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

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.

Solid-State Sensitized Heterojunction Solar Cells

Soo-Jin Moon

Dye-sensitized solar cells (DSCs) are considered as an emerging technology in order to replace conventional silicon solar cells or thin film solar cells such as amorphous silicon, CIGS, and CdTe. Liquid electrolytes containing iodide/triiodide redox couple have a durability problem due to the corrosion of metal contacts. In order to improve the long-term stability of DSC device it is important to find an alternate efficient redox couple. In search of this we are using 2,2',7,7'-tetrakis-(N,N-di-methoxyphenylamine)- 9,9'-spirobifluorene (spiro-OMeTAD) as a hole transport material for solid-state dye-sensitized solar cells (SSDSCs). In comparison to the liquid electrolytes the efficiencies of SSDSCs are inferior, they are around only 30% of the efficiencies obtained with the liquid electrolytes. In optimizing the device performance and stability of SSDSCs, various light harvesting systems are employed to enhance a photovoltaic performance and investigated their properties in SSDSCs. In SSDSCs we use thin TiO2 films to avoid the pore-filling problem of HTM. Hence it is critical to use high molar extinction coefficient dyes with an efficient light harvesting capability for SSDSCs. Representative ruthenium sensitizers such as N719 or Z907 have shown good and stable performances in liquid electrolyte-based DSCs. However, their performances are low in SSDSCs due to insufficient light harvesting in thin mesoporous TiO2 films. A new family of heteroleptic polypyridyl ruthenium sensitizers having thiophene units was employed to increase the light harvesting capabilities and their applicability in SSDSCs. These new dyes could improve the absorbed photon-to-current conversion efficiencies as well as power conversion efficiencies due to their high molar extinction coefficients. The thiophene units of the ancillary ligands not only enhanced molar extinction coefficients but also augmented electron lifetime in the devices. In general, ruthenium sensitizers possess lower molar extinction coefficients compared to organic dyes. In order to increase the molar extinction coefficients and bathochromic shift in the absorption spectra of organic dyes, we applied donor-acceptor concept in organic dyes with different π-conjugation bridges. Consequently, we achieved 6 % power conversion efficiency at AM 1.5G solar irradiation (100 mW/cm2) in a solid-state dye-sensitized solar cell. Transient photovoltage and photocurrent decay measurements showed that the enhanced performance of this device was ascribe to higher charge collection efficiency over a wider potential range. We also examined near infrared absorbing dyes and they could be employed to different device architectures such as tandem cells, Förster Resonance Energy Transfer, or co-sensitization to substantiate panchromatic response. Another interesting type of sensitizers is semiconductor or quantum dots due to their unique properties. However, the efficiency of the semiconductor-sensitized solar cells was only 1-2 % range. Recently, much improved efficiencies were reported with Sb2S3-sensitized cells using different hole conductors. The Sb2S3-sensitized cells with spiro-OMeTAD demonstrated a very high incident photon-to-conversion efficiency (IPCE) of 90 %. This excellent result shows that the semiconductor sensitizers are promising candidate as light absorbers for SSDSCs. Most of the standard ruthenium and organic dyes have the limited absorption in near infrared region of the solar spectrum. Porphyrin sensitizers possess strong absorptions in the visible and near infrared region and they have good chemical, photochemical and thermal stability. However, the power conversion efficiency of SSDSCs devices using a novel D-π-A porphyrin we reached only 1.6 %. In order to improve a cell performance, porphyrin was co-sensitized with an organic dye to increase light harvesting capability in the green wavelength region as well as to reduce the dye aggregation. Instead of spiro-OMeTAD a polymer hole conductor was applied, which had intense spectral response in the visible region. Interestingly, in this system the polymer hole conductor showed dual functions, as a light absorber and a hole transporter. This hybrid solar cell exhibited a clear panchromatic response and improved the power conversion efficiency of device compared to the cell with spiro-OMeTAD.

EPFL2011

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.

EPFL2018

Comprehensive control of voltage loss enables 11.7% efficient solid-state dye-sensitized solar cells

Heewon Bahng, Yiming Cao, Ulf Anders Hagfeldt, Ladislav Kavan, Yuhang Liu, Jingshan Luo, Jacques-Edouard Moser, Yasemin Saygili, Yinghui Wu, Chenyi Yi, Shaik Mohammed Zakeeruddin, Weiwei Zhang

The relatively large voltage loss (Vloss) in excitonic type solar cells severely limits their power conversion efficiencies (PCEs). Here, we report a comprehensive control of Vloss through efficacious engineering of the sensitizer and redox mediator, making a breakthrough in the PCE of dye-sensitized solar cells (DSSCs). The targeted down-regulation of Vloss is successfully realized by three valid channels: (i) reducing the driving force of electron injection through dye molecular engineering, (ii) decreasing the dye regeneration overpotential through redox mediator engineering, and (iii) suppressing interfacial electron recombination. Significantly, the ‘‘trade-off’’ effect between the dye optical band gap and the open-circuit voltage (VOC) is minimized to a great extent, achieving a distinct enhancement in photovoltaic performance (PCE 4 11.5% with VOC up to 1.1 V) for liquid junction cells. The solidification of the best-performing device leads to a PCE of 11.7%, which is so far the highest efficiency obtained for solid-state DSSCs. Our work inspires further development in highly efficient excitonic solar cells by comprehensive control of Vloss.