Quantum dot solar cells and electrochemical doping of QD films

Abstract

Quantum dots (QDs) are nano-crystal semiconductors (1-100 nm) in which charge carriers (electrons and holes) are confined in all 3 dimensions by potential barriers that cause them to behave differently from conventional bulk semiconductors. QD research in the past decade has progressed rapidly, allowing
a deeper understanding of the physics behind the functioning and the effective synthesis of such materials. The wide spread opto-electronic applications of such QD semiconductors in LEDs, lasers, electrochemistry and solar cells with the potential to outperform traditional bulk semiconductors has fuelled inspired research in this field.
Quantum dot solar cells (QDSC) are solution-based third generation solar cells that possess the potential to overcome the Shockley-Queisser limit using multiple exciton generation (MEG). The large Bohr radius, wide bandgap tunability and large light absorption coefficients of PbS QDs have made them the most common material used in the absorber layer of such solar cells and shall also be the material used in this thesis. PbS QDs are used in conjunction with an n-type metal oxide to form a heterojunction that enhances charge separation at the interface. ZnO and TiO2 are common metal oxides used for this purpose while different synthesis methods of the same metal has been observed to show different results. While different research groups have used different metal oxide layers, there has been no systematic study on the interaction of the metal oxide layer synthesized by different methods with the absorber layer. Such a study could help improve understanding of the heterojunction interface and shall
be briefly looked into in this thesis. Another area of improvement in device performance is the depletion region across the heterojunction. Varying the doping of the n-type material affects the depletion width which has been explored in the past by adding impurity atoms to the metal oxide. However, the
emergence of an alternate method to dope ZnO electrochemically has triggered an interesting novel pathway to integrate doped materials into solar cells. In this thesis, we have successfully fabricated PbS QD solar cells for the first time in TU Delft at the Synthesis lab of the Applied Sciences faculty with
power conversion efficiencies exceeding 5%. Additionally, a systematic study on the absorber layer and the metal oxide layer was also carried out to optimize the device performance and set protocols for any future work. Finally, electrochemical doping of the PbS absorber layer was attempted to develop
a precise doping mechanism for QD films.