Efficient, Large Area, and Thick Junction Polymer Solar Cells with Balanced Mobilities and Low Defect Densities

Abstract

molecular bulk heterojunction (BHJ), or be arranged in alternating thin layers to create a planar (or linear) heterojunction. The most effi cient OSCs also contain electron/hole blocking and transport layers to facilitate Ohmic extraction at the relevant electrodes. Laboratory-scale power conversion effi ciencies (PCEs) in OSCs now exceed 10% in both solution processed and evaporated junctions, with predictions of >13% within the next 2 years. [3][4][5] However, these PCEs have yet to be translated to the module-scale, with best efforts being serial or parallel connected narrow-strip "minimodules" from IMEC at ≈5-6% (16 cm 2 ), Heliatek 7.7% (140 cm 2 ) and Toshiba 9% (25 cm 2 ). [6][7][8] The narrow-strip architecture is a consequence of the relatively high sheet resistances ( R sh = 10-15 Ω/square) of currently available transparent conducting electrodes: predominantly indium tin oxide (ITO) or fl uorine doped tin oxide (SnO:F). Jin et al. recently showed that the TCE sheet resistance is a dominant scaling parameter controlling cell fi ll factor (FF) for carrier collection path lengths greater than ≈1.0-1.5 cm ( R sh ≈ 10 Ω/square). [ 9 ] Devices with collection path lengths >1 cm in any dimension suffer dramatic loss in FF and short circuit current density which have fi rst order effects upon cell effi ciency. This is clearly a major issue limiting the creation of high effi ciency OSC modules with large active areas (so called monolithic architectures). Connected narrow-strip geometries impose manufacturing complexity and substantial additional cost, plus lead to loss of active area (versus substrate usage) due to the need for extensive interconnection.In this regard, attempts to improve TCE performance have focused on three key strategies: i) The use of very thin, semitransparent metal layers sandwiched between transparent extraction and refractive index matching layers. [10][11][12][13][14][15] The idea behind these insulator/metal/insulator (IMI) stack electrodes originates from low emissivity coatings for windows. [ 16,17 ] By changing the thickness of the individual layers the electrical and optical properties of the stack can be adjusted, allowing for tuning of the sheet resistance and the optical transmission of the electrode. Unfortunately, it is not yet possible to achieve a high conductivity (sheet resistance <5 Ω/square) and a broadband optical transmission from 400-1000 nm for effi cient The high power conversion effi ciencies (PCEs) of laboratory-scale polymerbased organic solar cells are yet to translate to large area modules because of a number of factors including the relatively large sheet resistance of available transparent conducting electrodes (TCEs), and the high defect densities associated with thin organic semiconductor junctions. The TCE problem limits device architectures to narrow connected strips (<1 cm) causing serious fabrication diffi culties and extra costs. Thin junctions are required because of poor charge transport (imbalanced mobilities) in the constituent organic semiconduct...

On the unipolarity of charge transport in methanofullerene diodes

“…Numerous publications have reported electron and hole mobilities of different organic solar cell BHJ systems vs. the fraction of electron acceptor (fullerene) in order to investigate the role of the electron-hole mobility ratio, 22 or optimize the composition with respect to power conversion efficiency. 23 For example, Tuladhar et al 12 reported that hole mobilities increased for a BHJ film, and postulated that hole transport in the fullerene could play a role in this observation.…”

Efficient Large Area Organic Solar Cells Processed by Blade‐Coating With Single‐Component Green Solvent

“…For example, the optimal materials and device processing conditions for small area devices have been proved inapplicable to high‐performance large area OSC modules. One main reason is that, in most OSCs, the active layer processed by spin coating in the lab showed low compatibility with large area high‐throughput coating techniques, such as roll‐to‐roll coating . Because the optimal thickness of most reported light absorption materials should be precisely controlled, even a small variation in thickness across the device area often leads to a sharp decrease in device performance .…”

A new multi-functional conjugated polymer for use in high-performance bulk heterojunction solar cells

“…[31][32] In conclusion, a novel multi-functional conjugated polymer that incorporated DTBDT as the donor unit and DTP as the acceptor unit, PDTP-DTBDT, was developed for use in PSCs. The incorporation of the DTBDT and DTP units into PDTP-DTBDT resulted in a large internal dipole moment and improved the intermolecular stacking structure in the film state.…”