Tim Vangerven

The impact of precursor water content on solution-processed organometal halide perovskite films and solar cells

Perovskite solar cells are well known to degrade under post-fabrication stress, among others due to humidity as a consequence of the hydrophilic properties of the organic cation. On the other hand, it has been shown that the controlled addition of water molecules during the formation of the perovskite (while starting from water-free precursor materials) yields larger perovskite crystals with less defects, resulting in better device performance. One aspect still missing in this line of research is the water content of the perovskite precursors themselves: although most of them are prepared with anhydrous solvents as a precaution towards premature degradation, it is still unclear whether or not the precursors really need to be dry. In this paper, the impact of the perovskite precursors water content up to 10 vol% is investigated, in the form of a detailed study regarding the opto-electronic and morphological properties of the resulting films and devices. It is found that only modest changes occur in the films that do not affect the final photovoltaic performance, thus relaxing the conditions for large-scale production of this upcoming photovoltaic technology.

Fluorination as an effective tool to increase the open-circuit voltage and charge carrier mobility of organic solar cells based on poly(cyclopenta[2,1-b:3,4-b′]dithiophene-alt-quinoxaline) copolymers

The effect of fluorination on the optoelectronic properties and the polymer : fullerene solar cell characteristics of PCPDTQx-type (poly{4-(2′-ethylhexyl)-4-octyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-alt-2,3-bis[5′-(2′′-ethylhexyl)thiophen-2′-yl]quinoxaline}) low bandgap copolymers is reported. The introduction of fluorine atoms on the quinoxaline constituents is an effective way to lower the HOMO and LUMO energy levels of the alternating copolymers, resulting in an enhanced open-circuit voltage for the devices based on the fluorinated polymers (∼0.1 V per F added). Furthermore, fluorination also improves the charge carrier mobility in the bulk heterojunction blends. Despite the formation of unfavorable photoactive layer morphologies, the best solar cell performance is obtained for the copolymer prepared from the difluorinated quinoxaline monomer, affording a power conversion efficiency of 5.26% under AM 1.5G irradiation, with an open-circuit voltage of 0.83 V, a short-circuit current density of 11.58 mA cm−2 and a fill factor of 55%.

Absorption Tails of Donor:C60 Blends Provide Insight into Thermally Activated Charge-Transfer Processes and Polaron Relaxation

In disordered organic semiconductors, the transfer of a rather localized charge carrier from one site to another triggers a deformation of the molecular structure quantified by the intramolecular relaxation energy. A similar structural relaxation occurs upon population of intermolecular charge-transfer (CT) states formed at organic electron donor (D)-acceptor (A) interfaces. Weak CT absorption bands for D-A complexes occur at photon energies below the optical gaps of both the donors and the C60 acceptor as a result of optical transitions from the neutral ground state to the ionic CT state. In this work, we show that temperature-activated intramolecular vibrations of the ground state play a major role in determining the line shape of such CT absorption bands. This allows us to extract values for the relaxation energy related to the geometry change from neutral to ionic CT complexes. Experimental values for the relaxation energies of 20 D:C60 CT complexes correlate with values calculated within density functional theory. These results provide an experimental method for determining the polaron relaxation energy in solid-state organic D-A blends and show the importance of a reduced relaxation energy, which we introduce to characterize thermally activated CT processes.

A direct arylation approach towards efficient small molecule organic solar cells

Three extended molecular chromophores, differing in their central acceptor moiety and specifically designed as electron donor components for small molecule organic solar cells, are synthesized via a two-fold C–H arylation protocol. Upon removal of the side products inherent to the applied direct (hetero)arylation procedure, a record power conversion efficiency of 5.1% is achieved.

Impact of structure and homo-coupling of the central donor unit of small molecule organic semiconductors on solar cell performance

Currently, both low bandgap conjugated polymers and small molecule analogues are employed as electron donor components in state of the art bulk heterojunction organic photovoltaics, providing similar record efficiencies (∼10%). However, to evaluate molecular structure-device performance relations and (in particular) the effect of material purity, small molecule chromophores can be considered to be more versatile probes. In the present study, we have synthesized three small molecule donor materials with a varying central electron-rich building block, inspired by the well-known high-performance small molecule p-DTS(FBTTh2)2. The influence of this structural modification on the physicochemical material properties, electro-optical characteristics and solar cell performance is analysed. Most importantly, it is shown that the presence of homo-coupled side products generated during Stille cross-coupling reactions – which can be very hard to remove, even for small molecule semiconductors – is detrimental to solar cell performance, with a noticeable effect on the open-circuit voltage.