The technology behind a large area array of flexible solar cells with a unique design and semitransparent blue appearance is presented. These modules are implemented in a solar tree installation at the German pavilion in the EXPO2015 in Milan/IT. The modules show power conversion efficiencies of 4.5% and are produced exclusively using standard printing techniques for large‐scale production.
Krebs et al. Scalable, ambient atmosphere roll-to-roll manufacture of encapsulated large area, fl exible organic tandem solar cell modules
The tandem concept involves stacking two or more cells with complementary absorption spectra in series or parallel connection, harvesting photons at the highest possible potential. It is strongly suggested that the roll‐to‐roll production of organic solar cells will employ the tandem concept to enhance the power conversion efficiency (PCE). However, due to the undeveloped deposition techniques, the challenges in ink formulation as well as the lack of commercially available high performance active materials, roll‐to‐roll fabrication of highly efficient organic tandem solar cells currently presents a major challenge. The reported high PCE values from lab‐scale spin‐coated devices are, of course, representative, but not helpful for commercialization. Here, organic tandem solar cells with exceptionally high fill factors and PCE values of 7.66% (on glass) and 5.56% (on flexible substrate), which are the highest values for the solution‐processed tandem solar cells fabricated by a mass‐production compatible coating technique under ambient conditions, are demonstrated. To predict the highest possible performance of tandem solar cells, optical simulation based on experimentally feasible values is performed. A maximum PCE of 21% is theoretically achievable for an organic tandem solar cell based on the optimized bandgaps and achieved fill factors.
COMMUNICATION 1091 wileyonlinelibrary.com www.MaterialsViews.com www.advenergymat.dePhotovoltaic cells [ 1 ] based on organic and polymer materials (OPVs) are celebrated as being a possible solution to the energy needs of the future, [ 1b ] and, with record effi ciencies having breached the 10% milestone accompanied by emerging involvement from the materials industry, expectations are rapidly approaching reality. [ 1c ] However, in order to move OPVs beyond the individual laboratory and into a generally applied setting, scientists need to limit the gap between carefully prepared hero devices and the large-scale manufacture of thousands of devices. We propose that this is done by urgently attending to the challenges of scalability and reproducibility. In this communication, we demonstrate an approach using round-robin testing as a method to validate effi ciency measurements of OPVs based on semitransparent electrodes on fl exible substrates, with and without indium tin oxide (ITO). ITO-free substrates were rollto-roll coated under ambient conditions and were truly scalable. Our results demonstrate inherent uncertainties in the device-effi ciency data, with variations in the carefully measured effi ciency data for the same device between highly qualifi ed laboratories as high as 25%, depending on the substrate and its active area.Thus the concrete needs of society impose a broadening of the scientifi c perspective, but also a requirement for the validation and verifi cation of reports. The view should thus be that it is no longer enough to report very high effi ciency unless several independent laboratories report it. In a few instances, laboratories have obtained certifi ed effi ciency data for record devices, but, in practical terms, this effi cient solution represents a bottleneck, as very few laboratories have the capacity to offer certifi cation, and it would not be possible to certify all efficiency reports with the currently available laboratories. Instead, concerns of the validity of effi ciency data and reproducibility should be tackled by employing interlaboratory studies and round robins, in an effort to gain a consensus and gradually approach standardization. [2][3][4] Regarding the important issue of scalability, the average effi ciency of many devices prepared by large-scale methods is unlikely to rival current record effi ciencies. It has been argued that it is no longer enough to present high effi ciencies, focusing on the decimals of the reported numbers, without seriously addressing which processes and which materials can reasonably be included when fabricating polymer solar cells on what will eventually be a GW p per day scale. To do this, considerations of both the economic and environmental impact should be made. [ 1b ] In this regard, a systematic approach to such considerations as, for example, offered by life-cycle assessments is very important at this point. [ 1 , 5-7 ] Such studies reveal the favorite transparent electrode material, ITO, as being the most-critical bottleneck for state-...
One inherent limitation to the efficiency of photovoltaic solar cells based on polymer/fullerene bulk heterojunctions (BHJs) is the accumulation of positive charges at the anodic interface. The unsymmetrical charge collection of holes and electrons dramatically decreases the short-circuit current. Interfacial layers (IFLs) such as poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) have no effect on the unbalanced electron/hole transport across the BHJ. We report here on the use of dithiapyrannylidenes (DITPY), a new class of planar quinoid compounds, as efficient hole-transporting/electron-blocking layers in organic solar cells based on poly(3-hexylthiophene)/[6,6]-phenyl-C(61)-butyric acid methyl ester (P3HT:PCBM) BHJs. Inserting a 15-nm-thick IFL of 4,4'-bis(diphenyl-2,6-thiapyrannylidene) (DITPY-Ph(4)) between the indium-tin oxide electrode and the P3HT:PCBM BHJ prevents detrimental space-charge effects and favors recombination-limited currents. Current-sensing atomic force microscopy reveals a drastic increase of the hole-carrying pathways in DITPY-Ph(4) compared to PEDOT:PSS. In ambient conditions, photovoltaic cells using DITPY-Ph(4) exhibit an 8% increase in the current density, although the conversion efficiency remains slightly lower compared to PEDOT:PSS-based devices. Finally, we present a detailed analysis of the photocurrent generation, showing that DITPY-Ph(4) IFLs induce a transition from unproductive space-charge-limited currents to recombination-limited currents.
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