Strain engineering, a new way to break through the efficiency of perovskite

Prof. Qi Chen, Beijing Institute of Technology, and Prof. Huanping Zhou, Peking University, and Prof. Lijun Zhang, Jilin University, and other materials scientists have developed the high efficiency perovskite films with 20.7 % PCEs, which is certified by NIM, China, by modulating the status of residual strains in controllable manner via rational strain engineering.

The mixed halide perovskites have emerged as outstanding light absorbers for efficient solar cells. But it reveals inhomogeneity in these polycrystalline films due to composition separation, which leads to local lattice mismatches and emergent residual strains consequently. The team studies the evolution of residual strain over the films by depth-dependent grazing incident X-ray diffraction measurements and identifies the gradient distribution of in-plane strain component perpendicular to the substrate. Moreover, they reveal its impacts on the carrier dynamics over corresponding solar cells, which is stemmed from the strain induced energy bands bending of the perovskite absorber as indicated by first-principles calculations. The research"Strain engineering in perovskite solar cells and its impacts on carrier dynamics."was published in Nature Communications.

Measuring PL by SPCM-1000 Laser Confocal Scanning Image System
The research is revealed that hybrid perovskites with different lattice structures often exhibit different optoelectronic properties and it is expected to observe the gradient variation in terms of optoelectronic properties within the gradient phase structure of the film. Therefore, the team examined the depth-dependent photoluminescence (PL) spectra within the film by using Enli Tech SPCM-1000 confocal fluorescence microscope. With the increasing depth of the beam probe, a systematic red shift of PL spectra was observed (Fig. 1). Fitting with the Gaussian distribution function, they found the PL peak positions shifted from 781 to 788 nm, and their full width at half maximum (FWHM) decreased along the film thickness. Since the emission photon energy is determined by the bandgap of semiconductors, it indicates the perovskite film exhibits gradually decreased bandgap from the surface to the bottom vertically. In addition, narrower PL linewidth emission at the deeper region of perovskite film was observed, which may indicate weakened interaction between charge carriers and lattice vibrations (phonons) due to improved film homogeneity and lattice order.
Fig. 1. PL depth profile of confocal fluorescence microscope, the inset represents TOF-SIMS depth profiles of the perovskite film with tensile strain. (Cited from the research)

Strain engineering technology:
Modulating the residual strains via temperature gradient control
The research reveals that the observed residual strain gradient in perovskite films is closely related to lattice structure evolution due to detectable compositional inhomogeneity. However, it may not be the only contributor that governs the residual strain, given the largest tensile strain concentrated on the films surface. Interestingly, when examining the pure MAPbI3 perovskite thin film, the team still observed the existence of gradient residual strain. It is thus speculated that the thermal strain may take effects due to the temperature gradient during perovskite film fabrication. Therefore, the team designed a new annealing process to modulating the residual strains in perovskite films.

The efficiency performance of the device with/without strain
To investigate the impact of the gradient residual strain on the device performance, they first fabricated planar heterojunction solar cells by adopting the perovskite absorbers with/without residual strains. The device architecture follows the regular structure of ITO/SnO2/perovskite/Spiro-OMeTAD/Ag. Then they compare the J-V curves of the tensile-strained and the strain-free devices. To avoid possible misleading due to sample variation, the team fabricated 40 cells under optimal conditions in each batch. Figure 2a shows the histograms of PCEs for each batch of samples with/without strain.
The tensile-strained devices exhibited the PCE averaged around 18.7% with a wider distribution from 17.3% and 20.3%. In comparison, the strain-free devices achieved the averaged PCE of 19.8%, whose PCEs were distributed in a narrow range between 18.8% and 20.7%. The narrow distribution in PCEs of the strain-free devices stands for the good processing reproducibility. They also conducted the current–voltage (I-V) measurement for devices under different annealing conditions to preserve different tensile strains in the absorbers. It is found that the fill factor (FF) and the open-circuit voltage (VOC) have significantly improved with the increase of the flipped annealing time, wherein the surface tensile strain is gradually released through prolonged annealing at 120°C.
Fig. 2 Device performance and carrier dynamic behavior analysis. (a) Histograms of the PCEs for the devices with different strain conditions. (b) J–V curves of the tensile strain device and strain-free device. The inset is the stabilized current density measured at a bias voltage (0.94, 0.96V, respectively). (Cited from the research)

How to accurately and effectively measure the performance of the perovskite films with strain?
The team measured current-density voltage (J-V) characteristics of one of the best devices by using Enli Tech SS-F5-3A solar simulator, which provides simulated illumination of air mass (AM) 1.5 and 100 mWcm-2. This device generated a short-circuit current density (JSC) of 22.8 mAcm-2, a FF of 78.0%, an open circuit voltage (VOC) of 1.17 V, and a power conversion efficiency of 20.7%. The forward and reverse scanning current density-voltage (J-V) curves showed negligible hysteresis in the corresponding device, which was likely attributed to the improved carrier extraction at the interface. Also, the team uses Enli Tech QE-R quantum efficiency measurement system to measure the External quantum efficiency (EQE) spectra for the two types of devices. Compared to the tensile strain device, the strain-free device showed improved light harvesting efficiency along the entire absorption wavelength range of 350 to 800 nm. The integrated photocurrent densities were calculated to be 20.81, 22.7 mAcm-2, respectively, which was in good agreement with the JSC derived from the J–V measurement. Thus far, they observed the significant improvement in FF and VOC in strain-engineered devices.
Fig.3 (a) (b) (c) (d) Statistics of I-V performance parameters (Voc, FF, PCE, Jsc) for devices based on mixed FAMA perovskites with different flipped annealing time (e) J-V curve for the tensile-strain device and strain-free device under reverse and forward scan direction (f) External quantum efficiency (EQE) and integrated short-circuit current density for the tensile-strain device and strain-free device.(Cited from the research)

Measuring Perovskite films with high-end Enli Tech optical measurement equipment
To precisely measure the performance of mixed halide perovskite films, the team uses Enli Tech SPCM-1000 FLIM and PL Image Confocal Scanning Image System to examine the depth-dependent PL spectra within the film. SPCM-1000 provides Non-damage and high-resolution PL image and high-speed scanning techniques which can reduce the decay of perovskite materials up to 1000 times. Additionally, the team uses Enli Tech SS-F5-3A solar simulator which is perfectly integrated into a glove box. Then they do the measurement of different light intensity with its highly-precise light intensity adjustment function.

Strain engineering in perovskite solar cells and its impacts on carrier dynamics.
Cheng Zhu, Xiuxiu Niu, Yuhao Fu, Nengxu Li, Chen Hu, Yihua Chen, Xin He, Guangren Na, Pengfei Liu, Huachao Zai, Yang Ge, Yue Lu, Xiaoxing Ke, Yang Bai, Shihe Yang, Pengwan Chen, Yujing Li, Manling Sui, Lijun Zhang, Huanping Zhou & Qi Chen.
Nature Communications, volume 10, Article number: 815 (2019)