Inverted-type Organic Solar Cells Research Articles

All slot-die coated organic solar cells using an amine processed cathode interlayer based upon an amino acid functionalised perylene bisimide

Large area inverted type organic solar cells based upon a dye modified tin oxide electron transport layer are reported.

Boosting inverted type organic solar cell efficiency through the use of spray coated Y and Sn co-doped zinc oxide nanoparticles as an electron transport layers

Boosting inverted type organic solar cell efficiency through the use of spray coated Y and Sn co-doped zinc oxide nanoparticles as an electron transport layers

Cathode Interlayer Based on Naphthalene Diimide: A Modification Strategy for Zinc-Oxide-Free Inverted Organic Solar Cells.

Perylene diimide with ammonium oxide as a terminal group (named PDIN-O) is a well-known cathode interlayer in conventional-type organic solar cells (OSCs). Since naphthalene diimide exhibits a lower LUMO level than perylene diimide, we chose it as a core to further control the LUMO level of the materials. Small molecules (SMs) produce a beneficial interfacial dipole by the end of ionic functionality at the side chain of naphthalene diimide. With the active layer based on a nonfullerene acceptor (PM6:Y6BO), the power conversion efficiency (PCE) is enhanced by utilizing SMs as cathode interlayers. We discovered that the inverted-type OSC with naphthalene diimide with oxide as a counteranion (NDIN-O) shows poor thermal stability, which can cause irreversible damage to the interlayer-cathode contact, leading to poor PCE (11.1%). To overcome the disadvantage, we introduce NDIN-Br and NDIN-I with a higher decomposition temperature. An excellent PCE of 14.6% was achieved with the device based on NDIN-Br as an interlayer, which is almost the same as the PCE of the ZnO-based device (15.0%). The device based on NDIN-I without the ZnO layer exhibits an improved PCE of 15.4%, which is slightly higher than the ZnO-based device. The result offers a replacement of the ZnO interlayer, which is necessary to carefully manage the sol-gel transition by annealing temperatures as high as 200 °C and leading to low-cost manufacture of OSCs.

Computational and experimental investigations of a cyano group containing novel small molecule organic semiconductor

A novel cyano group containing small molecule, namely TAc-Cl was synthesized and characterized. We have performed experimental and Density Functional Theory (DFT) studies to investigate its vibrational frequencies, absorption characteristics and molecular orbital energy levels. Natural Bond Orbital (NBO) and Molecular Electrostatic Potential (MEP) studies through DFT calculations revealed TAc-Cl’s potential use as an acceptor molecule. According to electronic energy levels of TAc-Cl molecule obtained from DFT approach, poly(3-hexyl)thiophene (P3HT), which is a conjugated polymer, was selected to be used as donor with novel designed and synthesized molecule for organic solar cells. We have fabricated inverted type organic solar cells and investigated TAc-Cl as a potential acceptor using it as an active layer blended with P3HT in the bulk heterojunction concept.

Improved performance of inverted type organic solar cell using copper iodide-doped P3HT:PCBM as active layer for low light application

In this study, the effect of doping CuI into the active layer of poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) in inverted type organic solar cell for low light application was investigated. The CuI-doped and undoped solar cells displayed power conversion efficiency (PCE) of 2.04% and 1.44%, respectively, under 1-sun illumination. It is interesting to find that the solar cell with CuI doping achieved an even higher PCE of 4.84% compared to the undoped device (3.42%) under white LED light. This improvement is attributed to the CuI doping that improved the hole collection and reduced the charge recombination.

Synthesis of benzothiadiazole-based small molecule and its photovoltaic property

A1-D-A2-D-A1 type of structured BT-CMCN, which consists of an electron-deficient benzothiadiazole (BT) (A2), electron-rich thiophene (D) and electron-deficient dicyanomethylene-4H-chromene (CMCN) (A1) moiety, was synthesized. BT-CMCN showed pretty good thermal stability with the onset of decomposition temperatures of 304 °C at 5 wt % loss. The optical and electrochemical properties were investigated that reveal the HOMO/LUMO energy levels of BT-CMCN are −5.29 eV/−3.58 eV with the optical band gap of 1.72 eV. The HOMO/LUMO energy levels of BT-CMCN are higher than those of PC71BM. The best performance based on BT-CMCN/PC71BM in inverted-type organic solar cells (OSCs) exhibits the PCE of 0.69% with a short-circuit current density (Jsc) of 2.03 mA/cm2, Voc of 0.79 V, and fill factor (FF) of 42.7%.

Preparation of anatase form of TiO2 thin film at room temperature by electrochemical method as an alternative electron transport layer for inverted type organic solar cells

In this work, titanium dioxide thin films in rutile and anatase forms were prepared on pencil graphite and indium tin oxide electrodes, respectively by using an electrochemical method in one-step and room temperature. X-ray diffraction analyses were performed on the as prepared samples. The surface morphology of the electrodeposited TiO2 electrodes was characterized by scanning electron microscope and atomic force microscope. We have observed that the choice of the substrate affects the form of the electrodeposited thin TiO2 layers as rutile and anatase. When the pencil graphite electrode was used as a substrate rutile form of TiO2 was obtained on the electrode surface whereas anatase form of TiO2 was obtained when indium thin oxide electrode surface was used. We have also fabricated inverted type organic solar cells by employing TiO2 layer as an electron transport layer. We achieved a power conversion efficiency of 2.3%.

Effect of Ultrasonic Agitation and Spin Coating Speed on the Photovoltaic Properties of Inverted Organic Solar Cell Using Solution-Dispersed Copper Iodide as Anode Buffer Layer

Previous study showed that the introduction of solution-dispersed copper iodide (CuI) as anode buffer layer has improved the performance of inverted type organic solar cell. However, the CuI preparation parameters have yet to be optimized for the inverted type organic solar cell. In this work, the effect of ultrasonic agitation of CuI solution on inverted type organic solar cell based on zinc oxide (ZnO)/poly(3-hexylthiophene) (P3HT):(6,6)-phenyl-C61-butyric acid methyl ester (PCBM) has been studied. ZnO, acting as electron transporter, was spin-coated on top of fluorine tin oxide (FTO) substrate, while the active layer consisting of P3HT:PCBM as the electron donor and electron acceptor was spin-coated onto the ZnO layer and the top electrode, silver (Ag) was then thermally evaporated. CuI dispersed in acetonitrile solution was ultrasonicated with durations of 10, 20 and 30 min and then was spin-coated on the active layer. It was found that solution-dispersed CuI with longer ultrasonic agitation duration attained a relatively higher PCE than that with shorter duration and the highest PCE was 2.13%, achieved at 30 min ultrasonic agitation due to good film surface morphology. For further optimization, the deposition of CuI was carried out using different spin coating speeds of 1000 and 3000 rpm. The optimum PCE obtained was 2.29 at a spin coating speed of 3000 rpm as a result of uniform layer of CuI.

Electrochemical growth of Y doped ZnO nanorods for use in inverted type organic solar cells as electron transport layer

In this study, Yttrium (Y) doped and undoped Zinc oxide (ZnO) nanorods (NRs) were synthesized by the electrochemical method on indium tin oxide (ITO) substrate. Y doped and undoped ZnO-NRs were used as an electron selective layer in inverted type organic solar cell structure. Effect of Y doping in ZnO was investigated via x-ray diffraction (XRD) and scanning electron microscopy (SEM). Poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) was used as an active polymer blend. Inverted type organic solar cells were fabricated with the configuration ‘ITO/Y:ZnO-NRs/P3HT:PCBM/Ag’. Active layer was coated on ZnO-NRs by using drop-casting method to completely fill the gaps between the ZnO-NRs. Short circuit current density of the solar cells was increased from 10.6 to 16.4 mA cm−2 by using 3% Y doped ZnO-NRs as electron selective layer, where as the solar cell efficiency was changed from 2.35% to 3.87%. It was experimentally found that charge injection and selection in the Zn-ONRs were improved by doping the ZnO-NRs with Y atoms.

Performance improvement of organic solar cell via incorporation of donor type self-assembled interfacial monolayer

Surface engineering of solar cell subsrate plays an important role to achieve high performance organic solar cell (OSC). The surface modification of the electron selective electrodes on which active organic layers form is a main research issue to improve interface between the semiconductor and the electron selective electrode. Here an enhancement of performance of the inverted type organic solar cells (i-OSCs) was provided by the use of a facile method of titanium dioxide (TiO2) surface modification with triphenylamine based self-assembled monolayer (SAM) molecules. The surface potential as well the effective work function of SAMs treated TiO2 layer was better aligned to lowest unoccupied molecular orbital (LUMO) level of acceptor organic semiconductor for efficient electron charge extraction. As a result, the best maximum power conversion efficiency (PCE) of i-OSCs with and without SAMs modified TiO2 electrodes increased from 2.65% to 2.95%, respectively. The overall electrical parameters (short circuit current density, open circuit voltage and fill factor) of i-OSCs on the SAMS treated TiO2 electrode exhibited better results compared to unmodified TiO2 electrode. Series resistance (Rs) and shunt resistance (Rsh) of SAM treated devices was also evaluated to discuss an interface effects on the i-OSCs performance. These significant performance improvements reveal the potential application of SAMs for all solar cell technologies using TiO2 electrode.

Synthesis of Conjugated Materials Based on Benzodithiophene - Benzothiadazole and Their Application of Organic Solar Cells

Electron acceptor-donor-acceptor type oligomers based on benzothiadiazole (BT) unit and a (4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl) bis(trimethylstannane) (BDT) unit have been designed and prepared as electron-donating materials, which are 2,6-[5-(7-methyl-benzo[1,2,5]thiadiazol-4-yl)-thiophen-2-yl]-4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene) (BDT-TBT) and 2,6-[5′-(3′-hexyl-[2,2′]bithiophenyl-5-yl)-7-methyl-benzo[1,2,5]thiadiazole]-4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene) (BDT-THTBT), for Organic Solar Cells (OSCs) with PC71BM as electron-accepting materials. The HOMO energy level is elevated by the number of thiophene ring as a p-bridge, which lowers the band gap. Inverted-type organic solar cells (OSCs) with a configuration of ITO/ZnO/BDT-TBT (or BDT-THTBT):PC71BM/MoO3/Al are fabricated. OSC based on BDT-THTBT exhibits the highest power conversion efficiency (PCE) of 1.04% with the best J sc of 4.20 mA/cm2.

A-π-D-π-A type oligomer based on carbazole and benzothiadiazole for organic solar cells

ABSTRACTAn acceptor-donor-acceptor (A-p-D-p-A) type oligomer based on benzothiadiazole and 2,7 carbazole, 2,7-Bis(5-{7–2[-(4-decyloxy-phenyl)-vinyl]-benzo[1,2,5]thiadiazol-4-yl}-thiophen-2-yl)-9-(2-octyl-dodecyl)-9 H-carbazole (CzBT-DP), is synthesized by the Suzuki Coupling reaction. In addition, to extend the effective of conjugated length, we introduce thiophene (T) ring as a p-bridge and stilbene in the end of the conjugated chain. Optical, electrical and thermal properties and photovoltaic performances of CzBT-DP is systematically investigated. UV-vis absorption spectra and cyclic voltammograms (CV) reveal that HOMO/LUMO level of CzBT-DP is −5.37 eV/−3.38eV with an optical band gap of 1.99 eV, respectively, due to the addition of conjugated chain using stilbene compound system. The HOMO energies of CzBT-DP were calculated from the measured onset potential of oxidation by assuming the energy level of ferrocene (Fc) as −4.8 eV by CV. Addition of thiophene affects the HOMO level which makes CzBT-DP has high-lying HOMO level. Due to higher LUMO energy values above 0.3 eV than PC71 BM, CzBT-DP could be applied to BHJ OSCs as the donor [1]. Inverted type organic solar cells (OSCs) with a configuration of ITO/ZnO/CzBT-DP:PC71BM/MoO3/Al are fabricated. Figure 1 shows the current density-voltage (J-V) characteristics of the optimized BHJ OSCs based on CzBT-DP:PC71BM blend ratios from 3:2 to 3:6 (w/w). Maximum power conversion efficiency (PCE) is 0.64% with a short-circuit current density (Jsc) of 1.78 mA/cm2, Voc of 0.94 V, and fill factor (FF) of 38.3%. The best blend ratio between CzBT-DP and PC71BM having the best PCE appeared at 3:3 (w/w).

Performance enhancement of inverted type organic solar cells by using Eu doped TiO2 thin film

In the present work, europium (Eu) has been used as a dopant element in titanium dioxide (TiO2) crystal structure to enhance the photovoltaic performance of the inverted type organic solar cells fabricated with TiO2 thin film as an electron transport layer. Poly(3-hexylthiophene):phenylC61-butyric acid methyl ester (P3HT: PCBM) blend has been used as an active layer and Eu:TiO2 thin films have been fabricated with different Eu concentrations as 0, 1, 5, 7 and 10% (w/w) by sol-gel method. Effect of Eu doping concentration on the photovoltaic performance of the inverted type solar cells has been investigated. Inverted type organic solar cells using Eu doped TiO2 thin films as electron transport layer has been fabricated as a structure of ITO/Eu:TiO2/P3HT:PCBM/Ag. Power conversion efficiency of the fabricated solar cells increased from 1.16% to 2.47% at 5% Eu doping concentration. Electron transport and hole blocking properties of the TiO2 inter layer has been improved by Eu doping.

Synthesis and characterization conjugated oligomer based on phenothiazine derivative

ABSTRACTAn acceptor-donor-acceptor (A-D-A) type oligomer based on benzothiadiazole and phenothiazine, 3,7-bis-[5-(7-methyl-benzo[1,2,5]thiadiazol-4-yl)-thiophen-2-yl]-10-(2-octyl-dodecyl)-10H-phenothiazine (PTBT), is synthesized by the Suzuki Coupling reaction. In addition, we introduce thiophene (T) ring as a π-bridge to extend the effective conjugation length. To investigate the effect of combination between strong electron donor and strong electron withdrawing on the diverse properties of conjugated oligomer on PTBT, optical and electrochemical properties are examined using UV-Vis absorption spectra and cyclic volatommograms (CV) that revealing HOMO/LUMO level at −5.36 eV/−3.18 eV with an optical band gap of 2.18 eV, respectively. Inverted-type organic solar cells (OSCs) with a configuration of ITO/ZnO/PTBT:PC71BM/MoO3/Al are fabricated. Maximum power conversion efficiency of the organic solar cells (OSCs) based on PTBT is showed up to 0.44% with a short-circuit current density (Jsc) of – 1.63 mA/cm2, Voc of 0.87 V, and fill factor (FF) of 30.9%.

Effect of TiO2 modification with amino-based self-assembled monolayer on inverted organic solar cell

The effects of surface modification of titanium dioxide (TiO2) on the performance of inverted type organic solar cells (i-OSCs) was investigated in this study. A series of benzoic acid derivatized self-assembled monolayer (SAM) molecules of 4′-[(hexyloxy)phenyl]amino-3,5-biphenyl dicarboxylic acid (CT17) and 4′-[1-naphthyl (phenyl)amino]biphenyl-4-carboxylic acid (CT19) were utilized to modify the interface between TiO2 buffer layer and poly-3 hexylthiophene (P3HT):[6,6]-phenyl C61 butyric acid methyl ester (PC61BM) active layer having the device structure of ITO/TiO2/SAM/P3HT:PC61BM/MoO3/Ag. The work function and surface wetting properties of TiO2 buffer layer served as electron transporting layer between ITO and PC61BM active layer were tuned by SAM method. The solar cell of the SAM modified devices exhibited better performance. The power conversion efficiency (PCE) of i-OSCs devices with bare TiO2 electrodes enhanced from 2.00% to 2.21% and 2.43% with CT17 and CT19 treated TiO2 electrodes, respectively. The open circuit voltage (Voc) of the SAM treated TiO2 devices reached to 0.60V and 0.61V, respectively, while the Voc of untreated TiO2 was 0.57V. The water contact angle of i-OSCs with CT17 and CT19 SAMs was also higher than the value of the unmodified TiO2 electrode. These results show that inserting a monolayer at the interface between organic and inorganic layers is an useful alternative method to improve the performance of i-OSCs.

(Invited) Carbon Nanotube Films for Organic-Inorganic Hybrid Perovskite Solar Cells

A film of single-walled carbon nanotubes (SWNTs) can be a dual-functional layer as electron-blocking-layer and transparent-conductive electrode in various solar cells. We have demonstrated efficient SWNT/Si solar cells [1,2] and organic solar cells [3,4] using dry-deposited high-quality SWNTs and honeycomb-structured SWNTs. For organic solar cells, the SWNT doped by MoOxand PEDOT:PSS demonstrated a dual function, which can replace both ITO and electron-blocking-layer [3]. Using this advantage of the SWNT film, flexible organic solar cells were fabricated with ease [3]. Also, it was possible to replace electron-blocking-layer and metal electrode for inverted-type organic solar cells [4]. The removal of metal electrode enabled flexible, the semi-transparent window-like solar cells, which was inexpensive in their production process [4]. Here, this dual functional application of SWNT film is also demonstrated for organic-inorganic hybrid perovskite solar cells. Replacement of ITO in inverted-type perovskite solar cells, SWNTs/PEDOT:PSS/CH3NH3PbI3/PCBM/Al, is demonstrated by adjusting the surface energy of PEDOT:PSS [5]. The flexible application on polyethylene terephthalate (PET) is also demonstrated [5]. Furthermore, replacement of electron-blocking-layer and metal electrode in normal-type perovskite solar cells is demonstrated as well. They show high power conversion efficiency (PCE), cost-efficiency, and higher stability. Those devices can have comparable PCE as the conventional design with organic electron-blocking layer and top metal electrode. In addition to the expected lower cost and improved stability, These solar cells can be potentially semi-transparent when transparent SWNT films are used. This means that they can be illuminated from either cathode or anode side. The normal-type perovskite solar cell, composed of ITO/TiO2/CH3NH3PbI3/SWNTs, can achieve a PCE of 10 % without doping of SWNTs [6]. The PCE can be as high as 17 % with the preliminary doping of the film of SWNTs using Spiro-MeOTAD, which is the typical electron-blocking-layer used for the normal type perovskite solar cells. The PMMA layer on top of the film of SWNTs can also contribute as doping and protection layer. Furthermore, the modified structure with a perovskite layer sandwiched by C60 and SWNTs, i.e. ITO/C60/CH3NH3PbI3/SWNTs, can lead to the solar cells without hysteresis and with much improved air-stability [7]. The effective passivation of the degradation of perovskite material by moisture can be achieved with C60and SWNTs [7]. This device show about 13 % PCE without hole-transporting layer and 17% with spiro-MeOTAD this far [7]. This can be a good candidate for scale-up demonstration of practical perovskite solar cells. This work was supported by JSPS KAKENHI Grant Numbers JP25107002, JP15H05760, and IRENA Project by JST-EC DG RTD, Strategic International Collaborative Research Program, SICORP. Part of this work is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Direct observation of UV-induced charge accumulation in inverted-type polymer solar cells with a TiOx layer: Microscopic elucidation of the light-soaking phenomenon

The mechanism of light-soaking phenomenon in inverted-type organic solar cells (IOSCs) with a structure of indium-tin-oxide/TiOx/P3HT:PCBM/Au was studied by electron spin resonance (ESR) spectroscopy. Charge accumulation in the cell during UV-light irradiation was observed using ESR, which was clearly correlated with the light-soaking phenomenon. The origin of the charge accumulation is clarified as holes that are deeply trapped at p-type P3HT polymer-chain ends with bromine after hole transfer from the band excitation in the TiOx layer. The holes are considered to be electrostatically attracted to trapped electrons in the TiOx layer after the band excitation. These accumulated charges are the origin of the light-soaking phenomenon. Our results strongly suggest that passivation of the residual OH groups in the TiOx layer is needed to avoid the light-soaking phenomenon by preventing electron trappings, a step that is indispensable in the operation of highly stable IOSCs without UV-light irradiation based on a low-cost and low-temperature device fabrication process using flexible plastic substrates.

A-D-A type conjugated oligomers based on benzothiadiazole and their photovoltaic applications

To investigate the effect of combination between strong electron donor and strong electron acceptor on the diverse properties of conjugated small molecules based on carbazole (Cz) having 2,7-linkage as an electron donor and 2,1,3-benzothiadiazole (BT), 2,7-bis-[5-(7-methyl-benzo[1,2,5]thiadiazol-4-yl)-thiophen-2-yl]-9-(2-octyl-dodecyl)-9H-carbazole (Cz-BT) and 2,7-bis-[5′-(7-methyl-benzo[1,2,5]thiadiazol-4-yl)-[2,2′]bithiophenyl-5-yl]-9-(2-octyl-dodecyl)-9H-carbazole (CzT-BT), are synthesized by the Suzuki coupling reaction. In addition, we introduce thiophene (T) ring as the π-bridge to extend the effective conjugation length. Cz-BT and CzT-BT exhibit the band gap of 2.22 and 2.15eV, respectively, due to intermolecular charge transfer between Cz and BT. As increasing the number of T rings, the band gap is lowered and the HOMO and LUMO energy is increased. Inverted-type organic solar cells (OSCs) with a configuration of ITO/ZnO/Cz-BT or CzT-BT:PC71BM/MoO3/Al are fabricated. Maximum power conversion efficiency (PCE) of the OSCs based on small molecules is exhibited up to 1.58%.

Characterization of Inverted-Type Organic Solar Cell with Europium Doped Zinc Oxide as an Electron Transport Layer

Characterization of Inverted-Type Organic Solar Cell with Europium Doped Zinc Oxide as an Electron Transport Layer

Vertical phase separation and light-soaking effect improvements by photoactive layer spin coating initiation time control in air-processed inverted organic solar cells

Inverted type organic solar cells have been regarded more promising over normal type, owing to a favorable vertical phase separation and air-processability. Many techniques, such as thermal or solvent annealing, slow growth method, melting bi-layers etc. have been employed to improve the morphology of photoactive layer, which is one of the advantages of inverted type OSCs. As a result, the device performance is greatly enhanced which is also reflected in the current-density versus voltage curve. However, some of the methods presented above are difficult and costly to perform. In addition, inverted OSCs still employ certain metal oxide electron-transporting layers, which impose UV activation time due to light-soaking effect. Here we show a facile method that can control the vertical phase separation as well as minimize the light-soaking effect. By leaving the photoactive solution with a low volatile solvent in air before spin coating, conductive polymers (donor) and fullerene derivatives (acceptor) were separated vertically in a way that their charge extraction is enhanced. Besides, oxygen and impurities at the contact on ETL were removed by leaving the solution for a certain period. Mechanisms of these improvements were evidenced by various analytical techniques and empirical tests using different materials in different environments. We anticipate this finding can expedite the OSC research and its usage in all air-processed inverted OSCs.