A facile method to produce metal halide perovskite solar cells with improved stability and efficiency.

To date, the most efficient perovskite solar cells (PSCs) employ an n–i–p device architecture that uses a 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenyl‐amine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) hole‐transporting material (HTM), which achieves optimum conductivity with the addition of lithium bis(trifluoromethane)sulfonimide (LiTFSI) and air exposure. However, this additive along with its oxidation process leads to poor reproducibility and is detrimental to stability. Herein, a dicationic salt spiro‐OMeTAD(TFSI)2, is employed as an effective p‐dopant to achieve power conversion efficiencies of 19.3% and 18.3% (apertures of 0.16 and 1.00 cm2) with excellent reproducibility in the absence of LiTFSI and air exposure. As far as it is known, these are the highest‐performing n–i–p PSCs without LiTFSI or air exposure. Comprehensive analysis demonstrates that precise control of the proportion of [spiro‐OMeTAD]+ directly provides high conductivity in HTM films with low series resistance, fast hole extraction, and lower interfacial charge recombination. Moreover, the spiro‐OMeTAD(TFSI)2‐doped devices show improved stability, benefitting from well‐retained HTM morphology without forming aggregates or voids when tested under an ambient atmosphere. A facile approach is presented to fabricate highly efficient PSCs by replacing LiTFSI with spiro‐OMeTAD(TFSI)2. Furthermore, this study provides an insight into the relationship between device performance and the HTM doping level.

The search for lead‐free alternatives to lead‐halide perovskite photovoltaic materials resulted in the discovery of copper(I)‐silver(I)‐bismuth(III) halides exhibiting promising properties for optoelectronic applications. The present work demonstrates a solution‐based synthesis of uniform CuxAgBiI4+x thin films and scrutinizes the effects of x on the phase composition, dimensionality, optoelectronic properties, and photovoltaic performance. Formation of pure 3D CuAgBiI5 at x = 1, 2D Cu2AgBiI6 at x = 2, and a mix of the two at 1 < x < 2 is demonstrated. Despite lower structural dimensionality, Cu2AgBiI6 has broader optical absorption with a direct bandgap of 1.89 ± 0.05 eV, a valence band level at ‐5.25 eV, improved carrier lifetime, and higher recombination resistance as compared to CuAgBiI5. These differences are mirrored in the power conversion efficiencies of the CuAgBiI5 and Cu2AgBiI6 solar cells under 1 sun of 1.01 ± 0.06% and 2.39 ± 0.05%, respectively. The latter value is the highest reported for this class of materials owing to the favorable film morphology provided by the hot‐casting method. Future performance improvements might emerge from the optimization of the Cu2AgBiI6 layer thickness to match the carrier diffusion length of ≈40–50 nm. Nonencapsulated Cu2AgBiI6 solar cells display storage stability over 240 days.

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High efficiency and environmental stability are mandatory performance requirements for commercialization of perovskite solar cells (PSCs). Herein, efficient centimeter-scale PSCs with improved stability were achieved by incorporating an additive-free 2,2′,7,7′-tetrakis­[N,N-di­(p-methoxyphenyl)­amino]-9,9′-spirobifluorene (spiro-OMeTAD) hole-transporting material (HTM) through simply substituting the usual chlorobenzene solvent with pentachloroethane (PC). A stabilized power conversion efficiency (PCE) of 16.1% under simulated AM 1.5G 1 sun illumination with an aperture of 1.00 cm2 was achieved for PSCs using an additive-free spiro-OMeTAD layer cast from PC. X-ray analysis suggested that chlorine radicals from PC transfer partially to spiro-OMeTAD and are retained in the HTM layer, resulting in conductivity improvement. Moreover, unencapsulated PSCs with a centimeter-scale active area cast from PC retained >70% of their initial PCE after ageing at 80 °C for 500 h, in contrast with less than 20% retention for control devices. Morphological and X-ray analyses of the aged cells revealed that the perovskite and HTM layers remain almost unchanged in the cells with a spiro-OMeTAD layer cast from PC whereas serious degradation occurred in the control cells. This study not only reveals the decomposition mechanism of PSCs in the presence of HTM additives but also opens up a broad range of organic semiconductors for radical doping.

PSCs) are based on an architecture in which the perovskite light-absorbing layer is positioned between several other functional layers. [3][4][5][6] As the performance of these perovskite "sandwich"-type devices approach their limit based on the device architecture, the shading effect associated with intrinsic parasitic light absorption and the low defect tolerance of the layered device architecture (e.g., pinhole-related shunting) become a significant barrier to further performance increases.To overcome the shading effect, an alternative device architecture can be employed in which the anode and cathode are both positioned underneath the perovskite light-absorbing layer. This back-contact concept is conventionally employed in siliconbased solar cells and has been adapted recently to PSCs. [7] With the light-absorbing layer being exposed directly to sunlight, shading effects encountered in the sandwich-type architecture are avoided. In addition to removing this parasitic light absorption, the back-contact architecture has the additional benefit of eliminating the need for costly transparent conductive oxide layers.As the performance of organic-inorganic halide perovskite solar cells approaches their practical limits, the use of back-contact architectures, which eliminate parasitic light absorption, provides an effective route toward higher device efficiencies. However, a poor understanding of the underlying device physics has limited further performance improvements. Here a mesoporous charge-transporting layer is introduced into quasi-interdigitated back-contact perovskite devices and the charge extraction behavior with an increased interfacial contact area is studied. The results show that the incorporation of a thin mesoporous titanium dioxide layer significantly shortens the chargetransfer lifetime and results in more efficient and balanced charge extraction dynamics. A high short-circuit current density of 21.3 mA cm -2 is achieved using a polycrystalline perovskite layer on a mesoscopic quasi-interdigitated back-contact electrode, a record for this type of device architecture.

Spiro‐OMeTAD has been widely used as a promising hole conductor for metal halide perovskite solar cells due to its ability to deliver highly efficient devices. However, additives such as lithium salt and O2 exposure are still required to modify the electrical properties due to the poor conductivity of pristine spiro‐OMeTAD. In article number 1901519, Jianfeng Lu, Udo Bach and co‐workers employ the oxidized form of spiro‐OMeTAD as a dopant to improve the efficiency of the spiro‐OMeTAD‐based lithium‐free perovskite solar cells from 10% to 19.3% while simultaneously enhancing the device stability.