Raising the open-circuit voltage and ambient stability of all-inorganic CsSnI3 lead-free perovskite solar cells: suppressing tin-vacancy self-doping and Sn2+ oxidation beyond the SnF2 vacancy-modulation baseline

Kumar et al. (2014) demonstrated solution-processed, all-inorganic, lead-free CsSnI3 perovskite solar cells whose high photocurrents ($J_{sc}=22.7$ mA/cm$^2$, spectral response to 950 nm) came at the price of a very low open-circuit voltage ($V_{oc}\sim 0.24$ V), attributed to a high density of acceptor-type tin vacancies ($V_{Sn}$) that self-dope the absorber p-type. Adding SnF2 partially suppressed the vacancy density, but the voltage remained far below both lead-perovskite devices and the radiative-limit voltage implied by the ~1.27 eV bandgap, and devices survived only ~250 hours under inert atmosphere. Later work has largely resolved the analogous self-doping, oxidation, and degradation problems for organic-cation tin perovskites (the FASnI3 family, now above 14% certified efficiency) and has elucidated the Sn$^{2+}$ -> Sn$^{4+}$ oxidation and SnI4-driven degradation mechanisms. CsSnI3-specific work has also made major partial progress: efficiencies up to ~12.6% for pure CsSnI3, open-circuit voltages up to 0.75 V in Br-substituted CsSnI3-xBrx, and short-term (hours-to-weeks) ambient-air stability of unencapsulated devices. For the all-inorganic CsSnI3 system of the source paper, however, the central objective remains open: identify chemical routes and processing conditions that push tin-vacancy and Sn(IV) defect densities low enough to raise $V_{oc}$ toward the bandgap-implied radiative-limit voltage (the ~0.9 V class), and establish which combination of bulk-defect mitigation and interfacial passivation can deliver that voltage together with long-term unencapsulated ambient operational stability.

Background

Perovskite solar cells (PSCs) use an ABX3 metal-halide semiconductor as the light absorber. Lead-based PSCs exceed 25% certified power-conversion efficiency, but the toxicity of lead motivates replacing Pb$^{2+}$ with the similar-size Sn$^{2+}$. The all-inorganic compound CsSnI3 is an attractive candidate: its ~1.27 eV bandgap (the energy gap between valence and conduction band, setting both the achievable current and the maximum voltage) is near optimal, and it absorbs out to ~950 nm, supporting photocurrents comparable to lead perovskites. Its defining defect is the tin vacancy: unlike lead perovskites, Sn$^{2+}$ is easily oxidized, tin vacancies form with low energy, and the resulting $V_{Sn}$ acts as an acceptor, so as-grown CsSnI3 is strongly self-doped p-type with hole densities of order $10^{19}$ cm$^{-3}$. This high background carrier density causes the Fermi level to sit deep in the gap, so the built-in potential and the open-circuit voltage $V_{oc}$ (the voltage at which the photocurrent is exactly balanced by recombination) collapse even when the short-circuit current density $J_{sc}$ is high. In the source paper, Kumar et al. (2014) introduced 'vacancy modulation': adding 20 mol% SnF2 to the precursor lowered the hole density by roughly two orders of magnitude, rationalized thermodynamically by Materials-Project (a public database of density-functional-theory computed formation energies) calculations showing that coexistence of a SnF2 phase raises the tin-vacancy formation energy. The resulting champion device still gave only $V_{oc}\sim 0.24$ V, fill factor FF = 0.37 (the ratio of the maximum power point to the $J_{sc}$-$V_{oc}$ product), and 2.02% efficiency, versus >1 V and >15% for lead analogues, and degraded within ~250 hours even in a glovebox. The paper also identified tin oxidation-state control (Sn$^{4+}$ impurities in SnI2 precursors are detrimental) and hole-transport-material (HTM, the layer extracting holes) choice as further variables; the layer extracting electrons, the electron transport layer (ETL), was later engineered as well. Key relevant conventions: the voltage deficit is the difference between the bandgap energy per unit charge ($qE_g\approx 1.27$ V here) and the measured $V_{oc}$; 'radiative limit' denotes the $V_{oc}$ of an ideal Shockley-Queisser-type cell limited only by band-to-band radiative recombination, which for a ~1.27 eV bandgap lies in the ~0.9 V class. Subsequent literature (Marshall et al. 2015, 2016; Lee et al. 2017; Leijtens et al. 2017; Gupta et al. 2016; Lanzetta et al. 2021) built the mechanistic picture of tin oxidation, SnF2's role, and degradation; the organic-cation FASnI3 branch reached >14% certified efficiency; and CsSnI3-specific work climbed to ~12.6% efficiency and, in Br-substituted CsSnI3-xBrx, $V_{oc}$ up to 0.75 V with short-term unencapsulated ambient stability. The CsSnI3-specific voltage and stability problem framed by the source paper persists: reported voltages remain several hundred millivolts below the radiative limit implied by the ~1.27 eV bandgap, and no unencapsulated CsSnI3 device has demonstrated long-term ambient operation.

Problem Statement

For solution-processed all-inorganic CsSnI3 perovskite solar cells, with the Kumar et al. (2014) SnF2 vacancy-modulation configuration as the originating baseline: identify and demonstrate the chemical routes, precursor and oxidation-state control, and processing conditions that reduce tin-vacancy and Sn(IV)-related defect densities enough to raise the open-circuit voltage $V_{oc}$ toward the radiative-limit voltage implied by the ~1.27 eV bandgap of CsSnI3 (the ~0.9 V class, versus $V_{oc}\sim 0.24$ V in the source paper and at most 0.75 V reported since, in the larger-bandgap Br-substituted CsSnI3-xBrx), and determine quantitatively how much of the remaining recombination and voltage loss is bulk (self-doping and trap-assisted) versus interfacial (perovskite/HTM and perovskite/ETL), so that bulk-defect mitigation and interfacial passivation can be engineered jointly. The same defect and oxidation control must simultaneously deliver long-term device stability under ambient (oxygen- and moisture-containing) rather than inert-atmosphere operating conditions, extending the ~250 h glovebox-only result of the source paper and the ~120 h unencapsulated to ~45 day encapsulated ambient demonstrations reported since, toward operationally meaningful unencapsulated lifetimes, with the operative degradation mechanisms identified. These components are jointly required: a $V_{oc}$ gain together with the quantitative bulk-versus-interfacial voltage-loss split, unencapsulated ambient stability beyond the inert-atmosphere baseline, and an identified operative degradation mechanism.

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