Shading on photovoltaic modules is practically inevitable, especially in urban environments. The shadows cast by neighbouring objects on the solar panel force shaded solar cells to operate under reverse bias. In this case, instead of generating power, the shaded solar cell dissipates power, which is converted into heat and may induce the formation of hot-spots. Many attempts have been made to improve the shade tolerance photovoltaic modules. In this work, we focus on solar cells with low breakdown characteristics to build shade tolerant photovoltaic modules. These types of solar cells allow the current flow at low reverse bias voltages (around −4 V). The main design challenge is to maintain high conversion efficiencies while achieving low breakdown voltages.
In order to design shade tolerant photovoltaic modules, the carrier transport mechanisms in the solar cell under reverse bias conditions are firstly investigated. A robust simulation template is created in Sentaurus TCAD to perform a parametric evaluation, including both the structural and operating parameters, of the device I-V characteristics. The silicon heterojunction interdigitated back contact solar cell with a silicon oxide passivation layer is among the most promising cell structures to achieve both the low breakdown voltage and the high efficiency. Band-to-band tunneling happens between the heavily doped p+ and n+ regions at the rear side, which allow charge carriers to recombine without entering the bulk of the solar cell. We analyze the effect of the tunneling mass, the gap distance and the dopants penetration length on the forward and reverse I-V curves. Simulations suggest that it is possible to design high efficiency solar cells with breakdown voltages as low as −1.2 V. In addition, device performances under different temperature and irradiance conditions are analysed for the purpose of further investigations on the system level.
While this research study is mainly focused on the performance of solar cells, the results presented in this thesis facilitate comprehensive system level energy yield analyses of shade tolerant photovoltaic modules with low breakdown voltage solar cells.

In urban environments, more and more building added and building integrated photovoltaic (PV) systems are found. These systems use conventional PV modules which have a poor performance under non-uniform illumination conditions. When the module is partially shaded, either at least one subgroup is bypassed or the module current is limited by the current of the worst performing cell. This leads to significant and disproportionate power losses. One way to address this issue is to implement a dynamic module architecture which changes its electrical configuration depending on the shading profile. This report evaluates the potential and performance of reconfigurable modules in urban landscapes and describes the optimal module configuration. An electrical simulation framework was developed in MATLAB to trace the current-voltage (I-V) characteristics of several static and dynamic module architectures. For a PV module with 72 solar cells, it was found that a series-parallel connected reconfigurable architecture with six subgroups is the optimal trade-off between a high shading tolerability and a simple module design. The potential of these PV modules increases for locations further away from the equator since shadows are larger and occur more frequently. The annual energy yield of a shaded module of a typical PV system in an urban landscape located in Rotterdam (the Netherlands) can increase up to 9% when replacing a conventional with a reconfigurable module. Reconfigurable modules require additional components, but the negative effect on the power output under uniform illumination conditions is marginal. Consequently, reconfigurable PV modules make complex shading analysis for PV systems superfluous. Compared to a static parallel topology, the module current is generally much lower which in turn leads to lower power losses on system level. Other advantages of the reconfigurable PV module are the smaller seasonal fluctuations in energy yield and the absence of local maxima in P-V curves. Finally, a proof of concept was made to validate the simulation framework. Only minor deviations between the simulation results and the measurements were found. Experiments also showed that solar cells with a soft breakdown must be used to prevent the formation of hot-spots under all circumstances.