Decoupling radiative and non-radiative losses in lead free perovskite solar cells

Project Information

RADICEL

Grant agreement ID: 101030985

DOI

10.3030/101030985

Project closed

EC signature date 8 March 2021

Start date 1 October 2021

End date 30 September 2023

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 174 806,40

EU contribution

€ 174 806,40

Coordinated by

UNIVERSITAT KONSTANZ
Germany

Periodic Reporting for period 1 - RADICEL (Decoupling radiative and non-radiative losses in lead free perovskite solar cells)

Reporting period: 2021-10-01 to 2023-09-30

Summary of the context and overall objectives of the project

Lead (Pb) based perovskite solar cells (PSCs) demonstrate a certified power conversion efficiency (PCE of 26.1%). Their toxicity, however, raises environmental concerns and might hinder their commercial deployment. Quest for non-toxic PSCs is still at its infancy: The PCE in Pb-free PSCs is merely around 14%, which is about one third of their radiative limit, which is because of the high voltage loss and a low fill factor in Pb-free PSCs.

In this work, focusing on the properties of tin-perovskites, we first improved their bulk properties. We employed methylammonium chloride (MACl)-assisted crystallization as a route to improve stability and optoelectronic quality of quasi 2D/3D PEA0.08FA0.92SnI3perovskite. For an optimal additive amount (10 mol%), a 37% increase in power conversion efficiency is found. Notably, MACl enhances the films’ stability, evidenced by temporal photoluminescence tracking. We shows that MACl addition causes a shift in the optical bandgap and improves morphology, indicating effects in the bulk crystal structure. X-ray photoelectron spectroscopy confirms the presence of Cl on the surface, but no indication of MA is found. Intriguingly, UV photoelectron spectroscopy shows pronounced changes in the density of states. For the first time, it is shown that MACl promotes the formation of a two-dimensional layer via the surface accumulation of PEA. The MACl additive lowers the absorber’s ionization energy, possibly facilitating hole extraction.

In another work, we demonstrate the importance of energy alignment between charge transport layers and perovskite in further enhancing the photovoltaic performance of tin-based PSCs. We alter the properties of widely used PEDOT:PSS hole transport layer (HTL) via embedding Ti3C2Tx MXene and investigate its impact on the photovoltaic properties of PSCs. Both scanning electron and atomic force microscopies show improved perovskite film formation with reduced pinhole, while Kelvin probe force microscopy concluded a lower variation in contact potential difference when MXene is embedded into PEDOT:PSS HTL. The work function of the HTL was increased according to photoelectron spectroscopy in air measurement, leading to a favourable energy alignment with the HOMO of PEA0.2FA0.8SnI3 perovskite. Coupled with an improved average carrier lifetime, PSCs fabricated using MXene-embedded PEDOT:PSS HTL shows a 13.5% improvement in photovoltaic performance as compared to the pristine PEDOT:PSS HTL counterpart, while retaining ~90% of its initial PCE after 450 hours of storage in nitrogen atmosphere.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

In the first experiment focusing on improving bulk properties of tin perovskite, we provided a thorough investigation on the role of MACl on the crystallinity, film composition, surface density of states (DOS), and optoelectronic properties of PEA0.08FA0.92SnI3 perovskites. We demonstrate that the addition of MACl into a PEA0.08FA0.92SnI3precursor solution PEAþ=phenethylammonium promotes the formation of a PEA-rich(2D) surface that correlates linearly with the MACl concentration and is evident from the induced changes in the DOS measured using ultraviolet photoelectron spectroscopy (UPS). MACl inclsion also improves the morphology and crystallinity of thePEA0.08FA0.92SnI3films. An optimized MACl concentration of 10 mol% enhanced the carrier lifetime three folds and resulted in an improved photostability of perovskite films in air. Importantly, we find a 37% increase in the PCE compared to a MACl-free counterpart. Combining the MACl additive with a surface passivation strategy, we achieve a champion PCE of 8.25% in n–i–p PSCs. Remarkably, the devices were processed in a multipurpose glovebox with a relative high level ofO2(3–7 ppm) and H2O(5–8 ppm), differently from the sub-ppm values conditions reported by other groups.

In the second example, we investigated the effect of embedding Ti3C2Tx Mxene into the PEDOT:PSS HTL on the photovoltaic performance of the PEA0.2FA0.8SnI3 tin-based PSCs. A low concentration of 2D MXenes into the PEDOT:PSS HTL enhanced the ionic conductivity of the PEDOT:PSS HTL, leading to better photovoltaic performance through enhancing the charge extraction and transport. The PSCs with Ti3C2Tx-embedded PEDOT:PSS as the HTL also showed enhanced shelf-life stability than their pristine PEDOT:PSS counterparts at similar experimental conditions. A thorough investigation of the optoelectornic properties of Mxene embedding HTLs and their application as HTL in PSC is thoroughly explained.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The key findings of the project can be described in three main parts. In the first part, we focused on understanding and improving the energy alignment between charge transport layers and perovskite for further enhancing the photovoltaic performance of tin-based perovskite solar cells (PSCs). Scanning electron and atomic force microscopies show improved perovskite film formation with reduced pinhole, while Kelvin probe force microscopy concluded a lower variation in contact potential difference when MXene is embedded into PEDOT:PSS HTL. The work function of the HTL was increased according to photoelectron spectroscopy in air measurement, leading to a favorable energy alignment with the HOMO of PEA0.2FA0.8SnI3 perovskite. Coupled with an improved average carrier lifetime, PSCs fabricated using MXene-embedded PEDOT:PSS HTL shows a 13.5% improvement in photovoltaic performance as compared to the pristine PEDOT:PSS HTL counterpart, while retaining ~90% of its initial PCE after 450 hours of storage in nitrogen atmosphere.

In the second part, we stabilized the lead free perovskite via bulk passivation. This is because, the most common of the Pb-free perovskite i.e. tin (Sn) perovskites asuffer from inherent material instability and difficulty to control crystallization which leads to high defect density in the thin films. We demonstrate methylammonium chloride (MACl)-assisted crystallization as a route to simultaneously improve stability and optoelectronic quality of quasi 2D/3D PEA0.08FA0.92SnI3 perovskite. For an optimum value of 10% of this additive we find a 37% increase in power conversion efficiency. Notably, the MACl inclusion enhances ambient- and photo stability of the perovskite films, as evidenced by temporal PL tracking of non-encapsulated films in air. The MACl additive also leads to reduction in ionization energy from 5.4 eV in the reference samples to 5.2 eV for the 10% MACl counterpart, which could facilitate efficient hole extraction in the devices. Overall, our work highlights a facile route to control the crystallization of Sn-perovskites to simultaneously enhance their optoelectronic quality and ambient stability.

Finally, we extended the use of partially Pb free perovskite in making air stable and high efficiency all perovskite tandem solar cells. TWe investigate the interface between widely used hole transport layer i.e. PEDOT:PSS and narrow bandgap (NBG) Pb:Sn perovskite. We demonstrate that inclusion of interlayers between PEDOT:PSS and perovskite reduces leakage current and results in over 60 mV enhanced VOC, and also improves FF in the device. This together with surface treatment of Pb:Sn perovskite results in PCE approaching 20% in single-junction devices and exceeding 25% for all-perovskite TSCs.

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