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Abstract
Charge separation and extraction dynamics were investigated in high-performance bulk heterojunction solar cells made from the polymer PTB7 and the soluble fullerene PC71BM on a broad time scale from subpicosecond to microseconds using ultrafast optical probing of carrier drift and the integral-mode photocurrent measurements. We show that the short circuit current is determined by the separation of charge pairs into free carriers, which is strongly influenced by blend composition. This separation is found to be efficient in fullerene-rich blends where a high electron mobility of >0.1 cm2 V–1 s–1 is observed in the first 10 ps after excitation. Morphology optimization using the solvent additive 1,8-diiodooctane (DIO) doubles the charge pair separation efficiency and the short-circuit current. Carrier extraction at low internal electric field is slightly faster from the cells prepared with DIO, which can reduce recombination losses and enhance a fill factor.
Highlights
The limited supply of fossil fuels and rising energy demands have encouraged research into renewable energy sources, with photovoltaics proving to be a suitable candidate
One of the most efficient blends is of the polymer PTB7 and the soluble fullerene PC71BM, and it achieves high efficiency using a high-boiling-point solvent additive 1,8-di-iodooctane (DIO) for processing the active layer.[4−9] The influence of DIO on the morphology of PTB7:PC71BM blends has previously been investigated in detail, which indicated that the blends prepared without additive show pure fullerene clusters of 20−60 nm in size which form large agglomerates embedded in a polymer-rich matrix containing about 30 wt % of fullerene.[10−13] The addition of DIO to the casting solvent improves the miscibility of PC71BM with PTB7, dramatically shrinks the size of the clusters to several nanometers, and forms interpenetrating polymer-rich and fullerene-rich phases of tens of nanometers in size.[10,12]
We demonstrate that the increase of photocurrent in devices prepared with DIO occurs because of a higher dissociation efficiency of photogenerated charge pairs into free carriers
Summary
The limited supply of fossil fuels and rising energy demands have encouraged research into renewable energy sources, with photovoltaics proving to be a suitable candidate. Interest in organic and hybrid solar cells has grown due to their ability to produce flexible lightweight devices with low manufacturing costs and an abundant supply of environmentally friendly materials. Small-scale single-junction organic solar cells achieve power conversion efficiencies over 10%.1−4 The best performance is achieved using a bulk heterojunction where an electron donor (usually a conjugated polymer) and an acceptor (usually a fullerene) are blended to obtain a large interfacial area for charge carrier generation. This is consistent with the power conversion efficiency enhancements occurring in both the polymer and fullerene absorption regions, suggesting that the improvement results from reduced carrier recombination.[10,12] The origin of reduced recombination is not known; possible explanations include improved charge separation, higher carrier mobility, or reduced charge trapping.[12,14−16] It is clear that a detailed understanding of the free carrier generation, transport, and extraction in this important high-performance blend is lacking
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