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Characterisation of hybrid inorganic-organic solar cells by advanced spectroscopic methods

Final Report Summary - CHOIS (Characterisation of hybrid inorganic-organic solar cells by advanced spectroscopic methods)

The project had the following research objectives:
1. Characterise hybrid inorganic-organic solar cells using advanced spectroscopic methods to gain a better understanding of the working mechanism of the solar cells.
2. Compare hybrid solar cells consisting of different materials and prepared by different methods and identify loss processes in the solar cells.
3. Use the gained knowledge to propose new materials combinations and to obtain an optimum match of solar cell components. More efficient hybrid solar cells will be constructed.

In the first phase of the project, hybrid solar cells using blends of cadmium sulfide and the polymer, poly-3-hexylthiophene (P3HT) as active layers were characterised using femtosecond transient absorption spectroscopy. Cadmium sulphide and P3HT show complementary absorption spectra (Figure 1a) and either material can be excited to generate charges in the solar cell. If the P3HT is excited, excitons are formed in the material, which can diffuse to an interface with CdS and be separated into a hole on the P3HT and an electron on the CdS. If the CdS sulfide is excited, holes in the CdS can diffuse to an interface with P3HT and be transferred to the CdS. These processes are shown schematically in Figure 1b. Using transient absorption spectroscopy, it was shown that charge generation is independent of the composition ratio of the two materials when exciting the cadmium sulfide at 355 nm (Figure 1c). On the other hand, charge generation from P3HT is strongly composition dependent with smaller amounts of P3HT giving higher relative charge yields (excitation at 550 nm, Figure 1c). Using femtosecond transient absorption spectroscopy, it was shown that excitations in CdS are longer lived than excitations in P3HT. Charge generation from CdS is therefore not limited to small domain sizes. For excitation of P3HT, it was shown that there are two loss processes limiting charge generation: Some excitons decay without generating charges and there is some fast recombination between electrons in P3HT and holes in CdS across the hybrid interface. This part of the project therefore contributed to the first two research objectives. The use of an inorganic absorber with a broader absorption spectrum was suggested for the improvement of hybrid solar cells.

Another hybrid solar cell system, which was studied, consisted of mesoporous titanium dioxide substrates with metal sulfides as absorbers and organic hole conductors. In these systems, the metal sulfides transfer electrons to the TiO2 following excitations and holes to the hole conductor. Electrons are then transported through the TiO2 to a contact and holes through the hole conductor. Here, three different metal sulfides (cadmium sulfide, antimony sulfide and bismuth sulfide) were characterised and compared. Antimony sulfide and bismuth sulfide were identified as promising alternatives to cadmium sulfide as they have broader absorption spectra and are therefore able to harvest more of the solar spectrum (Figure 2a). Optimised antimony sulfide solar cells showed power conversion efficiencies of 3.5% with P3HT as the hole conductor and were significantly better than solar cells with cadmium sulfide due to a much improved photocurrent, matching the differences in light harvesting. Bismuth sulfide solar cells showed very poor performances with very limited photocurrent generation. These differences in performance were further elucidated using photoelectron spectroscopy in order to study the energy alignment and driving forces for charge separation in the solar cell (Figure 2b). Cadmium sulfide showed the largest driving force for both electron injection into titanium dioxide and for hole transfer to P3HT, resulting in a significant energy loss in these steps. Antimony sulfide showed a small but sufficient driving force for electron injection (200 meV) and a slightly larger driving force for hole transfer (400 meV). For bismuth sulfide there was no driving force for electron injection. This result explains why bismuth sulfide was not successful in solar cells in combination with titanium dioxide and suggest antimony sulfide as a good alternative solar cell material with minimised losses due to charge separation.

Antimony sulfide was used in the continuation of the project with the aim of constructing more efficient hybrid solar cells with new materials combinations. Films of mesoporous antimony sulfide were synthesised, which could absorb light as well as transport charges. Mesoporous TiO2 is no longer required when using these films and therefore energy losses due to the driving force of electron injection can be eliminated from the solar cell. Even though these films showed long-lived charge transfer yields to P3HT, no improvement in solar cell performance was achieved. Following on from this project, also dense layers of antimony sulfide were fabricated and tested in solar cells with P3HT as a hole conductor. These solar cells gave efficiencies of above 3%, therefore showing an improvement over cadmium sulfide as inorganic component of a solar cell and fulfilling the third research objective of the project of constructing more efficient hybrid solar cells. These solar cells follow a different working mechanism as other hybrid solar cells as the inorganic material is the main active component and can therefore also be classed as solution-processible inorganic solar cells. Such solar cells might offer a way of cheaply producing solar cells using stable inorganic materials.

The project has contributed towards furthering the understanding of hybrid solar cells and determining how to fabricate efficient hybrid solar cells. This understanding will help with future developments of hybrid and solution-processed inorganic solar cells and can therefore contribute to eventually bringing alternative, cheap solar cells into production.