The perovskite/transport layer interfaces dominate non-radiative recombination in efficient perovskite solar cells

By M. Stolterfoht, P. Caprioglio, Christian M. Wolff, José A. Márquez, Joleik Nordmann, Shanshan Zhang, D. Rothhardt, U. Hormann, A. Redinger, Lukas Kegelmann and 5 more. 2018

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Charge transport layers (CTLs) are key components of diffusion controlled perovskite solar cells, however, they can induce additional non-radiative recombination pathways which limit the open circuit voltage ( 𝑉 OC ) of the cell. In order to realize the full thermodynamic potential of the perovskite absorber, both the electron and hole transport layer (ETL/HTL) need to be as selective as possible. By measuring the quasi-Fermi level splitting (QFLS) of perovskite/CTL heterojunctions, we quantify the non-radiative interfacial recombination current for a wide range of commonly used CTLs, including various hole-transporting polymers, spiro-OMeTAD, metal oxides and fullerenes. We find that all studied CTLs limit the 𝑉 OC by inducing an additional non-radiative recombination current that is significantly larger than the loss in the neat perovskite and that the least-selective interface sets the upper limit for the 𝑉 OC of the device. The results also show that the 𝑉 OC equals the internal QFLS in the absorber layer of ( pin , nip ) cells with selective CTLs and power conversion efficiencies of up to 21.4%. However, in case of less selective CTLs, the 𝑉 OC is substantially lower than the QFLS which indicates additional losses at the contacts and/or interfaces. The findings are corroborated by rigorous device simulations which outline several important considerations to maximize the 𝑉 OC . This work shows that the real challenge to supress non-radiative recombination losses in perovskite cells on their way to the radiative limit lies in the suppression of carrier recombination at the perovskite/CTL interfaces. the heterojunction. Extensive drift diffusion simulations identify reasons for this mismatched QFLS- 𝑉 OC and highlight several important aspects for achieving high open-circuit voltages, including the role of energy level offsets and built-in voltage. By these means the presented results highlight that the primary non-radiative recombination loss channel of current perovskite cells is interfacial recombination at (or across) the perovskite/CTL interface, and that suppression of defect recombination in the perovskite bulk is an important yet secondary consideration today. This work represents a significant step forward towards understanding energy losses and the QFLS across the perovskite solar cells stack. Absolute Photoluminescence Imaging: Excitation for the PL imaging measurements was performed with a 445 nm CW laser (Insaneware) through an optical fibre into an integrating sphere. The intensity of the laser was adjusted to a 1 sun equivalent intensity by illuminating a 1 cm 2 -size perovskite solar cell under short-circuit and matching the current density to the 𝐽 SC under the sun simulator (22.0 mA/cm 2 at 100 mWcm -2 , or 1.375x10 21 photons m -2 s -1 ). A second optical fiber was used from the output of the integrating sphere to an Andor SR393i-B spectrometer equipped with a silicon CCD camera (DU420A-BR-DD, iDus). The system was calibrated by using a calibrated halogen lamp with specified spectral irradiance, which was shone into to integrating sphere. A spectral correction factor was established to match the spectral output of the detector to the calibrated spectral irradiance of the lamp. The spectral photon density was obtained from the corrected detector signal (spectral irradiance) by division through the photon energy ( ℎ𝑓 ), and the photon numbers of the excitation and emission obtained from numerical integration using Matlab. In a last step, three fluorescent test samples with high specified PLQY ( ~ 70%) supplied from Hamamatsu Photonics where measured where the specified value could be accurately reproduced within a small relative error of less than 5%. Measurement conditions: All films and cells were prepared fresh and immediately encapsulated in a glovebox after preparation with the exception of films and cells with spiroOMeTAD which require oxygen doping for enabling sufficient transport capability in the device (non-oxgygen treated spiroOMeTAD cells exhibited FFs below 20 % with negligible photovoltaic performance). Thus, films and cells with spiroOMeTAD were treated in atmosphere overnight at 25% relative humidity, and subsequently encapsulated before the PL measurements. The PL of the samples was readily recorded after mounting the sample after an exposure between 10-20 s to the laser light. Thus, the PLQY is obtained on timescales relevant to the 𝑉 OC measurements on the cells. all absolute PL measurements

Subfields: Electrical and Electronic Engineering, Polymers and Plastics

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Citations in the corpus

2D/3D Perovskite Engineering Eliminates Interfacial Recombination Losses in Hybrid Perovskite Solar Cells (2020, background): “This represents an additional interfacial recombination loss that is not captured by measuring the PL emission from the cell.(38,39) Considering that the internal QFLS is significantly higher than the external VOC in the reference device, an energy misalignment between theperovskite and the spiro-OM”
Imaging Grain Structure in Halide Perovskites: Local Crystal Misorientation Influences Non-Radiative Recombination (2019, background): “[9] Stolterfoht M, Caprioglio P, Wolff CM, Márquez JA, Nordmann J, Zhang S, et al.”