Modeling Back Contact Bulk Heterojunction Solar Cells

Conventional bulk heterojunction solar cells consist of a mixture of p- and n-region organic materials with electrodes on the top and bottom of the sample. The optimal thickness for power generation is often small, around 100 nm, which makes it challenging to maintain this thickness for large-area applications. An alternative solar cell design is the back contact solar cell, where both electrodes are on one side of the cell, separated by an insulator or wafer layer. We have modeled bulk heterojunction solar cells with 10 different designs for the back contacts using a code that discretizes the sample into p- and n- regions and describes the transport between them using either resistors or photodiodes. The power output for each design is computed as a function of thickness and material dependence and compared to conventional solar cells with electrodes on the top and bottom. For the back contact design, the power output does not depend on the thickness for sufficiently thick samples. Furthermore, the maximum power output is comparable to, or in some cases larger than, the corresponding optimal thickness of a conventional solar cell. In addition, it was found that the presence of an insulating layer, as well as the composition of the back contact, both contribute to the power output.