Solar cells loss mechanisms

Erese KW Jr (1982) A high efficiency single-crystal CdSe photoelectrochemical solar cell and an associated loss mechanism. Appl Phys Lett 40 275-277... 

For solar cells, the fill factor FF determines the position of the maximum power point in the 4th I/V quadrant of the illuminated diode and is therefore a quality sign of the photodiode. Besides the increased efficiency, the FF of a photodiode is also important when evaluating the proper function of the diode. High FF values are expected only for diodes with a strict selection principle for the separation of positive and negative carriers. There are several loss mechanisms for photodiodes that can reduce the FF in a photodiode ... 

Steady-state photocurrent-voltage behavior quantifies the bottom line efficiency of operational solar cells. Unfortunately, it provides little molecular information, particularly when the photocurrent yields are low. A variety of transient, time and frequency domain electrochemical techniques have been employed to provide further insights into the molecular origin of loss mechanisms. Space restrictions do not allow these electrochemical techniques to be discussed here [34-37]. 

After splitting of the exciton at the heterojunction and further separation of the charge-transfer state, the charges (or polarons) need to be transported within the semiconductor materials network to the respective electrodes. Despite the recognition that it is mainly the separation of the charge-transfer states that limits current polymer polymer solar-cell devices, charge transport also has to be considered, because it is another potential loss mechanism. We will briefly cover this topic the interested reader is referred to a recent review by Blom et al. [7]. 

In order to uncover the photophysical processes and the loss mechanism in polymeric solar cells, two main approaches can be distinguished electrical measurements and time-resolved optical spectroscopy. Electrical measurements have the fundamental disadvantage that they lack time and spatial resolution to probe the processes that occur directly after exciton ionization. To investigate these dynamics, time-resolved spectroscopy is a much more promising approach, because ultrafast laser systems allow observing these proeesses directly with subpicosecond time resolution. 

Probing the Major Loss Mechanism in Organic Solar Cells with High Open-Circuit Voltages... 

FIGURE 10.8 The energy diagram of the photoinduced electron transfer and the main energy loss mechanisms in donor-acceptor bulk heterojunction solar cells. 

To cover the maximum absorption in the white solar spectrum, two or three plates incorporated by the appropriate limiinescent species must be used. The only loss mechanism that cannot be circumvented is the Stokes shift that is necessary to avoid the overlapping of the absorption and emission. This is a thermodynamic requirement that was taken into account by calculating the efficiency of photovoltaic cells. 

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