By Jeroen Van Duren
Plastic solar cells have attracted considerable attention in the past few years owing to their potential of providing environmentally safe, flexible, lightweight, inexpensive, efficient solar cells. Especially, bulk-heterojunction solar cells consisting of a mixture of a conjugated donor polymer with a methanofullerene acceptor sandwiched between two electrodes (Figure 1) are considered as a promising approach. Fullerene-based bulk-heterojunction solar cells reached power conversion efficiencies of >9%. Improving the performance, stability, and lifetime of bulk-heterojunction solar cells requires more insight in the preparation, and operation of these devices.
The current status of PV is that it hardly contributes to the energy market, because it is far too expensive. The large production costs for the silicon solar cells is one of the major obstacles. Even when the production costs could be reduced, large-scale production of the current silicon solar cells would be limited by the scarcity of some elements required, e.g. solar-grade silicon. To ensure a sustainable technology path for PV, efforts to reduce the costs of the current silicon technology need to be balanced with measures to create and sustain variety in PV technology. It is, therefore, clear that ‘technodiversity’, implying new solar cell technologies, is necessary.1) In the field of inorganics, thin-film technologies based on cheaper production processes are currently under investigation. Another approach is based on solar cells made of entirely new materials, conjugated polymers and molecules.
Conjugated materials are organics consisting of alternating single and double bonds. The field of electronics based on conjugated materials started in 1977 when Heeger, MacDiarmid, and Shirakawa discovered that the conductivity of the conjugated polymer polyacetylene (PA, Figure 2) can be increased by seven orders of magnitude upon oxidation with iodine2), for which they were awarded the Nobel Prize in Chemistry in 2000.3)-6) This discovery led, subsequently, to the discovery of electroluminescence in a Poly[p-Phenylene Vinylene] (PPV, Figure 2) by Burroughes et al. in 1990.7),8)
The cost reduction mainly results from the ease of processing from solution. Solution processing requires soluble polymers. Poly[p-Phenylene Vinylene] (PPV, Figure 2) is hardly soluble. Attachment of side-groups to the conjugated backbone, as in poly[2-methoxy-5-(3′,7′- dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV, Figure 2), enhances the solubility of the polymer enormously. Furthermore, the nanoscale morphology, affecting the opto-electronic properties of these polymer films, can be controlled by proper choice of the position and nature of these sidegroups. The development of regio-regular polythiophenes, such as regio-regular Poly[3-Hexylthiophene] (P3HT, Figure 2) showed the huge effect side-groups can have on charge-carrier mobility.9)
Bulk-heterojunction solar cells made from MDMO-PPV and PCBM reach power conversion efficiencies of >2.5% under simulated solar light. It is shown for the first time that replacing the orange MDMO-PPV with a low-bandgap conjugated material results in a more red-shifted spectral response of these solar cells, necessary for a larger overlap with the solar emission spectrum. Additionally, in an attempt to control the nanoscale morphology of the photoactive layer the first example of a covalently linked donor polymer with pendant fullerenes incorporated in working solar cells is reported. Unfortunately, both experiments resulted in a decrease of the power conversion efficiency compared to the mixture of MDMO-PPV and PCBM. These results indicated that more fundamental questions concerning the operation of the device and the influence of morphology must be addressed, before a rational improvement can be expected.
Morphology strongly influences transport of charge carriers in organic semiconductor films. First, films of the pure components have been investigated. It has been found that the zero-field electron mobility in pure PCBM (2×10-3 cm2/Vs) is 4000 times larger than the zero-field hole mobility in pure MDMO-PPV (5×10-7 cm2/Vs). Furthermore, electron transport through spin cast PCBM films can consistently be described by the Correlated Gaussian Disorder Model (CDM). This model is based on energetic and spatial disorder. Transmission Electron Microscopy (TEM) shows that the spin cast films of pure PCBM are homogeneous in appearance, whereas Selected-Area Electron Diffraction (SAED) indicates that the films consist of a large number of randomly oriented nanocrystals. The latter, therefore, confirms the disorder as inferred from the CDM-behavior.
A preference for straight and worm-like conformations is observed with Scanning Force Microscopy (SFM) for cast films of symmetrically substituted Poly(p-Phenylene Vinylene)s (PPVs), compared to more spiraling conformations for asymmetrically substituted PPV. For thin cast films of symmetrically substituted PPVs, the larger tendency to aggregate into ordered domains and, consequently, the larger zero-field hole mobility observed for symmetrically substituted PPVs seem to result from more linear conformations, as compared to asymmetrically substituted PPVs.
Subsequently, the morphology and performance of solar cells based on mixtures of MDMO-PPV and PCBM have been investigated. A combination of techniques is used to resolve the morphology of these spin cast films on a nanometer scale. These investigations clearly show that a rather homogeneous polymer matrix containing tiny PCBM crystals is present up to 50 weight percentages (wt.-%) PCBM. Phase separation resulting in large, separate domains of rather pure
These results are related to the performance of the corresponding solar cells. Electrical characterization, under illumination and in the dark, of the photovoltaic devices revealed a strong increase of the power conversion efficiency when the phase-separated network develops, with a sharp increase of the photocurrent and fill factor between 50 and 67 wt.-% PCBM. As the phase separation sets in, enhanced electron transport and a reduction of bimolecular charge recombination provide the conditions for improved performance.
Finally, the applicability of the full-depth analysis of bulk-heterojunction solar cells has been investigated. A variety of analysis techniques is used to study interface integrity, layer-to-layer diffusion, and contamination levels. Enrichment of oxygen at the interface between Al and the photoactive layer is observed. Furthermore, depth-profiling showed that during preparation diffusion of indium into the PEDOT:PSS occurs under the influence of water, while the diffusion of aluminum into the polymer layers is negligible. TEM of cross-sections, as prepared with a focused ion beam, of fully processed photovoltaic cells, provide a clear view of the individual layers and their interfaces.
The work as described in this article clearly shows the necessity for control over nanoscale morphology to guarantee percolation and suppress recombination losses in order to improve the power conversion efficiency. Synergy of material design and film deposition will prove to be of paramount importance to create excellent opto-electronic properties on the one hand, and an optimum balance between a large charge-generation interface and minimal recombination losses in the photoactive layer on the other.
In order to improve performance, stability, and lifetime of bulk-heterojunction solar cells, researchers are faced with the following challenges. First of all, the operation principle of bulk-heterojunction solar cells needs to be unraveled to guide the way for future material and device design. So far, mainly materials from the fields of LEDs and FETs have been used in organic solar cells. It is obvious that more materials dedicated to PV have to be developed. Appropriate design and synthesis will prove to be of paramount importance for maturity of the plastic solar cell. Furthermore, control of the nanoscale morphology needs to be improved. For the latter, synergy of material design and film deposition is required. Finally, the rich physics and chemistry at the interfaces of the multiple-layered device structures needs to be understood and controlled.
Jeroen Van Duren is working as Head of PV Strategy and Technology, Intermolecular (www.intermolecular.com) achieving world-class efficiencies for 2-Step Sulfur-free CIGSe. He has over 12 years of experience in thin-film photovoltaic research and development as well as manufacturing. He started his career in organic, bulk-heterojunction, thin-film PV with a Ph.D. program at the University of Technology Eindhoven after receiving his B.S. and M.S. in Chemical Engineering & Chemistry in Eindhoven in the Netherlands. In March 2004, he joined Nanosolar, the U.S.A., as a Solar Cell Engineer, to develop an innovative CIGS roll-to-roll printing technology, achieving world-record printed CIGS solar cell efficiencies.
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