Regulating surface potential maximizes voltage in all-perovskite tandems.

The open-circuit voltage (VOC) deficit in perovskite solar cells is greater in wide-bandgap (over 1.7 eV) cells than in perovskites of roughly 1.5 eV (refs. 1,2). Quasi-Fermi-level-splitting measurements show VOC-limiting recombination at the electron-transport-layer contact3-5. This, we find, stems from inhomogeneous surface potential and poor perovskite-electron transport layer energetic alignment. Common monoammonium surface treatments fail to address this; as an alternative, we introduce diammonium molecules to modify perovskite surface states and achieve a more uniform spatial distribution of surface potential. Using 1,3-propane diammonium, quasi-Fermi-level splitting increases by 90 meV, enabling 1.79 eV perovskite solar cells with a certified 1.33 V VOC and over 19% power conversion efficiency (PCE). Incorporating this layer into a monolithic all-perovskite tandem, we report a record VOC of 2.19 V (89% of the detailed balance VOC limit) and over 27% PCE (26.3% certified quasi-steady state). These tandems retained more than 86% of their initial PCE after 500 h of operation.

Suppressed phase segregation for triple-junction perovskite solar cells.

The tunable bandgaps and facile fabrication of perovskites make them attractive for multi-junction photovoltaics1,2. However, light-induced phase segregation limits their efficiency and stability3-5: this occurs in wide-bandgap (>1.65 electron volts) iodide/bromide mixed perovskite absorbers, and becomes even more acute in the top cells of triple-junction solar photovoltaics that require a fully 2.0-electron-volt bandgap absorber2,6. Here we report that lattice distortion in iodide/bromide mixed perovskites is correlated with the suppression of phase segregation, generating an increased ion-migration energy barrier arising from the decreased average interatomic distance between the A-site cation and iodide. Using an approximately 2.0-electron-volt rubidium/caesium mixed-cation inorganic perovskite with large lattice distortion in the top subcell, we fabricated all-perovskite triple-junction solar cells and achieved an efficiency of 24.3 per cent (23.3 per cent certified quasi-steady-state efficiency) with an open-circuit voltage of 3.21 volts. This is, to our knowledge, the first reported certified efficiency for perovskite-based triple-junction solar cells. The triple-junction devices retain 80 per cent of their initial efficiency following 420 hours of operation at the maximum power point.

Effects of intrinsic and atmospherically induced defects in narrow bandgap (FASnI3)x(MAPbI3)1-x perovskite films and solar cells.

Narrow bandgap mixed tin (Sn) + lead (Pb) perovskites are necessary for the bottom sub-cell absorber in high efficiency all-perovskite polycrystalline tandem solar cells. We report on the impact of mixed cation composition and atmospheric exposure of perovskite films on sub-gap absorption in films and performance of solar cells based on narrow bandgap mixed formamidinium (FA) + methylammonium (MA) and Sn + Pb halide perovskites, (FASnI3)x(MAPbI3)1-x. Structural and optical properties of 0.3 ≤ x ≤ 0.8 (FASnI3)x(MAPbI3)1-x perovskite thin film absorbers with bandgaps ranging from 1.25 eV (x = 0.6) to 1.34 eV (x = 0.3) are probed with and without atmospheric exposure. Urbach energy, which quantifies the amount of sub-gap absorption, is tracked for pristine perovskite films as a function of composition, with x = 0.6 and 0.3 demonstrating the lowest and highest Urbach energies of 23 meV and 36 meV, respectively. Films with x = 0.5 and 0.6 compositions show less degradation upon atmospheric exposure than higher or lower Sn-content films having greater sub-gap absorption. The corresponding solar cells based on the x = 0.6 absorber show the highest device performance. Despite having a low Urbach energy, higher Sn-content solar cells show reduced device performances as the amount of degradation via oxidation is the most substantial.

Implications of Electron Transport Layer and Back Metal Contact Variations in Tin-Lead Perovskite Solar Cells Assessed by Spectroscopic Ellipsometry and External Quantum Efficiency.

The structural and optical properties of hybrid organic-inorganic metal halide perovskite solar cells are measured by spectroscopic ellipsometry to reveal an optically distinct interfacial layer among the back contact metal, charge transport, and absorber layers. Understanding how this interfacial layer impacts performance is essential for developing higher performing solar cells. This interfacial layer is modeled by Bruggeman effective medium approximations (EMAs) to contain perovskite, C60, BCP, and metal. External quantum efficiency (EQE) simulations that consider scattering, electronic losses, and the formation of nonparallel interfaces are created with input derived from ellipsometry structural-optical models and compared with experimental EQE to estimate optical losses. This nonplanar interface causes optical losses in short circuit current density (JSC) of up to 1.2 mA cm-2. A study of glass/C60/SnO2/Ag or Cu and glass/C60/BCP/Ag film stacks shows that C60 and BCP mix, but replacing BCP with SnO2 can prevent mixing between the ETLs to prevent contact between C60 and back contact metal and enable the formation of a planar interface between ETLs and back contact metals.

Urbach Energy and Open-Circuit Voltage Deficit for Mixed Anion-Cation Perovskite Solar Cells.

The Urbach energy indicating the width of the exponentially decaying sub-bandgap absorption tail is commonly used as the indicator of electronic quality of thin-film materials used as absorbers in solar cells. Urbach energies of hybrid inorganic-organic metal halide perovskites with various anion-cation compositions are measured by photothermal deflection spectroscopy. The variation in anion-cation composition has a substantial effect on the measured Urbach energy and hence the electronic quality of the perovskite. Depending upon the compositions, the Urbach energy varies from 18 to 65 meV for perovskite films with similar bandgap energies. For most of the perovskite compositions studied here including methylammonium (MA) + formamidinium (FA)-based Pb iodides, mixed Sn + Pb narrow-bandgap perovskites with low or intermediate Sn contents, and wide-bandgap FA + Cs- and I + Br-based perovskites, the correlation between the Urbach energy of the perovskite thin film and open-circuit voltage (VOC) deficit for corresponding solar cells shows a direct relationship with reduction of the Urbach energy occurring with a beneficial decrease in the VOC deficit. However, due to issues related to material quality, impurity phases and stability in laboratory ambient air, and unoptimized film processing techniques, the solar cells incorporating Cs-based inorganic and mixed Sn + Pb perovskites with a higher than optimum Sn content show a higher VOC deficit even though the corresponding films show a lower Urbach energy.

Impact of Humidity and Temperature on the Stability of the Optical Properties and Structure of MAPbI3, MA0.7FA0.3PbI3 and (FAPbI3)0.95(MAPbBr3)0.05 Perovskite Thin Films.

In situ real-time spectroscopic ellipsometry (RTSE) measurements have been conducted on MAPbI3, MA0.7FA0.3PbI3, and (FAPbI3)0.95(MAPbBr3)0.05 perovskite thin films when exposed to different levels of relative humidity at given temperatures over time. Analysis of RTSE measurements track changes in the complex dielectric function spectra and structure, which indicate variations in stability influenced by the underlying material, preparation method, and perovskite composition. MAPbI3 and MA0.7FA0.3PbI3 films deposited on commercial fluorine-doped tin oxide coated glass are more stable than corresponding films deposited on soda lime glass directly. (FAPbI3)0.95(MAPbBr3)0.05 films on soda lime glass showed improved stability over the other compositions regardless of the substrate, and this is attributed to the preparation method as well as the final composition.

Optical and Electronic Losses Arising from Physically Mixed Interfacial Layers in Perovskite Solar Cells.

Perovskite solar cell device performance is affected by optical and electronic losses. To minimize these losses in solar cells, it is important to identify their sources. Here, we report the optical and electronic losses arising from physically mixed interfacial layers between the adjacent component materials in highly efficient two terminal (2T) all-perovskite tandem, single-junction wide-bandgap, and single-junction narrow-bandgap perovskite-based solar cells. Physically mixed interfacial layers as the sources of optical and electronic losses are identified from spectroscopic ellipsometry measurements and data analysis followed by comparisons of simulated and measured external quantum efficiency spectra. Parasitic absorbance in the physically mixed regions between silver metal electrical contacts and electron transport layers (ETLs) near the back contact and a physical mixture of commercial indium tin oxide and hole transport layers (HTL) near the front electrical contact lead to substantial optical loss. A lower-density void + perovskite nucleation layer formed during perovskite deposition at the interface between the perovskite absorber layer and the HTL causes electronic losses because of incomplete collection of photogenerated carriers likely originating from poor coverage and passivation of the initially nucleating grains.