PEDOT:PSS as back contact for CdTe solar cells and the effect of PEDOT:PSS conductivity on device performance

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was studied as the back contact of Cadmium telluride (CdTe) solar cells and was compared with conventional Cu-based back contact. A series of PEDOT:PSS aqueous solutions with different conductivities were spin coated onto the glass/SnO2:F/SnO2/CdS/CdTe structures as back contact, and the PEDOT:PSS conductivity dependence of device performance was studied. It was found that PEDOT:PSS back contact with higher conductivity produces devices with lower series resistance and higher shunt resistance, leading to higher fill factor and higher device efficiencies. As the conductivity of PEDOT:PSS increased from 0.03 to 0.24 S/cm, the efficiency of the solar cell increased from 2.7 to 5.1 %. Methanol cleaning also played an important role in increasing the device performance. The efficiency of our best device with PEDOT:PSS back contact has reached 9.1 %, approaching those with conventional Cu/Au back contact (12.5 %).     

Effect of cadmium sulphide quantum dot processing and post thermal annealing on P3HT/PCBM photovoltaic device

The present study demonstrates the effect on photovoltaic performance of poly(3-hexylthiophene) (P3HT) on doping of cadmium sulphide (CdS) quantum dots (QDs). The P3HT/CdS nanocomposite shows a 10 nm blue shift in the UV-vis absorption relative to the pristine P3HT. The blue shift in the absorption of the P3HT/CdS nanocomposite can be assigned to the quantum confinement effect from the CdS nanoparticles. Significant PL quenching was observed for the nanocomposite films, attributed to additional decaying paths of the excited electrons through the CdS. Solar cell performance of pure P3HT and dispersed with CdS QDs have been studied in the device configuration viz indium tin oxide (ITO)/poly(3,4-ethylendioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS)/P3HT:PCBM/Al and ITO/PEDOT:PSS/ P3HT:CdS:PCBM/Al, respectively. Incorporation of CdS QDs in the P3HT matrix results in the enhancement in the device efficiency (?) of the solar cell from 0.45 to 0.87%. Postproduction thermal annealing at 150 °C for 30 min improves device performance due to enhancement in the device parameters like FF, V_OC and improvement in contact between active layer and Al.     

The use of aluminium doped ZnO as transparent conductive oxide for CdS/CdTe solar cells

CdTe/CdS and CdTe/ZnO thin film solar cells were grown with a high vacuum evaporation based low temperature process (≤ 420 °C). Aluminium doped zinc oxide (AZO) was used as transparent conducting oxide (TCO) material. AZO exhibited excellent stability during the solar cell processing, and no significant change in electrical conductivity or transparency was observed. The current density loss due to absorption in the 1 μm thick AZO layer with 5 Ω per square sheet resistance was found to be 1.2 mA/cm². We investigated the influence of an intrinsic ZnO layer (i:ZnO) in combination with various CdS thicknesses. The i:ZnO layer was found to significantly increase the open circuit voltage of the solar cells with very thin CdS layer. Increasing thickness of the i:ZnO layer leads to UV absorption losses, narrowing of the depletion layer width and hence reduced collection efficiency in the long wavelength (685-830 nm) part. With AZO/i:ZnO bi-layer TCO we could achieve cell efficiencies of 15.6% on glass and 12.4% on the flexible polyimide film.     

Characterization of CBD-CdS layers with different S/Cd ratios in the chemical bath and their relation with the efficiency of CdS/CdTe solar cells

In previous papers we have reported the improvement of the efficiency of CdS/CdTe solar cells by varying the thiourea/CdCl₂ ratio (R_tc) in the chemical bath solution used for the deposition of the CdS layers. In this work, a more complete study concerning the physical properties of Chemical Bath Deposited (CBD) CdS layers studied by photoluminescence, X-ray diffraction and optical spectroscopy are correlated to the I-V characteristics under AM 1.5 sunlight and the spectral response of CdS/CdTe solar cells. It is confirmed that the optimum R_tc for the CBD CdS films is R_tc = 5, since in this case the best solar cells were obtained and these films show the better optical and structural characteristics.     

A detailed study of the series resistance effect on CdS/CdTe solar cells with Cu/Mo back contact

CdS/CdTe thin film solar cells with an area of 1 cm² were obtained and studied in detail. A ZnO buffer layer was deposited by reactive RF-sputtering on commercial ITO substrates. The CdS layer was grown on ZnO also by using RF-sputtering and CdTe thin film was deposited by conventional CSS technique. The chlorination of the solar cells is performed into Freon atmosphere at 400 °C. The CdTe thin film surface was chemically etched by using Br-Methanol solution. The back contact was deposited using RF-sputtering from a pure Cu and Mo targets. The procedure developed in this work led us to make systematically solar cells with good efficiency. However, the series resistance has a high value for an area of 1 cm² (22 Ω cm²). In order to make more detailed study, the solar cell with an area of 1 cm² was divided in a 3 × 3 matrix. A good homogeneity in cell properties is observed and the efficiency increases to more than 11%, fundamentally through decreasing series resistance.     

Polymeric anodes from poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) for 3.5% efficient organic solar cells

We present highly efficient indium tin oxide free polymer solar cells based on poly-(3-hexylthiophene-2,5-diyl) and C₆₁-bis-butric-acid-methyl-ester (P3HT:bisPCBM) comprising a polymeric anode from highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) formulations. The film conductivity was optimized by various additives. We found conductivities of almost 600 S/cm upon the addition of dimethylsulfoxide. The wetting properties of different PEDOT:PSS formulations were investigated by contact angle measurements. The optimized high conductivity in combination with the good film forming properties allow for the fabrication of highly efficient organic solar cells with an external power conversion efficiency of 3.5% with PEDOT:PSS as polymeric anode.     

Dependence of efficiency of thin-film CdS/CdTe solar cell on parameters of absorber layer and barrier structure

Dependences of the open-circuit voltage, short-circuit current, fill factor, and efficiency of a CdS/CdTe solar cell on the resistivity and thickness of the p-CdTe absorber layer, the noncompensated acceptor concentration N_a-N_d, and carrier lifetime τ in CdTe, are investigated, and optimization of these parameters in order to improve the solar cell efficiency is performed. It has been shown that the observed low efficiency of CdS/CdTe solar cells is caused by the too short electron lifetime in the range of 10^− 10-10^− 9 s and too thin (3-5 µm) CdTe layer currently used for fabrication of CdTe/CdS solar cells. To achieve an efficiency of 28-30%, the resistivity and thickness of the CdTe absorber layer, the noncompensated acceptor concentration, and carrier lifetime should be ∼ 0.1 Ω·cm, ≥ 20-30 µm, ≥ 10¹⁶ cm^− 3, and ≥ 10^− 6 s, respectively.     

Metal-organic chemical vapor deposition of ultra-thin photovoltaic devices using a pyrite based p-i-n structure

Ultra-thin photovoltaic (PV) devices were produced by atmospheric pressure metal organic chemical vapour deposition (AP-MOCVD) incorporating a highly absorbing intermediate sulphurised FeS_x layer into a CdS/CdTe structure. X-ray diffraction (XRD) confirmed a transitional phase change to pyrite FeS₂ after post growth sulphur (S) annealing of the FeS_x layer between 400 °C and 500 °C. Devices using a superstrate configuration incorporating a sulphurised or non-sulphurised FeS_x layer were compared to p-n devices with only a CdS/CdTe structure. Devices with sulphurised FeS_x layers performed least efficiently, even though pyrite fractions were present. Rutherford back scattering (RBS) confirmed deterioration of the CdS/FeS_x interface due to S inter-diffusion during the annealing process.     

Preparation and characterization of regioregular poly(3-octylthiophene-2,5-diyl)/copper indium disenillide/titania heterojunction polymer solar cells

By optimizing the P3OT/CISe ratio, TiO₂ content in the P3OT/CISe active layer, annealing temperature and time, this study investigated hybrid Al/Ca/P3OT:CISe:TiO₂/PEDOT:PSS/ITO thin film solar cells with improved efficiency. Due to an increase in charge-carrier transport and a decrease of electron-hole recombination, it is possible to increase the efficiency of hybrid solar cells by adding TiO₂ nanoparticles to the P3OT:CISe active film. Also, performance enhancement of the solar cells can occur with an increase of CISe content in P3OT as well as the addition of a PEDOT:PSS layer to the cell structure. The optimum TiO₂ content in P3OT:CISe layer is 15 wt.%. The optimum annealing temperature and time are 125 °C and 30 min, respectively. The formation of large CISe and TiO₂ aggregates that reduce charge mobility may cause the decrease of efficiency. The rough surface may effectively reduce the charge-transport distance and provide nanoscale phase separation that further enhances internal light scattering and light absorption. The best results for the open circuit voltages (V_oc), short-circuit current density (J_sc), fill factor (FF), and efficiency (η_e) of Al/Ca/POCT15/PEDOT:PSS/ITO hybrid solar cells obtained at optimized conditions were V_oc = 0.49, J_sc = 3.20, FF = 42.96, and η_e = 0.674, respectively.     

Atmospheric-pressure argon plasma etching of spin-coated 3,4-polyethylenedioxythiophene:polystyrenesulfonic acid (PEDOT:PSS) films for cupper phtalocyanine (CuPc)/C60 heterojunction thin-film solar cells

Depth profiles of the optical constants, carrier mobility, and carrier density of spin-coated 3,4-polyethylenedioxythiophene:polystyrenesulfonic acid (PEDOT:PSS) films were investigated by real-time characterization by the spectroscopic ellipsometry (SE) during argon plasma etching at atmospheric pressure. Spectral analysis revealed that homogeneous etching occurred within 10-15 nm of the top surface, followed by the appearance of a conductive PEDOT phase and surface roughning, which originated from the depth profile of the PEDOT-to-PSS molar concentration ratio. The use of the plasma-etched PEDOT:PSS layer improved relatively the performance of the copper phtalocyanine (CuPc)/C₆₀ organic thin-films solar cells as a hole-transport layer with higher optical transmittance by adjusting the plasma etching condition.     

An effective method of Cu incorporation in CdTe solar cells for improved stability

Thin film CdTe solar cells of the superstrate configuration have been fabricated in order to study the effect of Cu on device stability. The study focused on two distinct sets of solar cells: in one set of devices Cu was introduced during the formation of the back contact, by sputtering a small thickness of Cu onto the CdTe surface prior to the application of a graphite electrode; for the second set of devices Cu was introduced in CdS by briefly immersing the CdS films in a CuCl solution prior to the deposition of CdTe with the back contact electrode being sputtered Mo. The solar cells were light soaked under approximately AM1.5 conditions for nearly 700 h during 4 h ON/4 h OFF cycles. Device degradation correlated well with the amount of Cu for the devices with Cu in the back contact. Cells with larger amounts of Cu exhibited larger degradation, suggesting that the amount of Cu utilized during the back contact formation must be minimized. On the other hand, a number of devices fabricated without any Cu in the back contact, but with Cu in the CdS, exhibited nearly no degradation during the light soaking process suggesting that in addition to the amount of Cu used for the fabrication of CdTe cells, the method of incorporating this element is also critical in achieving long term device stability.     

Overcoming Moisture-Induced Degradation in Organic Solar Cells

Unencapsulated organic solar cells are prone to severe performance losses in the presence of moisture. Accelerated damp heat (85 °C/85% RH) studies are presented and it is shown that the hygroscopic hole-transporting PEDOT:PSS layer is the origin of device failure in the case of prototypical inverted solar cells. Complementary measurements unveil that under these conditions a decreased PEDOT:PSS work function along with areas of reduced electrical contact between active layer and hole-transport layer are the main factors for device degradation rather than a chemical reaction of water with the active layer. Replacements for PEDOT:PSS are explored and it is found that tungsten oxide (WO₃) or phosphomolybdic acid (PMA)—materials that can be processed from benign solvents at room temperature—yields comparable performance as PEDOT:PSS and enhances the resilience of solar cells under damp heat. The stability trend follows the order PEDOT:PSS << WO₃ < PMA, with PEDOT:PSS-based devices failing after few minutes, while PMA-based devices remain nearly pristine over several hours. PMA is thus proposed as a robust, solution-processable hole extraction layer that can act as a one to one replacement of PEDOT:PSS to achieve organic solar cells with significantly improved longevity.     

Novel hybrid solar cells based on α-copper phthalocyanine–cadmium sulfide planar heterojunction

Cadmium sulfide (CdS) has been employed as an alternative acceptor for planar heterojunction solar cell based on copper phthalocyanine (CuPc). Spin-coated poly-3,4-ethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS) on indium tin oxide (ITO)-coated glass substrates was used for the vacuum deposition of CuPc and CdS planar heterojunction. In the present study, we have fabricated two different architectures of CuPc/CdS devices: (1) ITO/PEDOT:PSS/CuPc/CdS/Al and (2) ITO/PEDOT:PSS/CuPc/CdS/LiF/Al. Our results indicate that the CdS could effectively facilitate charge transport in the nanostructured network, and be a good acceptor. The fabricated bare CuPc/CdS device shows 0.13 % conversion efficiency while incorporation of LiF layer between CuPc/CdS and Al contact facilitates low-recombination rate results ~43 % enhancement in efficiency. The ITO/PEDOT:PSS/CuPc/CdS/LiF/Al device shows 0.30 % power conversion efficiency.     

The role of transparent conducting oxides in metal organic chemical vapour deposition of CdTe/CdS Photovoltaic solar cells

A systematic study is made between the relationship of Cd_0.9Zn_0.1S/CdTe photovoltaic (PV) device properties for three different commercial transparent conducting oxide (TCO) materials and some experimental CdO to determine the role of the TCO in device performance. The resistance contribution from the TCO was measured after depositing the gold contact architectures directly onto the TCOs. These were compared with the Cd_0.9Zn_0.1S/CdTe device properties using the same contact arrangements. Series resistance for the commercial TCOs correlated with their sheet resistance and gave good agreement with the PV device series resistance for the indium tin oxide (ITO) and fluorine doped tin oxide (FTO) 15 Ω/Sq. superstrates. The devices on the thicker FTO 7 Ω/sq superstrates were dominated by a low shunt resistance, which was attributed to the rough surface morphology causing micro-shorts. The device layers on the CdO substrate delaminated but devices were successfully made for ultra-thin CdTe (0.8 μm thick) and compared favourably with the comparable device on ITO. From the measurements on these TCOs it was possible to deduce the back contact resistance and gave an average value of 2 Ω.cm². The correlation of fill factor with series resistance has been compared with the predictions of a 1-D device model and shows excellent agreement. For high efficiency devices the combined series resistance from the TCO and back contact need to be less than 1 Ω.cm².     

Preparation and characterization of poly(3,4-ethylenedioxythiophene)–poly(styrene sulfonate) composite thin films highly loaded with platinum nanoparticles

In this work, we propose a simple and efficient, low-temperature (∼120 °C) process to prepare transparent thin films of poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS) loaded with high concentration (up to 22.5 wt%) of platinum (Pt) nanoparticles. Firstly, an improved polyol method was modified to synthesize nano-sized (∼5 nm) and mono-dispersed Pt particles. These nanoparticles were incorporated into the matrix of PEDOT:PSS thin films via a spin coating/drying procedure. The electrochemical activities of the PEDOT:PSS thin film modified electrodes with respect to the I⁻/I₃⁻ redox reactions were investigated. It was found that the modified electrode of PEDOT:PSS thin film containing 22.5 wt% Pt exhibited the electrochemical activity comparable to the conventional Pt thin film electrode, suggesting that this electrode has good potential to serve as a counter electrode in dye-sensitized solar cells.     

CdTe thin film solar cells with reduced CdS film thickness

A study was performed to reduce the CdS film thickness in CdTe thin film solar cells to minimize losses in quantum efficiency. Using close space sublimation deposition for CdS and CdTe a maximum efficiency of ~ 9.5% was obtained with the standard CdS film thickness of ~ 160 nm. Reduction of the film CdS thickness to less than 100 nm leads to poor cell performance with ~ 5% efficiency, mainly due to a lower open circuit voltage. An alternative approach has been tested to reduce the CdS film thickness (~ 80 nm) by depositing a CdS double layer. The first CdS layer was deposited at high substrate temperature in the range of 520-540 °C and the second CdS layer was deposited at low substrate temperature of ~ 250 °C. The cell prepared using a CdS double layer show better performance with cell efficiency over 10%. Quantum efficiency measurement confirmed that the improvement in the device performance is due to the reduction in CdS film thickness. The effect of double layer structure on cell performance is also observed with chemical bath deposited CdS using fluorine doped SnO₂ as substrate.     

Study of CSS- and HVE-CdTe by different recrystallization processes

CdTe polycrystalline thin film solar cells are highly scalable devices, showing stable performance and high efficiencies. A recrystallization treatment prior to the back contact deposition is essential for obtaining high efficiencies. Heat treatment in chlorine atmosphere (CdCl₂) promotes grain growth, intermixing between CdS and CdTe layers and passivation of the grain boundaries. An alternative method has been previously introduced on close space sublimated (CSS) CdTe devices, consisting in treating CdTe layers in the presence of a gas belonging to the Freon^© family at 400 °C. In this work, the application of this novel “activation process” on physical vapor deposited stacks is described.A comparison between physical/chemical features of the CdCl₂ and Freon treated layers is also presented.     

Back contact formation in thin cadmium telluride solar cells

We present a model describing the undesired roll-over which is a well-known phenomenon in the current-voltage characteristics of CdTe solar cells. Therein, the roll-over is ascribed to a Schottky barrier at the back contact which is effective as a reverse diode. The formation of this barrier is investigated depending on the CdTe absorber thickness as well as on the employed back contact metal. Computer simulations of the energy band diagram reveal that the back contact barrier can be reduced and even eliminated for sufficiently thin absorbers. The reason is the spatial overlap between the space-charge regions of the p-n heterojunction with the one of the back contact. This behaviour correlates with experimental current-voltage data of solar cells with a simple gold back contact. In the latter, the roll-over is considerable for absorbers with 3 to 5 μm thickness, diminishes when the absorber thickness is reduced and finally vanishes when the absorber thickness is approximately 1 μm. The investigations show that thickness reduction can be employed in order to suppress the roll-over phenomenon in CdTe solar cells.     

The structural and electrical properties of Zn-Sn-O buffer layers and their effect on CdTe solar cell performance

Thin films of zinc-tin-oxide (ZTO) have been deposited on SnO₂:F coated glass substrates by co-sputtering of SnO₂ and ZnO. The deposition conditions for ZTO were controlled in order to vary film stoichiometry. The electro-optical and structural properties of ZTO have been studied as a function of their stoichiometric ratio and post-deposition annealing conditions. The same films were subsequently utilized as part of a bi-layer transparent front contact for the fabrication of CdTe solar cells: glass/SnO₂:F/ZTO. The performance of these devices suggested that the ZTO deposition and cell processing conditions can be optimized for enhanced device performance in particular for devices with thin CdS. Specifically, high blue spectral response (> 70% at 450 nm), accompanied by high open-circuit voltages (830 mV), and fill factors (70⁺%) have been demonstrated. Best solar cell performance was obtained for multi-phase ZTO films deposited at substrate temperatures of 400°C and a Zn/Sn ratio of 2.0, and which contained the binary phase of ZnO₂.     

CdTe-CdS solar cells — Production in a new baseline and investigation of material properties

In this paper, we describe our new baseline for CSS-CdTe-CdS solar cells on 10 × 10 cm² substrates. The deposition of the p-n junction and all the following steps were performed at the Institut für Festkörperphysik (IFK) in Jena. Using the new baseline, we are already able to produce solar cells with similar properties as commercial ones. In the batch type process, all manufacturing steps can be investigated separately. We employ Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD) and external quantum efficiency (EQE) measurements to characterise the structure of the bulk materials and interfaces. It is demonstrated that by RBS the front contact becomes accessible for thinned CdTe films. At the back contact, RBS spectra show a tellurium accumulation which is due to etching. This tellurium rich layer is confirmed by XRD with Rietveld refinement. The intermixing at the CdS-CdTe interface caused by the activation step is quantified by a bandgap determination based on EQE measurements. From the bandgap energy of the CdTe_1 − xS_x compound, we calculated the sulphur fraction x at the interface. XRD measurements imply that the activation step induces a (111) texture in CdTe. With regard to an improved manufacturing process, our cells are compared to industrial cells produced by Antec Solar Energy.