Enhanced Open-Circuit Voltage of Wide-Bandgap Perovskite Photovoltaics by Using Alloyed (FA_1–xCs_x)Pb(I_1–xBr_x)₃ Quantum Dots

Abstract

We report a detailed study on APbX3 (A = formamidinium (FA+), Cs+; X = I–, Br–) perovskite quantum dots (PQDs) with combined A- and X-site alloying that exhibits both a wide bandgap and high open-circuit voltage (V oc) for the application of a potential top cell in tandem junction photovoltaic (PV) devices. The nanocrystal alloying affords control over the optical bandgap and is readily achieved by solution-phase cation and anion exchange between previously synthesized FAPbI3 and CsPbBr3 PQDs. Increasing only the Br– content of the PQDs widens the bandgap but results in shorter carrier lifetimes and associated V oc losses in devices. These deleterious effects can be mitigated by replacing Cs+ with FA+, resulting in wide-bandgap PQD absorbers with improved charge-carrier mobility and PVs with higher V oc. Although further device optimization is required, these results demonstrate the potential of FA1–x Cs x Pb­(I1–x Br x )3 PQDs for wide-bandgap perovskite PVs with high V oc.

Choose Your Own Adventure: Fabrication of Monolithic All‐Perovskite Tandem Photovoltaics

“…Since the V OC is limited by the E g of the perovskite, the fraction of the Shockley–Queisser V OC (% V OC‐SQ ) is a useful metric to compare across compositions, which is calculated by where the V Exp is the experimentally determined V OC from the solar cell and V SQ is the Shockley–Queisser V OC for the given E g of the material used in the solar cell. [ 111,112 ] We note that the % V OC ‐ SQ is heavily influenced by the exact E g used, which can vary depending on the exact measurement technique used. [ 113 ] Only two wide‐ E g perovskite solar cells have reached a % V OC‐SQ of ≥90%: CsPbI 3 quantum dots (QDs) have reached a V OC of 1.28 V and the BABr treated FA 0.83 Cs 0.17 Pb(I 0.6 Br 0.4 ) 3 reached a V OC of 1.31 V. [ 99,114 ] A total of 13 other perovskites with an E g ≥1.7 eV have reached a % V OC‐SQ of ≥85% [ 51,100,101,103–108,115,116 ] and 5 solar cells with an E g of ≥ 1.8 eV have % V OC ‐ SQ of >80%.…”

“…[ 73,146–148 ] Finally, some perovskites can be deposited using an orthogonal solvent system, such as methylamine/acetonitrile (MA/ACN) for MA‐based perovskites, such as MAPbI 3 [ 23 ] and MAPb 0.75 Sn 0.25 I 3 , [ 72 ] or octane in the case of perovskite QDs. [ 111,112,115,149–151 ] In lieu of using orthogonal solvent systems or solution‐free processing, most all‐perovskite monolithic tandems rely on the recombination stack (Section 3.3) being an effective solvent barrier.…”

“…[ 48,52,130 ] Lower processing temperatures can be reached mainly through the use of evaporated perovskites or orthogonal solvents as described above. [ 23,33,72,111,115,149–151 ] For example deposition of MAPbI 3 from a MA/ACN solvent system and the use of perovskite QDs eliminate the need for annealing. [ 23,150 ] However, simply lowering the annealing temperature or shortening the annealing time can significantly lower the device performance in more traditionally fabricated perovskites.…”

Metal Halide Perovskite Nanocrystal Solar Cells: Progress and Challenges

“…[ 46 ] Furthermore, the solubility limit of Br in the precursor solution (0.4–0.43 m in N , N ‐dimethylformamide (DMF)) is not a challenge for PNCs. [ 47,48 ] The flexible composition adjustment ensures that PNCs with various compositions can be synthesized [ 49 ] Particularly, for FA‐mixed type perovskites, PNCs exhibit more compositional tunability than that of their bulk phase counterparts. [ 46,50 ] Moreover, the synthesis and film deposition procedures for PNCs avoid the use of strong polar solvents, such as DMF and dimethyl sulfoxide (DMSO), with no need for subsequent annealing procedures.…”

“…They concluded that the replacement of I − with Br − resulted in fast carrier recombination and poor charge transport, thus resulting in severe V OC loss, but that the introduction of FA + mitigated the adverse effect. [ 49 ] Hazarika et al. also concluded that FA + controlled the V OC loss [ 84 ] and it has been proven that the formation energies of deep traps become large in FA‐containing PNCs.…”