No abstract

Photoinduced halide segregation currently limits the perovskite chemistries available for use in high-bandgap semiconductors needed for tandem solar cells. Here, we study the impact of post-deposition surface modifications on photoinduced halide segregation in methylammonium lead mixed-halide perovskites. By coating a perovskite surface with the electron-donating ligand trioctylphosphine oxide (TOPO), we are able to both reduce nonradiative recombination and dramatically slow the onset of halide segregation in CH 3 NH 3 PbI 2 Br films. This result, coupled with the observation that the rate of halide segregation can be tuned by varying the selective contact, provides a direct link between surface modifications and photoinduced trap formation. On the basis of these observations, we discuss possible mechanisms for photoinduced halide segregation supported by drift-diffusion simulations. This work suggests that improved understanding and control of perovskite surfaces provides a pathway toward stable and high-performance wide-bandgap perovskites for the next generation of tandem solar cells.

solar cells have a breakdown voltage (V BD ) at which current starts to flow in reverse bias. When current flows in reverse bias, the shaded cell dissipates power rather than producing it, and this can cause local heating, which damages the cell. [8] Silicon cells generally breakdown in reverse bias by avalanche breakdown; the carriers gain enough kinetic energy from the applied electric field to generate additional carriers through impact ionization. V BD s for silicon cells are typically >15 V. If the pn junction is highly doped, the depletion width can narrow enough to allow tunneling in reverse bias. With either mechanism, breakdown current can get localized by uneven doping, crystalline defects, trace processing contaminants, etch sites, or edge effects causing damaging hot spots. CIGS and CdTe exhibit V BD s < 10 V and a decrease in V BD under illumination. This has been attributed to tunneling through defects at the buffer layer/CIGS interface. [9] Partial shading has been shown to cause local current flow and the damage is exacerbated by the light dependence of the V BD , which causes even more of the current to selectively flow through the illuminated region. This can cause localized shunting those results primarily in a permanent decrease in fill factor. [10] Stability in reverse bias has not been explored for perovskite solar cells, but there have been studies of MAPbI 3 memristors, [11][12][13][14] which require biasing in both forward and reverse directions, and photodetectors, which function in reverse bias. [15] Some memristors operate via the formation of metallic filaments through the perovskite [11,12] and others seem to function based on mobile defects in the perovskite. [12,13] Photodiodes of the structure fluorine-doped tin oxide (FTO)/porous TiO 2 /MAPbI 3 / Spiro-OMeTAD/Au show current multiplication in reverse bias, which has been attributed to mobile ion accumulation. [15] Mobile ions have also been used to explain hysteresis in current-voltage measurements. [16,17] In most cells, preconditioning at lower voltages makes cells worse, which is why scans from J SC to V OC tend to give lower efficiencies. It has also been demonstrated that mobile ions can cause band bending that can turn a symmetric device with nonselective contacts into a diode that functions as a solar cell. [18] Mobile ions likely also play an important role in the behavior of perovskite solar cells in reverse bias.In this paper, we first present a phenomenological study of reverse bias breakdown in halide perovskite solar cells. We characterize cells that have been held at constant current in the dark as they would be in a series connected module if only one cell were completely shaded. We also provide constant voltage measurements. We show how the reverse breakdownThe future commercialization of halide perovskite solar cells relies on improving their stability. There are several studies focused on understanding degradation under operating conditions in light, but little is known about the stability of these solar cells u...

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sterilization techniques in the off-grid areas has been exposing human beings to high risk of various epidemic diseases. [3][4][5] Therefore, there have been efforts to develop various solar autoclaves, [6][7][8][9] aiming to provide a reliable off-grid sterilization solution.Recently, interfacial solar steam generation is attracting a lot of attention, with exciting progress in nanoscale designs of materials, light absorption, thermal management, and water supply, [10][11][12][13][14][15][16][17][18][19][20] promising for water purification, solar desalination, ground water extraction, power generation, and various other applications. [21][22][23][24][25][26][27][28][29][30][31][32][33] Most, if not all, of the previous works focus on the high solar to steam energy transfer efficiency, as the absorbers can concentrate the absorbed solar power into the top layers of water for vaporizing with minimized energy loss. In this work, we reveal the kinetic advantage of interfacial solar steam generation, and demonstrate that primarily because of much reduced thermal mass of interfacial heating, interfacial solar steam generation can enable fast responsive, energy efficient, and effective off-grid sterilization. In addition, the entire sterilization setup can be built with low cost and operated with minimum carbon footprint.A typical sterilization process includes heating phase (for steam temperature to increase from room temperature to sterilization temperature), exposure phase (for steam temperature to maintain at sterilization temperature), and cooling phase (for steam temperature to decrease from sterilization temperature to 100 °C to open the sterilizer). [34] As shown in Figure 1a, most, if not all, of the previous steam sterilization devices are based on volumetric heating, with the entire bulk water being heated for steam generation. As a result, during the heating phase, a long period of time (typically 30-60 min for commercialized autoclaves) together with high energy consumption per-unit volume (≈110 J mL −1 for steam of the commercialized autoclave reaching 121 °C) are necessary to offset the sensible heat stored in the bulk water. For exposure phase, the required duration for effective sterilization depends on the sterilization temperature. Steam with a higher temperature can enable effective sterilization within a shorter period, with typical sterilization conditions Steam sterilization is widely used as one of the most reliable sterilization methods for public health. However, traditional steam sterilization mainly relies on electricity, a constrained resource for many developing countries and areas. The lack of available and affordable sterilization techniques in these areas is exposing human beings to a high risk of various epidemic diseases, and calls for the development of off-grid sterilization solutions. For the first time, the kinetic advantages of interfacial solar steam generation is fundamentally revealed and it is demonstrated that interfacial solar steam generation can enable fast-responsive (a...