High open circuit voltages in pin-type perovskite solar cells through strontium addition

Abstract

The incorporation of even small amounts of strontium (Sr) into lead-based quadruple cation hybrid perovskite solar cells results in a systematic increase of the open circuit voltage (Voc) in pin-type perovskite solar cells. We demonstrate via transient and absolute photoluminescence (PL) experiments how the incorporation of Sr significantly reduces the non-radiative recombination losses in the neat perovskite layer and specifically at the perovskite/C60 interface. We show that Sr segregates at the perovskite surface, where it induces important changes of morphology and energetics. Notably, the Sr-enriched surface exhibits a wider band gap and a more n-type character, accompanied with significantly stronger surface band bending. As a result, we observe a significant increase of the quasi-Fermi level splitting in the neat perovskite by reduced surface recombination and more importantly, a strong reduction of losses attributed to non-radiative recombination at the interface to the C60 electron-transporting layer. The resulting solar cells exhibited a Voc of 1.18 V, which could be further improved to nearly 1.23 V through addition of a thin polymer interlayer, bringing the non-radiative voltage loss to only 110 meV. Our work shows that simply adding a small amount of Sr to the precursor solutions induces a beneficial surface modification in the perovskite, without requiring any post treatment, resulting in high efficiency solar cells with power conversion efficiency (PCE) up to 20.3%. Our results demonstrate very high Voc values and efficiencies in Srcontaining quadruple cation perovskite pin solar cells and highlight the imperative importance of addressing and minimizing the recombination losses at the interface between perovskite and charge transporting layer. IntroductionOrganic-inorganic halide perovskites are considered one of the most promising materials for photovoltaic applications due to their rather easy and low-cost fabrication, as well as outstanding optoelectronic properties. Notably, these semiconductors combine a high absorption coefficient with panchromatic absorption of light 1 with a long carrier diffusion length 2,3 , allowing efficient photon absorption and charge extraction for a typical active layer thickness of only 500 nm. Another peculiarity of hybrid perovskites is that defects create mostly shallow energy levels, allowing high open circuit voltage (Voc) and long carrier lifetime 4 . Moreover, perovskite materials can be obtained from in-nature abundant precursors, which potentially reduce further the costs of future large scale production. Despite the fact that the first full solid state perovskite solar cell was reported only in 2012, with a power conversion efficiency (PCE) of 9.7% 5 , this technology has experienced a tremendous improvement 6-8 , currently reaching a record PCE of 22.7% 9 . Regardless of the state of the art of perovskite solar cells and their astonishing performances, the metrics fill factor (FF) andVoc are currently still limiting their PCE. Thus, in order to achiev...

Conformal monolayer contacts with lossless interfaces for perovskite single junction and monolithic tandem solar cells

“…27 Thus, recent efforts were dedicated to reduce these losses through surface passivation, mostly by processing nanometer-thick interfacial layers between absorber and charge-selective contacts. [28][29][30][31][32][33][34] Recently, changes of the perovskite precursor (e.g., addition of Sr 35 or an organic molecule with passivating functional groups, 36 or a substitution 5 of PbI 2 ) led to open-circuit voltages of well over 1.20 V with comparable loss-in-potential values as obtained in best n-i-p PSCs. However, the mentioned strategies often require finely tuned processing that might be complicated to implement on a large scale.…”

On the Relation between the Open‐Circuit Voltage and Quasi‐Fermi Level Splitting in Efficient Perovskite Solar Cells

“…During the past years, many studies have evaluated recombination in perovskites layers and suggested that defects at the perovskite surface or at grain boundaries as possible reasons for nonradiative recombination in neat perovskite layers . More recently, several studies have additionally highlighted the thorny problematic of the interfaces between the perovskite and charge transport layers in the actual device, due to the fast nonradiative recombination of carriers at or across these interfaces . Therefore, a detailed understanding of the losses and the underlying physical processes is essential in order to exploit the full potential of these materials in solar cells.…”

Minimizing Voltage Losses in Perovskite Solar Cells

“…In this context, the V oc deficit ( V oc,def ), defined as E g / q − V oc ( q is the elementary charge), is termed to quantify the voltage loss. Tremendous efforts have been put to reduce their V oc,def , including (but not limited to) crystal growth control, [ 5–10 ] GB defect passivation, [ 11–15 ] interfacial defect passivation, [ 16–23 ] and band alignment optimization. [ 24–28 ] Up to now, V oc,def of the best‐performing PSCs can be 0.32–0.35 V, [ 9,16,29 ] which is still above the radiative loss (0.27 V) estimated by the S–Q model, as shown in Figure 1b and Table 1 .…”