The effects of sterics and molecular orientation on the open-circuit voltage and absorbance properties of charge-transfer states are explored in model bilayer organic photovoltaics. It is shown that the open-circuit voltage correlates linearly with the charge-transfer state energy and is not significantly influenced by electronic coupling.

A short series of alkyl substituted perylenediimides (PDIs) with varying steric bulk are used to demonstrate the relationship between molecular structure, materials properties, and performance characteristics in organic photovoltaics. Devices were made with the structure indium tin oxide/copper phthalocyanine (200 Å)/PDI (200 Å)/bathocuproine (100 Å)/aluminum (1000 Å). We found that PDIs with larger substituents produced higher open circuit voltages (VOC’s) despite the donor acceptor interface gap (ΔEDA) remaining unchanged. Additionally, series resistance was increased simultaneously with VOC the effect of reducing short circuit current, making the addition of steric bulk a tradeoff that needs to be balanced to optimize power conversion efficiency.

A benzannulated boron dipyrromethene (BODIPY, bDIP) molecule exhibiting strong absorption at 640 nm was synthesized. The organic dye was used in an organic solar cell as the electron donor with C60 as the acceptor. The BODIPY dye demonstrated the best performance in lamellar architecture (indium tin oxide (ITO)/bDIP/C60/bathocuproine/Al), giving power conversion efficiency up to 4.5% with short-circuit current (JSC) of 8.7 mA/cm(2) and an open-circuit voltage (VOC) of 0.81 V. Neutron reflectivity experiments were performed on the bilayer film to investigate the thickness dependence of JSC. A 13 nm mixed layer was found to be present at the donor/acceptor interface in the bilayer device, formed when the C60 was deposited onto a room temperature bDIP film. Planar-mixed heterojunction devices were fabricated to understand the extent of spontaneous mixing between the donor and acceptor materials. The native mixed region in the bilayer device was shown to most resemble 1:3 bDIP:C60 layer in the structure: (ITO/bDIP/bDIP:C60 blend/C60/bathocuproine/Al).

A critically important question that must be answered to understand how organic solar 3 cells operate and should be improved is how the orientation of the donor and acceptor 4 molecules at the interface influences exciton diffusion, exciton dissociation by electron transfer and recombination. It is exceedingly difficult to probe the orientation 6 in bulk heterojunctions because there are many interfaces and they are arranged with 7 varying angles with respect to the substrate. One of the best ways to study the 8 interface is to make bilayer solar cells with just one donor-acceptor interface. Zinc 9 phthalocyanine is particularly interesting to study because its orientation can be 10 adjusted by using a 2-nm-thick copper iodide seed layer before it is deposited. 11 Previous studies have claimed that solar cells in which fullerene acceptor molecules 12

A covalently linked multi-chromophoric array (BDP-Por) was used as the donor layer in planar hetero-junction organic photovoltaic (OPV) cells with the structure (ITO/BDPPor/C 60 /BCP//Al, BCP = bathocuproine). BDP-Por is a platinum tetrabenzoporphyrin core (Pt(TPBP)) bonded through the phenyl groups to the meso position of four 4,4-difluoro-3,5-dimethyl-4-bora-3a,4a-diaza-s-indacene (BODIPY) moieties. The OPV outperforms analogous OPVs which use only the Pt(TPBP) as the donor, predominantly by producing a higher short circuit current (J SC of 2.5 vs. 4.5 mA/cm 2 ), due to the enhanced absorptivity of the multi-chromophoric array. The open circuit voltages (V OC ) for Pt(TPBP) and BDP-Por based OPVs are both 0.65 volts. The system was further investigated by preparing and testing OPVs made from an analogous system with porphyrin and BODIPY components blended into a single homogeneous donor layer that absorbs similarly to the array. It is then found that the mixed system generates the same photocurrent as the array with a similar responsivity, but gives an unstable open circuit voltage that quickly degrades to 0.34 V. It is determined that the donor layer components undergo phase segregation at the interface upon illumination, resulting in the decreased voltage and highlighting an advantage of the multi-chromophoric array.