According to the detailed balanced limit for a singlejunction solar cell, tin−lead (Sn−Pb) perovskite solar cells (PSCs) can achieve power conversion efficiencies (PCEs) more than Pb-PSCs. However, the rise in PCE of Sn−Pb PSCs is limited by the choice of hole transport layer to PEDOT:PSS only. Inspired by the use of hole selective monolayers (HSM) in Pb only PSCs, here, we employed 2-(9H-carbazol-9-yl) ethyl] phosphonic acid (2PACz), leading to PCE (21.39%) comparable to PSCs fabricated on conventional PEDOT:PSS (21.37%). Moreover, we reported a small molecule, methyl phosphonic acid (MPA), employing which an equipotential performance (PCE= 21.08%) was obtained owing to its passivation effect on the transparent conducting oxide (TCO) layer. Furthermore, by taking motivation from the idea of cosensitization in dye sensitized solar cells, we explored the point that the coabsorption of 2-(9H-carbazol-9-yl) ethyl] phosphonic acid (2PACz) and a small molecule MPA on TCO glass led to the Sn−Pb PSC (1.25 eV) with a PCE of 23.3% and open-circuit voltage of 0.88 V.

Frequently observed high V loss in tin-lead mixed perovskite solar cells is considered to be one of the serious bottle-necks in spite of the high attainable Jsc due to wide wavelength photon harvesting. An amicable solution to minimize the V loss up to 0.50 V has been demonstrated by introducing an n-type interface with spike structure between the absorber and electron transport layer inspired by highly efficient Cu(In,Ga)Se solar cells. Introduction of a conduction band offset of ∼0.15 eV with a thin phenyl-C61-butyric acid methyl ester layer (∼25 nm) on the top of perovskite absorber resulted into improved V of 0.75 V leading to best power conversion efficiency of 17.6%. This enhancement is attributed to the facile charge flow at the interface owing to the reduction of interfacial traps and carrier recombination with spike structure as evidenced by time-resolved photoluminescence, nanosecond transient absorption, and electrochemical impedance spectroscopy measurements.

terms of employing the ideal bandgap absorber layer in PSC, the tin-lead (Sn-Pb) mixed PSCs are now getting attention with the additional benefit of utilizing them in tandem solar cell technology. [8][9][10] In recent years, various research groups have demonstrated power conversion efficiencies (PCEs) of more than 20% by employing Sn-Pb PSCs. [9,[11][12][13][14] However, PCE of Sn-Pb mixed PSCs is still lagging behind their Pb counterparts, especially in terms of open-circuit voltage (V oc ) loss, which is conventionally described as the deficit from the bandgap, is less than 0.3 V for the efficient Pb-PSCs. [4][5][6] Therefore, in the recent past, the efforts are directed toward finding the solutions to overcome the V oc loss. Researchers around the world are trying to address the problem by solving the issues related to the physical properties of the Sn-Pb perovskite films such as short carrier lifetime, [12] large Urbach energy, [11] high trap density, and most importantly the rapid oxidation of Sn 2+ to Sn 4+ . [9,13,14] The focus has been on improving the physical properties by employing different thin film formation strategies. Tong et al., [12] demonstrated the drastic improvement in optoelectronic properties such as the increase in carrier lifetime of more than 1 µs and carrier diffusion length of longer than 1 µm with the incorporation of bulky guanidinium thiocyanate (GuaSCN) into the perovskite films that led to the first research report of more than 20% PCE in Sn-Pb PSCs with a V oc loss of 0.42 V. The increase in PSC performance is assigned to the passivation by the formation of a 2D structure at the grain boundaries that also suppresses the formation of Sn vacancies. This kind of improvement in solar cell performance is the same observation as reported in the case of pure Pb-containing PSCs. [15] Our group also showed the effect of the decrease in trap densities, at the surface and bulk, by using strain engineering, steering to the PCE of 20.4%, and V oc loss of less than 0.50 V. [11] Recently, Li et al., [16] also demonstrated the importance of surface and grain boundary passivation by the formation of 1D pyrrolidine perovskite, a V oc loss of 0.41 V is reported. Lin et al., [9] addressed the oxidation problem in Sn-Pb precursor solution. The Sn metal is introduced as a reducing agent in precursor solution that decreased concentration of Sn 4+ (Sn 4+ + Sn→ 2Sn 2+ ) in the precursor solution before the film formation, Tin-lead perovskite solar cells (PSCs) show inferior power conversion efficiency (PCE) than their Pb counterparts mainly because of the higher open-circuit voltage (V oc ) loss. Here, it is revealed that the p-type surface of perovskite transforms to n-type, based on post-treatment by a Lewis base, ethylenediamine. This approach forms a graded band structure owing to the rise of the Fermi-energy level at the surface of the perovskite layer, and increases the built-in potential from 0.56 to 0.76 V, which increases the V oc by more than 100 mV. It is demonstrated that EDA can lower the ...

Tin−lead (Sn−Pb)-based perovskite solar cells (PSCs) still exhibit inferior power conversion efficiency (PCE) compared to their pure Pb counterparts because of high voltage loss (V L ) and high photocurrent loss in the infrared region. This study explores that a small amount of cesium ion (Cs + ) incorporation in the lattice of Sn−Pb perovskite can reduce the relative lattice strain, which in turn decreases the V L less than 0.50 V. Moreover, surface and bulk trap densities also seem to be reduced by Cs + addition, as concluded by thermally stimulated current measurements and increased carrier lifetime by photoluminescence study. It was discovered that a small amount of Cs + lowered the Urbach energy, which can be used as a signature to optimize the optoelectronic and the photovoltaic properties of multication perovskite materials. This study further demonstrates that a high external quantum efficiency (∼80% at 900 nm) can be obtained with fluorine-doped tin oxide (FTO) glass rather than frequently used indium tin oxide (ITO) glass. The strategies employed in the work improved the open-circuit voltage to 0.81 V and gave a photocurrent density of >30 mA/cm 2 and a PCE of >20% using a band gap of 1.27 eV.