The Role of Surface in Photovoltaic Perovskites: From Quantum Dots to Bulk Thin films
The surface of a semiconductor often has a key role in determining its properties. For metal halide perovskites, understanding the surface features and their impact on the materials and devices is becoming increasingly important. At length scales down to the nanoscale regime, surface features become dominant in regulating the properties of perovskite materials owing to the high surface-to-volume ratio. For perovskite bulk films in the micrometer range, defects and structural disorder readily form at the surface and affect device performance. Through concerted efforts to optimize processing techniques, high-quality perovskite thin films can now be fabricated with monolayer-like polycrystalline grains or even single crystals. Surface defects, therefore, remain the major obstacle to progress, pushing surface studies to the forefront of perovskite research. Hence, research towards fundamental understanding of perovskite surfaces and how they can affect the materials properties is crucial to further improving the perovskite-based optoelectronic devices.
In Chapter 2, surface energies will be shown to play an important role in affecting the phase stability of perovskite materials. Thermodynamic and crystallographic analyses revealed that enhanced contribution of the surface energy and lattice contraction contribute to the superior stability of colloidal quantum dots (CQDs). This approach provides a new route for achieving stable formamidinium lead iodide (FAPbI3) perovskite solar cells. Later on, in Chapter 3, a conjugated small molecule ITIC was introduced into the FAPbI3 CQDs perovskite solar cells to further enhance the power conversion efficiency of the devices.
In Chapter 4, I will show that in lead halide perovskites, how their “soft” nature renders them highly responsive to the external field, allowing for extended depth scale affected by the surface. By taking advantage of this unique feature of perovskites, I demonstrate a methodology for property manipulation of perovskite thin films based on secondary grain growth, where tuning of the surface induces the internal property evolution of the entire perovskite film. While in conventional microelectronic techniques secondary grain growth generally involves harsh conditions such as high temperature and straining, it is easily triggered in a perovskite thin film by a simple surface post-treatment, producing enlarged grain sizes of up to 4 μm. The resulting photovoltaic devices exhibit significantly enhanced power conversion efficiency and operational stability over a course of 1000 h and an ambient shelf stability of over 4000 h while maintaining over 90% of its original efficiency.
Surface trap-mediated non-radiative charge recombination is a major limit to achieving high-efficiency metal-halide perovskite photovoltaics. The ionic character of perovskite lattice has enabled molecular defect passivation approaches via interaction between functional groups and defects. However, a lack of in-depth understanding of how the molecular configuration influence the passivation effectiveness is a challenge to rational molecule design. In Chapter 5, the chemical environment of a functional group that is activated for defect passivation was systematically investigated with theophylline, caffeine and theobromine. When N-H and C=O were in an optimal configuration within the molecule, hydrogen-bond formation between N-H and I assisted the primary C=O binding with the antisite Pb defect to maximize surface-defect binding. A stabilized power conversion efficiency of 22.6% of photovoltaic device was demonstrated with theophylline treatment.