Resumen de Double resonant character in an optical cavity for high performance and stable polymer solar cells
Solution-processed thin film solar cells emerged as very promising photovoltaic technologies suitable for a low cost roll-to-roll upscale production. Such thin film character also ensures lightweight and flexibility for the solar cell modules, making them ideal for a wide variety of applications where silicon panels cannot be used. In addition to the above-mentioned advantages, common in all solution-processed thin film technologies, polymer solar cells (PSCs) have a unique semitransparency, which makes them very useful for solar window applications and very competitive in building integrated photovoltaics.
In recent years, a remarkable progress has been achieved in the field of PSCs. The power conversion efficiency of PSCs has already surpassed the 11% barrier. However, to be able to eventually compete with other solution-processed thin film technologies, such device efficiency must be further improved. Given the low charge carrier mobility in commonly used organic p-conjugated semiconductors, the tradeoff between optical absorption and charge collection, limits the thickness of the majority of photoactive layers currently being used to approximately 100 nm. To overcome the limited light absorption in such thin active layers, an adequate optical management becomes very important. Ideally, a light absorption or short-circuit current enhancement should be achieved without affecting the other photovoltaic parameters, such as the photovoltaic device open circuit voltage and fill factor.
In this thesis, we implement a one-dimensional new optical planar cavity that exhibits a resonant character at two different nonharmonic frequencies of each other, which we named two-resonance tapping cavity (TRTC). With such TRTC we demonstrate that one may reach an optimal broadband light trapping in thin film cells, largely improving the photocurrent of the solar without sacrificing the device electrical properties.
A limited stability is another obstacle that may prevent any industrial application of the PSC technology. In accordance, in parallel to a device efficiency increase one must address the problem of a short operational device lifetime. In the current thesis, we performed several experiments, which lead us to understand the physics behind the rapid destruction of the active layer nanomorphology under illumination. In addition, we propose and implement a new procedure based on the formation of a highly ordered PCBM phase to circumvent such degradation path, and achieve high-performance PSCs with long lifetimes.
The current thesis has been divided into five chapters. Chapter 1 briefly reviews some photon management approaches and some basics of degradation mechanisms in PSCs. Chapter 2 describes the TRTC concept and the experimental implementation of PTB7-Th:PC71BM cells integrated in such TRTC. In Chapter 3 we describe the use of the TRTC concept presented in Chapter 2 to achieve an optimal balance between open circuit voltage and photocurrent in flexible PBDBT:ITIC cells. In Chapter 4 we demonstrate an approach to increase the operational lifetimes of PSCs based on a UV treatment to actively remove chemisorbed oxygen on the ZnO interlayer. Finally, in Chapter 5, an in-depth study of the fast burn-in loss for PTB7-Th:PC71BM cells is given, and a new explanation to the such degradation path is proposed. In addition, we propose and implement an approach to circumvent such degradation and achieve long lifetime high efficiency solar cells.
