We demonstrate open-circuit voltages exceeding 1.26 V for CH3NH3PbI3 solar cells by careful process optimization of the perovskite and its interfaces to the electron and hole transport layers. This open-circuit voltage is the highest reported so far in a full MAPI cell stack and only 64 mV below the maximum open circuit voltage that is possible for this material. We confirm these values for the open circuit voltage by independent measurements of the external photoluminescence quantum efficiency reaching values of 5 % for the fully processed solar cell. We further find exceptionally long photoluminescence lifetimes in full cells and in layer stacks involving one or two contact layers. Numerical simulations reveal that these long photoluminescence lifetimes are only possible with extremely low interface recombination velocities between absorber and contact materials.
Nonradiative recombination processes are the biggest hindrance to approaching the radiative limit of the open‐circuit voltage for wide bandgap perovskite solar cells. In addition to high bulk quality, good interfaces and good energy level alignment for majority carriers at charge transport layer‐absorber interfaces are crucial to minimize nonradiative recombination pathways. By tuning the lowest‐unoccupied molecular‐orbital of electron transport layers via the use of different fullerenes and fullerene blends, open‐circuit voltages exceeding 1.35 V in CH3NH3Pb(I0.8Br0.2)3 device are demonstrated. Further optimization of mobility in binary fullerenes electron transport layers can boost the power conversion efficiency as high as 18.9%. It is noted in particular that the Voc fill factor product is >1.096 V, which is the highest value reported for halide perovskites with this bandgap.
The interfaces between absorber and transport layers are shown to be critical for perovskite device performance. However, quantitative characterization of interface recombination has so far proven to be highly challenging in working perovskite solar cells. Here, methylammonium lead halide (CH3NH3PbI3) perovskite solar cells are studied based on a range of different hole‐transport layers, namely, an inorganic hole‐transport layer CuOx, an organic hole‐transport layer poly(triarylamine) (PTAA), and a bilayer of CuOx/PTAA. The cells are completed by a [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM)/bathocuproine/Ag electron contact. Energy levels are characterized using photoelectron spectroscopy and recombination dynamics by combining steady‐state photoluminescence and transient photoluminescence with numerical simulations. While the PTAA‐based devices hardly show any interface recombination losses and open‐circuit voltages >1.2 V, substantial losses are observed for the samples with a direct CuOx/perovskite interface. These losses are assigned to a combination of energetic misalignment at the CuOx/perovskite interface coupled with increased interface recombination velocities at the perovskite/PCBM interface.
A highly transparent passivating contact (TPC) as front contact for crystalline silicon (c-Si) solar cells could in principle combine high conductivity, excellent surface passivation and high optical transparency. However, the simultaneous optimization of these features remains challenging. Here, we present a TPC consisting of a silicon-oxide tunnel layer followed by two layers of hydrogenated nanocrystalline silicon carbide (nc-SiC:H(n)) deposited at different temperatures and a sputtered indium tin oxide (ITO) layer (c-Si(n)/SiO2/nc-SiC:H(n)/ITO). While the wide band gap of nc-SiC:H(n) ensures high optical transparency, the double layer design enables good passivation and high conductivity translating into an improved short-circuit current density (40.87 mA cm−2), fill factor (80.9%) and efficiency of 23.99 ± 0.29% (certified). Additionally, this contact avoids the need for additional hydrogenation or high-temperature postdeposition annealing steps. We investigate the passivation mechanism and working principle of the TPC and provide a loss analysis based on numerical simulations outlining pathways towards conversion efficiencies of 26%.
Thermal admittance spectroscopy (TAS) is frequently used to analyze the properties of trap states in semiconductor devices. We perform detailed simulations in combination with experiments to understand the effect of low carrier mobility on the analysis of trap states by TAS. We show that the apparent characteristic peak in the differential capacitance spectra is strongly dominated by the dielectric relaxation (DR) peak caused by low carrier mobilities for the case of shallow traps and low trap densities. The model for the DR dominated case is successfully applied to interpret the experimental results from poly(3-hexylthiophene-2,5diyl) (P3HT) based diodes. In contrast, for deep states with high density of states, we are able to properly estimate the energetic position, but the low carrier mobility affects the correct determination of the attempt-to-escape frequency as well as the capture cross section. Our results reveal that low carrier mobilities cause inherent obstacles in accurately determining the trap properties and thereby affect the analysis of the origin and nature of the trap states by admittance spectroscopy.
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