Julian Perrenoud

Highly efficient Cu(In,Ga)Se2 solar cells grown on flexible polymer films

Solar cells based on polycrystalline Cu(In,Ga)Se(2) absorber layers have yielded the highest conversion efficiency among all thin-film technologies, and the use of flexible polymer films as substrates offers several advantages in lowering manufacturing costs. However, given that conversion efficiency is crucial for cost-competitiveness, it is necessary to develop devices on flexible substrates that perform as well as those obtained on rigid substrates. Such comparable performance has not previously been achieved, primarily because polymer films require much lower substrate temperatures during absorber deposition, generally resulting in much lower efficiencies. Here we identify a strong composition gradient in the absorber layer as the main reason for inferior performance and show that, by adjusting it appropriately, very high efficiencies can be obtained. This implies that future manufacturing of highly efficient flexible solar cells could lower the cost of solar electricity and thus become a significant branch of the photovoltaic industry.

Doping of polycrystalline CdTe for high-efficiency solar cells on flexible metal foil

Roll-to-roll manufacturing of CdTe solar cells on flexible metal foil substrates is one of the most attractive options for low-cost photovoltaic module production. However, various efforts to grow CdTe solar cells on metal foil have resulted in low efficiencies. This is caused by the fact that the conventional device structure must be inverted, which imposes severe restrictions on device processing and consequently limits the electronic quality of the CdTe layer. Here we introduce an innovative concept for the controlled doping of the CdTe layer in the inverted device structure by means of evaporation of sub-monolayer amounts of Cu and subsequent annealing, which enables breakthrough efficiencies up to 13.6%. For the first time, CdTe solar cells on metal foil exceed the 10% efficiency threshold for industrialization. The controlled doping of CdTe with Cu leads to increased hole density, enhanced carrier lifetime and improved carrier collection in the solar cell. Our results offer new research directions for solving persistent challenges of CdTe photovoltaics.

A comprehensive picture of Cu doping in CdTe solar cells

The importance of Cu for CdTe solar cell absorber doping has been increasingly recognized in recent years. Currently different models are being discussed how to understand the case of CuCd substitutional doping in polycrystalline CdTe solar cells. In this work, an understanding is developed, which is based on a low concentration deep acceptor doped CdTe layer (Na ∼ 5 × 1014 cm−3, Ea ∼ 300 meV above the valence band). Despite their non-shallow nature, CuCd acceptors are fully or at least heavily (>30%) ionized. The low hole concentration in CdTe (∼1 × 1014 cm−3) originates directly from low Cu solubility in CdTe bulk material and is not caused by partial ionization or compensation as proposed by earlier models. The three to four orders of magnitude difference between bulk acceptor concentration and average Cu concentration in polycrystalline CdTe is attributed to grain boundary segregation of Cu. Our model is derived from substrate and superstrate CdTe solar cell measurements, controlled CdTe doping and qu...

Optical, structural, and chemical properties of flash evaporated In2S3 buffer layer for Cu(In,Ga)Se2 solar cells

In2S3 layers were deposited by flash evaporation technique with varying flash rates. The optical constants of layers based on Tauc–Lorentz model dielectric function were extracted from spectroscopic ellipsometry measurements. X-ray photoelectron spectroscopic investigation revealed the presence of oxygen impurity in as-deposited and air-annealed layers with traces of Na inclusion in the layer grown at high flash rate. The enhancement in crystalline arrangement of as-deposited layer after air annealing was confirmed by Raman spectroscopy. Rutherford backscattering measurements revealed the growth of off-stoichiometric layers at all flash rates. An analytical layer growth model has been proposed supporting the results obtained by various layer characterization techniques. The solar cells were prepared with flash evaporated In2S3 buffer layers and their performances were compared with CdS reference solar cell. A significant gain in short-circuit current was obtained after air annealing of the complete device...

Application of ZnO 1−x S x as window layer in cadmium telluride solar cells

CdTe solar cells with ZnS window layer deposited by ultrasonic spray pyrolysis (USP) were grown. The current density of such solar cells reached 24.7 mA/cm2 without anti reflection coating (Voc 594 mV, FF 64.2%, η 9.4%). For CdTe solar cells with CdS and ZnS window layers we quantified the Jsc loss mechanisms in detail. In order to tune the conduction band alignment, and increase the Voc, ZnO1−xSx grown by atomic layer deposition (ALD) was used.

Flexible CdTe solar cells and modules: challenges and prospects

Thin film CdTe/CdS solar modules are currently the most promising photovoltaic technology for cost efficient solar electricity production. Solar modules on glass substrate with more than 10% efficiency are already available on the market. Substituting a flexible substrate material for the rigid and heavy glass will further reduce production costs due to manufacturing advantages and will allow novel applications due to product advantages. In this paper we are presenting the development of CdTe/CdS thin film solar cells with ZnO based TCO on commercially available polyimide. The polyimide is coated with Al doped ZnO as front electrical contact followed by a highly transparent and resistive ZnO buffer layer. Highest efficiency of 12.4% was achieved on 7.5 μm thin polyimide with 823 mV open circuit voltage, 19.6 mA/cm2 short circuit current density and 76.5% fill factor. Using 12.5 μm thick polyimide as substrate reduces the current density of the device by approximately 5%. No cracks in the layers or adhesion problems of the solar cell structure on the polyimide are observed even when rolled to small radius of curvatures (few mm).

Development of multijunction thin film solar cells

Due to the possibility of band gap engineering in Cu(In,Ga)Se2 (CIGS) absorbers and the spectral tunability of dye-sensitized solar cells (DSCs) with suitable choice of the sensitizer, this combination of solar cells could be ideal for the construction of dual junction photovoltaic devices. Factors for performance limitations of mechanically stacked tandem cells have been identified and alternative remedies are developed to overcome efficiency losses. Limiting factors of the efficiency have been determined. In a monolithical configuration, the issue of corrosion of the CIGS cell caused by exposure to the DSC electrolyte has been investigated.

CdTe thin films doped by Cu and Ag - a comparison in substrate configuration solar cells

One of the main problems to be solved in order to raise the efficiency of CdTe solar cells is the low acceptor concentration in polycrystalline CdTe layers which is limiting the open circuit voltage. The commonly used acceptor dopant Cu not only forms rather deep acceptor defects but is also likely to limit the net acceptor concentration due to the formation of compensating donor type defects. In this work, Ag is examined as a possible candidate for improving acceptor concentration in CdTe thin films. Hole density in CdTe layers with varying elemental Ag concentration follows a similar trend as previously reported for Cu doping. In finished solar cells efficiencies up to 11.6 % could be reached using Ag as acceptor dopant. Measurements of hole density, barrier height, space charge region width and depth dependent elemental distribution indicate the similarity of the Ag and Cu doping process.

Progress towards 14% efficient CdTe solar cells in substrate configuration

CdTe solar cells are conventionally grown in superstrate configuration. However, the growth in substrate configuration offers more control of junction properties as recrystallization of CdTe and junction formation with CdS can be decoupled. In this paper the influence of various annealing treatment conditions of the CdS layer on its morphology and phase and on the device properties is presented. The presence of CdCl2 during this annealing treatment is important for the phase change of the CdS layer to hexagonal wurtzite and for high efficiencies. A CdCl2 treatment of the CdS at 360 °C improves the efficiency of the device without the adverse effect of pinhole formation in the CdS. CdTe solar cells in substrate configuration with more than 13% efficiency are achieved as a progress towards 14% efficiency.

Flexible CdTe solar cells with high photovoltaic conversion efficiency

CdTe/CdS solar cells have been grown with a low temperature deposition process on flexible polyimide film and glass substrates in a superstrate configuration. Tin doped indium oxide (ITO) was used as transparent front electrical contact and a highly transparent and a resistive oxide layer was introduced between the ITO and CdS to enable the reduction of the CdS layer thickness resulting in increased current density, processing yield and reproducibility. The device structure consists of ITO, intrinsic ZnO, CdS, CdTe, Cu/Au back contact. The ITO and ZnO layers were deposited by rf sputtering while the other layers were grown by high vacuum thermal evaporation. During the whole process the substrate temperature did not exceed 420°C. The photovoltaic parameters of the flexible CdTe cells are comparable to CdTe/CdS cells on glass substrates except of a difference in the current density, which is due to the lower transmission of the polyimide substrate. Flexible CdTe/CdS cells on polyimide exceeding 10% efficiency were reproducibly produced. Nearly 9% efficiency was achieved with an in-line compatible pentacene gold electrical back contact without bromine methanol etching on glass substrate. No adhesion problem of the solar-cell structure on polyimide occurred even when rolled to small radius of curvatures. Simple processing and high performance of these high efficiency solar cells offer several advantages for manufacturing and applications.