I am PhD student at EPFL, Switzerland working on a new generation of solar cells.
I am making perovskite solar cells stable to make them commercially attractive.
Perovskite solar cells (PSCs) have attracted a substantial interest owing to a very fast achievement of efficiencies &gt;20 % within only several years of development. However, for any solar cell to become technologically viable, two additional milestones have to be achieved alongside high efficiency: means of industrial production and good operational stability. The former being mostly the focus of private sector, understanding degradation and improving the stability of PSCs has become the main research topic in the field of emerging photovoltaics. During my PhD, I first designed and built a dedicated setup to investigate stability of PSCs. Subsequently, I described the intrinsic instability of state-of-the-art PSCs and I showed how through incorporation of Cs into perovskite breakthrough room-temperature stability coupled with record efficiency can be achieved. Later, I discovered however, that these solar cells suffer from irreversible degradation if left at elevated temperatures. This is due to a vulnerability to Au diffusion from the electrode. I discovered, that by using a Cr diffusion barrier, one can circumvent this problem (albeit at considerable efficiency loss). Subsequently, I described an alternative solution to the same problem by using a polymeric hole transporting layer. This approach effectively stops Au diffusion without compromising device efficiency. Finally, I found a third approach: replacing the expensive Au electrode with one based on carbon nanotubes. In parallel, I also explained how mobile ions in the perovskite lead to a partial reversibility of losses in aged devices. This has far-reaching consequences for the way stability measurements of PSCs are conducted and how their lifetime is reported. Most recently, I described how different factors such as temperature, illumination, atmosphere and load on the device affect the stability of PSCs. Based on this, I recommend PSC stability measurement protocols as the first attempt to bring the community to a consensus on how to age PSCs. This is needed to streamline the efforts to create stable PSCs and to commercialize the technology.
Abstract: All of the cations currently used in perovskite solar cells (PSCs) abide by the tolerance factor for incorporation into the lattice. We show that the small and oxidation-stable 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 0.5 cm(2) cell) 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 loss-in-potential of 0.39 V, versus 0.4 V for commercial silicon cells. Polymer-coated cells maintained 95% of their initial performance at 85°C for 500 hours under full solar illumination and maximum power point tracking.
Pub.: 07 Oct '16, Pinned: 25 Aug '17
Abstract: 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 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, the 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, to in order to be commercially viable, need to withstand hard thermal stress tests.
Pub.: 18 May '16, Pinned: 25 Aug '17
Abstract: 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.
Pub.: 10 Jan '17, Pinned: 25 Aug '17
Abstract: Today's best perovskite solar cells use a mixture of formamidinium and methylammonium as the monovalent cations. With the addition of inorganic cesium, the resulting triple cation perovskite compositions are thermally more stable, contain less phase impurities and are less sensitive to processing conditions. This enables more reproducible device performances to reach a stabilized power output of 21.1% and ∼18% after 250 hours under operational conditions. These properties are key for the industrialization of perovskite photovoltaics.
Pub.: 16 Mar '16, Pinned: 25 Aug '17
Abstract: 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.
Pub.: 23 Feb '17, Pinned: 25 Aug '17
Abstract: Organic–inorganic lead halide perovskites have recently received significant attention as active materials for high-performance photovoltaics and photodetectors. However, the understanding of their operation mechanism remains limited. High-gain, low-voltage CH3NH3PbI3 photodetectors in various architectures are demonstrated herein. Photomultiplication in all structures with direct contact of fluorine-doped tin oxide (FTO) and perovskite with the highest responsivity 208 A W−1 corresponding to an incident photon-to-current efficiency of 47 000% is observed. Studying the dynamics and temperature dependence, a slow process with an activation energy of 420 ± 90 meV in the time scale of seconds is found, which is essential to photocurrent multiplication. A model based on ion migration to explain the observed transients and the photomultiplication is developed. The accumulation of negative ionic charge at the FTO/perovskite interface under reverse bias lowers the FTO work function allowing for direct hole injection into the perovskite valence band. Under illumination, the conductivity of perovskite is increased and the device behaves similar to a photoconductor.
Pub.: 20 Oct '15, Pinned: 25 Aug '17
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