Efficiency of bulk perovskite-sensitized upconversion: Illuminating matters

Abstract

Photon upconversion via triplet–triplet annihilation could allow for the existing efficiency limit of single junction solar cells to be surpassed. Indeed, efficient upconversion at subsolar fluences has been realized in bulk perovskite-sensitized systems. Many questions have remained unanswered, in particular, regarding their behavior under photovoltaic operating conditions. Here, we investigate the impact of repeated and continuous illumination on bilayer perovskite/rubrene upconversion devices. We find that variations of the underlying perovskite carrier recombination dynamics greatly impact the upconversion process. Trap filling and triplet sensitization are in direct competition: more saturated trap states in the perovskite and, thus, longer underlying perovskite photoluminescence lifetimes allow for an increased number of carriers to diffuse to the perovskite/rubrene interface and undergo charge extraction to the triplet state of rubrene. As a result, the upconversion efficiency is greatly influenced by the underlying trap density: the upconverted photoluminescence intensity increases by two orders of magnitude under continuous illumination for 4 h. This shows that the upconversion efficiency is difficult to define for this system. Importantly, these results indicate that perovskite-sensitized upconversion devices exhibit peak performance under continuous illumination, which is a requirement for their successful integration into photovoltaics to help overcome the Shockley–Queisser limit in single junction solar cells.

Ultrafast Triplet Generation at the Lead Halide Perovskite/Rubrene Interface

“…However, at later times, the dynamics slow and the underlying MAFA kinetics are recovered, indicating a population of carriers that are unaffected by the addition of rubrene, which we have previously attributed to charges in the bulk of the perovskite that do not reach the interface prior to recombination. 19,23 PIA1 grows in more rapidly for MAFA/rub than MAFA, initially recovers faster, and then appears to stagnate at later times. This is the result of multiple factors.…”

“…[10][11][12] Due to spin selectrion rules, the spin-triplet states required for TTA-UC are populated via energy transfer from triplet sensitizers that generate triplet states through intersystem crossing, 8,13,14 inherently exhibit an excitonic wavefunction that possesses both triplet and singlet character, 7,[15][16][17][18] or through asynchronous charge transfer (CT) processes from bulk lead halide perovskite materials. 12,[19][20][21][22][23][24][25] The long free carrier lifetimes, high absorption cross sections and facile bandgap tunability of perovskites, [26][27][28][29] which are desirable properties in photovoltaic applications also yield the required foundation for triplet sensitization. 12,[19][20][21][22][23] It is established that lead halide perovskites of varying halide and A-site compositions are capable of sensitizing the triplet state of rubrene.…”

“…12,[19][20][21][22][23][24][25] The long free carrier lifetimes, high absorption cross sections and facile bandgap tunability of perovskites, [26][27][28][29] which are desirable properties in photovoltaic applications also yield the required foundation for triplet sensitization. 12,[19][20][21][22][23] It is established that lead halide perovskites of varying halide and A-site compositions are capable of sensitizing the triplet state of rubrene.…”

Bulk Metal Halide Perovskites as Triplet Sensitizers: Taking Charge of Upconversion

“…To date, all perovskite-sensitized UC devices have been based on a bilayer structure consisting of various compositions of methylammonium and formamidinium lead triiodide perovskites and the organic annihilator rubrene doped with ∼1% dibenzotetraphenyl­periflanthene (DBP). − Rubrene, a tetracene derivative, the current workhorse of visible-to-infrared UC is chosen due to its history of efficient UC in both solution and solid-state UC configurations. ,, A representative device structure is shown in Figure a, where the perovskite thickness can be varied between ∼14 and 380 nm, , and rubrene layers are commonly ∼50–100 nm. Perovskite-based UC devices debuted in 2019 with an internal UC efficiency, η UC , of 3% (normalized to a 100% maximum), which allowed them to immediately rival previous PbS NC-based solid-state UC devices peaking at 7% .…”

“…Several unanticipated effects have been observed in this system, including (i) rapid yet reversible “photobleaching” of the upconverted emission upon initial illumination, due to a pre-charging effect stemming from interfacial band bending, (ii) two distinct rates of TTA resulting in a population-dependent UC efficiency, and (iii) distinct spatial variations in the UC efficiency which cannot simply be traced to a higher trap density of the perovskite or varying photoluminescence (PL) quantum yield (QY) of rubrene. , …”