Understanding Charge Transfer in Carbon Nanotube–Fullerene Bulk Heterojunctions

Abstract

Semiconducting single-walled carbon nanotube/fullerene bulk heterojunctions exhibit unique optoelectronic properties highly suitable for flexible, efficient, and robust photovoltaics and photodetectors. We investigate charge-transfer dynamics in inverted devices featuring a polyethylenimine-coated ZnO nanowire array infiltrated with these blends and find that trap-assisted recombination dominates transport within the blend and at the active layer/nanowire interface. We find that electrode modifiers suppress this recombination, leading to high performance.

Performance Enhancement of Polymer‐Free Carbon Nanotube Solar Cells via Transfer Matrix Modeling

“…The ideality factor was extracted from the slope of the dark current measurement (d d ( ) 1 V ln J − ) in the linear region of 0.28 to 0.38 V and 0.4 to 0.5 V, for the as-prepared and enriched (7 of 9) 1501345 wileyonlinelibrary.com (6,5) material, respectively. [ 38 ] Combining the increase in R SH and the signifi cantly smaller slope in the light-intensity-dependent V OC measurements indicates a marked reduction in trap-assisted recombination in solar cells made from the enriched (6,5) material, likely due to a reduction of the content of metallic SWCNTs. Having an ideality factor between 1 and 2 indicates recombination losses due to Shockley-Read-Hall (SRH) recombination, also called "trap charges," at or near the interface of the SWCNTs and C 60 .…”

“…The series resistance for the enriched (6,5) material is smaller but results only in a small increase in fi ll factor. [ 38 ] The increased slope for solar cells from as-prepared material is correlated with a reduction in V OC . [ 34 ] Reverse saturation current densities ( J 0 ) are on the same order for both materials.…”

Enhanced Uniformity and Area Scaling in Carbon Nanotube–Fullerene Bulk‐Heterojunction Solar Cells Enabled by Solvent Additives

“…The SWCNTs used in the devices presented in Figure 1 are purifi ed by density gradient ultracentrifugation (DGU). [ 9,[11][12][13]17 ] At these concentrations, SWCNTs act as the majority donor, and no signifi cant donor activity is observed for P3HT. [ 27 ] Figure 1 b shows the relative energy levels of the electrondonating SWCNTs and electron-accepting PC 71 BM.…”

“…[ 25,26 ] These SWCNTs have a diameter range of 0.8-1.2 nm, a broad chiral distribution enriched in the (8,7) chirality, and a semiconducting purity exceeding 97% as determined by optical absorbance spectroscopy. [ 17 ] Moreover, the high SWCNT:P3HT content allows the high stability and NIR absorption of SWCNTs to be realized in a thin-fi lm solar cell, [ 17 ] which further differentiates these active layers from the case where SWCNTs are only present in very low concentrations. Within the polychiral SWCNT distribution, smaller diameter (i.e., larger band gap) SWCNTs possess the desired type-II energy alignment for effi cient exciton separation, whereas larger diameter SWCNTs with narrower band gaps likely act as recombination sites.…”

“…[ 14,15 ] Despite these advances, SWCNT-fullerene solar cells have to date failed to demonstrate high performance over device areas larger than ≈1 mm 2 . [ 17 ] This recombination is in part due to the spatial inhomogeneities resulting from fullerene or SWCNT aggregates within the BHJ active layer. [ 15,16 ] Importantly, SWCNT-fullerene solar cells must maintain performance over areas on par with other TFPV technologies in order to exploit their exceptional absorption and chemical stability in hybrid or tandem photovoltaics, where an SWCNT-fullerene subcell would add value by harvesting NIR light and slowing degradation.…”