Abstract
ABSTRACT The possibility to manufacture perovskite solar cells (PSCs) at low temperatures paves the way to flexible and lightweight photovoltaic (PV) devices manufactured via high-throughput roll-to-roll processes. In order to achieve higher power conversion efficiencies, it is necessary to approach the radiative limit via suppression of non-radiative recombination losses. Herein, we performed a systematic voltage loss analysis for a typical low-temperature processed, flexible PSC in n-i-p configuration using vacuum deposited C60 as electron transport layer (ETL) and two-step hybrid vacuum-solution deposition for CH3NH3PbI3 perovskite absorber. We identified the ETL/absorber interface as a bottleneck in relation to non-radiative recombination losses, the quasi-Fermi level splitting (QFLS) decreases from ~1.23 eV for the bare absorber, just ~90 meV below the radiative limit, to ~1.10 eV when C60 is used as ETL. To effectively mitigate these voltage losses, we investigated different interfacial modifications via vacuum deposited interlayers (BCP, B4PyMPM, 3TPYMB, and LiF). An improvement in QFLS of ~30–40 meV is observed after interlayer deposition and confirmed by comparable improvements in the open-circuit voltage after implementation of these interfacial modifications in flexible PSCs. Further investigations on absorber/hole transport layer (HTL) interface point out the detrimental role of dopants in Spiro-OMeTAD film (widely employed HTL in the community) as recombination centers upon oxidation and light exposure.
Highlights
Organic-inorganic hybrid perovskite solar cells (PSC) have achieved a certified record efficiency of 23.7%, overtaking other more mature thin-film PV technologies, like CdTe and CIGS [1,2]
We observed an increase of ~30–40 meV in quasi-Fermi level splitting (QFLS) that is comparable to the improvements in the open-circuit voltage (Voc) observed when these interlayers are implemented in flexible PSCs
PSCs were grown on flexible foil which is used as a moisture barrier front sheet for encapsulation in flexible CIGS modules. 5 cm × 5 cm-size flexible substrates were washed by hand followed by ultrasonic soap and water baths
Summary
Organic-inorganic hybrid perovskite solar cells (PSC) have achieved a certified record efficiency of 23.7%, overtaking other more mature thin-film PV technologies, like CdTe and CIGS [1,2]. The community would benefit from interface engineering approaches for vacuum deposited C60 in n-i-p configurations (where the perovskite absorber is directly deposited onto the ETL), considering the potential large-area scalability of vacuum-based methods. After having identified the formation of the ETL/perovskite interface as a significant source of non-radiative recombination losses, we screened several possible interlayers to suppress interfacial recombination. We observed an increase of ~30–40 meV in QFLS that is comparable to the improvements in the open-circuit voltage (Voc) observed when these interlayers are implemented in flexible PSCs. Further investigations have shown that, the initially underestimated absorber/HTL interface represents a significant source of non-radiative recombination losses, potentially due to detrimental chemical modifications at the perovskite/HTL interface as a consequence of oxidation and light exposure of the doped HTL material
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