaction. On behalf of the good charge-transfer ability along one-dimensional alloy electrocatalyst and electrocatalytic activity toward liquid electrolyte, the resultant DSSCs with Ni1.1Pt and Co3.9Pt MT CEs display promising power conversion efficiencies of 8.29% and 7.73%, yielding 30.6% and 21.7% enhancements, respectively. Similarly importantly, the alloyed CEs are still scalable, other alloyed low-Pt or non-Pt nano/microstructures can also be realized using the method reported here.