Transient terahertz photoconductivity measurements of minority-carrier lifetime in tin sulfide thin films: Advanced metrology for an early stage photovoltaic material

Abstract

Materials research with a focus on enhancing the minority-carrier lifetime of the light-absorbing semiconductor is key to advancing solar energy technology for both early-stage and mature material platforms alike. Tin sulfide (SnS) is an absorber material with several clear advantages for manufacturing and deployment, but the record power conversion efficiency remains below 5%. We report measurements of bulk and interface minority-carrier recombination rates in SnS thin films using optical-pump, terahertz (THz)-probe transient photoconductivity (TPC) measurements. Post-growth thermal annealing in H2S gas increases the minority-carrier lifetime, and oxidation of the surface reduces the surface recombination velocity. However, the minoritycarrier lifetime remains below 100 ps for all tested combinations of growth technique and postgrowth processing. Significant improvement in SnS solar cell performance will hinge on finding and mitigating as-yet-unknown recombination-active defects. We describe in detail our methodology for TPC experiments, and we share our data analysis routines in the form freelyavailable software.For a more sophisticated solar cell figure of merit, we consider the dimensionless ratio (FPV) of minority-carrier diffusion length to optical absorption length:D is the minority carrier diffusivity,  is the bulk minority-carrier lifetime, and  is the optical absorption coefficient. In Figure 1b we present compiled data for FPV and solar cell efficiency, including results for wafer-based silicon technologies. With the exception of silicon, all of the materials represented strongly absorb light at energies above their respective band gaps. For each material we calculate FPV using  as measured at the knee the curve of log10((E)). For the thin film materials we require that lifetime, diffusivity, absorption coefficient, and device measurements were reported for samples that were synthesized in the same laboratory and using as close to the same procedure as is reasonably possible. This requirement greatly reduces the number of data points compared to Figure 1a. For crystalline silicon we assume the values D = 30 cm 2 s -1 and  = 300 cm -1 .SnS is an absorber with several inherent advantages compared to materials that are widely used in solar cells, but its demonstrated efficiency is too low for commercial relevance. It is composed of non-toxic, Earth-abundant and inexpensive elements. SnS is an inert and waterinsoluble semiconducting mineral (Herzenbergite) with an indirect bandgap of 1.1 eV, strong light absorption for photons with energy above 1.4 eV ( > 10 4 cm -1 ), and intrinsic p-type conductivity with carrier concentration in the range 10 15 -10 17 cm -3 . 5-7 SnS evaporates congruently and is phase-stable up to 600 °C. 8,9 This means that SnS thin films can be deposited by thermal evaporation and its high-speed cousin, closed space sublimation (CSS), as is employed in the manufacture of CdTe solar cells. It also means that SnS phase control is simpler than for most thin film PV materials, nota...

Charge Carrier Dynamics and Mobility Determined by Time-Resolved Terahertz Spectroscopy on Films of Nano-to-Micrometer-Sized Colloidal Tin(II) Monosulfide

“…22-24 Despite applicability to a wide variety of materials, only recently has the terahertz photoconductivity been reported in SnS thin films prepared by thermal evaporation and atomic layer deposition. 25 …”

Strongly Enhanced Photovoltaic Performance and Defect Physics of Air‐Stable Bismuth Oxyiodide (BiOI)

“…[11,15,16] This is significant because analysis of established thin-film solar absorbers has shown that materials with minority-carrier lifetimes of 1 ns or above have the potential to reach or exceed 10% efficiency. [17] Many of these bismuth halides and chalcohalides have also demonstrated improved air-stability compared to hybrid lead-halide perovskites. [11,13,15] However, devices based on these materials have low reported power conversion efficiencies under AM 1.5G illumination, between 0.1 and 1.2%.…”

Perovskite-Inspired Photovoltaic Materials: Toward Best Practices in Materials Characterization and Calculations

“…These include BiI 3 , 19,20 (CH 3 NH 3 ) 3 Bi 2 I 9 , [21][22][23][24] BiSI, 25,26 BiSeI, 25,26 K 3 Bi 2 I 9 , 27 Rb 3 Bi 2 I 9 , 27 Cs 3 Bi 2 I 9 , 27 BiOI, 28,29 Cs 3 Sb 2 I 9 , 30 SbSI, 31 SbSeI, 31 Sb 2 Se 3 , 32 CuSbSe 2 , 33,34 (CH 3 NH 3 ) 2 KBiCl 6 , 35 and SnS. 36 Many other recent works have been written on understanding the properties of hybrid lead-halide perovskites that make them excellent solar absorbers, e.g., energetic carrier screening via polarons, which may lead to new design rules. [37][38][39][40][41][42][43] In many cases, these materials are being investigated as solar absorbers for the first time.…”

“…UV-visible 3 spectrophotometry and ellipsometry can be used to determine the bandgap and absorption coefficient. The minority carrier lifetime and mobility can be measured through time resolved photoluminescence, 44 terahertz spectroscopy, 36,45,46 and photoconductivity decay measurements. 47 Challenges also exist in performing quantum mechanical calculations on some of these materials.…”