After demonstration of a 23% power conversion efficiency, a high operational stability is the next most important scientific and technological challenge in perovskite solar cells (PSCs). A potential failure mechanism is tied to a bias‐induced ion migration, which causes current–voltage hysteresis and a decay in the device performance over time. Herein, alkali salts are shown to mitigate hysteresis and stabilize device performance in n‐i‐p hybrid planar PSCs. Different alkali salts of potassium chloride, iodide, and nitrate as well as sodium chloride and iodide are deposited from aqueous solution onto the n‐type contact, based on SnO2, prior to deposition of the perovskite absorber Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3. Introduction of potassium‐based alkali salts suppresses the current–voltage hysteresis and stabilizes the operational device stability at the maximum power point. This is attributed to the suppression of hole trapping at the n‐type selective transport layer (SnO2)/perovskite interface observed by surface photovoltage spectroscopy, which is interpreted to reduce interfacial recombination and improve charge carrier extraction. The best and most stable performance of 19% is achieved using potassium nitrate as the interface modifier. Devices with higher and more stable performance exhibit substantially lower current transients, analyzed during maximum power point tracking.
The impact of sodium on the electrical properties of Cu(In,Ga)Se2 (CIGS) thin films and corresponding solar cells was investigated by preparing nearly alkali-free CIGS layers and doping them with different Na amounts via NaF post-deposition treatment (PDT) at temperatures between 110 and 400 °C. The mean Na concentrations in the CIGS layers ranged from 0.1 to 400 ppm. Sodium was found also in the grain interior even for the lowest PDT temperature. All samples were subjected to extensive electrical characterization: current–voltage, capacitance profiling, conductivity, steady-state, and transient capacitance spectroscopy. A continuous increase in open-circuit voltage VOC and fill factor FF, an accompanying increase in hole density and mobility, and a decrease in secondary barriers responsible for the distortion of current–voltage characteristics were observed with increasing sodium content. An abrupt change in defect spectra and a dominant transport mechanism was found for PDT temperatures T(PDT) of ≥150 °C. We attribute a further improvement in VOC observed above 150 °C PDT temperature to the reduced concentration of recombination centers with increased sodium content. An explanation of both gradual evolution and the abrupt change is proposed based on passivation of grain boundaries and interfaces by sodium.
The improvement of CIGSe solar cells efficiency due to potassium fluoride postdeposition treatment (KF‐PDT) is often interpreted as being caused by changes in the interface region. The impact of KF‐PDT on bulk properties of the CIGSe absorbers is investigated by measuring hole densities in the solar cells and conductivities of corresponding thin films and is then analyzed together with similar data for NaF‐treated absorbers and thin films. It is found that both alkali metals change the electrical properties of the whole CIGSe layer and not only the absorber surface. By measuring electrical conductivity and doping level in thermal equilibrium (relaxed) and metastable state (after light soaking) as well as their evolution with the amount of Na and K after postdeposition treatment (PDT) at various temperatures, one can observe effects that can only be explained by the influence of alkali metals on grain boundary properties. In contrast to Na, low concentration of potassium leads to a decrease in conductivity. Persistent photoconductivity exhibits a linear relationship with the relaxed conductivity in both NaF‐ and KF‐treated samples. These effects directly show that electrical transport in CIGSe is limited by potential barriers at the grain boundaries, which are impacted by the alkali treatment.
The electronic activity of defects and their impact on the efficiency of Cu(In,Ga)Se2 and CdTe solar cells is a subject of continuing interest and dispute in the photovoltaic community. However, after many years of research, the conclusions are far from satisfying yet. Here, the electrical defect spectroscopy results for Cu(In,Ga)Se2 and CdTe absorbers and devices are discussed with focus on findings that have been confirmed on many samples but still do not have a well-grounded interpretation. Charged grain boundaries are proposed as a possible source of some signatures observed in deep level spectra in both materials. Electrical nano-characterization methods combined with standard defect spectroscopy are suggested as a promising solution for unraveling the role and origin of dominating defects for solar cells efficiency.
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