Snaith group

Abstract:

Perovskite/perovskite tandem solar cells have recently exceeded the record power conversion efficiency (PCE) of single-junction perovskite solar cells. They are typically built in the superstrate configuration, in which the device is illuminated from the substrate side. This limits the fabrication of the solar cell to transparent substrates, typically glass coated with a transparent conductive oxide (TCO), and adds constraints because the first subcell that is deposited on the substrate must contain the wide-bandgap perovskite. However, devices in the substrate configuration could potentially be fabricated on a large variety of opaque and inexpensive substrates, such as plastic and metal foils. Importantly, in the substrate configuration the narrow-bandgap subcell is deposited first, which allows for more freedom in the device design. In this work, we report perovskite/perovskite tandem solar cells fabricated in the substrate configuration. As the substrate we use TCO-coated glass on which a solution-processed narrow-bandgap perovskite solar cell is deposited. All of the other layers are then processed using vacuum sublimation, starting with the charge recombination layers, then the wide-bandgap perovskite subcell, and finishing with the transparent top TCO electrode. Proof-of-concept tandem solar cells show a maximum PCE of 20%, which is still moderate compared to those of best-in-class devices realized in the superstrate configuration yet higher than those of the corresponding single-junction devices in the substrate configuration. As both the top and bottom electrodes are semitransparent, these devices also have the potential to be used as bifacial tandem solar cells.

Abstract:

We show that adding ethylenediamine (EDA) to perovskite precursor solutions improves the photovoltaic device performance and material stability of high-bromide-content, methylammonium-free, formamidinium cesium lead halide perovskites FA_1–xCs_xPb(I_1–yBr_y)₃, which are currently of interest for perovskite-on-Si tandem solar cells. Using spectroscopy and hyperspectral microscopy, we show that the additive improves film homogeneity and suppresses the phase instability that is ubiquitous in high-Br perovskite formulations, producing films that remain stable for over 100 days in ambient conditions. With the addition of 1 mol % EDA, we demonstrate 1.69 eV-gap perovskite single-junction p-i-n devices with a V_OC of 1.22 V and a champion maximum-power-point-tracked power conversion efficiency of 18.8%, comparable to the best reported methylammonium-free perovskites. Using nuclear magnetic resonance (NMR) spectroscopy and X-ray diffraction techniques, we show that EDA reacts with FA⁺ in solution, rapidly and quantitatively forming imidazolinium cations. It is the presence of imidazolinium during crystallization which drives the improved perovskite thin-film properties.

Abstract:

Successful manipulation of halide perovskite surfaces is typically achieved via the interactions between modulators and perovskites. Herein, it is demonstrated that a strong-interaction surface modulator is beneficial to reduce interfacial recombination losses in inverted (p-i-n) perovskite solar cells (IPSCs). Two organic ammonium salts are investigated, consisting of 4-hydroxyphenethylammonium iodide and 2-thiopheneethylammonium iodide (2-TEAI). Without thermal annealing, these two modulators can recover the photoluminescence quantum yield of the neat perovskite film in contact with fullerene electron transport layer (ETL). Compared to the hydroxyl-functionalized phenethylammonium moiety, the thienylammonium facilitates the formation of a quasi-2D structure onto the perovskite. Density functional theory and quasi-Fermi level splitting calculations reveal that the 2-TEAI has a stronger interaction with the perovskite surface, contributing to more suppressed non-radiative recombination at the perovskite/ETL interface and improved open-circuit voltage (V_OC) of the fabricated IPSCs. As a result, the V_OC increases from 1.11 to 1.20 V (based on a perovskite bandgap of 1.63 eV), yielding a power conversion efficiency (PCE) from ≈20% to 21.9% (stabilized PCE of 21.3%, the highest reported PCEs for IPSCs employing poly[N,N′′-bis(4-butylphenyl)-N,N′′-bis(phenyl)benzidine] as the hole transport layer, alongside the enhanced operational and shelf-life stability for unencapsulated devices.