Organic photovoltaic (OPV) devices have shown great potential as an inexpensive source of renewable energy. There are many factors that affect the efficiency of an OPV device, and understanding them can help the design of new architectures to boost the efficiency towards a commercially viable level. Multijunction OPV devices have been realised as a key route to higher efficiencies. The work presented in this thesis focuses on the design of multi heterojunction OPV devices based upon small molecule organic semiconductors. A small molecule tandem OPV device consisting of two donor materials with complementary absorption spectra, boron subphthalocyanine chloride (SubPc) and aluminium phthalocyanine chloride (ClAlPc) combined with C60 as the acceptor is used as a test device architecture. The recombination layer between the sub-cells was investigated first, with the layer thickness and deposition conditions optimised. Modification of the photoactive layers by co-deposition of the donor and acceptor, and the use of a templating layer is proven to increase the device efficiency. α-NPD co-deposited with MoOx was used as a hole transport and optical spacer layer in single junction and tandem OPV devices. The electrical, optical and morphological properties of the α-NPD and the co-deposited layers are investigated. The hole transporting properties of α-NPD:MoOx are shown in a SubPc / C60 device with the optical spacing properties shown for a ClAlPc:C60 bulk heterojunction (BHJ) device. Implementation of the α-NPD:MoOx optical spacer layer is shown as a tool for current matching in a SubPc:C60 BHJ tandem OPV device. An optical simulation predicts the use of an optical spacer will improve the device performance as it positions the sub-cells in an optimum optical field position for current matching. Experimental results show good agreement with the optical simulation and an improvement in short circuit current (JSC) is shown with the α-NPD:MoOx optical spacer layer. The tandem OPV device is also shown with a larger active area which displays good operational stability characteristics. A multi-heterojunction cascade device was designed using α-sexithiophene (α-6T) as an interfacial layer with phthalocyanine / C60 OPV devices. The α-6T is shown to dissociate excitons in the phthalocyanine layer and contribute to the photocurrent of the device without affecting the phthalocyanine / C60 interface. The dual exciton dissociation in the cascade device leads to an increased JSC compared to the bilayer devices.