Efficient Organic Photovoltaic Cells Research Articles

Spectroscopic Determination of Charge Formation Efficiency of Organic Photovoltaic Cells

The internal quantum efficiency (ΦIQ) of an organic photovoltaic (OPV) cell is governed by plural processes, i.e., the carrier formation process at the D/A interface and the carrier transfer process toward the collector electrode. Then, ΦIQ can be decomposed into the carrier formation (ΦCF) and carrier transfer efficiencies (ΦCT). By combination of femtosecond time-resolved and electrochemical spectroscopies, we determined absolute values of ΦCF of F8T2/PC71BM, P3HT/PCBM, and PTB7/PC71BM solar cells. We found that ΦCF at 400 nm of the F8T2/PC71BM cell is higher than those of the P3HT/PCBM, and PTB7/PC71BM cells, although ΦIQ at 400 nm is the lowest.

Controlling the Morphology of BDTT-DPP-Based Small Molecules via End-Group Functionalization for Highly Efficient Single and Tandem Organic Photovoltaic Cells.

A series of narrow-band gap, π-conjugated small molecules based on diketopyrrolopyrrole (DPP) electron acceptor units coupled with alkylthienyl-substituted-benzodithiophene (BDTT) electron donors were designed and synthesized for use as donor materials in solution-processed organic photovoltaic cells. In particular, by end-group functionalization of the small molecules with fluorine derivatives, the nanoscale morphologies of the photoactive layers of the photovoltaic cells were successfully controlled. The influences of different fluorine-based end-groups on the optoelectronic and morphological properties, carrier mobilities, and the photovoltaic performances of these materials were investigated. A high power conversion efficiency (PCE) of 6.00% under simulated solar light (AM 1.5G) illumination has been achieved for organic photovoltaic cells based on a small-molecule bulk heterojunction system consisting of a trifluoromethylbenzene (CF3) end-group-containing oligomer (BDTT-(DPP)2-CF3) as the donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor. As a result, the introduction of CF3 end-groups has been found to enhance both the short circuit current density (JSC) and fill factor (FF). A tandem photovoltaic device comprising an inverted BDTT-(DPP)2-CF3:PC71BM cell and a poly(3-hexylthiophene) (P3HT):indene-C60-bisadduct (IC60BA)-based cell as the top and bottom cell components, respectively, showed a maximum PCE of 8.30%. These results provide valuable guidelines for the rational design of conjugated small molecules for applications in high-performance organic photovoltaic cells. Furthermore, to the best of our knowledge, this is the first report on the design of fluorine-functionalized BDTT-DPP-based small molecules, which have been shown to be a viable candidate for use in inverted tandem cells.

Efficient polymer solar cells enabled by low temperature processed ternary metal oxide as electron transport interlayer with large stoichiometry window.

Highly efficient organic photovoltaic cells are demonstrated by incorporating low temperature solution processed indium zinc oxide (IZO) as cathode interlayers. The IZOs are synthesized using a combustion synthesis method, which enables low temperature processes (150-250 °C). We investigated the IZO films with different electron mobilities (1.4×10(-3) to 0.23 cm2/(V·s)), hydroxide-oxide content (38% to 47%), and surface roughness (0.19-5.16 nm) by modulating the ternary metal oxide stoichiometry. The photovoltaic performance was found to be relatively insensitive to the composition ratio of In:Zn over the range of 0.8:0.2 to 0.5:0.5 despite the differences in their electrical and surface properties, achieving high power conversion efficiencies of 6.61%-7.04%. Changes in composition ratio of IZO do not lead to obvious differences in energy levels, diode parameters and morphology of the photoactive layer, as revealed by ultraviolet photoelectron spectroscopy (UPS), dark current analysis and time-of-flight secondary ion mass spectrometry (TOF-SIMS) measurements, correlating well with the large IZO stoichiometry window that enables efficient photovoltaic devices. Our results demonstrate the robustness of this ETL system and provide a convenient approach to realize a wide range of multicomponent oxides and compatible with processing on flexible plastic substrates.

Organic Photovoltaics: Highly Thermal Stable and Efficient Organic Photovoltaic Cells with Crosslinked Networks Appending Open‐Cage Fullerenes as Additives (Adv. Funct. Mater. 2/2015)

Organic Photovoltaics: Highly Thermal Stable and Efficient Organic Photovoltaic Cells with Crosslinked Networks Appending Open‐Cage Fullerenes as Additives (Adv. Funct. Mater. 2/2015)

Electronic and optical excitations of the PTB7 crystal: First-principles GW-BSE calculations

Given the recent success in achieving efficient organic photovoltaic solar cells based on thieno[3,4-b]thiophene/benzodithiophene polymers (PTB7) and growing efforts to further improve the power conversion efficiency of the PTB7-based devices, a detailed atomic-scale picture of the electronic structure and the excitonic properties of PTB7 crystal is highly desirable. We report electronic and optical properties of PTB7 on the basis of first-principles density functional theory and GW many-body plus Bethe-Salpeter equation (GW-BSE) calculations. It is established that the first two highest valence bands (HVBs) and the first two lowest conduction bands (LCBs) originate from the benzodithiophene and thieno[3,4-b]thiophene functional units, respectively, thus confirming the donor-acceptor nature of PTB7. A significant difference of band splitting between HVBs and LCBs is found and its origins are explained. Our results strongly suggest that the strength of the interchain $\ensuremath{\pi}\text{\ensuremath{-}}\ensuremath{\pi}$ interaction is not only a function of interchain distance, but is also highly dependent on the nature of the fused rings. The experimental optical absorption spectrum of PTB7 is well reproduced and explained by our GW-BSE calculations. Further analysis shows that the nature of the lowest singlet (triplet) excitons in polymeric crystals such as PTB7 differs from that of organic molecular crystals. A possible reason is explored by combining BSE calculations with a simple Hamiltonian model.

Dispersion control of Ag nanoparticles in bulk-heterojunction for efficient organic photovoltaic devices

This research focuses on the effect of different capping agents on Ag nanoparticles (NPs), for the improved efficiency of organic photovoltaic cells. Ag NPs were produced by solution chemistry of the polyol process, and then successfully capped with oleylamine (OA), polyvinylpyrrolidone (PVP), or thiol terminated polystyrene (PS-SH), as proven by FT-IR spectra. These Ag NPs with different capping agents were finally embedded in the photoactive layer of poly(3-hexylthiophene):6,6-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) bulk heterojunction solar cells. Because of the presence of a suitable capping agent that prevents aggregation, the dispersity of the Ag NPs in organic solvent was significantly improved, in the sequence of OA, PVP, and PS-SH. The photovoltaic cells exhibit increased performance from 3.11% to 3.49%, at an optimized blend ratio of Ag NPs (2.5wt%) capped with PS-SH. This enhancement is mainly attributed to the improved short circuit current (increased from 8.49mA/cm2 to 9.29mA/cm2) and extinction with effective light scattering, caused by improved dispersion of the Ag NPs in BHJ films, through reducing unwanted particle aggregation.

Highly Thermal Stable and Efficient Organic Photovoltaic Cells with Crosslinked Networks Appending Open‐Cage Fullerenes as Additives

Highly thermal stable organic bulk heterojunction (OBHJ) photovoltaic cells are demonstrated with crosslinkable open‐cage fullerenes (COF) as additives in the active layer. Partial incorporation of COF, ≈10–15 wt% with weight ratio of P3HT:PC61BM = 1:0.9, builds up three‐dimensional local borders upon heating treatment at 150 °C for 10 min. This process induces crosslinking chemical reaction through activating the styryl moiety in COF and reduces phase aggregation rates of fullerenes materials. Supported by statistics of devices degradation data analysis and optical microscopy study, the devices with COF show longer lifetime with keeping their efficiency (t = 144 h) under accelerated heating test at 150 °C, while PCE of normal devices without COF drop dramatically. These results demonstrate that the thermally crosslinkable COF is an excellent additive for highly thermal stable and durable OPVs applications.

Effect of π-conjugated bridges of TPD-based medium bandgap conjugated copolymers for efficient tandem organic photovoltaic cells

Conjugated donor (D)–π–acceptor (A) copolymers, PBDT–TPD, PBDT–ttTPD, PBDTT–TPD, and PBDTT–ttTPD, based on a benzodithiophene (BDT) donor unit and thieno[3,4-c]pyrrole-4,6(5H)-dione (TPD) acceptor unit were designed and synthesized with different π bridges via Pd-catalyzed Stille-coupling. The π bridges between BDT and TPD were thiophene in PBDT–TPD and PBDTT–TPD, and 6-alkylthieno[3,2-b]thiophene in PBDT–ttTPD and PBDTT–ttTPD. The effects of the π bridges on the optical, electrochemical, and photovoltaic properties of the polymers were investigated, in addition to the film crystallinities and carrier mobilities. Copolymers with the 6-alkylthieno[3,2-b]thiophene π-bridge exhibited high crystallinity and hole mobility. Improved Jsc and FF were obtained to increase the overall power conversion efficiencies (PCE) in inverted single organic photovoltaic cells. A PCE of 6.81% was achieved from the inverted single device fabricated using the PBDTT–ttTPD:PC71BM blend film with 3 vol% 1,8-diiodooctane. A tandem photovoltaic device comprising the inverted PBDTT–ttTPD cell and a PTB7-based cell as the bottom and top cell components, respectively, showed a maximum PCE of 9.35% with a Voc of 1.58 V, a Jsc of 8.00 mA cm−2, and a FF of 74% under AM 1.5 G illumination at 100 mW cm−2. The obtained PCE of the bottom cell and FF of the tandem cell are, to the best of our knowledge, the highest reported to date for a tandem OPV device. This work demonstrates that PBDTT–ttTPD may be very promising for applications in tandem solar cells. Furthermore, 6-alkylthieno[3,2-b]thiophene π-bridge systems in medium bandgap polymers can improve the performance of tandem organic photovoltaic cells.

High efficiency organic photovoltaic cells based on inverted SubPc/C60/ITO cascade junctions

Conventional heterojunction organic photovoltaic cells typically involve the deposition of the electron donor layer (or donor–acceptor blend) on top of a transparent anode, with the cathode deposited last. Inverting the structure and deposition sequence usually worsens the performance characteristics, except device lifetime. We compare conventional (SubPc/C60) and inverted (C60/SubPc) junctions, the latter exhibiting a power conversion efficiency of 3.5%. We also find a significant trade-off between the open circuit voltage and short circuit photocurrent, potentially attributable to the formation of a C60/ITO Schottky junction, and a change from exciton-quenching to exciton-blocking behavior of the SubPc:MoOX interface in inverted devices.

Soluble squaraine derivatives for 4.9% efficient organic photovoltaic cells

A series of soluble squaraine derivatives has been designed and developed as a donor material for solution-processible organic photovoltaic cells. In combination with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an acceptor, we obtained a power conversion efficiency of 4.86% from bulk-heterojunction photovoltaic cells.

Effect of temperature on carrier formation efficiency in organic photovoltaic cells

The internal quantum efficiency (ϕIQ) of an organic photovoltaic cell is governed by plural processes. Here, we propose that ϕIQ can be experimentally decomposed into carrier formation (ϕCF) and carrier transfer (ϕCT) efficiencies. By combining femtosecond time-resolved and electrochemical spectroscopy, we clarified the effect of temperature on ϕCF in a regioregular poly(3-hexylthiophene) (rr-P3HT)/[6,6]-phenyl C61-butyric acid methyl ester blend film. We found that ϕCF (=0.55) at 80 K is the same as that (=0.55) at 300 K. The temperature insensitivity of ϕCF indicates that the electron-hole pairs at the D/A interface are seldom subjected to coulombic binding energy.

Short-lived charge-transfer excitons in organic photovoltaic cells studied by high-field magneto-photocurrent.

The main route of charge photogeneration in efficient organic photovoltaic cells based on bulk hetero-junction donor-acceptor blends involves short-lived charge-transfer excitons at the donor-acceptor interfaces. The cell efficiency is critically affected by the charge-transfer exciton recombination and dissociation processes. By measuring the magneto-photocurrent under ambient conditions at room temperature, we show here that magnetic field-induced spin-mixing among the charge-transfer exciton spin sublevels occurs in fields up to at least 8.5 Tesla. The resulting magneto-photocurrent increases at high fields showing non-saturating behaviour up to the highest applied field. We attribute the observed high-field spin-mixing mechanism to the difference in the donor-acceptor g-factors. The non-saturating magneto-photocurrent response at high field indicates that there exist charge-transfer excitons with lifetime in the sub-nanosecond time domain. The non-Lorentzian high-field magneto-photocurrent response indicates a dispersive decay mechanism that originates due to a broad distribution of charge-transfer exciton lifetimes.

7.9% efficient vapor-deposited organic photovoltaic cells based on a simple bulk heterojunction

Efficient organic photovoltaic cells (OPVs) based on vapor-deposited small molecule materials frequently utilize a planar-mixed heterojunction (PMHJ) architecture, in which a mixed layer of electron donating and accepting materials is sandwiched between neat, planar layers of the donor and acceptor. The neat layers contribute to photocurrent, help minimize the dark current and maximize the device open-circuit voltage. The use of organic semiconductors as the planar layers is not however without complication, as the low charge carrier mobility in these materials can hinder efficient charge collection. In this work, we examine OPVs where the neat donor layer between the anode and the mixture is replaced with a layer of MoOx. An average device power conversion efficiency of (7.7 ± 0.3)% is realized under AM1.5G (100 mW cm−2) solar simulated illumination in a uniformly mixed heterojunction OPV based on the donor–acceptor pairing of 2-{[7-(4-N,N-ditolylaminophenylen-1-yl)-2,1,3-benzothiadiazol-4-yl]methylene}malononitrile (DTDCPB)–C70 without any adjacent donor or acceptor layers. Correcting for spectral mismatch, an average device power conversion efficiency of (7.9 ± 0.3)% is obtained, with a champion device efficiency of 8.2%. The use of MoOxversus a neat donor layer is found to reduce the series resistance and facilitate charge collection from the mixed layer, leading to an increased fill factor and short-circuit current. To our knowledge, this result is among the highest single-cell efficiencies reported for vapor-deposited small molecule OPVs. We further find that mixed OPVs based on DTDCPB–C70 exhibit a promising shelf lifetime, showing no degradation in efficiency after more than seven months. Thus, the removal of the neat planar organic layer can favorably affect device efficiency without potentially adversely impacting stability.

UV/ozone‐treated WS2 hole‐extraction layer in organic photovoltaic cells

AbstractWS2 nanosheets obtained through a simple sonication exfoliation method are employed as a hole‐extraction layer to improve the efficiency of organic photovoltaic cells (OPVs). A reduction in the wavenumber difference in the Raman spectra, the appearance of a UV absorption peak, and atomic force microscopy images indicate that WS2 nanosheets are formed through the sonication method. The power conversion efficiency (PCE) values of OPVs with and without untreated WS2 layers are both 1.84%. After performing a UV‐ozone (UVO) treatment on the WS2 surface for 15 min, the PCE increases to 2.4%. Synchrotron radiation photoelectron spectroscopy data show that the work function of WS2 increases from 4.9 eV to 5.1 eV upon UVO treat‐ ment, suggesting that the increase in the PCE value is caused by the band alignment. Upon inserting poly(3,4‐ethylenedioxythiophene):poly(styrene‐sulfonate) (PEDOT: PSS) between the WS2 and the active layer, the PCE value of the OPV increases to 3.07%, which is superior to that of the device employing only PEDOT:PSS (2.87%). Therefore, it is considered that the use of UVO‐treated WS2 is able to improve the performance of OPV cells. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Templating Effects in Molecular Growth of Blended Films for Efficient Small-Molecule Photovoltaics

A strategy to control the molecular growth of coevaporated zinc phthalocyanine (ZnPc) and fullerene (C60) blended films for efficient organic photovoltaic (OPV) cells was demonstrated. Introduction of a 2,5-bis(4-biphenylyl)-bithiophene (BP2T) film or a ZnPc film on BP2T as nanostructured templates not only results in phase-separated domains in blended films with clear interpenetrating networks but also improves the crystallinity of ZnPc domains, both of which enhance photocurrent generation and charge carrier transport. Such morphology is strongly associated with the molecular growth of the templating layers. Roughness and adhesion of the templating layers are of great importance for the molecular growth of the blended films and in turn for cell characteristics. By carefully regulating the molecular growth of the blended films, the power conversion efficiency was improved by 125%, from 1.85 to 4.15% under 1 sun.

Fabrication and characterization of combined metallic nanogratings and ITO electrodes for organic photovoltaic cells

Organic photovoltaic devices are interesting alternatives to conventional silicon based photovoltaic cells, due to potentially lower material costs and energy consumption during the fabrication process. However, the energy conversion efficiency of organic photovoltaic cells may still be improved. One possible approach is a combination with metallic nanostructures to improve light absorption properties in the active layer. We report on the fabrication and characterization of metallic nanogratings integrated with an indium tin oxide (ITO) electrode to be combined with small-molecule organic photovoltaic cells. With respect to geometry and extinction properties gratings fabricated by two different fabrication methods, conventional electron beam lithography and nanoimprint lithography, are presented. Furthermore, gratings fabricated on top of ITO are compared to gratings integrated both below and within ITO electrodes. Fabricating structures below or within electrodes is advantageous for organic thin film techniques to avoid compromising the crystallinity of the organic thin film and short-circuiting across the thin layer.

Multilayer Epitaxial Growth of Lead Phthalocyanine and C70 Using CuBr as a Templating Layer for Enhancing the Efficiency of Organic Photovoltaic Cells

The molecular orientation and crystallinity of donor and acceptor molecules are important for high-efficiency organic photovoltaic cells (OPVs) because they significantly influence both the absorption of light and charge-transport characteristics. We report that the templating effect extends to multilayers to increase the crystallinity and to modify the orientation of the crystals of lead phthalocyanine (PbPc) and C70 layers at the same time by adopting CuBr as a new templating layer on indium tin oxide (ITO). The formation of a monoclinic phase with a preferred orientation of (320) for PbPc and a fcc phase with a preferred orientation of (220) for C70 on the PbPc layer is revealed by X-ray diffraction (XRD) patterns. The multilayer epitaxy results in an increase of the exciton diffusion lengths from 5.6 to 8.8 nm for PbPc and from 6.9 to 13.8 nm for C70 to enhance the power conversion efficiency (PCE) of the planar heterojunction OPVs composed of PbPc and C70 from 1.4 to 2.3%. The quasi-epitaxy model is proposed to explain the multilayer epitaxy.

Fluorine-functionalized and simultaneously reduced graphene oxide as a novel hole transporting layer for highly efficient and stable organic photovoltaic cells.

A one-step reduction and functionalization of graphene oxide (FrGO) was easily achieved using a novel phenylhydrazine-based reductant containing fluorine atoms, which can induce p-type doping due to its high electronegativity. The FrGO-based OPV exhibited a high power conversion efficiency of ∼6.71% and a superior OPV-stability to commercial PEDOT:PSS.

Fabrication conditions for efficient organic photovoltaic cells from aqueous dispersions of nanoparticles

We report the processing conditions for fabricating efficient organic solar cells from aqueous dispersions of conjugated polymer nanoparticles.

Organic photovoltaic cells with copper (II) tetra-methyl substituted phthalocyanine

Efficient heterojunction organic photovoltaic (OPV) cells are fabricated based on copper tetra-methyl phthalocyanine (CuMePc) as donor and fullerene (C60) as acceptor. The power conversion efficiency of CuMePc/C60 OPV cell (2.52%) is increased by 88% compared with that of the non-peripheral substituted copper phthalocyanine (CuPc)/C60 OPV cell (1.34%). The introduction of methyl substituent leads to stronger π—π interaction of CuMePc (∼ 3.5 Å) than that of CuPc (∼ 3.8 Å). The efficiency improvement is attributed to the enhanced carrier mobility of CuMePc thin film (1.1 × 10−3 cm2/V·s) and better film morphology by introducing methyl groups into the periphery of CuPc molecule.