Solvent-controlled growth of inorganic perovskite films in dry environment for efficient and stable solar cells

Is the inorganic perovskite solar cell a trend in the future?

In the highly competitive solar industry, perovskite solar cells have leapt toward commercialization, and challenged the silicon solar cells holding the largest market share at present. Perovskite solar cells have high optical absorption, low cost compared with silicon solar cells, and can be dissolved in solvent and then spray coated directly onto substrate. If its instability and deterioration on exposure to heat problems can be solved, perovskite solar cells could leap toward commercialization.

Jingbi You, a professor of the Institute of Semiconductors at Chinese Academy of Sciences, CAS, recently made a new progress in inorganic halide perovskite with solvent-controlled growth (SCG), and achieved 15.7 % high power conversion efficiency of CsPbI3 solar cell, which is the highest efficient reported for inorganic perovskite solar cell up to now.
Their work, “Solvent-controlled growth of inorganic perovskite films in dry environment for efficient and stable solar cells”, was recently published in Nature Communications.

The performance of perovskite solar cells has increased at high power conversion efficiency (PCE). Although the PCE of organic-inorganic hybrid perovskite solar cells has been jumped from 3.8 % to 20 %, the organic-inorganic hybrid perovskite materials also suffer from poor thermal stability due to easy evaporation of the organic parts.
The temperature of solar cells has been rising while exposed to sunlight for long time, and the high temperature will cause the PCE of solar cells to deteriorate. Therefore, improving the thermal stability of the materials is an essential key for improving photovoltaic devices.

The key factors: excellent thermal stability and SCG, to develop high quality, high stability and high efficiency of CsPbI3 Perovskite solar cells.

Fig. Solvent-controlled growth (SCG) for CsPbI3 deposition. a Schematic illustration of CsPbI3-perovskite crystallization procedures via solvent-controlled growth (SCG).

Compared to Inorganic perovskite materials, inorganic halide perovskites such as cesium lead halide are promising due to their excellent thermal stability, and become the trend of commercialization. Cesium lead iodide (CsPbI3) has a bandgap of 1.73 eV and is very suitable for making efficient tandem solar cells, either with low-bandgap perovskite or silicon. However, the phase instability of CsPbI3 is hindering the further optimization of device performance. However, it was found that the α-phase (black phase) of CsPbI3 could be rapidly degraded to nonphotoactive δ-phase (yellow phase) in an ambient environment with moisture. It has been explained that the moisture can effectively introduce vacancies in the crystal lattice and lower the free-energy barrier to nucleation, and trigger the phase transition of CsPbI3 perovskite even at room temperature.

There are several efforts to stabilize the α-phase of CsPbI3 to make efficient solar cells, such as tuning the tolerance factor of perovskite structure by partially substituting iodide with bromide to form CsPbI2Br or CsPbIBr2, reducing the crystal size, or introducing intermediate phase such as Cs4PbI6. All these efforts push the efficiency of inorganic perovskite solar cells to around 10%. Recently, during the preparation of this manuscript, significant progresses were witnessed, around 13% PCE of CsPbI3-based solar cells were reported by either doping B site in ABX3 perovskite structure or by passivating/stabilizing CsPbI3 quantum dot colloid via organic salt molecular. Even though, there is still a large room for further improving the PCE of CsPbI3 solar cells. To deliver higher efficiency of CsPbI3-based perovskite solar cells, two issues must be resolved.

● Forming stable α-phase of CsPbI3 films.
● Obtaining the high quality of CsPbI3 layer, the pinholes, and grain boundary in the active layer usually leading to serious recombination and also poor device performance.

The team show that high quality and stable α-phase CsPbI3 film is obtained via solvent-controlled growth of the precursor film in a dry environment. A 15.7% power conversion efficiency of CsPbI3 solar cells is achieved, which is the highest efficiency reported for inorganic perovskite solar cells up to now. And more importantly, the devices can tolerate continuous light soaking for more than 500h without efficiency drop.

Fig. b) Normalized absorption of CsPbI3-precursor films with and without SCG, inset shows the precursor film images without and with SCG. c) X-ray diffraction (XRD) pattern of CsPbI3-precursor films without and with solvent-controlled growth (SCG). Without SCG, the diffraction peaks are mainly from the δ-phase CsPbI3, while after SCG, part of δ- phase CsPbI3 was transferred into β-phase CsPbI3 (a slight distorted α-phase CsPbI3). The diffraction peaks labeled as are the diffraction peaks from the β-phase CsPbI3. d), e) Scanning electron microscopy (SEM) image of CsPbI3 perovskite precursor film without and with SCG, respectively, scale bar: 5μm. f), g) SEM images of annealed CsPbI3 perovskite precursor films without and with SCG, respectively, scale bar: 20μm.

Fig. Phase stability of α-CsPbI3 films in dry nitrogen environment. a) X-ray diffraction (XRD) of CsPbI3-precursor films annealed at 350°C for 10min, all the diffraction peaks from the α-phase of CsPbI3, and also the XRD pattern of α-CsPbI3 after storing in a dry nitrogen box for 7 days. b) Absorption of the α-phase of CsPbI3 films before and after 7 days of storage in dry nitrogen. c) Images of annealed CsPbI3 films stored in dry nitrogen box for different days.

Class AAA solar simulator, QE-R system for measuring EQE.  Fast and stable measurement.

Professor Yu Jingbi's team conducted CsPbI3 inorganic perovskite battery device measurement, using Enli Tech SS-3A-F5 solar simulator, measuring the light stability of the device under a sun (100mW/cm2) for 500 hours of continuous illumination; At the same time, the QE-R EQE measurement system is also used to obtain a photoelectric conversion efficiency of up to 15.7%.

For the research team, the 3A class solar simulator is used to adjust the light direction. In addition to the high-precision light intensity adjustment function, the researcher can adjust the light direction with the experimental requirements. In addition, the QE-R EQE measurement system's exclusive dual-channel dual-lock-in amplifier design greatly improves the accuracy and repeatability of the measurement results, and helps the research team to provide excellent signal-to-noise ratio in the test. The fastest and most stable measurement speed, the highest efficiency of CsPbI3 inorganic perovskite battery devices was measured.

Fig. Device performance of CsPbI3-based solar cells.

Fig. Photostability of the CsPbI3 solar cells. a) Photostability measurement of the devices under continuous one-sun illumination (100mWcm-2) with UV cut filter (420nm) in nitrogen glove box (temperature: approximately 25°C) for the unencapsulated devices. b) J–V curve of the devices under different continuous light-soaking time

Is the inorganic perovskite solar cell a trend in the future?

The answer is Yes.

There are more scientists get high hopes for inorganic perovskite solar cells. Under the efforts of Profe. You, conducted SCG, a 14.21% PCE of CsPbI2Br solar cells has been obtained. Using this SCG method, the team have also achieved as high as 16.14% and 9.81% PCE of CsPb(I0.85Br0.15)3- and CsPbBr3-based solar cells, respectively. These results indicated that our SCG method is universal at least for high-quality inorganic perovskite films growth and also for obtaining efficient solar cells. In conclusion, a 15.7% PCE of CsPbI3 solar cells have been achieved by SCG of the absorb layer, and the devices can tolerate above 500h of continuous light soaking. There is still a large room for device performance, especially on the open-circuit voltage, considering the bandgap of CsPbI3 (1.73eV); a 1.3V open-circuit voltage should be feasible for CsPbI3 solar cells if the contact and the defect can be perfectly controlled, and the efficiency will be close to or beyond 20%.

Cited from: Solvent-controlled growth of inorganic perovskite films in dry environment for efficient and stable solar cells, Pengyang Wang, Xingwang Zhang, Yuqin Zhou, Qi Jiang, Qiufeng Ye, Zema Chu1, Xingxing Li, Xiaolei Yang, Zhigang Yin & Jingbi You
Nature Communicationsvolume 9, Article number: 2225 (2018)