Jongwon Lee

Limiting efficiencies over 50% using multijunction solar cells with multiple exciton generation

The achievement of solar cells over 50% is a critical goal for photovoltaics. Multijunction solar cells over 5 junctions allow such efficiencies, but are severely limited by material constraints and growth requirements for lattice matching. Nanostructured approaches such as multiple exciton generation (MEG) potentially offer a route to higher efficiency but still require high values of sunlight concentration and large quantum yields. We show an approach that allows for higher efficiencies based on including MEG in a multijunction solar cell. We also present a thermodynamic model for multijunction solar cells with MEG that demonstrates possible improvements.

The thermodynamic limits of tandem photovoltaic devices with intermediate band

We present a hybrid thermodynamic model for multijunction solar cells with intermediate bands that demonstrates possible improvements to conventional multijunction photovoltaic systems. Applying this model to selected tandem cell structures shows that the performance of such hybrid solar cells is enhanced and that multiple transitions from intermediate bands can reduce the number of material stacks and boost overall efficiency. We demonstrate the results of detailed simulations for multiple numbers of stacks of hybrid multijunction solar cells. And, we can choose proper materials to compose intermediate band for each junction. Furthermore, we suggest other alternative hybrid solar cell systems to absorb moderate photon energy range and find appropriate materials for hybrid solar cells.

Limiting Efficiencies of Multijunction Solar Cells With Multiple Exciton Generation

The introduction of multiple exciton generation (MEG) processes with a low maximum quantum yield (QY) (maximum 200%) into a multijunction solar cell provides an efficiency increase up to 4.2% absolute for a triple-junction solar cell under concentration. In addition, the efficiency contour plots show increased flexibility in material choices, even for series connected devices. Importantly, the MEG QYs necessary to achieve these advantages are moderate and within experimental measured values, requiring no more than the generation of two electron hole pairs from a photon.

Development of Cu plating for silicon heterojunction solar cells

This paper reports the results of the study comparing various patterning and plating methods for the deposition of Cu electrodes on transparent conductive oxides for silicon heterojunction solar cells. We compared direct electroplating of Cu on different metal seeds (Ag, Ni, Cr and Ti deposited on transparent conductive oxide by physical vapor deposition) to the light induced plating of Ni/Cu directly on transparent conductive oxide. Patterning was done either using photoresists (formed by spin-on, screen printing or lamination) or lift-off of the PECVD dielectric using screen printed resist. The geometry of the fingers, line resistance, contact resistance and adhesion were used as comparative parameters. We identified direct electroplating of Cu on the sputtered Ag seed to achieve the lowest contact resistance and the best adhesion. All photoresists were able to achieve less than 60 micron resolution and could produce the fingers with the sought height (some, however, having a characteristic mushroom shape). The best silicon heterojunction cell with Cu contacts directly electroplated on the sputtered Ag seed achieved 21.9% efficiency on 153 cm2 area.

Impact of threshold energy of multiple exciton generation solar cells

The threshold energy (Eth) over 100% quantum yield (QY) is a key factor to determine the performance of multiple exciton generation (MEG) solar cells. By investigating non-idealities of MEG models, it is critical to consider non-idealities in Eth for the onset of MEG processes. Detailed balance calculations show that despite a large experimental emphasis on the maximum quantum yield, the threshold energy has a substantial and significant effect. The first effect is that at threshold energies between 2 and 3Eg (for one sun) or 3 and 4Eg for maximum concentration, even theoretical benefits of the MEG process disappear. Since measured values are within this range, this is an important effect to consider. The second effect is that the inclusion of non-ideal threshold energies increases the optimum band gap, moving to values consistent with silicon.

Limiting efficiency of silicon based nanostructure solar cells for multiple exciton generation

The materials for multiple exciton generation (MEG) solar cells have often focused on colloidal systems using low band gap materials such as PbSe. However, detailed balance calculations with non-ideal quantum yield (QYs) lead to higher band gaps, with silicon close to the optimum value. We calculate the conversion efficiency of MEG processes including non-idealities for nanostructured silicon. We also boost efficiency of MEG solar cells using multijunction solar cell configurations. Incorporating MEG into multijunction solar cells leads to increased calculated efficiencies due to QYs greater than unity in each junction. Here we have simulated the possible MEG enhanced QY of each junction and the corresponding conversion efficiencies for double junction hybrid solar cells. This hybrid structure extends the opportunities to maximize the MEG effect and also to select the appropriate effective bandgaps using silicon nanostructures.

Theoretical analysis for intermediate band and tandem hybrid solar cell materials

The efficiency limit of an intermediate band (IB) solar cell can be increased by a “tandem” configuration of multiple intermediate band devices. Thermodynamic models show that the efficiency of a two-stack tandem of IB devices achieves the efficiency of a six junction series connected solar cell. The efficiency of an IB in conjunction with a single or double stack tandem has similar efficiency advantages. Further, analysis of the materials which can be used to implement IB solar cells in a tandem configuration shows advantages relating to the ability to implement IB materials with quantum wells or quantum dots. For a single IB solar cell, a key difficulty is identifying materials for the barrier and the quantum well which have a small valence band offset and large conduction band offset (or the reverse). The use of an IB solar cell as the bottom solar cell of a tandem allows a larger range of materials with suitable barrier band gaps and a smaller ideal conduction band offset. A further theoretical advantage of such a structure is that it avoids the extremely low open circuit voltages achieved from pn junctions in low bandgap materials; for example, the thermodynamic optimum for a 6 junction tandem solar cell has its lowest bandgap below 0.4 eV. We present a thermodynamic model for IB hybrid tandem configurations which does not assume spectral selectivity among the different solar cells and predicts that a barrier/quantum dot structure can have an efficiency as high as 60 to 70 percent at 1000X blackbody radiation.

Limiting efficiencies of integrating single junction with intermediate band solar cells for multiphysics effects

The multi-physics aspects of hybrid solar cells has provided the numerous advantages. To overcome the disadvantage of tandem solar cells like three or four junctions, single junction and intermediate band solar cell (IBSC) integrated concept is useful alternatives to replace the tandem solar cells configuration. IBSC has triple carrier transitions so that integrating with single junction can show the similar characteristics of conventional four junction tandem solar cells. From the advantages of this hybrid solar cells, we investigates the theoretical approaches and the appropriate material selections.

The impact of quantum yield through limiting efficiency for multiple exciton generation with intermediate band solar cells

We develop the hybrid thermodynamic limit model using the intermediate band solar cells assisted with multiple exciton generation under blackbody radiation. For this hybrid solar cell model, we manage the spectral splitting to maximize the generated number of electron and hole pairs (EHP). First, we have separated two areas to explain the carrier transition. For regarding of quantum yield and charge neutrality, the multiple EHPs are generated at barrier bandgap and one carrier generation is in quantum dot. Thus, to extract additional carrier in quantum dot, it is required additional absorption paths or more photon energy. After studying the procedure of carrier multiplication in intermediate band solar cells, we have calculated the theoretical conversion efficiencies with number of generated EHPs. Its maximum theoretical efficiencies are increased and optimum bandgap is lowered compared to conventional intermediate band solar cells. And, based on these results, we can also choose the suitable material for these hybrid solar cells.

Limiting efficiencies of intermediate band solar cell assisted with multiple exciton generation

We propose the hybrid thermodynamic model using the intermediate band solar cells assisted with multiple exciton generation. We have calculated this thermodynamic model under blackbody radiation and standard AM1.5 spectrum and changed concentration to compare the conventional intermediate band solar cells. Because of multiple electron and hole pair at the conduction band edge, its maximum efficiencies and optimum bandgaps have been enhanced compared to conventional intermediate band solar cells. The maximum efficiencies of this solar cell are both 47.31 (Blackbody Radiation) and 49.85 (AM1.5G) percent for one sun and 65.07 (Blackbody Radiation) and 67.83 (AM1.5D) percent for maximum concentration. And its corresponding overall bandgap energies are also reduced because of multiple electron and hole pairs.