How to design a defect tolerant solar cell?

The term defect tolerance is widely used in literature to describe materials which exhibit long non-radiative lifetimes of carriers despite possessing a large concentration of point defects. The defect tolerance of a material is credited to the presence of an antibonding valence band that causes intrinsic point defects to lie either in the bands or at least not deep in the band gap. The family of lead-halide perovskites is one such material class with an antibonding valence band and solar cells made from these materials have shown a remarkable rise in photovoltaic performance in the past decade. However, most studies on defect tolerance are concerned with host material properties that affect the recombination coefficients of defects and not how the electrostatics and the design of the layer stack of a device affects the recombination activity.

Here we discuss defect tolerance from a device perspective and try to understand how certain device geometries will enhance or slow down non-radiative recombination via defects in the absorber or interface layers. The dependence of the recombination activity on device structure is a direct consequence of the fact that recombination inside a device is a function of both the properties of the defect and that of the device. The defect is characterized by the capture coefficients of the defect which encompass all the microscopic properties of the host material and the energetic position of the defect within the bandgap of the host material. The device structure constituting the absorber layer sandwiched between transport layers determines the carrier concentration inside the device as a function of the mobilities, workfunctions and doping concentrations of the different layers. The recombination efficiency of a defect, which is the number of recombination events occurring at a defect per unit time, is a function of both the capture coefficients and the carrier concentration inside the device. By studying different device geometries of a perovskite solar cell with an iodine interstitial defect level as the dominant recombination level, we explain how and why asymmetric thicknesses of hole and electron transport layers improve the solar cell performance. We also offer generic design principles which when implemented after identifying the dominant recombination levels will help reduce the recombination through the device.