As everyone knows the goal when constructing a photovoltaic cell is to maximize the power conversion efficiency (PCE). This is done through maximizing Voc and Jsc. I was just reading a paper that the authors' goal was to improve the Voc (by introducing buffer layer) as high as 1.2 V. However, the efficiency and FF of the cell is kept low. (Yun Seog Lee et al. Adv. Mater. 26 (2014) 4704)
So my question is: Voc or efficiency, which one to be maximized?
Basic maximum power transfer theorem says maximum power is transferred to load when load resistance is equal to source resistance in a resistive circuit. The resulting max P = V2/4R, R=load resistance, V= source voltage. This shows if V is increased, power transfer also increases. So does the efficiency. I hope this may help you.
Thanks for the reply. My question is about why some authors focus on maximizing Voc, while the efficiency they achieve is surprisingly low. This means that the Jsc of their cell would be low that results in a low power conversion efficiency. To conclude with, I just want to mention that as everyone knows, there is a trade-off between Voc and Jsc (increasing one would lead the other parameter to decrease). So, we have several parameters to optimize: Voc, Jsc, and efficiency. Why many authors focus on improving Voc rather than efficiency or Jsc? Is there any special reason for that?
I think that there is probably more potential to improve Voc than Jsc because Jsc is, for almost all cells, pretty close to the maximum - you have a situation where you have to put in a lot of effort and cost to get a small benefit. On the other hand, the energy in solar photons in eV is much higher than the usual 0.4V you get at MPP in silicon cells so the potential for improvement is correspondingly greater. But neither Voc or Jsc are easy to improve.
eff=FF X Voc X Jsc / G
increasing Voc increases the efficiency as I know.
plus the electric loss is low if the voltage is high P=IR2
Eff maximization is always the ultimate goal of solar cell development. One can choose to "divide and conquer", i.e. optimize Voc, Jsc, FF separately. Reasons for this are that different physical mechanisms underly each of these 3 parameters and that some of them can have more room for improvement compared to the theoretical limits than others. For example, for many cell types Voc is further away from the theoretical limit than Jsc, see e.g. this paper: http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7114196.
In the end, one will still need to provide good Jsc, Voc, FF to achieve high Eff cell. High Eff values may be achieved in different regions of the parameter space, e.g. higher Voc, lower Jsc, or higher Jsc, lower FF, etc.
The conversion efficiency is the main parameter. A large Voc with a poor photocurrent results in a poor conversion efficiency. Conversely a large photocurrent without photovoltage cannot delivers electrical energy to a load. The cell cannot work as a current source like a photodetector because the cell is firstly an energy source. The key parameter is the internal impedance of the cell which must be very close to the load impedance. In other words the photovoltaic conversion of solar energy needs that a photocurrent and a photovoltage are delivered to the load. The two parameters must be optimized simultaneously (junction without leakages, passivated surfaces, high bulk lifetime of minority carriers and so on).
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