Voc from a morphology point of view: On the open circuit voltage of polycrystalline organic heterojunction solar cells

Organic solar cells are a promising technology in the emerging field of renewable energies and enable potentially cheap harvesting of solar energy. Significant knowledge about the elementary processes in organic photovoltaic cells has been gained in recent years by the collaborative work of researchers around the world. The present work contributes with a special focus on the open circuit voltage of planar heterojunction solar cells and how it is influenced by the film morphology of the active layer materials. Diindenoperylene (DIP) is first introduced as a non-fullerene acceptor in combination with α-sexithiophene (6T) which yields extraordinarily high open circuit voltages (Voc) for single junctions between 1.2 V and almost 1.4 V, depending on the morphology of the active layers. Comparison of the rod-shaped DIP and the spherical C60 fullerene in planar heterojunctions with 6T then enables additional insight on the influence of the impact of molecular orientation on the open circuit voltage. As revealed by structural investigation this orientation can be tuned by the choice of the growth conditions. A mixture of upright standing and flat lying molecules is formed at room temperature (RT), while the lying orientation vanishes if the substrate is heated to 100 °C (HT) during the 6T growth. This structural change templates the subsequent acceptor layer. From a device perspective the different morphologies result in distinct changes of the open circuit voltage. In the case of 6T/C60 heterojunctions a significant reduction of Voc is observed if 6T is grown at an elevated temperature. This can unambiguously be attributed to increased recombination losses. For the 6T/DIP devices the Voc shift is in the opposite direction and the open circuit voltage is increased by the morphology that is induced by the high temperature 6T growth. Counterintuitively, it is found that the absolute recombination loss becomes even larger for this solar cell. Instead, temperature dependent device characterization indicates different photovoltaic energy gaps for the room temperature and the high temperature prepared device. Opposed to these findings, the intermolecular donor/acceptor energy gap remains unchanged as shown by ultraviolet photoelectron spectroscopy measurements. It is hence concluded that the optical gap of DIP takes the role of the photovoltaic gap in the high temperature grown 6T(HT)/DIP device. In order to explain this observation, a recently presented modification of the Shockley-Queisser theory for organic heterojunctions is reconsidered. A special focus is set on constellations where a linear extrapolation of the predicted temperature dependence of the open circuit voltage would indeed result in the optical gap of the absorber rather than in the intermolecular charge transfer (CT) energy gap. A temperature dependent competition between the recombination via the CT gap and the optical gap of the absorber is identified as the culprit. Hence, a transition temperature is introduced above which recombination is dominated by the optical gap and below which CT recombination prevails. It is shown that this transition temperature scales with the energy offset ∆E between the optical and the charge transfer gap and with the absorption strength of the CT state. Within this framework the extraction of the intermolecular gap for the room temperature prepared 6T(RT)/DIP solar cell but the optical gap of DIP for 6T(HT)/DIP from the temperature dependence of the open circuit voltage can be explained by a strongly reduced transition temperature for the HT device, which is below the operating temperature of the solar cell. This is attributed to significantly reduced intermolecular electronic coupling of 6T and DIP in standing/standing configuration in addition to the already small offset ∆E for this system. This demonstrates that recombination via the optical gap of the absorber, which has only been predicted for extreme parameters affecting the electronic coupling, may actually become relevant in real world devices