Small‐molecule photovoltaic cells using diindenoperylene (DIP) as a new donor material in combination with the fullerene C60 as an electron acceptor are demonstrated. In addition to the successful application in planar and bulk heterojunction devices, a comprehensive analysis including structural studies, the determination of the energy level alignment and electrical transport investigations is given, stressing the correlation between growth conditions, film morphology, and device performance. Due to pronounced crystallinity and a large surface area of DIP films grown at elevated temperature, exceptionally high fill factors of almost 75% are achieved in planar heterojunction cells. Bulk heterojunctions exhibit large‐scale phase separation forming a bicontinuous network of both molecular species, which enables efficient exciton dissociation and charge carrier transport. The high ionization potential of DIP and the favorable energy level alignment with the fullerene C60 yield large open circuit voltages close to 1 V and comparable power conversion efficiencies of about 4% in both cell architectures.
In organic photovoltaic (PV) cells, the well‐established donor‐acceptor (D/A) concept enabling photo‐induced charge transfer between two partners with suitable energy level alignment has proven extremely successful. Nevertheless, the introduction of such a heterojunction is accompanied with additional energy losses as compared to an inorganic homojunction cell, owing to the presence of a charge‐transfer (CT) state at the D/A interface. Based on the principle of detailed balance, a modified Shockley‐Queisser theory is developed including the essential effects of interfacial CT states, that allows for a quantitative assessment of the thermodynamic efficiency limits of molecular D/A solar cells. Key parameters, apart from the optical gap of the absorber material, entering the model are the energy (ECT) and relative absorption strength (αCT) of the CT state. It is demonstrated how the open‐circuit voltage (VOC) and thus the power conversion efficiency are affected by different parameter values. Furthermore, it is shown that temperature dependent device characteristics can serve to determine the CT energy, and thus the upper limit of VOC for a given D/A combination, as well as to quantify non‐radiative recombination losses. The model is applied to diindenoperylene (DIP)‐based photovoltaic devices, with open‐circuit voltages between 0.9 and 1.4 V, depending on the partner, that have recently been reported.
Low open-circuit voltages significantly limit the power conversion efficiency of organic photovoltaic devices. Typical strategies to enhance the open-circuit voltage involve tuning the HOMO and LUMO positions of the donor (D) and acceptor (A), respectively, to increase the interfacial energy gap or to tailor the donor or acceptor structure at the D/A interface. Here, we present an alternative approach to improve the open-circuit voltage through the use of a zinc chlorodipyrrin, ZCl [bis(dodecachloro-5-mesityldipyrrinato)zinc], as an acceptor, which undergoes symmetry-breaking charge transfer (CT) at the donor/acceptor interface. DBP/ZCl cells exhibit open-circuit voltages of 1.33 V compared to 0.88 V for analogous tetraphenyldibenzoperyflanthrene (DBP)/C60-based devices. Charge transfer state energies measured by Fourier-transform photocurrent spectroscopy and electroluminescence show that C60 forms a CT state of 1.45 ± 0.05 eV in a DBP/C60-based organic photovoltaic device, while ZCl as acceptor gives a CT state energy of 1.70 ± 0.05 eV in the corresponding device structure. In the ZCl device this results in an energetic loss between E(CT) and qV(OC) of 0.37 eV, substantially less than the 0.6 eV typically observed for organic systems and equal to the recombination losses seen in high-efficiency Si and GaAs devices. The substantial increase in open-circuit voltage and reduction in recombination losses for devices utilizing ZCl demonstrate the great promise of symmetry-breaking charge transfer in organic photovoltaic devices.
The film morphology and device performance of planar heterojunction solar cells based on the molecular donor material α-sexithiophene (6T) are investigated. Planar heterojunctions of 6T with two different acceptor molecules, the C 60 fullerene and diindenoperylene (DIP), have been prepared. The growth temperature of the 6T bottom layer has been varied between room temperature and 100 °C for each acceptor. By means of X-ray diffraction and X-ray absorption, we show that the crystallinity and the molecular orientation of 6T is influenced by the preparation conditions and that the 6T film templates the growth of the subsequent acceptor layer. These structural changes are accompanied by changes in the characteristic parameters of the corresponding photovoltaic cells. This is most prominently observed as a shift of the open circuit voltage (V oc ): In the case of 6T/C 60 heterojunctions, V oc decreases from 0.4 to 0.3 V, approximately, if the growth temperature of 6T is increased from room temperature to 100 °C. By contrast, V oc increases from about 1.2 V to almost 1.4 V in the case of 6T/DIP solar cells under the same conditions. We attribute these changes upon substrate heating to increased recombination in the C 60 case while an orientation dependent intermolecular coupling seems to change the origin of the photovoltaic gap in the DIP case.
Owing to the excitonic nature of photoexcitations in organic semiconductors, the working mechanism of organic solar cells relies on the donor-acceptor (D/A) concept enabling photoinduced charge transfer at the interface between two organic materials with suitable energy-level alignment. However, the introduction of such a heterojunction is accompanied by additional energy losses compared to an inorganic homojunction cell due to the presence of a charge-transfer (CT) state at the D/A interface. By careful examination of planar heterojunctions of the molecular semiconductors diindenoperylene (DIP) and C 60 we demonstrate that three different analysis techniques of the temperature dependence of solar-cell characteristics yield reliable values for the effective photovoltaic energy gap at the D/A interface. The retrieved energies are shown to be consistent with direct spectroscopic measurements and the D/A energy-level offset determined by photoemission spectroscopy. Furthermore, we verify the widespread assumption that the activation energy of the dark saturation current E and the CT energy E CT may be regarded as identical. The temperature-dependent analysis of open-circuit voltage V OC and dark saturation current is then applied to a variety of molecular planar heterojunctions. The congruency of E and E CT is again found for all material systems with the exception of copper phthalocyanine/C 60 . The general rule of thumb for organic semiconductor heterojunctions, that V OC at room temperature is roughly half a volt below the CT energy, is traced back to comparable intermolecular electronic coupling in all investigated systems.
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