Role of intrinsic photogeneration in single layer and bilayer solar cells with C 60

This chapter focuses on the examination of photogeneration in organic solar cells. The dissociation of excitons into free charges is usually considered to be located at the donor-acceptor interface and no other contributions are included. However, exciton dissociation can occur in the donor layer and in the acceptor layer as well. In normal low bandgap polymers, this intrinsic contribution to the external quantum efficiency is low, that means that an acceptor is needed anyway for an effective dissociation. In contrast, fullerenes show a high intrinsic dissociation. Thus, the intrinsic contribution of the acceptor materials C60 and PCBM to the photocurrent of organic solar cells is investigated in this work. We used single and bilayer geometries as they allow the basic study about the evaluation of the acceptor contribution. For the investigation of the dissociation behaviour in the acceptor materials, the intrinsic contribution from the applied donor materials should be insignificant. The novel PCDTBT derivative with and additional triphenyldiamine comonomer PCDTBT0.7/TPDDTBT0.3 exhibits such a negligible intrinsic dissociation. This polymer is referred to as PCDTBTco in this chapter. In

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addition, the small donor molecule Ph-TDPP-Ph was compared to the donor polymer. We investigated the correlation between the photogeneration in the fullerene acceptors and the excitation energy as well as the consequences for the efficiency of the solar cells.

In Figure 47, the dependence of the external quantum efficiency of single layer cells from C60

and PCBM on the excitation energy is shown. The intrinsic photogeneration in both acceptor materials is initiated at a photon energy of about 2.25 eV. In the low-energy range below 2.25 eV, the generated exciton is located on only one fullerene molecule and is tightly bound.

Thus, no contribution to the external quantum efficiency can be observed in this region. In contrast, excitation energies higher than 2.25 eV result in charge-transfer states that are delocalized over two fullerene molecules. This threshold value lies 0.4 eV higher than the first singlet excited state S1. The generated charge-transfer states in the acceptor material are short-lived and can either autoionize or relax to the S1 state consistent with the original Onsager theory. Therefore, the intrinsic dissociation yield increases with increasing excitation energy.

Figure 47: EQE spectra of single layer devices from C60 and PCBM in dependence on the excitation energy and schematic illustration of the generated states. The dotted line indicates the threshold value at about 2.25 eV for the intrinsic dissociation in the fullerene acceptors. Reprinted from Chapter 6.

The effect of the intrinsic contribution to the photocurrent of the C60 acceptor on organic bilayer solar cells is presented in Figure 48. Theexternal quantum efficiency is depicted in dependence of the internal electric field of the solar cell. By this method, the dissociation behaviour of C60

can be evaluated. As donor materials, the small molecule Ph-TDPP-Ph and the low bandgap copolymer PCDTBTco were used. The bilayer solar cells were excited at two photon energies. As the threshold for the intrinsic contribution of C60 lies at 2.25 eV, the first excitation is conducted at 2.14 eV which is below this threshold. Both donor materials show a high absorption at this wavelength. In consequence, the photocurrent at this excitation energy arises from the donor-acceptor interface. The second excitation is fixed at 2.94 eV for the small molecule donor and at 3.35 eV for the donor polymer. These energies exceed the threshold energy for the intrinsic dissociation of charge-transfer states within the bulk of C60 and both donor and the C60 acceptor absorb at these wavelengths.

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Figure 48: a) Chemical structures of the small molecule Ph-TDPP-Ph and the low bandgap copolymer PCDTBTco. b) EQE in dependence of the field for bilayer devices made from either PCDTBTco and C60 or Ph-TDPP-Ph and C60. The bilayer solar cells are excited at 3.35 eV and 2.14 eV in the case of the PCDTBTco donor and at 2.94 eV and 2.14 eV in the case of the Ph-TDPP-Ph donor. The intrinsic contribution of the C60 acceptor for excitation at 3.35 eV and 2.94 eV are indicated with black arrows. Reprinted from Chapter 6.

A saturation can be observed for both bilayer solar cells at a photon energy of 2.14 eV which mainly excites the donor materials, because the donors exhibit no significant intrinsic contribution. When exciting both donor and C60 at photon energies of 3.35 eV and 2.94 eV, an additional slope at high electric field strengths can be observed. This increase can be attributed to the intrinsic dissociation of C60 because the excitation energy is higher than the threshold value of 2.25 eV and charge-transfer states are generated in the C60 layer. Thus, both the donor-acceptor interface as well as the intrinsic dissociation in the C60 contribute to the solar cell performance in this energy range.

For the complete dissociation of photogenerated excitons into free charges, the charge-transfer states have to couple with the charge-transporting states. This coupling is by a factor of 3 lower in PCBM than in C60. By applying an Onsager fit to the data, the Coulomb binding energies, separation of the electron-hole pairs as well as the electrical gap can be estimated. We found that the binding energies of the charge-transfer states generated by optical excitation decreases with increasing photon energy from 220 meV to 100 meV for excitation energies between 2.25 eV and the electrical gap at 2.45 eV. This results in an increase of the electron-hole separation of the charge-transfer states from 2.0 nm to 2.5 nm which supports the delocalisation of the charge-transfer states over two fullerene molecules. The coupling of these states to charges-transporting states is achieved by ionisation upon thermal excitation. Charge-transfer states generated by excitation exceeding 2.45 eV undergo thermalization. In this case, the dependence of the electron-hole distance and the binding energy on the photon energy is weaker. By thermal excitation, the thermalized charge-transfer states also couple to charge-transporting states.

In conclusion, the dependence of the intrinsic dissociation on the excitation energy was examined in this work. By this means, we were able to evaluate the intrinsic contribution from the acceptors C60 and PCBM to the overall device efficiency of organic bilayer solar cells.

a)

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4.2.2 Monomolecular and bimolecular recombination of electron–hole pairs at the interface