Compositional and Interface Engineering of Organic-Inorganic Lead Halide Perovskite Solar Cells

Power conversion efficiency (PCE) of the perovskite solar cells (PSCs) has remarkably been increased from3.1%to 25.2%. The fast expansion of the PSCs has been along with the development of compositional and interface engineering, which has been playing a critical role. For the PSCs with record high-efficiency and stability, the perovskite absorber layer has been changed fromthe initialMAPbI3- to FAPbI3-based compositions. Owing to the enormous engineering works, perovskite absorber layers with monolithic grains could be achieved, in which the interior defects are negligible compared with the surface defects. Therefore, interface engineering, which can passivate the surface defects and/or isolate the perovskite from the environmental moistures, has been playing a more and more important role to further boost the PCE and stability of the PSCs. Herein, a compact review study of the compositional and interface engineering is presented and promising strategies and directions of the PSCs are discussed.

Organohalide lead perovskites for photovoltaic applications

Peng Gao, Michael Graetzel, Mohammad Khaja Nazeeruddin

There are only few semiconducting materials that have been shaping the progress of third generation photovoltaic cells as much as perovskites. Although they are deceivingly simple in structure, the archetypal AMX(3)-type perovskites have built-in potential for complex and surprising discoveries. Since 2009, a small and somewhat exotic class of perovskites, which are quite different from the common rock-solid oxide perovskite, have turned over a new leaf in solar cell research. Highlighted as one of the major scientific breakthroughs of the year 2013, the power conversion efficiency of the title compound hybrid organic inorganic perovskite has now exceeded 18%, making it competitive with thin-film PV technology. In this minireview, a brief history of perovskite materials for photovoltaic applications is reported, the current state-of-the-art is distilled and the basic working mechanisms have been discussed. By analyzing the attainable photocurrent and photovoltage, realizing perovskite solar cells with 20% efficiency for a single junction, and 30% for a tandem configuration on a c-Si solar cell would be realistic.

Royal Society of Chemistry2014

Strategy to Boost the Efficiency of Mixed-Ion Perovskite Solar Cells: Changing Geometry of the Hole Transporting Material

Ulf Anders Hagfeldt, Nikolaos Vlachopoulos, Jiahuan Zhang

The hole transporting material (HTM) is an essential component in perovskite solar cells (PSCs) for efficient extraction and collection of the photoinduced charges. Triphenylamine- and carbazole-based derivatives have extensively been explored as alternative and economical HTMs for PSCs. However, the improvement of their power conversion efficiency (PCE), as well as further investigation of the relationship between the chemical structure of the HTMs and the photovoltaic performance, is imperatively needed. In this respect, a simple carbazole-based HTM X25 was designed on the basis of a reference HTM, triphenylamine-based X2, by simply linking two neighboring phenyl groups in a triphenylamine unit through a carbon-carbon single bond. It was found that a lowered highest occupied molecular orbital (HOMO) energy level was obtained for X25 compared to that of X2. Besides, the carbazole moiety in X25 improved the molecular planarity as well as conductivity property in comparison with the triphenylamine unit in X2. Utilizing the HTM X25 in a solar cell with mixed-ion perovskite HC(NH2)(2)(CH3NH3)(0.15)Pb(I0.85Br0.15)(3), a highest reported PCE of 17.4% at 1 sun (18.9% under 0.46 sun) for carbazole-based HTM in PSCs was achieved, in comparison of a PCE of 14.7% for triphenylamine-based HTM X2. From the steady-state photoluminescence and transient photocurrent/photovoltage measurements, we conclude that (1) the lowered HOMO level for X25 compared to X2 favored a higher open-circuit voltage (V-oc) in PSCs; (2) a more uniform formation of X25 capping layer than X2 on the surface of perovskite resulted in more efficient hole transport and charge extraction in the devices. In addition, the long-term stability of PSCs with X25 is significantly enhanced compared to X2 due to its good uniformity of HTM layer and thus complete coverage on the perovskite. The results provide important information to further develop simple and efficient small molecular HTMs applied in solar cells.

Amer Chemical Soc2016

The Quest for Stability of Perovskite Solar Cells: Understanding Degradation, Improving Lifetimes and Towards Experimental Standards

Konrad Domanski

Perovskite solar cells (PSCs) have attracted a substantial interest owing to a very fast achievement of efficiencies >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 the private sector and industrially-oriented research institutes, understanding degradation and improving the stability of PSCs has become one of the major research topics in the field of emerging photovoltaics. This is also the focus of this thesis, where I investigate different degradation mechanisms in PSCs with the aim of extending their long-term stability. This dissertation can be divided into four parts: in the first one, I show how, through a series of prototypes, I designed and built a dedicated setup to investigate stability of PSCs. I also show how I developed its variation to improve by 700 % the efficiency of measuring current-voltage characteristics of our solar cells. Subsequently, I describe the intrinsic instability of, at the time, state-of-the-art PSCs. I show how, along with my colleagues, we managed to achieve breakthrough room temperature stability coupled with record efficiency through incorporation of Cs into perovskite. Next, I describe how and why these solar cells suffer from irreversible degradation if left at elevated temperatures however. This is due to a vulnerability to Au diffusion from the electrode, through Spiro-MeOTAD hole transporting layer into the perovskite. I show how, by incorporation of Cr diffusion barrier I managed to circumvent this problem (albeit at considerable efficiency loss). Subsequently, I describe an alterna-tive solution to the problem by substituting Spiro-MeOTAD with PTAA - a polymeric hole transporting layer. This approach effectively stops Au diffusion without compromising device efficiency, which was at the same time improved by incorporating Rb into the perovskite. Finally, a third approach is presented: replacing the Au electrode with one based on carbon nanotubes. This gives away with using Au and PTAA - both prohibitively expensive materials - and hence paves the way towards facile and inexpensive fabrication of stable PSCs. In the third part, I describe the effects of mobile ions in the perovskite on the behaviour of PSCs. First, I show how they lead to a partial reversibility of losses in aged devices. This has potentially far-reaching consequences for the way stability measurements of PSCs are conducted and how their lifetime is reported. Next, I show how the ionic movement in the perovskite can lead to PSCs with certain architecture to work as high-gain photodetectors. This is enabled by piling up of ions at the interfaces within the devices, which modifies the energy levels within. Finally, in the last chapter I describe systematically, how different factors such as temperature, illumination, atmosphere, load on the device and cycling of the environmental conditions affect the stability of PSCs. Based on this, I discuss the important parameters to control when ageing PSCs, as the first attempt to bring the community to a consensus on how to measure stability of PSCs. This is urgently needed to streamline the efforts to create stable PSCs and to commercialize the technology.