Andrea Pellegrino

From lab to fab: how must the polymer solar cell materials design change? – an industrial perspective

Organic photovoltaic (OPV) devices, in particular polymer solar cells, made by solution processed organic materials have shown great promise as a disruptive technology for affordable electricity. Even though recent advances look impressive on paper, until now the commercialization of OPV has been hampered by the difficulty of converting lab produced “champion” cell figures into reliable industrial-scale product performances. A key factor to achieve this condition is to develop OPV materials (polymer donors, acceptors, buffer materials, electrodes materials and encapsulants) exhibiting the required technical and economic characteristics to be conveniently used in an industrial environment. The well established strategies for the design of materials for efficient lab-scale OPV devices are not sufficient when large-area printed panels are concerned. A number of additional requirements, normally not addressed in the laboratory context, must be met: the materials must be easily accessible as pure compounds in few synthetic steps from cheap starting compounds, need to be stable and soluble enough to afford ink formulations processable with roll-to-roll compatible equipment; solvent and solvent additives should be easily removable after printing, and possibly should be environmentally friendly compounds; the layers should achieve a stable morphology under mild conditions (low temperatures and short times); the above mentioned materials can be screened on glass substrates, but should be finally tested on plastic films, protected through a scalable encapsulation technique. The more researchers adhere to these guidelines, the greater the possibility for OPV to demonstrate at last its enormous potential on the industrial scale.

Double acceptor D–A copolymers containing benzotriazole and benzothiadiazole units: chemical tailoring towards efficient photovoltaic properties

This study reports the design and synthesis of a series of D–A copolymers alternating benzothiadiazole and benzotriazole acceptors to a donor co-unit, and investigates their photovoltaic properties in bulk heterojunction solar cells with PC71BM. Successive modifications to the copolymers are carried out, passing from thiophene to benzodithiophene (BDT) donor co-units and from regular to random alternation of the accepting units. A copolymer containing a thiophene co-unit has reached a power conversion efficiency (PCE) of 1.88% in optimised devices. Moving from thiophene to BDT leads to some advantages, such as a lower optical energy gap and a lower confinement of the photo-generated charges, that positively affect the spectral coverage to solar radiation and the fill factor (FF) parameter in the devices. Passing from regular to random distribution of the accepting units, the BDT copolymer solar cells have reached a PCE of 5%. Such encouraging photovoltaic performances are combined with high solubility, high molecular weight and with an easy preparation procedure. These characteristics are necessary requirements to envisage the scale-up to industrial applications.

PBDTTPD for plastic solar cells via Pd(PPh3)4-catalyzed direct (hetero)arylation polymerization

Polymeric photovoltaics have attracted considerable academic and industrial interest over the last few years because of their unique features. However, they still suffer from some drawbacks which have prevented so far massive industrial production. These drawbacks include high costs related to the preparation of active materials and high toxicity of organo-tin compounds usually involved in synthetic routes. In this context, we report the synthesis of one of the most promising polymers for bulk heterojunction (BHJ) solar cells, PBDTTPD (poly[(benzo[1,2-b:4,5-b′]dithiophene)-alt-(4H-thieno[3,4-c]pyrrole-4,6(5H)-dione)]), via Direct (Hetero)Arylation Polymerization (DHAP) in the presence, for the first time, of a low-cost Pd-catalyst [Pd(PPh3)4]. The outcome of the polymerization process is discussed in comparison with the synthesis of the same polymer via the Stille polycondensation. The photovoltaic performances of the polymer synthesized via DHAP are compared with those of the same polymer obtained via the Stille coupling and a commercially available PBDTTPD. The polymer synthesized by Pd(PPh3)4-catalyzed DHAP blended with PC71BM exhibits a power conversion efficiency up to 5.3%, higher than that of the Stille reference polymer, and comparable with that of the commercially available PBDTTPD, under the same processing conditions. These results confirm the potential of DHAP as a convenient alternative for the synthesis of D–A copolymers for plastic solar cells.

Novel Terthiophene-Substituted Fullerene Derivatives as Easily Accessible Acceptor Molecules for Bulk-Heterojunction Polymer Solar Cells

Five fulleropyrrolidines and methanofullerenes, bearing one or two terthiophene moieties, have been prepared in a convenient way and well characterized. These novel fullerene derivatives are characterized by good solubility and by better harvesting of the solar radiation with respect to traditional PCBM. In addition, they have a relatively high LUMO level and a low band gap that can be easily tuned by an adequate design of the link between the fullerene and the terthiophene. Preliminary results show that they are potential acceptors for the creation of efficient bulk-heterojunction solar cells based on donor polymers containing thiophene units.

Design, Synthesis, Characterization and Use of Random Conjugated Copolymers for Optoelectronic Applications

We report the synthesis and the optoelectronic characterization of a new family of random conjugated copolymers based on 9, 9-bisalkylfluorene, thiophene and benzothiadiazole monomers unit synthesized by a palladium-catalyzed Suzuki cross-coupling reaction. The photophysical, thermal, electrochemical properties were investigated. The electronic structures of the copolymers were simulated via quantum chemical calculations. Bulk heterojunction solar cells based on these copolymers blended with fullerene, exhibited power conversion efficiency as high as 1% under illumination of 97 mWcm− 2. One of the synthesized copolymers has been successfully tested as active layer in simple light-emitting diode, working in the green spectral region and exhibiting promising optical and electrical properties. This study suggests that these random copolymers are versatile and are promising in a wide range of optoelectronic devices.