Efficient Organic Photovoltaic Cells Research Articles

Efficient Organic Photovoltaic Cells Based on Nanocrystalline Mixtures of Boron Subphthalocyanine Chloride and C60

AbstractThe electrical and structural behavior of uniformly mixed films of boron subphthalocyanine chloride (SubPc) and C60 and their performance in organic photovoltaic cells is explored. Device performance shows a strong dependence on active‐layer donor–acceptor composition, and peak efficiency is realized at 80 wt.% C60. The origin of this C60‐rich optimum composition is elucidated in terms of morphological changes in the active layer upon diluting SubPc with C60. While neat SubPc is found to be amorphous, mixed films containing 80 wt.% C60 show clear nanocrystalline domains of SubPc. Supporting electrical characterization indicates that this change in morphology coincides with an increase in the hole mobility of the SubPc:C60 mixture, with peak mobility observed at a composition of 80 wt.% C60. Organic photovoltaic cells constructed using this optimum SubPc:C60 ratio realize a power conversion efficiency of (3.7 ± 0.1)% under 100 mW cm−2 simulated AM1.5G solar illumination.

Highly Efficient PCDTBT:PC71 BM Based Photovoltaic Devices without Thermal Annealing Treatment

We propose an effective method to fabricate highly efficient organic photovoltaic cells based on poly [N-9"-heptadecanyl-2, 7-carbazole-alt-5,5-(4'7'-di-2-thienyl-2'1'3-b-enzothiadiazole):[6,6]-phenyl C71-butyric acid methyl ester (PCDTBT:PC71BM). A power conversion efficiency of as high as 5.6% and a fill factor of 53.7% are achieved from the optimized cells. The influence of surface morphology of the active layer on the performance of the cells is also investigated.

About the transparent electrode of the organic photovoltaic cells

Electrodes and the nature of their contact with organic materials play a crucial role in the realization of efficient optoelectronic components. Whether the injection (organic light-emitting diodes - OLEDs) or collection (organic photovoltaic cells - OPV cells) of carriers, contacts must be as efficient as possible. To do this, it is customary to refer to electrode surface treatment and/or using a buffer layer all things to optimize the contact. Efficiency of organic photovoltaic cells based on organic electron donor/organic electron acceptor junctions can be strongly improved when the transparent conductive anode is coated with a buffer layer (ABL). We show that an ultra-thin gold (0.5 nm) or a thin molybdenum oxide (3-5 nm) can be used as efficient ABL. However, the effects of these ABL depend on the highest occupied molecular orbital (HOMO) of different electron donors of the OPV cells. The results indicate that, in the case of metal ABL, a good matching between the work function of the anode and the highest occupied molecular orbital of the donor material is the major factor limiting the hole transfer efficiency. Indeed, gold is efficient as ABL only when the HOMO of the organic donor is close to its work function Phi(Au). MoO3 has a wider field of application as ABL than gold. The role of the oxide is not so clearly understood than that of Au, different models proposed to interpret the experimental results are discussed.

Study on the Output Characteristics of Organic Photovoltaic Cells

Based on the analysis of the equivalent circuit model of organic photovoltaic (OPV) cells, the explicit expression of current, short-circuit current and open-circuit voltage was obtained by means of W-function, and the effects of internal resistances and diode quality factor on the output characteristics of OPV cells were studied. The results demonstrate that the series resistance can effect short-circuit current but not open-circuit voltage, and the shunt resistance and diode quality factor can influence open-circuit voltage but short-circuit current unchanged. The decrease in series resistance and the increase in shunt resistance and diode quality factor can enhance the fill factor and energy conversion efficiency of OPV cells. This conclusion is essential in preparing OPV cells and improving device performance.

Doping-based control of the energetic structure of photovoltaic co-deposited films

Control of the energetic structure of photovoltaic co-deposited films consisting of fullerene and α-sexithiophene was demonstrated by ppm-level doping with molybdenum oxide (MoO3). The transition from an n-type Schottky junction via a metal/insulator/metal junction to a p-type Schottky junction by increasing the MoO3 doping concentration was verified by observing the photovoltaic properties. Direct ppm-level doping into photoactive co-deposited films could become a powerful tool for designing the appropriate built-in potential for efficient organic photovoltaic cells.

Enhanced efficiency of organic photovoltaic cells using solution-processed metal oxide as an anode buffer layer

We report performance improvement of organic (P3HT:PCBM) photovoltaic cell by introducing a solution-based WO x anode buffer layer, between the PH500 conducting polymer and the active layer. By introducing a solution-based WO x on PH500, the cell efficiency increases by 13% compared to that of the organic photovoltaic cell with only PH500. The organic cell exhibits J SC of 12.31 mA/cm 2, V OC of 0.61 V, FF of 58.22%, and the power conversion efficiency of 4.35%. The improved cell performance is due to effective hole collection injected from electron donors (P3HT) in P3HT:PCBM active layer by forming WO x nanoparticle on the conducting anode.

Flexible Ag electrode for use in organic photovoltaics

High efficiency organic photovoltaic cells discussed in literature are normally restricted to devices fabricated on glass substrates. This is a consequence of the extreme brittleness and inflexibility of the commonly used transparent conductive oxide electrode, indium tin oxide (ITO). This shortcoming of ITO along with other concerns such as increasing scarcity of indium, migration of indium to organic layer, etc. makes it imperative to move away from ITO. Here we demonstrate a highly flexible Ag electrode that possesses low sheet resistances even in ultra-thin layers. It retains its conductivity under severe bending stresses where ITO fails completely. A P3HT:PCBM blend organic solar cell fabricated on this highly flexible electrode gives an efficiency of 2.3%.

Phenanthrene-functionalized 3,6-dithiophen-2-yl-2,5- dihydropyrrolo[3,4–c]pyrrole-1,4-diones as donor molecules for solution-processed organic photovoltaic cells

3,6-Dithiophen-2-yl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP) based organic molecules and polymers have received strong attention as the donor materials of efficient organic photovoltaic cells (OPVs). This paper reports the synthesis and characterization of two DPP-based organic molecules functionalized with phenanthrene and ethynylphenanthrene and their application as the donor materials of solution-processed OPVs. They have band gaps of 1.6–1.8 eV and appropriate energy levels, and they can be soluble in common organic solvents such as chloroform and tetrahydrofuran. The C≡C triple bond in ethynylphenanthrene has a strong effect on the structure and properties of molecules. It increases the rigidity of molecule, decreases the solubility in organic solvents, and increases the crystallinity of the active layer and photovoltaic performance of the OPVs. The optimal photovoltaic efficiency of OPVs with ethynylphenanthrene-functionalized DPP is 1.71% under AM 1.5 G illumination (100 mW cm −2), more than double of that of the OPVs with phenanthrene-functionalized DPP.

Effect of ZnCdSe/ZnSe Core/Shell Quantum Dots on the Efficiency of Organic Photovoltaic Cells

Efficiency of Organic Photovoltaic Cells Kyoung Soon Choi, Yensil Park, Jae Soo Yoo,* Soo Young Kim, Heesun Yang, Gwan Ho Jung, and Jong-Lam Lee* School of Chemical Engineering and Materials Science, Chung-Ang University, Dongjak-gu, Seoul 156-756, Korea Department of Materials Science and Engineering, Hongik University, Seoul 121-791, Korea Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, Kyungbuk 790-784, Korea

Nanostructured donor/acceptor interfaces in photovoltaic cells using columnar-grain films of a cross-linked poly(fluorene-alt-bithiophene)

We report on the fabrication of photovoltaic cells, PVs, with controlled donor/acceptor interfaces using a process based on the phase separation between a cross-linkable polyfluorene and polystyrene. Robust, nanostructured columnar-grain layers of a conjugated cross-linked polymer, F8T2Ox1 (an oxetane-functionalized derivative of poly(9,9-dioctylfluorene-alt-bithiophene)) are obtained after removal of polystyrene. These layers are used, in combination with 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C61 (PCBM) deposited by spin coating, to define donor/acceptor interfaces, as PVs' active layers. The performance of these cells is dependent on the dimensions of the surface structures. In particular, a significant power conversion efficiency improvement is observed upon decrease of column diameter, reflecting an improvement of the exciton dissociation. We find, however, that these efficiencies still fall below those of the PVs based on blends of the same components, but are larger than the ones found for planar bilayer PVs. Furthermore, PVs based on blends of cross-linked F8T2Ox1 and PCBM showed enhanced efficiency and thermal stability with respect to PVs based on blends of PCBM and the non-cross-linkable analogue poly(9,9-dioctylfluorene-alt-bithiophene). Taking into account that the columnar-grain morphology is recognised as the “ideal” architecture for PVs' active layer provided the column radii are of the order of few nanometres, this work gives a new insight into how to achieve efficient organic photovoltaic cells through the use of cross-linkable conjugated polymers as the electron-donor component.

Effect of Morphology on Ultrafast Free Carrier Generation in Polythiophene:Fullerene Organic Solar Cells

Despite significant study, the precise mechanisms that dictate the efficiency of organic photovoltaic cells, such as charge separation and recombination, are still debated. Here, we directly observe efficient ultrafast free charge generation in the absence of field in annealed poly(3-hexylthiophene):methanofullerene (P3HT:PCBM). However, we find this process is much less efficient in unannealed and amorphous regiorandom blends, explaining the superior short-circuit current and fill-factor of annealed RR-P3HT:PCBM solar cells. We use transient optical spectroscopy in the visible and near-infrared spectral region covering, but not limited to, the previously unobserved and highly relevant time scale spanning 1 to 100 ns, to directly observe both geminate and nongeminate charge recombination. We find that exciton quenching leads directly (time scale less than 100 fs) to two populations: bound charges and free charges. The former do not lead to photocurrent in a photovoltaic cell; they recombine geminately within 2 ns and are a loss channel. However, the latter can be efficiently extracted in photovoltaic cells. Therefore, we find that the probability of ultrafast free charge formation after exciton quenching directly limits solar cell efficiency. This probability is low in disordered P3HT:PCBM blends but approaches unity in annealed blends.

Graded Donor‐Acceptor Heterojunctions for Efficient Organic Photovoltaic Cells

Graded Donor‐Acceptor Heterojunctions for Efficient Organic Photovoltaic Cells

Realization of efficient semitransparent organic photovoltaic cells with metallic top electrodes: utilizing the tunable absorption asymmetry

Efficient semitransparent organic photovoltaic (OPV) cells are presented in an inverted geometry employing ZnS/ Ag/ WO₃ (ZAW) as a top anode and ITO/ Cs₂CO₃ as a bottom cathode. Upon identification of the light absorption that differs depending on the illumination direction, the degree of the absorption asymmetry is tuned by varying the ZAW structure to maximize the efficiency for one direction or to balance it for both directions. Power conversion efficiency close to that of conventional opaque OPV cells is demonstrated in semitransparent cells for the ITO side illumination by taking advantage of the internal reflection occurring at the organic/ZAW interface. Cells with efficiencies that are reduced but balanced for both illumination directions are also demonstrated.

Improvement of efficiency in solar cells based on vertically grown copper phthalocyanine nanorods

We have fabricated efficient organic photovoltaic cells using copper phthalocyanine (CuPc) nanorods as donor and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as the acceptor material. Highly dense randomly oriented and vertically aligned nanorods with diameters of about 30–50 nm have been achieved from vacuum-deposited CuPc films by simple surface solvent treatment. X-ray diffraction confirms the polycrystalline nature of the CuPc nanorods. Significantly improved cell performance was observed with the change in shape and orientations of the nanorods. Maximum power conversion efficiency of 2.57% was obtained from the vertically aligned nanorods, which is a result of an increment in the donor–acceptor interface area and efficient photogenerated charge carrier transports.

Highly Efficient Organic Photovoltaic Cells with Molybdenum Oxide Buffer Layer

For improving the power conversion efficiency and fill factor of the organic photovoltaic cells (OPVs), molybdenum oxide (MoO3) can be used as a buffer layer. In this study, we fabricated OPVs by inserting a thin and transparent molybdenum oxide (MoO3) layer between the transparent indium tin oxide (ITO) and the organic photoactive layer, with the configuration of ITO/MoO3 (10 nm)/poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PC60BM)/lithium fluoride (LiF)/Al. By spin-coating, the device showed power conversion efficiencies as high as 4.2% under AM 1.5G (100 mW/cm2), enhanced fill factor of 0.67, a short circuit current density (Jsc) of 9.32 mA/cm2, and open-circuit voltage (Voc) of 0.67 V. In the case of flexible OPVs, device with 10-nm-thick MoO3 buffer or 45-nm-thick PEDOT:PSS buffer exhibited the power conversion efficiency (PCE) and fill factor (FF) of 1.54% and 0.44 or 0.76% and 0.30, respectively.

Improving Efficiency of Organic Photovoltaic Cells Using PEDOT:PSS and MWCNT Nanocomposites as a Hole Conducting Layer

In this study, polymeric nanocomposites of poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) and functionalized multi-walled carbon nanotubes (MWCNTs) were spin coated on a pre-patterned ITO glass and used as a hole conducting layer in organic photovoltaic cells. The multi-layered ITO/MWCNT-PEDOT:PSS/CuPc/C60/Al devices were fabricated to investigate the current density-voltage characteristics and power conversion efficiency. The power conversion efficiency obtained from the device with a concentration of 1.0 wt% MWCNT in the PEDOT:PSS layer was increased twice as those adopted from device without MWCNT doping in the PEDOT:PSS layer and current density-voltage characteristics was also improved well with incorporation of MWCNTs.

Magneto-optical studies of 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61; PCBM

In the quest for better power conversion efficiency of organic photovoltaic cells (OPVC) various architectures have been considered. Bulk heterojunction type OPVC proved to be the most successful, where the polymer as electron donor is blended with fullerene molecule as electron acceptor. With the goal of improving power conversion efficiency of OPVC we studied 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) as a film and dispersed in polystyrene matrix. We used a variety of continuous wave (cw) optical probes including photoinduced absorption (PA), photoluminescence (PL), and optically detected magnetic resonance (ODMR). The steady state photophysics of PCBM is dominated by triplet exciton. For both PCBM dispersed in polystyrene and PCBM film, the triplet exciton specie is characterized by a PA band at about 1.7 eV and spin 1 PA detected magnetic resonance (PADMR) powder pattern resonance around g ∼ 2. However, in PCBM film the occurrence of a polaron PA band at 1.2 eV and spin 1/2 PADMR are also noticed.

Efficient organic photovoltaic tandem cells with novel transparent conductive oxide interlayer and poly (3-hexylthiophene): Fullerene active layers

Efficient organic photovoltaic tandem cells consisting of P3HT:bis[60]PCBM and P3HT:[70]PCBM active layers and our original transparent conductive oxide interlayer are presented. The short circuit current and conversion efficiency are estimated to be more than 6 mA/cm 2 and 5% from simulations of devices with the thicknesses of both layers optimized. We clarify that actual devices fabricated utilizing nanoscale phase separation in the active layers and interlayer with indium tin oxide sandwiched by MoO 3 and LiF buffer layers exhibit characteristics almost the same as those simulated for the ideal tandem cell. This is attributed to improve the efficiencies of carrier transport in the active layers and recombination in interlayer.

Critical Interfaces in Organic Solar Cells and Their Influence on the Open-Circuit Voltage

Organic photovoltaics, which convert sunlight into electricity with thin films of organic semiconductors, have been the subject of active research over the past 20 years. The global energy challenge has greatly increased interest in this technology in recent years. Low-temperature processing of organic small molecules from the vapor phase or of polymers from solution can confer organic semiconductors with a critical advantage over inorganic photovoltaic materials since the high-temperature processing requirements of the latter limit the range of substrates on which they can be deposited. Unfortunately, despite significant advances, the power conversion efficiency of organic solar cells remains low, with maximum values in the range of 6%. A better understanding of the physical processes that determine the efficiency of organic photovoltaic cells is crucial to enhancing their competitiveness with other thin-film technologies. Maximum values for the photocurrent can be estimated from the light-harvesting capability of the individual molecules or polymers in the device. However, a better understanding of the materials-level processes, particularly those in layer-to-layer interfaces, that determine the open-circuit voltage (V(OC)) in organic solar cells is critical and remains the subject of active research. The conventional wisdom is to use organic semiconductors with smaller band gaps to harvest a larger portion of the solar spectrum. This method is not always an effective prescription for increasing efficiency: it ignores the fact that the value of V(OC) is generally decreased in devices employing materials with smaller band gaps, as is the case with inorganic semiconductors. In this Account, we discuss the influence of the different interfaces formed in organic multilayer photovoltaic devices on the value of V(OC); we use pentacene-C(60) solar cells as a model. In particular, we use top and bottom electrodes with different work function values, finding that V(OC) is nearly invariant. In contrast, studies on devices incorporating hole-transport layers with different ionization potentials confirm that the value of V(OC) depends largely on the relative energy levels of the donor and acceptor species that form the essential heterojunction. An analysis of the properties of solar cells using equivalent-circuit methods reveals that V(OC) is proportional to the logarithm of the ratio of the photocurrent density J(ph) divided by the reverse saturation current density J(0). Hence, an understanding of the physical origin of J(0) directly yields information on what limits V(OC). We assign the physical origin of J(0) to the thermal excitation of carriers from the donor to the acceptor materials that form the organic heterojunction. Finally, we show that the solution to achieving higher power conversion efficiency in organic solar cells will be to control simultaneously the energetics and the electronic coupling between the donor and acceptor materials, in both the ground and excited state.

High efficiency organic photovoltaic cells based on a vapor deposited squaraine donor

2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ) is used as a donor material in vapor deposited organic heterojunction photovoltaic cells. Devices with the structure indium tin oxide/SQ (x)∕C60 (400Å)/bathocuproine (100Å)∕Al (1000Å), where x=65, 110, 150, and 200Å were compared. Devices with x=65Å exhibited a power conversion efficiency of 3.1% under 1sun, AM1.5G simulated solar irradiation, giving an open circuit voltage of 0.76±0.01V, a short circuit current of 7.01±0.05mA∕cm2, and a fill factor of 0.56±0.05. Thicker SQ films lead to lower short circuit currents and fill factors, giving conversion efficiencies in the range of 2.6% to 3.2%. The demonstration of sublimable SQ as a donor material opens up a family of compounds for use in small molecule based heterojunction photovoltaics.