David McMeekin

Abstract:

Lead mixed halide perovskites are highly promising semiconductors for both multi-junction photovoltaic and light emitting applications due to their tunable band gaps, with emission and absorption energies spanning the UV-visible to near IR regions. However, many such perovskites exhibit unwanted halide segregation under photoillumination, the cause of which is still unclear. In our study, we establish crucial links between crystal phase stability, photostability and optoelectronic properties of the mixed-cation lead mixed-halide perovskite CsyFA(1-y)Pb(BrxI(1-x))3. We demonstrate a region for caesium content between 0.10 < y < 0.30 which features high crystalline quality, long chargecarrier lifetimes and high charge-carrier mobilities. Importantly, we show that for such high-quality perovskites, photoinduced halide segregation is strongly suppressed, suggesting that high crystalline quality is a prerequisite for good optoelectronic quality and band gap stability. We propose that regions of short-range crystalline order aid halide segregation, possibly by releasing lattice strain between iodide rich and bromide rich domains. For an optimized caesium content, we explore the orthogonal halide-variation parameter space for Cs0.17FA0.83Pb(BrxI(1-x))3 perovskites. We demonstrate excellent charge-carrier mobilities (11-40 cm2 V^−1 s^−1) and diffusion lengths (0.8 - 4.4 µm) under solar conditions across the full iodide-bromide tuning range. Therefore, the addition of caesium yields a more photostable perovskite system whose absorption onsets can be tuned for bandgap-optimized tandem solar cells.

Abstract:

Multi-junction solar photovoltaics are proven to deliver the highest performance of any solar cell architecture, making them ideally suited for deployment in an increasingly efficiency driven solar industry. Conventional multi-junction cells reach up to 45% efficiency, but are so costly to manufacture that they are only currently useful for space and solar concentrator photovoltaics. Here, we demonstrate the first four and two-terminal perovskite-perovskite tandem solar cells with ideally matched bandgaps. We develop an infrared absorbing 1.2eV bandgap perovskite, FA0.75Cs0.25Sn0.5Pb0.5I3, which is capable of delivering 13.6% efficiency. By combining this material with a wider bandgap FA0.83Cs0.17Pb(I0.5Br0.5)3 material, we reach initial monolithic two terminal tandem efficiencies of 14.0 % with over 1.75 V open circuitvoltage. We also make mechanically stacked four terminal tandem cells and obtain 18.1 % efficiency for small cells, and 16.0 % efficiency for 1cm^2 cells. Crucially, we find that our infrared absorbing perovskite cells exhibit excellent thermal and atmospheric stability, unprecedented for Sn based perovskites. This device architecture and materials set will enable “all perovskite” thin film solar cells to reach the highest efficiencies in the long term at the lowest costs, delivering a viable photovoltaic technology to supplant fossil fuels.

Abstract:

The power-conversion efficiency of perovskite solar cells has soared up to 22.1% earlier this year. Within merely five years, the perovskite solar cell can now compete on efficiency with inorganic thin-film technologies, making it the most promising of the new, emerging photovoltaic solar cell technologies. The next grand challenge is now the aspect of stability. The hydrophilicity and volatility of the organic methylammonium makes the work-horse material methylammonium lead iodide vulnerable to degradation through humidity and heat. Additionally, ultraviolet radiation and oxygen constitute stressors which can deteriorate the device performance. There are two fundamental strategies to increasing the device stability: developing protective layers around the vulnerable perovskite absorber and developing a more resilient perovskite absorber. The most important reports in literature are summarized and analyzed here, letting us conclude that any long-term stability, on par with that of inorganic thin-film technologies, is only possible with a more resilient perovskite incorporated in a highly protective device design.

Abstract:

Perovskite solar cells have shown phenomenal progress and have great potential to be manufactured as low‐cost large area modules. However, perovskite films often suffer from pinholes and the resulting contact between hole‐ and electron transporting layers provides lower resistance (shunt) pathways, leading to decreased open‐circuit voltage and fill factor. This problem is even more severe in large area cells and especially in the case of neutral color semitransparent cells, where a large absorber‐free area is required to provide the desired transparency. Herein, a simple, inexpensive, and scalable wet chemical method is presented to block these “shunting paths” via deposition of transparent, insulating molecular layers, which preferentially bind to the uncovered surface of the electron collecting oxide, without hindering charge extraction from the perovskite to the charge collection layers. It is shown that this method improves the performance in semitransparent cells, where the enhancement in open‐circuit voltage is up to 30% without negatively impacting the photocurrent. Using this method, we achieved an efficiency of 6.1% for a neutral color semitransparent perovskite cell with 38% average visible transmittance. This simple shunt blocking technique has applications in improving the yield as well as efficiency of large area perovskite solar cells and light emitting devices.

Abstract:

Metal halide perovskite photovoltaic cells could potentially boost the efficiency of commercial silicon photovoltaic modules from ∼20 toward 30% when used in tandem architectures. An optimum perovskite cell optical band gap of ~1.75 electron volts (eV) can be achieved by varying halide composition, but to date, such materials have had poor photostability and thermal stability. Here we present a highly crystalline and compositionally photostable material, [HC(NH2)2](0.83)Cs(0.17)Pb(I(0.6)Br(0.4))3, with an optical band gap of ~1.74 eV, and we fabricated perovskite cells that reached open-circuit voltages of 1.2 volts and power conversion efficiency of over 17% on small areas and 14.7% on 0.715 cm(2) cells. By combining these perovskite cells with a 19%-efficient silicon cell, we demonstrated the feasibility of achieving >25%-efficient four-terminal tandem cells.