Investigation of the Selectivity of Carrier Transport Layers in Wide‐Bandgap Perovskite Solar Cells

Abstract

Excellent contact passivation and selectivity are prerequisites to realize the full potential of high‐material‐quality perovskite solar cells, first to maximize the internal voltage (or quasi‐Fermi‐level separation) iV within the absorber, then to translate this high internal voltage into a high external voltage V. Experimental quantification of contact passivation and selectivity is, thus, key to improving device performance. Here, open‐circuit measurements of iVoc and Voc, combined with surface photovoltage measurements, are used to systematically quantify the passivation—using iVoc as a metric—and the selectivity—defined as Soc = Voc/iVoc—of a range of common carrier transport layers to wide‐bandgap (1.67 eV) perovskite absorbers. The resulting solar cells suffer from large voltage deficits, particularly when NiOx is used as the hole transport layer, even though it provides better passivation than its polymer‐based counterparts (PTAA and PTAA/PFN). This indicates a poor selectivity of NiOx (Soc < 0.81 for NiOx‐based devices), whereas devices using polymer‐based hole transport layers exhibit high selectivity (Soc = 0.94–0.95). In agreement with recent reports, this low selectivity is attributed to the formation of an interlayer of non‐perovskite material with high resistance to holes at the perovskite/NiOx interface. These measurements also imply that the selectivity of the C60‐based electron transport layers is relatively good.

Understanding Performance Limiting Interfacial Recombination in pin Perovskite Solar Cells

“…The results showed a ≈30 mV improvement in V OC but was quickly limited by the insulating nature of PMMA causing the V OC to drop again (further discussion in Figure S14, Supporting Information). [37] This validates the strategy but shows the need for a lithographic or wet chemical step to better pattern the insulator for it to reach its true potential (for the TC this would be roughly a 100 mV improvement over the control). Patterning on this length scale is typically achieved using photolithographic techniques, which one could envision causing some damage to the perovskite through deep-UV light exposure and subsequent etching steps.…”

Effect of Doping, Photodoping, and Bandgap Variation on the Performance of Perovskite Solar Cells

“…In theory, sufficiently high doping (> 10 17 cm -3 ) of surface regions of the perovskite would significantly reduce the concentrations of minority carriers at interfaces to the electrodes, thereby reducing non-radiative recombination and improving contact selectivity. [51,65] Furthermore, such an approach would mean that the perovskite itself provides functionality that is currently provided by the electrodes and other functional layers that affect the work-function difference between cathode and anode. A key open question is however whether such position dependent doping of the sub-surface areas of perovskites could be achieved in a way that it remains stable as a function of time and is not compensated by ionic movement.…”

Material, Phase, and Interface Stability of Photovoltaic Perovskite: A Perspective

“…can be created, especially under environmental stresses including heat, light, moisture, and oxygen. These imperfections have been identified as sources where nonradiative recombination, ionic migration, and degradation initiate. , Quantitative photoluminescence (PL) has been widely adopted to visualize the photovoltage losses at each interface. − Capacitance–voltage profiling (deep-level capacitance profiling, DLCP) based on device characterizations has also been developed for perovskite-based devices and returned spikes in defect density near the interfaces . Coupling PL with electron microscopy has also been reported to unravel the existence of nanoscale trap clusters at the grain boundary (Figure B) …”