Efficient Organic Photovoltaic Cells Research Articles

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.

Efficient organic photovoltaic cells with vertically ordered bulk heterojunctions

Nanoscale morphology has been proved to be the key parameter deciding the exciton dissociation and charge transportation in bulk heterojunction (BHJ) solar cells. In this paper, we report a kind of small molecular organic photovoltaic cell (OPV) with a vertically ordered BHJ prepared by the weak epitaxial growth method. By this method, zinc phthalocyanine (ZnPc) can easily be formed into a highly ordered and continuous thin film and C60 is inclined to become dispersed crystalline grains in ZnPc film. Furthermore, we can control both the size and distribution density of C60 crystalline grains in ZnPc thin film without destroying the order of the ZnPc thin film. The OPVs with the vertically ordered BHJ show a high fill factor and a power conversion efficiency over 3% has been achieved.

Revealing the Dynamics of Charge Carriers in Polymer:Fullerene Blends Using Photoinduced Time-Resolved Microwave Conductivity

During the past decade, time-resolved microwave conductivity (TRMC) has evolved to an established, powerful technique to study photoactive layers. With this feature paper, we aim to fulfill two goals: (1) give a full description of the photoinduced TRMC technique, including experimental details and data analysis, and discuss to what extent the TRMC technique differs from more conventional DC techniques and (2) illustrate the potential of this technique for probing charge carrier dynamics in photoactive materials. For these reasons recent studies on conjugated polymer:fullerene blends will be presented and discussed. The findings from these studies have advanced the insight into the mechanism of charge carrier generation and decay in polymer:fullerene blends, which allows us to improve the efficiency of organic photovoltaic cells based on this active layer architecture. In short, it is shown how the TRMC technique can be used as a versatile method to screen the potential of new photovoltaic materials.

Solution‐Processed Hydrogen Molybdenum Bronzes as Highly Conductive Anode Interlayers in Efficient Organic Photovoltaics

Highly efficient and stable organic photovoltaic (OPV) cells are demonstrated by incorporating solution‐processed hydrogen molybdenum bronzes as anode interlayers. The bronzes are synthesized using a sol‐gel method with the critical step being the partial oxide reduction/hydrogenation using an alcohol‐based solvent. Their composition, stoichiometry, and electronic properties strongly correlate with the annealing process to which the films are subjected after spin coating. Hydrogen molybdenum bronzes with moderate degree of reduction are found to be highly advantageous when used as anode interlayers in OPVs, as they maintain a high work function similar to the fully stoichiometric metal oxide, whereas they exhibit a high density of occupied gap states, which are beneficial for charge transport. Enhanced short‐circuit current, open‐circuit voltage and, fill factor, relative to reference devices incorporating either PEDOT‐PSS or a solution processed stoichiometric molybdenum oxide, are obtained for a variety of bulk heterojunction mixtures based on different polymeric donors and fullerene acceptors. In particular, high power conversion efficiencies are obtained in devices that employed the s‐HxMoO2.75 as the hole extraction layer.

Ultrafast intersystem-crossing in platinum containing π-conjugated polymers with tunable spin-orbit coupling

The development of efficient organic light-emitting diodes (OLED) and organic photovoltaic cells requires control over the dynamics of spin sensitive excitations. Embedding heavy metal atoms in π-conjugated polymer chains enhances the spin-orbit coupling (SOC), and thus facilitates intersystem crossing (ISC) from the singlet to triplet manifolds. Here we use various nonlinear optical spectroscopies such as two-photon absorption and electroabsorption in conjunction with electronic structure calculations, for studying the energies, emission bands and ultrafast dynamics of spin photoexcitations in two newly synthesized π-conjugated polymers that contain intrachain platinum (Pt) atoms separated by one (Pt-1) or three (Pt-3) organic spacer units. The controllable SOC in these polymers leads to a record ISC time of <~1 ps in Pt-1 and ~6 ps in Pt-3. The tunable ultrafast ISC rate modulates the intensity ratio of the phosphorescence and fluorescence emission bands, with potential applications for white OLEDs.

Enhanced efficiency of organic photovoltaic cells with Sr2SiO4:Eu2+ and SrGa2S4:Eu2+ phosphors

We report the effect of yellow Sr2SiO4:Eu2+ and green SrGa2S4:Eu2+ phosphors on the efficiency of organic photovoltaic (OPV) cells. Each phosphor was coated on the back side of indium tin oxide (ITO)/glass substrates by spin coating with poly(methyl methacrylate) (PMMA). The maximum absorption wavelength of the active layer in the OPV cells was ∼512nm. The emission peaks of Sr2SiO4:Eu2+ and SrGa2S4:Eu2+ were maximized at 552nm and 534nm, respectively. The short circuit current density (Jsc) and power conversion efficiency (PCE) of the OPV cells with Sr2SiO4:Eu2+ (8.55mA/cm2 and 3.25%) and with SrGa2S4:Eu2+ (9.29mA/cm2 and 3.3%) were higher than those of the control device without phosphor (7.605mA/cm2 and 3.04%). We concluded that phosphor tuned the wavelength of the incident light to the absorption wavelength of the active layer, thus increasing the Jsc and PCE of the OPV cells.

Solution‐Processed Nickel Oxide Hole Transport Layers in High Efficiency Polymer Photovoltaic Cells

AbstractThe detailed characterization of solution‐derived nickel (II) oxide (NiO) hole‐transporting layer (HTL) films and their application in high efficiency organic photovoltaic (OPV) cells is reported. The NiO precursor solution is examined in situ to determine the chemical species present. Coordination complexes of monoethanolamine (MEA) with Ni in ethanol thermally decompose to form non‐stoichiometric NiO. Specifically, the [Ni(MEA)2(OAc)]+ ion is found to be the most prevalent species in the precursor solution. The defect‐induced Ni3+ ion, which is present in non‐stoichiometric NiO and signifies the p‐type conduction of NiO, as well as the dipolar nickel oxyhydroxide (NiOOH) species are confirmed using X‐ray photoelectron spectroscopy. Bulk heterojunction (BHJ) solar cells with a polymer/fullerene photoactive layer blend composed of poly‐dithienogermole‐thienopyrrolodione (pDTG‐TPD) and [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM) are fabricated using these solution‐processed NiO films. The resulting devices show an average power conversion efficiency (PCE) of 7.8%, which is a 15% improvement over devices utilizing a poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL. The enhancement is due to the optical resonance in the solar cell and the hydrophobicity of NiO, which promotes a more homogeneous donor/acceptor morphology in the active layer at the NiO/BHJ interface. Finally, devices incorporating NiO as a HTL are more stable in air than devices using PEDOT:PSS.

Efficient Organic Photovoltaic Cells Using MoO3 Hole-Transporting Layers Prepared by Simple Spin-Cast of Its Dispersion Solution in Methanol

MoO3 is one of the efficient hole-transporting materials for organic photovoltaic cells (OPVs). Here, a facile method of preparing the MoO3 buffer layer will be introduced. MoO3 powder was added into methanol and ultrasonication of the dispersion solution was carried out. Then the solution was centrifuged, and the supernatant was spin-cast on an indium–tin-oxide anode. On it, a photoconversion layer was prepared by spin-casting a poly(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) composite solution. The fabricated OPVs revealed an efficiency as high as 3.05%, which is better than that of P3HT:PCBM OPV with a poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) hole-transporting layer.

Conjugated polyelectrolyte and zinc oxide stacked structure as an interlayer in highly efficient and stable organic photovoltaic cells

A highly efficient and stable inverted organic photovoltaic cell has been demonstrated by incorporating a zinc oxide (ZnO)/conjugated polyelectrolyte (CPE) stacked structure as an electron-transporting layer. The CPE not only improved the electron collection efficiency in inverted devices but also smoothened the ZnO surfaces, thereby reducing the leakage current and improving the device reliability. In this work, two CPEs, poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) and poly[3-(6-trimethylammoniumhexyl)thiophene] (PTMAHT), are used, and three different interlayers, ZnO, ZnO/PFN, and ZnO/PTMAHT, are systematically studied. By coating PFN onto the ZnO surface, a high power conversion efficiency of 8.54% (with a fill factor of 73%) and ∼17% efficiency improvement are achieved in thieno[3,4-b]-thiophene/benzodithiophene copolymer, PTB7, and [6,6]-phenyl C71 butyric acid methyl ester bulk heterojunction solar cells, along with excellent stability.

Improvement of the power conversion efficiency of organic photovoltaic cells with a P3HT layer fabricated by using a sonication process and having a vertically modulated nanoscale morphology

In organic photovoltaic (OPV) cells, the nanoscale morphology of the organic layer is an essential parameter governing their charge-transport properties. Optimization of the vertical composition gradient in the poly(3-hexylthiophene) (P3HT) active layer in an OPV cell with an enhanced efficiency was achieved by using different sonication times of 10, 15, 20, 25, and 30min. Atomic force microscopy images confirmed that the sonication process increased the roughness of the P3HT layer in a vertically modulated nanoscale morphology. Photoluminescence spectra exhibited a strong peak corresponding to the P3HT particles. The power conversion efficiency of the OPV cells with a vertically modulated P3HT nanostructural layer sonicated at 15min was enhanced by 1.04% due to an increase in the interfacial region between the acceptor and the donor materials.

Factors Limiting Device Efficiency in Organic Photovoltaics

The power conversion efficiency of the most efficient organic photovoltaic (OPV) cells has recently increased to over 10%. To enable further increases, the factors limiting the device efficiency in OPV must be identified. In this review, the operational mechanism of OPV cells is explained and the detailed balance limit to photovoltaic energy conversion, as developed by Shockley and Queisser, is outlined. The various approaches that have been developed to estimate the maximum practically achievable efficiency in OPV are then discussed, based on empirical knowledge of organic semiconductor materials. Subsequently, approaches made to adapt the detailed balance theory to incorporate some of the fundamentally different processes in organic solar cells that originate from using a combination of two complementary, donor and acceptor, organic semiconductors using thermodynamic and kinetic approaches are described. The more empirical formulations to the efficiency limits provide estimates of 10-12%, but the more fundamental descriptions suggest limits of 20-24% to be reachable in single junctions, similar to the highest efficiencies obtained for crystalline silicon p-n junction solar cells. Closing this gap sets the stage for future materials research and development of OPV.

MoO3 Thickness, Thermal Annealing and Solvent Annealing Effects on Inverted and Direct Polymer Photovoltaic Solar Cells

Several parameters of the fabrication process of inverted polymer bulk heterojunction solar cells based on titanium oxide as an electron selective layer and molybdenum oxide as a hole selective layer were tested in order to achieve efficient organic photovoltaic solar cells. Thermal annealing treatment is a common process to achieve optimum morphology, but it proved to be damageable for the performance of this kind of inverted solar cells. We demonstrate using Auger analysis combined with argon etching that diffusion of species occurs from the MoO3/Ag top layers into the active layer upon thermal annealing. In order to achieve efficient devices, the morphology of the bulk heterojunction was then manipulated using the solvent annealing technique as an alternative to thermal annealing. The influence of the MoO3 thickness was studied on inverted, as well as direct, structure. It appeared that only 1 nm-thick MoO3 is enough to exhibit highly efficient devices (PCE = 3.8%) and that increasing the thickness up to 15 nm does not change the device performance.

The Effect of Acceptor Content on the Efficiency of Organic Photovoltaic Cells

Phase separation of the poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester(PCBM) active layer (ATL) was investigated by varying their relative ratio in the organic solar cells (OSCs).With the help of the UV/visible spectrophotometer, optical microscopy and scanning electron microscope,we found that the cluster of PCBM at the interface or surface was affected by Al cathode, the composition of the blends and thermal annealing. The disc-like shape crystals of PCBM substituted for the needle-like ones at higher PCBM compositions at the ATL/Al interface, which led to stronger contacts and bigger contact area. It could make short circuit current density increase, but may affect the blend morphology and result in parallel resistance and open circuit voltage decreased with the PCBM ratio increasing from 40 to 60%. The microstructure of the P3HT:PCBM ATL, determined by the composition dependent phase separation, supported the optimized performance of the OSCs with the composition of 40–50% PCBM.

Phase separation of co-evaporated ZnPc:C60 blend film for highly efficient organic photovoltaics

We demonstrate phase separation of co-evaporated zinc phthalocyanine (ZnPc) and fullerene (C60) for efficient organic photovoltaic cells. With introducing a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) film and a crystalline copper iodide film on indium tin oxide, 20-nm-thick ZnPc film adopts a lying-down crystalline geometry with grain sizes of about 50 nm. This surface distributed with strong interaction areas and weak interaction areas enables the selective growth of ZnPc and C60 molecules during following co-evaporation, which not only results in a phase separation but also improve the crystalline growth of C60. This blend film greatly enhances the efficiencies in photocurrent generation and carrier transport, resulting in a high power conversion efficiency of 4.56% under 1 sun.

Improving the efficiency of organic photovoltaic cells by introducing an ultrathin modification layer with a strong dipole moment

A method to improve the efficiency of organic photovoltaic cells through inclusion of an ultrathin modification layer of Al2O3 or LiF sandwiched between poly(3,4-ethylenedioxythiophene)-polystyrene sulfonic acid (PEDOT:PSS) and indium tin oxide layers is developed. Because of the strong dipole moments of LiF and Al2O3, either can enhance the built-in electric field, which increases the probability of the carriers reaching the corresponding electrode. In addition, the low work function of PEDOT:PSS can decrease the energy barrier for carrier transmission. A 21.7% improvement in the power conversion efficiency of experimental devices was achieved, mainly because the short circuit current was enhanced by almost 30%.

The Influence of the HOMO Energy Level of the Organic Electron Donor on the Effectiveness of the Anode Buffer Layer in Photovoltaic Solar Cells

Efficiency of organic photovoltaic cells, based on electron donor/electron acceptor junctions (ED/EA) could be strongly improved when the transparent conductive anode, is covered with a thin anode buffer layer (ABL) such as Au or MoO3. To study the influence of the HOMO (Highest Occupied Molecular Orbital) value of the organic material on the effectiveness of the anode buffer layer (ABL), different ED have been probed: pentacene, copper phthalocyanine (CuPc), tetraphenyldibenzoperiflanthene (DBP) and, poly(benzo[c]thiophene-2-oxide) with 5 eV, 5.2 eV, 5.5 eV and 5.52 eV as HOMO value respectively. The effects of a metal (Au) or an oxide (MoO3) ABL are reported. The results indicate that the improvement of the cells performance depends on the value of the ABL work function. A good matching between the work function of the anode and the highest occupied molecular orbital of the organic electron donor is the most important factor limiting the hole transfer efficiency. It is shown that the MoO3 oxide has a wider field of application as ABL than the Au metal.

High internal quantum efficiency in fullerene solar cells based on crosslinked polymer donor networks

The power conversion efficiency of organic photovoltaic cells depends crucially on the morphology of their donor–acceptor heterostructure. Although tremendous progress has been made to develop new materials that better cover the solar spectrum, this heterostructure is still formed by a primitive spontaneous demixing that is rather sensitive to processing and hence difficult to realize consistently over large areas. Here we report that the desired interpenetrating heterostructure with built-in phase contiguity can be fabricated by acceptor doping into a lightly crosslinked polymer donor network. The resultant nanotemplated network is highly reproducible and resilient to phase coarsening. For the regioregular poly(3-hexylthiophene):phenyl-C61-butyrate methyl ester donor–acceptor model system, we obtained 20% improvement in power conversion efficiency over conventional demixed biblend devices. We reached very high internal quantum efficiencies of up to 0.9 electron per photon at zero bias, over an unprecedentedly wide composition space. Detailed analysis of the power conversion, power absorbed and internal quantum efficiency landscapes reveals the separate contributions of optical interference and donor–acceptor morphology effects.

Enhanced charge separation in organic photovoltaic films doped with ferroelectric dipoles

A key requirement for realizing efficient organic photovoltaic (OPV) cells is the dissociation of photogenerated electron-hole pairs (singlet-excitons) in the donor polymer, and charge-transfer-excitons at the donor–acceptor interface. However, in modern OPVs, these excitons are typically not sufficiently harnessed due to their high binding energy. Here, we show that doping the OPV active-layers with a ferroelectric polymer leads to localized enhancements of electric field, which in turn leads to more efficient dissociation of singlet-excitons and charge-transfer-excitons. Bulk-heterojunction OPVs based on poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester are fabricated. Upon incorporating a ferroelectric polymer as additive in the active-layer, power conversion efficiencies increase by nearly 50%, and internal quantum efficiencies approach 100% – indicating complete exciton dissociation at certain photon energies. Similar enhancements in bilayer-heterojunctions, and direct influence of ferroelectric poling on device behavior show that improved dissociation is due to ferroelectric dipoles rather than any morphological change. Enhanced singlet-exciton dissociation is also revealed by photoluminescence lifetime measurements, and predicted by simulations using a numerical device model.

Development of New Fullerene-based Electron Acceptors for Efficient Organic Photovoltaic Cells

ABSTRACTThis article describes design of fullerene-based electron-accepting materials to obtain high performance in organic thin-film photovoltaic devices. A 1,4-bis(dimethylphenylsilylmethyl)[60]fullerene gives higher open circuit voltage than 1,2-diadduct because of smaller π-conjugated systems, and enables columnar fullerene-core array for high electron mobility and thermal crystallization for ideal phase separation with electron-donor materials. A 56π-electron fullerene derivative possessing the dihydromethano group as the smallest carbon addend does not disrupt fullerene-fullerene contact in solid state, giving high open-circuit voltage without decreasing of short-circuit current density and fill factor.