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The incorporation of even small amounts of strontium (Sr) into lead-base hybrid quadruple cation perovskite solar cells results in a systematic increase of the open circuit voltage (V-oc) in pin-type perovskite solar cells. We demonstrate via absolute and transient photoluminescence (PL) experiments how the incorporation of Sr significantly reduces the non-radiative recombination losses in the neat perovskite layer. We show that Sr segregates at the perovskite surface, where it induces important changes of morphology and energetics. Notably, the Sr-enriched surface exhibits a wider band gap and a more n-type character, accompanied with significantly stronger surface band bending. As a result, we observe a significant increase of the quasi-Fermi level splitting in the neat perovskite by reduced surface recombination and more importantly, a strong reduction of losses attributed to non-radiative recombination at the interface to the C-60 electron-transporting layer. The resulting solar cells exhibited a V-oc of 1.18 V, which could be further improved to nearly 1.23 V through addition of a thin polymer interlayer, reducing the non-radiative voltage loss to only 110 meV. Our work shows that simply adding a small amount of Sr to the precursor solutions induces a beneficial surface modification in the perovskite, without requiring any post treatment, resulting in high efficiency solar cells with power conversion efficiency (PCE) up to 20.3%. Our results demonstrate very high V-oc values and efficiencies in Sr-containing quadruple cation perovskite pin-type solar cells and highlight the imperative importance of addressing and minimizing the recombination losses at the interface between perovskite and charge transporting layer.
Among various types of perovskite-based tandem solar cells (TSCs), all-perovskite TSCs are of particular attractiveness for building- and vehicle-integrated photovoltaics, or space energy areas as they can be fabricated on flexible and lightweight substrates with a very high power-to-weight ratio. However, the efficiency of flexible all-perovskite tandems is lagging far behind their rigid counterparts primarily due to the challenges in developing efficient wide-bandgap (WBG) perovskite solar cells on the flexible substrates as well as their low open-circuit voltage (V-OC). Here, it is reported that the use of self-assembled monolayers as hole-selective contact effectively suppresses the interfacial recombination and allows the subsequent uniform growth of a 1.77 eV WBG perovskite with superior optoelectronic quality. In addition, a postdeposition treatment with 2-thiopheneethylammonium chloride is employed to further suppress the bulk and interfacial recombination, boosting the V-OC of the WBG top cell to 1.29 V. Based on this, the first proof-of-concept four-terminal all-perovskite flexible TSC with a power conversion efficiency of 22.6% is presented. When integrating into two-terminal flexible tandems, 23.8% flexible all-perovskite TSCs with a superior V-OC of 2.1 V is achieved, which is on par with the V-OC reported on the 28% all-perovskite tandems grown on the rigid substrate.