Effective Exciton Dissociation and Reduced Charge Recombination in Thick‐Film Organic Solar Cells via Incorporation of Insulating Polypropylene

Abstract

In organic solar cells (OSCs), the sensitivity of device performance to active layer thickness is a limiting factor for the large‐scale manufacture and roll‐to‐roll production. To reduce the thickness‐dependent effects, a low‐cost insulating polypropylene (PP) material is incorporated into the system with a high crystallinity donor, poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione))]:[6,6]‐phenyl C71 butyric acid methyl ester (PBDB‐T:PC71BM) and the system with a low crystallinity donor, poly[[4,8‐bis[(2ethylhexyl)oxy]‐benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl][3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]thieno‐[3,4b]thiophenediyl] (PTB7):PC71BM. The devices based on PBDB‐T:PC71BM:PP (2 wt%) show a power conversion efficiency (PCE) of 7.6% at a thickness of 280 nm compared with the control device with a PCE of 7% at a thickness of 100 nm. The change is mainly due to the enhancement of the crystallinity of donors with high crystallinity. PTB7:PC71BM with 4 wt% PP shows higher PCE than the control device for the same thickness (100–300 nm) and reduced PCE with increasing thickness. These are mainly due to the reduced PC71BM aggregation. The optimization of crystallinity and morphology will further promote effective exciton dissociation, accelerated charge transport, and reduced charge recombination. It indicates that the low cost and simple method of adding PP is a promising option for roll‐to‐roll production in the future.

Diluted Organic Semiconductors in Photovoltaics

“…[ 1–5 ] To facilitate a straightforward fabrication of OSCs and tune the morphology of the active layer, [ 6 ] solid insulator additives such as polystyrene (PS) are used as an additional component in minute quantities. [ 3,4,7–10 ] These solid additives remain in the blend film as an additional (third) component after drying unlike solvent additives such as 1,8‐diiodooctane (DIO). [ 6,8,11 ] In contrast, when an additional donor or acceptor polymer/molecule are chosen as an optoelectronically active third component in ternary blends, high concentrations (>10%) are typically used.…”

Recent Progress of Organic Solar Cells with Insulating Polymers

“…[16,34,35] In our previous work, we fabricated thicknessinsensitive OSCs by introducing 2-4 wt% polypropylene (PP) in photovoltaic systems with different crystallinities and quantitatively explored the corresponding nanomorphology via grazingincidence wide (small)-angle X-ray scattering GIW(S)AXS. [36] For poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b 0]dithiophene))-alt-(5,5-(10,3 0-di-2-thienyl-5 0,7 0-bis(2-ethylhexyl)benzo[10,2 0-c:4 0,5 0-c 0]dithiophene-4,8-dione))]: [6,6]phenyl C71 butyric acid methyl ester (PBDB-T:PC 71 BM) blend systems with relatively high crystallinities, the addition of 2 wt% PP reduced PC 71 BM aggregation size and improved PBDB-T crystallinity ( Figure 3). These results showed increased exciton dissociation efficiency and accelerated and more balanced charge transportation.…”

“…As a result, these strategies provide a novel view for optimizing charge transportation and fabrication of thickness‐insensitive OSCs. [ 36 ]…”

A Critical Review on Efficient Thick‐Film Organic Solar Cells

“…The charge recombination will become serious along with the increase in active layer thickness, which is mainly due to the low charge mobility of organic photoactive materials and unfavorable morphology in thick active layers. [ 35–38 ] The severe charge recombination in thick active layers will drastically decrease the FF and PCE of thick‐film OSCs. [ 39,40 ] An urgent scheme is needed to find ways to reduce charge recombination in thick active layers for achieving highly efficient thick‐film OSCs.…”