Experimental Demonstration of Hot Carrier Solar Cells by Ultrafast Photovoltaic Spectroscopy

Despite over half a century of progress in silicon photovoltaics, their efficiency remains limited by the well-known Shockley–Queisser (SQ) limit, which arises from the thermalization of hot carriers. To surpass the SQ limit, the novel concept of hot carrier solar cells, which feature a similar and simple device architecture to single-junction devices, is among the most promising highly efficient technologies. However, the lifetime of hot carriers is extremely short, on the order of a few hundred of picoseconds, due to rapid interaction with phonons. As a result, carrier lifetime and dynamics are often characterized using optical spectroscopies such as pump-probe transient absorption, time-resolved photoluminescence, or up-conversion photoluminescence.<br/><br/>In this report, we have developed an ultrafast photovoltaic spectroscopy technique with sub-40 picosecond time resolution to directly capture the photocurrent while solar cells are operational. We have measured ultrafast photocurrents in perovskite solar cells with more than 20% alloyed cations. A typical time-resolved photocurrent exhibits a fast rise time of 40 picoseconds, followed by decay dynamics that are dependent on the applied bias voltage. By sweeping the voltage, we observed that the transient open-circuit voltage (Voc) exceeds the bandgap, which is a hallmark of hot carrier solar cells. Additionally, Voc decreases as the thickness of the perovskite layer increases. The hot carriers remain ‘hot,’ as the photocurrent peak is not sensitive to temperature changes. Our findings present the first experimental demonstration of hot carrier solar cells characterized by the unique ultrafast photovoltaic spectroscopy.