In-depth analysis of defects in TiO2 compact electron transport layers and impact on performance and hysteresis of planar perovskite devices at low light

Abstract

Properties of the electron transport layer (ETL) are known to influence the performance of lead halide perovskite solar cells (PSCs). But so far very little emphasis has been given on the increased impact of this layer at low light. In this work we compare the effect of thickness and coverage of a TiO2 compact layer on the performance and hysteresis of standard methyl ammonium lead iodide planar devices tested under 200 lux vs. 1 sun illumination. Standard TiO2 layers are produced with incremental thickness and coverage by application of spray pyrolysis of a Ti-acetylacetonate precursor (0-50 sprays, 1 spray ~ 1 nm TiO2). Thorough materials characterisation combining FEG-SEM, XPS, and cyclic voltammetry shows that a crystalline, nearly pin-hole free TiO2 layer is achieved by sequential deposition of ≥ 15 sprays over small to large areas (0.2 mm 2-1 cm 2). Device performance is affected by two main parameters, namely the coverage yield and thickness of the TiO2 layer, especially under 200 lux illumination. A 25 vs. 50 sprays-TiO2 layer is found to provide the best compromise between coverage and thickness to avoid charge recombination at the TiO2/perovskite interface whilst minimizing resistive losses with 11.7 % average PCE at 200 lux vs 7.8 % under 1 sun. Finally, the analysis of I/V forward vs. reverse scans and open circuit voltage decay data suggests that hysteresis is greatly affected by the capacitive properties of the ETL at low light, whilst other phenomena such as ion migrations may dominate under 1 sun.

Ultrafast and Scalable Laser‐Induced Crystallization of Titanium Dioxide Films for Planar Perovskite Solar Cells

“…[51,52] It is worth mentioning that although the laser-annealed TiO 2 film at 500-550 C shows no evidence of crystallization from the XRD and Raman results, it delivers a champion PCE of 13.2%, which is significantly higher than the device with the amorphous TiO 2 film dried on the hotplate at 120 C. This might be due to the partial crystallization of the TiO 2 film, which is challenging to detect by XRD. [41,53] The aforementioned study on using a UV (355 nm) laser to anneal TiO 2 also showed no peak corresponding to crystalline TiO 2 from the XRD patterns but achieved an equivalent PCE to the furnace-annealed sample. [32] However, strong hysteresis behavior was observed for their devices.…”

“…This might be due to the partial crystallization of the TiO 2 film, which is challenging to detect by XRD. [ 41,53 ] The aforementioned study on using a UV (355 nm) laser to anneal TiO 2 also showed no peak corresponding to crystalline TiO 2 from the XRD patterns but achieved an equivalent PCE to the furnace‐annealed sample. [ 32 ] However, strong hysteresis behavior was observed for their devices.…”

A Perspective on the Commercial Viability of Perovskite Solar Cells

“…[ 126,127 ] Several factors causing hysteresis were discussed previously, [ 128–130 ] with the space–charge accumulation at the interface pointed out as one of the main culprits. [ 131–133 ] The J–V hysteresis posted significant uncertainties in the measurement of the PV performance of PSCs; different efficiencies are obtained when measured under different measurement conditions. [ 134–139 ]…”

“…[126,127] Several factors causing hysteresis were discussed previously, [128][129][130] with the space-charge accumulation at the interface pointed out as one of the main culprits. [131][132][133] The J-V hysteresis posted significant uncertainties in the measurement of the PV performance of PSCs; different efficiencies are obtained when measured under different measurement conditions. [134][135][136][137][138][139] Following removal of the scaffold layer, researchers then demonstrated that PSCs without the HTL also function, resulting in HTL-free architecture (Figure 3e).…”

A Perspective on the Commercial Viability of Perovskite Solar Cells

“…[102,130] Several factors causing the hysteresis were discussed previously, [131][132][133] with the space charge accumulation at the interface pointed out as one of the main culprits. [134][135][136] The J-V hysteresis posted significant uncertainties in the measurement of the photovoltaic performance of the PSCs; different efficiencies are obtained when measured under different measurement conditions. [137][138][139][140][141][142] However, drawback from the J-V hysteresis is just limiting the efficiency determination by the most extended methodology but it is not affecting the solar cell working condition in DC.…”