• Energy Research

AbstractThin film hydrogenated nanocrystalline silicon (nc-Si:H) solar cells were deposited at a high growth rate of 5 nm/s by Very High Frequency Plasma Enhanced Chemical Vapor Deposition (VHF PECVD). A single deposition yielded 30 working cells out of 30 test cells. After characterization of J-V characteristics under AM1.5 conditions, spectral response and Fourier Transform Photocurrent Spectroscopy (FTPS) for defect density, the cells were subjected to a beam of 1 MeV protons, with different irradiation times for a series of cells, up to a maximum fluence of 1015 protons per cm2, to investigate the feasibility of using this type of solar cells in space. After degradation, the cells were characterized again. The highest fluence reduced the conversion efficiency of the solar cells by a factor of 10. These values are compared to a similar experiment in literature for thin film amorphous silicon cells. The spectral response shows that the quantum efficiency for low energy photons is drastically reduced, suggesting that the damage is mainly inflicted to the bulk of the absorber material. This is strongly supported by the FTPS results, which show a clear trend of increasing defect density with increasing fluence. The optical absorption coefficient at 0.8 eV increased by a factor of 20. After characterization, the cells were isochronally annealed, up to the deposition temperatures, to investigate if the original performance of the cell could be restored. The mid-gap absorption decreased by a factor of 5 after annealing.

Upconversion of subbandgap light of thin film single junction amorphous silicon solar cells may enhance their performance in the near infrared (NIR). In this paper we report on the application of the NIR–vis upconverter β-NaYF4:Yb3+(18%) Er3+(2%) at the back of an amorphous silicon solar cell in combination with a white back reflector and its response to infrared irradiation. Current–voltage measurements and spectral response measurements were done on experimental solar cells. An enhancement of 10 μA/cm2 was measured under illumination with a 980 nm diode laser (10 mW). A part of this was due to defect absorption in localized states of the amorphous silicon

Abstract The experimental observation of internal quantum efficiencies above unity in crystalline silicon solar cells has brought up the question whether the generation of multiple electron/hole pairs has to be taken into consideration also in solar cells based on direct gap amorphous semiconductors. To study photogenerated carrier dynamics, we have applied Intensity Modulated Photocurrent Spectroscopy (IMPS) to hydrogenated amorphous silicon p-i-n solar cells. In the reverse voltage bias region at low illumination intensities it has been observed that the DC quantum yield and the low frequency limit of the AC quantum yield Y increases significantly above unity with decreasing light intensity, indicating that more than one electron per photon is detected in the external circuit. This phenomenon can occur due to the presence of dangling bond defect centers in the band gap, which have an amphoteric character and can enhance the photocurrent due to trapping and thermal emission processes of photogenerated carriers.

Abstract Microcrystalline silicon germanium films showing excellent opto-electronic properties have been prepared at a substrate temperature of 195°C by radio frequency plasma enhanced chemical vapor deposition at 13.56 MHz. A white light (AM 1.5) photoconductivity of 5×10 −5 /Ω cm and ambipolar diffusion length of 114 nm (from SSPG) established the device quality. Films are intrinsic (Fermi level near midgap; activation energy E a (0.49 eV) is approximately half the band gap (1.01 eV)). Performance of preliminary n–i–p solar cells (with μc-SiGe:H i-layer) on stainless steel and molybdenum substrates justify their photosensitivities. A current density of 9.44 mA/cm 2 has been generated in an i-layer of only 150 nm thick without any back-reflector. A deposition rate of 0.75 A/s for such a thin layer gives this material much advantage than a μc-Si cell, where a thickness of >2 μm is needed. A high V oc of 0.43 eV has been achieved for such a low mobility gap cell (Ge fraction 60%).

ABSTRACTIn this study, we present a new light absorption enhancement method forp‐i‐nthin film silicon solar cells using pyramidal surface structures, larger than the wavelength of visible light. Calculations show a maximum possible current enhancement of 45% compared with cells on a flat substrate. We deposited amorphous silicon (a‐Si) thin film solar cells directly onto periodically pyramidal‐structured polycarbonate (PC) substrates, which show a significant increase (30%) in short‐circuit current over reference cells deposited on flat glass substrates. The current of the cells on our pyramidal structures on PC is only slightly lower than that of cells on Asahi U‐type TCO glass (Asahi Glass Co., Tokyo, Japan), but suffer from a somewhat lower open circuit voltage and fill factor. Because the used substrates have a locally flat surface area due to the fabrication process, we believe that the current enhancement in the cells on structured PC can be increased using larger or more closely spaced pyramids, which can have a smaller flat surface area. Copyright © 2012 John Wiley & Sons, Ltd.

Thepaperdescribesthewaytotransferprocesstechnologyofstate-of-the-arthighefficiencythinfilm siliconsolarcellsfabricationoncheapplastic(suchasPETorPEN)substrates,bytwocompletelydifferent approaches:(i)bytransferprocess(Helianthosconcept)ofthinfilmsiliconcellsdepositedathigh substratetemperature, Ts ( 200 1C) and(ii)directdepositionontemperaturesensitivesubstratesatlow Ts ( 100 1C).Adaptationoftheprocessparametersandcellprocessingtotherequirementoftheflexible/ plasticsubstrateisthemostcrucialstep.In-situdiagnosisoftheplasmahasbeendonetounderstandthe effectofinter-electrodedistance,substratetemperatureandhydrogendilutiononthegasphase conditions.Whereas,forthetransferprocess,theinter-electrodedistanceisacriticaldepositioncondition thatneedstobeadaptedfortheflexiblesubstrates,thedirectdepositiononplasticsubstrateshasan addedissueoflossinmaterialqualityandthedepositionrateduetodepositionsatlow Ts. Ourstudies indicatethationenergyiscrucialforobtainingcompactfilmsatlowtemperatureandhighhydrogen dilutionhelpstocompensatethelossofionenergyatlowsubstratetemperatures.Efficienciesof 5.9% and6.2%havebeenobtainedforn–i–ptypea-SicellsonPETandPENsubstrates,respectively,usingdirect deposition.Usinganadaptedinter-electrodedistance,ana-Si/nc-Sitandemcellonplastic(polyester) substratewithanefficiencyof8.1%hasbeenmadebyHelianthoscelltransferprocess.

Analyses of various impurities (O, C, W, Fe, Ni and Cr) in the poly-Si material (both in layers as well as in cell configurations) made by HWCVD have been carried out to judge the quality of this material for application in devices, SIMS analysis showed that the oxygen concentration in the bulk of a poly-Si film made at a low hydrogen dilution (Poly2) is 3 × 10 18 cm -3 and the oxygen content drops to this value within a depth of only 50 nm from the surface. On the other hand, a poly-Si film made at a high hydrogen dilution (Polyl) has a high and homogeneous oxygen content of more than 2 x 10 21 cm -3 , However, in a double-layer structure (Poly2 on top of Polyl), the oxygen content of the bottom layer (Poly1) is significantly smaller than the bare Polyl film, though this oxygen concentration is still much higher than that in the top Poly2 layer. We attribute this behaviour to the structural difference between these two films (2000 cm -1 Si-H IR vibration in low-dilution material and 2100 cm -1 vibration in the high-dilution material). We propose that the oxygen penetration in Polyl occurs by two processes: (1) oxygen incorporation during growth: (2) post-deposition oxygen intrusion. The first process occurs at a low deposition rate and is dependent on the type of growth process. The second process is due to the intrusion of water vapour into the film through the voids, which increases the conductivity of the film depending on the amount of intrusion. We have shown that our device quality compact poly-Si: H (Poly2) resists oxygen incorporation even when deposited in an oxygen-rich atmosphere.

AbstractReducing the optical losses and increasing the reflection while maintaining the function of doped layers at the back contact in solar cells are important issues for many photovoltaic applications. One approach is to use doped microcrystalline silicon oxide (μc‐SiOx:H) with lower optical absorption in the spectral range of interest (300 nm to 1100 nm). To investigate the advantages, we applied the μc‐SiOx:H n‐layers to a‐Si:H single junction solar cells. We report on the comparison between amorphous silicon (a‐Si:H) single junction solar cells with either μc‐SiOx:H n‐layers or non‐alloyed silicon n‐layers. The origin of the improved performance of a‐Si:H single junction solar cells with the μc‐SiOx:H n‐layer is identified by distinguishing the contributions because of the increased transparency and the reduced refractive index of the μc‐SiOx:H material. The solar cell parameters of a‐Si:H solar cells with both types of n‐layers were compared in the initial state and after 1000 h of light soaking in a series of solar cells with various absorber layer thicknesses. The measurement procedure for the determination of the solar cell performance is described in detail, and the measurement accuracy is evaluated and discussed. For an a‐Si:H single junction solar cell with a μc‐SiOx:H n‐layer, a stabilized efficiency of 10.3% after 1000 h light soaking is demonstrated. Copyright © 2015 John Wiley & Sons, Ltd.