Flexible, lightweight Cu(In,Ga)Se2 (CIGS) solar cells grown on polymer substrates are a promising technology with fast growing market prospects. However, power conversion efficiencies of solar cells grown at low temperatures (≈450 °C) remain below the efficiencies of cells grown at high temperature on glass substrates. This contribution discusses the impact on cell efficiency of process improvements of low‐temperature CIGS deposition on flexible polyimide and glass substrates. Different strategies for incorporation of alkali elements into CIGS are evaluated based on a large number of depositions. Postdeposition treatment with heavy alkali (here RbF) enables a thickness reduction of the CdS buffer layer and increases the open‐circuit voltage. Na supply during 3rd stage CIGS deposition positively impacts the cell performance. Coevaporation of heavy alkali (e.g., RbF) during capping layer deposition mitigates the adverse shunting associated with high Cu contents, yielding highest efficiencies with near‐stoichiometric absorber compositions. Furthermore, optimization of the deposition sequence results in absorbers with a 1 µm wide notch region with nearly constant bandgap minimum. The improved processes result in a record cell efficiency of 20.8% for CIGS on flexible substrate.

Record power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) have been obtained with the organic hole transporter 2,2′,7,7′-tetrakis( N , N -di- p -methoxyphenyl-amine)9,9′-spirobifluorene (spiro-OMeTAD). Conventional doping of spiro-OMeTAD with hygroscopic lithium salts and volatile 4- tert -butylpyridine is a time-consuming process and also leads to poor device stability. We developed a new doping strategy for spiro-OMeTAD that avoids post-oxidation by using stable organic radicals as the dopant and ionic salts as the doping modulator (referred to as ion-modulated radical doping). We achieved PCEs of >25% and much-improved device stability under harsh conditions. The radicals provide hole polarons that instantly increase the conductivity and work function (WF), and ionic salts further modulate the WF by affecting the energetics of the hole polarons. This organic semiconductor doping strategy, which decouples conductivity and WF tunability, could inspire further optimization in other optoelectronic devices.

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This work shows that vacuum decreases operational lifetime of perovskite solar cells (ITO/SnO 2 /perovskite/Spiro-MeOTAD/Au) by accelerating perovskite decomposition starting from the grain boundaries, accompanied by outgassing and defect formation. These defects further accelerate ion migration (Li + , Au + and/ or Au 3+ , I À , and Br À ) across the device. We propose a robust perovskite solar cell structure (ITO/PTAA/perovskite/PCBM/ZnO/AZO/[Ni/Al grid]) that effectively mitigates these degradation pathways, leading to a device showing a projected T 80 lifetime of 4,750 h at its maximum power point condition, 1-sun illumination at 50 mbar.

A key challenge that remains in perovskite light-emitting diodes is to achieve longterm operational stability. We find that the halide ions at perovskite surface migrate into the hole transport layer during operation, which works as one of the dominant device degradation pathways. Intriguingly, these ions can also gradually move back and consequently lead to the recovery of device performance. The repeatable performance recovery at room temperature can greatly help to enhance the long-term reliability of perovskite light-emitting devices in practical applications.