Light Management: A Key Concept in High-Efficiency Perovskite/Silicon Tandem Photovoltaics

The remarkable recent progress in perovskite photovoltaics affords a novel opportunity to advance the power conversion efficiency of market-dominating crystalline silicon (c-Si) solar cells. A severe limiting factor in the development of perovskite/c-Si tandems to date has been their inferior light-harvesting ability compared to single-junction c-Si solar cells, but recent innovations have made impressive headway on this front. Here, we provide a quantitative perspective on future steps to advance perovskite/c-Si tandem photovoltaics from a light-management point of view, addressing key challenges and available strategies relevant to both the 2-terminal and 4-terminal perovskite/c-Si tandem architectures. In particular, we discuss the challenge of achieving low optical reflection in 2-terminal cells, optical shortcomings in state-of-the-art devices, the impact of transparent electrode performance, and a variety of factors which influence the optimal bandgap for perovskite top-cells. Focused attention in each of these areas will be required to make the most of the tandem opportunity.

Strategies to Optimizing Dye-Sensitized Solar Cells

Sophie Wenger

Dye-sensitized solar cells (DSCs) constitute a novel class of hybrid organic-inorganic solar cells. At the heart of the device is a mesoporous film of titanium dioxide (TiO2) nanoparticles, which are coated with a monolayer of dye sensitive to the visible region of the solar spectrum. The role of the dye is similar to the role of chlorophyll in plants; it harvests solar light and transfers the energy via electron transfer to a suitable material (here TiO2) to produce electricity — as opposed to chemical energy in plants. DSCs are fabricated of abundant and cheap materials using inexpensive processes (e.g. screen-printing) and are likely to be a significant contributor to the future commercial photovoltaic technology portfolio. The work conducted during this thesis aimed at optimizing the DSC using three different strategies: The use of versatile organic sensitizers for stable and efficient DSCs, the study of tandem device architectures in combination with other solar cells to harvest a larger fraction of the solar spectrum, and the development of a validated optoelectric model of the DSC. Organic donor-π-acceptor dyes are an interesting alternative to the standard metal-organic complexes used in DSCs. Efficient photovoltaic conversion and stable performance could be demonstrated with three classes of donor systems, namely diphenylamine, difluorenylaminophenyl, and π-extended tetrathiafulvalene. The highest conversion efficiencies were obtained with a difluorenylaminophenyl donor system (η = 8.3 % with a volatile electrolyte and η = 7.6 % with a solvent-free ionic liquid, which was a new record for organic dyes at the time of publication). Surprisingly, efficient regeneration of the oxidized dye by the I-/I3- redox mediator was found with the π-extended tetrathiafulvalene system, even though the thermodynamic driving force was as low as 150 mV. So far driving forces of 300-500 mV had been regarded as necessary for efficient regeneration of the dye cation. Also, important structure-property relationships pertaining to the recombination of electrons with the electrolyte and to the stability of the device could be identified (i.e. effect of linear vs. branched structure, linker length, and moieties used). The power conversion efficiency of solar cells can be extended beyond the limit for a single cell (∼ 30 %) by using multiple cells with different optical gaps in a tandem device. DSCs and chalcopyrite Cu(In,Ga)Se2 (CIGS) solar cells have complementary optical gaps and are thus suitable systems for integration in a tandem device. It was shown that a monolithic DSC/CIGS tandem device has the potential for increased efficiency over a mechanically stacked device due to increased light transmission to the bottom cell, and a monolithic DSC/CIGS device with an initial efficiency of η = 12.2 % was demonstrated. The degradation of the devices — induced by the corrosion of the CIGS cell in contact with the I-/I3- redox mediator — could be retarded with a protective thin conformal ZnO/TiO2 oxide layer coated on the CIGS cell by atomic layer deposition. Finally, an experimentally validated optical and electrical model of the DSC has been developed to assist the optimization process, which is predominantly conducted by empirical means in the DSC research community. The optical model allows to accurately calculate the internal quantum efficiency of devices, i.e. the ratio of extracted electrons to absorbed photons by the dye, a crucial and so far difficult to determine characteristic. Intrinsic parameters — like injection efficiency, electron diffusion length, or distribution of trap states in the TiO2 — can be extracted from experimental steady-state and time-dependent data with the electric model. The model allows to make a comprehensive and quantitative loss analysis of the different optical and electric loss channels in the DSC. The model has been implemented with a graphical user interface for straightforward usage. All three optimization strategies — organic dyes, tandem architecture, and device modeling — developed during this thesis make a valuable contribution to the development and commercialization of inexpensive and high efficiency DSCs. They enable a comprehensive view of the system and pave the way for a systematic analysis and reduction of losses, which has been the ultimate route to success for several established photovoltaic technologies.

EPFL2010

High performance solid-state mesoscopic solar cells

Julian Burschka

Mesoscopic sensitized solar cells are one of the most promising third-generation photovoltaic technologies. Dye-sensitized solar cells (DSCs), imitating the photosynthesis of green plants, were the first photovoltaic devices to utilize a mesoscopic heterojunction for the conversion of solar irradiation into electrical power. Solid-state dye-sensitized solar cells (ssDSCs), that employ an organic hole-transporting material in place of a liquid redox electrolyte, have evolved as viable contenders to conventional liquid DSCs. A typical ssDSC is composed of a mesostructured wide-bandgap metal oxide semiconductor that is sensitized with a light-absorbing chromophore and infiltrated with a molecular organic hole-transporter, usually by solution-processing. In this device, photoexcitation of the sensitizer and subsequent ultrafast electron injection into the conduction band of the metal oxide semiconductor is followed by hole transfer from the oxidized sensitizer to the organic hole-transporter. Charge transport of both the electron and the hole through the two bicontinuous phases, followed by charge migration through the external circuit, completes the photovoltaic operation of the cell. Despite more than 15 years of development, ssDSCs have always been lagging behind their liquid counterparts in terms of both power conversion efficiency and long-term stability. My thesis presents three different approaches that are aimed at contributing to the development of high-performance solid-state mesoscopic solar cells. Firstly, I present a new class of Co(III) complexes as solution-processable p-type dopants for triarylamine-based hole-conductors such as the commonly employed 2,2’,7,7’-tetrakis-N,N-di-para-methoxyphenylamine-9,9’-spirobifluorene (spiro-MeOTAD). The proposed Co(III) complexes were characterized in detail using optical and electrochemical techniques. The application of the new p-dopants in ssDSC rendered it possible to directly relate the conductivity of the doped hole-transporter to the photovoltaic performance of the devices. The work shows that chemical p-doping is a powerful tool to control the charge transport properties of spiro-MeOTAD in ssDSCs, capable of replacing the commonly employed photo-doping, i.e. the light-assisted one-electron oxidation of spiro-MeOTAD by ambient oxygen. Combining this strategy with a state–of–the–art organic D–π–A sensitizer allowed us to achieve power conversion efficiencies of up to 7.2%—a new record for this kind of device architecture. Secondly, I investigated a series of new molecular hole-transporters based on the triarylamine-substituted 9,9’-spirobifluorene core. In total, we synthesized seven new materials and characterized them in detail using electrical and electrochemical techniques. Organic-field effect transistors were fabricated employing these materials in order to evaluate their hole mobilities. From the comparison of different substituents on the hole-conducting triarylamine moieties, a structure–property relationship was derived, highlighting the importance of processability and hole mobility of the hole-transporting material for an efficient application in a ssDSC configuration. We encountered the purity of the new materials as a key parameter for a proper functioning of the device and identified the use of functionalized metal-scavenging polystyrene beads as a valuable purification step. Thirdly, a new technique is introduced that allows to realize functional composites of a mesoporous metal oxide and the organic–inorganic hybrid perovskite CH3NH3PbI3. CH3NH3PbI3 has recently attracted great attention as a light-harvesting pigment in mesoscopic solar cells. We propose a two-step technique to deposit the perovskite absorber: PbI2 is first applied on a substrate by solution-processing and subsequently exposed to a solution of CH3NH3I. It was evidenced that the desired hybrid perovskite forms within seconds after contacting the two precursors. Scanning electron microscopy was employed to elucidate the changes in crystal morphology during the transformation reaction. The kinetics of the reaction was investigated in detail using X-ray diffraction and optical spectroscopy. Employing the two-step technique for the fabrication of mesoscopic perovskite-based solar cells enabled new record power conversion efficiencies of up to 15%. By following three different strategies, this thesis could contribute significantly to the development of high-performance solid-state mesoscopic solar cells. The power conversion efficiencies of 7.2% and 15%, achieved with a molecular dye and an organic–inorganic perovskite pigment, respectively, represent important milestones for the design of third-generation photovoltaic cells that hopefully one day can compete with the conventional silicon-based solar cell technology.

EPFL2013

Organic-inorganic halide perovskite/crystalline silicon four-terminal tandem solar cells

Julien Claude Bailat, Christophe Ballif, Stefaan De Wolf, Martin Ledinsky, Soo-Jin Moon, Sylvain Nicolay, Björn Niesen, Jun Ho Yum

Tandem solar cells constructed from a crystalline silicon (c-Si) bottom cell and a low-cost top cell offer a promising way to ensure long-term price reductions of photovoltaic modules. We present a four-terminal tandem solar cell consisting of a methyl ammonium lead triiodide (CH3NH3PbI3) top cell and a c-Si heterojunction bottom cell. The CH3NH3PbI3 top cell exhibits broad-band transparency owing to its design free of metallic components and yields a transmittance of >55% in the near-infrared spectral region. This allows the generation of a short-circuit current density of 13.7 mA cm(-2) in the bottom cell. The four-terminal tandem solar cell yields an efficiency of 13.4% (top cell: 6.2%, bottom cell: 7.2%), which is a gain of 1.8%(abs) with respect to the reference single-junction CH3NH3PbI3 solar cell with metal back contact. We employ the four-terminal tandem solar cell for a detailed investigation of the optical losses and to derive guidelines for further efficiency improvements. Based on a power loss analysis, we estimate that tandem efficiencies of similar to 28% are attainable using an optically optimized system based on current technology, whereas a fully optimized, ultimate device with matched current could yield up to 31.6%.