No abstract

The conventional picture of photocurrent generation in organic solar cells involves photoexcitation of the electron donor, followed by electron transfer to the acceptor via an interfacial charge-transfer state (Channel I). It has been shown that the mirror-image process of acceptor photoexcitation leading to hole transfer to the donor is also an efficient means to generate photocurrent (Channel II). The donor and acceptor components may have overlapping or distinct absorption characteristics. Hence, different excitation wavelengths may preferentially activate one channel or the other, or indeed both. As such, the internal quantum efficiency (IQE) of the solar cell may likewise depend on the excitation wavelength. We show that several model high-efficiency organic solar cell blends, notably PCDTBT:PC70BM and PCPDTBT:PC60/70BM, exhibit flat IQEs across the visible spectrum, suggesting that charge generation is occurring either via a dominant single channel or via both channels but with comparable efficiencies. In contrast, blends of the narrow optical gap copolymer DPP-DTT with PC70BM show two distinct spectrally flat regions in their IQEs, consistent with the two channels operating at different efficiencies. The observed energy dependence of the IQE can be successfully modeled as two parallel photodiodes, each with its own energetics and exciton dynamics but both having the same extraction efficiency. Hence, an excitation-energy dependence of the IQE in this case can be explained as the interplay between two photocurrent-generating channels, without recourse to hot excitons or other exotic processes.

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...