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The open-circuit voltage of organic solar cells is usually lower than the values achieved in inorganic or perovskite photovoltaic devices with comparable bandgaps. Energy losses during charge separation at the donor-acceptor interface and non-radiative recombination are among the main causes of such voltage losses. Here we combine spectroscopic and quantum-chemistry approaches to identify key rules for minimizing voltage losses: (1) a low energy offset between donor and acceptor molecular states and (2) high photoluminescence yield of the low-gap material in the blend. Following these rules, we present a range of existing and new donor-acceptor systems that combine efficient photocurrent generation with electroluminescence yield up to 0.03%, leading to non-radiative voltage losses as small as 0.21 V. This study provides a rationale to explain and further improve the performance of recently demonstrated high-open-circuit-voltage organic solar cells.

We show that converting the surfaces of lead halide perovskite to water-insoluble lead (II) oxysalt through reaction with sulfate or phosphate ions can effectively stabilize the perovskite surface and bulk material. These capping lead oxysalt thin layers enhance the water resistance of the perovskite films by forming strong chemical bonds. The wide-bandgap lead oxysalt layers also reduce the defect density on the perovskite surfaces by passivating undercoordinated surface lead centers, which are defect-nucleating sites. Formation of the lead oxysalt layer increases the carrier recombination lifetime and boosts the efficiency of the solar cells to 21.1%. Encapsulated devices stabilized by the lead oxysalt layers maintain 96.8% of their initial efficiency after operation at maximum power point under simulated air mass (AM) 1.5 G irradiation for 1200 hours at 65°C.

Avoiding buried voids The buried interfaces of perovskite solar cells are difficult to alter after synthesis. During manufacture, Chen et al . removed perovskite films with dimethyl sulfoxide solvent from the hole-transfer layer and observed a substantial void fraction that degraded film performance. Replacing most of the dimethyl sulfoxide with carbohydrazide, a lead-coordinating compound with a much higher boiling point, eliminated voids. Such solar cells maintained high power conversion efficiency after 550 hours of operation at 60°C. —PDS

To achieve efficient non-fullerene organic solar cells, it is important to reduce the voltage loss from the optical bandgap to the open-circuit voltage of the cell. Here we report a highly efficient non-fullerene organic solar cell with a high open-circuit voltage of 1.08 V and a small voltage loss of 0.55 V. The high performance was enabled by a novel wide-bandgap (2.05 eV) donor polymer paired with a narrow-bandgap (1.63 eV) small-molecular acceptor (SMA). Our morphology characterizations show that both the polymer and the SMA can maintain high crystallinity in the blend film, resulting in crystalline and small domains. As a result, our non-fullerene organic solar cells realize an efficiency of 11.6%, which is the best performance for a non-fullerene organic solar cell with such a small voltage loss.

Perovskite materials are good candidates for flexible photovoltaic applications due to their strong absorption and low-temperature processing, but efficient flexible perovskite modules have not been realized yet. Here, we demonstrate a record efficiency flexible perovskite solar module by blade coating high-quality perovskite films on flexible Corning® Willow® Glass using additive engineering. Ammonium chloride (NH 4 Cl) is added into perovskite precursor solution to retard the nucleation which prevents forming voids at the interface of perovskite and glass. The addition of NH 4 Cl also suppresses the formation of PbI 2 reduces the trap density in the perovskite films. The implementation of NH 4 Cl enabled fabricating single junction flexible perovskite solar devices with an efficiency of 19.72% on the small area cells and a record aperture efficiency of 15.86% on the modules with an area of 42.9 cm 2. This work provides a simple way to scale up high-efficiency flexible perovskite modules for various applications.

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Despite the fast progress of perovskite photovoltaic performances, understanding the crystallization and growth of perovskite films is still lagging. One unanswered fundamental question is whether the perovskite films are grown from top (air side) to bottom (substrate side) or from bottom to top despite 10 years of development. Here, by using grazing incidence x-ray diffraction and morphology characterizations, we unveil that the perovskite films prepared by one-step solution processes, including antisolvent-assisted spin coating and blade coating, follow the downward growth from intermediate phase during thermal annealing. Such a top-to-bottom downward growth is initialized by the evaporation of residual solvent from the top surface of “wet” films and is less sensitive to perovskite compositions and the wettability of underlying substrates. Addressing this fundamental question is important to understand the heterogeneity of perovskite films along the vertical direction, which markedly affects the efficiency and stability of perovskite solar cells.