High‐Efficiency, Vacuum‐Deposited, Small‐Molecule Organic Tandem and Triple‐Junction Photovoltaic Cells

Abstract

charge collection effi ciency while maintaining a high absorption, the PM-HJ also employs a neat (homogenous) donor and/ or acceptor layer, whose thicknesses are comparable to L D and that clad the mixed region.Unfortunately, such thick and highly absorbing sub-cells typically result in spectral overlaps between elements in the tandem that prevent photons from reaching the back sub-cell, ultimately limiting the total photocurrent. In this work, we overcome this defi ciency by using two relatively thick and strongly absorbing, vacuum-deposited small molecule PM-HJ sub-cells with considerable separation between their absorption maxima, thereby minimizing spectral overlap and maximizing photocurrent. [ 22 ] The front sub-cell adjacent to the transparent anode comprises the primarily orange-to-near infrared (NIR) absorbing donor, 2-((7-(5-(dip-tolylamino)thiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)methylene)malononitrile (DTDCTB) [23][24][25] blended with C 60 , paired with an ultraviolet-to-yellow absorbing DBP:C 70 back sub-cell. In the front cell, the C 60 intermolecular charge transfer (CT) absorption feature in the green is greatly reduced when diluted in DTDCTB, [ 26 ] thus providing a spectrally complementary system with the back sub-cell. The tandem solar cell broadly covers the solar spectrum from λ = 350 nm to 900 nm, achieving an effi ciency of 10.0 ± 0.2% under 1 sun, AM 1.5G illumination (25 ± 1 °C, 1000 W-m −2 , ASTM G173-03).To further improve the cell performance, a second DBP:C 70 sub-cell is placed immediately adjacent to the transparent anode at the second order optical interference maximum to form a triple-junction OPV. The cell exhibits a power conversion effi ciency of 11.1 ± 0.2%, with V OC of 2.58 ± 0.01 V and the external quantum effi ciency ( EQE ) close to 100% at short wavelengths. Further, this cell employs a thick stack structure that exploits the second order optical interference maximum. Together with a nearly optically lossless BPhen:C 60 electron fi ltering layer connecting the sub-cells, our design approach can lead to achieving even higher effi ciencies in the future.The low band gap DTDCTB absorbs at wavelengths as long as λ = 900 nm. To separate the front (i.e., that nearest the anode) and the back sub-cell (nearest the cathode) absorption spectra, the blue-green absorbing C 60 is used in the front cells as the acceptor, whereas the broadly absorbing C 70 is employed solely in the back sub-cell. The extinction coeffi cients ( k ) of the DTDCTB:C 60 fi lms measured as functions of volume ratio are shown in Figure 1 a. The absorption of a neat C 70 fi lm is also shown for comparison. The DTDCTB exhibits an absorption peak at λ = 700 nm, while C 60 shows two peaks at λ = 360 nm and λ = 450 nm, corresponding to Frenkel-type and intermolecular CT excitations. [ 27 ] The CT feature results from electrons excited from the highest occupied molecular orbital of one molecule to the lowest unoccupied molecular orbital of a nearby C 60 molecule , and hence is sensitive to C 60 concentration...

High‐Performance Ta2O5/Al‐Doped Ag Electrode for Resonant Light Harvesting in Efficient Organic Solar Cells

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Air-processed polymer tandem solar cells with power conversion efficiency exceeding 10%

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