Nanophotonic Emission Control for Improved Photovoltaic Efficiency

Abstract

With the necessary transition to renewable energy at hand, there is a renewed research focus on increasing solar cell efficiency in order to reduce the cost of electricity. Nanomaterials are promising candidates to contribute to a new generation of low cost and highly efficient solar cells. Due to their wavelength-scale dimensions, nanomaterials display exceptionally strong light− matter interactions that lead to large perturbations in absorption and emission compared to their bulk counterparts. Although most work on nanostructured solar cells has focused on increasing the absorption, emission control may have even greater potential for improving efficiency of state-of-the-art solar cells. In this Perspective, we describe how nanostructures can be applied to improve solar cell efficiency, focusing on emission control. First, we analyze the requirements for making the most efficient solar cell by looking at the thermodynamics of energy conversion. We show that an ideal solar cell at open circuit displays emission that is identical to its absorption. Comparing this to the emission of a typical silicon solar cell shows that there are three differences: the intensity, the angles in which light is emitted, and the spectrum. These differences lead to a reduction in efficiency, mainly due to a drop in open circuit voltage. For each loss mechanism, we discuss how nanomaterials can manipulate the emission and thereby reduce the voltage loss. Finally, we analyze the performance of two conceptual designs for solar cells based on nanomaterials. These give a large improvement in efficiency compared to conventional designs, showing the great potential of nanomaterials in solar cells.

Revealing the internal luminescence quantum efficiency of perovskite films via accurate quantification of photon recycling

“…Therefore, to further enhance the V OC of PSCs, it is crucial to (1) increase Q lum i by minimizing non-radiative recombination at the interfaces, 2,3,[17][18][19]80 which is regularly done in recent literature by chemical passivation and interface engineering approaches as well as by employing optimized charge transport layers 18,82,[85][86][87][88][89] ; and (2) minimize p a , which can, for example, be done by employing alternative transparent electrodes with reduced near-infrared absorption, such as indium-zinc-oxide or hydrogen-doped indium-oxide, 90 as well as by using either non-absorbing or very thin charge transport layers such as self-assembled monolayers. 82 Finally, we note that p e indeed should always be maximized even if this is at the expense of photon recycling, as recently discussed by Bowman et al 84 In that regard, nanophotonic emission control 8,83 is an as-yet underexplored way to fine-control the emission properties of PSCs and thereby could further boost the PCE; for example, by employing nanostructures. 71,75,91 While it is beyond the scope of the present work, accurately quantifying the values of p e ; p a , Q lum i , and DV PR OC for complete PSCs with various perovskite compositions, scattering properties, and transport/electrode layers, by implementing the above-described effects of parasitic absorption in our curve-fitting model, is goal of ongoing work.…”

“…8 Therefore, a reduction of p e either by a reduced scattering or, more importantly, by parasitic absorption of initially trapped PL as discussed above (Figure 4B) both result in a higher DV nrad OC (Figure 4D) and thus a lower overall V OC (Figure S28). 2,7,8,15,17,83,84 As an example related to the best 260 nm-thick MAPI film, assuming values of Q lum i = 78% (see vertical dashed line in Figure 4D), p e = 25% and p a = 0% in a complete PSC, DV nrad OC accounts for a very low $19.5 mV. In contrast, in case 15% of initially trapped PL would be parasitically absorbed before coupling out upon scattering (i.e., p e = 10% and p a = 15%), DV nrad OC increases to $43.2 mV.…”

Photonics for Photovoltaics: Advances and Opportunities

“…In all other materials, the open circuit voltage could gain >100 mV, corresponding to a >10% relative improvement in efficiency, by enhancing the radiative rate through enhanced light outcoupling. 10 , 63 − 65 Also, silicon solar cells, in which carrier recombination is ultimately governed by Auger recombination, could benefit substantially from a strong PL rate enhancement, as it would effectively drive them closer to the behavior of a traditional direct bandgap semiconductor.…”

“…Ideally, the cell couples in light with energy well above the bandgap over a wide angular range, while photons right at the bandgap (corresponding to PL energy) only couple strongly to the direction corresponding to the sun. 10 The latter avoids the entropy increase associated with the conversion from collimated light from the sun to a broad angular distribution of light inside the cell and allows the open circuit voltage to increase by >100 mV, while also allowing for better collection of diffuse light. One important consideration is that methods to control directional emission cannot come at the expense of photon absorption or enhanced surface recombination, otherwise, the efficiency gain is lost.…”

“…For example, altering the angular or spectral distribution of light entering or exiting the solar cell can lead to efficiencies well beyond 34%. 10 , 13 , 14 The most successful implementations so far involve using concentrating lenses and stacking solar cells in a tandem or multijunction configuration where high-energy light is absorbed in the high-bandgap solar cell placed in front of a low-bandgap cell that absorbs the low-energy light. 15 More exotic concepts, including up- or down-conversion of the solar spectrum to match it better to the solar cell bandgap, are predicted to have similar limits, but so far have not been successfully implemented to improve record solar cells.…”