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spiro­OMeTAD. More importantly, CO2 bubbling decreases the doping time to just one minute. Having such a short doping time will be essential for the commercial produc­ tion of perovskite solar cells. Because the lithium carbonate by­product is removed by filtration, the density of lithium ions in the resulting HTL is lower than that in HTLs made using conventional doping. Kong et al. show that solar cells prepared using their method are much more stable than are control devices prepared using conventional doping, consistent with the smaller amount of lith­ ium in the HTL. The authors’ solar cells retain about 80% of their initial power­conversion efficiency after 500 hours of continuous oper­ ation, whereas the efficiency of the control devices rapidly drops to less than 75% after just 6 hours. Even more excitingly, Kong and colleagues report that their CO2­bubbling method is not limited to small­molecule organic semi­ conductors such as spiro­OMeTAD — the conductivity of a broad range of polymeric organic semiconductors (mixed with LiTFSI) is between 2 and 100 times greater than that of polymer–LiTFSI films not treated with CO2. The power­conversion efficiency of perovskite solar cells made using the resulting doped polymeric HTLs is also substantially increased. Perovskite solar cells show great prom­ ise, and have the potential to find industrial applications in the next few years. However, mass­manufacturing methods must first be developed that produce devices with long­ term stability. This means that any compounds that reduce performance must be rigorously removed from HTLs during manufacturing. Kong and colleagues’ work is important because it shows the feasibility of removing stability­lowering compounds using poten­ tially scalable doping methods. So, although the power­conversion efficiency and stability of the authors’ devices are not the best in the field, the new findings will inspire the develop­ ment of other advanced doping strategies for rapidly producing clean films of organic semi­ conductors — thereby accelerating the pace of commercialization of perovskite solar cells.