Improving Si solar cell performance using Mn:ZnSe quantum dot-doped PLMA thin film
© Cheng et al.; licensee Springer. 2013
Received: 8 May 2013
Accepted: 5 June 2013
Published: 20 June 2013
Poly(lauryl methacrylate) (PLMA) thin film doped with Mn:ZnSe quantum dots (QDs) was spin-deposited on the front surface of Si solar cell for enhancing the solar cell efficiency via photoluminescence (PL) conversion. Significant solar cell efficiency enhancements (approximately 5% to 10%) under all-solar-spectrum (AM0) condition were observed after QD-doped PLMA coatings. Furthermore, the real contribution of the PL conversion was precisely assessed by investigating the photovoltaic responses of the QD-doped PLMA to monochromatic and AM0 light sources as functions of QD concentration, combined with reflectance and external quantum efficiency measurements. At a QD concentration of 1.6 mg/ml for example, among the efficiency enhancement of 5.96%, about 1.04% was due to the PL conversion, and the rest came from antireflection. Our work indicates that for the practical use of PL conversion in solar cell performance improvement, cautions are to be taken, as the achieved efficiency enhancement might not be wholly due to the PL conversion.
KeywordsSi solar cell Quantum dots Photoluminescence conversion Antireflectin 78.55.-m 84.60.Jt
Commercial solar cells employ only a small portion of the solar spectrum for photoelectric conversion, with the available wavelengths covering the visible to near-infrared (NIR) regimes . To fully use the solar emission energy, various light frequency-conversion approaches have been proposed [2–17], which convert IR or ultraviolet (UV) lights into visible ones, the so called up- and down-conversions, respectively. So far, the photoluminescence (PL) conversion, as a type of down-conversion, seems more potentially available in solar cell efficiency enhancement. However, its practical use is actually uncertain, as other factors such as antireflection (AR) might also contribute to the efficiency enhancement in addition to the PL conversion, making the assessment of real contribution from PL conversion doubtful [6, 9–14]. Although in our recent work , we have noticed this problem and tried to single out the contribution of PL conversion, systematic studies and convincing experimental facts are still lacking. This work aims to solve the puzzling problem by offering a combined approach and evaluating how important on earth the PL conversion could be in improving solar cell efficiency. We selected a material with high PL conversion efficiency (> 40%), i.e., Mn-doped ZnSe quantum dots (Mn:ZnSe QDs). The QDs were dispersed within an organic solution, poly(lauryl methacrylate) (PLMA), which was then spin-coated on the front surface of commercial Si solar cell. The photovoltaic (PV) responses to monochromatic and AM0 light sources were investigated, combined with reflectance and external quantum efficiency (EQE) measurements. With these, the real contribution from PL conversion to the solar cell efficiency enhancement was unambiguously identified and assessed.
Results and discussion
PV parameters for Si solar cells after treatments
Δ η/η0(%) (calculated)
CQD = 0
CQD = 1.6 mg/ml
CQD = 3.0 mg/ml
In this work, AM0 solar simulator rather than the more conventional AM1.5 one has been used. This is because the effect of PL conversion on the performance improvement of solar cell is more applicable in the environment with higher UV proportions. The UV proportion in the high altitude or outer space environment, which the AM0 condition mimics, is generally two to three times that in the normal AM1.5 one. On the other hand, from Figure 4, it is seen that the solar cell has high EQE in a broad wavelength range of approximately 450 to 1,000 nm; therefore, although for each wavelength, the corresponding reflectance changes with the changing film thickness due to the light interference, the overall efficiency enhancement is not sensitive to the film thickness, as what we found in our experiments for the film thickness in the range of 100 to 300 nm. However, it should be pointed out that to optimize the effect of PL conversion, the film thickness could be a concern in the work that follows; meanwhile, the side edges of PLMA film would be coated with SiO2 by a simple mask method so as to prevent visible lights due to the PL conversion escaping from the side areas and let them be absorbed by the solar cell underneath.
In this work, PLMA thin film doped with Mn:ZnSe QDs was spin-deposited on the front surface of Si solar cell in order to improve the solar cell efficiency via PL conversion. Significant efficiency enhancements (approximately 5% to 10%) were achieved indeed under AM0 conditions. Both the effects of AR and PL conversion contributed to the solar cell efficiency enhancements but that of PL took a small portion. A precise assessment of PL contribution to the efficiency enhancement was made by investigating the PV responses of Si solar cells coated with QD-doped PLMA to monochromatic and AM0 light sources as functions of QD concentration, combined with reflectance and EQE measurements. Our work shows that the real PL contribution might not be all that as reflected by the apparent efficiency enhancement, and cautions are to be taken when applying the PL conversion in this aspect. On the other hand, it indicates again that for practical use of PL conversion, high altitude or/and outer space environments are preferred where the UV proportion is high, and continuing to search for high PL efficiency materials and design efficient optical-coupling structures is still necessary.
This work was supported by the National Basic Research Program of China (973 Program) under the grant number 2012CB934303 and by the National Natural Science Foundation of China under the grant numbers 61275178, 10974034, and 60878044. Experimental assistances from Professors J. D. Wu, N. Xu, and J. Shen are gratefully acknowledged.
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