• Energy Research

Halide perovskite solar cells demonstrated a dramatic increase in efficiency in the last decade mainly thanks to improvements in the bulk material quality, as well as the choice of suitable selective carrier transport layers (SCTLs). More recently, improvements have come from interface optimization strategies, such as defect passivation. Nevertheless, the interactions between the perovskite (PSK) and SCTLs are still unclear and will become more and more important to improve both efficiency and stability of PSK solar cells.In our study, we quantified the optoelectronic interactions at the SCTL-PSK interfaces employing a lateral heterojunction device (LHJ) (Figure 1A). The device is composed of a glass substrate, on which two different SCTLs (TiOx and NiOx) are deposited in a lateral electrode configuration with a well-defined 100 µm – long channel between them. The two SCTLs sit on two separate ITO electrodes that serve as contacts. We measured a ≈600 meV work function (WF) difference between the SCTLs, which constitutes the built-in voltage (Vbi) of our device. To complete the structure, a Pb-based perovskite layer is deposited on top, thus filling the channel and creating a p-i-n device which develops in the horizontal direction, i.e. in the sample plane. We tracked the surface electrostatic potential variation along the PSK channel employing high-spatial-resolution (≈11 µm) X-ray photoemission spectroscopy (XPS) both in dark short circuit, and under applied bias, by recording the I3d5/2 core level binding energy (BEI) of the PSK.We discovered that the substrate built-in voltage was still present on top of the PSK surface: a linear potential drop (Vdrop) of about 600 meV was recorded across the channel by XPS in dark, short circuit conditions, implying a nearly undoped PSK. Indeed, by fitting the data with a 2D drift-diffusion (DD) model, we found Ndop=1011 cm-3 to be the maximum PSK doping density able to sustain such a long-range linear drop (Figure 1B).Furthermore, we could control (and even invert) the potential drop by applying a voltage bias (Vapp) to the electrodes, which adds up to Vbi (Vdrop= Vbi- Vapp) (Figure 1C). Our results indicate that the tested PSK is nearly intrinsic and its Fermi level is controlled by the substrate over which it is deposited, and can be modulated by an external bias voltage.

A low temperature (60 °C) encapsulation process based on a single thin (16 nm) coating of Al2O3prepared by atomic layer deposition.

Fabricating tin dioxide (SnO2) electron selective layers (ESL) by atomic layer deposition (ALD) can be of high interest for perovskite-based solar cell development since it offers a number of advantages over solution-based processes. However, ALD-grown SnO2 ESL has usually been reported to yield limited cell efficiency compared to solution-processed SnO2 ESL, without the causes being clearly identified. This is why we here conduct a thorough interface study using a set of complementary techniques. For this purpose, ALD-grown SnO2 thin films are characterized in a systematic comparison with reference solution-processed SnO2. Energetics analysis by ultraviolet photoelectron spectroscopy (UPS) points out an unfavorable band bending at the ALD-grown SnO2/perovskite interface. Chemical characterization by time-of-flight secondary ion mass spectroscopy (ToF-SIMS) and hard X-ray photoelectron spectroscopy (HAXPES) profiling unveils an unexpected lack of oxygen at the ALD-grown SnO2/perovskite interface, which may play a direct role in observed performance limitations.

Thin (approximately 10 nm) oxide buffer layers grown over lead-halide perovskite device stacks are critical for protecting the perovskite against mechanical and environmental damage. However, the limited perovskite stability restricts the processing methods and temperatures (<=110 C) that can be used to deposit the oxide overlayers, with the latter limiting the electronic properties of the oxides achievable. In this work, we demonstrate an alternative to existing methods that can grow pinhole-free TiOx (x = 2.00+/-0.05) films with the requisite thickness in <1 min without vacuum. This technique is atmospheric pressure chemical vapor deposition (AP-CVD). The rapid but soft deposition enables growth temperatures of >=180 ��C to be used to coat the perovskite. This is >=70 ��C higher than achievable by current methods and results in more conductive TiOx films, boosting solar cell efficiencies by >2%. Likewise, when AP-CVD SnOx (x ~ 2) is grown on perovskites, there is also minimal damage to the perovskite beneath. The SnOx layer is pinhole-free and conformal, which reduces shunting in devices, and increases steady-state efficiencies from 16.5% (no SnOx) to 19.4% (60 nm SnOx), with fill factors reaching 84%. This work shows AP-CVD to be a versatile technique for growing oxides on thermally-sensitive materials. R.D.R and R.A.J contributed equally. 23 pages. 6 figures

AbstractLead halide perovskites are semiconductor materials which are employed as nonintentionally doped absorbers inserted between two selective carrier transport layers (SCTL), realizing a p‐i‐n or n‐i‐p heterojunction. In our study, we have developed and investigated a lateral device, based on methylammonium lead iodide (MAPbI3) in which the p‐i‐n heterojunction develops in the horizontal direction. Our research suggests that the effective doping level in the MAPbI3 film should be very low, below 1012 cm−3. Along the vertical direction, this doping level is not enough to screen the electric field of the buried heterojunction with the SCTL. The perovskite work function is therefore affected by the work function of the SCTL underneath. From drift‐diffusion simulations, we show that intrinsic perovskite‐SCTL structures develop mV range surface photovoltages (SPVs) under continuous illumination. However, perovskite‐SCTL structures can develop SPVs of hundreds of mV, as confirmed by our measurements. We therefore analyzed the compatibility between low doping and low defect densities in the perovskite layer and such high SPV values using numerical modeling. It is shown that these high SPV values could originate from electronic processes due to large band offsets in the buried perovskite‐SCTL heterojunctions, or at the SCTL‐transparent conductive oxide (TCO) buried heterojunction. However, such electronic processes can hardly explain the long SPV persistence after switching off the illumination.

Indium phosphide (InP) surfaces are greatly affected by ionic bombardment. We investigate the resulting surface perturbation through the use of the complementary analytical techniques of electrochemistry and X-ray photoelectron spectroscopy (XPS). Following bombardment, modifications to the surface were identified by a reduction in the dark open circuit potential in comparison to the pristine state. Through XPS studies, it was found that the sputtered surface was enriched with a metallic-like In contribution, which oxidized upon exposure to air. Cyclic voltammetry measurements confirmed this observation, with initial cathodic features related to an oxidized metallic In-enriched layer on the InP surface. Repeated cyclic voltammetry experiments resulted in the formation of a more In-rich overlayer due to a specific oxidation/reduction phenomenon. This behavior is very similar to that obtained by cathodic decomposition on InP surfaces.