Advanced Functional Materials and Devices (AFMD) Group

Abstract:

A method for resolving the diffusion length of excitons and the extraction yield of charge carriers is presented based on the performance of organic bilayer solar cells and careful modeling. The technique uses a simultaneous variation of the absorber thickness and the excitation wavelength. Rigorously differing solar cell structures as well as independent photoluminescence quenching measurements give consistent results.

Abstract:

Optimization of the energy levels at the donor–acceptor interface of organic solar cells has driven their efficiencies to above 10%. However, further improvements towards efficiencies comparable with inorganic solar cells remain challenging because of high recombination losses, which empirically limit the open-circuit voltage (Voc) to typically less than 1 V. Here we show that this empirical limit can be overcome using non-fullerene acceptors blended with the low band gap polymer PffBT4T-2DT leading to efficiencies approaching 10% (9.95%). We achieve Voc up to 1.12 V, which corresponds to a loss of only Eg/q − Voc = 0.5 ± 0.01 V between the optical bandgap Eg of the polymer and Voc. This high Voc is shown to be associated with the achievement of remarkably low non-geminate and non-radiative recombination losses in these devices. Suppression of non-radiative recombination implies high external electroluminescence quantum efficiencies which are orders of magnitude higher than those of equivalent devices employing fullerene acceptors. Using the balance between reduced recombination losses and good photocurrent generation efficiencies achieved experimentally as a baseline for simulations of the efficiency potential of organic solar cells, we estimate that efficiencies of up to 20% are achievable if band gaps and fill factors are further optimized.

Abstract:

The design of novel nanostructures with tailored opto-electronic properties is a crucial step for third-generation photovoltaics, and the development of cheap and environmentally compatible materials is still a challenge. Carbon quantum dots (CQDs) emerged as promising candidates but usually a low processability and poor electron-donor properties hampered their photovoltaic applications. We tackle these issues through the synthesis and photophysical characterization of N-doped CQDs functionalized with different thiophene-containing groups. Functionalization was aimed at enhancing the electron donating properties of the carbon dots and improving the solubility in nonpolar solvents. The increased solubility in organic solvents allowed us to investigate the photoinduced interactions of the functionalized carbon dots with the fullerene derivative PCBM in solution and in solid blends. The investigation was carried out by cyclic voltammetry, photoluminescence spectroscopy and electron paramagnetic resonance (EPR). The remarkable oxidation potential shift of the functionalized carbon dots with respect to the pristine materials and the HOMO-LUMO energies strongly suggest them as good electron donors towards PCBM. The electron transfer process between CQDs and PCBM resulted in efficient fluorescence quenching in solution and in total quenching in solid blends. By using EPR spectroscopy in the solid blends, we demonstrated the efficient electron transfer by observing the photoinduced formation of a PCBM radical anion in the presence of functionalized CQDs. Time-resolved EPR allowed us to identify differences in the charge transport efficiency for different CQD:PCBM blends. The enhanced processability of CQDs with PCBM and the promising charge-generation and separation properties pave the way to the development of "all-carbon" photovoltaic devices.