Role of Intrinsic Photogeneration in Single Layer and Bilayer Solar Cells with C₆₀ and PCBM

Abstract

In an endeavor to examine how optical excitation of C60 and PCBM contribute to the photogeneration of charge carriers in organic solar cells, we investigated stationary photogeneration in single-layer C60 and PCBM films over a broad spectrum as a function of the electric field. We find that intrinsic photogeneration starts at a photon energy of about 2.25 eV, i.e., about 0.4 eV above the first singlet excited state. It originates from charge transfer type states that can autoionize before relaxing to the lower-energy singlet S1 state, in the spirit of Onsager’s 1938 theory. We analyze the internal quantum efficiency as a function of electric field and photon energy to determine (1) the Coulombic binding and separation of the electron–hole pairs, (2) the value of the electrical gap, and (3) which fraction of photoexcitations can fully separate at a given photon energy. The latter depends on the coupling between the photogenerated charge transfer states and the eventual charge transporting states. It is by a factor of 3 lower in PCBM. Close to the threshold energy for intrinsic photoconduction (2.25 eV), the generating entity is a photogenerated electron–hole pair with roughly 2 nm separation. At higher photon energy, more expanded pairs are produced incoherently via thermalization.

Charge Generation and Recombination in an Organic Solar Cell with Low Energetic Offsets

“…Indeed, time-dependent density functional theory (TD-DFT) calculations on the PIPCP Frenkel exciton and the CT state at the interface with PC 61 BM show that for certain PIPCP/PC 61 BM molecular configurations the E CT is higher in energy than the PIPCP S 1 . [43] Importantly, however, charge generation in PIPCP:PC 61 BM is not field dependent (Figure 3a), and the field dependence of the photocurrent which is observed in the EQE spectra does not appear to depend on excitation wavelength, until excitation energies where the fullerene may be directly excited [46] (Figure 3b, Figure S4b,c, Supporting Information). The temperature dependence of the EQE spectra may also be interpreted as resulting from "hot-generation," where electronic excess energy from exciting to S n>1 can assist in charge generation.…”

Charge and Triplet Exciton Generation in Neat PC70BM Films and Hybrid CuSCN:PC70BM Solar Cells

“…[1] Although fullerenes have been used widely in OPV devices, their (photo)excited state properties have received relatively little attention up to now. [2,[7][8][9][10] In fact, these studies suggest that fullerenes not only play a role as electron acceptor, but also can contribute significantly to light harvesting in the photoactive layer. [2,[7][8][9][10] In fact, these studies suggest that fullerenes not only play a role as electron acceptor, but also can contribute significantly to light harvesting in the photoactive layer.…”

“…[2][3][4][5][6] However, the role of fullerenes as light absorber contributing to the photocurrent generation has gained increasing recognition as several recent reports demonstrate. [2,3,[6][7][8]11] For instance, Burkhard et al showed that free charge carriers can be generated upon light absorption by fullerenes. [2,3,[6][7][8]11] For instance, Burkhard et al showed that free charge carriers can be generated upon light absorption by fullerenes.…”

“…Interestingly, only if charge carriers and singlet states are present at the same time, triplet excitons are generated in neat fullerene films. [2,6,8,9] Furthermore, optimized CuSCN/ PC 70 BM bilayer devices (PC 70 BM thickness: 40 nm) showed a PCE of more than 1%, while optimized CuSCN:PC 70 BM blends yielded a PCE of 5.4% (see red and black curves in Figure 1). This is in stark contrast to previous reports that assigned triplet generation in neat fullerenes to intersystem crossing from singlet excitons.…”

“…This is in stark contrast to previous reports that assigned triplet generation in neat fullerenes to intersystem crossing from singlet excitons. [2,13] To understand this remarkable improvement in short-circuit current density (J SC ) and PCE, we employed picosecond-microsecond transient absorption spectroscopy on neat fullerene films and on CuSCN:PC 70 BM blends. We find that the fraction of charge carriers outweighs largely the population of triplet excitons in both blends and neat films.…”