Reduced thermal degradation in perovskite solar cells by interfacial engineering using solution processed polymer layer

Abstract

Persistent R&D to get alternatives to the market-dominating expensive & heavy silicon solar cells has led to the development of thin film technologies, one of which is perovskite solar cells (PSC). After being introduced in 2009 they have shown an unprecedented increase in efficiency – from <10% in 2012 to over 23% [1-7] in just seven years. These solar cells can be manufactured with less energy and toxic material than traditional silicon solar cells. On the downside, PSCs have stability challenges with all major environmental stress factors namely oxygen, humidity, light and temperature. The purpose of this thesis is to produce high-performance perovskite solar cells and make them more robust to temperature degradation. We started with optimizing the cation mixture for our standard recipe and moved forward to introduce a new polymer i.e. PMMA to the recipe. This layer, when inserted between perovskite and HTL showed an increase in open-circuit voltage which motivated us to do more experiments with it. First, we added post-annealing in the spin-coating process of this layer and there was an increment in both power conversion efficiency and open-circuit voltage with it compared to the cells with non- annealed PMMA layer. This indicated the enhancement in thermal stability of cells because of PMMA. We went ahead to apply thermal stress to the cells by heating the perovskite with and without PMMA at 50oC for the first test and at 100oC for the second one before coating HTL. From these experiments, a clear improvement was visible in efficiency and open-circuit voltages of cells which can be attributed to PMMA helping the perovskite structure to maintain its crystallinity and reducing its degradation to PbI2. X-ray diffraction (XRD) and Field Emission Scanning Electron Microscope (FE-SEM) characterizations have been done on the overnight heated substrates, one with PMMA on top of perovskite and other with the only perovskite from both 50oC and 100oC heating experiments. The XRD plots clearly show the reduced degradation of perovskite when PMMA is used. From the FE-SEM images, the bigger grain size of perovskite with PMMA is clearly visible and also the difference in PbI2 content can be seen which is consistent with the XRD results. So we concluded that the addition of a PMMA layer between the perovskite and HTL (spiro –OMeTAD) has reduced the thermal degradation in PSCs till 100oC.