Konrad Domanski

Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance

Improving the stability of perovskite solar cells Inorganic-organic perovskite solar cells have poor long-term stability because ultraviolet light and humidity degrade these materials. Bella et al. show that coating the cells with a water-proof fluorinated polymer that contains pigments to absorb ultraviolet light and re-emit it in the visible range can boost cell efficiency and limit photodegradation. The performance and stability of inorganic-organic perovskite solar cells are also limited by the size of the cations required for forming a correct lattice. Saliba et al. show that the rubidium cation, which is too small to form a perovskite by itself, can form a lattice with cesium and organic cations. Solar cells based on these materials have efficiencies exceeding 20% for over 500 hours if given environmental protection by a polymer coating. Science, this issue pp. 203 and 206 The seemingly too small rubidium cation was successfully integrated into perovskite solar cells. All of the cations currently used in perovskite solar cells abide by the tolerance factor for incorporation into the lattice. We show that the small and oxidation-stable rubidium cation (Rb+) can be embedded into a “cation cascade” to create perovskite materials with excellent material properties. We achieved stabilized efficiencies of up to 21.6% (average value, 20.2%) on small areas (and a stabilized 19.0% on a cell 0.5 square centimeters in area) as well as an electroluminescence of 3.8%. The open-circuit voltage of 1.24 volts at a band gap of 1.63 electron volts leads to a loss in potential of 0.39 volts, versus 0.4 volts for commercial silicon cells. Polymer-coated cells maintained 95% of their initial performance at 85°C for 500 hours under full illumination and maximum power point tracking.

Not All That Glitters Is Gold: Metal-Migration-Induced Degradation in Perovskite Solar Cells

Perovskite solar cells (PSCs) have now achieved efficiencies in excess of 22%, but very little is known about their long-term stability under thermal stress. So far, stability reports have hinted at the importance of substituting the organic components, but little attention has been given to the metal contact. We investigated the stability of state-of-the-art PSCs with efficiencies exceeding 20%. Remarkably, we found that exposing PSCs to a temperature of 70 °C is enough to induce gold migration through the hole-transporting layer (HTL), spiro-MeOTAD, and into the perovskite material, which in turn severely affects the device performance metrics under working conditions. Importantly, we found that the main cause of irreversible degradation is not due to decomposition of the organic and hybrid perovskite layers. By introducing a Cr metal interlayer between the HTL and gold electrode, high-temperature-induced irreversible long-term losses are avoided. This key finding is essential in the quest for achieving high efficiency, long-term stable PSCs which, in order to be commercially viable, need to withstand hard thermal stress tests.

Highly efficient and stable planar perovskite solar cells by solution-processed tin oxide

Perovskite solar cells (PSCs) are one of the most promising lab-scale technologies to deliver inexpensive solar electricity. Low-temperature planar PSCs are particularly suited for large-scale manufacturing. Here, we propose a simple, solution-processed technological approach for depositing SnO2 layers. The use of these layers in planar PSCs yields a high stabilized power conversion efficiency close to 21%, exhibiting stable performance under real operating conditions for over 60 hours. In addition, this method yielded remarkable voltages of 1214 mV at a band gap of 1.62 eV (approaching the thermodynamic limit of 1.32 V) confirming the high selectivity of the solution-processed layers. PSCs aged under 1 sun illumination and maximum power point tracking showed a final PCE of 20.7% after ageing and dark storage, which is slightly higher than the original efficiency. This approach represents an advancement in the understanding of the role of electron selective layers on the efficiency and stability of PSCs. Therefore, the newly proposed approach constitutes a simple, scalable method paving the way for industrialization of perovskite solar cells.

Unbroken Perovskite: Interplay of Morphology, Electro-optical Properties, and Ionic Movement

Hybrid organic-inorganic perovskite materials have risen up as leading components for light-harvesting applications. However, to date many questions are still open concerning the operation of perovskite solar cells (PSCs). A systematic analysis of the interplay among structural features, optoelectronic performance, and ionic movement behavior for FA0.83 MA0.17 Pb(I0.83 Br0.17 )3 PSCs is presented, which yield high power conversion efficiencies up to 20.8%.

Migration of cations induces reversible performance losses over day/night cycling in perovskite solar cells

Perovskites have been demonstrated in solar cells with a power conversion efficiency of well above 20%, which makes them one of the strongest contenders for next generation photovoltaics. While there are no concerns about their efficiency, very little is known about their stability under illumination and load. Ionic defects and their migration in the perovskite crystal lattice are some of the most alarming sources of degradation, which can potentially prevent the commercialization of perovskite solar cells (PSCs). In this work, we provide direct evidence of electric field-induced ionic defect migration and we isolate their effect on the long-term performance of state-of-the-art devices. Supported by modelling, we demonstrate that ionic defects, migrating on timescales significantly longer (above 103 s) than what has so far been explored (from 10−1 to 102 s), abate the initial efficiency by 10–15% after several hours of operation at the maximum power point. Though these losses are not negligible, we prove that the initial efficiency is fully recovered when leaving the device in the dark for a comparable amount of time. We verified this behaviour over several cycles resembling day/night phases, thus probing the stability of PSCs under native working conditions. This unusual behaviour reveals that research and industrial standards currently in use to assess the performance and the stability of solar cells need to be adjusted for PSCs. Our work paves the way for much needed new testing protocols and figures of merit specifically designed for PSCs.

Identifying and suppressing interfacial recombination to achieve high open-circuit voltage in perovskite solar cells

With close to 100% internal quantum efficiency over the absorption spectrum, photocurrents in perovskite solar cells (PSCs) are at their practical limits. It is therefore imperative to improve open-circuit voltages (VOC) in order to go beyond the current 100 mV loss-in-potential. Identifying and suppressing recombination bottlenecks in the device stack will ultimately drive the voltages up. In this work, we investigate in depth the recombination at the different interfaces in a PSC, including the charge selective contacts and the effect of grain boundaries. We find that the density of grain boundaries and the use of tunneling layers in a highly efficient PSC do not modify the recombination dynamics at 1 sun illumination. Instead, the recombination is strongly dominated by the dopants in the hole transporting material (HTM), spiro-OMeTAD and PTAA. The reduction of doping concentrations for spiro-OMeTAD yielded VOCs as high as 1.23 V in contrast to PTAA, which systematically showed slightly lower voltages. This work shows that a further suppression of non-radiative recombination is possible for an all-low-temperature PSC, to yield a very low loss-in-potential similar to GaAs, and thus paving the way towards higher than 22% efficiencies.

High Temperature-Stable Perovskite Solar Cell Based on Low-Cost Carbon Nanotube Hole Contact

Mixed ion perovskite solar cells (PSC) are manufactured with a metal-free hole contact based on press-transferred single-walled carbon nanotube (SWCNT) film infiltrated with 2,2,7,-7-tetrakis(N,N-di-p-methoxyphenylamine)-9,90-spirobifluorene (Spiro-OMeTAD). By means of maximum power point tracking, their stabilities are compared with those of standard PSCs employing spin-coated Spiro-OMeTAD and a thermally evaporated Au back contact, under full 1 sun illumination, at 60 °C, and in a N2 atmosphere. During the 140 h experiment, the solar cells with the Au electrode experience a dramatic, irreversible efficiency loss, rendering them effectively nonoperational, whereas the SWCNT-contacted devices show only a small linear efficiency loss with an extrapolated lifetime of 580 h.

Additive-Free Transparent Triarylamine-Based Polymeric Hole-Transport Materials for Stable Perovskite Solar Cells.

Triarylamine-based polymers with different functional groups were synthetized as hole-transport materials (HTMs) for perovskite solar cells (PSCs). The novel materials enabled efficient PSCs without the use of chemical doping (or additives) to enhance charge transport. Devices employing poly(triarylamine) with methylphenylethenyl functional groups (V873) showed a power conversion efficiency of 12.3 %, whereas widely used additive-free poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) demonstrated 10.8 %. Notably, devices with V873 enabled stable PSCs under 1 sun illumination at maximum power point tracking for approximately 40 h at room temperature, and in the dark under elevated temperature (85 °C) for more than 140 h. This is in stark contrast to additive-containing devices, which degrade significantly within the same time frame. The results present remarkable progress towards stable PSC under real working conditions and industrial stress tests.

Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency††Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ee03874jClick here for additional data file.

Todays best perovskite solar cells use a mixture of formamidinium and methylammonium as the monovalent cations. Adding cesium improves the compositions greatly.

Interpretation and evolution of open-circuit voltage, recombination, ideality factor and subgap defect states during reversible light-soaking and irreversible degradation of perovskite solar cells

Metal halide perovskite absorber materials are about to emerge as a high-efficiency photovoltaic technology. At the same time, they are suitable for high-throughput manufacturing characterized by a low energy input and abundant low-cost materials. However, a further optimization of their efficiency, stability and reliability demands a more detailed optoelectronic characterization and understanding of losses including their evolution with time. In this work, we analyze perovskite solar cells with different architectures (planar, mesoporous, HTL-free), employing temperature dependent measurements (current–voltage, light intensity, electroluminescence) of the ideality factor to identify dominating recombination processes that limit the open-circuit voltage (Voc). We find that in thoroughly-optimized, high-Voc (≈1.2 V) devices recombination prevails through defects in the perovskite. On the other hand, irreversible degradation at elevated temperature is caused by the introduction of broad tail states originating from an external source (e.g. metal electrode). Light-soaking is another effect decreasing performance, though reversibly. Based on FTPS measurements, this degradation is attributed to the generation of surface defects becoming a new source of non-radiative recombination. We conclude that improving long-term stability needs to focus on adjacent layers, whereas a further optimization of efficiency of top-performing devices requires understanding of the defect physics of the nanocrystalline perovskite absorber. Finally, our work provides guidelines for the design of further dedicated studies to correctly interpret the diode ideality factor and decrease recombination losses.