Unravelling steady-state bulk recombination dynamics in thick efficient vacuum-deposited perovskite solar cells by transient methods

Abstract

By evaluating perovskite solar cells up to 820 nm thick using charge extraction and transient photovoltage, first and second order recombination dynamics can for the first time be identified under operational conditions in complete devices.

Transient Photovoltage Measurements on Perovskite Solar Cells with Varied Defect Concentrations and Inhomogeneous Recombination Rates

“…Such plots of τ versus n 0 are commonly discussed, where n 0 is obtained from charge extraction measurements. [9,12,18] To avoid the difficulties of this measurement, we can use V oc itself because for an unmodified band gap and density of states (and constant T), V oc (Equation 1) is a unique function of the charge carrier density (if it makes sense to define a homogeneous charge carrier density). Replotted versus V oc (Figure 2a), the trend of τ is indeed inverted, which might be taken as an indication that the recombination dynamics is indeed faster for the low-V oc device.…”

“…This explanation is not surprising given the recently reported results and discussions elaborated in the introduction. [15,18] This RC approach has also been formulated earlier for other solar cells and R is commonly approximated by the differential resistance of the diode curve. [14,27] As the RC time is decreasing with increasing voltage, criteria for the minimum light intensity can be formulated, under which recombination times could be seen.…”

“…Undoubtedly, corrections for capacitive effects have to be included in the analysis. This issue has been discussed in the literature various times [14,16,17] and has been recently revisited by several studies comparing different thinfilm technologies, [15] using thicker perovskite solar cells, [18] and looking at it from a more theoretical perspective. [19] The conclusion from these publications is that in most cases of perovskite solar cells measured under moderate light intensities, recombination dynamics cannot be captured by TPV.…”

Light-intensity and thickness dependent efficiency of planar perovskite solar cells: charge recombinationversusextraction

“…Although determination of the volume density of charge carriers [cm À3 ] remains challenging if their detailed spatial distribution in the device is unknown, such observations indicate the fraction of total charges that resides on or near the contact layers is considerable higher in 250 nm device than in 750 nm device, particularly at low light levels. 19,52,56,57 Charge carrier recombination kinetics at V OC can be determined from small-perturbation lifetimes obtained from fitting the transient photovoltage (TPV) transients with single exponential. The TPV lifetime (t Dn ), Fig.…”

“…[13][14][15][16][17] Studies on perovskite thickness variation have focused on the increase in light absorption for films up to 200 nm thick, 12,14 and for thicker films on film morphology 18 and device capacitance. 19 Yet whether there are different optimum photoactive layer thicknesses for different applications and the origin of such large thickness insensitivity under 1 sun irradiation remains largely unexplained. 17 One of the key determinants of photoactive-thickness dependencies in OSCs is the competition between charge transport to the charge extraction layers versus bimolecular charge recombination, 7,8 an issue which has received relatively little attention to date for PSCs (in this manuscript, the term bimolecular recombination is used to refer to both band to band recombination and non-radiative recombination via shallow trap states, a terminology commonly used in OSC literature).…”