Bulk-heterojunction Cells Research Articles

Optically tunable intramolecular charge transfer dyes for vacuum deposited bulkheterojunction solar cells

We present the tunability of the photophysical and electrochemical properties of aseries of intramolecular charge transfer compounds by facile molecular design andsynthesis. The photovoltaic performances based on these sublimable materials andC60 bulk heterojunction cells are compared and reported. The structural modification of thecharge transfer dyes altered not only the electronic properties, but also the morphology ofthe bulk heterojunction thin films, as revealed by AFM and SEM studies. Addition ofPEDOT:PSS between the ITO and the photoactive layer improved the hole injection fromthe photosensitizer into the anode, and the overall power conversion efficiency is alsoenhanced.

Origin of open circuit voltage in planar and bulk heterojunction organic thin-film photovoltaics depending on doped transport layers

The aim of this article is to investigate the origin of the open circuit voltage (Voc) in organic heterojunction solar cells. The studied devices consist of buckminsterfullerene C60 as acceptor material and an oligophenyl-derivative 4,4′-bis-(N,N-diphenylamino)quaterphenyl (4P-TPD) as donor material. These photoactive materials are sandwiched between indium tin oxide and p-doped hole transport layers. Using two different p-doped hole transport layers, the built-in voltage of the solar cells is independently changed from the metal contacts. The influence of the built-in voltage on the Voc is investigated in bulk and planar heterojunctions. In bulk heterojunctions, in which doped transport layers border directly on the photoactive blend layer, Voc cannot exceed the built-in voltage significantly. Though, in planar heterojunctions, Voc is identical with the splitting of quasi-Fermi levels at the donor-acceptor interface and is thus primarily determined by the difference of the lowest unoccupied molecular orbital of C60 and the highest occupied molecular orbital of 4P-TPD. In planar heterojunctions, the open circuit voltage can exceed the built-in voltage. Furthermore, the investigations show that the efficiency of organic solar cells can be improved by using p-doped charge transport layers with optimized energy level alignment to the active materials. The optimized planar heterojunction shows a fill factor of up to 65.5% and a Voc of 0.95 V. For solar cells with insufficient energy level alignment between the photoactive layer system and the hole transport layer, a reduced Voc in bulk heterojunction cells and a characteristic S shape of the I-V characteristics in planar heterojunction cells are observed.

Study of organic solar cells with stacked bulk heterojunction structure

Organic solar cells with stacked bulk heterojunction(BHJ) are investigated based on conjugated polymer. By using the solution spin-coating method, Poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and ZnO nanoparticles (50 nm) are mixed as the optical sense layer. Ag is used as inter-layer to connect the upper BHJ cell and the lower cell. The structures are ITO/PEDOT:PSS/MEH-PPV /Ag / MEH-PPV:ZnO /Al. The open circuit voltage (VOC) of a stacked cell is about 3.7 times of that of an individual organic solar cell (ITO/PEDOT:PSS/MEH-PPV /Al). The short circuit current (JSC) of a stacked cell is increased by about 1.6 times of that of individual one.

Organic solar cells: Structure, materials, critical characteristics, and outlook

This review surveys recent advances in the field of photovoltaic devices based on organic photoactive materials and used for converting solar energy into electricity. Different architectures of organic photovoltaic devices are considered: bilayer, bulk heterojunction, and tandem cells. Major groups of organic semiconductors are described together with some numerical data on their performance in solar cells. Possible ways of improving the efficiency of organic solar cells are discussed. The bibliography consists of 80 references.

P3HT/ZnO bulk-heterojunction solar cell sensitized by a perylene derivative

In this work, a soluble perylene-derivative dye, N, N′-didodecyl-3,4,9,10-perylene tetracarboxylic diimide (PDI), was used to improve the photovoltaic performance of poly(3-hexylthiophene) (P3HT)/ZnO bulk heterojunction cells through blending with the composite. Results show that by incorporation of PDI in the P3HT/ZnO composite, the light absorption and exciton separation can be significantly improved. The photocurrent under white-light irradiation can be increased from 6.35 to 9.55 mA/cm 2. Solar decay experiment shows that V OC of the ITO/PEDOT:PSS/P3HT:ZnO:PDI/Al device decreases rapidly to almost zero in 1 h under persistent white-light illumination. After placing a 420 nm cutoff filter between the cell and the xenon lamp, the stability of the cell can be significantly improved. The device performance can maintain about 80% of the original value within 30 h and I SC degraded to zero after 142 h. The addition of PDI into the P3HT/ZnO device up to 5 wt% does not show observable effect on the solar cell decay behavior.

High-efficiency hole extraction/electron-blocking layer to replace poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) in bulk-heterojunction polymer solar cells

An anode interfacial layer is reported for bulk-heterojunction (BHJ) polymer solar cells to replace the commonly used poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). A poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB)+4,4′-bis[(p-trichlorosilylpropylphenyl)phenylamino]biphenyl (TPDSi2) blend is crosslinked, forming robust ∼10nm thick films covalently bound to indium tin oxide, which transport holes while blocking misdirected electrons. The thermal stability and photovoltaic performance metrics of TFB:TPDSi2-modified BHJ cells are significantly greater than those of cells fabricated in parallel with PEDOT:PSS or with no interfacial layer. For a poly[2-methoxy-5-(3′,7′-dimethyloctyloxyl]-1,4-phenylene vinylene: methanofullerene[6 6]-phenyl C61-butyric acid methyl ester cell, Voc=0.89V, Jsc=4.62mA∕cm2, FF=54.4%, and ηp=2.23%.

Controlled Assembly of Hybrid Bulk−Heterojunction Solar Cells by Sequential Deposition

This work presents a technique to create ordered and easily characterized hybrid nanocrystal-polymer composites by sequential deposition of tetrapod-shaped cadmium telluride (CdTe) nanocrystals and poly(3-hexlythiophene). With controlled fabrication and composite morphology, these devices offer several advantages over traditional co-deposited hybrid cells and provide a model system for detailed investigation into the operation of bulk-heterojunction cells.

Open circuit voltage of stacked bulk heterojunction organic solar cells

We investigate the open circuit voltage (VOC) of stacked bulk heterojunction (BHJ) organic solar cells based on conjugated polymer and fullerene derivative by spin-coating method. Poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) blended with [6, 6]-phenyl C61-butyric acid methyl ester (PCBM) is used as a photoactive layer. The structure is ITO(lower BHJ cell)PEDOT:PSS/MDMO-PPV:PCBM/ITO/(upper BHJ cell)PEDOT:PSS/MDMO-PPV:PCBM/Al. VOC of the stacked cell is 1.34V and is about 1.6 times as large as that of single BHJ organic solar cell. VOC of the stacked cell is not proportional to the number of photoactive layers, but is the sum of the VOC values of the upper and the lower BHJ cells.

Enhanced Efficiency and Stability in P3HT:PCBM Bulk Heterojunction Solar Cell by using TiO2 Hole Blocking Layer

ABSTRACTInsertion of TiO2 layer between Al negative electrode and active layer of bulk-heterojunction gave a high solar conversion efficiency and high stability. The TiO2 layer was prepared by spin-coating titanium(IV)isopropoxide (Ti(OC3H7)4) on active layer of blended P3HT:PCBM. The parallel resistance (Rp) is increased by inserting TiO2 layer leading to an increase in Voc. The TiO2 layer works as a hole blocking layer and prevents the physical and chemical damages by the direct contact between P3HT:PCBM active layer and Al electrode resulting in improvement of the parallel resistance and rectification. The efficiency of P3HT:PCBM bulk heterojunction cell with TiO2 hole blocking layer attained 4.05% with a Isc 9.72 mA/cm2, Voc 0.60 V, and FF 0.70 by using optimized TiO2 film thickness and ratio of P3HT:PCBM condition.

An effective medium model versus a network model for nanostructured solar cells

In this paper, two methods are compared to model the I–V curves of nanostructured solar cells. These cells consist of an interpenetrating network of an n-type transparent semiconductor oxide (e.g. TiO2) and a p-type semiconductor absorber (e.g. CdTe, CuInS2), deposited on TCO covered glass. The methods are also applicable when a dye and an electrolyte replace the p-semiconductor, and even to organic bulk heterojunction cells. A network model (NM) with resistors and diodes has been published by us before. Another method which has been proposed in the literature is an effective medium model (EMM). In this model, the whole p–n nanostructure is represented by one single effective semiconductor layer, which then is fed into a standard solar cell device simulator, e.g. SCAPS. In this work, it is shown that the NM and the EMM can describe the same physical structure, when they are set up properly. As an illustration, some problems are described both by EMM and NM, and the results are compared. The EMM in this work confirms the results obtained earlier with a simplified NM (constant Rn, Rp): when illuminating the n-side, the structure is tolerant to Rn but not to Rp; the interpenetrating geometry is disadvantageous for Voc. To cite this article: B. Minnaert et al., C. R. Chimie 9 (2006). Deux méthodes sont comparées pour modéliser les courbes I–V des cellules photovoltaïques nanostructurées. Ces cellules consistent en un réseau interpénétré constitué d'un oxyde semiconducteur transparent de type n (c'est-à-dire TiO2) et un semiconducteur de type p absorbeur (c'est-à-dire CdTe, CuInSe2) déposé sur un substrat verre conducteur. Les méthodes peuvent aussi être appliquées lorsqu'un colorant et un électrolyte remplacent le semiconducteur de type p ou encore pour des cellules organiques avec hétérojonction. Un modèle de réseau (NM) avec des diodes et des résistors a été publié préalablement par nous-même. Une autre méthode proposée dans la littérature est celle du modèle de milieu effectif (EMM). Dans ce modèle, la nanostructure est représentée dans sa totalité par une simple couche d'un semiconducteur effectif, qui est ensuite introduite dans un simulateur d'un dispositif standard de cellule photovoltaïque. Il est montré ici que les deux modèles NM et EMM peuvent décrire la même structure physique quand ils sont bien utilisés. Comme illustration, quelques problèmes sont décrits, tant par le modèle EMM que NM, et les résultats sont comparés. Le modèle EMM utilisé dans ce travail confirme les résultats obtenus auparavant avec un modèle NM simplifié (Rn et Rp constants) : lorsque la face de type n est éclairée, la structure est tolérante vis à vis de Rn mais non de Rp ; la géométrie interpénétrée est désavantageuse pour Voc. Pour citer cet article : B. Minnaert et al., C. R. Chimie 9 (2006).

Theoretical Investigations of Polymer Based Solar Cells

AbstractWe investigate the behavior of a polymer blend (M3EH-PPV:CN-ether-PPV) bulk heterojunction solar cell using a numeric model that self-consistently solves Poisson's equation and the charge continuity equation while incorporating electric field dependent mobilities. We obtain good quantitative agreement with present experimental data for J-V curves and photocurrent action spectra. To reproduce experimental photocurrent action spectra, our model predicts 36% exciton dissociation efficiencies in the bulk of the polymer. We also study the limiting conditions of polymer solar cell development by simulating an ideal solar cell using an AM1.5 global spectrum and assuming all absorbed photons hitting a M3EH-PPV:CN-ether-PPV polymer blend (band gap ∼2.0 eV) based solar cell at normal incidence contribute to current. If such a solar cell has 100 nm length, open circuit voltage=0.6 V and 50% fill factor, then the maximum theoretical power conversion efficiency is ηp=5.6%. A similar analysis for a M3EH-PPV:PCBM bulk heterojunction cell yields, ηp=3.5%. These results further highlight the need to develop smaller band gap materials and help explain why the best polymer based solar cells have power conversion efficiencies that remain stuck at about 3%. Our model is used to investigate the important increase in power conversion efficiencies we can expect as lower band gap polymers become available.

Excitonic Solar Cells

Existing types of solar cells may be divided into two distinct classes: conventional solar cells, such as silicon p−n junctions, and excitonic solar cells, XSCs. Most organic-based solar cells, including dye-sensitized solar cells, DSSCs, fall into the category of XSCs. In these cells, excitons are generated upon light absorption, and if not created directly at the heterointerface as in DSSCs, they must diffuse to it in order to photogenerate charge carriers. The distinguishing characteristic of XSCs is that charge carriers are generated and simultaneously separated across a heterointerface. In contrast, photogeneration of free electron−hole pairs occurs throughout the bulk semiconductor in conventional cells, and carrier separation upon their arrival at the junction is a subsequent process. This apparently minor mechanistic distinction results in fundamental differences in photovoltaic behavior. For example, the open circuit photovoltage Voc in conventional cells is limited to less than the magnitude of the band bending Øbi; however, Voc in XSCs is commonly greater than Øbi. Early work on solid-state excitonic solar cells is described as are excitonic processes in general and the use of carrier-selective (energy-selective) contacts to enhance Voc. Then studies of DSSCs, which provide a particularly simple example of XSCs, are described. A general theoretical description applicable to all solar cells is employed to quantify the differences between conventional and excitonic cells. The key difference is the dominant importance, in XSCs, of the photoinduced chemical potential energy gradient ∇μhν, which was created by the interfacial exciton dissociation process. Numerical simulations demonstrate the difference in photoconversion mechanism caused solely by changing the spatial distribution of the photogenerated carriers. Finally, the similarities and differences are explored between the three major types of XSCs: organic semiconductor cells with planar interfaces, bulk heterojunction cells, and DSSCs.