PhD Candidate, Monash University
Studying new materials that will make organic solar cells cheaper and more efficient
Imagine a city powered completely by renewable energy. How is that power generated? If you live near the sea, it could be through off-shore wind farms, or through large-scale solar plants if you live near the desert. But what if you don’t live in close proximity to either? Up to 15% of electricity is lost in transmission between where power is generated and where it ends up being used. What if I told you scientists are working on a solution to that wasted energy – by developing new solar technology that can be integrated into existing infrastructure and will be so cleverly disguised, you won’t even realize it’s there.
Organic solar cells are inexpensive to produce, flexible and partially transparent, meaning they can be printed into unique shapes, similarly to how newspapers are produced. However, they currently show lower efficiencies and shorter lifetimes than commercial solar cells. This is where my PhD comes in – I’m studying new materials that are designed specifically to improve the efficiency and stability of organic solar cells. The biggest challenge when studying these materials is that the energy production takes place on very small length scales – on the order or a few tens of nanometers. This makes visualizing the physical landscape that charges must travel through tough. That’s where I rely heavily on synchrotrons, which are large particle accelerators that generate x-rays a million times brighter than the sun! Such bright light allows me to probe extremely small features in my films, giving me a greater understanding for how efficiently charges are able to first be generated and then extracted from the solar cell. Charges can only be extracted efficiently if they have clear pathways to the anode and cathodes.
Recently, I was able to measure three new materials that were all very similar, apart from one atom in each structure. Synchrotron data showed that one of the materials was much more structured than the other two, leading to efficient charge extraction through well defined pathways, however this material also exhibited the lowest cell efficiency – contrary to my initial hypothesis. Based on this information, I was able to perform additional lab-based measurements that confirmed that this highly ordered material also exhibits a much lower susceptibility to charge generation, explaining why it performed poorly. Keeping this in mind, we will be able to develop more efficient materials from the get-go, without the guess work!
Abstract: A new strategy for designing ternary solar cells is reported in this paper. A low-bandgap polymer named PTB7-Th and a high-bandgap polymer named PBDTTS-FTAZ sharing the same bulk ionization potential and interface positive integer charge transfer energy while featuring complementary absorption spectra are selected. They are used to fabricate efficient ternary solar cells, where the hole can be transported freely between the two donor polymers and collected by the electrode as in one broadband low bandgap polymer. Furthermore, the fullerene acceptor is chosen so that the energy of the positive integer charge transfer state of the two donor polymers is equal to the energy of negative integer charge transfer state of the fullerene, enabling enhanced dissociation of all polymer donor and fullerene acceptor excitons and suppressed bimolecular and trap assistant recombination. The two donor polymers feature good miscibility and energy transfer from high-bandgap polymer of PBDTTS-FTAZ to low-bandgap polymer of PTB7-Th, which contribute to enhanced performance of the ternary solar cell.
Pub.: 20 Jul '17, Pinned: 28 Aug '17
Abstract: Fullerene-free OSCs employing n-type small molecules or polymers as the acceptors have recently experienced a rapid rise with efficiencies exceeding 12%. Owing to the good optoelectronic and morphological tunabilities, non-fullerene acceptors exhibit great potential for realizing high-performance and practical OSCs. In this Review, recent exciting progress made in developing highly efficient non-fullerene acceptors is summarized, mainly correlating factors like absorption, energy loss and morphology of new materials to their correspondent photovoltaic performance.
Pub.: 25 Jul '17, Pinned: 28 Aug '17
Abstract: We investigated the electrical, optical, and structural properties of Al-doped ZnO (AZO)/Ag/AZO and indium gallium zinc oxide (IGZO)/Ag/IGZO multilayer films, and the performance of inverted organic solar cells (OSCs) fabricated with Sn-doped indium oxide (ITO), AZO/Ag/AZO, and IGZO/Ag/IGZO electrodes. Even though deposited at room temperature, AZO/Ag/AZO (36 nm/19 nm/36 nm) and IGZO/Ag/IGZO (39 nm/19 nm/39 nm) multilayer films showed sheet resistances of 4.1 and 4.2 Ω/sq, resistivities of 3.3 × 10−5 and 4.1 × 10−5 Ω cm, and transmittances of 93 and 88% at 550 nm, respectively. Figures of merit (FOMs) were obtained to be 99.9 and 49.9 × 10−3 Ω−1 for the AZO/Ag/AZO and IGZO/Ag/IGZO films, respectively. Inverted OSCs fabricated using the as-deposited AZO/Ag/AZO and IGZO/Ag/IGZO electrodes produced power conversion efficiencies (PCEs) of 2.4 and 2.6%, respectively, while that with high-temperature annealed ITO electrode gave a PCE of 3.2%.
Pub.: 24 Jul '17, Pinned: 28 Aug '17
Abstract: Rylene dimides are widely used as the building blocks for n-type semiconducting polymers due to the tunable electronic properties. To elucidate their potentials as the electron acceptors in all-polymer solar cells, systematic comparisons of the properties among the derivatives are necessary. Herein, we used perylene diimide (PDI), naphthalene diimide (NDI), and naphthodithiophene diimide (NDTI) with the same alkyl chains combined with dithienothiophene (DTT) unit to obtain three polymer acceptors PPDI-DTT, PNDI-DTT, and PNDTI-DTT, respectively. Light absorption, carrier mobility, film morphology, and molecular orientation were characterized and compared. The photovoltaic devices based on PPDI-DTT, PNDI-DTT, and PNDTI-DTT achieved power conversion efficiency (PCE) of 3.49, 2.50, and 5.57%, respectively, in combination with BDDT as the donor polymer. The high performance of PNDTI-DTT was attributed to the strong absorption profile in the near-infrared (NIR) region, high and balanced electron and hole mobilities, and the preferable face-on orientation for the polymer chains in the blend films. The results indicate that NDTI is a promising building block to construct n-type photovoltaic polymers, and higher photovoltaic performance is anticipated with the further development of novel NDTI-based polymers.
Pub.: 13 Apr '17, Pinned: 24 Aug '17
Abstract: Through the combination of central naphthalene diimide and terminal rhodanine or 1,3-indanedione functionalities, two new non-fullerene electron acceptors, coded as N3 and N4, were designed, synthesized and characterized. Both of the materials exhibited good solubility, thermal stability, and displayed energy levels matching those of the conventional and routinely used donor polymer poly(3-hexyl thiophene) (P3HT). A high power conversion efficiency of 4.76% was obtained in simple, solution-processable bulk-heterojunction devices (P3HT : N3 1 : 1.2) which is the best result for central NDI-based small molecular non-fullerene acceptors.
Pub.: 20 Jun '17, Pinned: 24 Aug '17
Abstract: Silicon (Si)-organic heterojunction solar cells suffer from a noticeable weakness of inefficient rear contact. To improve this rear contact quality, here, two solution-processed organic n-type donor-acceptor naphthalene diimide (NDI)-based conjugated polymers of N2200 and fluorinated analog F-N2200 are explored to reduce the contact resistance as well as to passivate Si surface. Both N2200 and F-N2200 exhibit high electron mobility due to its planar structure and strong intermolecular stacking, thus allow them to act as excellent transporting layers. Preferential orientation of the polymers leads to reduce contact resistance between Si and cathode aluminum (Al), which can enhance electron extraction. More importantly, the substitution of fluorine (F) atoms for hydrogen (H) atoms within the conjugated polymer can strengthen the intermolecular stacking, and improve the polymer-Si electronic contact due to the existence of F···H interactions. The power conversion efficiencies (PCE) of Si-PEDOT:PSS solar cells increased from 12.6% to 14.5% as a consequence of incorporating the F-N2200 polymer interlayers. Subsequently, in-depth density functional theory (DFT) simulations confirm that the polymer orientation plays a critical role on the polymer-Si contact quality. The success of NDI-based polymers indicates that planar conjugated polymer with a preferred orientation could be useful in developing high-performance solution-processed Si-organic heterojunction photovoltaic devices.
Pub.: 06 Jul '17, Pinned: 24 Aug '17