Boosting the efficiency of silicon solar cells

The efficiency of a solar cell is one of its most important parameters.

It indicates what percentage of the solar energy radiated into the cell is converted into electrical energy. The theoretical limit for silicon solar cells is 29.3 percent due to physical material properties. In the journal Materials Horizons, researchers from Helmholtz-Zentrum Berlin (HZB) and international colleagues describe how this limit can be abolished. The trick: they incorporate layers of organic molecules into the solar cell. These layers utilise a quantum mechanical process known as singlet exciton fission to split certain energetic light (green and blue photons) in such a way that the electrical current of the solar cell can double in that energy range.

The principle of a solar cell is simple: per incident light particle (photon) a pair of charge carriers (exciton) consisting of a negative and a positive charge carrier (electron and hole) is generated. These two opposite charges can move freely in the semiconductor. When they reach the charge-selective electrical contacts, one only allows positive charges to pass through, the other only negative charges. A direct electrical current is therefore generated, which can be used by an external consumer.

>Read more on the BESSY II at Helmholtz-Zentrum Berlin website

Picture: Darstellung des Prinzips einer Silizium-Multiplikatorsolarzelle mit organischen Kristallen
Credit: M. Künsting/HZB

Stable solvent for solution-based electrical doping…

… of semiconducting polymer films and its application to organic solar cells.

Controlled and stable electrical doping of organic semiconductors is desirable for the realization of efficient organic photovoltaic (OPV) devices. Thus, progress has been made to understand the fundamental doping mechanisms.1-3 In 2016, Aizawa et al. reported the use of 12-molybdophosphoric acid hydrate (PMA) to induce p-type doping and crosslinking of neat films of poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)](PCDTBT).4 Later on, a more general approach of sequential solution-based doping was presented, by post-process immersion of donor-like polymer films in PMA-nitromethane solutions.5 However, critical to the method is the use of nitromethane, a highly unstable solvent, to dissolve PMA and thus limited the applicability to large-scale fabrication of organic solar cells.

A collaboration between a team of researchers from the Kippelen Research Group at Georgia Tech and the Toney Research Group at SSRL developed a solution-based doping method using the highly stable solvent, acetonitrile. Figure 1a displays the chemicals used in this work. In Figure 1b, the optical properties of poly(3-hexylthiophene-2,5-diyl)(P3HT) films immersed for 30 min in a 0.5 M solution of PMA in acetonitrile (PMA-im-P3HT) were studied by comparing their transmittance spectra against pristine P3HT and P3HT immersed similarly in a 0.5 M solution of PMA in nitromethane. The normalized change of transmittance ΔT T-1 as a function of wavelength (inset of Fig.1b) reveals the same spectral signatures reported for PMA-im-P3HT films when PMA was dissolved in nitromethane. That is, changes in the region where ΔT T-1< 0 correlate with the P3HT polaron bands, and deviations in the region where ΔT T-1> 0 correlate to the bleaching of the main π-π* absorption bands.6 The data suggests electrical p-doping into the depth of the organic film. Figure 1c shows that the performance of PMA-doped OPV devices using PMA in acetonitrile is comparable to that of OPVs made using PMA in nitromethane or MoO3, under simulated AM 1.5G solar illumination. Furthermore, if the light soaking mechanism is used before each measurement, OPVs made using PMA in nitromethane or acetonitrile remain stable for up to 524 h in the air, retaining 80% of their initial power conversion efficiency (PCE).

>Read more on the Standfort Synchrotron Radiation Lightsource website

Figure: (extract) of GIWAXS data as measured on pristine and PMA doped P3HT, when using various solvents to dissolve the PMA. a, Two-dimensional GIWAXS data converted to q-space for pristine P3HT and P3HT immersed in PMA solutions in nitromethane, acetonitrile or ethanol for 60 seconds. b, One-dimensional scattering profiles (out-of-plane and in-plane profiles), obtained from the two-dimensional GIWAXS data.

Rational optimization of organic solar-cell materials

Phase diagram leads the way to tailored metamaterial responses

Modifications to novel non-fullerene small molecule acceptor in organic thin film

… for solar cells demonstrates improved power conversion efficiency.

Scientists from the Imperial College London, Monash University, CSIRO, and King Abdullah University of Science and Technology have reported an organic thin film for solar cells with a non-fullerene small molecule acceptor that achieved a power conversion efficiency of just over 13 per cent.

By replacing phenylalkyl side chains in indacenodithieno[3,2-b]thiophene-based non-fullerene acceptor (ITIC) with simple linear chains to form C8-ITIC, they improved the photovoltaic performance of the material.

C8-ITIC was blended with a fluorinated analog of the donor polymer PBDB-T to form bulk-heterojunction thin films.

The research was recently published in Advanced Materials.

Dr Xuechen Jiao of McNeill Research Group at Monash University carried out grazing incidence wide angle X-ray scattering (GIWAXS) measurements at the Australian Synchrotron to gain morphological information on pure and blended thin films.

“By changing the chemical structure of the organic compound, a promising boost in efficiency was successfully achieved in an already high-performing organic solar cells” said Jiao.

>Read more on the Australian Synchrotron website

 

Perovskite solar cells: perfection not required!

Experiments at BESSY II reveal why even inhomogeneous perovskite films are highly functional

Metal-organic perovskite layers for solar cells are frequently fabricated using the spin coating technique. If you follow the simplest synthesis pathway and use industry-relevant compact substrates, the perovskite layers laid down by spin coating generally exhibit numerous holes, yet attain astonishingly high levels of efficiency. The reason that these holes do not lead to significant short circuits between the front and back contact and thus high-rate charge carrier recombination has now been discovered by a HZB team headed by Dr.-Ing. Marcus Bär in cooperation with the group headed by Prof. Henry Snaith (Oxford Univ.) at BESSY II.

>Read more on the HZB website.

Bing-Joe Hwang received National Chair Professorship from Ministry of Education

Exceptional award for this NSRRC User

The Ministry of Education recently announced the recipients of the 21st National Chair Professorships and the 61st Academic Awards. Prof. Bing-Joe Hwang, a long-term user of NSRRC, was given the National Chair Professorship in the category of Engineering and Applied Sciences. Prof. Hwang is a Chair Professor in Chemical Engineering at National Taiwan University of Science and Technology. He is also an adjunct scientist of NSRRC. His research interests include electrochemistry, nanomaterials, nanoscience, fuel cells, lithium ion batteries, solar cells, sensors, and interfacial phenomena.

 

Approved! The EU INFINITE-CELL project

A large EU-sponsored research project on tandem solar cells in which HZB is participating begins in November 2017.

The goal is to combine thin-film semiconductors made of silicon and kesterites into especially cost-effective tandem cells having efficiencies of over 20 per cent. Several large research institutions from Europe, Morocco, the Republic of South Africa, and Belarus will be working on the project, as well as two partners from industry.

“We not only have detailed experience with kesterite thin films, but also a wide spectrum of analytical methods at our disposal to characterise absorber materials very thoroughly”, explains Prof. Susan Schorr. The FUNDACIO INSTITUT DE RECERCA DE L’ENERGIA DE CATALUNYA (IREC), Spain – a long-term collaborating partner of the HZB, is coordinating the entire project. The project begins with a kick-off workshop in Brussels in November 2017.

HZB launches the HI-SCORE international research school in collaboration with Israel

The Helmholtz-Zentrum Berlin is establishing the Helmholtz International Research School HI-SCORE, which will be oriented towards solar energy research.

To accomplish this, HZB is collaborating with the Weizmann Institute in Rehovot, the Israeli Institute of Technology (Technion) in Haifa, and three Israeli universities as well as universities in Berlin and Potsdam. The project is being funded by the Helmholtz Association.

The name “HI-SCORE” stands for “Hybrid Integrated Systems for Conversion of Solar Energy”. The research themes extend from novel solar cells based on metal-organic perovskites, to tandem solar cells, to complex systems of materials for generating solar fuels. These complex materials systems can convert the energy of sunlight to chemical energy so it can be easily stored in the form of fuel.

Joining forces to advance perovskite solar cells

Great Interest in the HySPRINT Industry Day

No fewer than 70 participants attended the first Industry Day of the Helmholtz Innovation Lab HySPRINT devoted to the topic of perovskite solar cells at Helmholtz-Zentrum Berlin (HZB) on 13 October 2017. This far exceeded the expectations of the event hosts. The knowledge shared on Industry Day will serve as the basis for deepening the collaboration even further with strategically important companies in the scope of HySPRINT.

“Seeing the industry partners’ active participation was very gratifying. We could feel in the lively discussions how there is great interest on both sides to collaborate even more closely on technology transfer,” says Dr. Stefan Gall, project manager of the Helmholtz Innovation Lab HySPRINT (“Hybrid Silicon Perovskite Research, Integration & Novel Technologies”). On the Industry Day, eight companies presented those topics that especially interest them. “From this, certain problems emerged that we are now going to work on targetedly with our industrial partners.”