Experimental demonstration of hot carrier solar cells by ultrafast photovoltaic spectroscopy

Experimental demonstration of hot carrier solar cells by ultrafast photovoltaic spectroscopy Despite more than half a century of advancements in silicon-based photovoltaics, their efficiency remains constrained by the well-established Shockley-Queisser (SQ) limit, which is primarily attributed to the thermalization of hot carriers. To overcome this limitation, the innovative concept of hot carrier solar cells has emerged as one of the most promising technologies for achieving high efficiency. These solar cells maintain a device architecture similar to conventional single-junction designs, making them an attractive alternative. However, a significant challenge lies in the extremely short lifetime of hot carriers, typically on the order of a few hundred picoseconds, due to rapid energy loss through interactions with phonons. Consequently, investigating carrier lifetimes and dynamics is critical for advancing this technology. These properties are often analyzed using advanced optical spectroscopic techniques, including pump-probe transient absorption, time-resolved photoluminescence, and up-conversion photoluminescence. 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.