S-Shaped Current-Voltage Characteristics in Solar Cells: A Review

S-shaped current-voltage (I-V) characteristics are a frequently occurring hurdle in the development of new solar cell material combinations and device architectures. Their presence points to the existence of a charge transport bottleneck that needs to be removed in order to unlock high fill factors and power conversion efficiencies. In this review, examples of studies in which s-shaped I-V curves have appeared are presented, and the cause and mitigation are discussed. Different solar cell material systems are often treated by separate communities, thereby, also the physics of s-shaped I-V curves have been treated separately. This review covers the main solar cell technologies-silicon, thin film, organic, hybrid-with the aim to provide an overarching picture of the common mechanisms and universal guidelines for mitigation of s-shaped I-V characteristics in emerging solar cell technologies. Except for a few studies on organic solar cells, s-shaped I-V curves are reported to result from charge transport barriers at one of the (selective) contact layers that can be overcome by interface engineering and doping.