Research reveals factors limiting the voltage of polycrystalline CdSeTe solar cells

• So far, solar cells based on polycrystalline cadmium selenide telluride (CdSeTe) have actually shown some crucial constraints, despite their low construction costs as well as useful qualities. Most notably, these solar cells display higher voltage losses than other mature photovoltaic or pv innovations (crystalline silicon, for example) as well as this considerably hinders their efficiency and also performance.

Scientists at Arizona State University (ASU), Colorado State University (CSU), the National Renewable Energy Laboratory (NREL) and also Very First Solar Inc. have actually just recently accomplished a research targeted at far better comprehending the factors for the voltage deficits observed in CdSeTe solar cells. Their paper, published in Nature Energy, provides brand-new insight that can help engineers to enhance the efficiency of these solar cells in the future.

" The idea for this project pertained to us throughout a previous partnership with CSU," Arthur Onno, Assistant Study Professor at ASU, told TechXplore. "Our group, which has its roots in silicon photovoltaic (PV) solar cells, is accustomed to parsing losses (including voltage losses) to recognize what is the issue limiting the effectiveness of our cells and also direct their optimization (think about determining the reduced hanging fruits). We recognized that the CdSeTe community did not have similar methods to systematically analyze voltage as well as effectiveness losses as well as were having a dead spot when attempting to optimize their solar cells."

While reviewing past efforts focused on creating CdSeTe solar cells, Onno and also his associates observed that typically designers developing these modern technologies linked voltage losses with the malfunctioning or constraints of particular cell components. However, extremely few researches attempted to clearly recognize the mechanisms behind observed voltage losses or quantify their effect.

To load this space in the literature, the researchers utilized an optical, non-destructive as well as contact-less technique, called External Radiative Efficiency (ERE) measurement. ERE is a valuable metric that explains exactly how great a solar cell goes to reemitting absorbed light at open circuit (when present is not drawn out from it). Thus, it quantifies how reversible the conversion of the solar resource into an electrochemical capacity is.

Onno and his coworkers accumulated ERE measurements on CdSeTe cells created at CSU. Surprisingly, they found that the main mechanism restricting their voltage is not recombination on problems within the bulk of the cell, as designers developing CdSeTe cells have long thought, yet rather an issue with selectivity. Much more specifically, the group observed that a non-negligible portion of electrons inside the cell appeared to move in the "incorrect" direction as well as thus canceled each other's cost while leaving the cell.

" This searching for is necessary, as it will ideally expand the technique that the CdSeTe community (market consisted of) takes to enhance their tools, with a deeper physical understanding of why the voltage of CdSeTe solar cells is not higher," Onno stated. "This is both a substantial problem and also a substantial possibility: as of today, CdSeTe lags behind many established solar cell modern technologies in terms of voltage losses, which is an issue, however with the largest headroom for improvement, which is a chance."

A solar cell's framework consists of a light-absorbing material sandwiched between semi-permeable membranes, which are known as "calls." The vital feature of these membranes is to make sure that ecstatic electrons leave the absorbing layer from just one of both electrodes in the cells, and that they only come back right into the cell from the other. When this does not take place (i.e., if electrons leave or get in the cell from the "incorrect" electrode), energy is lost.

" In the absorber, bound electrons get excited right into a free electron-hole set when they soak up photons (particles of light)," Onno explained. "The electron-hole set requires to return to the bound (ground) state eventually, we can not 'pump' them with light into the fired up free electron-hole pair state forever, there is and also must be a reciprocal process called recombination. This recombination can take place in the absorber, because case the energy of the taken in photon is shed, or we can collect the electron at an electrode, utilize a few of the energy in the form of electrical energy to power a tons, as well as restore the electron through the various other electrode at the other end of the cell where it will recombine with a free hole."

The speculative technique used by Onno and his coworkers permitted them to access the internal voltage of CdSeTe cells. A cell's interior voltage is the distinction in free energy (i.e., chemical potential) between populations of electrons as well as holes within the absorber.

" If electrons and also holes recombine with problems in the bulk of the absorber or at interfaces, this internal voltage lowers," Onno claimed. "So, the interior voltage is a procedure of exactly how problems decrease your voltage from the ideal thermodynamic restriction. The exterior voltage, on the other hand, corresponds to the interior voltage minus voltage losses as a result of the semi-permeable membranes being imperfect (i.e., losses due to electrons leaving at the wrong electrode or coming back at the wrong electrode).".

When a cell's semi-permeable membranes are operating completely, its internal and outside voltages match. If they are malfunctioning, however, the exterior voltage is lower than the interior voltage.

" In extreme cases, you can think of a symmetrical structure where the semi-permeable membranes coincide on both sides," Onno clarified. "In this case, symmetry dictates that the capacity will be the same on both sides of the cell and also the outside voltage will certainly be zero even if your internal voltage is high. The catch is that the external voltage can conveniently be determined with a voltmeter, but the inner voltage is more difficult to gain access to.".

One of the most significant achievements of the recent work by this group of scientists is that they introduced a basic technique to measure the internal voltage of solar cells. This allowed them to drop more light on the mechanisms underlying voltage losses in CdSeTe solar cells.

" With the support of a $1.5 million research study grant that we obtained from the united state Department of Energy, we first developed the strategy and revealed that it functioned well on silicon, a popular product system on which we might quickly cross-checked our results with advanced techniques," Onno included. "After that, with our partners at NREL and CSU, we began checking into CdSeTe solar cells produced at CSU, with the objective of comparing their internal and the external voltages. 3 years of work, 75 examples fabricated, and also near to 900 solar cells determined later on, we pertained to the verdicts outlined in our paper.".

The researchers also worked together with First Solar, the biggest U.S.-based solar module maker. First Solar sent them solar cell samples with various doping focus, which they might use to validate some of their findings.

In their experiments, Onno as well as his colleagues discovered that get in touches with to CdSeTe solar cells are not always perfect, as many scientists assumed, as they typically contribute to voltage losses in the devices. Furthermore, the results of their analyses suggest that the bulk absorber in these cells is very promising, yet its performance is not totally realized without superb semi-permeable membranes.

" Finally, we found that Se and As alloying have destructive material effects that restrict the attainable voltage independent of the other resources of losses," Onno said. "Understanding the origin of this issue and also mitigating it will certainly be key to push the CdSeTe innovation further.".

In the future, the methodology utilized by Onno and also his coworkers could be used by various other groups to examine the factors triggering voltage losses in a variety of various solar cells. In addition, the searchings for gathered in this research study could help to make extra reliable cells based upon CdSeTe.

The researchers are presently working to make the strategy they developed available to various other research study groups and firms worldwide, as an example by developing a reproduction of their measurement technique that could be utilized by solar cell makers. Their work lately obtained added funding from the State of Arizona's New Economic situation Campaign (NEI).

" We currently plan to expand our study to various other solar absorbers, such as perovskites, which are a warm study subject today," Onno wrapped up. "We also prepare to use the method on silicon solar cells because, as opposed to other comparable charaterization methods, our method can be used on any sort of examples (e.g., incomplete cell precursor, finished tool, cells encapsulated in modules, and so on). For instance, we can envision taking a module that has been generating power in the field for twenty years and exploring it with our technique.".