Charge-transfer state energy determines open-circuit voltage in organic photovoltaics

Organic photovoltaics (OPVs) are a promising alternative to inorganic photovoltaics because they are made out of cheaper and non-toxic materials combined with low-energy and low-cost processing techniques. However, large energy losses between the optical gaps of the absorbing materials and the open-circuit voltage (V_OC) limit how efficiently these OPVs convert solar energy into electricity.

Materials used in OPVs commonly have optical gaps of between 1.7 and 2.1eV. However, the V_OC seldom exceeds 1.0eV. This difference between the optical gap and qV_OC, the potential energy at open-circuit voltage, represents a loss of almost half the photon's original energy. In contrast, inorganic photovoltaic cells, such as polycrystalline silicon, gallium arsenide, and copper indium gallium selenide, show a difference of only 0.3–0.45eV between the material bandgap and qV_OC. For OPVs to reach higher power conversion efficiencies (PCEs), these energy losses must be decreased and the V_OC increased. This requires a better understanding of the origin of V_OC and energy-loss mechanisms.

The V_OC is the point at which the recombination current is equal to the photocurrent. It can be given by the equation J_rec = qk_recLnp, where k_rec is the recombination rate constant, L is the thickness of the recombination layer, and np is the product of electrons and holes (charges) in the device. The np product depends exponentially on the energy gap and applied or photogenerated voltage. Increases in the energy gap decrease np and the recombination current. The dependence of J_rec  on np results in a linear change in V_OC with the effective energy gap. A decrease in k_rec also lowers the recombination current. k_rec is determined by either the rate at which free charges meet in the device, or recombine once they form a charge pair. If the recombination rate of the charge pair is the rate-limiting step, then anything that reduces this rate increases the V_OC. On the basis of the Marcus theory of electron transfer, one way to decrease the charge-pair recombination rate is to decrease electronic coupling between electron donor and acceptor materials.

Figure 1. Chemical structures of the donor materials used in the organic photovoltaic devices reported in this article, tetracene and rubrene.

Reduced electronic coupling has previously been considered an explanation for increases in the V_OC. Using the charge-transfer state contribution to the external quantum efficiency allows for a deeper look at electronic coupling between donor and acceptor in the model systems of tetracene/C₆₀ and rubrene/C₆₀ bilayer OPVs. Owing to their energetic and structural similarities (see Figure 1), along with a previously reported large V_OC difference of 0.37V, these material systems provide an ideal platform to study the effects of sterics and electronic coupling on V_OC.

Analysis of these systems reveals that V_OC differences correspond directly with those of charge-transfer state energy (E_CT), with little or no influence from electronic coupling.¹ This result is consistent with trends where the qV_OC correlates linearly with E_CT (see Figure 2). Moreover, electronic coupling differences can be attributed to the edge-on and face-on orientations adopted by tetracene and rubrene, respectively. This is based on near-edge x-ray absorbance fine-structure spectroscopy and grazing incidence x-ray-scattering measurements to probe molecular orientation.

Figure 2. Plot of previously recorded qV_OC vs. E_CTvalues for organic photovoltaic devices (OPVs), as well as rubrene/C₆₀and tetracene/C₆₀ bilayer OPV devices analyzed in this work. Values taken from Vandewal et al.,^2–4Piersimoni et al.,⁵ Ko et al.,⁶Hoke et al.,⁷ and Graham et al.¹qV_OC: Potential energy at open-circuit voltage. E_CT: Charge-transfer state energy.

With the V_OC determined largely by E_CT, this energy must be increased to increase the V_OC. We probed the highest occupied molecular orbital levels and E_CT of both crystalline and amorphous systems to find out what determines E_CT. Results show that E_CT is influenced by the difference between the highest occupied and lowest unoccupied molecular orbital levels, as well as other factors, including the extent of crystallinity, donor-acceptor orientation, and interfacial dipoles.

In summary, our work shows that the V_OC is largely determined by E_CT, and that E_CT must be increased to strengthen the V_OC. This highlights the importance of understanding the factors that affect E_CT and also suggests that an increased understanding of the V_OC is necessary to further minimize voltage losses and increase OPV performance.

In the future, we will continue to work on ways to improve the efficiency of organic solar cells by investigating new ways to maximize the V_OC of devices, such as by looking at systems with different degrees of crystallinity and molecular conformation at the donor-acceptor interface.