Ultrafast Spectroscopy for Printable Photovoltaics

Abstract

The development of efficient and low cost photovoltaic technologies is key to a more sustainable energy pathway for future generations. Research efforts aimed at improving the performance of organic photovoltaic (OPV) materials have resulted in a continuous growth in power conversion efficiency (PCE) over time, with a recent maximum PCE value of 18.22% in a single bulk heterojunction device. However, further improved efficiency, stability and cost reduction are required in order for OPVs to succeed in the market.

To produce better performing OPV devices in a rational way, it is necessary to understand the relationships between material properties (e.g. energy levels, recombination rates, charge carrier mobilities) and the photovoltaic parameters. This requires combining different fundamental techniques, such as spectroscopic, electrical and structural studies of the materials. In this thesis work we contribute to the understanding of the mechanisms of charge photo-current generation in OPV layers by using transient absorption spectroscopy (TAS) to directly measure the fate of the photo-excited species created upon light absorption. In particular, we contribute to the understanding of the dynamical properties of tightly bound, interfacial charge-transfer (CT) states at the donor:acceptor heterojunction. We disentangle the contributions from individual transient species to the overall TAS signal via the soft-modelling algorithm known as Multivariate Curve Resolution by Alternating Least Squares (MCR-ALS), and we use simple kinetic models to retrieve associated kinetic rates.

Our first study explores the photo-physics of a family of polymers derived from the low-band-gap alternating copolymer PTBT where the sulphur atom in the thiadiazole unit was substituted with oxygen or selenium. The literature shows that replacing a single atom in the donor or acceptor unit of a polymer donor can cause large changes in the photovoltaic parameters, which cannot be explained considering only the variations in the optical band-gap. Opposite results have been reported on systems where a sulfur atom is replaced by selenium, and spectroscopic studies were lacking. Our TAS results on PTBO and PTBSe systems explain the superior photovoltaic performance of the original sulfur-containing variant PTBT, highlighting the low tolerance of these materials to backbone substitutions. In both PTBO and PTBSe systems, we identify strong recombination of geminate CT pairs as the major limiting factor of the Jsc and FF photovoltaic parameters. This is attributed to unfavourable electronic and conformational properties at the donor:acceptor interface. In the particular case of PTBSe:PC61BM, the recombination pathway of CT states with triplet character into the triplet exciton manifold is facilitated by the heavy atom effect, in addition to a highly intermixed morphology.

Our second study comprises the spectroscopic comparison between fullerene and nonfullerene (NFA) OPV layers. The PCE of OPV devices was reaching a plateau in past years, which was overcomed thanks to the development of high efficiency NFA acceptors. Here, we compare charge generation and recombination between three systems featuring the same polymer donor PPDT2FBT matched with three different acceptors, namely the fullerene acceptor PC70BM, the small molecule nonfullerene acceptor NIDCS-HO and the polymeric acceptor N2200. Our results provide insight on the processes that limit the performance of each device, showing that small molecule NFA are promising acceptors, since morphology and disorder, the factors that we have found to be limiting the device performance, could potentially be tuned for the development of more efficient materials. For the all-polymer device based on the N2200 acceptor, we find that both geminate and nongeminate recombination are limiting the photovoltaic performance.

Lastly, we investigate charge carrier dynamics in a series of solar devices composed predominantly of C60 and small amounts of organic small molecule donors, where their CT state energies are systematically varied. The well-defined microstructure in low-donor-content OPV blends makes it easier to correlate macroscopic properties to molecular parameters. Our results, in combination with time-delayed collection field (TDCF), and external quantum efficiency measurements (EQE) measurements at different bias performed by our collaborators, allow us to identify geminate recombination as the major loss channel. We find that the dynamics of the CT decay are connected to the CT state energy via the energy-gap law. In this way, the energy of the CT state is identified as the main parameter determining the efficiency of photocurrent generation in these morphologically well-defined donor:acceptor blends.

Overall, the contributions in this thesis work demonstrate how TAS measurements can provide valuable information to construct a comprehensive picture of the underpinning mechanisms of charge photo-current generation in OPV layers, in particular by isolating the dynamical properties of interfacial charge-transfer (CT) states at the donor:acceptor heterojunction via modelling.