Characteristics Of Bulk Heterojunction Solar Cells Research Articles

Impact of Nonfullerene Acceptor Core Structure on the Photophysics and Efficiency of Polymer Solar Cells

Small-molecule “nonfullerene” acceptors are promising alternatives to fullerene (PC61/71BM) derivatives often used in bulk heterojunction (BHJ) organic solar cells; yet, the efficiency-limiting processes and their dependence on the acceptor structure are not clearly understood. Here, we investigate the impact of the acceptor core structure (cyclopenta-[2,1-b:3,4-b′]dithiophene (CDT) versus indacenodithiophene (IDTT)) of malononitrile (BM)-terminated acceptors, namely CDTBM and IDTTBM, on the photophysical characteristics of BHJ solar cells. Using PCE10 as donor polymer, the IDTT-based acceptor achieves power conversion efficiencies (8.4%) that are higher than those of the CDT-based acceptor (5.6%) because of a concurrent increase in short-circuit current and open-circuit voltage. Using (ultra)fast transient spectroscopy we demonstrate that reduced geminate recombination in PCE10:IDTTBM blends is the reason for the difference in short-circuit currents. External quantum efficiency measurements indicate that...

Incorporation of Gold Nanodots Into Poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonate) for an Efficient Anode Interfacial Layer for Improved Plasmonic Organic Photovoltaics

In this paper, we demonstrate a simple strategy for the incorporation of gold nanodots (GNDs) into poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) ( PSS) films, towards fabrication of an efficient anode interfacial layer in order to improve the performance of bulk heterojunction (BHJ) solar cells that use a blend of poly(3-hexyl thiophene) and [6,6]-phenyl-C71 butyric acid methyl ester as the organic active layer. We synthesized citrate-stabilized GNDs, with sizes in the range of -20-30 nm, by initially blending them into PSS by aqueous dispersion. The influence of GNDs in the PSS layer on the photovoltaic characteristics of BHJ solar cells was investigated. Our results show that the improved performance is due to the enhanced conductivity and increased interfacial contact area between the PSS and organic active layer, owing to the inclusion of GNDs into the PSS. The BHJ solar cell included with GNDs (0.02 wt%) into PSS exhibited a power conversion efficiency (PCE) of 2.92% with an open circuit voltage of 0.61 V, fill factor of 50%, and a short-circuit current density of 9.51 mA/cm2, whereas the pristine device exhibited a PCE of 2.52%.

Fast Screening of the Optimal Polymer Ratio for Organic Solar Cells Using a Spray-Coating Deposition Method for the Fullerene Mixture

Bulk-heterojunction organic solar cells generate electrical power from light absorption in the active layer and consist of a mixture of electron donor and acceptor materials. The donor/acceptor ratio is one of the critical parameters that determines the electrical characteristics of bulk-heterojunction solar cells. Therefore, the optimal ratio of the donor/acceptor must be determined to achieve maximum efficiency in bulk-heterojunction solar cells. In this work, modified spray- or mist-coating methods are applied to control the donor/acceptor ratio systematically and continuously. The feasibility of this processing method for polymer-ratio screening is investigated by monitoring the morphological and optical properties of polymer solar cells based on poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl C61 butyric acid methyl ester (PCBM). Finally, the optimum ratio of P3HT/PCBM is obtained by increasing the ratio of P3HT/PCBM gradually. This study provides a strong and practical screening method to define the critical mixture ratio among the enormous number of polymer–fullerene combinations.

The Importance of Fullerene Percolation in the Mixed Regions of Polymer–Fullerene Bulk Heterojunction Solar Cells

AbstractMost optimized donor‐acceptor (D‐A) polymer bulk heterojunction (BHJ) solar cells have active layers too thin to absorb greater than ∼80% of incident photons with energies above the polymer's band gap. If the thickness of these devices could be increased without sacrificing internal quantum efficiency, the device power conversion efficiency (PCE) could be significantly enhanced. We examine the device characteristics of BHJ solar cells based on poly(di(2‐ethylhexyloxy)benzo[1,2‐b:4,5‐b′]dithiophene‐co‐octylthieno[3,4‐c]pyrrole‐4,6‐dione) (PBDTTPD) and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) with 7.3% PCE and find that bimolecular recombination limits the active layer thickness of these devices. Thermal annealing does not mitigate these bimolecular recombination losses and drastically decreases the PCE of PBDTTPD BHJ solar cells. We characterize the morphology of these BHJs before and after thermal annealing and determine that thermal annealing drastically reduces the concentration of PCBM in the mixed regions, which consist of PCBM dispersed in the amorphous portions of PBDTTPD. Decreasing the concentration of PCBM may reduce the number of percolating electron transport pathways within these mixed regions and create morphological electron traps that enhance charge‐carrier recombination and limit device quantum efficiency. These findings suggest that (i) the concentration of PCBM in the mixed regions of polymer BHJs must be above the PCBM percolation threshold in order to attain high solar cell internal quantum efficiency, and (ii) novel processing techniques, which improve polymer hole mobility while maintaining PCBM percolation within the mixed regions, should be developed in order to limit bimolecular recombination losses in optically thick devices and maximize the PCE of polymer BHJ solar cells.