Non-fullerene acceptors have significantly boosted the efficiencies of organic solar cells (OSCs) in the past few years. State-of-the-art OSCs have achieved a certificated power conversion efficiency of 17.4%. In spite of significant professes, there is still a gap between efficiencies of OSCs and those of traditional inorganic solar cells and emerging perovskite solar cells. One of the important reasons for this gap is the large voltage losses for OSCs. Understanding and reducing the voltage losses is of critical importance for further improving the performance of the OSCs. This thesis studies the voltage losses of OSCs based on non-fullerene acceptors.

The charge transfer (CT) state plays a critical role in the open-circuit voltage (VOC) of the OSCs. According to the reciprocity relation between the electroluminescence (EL) and the external quantum efficiency of solar cells (EQEPV), we know that the sub-bandgap absorbance (responsible for large radiative recombination voltage losses) and the weak emission of CT states (responsible for large non-radiative voltage losses) are the reasons for large voltage losses in fullerene-based OSCs. In addition, the driving force, defined as the difference between the energy of the singlet states and CT states, was considered to be essential for efficient charge generation, especially when the OSC field was dominated by fullerene acceptors. A series of polymer: non-fullerene pairs with different driving forces were studied by spectroscopy methods e.g. Fourier-transfer photocurrent spectroscopy (FTPS) and electroluminescence spectroscopy. It was demonstrated that both radiative recombination voltage loss and the non-radiative energy loss can be suppressed by reducing driving forces, resulting in overall decreased voltage losses of the OSCs.

Another question regarding the trade-off between the voltage losses and charge generation is still under debate – is the driving force essential for the efficient charge separation? A novel polymer: non-fullerene system with negligible offsets between both the lowest unoccupied molecular orbital (LUMO) and the highest unoccupied molecular orbital (HOMO) of the donor and acceptor was studied. Although the driving force for the new system is small, it works efficiently. It implies that efficient charge generation can occur with negligible driving forces for both electrons and holes, suggesting that the high VOC and efficient charge generation can be achieved at the same time for non-fullerene OSCs.

In addition to binary OSCs, the voltage losses in ternary OSCs are also studied in this thesis. It was found that the VOC of the ternary organic solar cells cannot be well interpreted by the widely used alloy or parallel model. The non-radiative voltage loss, which is not paid much attention in the two models, was found to play an important role in the tunable VOC of the ternary OSCs. We demonstrate that the non-radiative voltage losses in ternary OSCs is dependent on the radiative recombination rates and the energy levels of the CT states of the two constituting binary OSCs. Furthermore, the aggregation of the individual components can be decreased by adding the third component, suppressing the aggregation caused quenching and leading to a reduced non-radiative recombination voltage loss.

The non-fullerene based OSCs with small voltage losses show great potential for indoor applications. Although it might be difficult for OSCs to compete with commercial silicon solar cells for harvesting the solar energy, we demonstrate highly efficient and stable non-fullerene OSCs under indoor light, providing a unique application possibility where OSCs can out-compete other photovoltaic technologies. For the indoor application, the OSCs takes advantage of the easily tunable absorption range of the organic semiconductors, and avoids their drawbacks of the instability under strong outdoor light containing ultraviolet light.

Organiska solceller (OSC) har väckt mycket uppmärksamhet tack vare dess unika egenskaper såsom hög flexibilitet, låg vikt, möjlighet till lösningsmedelsbearbetning i skalbar rulle-till-rulle teknik samt stor ett stort urval av aktiva material. Dessa egenskaper ger OSC stora fördelar i applikationsområden såsom bärbar elektronik och arkitektur. Tillverkning via rulle-till-rulle teknik möjliggör en minskad energiförbrukning samt ett minskat utsläpp av koldioxid och diverse föroreningar. Via organisk syntes kan halvledarens egenskaper, primärt bandgapet, lätt ändras vilket gör OSC till konkurrenskraftiga kandidater även för inomhusapplikation.

Trots att OSC:s effektivitet har uppnåt 17,4% till följd av den snabba utvecklingen inom fältet, finns det fortfarande gott om utrymme för förbättringar av tekniken för att brygga effektivitetsgapet till traditionella oorganiska eller de nya perovskitbaserade solcellerna.

Solcellernas effektivitet bestäms av kortslutningsströmmen (JSC), fyllnadsfaktor (FF) samt öppenkretsspänning (VOC). Det huvudsakliga skälet till OSC:s låga effektivitet i förhållande till andra typer av solceller är den stora skillnaden mellan bandgapet och VOC, vilket ger upphov till en stor förlust av elektrisk spänning. Därför är det oerhört viktigt att förstå vilka faktorer som ger upphov till denna skillnad i syfte att minska spänningsförlusten.

Under de senaste åren har man visat att icke-fullerenbaserade OSC har en minskad spänningsförlust jämfört med de klassiska fullerenerna vilket banar en lovande väg för att öka OSC:s effektivitet. I denna avhandling utförs fundamentala studier rörande spänningsförlusten i nya icke-fullerenbaserade OSC. Forskningsresultaten i avhandlingen strävar att förbättra förståelsen för spänningsförlusten hos OSC: er vilket är ett fundament för att framställa nya OSC med förbättrade egenskaper och prestanda.