15 research outputs found
Emergence of Deep Traps in Long-Term Thermally Stressed CHâNHâPbIâ Perovskite Revealed by Thermally Stimulated Currents
Laminated Perovskite Photovoltaics: Enabling Novel Layer Combinations and Device Architectures
Highâefficiency perovskiteâbased solar cells can be fabricated via either solutionâprocessing or vacuumâbased thinâfilm deposition. However, both approaches limit the choice of materials and the accessible device architectures, due to solvent incompatibilities or possible layer damage by vacuum techniques. To overcome these limitations, the lamination of two independently processed halfâstacks of the perovskite solar cell is presented in this work. By laminating the two halfâstacks at an elevated temperature (â90 °C) and pressure (â50 MPa), the polycrystalline perovskite thinâfilm recrystallizes and the perovskite/charge transport layer (CTL) interface forms an intimate electrical contact. The laminated perovskite solar cells with tin oxide and nickel oxide as CTLs exhibit power conversion efficiencies of up to 14.6%. Moreover, they demonstrate longâterm and highâtemperature stability at temperatures of up to 80 °C. This freedom of design is expected to access both novel device architectures and pairs of CTLs that remain usually inaccessible
Drying Dynamics of SolutionâProcessed Perovskite ThinâFilm Photovoltaics: In Situ Characterization, Modeling, and Process Control
A key challenge for the commercialization of perovskite photovoltaics is the transfer of highâquality spin coated perovskite thinâfilms toward applying industryâscale thinâfilm deposition techniques, such as slotâdie coating, spray coating, screen printing, or inkjet printing. Due to the complexity of the formation of polycrystalline perovskite thinâfilms from the precursor solution, efficient strategies for process transfer require advancing the understanding of the involved dynamic processes. This work investigates the fundamental interrelation between the drying dynamics of the precursor solution thinâfilm and the quality of the blade coated polycrystalline perovskite thinâfilms. Precisely defined drying conditions are established using a temperatureâstabilized drying channel purged with a laminar flow of dry air. The dedicated channel is equipped with laser reflectometry at multiple probing positions, allowing for in situ monitoring of the perovskite solution thinâfilm thickness during the drying process. Based on the drying dynamics as measured at varying drying parameters, namely at varying temperature and laminar air flow velocity, a quantitative model on the drying of perovskite thinâfilms is derived. This model enables process transfer to industryâscale deposition systems beyond brute force optimization. Via this approach, homogeneous and pinholeâfree blade coated perovskite thinâfilms are fabricated, demonstrating high power conversion efficiencies of up to 19.5% (17.3% stabilized) in perovskite solar cells
VacuumâAssisted Growth of LowâBandgap Thin Films (FAMASnPbI) for AllâPerovskite Tandem Solar Cells
All-perovskite multijunction photovoltaics, combining a wide-bandgap (WBG) perovskite top solar cell (EG â1.6â1.8 eV) with a low-bandgap (LBG) perovskite bottom solar cell (EG 33%. While the research on WBG perovskite solar cells has advanced rapidly over the past decade, LBG perovskite solar cells lack PCE as well as stability. In this work, vacuum-assisted growth control (VAGC) of solution-processed LBG perovskite thin films based on mixed SnâPb perovskite compositions is reported. The reported perovskite thin films processed by VAGC exhibit large columnar crystals. Compared to the well-established processing of LBG perovskites via antisolvent deposition, the VAGC approach results in a significantly enhanced charge-carrier lifetime. The improved optoelectronic characteristics enable high-performance LBG perovskite solar cells (1.27 eV) with PCEs up to 18.2% as well as very efficient four-terminal all-perovskite tandem solar cells with PCEs up to 23%. Moreover, VAGC leads to promising reproducibility and potential in the fabrication of larger active-area solar cells up to 1 cmÂČ
Nanostructured front electrodes for perovskite/c-Si tandem photovoltaics
The rise in the power conversion efficiency (PCE) of perovskite solar cells has triggered enormous interest in perovskite-based tandem photovoltaics. One key challenge is to achieve high transmission of low energy photons into the bottom cell. Here, nanostructured front electrodes for 4-terminal perovskite/crystalline-silicon (perovskite/c-Si) tandem solar cells are developed by conformal deposition of indium tin oxide (ITO) on self-assembled polystyrene nanopillars. The nanostructured ITO is optimized for reduced reflection and increased transmission with a tradeoff in increased sheet resistance. In the optimum case, the nanostructured ITO electrodes enhance the transmittance by âŒ7% (relative) compared to planar references. Perovskite/c-Si tandem devices with nanostructured ITO exhibit enhanced short-circuit current density (2.9â
mA/cm2 absolute) and PCE (1.7% absolute) in the bottom c-Si solar cell compared to the reference. The improved light in-coupling is more pronounced for elevated angle of incidence. Energy yield enhancement up to âŒ10% (relative) is achieved for perovskite/c-Si tandem architecture with the nanostructured ITO electrodes. It is also shown that these nanostructured ITO electrodes are also compatible with various other perovskite-based tandem architectures and bear the potential to improve the PCE up to 27.0%
Optimization of SnO electron transport layer for efficient planar perovskite solar cells with very low hysteresisâ
Nanostructured tin oxide (SnO) is a very promising electron transport layer (ETL) for perovskite solar cells (PSCs) that allows low-temperature processing in the planar nâiâp architecture. However, minimizing currentâvoltage (JâV) hysteresis and optimizing charge extraction for PSCs in this architecture remains a challenge. In response to this, we study and optimize different types of single- and bilayer SnO ETLs. Detailed characterization of the optoelectronic properties reveals that a bilayer ETL composed of lithium (Li)-doped compact SnO (c(Li)-SnO) at the bottom and potassium-capped SnO nanoparticle layers (NP-SnO) at the top enhances the electron extraction and charge transport properties of PSCs and reduces the degree of ion migration. This results in an improved PCE and a strongly reduced JâV hysteresis for PSCs with a bilayer c(Li)-NP-SnO ETL as compared to reference PSCs with a single-layer or undoped bilayer ETL. The champion PSC with c(Li)-NP-SnO ETL shows a high stabilized PCE of up to 18.5% compared to 15.7%, 12.5% and 16.3% for PSCs with c-SnO, c(Li)-SnO and c-NP-SnO as ETL, respectively
From Groundwork to Efficient Solar Cells: On the Importance of the Substrate Material in CoâEvaporated Perovskite Solar Cells
Vacuumâbased deposition of optoelectronic thin films has a longâstanding history. However, in the field of perovskiteâbased photovoltaics, these techniques are still not as advanced as their solutionâbased counterparts. Although highâefficiency vacuumâbased perovskite solar cells reaching power conversion efficiencies (PCEs) above 20% are reported, the number of studies on the underlying physical and chemical mechanism of the coâevaporation of lead iodide and methylammonium iodide is low. In this study, the impact of one of the most crucial process parameters in vacuum processes â the substrate material â is studied. It is shown that not only the morphology of the coâevaporated perovskite thin films is significantly influenced by the surface polarity of the substrate material, but also the incorporation of the organic compound into the perovskite framework. Based on these studies, a selection guide for suitable substrate materials for efficient coâevaporated perovskite thin films is derived. This selection guide points out that the organic vacuumâprocessable hole transport material 2,2âł,7,7âłâtetra(N,Nâdiâpâtolyl)aminoâ9,9âspirobifluorene is an ideal candidate for the fabrication of efficient allâevaporated perovskite solar cells, demonstrating PCEs above 19%. Furthermore, building on the insights into the formation of the perovskite thin films on different substrate materials, a basic crystallization model for coâevaporated perovskite thin films is suggested
From Groundwork to Efficient Solar Cells: On the Importance of the Substrate Material in CoâEvaporated Perovskite Solar Cells
Vacuum-based deposition of optoelectronic thin films has a long-standing history. However, in the field of perovskite-based photovoltaics, these techniques are still not as advanced as their solution-based counterparts. Although high-efficiency vacuum-based perovskite solar cells reaching power conversion efficiencies (PCEs) above 20% are reported, the number of studies on the underlying physical and chemical mechanism of the co-evaporation of lead iodide and methylammonium iodide is low. In this study, the impact of one of the most crucial process parameters in vacuum processesâthe substrate materialâis studied. It is shown that not only the morphology of the co-evaporated perovskite thin films is significantly influenced by the surface polarity of the substrate material, but also the incorporation of the organic compound into the perovskite framework. Based on these studies, a selection guide for suitable substrate materials for efficient co-evaporated perovskite thin films is derived. This selection guide points out that the organic vacuum-processable hole transport material 2,2âł,7,7âł-tetra(N,N-di-p-tolyl)amino-9,9-spirobifluorene is an ideal candidate for the fabrication of efficient all-evaporated perovskite solar cells, demonstrating PCEs above 19%. Furthermore, building on the insights into the formation of the perovskite thin films on different substrate materials, a basic crystallization model for co-evaporated perovskite thin films is suggested
The Electronic Structure of MAPIâBased Perovskite Solar Cells: Detailed Band Diagram Determination by Photoemission Spectroscopy Comparing Classical and Inverted Device Stacks
High power conversion efficiency (PCE) perovskite solar cells (PSCs) rely on optimal alignment of the energy bands between the perovskite absorber and the adjacent charge extraction layers. However, since most of the materials and devices of high performance are prepared by solutionâbased techniques, a deposition of films with thicknesses of a few nanometers and therefore a detailed analysis of surface and interface properties remains difficult. To identify the respective photoactive interfaces, photoelectron spectroscopy measurements are performed on device stacks of methylammoniumâleadâiodide (MAPI)âbased PSCs in classical and inverted architectures in the dark and under illumination at openâcircuit conditions. The analysis shows that vacuumâdeposited MAPI perovskite absorber layers are nâtype, independent of the architecture and of the charge transport layer that it is deposited on (nâtype SnO or pâtype NiO). It is found that the majority of the photovoltage is formed at the nâMAPI/pâHEL (hole extraction layer) junction for both architectures, highlighting the importance of this interface for further improvement of the photovoltage and therefore also the PCE. Finally, an experimentally derived band diagram of the completed devices for the dark and the illuminated case is presented
The Electronic Structure of MAPIâBased Perovskite Solar Cells: Detailed Band Diagram Determination by Photoemission Spectroscopy Comparing Classical and Inverted Device Stacks
High power conversion efficiency (PCE) perovskite solar cells (PSCs) rely on optimal alignment of the energy bands between the perovskite absorber and the adjacent charge extraction layers. However, since most of the materials and devices of high performance are prepared by solutionâbased techniques, a deposition of films with thicknesses of a few nanometers and therefore a detailed analysis of surface and interface properties remains difficult. To identify the respective photoactive interfaces, photoelectron spectroscopy measurements are performed on device stacks of methylammoniumâleadâiodide (MAPI)âbased PSCs in classical and inverted architectures in the dark and under illumination at openâcircuit conditions. The analysis shows that vacuumâdeposited MAPI perovskite absorber layers are nâtype, independent of the architecture and of the charge transport layer that it is deposited on (nâtype SnOâ or pâtype NiOâ). It is found that the majority of the photovoltage is formed at the nâMAPI/pâHEL (hole extraction layer) junction for both architectures, highlighting the importance of this interface for further improvement of the photovoltage and therefore also the PCE. Finally, an experimentally derived band diagram of the completed devices for the dark and the illuminated case is presented