Search

8th World Conference on Photovoltaic Energy Conversion; 83-87, Aluminium doped zinc oxide (AZO) is an interesting transparent conductive oxide (TCO) for silicon heterojunction (SHJ) solar cells to replace indium tin oxide (ITO) to enable cost reduction and omit the use of the limited global indium reserve. However, AZO layers have a much higher sheet resistance and lower damp heat stability compared to ITO. This work shows that the AZO stability can be improved by using an ITO capping layer, which is also decreasing the sheet resistance of the AZO/ITO layer stack compared to a pure AZO layer. Here, high rate rotatable magnetron sputtering was used to deposit AZO and ITO films. Under these conditions ITO capped AZO layers deposited on a-Si/p can result in lower fill factors. It will be shown that heating before sputtering and introduction of a thin ITO layer between AZO and a-Si/P avoid the fill factor decrease and lead to fill factors above 80%. The properties of the solar cells with the ITO/AZO/ITO layer stack are comparable to those solar cells with the pure ITO reference layer. Similar solar cell properties with the same ITO/AZO/ITO layer stack on the a-Si/n of the front side have been achieved. Furthermore, the TCO materials costs of ITO, AZO/ITO and ITO/AZO/ITO layers stacks for industrial manufacturing are discussed.

Nanocrystalline silicon and silicon oxide nc Si Ox H layers grown by plasma enhanced chemical vapor deposition PECVD have shown low parasitic absorption and excellent contact properties when implemented as n type front surface field FSF contact in rear junction silicon heterojunction SHJ solar cells [1 3]. In this contribution we present results from the successful process transfer from the lab at PVcomB at the Helmholtz Zentrum Berlin HZB , to the industrial pilot line at Meyer Burger Germany GmbH MBG . Conversion efficiencies gt; 22.5 were demonstrated on SHJ cell 4 cm2 [2, 3]. The excellent cell performance in the lab and the potential to reduce parasitic absorption in the front stack by using nc SiOx H motivated the process transfer from HZB to MBG. Initial cross processing experiments on 244 cm2 wafers showed the benefit of using nc Si H as FSF layer. We here also emphasize the role of the Si texture on a fast nc Si H nucleation. After cross processing experiments a successful transfer of the nc Si H process and fine tuning resulted in a median cell efficiency of 23.4 . This is in the same range as the MGB reference on 244 cm2 cells, noteworthy, at the same throughput. Currently work is ongoing to further improve the optical performance of the cells by adding oxygen CO2

35th European Photovoltaic Solar Energy Conference and Exhibition; 1346-1351, Encapsulation materials are of critical importance for long-term reliability and safety of PV modules. Only months in the field may lead to drastic power output degradation for example due to PID and also in the longer run adhesion and discoloration issues (Hot Spot) can reduce power output or even may lead to critical electrical safety issues. These findings are not sufficiently covered by IEC 61215 and other standard testing. Additional and different types of tests are required for an in-depth understanding of the encapsulation material’s impact on PV module performance. PI Berlin has developed a test sequence that covers many field relevant aspects for PV encapsulations. For example: i) Potential Induced Degradation (PID), ii) Hot Spot Durability and iii) High Temperature Adhesion (delamination). Additionally, material properties which impact the processability in module manufacturing are tested. Also, very new aspects like the effect of encapsulation aging on PID are evaluated.

35th European Photovoltaic Solar Energy Conference and Exhibition; 456-459, Sputtered indium tin oxide (ITO) is a widespread used material for application as transparent conducting oxide as the front contact of rear-junction silicon heterojunction solar cells. Standard ITO films suffer from too high parasitic absorption in both the short- and the long-wavelength range. The aim of this contribution is to investigate lower absorbing TCOs deposited with industrial DC sputtering from rotatable targets. High Voc values >735 mV demonstrate that in spite of the high-power industrial DC sputter process no significant sputter damage is observed, independent of the target material. Cell efficiencies up to 23.3% are obtained with titanium-doped (ITiO) and tin-doped indium oxide (ITO 99/1). The highest short circuit current values of 39.6 mA/cm2 are reached with an ultimately low-carrier and high-mobility indium-oxide-based material. The cell efficiency in this case, however, is over-compensated by a too low FF, mostly due to a higher contact resistivity and TCO sheet resistances. We ascribe this effect to the very low carrier concentration.

35th European Photovoltaic Solar Energy Conference and Exhibition; 452-455, When designing silicon heterojunction (SHJ) solar cells with a rear-emitter (RE) configuration the requirements of the conductivity of the transparent conducting oxide (TCO) at the front side are relaxed due to the contribution of lateral current flow within the silicon wafer. In this study, we analyze two approaches that can be implemented to benefit from this. Firstly, the absorption of the TCO can be reduced by designing a thinner TCO layer than the single-layer anti-reflective optimum. In this case, a second anti-reflecting coating has to be deposited on top of the TCO to minimize reflection losses. Secondly, less conductive and more cost-effective materials can be used without strongly compromising the device’s series resistance Rs and FF and, hence, the cell performance. To quantify the optoelectronic potential of these approaches in detail, we performed a comparative study of three different TCOs: indium tin oxide (ITO), aluminum doped zinc oxide (ZnO:Al), and hydrogenated indium oxide (IO:H) on rear-emitter SHJ solar cells. From simulations and experimental results, we conclude that, as expected, solar cells with IO:H as the front TCO reach the higheest efficiency. However, low-cost and higher resistive materials such as ZnO:Al can be implemented, without having a mayor efficiency penalty making them, moreover, competitive to the mainstream used ITO.

35th European Photovoltaic Solar Energy Conference and Exhibition; 422-425, Nanocrystalline silicon and silicon oxide (nc-Si(Ox):H) layers grown by plasma-enhanced chemical vapor deposition (PECVD) have shown low parasitic absorption and excellent contact properties when implemented as n-type front surface field (FSF) contact in rear-junction silicon heterojunction (SHJ) solar cells [1-3]. In this contribution we present results from the successful process transfer from the lab at PVcomB at the Helmholtz-Zentrum Berlin (HZB), to the industrial pilot line at Meyer Burger Germany GmbH (MBG). Conversion efficiencies > 22.5 % were demonstrated on SHJ cell (4 cm2) [2, 3]. The excellent cell performance in the lab and the potential to reduce parasitic absorption in the front stack by using nc-SiOx:H motivated the process transfer from HZB to MBG. Initial cross processing experiments on 244 cm² wafers showed the benefit of using nc-Si:H as FSF layer. We here also emphasize the role of the Si texture on a fast nc-Si:H nucleation. After cross processing experiments a successful transfer of the nc-Si:H process and fine tuning resulted in a median cell efficiency of 23.4 %. This is in the same range as the MGB reference on 244 cm² cells, noteworthy, at the same throughput. Currently work is ongoing to further improve the optical performance of the cells by adding oxygen (CO2).

33rd European Photovoltaic Solar Energy Conference and Exhibition; 1057-1060, Tandem solar cells with a perovskite top cell and a silicon (c-Si) bottom cell are the most promising route to overcome the theoretical efficiency limit of silicon single-junction solar cells (29.4%), when the low-bandgap bottom cell should be based on silicon. To fully exploit the potential of two terminal perovskite-Si tandem cells, the device must be highly optimized from an optical point of view. In this contribution we present optical simulations for two designs of perovskite-Si tandem solar cells: regular structures with the hole-selective contacts of both subcells on the front side, and inverse structures with the hole-selective contacts on the rear sides of the subcells. As bottom cell we took a state-of-the-art silicon heterojunction (SHJ) cell with amorphous silicon layers on both sides. We generated absorption profiles with a coherent/incoherent algorithm based on the net-radiation method and used the so obtained absorption spectra of the perovskite and c-Si layers to extract the achievable photocurrent densities for the top and bottom cells.

33rd European Photovoltaic Solar Energy Conference and Exhibition; 715-719, We report on a systematical investigation of the influence of layer thickness and oxygen content of different silicon-based n-doped layers deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) on silicon heterojunction (SHJ) solar cell performance. N-type amorphous (a-), nanocrystalline (nc-) hydrogenated silicon (Si:H) and silicon oxide (SiOx:H) films were used as front surface field (FSF) layers on rear emitter (RE) SHJ solar cells. The benefit, both in electrical and optical properties, of using nc-silicon layers, is demonstrated. The best cell in the oxygen variation experiment was obtained with an (n)nc-SiOx:H FSF having a refractive index n of 2.72 at 633 nm, which leads to an increase in short circuit current density, jsc, of more than 1 mA/cm2 as compared to a reference cell with an a-Si:H FSF. This higher jsc obtained in our best nc-SiOx:H prototypes leads to a conversion efficiency, , of 21.4%. It is also remarkable our best cell result, certified by ISFH CalTeC, with an improved nc-Si:H FSF that enabled an improved jsc of 38.6 mA/cm2 and an of 22.5%.

Ultra thin silicon oxide a SiOx H films have been grown by means of plasma enhanced chemical vapor deposition PECVD to replace the standard hydrogenated amorphous silicon a Si H passivation layer for silicon heterojunction solar cells to reduce parasitic absorption. Additionally, silicon oxide surfaces are well known as superior substrates for the nucleation enhancement for nanocrystalline silicon doped films. Symmetrical passivation samples were fabricated with variable a SiOx H layers with a thickness of 10 1.5 nm and characterized after several annealing steps 25 650 C . The best value reached so far on lt;100 gt; oriented Si wafers is implied open circuit voltage of 686 mV and minority carrier lifetime of 1.6 ms after annealing at 300 C. Such values were found to be reproducible even for ultra thin a SiOx H layers 1.5 nm

32nd European Photovoltaic Solar Energy Conference and Exhibition; 753-755, This work gives an overview about the current status and the ongoing development of the silicon heterojunction (SHJ) solar cell process at HZB. Our research work focuses on the development of highly transparent nanocrystalline front surface field (FSF) layers and innovative transparent conductive oxide (TCO) layers as front electrodes for rear emitter SHJ solar cells. By introducing water vapor into the front ITO sputtering process we show an increase of 0.4 % absolute in conversion efficiency compared with our standard ITO process without water vapor. Furthermore first results for a rear emitter cell structure are presented with a conversion efficiency exceeding 21% for 4 cm² SHJ solar cells on n-type CZ-Si. Introducing further process optimizations, we expect to achieve conversion efficiencies above 22% for our baseline process sequence in the near future.

31st European Photovoltaic Solar Energy Conference and Exhibition; 713-715, We present a novel process scheme for emitter patterning for IBC-SHJ solar cells, employing laser based single step patterning of a dielectric hard mask, and self-alignment for further depositions. With the presented process sequence, we aim at developing a simple, lithography free and contactless emitter patterning method for industrial applications. Laser writing of the hard mask is performed using a picosecond laser operating at 355 nm to reduce the laser damage at the surface of the samples, which then allows for short damage etch durations. We demonstrate the feasibility of this process scheme for IBC-SHJ solar cells. Proof-of-concept cells on planar substrates show a short circuit current density of 33.2 mA/cm2, but still suffer from both low open circuit voltage and low fill factor, the latter being mainly due to a high series resistance. At the current stage of development, analysis of the etched contact groove reveals a rough surface in the overlap zone of the laser pulses, which very likely causes recombination and reduces both open-circuit voltage and fill factor.

29th European Photovoltaic Solar Energy Conference and Exhibition; 1614-1619, On the way to develop a very thin and highly efficient triple-junction thin-film solar cell in a-Si:H/ μc-Si:H/μc-SiGe:H configuration cell samples were prepared and characterized using an industrial relevant 13.56 MHz 0.5 nm/s process on 30 30 cm² PECVD tool. In order to improve the μc-SiGe:H absorbers single junction cells were processed carefully monitoring germanium content and crystallinity while varying pressure and germane flow. Homogeneity test on 30 cm x 30 cm deposition area were performed resulting in a good homogeneity for the thickness. Triple junction cells were fabricated by depositing μc-Si:H and μc-SiGe:H absorbers on state of the art tandem structures managed to reach 12.2 % efficiency. Our intention is to develop an industrial relevant process with attractive fabrication times by benefiting from the enhanced absorption of μc-SiGe:H compared to μc-Si:H.

28th European Photovoltaic Solar Energy Conference and Exhibition; 2691-2693, We present results on 1D-grating textures in ZnO:Al layers fabricated using direct pulsed laser interference texturing. A typical laser written grating texture showed a haze value of 9% at 700 nm. The total transmission of the film was not lowered compared to the film before texturing, while the sheet resistance increased moderately by 15%. A-Si:H/μc-Si:H solar cells with laser textured ZnO:Al front contact layers so far reach an efficiency of 10 % and current densities of 11.1 mA/cm², and 11.2 mA/cm² for top and bottom cell, respectively. This is an increase of 16% for the bottom cell as compared to flat ZnO:Al. This texturing method allows to write textures also in ZnO:Al films that cannot be texture etched in HCl. It can be improved further by using 2D periodic patterns and by optimizing the groove period and depth, and may be applicable also to other solar cell technologies.

28th European Photovoltaic Solar Energy Conference and Exhibition; 2537-2541, In this study, dark and illuminated lock-in thermography (DLIT/ILIT) is applied for the investigation of state of the art a-Si:H/μc-Si:H tandem modules. The recent progress in LIT technology enables the collection of spatially highly resolved thermographs that permit a precise location of individual shunts. The possibility of illumination at two different wavelengths was applied to determine the origin of the shunts. Additionally, quantitative information can be gained by the investigation of local heating at different working points of the module. For illuminated lock-in thermography a significant contribution of thermalization and Peltier cooling due to the lateral current flow was observed, which has to be considered in regard to quantitative analyses.

27th European Photovoltaic Solar Energy Conference and Exhibition; 2730-2734, In this paper, electroluminescence measurements in forward (EL) and reverse bias (ELR in microelectronics or ReBEL in photovoltaics) were applied on micromorph tandem solar cells. In both techniques the electrical energy was transferred into the light and then detected with a detector. But their device interaction mechanisms are completely different, EL yielding information on band to band recombination and ELR mainly information on the intraband relaxation. The investigated solar cell consists of a two p-i-n cell stack based on a-Si:H and μ:Si:H, respectively. A spectral analysis of the EL signal was performed to determine the layer dominating the emission. Simultaneously, the dark current-voltage characteristics of the device were monitored during the electroluminescence measurements. The combination of these three techniques allows a detailed investigation of a metastable shunt feature observed in this type of thin-film solar cells. Further, a monitoring of the shunt busting effect that typically improves the shunt resistance of micromorph solar cells was achieved. The ELR setup benefiting from high magnification objectives and a sensitive detector permits to localize the affected areas with very high resolution.

27th European Photovoltaic Solar Energy Conference and Exhibition; 44-49, Current solar cells are fundamentally limited by their inability to harvest photons with energies less than the absorber optical threshold. Thinfilm solar cells, usually having band gaps above the optimum singlethreshold value of 1.34 eV given by the ShockleyQueisser limit, and a much smaller volume to absorb the light, are particularly prone to this loss mechanism and thus suffer from imperfect usage of the solar spectrum. An elegant way towards overcoming this limitation and using a larger fraction of the incident light is the reshaping of the solar spectrum by upconversion (UC) of photons. In the present work we apply photochemical upconversion, as realized by triplettriplet annihilation in organic molecules, to amorphous silicon thin film solar cells, incoherently transforming light from the 600750 nm to the 550600 nm wavelength range. Employing a moderate concentration of 19 suns, we demonstrate a relative gain of up to 3% in quantum efficiency around 700 nm and a relative overall efficiency increase of 0.2%. We further pinpoint a pathway which will allow significantly increasing the gain by UC and which can adapted to various single threshold devices, including organic and dyesensitized solar cells.

26th European Photovoltaic Solar Energy Conference and Exhibition; 2511-2514, The Competence Centre Thin-Film- and Nanotechnology for Photovoltaics Berlin (PVcomB) has been established within the Helmholtz-Zentrum Berlin (HZB) in cooperation with TU Berlin. Two R&D lines for thin-film Si and CIGS based PV Modules on 30×30 cm² are currently built up. The goal is to facilitate technology transfer from R&D to industrial production. In this paper we report on the status of our thin film silicon baseline with the focus on in situ process monitoring by means of optical emission spectroscopy. The purpose is in-line monitoring of relevant process and plasma parameters during production and, finally, a better understanding of the plasma chemistry and thin film growth processes.

26th European Photovoltaic Solar Energy Conference and Exhibition; 101-104, We consider a scheme to increase the conversion efficiency of a single bandgap solar cell by applying photochemical upconversion (UC), i.e. joining two low energy photons together by incoherent means. Photochemical UC makes use of two molecular species; one for absorption of the low energy photons and one for UC via triplettriplet annihilation as well as emission of the higher energy photons. We describe a model wherein we place a (experimentally fully characterized) UC unit behind a thin a-Si bifacial solar cell. We calculate the relative increase of the photocurrent of the a-Si cell depending on the solar concentrations utilized.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 2896-2898, Sunfilm is the first producer of thin film silicon tandem junction modules on large substrate size (2,60x2,20 m2) using Applied Materials AKT PECVD systems. The company is scaling up its production facilities in Grossroehrdorf to a nominal output capacity of 2x60 MW/year. In April 2009, the final acceptance test (FAT) was achieved and production 24/7 has started in line 1. IEC product certification was achieved in June 2009. In January 2010 the FAT was achieved in line 2. Currently, Sunfilm is focusing on product development, line stabilization and cost reduction. The company has recently reached very high module efficiencies and offers a wide range of products. The line flexibility allows production of additional size formats between 1.4 and 5.7 m2. All Sunfilm products demonstrate better electrical yield kWh per installed MWp than conventional crystalline Si modules. In this paper the current status of module development and production at Sunfilm is presented. Special emphasis is given to the oneyear production and product experience.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 3158-3160, Module performance in large area thin film PV production crucially depends on homogeneous electrical performance of the module’s network of individual cells. We present I-V measurements of a standard a-Si/μc-Si tandem module and of its single cells. The distribution of single cell performance data is compared to module performance data to trace the influences of various inhomogeneities on overall panel performance. We estimate the theoretical potential of relative efficiency improvement resulting from removal of inhomogeneities to be 3% - 4%. Our study facilitates identification and minimization of performance-relevant inhomogeneities.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 3073-3075, Dielectric intermediate reflector layers in thin-film silicon tandem solar cells with randomly textured interfaces are used to optimize current matching and finally the energy conversion efficiency of the whole device. We integrated a 3D photonic crystal as an intermediate reflector layer between an amorphous silicon top cell and a microcrystalline silicon bottom cell. The reflectivity of the 3D photonic thin-film intermediate layer device is designed and optimized for a state-of-the-art tandem solar cell which leads to an enhancement of the external quantum efficiency within the amorphous silicon top cell. Our prototype shows an enormous gain of the external quantum efficiency in the in the weak absorption spectrum of the top cell and nearly no loss for short wavelength.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 3117-3119, In order to increase the deposition rate of hydrogenated amorphous silicon (a-Si:H) and microcrystalline (μc-Si:H) layers high plasma excitation frequencies ( > 13.56 MHz) are commonly used. However, scaling to squaremeter size in industrial production has been challenging, e.g. due to poor layer uniformity. In our inline PECVD production tool for a-Si/μc-Si tandem modules (1.78 m2) at Sunfilm AG we currently use RF (13.56 MHz) PECVD for both a-Si and μc-Si deposition. In this study, a-Si:H layers deposited by VHF PECVD at 40.68 MHz have been investigated in large area tandem modules. In order to maintain layer uniformity across the 1.78 m2 substrate at high deposition rates (0.3 – 0.4 nm/s), deposition conditions such like working pressure, hydrogen dilution ratio and VHF input power have been optimized. However, there were no hardware changes except generator and matching unit to the previous RF system. Compared to modules fabricated by RF-PECVD, 50 - 80% higher deposition rates have been achieved at the same stabilized total-area module efficiency. Additionally, light-induced degradation was smaller for the modules prepared by VHF-PECVD.