Thermally stable organic bulk heterojunction photovoltaic cells incorporating an amorphous fullerene derivative as an electron acceptor

A highly soluble amorphous fullerene derivative substituted with dihexylfluorene (DHFCBM) was synthesized and used as an electron acceptor material for P3HT-based bulk heterojunction solar cells. By fitting the experimental J-V curves with space charge limited current equation, the electron mobility of DHFCBM was determined to be 4×10⁻⁴ cm²/Vs, possibly leading to balanced charge transport with P3HT. From structural and morphological analysis using X-ray diffraction, UV-vis absorption, and atomic force microscopy, we found that the amorphous nature of DHFCBM stabilized the nanomorphology of P3HT:DHFCBM blend films under high temperature annealing. By optimizing blend ratios and annealing conditions, P3HT:DHFCBM-based solar cells yielded power conversion efficiencies in excess of 3%. In addition, the fabricated cells maintained their initial performances even after high temperature annealing for long times, as predicted from the stable nanomorphology. We believe that the use of thermally stable amorphous fullerene as an electron acceptor can be a promising strategy for commercialization of organic solar cells.     

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.     

Anthraquinone dyes as photosensitizers for dye-sensitized solar cells

Three anthraquinone dyes with carboxylic acid as anchoring group are designed and synthesized as sensitizers for dye-sensitized solar cells (DSSCs). Preliminary photophysical and photoelectrochemical measurements show that these anthraquinone dyes have very low performance on DSSC applications, although they have broad and intense absorption spectra in the visible region (up to 800 nm). Transient absorption kinetics, fluorescence lifetime measurements and density functional theory (DFT) calculations are conducted to investigate the cause of such low DSSC performance for these dyes. The results show that the strong electron-withdrawing character of the two carbonyl groups on anthraquinone framework may lie behind the low performance by suppressing the efficient electron injection from the dye to the conduction band of TiO₂.     

Optimal energy offsets for organic solar cells containing a donor/acceptor pair

Optimal energy levels and offsets of an organic donor/acceptor binary-type solar cell have been analyzed using the classic Marcus electron transfer theory to identify the most efficient photo-induced charge separation. This study reveals that, an exciton quenching parameter (Y_eq) yields one optimal donor/acceptor energy offset where the photo-induced exciton is converted to charges most efficiently, and a recombination quenching parameter (Y_rq) yields a second optimal donor/acceptor energy offset where the ratio of charge separation rate constant over charge recombination rate constant becomes maximum. Another energy offset is also identified where charge recombination becomes most severe. This information would be useful for evaluating and fine tuning frontier energy levels of a donor/acceptor pair for optimum solar cell applications.     

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.     

Fluctuation model for p–n heterojunction solar cells

Influence of the roughness (microrelief) of an active interface in p–n junction solar cells (SC) on the photovoltage (the open-circuit voltage V_oc) has been studied. Nonuniformity of contact potential difference between p- and n-regions leads to barrier height fluctuation that are exponentially enhanced when dealing with barrier current. This results in some decrease of the V_oc value. Three theoretical models of averaging open-circuit voltage were used. Experimental results on p⁺-Al_xGa_1−xAs/p⁺-n-GaAs heterostructure SC with various microrelief, obtained by the anisotropic chemical etching, are compared with theoretical calculations.     

Photocarrier generation in organic thin-film solar cells with an organic heterojunction

Photocurrent–voltage characteristics for organic solar cells with a heterojunction formed between copper phthalocyanine and a perylene derivative (or C₆₀) were studied. The photocurrent was observed under both reverse and forward biases. From the analysis of the photocurrent action spectra, the origin of the reverse photocurrent was attributed to the excitons formed in both the organic layers, whereas that of the forward photocurrent was attributed to the excitons formed in the perylene derivative (or C₆₀) layer. The photocurrent density under reverse bias increased at higher temperatures, suggesting that the charge recombination possibility was lowered at higher temperatures. On the basis of the time responses of the photocurrents observed after pulsed photoirradiation, the charge separation and transport processes are discussed.     

Preparation of (n) a-Si: H/(p) c-Si heterojunction solar cells

Heterojunction solar cells have been manufactured by depositing n-type a-Si: H on p-type 1–2Ω cm CZ single crystalline silicon substrates. Although our cell structure is very simple - neither a BSF nor a surface texturing is used - a conversion efficiency of 13.1% has been achieved on an area of 1 cm². In this paper the technology is described and the dependence of the solar cell parameters on the properties of the n-type a-Si: H layer is discussed. It is shown that this cell type exhibits no degradation under light exposure.     

Determination of photo-active region in organic thin film solar cells with an organic heterojunction

The photo-active region in the solar cells consisting of Cu-phthalocyanine (CuPc) and perylene-derivative (PV) layers was determined by using exciton blocking layers (EBLs) inserted in these layers. The photocurrent density was low when the EBL was placed near the CuPc/PV interface. With the increase of the distance between the EBL and the CuPc/PV interface, the photocurrent increased. However, when the distance reached a certain value, it leveled off owing to the limited diffusion length of excitons. From the analysis of the relationship between the position of EBL and the photocurrent density, the photo-active regions in the CuPc and PV layers were estimated to be 8 and 12 nm thick from the interface, respectively.     

An improved solar cell circuit model for organic solar cells

The validity of conventional circuit model for interpreting results obtained using organic solar cells is examined. It is shown that the central assumption in the model that photo-generated current remains constant from short-circuit to open-circuit condition may not hold for organic cells. An improved model based on the photovoltaic response of organic solar cells is proposed and a method of extracting the parameters of the model is presented.     

Laser fired contacts applied to the rear surface of heterojunction silicon solar cells

In this work, we fabricate heterojunction silicon solar cells on p-type substrates whose rear surface configuration is based on dielectric passivation and laser fired contacts (LFC cells). This is an alternative to boron-doped amorphous silicon film, with which we also fabricate solar cells for direct comparison (HJ cells). As substrates, 3.5 and 0.8 Ω cm p-type double-side polished FZ c-Si wafers are used. Regarding surface passivation for highly doped substrates, LFC configuration has some advantage due to the higher difficulty in creating an efficient amorphous back surface field. Additionally, those substrates are also more advantageous in terms of carrier injection when the rear surface is locally contacted. Thus LFC cells made on 0.8 Ω cm substrates reach V_oc values up to 680 mV, in the same range as that of their HJ cell counterpart, with better FF demonstrating that LFC configuration is a feasible alternative for highly doped substrates. Focusing on the impact of the distance between rear contacts on cell performance, we found a trade-off between open circuit voltage V_oc and fill factor FF. Finally electroluminescence characterization and the dependence of V_oc on pitch, modeled by Fischer's equation, indicate that the depassivated area due to the laser processing of the contacts is bigger than the contacted area.     

Dye-based donor/acceptor solar cells

We have fabricated organic donor/acceptor solar cells with three different architectures using soluble derivatives (dyes) of the molecular semiconductors phthalocyanine as electron donor (D), i.e. hole transport material and perylene as electron acceptor (A), i.e. electron transport material. These architectures comprise a blend and a double layer structure as well as the only recently reported laminated-device structure. The organic semiconducting films were deposited at room temperature via spin coating from solution. Current–voltage (I/V) characteristics and external quantum efficiency spectra will be discussed. The measured quantum efficiencies reach values between 0.3% and 1.1% with a photoresponse covering the entire spectrum of visible light. Our results show that together with insoluble small molecules (pigments) and conjugated polymers, dye molecules represent a new class of organic semiconducting materials that can be used to manufacture D/A solar cells.     

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.     

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.     

Utilization of natural carotenoids as photosensitizers for dye-sensitized solar cells

The dye-sensitized solar cells (DSCs) were assembled by using natural carotenoids, crocetin (8,8′-diapocarotenedioic acid) and crocin (crocetin-di-gentiobioside), as sensitizers and their photoelectrochemical properties were investigated taking a presence or absence of carboxylic group in the dye molecule into consideration. In these carotenoids, crocetin that has carboxylic groups in the molecule can attach effectively to the surface of TiO₂ film so that it performed the best photosensitized effect resulting in the short-circuit photocurrent with 2.84 mA under irradiation of 1.0 cm². On the other hand, crocin that has no carboxylic group in the molecule showed lower photoelectrochemical performance because of its lower affinity to the surface of TiO₂ film. These results indicate that it is possible to apply carotenoid as sensitizers for DSCs at the presence of effective function groups.     

Crystal network formation in organic solar cells

We have studied the effects of annealing on performance and morphology of photovoltaic devices using blends of two organic semiconductors: a conjugated polymer and a soluble perylene derivative. The efficiency of such photovoltaic cells has been determined. The effect of temperature on blend morphology has been investigated for actual device films. Annealing leads to the formation of micron size perylene crystals and an enhancement of the quantum efficiency. This enhancement has been attributed to the formation of an electron conducting perylene crystal network.     

Stacked organic solar cells based on pentacene and

In order to improve photon harvesting, two small molecule organic solar cells are placed in series on top of each other. These stacked cells need an efficient recombination center in between both cells. In this study we test vacuum deposited metal layers as recombination centers with pentacene and buckminsterfullerene (C₆₀) as donor and acceptor, respectively. S-shaped curves are visible in the I–V characteristics when using thin layers of aluminum, indicating a barrier for extraction inside the device. Thin metal layers of gold or silver result in an increased open-circuit voltage without the appearance of these S-shaped features.     

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.     

Bulk heterojunction organic photovoltaic based on polythiophene–polyelectrolyte carbon nanotube composites

It is shown that carbon nanotubes can be used to enhance carrier mobility for efficient removal of the charges in thin film polymer-conjugated/fullerene photovoltaic devices. The fabricated photovoltaic devices consist of poly(3-octylthiophene) (P3OT) polymer blended with undoped multiwalled carbon nanotubes (MWNTs) and carbon nanotubes doped with nitrogen (CNx-MWNTs). Nanophase formation and dispersion problems associated with the use of carbon nanotubes in polymer devices were addressed through the generation of functional groups and electrostatic attaching of the polyelectrolyte poly(dimethyldiallylamine) chloride (PDDA) in both MWNTs and CNx-MWNT systems. The resultant nanophase was highly dispersed allowing for excellent bulk heterojunction formation. Our results indicate that CNx-MWNTs enhance the efficiency of P3OT solar cells in comparison with MWNTs.     

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.