Monochromatic versus solar efficiencies of organic solar cells

The photovoltaic behavior in a perylene/phthalocyanine hetero-p/n-junction solar cell was investigated using intensity-dependent I/V-characteristics and short circuit photocurrent spectroscopy. It is concluded that the charge carrier generation occurs only in a very thin active region at the contact. By optimizing the light trapping, a maximum solar AM 1.5 efficiency of about 2% can be obtained. A further increase requires better material properties or new cell structures.     

A model to determine financial indicators for organic solar cells

Organic solar cells are an emerging photovoltaic technology that is inexpensive and easy to manufacture, despite low efficiency and stability. A model, named TEEOS (Technical and Economic Evaluator for Organic Solar), is presented that evaluates organic solar cells for various solar energy applications in different geographic locations, in terms of two financial indicators, payback period and net present value (NPV). TEEOS uses SMARTS2 software to estimate broadband (280-4000 nm) spectral irradiance data and with the use of a cloud modification factor, predicts hourly irradiation in the absence of actual broadband irradiance data, which is scarce for most urban locations. By using the avoided cost of electricity, annual savings are calculated which produce the financial indicators. It is hoped that these financial indicators can help guide certain technical decisions regarding the direction of research for organic solar cells, for example, increasing efficiency or increasing the absorptive wavelength range. A sample calculation using solar hats is shown to be uneconomical, but a good example of large-scale organic PV production.     

An improved method to estimate the equivalent circuit parameters in DSSCs

In this report, an improved method to estimate the equivalent circuit parameters in the dye-sensitized solar cells (DSSCs) is introduced. It is founded that, several different groups of values of equivalent circuit parameters can fit well to the same experiment-measured I-V curve. Furthermore, the gap between some parameter values in those different groups is so large that it reaches up to several orders of magnitude. To eliminate this uncertainty of parameter estimation, an improved method, which based on both the extention of measured range and the computer simulation of current-voltage (I-V) characteristic curves, is proposed to ensure uniquely the values of DSSCs equivalent circuit parameters. A series of I-V curves which derived from the estimated parameters by this improved method fit well to the corresponding experiment-measured I-V curves. The results indicate that, there exclusively exists one group of parameter values for a special DSSCs equivalent circuit, thus demonstrating the validity of the improved method proposed in this work.     

Vacuum-free processed transparent inverted organic solar cells with spray-coated PEDOT:PSS anode

Spray coating is a high throughput coating technique that is scalable and adaptable for organic photovoltaic manufacturing. To ensure uniform coating of the organic layers, the wettability, surface tension and boiling points of the solvents have to be optimized. Here, we used microscopic videos to understand the dynamics of the spray coating process. By optimizing the wettability and drying time of the PEDOT:PSS suspension on a hydrophobic surface, we attained a spray coated transparent anode without compromising on device performance. We further applied this vacuum-free process to a near infrared absorber to achieve a transparent organic solar cell with close to 60% transparency.     

A mathematical model for the anodic half cell of a dye-sensitised solar cell

We present a mathematical model of the steady-state current produced by the anodic half cell of a dye-sensitised solar cell (DSC) under both illuminated and non-illuminated conditions. A one-dimensional transport model that describes the transport of charged species via migration and diffusion within the electrolyte filled pores and the porous semiconductor that constitutes the porous anode of the DSC is given. This model is coupled to an interfacial model, developed previously by the authors, that describes charge transfer across the semiconductor–dye–electrolyte interface by explicitly accounting for each reaction at the interface involving dye molecules, electrolyte species, and semiconductor electrons. An equivalent circuit extension to the anode model (in the form of a boundary condition) is developed in order to validate some of the simulation results of the anode model with experimental results obtained from a full DSC specifically commissioned for the study. Parameter values associated with the model are obtained from the literature or experimentally from the specifically commissioned cell. A comparison of the numerical simulation results with experimental results shows a favourable correspondence without the need to fit parameter values.     

Space-charge-limited currents for organic solar cells optimisation

Basic suppositions and microphysical origin of the occurrence of the space-charge-limited currents (SCLC) are presented in general and for the temperature-modulated space-charge-limited currents (TM-SCLC) in particular. The criteria are given for the spectroscopical method TM-SCLC to be used for localized electron states elucidation in organic semiconducting materials for organic solar cells optimization and modelling. The “visibility “of the localized states by SCLC method, i.e. the power of the SCLC method to distinguish the localized states, is tested by the modelling, varying the temperature, energy position of localized states and their concentration. Generally, it was determined that the SCLC measurements results are more reliable with the increased energy of the states, with their increased concentration and with decreased temperature. The correlation (or its absence) between the measured current and activation energy on applied voltage, expressed by the dependence of preexponential factor of conductivity on activation energy (Meyer–Neldel rule), gives the possibility to determine the energy range where the reconstruction of density of localized states function is reliable.     

Degradation of organic solar cells due to air exposure

We present a study of dark air-exposure degradation of organic solar cells based on photoactive blends of the conjugated polymer, poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) with [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM). Photovoltaic devices were fabricated on indium tin oxide (ITO) glass with or without a layer of poly (3,4-ethylenedioxythiophene):poly(4-styrene sulfonate) (PEDOT:PSS), and were studied without encapsulation. Photovoltaic performance characteristics were measured as a function of time for different ambient conditions (under white light irradiation and in the dark, and under air, dry oxygen and humid nitrogen atmospheres). It was found that a key cause of degradation under air exposure is light independent and results from water adsorption by the hygroscopic PEDOT:PSS layer. Measurements of the charge mobility and hole injection after air exposure showed that the degradation increases the resistance of the PEDOT:PSS/blend layer interface.     

Progress in stability of organic solar cells exposed to air

In this paper, the stability of small-molecule organic solar cells based on copper phthalocyanine (CuPc) and fullerene (C₆₀) is investigated. The use of silver instead of aluminum as the metal electrode in these solar cells, with smaller grain size and grain boundaries as well as with more uniform grain size distribution in the film, results in significant improvement in the lifetime of the devices. The substantial role of silver in the protection of the cells against permeation of oxygen and/or water molecules into the organic thin films is confirmed. Substitution of a thin buffer layer (70 Å) of bathophenanthroline (Bphen) for bathocuproine (BCP), sandwiched between C₆₀ and the cathode, makes considerable progress in the lifetime of the device.     

Metal cluster enhanced organic solar cells

An enhancement of the photovoltaic conversion efficiency of an organic solar cell by incorporation of small metal clusters has been reported recently [1]. The enhancement is explained in terms of resonant light absorption in the metal cluster which is accompanied by a strengthened electric field in the vicinity of the particle. It is therefore assumed to be based on an enhanced absorption of the organic dye film. In contrast we will show here that an excited plasmon in a metal cluster is also capable to emit an electron directly in a preferential direction if the particles are placed inside an oriented electrical field like the one existing in the depletion layer of a Schottky junction. Thereby a primary photocurrent is observed in a spectral region without any direct absorption in the organic film. We will present results obtained at a Schottky junction formed at the interface of ITO and zinc phthalocyanine. In order to study the influence of the metal particles we evaporated a thin silver film on top of the ITO substrate and tempered the system in a vacuum, thereby forming small separated silver clusters. We investigated the influence of the silver clusters on the optical extinction spectra and on the short circuit photocurrent spectra of such constructed organic solar cells. The experimental data will be discussed using a qualitative energy diagram.     

Solution processable quinacridone based materials as acceptor for organic heterojunction solar cells

Two series of novel quinacridone (QA) based materials that combined a strong absorption over a broad range in visible region with good electrical characteristics, which were used as the new electron-accepting materials for organic solar cells, are explored. Unique cyclic compounds 1-6 are synthesized by incorporating electron withdrawing groups (CN, COOH) at carbonyl position of alkyl substituted quinacridones, which lead to the compounds possessing the characteristics of solution-processed and being suitable for photovoltaic applications. Heterojunction solar cells with simple device configuration using these soluble materials as acceptor and effective donor poly (3-hexyl thiophene) (P3HT) were fabricated. The maximum power conversion efficiency (PCE) achieved in the solar cell based on compound 5 is 0.42% under simulated AM 1.5 solar irradiation with J_sc=1.80 mA cm⁻², V_oc=0.50 V and FF=47%. Although the aimed devices just exhibit moderate PCE, our results clearly suggest that the new-type electron-accepting materials different from fullerene have great potential as acceptor in heterojunction solar cell due to many advantages of the QA derivatives such as relatively inexpensive, good electrochemical stability and could be readily modified.     

Synthesis and photovoltaic properties of soluble fulleropyrrolidine derivatives for organic solar cells

Organic solar cells made from bi-layer thin-film heterojunctions having poly((2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene)vinylene) (MEH-PPV) as an electron donor and fulleropyrrolidine derivatives as an electron acceptor were investigated. We synthesized soluble fulleropyrrolidine derivatives substituted different chain lengths for the organic solar cell. Due to the high solubility and sufficiently long chain length of fulleropyrrolidine derivatives, though those are monomers, a thin film (about 80–90 nm) could be fabricated individually by the spin-coating method. The fill factor of the bi-layer device was achieved to be 0.46, which is higher than that of the single-layer device by a polymer/fulleropyrrolidine derivative blend film of 0.37, due to the decrease of the recombination.     

Integration of an M-phthalocyanine layer into solution-processed organic photovoltaic cells for improved spectral coverage

Photovoltaic devices made from blended poly(3,3?-didodecylquaterthiophene) (PQT-12) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) incorporating an additional interlayer of M-phthalocyanine (M-Pc) have been characterized using current-voltage response, UV-visible absorption and external quantum efficiency. The introduction of H₂Pc, CuPc, ClInPc or TiOPc layers improves device performance compared to conventional bulk-heterojunction PQT-12:PCBM cells without M-Pc. Devices with M-Pc show increased absorption and free charge generation at longer wavelengths and have higher open circuit voltage. Polymorphic changes from solvent interaction are observed in TiOPc films during fabrication. Power conversion efficiencies of 0.79% are achieved for this modified bulk-heterojunction solar cell.     

Oxide/polymer interfaces for hybrid and organic solar cells: Anatase vs. Rutile TiO2

In this work, we study the effect of the transparent conducting oxide (TCO) and the polymer applied (MEH-PPV or P3HT) on the photovoltaic properties of TCO/TiO₂/polymer/Ag bi-layer solar cells. The solar cells were analyzed under inert atmosphere conditions resembling an encapsulated or sealed device. We demonstrate that the substrate applied, ITO or FTO, modifies the crystalline structure of the TiO₂: on an ITO substrate, TiO₂ is present in its anatase phase, on an FTO, the rutile phase predominates. Devices fabricated on an FTO, where the rutile phase is present, show better stability under inert atmospheres than devices fabricated on an ITO, anatase phase. With respect to the polymer, devices based on MEH-PPV show higher Voc (as high as 1 V), while the application of P3HT results in lower Voc, but higher J_sc and longer device stability. These observations have been associated to (a), the crystalline structure of TiO₂ and (b) to the form the polymer is bonded to the TiO₂ surface. In-situ IPCE analyses of P3HT-based solar cells show a red shift on the peak corresponding to TiO₂, which is not present on the MEH-PPV-based solar cells. The latter suggest that P3HT can be linked to the TiO₂ though the S-end atom, which results in devices with lower Voc. All these observations are also valid for devices, where the bare TiO₂ is replaced by an Nb-TiO₂. The application of an Nb-TiO₂ with rutile structure in these polymer/oxide solar cells is the reason for their higher stability under inert atmospheres. We conclude that the application of TiO₂ in its rutile phase is beneficial for long-term stability devices. Moreover there is an interplay between low Voc and J_sc in devices applying P3HT, since power conversion efficiency can be partially canceled by their lower Voc in comparison with MEH-PPV. These findings are important for polymer/oxide solar cells, but also for organic solar cells, where a layer of semiconductor oxides are in direct contact with a polymer, like in an inverted or tandem organic solar cells.     

Degradation mechanism of organic solar cells with aluminum cathode

Polymer-based solar cells with aluminum (Al) cathode often suffer from degradation in air. Here the study focuses on the degradation mechanism at the interface between Al and organic active layer. By performing interface modification combined with electrical and chemical characterization, it is demonstrated that the rapid degradation originates from the formation of a charge blocking layer between evaporated Al cathode and organic active layer. Insertion of a thin interfacial layer of thermally evaporated CrO_x between organic active layer and Al cathode can greatly improve the device stability. It is found that the CrO_x interfacial layer functions as a protective layer by stopping or minimizing penetration of thermally evaporated Al into the active layer to form a diffused organic-Al interface, which will then result in a large oxidized interfacial area upon air exposure.     

Evaluation of bis-dicyanovinyl short-chain conjugated systems as donor materials for organic solar cells

Three conjugated compounds based on carbazole, cyclopentadithiophene and dithienopyrrole substituted by branched alkyl chains have been synthesized by the Knoevenagel condensation of malonodinitrile with the appropriate dicarboxaldehyde. Electronic properties of the target compounds have been analyzed in solution by UV-vis absorption spectroscopy and cyclic voltammetry and their potentialities as a donor material in donor-acceptor heterojunction solar cells have been evaluated in bilayer devices involving both solution processed or vacuum deposited donor layers and thermally evaporated fullerene C₆₀ as an acceptor.     

Optimum two-dimensional short circuit collection efficiency in thin multicrystalline silicon solar cells with optical confinement

The two-dimensional short-circuit AM1.5 collection efficiency is studied in thin multicrystalline silicon solar cells with optical confinement. The collection efficiency is calculated by linking an optical analytical generation profile with the two-dimensional collection probability in pn junction solar cells. The calculations are carried out for variable grain boundary recombination velocity, cell thickness, grain width, diffusion length, and back surface recombination velocity. The role of optical confinement leading to a strong dependence of the collection efficiency on the cell thickness in very thin cells is confirmed. The optimum cell thickness for maximum collection efficiency increases in cells with low back reflection or poor back surface passivation. Also, the optimum thickness in very thin cells increases significantly with increasing the diffusion length. It is also found that the effect of grain boundary recombination is predominant if the cell thickness is larger than the diffusion length and if the diffusion length is larger than half the grain width, especially, in cells with unpassivated grain boundaries. On the other hand, back surface recombination dominates the response in cells with unpassivated back surface if the thickness is smaller than or comparable to the diffusion length.     

Abnormally high photocurrent of a degraded organic solar cell under chopped light

Photocurrent of an organic solar cell under chopped monochromatic light was studied at different stages, i.e. just after fabrication (without degradation) and after serious degradation. A large photocurrent under chopped light was observed for a seriously degraded device, unexpected from intuitionistic reckoning. This work demonstrates that both light bias and chopper frequency affect the photocurrent measurement of an organic solar cell, especially for a degraded device. The influence of light bias and chopper frequency is explained as the result of traps produced during the degradation of an organic solar cell. Thus measuring photocurrent under one sun light bias at low chopper frequency is suggested to more accurately determine the external quantum efficiency.     

Flexible, long-lived, large-area, organic solar cells

We report herein large area (>10 cm²), interconnected organic solar cell modules both on glass substrates as well as on flexible ultra-high barrier foils, reaching 1.5% and 0.5% overall power conversion efficiency under AM1.5 conditions. Series connection is described, as these modules consist of up to three cells. Using our flexible barrier material, a shelf lifetime of polythiophene-based solar cells of 6000 h could be realized. Furthermore, we compare the photovoltaic performance of efficient conjugated polymer:fullerene solar cell modules with established technologies. Under typical indoor-office lighting, our modules are competitive with these systems.     

Recent development of the inverted configuration organic solar cells

Recent years, the power conversion efficiency (PCE) of normal configuration organic solar cells (OSCs) has obtained rapid progress to reach more than 6% under standard illumination, which is reasonable value for the commercial criterion. More and more research attention has been paid on the stability and lifetime of OSCs. A novel structural OSCs with high work function metal or metal oxide as the top electrode and low work function metal as the bottom anode is proposed and named as inverted configuration OSCs. The inverted configuration OSCs with high work function metal as top cathode could improve OSCs's lifetime, i.e., protecting cells from the damage by oxygen and moisture in air. Furthermore, the inverted configuration OSCs is the appealing alternative to the conventional regular structure due to the inherent vertical phase separation in the polymer active layers and high stability or long device lifetime. The inverted configuration OSCs have not only achieved an impressive PCE of 4.4%, but also exhibited an exceptional device lifetime without encapsulation. In this review article, the recent developments and vital researches on the inverted configuration OSCs are summarized.     

Bulk heterojunction organic solar cells utilizing 1,4,8,11,15,18,22,25-octahexylphthalocyanine

Bulk heterojunction solar cells utilizing soluble phthalocyanine derivative, 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C6PcH₂) have been investigated. The active layer was fabricated by spin-coating the mixed solution of C6PcH₂ and 1-(3-methoxy-carbonyl)-propyl-1-1-phenyl-(6,6)C61 (PCBM). The photovoltaic properties of the solar cell with bulk heterojunction of C6PcH₂ and PCBM demonstrated the strong dependence of active layer thickness, and the optimized active layer thickness was clarified to be 120 nm. By inserting MoO₃ hole transport buffer layer between the positive electrode and active layer, the FF and energy conversion efficiency were improved to be 0.50 and 3.2%, respectively. The tandem organic thin-film solar cell has also been studied by utilizing active layer materials of C6PcH₂ and poly(3-hexylthiophene) and the interlayer of LiF/Al/MoO₃ structure, and a high V_oc of 1.27 V has been achieved.