Tutorial

Standard reporting conditions: AM 1.5.

  • integrated power density: 1000 W/m2 or 100 mW/cm2
  • solar spectrum: AM1.5 global (IEC 60904-3: 2008, ASTM G-173-03 global)
  • temperature: 25 °C

Main photovoltaic parameters.

(from review: Abbotto, A.; Manfredi N. “Electron-rich heteroaromatic conjugated polypyridine ruthenium sensitizers for dye-sensitized solar cells“, Dalton Trans., 2011, dx.doi.org/10.1039/C1DT10832H)

The performance of a photovoltaic cell is determined by measuring the overall power conversion efficiency η (sometimes referred to as PCE) from the ratio of maximum output power density (Pout, in W m-2) and the input light irradiance (Pin, in the same units) (eq. 1). Under standard reporting conditions the light intensity Pin is 1000 W m-2, the sun spectrum is AM 1.5 G, and the sample temperature is 25 °C. According to the Shockley-Queisser model the maximum theoretical efficiency for a single junction device under non-concentrated sunlight is ~ 30%. The maximum power point Pout of a cell is given by relationship (2) where Jmp and Vmp represent the current density and voltage at the maximum power point. By defining the fill factor ff as the ratio (values between 0 and 1) of Pout and the product of the maximum attainable voltage (open circuit conditions) Voc (in V) and current density (short circuit conditions) Jsc (in mA cm-2) (relationship 3), the efficiency relationship of eq (1) can be rewritten as eq 4, which is used to determine the cell performance. The Jsc, Voc, and ff values are measured by plotting the current density as the bias voltage is varied while irradiating the PV cell by means of a calibrated solar simulator. A typical diode current/voltage characteristic is shown in the Figure. DSC researchers usually report J and V as positive values, but other J/V curve notations are used as well.

External and Internal Quantum Efficiencies (EQE and IQE)

(from review: Abbotto, A.; Manfredi N. “Electron-rich heteroaromatic conjugated polypyridine ruthenium sensitizers for dye-sensitized solar cells“, Dalton Trans., 2011, dx.doi.org/10.1039/C1DT10832H)

An additional PV parameter which is routinely employed to determine the quality of a PV device is the external quantum efficiency (EQE), usually referred to as the incident photon-to-current conversion efficiency (IPCE) by the DSC community. IPCE(λ) is defined as the number of collected electrons under short circuit conditions per number of incident photons at a given excitation wavelength λ and gives the ability of a cell to generate current as a function of the wavelength of the incident monochromatic light. IPCE is calculated by measuring the short-circuit photocurrent as a function of the monochromatic photon flux. In a DSC cell EQE/IPCE is determined by the sensitizer light harvesting efficiency at λ (LHE), the quantum yield for electron injection from S* to the semiconductor oxide (Φinj), and the charge collection efficiency (ηcoll), the product of the latter two parameters giving the absorbed photon-to-current efficiency (APCE) or internal quantum efficiency. The integral of IPCE with the AM 1.5 G spectrum gives the photocurrent, which should match that measured under the solar simulator. Therefore higher IPCE and broader spectra correspond to higher Jsc.

Dye-Sensitized Solar Cells (DSC)

(from review: Abbotto, A.; Manfredi N. “Electron-rich heteroaromatic conjugated polypyridine ruthenium sensitizers for dye-sensitized solar cells“, Dalton Trans., 2011, dx.doi.org/10.1039/C1DT10832H)

A few basic concepts of DSCs are introduced. A DSC is a multi-component device comprising: a) a dye-sensitizer S; b) a n-type semiconductor metal oxide (typically TiO2); c) a p-type semiconductor or a redox electrolyte (typically the I-/I3- redox couple); d) a transparent working anode and a counter electrode (based on fluorine-doped tin oxide, FTO). In a typical cell assembly one or more mesoporous layers (ca. 10 μm) of transparent 20-nm and/or scattering 350-450-nm titania nanoparticles are deposited on a previously cleaned transparent FTO conducting glass sheet. After sintering, the FTO-TiO2 substrates are immersed into a solution (10-3 – 10-4 M) of the sensitizer for ca. 20-24 h. A Pt counter electrode is prepared by depositing a drop of a 10-2 – 10-3 M alcoholic solution of H2PtCl6 and firing to ca. 400 – 450 °C so to leave a thin layer of Pt catalyst. The dye-coated working electrode and the Pt counter electrode are then assembled in a sandwich type cell. After sealing with a hot-melt ionomer-class resin, the liquid electrolyte is introduced by vacuum backfilling via a pre-drilled hole. Under light irradiation the sensitizer S is promoted to its excited state S* from which an electron injection into the conduction band (CB) of TiO2 takes place, leaving the dye in its oxidized state S+. The collected electrons at the photoanode are then transferred through the external load to the counter electrode where, via Pt catalysis, reduce triiodide to iodide which, in turn, regenerates the sensitizer (S+ → S). If a p-type semiconductor is used in place of the electrolyte, dye-regeneration occurs via hole-transfer from S+ to the HOMO of the hole transporter. A DSC is a very efficient scheme where, formally, one photon is converted to one electron with no chemical change, in principle allowing an unlimited number of cycles. In addition to the main processes, a number of undesired pathways and losses are present including recombination of injected electrons from TiO2 to either S+ or the oxidized form of the electrolyte, incomplete light harvesting, and inefficient electron transfer from S*. The main source of loss-in potential of a DSC is the high overpotential needed to regenerate the dye, which strongly limit the maximum attainable photovoltage.