A strategy to optimize photocurrent in organic photovoltaics by suppressing recombination loss

• Organic photovoltaics (OPVs), solar technologies based on organic semiconductors, have actually revealed excellent promise both for the scalable provision of clean energy and also for the development of solar-powered wearable electronics. Regardless of their benefits, organic solar cells suffer from the so-called recombination loss, a loss of photocurrent resulting from the recombination of photocarriers.

Researchers at City University of Hong Kong, Nanjing University, as well as other universities worldwide have actually just recently recommended a new tool engineering strategy that can aid to suppress recombination loss in OPVs. This approach, introduced in a paper published in Nature Energy, requires using sequentially refined, planar-mixed heterojunction (PMHJ) styles.

" Organic products have reduced dielectric constant compared to the inorganic semiconductors such as silicon, therefore bound electron-hole pairs known as 'excitons' are produced in organics upon illumination instead of conveniently separated free charge carriers," Dr. Francis Lin and Prof. Alex Jen, 2 of the scientists who executed the study, informed TechXplore.

" To conquer this issue, engineers usually adopt two organic products with inherently different electron affinities to create a nanoscale donor/acceptor network in the OPV energetic layer. Photogenerated excitons can after that move to the donor/acceptor interfaces to develop charge-transfer states as well as dissociate right into free electrons as well as openings."

Researches revealed that using 2 organic materials with intrinsically various electron fondness benefits exciton dissociation, however does not totally deal with the issues associated with organic solar cells. As a matter of fact, if free fee service providers are not efficiently accumulated by the electrodes and meet again at donor/acceptor user interfaces, they can recombine and also develop a so-called low-energy spin-triplet exciton (T1). This irreversible procedure sets a limit on the optimum attainable power conversion efficiency (PCE) of OPVs.

" Scientists in the field have actually come up with various means to deal with the T1-related losses issue, nevertheless, they are normally from a product point of view which develops new problems when material buildings are altered," Lin and also Jen discussed. "In our work, we establish a new approach that can assist to resolve this issue by transforming the basic engineering of OPV active layer architectures."

In their previous work, the researchers showed that the transport of excitons as well as cost service providers in sequentially refined OPV cells can be significantly enhanced as well as with it their efficiency. Ultimately, they developed the solar cells based upon D18, a new donor product with longer exciton diffusion length (> 30 nm) than a lot of the reported products (15-- 25 nm).

" We observed that the photocurrent from the sequentially refined planar-mixed heterojunction (PMHJ) cells are regularly greater than those standard one-step processed bulk heterojunction (BHJ) cells," Lin and also Jen discussed. "We additionally determined that sequentially processed PMHJ can lower T1 focus as well as reduce the associated recombination losses in OPVs."

In their new paper, the researchers laid out to determine tool engineering techniques, materials and also processing procedures that could enhance the efficiency of OPVs better. Particularly, they introduced different nanoscale morphologies in the active layer of organic solar cells, utilizing a sequentially refined PMHJ architecture instead of the standard BHJ architecture processed in a solitary step.

" We utilize state-of-the art products with longer exciton diffusion sizes to make PMHJ films, in which the donor and also acceptor show larger domain name size as well as more unique phase separation than in BHJ," Lin and also Jen clarified.

" As a result, our approach can still warrant reliable exciton dissociation while we have less donor: acceptor user interfaces in PMHJ contrasted to BHJ to allow much less possibilities totally free providers to recombine at these user interfaces. Simply put, we limit the opportunities for apart electrons as well as openings to reencounter with each other in OPVs with minimized donor: acceptor interfaces, for that reason much more chances for them to be collected by electrodes to generate electricity."

The researchers utilized their style to develop a series of solution-processed organic solar cells and then examined their efficiency in a series of experiments. They located that their PMHJ solar cells accomplished high power conversion efficiencies of 18-- 19%, surpassing that of bulk heterojunction designs, which generally exhibit efficiencies below 17%.

" Our exploration may alter people's understanding on the donor: acceptor user interfaces in OPVs," Lin and Jen stated. "These donor: acceptor interfaces are basically essential for OPVs, however, having a lot of these user interfaces could be damaging to cell efficiency. Our work recommends that we must better establish materials with longer exciton diffusion lengths to enable reduced donor: acceptor user interfaces in OPVs."

This team of scientists was the initial to show that the T1 concentration in tools made up of similar products can be merely altered with gadget engineering approach. In the future, their work could assist to make best use of the photocurrent in OPVs without weakening their photovoltage, possibly getting rid of the notorious trade-off in between photovoltage and photovoltage in OPVs.

" We look forward to seeing OPV items in the consumables market soon, and later grow into a larger scale with the deployment of building-integrated photovoltaics," Lin and also Jen said.

Moreover, the searchings for gathered by Lin, Jen and their colleagues recommend that the T1 personality in organic products is not straight related to OPV photovoltage. Their monitorings could thus additionally motivate further studies investigating the role and also photophysical processes of T1 in OPVs, which could help the discovery of better fit organic semiconductors for solar cells.

" We have reached quite high efficiency in OPVs up until now," Lin and also Jen added. "As a next step, we ought to improve the stability of OPVs as well as scale-up of both materials as well as tool sizes. The bigger domain name sizes and also range of phase separation in our PMHJ cells can naturally forbid the diffusion of particles under thermal stress and illumination. Moreover, consecutive handling gives more room to optimize the large-area tool manufacture procedures utilizing printing methods."